CMP's overview

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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

• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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 gt1700 Å/min for SiO2 and gt2500
  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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Prestons Equation

• Simplest CMP model
• Expresses polishing rate in terms of applied
  pressure and relative velocity between polishing
  pad and wafer
• RR KpPS
• 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 ContaminationAdditives
to Alumina Slurry

• Isoelectric point of Alumina 8 - 9
• W 2.0 - 2.5
• SiO2 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 _at_ pH 1.5 -
  2.0)
• Both alumina and tungsten are negatively charged
  (anionic additives such as anionic surfactants
  and polyanions to slurries _at_ 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
• 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.

Support Circuits
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CMP Control Issues Removal Rate Drift

• 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 (contd)

• Brush cleaning
• 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
• 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
Edge Handling Receive Station
Wet Sand Indexer
Dual Brush Module
Rinse, Spin Dry Station (Megasonic)
User Interface
(OnTrak Systems, Inc.)
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Post CMP Cleaning (contd)

• 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 (contd)

• 1) List all unit operations in process of
  interest
• CMP unit operations
• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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
• 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 (contd)

• 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 (contd)

• 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 EIAs must be reviewed

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Environmental Impact Assessment (contd)

• 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 (contd)

• Criteria for choosing projects that require
  EIAs
• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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 (contd)

• 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