WCM   Wafer Charging Monitors, Inc.

Frequently Asked Questions
(Advanced FAQ's)

Wafer Charging: General

1.Q. What do the sensors on CHARM®-2 wafers actually measure?
A. The sensors on CHARM®-2 wafers measure:
  • the magnitude and polarity of charging potentials developed across gate oxides,
  • the magnitude and polarity of charge fluxes impinging on the wafer, and
  • the UV intensity.
In addition, CHARM®-2 wafers measure the J-V characteristics of the charging source experienced by devices on the surface of the wafer.

2.Q. Is it sufficient to only monitor wafer potentials?
A. No. It is also essential to know the charging currents, since they are responsible for device damage.

3.Q. Is it essential to use photoresist on CHARM®-2 wafers?
A. It depends on the application. For many applications, bare CHARM®-2 wafers may be used, such as: comparison of different process equipment, equipment monitoring, or equipment qualification. However, resist would be essential for applications in which you want to use the measured potentials and current densitites to predict device damage similar to that which a resist-patterned product wafer might experience.

4.Q. Can I use my product resist masks to pattern resist on CHARM®-2 wafers?
A. Yes. The potential measured using your product mask will be very similar to those obtained using WCM-provided CHARM®-2 compatible masks, but the J-V results will need to be "averaged" over several die to obtain useful J-V information.

5.Q. In high density plasmas where there may be a lot of UV, can CHARM®-2 sensors distinguish between charging damage and UV damage?
A. Yes. CHARM®-2 wafers contain not only charging sensors which do not respond to UV, but also UV sensors which do not respond to charging. Consequently, with CHARM®-2 wafers it is very easy to separate charging effects from UV effects.

6.Q. Can CHARM®-2 distinguish between a transient charging event and a steady-state charging situation?
A. Yes. This information can be deduced from the CHARM®-2 J-V plots.

7.Q. Do you have experience with monitoring drift in the charging characteristics of selected process equipment?
A. Yes. A good example of the benefits of using CHARM®-2 wafers to monitor equipment drift and the way this information can be used to improve maintenance procedures is described in the following paper:

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