BACKING PAD FOR MACHINING OPERATIONS This invention relates to the art of machining work pieces such as thin wafers or discs of silicon that are used for the fabrication of solid state circuit components. In machining processes which include polishing or planarization of thin work pieces such as silicon sub¬ strates or wafers with integrated circuits, a wafer is disposed between a load or pressure plate and a rotatable polishing wheel with the pressure plate applying a pressure so as to effect removal of rough spots from the wafer and to produce a surface of substantially uniform thickness on the wafer.

The preferred type of machine with which the present invention is used includes an abrading, lapping or polishing wheel assembly which is rotatably driven about a vertical axis such that work pieces may be engaged with the upper surface of the wheel assembly and polished or abraded by means of an abrasive slurry. It is known to employ pressure plates with such machines which are connected to vertically movable spindles and are adapted to have work pieces retained on the lower surfaces thereof for engage¬ ment with the polishing wheel assemblies. In such

polishing or planarization machining processes a backing pad is generally secured to the exposed outer surface of the pressure plate and the work piece to be machined contacts and is retained by the backing pad during the machining operation. The backing pads must have a surface of uniform thickness and a surface flatness profile necessary to produce the uniform flatness and thickness required in the final machined or polished wafer.

Backing pads used in machining thin work pieces are commercially available and representative of which are Rodel 40 film, Rodel DF200, Rodel BUP, Rodel B20, Rodel Polytex, Rodel FLEXMOUNT and Rodel IC Series available from Rodel Corporation, Newman PFG available from Newman Cor¬ poration and the like. Urethane based pads are generally preferred. The pads are commonly mounted on the pressure plates using heat and pressure. Other commercial backing pads are mounted on pressure plates merely by use of adhesive means. However, mounting of the pads using heat and pressure means are not altogether satisfactory since heating frequently distorts the surface profile of the backing pad and these techniques are difficult to carry out in a manufacturing environment, are time consuming and often adversely affect the pads with respect to hardness, density, compressibility and strength. Similarly, commer- cially available backing pads which are mounted on a pressure plate merely by use of an adhesive generally do not provide the flatness accuracies required for critical polishing operations, especially for chemical-mechanical planarization-type machining operations. In accordance with this invention there is pro¬ vided improved backing pads for use in machining thin work pieces. The invention involves a backing pad adapted for attachment to a flat, rigid pressure plate used in lapping machines and the like for machining thin work pieces, and the outer exposed surface of the pad having been abraded to eliminate surface irregularities and to achieve a high

degree of flatness on the pad surface. These important desiderata are accomplished by securing a backing pad to a flat, rigid pressure plate, such as a metal or ceramic plate, and then subjecting the exposed surface of the pad to an abrading or grinding action to remove surface ir¬ regularities and achieve surface flatness. Preferably, the abrading action is conducted using a lapping wheel with which the pad is to be used and to which is affixed a layer of an abrasive material. In this way, the surface of the finished pad has the surface profile of the lapping wheel.

The improved backing pads of the invention are composed of a relatively non-porous and non-absorbent soft material having sufficient rigidity and strength to be securable to a flat, rigid pressure plate and to firmly retain a thin work piece in position when the pressure plate moves a work piece into pressure contact with a lapping wheel for machining, i.e. , polishing or planarization of the work piece. By soft material is meant a material having sufficient resiliency so that when subjected to a compressive force assumes its original surface profile when the compressive force is released. Generally, the backing pad is sufficiently resilient for use when a uniform surface texture has been created and when subjected to a compressive pressure of 5-10 psi does not compress to more than about 1/5 of its thickness with the pad having the ability to spring back to its original state after pressure is released. The softness of the pad can be determined by compression and memory tests. Uniform physical characteristics throughout the material are desired, including compressibility, coefficient of fric¬ tion, hysterisis, non-porous structure and in-plastic state. The softness or resilient characteristic of the pad enables the pad to be fabricated so as to have a desired flatness and be free of surface irregularities.

