We have developed a silicon MEMS optical accelerometer in which the motion of the proof mass is mechanically
amplified using a V-beam mechanism prior to transduction. The output motion of the V-beam is detected using a Fabry-Pérot interferometer (FPI) which is interrogated in reflection mode via a single-mode optical fibre. Mechanical
amplification allows the sensitivity of the accelerometer to be increased without compromising the resonant frequency or
measurement bandwidth. We have also devised an all-optical method for calibrating the return signal from the FPI, based
on photothermal actuation of the V-beam structure using fibre-delivered light of a different wavelength. A finite-element
model has been used to predict the relationship between the incident optical power and the cavity length at steady state,
as well as the step response which determines the minimum time for calibration. Prototype devices have been fabricated
with resonant frequencies above 10 kHz and approximately linear response for accelerations in the range 0.01 to 15 g.
We report on a technology for multi-level microstructures manufacturing. Results are presented in the field
of multilevel diffractive optical elements (DOEs) fabrication. The DOEs presented as examples are Fresnel
lenses and Fourier computer generated holograms, calculated by means of a conventional Iterative Fourier
Transform Algorithm. The DOEs have a typical pixel dimension of 5x5 μm2 and are up to 512 by 512 pixels in
The fabrication technique is based on polymer laser ablation through a chrome-on-quartz half-tone mask
with a demagnifying high NA lens. In our case, the mask is imaged onto the polymer with a 5x, 0.13 NA
reduction lens. The experimental results are presented and discussed.
Multi-project-wafer (MPW) services provide an economical route for prototyping of new electronic circuit designs.
However, addition of MEMS functionality to MPW circuits by post-processing (also known as MEMS-last processing) is
difficult and inefficient because MPW typically yields individual dies. One solution to this problem is to embed the
MPW dies in a carrier wafer prior to MEMS processing. We have developed a process which allows 300 μm-thick
CMOS dies to be embedded in a BSOI (bonded silicon-on-insulator) carrier prior to low-temperature processing for
integration of metal MEMS. Deep reactive ion etching (DRIE) with an STS Multiplex ICP etcher is used to form
cavities in the device layer of a BSOI wafer. By adjusting the passivation and etching cycles, the DRIE process has been
optimized to produce near-vertical sidewalls when stopping on the buried oxide layer. The cavity sizes are closely
matched to the die dimensions to ensure placement of the dies to within ±15 μm. Dies are placed in all the cavities, and
then a photoresist layer is deposited by spin-coating and patterned to provide access to the required IC contact pads. The
photoresist has the dual role of securing the dies and also planarizing the top surface of the carrier. After an appropriate
baking cycle this layer provides a suitable base for multi-level electroplating or other low-temperature MEMS
Rotation of structures fabricated by planar processing into out-of-plane orientations can be used to greatly increase
the 3-dimensionality of microstructures. Previously this has been achieved by a self-assembly process based on surface
tension in meltable hinges. An important application is in fabricating vertical inductors on silicon, to reduce the substrate
coupling and thus increase quality factor and self-resonance frequency. Previous processes have used copper tracks, and
Pb-Sn hinges. However, the use of Cu limits potential applications because of oxidation, since the final structure is not
embedded. Moreover, a substitute hinge material is also required, as a result of legislative restrictions on Pb use. In this
paper, Au is used as an alternative to Cu for the fabrication of self-assembled 3D inductors. A process has been
developed to overcome photoresist deterioration problems due to the alkaline nature of Au electro-deposition solutions.
Furthermore, pure Sn is used instead of Pb-Sn as the hinge material. A Ni metal layer is introduced between the Au coils
and the Sn hinge to prevent inter-diffusion and formation of eutectic Au-Sn compounds. Finally a gold capping
technique is proposed to protect the Sn hinge from oxidation during hinge reflow. The fabrication techniques developed
here are compatible with post-processing on active CMOS circuits, and can be adopted for other MEMS applications.
Laser micromachining by ablation is a well established technique used for the production of 2.5D and 3D features in a
wide variety of materials. The fabrication of stepped, multi-level, structures can be achieved using a number of binary
mask projection techniques using excimer lasers. Alternatively, direct-writing of complex 2.5D features can easily be
achieved with solid-state lasers. Excimer laser ablation using half-tone masks allows almost continuous surface relief
and the generation of features with low surface roughness. We have developed techniques to create large arrays of
repeating micro-optical structures on polymer substrates. Here, we show our recent developments in laser structuring
with the combination of half-tone and binary mask techniques.
