Low-k Underfill Using Spray Coat Technology

Abstract

The purpose of this extended abstract is to present a new application of spray coating process which greatly improves the process of performing underfill of suspended structures (air bridges) with low-k polymer. By using spray coating, it is possible to realize as much as a nine-fold materials savings, plus a three fold time savings. This article describes the spray coating process, and how it applies to the underfill process.

Full text

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Low-k Underfill Using Spray Coat Technology

C. Brubaker*, M. Wimplinger*, G. Mittendorfer**, C. Thanner**

* EV Group Inc. 3701 E. University Dr. STE 300, Phoenix, AZ 85034 Phone: (602) 437-9492;
*Emails: c.brubaker@evgroup.com; m.wimplinger@evgroup.com
**EV Group DI Erich Thallner Strasse 1, A-4780 Schärding, Austria Phone: +43 7712-5311
**Emails: G.Mittendorfer@evgroup.com; c.thanner@evgroup.com

Keywords: Photoresist, Spray, Coating, BCB, Air Bridge, Low-k

Abstract
The purpose of this extended abstract is to present a new
application of spray coating process which greatly improves
the process of performing underfill of suspended structures
(air bridges) with low-k polymer. By using spray coating, it is
possible to realize as much as a nine-fold materials savings,
plus a three fold time savings. This article describes the spray
coating process, and how it applies to the underfill process.

INTRODUCTION:

Suspended interconnect pathways are a common
structure found in many devices requiring high power
applications. The reason that the structures exist is to
reduce the parasitic capacitance [1]. By filling the space
underneath with a low-k dielectric material, this reduction
is maintained, while reducing the chance of damage to the
fragile structures, effectively increasing yield. However,
filling this space can be time consuming and expensive.

This is a process that could be performed by spin
coating. However, there are some distinct disadvantages
to doing this. First of all, in many cases, the path from the
top of the substrate to the area under the suspended region
is relatively small (sometimes just a few micron). This
requires the use of a low viscosity material (<20 cSt), in
order for capillary effects to draw the substance under the
bridge. However, a significant amount of material needs
to be placed under the structures, as, oftentimes, the
structures are suspended 5 µm or more above the bulk
surface of the wafer.

Since a material with a viscosity of 20 cSt will result in
a film ~1 µm thick when spun on a blank wafer at ~2000
RPM, it takes multiple coats to deposit sufficient thickness
of material to completely fill in under the suspended
structure. This translates into lost resources, both in the
form of material lost and time spent.

Another method would be to use a photopatternable
low-k dielectric, such as Benzocyclobutene (BCB,
marketed by Dow chemical as Cyclotene®). Then, each
level of the suspended structure could be patterned
individually in the BCB, with successive layers stacking
one atop another. This method is used frequently for
redistribution layers in wafer level packaging.

The problem with this method is that, again, multiple
coats are being performed. Additionally, the resolution
capabilities and, in particular, the sidewall profile in this
case is less desirable than that found in other patterning
and layering methods.

In contrast, spray coating will make this a much
simpler process.




Figure 2: Spray Coating Process Dynamics

Figure 1: 30° Spray distribution

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SPRAY COATING

The key to this spray coating technology is the
ultrasonic spray nozzle. This nozzle oscillates to produce
microscopic resist droplets. The mean distribution of the
droplets’ diameter is typically around 20µm. In order to
achieve the proper droplet size distribution, the viscosity of
the material introduced to the nozzle must be below 20 cSt
(centistokes)[2]. Most commonly used photoresists can be
diluted with resist compatible solvents with the following
properties:

−
Same or similar chemistry as mean resist solvent
−
No chemical reaction with the resist before or during
the exposure
−
High vapor pressure

Table 1 shows a sample of materials that have been
used for spray coating processes:

Once the droplets have been produced by the ultrasonic
nozzle, they are accelerated toward the surface of the
substrate via a stream of clean, dry air (CDA) or nitrogen,
which can be adjusted directly within the software preset.
During the spray process, the substrate rotates with a very
low spinner speed (50 to 100rpm), which minimizes the
centrifugal forces on the outer substrate area. These
coating dynamics prevent coating defects and non-
homogeneous areas on the final coating layer. The
dynamics of the process are shown in Figure 2.

One of the most important dynamics in the spray
coating process is the velocity profile of the resist nozzle
as it sweeps over the wafer. Due to the decreasing area of
substrate requiring coverage as the nozzle nears the center
of the wafer, a constant sweep speed is not appropriate.
This would create a resist layer that thickens towards the
center of the substrate.

