Real time thin-film thickness monitoring sensors: a comparative study of dual mode Solidly Mounted Resonators with commercial Quartz Crystal Microbalances

Next generations of physical and chemical vacuum deposition systems require more accurate and faster sensors to monitor the deposition of very thin films (nm range). Acoustic sensors, particularly QCMs, have been typically used in such systems. However, their potential has reached certain limitations. They are bulky, they need a water-cooling system to work at higher temperatures, and for certain applications their response time is not fast enough. For many years, film bulk acoustic wave resonators (FBARs) working at higher frequencies have been studied as a higher sensitivity alternative to QCMs, particularly in biosensing applications. However, they have been never applied and simultaneously compared to QCMs in an actual PVD system so far. Sorex Sensors has developed an FBAR which not only offers higher mass sensitivity than QCMs but also allows discriminating temperature related effects within the same device without the need of a reference sensor. FBARs can also be integrated in arrays providing a cost-effective and more accurate solution for thin film monitoring. This sensor has been tested simultaneously with a commercial QCM in an evaporator, for benchmarking the characteristic of both technologies in terms of sensitivity.