Interferometric Lithography, High-Density Plasma Sources Featured at GEC '97

Interferometric lithography, high density plasma sources for flat panel displays and environmental applications of plasmas were among the highlights of the 50th Annual Gaseous Electronics Conference, a topical conference of the APS, held October 6-9 in Madison, Wis. In addition to the regular scientific program, there was a town meeting on Monday night to discuss the future organization of the GEC, including the possibility of forming a Topical Group on Gaseous Electronics within the APS. A new feature this year was the "Foundation Talk," a review paper aimed at educating students and individuals working outside the immediate field and given by Allan Gallagher, who spoke on gas-phase processes in low-pressure silicon deposition plasmas.

This year's program also included a special plenary session to recognize the 50th anniversary of the GEC's founding, with presentations on electric discharge light source R&D over the last 40 years, electron collisions in astrophysical plasmas, a historic perspective of gas phase lasers and the kinetics of swarms and discharges. There was also an historical exhibit on the achievements of Irving Langmuir, who first coined the word "plasma" to describe the bulk region of certain laboratory discharges and who essentially started the science of surface reaction processes and catalysis. His efforts resulted in such significant technological advances as the mercury diffusion vacuum pump, the gas-filled incandescent lamp, the thoriated electrode and the high-vacuum electron tube diode and amplifier.

Interferometric Lithography

The silicon industry continues to accelerate the historical rate of decrease in the critical dimension (CD), or smallest printed feature size, of ULSI circuits. Current estimates call for the introduction of 180-nm features in manufacturing by the year 2000, with succeeding generations of 130 nm and 100 nm to follow at three-year intervals.

According to S.R.J. Brueck of the University of New Mexico, this rapid pace of change has implications not only for lithography, but for all related manufacturing processes, including etching and deposition. In particular, the limits in depth-of-field and photoresist mechanical properties are likely to lead to thinner resist layers, requiring increased selectivity during etching. Brueck reviewed the current status of several potential future lithographic techniques, focusing on interferometric lithography, a newly developed optical technique that allows fabrication of the next several CD generations with currently available sources and photoresist systems, enabling immediate development of the necessary etch and deposition tools.

"Imaging interferometric lithography is.providing a true integration of imaging optical and interfermometric techniques that offers the potential of extending optical lithography beyond 70 nm," he said.

Environmental Applications

According to Louis Rosocha of Los Alamos National Laboratory, who spoke on Monday morning, electrical discharges in gases can be used to create nonthermal (or nonequilibrium) plasmas with energetic electrons at near-ambient background gas temperature. These low-temperature plasmas are an excellent source of free radicals and other active species useful for chemical conversion and synthesis. In particular, when reacted with volatile organic compounds, (VOC), these active species exhibited high reaction-rate constants for decomposition of the VOCs, making them very useful for pollution control.

David Green of NIST, who spoke at the same session, said he believes that better databases and models are required to evaluate the applicability of non-thermal plasma-based methods - such as electron beam irradiation or electrical discharge techniques - for the abatement of VOCs in contaminated humid gas streams.

"Much of the existing information on the energy efficiency of VOC destruction and the nature of byproducts is empirical, and the processes are so complicated that it is essential that predictive models and simulations be improved," he said.

The general data types needed include transport, thermodynamic, kinetic and electron interactions, and the nature of the data is dependent to a large extent on the concentration, pressure, temperature and degree of ionization.

High-Density Plasma Sources for FPDs

According to John Holland of Lam Research, the current trend in flat panel display (FPD) manufacturing is towards producing larger and higher quality displays, resulting in the increase of the size of the glass substrates used as the basis for displays as well as a need for improvements in the processing steps used to deposit and etch films on the glass. Typically either wet chemistry or low density, reactive ion etch (RIE) plasma sources have been used for these process steps, which are adequate for small display sizes. However, as the dimensions of the glass substrates are increased, these methods can be limited by their low processing rates and poorer process uniformities.

Holland maintains that a promising alternative, until recently unavailable commercially, is the use of a high-density plasma source for plasma processing of the large area panels. While these have become commonplace for wafer-based manufacturing, not all have been scalable for large area applications. A notable exception is the planar, inductively coupled plasma source, which has recently been demonstrated on a 600-mm substrate. In this way, issues with processing rates can be overcome, and process uniformities can be much lower than those achieved with either wet chemistry or RIE etching.

Planar Laser Induced Fluorescence

Low-pressure radio frequency (rf) fluorocarbon plasmas are extensively used for etching and chamber cleaning during microelectronics device fabrication. However, the models which are necessary to the understanding of these complex plasma processes require experimental input and must be rigorously verified by comparison to experimental measurements, according to Kristen Steffens of NIST, who spoke on Wednesday morning. She has found that planar laser-induced fluorescence (PLIF) is one of the most useful optical techniques for 2-D measurements of gas-phase species, relative densities and distributions.

"Because the entire 2-D map of the species density is obtained simultaneously, the need for multiple point measurements is eliminated," she said, adding that PLIF is also highly sensitive to free radical intermediates, which play crucial roles in plasma chemistry.