Micro-Processing Research Facility
The Mirco-Processing Research Facility at the University of Tennessee is a UT Core Facility and housed within the Joint Institute for Advanced Materials (JIAM). The Micro-Processing Research Facility (MPRF) provides researchers the ability to conduct micro-processing fabrication processes. Services include optical lithography, thin film deposition, capacitively coupled reactive ion etching, and silicon-based plasma enhanced chemical vapor deposition processes. This equipment is housed in a class 100 clean room with all necessary facilities and supporting process equipment. In combination with other JIAM facilities, the MPRF provides researchers with the means to conduct cutting-edge investigations in materials science and detector engineering. The Director of the MPRF is Dr. Eric Lukosi, and further information about the MPRF can be found here.
Material and Detector Characterization Laboratory
Dr. Lukosi's personal research laboratory in combination with the MPRF is designed to conduct cutting-edge development and application of radiation detection instrumentation, ranging from crystal growth, detector crystal packaging, front end electronic design and fabrication, and detector testing. The list of assets below represents the capabilities offered to collaborators. For further information contact Dr. Eric Lukosi.
The available equipment inludes
- Sula deep level transient spectroscopy (DLTS) measurement system
- Janis VPF-100 cryostat
- Keithley 6487 picoAmmeter
- Keithley 2410 high voltage SMU
- Lecroy 1.5 GHz, 8 Gs/s Oscilloscope
- Kulicke & Soffa 4523D wedge bonder
- SMT max reflow oven, drill press, and soldering station for circuit board fabrication
- Various dark boxes
- Probe station and various custom sample measurement units
- Micrometer-resolution xyz translation stage
- X-ray source
- CCD spectrometer
- Various front-end electronics, including preamplifiers, SiPMs, and PMTs
- NIM hardware (amplifiers, multi-channel analyzers, discriminators, etc.)
- VME hardware (digitizers, charge integrators, and pulse shape discriminators)
- Various radiation sources, including a 2 Ci Pu/Be source, 15 mCi Sr-90, and various electroplated alpha sources (Po-210, Am-241, Cm-244, Pu-239)
Some measurements that we are able to conduct with this equipment is described below.
Our laboratory is equipped to conduct essential measurement techniques for semiconductor device characterization, such as four point probe, Hall, Van der Pauw, current-voltage-temperature (IVT), and capacitance-voltage (CV) measurements. Some of the equipment used to make these measurements are shown to the left.
In addition to these techniques, charge carrier trap site energies, densities, and charge states located within the band gap of a semiconductor can be identified 
through additional capabilities within our laboratory. These techniques include DLTS, thermally stimulated current (TSC), charge collection efficiency (CCE), and the transient current technique (TCT). Our DLTS system is versatile, where additional measurement techniques include the current transient spectroscopy (CTS), charge transient spectroscopy (QTS), photo-induced transient spectroscopy (PITS), and constant capacitance DLTS (CCDLTS). Further information about DLTS and the purpose/capability of these additional options can be found here. Our thermally stimulated current measurement system is a simpler method to obtain information about charge carrier traps in semiconductors and is primarily used for highly resistive materials where a suitable capacitive junction can be made for DLTS measurements. A picture of the cryostat, the PITS optical system, and a deuterium lamp (used for TSC) is shown on the right.Finally, for wide bandgap semiconductors, DLTS and TSC are not suitable to quantify charge carrier traps below 1.5 eV. Cathodoluminescence is a technique that utilizes electrons to generate electron-hole pairs that are then trapped in these deep states, and if the trapping mechanism has the probability of emitting a visible photon, the wavelength and number of photons emitted can be used to quantify both the trap energy level and density within the sensor. In our laboratory, we utilize a low energy x-ray source to simulate an electron source via the photoelectric effect. As such, the deuterium lamp/x-ray source and spectrometer can also be used to measure scintillator emission spectra as well.
Semiconductor radiation spectrometers are often investigated through the use of charge collection efficiency (CCE) and transient current technqiue (TCT) measurements. In both techniques, the use of alpha particles is common, and the total integrated charge is measured in the CCE method and the transient current profile is measured via a wide bandwidth oscilloscope in the TCT method. CCE provides information on the total amount of charge collect at each event, and if the mean electron-hole pair formation energy is known, then this technique provides information on the mobility-trapping time consant of one of the two charge carriers as long as the depth of interaction (range of the alpha particle) is insignificant with respect to the total thickness of the sensor. Further, inspection of the signal from the preamplifeir provides information on the trapping and detrapping time constants and trap densities as well. With the measurment of the transient current profile in TCT, we are able to quantify the drift time, the dependence of the mobility on the strength of the electric field, and any polarization that may be taking place as a function of depth. This technique enables the direct measurement of the conductivity mobility (different than the Hall mobility). Both of these techniques are commonly conducted in a dark environment, but our custom setup enables us to use the CCE and TCT methods when the semiconductor is exposed to various light sources, which provides direct information on the effect of traps residing at different energy levels within the bandgap.
The CCE and TCT measurement techniques utilize equipment used in nuclear radiation instrumentation detection and spectroscopy. Therefore, this section of our laboratory is also used to conduct tests on both semiconductor and scintillation detectors for their performance in the detection of gamma-rays, beta particles, alpha particles, and neutrons. Quantities of common interest are long-term stability and energy resolution. The goal of these investigations is to investigate resolution enhancement techniques, both during the measurement and via post processing. Further, this measurement station provides the ability to test different weighting potentials with semiconductor detectors to define optimum performance. In this respect, we have the ability to many detector signals at the same time (pixels/stips on a single detector or many detectors). This allows us the additional capability to determine interpixel capacitance/cross talk and to investigate imaging sensors (coded aperture and time-of-flight-based systems).
For pixelated semiconductor detectors, the sensor must be properly patterned and packaged for testing. We are able to pattern detector substrates with any pattern down to one micrometer feature size. Packaging is done with wire bonding for electrode sizes exceeding 15 micrometers in size. In addition, the patterned semiconductor sensors require special front-end electronics. For a high density of channels, an ASIC is often required. However, for lower channel density detectors (~100 channels or less), we have the ability to fabricate custom signal processing boards to condition the signals necessary for digital pulse processing. In the image to the right, a sc-LISe sample is mounted to a custom circuit board and wire bonded to the readout channels. This imager was the first time that sc-LISe was ever used for neutron imaging, and we demonstrated an image resolution of ~300 micrometers, better than the pixel pitch by about of factor of two. This enhanced spatial resolution was achieved using a technique called supersampling, where the object-sensor space was translated during imaging at distances much less than the pitch of the pixels.
We have also developed our own position-sensitive testbed to measure the response of semiconductor detectors and scintillators. The system uses tungsten collimators, a gamma-ray source, and an xyz translation stage. The entire system is housed within a dark box. The spatial resolution of the system is about 1 mm. An image of the system is provided on the left.