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Part II: Wafer dicing System and Substrate Interaction Considerations

Part II: Wafer dicing System and Substrate Interaction Considerations

By John Boucher, President, Thermocarbon, Inc., Casselberry, Florida
Reprinted from Microelectronic Manufacturing and Testing May & June, 1986

 

Read first part of the article.
Part I: Introduction And Fundamental Requirements Of Diamond Dicing

 

While most end-users will take considerable steps in assuring the rigidity of the machine they purchase, they may overlook the critical mounting requirements necessary for the diamond blade. No matter how well the diamond blade was manufactured to run true, it can only run as accurately as the surfaces it comes into contact with. The bearing surfaces of the flanges or spacers must be flat, clean, and parallel. 

Accuracy in cut width and the ability to increase product output is a function of the tolerances used in manufacturing the resin-bonded diamond blade as well as the flanges used to mount them. A diamond blade, made to exact dimensions, can only run as true as the bearing surfaces that it comes into contact with.

 

Spacers used in gang cutting operations are generally made from aluminum or titanium carbide, depending on the application. Flanges or adapters for single-blade mounting are usually 4 made from stainless steel. The flanges will incorporate an undercut to reduce the bearing surface area in order to enhance intimate contact with the diamond blade. These surfaces, as well as the diamond blade surfaces, must be clean, with no loose particles present prior to assembly. This ensures proper fitting of the mating surfaces. 

Microscopic inspection is required of flange surfaces between blade changes to ensure their integrity. Any repairing of the bearing surfaces should be accomplished by lapping. In extreme cases, remachining of the flange may be necessary. All flanges and spacers have torque specifications to prevent distortion and ultimate separation of the bearing surfaces from the surfaces of the diamond blade. The most frequent cause of blade breakage and oversize cut widths with relation to blade thickness is improper flange torque or poor flange quality. 

lmproper handling or unclean assembly will hinder the performance of even the newest of flanges. Lapping the bearing surfaces of the flange for the removal of imbedded diamond particles or light burrs that may occur along the periphery will insure that the integrity of the flange/blade assembly is maintained for proper and long term performance.

 

Flanges and spacers must not induce vibration at operating spindle speeds. One method of determining whether or not the flange or spacer assembly is the source of vibration is to remove them from the spindle arbor, ramp up to operating speed, and see if the vibration is present. If no vibration exists then, the flanges or spacers are the problem. If the vibration persists, the spindle is in need of repair.

Reasons to pay attention to blade exposure

Dicing blade exposure, the distance that the cutting edge of the diamond blade protrudes from the flange or spacer periphery, is a critical item within the variables affecting the overall rigidity of the sawing system. Over-exposure may cause wider than desired kerf, excessive edge chipping, nonsquareness of cut, and blade breakage, while too little exposure of dicing blade can divert the critical coolant supply from the blade/material interface.

There exist no definitive rules for determining the optimum dicing  blade exposure for a specific application. Some blade manufacturers have published guidelines requiring ten times the blade thickness as the method of determining blade exposure. This type of approach does not take into consideration the depth of cut, dicing material hardness, or feed rate, and therefore is highly misleading. However, some rule of thumb is necessary to develop a starting point for determining the proper exposure of cutting blade.

The best results will usually be attained by adjusting the original “ten times” (six times the blade thickness for sapphire applications) blade thickness guideline in accordance with a prerequisite that at least 1 /3 to 1 /2 of the diamond blade’s exposure be buried into the cut. This prerequisite is the dominant variable in establishing proper blade exposure. This approach offers improved stability at the start, and depending upon material hardness and feed rates, can be finetuned with only minor adjustments. The tendency should be to expose the diamond blade at a minimum to attain maximum blade rigidity, with caution given in regard to a possible coolant cut-off or a collision of the flange with the workpiece. The alternative is to run a maximum exposure, within the guidelines, to reduce the amount of flange changes required to consume the entire working range of the self-sharpening diamond blade. 

