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Reclaiming Silicon Test Wafers Gets Updated
By Roger Tushingham, Product Manager, ATMI, Inc., Danbury, CT
When processed, a single silicon wafer will go through hundreds of individual steps over a period of several weeks, emerging as numerous individual semiconductors with a combined value of hundreds of thousands of dollars. To maximize productivity, quality and yield, manufacturing tools are constantly tested to ensure they are properly calibrated and that process flows are precise.

Due to the high cost of producing prime silicon wafers (approximately $300 for each 300mm wafer), semiconductor manufacturers typically use "dummy" or test wafers to optimize and monitor their manufacturing processes. A large semiconductor fabrication facility might spend $2 million a month on test wafers; while less expensive than prime wafers, they typically cost upward of $100 each. Because of the large volumes of prime and test wafers used, Silicon remains by far the greatest material expense in semiconductor manufacturing fabs.

Recycling Test Wafers

With continued pricing and margin pressure, anything manufacturers can do to reduce silicon costs by even one or two percent is important, thus, the popularity of silicon wafer reclamation — the practice of treating and recycling test wafers so they can be re-used multiple times. The global industry trade group SEMI estimates that semiconductor manufacturers spent $687 million on silicon wafer reclamation in 2007, and predicts growth to $859 million by 2010.

Reclamation can be performed in-house or outsourced to a specialized reclamation facility. It involves removing unwanted material from the wafer with a combination of dry and wet processes, and then grinding and polishing them to restore the silicon surface to a usable, Grade A condition. Due to the nature of the typical processes currently used, tens of micrometers (µm) of the silicon layer are lost during the polishing step. This means that after a relatively small number of reclaim cycles the test wafers become too thin to be used any longer and must be replaced by new ones. Also, while traditional reclamation techniques are certainly better than the alternative of disposing of a test wafer after a single use, the increased introduction of new materials into the semiconductor manufacturing process has made it increasingly difficult to successfully clean and re-use silicon wafers. Reclaim yields for test wafers with these new materials are characteristically low.

Improving Yields

Addressing the need for a process improvement, ATMI introduced a significant change in reclamation methodology in 2007 with RegenSi test wafer solutions. Using this approach, an all-wet process is used to strip away most films from the test wafer — including novel materials that manufacturers are evaluating for the first time — while limiting damage to the underlying silicon. As a result, the wafer requires less grinding and polishing to restore its surface to Grade A condition.

ATMI RegenSi solutions are employed in-house by semiconductor manufacturers or, more frequently, by sending batches of test wafers to an ATMI partner that specializes in reclamation services. The benefits of this approach quickly become apparent in case histories of actual customers. For example, semiconductor manufacturers have had great difficulty with internal reclamation of test wafers that have low-k dielectric material on them. Standard reclaim processes are not able to completely remove the low-k material, resulting in very low reclamation yields. After implementing an all-wet RegenSi solution for internal processing of low-k test wafers, a Taiwanese manufacturer reported that yields soared to 85 percent.

Significant Cost Savings

At a cost of $100 per new test wafer, the cost savings achieved by this customer are significant. Not surprisingly, the customer expanded the use of RegenSi to its other fabs, sending the majority of its external reclaim wafers to ATMI's reclaim partner.

In another example, a customer based in China reported that RegenSi helped it reduce the number and amount of chemicals used in a fab. Prior to applying RegenSi, this manufacturer used three separate chemicals to remove metal film stacks from its test wafers. The introduction of RegenSi led to replacing those three chemicals with one, helping reduce reclaim cycle time. This had the added benefit of limiting the amount of chemical waste for treatment and disposal. Finally, the amount of labor dedicated to the reclaim process could be reduced, so floor space formerly dedicated to reclaim could be transferred to revenue-producing manufacturing capacity.

As customer experiences show, the benefits of commercially engineered solutions such as RegenSi are compelling. For the new films being introduced in semiconductor processing, yields improve dramatically, Silicon loss as compared to traditional reclaim approaches is reduced by as much as 75 percent, enabling semiconductor manufacturers to get up to four times more life from each test wafer. This combination of higher yields, greater productivity and increased re-use leads to significant overall cost savings — and a competitive advantage for semiconductor manufacturers.

Contact: ATMI, Inc., 7 Commerce Drive, Danbury, CT 06810 888-301-4401 E-mail: sales@atmi.com Web: http://www.atmi.com

 
 
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