This document describes a method for fabricating both negative and positive patterns on a single layer of positive photoresist. The method involves locally crosslinking the photoresist using laser exposure, followed by a flood UV exposure and development. The crosslinked areas form a negative pattern, while the non-exposed areas form a positive pattern. Experimental results showed that the height of the negative pattern can be controlled by varying the laser exposure energy and other processing parameters.

1. Negative pattern fabrication using laser exposure of positive
photoresist
Boris Kobrin and Colleen Hagen
Rochester Photonics Corporation
330 Clay Road, Rochester, NY 14623
ABSTRACT
The method of simultaneous positive and negative pattern formation on a single positive photoresist layer
is described. A negative photoresist pattern was fabricated by using local laser exposure to crosslink a
positive resist layer, consecutive UV flood exposure, and resist developing. The positive pattern is
obtained on the same photoresist layer in the areas masked from the UV flood exposure. Effects of laser
energy and resist processing parameters on height and width of negative type resist structures were
investigated. Metal line grid structures with lines in the region of 3 to 30 pm in width were manufactured
on a 5"x 5" glass substrate using this technique. The proposed method of positive/negative pattern
formation significantly reduces the number of technological steps in the fabrication of diffractive elements
for dual-wavelength applications.
Key words: photoresist, negative pattern, positive pattern, laser exposure, crosslinking, diffractive
elements.
1. INTRODUCTION
Laser direct write lithography is a powerful method for fast prototype fabrication of arbitrary relief patterns
in photoresist. This technique does not require the manufacture of a photomask or series of masks, as in
the case of conventional contact printing or stepper exposure. Laser writing also does not require the high
vacuum processing and super-cleanliness that E-beam writing methods do. However, laser direct writing
techniques can have long write times depending upon the resolution required. Consequently, it is desirable
to decrease the writing time as much as possible. The laser writing time to manufacture a grid pattern, for
example, can be reduced drastically by using a negative type method of resist processing. In this case the
laser will only expose a small part of the entire area and the process can be accomplished much faster.
It is known that negative resists possess lower sensitivity than positive ones, and corner rounding is a very
pronounced feature of negative resists.' An image reversal (IR) scheme is an opportunity to obtain a
negative pattern using positive photoresist.
The JR capability is obtained by using a special crosslinking agent in the resist formulation which becomes
active only in the exposed areas of the resist. The crosslinking agent, together with the exposed
photoactive compound, leads to an almost insoluble substance in developer which is no longer light
sensitive. The unexposed areas still behave like normal unexposed positive photoresist. After a flood
exposure, these areas are dissolved in standard developer for positive photoresist, and the crosslinked areas
remain. The overall result is a negative image pattern.
Many different IR techniques are published. Among them are methods which use special chemical
formulations of resist that give positive or negative patterns depending on the type of developer solution
used.2'3 Other JR techniques treat the exposed areas of resist with different chemicals4'5 or an ion beam
* Further author information-
B .K.: E-mail: kobrin @rphotonics.com; BorisKobrin @worldnet.att.net;
C.H.: E-mail: hagen@rphotonics.com;
http://www.rphotonics.com Telephone: 716-272-3010 Fax: 716-272-9374
1426 I SPIE Vol. 3333 0277-786X198/$1O.OO

2. shower6 to change the type of resulting pattern. The image reversal scheme7 includes the image exposure of
a positive resist and subsequent UV flood exposure. The negative image is then obtained by using a
solvent to remove the resist material which was not exposed originally.
An article about the laser writing crosslinking process used for the fabrication of a negative mask for
Integrated Optics was published by OPN.8 A Korean team found that the laser beam crosslinking process is
not sensitive to dust, vibration, or intensity variation.
The goal for the present experiment is to perform an image reversal process on a positive photoresist layer
using laser beam exposure. This process utilizes the cross-linking characteristics of photoresist through
laser exposure. Different degrees of the cross-linking process can be achieved depending on the dosage of
laser exposure. The cross-linked areas of the resist are soluble in the developer at a rate that is much slower
than the regularly exposed areas. For extremely high dosages of crosslinking exposure, the crosslinked
areas are insoluble in the developer. As a result of further UV flood exposure of the resist, the regions
without laser exposure are removed by the developing process. Only the negative resist pattern remains on
the substrate.
'jU7 :1..J —) ..1
4- -: —:::
5 6
Fig. 1 Demonstration of the process sequence for the fabrication of negative and positive patterns on a
single photoresist layer.
SPIE Vol. 3333 / 1427
2. EXPERIMENTAL
The process sequence is shown in Fig. 1.
. (a)
(b)
(C)
(d)
(e)

