In applications such as target tracking the ability to steer the radiation pattern from an antenna array is required. This paper details the theory, design, fabrication and characterisation of a new type of reconfigurable planar antenna reflectarray for operation with circularly polarised signals. Each element in the array comprises a resonant dipole antenna that can be rotated about its axis, each element is positioned at an odd number of quarter wavelengths above a ground plane. An array of fixed antennas was fabricated on a high resistivity silicon substrate with measurements confirming that the silicon exhibits low absorption at quasi-optical frequencies. The demonstrator devices are designed for 100 GHz operation in order to facilitate fabrication of a usable array aperture on a single silicon wafer. Dimensions of the antennae and the thickness of the substrate were selected accordingly. The wide range of micro-machining techniques that are available for silicon based structures enabled the design of a demonstrator array where the elements can be rotated using a rack and pinion arrangement. In such structures, the actuating mechanism is positioned outside the radiating aperture of the array to exclude conducting elements that would otherwise impair antenna functionality. A method of fabricating the reconfigurable array has been developed and successfully implemented.
We describe the design, fabrication, and characterization of a buried polysilicon hot-wire anemometer with an integrated micromachined channel for fluid flow sensing. Hot-wire flow sensor elements and bulk micromachined channels are fabricated separately on device and channel wafers. The direct silicon wafer bonding technique is then used to integrate the flow sensors and channels. This is made possible by fabrication of the polysilicon resistor flow sensing element in a trench. Heavily doped polysilicon connections are also recessed into the device wafer. The attraction of using polysilicon for the flow sensing element and connections is the ability to adjust the resistivity and temperature coefficient of resistance (TCR) by control of the dopant concentration. The effect of doping on both these parameters is characterized in detail to enable selection of the appropriate doping concentration for each region of the device. Light doping is preferred for the resistor element, since this ensures high thermal sensitivity, while heavy doping is used to provide low resistance connections. The materials allow use of standard high-temperature fabrication processes compatible with standard integrated circuit (IC) technology and micromachining techniques, including silicon bonding. This offers the ability to integrate other microfluidic components and electronic control circuits.
We present a novel micromachined passive valve for fluid flow control within microfluidic systems. Forward flow through the valve is in the opposite direction compared to other published designs, and the valve thus has considerable potential for integrated microfluidic devices or systems. In applications such as micropumps, the inlet and outlet valves can be formed on the same surface, removing the need for precision processing on both wafer surfaces. The fabricated valve consists of three major layers, namely: a top bridge with an inlet aperture; a movable plate with four supporting arms; and an underlying substrate with an outlet aperture. The structure and operation of the device is presented, together with a detailed description of the fabrication process. The fabricated valves are characterized for fluid flow under forward and reverse pressures. In the forward direction, a water flow rate of 1920 µl/min is measured at a pressure of 14.1 kPa, and the reverse leakage is less than 8 µl/min.
A stacked thermal structure for fluid flow sensing has been designed, fabricated, and tested. A double-layer polysilicon process was employed in the fabrication. Flow measurement is based on the transfer of heat from a temperature sensor element to the moving fluid. The undoped or lightly doped polysilicon temperature sensor is located on top of a heavily doped polysilicon heater element. A dielectric layer between the heater and the sensor elements provides both thermal coupling and electrical isolation. In comparison to a hot-wire flow sensor, the heating and sensing functions are separated, allowing the electrical characteristics of each to be optimized. Undoped polysilicon has a large temperature coefficient of resistance (TCR) up to 7 %/K and is thus a preferred material for the sensor. However, heavily doped polysilicon is preferred for the heater due to its lower resistance. The stacked flow sensor structure offers a high thermal sensitivity making it especially suitable for medical applications where the working temperatures are restricted. Flow rates of various fluids can be measured over a wide range. The fabricated flow sensors were used to measure the flow rate of water in the range μl - ml/min and gas (Helium) in the range 10 - 100ml/min.
This paper investigates the use of electrical conductivity monitoring in silicon-based capillaries and the inherent problems therein. In comparison to reference glass devices, the conductance waveforms from the silicon devices were significantly distorted. This has been shown to be due to the profiles of the ends of the capillaries where single-sided etching was employed, and the silicon dioxide capacitance. Double-sided processing provides a solution to tapering of channel inlets, by reducing the time that the front side is exposed to the KOH solution. Models are developed for the devices, which identify degradation of the oxide isolation as another source of distortion. Matching of the experimental and simulated characteristics enables an estimation of the capacitance between the silicon and the bulk solution. Silicon nitride layers are shown to provide more effective isolation and greatly reduce the distortion observed during conductivity monitoring.
