Wafer Dicing: A Sticky Situation
Wafer dicing processes are no longer limited to the basic principle of separating a silicon wafer into individual die. As more packaging processes occur at the wafer level, and miniaturization is a must, different parameters must be taken into consideration when deciding which singulation process is best suited to the task at hand.
For example, singulating QFN packages requires methods that can address the ductile and brittle materials that compose the complex substrate. MEMS packages, on the other hand, contain such minute and delicate structures - cantilevers, bridges, hinges, gears, membranes, and other sensitive features - they require special handling and care.1
Silicon wafers thinned below 100 µm, and brittle, GaAs wafers pose other challenges to dicing methods - such as chipping, cracking, and debris generation. Wafer scribing and sawing, the two most common techniques used to separate a wafer into individual die, are most often performed with a diamond blade saw and diamond scribe tool, respectively.2 Innovations in laser technology have made laser scribing and dicing a viable option, especially in the case of blue LED packages and GaAs substrates.
Figure 1. Dicing process diagram using standard UV tape.
Regardless of which dicing process is selected, all methods require a wafer to be secured and, post dicing, for the die to remain intact in transport and storage through the die-attach processes. Options available include tape-based systems, jig-based systems, and tape-less systems that use alternative adhesives.
The standard dicing process begins with mounting thinned wafers, active side up, onto release tape fixed to a steel ring. This secures the wafer during the dicing process, and keeps die and packages aligned for easy transport to the next step in the process. Process limitations include issues around thinner wafers, difficulty removing the die from the tape after storage, tape being cut when diced with a laser, and damage caused by water used in the process.
When considering a tape-based system, it is important to consider the mounting system, as well as the type of tape best suited to the materials being diced. Various options exist for frame mounting systems. Semiconductor Equipment Corporation (SEC) has two models of wafer/film frame tape applicators that apply tape within controlled temperature and pressure parameters. Advanced Dicing Technologies’ (ADT) 966 Wafer Mounter is a high-volume automatic mounting system for blue and UV tape with uniform mounting and tape-tensioning that eliminates air bubble formation. It has a circular blade for cutting leftover tape and programmable temperature regulation. Disco Corporation designs different tools for package and wafer singulation, including dicing saws, blades, and dicing engines.
All dicing tape is made of three components - a base film plastic covered with pressure-sensitive adhesive film, and a release film. The two categories of tape used in most cases are blue (which is lower cost) and UV. Blue tape is suitable for dicing standard silicon wafers, and is sometimes used in conjunction with the more costly UV tape when dicing more delicate substrate materials such as GaAs. The substrate would be mounted on a piece of double-sided UV tape cut to size, which in turn is mounted onto the metal frame with blue tape.
Figure 2. This UV curing system is a table top unit in 200- or 300-mm configurations and can handle 50 wafers per hour at a wavelength of 365 nm.
When selecting the appropriate tape, it is crucial to consider tack, adhesion, and other mechanical properties. The goal is to have an adhesion level high enough to hold the die during sawing, and low enough for the die to be removed easily, without damage, for die-attach processes. If a coolant with lubricant is used during dicing, it is important to be sure that whatever additive is used doesn’t interact with the adhesive on the tape, or the die could move out of position. Most tapes have a limited shelf-life of up to one year. After that, the adhesive begins to break down.
Figure 3. Process flow for needle-less UV tape. (diagram source: Adwill/Lintec)
UV provides two levels of adhesion: stronger during the dicing process, and then reduced when cured with UV radiation for easy lift-off (Figure 1). ADT’s UV 955 Curing System is a compact, table-top system used to irradiate UV tapes post-dicing to facilitate die removal (Figure 2). Compliant with safety standards, it handles up to 50 wafers per hour at a wavelength of 365 nm. The system uses integrated lamp-life sensors with LED indicators to monitor performance at all times. UV tape, although more costly, is suitable for dicing sensitive substrates, such as GaAs and singulating optical components.
