Description RETAINING RING FOR CHEMICAL MECHANICAL POLISHING Technical Field [1] The invention relates to chemical mechanical polishing, and more particularly to a retaining ring used in a chemical mechanical polishing apparatus. Background Art [2] As the miniaturization and multi-level interconnection of semiconductor integrated circuits are demanded, there is a need at certain fabrication steps to planarize wafer surfaces or selectively remove conductive layers formed on wafer surfaces. To meet the need, recently chemical mechanical polishing (CMP) is widely used in the manu¬ facturing of semiconductor integrated circuits. Generally, in a chemical mechanical polishing (CMP) process, planarization of a wafer surface or selective removal of a conductive layer on a wafer is accomplished by spreading slurry on a polishing pad, bringing a wafer into contact with the polishing pad and moving the wafer and the polishing pad relatively to each other with a pressure applied to the wafer. [3] FIG. 1 represents a schematic cross-sectional view illustrating a wafer polishing process by a chemical mechanical polishing (CMP) apparatus. To describe it in detail, after attaching a polishing pad 12 on a platen 10, slurry 14 is spread on the polishing pad 12. Thereafter, a wafer 16 is made to contact the polishing pad 12 coated with the slurry 14, and the platen 10 is made in rotating or orbital motion. Then, planarization of the wafer surface contacting the polishing pad 12 is accomplished by the friction between the wafer 16 and the polishing pad 12. Here, an assembled part 18 which rotates the wafer 16 with a polishing pressure applied thereon during CMP, and which also carries the wafer 16 before and after CMP, is called a 'polishing head' or a 'carrier'. Hereinafter, the assembled part 18 will be referred to as 'carrier'. The carrier 18 comprises generally a carrier base 22 which receives a motive power from a drive shaft 20 and provides a space to fix other carrier components, a pressure applying means 24 such as a plate or a bladder which applies a polishing pressure onto the wafer 16 while rotating the wafer 16 by contacting the top surface of the wafer 16, and a retaining ring 26 which prevents the wafer 16 from slipping out from the beneath of the carrier 18 by supporting a side surface of the wafer 16. The function of the retaining ring 26 during CMP is not only to prevent the wafer 16 from slipping out but also to induce an uniform deformation of the polishing pad 12 near the edge of the wafer 16 by pressing the polishing pad 12 with a predetermined pressure, by which improved polishing uniformity can be obtained. Therefore, friction arises also between the retaining ring 26 and the polishing pad 12 coated with the slurry 14, which will cause abrasion of the bottom surface of the retaining ring 26. [4] FIG. 2 is an example of a retaining ring according to the prior art illustrating a retaining ring 30 wholly constructed of a single body. To fix the retaining ring 30 to a carrier base (not shown), screws are used generally. For this, screw holes 32 are formed on the upper surface of the retaining ring 30. The size of the retaining ring 30 may be defined in terms of the inner diameter, the width w, and the thickness h. The inner diameter of the retaining ring 30 is determined according to the diameter of wafers to be polished. The width w is in the range of 10-40mm and the thickness h is in the range of 10-30mm. The retaining ring 30 is generally formed of a plastic material such as polyphenylene sulfide (PPS) or polyetheretherketone (PEEK) which shows a low wear rate and good durability. [5] The replacement of a retaining ring is mostly due to the deterioration of polishing uniformity or due to the drop of wafer retaining capability caused by the wear of the bottom surface of the retaining ring. When the wear of the bottom surface reaches about 500 μ m, the polishing uniformity tends to be deteriorated and the retaining ring is replaced around at this point of time. Therefore, considering the thickness of retaining ring, only an extremely small portion is used, which results in unnecessary waste of resources and environmental pollution. [6] FIG. 3 is another example of a retaining ring according to the prior art illustrating a retaining ring 40 comprised of two parts; one part is an upper ring 50 which will be connected to a carrier base (not shown) and the other part is a lower ring 60 which will practically retain a wafer (not shown) during CMP by supporting a side surface of the wafer. On the upper ring 50, screw holes 52 necessary for fixing the retaining ring 40 to a carrier base are formed. On the bottom surface of the lower ring 60, grooves 62 may be formed to facilitate a slurry flow during CMP. The upper ring 50 is generally formed of a metal such as stainless steel showing a good corrosion resistance and machinability, while the lower ring 60 is formed of a plastic material such as polyphenylene sulfide (PPS) or