Description of Related Art In the manufacture of semiconductor wafers and integrated circuits, providing uniformity of planarity of the wafer surface is highly important. In this connection, when photolithographic processes are used, these processes are utilized close to the limit of resolution, and it is necessary that the wafer surface be highly planar in order to accurately focus the electromagnetic or other radiation used to create the integrated circuit in a single level and thereby provide precise imaging over the entire surface of the wafer. A wavy or curved or wedge-shaped semiconductor disk results in a lack of definition when a photosensitive resist is applied to the surface of the disk and exposed.

Accordingly, to achieve the extent of planarity necessary to produce ultra high-density integrated circuits, chemical-mechanical planarization processes are employed. These chemical mechanical planarization or polishing (CMP) processes involve pressing a semiconductor wafer against a moving polishing surface that contains an abrasive material or is wetted with a chemically reactive, abrasive slurry. The slurries are either basic or acidic and may contain alumina, silica or other abrasive particles. Typically, the polishing surface is a planar pad made of soft, porous material, such as polyurethane foam or non-woven fabric, and the pad is generally mounted on a planar platen.

A conventional method for effecting polish operations on semiconductor wafers is performed using commercially available polishing tools. The wafer is held to a carrier plate (or wafer carrier) by vacuum, or possibly by some other means such as an adhesive. A load is applied to the wafer through the carrier by a pressure plate to press the wafer into frictional contact with the polishing pad mounted on a rotating turntable. The wafer carriers on the polishing tools include an insert or carrier backing film, which acts as the holding device during polishing and transport of the wafer to and from the polishing pad. Possible carrier backing film materials include Rodel DF-200 and Rogers Poron, which have a sponge-like composition.

It has been found that during the polishing process, the load imposed on the wafer is not uniform, and causes center to edge non-uniformity in thickness as well as flatness of the wafer. During polishing, the surface life of the polishing cloth also affects the planarity of the polished wafer. In addition, frictional heat generated at the wafer surface accelerates chemical action of polishing fluid and thereby increases the polishing rate. However, the frictional heat can cause planarity problems unless the heat of friction is evenly transmitted over the surface of the wafer (typically, polishing systems utilize cooling systems to control the temperature of the polishing operation).

A well known phenomenon in chemical-mechanical polishing is the edge bead effect. The edge bead effect in oxide CMP is observed after polishing, where the remaining oxide thickness in an annular region 3 to 7 mm from the edge of the wafer is substantially greater than on the remainder of the wafer. The problem is worst when stiff pads, which are required for effective planarization, are used on top of soft pads, which enhance uniformity. Furthermore, CMP with conventional pads and backing films has a high polish rate in an annular region of radius 60 to 80 mm, on a 200mm wafer.

This nonuniform thickness causes yield loss, for example, when an oxide reactive-ion etch (RIE) process does not get all the way through the thick oxide at the edge of the wafer.

Workers in the field of CMP have found that the source of the edge bead effect is"pad dive" or tilting of the polishing pad. The standard process for oxide CMP is to use a hard pad, for good planarization, stacked on a soft pad, for good global uniformity. Polishing is done with the wafer surface extending beyond the surface of the surrounding carrier. As the pad passes under the carrier and under the wafer, the edge forces the pad to tilt locally. The pad pressure on the wafer is very high at the outer 2 mm of the wafer, but very low 3 to 7 mm in from the edge of the wafer. The low pressure results in low polish rate. The harder the pad, the larger the tilted region.

The standard wafer backing film has a soft porous top layer, which allows air applied to the backside of the wafer to escape through the outer edge of the film. When 200 mm wafers are being polished, the soft foam experiences compression set in the annular region with an inner radius 97 mm and therefore must be replaced frequently.

U. S. Patent 5,885,135 discloses a method for polishing semiconductor wafers that comprises securing the wafer to a lower surface of a wafer carrier means in which the lower surface has a non- uniform surface which when a force is applied to the upper surface of the wafer carrier to contact the wafer with a polishing pad. The wafer carrier provides a force, which varies across the wafer surface such that the polishing process is enhanced to impart a planar polished wafer surface.

