LABORATORY 2: The Thin-Film Silicon Pressure Sensor
Authors: A. K. Henning and C. G. Levey
Revision: January 1995
Purpose and Description:
This laboratory presents the fundamental operations in most electromechanical devices created using the techniques of bulk micro-machining.
In particular, a number of critical structures will be built, using an existing photolithographic mask. The central device will be a diaphragm pressure sensor, though other devices will also be built. One of the devices will be a result of your initial design.
As in Lab #1, some experimental variations will be utilized, to demonstrate how the behavior of micro-machines depends upon the properties of the constituent materials, which are determined by their material and physical structure.
Process Flow Representation (PFR):
General Step Specific Step Description Monitor/Measurement
1. Initialization: Starting material Four-point probe/Resistivity
75mm <100> silicon
2. Etch: RCA Clean
3. Diffusion: Wet Oxidation Ellipsometer/Film thickness
4a. Deposit: Spin on photoresist (backside) Ellipsometer/Film thickness
4b. Diffusion: Bake photoresist Time, Temperature
4c. Exposure: Pressure Sensor Cavity Defn. Time, Light Intensity
4d. Etch: Develop photoresist Microscope/Visual inspection
4e. Diffusion: Post-exposure bake photoresist Time, Temperature
5. Etch: Oxide etch Time, Temperature
6. Etch: Photoresist strip Time, Temperature
7. Diffusion: Boron doping (using BN disks) Time, Temperature
8. Etch: Oxide etch Time, Temperature
9. Etch: Cavity etch (KOH) Time, Temperature
Detailed Step Description (including cross-section [side view] and plan [top] view pictures):
2. Etch: RCA Clean
(using BN disks)
Cross Sectional View of diaphragm
Plan View of diaphragm
Structure Catalog (Mask Road Map):
Pressure structures (various sizes, shapes)
Etch structures (how does anisotropic etch perform)
In-Process Measurements and Post-Process Characterization:
Four-point probe resistivity measurement
Scanning Electron Microscopy
These problems must be turned in individually, prior to beginning this laboratory. Some problems will be assigned as a part of the regular homework.
1) Why is the boron doping done from the topside, and not the bottom side, of the wafer?
2) Using L-Edit, design (a) device(s) of your own for inclusion in the fabrication process. Describe the intended behavior of the device.
3) Describe quantitatively the deflection of the thin boron-doped silicon film, as a function of size parameters and pressure differential across the film.
© 1995 Trustees of Dartmouth College