1. ECE/ChE 4752: Microelectronics Processing Laboratory
Etching
ECE/ChE 4752: Microelectronics Processing Laboratory
Gary S. May
February 19, 2004

2. Outline
Introduction
Wet Chemical Etching
Plasma Etching

3. 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

4. Etching Hierarchy

5. Outline
Introduction
Wet Chemical Etching
Plasma Etching

6. Uses
Prior to thermal oxidation or epitaxial growth, wafers are chemically cleaned to remove contamination
Especially suitable for blanket etches (i.e., over the whole wafer surface) of polysilicon, oxide, nitride, metals, and III-V compounds.

7. Mechanism Reactants transported by diffusion to surface
Reactions occur at surface
Products from surface removed by diffusion

8. Uniformity
“Equality” of vertical etch rates at different sites on the wafer surface
This is actually non-uniformity
Alternative definitions:
s/m

9. Silicon Etching
Most etchants are mixtures of HNO3 and HF in water or acetic acid (CH3COOH).
HNO3 oxidizes silicon to form an SiO2 layer:
Si + 4HNO3 → SiO2 + 2H2O + 4NO2
HF is used to dissolve the SiO2 layer:
SiO2+ 6HF → H2SiF6 +2H2O
Water can be used as a diluent for this etchant, but acetic acid is preferred.

10. 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.

11. 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].

12. 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:
SiO2+ 6HF → H2SiF6 +2H2O
SiO2 can also be etched in vapor-phase HF.

13. Outline
Introduction
Wet Chemical Etching
Plasma Etching

14. where RL = lateral etch rate, RV = vertical etch rate
Anisotropy
Vertical features are desirable to increase circuit density.
Quantitatively:
where RL = lateral etch rate, RV = vertical etch rate

15. 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.

16. 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

17. Etch Equipment

18. Etch Mechanism Etchant species generated in plasma.
Reactant transported by diffusion to surface.
Reactant adsorbed on the surface.
Chemical reaction (along with ion bombardment) forms volatile compounds.
Compounds desorbed from surface, diffused into the bulk gas, and pumped out by vacuum system.

19. 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.

20. Laser Interferometry
Intensity of laser light reflected off thin film surface oscillates.
Period of the oscillation related to change in film thickness
where Dd = change in film thickness, l is the wavelength, and is the refractive index

21. Interferometry Example
Typical signal from a silicide/polycrystalline Si gate etch
Dd for polysilicon = 80 nm (measured by using a He-Ne laser with l = nm)