Characterization of microcantilever spring and force sensor using nanomanipulators

In this paper the authors present the development of a characterization process for microcantilever based spring and force sensor wherein fusion of real-time vision and force feedback is used. The process applies a very small force in micronewtons using MM3A nanomanipulators and senses the corresponding deflection using vision feedback, which produces direct characterization of microcantilever for evaluating its effective spring constant. The same process has been applied to find sensitivity of a microcantilever based force sensor. In the process force feedback values are viewed on a digital storage oscilloscope and once calibrated it is directly proportional to the applied force. By having known deflections (x) on images and known values of force (F) sensed by a force feedback sensor, the spring constant of microcantilever has been found as K = 8.75 μN/μm. Using the same procedure a microcantilever based force sensor has been characterized, the resulting sensitivity of force sensor has been found as 34.35 mV/μN.     

Fabrication and characterization of microcantilever integrated with PZT thin film sensor and actuator

The purpose is to find the potential application of PZT thin film for microsensor and microactuator. Pb(Zr_0.5Ti_0.5)O₃ thin film is deposited by sol-gel method. Piezoelectric microcantilever with two-segment top electrodes is fabricated using bulk micromachining techniques. Piezoelectricity of the deposited PZT thin film and sensing and actuation capability of the each segment of microcantilever is proved. Experiments are performed when one segment acts as an actuator or vibrator and another segment as force sensor. The results show that the proposed PZT thin film microcantilever can be used in force feedback and object manipulation simultaneously.     

High-rate deposition of amorphous silicon films using hot-wire CVD with a coil-shaped filament

To reduce the manufacturing cost of amorphous silicon (a-Si:H)-based photovoltaic devices, it is important to deposit high-quality a-Si:H and related materials at a high deposition rate. To this end, we designed and constructed a hot-wire deposition chamber with a coiled filament design and with multiple gas inlets. The process gas could be directed into the chamber through the filament coil and have maximum exposure to the high-temperature filament surface. Using such a chamber design, we deposited a-Si:H films at high deposition rates up to 800 Å s⁻¹ and dense, low-void a-Si:H at rates up to 240 Å s⁻¹.     

Hot-wire deposition of amorphous and microcrystalline silicon using different gas excitations by a coiled filament

Microcrystalline silicon (μc-Si:H) and amorphous silicon (a-Si:H) films were deposited using a hot-wire CVD (HWCVD) system that employs a coiled filament. Process gasses, H₂ and Si₂H₆, could be directed into the deposition chamber via different gas inlets, either through a coiled filament for efficient dissociation or into the chamber away from the filament, but near the substrates. We found that at low deposition pressure (e.g. 20 mTorr) the structure of the films depends on the way gases are introduced into the hot-wire chamber. However, at higher pressure (e.g. 50 mTorr), Raman measurement shows similar results for films deposited with different gas inlets.     

Fabrication of a near-infrared sensor using a polyaniline conducting polymer thin film

Near infrared (NIR) photo-responsive polyaniline-based conducting thin films are developed for sensor application. Upon NIR illumination (2.43 W/cm²), the electrical conductance of the polyaniline thin films was enhanced 5.9% and the response time was 20 s. NIR sensing performance of polyaniline conducting polymer thin film is comparable with that of bolometric carbon nanotube (CNT) network devices with the merits of polymers over CNTs such as processability, productivity and economy.     

High rate hot-wire chemical vapor deposition of silicon thin films using a stable TaC covered graphite filament

We grow silicon films by hot-wire/catalytic chemical vapor deposition using a new filament material: TaC-coated graphite rods. The filaments are 1.6 mm diameter rigid graphite rods with ~30 μm thick TaC coatings. Whereas heated W or Ta wire filaments are reactive and embrittle in silane (SiH₄), the TaC/graphite filament is stable. After > 2 h of exposure to SiH₄ gas at a range of filament temperatures, the full length of a TaC/graphite filament retains its shiny golden color with no indication of swelling or degradation. In comparison, a W wire exposed to SiH₄ under the same conditions becomes swollen and discolored at the cold ends, indicating silicide formation. Scanning electron microscopy images of the filament material are nearly identical before and after SiH₄ exposure at 1500-2000 °C. This temperature-independent chemical stability could enable added control of the gas phase chemistry during deposition that does not compromise the filament lifetime. The larger surface area of the 1.6 mm diameter TaC coated graphite filament (compared to the 0.5 mm W filament) allows for a ~ 2× increase in the deposition rate of Si thin films grown for photovoltaic applications.     

Fusion neutron detector calibration using a table-top laser generated plasma neutron source

Using a high intensity, femtosecond laser driven neutron source, a high-sensitivity neutron detector was calibrated. This detector is designed for observing fusion neutrons at the Z accelerator in Sandia National Laboratories. Nuclear fusion from laser driven deuterium cluster explosions was used to generate a clean source of nearly monoenergetic 2.45 MeV neutrons at a well-defined time. This source can run at 10 Hz and was used to build up a clean pulse-height spectrum on scintillating neutron detectors giving a very accurate calibration for neutron yields at 2.45 MeV.     

Deposition and structural characterization of poly-Si thin films on Al-coated glass substrates using hot-wire chemical vapor deposition

The growth of polycrystalline Si films onto Al-coated Corning 7059 glass substrates using hot-wire chemical vapor deposition (HW-CVD) was investigated. The crystalline fraction, grain structure and average grain size of the films were compared as a function of the growth rate and the Si/Al thickness ratio. Micrometre-size Si grains were achieved with a Si/Al ratio of 2 and Si thickness of 2 μm at a growth rate of 1 μm h⁻¹. It was found that the films had a bimodal grain size distribution, which included nanocrystalline Si, and that the growth of micrometre-size crystallites does not continue as the thickness of Si film increases. At a growth rate of 5 μm h⁻¹, films are similar to those grown on glass with an average grain size less than 60 nm and crystalline fraction of 75%.     

Ultrasensitive detection of CrO4(2-) using a microcantilever sensor

Microcantilevers modified with a self-assembled monolayer respond sensitively to specific ion concentrations. Here, we report the detection of trace amounts of CrO4(2-) using microcantilevers modified with a self-assembled monolayer of triethyl-12-mercaptododecylammonium bromide. The self-assembled monolayer was prepared on a silicon microcantilever coated with a thin layer of gold on one side. The microcantilever undergoes bending due to sorption of CrO4(2-) ions on the monolayer-modified side. It was found that a concentration of 10(-9) M CrO4(2-) can be detected using this technology in a flow cell. Other anions, such as Cl-, Br-, CO3(2-) (or HCO3-), and SO4(2-), have minimal effect on the deflection of this cantilever. The mechanics of the bending and the chemistry of cantilever modification are discussed.     

Accuracy of position detection using a position-sensitive detector

Position-sensitive detectors (PSD) provide four current outputs that are proportional to the two-dimensional center of mass of a light spot impinging on the face of the detector. This characteristic permits the PSD to be used for sensing the position of a light source. The application motivating the work reported here involved the tracking of a light source located on a surface with approximate dimensions 122 cm wide by 91 cm high located approximately 2 m from the detector. The diameter of the light source was a few millimeters. The work described here demonstrates the feasibility of using a PSD to track the position of a light source with accuracy better than 1 mm from this distance     

Highly sensitive ultraviolet detector using a ZnO/Si layered SAW oscillator

This study elucidates a highly sensitive ultraviolet light detector using the combination of an oscillator circuit with a high-frequency amplifier, a matching network and a layered surface acoustic wave (SAW) device. In this structure, a ZnO thin film is simultaneously used as an active layer for UV detection and a piezoelectric layer for exciting a high-order surface acoustic wave. The microstructure and crystallization of ZnO films were investigated using the scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The SAW oscillator shows a good performance with output power of − 1.14 dBm and phase noise of −94.7 dBc at 100 kHz. Firstly, the frequency shifts of the oscillator exhibit rapid increase with the intensity of the UV light. Then the increased shifts decayed at certain UV intensity due to the saturated photogenerated carriers. An extreme frequency shift of 1017 kHz was obtained as the UV intensity reached 551 μW/cm². The maximum sensitivity of 8.12 ppm/(μW/cm²) can be obtained in this detector.     

