Transcript Cleaning - MyCourses

Process integration
1: cleaning, sheet resistance and
resistors, thermal budget, front end
[email protected]
Wafer selection
• active role for the wafer ?
• passive role ?
– thermal conductivity
– optical transparency
– flat, smooth mechanical support
• compatibility with equipment ?
• thermal limitations ?
• contamination ? Especially glass in Si fabs !
Metal heater processing
1. Metal sputtering (or evaporation)
2. Lithography with resistor mask
3. Metal etching & resist stripping
Can be done on any wafer !
Glass wafers, polymer, ...
Diffused heater processing
1. Thermal oxidation
2. Lithography with heater mask
3. Oxide etching + resist strip
4. Diffusion (in furnace)
5. Oxide etching + resist strip
6. New thermal oxidation !
Only applicable on silicon wafers !
Diffused vs. metal resistor
Size determined by:
Lithography + diffusion
Always isotropic !!
Size determined by:
Lithography + etching
Can be anisotropic.
2 µm linewidth +
1 µm diffusion depth 
4 µm wide resistor
2 µm linewidth 
2 µm wide resistor
3rd option: polysilicon resistor
Oxide (insulation)
Poly deposition
Poly doping
Lithography
Poly etching
Resist strip
Why poly resistor option is useful ?
Because we can still thermally oxidize the wafer.
Sheet resistance: refresh
Rs  /T
Rs is in units of Ohm, but it is usually denoted by
Ohm/square to emphasize the concept of sheet
resistance. Resistance of a conductor line can now be
easily calculated by breaking down the conductor into n
squares: R = nRs
Aluminum film 1 µm thick, sheet resistance ?
Tungsten film, 1Ω resistance, thickness ?
Resistor sheet resistance
Figure 2.8: Conceptualizing metal line
resistance: four squares with sheet resistance
Rs in series gives resistance as R = 4Rs.
Resistance design
L
W
How to change resistor resistance ?
1. Change L: vary its length
2. Change W: vary its width
3. Change T: vary its thickness
4. Change ρ: choose a different material
T
Example:solar cell process flow
top metallization
n -diffusion
anti-reflective
coating (ARC)
p-substrate
p+ diffusion
Backside
metallization
The contact holes in anti-reflective coating are non-critical
The metallization alignment to contact holes is critical
(in case of misalignment, metal does not fully cover holes, and
gases, liquids, dirt can penetrate into silicon)
Front end processing
•wafer selection (thin p-type)
top metallization
•wafer cleaning
antireflection coating (ARC)
•thermal oxidation
n -diffusion
•photoresist spinning on front
p-substrate
•backside oxide etching
•resist stripping
p+ diffusion
•wafer cleaning
•p+ backside diffusion (boron 1019 cm-3)backside metallization
•front side oxide etching
•wafer cleaning
•n-diffusion (phosphorous 1017 cm-3)
FRONT END = STEPS BEFORE METALLIZATION
Backend processing
•resist spinning on front
•metal sputtering on back side
•resist stripping
•wafer cleaning (acetone + IPA)
•PECVD nitride deposition (ARC)
•lithography for contact holes
•etching of nitride
•resist stripping
•wafer cleaning
•metal deposition on front side
•lithography of front metal
•metal etching
•photoresist stripping
•contact improvement anneal
top metallization
antireflection coating (ARC)
n -diffusion
p-substrate
p+ diffusion
backside metallization
BACKEND = PROCESS
AFTER FIRST METAL
DEPOSITION
Active vs. passive cleaning
• Cleanroom (and its subsystems) provide
passive cleanliness
• Wafer cleaning provides active cleaning
Cleaning and surface
treatments
• Because atoms spread fast at higher temperatures, it is
essential to remove impurity atoms before subjecting the
wafers to high temperatures.
• Wet cleaning:
• RCA-1 (NH4OH-H2O2): removes particles and organic
materials
• RCA-2 (HCl-H2O2): removes metallic impurities
• HF: removes native oxide (SiO2)
• Rinsing and drying are integral parts of wet cleaning !!
TKK MICRONOVA,
2010
Microfabrication
14
Cleaning & surface treatments
(2)
-etch surface to undercut particles
(unfortunately rougheness increases)
-etch film to recover perfect surface (e.g. after oxidation)
-grow film to passivate surface (e.g. Al2O3 very sturdy)
-after cleaning, wafers are in known state.
-memory of previous process steps is eliminated.
-waiting time effects are eliminated.
Wafer cleaning
•
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removal of added contamination
ultrapure chemicals (very expensive)
particle-free (filtered 0.3 µm)
always includes rinsing & drying steps
(with ultrapure water and nitrogen)
Surface preparation
• leaves wafer in known surface condition
• eliminates previous step peculiarities
• eliminates waiting time effects
• Wafer cleaning is the same as surface
preparation; it is just a different viewpoint
of wafer cleanliness
Diffused heater processing
1. Cleaning
2. Thermal oxidation
3. Lithography with heater mask
4. Oxide etching + resist strip
5. Cleaning
6. Diffusion (in furnace)
7. Oxide etching + resist strip
8. Cleaning
9. New thermal oxidation !
Contact angle θ
Superhydrophilic (θ ~ 10o)
Hydrophilic (θ ~ 70o)
Hydrophobic (θ >90o)
If surface is hydrophobic, water-based
cleaning chemicals will be ineffective.
Cleaning in practise
RCA-1
RCA-2
HF-dip
Materials stability at high temperatures
• high temperature (>900°C; diffusion fast)
really only Si, SiO2, Si3N4, SiC
• intermediate temperature (450-900 °C)
refractory metals not in contact with Si
• metal compatible temperature (<450 °C)
Si/metal interface stable, glass wafers
• polymer compatible (<120 °C)
evaporation, sputtering (lift-off resist)
Thermal budget
Time-temperature limits that the device can endure.
High temperature causes:
-diffusion (in all atmospheres)
-oxidation (in oxidative atmosphere)
-damage recovery
Some of these are wanted effects, some are problems:
-implantation damage removed
-dopants driven deeper
-silicon oxidation competes with diffusion
Thermal budget 2
Concrete examples of junction depths,
Annealing effects: physical
•grain growth (in polycrystalline materials)
•crystallization (in amorphous materials)
•diffusion of dopants (e.g. boron in silicon)
•melting (e.g. aluminum melting point 653oC, very low)
•thermal expansion and thermal stresses
•desorption of adsorbed specie
Annealing effects: chemical
•oxidation of surface: Ti + O2  TiO2
•reactions between thin films (Al12W)
•reactions between substrate and thin film (TiSi2)
•dissolution (e.g. silicon dissolves into aluminum)
•corrosion (Cl residues: AlCl3)