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Photolithography Patterning

Photolithography is a technique

that is used to define the shape ofmicro-machined structures on a wafer.

Pattern process:

The first step in thephotolithography process is todevelop a mask, which will betypically be a chromium pattern on aglass plate.

Next, the wafer is then coated witha polymer which is sensitive toultraviolet light called a photoresist.

Afterward, the photoresist is thendeveloped which transfers thepattern on the mask to thephotoresist layer.

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Photolithography Photoresist

Two basic types of Photoresistsi) Positive resist & ii) Negative resist

Positive resists.Exposure to the UV light changes

chemical structure of resist so that it becomes moresoluble in developer.

The exposed resist is then washed away by the

developer solution.The mask, therefore, contains an exact copy of thepattern which is to remain on the wafer.

Negative resistsExposure to the UV light causes

negative resist to become polymerized, and moredifficult to dissolve.

it remains on the surface wherever it is exposedthe developer solution removes only the unexposed

portions.Masks used for negative photoresists, therefore, contain the inverse (or photographic "negative")of the pattern to be transferred.

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EtchingEtching is the process where

unwanted areas of films are removed by either dissolving them in a wetchemical solution (Wet Etching) or by reacting them with gases in aplasma to form volatile products ( DryEtching ).

- Resist protects areas which are to

remain. In some cases a hard mask,usually patterned layers of SiO 2 orSi3N4, are used when the etchselectivity to photoresist is low or theetching environment causes resist todelaminate.

Terminology Isotropic etch- a process that etches at the same

rate in all directions. Anisotropic etch- a process that etches only one

direction.

SiO2

SiO2

SiO2

(1)

(2)

photoresist

photoresist

SiO2

(1)

(2)

photoresist

photoresist

SiO2

SiO2

Isotropic etching Anisotropic etching

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EtchingWet Etching

- are in general isotropic(not used to etch features lessthan 3 m)

- achieve high selectivity formost film combinations

- capable of high throughputs- use comparably cheap

equipment

- can have resist adhesionproblems

- can etch just about anything

Examples wet process:

For SiO 2 etching- HF + NH 4F (1:7)(buffered oxide etch or BOE)

For Si 3N4- Hot phosphoric acid: H 3PO4 at 160-180 C- need to use oxide hard mask

Silicon- Nitric, HF, acetic acids- HNO 3 + HF + CH 3COOH + H 2O

Aluminum- Acetic, nitric, phosphoric (16:4:80) acids at35-45 C- CH 3COOH+HNO 3+H 3PO4

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Etching Dry Etching - also known as Plasma Etching, or

Reactive-Ion Etching , is anisotropic.- Plasma

is a partially ionized gas made upof equal parts positively andnegatively charged particles.

are generated by flowing gasesthrough an electric or magneticfield.

- Reactive Ion Etching (RIE)Directional etching due to ionassistance.

In RIE processes the wafers sit onthe powered electrode. Thisplacement sets up a negative bias onthe wafer which acceleratespositively charge ions toward thesurface. These ions enhance thechemical etching mechanisms andallow anisotropic etching.

- Silicon and its compounds can be etched

by plasmas containing F.- Aluminum can be etched by Cl.

SEM image shows 8m deep GaN RIE etch.

Wet etches are simpler, but dryetches provide better line widthcontrol since it is anisotropic.