Paul M. Dentinger

High aspect ratio patterning with a proximity ultraviolet source

A considerably less expensive option to synchrotron exposures of thick film photoresists is to use a proximity UV exposure tool. The use of UV radiation, however, is potentially limited by aerial image degradation as the image propagates through the thick photoresist layer. In addition to diffraction, run-out from uncollimated light or absorption in the resist easily dominate the aerial image problems for thick films. Alternatively, thick film lithography may be limited by the ability of present photoresists to print the aerial image. With some formulations, calculated line-and-space theoretical limits can be printed for 150-μm thick films. Typical aspect ratios for films greater than 200-μm thick exceed 20:1 with good process linearity and sidewall profiles and extension to 700-μm thick films is shown. We show that present commercial formulations of photoresists and not aerial image are likely the limiting factor in the practical resolution of final features from proximity UV printing. In particular, the redeposition of partially dissolved resist during drying after development leads to feature degradation. Released electrodeposited metal parts are also produced and demonstrated.

Photoresist film thickness for extreme ultraviolet lithography

The thickness of the photoresist directly impacts the etch stand off and may impact the number of defects in the spin- coated film. However, the maximum imaging layer thickness for extreme ultraviolet lithography (EUVL) is limited by absorption of the radiation. Attenuation in photoresist materials at relevant EUV wavelengths was calculated with atomic extinction coefficients provided from Henke et al. The calculations indicated that photoresist materials have an optical density (O.D.) of 4.0 micrometer-1 (base e) so that 100 nm thick imaging layers have approximately 67% transmission at 13.4 nm wavelength. Using Prolith/3DTM (Finle Technologies, Austin, TX) simulations of the effect of highly attenuating materials on sidewall slope were done and shown to be small. Imaging experiments were performed in a commercially-available DUV resist material on the 10 X II microstepper and with an improved EUV resist formulation. The imaging results agreed well with the calculations. Top down and cross-section images showed good sidewall profiles in 95 nm thick films at the nominal dose because over 68% of the energy was transmitted through the film. When the thickness of the film was increased, the dose was increased slightly to compensate for the absorption while good sidewall profiles and linearity were maintained. Photoresist thicknesses as high as 145 nm were imaged with a 35% increase in dose. Results are also shown for a single layer resist exposed at 175 nm thickness with only slight sidewall degradation. It is shown that the imaging layer thickness for 13.4 nm lithography is likely to be 120 +/- 15 nm. If 11.4 nm wavelength radiation is chosen for EUV lithography, it is shown that thicknesses of 170 nm is possible.

Photospeed considerations for extreme ultraviolet lithography resists

Photospeed is a prime consideration for wafer throughput of extreme ultraviolet (EUV) lithography. Faster photoresists additionally provide system advantages such as less thermal management of the mirrors and mask, and potentially increased component lifetimes. However, there are some predicted detrimental considerations when using fast photoresists such as shot noise. In this article, we report details of the formulation of photoresists exposed at 248 nm and identical formulations exposed at 13.4 nm. Compositions typically contained co- or terpolymers of poly-4-hydroxystyrene, t-butyl acrylate and as an option, styrene, a photoacid generator of bis-t-butylphenyl iodonium camphorsulfonate or perfluoroxbenzensulfonate and tetrabutyalummonium or triphenylsulphonium hydroxide base. With these formulations, the EUV photospeed was varied from 34 to 2.7 mJ/cm2. Scanning electron microscope analysis was done for all wafers at Sandia using GORA software to determine the line-edge roughness (LER). Identical formul...

Electrical breakdown across micron scale gaps in MEMS structures

Large voltage differences between closely spaced MEMS structures can cause electrical breakdown and destruction of devices 1-2. In this study, a variety of planar thin film electrode configurations were tested to characterize breakdown response. All devices were fabricated using standard surface micromachining methods and materials, therefore our test results provide guidelines directly applicable to thin film structures used in MEMS devices. We observed that planar polysilicon structures exhibit breakdown responses similar to published results for larger metal electrode configurations 3-6. Our tests were performed in air at atmospheric pressure, with air gaps ranging from 0.5 μm to 10 μm. Our results show a sharp rise in breakdown level following increases in gap width up to about 3 μm, a plateau region between 3 μm and 7 μm, and breakdown in gaps over 7 μm following the Paschen curve. This profile indicates an avalanche breakdown process in large gaps, with a transition region to small gaps in which electrode vaporization due to field emission current is the dominant breakdown process. This study also provides information on using multiple-gap configurations, with electrically floating regions located near the energized electrodes, and the added benefit this method may provide for switching high voltage with MEMS devices. In multiple-gap configurations, we noted a transition between direct tip to tip breakdown across electrode gaps of 40 μm, and a preferential breakdown path through the electrically floating contact head region for electrode gaps over 100 μm.

