For infrared imaging or thermography (pyrometry), it is known to use a detector operating at ambient temperature and comprising an array of elementary sensitive structures which use the variation according to temperature of a physical quantity of a material or assembly of appropriate materials. In the specific case of bolometric detectors, which are the most currently used, the physical quantity is the electric resistivity.
Such a bolometric detector usually comprises:                a substrate, typically made of silicon;        an array assembly of elementary sensitive structures, each comprising a so-called “bolometric” membrane (or “elementary bolometer”) suspended above the substrate by means of suspension arms having a strong thermal resistance. Such an assembly is commonly called “sensitive retina”;        electronic means for addressing in an array the elementary bolometers and for forming an electric signal from each elementary bolometer. These means are integrated to the substrate, and the substrate together with its electronic means is commonly called “read-out integrated circuit” (or ROIC);        a sealed package maintained under vacuum, or more exactly under a very low inner pressure (for example, a pressure lower than 10−3 mbar), having the sensitive retina housed therein. The package comprises a window transparent to the radiation of interest arranged opposite the sensitive retina. The package also comprises electric interconnects between the read-out integrated circuit and the outside of the package. The “vacuum” present in the package aims at making heat exchanges between the sensitive membranes and their environment via the ambient gas negligible as compared with the thermal conduction of the suspension arms.        
The detector thus formed is intended to be integrated in an imaging system (for example, a camera) provided with the electronics for controlling and processing the signals supplied by the detector read-out integrated circuit, and with optics adapted to the focusing on the sensitive retina of the thermal scene to be observed. Such a system thus generates a video signal representative of the temperature reached by each of said elementary detectors, that is, a “thermal image” of the observed scene.
The manufacturing techniques, essentially of microelectronic type, implemented to manufacture the retina (chemical and physical vapor deposition, photolithography, dry or wet etching, etc.) enable to obtain manufacturing costs compatible with a massive diffusion, but the cost of the hermetic package and of the vacuum assembly operations, in other words, of the “packaging”, remains significant and clearly dominates the general manufacturing cost of the detector.
With the development of room-temperature bolometric detectors, little after their commercial outbreak at the end of the 1990s, special attention has been paid to the package manufacturing in order to decrease the cost thereof. Various techniques have then been developed to limit the package cost, the first ones being so-called “Wafer Level Packaging” (WLP) techniques, where the assembly in a single operation of two substrates (a ROIC substrate comprising a plurality of sensitive retinas on one side, and a substrate comprising a plurality of windows on the other side) simultaneously forms a plurality of hermetic packages. Although such a collective manufacturing of detectors by substrate transfer enables to lower the manufacturing cost of each detector, the economical validity remains dependent on the cumulated efficiencies of the various operations, which nevertheless remain very complex to control and which require a series of high-cost equipment.
A way to partly overcome such limitations is described in document FR 2 822 541. This document of 2001, that is, a few years only after the outbreak of bolometric retinas, describes a technique of collective manufacturing of an object comprising microcavities or microcapsules formed above each detection site, or a plurality of sites simultaneously until the functional creation of vacuum by means of collective microelectronics techniques. According to this technology, called “monolithic”, the tight confinement is manufactured similarly to the manufacturing of the suspended membranes of the sensitive retina by using sacrificial layers having the cap-window of the “package” formed thereon (or more precisely: microcavity, or also capsule). Unlike “WLP” techniques, there is no further need for a second window substrate having a technology which is itself complex, nor for specific techniques and materials of hermetic assembly of two substrates, usually by fluxless soldering. The specific difficulties of “WLP” techniques and the use of many techniques unusual in microelectronics are thus suppressed, with a substantially decreased total number of operations. Further, the vulnerability of sensitive structures before the integration under vacuum and the particulate contamination during these operations, that is, the associated efficiency loss or the cost of the precautions necessary to limit the effect thereof can be considered as negligible. This results in a substantial gain in terms of manufacturing cost of the “packaging” over multiple-substrate techniques.
Such a manufacturing technique having a large number of advantages, the solution disclosed in document FR 2 822 541 has been and still is the object of continuous and intensive research (see, for example, French patent application FR 2 936 868 published in 2010 or also in document “Latest improvements in microbolometer thin film packaging: paving the way for low-cost consumer applications» de J. J. Yon et al., Proc. SPIE 9070, Infrared Technology and Applications XL, June 2014, Chinese patent application CN 102935994 published in 2013, or also International patent application WO 2014/100648 published in 2014.
