Optical sensor for use in high vacuums

Details Optical sensor for use in high vacuums Optical sensor for use in high vacuums

2002-04-26

2004-11-02

Allen, Stephone B. (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S222100, C250S239000, C362S267000

Reexamination Certificate

active

06812451

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical sensor, in particular for use in connection with high vacuum plants or ultrahigh vacuum plants.
In the following a high vacuum is understood as a vacuum with a residual gas pressure of less than 10
−5
mbar and an ultrahigh vacuum as a vacuum with a residual gas pressure of less than 10
−8
mbar.
High vacuum plants and ultrahigh vacuum plants are frequently used to produce, to treat or to examine articles whose surfaces must not be contaminated in these processes. The density of atoms or molecules in the plant is kept very low by the high vacuum so that only a few such atoms or molecules are deposited on the surface of the articles within a given period of time and thus contamination by undesired atoms or molecules is avoided. However, it can also come about that molecules are degassed from contaminated surfaces or from articles in such a high vacuum plant or ultrahigh vacuum plant and thus result in an increase in the density of unwanted atoms or molecules in the vacuum, For this reason, all possible parts which are not required in such a high vacuum plant or ultrahigh vacuum plant are arranged outside the plant to avoid contamination.
In this connection, particularly high demands are frequently made in the manufacture of wafers or microchips since the rejects connected with contamination result in very high costs.
When surfaces are examined in high vacuum or in ultrahigh vacuum, optical sensors are frequently used. Furthermore, optical sensors are also used to monitor handling in manufacturing processes of wafers or microchips, i.e. to monitor the transport and/or the positioning of these products, for example for light barriers or light sensors. To avoid contamination, in particular by the actual sensor elements such as photo-transistors, the sensors, and also the light sources, are generally arranged outside the high vacuum plant or the ultrahigh vacuum plant, with light being able to pass from the unit to the outside or from the outside into the unit through corresponding windows in the plant.
Due to the arrangement of the optical sensors outside the high vacuum plant or the ultrahigh vacuum plant, a comparatively large distance results between the optical sensor and the articles scanned, i.e. the article from which the light received by the optical sensor is transmitted, which results in a low detection precision. Furthermore, a complex holder of the optical sensor is required. Finally, windows are required for the optical sensors in the chamber of the high vacuum plant or of the ultrahigh vacuum plant, which can cause an increased effort in the building of the plant and further possible leak sources or which can result in the sensors not always being able to monitor the most favorable position for the examination in the plant.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an optical sensor which is suitable for monitoring even poorly accessible regions of a high vacuum plant or of an ultrahigh vacuum plant and which is simple to hold.
An optical sensor in accordance with the invention has a metal housing with a window permeable to the light to be detected. This metal housing with the window surrounds in a manner substantially impermeable to gas an interior space in which an optical sensor element is arranged for the detection of light to be detected which is incident through the window. A metal housing with window, from which no gas atoms or gas molecules at all exit at the pressures typical for high vacuum or ultrahigh vacuum of between 10
−5
mbar and 10
−10
mbar, is difficult to realize in practice; at least very low gas amounts exit over the course of time, in particular along the contact line between the metal housing and the window. Such low gas amounts can be tolerated in high vacuum plants or ultrahigh vacuum plants if the gas molecules which escape can again be pumped out of the plant, in which the sensor is used, with sufficient speed. “Substantially impermeable to gas” is therefore understood to mean that at most insubstantial gas amounts exit the housing, including the window, for outer pressures within a predetermined application range below 10
−5
mbar. The leak rate of gases from the interior space within the predetermined application range below 10
−5
mbar should preferably be lower than 10
−7
mbar·l/s, particularly preferably lower than 10
−9
mbar·l/s.
The metal housing is preferably produced from a metal which has only a very low sublimation rate in vacuum, whereby contamination of a high vacuum or of an ultrahigh vacuum by vaporizing atoms is reduced as far as possible. Furthermore, the metal should have sufficient strength to be able to absorb the forces arising due to the pressure difference between the interior space of the sensor and a high vacuum or an ultrahigh vacuum.
The window can generally be produced from any material permeable to the light to be detected and sufficiently impermeable to gas, e.g. from a corresponding glass, with further coatings on the side facing the interior space, for example, being able to be provided to avoid reflections, for example.
The sensor elements can be any preferred sensor elements which are sensitive for the light to be received and to be detected and which provide electrical signals corresponding to the light received. In particular, photo-resistors, photo-diodes, photo-transistors or CCD elements, optionally with integrated optical components, can be used. Due to the encapsulating of the sensor element in the metal housing, the selection of a suitable sensor element can take place irrespective of whether this would give off contaminating atoms or molecules to a large extent in a high vacuum or an ultrahigh vacuum.
A significant advantage of the sensor in accordance with the invention lies in the fact that to a substantial extent it can be used at virtually any position within a high vacuum plant or an ultrahigh vacuum plant due to its impermeability to gas, with no special windows having to be provided in the plant for its use. Furthermore, the installation within the plant allows a lesser distance between the sensor and the article to be scanned, which allows a higher detection precision both with respect to the spatial resolution and to the sensitivity towards interfering light. At the same time, high strain on the high vacuum plant or the ultrahigh vacuum plant due to possible atoms or molecules exiting the sensor element is largely avoided.
To keep the discharge of gases to the outside from the interior space of the sensor as low as possible, the window must be secured in a corresponding opening of the metal housing in a manner which is as impermeable to gas as possible. For this purpose, the window preferably has a region of metallized glass in which it is soldered to the metal housing. In this way, a particularly good tightness is ensured even with particularly low outside pressures. For this purpose, the use of Kovar® as the glass closing alloy for the housing is particularly suitable for a window of quartz glass.
The window can also be glued in. Furthermore, the surfaces of the glass for the window and the metal of the housing can be worked so precisely that a forcing open is possible.
Furthermore, the securing of the window in the metal housing can preferably be made such that it is self-sealing when the pressure in the interior space of the sensor is larger than that outside. This can, for example, take place in that the window is set against the metal housing from the inside such that the interior pressure presses the window against the wall of the metal housing and the seam position is thus better sealed. In this connection, the sealing effect particularly occurs at very high pressure differences between the interior space and the exterior space so that such sensors are particularly suitable for use in ultrahigh vacuum plants.
To increase its tightness, the metal housing is preferably made in one piece, but can also be made of many parts.
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