Instead, a varied sweep profile must be used. In order
to simplify this process, the sweep path is divided into
multiple parts, or indices, each with its own travel velocity.
By comparing the relative area of each of these indices, an
optimal velocity can be calculated for each index. A
general velocity profile can be seen in Figure 3.

In all, the film thickness, uniformity and roughness
depend on the following parameters:

−
The solid content (resist dilution)
−
The angle of the Spray nozzle
−
The resist dispense rate
−
The scanning speed of the atomizer (velocity profile)
−
The Spinner speed

All critical parameters can be adjusted via the recipe
and stored in the preset. The angle of the atomizer and the
resist dilution are normally fixed and kept constant for one
substrate or experiment. A recipe with optimized
parameters has been developed to give a reasonable
uniformity of the resist layer across the wafer. With these
features it is possible to get good repeatability, and also a
very good run to run standard deviation. Table 2 shows the
uniformity results of a 4.2 µm spray coated layer on a
series of blank 150 mm silicon wafers.

MATERIALS SAVINGS

Spray coating is an ideal process for the application of
the low-k underfill. Since this deposition method already
requires low viscosity materials, the capillary requirements
for underfill are met, and no difficulty is presented in
placing sufficient material in a single pass. This results in
direct time savings, as only a single coat is required to
properly underfill the structure. Material savings are also
produced, since only a single dispense is required.

It is well known that in spin coating processes, most of
the resist dispensed spins off of the wafer, and into the
bowl, eventually becoming waste. Even with the highest
level of process optimization, this remains true. Because
of the method of deposition used for spray coating, the vast
Material
Clariant AZ P4620
Shipley S1813
Clariant AZ nLOF 2070
MCC SU-8 2025
Dow Cyclotene™ BCB
Polyimide
PMMA
AGI Cytop™
Comments
Positive Photoresist
Positive Photoresist
Negative Photoresist
Negative tone epoxy
Low-k dielectric
Low-k dielectric
E-beam resist
Fluorinated Polymer
Spin on adhesive (Bond
intermediate layer)
Solder Mask
Bond intermediate layer
GenTak 130P
Epic LSF 60
UV curable adhesives
Table 1 – Example materials deposited via spray coating
Velocity Profile
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
1234567891011 12131415
Index
Motor Steps (=Velocity)

Figure 3: Example of a velocity profile

Page 3

Figure 5 – Syringe Dispense System
majority (typically >90%) of the material dispensed
remains on the substrate.

While this material savings is a good thing in any
situation, it becomes very important when applied to
precious material. A good example of this is BCB. This
material is finding a high degree of utilization as a material
for advanced packaging, due to both its mechanical
properties, and its utility as a low-k dielectric. Spray
coating can allow the use of this material to be extended to
cost sensitive devices, where standard coating methods are
too expensive to be justified.

A study was performed to determine the amount of
material savings that could be achieved when using spray
coating, as opposed to an optimized spin coating process,
as well as the resulting change in uniformity (see Figure 4
below).

For both 6” and 8” wafers, and for both resists and film
thicknesses, the amount of material savings was
approximately the same, at ~1:2.5. This can actually be
improved even more, considering that materials such as
BCB must be diluted prior to spray coating (the viscosity
of either Cyclotene® material used is greater than 300 cSt).
Because the thickness of film produced is a direct function
(linear) of the solids content of the diluted material, rather
than of the viscosity, it would be possible to use a higher
solids content material to create a thinner film, just by
diluting it more. Thus, using the Cyclotene® 4026-46 to
make a 5 µm film would result in requiring even fewer µl
of BCB to create the 5 µm film.

Furthermore, when used in conjunction with a syringe
dispense system (see
Figure 5), the material
savings are improved
even further. By using
the syringe system,
several benefits are
realized. First, the
amount of “dead
volume” (the volume of
material between source
and dispense point) is
greatly reduced, and can
even be as low as a
single milliliter.
Additionally, the syringe
allows the use of smaller
volumes of material at
one time, since many
BCB Coating tests on 6" Si-Wafer
21602160
849
667
2
1.6
2.5
3.5
0
500
1000
1500
2000
2500
3000
1234
BCB-Consumption [µl]
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Uniformity (±%]
BCB Coating tests on 8" Si-Wafer
3300
3000
3
1200
942
1.5
4
3.5
0
500
1000
1500
2000
2500
3000
3500
4000
1234
BCB-Consumption [µl]
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Uniformity [±%]