U. S. Patent 5,941,758 discloses a method of and apparatus for chemical mechanical polishing of a semiconductor wafer in which the substrate to be polished is placed face down in direct contact with a moving polishing pad. During polishing, a first portion of the substrate is pressed down against the polishing pad by a first fluid maintained at a first pressure applied directly to a first portion of the backside of the substrate. A second portion of the substrate is pressed down against the polishing pad by a second fluid maintained at a second pressure applied directly to a second portion of the backside of the substrate. A wear-resistant retaining ring adjacent to and surrounding the outer edge of the substrate is pressed down against the polishing pad with a third pressure applied by a mechanical force.

The substrate is rotated during polishing to help facilitate uniform polishing.

A wafer polishing apparatus that increases polishing uniformity by adjusting an abutting pressure on a retainer ring to an optimum value is disclosed in U. S. Patent 5,584,751. The abutting pressure of the retainer ring against the polishing pad is regulated by operating the ring pressure regulating mechanism that is provided separately from the carrier pressure regulating mechanism, thereby preventing the waving deformation of the polishing pad to avoid an excessive polishing of the outer peripheral portion of the wafer.

U. S. Patent 5,916,016 discloses methods and apparatus for programmably controlling the back pressure applied through a wafer carrier to the backside of a wafer during a chemical mechanical polishing operation. The system includes a mechanical arm and a carrier body that is configured to be coupled to the mechanical arm. The carrier body has a recessed portion for retaining a semiconductor wafer and the recessed portion has a carrier film that is in direct contact with a backside of the semiconductor wafer.

U. S. Patents 5,762,539 and 5,791,973 disclose apparatus and methods for polishing a wafer in which the apparatus has a source for supplying pressurized fluid and selective openings therefor, to permit uniform polishing of the semiconductor substrate even when the thickness of the substrate varies from one portion to another.

A process is needed to accomplish high polishing uniformity of a wafer that is simple and inexpensive to implement on a wide variety of polishing tools. Furthermore, it is desirable to have the wafer backing film act as a seal against backside air leakage at the edge of the wafer. In this regard, it should be noted that the Rodel DF-200 material permits outward air flow if the material is not sufficiently compressed. If the DF-200 material is used with too high a backside air pressure, this may result in uncontrolled and unstable release of air at the wafer edge,. leading to non-uniform air pressure and thus non-uniform polishing of the wafer. One way to obtain a sealing effect at the edge of the wafer is to use a carrier backing film made of two different materials where the material at the outer edge of the backing film is less porous than the material in the center. However, assembling each backing film requires several process steps and skill. In the current invention, we describe a backing film which create seal at the edge of the wafer but which are simpler to manufacture.

SUMMARY OF THE INVENTION The present invention provides a process for obtaining high uniformity in a chemical mechanical planarization or polishing process by uniformly and locally applying pressure of the wafer against the polishing pad without the use of a bi-material wafer carrier film. The considerable handling and reproducibility problems associated with bi-material films are therefore avoided.

In accordance with the invention, a single material wafer carrier film is modified by making the backing film substantially airtight at the wafer edge. This is done by: modifying a one-piece/one material backing film by abrading the one-piece/one material backing film mechanically in the center to increase the pressure at the edge of the wafer ; subjecting the one-piece/one material backing film edge to a thermal/chemical treatment to obtain increased material thickness, less porosity and a less uneven surface; applying a glue-like substance at the one-piece/one material backing film edge to inhibit air flow; or mechanically shaping the one-piece material backing film to render the film substantially airtight at the wafer edge.

According to another aspect of the invention, a wafer backing film for use in a chemical- mechanical polishing (CMP) apparatus is provided, which includes a single piece of porous material with a skin on at least one surface thereof. The piece of porous material has an inner portion and an annular outer portion, and a portion of the skin on the inner portion is treated so as to permit air flow through the piece of porous material. The skin at the inner portion may be treated by abrading, slicing, dicing or perforating, or alternatively may be removed; accordingly, the thickness of the film may be greater at the outer edge than in the center.

