Industrial Applications of Pulsed Lasers to Materials Microprocessing

Malcolm C Gower

Exitech Ltd Hanborough Park Long Hanborough Oxford OX8 8LH United Kingdom1

ABSTRACT The use of pulsed lasers for microprocessing material in several manufacturing industries is presented. Deep-uv photolithography, via hole and ink jet printer nozzle microdrilling, solar panel thin film scribing, texturing of hard disks, annealing amorphous silicon in flat panel displays, fiber Bragg grating production and micro-electro-mechanical system (MEMS) fabrication applications are discussed. Keywords: Materials microprocessing, pulsed lasers, laser ablation, industrial applications

1. INTRODUCTION

By providing solutions to critical problems in manufacturing integrated circuits, hard disks, displays, interconnects, desk top printers and telecommunication devices, pulsed laser materials processing is a key enabling technology allowing the current revolution in information technology to continue. The requirement for material processing with micron or submicron resolution at high-speed and low-unit cost is an underpinning technology in nearly all industries manufacturing hightech products. The combination of high-resolution, accuracy, speed and flexibility has allowed pulsed laser materials processing to gain acceptance by many industries(1). This paper reviews the background and market importance to several industries which use pulsed lasers to expose surfaces at sub- and above-threshold fluences for ablation. Areas like laser marking, coding, surface hardening, etc are considered to be on a macro scale and outside the scope of this paper.

2. DEEP-UV PHOTOLITHOGRAPHY FOR MANUFACTURING INTEGRATED CIRCUITS

At single-pulse fluences below the threshold for material ablation, the KrF excimer laser is becoming the light source of choice for performing the lithography step crucial to the mass production of large memory size integrated circuits (IC's), microprocessors and application-specific IC's (ASIC's)(2,3). Its short wavelength allows ever-smaller (down to ~0.14µ m) and greater densities of devices to be replicated. For more than 30 years the memory-size of Dynamic Random Access Memory (DRAM) chips has quadrupled every 3 years. Amazingly this trend, now called Moore's law, was predicted as long

1Further author information: Exitech Limited: Tel: 44-1993-883324; Fax: 44-1993-883334; e-mail: m.gower@exitech.co.uk Presented at SPIE's Symposium on 'High Power Laser Ablation', Santa Fe, New Mexico, USA, April 1998

ago as 1965 by Fairchild Semiconductor and Intel founder Gordon Moore - only 5 years after the first commercial planar transistor was introduced in 1959. This remarkable progress has been achieved by both decreasing the smallest circuit feature size by x√ 2 and doubling the chip area on a three-yearly cycle. Until at least 2005, there is little sign of this trend slowing down. With a total value of ~$150B/year which is expected to double by the year 2000, currently there are around 100 factories or "wafer fabs" making IC's. Equipment comprises ~85% of the total ~$1.5B fab building costs. Fab costs are projected to rise to >$4B by 2000 primarily because of increasing costs for lithography equipment. In the next ten years the electronics sector, of which the semiconductor content represents around 20%, is expected to grow to become the world's second largest industry after agriculture. Figure 1(a) shows the technology trends for photolithography as used in this industry.

17/07/02 Source: Cymer Inc.

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Source: Dataquest 2Figure 1. Lithography technology trends. Design-rule critical dimensions (CD) & global stepper sales

