lithography ia
TRANSCRIPT
Lithography
Lithography definitions Resist tone Introduction to the lithography
process Surface Preparation Photoresist Application Soft Bake Align & Expose Develop Hard Bake Inspection Etch Layer or Add Layer Resist Strip Final Inspection Clean- Room, Wafer Cleaning
Content CD and Tg
Making a Mask Moore’s ‘Law’
Photolithography -- Definitions
Photolithography is used to produce 2 1/2-D images using light sensitive photoresist and controlled exposure to light.
Microlithography is the technique used to print ultra-miniature patterns -- used primarily in the semiconductor industry.
Photolithography is at the Center of the Wafer Fabrication Process
*
Thin Films
Implant
Diffusion Etch
Test/Sort
Polish
PhotoPatterned
wafer
Photolithography -- Definitions
Resist Tone
Negative: Prints a pattern that is opposite of the pattern that is on the mask.
Positive: Prints a pattern that is the same as the pattern on the mask.
Negative Lithography
Island
silicon substrate
oxide
photoresist
Window
Areas exposed to light become polymerized and resist the develop chemical.
Resulting pattern after the resist is developed.
photoresistoxide
silicon substrate
Ultraviolet Light
Exposed area of photoresist
Shadow on photoresist
Chrome island on glass mask
Resist Tone
silicon substrate
oxide
photoresist
Positive Lithography
Island
Window
Areas exposed to light become photosoluble.
Resulting pattern after the resist is developed.
Shadow on photoresist
Exposed area of photoresist
Chrome island on glass mask
photoresist
silicon substrate
oxide
Ultraviolet Light
Resist Tone
Resist Tone
Photoresist profiles
– Overcut (LIFT-OFF)
– Vertical
– Undercut
Resist Tone
Photoresist profiles
– Overcut (LIFT-OFF)
– Vertical
– UndercutDose : High
Developer: Low
Dose : Medium
Developer: Moderate
Dose : Low
Developer: Dominant
Resist Tone
Ten Basic Steps of
Photolithography
1. Surface Preparation
2. Photoresist Application
3. Soft Bake
4. Align & Expose*
5. Develop
6. Hard Bake
7. Inspection
8. Etch
9. Resist Strip
10. Final Inspection* Some processes may include a Post-exposure Bake
Introduction to the Lithography Process
1. Surface Preparation (HMDS vapor prime)
Dehydration bake in enclosed chamber with exhaust
Clean and dry wafer surface (hydrophobic)
Hexamethyldisilazane (HMDS)
Temp ~ 200 - 250C Time ~ 60 sec.
HMDS
1. Surface Preparation (HMDS vapor prime)
1. Surface Preparation (HMDS vapor prime)
2. Photoresist Application
Wafer held onto vacuum chuck
Dispense ~5ml of photoresist
Slow spin ~ 500 rpm Ramp up to ~ 3000 - 5000
rpm Quality measures:
– time– speed– thickness– uniformity– particles & defects
vacuum chuck
spindleto vacuum
pump
photoresist dispenser
Resist spinning thickness T depends on:– Spin speed– Solution concentration– Molecular weight (measured by intrinsic
viscosity) In the equation for T, K is a calibration
constant, C the polymer concentration in grams per 100 ml solution, the intrinsic viscosity, and the number of rotations per minute (rpm)
Once the various exponential factors (, and ) have been determined the equation can be used to predict the thickness of the film that can be spun for various molecular weights and solution concentrations of a given polymer and solvent system
2. Photoresist Application
3. Soft Bake
Partial evaporation of photo-resist solvents
Improves adhesion Improves uniformity Improves etch resistance Improves linewidth control Optimizes light absorbance
characteristics of photoresist
4. Alignment and Exposure
Transfers the mask image to the resist-coated wafer
Activates photo-sensitive components of photoresist
Quality measures: – linewidth resolution– overlay accuracy– particles & defects
UV Light Source
Mask
ResistResistResistResist
Alignment errors (many different types)
Mask aligner equipment Double sided alignment
especially important in micromachines
4. Alignment and Exposure
4. Alignment and Exposure
4. Alignment and Exposure
Contact printing Proximity printing Self-aligned (see next) Projection printing : R = 2bmin =
0.6/NA
R = 2bmin 3 (s z
2)~
R = 2bmin 3 z
2)~
4. Alignment and Exposure
4. Alignment and Exposure
5. Develop
Soluble areas of photoresist are dissolved by developer chemical
Visible patterns appear on wafer– windows– islands
Quality measures:– line resolution– uniformity– particles & defects
to vacuum pump
vacuum chuck
spindle
developerdispenser
6. Hard Bake
Evaporate remaining photoresist
Improve adhesion
Higher temperature than soft bake
7. Development Inspection
Optical or SEM metrology Quality issues:
– particles
– defects
– critical dimensions
– linewidth resolution
– overlay accuracy
8. Plasma Etch-Or Add Layer
Selective removal of upper layer of wafer through windows in photoresist: subtractive
Two basic methods:– wet acid etch– dry plasma etch
Quality measures:– defects and particles– step height– selectivity– critical dimensions
Adding materials (additive) Two main techniques:
– Sputtering– evaporation
PlasmaPlasma
CF4CF4
8. Plasma Etch-Or Add Layer
9. Photoresist Removal (strip)
No need for photoresist following etch process
Two common methods:– wet acid strip– dry plasma strip
Followed by wet clean to remove remaining resist and strip byproducts
O2O2
PlasmaPlasma
10. Final Inspection
Photoresist has been completely removed
Pattern on wafer matches mask pattern (positive resist)
Quality issues:– defects– particles– step height– critical dimensions
Clean-rooms, Wafer Cleaning
Yellow light and low particle size/density curves
Cleaning steps
– RCA1-peroxides and NH3-removes organics
– RCA2-peroxide and HCl-removes metals
Dry vs. wet cleaning Supercritical cleaning-no liquid phase
Clean-rooms, Wafer Cleaning
Clean-rooms, Wafer Cleaning
Clean-rooms, Wafer Cleaning
Clean-rooms, Wafer cleaning
Yield is the reason for the clean-rooms-the smaller the features the more important the cleanroom
In the future people will work outside the cleanroom and only wafers will be inside the clean environment
At universities, modularity (many different materials and processes) is more important than yield
CD and Tg
CD (e.g. 90 nm) i.e. critical dimension (the smallest feature made in a certain process)
Glass transition temperature, above Tg the resist picks up dirt quite readily and the profile might get degraded
Making a Mask
Software Mask
Moore’s ‘Law’ Observation and self
fulfilling prophecy --not a physical law
Is it running out of steam?