Page 45 - Chip Scale Review Sep Oct_2022-digital

between -60°C and 100°C (Range 1) and
0.02°C between 101°C and 230°C (Range
2). The device’s uncertainty is, therefore,
maximally 0.02°C. A measurement wafer,
often consisting of up to 17 sensors, has
a significantly higher uncertainty of
approximately 0.05°C due to the sensor-
to-sensor accuracy deviation. It is also
necessary to factor in the absolute accuracy
of 0.03°C (taken from the wafer specification
sheet), which leaves you with a total device
uncertainty of 0.058°C—almost three times
more than the maximum uncertainty of the
single sensor. To calculate the calibration
reliability, the tool’s uncertainty is added to
the method (repeatability tests) uncertainty.
Figure 4: Over the course of three days, the temperature increased and decreased corresponding to the As a result, the calibration reliability
of the ERS system is 0.022°C, which is
temperature changes in the outside environment.
considerably lower than the reliability of the
increase of 0.2°C on the test surface was case, using an ERS ultra-low noise (ULN) measurement wafer (i.e., 0.065°C).
seen over five hours (Figure 3a)—this chuck led to a larger deviation than using
is called a temperature drift. However, a low-noise or high-thermal uniformity
looking again at a 30-minute interval (HTU) chuck. This is caused by the
in Figure 3b, the observed reliability additional ceramic plate in the ULN chuck,
was 10mK. which creates a larger separation between
To investigate the cause of the the top plate and the thermal plate.
temperature drift, an additional test
was done at 200°C, but this time a Measurement wafer vs. single-
measurement wafer was used. Again, the sensor probing tool
same gradual heating effect was observed. After obtaining and validating the test
Following this test result, the ProbeSense™ data, repeatability tests with a measurement Uniformity measurements. To compare
was again tested in a closed chamber, wafer were completed to compare the the uniformity measurements of the wafer
similar to a sealed prober chamber, over performance of the two devices. Because and ProbeSense™, it is first necessary to
three days. As seen in Figure 4, the the measurement wafer requires manual distinguish between static and dynamic
temperature increased and decreased at handling, it was placed and removed from measurements. The static measurement
regular intervals throughout the days, the chuck surface multiple times to simulate was done by placing the wafer on the chuck
corresponding with the temperature the touchdowns. Despite trying to realign and then measuring the temperatures
changes in the environment around the test the wafer the same way, the manual handling without moving the chuck. The dynamic
chamber, which was not air-conditioned. leads to an observed repeatability of 0.03°C, measurement was done by mounting our
After further tests, it was concluded that compared to the 0.01°C and 0.02°C values device and letting the chuck contact the
the temperature drift does not occur below recorded with the ProbeSense™. sensor in the same positions as with a
100°C and not if the test is performed in a Calibration reliability. The single sensor wafer on top. Tables 1a and 1b show the
climate-controlled environment. in ProbeSense™ is calibrated according static and dynamic measurements on two
It was also noted that the chuck different probers (Suppliers A and B) at three
construction is an influencing factor. In this to the ISO17025 standard down to 0.01°C temperature points.

Table 1: a) Static vs. dynamic measurements on a prober from Supplier A with a probe card size smaller than the chuck; b) Static vs. dynamic measurements on a prober from
Supplier B with a probe card size similar to the chuck.

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