Inkjet printing of conductive materials: A review

Abstract

Purpose
– The purpose of this paper is to present an exhaustive review of research studies and activities in the inkjet printing of conductive materials.

Design/methodology/approach
– This paper gives a detailed literature survey of research carried out in inkjet printing of conductive materials.

Findings
– This article explains the inkjet printing process and the various types of conductive inks. It then examines the various factors that affect the quality of inkjet printed interconnects such as printing parameters, materials and substrate treatments. Methods of characterising both the inkjet printing process and the electrical properties of printed conductive materials are also presented. Finally relevant applications of this technology are described.

Originality/value
– Inkjet printing is currently one of the cheapest direct write techniques for manufacturing. The use of this technique in electronic manufacturing, where interconnects and other conductive features are required is an area of increasing relevance to the fields of electronics manufacturing, packaging and assembly. This review paper would therefore be of great value and interest to this community.

Full text

Inkjet printing of conductive materials:
a review
Gerard Cummins and Marc P.Y. Desmulliez
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, UK
Abstract
Purpose – The purpose of this paper is to present an exhaustive review of research studies and activities in the inkjet printing of conductive materials.
Design/methodology/approach – This paper gives a detailed literature survey of research carried out in inkjet printing of conductive materials.
Findings – This article explains the inkjet printing process and the various types of conductive inks. It then examines the various factors that affect the
quality of inkjet printed interconnects such as printing parameters, materials and substrate treatments. Methods of characterising both the inkjet
printing process and the electrical properties of printed conductive materials are also presented. Finally relevant applications of this technology are
described.
Originality/value – Inkjet printing is currently one of the cheapest direct write techniques for manufacturing. The use of this technique in electronic
manufacturing, where interconnects and other conductive features are required is an area of increasing relevance to the fields of electronics
manufacturing, packaging and assembly. This review paper would therefore be of great value and interest to this community.
Keywords Inkjet printing, Manufacturing, Microfabrication, Direct write, Interconnects, Material characterisation, Conductive ink, Printing industry,
Ink jet printers
Paper type General review
1. Introduction
Since its foundations in the nineteenth century (Young, 1805;
Laplace, 1806; Savart, 1833) inkjet printing technology has
become one of the most widely used printing techniques
(Le, 1998) in the publishing and graphics industries,
especially for applications requiring short runs of varying
information, such as the printing of “sell-by” dates,
customised packaging, tailored advertising, office and
desktop publishing. However, in recent years this technology
has become of increasing interest as a manufacturing
technique for the deposition of functional materials. In that
regard, this paper aims to review the various inkjet printing
technologies used and the types of materials deposited, with
emphasis on the parameters that define a good print.
The beginnings of inkjet printing began with Felix Savart
who, in 1833, showed that the breakup of liquid jets into a
series of repeatable drops is governed by the laws of fluid
dynamics (Savart, 1833). Although reported earlier by both
Young (1805) and Laplace (1806), Savart neglected the role
that surface tension plays in the formation of drops. In 1856,
Plateau published an article about the formation of jets from
circular nozzles (Plateau, 1856) and later derived the
relationship between jet diameter and drop size. A series of
papers, written in 1878 by Lord Rayleigh on the instability of
jets, where he described the breaking up of inviscid liquid jets
into streams of droplets by the application of a transient
pressure pulse to the nozzle (Rayleigh, 1878). The formation
of droplets from the breakup of viscous liquid jets was later
explained by Weber (1931).
These seminal articles eventually lead to the production of
one of the first commercially available inkjet devices in 1951
by Rune Elmqvist of Siemens-Elma (Elmqvist, 1951). This
device was integrated into the mingograph, one of the first
commercial inkjet recorders of analogue voltage signals for
medical applications.
Dr Sweet of Stanford University produced one of the first
Continuous InkJet (CIJ) printers in 1965 (Sweet, 1965;
Kamphoefner, 1972). In CIJ printing, a force is applied to a
liquid jet to induce the stream to breakup into a series of
droplets of uniform size and spacing. Some of these droplets
are charged as they pass through an electric field and steered
onto the desired location on the substrate to form the printed
image. The uncharged droplets landed in a gutter for
recirculation. This work was later commercialised by IBM,
which introduced the IBM 4640 printer in 1976. Besides the
complexity of CIJ systems this method is not usually used for
the printing of functional materials as the recycled ink may
have become degraded after exposure to ambient conditions.
An alternative to CIJ is drop-on-demand (DOD) inkjet
printing, where droplets are only formed and ejected when
required. The complexity of a DOD printer is significantly
reduced relative to CIJ as the need for droplet charging,
deflection and ink recirculation are eliminated. The Radio
Corporation of America produced one of the first DOD
systems in the late 1940s, which was intended for use in a fax
machine. The device generated the transient pressure waves
through the use of mechanical deformation of a piezoelectric
disc upon application of an electrical driving voltage.
This printer never went into commercial production
(Wijshoff, 2008).
In 1965, a patent was awarded to Mark Naiman of the Sperry
Rand Corporation for thermal inkjet printing (Naiman, 1965).
A typical thermal inkjet print head is shown in Figure 1. In this
device, current is passed through a resistive heater located in the
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ink chamber near the nozzle. The resistor superheats the ink up
to the bubble nucleation temperature. The bubble insulates the
ink from the heater, preventing further heating of the ink.
The expansion of the bubble causes a change in volume within
the ink chamber and forces the ink out of the nozzle, thereby
forming a droplet. The heater is switched off causing the bubble
to collapse, which generates a transient pressure wave.
The droplet separates from the nozzle due to the momentum
imparted to it by this pressure wave. The ink channel refills by
capillary action. The entire cycle occurs in approximately
10
m
s. Thermal inkjet print heads have been sold by Hewlett-
Packard and Canon under the Thinkjet and Bubblejet brands.
These print heads are not generally used for the printing of
functional materials as the thermal cycling may lead to
degeneration of the ink and loss of the desired functional
properties.
In 1972 Zoltan of the Clevite company published a patent
for a DOD system composed of a hollow tube of piezoelectric
material (Zoltan, 1972). As shown in Figure 2, a voltage is
applied to these piezoelectric electrodes causing them to
contract, squeezing the ink chamber and forcing a droplet out
of the nozzle. Similarly designed print heads are referred to as
squeeze-mode print heads.
In bend-mode print heads, the bending of the ink chamber
wall induces droplet ejection. Such print heads have been
described in patents by Stemme of Chalmers University
(Stemme, 1972) and Kyser and Sears from the Silonics
company (Kyser and Sears, 1976). As shown in Figure 3 the
bend-mode DOD print head described by Kyser and Sears
consisted of a small rectangular chamber with a supply tube at
one side and exit for the nozzle at the opposite side. A 0.7 cm
diameter piezoelectric disk is mounted on one side of the
chamber. The electrically pulsed disk electrically causes
the chamber to flex inwardly, which reduces the volume of
the chamber. The sudden reduction in volume generates the
pressure pulse required to push a droplet out of the nozzle. Spot
sizes between 0.1 and 0.65 mm were produced on paper and the
jetting frequency was approximately 700 Hz. Bend-mode print
heads have been sold by Tektronix, Xerox, Kyocera and Epson.
The push mode piezoelectric DOD print head was patented
in 1984 by Stuart Howkins of the Exxon Company (Howkins,
1984). Push mode print heads, also called bump-mode print
heads, have a piezoelectric rod placed next to a membrane, as
shown in Figure 4. Electrical excitation of the element causes
it to expand and push against the ink chamber wall thereby
expelling a droplet. Trident, Brother, Hitachi and Epson have
further developed push mode piezoelectric print heads.
The last type of piezoelectric DOD print head to be
developed is the shear mode print head, which was patented by
Kenneth Fischbeck and Allen Wright of the Xerox Corporation
Figure 1 Schematic of a thermal inkjet print head
Figure 2 Schematic of a squeeze-mode print head
Figure 3 Schematic of a bend-mode print head
Figure 4 Schematic of a push mode print head
Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
Volume 38 · Number 4 · 2012 · 193 – 213
194

