Lecture 03 Photolithography

1

Photolithography

Himanshu J. SantFundamentals of Microfabrication

Source: R.B. Darling, M. Madou and F. Solzbacher

Fundamentals of Microfabrication

Learning Objectives• To understand important elements related to

photolithography• To review details on photoresists and resist tones • To review lithography steps• To highlight important features and limitations• To detail photomask patterning• To review post-processing and other lithography

techniques

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Photolithography• Photo-litho-graphy: latin: light-stone-writing• Photolithography is an optical means for transferring

patterns onto a substrate• It is essentially the same process that is used in

lithographic printing


Goals of Photolithography• Faithful transfer of CAD

drawing for mask pattern to the silicon wafer– From wafer to wafer– From device to device

• High resolution or minimum possible feature size– Critical dimension– Linewidth

• High throughput

3


Lithography Information Flow1. Design (CAD) file

– Mask writing tool and process

2. Mask (photomask)– Optical lithography tool

3. Exposure systems4. Photosensitive film (photoresist) application5. Alignment of mask and wafer (substrate)6. Exposure of the photoresist7. Development of patterns8. Post processing

• Etching (after postbake)• Lift-off

9. Physical structure on wafer

OpticsPhotochemistry

Mechanical


Photolithography -- Photomask

•Controlled light exposure using a mask•Wavelength of light?•Mask: Chrome on glass

•Imagable resist layer

4


Photolithography--Overview

*

Thin Films

Implant

Diffusion Etch

Test/Sort

Polish

PhotoPatterned

wafer

Photolithography is at the Center of the Wafer Fabrication Process

Courtesy: Mark Madou, UC Irvine


How does it work?• Patterns are first transferred to an

imagable photoresist (resist)layer.• Photoresist is a liquid film that can be

spread out onto a substrate, exposed with a desired pattern, and developed into a selectively placed layer for subsequent processing.

• Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image.

5


Photolithography-- a closer look


Photoresist 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.


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Negative Photoresist

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



Positive Photoresist

Positive Lithography

Areas exposed to light become photosoluble.

silicon substrate

oxide

photoresist

Island

Window

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


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Effect of Resist Tone


Photoresist Profiles• Vertical

– 75-950

• Overcut– 45-750

• Undercut– 95-1100

– Negative resist• Permanent

– Positive resist (LIFT-OFF)

Dose : High

Developer: Low

Dose : Medium

Developer: Moderate

Dose : Low

Developer: Dominant

Courtesy: Mark Madou, UC IrvineTable 1.6, Text for more info.

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Basic Photolithography Steps• Surface preparation• Coating (Spin casting)• Pre-Bake (Soft bake)• Alignment• Exposure• Development• Post-Bake (Hard bake)• Processing using the photoresist as a masking film• Stripping• Post processing cleaning (Ashing)


Surface Preparation: Cleaning • Typical contaminants that must be removed prior to

photoresist coating– dust from scribing or cleaving (minimized by laser scribing)– atmospheric dust (minimized by good clean room practice)– abrasive particles (from lapping or CMP)– lint from wipers (minimized by using lint-free wipers)– photoresist residue from previous photolithography

(minimized by performing oxygen plasma ashing)– bacteria (minimized by good DI water system)– films from other sources:

• solvent residue• H2O residue• photoresist or developer residue• oil• silicone

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How do we clean?• Standard degrease

– 2-5 min. soak in acetone with ultrasonic agitation– 2-5 min. soak in methanol with ultrasonic agitation– 2-5 min. soak in DI H2O with ultrasonic agitation– 30 sec. rinse under free flowing DI H2O– spin rinse dry for wafers; N2 blow off dry for tools and chucks

• For particularly troublesome grease, oil, or wax stains– Start with 2-5 min. soak in 1,1,1-trichloroethane (TCA) or

trichloroethylene (TCE) with ultrasonic agitation prior to acetone• Hazards

– TCE is carcinogenic; 1,1,1-TCA is less so– acetone is flammable– methanol is toxic by skin adsorption


Standard Cleaning Procedures• RCA clean: use for new silicon wafers out of the box

1. APW: NH4OH (1) + H2O2 (3) + H2O (15) @ 70°C for 15 min.– DI H2O rinse for 5 min.– Organic contaminants– Oxide formation/metallic contamination

