New cutting techniques using the latest fixed-diamond wire technology are speeding wafer production.



Throughout the world of high- performance materials, market demands constantly pressure manufacturers to offer better-performing products. Typically, suppliers respond with products that support higher levels of quality and productivity.

The electronics market probably best illustrates this trend. Cutting machine and abrasive manufacturers must continuously provide better ways to produce high- quality, ultra-thin wafers from blocks or ingots of hard, brittle crystals and ceramics. For example, producing the increasingly popular light emitting diodes (LEDs) used in a range of lighting and display applications requires the slicing of extremely thin wafers from rods of sapphire or single-crystal silicon carbide. These wafers are normally cut by annular (circular) sawing or by multi-wire slicing (MWS) using loose diamond abrasive. In recent years, alternative methods have been developed that involve fixed-diamond technology in single- or multi-wire machine set-ups.

Using diamond-coated wires offers many advantages, and slicing machine developers continue to make strides in the development of machines that fully leverage the benefits of fixed-diamond wires. Simultaneously, abrasive suppliers have improved diamond wires, with grain retention and distribution driving the bulk of the progress.

Shortcomings of Conventional Slicing Methods

Several disadvantages are associated with conventional methods of cutting thin wafers from crystal blocks. With annular sawing, also called "ID sawing," the wafers are cut individually from the work pieces. While the quality of ID-sawed wafers is good, this one-piece-at-a-time approach translates into long machining times, slow production rates and cutting widths rarely below 250-300 æm. Furthermore, the sizes of work pieces that ID saws can accommodate are limited. For instance, today's standard 300-mm (12-in.) silicon wafers cannot be cut using ID saws.

For these reasons, bare-wire and abrasive slurry MWS is the more commonly used method for producing thin wafers for the electronics industry. But while MWS enables cutting widths as low as 140-200 æm, this method also has disadvantages. After just a short period of time, loose abrasive dulls and slows the rate of material removal, so the slurry must be managed carefully on a continuous basis for grain sharpness and concentration. Furthermore, getting loose abrasive slurry consistently into very long or deep cuts (over 400 mm or 16 in.) presents significant application challenges. And since this slurry often has to be treated as waste, its disposal or recycling costs are high. Finally, this method can only achieve slow material removal rates, especially during the slicing of hard materials such as SiC or sapphire, where loose abrasive tends to wear the relatively soft steel wire before the much harder work piece.

A "FAST" Alternative

At Crystal Systems of Salem, Mass., a leading manufacturer of high-quality materials for the optical, laser and solar industries, the production of sapphire wafers is a major activity. The company grows sapphire boules and then slices them into wafers after core drilling.

Early on, the company's personnel came to understand the disadvantages of annular sawing and multi-wire slicing (MWS) with loose abrasive given the major grinding and finishing challenges posed by the anisotropy and high hardness and fracture toughness of sapphire. To overcome these challenges, Crystal Systems developed a system combining multi-wire kinematics with fixed-diamond technology for the mass production of wafers. Company engineers also determined that no suitable machines were available on the market for slicing sapphire with fixed-diamond wire, so they developed a completely new approach specifically for slicing with diamond-coated wire.

Known as "FAST" (fixed abrasive slicing technology), the system involves a sapphire rod fitted into a chuck, which rotates at speeds up to 5000 rpm. This design reduces the contact length so the force per diamond particle needed to slice with diamond-coated wire is achieved. At the same time, a wire pack consisting of up to 100 parallel, straight wires, each ~900 mm (36 in.) long and stretched on a frame, carries out a reciprocating movement up to 100 times per minute. In the FAST system, the rotating rod is fed downward into the wire pack. Depending on the material being cut and the machining quality desired, the downward feed rate varies from 0.1 to 10 mm/minute.

When the bulk of the rod is sliced, the operator stops the machine and glues a bar on top of the work piece to prevent the wafers from falling once the cut is fully completed. Of course, this action prevents the part from fully rotating. However, rotating back and forth less than 360 degrees during the balance of the cutting process ensures minimal contact between the wire and work piece for maximum slicing efficiency.

Getting Down to the Wire

To fully leverage the FAST machine, Crystal Systems needed to identify the best diamond wire for its highly demanding kinematics. As illustrated in Figure 1, only about 60 cm (2 ft) of each 90-cm (3-ft) wire comes in contact with the fast-spinning rod. FAST therefore requires the utmost in grain retention and concentration from the fixed-diamond wire used. High diamond concentration and excellent grit distribution along and around the wire are a must, as any bare spot will generate heat and friction instead of cutting action. The highest possible grit retention is also required-falling diamonds would lead to bare spots, rubbing instead of cutting, and ultimately wire breakage.

