Fei Feng

AFM probes fabricated with masked-maskless combined anisotropic etching and p + surface doping

The paper presents a newly developed high-yield micro-fabrication technology for single-crystalline silicon atomic force microscope (AFM) probes. Both the tips and the cantilevers are simultaneously formed by a masked?maskless combined anisotropic etching process. Compared to a conventional tip-to-cantilever sequential fabrication scheme, this tip-and-cantilever simultaneous formation can effectively increase fabrication yield by avoiding the tips damaged during the following processed photolithographic steps for defining the cantilevers. By heavy boron doping at the surface, the conductive AFM probe provides an electrical path to the electric ground of the AFM that helps to eliminate the electrostatic accumulation of charges and, therefore, eliminate undesirable electrostatic forces between the probes and the samples. A fabrication yield as high as 90% has been obtained for the AFM probes for 4 inch wafers. The tips after oxidation-sharpening treatment generally have a radius of 10?30 nm. The cantilever spring constant can be well controlled in the range of 0.025?40 N m?1. High-quality sample scanning results with the formed AFM probes are obtained with a slightly better resolution than that from commercial probes without surface conductive treatment.

CMOS MEMS-based thermoelectric generator with an efficient heat dissipation path

This paper presents a CMOS MEMS-based thermoelectric energy generator (TEG) device with an efficient heat dissipation path. For present CMOS MEMS-based thermoelectric generator devices, the output performance is greatly limited by the high thermal-contact resistance in the system. For the device proposed in the work, the silicon substrate is etched into two comb-shaped blocks thermally isolated from each other, which form the hot and cold sides. Thin-film-based thermal legs are densely located between the two blocks along the winding split line. Low internal thermal-contact resistance is achieved with the symmetrical thermal structure. When the TEG device is embedded between the heat source and heat sink, the heat loss can be well controlled with flat thermal-contact pads of the device. For a full device with 900 n/p-polysilicon thermocouples, the measured open-circuit voltage reaches as high as 146 mV K−1, and the power factor reaches almost five times higher value compared to the previously reported results. A test system integrated with a single device presents an open-circuit voltage of 110 mV K−1 when forcibly cooled by a Peltier cooler, or 26 mV K−1 when cooled by ambient air.

An uncooled optically readable infrared focal plane array

This paper reports a new uncooled optically readable infrared focal plane array (UOR-IRFPA), which is fabricated on a glass substrate. Compared with conventional UOR-IRFPA fabricated on a silicon substrate, visible light (readout light) can irradiate on visible reflecting layer through glass substrate and infrared light can directly radiate on infrared absorption layer from another direction. Therefore, the new UOR-IRFPA has near 100% infrared absorption efficiency without removing silicon substrate underneath UOR-IRFPA pixels by using expensive DRIE (deep reactive ion etching). UOR-IRFPA is successfully fabricated based on MEMS (micro-electro-mechanical system) process. Infrared image of human body is obtained by using fabricated UOR-IRFPA.

Investigation of the extinction coefficient of PECVD hydrogenated amorphous silicon nitride films

In order to study the infrared absorption characteristics of hydrogenated amorphous silicon nitride (a-SiNx:H) films prepared by plasma-enhanced chemical vapor deposition (PECVD), the influence of process parameters on the extinction coefficient (EC) of a-SiNx:H films is investigated in the spectrum from 714 to 1250 cm−1. In our experiments, the EC at Si–N stretching frequency ranges from 1.32 to 2.27 and the corresponding peak wave number shifts from 828 to 890 cm−1 by adjusting process parameters. The results show that the enhanced HF power and substrate temperature are the most important parameters for increasing the EC of the film, and increased NH3/SiH4 gas flow ratio is the key to the blueshift of peak wave number. Finally, the adjusted process parameters of a-SiNx:H film for application in an infrared focal plane array are selected: SiH4:NH3:N2 = 2:3:100, HF power: 45 W, LF power: 35 W, pulse time: HF/LF pulse time = 0.6, temperature: 350 and pressure: 650 mTorr.

Thin-film-based thermoelectric energy generator device with a card structure

This work presents a thermoelectric energy generator (TEG) device with a card structure which embeds a thin-film based TEG chip. The thermocouples are densely arranged between comb-shaped substrates in the chip. The chip is packaged vertically and heated/cooled by metallic plugs connected with the external heat source and heat sink. The packaged device has a footprint of 3 mm × 1.2 mm, while the height can be flexibly scaled by changing the dimensions of the metallic plugs, which indicates a potential use in the high density electronic devices. The n/p poly-silicon is employed as the thermoelectric material for the early-stage research of the new TEG structure. The measured open-circuit voltage for each TEG card reaches 110 mV·K-1. Being worn on human body, the output power on a matched external load reached 0.13 μW for 3 cards in parallel, which improves more than two orders of magnitude than traditional in-plane packaged TEG device with poly-silicon thermal legs.

