In-Situ Chemical Concentration control for Wafer Wet Cleaning
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ABSTRACT The continuously increasing integration of today's advanced semiconductors requires increasingly tight process control in the IC manufacturing steps. This paper demonstrates the use of conductivity sensors to monitor and control the chemical concentrations of RCA cleaning and HF etching solutions. Electrodeless conductivity sensors were used to monitor and regulate the concentration of these process chemicals. A linear relationship between the conductivity of the solution and the chemical concentration was obtained within the range studied. A chemical monitoring and concentration scheme (ICE-1TM) was developed. Different concentrations of RCA and HF solutions were investigated. Results show that these techniques are suitable for monitoring and controlling the concentration of chemicals in the process tanks for better process control. These techniques provide a lower cost of ownership of the process due to longer bath lives and the use of dilute chemicals. INTRODUCTION For many years, RCA-based wafer processing has adopted the use of high concentrations of chemicals at elevated temperatures and long process times. For example, the APM (SC 1) step is designed to remove particles from the wafer surface using a mixture of 1 part 28% NH4LOH : 1 part 30% H1202" 5 parts H120 at a temperature of about 85 °C for a process time of 10 minutes. Similarly, the HPM (SC2) process, consisting of 1 part 37% HC1 1 part 30% H1202 : 6 parts of H2 0), is used to remove metals. However, a major shortcoming is that these RCA-based cleaning solutions are typically replaced after short periods of time (2-4 hours) in order to obtain any degree of process uniformity. Similar problems exist with HFM (HF mix) processes which require strict control of SiO2 etch rates. Thin oxides are etched in dilute (200-100:1 H20 : HF) solutions while thick oxides are normally etched in concentrated (10:1 H20 : HF) solutions. Since etch rates depend on HF concentration, good process control and uniformity could only be obtained by limiting bath lives. This approach is not cost-effective for several reasons including the impact on the environment (due to high usage and waste of chemicals) and availability of equipment. Thus, if an accurate and continuous control of chemical concentration can be obtained, the lifetime of RCA cleaning and HFM etching baths can be considerably extended. Advanced semiconductors today require increasingly tighter process control of the wafer fabrication. RCA cleaning in wafer processing has proven to be a standard step for process engineers since it was developed and published by Kern 1960-1965'. Additionally, the etching of SiO2 layers from silicon surfaces is one of the most critical steps in wet processing technology 2 4 . Although numerous studies have been performed that analyze the mechanisms and kinetics of these processes, little attention, if any, has been given to monitoring and controlling the chemical concentrations in the process baths. Chemical concentration control is becoming crucial to wafer processing in
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