Download Semiconductor Processing and Microsystems Exam: Electronic Engineering, Autumn 2007 and more Exams Materials Processing in PDF only on Docsity!
Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering- Award
(NFQ Level 8)
Autumn 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) Illustrate bulk and surface micromachining processes. Compare compatibility
for CMOS integration of these processes. [30 marks] (b) Using a cubic crystal model, illustrate the crystal planes given by the Miller Indices (011), (111) and (010). [30 marks] (c) When etching (100) silicon wafers, the (111) crystal plane emerges at an angle of 54.7o^. If a mask with dimensions 993μm*818μm is used to etch a 10μm thick pressure membrane in a 500 μm thick wafer using KOH etching, determine the dimensions of the fabricated pressure membrane. Sketch the device etch profile. [40 marks]
Q2. (a)^ Describe^ and^ illustrate^ a^ bulk^ micromachining^ process^ using^ an electrochemical etch stop and bonding to glass to form an absolute pressure sensor. [30 marks] (b) Describe with the aid of a diagram the process of thin film formation using chemical vapour deposition. Outline the process parameters which most influence thin film properties.. [30 marks] (c) What are the sources of residual stress in MEMS thin films and what problems are associated with residual stress in MEMS devices? [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 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 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. [20 marks] (d) Describe ways in which the sensitivity of the designed accelerometer could be improved. [15 marks]
Required accelerometer resolution 0.25g Inertial mass area 300 μm * 200μm Polysilicon density 2330 kg/m^3 Capacitance detection 100 interdigitated fingers each of length 100 μm, width 1.5μm and spacing to substrate fingers of 1μm Minimum detectable capacitance change
35aF (*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
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
