1

Etching

Reading Assignments:

Plummer, Chap 10.1~10.4

2

Etching

• Process that removes material from surface • Chemical, physical or combination of the two• Selective or blanket etch• Selective etch transfers IC design image on the photoresist to the surface layer on

wafer • Other applications: Mask making, Printed electronic board, Artwork, etc.

Substrate

Film deposition Photoresist application

Deposited Film Photoresist

Exposure

Development Etching Resist removal

Mask

Etch mask

Light Important process consideration:

Etch Rate

Selectivity

Directional(CD and profile)

Uniformity

Damage (Dry etch)

3

• The process can remove the material from the surfaceis called “ETCHING”.

• “ETCHING” : Wet etching and Dry etching.

LiquidChemical solventChemical reaction.

Dry (or called Plasma Etching)GasPhysical bombardment. or Chemical reaction.(or combination)

A + B(s) AB

Jyyang in NDL

Etching Methods

4

Etch Rate

5

Selectivity• Selectivity is the ratio of etch rates of different

materials.• Very important in patterned etch• Selectivity to underneath layer and to photoresist

1

2

ERSER

=

6

Practical Etching Profiles

a)

b)

a) isotropic b) anisotropic c) completely anisotropic

More directional etching

over etch

lateral etch

Anisotropic Af = 1 – rlat/rver

7

Microloading Effect

wafer

wafer

Aspect Ratio Dependent Etch Rate

ARDE effectARDE effect

depth/width ; etch rate

Microscopic loading effectMicroscopic loading effect

slower etch ratefaster etch rate

Jyyang in NDL

8

Microloading Effect• Smaller hole has a lower etch rate than the

larger holes• Etchants are more difficult to pass through the

smaller hole• Etch byproducts are harder to diffuse out• Lower pressure can minimize the effect.• Longer MFP, easier for etchants reaching the

film and for etch byproducts to get out

9

Over Etch• Film thickness and etch

rate is not uniform• Over etch: removes the

leftover film• Selectivity of etched

film and substrate• Endpoit Detector• Optical Emission

Spectroscopy, OESOES• Mass Spectroscopy, MSMS• Laser-Induce

Fluorescence, LIFLIF

10

Mechanism

Main factors • reactant concentration• reaction time• reaction temperature

ReactantReactant

wafer

Film

PR

boundary layer

reaction

product

Wet EtchingIsotropic Step 1:

Diffusion (reactant)

Step 2:ReactionStep 3:Desorption (product)

Jyyang in NDL

11

Wet Etch• Chemical solution to dissolve the materials on the wafer

surface • The byproducts are gases, liquids or materials that are

soluble in the etchant solution. • Three basic steps : etch, rinse, and dry

Spin DryerSpin DryerEtchant Sink

D.I. Wafer Rinse

12

13

Wet Etch Profiles

7 - 8 μm

Photoresist

Substrate

3μmFilm

3 μm

PR

Substrate

Film

Etch Bias

• Can’t be used for feature size smaller than 3 μm

• Replaced by plasma etch for fine line patterning

Isotropic

14

Si Anisotropic Etching

• Si may be etched by direct dissolution of Si atoms.– The (111)-plane has higher Si bonds density

than the (100)-plane. The etch rate in (111)-plane is expected to be lower.

– Anisotropic etching or oriented dependent etching becomes possible.

• Etch (100)-oriented Si through a window creates V-groove (54.7o).– 19 wt% KOH AT 80°C, (100) : (110) : (111) =

100 : 16 : 1– 10 wt% KOH at 80°C, (100)-Si etch rate ~

1.1um/min, selectivity to SiO2>600.

lWlWWb 27.54cot2 00 −=−=

15

Plasma Etch or Dry Etch -Reactive Ion Etch (RIE)

• Combination of chemical and physical etch• Plasma process, ion bombardment (physical) plus free

radicals (chemical)• Misleading name, should be called ion-assisted etch or

ion-induced etch• High and controllable etch rate• Anisotropic and controllable etch profile• Good and controllable selectivity• All patterned etches are RIE processes in 8” fabs

16

Schematic of RIE System

Provide DC bias!

