industrial applications of laser micromachining

Industrial applications of laser micromachining

Malcolm C. GowerExitech Ltd

Hanborough ParkLong HanboroughOxford OX8 8LH

[email protected]

http://www.exitech.co.uk

Abstract: The use of pulsed lasers for microprocessing material in severalmanufacturing industries is presented. Microvia, ink jet printer nozzle andbiomedical catheter hole drilling, thin-film scribing and micro-electro-mechanical system (MEMS) fabrication applications are reviewed.©2000 Optical Society of AmericaOCIS codes: (140.3390) Laser materials processing; (220.4000) Microstructure fabrication

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drilling in printed circuit board materials,” CLEO Conference Summary (1982).6. M. N. Watson, “Laser drilling of printed circuit boards,” Circuit World, 11, 13 (1984).7. F. Bachman, “Excimer lasers in a fabrication line for a highly integrated printed circuit board,”

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micromachined structures and devices using excimer laser mask projection' in Micromachining andmicrofabrication process technology III, Proc. SPIE 3223, 26 (1997).

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(C) 2000 OSA 17 July 2000 / Vol. 7, No. 2 / OPTICS EXPRESS 56#22795 - $15.00 US Received May 24, 2000; Revised June 05, 2000

1. IntroductionAlthough material ablation by pulsed light sources has been studied since the invention of thelaser [1,2], reports in 1982 of polymers etched by uv excimer lasers [3,4] stimulatedwidespread investigations aimed at using the process for micromachining. In the interveningyears scientific and industrial research in this field has proliferated to a staggering extent –probably spurred on by the remarkably small features that can be etched with little apparentdamage to surrounding unirradiated regions of material. Manufacturing industry now useslaser micromachining in many high-tech application areas for which microfabrication is anenabling technology.

It is now known that clean ablative etching can also be achieved using pulsed lasersources at wavelengths other than the ultraviolet. Provided photons are absorbed strongly insubmicron depths in timescales less than the time it takes for heat to diffuse away from theirradiated region, then pulsed lasers like copper vapor (CVL), CO2 and Nd and its harmonicscan be as effective for ablative micromachining. For a particular micromachining applicationthe choice of laser is now judged as much by criteria such as process speed, part throughput,reliability, service intervals, capital and operating costs of the overall machine tool rather thansolely by the quality of the processed part.

The degree of industrial takeup of a technology is a good yardstick for assessing itsutility and state of maturity. This paper discusses some industrial applications of lasermicromachining and its market importance to several sectors. In this context 'industrialapplication' is taken to mean a process proven to add value to a manufactured product. Notdiscussed are areas which use pulsed lasers for non-ablative surface treatments likephotoresist exposure as used in deep-uv photolithography, marking, annealing, hardening,smoothing and secondary ablative processes like pulsed laser deposition of thin films, surfacecleaning and smoothing.

1. Hole drillingThe ability to drill ever smaller holes down to ~1µm diameter, is an underpinning technologyin many industries that manufacture high-tech products. By providing solutions to criticalproblems in manufacturing integrated circuits, hard disks, displays, interconnects, computerperipherals and telecommunication devices, laser micromachining is a key enablingtechnology allowing the current revolution in information technology to continue. Therequirement for material processing with micron or submicron resolution at high-speed andlow-unit cost is an underpinning technology in nearly all industries that manufacture high-tech products. The combination for high-resolution, accuracy, speed and flexibility isallowing laser micromachining to gain acceptance in many industries.

(a) (b)

Fig. 1. 100µm holes drilled in 75µm high-density polyethylene with (a) a twist drill bit (b) a KrF laser


As shown in Fig. 1, a practical illustration of laser micromachining can be seen whencomparing the quality of holes drilled by an excimer laser and a mechanical twist drill. Whilethe mechanically hole is close to the minimum size that can be drilled, the improved quality ofthis and much smaller holes drilled by lasers is obvious.

