Fabry-perot interferometer having a wide tuning range

Abstract

A method for producing a Fabry-Perot interferometer (FPI1) comprises: - forming a first module (MOD1), which comprises a first actuating electrode (E1), - forming a second module (MOD2), which comprises a movable mirror (M2) and a second actuating electrode (E2), and - bonding the first module (MOD1) to the second module (MOD2), wherein the first actuating electrode (E1) and the second actuating electrode (E2) are arranged to change the distance (dM) between the mirrors (M1, M2) of the interferometer (FPI1),10 wherein dimensions of the modules (MOD1, MOD2) are selected such that a distance (dE) between the first electrode (E1) and the second electrode (E2) is greater than or equal to 1.5 times a distance (dM) between the mirrors (M1, M2).

Description

[0001] FABRY-PEROT INTERFEROMETER HAVING A WIDE TUNING RANGE

[0002] FIELD

[0003] The present invention relates to a Fabry-Perot Interferometer, and to a method for producing the Fabry-Perot interferometer.

[0004] BACKGROUND

[0005] A known Fabry-Perot interferometer comprises actuating electrodes, which are implemented on the mirrors of the Fabry-Perot interferometer, so as to change the distance between the mirrors. The electrodes generate an electrostatic force in a situation where a non-zero voltage difference is applied between the electrodes. The electrostatic force pulls a movable mirror away from the normal position. The tuning range of the known Fabry-Perot interferometer may be limited by the pull-in effect. The pull-in effect refers to the situation where the electrostatic force generated by the electrodes rapidly exceeds the mechanical restoring force of the mirror, causing an accidental contact between mechanical parts. In particular, the pull-in effect may cause accidental contact between the mirrors, which typically causes permanent damage to the mirrors. The pull-in effect may limit the maximum tuning range. The maximum tuning range of the known Fabry-Perot interferometer may be e.g. less than 30% of the normal distance between the mirrors, due to the risk of the pull-in effect.

[0006] SUMMARY

[0007] An object is to provide a method for producing a Fabry-Perot interferometer. An object is to provide a Fabry-Perot interferometer.

[0008] According to an aspect, there is provided a method for producing a Fabry-Perot interferometer (FPI1 ), wherein the method comprises:

[0009] - forming a first module (MODI ), which comprises a first actuating electrode (E1 ), - forming a second module (MOD2), which comprises a movable mirror (M2) and a second actuating electrode (E2), and

[0010] - bonding the first module (MODI ) to the second module (MOD2), wherein the first actuating electrode (E1 ) and the second actuating electrode (E2) are arranged to change the distance (dM) between the mirrors (M1 , M2) of the interferometer (FPI1 ), wherein the dimensions of the modules (MODI , MOD2) are selected such that the distance (d ) between the first electrode (E1 ) and the second electrode (E2) is greater than or equal to 1.5 times a distance (dM) between the mirrors (M1 , M2).

[0011] According to an aspect, there is provided a method of claim 1 .

[0012] Further embodiments are defined in the other claims.

[0013] The scope of protection sought for various embodiments of the invention is set out by the independent claims. The embodiments, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

[0014] The Fabry-Perot Interferometer may be a tunable Micro-Electromechanical Fabry-Perot Interferometer (MFPI).

[0015] The Fabry-Perot Interferometer comprises a first optically semi-transparent mirror and a second optically semi-transparent mirror. The distance between the mirrors is adjustable. The mirrors may define together an optical interference cavity. At least one of the semi-transparent mirrors of the tunable Fabry-Perot Interferometer is a movable mirror. The movable mirror of the Fabry-Perot Interferometer may be based on an electrostatically actuated tensioned membrane. The other semi-transparent mirror may be a stationary mirror. Alternatively, also other semi-transparent mirror may be a movable mirror.

[0016] The Fabry-Perot Interferometer comprises a first semi-transparent mirror, a second semi-transparent mirror, a first actuating electrode, and a second actuating electrode. The first electrode and the second electrode may together operate as an electrostatic actuator, which pulls the second electrode towards the first electrode, in a situation where a voltage difference is applied between the first electrode and the second electrode. The second electrode may be pulled towards the first electrode by an electrostatic force, in a situation where a voltage difference is applied between the first electrode and the second electrode. The second mirror may be a movable mirror, and the second electrode may be attached to the second mirror, so as to move a movable portion of the second mirror with the electrostatic pulling force. The movable portion of the second mirror may be arranged to move together with the second electrode. The distance between the first mirror and the movable portion of the second mirror may be changed by using the electrostatic pulling force, which is generated by the actuating electrodes.

[0017] The second mirror has a movable portion. The second mirror has a movable central portion. The second mirror may also be called e.g. as the moving mirror or as the movable mirror. The second mirror comprises a tensioned membrane. The membrane is under lateral tension, so as to maintain the substantially planar shape of the central portion of the mirror. The perimeter of the tensioned membrane may be firmly attached to the supporting substrate.

[0018] The movable portion of the second mirror has a normal position in a situation where the voltage difference applied between the electrodes is equal to zero. The electrodes may generate an electrostatic force, which pulls the movable portion of the second mirror away from the normal position. The distance between the first mirror and the movable portion of the second mirror may be changed by using the electrostatic pulling force, which is generated by the actuating electrodes when a voltage difference AVE is applied between the actuating electrodes. The distance between first mirror and the movable portion of the second mirror may also be called e.g. as the mirror gap.

[0019] The second mirror may generate a mechanical restoring force when the movable portion is displaced away from the normal position. The second mirror is displaced to a stable position where the mechanical restoring force is equal to the electrostatic pulling force. The restoring force FR may depend on displacement Az and on the elastic properties of the materials of the second mirror. The pulling force Fp generated by the electrodes may depend on the voltage difference AVE and on the distance d between the first electrode and the second electrode.

[0020] The voltage difference between the electrodes generates an electrostatic force, which pulls the second mirror towards the first electrode. The elasticity of the second mirror provides a mechanical restoring force towards the normal position. When the applied voltage difference is lower than a pull-in threshold voltage, the electrostatic pulling force and mechanical restoring force are balanced. If the voltage difference exceeds the pull-in threshold voltage, then the electrostatic pulling force exceeds the mechanical restoring force, and the second mirror may move rapidly towards the first electrode, causing an accidental contact between mechanical parts.

[0021] The normal distance between the electrodes may be e.g. greater than or equal to 1 .5 times the normal distance between the mirrors. The normal distance refers to the distance in the situation where the voltage difference between the actuating electrodes is zero. The increased distance between the electrodes may reduce or avoid the risk of the pull-in effect. The tuning range of the distance between the mirrors M1 , M2 may be increased e.g. at least by 30%, when compared with a situation where the distance between the electrodes is equal to the distance between the mirrors.

[0022] The normal distance between the electrodes may be e.g. greater than or equal to 3 times the normal distance between the mirrors. Consequently, the distance between the mirrors may be changed e.g. by at least +90% or by at least -90%, while reducing or avoiding the risk of the pull-in effect. The change of +90% means that the electrostatic force is arranged to increase the distance. The change of -90% means that the electrostatic force is arranged to decrease the distance. The normal distance between the electrodes may be e.g. greater than three times the normal distance between the mirrors, so as to enable the use of the full range of mirror gap values.

[0023] The Fabry-Perot interferometer may be formed by bonding a first module to a second module. The first module comprises a first substrate and a first actuating electrode. The second module comprises a second substrate and the second actuating electrode. The first module may be implemented on a first substrate, and the second module may be implemented on a second substrate. The first substrate may be e.g. a silicon wafer, a fused silica wafer, or sapphire wafer. The second substrate may be e.g. a silicon wafer, a fused silica wafer, or a sapphire wafer.

[0024] Bonding of the first module to the second module may allow increased freedom to select the dimensions and the materials of the Fabry-Perot interferometer, when compared with a device where all structural material layers would be implemented on a single substrate. Bonding of a first wafer to a second wafer may facilitate production of Fabry-Perot interferometers, where the normal distance between electrostatic actuating electrodes is significantly greater than the normal distance between the mirrors.

[0025] The materials of the mating surfaces may also be selected such that the bonding forms a hermetic seal between the first module and the second module. The hermetic seal may e.g. enable maintaining controlled pressure and / or controlled gas composition in the space between the mirrors, after forming the hermetic seal. The controlled pressure in the space may be e.g. substantially equal to the normal atmospheric pressure (101 .3 kPa), or the space between the mirrors may have a reduced pressure. The internal space between the mirrors may have reduced pressure e.g. in order to facilitate modulating the distance between the mirrors at a high frequency. The gas composition may be different from the composition of the ambient air. The internal space may be e.g. filled with helium or hydrogen in order to facilitate modulating the distance between the mirrors at a high frequency. The controlled gas composition of the internal space may be selected to improve chemical stability of the Fabry-Perot interferometer. The materials of the hermetic seal may be selected so that the materials do not release vapors, which could contaminate the mirrors. The hermetic seal may effectively prevent any movement of the second module with respect to the first module. The hermetic seal may provide high mechanical stability for the Fabry- Perot interferometer. The hermetic seal may reduce effects of ageing on the Fabry-Perot interferometer.

[0026] The hermetic sealing may also facilitate further packaging with a camera sensor, with a detector or with light source, in particular at the wafer level. The first module may be implemented on a first wafer, and the second module may be implemented on a second wafer. The first module may be a portion of the first wafer, and the second module may be a portion of the second wafer. The first wafer may be bonded to the second wafer, so as to form a bonded combination of the first wafer and the second wafer. A Fabry-Perot Interferometer may be separated from the bonded combination by cutting the bonded combination.

[0027] The first wafer may comprise a plurality of first modules, and the second wafer may comprise a plurality of second modules. The first wafer may comprise an array of first modules, and the second wafer may comprise an array of second modules. One or more Fabry-Perot Interferometers may be separated from the bonded combination by cutting the bonded combination.