To achieve ever-smaller feature sizes, the performance of the step and repeat cameras or "steppers" used to replicate the circuit patterns from masks onto photoresist-coated silicon wafers has had to improve enormously in a very short time(4). Higher optical resolution using shorter wavelengths of illumination has enabled smaller circuit features to be produced. Increasing the image field diameter of the lens has allowed larger area chips to be fabricated. Figure 1 shows the annual global sales of steppers by type. In spite of higher plant costs, machine throughput must increase to allow the manufacturing cost of chips to remain roughly the same as it has for decades: ~$4/cm2 of processed silicon, of which ~35% is associated with lithography costs. Each generation of chip requires the development of increasingly complex imaging lenses which in terms of performance are already the most advanced optical elements every built. High-power mercury arc lamp sources are now the industry standard - first at 436nm (g-line), then 365nm (i-line). Because lamps cannot produce high-powers at shorter wavelengths, the industry is now turning to using excimer laser sources: currently KrF at 248nm; in the next century ArF at 193nm; followed possibly by F2 at 157nm - the technology route for achieving 0.1µ m design rules around 2007 is

still not yet decided(5). These lasers must produce up to 10W average power at 1kHz repetition rates in a FWHM linewidth of <1pm. The photoresist is exposed with multiple pulses at fluences ≤ 0.2mJ/cm2/pulse. Figure 2 shows 0.18µ m wide lines exposed in a chemically-amplified resist using an ArF laser source. In recent experimental work(6) 0.08µ m lines and spaces have been imaged using a 157nm laser source.

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Figure 2. (a) 193nm ArF laser lithography microstepper R&D tool. (b) 0.18µ m wide lines & spaces imaged in Shipley XP7022 resist. Courtesy of France Telecom CNET/DTM/TFM within the GRESSI consortium.

(b) 3 Larger than a garbage can, weighing ~ 1ton and costing ~$1M, current 248nm excimer laser stepper lenses image 0.25µ m features over a field diameter of ~35mm i.e a resolving power of >1010 pixels. Several critical levels of the Intel Pentium® II processor are made using KrF laser steppers. Such high-resolution optical systems are capable ultimately of imaging features in photoresist as small as half the wavelength of the light source. By 2000 each tool will cost ~$10M, making the total market for excimer laser steppers ~$5B/year. Since laser costs are $0.5-1M/tool, the ~$0.5B/year revenues generated will make this one of the most valuable sectors in the laser industry.

3. LASER DRILLING OF VIA HOLES IN INTERCONNECTION PACKAGES Almost as important as the rapid improvements in speed and memory of IC's are the parallel developments in interconnection packaging made during the last 20 years. So speed, power and area (real estate) are not compromised, packages on which chips are mounted for connection to other devices have had to keep pace with the rapid advances made in IC's. Thus there is a demand for an ever-increasing packing density of interconnections - for example mountings in current mobile phones and camcorders have around 1200 interconnections/cm2. There are now more than a dozen generic types of chip interconnection 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-conductor with electrical connection between layers made by drilling small holes (vias) through the dielectric and metal plating metal down the hole. Such blind via holes provide high-speed connections between surface-mounted components on the board and underlying power and signal planes while minimizing valuable real estate occupation. For example, due to difficulties in soldering IC's with greater than ~200 pins, peripheral lead mounting packages like TAB's must be made larger than the chip. By placing microvia connections in the package 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 100µ m diameter microvias on ~500µ m centers The cost for drilling these vias on such high-density packages can represent 30% of the overall cost of the board.

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

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(b) (c) 4Figure 3. (a) Nd:YAG & CO2 hybrid laser tool for microvia drilling.100µ m blind via in a PCB. (b) Step 1. Nd:YAG laser trepanned hole in top copper conductive layer.(c) Step 2. CO2 laser drilling of FR4 dielectric layer to copper below. Interconnection densities on rigid and flexible printed circuit boards (PCB's and FPC's) are also increasing, driving the requirement for drilling ever-smaller vias in these packages(10). In such lower cost packages the current common practice is to mechanically drill the vias. As diameters decrease to ≤ 100µ m it is generally recognized lasers will eventually displace mechanical drills, although for these packages excimer lasers are too slow and expensive. Because ~100µ m diameter tungsten-carbide drills are expensive, frequently break and rapidly wear, drilling costs skyrocket to several $ per 1,000holes. Using TEA, rf-excited slab CO2 or Q-switched Nd:YAG lasers, drilling speeds for precisely positioned vias

