Advanced plasma X-ray source using multi-reflecting optics: Recent results

MICROELECTRONIC ENGINEERING

ELSEVIER

Microelectronic Engineering 30 (1996) 231-234

Advanced plasma x-ray source using multi-reflecting optics: recent results

S. X~ Kang~ and I-[Y. Kang b ~Center for Fundamental Physics, University o f Science and Technology o f China, Hefei, Anhui

230026, P.R. China bDepartment of Materials Science and Engineering, University of Science and Technology, Hefei, Anhui 230026, P.R. China In this paper, we described an x-ray lithography system composed with an advance high brightness metal plasma source (Z-pinch) producing about 10 J radiation per pulse and 10 rrd/cm2 average x-ray power at 20 cm working distance, and an improved multi-reflecting x-ray lens with 79 mm focus distance and up to 30% transmittance having a modified construction by means of"whole body stretch" techniques. 0.3 gm resist patterns was successfully replicated in x-ray lithography. our experiments of x-ray exposure in 0.3 ~rn 1. INTRODUCTION linewidth scale. As a rapid development high technology, x-ray lithography is stepping into 0.1 ~tm scale as well as nanometer applications. The solid target xray source has been excluded in production due to its very low x-ray yield and poor intensity. The ideal and clean x-ray source is synchrotron radiation for its high intensity and parallel characteristics [ 1]. Its very high cost and stationnary beamline limit the practical volume throughout to most IC factories. Plasma sources, laser driven and discharge types, axe ones of just making up these disadvantages because of the lower cost, movability, acceptable xray intensity. Therefore plasma x-ray sources are very promising for many researchers. Although the metal plasma x-ray source can be directly used in x-ray exposure, as a point source, 2-3 mm diameter typically, will cause runout and penumbral blur to affect the minimum linewidth in x-ray lithography. Multi-reflecting lens can successfully resolve these problems due to its high effiecient, low cost and its focusing ability to broadband radiation, by means of which, we can obtain output near parallel radiation with little divergent angle. Here we reported a exposure system equiped with an advanced Fe plasma source combined with a new type multi-reflecting optics group, which let us obtain excellent performance in

2. PLASMA SOURCE The metal charged plasma (Z-pinch) soft xray source was developed. Anode is Fe/W materials of 4 mm diameter and 18 mm average length mounted on a copper disk support of 50 mm diameter. The plate shape cathode was also made of Fe/W with a 2 mm hole in center. The gap between anode and cathode was kept to 5.5 mm which affect strongly the x-ray radiation yield and the spatial position of plasma focus spot. The high power voltage was fixed at 20 kV. The discharge repeated frequency can be varied in the range 0.2 to 1 Hz, limited by the thermal shock to the mask. An YAG laser was used to trigger the pulse discharge. The laser beam passed through the center hole of the cathode perpendicular to x-ray beam. The energy was about 10 J per pulse, which can give us 8-10 rrd/Cm z power radiation per pulse at 20 cm working distance from the source. The average dispersion of focus spots for 1000 discharges was less than 2 mm diameter. The anode can steadily work more than 2000 discharges. The consumption rate of the anode is about 0.15 mm every 100 charges. A stepping motor was used to control the gap between anode

0167-9317/96/515.00 © 1996 - Elsevier Science B.V. All fights reserved. S S D I 0167-9317(95)00234-0

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S.X. Kang, H.Y. Kang / Microelectronic Engineering 30 (1996) 231-234

and cathode. A PTI Laser Energy Meter made by IOFM, Shanghai, served to measure the x-ray power on line. However, contamination and mask damage associated to target bombardment by debris emitted from the discharge process are always serious problems. As we measured, the charged pieces are 90% more of total debris. In our plasma source, a pair of permanent magnets of 4500 Gauss was put on aside of the path of x-ray beam. It can deviate almost all the charged heavy metal particles. In this way, it was effective to protect mask from damaged by charged particles. It also can reduce energy deposition on the mask, which will result in possible distorsion. Further more, to minhnize the influence of no-charged and left charged target debris, a 5 !am aluminium window was used to separate source and lens and 1 ~tm thickness aluminium filter was put on the front of the mask. ]'he window and filter can also cut off the longer wavelength x-ray above 2 nna, unused part of radiation. The peak wavelength is about 1.4 nm and typical spectrum distribution is shown in figure 1. 10

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composed with thousands of thin glass tube bending and forming a fixed shape. It has high transmutability in broadband x-ray spectrum based on total reflection, easily modulated structures by controlling bent shape of each tube to fit different requirements, e. g. x-ray focusing and beam expanding, large angular of radiation capture and low cost. These characteristics show that its very, promising application foreground. Here the lens we used in experiments was composed with about 5000 glass tubes of 120 ~tm outer diameter and 70 p.m inner diameter. Utilizing advanced "whole-body stretch" techniques, we successfully increased the transmitivity from 10% to 30%. The tuned array construction improved greatly the output intensity and uniformity. For our lens N°I, its diameter of the small end of the lens is 8.5 mm and the larger end is 11.7 mm. The total length of the lens is 77 mm. The rate of the occupy and the space is about 0.6. The focus length is 79 mm, typical transmitted efficiency is about 18-30%. Combined with the plasma source described above, it gives a near parallel x-ray beam with a divergence of about 2-4 mrad. After 10 cm far from the lens, the x-ray can be mixed up quite well. Figure 2 shows the x-ray intensity distribution (near Gaussian type) at the position of 14 cm far from the lens with a 5 ~tm AI filter, measured with a GaAs diode scanned across the section of the beam.