1.1 Microvia hole drilling in circuit interconnection packagesAlmost as important as the rapid improvements in speed and memory of integrated circuits(IC's) are the parallel developments in interconnection packaging made during the last 20years. So speed, power and area (real estate) are not compromised, packages on which chipsare mounted for connection to other devices have had to keep pace with the rapid advancesmade in IC's. Thus there is a demand for an ever-increasing packing density ofinterconnections - for example mountings in current mobile phones and camcorders havearound 1200 interconnections/cm2. There are now more than a dozen generic types of chipinterconnection packages which include multichip-modules (MCM's), chip-scale-packages(CSP's) like ball-grid-arrays (BGA's), chip-on-boards (COB's), tape-automated-bonds(TAB's). Generally these consist of multilayer sandwiches of conductor-insulator-conductorwith electrical connection between layers made by drilling small holes (vias) through thedielectric and metal plating metal down the hole. Such blind via holes provide high-speedconnections between surface-mounted components on the board and underlying power andsignal planes while minimizing valuable real estate occupation. For example, due todifficulties in soldering IC's with greater than ~200 pins, peripheral lead mounting packageslike TAB's must be made larger than the chip. By placing microvia connections in thepackage at the base of the chip instead of around its periphery, a BGA is no larger than 20%the size of the chip. Typically then the requirement is to drill 100µm diameter microvias on~500µm centers. The cost for drilling these vias on such high-density packages can represent30% of the overall cost of the board.

Drilling microvias by ablation was first investigated in the early 1980's using pulsedNd:YAG and CO2 lasers[5,6]. Excimer lasers led the way in applying it to volume productionwhen the Nixdorf computer plant introduced polyimide ablative drilling of 80µm diametervias in MCM's - as used to connect silicon chips together in high-speed computers [7]. Othermainframe computer manufacturers such as IBM rapidly followed suite and installed theirown production lines for this application [8,9]. With fewer process steps than other methods,laser-drilling is regarded as the most versatile, robust, reliable and high-yield technology forcreating microvias in thin film packages. Trillions of vias have now been drilled with excimerlasers at yields >99.99% whose mean time between failure (MTBF) has been logged at >1,000hours.

Interconnection densities on rigid and flexible printed circuit boards (PCB's andFPC's) are also increasing, driving the requirement for drilling ever-smaller vias in thesepackages [10]. In such lower cost packages the current common practice is to mechanicallydrill the vias. As diameters decrease to <100µm it is generally recognized lasers will displacemechanical drills, although for these packages excimer lasers are too slow and expensive.Because ~100µm diameter tungsten-carbide drills are expensive, frequently break and rapidlywear, drilling costs skyrocket to several $ per 1,000holes. Using TEA, rf-excited slab CO2 orQ-switched Nd:YAG lasers, drilling speeds for precisely positioned vias can be as high as200holes/sec at costs as low as 0.6¢ per 1,000holes. Trepanning the hole with a small focalspot under galvo-mirror scanner control allows hole positions and sizes to be programmed


from CNC drill files containing the circuit layout - see Figure 2(a). Since copper is highlyreflective at 10µm, Q-switched Nd:YAG lasers (fundamental or 3rd-harmonic) are used todrill the metal, while either CO2 (rf-excited or TEA) or 3rd-harmonic YAG lasers drill thedielectric material. When drilling blind microvias, CO2 lasers have the advantage that drillingnaturally self-terminates at the copper level below without damaging it. Holes definedpreviously in the top copper (either by a YAG laser or chemically etched usingphotolithography) can be used as a conformal mask for cleanly drilling the dielectric material- see Fig. 2(b).

(a) (b)

Fig.2. 100µm diameter blind microvia drilled in a PCB. (a) Step 1. Nd laser trepanned hole in top copper

conductive layer. (b) Step 2. CO2 laser drilling of fiber reinforced composite FR4 layer to copper below.

