Optics: measuring and testing – By shade or color – Fluid color transmission examination
Patent
1994-03-14
1996-06-11
McGraw, Vincent P.
Optics: measuring and testing
By shade or color
Fluid color transmission examination
2503561, 356 72, G01N 2185
Patent
active
055261221
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for determining the mass flow of gases on the basis of optical absorption.
2. Description of the Related Art
Fluid and solid initial materials or substances that have a low vapor pressure and a low vapor pressure in comparison to previously standard gases such as, for example, SiH.sub.4, B.sub.2 H.sub.6 are being increasingly employed in CVD coating processes (chemical vapor deposition) in many areas of technology, for example in microelectronics or in anti-wear protection. This development results, first, for safety reasons (if any contamination occurs at all given potential accidents, this should only be a local contamination) and, second, from method-conditioned simplifications. In the latter instance, layers should also be capable of being produced from compounds by simple, thermal decomposition of an initial substance. A typical example is the employment of TEOS (tetra ethyl ortho silicate) for the deposition of SiO.sub.2 in semiconductor technology. This fluid has a vapor pressure of a few hundred Pa at room temperature. Metallo-organic compounds for metal deposition are usually present in solid form at room temperature. The vapor pressure often lies below 100 Pa. Their reliable and time-constant dosing is an indispensable prerequisite for the reproducible deposition of layers.
Commercially available apparatus that are referred to as "mass flow controller" or "mass flow meter" are usually employed now for designational dosing of gaseous substances. These devices measure the mass flow of the gaseous substance via the thermal conductivity.
Such apparatus work on the principle that a pressure drop is produced in a line for the gaseous substance to be investigated, for example, by diaphragms or laminar elements. Dependent on the pressure difference, a part of the gaseous substance can flow via a bypass capillary arranged parallel thereto. Heating wires that typically heat the gaseous substance to 70.degree. through 100.degree. C. are attached to this capillary. Thermal elements that measure the temperature distribution are also thereby provided.
When no gas flows through the line, then a pressure drop does not arise in the system and, thus, a flow does not arise in the capillary. A uniform, symmetrical temperature distribution therefore derives. The deviation from this distribution that derives given a flow can form the basis for the mass flow as a quantity. Apparatus of the abovementioned type as well as their function are described, for example, in a company brochure "V-MAX The ultimate vapor source" of Tylan GmbH in Eching/Munich.
One problem given such apparatus is that an admission pressure of at least 10.sup.3 through 10.sup.4 Pa is required for the measuring principle in order to produce the pressure drop in the measuring system. This minimally required admission pressure can only be produced at higher temperatures of, for example, more than 100.degree. C. given many substances or materials, being produced via their vapor pressure. The following disadvantages derive due to the necessity of heating the materials or, substances:
In part, the materials are not adequately thermally stable and decompose before the necessary vapor pressure is reached.
Commercially available "mass flow controllers" are normally limited to an operating temperature of 70.degree. C. This temperature can be somewhat higher only in special designs.
Other disadvantages are that the measuring principle loses precision, the useful signal becomes extremely low and the apparatus can at most be operated only at a fixed temperature.
A further limitation in the use of mass flow controllers is that they must be calibrated to a specific material or, to a specific substance since each substance has a different thermal conductivity. This makes universal employment impossible or limits this decisively. Over and above this, the thermal conductivity data are often not known or are only very imprecisely known.
A further possibility of dosing
REFERENCES:
patent: 3654109 (1972-04-01), Hohl et al.
Patent Abstract of Japan, vol. 15, No. 133 (C-820) 2 Apr. 1991; JP A 30 16 555, NEC, Jan. 1991.
"Second Generation-Analyzer Cross Flow Modulation/Techniques Precision, Sensitive, Continuous Measurements", Adv. in Instrumentation, 42 (1987) Part 1, pp. 547-555, Research Triangle Park, NC, USA.
Company brochure, "Vaporizer Controllers et al.", Apr. 1987, pp. 5-9, 20 by Tylan GmbH, Eching/Munich, Germany.
Company brochure, "GASANALYSEGERATE ULTRAMAT 5 . OXYMAT 5", Siemens AG, Germany No. A19100-E681-A21-V4. No month & date available.
Company brochure, "V-Max The Ultimate Vapor Source", by Tylan GmbH, Eching/Munich, Germany. No month & date available.
Hieber Konrad
Intemann Andreas
Korner Heinrich
McGraw Vincent P.
Siemens Aktiengesellschaft
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