Optical endpoint detection for buff module on CMP tool

The invention includes a polishing station and a buffing station for chemically mechanically polishing a wafer. The buffing station preferably includes a pair of rotating opposing buffing pads that receive a portion of the wafer. In the buffing station, a measurement instrument may be positioned adjacent an exposed portion of the wafer for detecting an endpoint of the buffing process. A first slurry may be used in the polishing station and a second slurry may be used in the buffing station. For planarizing a wafer with a metal layer over a barrier layer, the metal layer may be removed at the polishing station while the barrier layer is removed at the buffing station. For planarizing a dielectric layer, the top portion of the dielectric layer may be removed at the polishing station with an additional amount removed at the buffing station thereby leaving a dielectric layer with a desired thickness.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS An improved method and apparatus utilized in the polishing of semiconductor substrates and thin films formed thereon will now be described. In the following description, numerous specific details are set forth illustrating Applicant's best mode for practicing the present invention and enabling one of ordinary skill in the art to make and use the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known machines and process steps have not been described in particular detail in order to avoid unnecessarily obscuring the present invention. The invention may be practiced with a chemical mechanical polishing (CMP) tool as shown in FIG. 4 . The general design of the CMP tool shown in FIG. 4 is similar to a 776 model sold by SpeedFam-IPEC which is headquartered in Chandler, Ariz. One or more cassettes 400 loaded with wafers (not shown) may be loaded onto the CMP tool 420 . A first robot 401 may slide along a track 401 as it removes wafers from the cassettes 400 and loads them into a holding station 403 . A second robot 404 may take the wafer from the holding station 403 and place the wafer in one of two positions in a wet bath 403 . A third robot 406 may remove the wafer from the wet bath 403 and transport the wafer to a carrier (not shown) associated with one of the four polishing stations 407 . The carrier presses the wafer against the polishing surface of the polishing station 407 as relative motion is created between the front surface of the wafer and the polishing surface. The relative motion is preferably created by holding the front surface of the wafer in the carrier stationary while the polishing pad is orbited. The polishing pad 104 may be orbited during the planarization process of the wafer 201 and rotated clockwise and counter-clockwise. FIG. 5 is a cross-sectional view of an exemplary motion generator 500 that may be used to generate an orbital motion for the platen 107 and polishing pad 104 . The motion generator 500 is generally disclosed in U.S. Pat. No. 5,554,064 Breivogel et al. and is hereby incorporated by reference. Supporting base 220 may have a rigid frame 502 that can be securely fixed to the ground. Stationary frame 502 is used to support and balance motion generator 500 . The outside ring 504 of a lower bearing 506 is rigidly fixed by clamps to stationary frame 502 . Stationary frame 502 prevents outside ring 504 of lower bearing 506 from rotating. Wave generator 508 formed of a circular, hollow rigid body, preferably made of stainless steal, is clamped to the inside ring 510 of lower bearing 506 . Wave generator 508 is also clamped to outside ring 512 of an upper bearing 514 . Wave generator 508 positions upper bearing 514 parallel to lower bearing 506 . Wave generator 508 offsets the center axis 515 of upper bearing 514 from the center axis 517 of lower bearing 506 . A circular platen 211 , preferably made of aluminum, is symmetrically positioned and securely fastened to the inner ring 519 of upper bearing 514 . A polishing pad or pad assembly can be securely fastened to ridge 525 formed around the outside edge of the upper surface of platen 211 . A universal joint 518 having two pivot points 520 a and 520 b is securely fastened to stationary frame 502 and to the bottom surface of platen 211 . The lower portion of wave generator 508 is rigidly connected to a hollow and cylindrical drive spool 522 that in turn is connected to a hollow and cylindrical drive pulley 523 . Drive pulley 523 is coupled by a belt 524 to a motor 526 . Motor 526 may be a variable speed, three phase, two horsepower AC motor. The orbital motion of platen 211 is generated by spinning wave generator 508 . Wave generator 508 is rotated by variable speed motor 526 . As wave generator 508 rotates, the center axis 515 of upper bearing 514 orbits about the center axis 517 of lower bearing 506 . The radius of the orbit of the upper bearing 517 is equal to the offset (R) 526 between the center axis 515 of upper bearing 514 and the center axis 517 of the lower bearing 506 . Upper bearing 514 orbits about the center axis 517 of lower bearing 506 at a rate equal to the rotation of wave generator 508 . It is to be noted that the outer ring 512 of upper bearing 514 not only orbits but also rotates (spins) as wave generator 508 rotates. The function of universal joint 518 is to prevent torque from rotating or spinning platen 211 . The dual pivot points 520 a and 520 b of universal joint 518 allow the platen 211 to move in all directions except a rotational direction. By connecting