High Aspect Ratio Micromachining Research Articles

UV-LIGA technique for ECF micropumps using back UV exposure and self-alignment

This paper proposes and develops a novel UV-LIGA technique using back UV exposure and self-alignment to realize high aspect ratio micromachining (HARM) in high power density electro-conjugate fluid (ECF) micropumps. ECF is a functional fluid designed to be able to generate strong and active jet flow (ECF jetting) between anode and cathode in ECF when high DC voltage is applied. We have developed high power density ECF micropumps consisting of triangular prism and slit electrode pairs (TPSEs) fabricated by HARM. The traditional UV-LIGA technique for HARM is mainly divided into two approaches: (a) single thick layer and (b) multiple thin layers. Both methods have limitations—deformed molds in the former and misalignment between layers in the latter. Using the finite element method software COMSOL Multiphysics, we demonstrate that the deformed micro-molds critically impair the performance of ECF micropumps. In addition, we experimentally prove that the misalignment would easily trigger electric discharge in the ECF micropumps. To overcome these limitations, we conceive a new concept utilizing the seed electrode layer for electroforming as the UV shield and pattern photoresist (KMPR) by back UV exposure. The seed electrode layer should be composed of a non-transparent conductor (Au/Ti) for patterning and a transparent conductor (ITO) for wiring. Instead of ITO, we propose the concept of transparency-like electrodes comprised of thin metal line patterns. To verify this concept, KMPR layers with thicknesses of 70, 220, and 500 µm are experimentally investigated. In the case of 500 µm KMPR thickness, the concept of transparency-like electrode was partially proved. As a result, TPSEs with a height of 440 µm were successfully fabricated. Characteristic experiments demonstrated that ECF micropumps (367 mW cm−3) fabricated by back UV achieved almost the same output power density as ECF micropumps (391 mW cm−3) fabricated by front UV. This paper proves that the proposed UV-LIGA technique, using back UV exposure and self-alignment, can effectively fabricate TPSEs for ECF micropumps with high output power density.

A mathematical model to predict overcut during electrochemical discharge machining

Overcut is a major reason for the dimensional deviations, especially during high aspect ratio micromachining in electrochemical discharge machining (ECDM). Since electrolyte concentration plays a vital role in material removal, by understanding the effect of electrolyte concentration on the machined feature, it is possible to reduce the overcut associated with the ECDM process. This reduction in overcut was used in the study to drill high aspect ratio microholes in glass. The effect of concentration on the overcut is explained with an analytical model. The model considers the thermal effects on material removal for ECDM assuming a high-temperature chemical etching mechanism for the material removal. It describes the effect of electrolyte concentration as well as machining time on material removal. Results reveal that at the microscale, tool diameter has minimal effect on overcut for a given machining voltage. Microholes with aspect ratios as high as 12 were produced in glass in this study.

High aspect ratio micromachining of glass by electrochemical discharge machining (ECDM)

Micromachining of glass is essential for several microfluidic components, micro-pumps, micro-accelerometers, micro-reactors, micro-fuel cells and several biomedical devices. Unique properties such as high chemical resistance, thermal stability and transparency give glass scope for additional applications. However, poor machinability of glass is a major constraint, especially in high aspect ratio applications of glass in microsystem technology. Micro electrochemical discharge machining (micro ECDM) is an emerging nontraditional fabrication method capable of micromachining ceramic materials like glass. While surface features less than 100μm have been successfully machined on glass, machining high aspect features is a challenge. Machining accuracy at high depths is severely affected due to overcut and tool wear. In this paper, high aspect ratio microtools fabricated in-house have been used for deep microhole drilling on glass using low electrolyte concentration. An aspect ratio of 11 has been achieved. The results show that lower electrolyte concentration reduced overcut by 22%, thus increasing the aspect ratio of the micro holes. Lowering the electrolyte concentration also reduced the tool wear and hole taper by 39% and 18% respectively.

Fabrication and quality factor control of a microelectromechanical system resonator with linear differential drive

The fabrication, characterisation and quality factor control of a microelectromechanical system resonator with differential drive and readout is reported. The differential drive results in a linear relationship between the AC drive voltage and the applied force. Furthermore, the static DC force and the undesirable force at the second harmonic frequency (introduced by other driving methods) are also reduced. Differential readout is employed in order to minimise undesirable common-mode signals in the detection electronics. The device has been fabricated in silicon on insulator (using the Integram high aspect ratio micromachining process at QinetiQ) and fully characterised. The ability to track the resonant frequency with fine resolution is related to the quality factor of the resonant system. Commonly used detection techniques such as slope detection and phase detection both require a high quality factor, however, resistance to motion caused by air damping results in a low quality factor – degrading the performance of the sensor. The low-pressure quality factor is measured as being >5500 and reduces to ∼10 at atmospheric pressure. The linear relationship between AC drive voltage and resultant force is exploited in conjunction with an electronic readout architecture yielding the device velocity to implement a force feedback technique – increasing the quality factor of the resonator.

