Transcript No Slide Title

Chemical Mechanical Polishing
(CMP) Overview
Dr. Stephen Beaudoin
Arizona State University
Dr. Duane Boning
Massachusetts Institute of Technology
Dr. Srini Raghavan
The University of Arizona
 1999 Arizona Board of Regents for The University of Arizona
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Outline
• CMP Basics
• CMP Process Optimization
• Environmental Issues in CMP
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Learning Objectives
• Gain the ability to discuss CMP with polishing experts
• Understand basic phenomena that occur during polishing and will be
able to explain why these phenomena occur
• Become aware of the processing and environmental challenges
associated with CMP
• Learn how to assess the environmental consequences of manufacturing
processes and how to compare the impacts of competing processes
• Gain experience in setting new, more environmentally sound polishing
practices
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Questions
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What is CMP?
How does CMP work?
Why do we need CMP?
How do we describe CMP?
What are the problems associated with the CMP process?
What are the environmental impacts of CMP?
How can we alter the environmental impacts of CMP?
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CMP Basics
• What is CMP?
– CMP is a physico-chemical process used to make wafer
surfaces locally and globally flat.
– Chemical action
• hydroxyl ions attack SiO2 in oxide CMP, causing surface
softening and chemical dissolution
• oxidants enhance metal dissolution and control passivation in
metal CMP
– Mechanical action
• polisher rotation and pressure
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CMP Basics (cont’d)
• How does CMP work?
– A rotating wafer is pressed face-down against a rotating
polishing pad; an aqueous suspension of abrasive
(slurry) is pressed against the face of the wafer by the
pad.
– A combination of chemical and physical effects
removes features from the wafer surface.
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CMP Apparatus
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CMP Basics (cont’d)
• Why do we need CMP?
– for precise photolithography for advanced devices
– for advanced multilevel metallization processes (Damascene)
• How is CMP described?
– key parameter: post-polish nonuniformity (NU)
• NU = ratio of the standard deviation of the post-polish wafer
thickness to the average post-polish wafer thickness
• caused by variations in local removal rate
– important parameter is removal rate (RR)
• RR = average thickness change during polishing divided by polishing
time
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Metal Damascene Process
• Trenches/vias etched into ILD (interlayer dielectric)
• Metal deposition
• Metal CMP
• Repeat for multiple levels of metal
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CMP Consumables
• Slurries for oxide (SiO2) polishing
– colloidal suspension of silica particles in alkaline medium
– hydroxyl ions attack SiO2, causing softening and chemical
dissolution (mechanism unverified)
– particles range from 10 to 3000 nm, mean size 160 nm
– 12% (wt) particles, KOH used to set pH ~11
– other concerns: particle size distribution (scratching), particle
shape, particle agglomeration
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CMP Consumables (cont’d)
• Slurries for metal (W, Al, Cu) polishing
– oxidants cause metal dissolution and passivation (reactions to
form protective layer on metal surface)
– typically alumina particles (a or g), 100 to 2000 nm in diameter,
12% (wt) particles, pH 3 to 4
• alumina-peroxide
– 1 part slurry, 1 part 50% H2O2, pH 3.7-4.0
• alumina-ferric nitrate
– 6% alumina solids, 5% ferric nitrate, pH 1.5
• alumina-potassium iodate
– 6% alumina solids, 2-8% potassium iodate, pH 4.0
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CMP Consumables (cont’d)
• W polishing
– pH 4 with H2O2 or KIO3
– pH 1.5 with ferric nitrate
– pH 6 with potassium ferricyanide, potassium acid
phosphate and ethylene diamine
• Al polishing
– peroxide or iodate-based slurries
• Cu polishing
– ammonia-based solutions, passivating agents
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CMP Consumables (cont’d)
• Polish pads
– cast polyurethane or felt impregnated with polyurethane,
thickness~ 1-3 mm
– hardness affects planarization and nonuniformity
– surface treatment (conditioning) required to control polish
rate and slurry transport
• scraping pad surface with hard edge to remove debris, open pores
– pads wear out quickly (100-1000 wafers/pad!)
