IC 制造中的 HF 浓度控制

ABSTRACT  

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 ranged from 6600 to 6800 µS/cm at 21°C. In addition, the bath’s conductivity and  temperature were recorded by a PC with automated data acquisition software.

For the test of etching process characterization, a lot of 50 oxide wafers were  immersed in the HF bath for 3 hours, without deionized water or HF injection, followed  by rinse and dry. The characteristic curve of conductivity versus etch time under the  specific system setting was developed and used as a key parameter for the control  scheme.

For the test of new algorithm evaluation, a batch of 50 oxide wafers was  processed in the HF bath followed by rinse and dry. This process sequence was repeated,  without chemical changeout, for 23 test runs. The chemical process time of each run was  25 minutes, which was estimated to have etched the oxide of 560 Å in thickness in a fresh  HF bath. The actual etch rate for each run, however, was obtained using an ellipsometer  to measure the oxide thickness change of the test wafers. The control system was  activated to let the system spike whenever needed.

RESULTS AND DISCUSSION  

Results showed that the etch by-products have an effect on the linear dependence  of conductivity and HF concentration. Since HF is consumed in the reaction, one would  expect that the conductivity should drop as wafers are introduced into the bath. However,  it was observed that conductivity increased with the amount of SiO2 etched in the bath  while the oxide etch rate drops as shown in figure 1. 

Fig 1

This observation was again  confirmed by immersing 50 oxide wafers (8”) in the 0.5% HF bath for an extended time  and monitoring the change in conductivity as illustrated in Fig. 2. The amount of SiO2 can  be calculated and the relationship of conductivity versus the amount of dissolved SiO2 can be simulated as shown in Fig. 3.