Richard McWilliam

Nonplanar photolithography with computer-generated holograms.

We outline a method for accomplishing photolithography on grossly nonplanar substrates. First we compute an approximation of the diffraction pattern that will produce the desired light-intensity distribution on the substrate to be patterned. This pattern is then digitized and converted into a format suitable for manufacture by a direct-write method. The resultant computer-generated hologram mask is then used in a custom alignment tool to expose the photoresist-coated substrate. The technique has many potential applications in the packaging of microelectronics and microelectromechanical systems.

Designing convergent cellular automata.

Cellular automata (CA) have been used by biologists to study dynamic non-linear systems where the interaction between cell behaviour and end-pattern is investigated. It is difficult to achieve convergence of a CA towards a specific static pattern and a common solution is to use genetic algorithms and evolve a ruleset that describes cell behaviour. This paper presents an alternative means of designing CA to converge to specific static patterns. A matrix model is introduced and analysed then a design algorithm is demonstrated. The algorithm is significantly less computationally intensive than equivalent evolutionary algorithms, and not limited in scale, complexity or number of dimensions.

Fabrication of a 3D electrically small antenna using holographic photolithography

We describe the novel fabrication of a 3D electrical small antenna and its subsequent characterization. The patterning of meander lines conformed onto a hemispherical substrate is achieved by 3D holographic photolithography, which uses time-division multiplexing of a series of iteratively optimized computer-generated holograms. The meander lines have a line width of 100 µm and line separation of 400 µm, with a line pitch of 500 µm and a total meander length of 145 mm. The working frequency is found to be 2.06 GHz, with an efficiency of 46%. This work demonstrates a new method for the fabrication of 3D conformal antennas.

Concepts of Self-Repairing Systems

Systems fail. Period. No matter how much planning and fault analysis is performed, it is impossible to create a perfectly reliable machine. The existing approach to improving reliability invariably involves advances in fault prediction and detection to include specific mechanisms to overcome a particular failure or mitigate its effect. While this has gone a long way in increasing the operational life of a machine, the overall complexity of systems has improved sharply, and it is becoming more and more difficult to predict and account for all possible failure modes. What is discussed here is a possible shift in approach from specific repair strategies to autonomous self-repair. Rather than focusing on mitigating or reducing the probability of failure, the focus is instead on what can be done to correct a failure that will invariably occur at some point during operation. By taking this approach, it is not just expected failure that can be designed for, unexpected failure modes are also inherently compensated for, extending the potential life of a system and reducing the need for through-life servicing.

Controlled-width track in through silicon via using 3D holographic photolithography with modified electrodepositable photoresist

We present a novel lithographic process for patterning controlled-width tracks onto anisotropically micromachined silicon. The technique is based on the use of computer-generated holographic masks with a custom alignment and exposure tool. Experimental and simulation results are presented. 3D holographic photolithography significantly reduces the problem normally present with photolithography on non-planar surfaces—namely diffractive line broadening. A negative-acting electrodepositable photoresist (InterVia 3D-N) is used in the study. Its deposition onto the 3D substrate is optimized by modification of coating temperature and thickness and of pre-exposure bake conditions. We show the successful patterning of a constant-width 8 µm line down the sloping sidewall of a 500 µm thick silicon wafer. This is beyond the conventional resolution limit and indicates the potential of the technique for realizing high-density vertical routing in electronic packages and MEMS.

Photolithographic patterning of bihelical tracks onto conical substrates

We demonstrate the direct photolithographic patterning of a grossly nonplanar substrate by creating 62-m helical tracks on a 22- mm-high cone. The projection of focused light onto the 3-D surface is achieved using a computer-generated hologram CGH suitably illumi- nated so as to create the required pattern on the photoresist-coated surface. The approach adopted forms the basis of a novel method for patterning nonplanar structures. We address the key challenges encoun- tered for the implementation of holographic photolithography in three dimensions, including mask design and manufacture, exposure compen- sation, mask alignment, and chemical processing. Control of linewidth and resolution over the nonplanar surface is critical. We describe the methods adopted and critically assess the structures created by this pro- cess. The bihelical cone is representative of a broadband, high- frequency coil-like structure, known in wireless communications as a log- periodic antenna.

Three-dimensional holographic lithography by an iterative algorithm

We have applied an iterative algorithm for hologram design with multiple output image planes arranged in close proximity to create continuous patterns within an imaging volume. These holograms have been designed for photolithography on three-dimensional surfaces. The influence of simulated image plane separation on the final image, and its suitability for lithography, is assessed. Results are presented and the most suitable case is demonstrated experimentally.

Photolithography on Grossly Non-Planar Substrates

Within the electronics industry, photolithography is the primary technique by which patterns are transferred from mask to substrate. The substrates are either semiconductor wafers or printed circuit boards, both of which are nominally flat. The growth of micro-electro-mechanical and micro-electro-opto-mechanical systems (MEMS, MOEMS) and the search for higher-density electronics packaging solutions is leading to the requirement to pattern fine features onto grossly non-planar substrates. Standard photolithographic techniques cannot be used with these surfaces because the unavoidably large gap between mask and substrate allows diffractive line broadening, with a consequent loss of resolution. In this paper, we outline a method for realising photolithography on grossly non-planar substrates by using computer-generated holographic (CGH) masks. The technique that we describe enables photolithography to be realised in three dimensions, leading to the potential for a wide range of novel microelectronics packaging schemes to be realised

High-contrast pattern reconstructions using a phase-seeded point CGH method.

A major challenge encountered in digital holography applications is the need to synthesize computer-generated holograms (CGHs) that are realizable as phase-only elements while also delivering high quality reconstruction. This trade-off is particularly acute in high-precision applications such as photolithography where contrast typically must exceed 0.6. A seeded-phase point method is proposed to address this challenge, whereby patterns composed of fine lines that intersect and form closed shapes are reconstructed with high contrast while maintaining a phase-only CGH. The method achieves superior contrast to that obtained by uniform or random seeded-phase methods while maintaining computational efficiency for large area exposures. It is also shown that binary phase modulation achieves similar contrast performance with benefits for the fabrication of simpler diffractive optical elements.

Applying field mapping refractive beam shapers to improve holographic techniques

Performance of various holographic techniques can be essentially improved by homogenizing the intensity profile of the laser beam with using beam shaping optics, for example, the achromatic field mapping refractive beam shapers like πShaper. The operational principle of these devices presumes transformation of laser beam intensity from Gaussian to flattop one with high flatness of output wavefront, saving of beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field, negligible residual wave aberration, and achromatic design provides capability to work with several laser sources with different wavelengths simultaneously. Applying of these beam shapers brings serious benefits to the Spatial Light Modulator based techniques like Computer Generated Holography or Dot-Matrix mastering of security holograms since uniform illumination of an SLM allows simplifying mathematical calculations and increasing predictability and reliability of the imaging results. Another example is multicolour Denisyuk holography when the achromatic πShaper provides uniform illumination of a field at various wavelengths simultaneously. This paper will describe some design basics of the field mapping refractive beam shapers and optical layouts of their applying in holographic systems. Examples of real implementations and experimental results will be presented as well.