Laser micromachining for manufacturing MEMS devices Malcolm Gower

Exitech Ltd, Hanborough Park, Long Hanborough, Oxford OX8 8LH, England

ABSTRACT

Applications of laser micromachining to the manufacture and prototyping of MEMS and MOEMS devices are presented. Examples of microturbines, biofactory on a chip, microfluidic components and microoptical elements manufactured by laser micromachining are described. Key Words: Laser micromachining, MEMS, MOEMS, Biofactory-on-a-chip, Microoptics

1. INTRODUCTION

Adaption of silicon lithography and batch etch processing as developed by the semiconductor industry is currently the dominant MEMS fabrication method. However being restricted to predominately just one material surface bulk etched in only 3 directions - along (110), (100) and (111) silicon crystallographic-planes, other more flexible micromachining methods including pulsed laser ablation are of growing interest for manufacturing MEMS and MOEMS devices (1). The perceived advantages of laser micromachining are many: (i) few processing steps, (iii) highly-flexible CNC programming of shapes for engineering prototyping, (iii) capable of serial and batch-mode production processing, (iv) no major investment required in large clean-room facilities and many expensive process tools, (v) applicable to virtually any material including polymers, ceramics, glasses, crystals, insulators, conductors, piezomaterials, biomaterials, non-planar substrates, thin and thick films, (vi) compatibility with lithographic processes and photomask making.

2. LASER MICROMACHINING A wide variety of lasers can be used for micromachining – from microsecond pulsed infrared CO2 gas lasers at wavelengths between 9.3-11µm to nanosecond pulsed excimer gas lasers in the 157-353nm uv wavelength range and femto to nanosecond pulsed solid state lasers between wavelengths of 266-1060nm. Depending on the materials being processed and the application involved, each laser has its own merits and demerits. In general, optimal laser micromachining is obtained when photons are strongly absorbed in submicron depths at the surface of a material. Furthermore, if these photons are delivered in a short duration burst, a miniexplosion is created ejecting solid and gaseous particulates from the irradiated site without significant thermal degradation (melting, spatter, recrystallization, etc) occurring to the surrounding region. Since most materials absorb short wavelengths much more strongly than long wavelengths, ultraviolet lasers are in general much better suited for micromachining applications than infrared lasers. Greater depth control can be achieved and since short wavelengths are diffracted less, optical beam delivery systems can have greater resolution that allows smaller lateral feature sizes to be machined. The high intensities provided by pulsed ultrashort femtosecond focused laser beams can induce strong nonlinear optical absorption of photons in materials that might otherwise be highly transparent to photons at much lower intensities. With the correct choice of laser matched to the material and processing conditions, virtually any material can now be laser micromachined with submicron precision. By way of example Figure 1, shows a silicon wafer which has been laser micromachined using 10nsec pulses of second and third harmonic radiation at wavelengths of 532nm and 355nm respectively from a Q-switched diode-pumped Nd:YAG laser.

(a) λ = 532nm (b) λ = 355nm

Figure 1. Silicon wafer micromachined using 10nsec duration pulses from Nd:YAG laser. I ~ 3GW/cm2

In contrast Figure 2 shows fused silica and diamond that has been micromachined using 100fsec pulses from a mode locked Ti:Al2O3 laser. While these materials are normally very transparent to ~800nm wavelength radiation, at an intensity of I~2.8x1014W/cm2 used here strong nonlinear multiphoton absorption dominates the transmissive properties and the material becomes opaque. The intensity used in Fig 2 is more than four orders of magnitude higher than that used to obtain the results in Fig 1. (a) Fused silica SiO2 (b) CVD deposited Diamond film

Figure 2. Trenches and cuts micromachined using 110fsec duration pulses from Ti:Al2O3 laser. λ = 800nm, I ~ 280TW/cm2, 3kHz

As illustrated in Figure 3, controlled 3D-structuring of materials by nanosecond pulse excimer laser etching(2-4) can produce the basic building blocks of bridges, diaphragms, pits, holes, ramps, cantilevers, etc needed for microengineering many types of MEMS devices. Rather than being limited to machining only along straight planar directions, laser micromachining can be used to fabricate surface structures having rounded parabolic shapes.

3. MEMS DEVICE STRUCTURES MEMS device structures can be fabricated by direct laser ablative removal of material (5). Examples of this technique are a biofactory-on-a-chip (BFC) described below, shaped bimorph actuators in silicon (6) and patterning of magnetic multilayer actuators (7).