3. a) The substrate ( I) with a metal layer (2) is coated with positive pliotoresist (3
h The laser beam (4) exposes photoresist locally in accordance with the negative (5) and positive (6)
pattern structures.
c) The photoresist layer is exposed with IJV light (7> through a pholomask (8 . The photomask has
transparent (9) and opaque (10) areas which correspond to the negative (5) and posItive(6) type pattern areas
needed.
After development of the photoresist layer, negative ( I I) and positive (12) pliotoresist patterns arc
created in resist on the metal surface.
The patterns are etched into the metal layer. The negative (13) and positive ) 14) type metal patterns are
created on the substrate surface.
Standard glass plates. I 'x 1" to 5" x 5" in site were used. The substrates were cleaned and dehydration
baked. Next the substrates were spin coated with positive photoresist. Different layer thicknesses were
applied by varying the spin speed and resist viscosity. In the current RP(' laserwriting system. a helium-cadmium
laser operating at 441 nm is used to expose the resist coated substrate, which is niounted on an
r-O air hearing spindle. Machine hardware and environment are sublect to closed-loop control to maintain
feature placement accuracy in the PPfl1 range. A 0.7pin laser beam spot site was used and the exposure
energ varied from 0.05 J/cm to 2.4 J/cm. Theexposure energy at fects the width of the exposed area as
well as the height of the negative photoresist pattern on the substrate. For experimental parts the
laserwriter was programmed to write a (qtni wide ring every S0pni.
The parts covered by chrome and gold layers were etched in chromium and gold etch solutions, respectively.
Results were obtained using optical profilometrv Zygo New View white-light surface profilomctem1 and
optical microscopy (Nomarski microscope).
3. RESULTS AND DISCUSSION
Figure 2 shows the results of an experimental part. The trenches on the positive pattern (bottom of the
picture) become the rectangular steps on the negative pattern (top of the picture. The top halt of the
pattern I Fig. 2h)] is the portion where the crosslinked resist behaves like a negative pattern The bottom
half of the pattern Fig. 2(c )I was masked during the UV flood exposure. The laser written pattern on this
part of the sample behaves like positive tone resist.
Fig. 2(a) Optical profilomctrv data of an experimental part.
(428 .SP)L t'oI. 3333

4. The height of the negative pattern is O.7311m [Fig. 2(h)], while the depth of the positive pattern is I
[Fig. 2(c)]. This gives a ratio of 1:2.7 for the negative to positive step tahncatcd by the laser beam
crosslinking method. This ratio can he controlled by varying the parameters of the process. e.g. the laser
exposure energy. the intermediate hake temperature and duration, and the UV 1100(1 exposure energy. The
intersect region between the positive and negative pattern areas can he inininiiied by providing a vacuum
contact between the mask and substrate during the UV flood exposure step. Various resist thicknesses.
writing parameters. intermediate hake parameters. flood exposure dosages. and developriient times were used
for process optimization. Because the height of the negative resist pattern that can be obtained with this
method is typically only a fraction (1/2 to 1/3) ot the initial resist layer thickness, it is difficult to control
the exposure and development parameters required to produce resist layers that are less than lllin in
thickness. On the other hand, for resist layers greater than 5pm in thickness, it is difficult to clear the flood
exposed areas completely because of resist hardening during the intermediate hake. A resist thickness of 3
to 511m is optimal for the reliable fabrication of the negative-type resist patterns.
The results in Fig. 3 show how laser power affects the negative pattern height and line width. For the
range of laser energies investigated, the height changed by ôO'3 and line width by 7ft/ . Further
investigations showed that this effect is stronger when the resist layer is spun on a metal-coated substrate.
Fig. 4 displays the results of such dependence for the same type of resist as in Fig. 3. hut spun on a
substrate coated with a 200 nm thick cold layer. The gold layer causes approximately a 5 times larger
iii i.
)ptical profilometry data of the UV flood exposed area (negative-type pattern.
Fig. 2(c) Optical profilometrv data of the unexposed UV flood exposure area positi'e-type pattern).