[0028] In an embodiment, the mirror of the first module and the mirror of the second module may be formed simultaneously on two separate wafers, in the same run. The mirrors may be formed simultaneously such that the first wafer and the second wafer are simultaneously in the same deposition chamber. Consequently, both mirrors may also have similar or identical optical properties. Consequently, the time needed for producing the interferometer may be reduced. Consequently, the rate of producing the both mirrors for the Fabry-Perot interferometers may be increased. The increased production rate may also facilitate compensating a production delay if one or more tests indicate that there has been a failure in the mirror deposition process.

[0029] Both mirrors of the Fabry-Perot interferometer may be formed at the same time on two separate wafers. This saves the total time needed for producing the mirrors of the Fabry-Perot interferometer. Forming both mirrors at the same time on two separate wafers may also ensure that both mirrors have similar optical properties. The mirrors may also be tested before bonding the wafers together. The mirrors may be tested in order to determine whether the properties of the mirrors fulfil one or more predetermined criteria regarding e.g. spectral reflectivity and / or flatness. Modules on either of the wafers which do not fulfil the predetermined criteria may be e.g. rejected or used as parts for a less demanding application. The distance between the first electrode and the second electrode may also be called e.g. as the electrode gap. Bonding of the first module wafer to the second module wafer facilitates providing an electrode gap, which is substantially greater than the mirror gap. In particular, the normal distance between the electrodes may be e.g. greater than three times the normal distance between the mirrors.

[0030] Bonding the module wafers together also allows providing a controlled pressure and / or controlled gas composition in the space between the mirrors. Bonding of the first module wafer to the second module wafer may also facilitate providing a low pressure or a vacuum in the space between the mirrors.

[0031] In an embodiment, the movable mirror of the Fabry-Perot interferometer may be arranged to operate in a vacuum in the hermetically sealed interior of the Fabry- Perot interferometer, e.g. in order to increase scanning speed, in order to reduce (acoustic) resonances and / or in order to protect the movable mirror from ambient gas.

[0032] In an embodiment, a hyperspectral imaging device may comprise the Fabry-Perot interferometer and an image sensor. The optical aperture of the Fabry-Perot interferometer may be e.g. in the range of 0.5 mm to 6 mm, so as to facilitate operation of the hyperspectral imaging device.

[0033] In an embodiment, a spectrometer may comprise the Fabry-Perot interferometer and a detector. The mirrors may comprise e.g. stacked layers of silicon (Si) and silicon dioxide (SiC ). The spectral operating range of the Fabry-Perot interferometer may cover e.g. the range of 1350 nm to 1950 nm.

[0034] In an embodiment, the low absolute pressure of the internal free space may facilitate fast scanning of the mirror gap of the Fabry-Perot interferometer FPI1 .

[0035] The structure of the mirrors may be selected according to the operating wavelength range. The structure may be selected e.g. from the following alternatives:

[0036] - Bragg reflector comprising or consisting of layers of AI2O3 and TiC (suitable e.g. for spectral range 350 nm to 760 nm), - Bragg reflector comprising or consisting of layers of polysilicon separated by layers of vacuum or a gas (suitable e.g. for spectral range 1.1 pm to 12 pm),

[0037] - Bragg reflector comprising or consisting of layers of polysilicon and layers of silicon nitride (suitable e.g. for spectral range 600 nm to 4500 nm),

[0038] - Metallic semi-transparent mirror comprising or consisting of a layer of silver. The silver layer may be protected e.g. by aluminum oxide AI2O3 (suitable e.g. for spectral range 400 nm to 2500 nm),

[0039] - Metallic semi-transparent mirror (the reflective layer may be a metal layer, e.g. silver or gold).

[0040] In an embodiment, the mirrors may be implemented e.g. as Poly-Si-Air Bragg mirrors. A Poly-Si-Air Bragg mirror may comprise two or more stacked layers of polysilicon separated by air layers (or by vacuum layers). The mirror distance may be varied e.g. in the range of 500 nm to 1520 nm. The first mirror may comprise e.g. two or three layers of polysilicon, which are separated by air layers (or by vacuum layers). The operation of the Poly-Si-Air Bragg mirrors may be optimized e.g. for the wavelength of 1550 nm. The spectral operating range of the interferometer may be e.g. in the range of 1185 nm to 2500 nm. The normal distance between the electrodes may be e.g. substantially equal to 4600 nm.

[0041] A Poly-Si-Air Bragg mirror may comprise two or more stacked layers of polysilicon separated by air layers (or separated by vacuum layers). Adjacent layers of polysilicon may define a layer of air or a layer of vacuum between them. The mirror may comprise a plurality of microscopic columns which mechanically connect a lower layer of polysilicon to an upper layer of polysilicon. The columns may define the height of the air layer between the adjacent layers of polysilicon. One or more layers of polysilicon may comprise microscopic release holes for etching. The air layer may be formed by etching sacrificial material away through the release holes.

[0042] In an embodiment, the first mirror and / or the second mirror may be implemented e.g. as Poly-Si-SiNx Bragg mirrors, i.e. the structure may further comprise silicon nitride. A conductive layer for coupling the voltage to the electrode may also be formed without ion implantation, e.g. to simplify production of the interferometer. In an embodiment, the first mirror and / or the second mirror may be e.g. a Bragg mirror, which comprises e.g. three layers of silica (SiC ) separated by layers of silicon (Si). The mirror may be optimized for the wavelength 1550 nm. The distance between the mirrors may be varied e.g. in the range of 500 nm to 1050 nm. The substrate of the first module may be e.g. silicon or SiO2. The substrate of the second module may be e.g. silicon or SiO2.

[0043] According to a comparative example, the distance between the electrodes is equal to the distance between the mirrors, and the distance between mirrors must be limited to the range of 700 nm to 1050 nm due to the pull-up effect.

[0044] In an embodiment, the increased distance between the electrodes may be utilized for measuring the dark current of a detector. A spectral measuring apparatus may comprise the Fabry-Perot interferometer and a detector. The capability to provide small mirror gap values without the risk of the pull-in effect may facilitate measuring the dark current of the detector. The distance between the mirrors may be decreased so that the spectral transmittance band(s) of the Fabry-Perot interferometer is shifted outside the spectral detection range of the apparatus. The Fabry-Perot interferometer may operate as an optical shutter. The dark current of detector may be measured when the spectral transmittance band(s) of the Fabry-Perot interferometer are outside the spectral detection range of the apparatus.

[0045] BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In the following examples, several variations will be described in more detail with reference to the appended drawings, in which

[0047] Fig. 1 a shows, by way of example, in a cross-sectional view, a Fabry-Perot interferometer,

[0048] Fig. 1 b shows, by way of example, in a cross-sectional view, the modules of the interferometer of Fig 1 a before bonding, Fig. 1 c shows, by way of example, in a three-dimensional view, the modules of the interferometer of Fig 1a before bonding,

[0049] Fig. 2a shows, by way of example, in a cross-sectional view, a Fabry-Perot interferometer,

[0050] Fig. 2b shows, by way of example, in a cross-sectional view, the modules of the interferometer of Fig 2a before bonding,

[0051] Fig. 3a shows, by way of example, in a cross-sectional view, a Fabry-Perot interferometer,

[0052] Fig. 3b shows, by way of example, in a cross-sectional view, the modules of the interferometer of Fig 3a before bonding,

[0053] Fig. 4a shows, by way of example, in a cross-sectional view, a Fabry-Perot interferometer,

[0054] Fig. 4b shows, by way of example, in a cross-sectional view, the modules of the interferometer of Fig 4a before bonding,

[0055] Fig. 4c shows, by way of example, in a three-dimensional view, the Fabry- Perot interferometer,

[0056] Fig. 5a shows, by way of example, in a three-dimensional view, bonding an array of first modules to an array of second modules,

[0057] Fig. 5b shows, by way of example, in a three-dimensional view, an array of Fabry-Perot interferometers,

[0058] Fig. 6a shows, by way of example, in a three-dimensional view, bonding a first wafer to a second wafer,

[0059] Fig. 6b shows, by way of example, in a three-dimensional view, a bonded combination of wafers, Fig. 7a shows, by way of example, a spectroscopic measuring device,

[0060] Fig. 7b shows, by way of example, an adjustable spectral light source,

[0061] Fig. 8a shows, by way of example, in a cross-sectional view, modules of a

[0062] Fabry-Perot interferometer, and a detector module,

[0063] Fig. 8b shows, by way of example, in a three-dimensional view, bonding a detector wafer to an interferometer wafer,

[0064] Fig. 8c shows, by way of example, in a cross-sectional view, a spectroscopic measuring device formed by bonding the modules of Fig. 8a,

[0065] Fig. 9 shows, by way of example, in a cross-sectional view, a Fabry-Perot interferometer, which comprises an electrode formed by ion implantation,

[0066] Fig. 10a shows, by way of example, in a cross-sectional view, modules of a Fabry-Perot interferometer, wherein an electrode has been formed by ion implantation,

[0067] Fig. 10b shows, by way of example, in a cross-sectional view, modules of a Fabry-Perot interferometer, wherein an electrode has been formed by ion implantation.

[0068] DETAILED DESCRIPTION

[0069] Referring to Fig. 1 a, the Fabry-Perot interferometer FPI1 has a first semitransparent mirror M1 , a second semi-transparent mirror M2, a first actuating electrode E1 , and a second actuating electrode E2. The electrodes E1 , E2 may have e.g. an annular form, when viewed in the direction of the central axis AX1 .

