Lithography

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• Lithography
• Photolithography (pattern creation) process
• Photoresist chemistry
• Ra, Hyun-Wook

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Photolithography

• Introduction
• Transfer the image of the photomask to the wafer
• Remove specific portion of the layer on wafer
  surface (oxide, dielectrics, metal)
• Most critical operation in the processing
  control of dimension and alignment
• Determine the image size of feature size
  (measured by resolution)
• Align a pattern on previous pattern (alignment or
  registration)
• Overview
• Coating photoresist (PR) on wafer
• Transfer the photomask pattern to PR
• Develop (pattern generation on PR)
• Etch the layer underlying the PR/Remove PR

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Photomask

• A glass plate with pattern
• Pattern is generated by selective etching of Cr
  thin layer (70nm)
• Cr layer prevent transfer of light, but the clear
  region is free to light transfer
• Light-field and dark-field masks
• Reticle unit mask pattern of usually magnified
  of the real pattern size (10X, 5X)
• Photomask repeated pattern in real size

Reticle(10X)
Photomask
Light field
Dark field
Cr
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Photoresist

• Photoresist composition
• 1. Light sensitive polymer
• Polymer a group of large molecules with
  cross-linked chain structure
• Two kinds of polymer materials
• Soluble polymer that change into insoluble
  polymer under light exposure (negative PR)
  polyisoprene type
• Insoluble polymer that change into soluble
  polymer under light exposure (positive PR)
  phenol-formaldehyde

• Most popular light source is ultra-violet (UV)
  light
• There are some reaction under visible light, thus
  yellow light for photomask-room
• 2. Solvent Make the resist a liquid thus
  allowing thin layer coating of the resist on
  wafer by spinning
• For negative aromatic type, xylene
• For positive ethoxyethy acetate, 2-methoxyethyl
• 3. Sensitizer Control the polymer response to
  the light spectrum range
• 4. Additive Various added chemical to achieve
  process results
• dye for absorption of light in specific
  wavelength
• PR dissolution inhibitor

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Photoresist

• PR Performance
• 1. Resolution capability (reproduction of mask
  dimension)
• Thinner PR reproduce smaller opening
• Trade off between resolution and etch barrier
  performance
• Aspect ratio (w/t) measure the capability
• Positive PR has higher AR

• 2. Etch barrier
• Mechanical strength in etchant
• Adhesion image distortion
• Negative PR has better adhesion
• Pinhole (small voids) free thicker PR has less
  pinholes
• Step coverage
• Exposure sensitivity source
• Common PRs respond to UV and deep UV (DUV)
• Some PRs respond to particular wavelength peaks
  (G, H, I lines)
• Some respond to x-ray or e-beam

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Photoresist

• Positive and Negative PR
• Negative PR limit 25 mm feature size. With
  feature size becomes smaller, positive PR should
  come
• N-PR gives smaller opening, while P-PR does
  larger opening than the mask dimension due to
  diffraction at the image boundary

• Also N-PR mask is
• Light-field mask (P-PR mask is dark-field mask) ?
  More glass damage for N-mask (F8.20)
• PR oxygenation in air ? thinned PR layer
• P-PR has lower selectivity in polymerized and
  unpolymerized area during development ? need
  careful control of developer and temperature

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Photoresist

• Physical properties
• Solid content 2040
• Viscosity
• Measure of liquidity
• Related to solid content
• Determine thickness during spinning
• Surface tension
• index of refraction

• Storage and control
• Light heat sensitivity
• Self-polymerization or photosolubilization
• Yellow room light, brown bottle
• Keep in refrigeration
• Shelf life

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Patterning Process
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Surface Prep/Spin Coat

• 1. Surface preparation
• Particle removal
• High pressure nitrogen blow-off
• Wet chemical cleaning
• Rotating brush scrubber
• High pressure water stream
• Dehydration baking
• Most wafers-in are in hydrophilic (hydrated
  water molecules attached) surface condition
• Remove moisture on wafer surface by heating
• 150200 ? heating evaporate water vapor
• Priming
• Primer PR adhesion promoter by chemically
  tying-up molecular water on the wafer surface
• HMDS (hexamethyldislazane)

