Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/84—Systems specially adapted for particular applications
  • G01N21/88—Investigating the presence of flaws or contamination
  • G01N21/94—Investigating contamination, e.g. dust
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/84—Systems specially adapted for particular applications
  • G01N21/88—Investigating the presence of flaws or contamination
  • G01N21/8806—Specially adapted optical and illumination features

Definitions

• Apparatus for measuring particle fallout on a surface, using a witness plate Apparatus for measuring particle fallout on a surface, using a witness plate.
• the invention relates to an instrument for measuring particle fallout on a surface, using a witness plate.
• control plates have been proposed to measure the contamination of surfaces due to the fallout of particles by using sample surfaces in the form of control plates. These control plates are placed in the immediate vicinity of the product during manufacture or of the device during assembly. The particles in suspension can therefore be freely deposited on these control plates for a determined period of time. At the end of this determined period, the plates above are collected and the particle deposition is measured using an optical device based on a photometer.
• the accuracy of the measurements is greatly improved: it allows in particular to take into account the heaviest particles produced by the operators, since the plates are also placed close to them when they are working on a product or a given device. It is simple to implement, it is less expensive than the methods previously used. It also measures the quality of the renewed air. It is enough to carry out measurements during periods of inactivity.
• a device allowing measurements according to this method has been developed by the company SAAB AKTIEBOLAG and is described in British patent application GB-A-1 145 657.
• FIG. 1 schematically illustrates the operating principle of such a measuring device.
• the control plate P ⁇ a is contained in a plate holder (not shown) to avoid any accidental contamination. During the measurement, it is placed on a movable drawer T j (position I: outside the measuring device, position II: measuring position).
• the control plate Pj a once introduced into the PFO measuring device, is illuminated, in grazing light, by two intense light beams, f j and ⁇ 2- These two beams, fi and ⁇ 2, come from a single light source SQ, of the halogen lamp type.
• a set of mirrors Mi to M3, suitably oriented, and focusing lenses, L1-L2 and L3-L4, makes it possible to divide the main beam f, emitted by the single source SQ, into two sub-beams, f
• the particles deposited on the witness plate P ] a scatter the incident light in all directions.
• a photoelectric cell P H0 of the cadmium sulphide type, measures the light scattered in a direction orthogonal to the surface of the control plate P] a , that is to say at a scattering angle of 90 °.
• the light intensity causes the variation of its internal resistance.
• a mirror Mjvj is provided, movable in rotation about an axis parallel to the surface of the control plate P] a .
• An appropriate mechanism M e drives the mirror M ⁇ . This is raised (position II) to allow observation using the microscope M j .
• the mirror M ⁇ makes an angle of 45 ° (position II) with the surface of the witness plate P
• the output signals are transmitted to electronic circuits A j p which include a measurement amplifier and circuits for controlling the light source SQ.
• the amplitude of the electrical signals at the output of the measuring amplifier are transmitted to a measuring device. These are representative of the quantity of particles deposited, per unit of surface, on the surface of the control plate P ja . Knowing its surface and its exposure time, it is possible to deduce the degree of contamination of the objects near which it had been placed. If more than one plate has been used, an average can be taken to increase the accuracy of the measurement.
• the apparatus described comprises three measurement ranges, the measurement amplifier A ⁇ p being provided with automatic switching electronic circuits.
• the latter is supplied with a stabilized voltage so that the measurements are insensitive to variations in the mains and temperature variations.
• the light flux of the SQ halogen lamp is stabilized using an additional photodiode (not shown) placed in a feedback circuit.
• a calibration circuit (not shown) is also made necessary.
• a potentiometer (not shown) is used to adjust the measurements, during the calibration phase. This type of measuring device represented real progress compared to previously known measuring methods. However, it is not completely free from drawbacks. These can be summarized as follows:
• the useful measurement area is small: substantially limited to a circle of 15 mm; -
• the alignment of the incident light is difficult, in particular when the device has been moved: the origin of these alignment problems lies in the fixing of the halogen lamp and the four mirrors;
