KrF EXCIMER LASER LITHOGRAPHY TECHNOLOGY FOR 64MDRAM

Transcription

1 Journa' of Photopolymer Science and Technology Volume 4, Number 3 (1991) KrF EXCIMER LASER LITHOGRAPHY TECHNOLOGY FOR 64MDRAM MASAYUKI ENDO, YOSHIYUKI TAM, TOSHIKI YABU, SHOZO OKADA MASARU SASAGO and NOBORU NOMURA Semiconductor Research Center, Matsushita Electric Ind. Co., Ltd. Moriguchi 570, JAPAN New KrF excimer laser resist was developed. The chemically amplified positive resist, named ASKA, attained high sensitivity (3OmJ/cm2), high resolution (0.3 micron) and high stability. This resist was successfully applied to the fabrication of 64M DRAM with KrF excimer laser stepper (N.A 0.42). This paper describes the chemistry and lithographic characteristics of ASKA, followed by the demonstration of actual pattern fabrication of 64M DRAM. i 1. Introduction As the density of LSI, or DRAM increases, the minimum feature size patterns required for lithography technology is becoming below 0.4 micron. KrF excimer laser lithography has been one of the most promising technology since AT&T published their paper in 1986[1]. There are lots of efforts for this lithography and it was reviewed[2]. Some problems, however, are retarding the accomplishment and production-use of KrF excimer laser lithography. The low power and unstability of the KrF excimer laser is not enough due to the short life time of a narrow band system of a laser. The present deep UV resist cannot be applicable to KrF excimer laser lithography because it has low sensitivity and low resolution. The alternative new lithography technology, which is phase-shift mask[3] or EB cell projection [4], has been proposed. There are still many difficulties such as huge data treatments or expensive machine. Received Accepted April 21, 1991 May

2 We have developed new excimer laser and excimer laser resist to realize the actual use of KrF excimer laser lithography. The laser is PCR ( Poralization Coupled Resonator ) type laser[5], which attained high power and long life time. The resist is positive chemically amplified resist[6], which achieved high sensitivity, high resolution and high stability. In this paper, we report on the characteristics of the new resist and the success of the fabrication of 64M DRAM using this resist. 2. Chemistry of the positive chemically amplified resist The chemically amplified positive resist developed here is named ASKA (Alkaline Soluble Kinematics using Acid generating positive resist). ASKA is composed of a polymer, a photo acid generator and a solvent. The polymer is polyvinylphenol-type polymer, which becomes alkaline soluble by an acid through the photo acid generator upon KrF excimer laser irradiation. The transmittance of ASKA for 248nm is 44% for ASKA 1 (Fig. 1) and 65% for ASKA2 (2nd ASKA version) [7] (Fig.2). Fig. 1 UV Characteristics of ASKA 1 Fig.2 UV Characteristics of ASKA2 362

3 3. Experimental ASKA1 or ASKA2 was spun on a substrate to a thickness of 1.0 micron. Exposure was done on a KrF excimer laser stepper (Nikon NSR-1505EX, N.A0.42) using our developed PCR type excimer laser. After the post exposure baking of 90sec. at 95 deg., resist pattern was fabricated by 60sec. development of TMAH developer. 4. Lithographic characteristics of the positive chemically amplified resist Sensitivity Fig.3 shows sensitivity of ASKA2. It is found that the resist has high sensitivity (25mJ/cm2) and high gamma value(3.0). Resolution and pattern profile Resist pattern profiles of ASKA1 or ASKA2 are compared in Fig.4 or Fig.5, respectively. These SEM photographs represent 0.4 micron down to 0.3 micron line-and-space patterns on Si substrate. Both resists can successfully fabricate sub-half micron patterns with high aspect ratio. ASKA2 exhibits rectangular pattern profile without resist thickness loss, to which the higher transmittance of ASKA2 contributed. Fig.6 shows contact hole patterns of ASKA2. Straight 0.4 micron contact hole patterns, which are applicable to making 64M DRAM, were realized with large depth of focus range of 1.2 micron. Linewidth control The linewidth control of ASKA2 on depth of focus and exposure energy are investigated in Fig.7 and Fig.8, respectively It is seen that there is 2.1 micron budget of depth of focus and 5mJ/cm2 budget of exposure energy for 0.4 micron line-and-space patterns (0.4 micron+/-10% linewidth change allowed). These values are much lager than a case of exposure by g-line or i-line. ASKA2 attained the advantage of KrF excimer laser exposure. Mask linearity The relationship between the mask and pattern size was linear down to 0.4 micron on ASKA2(Fig.9).10% mask vias can lead it to 0.3 micron pattern fabrication. Stability Unstability of a chemically amplified resist on a delay time after exposure has been indicated[8]. Fig. 10 shows the excellent stability of ASKA2. The 0.4 micron patterns of ASKA2 do not change at all during a half hour delay. This means that this resist could be easily used for production of LSI. 363

