FR3089022A1 - Installation of an interferometer and its production process - Google Patents

Abstract

Title: Interferometer installation and its production method Interferometer installation (1) comprising a substrate (2) having an optical zone (OB) for the passage of light rays, a first mirror installation (SP1), and a second mirror installation (SP2). The first and second mirror installations (SP1; SP2) are parallel and superimposed above the optical zone (OB), being at least zone movable with respect to each other. One of the two mirror installations (SP1; SP2) has a first mirror layer (SpSa) and parallel to it a second mirror layer (SpSb). The first or the second mirror layer (SpSa; SpSb) have, in an edge zone (RB) outside the optical zone (OB), an edge thickness (DR) greater than the thickness (D1) of the other mirror layer (SpSa). Figure 1

Description

Description

Title of the invention: Installation of an interferometer and its production process

Field of the Invention The present invention relates to an interferometer installation and to its production method.

State of the art Micromechanical Eabry-Pérot interferometers (PPE interferometers), usually use batteries of DBR mirrors (Bragg reflectors or Bragg gratings) comprising layers of materials with low refractive index and materials with high refractive index. This allows the thickness of the mirror layers to be chosen so that the optical thickness of the DBR layers corresponds respectively to a quarter of the target wavelength which must pass through the interferometer installation. Unlike the refractive index between the layers, the reflectance can increase and this also with the number of pairs of layers made of a material with a high refractive index and a low refractive index. This is why you can combine thin layers with a high refractive index and thick layers with a low refractive index. The high refractive index layers can thus provide better mechanical stability to the mirrors of the FPI interferometer. A high reflectance, in particular also a high spectral bandwidth, among semiconductor compatible materials, develops a system of silicon and air mirrors with a large difference in refractive index. For Si-air-DBR type mirrors of FPI interferometers, it is possible, depending on the range of target wavelengths, to have thicknesses of Si layers for the different mirror layers, less than 200 nm. Spectrometric applications advantageously require large apertures to achieve significant signal / noise ratios. It is desirable to find a compromise between mechanical robustness (diameter of the opening) and optical functionality (thickness of the layers) as well as the possibility of production (number of pairs of layers.

Document US 6,400,738 B1 describes a Fabry-Pérot interferometer device which includes mirrors with several layers having different refractive indices; to stabilize with respect to mechanical vibrations, the distance between the mirrors is modified.

SUMMARY OF THE INVENTION The present invention relates to an interferometer installation comprising a substrate having an optical zone for the passage of light rays, a first mirror installation and a second mirror installation, the first and the second. mirror installations being parallel and superimposed above the optical zone, being at least mobile by zone with respect to each other and at least one of the two mirror installations has at least a first layer of mirror and parallel thereto, a second mirror layer, the first or second mirror layer having in an edge region external to the optical zone, an edge thickness greater than the thickness of the other mirror layer.

The invention also relates to a method for producing an interferometer installation comprising the following steps consisting in: providing a substrate; applying a first sacrificial layer on the substrate, applying a first installation of mirrors on the first sacrificial layer, applying a second sacrificial layer applying a second installation of mirrors on the second sacrificial layer, the first and / or the second mirror installation being parallel and superimposed above the optical zone, being at least mobile by zone relative to one another and at least one of the two mirror installations has at least a first layer of mirror and parallel to it ci a second mirror layer, the first or second mirror layer in an edge area outside the optical area having an edge thickness greater than the thickness of the other mirror layer and removing at least the second sacrificial layer at least on the optical zone.

Advantages of the invention [0007] The basic concept of the invention consists in developing an interferometer installation which is distinguished by a more mechanically robust mirror system while retaining the advantageous optical properties. The mirror systems of such interferometer installations are distinguished by their robustness to external influences, such as the action of forces, while being able to be carried out in micromechanical construction and suitable for specific applications and products.

According to the invention, the interferometer installation comprises a substrate having an optical zone designed to let the light rays pass; a first mirror installation and a second mirror installation; the first and second mirror installations are parallel and superimposed on the optical zone; these mirror installations are movable with respect to each other, at least by zones, and at least one of the two mirror installations has a first mirror layer and parallel to this a second mirror layer . The first and second mirror layers have an edge thickness in the area of their edge outside the optical area which is greater than the thickness of the remainder of the mirror layer.