in 1986 (Fischbeck and Wright, 1986). Shear mode print heads
are designed such that shear deformation in the piezoelectric
element is used to deform the upper half of the channels, as
seen in Figure 5. This deformation is mirrored in the lower
half of the channel forcing the channel into a chevron shape.
Flexing of the channel induces droplet ejection. Xaar and
Fujifilm Dimatix sell shear mode print heads.
As seen above, the two most dominant DOD print head
technologies are based on thermal and piezoelectric effects.
Initially, thermal print heads were more dominant due to the
low cost of production, but further development of
piezoelectric technology; the associated drop in cost of
manufacturing in addition to its ability to deposit a wider
variety of materials on various substrates have levelled the field
between the two DOD print head technologies. Piezoelectric
print heads are more commonly used when printing functional
materials, as there is no risk of thermal degradation of the ink
or need to use only solvents with a specific nucleation
temperature.
Some of the earliest uses of inkjet printing of conductive
materials include the work by Vest et al. (1983), Teng and Vest
(1987) and Wallace (1989). The first demonstration of high
resolution piezoelectric DOD inkjet printing of a functional
material was conducted by Sirringhaus et al. (2000) with
minimum feature sizes of 5
m
mthroughtheuseof
hydrophobic surface treatments.
Before the adoption of inkjet printing, the deposition of
functional soluble materials was carried out by screen-printing
and spin coating. Since then inkjet printing has been used to
write electrical interconnects using a variety of conductive
inks on various flexible and rigid substrates. Applications that
have used this direct write technology include printed circuit
board (PCB) manufacturing, radio frequency identification
(RFID) tags, solar cells, batteries, displays and many more.
Inkjet printing, like some other printing techniques,
enables low cost, low material waste, low temperature and
flexible production of electronic components. It should not
be considered a replacement to conventional silicon
microfabrication techniques, where feature sizes of 28 nm
can currently be reached. The narrowest feature line width
obtained with inkjet printing without the use of surface
treatments lies between 14 and 25
m
m (Meier et al., 2009),
a situation comparable to the semiconductor industry in the
1970s. Furthermore, the manufacturing technology is
immature and the development of functional inks with
properties comparable to those found in the semiconductor
industry has yet to be realised.
2. Conductive inks
Different types of conductive inks include colloidal
suspensions of nanoparticles, organometallic compounds in
solution and conductive polymers (Bidoki et al., 2007).
Whatever the conductive component or solvent used these
inks will often contain other constituents such as dispersants,
adhesion promoters, surfactants, thickeners, stabilizing agents
and other additives (Gamota et al., 2004; Magdassi, 2010).
The various conductive inks are reviewed in the following
sections.
2.1 Nanoparticle inks
Nanoparticle inks are a suspension of nanoparticles either in
water or an organic solvent such as toluene, ethylene glycol or
cyclohexanone. The solvent chosen must readily evaporate
once deposited but not so quickly that it dries out at the
nozzle when idle for short periods of time and forms a viscous
film that prevents drop ejection. Nanoparticle inks are widely
used as the nanoparticles can be produced in large quantities,
dispersed in high concentrations and produce relatively good
electrical conductivities. As a general rule of thumb, the
particle size should be less than 100th the size of the nozzle;
particles should also have a narrow distribution of sizes and be
homogeneously distributed throughout the solution to ensure
good jetting.
The high surface area to volume ratio of these particles
enables sintering at temperatures lower than that of the bulk
material. For example, gold nanoparticles with diameters less
than 5 nm are predicted to melt at 300-5008C, which is
considerably lower than the 1,0638C required to melt bulk
gold (Magdassi, 2010).
However, these inks are susceptible to agglomeration of the
suspended particles, which leads to an increase in viscosity
and can lead to clogging of the print head nozzles. The use of
specialized polymer coatings on the nanoparticles can prevent
agglomeration by ensuring stability and dispersion of the
particles. These coatings are usually removed through the use
of high sintering temperatures (.3008C) making them
unsuitable for use with flexible substrates. The use of a
photonic or microwave flash sintering step in conjunction
with a thermal sintering step may enable the removal of this
organic coating at lower temperatures.
The electrical conductivity and air stability of these
materials are high relative to conducting organic polymers.
However, the conductivity of sintered nanoparticle metals
rarely matches those achievable with the equivalent bulk form
(Gamota et al., 2004; Kamyshny et al., 2011). Although
nanoparticles have been widely used to manufacture silver
and gold inks they are not limited solely to these materials.
Silver and gold are widely used due to their high chemical
stability, low chemical reactivity and high electrical
conductivity. Copper and nickel inks have also been
produced, however their tendency to oxidise can affect the
lifetime of the ink or require the use of additional specialized
coatings or printing in inert atmospheres.
2.2 Organometallic inks
The reduction of organometallic inks to the metallic species is
carried out either optically or thermally. These inks have the
Figure 5 Top down schematic of a shear mode print head
Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
Volume 38 · Number 4 · 2012 · 193 – 213
195

advantage of being in the form of a solution and not a
particulate suspension, thereby removing the risks of
agglomeration and subsequent nozzle blocking. Higher
conductivities have been achieved using organometallic inks
compared to nanoparticulate equivalents as the nanoparticles
can be formed on the substrate (Tekin et al., 2008).
Conductivity values between 11 and 53 per cent of bulk
values have been reported for some silver inks after sintering
at temperatures of 1508C and above. Organometallic inks can
be reduced and sintered at temperatures of less than 1508C
(Dearden et al., 2005), which is generally lower than that
reported for sintering nanoparticles in organic solvents (Smith
et al., 2006). Silver is one of the more commonly produced
organometallic inks (Smith et al., 2006) as it can form a wide
range of compounds soluble in organic solvents. Other inks
have been formulated using platinum (Cummins et al., 2011),
gold (Nur et al., 2002), copper (Rozenberg et al., 2002),
nickel (Rivkin et al., 2002) and aluminium (Curtis et al.,
2009). Organometallic precursors have also been reported.
2.3 Conductive polymers
Conductive polymers such as PEDOT, polypyrrole and
polyaniline have also been used with inkjet printing.
A common feature of these materials is the presence of a
conjugated pi-electron system present throughout the polymer,
which gives them their conducting properties. These polymers
have been widely used in sensing applications as their electrical
or electrochemical properties can be sensitized to specific
environmental or chemical factors by using an appropriate
conducting polymer backbone or molecular dopant (Weng
et al., 2010; Persaud, 2005). These materials have also been
used in applications such as electrochromic displays (Mortimer
et al., 2006), batteries (Ryu et al., 2007), fuel cells (Wegner,
2006) and anti-static layers (Angelopoulos, 2001).
These polymers typically have lower conductivities than
metallic inks (Gamerith et al., 2007), and may require the use
of inert atmospheres due to their high susceptibility to ambient
humidity and their reactivity with oxygen. Inkjet printing is
further complicated as these materials can exhibit non-
Newtonian behaviour (Meixner et al., 2008). Under these
conditions, a droplet of polymer solution remains attached to
the nozzle by a long tail for several hundred microseconds.
The tail will eventually detach from the nozzle when a pinch
point is exceeded and will either collapse into the main droplet
or disintegrate into a series of satellite droplets. The cause of
the non-Newtonian behaviour is not solely attributed to
increased polymer concentration in the ink but is also due to
elastic stresses caused by extensional flow in the nozzle.
The conductivity of these materials is susceptible to the
substrate roughness. Xia and Friend produced photovoltaic
devices using either spin coating or inkjet printing and showed
that the devices prepared with inkjet printed polymer films had
better performance and higher efficiency than those produced
with spin coating. This was attributed to the finer phase
separation that occurred due to the rapid drying of inkjet
printed droplets (Xia and Friend, 2005).
2.4 Other types of conductive inks
Graphene oxide
Graphene oxide inks have also been produced (Eda and
Chhowalla, 2010; Guo et al., 2011; Jo et al., 2011; Huang
et al., 2011; Eda et al., 2008). This material can be easily
dispersed in water and the high electrical conductivity
associated with graphene can be partially restored by
chemical or thermal reduction (Dai et al.,2011),as
reduction of graphene oxide to graphene only affects the
surface layer of graphene oxide (Li et al., 2008). This leaves
the underlying material as an insulator. Thermal reduction
has been shown to be a more effective reduction mechanism
than chemical reduction, but it can involve high temperatures
(.5508C), which prohibits its use for flexible substrates or
CMOS electronics (Eda et al., 2008). Graphene oxide when
printed can form a polycrystalline film, where the single
crystals are the individual flakes of graphene oxide previously
in suspension. The unique properties of these films make it of
increasing interest for a wide variety of solution processing
based applications such as transparent conductors (Hecht and
Kaner, 2011), photovoltaics (Yun et al., 2011) and chemical
and biological sensors (Huang et al., 2011; Shao et al., 2010;
Robinson et al., 2008; Dua et al., 2010).
Carbon nanotubes
Another conductive material that has been printed are carbon
nanotubes (CNTs). This material is considered a good
alternative to copper for future interconnects due to its
tolerance to higher current density (.1£10
9
Acm
22
) and
thermal conductivity (3,000-5,800 Wm
21
K
21
)(Banerjee
et al., 2008). CNTs consist of one or more sheets of
graphene rolled up into hollow cylinders. The number of
sheets and the direction these sheets have been rolled up
determine the properties of the CNTs. Single-walled CNTs
(SWCNTs) are composed of a single shell of graphene and
typically have diameters ranging from 0.4 to 4 nm. Multi-
walled CNTs (MWCNTs) are composed of multiple,
concentric shells of graphene and can have diameters ranging
from several to tens of nanometres. MWCNTs always have
metallic properties due to the direction they get rolled up.
SWCNTs can be rolled up in multiple ways, which can lead to
some having semiconducting properties and others having
metallic properties. CNTs are usually grown using chemical
vapour deposition (CVD) but other production methods such
as arc discharge and laser ablation do exist. Further
information about CNT synthesis and properties can be
found in reviews by Hu et al. (2010) and Popov (2004).
The deposition of these CNTs using solution based
processing techniques such as inkjet printing has also been
investigated. A low temperature solution based process has
many advantages: it is compatible with plastic substrates not
suitable for CVD, there is no need for expensive vacuum
systems, selective deposition of CNTs and increased device
throughput can be achieved with roll-to-roll printing.
However, dispersion of CNTs in solution is a major
challenge for inkjet printing or any other solution based
processing technique. CNTs have a tendency to stick together
and form large bundles due to their large aspect ratios,
making them susceptible to large van der Waals forces
(Hu et al., 2010). The formation of these bundles can impair
electrical conductivity as current only flows on the outermost
tubes of the bundle. Various methods of dispersing the CNTs
without the use of harsh conditions or chemistries that would
lower their electrical conductivity have been proposed. These
include the addition of surfactants, polymer coatings, CNT
functionalization and other methods. The use of surfactants is
the most widely used method to maintain the separation of
CNTs in solution (Rastogi et al., 2008).
Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
Volume 38 · Number 4 · 2012 · 193 – 213
196

