NL2022390B1

NL2022390B1 - Monolithic semiconductor device for optical sensing.

Info

Publication number: NL2022390B1
Application number: NL2022390A
Authority: NL
Inventors: Gruca Grzegorz; Rijnveld Niek
Original assignee: Optics11
Priority date: 2019-01-14
Filing date: 2019-01-14
Publication date: 2020-08-14
Also published as: WO2020149739A1; NL2022390B9

Abstract

A monolithic semiconductor device (101) forms a Fabry-Perot sensor (503) when coupled to an optical fiber (501). The monolithic semiconductor device (101) comprises a cavity (108) that extends from a back side (105) to a front side (106) of the monolithic semiconductor device (101). The back side (105) of the monolithic semiconductor device (101) can be coupled to an end (504) of the optical fiber (501). The end (504) of the optical fiber (501) then seals off the cavity (108) in the monolithic semiconductor device (101) at the back side (105). The monolithic semiconductor device (101) comprises a light reflecting membrane (109) that seals off the cavity (108) in the monolithic semiconductor device (101) at the front side (106). The light reflecting membrane (109) is comprised of silicon nitride.

Description

Monolithic semiconductor device for optical sensing.

FIELD OF THE INVENTION An aspect of the invention relates to a monolithic semiconductor device adapted to form a Fabry-Perot sensor when coupled to an optical fiber. The Fabry-Perot sensor may be used, for example, for measuring a physical quantity, such as, for example, pressure. Further aspects of the invention relate to an optical sensor assembly, an optical sensor system, and a method of manufacturing a monolithic semiconductor device.

BACKGROUND ART Patent publication FR 2 676 539 discloses a membrane sensor comprising a cavity sealed by a membrane, and an optical fiber terminating inside the cavity. The membrane is made of semiconductor material, silicon or gallium-arsenide, and its manufacture is derived from semiconductor technologies. The walls of the cavity comprise the same material as the membrane and form a single article with the membrane. In preferred embodiments, the membrane is of mono-crystalline silicon, which provides advantages of a mechanical nature. The following applications are indicated: pressure sensor, charge sensor, temperature sensor, frost sensor.

SUMMARY OF THE INVENTION There is a need for an improved solution that allows cost-efficient and stable manufacture of a monolithic semiconductor device of the type concerned that has a relatively thin membrane, In accordance with an aspect of the invention as defined in claim 1, there is provided a monolithic semiconductor device adapted to form a Fabry-Perot sensor when coupled to an optical fiber, the monolithic semiconductor device comprising: - a cavity extending from a back side to a front side of the monolithic semiconductor device, the back side of the monolithic semiconductor device being adapted to be coupled to an end of the optical fiber so that the end of the optical fiber seals off the cavity in the monolithic semiconductor device at the back side and

- a light reflecting membrane that seals off the cavity in the monolithic semiconductor device at the front side, - wherein the light reflecting membrane is comprised of silicon nitride.

Such a monolithic semiconductor device can be manufactured in a cost- efficient and reproducible, thus stable manner. The light reflecting membrane of the monolithic semiconductor device may be relatively thin. Accordingly, the monolithic semiconductor device may form a Fabry-Perot sensor allowing relatively precise measurements, which, moreover, may exhibit relatively high sensitivity. These advantages are related to the light reflecting membrane being comprised of silicon nitride.

In an embodiment as defined in claim 2, the light reflecting membrane, which is comprised of silicon nitride, has a thickness comprised between 0.1 and 10 um.

In an embodiment as defined in claim 3, the monolithic semiconductor device has a length, which extends between the back side and the front side, comprised between 0.1 and 1 mm, In an embodiment as defined in claim 4, the cavity in the monolithic semiconductor device is cylindrical and has a diameter comprised between 0.05 and 0.5 mm In accordance with a further aspect of the invention as defined in claim 5, there is provided an optical sensor assembly comprising an optical fiber and a monolithic semiconductor device as defined hereinbefore that is coupled the optical fiber so as to form a Fabry-Perot sensor.