Thanks to their better mechanical stability, such mirror installations have the advantage of being insensitive to shocks and drops (drop test) of the interferometer installation. The installation of an interferometer can be done in micromechanical technique or as a microspectrometer. The mirror installations are practically parallel to each other or overlapping plane-parallels. The first and second mirror layers are made of the same or different materials.

The edge area is located outside the optical area.

According to a preferred development of the interferometer installation, the median zone above the optical zone of the first and / or the second mirror layer has a median optical thickness equal to an odd multiple of a quarter of wavelength of the transmission wavelength of the first and second mirror installations. Here and in the following description, the thickness is advantageously the optical thickness, that is to say the product of the refractive index by the actual thickness of the layer.

The transmission wavelength is the target wavelength which must pass through the interferometer installation.

According to a preferred development of the interferometer installation, the second mirror layer is opposite the optical zone (not turned towards the optical zone) and the first mirror layer is located in one of the mirror directions between the second mirror layer of this mirror installation and the substrate.

The second mirror layer may in particular be opposite the resonator interval between the mirror installations.

According to a preferred embodiment of the interferometer installation, the first and second mirror installations are embedded by their edge zone in an intermediate layer zone.

According to a preferred embodiment of the interferometer installation, the second mirror layer present in the optical zone, at least in part, an optical thickness greater than the thickness of the other mirror layer.

According to a preferred embodiment of the interferometer installation, the first and / or the second mirror layer have an extra thickness at least by zones.

According to a preferred embodiment of the interferometer installation, the optical thickness of the first mirror layer in its median zone and the median optical thickness of the second mirror layer correspond to an odd multiple of a quarter wavelength of the transmission wavelength of the first and second mirror installations.

According to a preferred embodiment of the interferometer installation, the sum of the optical thickness of the first mirror layer in the middle zone and the median optical thickness of the second mirror layer correspond to a odd multiple of a quarter wavelength of the transmitting wavelength of the first and second mirror installations.

According to the invention, the method for producing an interferometer installation consists in using a substrate and in applying thereon a first sacrificial layer, then in applying thereon a first mirror installation and then a second sacrificial layer on which a second mirror installation is placed. The first and / or the second mirror installation are mutually parallel and are superimposed on the optical zone; these mirror installations are at least per zone, movable with respect to one another. At least one of the two mirror installations comprises at least a first mirror layer and, in parallel to this, a second mirror layer. The first and second mirror layers have in their edge zone, beyond the optical zone, an edge thickness greater than the thickness of the other mirror layer; then at least the second sacrificial layer is eliminated at least above the optical zone.

The method has the advantages and characteristics already presented by the installation of interferometer and vice versa.

According to a preferred embodiment of the method, in the median zone above the optical zone, the first and / or the second mirror layer is produced with a median thickness which optically corresponds to an odd multiple of a quarter of wavelength of the transmission wavelength of the first and second mirror installations.

The layer of mirrors of increased thickness or which is thicker may be optically thicker than the other layer of mirror according to a multiple of the half-wavelength. The first layer of mirrors will, for example, have a thickness equal to λ / 4 and the mirror layer of increased thickness, will have a thickness equal to (2η + 1) * λ / 4, n being an integer.

According to a preferred embodiment of the method, the mirror layer is reinforced in thickness by an extra layer in the edge area.

The extra layer can be made with the same material or another material as the mirror layer which can be reinforced in thickness.

According to a preferred embodiment of the method, the thickness of the second mirror layer is reinforced in the median zone so that in this median zone, the optical thickness of the first mirror layer in the median zone and the median optical thickness of the second mirror layer corresponds respectively to an odd multiple of a quarter wavelength, of the transmission wave of the first and of the second mirror installation.

Presentation of the drawings The present invention will be described below, in more detail with the aid of an example of an interferometer installation and its re5 production method presented in the attached drawings in which:

[Fig. 1] is a schematic side view of an interferometer installation according to an exemplary embodiment of the present invention, [fig. 2] schematic side view of an interferometer installation according to another exemplary embodiment of the present invention, [fig.3a] [0030] [fig.3b] schematic side view from above of an interferometer installation according to another exemplary embodiment of the present invention, [0031] [fig. 4] curves showing the reflectance of the wavelengths of different configurations of mirrors of the interferometer installation, [0032] [fig.5] block diagram of a method for producing an interferometer installation according to a exemplary embodiment of the invention.

