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Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering – Award
(NFQ - Level 8)
Autumn 2006
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. G. Hurley Dr. S. Foley
Q1. (a) Illustrate bulk and surface micromachining processes. Compare compatibility
for CMOS integration of these processes. [33 marks]
(b) Describe how a pressure sensor with piezoresistive readout could be fabricated both surface and bulk micromaching processes. Use diagrams to illustrate the device construction. [34 marks]
(c) Using a cubic crystal model, illustrate the crystal planes given by the Miller Indices (110), (100) and (111). [33 marks]
Q2. (a) Describe isotropic and anisotropic etching for bulk micromachining. [40 marks]
(b) Anisotropic KOH etching of silicon is widely used for feature definition. What are the main parameters that determine the etch rate? [20 marks] (c) When etching (100) silicon wafers, the (111) crystal plane emerges at an angle
of 54.7o^. What width of mask area is required for a 100 μ m deep via profile
etched in a (100) silicon wafer using KOH etching. The profile is shown.in Fig. Q2(c) [30 marks] (d) If the via is to be etched through the wafer, how can wafer thinning assist in reducing costs. [10 marks]
100 μm
(100)
(111)
100 μm
(100)
(111)
Figure Q2(c)
Q3. (a) Describe a process flow for the fabrication of a post-CMOS surface
micromachined metal-oxide switch. In particular pay attention to process restriction and the effect of stress and stress gradient. [35 marks]
(b) For the capacitive accelerometer (in a 1.5μm polysilicon process) shown in the layout in Fig. Q3(b) calculate the required spring constant of the beams given the information and requirements in the table below? [40 marks] (c) Calculate suitable dimensions for the spring flexures for this requirement. [25 marks]
Required accelerometer resolution 0.4g Inertial mass area 400 μm * 200μm Silicon density 2330 kg/m^3 Capacitance detection 60 interdigitated fingers each of length 100 μm, width 2μm and spacing to substrate fingers of 1μm Minimum detectable capacitance change
150aF (*10 -18^ )
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 3(b)
Accelerometer anchor
Inertial Mass
Support flexure
Static capacitor fingers (^) Moving capacitor fingers
Accelerometer anchor
Inertial Mass
Support flexure
Static capacitor fingers (^) Moving capacitor fingers
Course DLX
Summer 2005
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
Axial compression:
L
L
E E
A
F δ σ = = ε= EA
LF
δ L = L
Ebh L
EA
K (^) x = =
2 2 2
h k l
a
d hkl
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
Polysilicon Properties
Elastic Modulus = 165 GPa
Density = 2330 kg/m^3
Aluminium Properties
Elastic Modulus = 77 GPa
Density = 2700 kg/m^3
Silicon Dioxide Properties
Relative Permittivity = 3.
ε th = (αf − αs)(Tdep − Troom )
ν t
E t
s f
s s f
R −
2
( v)L
E
dh
dσ