Figure 4 – BCB Consumption and Uniformity on 6” and 8” wafers –
1 - 10µm 4026-46 spin; 2 – 5 µm 4024-40 spin
3 – 10 µm 4026-46 spray; 4 – 5 µm 4024-40 spray
Point
1
2
3
4
5
6
7
8
9
Average
Range
%Unif
Table 2 – Spray Coating Uniformity Data
123456
4.201
4.177
4.088
4.180
4.160
4.051
4.148
4.151
4.128
4.143
0.150
1.81%
4.307
4.298
4.321
4.269
4.207
4.202
4.155
4.147
4.163
4.230
0.174
2.06%
4.307
4.324
4.329
4.316
4.189
4.077
4.062
4.225
4.232
4.229
0.267
3.16%
4.021
4.128
4.218
4.326
4.342
4.217
4.150
4.258
4.229
4.210
0.321
3.81%
4.412
4.369
4.309
4.245
4.245
4.344
4.390
4.206
4.304
4.314
0.206
2.39%
4.066
4.402
4.386
4.392
4.390
4.132
4.084
4.089
4.369
4.257
0.336
3.95%
4.230
0.171
2.02%
Wafer
Wafer
To
Wafer

Page 4

materials (BCB included) have a sharply reduced lifetime
at room temperature. (BCB expires within 5 days when
stored at room temperature.) Finally, the syringe system
allows rapid changeover between resists (or different
dilutions), when used in an R&D environment.

Cost data from work with a major power devices
manufacturer shows the benefits of spray coating (Table 3)

UNDERFILL COATING

In contrast to spin coating, spray coating will make the
underfill process much simpler.. It also allows great
economic benefit, both from reduced process time and
reduced materials usage.

As mentioned above, spray coating already requires
low viscosity materials, thus satisfying the requirement to
allow for capillary effects to draw the material under the
structures. Since there is no concern with the low-k
material flowing off of the wafer, no difficulty is presented
in placing sufficient material in a single pass. This results
in direct time savings, as only a single coat is required to
properly underfill the structure, while in the case of spin
coating, at least three coats are usually required..

Material savings are found in two ways. First of all,
the amount of material required to fully coat the substrate
via spray coating is less than the material required for a
single spin coat (already over 70% savings). Since
multiple spin coats (~3) would be necessary to match a
single spray coat process, the material savings would
multiply (>90%). In addition, the spin coating process
tends to leave the top surface of the material (above the
suspended structure) with limited planarity, while the
spray coating method has demonstrated a high degree of
planarization (see Figure 6).

CONCLUSION

This article has described a new process for the
deposition of dielectric material. This process is very
effective a creating a layer of low-k material sufficient to
serve as effective protection for the fragile structures that
are encapsulated. It was shown that spray coating can be
used to not only provide a superior quality of coating as
compared to spin coating, but at a great reduction in
materials cost and process time.

By performing this underfill process, the power
performance benefits of air bridges can be realized at high
yield with minimum cost per wafer

REFERENCES

1. J. Park, M. Allen, “Pacakaging Compatible High
Q Microinductors and Microfilters for Wireless
Applications,” IEEE Transactions on Advanced
Packaging, vol 22, No 2, May 1999, pp 207 – 213
2. B. Wieder, C. Brubaker, T, Glinsner, P. Kettner,
N. Nodes, “Spray Coating for NEMS, MEMS,
and Microsystems,” Pacific Rim Workshop on
Transducers and Micro/Nano Technologies 2002

Figure 6 – Underfill Process a) with Spin Coat b) with Spray Coat. Images courtesy of Agilent Technologies
Spin
3
1.4
Spray
3
Spin
0.16125
2.24000
Spray
0.16125$
$
0.64000$
0.09405$
$
1.37250 $
$
$
Prime AP3000
BCB4024-40
BCB4026-46
ml
ml
ml
ml
ml
ml
0.05375$
1.60000$
1.60000 $
0.02939$
0.05046$
0.04575$
$
$
$
$
$
$
$
$
-
0.4
3.2
0
30
-
-Spray Diluents
EBR
Develop
0
T1100
DS2100
15
30
Cost per wafer - Coating
Cost per wafer - Full Process
= 76% savings per wafer for coating only
= 70% savings per wafer for full process
0.75690
1.37250
2.9969
7.5276
-
0.7340
2.2678
BCB Coat
Units
Cost per
Unit
Units per WaferCost per Wafer
Table 3: Chemical costs for 4µm BCB film