According to a further aspect of the invention, a wafer backing film arrangement for use in a chemical-mechanical polishing (CMP) apparatus is disposed between a backing plate of the apparatus and a wafer. This arrangement includes a wafer backing film (a single piece of porous material) and an annular shim between the backing film and the backing plate. The shim at the outer edge of the wafer backing film provides an edge backing film thickness greater than the center film thickness.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be readily understood by reference to the following detailed description in conjunction with the accompanying drawings.

FIG. 1 shows a prior art wafer carrier provided with air holes for the application of back pressure through a standard carrier/backing film and onto a wafer substrate.

FIG. 2A shows one embodiment of the invention to make the backing film substantially airtight by abrading a one-piece/one material backing film in the center to increase the pressure at the edge of the wafer.

FIG. 2B shows a wafer backing film including a piece of porous foam with a skin which is removed or treated in the center portion thereof, according to another embodiment of the invention.

FIG. 2C shows a wafer backing film arrangement including a wafer backing film and an annular shim to provide a raised edge profile, according to a further embodiment of the invention.

FIG. 3 shows the invention modification of a one-piece/one-material backing film that has been subjected to a thermal/chemical treatment to increase material thickness and provide less porosity and a less uneven surface at the outer edge of the film.

FIG. 4 shows the invention modification of a one-piece/one-material backing film to make the film airtight at the wafer edge by applying a glue-like substance at the backing film edge to inhibit air flow.

FIG. 5 shows the invention modification of a one-piece/one-material backing film by a mechanical shaping to make the backing film airtight at the wafer edge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, polishing operations performed in the processing of semiconductor wafers utilize polishers that have wafer carriers, as shown in the prior art depiction of Figure 1. In Figure 1, the wafer carrier 10 comprises a backing plate 11 made of metal or a ceramic material, and a wafer backing film 12 with a sponge-like composition, such as Rodel DF-200. Typically, a removable adhesive 14 is used to attach the backing film to the backing plate. The carrier/backing film is a holding device that holds the wafer 13 during transport of the wafer to and from the polishing pad. Air holes 15 are disposed throughout the carrier/backing film for application of back pressure to obtain the desired air pressure distribution.

Referring now to Figure 2A, there is shown a backing film 20 of a wafer carrier/backing film arrangement in accordance with an embodiment of the present invention. The backing film 20 is of one piece and one material. The film 20 is shown before and after being mechanically abraded in the center. The mechanical abrasion of the backing film in the center reduces the thickness of the film in the center relative to the edge, so that the backing film has a raised profile at the edge of the wafer.

Removable adhesive is used to afix the backing film to a carrier backing plate.

In another embodiment, shown in Figure 2B, the wafer backing film 20 comprises a single piece of porous foam, with a skin 21 on the surface facing the wafer. A preferred material for this embodiment is Rogers Poron with an overall thickness of 1/32 inch. The skin in the center portion 21c of the film is removed or treated so as to expose the porous foam, as shown schematically in Figure 2B; the skin at the edge (in the peripheral annular portion 21 a) remains untreated. The porous foam in the center portion 21c may be exposed by removing the skin, by perforating or slicing the skin, or by various other treatments. Accordingly, the film 20 at the edge is at least as thick as at the center. The wafer backing film, when treated in this manner, seals in backside air with the skin at the peripheral portion 21 a. In the center portion 21c, the air channels 14 conduct air through the exposed porous foam facing the wafer, so that backside air pressure is distributed evenly across the wafer. The backside air assures that an even downforce is exerted across the central portion of the wafer. Again, removable adhesive is used to afix the film to the carrier backing plate.