2. 10:1 BOE for 1 min.– DI H2O rinse for 5 min.– Oxide removal

3. HPW: HCl (1) + H2O2 (3) + H2O (15) @ 70°C for 15 min.– DI H2O rinse for 5 min.– Metallic contamination removal

4. Spin & rinse dry

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Surface Preparation: Wafer Priming • Adhesion promoters are used to assist resist coating.• Resist adhesion factors

– moisture content on surface– wetting characteristics of resist– type of primer– delay in exposure and prebake– resist chemistry– surface smoothness– stress from coating process– surface contamination

• Ideally want no H2O on wafer surface– Wafers are given a “singe” step prior to priming and coating

• 15 minutes in 80-90°C convection oven


• Used for silicon:– primers form bonds with surface and produce a polar

(electrostatic) surface– most are based upon siloxane linkages (Si-O-Si)

• 1,1,1,3,3,3-hexamethyldisilazane (HMDS), (CH3)3SiNHSi(CH3)3• trichlorophenylsilane (TCPS), C6H5SiCl3• bistrimethylsilylacetamide (BSA), (CH3)3SiNCH3COSi(CH3)3

• Used for gallium arsenide:– GaAs already has a polar surface

• monazoline C• trichlorobenzene• xylene

Surface Preparation: Wafer Primers

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• Dehydration bake in enclosed chamber with exhaust

• Clean and dry wafer surface (hydrophobic)

• Hexamethyldisilazane (HMDS)– Temp ~ 200 - 250C– Time ~ 60 sec.

HMDS

Surface Preparation: HMDS Vapor Prime




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Photoresist Coating (Spin Casting)• Wafer held onto vacuum

chuck• Dispense ~5ml of photoresist• Slow spin ~ 500 rpm• Ramp up to ~ 3000 - 5000

rpm• Quality measures:

– thickness– time– speed– uniformity– particles & defects

• What is the ideal thickness?– 1-2 µm thickness for

commercial Si processes.

vacuum chuck

spindleto vacuum

pump

photoresist dispenser


Photoresist Thickness• Resist spinning thickness T depends on:

– Spin speed– Solution concentration– Molecular weight (measured by

intrinsic viscosity• K is a calibration constant, • C the polymer concentration in grams per

100 ml solution,• h the intrinsic viscosity, • w the number of rotations per minute (rpm)• Once the various exponential factors (a,b

and g) 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

• Etching (post-lithography) depends on resist quality and thickness

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Typical Spin Coating Program


Spin Stages

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Spinning Artifacts• Striations

– ~ 30 nm variations in resist thickness due to nonuniform drying of solvent during spin coating

– ~ 80-100 mm periodicity, radially out from center of wafer• Edge Bead

– residual ridge in resist at edge of wafer– can be up to 20-30 times the nominal thickness of the resist– radius on wafer edge greatly reduces the edge bead height– non-circular wafers greatly increase the edge bead height– edge bead removers are solvents that are spun on after resist

coating and which partially dissolve away the edge bead• Streaks

– radial patterns caused by hard particles whose diameter are greater than the resist thickness


Dry Resist?

Good or bad?

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Soft bake

• Partial evaporation of photo-resist solvents– Improves adhesion– Improves uniformity– Improves etch resistance– Improves linewidth control– Optimizes light absorbance

characteristics of photoresist– Densifies photoresist

• Hotplate preferred– Doesn’t trap solvent like

convection oven baking

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Convection or Conduction?• Convection ovens

– Solvent at surface of resist is evaporated first, which can cause resist to develop impermeable skin, trapping the remaining solvent inside

– Heating must go slow to avoid solvent burst effects• Conduction (hot plate)

– Need an extremely smooth surface for good thermal contact and heating uniformity

– Temperature rise starts at bottom of wafer and works upward, more thoroughly evaporating the coating solvent

– Generally much faster and more suitable for automation


Soft bake Parameters• A narrow time-temperature

window is needed to achieve proper linewidth control.

• The thickness of the resist is usually decreased by 25 % during prebake for both positive and negative resists.

