spin_coating_photoresist (1063542)
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Spin Coating ofPhotoresistsRevised: 2013-11-07Source:www.microchemicals.com/downloads/application_notes.htmlBasics of Spin CoatingDuring spincoating, the centrifugal force of the substrate spinning with several 1.000 roundsper minute (rpm) distributes few ml of resist over the substrate.Advantages: The high resist film thickness homogeneity as well as the short coating timesmake spincoating the most-applied coating technique at least in microelectronics.
The factthat the coated resist already lost a significant amount of its solvent after spin coating, notime-consuming delays have to be kept before further process steps (handling, softbake).Limitations and disadvantages: In case of non-rotation-symmetric substrates, the resistforms a pronounced edge bead near the substrate edges due to the strong air turbulences.On textured substrate, the attained edge coverage of the textures is rather small, and partsof the topology might not be coated at all. With only few %, the resist yield is comparablelow.Nominal Resist Film ThicknessThe resist film thickness attained by spin coating represents the equilibration between centrifugal force and solvent evaporation, both increasing with spin speed.
Generally, the lasttwo digits of the resist name (e. g. AZ® 6632) indicate the film thickness attained by spincoating (without gyrset) at v = 4.000 rpm in 100 nm units. Some US AZ®-resists such as theAZ® 9260, the 701 MiR, or nLOF 2000 refer to 3.000 rpm.The resist film thickness approximately decreases with the reciprocal square-root of the spinspeed (e. g. AZ® 4533: 4.0 µm ... 3.3 µm ... 2.3 µm at 2000 ... 4000 ... 6000 rpm). This allows to realize a certain range in the film thickness with one resist.Influences on the Resist Film Thickness16141210840353025030 60 90 120spin coating time (s)after coatingafter softbake (20 min 100°C)6solvent conc. aftercoating (%)18AZ® 9260resist film thickness in µm___2045coating (%)22solvent conc.
afterresist film thickness in µm___The resist film thickness given for a certain spin speed holds for sufficiently long spin times,when the resist spin-off is completed. As the figure below shows, the spin times required toattain the final film thickness increases with the resist viscosity. Low spin speeds also increase the time for a constant resist film thickness.765AZ® 6632353025200415304560spin coating time (s)after coatingafter softbake (20 min 100°C)30306090120sping coating time @ 3000 rpm (s)015304560spin coating time @ 4000 rpm (s)Resist film thickness and remaining solvent concentration after different long spin coating for a thin(left) and thick resist (right) before (black) and after (blue) softbaking.Photoresists, wafers, plating solutions, etchants and solvents ...Phone: +49 731 977343 0www.microchemicals.eusales@microchemicals.euMicroChemicals GmbH - Spin Coating of PhotoresistsThis time-dependency not only allows the film thickness to be adjusted for spin speed, butalso the spin time.
However, this is only a reasonable procedure for attaining thick and verythick films. For thin films (approx. < 10 µm), the spin times should always be ‘sufficientlylong’ to make the result more reproducible.The remaining solvent concentration in the resist film also impacts on its thickness. Duringcoating, the solvent concentration drops and saturates at a value of approx. 15-25 % whichdepends on the film thickness and is higher for thicker films. The subsequent softbake reduces the remaining solvent concentration to values of typ. 5 %.A high ambience solvent saturation in the spin coater reduces the attained resist film thickness due to a suppressed solvent evaporation from the resist.
Therefore, the first few wafersof a coating series might reveal a slightly higher film thickness as compared to wafers subsequently coated with a certain amount of fresh resist in the spin-coater. In this case, coating some dummy wafers at the beginning of a coating series would be beneficial by means ofconstant resist film thicknesses.The equipment also impacts on the attained resist film thickness: A gyr-set may reduce thefilm thickness by even a factor of two as compared to standard systems.Edge BeadEspecially in the case of coating thick resist films, a so-called edge bead forms which maycause sticking to the mask as well as an undesired proximity-gap during exposure (with areduced lateral resolution as a consequence). If no automatic edge bead removal is possible,initial stages for solution to avoid/lower the edge bead are: Circular substrates: Manual edge bead removal at approx. 500 rpm with AZ® EBR Solvent(not acetone or other low-boiling solvents!) dispensed onto the edge of the rotating substrate An elevated spin-speed for a shorter time A ‘spin-off’ of the edge bead by abruptly increasing the spin speed at a certain stage ofspin coating: The resist film should be solvent-poor enough to prevent further thinning,while the edge bead needs a viscosity still low enough for proper spin-off.
For this purpose, the optimum spin profile has to be found individually. A multiple coating with an elevated spin speed for each coating cycle For thick or solvent-rich resist films, a delay between coating and softbake prevents theedge bead from growing during softbake due to the thermally reduced viscosity. The delay time depends on the resist film thickness and can be shortened by applying moderatetemperatures (e. g.
stepwise: room temperature ... 50°C ... 95°C). A well-adjusted cavity in the substrate holder with the substrate as close inlay Edged substrates: If applicable, removing (breaking) the outer pieces of the substratebearing the edge bead; alternatively manual cleaning of the substrate from the edgebead with cleanroom wipes. Exposure and development of the edge bead is also possible,but may cause T-topping.Resist Striations and ‘Comets’The typical comet-shaped inhomogeneities inthe resist film thickness often have their originin either particles stemming from an insufficientsubstrate cleaning, or an expired resist.
Bubbles(N 2 from bottles closed for a long time, or airbubbles from bottles moved before resistdispension) enclosed in the resist film will causesimilar inhomogeneities. In this case, a certaindelay before dispension to outgas the bubbleswill help.A too small resist volume can also be the reasonfor uncoated areas on the substrate. DependingDecreasingaccelerationduring spincoating: ‘Comets’ becomemore pronouncedPhotoresists, wafers, plating solutions, etchants and solvents ...Phone: +49 731 977343 0www.microchemicals.eusales@microchemicals.eu2-MicroChemicals GmbH - Spin Coating of Photoresistsdgeer efawwafer edgethinned areaair bubble beneath resist filmbead10 mm100 µmInhomogeneities in the coated resist film (left) often misinterpreted as particles, mostly originate fromair or N2 bubbles (right).on the resist viscosity, the desired resist film thickness, and the substrate size, 1-5 ml resistper substrate is recommended.
Dispensing the resist as central as possible will improve theresult.In case of smooth substrates, a high acceleration (> 1000 rpm/s) towards the final spinspeed suppresses the appearance of uncoated parts on the substrate. On the other hand,textured substrates may require a two-step spin profile: After distributing a comparable highresist volume at low spin speeds (e. g. 1000 rpm) for a few seconds, ramping upwards tothe final spin speed adjusts the desired resist film thickness.Particles, Air or Nitrogen Bubbles?The appearance of air bubbles or particles in the coated resist film is very similar to eachother (figure above).Particles often stem from an insufficient substrate cleaning, or an expired resist.Air bubbles in the resist are often caused by transport, refilling or dispensing of the resist.N2-bubbles originate from the gradual decomposition of the photo active compound in theliquid resist. If - after storage for a certain time - the resist bottle is opened, the N2 dissolvedin the resist may form bubbles.In case of either air or N2 bubbles, a delay before dispensing of - dependant on the resistviscosity - a few hours will help for outgasing the bubbles.A locally reduced resist wetting can stem from previous processes or insufficient substratecleaning, or incorrect HMDS application.Particles and Bubbles in Etching MasksParticlesSecond resist filmParticles or bubbles in the resist film may act as channel forwet etches or during electroplating.
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