Equally important to rigid blade mounting procedures are the workpiece mounting methods. These two variables of the sawing system are the closest in proximity to the desired finished parts and warrant proper attention. The workpiece is normally mounted on an intermediate carrier, which is then mounted onto the chuck surface by vacuum or mechanical means. This enables the user to cut completely through the workpiece without causing damage to the chuck surface. Vacuum chuck systems require a vacuum gauge to indicate holding stability and assure operating safety. 

The two most common intermediate carriers are tape and glass. The workpiece is held to the “tacky” side of the tape, while wax is used as the bonding medium for mounting on glass. Either system is an acceptable method, but like anything else, they each provide advantages for specific applications. The two systems are not competitive, but rather complementary for cutting a wide variety of hard, brittle materials. 

Tape or film is normally supplied with one side possessing a light adhesive to provide fair holding power to the workpiece during the cutting process, yet still providing ease of part removal after processing. These tapes are conducive to easy handling and process automation.

The fact that these tapes are plastic and exhibit elastic qualities explains why they should only be used in light-cutting applications. Rigidity is paramount to cutting hard materials, and mounting a part on plastic that will stretch or shift during cutting defeats the system rigidity concept. Secondly, diamond blades do not cut plastic, but rather displace the plastic material, creating a burr. This plastic burr, having no place else to go, is forced under the edges of the material being processed, which in turn pushes the material upward along the cut edge. Subsequently, when cutting 90 degrees to the original cuts the strips of the material being processed will either break loose or vibrate against the cutting edge of the diamond blade.

Diamond dicing blade thickness: How to process wafer material efficiently?

Another concern with using tapes is the thickness of the diamond blade being used for processing a given dicing material. Not only is thickness a function of the amount of plastic being displaced, but also the radius formed on the blade edge. Diamond blades designed for hard material cutting, such as the resin-bonded type, form a natural radius at its comers along the cutting edge. This radius provides stress-free cutting of hard materials, however, its profile is propagated to the material being processed if it is not positioned well below the material being cut. The thicker the blade is, the larger the radius. The tapes which are normally only three mils in thickness do not allow passing the cutting edge of the diamond blade adequately below the bottom of the substrate.

Doubling the tape to a thickness of six mils, or acquiring a thicker tape, only compounds the plastic displacement problem, and will further hinder system rigidity by providing additional plastic qualities to the tape. Blade wear becomes excessive when cutting deep into tapes because the plastic will tend to melt around the cutting edge, thus preventing efficient cooling, and causing loading of the diamond blade. The blade will break down rapidly until only about one mil of the blade is into the tape. As the blade continues to cut the substrate, (depending on the number and length of cuts), it will lose that remaining one mil buried into the tape, and will not cut completely through the material for the remaining cuts.

Tape should only be used for scribing and light cut-through applications. Dicing completely through 0.020 in silicon or 0.015 in thick ceramic materials are examples of light applications for use with tape systems. Even in these light applications where cut-through is mandatory, certain precautions must be taken to ensure the desired results. The tape must be applied air bubble and dirt-free to ensure solid mounting to the chuck surface. Secondly, to prevent parts from being lifted off the tape while dicing, mount the substrate 24 hours prior to processing, or use heat when applying the tape to the clean substrate surface. The tape should always be applied under a fully prestretched condition through the use of mounting ring systems or various other acceptable mounting stations presently available. This pre-stretching reduces the loading of the plastic material onto the blade edge and will prevent the plastic burr from forcing itself under the workpiece.

Methods to cut thicker materials with dicing blades

Generally, thicker dicing materials must be cut on glass or graphite carriers to assure cutting completely through the workpiece consistently. In critical applications, this method will reduce the cutting forces required to process the workpiece. The latter requires that the carrier be softer than the workpiece being processed. Glass possesses a dressing property conducive to diamond grinding, and is acceptable for most applications. Soda-lime, or simply window glass is sufficient, if relatively flat to ensure a proper fit to the chuck surface.

