L. R. Harriott

Scanning Hall probe microscopy

We describe the implementation of a scanning Hall probe microscope of outstanding magnetic field sensitivity (∼0.1 G) and unprecedented spatial resolution (∼0.35 μm) to detect surface magnetic fields at close proximity to a sample. Our microscope combines the advantages of a submicron Hall probe fabricated on a GaAs/Al0.3Ga0.7As heterostructure chip and the scanning tunneling microscopy technique for precise positioning. We demonstrate its usefulness by imaging individual vortices in high Tc La1.85Sr0.15CuO4 films and superconducting networks, and magnetic bubble domains.

Limits of lithography

Lithography technology has been one of the key enablers and drivers for the semiconductor industry for the past several decades. Improvements in lithography are responsible for roughly half of the improvement in cost per function in integrated circuit (IC) technology. The underlying reason for the driving force in semiconductor technology has been the ability to keep the cost for printing a silicon wafer roughly constant while dramatically increasing the number of transistors that can be printed per chip. ICs have always been printed optically with improvements in lens and imaging material technology along with decreases in wavelength used fueling the steady improvement of lithography technology. The end of optical lithography technology has been predicted by many and for many years. Many technologies have been proposed and developed to improve on the performance of optical lithography, but so far none has succeeded. This has been true largely because it has always been more economical to push incremental improvements in the existing optical technology rather than displace it with a new one. At some point in time, the costs for pushing optical lithography technology beyond previously conceived limits may exceed the cost of introducing new technologies. In this paper the author examines the limits of lithography and possible future technologies from both a technical and economic point of view.

Scattering with angular limitation projection electron beam lithography for suboptical lithography

There are several candidate lithography technologies for the postoptical era early in the next century. The scattering with angular limitation projection electron-beam lithography (SCALPEL) approach combines the high resolution and wide process latitude inherent in electron beam lithography with the throughput of a parallel projection system. In the SCALPEL system, a mask consisting of a low atomic number membrane and a high atomic number pattern layer is uniformly illuminated with high energy (100 keV) electrons. The entire mask structure is essentially transparent to the electron beam so very little of the beam energy is deposited in it. The portions of the beam which pass through the high atomic number pattern layer are scattered through angles of a few milliradians. An aperture in the back focal plane of the electron projection imaging lenses stops the scattered electrons and produces a high contrast image at the plane of the semiconductor wafer. This article describes how a lithography system based o...

Micromachining of integrated optical structures

Three‐dimensional features have been milled into optical materials by scanning a submicron focused gallium ion beam. Different shapes are obtained using computer controlled beam placement and dwell time during sputtering. We have used this technique to create micron‐sized facets and reflectors in the active areas of semiconductor lasers. Light output and quantum efficiency measurements indicate that these features are of sufficient quality to fabricate monolithic integrated optical devices. Some of the applications currently being investigated are laser‐detector pairs, coupled cavity lasers, lasers with integral lenses, distributed feedback lasers, confocal cavities, and laser cavity length tuning.

High‐resolution patterning of high Tc superconductors

We have used a 20 keV Ga focused ion beam to pattern superconducting submicrometer bridge structures in thin films of Ba2YCu3O7 material by physical sputtering. The technique can produce structures down to 0.5 μm or less in epitaxial films with no degradation in superconducting transition temperature (Tc) or critical current density (Jc). Photolithography was used to define a coarse pattern of 20‐μm‐wide and 50‐μm‐long strips, each wired for four‐terminal resistance measurements. Submicrometer constrictions were then milled by the focused ion beam to form weak‐link junctions with roughly 0.3 μm separating the superconducting banks. We have demonstrated that focused ion beam micromachining is capable of producing submicrometer‐sized superconducting structures.We have used a 20 keV Ga focused ion beam to pattern superconducting submicrometer bridge structures in thin films of Ba2YCu3O7 material by physical sputtering. The technique can produce structures down to 0.5 μm or less in epitaxial films with no degradation in superconducting transition temperature (Tc) or critical current density (Jc). Photolithography was used to define a coarse pattern of 20‐μm‐wide and 50‐μm‐long strips, each wired for four‐terminal resistance measurements. Submicrometer constrictions were then milled by the focused ion beam to form weak‐link junctions with roughly 0.3 μm separating the superconducting banks. We have demonstrated that focused ion beam micromachining is capable of producing submicrometer‐sized superconducting structures.

