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PROCEEDINGS OF SPIE
SPIEDigitalLibrary.org/conference-proceedings-of-spie
Tailored multiphoton polymerization
for functional microstructures
Prem Prabhakaran, Cheol Woo Ha, Kwang-Sup Lee,
Xiangming Cheng
Prem Prabhakaran, Cheol Woo Ha, Kwang-Sup Lee, Xiangming Cheng,
"Tailored multiphoton polymerization for functional microstructures," Proc.
SPIE 11098, Molecular and Nano Machines II, 110980M (30 August 2019);
doi: 10.1117/12.2529977
Event: SPIE Organic Photonics + Electronics, 2019, San Diego, California,
United States
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Tailored Multiphoton Polymerization for Functional Microstructures
*Prem Prabhakarana, Cheol Woo Hab, Kwang-Sup Leea, Xiangming Chenga
aDepartment of Advanced Materials, Hannam University, Daejeon 305-811, Republic of Korea.
bKorea Institute of Industrial Technology (KITECH), Gyeonggi-do, Incheon,
e-mail: premp@hnu.kr
ABSTRACT
In this presentation we discuss our attempts at balancing the desirable, and often mutually exclusive properties of
resolution and mechanical stability with radical polymerizable photoresists in two-photon lithography (TPL). The
photopolymerization dynamics at and around voxels (volume pixels) were controlled by the combined action of a co-
initiator and a radical quencher added into the photoresist. In the second part of the manuscript longitudinal scanning
during microfabrication is studied as a method to achieve fast fabrication of hierarchical microstructures with hexahedral
unit cells.
Keywords: 3D printing, two-photon lithography, non-linear optics, polymerization, two-photon polymerization
1. INTRODUCTION
Two-photon lithography is strong candidate for 3D printing of functional microstructures with sub-micrometer to
micrometer resolutions.[1] Two-photon lithography (TPL) is based on the nonlinear optical phenomenon two-photon
absorption.[2] Two-photon absorption is dependent on the square of laser intensity and involves the simultaneous
absorption of two photons of same or different wavelengths. Two-photon absorption results in highly confined
fluorescence in the photoactive medium. The energy released as a result of two-photon fluorescence can then be used to
initiate diverse chemical reactions. Cationic and radical polymerizations have been most widely triggered chemical
reactions. Many research groups have also reported two-photon photoreduction of metals and anionic polymerization by
two-photon excitation.[3-5] A depiction of two-photon lithographic system can be seen in Figure 1. In a typical
fabrication system, a pulsed laser source is focused into a sample placed on a substrate using a high numerical aperture
(NA) lens. The sample could be a photoactive media constituting polymeric, metallic, ceramic or biologically derived
precursors. The position of the focus inside the photoactive medium can be three-dimensionally manipulated by piezo
stages or Galvano-mirrors. Samples with high two-photon sensitivity give highly specific chemical reactions upon two-
photon excitation. Small amounts of highly two-photon sensitive materials called two-photon absorbing (TPA)
molecules are often added to the photoactive medium to improve the two-photon process.[6]
Figure 1. Scheme of the two-photon lithographic system.
Molecular and Nano Machines II, edited by Zouheir Sekkat, Takashige Omatsu, Proc. of SPIE Vol. 11098,
110980M · © 2019 SPIE · CCC code: 0277-786X/19/$21 · doi: 10.1117/12.2529977
Proc. of SPIE Vol. 11098 110980M-1
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2. TIGHTLY POLYMERIZED VOXELS
Attributes of microfabricated structure like the resolution of its structure motifs, their mechanical properties and
chemical tendencies govern their application. Resolution can be understood as the closest distance at which two well-
formed structural motifs can exist. Mechanical strength, flexibility and adhesion are different mechanical qualities that
may be demanded of a microstructure. Generally, resolution and mechanical sturdiness seem to conflict each other. High
resolution means thinner structures and such structures may lack the mechanical stability to withstand the development
process after two-photon polymerization. The basic building block of a microstructure fabricated by TPL is called a
volume pixel (voxel). In TPL the voxel is the optical image of the laser focus produced by chemical changes at the focal
point inside a photoactive medium. Two-photon absorption is dependent on the intensity of light and is initiated at the
focus of the laser where light intensities are the highest. The dimension of the voxel is governed by photochemistry of
the photoactive medium, its rheological properties, the laser intensity and time of laser exposure.[7, 8]
Figure 2. The concept of tightly polymerized voxels. (a) summarizes photochemical and diffusive processes in and around the voxel
(b) summarizes the concept of tightly polymerized voxels, the vertical and horizontal axes describe the effect of the radical quencher
and co-initiator respectively, the diagonal describes combined effect of the quencher and the co-sensitizer to achieve leaner
polymerization at the periphery of the voxel.
