Lianwei Wang

Electrochemically-deposited nanostructured Co(OH)2 flakes on three-dimensional ordered nickel/silicon microchannel plates for miniature supercapacitors

Silicon microchannel plates (Si-MCPs) coated with a nickel layer are an excellent substrate in miniature supercapacitors. Nanometer-sized Co(OH)2 flakes serving as the active materials are electrodeposited on ordered three-dimensional (3D) Ni/Si-MCPs and the Co(OH)2 flakes have different structures depending on the solvent used. The cobalt hydroxide synthesized from a de-ionized water solvent is composed of compact nano-flakes, whereas that synthesized from an alcohol containing solvent is composed of loosely packed nano-flakes, and that from acetone are nano-flakes with nano-particles. The three types of electrode materials are investigated from the perspective of electrochemical capacitors by means of cyclic voltammogram, galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy. The highest specific capacitance of 6.90 F cm−2 is achieved from the samples prepared in acetone at a discharge current density of 10 mA cm−2 and it is much better than the 1.46 F cm−2 observed in previous studies, thus demonstrating excellent capacity retention.

Obtaining a high area ratio free-standing silicon microchannel plate via a modified electrochemical procedure

A silicon microchannel plate (Si MCP) is of large interest for electron multiplication. Conventional fabrication processes utilize an etching process on the front side and wafer thinning on the backside. In this note, a process based on electrochemical etching, developed to generate a gap between the device layer and the substrate, is presented. A sample containing a microchannel through-hole structure can be directly obtained with a laser cut by scanning the laser beam on the surface without cutting through the wafer. An oxidation step is used to increase the MCPs electrical resistance. After dry oxidation at 900 °C, structure distortion is observed in the free-standing sample. Thus, oxidizing before cutting the microchannel plate from the substrate is recommended.

Evolution of hydrogen and helium co-implanted single-crystal silicon during annealing

H+ was implanted into single-crystal silicon with a dose of 1×1016/cm2 and an energy of 30 KeV, and then He+ was implanted into the same sample with the same dose and an energy of 33 KeV. Both of the implantations were performed at room temperature. Subsequently, the samples were annealed in a temperature range from 200 to 450 °C for 1 h. Cross-sectional transmission electron microscopy, Rutherford backscattering spectrometry/channeling, elastic recoil detection, and high resolution x-ray diffraction were employed to characterize the strain, defects, and the distribution of H and He in the samples. The results showed that co-implantation of H and He decreases the total implantation dose, with which the surface could exfoliate during annealing. During annealing, the distribution of hydrogen did not change, but helium moved deeper and its distribution became sharper. At the same time, the maximum of the strain in the samples decreased a lot and also moved deeper. Furthermore, the defects introduced by ion i...

Oxygen-induced nickel segregation in nitrogen plasma implanted AISI 304 stainless steel

Austenite stainless steel is widely used commercially due to its superior corrosion resistance. Plasma surface treatment has been shown to improve the wear resistance of the materials without degrading the corrosion resistance. Plasma immersion ion implantation (PIII) is a special form of plasma treatment in which the ion energy can be adjusted easily and its non-line-of-sight characteristic makes it suitable for large industrial components possessing an irregular geometry. We observe nickel segregation beneath the top surface in nitrogen plasma immersion ion implanted AISI 304 stainless steel. The amount of segregated nickel and the location depend on the implantation conditions. The phenomenon can be attributed to oxygen-induced surface segregation despite the use of high-purity (99.999%) nitrogen in our experiments. The Auger results indicate that the sample surface has been unexpectedly oxidized in spite of a very small amount of oxygen in the residual vacuum. This is due to the non-UHV (ultra-high vacuum) nature of PIII instruments and the reactive plasma environment. It is believed that the movement of the nickel atoms away from the surface is due to the higher affinity of oxygen to Cr or Fe than Ni. Our investigation also shows that the phenomenon is not related to nitrogen incorporation. As the properties of the treated sample depend on many factors, nickel segregation must be considered in designing PIII experiments.

Silicon micromachining of high aspect ratio, high-density through-wafer electrical interconnects for 3-D multichip packaging

This paper presents a novel silicon micromachining method, which combines tetra methyl ammonium hydroxide (TMAH) etching and deep-reactive ion etching (DRIE) along with bottom-up copper electroplating, to fabricate high-density and high-aspect ratio through-wafer electrical interconnects (TWEIs) for three-dimensional multichip packaging. The silicon wafer was locally etched with TMAH from the backside until the desired membrane thickness was reached, and then DRIE was performed on the membrane until the holes were etched through. TMAH etching preserved large areas of the wafers at the original thickness, thus, ensuring relatively strong mechanical strength and manipulability. DRIE made it possible to realize high-aspect ratio holes with minimized wafer area consumption. A new bottom-up copper electroplating technique was developed to fill the high-aspect ratio through-wafer holes. This method can avoid seams and voids while achieving attractive electrical features. Through-wafer holes, as small as 5 mum in diameter, have been realized by using the combination of TMAH and DRIE, and have been completely and uniformly filled by using bottom-up copper electroplating

