Yield and quality control

Check the basics of quality control — yield calculations, the relationship between defect density and chip area, and control charts — with multiple-choice questions.

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Yield and quality control: key points

Yield is the proportion of good products among those started, and it greatly affects a fab's profitability. With the same defect density, larger chips are more likely to contain a defect, so their yield is lower.

To keep processes stable, data is monitored with statistical methods such as control charts (SPC), so that abnormalities are found and fixed quickly.

All Yield and quality control questions (50)

  1. A wafer yields 500 chips, of which 450 are good. What is the yield of this wafer?
  2. With the same defect density, what generally happens to yield when chip area is increased?
  3. In control charts (Shewhart control charts) used for process control, how are the control limits generally set?
  4. Which correctly describes the purpose of a Pareto chart used in quality control?
  5. Which formula gives the process capability index Cp (when the process mean coincides with the center of the specification)? (σ: process standard deviation)
  6. Among the causes of yield loss, what are defects called that recur in specific patterns due to, for example, interactions between design and manufacturing processes?
  7. Which diagram organizes the relationship between a problem (effect) and the factors that influence it in a shape like a fish skeleton?
  8. What is the main purpose of analyzing wafer maps, which show wafer test results as pass/fail for each chip position on the wafer?
  9. With all else equal, about how many times larger does wafer area become when wafer diameter is increased from 200 mm to 300 mm?
  10. In a control chart, which is an example that should be judged abnormal even though all points are within the control limits?
  11. In the Poisson model Y = exp(−D × A), with defect density D = 0.1 /cm² and chip area A = 1 cm², approximately what is the yield?
  12. Among defects on a wafer, what are those called that actually cause electrical failure of a chip?
  13. Which chart divides data into intervals and shows the frequencies as bars to see the shape of the distribution of variation?
  14. Which process capability index is used when the process mean is offset from the center of the specification?
  15. Which type of control automatically adjusts equipment processing conditions for the next lot or wafer based on, for example, measurement results from earlier steps?
  16. Which technique identifies possible failure modes in a product or process, evaluates their effects and severity, and takes countermeasures in advance?
  17. Which chart plots pairs of two characteristic values to examine their correlation?
  18. What is it called that shows yield improving over time as defect causes are analyzed and addressed after a new process is launched?
  19. What is the purpose of evaluating manufacturing variation separately by factor, such as lot-to-lot, wafer-to-wafer and within-wafer?
  20. Which correctly describes the idea behind the quality management activity called 'Six Sigma'?
  21. On the same wafer, why does making chip area smaller greatly increase the number of good chips per wafer?
  22. Which control chart is used to control the fraction defective?
  23. What is the purpose of inspecting for defects after each step and extracting only the defects newly added since the previous step?
  24. Which concept expresses, from particle size and wiring layout, the area in which a falling particle could cause a failure?
  25. Which technique digs into a problem by repeatedly asking 'why' to find its true cause?
  26. A wafer has 600 chips and the yield is 80%. How many good chips are there?
  27. If wafer process yield is 95% and wafer test yield is 90%, approximately what is the overall yield obtained by multiplying them?
  28. What is the system called that tracks each product's manufacturing history, needed to identify products that may have been affected when an abnormality is found?
  29. Which technique rejects chips that are within specification in test but show values far outside the distribution of the same lot?
  30. Which correctly describes the difference between the control limits of a control chart and a product's specification limits?
  31. What is the correct order of the PDCA cycle used in quality control and process improvement?
  32. Which of the seven QC tools is the approach of dividing data into groups, such as by machine, operator or material, and comparing them to find the causes of differences?
  33. Which of the seven QC tools is a pre-prepared table of items to check that makes it easy to record and tally inspection results and defect occurrences?
  34. Which is the general explanation of the difference between a product's "quality" and its "reliability"?
  35. What is the approach called that emphasizes, when a problem occurs, going to the actual place, looking at the actual thing and confirming the actual situation?
  36. Which technique varies several process conditions (temperature, time, gas flow and so on) simultaneously and systematically to efficiently determine each factor's effect with few experiments?
  37. What is the system called that monitors equipment sensor data (temperature, pressure, power and so on) during processing, detects unusual states and flags abnormalities early?
  38. Which metric evaluates equipment productivity by multiplying the fraction of time running, the performance (speed) and the good-product rate?
  39. In a semiconductor fab, what is the time from wafer start to completion called?
  40. Which control chart monitors shifts in a process's mean and changes in its spread at the same time by plotting each sample group's mean and range (max − min) over time?
  41. Which system manages which lot is at which step and tool, under what conditions it was processed and so on, directing and recording the production flow in a factory?
  42. In sampling inspection, which judges an entire lot as accepted or rejected by inspecting a sample, which metric expresses the quality level considered acceptable?
  43. When analyzing yield data, which example correctly illustrates that "a correlation between two quantities does not mean one causes the other"?
  44. Which correctly describes efforts to raise yield by analyzing the large amounts of equipment data, metrology data and test results accumulated in a fab with machine learning and similar methods?
  45. Which yield model accounts for defects that occur clustered unevenly on the wafer?
  46. What is the quantity called that, for each type of defect found by inspection, gives the fraction of chips containing that defect that actually fail electrically?
  47. Which correctly states Little's law, relating work in progress (WIP), throughput and cycle time in a factory?
  48. What is the analysis called that statistically identifies, from the manufacturing histories of many lots, the tools, chambers, materials and the like commonly used by low-yield lots?
  49. Which statistical process control technique monitors several mutually correlated characteristics together, rather than one at a time, to detect abnormal combinations?
  50. When there are several tools of the same type, what is it called to adjust and manage them so that processing results differ little between tools and chambers?

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