Effect of Ultra-Dilute RCA Cleans on the Integrity of Thin Gate Oxides
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Tushar Dhayagude, Weidong Chen and Mohsen Shenasa' National Semiconductor Corp., Advanced Technology Group, Santa Clara, CA 95052 David Nelms and Mike Olesen VERTEQ Inc., Santa Ana, CA 92705 ABSTRACT
Comparative studies on the effect of Ultra-dilute RCA cleans, chemical ratios in excess of 300:1, and Dilute RCA cleans, chemical ratios around 50:1, on the integrity of thin gate oxides have been performed. Ultra-dilute RCA chemistries have shown particle removal efficiency, metallic contamination removal, surface roughness, Qbd, BVox and defect density equivalent to those obtained using dilute RCA chemistries. Furthermore ultra-dilute chemistries use less chemical leading to shorter rinse times and thus increased throughput as compared to the dilute RCA chemistries.
INTRODUCTION
As device geometries are scaled below the sub-half micron regime and wafer is size increased, the pre-gate wafer cleans become increasingly important and a substantial part of the total cost. The standard RCA clean, developed by W. Kern and D. Puotinen in 1965 [1], is extremely effective in removing contamination from silicon wafer surfaces and has become the industry standard for all pre-diffusion cleans, including pre-gate clean. It is well known that SC I is used for particle removal and SC2 is used for metal removal and that improper ratios and sequencing may be ineffective or even detrimental. Recently dilute RCA cleans, chemical ratios
50:1 have been introduced to reduce cost and facilitate cleaning tools with less number of chemical baths. In this paper, we report an entire ultra-dilute RCA clean process for the first time and discuss its effect on the integrity of thin gate oxides. A comparative analysis of the data obtained from ultra-dilute and dilute RCA cleans is also presented. RECIPE DETAILS
The ultra-dilute clean, using a combination of uniquely formulated dilutions and sequencing, provides a low cost, high throughput and less roughening alternative to traditional cleans [2]. First the wafer is cleaned using the ultra-dilute SC 1 chemistry followed by a 100:1 HF dip at 25°C to remove the KOOI oxide. A unique SCI sequence is then performed as described. Water and H20 2 are introduced at low temperature to grow an ultra clean thin chemical oxide of -10A. NH 4OH is then introduced along with DI water at 60'C. This chemical sequence ensures that the silicon surface is not exposed directly to NH 4OH. Also the concentration of NH 4OH is lower than peroxide to prevent roughening [3]. After NH 4OH in the solution stabilizes, megasonic energy is applied. The final SCI concentration is around 1000:5:1. It has been shown 'Currently at Fujitsu Microelectronics, San Jose, California 95134 217 Mat. Res. Soc. Symp. Proc. Vol. 477 ©1997 Materials Research Society
by Steve Verhaverbeke et. al., that the [OH-] concentration and ionic strength are critical for an SC1 solution [4]. Thus the ratio of H20 2:NH 4OH which determines the [OH-] concentration was found to be an important parameter for optimizing SCI solutions. Dilutions of 100:2:1
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