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Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering – Award
(Bachelor of Engineering in Electronic Engineering - Award)
(NFQ – Level 8)
Summer 2005
Semiconductor Processing and Microsystems
(Time: 2 Hours)
Answer any three questions [each 100 marks]. Maximum available marks is 300.
All questions carry equal marks.
Examiners: Mr. Martin Hill Prof. Cyril. Burkley Dr. John. Ryan
Q1. (a) Outline the main advantages and disadvantages of silicon as a material for
microsystems fabrication. [20 marks] (b) Describe bulk and surface micromachining processes and compare compatibility for CMOS process integration. [25 marks] (c) A 10mm length of silicon which has an elastic modulus of 150GPa is eleongated during thermal cycling by 4μm. Assuming elastic behaviour, calculate the resulting stress in the film. Describe one possible problem arising from thermal expansion in MEMS processing. [25 marks] (d) Using a cubic crystal model, illustrate the crystal planes given by the Miller Indices (111), (100) and (101). [30 marks]
Q2. (a) Describe and illustrate a bulk micromaching process using an electrochemical
etch stop and bonding to glass to form an absolute pressure sensor. (^) [25 marks] (b) The cycle of deposit/implant, pattern and etch are used in all semiconductor processes. Describe with the aid of diagrams how these processes are sequenced to fabricate a PMOS transistor from a <100> N-type silicon starting wafer. [35 marks] (c) Describe with the aid of a diagram the process of thin film formation using sputtering. Indicate configuration options in the process and the parameters most likely to yield useful MEMS device films. [40 marks]
Q3. (a) Describe the requirements for fabrication of a capacitive polysilicon surface
micromachined accelerometer. In particular pay attention to location in the process flow, CMOS compatibility, effect of stress and stress gradient. [25 marks]
(b) For the capacitive accelerometer (in a 2μm polysilicon process) shown in layout in Figure 1, calculate the required spring constant of the beam given the following information and requirements. [40 marks] (c) Calculate suitable dimensions for the spring flexures for this requirement. [25 marks] (d) How could the process be modified to increase the accelerometer resolution? [10 marks]
Required accelerometer resolution 0.25g Inertial mass area 400 μm * 150μm Polysilicon density 2330 kg/m^3 Capacitance detection 50 interdigitated fingers each of length (^110) μm, width 1.5μm and spacing to substrate fingers of 1μm Minimum detectable capacitance change
50aF (*10 -18^ )
Polysilicon Elastic Modulus 165*10^9 Pa Air permittivity (^) ε 0 8.85 * 10-12^ F/m
Note : Mass is sum of inertial mass and mass of fingers. All fingers not shown in diagram.
Figure 1
Accelerometer anchor
Inertial Mass
Support flexure
Static capacitor fingers (^) Moving capacitor fingers
Accelerometer anchor
Inertial Mass
Support flexure
Static capacitor fingers (^) Moving capacitor fingers
Subject Semiconductor Processing and Microsystems
Useful Formulae and Constants
Interatomic Spacing
Piezoresistive strain gauge factor
R
δR
G
Moment/curvature relationship
EI
M
For an isotropic beam I, the moment of inertia of the beam cross-section is
bh^3 I =
Cantilever beam bending under end loading with a point force F at endpoint x=L
EI
Fx L x y x 6
EI
FL
y L 3
3 = (^3)
3 (^3 )
( ) L
Ebh L
EI
yL
F
K (^) y = = =
Cantilever beam bending under end moment M
EI
Mx y x 2
2
By superposition with a combined end force and moment
EI
Mx EI
Fx L x y x 6 2
2 2
2 2 2
h k l
a
d hkl
Axial compression:
L
L
E E
A
F δ
EA
LF
δ L =
L
Ebh L
EA
K (^) x = =
Stress in thin films
σ total = σ thermal + σ internal + σ external
Thermal strain
Stoney Formula –Curvature due to thin film stress
Stress Gradient Bending
Electrostatic MEMS Actuation Force
2
2
2 d
AV
F
ε o
Polysilicon Properties
Elastic Modulus = 165 GPa
Density = 2330 kg/m^3 Aluminium Properties
Elastic Modulus = 77 GPa
Density = 2700 kg/m^3
ε th = (αf − αs)(Tdep − Troom )
ν t
E t
s f
s s f
R −
2
( v)L
E
dh
dσ
2
