Jason P. Hayes

X-ray phase imaging: Demonstration of extended conditions with homogeneous objects

We discuss contrast formation in a propagating x-ray beam. We consider the validity conditions for linear relations based on the transport-of-intensity equation (TIE) and on contrast transfer functions (CTFs). From a single diffracted image, we recover the thickness of a homogeneous object which has substantial absorption and a phase-shift of --0.37 radian.

Microwave welding of polymeric-microfluidic devices

This paper describes a novel technique for bonding polymeric-microfluidic devices using microwave energy and a conductive polymer (polyaniline). The bonding is achieved by patterning the polyaniline features at the polymer joint interface by filling of milled microchannels. The absorbed electromagnetic energy is then converted into heat, facilitating the localized microwave bonding of two polymethylmethacrylate (PMMA) substrates. A coaxial open-ended probe was used to study the dielectric properties at 2.45 GHz of the PMMA and polyaniline at a range of temperatures up to 120 °C. The measurements confirm a difference in the dielectric loss factor of the PMMA substrate and the polyaniline, which means that differential heating using microwaves is possible. Microfluidic channels of 200 µm and 400 µm widths were sealed using a microwave power of 300 W for 15 s. The results of the interface evaluations and leak test show that strong bonding is formed at the polymer interface, and there is no fluid leak up to a pressure of 1.18 MPa. Temperature field of microwave heating was found by using direct measurement techniques. A numerical simulation was also conducted by using the finite-element method, which confirmed and validated the experimental results. These results also indicate that no global deformation of the PMMA substrate occurred during the bonding process.

Nanometer thickness laser ablation for spatial control of cell attachment

We demonstrate here a new method to control the location of cells on surfaces in two dimensions, which can be applied to a number of biomedical applications including diagnostic tests and tissue engineered medical devices. Two-dimensional control over cell attachment is achieved by generation of a spatially controlled surface chemistry that allows control over protein adsorption, a process which mediates cell attachment. Here, we describe the deposition of thin allylamine plasma polymer coatings on silicon wafer and perfluorinated poly(ethylene-co-propylene) substrates, followed by grafting of a protein resistant layer of poly(ethylene oxide). Spatially controlled patterning of the surface chemistry was achieved in a fast, one-step procedure by nanometer thickness controlled laser ablation using a 248 nm excimer laser. X-ray photoelectron spectroscopy and atomic force microscopy were used to confirm the production of surface chemistry patterns with a resolution of approximately 1 µm, which is significantly below the dimensions of a single mammalian cell. Subsequent adsorption of the extracellular matrix proteins collagen I and fibronectin followed by cell culture experiments using bovine corneal epithelial cells confirmed that cell attachment is controlled by the surface chemistry pattern. The method is an effective tool for use in a number of in vitro and in vivo applications.

Observation of an x-ray vortex

Phase singularities are a ubiquitous feature of waves of all forms and represent a fundamental aspect of wave topology. An optical vortex phase singularity occurs when there is a spiral phase ramp about a point phase singularity. We report an experimental observation of an optical vortex in a field consisting of 9-keV x-ray photons. The vortex is created with an x-ray optical structure that imparts a spiral phase distribution to the incident wave field and is observed by use of diffraction about a wire to create a division-of-wave-front interferometer.

Semi-automatic calibration technique using six inertial frames of reference

A triaxial accelerometer calibration technique that evades the problems of the conventional calibration method of aligning with gravity is proposed in this paper. It is based on the principle that the vector sum of acceleration from three sensing axes should be equal to the gravity vector. The method requires the accelerometer to be oriented and stationary in 6 different ways to solve for the 3 scale factors and 3 offsets. The Newton-Raphson method was employed to solve the non-linear equations in order to obtain the scale factors and offsets. The iterative process was fast, with an average of 5 iterations required to solve the system of equations. The accuracy of the derived scale factors and offsets were determined by using them to calculate the gravity vector magnitude using the triaxial accelerometer to measure gravity. The triaxial accelerometer was used to measure gravity 264 times to determine the accuracy of the 44 acceptable sets of scale factors and offsets derived from the calibrations (gravity was assumed to equal 9.8000 ms-2 during the calibration). It was found that the best calibration calculated the gravity vector magnitude to 9.8156 ± 0.4294 ms-2. This equates to a maximum of 4.5% error in terms of a constant acceleration measurement. Because of the principle behind this method, it has the disadvantage that noise/error in only one axis will cause an inaccurate determination of all the scale factors and offsets.

