Patent Application
20090176445
The inventions described below relate the field of wafer carriers and particularly to wafer carriers used during chemical mechanical planarization of silicon wafers.
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 is therefore achieved by a combination of chemical softener and physical downward force that removes material from the wafer or wafer layer.)
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 wafer 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 a membrane carrier tends to avoid or limit 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.
In our prior patents, Fuhriman, et al., Wafer Carrier with Pressurized Membrane and Retaining Ring Actuator, U.S. Pat. No. 7,238,083 (Apr. 25, 2006) and Spiegel, Independent Edge Control for CMP Carriers, U.S. Pat. No. 7,033,252 (Jun. 20, 2006) we disclose CMP systems which employ flexible membrane assemblies, and disclose inventive features which provide for enhanced control of the CMP process to limit the edge effect. The flexible membrane assemblies comprise a round pan-like assembly, constructed of a single piece of synthetic rubber or other pliable material. The membrane portion of the pan (the bottom) is held in place within the wafer carrier by its cylindrical side-wall and its flange which are trapped within other components of the wafer carrier. Along with the advances shown in our prior patents, this construction aids in the reduction of the edge effect which limits yield in CMP processes.
The methods and devices described below provide for a wafer carrier adapted to further reduce the edge effect and allow a wafer to be uniformly polished across its entire surface. A flexible membrane assembly is provided for use in the wafer carrier, upon which pressurized air and/or pressurized bladder act to control wafer backpressure during polishing. The flexible membrane assembly comprises a flat flexible membrane, a relatively rigid cylindrical side-wall, and a flexible flange for interconnection with the wafer carrier components. This construction of the membrane assembly helps reduce the edge-effect in the CMP process and may also reduce vibration in the CMP process. The construction is also easier to make because it is easier to control the dimensions of the rigid cylindrical side-wall than it is to control the dimensions of a molded single piece membrane assembly. The construction also makes it practical to use fluorelastomers, which are very difficult to mold to the close tolerances required in CMP wafer carrier components, as the membrane material, and the membranes can be cut from sheets of known thickness.
The flexible membrane 29 extends horizontally over a peripheral portion of the backside of the wafer 3 and extends vertically between the side of the piston plate 27 and the retaining ring 25 and gimbal plate 28. An extension of the membrane 29 projects into an annular space 32 provided in the gimbal plate 28. Thus, the pressure-regulated flexible membrane 29 moves with the wafer and the piston plate but, during polishing, moves independently of the movement of the gimbal plate 28 and the retaining ring 25. Pressure in the flexible membrane is adjusted by a control computer to apply downward force to the backside 33 of the wafer and to ensure that the rate at which material is removed from the front side 34 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. 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 28 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. Using a control computer, pressure in the retaining ring actuator 26 is regulated independent of the pressure in the inflatable membrane 29. 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.
The dimensions of the membrane assembly and its components can varied to fit various wafer carriers. For use in Strasbaugh™ 200 mm wafer carriers, the sidewall has an overall diameter of 200 mm (7.86″), a wall height of 1.73 cm (0.682″) and a wall thickness of 2 mm (0.080″). The bead and flange are sized and dimensioned, as shown in
The sidewall is made of a rigid or inelastic material, such as ABS plastic, polyethylene terephthalate (PET), polyurethane, polyvinyl chloride, polymethyl methacrylate (Lucite®, Plexiglas®), polycarbonate (Lexan®), and may be furthered stiffened with the addition of carbon fibers or metal layers. The membrane is made of a flexible, elastic material such as rubber, synthetic rubber (neoprene, for example), silicone rubber, nitrile, fluorelastomers (Viton®), urethane and polyurethane foams (Poron®), hydrated acrylonitrile butadiene rubber (HNBR), vinyl, TPE (thermoplastic elastomer). The cylindrical sidewall is most conveniently made by cutting pre-formed cylinders of plastic. The membrane assembly is constructed by cutting the circular membrane from a flat sheet of material, and gluing or melting the flat sheet to the bottom edge of the cylindrical sidewall. The membrane may be cut to size either before or after it is secured to the cylinder. The membrane may be pre-tensioned (stretched) prior to securing it to the cylindrical sidewall, if necessary to prevent droop of the membrane during use which might interfere with wafer loading and sensing. The flange and bead are preferably made of a flexible, elastic material such as synthetic rubber, silicone rubber, nitrile, Viton, Poron, HNBR, Vinyl, TPE (thermoplastic elastomer), formed by injection molding or any other suitable manner, and may also be joined to the cylindrical wall by gluing or melting the two together. The flange and bead components may be varied, for example by providing the flange as an inwardly extending flange, or providing additional structures, to provide mounting or retaining structures suitable for a variety of different wafer carrier constructions (see
While the preferred embodiments of the devices and methods have been described in reference to the environment in which they were developed, they are merely illustrative of the principles of the inventions. Other embodiments and configurations may be devised without departing from the spirit of the inventions and the scope of the appended claims.