In an embodiment as defined in claim 6, the optical fiber comprises an additional Fabry-Perot sensor formed by at least one reflective structure in the optical fiber to which the monolithic semiconductor device is coupled.

In an embodiment as defined in claim 7, the additional Fabry-Perot sensor comprises two Bragg gratings in the optical fiber.

In an embodiment as defined in claim 8 the additional Fabry-Perot sensor is formed by a splice connection and a transversal end face of the optical fiber to which the monolithic semiconductor device is coupled.

In an embodiment as defined in claim 9, the additional Fabry-Perot sensor is formed by a local modification f of a core of the optical fiber and a transversal end face of the optical fiber to which the monolithic semiconductor device is coupled.

In an embodiment as defined in claim 10, the Fabry-Perot sensor formed by the monolithic semiconductor device coupled to the optical fiber on the one hand, and the additional Fabry-Perot sensor formed by the at least one reflective structure in the optical fiber, on the other hand, have a difference in nominal optical length of at least 10 um.

In an embodiment as defined in claim 11, the optical fiber is of the single mode type.

In accordance with a yet further aspect of the invention as defined in claim 12, there 1s provided an optical sensor system comprising at least one Fabry-Perot sensor formed by a monolithic semiconductor device as defined hereinbefore coupled to an optical fiber and an optical read-out arrangement adapted to measure an optical length in the Fabry-Perot sensor.

In an embodiment as defined in claim 13, the optical read-out arrangement comprises: - a spectral acquisition arrangement adapted to acquire successive spectral responses from the Fabry-Perot sensor during successive time intervals; and - a spectral analysis arrangement adapted to detect a periodicity in at least one of the successive spectral responses that have been acquired and to detect a phase evolution of the periodicity throughout the successive spectral responses.

In accordance with a yet further aspect of the invention as defined in claim 14, there is provided a method of manufacturing a monolithic semiconductor device as defined hereinbefore. the method comprising: - a silicon nitride depositing step in which a silicon nitride layer is deposited on at least a portion of at least one main face of a semiconductor substrate; and - a cavity forming step in which a main face of the semiconductor substrate is exposed to at least one etching substance so as to form a cavity that extends from the face of the semiconductor substrate that is exposed, to a silicon nitride layer that has been deposited on an opposite main face of the semiconductor substrate, the silicon nitride layer forming the light reflecting membrane.

In an embodiment as defined in claim 15, the semiconductor substrate is a silicon substrate that comprises two main faces that are at least partially covered by a silicon oxide layer and wherein, in the cavity forming step, the silicon oxide layer that covers the silicon nitride layer within the cavity is removed.

For the purpose of illustration, some embodiments of the invention are described in detail with reference to accompanying drawings. In this description, additional features will be presented and advantages will be apparent.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view of a monolithic semiconductor chip that incorporates a monolithic semiconductor device adapted to form a Fabry-Perot sensor when coupled to an optical fiber. FIG. 2 is a schematic perspective bottom view of the monolithic semiconductor chip. FIG. 3 is a bottom view photo of the monolithic semiconductor chip comprised in a wafer. FIG. 4 is a schematic perspective bottom view of a portion of a wafer comprising a plurality of monolithic semiconductor chips incorporating monolithic semiconductor devices adapted to be coupled to an optical fiber. FIG. 5 is a schematic cross-sectional diagram of a first exemplary optical sensor assembly comprising the monolithic semiconductor device of the semiconductor chip illustrated in FIGS. 1-3. FIG. 6 is a schematic cross-sectional diagram of a second exemplary optical sensor assembly comprising the monolithic semiconductor device of the semiconductor chip illustrated in FIGS. 1-3. FIG. 7 is a schematic cross-sectional diagram of a third exemplary optical sensor assembly comprising the monolithic semiconductor device of the semiconductor chip illustrated in FIGS. 1-3. FIG. 8 is a schematic cross-sectional diagram of a first alternative optical sensor assembly. FIG. 9 is a schematic cross-sectional diagram of a second alternative optical sensor assembly. FIG. 10 is a schematic cross-sectional diagram of a third alternative optical sensor assembly. FIG. 11 is a schematic block diagram of an optical sensor system comprising a plurality of Fabry-Perot sensors, any of which may be formed by a monolithic semiconductor device illustrated in FIGS. 1-3.