In situ pattern formation and high quality overgrowth by gas source molecular beam epitaxy

We demonstrate a combination of focused Ga beam writing and dry etching techniques to pattern InP wafers in a common vacuum chamber. Surface steps on the order of 1000 A can be efficiently prepared using moderate Ga ion fluences. The implanted areas exhibit a faster etch rate, even for Ga doses below ∼1014 cm−2. The implantation damage is removed by the low‐energy Cl‐assisted ion beam etching as shown by the high quality of p‐n junctions grown on etched surfaces. GaInAs/InP heterostructures grown on in situ patterned substrates show excellent morphology and high luminescence efficiency.

Stochastic scattering in charged particle projection systems: A nearest neighbor approach

Image blurring as a result of stochastic particle–particle interactions has been investigated for projection electron‐ and ion‐beam lithography systems. A comparative analysis of the currently available analytical theories is presented. The results from these theories are also compared with Monte Carlo simulation results and experimental data. Large variations in results and serious disagreements between the different theoretical approaches are found. We have formulated a new theory on the basis of a simple, analytical approach that overcomes most of the difficulties experienced by earlier theories with two key concepts: consideration of nearest‐neighbor interactions only, and a randomization length, over which the interactions are correlated. Our model displays satisfactory functional and numerical agreement with Monte Carlo simulation results over a large range of beam currents, as well as with the only available experimental data. The physical basis of our model also enables us to understand the origin...

Focused ion beam stimulated deposition of aluminum from trialkylamine alanes

Focused ion beam stimulated deposition of aluminum from trimethylamine alane, a white solid, and triethylamine alane, a colorless liquid, is reported. Initiation of growth on Si and SiO2 substrates is enhanced by in situ sputter cleaning of the surface with the Ga+ beam prior to introduction of the metallo‐organic. Alternatively gas phase chemical activation with a silane coupling agent can enhance nucleation. Uniform nucleation on Al surfaces does not require any pretreatment. The Al features are electrically conducting, but incorporation of C and N from the amines leads to a resistivity approximately 300 times that of bulk Al. A qualitative model is presented that describes the balance condition for net material deposition as opposed to sputtering in terms of precursor flux and sticking probability as well as ion beam current density and beam scanning parameters. Film morphology and composition are also discussed.

Designing CMOS/molecular memories while considering device parameter variations

In recent years, many advances have been made in the development of molecular scale devices. Experimental data shows that these devices have potential for use in both memory and logic. This article describes the challenges faced in building crossbar array-based molecular memory and develops a methodology to optimize molecular scale architectures based on experimental device data taken at room temperature. In particular, issues in reading and writing such as memory using CMOS are discussed, and a solution is introduced for easily reading device conductivity states (typically characterized by very small currents). Additionally, a metric is derived to determine the voltages for writing to the crossbar array. The proposed memory design is also simulated with consideration to device parameter variations. Thus, the results presented here shed light on important design choices to be made at multiple abstraction levels, from devices to architectures. Simulation results, incorporating experimental device data, are presented using Cadence Spectre.

Decomposition of palladium acetate films with a microfocused ion beam

Submicron Pd features have been fabricated on Si and SiO2 substrates by microfocused Ga+ ion beam exposure of spin‐on palladium acetate, [Pd(O2CCH3)2]3, films. Electrical conductivity measurements were made on the exposed features as a function of ion dose for nominal linewidths of 1 and 10 μm. The sheet conductivity in the two cases is comparable and increases dramatically in the exposure dose range between 2×1014 and 5×1014 ions/cm2. The conductivity of the exposed lines is further increased after heating in a hydrogen furnace. Potential applications of this process include mask repair and integrated circuit modification.