Michel Despont

SU-8: a low-cost negative resist for MEMS

This paper describes the characterization of a home-made negative photoresist developed by IBM. This resist, called SU-8, can be produced with commercially available materials. Three blends were prepared for this article and some of its optical and mechanical properties are presented. One of its numerous advantages is the broad range of thicknesses which can be obtained in one spin: from 750 nm to with a conventional spin coater. The resist is exposed with a standard UV aligner and has an outstanding aspect ratio near 15 for lines and 10 for trenches. These ratios combined with the electroplating of copper allow the fabrication of highly integrated electromagnetic coils.

The Millipede: more than one thousand tips for future AFM data storage

We report on a new atomic force microscope (AFM)-based data storage concept called the “Millipede” that has a potentially ultrahigh density, terabit capacity, small form factor, and high data rate. Its potential for ultrahigh storage density has been demonstrated by a new thermomechanical local-probe technique to store and read back data in very thin polymer films. With this new technique, 30–40-nm-sized bit indentations of similar pitch size have been made by a single cantilever/tip in a thin (50-nm) polymethylmethacrylate (PMMA) layer, resulting in a data storage density of 400–500 Gb/in. 2 High data rates are achieved by parallel operation of large two-dimensional (2D) AFM arrays that have been batch-fabricated by silicon surface-micromachining techniques. The very large scale integration (VLSI) of micro/nanomechanical devices (cantilevers/tips) on a single chip leads to the largest and densest 2D array of 32 × 32 (1024) AFM cantilevers with integrated write/read storage functionality ever built. Time-multiplexed electronics control the write/read storage cycles for parallel operation of the Millipede array chip. Initial areal densities of 100–200 Gb/in. 2 have been achieved with the 32 × 32 array chip, which has potential for further improvements. In addition to data storage in polymers or other media, and not excluding magnetics, we envision areas in nanoscale science and technology such as lithography, high-speed/large-scale imaging, molecular and atomic manipulation, and many others in which Millipede may open up new perspectives and opportunities.

High-aspect-ratio, ultrathick, negative-tone near-UV photoresist and its applications for MEMS

Abstract Detailed investigations of the limits of a new negative-tone near-UV resist (IBM SU-8) have been performed. SU-8 is an epoxy-based resist designed specifically for ultrathick, high-aspect-ratio MEMS-type applications. We have demonstrated that with single-layer coatings, thicknesses of more than 500 μm can be achieved reproducibly. Thicker resist layers can be made by applying multiple coatings, and we have achieved exposures in 1200 μm thick, double-coated SU-8 resist layers. We have found that the aspect ratio for near-UV (400 nm) exposed and developed structures can be greater than 18 and remains constant in the thickness range between 80 and 1200 μm. Vertical sidewall profiles result in good dimensional control over the entire resist thickness. To our knowledge, this is the highest aspect ratio reported for near-UV exposures and the given range of resist thicknesses. These results will open up new possibilities for low-cost LIGA-type processes for MEMS applications. The application potential of SU-8 is demonstrated by several examples of devices and structures fabricated by electroplating and photoplastic techniques. The latter is especially interesting as SU-8 has attractive mechanical properties.

A cantilever array-based artificial nose

We present quantitative and qualitative detection of analyte vapors using a microfabricated silicon cantilever array. To observe transduction of physical and chemical processes into nanomechanical motion of the cantilever, swelling of a polymer layer on the cantilever is monitored during exposure to the analyte. This motion is tracked by a beam-deflection technique using a time multiplexing scheme. The response pattern of eight cantilevers is analyzed via principal component analysis (PCA) and artificial neural network (ANN) techniques, which facilitates the application of the device as an artificial chemical nose. Analytes tested comprise chemical solvents, a homologous series of primary alcohols, and natural flavors. First differential measurements of surface stress change due to protein adsorption on a cantilever array are shown using a liquid cell.

Ultrahigh-density atomic force microscopy data storage with erase capability

We report a simple atomic force microscopy-based concept for a hard disk-like data storage technology. Thermomechanical writing by heating a Si cantilever in contact with a spinning polycarbonate disk has already been reported. Here the medium has been replaced with a thin polymer layer on a Si substrate, resulting in significant improvements in storage density. With this new medium, we achieve bit sizes of 10–50 nm, leading to data densities of 500 Gbit/in.2. We also demonstrate a novel high-speed and high-resolution thermal readback method, which uses the same Si cantilevers that are used in the writing process, and the capability to erase and rewrite data features repeatedly.

