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Thin Films. Polish. Patterned wafer. Etch. Photo. Diffusion. Test/Sort. Implant. *. ME 598 – Lecture 2 - Photolithography. Ultraviolet Light. Chrome island on glass mask. Exposed area of photoresist. Window. photoresist. Shadow on photoresist. photoresist. oxide. oxide.

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slide1

Thin Films

Polish

Patterned wafer

Etch

Photo

Diffusion

Test/Sort

Implant

*

ME 598 – Lecture 2 - Photolithography

negative tone

Ultraviolet Light

Chrome island on glass mask

Exposed area of photoresist

Window

photoresist

Shadow on photoresist

photoresist

oxide

oxide

silicon substrate

silicon substrate

Negative Tone

Areas exposed to light become polymerized and resist the develop chemical.

Island

Resulting pattern after the resist is developed.

positive tone

Ultraviolet Light

Chrome island on glass mask

Shadow on photoresist

photoresist

Exposed area of photoresist

photoresist

oxide

silicon substrate

Positive Tone

Areas exposed to light become photosoluble.

Island

Window

oxide

silicon substrate

Resulting pattern after the resist is developed.

photoresist side profiles
Photoresist Side Profiles

Overcut (aka lift-off):

Used for material deposition after photolithography step

    • Vertical (aka normal):
  • Used for material removed after photolithography step
    • Undercut:
  • Typical result when trying to achieve highest resolution
development profiles
Development Profiles

PR

PR

Substrate

Substrate

Normal Development

Incomplete Development

PR

PR

Substrate

Substrate

Under Development

Over Development

resolution
Resolution
  • Critical Dimension (CD) refers to a specific “minimal” feature size and is a measure of the resolution of the lithographic process
  • Theoretical limitations can be calculated based on experimental measures (wavelength, gap size, PR thickness…)
az 1518 exposure dose
AZ 1518 Exposure Dose

1 μm trench 2 μm mask lines

270 mJ/cm2

250 mJ/cm2

310 mJ/cm2

290 mJ/cm2

shadow printing resolution

UV light 

Mask

3

Resist thickness z

s

b

Z

2

2

Wafer

(S + )

R = Bmin =

Shadow Printing Resolution

Shadow printing (Contact and Proximity)

  • Factors that affect resolution
    • Separation between the wafer and the mask
      • Nonuniformities in wafer/mask flatness
      • Debris on mask or PR layer
      • Size of the PR edgebead
    • Photoresist thickness
    • Exposure wavelength
  • = wavelength

s = separation

z = resist thickness

What happens when s goes to 0 (contact printing)?

What happens when s>>z (proximity printing)?

What is the difference between AZ 1518 and AZ 4620?

standing wave effect on photoresist
Standing Wave Effect on Photoresist

l/nPR

Photoresist

Substrate

Overexposure

Underexposure

phase shift masks
Phase Shift Masks
  • Extends resolution capability of current optical lithography
  • Takes advantage of the wave nature of light
  • PSM changes the phase of light by 180° in adjacent patterns leading to destructive interference rather than constructive interference
  • Improves MTF of aerial image on wafer. Making k1 smaller.
optical proximity correction
Optical Proximity Correction
  • Optical Proximity Correction (OPC) can be used to compensate somewhat for diffraction effects.
  • Sharp features are lost because higher spatial frequencies are lost due to diffraction. These effects can be calculated and can be compensated for. This improves the resolution by decreasing k1.
immersion photolithography
Immersion Photolithography
  • NA is determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens. The physical limit for an air based system is clear, but what if a medium with a higher index of refraction is substituted for air? Microscopy has for years used oil between the lens and the sample being viewed for resolution enhancement and it is somewhat surprising that the semiconductor industry has taken this long to seriously consider the merits of replacing air with an alternative.
immersion photolithography1
Immersion Photolithography
  • The medium between the lens and the wafer being exposed needs to have an index of refraction >1, have low optical absorption at 193nm, be compatible with photoresist and the lens material, be uniform and non-contaminating. Surprisingly, ultrapure water may meet all of these requirements. Water has an index of refraction n = 1.47, absorption of <5% at working distances of up to 6mm, is compatible with photoresist and lens and in it’s ultrapure form is non-contaminating.
use water for immersion lithograph
Use Water for Immersion Lithograph?
  • Index of refraction of air? 1.000
  • Index of refraction of glass? 1.517
  • Index of refraction of water? 1.332

website simulator:

http://interactagram.com/physics/optics/refraction/

immersion photolithography2
Immersion Photolithography

what does immersion lithography do to DOF? http://spie.org/x8368.xml?ArticleID=x8368

anyone decided
Anyone Decided?

Topic can NOT be your thesis work.

  • MEMS batteries (micro-batteries)
  • MEMS gas turbine
  • MEMS microphone or speaker
  • MEMS super-capacitor
  • Micro-robot or MEMS walker
  • Biosensing methods for microfluidics
  • Microfluidic cell sorting methods
why will we be using an sem
Why will we be using an SEM?
  • Because the wavelength of the visible light, which is from 3900 Å (violet) to 7500 Å (red), is larger than many feature aspects. We need smaller wave length microscope such as electron microscope to investigate the small feature size pattern.
quotes
Quotes
  • Listen, opportunity sometimes knocks very softly.
  • Strive for excellence, not perfection.
  • Be tough minded, but tenderhearted.
  • Never underestimate your power to change yourself.
  • The race is not always to the swift, but to those who keep on running
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