Gas flow calibration of mass flow controllers

Details Gas flow calibration of mass flow controllers Gas flow calibration of mass flow controllers

2000-01-05

2001-12-25

Williams, Hezron (Department: 2856)

Measuring and testing

Instrument proving or calibrating

Volume of flow, speed of flow, volume rate of flow, or mass...

Reexamination Certificate

active

06332348

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to semiconductor processing and, more particularly, to a method, system, and storage medium for gas flow calibration of mass flow controllers.
2. Description of the Relevant Art
Many process tools used in semiconductor fabrication require regulated flow of various gases into the process tool. Mass flow controllers (“MFCs”) are ubiquitous in semiconductor fabrication for regulating the flow of gases into process tools. Each process tool typically has a gas tray which includes a MFC for each gas used by the process tool. Accurate processing often requires that the gas flow be carefully regulated. Failure to accurately control the gas flow may result in the fabrication of defective integrated circuits. To ensure accurate gas flow requires that the calibration of each MFC be checked on a regular basis. For example, the calibration may be checked as often as once every two weeks.
MFCs are often calibrated using mass flow meters (MFMs). The MFMs may be located in an external test rig which can be attached to the gas tray to calibrate the MFCs or, preferably, the MFMs may be located on the gas tray thereby allowing more frequent checks on the MFC calibrations. MFMs are typically only accurate between 5% and 95% of their full scale readings. Process tools, however, often require flow rates of different gases that differ by two or more orders of magnitudes. For example, process tools may have MFCs with full scale ranges of 20 slm and 200 sccm on the same gas tray. To calibrate a 20 slm MFC typically requires a 20 slm MFM, however, a 200 sccm MFC has a full scale range that is only 1% of the full scale range of the 20 slm MFM and, therefore, the 200 sccm can not be calibrated accurately using the 20 slm MFM directly. To calibrate both a 20 slm MFC and a 200 sccm MFC on the same gas tray requires the gas tray contain multiple MFMs. Having multiple MFMs on a gas tray increases the complexity and cost of the gas tray.
It is therefore desired to have a gas tray that requires only a single MFM on the gas tray to calibrate MFCs that have vastly different flow rates. It is also desired that the calibration be easy to perform quickly thereby allowing the calibration to performed more frequently.
SUMMARY OF THE INVENTION
The problems outlined above are in large part addressed by a method for gas flow calibration that involves “gas biasing” of a MFM to allow that MFM to be used to calibrate MFCs that have full scale ranges substantially less than the full scale range of the MFC. Calibrating a MFC using a MFM with a full scale range substantially greater than that of the MFC allows for gas trays with only a single MFM. Utilization of such a method allows gas trays to be produced having only a single MFM thereby advantageously reducing the cost of those gas trays. Additionally, such a gas tray implementing the calibration method recited herein allows all MFCs on the gas tray to be calibrated on a regular basis thereby ensuring accurate delivery of process gases to the process tool.
The method contemplated herein for calibrating a MFC having a full scale range substantially less than the full scale range of the MFM includes the utilization of two MFCs and a MFM. The first MFC has a full scale range substantially similar to that of the MFM. The second MFC, which is the MFC to be calibrated, has a full scale range substantially less than the full scale range of the MFM. The first MFC is already present on the gas tray and is also used for regulating the flow of a process gas to the process tool. The first MFC is used to “gas bias” the MFM. The first MFC is set such that a first gas flow through the MFM at a first flow rate results. The first flow rate is approximately 40% to 60% of the full scale range of the MFM and is preferably approximately 50% of the full scale range of the MFM, which corresponds to the flow rate at which MFMs are typically the most accurate. A first reading of the MFM is then recorded. The second MFC is then set such that a second gas flow of gas through the MFM at a second flow rate results. The second MFC may be set such that flow rate of gas through the second MFC is equal to the full scale range of the second MFC. A second reading of the MFM is then recorded. The difference between the second and first readings is then compared to the set flow rate of the second MFC to determine the calibration of the second MFC. The full scale range of the second MFC may be less than 1% of the full scale range of the MFM. This method may be repeated for the second MFC for different flow rates through the second MFC to determine the calibration of the second MFC over a range of flow rates.
A system, which includes a computer system, a MFM, and a plurality of MFCs, is contemplated herein for implementing the method described above. The system includes a database of the calibration of each of the plurality of MFCs. A program executing on the system controls the MFM and the MFCs to perform the method described above. The program may then modify the database of calibration results.
A computer-readable storage medium is also contemplated herein. The storage medium contains program instructions that can be implemented by a execution method to determine the calibration of a MFC according to the method described above. The storage medium also contains calibration data for the MFC.


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patent: 3330156 (1967-07-01), Thomas
patent: 5648605 (1997-07-01), Takahashi
patent: 5744695 (1998-04-01), Forbes
Wolf et al.,Silicon Processing for the VLSI Era, vol. 1: Process Technology, © 1986 by Lattice Press, pp. 165-166.

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