A blog to explore whole VLSI Design, focused on ASIC Design flow, Physical Design, Signoff, Standard cells, Files system in VLSI industry, EDA tools, VLSI Interview guidance, Linux and Scripting, Insight of Semiconductor Industry and many more. Happy learning!
08 September
Interview questions for experienced Physical Design Engineer, Question set - 9
04 August
Physical Design Interview Questions for 3 years experience , Question set - 8
Code: EXIM4Y062021PD
- Brief Introduction and major projects?
- Tell me the most challenging part of your recent project
- How does the lockup latch help to fix hold violations?
- If we add a lockup latch, it might violate the setup? How will we fix it further?
- How did you fix SigEM? What are patch wires?
- What CTS constraints have you used?
- How did you fix the setup violation?
- Apart from setup and hold, what other checks do we perform in timing signoff?
- What are the PV checks?
- What are the sanity checks we do before starting PnR?
- What are the reports of synthesis we check before PnR?
- What are the physical cells we have used in PD and what are the uses of all those?
- What is the latch-up issue and how well tap cells prevent latchup?
- What is the endCap cell and what is the purpose of using that?
- What is Dcap Cell and why do we use it?
- What is the antenna effect?
- What are the ways to fix the antenna effect?
- How do antenna diodes help to fix the antenna violations?
- If we have timing criticality and we can't use antenna diodes or floating gates, How can we fix the antenna?
- If antenna violation is already the highest metal layer and we can use higher metal for metal hopping, how will fix the antenna?
- How will you fix the antenna violations on via?
- What is a metal cut layer?
- What is the crosstalk delay?
- What is the crosstalk noise?
Physical Design Interview Questions : Question set -7
Code: CDN5Y062021PD
Experience level: 5 YearsFor Application Engineer
- What are the major differences between 7nm and 12/14nm technology nodes?
- What are the new DRC rules in the 7nm technology node?
- What is the via-piller?
- What is double patterning?
- How many layers have double patterning in the 7nm node?
- How tool performs placement steps?
- Why do we perform scan chain reordering?
- What is scan mode, why do we need that?
- What is ECF (Early Clock Flow) flow?
- What are the benefits of ECF flow?
- Can you explain the CTS flow?
- What are the low power techniques used in data and clock paths?
- Where does the clock-gater use?
- Have you built a custom clock tree?
- What are the constraints you have given to the clock tree?
- How did you solve max_trans violations in the clock path?
- How to provide different clock tap points in H-Tree?
- How many clocks were there in your block?
- How were they related?
- How did you analyze the clock domain crossing paths?
- What is a lock-up latch and how does it helps in hold fixing?
- What was the target skew in your block?
- What value of skew you achieved?
28 April
Interview questions asked for DFT Engineer (Fresher) - Question Set - 07
Code: INTL0Y032021DFT
This interview was held for the position of DFT Engineer with 0 years of experience. I personally felt the questions are good and generic which will help the freshers in other domains too.
Round-1:
1) Differences between MOSFET and FINFET ?
2) Puzzle: A blind man walking in a desert has 2 red pills and 2 blue pills with him. He has to take one red pill and one blue pill each per day. How can he do that correctly?
3) Limitations of MOSFET
4) Puzzle: If a man climbs 15 m well. He climbs 4 m every day and slips 3 m . How many days it takes for him to get outside of the well?
5) What is set up time and hold time?
6) What is set up time and hold time violations?
7) What is metastability and do you know its physical significance?
8) What is a critical path?
9) How can you improve the timing?
10) How can you fix the setup time violations?
11) Implement a 2:1 mux for AND gate?
12) Draw the timing diagram of half adder?
13) What is blocking and non-blocking assignments?
14) What is synchronous and Asynchronous reset?
15) Why do we need DFT and what do you mean by that ?
16) Explain briefly the methods of DFT?
17) Why do you need to make flip flops initialized in an ad hoc technique?
18) What are controllability and observability?
19) If you fabricate a full adder and do the functional test and verify outputs are correct, then do you still require DFT also?
20) What is BIST?
21) What is JTAG?
22) Difference between verification and DFT?
23) What is yield?
24) Explain your course works and the projects.
25) Difference between RAM, ROM.
26) What is DMA?
1) Short channel effects in MOSFET.
2) Brief about JTAG.
3) Any insight into BSDL.
4) What are the different faults that occur while fabrication?
5) What is bridging fault?
6) What is fault collapsing?
7) Draw a D-flipflop along with a timing diagram.
8) What is Stuck at faults?
9) What do you mean by clock skew?
10) Is Hold time dependent on clock frequency?
11) Is clock skew an advantage or not?
12) Current technology node used in industry?
13) Is set up time-dependent on clock frequency?