X-ray phase vortices: theory and experiment.

We review the current work on x-ray phase vortices. We explain the role of an x-ray vortex in phase recovery and speculate on its possible applications in other fields of x-ray optical research. We present our theoretical understanding of the structure of phase vortices and test these predictions against experiment. We present experimental observations of phase vortices with charge greater than 3 and observe that their propagation appears to be consistent with our theoretical models.

AFM-measured surface roughness of SU-8 structures produced by deep x-ray lithography

Deep x-ray lithography is a well-known technique used to pattern ultra high aspect ratio microstructures. It relies on the fact that higher energy synchrotron x-rays have the ability to penetrate millimeters of resist layers. However, the spectral shape of the beam will vary as a function of penetration depth, sometimes by design, so as to distribute the dose differently for different thickness structures and always as a result of filtering of lower energies. Some studies have shown that in PMMA sidewall roughness can be affected by spectral issues. SU-8 is now the resist of choice for certain high aspect ratio structures due to its high sensitivity and contrast. As sidewall roughness is a key parameter in several potential applications of high aspect ratio structures, we therefore investigated the surface roughness of 500 µm thick SU-8 structures exposed using beam spectra with peak energies between 3 keV and 12 keV. Results indicate that as the x-ray energy increases so too does the surface roughness. The surface roughness also increases as a function of feature depth. We attribute this to the random secondary physical processes of photo and Auger electron scattering both of which are strongly energy dependent.

Excimer laser micromachining of TiN films from chromium and copper sacrificial layers

This paper presents results on the laser micromachining of TiN films. Machining performance was evaluated in terms of patterning quality and the ability to remove TiN with minimal interference with an underlying sacrificial layer. TiN was arc-deposited onto (100) silicon substrate with chromium (Cr) and copper (Cu) sacrificial layers. Films were also deposited onto bare silicon substrates under the same conditions. These films were analysed for their composition and structure using Rutherford backscattering spectroscopy and x-ray diffraction techniques. Laser micromachining was performed using a KrF excimer laser at 248 nm. The effect of fluence and number of shots on the machined features has been investigated in detail. The patterned features were examined using optical, confocal and scanning electron microscopes. The characteristics observed were analysed and compared in all three sets of samples. The results showed selective removal of TiN films from Cr and Cu sacrificial layers under different conditions. The machining of TiN from (100) silicon showed relatively poor definition of patterned features. The analysis of these results indicated that laser machining of TiN from Cr and Cu layers is best explained using the explosion mechanism of removal.

Specification of mechanical support structures to prevent SU-8 stiction in high aspect ratio structures

Densely packed high aspect ratio structures are difficult to fabricate due to surface adhesion effects which lead to pattern collapse during processing. However, it is possible to fabricate such structures in SU-8 by means of a top-plate support member which stiffens the overall structure and prevents pattern collapse. We have fabricated a variety of structures in SU-8 up to 1.5 mm high and with feature sizes in the 10–50 µm range. We investigate theoretical, computational and experimental approaches for predicting the thickness of the top-plate supports as a function of the aspect ratio and spacing of the structures. We demonstrate the fabrication of a number of densely packed high aspect ratio structures with top-plate supports that approach the minimal thickness sufficient to prevent pattern collapse.

Disposable biochip fabrication for DNA diagnostics

The need for disposable diagnostic sensors in the health care industry has been a major factor in the development of low-cost microfluidic devices. Polymer materials have been the obvious choice due to their cost effectiveness. However, these materials often do not possess the desired properties for biochip operation, such as their high non-specific binding and poor electroosmotic flow characteristics. Various fabrication techniques have also been developed for polymeric chips over the past few years due to their incompatibility with the traditionally preferred micromachining technologies associated with glass and silicon. This paper presents a method for constructing microfluidic devices in Poly(ethylene terephthalate) (PET) using a direct-write Neodymium Yttrium Aluminium Garnet (Nd:YAG) laser system. Issues involving the operation and fabrication of such disposable devices, with particular emphasis on the development of a bio-chip for DNA diagnostics, are discussed.