Photolithography

1
Photolithography

• ECE/ChE 4752 Microelectronics Processing
  Laboratory

Gary S. May January 22, 2004
2
Outline

• Introduction
• Clean Rooms
• Exposure
• Masks
• Photoresist
• Pattern Transfer
• E-Beam Lithography

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Definition

• Photo-imaging method by which geometric patterns
  are transferred from a mask to the substrate
  (wafer).
• Uses photosensitive polymer (called
  photoresist).
• Features transferred to substrate surface by
  shining light through glass plates (called
  masks).

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BasicProcess Flow
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Process Sequence

• 1) Clean wafer surface
• bake (get rid of H2O)
• RCA clean
• apply adhesion promoter (HMDS
  hexi-methyl-di-silizane)
• 2) Deposit photoresist (usually by spin-coating)
• 3) Soft bake (or pre-bake) - removes solvents
  from liquid photoresist
• 4) Exposure (pattern transfer)
• 5) Development - remove soluble photoresist
• 6) Post bake (or hard bake) - desensitizes
  remaining photoresist to light
• 7) Resist removal (stripping)

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Outline

• Introduction
• Clean Rooms
• Exposure
• Masks
• Photoresist
• Pattern Transfer
• E-Beam Lithography

7
The Need

• Electronics fabrication requires a clean
  processing environment for lithography.
• Goal minimize dust particles that can settle on
  substrates or masks and cause DEFECTS.
• Dust on a mask looks like an opaque feature will
  get transferred to underlying layers can lead to
  short circuits or open circuits.

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Graphic Illustration

• Particle 1 may result in formation of a pinhole
  in underlying layer.
• Particle 2 may cause a constriction of current
  flow in a metal runner.
• Particle 3 can lead to a short between the two
  conducting regions.

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Class

• Numerical designation taken from maximum
  allowable number of particles 0.5 mm and larger
  per ft3 (English system).
• For IC fabrication, a class 100 clean room is
  required (about four orders of magnitude lower
  than ordinary room air).
• For photolithography, class 10 or better is
  required.

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Particle Size Distribution Curve
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Sample Problem

• A 300 x 300 mm square substrate is exposed for 1
  minute under laminar flow at 30 m/min. How many
  dust particles will land on this substrate in a
  Class 1000 clean room?

• SOLUTION
• Class 1000 gt 35,000 particles/m3 (from graph)
• Air flow volume over wafer/min 30 m/min (0.3m x
  0.3m) 2.7 m3
• of particles 35,000 x 2.7 94,500!!!
• If each of these causes a defect, we are in
  serious trouble!

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Outline

• Introduction
• Clean Rooms
• Exposure
• Masks
• Photoresist
• Pattern Transfer
• E-Beam Lithography

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Performance Metrics

• Resolution minimum feature dimension that can be
  transferred with high fidelity to a resist film.
• Registration how accurately patterns on
  successive masks can be aligned (or overlaid)
  with respect to previously defined patterns.
• Throughput number of wafers that can be
  exposed/unit time for a given mask level.

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Shadow Printing

• Mask and wafer in direct contact (contact
  printing) or
• Mask and wafer in close proximity (proximity
  printing).

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Contact Printing

• Contact between the resist and mask provides a
  resolution of 1 mm.
• Drawback dust particles on the wafer can be
  imbedded into mask where mask makes contact with
  the wafer.
• Imbedded particles cause permanent damage to mask
  and result in defects with each succeeding
  exposure.
• We use this in lab.

