Lithography beyond 64Mb

Microelectronic Engineering 21 (1993) 3-10 Elsevier

Lithography

3

beyond 64Mb

N. Nomura, K. Yamashita, M. Endo, and M. Sasago Semiconductor

Research Center, Matsushita Electric Ind. Co., Ltd.

3- 1- 1 Yagumo-nakamachi,

Moriguchi, Osaka 570, Japan

Abstract The delay in optical lithography development,

which has contributed miniaturization of LSI,

is pushing the R&D trend of DRAM development one year behind from the previous trend. The application of KrF excimer laser lithography which has been the leading optical lithography is successfully

achieved

with enough

process latitude for 64M bit DRAM fabrication.

KrF

excimer laser lithography will become a factory use technology. The extension technologies of KrF

and

ArF excimer

developments. 1. Impact

greatly

of delay

manufacturing

contributed Figure

lithography

0.25-0.15

micron

rule

device

developments

has progressed for last three decades, and has offered industry.

The break-throughs

in optical lithography

an

have

to the miniaturization of LSIs and have realized the down sizing of 1 shows

the trends of lithography

is facing major limitations in resolution

significant exposure

in lithography

growth in electronics

computer.

will support

Optical lithography will provide 0.25-0.2 micron factory use lithography.

Semiconductor enormous

laser lithographies

trend in lithography

and DRAM developments.

Optical

and depth of focus (DOF). The most

over the last decade has been the gradual replacement

of

wavelength from g-line (436nm) to i-line (365nm) and further shorter wavelength

light, such as KrF (248nm)” and ArF (193nm)” excimer laser. The replacement has been very slow. The progress of lithography has been mainly affected by the development projection

cycle of the

lens. The reduction rate in resolution has been 82 % per 3 years. The trend of

lithography development has crossed over the R&D trend of DRAM development last year. The development

for supplying 64M bit DRAM commercial sample (CS) will delay for over one

year to the previous trend.

0167-9317/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.

4

N. Nomura et al.

On the other hand, DRAM has been the business driver and miniaturization driver. The miniaturization speed of device feature size has been 70 % per 3 years. In the early 80’s, the investment for a semiconductor factory was the 30-40 million dollars range. A factory invested around the mid 90’s for the production of LSIs with less than 0.4 micron feature size will far exceed the 1 billion dollars. The investment for 16M bit DRAM factories delayed for 1 year. Figure 2 shows the trend of normalized wafer cost. Normalized wafer cost has been growing at an alarming rate beginning with the 1 micron technology one-wafer-at-a-time

generation.

The low throughput

processes such as dry etching and CVD became dominant from 4M bit

DRAM manufacturing.3’

Also, process complexity

multi-layer interconnection.

has been constantly

increasing,

such as

It can be extrapolated that at this rate, capital cost will completely

dominate wafer cost by the end of the century. People begin to think it is very difficult to expect a healthy return from DRAM business. The paradigm of DRAM development has to change to a new paradigm of alternative devices!? 2. Status

of 0.35

micron rule lithography

The annular illumination4’ enhances the resolution limit and DOF latitude of i-line lithography down to 0.4 micron without using a phase shift mask (PSM).S’ The PSM methods also enable to improve

resolution

and DOF. However,

in those techniques,

the projected patterns are

limited to special patterns, or unnecessary patterns are formed at the shifter edges. A projection capability of voluntary 0.35 micron features is required for the production of 64M bit DRAM. Table 1 shows the specifications of the new in-house KrF excimer laser lithography system” with wide projection field, high alignment accuracy and a new in-house positive ASKA resist” with high sensitivity and stability using photo acid generator (PAG). The system employed a quartz lens with NA of 0.42 and 20 mm square field, a new heterodyne holographic “through the reticle” alignmen?’ system having 9-axis stage with high precision of 30 nm and chromatic compensation optics for referencing the reticle by He-Ne laser wavelength, and the high quality beam KrF excimer laser with 5W at 200 Hz. Overlay accuracy of the system is 0. I micron. The system features high throughput

as I5 slices/hour

in case of 8” wafer by using the high

sensitivity and high stability chemical amplified (CA) positive resist with 30 mJ/cm2. Patterns with 0.35 micron features were delineated with wide DOF latitude of about 1.8 micron at whole field. Figure 3 shows an SEM photograph of a 64M bit DRAM polycide bit-line. Figure 4 shows overlay results of processed wafers. Application to 64M bit DRAM fabrication will be successfully achieved with enough process latitude. KrF excimer laser lithography will become a factory use technology.

