John T. Hachman

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.

Transport limitations in electrodeposition for LIGA microdevice fabrication

To better understand and to help optimize the electroforming portion of the LIGA process, we have developed one and two- dimensional numerical models describing electrodeposition of metal into high aspect-ratio molds. The one-dimensional model addresses dissociation, diffusion, electromigration, and deposition of multiple ion species. The two-dimensional model is limited to a single species, but includes transport induced by forced flow of electrolyte outside the mold and by buoyancy associated with metal ion depletion within the mold. To guide model development and to validate these models, we have also conducted a series of laboratory experiments using a sulfamate bath to deposit nickel in cylindrical molds having aspect ratios up to twenty-five. The experimental results indicate that current densities well in excess of diffusion-limited currents may still yield acceptable morphologies in the deposited metal. However, the numerical models demonstrate that such large ion fluxes cannot be sustained by convection within the mold resulting from flow across the mold top. Instead, calculations suggest that the observed hundred-fold enhancement of transport probably results from natural convection within the molds and that buoyancy-driven flows may be critical to metal ion transport even in micron-scale features having very large aspect ratios. Taking advantage of this enhanced ion transport may allow order-of-magnitude reductions in electroforming times for LIGA microdevice fabrication.

Experimental Study of the Microstructure and Stress of Electroplated Gold for Microsystem Applications

Gold, used as an X-ray absorber for lithography and as a structural or conducting material in microsystems, was deposited from a commercially available sulfite electrolyte. The effects of current type and an arsenic additive on the gold microstructure and residual stress were studied. Using a simple bent strip method, good agreement with literature stress values determined via other methods was obtained. Without arsenic, residual stresses are tensile (∼50 MPa) regardless of current type. With 25 ppm arsenic, the stress is compressive for direct current (ca. -30 MPa), with pulsed current reducing the stress to <-5 MPa compressive; this has been noted previously with thallium as an additive. The addition of arsenic does not appreciably change the film grain size but does lower the twin density and modify texture. Pores were observed in all the gold films, but were larger and less concentrated for films produced with arsenic; for the case of direct current with arsenic, the pores were concentrated at the grain boundaries. Consistencies in the microstructural observations and thin film stress theories are discussed.

Resist substrate studies for LIGA microfabrication with application to a new anodized aluminum substrate

Resist substrates used in the LIGA process must provide high initial bond strength between the substrate and resist, little degradation of the bond strength during x-ray exposure, acceptable undercut rates during development and a surface enabling good electrodeposition of metals. Additionally, they should produce little fluorescence radiation and give small secondary doses in bright regions of the resist at the substrate interface. To develop a new substrate satisfying all these requirements, we have investigated secondary resist doses due to electrons and fluorescence, resist adhesion before exposure, loss of fine features during extended development and the nucleation and adhesion of electrodeposits for various substrate materials. The result of these studies is a new anodized aluminum substrate and accompanying methods for resist bonding and electrodeposition. We demonstrate the successful use of this substrate through all process steps and establish its capabilities via the fabrication of isolated resist features down to 6 µm, feature aspect ratios up to 280 and electroformed nickel structures at heights of 190 to 1400 µm. The minimum mask absorber thickness required for this new substrate ranges from 7 to 15 µm depending on the resist thickness.

LIGA: metals, plastics, and ceramics

LIGA, an acronym from the German words for Lithography, Electroforming, and Molding, is being evaluated worldwide as a method to produce microparts from engineering materials. Much of the work to date in LIGA has focused on producing metal microparts, with nickel as the most common material of choice. There is a growing interest in producing plastic parts replicated from LIGA metal masters due largely to microanalytical instrumentation and medical applications. These plastic replicates are generally made by either hot embossing or injection molding. Ceramic replication, of particular interest for high temperature applications or to produce piezoelectric or magnetic microparts, is also emerging as an area of interest. In this paper, a model of the LIGA exposure and development processes is presented along with the result of numerical optimization of mask design and process cost. The baseline processes for a cost- effective method to produce metal microparts are discussed, along with replication methods and result for plastics and ceramics.

Improvement of current distribution uniformity on substrates for microelectromechanical systems

The employment of an insulating shield for the improvement of the current distribution on 3-in. wafer substrates is considered. Numerical analysis is used to evaluate the influence of shield shape and position on the deposition uniformity, and the simulation results are compared to experimental data for nickel deposition from a Ni sulfamate bath. The use of a shield is shown to be an effective and simple way to improve current distribution uniformity, reducing the measured disparity between the average current density and the current density at the substrate center from ∼35% to less than about 10% for the cases studied.

Nucleation and Adhesion of Electrodeposited Copper on Anodized Thin-Film Aluminum for LIGA Microfabrication

The nucleation and electrodeposition of copper from a pyrophosphate electrolyte onto anodized thin-film aluminum was used to provide well-adhering electroforms in high aspect-ratio resist molds made by X-ray lithography. Cyclic voltammetry and chronoamperometry methods were employed to characterize the impact of process variables on initiating copper deposition onto an anodized aluminum film. Adhesion of the electroplated films was evaluated using a 90° peel test. Several aluminum films were studied to elucidate the influence that specific alloying elements have on the electrodeposition after anodization. The alloy content of the aluminum, specifically, small amounts of copper (1-4%), was found to significantly enhance nucleation. Under certain anodization conditions, small amounts of copper in the original aluminum film provided facile nucleation while aluminum alloys without copper could not be electroplated. The addition of copper to the aluminum alloys anodized at room temperature reduced the nucleation overpotential by more than 300 mV and increased the nuclei densities by two orders of magnitude. With sufficient adhesion of the electrodeposited metal to withstand both plated-metal stresses and planarization, through-mold electrodeposits were fabricated within resist molds ranging in height from 250 to 2800 μm, possessing features having lateral dimensions down to 10 μm.

Polymeric microfluidic devices for the monitoring and separation of water-borne pathogens utilizing insulative dielectrophoresis

We have successfully demonstrated selective trapping, concentration, and release of various biological organisms and inert beads by insulator-based dielectrophoresis within a polymeric microfluidic device. The microfluidic channels and internal features, in this case arrays of insulating posts, were initially created through standard wet-etch techniques in glass. This glass chip was then transformed into a nickel stamp through the process of electroplating. The resultant nickel stamp was then used as the replication tool to produce the polymeric devices through injection molding. The polymeric devices were made of Zeonor 1060R, a polyolefin copolymer resin selected for its superior chemical resistance and optical properties. These devices were then optically aligned with another polymeric substrate that had been machined to form fluidic vias. These two polymeric substrates were then bonded together through thermal diffusion bonding. The sealed devices were utilized to selectively separate and concentrate a variety of biological pathogen simulants and organisms. These organisms include bacteria and spores that were selectively concentrated and released by simply applying D.C. voltages across the plastic replicates via platinum electrodes in inlet and outlet reservoirs. The dielectrophoretic response of the organisms is observed to be a function of the applied electric field and post size, geometry and spacing. Cells were selectively trapped against a background of labeled polystyrene beads and spores to demonstrate that samples of interest can be separated from a diverse background. We have implemented a methodology to determine the concentration factors obtained in these devices.

Microstructure and mechanical properties of nickel microparts electroformed in replicated LIGA molds

A novel process for the rapid replication of electroforming plastic micromolds has been developed and is now being used to produce plated nickel test specimens. The process combines hot embossing or injection molding with metallic microscreens to produce sacrificial electroforming molds with conducting bases and insulating sidewalls. The replicated micromolds differ from standard LIGA molds in that the holes in the microscreen act as insulating defects in the electroforming base. The effects of such defects on the materials properties of electroformed microparts will be discussed and it will be shown that when the surface irregularities corresponding to the microscreen holes are removed, mechanical properties are experimentally indistinguishable from those found in conventionally processed LIGA specimens.

Fabrication and characterization of polymer microfluidic devices for bio-agent detection

Sandia and Lawrence Livermore National Laboratories are developing a briefcase-sized, broad-spectrum bioagent detection system. This autonomous instrument, the BioBriefcase, will monitor the environment and warn against bacterium, virus, and toxin based biological attacks. At the heart of this device, inexpensive polymer microfluidic chips will carry out sample preparation and analysis. Fabrication of polymer microfluidic chips involves the creation of a master in etched glass; plating of the master to produce a nickel stamp; large lot chip replication by injection molding; and thermal chip sealing. Since the performance and reliability of microfluidic chips are very sensitive to fluidic impedance and to electromagnetic fluxes, the microchannel dimensions and shape have to be tightly controlled during chip fabrication. In this talk, we will present an overview of chip design and fabrication. Metrology data collected at different fabrication steps and the dimensional deviations of the polymer chip from the original design will be discussed.