However, the monolithic confinement technique, in its original version of document FR 2 822 541, as well as in its improved variations, such as for example that described in document FR 2 936 868, has defects. Indeed, such a technique induces an imperfect optical transmission of the “window” portion of the capsules. The general principles of this technique and its specific limitations are now described in relation with the simplified cross-section views of FIGS. 1 and 2. In these drawings, the forming of a hermetic individual capsule is illustrated for each suspended membrane of the sensitive retina. This characteristic is however not essential herein, the capsule being for example capable of housing a plurality of membranes or of being a single structure which entirely houses the retina, the positioning, the distribution, the dimensions, the structure of the lateral walls being further likely to be very varied. Similarly, FIGS. 1 and 2 illustrate a type of mechanical support of the sensitive membranes by means of pillars. The membrane support is not an essential characteristic either, other types of mechanical support being possible, for example by means of arms embedded in the lateral walls of the hermetic packages or of reinforcing structures thereof, such as for example described in document FR 2 930 639.
FIG. 1 illustrates the final detector manufactured according to the monolithic technique, comprising substrate/read-out integrated circuit 10, the bolometric membranes 12 suspended above substrate 10 by pillars 14 also ensuring the electric continuity between the membranes and the substrate, and a microcavity 16, for example, hermetic, formed of substrate 10, of lateral walls attached to the substrate and of cap/window 20 attached to lateral walls 18.
According to the monolithic technique, all the elements just described are manufactured by successive depositions of layers of various materials, without ever transferring one element onto another, in other words, without assembling two elements previously manufactured independently from each other, and then attached to each other for example by means of a welding, a soldering, an adhesive, a molecular bonding, etc. The monolithic technique is conversely based on temporary layers of material (more commonly called “sacrificial layer”). Particularly, the monolithic manufacturing of the packages advantageously continues the monolithic manufacturing of the suspended bolometric membranes of the sensitive retina illustrated in FIG. 2A. As known, and for example as described in document FR 2 752 299, a sacrificial layer 22 is deposited on substrate 10 and the various layers forming membranes 12 are formed thereon, pillars 14 being further manufactured after the etching of openings (or “vias”) in sacrificial layer 22.
For the monolithic manufacturing of package 16, a second sacrificial layer 24 is deposited on first sacrificial layer 22 and membranes 12 (FIG. 2B). A first layer 26 of a material transparent to the radiations of interest is then deposited on second sacrificial layer 24 (FIG. 2C).
According to the state of the art, the first layer of material 26 is considered of major importance in monolithic manufacturing, and should implement at least six functions:    1. it plays the role of a “hard mask” aiming at protecting the surface of the second sacrificial layer 24 during the subsequent (anisotropic) directional etching of the two sacrificial layers 22 and 24 to form lateral walls 18, the surface thereof being by definition exposed to the etching purposefully selected to etch the material of layers 22, 24;    2. it is inert to the process of removing sacrificial layers 22, 24. These layers being intended to be totally removed and to leave no trace are generally of organic nature (for example, made of polyimide) and are in this case removed by means of an oxygen-based plasma, which is particularly aggressive. As a variation, SiO-type mineral sacrificial materials may be preferred, in which case vapor-phase hydrofluoric acid (HFv), which is very aggressive for many usual materials of microbolometric assemblies, is used for the removal thereof.    3. it is optically appropriate and more specifically:            transparent to let through the radiation to be detected; and        naturally with no local defect (continuity, gaps, or perforations called “pinholes”) or topological defect (excess thicknesses, inclusions or adhering or included particles disturbing the optical transmission, which would compromise the optical quality of window 20);            4. it is hermetic, which means that it does not have the structural defects listed in the above point, which also compromise the final tightness of the package;    5. its adherence to organic or mineral upper sacrificial layer 24 is of very good quality;    6. it enables to form openings by etching of the vents, such as described hereafter, of sub-micron dimensions to maximize the useful surface area of collection of the incident radiation energy on the area occupied by each membrane of the sensitive retina.
The manufacturing carries on (FIG. 2D) with the anisotropic etching of layer 26 and of sacrificial layers 22, 24 all the way to substrate 10, to form the separations or vias 28 where lateral walls 18 are formed, for example, a trench formed all around each membrane 12. A second layer 30 of material transparent to radiations of interest is then deposited to at least cover, or even totally fill, vias 28, thus forming lateral walls 18 delimiting the elements or groups of sensitive elements, and/or pillars for attaching cap/window 20 to substrate 10 (FIG. 2E). Second layer 30 is also considered important in the state of the art since it should fulfill or at least complete functions 2 to 6 described hereabove.
