Negative pattern fabrication using laser exposure of positivephotoresist

Boris Kobrin and Colleen Hagen

Rochester Photonics Corporation330 Clay Road, Rochester, NY 14623

ABSTRACT

The method of simultaneous positive and negative pattern formation on a single positive photoresist layeris described. A negative photoresist pattern was fabricated by using local laser exposure to crosslink apositive resist layer, consecutive UV flood exposure, and resist developing. The positive pattern isobtained on the same photoresist layer in the areas masked from the UV flood exposure. Effects of laserenergy and resist processing parameters on height and width of negative type resist structures wereinvestigated. Metal line grid structures with lines in the region of 3 to 30 pm in width were manufacturedon a 5"x 5" glass substrate using this technique. The proposed method of positive/negative patternformation significantly reduces the number of technological steps in the fabrication of diffractive elementsfor dual-wavelength applications.

Key words: photoresist, negative pattern, positive pattern, laser exposure, crosslinking, diffractiveelements.

1. INTRODUCTION

Laser direct write lithography is a powerful method for fast prototype fabrication of arbitrary relief patternsin photoresist. This technique does not require the manufacture of a photomask or series of masks, as inthe case of conventional contact printing or stepper exposure. Laser writing also does not require the highvacuum processing and super-cleanliness that E-beam writing methods do. However, laser direct writingtechniques can have long write times depending upon the resolution required. Consequently, it is desirableto decrease the writing time as much as possible. The laser writing time to manufacture a grid pattern, forexample, can be reduced drastically by using a negative type method of resist processing. In this case thelaser 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 verypronounced feature of negative resists.' An image reversal (IR) scheme is an opportunity to obtain anegative pattern using positive photoresist.

The JR capability is obtained by using a special crosslinking agent in the resist formulation which becomesactive only in the exposed areas of the resist. The crosslinking agent, together with the exposedphotoactive compound, leads to an almost insoluble substance in developer which is no longer lightsensitive. The unexposed areas still behave like normal unexposed positive photoresist. After a floodexposure, these areas are dissolved in standard developer for positive photoresist, and the crosslinked areasremain. The overall result is a negative image pattern.

Many different IR techniques are published. Among them are methods which use special chemicalformulations of resist that give positive or negative patterns depending on the type of developer solutionused.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

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'\j7 1J)U:.. —. .1

- - —4::::5 6

Fig. 1 Demonstration of the process sequence for the fabrication of negative and positive patterns on asingle photoresist layer.

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shower6 to change the type of resulting pattern. The image reversal scheme7 includes the image exposure ofa positive resist and subsequent UV flood exposure. The negative image is then obtained by using asolvent 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 forIntegrated Optics was published by OPN.8 A Korean team found that the laser beam crosslinking process isnot sensitive to dust, vibration, or intensity variation.

The goal for the present experiment is to perform an image reversal process on a positive photoresist layerusing laser beam exposure. This process utilizes the cross-linking characteristics of photoresist throughlaser exposure. Different degrees of the cross-linking process can be achieved depending on the dosage oflaser exposure. The cross-linked areas of the resist are soluble in the developer at a rate that is much slowerthan the regularly exposed areas. For extremely high dosages of crosslinking exposure, the crosslinkedareas are insoluble in the developer. As a result of further UV flood exposure of the resist, the regionswithout laser exposure are removed by the developing process. Only the negative resist pattern remains onthe substrate.

2. EXPERIMENTAL

The process sequence is shown in Fig. 1.

. (a)

(b)

(C)

(d)

(e)

a) The substrate ( I) with a metal layer (2) is coated with positive pliotoresist (3h 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 areasneeded.

After development of the photoresist layer, negative ( I I) and positive (12) pliotoresist patterns arccreated 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 dehydrationbaked. Next the substrates were spin coated with positive photoresist. Different layer thicknesses wereapplied 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 anr-O air hearing spindle. Machine hardware and environment are sublect to closed-loop control to maintainfeature placement accuracy in the PPfl1 range. A 0.7pin laser beam spot site was used and the exposureenerg\ varied from 0.05 J/cm to 2.4 J/cm. The exposure 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 thelaserwriter 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 bottomhalf 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.

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Fig. 2(a) Optical profilomctrv data of an experimental part.

