碳化硅器件加工过程化学浓度控制

Introduction

For the last 20 years, SiC has been investigated as a  replacement for traditional etch stops in bulk micromachining  due to its properties of being chemically inert in conventional  etching solutions.1 It is also a desired material for high  temperature devices due to its thermal properties. Applications  for creating such devices as fuel atomizers, pressure sensors,  and microfabricated molds can use typical wet etching  techniques and take advantage of the properties of the SiC layer  which make it chemically resistant.1 Particularly for SiCMEMS devices, large area substrates are essential. Due to the  difficulty in manufacturing single growth crystal substrates,  there has been much interest in epitaxial growth of single and  poly-crystalline SiC layers on silicon. After the deposition,  bulk etching is able to create microstructures and patterns  suitable for the desired devices.

With bulk etching, one major concern is that the byproducts  are released from the wafer into the bath. Depending on the  open surface area of the patterned wafer, large amounts of  silicates can be introduced. This can then create unwanted side  effects such as decreased etch rate. By creating stable chemical  conditions inside of the bath, it not only allows for consistent  etch depth within and between lots, but it also allows for  increased bath life which will lessen the cost of ownership for  the process.

Experimental 

Wet chemical processes were conducted on a fullyautomated GAMA™ wet processing station using 200mm  wafers with a typical cavity structure. TMAH was used as an alkaline etchant at a concentration suitable for achieving a  maximum etch rate. KOH was used as an alkaline etchant  during subsequent testing on Solar wafers following the same  methods outlined. Silicon etching processes were conducted  with the aid of Naura-Akrion’s in-situ chemical concentration  control system. Occasionally, samples of the baths were taken  and titrated for comparison to the system’s readings. The goal  for the TMAH testing was to fully etch through the wafers by  maintaining a consistent etch rate throughout the entire process.  For the KOH testing, a consistent etch rate and reflectance were  measured to ensure the versatility of the system.

Results and Discussion 

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). For the purpose of this  study, the starting setpoint concentration of TMAH was 5% and  allowed to drop to a minimum level of 3% whereafter the etch  rate was kept constant. Through the course of the experiment,  it was discovered that the generally held overall reaction  mechanism for the etching of silicon with TMAH, as is shown  in equation 1, did not match the real data.

Fig1

As can be seen in Figure 3, when the system initiates the  concentration control process, the bath is then left at a steady  state position. Therefore, it is important to set all parameters for  upper and lower bounds to allowable values for maintained  chemical concentration. In Figure 3, when the Si concentration  reaches 5 g/L, the balanced volume of chemical being drained  from the tank and added to the bath allows the concentration to  stay at 5 g/L. The system does not correct itself to lessen the  amount of Si in the bath significantly, as this would provide too  much disturbance to the process. The TMAH concentration  during the process is also maintained at a steady concentration,  thus allowing the bath to sustain itself in an equilibrium  between TMAH concentration and Si concentration for an  extended period of time. The constant concentration of TMAH  also allows the system to keep a constant etch rate, which was  monitored before and after the system was activated. The  production etch rate was the same across the experiment, and  the qual etch rate was comparable to that of normal conditions.  This dynamic equilibrium can maintain itself for weeks or even  months depending on the process and the needs of the fab, as  shown in the chart where the data is collected for 11 days with  consistent results. A benefit of the SiC etch processes is that the  material itself is inert to the normal chemical concentrations  used for bulk etching, therefore the models necessary to track  the silicate byproducts in the bath for typical bulk Si etching  have already been created.

Fig3

Conclusions 

Results have shown that Naura-Akrion’s novel closed loop  concentration control system allows for a minimum usage of  etchant, such as TMAH or KOH, while still maintaining the  desired etch rate. This reduces the amount of chemical sent to  the waste stream, decreasing the overall cost of ownership for  the process. It also allows for consistent results within lots and  between lots for wafer processing, effectively reducing the  need to changeout baths after specific number of lots or time,  depending on the demands of the process. This not only saves  on more chemical cost, but also reduces the total time taken to  qualify the baths in the long run after a chemical changeout  occurs. The current state of SiC device manufacturing involves  deposited films on bulk Si which must be etched. With closed  loop concentration control, the process is more robust and  results in less costly manufacturing.