This invention relates to technology for removing unwanted metal from semiconductor wafers. More particularly, the invention pertains to wafer chucks used in metal etching modules to align, rotate and clamp the semiconductor wafer while etchant is applied. The modules used to perform the metal etch are typically edge bevel removal (EBR) modules specifically designed to perform the metal etch or integrated electrofill modules that are used to perform both metal deposition and etching.
Damascene processing is a method for forming metal lines on integrated circuits. It is often a preferred method because it requires fewer processing steps than other methods and offers a higher yield. In Damascene processing, as well as other integrated circuit manufacturing processes, the conductive routes on the surface of the circuit are generally formed out of a common metal, traditionally aluminum. Copper is a favored metal because of its higher conductivity and electromigration resistance when compared to aluminum, but copper presents special challenges because it readily diffuses into silicon oxide and reduces its electrical resistance at very low doping levels. During integrated circuit fabrication, conductive metal is needed on the active circuit region of the wafer, i.e., the main interior region on the front side, but is undesirable elsewhere. In a typical copper Damascene process, the formation of the desired conductive routes generally begins with a thin physical vapor deposition (PVD) of the metal, followed by a thicker electrofill layer (which is formed by electroplating). The PVD process is typically sputtering. In order to maximize the size of the wafer""s useable area (sometimes referred to herein as the xe2x80x9cactive surface regionxe2x80x9d) and thereby maximize the number of integrated circuits produced per wafer), the electrofilled metal must be deposited to very near the edge of the semiconductor wafer. Thus, it is necessary to allow physical vapor deposition of the metal over the entire front side of the wafer. As a byproduct of this process step, PVD metal typically coats the front edge area outside the active circuit region, as well as the side edge, and to some degree, the backside. Electrofill of the metal is much easier to control, since the electroplating apparatus can be designed to exclude the electroplating solution from undesired areas such as the edge and backside of the wafer. One example of plating apparatus that constrains electroplating solution to the wafer active surface is the SABRE(trademark) clamshell electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif. and described in U.S. patent application Ser. No. 08/969,984, xe2x80x9cCLAMSHELL APPARATUS FOR ELECTROCHEMICALLY TREATING SEMICONDUCTOR WAFERS,xe2x80x9d naming E. Patton et al. as inventors, and filed Nov. 13, 1997, which is herein incorporated by reference in its entirety.
The PVD metal remaining on the wafer edge after electrofill is undesirable for various reasons. One reason is that PVD metal layers are thin and tend to flake off during subsequent handling, thus generating undesirable particles. This can be understood as follows. At the front side edge of the wafer, the wafer surface is beveled. Here the PVD layers are not only thin, but also unevenly deposited. Thus, they do not adhere well. Adhesion of subsequent dielectric layers onto such thin metal is also poor, thus introducing the possibility of even more particle generation. By contrast the PVD metal on the active interior region of the wafer is simply covered with thick, even electrofill metal and planarized by CMP down to the dielectric. This flat surface, which is mostly dielectric, is then covered with a barrier layer substance such as SiN that both adheres well to the dielectric and aids in the adhesion of subsequent layers. Another reason to remove the residual PVD metal layers in the wafer edge area is that the barrier layers underneath them are also thin and uneven, which may allow migration of the metal into the dielectric. This problem is especially important when the metal is copper.
To address these problems, semiconductor equipment may have to allow etching of the unwanted residual metal layers. Various difficulties will be encountered in designing a suitable etching system.
One such difficulty involves the design of wafer chucks that hold the semiconductor wafer during the metal etch. First, the system must align the wafer on chuck for rotation. Conventionally, such alignment is done by placing the wafer in a separate alignment module and then transporting it the chuck. Unfortunately, this approach involves a separate alignment step that can add expense and affect IC throughput. Further, the wafer chuck should not contact the wafer edges during the actual etching of unwanted metal from those regions. Otherwise, the viscous etchant would not be able flow over the side edge of the wafer unimpeded. Still further, the chuck should be able to clamp the wafer during high-speed rotation (e.g., greater than 750 rpm), such as is typically used in drying operations. The chuck must be made of materials that are resistant to the etching properties of the etchant. The chuck should also be designed to facilitate wetting and rinsing operations, and to allow unimpeded application of the etchant to the backside of the wafer.
For these reasons, an improved wafer chuck design is required for etching unwanted metal from semiconductor wafers.
The present invention provides wafer chucks designed to solve the above-described problems, among others. Such wafer chucks may include alignment members that allow the wafer to be properly aligned on the chuck without using a separate alignment stage. The alignment members may be cams, for example, attached to arms of the wafer chuck. These members may assume an alignment position when a robot arm places the wafer on the chuck. In this position, they guide the wafer into a proper alignment position with respect to the chuck. During rotation at a particular rotational speed, the alignment members move away from the wafer to allow liquid etchant to flow over the entire edge region of the wafer. At still higher rotational speeds, the wafer is clamped into position to prevent it from flying off the chuck. A clamping cam or other device (such as the alignment member itself) may provide the clamping. While the wafer chuck is described in the context of EBR modules, similar designs may be employed with other types of semiconductor processing modules. It is obvious to one of ordinary skill in the art that various specifications, alignment and clamping timings/rotational speeds can be modified to adapt the wafer chuck of the present invention to other semiconductor wafer processes.
One aspect of the invention provides for a wafer chuck with alignment members and clamping members on the ends of chuck arms. The wafer sits on support pins on the chuck arms. When the chuck is at low rotational speed (below EBR speed) or at rest, the alignment members are in an inward, alignment position that keeps the wafer aligned by contacting its edge. Above a certain rotational speed, the alignment members move away from the edge of the wafer, thus allowing the EBR etchant to flow over the edge of the wafer unimpeded. At these speeds, the clamping members are in an outward, non-clamping position. At still higher rotational speeds (e.g., drying speeds), the clamping members move inward to clamp the wafer by contacting its edge. The invention provides for alignment and clamping members that are cams. The cams will typically be designed so that they will automatically move inward or outward about pivot pins, based on the direction of the centripetal force on the cams.
An alignment member and a clamping member can be placed on the same chuck arm, so that one sits inside the other, though the manner of their operation is essentially the same as in the above-described aspect. The clamping member can be designed with a cross-member to help constrain the alignment member to its alignment position. In an alternative embodiment, a pneumatic mechanism can also be used to align and clamp the wafer. The pneumatic mechanism keeps the wafer in alignment while it is at rest or at low rotational speed by constraining it with alignment pins. The pneumatic mechanism then retracts at EBR speeds to allow the etchant to flow over the edge of the wafer. The pneumatic mechanism clamps the wafer at higher rotational speeds (drying speeds) by again contacting the edge of the wafer with the alignment pins.
Another aspect of the invention provides for a method of removing metal from a semiconductor wafer by aligning the wafer in a rotatable chuck of an etching module, aligning and rotating the wafer, performing the metal etch, and clamping down the wafer during rotation at higher speeds (e.g., for drying). The method can be practiced in a stand-alone etching module, or in an integrated module that is designed to do both metal deposition and etching. The metal that is involved is typically copper. The method is practiced with alignment members that align the wafer by contacting its edge, and clamping members that clamp the wafer by contacting its edge.
Another aspect of the invention provides for pre-aligning a semiconductor wafer using a pre-alignment module or pre-alignment section of the etching module, so that the semiconductor wafer is attached at a precise position on a robot arm. The robot arm can then be moved to a predetermined precise position in an etching module, at which time the wafer is released, thus placing it in precise alignment within the etching module.
These and other features and advantages of the present invention will be described in more detail below with reference to the associated drawings.