Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
1999-11-19
2003-02-25
Truong, Duc (Department: 1711)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S3550RA, C428S343000, C427S208400, C427S208000
Reexamination Certificate
active
06524701
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a pressure sensitive adhesive sheet. More particularly, the present invention relates to a pressure sensitive adhesive sheet which, at the time of working of the back of an adherend having a surface whose unevenness (irregularity) height differences are large, is preferably stuck to the adherend surface to thereby protect it during the working.
BACKGROUND OF THE INVENTION
In the step of grinding the back of a semiconductor wafer, the surface thereof having electrical circuits formed thereon is protected by a pressure sensitive adhesive sheet. The height differences of customary circuits attributed to electrode elements have ranged from about 5 to 20 &mgr;m. For the wafer having these customary circuits formed thereon, the conventional surface protective sheets can fully protect the circuits without the occurrence of circuit breakage or wafer cracking and thus have been satisfactory.
However, in recent years, the method of mounting IC chips has become diversified. For example, a packaging method has been developed in which an IC chip circuit surface is arranged in a downward direction. In this packaging method, electrode elements protrude from the circuit surface and the height differences thereof are 30 &mgr;m or more, occasionally greater than 100 &mgr;m. Protrudent portions formed on the surface of semiconductor wafers like those mentioned above are referred to in the art as bumps.
At least two bumps are generally formed per chip. When the number of bumps is large, the bump pitch (distance between bumps) may be hundreds of microns. Depending on bump pitch pattern and chip arrangement, there occur dense portions where the bumps gather and portions where the bump pattern is sparse. In particular, the periphery of wafers is likely to become sparse in bump portions because of the absence of chips. With respect to wafers having the pressure sensitive adhesive sheet stuck thereto, the thicknesses thereof at dense bump sites and sparse bump sites conspicuously differ. The grinding of such wafers having conspicuous thickness differences would cause a problem because the thickness differences lead to a difference in the ground wafer thickness.
When the back of a wafer carrying such bumps is subjected to grinding while the wafer surface is protected with a conventional surface protective sheet, the wafer back is ground deeply in conformity with the configuration of the bumps which forms recesses (dimples) on the back of the wafer with the result that the thickness of the wafer becomes nonuniform. Further, cracking has been encountered, which occurs from dimple portions, resulting in breakage of the wafer.
A similar cracking problem is encountered with respect to the ink marking of failure circuits (bad mark) after the inspection of the wafer circuits.
For semiconductor wafers having large bumps, preventive measures have been taken, which include, for example, decreasing the hardness of the substrate film of the surface protective sheet or increasing the thickness of the pressure sensitive adhesive sheet. These measures, however, have been unsatisfactory, and the above problems have not yet been resolved.
The present invention overcomes the problems found in the prior art and provides a pressure sensitive adhesive sheet which, at the time of working of the back of an adherend having a surface whose unevenness height differences are large, is preferably stuck to the adherend surface to thereby protect it during the working. In particular, the present invention provides a pressure sensitive adhesive sheet which, when the adherend is to be ground to an extremely small thickness, enables performing the grinding at a uniform thickness without the occurrence of dimples.
SUMMARY OF THE INVENTION
A first pressure sensitive adhesive sheet of the present invention comprises a substrate, an intermediate layer superimposed thereon and a pressure sensitive adhesive layer superimposed on the intermediate layer,
the above pressure sensitive adhesive layer exhibiting an elastic modulus at 23° C. ranging from 5.0×10
4
to 1.0×10
7
Pa, the above intermediate layer exhibiting an elastic modulus at 23° C. which is not greater than the elastic modulus at 23° C. of the pressure sensitive adhesive layer.
A second pressure sensitive adhesive sheet according to the present invention comprises a substrate, an intermediate layer superimposed thereon and a pressure sensitive adhesive layer superimposed on the intermediate layer,
the above intermediate layer exhibiting an elastic modulus at 40° C. of less than 1.0×10
6
Pa.
In the present invention, it is preferred that the substrate exhibit a maximum value of tan &dgr; of dynamic viscoelasticity of at least 0.5 at a temperature ranging from −5 to 80° C.
The substrate preferably has a thickness and Young's modulus whose multiplication product is in the range of 0.5 to 100 kg/cm.
Moreover, the method of using the pressure sensitive adhesive sheet according to the present invention comprises sticking the pressure sensitive adhesive sheet to a surface of an adherend and working the adherend at its back while protecting the adherend surface by means of the pressure sensitive adhesive sheet.
Thus, the present invention provides a surface protective pressure sensitive adhesive sheet which desirably follows unevennesses formed at an adherend surface to thereby absorb unevenness height differences, so that a smooth back grinding of the adherend can be performed without being influenced by the surface unevennesses and, hence, without a thickness difference. Further, the present invention provides a method of using a pressure sensitive adhesive sheet, characterized by the use of this surface protective pressure sensitive adhesive sheet.
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be described in greater detail below. First of all, the first pressure sensitive adhesive sheet according to the present invention will be illustrated.
The first pressure sensitive adhesive sheet of the present invention comprises a substrate, an intermediate layer and a pressure sensitive adhesive layer arranged in this ordered array.
The pressure sensitive adhesive layer can be formed from various conventional pressure sensitive adhesives. The elastic modulus at 23° C. of the pressure sensitive adhesive layer is in the range of 5.0×10
4
to 1.0×10
7
Pa, preferably 6.0×10
4
to 5.0×10
6
Pa, and still more preferably 8.0×10
4
to 1.0×10
6
Pa. When the pressure sensitive adhesive layer is composed of an energy radiation (energy beam) curable pressure sensitive adhesive described later, the elastic modulus is that value exhibited by the pressure sensitive adhesive layer prior to the energy radiation irradiation.
When the elastic modulus at 23° C. of the pressure sensitive adhesive layer is lower than 5.0×10
4
Pa, it is likely that the pressure sensitive adhesive will ooze from the edges of the pressure sensitive adhesive sheet, or it is likely for the pressure sensitive adhesive to suffer from shearing deformation caused by the grinding force because of insufficient cohesive strength, with the result that the difference of ground wafer thickness is increased. Further, if a shear force applied to the pressure sensitive adhesive forces the adhesive into bump recesses, the danger of the pressure sensitive adhesive remaining on the wafer surface is high. On the other hand, when the elastic modulus at 23° C. of the pressure sensitive adhesive layer is higher than 1.0×10
7
Pa, the pressure sensitive adhesive layer becomes hard and its capability for following bump unevennesses is deteriorated. Consequently, problems such as an increase in the difference of ground wafer thickness and an invasion of cooling water for grinding through gaps between bumps and the pressure sensitive adhesive sheet are likely to occur.
The pressure sensitive adhesive, although not limited at all, can be selected from among, for example, rubber, acrylic, silicone and polyv
Kondo Takeshi
Mineura Yoshihisa
Takahashi Kazuhiro
Lintec Corporation
Truong Duc
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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