[0070] The symbol dM denotes the distance between the mirrors M1 , M2. The symbol d denotes the distance between the electrodes E1 , E2. The electrodes E1 , E2 may together generate a pulling electrostatic force, which pulls the second mirror M2 towards the first mirror M1 . The electrodes E1 , E2 may pull the second mirror M2 so that the distance dM between the mirrors M1 , M2 is smaller than the normal distance dM.o between the mirrors M1 , M2. The Fabry-Perot interferometer FPI1 of Fig. 1 a has a contracting mirror gap dM. The contracting mirror gap dM means that applying a voltage difference V2-V1 between the actuating electrodes E1 , E2 reduces the mirror gap dM from a normal mirror gap value dM.o. The interferometer FPI1 has the normal mirror gap dM.o and a normal electrode gap d .o in a situation where the voltage difference V2-V1 applied between the electrodes E1 , E2 is zero. Applying a non-zero voltage difference V2-V1 between the electrodes E1 , E2 pulls the second electrode E2 towards the first electrode E1 , and also pulls the second mirror M2 towards the first mirror M1 .

[0071] The first electrode E1 may be implemented e.g. on a bottom surface of a recess REC1 , so that the normal electrode gap d .o may be substantially greater than the normal mirror gap dM.o.

[0072] The Fabry-Perot interferometer FPI1 may be formed by bonding a first module MODI to a second module MOD2 (Figs. 1 b, 1 c). The first module MODI may comprise a first substrate SLIB1 , the first electrode E1 , and the first mirror M1. The first substrate SLIB1 may be optically transparent in the spectral operating range of the Fabry-Perot interferometer FPI1. The second module MOD2 may comprise a second substrate SLIB2, the second electrode E2, and the second mirror M2.

[0073] The first module MODI may be portion of a first wafer WAF1 , and the second module MOD2 may be portion of a second wafer WAF2 (Fig. 6a). The first wafer WAF1 may be bonded to the second wafer WAF2.

[0074] The dimensions of the modules MODI , MOD2 may be selected such that the normal distance d .o between the electrodes E1 , E2 may be e.g. greater than 1 .5 times the normal distance dM.o between the mirrors M1 , M2. The normal distance d .o between the electrodes E1 , E2 may advantageously be e.g. greater than or equal to 3 times the normal distance dM.o between the mirrors M1 , M2.

[0075] The first mirror M1 may be supported by the first substrate SLIB1. The second mirror M2 may be supported by the second substrate SLIB2. The entire area of the first mirror M1 may be supported on the first substrate SLIB1. The entire perimeter of the second mirror M2 may be supported by the second substrate SUB2. The second mirror M2 may be under lateral tension, so as to keep the second mirror M2 as planar as possible. The second substrate SLIB2 may keep the second mirror M2 under lateral tension. Implementing the movable mirror as a tensioned membrane may provide a structure, which is not highly sensitive to gravity, external vibration, and / or mechanical shocks.

[0076] The assembled interferometer FPI1 comprises a free space SPC1 between the first mirror M1 and the movable portion of the second mirror M2, so as to allow vertical movements of the movable portion of the second mirror M2. In particular, the free space SPC1 may allow vertical movement of the electrode E2 and the central portion of the second mirror M2, with respect to the first mirror M1 .

[0077] WFLEX denotes the width or diameter of the moving portion of the second mirror M2. The diameter WFLEX may be defined e.g. by the diameter of the free space SPC1 and / or by the diameter of an opening OPE2 of the second substrate SLIB2. WARE denotes the diameter of the optical aperture APE1 of the interferometer FPI1 . The diameter WARE may be defined e.g. by an opening of the electrode E2 and / or by the diameter of the first mirror M1 .

[0078] The first electrode E1 may be opaque, transparent or semi-transparent. The second electrode E2 may be opaque, transparent or semi-transparent. A transparent or semi-transparent electrode may or may not limit the width WARE of the optical aperture.

[0079] The central portion of the second mirror M2 may be arranged to move with respect to the first mirror M1 , with respect to the first substrate SLIB1 , and with respect to the second substrate SLIB2. The first mirror M1 may be a stationary mirror. The second mirror M2 may be a movable mirror. The second mirror M2 may be arranged to move together with the second electrode E2. The second electrode E2 may be attached to the second mirror M2 such that the central portion of the second mirror M2 may move together with the second electrode E2. The Fabry-Perot interferometer FPI1 may be produced by implementing the first mirror M1 and the first electrode E1 at different heights on the first substrate SLIB1 . The first substrate SLIB1 may have an annular recess REC1 , and the first electrode E1 may be implemented on the bottom of the annular recess REC1. The first substrate SLIB1 may have a central elevated portion PED1 , and the first mirror M1 may be implemented on the central elevated portion PED1 . The central elevated portion PED1 may also be called e.g. as a platform.

[0080] The first module MODI may comprise a central elevated portion PED1 , the first mirror M1 may be implemented on the central elevated portion PED1 , the central elevated portion PED1 may be surrounded by a recessed portion REC1 , and the first electrode E1 may be implemented on the recessed portion REC1 .

[0081] A first voltage Vi may be applied to the first electrode E1 e.g. via a conductive layer C1 and / or via a connection terminal T1. The recess REC1 may optionally have a sloped portion, e.g. in order to facilitate forming of the conductive layer C1. A portion of the conductive layer C1 may be implemented on the sloped portion. A second voltage V2 may be applied to the second electrode E2 e.g. via a conductive layer C2 and / or via a connection terminal T2.

[0082] The connection terminal T1 and / or T2 may be implemented through the first substrate SLIB1. The connection terminal T1 and / or T2 may be implemented through the second substrate SLIB2.

[0083] A connection terminal implemented through a substrate may be optionally surrounded by an electrically insulating element 122, so as to provide additional electrical insulation between the connection terminal and the substrate. For example, the terminal T2 may be implemented through the substrate SLIB2, and the terminal T2 may be surrounded by an element 122, which may insulate the terminal 122 from the substrate SLIB2.

[0084] The recess REC1 may have e.g. a vertical circumferential surface portion or a sloped circumferential surface portion. An angle between the surface normal of the sloped portion and the surface normal of the sloped portion may be e.g. in the range of 5° to 70°. The sloped surface portion may facilitate forming the conductive layer C1 on the sloped portion. The vertical surface portion may be substantially perpendicular to the mirrors. The connection terminal T1 may be implemented through the first substrate SLIB1 e.g. in a situation where the recess REC1 does not comprise the sloped portion.

[0085] Referring to Figs. 1 b and 1 c, a first module MODI may comprise the first mirror M1 , the first electrode E1 , and the first substrate SLIB1 . The first substrate SLIB1 may comprise an annular recess and a central elevated portion PED1 . The first electrode E1 may be implemented on the bottom surface of the recess REC1 . The first mirror M1 may be implemented on the central elevated portion PED1. The first module MODI may comprise a conductive layer C1 for coupling the first voltage Vi to the first electrode E1 . The first module MODI may comprise one or more insulating layers 111 e.g. to improve electrical insulation between the electrodes E1 , E2.

[0086] A second module MOD2 may comprise the second mirror M2, the second electrode E2, and the second substrate SLIB2. The second substrate SLIB2 may have an opening OPE2 to allow movements of the second mirror M2. The second electrode E2 may be implemented e.g. on the second mirror M2. The second module MOD2 may comprise one or more insulating layers 211 ,212 e.g. to improve electrical insulation between the electrodes E1 , E2.

[0087] The normal distance dM.o between the mirrors M1 , M2 may be defined e.g. by the dimensions of the substrate SLIB1 , by thickness of the material layer C2 and by the thickness of the material layer 111. The normal distance d .o between the electrodes E1 , E2 may also be defined by the dimensions of the substrate SLIB1 , by thickness of the material layer C2 and by the thickness of the material layer 111.

[0088] The first module MODI and / or the second module MOD2 may comprise one or more connection terminals T1 , T2 for applying the voltage Vi to the electrode E1 and / or for applying the voltage V2 to the electrode E2.

[0089] The Fabry-Perot interferometer FPI1 of Fig. 1 a may be formed by bonding the first module MODI to the second module MOD2. The first module MODI may be bonded to the second module MOD2. A first wafer WAF1 comprising the first module MODI may be bonded to a second wafer WAF2, which comprises the second module MOD2.

[0090] The first module MODI may have a first mating surface SRF1. The second module MODI may have a second mating surface SRF2. The first mating surface SRF1 may comprise e.g. material of the conductive layer C1 and / or material of the insulating layer 111. The second mating surface SRF2 may comprise e.g. material of the conductive layer C2 and / or material of the insulating layer 211. The first mating surface SRF1 may be bonded to the second mating surface SRF2.

[0091] A method for producing the Fabry-Perot interferometer FPI1 may comprise:

[0092] - forming the first module MODI , which comprises the first mirror M1 and the first actuating electrode E1 ,

[0093] - forming the second module MOD2, which comprises the second mirror M2 and the second actuating electrode E2,

[0094] - bonding the first module MODI to the second module MOD2, wherein the second mirror M2 is arranged to move together with the second electrode E2, wherein the distance d between the first electrode E1 and the second electrode E2 is greater than 1.5 times a distance dM between the first mirror M1 and the second mirror M2.

[0095] A method for producing Fabry-Perot interferometers FPI1 may comprise:

[0096] - forming a plurality of first modules MODI on a first wafer WAF1 , each first module MODI comprising a first mirror M1 and a first actuating electrode E1 ,

[0097] - forming a plurality of second modules MOD2 on a second wafer WAF2, each second module MOD2 comprising a second mirror M2 and a second actuating electrode E2,

[0098] - bonding the first wafer WAF1 to the second wafer WAF2, each first module MODI being bonded to a corresponding second module MOD2.

[0099] Each second mirror M2 may be arranged to move together with a corresponding second electrode E2. The distance d between each first electrode E1 and the corresponding second electrode E2 may be e.g. greater than 1 .5 times a distance dM between the first mirror M1 and the second mirror M2. The first mirror M1 and the second mirror M2 may be formed simultaneously in the same run on two separate wafers. The first substrate SLIB1 may be a first portion of a first wafer, and the second substrate SLIB2 may be a second portion on a second wafer. The optical properties of the mirrors M1 , M2 may be optionally measured before bonding the wafers together. In an embodiment, the wafers may be bonded together only if the optical properties of the mirrors M1 , M2 fulfil one or more predetermined criterions.

