Industrial Micro Machining Applications of Excimer Lasers

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Industrial micromachining applications of excimer lasersDr M C Gower

Exitech LidHanborough Park , Long Hanborough.

Ogord OX8 8LH.England

The ability of the excimer laser to dry etch and micromachine materials such as polyme rs, crystals and cera mics with extremelyhigh precision i s now well establish ed'. The first true industrial implem entation of the excimer laser onto production lines hasoccurred since 1988 in the microelectronics industry for drilling small feedthrough via holes in insulating layers in multichipmodules. The excimer laser has proved to be. well suited to this industry for which microfabrication is an underpinningtechnology. Currently the most active areas of research on the indu strial uses of excimer lasers are also in microelectronics - likefo r example laser-assisted C V D and ion implantation, chip circuit and mask repair, TFT silicon annealing, photolithography,wafer marking, planarisation etc. Microfabrication is also at the heart of many products in other areas like, for example, thebiomedical and senso r industries. While just beginning to be applied to these areas, the excimer laser could soon find widespreaduse in the fabricatio n of produc ts such as contact lenses; medical pr obes for monitoring bodily functions; microsurgical and bodyimplant devices; environm ental monitors of trace elements, impurities and microbe s in fluids like water.1. M I C R O M A C H I N I N G A P P L I C A T I O N S I N T H E M I C R O E L E C T R O N I C S I N D U S T R YExcimer lasers excel at microprocessing materials. The most active and demanding industry for microfabrication is in themanufac ture of sm all electrical structures such as printed circuit boards or in microelectronic semiconductor devices in the form o fintegrated circ uils (silicon ch ips). Thus it is no surprise that many research studies and feasibility trials have been carried out onusing ex cimer lase rs for a variety of the production steps used by this industry. Because of the tight tolerances and high productionvolumes involved, the potential application of excimer lasers to microelectronics fabrication is one of the most demanding onlaser perform ance in terms of pow er, beam quality and reliability. Laser manufacturers are addressing these stringent specificationsby cayloring devic es to suit the particular needs of the application.

The speed at which modem computers operate is often limited by the time delays produced by the intricate interconnectionsbetween integ rated circuits rather than by the chips themselves. Thus there is great emphasis placed on de veloping efficient highspeed chi p interconnection packaging. One recent method uses printed circuit boards comprising of mutilayer sandwiches of acrylicresin, polyimide and c opper on which can be mounted and interconnected over 100chips. The package relies on building up layerby layer copper/acrylic/polyimide aminations with as many as 10,000 interconnection via holes drilled between conductivemetallic layers. After drilling, circuits on one layer are connected to the one below by electroless plating of cop per down the viaholes. Further laminations are then added on top and via hole drilling repeated. The - 8 0 p diameter via holes must be cleanlydrilled with aspe ct ratios clos e to unity through each acrylic/polyimide la minate. The excimer laser has been found to be the onlysatisfactory method for cleanly drilling these small holes. Using a laser fluen ce of between 0.5-0.8J/cm2 that does not damage thecopper, it lakes between 300 o 400 pulses to simultaneously drill up to 100 holes through the polymer laminate with either econformal contact mask or a projected mask image that defines the hole pattem. Since there is insufficient fluence to remove thecopper, once thro ugh to th e next layer the drilling stops automa tically. After initial validation, drilling such compo nents with 12cxcimcr lasers has been used successfully on a production line on a 24 hours a day , 6 day a week basis since 1988 and representsthe first true use of an excimer laser on a fu l ly automated basis2.

Authorized licensed use limited to: KING SAUD UNIVERSITY. Downloaded on February 7, 2009 at 04:49 from IEEE Xplore. Restrictions apply.


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2. M I C R O F E N E S T R A T I O N O F C O N T A C T L E N S E S B Y E X C I M E R L A S E R SThe degree of comfort experienced by wearers of contact lenses over an extende d period of time depends on the level of tears an doxygen that reaches the c o m a of th e eye. Soft contact lenses with a high water content improve oxygen permeability but aremore delicate and prone to produ cing eye infections than hard (PMMA derivative) lenses.Microfenestration of hard contact lenses,whereby small holes are fabricated into the lens outside the field of view, has be en studied for several years as a technique forimproving the passage of tears and ox ygen to the com ea. However the difficultie s in fabricating the small holes economically andwithout weakening the mechanical integrity of the lens have prohibited its inuoduction as a consumer product. The techniquedescribed here of using excim er lasers to microfenestrate contact lenses has been deve loped by Award Technology Lid and ExitechLtd in collaboration with British Technology Grou p Ltd who hold extensive worldw ide paten t applications.

