Physical Design Engineer Interview Questions & Answers

Question 1. Why Metal Density Rules Are Important?

Answer :

Metal Density rules take care of metal over-etching and metal lift off issues encountered durinf manufacturing process.

Question 2. Why Power Stripes Routed In The Top Metal Layers?

Answer :

Power routes generally conduct a lot of current. In order to reduce effect of IR drop, we need to make these routes less resistive. Top metal layers are thicker and offer lesser resistance. This helps to reduce IR drop.

Question 3. Types Of Checks That Can Be Done In Prime Time ?

Answer :

Timing (setup, hold, transition), design constraints, nets, noise, clock skew and analysis coverage.

Question 4. How Do You Validate Your Floorplan And What Analysis You Do During Floorplan?

Answer :

1. Overlapping of macros.
2. Global route congestion -> in order to finalize Min. Channel spacing.
3. Allowable IR drop.
4. Physical information of the design (report_design_physical)

Question 5. How Many Clocks You Had In Your Designs? How Did You Do Cts For The Same?

Answer :

I had 5 clocks in my designs, sys_clk, sys_rclk, uart_clk, g_clk and scan_clk, where sys_clk, g_clk and uart_clk logically exclusive to scan_clk.

Question 6. Did You Get Antenna Problem In Your Project For All The Metal Layers? How Did You Fix Them?

Answer :

Metal Jumper and Antenna diode are two methods to resolve Antenna violations. But Metal Jumper is preferred approach as it does not need change to the Netlist and placement. This methodology works for antenna violations on all metal layers except for the top most layer. In this methodology, we will switch the small portion of routing to higher level metal close to the location of failing gate. This will make sure that accumulated charges on metal layer does not affect the gate as gate will not be connected to the charge carrying metal route until higher level metal is manufactured.

For example, lets say antenna violation is in M2. This means that M2 has enough area to accumulate large charge that induces high electron voltage to destroy the gate. To solve this problem, we cut a portion of M2 close to failing gate and move the routing to M3. This makes sure that when M2 is being manufactured, it does not get connected to gate. Connection happens only when M3 gets manufactured which is much later in time. By then charges on Metal M2 would have leaked away.

When metal jumper is not possible to implement (probably due to routing congestion or violation happening in top most layer) we try to fix it by inserting antenna diode closed to gate failing antenna. Antenna diode provide electrical path for safe conduction of accumulated charges to the substrate. Antenna diode is a reversed biased diode but acts like resistor during manufactured process (CMP) due to high temperature environment.

Question 7. How Do You Reduce Power Dissipation Using High Vt And Low Vt On Your Design?

Answer :

1. Use HVT cells for timing paths having +ve slacks.
2. Use LVT cells for timing paths having -ve slacks.

 HVT cells have a larger delay but less leakage. +ve slack in a design is not useful as having only some paths working faster will not help overall design. We are good if the slack is 0. In such cases give up the slack by using HVT cells but gain on power dissipation.

 LVT cells are very fast but very leaky. Limit the use of LVT cells to only those paths that have difficulty in closing time.

Question 8. What Is Electromigration And How To Fix It?

Answer :

Electromigration (EM) refer to the phenomenon of movement of metal atoms due to momentum transfer from conducting electrons to metal atoms. Current conduction over a period of time in a metal route causes opens or shorts due to EM effect. EM effect cannot be avoided.

 In order to minimize its effect, we use wider wires so that even with EM effect wire stays wide enough to conduct over the lifetime of the IC.

Question 9. What Are The Various Statistics Available In Ir Drop Reports?

Answer :

1. IR drop info for VDD/ VSS.
2. Maximum current through VDD/VSS.
3. Number of current sources for VDD/VSS.
4. Utilization of metal layers used.
5. EM information for signal and via.

Question 10. What Is The Importance Of Ir Drop Analysis?

Answer :

IR drop determines the level of voltage at the pins of standard cells. Value of acceptable IR drop will be decided at the start of the project and it is one of the factors used to determine the derate value.

 If the value of IR drop is more than the acceptable value, it calls to change the derate value. Without this change, timing calculation becomes optimistic. For example setup slack calculated by the tool is less than the reality.

Question 11. In Which Field Are You Interested?

Answer :

• Answer to this question depends on your interest, expertise and to the requirement for which you have been interviewed.
• Well..the candidate gave answer: Low power design

Question 12. Can You Talk About Low Power Techniques? How Low Power And Latest 90nm/65nm Technologies Are Related?

Answer :

Refer here and browse for different low power techniques.

Question 13. Do You Know About Input Vector Controlled Method Of Leakage Reduction?

