Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1998-04-07
2000-09-19
Font, Frank G.
Optics: measuring and testing
By particle light scattering
With photocell detection
356337, G01N 2100
Patent
active
061220531
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns an instrument for measuring particle fallout on a surface using a telltale plate.
2. Related Art
In many industries it is necessary to protect all kinds of products, objects, devices, etc from contamination, in particular by particles. This is the case in particular in the fabrication of integrated circuits, measuring instrumentation for aircraft or spacecraft, medical instrumentation, thin film devices, pharmaceutical preparations, photographic emulsions, etc. In all such applications the slightest contamination can reduce output or shorten service life. Monitoring contaminants that can be deposited on a product is therefore vital for this type of industry.
For this reason, production and inspection are carried out in "white rooms" and the air entering such rooms is filtered to minimize the risk of contamination. Additionally, the pressure is raised relative to the external surroundings. However, because of the activity of the personnel present, a large quantity of particles is emitted into the environment of such rooms. The effect of gravity on such particles causes them to settle on all surfaces of the room and, more importantly, on the products being fabricated or assembled.
Initially the contamination of products in "white rooms" was not measured directly. Instead of such direct measurement the size and the number of particles in suspension per unit volume were measured. However, this method could give only a highly uncertain indication of the exact degree of contamination of the products themselves due to the fallout of particles. The larger particles generated by a given operative tend to settle faster in the immediate vicinity of the operative in question. What is more, she operative is likely to move around inside the protected enclosure. Th above method had major drawbacks.
It has been proposed to measure the contamination of surfaces due to the fallout of particles using sampling surfaces in the form of telltale plates. These telltale plates are placed in the immediate vicinity of the product during fabrication or the device during assembly. Particles in suspension can therefore settle freely on the telltale plates for a specified time period. At the end of this specified time period the aforementioned plates are collected and the particle deposit is measured using optical apparatus based on a photometer.
The above method has a number of advantages. Firstly, the accuracy of the measurements is greatly enhanced: in particular it allows for the heaviest particles produced by the operatives, since the plates are disposed near them when they are working on a given product or device. It is simple to use and less costly than methods previously used. It can also measure the quality of the renewed air, for which purpose it is sufficient to carry out measurements during periods of inactivity.
Apparatus for carrying out measurements by the above method has been developed by SAAB AKTIEBOLAG and is described in British patent application GB-A-1 145 657.
FIG. 1 is a diagrammatic illustration of the operating principle of measuring apparatus of the above kind.
The telltale plate P.sub.la is contained in a plate-carrier (not shown) to prevent accidental contamination. During the measurement it is placed on a mobile drawer T.sub.i (position I: out of measuring apparatus, position II: measuring position). Once introduced into the measuring apparatus PFO the telltale plate P.sub.la is illuminated at grazing incidence by two intense light beams f.sub.1 and f.sub.2. The two beams f.sub.1 and f.sub.2 are from a single light source S.sub.O of the halogen lamp type. A set of appropriately oriented mirrors M.sub.1 through M.sub.3 and of focusing lenses L.sub.1 -L.sub.2 and L.sub.3 -L.sub.4 splits the main beam f emitted by the single source S.sub.O into two sub-beams f.sub.1 and f.sub.2 and directs them onto the telltale plate P.sub.la at grazing incidence in two mutually orthogonal directions.
The particles deposited on the telltale plate P.
REFERENCES:
patent: 4614427 (1986-09-01), Koizumi et al.
patent: 5046847 (1991-09-01), Nakata et al.
Agence Spatiale Europeene
Font Frank G.
Stafira Michael P.
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