FIG. 12 is a cross-sectional diagram of a semiconductor substrate that will undergo a method of manufacturing a monolithic semiconductor device adapted to form a Fabry-Perot sensor when coupled to an optical fiber FIG. 13 is a cross-sectional diagram of a one step processed semiconductor 5 substrate that is obtained by applying an oxidation step to the silicon substrate.

FIG. 14 is a cross-sectional diagram of a two step processed silicon substrate that 1s obtained by applying a silicon nitride deposition step to the one step processed silicon substrate.

FIG. 15 is a cross-sectional diagram of a three step processed silicon substrate that is obtained by applying a photolithographic mask deposition step to the two step processed silicon substrate.

FIG. 16 is a cross-sectional diagram of a four step processed silicon substrate that is obtained by applying an etch step to the three step processed silicon substrate.

FIG. 17 is a cross-sectional diagram of a five step processed silicon substrate that is obtained by applying a photoresist removal step to the four step processed silicon substrate.

FIG. 18 is a cross-sectional diagram of a six step processed silicon substrate that 1s obtained by applying a silicon oxide removal step to the five step processed silicon substrate

DESCRIPTION OF SOME EMBODIMENTS FIG. 1 schematically illustrates a monolithic semiconductor chip 100 that incorporates a monolithic semiconductor device 101. FIG. 1 provides a schematic cross- sectional view of the monolithic semiconductor chip 100. FIG. 2 provides a schematic perspective bottom view of the monolithic semiconductor chip 100. The monolithic semiconductor device 101 incorporated therein can be coupled to an optical fiber so as to form a Fabry-Perot sensor. The monolithic semiconductor device 101 will therefore be referred hereinafter as Fabry-Perot device 101 for the sake of convenience.

The monolithic semiconductor chip 100 comprises a bulk substrate 102 in which the Fabry-Perot device 101 has been formed. The bulk substrate 102 may comprise silicon. The Fabry-Perot device 101 may be formed in this material. The Fabry-Perot device 101 is coupled to the bulk substrate 102 by means of a coupling membrane 103, which may comprise, for example, silicon nitride, the coupling membrane 103 allowsseparating the Fabry-Perot device 101 from the bulk substrate 102 in a relatively easy and secure manner. To that end, a technique such as, for example, laser ablation may be used.

In this embodiment, the Fabry-Perot device 101 has a cylindrical shape. The Fabry-Perot device 101 may have a diameter that is comprised between, for example, 0.2 and 0.3 mm, 0.25 mm being a suitable nominal diameter. The Fabry-Perot device 101 is contained in a cylindrical cavity 104 in the bulk substrate 102. This cavity 104 will hereinafter be referred to as device-containing cavity 104 for the sake of convenience. The device-containing cavity 104 may have a diameter that is slightly larger than that of the Fabry-Perot device 101. For example, the diameter of the device-containing cavity 104 in the bulk substrate 102 may be, for example, 0.27 mm in case the diameter of the Fabry- Perot device 101 is 0.25 mm.

The Fabry-Perot device 101 has a length, which extends between a back side 105 and a front side 106 as illustrated in FIG. 1. The length of the Fabry-Perot device 101 may be comprised between 0.1 and 1 millimeter (mm). More specifically, the length of the Fabry-Perot device 101 may be comprised between, for example, 0.2 mm and 0.4 mm,

0.3 mm being a suitable nominal length.

The Fabry-Perot device 101 essentially comprises a support structure 107 that encloses a cavity 108. The cavity 108 in the Fabry-Perot device 101 will hereinafter be referred to as elementary Fabry-Perot cavity 108 for the sake of convenience. The elementary Fabry-Perot cavity 108 essentially extends between the back side 105 and the front side 106 of the Fabry-Perot device 101. The elementary Fabry-Perot cavity 108 may have a cylindrical shape. The elementary Fabry-Perot cavity 108 may have a diameter comprised between 0.05 and 0.5 mm. More specifically, the diameter of the elementary Fabry-Perot cavity 108 may be comprised between, for example, 0.1 and 0.15 mm; 0/127 mm being a suitable nominal diameter.

