Nearly all semiconductor devices, such as microprocessors and memory chips, are fabricated in part by low temperature plasmas for etching, deposition and cleaning silicon wafers. These processes are performed in reactors excited by radio frequency voltages. Despite the great importance of this field, the complex plasma transport occurring within these reactors is still not well understood. This project involves a collaboration between experimental plasma physicists at the University of California at Los Angeles and computer modeling experts at the University of Michigan during which fundamental properties of the boundary layers of these plasmas will be investigated. The boundary layers (called sheaths) are responsible for accelerating charged ions into the wafers. The collaboration is unique in that it utilizes a state-of-the-art industrial plasma processing tool modified to incorporate extremely sophisticated diagnostics. The plasma is produced with a pulsed 600 kHz inductive coil and the silicon wafer on which the devices are fabricated is capacitively biased with multiple frequencies (2 and 60 MHz), which can also be pulsed. This research will focus on understanding and characterizing the ion motion and sheath dynamics for both continuous and pulsed (inductive and capacitive) plasmas. The ion velocity distribution function, contains all the information on the motion and location of ions used to activate etching or deposition on the Si wafer. Using laser induced fluorescence (LIF), it will be measured in a plane above and perpendicular to the wafer in Ar and Ar/O2 and Ar/Xe gas mixtures as a function of space and time in the sheath. It will be measured at thousands of spatial locations with a spatial resolution of less than 100 micrometers and time resolution of < 3 ns to map out the dynamics of the sheath. Emissive probes will measure the 3-d structure of recently discovered waves above the wafer.
This project provides a rare opportunity for a synergistic collaboration in experimental modeling and this will make a broad impact on the processing field. The University of Michigan has developed computer models that will be further improved and applied to the analysis and interpretation of the measurements. The project will focus attention on problems that not only have cutting edge physics challenges, but also are of relevance to the semiconductor industry. The project will involve significant outreach. We will collaborate with the plasma physics groups at Ruhr University in Bochum, Germany who are leaders in optically diagnosing transients in these plasmas. In addition, this project will involve significant educational outreach. Prof. Gekelman is one of the founders of LAPTAG (Los Angeles Physics Teachers Alliance Group), which now has contacts with over twenty local high schools and community colleges, including underrepresented high school students and teachers. LAPTAG has built a dedicated high school plasma physics laboratory (http://coke.physics.ucla.edu/laptag) with funding from the Department of Energy. The outreach will involve high school students and teachers in research internships on this project as well using as the LAPTAG plasma source.
Low temperature plasmas are ionized gases which produce reactive species capable of modifying and delivering activation energy to the surfaces of materials. Plasma processing of semiconductor materials is an essential process in the production of microelectronics components. This is particularly true in plasma etching, a process that creates the very small features (a few billionths of a meter) that comprise state of the art microprocessors. In plasma etching, control of the distribution of ion energies onto the semiconductor wafer is extremely challenging, and this control is necessary to optimize the microelectronics components. In this research project, state of the art experiments and computer modeling were applied to investigating the transport of ions through the boundary layers of plasma processing reactors – a region called the sheath. Ions are accelerated to high energy in the sheath, and the dynamics of ions passing through the sheath in large part determine the quality of the etch and of the final microprocessor. We determined the manner in which the ion flux transitions from the bulk plasma, through the presheath and the sheath to form unique distributions of ion energies. This improved knowledge will aid in the development of new plasma processing techniques to better control materials processing, both in the microelectronics industry and in other industries using low pressure plasmas for materials modification. During this project, two graduate students performed the majority the research that will become their PhD thesis. Outreach included research projects and demonstrations that involved high school students and teachers.
