U. Staufer

Advanced deep reactive ion etching: a versatile tool for microelectromechanical systems

Advanced deep reactive ion etching (ADRIE) is a new tool for the fabrication of bulk micromachined devices. Different sensors and actuators which use ADRIE alone or combined with other technologies such as surface micromachining of silicon are presented here. These examples demonstrate the potential and the design freedom of this tool, allowing a large number of different shapes to be patterned and new smart devices to be realized.

Near-field fluorescence imaging with 32 nm resolution based on microfabricated cantilevered probes

High-resolution near-field optical imaging with microfabricated probes is demonstrated. The probes are made from solid quartz tips fabricated at the end of silicon cantilevers and covered with a 60-nm-thick aluminum film. Transmission electron micrographs indicate a continuous aluminum layer at the tip apex. A specially designed instrument combines the advantages of near-field optical and beam-deflection force microscopy. Near-field optical data of latex bead projection patterns in transmission and of single fluorophores have been obtained in constant-height imaging mode. An artifact-free optical resolution of 31.7±3.6 nm has been deduced from full width at half maximum values of single molecule images.

Integrated atomic force microscopy array probe with metal–oxide–semiconductor field effect transistor stress sensor, thermal bimorph actuator, and on-chip complementary metal–oxide–semiconductor electronics

A microfabricated 2×1 array of active and self-detecting cantilevers is presented for applications in atomic force microscopy (AFM). The integrated deflection sensor is based on a stress sensing metal–oxide–semiconductor transistor. Full custom complementary metal–oxide–semiconductor amplifiers for signal readout are combined on the same chip. A sensor sensitivity of 2.25 mV/nm, or a change in current ΔId/Id=2.8×10−6/nm, was obtained at the final output stage. Three Al–Si thermal bimorph actuators are integrated on each cantilever for self-excitation and feedback actuation. The efficiencies of the heaters are 2.4–4.7 K/mW. In the experimental setup, a maximum displacement of 8 μm was achieved at 45 mW input. A pair of parallel AFM images in the constant height mode, a typical tapping mode image, and a constant force image with 1.3 μm high features have been successfully taken with the array probe.

Nanoscale dispensing of liquids through cantilevered probes

Nanoscale dispensing is a novel technique to deposit material and create structures at dimensions of 100 nm and below. It has great flexibility in feature shape and choice of deposited material. Due to its potential low cost and lack of time consuming steps, it represents an interesting complementary tool to standard lithographic processes. The key feature of nanodispensing is deposition of liquids through an apertured scanning force microscopy probe tip. In the first experiments, liquid is manually loaded into a hollow pyramidal probe tip. Upon contact of the tip and the substrate, liquid at the end of the tip is transferred to the substrate surface. Moving the sample during contact allows to write features with sizes that can be as small as 100 nm and below, largely dependent on the aperture diameter. This approach is novel, and has recently been demonstrated in our laboratory for the first time, with feature sizes still well above 1 µm.

Wafer- and piece-wise Si tip transfer technologies for applications in scanning probe microscopy

A novel tip transfer technology is proposed for applications in scanning probe microscopy (SPM). The technology is based on the concept of fabricating tips on an independent wafer and transferring them onto the target wafer. The transfer is also feasible on a full 4-in wafer scale. This is especially attractive for postprocessing CMOS wafers, e.g., for atomic force microscopy chips with integrated electronics. A yield of more than 90% has been achieved in a first experimental set-up. Moreover, a piece-wise tip transfer onto a free-standing cantilever is also shown. During this transfer, the tip is completely encapsulated in a resist post and, hence, protected against mechanical impact. This technology can be applied not only to SPM probe fabrication but also to create a new kind of MEMS device.

Piezoresistive cantilever array for life sciences applications

Atomic Force Microscopy (AFM) techniques are used with one- or two-dimensional arrays of piezoresistive probes for parallel imaging. We present a newly designed AFM platform to drive these passivated piezoresistive cantilever arrays in air and liquid environments. Large area imaging in liquid as well as qualitative and quantitative analysis of biological cells are demonstrated by the means of piezoresistive cantilever for the first time to our knowledge. Noise limitations in topography and force resolutions of these piezolevers are quantified.

Micromachined photoplastic probe for scanning near-field optical microscopy

We present a hybrid probe for scanning near-field optical microscopy (SNOM), which consists of a micromachined photoplastic tip with a metallic aperture at the apex that is attached to an optical fiber, thus combining the advantages of optical fiber probes and micromachined tips. The tip and aperture are batch fabricated and assembled to a preetched optical fiber with micrometer centering precision. Rectangular apertures of 50 nm X 130 nm have been produced without the need of any postprocessing. Topographical and optical imaging with a probe having an aperture of 300 nm demonstrate the great potential of the photoplastic probe for SNOM applications.

Atomic force microscope for planetary applications

We have developed, built and tested an atomic force microscope (AFM) for planetary science applications, in particular for the study of Martian dust and soil. The system consists of a controller board, an electromagnetic scanner and a micro-fabricated sensor-chip. Eight cantilevers with integrated, piezoresistive deflection sensors are aligned in a row and are engaged one after the other to provide redundancy in case of tip or cantilever failure. Silicon and molded diamond tips are used for probing the sample. Images can be recorded in both, static and dynamic operation mode. In the latter case, excitation of the resonance frequencies of the cantilevers is achieved by vibrating the whole chip with a piezoelectric disk.

Atomic force microscopy using an integrated comb-shape electrostatic actuator for high-speed feedback motion

A microfabricated cantilever with integrated comb-shape electrostatic actuator is proposed to yield a high-speed feedback motion in atomic force microscopy with optical deflection detection. The actuator has a linear response to the driving voltage. The dynamic range of tip amounts to 1 μm with an actuation efficiency of 11.4 nm/V. Using this cantilever, an imaging bandwidth of ∼80 kHz was obtained. High-speed constant force imaging with a tip velocity of 1.22 mm/s is demonstrated. The tip-sample force is verified to be constant by using a cantilever with an integrated piezoresistor as the scanned sample.

Fabrication and characterization of a silicon cantilever probe with an integrated quartz-glass (fused-silica) tip for scanning near-field optical microscopy

A cantilever-based probe is introduced for use in scanning near-field optical microscopy (SNOM) combined with scanning atomic-force microscopy (AFM). The probes consist of silicon cantilevers with integrated 25-mum-high fused-silica tips. The probes are batch fabricated by microfabrication technology. Transmission electron microscopy reveals that the transparent quartz tips are completely covered with an opaque aluminum layer before the SNOM measurement. Static and dynamic AFM imaging was performed. SNOM imaging in transmission mode of single fluorescent molecules shows an optical resolution better than 32 nm.