5. height and 2 times wider line width for the same laser exposure energy. The metal layer effect was
investigated further using a substrate coated with a l5Onm thick Cr layer. The positive resist was exposed
with a wide range of laser energies. Fig. 5 displays the line width measurements. Resist and chrome line
width increases more than 8 times with laser energy. This effect can be explained by the lateral thermal
diffusion from the laser exposed area, which causes the lateral expansion of the resist crosslinked area. The
temperature profile during laser exposure depends on the laser power, the efficiency of optical absorption,
and the thermal properties of the materials. The diffusivity parameter determines how far heat can flow
during the laser exposure. The values for polymers, glasses, and metals are 0.001, 0.01 and 1 cm2/s,
accordingly.9 The difference of 102 in thermal diffusivity between glass and metal materials is the cause of
the difference in lines widths seen in this experiment.
5
4
E
:1
.c:
C)
ci)
C
ci)
cci
C
cci
0
ci) C
-J
3
2
Fig. 4 Negative-resist pattern geometry fabricated on Au coated glass substrate.
1430 / SPIE Vol. 3333
1.0 1.5 22.0
Laser energy (J/cm
—II— Line width
-A- Pattern height
Positive Resist
4 im thick
RPC
RTXY-072
Fig. 3 Negative-resist pattern geometry fabricated on glass substrate.
E
C)
ci) .C
C
—— Line width
C 4 - A- Pattern height
2 Positive Resist
4 im thick
ci) C
-J
RPC
RTXY-085
0.4 0.8 1.2
Laser Energy (J/cm2)
1.6

6. 80 RPC
RTXY-086
Fig. 5 Negative—resist pattern Oeonietry fabricated on Cr coated class suhstratc.
Special attention should he paid to the narrowing of lines for the regions of low laser energy obtained with
the resist deposited on metal layers. With such low exposure energies. the metal layer acts as a heat sink
and decreases the effective temperature of the exposed resist layer. This inhibits the crosslinkinc process.
and consequently decreases the height and line width of the negative-type resist pattern.
4. APPLICATIONS
Different patterns were fabricated on glass substrates using the developed technique. Portions of the chrome
grid patterns, written on a 5x5" glass plate. are presented in Fig. 6.
F'ig. 6 Chrome grid line pattern fabricated by laser image reversal technique on 5x5" glass plate: a) 7piii
line width. 50 pni and IOU pm radii h 4 pm and IS pm line v idths.
4.2 Dual-wavelength diffractive elements
An exaniple of a diffractive element designed for operation at wavelength i is shown in Fig. 7(a). Another
diffractive element for a shorter wavelength X is shown in Fig. 7(h. The superposition of two such
elements is shown in Fig. 7(c).
SPIt 3 14 /
E
r'0
ci,
-J
25
20
15
10
5
—U-- Line width
Positive Resist
4 pm thick
0 20 40 60
2 Laser Energy (J/cm
4.1 Reticles and IR filters and polarizers

7. I I I I I I
(a)
I Ii fl fl fl fl fl rir n n n n nfl n n n n nfl n n n n ii n
_____________________________________I
ptInntl poonci panuq
ru Innnnnl Innnnnl Ii,
(c)
Fig. 7 Schematic representation of diffractive optical elements (DOEs): a) DOE for relatively long
wavelength; b) DOE for relatively short wavelength; c) dual-wavelength DOE.
The conventional method of fabrication for such elements consists of two lithography steps with an
alignment process in-between, and two etching processes:
First cycle:
1 . The substrate is coated with a resist layer and softbaked.
2. The resist layer is exposed through the first photomask, corresponding to pattern P2
3. The resist layer is developed in developer solution and hardbaked.
4. The substrate is etched by either a wet or dry etching process.
5. The resist is stripped from the surface of the substrate.
Second cycle:
6. The substrate is coated with a resist layer and softbaked.
7. The resist layer is exposed through the first photomask, corresponding to pattern P1.
8. The resist layer is developed in developer solution and hardbaked.
9. The substrate is etched in a wet or dry etching process.
10. The resist is stripped from the surface of the substrate.
Using the proposed method of positive/negative pattern fabrication on a single resist layer, the dual-wavelength
diffractive element can be manufactured by following this sequence of steps (Fig. 8):
1. The substrate is coated with a resist layer and softbaked.
2. The resist layer (1) is exposed with a laser beam (2) which creates the pattern (3) corresponding
to period P2 [Fig. 8(a)].
3. The resist layer is baked.
4. The resist layer is exposed by UV light (4) through a photomask (5), which contains opaque
regions (A) and transparent regions (B), corresponding to pattern P1 [Figure 8(b)];
5. The resist layer is developed in a developer solution and hardbaked [Fig. 8(c)].
6. The resist pattern is etched into the substrate by a dry etching process [Fig. 8(d)].
1432/SPIE Vol. 3333

