Study On Laser-MicroPen/MicroJet Direct Writing Technology For Integrated Fabrication Of MEMS Micro-Structures

In recent years,flexibility,customization,integration and intelligence has become thedevelopment trends in the fabrication field of Micro Electronic Mechanical System(MEMS).As a new type of micro-fabrication technology,the direct-writing technology,which based on"free deposition/removal"principle,has absorbed extensive concerns.This is a excellcent opportunity to develop direct writing micro-fabrication technology andequipments with own intellectual property rights,which would be helpful to promote thetechnology level and core competitiveness in the field of MEMS manufacturing in China.In this dissertation,a novel direct writing technique,named Laser-MicroPen/MicroJet(LMM) direct writing technology,has been developed for integrated fabrication of MEMSmicro-structures and the equipment,materials and processing mechanisms have beenstudied systematically.Typical MEMS micro-structures,such as signal electrode pattern,films,cross cantilevers,high depth-width ratio wall/needles,and suspended air-bridges,are successfully fabricated on the Al₂O₃ ceramic or quartz glass substrate by thecombination of Laser Micro-Cladding process and MicroPen/MicroJet direct-writedeposition process,which provide the powerful proof that the idea of LMM integratedfabrication can be realized in the fabrication of MEMS micro-structures.Following are themain results of this dissertation:Firstly,a multi-functional direct-writing micro-manufacturing system (M-DMS) isdesigned and implemented,which integrates the Laser system,MicroPen and Micro Jetdirect-write processing tools together,with flexible and integrated,multi-functional andexpandable characteristics.The systems configuration,custom CAD/CAM software'sdesign,essential module and algorithms are introduced in detail.Based on the requirements of material's structural property and LMM direct-writeprocess compatibility,two different kinds of"sacrificial layer/structural layer/substrate"material system and the corresponding processing schemes are designed,including:1)"spin-on-glass (SOG)/conductive electronic paste/Al₂O₃ ceramic substrate"materialsystem.The SOG films are deposited on 99%wt.Al₂O₃ ceramic substrates by spin-onprocess or dip-coating process,then patterned by laser direct-writing densification processand wet chemical etching technology;The conductive electronic paste structural layers arefabricated by MicroPen/MicroJet direct-writing deposition process or laser micro-claddingpreset paste film process;finally,structural layers are released by wet chemical etching process.2)"Polyimide (PI)/conductive gold (Au) paste/quartz glass substrate"materialsystem.Fluxible PAA solution is deposited on quartz glass substrate by MicroPendirect-writing deposition process and imidized to polyimide (PI) sacrificial layer in oven;Au paste structural layers are fabricated by MicroPen direct-writing deposition technology,and finally released by oxygen plasma dry etching process.Technological procedures and mechanisms of the LMM direct writing technology arestudied systematically by theory analysis and experiments.The results shows that:(1) Outlet diameter of the MicroPen tip is a decisive factor to the depositionlinewidth,as the linewidth decreases linearly when reducing the outlet diameter.In orderto restrain extrusion swell behavior of the deposition material,it is essential to controlMicroPen tip-to-substrate distance appropriatly.When the material rheological propertiesand outlet diameter of MicroPen tip are selected,the volume of the material extruded perunit time increases with the increase of the extrusion air-pressure.The linewidth andthickness of the deposition pattern can be controlled by both extrusion air-pressure anddirect-writing speed.(2) It is very critical to adjust the atomizing pressure and deposition spray pressure tocontrol the film thickness and density properties in Micro Jet direct-writing depositionprocess.If the pressure is too big or too small,the fabricated deposition films will bocomeloose and have unqualified electrical properties.Due to the cone-shaped Micro Jet nozzlehole,the atomized paste flows along the nozzle wall and injects out of the export,creates aclustering region at a certain distance under the nozzle hole.Thus,with the help ofspraying clustering effect of the cone-shaped Micro Jet nozzle,it is possible to deposite thepaste patterns with linewidth less than the outlet diameter of the nozzle.It is veryimportant to keep appropriate nozzle-to-substrate distance and appropriate depositionspray pressure to fabricate the lines with fine depsotion linewidth.(3) The essential mechanism of the Laser Micro-Cladding Electronic Paste (LMCEP)process is rapid sintering of the electronic paste by laser beam.The scanning speed of thefocused laser beam and power density are the two main process factors which determinethe sintering time and temperature.Electrical patterns with linewidth less than thelaserbeam spot size could be fabricated by utilizing the gaussian energy distributioncharacteristics of the laser beam and ultrasonic clean.In order to ensure full removal of theorganic matter within the pool and ensure glass binder ingredients melting,and wetting thesubstrate full,the laser scanning speed should be low.When the scanning speed is biggerthan 5mm/s,the melted glass binder ingredients can not infiltrate and wet substrate completely,which cause the adhesion strength between the film and substrate be lowerthan the critical strength for practical applications.Moreover,if the laser scanning speed istoo faster,the organic matter within the coatings may not elininate fully during the laserirradiation period.When the molten pool begin to solidify,the trapped residual organicmatter may burn,carbonize and boil within the cooling pool,which results in trappedbubble defects inside on the surface of the sintered electronic paste film.As an application example,a cross-bridge structure RF MEMS electrostatic driventunable capacitor is fabricated by the Laser-MicroPen direct-writing integrated fabricationtechnology.The results of electrical properties tests show that when the tuning voltageincreases from 0 to 38V,the capacitance increases from 0.1705pF to 0.1919pF,and thetuning range is 12.55%;the Q value under the 1GHz frequency is 20.Although theprocessing accuracy and electrical properties of the LMM direct-writing integratedfabrication technology is needed to be improved compared with the traditional MEMSmanufacturing process,the LMM direct-writing integrated fabrication technology stillshows powerful potential prospects in engineering application in the componentsmanufacturing and packaging areas of MEMS with its outstanding characteristics such asflexible processing,wide range selection of materials and compatibility with LIGAtechnology.