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Gas Flow and Properties: Standards and Models for Flow Metering Semiconductor Gases at Flows from 10 μL/min to 1 L/min
Summary
Numerous gases are used in the production of semiconductor chips, and the accurate metering of these gases into the process chamber is critical for maximizing device throughput and yield. Many of these gases are hazardous, and flow standards for hazardous gases do not exist. Therefore, the semiconductor industry relies on approximate “meter factors” to extrapolate a meter calibration.
This project will provide standards for the metering of hazardous gases and enable higher chip throughput and yield.
Grand Challenge 1: Metrology for Materials Purity, Properties, and Provenance
Description
Today the control of process gases in semiconductor fabrication is highly empirical. 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. However, the correction method lacks standardization and SI traceability and introduces significant errors depending on the flow and the gas type. A national flow standard will remove bias between different meter manufacturers, resulting in more accurate MFCs. This project will develop physics-based and experimentally verified models of the flow meters, measure gas properties to state-of-the-art accuracy, and validate the resulting calibration chain with flow measurements on a variety of industrially relevant process gases.
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.