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Four-point probes (4PPs) are used for the resistivity measurement of thin metal or semiconductor films. There is an interest in miniaturization of 4PPs to obtain higher surface sensitivity, increased spatial resolution and less damage to the sample. Several attempts have been made to reduce the probe spacing, e.g. a four-tip scanning tunneling microscope (STM) was reported and a 4PP based on silicon cantilevers was designed. This paper shows design, fabrication and characterisation of a micrometer scale 4PP (micro4PP) device based on SU-8-cantilevers. The photoresist SU-8 has appropriate mechanical properties that make it an excellent candidate material for active parts in selecterd micro-mechanical applications such as cantilevers for scanning probe methods.

Conventional Hall devices are widely employed to measure magnetic fields produced by millimetre sized (or larger) current-caring conductors or permanent magnets with a standard active area of about (100 um)^2. Micro-Hall sensors have recently emerged as a powerful and sensitive tool for the investigation of local magnetic field properties of the micro and nano-structures. The conventional Hall effect devices are based on non-magnetic materials (typically semiconductors e.g: AlGaAs/GaAs and InSb or semimetals such as Bi). Bismuth is a semi-metal that presents a very long mean free path, a large spatial extension of the electron wave function, and a low electron density. This explains the amazing galvanomagnetic properties of bismuth nanostructures. Recently, Hall sensors having dimensions down to (100 nm)^2 have been fabricated. By mounting them on micro-scanning systems, a new magnetic imaging method, called Scanning Hall Probe Microscopy (SHPM), has been introduced for simultaneous magnetic and topography imaging. Miniaturized Hall sensors are employed to measure highly inhomogeneous magnetic fields such as those produced by ferromagnetic nanoparticles, vortices in superconductors, magnetic domains in ferromagnetic materials, magnetic force microscopy tips, and magnetic recording media. We have developed an alternative route to fabricate micro and nano Hall sensors on cantilevers. The approach was to incorporate bismuth Hall sensors of dimensions down to 100 nm on photo-plastic cantilevers made with SU-8, a material already used for the fabrication of atomic force microscopy (AFM) and near field scanning optical microscopy (NSOM). The smallest active area of the Hall sensors obtained by the focused Ion beam (FIB) is (60x60) nm^2. The electrical characteristics of our bismuth Hall sensors are comparable to the best results reported up to now. For instance, a device with an active area about (0.1x0.1) um^2 the current sensitivity, the Hall coefficient and the field resolution measured are 2 V/AT, 180x10^-9 m3/C and 120 uT/Hz^1/2, respectively.

Cantilever-based detection of ultrasmall forces is of great interest for several applications such as magnetic resonance force miscroscopy (MRFM) where an ultrahigh sensitive cantilever is used to detect magnetic resonance in small ensembles of electron or nuclear spins. Detecting the smallest possible forces requires ultrahigh sensitive cantilevers with low spring constant (k < 0.1 mN/m), resonance frequency f > 1 kHz and high quality factor (Q > 30000 in vacuum). Considerable progress has been made in fabricating attonewton sensitive cantilever but the yield of batch fabrication remains limited due the fragility of the cantilever during the fabrication. A new process fabrication of single-crystal silicon (Si) cantilevers is presented here that lead to a yield superior to 70%. It is based on a combination of two novel process steps for ultra-thin MEMS/NEMS fabrication, i.e. i) protective embedding of the structural Si device in aluminium (Al) during critical process steps and ii) HF vapour release to avoid the stiction effect. SOI wafers with single-crystal silicon thickness of 340 and 500 nm were used. Cantilevers were patterned with photolithography and dry etch process. 2-um-thick Al films were deposited on the front and backside of the wafers. The Al backside layer, patterned with photolithography and Al wet etching, acts as a mask for the deep backside etching while the Al top layer protects the Si cantilever and reinforces the SiO2 etch stop layer. Deep dry etching of the wafer backside is performed in a high inductive coupled plasma reactor. After removing the Al by wet etching the thin membrane of SiO2 is etched with HF vapour, a convenient technique to release fragile, suspended structures and which does not require fluidic water cleaning. This allows increasing the yield to more than 70%. A laser beam deflection setup implemented in a vacuum chamber is used to measure f and Q. The ring down method is used to calculate the Q factor. The cantilever is excited at its natural resonance frequency with a piezoelectric actuator with a precise pulse duration. After the excitation is interrupted, the decay time is measured and the factor Q is deduced. Several measurements have been carried out in different conditions: air, vacuum, with and without annealing. The characterized cantilevers show promising results in vacuum at room temperature, e.g. a cantilever (L=500um, w=10um, t=0.5um), respectively has f = 2kHz, k = 0.0004 N/m and Q > 100000. The force sensitivity has been calculated from the expression given by F~ (kT/Qf)^1/2 and is 6x10-17 N/Hz^1/2. The presented technology that combines embedding with Al thin films and the HF vapour releasing is a promising generic method for improving the fabrication yield of fragile freestanding structures. Annealing rearranges the atomic structures, reduces the dissipation energy and allows increasing the Q factor significantly.

A technology for the fabrication of bismuth Hall sensors integrated on SU-8 cantilevers has been developed in this project. Hall sensors with active area of 2 x 2 μm2 (Fig.2) are produced with lift-off and resized to 200 x 200 nm2 (Fig.3) with the FIB. SU-8 cantilevers containing the sensors are characterized as well as the magnetic properties of the sensors.

