Patent Publication Number: US-2012040591-A1

Title: Replaceable cover for membrane carrier

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of wafer carriers, and in particular, to removable covers added to wafer carrier membranes used during the processing of silicon wafers. 
     2. Description of the Prior Art 
     Integrated circuits, including computer chips, are manufactured by building up layers of circuits on the front side of silicon wafers. An extremely high degree of wafer flatness and layer flatness is required during the manufacturing process. Chemical-mechanical planarization (CMP) is a process used during device manufacturing to flatten wafers and the layers built-up on wafers to the necessary degree of flatness. 
     Chemical-mechanical planarization is a process involving polishing of a wafer with a polishing pad combined with the chemical and physical action of a slurry pumped onto the pad. The wafer is held by a wafer carrier, with the backside of the wafer facing the wafer carrier and the front side of the wafer facing a polishing pad. The polishing pad is held on a platen, which is usually disposed beneath the wafer carrier. Both the wafer carrier and the platen are rotated so that the polishing pad polishes the front side of the wafer. A slurry of selected chemicals and abrasives is pumped onto the pad to affect the desired type and amount of polishing, (CMP) being thus achieved by a combination of chemical softener and physical downward force that removes material from the wafer or wafer layer. The downward force is split in the wafer carrier to a retaining ring force and a wafer force. 
     Using the CMP process, a thin layer of material is removed from the front side of the wafer or wafer layer. The layer may be a layer of oxide grown or deposited on the wafer or a layer of metal deposited on the wafer. The removal of the thin layer of material is accomplished so as to reduce surface variations on the wafer. Thus, the wafer and layers built-up on the wafer are very flat and/or uniform after the process is complete. Typically, more layers are added and the chemical mechanical planarization process repeated to build complete integrated circuit chips on the wafer surface. 
     A variety of wafer carrier configurations are used during CMP. One such wafer carrier configuration is the hard backed configuration. The hard backed configuration utilizes a rigid surface such as a piston or backing plate against the backside of the silicon wafer during CMP forcing the front surface of the silicon wafer to the surface of the polishing pad. Using this type of carrier may not conform the front wafer surface of the wafer to the surface of the polishing pad resulting in planarization non-uniformities. Such hard backed wafer carrier designs generally utilize a relatively high polishing pressure. These relatively high pressures effectively deform the wafer to match the surface conformation of the polishing pad. When water surface distortion occurs, the high spots are polished at the same time as the low spots giving some degree of uniformity but also resulting in poor planarization. Too much material from some areas of the wafer will be removed and too little material from other areas will also be removed. In addition to wafer distortion, the relatively high pressure also results in excessive material removal along the edges of the silicon wafer. When the amount of material removed is excessive, the entire wafer or portions of the wafer become unusable. 
     In other wafer carrier configurations, the wafer is pressed against the polishing pad using a membrane or other soft material. Use of membrane carriers tend not to cause distortion of the wafer. Lower polishing pressures may be employed, and conformity of the wafer front surface is achieved without distortion so that both some measure of global polishing uniformity and good planarization may be achieved. Better planarization uniformity is achieved at least in part because the polishing rate on similar features from die to die on the wafer is the same. 
     While many soft backed wafer carrier configurations are used in CMP, their use has not been entirely satisfactory, In some carrier designs, there have been attempts to use a layer of pressurized air over the entire surface of the wafer to press the wafer during planarization. Unfortunately, while such approaches may provide a soft back for the wafer carrier, it does not permit independent adjustment of the pressure at the edge of the wafer and at more central regions of the wafer to solve the wafer edge non-uniformity problems. 
