This invention relates in general to sputtering deposition and relates more particularly to the problems of thickness uniformity and step coverage of coats produced by planar magnetron sputtering.
In a sputtering device, a target is subjected to bombardment by high energy ions to dislodge and eject material from the target onto a workpiece, such as a semiconductor wafer. In general, the target and wafer are placed into a vacuum chamber that is evacuated to a pressure typically 3.multidot.10.sup.-4 Torr and below. In physical sputtering, the bombarding particles are generally ions of a heavy, inert gas such as argon and are accelerated to high velocities in directions that are substantially perpendicular to an exposed front surface of the target.
For industrially acceptable sputtering rates, it is important to produce a relatively high density of ionized bombarding particles. The most cost-effective source of ions is a low pressure glow discharge. In electrically biased glow discharge sputtering devices, the wafer is mounted on a pedestal that is biased as the sputtering reactor anode or separate anode and the target is biased as the sputtering reactor cathode. This produces at the target an electric field that is substantially perpendicular to the exposed front surface of the target and that accelerates the ions into the exposed surface of the target along trajectories that are substantially perpendicular to the front surface of the target.
Collisions between high energy electrons and gas molecules ionize some of these neutral particles to replace the ions that bombard the target. This ionization produces within the reactor chamber a conductive plasma body that is separated from the cathode and anode by free-electron deficient regions called plasma sheaths. Because the plasma body is conductive, the accelerating electric fields are substantially restricted to the plasma sheath regions at the wafer and target.
In a planar magnetron sputtering device, ion densities at the target are enhanced by producing within the target sheath a magnetic field that helps trap and direct electrons near the target. The increased electron density in these traps increases the rate of ion generation within these regions, thereby increasing the ion density at the target. Because the electric field E within the target sheath is substantially perpendicular to the exposed surface of the target, in those regions where the magnetic field E is substantially parallel to the target surface, the E.times.B drift field pushes electrons parallel to the surface of the target. The electrons are trapped by configuring the magnetic fields so that these regions in which the E.times.B drift field is parallel to the target surface form closed paths within which the electrons are then trapped. This configuration of the drift fields is achieved by cooperative selection of the locations and shapes of magnetic fields.
Typically, the target material is attached to a backing plate to mount the target within the reactor. It is important that sputtering of the target does not reach this backing plate because particles sputtered from the backing plate would contaminate the integrated circuit fabrication process. Therefore, a target is typically replaced before sputtering of the backing plate can possibly happen. Unfortunately, this results in the replacement of targets that still contain a significant fraction of the original target material.
Because many targets are composed of an expensive material, such as gold, and because of the significant expense of lost production time to replace a target, it is important to maximize the amount of sputtering that is allowed before the target must be replaced. In German Offenlegungsschrift 27 07 144 entitled Kathodenzerstaeubungsvorrichtung filed by Sloan Technology Corp on Feb. 18, 1977, magnet/magnetic pole assemblies are swept either longitudinally in x- and y- directions or off-axis pole assemblies are swept radially about a rotation axis to produce a pattern of target sputter that has an increased time-averaged uniformity of sputter.
Although there are clear cost benefits to improving the efficiency of utilization of a target, it is also important to produce wafer coatings that have very uniform thickness, excellent step coverage and excellent step coverage uniformity. This is especially true in submicron integrated circuit fabrication. When a layer is patterned, if portions of the layer are thicker than other portions, then each of these portions will achieve pattern completion at different times. If some regions are over-etched to achieve etch completion in other regions, then linewidths in the over-etched regions will be reduced below designed values. The layer uniformity and step coverage of these systems are therefore inadequate for most sputtering applications in current state of the art circuits. Thickness variations on the order of 25% can result from the above-discussed process, making it suitable substantially only for deposition of the back ground plane of the wafer.
Uniform coatings are also important for preserving accurate mask alignment. The alignment marks typically consist of raised features on a wafer so that they remain visible even after deposition of several layers on the wafer. Asymmetric coating at alignment marks can produce spurious results in the apparent location of these marks, thereby misaligning subsequent patterning.