polyetheretherketone (PEEK). FIG. 4 is a cross- sectional view cut through AA' in the retaining ring 40 of FIG. 3 illustrating the upper ring 50 and the lower ring 60 are attached by an adhesive material 70 such as epoxy. The thickness t of the lower ring 60 is generally about 5mm. In this case also, the retaining ring 40 is replaced when the wear of the lower ring 60 reaches about 500 μ m. Therefore, considering the thickness of the lower ring 60, only a small portion is used. [7] The difference of the retaining ring 40 comprised of the two parts (the upper ring 50 and the lower ring 60) compared to the retaining ring 30 constructed of a single body in FIG. 2 is that the upper ring 50 is reusable after detaching the lower ring 60 as shown in FIG. 5 when the lower ring 60 is worn. One of the methods to separate the upper and lower rings 50, 60 is to heat the retaining ring 40 to a high temperature (for example a temperature in the range of 200-300 0C ) so that an adhesive material joining the two rings becomes melted or thermally decomposed. When the retaining ring 40 is heated, if the material forming the upper ring 50 has a different thermal expansion co¬ efficient from that of the material forming the lower ring 60, then heated upper and lower rings 50', 60' illustrated schematically as dotted lines in FIG. 6 have different diameters although the upper and lower rings 50, 60 have a same diameter at room temperature. This size difference upon heating for the separation of the upper and lower rings 50, 60 may cause distortions in the two rings. Particularly, the distortion in the reusable upper ring 50 may degrade the dimensional accuracy of a refurbished retaining ring thereby resulting in deteriorated polishing uniformity. Disclosure of Invention Technical Problem [8] As described above, retaining rings according to the prior arts are too much material consuming in that only small portions of the rings are used for the CMP process and then discarded. Technical Solution [9] To solve the above problem, it is an object of the present invention to provide a retaining ring which can reduce an unnecessarily discarded portion, and can improve polishing uniformity by eliminating distortions due to the difference of thermal expansion coefficient between discarded and recycled portions. [10] According to the present invention to achieve above objet, a retaining ring for a chemical mechanical polishing apparatus comprises a plurality of retaining pieces to prevent a wafer from slipping out during chemical mechanical polishing by supporting a side surface of the wafer, and a substantially annular base ring where the plurality of retaining pieces are attached. Description of Drawings [11] The above-mentioned object and advantage of the present invention will be more clearly understood when considered in conjunction with the accompanying drawings, in which: [12] FIG. 1 represents a schematic cross-sectional view illustrating a chemical mechanical polishing process; [13] FIG. 2 represents a perspective view of an example of a retaining ring according to the prior art; [14] FIGS. 3 through 6 are drawings of another example of a retaining ring according to the prior art illustrating an upper ring and a lower ring which compose the retaining ring; [15] FIGS. 7 and 8 are perspective views illustrating a retaining ring according to an embodiment of the present invention; [16] FIGS. 9 through 11 are perspective views illustrating examples of a retaining piece a plurality of which constitute a retaining ring according to the present invention; [17] FIGS. 12 and 13 are cross-sectional views schematically comparing the de¬ formation of a retaining piece according to the thickness thereof; [18] FIG. 14 represents a perspective view of a retaining ring according to an alternate embodiment of the present invention; [19] FIGS. 15 through 20 are perspective views illustrating protruded and engraved structures formed on a retaining piece, and on a base ring; [20] FIGS. 21 through 24 are bottom views of a retaining ring according to the present invention illustrating examples of retaining piece shape according to the total number of retaining pieces, and also illustrating the direction of gaps between two adjacent retaining pieces; [21] FIGS. 25 and 26 are perspective views illustrating an example of grooves formed on a base ring. Best Mode [22] Hereinafter, retaining ring for chemical mechanical polishing according to the present invention will be described in detail with reference to the attached drawings. It should be noted, however, that embodiments of the present invention are provided so that this disclosure will be through and complete, and will fully convey the scope of invention to those skilled in the art. Accordingly, in the drawings, the relative size and shape of members may be exaggerated for clarity and like reference characters refer to the same members. [23] FIGS. 7 and 8 illustrate a retaining ring 90 according to an embodiment of the present