Reactive inks
An alternative means of metallising a surface using inkjet
printing involves the deposition of two inks, which react to
form the metallised pattern (Johnson and Dameral, 2003;
Teranishi, 2002). The main advantage of this reactive inkjet
printing technique is that it allows different materials to be
selectively produced or removed. The method has been
successfully used to fabricate silver-silver chloride electrodes
(Bidoki et al., 2007), etch silicon dioxide (Lennon et al.,
2009) and form cadmium sulphide films on paper substrates
(Teranishi, 2002).
2.5 Properties of inks
The most important parameters for selecting an ink are its
fluid properties such as viscosity, density and surface tension.
These properties dictate the “jettability” of the material,
influence the size and shape of the deposited droplets and
indicate the wetting of the substrate and the presence of
satellite drops. The viscosity of the ink must be low enough to
enable the refill of the ink reservoir in about 100 ms and the
expulsion of a drop out of the nozzle by the transient pressure
pulse. The surface tension must be high enough to prevent
unwanted dripping from the nozzle but low enough such that
the ejected droplet can break away from the nozzle.
The recommended values of these properties vary from
printer to printer but generally for inkjet printing the viscosity
should be within the range of 1-25 mPa · s and the surface
tension would be between 25 and 50 mNm
21
.
The effect of these fluid properties and other factors such as
the nozzle diameter on the “jettability” of the material can be
quickly assessed using the dimensionless Reynolds (Re),
Weber (We ) and Ohnesorge (Oh) numbers. The Reynolds
number is the ratio of the inertial forces to the viscous forces
and is defined by:
Re ¼
r
VL
h
The Weber number is the ratio of the inertial forces to the
surface tension forces and is calculated as:
We ¼
r
V2L
s
The Ohnesorge number describes droplet formation and is the
ratio of the viscous forces to the surface tension and inertial
forces such that:
Oh ¼ffiffiffiffiffiffiffi
We
p
Re ¼
h
ffiffiffiffiffiffiffiffiffi
r
L
s
p
where
r
,
h
,
s
,Land Vare the density, viscosity, surface
tension, nozzle diameter and droplet velocity, respectively.
Fromm was one of the first to use dimensionless numbers to
characterize the suitability of inks for drop formation (Fromm,
1984). He proposed that if the reciprocal of the Ohnesorge
number, Z, is larger than 2 then the generation of stable
droplets could occur. This was further refined by Reis and
Derby (2000), who showed using numerical simulations that a
value of Z such that 1 ,Z,10 is acceptable for stable droplet
generation.
The Weber and Reynolds numbers can also be used to
define a window of jettability for inks to be used in DOD
printing as shown in Figure 6 (Derby, 2010). The validity of
this method has been demonstrated for a wide variety of inks
(Derby and Reis, 2003).
3. Principles of inkjet printing
Inkjet printing relies on the generation of sequences of
droplets. This can be achieved either by continuous (CIJ) or
DOD inkjet. Only DOD printing will be considered from here
onwards, since CIJ is not commonly used in printing for
manufacturing because many functional inks can be affected
by environmental exposure that occurs during ink recycling
from the gutter.
The DOD inkjet printing process consists of five stages:
drop ejection, drop flight, drop impact, drop spreading and
drop solidification (Singh et al., 2010). The minimum feature
size depends on drop volume, placement accuracy and
substrate-ink interactions. A greater understanding of these
five stages improves the quality and reduces the line width of
printed features.
3.1 Droplet ejection and flight
Droplet ejection involves the generation and breaking away of
drops at the print head. Droplets are formed when an
electrical signal is applied to a piezoelectric element contained
within the ink reservoir. The sudden change in volume caused
by the flexing element induces a transient pressure wave that
travels along the capillary to the nozzle where fluid is pushed
outwards to form a drop. The droplet deforms to form a thin
neck between the droplet and the nozzle plate. The droplet
breaks away from the nozzle plate if the kinetic energy
imparted by the pressure wave is sufficient to overcome the
surface tension keeping it attached to the nozzle. The droplet
velocity is determined by the amount of kinetic energy
transferred. The velocity of the droplet must be several metres
per second to overcome drag due to air.
The performance of the droplet ejection process can be
characterized by responses such as the droplet velocity, volume,
consistency, shape and directionality. These parameters are
affected by factors such as the fluid properties, nozzle geometry
and waveform design (Kwon, 2009, 2010; Shin and Sung,
2009; Gan et al., 2009; Liou et al., 2010; Wijshoff, 2010;
Tekin et al., 2008). The effects of these factors have been
the subject of many experimental (Dong et al., 2006a, b)
Figure 6 Graphical means of assessing ink suitability using non-
dimensional numbers
Source: Reproduced from Derby (2010)
Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
Volume 38 · Number 4 · 2012 · 193 – 213
197

