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Laser micromachining for manufacturing MEMS devicesMalcolm 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 arepresented. Examples of microturbines, biofactory on a chip, microfluidic components and microoptical elementsmanufactured 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 thedominant MEMS fabrication method. However being restricted to predominately just one material surface bulk etchedin 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)

. Theperceived advantages of laser micromachining are many: (i) few processing steps, (iii) highly-flexible CNCprogramming 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 virtuallyany 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 MICROMACHININGA wide variety of lasers can be used for micromachining from microsecond pulsed infrared CO 2 gas lasers atwavelengths between 9.3- 11m to nanosecond pulsed excimer gas lasers in the 157-353nm uv wavelength range andfemto to nanosecond pulsed solid state lasers between wavelengths of 266-1060nm. Depending on the materials beingprocessed and the application involved, each laser has its own merits and demerits. In general, optimal lasermicromachining 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 andgaseous 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 longwavelengths, 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 systemscan have greater resolution that allows smaller lateral feature sizes to be machined. The high intensities provided bypulsed ultrashort femtosecond focused laser beams can induce strong nonlinear optical absorption of photons inmaterials 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 withsubmicron precision.

By way of example Figure 1, shows a silicon wafer which has been laser micromachined using 10nsec pulses of secondand third harmonic radiation at wavelengths of 532nm and 355nm respectively from a Q-switched diode-pumpedNd:YAG laser.

MEMS Components and Applications for Industry, Automobiles, Aerospace, and Communication,Henry Helvajian, Siegfried W. Janson, Franz Lrmer, Editors, Proceedings of SPIE Vol. 4559 (2001)

2001 SPIE 0277-786X/01/$15.0053

Invited Paper

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(a) = 532nm (b) = 355nmFigure 1. Silicon wafer micromachined using 10nsec duration pulses from Nd:YAG laser. I ~ 3GW/cm 2

In contrast Figure 2 shows fused silica and diamond that has been micromachined using 100fsec pulses from a modelocked Ti:Al 2O3 laser. While these materials are normally very transparent to ~800nm wavelength radiation, at anintensity of I~2.8x10 14W/cm 2 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 thatused to obtain the results in Fig 1.

(a) Fused silica SiO 2 (b) CVD deposited Diamond filmFigure 2. Trenches and cuts micromachined using 110fsec duration pulses from Ti:Al 2O3 laser. = 800nm, I ~

280TW/cm 2, 3kHz

As illustrated in Figure 3, controlled 3D-structuring of materials by nanosecond pulse excimer laser etching (2-4) canproduce the basic building blocks of bridges, diaphragms, pits, holes, ramps, cantilevers, etc needed formicroengineering many types of MEMS devices. Rather than being limited to machining only along straight planardirections, 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 techniqueare a biofactory-on-a-chip (BFC) described below, shaped bimorph actuators in silicon (6) and patterning of magneticmultilayer actuators (7).

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(a) Beam structure (b) Ramps, channels and barsFigure 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) orinjection 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 LaserLIGA. Once a master has been made by excimer laser micromachining such methods allow high volumes of replicaparts to be manufactured at low unit costs.

Figure 4. Nickel rotor turbine made by KrF laser LIGA. 470 m diameter, 130 m heightCourtesy 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 differentdiagnostic devices that can perform a variety of analysis functions. The applications of such diagnostic 'chips' iswidespread 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-calledbiofactory-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 befabricated from other materials. Because many biodegradable or compound materials are not well-suited toconventional chemical and plasma etching processing, laser machining of these devices is becoming ever moreimportant. A biochip manufactured using laser technology is shown in Figure 5.

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Figure 5. Biochip manufactured using excimer laser micromachining (13)

This device consists of a multilevel layout of 2 1 / 2D laminations of gold electrode conveyor tracks, channels andchambers 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 10 m wide electrodes as well as tomachine the interconnect microvia holes in a dielectric polymer layer. Using the process of travelling-wavedielectrophoresis, a simple low voltage AC power supply applied to the electrode structure controls the sample fluidicmotion. Microchannels for transporting the sample from the inlet ports to the analysis sites are excimer lasermicromachined (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 anyadditional processing steps. Hence there is great flexibility in the design of the chip layout.

4. MICROOPTICAL ELEMENTS

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Examples of some types of micro-optical surfaces that can be structured by excimer laser micromachining are shown inFigure 7. Blazed gratings and microlens arrays used to guide and control light for LED and LCD displays, for couplinglight to and from LEDs and optical waveguides can be fabricated on surfaces by mask imaging techniques (14, 15) .

(a) Blazed grating (b) Moths 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 polycarbonateexcept (b) which is in infrared transmitting CsI crystal.

The black anti-reflective property of the moths-eye type of surface machined on the CsI crystal shown in Figure7(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 workpiecehaving a grey intensity scale in a similar fashion to images printed in newspapers. This grey scale can be used tocontrol the excimer laser etch depth. As shown in Figure 8, such masks can be used to produce structures likediffractive optical elements ( DOE';s ) that have 2 1 / 2D depth topography

(16) .

5. SUMMARYLaser micromachining has developed into a viable process for manufacturing many MEMS and MOEMS devicestructures particularly for applications in which silicon is not the preferred material base. In some areas such asmicrofluidics and microoptics, laser micromachining is likely to be used for manufacturing masters after which massproduction is accomplished by replication techniques.

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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 newmanufacturing capabilities. Wavelengths from the vacuum ultraviolet (157nm) to infrared (10 m), pulse durations frommicroseconds to femtoseconds and reliable compact all solid-state/sealed gas laser constructions are enabling anincreasingly 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 andfunctions from a variety of processes are incorporated.

ACKNOWLEDGEMENTSIt 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 originalexperimental material contained in this paper.

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13. R Pethig, et al, 'Development of Biofactory on a chip technology using excimer laser micromachining' , J.Micromech. Microeng. 8, 57 (1999)

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