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ETCHING

E5163– ICDESIGN

UNIT 2 : INTEGRATED CIRCUIT

FABRICATION PROCESS

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LEARNING OUTCOMES

At the end of this topic, student should be ableto:

• Define etching in integrated circuitfabrication.

• Explain why etching is used in waferfabrication.

• Give examples of etchant and etched layer.

• Compare between wet etching (chemical

etching) and dry etching (plasma etching).

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• Etching is the process of using strong acid or

etchant to cut into the unprotected parts of ametal surface to create a design.

• Etching is used in microfabrication to

chemically remove layers from the surface of a

wafer during manufacturing.• Etching is a critically important process module,

and every wafer undergoes many etching steps

before it is complete.

• For many etch steps, part of the wafer isprotected from the etchant by a "masking"

material which resists etching. the masking

material is a photoresist which has been

patterned using photolithography.

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• Two main methods of etching:

• wet etching

• dry etching

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1. In wafer formation process, after slicing ingot

to removed any remaining damaged and

contaminated regions.2. In photolithography process to removed

unnecessary photoresist.

3. In metallization process to removedunnecessary metal surface for making

contact between devices.

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 – Use of chemicals where a batch of wafers is dipped into a highlyconcentrated pool of acid and the exposed areas of the wafer are

etched away.

 – Wet etching is good and fairly cheap and capable of processing

many wafers quickly. – The disadvantage is that wet etching does not allow the smaller

critical geometries that are needed for today chips.

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• Dry etching refers to any of the methods of etching that use gas instead of chemical

etchants.• Dry etching is capable of producing critical

geometries that are very small.

• Ex: Plasma Etching

 – Reactive Ion Etching in 80’s

• Ex: Electron Cyclotron Resonance (ECR) andInductively Coupled Plasmas (ICP) in 90’s.

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PLASMA ETCHING

• Plasma etching uses a gas that is subjected to anintense electric field to generate the plasma state.

• The electric field is produced with coils that arewrapped around the chamber and exposed to a highlevel RF source.

• There are two different versions of this type of etchingbased on the shape of the chamber used.

1. One consists of a barrel type chamber where the wafersare placed sitting up while the gas is flowed over thewafers and out through an exhaust pipe.

2. The second process uses a parallel plate reactor.

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BARREL CHAMBER

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PARALLEL PLATE REACTOR

• There are two plates that are used to give the gas the electric

field rather than the coil that is wrapped around the barrelchamber.

• In plasma form, the gases used are very reactive, providing

effective etching of the exposed surface. Plasma etching

provides good critical geometry but the wafer can bedamaged from the RF radiation.

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ETCHANT and ETCHED LAYER

Material to be etched Wet etchants Dry / Plasma etchantsSilicon (Si)

Nitric acid (HNO3) + hydrofluoric

acid (HF)[3]

•CF4, SF6, NF3[4]

•Cl2, CCl2F2[4]

Silicon dioxide (SiO2)

•Hydrofluoric acid (HF)[3]

•Buffered oxide etch [BOE]:

ammonium fluoride (NH4F) and

hydrofluoric acid (HF)[3]

CF4, SF6, NF3[4]

PhotoresistPiranha etch: sulfuric acid (H2SO4)

+ hydrogen peroxide (H2O2)O2 (ashing)

Aluminium (Al)

80% phosphoric acid (H3PO4) + 5%

acetic acid

+ 5% nitric acid (HNO3) + 10%

water (H2O) at 35 –45 °C[3]

Cl2, CCl4, SiCl4, BCl3[4]

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