and computational studies (Bogy and Talke, 1984; Xu and
Basaran, 2007).
The size, shape and volume of the ejected droplets are
affected by factors such as the nozzle size, fluid viscosity and
surface tension, as demonstrated by the work of Fromm,
Derby and Reis and others. As explained previously, if the
reciprocal of the Ohnesorge number, Z, which is defined by
the ink viscosity, surface tension and density, as well as the
nozzle diameter, is between 1 and 10 then stable droplet
generation can occur for this ink. Satellite droplets are formed
beyond the upper limit of this range, whilst the droplet will
not break free from the nozzle below this range.
The viscosity of the fluid impacts the operational jetting
frequency and hence the printing speed. Reservoir filling rate is
decreased for highly viscous ink, which reduces the jetting
frequency. High frequency jetting can potentially cause
unstable droplet ejection if the ink is not viscous enough
since the transient pressure wave will not have been sufficiently
dampened in time for the arrival of the next pulse. This can
result in the previous and current waves interfering with each
other in the print head leading to unstable droplet ejection
(Antohe and Wallace, 2002).
The Ohnesorge values required for stable DOD inkjet
printing (Oh ,1) demonstrate the importance of surface
tension to droplet formation and ejection. Fluid/air surface
tension at the nozzle can prevent droplet generation and
ejection under certain conditions. Duineveld indicated that
We .4 for the droplet velocity to be sufficient enough to
overcome surface tension:
We ¼vmin ffiffiffiffiffiffi
r
L
s
r.4
The minimum velocity for droplet ejection should therefore be:
vmin ¼ffiffiffiffiffiffi
4
s
r
L
s
Droplet velocity should be between 1 and 30 ms
21
depending
on the fluid properties of the ink (Derby, 2010; Yarin, 2006).
Too high a velocity can result in splashing of the droplet upon
impact with the substrate. Too low a velocity and the droplet
will be decelerated by air drag before it impacts the surface.
The droplet shape should be nearly spherical. Sometimes a
long tail may form from the droplet due to the droplet ejection
conditions. This tail may eventually collapse back into the
body of the main droplet or it may separate and break into a
stream of smaller droplets, called satellite droplets.
The avoidance of such droplets is a commonly encountered
challenge during drop ejection and flight. Satellite droplets are
unwanted and can lead to poor quality phenomena such as
unwanted drops and blurring of line edges and even faulty
device behaviour.
Smaller nozzle diameters can produce droplets with lower
volumes. Current print heads produce droplets with volumes
between 1 and 100 pL. The smaller the droplet volume, the
smaller the minimum feature size achievable. However, as
droplets get smaller, the energy required for them to break
away from the nozzle plate increases due to increased inertial
and capillary forces. These smaller droplets also decelerate
quicker, requiring that the distance between the print head
and substrate be reduced (Magdassi, 2010).
The shape, intensity and frequency of the electrical
excitation signal also affect the characteristics of the formed
droplets. There have been several experimental studies on
waveform design but the results of these are often print head
specific (Hwang et al., 2007; Kwon, 2010; Dong et al., 2006b;
Chen and Basaran, 2002). The acoustic response of the print
head affects the waveform design. Optimal waveform design
can be used to generate droplets with diameters an order of
magnitude less than that of the nozzle (Chen and Basaran,
2002). The desired size of the droplets depends upon the
application. Larger droplet sizes result in quicker printing for
large area applications, whereas small droplet sizes are ideal
for applications where high resolutions and small dimensions
are needed but are more susceptible to deflection by air
currents. Incorrect design of the waveform can result in a
range of undesirable phenomena such as puddle formation on
the nozzle plate, which can prevent ejection or satellite droplet
formation resulting in imprecise drop deposition and poor
print quality.
Accurate and reproducible droplet positioning requires that
the path of the droplets be perpendicular to the substrate. As a
rule of thumb, the drop placement accuracy should be one
tenth of the line width. The distance between the nozzle and
the substrate is called the throw distance. Long throw distances
can cause increased inaccuracy of droplet placement due air
currents, drop velocity, variation of nozzle behaviour over time
and other factors.
The volatile components of the ink at the nozzle may
evaporate if the nozzles are idle. This can lead to a higher
viscosity of the ink at the nozzle compared to that in the
reservoir. This disparity can lead to clogging of the nozzle or a
shift in the properties required for droplet formation and
ejection. This is referred to as the “first drop problem” (Famili
et al., 2011; Calvert, 2001). Regular cleaning of the nozzles
and capping of the print head when not in use to ensure
constant wetting of the nozzles can address this problem.
3.2 Droplet impact and spreading
Droplet impact and spreading affect the minimum feature size
that can be achieved. Droplet deposition can be divided into
two stages. The first stage of droplet deposition is an impact
driven stage of less than 1
m
s duration, where the kinetic
energy of the impact is partially dissipated by viscous forces.
The rest of this energy is converted into surface energy, which
spreads the droplet to a diameter determined by the relative
surface energy between the ink and the substrate in the
second stage of the deposition process (Stringer and Derby,
2009; Yarin, 2006). If the initial kinetic energy of the droplet
is high then the droplet may overshoot the diameter, causing
the droplet diameter to oscillate until the energy is dissipated,
whereupon the droplet diameter stabilises. The surface energy
driven stage takes place between 0.1 and 1 ms after impact.
Dimensionless parameters such as the Reynolds, Weber and
Ohnesorge numbers can be used to determine which of these
two stages dominates the overall behaviour of a liquid drop on
a surface. Schiaffino and Sonin demonstrated that the Weber
number indicates the dominant force of the spreading process
(Schiaffino and Sonin, 1997). At a high Weber number the
droplet is pushed radially outwards by the impact induced
pressure gradients, whereas capillary force pulls the droplet at
a low Weber number. The Weber number can also identify the
point at which droplet behaviour switches between these
two stages.
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The ink’s resistance to spreading can be characterised by the
Ohnesorge number. A high number indicates that resistive
force is due to viscosity, whereas for a low number it is due to
inertial forces. Schiaffino and Sonin devised a graphical means
of identifying the driving forces for droplet spreading.
The graph is shown in Figure 7 and is defined by axes for
the Ohnesorge and Weber numbers. The parameter space for
good DOD inkjet printing is represented by the shaded area.
The effect of gravitational forces on the droplet, if any, can
be assessed using the Bond number, which is defined as:
Bo ¼
r
gL2
s
The Bond number is usually 10
22
to 10
23
for inkjet printing,
so it can be assumed that gravitational forces are negligible
(Stringer and Derby, 2009). Under these conditions a droplet
of a given volume will form a truncated spherical cap with
contact angle
u
on the substrate upon reaching equilibrium.
This contact angle is defined by the Young-Dupre equation:
cos
u
¼
s
SV 2
s
LS
s
LV
where
s
SV is the interfacial energy between the substrate and
the air,
s
LS is the interfacial energy between the droplet and the
substrate and
s
LV is the interfacial energy between the droplet
and the air. Assuming that the volume of the ejected droplet is
conserved, the degree of droplet spreading can be calculated
using:
b
eqm ¼d
deqm ¼8
tanð
u
=2Þð3þtan2ð
u
=2ÞÞ

1=3
where
b
eqm is the ratio of the initial droplet diameter, d, to the
diameter of the droplet on the substrate at the equilibrium
contact angle, deqm.
The print head velocity and the diameter, pitch and contact
angle of the droplet are important factors in ensuring the
correct coalescence of the droplets. If the droplet pitch, p,is
larger than the diameter of the droplets on the substrate, deqm,
then as shown in Figure 8(a), a train of separate, isolated
droplets will be formed. As the droplet pitch is decreased the
droplets begin to overlap and coalesce, first forming a scalloped
line, such as that shown in Figure 8(b). As seen in Figure 8(c),
further reduction of the droplet pitch results in the edges of the
line becoming smoother and the width of line being more
uniform (Soltman and Subramanian, 2008). Derby and
Stringer derived a model for a bead of ink with a circular
cross section formed from the coalescing of a train of droplets
(Stringer and Derby, 2009). This dimensionless model was
defined as:
w
b
eqmd¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
p
d
3p
b
2
eqmðð
u
=sin2
u
Þ2ðcos
u
=sin
u
ÞÞ
s
where wis the width of the formed bead and pis the pitch
between droplets. The smallest bead size possible is equal to
the diameter of a single droplet at equilibrium on the substrate.
The maximum droplet pitch, pmax, which can produce lines
with parallel edges, is given by:
pmax ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
p
d
3
b
2
eqmðð
u
=sin2
u
Þ2ðcos
u
=sin
u
ÞÞ
s
Another limit to droplet pitch is encountered as the droplet
spacing and print head velocity are reduced (Duineveld, 2003).
A series of periodic bulges, such as those shown in Figure 8(d),
appear along the line at this limit. These bulges are connected
together by a ridge whose width is equal to the diameter of an
individual printed droplet d
0
. Duineveld investigated this
bulging instability and found that the newly deposited spherical
droplet will have a greater curvature at the liquid/air interface
than the printed cylindrical liquid bead when droplet spreading
Figure 7 Graphical representation of influence of ink properties on
droplet spreading
Figure 8 Examples of printed line morphologies
(a)
Source: Reproduced from Soltman and
Subramanian (2008)
(b) (c) (d)
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is slow and a small drop pitch and slow print head velocity are
used. This creates a Laplace pressure that drives liquid to the
front of the droplet from the printed bead and prevents the
depleting bead from spreading, as its contact angle is lower
than the advancing contact angle. Soltman and Subramanian
(2008) identified the dynamic nature of this limit by proposing
a qualitative parameter space with drop spacing on one axis and
time between droplets on the other. Stringer and Derby (2010)
built upon this work and using an analytical approach were able
to derive a non-dimensional formula to calculate the onset of
this dynamic instability. The maximum limit of the print head
velocity is given as:
U*
Tmax ¼
h
UTmax
s
LV
where UTmax is the maximum print head velocity possible with
the printer. The dimensionless limit for the drop pitch was
given as:
gp
*
max;
u
a

