BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of CMP wherein the polishing pad is partially or wholly replaced by a counter face made of a smooth hard material and beads or particles of a hard material are suspended in the slurry to provide asperities for the polishing process.

FIELD OF THE INVENTION

[0002] When integrated circuits (ICs) are constructed in the semiconductor industry and related industries, a process called chemical-mechanical planarization, or CMP, is typically used numerous times during manufacturing to planarize the wafer surface on which the circuits are being built. Planarization is essential for the construction of the wiring, or interconnects, that are used in circuits, and it also can be an important step in forming transistors and other electronic components. Non-planar surfaces present difficulties for the application of lithographic tools, which are used to create patterns on the wafer and which have a limited depth of focus. CMP in the last 20 years has in fact become a key enabling technology that has made possible essentially unlimited complexity in integrated circuit design. IC fabrication facilities therefore typically have large numbers of CMP tools and incur substantial costs running them.

[0003] In a conventional CMP process, a silicon wafer with integrated circuit chips under construction is held upside down in a rotating wafer carrier and is pressed with a controlled force against a large rotating polishing platen. The platen is covered with a thin polyurethane polishing pad, typically up to a meter in diameter and 1 -2 mm thick. Microscopic protuberances on the pad surface, also known as asperities or summits, make contact with the wafer, and, with the assistance of polishing slurry containing chemistry and abrasive particles, effect the removal of material from the wafer surface.

[0004] The polishing slurry is usually applied at a slow, continuous rate to the pad in the vicinity of the wafer using a drip or spray system. Because polyurethane pad asperities may be irreversibly deformed by contact with the wafer, or may be abraded by the slurry particles, the pad surface must be continuously renewed in order to sustain a stable CMP process. Pad renewal is usually accomplished with a circular diamond cutting tool, called a pad conditioner or pad dresser. During CMP tool operation, the pad conditioner is swept back and forth across the pad surface under a light applied load. The diamonds cut the pad surface at a slow rate, eliminating old asperities and creating new ones in the process. Since they cut the pad, conditioners produce small particles of pad debris, many of which are washed off of the pad surface, but some of which also come into contact with the wafer. The latter are suspected to be a source of defects that affect integrated circuit viability and reliability. [0005] Conditioning also gradually thins the pad, which eventually forces pad replacement. Conventional CMP processing, as described above, has several weaknesses. One is that the pad asperities so essential to the process are in fact not very well controlled.

[0006] We find that asperities on commercial pad surfaces produced by commercial conditioners under normal polishing conditions can be highly variable in height, in curvature or sharpness at the point of contact, and in area density on the pad. As a result, contact area distributions are highly variable. Substantial evidence of pad fragments from the conditioner that are either loosely connected to the pad (hanging chads) or are totally disconnected (pad debris) are also observed.

[0007] This variability becomes visible to the user of CMP in the form of wafer-to- wafer variations in the material removal rate, in removal rate non-uniformity across the wafer surface, and in micro-scratches on the polished surface. Pad surface variability will become an increasing concern over time since some materials being considered for incorporation in future generations of ICs are very fragile and easily damaged by the occasional large asperity.

[0008] Many of the large spots can be matched with the underlying topography and can be identified as hanging chads or pad debris. Cost is also an issue in an industry that has had to produce increasing performance at continuously decreasing cost for several decades. Polishing pads typically cost a few hundred to in excess of a thousand dollars each. A commercial polisher may use up to three pads simultaneously, and the useful life for each pad is often only two or three days of continuous use. Each CMP tool therefore-uses hundreds of pads annually, and since wafer fabrication facilities can have dozens of tools, the total cost for pads alone is substantial. Since it can take several hours to remove a pad and install a new one, and dozens of monitor wafers to qualify the new pad for production worthiness, the engineering and product loss cost of tool down time can be significant.

SUMMARY OF THE INVENTION PROBLEM THE INVENTION WAS INVENTED TO SOLVE

[0009] Similar issues occur with pad conditioners, which cost several hundred dollars each. Even though they are often constructed with thousands of diamonds embedded in a metal or ceramic matrix, it has been found through extensive studies for diamond conditioner

manufacturers U.S. Patent Application No. 12/359,772, incorporated herein by reference in its entirety and made a part hereof, that only a few hundred diamonds actually actively cut the pad surface and that only a small number of these, the aggressive diamonds, account for most of the cut rate. The small number of aggressive diamonds can result in large differences in the rate at which nominally identical conditioners cut the pad. When a few of the aggressive diamonds wear out or break, the conditioner has to be replaced even though 99.99% of the diamonds are still in usable condition. Because of the small number of diamonds involved, and sometimes because of the chemical environment, conditioner replacement usually occurs after a few tens of hours of use. Thus, conditioners also create both process variability and a substantial consumables cost.