Representative materials preferred for use as backing pads

in accordance with this invention are Rodel B20, BUP and FLEXMOUNT available from Rodel Corporation, Scottsdale, Arizona; Newman PFG5 available from Newman Corporation, Boston, Massachusetts; and UT5 through UT35 pads available from UTEC Corporation, Chandler, Arizona.

The size of the backing pad can vary depending upon the size of the work piece to be machined such as, for example, 5, 6 or 8 inches in diameter, to accommodate similarly sized silicon wafers and other similar thin work pieces.

To prepare the improved backing pads of this invention, the pads are mounted on the surface of a pressure plate of a lapping machine. Lapping machines and pressure plates used therein are well known and the improved backing pads of this invention can be advan¬ tageously utilized with conventional lapping machines such as disclosed, for example, in United States patent No. 4,270,314 and United States patent No. 4,141,180.

In fabrication, a backing pad is secured to the outer exposed surface of a pressure plate by any suitable means, such as by the use of adhesives or double sided adhesive tapes. Excess adhesive and air are expressed from between the pad and pressure plate by expedients such as moving a roller or the like over the surface of the pad. Suitable adhesives for securing the pad to the pressure plate are, for example, double sided adhesive tapes, spray contact cements, carpet tape and the like.

After mounting of the pad to the pressure plate, the exposed outer surface is then subjected to the abrading action of a dry abrasive material affixed preferably to a conventional lapping wheel so as to conform thereto and present the same surface contour of the lapping wheel. As is known, lapping wheels can be essentially planar or they can have either a convex or concave configura ion or profile. Suitable abrasive materials include, for example, silicon carbide sandpaper, mylar impregnated abrasive film

and the like, or a diamond plated lapwheel can be used in this operation.

The abrading action is conducted under suitable pressure for a time sufficient to remove surface irregu- larities and to impart a desired flatness to the pad.

Preferably, backing pads used for machining flat silicon wafers are abraded so as to produce a surface having a high degree of flatness. Preferably, the flatness is such that no part of the pad surface varies from a true plane no greater than plus or minus 0.000040 inch and more prefer¬ ably the surface variation is no greater than plus or minus 0.000010 inch. In any event, abrading of the pad is con¬ ducted so as to produce a desirably flat surface free of surface irregularities over the exposed surface of the pad and the thickness of the pad throughout does not vary more than about 10%. Abrading the exposed surface of the backing pad contours the pad without requiring the use of heat. Grinding or abrading the backing pad generates a uniform surface texture or nap on the pad which prevents a work piece from sliding under polishing side forces. Also, abrading provides surface roughness on the pad so that water disperses in a uniform layer under a wafer during a polishing operation. The abrading action also exposes and corrects surface defects and deburrs vacuum hole passages thereby enhancing the seal between the pad and work piece at the location of the vacuum holes thus promoting improved handling of the work piece. Moreover, abrading the pad enhances the vacuum holding characteristics of the pad by increasing the surface area exposed to vacuum. After abrading, the surface of the abraded backing pad is cleaned by use of a water or air spray or other known cleaning means. The backing pad attached to a pressure plate is then ready for use in a SpeedFa Corpora¬ tion CMP-V planarization machine or other polishing machine to polish or planarize thin work pieces such as silicon wafers.

The present invention and the advantages thereof will be further apparent from the following description taken in conjunction with the drawings wherein:

Figure 1 is a perspective view of a typical backing pad used in machining thin work pieces, such as silicon wafers.

Figure 2 is a perspective view showing application of a typical backing pad to a pressure plate used in conventional lapping machines. Figure 3 is a perspective view showing one means of flattening the backing pad against a pressure plate.

Figure 4 is a fragmentary view of a conventional lapping wheel which carries on its upper surface a layer of an abrasive material and showing a backing pad attached to a pressure plate in position for abrading by rotation of the lapping wheel.