Improved separation of micro gas chromatographic column using mesoporous silica as a stationary phase support

In this paper, a novel and facile way to improve the separation of micro gas chromatographic column is presented which utilizes the mesoporous silica thin film as the stationary phase support. A serpentine semi-packed column is fabricated based on a micro-electro-mechanical system (MEMS) technology and polydimethylsiloxane is used as the stationary phase. The chromatographic resolution of C6-C7 increases from unseparated to 7.44 after depositing mesoporous silica thin film as the stationary phase support in the separation of a mixture of heavy hydrocarbons (C6-C10), and the separation efficiency is as high as 9290 plates/m. Meanwhile, in the separation of a mixture of benzene series (gas mixtures of benzene, toluene and paraxylene), the chromatographic resolution of benzene and toluene can also be increased by 483%. Those outstanding results indicate that using the mesoporous silica as the stationary phase support is an effective way to improve the separation efficiency of the gas chromatographic column.

Pore size effect of mesoporous silica stationary phase on the separation performance of microfabricated gas chromatography columns

Microfabricated semi-packed gas chromatography (GC) columns coated with two kinds of mesoporous silica (MS) stationary phase have been explored and compared in this paper. The micro GC (μGC) columns are fabricated by microelectromechanical system (MEMS) technique. MSs with pore size of 2 and 5 nm are prepared using two different soft-templates and deposited on the inner surface of the μGC columns by dip-coating. For the first time, the influence of pore size of MS stationary phase on the separation performance in a μGC column has been investigated. As demonstrated, the mixtures of light alkanes (C1-C4) or heavy alkanes (C5-C10) can be well separated in both columns, where the separation resolution is always higher than 1.25 except for C1-C2 and C5-C6. Considering the pore size effect on the separation performance, it is shown that analytes, especially those with bigger size, display longer retention time in the column with larger pore size. For C10, the retention time in 5 nm-MS column is 200% longer than that in 2 nm-MS one.

A micro gas chromatographic column with embedded elliptic cylindrical posts

In this paper, a novel embeded elliptic cylindrical posts (ECPs) with large surface area and wide effective width, which could support more stationary phase and decrease the pressure drop, is applied on the micro-fabricated gas chromatographic (μGC) column. Compared with μGC column with cylindrical posts (CPs), the surface area and effective width of μGC column with ECPs are increased by 29% and 30%. Separation experiments are performed under the same head pressure at column inlet: in experiments of separating mixture 1, the column efficiency of the μGC column with ECPs for C9 has a 76% improvement, and the separation resolution between C8 and C9 also has a 34% improvement; in experiments of separating mixture 2, seven kinds of analytes can be identified by the μGC column with ECPs less than ten minutes, while, only six kinds can be identified by the μGC column with CPs in almost the same time. In detail, the column efficiency of the μGC column with ECPs for toluene has a 129% improvement, and the separation resolution between benzene and toluene also has a 56.4% improvement. Hence, the μGC column with the inner structures of ECPs is a valid means to improve column efficiency and resolution in a lower pressure drop.

High sensitive micro thermal conductivity detector with sandwich structure

This paper reports a high sensitive micro thermal conductivity detector (μTCD) with sandwich structure, which is composed of glass substrate, silicon-on-insulator (SOI) with micro channels and glass with micro channels. A suspended cross-mesh structure includes a Pt thermistor protected by two SiNx layers and a cross-mesh support made of top silicon of SOI, and it is released by deep reactive ion etching (DRIE) substrate silicon of SOI. Our new μTCD has the better reliability, smaller excess dead volume and less process time compared to the state of the art. Experimental results show that this detector demonstrates a high sensitivity without pre-concentrator.

Effect of pore size of stationary phase on the separation performance of chip-based gas chromatography columns

Chip-based semi-packed gas chromatography (GC) columns coated with two kinds of mesoporous silica (MS) stationary phase have been explored and compared in this paper. For the first time, the influence of pore size of the stationary phase on the separation performance in a micro GC (μGC) column has been investigated. As has been demonstrated, full separation of heavy hydrocarbons (C5-C10) can be achieved in both columns. Moreover, compared with stationary phase of smaller pores, that of bigger ones leads to obviously increased retention time of C9 and C10.