14) Why do you need a reset in flip flop?
15) What is the multicycle path?
16) Any tools for DFT?
17) Why DFT is in the front end?
18) VLSI design flow.
19) OR gate using 2:1 mux
20) How to do if statement synthesis?
21) How do a case statement in Verilog synthesis?
22) What is Inferred latch?
23) List out a few Power optimization methods in STA.
13 December
Written Test Question for Physical Design Engineer: Question Set - 6
Code: ALTRN0Y112020PD
Some companies take a written test sometimes to shortlist the candidates before the interview process. This is a common process if the applicants are large in numbers. In such written test, the format is MCQ and some short questions.
We are very thankful to one of our follower who has appeared in this test and shared these questions based on memory. The purpose of sharing this question is only to provide the right guidance to the candidate who is going to appear in such a written test. I would encourage all the freshers to practice these question for their future test/Interview. I would also encourage to find the answer to these question either by own. There is no sense of proving the answer to these questions here. But if someone finds any question really difficult to solve and open discussion in the comment section.
1. Hold slack equation
a) half cycle pathb) single cycle pathC) multicycle pathd) it never depends
a) max launch path delay and min capture path delayb) min launch path delay and max capture path delayc) max launch and capture path delayd) min launch and capture path delay
a) decreasing the space between metal layersb) decreasing strapsc) increasing drive strength of the driverd) increasing space between metal layers
a) to maintain well continuityb) to prevent latch upc) to prevent DRC'sd) all of the above
a) to ensure etching properlyb) to ensure density issuesc) to increase the area of the chipd)to ensure well continuity
a) opensb) shortsc) ERCd) all of the above
a) setup violationb) hold violationc) DRcsd) all of the above
a) trueb) falsec) based on SDCd) based on clock frequency
a) trueb) false
a) static stateb) when coolingc) when warmingd) all of the above
a) ratio of required layers and available layersb) ratio of available and requiredc) depending on availability of metalayersd) none of the above
a) doping concentrationb)distance between source and drainc) temperatured) all of the above
a) isolation cellsb) retention cellsc) level shiftersd) all
a) double BACK + channel width spacingb) without double back + channel width spacingc) double back + row alignmentd) none of the above
a) time borrowingb) managing skewc) maintaining timingd) none of the above
a) macro + blockage + standard cell (area)b) macro + blockage (area)c) macro + physical cells + standard cells (area)d) all of the above
a) spice + netlist + rule deckb) spice + rule deck + lib filesc) OASIS + netlist + libd) none of the above
a) 1 and 3b) 2 and 3c) 3 and 4d) none
a)to ensure chip fabricationb)to ensure chip doesn't get heatedc)to verify parasitics on chipd) all of these
a) to protect the gateb) protect VDD and VSSc) to protect the substrated) all of the above
a) to fix timing violationsb) to fix DRCc) to fix parasitic valuesd) all
TrueFalse
a) delay of net decreasesb) delay of net increasesc) delay doesn't effectd) none of the above
a)timing failureb) functional failurec) substrate failured)all of these
a) gate array filler cellsb) filler cellsc) metal fillsd)decap cells
a) cell which doesn't have any functionalityb) cell is not synthesizedc) cell which is inserted only in layoutd) all of the above
a)static powerb)leakage powerc)internal powerd) all of the above
a) max capb) max transc) max current densityd) max fanout
a) ground bounceb) supply bouncec) glitchD) none
a)Hvt to Svtb) Lvt to HvtC)Rvt to Lvt.d) svt to lvt
a) floating inputsb) floating outputsC) floating metalsd) both b and c
a) 80 psb) 100 psc) 20 plsd) 180 ps
a) 25b) 10c) 2d) 16
a) serialb) digitalC) slow speedd) none
a)z- routeb)s-routec)trail routed) nano- route
a) empty listb) [a b]c) [a b c]d) none
[ Note: If you find any question has typo error or wrong, comment for the correction.]
Thank you.
Synthesis and Physical Design Interview Questions: Question Set -5
Code: CYPR2Y102020PD
Introduction and Experiences
- Self Introduction
- Explain about the projects that you have worked on. (Type of work and tools used)
Synthesis
- Explain about Synthesis flow and what happens at each stage. (Inputs required, elaboration, generic stage, mapping and optimization stages)
- Explain about Synthesis Inputs.
- Differentiate between Logical and Physical Synthesis. (QoR impact between them)
- Wire load model(WLM), Mode, Types of trees
- Delay Calculation in WLM method. (Fanout based delay calculation)
Place and Route
- What are Inputs for PnR and Initial Checks need to be done?
- How to check for uniqueness of Netlist?
- Explain stages in PnR.
- Explain about useful skew and how it impacts the design.
- How do we achieve a better insertion delay?
