Patent Application
20090047884
The present invention relates to chemical mechanical polishing (CMP) pad structures, and more particularly the present invention relates to CMP pad structures that minimize trapped air and polishing fluid intrusion.
Chemical-mechanical planarization or Chemical-mechanical polishing, commonly abbreviated CMP, is a technique used in semiconductor fabrication for planarizing the top surface of an in-process semiconductor wafer or other substrate. The process typically uses an abrasive and reactive chemical slurry, commonly a colloid, (collectively called the polishing fluid) in conjunction with a polishing pad and retaining ring, typically of a greater diameter than the wafer. The pad and wafer are pressed together by a dynamic polishing head and held in place by the retaining ring. The dynamic polishing head and pad are rotated, with different axes of rotation (i.e., not concentric) to remove material and tends to even out any irregular topography, making the wafer flat or planar. This is generally necessary in order to set up the wafer for the formation of additional circuit elements. For example, this might be necessary in order to bring the entire surface within the depth of field of a photolithography system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to nanometer levels. Other systems using polishing belts have also been proposed, but rotary polishing pads are more common in the industry.
A polishing pad for a rotary CMP polishing apparatus is a relatively thin, planar, disk-shaped article. The pad includes at least a polishing layer having a polishing surface on a first end of the polishing pad and a polishing layer peripheral edge extending away from the polishing surface. If only the polishing layer is present the pad is typically referred to as a “single layer pad”. The pad may be comprised of multiple layers which is sometimes referred to as a stacked pad construction. Within the meaning of this application each layer of a pad will have two opposed surfaces or faces, which are the circular faces of the layer and the surfaces are connected by the peripheral edge of the layer which forms the side of the layer. The “ends” of the pad are, within this application, the outer facing surfaces of the pad. A “peripheral edge” of the pad, within this application, extends from the polishing surface to other outer facing surface and is comprised of the peripheral edges of each layer of the pad (which may be one or more). Examples of single layer and stacked pads are commercially available, for example, from PPG Industries and Rohm & Haas Electronic Materials (R&H).
The CMP polishing pad typically is placed on a platen of a polishing machine, or on another mounting surface (herein collectively called platens), and secured to the platen by a pressure sensitive adhesive on the back surface of an attaching layer on the polishing pad. The attaching layer typically is a gas impermeable layer such as a polyethylene terephthalate (PET) type layer. Examples of attaching layers are sold by 3M, Inc., Adhesives Research, Inc. and Adchem, Inc., for example. As the polishing pad is placed on the platen, bubbles of air tend to get trapped between the attaching layer and the platen. Any trapped air can distend the relatively thin pad, thereby causing raised areas or bulges in the polishing surface of the polishing pad. The presence of trapped air prevents the air entrapping portion of the attaching layer from contacting and adhering to the platen. These bulges are very difficult to eliminate by conventional means, e.g., by forcing the air bubbles out from under the pad with a roller. Some sources have suggested that the bulges be manually pierced with a hand tool and then the pad around the pierced bulge can be pressed flat against the platen or other mounting surface to obtain the flattest possible polishing surface. This proposed manual process would be time-consuming, and some small bulges that remain undetected would cause bulges to remain in the polishing surface. Any bulges in the polishing surface of the pad tend to generate non-uniformities on the polished surface of the wafer work piece during polishing, thereby causing defects in the polished surface of the wafer. Also, bulges in the polishing surface of the pad can cause the surface to wear at a faster rate than the remainder of the pad, thus resulting in shortened pad life.
Other features which are included in some polishing pads for specific purposes include “windows” that allow for “through-the-pad” measurement of wafer flatness during polishing. Early versions of this feature merely consisted of an opening completely through the pad such that metering equipment within or below the platen can measure through the opening. In some instances the opening through the pad contains a transparent window member that will prevent polishing fluid from filling the window opening thereby disrupting the polishing process in the area of the window opening. It is difficult to design transparent window members in the window opening that allow for the through-the-pad wafer measurement but do not detrimentally effect the pad polishing. Regarding the focus of the present invention such a window opening, even when not filled with a window member, fails to adequately address the air entrapment during pad attachment as the window opening is limited to a single spot on the pad (i.e., the spot that must be aligned with the measuring equipment in the platen).