[0100] The recess REC1 for the electrode E1 may be formed e.g. by etching. The recess REC1 may be formed by dry etching or by wet etching. Forming of the recess REC1 may comprise e.g. local oxidation of silicon (LOCOS) and / or anisotropic wet etching with tetramethylammonium hydroxide (TMAH). The local oxidation of silicon is a microfabrication process where silicon dioxide is formed in selected areas on a silicon wafer having a Si-SiO2 interface at a lower point than the rest of the silicon surface. The method may comprise forming a slanted sidewall of the recess REC1 for the conductive layer C1. An angle between the slanted sidewall and the central axis AX1 may be greater than zero.

[0101] One or more slanted sidewalls for the recess REC1 and / or for the elevation PED1 may be formed e.g. by anisotropic wet etching.

[0102] Alternatively, one or more straight sidewalls for the recess REC1 and / or for the elevation PED1 may be formed by dry etching. The straight sidewalls may be substantially parallel with the central axis AX1 .

[0103] The opening OPE2 of the substrate SLIB2 may be formed e.g. by dry etching or by TMAH etching. The opening OPE2 may allow free movement of the second mirror M2.

[0104] The electrode E1 and / or the conductive layer C1 may also be formed e.g. by using conductive silicon. The wafer may be e.g. a Silicon-on-lnsulator wafer (SOI).

[0105] The wafer may comprise an insulating layer. A connecting terminal T1 to the conductive layer (E1 , C1 ) may be formed e.g. through the insulating layer. The first module MODI and / or the second module MOD2 may comprise one or more terminals T1 , T2 for coupling the actuating voltage V1 , V2 to the electrodes E1 , E2. The terminal T1 , T2 may optionally pass through the substrate SLIB1 , SUB2.

[0106] The first module MODI may have a first mating surface SRF1. The second module MOD2 may have a second mating surface SRF2. The first mating surface SRF1 may be bonded to the second mating surface SRF2.

[0107] The first mating surface SRF1 may be bonded to the second mating surface SRF2 at the wafer level. The modules MODI , MOD2 may be bonded together at the wafer level.

[0108] A hermetic joint H1 may be formed at a plane defined by the mating surfaces SRF1 , SRF2. The modules MODI , MOD2 may be hermetically bonded together, so as to hermetically seal the space SPC1 and the recess REC1 from the environment ENV1 of the interferometer FPI1 . The internal space SPC1 and the recess REC1 between the mirrors M1 , M2 may have a controlled pressure and / or a controlled gas composition, after the hermetic bonding. The gas may have predetermined chemical composition. The space SPC1 and the recess REC1 may have a reduced absolute pressure pi after the hermetic bonding. The pressure pi may be e.g. smaller than 10 kPa, smaller than 1 kPa, or even smaller than 100 Pa. The recess REC1 and the space SPC1 may have a vacuum.

[0109] In an embodiment, the Fabry-Perot interferometer FPI1 may further comprise one or more venting holes, so that the internal space SPC1 is in fluid communication with the environment ENV1 .

[0110] Referring to Figs. 2a and 2b, the electrodes E1 , E2 of the Fabry-Perot interferometer FPI1 may also be positioned such that the electrostatic force of the electrodes E1 , E2 pulls the movable portion of the second mirror M2 away from the first mirror M1. The second mirror M2 may be arranged to operate as the movable mirror. The first mirror M1 may be arranged to operate as the stationary mirror. The electrostatic force generated by the electrodes E1 , E2 may actuate the Fabry-Perot interferometer FPI1 from the (minimum) normal mirror gap towards a larger mirror gap. The electrostatic force may pull the mirror M2 such that the mirror distance dM is greater than the normal distance dM,o.

[0111] The dimensions of the modules MODI , MOD2 may be selected such that the normal distance d .o between the electrodes E1 , E2 may be e.g. greater than 1 .5 times the normal distance dM.o between the mirrors M1 , M2. The normal distance d .o between the electrodes E1 , E2 may advantageously be e.g. greater than or equal to 3 times the normal distance dM.o between the mirrors M1 , M2.

[0112] A first voltage Vi may be applied to the first electrode E1 e.g. via a conductive layer C1 and / or via a connection terminal T1. The recess REC1 may have a sloped portion, e.g. in order to facilitate forming of the conductive layer C1. A portion of the conductive layer C1 may be implemented on the sloped portion. A second voltage V2 may be applied to the second electrode E2 e.g. via a conductive layer C2 and / or via a connection terminal T2.

[0113] The connection terminal T1 and / or T2 may be implemented through the first substrate SLIB1. The connection terminal T1 and / or T2 may be implemented through the second substrate SLIB2.

[0114] The Fabry-Perot interferometer FPI1 may comprise one or more electrically insulating material layers 121 , 122, 223 to electrically insulate the electrodes E1 , E2 from each other.

[0115] The Fabry-Perot interferometer FPI1 may be produced by bonding a first module MODI to a second module MOD2.

[0116] The first module MODI may comprise the first electrode E1 . The first electrode E1 may be attached to the first substrate SLIB1. The first module MODI may comprise a recess REC1. The first electrode E1 may be located on the recess REC1 . The first module MODI may comprise a recessed portion REC1 , and the first electrode E1 may be implemented on the recessed portion REC1. The substrate SLIB1 may comprise the recess REC1. The substrate SLIB1 may be optically transparent in the spectral operating range of the Fabry-Perot interferometer FPI1 . The recess REC1 may be formed e.g. by etching. The recess REC1 may be formed e.g. by dry etching or by wet etching. Forming of the recess REC1 may comprise e.g. local oxidation of silicon (LOCOS) and / or anisotropic wet etching with tetramethylammonium hydroxide (TMAH). The method may comprise forming a slanted sidewall of the recess REC1 for the conductive layer C1 . One or more slanted sidewalls for the recess REC1 may be formed e.g. by anisotropic wet etching.

[0117] The second module MOD2 may comprise both mirrors M1 , M2. Both mirrors M1 , M2 may be supported by a second substrate SUB2.

[0118] The empty space SPC1 between the mirrors M1 , M2 may be formed e.g. by

[0119] - forming the first mirror M1 on the substrate SLIB2,

[0120] - depositing sacrificial material layer (e.g. 222) on the first mirror M1 ,

[0121] - forming the second mirror M2 on the sacrificial material layer,

[0122] - forming microscopic holes OP2 in the second mirror M2, and

[0123] - removing the sacrificial material away from the space SPC1 between the first mirror M1 and the second mirror M2, by etching via the microscopic release holes OP2.

[0124] The module MOD2 may be produced by forming the first mirror M1 on the optically transparent substrate SLIB2, forming a sacrificial material layer (222) on the first mirror M1 , forming a second mirror M2 on the sacrificial material layer (222), and forming an internal cavity SPC1 between the first mirror M1 and the second mirror M2 by etching the sacrificial material away (222) from the internal space SPC1 . The sacrificial material is removed by etching through the release holes OP2 of the second mirror M2.

[0125] The second module MOD2 may comprise the first mirror M1 and the second mirror M2, wherein the second mirror M2 may comprises a plurality of release holes OP2, wherein the space SPC1 between the mirrors M1 ,M2 may be formed by removing sacrificial material away via the release holes OP2 before said bonding. The sacrificial material layer (222) may comprise or consist of e.g. silicon oxide (SiO2). The sacrificial layer may be formed e.g. by plasma enhanced chemical vapor deposition (PECVD) from tetraethoxysilane (TEOS).

[0126] The sacrificial material layer may temporarily support the second mirror during fabrication of the module MOD2. The peripheral region of the sacrificial material layer (222) may remain between the mirrors M1 , M2 after the etching. The peripheral region of the sacrificial material layer (222) may surround the internal space SPC1. The peripheral region of the sacrificial material layer (222) may define the normal distance dM between the mirrors M1 , M2, and the width WFLEX of the moving region of the mirror M2. The thickness of the sacrificial material layer (222) may define the normal distance dM between the mirrors M1 , M2.

[0127] The diameter of the release holes OP2 may be so small that they do not significantly affect the optical performance of the Fabry-Perot interferometer FPI1 . The diameter of the release holes OP2 may be e.g. smaller than 3 .m. The sacrificial material layer (222) may be etched away via the holes OP2 e.g. by using hydrofluoric acid (HF).

[0128] The second module MOD2 comprises a spacer layer 222 to define the distance between the mirrors M1 , M2, to mechanically support the second mirror M2, and to maintain the lateral tension of the second mirror M2. A peripheral portion of the sacrificial material layer may optionally remain as the spacer 222.

[0129] The second module MOD2 may optionally comprise one or more supporting material layers 221 between the substrate SLIB2 and the first mirror M1 .

[0130] The normal distance dM.o between the mirrors M1 , M2 may be defined e.g. by the thickness of the spacer 222. The normal distance d .o between the electrodes E1 , E2 may be defined by the dimensions of the substrate SLIB1 , by thickness of the material layer C1 and by the thickness of the material layer 223.

[0131] The first module MODI may have a first mating surface SRF1. The second module MOD2 may have a second mating surface SRF2. The first mating surface SRF1 may be bonded to the second mating surface SRF2. The first substrate SLIB1 and the second substrate SLIB2 may be formed from different wafers. For example, the interferometer FPI1 may comprise a silicon substrate and SiO2 substrate on opposite sides.

[0132] The movement from the normal mirror gap to a maximum mirror gap may enable accurate control of the movement near the normal mirror gap. The bending of the mirror M2 may be lowest near the normal mirror gap.