In the different figures we will use the same references for identical elements or the same function.

Description of the embodiment of the invention [0035] Figure 1 is a schematic side view of an interferometer installation according to an exemplary embodiment of the present invention.

The interferometer installation 1 comprises a substrate 2 with an optical zone OB designed for the passage of light rays, a first installation of mirror SP1 and a second installation of mirror SP2; the first and second mirror installations SP1; SP2 are parallel and superimposed above the optical zone OB; these installations are movable relative to each other, at least by zone, and at least one of the two mirror installations SP1, SP2 comprises at least one first layer of mirror SpSa and, parallel to this, a second layer of SpSb mirror. The first and second layers of SpSa mirror; SpSb have in the area of their edge RB outside the optical zone OB, an edge thickness DR greater than the thickness Dl of the other layer of mirror SpSa.

The increase in edge thickness DR ensures better mechanical robustness for mirror installations SP1, SP2. The layers with a high refractive index will advantageously be mechanically stronger and will have a higher mechanical modulus and, where appropriate, a higher critical stress than the layers with a lower refractive index; this is, for example, the case of silicon-air mirrors; silicon then constitutes the medium with a high refractive index. Thus, for the same increase in robustness, it is advantageous to have layers with a high refractive index which are made thicker in the mirror installation than the layers or the layer with low refractive index.

The mirror installations and / or the mirror layers of an interferometer installation can be coupled to each other from the fluidic point of view by a residual gas in the interval forming the resonator, c ' that is, the interval between the mirror layers. If in the two mirror installations SP1, SP2, one of the mirror layers SpSa, SpSb is thicker, the two mirror installations are thus better protected since the reinforcement of one of the mirror installations by the spring coupling pneumatic automatically affects the other mirror installation and mechanically reinforces it against external stresses; this can play a role especially vis-à-vis rapid stimuli, for example, in the event of falls. This may be due to the fact that the gas spring effect produced by a residual gas plays an important role on short time scales. Thus, a film of gas can be trapped between the two mirror installations and, thanks to the high aspect ratio, it has a very high fluid resistance.

If the thickness of a mirror layer is increased opposite the gap forming the resonator (optical gap) in a stack of mirrors (mirror installation) or if this is done directly (optically λ / 4) then the protective effect is particularly effective because for the robustness, the tensile stress which occurs, for example, the shear bending in the edge area RB (in the case of an embedding) is decisive or has a limiting effect. The thicker the layer of thickness at this point, the higher the tensile strength, for example, breaking by shear bending.

The middle zone MB advantageously extends throughout the zone between the edge zones RB. The mirror installations SP1, SP2 and in particular the mirror layers can also comprise a succession of several layers, for example, an alternation of layers with a high refractive index and of layers with low refractive index and instead of a layer with low refractive index, one can also have a gas or vacuum. Such mirror installations can be produced in the form of dielectric Bragg mirrors. The mirror installations SP1 and SP2 can be separated by a first distance dl2 which varies due to the mobility of at least one of the mirror installations. In addition, the mirror installations SP1 and SP2 can have one or both respectively, an optical zone OB released by an engraving process and which can be actuated. The edge area RB advantageously includes any thickness reinforcement, for example, with an overthick layer AS because it is beyond the optical area OB and does not have to intervene in the optical transmission properties. The reinforcement in thickness of the edge zone advantageously serves only to ensure the mechanical stability of the installation of mirror SP1 and / or SP2. Mechanical robustness increases with the thickness of the mirror layers, for example, to withstand cases of over stress. Increasing the thickness of the layer reduces the internal mechanical stresses of the mirror installation.

Such an interferometer installation 1 constitutes a Fabry-Pérot filter and can be carried out in micromechanical technique.

The increase in the thickness of the edge is related to the remainder of the thickness of the mirror of the same mirror installation. The choice of the mirror layer SpSa, SpSb which must be reinforced in thickness is arbitrary. In a particularly advantageous manner, however, the thickness of the mirror layer which is opposite the gap forming the resonator between the two mirror installations is reinforced. In FIG. 1, for this, the first mirror installation SP1 has its lower mirror layer (first layer) SpSa with an extra thickness DR in the edge area RB; in the second mirror installation SP2 which is above the first mirror installation SP1, the upper mirror layer (second layer) SpSb has a thickness allowance DR in the edge area RB. These zones can be equal or different depending on the mechanical stability required locally.

