Transcript Figure 5.1 Relative size of one micron.

Figure 5.1 Relative size of one micron.
• Major contaminants:
1. Particles
2. Metallic ions
3. Chemicals
4. Bacteria
•
Bacteria: Need const. running
water to prevent bacterias
Figure 5.2 Relative size of contamination. (Hybrid Microcircuit
Technology Handbook)
Figure 5.3 Relative size of airborne particles and wafer dimensions.
• As lines get smaller, particle
control becomes more
important.
• Killer defects: particles in
critical location
Contamination caused problems:
Device processing yield
Device performance
Device reliability
Contamination sources
1. Air
2. The production facility
3. Clean room personnel
4. Process water
5. Process chemicals
6. Process gases
7. Static charge
Figure 5.4 Example resist stripper trace metal contents (EKC
Technology—830 Photoresist stripper)
Process chemicals and process water can be
contaminated with trace chemicals from
wafer process.
• MOS Grade = Low sodium
grade (sodium is the most
prevalent mobile ionic
contaminant)
• MICs: Mobile ion contaminants
• Metals in an ionic form can
cause electrical failure even
after final test since they are
movable.
• Exist in most chemical
• Need to be < 1010 atoms/㎝2
Figure 5.5 Relative size of airborne particulates (microns).
Figure 5.6 Air cleanliness classes standard
209E.
•regular air in city: 5M /ft3
Figure 5.7 Typical class numbers for various environments.
• Clean room design strategies:
1. Clear air station
2. Tunnel design
3. Total clean room
4. Mini environments
Figure 5.8 Hepa filter.
• Clean room starts with space
program: NASA, a single speck
can cause satellite to fail.
•
•
•
Fragile fiber with small holes in
accordion design (手風琴)
Air pass with large volume and low
velocity (not to cause air currents)
air flow → 90 - 100 ft/min
Figure 5.9 Cross section of VLF hood.
• HEPA (High Efficiency
Particulate Attenuation) ~
99.99+ efficiency
• (work station) vertical laminar
flow
• ＊important: (1) HEPA (2)
positive pressure
Figure 5.10 Cross section of a VLF-fume-exhaust hood.
Safety + No contamination
•
Wet Chemical Process Hood
wafer storage
Figure 5.11 Cross section of clean-room tunnel.
Divide fabrication area into separate
tunnels or bays
•
To prevent contamination from too
many people working in the same
room: use Tunnel/Bay concept
Fewer people work in one bay
Figure 5.12 Cross section of laminar flow clean room. (Courtesy of
Semiconductor International.)
• Recovery: the time required for
the filters to return the area to
acceptable condition after a
shift start, personnel break or
other disturbance. Class 1:6
seconds
Figure 5.12
air return
open work station with perforations
Figure 5.13 Wafer transfer microenvironment.
• Cost billion USD to build clean
room. Use micro / mini
environments to reduce cost →
isolate the wafer in as small an
environment as possible.
Figure 5.13
pressure air / N2
Figure 5.14 Minienvironment system elements.
• (WIT): wafer isolation
technology
• low construction and operating
cost
• with > 8’wafer → too heavy to
carrier, too expensive to drop
Figure 5.14
• Mechanical interface
Figure 5.15 Fab area with growing area, air showers, and service aisles.
• Temperature: 72℉±2℉ (stable
chemical reaction)
• Humidity: important for
polymer (too wet, polymer is
not sticky) (too dry,static charge)
• relative humidity: 15 ~ 50%
• smog control: ozone filtered by
carbon
Figure 5.15
Service bay
Positive air pressure
static control
Double door
Shoe glove cleansers
Adhesive flow mats
Figure 5.16 Triboelectric series. (Hybrid Circuit Technology Handbook,
Noyes Publications)
• Static charge formed by
triboelectric charge (formed
when two materials initially in
contact are separated)
• One surface loses e• One surface gains e-
Figure 5.16
• loses e-
Figure 5.16
• gains e-
Figure 5.16
• (1) High density circuits with submicron feature size vulnerable to
smaller particles attracted by static charge. Static charges build up on
wafer storage boxes, work surface, equipments. Can be as high as
50,000 volt to attract aerosols from air or personnel garments. Very
difficult to remove.
• (2)ESD (Electric Static Charge) can destroy devices. Need to package
devices with antistatic materials
Figure 5.17 Static-charge reduction techniques.

Preventions:
Use antistatic materials in
garments and storage boxes.
(2) Use antistatic solution
(apply to the wall),but not in critical
area to prevent contamination from the
solution
(3) Grounded static discharge straps.
(4) Ionizer (underneath the Hepa or
close to the Nitrogen blow gun) to
neutralize the charge built up in the
filtered air.
