This invention relates in general to a wafer processing system and relates more particularly to a wafer processing plasma reactor in which the plasma is generated primarily by inductively coupled power.
Plasma etching or deposition 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 dielectric coated metal wall 11 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.
RF power at 13.56 MHz is predominantly utilized in plasma reactors because this frequency is an ISM (Industry, Scientific, Medical) standard frequency for which the government mandated radiation limits are less stringent than at non-ISM frequencies, particularly those within the communication bands. This substantially universal use of 13.56 MHz is further encouraged by the large amount of equipment available at that frequency because of this ISM standard. Other ISM standard frequencies are at 27.12 and 40.68 MHz, which are first and second order harmonics of the 13.56 MHz ISM standard frequency.
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 bias is a consequence of the unequal electron and ion mobilities and the inequality of the sheath capacitances at the electrode and wall surfaces. The magnitudes of the sheath capacitances are a function of the plasma density as well as the chamber geometry and the relative areas of the electrode and wall within the plasma chamber. Sheath voltages at the powered electrode on the order of several hundreds of volts are commonly produced (see, for example, J. Coburn and E. Kay, Positive-ion bombardment of substrates in rf diode glow discharge sputtering, J. Appl. Phys., 43, p. 4965 (1972).
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, in a plasma etching process, a wafer 17 to be etched 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, thereby producing substantially vertical etching of unprotected regions of the wafer. These high sheath voltages (and high discharge voltage) are needed in some processes (like SiO.sub.2 etching) to produce etch rates that are required for a commercial etch process.
Transistor speed specifications and high device densities in the most modem MOS integrated circuits have required the use of shallow junctions and thin (on the order of 100 .ANG.) gate oxides under polysilicon gates that are thousands of Angstroms thick. Unfortunately, such IC structures are sensitive to ion bombardment by high energy (&gt;100 ev) ions such as in the conventional plasma etch apparatus of FIG. 1 so that, during the step of etching the polysilicon layer to form the gate, it is difficult to avoid damaging the gate oxide. Because wafer damage decreases with decreasing ion energy and associated sheath voltage, it would be advantageous to operate at smaller discharge power levels and voltages. Unfortunately, for capacitively coupled power at 13.56 MHz, this reduction of voltage results in a proportionately slower etch rate for many processes, which thereby significantly degrades process throughput.
Etch rates for SiO.sub.2 and some Si etch processes are a function of the ion bombardment power density transmitted from the plasma to the wafer. Since this power is equal to the product of the sheath voltage at the powered electrode and the ion current density at the wafer, the ion current density at the wafer must be increased to maintain substantially constant etch rate with decreased sheath voltage. This requires that the plasma ion density near the wafer be increased. Unfortunately, in a conventional plasma etcher, both the sheath voltage of the powered electrode and the ion density near that electrode are proportional to each other and are monotonically increasing functions of the amplitude of the rf voltage applied to the powered electrode. Thus, if the sheath voltage is decreased by decreasing the voltage of the rf signal, then the current density of the ion beam at the wafer also decreases thereby producing an even greater percentage decrease in etch rate than in either the sheath voltage or the ion current. It would therefore be advantageous to be able to control independently the sheath voltage and ion density at the wafer so that a soft etch process (i.e., an etch process with reduced sheath voltage at the wafer) can be implemented that has a commercially adequate etch rate.
One method of increasing etch rate by enhancing 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. Unfortunately, nonuniformities of the magnetic containment field of such "magnetically enhanced" plasma etching systems decrease etch rate uniformity over the surface of the wafer. The E.times.B drifts due to the electric field in and near the sheath also reduces etch rate uniformity in such systems. To improve uniformity over the surface of the wafer in one such system, the wafer is rotated about an axis that is perpendicular to and centered over the surface of the powered electrode. This produces at the wafer surface a cylindrically symmetric time-averaged field that has improved average uniformity over the wafer, thereby producing increased etch uniformity. However, this rotation produces within the plasma chamber undesirable mechanical motion that can produce particulates and increase contamination.
Another technology that has potential for producing acceptable etch rates at low ion bombardment energy is the recently developed technique of electron cyclotron resonance plasma production. This technique has application to wafer cleaning, etching and deposition processes. In this technique, a plasma is produced by use of a microwave source and a magnetic containment structure. Unfortunately, this technique, when applied to etching or chemical vapor deposition, exhibits high levels of particulate formation, poor radial etch rate uniformity and low throughput. The fraction of energy coupled into production of radicals increases very rapidly above about 1 milliTorr so that the pressure in this system must be kept below this level. This requires expensive hardware that includes: (1) a very large pumping speed (&gt;3,000 liters per second, which is 10 times typical values) vacuum pumping system to produce the very low (0.1-1 milliTorr) pressures required for this process; and (2) a large magnetic containment system that sometimes includes large electromagnets.
Another technique for increasing the ion density utilizes a microwave plasma generator to generate ions in a region at least 10 cm above the wafer. These ions flow into the volume above the wafer and therefore contribute to the ion density at the wafer. However, this approach tends to produce copious amounts of free radicals and produces no more than a few milliamps/cm.sup.2 ion current density at the wafer.