can be as high as 200holes/sec at costs as low as 0.6¢ per 1,000holes. Trepanning the hole with a small focal spot under galvo-mirror scanner control allows hole positions and sizes to be programmed from CNC drill files containing the circuit layout - see Figure 3(b). Since copper is highly reflective at 10µ m, Q-switched Nd:YAG lasers (fundamental or 3rd-harmonic) are used to drill the metal, while either CO2 (rf-excited or TEA) or 3rd-harmonic YAG lasers drill the dielectric material. When drilling blind vias, CO2 lasers have the advantage that drilling naturally self-limits at the copper level below without damaging it, and holes defined previously in the top copper (either by a YAG laser or photolithography) can be used as a conformal mask for cleanly drilling the dielectric material - see Figure 3(c). Although there is a hugh potential market for laser via drilling tools, it is still at the proving stage. Nevertheless, in Japan companies are now using pulsed CO2 lasers for drilling 80-100µ m 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, mobile phones, photocopiers, fax machines, notebook and palmtop PC's. With laser-drilling now producing twice as many microvias than any other method, the annual market for laser-drilling tools in Japan alone is estimated to reach ~400 by 1999.

4. LASER DRILLING OF INK JET PRINTER NOZZLES Inkjet printers comprise a row of small tapered holes through which ink droplets are squirted onto paper. Adjacent to each nozzle, a tiny resistor rapidly heats and boils ink forcing it through the orifice. Increased printer quality is achieved by simultaneously reducing the nozzle diameter, decreasing the hole pitch and lengthening the head. Modern printers like HP's Desk Jet 800C and 1600C have 300x 28µ m input diameter nozzles giving a resolution of 600 dots-per-inch (dpi). Earlier 300dpi printers consisted of a 100 nozzle row of 50µ m diameter holes made by electroforming thin nickel foil. Trying to fabricate more holes with smaller 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 routinely used for drilling arrays of nozzles each having identical size and wall angle(11). Most of the ink jet printer heads sold currently (e.g by HP and Canon) are excimer laser drilled on production lines in the US and Asia. Figure 4(a) shows some excimer laser drilled nozzles in a modern printhead.

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Figure 4 (a) Array of 30µ m diameter ink jet printer nozzles drilled in polyimide.

(b) Array of nonlinear tapered nozzles aiding laminar fluid flow

(b) 5 Figure 4(b) shows nozzles with nonlinear tapers to aid the laminar flow of the droplet through the orifice. More advanced printers sometimes use piezo-actuators. Rather than being constrained to give shapes characteristic of the process, excimer laser micromachining tools with appropriate CNC programming can readily engineer custom-designed 21/2D and 3D structures. Figure 5(a) shows an example of a rifled tapered hole which spins the droplet to aid its accuracy of trajectory, while Figure 5(b) shows an array with ink reservoirs machined behind each nozzle.

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Figure 5

(a) Tapered nozzle with rifling.