Also known is a type of pad used on polishing tools that pumps polishing fluid through the platen and through polishing fluid holes in the pad. In such a pad polishing fluid holes are provided in a grid work of about tol inch spacing along the pad. The holes extend entirely through the pad and generally must be aligned with associated polishing fluid supply features in a specialized platen. These polishing fluid holes, however, generally are spaced too far apart to adequately minimize air entrapment during attachment. Regardless of other design features, the issue of air entrapment remains.
U.S. Pat. No. 6,699,104 attempted to address this trapped air problem by providing a polishing pad having a polishing layer and a bottom attaching layer below the polishing layer. The attaching layer has an adhesive bottom surface and a plurality of hollow air transmitting pathways or channels in the attaching layer extending to the peripheral edge of the adhesive layer. When the attaching layer bottom surface is applied to a platen, air which is trapped between the adhesive backed attaching layer and the platen can escape through the channels. The channels however must be “sealed” with a special edge sealing tool that minimizes the channel openings at the peripheral edge such that air can escape but the channel is below a minimum capillary level to allow polishing fluid to enter, based upon the “surface tension” of a given polishing fluid. This edge sealing technique seems to be too time consuming or impractical to be widely commercially adopted.
U.S. Pat. No. 6,197,397 discloses an adhesive that has a “micro-replicated topography” prepared from contacting a micro-embossed pattern to a layer of adhesive. The patent proposes that when an adhesion interface is established between the layer of adhesive and a supporting substrate, the topography of the adhesive surface controls the performance of the adhesion interface. The reference suggests that the adhesive surfaces also have the advantage of providing micro-channels for fluid egress for an effective period of time. In practice, however, it has been noted that upon application of CMP pads with pressure sensitive adhesives such channels in the adhesive itself will be “rolled” out with the application of the pad and therefore will not be effective to allow trapped air to escape, for example, where the pad is constructed of a flexible material.
Thus there remains a need to expand the development of CMP pad structures that minimize trapped air during pad attachment and minimize polishing fluid intrusion at the pad and platen interface.
It is noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless expressly and unequivocally limited to one referent. The “top” of a CMP pad will reference the polishing layer end of the pad, and the “bottom” of a CMP pad will reference the attaching surface end of the pad, but “top” and “bottom” are merely used for relative orientation and not intended to define operational orientation of the pad itself (e.g. the polishing layer “top” may be vertically below the attaching layer “bottom” of the pad in certain polishing machines). The reference to “through” a pad layer indicates that the structure (e.g. an opening) is extending from one surface of the layer to the opposed surface of the layer. The reference to “across” a pad layer will indicate that the structure (e.g. a channel”) is extending along a given surface of the pad layer but not “through” the layer as defined herein.
For the purposes of this specification, unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and other parameters used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
All numerical ranges herein include all numerical values and ranges of all numerical values within the recited numerical ranges. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
The various embodiments and examples of the present invention as presented herein are each understood to be non-limiting with respect to the scope of the invention. In accordance with one embodiment of the invention a polishing pad is configured to be removably attached to a platen in a manner minimizing trapped air and polishing fluid intrusion. The pad comprises a polishing layer having a polishing surface on a first end of the polishing pad and a polishing layer peripheral edge extending away from the polishing surface; a gas impermeable attaching layer defining an attachment surface for securing the polishing pad to a platen at a second end of the polishing pad which is opposed from the first end, the attaching layer having an attaching layer peripheral edge extending away from the attachment surface, wherein a peripheral edge of the pad extends from the polishing surface to the attaching surface formed of the peripheral edges of each layer of the pad; and a plurality of openings extending through the attaching layer. During pad attachment trapped air can flow through the openings in the attaching layer and out of the peripheral edge of the pad at a position spaced from the attaching layer peripheral edge. The attaching layer peripheral edge has no openings which minimize the polishing fluid from being directed to the attaching layer and platen interface.
These and other advantages of the present invention will be described in the following description taken together with the attached figures in which like reference numeral represent like elements throughout.
The pad 5 is a stacked pad formed of several pad layers. Within this application, adjacent pad layers are attached to each other in conventional fashion. Adhesive or direct casting (layers directly bonded together) are typically utilized to attach adjacent pad layers, but any conventional attachment technology can be used for adjacent layers (e.g. ultrasonic welding of certain materials).