Simple window glass provides excellent characteristics for the cutting through of hard/ brittle materials. Glass carriers exhibit dressing properties for the resin-bonded diamond blade and allows for cutting well below the workpiece for insuring straight parallel edges on the finished parts.

 

It also is good practice to cut a minimum of five mils (0.127mm) into the glass carrier for most applications. This will ensure straight, parallel cut edges in the workpiece, and provide adequate assurance of cutting completely through the substrate up to the final cut. With thicker materials (i.e. 50 to 100 mils), it is recommended to cut 10 to 15 mils or more into the glass carrier in order to reduce the amount that the cutting edge of the diamond blade is in contact with the harder workpiece. This method is very efficient when cutting through extremely hard materials such as GaAs, sapphire, or high-density ceramics. 

The most common problem associated with the wax mounting of materials for cutting is the choice of wax. Most waxes result in “loading,” so that caution should be used in their selection. Most brown waxes are acceptable, but waxes such as beeswax or those containing paraffin should be avoided. Wax should always be kept at a minimum, with any excess removed prior to cutting. 

Wax loading of the diamond blade is easily detected by inspecting the cutting edge of the blade for a glossy appearance. Wax fills the voids generated by the diamond blade as used abrasives are pulled from the cutting face. These microscopic voids are necessary for the diamond blade to carry coolant into the cut, and to serve as a conduit for removing particles of material generated during the abrasive machining process. Interference with this self-sharpening process of abrasive machining will result in excessive heat in the cut, and eventual blade breakage.

Operating within an Efficient Dicing System - Where do I Start?

Determining the work to be done and applying the guidelines for building an efficient sawing system, covered in part one of this article, will dramatically increase the chances for success in processing any given material. However, there is a need to understand how the remaining variable process components such as the diamond blade, and system operating parameters, can affect the total system. 

The following sections will deal with hard, brittle material processing and the resin-bonded diamond blade. The case histories described will only list the variables necessary for appropriate background and understanding as to why a given variable component was selected or modified. 

The resin-bond blade matrix is a very complicated composite system made up of abrasives, fillers, and resin, of which the combinations are infinite. No one combination exists that can dice all materials successfully. Internal variations of diamond grit, resin strength, concentration, and fillers, all affect, differently, the results attained in a specific application.

Main criteria to choose a proper diamond dicing blade

Selection of the proper diamond blade for a specific application is an essential step in achieving an efficient diamond grinding process. The optimal diamond blade is one in which the bond will wear at the same rate as the diamond particles dull during the cutting operation at a given set of parameters. The problem is twofold: For each diamond blade matrix, there exists an optimal operating range. A simple approach to determining this “steady state” for a given diamond blade is to collect the data with regard to the amount of blade wear realized over a given distance. Increasing the distance over which the blade wear can be significantly measured will improve the accuracy and validity of the results 

Premature diamond particle pull-out from the bond is the characteristic of a bond too soft for the application. Such bonds will cause premature blade failure and be susceptible to loading at the cutting edge. A bond that tends to hold the diamond particles beyond their usefulness is considered too hard for a specific application. A bond that is too hard will cause burning in the cut and glazing of the blade’s cutting edge. Exactly how the diamond blade will act - hard or soft - is also determined by the parameters under which it is operating (e.g. feed rate and spindle speed). 

The diamond particle size, or grit, within the blade’s matrix, will also affect the hard or soft acting characteristics of a diamond blade. Coarser diamonds are best suited to heavy or faster stock removals, while finer diamond sizes perform best in lighter applications and where a fine finish on the workpiece is desired. Generally, finer diamond sizes demand higher horsepower than coarser diamonds and generate higher heat levels in the cut. A normal approach to diamond size selection is to choose the largest diamond size available for a particular blade thickness, and work down in size until the desired finish is obtained. 

The final criteria in blade selection are the tolerances employed at the manufacture of the blade. Thickness tolerances must be specified to the end-user so that he is aware of the blade’s limitations to run true. Loose thickness tolerances will not allow proper seating of the bearing surfaces for even the finest of flanges or spacers.

 

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