• 2. Spin-coating
• Thin(0.51.5 mm), uniform (0.01 mm, 1),
  defect-free PR film coating on the wafer surface
• Reduce edge-bead (PR pile-up at the edge)

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Soft Bake

• 3. Soft bake
• Heating for evaporation of solvent in the PR film
• Solvent absorbs exposure light
• Better adhesion
• Under-baking causes incomplete image formation
  and film lifting
• Over-baking causes polymerization before
  radiation
• Control by temperature and time
• N-PR in nitrogen, but P-PR in air allowed

• Methods
• Convection
• Hot nitrogen flowing oven
• Batch process
• Crust traps solvent on the film
• Conduction
• Hot plate
• Conduction from the back-side, thus no crust
  formation
• Low productivity ?? moving-belt hot plate
• Radiation
• IR or microwave radiation
• Productivity by moving-belt
• No crust by bottom-up heating
• Very short time for microwave due to high energy

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Alignment Exposure

• 4. Alignment Exposure
• Positioning (alignment) of a image on the wafer
  surface
• Encoding of the image in the PR layer by
  radiation
• Alignment selection criteria
• Resolution capability/limit
• Alignment accuracy
• Contamination level
• Reliability
• Productivity
• Cost
• Exposure source and alignment system

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Alignment Exposure

• Exposure sources
• How image quality improved?
• 1) Shorter and single wavelength reduce
  diffraction
• 2) Collimated light by using mirrors and lenses
• High-pressure mercury lamp
• 365 (i), 405 (h), and 436 (g) nm peaks
• Using a single peak, improve resolution and
  faster exposure from the high-energy portion

• Mid- (313 nm) or deep UV (254 nm)
• Filter with UV source
• Eximer gas laser XeF (351), XeCl(308),
  KrF(248), ArF(193 nm)
• Focussed ion beam low scattering and high
  energy
• Numerical aperture (NA)
• Issue in projection type (mask-wafer separated)
• Loss of resolution during projection
• NA of lens is the ability to gather light
• with sminimum feature size
• lwavelength
• NA increase, however, reduce the depth of focus
  and field of view (trade-off)

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Alignment Exposure

• Alignment
• Use alignment marks
• Alignment errors (misalignment)
• Linear
• Rotational
• Run-out/in (stepper)
• Aligners
• Types
• 1) Optical - Contact, Proximity, Projection,
  Stepper
• 2) Non-optical - X-ray, E-beam

• Contact aligner
• Full wafer size photomask
• x, y, z, rotation movement
• Split-field objective microscope
• Mask-to-wafer contact ? particle contamination,
  damage in PR and mask
• High defect level, mask cost, resolution,
  alignment limitation

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Alignment Exposure

• Proximity aligner
• Soft-contact ? trade-off between resolution and
  defect density
• Scanning projection aligner
• Projecting mask image on wafer
• Need excellent optical system
• Stepper
• Stepping image using reticle
• Better overlay and alignment by individual
• 5x or 10x reticle use
• Resolution improvement
• g- or i-line
• Automatic alignment system

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Alignment Exposure

• X-ray aligner
• For higher resolution (l450 Å)
• Projection using 1x mask
• Small reflection and scattering, lower level of
  defects (x-ray pass through dusts)
• X-ray mask, resist development required

• E-beam aligner
• In use for reticle and mask fabrication
• No mask is necessary direct writing from CAD
  station
• Raster or vector scanning

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Alignment Exposure

• Aligner system comparison

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Development

• 5. Development
• Reticle pattern transfer to PR
• Chemical resist dissolution (develop) of
  unpolymerized region
• Rinse stop developing and remove partially
  polymerized region
• Negative PR
• Exposed region polymerize
• Developer xylene
• Rinser n-butylacetate
• Positive PR
• Exposed region unpolymerize
• Developer alkaline-water (NaOH, KOH), nonionic
  soln (TMAH)
• Rinser water
• Dissolving rate of exposed unexposed 41,
  thus PR thinning occurs
• More sensitive than negative PR

