The move to 300-mm wafers: A prime time to consider reclaim
02/01/1998
The move to 300-mm wafers: A prime time to consider reclaim
Joyce Jensch, Spectrum Technologies, Healdsburg, California
Cliff Thomson, Exsil Inc., San Jose, California
The semiconductor industry`s acceptance and move to the use of 300-mm wafers provides an excellent opportunity to consider the virtues of using wafer reclamation services to decrease the overall cost of the transition. During a recent I300I/J300 Global Joint Guidance conference, Paolo Gargini, Intel`s director of technology strategy, estimated the industry`s transition costs to be upwards of $14 billion.
One way to minimize this cost is with the use of reclaim wafers as an alternative to prime test wafers and monitor wafers. In 1996, the worldwide market for reclaim material was estimated to be 313 million in.2 of silicon for a value of $190 million. A more recent report was released by market researchers Rose Associates (Table 1).
The wafer reclaim business is forecast to continue to grow in every region of the world. Reclaim operations have recently been established to support the strong growth in the Asian market. US- and European-based companies have constructed facilities in Singapore and Taiwan to provide reclaim services to this region.
The actual number of prime wafer starts a device manufacturer may have to test wafer usage, including reclaimed wafers, varies depending on the type of device and technology being used. Commonly accepted ratios exist of one test wafer/six product wafers to as high as three test wafers/one product wafer. In general, companies that tend to produce more value-added products use more test wafers.
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Typically, 25-30% of the total test wafer usage is reclaim material. "Best in class" device manufacturers may have a reclaim usage of as great as 70%. This translates into significant cost savings for the device manufacturers. A 200-mm fab with approximately 20,000 wafer starts/month can save up to $5 million/year if 50% of its test wafer requirements use reclaim material. A 200-mm reclaim wafer costs approximately $40. Specifications for this material would be 50 particles at 160 nm (0.16 ?m). This is in comparison to a test wafer with specifications of 50 particles at 200 nm for $80+. Tighter wafer specifications are available for both reclaim and test material at a 10-15% premium. A current market price for a prime 200-mm wafer ranges from $120 to $175 (Table 2).
How much cost saving will the use of 300-mm reclaim have? As stated in the recent SEMI-sponsored newsletter Global 300-mm Report, "Projections are difficult at this point because a wafer reclaim standard does not yet exist for 300-mm substrates." According to SEMATECH`s Howard Huff, "No issue can have as great an impact on cost of ownership for the industry as a good reclaim wafer specification." However, reclaimed 300-mm wafers are expected to be priced considerably less than prime virgin wafers, considering that prime wafers are currently going for between $1000 and $1500 apiece. If the price for reclaiming 300-mm wafers is even five times the 200-mm price, the savings over a prime wafer will still be between $750 and $1000.
Device manufacturers` attitudes are changing regarding wafer quality. It`s not enough just to be shiny on one side. Reclaimers have improved their process to provide reclaim wafers that meet the quality requirements of prime test wafers. These higher quality products have greatly increased their applications in device manufacturing (Fig. 1).
With higher quality, reclaim can be used in more applications and become a greater percentage of the overall quantity of test wafers. As reclaim moves from the more conventional applications of cold processing, to deposition, furnace, and ion implantation applications, the percentage of reclaim can go as high as 70%. This could decrease the total cost of test wafers by more than 50%.
There is still some reluctance to reclaim metal layer wafers because of fear of contamination. However, reclaim operations are able to provide metal analysis using total reflectance x-ray fluorescence or graphite furnace atomic absorption spectroscopy. Typically, trace metals in the low E10 atoms/cm2 can be achieved.
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Figure 1. Improved quality extends usage.
Typical reclaim wafer applications include:
particle monitors,
thin film thickness measurements,
oxide growth monitors,
etch rate monitors,
equipment characterization,
implant dosing, and
furnace baffles.
As device manufactures begin the characterization of their 300-mm processes, almost every department in the fab will use reclaim. In the thin film department, high quality reclaim wafers will be used for LTO, PSG oxide, nitride, poly, and metals monitoring. Additionally, wafers to check spin on layers may be reclaim material. During furnace operations, reclaim will be used to check oxide thickness and diffusion depths, as well as sodium levels in CMOS applications.