[0010J The inventors of the present invention have made various and earnest researches into the problem hereinabove described and have discovered that if the asperity containing polishing pad and conditioner are largely or entirely eliminated from the CMP apparatus and in place of the asperities on the pad surface, microscopic beads or particles comprising suitably durable and chemically stable materials such as polyethylene are used within a range of micron sizes comparable to the size of conventional polishing pad asperities between about 2 microns and 50 microns suspended in polishing slurry with the aid of a surfactant or other agent as needed which would come into contact with the wafer surface by being pressed between said wafer surface and a counter surface made of a durable chemically stable material such as polyethylene, polishing will occur in much the same way as conventional asperities. Since the size of the beads, the size distribution, and the concentration of beads in the slurry can easily be adjusted, it is possible to control the height, curvature, and area density of the "summits" that come in contact with the wafer. This should substantially reduce the process variability associated with conventional asperity contact since we will have almost total control of the contact conditions.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0011] The method of the present invention has been developed in response to the present state of the art, and, in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available CMP methods for reducing the level of defects to the semiconductor wafer and inconvenience, inefficiency and process waste to the CMP process caused by the use of polishing pads with fixed asperities and using conditioning discs. Thus it is an overall objective of the present invention to provide a method for CMP that achieves significant reduction in the level of defects to the semiconductor wafer and inconvenience, inefficiency and process waste to the CMP process caused by the use of polishing pads with fixed asperities and using conditioning discs during the CMP process.

[0012] The purpose of the method is to allow the consistent production of a significantly higher quality of semiconductor wafer product for lower cost.

[0013] Through application of the method of the present invention, the consistent production of a significantly higher quality of semiconductor wafer product with lower expense or difficulty of operation of the CMP process has been achieved.

[0014] All dimensions in the present invention are based on a polishing pad size or as the case may be counter face size of about 20" to 30" in diameter and a wafer size of between [ 8"] and [ 12"] in diameter and may be altered as needed in proportion to changes in the size of the polishing pad and wafer used. The specific dimensions given herein are in no way limiting but are by way of example to demonstrate an effective embodiment of the invention. For the avoidance of doubt, dimensions include, without limitation, dimensions of parts, flow rates, measurement of damage, rates of rotation and velocities.

(00151 The beads of the present invention are not particularly limited and the material used for the beads may be any durable material that is flexible enough to withstand the pressures involved in CMP and at the same time is chemically stable in the respective chemical environments in which CMP is carried out. Beads or particles made of plastic, ceramic, glass, mineral, metal or the like may be used. Polyethylene beads are preferred.

[0016] The beads of the present invention and especially the active beads (those involved in polishing) are compressed between the wafer surface and the counter face and slide or roll along the wafer surface. Consequently, although there is no limitation, a certain degree of

compressibility is desirable.

[0017] The size if the beads used is not particularly limited and any diameter that corresponds roughly to the dimensions of the asperities on commercial polishing pads sought to be replaced by the present invention may be used. Sizes within a range of between one and 50 microns are preferred. The particles or beads may be uniform in size or they may vary either in two or more discrete variations of size in any proportion or quantity. The distribution of sizes may also be continuous or irregular, and where distribution of size is irregular, the particle size distributions are not particularly limited, however, particle size distributions represented by Gaussian curves corresponding roughly to the distribution of dimensions of asperities on commercial polishing pads are preferred.

[0018] The shape of the beads is not particularly limited and the beads may be in any shape or configuration including even or uneven and asymmetric shapes. Non-angular or angular shapes may be used without limitation and among non-angular shapes spherical or ovoid shaped beads or particles are preferred and spherical shapes are more preferred.

[0019] The roughness of the surface of the particle is not particularly limited and the surface may be either smooth or rough.

[0020] The concentration of the beads in slurry is not particularly limited and any suitable concentration may be used provided, however, that a concentration of between 0.1 weight percent and 30 weight percent of beads is preferred. The concentration of beads in the slurry is determined by the area density required on a pad surface at a given load.

[0021] The composition of the slurry is not particularly limited and any slurry and, particularly, any commercial CMP slurry may be used provided, however, that slurries that easily wet and suspend the beads or particles without surfactants or other agents are preferred. The range of concentrations that may be used is very large. However, the variability in asperity topography versus the size of the beads may be correspondingly large as well. Bead surface character as opposed to slurry and pad effects determines what weight percent of beads may optimize polishing effects.

[0022] The counter face material of the present invention is not particularly limited and any hard plastic, ceramic, mineral, metal or other suitable material may be used but polyurethane is preferred. The shape of the counter face is not particularly limited but it could be a plain, smooth solid object, a conventional pad with a skived, or planarized surface, or even a roughened surface, or a smooth alternative plastic or polymer.

[0023] The size of the counter face of the present invention is not particularly limited though in general it should not be larger than the polishing pad used on the relevant CMP tool or smaller than the wafer. A counter face the same size as the polishing pad is preferred.

