Integrated Circuits Cycle Time at Test Operation Estimation for Production Planning using Computer Simulation By Jitrayut Junnapart

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Integrated Circuits Cycle Time at Test Operation
Estimationfor Production Planningusing Computer
SimulationByJitrayut Junnapart
2
Introduction
3
Introduction(1) Assembly Process

• General Process of IC manufacturing

4
Introduction(2) Electrical Test Process

• All products must be 100 tested before deliver
  to customers
• Electrical Test process is shown below

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Introduction(3) Electrical Test Time

• Test Time depends on
• Test Rate at main flow (in Units Per Hour)
• First Pass Yield (0 to 1 for 0 to 100)
• Test Rate at rescreen flow (in UPH)

To P.15
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Introduction(4) An Example

• Suppose one production lot of
• 3,000 units
• Main Flow Test Rate 1,000 UPH (Units Per
  Hour)
• Rescreen Flow Test Rate 300 UPH
• First Pass Yield 90

• Calculate Cycle Time

Main Flow Test Time 3,000 / 1,000
3 hours Rescreen Flow Test Time 3,000 (1 -
0.9) / 300 1 hour Total Test Time 3
hours 1 hour 4 hours
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Introduction(5) Why interesting?

• Electrical test time is very important because it
  dominates overall IC manufacturing cycle time
• There are methods to calculate cycle time
• Use average values (of UPHs, FPY)
• Apply simulation method
• Others

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Literature Review
9
Literature Review(1)

• This project applies Monte Carlo Simulation based
  on historical data of
• Main flow test rate
• Rescreen flow test rate
• First pass yield
• to simulate for total cycle time of one
  production lot

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Literature Review(2)

• Monte Carlo simulation uses random number to
  pick one value of data as a representative for
  further calculation

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Literature Review(3) Example

• Suppose we want to simulate main flow test time
• And partial of historical data of main flow test
  time (in UPH) looks like

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Literature Review(4) Step 1

• Rearrange the numbers into frequency and
  probability table

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Literature Review(5) Step 2

• Construct cumulative probability table from
  probability table

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Literature Review(6) Step 3

• Set up probability intervals for ranges of UPH

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Literature Review(7) Step 4

• Generate a random number and map onto the
  appropriate interval to find a simulated value of
  main flow UPH

Random No. generated is 0.5934
Simulated UPH 160
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Literature Review(8) Step 5

• Repeat step 1-4 but for
• First Pass Yield and
• Rescreen Flow UPH

• Use equation in slide 5 to calculate Simulated
  Total Test Time

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Literature Review(9)

• BUT real test rate (UPH) data is continuous
  number (appendix C,p79)
• We need to group them first into ranges
• Then count number of data that are in each range
• And find the center value of the group as a
  representative number of the group

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Literature Review(10)

• Number of group can be chosen
• Groups shall cover every data

• See appendix D, p.86 for detail

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Project Development
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Project Development (1)

• Problem Definition
• Electrical test time is the main dominator of
  overall production cycle time in back-end IC
  manufacturing
• Cycle time affects shipping commitment to
  customers
• Using average test time is not effective because
  the numbers vary over wide range

Production control people wants a tool to
forecast electrical test time of the IC
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Project Development (2)

• Objectives
• To develop computer software that processes
  historical test rate data (in Units Per Hour -
  UPH) at main flow and rescreen flow, and first
  pass yield (retest rate-related) from production
  and simulate for probability to finish testing a
  production lot with various allocated time.
• The computer software must be separated into main
  program and auxiliary data files. The main
  program will work with various environments
  without modification.
• The developed simulation program code must work
  on both Personal Computer (PC) and larger
  computer running on UNIX operating system.

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Project Development (3)

• Scope and limitations of the project
• It is considered that main flow test rate,
  rescreen flow test rate, and first pass yield
  being input of this project are available
  beforehand. They are stored in separated text
  files which, in fact, easy to develop the system
  further.
• This project gives calculation output as a text
  file for easy to use in the future. Example of
  this is putting into Hypertext Markup Language
  (HTML) and post on facilitys Intranet.
• The software in this project will calculate for
  success probability to finish electrical test
  process based on a certain size of production
  lot. User can later use linear relationship to
  calculate for other size.

