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Differences between Various AMS Models
There are at least 10 different AMS models that utilize the same basic electronics. Most of these models contain a
single central humidity/temperature sensor designated as HT1. However, there is one model, the AMS-300C that
contains four additional sensors near the edge of the AMS. The five humidity/ temperature sensors in the AMS-
300C allow you to characterize a large wafer area for variations in humidity and temperature.
Storage and Handling of the AMS
Because of the polymer sensing element in the humidity/temperature (HT) sensors, the AMS requires special care
in handling. Prior to calibration at CyberOptics, all the HT sensors are reconditioned to bring them back to their
original, uniform response. This reconditioning consists of a mild bake out to 82 °C in order to evaporate any
contamination hydrocarbons, followed by a re-hydration step at greater than 74% RH in order to return the
polymer to its original hydrated state. Under normal operating conditions this reconditioning is not required again.
CAUTION.
Do not place the AMS in close proximity to volatile chemicals such as solvents or other
organic compounds. Also avoid exposure to ozone, acids and bases, and cleaning agents. Do not wipe
the top surface of the AMS with IPA. In general, if a material emits a strong odor, it should not be used
in close proximity to the HT sensors.
CAUTION.
Do not direct strong air blasts into the HT sensors.
CAUTION.
Store the AMS unit at a temperature of 10
–
50 °C and in the range of 20
–
60% RH. Short
times (up to four hours) at RH levels outside of this range are acceptable. If the AMS is stored at RH
levels outside of this range for longer than four hours, the AMS should be returned to an environment
of 20
–
60% RH for 12 or more hours to return the AMS to its normal sensitivity and accuracy.
CAUTION.
Do not store the AMS unit in polyethylene antistatic bags (light blue, pink, or rose color). If it
is necessary to use tape in the vicinity of the HT sensors, use only Kapton Electrical Tape 92
manufactured by 3M.
Hysteresis and Long-Term Stability of the RH Readings
Almost all humidity sensors exhibit hysteresis. That is, the RH reading will vary depending on the recent exposure.
Sensors with a dry history carry some negative offset, while sensors with a humid history carry some positive
offset
—
that is, the reading will be slightly higher than the actual RH. The amount of hysteresis is roughly 1 %RH in
the range of 40
–
80% RH, and smaller at RH levels above and below this range. The HT sensors are calibrated by
starting at low RH values and moving upwards. So maximum accuracy is obtained by following the same procedure
during measurement. Try to acclimate the AMS at RH levels lower than those expected during measurement. The
long-term stability of the HT sensors is better than 0.25% RH per year, and for temperature is better than 0.03°C
per year.
