Development of lead-free thin-film dielectrics for capacitor applications

This PhD project aims to develop lead-free thin-film dielectric materials for fixed value, voltage tunable and high-k zipping variable capacitors using growth techniques that can be scaled for silicon batch fabrication. The thesis specifically details the growth and characterisation of barium zirconate titanate (BZT) and bismuth zinc niobate (BZN) dielectric thin films. Fixed value and tunable capacitors have been realised through the use of low and high permittivity dielectric thin film materials in both the amorphous and crystalline states. Planar devices fabricated using BZT and BZN thin-film dielectrics were grown by sol-gel and RF magnetron sputtering, respectively. The effects of different bottom electrodes were also investigated. Capacitors in metal-insulator-metal (MIM) structure have been fabricated to characterise the dielectric films at low frequency (to 300 kHz). Finding alternative higher permittivity dielectrics to SiO2 for various capacitor and isolation layer applications can be a challenge. Trials were conducted that looked at using amorphous BZT and nano-crystalline/crystalline BZN as a low-k dielectric insulator. Dielectric constants of ~50 were typical for BZN, but much lower permittivity was achieved for amorphous BZT, between 2 and 15 when deposited on Cr/Au bottom electrode. Breakdown strength of amorphous BZT was extremely high (2.0 MV/cm) and far superior to that of BZN (0.35 MV/cm). The dielectric strength of BZN was increased to 0.56 MV/cm when BZN was grown on a BZT seed layer due to a change in the microstructure of the BZN film from granular to columnar. The development of a suitable dielectric BZN for use with polymer substrate was also investigated and MIM capacitors fabricated by sputter deposition. Preliminary results for nano-crystalline BZN thin film growth on gold coated liquid crystal polymer (LCP) substrates appeared encouraging with dielectric constant of 27 and loss 0.005. Crystallised BZT thin films can be used to good effect as lead-free dielectric material in tunable devices. For BZT in the ferroelectric phase, excellent tunabilities of 80% were realised when deposited on platinised silicon. This wasfound not to be the case for BZT in the paraelectric phase where tunabilities tended to be ~60% at best. The dielectric properties of thin-film MIM capacitors can be enhanced by the use of lower resistivity bottom electrode such as gold. Previous research has failed to demonstrate growth and crystallisation of BZT on gold electrode due to the fact that it is technically difficult using the sol-gel method. Films tend to crack after annealing. I have found that films can be stabilised, and the tunability of BZT thin film in the paraelectric phase can be increased significantly, by growing BZT on gold bottom electrode using a BZN buffer layer 25nm thick. A peak tunability of 83% was achieved while maintaining a low dielectric loss of ~0.01. Novel BZT multilayer structures incorporating both ferroelectric and paraelectric compositions were grown on platinised silicon resulting in a tunability of 82% at a bias field 600 kV/cm. Based on the success of growing good quality BZN thin films on gold bottom electrode, it was decided to use BZN thin film as one of the high-k dielectrics in the zipping varactor, a miniature MEMS tunable device. Trials were performed that looked at depositing BZN on thick (800nm) gold coated silicon and glass. This was successful on small sample pieces but failed when scaled up to full device wafer size (100 mm diameter) due to Cr/Au diffusion into the dielectric layer. To overcome this, a 300nm thick BZN film was sputter deposited on Ti/Pt and Ti/Au/Pt coated 100 mm glass device wafers and processed to form the dielectric layer and bottom electrode of the capacitor. As part of the process the BZN layer required patterning. Wet etching of the small features was inappropriate due to undercutting of the structure; dry etching was therefore necessary. Prior to this work there had been nothing reported on the dry etching of BZN, only wet etched using a 1:10 HF-deionised H2O solution. In this work, thin-film BZN was reactively ion etched in Ar/CHF3 plasma at a rate of 6nm per minute with excellent selectivity over platinum of 10:1. Fabrication of the curved top electrode, final assembly and device testing were undertaken by a group at Imperial College London who were collaborators on a work programme entitled, “Integrated Functional Materials for System-in-Package Applications”.