Homework 3:
| Assigned: | Feb 13, 2003 |
| Due: | Feb 24, 2003 |
Individual Assignments:
1) Process and Mask Determination:
You are asked to develop a process flow - the processing steps and masks - to fabricate a simple active mirror device, shown in the figure above. The device consists of a suspended polysilicon beam mounted above the substrate. A voltage is applied between the beam and the substrate which results in an electrostatic attraction which deflects the beam, thus deflecting the light that shines down from above.
a) work out the processing steps necessary to fabricate this device. Be as specific as possible and include the following: Process steps, chemistry, times (approximate),
b) submit sketches (of reasonable quality) of the mask set needed to fabricate this device
2) Vacuum Sealing of a cavity.: Consider a surface micromachined cavity as shown above. The cavity is 3 microns deep and is a circle of radius 200 microns. The device is made by first growing oxide in a defined region, and then depositing a polysilicon cap layer which covers the oxide. A hole in the polysilicon is then etched and the oxide is removed from inside using BOE through the etch hole.
The cap layer is made by 1 micron thick polysilicon with minimal residual stress. We wish to seal the cavity so that the sealed cavity is at low pressure- this is accomplished by oxidizing the poly in a furnace with a 1000 degrees C, 80% oxygen, 20% nitrogen, 1 atm environment. In this way, the etch holes grows closed as the oxide foms in the polysilicon, and the oxygen in the cavity is consumed, leaving a sealed cavity.
a) how long will it take to complete the sealing process.
b) how much oxide grows on the walls inside the cavity
c) how big should the etch holes be to ensure that they seal during this oxidation
d) what is the pressure inside the cavity at the end of the oxidation but before the wafer is brought out of the furnace
e) what is the pressure inside the cavity when the wafer is cooled to room temperature
f) what is the deflection of the membrane at this final stage (sealed, room temperature). You can assume that the membrane is rigidly anchored to the substrate at the sides.