Patent Application
20060046433
This invention relates generally to thinning semiconductor wafers.
Thinning semiconductor wafers may be desirable, for example, because thinner wafers may have reduced thermal resistance and increased reliability. Thinning the wafer may reduce the die stress from thermal effects, thereby increasing reliability. The thickness of the wafer may also affect the package size adversely. In some cases, the thickness of the wafer may adversely affect performance. Thus, for a variety of reasons it is desirable to thin semiconductor wafers.
As wafers are made thinner and thinner still, the severity of wafer damage may increase dramatically. At thicknesses below 5 mils, wafers are prone to cracking, surface burnishing, and surface irregularities. Bumped wafers may have additional problems related to surface pitting in areas between bumps. Especially with very thin wafers, the wafers are prone to damage during removal from the thinning fixture.
Thus, there is a need for better ways to thin semiconductor wafers.
Referring to
A fixture 10 may hold the wafer 12 during processing. It may have a plurality of protrusions 16 that, in one embodiment, may also be solder balls or surface mount features. In another embodiment, the protrusions may be integral with the fixture 10.
As shown in
Where the protrusions 16 are integral with the fixture 10, the fixture 10 may be formed, for example, of etched silicon, borosilicate glass, or alumina. The protrusions 16 may have dimensions similar to the features 14 on the wafer 12 with corresponding spacing and pitch. In some embodiments, the features 14 may be offset such that the protrusions 16 lie in interstitial areas between features 14. As a result of the interference fit, the wafer 12 may be firmly secured while it is subjected to rotary grinding motion indicated by the arrow A in
Eventually, the wafer 12 may be thinned as indicated in
In accordance with another embodiment of the present invention, shown in
For example, the adhesive 20 may be a thermoplastic resin initially heated to its melting point. Once it reaches its melting point, it can be engaged by the wafer 12 and, particularly, by the features 14. Once the two elements are engaged, as shown in
Referring to
Referring to
The openings 26 may correspond generally to the horizontal dimensions of the features 14. Initially, the openings 26 may be slightly larger than the features 14 in order to allow a feature 14 to be inserted into an opening 26. Thus, as shown in
In some embodiments, the film 24 may be made of a sheet of Nitinol metal or from woven Nitinol structure with the openings in the woven pattern for forming the openings 26 for capturing the features 14.
The shape memory alloys may be alloys that undergo a martensitic phase transformation that yields a thermoelastic martensite phase exhibiting high elongation characteristics. This phase typically appears as a herringbone structure formed from an assembly of alternately sheared platelets. As the temperature increases, the martensite phase transforms to the austenite, decreasing the starting dimensions of the film 24 openings 26.
This transformation may not occur at a single temperature, but, instead, may occur over a range of temperatures that varies with each alloy system. Most of the transformation may occur over a relatively narrow temperature range, although the beginning and ending of the transformation, during heating or cooling, actually extends over a much larger temperature range. The transformation may also exhibit hysteresis in that the transformations on heating and on cooling do not overlap.
Referring to
In one embodiment of the present invention, Nitinol shape memory alloy having an austenitic phase at room temperature and a martensitic phase transformation in the range of 40° C. to 100° C. may be utilized. The diameter of the openings 26, in the film 24, may be from about 1 to about 8 percent greater than the feature 14 diameter, to allow easy engagement and self-alignment of the features 14 and the fixture 22. Upon wafer thinning, the heat generated due to the frictional forces causes heating to temperatures greater than 40° C. As a result, the film 24 transforms to the martensitic phase, in which its openings 26 are smaller, reducing the diameter of the openings 26 and engaging the features 14.
In some embodiments, the fixture 22 may also be equipped with a heater that heats the film 24 in order to positively engage the features 14. After thinning, the structure may be cooled to room temperature, causing the openings 26 to increase in diameter, thereby releasing the wafer 12 that has been thinned, as shown in
Thus, in some embodiments of the present invention, wafers to be thinned may be more effectively held. This may reduce wafer damage during thinning operations such as when wafers are thinned to 5 mils or less.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
