Problems and Issues

Question 1

What’s a “soft short”?

Answer 1

A short between two or more signals through high resistive materials such as nwell or p-substrate.

Question 2

What’s a “soft connect” or “weak connect”?

Answer 2

A “correct” connection between two or more signals through high resistive materials such as nwell or p-substrate. It will not create an LVS problem, but may cause circuit failure.

Question 3

What is the “antenna effect”? What two parts of the manufacturing process can cause the antenna effect?

Answer 3

Due to plasma etching or ion implantation equipment, charges can build up on circuit areas and then discharge, causing permanent damage to physical structures, especially gate oxide.

Question 4

How can you tell if a layout may have an antenna effect problem?

Answer 4

Run the verification tool known as the Antenna Rule Check.

Question 5

What calculation does the Antenna Rule Check generally use?

Answer 5

The “antenna ratio”, which is the ratio between the physical area of an interconnect to the total gate oxide area to which the interconnect is connected.

Question 6

What are two common solutions to an Antenna Rule Check failure?

Answer 6

1. Metal jumper insertion: break up a long wire and connect with higher-layer metals.
2. Antenna Diode Insertion: Add protection diodes, or “Antenna Diodes”. If the voltage along the wire reaches the breakdown voltage (approx. -8V) Avalanche Breakdown occurs and causes the PN junction to short and bleed-off the charges to the grounded substrate.

Question 7

What’s the electro-migration issue?

Answer 7

Current density can move atoms in conductor.

Question 8

What’s the typical current density minimums?

Answer 8

Metal <= .5mA / um

Question 9

What are two solutions to electro-migration issue?

Answer 9

Wider wires / higher metal layer (thicker)

Question 10

What is the solution to a layer density problem?

Answer 10

Dummy layer pattern filling.

Question 11

What’s the difference between “global density” and “local density”?

Answer 11

Global = die level, local = portions of die (100um X 100um).

Question 12

What’s the metal stress issue?

Answer 12

Due to thermal stress, wide metal traces (>35um) can develop cracks.

Question 13

What the solution to the metal stress issue?

Answer 13

Adding metal slots

Question 14

What are typical metal slot rules?

Answer 14

Metal slots must be placed for line > 35um.
Minimum slot density (slot area / total metal area) >= .015
Slot width > 1um
Slot height > 10um
Slot spacing > 1um

Question 15

What’s the typical Percentage of Utilization for poly and metals?

Answer 15

30% < density > 80%

Question 16

How do metal slots affect branches?

Answer 16

Branches must be between slots

Question 17

What are the terms referring to a guard ring that’s fully strapped with contacts to M1, vs. a ring where the M1 strap is broken in places to allow M1 lines to run into the ring?

Answer 17

“Hard tie” and “soft tie”.