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Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering- Award
(NFQ Level 8)
Summer 2007
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. M. Hill Prof. G. Hurley Dr. S. Foley
Q1. (a) Discuss the requirement and constraints on a MEMS process if it is to be
integrated in a CMOS process flow and compare Bulk and Surface micromachining suitability for such integration. [35 marks] (b) Describe how an accelerometer with piezoresitive readout could be fabricated in a bulk micromachining process and identify the parameters which most influence device performance. [35 marks] (c) When etching (100) silicon wafers, the (111) crystal plane emerges at an angle of 54.7 o^. What width of mask area is required to fabricate a 300μm wide 50μm thick pressure membrane in a 300μm thick wafer using KOH etching? Sketch the device etch profile. (^) [30 marks]
Q2. (a) Describe with the aid of diagrams how the fabrication process flow to fabricate a PMOS transistor from a <100> N-type silicon starting wafer. [30 marks] (b) Describe with the aid of a diagram the process of thin film formation using sputtering. Compare this deposition method with alternative processes and identify the deposition parameters that most influence MEMS films. [30 marks] (c) What are the sources of stress gradients in MEMS thin films and what problems are associated with process stress gradients? [20 marks] (d) A surface micromachined aluminium MEMS cantilever had a measured tip deflection of 50μm after sacrificial layer etch. Given the following properties of aluminium calculate the linear thin film stress gradient in MPa/μm. Beam length - 500μm. Elastic Modulus – 77GPa. Poisson’s ratio 0.29. [20 marks]
Q3. (a) Describe a metal/dielectric process flow for post-CMOS MEMS. What are the main considerations in location in the process flow? [25 marks]
(b) For the capacitive accelerometer (in a 1.2μm polysilicon process) shown in layout in Figure 1, calculate the required spring constant of the support beams given the following information and requirements. [40 marks] (c) Calculate a suitable length for the spring flexures if the flexure width is 1μm and for this design determine the cross-axis selectivity of the device. [20 marks] (d) Briefly describe a differential measurement circuit for this application that can reduce cross-axis sensitivity. [15 marks]
Required accelerometer resolution 0.3g Inertial mass area 240 μm * 180μm Polysilicon density 2330 kg/m^3 Capacitance detection 80 interdigitated fingers each of length 110 μm, width 2μm and spacing to substrate fingers of 1μm Minimum detectable capacitance change
30aF (*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
Course DLX
Summer 2007
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
2
