Flow Metering and Properties for Semiconductor Process Gases

A type of flow meter called a mass flow controller (MFC) is used to regulate gas flow in order to produce the desired structures during chip fabrication. As semiconductor manufacturing advances, the requirements on MFC performance are increasingly strict: any process variation can reduce device yield. In the year 2000, flow uncertainty of 1 % of full scale was acceptable, now 0.1 % of reading is desired. The industry requires interchangeability of MFCs: if an MFC fails, they need to replace it with a flow meter that provides the same flow behavior in order to successfully apply the same chip manufacturing recipe. Chip manufacturers require new process gases, lower flows, and better response times from MFCs to support new chip technologies.

Each model of MFC has a list of gas correction factors or functions to convert a calibration in nitrogen to a calibration in some other gas. For many gases, this is grossly inadequate to correct the performance of the flow meter. The MFC calibration curve is complex and poorly understood for many gases of interest (SF₆, C₄F₈, NF₃), partially because of a lack of primary flow standards that can safely handle semiconductor gases and a lack of low uncertainty gas property values. Experimentally validated physical models for the relevant flow meters coupled with accurate gas properties (e.g., viscosity, density, thermal conductivity) allow the chip industry to extrapolate a nitrogen calibration to the new, often toxic gases used to produce semiconductors. The goal of this project is to design and build a gas flow standard combined with new gas property data to realize flows of semiconductor process gases from 0.01 cm³/min to 1 L/min. The standard and property data will be used to develop physics based models that explain how MFCs work across a wide range of gas species in the semiconductor industry.

Capabilities

The Fluid Metrology Group (FMG) in the Sensor Science Division of the Physical Measurement Laboratory has best in the world capabilities over the range of gas flows of interest to chip manufacturers in non-hazardous gases. The 34 L and 677 L pressure-volume-temperature-time (PVTt) standards measure flow with uncertainty < 0.025 %. NIST research on the rate-of-rise (RoR) gas flow measurement method extended capabilities down to 0.1 cm³/min, meeting the needs of the chip industry for ever smaller flows. The gas flow capabilities have been proven by numerous interlaboratory comparisons. These capabilities are the foundation for traceability and uncertainty for the tens of thousands of MFCs used in the chip fabrication industry.

Recently, the FMG has built a new RoR gas flow standard that can measure gas flow down to 0.01 cm³/min. The new flow standard is called SLowFlowS for Semiconductor Low Flow Standard. The new standard uses an air bath for temperature stability and was designed to accurately determine the gas temperature during filling with an expanded uncertainty of 0.02 % of reading. The unique design allows for low gas flow measurement within 0.06 % of the flow in a matter of hours, not days. Figure 1 shows SLowFlowS.

NIST gas flow standards are a critical resource for research on the flow meter physical models. An experimentally validated physical model allows meter manufacturers and users to understand the performance of meters in new applications, such as a new gas or a gas at a different temperature or pressure condition than those used during calibration of the meter. The NIST Sensor Science Division is the world leader in developing physical models for flow meters, including laminar flow meters, critical flow venturis, and Coriolis meters. NIST collaborates on calibration standards and flow meter research with MFC manufacturers and end users. For example, we have worked with a meter manufacturer to share experimental data and apply a NIST-developed laminar flow meter model to reduce the uncertainty of their products in new gases for the semiconductor industry.

Along with the work the FMG is doing, the Thermophysical Properties of Fluids Group in the Applied Chemicals and Materials Division of the Materials Measurement Laboratory are performing new measurements on the properties of semiconductor gases. This work is a collaboration as accurate thermophysical properties are necessary for the flow standard and physical models. Density is needed directly or the RoR flow standard, whereas viscosity and thermal conductivity are important for the generation of the physical models.