Recent advances in plasma chemical vapor deposition (CVD), etch tools, and plasma display technologies were among the highlights of the 49th Annual Gaseous Electronics Conference, held 20-24 October in the conference facilities of the Advanced Photon Source at Argonne National Laboratory. The meeting was on endorsed topical conference of the APS. In addition to the standard technical program, the meeting featured the 1996 APS Will Allis Prize Lecture and a conference banquet on Tuesday evening, with a keynote address by Fermilab's Michael Turner on "The Age of the Universe and Other Large Questions."
Plasma CVD and Etching
According to John Arnold of Motorola's Advanced Products R&D Laboratory, as device geometries shrink and cost and performance demands increase, the semiconductor industry must find ways to etch dielectric materials with unprecedented levels of feature aspect ratio, mask and underlying film selectivity, and throughput. High density plasma (HDP) etch tools have demonstrated great promise for meeting present and future needs.
However, "The simultaneous deployment of this new technology and migration to unfamiliar and more complex etching chemistries has left a growing chasm between industrial practice and theoretical understanding of process mechanisms," said Arnold. To address this problem, he has applied a series of diagnostic tools to a commercial HDP etcher in a semi-production environment, which resulted in a qualitative but coherent model of the etching process. Of particular importance to controlling etch behavior are thermal transients, polymer deposition and removal and chamber wall interactions.
Characterization of gas phase species densities is also important to the understanding of complex plasma chemistry. According to Harold Anderson of the University of New Mexico, infrared diode laser absorption is particularly well-suited for aiding in process development in HDP etch tools. He has developed a single-pass, FM diode laser absorption spectroscopic system to measure chlorine and fluorocarbon dissociation in inductively coupled discharges in both laboratory and commercial plasma etchers. A relatively small, portable version of his system has been used on site at semiconductor lab facilities for etch tool process characterization.
Neutral radicals play key roles in plasma CVD and etching, but modelling and controlling such processing requires cross-section data for electron-impact dissociation of molecules into neutral radicals. Earlier electron impact studies resulted in extensive data of ionization cross sections, but little is known about dissociation into neutral fragments because their detection is extremely difficult. However, scientists at Japan's Nagoya University have developed a highly sensitive technique for radical detection - appearance mass spectroscopy - which they have used to measure cross sections for neutral radical yield from several species of molecules important for CVD and etching.
The predominance of multi-level metallization schemes in advanced integrated circuit manufacturing has greatly increased the importance of plasma-enhanced CVD and, in turn, in situ plasma chamber cleaning, according to John Langan of Air Products and Chemicals, Inc., in Allentown, Pennsylvania. "In order to maintain the highest throughput for these processes, the clean step must be as short as possible," he said, adding that it is also desirable to minimize fluorated gas usage during cleaning while maximizing efficiency, not only to achieve lower costs but also because many of the gases used in the process are global warming compounds. Using electrical impedance analysis and optical emission spectroscopy, he has studied the fundamental properties of various discharges emitted during cleaning to develop numerous strategies to optimize chamber clean processes.
Plasma display panels (PDPs) are arrays of individually addressable microdischarges that produce ultraviolet radiation which is, in turn, converted to visible radiation by a phosphor. In ac PDPs, the electrodes are coated with dielectrics, and discharges are initiated and extinguished on each half cycle as the dielectric surfaces become charged. To better enhance the understanding of the phenomena involved, scientists at the CPAT facility in Toulouse, France, have developed two-dimensional models to study the effects of changing discharge conditions on the characteristics of PDP cells in complex geometries. In addition, G.J. Parker of Lawrence Livermore National Laboratory has developed two-dimensional fluid/kinetic hybrid models for plasma switches, which have been introduced as replacements for thin-film transistors in liquid crystal displays, facilitating the manufacture of large-area devices suitable for TV and high-resolution computer terminals.
Plasma processing is also being used in the fabrication of liquid crystal flat-panel displays, which has much in common with integrated circuit processing on large diameter silicon wafers. However, according to Amy Wendt of the University of Wisconsin, Madison, it also presents new challenges, since it involves plasma processing for sputter deposition, plasma-enhanced CVD, etching, and hydrogen passivation for displays using polycrystalline silicon. She has studied plasma immersion as a possible technique for the introduction of hydrogen into the polysilicon film to passivate defects, demonstrating that high rates of hydrogen passivation can be obtained using electron cyclotron resonance discharges.
Plasmas in Growth of Thin Films
Pulsed laser deposition (PLD) uses laser ablation of a solid target to accelerate atoms in a high- density plasma to superthermal kinetic energies, which have proven essential for the formation of thin films of new ultrahard metastable phases, such as the synthesis of amorphous diamond from the laser ablation of pyrolytic graphite in a vacuum. However, in low-pressure background gases, the kinetic energy of the plume atoms is moderated by collisions with the background gas.
David Geohegan of Oak Ridge National Laboratory has employed a combination of fast plasma diagnostics to characterize fundamental collisional phenomena relevant to PLD film growth in vacuum and background gases, including optical emission spectroscopy, optical absorption spectroscopy, and fast Langmuir probe analysis. These phenomena are highly relevant to understanding cluster formation and aggregation by ablation into background gases, which in turn are of practical importance for the current synthesis of nanocrystalline and composite materials.
In addition, Dieter Gruen of Argonne National Laboratory has accomplished the first synthesis of diamond films using fullerene (C60) precursors in an argon microwave plasma without the addition of hydrogen, which strongly suggests that the diamond phase grows by a new and hitherto unexplored mechanism. According to Gruen, diamond films produced this way are nanocrystalline, smooth, and highly reflective, and maintain their nanocrystallinity to thicknesses of more than 20 mm. "Diamond nucleation occurs in preference to graphite nucleation as a result of the thermodynamic stability of nanocrystalline diamond," he said. "Once established at the level of supercritical or embryonic nuclear size, growth continues even in the absence - or virtual absence - of hydrogen because of the large energy barrier for the solid-state nucleation of the stable bulk graphite phase."
Scientists at the Naval Research Laboratory are investigating the application of plasma arc technology for the on-board remediation of waste material generated by sea faring ships. They use a 150kW arc torch within a one-meter diameter chamber for the pyrolysis of liquid and solid material to simulate the waste stream. According to NRL's John Giuliani, the greatest challenge for a shipboard plasma remediation is the overall size of the system imposed by the limited confines of a ship. "The research component of NRL's program is to characterize the gaseous byproduct emanating from the remnant slag and the plasma arc through systematic experiments, as well as to model the plasma dynamics and chemistry within the chamber," he said, adding that spectroscopy offers the potential of a non-invasive diagnostic to eventually be used for on-line process control, a necessary feature for a operating system because of the heterogeneous waste stream.