chapter 5: surface MICROMACHINING

5.1. In the context of using piezoresistivity to sense strain, what are the advantages/disadvantages of using silicon versus silicon nitride for pressure sensor diaphragms? How does residual stress affect the sensitivity?

5.2. For pressure sensing diaphragms of the order of 100 microns square, why is it impractical to use capacitance to measure deflection?

5.3. Discuss the advantages/disadvantages of using surface versus bulk micromachining.

5.4. Design at least 3 micromachines enabling the testing of thin film mechanical properties.

5.5 Consider a MEMS-based condensor acoustic transducer as shown in the figure below. The diaphragm is made of silicon nitride and the backplate is perforated. A stress gauge was used to measure the stress in the silicon nitride and was found to be 1.5X108N/m2.

Problem Figure 5.5

Thickness of air gap = 5 ?Hm

Thickness of diaphragm = 5 ?Hm

Density of Si3N4 = 3 ?? 103kg/m3

-        Write the differential equation governing the dynamic behavior of the movable diaphragm

-        Solve the differential equation under the 3 cases of overdamped, underdamped, and critically damped conditions

-        Calculate the mass of the diaphragm, the small signal capacitance, and the air-streaming resistance as per the layout shown if the device was to be operated in air and water

-        Calculate the cut-off frequency for the overdamped case in air and in water.

Professor Rashid Bashir, Purdue University

5.6 View the Sandia WWW page http://www.mdl.sandia.gov/Micromachine/images.html.  Many of the gears have tiny holes in their surfaces. Why are they there?

5.7 What is a sacrificial layer? In conjunction with what is it used? Give examples of materials used as sacrificial layers.

5.8 Design a comb drive for the MUMPs process.

(a) Show the mask layouts (top view) of the relevant layers.

(b) Calculate the maximum force, maximum displacement, and resonance frequency that you expect to achieve with this design.

Make sure the following constraints are satisfied:

?        The total number of comb fingers should be less than 80

?        The device fits into a 200 ?? 200 mm2 square area

?        The maximum applied voltage is 30V

?        The device satisfies all design rules for MUMPs processing.

5.9 Consider the scanning probe cantilever shown below.

Problem Figure 5.9

πL = 71.8 ?? 10-11 /Pa

πT = -66.3 ?? 10-11 /Pa

E = 169 GPa

-        Find a closed form solution for the relationship of piezoresistive displacement and force sensitivity (find the expression for the maximum stress).

-        What key assumptions are used

-        What is the best way to maximize the force sensitivity

-        What is the best way to maximize the displacement sensitivity

-        Calculate the piezoresistive displacement and force sensitivity for L = 2000 ?Hm, W = 240 ?Hm, and t = 20 ?Hm.

Professor Rashid Bashir, Purdue University

5.10 Prepare a table with the most design parameters for making a very sensitive piezoresistive Si cantilever to measure bending induced by differential surface stress.

5.11 Use surface micromachining to fabricate the butterfly wing shown below. How would you make the wing move up and down? What are the best materials to use to give the wing the best movability?

Problem Figure 5.11

5.12 Define the following terms in 3-4 lines with examples:

LPCVD

PECVD

DRIE

Difference between RIE and plasma etching

LIGA process

SCREAM process

Silicon fusion bonding

Dissolved wafer process

Professor Rashid Bashir, Purdue University

5.13 In silicon-on-insulator (SOI) technology, fabrication of devices on a thin layer of Si separated from the rest of the Si substrate by a buried insulator layer (typically SiO2) allows circuit speed, device density, and radiation hardness to be increased. One method for fabrication of SOI starting wafers involves high-energy implantation of a Si wafer with oxygen, followed by annealing to form a buried layer of SiO2. This technique for forming SOI wafers is called Separation by IMplantation of OXygen (SIMOX). Suppose oxygen is implanted in a Si wafer to produce a buried 0.2 ?Hm SiO2 layer.

(a) Assuming that all the implanted oxygen is incorporated into the SiO2, calculate the

oxygen implant dose required to form the buried SiO2 layer. How does this dose compare with typical doses for dopant implants?

(b) What ion beam current would be required if a 125 mm diameter wafer is to be implanted with O+ ions in 15 minutes? If the implant energy is 300keV, how much power is supplied to the ion beam, and what effect might this have on the wafer?

5.14 A certain process forms Al contacts to n+ Si through a 10 ?Hm ?? 10 ?Hm contact window resulting in a contact resistance of 0.5 ohms.

(a) What is the specific contact resistivity for this contact?

(b) What will the contact resistance be if the contact windows are reduced to 1 ?Hm ?? 1 ?Hm?

5.15 Describe 5 methods you could implement to prevent stiction between surface micromachined components.

5.16 Describe the merits and problems associated with micromachining in GaAs, Si, diamond, and 3C-SiC.

5.17 List 5 methods you could use to release stress in a poly-Si comb resonator and describe the relative merits of each approach.

5.18 Why did surface micromachining catch on so fast with the IC industry?