EP0855021A1

EP0855021A1 - Apparatus for measuring particle fallout on a surface using a telltale plate

Info

Publication number: EP0855021A1
Application number: EP96932658A
Authority: EP
Inventors: Arie Zwaal
Original assignee: Agence Spatiale Europeenne
Current assignee: Agence Spatiale Europeenne
Priority date: 1995-10-09
Filing date: 1996-09-26
Publication date: 1998-07-29
Also published as: JPH11513494A; FR2739692B1; US6122053A; WO1997014030A1; FR2739692A1

Abstract

A measuring apparatus including a ring-shaped base (11) surrounding a telltale plate (P1a) contaminated by particles. Uniformly distributed LEDs (D1, Dx) emitting on a wavelength of 630 nm are attached to the plate. In a preferred embodiment, twelve LEDs (D1, Dx) illuminate the top surface of the telltale plate (P1a) at a low angle of incidence. The lower ring-shaped structure (11) supports a cylindrical optical enclosure (10) underneath an upper ring-shaped structure (12) which in turn supports a CCD detector (122) for measuring the light diffused by the particles, and an optical filter (124) centred on the 630 nm wavelength. The filter (124) and the detector (122) may be removed and replaced with a microscope.

Description

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.

In many industries it is necessary to protect all kinds of products, objects, devices, etc. from contamination, especially by particles. This is particularly the case for the manufacture of integrated circuits, measuring instruments intended for airplanes or space applications, medical instrumentation, thin films, pharmaceutical preparations, photographic emulsions, etc. For all these applications, the least contamination can lead to a lower yield or a shorter service life. Controlling the contaminants that can deposit on a product is therefore vital for this type of industry. For this reason, production and examinations are carried out in "clean rooms" whose air renewal is filtered so as to minimize the risk of contamination. In addition, an overpressure with respect to the external environment is maintained. However, due to the activity of the personnel present, a large quantity of particles is emitted into the environment of these rooms. The action of gravity on these particles causes it to deposit on all surfaces of the room and, more importantly, on the products being manufactured or assembled.

Initially, contamination of products in the "clean room" was not measured directly. Instead of this direct measurement, the size and number of suspended particles per unit volume were measured. However, this method could only give a very uncertain indication of the exact degree of contamination of the products themselves due to the fallout of the particles. Indeed, the largest particles, generated by a given operator, tend to fall quickly and, this, in the immediate vicinity of the operator in question. Furthermore, it is likely to move within the protected enclosure. This method had major drawbacks.

It has 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.

This method has several advantages. First of all, 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.

Figure 1, annexed to this description, schematically illustrates the operating principle of such a measuring device. The control plate Pι _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 _| and Ï2, and direct them, in grazing incidence, on the control plate P] _a , in two directions orthogonal to each other. 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. So as to also be able to observe the light scattered along this measurement axis using a microscope Mj, 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 . On the other hand, the mirror M ^ makes an angle of 45 ° (position II) with the surface of the witness plate P | _a , and also with the measured _scattered beam L _us , to allow measurement, by 90 ° deflection of the beam L _us towards the photoelectric cell P ^ o 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 use of a white light source restricts the measurement to non-fluorescent particles; - The optical background is typically limited to around 10 ppm on uncontaminated plates, due to stray light problems.

The invention, while retaining the advantages of the device according to the known art which has just been described, sets itself the aim of overcoming the disadvantages, some of which have just been recalled. To do this, according to an important characteristic of the invention, 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. In a preferred variant of the invention, 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 structure of the measuring device of the invention has many advantages and among which:

- 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 use of mirrors is no longer necessary since the annular structure of light-emitting diodes can be directly fixed to a frame, also annular, which surrounds the witness plate. The angle of incidence is fixed. It is then possible to determine, once and for all, an optimized structure in order to obtain a maximum of scattered light for the aforementioned area and the lowest possible stray light acquisitions. This structure avoids the readjustment of the optical paths (beams f _j and f ₂ in FIG. 1);

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;

- As will be described in detail below, 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; - 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 ^). The invention will be better understood and other characteristics and advantages will appear on reading the description which follows with reference to the appended figures, and among which:

- Figure 1 schematically illustrates an example of an apparatus for measuring particle fallout on a surface, using a witness plate, according to known art;

- Figure 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 | _a is illuminated by several incident beams, in grazing light.