• Less prebake increases the development rate

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Alignment/Exposure/Development

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Exposure Systems

Alignment Advantages DisadvantagesContact Sub µm resolution •Defects in photoresist

•Contamination and damage to mask

Proximity No defects caused in resist or mask

Light defraction reduces resolution 10-30 µm

Exposure

Mask aligner Projection exposure

Contact exposure Proximity exposure


Mask Aligner—Contact and Proximity

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Alignment & Exposure Hardware• For simple contact, proximity, and projection systems, the

mask is the same size and scale as the printed wafer pattern. i.e. the reproduction ratio is 1:1.

• Projection systems give the ability to change the reproduction ratio. Going to 10:1 reduction allows larger size patterns on the mask, which is more robust to mask defects.– Mask size can get unwieldy for large wafers.– Most wafers contain an array of the same pattern, so only one cell of

the array is needed on the mask. This system is called Direct Step on Wafer (DSW).

– These machines are also called “Steppers”– Example: GCA-4800 (original machine)

• Advantage of steppers: only 1 cell of wafer is needed• Disadvantage of steppers: the 1 cell of the wafer on the mask must• be perfect-- absolutely no defects, since it gets used for all die.


Step and Repeat (Stepper) Lithography Systems

• Conventional refractive optics • Can produce image smaller than object

• Cannot make lens with sufficient resolution to project image over whole wafer

• “pixel” count: field size / (lmin)2

• 1 cm2 / (0.5 μm)2 = 4 x 108

• Requires mechanical translation (step) of wafer under lens

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Projection Exposure


Resolutions in Projection Imaging Systems

• Diffraction limits passband of system– minimum geometry or Resolution ~ k1 λ/ NA

• k1 ~ 0.5 to 1, typically ~0.8• λ : exposure wavelength• NA: numerical aperature (typically NA = 0.5)

– related to quality and “size” (entrance/exit pupil) of imaging system– collected light by condenser or objective lens– For objective, NA is also a ratio of focal length to aperture

• Challenges– Need high NA, low aberrations, short wavelength but:

• depth of focus ~ k2/ 2(NA)2• k2 ~ 1

• Restricted set of transparent materials for λ= 350nm– Optics start absorbing

• Very difficult to get large field size and high NA

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Photolithography-Depth of Focus• The defocus tolerance (DOF)

• Much bigger issue in miniaturization science than in ICs

A small aperture was used to ensure the foreground stones were as sharp as the ones in the distance.

What you need here is a use a telephoto lens at its widest aperture.

Source: M. Madou


Image Characteristics• Contrast

– intensity based: scalar quantity• “incoherent” imaging• diffused or blurry edges

• Electric field based: magnitude AND phase– interference effects should be included in “coherent” imaging

system• Spatial variations in image

– measure of how “fast” image varies• Line pairs per unit distance is “digital” analogy

– test pattern made up of periodic clear/opaque bars with sharp edges

• Frequency domain analogy: spatial frequency– Test pattern is sinusoidal variation in optical transparency

• No complete darkness or maximum brighness

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• Image formed by a small circular aperture (Airy disk) as an example

• Image by a point source forms a circle with diameter 1.22lf/dsurrounded by diffraction rings (airy pattern)

• Diffraction is usually described in terms of two limiting cases– Fresnel diffraction - near field.– Fraunhofer diffraction - far field.

Photolithography-Diffraction



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• The angle q in the figure is the maximum angle for which diffracted light from the mask will be collected for imaging by the lens.

• With sin q = N l/2b* now, only those values of N for which the term on the right is less than sin q are allowed. Thus, as the grating period “2b” gets smaller (l/2b gets larger), N gets smaller (i.e. lower diffracted orders).– Larger the cone of acceptance, higher

the resolution• The figure on the right shows the spread

of the diffracted orders for a decrease in relative slit width.

• Because of this spreading effect, fewer diffracted orders form the image. This means that information about the pattern is being lost.

Photolithography-Fraunhofer Diffraction

* Madou text pg 26


Potential Exposure Problems• Substrate induced

reflections– Multiple reflections induced

standing wave pattern• Destructive interference:

underexposed– Primarily an issue near an

edge– For metals, BCs require

“zero” tangential E field at interface!

