No abstract

Inductively coupled plasma ͑ICP͒ etching reactors are rapidly becoming the tool of choice for low gas pressure, high plasma density etching of semiconductor materials. Due to their symmetry of excitation, these devices tend to have quite uniform etch rates across the wafer. However, side to side and azimuthal variations in these rates have been observed, and have been attributed to various asymmetries in pumping, reactor structure and coil properties. In this article, a three-dimensional computer model for an ICP etching reactor is reported whose purposes is to investigate these asymmetries. The model system is an ICP reactor powered at 13.56 MHz having flat coils of nested annuli powering Ar/N 2 and Cl 2 plasmas over a 20-cm diam wafer. For demonstration purposes, asymmetries were built into the reactor geometry which include a wafer-load lock bay, wafer clamps, electrical feeds to the coil, and specifics of the coil design. Comparisons are made between computed and experimentally measured ion densities and poly-silicon etch rates in Cl 2 plasmas. We find that the electrical transmission line properties of the coil have a large influence on the uniformity of plasma generation and ion fluxes to the wafer.

A tuned, cylindrical Langmuir probe was used to measure current-voltage traces in a planar, inductive oxygen, radio frequency glow discharge at several pressures ranging from 0.5 to 10 mT. The plasma potentials were determined from the zero crossings of the trace second derivatives. Positive ion densities were evaluated using orbit motion limited probe theory; electron densities were estimated by integrating the area under the unnormalized distribution function. By applying the Druyvesteyn formula to the digitized probe traces, the electron energy distribution functions were obtained. The distribution functions ranged from Maxwellian at 0.5 mT to almost Druyvesteyn-like at 10 mT.

A novel plasma source combining rf inductive drive and multipole plasma confinement has been constructed to process advanced semiconductor materials. Measurements show a linear dependence of density with input power. Ion current efficiencies of 1 A per 150–300 W of input power can be achieved in argon, with lower efficiencies in electronegative gases. Applying an rf bias to a substrate immersed in the plasma allows the sheath voltage to be controlled between 8 and 300 V. Insight into the rf induction process can be gained by a simple circuit model, which represents the induction process with a transformer. The physical quantities describing the transformer can be obtained from numerical calculation of the fields of the induction coil. This plasma source can etch thin films at rates exceeding 1 μm/min.

Large-signal time-domain modeling (simulation) of rf glow discharges is a very useful and potentially accurate tool for the study of low-pressure (50–500-mT) gaseous electronics at high frequencies. Unfortunately, the computational limitations imposed for stability, accuracy, and efficiency can often hinder the production of useful, cost-effective results. This paper describes a self-consistent argon rf glow-discharge simulation at 13.56 MHz for equal- and unequal-area parallel-plate electrode geometries. Some of the numerical problems associated with this type of simulation are identified and the numerical methods used to overcome them are described. To illustrate the usefulness of this modeling scheme, the plasma potential and the cathode dc bias are examined as functions of electrode area ratio and rf power.