MSN 551 Class Notes

LITHOGRAPHY

Why do we need lithography?

Why do we need lithography?

LITHOGRAPHY

It is a general name given to processes used to transfer patterns on to a substrate to define structures that make up devices

Optical lithography: Uses light Electron Beam lithography: Uses electrons Ion beam lithography: Uses energetic ions to

bombard and pattern surfaces Soft lithography: Uses mechanical contact

indentation to transfer patterns

Optical Lithography: Summary

A photoresist is exposed with light Photoresist undergoes chemical modification A developer is used to etch away modified (or

unmodified) parts of the photoresist Pattern transfer is achieved.

coat expose develop

Main performance paremeters for lithography

Resolution: How small features can you write

Registration: How tight can you align different layers (about 1/3 resolution for optical lithography)

Throughput: How fast (cm^2/minutes) can you expose?

Coating resists: Spin coating Easiest method of coating a flat substrate The wafer (substrate) is placed on a rotating

table (chuck). The chuck is rotated at a fixed rotational speed

(rpm, rotations per minute) for a fixed amount of time.

Depending on viscosity of the resist, a certain thickness of film remains on the substrate.

We want uniform (constant thickness) films that conform the surface (coat sharp edges and deep groves)

Coating resists: Spin coating

Coating: Alternatives Spray coating

− Resist is sprayed onto the substrate Dip coating

− Substrate is dipped and then the resists is let to flow with gravity

Meniscus coating− Substrate is inverted and pulled over a resist bath

Plasma coating− Thin photoresist films can be deposited by certain

plasmasSuccess of each method depends on wettability and viscosity

Spin coating Issues You can't spin coat everything on everything

− Wetting issues− Fluid dynamical instabilities may cause nonuniform

or noncontinous films− Surface treatment is a solution.

Conformity− Works best when surface topographic variations are

less than the film thickness− If deep feature is what you have you may want to

try other coating techniques.

Spray coated substrate with deep features

Spin coating nonplanar topography

Spin coating: Cleanliness

Spin coating: Thickness Inverse Square Root law

Spin coating recipe

Surface treatment: HMDS Problem: nanometer

thick water film on surfaces degrades photoresist sticktion

Surface treatment: HMDS

Light sources Arc Lamp

Mask materials

Alignment marks

Alignment marks

Photoresist Chemistry Polymer with photosensitive crosslinkers

Bisazide crosslinking

Photoresist Chemistry Polymer with photosensitive crosslinkers

Poly(vinyl cinnamate)

Photoresist Chemistry

Exposure process: Thin film interference

Exposure process: Thin film interference

Exposure Process

We can write a differential equation to estimate the dose received by

each point

Exposure process: Thin film interference

Edge beadsNear the edges,photoresist piles up

A mask in contactcan be tens of micronsaway from the substrate plane

Edge beads: Removal

develop

Diffraction effects

Diffraction effects

Diffraction effects

Phased Masks

Standard binary mask Phase shifted masks

Resist polarity

Resist contrast

Resist Profiles

Depending on polarityof resist, inverted trapezoidcan be printed.

Simulations can predict development

Resist Profiles

Negative resist, exposed with lines

Resist Profiles

Negative resist, exposed with lines

Thin film interference effects

Post exposure bake (P.E.B.) reflows resist and smoothens out walls

Post exposure Bake

Why is it important to control the profiles

Lift-off− A name given to process where an evaporated

material is patterned by the shadowing effect of the photoresist barrier.

Conformal coatings

Lift-off process

Lift-off process: Problems

Lift-off process

Multiple layer resist coatings

Bi-layer coating: T shaped profiles

Bi-layer coating: T shaped profiles

Chlorobenzene treatment

Soak sample in chlorobenzene for 10-20 minutes after exposure.Chlorobenzene is toxic, and is not being widely used anymore!

Other Exposure types Direct laser writing

Interference lithography

Interference lithography

Interference lithography

Gray scale lithography Location dependent exposure dose may be

used to fabricate 3D structures with arbitrary profiles.

Photoresist cleaning Photoresist removers Sulfiric acid and Hydrogen peroxide Oxygen plasma