(b) Nozzle array with machined reservoirs

(b) 6

5. LASER ANNEALING OF FLAT PANEL DISPLAYS In 1997 the world market for flat panel displays (FPD's) was worth ~$14B and predicted to grow to $22B by the end of the century. Active matrix liquid crystal displays (AMLCD's) are gradually displacing the supertwisted nematic devices from the FPD market because they have a much wider viewing angle, can be viewed in bright sunlight and are insensitive to extremes of temperature. Because of the greater costs and manufacturing difficulties inherent to AMCLD's, this change has been slower than expected originally. Each pixel in an AMCLD has a small thin-film transistor (TFT) located in its corner that switches and holds the state of polarization for light transmitted through the display. For reasons of speed on displays larger than 17", TFT's are sometimes also incorporated on peripheral driver circuitry used to address the pixels. Silicon must have a high-carrier mobility and low-leakage current to achieve high-switching speeds and long-hold times of TFT's. With plasma-enhanced and low-pressure chemical vapor deposition (PECVD and LPCVD) methods for depositing silicon films, the amorphous (a-Si) state is produced having a low channel mobility (≤ 1cm2/V.s). The mobility can be increased by melting and recrystallizing a-Si (annealing) to form polycrystalline material (poly-Si). However placing the panel in a furnace to anneal the film requires temperatures between 600-1400oC which is sufficient to cause softening of borosilicate glass substrates. Thermally-resistant materials like quartz or high-temperature glasses are both expensive and available in small sizes only. For example, blanks of fused silica cost ~$1.60/cm2 so for an A4 size display the cost of the substrate alone would be ~$900. While these materials are satisfactory for very small displays like camera viewfinders, they are not cost-effective for ones larger than ~1". Since heat transfer to the substrate is minimal when annealing the film with an excimer laser beam, inexpensive float-glass substrates can be used for the display. As well as lower material costs, laser annealing of FPD's also provides flexibility. Selected regions can be annealed allowing a-Si and poly-Si devices to be incorporated on the same panel. Also, throughputs are higher than for furnace annealing. Since the absorption depth for uv photons in a-Si is only 6-10nm and film thicknesses range between 20-150nm, excimer laser pulses are almost completely absorbed in the film(12). For ~20nsec duration pulses, the heat affected zone (HAZ) as determined by the heat diffusion length is also small (~100nm). Below the threshold fluence for ablation, at 180-250mJ/cm2/pulse the film melts and on recrystallization forms poly-Si grains. The average grain size and the carrier density are increased, both of which contribute to raising the mobility of the film. Depending on the laser fluence, number of pulses, film thickness and type (n or p-doped), poly-Si grain sizes range between 30-300nm with channel mobilities >100cm2/V.s - well exceeding the 20-30cm2/V.s values obtained in thermal annealing. As shown in Fig 6(a), larger grain sizes are produced when annealing thinner films and applying multiple pulses enlarges their growth to >3µ m. Figure 6(b) shows the variation of electron mobility with single pulse 248nm laser fluence as the beam is scanned across n-doped films

of different thicknesses. Region II corresponds to conversion of a-Si to poly-Si at only the surface of the film with grain sizes of ~100nm. At greater fluences (Region III), the complete film is melted producing larger grains with higher film mobility.

17/07/02(a) (b) Figure 6. (a) As deposited 50nm a-Si film (b) 1 - 5 µ m crystal grains laser annealed with 128pulses(13). (c) Electron field mobilities for n-type TFT(14)

(c) 7 As illustrated in Figure 7(a), the beam from a high-repetition rate 200W excimer laser (either 248 or 308nm) is typically configured in the form of a long thin stripe up to 300mm long which is then scanned in a raster fashion across the complete area of the panel.

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(b) 7Figure 7. (a) Raster scanning of FPD with homogenized excimer line beam (b) Annealing system with vacuum chamber

Since small changes of the degree of crystallization can produce large variations in mobility, the single pulse spatial intensity beam uniformity and integrated dose must be tightly controlled. Spatial beam homogenization is used to provide a uniform intensity in the long direction, while beam overlap and multipulse exposure provide temporal homogenization during processing. TFT's having an industrially acceptable mobility uniformity of <± 10% are now being routinely produced(15). Uniformity of dose control and lack of stitching seams make this scanning method preferable to step-and-

repeat types of exposure. Although annealing with a few pulses from a low repetition-rate high-pulse energy excimer laser is being investigated(16), dose uniformity control and the immaturity of this laser technology applied to manufacturing industry remain critical issues. In Japan, Sony now use excimer laser annealed TFT displays on their camcorders, while Toshiba recently announced it will soon begin production of 400 x 500mm excimer laser-annealed FPD's at a rate of ~6,000 panels/month.

