New Maskless Lithography System for Fabricating Biodevices using

Light-Emitting Diodes and Squared Optical Fibers

Jun Watanabe, Jun-ya Iwasaki and Toshiyuki Horiuchi

Tokyo Denki University, 5 Senju-Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan

Keywords: Light-Emitting Diode, Squared Optical Fiber, Projection Exposure, Biodevice, Cell Array, Micro-Fluidic

Path.

Abstract: A new low-cost lithography system convenient for fabricating biodevices was developed. Using the new

system, various patterns of cell arrays, chamber arrays, flow paths of micro-fluidic devices, and others were

easily printed without preparing any reticles or masks. In the system, light-emitting diodes (LEDs) were used

as exposure sources and squared optical fibers arrayed in a 10×10 matrix were used as the combination of a

secondary light source and a reticle. Light rays emitted from each LED were individually led to each fiber,

and bright or dark of each LED was assigned by a personal computer. As a result, it became possible to print

arbitrary patterns without preparing any reticles or masks. In addition to the ordinary patterning using various

lightening maps of LEDs and their stitching, scan exposure was also tried. When bright images of optical

fiber ends were scanned on a resist film by moving the wafer stage, long patterns appropriate for micro fluidic

paths were very smoothly formed.

1 INTRODUCTION

Various researches are vigorously practiced for

supporting medical treatments and diagnosises, and

clarifying causes of diseases. As techniques or

procedures of such researches, chemical or medical

analysises using cell arrays and micro-fluidic devices

are frequently used. For example, functions of cells

and DNAs are often investigated. To speak about

researches on cells, for example, when and in what

state, what cells work and how to work are the

subjects (Arnandis et al., 2012; Choi et al., 2010; Kim

et al., 2013). It is often investigated what proteins are

made from particular DNAs also (Gach et al., 2014;

Schmidt et al., 2013). In such cases, it is important to

evaluate a lot of cell and DNA samples efficiently.

For this reason, in many cases, it is required to

develop special devices for analyzing them using

minute volume samples. Micro-chamber array chips

or micro-cell array chips are the most simple devices

for such usage (Bianchi et al., 2013; Lei et al., 2014).

That is, the micro-chambers or micro-cells work as a

lot of test tubes. Accordingly, by minimizing the sizes

of micro-chambers or micro-cells, it becomes

possible to reduce costs and analyzing time, and save

reagents. Micro-fluidic devices are also used for

analyzing various mixing and separation of small

quantity liquids (Wang and Hu, 2010).

As a fabrication method of such devices, optical

lithography is usually used (Boero et al., 2014; Ilyas

et al., 2014; Li et al., 2005; Malainou et al., 2012;

Moraes et al., 2010; Negrete and Cerrina, 2008; Tanii

et al., 2004). However, exposure systems for

lithography are very expensive even for printing very

large patterns with sizes of several tens or hundreds

microns. In addition, conventionally available mask

aligners and projection exposure systems with large

numerical apertures are not suitable for printing thick

resist patterns required for fabricating above

mentioned biodevice patterns. Because depth of focus

is generally small in such systems, it is difficult to

print thick resist patterns with almost vertical side

walls. In addition, because masks and reticles are also

expensive, it is not easy to change or improve pattern

shapes after once they are decided.

In this paper, a new exposure system is developed

using light-emitting diodes (LEDs) as exposure

sources. It has great advantages from the viewpoints

of cost and simplicity. Maintenance times are also

much reduced. In addition, optical fibers with squared

ends are developed, and the squared optical fibers

were bound in a matrix for printing smoothly stitched

patterns. Because bright or dark are directly assigned

by simply lighting and extinguishing LEDs, this

Watanabe, J., Iwasaki, J-y. and Horiuchi, T.

New Maskless Lithography System for Fabricating Biodevices using Light-Emitting Diodes and Squared Optical Fibers.