¼0:013832
b
3
eqmSsin
u
a
K1
u
a
sin2
u
a
2cos
u
a
sin
u
a

2
where
u
ais the advancing contact angle, K1is a constant and
Sis a function of the dynamic contact angle
u
cused to describe
the shape of the ridge and defined as:
S¼
u
c2sin
u
ccos
u
c
8ðsin
u
cþ
u
cÞ2
Stringer and Derby also plotted these equations to produce a
quick graphical method of determining the stability limits for
an ink/substrate system.
One commonly used method to ensure high resolution
features is by using surface treatments to increase the contact
angle between the ink and the substrate (Lim et al., 2006).
One widely used surface treatment involves the use of
fluorocarbon plasmas (Sele et al., 2005).
3.3 Solidification
A solid deposit is produced by the phase change of the ink due
to mechanisms such as evaporation or polymerization, for
example. Which mechanism occurs is determined by the type
of ink (Le, 1998) and whether the deposition process is an
impact (solidification) or surface energy (evaporation) driven
process (Schiaffino and Sonin, 1997; Deegan et al., 2000).
Solidification of the droplet is often accompanied by a
reduction in volume, which can be significant if solidification
is obtained due to solvent evaporation.
Solvent evaporation can influence the profile of the
solidified droplet depending on the distribution of the
solute. For example, the edges of evaporated droplets can
have a higher concentration of solutes with little or no solutes
elsewhere in the drop footprint. This ring like profile formed
at the perimeter of the dried droplet is commonly
encountered during the printing of colloidal suspensions
such as nanoparticle inks and is commonly referred to as
“coffee staining” (Adachi et al., 1995; Deegan et al., 1997;
Deegan, 2000; Shmuylovich et al., 2002; Fukai et al., 2006).
Coffee staining occurs due to the pinning of the contact
line, where the surface, air and liquid interfaces meet, of the
drying drop and the increased rate of evaporation of liquid at
the drop edges, which sets up an internal flow that deposits
solute at the edges. Rings formed from metallic nanoparticle
inks have been shown to be densely packed and capable of
conductivities equivalent to 14 per cent of bulk values when
formed at room temperature without additional sintering
(Magdassi et al., 2005). Multiple concentric rings of solute
can also occur if the contact line exhibits stick-slip motion
where the contact line is discretely pinned and depinned
(Adachi et al., 1995; Deegan et al., 2000; Shmuylovich et al.,
2002). Shmuylovich and Deegan showed that the size of any
particles suspended in the drying droplet determines where
they are deposited during the coffee staining effect. It may be
necessary to minimise this effect to reduce faulty device
behaviour. Several studies have developed means to prevent
coffee staining from occurring. These include decreasing the
substrate temperature (Kim et al., 2012a, b; Soltman and
Subramanian, 2008) to reduce edge evaporation, preventing
pinning by printing on Teflon in a solvent saturated
atmosphere (Deegan et al., 2000), using a solvent with a
high boiling temperature and low surface tension (Kim et al.,
2006) or reformulating the ink to use both a high boiling
point and low boiling point solvent (de Gans and Schubert,
2004; Park and Moon, 2006; Shin et al., 2011).
3.4 Sintering
Printed metals require a sintering stage after solidification to
create a continuous conductive path. Sintering also promotes
grain growth, formation and coalescence within printed metal
features, which further improve electrical conductivity and
mechanical adhesion. This is especially true for nanoparticle
inks, as they require sintering to form the connections
between neighbouring particles necessary for achieving a
highly conductive film. Often to form these conductive films it
is first necessary to either remove the organic coatings of
nanoparticles or decompose organometallic inks into metal.
Therefore, the lowest sintering temperature required is
typically that at which these organic materials are removed.
Although sintering is commonly carried out by heating
(Jung et al., 2007) it is not the only method that can be used.
Other techniques for sintering printed metals include
microwaves, plasmas and light sources such as lasers and
flash lamps. These techniques are explained in detail below.
Thermal sintering
Thermal sintering usually involves long sintering times,
typically above 30 min or temperatures in excess of 2008C.
These characteristics can limit their compatibility with some
polymer substrates and fast roll-to-roll printing. Thermal
sintering is not area specific and requires the entire substrate
to be heated. This can lead to warping and shrinking of some
substrates as well as gas emission. Additionally, cracks may
form in the printed layer due to the large reduction in volume
during sintering (Lee et al., 2010; Kang et al., 2010; Tobjo¨rk
et al., 2012). This effect may be especially exacerbated due to
either thermal coefficient of expansion mismatch between the
substrate and metal, pre-existing cracks (Bordia and Jagota,
1993), a non-uniform stress distribution or if the printed film
thickness is large (Kang et al., 2010).
Electrical sintering
Electrical sintering refers to the use of Joule heating to sinter a
nanoparticle film (Allen et al., 2008). A voltage is applied
across the printed film causing a current to flow through
the nanoparticle film, which leads to heat being generated
within the film. The initial resistivity and dimensions of the
patterned film determine the sintering voltage used.
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The advantages of this method are fast sintering times, which
can be less than 1 min, depending on geometry of the printed
area, reduced damage to the substrate due to area specific
heating and real-time monitoring of the sintering process
through current measurement. This ability to monitor while
sintering allows a desired conductivity to be repeatedly
achieved.
However, electrical sintering requires contact with the
printed film, which may not be ideal for some applications
and the localised nature of this method limits it to use with
small areas. It is also limited to use with narrow features such
as interconnects as thermal images from sintering experiments
have shown that electrical sintering forms sintered, narrow
conductive tracks between the two electrodes with the
surrounding nanoparticle film un-sintered (Alastalo et al.,
2011). This is obviously problematic for large areas, which
would require the use of more complicated, contactless AC
electrical sintering methods (Allen et al., 2011).
Photonic sintering
The use of lasers, flash lamps and other light sources is referred
to as photonic sintering (Yun et al., 2008a, b; Tobjo¨rk et al.,
2012; Rapp et al., 2011; Kang et al., 2011). Absorption of light
by the printed material causes heating through the dissipation
of non-radiative energy and exothermic photochemical
reactions. This heat is confined within the individual
nanoparticles and transferred to neighbouring particles
leading to localised, rapid heating of the film. This heating
causes evaporation of solvent and eventually sintering of
adjacent particles.
Flash lamps have been used to sinter silver and copper
nanoparticle inks (Yung et al., 2010; Kang et al., 2011;
Kim et al., 2009a, b). Yung et al. (2010) showed that flash lamps
directly sinter the surface of the printed nanopar ticle film due to
the photo-thermal effect experienced by surface nanoparticles.
Heat transfer from the surface sinters the internal material of
the nanoparticle film. This results in a denser surface layer and
a relatively porous internal bulk (Lee et al., 2011; Yung et al.,
2010). Localised heating results in some heating at the
substrate/film interface, which was shown to improve film
adhesion with increasing flashes (Kang et al., 2011).
The effectiveness of the sintering process is determined by
factors such as the properties of the printed material (thermal
conductivity, light absorption), the thermal conductivity of
the substrate, the flash exposure time, lamp intensity and the
number of flashes. Flash durations are short, typically being
on the order of milliseconds for film thicknesses usually
achieved with inkjet printing and the intensity of flash lamps
are typically equal to or greater than 25 kW cm
22
(Tobjo¨rk
et al., 2012). Flash lamp sintering is ideal for large areas on a
variety of substrates; this, coupled with the short sintering
times, makes it an attractive alternative to thermal annealing.
Lasers have also been used to sinter printed metallic films
(Chung et al., 2004; Ko et al., 2007; Rapp et al., 2011; Cai
et al., 2011). This method is better suited for small areas, as
the spot size is typically between 20 and 40
m
m and the scan
speed can be as low as 0.2 mms
21
. However, the use of lasers
is particularly advantageous when compared to thermal
sintering due to the reduction in sintering time and the heat
affected zone, which can minimise damage to low
temperature substrates. The efficiency of laser sintering and
other photonic sintering methods depends on whether the
source wavelength is matched to the absorption spectra of the
metal nanoparticle.
Microwave sintering
Microwave sintering is a rapid sintering method which, as the
name suggests, involves the use of microwave radiation at
around 2.54 GHz (Perelaer et al., 2006). The penetration
depth of approximately 2
m
m at that frequency dictates the
thickness of the film that can be successfully sintered. Such a
thickness is not a limitation for most printed metal films. This
method is particularly attractive if polymer substrates are
used, as they are almost transparent to microwaves. This
ensures that the microwaves are absorbed mostly by the
metallic nanoparticles, leading to localised heating with
ensuing minimum damage to the substrate (Perelaer et al.,
2012). Schubert used microwave radiation to sinter silver
nanoparticle ink onto a polyimide substrate. The conductivity
of the sintered film was 5 per cent of bulk silver, which was
the same for a similar film sintered using conventional thermal
methods. However, microwave sintering only took 3 min,
down from the 60 min required by thermal sintering (Perelaer
et al., 2006).
4. Characterisation of the inkjet printing process
Characterisation of inkjet printer performance is required for
benchmarking, maintenance of the process over time, as well
as assessment of this technology for higher resolution and
novel functional inks. Various methods of printer
characterisation have been proposed. Such methods can be
classified in two categories. The first category observes and
quantifies droplet ejection, often through use of high-speed
video recording and image processing. The second category
characterises printer performance with regards to ink/
substrate interactions by examining printed test patterns on
the substrate. Each method has advantages and disadvantages,
which are described below. However, both characterisation
techniques are needed to cover the entire inkjet printing
process.
4.1 Droplet ejection
This method relies on measurement of the shape, volume,
velocity and consistency of the ejected droplets. Consistency
and stability of the droplet ejection process are important
indicators of performance for this printing technology.
Whereas some processes can tolerate a few failures over a
certain number of cycles, failures when printing conductive
inks may result in either the presence of discontinuities along
the electrical interconnects or the formation of shorts between
neighbouring interconnects if the flight of the droplet is not
perpendicular to the surface of the substrate.
Drop formation, ejection and impact normally lasts less than
100
m
s for typical micrometre scale droplets formed by DOD
inkjet printing. Processes occurring over such timescales are
commonly measured using stroboscopic illumination in
conjunction with high-speed camera recording, with the
strobe frequency set equal to the drop ejection frequency
(Dong et al., 2006a, b, 2007). Stroboscopic recording only
captures the reproducible aspects of droplet ejection and
formation, as the images formed are the result of integration
over many droplet ejection cycles. However, this is not a
disadvantage for most applications.
Image processing techniques are required to measure the
drop properties from these images. Drop velocity can be
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measured by noting the distance a drop travels between
successive frames. Drop volume can be inferred by measuring
the number of pixels and drop direction or position can be
found by calculating the centre of mass. Effects such as
refraction, droplet flattening during flight and fluid oscillation
may lead to errors creeping into volume or mass extraction of
up to 10 per cent. Despite this the data provided by this
method is invaluable when developing a process for ejection of
an ink.
Gravimetric methods have also been used to characterise
droplet ejection (Verkouteren and Verkouteren, 2009, 2011).
Droplets are ejected for a defined period of time at a known
ejection frequency onto a pan placed on top of a quartz crystal
microbalance. The change in mass of the pan before and after
ejection is divided by the number of droplets ejected to get the
average droplet mass. The initial variation in droplet mass due
to the first drop problem is averaged out and measurement
uncertainty due to droplet flattening, refraction, satellite
droplets and other geometrical factors are removed.
Uncertainty due to fluid evaporation can be minimised by
conducting the measurement inside a closed solvent saturated
environment. Total relative uncertainties with the Verkouteren
system have been quantified at around 1 per cent. This system
is not suitable for processes that result in unstable droplet
ejection and measurements of droplet shape, velocity and
direction still require the use of an optical system.
4.2 Substrate-ink interactions
Substrate-ink interactions are primarily affected by the
wetting of the ink on the substrate. This in turn determines
the optimal drop pitch, which affects the print quality. The
wetting of the substrate is usually measured in turns of the
contact angle between the droplet and the substrate. Contact
angle can be measured using a goniometer. Besides the static
contact angle, the advancing and receding contact angles
should also be measured due to their influence on drop
behaviour.
Droplet pitch has been shown to have a significant influence
over line width. Doggart suggested that the optimal drop
spacing can be determined by measuring the uniformity of the
line edge (Doggart et al., 2009). This was measured using the
ratio of the average line thickness of the centre 50 per cent of
the line to the average thickness of the outer 50 per cent of
line. An optimal drop pitch, where the thickness is uniform
along the length of the line would give a ratio of 1.
The graphics printing community have used computer
vision and image processing to develop various statistical
measurements (Streckel et al., 2003; Briggs et al., 2000), test
patterns (Forrest et al., 1998; Pascal and Patrice, 1999;
Guiping et al., 2010) and standards (Briggs et al., 1999) for
print quality. However, to the best of the authors’ knowledge
similar work has not been developed for functional material
printing.
5. Electrical characterisation of printed
conductive patterns
One of the most common methods of measuring the electrical
resistivity of printed films is the four-probe technique, which
is also referred to as Kelvin measurements. By using four-
probes instead of two the effect of parasitic cabling and
contact resistances are negligible compared to that of the
device under test (Schroder, 2006). Due to the difficulty in
measuring the thickness of thin films the resistivity is often
related to the sheet resistivity (R
S
), which is calculated as:
RS¼
r
t
p
lnð2Þ
DV
I
In the semiconductor industry the sheet resistivities of thin
films are usually measured using Van der Pauw or similar
electrical test structures (Enderling et al., 2006; Pynttari et al.,
2010). These structures require that the film be uniform in
thickness and resistivity.
The resistance of the structure shown in Figure 9, with the
contacts A-D is defined as:
RðAB;CDÞ¼VD2VC
IAB
The general Van der Pauw formula for a structure that is
symmetric along both axes with equally spaced contacts is
given by:
RS¼
p
lnð2ÞRðAB;CDÞ
The Greek cross structure shown in Figure 10 is a type of Van
der Pauw structure. This structure was designed to measure
the resistivity of discrete samples of semiconductor and
conductor materials, manufactured using microfabrication
techniques and be on the same scale as microelectronic
devices (David and Buehler, 1977). In addition, the design of
this structure is governed by rules that can be used to reduce
any error due to non-ideal and finite contacts that can occur
with traditional Van der Pauw structures. The ratio of arm
length (L) to width (W) should be at least a factor of 2 for
accurate sheet resistance measurements (Buehler et al., 1978).
The sheet resistance is calculated from this structure using
four resistance measurements. Two of these measurements
are taken at the 08orientation:
R0ðþIÞ¼VD2VC
IAB
R0ð2IÞ¼VC2VD
IBA
Figure 9 Van der Pauw sheet resistance test structure
Inkjet printing of conductive materials
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and two are taken at the 908orientation. This is necessary to
highlight any offsets in the measurement equipment that may
affect the accuracy of the results:
R90ðþIÞ¼VC2VB
IDA
R90ð2IÞ¼VB2VC
IAD
The average of these measurements ðRð^IÞÞ is used to
calculate the sheet resistance of the structure:
RS¼f
p
Rð^IÞ
lnð2Þ

