Shamus McNamara

On-chip vacuum generated by a micromachined Knudsen pump

This paper describes the design, fabrication, and testing of a single-chip micromachined implementation of a Knudsen pump, which uses the principle of thermal transpiration, and has no moving parts. A six-mask microfabrication process was used to fabricate the pump using a glass substrate and silicon wafer. The Knudsen pump and two integrated pressure sensors occupy an area of 1.5 mm /spl times/ 2 mm. Measurements show that while operating in standard laboratory conditions this device can evacuate a cavity to 0.46 atm using 80 mW input power. The pumpdown time of an on-chip chamber and pressure sensor cavity with a total volume of 80 000 cubic micrometers is only 2s, with a peak pump speed of 1/spl times/10/sup -6/ cc/min. High thermal isolation is obtained between the polysilicon heater and the rest of the device.

Scanning thermal lithography: Maskless, submicron thermochemical patterning of photoresist by ultracompliant probes

This article introduces a scanning probe lithography technique in which ultracompliant thermal probes are used in the selective thermochemical patterning of commercially available photoresist. The micromachined single-probe and multiprobe arrays include a thin-film metal resistive heater and sensor sandwiched between two layers of polyimide. The low spring constant (<0.1N∕m) and high thermal isolation provided by the polyimide shank is suitable for contact mode scanning across soft resists without force feedback control. The probes provide what is effectively a spatially localized postexposure bake that crosslinks the photoresist in the desired pattern, rendering it insoluble in developer. For 450-nm–1400-nm-thick AZ5214E (Clariant Corp.), line and dot features with sizes of 450–1800nm can be printed using probe powers of 13.5–18mW, and durations of 1–60s per pixel. Variation of feature sizes with process parameters is described.

Knudsen pump driven by a thermoelectric material

The first use of a thermoelectric material in the bidirectional operation of a gas pump using thermal transpiration has been demonstrated. The thermoelectric material maintains a higher temperature difference which favors thermal transpiration and thus increases the efficiency of gas pumping. Since the hot and cold sides of the thermoelectric material are reversible, the direction of the pump may be changed by reversing the electrical current direction. Two different pump designs are presented that illustrate some of the design tradeoffs. The pumps are characterized by measuring the pressure difference that may be generated and by measuring the flow rate in the forward and reverse directions. For a pump composed of a porous material with a pore size of 100 nm, a maximum flow rate of 0.74 cm3 min−1 and a maximum pressure of 1.69 kPa are achieved.

Ultracompliant thermal probe array for scanning non-planar surfaces without force feedback

This paper describes an array of micromachined thermal probes for scanning thermal microscopy for which the structural design and choice of materials virtually eliminate the need for z-axis mechanical feedback in contact mode scans. The high mechanical compliance accommodates significant topographical variation in the sample surface and prevents damage to soft samples. Thin film metal bolometers are molded into tips at the end of each cantilever in the array, and are sandwiched between two layers of polyimide that serves as the structural material. The probes overhang a Si substrate on which they are fabricated. Since integrated actuators and accompanying circuitry are no longer required, the prospect of scaling from the present eight-probe version to large numbers of probes for high speed, high resolution thermal mapping of large areas with simple detection circuitry is enhanced. The scalability and performance of the eight-probe prototype are evaluated, addressing issues of speed versus resolution, and thermal and mechanical decoupling. The results demonstrate that contact mode scans can provide better than 2 µm spatial resolution at speeds greater than 200 µm s−1 and show a 6.5 bit topographical resolution over a 7 µm dynamic range. Line scans obtained with a single-shank probe suggest that there are good prospects of obtaining images showing a lateral spatial resolution of less than 50 nm.

A micromachined Knudsen pump for on-chip vacuum

This paper describes a single-chip micromachined implementation of a Knudsen pump-a type of vacuum pump that works by the principle of thermal transpiration, has no moving parts, and consequently offers high reliability. A 6-mask process was used to fabricate the pump from a glass substrate and a silicon wafer. A single stage pump and two integrated pressure sensors occupy 1.5 mm/spl times/2 mm. Measurements show that this device can evacuate a cavity to 0.46 atm while operating at atmospheric pressure and using 80 mW input power. Temperature measurements show thermal isolation on the order of 10/sup 4/ K/W between the polysilicon heater used to operate the pump and the rest of the device.

