Abstract:
A method and apparatus for improving uniformity of the rate of removal of material from the surface of a semiconductor substrate by chemical mechanical polishing. In accordance with the invention, the semiconductor substrate is subjected to a combination of polishing motions, including orbital motion, and at least one additional polishing motion selected from rotational, oscillating, sweeping, and linear polishing motions. The invention also provides an improved method for conditioning polishing pads to provide more uniform conditioning and to extend their useful life span.

Description:
FIELD OF THE INVENTION 
     The invention relates to integrated circuit manufacturing technology, and more specifically, to processes for planarizing surfaces of wafer-type semiconductor substrates, such as semiconductor wafers, through chemical mechanical polishing. 
     BACKGROUND OF THE INVENTION 
     Photolithographic optics-based processes are used in the manufacture of integrated circuits, and since these processes require accurate focusing to produce a precise image, surface planarity becomes an important issue. This is becoming increasingly critical as line widths are being reduced in size in order to make semiconductor devices even more compact, and to provide higher speeds. More accurate optical focusing for finer line widths results in a loss of “depth of field”(i.e., the focusing is very accurate only in a plane of very limited depth). Accordingly, a planar surface is essential to ensure good focusing to enable the photolithographic process to produce fine line width, compact high speed semiconductor devices. 
     There are several techniques for planarizing the surface of a semiconductor wafer. One of these is chemical mechanical polishing (CMP). As indicated in an article entitled “Chemical Mechanical Polishing: The Future of Sub Half Micron Devices,” Dr. Linton Salmon, Brigham Young University (Nov. 15, 1996), CMP is now considered the most effective method yet for planarizing wafers with sub micron lines. In this process, a wafer is mounted on a rotary carrier or chuck with the integrated circuit side facing outward. A polishing pad is then brought into contact with the integrated circuit side. Pressure may be applied by the carrier and/or the platen to effectuate polishing. According to Salmon, in some CMP machines the wafer rotates while the polishing pad is stationary, in others the pad rotates while the wafer carrier is stationary, and in yet another type both the wafer carrier and the pad rotate simultaneously. The polishing pad may be pre-soaked and continually re-wet with a slurry that has a variety of abrasive particles suspended in a solution. Typically, the particles range in size from 30 to 1,100 nanometers. After planarization through polishing, the wafers go through a post-CMP clean up to remove residual slurry, metal particles, and other potential contaminants from its surface. 
     An important variable in planarization through CMP is “removal rate” which is the rate of removal of material from the surface of the semiconductor wafer being polished. Preferably, the rate of removal should be such that any surface peaks are preferentially flattened and the resultant surface should be as near perfectly planar as possible. There are several factors that may affect the rate of removal. For example, the nature of the slurry can have a dramatic effect. The slurry includes abrasive particles suspended in a solvent which selectively may soften certain features of the pattern on the semiconductor wafer surface, thereby affecting the relative rate of removal of those features vis-à-vis others. As indicated in the above article, “The purpose of the slurry is simple, yet understanding and modeling all the mechanical and chemical reactions involved is nearly impossible.” Accordingly, development of the CMP process has proceeded on a “trial and error basis.” 
     Among the more advanced CMP machines presently available are the AvantGaard Model 776 of IPEC of Phoenix, Ariz. In this CMP apparatus, the lower head (containing the polishing pad) orbits, while the carrier holding the wafer rotates about a central axis. Polishing fluids (slurry) are introduced to the wafer directly through the polish pads with point-of-use mix, which results in better wafer uniformity and reduced slurry consumption. 
     There continues to be multiple challenges in CMP, making the polishing and planarization faster, more uniform across a wafer, and improving the variation seen in wafer to wafer results. The polishing motion of the pad and carrier play a crucial role in the CMP process along with the quality of the polishing pad over its life. 
