Recycled silicon wafers lower tool design costs

Providence, RI--When the much discussed multi-billion dollar factories for producing 300-mm wafers eventually come on line with serious throughputs, the need for wafer-cost containment will become even greater. With 300-mm sizes, wafers alone are expected to represent about 30% of the cost of fabrication, second only to equipment at 32%.

Reclaim of partially and completely processed wafers can dramatically cut wafer expense for both chip makers and semiconductor equipment manufacturers. Manufacturers of the various tools used in wafer processing, such as wafer steppers, etchers, and ion implanters, can cut costs by using reclaimed wafers in such applications as:

- Particle monitors

- Diffusion/CVD

- Equipment set-up/

Qualifications

- Equipment demonstrations/Etch rate sampling

- Epi, Resistivity, Thickness

- Furnace fillers

Reclaim is typically less than one-third to one-fourth the cost of new prime wafers and is cost-effective for 100-mm through 300-mm sizes.

In mainstream 100-mm to 200-mm wafer fabrication plants worldwide, major semiconductor device manufacturers are now realizing cost savings by using a mix of reclaim and prime wafers instead of all prime. In some circumstances, reclaim wafers represent 40 to 50% of the actual good processed-wafer throughput.

The wafer reclaim process begins with a visual inspection of incoming wafers for such flaws 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 smoother polishing steps until the surface is once again mirror-like.

Following the polishing sequence, the wafer is 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 metrology tools. Packaging takes place within a Class One cleanroom, and the "like prime" wafers are ready for shipment to the customer.

Reclaim wafers lower throughput costs without degrading device performance because, in some cases, reclaim wafers are even better than new for flatness, cleanliness, and process readiness. Since reclaim wafers can meet or exceed Semiconductor Equipment and Materials International's (SEMI) re-quirements for prime wafers, most are suitable for any type of process application.

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.

There are several classes of reclaim wafers, enabling a variety of reuse applications. The highest quality reclaim wafers are suited for all but a few applications where sheet resistance or gate oxide integrity are extremely critical. For second-best quality reclaim (P+), fabrication plants should avoid processing above 800C. Third-best quality wafers (N or P types) are typically used as "dummies" in restricted applications.

Providence, RI-based American Silicon Products, Inc. (ASP) is a leading global provider of silicon wafer-reclaim services. According to ASP, a specialty-device three-fab operation, with one development and two production lines, uses 132,000 wafers per year. Development effort requires 6,000 wafers per month while each manufacturing line uses 2,500 per month. The annual silicon cost, using new wafers, was $12.5 million. By implementing a reclaim program and using some reclaimed wafers, with a reclaim yield of 80%, costs dropped significantly.

A combination of new wafers and single-time reclaim of what had been rejects cost $8.7 million, a 30% reduction, the company says. Total wafer cost, when based upon three-time reclaim, results in a 43% cost savings. When the reclaim yield later improved to 90%, annual savings reached $6.7 million, according to ASP.

Today, the opportunity to reduce costs is significant with any wafer size.