- Explain about the CTS issues that you solved.
- Explain Physical Cells at the transistor level.
- Explain UPF, power domains, supply sets, isolation cells, retention registers.
- Feedthru insertion procedure and minimizing them. How you did Partitioning and improved partition related size, ports creation, Congestion.
RTL
- RTL Code for synchronous rst and asynchronous rst. (How do we write always block for this?)
- How do you write RTL code for FSM? (Explain about the number of always blocks required and significance of each one)
- Write RTL code for the traffic light system.
SignOff
- Explain about the LEC procedure and issues faced and solved with respect to non-equivalent points.
- How do we fix Setup and Hold time violations?
- Explain about Dynamic and Leakage Power Consumption and methods to reduce them.
- If the chip is fabricated and hold violation exists. So what will you do? (other than saying that chip won't work)
- Scripting related to finding empty modules, Unix commands.
Post Credit:
These questions are shared by one of our active group members. Thanks a lot for your contribution!!!
04 October
Physical Verification Interview Questions : Question set - 4
Code: QLCM2Y062020PV
General Questions
- What are lambda based design rules?
- What is nm in 10nm technology node?
- Do you know about under bum density?
- Any prior experience of PnR?
- How do you import design in PnR?
- Why ID layers have been given in lower nodes (10nm)?
- Why nwell continuity is required?
- Have you done scripting?
- Can you write an algorithm?
- What is drive strength?
- Why the industry is moving towards lower technology nodes like 7nm or 5nm?
- What are challenges seen in lower technology node?
Physical Only Cell
- What is a tap cell?
- What are the uses of tap cells?
- What is Endcap or boundary cell?
- How does halo cell avoid DRCs t boundary? What is there inside the halo cell that prevents DRCs ? How it is different from standard cells?
- What difference it makes when you add halo cell in boundary instead of standard cell?
- Why is the standard cell nwell bigger in size and slightly coming out of the standard cell? It is not so in halo cell.
- Suppose a tap cell is covering 10um distance, then where should the next tap cell be placed in the same row?
- How do you calculate the distance between tap cells in a row?
DRC
- What kind of base DRCs have you encountered?
- Have you seen DRCs between the standard cell and filler cell of the same height?
- What is Endcap or boundary cell?
- How did you solve DPT (double patterning)?
- What metal DRC have you encountered?
- What kind of via DRCs have you come across?
- What will happen if GDSII has DRC unfixed and sent to Fab?
- Is there any problem if the ID layers are interchanged?
- Which ID layer is manufactured first?
LVS
- Have you analyzed the LVS result or someone else analyzed the result and you fixed?
- Apart from short and opens what errors have you seen in LVS and How did you resolve?
- What kind of issues are seen in LVS?
- What is priority errors in LVS? Could there be false violations?
- Where does the tool start to calculate LVS from?
Post Credit:
These questions are shared by Karthik K Umesh, one of our group member. Thanks, Karthik for sharing and helping people.
6.
11
Physical Verification Interview Questions : Question set - 3
Code: INTL2Y052020PV
Floorplan
- What are the inputs for floorplan?
- In order to make sure integration is DRC clean, what rules or guidelines need to be followed at the floorplan stage?
- Can we abut macros on par boundary?
- How will judge the congestion between two IPs during floorplan stage without actual routing being done?
- Apart from IP alignment, what analysis have you done?
- What do you check at the placement stage?
Physical Only Cell
- What are the types of physical only cells in the design?
- In what stage are physical only cells placed in the design?
- Why are different types of physical only cells needed in design?
Latch-up
- Are you aware of latch-up in CMOS, can you elaborate?
- Explain how the parasitics are formed?
Antenna Effect
- What is the antenna effect?
- How can antenna effect be solved?
- Why metal jogging is done in the higher layer during antenna fixing?
- Which diode is used as an antenna diode?
- What parameters are considered while choosing the cutoff voltage of an antenna diode?
- How do you decide the number of diodes that needs to inserted for the failing net?
- On what basis have you inserted diodes?
- If M12 is the highest metal in the design and it has antenna, how do you fix it?
- Have you heard of nwell antenna?
- Can power net have an antenna effect?
- Can the antenna diode be placed in a different power from that of the affected cell?
- What is the impact of placing the antenna diode on timing?
- Where are the antenna diode terminals being connected?
- Voltage or current that is causing the antenna effect?
- How the gate area can be increased in antenna affected cell?
- How are you adding the antenna in antenna affected cell?
- What is the drive strength?
- What is the W/L ratio?
- Which layers are most susceptible to antenna violations, higher or lower layers?
- How do you handle antenna in clock net?
ECO Implementation
- What are types of ECOs?
- Where the ECOs generated and given to you or you prepared ECOs?