Lithography beyond 64Mb

3. Lithographies

for 0.25 micron

5

rule devices

We have made three attempts to obtain 0.25-0.20 micron pattern fabrication. The first attempt was an extension technology of the most simple KrF excimer laser lithography with a new CA positive single layer resist.” Also, to accomplish the critical dimension control, the resist was used in cooperation with overcoat film and/or anti-reflective coating on high reflective surface. The new resist developed here is composed of an alkaline-protected polymer, a PAG and surface-inhibition exposure,

the deprotection

polyvinylphenol

based

reducing reagent. Upon the generated photo acid by an

of the polymer begins. In which, the surface-inhibition

reducing

reagent plays an important role. It prevents the formation of the insoluble surface skin on the exposed resist, which leads to the high resolution and the stable pattern fabrication in a time delay between the exposure and the following post exposure bake. At last, the positive tone patterns are fabricated by an alkaline developer. The over coat film is made of water-soluble polyvinylalcohol

derivative.

The refractive

index of this polymer is 1.3, which is suitable for the new resist ( index 1.7 ) under KrF excimer laser exposure. composed

The under-lying

of polymethacrylate

24&m-absorption

film of a resist as an anti-reflective

as a binding

polymer

and diazonaphthoquinone

coating

is

as an

dye. The experimental conditions are listed in table 2.

Figure 5 shows a SEM photograph of the 0.25 micron lines/spaces patterns of the chemically amplified resist. The rectangular patterns on a silicon substrate are clearly shown. 0.35 micron contact hole patterns without re-entrant profile are deliniated in this resist. The DOF for this resist is 0.9 micron at 0.25 micron patterns (Fig.6). The stability of the resist after exposure is shown in Fig.7. The time delay effect ( the critical dimension

change due to the time between exposure

and post exposure

bake) was much

reduced, as compared to our former resist. The dimensional change of within 0.02 micron at 0.25 micron patterns for more than 100 hours is accomplished. Figure 8 shows the multiple interference effect due to the reflection from the air and substrate. Even if this advanced positive resist is used, there is 0.2 micron critical dimension variation of 0.25 micron patterns due to the resist thickness variation (a). In order to solve this problem, we tried two approaches.

Both of the overcoat film (b) and anti-reflective coating layer (c) much

reduced the swing curve of the ciritical dimension variation of the resist. It is expected that the combination of these methods is more effective for the multi interference effect. The second approach studied here is surface imaging (SI) process. Silylation process is one of the most attractive SI techniques for deep-UV lithography. We have demonstrated the quarter

6

N. Nomura

et al.

micron pattern fabrication by using KrF excimer laser lithography with silylation SI process. By combining the silylation process with PSM, excellent resolution of 0.2 micron and DOF of about 1.2 micron are obtained.‘“’ Experimental conditions are shown in table 3. Figure 9 shows the exposure latitude for silylation process. The latitude of *lo% nominal linewidth was ? 10%

change in

for 0.3 micron and * 7% for 0.25 micron lines/spaces.

The

values were large enough for the stable pattern fabrication. Figure 10 shows SEM photographs of 0.3-0.225

micron lines/spaces patterns in 1.25 micron thick PLASMASK 30 1U (UCB-JSR

Electronics). Rectangular patterns with high aspect ratio were obtained. Mask linearity down to 0.225 micron was achieved. The combination of SI and PSM is the most advanced lithography in resolution. shows SEM photographs

Figure 11

of 0.20 micron lines/spaces as a function of focus offset for KrF

excimer laser lithography with silylation process. The DOF for 0.2 micron lines/spaces for the combination

technology

was 1.2 micron as shown

in figure

12. Figure

enhancement of focus latitude among four different technologies.