Manufacturing costs for microsystems/MEMS using high aspect ratio microfabrication techniques

The emphasis on high aspect ratio micromachining techniques for microsystems/MEMS has been mainly to achieve novel devices with, for example, high sensing or actuation performance. Often these utilize deep structures (100–1,000 μm) with vertical wall layers but with relatively modest spatial resolution (1–10 μm). As these techniques move from research to industrial manufacture, the capital cost of the equipment and the cost of device manufacture become important, particularly where more than one micromachining technique can meet the performance requirements. This paper investigates the layer-processing costs associated with the principal high aspect ratio micromachining techniques used in microsystems/MEMS fabrication, particularly silicon surface micromachining, wet bulk etching, wafer bonding, Deep Reactive Ion Etching, excimer laser micromachining, UV LIGA and X-ray LIGA. A cost model (MEMSCOST) has been developed which takes the financial, operational and machine-dependent parameters of the different manufacturing techniques as inputs and calculates the layer-processing costs at the wafer and chip level as a function of demand volume. The associated operational and investment costs are also calculated. Cost reductions through increases in the wafer size and decreases in chip area are investigated, and the importance of packaging costs demonstrated.

Design, fabrication, and testing of a 3-DOF HARM micromanipulator on (1 1 1) silicon substrate

In this study, a novel HARM (high aspect ratio micromachining) micromanipulator fabricated on (1 1 1) silicon wafer is reported. The micromanipulator consists of a positioning stage, a robot arm, supporting platforms, conducting wires, and bonding pads. These components are monolithically integrated on a chip through the presented processes. The three-degrees-of-freedom (3-DOF) positioning of the micromanipulator is realized by using the integration of two linear comb actuators and a vertical comb actuator. The robot arm is used to manipulate samples with dimension in the order of several microns to several hundred microns, for instance, optical fibers and biological samples. The robot arm could be a gripper, a needle, a probe, or even a pipette. Since the micromanipulator is made of single crystal silicon, it has superior mechanical properties. A micro gripper has also been successfully designed and fabricated.

Partially hydrogenated poly(vinyl phenol) based photoresist for near UV, high aspect ratio micromachining

A negative resist platform based on a low Mw epoxy novolac polymer, partially hydrogenated poly(vinyl phenol)s and suitable onium salt photoacid generators is presented and evaluated for high aspect ratio micromachining. The high content of hydroxyl groups in this resist system allows development in standard aqueous base solutions (TMAH 0.26 N). The effects of material parameters such as the degree of poly(vinyl phenol) hydrogenation and the onium salt hydrophobicity on the development process are examined. The cross-linked areas of this resist can be easily removed by commercial wet photoresist strippers after processing. Experimental data demonstrate a promising lithographic performance with an aspect ratio of 7:1 for 5 μm closely spaced lines under exposure at 365 nm.

A boron etch-stop assisted lateral silicon etching process for improved high-aspect-ratio silicon micromachining and its applications

High-aspect-ratio micromachining (HARM) developed in recent years has made many devices more versatile compared to the surface micromachining process, but it has also met some challenges which have not occurred before. Those issues made many restrictions on HARM structure design, including structure thickness/width limitations, anchor-induced design problems, thickness uniformity and sidewall conductivity problems, which are discussed in this paper. Accordingly, we propose a novel boron etch-stop assisted lateral silicon etching (BELST) process which employs the (111) wafer process and heavy boron diffusion. Many design constraints have been reduced through some delicate designs of the BELST process. Furthermore, the process is capable of various applications, and has been applied in fabricating a dual mass-spring resonator and a micro vibrating gyroscope in this work. In summary, the developed BELST process can possess most existing merits as well as reducing many design constraints in the existing HARM process, and is expected to contribute in making HARM more competitive and convenient.

Strippable aqueous base developable negative photoresist for high aspect ratio micromachining

The patterning of high aspect ratio structures in ultra thick photoresist films is extremely challenging. Photoresists used should provide low absorption in order to achieve vertical sidewalls and easy stripping after the pattern transfer process. In the current work, a novel resist platform is presented and applied for lithographic evaluation in film thickness up to 60 μm. The resist formulation consists of a mixture of an epoxy novolac oligomer, partially hydrogenated poly(4-hydroxystyrene) and onium salt photoacid generators. Since the resist is rich in hydroxyl groups the unexposed regions can be developed in standard aqueous base solutions (TMAH, 0.26 N). Additionally since the required crosslink density for solubility change is limited, the crosslinked regions can be easily removed by commercial wet photoresist strippers. First experimental data revealed promising lithographic performance with resolution down to 20 μm with an aspect ratio 3:1.

High aspect-ratio micromachining of polymers with an ultrafast laser

Laser ablation of various polymers has been studied using a beam of 100 fs laser pulses at a wavelength of 800 nm. For a beam focussed by a plano-convex lens, high-quality machining of holes with diameters 20–40 μm and depths 300–400 μm (aspect ratio greater than 10) has been achieved. When projection patterning is carried out with a rectangular pinhole, arrays of micro-strings have been observed deep in the bulk of the sample. The micro-strings have diameters as small as 2 μm (about three times the wavelength) and lengths greater than 10 mm (aspect ratio 5000). The achievement of the high aspect ratios and the formation of the arrays of micro-strings are discussed.

