spray_coating_photoresist (1063651)
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Spray Coating ofPhotoresistsRevised: 2013-11-07Source:www.microchemicals.com/downloads/application_notes.htmlSpray Coating: Basics and MotivationSpray coating denotes the formation of a resist film via millions of µm-sized resist dropletsmoving towards the substrate. This coating technique allows a - at least in principle - lowerresist consumption as compared to spin coating.
The main advantage, however, is the possibility to coat arbitrary shaped and textured substrates where spin coating cannot attain satisfying results by means of film thickness homogeneity and edge coverage.Atomized Spray FormationEach technology for droplet generation requires a certain low resist viscosity of usually a fewcSt. Varying the resist viscosity impacts on the dropletgeneration rate as well as the droplet diameter distribution.When diluting resists with a solvent, one has to considerSpraypossible incompatibilities of certain solvents with the renozzlesist, as well as the fact that highly diluted photo resistsgenerally reveal an accelerated ageing of the resist in thediluted state with particle formation as a consequence.Solvent Evaporation during FlightA certain solvent evaporation out of the droplets duringflight (between spray nozzle and substrate) is required fora sufficient resist edge coverage onto textured substrates:The hereby increased resist viscosity prevents the resistfrom macroscopically flowing on the substrate.If, however, too much solvent evaporates during flight,this prevents the droplets from sticking to the substrate orat least causes a rough surface.The parameters temperature, droplet velocity (relative tothe ambient air), and air solvent saturation, as well as thesolvent composition and concentration determine theevaporation rate for each droplet as a function of thedroplet surface.
This surface concentration again dependson the temperature- and solvent concentration dependantdiffusion constant of the solvent from the droplet bulk toits surface.ResistdropletsSubstrateWetting and Edge CoverageA homogeneous resist film requires a certain flowing of the resist film on the substrate for atleast few µm, thus defining an upper limit for the resist viscosity or, respectively, a certainminimum for the remaining solvent concentration. A viscosity, which is too low (or, respectively, a remaining solvent concentration that is too high) causes macroscopic resist flowingthereby reducing the edge coverage of the resist film in the case of textured substrates.The parameters wetting, edge coverage, and homogeneity, which strongly impact the spraycoating performance, depend on the initial coverage of the droplets from the atomized spray,the resist adhesion to the substrate, the resist surface tension, and its viscosity.Therefore, besides the chemical and physical resist properties, the atomized spray formationPhotoresists, wafers, plating solutions, etchants and solvents ...Phone: +49 731 977343 0www.microchemicals.eusales@microchemicals.eu761,274Solvent conc.
(%)___1,410,80,60,40,27270686664620The bar graphs to the right plot the results fromnumerical simulation of a spray fog consisting ofdroplets with a certain (histogram above) radiusdistribution.If MEK is used as diluent (upper right), evaporation causes a significant loss in the droplet solventconcentration in much less than a second, whilePGMEA (lower right) evaporates much slower.Since the solvent concentration of the droplets hitting the substrate reflects the solvent concentration of the resist film formed, choosing the propersolvent(s) strongly impacts the resist edge coverage. Please not the different scale in the y-axis!Solvent conc.
(%)_____Droplet Start Radius (µm)MEK20°C 1 m/s602,732,462,191,921,651,391,120,850,5800,31Frequency (a. u.)___MicroChemicals GmbH - Spray Coating of Photoresists0,050,10,15Time (seconds)7574,974,874,774,674,574,474,374,274,1740,2PGMEA20°C 1 m/s00,050,10,15Time (seconds)0,2Few overlapping dropletsResist film not completely closedClosed 1 µm film, roughness approx. 300 nmClosed 3 µm film, roughness approx.
150 nmThe figures above show snapshots during the formation of a resist film via spray coating.Photoresists, wafers, plating solutions, etchants and solvents ...Phone: +49 731 977343 0www.microchemicals.eusales@microchemicals.eu2-MicroChemicals GmbH - Spray Coating of PhotoresistsResistSubstrateModeled cross sections of resist flowing on a textured substrate. The bottom left image reflects optimum edge coverage, while the other two images show what happens when the resist adhesion is toolow (right) or, respectively, a resist viscosity too low for too long time (center).mechanism (distribution of the droplet sizes), the flight of the droplets to the substrate(evaporation), and the solvent evaporation out of the growing/grown resist film (time dependant surface tension and viscosity) determine the spray coating result with respect tohomogeneous, smooth, and closed films.Spray Coating Resists and Process OptimizationFor spray coating, we recommend the spray coating resists TI Spray and AZ® 4999 with asolvent composition optimized for typical spray coating equipment.Please contact us if you would like us to assist you in improving your (scheduled?) spraycoating process!Photoresists, wafers, plating solutions, etchants and solvents ...Phone: +49 731 977343 0www.microchemicals.eusales@microchemicals.eu3-.
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