Jill M. Hruby

Masks for high aspect ratio x-ray lithography

The requirements for deep x-ray lithography (DXRL) masks are reviewed and a recently developed cost effective mask fabrication process is described. The review includes a summary of tabulated properties for materials used in the fabrication of DXRL masks. X-ray transparency and mask contrast are calculated for material combinations using simulations of exposure at the Advanced Light Source (ALS) at Berkeley, and compared to the requirements for standard x-ray lithography (XRL) mask technology. Guided by the requirements, a cost-effective fabrication process for manufacturing high contrast masks for DXRL has been developed. Thick absorber patterns () on a thin silicon wafer (m) were made using contact printing in thick positive (Hoechst 4620) and negative (OCG 7020) photoresist and subsequent gold electrodeposition. Gold was deposited using a commercially available gold sulphite bath with low current density and good agitation. The resultant gold films were fine-grained and stress-free. Replication of such masks into thick acrylic sheets was performed at the ALS.

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.

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.

Commercial importance of a unit cell: nanolithographic patenting trends for microsystems, microfabrication, and nanotechnology

Microsystems and nanosystems hold the promise of new and much more effective approaches to both commercial and national security applications. The patenting rate in nanotechnology is exploding, underscoring its commercial and scientific potential. Yet how much of this effort is focused on nanopatterning or a top-down approach to nanofabrication? Nanopatterning in semiconductor microfabrication has already furthered Moores law, facilitating the transistor as that mediums unit cell. Yet the search for a unit cell for the other two small technical markets (microsystems and the more broadbased nanotechnology) has proven much more elusive. Do nanopatterning advances hold the key to these technology bases finally obtaining a unit cell? We explore the intellectual property base of nanopatterning and how it pertains to semiconductor microfabrication, microsystems, and nanotechnology.

Overview of LIGA Microfabrication

This paper is an overview of the LIGA technique, an increasingly accepted approach for fabricating metal, ceramic or plastic microdevices. The LIGA technique was invented in Germany in the early 1980s and the acronym derives from the words LIthographie, Galvanoformung, Abformung meaning Lithography, Electroplating, and Molding in English. The paper is presented as an abbreviated set of annotated overheads used for the conference presentation and some summary remarks.

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.

Deep etch x-ray lithography at the Advanced Light Source: First results

Deep etch x‐ray lithography permits the manufacture of very accurate high‐aspect‐ratio microstructures, which can be used as master templates for subsequent replication by electroforming and/or molding processes. This allows for mass production of three‐dimensional microstructures in a variety of materials. In this article we report on the first results using x rays from the Advanced Light Source (ALS) at the Lawrence Berkeley Laboratory, as well as on the processing and technology developed to produce high‐aspect‐ratio microstructures. The first masks used were simple stencil masks chemically or laser etched in thick metal sheets. For resist, we used commercial acrylic cast sheets. Microstructures 840 μm thick were fabricated by deep x‐ray lithography and used as templates for copper electroforming. A technology for the high contrast masks required to work at these short wavelengths is being developed and a deep etch x‐ray lithography facility is under construction at the ALS.

Optimum doses and mask thickness for synchrotron exposure of PMMA resists

Minimizing mask absorber thickness is an important practical concern in producing very small features by the LIGA process. To assist in this minimization, we have developed coupled numerical models describing both the exposure and subsequent development of a thick PMMA resist. The exposure model addresses multi-wavelength, 1D x-ray transmission through multiple beam filters, through the mask substrate and absorber, and the subsequent attenuation and photon absorption in the PMMA target. The development model describes 1D dissolution of a feature and its sidewalls, taking into account the variation in absorbed dose through the PMMA thickness. These exposure and development models are coupled in a single interactive code, permitting the automated adjustment of mask absorber thickness to yield a prescribed sidewall taper or dissolution distance.We have used this tool to compute the minimum required absorber thickness for exposures performed at the ALS, SSRL and NSLS synchrotron sources. Results are presented as a function of the absorbed dose for a range of the prescribed sidewall tolerance, feature size, PMMA thickness, mask substrate thickness and the development temperature.

Fabricating subcollimating grids for an x-ray solar imaging spectrometer using LIGA techniques

We are fabricating sub-collimating X-ray grids that are to be used in an instrument for the High Energy Solar Spectroscopic Imager (HESSI), a proposed NASA mission. The HESSI instrument consists of twelve rotating pairs of high aspect ratio, high Z grids, each pair of which is separated by 1.7 meters and backed by a single Ge detector. The pitch for these grid pairs ranges from 34 micrometers to 317 micrometers with the grid slit openings being 60% of the pitch. For maximum grid X-ray absorbing with minimum loss of the solar image, the grid thickness-to-grid-slit ratio must be approximately 50:1, resulting in grid thicknesses of 1 to 10 millimeters. For our proof-of-concept grids we are implementing a design in which a 34 micrometers pitch, free-standing PMMA grid is fabricated with 20 micrometers wide slits and an 800 micrometers thickness. Stiffeners that run perpendicular to the grid are placed every 500 micrometers . After exposure and developing, metal, ideally gold, is electrodeposited into the free-standing PMMA grid slits. The PMMA is not removed and the metal in the slits acts as the X-ray absorber grid while the PMMA holds the individual metal pieces in place, the PMMA being nearly transparent to the X-rays coming from the sun. For optimum imaging performance, the root-mean-square pitch of the two grids of each pair must match to within 1 part in 10000 and simultaneous exposures of stacked sheets of PMMA have insured that this requirement is met.

Fabrication of miniaturized electrostatic deflectors using LIGA

We are currently investigating the fabrication of high precision, miniaturized, electrostatic deflectors for use in electron or ion beam micro-columns. These columns can be used in a broad array of applications including microscopy, spectroscopy and lithography. Typically, micro-columns consist of a field emitter tip, a set of micromachined miniaturized lenses and one or more electrostatic deflectors. Miniaturization of the column allows the use of simple electrostatic lenses to achieve very high performance in a package that is just a few millimeters in length. Presently, all reported microcolumns have included miniaturized but conventionally-machined octupole deflector plates. If micromachined plates are used instead, lower deflection voltage is required for deflection, and the system becomes more amenable to very high speed operation. In addition, some reduction in scan field distortion is expected. These improvements results directly from the higher degree of miniaturization, tighter dimensional control, better placement accuracy, and smoother facets offered by micromachining. Given the dimensions (100 micrometers - 1000 micrometers thick) and tolerances (1 - 10 micrometers ) required, LIGA is well suited to fabricate such miniature deflectors. This paper will describe the fabrication of the deflectors using LIGA. The Center for X-ray Optics has built an endstation at Lawrence Berkeley National Laboratorys Advanced Light Source suitable for LIGA X-ray exposures.