     In order to correct or compensate for edge polishing effects, attempts have been made to adjust the shape of the retaining ring and to modify a retaining ring pressure so that the amount of material removed from the wafer near the retaining ring was modified. Typically more material is removed from the edge of the wafer resulting in over polishing. In order to correct this over polishing, the retaining ring pressure is adjusted to be somewhat lower than the wafer backside pressure so that the polishing pad in that area was somewhat compressed by the retaining ring and less material was removed from the wafer within a few millimeters of the retaining ring. These attempts, however, have not been entirely satisfactory as the planarization pressure at the outer peripheral edge of the wafer was only indirectly adjustable based on the retaining ring pressure. It was not possible to extend the effective distance of a retaining ring compensation effect an arbitrary distance into the wafer edge. Neither was it possible to independently adjust the retaining ring pressure, edge pressure, or independently adjust backside wafer pressure with respect to retaining ring pressure to achieve a desired result. 
     Thus there remained a need for a membrane backed wafer carrier having independent control of both the membrane pressure and retaining ring pressure providing excellent planarization, control or edge planarization effects, and adjustment of the wafer material removal profile to compensate for non-uniform deposition of the structural layers on the wafer semiconductor substrate. 
     U.S. Pat. No. 7,238,083 to Fuhriman et al describes a wafer carrier adapted to reduce the edge effect thus enabling a wafer to be uniformly polished across its entire surface. The wafer carrier has a pressure-regulated soft membrane behind the wafer, a retaining ring having a retaining ring actuator, and a pressurized edge control bladder or resilient ring used during CMP. Pressures behind the soft membrane, within the retaining ring actuator, and within the edge control bladder are regulated independently from one another. This enables the wafer carrier to account for non-uniformities on the wafer surface, changes in the retaining ring, and edge effect. 
     Although the system described in the &#39;083 patent provides excellent results, a problem still exists. In particular, membranes are replaced periodically (typically after 1500 wafers) due to contamination by slurry, slurries or abrasives. Specifically, the abrasive particles become lodged in the elastomer and causes scratching or cleaning issues. Since membranes are expensive, the wafer process as a result can be costly. 
     What is desired is to provide a membrane that can be used to process more wafers than currently possible without substantially increasing the cost of wafer processing. 
     SUMMARY OF THE INVENTION 
     The present invention provides a thin, peel off skin member at the membrane surface that contacts the wafer thereby protecting the membrane surface for an extended period time. If the member itself becomes damaged, it is removed and replaced with a similar member. Since the member is less expensive than the membrane, the operating cost of the wafer process is substantially reduced. Although the invention is primarily directed to processing a silicon wafer, other wafer materials can be used to fabricate items other than chips, such as magnetic disk heads and LEDs. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawing therein: 
         FIG. 1  shows a cross-sectional view of a prior art wafer carrier having a pressure-regulated soft membrane, retaining ring actuator; 
         FIG. 2  is a cross-sectional view of a wafer carrier modified in accordance with the teachings of the present invention; 
         FIG. 3  shows a detail of the cross-sectional view shown in  FIG. 2 ; 
         FIG. 4  is a view of a tool used to join the membrane to the protective cover; 
         FIG. 5  is a top view of the tool shown in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view along line BB of  FIG. 5 ; and 
         FIG. 7  is a detail of a portion of the view shown in  FIG. 6 . 
     
    
    
     DESCRIPTION OF THE INVENTION 
     In order to put the present invention in perspective, the description of the wafer carrier described in the &#39;083 patent will be set forth herein since the basic structure of that wafer carrier is the same as that utilized to implement the present invention. The teachings of the &#39;083 patent necessary for the understanding of the present invention is incorporated herein by reference. The same reference numerals shown in the figures identify the same components. 
       FIG. 1  shows a cross section of a wafer carrier  2 . The wafer carrier  2  includes, a top plate  23  coupled to a spindle (not shown)  8 , a housing  24  coupled to top plate  23 , a gimbal plate  27  coupled to housing  24 , a retaining ring  25  coupled to the gimbal plate  27 , a retaining ring actuator  26  disposed in the retaining ring  25 , a piston plate  28  having one degree of freedom in the vertical direction coupled to the gimbal plate  27 , and a pressure regulated soft membrane  29 . The membrane may be made of a synthetic rubber or other pliable material. The piston plate  28  is disposed within the inner diameters of the membrane  29  and gimbal plate  27 . When pressurized fluid is applied, the pressurized fluid flows through the passage to the recessed regions in the lower face  30  of the piston plate  28  forcing the soft membrane  29  downwardly away from the lower face  30  of the piston plate  28 . At the same time, the pressurized fluid pushes the piston plate  28  upward (note that piston plate  28  is only utilized for wafer pick up and not critical to the operation of the carrier for polishing). 