DOI: 10.5220/0005757802030208

In Proceedings of the 9th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2016) - Volume 1: BIODEVICES, pages 203-208

ISBN: 978-989-758-170-0

203

combination of LEDs and a fiber matrix is very

effective for easily printing arbitrary patterns as if

they are pixel arts without using any reticles. On the

other hand, it is also possible to print oblique and

curved patterns if the wafer stages are

programmatically scanned. It is demonstrated that

various patterns of biodevices are fabricated using the

exposure system.

2 NEW EXPOSURE SYSTEM

2.1 LED Exposure Optics

The newly developed exposure system is shown in

Figure 1. The sizes of the system were 600 mm wide,

400 mm deep, and 2,000 mm high. Bullet-type LEDs

with a central wavelength of 405 nm (OptoSupply,

OSSV5111A) were used as exposure sources, and

100 pieces of LED were attached to a universal board

in a 10×10 matrix. Assignments of ON or OFF were

given to all the LEDs using a personal computer.

Accordingly, arbitrarily designed patterns were

obtained without using reticles. The projected field

size was approximately 1×1 mm

, and the element

pattern size was 100×100 μm

because the projection

ratio was 1/10, and the squared fiber matrix size of

10×10 mm

was reduced by a factor of 10 through the

projection lens. The numerical aperture of the lens

was 0.4. Two-axis automatic stage (SIGMAKOKI,

SHOT202) was used as the wafer stage. The stage

was used for both step-and-repeat and scan exposures.

Ultra-high pressure mercury lamps are often used as

exposure sources in usual optical projection exposure

system. However, there is fear of lamp burst. In

addition, an expensive elliptical collector mirror is

Figure 1: Exposure system used for the research.

required. Without using these optical components, the

cost of exposure-source was much reduced.

2.2 Squared Optical Fiber

At the early stage of the past research, patterns were

printed directly using LEDs arrayed in a matrix.

However, because each LED had a strong intensity

distribution of emitted light, it was difficult to print

element patterns with a uniform shape. For this reason,

to obtain a uniform surface emission, optical fibers

were used. Collective-lens optics were designed and

inserted between each couple of an LED and a fiber.

In addition, to collect light rays emitted from LEDs

efficiently, plastic optical fibers (Mitsubishi Rayon,

Eska CK-40E1) with a diameter of 1 mm and clad

thickness of 10 μm were used. The end of each fiber

was transformed to a square shape in approximately

20 mm length from the fiber end. The sequence used

to deform circular optical fibers to square ones is

shown in Figure 2. First of all, an optical fiber was

inserted into a square clearance of a deforming

instrument and clamped using screws (a). The square

clearance was controlled to a size of 1×1 mm

. Next,

the instrument was heated at 150 °C on a hot plate for

5 min (b). Though the end part of the fiber heated in

the square space was expanded to fill the space, it

became shorter in the longitudinal direction. After

heating and deforming the fiber to a square cross

section, the fiber was cooled down in the instrument

(c), and taken out (d). Finally, the squared fiber end

was polished with emery papers. As a result, the cross

section of the optical fiber was deformed to a square,

as shown in Figure 3.

Figure 2: Method used for deforming optical fiber ends to

square shapes.

Optics for leading light rays from LEDs to

fibers through collective lenses

Fibers arrayed in 10×10 matrix

Shutter

1/10 projection lens

Wafer

Personal computer used for

exposure control

XYZ wafer stage

XY stage controller

200 mm

(d) Take out

(a) Clamping (b) Heating

Optical fiber

BIODEVICES 2016 - 9th International Conference on Biomedical Electronics and Devices

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Figure 3: Fiber ends before and after squared.

2.3 Fiber Matrix

Sizes of all the squared fibers were measured using a

micrometer, and fibers with regular shapes and sizes

in a permissible range of within 1 ± 0.01 mm in both

vertical and horizontal directions were selected. Next,

the selected 5 optical fibers were simultaneously

inserted in a bundling instrument with a 5-mm

opening space, as shown in Figure 4, after coating

adhesive (Loctite LPP-005), and aligned in a line.

