Dry Etch

Question 1

Etch rate can:

Answer 1

• vary across a substrate
• can change with etch depth
• can vary with etch direction

Question 2

Mean free path is:

Answer 2

The average distance travelled by a particle between interactions with other particles. It is dependent on pressure, particle diameter and temperature

Question 3

What are four types of dry etching?

Answer 3

1) Sputtering (directional etch)
2) Chemical (isotropic etch)
3) Ion-enhanced energetic (vertical etch)
4) Ion-enhanced inhibitor (vertical etch)

Question 4

Isotropic etching is

Answer 4

The same in all directions

Question 5

Sputtering (Ion milling) uses

Answer 5

high energy ions to bombard the material and do ballistic damage

Question 6

What are the benefits and disadvantages of sputtering?

Answer 6

Benefits: mechanical, not chemical so it can etch almost any material
Disadvantages:
- slow
- causes crystal damage
- poor selectivity
- requires higher vacuum

Question 7

What are the benefits and disadvantages or chemical plasma etch processes?

Answer 7

Benefits:
- can achieve excellent material selectivity
- Isotropic or weakly anisotropic
Disadvantages
- isotropic
- loading induced anisotropy

Question 8

Vapor phase etching benefits and disadvantages

Answer 8

Benefits:
- Chemical not mechanical, can achieve excellent selectivity
- Isotropic or anisotropic
- Inexpensive
- doesnt require plasma
Disadvantages
- loading induced anisotropy
- dangerous gaseous byproducts such as HF

Question 9

Reactive Ion etching uses

Answer 9

Both plasma and vapor phase etching, by accelerating reactive species in a plasma towards a substrate. Crystalline / mechanical damage associated with the ion bombardment enhances the chemical etching.
- Mixtures of gasses are often used to simultaneously etch and passivate (increases anisotropy, by protecting side walls with passivating compounds)

Question 10

Deep reactive Ion Etching (DRIE)

Answer 10

Uses passivation to increase the anisotropy of the etching process, enables AR up to 30:1.
Cryogenic:
- cools substrate, etch products get stuck to substrate side walls improving passivation
- large thermal stresses
Bosch Process
- Repeated alternating etch and deposition steps
- Uses a deposited polymer to protect sidewalls while etching bottom with ion bombardment

Question 11

Issues with DRIE

Answer 11

• side wall scalloping
• Rough side walls, potential for crack propagation
• high AR makes it stiff
• High AR etches very slowly