After years of in-house diamond wire development and extensive testing of commercially available wires from numerous U.S. and overseas suppliers, Crystal Systems discovered a fixed-diamond wire* that stands up to the high demands of the FAST system. The wire is coated with a continuous layer of synthetic or natural diamond that is evenly distributed around and along the wire in high concentrations, similar to those of annular saws (see Figure 2).

The grain is bonded by a single layer of nickel electroplated directly onto the steel wire. Unlike other diamond wires, the nickel plating takes place directly on the wire's steel core, without any copper sheath between the steel and nickel-plated layer. Because of the basic properties of copper, such layers can increase kerf loss and lower the wire's heat and wear resistance. While avoiding the use of copper results in a slower diamond wire manufacturing process, the effort provides significant advantages in wire life and breakage frequency. Tests comparing the fixed-diamond wire to a wire using a copper sheath on a single-crystal SiC material showed that the fixed-diamond wire lasted 70-80% longer and eliminated the random breakages that previously forced the operator to stand by the wire saw at all times.

With steel cores ranging from 150 to 400 µm in diameter and grit sizes between D7 and D91 in natural or synthetic diamond, the fixed-diamond wire can be used for just about any cutting and slicing application.

Putting it all Together

Today, Crystal Systems operates several FAST machines using the fixed-diamond wires. The machines run on a continuous basis for sapphire wafer production. In a typical operation, a machine cuts sapphire rods up to 100 mm (4 in.) in diameter and 100 mm long. With the blank rotating up to 5000 rpm and the wire reciprocating up to 100 times per minute, the blank is cut in less than eight hours for 50-mm rods, and 14 hours for 100-mm rods. The total thickness variation (TTV) and bow values of the 50-mm wafers are less than 0.020 mm, and the surface finish measured as average roughness (R_a) is less than 0.001 mm. With the FAST technology, the fixed-diamond wires last long enough to slice four 2-in. wafers per ~900-mm (36-in.) wire used.

The FAST machine/fixed-diamond wire combination produces even better results when slicing silicon carbide. In tests, the FAST system cut standard SiC blanks, 50 mm in diameter, with a yield of 100% TTV, and bow values were under 0.015 mm and 0.020 mm, respectively, with an R_a below 0.007 mm.

Because of the high cutting speeds and the short contact area, the diamond wire suffers less thermal damage than in other processes. As a result, the wire maintains its cutting ability longer than conventional wires. Overall, wafer slicing costs have been considerably reduced compared to conventional systems due to lower consumable costs, higher material removal rates, high machining quality, low losses of work piece material, little damage to the wafer surfaces and edge zones, and a low reject rate. The technology has also been integrated with a touch-screen computer system to improve process repeatability and minimize labor requirements.

While studies and production runs with sapphire and SiC demonstrate the superior economics of using FAST technology and the fixed-diamond wire in those processes, the combination lends itself to cutting other work piece materials as well, including quartz and various other ceramics. Interestingly, the parts to be cut do not have to be round, as FAST has successfully sliced square blocks or clusters of small rods of various geometries.

With the FAST technology and advanced fixed-diamond wire, manufacturers can achieve accurate, efficient cutting in a wide range of high-tech applications.

For more information about the FAST system, contact Crystal Systems, 27 Congress St., Salem, MA 01970; (978) 745-0088; fax (978) 744-5059; e-mail jack@crystalsystems.com ; or visit http://www.crystalsystems.com .

For more information about the fixed-diamond wire, e-mail electronics@saint-gobain.com , visit http://www.diamondwire.saint-gobain.com , or contact:

• Saint-Gobain Abrasives, Inc., 21 Saddleback Cove, Travelers Rest, SC 29690; (864) 834-4145; fax (864) 834-3730
• Saint-Gobain Diamantwerkzeuge GmbH & Co. KG, Schützenwall 13-17, D-22844 Norderstedt, Germany; (49) 40-52-580; fax (49) 40-52-58-347
• Saint-Gobain Abrasives (Singapore) Pte. Ltd. 15 Beach Rd., #04-03 Beach Centre, Singapore 189677; (65) 337-22-76; fax (65) 337-22-47

SIDEBAR: Improving Reel-to-Reel Multi-Wire Saws

Beyond the Crystal Systems FAST machine, direct-bonded fixed-diamond wires are compatible with a range of conventional single- and multi-wire saws. While work with fixed-diamond wires on such spool-to-spool MWS platforms is relatively recent, machine manufacturers and end users are highly interested in transitioning from loose to fixed abrasive processes to speed up cut rates.

In a first set of preliminary tests on materials ranging from quartz and sapphire to polycrystalline silicon carbide, direct-bonded fixed-diamond wires have demonstrated three to five times faster slicing rates vs. traditional bare wire and slurry MWS technology. Leading manufacturers already offer systems capable of running loose or fixed MWS processes, and additional application developments are ongoing-both on the machine and wire sides-to fully leverage the advantages of fixed-diamond technology on such platforms.