湿法蚀刻MEMS硅腔的工艺控制

Introduction  

Anisotropic etching of silicon refers to the directional-dependent etching, usually by alkaline  etchants like aqueous KOH, TMAH and other hydroxides like NaOH. With the strong dependence  of the etch rate on crystal orientation and on etchant concentration and temperature, a large variety  of silicon structures can be fabricated in a highly controllable and reproducible manner. Hence,  anisotropic etching of <100> silicon has been a key process in common MEMS based technologies  for realizing 3-D structures [1-4]. These structures include V-grooves for transistors, small holes for  ink jets and diaphragms for MEMS pressure sensors as shown in Figure 1 [1]. The actual reaction  mechanism has not been well understood and comprehensive physical and chemical models for the  process have not yet been developed. With increasing numbers of MEMS applications, interest has  grown in recent years for process modelling, simulation and software tools useful for the prediction  of etched surface profiles [4-6].

In this study, test wafers were processed in dilute TMAH in order to etch deep cavities similar to  the ones shown below in Figure 1. Due to the large surface areas being etched from the wafers‘  surface, etch byproducts presented a challenge to the process control that produced undesired  results. In-line sensors were installed to monitor and control the chemical’s concentration and mass  of etch byproducts in real-time. Algorithms were developed to offset the effect of these factors and produced consistent results. The purpose was also to lower the overall cost of ownership (COO) of  the process.

Figure 1

Experimental  

Wet chemical processes were conducted on a fully-automated GAMA™ wet processing station  using 200mm wafers with a typical cavity structure. TMAH was used as the alkaline etchant at a  concentration suitable for achieving a maximum etch rate. Silicon etching processes were  conducted with the aid of Akrion Systems’ in-situ chemical concentration control system.  Occasionally, samples of the baths were taken and titrated for comparison to the NIR results. The  goal was to fully etch through the wafers by maintaining a consistent etch rate throughout the entire  process.  

Results and Discussion  

Silicon Etch Rates, Theory vs. Experiment. 

The chemical reaction for the anisotropic alkaline  etching of silicon is well known and a variety of etchants can be used for the process (KOH, NaOH,  TMAH). However, CMOS fabs typically cannot use metal hydroxides as the metal ions will  (e)  Solid State Phenomena. As a result, TMAH is often used when the metal  hydroxides are not allowed.