Before you start using a parametric measurement unit PPMU, you need to be aware of measurement terminology. This is important to collect measurement data. You need to understand the meaning of the following terms:
- Accuracy
- Repeatability
- Resolution
You cannot make efficient use of a parametric measurement unit PPMU and understand parametric tests and take measurements.
Accuracy and Repeatability
These terms are both different and related. Accuracy reflects the degree of closeness of calculated or measured value to the actual value. Also known as precision, repeatability is the degree to which you get similar or same results after repeated calculations or measurements. There is no innate correlation between accuracy and repeatability. The results you get can have:
- Low accuracy and low repeatability
- Low accuracy and high repeatability
- High accuracy and low repeatability
- High accuracy and high repeatability
These terms are explained using the bull’s eye target visual analogy. When there is a large space between points and they are close to the center of the bull’s eye target, accuracy is high and repeatability is low. When there is less space between these points and they are far from the center of the bull’s eye target, accuracy is low and repeatability is high.
Absolute accuracy is less important than repeatability in production parametric tests. It is because, in parametric testing, the concern is monitoring trends. Therefore, highly repeatable measurements are of extreme importance.
Resolution
The parametric test requires resolution to gauge accuracy and repeatability. Resolution is one of the most important parametric testing specifications. The smallest change in data that can be displayed by an instrument is resolution.
See it in terms of an ADC (analog-to-digital converter) circuit to easily think about this. The fineness of distinguishable measurement details is determined by the number of bits available to the converter. The measurement resolution can be expressed in terms of voltage or current. You are operating in a measurement range. You need to know that range. Analog-to-digital converter measures from –5 V to +5 V in a measurement range of 5 V. Assume that it is a 20-bit ADC. It yields (5 5)V/1048576≈ µV measurement resolution.
However, due to factors like noise, the resolution in a data sheet and the readable resolution are not the same.
Traceability
A company that manufactures measurement equipment must establish traceability according to both international and national standards. This ensures that the user gets accurate measurements.
In addition to being aware of these basic terms associated with parametric measurement, you need to choose the right parametric measurement unit PPMU and other instruments used in semiconductor testing.
