1. Process integration 1: cleaning, sheet resistance and resistors, thermal budget, front end

2. 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 !

3. Metal heater processing
Metal sputtering (or evaporation)
Lithography with resistor mask
3. Metal etching & resist stripping
Can be done on any wafer !
Glass wafers, polymer, ...

4. Diffused heater processing
Thermal oxidation
Lithography with heater mask
Oxide etching + resist strip
Diffusion (in furnace)
New thermal oxidation !
Only applicable on silicon wafers !

5. Diffused vs. metal resistor
Size determined by:
Lithography + diffusion
Always isotropic !!
2 µm linewidth +
1 µm diffusion depth 
4 µm wide resistor
Size determined by:
Lithography + etching
Can be anisotropic.
2 µm linewidth 
2 µm wide resistor

6. 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.

7. 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 ?

8. Resistor sheet resistance
Figure 2.8: Conceptualizing metal line resistance: four squares with sheet resistance Rs in series gives resistance as R = 4Rs.

9. Resistance design How to change resistor resistance ?L W
How to change resistor resistance ?
Change L: vary its length
Change W: vary its width
Change T: vary its thickness
Change ρ: choose a different material

10. Example:solar cell process flow
top metallization
anti-reflective
coating (ARC)
Backside metallization
p-substrate
p+ diffusion
n -diffusion
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)

11. Front end processing wafer selection (thin p-type) wafer cleaning
thermal oxidation
photoresist spinning on front
backside oxide etching
resist stripping
p+ backside diffusion (boron 1019 cm-3)
front side oxide etching
n-diffusion (phosphorous 1017 cm-3)
FRONT END = STEPS BEFORE METALLIZATION
backside metallization
top metallization
antireflection coating (ARC)
p-substrate
p+ diffusion
n -diffusion

12. 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
wafer cleaning
metal deposition on front side
lithography of front metal
metal etching
photoresist stripping
contact improvement anneal
backside metallization
top metallization
antireflection coating (ARC)
p-substrate
p+ diffusion
n -diffusion
BACKEND = PROCESS AFTER FIRST METAL DEPOSITION

13. Active vs. passive cleaning
Cleanroom (and its subsystems) provide passive cleanliness
Wafer cleaning provides active cleaning

14. 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

15. 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.

16. Wafer cleaning removal of added contamination
ultrapure chemicals (very expensive)
particle-free (filtered 0.3 µm)
always includes rinsing & drying steps (with ultrapure water and nitrogen)

17. 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

18. Diffused heater processing
Cleaning
Thermal oxidation
Lithography with heater mask
Oxide etching + resist strip
Diffusion (in furnace)
New thermal oxidation !

19. Contact angle θ
Superhydrophilic (θ ~ 10o) Hydrophilic (θ ~ 70o) Hydrophobic (θ >90o)
If surface is hydrophobic, water-based cleaning chemicals will be ineffective.

20. Cleaning in practise
RCA-1
RCA-2
HF-dip

21. Materials stability at high temperatures
high temperature (>900°C; diffusion fast)
really only Si, SiO2, Si3N4, SiC
intermediate temperature ( °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)

22. 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

23. Thermal budget 2
Concrete examples of junction depths,

24. 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

25. 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)