17

Time

Vol

t

DC Bias RF potential

Plasma Potential

Time

Vol

t

DC Bias RF potential

Plasma Potential

RF Source

Potential Distribution – RF Biased

1 2

2 1

mV AV A

⎛ ⎞= ⎜ ⎟⎝ ⎠

18

Ion-assisted Etch

Free radical only

(isotropic)

Ion Bombardment only

(anisotropic)

19

Ion-assisted Etch

(anisotropic) (anisotropic)

20

Ion-induced Etching• Etch rate of reactive radicals with etched material ~ 0• Heavy ion bombardment damages chemical bonds• Exposed surface atoms are easier to react with reactive radicals,

i.e., etch reaction is induced by ion bombardment • Ion bombardment is mainly in vertical direction• Etch conducted only on vertical direction → anisotropic etch• Ex. Poly-Si gate etching

21

4F + SiO2(s) SiF4(g) + O2(g)

Etch chemistry:

Byproduct volatility

Selectivity

Anisotropy

22

Inhibitor vs. Etch ProfileFilmMask

FilmMask

Inhibitordeposition

or formation

Etch

Etch

Finalprofile

a. Inhibitor deposition rate fast compared to etch rate

b. Inhibitor deposition rate relatively slow compared to etch rate

. . . . . .

Inhibitordeposition

or formation

Inhibitor: etch byproducts that impede further etch process. Important for the control of etching profile.

23

Poly-silicon etch SiO2 etch

C2H6 increase the sidewall inhibitor (polymer ) deposition.

Higher C/F ratio increase the sidewall inhibitor (polymer) deposition. O2reduces the polymer deposition.

Inhibitor vs. Etch Profile

24

High-Density Plasma Etching

Sputter Etching and Ion Beam Milling

High Density Plasma Etching

Reactive Ion Etching

Plasma Etching

Wet Chemical Etching

Pres

sure

Sele

ctiv

ity

Ani

sotr

opy

Ene

rgy

PhysicalProcesses

ChemicalProcesses

gas inlet

plasma

magnetic coil

RFbias supply

(13.56 MHz)

wafer

gas outlet,pump

Microwavesupply

(2.45 GHz)

In conventional RIE (10-100 mtorr),• For more directed etching, need stronger ion bombardment. Possible more damage to substrate.• Lower pressures can be used to attain more directional etching and less microloadingeffect, but reduces the plasma density and thus the etching rate.

In HDP etching (1-10 mtorr) ,• Uses separate RF source as wafer bias. This separates the plasma power (density), from the wafer bias (ion accelerating field).• High density plasma can be obtained at low pressure which improves both the directional etching and reduces the substrate damage while keeping the compatible etching rate.

25

Endpoint Detect• Each atom has its own emission wavelength• Color of plasma changes when etch

different materials• Optical sensors can be used to detect the

change and indicate the endpoint for plasma etch processes

excited species`

excited species``

hvhv

26

Residue• Unwanted leftovers• Causes

–insufficient over etch–non-volatile etch byproducts

• Sufficient ion bombardment to dislodge• Right amount of chemical etch to scoop

• Oxygen plasma ashing: Organic residues• Wet chemical clean: inorganic residues

27

Post Etch Clean• Remove PR

– O2 plasma ashing (typical method)– UV + O3

– H2SO4: H2O2=3:1 or 5:1 (before metal deposition)– Solvents rinse (after metal deposition)

• Remove polymer– H2SO4: H2O2=3:1 or 5:1 (before metal deposition)– Solvents rinse (after metal deposition)– H2SO4+H2O2+HF(ppm)+H2O

• Remove radicals– H2O rinse– Solvents rinse

• Remove particles– SC1 + megasonic clean– Scrubber clean

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100

O2/He Ratio (%)

Ash

ing

Rat

e (u

m/m

in)

0

5

10

15

20

25

Uni

form

ity (%

)