Pneumatic pumping of liquids using thermal transpiration for lab-on-a-chip applications

This paper reports, for the first time, the pumping of water in microfluidic glass channels due to a pressure difference generated by an integrated Knudsen pump. The Knudsen pump operates by the principle of thermal transpiration which generates a pressure difference along a channel with a thermal gradient, and whose channel height is on the order of the mean free path of air (60 nm at 1 atm). A two mask process using microscopic glass slides is used to form hydrophobic, sealed channels with two heights. Shallow channels (80 nm) are used for the Knudsen pump, and water flows in the deep channels (12 µm). The hydrophobic coating repels water from the channels until a thermal gradient is applied across the shallow channel, reducing the pressure in the shallow channel by thermal transpiration, and drawing water into the deep channel. This pump can be readily integrated into many other microfluidic platforms because it has no moving parts, uses low voltages, and requires no additional materials.

Maskless Grayscale Lithography Using a Positive-Tone Photodefinable Polyimide for MEMS Applications

The novel use of a positive-tone photosensitive polyimide for the rapid production of grayscale features using a maskless lithography system is demonstrated. The removal rate of the polyimide, HD-8820, is characterized as a function of exposure dose. A broad contrast curve is found that is suitable for grayscale lithography. Three-dimensional polyimide structures up to 22 μm thick are demonstrated, and the surface roughness after the final cure is Ra = 4.4 nm, which is suitable for many microelectromechanical systems (MEMS) applications, including many optical applications. Tensile testing of 63 polyimide samples shows excellent mechanical properties for four different polyimide thicknesses produced with grayscale lithography. The modulus of elasticity is found to be 1.92 GPa, the yield strength to be 103 MPa, the fracture strength to be 133 MPa, and the percent elongation to be 51%. The test results show that the mechanical properties are consistent and do not change due to a partial exposure to UV light. The entire fabrication sequence, from computer-aided design file to cured structure, can be performed in less than 4 h, making this a fast low-cost method of producing polymer MEMS devices with excellent mechanical properties.

Efficiency derivation for the Knudsen pump with and without thermal losses

This article describes the derivation of the efficiency for the Knudsen pump for two cases. First, by neglecting the thermal losses to calculate the maximum efficiency, and second, by including the thermal losses along the length of the channel. The Knudsen pump is a thermally driven pump with no moving parts and which functions on the principle of thermal transpiration. Diffusion is the main mode of mass transport, as opposed to bulk fluid motion. The pump functions when the gases are in a rarefied state. The efficiency without thermal losses is shown to asymptotically approach the value of the ideal gas constant divided by the specific heat capacity, while the efficiency with thermal losses is given for rectangular and circular cross sections and is shown to be independent of length.

A fabrication process with high thermal isolation and vacuum sealed lead transfer for gas reactors and sampling microsystems

This paper describes a six-mask fabrication process for vacuum-sealed microsystems including pressure and flow sensors, reaction chambers and reservoirs, and channels ranging from 100 nm to 10 /spl mu/m in hydraulic diameter. All structures are countersunk into a glass substrate, resulting in a planar upper surface that eliminates stress concentrations and also facilitates further lithographic processing. The material choices and structural arrangements are designed to. provide high thermal isolation (/spl sim/2 /spl times/ 10/sup 5/ K/W). Lead transfer between the three available levels of interconnect are accomplished with minimal parasitic capacitance (< 1 fF), and low resistance (/spl sim/1 /spl Omega/). A variety of test structures have been successfully fabricated.

The study of radiation effects in emerging micro and nano electro mechanical systems (M and NEMs)

The potential of micro and nano electromechanical systems (M and NEMS) has expanded due to advances in materials and fabrication processes. A wide variety of materials are now being pursued and deployed for M and NEMS including silicon carbide (SiC), III–V materials, thinfilm piezoelectric and ferroelectric, electro-optical and 2D atomic crystals such as graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2). The miniaturization, Semiconductor Science and Technology Semicond. Sci. Technol. 32 (2017) 013005 (14pp) doi:10.1088/1361-6641/32/1/013005 15 Author to whom any correspondence should be addressed. 0268-1242/17/013005+14