The precision study was done by studying the "discriminating power" of the two instruments. The discriminating power of an instrument is its ability to differentiate between different pieces of product (in this case 24 measurement locations on the same wafer) by making multiple measurements on each piece and studying the range of measurement error. Discriminating power is defined in detail in the AT&T Statistical Quality Control Handbook. The comparison results showed that in respect to the application of the eddy current probe to MMIC yield enhancement the eddy current probe is the more precise and accurate sheet resistance probe. It is more suitable for yield enhancement applications where a variation in implantation dose, substrate donor or acceptor level, anneal furnace temperature, or other process variation must be detected to improve yield. In fact, the eddy current probe was used to measure blanket (unpatterned) channel implants into substrates from various suppliers. The probe determined that substrates manufactured by Supplier A produced a factor of two better across-wafer uniformity in sheet resistance than substrates manufactured by Supplier B. A total of 191 MMIC wafers were then fabricated on wafers from both suppliers. Each MMIC wafer had nine van der Pauw structures located across it for the determination of the channel layer sheet resistance and its uniformity. The sheet resistance for this channel layer is approximately 600 ohms/square.

The median standard deviation was 14 and 31 ohms/square for Supplier A and B substrates, respectively. MMIC yield on Supplier A substrates was higher, in agreement with the eddy current probe and van der Pauw structure measurements.

The following quotation [4] from Dr Martin Brophy of TriQuint describes an application in which a contactless sheet resistance probe has helped save a customer over $250,000 yearly:

"Expedient utilization of measurements are used to qualify the important steps of the ion implant process detecting unwanted contamination in CVD processes and non uniform cleaning and etching. This has saved TriQuint more than $250,000 per year by helping increase yields. Every day, a normal semi-insulating GaAs wafer is sent through our production front end, consisting of wafer prep, cap deposition, blanket FET channel implant and rapid thermal anneal. The wafer is then tested on the Lehighton 1310 using a 35 point map. If the average sheet resistance and on-wafer uniformity fall within their SPC limits, the front end of the process is considered OK and the day's fab runs can be started. Otherwise, the front end is shut down for production and we mobilize to determine the source of the discrepancy and ameliorate it."

The maps are an integral part of these savings. They reveal an uneven hard spray rinse (Fig. 1) and three severe spots after processing in a plasma machine, showing a detectable contamination signature (Fig. 2).

Further, Brophy says [4], "In addition to production use, our Lehighton systems are also widely used for rapid process development in implant and anneal and for potential new substrate suppliers."

The following excerpts are from papers presented by some of our current and prospective customers at GaAs symposia. They state how the eddy current sheet resistance measurements have improved their process and yields.

Wilson et al., of Motorola Semiconductor Products Sector, Tempe, AZ, report [5] that

"to address the difficult task of low dose ion implant monitoring for GaAs we have developed a technique that is very simple and elegant, but which is highly accurate for monitoring implant matching as well as absolute dose control. We dedicate an entire boule (typically greater than 150 wafers) for ion implanter monitoring. When the implanter is in a known qualified state, we utilize custom fixtures to implant one half of all wafers with Be and Si doses that are typical of our channel implants."

Above left - Fig. 1, Sheet resistance map of wafer hard spray rinse

Left - Fig. 2, Sheet resistance map of wafer, showing three severe spots due to plasma machine