[0024] The means of attachment of the counter face of the present invention to the CMP apparatus is not particularly limited however, the attachment of the counter face to the polishing platen in the same way as a commercial polishing pad is attached, using a double-sided adhesive film is preferred. The distance between the counter face surface of the present invention and the wafer during CMP is not particularly limited however it is preferred that it should be about the same as the average diameter of the beads or particles used or slightly less.

[0025] The counter face of the present invention may be hydrophilic or hydrophobic without limitation but a hydrophilic counter face is preferred. The counter face of the present invention develops a surface charge when slurry is applied that will attract the beads. In a modification upon the process of the present invention, the counter face is wettable by the slurry and the suspended beads deposit on the counter face surface at a slow rate. In this case, many of them then encounter the wafer during CMP and effect polishing. Since these beads are deposited continuously and active beads are then crushed or deformed by the wafer, bead removal from the counter face is a necessity. This means of accomplishing this are not particularly limited but removal may be accomplished by means of a soft brush, which would replace the conditioner in the normal process.

[0026] The surfactant used to suspend beads in commercial slurry is not particularly limited and may be a commercially available surfactant like Tweon 20 or the like.

[0027] One embodiment of the method of the present invention comprises removal of the commercial polishing pad from the CMP polishing tool to be used, addition of a counter face affixed to the CMP polishing tool according to the methods of the present invention and polishing a wafer under normal conditions. In this case, however, the beads of the present invention are suspended in the slurry. The method of introduction of beads thus suspended is not particularly limited and they may be introduced either through the normal slurry input tube or by means of an independent tube or device.

EXAMPLES

[0028] About 1 kg of high-grade 15 micron polyurethane beads and several commercial slurries with different pH values and chemical compositions were obtained and used in the practice examples of the present invention. Determination of the ability of a particular commercial slurry to suspend the beads evenly was determined by means of a beaker and a stirring unit. About 50 grams of beads were placed in 200 ml of slurry and the mixtures were stirred for 10 minutes. Beads were also tested to confirm that they would not flock on the surface. Although commercial slurries do not naturally support a suspension of the beads, a minimal amount, eg 1- 5 ml/liter, of a surfactant, such as Tweon 20 or other additive to help promote suspension may be added with or without stirring to effect even suspension of the beads. In this way a commercial slurry requiring the least modification is selected.

[0029] At the end of this initial step, the beads were suspended in a relatively stable smooth suspension with no flocking and a low settlement rate.

[0030] Several materials were used for the counter face sheet to determine which would be unsuitably hydrophilic, as measured by the slurry contact angle with the material, to determine which materials might make a superior counter face.

[0031] Using a beaker test it was determined whether in the presence of slurry the counter face material has a surface charge that promotes or retards bead adsorption. For 15 micron beads, the determination can be made with optical microscopy. A simple test was also carried out to assess the difficulty with which adsorbed beads are removed from the counter face material with a soft brush. At the end of this second step, one or two materials may be selected for further testing as the counter face.

[0032] One or more counter faces was prepared for use on an R&D polishing tool, the Araca APD-500. The precise grooving of the counter face was done by Toho Engineering to match M type concentric grooving. Then a series of polishing experiments at a single polishing pressure and sliding speed under conditions used in commercial polishing to measure the blanket material removal rate of either silicon dioxide or copper, depending on the slurry selected. The blanket rate is measured on wafers without circuit patterns).

EFFECTS OF THE INVENTION

[0033] In addition to the reduction of defects in the product semiconductor wafers already referred to above, from an environmental perspective, pad debris in the liquid waste stream that has a distribution of sizes ranging from submicron to tens of microns in the conventional process is traded for polyurethane micro-beads in the waste stream that have a much tighter size distribution. The latter may therefore be more easily filtered or made to float, via an anti-wetting agent, thus allowing easy recovery of the beads. In addition, it is possible to hold the mass fraction of beads to a value no greater than the mass fraction of debris currently in the waste slurry, so that the rate of waste production in the modified process will not increase over the current process. Controlling the size and usage rate and filtering or removing the waste will help to insure that the beads do not enter the rivers and oceans.

[0034] Since the modified process also does not require periodic disposal of 2/3 of the original pad, we will produce a net reduction in polyurethane use by a factor of 3. Micro-beads are also cheaper and much easier to produce and transport than polyurethane polishing pads, so there are additional economic and environmental benefits to their use. Furthermore, with conventional pads, both the asperities as well as the land and valley areas act as absorption sites for unwanted and environmentally harmful polishing by-products such as copper and ruthenium and in the future, arsenic. These elements remain within the pad matrix after the pad has been

decommissioned and discarded in landfills. On the other had, the beads, having a much lower number of absorption sites for the metallic by-products, will likely force the by-products to remain in solution and be subsequently treated by electro-coagulation or other chelating or sedimentation means. Similarly, the modified process will replace a technologically complex diamond conditioner, which uses artificial diamonds and is made using processes that include electroplating, sintering, brazing or chemical vapor deposition, with a relatively environmentally friendly soft brush.