The intermediate zone ZB advantageously remains as an edge zone RB after removal of the first and second sacrificial layer 01 and 02 in the middle zone MB and allow the installation of the mirror installations. In the embodiment of FIG. 1, the middle zones MB of the mirror installations SP1 and SP2 as well as their mirror layer SpSa, SpSb (here these are the two mirror installations of the same shape, but which can also be different) have mirror layers reinforced in thickness such as a median thickness DM which corresponds, for example, from the optical point of view, to a quarter wavelength to be transmitted. The mirror installations can be electrically connected to be actuated by external contacts K. In one or both mirror installations, AN spacers can be provided which keep the first and second mirror layers apart.

The optical zone OB can be a recess made in the substrate 2. If the substrate itself is transparent, it is possible to remove the recess from the substrate.

The mirror installations can be fixed and advantageously embedded in the edge area RB of the intermediate layer ZB.

Figure 2 is a schematic side view of an interferometer installation according to another embodiment of the present invention.

The embodiment of the interferometer installation 1 of FIG. 2 differs only from that of FIG. 1 in that the second mirror installation (upper installation) SP2 also has an overthickness AS or an increased thickness in the middle zone MB. Advantageously, the median thickness DM is chosen so that the optical transmission properties of the two mirror installations are modified or are not modified in their cooperation. Such a determined choice of the median thickness DM is, for example, a thickness equal to an odd multiple of a quarter of the transmission wavelength of the first and / or the second mirror installation; the transmission properties of one or both mirror installations can be taken into account for several wavelengths. In a mirror installation such as installation SP2, the first layer of mirror SpSa is turned towards the resonator interval and optically the median thickness DM corresponds to a quarter of the transmission wavelength. The second SpSb mirror layer does not face the resonator interval and its median optical thickness DM corresponds, for example to three quarters or five quarters of the transmission wavelength. The resonator interval between the two mirror installations SP1, SP2 corresponds to a first distance dl2, for example equal to (2η + 1) * λ / 2, relation in which n is an integer and λ is the length of transmission wave.

The mirror layers can have a high refractive index; they advantageously have a refractive index higher than that of air, vacuum or an intermediate material which is in the interval between the two mirror layers of a mirror installation. The two mirror layers and the gap can have an optical thickness or height equal to λ / 4, 3λ / 4, 5λ / 4 or greater multiples.

By taking into account the optical properties of the mirror installations for thicker embodiments of one or more mirror layers, it is possible to limit the useful optical range or the optical characteristics.

We will have the greatest robustness (mechanical stress) with thicker mirror layers over their entire surface; but it is also possible to strengthen the thickness only of the edge zone RB which already offers greater robustness because at this point, in the transient zone, between the recessed zone and the free zone, the strongest mechanical stresses are encountered such as, for example, breaking by bending.

If the middle zone MB is mechanically more rigid (reinforced in thickness) than the edge zone RB and / or the embedding zone, there will be better plane parallelism which results in a higher resolution of the installation of interferometer.

Advantageously, the mirror layers opposite the resonator gap (layer not turned towards this gap) of the two mirror installations can have a greater thickness, that is to say have a reinforcement in thickness which makes it possible to have a structure having an optical symmetry.

We can reduce the bandwidth (depending on the wavelength, as shown in Figure 4) of the reflectance of a mirror installation, for example a DBR mirror by so-called mirror layers “Λ / 4 layer”, by adding 3λ / 4 layers or with multiples greater than a quarter of the transmission wavelength, according to FIG. 4, the high and low wavelengths give a lesser reflectance for thicker mirror layers than thinner layers.

This is why it may be necessary to find a compromise between the robustness provided by large thicknesses of the layers and the optical properties. Reflectance is the ratio between the amount of light reflected by an illuminated body and the intensity of the light source or that of the incident radiation.

Figure 3a, b is a schematic top view of an interferometer installation corresponding to an exemplary embodiment of the present invention.