8. 3
IIIIMUIHNHHHUII 1
5
(a)
H
_ _ _ _ —
....
— — — — —
....
—
DD
P2
D—ODøDIt--- DODD h
Irinrinril I nnn nfl In
Lm Pi .I
(c)
(d)
Fig. 8 Schematic representation of the process sequence for dual-wavelength diliractive element
fabrication by negative/positive patterning technique: (a) laser writing: (h UV exposure: (c developing: tdt
etching.
The proposed method of fabrication decreases the number of steps which are necessary to nianulacture such
elements. The conventional method uses 2 resist coating. 2 exposures. 2 developing, and 2 etching steps.
The proposed method uses only I resist coatIng. 2 exposure. I developing, and I etching process.
The required heights of the pattern Ii in areas A and B could he obtained through optical design and process
optimization. The required step height 11 is achieved h' providing the appropriate thickness of resist. '['he
period sites of patterns P1 and I' depend on the required angle of the diffracted beam, hut the ratioJ'1/P is
proportional to HI.
l-'ocusing and fan-out elements for the infrared and I.TV regtons of the spectrum with a visible diffractive
pattern for visualization and alignment purposes are applications for dual—wavelength dill raclive optics.
I'/I oI 14
A
-B'__
IImwulluII1 i nmnt
(b)

9. For example, CO2 laser material processing with 2 = 1O.2im requires P1/P,=16.l1 for pointing with
HeNe laser light. Such a dual-wavelength diffractive element can be fabricated in ZnSe material. In this
case, the step height H is 3.64tm and the pattern height h is n x O.l97jtm, where n=l,3,... Another
example of an JR/visible pair is Nd:Yag and HeCd, where ?1=l.O6.tm and A2=O.442im. This element can
be fabricated in IR-grade Fused Silica with the corresponding parameters of the structure P1/P2=2.4,
H=1.l8im, and h=n x O.48im. An example of a UV/visible pair is an ArF excimer laser with
A1=O.193jim and HeNe a laser with 22=O.6328im. A diffractive optical element fabricated in UV-grade
fused silica will have the following parameters: P1/P2=3.28, H=O.69im, and h=n x O.l82im.
5. SUMMARY
Photoresist crosslinking is a practical method to reduce the writing time of some phase masks. By
exposing a pattern with laser light and then backflooding the pattern with UV light, a negative image is
formed on a positive photoresist layer. The area of resist inversion can be controlled with a mask during
the UV flood step of the process. This allows the laserwriter beam to cover the smallest amount of area for
a desired pattern. By utilizing this technique, the manufacturing time of wire grid patterns can be
significantly reduced. Similarly, the manufacturing steps necessary to produce a multiple wavelength
diffractive element can be reduced.
The use of different materials and thickness for the metal layer under the resist gives additional flexibility in
the process design of negative-type resist pattern fabrication. The line width of the pattern can be controlled
not only by the laser scanning program, but also by using the effect of the metal layer. In the case of line
widening it can be used to conserve laser writing time. The effect of line narrowing will provide the
possibility to reach submicron resolution for laser written patterns, which is the subject of future
development.
Image reversal was used in this experiment to successfully create metal grid patterns on glass substrates.
By controlling the process parameters, the grid patterns varied in width from 3 to 3Oim.
6. ACKNOWLEDGMENTS
The authors would like to thank Dr. D. Raguin for his insightful discussions concerning this work.
This work was supported by the Physical Optics Program of the DARPA consortium (Contract MDA 972-
96-C-0801).
7. REFERENCES
1 . P. I. L. Hagouel, A. R. Neureuther, and A. M. Zenk, "Negative resist corner rounding. Envelope
volume modeling", J. Vac. Sci. Technol, B14, pp. 4257, 1996.
2. T. Yoichi and M. Saito, "Method for making a dry etching resistant positive and negative photoresist",
U.S. Patent 5,550,008, Aug. 27, 1996.
3. Y. Yamashita, R. Kawazu, T. Itoh, T. Asano, and K. Kobayashi, "Method of forming a photoresist
pattern", U.S. Patent 4,801,518, Jan 31, 1989.
4. M. McFarland, "Image reversal", U.S. Patent 4,775,609, Oct. 4, 1988.
5. J. Thackeray and A. W. McCullough, "Method of forming positive images through organometallic
treatment of negative acid hardening cross-linked photoresist formulations", U.S. Patent 5,079,131, Jan. 7,
1992. .
1434 /SPIE Vol. 3333

10. 6. K. Hashimoto, K. Yamashita, and N. Nomura, "Fine pattern forming method", U.S. Patent 5,186,788,
Feb. 16, 1993.
7. H. Moritz and G. Paal, "Method of making a negative photoresist image", U.S. Patent 4,104,070, Aug.
1, 1978.
8. K. H. Park, Y. T. Byun, M. W. Kim, S. H. Kim, S. S. Choi, W. R. Cho, S. H. Park, and U. Kim,
"Negative mask fabrication technique by laser writing for integrated optics", OPN 8, pp. 49, 1997.
9. A. Marchant, "Optical Recording", Addison Wesley, 1990.
SPIE Vol. 3333 / 1435