Retrofitting an older sputtering tool with an automatic wafer ID system greatly reduces misprocessing. ILLUSTRATION BY JAMES SCHLESINGER

The Anelva, like most older wafer processing equipment, was not previously equipped with an automatic wafer ID capability, posing a line-yield risk. "We wanted to automatically read the lot number on the wafers and stop processing if the actual lot number read did not match the expected lot number," explains Mark Bradford, a senior engineer at AMD. "The wafers are transported from operation to operation in boxes labeled with their lot number. On occasion, someone will inadvertently mix wafers between boxes. If this is not realized at the next operation, the wafers could receive the wrong processing. In the case of the Anelva metal deposition process, once you put the wrong metal on the wafers, they're trash.

"By adding wafer reading to the tool, we now have a way to stop the tool if the wrong wafers are loaded, so scrap due to this type of misprocessing is [all but] eliminated."

Bradford evaluated other wafer ID products and ultimately selected an In-Sight 1700 wafer-reading sensor from Cognex (Natick, MA). The self-contained unit combines advanced recognition software, image processing, and LED-based image formation in a handheld enclosure. He chose the sensor primarily because of its ability to read OCR scribes, regardless of their appearance. "Sometimes there will be edge-bead removal lines that run right through the middle of the scribe, and the other systems we tried couldn't handle that very well," the AMD engineer explained. "The others also had trouble reading very light scribes that sometimes occur when the laser scribe is not set up properly."

Before wafers are transported by belt into the sputtering station on the tool, they are first positioned onto a flat finding station where each wafer is mechanically aligned. The wafer reader is mounted directly above the station, roughly 10 mm above the wafer surface, and is triggered immediately after alignment. In less than a second, the sensor illuminates the wafer using built-in LEDs, captures an image of the scribe, and analyzes the image using recognition algorithms. An automatic image-tuning feature dynamically balances illumination and image processing based on the specific visual characteristics of the scribe being read. This feature eliminates the need for filters and other image preprocessing tools often used to handle image variations.

"The 1700 can read anything we throw at it," claims Bradford. "It also provides us with a large read area, which is important because the flat align does not have to be precise."

As each cassette on the sputtering tool starts, the expected lot number for that cassette is transmitted to the wafer ID system by a custom Visual Basic (VB) program that Bradford developed. Once the scribe is read, the sensor unit compares the expected lot number and the actual lot number. The result of this comparison, as well as the read information and the score of the read, are passed back to the VB application. If the comparison fails, the program automatically stops processing on the tool.

AMD saw the ID unit's Ethernet connectivity as another important feature. The entire fab runs on an Ethernet network, so it was advantageous to find a wafer reader that could link directly to the network. The sensor can be set up, monitored, and modified using any Ethernet-connected PC in the facility, and the link between real-time ID information with work-in-process control software is seamless. The sensor also provides a familiar Web browser interface that allows fab workers to adjust reading parameters using menu-driven dialogue boxes as well as a live image display of the wafers being read.

Another key feature of the sensor is its ability to send the data read to a file, and thereby directly track the order in which the wafers are being processed through the equipment. The factory tracks this information indirectly by reading the wafer IDs at a separate workstation. The fab typically runs 48-wafer lots, with two cassettes of 24 wafers each. The current process calls for the operator to read the left cassette first, then the right cassette. The lot is then supposed to be processed in the same way (left first, then right). The assumption is then made that the order in which the wafers were processed through the equipment is the same order in which they were read.

"There is always some uncertainty about whether the left cassette gets processed before the right one, or whether wafers get mixed up between the wafer read step and the actual process step," according to Bradford. "Knowing the exact order that specific wafers are processed through a given tool is critical for troubleshooting when there are process or equipment problems."

After successfully retrofitting the sputtering tool with the wafer sensor unit, AMD is looking into upgrading a wide variety of other fab equipment with automatic ID for improved wafer line yield and traceability.