• Can cause underexposure over metals

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Interference Effects• Step edges also produce non-uniform resist thickness andexposure• Resist thinning


Interference Effects : Fixes•Post exposure bake

• Try to diffuse exposed PAC (photo active compund)• Increase sensitivity

•AR coating• Place highly absorbing layer under PR• Must then be able to pattern AR layer•planarize!

• Multi-layer resist schemes• Portable conformal mask (PCM)

• Thin “normal” PR on top of thicker, planarizing deep UV PR•Expose/develop thin layer normally•Use as “contact” mask for DUV exposure of underlying layer

•Contrast enhancement materials (CEM)• photo-bleachable material with VERY sharp threshold placed above PR

• For energies below threshold PR is “masked”• Above threshold CEM becomes transparent, resist below exposed• Sharpens edges

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Optical Proximity Correction• 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


Phase shift mask

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Exposure Radiation/Wavelength Choices

• Want short wavelength• Electromagnetic

radiation– “optical”– near UV: high pressure

mercury arc lamp• g-line: 436 nm• i-line: 365 nm

• mid UV: xenon arc lamps– 290-350 nm

• Deep UV: excimer laser– 200-290 nm

• XeCl: 308 nm• KrF: 248 nm• F2: 157 nm

• x-ray: synchrotron, plasma– 0.4 - 5 nm

• Particles: very short de Broglie wavelength – electron beam (~50eV electron, λ

~ 1.5Å)– ion beam


Resolution: Effect of Radiation Wavelength

UV 400 UV 300 UV 250

Soft Contact < 2.5 µm < 2.0 µm --

Hard Contact

< 2.0 µm < 1.5 µm --

Vacuum Contact

< 0.8 µm < 0.6 µm < 0.4 µm

Proximity < 3.0 µm < 2.5 µm --

Source: F. Solzbacher

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Photomasks• Master patterns which are transferred to

wafers• Types

– photographic emulsion on soda lime glass (cheapest)

– Fe2O3 on soda lime glass– Cr on soda lime glass– Cr on quartz glass (most expensive, needed

for deep UV litho)• Dimensions

– 4” x 4” x 0.060” for 3-inch wafers– 5” x 5” x 0.060” for 4-inch wafers

• Polarity– “light-field” = mostly clear, drawn feature

(data) = opaque– “dark-field” = mostly opaque, drawn

feature (data) = clear


Basic Mask Structure

• Absorbing Layer– optical, UV wavelengths

• photographic emulsion• thin metal films

– chrome, white and black, iron oxide, silicon

– x-ray wavelengths• “thick,” high Z metals: gold

• Blanks• Optical, UV

wavelengths: glass• Soda-lime, borosilicate,

quartz

• X-ray: thin dielectric• Boron nitride

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Blank: Problem Areas• Surface flatness

– Gravitational sag• hold mask vertically rather than horizontally

• Optical transparency– For wavelengths < ~350nm: quartz

• for wavelengths < ~200nm can have significant absorption

• Thermal expansion– For 100 mm separation, 1°C ΔT

• Soda-lime: 0.9 μm• Fused silica (quartz): 0.05 μm• Silicon: 0.2 μm

– Traceable temperature control is essential


Mask Pattern Generation• E-beam pattern generator

– Can expose very small features– Direct write

• slow, sequential exposure of pattern• ok for mask generation

• Absorbing layer : problem areas– thin compared to feature width for ease of etching

• more difficult as dimensions shrink,• x-ray exposure requires ~micron thick metal layer: hard to make small!

• Defect density– yield formula

• Do: # of fatal defects/unit area• A: die area• mask must be “perfect” so “repair” is essential• laser etch / deposition (focussed ion beam)

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Mask Making• Pathway pattern transfer• Emulsion mask and Chrome mask• Fabrication steps

1. Substrate preparation2. Pattern writing or exposing3. Pattern processing4. Metrology5. Inspection for pattern integrity6. Cleaning7. Repair8. Pellicle attachment9. Final defect inspection


Pathway from Pattern Design to Pattern Transfer

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Mask Design• Device design• Pattern design• Process design• Drawing your mask pattern

– Finest line’s line width– Pattern arranged uniformly– Positive/negative photoresist– Alignment marks


Mask Aligner Technology• Requirements

– faithfully reproduce master mask pattern on wafer

• High resolution• Low distortion errors

– Allow accurate alignment between pattern on wafer and mask (low registration errors)

• Overlay error = 1/3 - 1/5 resolution

• A mechanical process!– Wafer steppers < 40 nm

• Throughput!!!

