Defects

Question 1

What is wafer yield?

Answer 1

(starting # of wafers - rejected wafers) / (starting # of wafers)

Question 2

What is die yield?

Answer 2

The proportion of good dice relative to the total number of dice in the wafer, before packaging

Question 3

What is assembly yield?

Answer 3

The yield after packaging

Question 4

What is burn-in?

Answer 4

Some proportion of dice will fail at an early stage. Burn-in is a series of electrical tests meant to accelerate the failure of these devices, so that they can be found

Question 5

What is the total yield?

Answer 5

The product of the:

1. Wafer yield
2. Die yield
3. Assembly yield
4. Burn-in yield

Question 6

What types of yield loss are there?

Answer 6

1. Parametric yield loss - errors in measurable targets such as film thickness, dopant concentration, feature size, etc
2. Defect yield loss - yield loss due to defects. Systemic defects are common in new processes and can be eliminated, but some extent of random defects also occur. Defects include contaminants, electrostatic discharge, crystal defects, scratches, etc.

Question 7

How are defects prevented by controlling the process?

Answer 7

1. Environment - clean rooms, HEPA filters, “bunny suits”
2. Automation of handling
3. Monitor defects to identify and eliminate sources

Question 8

Based on the defect model, how can defects be decreased?

Answer 8

1. Reducing the critical area (“design for reliability”). For example, this can be achieved by wire spreading or by depositing redundant vias
2. Reducing the defect density by classifying, analyzing, and working to prevent defects. The defects must be found, classified, and analyzed to understand at what stage they developed