Transcript Thomas Roche Member of the Technical Staff Motorola MOS 12

Surface Preparation
and
Wet Processing
Thomas S. Roche, Ph.D.
Motorola Corporation
 1999 Arizona Board of Regents for The University of Arizona
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Outline
• Why wet processing?
– Etching and cleaning
• What chemicals are used?
– Chemistry of the processes
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Why Wet Processing?
• Isotropic
• Non-damaging
• Selective
• Clean - particle and metal contaminant removal
• Well understood
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Isotropic
• Isotropic = the same in all directions
Photoresist
Oxide
Silicon
Isotropic Etch
Anisotropic Etch
• Is anisotropic etching always better?
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Anisotropic Etch
• Spacer Etch
This method of forming spacers uses the anisotropic nature of the reactive ion
etching (RIE) process with no masking steps. This process could not be performed
with wet etchants.
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Anisotropic Etch
• Stringer Formation
Anisotropic etch can cause problems with strips of deposited layers.
Like the spacer formation, small “stringers” can be left if the strip is
done anisotropically.
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Non-Damaging
• Reactive ion etching and plasma etching can damage
silicon surfaces.
– These techniques also have some etching effect on the silicon
surface.
• HF solutions show better selectivity of oxide to silicon
than plasma.
– An HF solution has fewer chemical species than a fluorine
based plasma. It is a better controlled reactant than a plasma.
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Wet Etch Solutions
• HF and Buffered Oxide Etchant - oxide etch
• Phosphoric Acid - nitride etch
• Nitric Acid and HF - polysilicon etch
• Selective metal etches
– Al etch - nitric acid, phosphoric acid, acetic acid
Ti etch - ammonium hydroxide and hydrogen peroxide
W etch - hydrogen peroxide and ammonium hydroxide
• Potassium hydroxide solution - silicon etch
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HF Etches
• HF based solutions (Dilute HF or BOE) etch silicon
dioxide with a rate dependent on the concentration of
HF in the solution.
– The actual etchant in more concentrated fluoride solutions is
HF2, which is a result of the equilibria:
HF
H + +F HF + F HF 2-
• Very selective
• No attack on silicon or silicon nitride
• However, photoresist or thin polysilicon are not
always adequate masks.
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BOE Solution
• BOE is a solution made by mixing 40% NH4F solution
with 49% HF solution.
• Typically used for removal of large amounts of oxide
• Can be used with photoresist
• Higher capacity for Si, since it can tie up the etch
product as SiF62In HF Solution:
In BOE:
Si02 + 6HF
SiO2 + 4 HF + 2 NH4F
2 H2O + H2SiF6
2 H2O + (NH4)2SiF6
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Nitride Etch
•
•
•
•
•
Concentrated (85%) Phosphoric acid
High temperature (> 145 °C)
Etches oxide at a slow rate (selectivity > 30)
Can also etch polysilicon
Difficult to operate
– Influenced by the water content
– Water is injected or dripped in
– Bath is covered and fitted with a condenser
• Difficult to rinse
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Polysilicon Etch
• Solutions of nitric acid and HF
– The higher the nitric:HF ratio, the slower and more
selective the etch
• Typical use - 300:1 ratio for removal of poly on
product wafers and 5:1 ratio for removal of poly on
dummy wafers
• With the more dilute and selective etches, it is
important that no oxide be present on top of the poly
since that will inhibit the etch of the polysilicon.
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Cleans
• Most wet processes in modern fabs involve cleaning
of surfaces. Some of these cleans also involve
etching.
• The number of cleans in a process flow is typically
20-25% of the total steps in the process and involve all
parts of the flow.
• Cleans typically precede all diffusion steps and film
deposition steps, and they follow all RIE or plasma
etch steps and plasma strip processes.
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Silicon Surfaces
• Bare silicon surfaces - unstable
– Hydrogen terminated
– React with oxygen to form “native” oxide
– Hydrophobic
• Native oxide - stable
– Not equivalent to thermal oxide
– Thickness = about 10Å
– Self limiting
– Grown by exposure to air or oxidizing solution
– Hydrophilic
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Surface Characteristics
• Silicon surfaces etched in HF become hydrophobic if
no oxide is left on the surface.
• All other surfaces should be hydrophilic.
Hydrophilic
Hydrophobic
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Surface Characteristics
• Surfaces with oxygen termination are hydrophilic
since they interact with water molecules
• Surfaces with hydrogen termination are similar to
organic surfaces and do not interact with water. They
are hydrophobic.
