FR2923009A1 - Sealably or hermetically encapsulated detector e.g. microbolometric infrared imaging retina, for camera, has carrier with corrugation extended along direction other than that of plane in which window is fixed or plane parallel to window - Google Patents

Abstract

The detector (3) has a silicon substrate (2) receiving the detector. The substrate has a base (4) and lateral walls (5) to define a cavity (8) closed by a transparent silicon window (6). The window is connected on a window carrier (7) that is hermetically sealed on an upper edge of the walls. The carrier is not placed in a unique plane and provided with a corrugation (9). The corrugation is peripheral with respect to the window and absorbs constraints. The corrugation is extended along a direction other than that of a main plane in which the window is fixed or a plane parallel to the window.

Description

FIELD OF THE INVENTION The invention relates to an electromagnetic radiation detector, and more particularly to an infrared radiation detector. It relates more specifically to such detectors to be encapsulated under vacuum or controlled atmosphere. PRIOR STATE OF THE TECHNIQUE

In the traditional way, the electromagnetic radiation detectors are formed on a substrate of an appropriate nature, for example of the semiconductor type (silicon, etc.).

Some of these detectors require, for their operation, to work under vacuum or under a controlled atmosphere (reduced pressure of heavy gas for example, etc.) and generally, hermetically with respect to the ambient atmosphere. In fact, a housing or protective cover is added to them to define a cavity above said detector, enclosing the appropriate controlled atmosphere or vacuum. This housing includes a window transparent to the radiation to be detected by the detector, or even a radiation concentration lens at said detector.

The implementation of such an encapsulation must be compatible with a certain number of economic and technical characteristics, among which: the size: the encapsulation boxes must be the smallest possible, notably because of their integration into devices for which by nature or by destination, congestion is a permanent concern; - the cost, defined by the cumulative costs of the components and assembly techniques, as well as the efficiency of these technologies; - the service life due to the pressure evolution within the housing which must remain sufficiently low throughout the life of the product.

These various requirements are particularly proven in the field of optical imaging chips, and in general, infrared imaging retinas based on microbolometers. FIGS. 1 and 2 show a diagrammatic sectional view of such a principle of encapsulation of detectors.

Basically, the actual detector (3) is deposited on a substrate (2), itself deposited on the base (4) of a housing (1) forming a cavity (8). This cavity is hermetically sealed by a window (6) transparent to the wavelength radiation to be detected, said window being capable of being held in place by means of a French window (7) hermetically sealed to the housing.

It is desired to have, after sealing, a leakage rate of less than 10-12 millibars.litre / second to helium. The cavity is packaged under vacuum at a pressure of less than 10-2 millibars throughout the life of the product.

In order to guarantee a satisfactory lifetime of the detector thus encapsulated, and typically of the order of ten years at ambient temperature, the vacuum packaging of the detectors, especially infrared detectors, is carried out according to a degassing cycle under a high vacuum of the order of 10-6 millibars at a given temperature. This temperature is limited by the subassemblies constituting the detectors and by the assembly and sealing technologies used.

Typically, the temperature range used for the infrared detector degassing processes ranges from 70 ° C to 350 ° C. The higher the degassing temperature, the more the degassing cycle can be short, which directly impacts the production capacity, and consequently, improves the manufacturing costs of such a detector.

Moreover, the formats of such components being increasingly important in terms of number of pixels (format), it is necessary to assemble pieces of housing of increasingly important dimensions. In addition, the assembled materials sometimes have significant differences in the coefficient of thermal expansion. Thus, and in the case of infrared detectors in particular, it is commonly used, for the realization of the housing, an alloy of iron, nickel and cobalt with a low coefficient of thermal expansion and marketed for example under the trademark KOVAR, or alumina , and for the window transparent to infrared rays, germanium. Indeed, the latter has, in known manner, good optical characteristics in the wavelength range of the infrared, in addition to a coefficient of expansion adapted to the constituent materials of the housing.