platen 211 to the inner ring 519 of upper bearing 514 and by connecting universal joint 518 to platen 211 and stationary frame 502 the rotational movement of inner ring 519 and platen 211 is prevented and platen 211 only orbits as desired. The orbit rate of platen 211 is equal to the rotation rate of wave generator 508 and the orbit radius of platen 211 is equal to the offset of the center 515 of upper bearing 514 from the center 517 of lower bearing 506 . It is to be appreciated that a variety of other well-known means may be employed to facilitate the orbital motion of the platen 211 . While a particular method for producing an orbital motion has been given in detail, the present invention may be practiced using a variety of techniques for orbiting the platen 211 . The platen 211 is preferably orbited with a radius between about 20 mm and 5 mm. It should also be understood that while an orbital motion for the platen 211 and polishing pad is preferred, the polishing pad may also be moved in other ways, e.g. linearly or rotationally. With reference back to FIG. 4, a slurry may be introduced between the wafer and the polishing pad, preferably by pumping the slurry through the polishing pad directly to the wafer-polishing pad interface. A slurry that is reactive to the material being removed from the front surface of the wafer may be used to increase the removal rate of the material. A typical slurry may be SSW2000 for tungsten or SS12 for oxide; both manufactured by Cabot Microelectronics, headquartered in Aurora, Illinois. The third robot 406 may be used to take the wafer from the carrier in one of the polishing station 407 and transport the wafer to one of two buff stations 408 . FIGS. 2 and 3 illustrate a simplified view of a buffing station 408 . The wafer 100 is inserted between buff pads 200 a and 200 b . The buff pads 200 a and 200 b may be made of polyurethane, have a textured surface and have a diameter of about 130 mm for a 200 mm wafer. The buff pad 200 a and 200 b may be, for example, a Politex Supreme model manufactured by Rodel Inc., headquartered in Phoenix, Ariz. The buff pads 200 a and 200 b may be connected to one or more corresponding motors 203 a and 203 b via shafts 201 a and 201 b . The motors 203 a and 203 b preferably drive the buff pads 200 a and 200 b counterclockwise at 300 rpms as shown by arrows A 2 and A 3 . The rotation of the buff pads 200 a and 200 b will rotate the wafer 100 supported by freely rotating stanchions 202 as shown by arrow A 1 . Slurry 208 from a holding tank 207 may be used to wet the buffing pads 200 a and 200 b and improve the buffing process. The slurry 208 may advantageously be selected to be reactive with areas where material should be removed and less reactive with areas where material should not be removed. The slurry 208 will generally not be as reactive with the material on the front surface of the wafer as the slurry from the polishing station. Examples of slurries that may be used are SSW2000 or deionized water. An optical probe 206 may be placed close to the front surface of the wafer 100 . The layout of the buffing station 408 as described allows the probe 206 easy access to portions of the wafer 100 not covered by the buff pads 200 a and 200 b . A measuring instrument 204 , such as a spectrometer or interferometer, may receive reflected light from the front surface of the wafer 100 and probe 206 via a fiber optic cable 205 . The probe 206 , fiber optic cable 205 and measurement instrument 204 may be part of, or replaced by, an endpoint detection system. For example, a Sentinal optical endpoint system manufactured by SpeedFam-IPEC, headquartered in Chandler, Ariz. may be used to detect endpoint on ILD, STI, tungsten and copper applications. Applicants have discovered that due to the slow removal rate and small amount of material removed, extremely accurate readings may be made in real time. The accurate determination of the endpoint greatly reduces dishing and erosion of metal lines and greatly enhances the accuracy of the thickness of a dielectric layer. The endpoint system for tungsten vias and dual damascene polishing would reduce defectivity and improve dishing and erosion and interlayer dielectric (ILD) would benefit by a controlled final wafer oxide thickness. Referring back to FIG. 4 , while the wafer was buffed in one of the buff stations 408 , the polishing pad in one or more of the polishing stations may be conditioned by a polishing pad conditioner 409 sweeping across the surface of the polishing pad. After the wafer has been buffed at one of the buffing stations 408 , the wafer may be transported by the third robot 406 back to one of the wet baths 405 . The second robot 404 may then remove the wafer from the wet bath 405 and transport the wafer to a first cleaning position 410 within a cleaning station 414 . After an initial cleaning in the first cleaning position 410 , a fourth robot 412 may transport the wafer to a second cleaning position 411 . Cleaning positions 410 and 411 may be of types known in the art and may be, for example, similar in layout to the buffing stations 408 already described. If the cleaning positions 410 and 411 are similar to the buff stations 408 , softer pads and cleaning solutions will be used. Alternatively, the cleaning positions 410 and 411 may comprise a plurality of pairs of opposing rollers aligned so that the wafer may be pulled through the center of them. After cleaning in cleaning positions 410 and 411 , the fourth robot will move the wafer to a drying unit 