Clear adhesive sheet

Details Clear adhesive sheet Clear adhesive sheet

2000-07-13

2002-10-29

Zirker, Daniel (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Adhesive outermost layer

C428S345000, C428S3550AC, C428S3550EN, C428S901000

Reexamination Certificate

active

06472065

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to adhesive coated sheets used during fabrication of electronic devices and more particularly to adhesive coatings having a low level of haze and reduced levels of tack after exposure to suitable radiation.
BACKGROUND OF THE INVENTION
The production of semiconductor dies, approximately 100 &mgr;m to 175 &mgr;m thick, requires a sequence of process steps for converting a silicon wafer into relatively tiny integrated circuit chips. Conventional processing of silicon wafers uses adhesive coated sheet material during grinding of full-thickness wafers to a target overall thickness, and while dicing a wafer into individual semiconductor dies.
Wafer dicing requires delicate control of equipment since the dicing process is one of the most aggressive operations in silicon wafer manufacture. Individual wafers acquire a great deal of value during processing before wafer dicing and any damage occurring at this stage is very costly in terms of die scrapped and production costs associated with die replacement. Dicing techniques and processes can introduce chipping into both the front and backside edges of the die due to such factors as blade loading, depth of the cut, feed rates, blade width, coolant, blade orientation with reference to the crystal lattice, tape adhesion, and die rotation.
The use of radiation-curable adhesive sheets offers several advantages over non-curing types of adhesive sheet material. Ultraviolet (UV) wafer dicing tape, for example, provides the benefit of high holding power during dicing, which serves to reduce die rotation. Die rotation is one cause of chipping of semiconductor dies. The high holding power of the UV wafer dicing tape thus reduces damage to individual dies. Damage to dies may be further reduced, after dicing, by exposing adhesive to UV radiation to greatly lower tape adhesion. Lowering of tape adhesion, i.e. level of tack, reduces the strength of the bond between the exposed adhesive and the back of the processed wafer. This lessens ejector pin scratches, chipping and other forms of die damage by reducing the force necessary to eject or remove a die from the surface of an adhesive tape.
U.S. Pat. No. 5,480,842 refers to the use of UV adhesive tapes during wafer grinding and wafer dicing operations of semiconductor die fabrication. No compositional information is provided for the UV sensitive adhesive. Another U.S. Pat. No. 6,039,830 describes the use of an UV tape comprising a base film of polyolefin coated on one side with an acrylate-containing adhesive having a photoinitiator. This limited information only places the adhesive material within the general category of acrylate adhesives that includes a wide variety of compositions differing in properties. The description of a radiation curable adhesive tape, in U.S. Pat. No. 4,999,242, provides evidence of the use of copolymers including acrylate ester monomers, and acrylic acid. Copolymers formed from these monomers may be mixed with urethane acrylate oligomers to provide adhesive formulations that lose adhesion during exposure to ultraviolet radiation. There is no indication of the actual proportions of monomers used to produce the acrylate copolymers, and thus no evidence of property changes, such as transparency of the adhesives, due to change in proportions of selected monomers. U.S. Pat. No. 4,756,968 uses an adhesive layer consisting of an adhesive and a radiation polymerizable compound. The radiation polymerizable compound is a urethane acrylate oligomer having a molecular weight of 3,000-10,000. This oligomer may be used with an equal amount of acrylate copolymer adhesive to provide a radiation sensitive composition in the presence of a benzophenone type of free radical initiator. Upon irradiation with a suitable form of radiant energy, the adhesion level of the adhesive decreases. This property has value for wafer dicing processes, as previously discussed.
A material that changes color during radiation may be added to curing adhesive compositions, described in U.S. Pat. No. 4,756,968, to facilitate detection and picking of diced dies by automatic equipment. In the absence of color change, the photosensor loses discriminatory power causing errors in the automatic picking of separated dies. Automated equipment processes for fabricating semiconductor dies include steps for inspecting silicon wafers at various stages of manufacture. Inspection equipment will, at times, be required to assess the condition of a wafer covered and protected by an adhesive sheet. A highly transparent sheet will reduce die sensing and die picking errors and facilitate inspection of a wafer for grinding and sawing damage . Transparency of adhesives and adhesive sheets, which is important to wafer processing, may be altered during exposure of an adhesive to radiation. The radiation produces a curing reaction and adhesive polymerization with haze development that reduces sheet transparency.
U.S. Pat. No. 6,013,722 describes water resistant, low haze acrylic emulsion pressure sensitive adhesive compositions. Although apparently resistant to one-time application of water, during lamination, there is no evidence to show that the low haze emulsion adhesives described as “water resistant,” could withstand repeated water impingement from the high-pressure water jets used during wafer dicing. It is known that, in contact with water, emulsion polymers typically exhibit haze characteristics described as “cloudy,” “milky,” and “white.”
A low haze radiation curable coating, described in U.S. Pat. No. 5,907,023, comprises a urethane oligomer, including urethane acrylates, as a primary ingredient of a radiation polymerizable liquid composition. Visual determination of haze shows that lowering the proportion of urethane oligomer, in the curable compositions, produces more haze. The observed increase in haze may result, for example, from incompatibility among formulation components.
Significant reduction or substantial elimination of haze would be advantageous to allow better inspection of wafers during processing. Improvement towards this objective could lead to greater effectiveness and efficiency of automated semiconductor die manufacture, thereby reducing cost.
SUMMARY OF THE INVENTION
The present invention provides adhesive compositions suitable for coating on transparent substrates to provide adhesive sheets exhibiting relatively high initial adhesion and low haze. Exposure to suitable radiation induces curing of clear adhesive sheets, according to the present invention, to a condition having adhesion values significantly lower than an uncured adhesive. Reduction of adhesion, also referred to herein as adhesive detackification, proceeds without significant change in sheet transparency. Adhesion reduction without increase in haze favors the use of clear adhesive sheets according to the present invention in automated processes, during semiconductor die manufacture, and particularly during inspection of diced wafers for cracks and chipping.
High initial adhesion of UV detackifiable wafer-dicing adhesive sheets, according to the present invention, facilitates dicing without wafer edge damage or die losses. Controlled lowering of adhesion also facilitates die removal or pick from the adhesive coating by lowering the force required to remove individual dies. The use of clear tape allows inspection of die edges before picking. Die inspection, whether visual or machine automated, identifies damaged dies for rejection to avoid defective die failures in subsequent operations.
Adhesive compositions according to the present invention provide wafer-dicing tapes having greatly improved clarity and initial adhesion levels sufficiently high for effective bonding to silicon wafers. A process of adhesive detackification uses ultraviolet radiation to crosslink an adhesive composition, to lower the level of adhesion or tack and thereby allows low force, clean removal of individual dies produced during the diamond sawing, and wafer dicing operation.
More particularly the present invention provides a rad

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