[0133] A hermetic joint H1 may be formed at a plane defined by the mating surfaces SRF1 , SRF2. The modules MODI , MOD2 may be hermetically bonded together, so as to hermetically seal the space SPC1 and the recess REC1 from the environment ENV1 of the interferometer FPI1. The hermetic joint H1 may hermetically seal the space SPC1 and the recess REC1 from the environment ENV1. The internal space SPC1 and the recess REC1 between the mirrors M1 , M2 may have a controlled pressure and / or a controlled gas composition, after the hermetic bonding. The gas may have predetermined chemical composition. The space SPC1 and the recess REC1 may have a reduced absolute pressure pi after the hermetic bonding. The pressure pi may be e.g. smaller than 10 kPa, smaller than 1 kPa, or even smaller than 100 Pa. The recess REC1 and the space SPC1 may have a vacuum. The first mirror M1 and the second mirror M2 may define a first internal space SPC1 . The recess REC1 and the second mirror M2 may define a second internal space SPC2. Both spaces SPC1 , SPC2 may have a vacuum. The movable mirror M2 may be arranged to operate in a vacuum.

[0134] In an embodiment, the Fabry-Perot interferometer FPI1 may further comprise one or more venting holes, so that the internal space SPC1 is in fluid communication with the environment ENV1 .

[0135] Referring to Figs. 3a and 3b, the first module MODI may comprise the first mirror M1 and the first electrode E1. The second module MOD2 may comprise the second mirror M2 and the second electrode E2. The Fabry-Perot interferometer FPI1 may be produced by bonding the first module MODI to the second module MOD2.

[0136] The electrostatic force generated by the electrodes E1 , E2 may be arranged to pull the movable portion of the second mirror M2 towards the first mirror M1 . The electrodes E1 , E2 of the Fabry-Perot interferometer FPI1 may be positioned such that the electrostatic force of the electrodes E1 , E2 pulls the movable portion of the second mirror M2 towards the first mirror M2. The second mirror M2 may be arranged to operate as the movable mirror. The first mirror M1 may be arranged to operate as the stationary mirror. Alternatively, also the first mirror M1 may be a movable mirror. The electrostatic force may pull the mirror M2 such that the mirror distance dM is smaller than the normal distance dM.o.

[0137] The first module MODI may comprise a spacer material layer 132 between the first mirror M1 and the first electrode E1. The spacer material layer 132 may define a distance between the first mirror M1 and the first electrode E1. The spacer material layer 132 may also operate as an electrically insulating layer, to electrically insulate the first electrode E1 from the second electrode E2. The thickness of the insulator layer may be determined also based on the relative permittivity (i.e. dielectric constant) of the insulator material. The insulator material layer 132 may comprise or consist of a material, which has low relative permittivity (e.g. lower than 3.0), so as to maximize the electrostatic force. The insulator material layer 132 may comprise e.g. porous silicon dioxide.

[0138] The first module MODI and / or the second module MOD2 may comprise a spacer material layer 231 to define the normal distance dM.o between the mirrors M1 , M2. The spacer material layer 231 may partly or completely remain between the mirrors M1 , M2 after the bonding. The spacer material layer 231 may comprise or consist of an electrically insulating material to insulate the first electrode E1 from the second electrode E2.

[0139] The normal distance dM.o between the mirrors M1 , M2 may be defined e.g. by the thickness of the spacer material layer 231 . The normal distance d .o between the electrodes E1 , E2 may be defined by the thickness of the spacer material layer 231 , by the thickness of the mirrors M1 , M2, and by the thickness of the insulator material layer 132.

[0140] The dimensions of the modules MODI , MOD2 may be selected such that the normal distance d .o between the electrodes E1 , E2 may be e.g. greater than 1 .5 times the normal distance dM.o between the mirrors M1 , M2. The normal distance d .o between the electrodes E1 , E2 may advantageously be e.g. greater than or equal to 3 times the normal distance dM.o between the mirrors M1 , M2.

[0141] The second mirror M2 may be implemented on a second substrate SLIB2. The second substrate SLIB2 may comprise an opening OPE2 to allow movements of the movable portion of the second mirror M2. The substrate SLIB2 may support the perimeter of the second mirror M2. The substrate SLIB2 may fix the vertical and lateral position of the second mirror M2. The substrate SLIB2 may maintain the lateral tension of the second mirror M2.

[0142] The first mirror M1 may be implemented on a first substrate SLIB1. The first substrate SLIB1 may comprise an opening OPE1 e.g. to allow forming the first electrode E1 on the spacer layer 132. Also the first mirror M1 may be a movable mirror. The opening OPE1 may allow movements of the movable portion of the first mirror M1 . The substrate SLIB1 may fix the vertical and lateral position of the first mirror M1. The substrate SLIB1 may maintain the lateral tension of the second mirror M1 .

[0143] The first module MODI may have a first mating surface SRF1. The second module MOD2 may have a second mating surface SRF2. The first mating surface SRF1 may be bonded to the second mating surface SRF2.

[0144] The module MODI may be formed on a first wafer WAF1 . The first wafer WAF1 may be e.g. a silicon wafer, a fused silica wafer, a sapphire wafer, or a silicon-on insulator wafer. The module MOD2 may be formed on a second wafer WAF2. The second wafer WAF2 may be e.g. a silicon wafer, a fused silica wafer, a sapphire wafer, or a silicon-on insulator wafer.

[0145] The thickness of the wafer WAF1 , WAF2 may be e.g. in the range of 200 Lim to 4000 .m. The combined thickness of the material layers deposited on the wafer WAF1 , WAF2 may be e.g. in the range of 0.5 Lim to 20 .m.

[0146] The mirrors M1 , M2 may be e.g. Bragg mirrors or metallic mirrors. A metallic second mirror M2 may also be arranged to operate as the second electrode E2. The first mirror M1 and the second mirror M2 may be formed e.g. at the same time, on the different wafers WAF1 , WAF2. The substrate SLIB1 , and / or SLIB2 may be etched e.g. by using TMAH to enable sloped sidewalls of the openings OPE1 , OPE2. The sloped sidewall may facilitate forming the contact layer C1 , C2 on the sidewall.

[0147] A hermetic joint H1 may be formed at a plane defined by the mating surfaces SRF1 , SRF2. The modules MODI , MOD2 may be hermetically bonded together, so as to hermetically seal the space SPC1 from the environment ENV1 of the interferometer FPI1 . The hermetic joint H1 may hermetically seal the space SPC1 from the environment ENV1. The internal space SPC1 between the mirrors M1 , M2 may have a controlled pressure and / or a controlled gas composition, after the hermetic bonding. The gas may have predetermined chemical composition. The space SPC1 may have a reduced absolute pressure pi after the hermetic bonding. The pressure pi may be e.g. smaller than 10 kPa, smaller than 1 kPa, or even smaller than 100 Pa. The space SPC1 may have a vacuum.

[0148] In an embodiment, the Fabry-Perot interferometer FPI1 may further comprise one or more venting holes, so that the internal space SPC1 is in fluid communication with the environment ENV1 .

[0149] Figs. 4a and 4b show a variation of the interferometer of Fig. 3a, wherein the first module MODI may be similar or identical to the second module MOD2. The first module MODI comprises a first mirror M1 supported by the first substrate SLIB1 . The first electrode E1 is attached to the movable portion of the first mirror M1 via an insulator layer 132. The first substrate SLIB1 comprises an opening OPE1 to allow movements of the movable portion of the mirror M1. The voltage V1 may be coupled from the terminal T1 to the electrode E1 via the conductor layer C1 . The opening OPE1 may have a sloped sidewall to facilitate implementing the conductor layer C1 .

[0150] The second module MOD2 comprises a second mirror M2 supported by the second substrate SLIB2. The second electrode E2 is attached to the movable portion of the second mirror M2 via an insulator layer 232. The second substrate SLIB2 comprises an opening OPE2 to allow movements of the movable portion of the mirror M2. The voltage V2 may be coupled from the terminal T2 to the electrode E2 via the conductor layer C2. The opening OPE2 may have a sloped sidewall to facilitate implementing the conductor layer C2.

[0151] The first module MODI may comprise a first spacer layer 131 , which has a mating surface SRF1. The second module MOD2 may comprise a second spacer layer 231 , which has a mating surface SRF2. The interferometer FPI1 may be assembled by bonding the mating surfaces SRF1 , SRF2 together. The normal distance dM.o between the mirrors M1 , M2 may be defined by the thickness of the spacer layers 131 , 231 . The normal distance d .o between the electrodes E1 , E2 may be defined by the thickness of the spacer layers 131 , 231 , by the thickness of the mirrors M1 , M2, and by the thickness of the insulator layers 132, 232.

[0152] The dimensions of the modules MODI , MOD2 may be selected such that the normal distance d .o between the electrodes E1 , E2 may be e.g. greater than 1 .5 times the normal distance dM.o between the mirrors M1 , M2. The normal distance d .o between the electrodes E1 , E2 may advantageously be e.g. greater than or equal to 3 times the normal distance dM.o between the mirrors M1 , M2.

[0153] Fig. 4c shows, in a three-dimensional view, the Fabry-Perot interferometer FPI1 of Fig. 4a. The movable region of the mirror M2 may be substantially circular.

[0154] A hermetic joint H1 may be formed at a plane defined by the mating surfaces SRF1 , SRF2. The modules MODI , MOD2 may be hermetically bonded together, so as to hermetically seal the space SPC1 from the environment ENV1 of the interferometer FPI1 . The hermetic joint H1 may hermetically seal the space SPC1 from the environment ENV1. The internal space SPC1 between the mirrors M1 , M2 may have a controlled pressure and / or a controlled gas composition, after the hermetic bonding. The gas may have predetermined chemical composition. The space SPC1 may have a reduced absolute pressure pi after the hermetic bonding. The pressure pi may be e.g. smaller than 10 kPa, smaller than 1 kPa, or even smaller than 100 Pa. The space SPC1 may have a vacuum.

[0155] In an embodiment, the Fabry-Perot interferometer FPI1 may further comprise one or more venting holes, so that the internal space SPC1 is in fluid communication with the environment ENV1 . T1

[0156] Referring to Figs. 5a and 5b, an array ARR3 of Fabry-Perot interferometers FPI1 may be formed by bonding a first array ARR1 of first modules MODI to a second array ARR2 of second modules MOD2. The first array ARR1 may comprise two or more first modules MODI , which are joined together. The second array ARR2 may comprise two or more second modules MOD2, which are joined together.

[0157] Bonding the first array ARR1 to the second array ARR2 may e.g. ensure that the mirrors M1 , M2 are parallel with each other. Handling the larger arrays may be easier than handling individual small modules. Positioning of the larger arrays may be more accurate than positioning of smaller individual modules. Bonding the first array ARR1 to the second array ARR2 may increase production rate of the Fabry-Perot interferometers FPI1 , as the time needed for bonding the arrays together may be shorter than or equal to the time needed for bonding individual modules together.

[0158] The first array ARR1 may be formed on a first wafer WAF1 , and the second array ARR2 may be formed on a second wafer WAF2. The first array ARR1 may be a portion of the first wafer WAF1 . The second array ARR2 may be a portion of the second wafer WAF2.

[0159] One of the arrays ARR1 , ARR2 may be flipped with respect to the other array. One of the wafers (WAF2) may be flipped with respect to the other wafer (WAF1 ). The mating surface SRF1 , SRF2 of the arrays ARR1 , ARR2 may be bonded together, so as to form an array ARR3 of Fabry-Perot interferometers FPI1 .

[0160] If desired, one or more individual Fabry-Perot interferometers FPI1 may be subsequently separated from the array ARR3 e.g. by cutting along one or more cutting lines CIIT1 , CIIT2.

[0161] If desired, one or more smaller arrays of Fabry-Perot interferometers FPI1 may be subsequently separated from the array ARR3 e.g. by cutting along one or more cutting lines CLIT1 , CLIT2.

[0162] The first modules MODI of the first array ARR1 may be different from the second modules MOD2 of the second array ARR2, e.g. as shown in Figs.1 b, 2b, 3b. The first modules MODI of the first array ARR1 may be identical to or similar to the second modules MOD2 of the second array ARR2, e.g. as shown in Fig. 4b.

[0163] For certain applications, it may be advantageous to keep the Fabry-Perot interferometers FPI1 joined to each other, so that the same substrates SLIB1 , SLIB2 extend over the whole array ARR3. In other words, it is not always necessary to separate individual Fabry-Perot interferometers FPI1 from the bonded combination. For example, a plurality of Fabry-Perot interferometers FPI1 may remain joined together and may be arranged in the rectangular array ARR3, which has Fabry-Perot interferometers FPI1 in M columns and in N rows. The number M of the columns may be e.g. in the range of 2 to 20, and the number M of the rows may be e.g. in the range of 1 to 20.

[0164] Referring to Fig. 6a, a first wafer WAF1 may comprise a plurality of first modules MODI , and a second wafer WAF2 may comprise a plurality of second modules MOD2. The first wafer WAF1 may comprise an array ARR1 of first modules MODI , and a second wafer WAF2 may comprise an array ARR2 of second modules MOD2. The mating surfaces SRF1 , SRF2 of the wafers WAF1 , WAF2 may be bonded together, so as to form a bonded combination CMB1 of the wafers WAF1 , WAF2. The mating surface SRF1 of each first module MODI may be bonded to the mating surface SRF2 of the corresponding second module MOD2. The second modules MOD2 may be positioned accurately respect to the first modules MODI before the bonding.

[0165] The mating surfaces SRF1 , SRF2 of the wafers WAF1 , WAF2 may be bonded together, so as to form a bonded combination CMB1 of the wafers WAF1 , WAF2. The mating surface SRF1 of each first module MODI may be bonded to the mating surface SRF2 of the corresponding second module MOD2. The second modules MOD2 may be positioned accurately respect to the first modules MODI before the bonding.

[0166] The method may comprise forming a first array ARR1 of modules MODI on a first wafer WAF1 , forming a second array ARR2 of modules MOD2 on a second wafer WAF2, and bonding the first wafer WAF1 to the second wafer WAF2. The first array ARR1 may comprise the first module MODI of a Fabry-Perot interferometer FPI1 , and the second array ARR2 may comprise the second module of the Fabry-Perot interferometer FPI1 .

[0167] The combination CMB1 may comprise e.g. modules described with reference to Figs. 1 a and 1 b, modules described with reference to Figs. 2a and 2b, modules described with reference to Figs. 3a to 3c, or modules described with reference to Figs. 4a to 4c.

[0168] A hermetic joint H1 may be formed at a plane defined by the mating surfaces SRF1 , SRF2. The materials of the mating surfaces SRF1 , SRF2 may be selected so as to form a hermetic joint H1 between the mating surfaces SRF1 , SRF2. The modules MODI , MOD2 of the arrays may be hermetically bonded together, so as to hermetically seal the space SPC1 from the environment ENV1. The hermetic joint H1 may surround the space SPC1. The bonding may form a hermetic joint H1 , which hermetically seals the space SPC1 between the first mirror M1 and the second mirror M2 from the environment ENV1 of the interferometer FPI1 .

[0169] Referring to Fig. 6b, the bonded combination CMB1 of the wafers WAF1 , WAF2 may comprise an array ARR3 of Fabry-Perot interferometers FPI1 . One or more Fabry-Perot interferometers FPI1 may be separated from the combination CMB1 by cutting the combination CMB1 . The combination CMB1 may be cut e.g. along one or more cutting lines CUT 1 , CUT2.

[0170] Bonding of the first wafer WAF1 to the second wafer WAF2 may e.g. ensure that the mirrors M1 , M2 are parallel with each other. Bonding of the first wafer WAF1 to the second wafer WAF2 may e.g. improve the accuracy of positioning the first modules MODI with respect to the second modules MOD2. Bonding of the first wafer WAF1 to the second wafer WAF2 may speed up production, as all interferometers FPI1 of the array ARR3 may be formed substantially simultaneously. A complete wafer WAF1 may be bonded to a complete wafer WAF2, without cutting the wafers before the bonding. There is no need to cut the wafers WAF1 , WAF2 before the bonding. Production of dust particles may be minimized or avoided before the wafers WAF1 , WAF2 are bonded together in a processing chamber. A risk of trapping harmful particles to the internal space SPC1 may be minimized. The bonded combination CMB1 may be cut along one or more cutting lines CLIT1 , CLIT2 after the bonding. The cutting may involve a risk of producing harmful particles. However, the internal space SPC1 may already by hermetically closed, and trapping of the particles to the internal space SPC1 may be avoided.

[0171] The bonded wafers WAF1 , WAF2 may comprise a plurality of hermetic joints H1 , which hermetically bond each first module MODI of the first array ARR1 to the corresponding second module MOD2 of the second array ARR2. The joints H1 may hermetically bond each first module MODI to the corresponding second module MOD2 also after the Fabry-Perot interferometers FPI1 of the array ARR3 have been separated from each other. A hermetic joint H1 may surround the internal space SPC1 of each Fabry-Perot interferometer FPI1 of the array ARR3. A hermetic joint H1 may surround the internal space SPC1 of each Fabry-Perot interferometer FPI1 , which has been separated from the array ARR3.

[0172] In an embodiment, an array ARR1 of first modules MODI , and an array ARR2 of second modules MOD2 may also be formed on the same wafer. A first portion of the wafer may comprise the first array ARR1 , and a second portion of the same wafer may comprise the second array ARR2. The second array ARR2 may be separated from the wafer by cutting, the arrays ARR1 , ARR2 may be cleaned (if needed), the second array ARR2 may be flipped, the second array ARR1 may be bonded to the first array ARR1 , and one or more interferometers FPI1 may be separated from the bonded combination. This embodiment may allow producing a plurality of interferometers FPI1 from the same wafer, e.g. such that the first modules MODI are identical to the second modules MOD2. The mirrors M1 , M2 may have similar optical properties. However, this embodiment may involve more work and / or may involve a higher risk of trapping particles to the internal space SPC1 , when compared with bonding a first complete wafer to a second complete wafer.

[0173] Two or more wafers WAF1 , WAF2 may be bonded together. The bonding may be selected e.g. from the following list: direct fusion bonding, anodic bonding, and adhesive bonding. The choice of bonding method may depend e.g. on the materials, which are bonded together, and on the intended operating conditions of the Fabry-Perot interferometer FPI1 . In general, the hermetic bonding forms a gas-tight joint between the mating surfaces SRF1 , SRF2. The hermetic bonding may be carried out e.g. such that the change of pressure pi of the internal space SPC1 during a time period of one year is lower than 10-2times the difference (po-pi) between the pressure po of the environment ENV1 and the pressure pi of the internal space SPC1 . The hermetic bonding may be carried out e.g. such that the absolute pressure pi of the space SPC1 remains lower than 1 kPa during the time period of 1 year, in a situation where the absolute pressure po of the environment ENV1 is equal to 100 kPa.

[0174] The first mirror M1 may have a first planar solid-gas interface and the second mirror M2 may have a second planar solid-gas interface. In case of a vacuum, the first mirror M1 may have a first planar solid-vacuum interface and the second mirror M2 may have a second planar solid-vacuum interface. The distance dM may refer to the adjustable distance between the first interface and the second interface. The distance dM between the mirrors M1 , M2 may be defined herein so that dM = 0 in a hypothetical situation where the mirror M1 would be mechanically in contact with the mirror M2. In other words, the zero distance may refer to the hypothetical situation where the mirrors would mechanically contact each other. The actual reflection of light may take place slightly below the solid-air interface of the mirror.

[0175] The substrate SLIB1 may be e.g. a silicon wafer, a fused silica wafer or a sapphire wafer (AI2O3). The substrate SLIB2 may be e.g. a silicon wafer, a fused silica wafer or a sapphire wafer (AI2O3). The substrate SLIB1 and / or SLIB2 may also be e.g. a coated substrate, i.e. the substrate may optionally comprise e.g. a stress buffer layer. The stress buffer layer may e.g. reduce internal stress between the bulk material of the substrate and the mirror M1 or M2. The modules MODI and / or MOD2 may optionally comprise an antireflection coating.

[0176] The semi-transparent reflectors of the mirrors M1 , M2 may be implemented by using dielectric multilayer coatings, or by metal layers. Dielectric multilayer coatings may provide low loss. Metallic reflectors may provide a wide spectral operating range. A metallic semi-transparent reflector of the mirror M1 and / or M2 may e.g. consist of a metal, e.g. silver or gold. The first electrode E1 and / or the second electrode E2 may have e.g. an annular shape, when viewed in the direction of the central axis AX1 , in order to maximize the pulling force generated by a given voltage difference V2-V2.

[0177] The first electrode E1 and / or the second electrode E2 and / or the conductive layer C1 and / or the conductive layer C2 may be formed e.g. by depositing additional conductive material on an underlying material layer.

[0178] The first electrode E1 and / or the second electrode E2 and / or the conductive layer C1 and / or the conductive layer C2 formed e.g. by doping a region of the underlying material layer such that the region becomes electrically conductive.

[0179] The first electrode E1 and / or the second electrode E2 and / or the conductive layer C1 and / or the conductive layer C2 may comprise or consist of a metal, e.g. silver or gold. A region of a metallic semi-transparent reflector may also operate as an electrode E1 , E2.

[0180] The first electrode E1 may be implemented by depositing conductive material on the substrate SLIB1 , or by doping a region of the substrate SLIB1 .

[0181] The second electrode E2 may be implemented by depositing conductive material on the second mirror M2 or by doping a region of the second mirror M2.

[0182] The first electrode E1 may be opaque, transparent or semi-transparent. The second electrode E2 may be opaque, transparent or semi-transparent. A transparent or semi-transparent electrode may or may not limit the width WARE of the optical aperture APE1 .

[0183] The first module MODI and / or the second module MOD2 may comprise one or more terminals T1 , T2 for coupling the actuating voltage V1 , V2 to the electrodes E1 , E2. The terminal T1 , T2 may optionally pass through the substrate SLIB1 , SUB2.

[0184] The first module MODI and / or the second module MOD2 may optionally comprise one or more antireflection layers. The optical aperture of the Fabry-Perot interferometer may be e.g. in the range of 0.5 mm to 6 mm. The width WFLEX of the moving portion of the movable mirror may be e.g. in the range of 0.5 mm to 10 mm.

[0185] The first substrate and / or the second substrate may have an opening, which allows light to be transmitted through the Fabry-Perot interferometer. In case of an opaque material, the first substrate and / or the second substrate may have an opening, which allows light to be transmitted through the Fabry-Perot interferometer. The material of the first substrate and / or the material of the first substrate may also be selected such that the material is optically transparent in the spectral operating range of the Fabry-Perot interferometer. In that case the first substrate and / or the second substrate do not need to comprise the opening.

[0186] WFLEX may denote the diameter of the movable portion of the second mirror M2. WARE may denote the diameter of the optical aperture of the Fabry-Perot interferometer FPI1. AX1 may denote a central axis of the Fabry-Perot interferometer FPI1. SX, SY, and SZ denote orthogonal directions. The mirrors M1 , M2 may be parallel with a plane defined by the directions SX and SY. The central axis AX1 may be parallel with the direction SZ. The central axis AX1 may be perpendicular to the mirrors M1 , M2. The movable portion of the mirror M2 may be substantially circular, when viewed in the direction of the axis AX1 .

[0187] Referring to Figs. 7a and 7b, a spectroscopic device 500 may comprise e.g. the Fabry-Perot interferometer FPI1 described with reference to Fig. 1 a, 2a, 3a, or 4a. The optical aperture of the Fabry-Perot interferometer may be e.g. in the range of 0.5 mm to 6 mm.

[0188] The device 500 may comprise a driving unit DU1 to provide a driving voltage signal HV1 for the electrodes E1 , E2 of the Fabry-Perot interferometer FPI1 . The driving unit DU1 may form a first voltage Vi for the first electrode E1 , and a second voltage V2 for the second electrode E2. The first voltage Vi may be coupled to the first electrode E1 e.g. via a first conductor CON1. The second voltage V2 may be coupled to the second electrode E2 e.g. via a second conductor CON2. The Fabry-Perot interferometer FPI1 may optionally comprise connection terminals T1 , T2 to facilitate coupling of the voltages Vi, V2 to the electrodes E1 , E2. The connection terminal T1 and / or T2 may be implemented e.g. as a through- silicon-via (TSV) or as a through-glass-via (TGV). The wafer WAF1 and / or WAF2 may also comprise pre-fabricated vias.

[0189] The module MODI and / or the module MOD2 may comprise a lateral connection terminal T1 , T2, which may be implement e.g. by locally removing material so that the conductive layer C1 , C2 is exposed. In other words, the conductive portion of the terminal T1 , T2 does not need to extend through the substrate SUB1 , SUB2.

[0190] Electrical connection to the conducive layer C1 may be provided e.g. via an opening OPE11 , which extends through the substrate SLIB1 or SLIB2 (Fig. 8c). A region of the conducive layer C1 may be arranged to operate as a connection terminal T1 . Electrical connection to the conducive layer C2 may be provided e.g. via an opening OPE12, which extends through the substrate SLIB1 or SLIB2. A region of the conducive layer C2 may be arranged to operate as a connection terminal T2.

[0191] The distance dM between the mirrors M1 , M2 may be changed by changing the voltage difference V2-V1. The driving unit DU1 may form the voltage signal HV1 according to a control signal SG. The control signal SG may be e.g. a digital signal or an analog signal. The device 500 may comprise a control unit CNT1 for providing the control signal SG. The control unit CNT 1 may be arranged to change the mirror gap dM by changing the control signal SG.

[0192] The distance between the mirrors may be changed e.g. by at least +90% or by at least -90%, while reducing or avoiding the risk of the pull-in effect.

[0193] The reduced risk of the pull-in effect may allow using a large movement of the second mirror M2. The control unit CNT1 may be arranged to change the mirror gap dM so that the absolute value of the difference between the mirror gap dM and the normal mirror gap dM.o is e.g. greater than 80% of the normal mirror gap dM,o. The control unit CNT1 may be arranged to change the mirror gap dM so that the minimum distance between the mirrors M1 , M2 is kept e.g. greater than 2% of the normal mirror gap dM,o. Referring to Fig. 7a, the spectroscopic device 500 may be e.g. a measuring device, which comprises the Fabry-Perot interferometer FPI1 and a detector DET1. The Fabry-Perot interferometer FPI1 may form transmitted light B2 by filtering input light B1. The detector DET1 may provide a detector signal SDETI , which may be indicative of the intensity of the transmitted light B2. The wavelengths of the passbands of the Fabry-Perot interferometer FPI1 may depend on the distance dM between the mirrors M1 , M2. The detector DET 1 may detect the intensity of the transmitted light B2. The detector SET1 may be e.g. an image sensor, wherein the device 500 may be an imaging spectrometer. The device 500 may optionally comprise optics LNS1 e.g. for focusing transmitted light B2 to the detector DET1 . The control unit CNT1 may be arranged to change the control signal SG, and control unit CNT1 may be arranged to record the detector signal SDETI as a function SDET-I(SG) of the control signal SG. The function SDET-I (SG) may represent a spectral intensity distribution of the input light B1 . The function SDET-I (SG) may be called e.g. as a spectrum. The control unit CNT1 may measure the function SDET-I(SG) by executing computer program code PROG1. The device 500 may comprise a memory MEM2 for storing the program code PROG1. The device 500 may comprise a memory MEM1 for storing the measured data SDET-I (SG). The device 500 may comprise a communication unit RXTX1 for sending and / or receiving data. For example, the communication unit RXTX1 may send measured data SDET-I (SG) via a communication network, e.g. via the Internet. The detector DET1 may also be a non-imaging detector, and the device 500 may be a non-imaging spectrometer.

[0194] Referring to Fig. 7b, the spectroscopic device 500 may be e.g. an adjustable spectral light source. The device 500 may comprise a light source LED1 to provide broadband input light B11. The light source LED1 may be e.g. a light emitting diode, an incandescent lamp, or a gas discharge lamp. The device 500 may optionally comprise optics LNS1 e.g. for collimating input light B11. The Fabry-Perot interferometer FPI1 may form transmitted light B12 by filtering input light B11 . The wavelengths of the passbands of the Fabry-Perot interferometer FPI1 may depend on the distance dM between the mirrors M1 , M2. The wavelength of the transmitted light B12 may be changed by changing the distance dM. The control unit CNT1 may be arranged to modulate the control signal SG e.g. as a function Sc(t) of time t. The intensity of the input light B11 may be optionally controlled by providing a signal SLEDI to the light source LED1. The light source LED1 may e.g. adjust the intensity of the input light B11 according to the signal SLEDI . The light source LED1 may e.g. enable or prevent emission of input light B11 according to the signal SLEDI . The control unit CNT1 may modulate the control signal SG by executing computer program code PROG2. The device 500 may comprise a memory MEM12 for storing the program code PROG2. The device 500 may comprise a memory MEM11 for storing the modulation function Sc(t). The device 500 may comprise a communication unit RXTX1 for sending and / or receiving data. For example, the communication unit RXTX1 may receive the modulation function Sc(t) via a communication network, e.g. via the Internet.

[0195] Referring to Fig. 8a, also a third module MOD3 may be bonded to the first module MODI or to the second module MOD2 of the Fabry-Perot interferometer FPI1. The third module MOD3 may comprise a substrate SLIB3. The substrate SLIB3 may be transparent in the spectral operating range of the Fabry-Perot interferometer FPI1. The substrate SLIB3 may be e.g. a silicon wafer, a fused silica wafer or a sapphire wafer (AI2O3). The third module MOD3 may further comprise a detector DET1. The third module MOD3 may be called e.g. as a detector module. A mating surface SRF4 of the detector module MOD3 may be bonded to a mating surface SRF3 of the module MODI or MOD2. The detector DET1 may detect light at one or more wavelengths, which are in the spectral operating range of the Fabry-Perot interferometer FPI1 . The detector DET1 may be an image sensor or a non-imaging detector. The image sensor may comprise an array of light-detecting pixels. The third module MOD3 may comprise the detector DET1 already before the third module MOD3 is bonded to the module MODI or MOD2. The detector module MOD3 may be hermetically bonded to the module MODI or MOD2.

[0196] The detector DET1 may be implemented e.g. on the inner side or on the outer side of the substrate SLIB3. One or more parts of the detector DET1 may optionally extend through the substrate SLIB3.

[0197] Electrical connection to the conductive layer C1 may be provided e.g. via an opening OPE11 of the substrate SLIB1 or SLIB2. A region of the conductive layer C1 may operate as a connection terminal T1 . Electrical connection to the conductive layer C2 may be provided e.g. via an opening OPE12 of the substrate SLIB1 or SLIB2. A region of the conductive layer C2 may operate as a connection terminal T2.

[0198] Referring to Fig. 8b, an array ARRD of detector modules MOD3 may be bonded to the array ARR1 or ARR2. A detector wafer WAF3 may comprise an array ARRD of detector modules MOD3. The detector wafer WAF3 may be bonded to the wafer WAF1 or WAF2. In particular, the detector wafer WAF3 may be hermetically bonded to the wafer WAF1 or WAF2.

[0199] Referring to Fig. 8c, the Fabry-Perot interferometer FPI1 may comprise the first module MODI , the second module MOD2, and the detector module MOD3. A first joint H1 may be formed by bonding a mating surface SRF1 of the first module MODI to a mating surface SRF2 of the second module MOD2. A second joint H2 may be formed by bonding a mating surface SRF3 of the module MOD2 (or MODI ) to a mating surface SRF4 of the detector module MOD2. The joint H1 and / or the joint H2 may be a hermetic joint.

[0200] The first module MODI may comprise a first internal space SPC1. The opening OPE2 of the second module MOD2 may define a second internal space SPC2 together with the detector module MOD3 and with the second mirror M2. The first space SPC1 may have a pressure pi. The second space SPC2 may have a pressure p2. The pressure pi and / or the pressure p2 may be e.g. smaller than 1 kPa. The first space SPC1 and the second space SPC2 may have a vacuum. Both sides of the movable mirror M2 may be arranged to operate in a vacuum.

[0201] Referring to Fig. 9, an electrode of the Fabry-Perot interferometer FPI1 may also be formed by ion implantation. For example, the electrode E2 of the movable mirror M2 may be formed by doping a region of a material layer of the mirror M2, so that said region becomes electrically conductive. Fig. 10a shows, by way of example, modules MODI , MOD2 for producing the Fabry-Perot interferometer FPI1 discussed with reference to Fig. 1 a. The electrode E1 and / or E2 may be formed by ion implantation. The electrodes E1 , E2 may be implemented so that they do not extend to the area of the optical aperture. The conductive layer C1 and / or C2 may be formed by ion implantation. For example, the mirror M2 may comprise an electrically conductive electrode region (E2), which has been formed by ion implantation. For example, the electrode E2 may be formed by doping one or more regions of an aluminum oxide layer of the mirror M2. The connection terminals T1 , T2 may be provided e.g. through the substrate SLIB2 of the second module MOD2.

[0202] Fig. 10b shows, by way of example, modules MODI , MOD2 for producing the Fabry-Perot interferometer FPI1 discussed with reference to Fig. 1 a. The electrode E1 and / or E2 may be formed by ion implantation. The electrodes E1 , E2 may be implemented so that they do not extend to the area of the optical aperture. The conductive layer C1 and / or C2 may be formed by ion implantation. For example, the mirror M2 may comprise an electrically conductive electrode region (E2), which has been formed by ion implantation. For example, the electrode E2 may be formed by doping one or more regions of an aluminum oxide layer of the mirror M2. The connection terminals T1 , T2 may be provided e.g. through the substrate SLIB1 of the first module MOD2.

[0203] The connection terminal T1 may be provided through the substrate SLIB1 or through the substrate SLIB2. The connection terminal T2 may be provided through the substrate SLIB1 or through the substrate SLIB2.

[0204] For the person skilled in the art, it will be clear that modifications and variations of the devices and methods according to the present invention are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

CLAIMS1 . A method for producing a Fabry-Perot interferometer (FP11 ) comprises:- forming a first module (MODI ), which comprises a first substrate (SLIB1 ) and a first actuating electrode (E1 ),- forming a second module (MOD2), which comprises a second substrate (SLIB2) and a second actuating electrode (E2),- bonding the first module (MODI ) to the second module (MOD2), wherein the Fabry-Perot interferometer (FPI1 ) comprises a first mirror (M1 ), wherein the second module (MOD2) comprises a second mirror (M2), wherein the second mirror (M2) is arranged to move together with the second electrode (E2), wherein the first actuating electrode (E1 ), and the second actuating electrode (E2) are arranged to change the distance (dM) between the mirrors (M1 , M2), wherein the second mirror (M2) is arranged to move together with the second electrode (E2) in a situation where the distance (dM) between the mirrors (M1 , M2) is changed, wherein dimensions of the modules (MODI , MOD2) are selected such that the normal distance (d ) between the first electrode (E1 ) and the second electrode (E2) is greater than or equal to 1 .5 times the normal distance (dM) between the first mirror (M1 ) and the second mirror (M2).

2. The method of claim 1 , comprising forming a first array (ARR1 ) of modules (MODI ) on a first wafer (WAF1 ), forming a second array (ARR2) of modules (MOD2) on a second wafer (WAF2), and bonding the first wafer (WAF1 ) to the second wafer (WAF2), wherein the first array (ARR1 ) comprises the first module (MODI ), and wherein the second array (ARR2) comprises the second module (MOD2).

3. The method of claim 1 or 2, wherein the bonding forms a joint (H1 ), which hermetically seals a space (SPC1 ) between the first mirror (M1 ) and the second mirror (M2) from an environment (ENV1 ) of the interferometer (FPI1 ).

4. The method according to any of the claims 1 to 3, wherein the normal distance (d ) between the first electrode (E1 ) and the second electrode (E2) is greater thanor equal to 3 times the normal distance (divi) between the first mirror (M1 ) and the second mirror (M2).

5. The method according to any of the claims 1 to 4, wherein the first module (MODI ) comprises the first mirror (M1 ).

6. The method of claim 5, wherein the first module (MODI ) comprises a central elevated portion (PED1 ), the first mirror (M1 ) is implemented on the central elevated portion (PED1 ), the central elevated portion (PED1 ) is surrounded by a recessed portion (REC1 ), and the first electrode (E1 ) is implemented on the recessed portion (REC1 ).

7. The method of claim 5, wherein the first module (MODI ) comprises an insulating spacer layer (132) between the first mirror (M1 ) and the first actuating electrode (E1 ).

8. The method according to any of the claims 1 to 7, wherein the first mirror (M1 ) and the second mirror (M2) are formed at the same time.

9. The method according to any of the claims 1 to 4, wherein the second module (MOD2) comprises the first mirror (M1 ) and the second mirror (M2), wherein the second mirror (M2) comprises a plurality of release holes (OP2), wherein the space (SPC1 ) between the mirrors (M1 ,M2) is formed by removing sacrificial material away via the release holes (OP2) before said bonding, wherein the first module (MODI ) comprises a recessed portion (REC1 ), and wherein the first electrode (E1 ) is implemented on the recessed portion (REC1 ).

10. The method according to any of the claims 1 to 9, comprising:- forming a first array (ARR1 ) of first modules (MODI ),- forming a second array (ARR2) of second modules (MOD2), and- bonding the first array (ARR1 ) to the second array (ARR2).11 . A Fabry-Perot interferometer (FPI1 ), comprising:- a first mirror (M1 ),- a second mirror (M2),- a first module (MODI ), and- a second module (MOD2), wherein the first module (MODI ) is bonded to the second module (MOD2), wherein the first module (MODI ) comprises a first actuating electrode (E1 ), wherein the second module (MOD2) comprises the second mirror (M2) and a second actuating electrode (E2), wherein the electrodes (E1 ,E2) are arranged to change the distance (divi) between the mirrors (M1 , M2), wherein the second mirror (M2) is arranged to move together with the second electrode (E2) in a situation where the distance (divi) between the mirrors (M1 , M2) is changed, and wherein the dimensions of the modules (M0D1 , MOD2) are selected such that the distance (d ) between the first electrode (E1 ) and the second electrode (E2) is greater than or equal to 1 .5 times a distance (divi) between the first mirror (M1 ) and the second mirror (M2).

12. A spectrometer device (500), comprising the Fabry-Perot interferometer (FPI1 ) of claim 11 to form filtered light (B2) by filtering input light (B1 ), a detector (DET1 ) to detect the filtered light (B2), and a control unit (CNT1 ) to change the mirror gap (dM) so that the absolute value of the difference between the mirror gap (di ) and the normal mirror gap (div.o) is greater than 80% of the normal mirror gap (dM,o).

13. A method for using the spectrometer device (500) of claim 12, comprising changing the mirror gap (div) so that the absolute value of the difference between the mirror gap (div) and the normal mirror gap (div.o) is greater than 80% of the normal mirror gap (di ,o).

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