Such 5-fiber alignment and bonding were repeated

4times more. As a result, 5×5 fiber matrix was

obtained after the adhesive was hardened. 10×10

matrix was fabricated by bonding such 4 pieces of

5×5 matrices using the same adhesive, as shown in

Figure 5. The matrix end was polished again with

emery papers. Measured maximum gap between

fibers was 49 μm, and it was approximately 5% of the

square size. Accordingly, the maximum gap was

sufficiently small to use the fiber matrix as an

exposure tool.

Figure 4: Bundling instrument used for aligning and

bonding squared 5 fibers, and piling them.

Figure 5: Fibers arrayed in a 10×10 matrix and fixed to the

exposure system.

3 EXPERIMENTS

3.1 Patterning by using Exposure Maps

Resist patterns formed by lithography are variously

applicable to biodevices. If thick patterns are obtained,

they are directly available to cell or chamber arrays,

and micro-fluidic paths. They are also applicable to

molds of soft lithography used for replicating them to

poly dimethyl siloxane (PDMS) (Lyu et al., 2014). On

the other hand, if the resist patterns are used as

etching masks, thin resist patterns are also applicable.

Here, considering the wide varieties of application,

thick resist patterning was investigated. To print thick

resist patterns, negative-type PMER N-CA3000PM

(Tokyo Ohka Kogyo) was spin-coated on Si wafers in

a thickness of approximately 30 μm.

At first, checker patterns were printed by giving

assignments of lighting LEDs alternately. It was

anticipated that all the square resist patterns were

slightly separated from neighbored patterns at every

corner caused by the gaps between fibers explained in

section 2.3 if fiber-matrix elements were faithfully

printed in square shapes. For this reason, sufficient

exposure dose was given, and the gaps between

elements were eliminated by excessively sensitizing

the resist. As a result, the checker patterns were

connected at corners, as shown in Figure 6. It is

thought that the patterns are usable as a micro-cell

array. Though the ordinary exposure time was 30-40

s, it took 80 s to print sufficiently connected checker

patterns. The patterning characteristics that the

projected element-pattern sizes were controllable by

changing the light intensity of LED and the exposure

time were almost similar to the conventional

lithography using lamps or lasers as the exposure

sources. Pattern width controllability by exposure

time is shown in Figure 7.

Figure 6: Printed checker patterns usable as a cell array.

Other kinds of chamber patterns were also

fabricated by variously assigning the lighting LEDs,

as shown in Figures 8-10. The exposure time was set

for 70 s in these experiments. Figure 8 shows a pattern

New Maskless Lithography System for Fabricating Biodevices using Light-Emitting Diodes and Squared Optical Fibers

205

Figure 7: Pattern size controllability by adjusting the

exposure time.

fabricated by lighting only the fiber elements at outer

periphery. It has a 800-μm square hollow chamber

surrounded by a 100-μm wide outline fence. Figure 9

shows a cell array fabricated by lighting LEDs

corresponding to 3×3 fiber elements except the center,

and repeating exposures and step movements.

Repeated 300-μm square cell patterns with 100-μm

square hollows were regularly printed.

In Figure 8 and 9, concave chamber and cells were

directly fabricated. However, it is also possible to

print convex resist patterns at first, and replicate the

patterns to PDMS using soft lithography (Wang et al.,

2008). Figure 10 shows an example of convex resist

pattern array. Square dot patterns with a size and a

pitch of 100 and 200 μm were regularly printed in a

wide area. Cell arrays would be fabricated by

replicating them to PDMS using soft lithography.

Thus, it was demonstrated that various chamber

and cell array patterns were easily fabricated by the

new exposure system without using reticles.

Figure 8: Chamber pattern with 800-μm square chamber

and 100-μm wide fence.

3.2 Scan Exposure

It was demonstrated that arbitrary patterns were

printable by assigning LED lighting maps. However,

curved or oblique patterns were not printable by the

direct exposure using the squared fiber matrix. For

this reason, scan exposure using automatic stage

control was investigated next. Exposure light spots

with a 100-μm square shape were relatively moved in

X and Y directions by scanning the wafer stage. Scan

speeds were changed between 5 μm/s and 20 μm/s.