Radical polymerizable photoresists are widely studied for many applications. We tried to evolve a method to control the
photopolymerization at the focus of the laser such that the voxels formed have a high degree of polymerization without
considerable loss of resolution. As mentioned above two-photon polymerization is initiated at the focus of the laser used
for fabrication. Depending on the time elapsed after exposure many dynamic processes are at play in shaping the voxel.
Such processes in case of radical polymerization are summarized in Figure 2(a). During photoexposure high densities of
initiating species are formed at the focus of the laser. The high repetition rate of the laser makes the exposure extremely
short and extremely powerful. Inhibitors like oxygen dissolved in the photoresist or other radical quenching species will
start affecting the polymerization of the voxel in a few microseconds time due to their diffusion from around the
voxel.[8, 9] At the same time radicals diffuse out of the voxel creating polymerizations in its periphery. The complex
dynamics of these diffusion processes govern the formation and properties of the voxel. The degree of polymerization at
the center of the voxel depends on the spatial density of the initiating species and the local distribution of radical
quenchers. The voxels are chemically active even after the laser exposure is done, this is often termed as dark
polymerization. During dark polymerization phase various diffusion processes become relevant at the periphery of the
voxel. A fraction of radicals formed at the center of voxel during photoexposure migrate to its periphery creating a
region of lesser degree of polymerization indicated in blue in Figure 2(a). Radical quenchers like dissolved oxygen
diffuse towards the voxel from the bulk of the polymerizable material around the voxel. The blue region features many
dangling bonds containing radical terminal groups capable of undergoing polymerization. Then there is the generation of
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heat of polymerization which itself diffuses around the voxel influencing various chemical processes. To create tightly
polymerized voxels, we combined the photochemical action of a co-initiator Michler’s Ketone (MK) and a radical
quencher 2,2,6,6-Tetramethyl-1-piperidinyloxy free radical (TEMPO).[10] The presence of the co-initiator at the center
of the voxel increases the degree of polymerization, while a controlled amount of radical quencher improves the spatial
resolution of the voxel by limiting polymerization at the periphery during dark reaction. This approach is summarized in
Figure 2(b). A commercial urethane acrylate photoresist SCR 500 was used for these studies. Highly efficient two-
photon absorbing (TPA) dye given in Figure 3(a) was used as the two-photon sensitizer. A bezophenone derivative MK
containing a tertiary amine group seen in Figure 3(b) was used as the co-initiator. The structure of the radical quencher
TEMPO can be seen in Figure 3(c).
Figure 3. Materials used to control the initiation and course of photopolymerization at the voxel (a) highly efficient two-photon
absorber (b) co-initiator Michler’s Ketone (b) radical quencher TEMPO
2.1. Effect of Co-initiator
The base resist consisted of SCR500 sensitized with 0.1 wt% of the two-photon absorbing (TPA) dye. The effect of MK
on two-photon polymerization was explored by formulations with different concentrations of MK while the
concentration of the TPA dye was constant. Suspended lines supported by triangular pillars were fabricated to assess the
effect of MK on the vertical and horizontal dimensions of the fabricated lines. It is well known from the study of voxels
that in the case of increase of laser power or exposure time, the vertical dimensions of the voxels grow at a faster rate
than their horizontal dimensions.[7] Results from three resists containing 0.2, 0.4 , 0.8 wt% MK called M1, M2 and M3
respectively are summarized in Figure 4. The threshold power for fabrication defined as the minimum laser power
required to stable well developed lines, decreased with the increase in concentration of MK in the photoresist.
Photoresist that were formulated with only MK without the TPA dye showed very high threshold powers.[10] This
means that the two materials act in tandem to bring about the increased sensitivity to the polymerization process. The
structures formed were studier for the same fabrication power as the MK concentration increased. The horizontal
dimension of the fabricated line structure is termed as linewidth and the vertical dimension is termed as line height. The
base resist containing only the TPA dye gave a line height of 438 nm at 60 mW fabrication power. The line heights
given by M2, M1 and M3 at the same power were 564, 705 and 1023 nm respectively. The threshold power for
fabrication was reduced to as low as 30 mW in the case of M2 and M3. Thus, sturdy and stable structures could be
fabricated by the addition of MK, but high concentrations of MK lead to loss of vertical resolution due to increase in the
height of the lines. Further addition of large excess of MK showed a decrease in Yong’s modulus in tests carried out
with UV polymerized films. Michler’s Ketone has been used as a co-synergist in many polymerization reactions. A co-
synergist acts together with the initiator to aid the polymerization process. MK is also known to have prolonged excited
states; this might also be a factor in the high degree of polymerization in the fabricated structures.