Several manufacturers offer premium specialty tapes to serve a smaller market of specific requirements. Nitto-Denko manufactures a thermal-release tape that uses heat for irradiation instead of UV curing. When heat is applied, the tape releases its grip on the substrate. Adwill offers a needle-less dicing tape that allows pick-and-place machines to pick the die off without using needles from below (Figure 3). The tape maintains strong adhesion during the dicing process, and the chip distance is self-expanded by UV irradiation and heat after dicing. There is no needle-induced device breakage. Furokawa Electric manufactures an electrostatic discharge (ESD) tape which reduces contamination and is intended for singulation of sensitive devices such as MEMS and image sensors. AI Technology, Inc. manufactures a dicing and die-attach film (DDAF) that combines high-temperature, antistatic, ultra-low residue dicing tape with electrically conductive, high bond strength die-attach epoxy, which enables low residue trasfer of the adhesive from the tape. (Figure 4).
Using Tape with Laser Processes
Wafer dicing is a traditional industry, and currently most dicing and scribing is done using mechanical saws and scribes, according to Ramon Albalak, Ph.D., engineering manager, ADT. While ADT has a laser scribing system available, Albalak says that lasers are a relatively new player in the dicing and singulation markets.
Figure 4. This combination dicing tape and die-attach film is available in control- and UV-release varieties.
According to Doug Pulfer, business manager at JP Sercel Associates, customers using JPSA ChromaDice, a dry-etch laser scribe process, do not encounter issues with standard dicing tape. The wafers are scribed to a depth that produces the best break. High throughput and best yield are achieved by balancing the depth of scribe with the breaking. The shallower scribe depth that will produce a 100% yield on breaking is the best scribe depth for maximum throughput (Figure 5).
Figure 5. Smaller kerfs, such as this 2.5-µm-width kerf created with a laser, and shallower scribe depths produce less debris.
However, when using the laser to dice wafers - cutting all the way through and eliminating the break step - the tape becomes an issue, as the laser will cut through the tape. But cutting nearly all the way through the wafer will permit singulation on the tape without the breaking machine. The material left in the bottom of the vee-groove is only microns thick and will break completely by simply moving your hand under the backside of the tape.
Synova, who has developed a water-jet guided laser process for dicing, has also formulated LaserTape, which retains singulated parts during the laser cutting. Because it doesn’t absorb laser light, it isn’t cut during the dicing process. It is porous to allow water to pass through without damage. Work pieces can be picked up easily after UV-curing, because the glue isn’t adhered on the back-side of the wafer. When used in conjunction with the company’s proprietary water-jet guided technology, which uses a hair-thin water jet to guide a laser beam and eliminates heat damage and contamination, it is said to be a reliable method of singulation.
Alternatives to Tape-based Systems
Tape-based systems are still the method-of-choice when dicing silicon in low-volume operations, and when diced wafers are being transported and stored for a period of time prior to the die-attach process. However, when dicing more robust packages in high-volume, such as BGA, QFN, or CSPs, a tape-less, jig-based system that uses a vacuum system to secure and transport the wafer through an integrated system is recommended. The jig is matched to the substrate, and a layer of rubber holds the wafer in place. Under each die or package is a vacuum hole. When the wafer is diced, the die or packages remain in the array until the next step in the process - pick-and-place or die bonding.
One alternative to tape-based dicing, is a thick-wafer scribing process. The wafer is scribed before thinning and can be held in place by a vacuum chuck, rather than tape, and thinned later to the appropriate thickness for breaking along the already-made scribes. The thinning process can cause wafers to experience bowing or cupping, making it difficult to scribe or dice accurately with any method. If the wafer is thinned after scribing, any resulting bowing or cupping may be less of a problem.
Another method of tape-less dicing involves alternative means of adhesion. For instance, sensitive substrates may be coated with a wax adhesive and mounted on glass. The dicing saw cuts through the substrate and into the glass, which holds the singulated die and eliminates movement. This alternative mounting system is generally seen in R&D or low-production settings, or when the substrate in question is very expensive and die are small.
Although traditional sawing and tape-based systems are still the mainstay of wafer dicing technologies, innovations in materials and methods - such as laser dicing and scribing, specialty tapes, and vacuum-based systems - provide viable options for sensitive package singulation.
- Ramon J. Albalak, Ph.D. “Dicing MEMS” Advanced Dicing Technologies, Haifa, Israel.
- Richard K. Ulrich and William D. Brown, Advanced Electronic Packaging, 2nd Edition, p. 402.
The author would like to thank Ramon J. Albalak, Ph.D., engineering manager at Advanced Dicing Technologies, and Doug Pulfer, sales manager at J.P. Sercel Associates, for their contributions to this article.