The flatness of silicon wafers used to manufacture integrated circuits is controlled to tight tolerances to help ensure that the whole wafer is sufficiently flat for lithographic processing. To ensure your wafer manufacturing process is within tolerance, you need to measure the wafers being produced. Various measurement techniques exist to verify process tolerances and eliminate expensive scrap wafers.
Non-contact capacitive sensing is a highly precise and cost-effective means to measure wafer flatness, bow, warp, and total thickness variation (TTV). MTI Instruments’ Proforma 300i and 300iSA systems use capacitance sensors to improve yields while reducing costs through better dimensional control of critical wafer parameters. These non-contact systems provide highly precise measurements of semiconducting and semi-insulating wafers up to 20k Ohm-cm.
Proforma technology has a wide wafer thickness measurement range and provides excellent repeatability. It is ideal for measurements of Silicon and other semiconductor materials.
Capacitance sensing can be used to measure wafer thickness, bow, warp, and total thickness variation (TTV) as represented in Fig 1. Capacitance probes use low voltage electric fields to measure the distance from the probe to the wafer.
Fig 1 Representation of Wafer TTV, Bow and Warp
The wafer is located between two probes. The distance between these probes “G” is accurately known, and the gap “A” and “B” between the wafer and the probes is obtained from the capacitance at each measurement point. In Figure 2 below, the Proforma 300iSA semi-automated metrology system is on the left and the Proforma 300i manual semiconductor metrology system is on the right. When the tool is automated, the wafer shape is characterized with non-contact capacitive scanners, which can measure the thickness and shape of the rotating wafer.
Fig 2 MTI Wafer tools use opposing capacitance probes to measure flatness, thickness
Capacitance probes cost significantly less than laser interferometers, which is another type of wafer measurement technology, yet capacitance sensors often exceed the performance of expensive interferometers with their stability, precision, and resolution. For a lower-cost, higher-precision solution, capacitance probes support a faster ROI and proven capabilities.
Three System Solutions to Wafer Measurement Challenges
MTI Instruments of Albany, New York (USA) provides three capacitance-based metrology systems for non-contact wafer measurement.
- The Proforma 300i is a manual metrology system that measures thickness, 5 point TTV, and bow of semiconducting and semi-insulating wafers (up to 20k Ohm-cm).
- The Proforma 300iSA is a semi-automated metrology system for semiconducting and semi-insulating wafers (up to 20k Ohm-cm) that performs full-surface scanning/mapping for thickness, TTV, bow, and warp (Fig 3 ) .
- MTI’s photovoltaic/solar metrology system provides in-process monitoring for solar/PV wafers and measures thickness, TTV and bow.
Fig 3 Proforma 300iSA Wafer mapping
Additionally, MTI’s Digital Accumeasure Gen 3 capacitance amplifier and capacitance probes can be used to build custom OEM wafer metrology systems. The SDK and Amplifier take a lot of the hard work out of building an OEM system.
ASTM standards provide the definitions for specific wafer characteristics. MTI’s application note contains these definitions, includes calculations, and explains how its capacitance-based technologies work. The first two sections explain how to measure thickness, TTV, bow and warp between two capacitance-probes. The third section is specifically for the solar industry. It describes the difference between bow and warp and explains two ways to construct a reference plane for warp measurements.