300mm wafers: Reclaimer's role and challenges
08/01/1999
A recent report by industry tracker Allied Business Intelligence Inc. (ABI) predicts a wafer shortage and steep price increases by the year 2000. ABI states that with silicon remaining the dominant material for substrates and wafers, demand for the material could reach $7 billion this year. The reclaimed wafer market is beginning to emerge as an important factor, and savings in the use of reclaimed wafers is expanding opportunities for test wafers as replacements for more expensive prime wafers. Reclaim wafers now comprise 10% of the market.
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Although ABI's interviews with leading industry players appear to indicate that the transition from 200 to 300mm may be delayed by a number of issues such as the industry's slump in revenues, the lack of available processing equipment, the cost of constructing new fabs, and other uncertainties, it is evident that the wafer reclaimer's role in assisting in the transition will be critical.
The market for reclaim
During the development and marathon testing of wafer-manufacturing tools, reclaim wafers provide an excellent opportunity to re-use materials. The worldwide demand for these wafers is increasing in new applications as their quality ap proaches that of prime monitor. Reducing costs in a competitive marketplace also continues to be a requirement; these factors will drive the move to 300mm wafers.
In contrast, the maturation of 200mm processes has greatly reduced the need for test wafers. The ratio of test to prime usage has moved from 1:1 in 1995 to 0.6:1 in 1998. Due to the current industry slowdown, substantial cost-saving measures are being put in place this year. The ratio of 200mm test to prime wafers now approaches 0.3:1.
The table provides a worldwide reclaim market estimate and forecast, provided by market research firm Rose Associates.
The use of reclaim wafers provides an excellent opportunity to reduce development costs and, therefore, the final cost of the tool. As the figure shows, the money saved using reclaim is substantial when compared to money spent on virgin test wafers.
 Relative cost comparison between virgin test and reclaim wafers. |
Partnering with equipment makers and device manufacturers has not only reduced wafer cost, but also cost of ownership, since wafers are used more times in more areas. With shared process information, the reclaimer has a better understanding of the types of films, implants, and diffusions used on and in the wafer. The reclaimer can then optimize the process of returning the wafers to a useable condition.
The wafer reclaim process
The reclaim process is similar to prime wafer manufacturing, except that crystal growing and wafer modification are replaced by sorting, film removal, and etching.
During the process, material is introduced into the reclaim line and assigned a traceability number that correlates to the original customer shipment. At incoming inspection, the wafers are sorted into groups of materials depending on films, diffusions, implantations, and defects observed. These groups are then moved through multiple processes such as a combination of chemical and mechanical film removal. The groups are then inspected to verify that all of the films have been removed and that defects not observed at incoming inspection are also removed.
About the authors
Cliff Thomson is technical services manager and Harald Mynster is VP of sales and marketing at Exsil Inc., 6541 Via Del Oro, San Jose, CA 95119; ph 408.629.3142, fax 408.629.3168, www.exsil.com.
 Relative cost comparison between virgin test and reclaim wafers. |
Final end product variation depends on customer needs, sorting, and the disposition of reject material. For specific applications, the product may be sorted not only at incoming inspection, but also within processing or at final inspection in order to meet customer needs. This creates another variation on the prime wafer-manufacturing process. Metal deposition wafers, for example, must be sorted from nonmetal wafers to minimize the risk of contamination.
Wafer reclaim challenges
Wafer reclaim presents different challenges than does prime wafer manufacturing. First, the final reclaim product must be at prime monitor specifications for light-scattering events, surface metals, and flatness. At the same time, silicon removal must be minimized to provide the greatest number of reclaim cycles. The reclaimer must balance the processes used for the effective removal of various defects with meeting stringent final product specifications. Minimizing chemical exposure to process people, feedstock (or starting material) variations, and end product variations are other challenges faced by wafer reclaim operations.
Aggressive proprietary chemistries are used to remove the variety of films found on wafers. As new processes and multilayer devices are developed, however, there is a need for even more complex film removal technology. A related requirement is that decomposition products resulting from film removal, especially those from copper films, be safely contained.
The variation of reclaim feedstock between and even within device manufacturers is another issue that the reclaimer must address. This variation occurs because of the nature of the different devices being manufactured. For example, in one case, a device manufacturer sent wafers for reclaim that had poly layers totaling more than 60µm on top of the original substrate.
Today's reclaimed wafers, then, are not processed with just one or two films, nor have they been used in only one part of the fab. Device-manufacturing and engineering personnel have made more efficient use of test
eclaim wafers by re-using them several times. For example, a diffusion test wafer - where an oxide has been grown on the wafer to monitor the oxide growth in the furnace - can also be used for etch rate monitoring. That wafer can then be used in another operation or testing process. Again, partnerships between the device manufacturers and the reclaimer are necessary, since the more efficient the removal process of silicon, the more cost effective the system becomes.
Flexibility needed
Silicon reclaim operations must have flexible capabilities that can respond to the semiconductor industry's changes. Due to the need to reduce costs, the reclaimer works with equipment suppliers to provide re-use of wafers for tool development and to determine the processes that will be required to remove next-generation films.