Microwave dissipation in arrays of single-wall carbon nanotubes

The transmission and reflection scattering parameters of arrays of single-wall carbon nanotubes (SWCNTs) directly assembled onto coplanar waveguides (CPWs) have been measured from 0.01to50GHz at room temperature. Typical arrays consisted of roughly ∼103 SWCNTs aligned parallel to the electric field polarization of the propagating field. Scattering parameters were measured on CPWs both before and after SWCNT assembly, allowing separation of SWCNT effects from the characteristics of the bare CPWs. Additional frequency-dependent power dissipation was consistently observed after assembly of SWCNT arrays.

Assembly and electrical characterization of DNA-wrapped carbon nanotube devices

In this article we report on the electrical characteristics of single wall carbon nanotubes (SWCNTs) wrapped with single-stranded deoxyribonucleic acid (ssDNA). We fabricate these devices using a solution-based method whereby SWCNTs are dispersed in aqueous solution using 20-mer ssDNA, and are placed across pairs of Au electrodes using alternating current dielectrophoresis (ACDEP). In addition to current voltage characteristics, we evaluate our devices using scanning electron microscopy and atomic force microscopy. We find that ACDEP with ssDNA based suspensions results in individual SWCNTs bridging metal electrodes, free of carbon debris, while similar devices prepared using the Triton X-100 surfactant yield nanotube bundles, and frequently have carbon debris attached to the nanotubes. Furthermore, the presence of ssDNA around the nanotubes does not appear to appreciably affect the overall electrical characteristics of the devices. In addition to comparing the properties of several devices prepared on no...

Extreme ultraviolet lithography based nanofabrication using a bilevel photoresist

We describe the use and characterization of a bilevel photoresist for extreme ultraviolet lithography (EUVL). The bilevel photoresist consists of a combination of a commercially available polydimethylglutarimide (PMGI) bottom layer and an experimental EUVL photoresist top (imaging) layer. We measure the sensitivity of PMGI to EUV exposure dose as a function of photoresist prebake temperature, and using this data, optimize a metal liftoff process. Reliable fabrication of 700 A thick Au structures with sub-1000 A critical dimensions is achieved, even without the use of a Au adhesion layer, such as Ti. Using the bilevel photoresist process, we fabricate an electrode array test structure, designed for electrical characterization of molecules and nanocrystals.

Low-power electrothermal actuation for microelectromechanical systems

Abstract. Electrothermal actuation has been used in microelectrome-chanical systems where low actuation voltage and high contact force arerequired. Power consumption to operate electrothermal actuators hastypically been higher than with electrostatic actuation. A method of de-signing and processing electrothermal actuators is presented that leadsto an order of magnitude reduction in required power while maintainingthe low voltage, high force advantages. The substrate was removed be-neath the actuator beams, thereby discarding the predominant powerloss mechanism and reducing the required actuation power by an orderof magnitude. Measured data and theoretical results from electrother-mally actuated switches are presented to confirm the method.

Electrical Breakdown Response for Multiple-Gap MEMS Structures

We characterize the electrical breakdown response for planar structures, fabricated using microelectromechanical systems (MEMS) methods and materials, to enable design of high voltage microswitches. Electrode configurations that use multiple air gaps provide voltage division between electrodes and allow large voltage holdoff values in microswitch contact configurations with short actuation distances. The comparatively large benefits gained from very small air gaps (4 to 7 mum) help to enable high holdoff values, particularly when multiple gaps in this range are added in series. The capacitive effect in multiple gaps can lower breakdown levels, but sufficient electrode spacing reduces this effect

Robustness of nanotube electronic transport to conformational deformations

We present experimental observation and theoretical analysis of looping carbon nanotubes connecting two electrodes. The measured conductance of the nanotubes is not strongly affected by the presence of these conformational defects, a result that is confirmed by quantum transport calculations. Our work indicates that solution-based fabrication methods for carbon nanotube devices can have high conformational defect tolerance, except for defects with 5–10nanometer bending radius.