A HIGH-SPEED HIGH-SENSITIVITY ACOUSTIC CELL FOR IN-LINE CONTINUOUS MONITORING OF MOCVD PRECURSOR GASES

We describe a continuous wave resonant acoustic sensor that has been o ptimized as a very sensitive in-line monitor for measuring the composi tion of precursor gases used in MOCVD processes. The precursor/carrier gas mixtures flow through a compact stainless steel acoustic chamber that is isolated from the acoustic transducers by a set of metallic di aphragms. The sensor has been successfully operated at supply line pre ssures from atmosphere down to 50 Torr with gas flow rates of up to 16 00 seem. The accuracy of the speed of sound measurement for hydrogen g as is better than 0.005%, even in a high noise and low pressure enviro nment. Hydrogen, as well as nitrogen or argon carrier gases, are accom modated within the instrument's 1-5 kHz working frequency range. The i nstrument's sensitivity and stability are demonstrated with the labora tory data. Measurements of the dynamic response characteristics of the metalorganic bubbler lines at low pressure are also be presented. App lication of the cell is general, encompassing any of the metalorganic and hydride materials typically used in MOCVD processes.