9. LASER TEXTURING OF HARD DISKS Melting surfaces with pulsed lasers is being used in computer hard-disk manufacturing. The inductive read-write head in a hard-drive floats on the boundary layer of air ~50nm above the nickel-phosphorous coated surface of the aluminium disk as it rotates at several thousand rpm. When power is turned off, the slider on which the ~3mm head is located lands on the surface of the disk. If the two surfaces were perfectly smooth, stiction forces on restarting would prevent the disk spinning-up leading to catastrophic damage of head and disk. To prevent this, the disk surface is deliberately microroughened using an abrasive polish of diamond grit slurry. Random bumps are produced with peak-to-valleys typically averaging ~20nm. Any higher peaks which may hit the head limit the reliability and lifetime of the drive, although rarely cause a loss of data. As fly heights reduce proportionally to accommodate the ~60%/year increase in disk storage density, so there is a continuous drive to reduce bump heights.

In 1994 IBM first introduced the use of pulsed lasers to texture of disks with a greater degree of process control(17). Using 4mJ, 40nsec pulses from a 10kHz pulsed Q-switched diode-pumped Nd:YLF laser focussed to 3-15µ m spots on the disk surface, craters with peak-to-valley heights of ≤ 10nm are created with great reproducibility. In forming the crater, material reflows rather than ablatively removed so no contaminants are generated. As the disk rotates at several thousand rpm with the laser operating at a fixed repetition rate, a spiral track of around half a million bumps is created as the beam is moved slowly outwards. Their ~50µ m separation is controlled by adjusting the rotation speed of the disk. As well as providing smaller bump heights, the reliability and durability of the drive is improved further by the lower stiction forces associated with the regular pattern of bumps. To maximize process speed, Nd:Vanadate sources operating at >100kHz repetition rates are now incorporated into laser-texturing machines. Unlike mechanical-roughening methods, laser-texturing readily lends itself to treating only selected regions of the disk. With throughputs up to 300 disks/hour, laser texturing tools are up to ~x3 faster than mechanical machines and being a clean, dry one-step process without consumables are cheaper to operate. When the power is turned off, only a region the width of the head located on the inner diameter of the disk need be used to land and park the slider. Thus the remainder of the disk can be kept highly polished allowing fly heights to be reduced even further as storage densities increase. Because of higher sensitivity and storage density capability, magneto-resistive (MR) read-write heads are now replacing inductive devices. However in occasional collisions bump peak collisions, MR heads are more prone to losing data in soft crashes of the drive. Hence the data region on disks used with MR heads must be highly polished making selective texturing of the disk essential. It is predicted by the end of the century nearly all the estimated market of 600 million disks/year will be laser textured.

8. LASER SCRIBING OF THIN FILM SOLAR PANELS The maximum intensity of solar radiation incident on the earth is ~1kW/m2 which when spatially and temporally averaged over the entire surface and an annual cycle reduces to ~0.3kW/m2. Thus a 140x140km collection area could generate the entire US electricity requirements of 2.5x1012kWhr/year assuming an overall plant efficiency of 10%(18). Currently solar power is mostly limited to supplying power to remote objects such as satellites, oil and gas platforms, telecoms equipment, railway signalling and consumer products like wristwatches, calculators, battery chargers, ventilators, etc.