     The polishing pad should be “conditioned” after a period of use to provide for a more uniform polishing rate, from wafer to wafer, and to provide for better planarization uniformity across a single wafer. During the pad conditioning process, a pad conditioner arm with an abrasive lower surface is forced to come in contact with the pad upper surface while the pad oscillates and the conditioner arm moves back and forth in an arc about a pivot axis outside of the circumference of the polish pad. The combined pad oscillation and the conditioning arm arc motion during conditioning results in non-uniform pad surface removal and roughing. Areas closer to the arm arc pivot are conditioned at a higher rate than the areas more distant from the arc pivot. Over time, this non-uniform pad conditioning results in poorer polishing uniformity on the semiconductor wafers. 
     Semiconductor manufacturers consistently require CMP processes to improve over time. As semiconductor devices become ever more complex and device geometry becomes ever so much smaller, there exists a need to make the CMP removal rate more consistent from wafer to wafer and wafer lot to wafer lot, while also making the polishing results more uniform across the entire surface of a wafer. Furthermore, there is also a need for a method to provide better and more uniform conditioning of CMP pads during their lifetime. 
     SUMMARY OF THE INVENTION 
     The invention provides a method of improving the uniformity of the rate of removal of material from the surface of a semiconductor substrate, such as a wafer having integrated circuits formed thereon. The invention also provides a method for better and more uniform conditioning of chemical mechanical polishing (CMP) pads to extend the useful lifetime. 
     The objective of the invention is achieved through use of a combination of polishing motions applied to the surface of the semiconductor substrate or cleaning motions applied to the polishing pad. These motions are selected from combinations of the following: rotational, orbital, oscillating, sweeping and linear movement. As explained in more detail herein, the combination of motions may be achieved through permutations of movement of the polishing platen and wafer carrier, in the case of semiconductor substrate surface polishing; and through permutations of the movement of the polishing pad and conditioning surface, during polishing pad conditioning. 
     In accordance with one embodiment of the method of the invention, a wafer held in a carrier (which may rotate about a central axis, or which may be stationary) is brought into contact with a polishing pad that is rotating or oscillating (i.e., at least partially rotating in alternating directions) about its central axis, while the pad is simultaneously orbiting around an orbital axis. The clockwise and counter-clockwise rotational oscillations of the polishing pad about its central axis may range through angles of less than 360 degrees to more than 360 degrees in each direction. Continuous rotation of the polishing pad about its central axis may also be imparted in certain embodiments to improve the surface characteristics of the semiconductor wafer. The wafer carrier may be rotated or oscillated about an axis or held stationary. A polishing slurry is applied, either through the pad itself or through distribution onto the pad to allow infiltration between the pad and the wafer surface being polished. The polishing is maintained while applying a sufficient pressure to polish the semiconductor wafer surface to a desired degree of planarity. 
     In accordance with another embodiment of the method of the invention, a wafer held in a carrier is brought into contact with a polishing pad that is moving linearly, relative to the wafer surface. The wafer carrier on the other hand both orbits about an axis, and oscillates about a second axis, offset from the first axis. Alternatively, the polishing pad may rotate about a central axis. 
     The current embodiment of the invention also provides an apparatus for polishing semiconductor wafers to planarize the surfaces of the wafers. The apparatus includes a carrier adapted for securely holding at least one semiconductor wafer to expose the back surface of the wafer to be polished on an underside of the carrier, and the front surface of the wafer to be polished to a polishing pad, supported on a platen, spaced from the carrier underside. But one with ordinary skill in the art could orient the apparatus such that the carrier was below the platen. The apparatus includes mechanical means for imparting orbital motion to the platen. Such means, for example, may include a stacked pair of rotary bearings with an upper bearing fixedly mounted to the platen and an upper portion of a cylindrical sleeve, that has central axes in its upper and lower portions offset from each other, extending vertically below the platen. A lower bearing is mounted to the lower portion of the cylindrical sleeve and housing of the apparatus, such that the axes of rotation of the bearings are offset. A drive motor rotates the sleeve, thereby causing the platen to orbit about an orbital axis. The apparatus of the invention further includes a shaft having a first end coupled to the platen supporting the polishing pad, and a second end coupled to means for imparting rotating or oscillating motion to the shaft. These means may include, for example, a drive motor with a gear box to rotate the shaft and a motor controller to control degrees of rotational output of the motor. Alternatively, a mechanical stop means may limit the arc of rotation of the shaft, and an electrical stop may reverse oscillatory motion of the shaft when the stop has been reached. Other mechanical devices for controlling degrees of shaft rotation or oscillation are clearly also useful. The wafer carrier may be rotated or oscillated about its axis by suitable means, or remain stationary. 