Figure 5 is a fragmentary side view of a typical commercial backing pad attached to a pressure plate which is not fabricated in accordance with the invention. Figure 6 is a view similar to Figure 5 showing the improved exposed surface of a backing pad when fabri¬ cated in accordance with the invention.

Figure 7 is a side schematic view showing the positioning of a pressure plate and backing pad within a typical chuck used in lapping and polishing machines.

Figure 8 is a fragmentary view of a conventional lapping wheel carrying on the upper surface a layer of an abrasive material and showing one means for reducing the coarseness of the abrasive layer. Referring to the drawings (Figure 1) numeral 11 refers to a 8-inch diameter backing pad having a thickness of .020 inch composed of white urethane product, B-20 with PSA II adhesive and available commercially from Rodel Corporation. Vacuum access apertures 12 (Figure 2) are provided in the pad after mounting on a pressure plate to permit a vacuum to be applied through the pad so as to hold

a work piece during wafer handling. The use of vacuum means for retaining work pieces such as silicon wafers during handling is commonly employed.

The pad 11 is secured to a rigid, flat metal or ceramic pressure plate 14 (Figure 2) by use of an adhesive 13 such as PSA II or double side adhesive tape. The pressure plate 14 has a diameter corresponding to the backing pad and is utilized for machining work pieces of various sizes, illustratively an 8-inch diameter silicon wafer. Vacuum access apertures are provided in pressure plate 14 to permit use of vacuum holding means.

After placing the pad 11 on the pressure plate, excess adhesive and air are expressed from the laminated structure such as by rolling the surface of the pad with a roller 15 (Figure 3) or similar flattening element.

The next step in preparing the advantageous backing pad of this invention is to abrade or grind the exposed outer surface of the pad. Thus, as shown in Figure 4, the pressure plate 14 and pad 11 are placed within a suitable retaining ring 16 which is guided by a pair of rigid and adjustable holding arms 18. The pressure plate and pad are placed on a lapping wheel in an inverted position as compared to Figure 3, that is, the pad is at the bottom so as to contact the abrasive material layer 21 which is affixed to the surface of lapping wheel 22.

Abrading of the exposed surface is conducted by rotating the lapping wheel under dry conditions for a sufficient time to obtain the flatness required for the backing pad and to free it of substantially all surface irregularities. Typically, the lapping wheel carrying the abrasive material 21 is operated for a period of say 5 to 10 minutes depending upon the flatness of the original pad and the flatness desired in the finished pad. Representative of the abrasive materials 21 which are affixed to the lapping wheel 22 are 220 mesh silicon carbide sanding paper.

If desired, before abrading the backing pad the abrasive surface layer 21 can be somewhat polished or sized to reduce its coarseness by lapping it with a cylindrical cast iron truing ring 25 (Figure 8) . By oscillating arm 18, the truing ring traverε_-_ _* th*. entire surface of the abrasive layer 21 on the lapping wheel. Dressing the abrasive surface 21 is preferable when the abrasive is of very coarse nature.

After abrading the pad, it can be cleaned by use of vacuum or air spray and its flatness profile checked by means of a sphero eter as is known in the art. The pad firmly affixed to the pressure plate is then ready for in¬ sertion into a polishing or planarization machine chuck 25 (Figure 7) and it is then ready for use in a polishing operation.

Commercially available backing pads, while ostensibly flat, do not possess the extreme flatness characteristics required for critical polishing of work pieces such as thin silicon wafers. The typical commercial backing pad as illustrated in iyjre 5 has a total out of flatness of 0.0003 inch to 0.0005 nch., wherea:. the backing pad fabricated in accordance with this invention, as illus¬ trated in Figure 6, has a total out of flatness of say 0.000020 inch (i.e. 1/2 micron). Those modifications an squivaients which fall within the spirit of the inv≤ntion are to be considered a part thereof.