520.495/580.495
Fall 1998 Microfabrication Laboratory
Homework Assignment #4
Reading:
1. "The mechanism of anisotropic silicon etching and its relevance to micromachinining" by H. Seidel
2. Product information for spin-on Boron dopants by Fimtronics Inc.
Problem:
Suppose that you are designing a course in microfabrication laboratory and suppose that you are going to build a bulk micromachined, pressure sensor like the ones we are doing in this class. We want to diffuse piezo-resistors in the front and etch "pits" in the back to form the membranes.
We know that the wafers are about 425 microns thick and that the membrane thickness will be approximately 55 microns. We will use SiO2 as the masking material and we will us some alkaline solution like KOH or LiOH or NaOH, or EDP to etch the "pit".
Design the processing schedule, i.e. do a calculation of the processing parameters (time, temperature, concentration of chemicals e.t.c.) up to the point that you form successfully pits in the back of the wafer.
When you finish your design, contrast your processing schedule with the one we are doing in the class.
Processing parameters are temperatures of the diffusion oven, composition and temperature of the etching solutions, time in oven and etching solutions e.t.c.
Hints:
- We need to grow oxide of certain thickness to mask the diffusion step.
- We need to diffuse Boron.
- We need to remove any Boron oxide from the surface of the wafer.
- We need to re-grow oxide after Boron diffusion, while at the same time worrying a little bit that we don’t consume our Boron dopants (remember oxide grows at the silicon-silicon dioxide interface.
The reading material and references therein provides a wealth of experimental data on etching rates of various chemicals.