     The soft membrane  29  extends horizontally over a peripheral portion of the backside of a wafer  3  and extends vertically between the side of the piston plate  28  and the retaining ring  25  and gimbal plate  27 . An extension of the membrane  29  projects into an annular space  31  provided in the gimbal plate  27 . Thus, the pressure-regulated soft membrane  29  moves with the wafer and the piston plate but, during polishing, moves independently of the movement of the gimbal plate  27  and the retaining ring  25 . Pressure in the soft membrane is adjusted by a computer to apply downward force to the backside  32  of the wafer and to ensure that the rate at which material is removed from the front side  33  of the wafer is uniform across the entire front side of the wafer. 
     The retaining ring actuator in the wafer carrier  2  is independently controlled and affects the amount of force being applied behind the retaining ring  25 . A retaining ring actuator  26  is provided within the retaining ring  25  (note that the retaining ring can be fixed, or static). When the actuator is pressurized, it extends against the retaining ring and increases the amount of force being applied to the polishing pad by the retaining ring relative to the rest of the wafer carrier  2 . The retaining ring  25  is attached to the gimbal plate  27  in such a manner that allows the pressure inside the retaining ring actuator  26  to be increased or decreased. Change of pressure within the retaining ring actuator will influence the amount of force acting on the polishing pad by the retaining ring. The computer regulates the pressure in the retaining ring actuator  26  independent of the pressure in the inflatable membrane  29  and pressure in the edge control bladder  37 . Pressure inside the retaining ring actuator  26  is used to force the retaining ring  25  downwardly as material is removed from the bottom surface of the retaining ring  25 . 
     Polishing removes material from the bottom surface of the retaining ring, particularly over the course of multiple polishing runs. When the carrier  2  is in use, the soft membrane pressure, retaining ring actuator pressure, and edge control bladder pressure can all be regulated independently. This enables an operator to account for non-uniformities on the wafer surface, changes in the height of the retaining ring, and edge effect while using a CMP tool. Thus, the front side  33  of the wafer will remain substantially co-planar with the bottom surface of the retaining ring even as material is removed from the bottom surface of the retaining ring. The retaining ring actuator  26  and the fluid inside it allow the retaining ring  25  to move independently of the wafer  3  and the inflatable membrane  29 . 
     Referring now to  FIGS. 2 and 3 , the present invention provides a cover  50  for protecting the surface of the membrane for the reasons noted hereinabove. Covers can be made out of most any elastomer including: Buna-N, Butadiene, Butyl, Chlorinated Polyethylene, EPDM, HNBR, Hypalon, Kalrez, Neoprene, Nitrile, Polyruethane, Silicone, Viton, etc. The cover material is chosen as to protect the base membrane material from the chemistry at hand. For example, if the first step in the CMP process is basic, the protective material then can be geared toward withstanding basic chemistry used in the process. If the process changes to acidic, then the cover can be so chosen to protect against low PH chemistries. 
     Cover  50  is manufactured with a suitable pressure sensitive adhesive on one side. The adhesive is formulated to adhere the chosen protective cover material to the membrane face surface and could be acrylic or silicone based. The material can be purchased in sheet form with the adhesive pre-applied. The shape can be pre-cut with a steel rule die or with a laser cutting tool or water jet cutting tool. 
     Although the cover is preferably of a uniform thickness and stiffness (thickness in the range between 1/64″ and ⅛″, the preferred thicknesses are 1/32″ and 1/16″), the cover could have a non-uniform thickness or non-uniform stiffness. This would provide a variable down force on the wafer locally (even with a uniform air pressure behind/above the membrane) in order to remove polish local zones in a multi-zone processing system. 
