Dr. Vincent M. Donnelly
Moores Professor
My research interests are mainly in the field of semiconductor device materials processing, and in particular plasma processing. In the semiconductor industry, plasmas are widely used to deposit and etch thin films in integrated circuits. In this regard, plasmas have played and will continue to play a major role in the continuing nano-technology revolution in electronic devices. Micro-plasmas are also increasingly finding applications in such diverse areas as high-resolution displays and bioengineering. In the plasma etching process, bombardment by positive ions normal to the wafer surface allows fine-line patterns to be transferred into thin films on the wafer surface, making it possible to fabricate integrated circuits with device dimensions of only a few hundred atoms. Such precise control of this pattern transfer process has been realized through advances in our understanding of the underlying plasma physics and chemistry, made possible through a combination of plasma experiments and modeling. Nonetheless, the lack of a more thorough understanding of the process, and especially the need for applicable diagnostic techniques, continue to hamper our ability to extend this process to ever smaller, nanometer-scale features. Our group is also interested in using plasmas in new ways. Some of our current studies include:
Nanopantography
In a joint effort with Profs. Demetre Economou and Paul Ruchhoeft (Electrical and Computer Eng.), we are exploring a new technique for massively parallel nanopatterning over large areas. A broad ion beam is directed at an array of sub-micron-diameter electrostatic lenses fabricated on the conductive substrate (e.g., doped Si wafer). By applying appropriate voltages to the lens electrodes, the "beamlets" entering the lenses are focused to spots that can be 100X smaller than the diameters of the lenses. Each lens writes identical nanofeatures on the substrate, therefore nanopantography is a parallel process, very different from focused electron beam or ion beam writing. When the substrate is tilted with respect to the ion beam axis, the focal points of the off-normal ion beamlets are scanned across the centers of the lens bottoms, making it possible to “write” any arbitrary nanopattern, as shown in the figure.
Plasma-surface interactions at a “Spinning Wall”
Some of the most important but least understood aspects of plasma processing are the interaction at the surface of the substrate (e.g. silicon wafer) and the walls of the reactor. These elusive processes have been difficult to study on real surfaces while they are exposed to the harsh plasma enviroment. We have begun studies of plasma chemistry on dynamic, reactive surfaces, using a novel spinning substrate method. In this case, the spinning wall is rapidly spun (e.g. 40,000 rpm) to bring the surface from the plasma to an analysis chamber in times of less than 1 millisecond. In the analysis chamber, mass spectrometry is used to observe short-lived species that are desorbing, while Auger electron spectroscopy is used to detect such species before they leave the surface. By varying the rotation frequency, surface reaction kinetics can be determined.
Plasma etching of new materials such as hafnium oxide
Hafnium oxide is replacing SiO2 as the dielectric layer below gate electrodes in complementary metal oxide semiconductor (CMOS) field effect transistors (FETs). We are investigating etching of this important material in boron trichloride plasmas, using a variety of diagnostic methods, including x-ray photoelectron spectroscopy (XPS). As shown below, etching in a BCl3 plasma removes the HfO2 layer, but leaves an undesirable boron residue on the underlying silicon substrate. We can successfully remove this layer with a subsequent hydrogen plasma cleaning process, as confirmed by XPS measurements such as those reproduced below.
Atmospheric pressure micro-discharges
In a joint effort with Prof. Demetre Economou, we are exploring the physics, chemistry and applications of atmospheric pressure micro-discharges. Unlike conventional high pressure plasmas, these discharges operate in a stable glow mode. We have constructed an optical emission spectroscopy microscope and a focused laser microprobe coupled with a triple grating spectrometer to obtain 3-D imaging of gas temperature, electron temperature, electron density and electric fields, and compare these quantities to model predictions. We are also starting to explore the use of micro-discharges for trace chemical sensors.
Neutral beam nano-processing
In a joint effort with Prof. Demetre Economou, new methods are being explored for nanometer scale patterning of electronic materials using energetic neutral beams.