     The invention also provides an apparatus in which the wafer carrier undergoes orbital and rotational motion, or orbital and oscillating motion; while the pad in contact with the semiconductor substrate held in the carrier either rotates, or is held stationary. In accordance with this apparatus, the mechanical means for imparting orbital and rotational or oscillating movement to the carrier substantially corresponds to the above-described apparatus for imparting such motion to the platen. The platen holding the polishing pad, in accordance with this embodiment, has a central shaft that may be rotated at a controlled rate by an electrical motor, or maintained stationary. Thus, a substrate held in the wafer carrier has a surface subjected to potentially one of four types of permutations of polishing motion: (1) orbital and rotational (with platen stationary); (2) orbital and rotational and sweeping (with platen rotating); (3) orbital and oscillating (with platen stationary); and (4) orbital, oscillating, and sweeping (with platen rotating). 
     In a yet further embodiment of the invention, the pad is a continuous belt mounted over a pair of rollers, and has a backing slide plate to allow pressing of the belt pad against a semiconductor substrate held in a wafer carrier. In this embodiment, the wafer carrier is able to produce orbital motion, and either oscillating or rotational motion. Thus, when the continuous belt is driven linearly, the surface of the semiconductor substrate is subjected to one of the following two polishing motions: (1) a combination of orbital oscillation and linear movement; and (2) a combination of orbital, rotational and linear polishing movement. 
     Using the apparatus of the invention, and applying the method of the invention, semiconductor wafers are produced that are more planar across the entire surface area, than wafers that are polished without the rotary or oscillatory motion of the invention. The removal rate of the method and apparatus of the invention is more uniform across the wafer. 
     The invention also provides, through the at least partial rotational movement and simultaneous orbital movement of the pad, a method for improving pad conditioning. Usually, as explained before, in the prior art pad conditioning process, a pad conditioner arm with an abrasive lower surface is brought into contact with the pad upper surface while the pad oscillates and the conditioner arm moves back and forth in an arc about a pivot axis outside of the circumference of the polish pad. The combined pad oscillation and the conditioning arm motion during conditioning results in non-uniform pad surface removal and roughing. Over time, this non-uniform pad conditioning results in poorer polishing uniformity on the semiconductor wafers. In accordance with the invention, the rotation or oscillation of the pad greatly enhances the conditioning process by allowing the areas that ordinarily are less conditioned in the prior art to move into regions of higher conditioning while the more heavily conditioned areas move into the regions of lower conditioning. Thus, uniform conditioning across the pad may be achieved through the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings which are schematic and not to scale, wherein: 
     FIG. 1 is a schematic partial side view in cross section of a preferred embodiment of the apparatus of the invention; 
     FIG. 2 is a schematic partial side view in cross section of another embodiment of the invention; 
     FIG. 3 is a perspective view of the apparatus required to impart oscillatory motion to an orbiting platen, in accordance with the embodiment of the invention of FIG. 2; 
     FIG. 4 is a schematic partially exploded view showing mechanical stop details of an embodiment of the invention of FIG. 3 for providing oscillatory motion to an orbiting platen; 
     FIG. 5 is a schematic illustration showing a side view, in partial cross-section to show details of an alternative embodiment of the invention, wherein a wafer carrier is equipped to both oscillate and orbit, or rotate and orbit, against a polishing pad that is either stationary or rotating; 
     FIG. 6 is a schematic diagram, in partial side cross-section to show detail, illustrating an alternative embodiment of the invention, wherein the wafer carrier is equipped to either orbit and oscillate, or orbit and rotate; while the wafer is brought into contact with a continuous belt polishing pad that slides linearly across the surface of the semiconductor substrate; and 
     FIG. 7 is a schematic diagram to illustrate the polishing pad conditioning process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     U.S. Pat. No. 5,554,064 entitled “Orbital Motion Chemical-Mechanical Polishing Apparatus and Method of Fabrication,” discloses an orbital chemical-mechanical polishing apparatus, and is hereby fully incorporated by reference. The apparatus of the present invention adds an additional type of motion to the polishing pad of the apparatus: namely, rotation or oscillation achieved by rotating the platen with its polishing pad, in the preferred embodiment, in alternating clockwise and counterclockwise directions. These rotations or oscillations of the platen with its polishing pad during CMP enhance the polished wafer surface by reducing polish variations as compared to a surface obtained using orbital motion only. 