413 . The drying unit 413 is preferably a spin drier that dries the wafer by rapidly spinning the wafer and removing the fluids on the wafer by centrifugal force. The dried wafer may be removed from the cleaning station 414 by the first robot 401 and replaced into one of the cassettes 400 . A detailed layout of one possible CMP tool has thus been described. Of course, many variations in the CMP tool design with, for examples, a different number of robots, polishing station and/or buffing stations or a different layout may also be used. A description of a process for removing a deposited metal layer from the top surface of a wafer will now be described with continuing references to FIGS. 1A, 1B , 1 C, and 6 . The wafer 100 in FIG. 1A has had grooves etched from a dielectric layer 103 and a thin barrier layer 102 , e.g. Ti or TiN, is deposited over the grooves and dielectric layer 103 . The barrier layer 102 prevents the metal in the metal lines 101 b from migrating into the dielectric layer 103 and possible causing current leaks between lines 110 b . A metal layer, e.g. aluminum, tungsten or copper, is deposited over the barrier layer 102 . The metal layer thus comprises the desirable metal lines 101 b that have been deposited in the grooves and the undesirable metal overburden 110 a. The metal overburden 110 a shown in FIG. 1A may be removed by chemical mechanical polishing of the wafer 100 at a polishing station leaving the barrier layer 102 and metal lines 110 b in the grooves as shown in FIG. 1B . The slurry used at the polishing station may advantageously be chosen to assist in the removal of the metal overburden 110 a . (Step 600 ) Care should be taken to terminate the CMP process before dishing or erosion 104 occurs as shown in FIG. 1D . Erosion 104 is a problem and may easily occur because the slurry for the polishing station will generally be reactive with the metal lines 101 b. The wafer may be transported to a buffing station to remove the barrier layer 102 . A slurry may be chosen for the buffing station that will assist in removing the barrier layer 102 while minimizing the removal of the metal lines 101 b as shown in FIG. 1C . (Step 601 ) After buffing, the wafer 100 may be cleaned and dried (Step 602 ). A description of a process for leaving a desired thickness of a deposited dielectric layer on the top surface of a wafer will now be described with continuing references to FIGS. 1E, 1F , 1 G, and 7 . The wafer 100 in FIG. 1E has had a dielectric layer 103 a , typically an oxide, deposited onto the top surface of the wafer 100 after the previously described process of planarizing a metal layer on a wafer 100 . The dielectric layer 103 a will be used to electrically separate the lines of wire 101 from one level from the lines of wire in another level. A portion of the dielectric layer 103 b shown in FIG. 1F may be removed by chemical mechanical polishing of the wafer 100 at a polishing station leaving only slightly more than the final desired thickness of the dielectric layer 103 a . The slurry used at the polishing station may advantageously be chosen to assist in the efficient removal of the dielectric layer 103 b . (Step 700 ) However, slurries that assist in the efficient removal of the dielectric layer 103 b are also likely to cause microscratches and defectivities. The wafer may be transported to a buffing station to remove a thin layer of the dielectric layer 103 c . By removing this thin layer 103 c , the microscratches and defectivities in the dielectric layer 103 a may also be removed. Another slurry may be chosen for the buffing station that will remove the microscratches and defectivities without causing new microscratches or defectivities as shown in FIG. 1G . (Step 701 ) This second less reactive slurry may be used in the buffing station because less material needs to be removed from the dielectric layer. After buffing, the wafer 100 may be cleaned and dried (Step 702 ). By removing the barrier layer 102 for metal planarization or microscratches and defectivities for dielectric planarization at the buffing station, Applicants have discovered several advantageous. One advantage is that a first slurry may be used on the polishing station that is more reactive while a second slurry may be used on the buffing station that is less reactive. This allows greater control over the important final phase of material removal. By separating the point of use for the different slurries, the plumbing is simplified and minimal mixing of the slurries occurs as typically happens when both slurries are used at the same station. Another advantage is that greater through-put may be achieved by many CMP tools where the other stations in the CMP tool wait on the polishing stations, i.e. the polishing stations are a bottle-neck. The increase in throughput occurs since the buffing stations take over some of the workload from the polishing stations. Another advantage is that a more accurate termination of the planarization process, i.e. endpoint, may be determined at the buffing stations instead of at the polishing stations. The material removal rate is much slower at the buffing stations making it easier to determine endpoint in real time. In addition, when using a buffing station as previously described, much of the front surface of the wafer is exposed during the buffing process making it easier to take accurate measurements to determine endpoint. While the invention has been described with regard to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention.