Research & Development, Analytical Services and Consulting in Advanced Sensors, Materials and Fuels
Electronic Materials & Devices
Film Deposition System - AFR, Inc. has assembled a complete system for UV laser ablation and film deposition (referred to as Pulsed Laser Deposition, PLD). The system is based on a Continuum Corp. Nd:YAG laser, model NY-61-10. This laser has both oscillator and amplifier rods, plus second, third and fourth harmonic generators capable of providing UV light with 170 mJ/pulse at 355 nm, and 60 mJ/pulse at 266 nm. Q-switching provides 4-ns pulses (266 nm) at 10 Hz repetition rate. A Scientech volume-absorbing UV power meter is used for tuning the laser. A motor-driven dichroic mirror scanner and a focusing lens direct the laser beam onto the target through a Suprasil UV-quartz window. Two vacuum deposition chambers are available for PLD. Both are designed around a 6-inch ID, 6-way cross with ISO flanges, for flexibility in configuration. The first is pumped through a 4-inch ID gate valve by a Balzers turbomolecular system to a base pressure of 3x10-7 mbar. The turbo pump is backed by a liquid-nitrogen cold trap and membrane forepump to ensure entirely oil-less pumping. A micrometer leak valve allows process gases to be admitted to the chamber, with flow through to the pump, during film deposition. Oxygen, with a very low carbon content, is used for oxide (e.g., ceramic superconductor YBaCuO) film depositions. Alternatively, an effusion cell can direct a beam of thermally evaporated mercury, or other low vapor pressure, atoms onto the substrate during the PLD process (e.g., for growth of HgCdTe semiconductor films). This chamber also has a regulated and programmable substrate heater system, with adjustable target-substrate distances. Direct heating of substrates in the range of 30-850 oC is provided by a silicon-carbide bar element. Presently, substrates are single-crystal wafers of metal oxides (e.g., LaAlO3, YSZ), and semiconductors (e.g., Si, GaAs, and CdTe) up to 10x10 mm2. A modification is being developed for wafer substrates 50 mm in diameter. Target mounting in the chamber is on a translatable finger, allowing multitarget operation during a single deposition routine. The finger is pneumatically actuated and can rapidly position various targets for ablation, allowing, for example, multilayers to be deposited onto a buffer layer film (i.e., total of three film types). In the second chamber a 6-inch ID diffusion pump and single target operation are used. This system has been used successfully to grow epitaxial films of diamond-structure b-SiC on Si wafers by PLD. The chamber is also equipped to deposit diamond films by the tungsten-filament CVD method, and various films by rf-plasma CVD.
Rapid Thermal Processing (RTP)/Deposition Tool - The RTP tool is a water-cooled, 12- inch diameter x 14-deep stainless steel cylindrical chamber. The RTP system includes a rotating wafer support for rotating 150 mm wafers or smaller wafers during processing up to 120 rpm. The substrate heater consists of a six-zone array of tungsten-halogen linear lamps and a water-cooled aluminum reflector located outside of the chamber, separated by a 0.75-inch thick x 10-inch diameter quartz window. With this configuration, silicon wafer temperatures up to 900 °C are possible during processing. The RTP system is currently configured with a two-inch magnetron sputter source (AJA International model ST20) in an upwards deposition geometry. The dc power supply is an Advanced Energy MDX 500 1 kV (500 W) power supply with arc suppression. A gas ring and chimney on the sputter source minimize deposition on chamber ports and walls. Metal oxides can be deposited by reactive sputtering in an argon/oxygen environment.
Deposition Monitors - Vacuum and process gas pressures are measured with a CVC, Inc. model GPT-450 gauge covering the range of 1 atm to 10-8 mbar. A Minolta-Land, Cyclops 52, pyrometer is used for direct observations of the substrate surface temperature. A pair of IR access windows on the chamber allow reflection Fourier-Transform Infrared Spectroscopy (FT-IR) in situ monitoring of film growth surface temperature, composition, and thickness. The spectrometer is a Bomem 102, modified to use two MCT detectors allowing simultaneous emission and reflectance with 4 cm-1resolution. On-line computer control of the laser, its shutter, the temperature regulator, and the FT-IR allows for efficient execution and monitoring of elaborate deposition procedures.
Ablation Targets - AFR has a full wet lab for superconductor precursor material processing. A ten-ton pill press is used to form the 0.5 inch diameter targets. A tubular muffle furnace, capable of sustained temperatures up to 1050 oC in flowing oxygen, is used to sinter the targets. This system allows, for example, the fabrication of YBaCuO targets with on or off-stoichiometry and cation substitutions, or targets of zirconia, magnesia or ceria with various compositions, e.g., yttria-stabilized zirconia (YSZ) with 9 M% yttria in the cubic-fluorite phase. Presently, we also use commercial targets from various sources.
Substrate Preparation and Film Patterning - To prepare silicon-wafer substrates for film deposition, cleaving tools, a wet bench and dry glove box for etching are used. This latter process is adapted from the Xerox PARC and JPL methods to spin etch Si wafer surfaces with a solution of HF in ethanol, which produces atomically clean, passive and oxide-free surfaces. After deposition, superconductor-oxide films are patterned in a yellow room with a spin-on positive photoresist (Shipley 1400 series), contact masking and wet etching with nitric or EDTA acids, in an attenuated ultrasonic bath. A class-100 clean bench is used for the lithography. The masks are drawn by CAD, laser printed, and photo-reduced (typically, by ~20:1 or 100:1) onto Kodak 35-mm Technical Panfilm. Full in-house facilities allow rapid device-pattern design and modifications. The current design limit for patterned lines is ~3 mm, and patterned gold films have been realized with edges resolved to 1-2 mm. Electrical contact metallizations are made by gold or silver which is DC-sputtered, slurry-painted, or PLD-ablated (through contact shadow masks) onto the superconductor films. Superconductor films are passivated by a protective coating of dense, transparent YSZ deposited in the PLD system, onto near-room-temperature film devices.
Device-Testing & Cryostats - For superconductor device characterization, AFR has two, variable-temperature, cryostats and associated electronics for measuring DC and AC current-voltage (I-V), electrical noise, and IR-visible radiation-response characteristics. In one cryostat, samples, up to 1x1 cm2 in size, are mounted at the bottom and face up the bore axis of the cryostat tube for operation from 4.2 to 300 K. Quick contact is made in a jig by gold or silver spring pins. The optical (IR) characteristics of this apparatus have been calibrated with an FT-IR spectrometer for use with a KRS-5 (TlBrI) window and a Globar thermal source of known emission spectrum and sample irradiance. The other cryostat is a Janis Research, model VFP-100-LN2-SSVT, with 77 to 300 K operation, high lead-wire count, four windows and 3-inch ID bore. The testing electronics include a custom low-noise, wide-range constant current source, a Keithley model 224 programmable constant-current source, a Fluke model 8842A programmable digital microvoltmeter, a Lake Shore model DRC-91C programmable temperature regulator, Tektronics model 7100 oscilloscope with high-gain differential preamplifiers and Tektronics model 2221A 100-MHz digital storage scope, and a PAR model 5210 programmable lock-in amplifier. The later is used for AC characterization (e.g., photoresponse or switching) and narrow-band noise measurements from 0.5 Hz to 100 kHz. This system has an IEEE-488 bus for computer (PC-386) control and data logging of measurements in the cryostats. The cryostats are also used for pulsed light (xenon strobe and YAG laser) illumination, modulated microwave (11.5 GHz waveguide), and FT-IR spectroscopy of samples and detectors.
R&D 100 Award Winner 2012, 2008, 2001, 1996, and 1995