Figure 3a is a top view of a second SP2 mirror installation. The second mirror layer SpSb of the second mirror installation SP2, not turned towards the resonator interval, can be located above the first mirror layer according to FIG. 2. The arrangement of FIG. 3a thus differs from the FIG. 2 in that the second mirror layer SpSb, in the middle zone MB, is only partially reinforced with respect to the thickness λ / 4. The partial thickness reinforcement can be done according to the pattern presented in Figure 3a; this pattern consists of radial arms and a central circle above the middle zone MB and the optical zone OB. Such a shape with local reinforcement in thickness of the second layer of mirror SpSb ensures greater mechanical stability while taking into account the properties of optical transmissions. The radial structure can be combined with the edge zone RB which also has an increased thickness and is anchored in the intermediate zone ZB. Below the dashed line, between the edge zone and the intermediate zone ZB, the first and second SpSa mirror layers, SpSb, are radially inward.

Figure 3b is a top view similar to that of Figure 3a. However, the second layer of SpSb mirror has another form of thickness reinforcement, in particular a form of closed surface. The second mirror layer has a median optical thickness which is, for example, equal to three quarters of the transmission wavelength completely covering the optical zone OB. The edge area RB has a thickness similar to or equal to that of the second mirror layer in the middle area; in the meantime, the applied thickness may be less, for example, equal to λ / 4. It is also possible that in the case of FIG. 3a, the second mirror layer SpSb has radially inside the edge region RB, an optical thickness equal to λ / 4 and an optical thickness of 3λ / 4 or more . The edge area can also be different from a round shape.

FIG. 4 shows the reflectance curves as a function of the wavelength for different mirror configurations of the interferometer installation.

We have, for example, the relationship between the reflectance of a mirror installation with two mirror layers with a high refractive index such as silicon layers and an air gap or vacuum between the layers; in the first case, the two mirror layers have an optical thickness equal to λ / 4; in the second case, the mirror layer turned towards the incident side has an optical thickness equal to 3λ / 4 and the other, an optical thickness equal to λ / 4; in the third case, the mirror layer not turned towards the incident side has an optical thickness equal to 3λ4. In the 4th case, the two mirror layers have an optical thickness equal to 3λ / 4; in the 5th case, the two mirror layers and the gap (air / vacuum) have an optical thickness equal to 3λ / 4. It appears that the bandwidth (as a function of wavelengths) of the reflectance of the single mirror decreases with each layer increasing the layer by 3λ / 4 at the edges of the reflectance range; the choice of coverage over the entire surface of a mirror layer requires a compromise between mechanical robustness and optical bandwidth.

For this compromise, it must be examined whether optical functionality and / or mechanical robustness are the main objective, each of these functionalities being important. This is why the increase in thickness is limited to the edge area which is not affected by the optical transmission and / or, in addition, the optical limit condition is taken into account and by choosing the reinforcement of the thickness. , the half wavelength of the transmission wave (central wavelength of the mirror installation or both (for example, in the case of more than two mirror installations 3λ / 4, 5λ / 4 or more ).

FIG. 5 is a block diagram of a method for producing an interferometer installation according to an exemplary embodiment of the invention.

The method for producing an interferometer installation consists in providing (SI) a substrate, in installing (S2) a first sacrificial layer on the substrate, in applying (S3) a first mirror installation on the first layer sacrificial; then apply (S4) a second sacrificial layer and on it, apply (S5) a second mirror installation; the first and second mirror installations are parallel and are superimposed on the optical zone; these two mirror installations are movable with respect to each other at least in zones and at least one of the two mirror installations comprises at least a first layer of mirror and, parallel to this, a second layer of mirror; the first or second mirror layer has at least in the edge zone, beyond the optical zone, an edge thickness greater than the thickness of the other mirror layer; finally, removing (S6) at least the second sacrificial layer at least above the optical zone.