At least one opening (or “vent”) 32 per package (per capsule) of small dimensions, typically having a width or diameter smaller than 0.5 μm, is then formed by directional etching through layers 26 and 30 (FIG. 2F). The inside of each package is then emptied of any sacrificial material through vent 32 by means of a complete isotropic etching of sacrificial layers 22 and 24 (FIG. 2G). Sensitive membranes 12 remain held by means of pillars 14 attached to substrate 10. Membranes 12 are then “free” but are protected from mechanical aggressions and from ambient particulate contamination since they are housed in a cavity 34 delimited by substrate 10, lateral walls 18, and upper portion 26, 30 forming the base of the future window 16.
The process carries on with the creation of vacuum in cavities 34 by placing the obtained assembly in a pumped-out enclosure, and then by depositing without breaking vacuum a third layer 36 transparent to radiations of interest, to hermetically close vents 32 (FIG. 2H). Thermal evaporation is however preferred for such a critical operation, due to the very low operating pressures both characteristic of such processes and typically eventually required inside of the capsules (lower than 10−3 mbar). Other closing architectures are for example provided in documents describing the reference technique, but the mode retained herein, described by WO 2013/079855 A1, is preferred for its simplicity and a negligible optical disturbance rate (occultation, diffusion, deflection) of its window in its portion essentially parallel to the substrate. The inner cavity 34 of each package is thus definitively sealed, and thus hermetic, at the pressure desired for the detector operation.
Optionally, to improve the optical quality of the stack of the first and second layers 26, 30 belonging to the window vertically in line with membranes 12, a fourth layer 38, also transparent to the radiation of interest, is deposited on second layer 36, for example, by thermal evaporation.
The detectors thus advantageously collectively, and thus economically, integrated under vacuum on a substrate of standard shape can then be “individualized” (separated by conventional cutting) and then integrated in their final packaging (package, support, PCB, etc.).
In practice, first layer 26, but also second layer 30 and lateral walls 18, are made of amorphous silicon (“a-Si”) obtained by CVD (for “Chemical Vapor Deposition”). Indeed, the amorphous silicon thus obtained is particularly appropriate for the forming of layers 26 and 30, due to its very acceptable transparency in infrared, its simplicity of implementation, the absence of local and topological defect. Further, the adherence of amorphous silicon appears to be very satisfactory at the surface of usual sacrificial materials such as a polyimide or silicon oxide SiO, or between successive layers of same nature or related. Amorphous silicon is further characterized by an excellent selectivity over usual methods of oxygen plasma etching (ashing) of the sacrificial organic materials or under HFv for SiO-type mineral materials, and its perfect adequation to a definition by lithography and dry etching (RIE) of vias 28 and vents 32. Indeed, the latter should typically be of very small size (micron, or sub-micron for vents 32), to maximize the useful surface area of collection of the incident radiation energy on the area occupied by each elementary point of the retina. Amorphous silicon is also inert to vapor-phase hydrofluoric acid, in the case of the use of a SiO-type material as a variation of organic sacrificial layers.
Finally, layer 26 should be mechanically resistant, non-porous, and perfectly adherent to the materials and structure preformed at the surface of substrate 10. The CVD amorphous silicon adds to these qualities a specific ability to cover depressions of high form factor (depth-to-width) of patterns 28, “conformally”, that is, with no continuity flaw and with a practically constant thickness. Actually, this material is naturally capable of fulfilling at a limited cost all the previously-specified functions and constraints, and no material better suited to the forming at least of the base of window 16 in the present context is known.
Third layer 36 is usually obtained by evaporation of germanium (Ge). The advantage of evaporated germanium, well known in the forming of conventional multiple-layers intended for the antireflection and/or bandpass treatments of thick windows usually implemented in the field of all detectors operating in infrared, is its high optical index (4-4.2) and its very good transparency. Further, and decisively, the hermetic closing of vents 32 by evaporation of germanium is recognized as satisfactory.
Fourth layer 38 is made of zinc sulfide (ZnS). Zinc sulfide is also commonly used in the field of infrared optical multilayers for its relatively low refraction index (2.2-2.3) despite its limitations in terms of mechanical constants (hardness, abrasion resistance) and of thermal stability, which is a problem beyond 300° C.
The material optical structure of windows of the state of the art is thus formed of an assembly of three basic materials (a-Si, Ge, ZnS), the selection of which is essentially dictated by a complex tradeoff induced by the monolithic manufacturing technique.
However, the thickness of the various layers present also defines the transmittance of the window and particularly the quality of this transmittance. More particularly, a transmittance between 8 and 14 μm is desired for thermal infrared (LWIR), which:                has a high average value in the waveband of interest; and        has minimum variations in said band, or in other words the “flattest” possible transmittance in said band.        