– perforated, grooved pads coming into use (improved slurry
transport/uniformity)
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CMP Consumables (cont’d)
• Carrier Films
– hold wafers onto polish head (carrier)
– porous polymeric materials
• held onto carrier by vacuum, thermal processing, adhesive
– average roughness ~1-20 microns
– compressibility range 1-25% under 10 psi load (typical
of CMP conditions)
– thickness ~ 0.1-1 mm
– profound effect on polishing performance
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CMP Requirements
• Stable, predictable, reproducible process
• Removal rates >1700 Å/min for SiO2 and >2500 Å/min for W
• Independent of device/circuit design, substrate
– good selectivity between metal and dielectric and similar
polishing rates for metals and liners
• Few defects (scratches, peeling, particles)
• Low NU
– less than 5% variation in film thickness across wafer
• 3-6 mm edge exclusion
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Preston’s Equation
• Simplest CMP model
• Expresses polishing rate in terms of applied pressure and
relative velocity between polishing pad and wafer
– RR = Kp•P•S
• Kp = Preston coefficient (inversely proportional to elastic
modulus of material being polished)
• P = down pressure
• S = pad-wafer relative speed
– can predict general trends
– observed RR usually proportional to P and S
– cannot predict within wafer NU, feature effects, or
variations due to pattern density effects
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CMP Process Variables
• Tool
– Pressure (down force)
– Platen and carrier speeds
– Platen temperature
• Slurry
– Flow rate (150-300 ml/min)
– Slurry age
– Temperature
• Pad conditioning
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CMP Processing Problems
• Particle contamination on wafers
– slurry particles, pad material, abraded films
• Chemical contamination on wafers
– metal ions (K+, Fe3+, Ni2+)
– anions (SiO32-, WO42-, IO32-)
– surfactants
• Mechanical damage to wafers
• Nonuniform polishing
• RR variations with time during processing
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Particle Contamination
• Electrostatic effects can cause particles to be attracted to wafer
– depends on zeta potential of particle, pH, ionic strength of
solution
– can be attractive or repulsive
• Once particles are near wafer, Van der Waals interactions
(always attractive) enhance adhesion
• To minimize particle contamination, particle and surface must
have same charge
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Minimization of Particle Contamination:
Additives to Alumina Slurry
• Isoelectric point of:
Alumina
W
SiO2
8-9
2.0 - 2.5
2-3
Minimization of particulate contamination may be achieved by
choosing a pH such that the surface charge (and zeta potential) of
tungsten, silica, and alumina bear the same sign.
• Two strategies possible:
– Both alumina and tungsten bear a positive surface charge (ferric
nitrate based slurries @ pH 1.5 - 2.0)
– Both alumina and tungsten are negatively charged (anionic
additives such as anionic surfactants and polyanions to slurries @
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pH 3.5 - 4.0)
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Mechanical Contamination
• CMP can induce rearrangement of the structure of the
metal or SiO2 wafer surface
• Can extend tens of nm into the wafer
• Highly strained structures, broken networks and loss of Si
atom tetrahedral coordination
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Chemical Contamination
• Chemicals in solution change oxidation state based on pH,
potential of the solution
• Reactivity also changes
• Solubility and partitioning of chemical species can vary
considerably with oxidation state and reactivity changes
• Corrosion may occur depending on redox potential of
exposed metals (TiN-W system of concern)
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CMP Control Issues:
Polishing Nonuniformities
• Dishing
– reduction in thickness of large metal features towards the center of
the features
– caused by differences in polishing rates of metal, liner, and
insulator
• Pattern erosion
– thinning of oxide and metal in a patterned area
– increases with pattern density
• Edge effect, “racetrack” NU
– variations in removal rate due to stress variations with radial
distance across wafer
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Pattern Erosion and Large Feature Dishing
• Dense SRAM Array
• Dishing
Support Circuits
– Erosion is the thinning of oxide and metal in a patterned area,
while dishing is a reduction in the thickness of a large tungsten
feature toward the center of that feature.