There is a large potential market for laser microvia drilling tools. Many companies nowuse pulsed CO2 lasers for drilling 80-100µm blind vias through dielectric layers on MCM's,CSP's, and TAB's which then get incorporated into flat panel displays, hard-disk drives,printers, cameras, cellular phones, photocopiers, fax machines, notebook and palmtop PC's.With laser-drilling now producing twice as many microvias than any other method, the annualmarket for laser-drilling tools in Japan alone is estimated to be ~600 units.

Fig. 3. Nd:YAG & CO2 hybrid laser tool for microvia drilling.

1.2 Ink jet printer nozzle drillingInkjet printers comprise a row of small tapered holes through which ink droplets are squirtedonto paper. Adjacent to each nozzle, a tiny resistor rapidly heats and boils ink forcing itthrough the orifice. Increased printer quality is achieved by simultaneously reducing thenozzle diameter, decreasing the hole pitch and lengthening the head. Modern printers likeHP's Desk Jet 800C and 1600C have 300x 28µm input diameter nozzles giving a resolution of600 dots-per-inch (dpi). Earlier 300dpi printers consisted of a 100nozzle row of 50µmdiameter holes made by electroforming thin nickel foil. Trying to fabricate more holes withsmaller diameters reduced even further the already low 70-85% production yield. Laser-drilling of nozzle arrays allowed manufacturers to produce higher performance printer heads


at greater yields. At average yields of >99%, excimer laser mask projection is now routinelyused for drilling arrays of nozzles each having identical size and wall angle [11]. Most of theink jet printer heads sold currently are excimer laser drilled on production lines in the US andAsia. Figure 4(a) shows some excimer laser drilled nozzles in a modern printhead.

(a) (b)

Fig. 4. (a) Array of 30µm diameter ink jet printer nozzles drilled in polyimide. (b) Array of nonlinear tapered

nozzles aiding laminar fluid flow

Fig. 4(b) shows nozzles with nonlinear tapers to aid the laminar flow of the dropletthrough the orifice. More advanced printers sometimes use piezo-actuators. Rather than beingconstrained to give shapes characteristic of the process, excimer laser micromachining toolswith appropriate CNC programming can readily engineer custom-designed 2½D and 3Dstructures. Fig. 5(a) shows an example of a rifled tapered hole that spins the droplet to aid itsaccuracy of trajectory, while Figure 5(b) shows an array with ink reservoirs machined behindeach nozzle.

(a) (b)

Fig. 5.(a) Tapered nozzle with rifling. (b) Nozzle array with machined reservoirs

1.3 Hole drilling in biomedical devicesAs in microelectronics and its associated technologies, the drive for increasing miniaturizationwith improved device functionality is crucial to the rapid progress being made in thebiomedical industry [12]. Precision microdrilling with excimer lasers is routine when makingdelicate probes used for analysing arterial blood gases (ABG's) [13]. ABG sensors measurethe partial pressures of oxygen (PaO2), carbon dioxide (PaCO2) and hydrogen-ionconcentration (pH) used for monitoring the acid-base concentration essential for sustaininglife. In intensive care units, ABG results are used to make decisions on patient's ventilatorconditions and the administration of different drugs. The use of fiber-optic sensors for ABGanalysis provide clinical diagnostics at the patient's bedside without the need for taking bloodsamples [14].


Fig. 6(a) shows an example a ABG catheter for monitoring blood in prematurelyborne babies. The hole at the side of the PVC bilumen sleeving tube through which blood isdrawn is machined using a KrF excimer laser. In this case the clean cutting capability of thelaser provides the necessary rigidity that prevents kinking and blockage of the tube wheninserted into the artery.

(a) (b)

Fig. 6 (a). Hole in the side of a bilumen catheter (b) Automated reel-to-reel excimer laser workstation for

simultaneous hole drilling in optical fibers.