O O O O O O O O O
SI SI SI SI SI SI SI SI SI
HYDROPHILIC
H H H H H H H H H
SI SI SI SI SI SI SI SI SI
HYDROPHOBIC
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Particle Attachment
Hydrophobic wafers
tend to pick up any
particles on the surface
of the solution as they
are withdrawn through
the liquid-air interface.
Hydrophobic
Surface
Hydrophilic
Surface
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Oxide Surfaces
• Oxide surfaces can be formed:
– by room temperature oxidation of “bare” silicon
surfaces (native oxide)
– by heating silicon in an oxygen atmosphere
(thermal oxide)
– by deposition in a low pressure furnace
(LPCVD oxide)
– by deposition from decomposition of TEOS in a
plasma reactor (plasma TEOS or plasma oxide)
• All oxide surfaces are hydrophilic, but they etch at
different rates in HF.
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Cleaning Solutions
• Sulfuric Acid-Hydrogen Peroxide (aka SPM or
Piranha) solution
 Organic removal
• Ammonium Hydroxide-Hydrogen Peroxide (aka SC1
or APM)
 Particle removal
• Hydrochloric Acid-Hydrogen Peroxide (aka SC2 or
HPM)
 Metallic contamination removal
• Dilute HF solutions
 Metallic contamination removal with oxide
removal
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Sulfuric Acid-Hydrogen Peroxide
(SPM/Piranha)
• Operated at > 100°C
• Useful for its oxidizing (organic destruction)
capabilities
• Will grow an oxide on bare silicon but will not grow
oxide on oxidized surfaces
• Peroxide has a limited lifetime, replenished by spiking
with added peroxide
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Sulfuric Acid-Hydrogen Peroxide (cont’d)
• Sulfuric acid disadvantages
– Difficult to remove from surfaces
– Water used for rinsing is usually very hot
• Amount of sulfur left on surfaces is very high.
– Removed easily with a brief HF dip
• Solutions also effective in removal of some metal
contamination
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Sulfuric Acid-Ozone
• Some facilities use sulfuric acid with ozone bubbled
through it
• Effective solution
– Long bath life - no water added
– Easily overwhelmed by organics
• Another variant - ozone in cold water
– Good oxidant
– Rate of resist removal is fairly slow
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NH4OH-H2O2
(SC1/APM)
• Typically used with a Megasonic unit for particle
removal
• Temperature range of 50-80°C
• Standard solution is 5:1:1 of water:ammonia:peroxide
• Problems
– Peroxide is unstable in basic solution, especially in the
presence of metallic impurities.
– Ammonia has high volatility.
– Metallic impurities readily adsorb onto surfaces.
– Presence of the oxidant is critical!
» If lost, solution will etch silicon
» Maintained by spiking
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NH4OH-H2O2 Details
• Can grow a native (chemical) oxide on a bare silicon
– Also etches oxides at a low rate (~1Å/min),
therefore can etch silicon
• Dilutions down to 50:1:1 probably as effective
– May have problems the current solutions do not
• Al and Fe contamination - typical problem
– Other transition metals can also deposit
– Can also result in surface roughening
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NH4OH-H2O2 Details (cont’d)
• Lower concentrations
– More likely to deposit metals
– Control of the concentration is critical
• Tradeoffs to be considered with lower concentrations:
– higher changeout frequency/shorter bath life
– lower particle removal efficiency
– metallic contamination
– ammonia evaporation and peroxide decomposition
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HCl-H2O2
(SC2/HPM)
• Temperature range of 50-70°C
• Standard dilution is 5:1:1 of water:HCl:peroxide
• Mechanism for operation - chlorides of most metallic
contaminants are soluble in an acidic solution
• Peroxide may not be needed at low metallic
contamination levels.
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Solvent Based Cleaning Solutions
• Resist strippers - proprietary mixtures of organic
solvents and active ingredients
–
–
–
–
The solvents are typically NMP (N-methyl pyrrolidone)
NMP is water soluble and neutral in aqueous solution
“Active ingredients” are typically amines
Attack the resist and dissolve into the solvent media.
• New strippers - “semi-aqueous”, containing both
organic solvents and water along with “active
ingredients” and corrosion inhibitors
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Solvent Based Cleaning Solutions:
Problems
• Corrosion of metal lines due to the transition of the
surface from the organic solution to rinse water
• Any “active ingredients” left on the surface attack the
metal in water
– Solution- use an intermediate rinse solvent
• Metallic contamination (especially sodium)
– Solution - use only low sodium strippers
– Less of a problem with “semi-aqueous” strippers
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Solvent Modified Cleans
e.g. EG-BOE
• Residues after metal etch processes often contain
compounds of the metal etched, along with other
species.