invention. The retaining ring 90 comprises a plurality of retaining pieces 100 and a substantially annular base ring 200 where the plurality of retaining pieces 100 are attached. The retaining pieces 100 play a role of preventing a wafer (not shown) from slipping out during CMP by supporting a side surface of the wafer. As shown in FIG. 8 which is a flipped drawing of FIG. 7, the base ring 200 where the retaining pieces 100 are attached has screw holes 210 on the top surface thereof so that a carrier base (not shown) can fix the retaining ring 90 with screws. Another role of the retaining pieces 100 is to make a uniform deformation of a polishing pad (not shown) near the edge of a wafer during CMP by pressing the polishing pad. [24] FIGS. 9 through 11 are perspective views illustrating examples of a retaining piece a plurality of which constitute a retaining ring according to the present invention. First, referring to FIG. 9, a retaining piece 100 comprises a curved inside surface defined by an arc of radius Ri and a curved outside surface defined by an arc of radius Ro. Here, the inside surface defined by an arc of radius Ri will contact and support a side surface of a wafer during CMP. Therefore, it is preferred that the radius Ri is larger than the radius of wafers to be polished by about 0.5-2mm. The radius Ro may be larger than the radius Ri by about 10-40mm. The thickness t of the retaining piece 100 can be in the range of 0.5-5mm. FIG. 10 illustrates another example of a retaining piece 102. While the inside surface of the retaining piece 102 is defined by an arc of radius Ri as in the above example, the outside surface of the retaining piece 102 may comprise planes (Pl and Pl for example) instead of a curved surface defined by an arc. FIG. 11 illustrates another example of a retaining piece 103. Here, the inside surface of the retaining piece 103 is defined by a plane P3 and the outside surface thereof is defined by another plane PA, which is an example of a case that neither the inside surface nor the outside surface of a retaining piece is defined by an arc. For this case of a flat inside surface like P3, it is preferred that the angle θ made by two lines (denoted as dotted lines) from the center C of a retaining ring (not shown) to both extremes of the retaining piece 103 is smaller than 20 degrees. In other words, when a retaining ring comprises not less than 18 retaining pieces, the inside surface of the retaining piece can be composed of a plane like P3 for example. [25] Compared to a form of ring used in the prior art, a form of piece in the above examples is smaller, and therefore easier to fabricate. Especially, when the thickness of a ring is required to be thin (for example a thickness less than 2mm), it is difficult to fabricate the ring. Although the ring is successfully fabricated, it can be easily broken due to the thinness. Contrary to a ring, fewer problems are encountered in fabricating thin pieces. Considering that retaining rings are replaced after a wear of about 500 μ m, a discarded portion can be cut down by composing a retaining ring with thin (for example a thickness less than 2mm) retaining pieces attached to a base ring, and after use discarding only worn retaining pieces. [26] Another advantage of using thin retaining pieces is that it allows a material having good wear resistance but easily deformable due to its high compressibility to be used for retaining pieces because the thickness variation with the pressure during CMP is small in thin retaining pieces. To explain this with drawings, as shown in FIG. 12, under a pressure P a retaining piece 120 made of a material with high compressibility becomes a retaining piece 120' deformed in the parallel and perpendicular directions of the applied pressure P. When the thickness of the retaining piece 120 is thick, the degree of thickness reduction and side surface swelling of the retaining piece 120 is large, which causes problems in wafer retaining during CMP. On the other hand, as shown in FIG. 13, when the thickness of a retaining piece 130 is thin, the degree of thickness reduction and side surface swelling of the retaining piece 130 by the pressure P is small, which will be helpful in wafer retaining. [27] 27Retaining pieces can be formed of a plastic which shows good wear resistance and chemical inertness such as PPS, PEEK, polyester, polyamide-imide, polyurethane or the like. [28] A Base ring where retaining pieces are attached can be formed of a corrosion resistant metal such as stainless steel, a plastic material such as PPS, or a ceramic material such as alumina. A base ring can be constructed of a single body as the base ring 200 of FIG. 7. Plural rings attached together may form a base ring as shown in FIG. 14 which illustrates a retaining ring 90 according to an alternate embodiment of the invention. Here, a base ring 206 comprises a lower base ring 202 and an upper base ring 204. The upper base ring 204 where retaining pieces 100 