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CMP Control Issues:
Removal Rate Drift
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As pads wear, RR decreases
Occurs even with conditioning
Coincident with increasing NU over time
Solutions
– substantial use of monitor wafers to check performance
– increase polish time over time to achieve desired
removal
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Post CMP Cleaning
• Remove particles and chemical contamination following
polishing
• Involves buff, brush clean, megasonic clean, spin-rinse dry steps
• Buffing
– after main polish , wafers “polished” using soft pads
– used following metal CMP
– oxide slurries, DI water, or NH4OH used
• changes pH of system to reduce adhesion of metal particles
• removes metal particles embedded in wafers
– can reduce cleaning loads
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Post CMP Cleaning (cont’d)
• Brush cleaning
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brushes made from PVA with 90% porosity
usually double sided scrubbing, roller or disk-type
brushes probably make direct contact with wafer
NH4OH (1-2%) added for particle removal (prevents redeposition), citric
acid (0.5%) added for metal removal, HF etches oxide to remove subsurface
defects
• Megasonic cleaning
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sound waves add energy to particles, thin boundary layers
cleaning chemicals added (TMAH, SC1, etc.)
“acoustic streaming” induces flow over particles
importance uncertain
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Brush Box
Upper Brush Assembly
Chemical Drip Manifold
Lower Brush Assembly
Roller
Water Inlets
Rotating Wafer
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Double Side Scrubbing (DSS)
System Configuration
Wet Sand
Indexer
Dual Brush
Module
Rinse, Spin
Dry Station
(Megasonic)
Edge
Handling
Receive
Station
(OnTrak Systems, Inc.)
User
Interface
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Post CMP Cleaning (cont’d)
• Spin-rinse drying
– following cleaning, wafers rotated at high speed
– water and/or cleaning solution (SC1) sprayed on wafer at
start
– hydrodynamics drain solutions from wafer
– probably no effect on cleaning, but ensures that particles
dislodged from wafer during preceding steps do not resettle
on wafer
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CMP Environmental Problems
• Huge quantities of waste generated
• Polishing
– consumables (slurry, pads, water, chemicals)
– monitor wafers (used for testing purposes)
– killed wafers
– rinse water used during process
• Post-CMP cleaning
– consumables (chemicals, water, brushes, buff pads)
– post-CMP cleaning rinse water
– killed wafers
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Waste Problems
• Slurry
– solids present in waste
– highly basic or acidic solutions cause pH changes in
natural waters
• kills organisms
• enhances sediment dissolution, diminishes precipitation
– oxidizers toxic to wildlife
• Rinse waters
– large volumes tax wastewater treatment systems
– water purification wastes are significant (ion exchange
wastes, membranes, energy)
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Quantities of Wastes
• Typical polisher processes 40 wafers/hr. with 65% overall
equipment efficiency
• Aqueous process wastes
– 190 gallons slurry/day/machine
– 180 gallons DI rinsewater/day/machine
• Solid wastes
– 3-4 monitor wafers/pad for break in (RR drift?)
– 1-2 pads/machine/day (not including buff pads)
• Cleaning wastes
– 190 gallons rinsewater/day/machine
– cleaning chemicals highly variable
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Subtle Concerns
• Energy, materials required to manufacture consumables
• Energy, materials required to manufacture monitor and lost
wafers
• Long and short term environmental impacts
• Effects of process improvements
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Evaluating Environmental
Aspects of Manufacturing
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The Million Dollar Questions
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$1,000,000 Questions
• How does one assess environmental “soundness” of
exisiting processes?
– Waste Audit
• How does one assess environmental consequences of
processes?
– Environmental Impact Assessment (EIA)
• How does one assess and compare environmental impacts
of real and proposed/improved processes?
– Life Cycle Analysis (LCA)
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Waste Audits - Objectives
• Develop understanding of the actual operating processes in
a facility or unit operation
• Identify regions where waste is generated
• Guide to environmental optimization of process
• 6 steps
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Waste Audits (cont’d)
1) List all unit operations in process of interest
• CMP unit operations:
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DI water preparation
slurry mixing
chemical mixing
polish tool
buff tool
wafer transport line
brush cleaning tool
megasonic tank
SRD (spin rinse dryer)
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Waste Audits (cont’d)
2) Construct process flow diagrams
• easy for case of CMP and post-CMP cleaning
3) Determine resource usage
• raw materials/feeds used in each process/unit operation
• analysis of process specifications and actual process data
• many subtle materials (air, water)
• startup wastes
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Waste Audits (cont’d)
4) Determine storage/handling losses
• invoices can be compared to actual operating data
• spillage, spoilage, bad feed wastes identified
5) Quantify levels of waste reuse
• easy for CMP (none)
6) Quantify process outputs
• products, wastes
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Waste Audits (cont’d)
• Results
– awareness of wastes, both obvious and hidden, in process
– ability to optimize process to minimize environmental impact
• Questions: waste audit of CMP/post-CMP train
– Where do wastes come from in CMP/post-CMP cleaning?