• the halogen lamp is replaced by a ring of light sources formed by light-emitting diodes illuminating the surface of the control plate at a grazing incidence, in more than two distinct directions.
• the light-emitting diodes are twelve in number, equi-distributed on a circle.
• the subject of the invention is therefore an apparatus for measuring particle fallout on a surface, of the type using a witness plate of determined dimensions, exposed for a determined period of time to said fallout of particles, comprising means of illumination. of said witness plate when it is introduced into the measuring device, opto-electronic detection means, in a direction orthogonal to the surface of said witness plate, of the light scattered by said particles illuminated by said illumination means , so as to measure the intensity of the light scattered in said orthogonal direction and to deduce therefrom the degree of contamination, per unit area, of the control plate by said particles, characterized in that it comprises a lower annular structure, surrounding said control plate, and supporting an optical enclosure and in that said means for illuminating the control plate are constituted by at least s two monochromatic optical sources fixed to the annular structure and arranged in space so as to illuminate the upper surface of the control plate in as many distinct directions and at low incidence so as to obtain grazing light.
• the useful measurement area can be increased from a circle with a diameter of 15 mm to, typically, a circle with a diameter of 30 mm, while using a control plate of the same dimensions. There follows an increase factor equal to four, which is very important for statistical interpretations;
• the light emitting diodes generate monochromatic light, advantageously on a wavelength 630 nm, which allows elimination of fluorescence effects when a 630 nm optical filter is placed opposite the electro-optical detector;
• the optical enclosure is painted black which allows typical optical acquisitions less than or equal to 1 ppm (which must be compared with the typical value of 10 ppm mentioned above), "ppm” being the abbreviation of "parts per million ".
• the structure of the device according to the invention allows very simple electronics associated with very low energy consumption and, as a direct consequence, a very short thermal stabilization time. It is also not necessary to have a very stable base, since the adjustment of the mirrors is no longer necessary, they no longer exist as indicated above. Finally, due to the miniaturization made possible, both from a mechanical and electronic point of view, the device has a much smaller footprint than the devices of the known art according to Figure 1;
• the overall structure of the measuring device comprises two main compartments: an optical enclosure and a detector part.
• This arrangement authorizes, as in the known art, either the use of a detector, advantageously in the case of the invention of the so-called “CCD” type (from the Anglo-Saxon “Charge Coupled Device” or “Device to Load Coupled "), ie the use of a microscope; -
• CCD Charge Coupled Device
• the complex internal calibration system is also no longer necessary. This makes it possible to avoid additional causes of errors in the measurements, due to the presence of a complicated mechanism necessary for adjusting the deflection angle of the mirrors (acting on the beams f ] and f 2 ) and of a mirror. removable (M ⁇ ).
• FIG. 1 schematically illustrates an example of an apparatus for measuring particle fallout on a surface, using a witness plate, according to known art
• FIG. 2 schematically illustrates an example of a device for measuring particle fallout on a surface, using a witness plate, according to a preferred embodiment of the invention.
• the measuring device according to the invention will now be described with reference to FIG. 2, which illustrates a preferred embodiment of such a device.
• the measurement method in itself, does not differ from the measurement method used in known art and which has been recalled. There is therefore no need to rewrite it in detail.
• the control plate Pj a is exposed, for a determined period, to the fallout of particles. The contamination will then be measured, by measuring the light scattered by these particles, when the plate P
• the apparatus 1 essentially comprises two parts: - a lower part comprising an annular structure 1 1, support for the light-emitting diodes, Dj and D x , and an optical enclosure 10 placed above this structure 11; - And an upper part 12, advantageously of annular structure, intended to support optoelectronic detection members 122.
• the lower structure 11 has a lower opening 1 10, through which the control plate P
• the introduction of the witness plate is carried out in a similar manner to that which is provided for the device which is the subject of the abovementioned British patent application.