The application of precision grinding for the formation of a silicon diaphragm is investigated. The test structures involved 2-6 mm diam diaphragms with thicknesses in the range of 25-150 μm. When grinding is performed without supporting the diaphragm, bending occurs due to nonuniform removal of the silicon material over the diaphragm region. The magnitude of bending depends on the final thickness of the diaphragm. The results demonstrate that the use of a porous silicon support can significantly reduce the amount of bending, by a factor of up to 300 in the case of 50 μm thick diaphragms. The use of silicon on insulator (SOI) technology can also suppress or eliminate bending although this may be a less economical process. Stress measurements in the diaphragms were performed using x-ray and Raman spectroscopies. The results show stress of the order of 1×107-1×108 Pa in unsupported and supported by porous silicon diaphragms while SOI technology provides stress-free diaphragms. Results obtained from finite element method analysis to determine deterioration in the performance of a 6 mm diaphragm due to bending are presented. These results show a 10% reduction in performance for a 75 μm thick diaphragm with bending amplitude of 30 μm, but negligible reduction if the bending is reduced to <10 μm.
This paper investigates modeling of the conductance of liquid in a microchannel, using SPICE. When more than one liquid is present, conductance monitoring is an effective technique to measure electroosmotic flow rates. In micromachined silicon capillaries, the technique is hampered by the capacitance that exists between the bulk fluid and the silicon substrate, and leakage currents due to the thin insulating oxide layer. A SPICE model is used to simulate conductance waveforms, by using MOS transistors to model the time dependant resistance of the channel. The simulation results are used to determine the capacitance and explain the conductance waveforms measured for micromachined silicon channels.
Silicon is being investigated as a low cost, low loss substrate for MMICs. The conflicting requirements of low resistivity silicon for active device fabrication and very high resistivity silicon for low microwave transmission losses have been met by two differing technologies. In one technology the low loss CPW lines are fabricated on oxidized porous silicon (OPS) formed on 1-3 (Omega) -cm (100) silicon substrates. In the other technology SOI substrates are produced by bonding 1-3 (Omega) -cm silicon wafers to 2-4 k(Omega) -cm handle wafers which are covered with a layer of silicon dioxide on a layer of polycrystalline silicon. To minimize bowing of the silicon substrate it was found necessary to limit the OPS thickness to 10 micrometers . For the CPW lines the microwave losses on the OPS substrates were 8.5 dB/cm at 30 GHz and on the SOI wafers they were 2.2 dB/cm. The SOI wafers offer considerable promise for reliable low cost low loss MMIC substrates.
The application of precision grinding for silicon diaphragm formation is investigated. The test structures involved 6 mm diameter diaphragms with thickness in the range 25 micrometers - 150 micrometers . When grinding is performed without supporting the diaphragm, buckling occurs due to non-uniform removal of the silicon material over the diaphragm region. The magnitude of buckling depends on the final thickness of the diaphragm. Results obtained from using FEM analysis to determine deterioration in performance of the diaphragm performance due to buckling are presented. These results show a 10 percent reduction in performance for a 75 micrometers thick diaphragm with a buckling amplitude of 30 micrometers , but negligible reduction if the buckling is reduced to < 10 micrometers . It is shown that the use of a porous silicon support can significantly reduce the amount of buckling, by a factor of 4 in the case of 75 micrometers thick diaphragms. The use of SOI technology can also suppress or eliminate the buckling although this may be a less economical process.
This paper investigates methods of flow rate quantification in micro- fluidic devices, using electrodes to measure the conductivity of solution. Conductivity changes occur when liquid flow causes movement of the boundary between two solutions of differing conductivity. The fabrication technology for the micromachined silicon structures is based on anisotropic etching and anodic bonding to glass. The silicon processing is simplified by using a single-mask process, whereby 9 - 15 mm long, 50 - 100 micrometers wide capillaries and access through-holes are created with a single etch step. Thin film gold electrodes patterned on the glass provide contact with the liquid in the capillary. The current monitoring method, used in capillary electrophoresis, is employed to determine conductance-time waveforms during electroosmotic pumping. The waveforms for silicon based devices are distorted due to oxide capacitance and the profiles of the ends of the channel. The transitions are much more linear for reference devices formed using standard glass capillary tubing. Electrical models are developed for the devices and these are used to determine flow velocities and hence volume flow rates of liquid.