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Although the cost for producing power from the best large-area thin-film silicon (TFS) solar cells is still ~25¢ /kWhr and more than three times that for fossil and nuclear fuel power stations, recent technological improvements in cell design have stimulated a rapid growth for domestic and commercial use on buildings as a local source of electricity. Compared to crystalline devices, TFS panels use far less active material and because interconnections between cells are intrinsic to their fabrication are cheaper to manufacture. First introduced commercially in the early 1990's, TFS solar cells comprise a 5-layer thin-film sandwich on a float-glass substrate with each layer only a fraction of a micron thick. Light passes through the glass and the first film of a transparent conductive oxide (TCO) material like indium tin oxide (ITO). As illustrated in Figure 8, electron-hole pairs and a photovoltaic voltage are generated between p-i-n Si-diode junction layers. Individual cells are segregated and interconnected by scribing narrow isolation tracks in each film and collecting the photocurrent at the end of the panel. The fabrication steps are: (i) chemical vapor deposition of TCO; (ii) cell segregation by laser-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 of top conductor; (vi) laminate protective plastic or glass covering on top. In general the cell width is varied to give the required voltage while its length is changed to produce the requisite current. For incorporating into individual products completed panels are cut into smaller sizes. To maximize efficiency, isolation tracks need to be kept as narrow as possible conducive with maintaining high electrical resistivity between the 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(19). Tracks are typically ~25-50µ m wide displaced by ~30-50µ m in each film giving interstrip impedances >1MΩ. Glass panels to ~0.5m size are processed with the laser operating 'on the fly' taking typically ~1min/layer to scribe. The efficiency for generating electricity from the best TFS cells is now ~7%, so in full sunlight ~70W is produced per square meter of cell. Currently the world manufacturing capacity for TFS cells is ~40MW. Sanyo who are the world's leading producer, manufacture ~0.4m size roof tiles and have an extensive domestic market through the Japanese government-sponsored 7000 Roof Program. A machine incorporating a Q-switched Nd:YAG laser for scribing TFS cells is shown in Figure 9.

17/07/02 Figure 9 Nd:YAG pulsed-laser solar-panel scribing machine 10

9. LASER WRITING OF FIBER BRAGG GRATINGS

As a means of greatly increasing the bandwidth for data transmission through optical fibers, wavelength division multiplexing (WDM) of signals is currently experiencing extremely rapid growth in the telecommunications sector. Early single-mode fibers were designed to have zero dispersion at 1.3µ m - the best source wavelength at the time. With ensuing improvements to laser diodes at longer wavelengths, modern fibers are designed to operate in the region of lowest attenuation in silica at 1.5µ m. To double the bandwidth of older fibers, both wavelengths can be copropagated in the fiber. Until 1994, WDM for data transmission was limited to just these two channels at rates up to 2.5Gb/sec/channel. In current communication systems this has expanded to 16 channels and is expected to rise to ~40 wavelengths within a year. With single-channel capacity also rising to ~10Gb/sec, WDM will soon enable a single fiber to carry rates of ~160Gb/sec. Key parallel developments which are making WDM technology so successful are (a) stable output at specific wavelengths from DFB laser sources (b) erbium-doped fiber-amplifiers for signal regeneration without electronic conversion (c) external modulators for reducing crosstalk between signals at closely spaced wavelengths (d) fiber Bragg gratings (FBG's) for efficient separation (demultiplexing) of closely spaced wavelengths. In 1997, Ciena Corp a manufacturer WDM devices, became the largest public startup company in corporate history and with first year earnings of ~$200 million had the fastest revenue track ever(20). FBG's are manufactured by exposing the fiber core through the side cladding with a spatially-modulated intensity distribution from a uv laser - sometimes 2nd-harmonic cw Ar+, but increasingly pulsed 193 and 248nm excimer and 266nm 4th-harmonic Nd:YAG or Nd:YLF(21). Although not yet fully understood, it is believed that the uv light breaks oxygen-vacancy defect bonds in the (germanium or cerium) doped core. Liberated electrons migrate and retrap at other color center sites leading to permanent intensity-dependent changes of ~1% to the refractive index. The most simple and popular method for producing the intensity modulation is to use the pattern created by the interference between ± 1 diffracted orders from a transmission phase mask designed to suppress light in the 0th-diffracted order. When the mask is placed close to the side of the fibre at average illumination intensities of 10-20W/cm2, gratings can be recorded in a few minutes depending on the photosensitivity of the fiber used. For reflectivity at 1.5µ m wavelengths, the FBG pitch needs to be 0.5µ m. As shown in Figure 10(b), FBG's several millimeters in length can produce reflection filters with bandwidths <100GHz (≤ 0.8nm).