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Figure 4. Two different perspectives of suspended line structures fabricated from M1 (MK:0.2 % wt), M2 (MK:0.4 % wt) and
M3(MK:0.8 % wt) at 60 mW power.
2.2. Effect of Radical Quencher
Radical quenchers are molecules capable for chemically consuming radicals during a reaction. The radical quencher
might itself be a stable radical capable of reacting easily with free radicals.[11] Widely used radical quencher 2,2,6,6-
Tetramethyl-1-piperidinyloxy (TEMPO) was added to the base photoresist to investigate its effect on the fabricated
lines. The results form fabrications involving three different concentrations of TEMPO are summarized in Figure 5. The
resists T1 and T2 contain 0.2 and 0.4 wt% of TEMPO respectively. The threshold power for fabricating was increased
on addition of TEMPO because many structures did not survive the developing process. The photoresist T1 had a
threshold of 150 mw as opposed to 60 mW of the base resist. The second photoresist T2 did give stable structures even
at 200 mW. The voxels formed when the concentration of TEMPO was increased to 0.4 wt% is so thin that the scanning
path of the laser is clearly visible in the pillar structure. The line heights of structures fabricated from T1 at 150 mW and
200 mW were 125 nm and 475 nm respectively. This clearly demonstrated the potential of TEMPO in limiting
polymerization at the voxel periphery. The lesser extent of polymerization around the voxel is validated by the non-
bound scan lines in the structure fabricated with T2 given in Figure 5(c). The structure fabricated with T1 and T2 are
mechanically unstable because of the lesser degree of polymerization.
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Figure 5. Two different perspectives of suspended line structures fabricated from T1 (TEMPO: 0.2 wt%) and T2(TEMPO: 0.4 wt%)
at different powers.
2.3. Tightly Polymerized Voxels
The co-sensitizer and the radical quencher were combined to formulate photoresists capable of forming highly
polymerized center and limited polymerization in the region of diffused polymerization around the voxel. The best
formulation constituted 0.4 wt% of MK and 0.2 w% of TEMPO along with the TPA dye. The voxels formed with this
resist is smaller than those formed my M2 and bigger than that formed the base resist and T2. Woodpile structures
fabricated from different photoresists can be seen in Figure 6. The SEM images in Figure 6(a-b) compares woodpile
structures fabricated with the base to that of M2 at the same fabrication power 150 mW. The details of the top view of
the structures can be seen in Figure 6(c-d). The slight bucking of the structures fabricated from the base resist can be
seen in both Figure 6(a) and (c). Addition of MK improves the form of the lines and the overall structure. However, at
higher powers the vertical resolution is lost for M2. The loss of resolution can be clearly seen in the vertical walls of the
woodpile structure in Figure 6(e). The spaces between the vertical lines are filled over by excess polymerization. This
problem is overcome by the introduction of the radical quencher which limits polymerization at the periphery of the
voxel. The restored vertical resolution can be seen in Figure 6(f). Hence the synergistic action of the co-initiator and the
quencher leads to tightly polymerized voxels. Micro-FT-IR measurements carried out on woodpile structures fabricated
with T2 photoresist showed the lowest extent of polymerization at 32%, the base resists showed a conversion of 50%.
The presence of MK in photoresist M2 boosted the extent of polymerization to 64%. Finally, the presence of 0.4 wt% of
MK and 0.2 wt% of TEMPO giving tightly polymerized voxels gave structures with 57% conversion. The micro-FT-IR
measurements showed that the combination of MK and TEMPO helps achieve quite high degree of polymerization at the
voxel while maintaining a degree of resolution in the vertical walls of the microstructure. Both MK and TEMPO are
commercially available materials so this approach can be easily translated to other photoresists undergoing radical
polymerization. We demonstrated this using MK-TEMPO combination in the organic-inorganic hybrid photoresist
ORMOCER (ORMOCOMP). Structures with over-hang features like in the case of the toroid structure see in Figure
6(g-h) requires the polymerized structure to have good mechanical stability. Toroid fabricated with off the shelf
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ORMOCOMP lead to a collapsed wing like features seen in Figure 6(g). The addition of MK-TEMPO combination
leads to stable structures seen in Figure 6(h). Thus, optimization of photoresists to form tighly polymerized voxels can
lead to stabler functional microstructures by controlling the polymerization dynamics at the voxel.