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 beamcrosslinking method. This ratio can he controlled by varying the parameters of the process. e.g. the laserexposure energy. the intermediate hake temperature and duration, and the UV 1100(1 exposure energy. Theintersect region between the positive and negative pattern areas can he inininiiied by providing a vacuumcontact 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 usedfor process optimization. Because the height of the negative resist pattern that can be obtained with thismethod is typically only a fraction (1/2 to 1/3) ot the initial resist layer thickness, it is difficult to controlthe exposure and development parameters required to produce resist layers that are less than lllin inthickness. On the other hand, for resist layers greater than 5pm in thickness, it is difficult to clear the floodexposed areas completely because of resist hardening during the intermediate hake. A resist thickness of 3to 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 therange of laser energies investigated, the height changed by ôO'3 and line width by 7ft/ . Furtherinvestigations 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 asubstrate 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).

height and 2 times wider line width for the same laser exposure energy. The metal layer effect wasinvestigated further using a substrate coated with a l5Onm thick Cr layer. The positive resist was exposedwith a wide range of laser energies. Fig. 5 displays the line width measurements. Resist and chrome linewidth increases more than 8 times with laser energy. This effect can be explained by the lateral thermaldiffusion from the laser exposed area, which causes the lateral expansion of the resist crosslinked area. Thetemperature 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 flowduring 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 ofthe difference in lines widths seen in this experiment.

Fig. 4 Negative-resist pattern geometry fabricated on Au coated glass substrate.

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5

4

E:1

.c:C)ci)

Cci)

cci

Ccci

0ci)C-J

3

2

1.0 1.5 22.0Laser energy (J/cm

—II— Line width-A- Pattern height

Positive Resist4 im thick

RPCRTXY-072

Fig. 3 Negative-resist pattern geometry fabricated on glass substrate.

E

C)ci).CC

—— Line width

C 4 - A- Pattern height

2 Positive Resist4 im thick

ci)C-J

RPCRTXY-085

0.4 0.8 1.2Laser Energy (J/cm2)

1.6

80 RPCRTXY-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 withthe resist deposited on metal layers. With such low exposure energies. the metal layer acts as a heat sinkand 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 chromegrid 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) 7piiiline 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). Anotherdiffractive element for a shorter wavelength X is shown in Fig. 7(h. The superposition of two suchelements is shown in Fig. 7(c).

SPIt 3 14 /

E

r'0

ci,

-J

25

20

15

10

5

—U-- Line width

Positive Resist4 pm thick

0 20 40 602

Laser Energy (J/cm

4.1 Reticles and IR filters and polarizers

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 panuqru Innnnnl Innnnnl Ii,

(c)

Fig. 7 Schematic representation of diffractive optical elements (DOEs): a) DOE for relatively longwavelength; b) DOE for relatively short wavelength; c) dual-wavelength DOE.

The conventional method of fabrication for such elements consists of two lithography steps with analignment 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 P23. 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) correspondingto 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 opaqueregions (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)].

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3

IIIIMUIHNHHHUII 1

5

(a)

H

_ _ _ _ —....

— — — — —

....—

DDP2

—øIt---DODD DODD hIrinrinril I nnn nfl InLm Pi .I

(c)

(d)

Fig. 8 Schematic representation of the process sequence for dual-wavelength diliractive elementfabrication 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 suchelements. 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 processoptimization. The required step height 11 is achieved h' providing the appropriate thickness of resist. '['heperiod 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 diffractivepattern for visualization and alignment purposes are applications for dual—wavelength dill raclive optics.

I'/I oI 14

A

B-'__IImwulluII1 i nmnt

(b)

For example, CO2 laser material processing with 2 = 1O.2im requires P1/P,=16.l1 for pointing withHeNe laser light. Such a dual-wavelength diffractive element can be fabricated in ZnSe material. In thiscase, the step height H is 3.64tm and the pattern height h is n x O.l97jtm, where n=l,3,... Anotherexample of an JR/visible pair is Nd:Yag and HeCd, where ?1=l.O6.tm and A2=O.442im. This element canbe 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 withA1=O.193jim and HeNe a laser with 22=O.6328im. A diffractive optical element fabricated in UV-gradefused 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. Byexposing a pattern with laser light and then backflooding the pattern with UV light, a negative image isformed on a positive photoresist layer. The area of resist inversion can be controlled with a mask duringthe UV flood step of the process. This allows the laserwriter beam to cover the smallest amount of area fora desired pattern. By utilizing this technique, the manufacturing time of wire grid patterns can besignificantly reduced. Similarly, the manufacturing steps necessary to produce a multiple wavelengthdiffractive element can be reduced.

The use of different materials and thickness for the metal layer under the resist gives additional flexibility inthe process design of negative-type resist pattern fabrication. The line width of the pattern can be controllednot only by the laser scanning program, but also by using the effect of the metal layer. In the case of linewidening it can be used to conserve laser writing time. The effect of line narrowing will provide thepossibility to reach submicron resolution for laser written patterns, which is the subject of futuredevelopment.

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. Envelopevolume 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 photoresistpattern", 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 organometallictreatment of negative acid hardening cross-linked photoresist formulations", U.S. Patent 5,079,131, Jan. 7,1992. .

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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.

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