13 shows

the

The focus latitude of KrF

excimer laser lithography with PSM+SI is enhanced about 0.5 micron than that with PSM and is enhanced about 1.2 micron than that with an ASKA single layer resist. ArF excimer laser lithography has been considered to be one of the candidates to achieve quarter micron pattern fabrication. Resolution of ArF excimer laser lithography was compared with KrF excimer laser lithography

with PSM. The image contrast of ArF excimer

laser

lithography was enough high to realize 0.25 micron lines/spaces pattern without any pattern restrictions.

The main technical inhibitor of ArF excimer laser lithography

absorption

of resist material, lens material, and optical path in the projection

improve the transmittance of projection lens, we proposed

is the optical

a new monocromatic

system.

To

aspherical

projection lens system as shown in figure 14.2’The lens system is composed of 7 elements with 15 mm square field and NA 0.45. The total lens thickness of the aspherical lens was decreased to 16 cm. A new ArF excimer laser lithography with a new CA positive resist have been developed for 0.20

micron

polymethacrylate

pattern

fabrication.

The

resist

is composed

of

alkaline-soluble-protected

and PAG. To prevent large photo absorption at 193 nm, the aromatic molecule

was excluded in both of the polymer and PAG. An extreme high transmittance of 75% in 1 micron thick resist was obtained at 193 nm. After the 193nm exposure and proceeding

post

baking, the resist becomes alkaline-soluble as the results of the photo cleave of the polymer. Owing to the high transmittance and high sensitivity ( 15 mJ/cm2) of the resist, rectangular 0.2 micron lines/spaces patterns were obtained using ArF excimer laser stepper with 0.45 N.A

Lithography beyond 64Mb

7

(figure 1.5). The etching ratio of this resist was very comparable

with that of conventional

novolac resin type resist. The DOF for 0.2 micron lines/spaces was 1.O micron. ArF excimer lithography

will provide 0.2.5-0.2 micron factory use lithography,

but many inhibitors are

obstructing before the solutions. The extension technologies of ArF excimer laser lithography may realize 0.15 micron resolution. 4. Summary Table 4 shows the summary of KrF and ArF excimer laser lithographies.

KrF excimer laser

lithography has enough process latitude for 0.35 micron rule device fabrication and will become a factory use technology. The extension technologies of KrF excimer laser lithography,

such

as KrF+SI

KrF

and KrF+SI+PSM,

excimer laser lithographies

are usefull for 0.30-0.20

are promising

micron pattern fabrication.

for the development

tools of 0.25 micron rule

devices. ArF excimer lithography will provide 0.25-0.2 micron factory use lithography. Acknowledgements The authors would like to thank Dr.T.Takemoto

and Mr.H.Esaki for their encouragements

and supports on this work. References l)M.Sasago et al., Extended Abstracts of the 1991 Int. Conf. on Solid State Devices and Materials, p472( 199 1) 2)N.Nomura et al., Microelectronic

Eng., u, 183( 1990)

3)P.Gargini et al., Proc. SEMICON/Kansai-Kyoto

92 Technology

Seminor, p15(1992)

4)S.Matsuo et al., Tech. Dig. of IEDM, p970( 199 I) S)M.D.Levenson

et al., IEEE Trans., ED-29 3828(1982)

6)T.Sato et al., Dig. of MicroProcess Conf..‘p72( 199 1) 7)Y.Tani et al., Dig. of Sympo. on VLSI Technol., p7( 1990) 8)N.Nomura et al., Microelectronic

Eng., u,l33(

1990)

9)M.Endo et al., Dig. of Sympo. on VLSI Technol., pl lO(1992) lO)T.Matsuo et al., J.Photopolymer

Sci. and Technol., 5,14 1( 1992)