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 | _a . 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 _{| a} at a very low incidence α, that is to say in grazing light. Typically, the angle α is of the order of 4 degrees.

Advantageously, the number of light-emitting diodes, that is to say of light sources 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 Δ ₀ , is fixed once and for all on the annular base 11.

The upper part 12, disposed on the optical enclosure 10, supports, in addition to the above-mentioned optoelectronic detection members 122, an optical filter 121, placed under the detector 122 and, at its lower part, a focusing lens 120. This (in the example described in FIG. 2), is located at the junction of the annular structure 12 and the optical enclosure 10.

The filter 121 is of the bandpass type, with a narrow band. As indicated, 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 ΔQ), 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. However, due to the provisions specific to the invention, 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.

As previously indicated, the apparatus according to the invention remains compatible with the use of a microscope. In this case, it suffices to disassemble the detection member 122 and the associated filter 124, using screw fixing means 123. 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.

A complicated calibration system is no longer required. Simple protocols can be implemented. They consist, for example, in the quantified deposition of spheres of standardized polystyrenes, of diameter substantially equal to 40 μm, on a control plate P | _a .. Metal particles can also be used in a molten glass substrate. On reading the above, it can easily be seen that the invention does indeed achieve the aims which it has set for itself and has the advantages which have been recalled in the preamble to the present description.

In particular, 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.

It should be clear, however, that the invention is not limited to only the examples of embodiments precisely described, in particular in relation to FIG. 2.

In particular, as has been indicated, the number of light-emitting diodes is not limited to twelve, although this number ensures good illumination and a large diffusion of light. Finally, 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.

If the use of a microscope is not desired, 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.

Claims

1. Apparatus for measuring (1) particle fallout on a surface, of the type using a witness plate (Pj _j ) of determined dimensions, exposed for a determined time interval to said particle fallout, comprising means for illumination of said witness plate (P _{] a} ) when it is inserted into the measuring device (1), opto¬ electronic detection means, in an orthogonal direction (Δ ₀ ) at the surface of said witness plate (Pja ), 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 (ΔQ) and to deduce therefrom the degree of contamination, per unit area, of the control plate (P ^ by said particles, characterized in that it comprises a lower annular structure (11), surrounding said control plate (PQ, and supporting an optical enclosure (10) and in that said means for illuminating the plaque plaque oin consist of at least two monochromatic optical sources (Dj, D _x ) fixed to the annular structure (11) and arranged in space so as to illuminate the upper surface of the control plate in as many distinct directions and under low incidence so as to obtain a grazing light.

2. Apparatus according to claim 1, characterized in that said monochromatic sources are light-emitting diodes (D _j , D _x ) whose emission is centered on a wavelength of 630 nm.

3. Apparatus according to claim 2, characterized in that said light emitting diodes (Dj, D _x ) are twelve in number, equi-distributed over said annular structure (11).

4. Apparatus according to claim 1, characterized in that said angle of incidence is substantially equal to 4 degrees.

5. Apparatus according to claim 1, characterized in that said optical enclosure (10) is provided with a cylindrical structure of axis of symmetry (Δ "Q) orthogonal to the surface of said control plate (Pja) and in that the internal walls of this enclosure are covered with a matt black paint so as to avoid parasitic reflections.

6. Apparatus according to claim 5, characterized in that said enclosure is surmounted by an upper annular structure (12), in that this structure

(12) supports said optoelectronic detection means and in that these optoelectronic detection means include charge coupled semiconductor circuits (122).

7. Apparatus according to claim 6, characterized in that said upper annular structure (12) also supports an optical filter disposed under said optoelectronic detection means (122), of the bandpass type, the central wavelength of which is matched at the emission wavelength of said monochromatic optical sources (D _j , D _x ).

8. Apparatus according to claim 6, characterized in that said upper annular structure (12) further supports, in its lower part, an optical lens (120) focusing the diffuse light on said optoelectronic detection means (122).

9. Apparatus according to claim 7, characterized in that said upper annular structure (12) is provided with removable fixing means of said optoelectronic detection means (122) and of said filter (121) are removable, so as to replace them with a direct observation microscope of the control plate (Pja).

10. Apparatus according to claim 9, characterized in that said removable fixing means comprise screwing and elastic retaining means with leaf spring (123) of said optoelectronic detection means (122) and of said filter (121) to the interior of a housing (124) formed in the upper part of said upper annular structure (12).