ECE6397 MEMS, NEMS, and NanoDevices

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ECE6397MEMS, NEMS, and NanoDevices
Introduction to Microfabrication
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Overview of Microfabrication

• MEMS technology was implemented directly from IC
  fabrication. Si was the leading material and the
  same processes have been used. Many others and
  other materials are included to meet specificity
  of MEMS.

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Overview of Microfabrication
go to http//www.engr.uiuc.edu/OCEE/outreach.htmto
see video of fabrication
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Thermal Oxidation and SiO2 Interface
MEMS use oxides of various thicknesses
Applications in microeletronics
SiO2 grows on Si (also _at_ RT) enables very easy
IC formation ensures stability and reliability.
Lower thermal budget
1 2 nm
New dielectrics ? ? to avoid tunneling. (high K)
Low K dielectrics
Plummer et al.
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Historical Development and Basic Concepts
Oxide growth using O16 and O18 isotopes
identifies the mechanism.
Neutral O2 and H2O and/or OH are dominant species
in oxidation, not atoms or ions O, O- , O2-,
Volume of SiO2 is 30 larger than Si.
(1.3)3 2.2 volume of the oxide cannot be
accommodated in Silicon
Plummer et al.
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Effect of Volume Mismatch in Si/SiO2 System
Recessed LOCOS
H2O_at_1000C
2.2X volume expansion -gt 45xoxxSi
y/.45y0.5µm y0.41µm
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Silicon Consumption During Oxidation (LOCOS)
Nonplanar structures form due to Si consumption -
stress between Si and SiO2
Birds Beak formation Stress at the Si/Si3N4
interface
Plummer et al.
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Structure of Silica Glass
Short range order maintained
Amorphous material
Non-bridging oxygen in fused Silica (not present
in crystalline SiO2)
Si can be replaced by deposits. B,P,As or Sb
network modifiers.

• large compressive stress (5109 dynes/cm2)
  exists in SiO2. High temperature can relief
  stress by viscous flow.
• Large difference in the thermal expansion
  coefficients of Si and SiO2.

Silicon in tension ? refer to curvature Properties
of OXIDES such as stress, porosity are important
in MEMS
Plummer et al.
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SiO2/Si System Structure and Charges
When charges are important in MEMS? In capacitive
sensors (impedance measurements)
Amorphous/crystalline interface is flat. (TEM)
Roughness ? with ? growth rate and ? T.
Detect density at the interfaces is 109 1011
cm-2.
Fixed charge 109 1011 cm-2 is and does
not change in device operation.
Interface charge traps due to dangling Si bonds
? change in operation Qp?Qit both related to
unoxidized Si atoms.
Reduce charges since they degrade device
operation ? ? T , H2 anneal..
Plummer et al.
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Oxide Charges and Their Annealing
Much more important in Si devices than in MEMS
Increasing surface roughness increases charges
Plummer et al.
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Manufacturing Methods and Equipment Typical for
ICs and for MEMS
Vertical furnaces are also used.
3 zones
0.5 C
Dry or wet oxidation
Ramping of T from/to 800 C ( 10 C/sec) Add HCl
or TCA for gettering purpose (metals, Na )
Plummer et al.
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Models and Simulation First -Order Planar Growth
Kinetics - Linear Prabolic Model
Deal and Groove Model
Transport to Si Diffusion through the oxide
Reaction at the Si surface
In steady state F1F2F3
Transport of the oxidant to the oxide surface.
Plummer et al.
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Deal-Grove Model for Wet and Dry Oxidation
Slow growth rates
Dry oxidation - used up to 100-200 nm
Fast growth rates.
Wet oxidation - used for thicker oxides
CwetgtgtCdry
MEMS usually use thick oxides
Plummer et al.
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Orientation Effects in Oxidation
Orientation effects are important in MEMS
(100), (111), and Polysilicon
Density of atoms (bonds) in (111)gt(100)
No effect of orientation for thick oxides Strong
effect of orientation for thin oxides
Simulated oxide growth
Related to stress
Plummer et al.
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2D SiO2 Growth Kinetics
Difference in volume -gt problems when expansion
is restricted (SiO2 confined)

• Experiments by Kao et al.
• Retardation at sharp corners (2X for 500 nm SiO2)
• Retardation larger _at_ low T (no effect _at_ 1200 C)
• Interior (concave) corners oxidize slower than
  exterior (convex) but both slower than flat Si
• Reasons
• Crystal orientation
• Diffusion of oxidant through amorphous SiO2 is
  the same -gt no dependence on direction
• Stress (volume difference) SiO2 under large
  compressive stress -gt affect both oxidant
  transport and reaction at the Si surface

