Raul E. Riveros

Novel ultra-lightweight and high-resolution MEMS x-ray optics

We have been developing ultra light-weight X-ray optics using MEMS (Micro Electro Mechanical Systems) technologies.We utilized crystal planes after anisotropic wet etching of silicon (110) wafers as X-ray mirrors and succeeded in X-ray reflection and imaging. Since we can etch tiny pores in thin wafers, this type of optics can be the lightest X-ray telescope. However, because the crystal planes are alinged in certain directions, we must approximate ideal optical surfaces with flat planes, which limits angular resolution of the optics on the order of arcmin. In order to overcome this issue, we propose novel X-ray optics based on a combination of five recently developed MEMS technologies, namely silicon dry etching, X-ray LIGA, silicon hydrogen anneal, magnetic fluid assisted polishing and hot plastic deformation of silicon. In this paper, we describe this new method and report on our development of X-ray mirrors fabricated by these technologies and X-ray reflection experiments of two types of MEMS X-ray mirrors made of silicon and nickel. For the first time, X-ray reflections on these mirrors were detected in the angular response measurements. Compared to model calculations, surface roughness of the silicon and nickel mirrors were estimated to be 5 nm and 3 nm, respectively.

Magnetic field assisted finishing of ultra-lightweight and high-resolution MEMS X-ray micro-pore optics

In recent years, X-ray telescopes have been shrinking in both size and weight to reduce cost and volume on space flight missions. Current designs focus on the use of MEMS technologies to fabricate ultra-lightweight and high-resolution X-ray optics. In 2006, Ezoe et al. introduced micro-pore X-ray optics fabricated using anisotropic wet etching of silicon (110) wafers. These optics, though extremely lightweight (completed telescope weight 1 kg or less for an effective area of 1000 cm2), had limited angular resolution, as the reflecting surfaces were flat crystal planes. To achieve higher angular resolution, curved reflecting surfaces should be used. Both silicon dry etching and X-ray LIGA were used to create X-ray optics with curvilinear micro-pores; however, the resulting surface roughness of the curved micro-pore sidewalls did not meet X-ray reflection criteria of 10 nm rms in a 10 μm2 area. This indicated the need for a precision polishing process. This paper describes the development of an ultra-precision polishing process employing an alternating magnetic field assisted finishing process to polish the micro-pore side walls to a mirror finish (< 4 nmrms). The processing principle is presented, and a polishing machine is designed and fabricated to explore the feasibility of this polishing process as a possible method for processing MEMS X-ray optics to meet X-ray reflection specifications.

Development of an alternating magnetic-field-assisted finishing process for microelectromechanical systems micropore x-ray optics.

X-ray astronomy research is often limited by the size, weight, complexity, and cost of functioning x-ray optics. Micropore optics promises an economical alternative to traditional (e.g., glass or foil) x-ray optics; however, many manufacturing difficulties prevent micropore optics from being a viable solution. Ezoe et al. introduced microelectromechanical systems (MEMS) micropore optics having curvilinear micropores in 2008. Made by either deep reactive ion etching or x-ray lithography, electroforming, and molding (LIGA), MEMS micropore optics suffer from high micropore sidewall roughness (10-30nmrms) which, by current standards, cannot be improved. In this research, a new alternating magnetic-field-assisted finishing process was developed using a mixture of ferrofluid and microscale abrasive slurry. A machine was built, and a set of working process parameters including alternating frequency, abrasive size, and polishing time was selected. A polishing experiment on a LIGA-fabricated MEMS micropore optic was performed, and a change in micropore sidewall roughness of 9.3+/-2.5nmrms to 5.7+/-0.7nmrms was measured. An improvement in x-ray reflectance was also seen. This research shows the feasibility and confirms the effects of this new polishing process on MEMS micropore optics.

Novel ultra-lightweight and High-resolution MEMS X-ray optics for space astronomy

We report a novel micromachined X-ray optics using DRIE formed micro mirrors for future space astronomical missions. We have fabricated a test optics and measured its imaging quality using X-rays. We have successfully verified the X-ray focusing using this type of optics for the first time in the world. The obtained angular resolution was about 20 arcmin and 3.1 mm. This result was consistent with the expected angular resolution based on the surface roughness of the DRIE formed side walls. To achieve a better angular resolution, we will condition annealing and magnetic field assisted finishing processes.