• Cleaners contain fluoride and a polar solvent such as
ethylene glycol to dissolve these metallic compounds.
• Involves a balance of acidity and etching of the
fluoride with the inhibition of metal attack by ethylene
glycol
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Water
• Most heavily used chemical - 2000 gal/wafer
• Used mainly for removing other chemicals
• Rinse times
– until solution reaches high resistivity
– given time period
• Problems
– Bacteria
– TOC
– Metallic contamination
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Wafer Rinsing
• Continuous flow of water past wafers
• Completion of rinse is determined by measuring the
resistivity of the water
(Resistivity = very sensitive method of determining the
presence of ions in solution)
• Dump rinsing - used for wafers that are hydrophilic
• Hot water - used after some processes (but not HF
treated surfaces)
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Wafer Rinsing, cont’d
Resistivity
1800
1600
1400
Resisitivity, E4-Ohm-cm
1200
1000
piranha49
800
piranha50
piranha51
600
piranha53
Spiked H2SO4_a
400
Spiked H2SO4_b
Spiked HF
200
0
0
100
200
300
400
500
Time, sec
• Resistivity measurements describe the purity of the water in the
rinse bath - not the amount of chemical left on the wafer.
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Bacterial Contamination
• All DI systems contain bacteria, which can grow and
contaminate systems if nutrients are provided.
• Ozonation or hydrogen peroxide (oxidizing agent) is
typically employed to keep down bacterial levels.
• Once bacterial contamination is established, it is
difficult to remove since the bacteria form colonies
which are difficult to completely destroy with oxidant
solutions.
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Drying
• Critical step in cleaning
• Objective - remove as much of the water as possible
before it can dry on the surface and leave residues
from any impurities in the water
• Two types of dryers
– Spin rinse dryer (SRD) - throw off as much water
as possible
– IPA vapor - displace water with IPA
» IPA vapor dryer styles
• Tank
• Vapor jet (VJD)
• Marangoni-type
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IPA “Vapor Jet” Dryer
T OP
VIEW
SIDE
VIEW
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Marangoni Drying
A low concentration
of IPA vapor
condenses on the
water surface.
As the wafers are withdrawn
slowly from the water, the IPA
layer displaces the water.
The IPA evaporates from the
wafers and they are dried.
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Types of Cleans
• Resist Strips - post etch and post implant
• Etch residue removal
• Pre-Metal deposition cleans
• Pre-diffusion cleans
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Resist Strip
• Resist removed by ashing - a plasma process which
“burns” the resist off the surface
(Note - Resist contains measurable amounts of impurities
such as sodium.)
• Ashing concentrates impurities on the surface.
– Must be followed by a wet process to remove
contaminants
• Sulfuric acid-hydrogen peroxide used before metal is
present on the wafer
• Solvent strippers used after metal deposition
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Post-Etch Cleans
• Reactive ion etching (RIE) = a physical-chemical
process which can leave residues
• Difficult to remove (can be fluorinated polymers) so
both ash and wet clean are usually used
– Residues are created to inhibit attack of the plasma on the
sidewall of the etched film
– Changes in the etch process or the material being
etched can require changes in the clean
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Residues from RIE Processes
Post Etch Residue
Post Ash Residue
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Metal Etch Veils
Post Etch
Post Clean
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Post Implant Resist Strip
• Implantation sometimes uses resist masks
• High dose implants can convert photoresist to a coaltype material
– Can be ashed if it is done carefully, but problems
can result
• Residue can be more complex since it can contain
residue from the implant
• May require more than a sulfuric-peroxide clean
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Pre-Metal Contact Clean
• Clean before metal is deposited
• Connects the silicon circuit to the metal lines on top of
the die
• Nature of the silicon surface is critical to the metal
contact resistance
• Performed with HF containing etchant
– Important consideration - selectivity of the etchant
to the films present (e.g. oxide, BPSG, TEOS)
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Metal Contact
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Pre-Diffusion Clean
• Cleans before furnace operations are critical
– Defects or contamination will be incorporated into
the wafer if not removed prior to furnace
• Most important steps - those where bare silicon is
present
– Pre-initial
– Pre-sac gate
– Pre-gate
• Typically full cleans (Piranha+BOE+SC1+SC2)
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Gate Oxide Stack
Polysilicon
Gate Oxide
Silicon
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Silicon Oxide Interface
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