R. L. Kubena

A low magnification focused ion beam system with 8 nm spot size

A 50 keV Ga+ beam has been focused to a spot diameter of 8 nm (full width at half‐maximum) in our two‐lens microprobe system by reducing the contributions of both chromatic aberration and the virtual ion source size to the final image size. Features as small as 6 to 8 nm were distinctly visible in scanning ion images. To our knowledge, this is the smallest focused beam of ions produced to date. The limiting resolution in 30‐nm thick films of poly(methylmethacrylate) exposed with this beam was approximately 8 to 10 nm. Effects such as ion scattering, atomic recoil, and statistical dose fluctuations during exposure are believed to set inherent limits to the lithographic resolution.

Shot-noise and edge roughness effects in resists patterned at 10 nm exposure

The experimental shot-noise effects and line-edge roughness are reported for two positive and two negative tone chemically amplified resists (IBM Apex-E, Shipley UVIIHS, IBM ENR, and Shipley SAL-601, respectively) produced by high resolution (10 nm) focused ion-beam exposure. Scanning electron micrographs at the resolution limit for each resist (50–70 nm) showed that the positive resists became negative in tone and that edge roughness was reasonable. Shot-noise effects causing arrays of 10 nm posts to print or not to print at exposure events of 7, 14, and 28 average ions per post were observed in SAL-601 and agree with Poisson statistics. Single exposure events were not observed in any resist possibly owing to the fact that the working minimum exposure level at the resolution limit of the resist material required several overlapping events to print.

Sub‐20‐nm‐wide line fabrication in poly(methylmethacrylate) using a Ga+ microprobe

A 50 keV focused Ga+ microprobe has been formed with a full width at half‐maximum of 15 nm and a 800 nm diameter at 10−5 below the peak current density. This probe was used to expose patterns in both positive and negative resist films. Features as small as 12 nm on sub‐100‐nm periods were delineated in thin poly(methylmethacrylate) (PMMA) resist on thick GaAs substrates. Although only about 25 ions irradiated each pixel, these features exhibited smooth edges. Using the ion beam exposed PMMA patterns as a mask, Al lines 50 nm wide on 85 nm periods were successfully fabricated using a lift‐off procedure.

A new tunneling-based sensor for inertial rotation rate measurements

Micro-electro-mechanical (MEM) technology promises to significantly reduce the size, weight, and cost of a variety of sensor systems. For vehicular and tactical-grade inertial navigation systems, high-performance MEM gyroscopes are required with 1 to 100°/h resolution and stability. To date, this goal has proven difficult to achieve in manufacturing for many of the previous approaches using Coriolis-based devices due, in part, to the need to precisely tune the drive and sense resonant frequencies. We have designed, fabricated, and tested a new highly miniaturized tunneling-based sensor that employs the high displacement sensitivity of quantum tunneling to obtain the desired resolution without the need for precise mechanical frequency matching. Our first tested devices with 300-μm-long cantilevers have demonstrated 27°/h/√Hz noise floors. Measurements indicate that this number can be reduced to near the thermal noise floor of 3°/h/√Hz when a closed loop servo, operating at the devices oscillation frequency, is implemented around the sensor.

A new high‐performance surface‐micromachined tunneling accelerometer fabricated using nanolithography

We have fabricated a new class of high‐performance tunneling accelerometers using surface micromachining. The accelerometer structures are fabricated on the surface of a single silicon wafer and consist of a single cantilevered beam with electrostatic deflection electrodes and a sub‐100‐nm‐diam tunneling tip underneath. The noise level resolutions in air of 100‐ and 250‐μm‐long cantilever devices are 8.3×10−4 and 8.5×10−5 g/Hz1/2 at 500 Hz, respectively. The devices are operated in a force rebalance feedback mode using a low noise automatic servo‐control circuit, providing a dynamic range of over 104 g. This new accelerometer technology provides devices with extremely high sensitivity, high bandwidth, and wide dynamic range, in an ultracompact, low‐cost package that is easily integrated with silicon control electronics.

