Herbert Thanner

Material properties of GaPO4 and their relevance for applications

Abstract A set of physical constants of gallium(ortho-)phosphate GaPO 4 has been determined in recent years. These constants allow to calculate precisely the properties of the most important applications of GaPO 4 such as BAW and SAW devices.

GaPO4 high temperature crystal microbalance demonstration up to 720°C

Quartz-homeotypic gallium (ortho-) phosphate, GaPO4, is of special interest for resonator applications asking for temperature compensated cuts with higher electro-mechanical coupling than quartz and operational temperatures up to 970°C. The crystal microbalance technique, well known for quartz (QCM) which can be used only at moderate temperatures, can now be extended to much higher temperatures using GaPO4 crystals, benefiting from all three advantages mentioned above. Two different experiments were done to demonstrate the advantages of a crystal microbalance based on GaPO4. First, the GaPO4 resonator was used for film thickness determination and compared with a commercial QCM. This experiment demonstrated that the measuring range can be extended by using GaPO4 resonators instead of quartz. The second experiment demonstrates the possibility for thermogravimetric analysis up to 720°C by using a new concept for resonator mounting.

GaPO4 high temperature crystal microbalance with zero temperature coefficient

Abstract Quartz-homeotypic gallium (ortho-) phosphate, GaPO 4 , is of special interest for advanced crystal microbalance applications in particular for thin film deposition monitoring and related surface phenomena investigations. One of the most important requirements for crystal microbalance applications is the temperature compensated behaviour of the resonant frequency. GaPO 4 based crystal microbalances can be designed with much better temperature stability than quartz crystal microbalances (QCM) in a temperature range up to 970°C. The higher electromechanical coupling coefficient allows the determination of higher film thicknesses than it is possible with the commercially available QCM. Two different experiments were done to demonstrate the principal function of a microbalance, based on GaPO 4 thickness shear resonators. First, the GaPO 4 resonator was mounted in a standard microbalance holder for monitoring the film thickness during a sputter process. The results show an excellent agreement with the so-called “Z-match theory.” Second, a GaPO 4 thickness shear resonator, mounted in a new microbalance holder, which was developed for very high temperature operation, was contaminated with lube oil on one surface. Heating up the resonator to 750°C, the evaporation process of the lube oil could be studied, up to the complete cleaning of the resonator surface.

GaPO/sub 4/ resonators with Q factors of some millions in the fundamental mode

Gallium orthophosphate (GaPO/sub 4/) is a novel, synthetic piezoelectric crystal which is homeotypic to quartz. Most physical properties are stable until 970/spl deg/C. New applications are thickness shear resonators (fundamental mode) with a cut angle near Y-84/spl deg/ and very low coupling coefficients which have shown very high Q factors (Reiter et al, Proc. 15th Euro. Freq. and Time Forum, pp. 50-54, 2001), and thus it is called the HiQ cut. The temperature behavior and the electrical equivalent circuits of some new HiQ resonators are investigated with a HP network analyzer and an oscillator. The oscillator had a special design because of the high motional resistances of the HiQ resonators (several k/spl Omega/ at normal pressure conditions which decrease to several hundreds of Q under vacuum conditions). The highest Q factors (up to 13/spl times/10/sup 6/) are obtained with the network analyzer at a pressure of about 0.04 mbar. The next measurements are focused on short term stability to obtain oscillators with very low noise.

High temperature microbalances based on GaPO4

Abstract Two different microbalance applications, based on temperature compensated thickness shear mode resonators (single rotated Y-cuts), were studied. In the first case, the film thickness growth during the sputter process was monitored. The measured frequency shift was in excellent agreement with the applied theory. In the second case, the selfcleaning by heating the crystal up to 650 °C after contamination with particulates was demonstrated.

The temperature-stable piezoelectric material GaPO 4 and its sensor applications

Gallium (ortho) phosphate (GaPO4) has a similar structure as quartz, but the high thermal stability up to 970 °C and the high sensitivity make it a very attractive choice for a wide range of uncooled high temperature applications. Furthermore it shows no pyroelectricity, no outgassing and a high electric resistance to guarantee high precision piezoelectric measurements. GaPO4 offers good temperature compensation for a new generation of crystal microbalances operating at temperatures up to at least 700 °C as shown in thermogravimetric and film thickness measurements. It also allows the production of SAW sensors with a very high thermal stability.