Etching

1
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

1. Reactants transported by diffusion to surface
2. Reactions occur at surface
3. 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
  100161.

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
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 1960s and 70s

17
Etch Equipment
18
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.

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 632.8 nm)