Patent Application
20060211349
The present invention relates to a polishing template and method of utilizing the polishing template. More specifically, the invention relates to improving the flatness and wafer shape on the peripheral edge of semiconductor wafers during a polishing process.
In the production of semiconductor wafers, such as silicon wafers, it is important to produce wafers having very flat, smooth, and parallel surfaces. These wafers serve as the substrate for a wide variety of microelectronic devices. As such, any irregularities introduced during the production of these wafers may ultimately cause defects and failure in the microelectronic or semiconductor devices manufactured on them. Therefore, the minimization of surface defects is of the utmost importance.
Chemical Mechanical Polishing, or CMP, is the most common method of polishing wafer surfaces in order to produce the desired flat, smooth, and parallel surfaces. In CMP, a slurry comprising etching chemicals and abrasive particles is applied to the surface of wafers through a porous polishing pad. A number of wafers are secured to individual polishing templates that are in turn mounted to a platen. The polishing pad containing the slurry is then brought into contact with the wafers.
As the name suggests, CMP acts through both chemical and mechanical action. The chemicals in the slurry typically are used to oxidize the surface of the wafer, causing the surface of the wafer to corrode. After corrosion occurs, the mechanical action of the rotating polishing pad and the abrasion by the particles within the slurry act to mechanically remove the corroded surface.
In practice, several polishing steps may be involved in polishing the surface of the wafer, with each successive step utilizing smaller and smaller abrasive particles. Between each step, the wafers are rinsed with water to stop the chemical action and to remove any remaining abrasive particles. The result of this process is the ability to start with a rough surface and end with a very finely polished, smooth surface.
In order to successfully produce large quantities of wafers with consistent high and similar quality, many procedures have been developed. One critical parameter is operating temperature. When a polishing machine sits idle and has not processed wafers for an extended amount of time, the platen and polishing pads cool down. Temperature significantly affects the repeatability of the polishing process, and as such, it is common in the polishing process to either heat the platen with heat exchangers, to run a number of “dummy” loads of wafers to increase the temperature up to the normal operating process, or a combination of both. Naturally, running “dummy” loads increases production costs and reduces productivity of the machines. Similarly, running heat exchangers to heat the platen increases production costs.
Unfortunately, notwithstanding the careful design of the polishing process, it has generally been difficult to polish wafers that have both a consistent thickness and a flat surface near the peripheral edge of the wafers. Instead, many wafers suffer from either edge flip in which the peripheral edge have a greater thickness than the medial portion of the wafer, or edge roll in which the peripheral edge has a reduced thickness relative to the medial portion of the wafer. In either situation, the peripheral edge of the wafer is not flat in comparison to, or planar with the medial portion of the wafer. Edge flip and edge roll are attributed to the different mechanical forces exerted on the peripheral portion of the wafer compared to those forces exerted on the medial portion.
Traditionally, the specifications associated with wafer fabrication included a so-called “edge exclusion”, wherein the outer periphery of the wafer was excluded from the stringent flatness and specifications, as it was not contemplated that devices would be fabricated on this annular peripheral region.
Over time, and with the increase in the diameter of wafers, the width of this edge exclusion has been shrinking. It has increasingly become important to improve the template polishing process to improve the peripheral edge profile of the wafers to match that of the medial portion.
Polishing templates used in the polishing of semiconductor wafers, and associated methods of using the templates, are provided. One aspect of the present invention provides a novel template that improves the consistency between the peripheral edge portion of a polished wafer and the medial portion of that same wafer. According to this aspect, by changing the shape and profile of the wafer template, the resulting shape of the polished wafer can be altered and better controlled. In this regard, the usable surface area of the wafer can be increased and the edge exclusion requirement can be reduced or eliminated.
Another aspect of the present invention provides methods of altering or manufacturing templates suitable for reducing edge roll and edge flip. According to this aspect, templates may be readily changed from one shape to another to assist in optimizing the edge profile. The ability to temporarily modify the shape of the template provides added flexibility in the polishing process, and allows for process optimization based on parameters such as platen temperature, platen pressure, and polishing pad life.
The present invention now will be described more fully hereinafter in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to convey the scope of the invention to those skilled in the art.
Wafer templates used in polishing processes for semiconductor wafers are made of a plastic material such as PVC or the like. The template is manufactured with tight thickness control, and is free of blemishes or other disfigurations that could transfer through to the wafer being polished. The template is somewhat larger in diameter than the wafer to be polished, and contains a front and backside. The front side of the template contains an annular receptacle ring extending from the outer edge of the template inward to the diameter of the wafer to be polished, and somewhat thinner that the final thickness of the wafer, thereby forming a cavity to receive the wafer to be polished. A soft cloth, pad, or other frictionless and soft material is on the template, such that the wafer and frictionless material will stick together when wetted, without the use of adhesives such as wax or the like, and will be surrounded circumferentially in the cavity formed by the annular receptacle ring such that the wafer cannot slide laterally off of the template.
The backside of the template contains an adhesive sheet over essentially the entire surface that is used to stick the template and wafer to a platen used in polishing. The backside also contains a thin edge enhancement ring that extends from approximately 5 millimeters smaller than the wafer diameter to approximately 5 millimeters larger than the wafer diameter. For example, if the wafer to be polished is 200 mm in diameter, the template may be approximately 220 millimeters in diameter, with the edge enhancement ring having an annular shape with an inner diameter of approximately 190 mm in diameter and an outer diameter of approximately 210 mm. Optionally, the edge enhancement ring may go out to the edge of the template, but in any case the outer edge of the template must have sufficient adhesive to stick to the platen without coming loose. The thickness of the edge enhancement ring is in the range of 70 to 130 microns thick, optimized depending upon the polishing pad used, but preferably approximately 95 microns in thickness.
The edge enhancement ring may be manufactured as part of the template itself. This may be accomplished by adding an extra layer or layers of adhesive on the back surface in the desired area to form the edge enhancement ring. In this instance the added layer or layers may be placed either before or after the adhesive sheet. It may also be added thickness of the plastic sheet itself.
The edge enhancement ring may also be an add-on piece such as, for example, double side stick tape meeting the desired thickness and diameter specifications may be utilized. In the case that an add-on piece is utilized for the edge enhancement ring, care must be taken to ensure it is secured in a flat manner without any folding, wrinkles, or the like. Also, if an add-on is utilized, it may be replaced after each run or as needed.
Other embodiments of the present invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification be considered in all aspects as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the forgoing description. All changes, which come within the meaning and range of the equivalence of the claims, are to be embraced within their scope.
