- SEMI MF1529 - Test Method for Sheet Resistance Uniformity Evaluation by In-Line Four-Point Probe with the Dual-Configuration Procedure
This Standard was technically approved by the Silicon Wafer Global Technical Committee. This edition was approved for publication by the global Audits and Reviews Subcommittee on August 31, 2015. Available at www.semiviews.org and www.semi.org in November 2015; originally published by ASTM International as ASTM F1529-94; previously published November 2010.
NOTICE: This Document was reapproved with minor editorial changes.
The sheet resistance of epitaxial, implanted, diffused or deposited films is an important materials acceptance and process control parameter. The uniformity across a wafer of the sheet resistance resulting from any of these processes is important for the equivalence of performance of devices or circuits made from various regions of the wafer.
This Test Method uses a four-point probe in a manner different from that of other ASTM methods for the measurement of the resistivity or sheet resistance of semiconductors. In this Test Method, two different ways (configurations) of connecting the probe pins to the electronics that supply current and measure voltage are used at each measurement location on the specimen. This use of a four-point probe is often referred to as ‘dual-configuration’ or as ‘configuration switched’ measurements.
There are three benefits that result from the second measurement configuration at each location: (1) the probe no longer needs to be in a high symmetry orientation on the specimen, that is, being perpendicular or parallel to the radius on a circular wafer or to the length or width of a rectangular specimen, as long as it is a modest distance from the edge of the wafer, (2) the lateral dimension(s) of the specimen, and the exact location of the probe on the specimen no longer have to be known—the geometric scaling factor results directly from the two sets of electrical measurements at each location, (3) the two sets of measurements self-correct for the actual separations between the probe pins in a manner that has been shown to be more effective than measuring probe impressions made on a piece of polished material. As a result, high precision measurements can be made with smaller probe separations than is possible with single configuration use of a four-point probe, thus allowing higher spatial resolution of wafer sheet resistance variations.
This Test Method is intended primarily for assessing the uniformity of layers formed by diffusion, epitaxy, ion implant and chemical vapor, or other deposition processes on a silicon substrate. The deposited film, which may be single crystal, polycrystalline or amorphous silicon, or a metal film, must be electrically isolated from the substrate. This can be accomplished if the layer is of opposite conductivity type from the substrate or is deposited over a dielectric layer such as silicon dioxide. This Test Method is capable of measuring films as thin as 0.05 µm, but particular care is required for establishing reliable measurements for most films in the range below 0.2 µm. Films that have a thickness up to half the probe separation can be measured without the use of a thickness-related correction factor. It may give misleading results for films formed by silicon on insulator technologies because of charge or charge trapping in the insulator.
This Test Method can be used to measure the sheet resistance uniformity of bulk substrates. However, the thickness of the substrate must be known to be constant or must be measured at all positions where sheet resistance values are measured in order to calculate relative variations in resistance reliably.
This Test Method is suitable for use in materials acceptance, equipment qualification, process control, research, and development.
Referenced SEMI Standards
SEMI C19 — Specification for Acetone
SEMI C23 — Specifications for Buffered Oxide Etchants
SEMI C41 — Specifications and Guidelines for 2-Propanol
SEMI M59 — Terminology for Silicon Technology
SEMI MF42 — Test Methods for Conductivity Type of Extrinsic Semiconducting Materials
SEMI MF81 — Test Method for Measuring Radial Resistivity Variation on Silicon Wafers
SEMI MF84 — Test Method for Measuring Resistivity of Silicon Wafers with an In-line Four-Point Probe
SEMI MF374 — Test Method for Sheet Resistance of Silicon Epitaxial, Diffused, Polysilicon, and Ion-Implanted Layers Using an In-Line Four-Point Probe with the Single-Configuration Procedure
SEMI MF1618 — Practice for Determination of Uniformity of Thin Films on Silicon Wafers