This article describes a new negative-tone photoresist, SU-8, for ultrathick layer applications. An aspect ratio of 10:1 has been achieved using near-ultraviolet lithography in a 200-μm-thick layer. The use of this resist for building tall micromechanical structures by deep silicon reactive-ion etching and electroplating is demonstrated. Using SU-8 stencils, etched depths of ≳200 μm in Si and electroplated 130-μm-thick Au structures with near-vertical sidewalls have been achieved.

We have used a simple self-aligned process to fabricate magnetic tunnel junctions down to submicron sizes. Optical and electron-beam lithographies were used to cover a range of areas spanning five orders of magnitude. The bottom magnetic electrodes (Co or permalloy) in our junctions were exchange biased by an antiferromagnetic layer (MnFe). The top electrodes were made of soft magnetic materials (Co or permalloy). We have consistently obtained large magnetoresistance ratios (15%–22%) at room temperature and in fields of a few tens of Oe. The shape of the field response of the magnetoresistance was varied from smooth to highly hysteretic by adjusting the shape anisotropy of one junction electrode.

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A novel spectrometer is employed to study the spectrum of heavily doped quantum dots. A single-particle discrete spectrum is found to exist only in close vicinity to the Fermi energy. Levels further away are broadened beyond the average level spacing and merge to form a quasi-continuous spectrum. The broadening is traced to electron-electron interaction in the dot. For the discrete part of the spectrum, level statistics is studied as a function of magnetic field and found to agree remarkably well with recent calculations.

A near-field capacitance microscope has been demonstrated on a 25 nm scale. A resonant circuit provides the means for sensing the capacitance variations between a sub-100-nm tip and surface with a sensitivity of 1×10−19 F in a 1 kHz bandwidth. Feedback control is used to scan the tip at constant gap across a sample, providing a means of noncontact surface profiling. Images of conducting and nonconducting structures are presented.

The exposure distribution function in electron beam lithography, which is needed to perform proximity correction, is usually simulated by Monte Carlo techniques, assuming a Gaussian distribution of the primary beam. The resulting backscattered part of the exposure distribution is usually also fitted to a Gaussian term. In this paper we demonstrate a technique, using a very high contrast resist, whereby the normalized point exposure distribution can be measured experimentally, both on solid substrates which cause backscattering, and on thin substrates where backscattering is negligible. The data sets so obtained can be applied directly to proximity correction and represent the practical conditions met in pattern writing. Results are presented of the distributions obtained on silicon, gallium arsenide, and thin silicon nitride substrates at different beam energies. Significant deviations from the commonly assumed double Gaussian distributions are apparent. On GaAs substrates the backscatter distribution cannot adequately be described by a Gaussian function. Even on silicon a significant amount of exposure is found in the transition region between the two Gaussian terms. This deviation, which can be due to non-Gaussian tails in the primary beam and to forward scattering in the resist, must be taken into account for accurate proximity correction in most submicron lithography, and certainly on the sub-100 nm scale.

We have investigated the effects of particle size and aspect ratio on the hysteresis in controlled arrays of small magnetic particles. The arrays of permalloy particles were fabricated via electron-beam lithography. Each array consists of -lo6 identical, uniformly spaced particles. Hysteresis loops measured with an alternating-gradient magnetometer for particles -5-0.1 pm are presented. We find an increase in the coercive force as the particle width decreases below 0.3 pm due to a change in the switching mechanism from domain-wall nucleation and wall motion to vortex nucleation and vortex motion. A novel angular dependence of the loops is described in detail. Results from ab initio micromagnetic calculations on isolated rectangular Permalloy particles are compared, where applicable, with the measurements. We find excellent qualitative and, in selected cases, quantitative agreement between the experiments and calculations. 5262

We show that shape anisotropy can be used to control the response characteristics of magnetic tunnel junctions. By varying the junction shape, the resistance versus field curve was made to vary from a nonhysteretic linear curve with a high-field sensitivity (0.3%/Oe) to a hysteretic response curve with high squareness.