Using imaging of a mask illuminated with an ArF laser, m icrofenestration of contact lenses has been carried out. In a hydrophiliclens, -Sopm diameter hole arrays have been drilled in 40 sectors around the no n-optic al periphcry of the lens. The original shape,form and handling characte ristics of the lens have been retained. The cxcim er laser allo ws the holes or 'pores' to be formed withoutdebris or burrs in all polyme r form ulations used for fabricating contact lenses. Clinic al trials have shown that the oxygenpermeability of lenses with a low water content of -38% was raised to that of lenses having a -80% water con tent. Clinical trialscurrently in progress initially show a greatly improved wearing coml"ort level for exc ime r laser microfenestrated lenses.3. M I C R O D I S C A R R A Y C H E M I C A L S E N S O R SThe electrochemical technique of an odic srripping vol/amme/ry AS V) is currently on e of the mo st sensitive for detecting heavymetal ions in fluids. Sweep ing the m agnitude of a bias voltage applied to a m ercury e lectrod e imm ersed in a fluid strips ions fromthcir ligands that then conc entrate in the electrode as a mercury amalgam. Pe aks in th e measured current are specific to the ion ofinterest and allow trace metals in the solution to be detected at the ppb level. Modem electronic instrumentation for analysis andreadout is compact and computerised. The electrodes used are usually eithe r a hanging mercury drop or a mercury film depositedonto vitreous carbon. The mercury drop is formed from a fine capillary which itself requires particular attcntion and is oftencumbcrsome. On the other hand to increase planar diffusion of reagents to the surface, single film electrodes have the drawbackthat either they must be rotated or the solution shrred. Howeve r with arrays of sm all electro des fabricated by excimer laser etching- a process developed and p atented by EcosseSen sors and Exitech Ltd, mass transport to the electroactive surface occurs bycylindrical/spherical diffusion so no stirring or electrode rotation is required. Mercury is depo sited by electroylsis of the reagent.Since the sensitivity of the senso r increases linearly with the number 0 1 electrodes, the microdisc array electrode array detectsmicroamp currents rather the pA levels encountered when using a single large electrode. Reduction of oxygen, which is aninterfering electrode reaction in classical polarography and requires a thorough and lengthy deoxygenation step prior toelectroanalysis, is much less pronounced when using microelectrode arrays. Between measurements mercury film electrodes mustbeeirher polished or the stripping process stopped before dissolution of the mercury occurs. With a disposable clectrode like themicrodisc array or the mercury drop, a fresh surface is used for each m easurement. This disposable sensor has been developed toprovidepolarography on a sfrip for the field analysis of heavy metal ions at the ppb level.

A carbon electrode is first screen printed on to a plastic or ceramic substrate. A thin pinhole-free layer of vapour-polymerisedparylene is then deposited o ve r part of this electrode. An array o f -15pm diameter holes is then drilled through this dielectric layerusing an ArF laser and projection mask imaging techniques. The reference electrode (silver or other metals) is then screen printedonto the dielectric to obtain a disposa ble PCB type cell that simply plugs into an electro nic biasing-readout controller.



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The pro perties of this sensor have been characterised using artificial buffer solutions. The fabrication of thc microelectrode m a ysensor inheren tly relies on the excim er laser's ability to accurately, reliably and che aply fabricate disc/interdisc dimensions in aregular geometric pattern optimised for the particular analytical performance.

4. B I O M E D I C A L B L O O D SENSORSExcimer lasers are currently used to drill a small hole in the sidewall of PVC tubes through which blood is drawn in bilumenmedical cath eters that detect with electrical sensors the oxygen content of the blood of prematurely borne babies. In this case theclean cutting capability of the excimer laser provides the necessary rigidity that prevents kinking and blockage of the tube wheninserted into the child.

In addition, prior to soldering the melallic conducting cores of wires that form an electrical scnsor in this catheter. the plasticinsulation sleev ing is cleanly suipped aw ay by a an excimcr laser. The process relies on the threshold for excime r laser ablation ofthe polymer being much lower than that for damaging the copper or silver core. Using a fu l ly automated computer-controlled reelto reel wire-feed system wires with diameters as small as 50pm have been stripped routinely on a production basis. Anothercompon ent of this catheter is a blood optical sensor that consists of a 200pm diam eter plastic optical fibre with a spiral of up tosix -5Ox15pm rectangular holes machined simultan eously using an ArF laser. Using the f u l l y automated workstation that hascomputer controlled rcel to reel hbre feeding and laser firing, this drilling operation has also been in production for the past 2years. The holes are produced by spatially multiplexing a shapcd excimer lascr beam into 6 smaller bcams and imaging an aperturcmask onto the fibre. The drilling process is highly reliable. For quality control, CCD cameras coupled to microscopes providereal-timc in line viewing of the drilling operation and the finished fibre.5 . B I O M E D I C A L M E S H E SRobust high transmissivity meshes and membranes are of great in terest to many a r m n biomedicine. T h i n film panicle andbacterial filtershaving mesh sizes down to 0 . 5 ~an be readily fabricated with excimer lasers. Precisely controlled hole sizes canbe produced for calibration and cxact dosimeuy applications. Small holes can also substan ually increase the passage of fluid andoxygen through a film to increase 11s biocompatibility.A mesh array of tapering 8 0 p holes in a th in film of biodegradablepolymer is now fabricated in production on a single shifVday basis using an ArF aser operating at 100Hz.The successful implementation of excimer laser processing in to a product relies not only on the ability to produce a reliable highquality result, but that such a processing step be cost effective in relation to the final selling price of the component. Applicationsof cxcimer lasers to industry are often at the forefront o f research and development in high-tech industries and form part ofsensitive key tech nologies the details of which ar e necessarily kept confidential by the com panies involvcd. We have outlinedsome industrial applications of excime r lasers that arc being used succcssfully by com panies. As the potential for excimer laserprocessing is realised by industry - particularily in the area of microfabrication, applications w ill continue to develop rapidly andoften involv e the fabrication of entirely new products that could not cven be contemplated by o ther mechanical, chemical or lascrfabrication mcthods.

1) M C Gower. 'Excimer lasers: Their current and future uppl icat ions to i ndustr y and medicine' . Lascr Processing inManufacturing. E&. R C Crafer and P J Oakley, Chapman and Hall (1992).

2) F Bachmann. 'Excimer Lasers in a fabrication line for a highly integrated printed circuit board, Chemtronics, Vol 4, 149(1989)

B 993T he Institutionof Electrical Engineers.Printed and published by the IEE. Savoy Place, LondonWCPR OBL, UK .

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Industrial Micro Machining Applications of Excimer Lasers