Fabrication and characteristics of n-Si/c-Si/p-Si heterojunction solar cells using hot-wire CVD

A double-side (bifacial) heterojunction (HJ) Si solar cell was fabricated using hot-wire chemical vapor deposition. The properties of n-type, intrinsic and p-type Si films were investigated. In these devices, the doped microcrystalline Si layers (n-type Si for emitter and p-type Si for back contact) are combined with and without a thin intrinsic amorphous Si buffer layer. The maximum temperature during the whole fabrication process was kept below 150 °C. The influence of hydrogen pre-treatment and n-Si emitter thickness on performance of solar cells have been studied. The best bifacial Si HJ solar cell (1 cm² sample) with an intrinsic layer yielded an active area conversion efficiency of 16.4% with an open circuit voltage of 0.645 V, short circuit current of 34.8 mA/cm² and fill factor of 0.73.     

Deposition of HWCVD poly-Si films at a high growth rate

The process parameters for high growth rate poly-silicon films by hot-wire chemical vapour deposition have been explored. A four-wire hot wire assembly has been employed for this purpose. High silane to hydrogen flow ratios and high wire temperatures are the key process parameters to achieve high growth rate and growth rates higher than 5 nm/s can be achieved. The process conditions to incorporate high hydrogen content into the material for passivation of defects and donor states have been identified as high hydrogen dilution and lower wire temperature. With these procedures poly-Si films deposited at 1.3 nm/s showed high ambipolar diffusion length of 132 nm. Incorporating such poly-Si films as i-layer, n–i–p solar cell on stainless steel substrate without back reflector showed an efficiency of 4.4%.     

Growth of c-GaN films on GaAs(100) using hot-wire CVD

Cubic gallium nitride (GaN) films were grown on nitrided layers of GaAs(100) by hot-wire chemical vapor deposition. The nitrided layer was also formed by NH_x radicals generated on a tungsten hot-wire surface. Nitridation conditions for the growth of GaN with a cubic-type structure were investigated. As a result, GaN film with a preponderant cubic phase was grown on the GaAs surface layer nitrided at a substrate temperature of 550 °C, a filament temperature of 1200 °C and an ammonia (NH₃) pressure of 1 Torr.     

Preparation of n-type nanocrystalline 3C-SiC films by hot-wire CVD using N2 as doping gas

N-type nanocrystalline 3C-SiC films were prepared by hot-wire chemical vapor deposition from SiH₄/CH₄/H₂ and N₂ as a doping gas and the structural and electrical properties were investigated. The gas flow rates of SiH₄, CH₄ and H₂ were 1, 1 and 200 sccm, respectively. As the N₂ gas flow rate was increased from 0 to 10 sccm, the conductivity and the activation energy improved from 0.05 to 0.3 S/cm and from 45 to 28 meV, respectively. The Hall Effect measurement proved that the improvement of the electrical properties was caused by the increase in the carrier concentration. On the other hand, in the N₂ gas flow rate between 10 and 50 sccm, the conductivity and the activation energy remained unchanged. The crystallinity deteriorated with increasing N₂ gas flow rate. This gave rise to the unchanged electronic properties in spite of the increase in the intake of N atoms.     

Growth of GaN films on nitrided GaAs substrates using hot-wire CVD

Epitaxial growth of cubic-type gallium nitride (c-GaN) by hot-wire CVD on GaAs(100) substrates was investigated. Prior to the epitaxial growth, a nitridation layer was formed using ammonia plasma generated by electron cyclotron resonance (ECR). It was found that the crystal phase of the epitaxial layer was predominantly determined by that of the nitrided layer. The best nitridation condition using ECR plasma for the growth of the GaN films with preponderant cubic-type structure was obtained.