However, the cost of germanium is high, so that it is increasingly replaced by silicon for economic reasons. However, silicon has a coefficient of thermal expansion less well suited to the materials usually used for the housing, typically of the order of 3 ppm per degree for silicon, against 6 ppm per degree for the housing materials and germanium. In doing so, the stresses and deformations generated by these silicon / KOVAR or silicon / alumina assemblies in the context of the production of such housings, reduce the reliability or even make hermetic assemblies impossible, especially since the assembly temperatures or degassing are high.

However, it is recalled that the assembly temperature, including degassing, must be the highest possible to ensure a satisfactory life of the detector.

The present invention aims to overcome these disadvantages and to enable the realization of such encapsulated detectors having high reliability, in addition to a longevity compatible with economic data in force.

SUMMARY OF THE INVENTION The invention therefore relates to a detector for electromagnetic and particularly infrared radiation with sealed or hermetic encapsulation, comprising a support intended to receive one or more detectors, said support comprising a base and side walls defining a cavity, said cavity being closed by a transparent window in the range of sensitivity wavelengths of the detector (s).

According to the invention, said window is attached to a window-door hermetically sealed to the upper edge of the side walls of the support, said window door not being inscribed in a single plane and being provided with a stress absorption zone , peripheral to the window, and extending in a direction other than those of the main plane in which the window is inscribed or a plane parallel thereto. This zone is described as corrugated, or as corrugation in the remainder of the document. In other words, the invention consists in giving the window-door a shape that is suitable for absorbing thermal expansion phenomena arising from associated degassing and sealing procedures used for encapsulation and resulting from the use of different thermal coefficient of expansion materials while maintaining the hermetic seal required for the intended application.

According to the invention, the corrugated zone of the French window preferably extends along a closed perimeter, that is to say extends continuously over the entire periphery of the window.

According to another variant of the invention, the French window extends in two parallel planes interconnected by a linear or nonlinear zone, said zone being intended to allow the absorption of thermal expansion phenomena.

According to another characteristic of the invention, the base and the side walls of the cavity are made of iron alloy, cobalt and nickel and in particular KOVAR or alumina, and the French window is made of metal, typically an alloy of the FeNi36 type. or FeNi42 according to the standard terminology of the trade. BRIEF DESCRIPTION OF THE FIGURES

The invention will be better understood on reading the description which will follow, given solely by way of example, and made with reference to the accompanying drawings, in which: FIGS. 1 and 2 are, as already stated, views diagrammatic section of an encapsulation box of a detector according to the prior art; Figure 3 is a schematic representation in section of an encapsulated detector according to the invention; Figure 4 is a schematic sectional representation of another embodiment of an encapsulated detector according to the invention.

EMBODIMENT OF THE INVENTION

FIGS. 3 and 4 show the encapsulated detector according to the invention. Basically, the actual detector (3) is attached or formed on a substrate (2) for example silicon, itself fixed by bonding or brazing on the base (4) of a housing (1). The base (4) is further surmounted by peripheral side walls (5), said walls (5) being made either by transfer of a metal frame or iron alloy, cobalt, KOVAR type nickel, or by construction of ceramic layers (alumina), or even a combination of both.

The housing thus defines a cavity (8) sealed hermetically by a transparent window (6) in the wavelength range of the detector (3), in this case infrared, and more particularly between 8 and 14 m, said window being itself reported on a window-door (7), metallic, hermetically sealed on the rim or upper edge of the side walls (5).

According to the invention, this French window (7) has a corrugation (9) as can be seen in FIG.

This corrugation, formed at the periphery of the window (6), extends in a direction other than those of the main plane in which the window (6) is inscribed or a plane parallel thereto. In this case, the corrugation (9) extends in a vertical plane and runs laterally (parallel to the main plane) preferentially over the entire periphery of the window.