High aspect-ratio combined poly and single-crystal silicon (HARPSS) MEMS technology

This paper presents a single-wafer high aspect-ratio micromachining technology capable of simultaneously producing tens to hundreds of micrometers thick electrically isolated poly and single-crystal silicon microstructures. High aspect-ratio polysilicon structures are created by refilling hundreds of micrometers deep trenches with polysilicon deposited over a sacrificial oxide layer. Thick single-crystal silicon structures are released from the substrate through the front side of the wafer by means of a combined directional and isotropic silicon dry etch and are protected on the sides by refilled trenches. This process is capable of producing electrically isolated polysilicon and silicon electrodes as tall as the main body structure with various size capacitive air gaps ranging from submicrometer to tens of micrometers. Using bent-beam strain sensors, residual stress in 80-/spl mu/m-thick 4-/spl mu/m-wide trench-refilled vertical polysilicon beams fabricated in this technology has been measured to be virtually zero. 300-/spl mu/m-long 80-/spl mu/m-thick polysilicon clamped-clamped beam micromechanical resonators have shown quality factors as high as 85 000 in vacuum. The all-silicon feature of this technology improves long-term stability and temperature sensitivity, while fabrication of large-area vertical pickoff electrodes with submicrometer gap spacing will increase the sensitivity of micro-electromechanical devices by orders of magnitude.

High aspect ratio micromachining (HARM) technologies for microinertial devices

In this paper, we review work on novel, high aspect processes for microinertial components at the Defence Evaluation and Research Agency (DERA). High aspect components may lead to significant cost-performance improvements in both accelerometers and gyroscopes. We have evaluated 3 low temperature process technologies – silicon on insulator (SOI) HARM, UV electroforming and bulk HARM. Prototype microinertial devices fabricated in these technologies are also presented. The potential of the processes for integration with on-chip CMOS electronics is assessed which may be either as part of a fully integrated MEMS process or as “value-added” post-processing on commercial CMOS wafers. Bonded SOI (BSOI) materials has been specially designed for micromachining applications to give a low stress material that is optimised for a sacrificial release process. Trench isolation is achieved by deep dry etching to the buried dielectric. These trenches may be refilled to allow metallisation to reach isolated components. Structures with aspect ratios of up to 50:1 have been realised using a combination of photolithography, deposition and deep dry etching. CMOS compatibility has been demonstrated. The process is an attractive manufacturing technology. Electroforming of nickel in resist moulds formed using conventional UV photolithography has also been investigated. Some of the early limitations with this technology have been overcome by using a new resist technology, SU8. The process needs to mature further, but remains a promising candidate. Bulk HARM uses deep dry etching of a bulk silicon membrane which is defined using wet etching. Device isolation is difficult and process control complex making this the least attractive of the technologies.

High aspect ratio micromachining by synchrotron radiation direct photo-etching

We have carried out micromachining of Teflon-polymers such as PTFE, PFA and FEP as well as optical crystal such as NaCl and LiF by synchrotron radiation direct (without any chemicals) photo-etching and succeeded in creating microstructures with very high aspect ratios. The maximum aspect-ratio achieved was 50 and the maximum processing depth was 1500 microns. Dependence of the etching rate on the synchrotron beam current and on the substrate temperature was studied. Based on the study, we could use only x-rays from the synchrotron radiation so as to apply x-ray lithography technology (such as using an x-ray mask and processing in He atmosphere) to our process. A rise in the sample temperature results in significant enhancement of the etching rate. The etching rate measured was on the order of a few 100 μm/min. So that this process is much faster than hard x-ray deep lithography for the processing of more than 100 μm deep microstructures.

X-ray exposure system for induced chemistry and dry processes in microlithography

A new x-ray microlithography exposure system has been designed and built at the Center for Advanced Microstructures and Devices (CAMD) to meet the specific demands of synchrotron radiation assisted processing for microfabrication and high aspect ratio micromachining. The system consists of a broadband transmission (1.2–6.5 Å) beamline and a multichamber x-ray exposure station which can simultaneously provide x-ray exposure and activation, plasma, or reactive ion etching (RIE). The beamline is operational at the CAMD 1.3–1.5 GeV synchrotron storage ring. The x-ray exposure station includes filter and exposure chambers. A filter chamber is incorporated into the exposure system to optimize the radiation spectra for thick resist (in excess of 1 mm) exposures and to protect the Be window from reactive gases during exposure. Two assisting agents, x-ray irradiation and rf (13.53 MHz) induced plasma for RIE, can be used separately or in combination. Samples of ∼500-μm-thick PMMA resist have been exposed in neutral and reactive atmospheres and successfully developed. The preliminary results indicate that He gas acts as a ‘‘catalyst’’ for PMMA losses during exposure.