Printed conductive films have also been characterised using
AC or high frequency electrical measurements. Unlike DC
measurements such as sheet resistance, these techniques can
determine the magnitude of loss mechanisms caused by
porosity, surface roughness and grain boundaries. This
information would be useful when printing RFIDs, which
are an increasingly common application of printed electronics.
Pynttari used transmission line measurements to characterise
the loss mechanisms in an inkjet printed co-planar waveguides
(CPW) and co-planar waveguides with ground (CPWG)
(Pynttari et al., 2010). This, and similar methods, have been
used for a variety of conductors and substrates (Pynttari et al.,
2007, 2008; Sillanpaa et al., 2009; Makinen et al., 2009).
A vector network analyser (VNA) with ground-signal-
ground (GSG) microwave probes was calibrated using the
line-reflect-reflect-match (LRRM) method to obtain the
complex propagation constant (
g
)andcharacteristic
impedance (Z
0
) of the line from the scattering parameter
measurements. The R, C, L, G transmission line parameters
were, in turn, extracted from the impedance of the line.
The conductor and dielectric losses are related to R and G,
allowing the separation of loss mechanisms. Varying printing
parameters, inks and substrates were found to affect the
surface roughness of the waveguide structures, which was
observable in the high frequency measurements obtained with
these patterns.
6. Applications for inkjet printing of conductive
materials
Inkjet printing of conductive materials has been used in a
wide range of applications, ranging from the manufacturing of
display backplanes, RF antenna, electronic packaging, solar
cells and sensors. Some of these applications are reviewed in
this section.
6.1 Sensors
Inkjet printing has been used to print a variety of sensors.
Besides the reduced cost of fabrication, the ability to deposit
small volumes of material accurately and repeatedly using
inkjet printing has enabled multi-analyte detection systems to
be easily fabricated (Beccherelli et al., 2010; Li et al., 2007).
The combinatorial nature of these systems can enhance the
selectivity and sensitivity towards different analytes.
The ability to deposit small volumes of material has also
been used to functionalise micromachined cantilevers for the
detection of various analytes (Bietsch et al., 2004; Ness et al.,
2012). In many applications the cantilevers are commonly
coated with a thin layer of gold on one side to improve the
optical detection of cantilever deflection and the adsorption of
functionalised layers using thiol chemistry.
Conductive polymers have been widely used in gas sensors
as these materials readily absorb gases into their interior
causing a change in their electrical or electrochemical
responses (Weng et al., 2010; Janata and Josowicz, 2003;
Persaud, 2005). The sensitivity and selectivity responses of
these materials can be tailored for different analyte volatile
compounds. Additionally sensors composed of these materials
display low power consumption and can operate at room
temperature (Bai et al., 2007) unlike metal oxide gas sensors
that require elevated operating temperatures. Various types of
gas sensors have been fabricated with these materials, ranging
from simple chemoresistors to more complicated chemFETs
and capacitive sensors. These will be discussed in the
following sections.
Resistor based chemical sensors
The structure of chemoresistors is as shown in Figure 11 and
consists of a narrow layer of conductive polymer on an
insulating substrate between two gold electrodes. They operate
by sensing a change in polymer conductivity due to the
donation or acceptance of charge carriers from an analyte gas.
These sensors can have time constants between tens of seconds
to minutes and often exhibit hysteretic behaviour, as the
response depends on changes of the bulk conductivity
Figure 10 Greek cross structure for sheet resistance measurement
Figure 11 Schematic of chemoresistor structure
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(charge mobility and concentration) due to the slow
penetration of gaseous species into the polymer. The recovery
time, linearity and sensitivity of conductive polymer sensors
were found to depend on the number of printed layers
(Crowley et al., 2008) and film morphology (Torsi et al., 2002).
Irrespective of the presence of the gas, the bulk conductivity
of these polymers is also affected by the thickness and
morphology of the printed film, which can affect the work
function at the metal-polymer interface and hence the
electrical behaviour of the sensor. The metal-insulator
interface can also be problematic if water vapour is present
during fabrication but steps such as use of a hydrophobic
surface treatment can be used to alleviate this issue.
Chemical fuse sensors, which are similar in structure to
chemoresistive sensors, were printed using ultra-thin films of
PEDOT:PSS to detect methanol and ethanol. These sensors
exhibit a sharp and irreversible change in conductivity upon
exposure to these gases due to a change in film morphology
that disrupts the current flow (Mabrook et al., 2005).
Response times of 8 and 6 s and sensitivities of the 0.05 and
0.03 per cent ppm
21
were measured for methanol and
ethanol, respectively, (Mabrook et al., 2006a). Methanol has
also been detected using polypyrrole sensors (Mabrook et al.,
2006b).
Electronic based chemical sensors
Various structures for chemFET sensors have been proposed,
such as those shown in Figure 12. Despite their structural
differences they all operate on the principle of work function
modulation. In thin film transistor ChemFETs, shown in
Figure 12(a), the current flows through the conductive polymer
whose conductivity varies based on interaction with the analyte
and/or the electric field. The response of this device is a
function of the changing conductivity and work function of the
polymer, which makes interpretation more difficult.
Such a problem does not exist if the polymer acts as the gate
conductor of a traditional silicon transistor, which is shown in
Figure 12(b). In this device the conductivity of the polymer has
no effect on the sensor response as the current flows through the
silicon channel. Hence the sensor response is determined only
by the effect of the analyte on the work function of the
conductive polymer.
Similar to chemFET sensors are devices utilising a
MOS capacitor structure, where the presence of an analyte
causes a change in the capacitance-voltage curve of the device.
Although such sensors have a simpler structure, they require
the use of more complicated circuitry in order to acquire and
extract data.
Sensing materials
Polyaniline (PANi) is a commonly used conductive polymer for
gas sensing applications. It is typically used for ammonia
sensing (Loffredo et al., 2007; Crowley et al., 2008) but its
sensitivity to different gases can be adjusted through the use of
additives. PANi has been modified to detect urea by blending it
with Nafion (Cho and Huang, 1998), carbon monoxide
(Wanna et al., 2006) and alcohol were detectable when it was
mixed with CNTs, ammonia and carbon monoxide when
mixed with titanium dioxide particles (Tai et al., 2007). Printing
alternate layers of PANi with copper chloride was used to
fabricate a gas sensor for detecting hydrogen sulphide (Crowley
et al., 2010).
Other conductive materials that have been used to print
sensors include CNTs, graphene oxide and noble metal
nanoparticles.
The large surface area of CNTs, in addition to the change in
electrical conductivity from exposure to reductive or oxidative
gases, makes them attractive for gas sensing applications
(Zhang et al., 2008; Wang and Yeow, 2009; Bondavalli et al.,
2009). CNT inks have also been used by various research
groups to print gas sensors for detecting analytes such as
methanol (Mabrook et al., 2009), nitrogen dioxide (Yun et al.,
2008a, b), carbon monoxide (Kim et al., 2009a, b) and
hydrogen sulphide (Ma¨klin et al., 2008). Graphene oxide has
been used in chemoresistive and electrochemical sensors
(Huang et al., 2011; Shao et al., 2010).
Silver and gold nanoparticles have also been printed to form
chemoresistors (Chow et al., 2009; Raguse et al., 2007;
Cooper et al., 2010), electrochemical sensors (Jensen et al.,
2011) and surface enhanced Raman spectroscopy (SERS)
based sensors (Yu and White, 2010). Raman spectroscopy
measures the frequency of photons scattered by molecules
adsorbed on the surface. The frequency of the scattered
photons corresponds to the vibrational energy of the
molecules. The signals produced by standard Raman
spectroscopy are too weak to be of use in sensing systems.
However, the use of a noble metal surface can significantly
augment the Raman signal to enable the detection of single
molecules (Cialla et al., 2012). Over the years SERS has been
utilised in various microsystems but these require expensive
microfabrication techniques. Recently, inkjet printing has
been used to print more affordable SERS sensors (Eshkeiti
et al., 2011; Yu and White, 2010). The sensitivity of the
Figure 12 Structure of conductive polymer chemFET sensors
(a)
(b)
Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
Volume 38 · Number 4 · 2012 · 193 – 213
204

