Etching

ECE/ChE 4752: Microelectronics ECE/ChE 4752: Microelectronics Processing LaboratoryProcessing Laboratory

Gary S. May

February 19, 2004

Outline

IntroductionIntroduction Wet Chemical EtchingWet Chemical Etching Plasma EtchingPlasma Etching

Definition

Recall: Photolithography = process of transferring patterns (on masks) onto a thin layer of photoresist

Photoresist patterns must be transferred once more onto the underlying layers to produce circuit features

Pattern transfer accomplished by selectively etching unmasked portions of a layer

Etching Hierarchy

Etching

Wet Dry

isotropic anisotropic anisotropic(plasma etching)

Outline

IntroductionIntroduction Wet Chemical EtchingWet Chemical Etching Plasma EtchingPlasma Etching

Uses

Prior to thermal oxidation or epitaxial Prior to thermal oxidation or epitaxial growth, wafers are chemically cleaned to growth, wafers are chemically cleaned to remove contamination remove contamination

Especially suitable for blanket etches (i.e., Especially suitable for blanket etches (i.e., over the whole wafer surface) of over the whole wafer surface) of polysilicon, oxide, nitride, metals, and III-V polysilicon, oxide, nitride, metals, and III-V compounds. compounds.

Mechanism

1. Reactants transported by diffusion to surface

2. Reactions occur at surface

3. Products from surface removed by diffusion

Uniformity “Equality” of vertical etch rates at different sites

on the wafer surface

This is actually non-uniformity Alternative definitions:

/

100% rateetch minimum rateetch maximum

rate)etch minimum - rateetch (maximum (%) uniformity rateEtch

%100||

center

edgecenter

R

RRU

Silicon Etching

Most etchants are mixtures of HNOMost etchants are mixtures of HNO33 and HF in and HF in

water or acetic acid (CHwater or acetic acid (CH33COOH). COOH).

HNOHNO33 oxidizes silicon to form an SiO oxidizes silicon to form an SiO22 layer: layer:

Si + 4HNOSi + 4HNO33 → SiO → SiO22 + 2H + 2H22O + 4NOO + 4NO22

HF is used to dissolve the SiOHF is used to dissolve the SiO22 layer: layer:

SiOSiO22+ 6HF → H+ 6HF → H22SiFSiF66 +2H +2H22OO

Water can be used as a diluent for this etchant, but Water can be used as a diluent for this etchant, but acetic acid is preferred.acetic acid is preferred.

Orientation-Dependent Etching

Some etchants dissolve a certain crystal plane of Si faster than another plane

For Si, the (111) plane has more available bonds per unit area than the (110) and (100) planes

Therefore, etch rate is slower for the (111) plane.

KOH Etching

KOH is an orientation-dependent etchant for Si. Solution with 19 wt % KOH in deionized water at

80 oC removes the (100) plane at a much higher rate than the (110) and (111) planes [ratio of etch rates for (100):(110):(111) planes = 100:16:1].

SiO2 Etching

Commonly etched in a dilute solution of HF with or without NH4F

Adding NH4F is called a buffered HF solution (BHF), also called buffered-oxide-etch (BOE)

Reaction for SiO2 etching:

SiOSiO22+ 6HF → H+ 6HF → H22SiFSiF66 +2H +2H22OO

SiO2 can also be etched in vapor-phase HF.

Outline

IntroductionIntroduction Wet Chemical EtchingWet Chemical Etching Plasma EtchingPlasma Etching

Anisotropy Vertical features are desirable to increase circuit density.

Quantitatively:

where RL = lateral etch rate, RV = vertical etch rate

%1001

V

Lf R

RA

photoresist

etched material

substrate

photoresist

etched material

substrate

undercut

Anisotropic Isotropic

Plasma Fundamentals Plasma: ionized gas composed of equal

numbers of positive and negative charges and a different number of unionized molecules

Produced when electric field is applied to a gas, causing gas to break down and become ionized

Initiated by free electrons that gain kinetic energy from electric field, collide with gas molecules, and lose energy.

Energy transferred causes the gas molecules to be ionized (i.e., to free electrons).

Free electrons gain kinetic energy from the field, and the process continues.

Plasma Etching

Plasma etching - chemical reaction combined with physical ion bombardment

Other names: ion milling sputter etching reactive ion etching reactive ion beam etching

First explored as a cheaper alternative to wet solvent resist stripping in 1960’s and 70’s

Etch Equipment

Etch Mechanism

1. Etchant species generated in plasma.2. Reactant transported by diffusion to surface. 3. Reactant adsorbed on the surface.4. Chemical reaction (along with ion bombardment) forms volatile compounds.5. Compounds desorbed from surface, diffused into the bulk gas, and pumped out

by vacuum system.

x

CDF

End-Point Control

Dry etching has less etch selectivity than wet.

Plasma reactor must be equipped with a monitor that indicates when the etching process is to be terminated (“end point detection” system).

Laser interferometry is used to determine the end point.

tDx sj 1.1

Laser Interferometry

Intensity of laser light reflected off thin film Intensity of laser light reflected off thin film surface oscillates. surface oscillates.

Period of the oscillation related to change in Period of the oscillation related to change in film thicknessfilm thickness

where where dd = change in film thickness, = change in film thickness, is the is the wavelength, and is the refractive index wavelength, and is the refractive index n

nd 2/ nd 2/

nd 2/

Interferometry Example

Typical signal from a silicide/polycrystalline Si gate etch

d for polysilicon = 80 nm (measured by using a He-Ne laser with = 632.8 nm)