Sticky Issues With Semiconductor Processing Tape

The degree of success or failure in applying plasticized PVC film tape for semiconductor processing is determined by whether the right tape is used for the job.  There are three levels of tape quality suitable for use: industrial, electronic, or select.  The degree of uniformity determined the tape grade, which is defined by the level of adhesion, material thickness, and elasticity in both X and Y directions.

Industrial vs. Electronic
Industrial grade tape is not an acceptable level of quality for semiconductor manufacturing use.  This tape should never make direct contact with either the wafer or die because the rolls contain unacceptable pockets of entrapped air.  The adhesion levels actually are reduced in those areas of the tape roll containing the entrapped air.  If the diameters of the sir bubbles are larger than the size of the die, the die might fly off during the sawing process.  The overall result will be smaller yields.  Unless the air bubbles are noticed while the tape is still on the roll so that portion of the can be discarded, there is little that can be done.  Once the tape has been applied, those air bubbles will become invisible and the areas where they occurred will become areas of low adhesion.

Tape procurers must be wary that the tape they ate purchasing is at least the electronic grade.  In times of high demand, electronic and select grades are hard to obtain.  Tape manufacturers can make more square feet of industrial grade tape per unit of time, and there is a larger market for it.  Thus, their production priority is on industrial tape.  Users of higher tape grades burden their suppliers since their usage changes, sometimes drastically unpredictably.

To avoid unknowingly buying tape that is industrial grade, the user should only deal with a manufacturer or distributor who will provide test and inspection results by lot number with the tape it ships.  This certifies the tape to be electronic grade by being free of entrapped air bubbles.  Also, when the tape manufacturer sets up a line for an electronic grade run, a core material such as plastic is substituted for cardboard to help distinguish it.  A plastic core is dust-free to support the cleanroom environment.

Industrial grade tape can also come from the same roll as electronic grade tape.  The difference between the two grades takes place when the tape is rolled into lengths.  It is virtually impossible to roll the tape evenly, especially in the middle.  Since the middle portion is less tightly rolled, it contains entrapped air.  The two outer portions, made up of tightly wound, bubble-free tape, are cut off and become electronic grade tape.  The middle portion is sold as industrial grade.  It is that middle from the industrial grade run that is sometimes sold as electronic grade, with or without the premium charge the latter commands.

Electronic grade tape is available in rolls and pre-cut squares.  The squares are a handy way for users without tape handling equipment to dispense tape.  Tape rolls are used in automatic and manual wafer mounting systems to mount tape onto wafer/film frame assemblies containing silicon wafer tapes most commonly used are blue, black, and clear adhesive tape with different levels of stickiness, and non-adhesive clear tape.  (Fig.1).

Figure 1: Electronic grade tape, left to right: (back row) medium tack blue, low tack blue; (middle row) low tack clear, high tack black, mounted wafer/film frame; (front row) black, clear and blue tape in square formats.

Adhesion Levels
Adhesion levels are stated in ounces of adhesion per inch of tape width or in ounces of force required to unpeel the tape per inch of width.  For example, electronic grade blue tape is tested according to Mil Spec ASTM D-1000.  To determine whether the specification is met, a piece of tape is applied to a piece of stainless steel having a certain finish and cleanliness.  After the tape is left on the steel for 20 minutes, the adhesive strength is measured by unpeeling the tape 180° back across itself at a specific speed.

Low tack 3-mil blue tape is ideal when sawing wafers with large die on them.  The adhesion level (2 oz/in. of width nominal) is high enough to hold the die firmly during sawing, but low enough for the die to be easily removed by die bonders or pick and place equipment.  Other uses for this have included removal of foreign particles from the face of wafers and covering ink jet printer heads.  Low tack 4-mil clear tape is used when sawing silicon wafers where a thicker tape is required with the same adhesion level.

The most popular tape is medium tack 3-mil blue, which is used mostly on silicon wafers having smaller die of <180mil².  The adhesion level (5oz/in. of width) is slightly greater than low tack blue to more firmly  hold the die to the tape when sawing, but it is low enough that the die can be easily removed at the next stage of processing.

High black tape is used when sawing ceramic and other hard-to-cut material or when slightly, more adhesion is required – for thick, heavy substrates, for example.  The adhesion level of black tape is 8oz/in. of width.  This tape is made thicker (5mils) in order to accommodate deeper dicing saw penetration and the contour of the saw blade at the two outer cutting edges.  The black tape is popular with manufacturers who work with very small ceramic die.  However, there is a tendency for some of the adhesive to be transferred onto the small die.  In cases where this would be a problem, it is best to use one of the more expensive select grade tapes.

Non-adhesive 4-mil clear tape is a flexible poly vinyl chloride with excellent clarity.  It is tough, has high tear strength and may be elongated up to 200 percent.  Available only in squares mounted on non-contaminating backing paper, it is used extensively for wafer fracturing after scribing, expanding and wafer surface protection.

