Non-Contact Sheet-Resistance Measurement on Semiconductor Wafers

Delcom Non-Contact Measurement for Semiconductor Wafers

Semiconductor wafers require electrical properties suited to their intended applications. A non-contact sensor can measure a wafer’s sheet-resistance response, supporting evaluation across different doping types and process technologies such as diffusion and ion implantation.

Wafer Doping and Deposition Processes

Measurements may be considered for:

• Overall n-type or p-type doping in wafer manufacturing
• Localized doping introduced through diffusion
• Localized doping introduced through ion implantation
• Doping introduced using alloying processes
• Deposition processes such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD)

Delcom sensors may also be evaluated for wafer substrate materials including silicon (Si), gallium arsenide (GaAs), gallium nitride (GaN), silicon carbide (SiC), sapphire, and glass. Suitability depends on the material’s electrical range, geometry, and the specific instrument configuration.

Sheet resistance can change after thermal treatment and physical processing, including polishing, annealing, etching, and oxidation. Consider measuring at critical manufacturing steps to monitor these changes. A measurement plan should define the required property, calibration, and process conditions.

Figure 1: Example — PV wafer after doping

Why Use Delcom to Measure Wafer Sheet Resistance?

Because an eddy-current instrument reads through electromagnetic coupling, non-contact measurement can offer advantages over a contacting four-point probe in suitable applications:

• Non-destructive measurement without probe marks
• Measurement through an insulating layer, where the sample and instrument configuration allow
• Ability to measure moving material in a configured production process
• Near-real-time readings
• Potential support for in-line process inspection

How Does a Delcom Sensor Work?

An eddy-current sensor can measure the overall sheet-resistance response of a uniformly doped wafer. The wafer is placed in the sensor’s magnetic field, and the instrument reads the electrical response.

Figure 2: Sheet-resistance measurement of a bulk-doped wafer

The changing magnetic field couples with conductive layers in the material. In a multilayer structure with insulating layers between conductors, the sensor may respond to the combined sheet conductance of the conductive layers (a superposition effect). The response should be interpreted according to the instrument’s calibration and the sample structure.

Two-Step Eddy-Current Measurement

For a wafer with a substrate doping layer and an additional doped layer, a two-step method can help estimate the added layer’s contribution:

Step 1: Measure the wafer before the second doped layer is formed.
Step 2: Measure it again after the second doped layer is added.

Figure 3: Two-step eddy-current method

Compare the sheet-conductance readings to estimate the added layer’s contribution. The reciprocal of sheet conductance corresponds to sheet resistance for a uniform layer. This method depends on consistent measurement conditions and a suitable model for the layered wafer.

Embedded Sensor Setups for Delcom

A Delcom sensor can be integrated into a production line using one or more sensor arrangements.

A single-sensor setup is generally more cost-effective and can quickly measure the same fixed location. A dual-sensor setup can compare electrical responses before and after a process step, helping identify process-related changes.

The right configuration depends on the wafer, process flow, measurement range, target locations, and whether the goal is a fixed-point check or before-and-after monitoring. Confirm the setup with sample measurements before implementation.

Need help evaluating wafer sheet resistance?

Contact Fulcrum Technology to discuss your wafer material, doping or deposition process, target electrical range, and measurement setup.