GaPO4 sensors for gravimetric monitoring during atomic layer deposition at high temperatures

The quartz crystal microbalance is extremely useful for in situ monitoring of thin-film growth by atomic layer deposition (ALD) in a viscous flow environment. Unfortunately, conventional AT-quartz sensors are limited to growth temperatures below similar to 300 degrees C. Gallium orthophosphate (GaPO(4)) is an alternative piezoelectric material offering much greater high-temperature frequency stability than AT-quartz (SiO(2)), Our measurements reveal that the temperature coefficient for Y-11 degrees GaPO(4) decreases linearly with temperature reaching 3 Hz/degrees C at 450 degrees C. In contrast, the temperature coefficient for the SiO(2) sensor increases as the cube of the sensor temperature to 650 Hz/degrees C at 390 degrees C. To examine the effect of temperature fluctuations on the sensor frequency, we exposed the SiO(2) and GaPO(4) sensors to helium pulses at 400 degrees C. The resulting frequency change measured for the SiO(2) sensor was similar to 40 times greater than that of the GaPO(4) sensor. Next, we performed Al(2)O(3) ALD using alternating trimedlylaluminum/water exposures at 400 degrees C and monitored the growth using the SiO(2) and GaPO(4) sensors. The GaPO(4) sensor yielded well-defined pulse shapes in agreement with predictions, while the SiO(2) pulses were severely distorted. Measurements during TiO(2) ALD using alternating titanium tetrachloride/water exposures at 450 degrees C with the GaPO(4) sensor also showed well-defined ALD mass steps.