sensor printed by Yu was sufficient to detect ten femto-moles
of a Rhodamine 6G dye. However, it is suspected that
printing parameters could be a factor in the variation in the
signal intensity measured across the sensor.
6.2 Solar cells
Solar cell production is one of the earliest applications of the
inkjet printing of conductive materials (Teng and Vest,
1988a, b). The process has been used to deposit the front
metallization (Teng and Vest, 1988b; Rivkin et al., 2002;
Curtis et al., 2006; Kaydanova et al., 2003; Liu et al., 2009;
Gizachew et al., 2011), to texture the surface for improved
efficiencies and to form openings to the semiconductor layers
(Lennon et al., 2008; Utama et al., 2008).
The collector grid pattern on the front side of the solar cell is
most often screen printed in industry. However, the ever-
increasing demand for increased fill factor has lead to a growing
interest in using inkjet printing instead, as this technique can
typically print narrower, thinner features thereby allowing
lower shadowing losses and lower manufacturing costs.
Inkjet printing has also been used to print transparent
electrodes onto the surfaces of solar cells, which can be used
to achieve greater energy conversion efficiencies as the entire
surface is exposed to the sun. Currently, most transparent
electrodes are fabricated by sputtering from an indium tin
oxide (ITO) target, which produces good quality, non-porous
thin films with a high optical transparency (.85 per cent),
low sheet resistance (20-30 V/A)andgoodadhesion.
However, with the increasing cost of ITO due to a
dwindling supply, expensive processing requirements and
the fact that sputtering can result in up to 65 per cent of a
target being unused, there is a need for a low material waste
deposition method such as inkjet printing.
Besides ITO (Jeong et al., 2010; Puetz and Aegerter, 2008;
Hong et al., 2008), other nanoparticle inks have been
developed for printing transparent electrodes and include
indium zinc tin oxide (Kim et al., 2012a, b) and antimony tin
oxide (Cranton et al., 2007). The sheet resistances and optical
transparency of thin films printed with these inks are not
comparable with what can be achieved by sputtering. ITO
inks can produce films with sheet resistances ranging from
202.7 (Jeong et al., 2010) to 400 V/A(Puetz and Aegerter,
2008) and optical transparencies of around 84 per cent or
more for visible wavelengths. Recent work by Kim with an
indium zinc tin oxide ink produced films with a sheet
resistance of 20.6 V/Aand an optical transparency of 81.26
per cent. However, the porous microstructure of the rapidly
annealed printed film was found to be the cause of the low
power conversion efficiencies (0.81 per cent) measured. Work
on printable transparent conducting oxides for solar cells is
however still in its infancy.
Other alternative materials for the inkjet printing of
transparent conductive electrodes include graphene, CNTs
and conductive polymers such as PEDOT:PSS. Graphene
can produce films with a lower sheet resistance, higher optical
transparency and greater mechanical flexibility than ITO films
(Jo et al.,2012).However,todatemosttransparent
conducting electrodes for solar cell applications have been
produced using CVD and not inkjet printing (Pan et al., 2012;
Wan et al., 2011; Jo et al., 2012) due to the lower sheet
resistance and improved film quality that can be achieved with
the CVD process. Some research groups have deposited
graphene oxide solutions using spin coating to form
transparent electrodes for solar cell applications (Becerril
et al., 2008; Wu et al., 2008), with post-reduction film sheet
resistances ranging from 100 to 1,000 V/Abeing reported.
The conductivity of the printed graphene films needs further
optimisation to meet the properties of ITO through further
improvements to the thermal reduction process as suggested
by several of these research groups.
CNT films also exhibit high transparency to visible light but
their conductivity does not match that of ITO because of the
high resistance between nanotube junctions and the need for a
certain nanotube density to form a conductive path (Pang
et al., 2011; Ho et al., 2001; Tapaszto
´et al., 2006; Dan et al.,
2009).
6.3 Microelectronic packaging
Printing techniques such as screen-printing are already widely
used in the assembly and manufacturing of microelectronic
packaging (Chang and Sze, 1996). Inkjet printing has several
characteristics that make it an attractive technology for
meeting the packaging needs of future products. The push for
ever decreasing packaging dimensions and increased packing
densities from the semiconductor industry could be partially
achieved by inkjet printing for applications such as wiring in
superfine packaging (Hayes et al., 1999; Mengel and Nikitin,
2010). The non-contact nature of inkjet printing would allow
novel packaging solutions for flexible electronics or non-
planar topologies. The flexibility and digital nature of inkjet
printing can enable rapid re-wiring of bare die to substrates
without the need expensive tooling changes (Miettinen et al.,
2008).
The use of inkjet printing in electronic packaging also offers
challenges. For example, packages are designed to prevent
device failure and they undergo multiple, well-established
tests such as temperature cycling to ensure error-free
operation over various environmental conditions. Any ink
used in such applications must not degrade under testing in
addition to matching viscosity, surface tension and density
required for inkjet printing. Inks used for packaging
applications should have a high thermal stability, a low
thermal coefficient of expansion and good adhesion to the
various materials commonly found in packages such as copper
and ceramics. The adhesion of the inks is related to the
contact angle of the ink on the substrate. Low contact angles
and hence good wetting and adhesion are found when the
surface has a high relative surface energy. High contact angles
may be necessary to reduce wetting for applications where
high density interconnects are required, but these must be
traded off against good adhesion properties (Mengel and
Nikitin, 2010). The low temperature sintering associated with
nanoparticle inks can minimise the thermal stresses and
strains acting on packages (Miettinen et al., 2008; Buffat and
Borel, 1976). Insulating inks must also have a high dielectric
strength, low water absorption, low ionic content and a lack of
halogens due to environmental legislation (Mengel and
Nikitin, 2010; Pekkanen et al., 2010). Polyimide has been
found to be one material that meets these requirements
(Miettinen et al., 2009).
Much of the work to date concerning the use of inkjet
printing in packaging has been carried out at the Tampere
University of Technology in applications such as electrical
interconnects, embedded die, passive component fabrication
Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
Volume 38 · Number 4 · 2012 · 193 – 213
205