The term “Layer” within this application defines a structural pad layer, which is generally a film, foam, fiber structure, or the like. The term “layer” does not include the typical adhesive material (pressure sensitive adhesives) used solely to bond adjacent structural pad layers together, even though such materials are sometimes referenced in the art as “adhesive layers”, “adhesive films” and “adhesive film layers.” The pressure sensitive adhesives typically have no meaningful material strength by themselves, i.e. no significant tensile strength, no significant modulus, etc. The adhesive is present only to hold the structural pad elements or pad layers together and does not generally form a structural aspect of the pad. Pressure sensitive adhesives form a bond by the application of light pressure to marry the adhesive with the adherend(s). They are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow, or wet, the surface(s) to be adhered. The bond has strength (i.e., bond strength) because the adhesive has sufficient resistance to flow when stress is applied to the bond. Once the adhesive and the surface(s) to be adhered are in close proximity, molecular interactions, such as van der Waals forces, become involved in the bond, contributing significantly to its ultimate strength. Pressure sensitive adhesives (PSAs) are designed for either permanent (pad layer to pad layer) or removable applications (pad to platen). Consequently the term “layer” within the meaning of this application is a structural pad layer, such as film, foam, fibers and the like, having no substantial flow (ability to wet a bonding surface) under ambient conditions.
It should also be apparent that a pad “layer” within the meaning of this application may be attached to an adjacent pad “layer”, as noted above, without a separate “adhesive”. For example a pad layer can be a thermoplastic resin that is placed in contact with an adjacent layer or layers in a heated state and will bond directly thereto as the resin sets (e.g. a hot melt adhesive).
The pad 5 includes a polishing layer 12 having a polishing surface on a first end of the polishing pad 5 and a polishing layer peripheral edge extending away from the polishing surface.
The polishing layer 12 may be formed in any conventional fashion, and can have, for example, an open cell structure or a closed cell structure. Representative non limiting examples of the material for the polishing layer 12 of the CMP pad 5 include but are not limited to felts and polymer impregnated felts, such as those described in U.S. Pat. No. 4,728,552, commercially available under the trade name SUBA™ IV pads from R&H; microporous synthetic leather, available as POLITEX™ pads from R&H; porous polymer sheet, available as IC1000 and IC1010 from R&H, and FastPad™ CS7 and FastPad™ S7 available from PPG Industries, Inc.; and unfilled textured polymer sheet. The polishing layer typically will be 0.050 to 0.080 inches in thickness, but any desired polishing layer thickness can be utilized.
Adjacent the polishing layer 12, in pad 5, is a barrier layer 14 that is attached to the polishing layer 12. The barrier layer 14 typically is a gas and liquid impermeable layer provided between the polishing layer and a sub-pad layer 16, and minimizes the movement of polishing fluid into the sub-pad layer 16. Where the barrier layer 14 is formed as a double sided adhesive structure the layer 14 serves as an attaching structure for easily assembling the stacked pad 5.
Non-limiting examples of materials suitable for use as the barrier layer include polymer and metallic films and foils. Examples of polymers that may be used as the barrier layer in the present invention include polyesters and co-polyesters, such as PET and PET glycol; polyolefin, such as low density polyethylene, high density polyethylene ultra-high molecular weight polyethylene and polypropylene; polyvinylchloride; cellulose-based polymers, such as cellulose acetate and cellulose butyrate; acrylic; polycarbonate; and polyamide, such as nylon 6/6 and nylon 6/12; Examples of metallic films that may be used in the barrier layer of the present invention include, but are not limited to: aluminum; copper; brass; nickel; and stainless steel. The barrier layer 14 typically will be 1 to 4 mils (about 25 to 100 micrometers) in thickness, but any desired barrier layer 14 thickness can be utilized.
A gas permeable sub-pad layer 16 is adjacent the barrier layer 14. The gas permeable sub-pad layer 16 will allow gas, such as trapped air that enters the sub-pad layer 16 to flow out of the sub-pad layer peripheral edge (e.g. the side) of the pad 5. The gas permeable sub-pad layer 16 can be formed as an open cell foam or non-woven fiber mat or any other known gas permeable CMP pad layer constructions.
Non-limiting examples of materials suitable for the gas permeable sub-pad layer include but are not limited to non-woven fabric impregnated with polyurethane as describe in U.S. Pat. No. 4,728,552, i.e. polyurethane impregnated felt, and open cell foam sheet made of polyurethane, natural rubber, synthetic rubber, and thermoplastic elastomer. An example of a suitable commercial material is a subpad which is available from R&H under the tradename SUBA™ IV. The sub-pad layer 16 typically will be 0.030 to 0.060 inches in thickness, but any desired sub-pad layer 16 thickness can be utilized.