(a) Beam structure (b) Ramps, channels and bars

Figure 3. 21/2D-structures in polycarbonate using CNC KrF laser micromachining tool. Mass production using batch type processing methods can then be achieved using electroforming LIGA (8-10) or injection moulding processes (11,12). Figure 4 shows examples of uv-laser micromachined 3D-structures in polymers which when used with the LIGA process of electroforming, can be replicated in metal - a process now known as Laser LIGA. Once a master has been made by excimer laser micromachining such methods allow high volumes of replica parts to be manufactured at low unit costs.

Figure 4. Nickel rotor turbine made by KrF laser LIGA. 470µm diameter, 130µm height

Courtesy of Dr A Holmes Imperial College, University of London (5,9) Recent advances in personal healthcare and environmental monitoring have led to the development of different diagnostic devices that can perform a variety of analysis functions. The applications of such diagnostic 'chips' is widespread and includes food and water supplies, drug delivery systems, personal drug administration, DNA analysis, blood monitoring, cell sorters, pregnancy testing, etc. One of the most researched areas has been the so-called biofactory-on-a-chip comprising compact discrete devices for micro-monitoring chemical analysis in the environment and medical areas. Because of the inherent bio-incompatibility of silicon, in general biomedical sensors must be fabricated from other materials. Because many biodegradable or compound materials are not well-suited to conventional chemical and plasma etching processing, laser machining of these devices is becoming ever more important. A biochip manufactured using laser technology is shown in Figure 5.

Figure 5. Biochip manufactured using excimer laser micromachining (13) This device consists of a multilevel layout of 21/2D laminations of gold electrode conveyor tracks, channels and chambers sandwiched between insulating polymer layers. The process of manufacture has been presented elsewhere (13). In general an excimer laser is used both to ablate a gold layer to leave behind 10µm wide electrodes as well as to machine the interconnect microvia holes in a dielectric polymer layer. Using the process of travelling-wave dielectrophoresis, a simple low voltage AC power supply applied to the electrode structure controls the sample fluidic motion. Microchannels for transporting the sample from the inlet ports to the analysis sites are excimer laser micromachined (see Figure 6).

Figure 6. KrF laser micromachined microfluidic channels and mixing chamber in polyester When connected to external power and optical recognition devices the overall dimension of the chip is 55mm x 40mm. With such a laser-based micromachining method all steps are carried out using the same tool obviating the need for any additional processing steps. Hence there is great flexibility in the design of the chip layout.

4. MICROOPTICAL ELEMENTS

Examples of some types of micro-optical surfaces that can be structured by excimer laser micromachining are shown in Figure 7. Blazed gratings and microlens arrays used to guide and control light for LED and LCD displays, for coupling light to and from LED’s and optical waveguides can be fabricated on surfaces by mask imaging techniques (14, 15). (a) Blazed grating (b) Moth’s eye AR surface (c) Cylindrical microlens array (d) Fresnel microlenses

Figure 7. KrF laser produced microoptical surfaces produced using mask imaging techniques. All are in polycarbonate

except (b) which is in infrared transmitting CsI crystal.

The ‘black’ anti-reflective property of the ‘moth’s-eye’ type of surface machined on the CsI crystal shown in Figure 7(b) is being used to prevent ghost images in very large infrared optical telescopes. Half-tone masks comprising variable dot size and density features can be used to produce images on a workpiece having a grey intensity scale – in a similar fashion to images printed in newspapers. This grey scale can be used to control the excimer laser etch depth. As shown in Figure 8, such masks can be used to produce structures like diffractive optical elements (DOE';s) that have 21/2D depth topography (16).

5. SUMMARY Laser micromachining has developed into a viable process for manufacturing many MEMS and MOEMS device structures particularly for applications in which silicon is not the preferred material base. In some areas such as microfluidics and microoptics, laser micromachining is likely to be used for manufacturing masters after which mass production is accomplished by replication techniques.

Figure 8. 8-level diffractive optical element. Produced using half-tone masks to control feature depth. As laser technologies continue to mature the ever-widening choice of wavelengths and pulse durations will open up new manufacturing capabilities. Wavelengths from the vacuum ultraviolet (157nm) to infrared (10µm), pulse durations from microseconds to femtoseconds and reliable compact all solid-state/sealed gas laser constructions are enabling an increasingly wider range of micromachining capabilites to be developed for MEMS applications. More mix-and-match of complementary techniques involving chemical - plasma – laser etching will likely be used where materials and functions from a variety of processes are incorporated.

ACKNOWLEDGEMENTS

It is a pleasure to thank P Rumsby, N Rizvi and J Fieret of Exitech, A Holmes of Imperial College, University of London, R Pethig and J Birt of the University of Wales, Bangor who made many contributions to the original experimental material contained in this paper.

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