     Referring to FIG. 1, the preferred embodiment of the apparatus of the current invention, the apparatus includes a frame  100  onto which is mounted a platen  102  that is equipped with a polishing pad  104 . 
     The apparatus includes a pair of rotary bearings, the upper rotary bearing  106  is fixedly mounted to an underside of the platen  102 , and a rotatable “wave generator”  110  that includes a substantially cylindrical sleeve  111  extending downward under the platen  102 . A first central axis Co of the upper rotary bearing  106  of the wave generator  110  is offset from the second central axis Cc of the lower rotary bearing  108 . The lower rotary bearing  108  is fixedly mounted to the lower portion of the sleeve  111 , and to the supporting frame  100  of the apparatus. Thus, when the wave generator  110  is brought into rotational motion, the first central axis Co orbits about the second central axis Cc of the lower rotary bearing  108  at a rate equal to the rotation rate of the wave generator  110 . The radius of orbit of the first central axis Co of the upper rotary bearing  106  is equal to the parallel offset between the first central axis Co and the second central axis Cc. This causes the platen  102  and pad to orbit. As indicated in FIG. 1, rotary motion is imparted to the wave generator  110  by means of a drive belt  112  that embraces sleeve  111  and that extends over a pulley  114  coupled to a drive motor  116 . More detail about the orbital motion is found in U.S. Pat. No. 5,554,064 previously incorporated by reference. 
     According to the invention, a shaft  118  extends from an underside of the platen  102  where it is fixedly attached, through the annular space of the sleeve  111  of the wave generator  110  downward to a mechanism for imparting rotary or oscillatory motion to the platen  102 . The shaft  118  includes an upper pedestal  120  fixedly attached to the underside of the platen  102 . Extending downward from the pedestal  120 , the shaft includes an upper universal joint  122   a  and a lower universal joint  122   b , spaced from the upper universal joint  122   a.    
     A variety of mechanisms that may be used to impart rotational or oscillatory motion of the invention will become clear to one of skill in the art who has read this disclosure. In the preferred embodiment of FIG. 1, a drive shaft  124  is coupled to the lower universal joint  122   b  at one of its ends, and to gear box  126  at its other end. The axis of drive shaft  124  is along the same axis of rotation of the second center axis Cc of the lower rotary bearing. The gear box is driven by a step motor  136 , that is controlled by a motor controller  138 . The motor controller controls the degree of rotation imparted by the motor to shaft  124 . Thus, by adjusting the motor controller, the arc may be varied within the range from about −360 to about +360 degrees for oscillatory motion. For rotational motion, the motor may be allowed to continuously rotate shaft  124  thereby causing continuous rotation of pad  104 . 
     Other mechanisms may also be utilized to impart oscillatory (partial rotational movement) or rotational movement to the pad  104 . For example, in the alternative embodiment of the invention shown in FIG. 2, oscillatory motion is produced by a combination of a drive motor and mechanical and electrical stops that cause the shaft to move in alternate counterclockwise and clockwise motion, limited by the mechanical stop. Thus, referring to FIGS. 2,  3 , and  4 , a substantially vertical shaft  124  is coupled to and extends downward from below the lower universal joint  122   b,  and into a hard stop box  140 . As shown, the shaft  124  has a radial leg  128  that sweeps the interior of surrounding cap  142  when the shaft  124  is rotated. To limit rotation of shaft  124 , one or more mechanical stops are placed in the cap  142  to arrest rotational movement of the shaft by blocking movement of the radial leg. A pair of electrical sensors or stops (not shown) are located on the outside of each side of the mechanical stop  130  so that the radial leg  128  will encounter the electrical stops before being blocked by the mechanical stop. 
