CMP

Question 1

Why is planarization needed?

Answer 1

Patterning leaves topography on the wafer. If the topography is not planarized, subsequent steps will be affected by poor step coverage, shadowing, and loss of DOF in lithography.

Also, sometimes it is more economical to deposit a large amount of material and then use CMP to remove excess

Question 2

What processes use planarization?

Answer 2

1. Deposition of W plugs
2. Shallow trench isolation
3. Cu dual Damascene process
4. Ensure the precise thickness of oxide layers + improve deposition of future layers

Question 3

What local planarization techniques exist? What issues exist with them?

Answer 3

1. Reflow - Add dopants to glass to lower Tg and reflow over layer. This provides “surface smoothing” that has bumps where it is deposited over topograpy
2. Etchback - coat the wafer with a planarizing resist, then sputter etch. This proves “local planarization” where areas with topography are higher than other regions

Question 4

What type of planarization is achieved by CMP?

Answer 4

Global planarization - the entire surface is a single, even, level layer

Question 5

How is CMP carried out?

Answer 5

The wafer is placed on a rotating table, and a polishing head/pad rotates and polishes the wafer, using a slurry with some chemical and abrasive element.

The pad and the wafer rotate in the same direction and different speeds.

Question 6

What factors control the polish rate in CMP?

Answer 6

1. The relative rotation speed of the wafer and the polishing pad
2. The pressure

As the speed difference increases, polish rate increases but uniformity decreases.

Question 7

What is the polishing pad made of?

Answer 7

Some type of half-rigid polymer, with pores or grooves.

Question 8

What is the CMP slurry made of generally? How is it composed for metals and oxides?

Answer 8

1. An abrasive element - “mechanical polish”
2. A chemical element - “chemical polish”

For metals: alumina and low pH
For oxides: silica and high pH

Question 9

What issues can arise with CMP? How are they caused?

Answer 9

1. Dishing - over-polish that occurs when the plug has a higher polish rate than the surrounding material
2. Erosion - removal of unwanted material. CMP is moderately selective due to the chemical element, but the abrasive element is non-selective

Question 10

What is the purpose of a pad conditioner?

Answer 10

Keeping the pad surface rough