and reliability of inkjet printed interconnects (Ma¨ntysalo and
Mansikkama¨ki, 2007; Ma¨ntysalo et al., 2007, 2009; Caglar
et al., 2008, 2009; Miettinen et al., 2008, 2009; Kaija et al.,
2010). Besides conventional two-dimensional packaging, inkjet
printing could also be used to produce complex layouts with
multiple, small geometry 3D interconnections between
components, as found in system-in-package (SiP) and multi-
chip packages (MCP). SiPs contain bare integrated die and
discrete passive components that have been encapsulated with a
resin moulding. The resin also acts as a substrate for
interconnections between the components and the I/O pins,
which are deposited on top of the mould. Ma¨ntysalo and
Mansikkama¨ki printed silver nanparticle interconnections that
were covered with a printed dielectric layer (Ma¨ ntysalo and
Mansikkama¨ki, 2007; Ma¨ntysalo et al., 2007). A low viscosity
ink, surface treatments and use of a sequential printing
algorithm were recommended to ensure high resolution
interconnects compatible with the pitch size (40
m
m) and pad
size (60
m
m) of current integrated circuits. Factors that can
adversely affect the reliability of inkjet printed interconnects on
moulded packages were identified by Miettinen et al. (2009)
and include misalignment of the interconnects due to thermally
induced warping of the package and surface contamination
from adhesive carrier tape.
The non-contact nature of inkjet printing has also been
exploited to form structures on non-planar or vertical
surfaces. For example, Mengel printed a dielectric layer
down an edge of a die using inkjet printing (Mengel and
Nikitin, 2010). The reliability of inkjet printed interconnects
for packaging applications has been investigated by Joo at the
Georgia Institute of Technology (Baldwin, 2008; Joo and
Baldwin, 2007, 2010) and the Tampere group. Silver
nanoparticle inks were used to print interconnects in both
groups. According to Joo, the contact height of printed
interconnects is negligible compared with wire bonding and
flip chip bonding, which would enable thinner packages and
improved high frequency operation (Joo and Baldwin, 2007).
These interconnects were also found to be robust, being able
to withstand up to 2,940 cycles of thermal cycling between
225 and 1258C before failure when printed on a stainless
steel substrate.
7. Conclusions
Inkjet printing has traditionally been used in the graphical and
publishing industries. Recently this technology has been
applied for use in manufacturing due to the flexible and cheap
nature of this deposition technology and the low material
wastage. The use of inkjet printing is still in its infancy and
there is still significant scope for use of this technology in
advanced applications such as sensors, displays, renewable
energy and microelectronics. However, challenges such as
coffee staining and the limits to feature resolution need to be
addressed by improving the understanding of fluid behaviour
throughout the various stages of inkjet printing.
Inkjet printing with conductive inks is a novel method for high
quality and low cost conductive films on various substrates.
Further development of these materials is also required to meet
the properties of thin films processed using traditional vacuum
deposition methods at lower cost and temperature.
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Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
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About the authors
Gerard Cummins obtained an honours Bachelor’s
degree in Electrical and Microelectronic Engineering from
University College Cork, Ireland and his doctoral degree from
the University of Edinburgh, during which he was the
recipient of the Wolfson Microelectronics scholarship.
He is currently a Research Associate at Heriot Watt University.
Gerard Cummins is the corresponding author and can be
contacted at: G.Cummins@hw.ac.uk
Marc P.Y. Desmulliez is Head of the Microsystems
Engineering Centre at Heriot-Watt University. He is one of
the founders of MicroStencil Ltd, a company that specialised
in the formation of electroformed stencils. Professor
Desmulliez has co-authored over 320 publications in the
field of MEMS, opto-electronics and advanced manufacturing
technologies. He is a Fellow of the IET, the IOP, a Chartered
Physicist and a Chartered Engineer.
Inkjet printing of conductive materials
Gerard Cummins and Marc P.Y. Desmulliez
Circuit World
Volume 38 · Number 4 · 2012 · 193 – 213
213
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