NOMENCLATURE OF THE MAIN ELEMENTS [0063] 1 Installation of interferometer [0064] 2 Substrate [0065] AS Extra layer [0066] DM Median optical thickness [0067] DR Edge thickness [0068] Dl Thickness of the other mirror layer [0069] MB Middle zone [0070] OB Optical zone [0071] 01 First sacrificial layer [0072] 02 Second sacrificial layer [0073] RB Edge zone [0074] SP1 Mirror installation [0075] SP2 Mirror installation SpSa First mirror layer [0077] SpSb Second mirror layer [0078] S1-S6 Stages of the process for producing an interferometer installation [Z79] Intermediate layer zone

Claims (1)

Claims [Claim 1] Interferometer installation (1) comprising a substrate (2) having an optical zone (OB) for the passage of light rays, a first mirror installation (SP1), and a second mirror installation (SP2), the first and the second mirror installations (SP1; SP2) being parallel and superimposed above the optical zone (OB), being at least mobile by zones with respect to one another and at least one of the two installations of mirror (SP1; SP2) has at least a first mirror layer (SpSa) and parallel to it, a second mirror layer (SpSb), the first or second mirror layer (SpSa; SpSb) having in an area edge (RB) outside the optical zone (OB), an edge thickness (DR) greater than the thickness (Dl) of the other mirror layer (SpSa). [Claim 2] Interferometer installation (1) according to claim 1, according to which in a central region (MB) above the optical region (OB), the first and / or the second mirror layer (SpSa; SpSb) has a thickness median optics (DM) equal to an odd multiple of quarter wavelength of a transmission wavelength of the first and second mirror installations (SP1; SP2). [Claim 3] Interferometer installation (1) according to claim 1 or 2, according to which the second layer of mirrors (SpSb) is opposite the optical zone (OB) and the first layer of mirrors (SpSa) is in one directions of the mirror between the second layer of mirrors (SpSb) of the same mirror installation (SP1; SP2) and the substrate (2). [Claim 4] Interferometer installation (1) according to one of claims 1 to 3, according to which the first and second mirror installations (SP1; SP2) are embedded by their edge region (RB) in an intermediate layer region (ZB ). [Claim 5] Interferometer installation (1) according to one of claims 1 to 4, according to which the second mirror layer (SpSb) has in the optical zone (OB), at least in part, an optical thickness greater than the thickness ( Dl) of the other mirror layer (SpSa). [Claim 6] Installation of interferometer (1) according to one of claims 1 to 5, according to which the first and / or the second mirror layer (SpSa; SpSb) comprises at least per zone a layer in excess thickness (AS). [Claim 7] Interferometer installation (1) according to one of claims 1 to 6, according to which the optical thickness of the first mirror layer (SpSa) in the middle region (MB) and the median optical thickness (DM) of the second mirror layer (SpSb) correspond to an odd multiple of a quarter wavelength of a wavelength of transmission of the first and second mirror installations (SP1; SP2). [Claim 8] Method for making an interferometer installation (1) comprising the following steps: provide (SI) a substrate (2), apply (S2) a first sacrificial layer (01) on the substrate (2), apply (S3) a first mirror installation (SP1) on the first sacrificial layer (01), applying (S4) a second sacrificial layer (02), applying (S5) a second mirror installation (SP2) on the second sacrificial layer (02), the first and / or the second mirror installation (SP1; SP2) being parallel and superimposed above the optical zone (OB), being at least by zone, movable with respect to each other and at least one of the two mirror installations (SP1; SP2) has at least a first mirror layer (SpSa) and parallel to it, a second mirror layer (SpSb), the first or second mirror layer (SpSa ; SpSb) in an edge zone (RB) outside the optical zone (OB) having an edge thickness (DR) greater than the thickness (Dl) of the other mirror layer (SpSa; SpSb) , and removing (S6) at least the second sacrificial layer (02) at least on the optical zone (OB). [Claim 9] Method according to Claim 8, in which in the median zone (MB) above the optical zone (OB), the first and / or the second mirror layer (SpSa; SpSb) is formed with a median thickness (DM) which optically corresponds to an odd multiple of a quarter wavelength of a transmission wavelength of the first and second mirror installations (SP1; SP2). [Claim 10] Method according to Claim 8 or 9, in which the second mirror layer (SpSb) is thickened in the edge region (RB) by an extra thickness layer (AS). [Claim 11] Method according to one of claims 8 to 10, in which the second mirror layer (SpSb) is thickened in the middle region (MB) so that in this middle region, the optical thickness of the first mirror layer (SpSa ) in the middle zone (MB) and the thickness optical median (DM) of the second mirror layer (SpSb) correspond respectively to an odd multiple of a quarter wavelength of a transmission wavelength of the first and second mirror installations (SP1 ; SP2).