More important components of this catheter are the PaO2 and PaCO2 sensors. These consist ofa spiral of up to five ~50x15µm rectangular holes machined in a 100µm diameter acrylic(PMMA) optical fiber with an ArF laser. The holes are filled with a reagents whose opticaltransmission depend on the PaO2 and PaCO2 levels of the surrounding blood. Using a fully-automated workstation shown in Figure 6(b) that has computer-controlled reel-to-reel fiber-feeding and laser-firing, all five holes shown in Figure 7(a) are drilled in the fiber. Byspatially-multiplexing a single excimer beam into five smaller ones, holes are drilledsimultaneously through the fiber.

(a) (b)

Figure 7. (a) Rectangular 50x20µm holes drilled in 100µm fibers for PaO 2 & PaCO2-sensors. (b) Laser stripped

insulation from 100µm diameter pH-sensor wire

Preferential excimer laser etching of plastics compared to metals is applied to the stripping ofinsulation from fine diameter wires prior to soldering connections. The process relies on thethreshold for excimer laser ablation of the polymer being much lower than for damaging thecopper or silver core. As shown in Figure 7(b), excimer lasers are used to cleanly strip awaythe polyurethane insulation sleeving of wires which form the pH resistivity sensor in the ABGcatheter above. Such pulsed laser wirestripping is also in widespread use for preparingconnection wires to computer hard-disk reader heads.


2. Laser scribing of thin films

Although the cost for producing power from the best large-area thin-film silicon (TFS) solarcells is still ~25¢ /kWhr which is more than three times that for fossil and nuclear fuel powerstations, recent technological improvements in cell design have stimulated a rapid growth fordomestic and commercial use on buildings as a local source of electricity. Compared tocrystalline devices, TFS panels use far less active material and because interconnectionsbetween cells are intrinsic to their fabrication are cheaper to manufacture. First introducedcommercially in the early 1990's, TFS solar cells comprise a 5-layer thin-film sandwich on afloat-glass substrate with each layer only a fraction of a micron thick. Light passes throughthe glass and the first film of a transparent conductive oxide (TCO) material like indium tinoxide (ITO).

Figure 8. Laser scribing of thin films on solar panels and completed TFS panel

As illustrated in Figure 8, electron-hole pairs and a photovoltaic voltage are generatedbetween p-i-n Si-diode junction layers. Individual cells are segregated and interconnected byscribing narrow isolation tracks in each film and collecting the photocurrent at the end of thepanel. The fabrication steps are: (i) chemical vapor deposition of TCO; (ii) cell segregation bylaser-scribing; (iii) plasma deposition of p-i-n doped amorphous Si films, each layer laser-scribed; (iv) deposition of conductive film of aluminum or TCO material; (v) laser-scribing oftop conductor; (vi) laminate protective plastic or glass covering on top.

The cell width is generally varied to give the required voltage while its length ischanged to produce the requisite current. For incorporating into individual productscompleted panels are cut into smaller sizes. To maximize efficiency, isolation tracks need tobe kept as narrow as possible conducive with maintaining high electrical resistivity betweenthe collection strips. The ability to achieve high lateral spatial resolution with precision depth-control without inducing damage to underlying layers are the reasons pulsed excimer and Q-switched YAG laser ablation is used for scribing cells [15]. Tracks are typically ~25-50µmwide displaced by ~30-50µm in each film giving inter-strip impedances >1MΩ. Glass panelsto ~0.5m size are processed with the laser operating 'on the fly' taking typically ~1min/layerto scribe. The efficiency for generating electricity from the best TFS cells is now ~7%, so infull sunlight ~70W is produced per square meter of cell. Currently the world manufacturingcapacity for TFS cells is ~40MW.

