1. FRONT END PROCESSES - CLEANING, LITHOGRAPHY, OXIDATION
Chapter 4
FRONT END PROCESSES - CLEANING, LITHOGRAPHY, OXIDATION
ION IMPLANTATION, DIFFUSION, DEPOSITION AND ETCHING
• Cleaning belongs to front end processes and is an important part of fabrication.
• Reference - ITRS Roadmap for Front End Processes (class website).

2. Semiconductor Manufacturing Clean Rooms, Wafer Cleaning and Gettering
Importance of unwanted impurities increases with shrinking geometries of devices.
75% of the yield loss is due to defects caused by particles (1/2 of the min feature size)
Crystal originated (45-150nm) particles (COP) ~1,000Å=void with SiOx -> affect GOI
-> anneal in H2 -> oxide decomposes and surface reconstructs! & oxide precipitates from deep depth in Si.
Yield -> 90% at the end -> each step

3. SEMICONDUCTOR MANUFACTURING - CLEAN ROOMS, WAFER
CLEANING AND GETTERING
• Modern IC factories employ a three tiered approach to controlling unwanted
impurities: 1. clean factories 2. wafer cleaning 3. gettering
Up till 2018
2003 ITRS Front End processes - see class website
• Contaminants may consist of
particles, organic films (photoresist),
heavy metals or alkali ions.
swirls

4. Historical Development and Basic Concepts
Contaminants and their role in devices (various elements, various films)
Particles cause defects ! QM !
Life time killers ! !
Poly-Si, silicides
Na+, Ka+
XOX ~10nm QM≈ 6.5x1011cm-2, VTN=0.1V (equivalent to 6.7*1017 cm-3 or 10 ppm contaminations)

5. Dynamic Random Access Memory
Vth~107cm/sec
write, read
~10-15cm-2
Deep-level traps (Cu, Fe, Au etc.)
Pile up at the surface where the devices are located.
Leakage currents discharge the capacitor (mechanism SRH) refresh the charge storage (time ~ a few msec)
Lifetime must be > ~ 25 µsec
Use gettering to keep Nt <1012 cm-3 (Nt< ppb) -> G≈100µsec

6. Role of Surface Cleaning in Processing
Oxide thickness [Å]
Residual contaminants, layers affect kinetics of processes.
Surface effects are very important (MORE) in scaled down devices

7. Level 1 Contamination Reduction: Clean Factories
• Air quality is measured by the
“class” of the facility.
Ex. Class 100 -> 5 particles/cm, >0.1 µm in 1hr.
Small particles remain in air (long) coagulate  large ones precipitate quickly and deposit on surfaces by (small) Brownian motion and gravitational sedimentation (larger).
Class 1-100,000 mean number of particles, greater than 0.5 m, in a foot of air
Use local clean rooms
from
Particles > people , machines, supplies
suits
Material filters
Chemicals, water (use DI)
Factory environment is cleaned by:
• Hepa filters and recirculation for the air,
• “Bunny suits” for workers.
• Filtration of chemicals and gases.
• Manufacturing protocols.
(Photo courtesy of Stanford Nanofabrication Facility.)

8. Level 2 Contamination Reduction: Wafer Cleaning
Front End Process
Back End
Oxygen plasma
H2S04 +H2O2
Oxygen plasma
Organic strippers
(do not attack metals)
Good clean for high T steps
Low T - less critical
Oxidizes organic films
Oxidizes Si and complexes metals
5 H20 + H2O2 + NH4OH SC1
Small content reduces Si etch (0.05%)
Removes alkali ions & cations Al3+, Fe3+, Mg3+ (insoluble in NH4OH - SC1)
6H2O : H2O2 : HCl SC2
Ultrasonic and now megasonic cleaning for particulates removal (20-50 kHz)
DI water is necessary: H2O<-> H++OH [H+]=[OH-]=6x10-13cm-3
Diffusivity of: H+≈9.3x10-5cm2s-1 -> µH+=qD/kT=3.59cm2V-1s-1
of : OH-≈5.3x10-5cm2s-1 -> µOH-=qD/kT=2.04cm2V-1s-1

9. Level 2 Contamination Reduction: Wafer Cleaning
Piraha Solution
with all contaminants -> H passivation (or F!)
NH4OH small -> reduce surface roughness
• RCA clean is “standard process”
used to remove organics, heavy
metals and alkali ions.
• Ultrasonic agitation is used to
dislodge particles.
Not removed by SC1
HF dip added to remove oxide

10. Level 3 Contamination Reduction: Gettering
• Gettering is used to remove metal ions and alkali ions from device active regions.
• For the alkali ions, gettering generally uses dielectric layers on the topside
(PSG or barrier Si3N4 layers).
• For metal ions, gettering generally uses traps on the wafer backside or in the
wafer bulk.
• Backside = extrinsic gettering. Bulk = intrinsic gettering.

11. Gettering Concepts: contaminants freed  diffuse  become trapped
PSG (for alkali ions Na+, K+ and metals) affects E fields (dipoles in PSG) and absorbs water leading to Al corrosion (negative effects)
Fast Diffusion of Various Impurities
or Si3N4
metals
Closer to devices than to a backside layer -> high efficiency
Metal contaminants will be trapped by dislocations and SF (decorate) and far away from ICs
• Heavy metal gettering relies on:
• Metals diffusing very rapidly in silicon.
• Metals segregating to “trap” sites.