     Cover  50  is applied to the surface of membrane  29  by having the end user remove the carrier from the polishing machine and positioning it on a surface with the membrane cover pointing up. The retaining ring is then removed to facilitate access to the membrane. The old cover is peeled off and a new one applied. If this is not easily accomplished, the membrane will be removed from the carrier. A tool that is basically just a plate with a hole the same diameter as the cover and membrane is placed in front of the operator. The cover is then placed in the hole, adhesive side up and the membrane placed in the tool on the top cover. Since the bore of the tool is the same size as the parts to be assembled, the parts necessarily will come out of the tool concentric as is desired. 
     A vacuum port with a tube connected to a vacuum source can be employed to evacuate the bore and minimize the potential for air bubbles to form between the membrane face surface and the cover (the tool is shown in  FIGS. 3-7 ). 
     The purpose of the membrane cover  50  is to protect the membrane from damage and aging since the cost of the membrane is typically 20 times the cost of the cover. The membrane  29  is subject to abrasion and tearing when a wafer breaks. If a wafer breaks and pieces of wafer lodge in the cover, it can be peeled off and replaced. The membrane is also subject to chemical attack at noted hereinabove. The membrane is also subject to the accumulation of the abrasive particles suspended in polishing slurries which the cover can absorb. Thus, the cover can be replaced at roughly 1/20 the cost of replacing the membrane thus reducing the overall cost of the process to the end user. 
     Referring now to  FIGS. 4-7 , a tool  52  utilized to prepare a membrane having a protective cover formed thereon is illustrated. 
     In particular, protective cover  50  is placed adhesive side up in the bore of tool  52 . 
     The tool bore is sized unilaterally larger in diameter than the diameter of cover  50  by a predetermined amount. The cover  50  and the membrane  29 , nominally the same diameter (and corresponding to the wafer diameter), causes the two components to be concentric within the aforementioned range. The diameter of protective cover  50  (and thus the diameter of the wafer) is in the range between 100 mm and 450 mm. 
     Membrane  29  is placed in the bore of tool  52  at an angle such that the point furthest from the vacuum port touches first. Membrane  29  is then tilted down toward the vacuum port  54  allowing the membrane  29  to come into contact with the adhesive surface of cover  50  while the volume between the parts is evacuated. 
     A disk, or mandrel,  70  may be placed in the membrane bore to apply pressure to the backside of membrane  29  to facilitate adhesion of the protective cover  50  thereto. 
     Tool  52  is preferably made from polyolefin material since these materials repel adhesion; therefore, any glues on the parts being assembled have less chance of sticking to the tool surface thus making it easier to clean. Alternatively, tool  52  could be fabricated of a metal for better size control, hence potentially better concentricity. 
     Membrane  29  is placed at an angle for two reasons. The first is to allow for a gradual, controlled application of cover  50 , helping the operator to maintain the concentricity of parts. If the membrane is put in parallel to the cover, it needs to be correctly lined up, or likely the operation would have to be repeated. The second reason is to start opposite the vacuum port and push any air that could be trapped toward the vacuum port. Otherwise, there is a risk of blocking the port and still trapping air. 
     Since the membrane is flexible, it may be hard to control in its free state. The outer ring is relatively stiff as compared to the membrane, but can be distorted. Membrane  29  can droop or be forced in the other direction due to trapped air. The addition of disk, or mandrel  70 , will keep the outer ring round and help to push the air out toward the vacuum port. It could be most any plastic material, either a polyolefin, polyurethane, Nylon, Delrin or PET. Metal is also possible, however, plastic would be preferred to keep the weight down for the operator. The disk  70  is smaller than the inside diameter of the membrane ring, by just a few thousandths of an inch and would slip easily into the internal diameter of the membrane and be removed just as easily. 
     While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.