There is intense ongoing research on various types of flat panel display devices(FPD's) which will supersede conventional cathode-ray tubes (CRT's) in most high-definition


and large area applications. These include active matrix liquid crystal displays (AMLCD's),polymer light-emitting diodes including polymer devices (LED's and pLED's) and plasmadisplays panels (PDP's). This display market is now worth around $30B/year. As the demandfor larger panels grows more conventional lithography-etch methods of production becomeproblematic and expensive. Lasers are increasingly being used to process FPD's in scribingoperations that segment and define interconnect electrode circuitry in a similar manner to that

used for scribing TFS solar panels. (a) (b)

Figure 9. (a) 25µm wide tracks in ITO layer. (b) Nd pulsed laser panel scribing machine

Fig. 9(a) shows a thin film of ITO on glass scribed with 25µm wide tracks by the Q-switched3rd-harmonic Nd laser machine in Fig 9(b). In addition excimer lasers are now commonly usedfor low temperature annealing of the silicon layer in thin-film transistors (TFT's) used toswitch and hold the transmissive state of pixels on AMLCD's.

3. Future trends in laser micromachining

'Micro-electro-mechanical systems' (MEM's) or 'Microsystems technologies' (MST) bringtogether mechanical, electrical and optical technologies to create an integrated device thatemploys miniaturization to achieve high-complexity in a small volume [16]. This generallyinvolves fabricating mm-µm size structures with µm-nm tolerances. Existing products includedevices such as computer hard-disk drive heads, inkjet printer heads, heart pacemakers,hearing aids, pressure and chemical sensors, infrared imagers, accelerometers, gyroscopes,magnetoresistive sensors and microspectrometers. Recent market analysis reports predict themarket for MEM's products will continue to grow at a rate of 18%/annum reaching a value of$34B/year by 2002. Emerging products like drug delivery systems, magneto-optical heads,optical switches, lab on a chip, magneto-optical heads and micromotors will add an additional$4B to this market. The success of microengineering comes from miniaturization and itsconsequences: high-sensitivity, short-measurement times, low-energy consumption, good-stability, high-reliability, self-calibration and testing. Microsensors detecting local parameterslike pressure, flow, force, acceleration, temperature, humidity, chemical content etc, have inthe last decade been engineered into the engine and performance management systems of carsand aircraft. They also provide the key to electromechanical microcomponents such as ink jetprinter nozzles, gas chromatographs, gyroscopes, galvanometers, microactuators,micromotors, micro-optics etc. Devices like implantable drug delivery systems containingsensors, valves and control system with power source capable of operating for many years arebeing developed. There is no doubt microengineering will be a key underpinning technologyof the 21st century.


Examples of the types of surfaces that can be structured by excimer laser ablation areshown in Fig. 10. Blazed grating and pyramid-like structures can be readily fabricated onsurfaces by mask-dragging techniques [17].

(a) Blazed grating (b) Pyramids.

Figure 10. KrF laser produced surfaces in polycarbonate produced using mask-dragging techniques.

Such methods can be used for making micro-optical surfaces like those shown in Figure 11.The ‘black’ anti-reflective property of the ‘moth’s-eye’ type of surface machined on the CsI

(a) Cesium Iodide far-infrared optical crystal (b) Polycarbonate

Figure 11. Micro-optical surfaces fabricated by KrF laser micromachining and orthogonal mask-dragging

crystal substrate shown in Fig. 11(a) is being used to prevent ghost images in very largeinfrared optical telescopes. The microlens array shown in Fig. 11(b) is used for shaping beamsfrom laser diodes. Each lenslet in this array has a focal length of 1mm.