Microelectromechanical tunneling sensor fabrication and post-processing characterization using focused ion beams

Focused ion beams(FIBs) have been previously used as tools for such diverse tasks as high-resolution lithography, where their sub-10 nm spot sizes enable the patterning of diverse nanostructures, and surface compositional analysis, where their ability to sputter material in a localized area allows discrete components of a device or circuit to be characterized. Recently, the authors have capitalized on the FIB’s versatility by using it for microelectromechanical sensor fabrication as well as post-processing device characterization. The HRL FIB nanoprobe system has been used in the fabrication of high-performance surface-micromachined accelerometers operating on the principle of tunneling between a cantilevered beam and a sub-0.1-μm-diam tip lying beneath it on a Si substrate. The 8-nm-diam FIB has been used to pattern small dots in a bilevel negative-positive resist layer which are then transferred into a Au layer to form pyramid-shaped tunneling tip structures whose narrow dimensions are essential to high device performance and stability. High-purity, contamination-free Au on both the tunneling tip and cantilever underside is also critical to high-sensitivity tunnelingdevices. Because the undersides of the beams cannot be viewed with a scanning electron microscopy, even at high mechanical tilt angles, the cantilevers must be physically peeled back in order to expose their bottom surfaces and analyze them for cleanliness. Depending on the material used for fabricating the cantilevers, the rigidity of the structures can render them difficult to bend. We have used a commercial FIBmilling system to cut through a portion of the cantilever width, thus creating a hinge, which facilitates the subsequent peeling back of the structure. Comparison of Auger spectroscopy data on peeled-back beams with and without a FIB-milled hinge shows similar surface contamination levels, indicating that redeposited material due to ion milling is localized enough to not affect the compositional analysis of the tunneling region.

30 nm resolution zero proximity lithography on high‐Z substrates

In order to fabricate masks for x‐ray lithography, there is growing interest in subtractive patterning of high atomic mass (high‐Z) materials such as tungsten or gold. Favorable writing speeds and sub‐50 nm resolution without proximity effects combine to make heavy ion focused ion beam lithography an ideal candidate for this area of nanofabrication. Using a 50 keV Ga+ beam with an 8 nm spot diameter, we have exposed a variety of proximity effect test patterns in 60 nm thick PMMA on 0.5 μm thick tungsten films. The results indicate that 30‐nm resolution or better is possible at line/space pitches as small as 80 nm. The test patterns show no apparent proximity effects at these dimensions. An anomalous ‘‘inverse proximity effect’’ was observed, and was determined to be an artifact of the scanning electron microscope technique used to observe the PMMA resist.

New miniaturized tunneling-based gyro for inertial measurement applications

Microelectromechanical (MEM) technology promises to significantly reduce the size, weight, and cost of a variety of sensor systems. For vehicular, tactical, or personal inertial/GPS navigation systems, high performance MEM gyroscopes are required with 1–100°/h resolution and stability. To date, this goal has proven difficult to achieve with low cost manufacturing for many of the previous approaches using Coriolis-based devices due, in part, to the need to precisely tune the drive and sense resonant frequencies or to employ large millimeter-size structures. We have designed, fabricated, and tested a new highly miniaturized tunneling-based gyro that employs the high displacement sensitivity of quantum tunneling to obtain the desired resolution without the need for precise mechanical frequency matching. Our first tested devices with 300-μm-long cantilevers have demonstrated 27°/h/√Hz noise floors. Measurements indicate that this number can be reduced to near the thermal noise floor of 3°/h/√Hz when a closed ...

An ion counting apparatus for studying the statistics of ion emission from liquid metal ion sources

We have constructed a new ion counting apparatus for measuring the noise properties of liquid metal ion sources operating at low extraction currents. With this apparatus we have measured the noise properties of a gallium liquid metal ion source operating at source currents of 2, 5, and 10 μA. We have found that the spectral density of the current fluctuations corresponds to ‘‘white noise,’’ which is flat from dc out to our maximum measured frequency of 400 kHz. We also find that the variance of the current fluctuations is approximately 50% to 100% greater than would be expected for pure shot noise. These results imply that the ions do not arrive at the target at totally random times, but are partially correlated, and therefore exhibit properties of a slightly bunched beam.

Low-cost tunneling accelerometer technology for high dynamic range applications

We have developed high dynamic range (10/sup 5/-10/sup 6/ gs) tunneling accelerometers that may be ideal for smart munitions applications. These highly miniaturized surface-micromachined devices can be manufactured at very low cost and integrated on chip with the control electronics. Fully integrated sensors may provide advantages in minimizing microphonics for high-g applications. Previously, we described initial test results using electrostatic forces generated by a self-test electrode located under a Au cantilever. We describe the testing of Ni and Au cantilever devices on a shaker table using a novel, low input voltage (5 V) servo controller on both printed wiring board and surface-mount control circuitry. In addition, we report our initial test results for devices packaged using a low-temperature wafer-level vacuum packaging technique for low-cost manufacturing.