Pulsed UV laser machining is an established method for production of 2.5D and 3D features in a wide variety of materials. In addition to direct laser patterning by ablation, exposure of photoresist using pulsed lasers can eliminate the need for large area contact photomasks. Half-tone machining, either by ablation or exposure, allows the production of high quality shallow features where the surface roughness from other laser machining techniques would be unacceptable. Such features could be used as anti-reflection surfaces for mobile display devices. Features produced by lithography typically exhibit low surface roughness but have more complex fabrication processes. Here, the surface roughness of shallow features produced by half-tone lithography and half-tone ablation is investigated for a photoresist. Similar surface profiles are achieved for each technique and roughness levels are comparable for both.
Laser micromachining by ablation is an established technique for the production of 2.5D and 3D features in a wide
variety of materials. Mask projection techniques using excimer lasers have been used to fabricate microstructures on
large panels where diamond turning and reflow techniques have reached their limits. We have developed 3D structuring
tools based upon UV laser ablation of polymers to create large arrays of repeating micro-optical features.
Synchronization of laser pulses with workpiece movement allows layer-by-layer growth of deep structures with
outstanding repeatability. Here, we show recent developments in laser structuring with the combination of half-tone and
binary mask techniques. Significant improvements in surface quality are demonstrated for a limited range of structures.
Laser micromachining has great potential as a MEMS (micro-electro-mechanical systems) fabrication technique because of its materials flexibility and 3D capabilities. The machining of deep polymer structures with complex, well-defined surface profiles is particularly relevant to microfluidics and micro-optics, and in this paper we review recent work on the use of projection ablation methods to fabricate structures and devices aimed at these application areas. In particular we focus on two excimer laser micromachining techniques that are capable of both 3D structuring and large-area machining: synchronous image scanning (SIS) and workpiece dragging with half-tone masks. The methods used in mask design are reviewed, and experimental results are presented for test structures fabricated in polycarbonate. Both techniques are shown to be capable of producing accurately dimensioned structures that are significantly deeper than the focal depth of the projection optics and virtually free from fabrication artifacts such as the steps normally associated with multiple-mask processes.
Measurements of ablation rate have traditionally been carried out only at normal incidence. However, in real-world applications ablation is often carried out at oblique angles, and it is useful to have prior knowledge of the ablation rate in this case. Detailed information about the angular dependence is also important for the development of ablation simulation tools, and can provide additional insight into the ablation mechanism. Previously we have reported on the angular dependence of direct-write ablation at 266 nm wavelength in solgel and polymer materials. In this paper we present a systematic study of angular dependence for excimer laser ablation of two polymer materials of interest for microfabrication: polycarbonate and SU8 photoresist. The results are used to improve simulation models to aid in mask design.
An efficient simulation method has recently been developed for multi-pulse ablation processes. This is based on pulse-by-pulse propagation of the machined surface according to one of several phenomenological models for the laser-material interaction. The technique allows quantitative predictions to be made about the surface shapes of complex machined parts, given only a minimal set of input data for parameter calibration. In the case of direct-write machining of polymers or glasses with ns-duration pulses, this data set can typically be limited to the surface profiles of a small number of standard test patterns. The use of phenomenological models for the laser-material interaction, calibrated by experimental feedback, allows fast simulation, and can achieve a high degree of accuracy for certain combinations of material, laser and geometry. In this paper, the capabilities and limitations of the approach are discussed, and recent results are presented for structures machined in SU8 photoresist.
Laser ablation using diode pumped solid state lasers shows great potential for a wide range of micromachining applications. We have been using a frequency quadrupled Nd:VO4 laser (266 nm wavelength), with a pulse duration < 30 ns, to ablate a sol-gel Ormocer material. With a pulse energy of around 20 μJ, and a focal spot of the order of 10 μm diameter, single pulses were found to produce craters a few microns in depth and ~10 μm in diameter. A study of the variation of the crater profile with pulse energy and angle of incidence to the surface has enabled the development of an efficient method to simulate the ablation for a series of consecutive shots constituting a toolpath. Multiple pulses with varying degrees of overlap were simulated, and compared with experiment. Results show that the model accurately predicts the profiles of trenches and pocketed surfaces given parameters obtained from a single crater machined at normal incidence. The "self calibrating" feature of our approach significantly reduces the number of input parameters required for adequate simulations. In particular, it does not require knowledge of the beam profile or material ablation curve. The simplicity and practicality of the method make it promising for use in an industrial environment.