Figure 6. a) woodpile structure (line distance 2 μm, layer distance 500 nm) fabricated from B (laser power :150 mW) the structure can
be seen collapsing toward the corners. b) Woodpile structure fabricated from M2 at the same power. c) close up image of the
woodpile structure in (a); d) close up image of the woodpile structure in (b); (e) at higher powers the vertical wall resolution is lost for
structure fabricated with M2 (f) the vertical resolution can be restored by combining both MK (0. 4 wt%) and TEMPO (0.2 wt%) in
the same photoresist. (g) An overhang structure fabricated from a commercially available ORMERCER photoresist sensitized with
TPA dye (h) the same structure fabricated by adding the MK-TEMPO combination into the ORMOCER resist. The fan like arms of
the undercut structure remains flared and intact showing the better mechanical stability of the structure.
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3. LONGITUDINAL SCANNING FOR FASTER FABRICATION
Fast fabrication of functional microstructures is a prerequisite for their application in any practical application.
Consistent production of 3D structures would lead to their widespread use in photonics and biology. Recently the use of
TPL has been introduced to the fabrication of 2D and 2.5 D optical interconnects and industrial scale fabrication systems
have started appearing in the market. Scaling speeds of fabrication presents a multi-faceted problem. Factors like optical
or mechanical hardware, software conversion of a 3D design to fabrication data, actual fabrication scheme and materials
used for fabrication all affect the rapidity of the process. Hexahedral shapes with six faces are fundamental in many
functional microstructures. Many functional microstructures consist of a spatially repeated array of hexahedral unit cells.
Such structures consist of both vertical pillars and horizontal frames as seen in the illustration to the top left of Figure 7.
Horizontal frames are fabricated by x-y movements of the piezo stages of the fabrication system seen in Figure 1.
Traditionally the vertical pillar structures are fabricated through a point-to-point scanning method seen in Figure 7 (a).
The laser progressively moves through an upward loop adding a voxel at each corner. This eventually generates the
vertical pillars of the hexahedral shape.
Figure 7. Comparison between (a) point to point scanning and (b) longitudinal scanning. (c) The comparison between the scanning
length for fabrication with point to point scanning L1 with that of longitudinal scanning L2, for larger structure the longitudinal
scanning has a shorter scanning length and hence faster fabrication time. For L2/L1=72 times for a hexaheadral structure with sides
w=l=20 μm.
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The total scanning length of the hexahedral shape with point to point scanning is L1=2n(w+l) where w and l are the
width and length of the square as seen in Figure 7 (a).[12] The integer ‘n’ specifies the number of steps the laser moves
to upwards to complete the pillar structure. First the horizontal frame on the surface of the substrate is fabricated. Then
the system proceeds to fabricate the vertical pillars by moving in a square loop with a voxel being fabricated at a corner
and the laser moving to the next corner to fabricate a voxel at the next corner. This process consumes time and leads to
slower fabrication of structures. To improve on this, we developed a longitudinal scanning scheme where a vertical pillar
is fabricated by one sweeping movement of the piezo system. The laser scanning length and hence the time of
fabrication can be considerably reduced with this approach. This approach is illustrated in Figure 7 (b), the laser
scanning length for the longitudinal scanning scheme is L2=4(w+l)_+4h. The longitudinal scanning approach offers
greater advantage when larger hexahedral structures are fabricated. The scanning length of point-to-point scanning
method is compared to that of longitudinal scanning in Figure 7 (c). In the figure the scan speed is compared depending
on the total height of the eventual microstructure. For a structure with a total height of 100 µm and with a hexahedral
unit of equivalent length and width of 20 µm, the longitudinal scanning method is up to 72 times faster than the point-to-
point scanning method.
The distribution of laser intensity around the laser focus and the shape of the voxel formed can be seen in Figure 8(a).
The power of laser is not uniform for movement of the sample along different directions. The X, Y and Z axes as well as
the XY plane are the important directions of movement for the laser. The energy intensity of the laser varies depending
on the direction of scanning because of the inherent difference in the distribution of laser power at the focus of the laser
power. Hence the speed of scanning along different directions must be optimized to efficiently manufacture hexahedral
microstructures. Results from different scans speeds in different directions are summarized in Figure 8(b-e). The scan
speed along X, Y and XY directions are studied with Figure 8(b-c). A good line along a given direction is defined as a
well-formed line after fabrication and development. The laser power during these experiments was kept constant at 300
mW. The best scan speeds for fabrication along X, Y and XY directions are indicated by the boxes I, II and III
respectively. The numbers next to the line is writing speed in nm/ms. The optimum writing speeds for X, Y and XY
directions are 3 nm/ms or 3 µm/s. The lateral dimensions of lines formed at 3 nm/ms writing speed for X, Y and XY
directions is given in Figure 8(c). Results from scan speed test along Z-direction is summarized in Figure 8(d-e). The
variable distribution of laser power at the focus of the laser can be seen in Figure 8(a). A higher density of laser power is
distributed along the vertical direction (direction of propagation of laser) than the horizontal direction. Physically this
means that a larger region in the photoactive materials is active along the horizontal direction than the vertical direction.
