Chantal G. Khan Malek

Nanometer X-ray lithography

New developments for x-ray nanomachining include pattern transfer onto non-planar surfaces coated with electrodeposited resists using synchrotron radiation x-rays through extremely high-resolution mask made by chemically assisted focused ion beam lithography. Standard UV photolithographic processes cannot maintain sub-micron definitions over large variation in feature topography. The ability of x-ray printing to pattern thin or thick layers of photoresist with high resolution on non-planar surfaces of large and complex topographies with limited diffraction and scattering effects and no substrate reflection is known and can be exploited for patterning microsystems with non-planar 3D geometries as well as multisided and multilayered substrates. Thin conformal coatings of electro-deposited positive and negative tone photoresist have been shown to be x-ray sensitive and accommodate sub-micro pattern transfer over surface of extreme topographical variations. Chemically assisted focused ion beam selective anisotropic erosion was used to fabricate x-ray masks directly. Masks with feature sizes less than 20 nm through 7 microns of gold were made on bulk silicon substrates and x-ray mask membranes. The technique is also applicable to other high density materials. Such masks enable the primary and secondary patterning and/or 3D machining of Nano-Electro-Mechanical Systems over large depths or complex relief and the patterning of large surface areas with sub-optically dimensioned features.

Mask prototyping for ultradeep x-ray lithography: preliminary studies for mask blanks and high-aspect-ratio absorber patterns

The use of hard X-ray energies for ultra-deep X-ray lithography requires a thorough re-investigation of all issues associated with the LIGA technology materials issues and processes, in particular for the manufacture of high-energy-X- ray masks. Calculations were performed to compare various mask blanks in particular thick Kapton R and thinned silicon blanks. Absorber pattern formation schemes have been investigated using UV contact printing or X-ray lithography with SU8 photoresist. SU8 photoresist also offers an improved X-ray sensitivity over PMMA resist. Resist patterns over 500 micron deep with aspect ratio over 10 and vertical sidewalls were achieved in SU-8, allowing the use of medium energy range X-rays to obtain high quality patterns of much greater resist thickness.The use of hard X-ray energies for ultra-deep X-ray lithography requires a thorough re-investigation of all issues associated with the LIGA technology materials issues and processes, in particular for the manu[1cture of high-energy-X-ray masks. Calculations were performed to compare various mask blanks, in particular thick Kapton and thinned silicon blanks. Absorber pattern formation schemes have been investigated using UV contact printing or X-ray lithography with SU8 photoresist. SU8 photoresist also offers an improved X-ray sensitivity over PMMA resist. Resist patterns over 500 micron deep with aspect ratio over 10 and vertical sidewalls were achieved in SU-8, allowing the use of medium energy range Xrays to obtain high quality patterns ofmuch greater resist thickness.

Focused ion beam direct micromachining of DOEs

We discuss here the capability of direct manufacture of various high- resolution diffractive optics, in particular regarding micromachining of DOEs in 3D. Preliminary demonstrations were made in 2-D using an automated FIB system operated at 30 KeV with a Gallium liquid metal ion source and equipped with a gas injection system (GIS). Gratings with a 20 nm line width and zone plates with 32 nm outer ring were milled in a reactive atmosphere (iodine) directly through 3.5 (mu) m and 800 nm of gold respectively. Plans for combining FIB and X-ray lithography to make diffractive optical elements (DOEs) for JPL are also mentioned.

Dimensional variation and roughness of LIGA fabricated microstructures

We have measured the dimensional variation and sidewall roughness of features on PMMA microcomponents fabricated by deep x-ray lithography in order to assess the effect of dimensional variation on subsequent assembly operations. Dimensional measurements were made using a stylus profilometer with a repeatability in step height of better than 0.01 micrometers . Roughness measurements were made with the same profilometer scanning in a direction perpendicular to the length of the parts. 22 micrometers and 54 micrometers features exhibited dimensional variations described by a Gaussian distribution with standard deviations of 0.202 micrometers and 0.381 micrometers , respectively. This corresponds to a maximum relative variation of between 0.6% and 0.9%. Sidewall roughnesses were found to be in the range of 0.02 micrometers to 0.03 micrometers , an insignificant contribution to the total variation when compared to overall dimensional variation. Several potential sources of this variation are discussed, but no single cause was identified as the source of the significant dimensional variation observed here.