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Project Development (4)

• Project Time Line
• The project consists of 9 major tasks
• Total time taken is 7.5 months

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Project Development (5)

• Feasibility Study
• Technical Feasible
• The champion has knowledge of simulation and
  programming language
• Economic Feasible
• The project does not require special expenses
• Operational Feasible
• The project does not interfere the normal work
  practice of concerning people
• Legal Feasible
• Software used in development is legal and all
  data is legally used

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Project Implementation
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Project Implementation (1)

• Concerns

1. Output must be easy for user to understand
2. Test time depends on lot size, and it is not
   feasible to provide all lot sizes output
3. User should have control over some essential
   simulation parameters
4. Change in source code is undesirable

1,2
3
4
Need modifications
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Project Implementation (2)

• To solve concerns 1 and 2

To concerns

1. An allocated time is set
2. The simulated test time for a certain lot size is
   calculated and compared to a allocated time. If
   test time is shorter than allocated time, the
   result is possible. This 'success number' is
   counted.
3. Repeat step 2 for many times and calculate
   success chance from success times and total
   simulation times and report this chance to user.
4. Increase the allocated time by delta defined
5. Repeat step 2 to 4 for certain rounds defined

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Project Implementation (3)

• To solve concerns 3

To concerns

• Store simulation parameters in file para.txt
• Number of types of IC to perform simulation
• Number of divisions to group historical data
• Lot size that simulation is based on
• Number of repetitive simulation for one allowable
  time
• Start point of allowable time
• Increment of allowable time
• Steps of increment of allowable time

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Project Implementation (4)

• To solve concerns 4

To concerns

• All input data are stored in separated files
• Historical files of Main flow UPH (.mnf)
• Historical data of Rescreen flow UPH (.rsf)
• Historical data of First pass yield (.rrf)
• List of IC types file (progname.lst)

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Project Implementation (5) Files

• Main flow UPH file
• icname.mnf

• Rescreen flow UPH file
• icname.rsf

• First pass yield file
• icname.rrf

• Parameters file para.txt

• List of icname file progname.txt

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Project Implementation (6)
Open file progname.lst and para.txt that
stores test program names to simulate and read
all the names and parameters
Processed for every test program?
End
Yes
No
1.Open file that stores UPH Main flow, and
read all main flow UPH for next program 2. Count
number of data 3. Divide into divisions 4.
Calculate for accumulated probability 5. Repeat
step 1-4 but for rescreen rate and UPH of
rescreen flow
1.Simulate total time used to test the production
lot 2.Count for number of success that the time
used is less than allocated time 3. Repeat 1
and 2 and calculate success probability 4.Record
result in output file 5.Increase allocated time
Simulation Method
Allocated time gt limit?
No
Yes
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Project Implementation (7)

• Programming
• Coding is done in ANSI C language
• Length of the program 900 lines
• Special statements and commands are avoided for
  the program can run both on PC and UNIX based
  computer system

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Simulation Result
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Simulation Result (1)

• PC based result

1) Screen output
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Simulation Result (2)

• PC based result

2) File output (result.txt)
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Simulation Result (3)
Compiled program

• Unix
• Based
• result

Control parameters input Printed out for
monitoring
Screen output
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Simulation Result (4)

• Unix
• Based
• result

Output file Generated
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Simulation Result (5)

• Unix
• Based
• result

bkkeispemg34 vi result.txt
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Project Evaluation
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Project Evaluation (1)

• Verification of Objectives
• Output is easy for non-engineering people to
  understand ? Yes
• Control parameters used in simulation can be
  input to program through external file ? Yes
• Various simulations can be performed without any
  changes to main program ? Yes
• The source program can be compiled to work on
  both PC and UNIX based computer ? Yes

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Project Evaluation (2)

• Verification of Result

• The simulated result is verified back to
  historical data

• The result is 3-5 deviation
• This is very good for simulation (probabilistic
  work)

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Conclusions

• Simulation technique can generate result that
  gives adequate accuracy (3-5 error in this
  work). The result can be transform in the way
  that it is easy to understand by end user.
• The planner (user) can assign allowable time to
  test production lot based on his/her confidence
  level that the lot will finish in time
• In this particular work, essential parameters in
  simulation can be changed easily in control files
  without modification to the main program

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Random Number Generation

• Confirm the even distribution by JMP

1,000 numbers
10,000 numbers