Peter W. Krempl

Gallium phosphate, GaPO4: a new piezoelectric crystal material for high-temperature sensorics

Abstract Single-crystalline GaPO 4 , gallium phosphate (more precisely gallium orthophosphate), is a new piezoelectric material which is very similar to quartz in its crystal structure but has a much higher thermal stability, a higher piezoelectric effect, larger electromechanical coupling constants and temperature-compensated cuts for bulk acoustic wave (BAW) and surface acoustic wave (SAW) applications. It is thus a promising material for sensor applications in the temperature range up to 900°C. The first products are already on the market: uncooled pressure sensors for combustion engines, with sensitivity and stability surpassing those attainable with quartz. As the crystal growth process is reaching larger quantity and higher quality, gallium phosphate will be the material for further new applications exceeding the possibilities of conventional materials.

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.

High-temperature 434 MHz surface acoustic wave devices based on GaPO/sub 4/

Research into surface acoustic wave (SAW) devices began in the early 1970s and led to the development of high performance, small size, and high reproducibility devices. Much research has now been done on the application of such devices to consumer electronics, process monitoring, and communication systems. The use of novel materials, such as gallium phosphate (GaPO4), extends the operating temperature of the elements. SAW devices based on this material operating at 434 MHz and up 800degC, can be used for passive wireless sensor applications. Interdigital transducer (IDT) devices with platinum/zirconium metallization and 1.4 mum finger-gap ratio of 1:1 have been fabricated using direct write e-beam lithography and a lift-off process. The performance and long-term stability of these devices has been studied, and the results are reported in this paper

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.

Measurement of the piezoelectric and electrooptic constants of GaPO4 with a michelson interferometer

Abstract A new measurement system to determine the piezoelectric and electrooptic constants of gallium orthophosphate (GaPO4) by the inverse piezoelectric effect was developed. The change in length due to the applied voltage is measured with a Michelson interferometer. With this method the piezoelectric constants are determined without the knowledge of any other material constant and in a wide temperature range. The temperature dependence of the longitudinal piezo constant (d11 = 4.5 pm/V) from room temperature up to 550°C is very low. Therefore this new material is a good choice for piezoelectric applications in a high temperature environment.

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.

Segregation forming and growth defect characterization by heat treatment of hydrothermally grown GaPO4

Abstract Quartz-type gallium (ortho-)phosphate (GaPO 4 ) is grown hydrothermally from acid solutions. By heating to 600–800°C for several hours, defects containing OH groups segregate in submicroscopic bubbles which form various patterns relating to growth defects. The formation of oriented disks and star-like forms, as well as of helices, is attributed to passive climbing of step dislocation loops, sometimes originating at local growth defects. Growth dislocations are decorated and show effects of dislocation climbing. Boundaries between growth zones and Brazil twin boundaries become clearly visible. Bands and growth striations are attributed partly to growth condition fluctuations, partly to macrostep propagation. The microscopic investigation of patterns created by heat treatment thus has been shown to be a convenient method for characterization and the study of growth defects and offers insight into segregation formation and dislocation dynamics of GaPO 4 .

Temperature sensors based on GaPO/sub 4/

Results of investigations of temperature sensors using singly rotated Gallium Orthophosphate (GaPO/sub 4/) Y-cut resonators are presented in this paper. Depending on rotating angle the most important parameters - sensitivity, resolution and linearity differ in a wide range. Theoretical calculations leads to cuts owning nearly a linear response with sensitivity between 44.5 ppm//spl deg/C and 46.5 ppm//spl deg/C within a temperature range from room temperature to 600/spl deg/C with a high resolution of about 10/sup -6//spl deg/C. First measurements confirm the theoretically predicted sensitivity. The broad thermal stability range of GaPO/sub 4/ can also be used for temperature sensors based on surface acoustic wave devices. By theoretical calculations, suitable orientations with high temperature coefficients are identified.