High-aspect-ratio electroformed Ni-Co microstructures with improved mold adhesion using a LIGA-like process and a Novolak sublayer

This work relates to a method for increasing the adhesion of polymer resist to electrically conductive substrates which is an important step in the lithographic steps for creating high-aspect-ratio micro structures. Here we are particularly interested in plating Ni-Co into the very tall high-aspect- ratio accurately patterned polymethyl methacrylate (PMMA) micromolds. They were made by deep x-ray lithography for primary or secondary metal structures or metal mold inserts within the framework of the LIGAS process. We investigated the effect of using a Novolak intermediate layer on various substrates to consolidate the adhesion of the relatively weak PMMA-metal interface. Modifying our process by introducing this intermediate Novolak sublayer improved the adhesive properties overall throughout the whole process including the planarization step, leading to a more reliable process with better yield as well an increase in the quality of the Ni-Co parts. The increase of bond resistance to heat and x-rays was evaluated by shear stress measurements.

Analysis and enhancement of bond strength of acrylic sheets to metallic substrates for use in LIGA-type processing

In thick photoresist applications, commercially available acrylic sheets are bonded to a substrate as an alternative to the casting and in-situ polymerization of PMMA. The factors affecting the adhesion of a thick acrylic sheet to different substrates have been studied. In case of copper and titanium substrates and bond-strength can be improved by roughening the surface through chemical oxidation which then provides a mechanical interlocking between the resist and substrate surfaces. Annealing of PMMA sheet before gluing and use of adhesion promoter such as organosilane further improves the bond strength at the resist-substrate interface. The resist adhesion to various substrates is evaluated by measuring the debonded length of the acrylic sheet during a mechanical cleaving test.

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.

Maskless 3D micromachining with focused ion beam of high-resolution diffractive optics

We discuss here the capability and details of direct manufacture of various high-resolution diffractive optics using focused ion beam (FIB) micromachining. In preliminary demonstrations illustrating the capability of the technique, zone plates and gratings were milled in gold using a reactive atmosphere in an automated FIB system operating at 30 keV with a Gallium liquid metal ion source equipped with an iodine beam gas injection system (GIS). Gratings with 20 nm line width and zone plates with 32 nm outer ring were milled directly through 3.5 micrometer and 800 nm of gold respectively.

X-ray exposures of electrodeposited photoresist for conformal lithography on corrugated surfaces

Proximity printing using synchrotron x-ray lithography provides high resolution pattern transfer with large depth of field, low diffraction effects and no reflection form the substrate. Electro-plating of photo-resist allows deposition of thin, uniform films over geometrically complex and topographically diverse, electrically conductive surfaces. Two electro-deposited photoresists produced by Shipley, EAGLE 2100 ED negative tone and PEPR 2400 positive tone resist, have been tested with x-rays demonstrating micron pattern transfer over depths-of-field in fractions of millimeters.

X-ray micromachining of high-aspect-ratio MEMS using SU8 ultrathick photoresist

In this work, a negative-tone photoresist, SU8, used in UV- based micromachining of high-aspect-ratio MEMS has been tested using proximity X-ray printing. Very thick (a few hundreds of micrometers) SU8 resist layers were processed with standard cleanroom equipment and exposed with 1 - 10 keV X-rays at a beamline of the CAMD synchrotron radiation facility. It showed a large increase in sensitivity in deep X-ray lithography compared to the standard poly(methyl-methacrylate) (PMMA) resist, resulting in increased throughput potential. Resist microstructures with aspect-ratio as high as 50 (height 350 : width 7) and vertical sidewalls, were produced. The benefits of using such X-ray resist in X-ray manufacturing are discussed.In this work, a negative-tone photoresist, SU8, used in UV- based micromachining of high-aspect-ratio MEMS has been tested using proximity X-ray printing. Very thick (a few hundreds of micrometers) SU8 resist layers were processed with standard cleanroom equipment and exposed with 1 - 10 keV X-rays at a beamline of the CAMD synchrotron radiation facility. It showed a large increase in sensitivity in deep X-ray lithography compared to the standard poly(methyl-methacrylate) (PMMA) resist, resulting in increased throughput potential. Resist microstructures with aspect-ratio as high as 50 (height 350 : width 7) and vertical sidewalls, were produced. The benefits of using such X-ray resist in X-ray manufacturing are discussed.