: Silicon VLSI Technology Fundamentals, Practice and Modeling by J. D. Plummer, M. D. Deal, and P. B. Griffin

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Silicon VLSI TechnologyFundamentals, Practice
and Modelingby J. D. Plummer, M. D. Deal, and
P. B. Griffin

• ECE 6466 IC Engineering
• Dr. Wanda Wosik

Chapter 4 Cleaning Processes in Si Technology
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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).
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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) 1,000Åvoid with SiOx
-gt affect GOI -gt anneal in H2 -gt oxide decomposes
and surface reconstructs! oxide precipitates
from deep depth in Si.
Yield -gt 90 at the end -gt 99 _at_ each step
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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.
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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, ??VTN0.1V
(equivalent to 6.71017 cm-3 or 10 ppm
contaminations)
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Dynamic Random Access Memory
Vth107cm/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 gt 25 µsec
Use gettering to keep Nt lt1012 cm-3 (Ntlt ppb) -gt
?G100µsec
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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
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Level 1 Contamination Reduction Clean Factories
Air quality is measured by the class of
the facility.
Ex. Class 100 -gt 5 particles/cm, ?gt0.1 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 ---gt 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.)
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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 H2Olt-gt HOH-
HOH-6x10-13cm-3 Diffusivity
of H9.3x10-5cm2s-1 -gt µHqD/kT3.59cm2V-1s-1
of OH-5.3x10-5cm2s-1 -gt
µOH-qD/kT2.04cm2V-1s-1
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Level 2 Contamination Reduction Wafer Cleaning
Piraha Solution
with all contaminants -gt H passivation (or F!)
NH4OH small -gt 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
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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.
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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 -gt
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.
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Intrinsic Gettering
Oxygen 1018 cm-3 15-20 ppm Oigt20ppm -gt too
much precipitation-gt strength decreases and
warpage increases Oilt10ppm -gt no precipitation-gt
no gettering
gtgt Ddopants but D0ltlt Dmetals
denuded zone oxygen free thickness several
tens of µm
Slow ramp
50-100 µm in size
1-3 nm min size of nuclei, concentrations
1011cm-3
Trap sites can be created by SiO2
precipitates (intrinsic gettering), or by
backside damage (extrinsic gettering). In
intrinsic gettering, CZ silicon is used and SiO2
precipitates are formed in the wafer bulk
through temperature cycling at the start of
the process.
SiO2 precipitates (white dots) in bulk of wafer.
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Intrinsic Gettering Due to Oxide Precipitates
Precipitates (size) grow _at_ high T Density of
nucleation sites grow _at_ low T
The largest the most dense defects -gt the most
efficient gettering
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Measurement Methods
Clean factories particle control. Detect
concentrations lt 10/wafer of particles smaller
than 0.1 µm

• Unpatterened wafers (blank)
• Count particles in microscope
• Laser scanning systems -gt maps of particles
  down to 0.2 µm
• Patterned wafers
• Optical system compares a die with a known good
  reference die (adjacent die, chip design - its
  appearance)
• Image processing identifies defects (SEM)
• Test structure (not in high volume
  manufacturing)

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Test Structures
Trapped charge QT ? VTH change Dielectric
breakdown due to particles, metals etc.
Models relate type of defects (typical for
processes) with yields
Water measure water resistivity ? Deionized
Water r18.5 M?
H2O ? H OH-
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Monitoring the Wafer Cleaning Efficiency
Concentrations of impurities determined by
surface analysis
works with SEM
He 1-3 MeV
O or Cs sputtering and mass analyses

• Excite
• Identify (unique atomic signature)
• Count concentrations

emitted
Primary beam e - good lateral
resolution Detected beam e good depth
resolution and surface sensitivity X-ray poor
depth resolution and poor surface
sensitivity ions (SIMS) excellent ions
(RBS) good depth resolution, reasonable
sensitivity (0.1 atomic)
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Electrons in Analytical Methods
(as in SEM)
Inelastic collisions with target electrons, which
are then emitted from the solid
Elastic collision of incoming electrons with
atoms (reflected back) the same energy as for
the incoming electrons
5 eV
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Analytical Techniques

• If X-Ray is at the input
• el. Emitted X-ray Photoelectron Spectroscopy
  (XPS)
• X-ray emitted X-ray Fluorescence (XRF)
• XPS usually more dominant for lighter elements,
  XRF for heavier

This scheme is for lighter elements (Z33 as is
crossover b/w Auger and X-Ray
Several keV
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3
Several keV
The core electron energy levels
2
2
1
kicked out a core electron
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Electron Microprobe
X-Ray Electron Spectroscopy
Auger El. Spectroscopy
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Monitoring of Gettering Through Device Properties
and Dielectric
p n leakage, refresh time DRAM junction and
dielectric breakdown, b of n-p-n
emissionlt-gt capture
Material properties ??G(gtgt?R) in the bulk and
on the surface
Photoconductive Decay Measurements
?ngoptG

• Carriers are generated due to light
• Decrease resistivity
• Recombine

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Carrier Generation Lifetime


inversion

DL
Deep Depletion - Return to Inversion via Carrier
Generation (measure tG) and surface recombination
(s)
Zerbst technique
sf(NST, ?)
if plotted vs. (Cmin/C)-1
?Capture cross section
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Lifetime Measurements Open Circuit Voltage Decay
t0
0.7V
Diode switched from ON VD when carriers
recombine
off
for t??gt4
Measurements include surface and bulk
recombination
Use also DLTS identifies traps (Et) and
concentrations Thermal or photoexcitation
processes in voltage modulable space-charge
region (Schottky Diode, p-n junction, MOS
Capacitor) Measured capacitance, currents or
conductance
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Excess Carrier Concentrations Decays minority
carriers
Mobility of minority carriers
Experiment to calculate the diffusion constant
Dp, (n) for minority carriers (dpn) -gt µp, (n)
Driftdiffusion
diffusion
DxDtL/td
oscilloscope screen
Pulse
Drift vdL/td µpvd/E
Mobility of minority carriers
vd-gt µ