4 Multiple interference effect and bulk effect on patterns Linewidth change of ASKA2 versus resist thickness change on Si or SiN substrate is shown in Fig. 11 or Fig. 12. As found in Fig. 13, high reflectivity of the substrate for KrF excimer laser wavelength (248 nm) enhances the effect of multiple interference between the resist film and the substrate, which enlarges the Linewidth change. This problem is connected with the short wavelength of KrF excimer laser and it has to be resolved for actual device fabrication. Fig.14 is the case on TiNJAISiCu. TiN as the anti-reflective layer decreases the reflection from the substrate, so the linewidth change of ASKA2 decreased drastically. This approach(anti-reflective layer) might be a candidate for the multiple interference effect. Due to the high aspect ratio pattern profile of ASKA2, the bulk effect(total tendency of the linewidth change on the resist thickness change) is very small for any substrate (Fig.11,12,14). ".r.,--... _..,,. 3J '..._.~.~..,..., Fig.3 Exposure Characteristics of ASKA2 Fig.5 Line-and-Space Patterns of ASKA2 Fig.4 Line-and-Space Patterns of ASKA 1 364

5 j Photopolym. Sci. Technol., Vol. 4, No.3, 1991 Fig.9 The Relationship Between Mask Size and Pattern Size of ASKA2 Fig.6 Contact Hole Patterns of ASKA2 Fig.7 The Relationship Between Depth of Focus and Linewidth of ASKA2 Fig.8 The Relationship Between Exposure Energy and Linewidth of ASKA2 365

6 Fig. 10 Delay Time Comparison of ASKA2 Fig.13 Reflectivity of Substrates Fig. 1 l The Relationship Between Resist Thickness and Linewidth of ASKA2 on Si Substrate Fig. 12 The Relationship Between Resist Thickness and Linewidth of ASKA2 on Silo Substrate 366

7 Fig.14 The Relationship Between Resist Thickness and Linewidth of ASKA2 on TiN/A1SiCu Substrate 5. Application of the positive chemically amplified resist to the actual fabrication of 64M DRAM Specification of the 64M DRAM The specification of 64M DRAM developed in our laboratory is as followings[9]: * Design rule micron * Cell size x 2.Opm2 * Cell type stacked storage * Chip size x 21.60mm2 Actual device patterns Fig. 15 demonstrates the separation pattern of ASKA2. Rectangular 0.4 micron line and 0.5 micron space patterns on SiN/Si02 were fabricated. 0.4 micron word line patterns were achieved on HTO/poly-Si(Fig.16). The high resolution of ASKA2 was not prevented from notching by the topography of the separation pattern. 0.4 micron bit line patterns on polyside were similarly obtained(fig.17). Tunnel type storage electrode patterns after etching was shown in Fig micron storage pattens and 0.4 micron spaces were successfully fabricated. Fig.19 exhibits 0.4 micron line-and-space connection patterns of ASKA2 on TiN/A1SiCu. This SEM shows patterns on 0.7 micron step. One of the most difficult pattern fabrication in making 64M DRAM was easily done with steep wall profile. It is found in Fig.20 that the excellent etching patterns reproduced the straight resist pattern profile. 367

8 I Photopolym. Sci. Technol., Vol. 4, No.3, 1991 Fig. 15 Separation Patterns of ASKA2 Fig.16 Word Line Patterns of ASKA2 Fig.17 Bit Line Patterns of ASKA2 Fig. 18 Storage Cell Patterns After Etching Fig. 19 Connection Patterns of ASKA2 Fig.20 Connection Patterns After Etching 368

9 6. Conclusions We have developed high performance KrF excimer laser resist. The positive chemically amplified resist, ASKA, showed excellent lithographic characteristics(sensitivity, resolution, linewidth control, mask linearity, stability, bulk effect), which meet the requirements for the fabrication of 64M DRAM. 0.4 micron-rule 64M DRAM was successfully fabricated on KrF excimer laser lithography. ASKA was applied to any critical layer of 64M DRAM including severe topography and reflection and it resulted in great success. We verified KrF excimer laser lithography is very promising for 64MDRAM production. We'll develop 256M DRAM with the extension of this excimer laser lithography. Acknowledgements The authors thank Dr.S.Horiuchi and Dr.T.Takemoto for their encouragement and support on this work. The authors also thank Mr.Y.Naito, Mr.S.Matsumoto and Mr.Y.Nagai for their useful discussions. References 1. V.Pol et al., Proc.of SPIE, 633 (1986) D.J.Ehrlich et al., J. Vac. Sci. Technol., B 3 (1985) M.D.Levenson et al., IEEE, ED-29 (1982) Y.Nakayama et al., J. Vac. Sci. Technol., B8 (1990) N.Furuya et al., Proc. of SPIE,1264 (1990) Y.Tani et al., Tech. digest of VLSI Sympo., (1990) p M.Endo et al., Proc. IEICE, (1991) SDM (in Japanese). 8.O.Nalamasu et al., Proc. of SPIE,1262 (1990) T.Yamada et al., ISSCC Digest of Tech. Papers, (1991) p