The aim of the project is the development of a new technology for the fabrication of bismuth (Bi) micro-nano Hall sensors integrated on plastic cantilevers. Bi, a semi-metal, is deposited by evaporation and structured by lift-off. Four electrodes made with Ti-Al are deposited and structured in the same way as Bi. SU-8 cantilevers and carrier chip are structured by photolithography process. The smallest size of the Hall sensor active area obtained with the photolithography was 2 x 2 um2 and was reduced to the nanometer scale (200x200 nm2) by the mean of the focused ion beam (FIB).

The goal of the project is to fabricate ultra sensitive cantilevers for magnetic resonance force microscopy. Such devices have been produced using SOI wafers and a novel fabrication process combining Al encapsulation and HF-vapor release. The realized cantilevers are 200, 350 or 500 µm long, 10 or 20 µm wide and 500 or 340 nm thick. The characterization shows promising results in vacuum at room temperature, e.g. a cantilever annealed at 800°C (L=500um, w=10um, t=0.5um) has a resonance frequency f = 2kHz, a spring constant k = 0.0004 N/m and a quality factor Q > 100000.

Innovative surface imaging techniques allowing parallel magnetic and topographical microscopy with nanometer scale spatial resolution are developed in this project. Nanometer scale magnetosensitive devices on Atomic Force Microscopy (AFM) tips will be studied and fabricated, allowing quantitative and noninvasive magnetic field imaging with spatial resolution better than 100 nm and magnetic field resolution better than 100 mT/Hz. Such performance will allow one to perform magnetic microscopy studies, which are impossible with present magnetic imaging techniques. Magnetic imaging with spatial resolution better than 100 nm is currently performed by magnetic force microscopy (MFM). However, the MFM technique has three major drawbacks: it is hardly quantitative, invasive, and not fully "decoupled" from the AFM topography. The approach proposed in this project allows quantitative, more sensitive and non-invasive magnetic field measurements. Additionally, totally "decoupled" parallel AFM imaging is possible.

We report here on a new method to release functional SU-8 micro-components with embedded and integrated metal electrodes from the surface, based on isotropic SF6 dry etching of a poly-silicon sacrificial layer. This new fast and clean release technique allows a reproducible fabrication of SU-8 structures with integrated metal devices without risk of damage or altering their performance.

The continuous improvements in conventional lithography methods for integrated circuit hardware allow further device shrinkage deep into the sub-micron dimensions. Two major drawbacks of these high-end patterning methods based on photoresist technologies are i) their high process costs and ii) their limited process versatility. Increased flexibility, however, becomes important for the engineering of advanced nano/micro-electro-mechanical systems (NEMS/MEMS), such as ultra-thin solid-state membrane or cantilever devices, polymer-based devices, micro/nanofluidics, and bio-analytical systems. Often standard lithography methods cannot be applied on unconventional materials, because the surfaces to be structured are either mechanically unstable and/or (bio-) chemically functionalized. Bottom-up methods relying on self-assembling strategies are making tremendous progress to form ordered structures with nanometer precision, but they still lack control on a larger scale. The key aspect in the near future is to find new ways to tie top-down and bottom-up methods together, and to connect structures across the nano/micro gap so that the improved effects relying on nanometer dimensions can be efficiently scaled-up. Recently, a series of new, alternative surface patterning methods related to MEMS technology have been developed, such as molecule delivery via DipPen/NADIS lithography or soft-lithography, thermo-mechanical indentation by Nanoimprint lithography, or local deposition via nanostencils. Due to their versatility and multiple length-scale capabilities they have great potential to form bridges between nano and micro-world. We will present some examples of our contribution to the field of MEMS and Nanotechnology, such as large area nanostencil based on a DUV/MEMS membrane, NEMS devices, functional SAM layers for MEMS, self-assembled nanowire sensors, nanoscale Hall-probe on surface released polymer cantilevers, and focused ion beam (FIB) and focused electron beam (FEB) nanofabrication. Some of these examples are presented in detail during TNT’04.

We report here a new method to release functional SU-8 micro-cantilevers with embedded metal electrodes from the surface, based on isotropic SF6 dry etching of a poly-silicon sacrificial layer. This new fast and clean release technique allows a reproducible fabrication of SU-8 structures with integrated metal devices without risk of damage or altering their performance.

Our research is centered on the engineering of structures at the mesoscopic length scale. Patterns between the micrometer and nanometer bridge molecules and biological cells to microelectronics and microsystems, and further to the macroworld, and vice versa. Standard photo-resist based lithography and micro/nano-fabrication methods, however, reach limits when applied to advanced micro- and nano-electromechanical systems (MEMS/NEMS). New alternative nanopatterning methods, such as local vacuum deposition through nanostencils, scanning probe methods, as well as nanostrucuturing with focused ion beam (FIB), are therefore being further developed in our lab. Some of our research activities, which cover the mesoscopic length scale that can bridge the “Top-Down” engineering methods and the “Bottom-Up” self-organization strategies, will be presented in this talk.

Micro-Hall sensors made of Bi have been previously fabricated and have shown interesting properties for magnetic field detection. In this project, new ideas to improve the performance of these Hall sensors and to explore ferromagnetic metals are developed. Ni and Bi protected with Al2O3 are used as materials to realize the sensing element. Different thicknesses between 10 and 100 nm of Ni and Bi are deposited by evaporation and structured with lift-off. The electrodes are made of Ti/Al. A Pt thin film is deposited between the sensor and the electrodes to improve electrical contact. Two SU-8 layers are structured with the standard photolithography process to build a cantilever and a carrier chip. Release of the sensor chip is performed with ICP SF6 plasma to etch the poly-Silicon layer.