Resistance

Question 1

What are two applications of resistors?

Answer 1

1. Current limiting (regulate amount of current)

2. Voltage division

Question 2

What is Ohm’s Law?

Answer 2

V = IR, or R = V/I or I = V/R

Question 3

How to calculate resistance of resistors in series?

Answer 3

Rab = R1 + R2 + R3

Question 4

How to calculate resistance of resistors in parallel?

Answer 4

1/Rab = 1/R1 + 1/R2 + 1/R3

Question 5

1. What are three names for what ρs stands for?

1. What is the standard unit for ρs?

Answer 5

1. Sheet rho, sheet resistivity, sheet resistance.

1. Ohms per square.

Question 6

What are Kirchoff’s Laws?

Answer 6

Kirchoff’s Voltage Law = Vt = V1 + V2 + V3 + …

Kirchoff’s Current Law = It = I1 + I2 + I3 + …

Question 7

What is the basic sheet resistivity equation?

Answer 7

Rab = ρs * (L / W) Ω

Question 8

What are the two main types of fabricated resistors?

Answer 8

1. Deposited resistors such as poly or thin-film resistors.

2. Implanted resistors such as N+, P+, N- (nwell) and P- (pwell) resistors.

Question 9

What is a “device recognition layer”?

What are common names for resistor and capacitor device recognition layers?

Answer 9

1. A layer commonly added on layouts to help programs such as LVS and DRC to recognize certain devices.
2. rmask or rdef (resistor definition layer), cmask or cdef

Question 10

What is a thin-film resistor commonly made out of?
What is the typical range of sheet rho?
What is an advantage?
What is a disadvantage?

Answer 10

1. nickel- or silicon-chromium.
2. a few to several thousand ohms per square.
3. On same level as a metal, so no contacts needed.
4. Additional costly specific process steps.

Question 11

What are two ways to increase the accuracy of a resistor when using the basic resistor equation?

Answer 11

1. Avoid using narrow diffusions, to avoid large out-diffusion effect.
2. Use more terminal contacts to reduce ohmic contact resistance.

Question 12

What are two issues that complicate the basic resistance equation?

Answer 12

1. The out-diffusion effect or “etching effect” (diffusion tends to be wider than drawn)
2. The ohmic contact resistance (the contacts have some resistance as well).

Question 13

What is the more advanced resistance equation?

Answer 13

Rab = (ρs * (L / W+2ΔW)) + 2Rcontact Ω

Question 14

What equation describes the switching speed of a logic? AKA propagation delay?

Answer 14

Fs = 1 / 2RC
Fs = Switching frequency (per second?) Propagation delay
R = Load resistance
C = Load capacitance
delta t = RC (RC time constant)

Question 15

Name five ways to reduce parasitic resistance

Answer 15

1. Use good conductors to form connections, especially for long and heavily-loaded lines. Higher layer metals are preferred because they are thicker (reduced sheet rho) and less capacitance with surrounding layers.
2. Shorting bar technique, strap poly with metal.
3. More contacts and vias
4. Minimum spacing between diffusion contacts and active gate area.
5. Fabricate low resistance polys (silicidation).

Question 16

1. What metals are typically used for silicidation?

2. What are the resulting polys or diffusions called?

Answer 16

1. Tungsten, titanium, molybdenum

2. Silicided or clad polys, clad diffusions or moats

Question 17

What are two common names for the method of ignoring silicide to create resistors?

Answer 17

1. Silicide-Block Layer

2. RPO (Resist Protection Oxide) in TSMC design rules

Question 18

What does Ids stand for?

Answer 18

Current from source to drain.

Question 19

What are the terms for running metal over, or running diffusion under?

Answer 19

“jumper” and “tunnel”

Question 20

What is the name for a “fingered” resistor?

Answer 20

“Segmented” resistor.

Question 21

What the name for moving a chip to a new technology?

What are easier to migrate, resistors or capacitors, and why?

Answer 21

1. “Technology migration”.

2. Resistors, because the resistance equation is based on ratios that don’t change when you scale them.

Question 22

What is an advantage of implanted resistors over deposited resistors?

Answer 22

They disperse heat better.

Question 23

What is a case when you will create long diffusions?

Answer 23

Guard rings

Question 24

What does RPO stand for?

Answer 24

Resist Protection Oxide (for creating non-silicided materials)