Hf Concentration Control in Ic Manufacturing

The etching of SiO2 layers from silicon surfaces is one of the most critical steps in wet processing technology. Although numerous studies have been performed to analyze the mechanisms and kinetics of these processes, little attention has been given to monitoring and controlling the chemical concentrations in the process baths. Chemical concentration control is becoming crucial to wafer processing in order to obtain consistency and more cost-effective IC manufacturing. This paper demonstrates the use of conductivity sensors to monitor and control the concentration of HF etching solutions. Effects of etch byproducts on the conductivity measurements have been investigated. Once the etch byproducts are characterized and accounted for, results showed that a much more stable etch process can be obtained and the bath life can be extended even in the presence of etch by products. INTRODUCTION Life of etching baths can be extended for longer periods of time if an accurate and continuous control of chemical concentrations is provided [1]. Compared to standard analytical techniques for chemical concentration e.g. NIR, UV spectroscopy [1], conductivity cells provide fast, very cost-effective, and real-time control of HF concentration. If the temperature is held constant, and the conductivity of the HF solution is maintained, the etch rate of SiO2 can be accurately controlled as well [1]. Electrodeless conductivity sensors were used to accurately monitor and control the concentration of HF acid during the etching of thermal oxide from silicon surfaces [1]. However, the etch by-products can affect the linear conductivity-concentration relationship. This effect is magnified when etching thick oxide layers in dilute HF baths. A correction must be developed to correlate the amount of SiO2 etched with the change in conductivity [2]. Results showed that these techniques are suitable for monitoring and controlling the etch rate in IC manufacturing environment. EXPERIMENTAL The experiment was performed on Akrion’s Fully Automated GAMA wafer processing station in the Class 1 Application Laboratory at Akrion. Fifty 200-mm wafers with sufficient amounts of thermal oxide on both sides were prepared for the test. A standard HF process tank was used. The etching process was conducted at 21°C in an initially mixed 100:1 (H2O:HF) HF bath. The conductivity sensor for monitoring the HF bath was calibrated and the reading for the aforementioned HF concentration typically