Direct patterning of photosensitive chemical solution deposition PZT layers

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Abstract

A highly concentrated solution for producing photopatternable layers of lead zirconate titanate (PZT) was prepared by dissolving into acrylic acid an amorphous PZT powder which was obtained by a sol-gel process. The solution was found to be suitable for spin-coating PZT layers that are photosensitive to UV radiation. After deposition and subsequent exposure the PZT film showed a decreased solubility in several organic compounds. These properties were exploited to create features on platinised silicon (Si) substrates. A thermogravimetric analysis was performed on the solution to determine the best thermal profile for the burn out of the organics. PZT features up to 2 μm thick were obtained after the firing process.

Introduction

Chemical Solution Deposition (CSD) has several advantages over other fabrication methods. It permits coating of complex geometries and offers a very good control of the material composition. Ceramic films have been deposited in a single layer with sol-gel techniques up to about 1 μm thick.1 Thicker films are normally produced by successive coatings.2 With a ceramic/sol-gel composite route3 the thickness achievable by a single deposition step is usually greater than 1 μm but the resulting film is very porous and needs to be infiltrated with sol to increase the density and to improve the electrical properties.4 In both cases a long time is required to finish the whole process; therefore the manufacture of a dense thick layer in one stage would offer great time savings. In most practical applications it is also necessary to pattern layers by conventional lithography. This usually entails the deposition and exposure of a photoresist layer followed by development and chemical etching. It would be highly desirable if the PZT layer could be patterned directly by UV exposure, both because it would eliminate the need for further lithographic processing and because PZT materials are quite difficult to etch. The development of such a process is one of the objectives of the research described here. A further significant issue related to the thick film manufacture is the cracking that occurs during the thermal treatments for drying and firing. The shrinkage that takes place when the organics are removed from the film creates a stress that is released by the formation of cracks. In order to exploit the desired properties of the ceramic, it is naturally essential that the ceramic film should be crack-free. The production of small patterned areas on the substrate by direct patterning offers a possibility for reduction of the stresses associated with shrinkage and thus cracks elimination.

This paper reports a method for the production of directly patterned features up to 1.9 μm thick using a single deposition step with highly concentrated lead zirconate titanate photosensitive solutions. The films were patterned with UV light before any firing process. High concentrations were used in order to reduce the organic loss during the thermal treatments and the subsequent shrinkage.

Section snippets

Experimental

An amorphous PZT powder was prepared by a sol-gel process. An ethanol-based sol used in this route gives a PZT powder with composition Pb1.1[Nb0.02(Zr0.52Ti0.48)0.98]O3. The starting materials were lead acetate trihydrate, titanium iso-propoxide, zirconium n-propoxide and niobium ethoxide.

The lead solution was prepared with 10 mol% excess lead by dehydrating lead acetate and dissolving it in ethanol. Mono-ethanolamine was then added and the mixture was gently heated in a nitrogen atmosphere to

Results and discussion

Fig. 1 shows the result of the thermogravimetric analysis performed on the solution. The results are expressed in terms of weight loss (% of mass) versus temperature of the PZT solution. The derivative of the TG curve is also shown to highlight the temperatures at which the major weight losses occur. The coupling of TGA with mass spectroscopy allowed the volatile compounds to be identified in the four main steps.

The decomposition of the PZT solution is initially dominated by the release of

Conclusions

A photopatternable solution has been developed and spun onto platinised silicon substrates. Optimal experimental conditions were found to prebake and expose the film in order to obtain a good quality pattern and to minimise the exposure time. These conditions were found to be 70 °C prebaking temperature and 20 min exposure time. The concentration of the spun solution was increased to obtain patterned ceramic films up to 1.9 μm thick.

Acknowledgments

EPSRC funding under grant GR/N05970 are gratefully acknowledged.

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