MFA1M Lithography 1

8/8/2019 MFA1M Lithography 1

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MFA1M

Lithography – part 1 (of 3)

Overview of process steps

Xuuyan Chen, Nils Hoivik



Photolithography

*

Thin Films

Implant

Diffusion Etch

Test/Sort

Polish

PhotoPatternedwafer

Photolithography is at the Center of theWafer Fabrication Process



• 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

• CD and Tg

• Making a mask

Content



Photolithography - Definitions

Photolithography is used to produce 2 1/2-D images usinglight sensitive photoresist and controlled exposure to light.

Microlithography is the technique used to print ultra-

miniature patterns -- used primarily in the semiconductor

industry.



Resist ToneNegative: 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.



Positive Lithography

Island of resist

silicon substrate

oxide

photoresist

Areas exposed to light become soluble inthe develop chemical.

Resulting pattern after the resist isdeveloped.

photoresistoxide

silicon substrate

Ultraviolet Light

Exposed areaof photoresist

Shadow onphotoresist

Chrome island

on glass mask

Resist Tone - Positive



silicon substrate

oxide

photoresist

Area with resist

Window

Exposed regions become insoluble in thedevelop chemicals.

Resulting pattern after the resist isdeveloped.

Shadow onphotoresist

Exposed areaof photoresist

Chrome island

on glass mask

photoresist

silicon substrate

oxide

Ultraviolet Light

Resist Tone - Negative



Desired photoresist structure to be

printed on wafer

Window in Quartz

Substrate

Island of photoresist

QuartzChrome

Island

Mask pattern required when using

negative photoresist (opposite of

intended structure)

Mask pattern required when using

positive photoresist (same as

intended structure)

Relationship Between Mask and Resist



Photoresist profiles

• cause?

• structure?

Resist Tone

• Reflective Interface

• Photons strike in differentangles.

• Overexpose …..

• Overcut• Vertical

• Undercut(LIFT-OFFapplication)



• Photoresist profiles

– Overcut

– Vertical – Undercut

Dose : High

Developer: Low

Dose : Medium

Developer: Moderate

Dose : Low

Developer: Dominant

Resist Tone• For positive photoresist

(refer: page 18, fig 1.8)



R e f

e r t o p 1 9 ,

f i g u r e

1 . 9 .



10 Steps of Photolithography

10) Develop inspect

?) Post-exposurebake

5) Develop 6) Hard bake

UV Light

Mask

4) Alignmentand Exposure

Resist

2) Spin coat 3) Soft bake1) Vapor prime

HMDS

7) Inspection

8) Etch

9) Resist Strip



1. Surface Preparation (HMDS vapor prime)

• Dehydration bake in enclosed chamber with exhaust• Clean and dry wafer surface

• Hexamethyldisilazane (HMDS - to make wafer surface hydrophobic)

• Temp ~ 200 - 250°C

• 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

photoresistdispenser



• Resist spinning thickness T depends on:

– Spin speed – Solution concentration

– Molecular weight (measured by intrinsicviscosity)

• In the equation for T, K is a calibration constant,C the polymer concentration in grams per 100

ml solution, the intrinsic viscosity, and thenumber 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 thatcan be spun for various molecular weights and

solution concentrations of a given polymer andsolvent system

2.Photoresist Application



Patent pending

Substrate

Chuck with insert

Processbowl

Saturatedatmosphere

Rotating cover

Closed and stable atmosphere between rotating chuck androtating cover generates :

Venturi effect produces a stable under pressure in the closed atmosphere

solvent saturated atmosphere

no turbulences around the Substrate that could cause corner effects

no resist contamination on the rotating cover

self cleaning of drainage holes by venturi effect

allows rotation during closing of the cover

Covered Chuck Technology



3. Soft Bake

• Partial evaporation of photo-resist solvents

• Improves adhesion

• Improves uniformity

• Improves etch resistance

• Improves linewidth control

• Optimizes light absorbancecharacteristics of photoresist



4. Alignment and Exposure

• Transfers the mask imageto the resist-coated wafer

• Activates photo-sensitivecomponents of

photoresist• Quality measures:

– linewidth resolution

– overlay accuracy

– particles & defects

UV Light Source

Mask

Resist



• Alignment errors (many different

types)

• Mask aligner equipment

• Double sided alignment especially

important in micromachines





365.4 I-line

404.7 H-line

435.8 G-line




http://www.ece.gatech.edu/research/labs/vc/theory/photolith.html

Contact: contact for expose, separate for align

Proximity: Less wear on mask, but poorer image than from contact aligner

Projection: Less demand on mask feature dimensions, but higher demand on equipment



Printing resolution : R = bmin

)2

(32bmin

zs

22

3b=Rmin

z


Thickness

Z

s2

3b=Rmin

Contact printing

Proximity printing

Projection printing

NA2

6.0b=Rmin NA

k 1 =R

From Airy disc

In general andPractical

Olympusmicro.com




m



• The defocus tolerance (DOF –

depth of the field)

• k 2 varies around 0.5

• Much bigger issue in

miniaturization science than in ICs

• Will have a great effect on thick

resist layers

4.Alignment and Exposure

2

2

)( NA

k DOF

2

1

2

2

k

Rk DOF

Trouble areas:Device topographyWafer bow (flatness)Resist thicknessFocus error



4. Alignment to previous features

Fiducial marks are used to align the nextmask to the previously patterned

features/layer.

Ensures that the next layer is preciselyaligned and “overlays” the previous layer.

Picture courtesy of Deokki Min

VerniersFeatures on wafer

Features on mask

Mask over wafer

Alignment marksused to register twolayer, wafer ready forexposure



5. Develop

• Soluble areas of photoresist aredissolved by developer chemical

• Visible patterns appear on wafer

– windows

– islands

• Quality measures: – line resolution – uniformity

– particles & defects

to vacuumpump

vacuum chuck

spindle

developerdispenser



6. Hard bake

• Evaporate remainingphotoresist

• 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


• Adding materials (additive)• Two main techniques:

– Sputtering – evaporation

Plasma

CF4



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

O2

Plasma



10. Final Inspection

• Photoresist has been

completely removed• Pattern on wafer matches

mask pattern (positive resist)

• Quality issues:

– defects

– particles – step height




Movies on lithography

• Single layer photolithography (Cornell) – http://www.youtube.com/watch?v=9x3Lh1ZfggM&feature=related

• ASML – Next phase of semiconductor manufacturing

– http://www.youtube.com/watch?v=ShYWUlJ2FZs

http://www.youtube.com/watch?v=9x3Lh1ZfggM&feature=related

http://www.youtube.com/watch?v=ShYWUlJ2FZs

http://www.youtube.com/watch?v=ShYWUlJ2FZs

http://www.youtube.com/watch?v=9x3Lh1ZfggM&feature=related



Next lecture(s)

• Part 2

– Optical theory

• NA

• DOF

•

• Part 3

– Next generation

lithography

• E-beam

• X-rays

• Nano imprint

• 3D lithography

MFA1M Lithography 1

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