Figure 11 shows the relationship between the scan

speed and measured pattern width. Pattern width was

controllable between 93 and 115 μm by changing the

scan speed.

Figure 9: Dispersively arrayed 300-μm square patterns with

100-μm square hollow cells.

Figure 10: Arrayed 100-μm square pillars. If patterns were

replicated to PDMS using soft lithography, a cell array

would be obtained.

Next, a flow-path mold of a micro-mixer with a

width of 100 μm was fabricated by scanning a light

spot from one LED at a scan speed of 10 μm/s.

However, at the inlets and outlet of the mixer, all the

LEDs were lit, and 1-mm square patterns were printed

without scanning the stages. Fabricated flow path

pattern is shown in Figure 12. Patterns were

successfully delineated in 6×12 mm

area.

Pattern widths were measured at 15 points, as

shown in Figure 13. Figure 14 shows the results.

Figure 11: Pattern width control by adjusting the scan

speed.

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Figure 12: Micro-fluidic device pattern.

Figure 13: Points selected for measuring pattern widths.

Figure 14: Width homogeneity of fabricated micro-fluidic

device patterns.

Pattern widths of vertical parts and horizontal parts

were almost uniform. In addition, widths of 45°

oblique patterns became almost the same as those of

the vertical and horizontal patterns. Deviation of line

widths for all the measurement points was as small as

less than 1μm.

4 EXTENDABILITY

It was demonstrated that various biodevice patterns

were easily fabricated using the new system.

However, the minimum pattern size was decided by

the fiber size, and it was approximately 100 μm. In

some cases, patterns with smaller sizes are required.

For this reason, it was investigated whether finer

squared fibers were obtained or not. Figure 15 shows

a cross section of 500-μm square fiber. The fiber was

fabricated by deforming the end of a circular fiber

with a diameter of 500 μm. It was verified that the

profile and corner roundness were almost the same as

those of 1-mm fiber. Accordingly, it is probably

possible to reduce the minimum pattern size at least

to 50 μm. If the element fiber size is reduced in a half,

and LED light rays are efficiently collected in the

fiber, the intensity of light spot on a wafer is increased

by a factor of 4, and the exposure time is reduced to

1/4.

On the other hand, if the new system is applied to

fabrication processes of biodevices using etching, and

resist patterns are used as etching masks, highly

sensitive thin positive resist is applicable. In such

cases, exposure time becomes considerably short. For

example, even using 1-mm square fiber-matrix,

patterns were printable within 4 s using 1-μm thick

THMR iP-3300PM (Tokyo Ohka Kogyo). The new

exposure system is very flexible and has a wide

extendability.

Figure 15: Half-size squared optical fiber with a cross

section profile similar to that of current size fiber.

5 CONCLUSIONS

Applicability of the newly developed low-cost

maskless exposure system to the fabrication of

biodevices was investigated. In the new system,

LEDs were used as original exposure sources, and a

squared optical fiber matrix was used as a component

combining a secondary light source and a reticle with

bright and dark parts. It was demonstrated that

various chamber or cell arrays were directly

fabricated using 30-μm thick negative resist and LED

lighting control. In addition, combining with the

scanning exposure, a flow path pattern applicable to

a replica mold for soft lithography using PDMS was

fabricated. It was verified that the new exposure

system was prospective for the use of biodevice

fabrication. If optical fibers with a smaller diameter

are used, and the matrix size is enlarged in future,

convenience and applicability will be extended

further. The minimum element pattern size, pattern

size accuracy and repeatability, and other

performances of the exposure system should be

investigated further more hereafter.

New Maskless Lithography System for Fabricating Biodevices using Light-Emitting Diodes and Squared Optical Fibers

207

ACKNOWLEDGEMENTS

This work was partially supported by Grant-in-Aid

from J I Engineering, and Research institute for

Science and Technology of Tokyo Denki University,

Grant Number QT15-03.

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