Patent Application
20140057531
1. Field of the Invention
The present invention is utilized in the manufacture of semiconductor material wafers without components on the front side and in particular, a method and apparatus for preventing the wafer from distorting during the planarization steps by attaching a grindable resilient tape to a porous grind chuck or hard chuck with discrete vacuum holes, grinding the tape and then placing a wafer on the chuck for processing.
2. Description of the Prior Art
The semiconductor industry has, and is, interested in producing material wafers with very low total thickness variation (TTV) from wafers that are uneven in thickness, post wire sawing for example. Most wafers are cut from their growth boule using a high production wire-sawing method that inherently produce waviness and high TTV due to the irregular motion of the wire position as the boule is turned. This waviness and sometime highly irregular wafer shape after sawing presents subsequent difficulties especially for batch processes.
U.S. Pat. No. 6,866,564 to Strasbaugh et al, assigned to the assignee of the present invention, discloses a method of back-grinding device-mounted wafers wherein a resilient back grinding component, such as a tape or pad, is applied to the surface of a solid chuck. The component is held to the chuck surface as each wafer is sequentially placed on the component and after grinding the backside of the wafer, each wafer is removed from the chuck. This patent applies to thinning wafers, such as those made from silicon, for final packaging to facilitate the process by eliminating the taping and de-taping of thin device wafers, and to save the cost associated with this step.
U.S. Pat. No. 5,964,646, assigned to the assignee of the present discloses a method and apparatus for planarizing silicon wafers initially having wavy surfaces. A vacuum is applied to one side of a porous ceramic plate, and a perforated resilient pad is affixed to the opposite side of the porous ceramic plate. A vacuum extends through the perforations on the resilient pad to permit a wafer to be mounted on the exposed side of the resilient pad. The wafer is not deformed while being held in place for grinding and thus the wafer has no tendency to spring back to its original wavy shape. Once one side of the wafer has been planarized, the second wafer surface is planarized on a flat chuck to provide a wafer that has uniform thickness.
U.S. Pat. No. 8,025,533 to Seklya discloses a method for back grinding a wafer wherein the face-side surface of the wafer is covered with a resin film, the surface of the resin film being cut to form a flat surface parallel to the face-side surface of the wafer. The wafer is held with the surface of the resin film in contact with a suction surface of a chuck table in a grinding apparatus, and the exposed backside surface of the wafer is ground. Unevenness in thickness of the resin film is suppressed, the thickness of the wafer subjected to back grinding as a result being made uniform.
Although the above-noted patents disclose various techniques for maintaining a uniform wafer thickness, the total thickness (TTV) variation is too high to meet the requirements of new chip technology.
What is desired is to provide a method for treating wafers wherein the TTV is reduced to the point that it meets the requirements of thin semiconductor manufacturers for producing semiconductor chips that can be used in various devices, such as MP3 players, televisions, etc.
The present invention provides a method of producing semiconductor material wafers with very low TTV from wafers that are uneven in thickness.
Specifically, a soft chuck surface is prepared with the soft chuck being of semiconductor production compatible clean-room material whose surface characteristics can be altered with holes for facilitating vacuum and via grinding and/or polishing to form a planar surface parallel to the cutting plane of either grinding (grind wheel plane) or polishing (pad plane).
This method facilitates and improves the first side planarization of a substrate of any shape by adhering traditional resilient back-grind tape (such as Ultron 1034R-9.8 or Ultron 1044R-9.8) that has been perforated for example, by a laser punching over the area that will contact the wafer on to the ceramic porous grind chuck, so as to allow vacuum to be applied to the wafer back surface (i.e., surface facing the chuck) to hold the wafer for processing. The tape adhering to the chuck is ground with an abrasive grind wheel, bringing the surface of the tape parallel to the chuck surface which was previously ground by the same grinding wheel. The act of grinding the tape while it is mounted on the chuck establishes the plane perpendicular to the grind spindle, removes the bumpiness from the perforation holes and evens out the non-uniformity of the tape application onto the chuck, which attachment normally being highly operator dependent, resulting in improved wafer first side grinding and thus good site flatness.
For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawing therein:
a)-1(f) illustrate the steps of backgrinding wafers in accordance with the teachings of the present invention; and
Referring to
A porous ceramic chuck 20 is first ground to the desired shape (
The upper surface 28 of tape 22 is then ground by grinder 21 parallel to surface 26 (
The purpose of grinding tape 22 is to correct any anomaly that might be associated with applying the tape to the chuck surface since this is usually a manual step depending on the skill of the operator performing the grinding step. In addition, the tape itself typically has some degree of non-uniformity that is corrected by the grinding step.
A more detailed description of the process set forth hereinabove follows: The grind chuck 20, porous as illustrated, is prepared by pre-grinding a small amount from the surface using the abrasive grind wheel 37 (
The film that protects the adhesive on the back of the tape (not shown) is then removed and the perforated tape 22 is then attached to the entire surface of grind chuck 20 with the perforated area aligned where the wafer 32 will be attached by vacuum for surface grinding or polishing. Alternatively, perforated tape holes can be aligned with holes in the top surface of a hard chuck so as to facilitate vacuum attachment onto a solid chuck rather than a porous ceramic chuck. The top tape surface is then ground with a light down force (typically less than 4 pounds) to remove bits of debris caused by perforation holes generated by the laser and to shape the top surface of the tape. The wafer 32 to be ground is then placed on the perforated portion of the tape and attached by applying vacuum from below the chuck. With the wafer held firmly, a predetermined amount of wafer material from the top side of the wafer is ground so that the wafer achieves its desired shape. The wafer 32 is then removed from the chuck, turned over and then placed on the upper surface of the chuck. The exposed surface of the wafer is then ground to a predetermined thickness.
The grinding platform utilized in the Strasbaugh 7AF grinder may be utilized to grind the tape. Using the same grinder to shape the tape that was used to shape the grind chuck enables a precise tape surface parallel to the contacting chuck surface to be fabricated which meets the stringent dimensional requirements of wafer manufacturers.
Typical resilient back-grind tape that has been successfully utilized is Ultron® tape 1034R-9.8 and Ultron® tape 1044R-9.8, distributed by Ascend Performance Materials Operations LLC, Houston, Tex.
The process of the present invention thus enables semiconductor wafers to be produced of superior uniform thickness from various materials.
It should be noted that this planarization concept also applies to polishing whereby the carrier film that is currently used for wafer backing during polish, such as the Strasbaugh DF200, can be replaced with a malleable tape that can be polished to remove the non-uniformity associated with material and operator applications.
While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.