By using a backing film with a raised profile at the edge, the pressure at the edge of the wafer (that is, the downforce of the wafer against the polishing pad) is increased, and the seal effect at the edge of the wafer is improved. The sealing effect causes the air pressure applied through the holes 14 in the film to be equally distributed across the central portion of the wafer backside, thus causing uniform polish across the wafer frontside. At the edge of the wafer, the elevated portion of the backing film applies enough pressure to compensate for pad dive, thus ensuring a similar polishing rate at the edge and central regions of the wafer. The edge bead and high polish rate regions may thus be eliminated without adding process time, complexity or cost.

A raised edge profile in the backing film may be obtained by reducing the thickness of the backing film in the center, as shown in Figure 2A. Alternatively, as shown in Figure 2C, an annular shim 22 may be placed between the backing film 20 and the backing plate 11 of the wafer carrier. This arrangement effectively increases the thickness of the film 20 at the edge. For example, when polishing a 200 mm wafer, the thickness should be increased by at least 0.5 mil in the region 95 to 100 mm from the center. This increases the pressure at the edge of the carrier transmitted through the backing film, causing the wafer to conform to the polishing pad and resulting in uniform polishing to within 3 mm of the edge of the wafer. The shim may vary in edge thickness and radius; generally, the shim should be in the range of 0.5 to 2.0 mil thick at the edge and the inner radius should be in the range 90 to 95 mm, when polishing a 200 mm diameter wafer. The shim may advantageously be modified to have a tapered, rounded or other non-flat profile. A preferred material for this embodiment is Rodel DF-200. The outward air flow is restricted due to compression of the film at the outer edge.

Another embodiment of the invention, in which the wafer backing film is made airtight at the wafer edge, is shown in Figure 3. In this embodiment, a one-piece/one material backing film 30 is subjected to a thermal/chemical treatment at the edge region thereof, to obtain increased material thickness, lower porosity and a less uneven surface D. The untreated Rodel DF-200 material is rough on a scale of 20 u. m. This treatment of the backing film material at the edge region permits highly uniform air pressure to be established on the back side of the wafer, which in turn enables attainment of high polishing uniformity upon subjecting the carrier holding the wafer to a polishing pad.

In Figure 4, a one-piece/one-material backing film 40 is made airtight at the wafer edge by applying a glue-like substance E at the edge region thereof. This inhibits outward air flow at the wafer edge. As in the previous embodiment, this permits highly uniform air pressure to be established on the back side of the wafer, which in turn enables attainment of high polishing uniformity upon subjecting the carrier holding the wafer to a polishing pad. A preferred material for this embodiment is Rodel DF- 200.

A mechanical shaping F of the material of the backing film 50 at the edge region thereof is shown in Figure 5. This mechanical shaping renders the backing film airtight at the wafer edge to permit high polishing uniformity by virtue of highly uniform pressure on the back side of the wafer.

An advantage of the present invention is that the wafer backing film is a single piece of a single material, as opposed to a two-piece, bi-material backing film where the materials have different compressibilities and thicknesses.

The invention process of utilizing the modified one-piece/one-material backing film enables the application of highly uniform air pressure at the back of the wafer keeping the air flow behind the wafer and limiting it at the edge of the wafer and wafer backing film ; this in turn provides high polishing uniformity.

The invention is practiced by: (1) mounting the upper surface of a rotatable workpiece to the lower surface of a carrier comprising a structure of an upper surface of a metal or ceramic material and a lower attached surface of a one-piece/one-material backing film means; wherein the one-piece/one- material backing film means is modified to provide an airtight function and/or increased step height at edges of the one-piece/one-material backing film means; (2) contacting the rotatable workpiece with the one-piece/one-material backing film means prior to applying a rotating force to the upper surface of the carrier; (3) applying a rotating force to the upper surface of the carrier; and (4) subjecting the rotating workpiece to a polishing pad; and (5) applying sufficient air pressure to the backside of the wafer to cause uniform polishing across the body of the wafer.

While the present invention has been described in conjunction with specific preferred embodiments, it would be apparent to those skilled in the art that many alternatives, modifications and variations can be made without departing from the scope and spirit of the invention. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.