This invention relates in general to a magnetically enhanced wafer processing system and relates more particularly to a wafer processing plasma reactor in which the processing rate at the wafer is adjusted by means of magnetic material included within the reactor processing chamber.
Plasma processing, such as plasma etching and deposition processing, in the fabrication of circuits is attractive because it can be anisotropic, can be chemically selective and can produce processing under conditions far from thermodynamic equilibrium. Anisotropic processing enables the production of integrated circuit features having sidewalls that extend substantially vertically from the edges of a the masking layer. This is important in present and future ULSI devices in which the depth of etch and feature size and spacing are all comparable.
In FIG. 1 is shown a typical wafer processing plasma reactor 10. This reactor includes a metal wall 1 1 that encloses a plasma reactor chamber 12. Wall 11 is grounded and functions as one of the plasma electrodes. Gases are supplied to chamber 12 from a gas source 13 and are exhausted by an exhaust system 14 that actively pumps gases out of the reactor to maintain a low pressure suitable for a plasma process. An rf power supply 15 connected to a second (powered) electrode 16 capacitively couples power into a plasma in chamber 12. A wafer 17 is positioned on or near powered electrode 16 for processing. Wafers 17 are transferred into and out of reactor chamber 12 through a port such as slit valve 18.
A plasma consists of two qualitatively different regions: a quasineutral, equipotential conductive plasma body 19 and a boundary layer 110 called the plasma sheath. The plasma body consists of substantially equal densities of negative and positive charged particles as well as radicals and stable neutral particles. RF power coupled into the reactor chamber couples energy into the free electrons, imparting sufficient energy to many of these electrons that ions can be produced through collisions of these electrons with gas molecules. The plasma sheath is an electron deficient, poorly conductive region in which the gradient in the space potential (i.e., the electric field strength) is large. The plasma sheath forms between the plasma body and any interface such as the walls and electrodes of the plasma reactor chamber.
When the powered electrode is capacitively coupled to the rf power source, a negative dc component V.sub.dc, of the voltage at this electrode (i.e., the dc bias) results (see, for example, H. S. Butler and G. S. Kino, Physics of Fluids, 6, p. 1348 (1963). This dc component, typically several hundred volts, is a consequence of the unequal electron and ion mobilities and the inequality of the sheath capacitances at the electrode and wall surfaces.
The dc component of the sheath potential at the powered electrode is useful in accelerating ions to higher energy in a direction substantially perpendicular to the powered electrode. Therefore, a wafer 17, to be processed by ions from the plasma, is positioned on or slightly above the powered electrode 16 so that this flux of positive ions is incident substantially perpendicular to the plane of the wafer.
The process rate is dependant on the dc component of the sheath potential and on the density of ions in the plasma near the sheath. To achieve high process throughput, it is advantageous to have an increased density of ions near this sheath. One method of increasing plasma ion density near the wafer utilizes magnets to produce a magnetic containment field that traps electrons within the vicinity of the wafer, thereby increasing the ion production rate and associated density at the wafer. The magnetic containment field confines energetic electrons by forcing them to spiral along helical orbits about the magnetic field lines, caused by a Lorentz force F=q(E+vxB), where E is the electric field vector and B is the magnetic field vector. Unfortunately, nonuniformities of the magnetic containment field of such "magnetically enhanced" plasma processing systems result in increased nonuniformity over the surface of the wafer.
To reduce the spatial variation of process rate uniformity over the surface of the wafer, some systems, such as the model 5000E from Applied Materials, Inc., produce a magnetic field that is substantially parallel to the surface of the wafer along a direction that rotates around an axis that is concentric with and perpendicular to the wafer. This rotation produces at the wafer surface an approximately cylindrically symmetric time-averaged field that has improved average uniformity over the wafer, thereby producing improved process uniformity.
Although the rotating magnetic field is to produce a uniform processing rate across the entire surface of the wafer, tests of the process rate at various points of the wafer exhibit some significant nonuniformity. It is therefore advantageous to identify the source of this nonuniformity and to provide a mechanism for eliminating or compensating for this nonuniformity.