VIP_Conformal_Photoresist_Coatings_for_H igh_Aspect_Ratio_Features_09 (1063537)

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CONFORMAL PHOTORESIST COATINGS FORHIGH ASPECT RATIO FEATURESKeith A. Cooper, Clif Hamel and Bill WhitneySUSS MicroTecWaterbury Center, VT, USAkcooper@suss.com, chamel@suss.com, bwhitney@suss.comKatrin Weilermann and K. Josef KramerSUSS MicroTec LithographyGarching, GermanyYoungjun Zhao and Harold GentileSeagate ResearchPittsburgh, PA, USAABSTRACTThe recent proliferation in advanced packaging and MEMSapplications has created a need for high aspect ratiolithography processes. This paper will explore some of thealternatives for coating photoresists and dielectrics ontosuch structures, including conventional spin coating,electrodeposition, and more recently, spray coating.

Datawill be presented and discussed regarding the relative meritsof the competing coating technologies, with real-worldapplication examples for spray coating.This noveltechnique will be shown to deliver highly conformal resistcoatings not only over etched v-groove structures commonlyfound in the MEMS world, but also over deep trenches andwells with high aspect ratios. The paper will emphasizerecent improvements to both hardware and processmethodology in an effort to broaden the scope of structuresand materials suitable for spray coating.Practicalextensions of this new technology will be explored anddiscussed, along with an assortment of new structures andapplications that spray coating has enabled.Keywords: lithography, photoresist, conformal coatings,MEMS, wafer-level packagingtopography.

So inefficient are most dispense methods thatCurtis, et al have wryly noted that “the probability that amolecule of resist will proceed from the bottle to softbake isunacceptably small.”1. In his paper, he goes on to outline anovel method of photoresist dispense dubbed “reverse radialdispense” which has been shown to greatly reduce resistconsumption, particularly for certain resists and dielectrics.Spin coating is governed by several physical phenomena,including the rheology of the resist, the spin dynamics andthe dynamics of solvent transport. These phenomena havebeen outlined by Curtis, et a1 to describe the typicalbehavior of a photoresist or other such material in a spincoating cycle.

In general, a photoresist spin coating processcan be expected to process good uniform coatings onrelatively planar surfaces largely due to the spin mechanicsand the tendency of the resist in its fluid state to minimizethe surface area. But these same flow characteristics willlikely cause the film to poorly coat a non-uniform surface,primarily in response to effects from spin dynamics, gravityResist film tends to tearat the topography edgesINTRODUCTIONThe fabrication of microelectronic devices has typicallyinvolved the lithographic patterning and etching ofrelatively planar substrates with topography less than a fewmicrometers in height.

Lithography for these planarsubstrates has been and still is primarily carried out by spincoating a photosensitive resin onto the substrate to form athin, uniform film which is subsequently baked, exposedand developed to form the desired circuit patterns.While this historical spin-coating method is still a viabletechnique for many or most applications, it is not without itschallenges or limitations, including inefficient utilization ofthe photoresist and difficulties in coating various degrees ofResist begins to pool at thebottom of the topographyFigure 1: Sketch of topography with insufficientcoating uniformityand the surface tension of the resist.2 When spin coating awafer with topographical steps such as trenches, grooves ormesas, the photoresist will tend to draw back from edgesand corners as shown in Figure 1.

This result is highlydetrimental and cannot be tolerated in lithographyprocessing.The need for coating high topography has grownsubstantially over the last several years primarily due to theproliferation of applications for wafer-level packaging, 3Dimensional Integrated Circuits, MEMS devices, and manyothers. Within these varied applications, manytopographical steps exist in the form of v-grooves, mesas,cantilevered structures, deep trenches and cylinders, bumps,and tall multi-level metal structures, just to name a few.The need to perform lithographic patterning on and overthese structures has created the demand for conformal resistcoatings which protect the underlying layers fromsubsequent etch, deposition, or any other follow-onprocessing step.One way of creating conformal photoresist coatings is bymeans of electrodeposited resist.

