Etching

Basics of Etching technology

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

I t i & i t i t hi-----Isotropic & anisotropic etching

Dry etching (RIE)

Electrochemical polishing

Etching

Three steps in etching process?

Two kinds of Etching?

(2) Reaction(1) Diffusion (3) Diffusion

(2) Reaction

dry & wet etching

Gas phase (chemical, and physical)

Liquid phase (chemical)

Etching profiles

Profiles: isotropic & anisotropicApplications: Si, Silicon nitride, silicon dioxide, metalControls: doping, electrochemical, film quality, mask materials

Wet Etching of MetalsAluminum - “Aluminum Etchant Type A” from Transene Co., Inc. is a mixture of phosphoric acid, acetic acid and nitric acid. Etch Rate is about 1 min for 2000 Å.

For multilayer metal processes it is often necessary to etch through an insulatingFor multilayer metal processes it is often necessary to etch through an insulating interlevel dielectric. When the underlying layer is aluminum and the insulating layer is glass the preferred etchant is 5 parts BOE and 3 parts Glycerin. (straight BOE etches aluminum)BOE 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 hot plate and heat to 50 C (plate Temp set at 100 C) Etch Rate 1 66 µm/minhot plate and heat to 50 C (plate Temp set at 100 C) Etch Rate ~ 1.66 µm/min Copper Coated Board about 30 min.

Platinum –7ml HCl, 1ml HNO3, 8 ml H2O at 85 C approximate etch rate 450 ÅÅ/min.

Chromium - CR-9 Etch, Cyantek Corp.

Gold Gold EtchGold - Gold Etch.

Profiles: isotropic & anisotropicApplications: Si, Silicon nitride, silicon dioxide, metalControls: doping, electrochemical, film quality, mask materials

Wet Etching of Dielectrics

Wet Etching of Silicon OxideWet Etching of Silicon OxideBY HF with or without the addition of ammonium flouride (NH4F).•The addition of ammonium flouride creates a buffered HF solution (BHF) also called buffered oxide etch (BOE).

•The addition of NH4F to HF controls the pH value and replenishes the depletion of h fl id i h i i i bl hthe fluoride ions, thus maintaining stable etch 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 treatmentsf f

•HF/HCl or HF/Glycerin mixtures – special applications

Buffered HF Etchingg

7:1 NH4F/HF gives about

1000 Å/min etch rateat room temperature

}}Etching tools - HF

SiO2 etching is performed on HF bench using Teflon tools

Teflon Chemical Process Wafer Cassette & container

•Fluoroware A182-60MB or PerFluoroAlkoxy (Teflon®)

•High resistance to chemicals and gtemperature

•Can be used in wet chemistry processes (RCA clean, BOE etch,

HF Bench

wet nitride etch, wet aluminum etch)

Silicon Nitride EtchingSilicon Nitride Etching •BOE (7:1) 20A/min,

•1:1 HF:HCL 120A/min,

•49% HF 140 A/min

•165°C Phosphoric Acid 55A/min (BOE dip first to remove oxynitride layer), etches silicon dioxide at 10 Å/min and ilisilicon

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

Silicon Nitride Etching with Hot H3PO4

Etching tools – Hot PhosWarm up Hot Phosphoric Acid

pot to 175ºC

• Use Teflon boat to place wafersHot Phos Bench

Use Teflon boat to place wafers in acid bath

• 3500Å +/-500 à 50 minutes

• 1500Å +/- 500 à 25minutes 1500Å / 500 à 25minutes

• Etch rate of ~80 Å/min

• Rinse for 5 min.

Water rinse

Wet Etching of Silicon

Acid:

gAnisotropic etching

Acid:

•HNA (Hydrofluoric acid + Nitric acid + Acetic acid)

Base:

•KOHetch

•NaOH etch

•EDP (Ethylene Diamine Pyrocatechol) etch

•TMAH (Tetra Methyl Ammonium Hydroxide) etchTMAH (Tetra Methyl Ammonium Hydroxide) etch

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OrientationPrimary flat orientation <110>Secondary flat locations<111> p-type no secondary flat<111> p-type no secondary flat<100> p-type 90º clockwise from primary flat<111> n-type 45º clockwise from primary flat<100> n-type 180º clockwise from primary flat

Anisotropic Wet etching

Convex corners are undercut

Concave corners stop at [111] intersection

100100

(100) Si Etching rate

Wh SiO i d ki l

Etching selectivity•When SiO2 is used as a masking layer with a KOH solution both temperature and concentration should be chosen as low as possible. p

•KOH etch rate is about 50 to 55 µm/min at 72 °C and KOH concentrations between 10 and 30 weight %. g

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

•The relative etch rate of doped silicon to•The relative etch rate of doped silicon to 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 is 50.6°)

•Si3N4 is the perfect masking material for KOH etch solution. The etch rate for Silicon Nitride appears to be zero.

KOH etching rate on B-doping Doping

Revisit:

2+Si + 2 (OH)- = Si(OH) 2+ + 4e-Si SiHO OH[ ]2+Si + 2 (OH) = Si(OH)2 + 4e

4H2O + 4e- = 4(OH)- + 2H22 2

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

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

Ec

y, y

Transfer to conduction band

Ev

recombination

Ev

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