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2 3. MULTI-REFLECTING OPTICS In order to obtain near parallel x-ray beam which is very important to have uniform exposure dose on a large area, a broad wavelength reflecting x-ray lens, "Kumakhov lens" [2], was fabricated and was used as x-ray optics in our system. This kind of lens is

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S.X. Kang, H.Y. Kang / Microelectronic Engineering 30 (1996) 231-234

From figure 2, one can find the uniformity of the intensity is better than 90% in a 5x5 n l l ' n 2 field in the case of our test with output lens diameter of 11.7 mm and the uniformity remain better than 90% in a 10xl0 mm2 field with the output lens diameter of 22 ram. 4. EXPERIMENTS AND RESULTS The plasma source was working in a vacuum chamber pumped with a 500 liter ion pump up to 10-4 Torr. Exposure was also carrried out in vacuum at 10-4 Torr. The 5 grn AI window was located 12 cm apart the source and the lens was placed 8 cm from the window. The mask and wafer were 15 cm down stream of the output end of the lens. About 1.2 mJ/cm2 average x-ray energy can be obtained on the surface of the resist. Figure 3 shows the exposure system diagram.

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the focus length in our source as well as the minimum replicating patterns of 0.3 gm, to minimize diffraction effect and nln-out blur, the gap between wafer and mask was kept at 10 gm. Several kind of resists have been tested and evaluated in our experiments, including PMMA KTI, RAY-PF, RAY-PN, FBM-120 and some negative and positive resists supplied by Wuxi, China. We found RAY-PN and FBM-120 resists have the best resolution, acceptable sensitivity, excellent process latitude and high heat stability. For RAY-PN resist the thickness was about 0.8 gm spinned onto the wafer. The exposure doses were 80-120 pulses corresponding to 100-150 mJ/cm2 onto the surface of the resist. Under optimized conditions [3] [4], the developing time was 120 seconds. With the PMMA-KTI resist (Molecular weight 495k, 7%), the exposure dose for 0.5 gm thick resist was 1200-1500 pulses corresponding to 1500-1800 mJ/cmz and development time was 45 seconds. This exposure doses ask for too large exposure time for practical application. For positive resist made by Wuxi, P(MMA-COMMA) the exposure doses was about 1000-1200 pulses corresponding to 1250-1500 rnJ/cm2. Figure 4 shown the SEM photograph of patterns replicated onto RAY-PN resist. The minimum linewidth is 0.3 gin

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According to the spectrum range of 0.8-2 nm in our case, which is softer than that used with the synchrotron radiation, 0.4-1.4 nm typically, we chose only 1 grn thick Si3N4 as mask membrane in the way to reduce x-ray absortion. The mask absorber in our case is 0.6 gin thick gold. The minimum structure transfered onto the mask is 0.3 ~tm. Mask diameter is 50 mm. A zoneplate mask with 0.5 gm thick Kapton membrane and 0.5 gm gold absorber was also used in tests. Considering the x-ray beam structure and

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Replication pattern in RAY-PN? resist; the minimum linewidth is 0.3 ~tm on 0.8 ixm resist thickness.

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S.X. Kang, H.Y. Kang / Microelectronic Engineering 30 (1996) 231-234

5. CONCLUSION Experiments have shown that the advanced metal plasma x-ray source is highly steady and has good repeatability. The multi-reflecting optics has demostrated its excellent performance in high efficiency and high transmitivity of 30%. This lithography system, combining the above x-ray source and lens, can be suitable for practical application. Resist pattern of 0.3 Ixm minimum linewidth have successfully been replicated. REFERENCES

1. E.Burattini, A. Grilli, A. Balema, E. Bemieri, S. Simeoni, L. Mastrogiacomo, M. Gentili, A. Raco, Kang Shixiu, "X-ray lithography at Frascati: first results" 2nd European Conference on Progress in X-ray Synchrotron Radiation Research Vol 25 (1989) 25 2. I~A. Kumakhov, F.F. Komarov, "Multiple reflection f~om surface x-ray optics" Physics Reports N°5 (1990) 289-350 3. Kang Shixiu "Resist process in synchrotron radiation x-ray lithography" Proceedings of the Third International Conference on Solid State and Integrated Circuit Technology (1992) 157-159 4. Kang Shixiu, A. Grilli, A. LAco, "X-ray mask copying based on negative resist RAY-PN" Microelectrouic Engineering 23 (1994) 231-234