During the photolithography process, reclaim will provide significant cost savings as it is used for spin checks and particle monitoring. Etch rates will also be characterized using 300-mm reclaimed material. Generally, reclaim will be used for cleanline particle counters and witness wafers, handling checks, and equipment set-ups. Equipment suppliers are already looking for significant volumes of reclaim material as they continue to develop 300-mm tools.
As shown in Fig. 2, the first step of the wafer recycling process is to inspect wafers sent from customers` operations. Wafers are sorted by the various types of films that need to be removed. Multilayered films, such as TEOS, polysilicon, tungsten, or nitrides, need to be completely removed before further processing.
Once the wafers are sorted, they are routed through a chemical striping process that eliminates all existing patterns and films. The wafers are then inspected again and sorted according to thickness. The practice of processing wafers of similar thicknesses assures consistent removal across the wafer and from wafer to wafer.
A multistep polishing process using state-of-the-art equipment produces wafers with a surface comparable to prime test wafers. Low removal polishing technology insures that the reclaimed wafers will meet conditions of flatness and micro-roughness for a wide range of applications within the device manufacturing process (Fig. 3).
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Figure 2. Process flow. Step 1: Assign wafer log number. Step 2: Sort for thickness, metal, oxides, silicides, cosmetics. Step 3: Chemically strip surface. Etch to remove backside and edge diffused regions. Step 4: Inspect for visual defects. Step 5: Verify compliance with front-end specifications. Step 6: Stock removal polish to restore surface quality. Step 7: Eliminate surface contaminates and trace metals. Steps 8 & 9: Inspect for surface abnormalities, type, resistivity, thickness, and dimensional parameters. Step 10: Inspect for all visual and LSE defects.
The number of possible reclaim cycles depends on the original thickness of the wafer, which is typically specified by the customer. Different thicknesses of wafers can be used in different applications.
After polishing, the wafers are cleaned to guarantee that final traces of impurities are removed. The reclaimed wafers are inspected using automated mechanical equipment that measures bow, warp, thickness, resistivity, and flatness. In addition, the surfaces are automatically examined using laser particle counters. Particles, or light scattering events (LSEs), can be measured to 0.12 ?m or lower. However, at the lower LSE levels, it is not clear what is really being measured - particles or crystal defects. The reclaimed wafers are then packaged to provide maximum protection and to maintain cleanliness during subsequent handling and transportation.
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Figure 3. A Laporte Electronics technician loads a Speedfam precision polishing machine. (Photo courtesy of Laporte plc.)
Joyce Jensch is principal of Spectrum Technologies, 1532 Chablis Rd., Suite #101, Healdsburg, CA 95448; ph 707/433-8990, fax 707/433-5383.
Most major wafer reclamation services do not process gallium arsenide (GaAs) wafers in their operations. The possibility of cross contamination between the silicon wafers and GaAs would be a problem. However, because of the initial cost and value of GaAs wafers, a few companies do provide reclaim services to this new niche market.
Silicon suppliers constantly evaluate the viability of the reclaim market. With the forecast requirements of 300-mm wafers, the return on investment needed for these companies to service this market may prove favorable. However, when resources are limited and silicon is in demand, most manufacturers continue their primary investment in "prime."
Another consideration is that device manufacturers may perform their reclaim in-house. The original reclaim process and equipment were the basis for the development of chemical mechanical polishing (CMP), which has emerged as the key enabling technology for the new generation of devices with sub-0.25-?m critical dimensions and multiple layers of interconnect.
Why wouldn`t device companies who have developed CMP consider using it for reclaim as well? As it exists today, the cost of ownership of the equipment and particularly the consumables for the CMP process is very high. The value added from the CMP process makes it a reasonable expense on a per-die basis. It is also necessary to have in-house CMP capability as part of the actual device manufacturing process. However, the economics are not favorable for device companies to dedicate the manpower, capital equipment, and cleanroom space to provide this service in-house. Wafer reclamation operations continue to remain a competitively priced alternative.
Cliff Thomson is corporate technology manager at Exsil Inc., a supplier of wafer reclamation services. Exsil Inc., 6541 Via Del Oro, San Jose, CA 95119; ph 408/629-3142, fax 408/629-3168.
Joyce Jensch is principal of Spectrum Technologies, 1532 Chablis Rd., Suite #101, Healdsburg, CA 95448; ph 707/433-8990, fax 707/433-5383.