(1)
Other static change examples:
(1) Photomask and rectile damage. ESD discharge can vaporize and
destroy the chrome pattern.
(2)ESD discharge between package material (PFA) for wafer and
equipment produce EM interference with machine operation.
Static Charge Prevention
(1) Use antistatic materials in garments and storage boxes.
(2) Use antistatic solution (apply to the wall)
(3) Grounded static discharge straps.
(4) Ionizer (underneath the Hepa) to neutralize the charge built up in the
filtered air.
Figure 5.18 Activity-caused increase in particles.
over background=1
(Hybrid Microcircuit Technology Handbook,Noyes Publications)
• Human: biggest source of
contamination
• After showering and sitting,
gives off 100,000 ~ 1,000,000
particles/
min
• Hair spray cosmetics, smoking,
pencil must be prevented.
• Gowning from top to bottom.
Undress from bottom to top.
Figure 5.19 Resistivity of wafer versus concentration of dissolved solids.
A Fab uses millions of gallons of water
per day.
Water contaminants if nor processed:
1.Dissolved minerals
2.Particulates
3.Bacteria
4.Organics
5.Dissolved oxygen
6.Silica
D.I water 18,000,000 Ohms-cm at 25
degree C
Process chemicals
• Contaminants: metallic; particulates; chemicals
• Grades:
commercial ~ too dirty for IC
reagent ~ too dirty for IC
electronic ~ cleanliness depends on manufacturer
semiconductor ~ cleanliness depends on manufacturer
• usually MIC level 1 ppm, some supplier provide 1 ppb
• particle filtering level 0 - 2μm or lower.
• Usually purchase bulk quantities
(prevent container contamination)
Process for Cleaner Chemicals
• BCDS (Bulk Chemical Distribution Systems) →cleaner chemical /
lower cost
• Point of use (POU), mix chemical at process vessel.
• Point of use chemical generation (POUCG)
Chemical made at process station, to reduce contamination and cost. (such
as NH4OH,HF, H2O2)
Gas Quality
1. Purity
2. Water vapor contents
3. Particulates
4. Metallic ions
Process with gas reactions
Oxidation
Reactive ion etch
Sputtering
Plasma etch
CVD
Contamination Control
•
•
•
•
•
Contamination may change the chemical reaction
Gas Purity 99.99 ~ 99.999999%(Highest purity, with six 9’s)
Water vapor is limited to 3 - 5 ppm, or it can oxide the Si surface
Gas filtered (Particulates ~ 0.2μm)
MIC < ppm
Water Requirements
•
1.
2.
3.
4.
5.
6.
•
•
Regular wafer contains:
Dissolved minerals ~ removed by ion exchange system.
Particulates ~ removed by sand, earth, membrane filtration.
Bacteria ~ removed by sterilizer
Organics ~ removed by carbon bed filtration.
Dissolved oxygen ~ removed by decarbonator & vacuum degasifier
Silica
Monitor water resistivity in several points in fabrication area.
Standard 18,000,000Ω-㎝ at 25℃ (18 megohm 18mΩ) water
Clean Room Materials and Supplies:
Notebook
Tools
Pencils
Storage boxes
Cartwheels
Need special materials, which don’t generate particles
Clean Room Maintenance:
Cleaner
Applicator
Wiper
Vacuum cleaner with Hepa filter
all need special materials
Wafer Surface Cleaning
• Wafer surface contamination:
1. Particulates
2. Organic residues
3. Inorganic residues
4. Unwanted oxide layers
• Wafer surface roughness requires 0.15 nm (nmRMS) root mean square
of vertical surface roughness.
• In 2010 (nmRMS)<0.1nm
• Excess surface roughness effect device performance and layer
unifromaity
Surface Contamination
• Surface contaminant type
1.Particulates
2.Organic residues
3.Inorganic residues
4.Unwanted oxides
• Gate oxide need <0.02 defects/㎝2 when tested at 5MV/㎝ 30 secs
• Zn, Na, Fe, Ni, Ca<2.5*109 atoms/㎝2
• Al,Ca < 5*109 atoms/㎝2
• FEOL: Front End of the Line (from active layer)
• BEOL: Back End of the Line
Figure 5.20 DRAM water specs. (Semiconductor International, July 1994,
p. 178)
Figure 5.21 Typical deionized water system.
Water stored is blanked with nitrogen to prevent the
Absorption of carbon dioxide.Carbon dioxide
interfere with resistivity may cause wrong reading
Figure 5.22 Sources of particulate contamination. This analysis, shown at
SEMI Forecast by Dr. C. Rinn Cleavelin, Texas Instruments, revealed
equipment-generated particles as the top enemy in 1985.