Currently most MST devices are manufactured using photolithography to define thesurface shape of silicon or quartz material that is selectively removed below in a subsequentchemical or plasma etching process. Material removal is almost always used to achieve thetopography of the MST device being fabricated. Microparts having a 2½D topology arefabricated by undercutting material from the planar surface using anisotropic etching alongcrystallographic planes. Upward building is achieved by depositing additional layers on top ofprocessed ones below and repeating the lithography-etch cycle. Adaption of siliconlithography and etch batch-processing as developed by the semiconductor industry iscurrently the dominant MEMS fabrication method. However being restricted to just onematerial (silicon) surface and bulk etched in only three directions - along (110), (100) and(111) crystallographic-planes, other more flexible micromachining methods including pulseduv-laser ablation are being evaluated for MEMS. The advantages of laser micromachiningare: (i) few processing steps, (iii) highly-flexible CNC programming of shapes for engineeringprototyping, (iii) capable of serial and batch-mode production processing, (iv) no majorinvestment required in large clean-room facilities and many expensive process tools, (v)


applicable to a wide range of polymers, ceramics, glasses, crystals, insulators, conductors,piezomaterials, biomaterials, non-planar substrates, thin and thick films, (vi) compatible withlithographic processes and photomask manufacture.

Advances in personal healthcare and environmental monitoring are driving thedevelopment of various diagnostic chip-based devices for performing analytic functions.Applications include diagnostics of food and water supplies, drug delivery systems, personaldrug administration, DNA analysis, pregnancy testing and blood monitoring. Although manysensor device technologies are well developed, the inherent bio-incompatibility of silicon isdriving the move towards developing microengineering techniques suitable for use with othermaterials. Since most biocompatible materials do not lend themselves to lithography-etchprocessing laser machining of biodevices is becoming increasingly important. Fig. 12 showsthe layout of a 55x40mm biochip that uses travelling-wave dielectrophoresis to sort and sensecells [18]. It comprises 2½D laminations of channels, chambers and electrode conveyortracks. An excimer laser is used to both ablate a conductive layer of gold to leave 10µm wideelectrode tracks as well as drill microvia holes in the polymer layer [18].

Figure 12. Biochip manufactured using laser micromachining.

Using only a low-voltage AC power supply, dielectrophoresis is used to control celltransport in the chip. Microfluidic channels and ramps like those shown in Fig. 13 are used totransport the sample from inlet ports to analysis sites.

Figure 13. KrF laser micromachined microfluidic channels in polyester

Constant and varying depth laser-micromachined channels are also used for locatingand securing fiber optics in telecommunication devices as shown in Fig. 14.


Figure 14. KrF laser micromachined fiber holders in polyester

As illustrated in Fig. 15, controlled 3D-structuring of materials by excimer laseretching [17] can produce the basic building blocks of bridges, diaphragms, pits, holes, ramps,cantilevers, etc needed for microengineering devices like gyroscopes, galvanometers, gaschromatographs, microactuators, micromotors, etc.

(a) Beam structure (b) Ramps, channels and bars (c) Descending column staircase

Figure 15. KrF laser-machined 3D-structures in polycarbonate.

Figure 16 shows examples of uv-laser micromachined 3D-structures in polymerswhich when used with the LIGA process of electroforming (from the German acronym:lithographie galvanoformung abformung), can be replicated in metal - a process now knownas laser-LIGA[19]. Once a master has been made by excimer laser micromachining suchmethods allow high volumes of replica parts to be manufactured at low unit costs.

(a) KrF laser machined 100µm fiber clamp (b) 470µm diameter, 130µm height nickel intravascular

rotor microturbine. Replicated by electroplating from

ArF laser machined PMMA master. Courtesy of the

Rutherford Appleton Laboratory

Figure 16. MEMS devices fabricated by excimer laser micromachining


Already recognized by government-supported initiatives in Japan and the EuropeanUnion, laser micromachining will be a key manufacturing tool in emergingnanotechnologies[20]. The economic advantages of mass production at low unit cost is of thehighest importance and will open up many new industrial application areas.

4. AcknowledgementsIt is a pleasure to thank J Fieret, D Milne, N Rizvi, P Rumsby, and D Thomas of Exitech Ltdwho contributed much of the original experimental material contained in this paper.


industrial applications of laser micromachining

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