• SiC, GaAs, InP, GaN and other semi-insulating materials
• Resistivity: 1E5 - 1E12 ohm-cm (Extended Range Version)
• Fast, 1% Repeatability
• Non-Destructive Measurements

• No Sample Preparation
• Up to 200mm Wafer Size
• High Resolution Topograms
• For Wafer Production and Process Development
• High Speed - Approx. 20 minutes per wafer

The rapid growth of the telecommunications market has dramatically increased the demand for high-speed analog and digital microelectronic devices. These circuits are typically fabricated on semi-insulating substrates. As a result, sales of GaAs, InP, SiC and GaN wafers are growing rapidly. The progress of the crystal growth and wafer fabrication technologies steadily yields increased wafer diameters and better material quality. However, due to the high basic cost of compound semiconductor materials, these improvements must be achieved while keeping steady or even reducing the cost per substrate area. Production compatible characterization equipment offering rapid, comprehensive, inexpensive, and nondestructive material quality assessment is indispensable to meet these challenges.

The electrical resistivity of semi-insulating GaAs and InP wafers typically ranges from about 1E6 to 1E9 ohm-cm. Both the absolute value and the lateral variation across the wafer area must routinely be assessed. A resistivity topogram adequately reproducing the lateral variation of state-of-the-art material requires several thousand individual measurements. The data must be generated with a repeatability better than 1%. In addition, the entire wafer evaluation must be done rapidly enough to be compatible with production requirements.

The COntactless REsistivity MApping measurement system COREMA - WT is designed to meet these requirements. It is based on a capacitance technique originally developed at the Fraunhofer Institute IAF in Germany and now licensed to SemiMap Scientific Instruments GmbH. The arrangement of the wafer sample and a non-contacting metal electrode defines an RC circuit. A voltage step is applied and the dynamic charge redistribution is subsequently measured. The evaluation, based on first principles, yields absolute resistivity values and does not require calibration standards.

The system is typically used by vendors and customers for routine production control and quality assessment of semi-insulating wafers. Materials other than GaAs and InP can also be evaluated. For SiC and other high resistivity materials, the system can optionally be ordered with enhanced software and hardware and with an expanded resistivity range of 1E5 to 1E12 ohm-cm.

The COREMA - WT is most valuable for monitoring and optimizing the fabrication process steps that determine the lateral homogeneity of the wafer. One important application is monitoring of post-growth thermal annealing. Powerful software allows the user to analyze local inhomogeneities resulting from a mixture of material phases with different resistivities.

The complete measurement process, including data evaluation and reporting is supported and controlled by user-friendly software based on Microsoft Windows. The software interface includes setup of the desired measurement routine, online supervision of the measurement process, display and statistical evaluation of the data. Various topographic displays including gray-scale or color as well as a pseudo-3D representation are included. Integration with customer-specific data management systems is possible.

The performance parameters of the standard configuration are specified below. Customer-specific upgrades, e.g. a smaller probe (500 µm) for higher lateral resolution, are available upon request. With these smaller probes, very fine details of the resistivity variations and the electrical material structure can be discerned.

The individual measurement takes only about 100 ms, hence the total time needed to acquire repetitive data points is essentially determined by the speed of the mechanical stepping. For instance, a 100 mm diameter wafer topogram with a 1.4 mm step size (approx. 4000 data points) is obtained within 20 minutes.