Negative
Positive
Incomplete develop
Under- develop
Over- develop
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Development

• Develop methods
• Immersion
• Chemical penetration into small opening
• Partially dissolved PR piece cling to wafer
  surface
• Contamination in tank
• Chemicals dilution
• High cost if frequent change of the soln
• Spray N-PR
• Puddle spray P-PR

• Develop parameters soft-bake temp, exposure,
  developer conc, time, temp, method

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Hard Bake/Develop Inspect

• 6. Hard bake
• Similar to Soft-bake process
• Harden PR by solvent evaporation
• Achieve good adhesion (by dehydration
  polymerization)
• 130200 ? / 30 min in convection oven
• Too high temp causes PR flow resulting in
  dimension change
• PR may reabsorb water, thus store in nitrogen,
  rebake, or quick processing required

• 7. Develop inspection
• Check photomasking process performance
• Pattern dimension
• Misalignment, distortion, etc
• Surface problems (contamination, scratch, )
• Select rework wafers

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Etch

• 8. Etch
• Image transfer to wafer surface film
• Wet Dry etch
• 1) Wet etch
• Immersion in chemicals or spray
• For feature size gt 3 mm
• Uniformity increase by heating, agitation
• Usually isotropic etching occurs
• Problems
• Incomplete etch
• Overetch excess undercutting
• Resist lifting
• Selectivity
• Etch rate ratio of the layer material to
  underlying material
• High selectivity etchants are required
• Si, SiO2, Si3N4, Al, etc
• Si HNO3HFwater

• SiO2 HF based
• Buffered oxide etch (BOE) HF NH4F

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Etch

• Aluminum
• H3PO4 based soln
• H2 bubble block the etch action and causes short
• Agitation helps
• Deposited passivation oxide
• Different etch rate
• Need caution not to attack underlying aluminum pad

• Si nitride
• Hot H3PO4 (180 ?)
• Need blocking oxide (PR does not resist at 180 ?)

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Etch

• 2) Dry etch
• Limit of wet etch
• Pattern size 3 mm
• Isotropic sloped wall
• Undercutting
• Hazardous, toxic chemical handle
• Dry etch uses gases
• Plasma, reactive ion etch (RIE), ion milling
• Plasma etch
• System chamber, vacuum system, gas supply, RF
  power supply
• Plasma neutral gas positive ions electrons
• CF4 for SiO2 etch
• Directional etching ions result in vertical
  side-wall (anisotropic)

• For smaller feature size etch (lt0.35 mm), ECR
  (electron cyclotron resonance), ICP(inductively
  coupled plasma)
• Etch rates varying with ion density, pressure
• High density low pressure enhance the rate
• 0.450 mtorr
• Rate 60200 nm/min

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Etch

• Ion beam etching
• High energy Ar-ion physically blast the wafer
  surface
• Sputter etching, ion milling
• Good definition for small opening by directional
  beam
• Poor selectivity
• RIE
• Plasma etch ion beam etch
• High selectivity for SiO2/Si

• High energy particles in plasma cause damage in
  semiconductor
• Surface leakage
• Electrical parameters change
• Films degradation
• Resist hardening
• Selectivity
• Layer material to underlying material
• Resist to Layer material, particularly for high
  aspect ratio patterns (etch slows down in deep of
  narrow hole)
• Miroloading effect local etch rate variation
  phenomenon depending on the pattern size to be
  etched (contact hole)
• Contamination, residue, corrosion
• Side chemical reactions take place
• Contaminate by forming metal halide (AlF3, WF5,
  WF6) and oxides (TiO3, TiO2, WO2)
• Post-etch residue (Cl containing) corrodes
  underlying metal

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Etch
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PR Strip

• 9. PR strip
• Wet chemical removing strip, rinse, and dry
• Cost effective
• Metallic ion removal
• No plsama damage
• Distinguish metallic and non-metallic surface
• Plasma O2 strip
• CxHy (resist) O2 ? CO CO2 H2O

• 10. Final inspection
• Etch pattern quality inspection
• Similar to develop inspection process

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Mask Making Process

• Identical with wafer patterning process
• 100 nm Cr-coated glass