Wednesday, January 23, 2013

Parasitic Extraction Based Interview Questions

NOTE: I have updated this post recently. Few more questions you will find in another post and I will post the link of that asap.
In this post we are discussing about the Parasitic Extraction related Interview Questions. Remember- I am also listing the Expected Ans in one word or one sentence. But expect the WHY/HOW as part of follow-on question. I have mentioned few follow-on questions also (without Ans). For the detailed ans of any topic, please read the different articles in the ( VLSI CONCEPTS ) or refer any other book/weblink.
 Parasitic Extraction related

  • Q: What are the extraction tools you know/you experienced?
  • Expected answer (any): Synopsys(StarRC), Cadence(QRC), Mentor Calibre/xRC, etc.
  • Follow On Questions:
    • How much comfortable you are in a specific tool?
    • What's the difference across the different tools? (if you have know more then 1 tool)
    • List down 1 good and 1 bad thing about a specific tool?
    • Which one is the industry standard one?
    • What is the role of extraction tools in design?
  • Q: What are the different inputs of extraction tool?
  • Expected answer: Atleast 1 from this "LEF/DEF, mwlib, gds" (or similar type of any physical view), extraction model (like nxtgrd, tluplus), general setup scripts.
  • Q: What are the different types/Formats of output of the Extraction Tool or the Parasitic Data?
  • Expected Answer:  SPF, DSPF, RSPF, SPEF, SBPF, SPICE, and SSPEFBinary interface.
  • Follow on Question:
    • What is the difference in all these formats?
    • Which one is more accurate?
  • Q: What type of information, we can obtain after running the extraction tool?
  • Expected answer: Resistance and Capacitance of wires (either lumped or distributed)
  • Q: How many different type of capacitance from design point of view?
  • Expected answer: capacitance to ground - (Ground cap), capacitance between 2 metal wire of same type (coupling) , Fringe Cap, capacitance between 2 metal wire of different layers
  • Follow On Question:
    • What do you mean by Ground Cap?
    • What do you mean by Fringe cap?
    • There are 3 Metals- M1, M2 and M3. How many different types of Cap will be here?
    • What do you mean by Area capacitance or Surface Capacitance?
    • What do you mean by Total Capacitance?
  • Q: What’s the Formula of the Capacitance?
  • Expected Answer: C=εA/D=εW.L/D
  • Follow On Question:
    • Please identify the different parameter in the above Formula for Ground Cap and Coupling cap in terms of Design parameters.
      • Expected Ans:
        • For Ground Cap : D -> Distance between the two metal plates -> thickness of the dielectric material between the 2 Metals. ; W -> Width of the metal Wire.
        • For coupling Cap : D -> Space between the 2 metal plates. W -> Thickness of the Metals.
    • For coupling cap and ground cap- is there different dielectric constant or it’s same? If it’s different why?
    • What are the different component / parameter with in the design on which Total capacitance has dependency?
    • What’s the concept behind the low K dielectric in the parasitic extraction?
    • In lower technology, coupling capacitance is always a challenge. For reducing the coupling capacitance what technique we are using right now?
  • Q: What’s the formula of the resistance?
  • Expected Answer: Resistance = ρL/(width of metal)*(thickness of metal)
  • Follow On Questions:
    • Which material, we are using for Metal now a day?
    • Before 0.18μm technology, which material we were using for Metal wire?
  • Q: What’s the significance of calculating the cap value in the design
  • Expected Ans: RC value of the wire/net is directly proportional to the delay of the wire/net. Delay has significance in the timing calculation. So RC value has significance in STA and SI.
  • Follow on Questions:
    • How are we using these cap and res values in the STA and SI?
    • Which tool are you using for this

  • Q: What do you mean by RC/ Extraction corners
  • Q: What’s the Difference between Cworst and RC worst corner?
  • Expected Ans: Cworst- Interconnect capacitance is worst , and in RC worst – product of Interconnect R and Interconnect C become worst (Not like both R & C worst- because that’s can’t possible).
  • Follow On Questions:
      • What are other corners?
      • What’s the significance of RCworst / RCbest corner?
      • How R and C plays their roles in deciding the sighoff corners.
      • For signoff the design (closing the timing and all) how many parasitic corners are required?
      • Are these corners has any dependency on the technology and if yes, then how?
  • Q: What are the process variations that contribute to RC corners in parasitic extraction?
  • Expected answer:  Metal (copper) thickness, line width, dielectric thickness, etc. Process variation can be classified into two categories.  One is deterministic, the other is random. All the  manufacturing effects, such as wire width changes not corrected by Optical Proximity Correction (OPC), wire  thickness changes due to Chemical Mechanical Polishing (CMP), and effective resistance changes due to the cladding on copper interconnects and dishing of wide metal lines, can be accounted in extraction tools. etc
  • Follow On Question:
    • Why these are contributing?
    • One by one interviewer can ask the details of the Process-variation parameters.
    • If we will increase the widths of the metal – how it will effect on the cap and res value of metal?

In the next post, I will discuss few other questions related to face to face interview.


  1. R and C worst is possible if you have thin metal (R worse) and poor resolution in patterning metal lines causing wide wire routes or narrower
    metal line spacing(C worse)

  2. Here's some more I like to ask:

    (1) When does inductance become important
    (2) What is skin effect
    (3) What is typical cladding the use on Cu BEOL process and why
    (4) How might dual damascene versus single damascene impact RC's
    (5) What kind of min/typ/max variance might be seen in contacts/vias
    (6) Explain typical RC extraction tool (StarRC, etc.) technology file creation, how is a field solver used in it's generation?
    (7) Explain the difference between BEM, FEM, and FDM
    (8) and lot's lot's more :-)

    1. Hi David,

      Nice one and good one ... I will talk to you regarding this on your mail id....
      Please provide me that ...


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