The French window (7) provided with its corrugation (9) being made of a material, and in particular of metal capable of deforming in its elastic domain under the constraints generated by the differences of the coefficients of thermal expansion between the window (6) and the housing (1) thus makes it possible to absorb these phenomena of thermal expansion during the cooling phase after degassing and sealing, printed on the assembly and as mentioned above.

In doing so, it retains the tightness or hermeticity required for the realization of such detectors. In other words, the French window (7) can deform freely in its plane without reporting constraints generated by the housing to the window (6).

In doing so, it becomes possible to implement the most suitable materials for the window, typically silicon, whatever the material or materials constituting the support (4, 5) and without observing the mechanical problems of rupture, loosening, fatigue under repeated solicitation.

In fact, there is no longer any loss of physical integrity, or even leakage, even of the component thus produced.

In addition, it becomes possible to implement higher temperatures for the assembly of components, in addition to the degassing and sealing phase. Indeed, even when the assembly is carried out at high temperature on large parts, the high mechanical stresses occur during cooling as soon as all the fusible joints are solidified, in particular between the window (6) and the housing (4). , 5), stresses which are absorbed by the French window (7) which deforms at its corrugation (9).

In addition, the corrugation (9) of the French window may exceed the upper plane of said window. In doing so, it also ensures the protection of the window against external mechanical aggression arising from shocks and other friction during camera integration phase for example.

According to another embodiment of the invention shown in FIG. 4, the corrugation (9) is replaced by the following architecture: the French window (7) extends along two main planes (10) and (11) ) interconnected by an intermediate plane (12). More specifically, the window (6) is attached to the lower plane (11) of the French window (7), the lower plane (11) which, connected to the upper plane (10), allows its hermetic sealing on the upper edge of the walls (5) of the support. The deformation of the French window therefore intervenes at the level of the connection zone (12) which makes it possible to absorb the mechanical stresses arising from the thermal expansion.

Again, in this embodiment, the window (6) is mechanically protected from external mechanical attack due to its positioning according to a volume inscribed in the volume defined by the variation of the constituent plane of the French window (7).

It is therefore conceivable that, because of the particular structure of the window door of the invention, it becomes possible to overcome the problems arising from the thermal expansion phenomena, while using less expensive materials than those set in motion. work in the prior art to oppose such phenomena. This guarantees the required longevity of detectors operating under a controlled atmosphere or under vacuum, and in particular infrared detectors.

Claims (7)

A detector for electromagnetic and particularly infrared radiation with sealed or hermetic encapsulation comprising a support (4, 5) for receiving one or more detectors (2, 3), said support comprising a base (4) and side walls (5) defining a cavity (8), said cavity being closed by a window (6) transparent in the range of sensitivity wavelengths of the at least one detector, characterized in that said window (6) is attached to a French window ( 7) hermetically sealed on the upper edge of the side walls (5) of the support, said window door (7) not being inscribed in a single plane and being provided with a stress absorption zone (9, 12), peripheral to the window, and extending in a direction other than those of the main plane in which the window is inscribed or a plane parallel thereto. 2. An electromagnetic radiation detector according to claim 1, characterized in that the window door (7) is substantially flat and whose stress absorption zone consists of a corrugation (9) extending out of said plane. 3. An electromagnetic radiation detector according to claim 1, characterized in that the window door extends substantially in two planes (10, 11) parallel to each other, and in that the stress absorption zone consists of a zone (12) connecting the two planes (10, 11). 4. Electromagnetic radiation detector according to one of claims 1 to 3, characterized in that the stress absorption area (9, 12) of the window door (7) extends along a closed perimeter. 30 5. Electromagnetic radiation detector according to one of claims 1 to 4, characterized in that the base (4) and the side walls (5) defining the cavity (8) are made of iron alloy, cobalt and nickel and in particular KOVAR or alumina. 35 6. Electromagnetic radiation detector according to one of claims 1 to 5, characterized in that the French window (7) is made of iron-nickel alloy metal. 25 7. Electromagnetic radiation detector according to one of claims 1 to 6, characterized in that the window (6) is made of silicon.