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Proximity Printing

• Small gap (10 50 mm) between the wafer and the
  mask.
• Minimizes mask damage, but
• Gap results in optical diffraction at feature
  edges that degrades resolution to 25 mm.
• Minimum linewidth (or critical dimension)

when l wavelength and g gap
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Projection Printing

• Wafer many centimeters from mask
• To increase resolution, only small portion of the
  mask is exposed at a time.
• Small image area is scanned or stepped over the
  wafer to cover the entire wafer surface.
• After exposure of one site, wafer is moved to
  next site and the process is repeated.
• Called step-and-repeat projection, with a
  demagnification ratio M1

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Step and Repeat Projection

• After exposuring one site, wafer moved to next
  site and the process repeats.
• Demagnification ratio M1

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Resolution

• Given by
• where k1 is a process dependent factor and
• NA numerical aperture, which is

where is the index of refraction
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Depth of Focus

• Expressed as
• where k2 is another process-dependent factor

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Outline

• Introduction
• Clean Rooms
• Exposure
• Masks
• Photoresist
• Pattern Transfer
• E-Beam Lithography

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Making Masks

• CAD system used to describe the circuit patterns
  electrically.
• Digital data produced by CAD system drives a
  pattern generator that transfers the patterns
  directly to electron-sensitized mask.
• Mask consists of a fused silica substrate covered
  with chrominum.
• Circuit pattern is first transferred to the
  electron-sensitized layer (electron resist),
  which is transferred into the underlying
  chrominum layer for the finished mask.

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Use of Masks

• Patterns on a mask represent one level of an IC
  design.
• Composite layout is broken into mask levels that
  correspond to the manufacturing process sequence.
  
• 15 20 different mask levels are typically
  required for a complete IC process.

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Mask Composition

• Fused silica plate 15 ? 15 cm, 0.6 cm thick
• Accommodates lens field sizes for 41 or 51
  optical exposure tools

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Outline

• Introduction
• Clean Rooms
• Exposure
• Masks
• Photoresist
• Pattern Transfer
• E-Beam Lithography

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Definition

• Photosensitive polymer compound that either gets
  more or less soluble when exposed to light.
• Photolithography labs have yellow light because
  photoresist is sensitive to wavelenghts gt 500 nm.

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Types

• Positive gets more soluble after exposure
• Negative gets less soluble after exposure.

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Development
More exposure energy vs. Higher resolution
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Contrast Ratio

• where ET sensitivity or threshold energy
  (where resist becomes completely soluble)
• E1 energy to reach 100 resist thickness (50
  for negative resist)

• Larger g gt higher solubility of resist and
  sharper images
• ET and E1 interchanged for negative resists

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Outline

• Introduction
• Clean Rooms
• Exposure
• Masks
• Photoresist
• Pattern Transfer
• E-Beam Lithography

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Steps

1. Apply adhesion promoter (HMDS)
2. Spin coat photoresist at 1000 10,000 rpm
3. Soft bake (90 120C for 60 120 sec) to
   remove solvent
4. Alignment
5. Exposure
6. Development
7. Post bake (100 180C) to increase adhesion
8. Etch exposed regions
9. Strip resist

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Illustration
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Alignment

• Mask for each layer must be aligned to previous
  layer patterns
• For a minimum feature size 1 mm gt alignment
  tolerance should be /- 0.2 mm
• To align, wafer is held on vacuum chuck and moved
  around using an xyz stage
• Alignment marks special patterns on mask used to
  facilitate accurate alignment.

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Outline

• Introduction
• Clean Rooms
• Exposure
• Masks
• Photoresist
• Pattern Transfer
• E-Beam Lithography

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Limitations of Optical Lithography

• Resolution becoming a challenge for
  deep-submicron IC process requirements
• Complexity of mask production and mask inspection
  
• High cost of masks

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Electron Beam Lithography

• Involves direct exposure of the resist by a
  focused electron beam without a mask
• Currently used to primarily produce photomasks
• Resolution as low as 10 25 nm

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Schematic

• Electron gun generates beam of electrons
• Condenser lenses focus the e-beam
• Beam-blanking plates turn beam on and off

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Advantages

• Generation of submicron resist geometries
• Highly automated and precisely controlled
  operation
• Greater depth of focus than that available from
  optical lithography
• Direct patterning on wafer without using a mask

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Scanning

• Raster beam scans sequentially over every
  possible location on the mask and turned off
  where no exposure is required
• Vector beam directed only to requested features,
  jumps from feature to feature

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Disadvantages

• Low throughput
• Expensive resists
• Proximity effect backscattering of electrons
  irradiates adjacent regions and limits minimum
  spacing between features