We have used KrF excimer laser ablation in the fabrication of a novel MEMS power conversion device based on an axial-flow turbine with an integral axial-flux electromagnetic generator. The device has a sandwich structure, comprising a pair of silicon stators either side of an SU8 polymer rotor. The curved turbine rotor blades were fabricated by projection ablation of SU8 parts performed by conventional UV lithography. A variable aperture mask, implemented by stepping a moving aperture in front of a fixed one, was used to achieve the desired spatial variation in the ablated depth. An automatic process was set up on a commercial laser workstation, with the laser firing and mask motion being controlled by computer. High quality SU8 rotor parts with diameters of 13 mm and depths of 1 mm were produced at a fluence of 0.7 J/cm2, corresponding to a material removal rate of approximately 0.3 μm per pulse. A similar approach was used to form SU8 guide vane inserts for the stators.
A facility for rapid prototyping of MEMS devices is crucial for the development of novel miniaturized components in all sectors of high-tech industry, e.g. telecommunications, information technology, micro-optics and aerospace. To overcome the disadvantages of existing techniques in terms of cost and flexibility, a new approach has been taken to provide a tool for rapid prototyping and small-scale production: Complex CAD/CAM software has been developed that automatically generates the tool paths according to a CAD drawing of the MEMS device. As laser ablation is a much more complicated process than mechanical machining, for which such software has already been in use for many years, the generation of these tool paths relies not only on geometric considerations, but also on a sophisticated simulation module taking into account various material and laser parameters and micro-effects. The following laser machining options have been implemented: cutting, hole drilling, slot cutting, 2D area clearing, pocketing and 2½D surface machining. Once the tool paths are available, a post processor translates this information into CNC commands that control a scanner head. This scanner head then guides the beam of a UV solid-state laser to machine the desired structure by direct laser ablation.
This paper discusses the use of excimer lasers in the manufacture of microelectromechanical devices and systems, with emphasis on two application areas: laser micromachining of polymer masters for replication in metal by electroplating (Laser-LIGA), and laser-assisted manipulation of microparts for hybrid assembly. As a master fabrication method, laser micromachining offers advantages over conventional UV lithography in terms of materials flexibility and 3-dimensional capability. However, these advantages are offset by higher cost and lower throughput. We have been using a combination of laser micromachining an UV lithography to produce relatively complex multi-level fluidic devices, with laser micromachining being used only for layers requiring greater structural height and/or 3D profiling. Process details and examples of prototype devices are presented. Laser-assisted assembly is a new technique based on release and transport of parts by ablation of a sacrificial layer, using light incident through the substrate. We have been using this approach to assemble microelectromechanical devices from parts fabricated on separate substrates. Fundamental aspects of the process are discussed, and results are presented for hybrid electrostatic micromotors assembled by laser-assisted transfer of nickel parts.
A novel method is presented to manufacture multilevel diffractive optical elements (DOEs) in polymer by single- step KrF excimer laser ablation using a halftone mask. The DOEs have a typical pixel dimension of 5 micrometers and are up to 512 by 512 pixels in size. The DOEs presented are Fresnel lenses and Fourier computer generated holograms, calculated by means of a conventional iterative Fourier transform algorithm. The halftone mask is built up as an array of 5 micrometers -square pixels, each containing a rectangular or L- shaped window on an opaque background. The mask is imaged onto the polymer with a 5x, 0.13 NA reduction lens. The pixels are not resolved by the lens, so they behave simply as attenuators, allowing spatial variation of the ablation rate via the window size. The advantages of halftone mask technology over other methods, such as pixel-by-pixel ablation and multi-mask overlay, are that it is very fast regardless of DOE size, and that no high-precision motion stages and alignment are required. The challenges are that the halftone mask is specific to the etch curve of the polymer used, that precise calibration of each grey-level is required, and that the halftone mask must be calculated specifically for the imaging lens used. This paper describes the design procedures for multilevel DOEs and halftone masks, the calibration of the various levels, and some preliminary DOE test results.
This paper discusses the use of high power lasers in the manufacture of microelectromechanical systems (MEMS). The ability to process a wide range of materials, and to produce truly three-dimensional structures with tolerances at the micron or sub-micron level, give laser micromachining some key advantages over other more established micromachining techniques. Previous work in this area is reviewed, covering the following topics: use of ablation in the direct fabrication of MEMS devices and to define polymer masters for subsequent replication by electroforming and moulding (the so-called Laser-LIGA process); laser-assisted deposition and etching on planar and non-planar surfaces; laser-assisted manipulation of microparts and laser-assisted assembly.