Use of reclaimed wafers offers cost savings
Process can cut costs by one-third in applications requiring test wafers, among others
By Marilyn Bollengier
American Silicon Products
The recent downturn in the semiconductor market has forced many chip makers and semiconductor manufacturing equipment developers to sharpen their pencils and find new ways to reduce costs without compromising product quality. As a result, more and more companies are discovering that wafer reclamation is an effective cost-cutting measure. In fact, some of the biggest wafer manufacturers, under pressure since the first quarter of 1996 when dynamic random-access memory (DRAM) prices dropped dramatically, have been pursuing reclaim strategies of their own, chasing the niche marketers who specialize in the field, explains the research director of one industry trade group.
For over a decade, recycling or reclaiming silicon wafers for reuse has proven an effective option for containing costs and optimizing materials utilization associated with semiconductor manufacturing. Reclaim wafers are carefully reconditioned process test wafers that meet prime specifications, and thus, lower throughput costs without degrading device performance. And since reclaim wafers can meet or exceed Semiconductor Equipment and Materials International`s (SEMI) requirements for prime test wafers, they are suitable for numerous process applications.
SEMI`s proposed standards1 for uses of reclaim wafers include:
Furnace wafer for monitoring thermal processes or as an implant monitor;
Lithography wafer for testing lithography equipment wherein surface flatness is key to process monitoring;
Mechanical wafer suitable for equipment or process testing; and
Particle wafer for monitoring area or process cleanliness.
The process
The wafer reclaim process begins with a visual inspection of incoming wafers for fatal flaws such as chipped edges or serious scratches. Wafers that pass this test are then measured for thickness to ensure that sufficient material exists to permit successful reclaim.
After inspection, wafers pass through a chemical stripping process, which removes the thin films from the wafer processing steps. This is followed by a series of progressively finer polishing steps, which restores the surface of the wafer to prime levels.
Following the polishing sequence, the wafer is chemically cleaned, washed, rinsed with ultra-pure deionized water, and dried using both industry standard and proprietary techniques, completing the reclaim process. Prior to packaging, the wafer surfaces are inspected with state-of-the-art metrology tools. Final processing and packaging takes place within a Class 1 cleanroom, and the “like prime” wafers are ready for shipment to the customer.
Progression in specifications
Critical to the success of the reclaim process is the quality of the various tools and systems used. A reclaim service provider must have “customer equivalent or better” technologies — from incoming inspection, cleaning and polishing, to testing and packaging — in order to satisfy the demands of semiconductor device and equipment manufacturers.
Thus, world-class wafer reclaim service providers have intensified capital investment in such state-of-the-art equipment and facilities to meet or exceed wafer reclaim standards and specifications.
As an example, American Silicon Products Inc. (ASP/Providence, RI) uses the same wafer cleaning and measurement equipment found in the most sophisticated semiconductor fabs, such as Ontrak wafer scrubbers and Tencor`s laser inspection tools. In addition, ASP`s facilities meet the highest standards, including Class 1 and Class 10 cleanrooms and an advanced, 18.4 megohm deionized (DI) water system. The DI water system provides the highest quality rinse water available with the key measurements of resistivity and total organic carbon tracked and validated by statistical process control.
According to ASP customer Marylou Meloni, applications laboratory manager, process development at Varian Ion Implant Systems (Gloucester, MA), Varian has been using reclaimed wafers for more than 10 years. Through close collaboration with its wafer reclaim vendor ASP, Varian has witnessed and influenced the progression of the particle spec for wafer reclaim from a decade ago (before there was a spec) to 0.3 micron in 1995 to 0.13 micron today (see Table 1).
Quality control
Advancement in specifications and capital investment on the part of reclaim service providers has generated wider acceptance of the reclaim concept. But there remains a level of uncertainty among those who are unfamiliar with the process. Meloni recalls when internal company skepticism relative to quality was a major hurdle to implementing a wafer reclaim program.
“We`ve been reclaiming wafers for more than 10 years,” according to Meloni. “Initially, it was an uphill battle in selling the concept that reclaim is as good as prime. We had to build confidence internally through quality control and particle measurement. We continue to do periodic tests on incoming reclaim wafers to ensure that the material meets our specifications.”
Applications for wafer reclaim
The highest quality reclaim wafers are suited for all test wafer applications with few exceptions. Specific applications for reclaim wafers include:
particle monitors;
diffusion/CVD/implant;
equipment set-up/qualifications;
equipment demonstrations/etch rate sampling;
epi/resistivity/thickness; and
furnace fillers.
Increased cost savings
Reclaim of partially and completely processed wafers can dramatically cut wafer expense for both chip makers and semiconductor equipment manufacturers. Reclaim is typically less than one-third to one-fourth of the cost of new prime test wafers and is cost-effective for 100-mm through 300-mm wafer sizes. Major semiconductor device manufacturers worldwide are now realizing significant cost savings by using a mix of reclaim and prime wafers instead of all prime.
Expanded use of reclaim creates a proportional increase in savings. Wafer reclaim customers have been able to double the use of reclaim and likewise double the cost savings.
Depending on starting thickness, most wafers can be reclaimed several times, which results in additional cost savings. Since reclaim typically removes less than 30 microns of material, three reclaims are common.
“At Varian, we use the same wafer a minimum of five times for qualifying our ion implanters,” explains Meloni. “We experience additional cost savings by using these same wafers again as furnace dummies or in other areas where thickness is not an issue.”
Test wafer-to-product wafer ratios and costs vary considerably from company to company, depending on efforts to reduce test wafer usage per application, greater use of internal recycle loops and optimized reclaim programs such as 3-times reclaim and the reclaim of P+ wafers.
The wafer reclaim cost model (see Table 2) assumes one 200-mm fab, with a total annual virgin test wafer demand of 60,000 (5,000 wafers per month). By reclaiming less than half its total annual yield, this fab experiences a cost savings of more than $1.3 million annually.
Continued process development
ASP`s process development is focused on establishing a high yield process for 0.12 micron particle size, continuous reduction of trace metals to the detection limits of the analytical tool, optimization of wafer flatness and the development of chemical processes that maximize reclaim potential and are environmentally sound.
Wafer reclaim service providers are working together with semiconductor companies and Sematech to further refine the wafer reclaim process. Currently, ASP is working closely with Varian and others in developing specifications for 300-mm wafer polishing and cleaning.
Today, the opportunity to reduce costs is significant with any wafer size. With 300-mm wafer production, wafers alone are expected to represent about 30 percent of the cost of fabrication, second only to equipment at 32 percent.2 When 300-mm wafer fabs eventually come on line with serious throughputs, the need for wafer cost containment will become even greater. This bodes well for wafer reclaim and those who depend on it, according to industry analysts, some of whom expect significant growth based upon the inherent economic realities of 300 mm. Wafer reclaim technologies can be expected to keep pace and serve as a viable strategic alternative for fab managers who want to get out in front of ever-present cost-reduction pressures.
Marilyn Bollengier is the manager of customer applications engineering at Providence, RI-based American Silicon Products (ASP). She has been instrumental in the development of ASP`s wafer reclaim process for more than 10 years.
References:
1. SEMI Blue Ballot Specification for Polished Reclaimed Silicon Wafers, 1998.
2. “Silicon: The Conflict Between Price and Perfection,” Daniel J. Rose and L. Louise Sheet, Rose Associates, SEMI`s Channel, January 1998.
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