Bias is the difference between the mean of many measurements of a reference part and that part's accepted value. Unlike random noise it does not shrink with averaging, so it has to be found by measuring a calibrated standard and then removed by calibration or an offset. Linearity is bias that changes across the measuring range, such as a gauge reading true at the low end and 2 microns high at the top.
There's a 1.2 nm bias between the two SEMs. Same wafers, same sites, tool B always reads high.
Also heard as
- offset
- systematic error
Do not mix it up with
- Repeatability: Repeatability is random scatter around the gauge's own average; bias is how far that average sits from the true value.
Related terms
Repeatability
Scatter in the readings when one person and one gauge measure the same part over and over with nothing else changed.
Tool matching
Making several tools or chambers that run the same step produce the same results on the same material.
MSA
Measurement systems analysis: the set of studies that prove a gauge is fit for the decisions its data will drive.
Golden wafer
Reference wafer or part kept aside and measured on a schedule to check that a tool or gauge has not drifted.