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[1]):

General Step Specific Step Description Monitor/Measurement

1. Initialization: Starting material Four-point probe/Resistivity

75mm <100> silicon

2. Etch: RCA Clean[2]

3. Diffusion: Wet Oxidation Ellipsometer/Film thickness

4a. Deposit: Spin on photoresist (backside) Ellipsometer/Film thickness

Microscope/Visual inspection

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

Microscope/Visual inspection

6. Etch: Photoresist strip Time, Temperature

Microscope/Visual inspection

7. Diffusion[3]: Boron doping (using BN disks) Time, Temperature

Microscope/Visual inspection

Four-point probe/Resistivity

8. Etch: Oxide etch Time, Temperature

Microscope/Visual inspection

9. Etch: Cavity etch (KOH) Time, Temperature

Microscope/Visual inspection

Detailed Step Description (including cross-section [side view] and plan [top] view pictures):

2. Etch: RCA Clean

(backside)

photoresist

Cavity Definition

(using BN disks)

Final Structure:

Cross Sectional View of diaphragm

Plan View of diaphragm

Structure Catalog (Mask Road Map):

(Under Construction)

Pressure structures (various sizes, shapes)

Etch structures (how does anisotropic etch perform)

In-Process Measurements and Post-Process Characterization:

(Under Construction)

Four-point probe resistivity measurement

Ellipsometry

Thin-film stress

Scanning Electron Microscopy

Pre-Lab Problems:

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.