In most cases, a criterion of wavelength filtering outside of the band of interest adds to these two criteria, which impact the sensitivity and the linearity of the detector response. For example, in the case of a LWIR bolometric detection, a transmittance which clearly and deeply decreases at least on the side of short wavelengths (<8 μm), the window thus playing the role of a low-pass filter greatly improving the quality of the detection.
It is possible to simultaneously fulfill these various spectral features when the designer admits the use of a “thick” optical stack formed of many stacked layers having a total optical thickness way beyond the central wavelength λ10 of the band of interest. Indeed, the highly complex multiple interferences of such stacks provide many degrees of liberty for the adjustment of the transmittance spectrum, so that there are no predefined criteria, principles, or laws (except for a negligible optical absorption, in the band of interest of all the layers used for their respective thicknesses) to obtain the best tradeoff. Such a search for a specific transmittance spectrum actually comes under “random” digital simulation and the designer's know-how.
In the case of simple stacks limited to a few layers only, which is typical of and specific to the present context, the designer should favor the first criterion of high transmittance in the band of interest, by means of simplified design rules usual in the field of multilayer filters. In particular, he/she is led, for each of the layers made of a material of optical (or refraction) index ni, to preferring a thickness close to pi·λ10/4ni, where pi is an integer and λ10 is a wavelength of the band of interest having the multilayer tuned thereto (usually the central wavelength of said band). In the state of the art, such preferred criteria lead to retaining thicknesses respectively in the range from 0.6 to 1 μm for amorphous silicon (sum of layers 26 and 30), from 1 to 2 μm for germanium (layer 36), of approximately 1.2 μM for zinc sulfide (layer 38), to form structures adapted to a detection between 8 μm and 14 μm.
An example of a transmittance spectrum (ratio I/IO, IO being the intensity of a radiation of normal incidence on layer 38 and I being the intensity of the transmitted radiation coming out beyond layer 26) of the window alone is illustrated in FIG. 3A with thicknesses selected according to rule pi·λ10/4ni, resulting in a stack of 0.7 μm of a-Si (n=3.65), of 0.6 μm of Ge (n=4.2), and of 1.2 μm of ZnS (n=2.2), that is, three quarter-wave layers (pi=1) tuned to 10 μm. It can be observed that the average transmittance over the [8; 14 μm] band of interest is close to 83%. All the other thickness combinations, with a quarter-wave (or multi-quarter wave, that is, with pi>1) tuning or non-tuned, result in non-competitive spectrums, that is, spectrums having a greater intra-band amplitude dispersion or/and having a smaller average value.
To make the issue even more complex, the above thicknesses, considered as the best regarding the “pi·λ10/4ni” rule, are not adapted to monolithic manufacturing. In particular, the 0.7-μm thickness of a-Si (layer 26+30) may be insufficient to form lateral walls 18 having a satisfactory mechanical resistance. Similarly, a 0.6-μm thickness of Ge of third layer 36 is not adequate to safely close vents 32, so that a much larger thickness beyond 1 μm is preferable for this layer. The transmittance should thus be “degraded” regarding the “pi·λ10/4ni” rule to fully satisfy the mechanical and tightness constraints. FIG. 3B thus illustrates an example of transmittance of a window satisfying said constraints, formed of 0.8 μm of a-Si, of 1.6 μm of Ge, and of 1.2 μm of ZnS. A transmittance spectrum well positioned between 8 and 14 μm, but which has significantly stronger oscillations and a substantially less favorable average (˜73%) than the “ideal spectrum” of FIG. 3A, is thus obtained.
Further, since in such a simplified optical context, the low-pass filtering cannot be optimized together with the transmittance in the band of interest, there remains a risk of too high a sensitivity to short wavelengths (for example, below 8 μm for the LWIR band), due to possible “unfortunate” matches of the respective maximum values of the spectral transmittance of the window and of the absorption spectrum of the sensitive membrane. Indeed, incidental strong radiations at wavelengths smaller than 8 μm are for example likely to induce a problematical electric drift of the sensitive retina. In particular, the spectrum of FIG. 3A for example exhibits a plurality of narrow peaks of very high transmittance, which are thus particularly harmful, in the band under 7 μm where a very low transmittance is preferred.
All these issues impose taking specific measures, for example, in terms of transmittance of the window and/or of mechanical resistance.
As a summary, the adjustment of the transmittance in a band of a package manufactured according to the monolithic technique is very complex since there exists a large number of constraints of different nature to be taken into account. Of course, although the monolithic manufacturing of a hermetic package has been illustrated hereabove in relation with a bolometric infrared detector, this issue also arises for any type of detector having its package manufactured according to the monolithic technique.
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