     A motor  136 , able to impart rotary motion, is mounted to a supporting frame  100  of the apparatus, and is mechanically coupled to the gear box  126 . Thus, the motor  136  through gear box  126  rotates shaft  124  and, hence, shaft  118  counterclockwise, thereby causing the platen to rotate in the same direction, until the radial leg  128  of the shaft  124  is stopped by the mechanical stop  130 . Then, due to electrical contact with electrical sensor  132 , direction of rotation is reversed to a clockwise direction. Again, shaft  118  and platen  102  also rotate clockwise until the radial leg  128  of shaft  124  is limited by mechanical stop  130 . Contact with the other electrical stop  132  causes reversal of the rotational movement, as described above. Thus, the apparatus provides clockwise and counterclockwise oscillatory movement in an arc determined by the location of the mechanical stop. 
     As explained above, in accordance with the invention, the pad is simultaneously subject to at least partial rotational movement and orbital movement. For complete rotational movement, in those apparatus where the supply of polishing slurry is applied through the pad, the slurry supply lines (and any other supply lines) should be supplied with rotatable couplings so that the supply lines do not twist around the shaft. Obviously, for partial rotational movement or oscillation, such rotational couplings may not be needed, as long as the supply lines are of adequate length. 
     In a preferred embodiment of the invention, developed for polishing standard 8 and 12-inch wafers, the platen and pad orbit such that the locus of the center of the pad describes a circle with a diameter from about ½ of the wafer diameter to about 0.1 inches with the preferred orbit diameter of 1.25 inches. The center of orbit of the carrier is offset from the center of the orbit of the platen by from about 0 to about 1 inch with a preferred offset of about ⅜ inches. 
     Typically, in accordance with the invention, the pad and platen orbit at speeds of at least 300 revolutions per minute, more preferably in the range 300-600 revolutions per minute, but the range can be as much as 200-2000 revolutions per minute. The wafer carrier  150  may rotate or oscillate about its axis or remain stationary. 
     In accordance with the invention, it is preferred that the polishing pad be rotated or oscillated an integral number of times during each polish cycle. The duration of a polish cycle depends upon several factors, and typically varies in the range from about one to about four minutes. It is preferred to have from about 1 to about 6 complete oscillations per polish cycle. 
     While the arc through which the polish pad  104  rotates or oscillates may vary, it is preferred to oscillate continuously. It should preferably be able to oscillate through the range from about −180 degrees (counterclockwise) to about +180 degrees (clockwise). Oscillatory motion in the region from about −135 degrees to about +135 degrees is useful, but lesser or greater angular rotation may also be beneficial. 
     It will be readily apparent that in the above embodiment of the apparatus of the invention, the surface of a semiconductor substrate being polished may be subjected to a combination of several kinds of motion, depending upon mode of operation of the apparatus. For example, when the platen both orbits and oscillates, and the wafer carrier rotates, the wafer surface is subjected to orbital, rotational and oscillating polishing movement. On the other hand, when the platen orbits and rotates, while the wafer carrier rotates, the wafer surface is subjected to orbital polishing movement along with two kinds of rotational polishing movement. When the wafer carrier is stationary, the wafer surface is subjected to either orbital and rotational polishing movement, or orbital and oscillating polishing movement, depending upon mode of operation of the apparatus. In accordance with term usage of this document, “an oscillating polishing movement” refers to movement of the device (carrier or platen) and not the actual movement experienced (or traced) by a locus on the wafer surface; the same applies to “linear”, “rotational”, “sweeping” and “orbital polishing movements”. 
