New Advances in Forming Functional Ceramics for Micro Devices

Micro electromechanical systems (MEMS) are finding uses in an increasing number of diverse applications. Currently the fabrication techniques used to produce such MEMS devices are primarily based on 2-D processing of thin films. The challenges faced by producing more complex structures (e.g. high aspect ratio, spans, and multi-material structures) require the development of new processing techniques. Potential solutions to these challenges based on low temperature processing of functional ceramics, selective chemical patterning, and micro-moulding are presented to show that it is possible to create complex functional ceramic structures which incorporate non-ceramic conducting and support structures. The capabilities of both techniques are compared and the relative advantages of each explored. Introduction Piezoelectric functional ceramics, such as lead zirconate titanate (PZT), find applications in a range of devices including ultrasound transducers, actuators, and sensors [1, 2]. There exists a desire to reduce the size of these devices to increase performance, portability and applications. While large scale devices can be fabricated with ease using bulk materials that are shaped and assembled together, it becomes increasingly difficult to produce such structures at progressively smaller scales. To achieve the required reductions in size will require the use of microscale assembly techniques instead of traditional bulk processing routes. With microscale assembly techniques multiple materials are typically integrated and processed simultaneously. One of the primary issues that needs to be overcome when integrating ceramics with other materials typically employed in micro electromechanical systems (MEMS) (e.g. silicon, metals, glass) is the requirement for high temperatures to bring about densification of the ceramic material [2, 3]. The authors have combined multiple approaches for ceramic processing to develop a technique for producing structured thick (10 – 50μm) films. By combining low temperature sol gel processing with sintering aid assisted powder processing it has been possible to produce ceramic films with densities in the region of 80-95% theoretical density, at temperatures between 600 and 720oC [4]. Such low sintering temperatures are possible due to the use of the sol gel material which both increases the green density of the film beyond that achievable using powder processing alone, and fuses the powder particles together with a compositionally matched material. The incorporation of the small amount of sintering aid is then sufficient to still further increase the density of the film by 5-10% and consolidate the sol gel phase. Figure 1 shows an example of the cross-section of such a composite sol gel film showing the incorporation of a thin metallic electrode structure between two layers of ceramic. The figure demonstrates that it is possible to co-process ceramic and metallic materials on a silicon substrate. Journal Advances in Science and Technology Vol. 45 (2006) pp. 2440-2447 Fig. 1 – SEM photomicrograph showing a fracture cross section of a multilayer composite sol gel film, on a silicon substrate, incorporating multiple metal electrode layers each approximately 100nm thick. The combined use of sol gel and powder processing has been demonstrated to be capable of realising integrated film structures with the potential to use such technology to produce functional integrated structures at low temperatures. The ability to integrate these materials to produce layered structures is, however, not sufficient to realise a functioning device. In order to produce a functioning device is also necessary to structure these films on a fine scale. This can be technically challenging due to the chemical inertness of many of these materials – especially ceramic materials. This paper will present and compare two approaches to structuring functional ceramics. The two approaches described are both based on the application of the composite sol gel technique, with either material removal or moulding being used to create structured features. Experimental Composite sol gel. The composite sol gel approach has been described in detail in previous publications [4]. Briefly, a sol is mixed with ceramic powder (and a sintering aid if required) to produce a slurry which can be spin coated onto a substrate. Table 1 gives details of the composition of the sol and suspension used for the two approaches. For the purposes of this comparison the slight differences in the compositions of the two systems can be ignored as they do not influence the processing conditions used. Table 1 – Composition of the sol gel and slurry used Wet etching [5] Micro-moulding [6] Sol composition Pb(Zr0.49 Ti0.51)O3 lead acetate, zirconium isopropoxide, titanium propoxide, acetic acid/2-methoxyethanol solvent Pb(Zr0.5 Ti0.5)O3 lead acetate, zirconium isopropoxide, titanium propoxide, acetic acid/1-propanol solvent Slurry composition 1.5 g of PZ26 powder (Ferroperm): 1ml sol Cu2O + PbO (sintering aid) KR55 (DispersantKenrich petrochemicals) 1.5 g of PZ26 powder (Ferroperm): 1ml sol KR55 (DispersantKenrich petrochemicals) Journal Advances in Science and Technology Vol. 45 (2006) pp. 2440-2447 Wet etching. Continuous films were fabricated using the sol gel approach where the substrate was coated with the slurry which was then spun off at 2000rpm for 30 seconds to produce a uniform film approximately 2.5 μm thick. The film was then dried at 200oC and pyrolysed at 450oC to convert the sol gel phase to an amorphous oxide ceramic. The density of the composite layer was then increased by infiltrating the layer with pure sol which was again spun at 2000rpm, dried and 200oC and pyrolysed at 450oC. This process was repeated either 2, 3 or 4 times to produce films of different densities. The film thickness was increased by depositing further composite/sol layers until the required film thickness had been achieved. To structure the film a 5μm thick layer of photoresit (AZ4562) was deposited and selectively exposed to UV light, and developed to create a patterned mask on the surface of the PZT layer. A HCl/HF etchant was then used to etch the exposed areas of PZT and in doing so produce structured PZT micro-features. Micro-moulding. To produce the micro-moulds a thick layer of photoresit (AZ4562) was first deposited onto the surface of the substrate. The phororesit was then selectively exposed to UV and developed to create the micro-moulds. The micro-moulds were filled by coating the wafer with the composite slurry and spinning the substrate at 2000rpm to remove the excess. Instead of drying the composite slurry at 200oC the slurry was allowed to dry at room temperature prior to another composite layer being deposited. The high temperature drying stages were not conducted at this intermediate stage as they would have resulted in damage to the phororesist micro-moulds. Once the micro-moulds had been filled the wafer was dried and pyrolysed in a furnace using a ramp rate of 2oC/min and a hold temperature of 600oC for 20 minutes. This drying/pyrolysis stage was conducted with the wafer suspended upside down to that any unwanted PZT would fall away from the sample and not contaminate the sample.