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11Figure 10 Proximity mask aligner for writing FBG's & 0.6nm bandwidth reflection peak at 1.6µ m of 248nm-written FBG.

FBG's can be used for other signal management operations in telecommunications. Due to the wavelength variation of refractive index, normally different wavelengths will travel at different speeds in the silica core which because its increasing bandwidth with distance adds noise to the signal. By reflecting different wavelengths from different regions, a chirped FBG in which the grating period changes along its length can be used to compensate for this dispersion. This application is important both for long-haul communications when nonlinear two-photon dispersion effects become important, and when using low-loss 1.5µ m wavelengths in older 1.3µ m fibres. By allowing each propagating wavelength to experience similar amplification characteristics in wide-band WDM operation, FBG's can be used to flatten the gain from fiber-amplifiers. In addition, FBG's can act as frequency and mode-selective mirrors for erbium fibre and diode lasers, narrow-bandwidth transmission filters and wavelength-selective taps for network monitoring. FBG's can also be used as optical sensors for the remote detection of local stress, strain and temperature in engineering and medical environments. A change in one or more of these properties changes the period of the FBG which in turn modifies its reflection properties for laser diode probe sources. Reflected wavelength shifts of <1pm at 1.5µ m correspond to grating optical length changes of <10-6% induced by local strain and temperature fluctuations of <1µ strain and <0.1oC respectively. Their small size, corrosion resistance, chemical inertness, electrical nonconductivity and immunity to electromagnetic interference make FBG's ideal for embedding in civil engineering structures like buildings, bridges and dams; transportation systems like aircraft, trains and boats; military systems like sonar arrays; terrestrial intruder alarms and medical catheters.

10. LASER MACHINED BIOMEDICAL DEVICES As in microelectronics and its associated technologies, the drive for increasing miniaturization with improved device functionality is crucial to the rapid progress being made in the biomedical industry(22). Precision microdrilling with excimer lasers is routine when making delicate probes used for analysing arterial blood gases (ABGs)(23). ABG sensors measure the partial pressures of oxygen (PaO2), carbon dioxide (PaCO2) and hydrogen-ion concentration (pH) used for monitoring the acid-base concentration essential for sustaining life. In intensive care units, ABG results are used to make decisions on patient's ventilator conditions and the administration of different drugs. The use of fiber-optic sensors for ABG analysis provide clinical diagnostics at the patient's bedside without the need for taking any blood samples(24).

Figure 11 shows an example a ABG catheter for monitoring blood in prematurely borne babies. The hole at the side of the PVC bilumen sleeving tube through which blood is drawn is machined using a KrF excimer laser. In this case the clean cutting capability of the laser provides the necessary rigidity that prevents kinking and blockage of the tube when inserted into the artery. 17/07/02 Figure 11. Hole in the side of a bilumen catheter

Figure 12.Automated reel-to-reel excimer laser workstation for simultaneous hole drilling in optical fibers.

12 More important components of this catheter are the PaO2 and PaCO2 sensors. These consist of a spiral of up to five ~50x15µ m rectangular holes

machined in a 100µ m diameter acrylic (PMMA) optical fiber with an ArF laser. The holes are filled with a reagents whose optical transmission depend on the PaO2 and PaCO2 levels of the surrounding blood. Using a fully-automated workstation shown in Figure 12 that has computer-

controlled reel-to-reel fiber-feeding and laser-firing, all five holes shown in Figure 13 are drilled in the fiber. By spatially-multiplexing a single excimer beam into five smaller ones, holes are drilled simultaneously through the fiber.

17/07/02Figure 13.

Rectangular 50x20µ m holes drilled in 100µ m fibers for PaO2 & PaCO2-sensors

Figure 14.