Large-size P-type silicon microchannel plates prepared by photoelectrochemical etching

The influence of backside illumination and temperature on the fabrication of large and high aspect ratio silicon microchannel plates (MCPs) by photoelectrochemical (PEC) process is described. Backside illumination is provided by three 150-W tungsten halogen lamps with a feedback loop, keeping a constant current density. The etching temperature is maintained by a circulation system. Proper backside illumination and the lower temperature can provide better integrated etching conditions compared to that without illumination and temperature control. Etching under the improved conditions results in smoother undercutting and better surface topography for large (effective diameter of about 80 mm for 4-inch silicon substrates) silicon microchannel plates. Enhancing the backside illumination within the etching temperature range ensures that the aspect ratio is more than 40, boding well for applications of silicon microchannel plates.

Miniature supercapacitors composed of nickel/cobalt hydroxide on nickel-coated silicon microchannel plates

Silicon microchannel plates with a large surface area coated with a nickel layer are excellent templates for miniature supercapacitors. Three-dimensional Si-MCP/Ni/Ni(OH)2 and Si-MCP/Ni/Co(OH)2 sandwiched structures are produced with the Ni(OH)2 and Co(OH)2 thin films serving as the active materials in the supercapacitors which consist of microcrystalline and nano-sized flakes or rods. The specific capacitances derived from the CV and typical charge/discharge curves exhibit good consistency. The highest specific capacitance of 3.75 F cm−2 is obtained at a discharge current density of 10 mA cm−2 from Si-MCP/Ni/Ni(OH)2 and 1.46 F cm−2 from Si-MCP/Ni/Co(OH)2 at a scanning rate of 10 mV s−1. At high rates, the specific capacitances measured from both samples are relatively low, but the excellent capacitive properties are restored at low rates after being subjected to several high rate charge/discharge cycles. Owing to the large specific capacitance per unit area, good cycling performance, small volume, and unique three-dimensional structure, these electrodes are excellent supercapacitors suitable for secondary power sources. In addition, the potential of nickel coated Si-MCPs as templates for miniaturization and integration of devices in practical applications is discussed and demonstrated.

High aspect ratio through-wafer interconnections for 3D-microsystems

Closely spaced, through-wafer interconnects are of large interest in RF MEMS and MEMS packaging. In this paper, a suitable technique to realize large arrays of small size through-wafer holes is presented. This approach is based on macroporous silicon formation in combination with wafer thinning. Very high aspect ratio (/spl ges/ 100) structures are realized. The wafers containing the large arrays of 2-3/spl mu/m wide holes are thinned down to 200-150/spl mu/m by lapping and polishing. Copper electroplating is finally employed to realize arrays of high aspect ratio Cu plugs.

Intense blue-light emission from carbon-plasma-implanted porous silicon

We have investigated the room-temperature photoluminescence (PL) characteristics of porous-silicon plasma implanted with carbon. Before implantation, the porous silicon made by anodizing emits intense orange light. After carbon-plasma-immersion ion implantation, the orange light disappears and blue light appears. Furthermore, intense blue light is obtained after annealing at 400 °C for 30 min. Analytical results show that the quenching of orange light and appearance of blue light are due to the reduction of the size of nanocrystallites caused by implantation. The effects of different annealing temperature on the light-emission properties of the implanted porous silicon are also studied. The intensity decreases with increased temperature from 600 to 1000 °C, but the PL intensity increases drastically again after annealing at 1250 °C due to the formation of a substance.

Hierarchical 3-dimensional CoMoO4 nanoflakes on a macroporous electrically conductive network with superior electrochemical performance

Nanoscale cobalt molybdate (CoMoO4) particles are fabricated hydrothermally on the surface and sidewall of three-dimensional nickel-coated silicon microchannel plates (also called macroporous electrically conductive network, MECN) as the active electrode in a miniature energy storage device. The relationships between the reaction time, morphology, formation mechanism of the CoMoO4 nanostructure, and electrochemical performance are studied. After an optimal hydrothermal synthesis time of 2.5 h, the CoMoO4 electrode has a capacity of 32.40 mA h g−1 (492.48 μA h cm−2) at constant current density of 1 A g−1 and retention ratio of 85.98% after 5000 cycles. The large specific capacity and excellent rate capability can be attributed to the unique 3D ordered porous architecture which facilitates electron and ion transport, enlarges the liquid–solid interfacial area, and enhances the utilization efficiency of the active materials. Furthermore, the weight and size of the device are reduced. By using the CoMoO4/MECN electrode as the positive electrode and carbon/nickel foam (carbon/NF) as the negative electrode, the faradaic electrode was packaged by a CR2025 battery cell as the miniature hybrid device exhibits stable power characteristics (5000 cycle times with 71.82% retention). After charging each hybrid device for 10 s, three devices in series can power two 5 mm diameter light-emitting diodes (LED) efficiently.