Thin Film Deposition

1
Thin Film Deposition

• Quality composition, defect density,
  mechanical and electrical properties
• Uniformity affect performance (mechanical ,
  electrical)

Thinning leads to ? R
Voids Trap chemicals lead to cracks (dielectric)
large contact resistance and sheet resistance
(metallization) AR (aspect ratio) h/w ?
with ? feature size in ICs.
Plummer et al.
2
Chemical Vapor Deposition
Methods of Deposition Chemical Vapor Deposition
(CVD), Physical Vapor Deposition (PVD
evaporation, sputtering)
Cold wall reactor
Atmospheric Pressure APCVD Cold wall reactors
(walls not heated only the susceptor)
Flat on the susceptor
Low pressure LPCVD batch processing.
Hot wall reactor
Plummer et al.
3
Plasma Enhanced Chemical Vapor Deposition(PECVD)

• Used when
• Low T required (dielectrics on Al, metals) but
  CVD at decreased T gives increased porosity, poor
  step coverage.
• Good quality films energy supplied by plasma
  increases film density, composition, step
  coverage for metal decreases but WATCH for damage
  and by product incorporation.

13.56 MHz
P ? 50 mtorr - 5 torr Plasma ionized excited
molecules, neutrals, fragments, ex. free radicals
? very reactive reactions _at_ the Si surface
enhanced ? increase deposition rates
Ions, electrons, neutrals bombardment
Outgassing , peeling , cracking stress.
200- 350 C
Plummer et al.
4
Physical Vapor Deposition (PVD) no chemical
reactions (except for reactive sputtering)
Evaporation

• Advantages
• Little damage
• Pure layers (high vacuum)
• Disadvantages
• Not for low vapor pressure metals
• No in-situ cleaning
• Poor step coverage

Very low pressure (P lt 10 5 torr) - long mean
free path.

• purer no filaments, only surface of the source
  melted
• X-rays generated ? trapped charges in the gate
  oxides ? anneal it !

Plummer et al.
5
Evaporation
Partial Pressure of the source (target)?
Needed for reasonable v ? 0.1 - 1?m/min
No alloys partial pressure differences
Use separate sources and e-beam
?
?

• Step Coverage Poor
• Long mean free path (arrival angle not wide
  small scattering) and low T (low energy of
  ad-atoms)
• Sticking coefficient high (_at_? T) no desorption
  and readsorption ? poor step coverage
• Heating can increase Sc but may change film
  properties (composition, structure)

Sticking coefficient
Plummer et al.
6
Sputter Deposition
Higher pressures 1 100 mtorr ( lt 10-5 torr in
evaporation)
Major Technique in Microelectronics for
DC Sputtering (for metal)

• Alloys (TiW, TiN etc)
• good step coverage
• controlled properties

Conductive
Al, W, Ti
Ar inert gas at low pressure. No free radicals
formed by Ar (ex. O, H ,F as was for PECVD)
Plummer et al.
7
RF Sputter Deposition
Dielectric
13.6 MHz RF coupled capacitively to plasma
DC sputter cannot be used for dielectrics
secondary e-
plasma extinguished (VZ )
More
on the walls
charge built-up potential VP potential _at_ the
target ( area)
-
e-
e- charge
faster, smaller
several 100V
tenths of volts
A1
A2
wafers
NON-CONDUCTING
A2
Oscillating (with RF) e-
ionization yield
pressure
can be used
large A1 area

magnet e- trajectory Magnetron
Sputter Deposition have better ionization yields

deposition rates (10-100X)
better film quality (Ar needed)
use in DC RF ( heating of the target since
I )
Plummer et al.
8
Polysilicon - Very Widely Used in MEMS
As, P segregate _at_ the grain boundaries
Low T gives more amorphous layers
r ( B does not ! )
columnar structure
625C
As P deposition rate of poly Si
use doping after poly deposition
B Vpoly - Si
Plummer et al.
9
Various Aspects of Deposition Processes
C. Liu
10
Etching
DRY ETCH
ANISOTROPIC
Plummer et al.
11
Etching Profiles
PR Mask
Lateral etching chemical good
selectivity
Lateral etching poor selectivity
Rounded sloped PR
Required for scaled down devices
Plummer et al.
12
Wet Etching Isotropic Etch
Plummer et al.
13
Plasma Etching
Replace wet processes in VLSI directional
etching, faster, (less) selective but does not
degrade PR adhesion as some wet steps do. MEMS
use plasma etching widely (deep etch, highly
anisotropic)
Parallel plate system

• Reactive chemical components
• Ionic components

!
Low pressure 1mtorr-1torr
As in CVD or sputtering (here RF electrode was
much smaller and neutral gas Ar)
Plummer et al.
14
Physical Etching Ion bombardment Degrades
selection sputter etch
Chemical Etching
Isotropic arrival angle
volatile
Cl
Low sticking coefficient
ISOTROPIC ETCH
O2 F recombination with CF3
CF4 F etch
rates _at_ small amounts of O2 but _at_ large O2 etch
rates decreasesoxidation of Si takes place
Free radicals S But in practice S is low
(0.01-0.05F - Si)
Plummer et al.
15
Ion-Enhanced Etching
No plasma
Sputtering
Chemical component
selectivity Physical component
anisotropy
Etching Enhancement by ions
Role of ions Adsorption,
Reaction, Formation of
byproducts, and their removal
Polymer formation on all walls but removed at
the bottom by bombardment
volatility of byproducts
Plummer et al.
16
Anisotropic Etch
Fast formation of the polymer
Slow polymer formation
May contain byproducts of etching, various layers
including resist
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MEMS call for optimization of cross-reactivity of
various materials (layers) and processes
18
Silicon-Based MEMS Processes

• Bulk micromachining (historically the first)
  silicon substrate is the main active part of the
  MEMS structures

Etch silicon
Wafer bonding
Oxide growth or nitride deposition (if needed)
Strip PR
Wafer thinning by Chemical Mechanical Polishing
to leave a thin membrane Make piezoresistors
(deposition, patterning, doping) to measure
stress (use Wheastone bridge etc.)
Expose PR
Develop PR
Si etched
Oxide etch Or nitride if used as a mask for Si
etching
C. Liu
19
Bulk Micromachining

• Fabrication of pressure sensors seen in
  cross-sections

Membrane made of poly-Si, Si-nitride, or of oxide
but also from polymers
C. Liu
20
Surface Micromachining
Historically - the later process. Relies on the
sacrificial layers deposited and etched
selectively
etching
C. Liu
21
LIGA process

• Three dimensional metallic and polymer structures
  500µm deep (up to 6cm?!) require deep etching,
  molding, plating etc.
• LIGAX-ray Lithography, electroplating (galvo)
  and injection molding (abformung) and damascene
  processes are widely used. Now UV-LIGA is used
  more frequently.

500-60,000µm
LIGA integration with CMOS via Post-processing
approach Preprocessing approach Side-by-si
de processing
C. Liu
22
New Materials and Fabrication Processes

• Materials Silicon was the main material but
  others are also widely used
• Polymers as active structures optical
  transparency, biocompatibility
• Polymers as protection and sealing layers
• High T and corrosive operation conditions
  (silicon carbide, diamond, nitrides )
• Other semiconductors (optical operation)
• Processes traditional IC fabrication and other
  complementary/new processes (for nanoscale
  dimensions) and/or complementary materials
• Self assembly
• New lithography processes (molding, imprints )