With this process flow, theuser creates a conductive film on the wafer, often throughsputtering a seed layer such as chromium or other metals3,then electroplates the photoresist onto the wafer surfaceusing a bath setup. While this method produces conformalphotoresist coatings even on severe steps, the need for aconductive underlying layer and the inefficiencies andtherefore high costs associated with this method limit itsapplication.To create alternatives to conventional spin coating andelectrodeposition of photoresists, this investigation set out toexplore two novel methods of coating the resist: closedcover spinning with a co-rotating chamber and spraycoating of the materials.EXPERIMENTALWith conventional spin coating of photoresist, the spinningwafer resides in an open, uncontrolled atmosphere.

Thismeans that during the material spreading and drying cycle,the carrier solvent in the resist is allowed to evaporate freelyinto the atmosphere. Depending on several process factorsincluding the volatility of the solvent, the relative humidityand velocity of the vertical laminar flow air, and therotational velocity of the wafer, this evaporation could bevery rapid, creating a skin on the resist surface.For applications involving relatively planar surfaces, thismight not present a problem in coating the wafer uniformly,but for wafers with some topography, the solvent needs toremain in the resist for a longer period of time to avoidcreating striations around some of the topographical steps.To avoid this uncontrolled drying of the resist, a closedchamber has been created by means of a co-rotating cover.Dubbed the GyrsetTM coater, the system is depicted inFigure 2 which creates a small solvent-rich environmentabove the spinning wafer surface.

With the chuck and theGyrset cover rotating synchronously, air turbulence iseliminated since the air is spinning with the substrate, andthe evaporating solvent is captured, creating a solvent-richenvironment. With this apparatus, the resist remains in thefluid state for a longer portion of the spread cycle, allowingit to flow more easily around obstructions such as corners ofthe topography. Avoiding this skin effect also produces anancillary benefit of reducing the volume of resist materialrequired, especially for materials with highly volatilesolvents.

Good results are readily achieved on moderatetopography up to 10µ in height.TurbulenceflowGYRSETSolvent saturated atmosphereSubstrateCompound chuckFigure 2: Schematic of Closed Chamber coating systemBut as the Gyrset system is employed for higher and highertopography, gravity and the surface tension of the flowableresist become the dominant effects, and the feature edgescreated by the topographical steps are no longer sufficientlycovered by the resist. The resist in its liquid state will flowdown the trenches, creating open areas on the corners andedges of features. Though the coating results with theGyrset spinner are superior to conventional open-bowl spincoating, high topography of several 10’s of microns cannotbe adequately coated by the Gyrset concept, and anothermethod must be used.To coat high topography, a new method of depositing resistis required which will circumvent the fundamentallimitations of spin coating.

As long as the resist is in itsliquid state, it will flow around obstructions such astopographical features in response to the centrifugal force,and it will flow down inside features in response to gravity.A new method of coating photoresist onto high topographyhas been proposed and investigated which does not employany type of spinning of the substrate.

Rather, a spraydeposition method is employed which deposits the resist asan aerosol onto the surface of the stationary wafer, avoidingthe problems with resist flow described above.The apparatus is depicted in Figure 3 and employs a binarynozzle for creating the resist aerosol, an XY stage, and achuck to hold the wafer. The binary nozzle is mounted toan arm a certain distance above the substrate, and isnot at all.X-StageY-StageSpray NozzleRaster PathWhen the spray nozzle is directed across a surface with asingle pass, the resulting photoresist distribution will looklike Figure 4.

Roughly a Gaussian distribution, the spraypattern will have a characteristic width and densitydepending on the process parameters and the particularmaterial being sprayed.Since the goal of the coating process is to have a uniform,conformal layer of material on the topography, the sprayhead is directed across the entire surface of the substratesingle spray pathsuniform coatingFigure 3: Layout of Spray Systemsupplied with a flow of photoresist and nitrogen pressure.As controlled volumes of photoresist are delivered to thenozzle, the nitrogen is mixed with the resist to create verysmall droplets as the nitrogen pressure forces the resistthrough the orifice.

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