are attached is preferred to be formed of a plastic material such as PPS because it will contact slurry. The lower base ring 202 which is to be connected to a carrier base is preferred to be formed of a metal such as stainless steel. Hereinafter, base rings constructed of a single body as in FIG. 7 will be illustrated for the simplicity of drawings. [29] To attach retaining pieces to a base ring, an adhesive material such as epoxy, silicone, or paraffin can be used. For recycling a base ring, it is desired that an adhesive material can be melted or decomposed at a high temperature (for example a temperature in the range of 200-300 0C ), or can be dissolved in a specific solvent (for example trichloroethylene) to separate worn retaining pieces from the base ring. [30] When retaining pieces are being attached to a base ring, they should be aligned to the base ring to maintain the dimensional accuracy. To align retaining pieces fast to a base ring, as shown in FIG. 15, protruded structures 140 can be formed on the attachment surface of a retaining piece 104. FIG. 16 illustrates engraved structures 150 formed on the attachment surface of a retaining piece 104 for the same purpose. FIGS. 17 and 18 show a base ring 200 having matching structures to the protruded and engraved structures 140, 150 of the retaining piece 104. Here, as shown in FIG. 17, engraved matching structures 220 are formed on the attachment surface of the base ring 200, while in FIG. 18 protruded matching structures 230 are formed on the attachment surface of the base ring 200. Once the protruded structures 140 of the retaining piece 104 coincide with the engraved matching structures 220 of the base ring 200, or the engraved structures 150 of the retaining piece 104 coincide with the protruded matching structures 230 of the base ring 200, the retaining piece 104 is alined to the base ring 200. The protruded structure may have a shape of cylinder or square pillar. Accordingly, the engraved structure may have a shape of round or square hole. FIGS. 19 and 20 show another example of a ridge-shaped protruded structure 142 and a trench-shaped engraved structure 152, respectively. [31] FIGS. 21 through 23 are bottom views of a retaining ring 90 illustrating examples of retaining piece shape according to the total number of retaining pieces 106, 108, 109 attached to a base ring 200. The total number of retaining pieces can be from 4 to 72 according to the size of a retaining ring, the thickness of retaining pieces, and the kind of a material of which retaining pieces are formed. As the size of a retaining ring gets larger, in other words when the size of wafers to be polished is larger, and as the thickness of retaining pieces gets thinner, it is desirable to increase the total number of retaining pieces. Referring to FIG. 23, when many retaining pieces (for example not less than 18 pieces) constitute a retaining ring 90, the inside surface of retaining pieces 109 which will contact and support a side surface of a wafer during CMP may be comprised of a plane. Referring to FIGS. 21 through 23, it is desirable that the sizes of the retaining pieces 106, 108, 109 are determined so that adjacent two retaining pieces can be separated by a gap 150 when all the retaining pieces are attached to the base ring 200. The gaps 150 provide spaces for easy detaching of the retaining pieces 106, 108, 109 from the base ring 200, and for accommodating a lateral expansion of the retaining pieces 106, 108, 109 due to thermal expansion. The gaps 150 also facilitate a slurry flow during CMP. It is desirable that the width of the gaps 150 is in the range of l-5mm. The direction of the gaps 150 points to the center of the retaining ring 90 as shown in FIGS. 21 through 23. However, as shown in FIG. 24, when asymmetric retaining pieces 110 are used, the direction of a gap 160 may deviate from the center of a retaining ring 90. [32] FIG. 25 represents a perspective view of a base ring 250. Here, the base ring 250 has grooves 260 on the top surface where retaining pieces are to be attached. In case that the thickness of retaining pieces is thin, the depth of a gap between two adjacent retaining pieces will be shallow if the retaining pieces are attached to a flat top surface of a base ring. This may cause an insufficient slurry flow to the inner part of a retaining ring during CMP. To increase the depth of a gap, the grooves 260 are formed first on the base ring 250, and retaining pieces 100 are attached thereon as illustrated in FIG. 26. Then, a gap 270 between two retaining pieces 100 has a depth larger than the thickness of the retaining piece 100. [33] While the principles of the present invention have been described with reference to certain examples of embodiments illustrated herein, it will be understood by those skilled in the art that various changes in form and in detail may be made thereto without departing from the spirit and scope of the invention, as covered by the following claims.