1 Photolithography Himanshu J. Sant Fundamentals 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
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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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
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
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
Photolithography -- Photomask
•Controlled light exposure using a mask•Wavelength of light?•Mask: Chrome on glass
•Imagable resist layer
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
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.
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Fundamentals of Microfabrication
Photolithography-- a closer look
Fundamentals of Microfabrication
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.
Courtesy: Mark Madou, UC Irvine
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Fundamentals of Microfabrication
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
Courtesy: Mark Madou, UC Irvine
Fundamentals of Microfabrication
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
Courtesy: Mark Madou, UC Irvine
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Fundamentals of Microfabrication
Effect of Resist Tone
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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)
Fundamentals of Microfabrication
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:
– 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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
• Used for silicon:– primers form bonds with surface and produce a polar
(electrostatic) surface– most are based upon siloxane linkages (Si-O-Si)
• What is the ideal thickness?– 1-2 µm thickness for
commercial Si processes.
vacuum chuck
spindleto vacuum
pump
photoresist dispenser
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
Typical Spin Coating Program
Fundamentals of Microfabrication
Spin Stages
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
Dry Resist?
Good or bad?
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Fundamentals of Microfabrication
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)
characteristics of photoresist– Densifies photoresist
• Hotplate preferred– Doesn’t trap solvent like
convection oven baking
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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)
Fundamentals of Microfabrication
Alignment/Exposure/Development
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
Mask Aligner—Contact and Proximity
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Fundamentals of Microfabrication
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.
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
Projection Exposure
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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.
• 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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Fundamentals of Microfabrication
• 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
Fundamentals of Microfabrication
Photolithography-Diffraction
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Fundamentals of Microfabrication
• 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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
Interference Effects• Step edges also produce non-uniform resist thickness andexposure• Resist thinning
Fundamentals of Microfabrication
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!
• 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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Fundamentals of Microfabrication
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)
Fundamentals of Microfabrication
Photolithography-Diffraction
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
Phase shift mask
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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
1. Substrate preparation2. Pattern writing or exposing3. Pattern processing4. Metrology5. Inspection for pattern integrity6. Cleaning7. Repair8. Pellicle attachment9. Final defect inspection
Fundamentals of Microfabrication
Pathway from Pattern Design to Pattern Transfer
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
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
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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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
• 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
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
Photoresist Removal• Want to remove the photoresist and any of its residues• Simple solvents are generally sufficient for non-postbaked
• 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!
Fundamentals of Microfabrication
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
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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
?
?
?
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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Fundamentals of Microfabrication
Other Terms used to Describe Photolithography
• masking• photomasking• photo• lithography• litho• yellow room• dark room
Fundamentals of Microfabrication
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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Fundamentals of Microfabrication
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
Fundamentals of Microfabrication
2. Photoresist Application• Wafer held onto vacuum
chuck• Dispense ~5ml of
photoresist• Slow spin ~ 500 rpm• Ramp up to ~ 3000 - 5000