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Mask to Wafer Alignment• 3 degrees of freedom between mask and wafer: (x,y,q)• Use alignment marks on mask and wafer to register patterns

prior to exposure.• Modern process lines (steppers) use automatic pattern

recognition and alignment systems.– Usually takes 1-5 seconds to align and expose on a modern stepper.– Human operators usually take 30-45 seconds with well-designed

alignment marks


Alignment Marks • Normally requires at least two alignment mark sets on opposite

sides of wafer or stepped region• Use a split-field microscope to make alignment easier

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Visual Alignment• Process of getting wafer coarsely centered under

mask• All that is needed for the first mask of the set, since

no patterns on the wafer exist yet– Accomplished by special windows on a dark field mask


Double-sided Alignment• For many MEMS devices patterns exist on BOTH sides

of the substrate• Typically contact aligners in current use• EVG double-sided optical system

• Use microscopes indexed mechanically to both sides of wafer• Requires transparent wafer chuck

http://www.evgroup.com/products/precisionalignment.htm

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Photoresists• Negative: exposed regions REMAIN after

development– One component: PMMA, COP (e-beam resist)– Two component: Kodak KTFR– Dominant PR until early 1980’s– Better etch resistance

• Positive: exposed regions REMOVED after development– One component: acrylates– Two components: diazoquinone / novolac resin– Higher resolution, but “slower”

• Sub-micron resolution– Largely supplanted negative resists in 80’s

• Better step coverage


Photoresist Exposure Properties• Full exposure is set by energy

threshold– Time * Intensity = Energy– ~linearly increases with resist

thickness• ~ 20 mJ / μm of thickness

• Can NOT easily compensate for underexposure by overdevelopment

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Postbake (Hard Bake)• Used to stabilize and harden the developed photoresist

prior to processing steps that the resist will mask• Main parameter is the plastic flow or glass transition

temperature– Beyond glass transistion, dust accumulation increases

• Postbake removes any remaining traces of the coating solvent or developer

• This eliminates the solvent burst effects in vacuum processing

• Postbake introduces some stress into the photoresist• Some shrinkage of the photoresist may occur• Longer or hotter postbake makes resist removal much

more difficult


Postbake Characteristics• Firm postbake is needed for acid etching, e.g. BOE

– Re-baking after few minutes of etching • Postbake is not needed for processes in which a soft resist

is desired, e.g. metal liftoff patterning.• Photoresist will undergo plastic flow with sufficient time

and/or temperature:– Resist reflow can be used for tailoring sidewall angles

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Photoresist Removal• Want to remove the photoresist and any of its residues• Simple solvents are generally sufficient for non-postbaked

photoresists– Positive photoresists

• acetone• trichloroethylene (TCE)• phenol-based strippers (Indus-Ri-Chem J-100)• Lift-off: acetone and ultrasonicator

• – Negative photoresists• methyl ethyl ketone (MEK), CH3COC2H5• methyl isobutyl ketone (MIBK), CH3COC4H9

• Plasma etching with O2 plasma (ashing) is also effective for removing organic polymer debris– Also: Shipley 1165 stripper (contains n-methyl-2-pyrrolidone),

which is effective on hard, postbaked resist


Basics of Photolithography for Processing

• Microfabrication processes:– Additive deposition– Subtractive etching– Modifying doping, annealing, or curing

• Two primary techniques for patterning additive and subtractive processes:– Etch-back:

• photoresist is applied overtop of the layer to be patterned• unwanted material is etched away

– Lift-off:• patterned layer is deposited over top of the photoresist• unwanted material is lifted off when resist is removed

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Etch-back


Lift-off

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Lithography Information Flow– a closer view

• Design (CAD) file– Mask writing tool and process

• Mask– Optical lithography tool, types

• Exposure– Type, λ, NA, field of view

• Aerial image– Focus, dose, wafer topography, reflections, thin film interference