In terms of fabrication this means that the useful structures can be fabricated at a faster rate in the vertical direction.
Vertical pillars were fabricated by scanning along Z- axis in Figure 8(d), but the polymeri is not mechanically strong
enough to stand by itself after developing. Still well-formed structures can be fabricated at a scan speed of around 10
nm/ms. The dimensions of the structure formed at 10 nm/ms is 770 nm which is thinner than the lines fabricated at the
optimum scan speeds along X, Y and XY directions. This is good thing in the case of many applications because vertical
resolution of structures is crucial for many applications.
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Figure 8. Parameter study to find best speeds of fabrication that can be achieved along different directions (a) shows the distribution
of laser power along the voxel, the power density is higher along the Z axis than along the X and Y axes. (b) Optimization of scan
speeds along X, Y and XY directions. The numbers next to lines give scan speeds at nm/ms or µm/s; (c) shows optimum structures
along X, Y and XY directions their scan speeds and dimensions. (d)-(e) shows pillar structures fabricated along Z direction, the pillars
are all collapsed during post fabrication development due to forces of viscosity from the developing solvents, but their dimensions can
be measured well from SEM measurements.
The adaptation of the longitudinal scanning scheme allows faster fabrication of array structures constituting taller
hexahedral unit cells. The fabrication of 3D structures constitutes the laser scanning along X, Y, XY and Z directions.
Due to many scanning paths the threshold speed for fabrication of 3D structures can be higher than fabricating 2D
structures. This is because of the high relative density of laser powers deposited in a three-dimensional area during
fabrication of 3D structure. We proposed a fabrication window based on the study of scanning parameter along the
vertical direction. The scanning window provides parameters for optimum fabrication conditions of structures with
heights up to 20 µm at a scanning speed of 14 µm/s (14 nm/ms) at different laser powers. The fabrication window is
depicted in Figure 9. This fabrication window can be used to select fabrication parameters for various structures.
Figure 9. Optimum fabrication conditions for structures with different vertical dimensions.
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The difference between point to point scanning and longitudinal scanning in the case of an array structure is visualized in
Figure 10(a-d). The microstructure consists of a cubic unit cell of side 12 µm. The first level has 8 cells the second level
has 6 cells and the third level has 4 cells. Point to point scanning involves scanning of the planar mesh followed by a
step by step fabrication along Z-direction where all points with the same Z is simultaneous scanned before moving up to
the next Z-value. This process is tedious and takes long periods of time to fabricate structures along the Z-direction.
Contrastingly the longitudinal scanning technique uses 3D data and fabricates the structure of a unit cells along the Z
direction with a single movement. Thus, all the vertical pillars of the unicells constituting a layer are fabricated
consecutively leading to faster fabrication. Examples of structures fabricated by longitudinal scanning can be seen in
Figure 10(e-g).
Figure 10. Different microstructures fabricated by longitudinal scanning.
4. CONCLUSIONS
Fast fabrication of mechanically stable structures with fine features is a prerequisite for many functional microstructures
fabricated by TPL. In this brief proceeding we have discussed two different problems; the first one is related to stability
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of fabricated microstructures and the second connected to fast reproduction of prototype. The importance of controlling
the polymerization dynamics in each photoresist to achieve stable structures without the loss of resolution was
demonstrated with the used of MK and TEMPO in a photoresist. These materials succeed in creating leaner yet sturdy
voxels leading to minimal loss of resolution. Both MK and TEMPO are commercially available materials, hence this
approach can be easily translated to other photopolymers undergoing radical polymerization. This was demonstrated by
applying them into ORMOCER photoresists. The development of longitudinal scanning allows swifter fabrication of
hierarchical structures with hexahedral unit cells. The dramatic reduction in fabrication times in many cases leads to fast
replication of many structures facilitating their wider applications.
ACKNOWLEDGEMENTS
This research described in this work was supported by Basic Science Research Program (2017R1C1B5077130) of the
National Research Foundation of Korea (NRF) funded by the MEST.
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