Millipede: a MEMS-based scanning-probe data-storage system

Ultrahigh storage densities of up to 1 Tbit/in./sup 2/ or more can be achieved by local-probe techniques to write, read back, and erase data in very thin polymer films. The thermomechanical scanning-probe-based data-storage concept called Millipede combines ultrahigh density, small form factor, and high data rate. After illustrating the principles of operation of the Millipede, we introduce system aspects related to the read-back process, multiplexing, and position-error-signal generation for tracking.

Nanoscale Three-Dimensional Patterning of Molecular Resists by Scanning Probes

Patterning a Molecular Glass Lithographic patterning for device fabrication is usually based on initiating polymerization reactions with photons or electrons in a molecular resist. However, patterning can be achieved by mechanically removing a hard resist with scanning probe microscopy tips, but in many cases the resolution is low and excess material is left on the surface. Pires et al. (p. 732, published online 22 April) found that thin films of organic molecules could form glasses through weak interactions and be patterned to tens of nanometers with a heated scanning probe tip. These patterns could be transferred to other substrates or sculpted into three-dimensional shapes by successive rounds of patterning. A molecular glass can be patterned to dimensions of tens of nanometers with a heated scanning probe tip. For patterning organic resists, optical and electron beam lithography are the most established methods; however, at resolutions below 30 nanometers, inherent problems result from unwanted exposure of the resist in nearby areas. We present a scanning probe lithography method based on the local desorption of a glassy organic resist by a heatable probe. We demonstrate patterning at a half pitch down to 15 nanometers without proximity corrections and with throughputs approaching those of Gaussian electron beam lithography at similar resolution. These patterns can be transferred to other substrates, and material can be removed in successive steps in order to fabricate complex three-dimensional structures.

Atomic force microscope cantilevers for combined thermomechanical data writing and reading

Heat conduction governs the ultimate writing and reading capabilities of a thermomechanical data storage device. This work investigates transient heat conduction in a resistively heated atomic force microscope cantilever through measurement and simulation of cantilever thermal and electrical behavior. The time required to heat a single cantilever to bit-writing temperature is near 1 μs and the thermal data reading sensitivity ΔR/R is near 1×10−4 per vertical nm. Finite-difference thermal and electrical simulation results compare well with electrical measurements during writing and reading, indicating design tradeoffs in power requirements, data writing speed, and data reading sensitivity. We present a design for a proposed cantilever that is predicted to be faster and more sensitive than the present cantilever.

Ultralow nanoscale wear through atom-by-atom attrition in silicon-containing diamond-like carbon

Understanding friction and wear at the nanoscale is important for many applications that involve nanoscale components sliding on a surface, such as nanolithography, nanometrology and nanomanufacturing. Defects, cracks and other phenomena that influence material strength and wear at macroscopic scales are less important at the nanoscale, which is why nanowires can, for example, show higher strengths than bulk samples. The contact area between the materials must also be described differently at the nanoscale. Diamond-like carbon is routinely used as a surface coating in applications that require low friction and wear because it is resistant to wear at the macroscale, but there has been considerable debate about the wear mechanisms of diamond-like carbon at the nanoscale because it is difficult to fabricate diamond-like carbon structures with nanoscale fidelity. Here, we demonstrate the batch fabrication of ultrasharp diamond-like carbon tips that contain significant amounts of silicon on silicon microcantilevers for use in atomic force microscopy. This material is known to possess low friction in humid conditions, and we find that, at the nanoscale, it is three orders of magnitude more wear-resistant than silicon under ambient conditions. A wear rate of one atom per micrometre of sliding on SiO(2) is demonstrated. We find that the classical wear law of Archard does not hold at the nanoscale; instead, atom-by-atom attrition dominates the wear mechanisms at these length scales. We estimate that the effective energy barrier for the removal of a single atom is approximately 1 eV, with an effective activation volume of approximately 1 x 10(-28) m.

Design of atomic force microscope cantilevers for combined thermomechanical writing and thermal reading in array operation

In thermomechanical data writing, a resistively-heated atomic force microscope (AFM) cantilever tip forms indentations in a thin polymer film. The same cantilever operates as a thermal proximity sensor to detect the presence of previously written data bits. This paper uses recent progress in thermal analysis of the writing and reading modes to develop new cantilever designs for increased speed, sensitivity, and reduced power consumption in both writing and reading operation. Measurements of cantilever electrical resistance during heating reveals physical limits of cantilever writing and reading, and verifies a finite-difference thermal and electrical simulation of cantilever operation. This work proposes two new cantilever designs that correspond to fabrication technology benchmarks. Simulations predict that the proposed cantilevers have a higher data rate and are more sensitive than the present cantilever. The various cantilever designs offer single-bit writing times of 0.2 /spl mu/s-25 /spl mu/s for driving voltages of 2-25 V. The thermal reading /spl Delta/R/R sensitivity is as high as 4/spl times/10/sup -4/ per vertical nm in near steady-state operation.