– What could have the highest impact for reducing waste?
– Would process performance be affected?
– Which change could reduce the waste with the least impact?
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Environmental Impact
Assessment
• Prioritization of concerns for environmental impacts of
processes and appropriate planning to minimize impacts
• Required by law in U.S. for many new manufacturing
projects
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mandated contents
interpreted and enforced by courts
government approves or disapproves project
public can challenge in court
• 4 stages
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Environmental Impact
Assessment (cont’d)
1) Process screening - determines which aspects of existing
or planned process must be evaluated
• a process step that generates slurry waste may be more
important that one that generates DI water waste
2) Scoping - determines key issues to be considered
• CMP generates basic wastewater
• immediate concern: effect of pH on natural waters or
treatment loop
• long-term concern: effects of neutralization wastes
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Environmental Impact
Assessment (cont’d)
3) Statement Preparation - the impact of each waste is
assessed
• soil, water, air, wildlife, and people considered
• evaluated over appropriate time scales
4) External review - the community evaluates the EIA
• independent review by local community, government,
academia
– ensures that the statement is accurate, objective
• all EIA’s must be reviewed
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Environmental Impact
Assessment (cont’d)
• Mandated contents of EIA:
– state of present environmental condition
– features of project
– effects of project
– ways to minimize effects
– residual impacts of project
• Must be comprehensible to the general public
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Environmental Impact
Assessment (cont’d)
• Criteria for choosing projects that require EIA’s:
– Lists: certain types of projects always require EIAs
– Project thresholds: exceeding threshold values of project cost,
production, or land use can mandate EIA
• Sensitive area criteria - based on ability of environment to
handle project and wastes
• Matrix criteria - all project activities and impacts listed on
a matrix
– activities: site investigation, preparation, construction, operation
and maintenance, future and related activities
– impacts: physical, chemical, ecological, aesthetic, social
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Environmental Impact
Assessment (cont’d)
• Sensitive area criteria - based on ability of environment to
handle project and wastes
• Matrix criteria - all project activities and impacts listed on
a matrix
– activities: site investigation, preparation, construction,
operation and maintenance, future and related activities
– impacts: physical, chemical, ecological, aesthetic,
social
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Life Cycle Analysis
• Evaluation of entire life of a product
– cycle = material acquisition to final product disposal
• Tool to identify and evaluate opportunities to reduce
environmental impacts of products, processes, packaging,
materials, and activities
• Important in ISO 14000, Product Stewardship
• 7 steps
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Life Cycle Analysis (cont’d)
1) Define scope and purpose of process
2) Set system boundaries
• primary systems: activities that directly contribute to
making, using or disposing of a product
• secondary systems: auxiliary processes that contribute to
making or doing something in the primary sequence
• Good use of LCA - to assess environmental impacts of
changes in CMP processing methods
• Question: What is a primary, secondary and ternary
process for CMP?
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Life Cycle Analysis (cont’d)
3) Inventory checklist
• outlines all decision areas to be considered in the
analysis
• Guides data collection and analysis
• Decision areas:
– purpose, system boundaries, geographic scope, types of data
used, data collection or synthesis methods, data quality
measures, presentation of results
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Life Cycle Analysis (cont’d)
4) Peer review
• guarantees validity of study
• internal or external reviewers
• financially supported by EPA
• possible comment areas:
– scope/boundaries methodology, data acquisition/compilation
methodology, validity of assumptions and results, method of
communication of results
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Life Cycle Analysis (cont’d)
5) Gather data
• depending on scope and boundaries, may have to go all
the way to raw materials acquisition for each chemical
used in process
• remember to include data on materials required to
maintain and use your product, and on the final fate of
your product
6) Normalize data
• all data must be evaluated on a common scale (per
wafer, per machine per wafer, per hour, per liter
slurry...)
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Life Cycle Analysis (cont’d)
7) Generate mathematical model of process
• allows effects of changes in operating techniques to be
compared in terms of their environmental impacts
• Question: outline the LCA for an oxide polishing
process
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