A gas impermeable attaching layer 18 is adjacent the sub-pad layer 16 in pad 5 and defines an attachment surface for securing the polishing pad 5 to the platen 20 at a second end of the polishing pad 5 (which is opposed from the first end of the pad 5 at the polishing surface). The attaching layer 18 has an attaching layer peripheral edge extending away from the attachment surface. The peripheral edge of the pad 5 extends from the polishing surface to the attaching surface and is formed of the peripheral edges of each layer of the pad 5, namely the peripheral edges of the polishing layer 12, the barrier layer 14 the sub-pad layer 16 and the attaching layer 18. The gas impermeable attaching layer 18 will, without other structure, trap air between the layer 18 and the platen which can be detrimental to the operation of the pad 5 as discussed above.
The attaching layer 18 can, effectively, be formed substantially the same as barrier layer 14 discussed above. The attaching layer 18 will typically be 1 to 4 mils in thickness, but any conventional or desired attaching layer 18 thickness can be utilized.
The present invention provides a plurality of openings 22 extending through the attaching layer 18, whereby trapped air, during pad attachment, can flow through the openings 22 in the attaching layer 18 and out of the peripheral edge of the pad 5 at a position spaced from the attaching layer peripheral edge. The plurality of openings 22 may form a pattern of openings 22 on the attaching layer 18 such as the grid work of openings 22 as shown in
Preferably the size of the openings 22 should be large enough to allow the free flow of trapped air through the openings 22 during attachment of the pad 5, and small enough not affect the support or attachment of the pad 5. The above sizes and spacing for the opening 22 have been found to be effective. During attachment the pressure sensitive adhesive may cover an opening 22, but the air pressure of the trapped air beneath the pad 5 adjacent the opening 22 will force through the opening 22 as the adhesive has no significant material strength by itself. Where the openings 22 are made extremely small (i.e. smaller than a minimum effective area) the adhesive may contain enough strength to keep the opening 22 sealed and keep air trapped. The minimum effective area of the opening 22 will likely vary with different adhesives and attaching layer 18 forming material. The specific ranges of the minimum and maximum effective areas of openings 22 for any given adhesive and layer 18 are believed to be discovered through routine experimentation.
The openings 22 may be formed in the layer 18 after the formation of the pad 5 or after formation of the layer 18, such as using a perforating roller on either the pad 5 or the layer 18 (before layer 18 is attached to the pad 5). Alternatively the opening 22 could be formed in the layer 18 with the formation of the layer 18, such as casting the openings in place (e.g. removable pins). However, a perforating roller on the formed layer 18, or formed pad 5, seems more practical.
The pad 5 as shown and described minimizes trapped air and the intrusion of polishing fluid into the interface between the platen 20 and attaching layer 18. The polishing fluid intrusion is minimized because the attaching layer 18 does not have openings or channels at its peripheral edge. The trapped air is directed “up and out” in the present invention. This construction will help avoid having the polishing fluid that may be sitting on the platen 20 adjacent the attaching layer 18 from being channeled across the interface between the platen 20 and attaching layer 18.
All of the above embodiments illustrate a construction where the pad layer adjacent the attaching layer 18 is gas permeable. There are pad layers that are gas impermeable and such layers can be positioned adjacent the attaching layer 18.
An example of the present invention was formed by using a pad roller perforator on top of an attaching layer 18 of a stacked pad (similar to pad 5 above) wherein the perforator penetrated the layer 18 but not the sub-pad layer 16, whereby any trapped air during pad mounting can escape through perforations into the sub-pad and out. The Pad perforator is a hand held roller about 4.0″ wide, 15 point row, with points ¼″ apart. A 3/16″ thick sheet of clear acrylic was used as a representative platen 20 for mounting purposes to allow for observation.
The Pad was placed on laboratory bench top with attaching layer 18, and release liner that will be removed upon pad attachment, facing up. The Perforator applied to about ½ of pad to be observed. A piece of opaque tape applied to other side of clear acrylic sheet to show perforated & un-perforated side of experimental pad. The experimental Pad was applied to the simulated plexiglass platen while pulling liner off. A one Kilogram weight was used to force any trapped air pockets out (to the extent possible) by moving it along the top (polishing) surface of the experimental pad as the liner was being pulled off. The results, illustrated in
Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.
