Multi-gas measurement technology

1. Background

　　Gas mass flow controllers use a wide variety of gases, and different customers have different gas requirements. Additionally, many gases in the semiconductor industry are toxic or flammable/explosive. When calibrating flow controllers, it's impossible to use actual gases for all calibrations to simplify production and ensure feasibility and safety.

　　Therefore, it is very necessary to establish a gas calibration platform that can provide various basic gases (N2, H2, He, SF6, etc.) based on the compression characteristics of gases. For other gases, the actual flow rate can be obtained by using conversion coefficients based on the measurement of these basic gases. Most commonly, N2 is used as a substitute gas, and the calibration is performed by multiplying by a conversion coefficient (the conversion coefficient between the target gas and nitrogen can be measured experimentally).

2. Basic Principles of Gas Conversion

　　The theoretical gas flow conversion coefficient depends on the gas's density and specific heat at standard conditions, as well as its molecular composition coefficient. For common single-component gases, the conversion coefficient can be found in the manufacturer's product technical specifications; if the gas's density and specific heat are known, it can also be calculated using the basic formula below.

　　Basic formula for gas mass flow conversion coefficient C:

　　C=0.3106 N /ρ(Cp)

　　Where: ρ — is the gas density

　　CP — is the gas's specific heat at constant pressure

　　N — is the gas molecular composition coefficient (related to the gas's molecular components, see table below)

Table 1 Gas Molecular Composition Coefficient Table

　　If it is a multi-component mixed gas (assuming it consists of n types of gases), calculate its conversion coefficient C according to the following formula:

　　0.3106 [N1 (ω1/ωT )+N2 (ω2/ωT ) + ··· + Nn (ωn/ωT )]

　　C = ———————————————————————— ———

　　ρ1Cp1 (ω1/ωT )+ ρ2Cp2 (ω2/ωT )+ ··· + ρnCpn (ωn/ωT)

　　Where: ω1 …ωn — are the flow rates of the respective gases

　　　　　ωT — is the flow rate of the mixed gas

　　　　　ρ1…ρn — are the densities of the respective gases (values can be found in the product technical specifications)

　　　　　CP1…CPn — are the specific heats at constant pressure of the respective gases (values can be found in the product technical specifications)

　　　　　N1 … Nn — are the molecular composition coefficients of the respective gases

　　Note: If the mixing ratio of the mixed gas is not fixed, the conversion coefficient cannot be calculated.

3. Application of Gas Coefficients

　　The method of using gas coefficients can significantly reduce the difficulty of calibrating and using multi-gas products. Combined with digital product technology, a single product can be used with various different gases, solving the problem of a wide variety of gases used in the semiconductor industry and significantly reducing customer inventory and management complexity.