Many semiconductor processes require that stable and known ﬂows of gas be delivered to the processing chamber. The thermal mass ﬂow meter (TMFM) is used almost exclusively in the semiconductor industry for the admission of process gases. While TMFMs have been used in the semiconductor industry for over twenty years, much still remains to be understood about their behavior. The abundance of TMFM manufacturers that make instruments which are supposedly interchangeable complicates the use of TMFMs because the instruments generally have different designs and performance. While some attempt has been made via written standards to address the speciﬁcations of the instruments, these standards do not address all performance issues and cannot eliminate the systematic errors in the original manufacturers calibration of the TMFM's. Further, the TMFM's used to measure the process gases are generally calibrated with nitrogen and "corrected" for other gases, but the correction factors are not well understood and are of questionable reliability. It is also important to understand how the TMFM's perform under conditions that differ from the laboratory conditions where they were calibrated and the measurement errors that are introduced as a result of these different operating conditions. This article presents data on the performance of ﬁve low-ﬂow TMFM's, from different manufacturers, with full scale ranges of 1.5x10^-6-3.7x10^-6 mol/s (2-5 sccm).
A critical evalution of thermal mass flow meters, Journal of Vacuum Science & Technology A, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=920804
(Accessed June 1, 2023)