Thin Film Deposition

1
Thin Film Deposition

• Topics
• Chemical Vapor Deposition
• Physical Vapor Deposition
• Evaporation
• Sputtering
• Strengths and Weaknesses
• Basic Calculations

2
Issues related to thin film deposition

• Quality
• Composition
• Defect density (e.g. pinholes)
• Contamination
• Mechanical and electrical properties
• Good adhesion
• Minimum stress
• Topography
• Uniform thickness on non-planar surfaces
• Step coverage
• Conformal coverage uniform
• Space filling in holes, channels
• Voids

3
Thin film filling issues (a) shows good metal
filling of a via or contact hole in a dielectric
layer (b) silicon dioxide dielectric filling the
space between metal lines, with poor filling
leading to void formation (c) poor filling of
the bottom of a via hole with barrier or metal
SEM photo showing typical coverage and filling
problems
4
Two types of thin film deposition CVD and PVD

• CVD
• Reactive gases interact with substrate
• Used to deposit Si and dielectrics
• Good film quality
• Good step coverage
• PVD
• Used to deposit metals
• High purity
• Line of sight

5
CVD
Gases react with substrate Various types of
CVD Atmospheric pressure APCVD Low pressure
LPCVD Plasma enhanced PECVD High density plasma
- HDPCVD

• CVD systems
• APCVD w/cold wall for deposition of epitaxial
  silicon
• LPCVD w/hot wall for depositing polycrystalline
  and amorphous silicon

6
Steps in CVD

1. Transport reactants via forced convection to
   reaction region
2. Transport reactants via diffusion to wafer
   surface
3. Adsorb reactants on surface
4. Surface processes chemical decomposition,
   surface migration, site incorporation, etc.
5. Desorption from surface
6. Transport byproducts through boundary layer
7. Transport byproducts away from deposition region

7
Flux through boundary layer F1 hG(CG-Cs)
(molecules/cm2/s)
Flux of reactants consumed at surface F2 ksCs
(molecules/cm2/s)
Process is limited by slowest step, thus F F1
F2
8
Film Growth
Define Y CG/CT PG/Ptotal
If ks ltlt hG, then v CT/N ksY
If hGltlt ks , then v CT/N hGY
ks hGexp(-Ea/kT) Ea 1.6 eV
9
(No Transcript)
10
hG DG/ds
The position of the boundary layer changes wrt x
Boundary layer velocities along susceptor. ds is
the thickness of the boundary layer. The
boundary layer increases with distance in the
direction of gas flow
m viscosityr density of gasU gas velocity
The susceptor in a horizontal epitaxial reactor
is tilted so that the cross-sectional area of the
chamber is decreased, increasing the gas velocity
along the susceptor. This compensates for both
the boundary layer and depletion effects.
11
Autodoping
Gases may be doped, e.g., AsH3, PH3, B2H6
Autodoping occurs when dopant atoms adsorbed on
(1) wafer frontside (2) wafer backside and edges
(3) other wafers and (4) susceptor are reemitted.
12
LPCVD
Recall that
Growth velocity vs 1/T for APCVD (760 torr) and
LPCVD (1 torr) systems. The lower total pressure
(with PG and CG fixed) shifts the hG curve
upward, extending the surface reaction regime to
higher temperatures.
Decreasing Ptotal increases DG, hG and v
13

• Advantages of LPCVD
• Faster growth
• Less autodoping
• Little diluent gas needed
• Lower gas consumption
• Fewer byproducts (particles)
• Disadvantages
• Line of sight
• Poorer step coverage
• shadowing

14
Plasma Enhanced CVD (PECVD)
Good when temperature is restricted Provides
reasonable deposition rates Good film
quality Conformal
May leave unwanted byproducts on film
15
Physical Vapor Deposition PVD
2 types evaporation and sputtering
Advantages Versatile deposits almost any
material Very few chemical reactions Little wafer
damage Limitations Line-of-sight Shadowing
Thickness uniformity Difficult to evaporate
materials with low vapor pressures
16
Flux from a point source
Deposition rate from a surface source
Geometries of flux and deposition of small areas
on a flat wafer holder for (a) a point source and
(b) a small planar surface source
17
Deposition rate of evaporated film as function of
position on substrate for point and surface
sources. qi qk in this configuration for both
point and surface sources.
18
Mean free path l
k 1.36 x 10-2 erg/at-K d.4 x 10-8 cm Pe
partial pressure (torr)
Vapor pressure as a function of temperature of
commonly evaporated metals
19
Sticking coefficient Sc Freacted/Fincident
The depositing species have a high sticking
coefficient (close to 1) in (a), so that they are
deposited where they first strike. In (b) the
depositing species have a low sticking
coefficient (ltlt1) so that man are reemitted and
deposit elsewhere on the topography, such as the
sidewalls.
20
Sputtering
Schematic diagram of DC-powered sputter
deposition equipment
Plasma structure and voltage distribution in DC
sputter system
21
Distribution of arrival fluxes for (a) uniform or
isotropic arrival distribution and (b) directed
or anisotropic arrival distribution. Arrival
angle distribution (cosnq) is defined by arrival
flux relative to unit surface area. This flux is
equal to the normal component of incoming flux,
relative to the vertical direction for a
horizontal surface.
Processes in sputter deposition
22
Effect of arrival angle distribution of
depositing species on filling trenches or holes.
In (a) a relatively wide arrival angle
distribution leads to poor bottom filling or
coverage, while (b) a narrower arrival angle
distribution leads to better bottom filling. The
higher the aspect ratio of the feature, the
narrower the arrival angle distribution must be
for adequate coverage.
Schematic diagram of ionized sputter deposition
system (ionized PVD) showing atomic flux lines
23
(No Transcript)