Photolithographic mkromolding of ceramics using plasma etched polyimide patterns

Photolithography is a highly advanced method for fabricating features in the micron and submicron size range. Semiconductor processing methods have been applied to fabricate microsensors and microactuators with high throughput and excellent dimensional control of small features, usually from silicon. It is clearly desirable to extend these methods to microfabricate other materials, such as piezoelectric ceramics, where such fine scale high aspect ratio features are required for high resolution ultrasonic imaging.’ We have explored the extensions of these methods to ceramic materials~ using, a photolithographically fabricated pattern as a ?nicromold” for forming small features in a ceramic. We find it possible to replicate deep features from the polyiinide onto a conventional ceramic green tape made from submicron powder. The technique is applicable to any powder-processed ceramic. Fine features with a high aspect ratio are a particular goal of this work. Fabrication of such features in polymer has been demonstrated by using the lithographic galvanoformung abformung (LIGA) process* and reactive ion etching ( RIE).3 Although LIGA can form high aspect ratio structures in polymer, it requires a high intensity synchrotron x-ray source which is costly and not readily available.. It is also difficult to generate high aspect ratio structures with conventional RIE, because of the slow etch rate and low selectivity. In this work a multipolar electron cyclotron resonance (ECR)source is used to generate a high density plasma with low ion energy.” This plasma system provides a fast etch rate, residue-free surface, and anisotropic etch profile for fabrication of high aspect ratio structures. We pattern ceramic by impressing a plasma-etched polyimide pattern. onto a soft ceramic “green tape.” We made the green tape from a high quality submicrometer powder of a ceria-zirconia alloy, chosen as a convenient model system. The ceramic green tape was a flexible film, 280 ,um thick, consisting of a 55 ~01% submicron ceramic powder in 45 vol % plasticized ethylmethacrylate (EMA). The ceramic was a Tosoh TZ-12CE, a 12 mol % Ce02-88 mol % ZrG2 powder with a specific surface area of 9.4 rn’ g and an average particle size of 0.3 pm. A conventional tape casting slip was prepared by dispersing this powder in a 60 wt % ethanol-40 wt % methylethylketone (MEK) solution at a solid loading of 30.2 vol %, using a Witco Emphos PS-21A phosphate ester dispersant at a level of 0.44 wt % of the dry ceramic. To 100 parts by weight of this dispersion, we added 25.5 parts of an Rohm and Haas Acryloid B-7 EMA binder solution (MW,= 124 000 Daltons by gel permeation chromatography), 3.56 parts of butyl benzyl phthalate plasticizer, and 3.56 parts of 400 Dalton polyethylene glycol. The complete slip was ball milled about 4 h to homogenize it prior to tape casting. This slip was doctor-bladed onto a mylar substrate using a ceramic tape casting machine, and dried at room temperature. High aspect ratio structures were generated in polyimide by etching in an O2 plasma with Ti as an etch mask using a trilayer resist scheme. The trilayer resist consists of DuPont Pyralin PI-261 1 polyimide, Ti, and Shipley AZ5214 photoresist. The polyimide film was coated on lOO-mm-diam wafers with f 5% uniformity across the wafer. A 0.2~pm-thick Ti layer was evaporated on top of the polyimide as a mask for the subsequent 0, plasma etching of polyimide. To define patterns on the Ti mask, a 1.4+mthick AZ5214 photoresist was spun on the Ti mask and patterned using conventional optical lithography. The Ti layer was etched by RIE in an Ar/BCl, plasma with the developed photoresist as an etch mask. In an O2 plasma, Ti is oxidized to form TiO,, serving as a durable mask since TiO, has a very low sputtering yield. Polyimide was etched in the high density and low perssure O2 plasma generated by a multipolar ECR source excited by a 2.45 GHz microwave power supply. The wafer was placed on top of a 13.56 MHz rf powered stage with the temperature of the stage controlled at 30 “C!. A typical etch condition is 750 W microwave power and 300 W rf power at 0.5 mTorr and 20 seem Oz. The polyimide etch rate is 0.9 pm/min with an etch selectivity of better than 1OOO:l between polyimide and the Ti mask. Anisotropic profiles and smooth surface morphology can be obtained by adjusting the etch conditions.