• Intensity image in resist– Resist photochemistry, post-exposure bake

• Latent image– Development

• Resist image– Etching

• Physical structure on wafer


Review- Wafer Conditions Prior to Patterning

• Surface conditions include:– film composition, e.g.: silicon, nitride, polysilicon,

metal, etc.– bare surface vs. patterned surface– surface reflectivity

• Surface conditions may affect – photoresist-to-wafer adhesion– alignment accuracy– linewidth resolution– exposure settings– bake time

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Review-Wafer Conditions after Photolithography

• Resist coated wafer• Patterned resist layer• Withstands etching process• Withstands ion implanting• Quality measures

– linewidth resolution– overlay accuracy– particles & defects


Ten Basic Steps of Photolithography1. Surface Preparation2. Photoresist Application3. Soft Bake4. Align & Expose*

5. Develop6. Hard Bake7. Develop Inspection8. Etch9. Resist Strip10. Final Inspection

* Some processes may include a Post-exposure Bake

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Number and Label 10 Basic Steps of Photolithography

HMDS

UV Light Source

Mask

Resist

l

l

4Alignment &

Exposure

Plasma

CF4O2

Plasma


Label the 10 Basic Steps of Photolithography

HMDS

UV Light Source

Mask

Resist

l

l

Plasma

CF4 O2

Plasma

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Alignment Exposure Hardware —Alternatives (Beam Processing)

• Higher end research systems go one step further and use Direct Write on Wafer (DWW) exposure systems.– This can be accomplished using:

• Excimer lasers for geometries down to 1-2 µm• Electron beams for geometries down to 0.1-0.2 µ m• Focused ion beams for geometries down to 0.05-0.1 µm

– No mask is needed for these technologies.– These are serial processes, and wafer cycle time is

proportional to the beam writing time-- the smaller the spot, the longer it takes!


Other Approaches to High Resolution Lithography

• E - beam systems (“direct - write”):– high resolution (< 0.2 μm )– no mask requirement– low throughput

• E - beam proximity printers:– requires mask but has high throughput potential

• X - ray systems (proximity - type contact printers):

– high resolution if λ is small– for g ~ 10 µm, λ ~ 10 Å ? lmin ~ 0.15 µm– may also be overlay limited

• not clear if sub 0.2-ish micron possible– mask technology very complex– low throughput until brighter sources are found

42


Electron Beam Exposure Systems• Dominant mask making tool.• Potential < 0.1 µm resolution (on flat, uniform substrates).

– Usually step - and - repeat format, e - beam computer driven• Typical resist:

– poly (methyl methacrylate)• Low throughput• Problem in electron beam systems:

– most electrons do not stop in the photoresist:– potential damage problem– back scattered electrons cause pattern edges to blur– most e- beam pattern generators contain computer code to reduce

dose near edges to control proximity effects


?

?

?

silicon substrate

Silicon nitride

photoresist

?

Resist tone?

silicon substrate

Silicon nitride

photoresist

?

photoresist

?

Resist tone?

photoresistoxide

silicon substrate

?

?

Silicon nitride

Identify and label each component

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Other Terms used to Describe Photolithography

• masking• photomasking• photo• lithography• litho• yellow room• dark room


Types of Photolithography Processes

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.

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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


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

45


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

Resist

l

46


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

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7. Develop Inspect• Optical or SEM metrology • Quality issues:

– particles– defects– critical dimensions– linewidth resolution– overlay accuracy


8. Etch• Selective removal of upper layer of

wafer through windows in photoresist

• Two basic methods:– wet acid etch– dry plasma etch

• Quality measures:– defects and particles– step height– selectivity– critical dimensions

Plasma

CF4

48


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– critical dimensions

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k1

• The third element in the Rayleigh equation is k1.

• k1 is a complex factor of several variables in the photolithography processes– quality of the photoresist – Use of resolution enhancement

techniques• phase shift masks, • off-axis illumination (OAI)• optical proximity correction

(OPC). – While exposure wavelengths have

been falling and NA rising, k1 has been falling as well

– Practical lower limit for k1 ~ 0.25

Textbook Topics• Photolithography

overview• Wafer cleaning• Masks• Photoresist

deposition• Exposure and

postexposuretreatment

• Development• Descumming and

postbaking• Resists• Critical dimension,

overall resolution• Resist profiles • Resolution• Improved resolution


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Lecture 03 Photolithography