• PWP: Particles per Wafer Pass
• Equipment need material and
design selection and assembled
in a clean room environment.
Figure 5.23 Typical FEOL cleaning process steps.
Standard clean
Particulate removal
Small particulate held to surface by :
1.van der Waals force ( strong inter-atomic attraction between the
electrons of one atom and nucleus of another)
2. Capillary force (Occurred when there’s liquid bridge between
particle and the Surface)
Zeta Potential: arises from a charge zone around particles that is
Balanced by opposite charge zone in the cleaning liquid
Van der Waals force can be minimized by Zeta potential
The charge in the liquid varies with cleaning liquid speed, PH of the
solution, concentration of the electrolytes in the solution, additives
in the solution, such as surfactant. These conditions create a large number of
charge that has same polarity of the wafer and create repulsive force to
keep the particle in the solution and off the wafer surface
Surfactant and mechanical assist tool (such as megasonics)
are used to dislodge the particle attached to the surface by capillary force
Figure 5.24 Capillary force from film.
Occurred when there’s liquid
bridge between particle and
the surface
Surfactant and mechanical
assist are used to dislodge
the particle
Most commonly used cleaning process
Nitrogen blow off:
1. Spray of filtered high pressure nitrogen
2. Ionizer strip static charges from the nitrogen
stream and neutralize the wafer surface
Figure 5.25 Mechanical scrubber.
Wafer hold by a rotating vacuum
Chuck
Brush and wafer rotation create high
energy cleaning action
Liquid forced between the space with
high velocity adds to cleaning
Dilute NH4OH is added to the cleaning
solution to control zeta potential
RCA clean
SC1H2O,H2O2,NH4OH ratio from 5:1:1-7:2:1
Used at 75-85 degree C
Oxide keeps forming and dissolving
Removes organic residues and sets up a condition for desorption
of trace metal from the surface
SC2H2O,H2O2, HCl ratio from 6:1:1 to 8:2:1
Used at 75-85 degree C
Removes alkali ions, hydroxides and complex residue metal
If oxide free surface is needed, HF etch is added before or after RCA clean
Metal ions are not dissolved in most cleaning solution, need to add chelating
agent (ethylenediamine-tetra-acetic acid)
Dilute solutions, SC1 (1:1:50), SC2 (1:1:60) are usually used with same
effectiveness and with less roughness on the surface.
Photomask Cleaning
High pressure water spray (2000-4000 psi)
Add small amount of surfactant as destatic
agent
Organic residue removal
Use TCE, Aceton, Alcohol
Problems:
Solvent cleaning is difficult to dry and contains
contaminants
Chemical Cleaning
H2SO4 + Oxidant { (H2O2, [(NH4)2S2O3],HNO3,
OZONE)}
H2SO4 is effective cleaner for inorganic residues
and particles from 90o C to 120oC
Oxidants are added to remove carbon residues by
converting C to CO2 which leaves as vapor
 C+O2=CO2 (gas)
Chemical Cleaning (CONTINUE)
H2SO4 + 30% H2O2 (by volume)
(Carro’s acid or Piranha acid)
Used for all stages of processing and
photoresist stripper
Exothermic reaction, T=110-130 degree C
Need to add H2O2 to maintain the cleaning rate
(As time proceeds, temperature falls and reaction
rate falls)
Ozone addition
Ozone can be used in the sulfuric acid
instead of oxidant additive
Ozone can also be added to D.I. Water (1-2ppm)
to provide a cleaning solution for light
organic contaminants (for 10 minutes at room
temperature)
Oxide layer removal
Thin oxide (100-200Å) formed in air or in the heated
chemical bath with the presence of oxygen
Thin oxide is an insulating layer which prevents
Electrical contact between Si and metal, also prevents
silicon surface from chemical processes
Hydrascopic-silicon surfaces with oxide
Hydrophobic-silicon surfaces that are oxide free
Oxide layer removal (continue)
Before Oxidation process Si surface is cleaned in 49%
HF, which etches oxide but not Si
In later process, oxides in patterned holes are etched
in water and HF solution (strength from 100:1 to 10:1)
Strength is chosen that solution will etch oxide in the
Hole, but not the silicon (typical dilutions from
1:50-1:100)
Pregate cleaning uses HF as the last chemical step
(HF-last)- surface is hydrophobic and low metal
contamination
Figure 5.26 RCA clean formulas.
Developed by
RCA engineer
Werner Kern
in mid 1960s
to remove
Organic and
inorganic residues
from silicon surface
Figure 5.27 Experimental room temperature cleaning process.
