This invention relates to an electrostatic chuck for holding a substrate in a process chamber containing a magnetic flux.
In semiconductor fabrication processes, electrostatic chucks are used to hold substrates, such as silicon wafers, during processing of the substrate. Electrostatic chucks are generally described in, for example, U.S. Pat. No. 4,184,188, to Briglia; U.S. Pat. No. 4,399,016, to Tokuda; U.S. Pat. No. 4,384,918, to Abe; and U.S. patent application Ser. No. 08/189,562, entitled "Electrostatic Chuck" by Shamouilian, et al., filed on Jan. 31, 1994--all of which are incorporated herein by reference.
A typical electrostatic chuck comprises a base adapted to be secured to a support in a process chamber. An electrically insulated electrode is on the base, and underside of the base typically comprises a cavity. An electrical connector electrically connects the insulated electrode on the base to an electrical contact on the bottom of the base. The electrical contact is supported by a support member which extends into the cavity of the base.
During its use, the chuck is secured to a support in a process chamber, so that the electrical contact on the bottom of the base electrically contacts a voltage supply connector on the support. A substrate is placed on the upper surface of the chuck, and the electrode in the chuck is electrically biased with respect to the substrate by a voltage applied via the electrical contact of the chuck. The applied voltage causes electrostatic charge to accumulate in the insulated electrode of the chuck and in the substrate, and the resultant electrostatic force holds the substrate to the chuck.
Electrostatic chucks are used in semiconductor fabrication processes, such as deposition processes and etching processes. In a typical etching process, etchant process gas is introduced in the process chamber, and a plasma is formed from the process gas for etching the substrate. During the etching process, a magnetic flux can be generated in the process chamber to enhance the uniformity and rate of etching of the substrate. Typically, the magnetic flux has a component which is parallel to the surface of the substrate. The magnetic field confines electrons and reactive species near the surface of the substrate thereby increasing ionizing collisions. In such etching processes, the peripheral portions of the substrate are typically etched faster than the central portions of the substrate, because etchant gas ingress, and gaseous byproduct removal, are faster at the peripheral substrate regions.
In such etching processes an annular magnetic shunt, positioned in the cavity on the underside of the base can be used to control the rate of etching of the peripheral portions of the substrate which are adjacent to the annular magnetic shunt. The magnetic shunt typically comprises a ferromagnetic material which attracts the magnetic flux, thereby causing the magnetic flux above the shunt to be redirected toward the shunt, instead of parallel to the substrate surface. The resultant depletion of magnetic flux above the peripheral portions of the substrate reduces the rate of etching of the substrate at these regions. In this manner, magnetic shunts are used to control etch rates across the substrate surface, to obtain more uniform etching of the substrate. More uniform substrate etching provides higher IC chip yields and allows utilization of the entire substrate surface.
However, there are several problems with conventional magnetic shunt configurations. One problem is that conventional magnetic shunts do not uniformly deplete the magnetic flux above the substrate. It is believed that the non-uniform depletion results from discontinuities and cutaways in the conventional shunts. Conventional shunts are typically positioned in the cavity of the base, and are configured to circumvent projections, such as supports, projections, water cooling tubes, and screw holders in the base. The discontinuities in the magnetic shunt result in non-uniform etch rates across the periphery of the substrate.
Another problem with conventional magnetic shunts is that the shunts result in non-uniform heat transfer rate between the substrate and the support, because the heat transfer rates differ between the continuous and discontinuous portions of the shunt. Non-uniform heat transfer rates result in non-uniform temperatures across the surface of the substrate, resulting in non-uniform etch rates.
Thus, it is desirable to have electrostatic chuck and magnetic shunt configuration that allows uniform magnetic shunting across the surface of the wafer, and allows uniform heat transfer between the substrate and the support. It is also desirable for the magnetic shunt to be disposed proximate to the substrate to enhance magnetic shunting through the substrate.