Answer :

Leakage current of a gate is dependant on its inputs also. Hence find the set of inputs which gives least leakage. By applyig this minimum leakage vector to a circuit it is possible to decrease the leakage current of the circuit when it is in the standby mode. This method is known as input vector controlled method of leakage reduction.

Question 14. How Can You Reduce Dynamic Power?

Answer :

• Reduce switching activity by designing good RTL
• Clock gating
• Architectural improvements
• Reduce supply voltage
• Use multiple voltage domains-Multi vdd

Question 15. What Are The Vectors Of Dynamic Power?

Answer :

Voltage and Current

Question 16. How Will You Do Power Planning?

Answer :

Refer here for power planning.

Question 17. If You Have Both Ir Drop And Congestion How Will You Fix It?

Answer :

• Spread macros
• Spread standard cells
• Increase strap width
• Increase number of straps
• Use proper blockage

Question 18. Is Increasing Power Line Width And Providing More Number Of Straps Are The Only Solution To Ir Drop?

Answer :

• Spread macros
• Spread standard cells
• Use proper blockage

Question 19. In A Reg To Reg Path If You Have Setup Problem Where Will You Insert Buffer-near To Launching Flop Or Capture Flop? Why?

Answer :

• (buffers are inserted for fixing fanout voilations and hence they reduce setup voilation; otherwise we try to fix setup voilation with the sizing of cells; now just assume that you must insert buffer !)
• Near to capture path.
• Because there may be other paths passing through or originating from the flop nearer to lauch flop. Hence buffer insertion may affect other paths also. It may improve all those paths or degarde. If all those paths have voilation then you may insert buffer nearer to launch flop provided it improves slack.

Question 20. How Will You Decide Best Floor Plan?

Answer :

Refer here for floor planning.

Question 21. What Is The Most Challenging Task You Handled? What Is The Most Challenging Job In P&r Flow?

Answer :

• It may be power planning- because you found more IR drop
• It may be low power target-because you had more dynamic and leakage power
• It may be macro placement-because it had more connection with standard cells or macros
• It may be CTS-because you needed to handle multiple clocks and clock domain crossings
• It may be timing-because sizing cells in ECO flow is not meeting timing
• It may be library preparation-because you found some inconsistancy in libraries.
• It may be DRC-because you faced thousands of voilations

Question 22. How Will You Synthesize Clock Tree?

Answer :

• Single clock-normal synthesis and optimization
• Multiple clocks-Synthesis each clock seperately
• Multiple clocks with domain crossing-Synthesis each clock seperately and balance the skew

Question 23. How Many Clocks Were There In This Project?

Answer :

• It is specific to your project
• More the clocks more challenging

Question 24. How Did You Handle All Those Clocks?

Answer :

• Multiple clocks-->synthesize separately-->balance the skew-->optimize the clock tree

Question 25. Are They Come From Separate External Resources Or Pll?

Answer :

• If it is from separate clock sources (i.e.asynchronous; from different pads or pins) then balancing skew between these clock sources becomes challenging.
• If it is from PLL (i.e.synchronous) then skew balancing is comparatively easy.

Question 26. Why Buffers Are Used In Clock Tree?

Answer :

To balance skew (i.e. flop to flop delay)

Question 27. What Is Cross Talk?

Answer :

Switching of the signal in one net can interfere neigbouring net due to cross coupling capacitance.This affect is known as cros talk. Cross talk may lead setup or hold voilation.

Question 28. How Can You Avoid Cross Talk?

Answer :

• Double spacing=>more spacing=>less capacitance=>less cross talk
• Multiple vias=>less resistance=>less RC delay
• Shielding=> constant cross coupling capacitance =>known value of crosstalk
• Buffer insertion=>boost the victim strength

Question 29. How Shielding Avoids Crosstalk Problem? What Exactly Happens There?

Answer :

• High frequency noise (or glitch)is coupled to VSS (or VDD) since shilded layers are connected to either VDD or VSS.
• Coupling capacitance remains constant with VDD or VSS.

Question 30. How Spacing Helps In Reducing Crosstalk Noise?

Answer :

width is more=>more spacing between two conductors=>cross coupling capacitance is less=>less cross talk

Question 31. Why Double Spacing And Multiple Vias Are Used Related To Clock?

Answer :

• Why clock.-- because it is the one signal which chages it state regularly and more compared to any other signal. If any other signal switches fast then also we can use double space.
• Double spacing=>width is more=>capacitance is less=>less cross talk
• Multiple vias=>resistance in parellel=>less resistance=>less RC delay

Question 32. How Buffer Can Be Used In Victim To Avoid Crosstalk?

Answer :

Buffer increase victims signal strength; buffers break the net length=>victims are more tolerant to coupled signal from aggressor.