Plummer et al.
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Oxidation of Non-Planar Structures
Stress retards oxidation _at_high T viscoelastic
flow relaxes stress Oxide viscosityf(stress, T)
no stress
Stress included
Stress _at_?Tgt Stress _at_?T
Large stress can bend thin mechanical structures
in MEMS Cantilevers etc.
History of Stress VERY IMPORTANT Stress in an
oxide depends on growth T. In sequential
processing, transient will appear in the next
step _at_ higher T from the original stress
(higher at lower T) which sets the oxide growth
rate below that at high T (lower stress).
Also TF(irst)gtTS(econd) StressFltStressS GrowthFgtGr
owthSquilibrium
Plummer et al.
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Segregation of Dopants at the Si/SiO2 Interface
Highly doped layers are used in MEMS so the role
of oxidation on dopant distribution is important
Plummer et al.
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Oxidaion Enhanced Didiffsion
Oxidation can change dopant diffusion - affect
layer thickness.
Substrate Doping Effects Concentration Enhanced
Oxidation (CEO)
Low T
High T
CEO stronger for N than P
Plummer et al.
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Beginning of Integrated Circuits in 1959 Kilby
(TI) and Noyce (Fairchild Semiconductors)
Photolithography used for Pattern Formation
Plummer et al.
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Exposure
Exposure system gives sharp contrast
Will develop
Resist has to respond with changes
Plummer et al.
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Wafer Exposure Systems
25-50 wafers/hr
Degradation of patterns by diffraction
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Basic properties and characterization of results
Contrast allows distinguishing light and dark
areas on the mask. DUV resists have better
contrast and better sensitivity because of
chemical amplification.
Affected by processing conditions also CONTRAST
SLOPE
Plummer et al.
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• Photoresists
• Negative (older resolution limited by
  swelling)- more soluble when not exposed.
• Positive more soluble when exposed. Important
  parameters
• Sensitivity how much time is reqd. for changes
  mJcm-2 (Ex. 100 mJcm-2 for g line and i-line
  resists, newer down to 20 mJcm-2 .
• Resolution
• Robustness to etching.
• Photoresists for g-line and i-line a
  hydrocarbon inactive resin, a (hydrocarbon)
  photoactive compound(PAC), and a solvent PAC
  replaced by a Photo Acid Generator (PAG) to act
  as a chemical amplifier.

Plummer et al.
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Historical Development and Basic Concepts of
Doping
1960

• Development (40 years) in predeposition
• Solid-phase diffusion from glass layer.
• Gas phase deposition at high temperatures (B2H6,
  PH3, AsH6) ? reproducibility good only for solid
  sol. (too high Ns)
• Replace predeposition by ion implantation good
  for bigger devices but difficult for small ones
  (TED)
• Return to diffusion

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Evolution of the Fabrication Process The Planar
Design of Bipolar Transistors
Beginning of Silicon Technology and End of Ge
devices
Implementation of a masking oxide to protect
junctions at the Si surface
Oxidation possible for Si not good for Ge
Lithography to open window in SiO2
Boron diffusion
SiO2 Mask
Phosphorus diffusion through the oxide mask
Oxidation and outdiffusion
Plummer et al.
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Dopant Diffusion
Concept of Sheet Resistance of doped
layers. Higher doping lower the resistance
W
t
?s?/sq.? 4 point probe or van der Pauw
L
Resistivity
Sheet resistance
In MOSFETs Rcontact Rsource Rext lt 10 Rchen
?s?? but keep xj small to avoid DIBL (conflicting
requirements
Plummer et al.
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Junction Formation Process Choice
Plummer et al.
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Dopant Solid Solubility
Concentrations above SS limits result in inactive
coplexes (defects,precipitates)
Metastable electrical activation
Practical concenrations for active P and As
As complexes
Plummer et al.
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Intrinsic Diffusion coefficients of Dopants in
Silicon
Arrhenius fit
Fast Diffusers
Slow Diffusers
_at_ High dopant concentrations the diffusion is
enhanced.
Plummer et al.
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Successive Diffusion Steps
Dt is a measure of thermal budget
T1 followed by T2
Equiv. time
Transient Enhanced Diffusion (TED) and
Concentration Enhanced Diffusion (CED) when D
increases with C and/or crystallographic/point
defects (can be related to damage)
Plummer et al.