Progress with MEMS x-ray micro pore optics

Our development of ultra light-weight X-ray micro pore optics based on MEMS (Micro Electro Mechanical System) technologies is described. Using dry etching or X-ray lithography and electroplating, curvilinear sidewalls through a flat wafer are fabricated. Sidewalls vertical to the wafer surface are smoothed by use of high temperature annealing and/or magnetic field assisted finishing to work as X-ray mirrors. The wafer is then deformed to a spherical shape. When two spherical wafers with different radii of curvature are stacked, the combined system will be an approximated Wolter type-I telescope. This method in principle allows high angular resolution and ultra light-weight X-ray micro pore optics. In this paper, performance of a single-stage optic, coating of a heavy metal on sidewalls with atomic layer deposition, and assembly of a Wolter type-I telescope are reported.

Magnetic Field Assisted Finishing of Silicon MEMS Micro-Pore X-Ray Optics

An alternating magnetic field assisted finishing (MAF) technique has been developed to finish the 5–20 μm wide pore sidewalls of micro-pore X-ray focusing optics fabricated using micro-electro-mechanical systems (MEMS) techniques. To understand the material removal mechanism, this MAF technique is used to finish a silicon MEMS micro-pore X-ray optic that had previously undergone a hydrogen annealing treatment. Compared to the unfinished surface, distinctive surface features are observed on the finished surfaces using scanning electron microscopy, optical profilometry, and atomic force microscopy. This demonstrates the finishing characteristics and reveals the material removal mechanism on the nanometer scale. Moreover, the representative unfinished and finished micro-pore sidewall surfaces show a reduction in roughness due to finishing from 1.72 to 0.18 nm Rq.Copyright

Extension of a Microscale Indentation Fracture Model to Nanoscale Contact in Purview of Mechanical Nanofabrication Processes

In this work, we investigate the extension of the Lawn and Evans indentation fracture model, developed primarily for microscale contact, to nanoscale contacts. Systematic nanoindentation fracture experiments are performed on Si (100) using a sharp diamond cube corner (radius, r = 32 nm) indenter as a function of load, load cycles, contact dimension, and contact separation. Atomic force microscopy is used to image and measure contact deformation and fracture. The experimental results show that the threshold load for fracture was 290 μN, which is lower than previously reported. Adjacent indents separated by less than three times the radius of each indent were observed to interact with each other, such that second indents were consistently deeper than the first at the same loads. There was an increase in crack length for pairs of indents that were separated by equally small distances (<3 indentation radii). These results have clear implications for nanofabrication where stress field interactions impose limits on the closeness (resolution) to which features can be generated and to free abrasive machining where stress field interactions enhance the ability to machine below the threshold load.Copyright

Measurement of the taper angle and X-ray reflectivity of MEMS-based silicon mirrors

A taper angle and X-ray reflectivity of MEMS-Based silicon X-ray mirrors is quantitatively measured using a parallel X-ray beam at Al K<inf>α</inf> 1.49 keV.

Progress on the magnetic field-assisted finishing of MEMS micropore x-ray optics

Microelectromechanical systems (MEMS) micropore X-ray optics were proposed as an ultralightweight, high- resolution, and low cost X-ray focusing optic alternative to the large, heavy and expensive optic systems in use today. The optics monolithic design which includes high-aspect-ratio curvilinear micropores with minimal sidewall roughness is challenging to fabricate. When made by either deep reactive ion etching or X-ray LIGA, the micropore sidewalls (re ecting surfaces) exhibit unacceptably high surface roughness. A magnetic eld-assisted nishing (MAF) process was proposed to reduce the micropore sidewall roughness of MEMS micropore optics and improvements in roughness have been reported. At this point, the best surface roughness achieved is 3 nm Rq on nickel optics and 0.2 nm Rq on silicon optics. These improvements bring MEMS micropore optics closer to their realization as functional X-ray optics. This paper details the manufacturing and post-processing of MEMS micropore X-ray optics including results of recent polishing experiments with MAF.

X-ray imaging test for a single-stage MEMS X-ray optical system

An X-ray imaging test for an X-ray optical system based on MEMS technologies was conducted at the ISAS 30 m beamline. An X-ray reflection and focusing were successfully verified at Al Kα 1.49 keV for the first time. The image quality estimated as a half power diameter was ~20 arcmin. This was consistent with the angular resolution estimated from the surface roughness of 200 nm rms at 100 μm scale. In this paper, the experimental setup and the result of X-ray imaging analysis are reported.