Laser stripped insulation from 100µ m diameter pH-sensor wire

13 Preferential excimer laser etching of plastics compared to metals is applied to the stripping of insulation from fine diameter wires prior to soldering connections. The process relies on the threshold for excimer laser ablation of the polymer being much lower than for damaging the copper or silver core. As shown in Figure 14, excimer lasers are used to cleanly strip away the polyurethane insulation sleeving of wires which form the pH resistivity sensor in the ABG catheter above. Such pulsed laser wirestripping is also in widespread use for preparing connection wires to computer hard-disk reader heads. 11. LASER MICROMACHINING IN MICRO-ELECTRO-MECHANICAL SYSTEMS 'Micro-electro-mechanical systems' (MEMS) bring together mechanical, electrical and optical technologies to create an integrated device that employs miniaturization to achieve high-complexity in a small volume. This generally involves fabricating mm-µ m size structures with µ m-nm tolerances. In Europe MEMS is referred to as 'Microsystems technology' (MST), in the UK as 'Microengineering' and in Japan as 'Micromachines', and is predicted to grow into a $4b/year industry by the end of the century. The success of microengineering comes from miniaturization and its consequences: high-sensitivity, short-measurement times, low-energy consumption, good-stability, high-reliability, self-calibration and testing. Microsensors detecting local parameters like pressure, flow, force, acceleration, temperature, humidity, chemical content etc, have in the last decade been engineered into the engine and performance management systems of cars and aircraft. They also provide the key to electromechanical microcomponents such as ink jet printer nozzles, gas chromatographs, gyroscopes, galvanometers, microactuators, micromotors, micro-optics etc. Devices like implantable drug delivery systems are being developed containing sensors, valves and control system with power source capable of operating for many years. There is no doubt microengineering will be a key underpinning technology of the 21st century. Adaption of silicon lithography and etch batch-processing as developed by the semiconductor industry is currently the dominant MEMS fabrication method. However being restricted to just one material (silicon) surface and bulk etched in only 3 directions- along (110), (100) and (111) crystallographic-planes, other more flexible micromachining methods including pulsed uv-laser ablation are being evaluated for MEMS. The perceived advantages of uv-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) can be applied to a wide range of polymers, ceramics, glasses, crystals, insulators, conductors, piezomaterials, biomaterials, non-planar substrates, thin and thick films, (vi) compatible with lithographic processes and photomask making.

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Figure 15 Plastic 100µ m fiber clamp

Figure 16470µ m diameter, 130µ m height nickel intravascular rotor microturbine. Replicated by electroplating from an excimer laser machined PMMA

master. Courtesy of Rutherford Appleton Laboratory.

14

Controlled 3D-structuring of materials by excimer laser etching(25) can produce the basic building blocks of bridges, diaphragms, pits, holes, ramps, cantilevers, etc needed to microengineer devices like gyroscopes, galvanometers, gas chromatographs, microactuators, micromotors, micro-optics etc. Figures 15 and 16 show examples of uv-laser micromachined 3D-structures in polymers which when used with the LIGA (from the German acronym: lithographie galvanoformung abformung) process of electroforming, can be replicated in metal - a process now known as Laser LIGA. Excimer laser ablation is being used to manufacture 'biofactory-on-a-chip' (BFC) travelling-wave dielectrophoresis cell-sorters and sensors that consist of 21/2D laminations of channels, chambers and electrode conveyor tracks(26). Already recognized by government-supported initiatives in Japan and the European Union, pulsed lasers will be a key manufacturing tool in emerging nanotechnologies(27). The economic advantages of mass production at low unit cost is of the highest importance and will open up many new industrial application areas.

11. ACKNOWLEDGEMENTS It is a pleasure to thank J Cashmore, E Harvey, J Fieret, D Milne, N Rizvi, P Rumsby, D Thomas and M Stallmach of Exitech Ltd who contributed much of the original experimental material contained in this paper.

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