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.