Combine Water,
HF + Ozone in
Megasonic for
cleaning
Spray cleaning
Standard cleaning process using immersion in chemical
baths performed in wet bench
Immersion process is expensive (needs a lot of solution),
causes redeposition of contaminants on the surface, and
smaller and deeper patterns are difficult to clean
Spray cleaning advantages:
Chemical costs are down, cleaning efficiency is low,
less recontamination due to spray
Pressured spray assists in cleaning small patterns
Rinsing after cleaning in the same machine without
separate station
Figure 5.28 CO2 “SNOW” cleaning (Courtesy of Walter Kern).
High pressure CO2 is directed
at the surface from the nozzle
Pressure drop causing rapid
cooling and forms CO2 particles
and snow
Impinging particles dislodge the
surface particles and flow carry
them away
Can also use argon:
Argon aerosol is large and
heavy can dislodge the particle
when directed to the wafer
under pressure
Nitrogen/argon can also be used
for this technique:
(cryokinetic)
Water Rinse
Wet cleaning is followed by rinse in D.I. Water
Rinsing functions:
Removing cleaning chemicals
Stop oxide etch
Future direction:
Higher rinse efficiency, from 30 gal/sq.in. of Si to
2 gal/sq.in. of Si in 2012
Dry cleaning
Vapor or gas phase cleaning
(E.g. HF/water mixtures)
Plasma etch
UV ozone cleaning: oxidize and photo-dissociate
contaminants from the wafer surface
Figure 5.29-(1)
Rinse systems: (a) single overflow
D.I.water from bottom
flow through around the
wafer, exitiy into over
a dam into drain system
Nitrogen bubbles up
through the water, aids th
mixing of the chemical with the water on
wafer surface (bubbler)
Figure 5.29-(2)
Rinse systems: (b) three-stage overflow
Figure 5.30 Parallel down flow rinsing (Courtesy of Walter Kern).
Water brought into the system from
outside the rinser and flow down
through the wafer
Rinse usually take 5 minutes with water
flow rate equivalent to 5 times the
volume of the rinser per minutes
(5V/min)
Rinse time can be determined by
measuring the resistivity of the water
(water resistivity meter is used, usually
exiting water is 15-18 megohm)
Figure 5.31 Spray-dump rinser.
Overflow rinser with spray capability
Spray rinsing
Flow water removes water soluble chemicals and
carries the chemicals away
Faster flow rate will speed up the rinsing process
Spray rinsing removes the chemical with a physical
force from momentum and has a faster rinse rate
Advantages:
Faster rinse rate, more efficient rinsing
Use less water
Disadvantage:
Carbon dioxide from air get trapped in the spray
and form charged particles and resistivity meter reads
them as contaminants
Figure 5.32 Ultrasonic/megasonic wafer cleaning/etching bath.
Ultrasonic:20,000-50,000 Hz,waves passes liqyid causing microscopic bubbles to form and collapse
rapidly creating scrubbing action that dislodge the particles (cavitaion)
Megasonic:850KHz, small particles held on the wafer surface due to slow moving boundary layer
on the wafer surface, leaving the particle unexposed to the cleaning chemicals,megasonic energy reduces
the stagnant layer on the wafer surface, exposing particles to the cleaning solutions,also,acoustic streaming
fosters an increase in the velocity of the rinse and cleaning solutions past the wafer surface, increasing
cleaning efficiency
Figure 5.33-(1)
Spin rinse dryer styles. (a) Multiboat
Rinse the wafer with
Water from central
pipe, than rotate with
high speed with hot
nitrogen from the
center pipe.The rotation
throw water off the wafer
And the hot nitrogen
remove the water droplet
Drying Technique
• Spin Rinse dryers
• Isopropyl Alcohol (IPA) Vapor dry
• Surface Tension/ Marangoni Drying
Figure 5.33-(2)
Spin rinse dryer styles. (b) single boat axial
Axial dryer:water
and nitrogen come
through the side,
rinse and drying
take place while
spinning
Figure 5.34 Vapor dry (Courtesy of Walter Kern).
Alcohol drying:
Heated reserve of
Liquid IPA with vapor
Cloud above it. When
Wafer with residual
Water on surface is
Suspended in the
Vapor zone, the IPA
Replaces the water,
Chilled coils around
the vapor zone condense the water vapor
Out of the IPA, leaving
The surface water free
Figure 5.35 SIA Roadmap Projections (Micro October 1998 p. 54).
Surface Tension/Marangoni Drying
• Surface tension draws the water away from the
surface, leaving it dry.
• IPA and nitrogen are directed at wafer water level
interface.
• IPA/Nitrogen flow created a surface tension
gradient causing a water flow from surface into
the water. This internal flow further enhances the
removal of water from the wafer.