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Etching

Basics of Etching technology

Wet etching-----dielectrics, semiconductors, & metal

----- so rop c an so rop c e c ng

Dry etching (RIE)

Electrochemical polishing

Etching

Three steps in etching process?

Two kinds of Etching?

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(1) Diffusion (3) Diffusion

eac on

dry & wet etching

Gas phase(chemical, and physical)

Liquid phase(chemical)

Etching profiles

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Profiles: isotropic & anisotropic

Applications: Si, Silicon nitride, silicon dioxide, metal

Controls: doping, electrochemical, film quality, mask materials

Wet Etching of Metals

Aluminum - Aluminum Etchant Type A from Transene Co., Inc. is a mixture ofphosphoric acid, acetic acid and nitric acid. Etch Rate is about 1 min for 2000 .

interlevel dielectric. When the underlying layer is aluminum and the insulatinglayer is glass the preferred etchant is 5 parts BOE and 3 parts Glycerin. (straightBOE etches aluminum

Copper - Ferric Chloride or mix Etchant from 533 ml water, add 80 ml Na2S2O3,Sodium Persulfate, (white powder, Oxidizer), prepare in glass pan, place pan on

~ .Copper Coated Board about 30 min.

Platinum7ml HCl, 1ml HNO3, 8 ml H2O at 85 C approximate etch rate 450/min.

Chromium - CR-9 Etch, Cyantek Corp.- o c .

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Profiles: isotropic & anisotropic

Applications: Si, Silicon nitride, silicon dioxide, metal

Controls: doping, electrochemical, film quality, mask materials

Wet Etching of Dielectrics

Wet Etching of Silicon OxideWet Etching of Silicon Oxide

BY HF with or without the addition of ammonium flouride

(NH4F).

The addition of ammonium flouride creates a buffered HF solution (BHF) alsocalled buffered oxide etch (BOE).

The addition of NH4F to HF controls the pH value and replenishes the depletion oft e uor e ons, t us ma nta n ng sta e etc rate.

SiO2 + 6HF = H2SiF6 + 2H2O

Types of silicon dioxide etchants:

49% HF - fast removal of oxide, poor photoresist adhesion

BHF - medium removal of oxide, with photoresist mask

Dilute HF - removal of native oxide, cleans, surface treatments

HF/HCl or HF/Glycerin mixtures special applications

Buffered HF Etching

7:1 NH4F/HFgives about

1000 /min etch rateat room temperature

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Etching tools - HF

SiO2 etching is performed on HF bench using Teflon tools

Teflon Chemical Process Wafer

Cassette & container

Fluoroware A182-60MB orPerFluoroAlkoxy (Teflon)

High resistance to chemicals and

temperatureCan be used in wet chemistryprocesses (RCA clean, BOE etch,

HF Bench

wet nitride etch, wet aluminumetch)

BOE (7:1) 20A/min,

1:1 HF:HCL 120A/min,

49% HF 140 A/min165C Phosphoric Acid 55A/min (BOE dip first to removeoxynitride layer), etches silicon dioxide at 10 /min ands con

Hot phosphoric acid etch of nitride can not use photoresist as an etchmask. One can use a thin patterned oxide (or oxynitride) to act as the etchmask. Etch rate for silicon is even lower than the etch rate of oxide.

Silicon Nitride Etching with Hot H3PO4

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Etching tools Hot Phos

Warm up Hot Phosphoric Acid

pot to 175C

Use Teflon boat to lace wafers

Hot Phos Bench

in acid bath 3500 +/-500 50 minutes

1500 +/- 500 25minutes

Etch rate of ~80 /min

Rinse for 5 min.

Water rinse

Wet Etching of Silicon

Anisotropic etching

HNA (Hydrofluoric acid + Nitric acid + Acetic acid)

Base:

KOHetch

NaOH etch

EDP (Ethylene Diamine Pyrocatechol) etch

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Orientation

Pr imary f lat or ien tat ion

Secondary flat locations--

p-type 90 clockwise from primary flat

n-type 45 clockwise from primary flat

n-type 180 clockwise from primary flat

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Anisotropic Wet etching

Convex corners areundercut

Concave corners stopat [111] intersection

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100

(100) Si Etching rate

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Etching selectivity en 2 s use as a mas ng ayerwith a KOH solution both temperature

and concentration should be chosen as low

as ossible.

KOH etch rate is about 50 to 55 m/minat 72 C and KOH concentrations between

10 and 30 weight %.

The Si/SiO2 etch ratio is 1000:1 for 10%KOH at 60 C, at 30% it drops to 200:1.

lightly doped silicon decreases for doping

concentrations above 1E19 and at 1E20

the relative etch rate is 1/100 for 10%

concentration. (on (100) wafer the angle is50.6)

Si3N4 is the perfect masking material forKOH etch solution. The etch rate for

Silicon Nitride appears to be zero.

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KOH etching rate on B-dopingDoping

Revisit:

- 2+ -

Si SiHO OH[ ] 2

4H2O + 4e- = 4(OH)- + 2H2

Si + 2 (OH)- = Si(OH)22+ + 4e- (1)

In Boron doped Si, particularly the degenerated doped Si (p++ - Si),Fermi level close to valance band and man , man holes in Ev

Ec

Transfer to conduction band

recombination

4H2O + 4e- = 4(OH)- + 2H2 (2)

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