• the light-emitting diodes twelve in number in a preferred embodiment of the invention, are evenly distributed all around the control plate Pj a to be lit.
• the light beams, for example fi and f x produced by the light-emitting diodes, D j and D x , illuminate the control plate P
• the angle ⁇ is of the order of 4 degrees.
• the number of light-emitting diodes is high, greater than two and preferably, as has been indicated, equal to twelve.
• the subscript " x " associated with D x is an arbitrary number depending on the total number of light emitting diodes.
• Conventional circuits 13 supply electrical energy to the light-emitting diodes, Dl - Dx.
• the optical enclosure 10, of axis of symmetry ⁇ 0 is fixed once and for all on the annular base 11.
• the filter 121 is of the bandpass type, with a narrow band.
• the light emitting diodes for example D j and D x , emit a substantially monochromatic light, centered on a wavelength of 630 nm.
• the passband of the filter 121 will also be centered on this wavelength.
• Removable means 123 are also provided for fixing the detector 122 - filter 121 assembly, for example of the conventional type with screw and leaf spring pressing on the abovementioned assembly to hold it in a housing 124 formed in the upper part of structure 12.
• the interior of the optical enclosure 10 is advantageously painted in matt black to avoid parasitic reflections.
• the optoelectronic detection members 122 are advantageously constituted, as has been indicated, by circuits of the so-called "CCD" type.
• the electrical signals due to the conversion of the measured flux, at incidence equal to 90 ° (along the axis of symmetry ⁇ Q), of the light scattered by the particles contaminating the surface of the witness plate Pj a are transmitted to electronic circuits detection 14. These include conventional amplification circuits (not shown).
• the signals thus amplified are, as before ( Figure 1) representative of the number of particles deposited per unit area, and thereby the degree of contamination.
• the useful measurement area is typically a circle 30 mm in diameter, for dimensions of control plate P] a unchanged, which constitutes an enlargement factor of four.
• the amplified signals can be processed as they are and transmitted to an analog measuring device or, before undergoing an analog-digital conversion, also in a conventional manner, to be processed by a digital measuring device, or even be processed and / or saved in signal processing circuits, for example a microcomputer.
• the apparatus according to the invention remains compatible with the use of a microscope.
• the base of the microscope (not shown) must naturally be provided to allow its introduction into compartment 124 and its fixing using the above-mentioned screw fixing means 123.
• Simple protocols can be implemented. They consist, for example, in the quantified deposition of spheres of standardized polystyrenes, of diameter substantially equal to 40 ⁇ m, on a control plate P
• a .. Metal particles can also be used in a molten glass substrate.
• the device according to the invention does not have any removable or adjustable parts, at least inside the optical enclosure, including the support for light-emitting diodes.
• the light-emitting diodes are fixed once and for all and their large number allows uniform illumination and maximum diffusion, without requiring a great energy power.
• the optical paths are not critical and do not require in situ adjustments. The structure as a whole is therefore much less complex and the device can be miniaturized.
• Fluorescence effects are minimized, which expands the range of types of measurable particles.
• the number of light-emitting diodes is not limited to twelve, although this number ensures good illumination and a large diffusion of light.
• the use of light-emitting diodes of wavelength 630 nm is particularly suitable, it is however quite possible to replace them with other types of monochromatic light sources: laser diodes for example, although the latter are more expensive.
• the "CCD" detector and filter assembly can be fixed permanently to the upper support, which is likely to make the measuring device more robust according to the invention.

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• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Immunology (AREA)
• Pathology (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)

Applications Claiming Priority (3)

Publications (1)

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• 1995
  • 1995-10-09 FR FR9511869A patent/FR2739692B1/fr not_active Expired - Lifetime
• 1996
  • 1996-09-26 JP JP9514755A patent/JPH11513494A/ja active Pending
  • 1996-09-26 EP EP96932658A patent/EP0855021A1/de not_active Withdrawn
  • 1996-09-26 WO PCT/FR1996/001505 patent/WO1997014030A1/fr not_active Application Discontinuation
  • 1996-09-26 US US09/051,415 patent/US6122053A/en not_active Expired - Lifetime

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