     It will be readily apparent to one of skill in the art who has read this disclosure, that mode of movement of the carrier and platen can be reversed, i.e., the wafer carrier may be equipped with mechanical means to generate orbital and either oscillating or rotational movement; while the platen may be retained stationary or may rotate. Accordingly, the invention also provides an apparatus for carrying out this “reverse” application of polishing movement, through the embodiment illustrated in FIG.  5 . Since many of the component parts of the apparatus are similar to that of the above-described embodiment, the same numerals are used for simplicity. In this instance, the wafer carrier  150  is linked to a wave generator  110 , that is similar to the wave generator described above in that it is comprised of two bearings  106 ,  108  spaced vertically from each other, and with centers of rotation offset. The lower bearing  108  is mounted to a support structure, such as the housing  154 , which is in turn supported by a support structure  156 . One end of the wave generator has a cylindrical sleeve  111  which is driven by a belt  112  that passes over a drive pulley  114  of an electrical motor  116  which preferably has speed control. Once again, a central shaft  118  extends in the annular space of the wave generator and the pedestal  120  at its lower end is mounted to the upper surface of the wafer carrier  150 . The shaft  118  is equipped with at least two universal joints,  122   a  and  122   b , one at each of its ends. A drive shaft  124  is mounted to an upper end of the shaft  118 , above the upper universal joint  122   b , and is driven through gear box  126  by motor  136  which is in turn controlled by motor controller  138 . Thus, the apparatus for imparting orbital and rotational or oscillating movement to the wafer carrier  150  is similar to the apparatus described above for imparting such motion to the polishing pad platen. 
     In this instance, the wafer carrier, when it contains a wafer  152 , is brought into contact with the pad  160  which is supported on platen  166 , which may rotate or which may be held stationary. When the platen rotates, the pad sweeps across the face of the wafer being polished in a “sweeping motion.” At the same time, operation of the above-described apparatus imparts an orbital motion to the wafer carrier (and hence to the wafer) along with either complete rotation of the carrier around its central axis, or oscillation about that access. Thus, the apparatus provides for several permutations of polishing movement on the surface of the wafer: (1) orbital, rotational and sweeping polishing movement; (2) orbital, oscillation and sweeping polishing movement; (3) orbital and oscillating polishing movement; and (4) orbital and rotational polishing movement. 
     The embodiment of FIG. 6 provides yet another variation of the above-described invention. In this instance, the polishing pad is in the form of a continuous belt  160  that passes over to rollers  162   a  and  b , one of which is a drive roller. Thus, the polishing pad moves linearly relative to the wafer carrier  150  at a controlled rate. Preferably, the polishing pad moves at a rate of 100 to about 200 centimeters per second . The polishing pad is preferably backed with a rigid backing slide plate  164  that is mounted to a support the pad and allow controlled pressing of the wafer surface against the pad, without untoward yielding of the moving continuous belt pad  160 . In accordance with this embodiment of the invention, a wafer surface being polished may be subject to rotational, orbital and linear polishing movement; or orbital, oscillation and linear polishing movement; or orbital and oscillation polishing movement; or orbital and rotational polishing movement. 
     In accordance with the invention, pad conditioning is also substantially improved and enhanced. As illustrated in FIG. 7, a polishing pad  200  is conditioned by a conditioning arm  204  that carries an abrasive conditioning surface and that pivots about point  202 . In prior art, as a consequence of the motion of the arm in an arc and of the pad as it orbits, a lower conditioning region  208  arises at the locations farthest from the pivot point of the arm, and a higher conditioning region  210  arises at locations nearest the pivot point of the arm on the polishing pad. Moreover, in the prior art, two non-conditioned regions  206  may also arise. In accordance with the current invention, the entire pad is more uniformly conditioned. Due to oscillation or rotation of the polishing pad, those regions that may have been subjected to lower conditioning rotate to positions closer to the pad conditioner arm pivot and are then subjected to higher conditioning. Likewise, of course, those areas previously with high conditioning are then rotated to zones with lower conditioning. Thus, on the average, each region of the pad may be subjected to the same average conditioning. Accordingly, more uniform pad conditioning is obtained. 
     One skilled in the art may realize that it is the combinations of the respective motions that produces the desired results. The invention provides methods and apparatus which allow the selection of a range of permutations of polishing movement on the surface of a wafer being polished. Thus, the invention allows customization of polishing to meet specific requirements, and provides, for the first time, significant added flexibility to the operator to select polishing motion combinations for achieving the best result. Also, please note that the invention has been described in terms of a polish pad and a slurry with abrasive particles. The current invention will work equally as well with a slurryless pad, where the abrasive is embedded into the pad. Such pads are commercially available from 3M Products 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.