Statistical Engineering Division, ITL
Semiconductor Devices Division, EEEL
The purpose of this project is to produce seven issues of SRMs with certified resistivities at 200, 100, 25, 10, 1, 0.1 and 0.01 ohm.cm levels. The SRMs come from crystals that have been grown from liquid silicon doped with phosphorous.
The SRMs are intended for on-line calibration of instruments used in semiconductor fabrication. The project began several years ago with the validation of a more precise method (relative to the existing ASTM standard method) for measuring resistivity (or sheet resistance) of silicon wafers with probing instruments.
This is a classic situation of a unit of measurement that is defined solely by a measurement method (instrumention and procedures) together with certified artifacts disseminated by a national laboratory. The research phase for this project studied failure modes, wafer stability, instrument geometries, effects of wear on probes, effects of repeated probing on wafer surfaces, and photo-electric effects (exposure to light). These experiments uncovered some mechanisms which are not yet fully understood, although physical arguments can be made by way of explanation, and suggested sources of uncertainty to be examined during the certification process.
For the certifications, the same experimental design was applied to all seven issues. A check wafer, chosen at random from the batch of approximately 150 SRMs from a single crystal, was measured daily to estimate components of variance. A pre-certification experiment on five wafers was conducted to: identify the probe with the best precision, test the difference between two wiring configuations for that probe, estimate systematic differences among the five NIST probes, and estimate temporal components of variance. This phase was followed by the certification where all wafers from the crystal were measured with a single probe. This, in turn, was followed by a post- certification experiment, identical to the pre-certification experiment, that checked for drift in the process.
Sources of type A uncertainties are: 1) probe precision; 2) run-to-run variability; 3) long-term variability in the measurement process; 4) bias in the certification probe; and 5) differences between wiring configurations for a single probe.
Type B uncertainties arise from uncertainties in the calibrations associated with the measurements of ratio of current to voltage, temperature, and wafer thickness. Problems encountered in the analyses include the effect of debris on the surface of some wafers from repeated probing and significant photo-electric effects.
Figure 1: Differences from the wafer mean (ohm.cm) for each of 5 wafers show systematic differences among the 5 NIST probes coded (1-5).
Date created: 7/20/2001