In addition to die size, other characteristics determine the required tackiness of the tape.  The surface finish of the wafer back is one factor, since shiny smooth surfaces enhance the adhesion qualities of the tape.  Other factors are the efficiency of the dicing equipment at cleanly separating the die and whether the taped wafer is subjected to rigorous operations after dicing that include high pressure washing, rinsing, and drying.

Select Grade
Select grade tape is used for special high-grade devices or applications, such as very thin or very thick wafers, very large or very small chips, and when the tape is left on for a long time.  Because these tapes feature superior characteristics in uniformity and low ionic contamination levels, they command four to 10 times the prices of electronic grade tape.

When processing delicate wafers, one type of select grade tape is applicable.  This tape releases more readily than electronic grade tapes when exposed to UV light.  Such exposure breaks down the adhesive strands.  A new select grade tape releases when exposed to UV light.  Such exposure breaks down the adhesive strands.  A new select grade tape releases when exposed to a mildly elevated temperature.  Another select grade tape is available for use when backlapping.  This grade of tape has fewer trace elements of metal and other contaminants which can transfer onto the active face of the wafer.  Select grade tapes require storage below 40°C and protection from direct sunlight.

Tape Performance Factors
A peculiarity of all tape is the natural phenomenon of post-curing.  This is the variation in adhesion strength after the tape has been shipped and put in storage.  As Fig. 2 shows, the adhesive strength decreases to a lower value than what is stated on the tape manufacturer’s quality inspection sheets in days following the tape’s production.  This slight decline does not affect its performance as specified by the manufacturers, because once the tape is used the level of adhesive strength increases.

Figure 2: post-curing causes a drop in adhesion strength after the tape has been shipped and put in storage.

The longer the die are on tape, the more they adhere.  Even if the tape is chosen for its low adhesive level over time, at some point it will be difficult to remove the die.  The typical production facility will mount the tape, dice and clean the die, and remove them within one day or, at the maximum, on the following day.  After that, the adhesion level starts to increase to where after a month it becomes difficult to remove the die.

The tape should not be exposed to ultraviolet light sources such as sunlight or fluorescent lights.  When put in storage at between 10-25°C and in humidity of 60±15%, the shelf life of electronic grade tape is usually one year from the date of manufacture.  After that time the adhesion of the tape becomes unreliable due to certain ingredients in the tape leeching out to the surface.  After several years, the backside of the tape can take on a gummy appearance because of this chemical leeching.

Tape Mounting Process
Control of the mounting process temperature helps control the amount of adhesion.  The best heat setting on the wafer/frame tape applicator is 60°C.  This temperature makes the tape more flexible, allowing wafer contact to be uniform and conformal.  Expansion of the tape and subsequent contraction upon cooling helps to cause evenly distributed tape tension on the wafer in the film frame.  This effect will occur if the tape has uniform elasticity.

The amount of tension needs to be even in both X and Y directions.  Thus, the mounting system used should feature a means of evenly supporting the tape around  the perimeter of the film frame as the tape is being mounted.  The wrong amount of tension in either direction will result in load shifting, leading to crooked die rows when the tension is released on the second saw cut.

Once the tape has been mounted, it should not be sawed immediately.  About 30 minutes of dwell time should be built into the period between the mounting and dicing steps to allow time for the process to stabilize and provide uniform adhesion.  The time interval should be the same for every film frame.

Various mounting system designs are available.  Some merely stick the tape on the wafer film frame, as opposed to rolling it on.  Most make use of the hand roller, similar to what a printer uses when inking.  A few employ a vacuum to draw the tape down onto the wafer and require a hand roller, similar to what a printer uses when inking.  A few employ a vacuum to draw the tape down onto the wafer and require a hand roller.  Others require two pieces of equipment to mount the wafer/film frame: one machine to mount the tape to the ring set and another to press the wafer onto the mounted tape.

It is important to select an applicator that allows the operator to mount the tape to the wafer and film frame quickly, easily, uniformly, and in a bubble-free manner.  An applicator that allows air to be trapped requires using a knife to puncture the sir bubbles by hand and flattening out the tape in those areas.

One of the most comprehensive applicator designs is shown in Fig 3.  This particular tape applicator works with all standard film frames and features a captive roller, cutting blades that automatically retract when not in use and the option of a non-contact wafer platen.  The applicator also has a built-in tensioning frame, closed loop temperature control and a spring-loaden platen working in concert with the roller to apply the mounting pressure.

Figure 3: Tape being pulled from an internal storage position over wafer/film frame mounting platen in a tape applicator from Semiconductor Equipment Corporation.

Acknowledgements:
The author wishes to thank Nobby Abe of Nitto Denko America Inc. and Michele Bindler and Kristen Kenlin of Permacel Inc. for their technical contributions.