The Fabry-Perot device 101 comprises a light-reflecting membrane 109 that seals off the elementary Fabry-Perot cavity 108 at the front side 106. The light-reflecting membrane 109 is essentially comprised of silicon nitride. The light-reflecting membrane 109 may entirely be comprised of silicon nitride, in particular for applications where light reflection that silicon nitride provides is sufficient. Alternatively, the light-reflecting membrane 109 may comprise a base membrane comprised of silicon nitride that is provided with a light reflecting coating on an inner side, which interfaces with the elementary Fabry-Perot cavity 108. Such a coating may comprise, for example, a composition of chromium and gold.

The light-reflecting membrane 109 may have a thickness comprised between 0.1 and 10 um.

More specifically, the thickness of the light-reflecting membrane 109 may be comprised between, for example, 0.5 and 2 um; 1 um being a suitable nominal thickness.

In case the light-reflecting membrane 109 comprises a coating, this coating may have a thickness in the order of, for example, tens of nanometers, FIG. 3 illustrates the monolithic semiconductor chip 100 comprised in a wafer 300. FIG. 3 is a photo of a back side 105 of the monolithic semiconductor chip 100, which includes the back side 105 of the Fabry-Perot device 101. The bulk substrate 102, the support structure 107, and the elementary Fabry-Perot cavity 108 are visible on the photo.

The photo shows that the wafer 300 comprises trenches 301 that surround the monolithic semiconductor chip 100 incorporating the Fabry-Perot device 101. These trenches 301 allow the monolithic semiconductor chip 100 to be separated from the wafer 300 in a relatively easy and secure manner.

FIG. 4 illustrates a portion of a wafer 400 comprising a plurality of monolithic semiconductor chips 401-416 incorporating Fabry-Perot devices.

FIG. 4 provides a schematic perspective bottom view of the portion of the wafer 400. The monolithic semiconductor chips 401-416 may each be similar to the monolithic semiconductor chip 100 presented hereinbefore with reference to FIGS. 1-3. FIG. 5 illustrates a first exemplary optical sensor assembly 500 comprising the Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3. FIG. 5 provides a schematic cross-sectional view of this exemplary optical sensor assembly 500. The first exemplary optical sensor assembly 500 will hereinafter be referred to as first optical sensor assembly 500 for the sake of convenience.

The first optical sensor assembly 500 comprises an optical fiber 501 to which the Fabry-Perot device 101 has been coupled.

More precisely, an end portion 502 of the optical fiber 501 is received in the elementary Fabry-Perot cavity 108 of the Fabry- Perot device 101 at the back side 105. The Fabry-Perot device 101 may be coupled to the optical fiber 501 by means of, for example, gluing.

To that end, an ultraviolet curable glue may be used, such as, for example, a glue designated by one of the following references: NOA81 and NOA 63. A cyanoacrylate glue or an epoxy glue may also be used.

Another technique for coupling the Fabry-Perot device 101 to the optical fiber 501 may involve glass solder preforms.

Yet another technique that may be used is anodic bonding.

The Fabry-Perot device 101and the optical fiber 501 jointly form a Fabry- Perot sensor 503 that has two light reflecting surfaces.

One light reflecting surface isprovided by the light-reflecting membrane 109 of the Fabry-Perot device 101, which seals off the elementary Fabry-Perot cavity 108 therein at the front side 106. The other light reflecting surface, which is partially light reflecting, is provided by an a transversal end face 504 of the optical fiber 501, which seals off the elementary Fabry-Perot cavity 108 in the Fabry-Perot device 101 at the back side 105. These two light reflecting surfaces provide Fabry-Perot interferometry, which depends on an optical path length between the two light reflecting surfaces. Light that travels through the optical fiber 501 and exits the transversal end face 504 of the optical fiber 501 undergoes this Fabry-Perot interferometry. A portion of the light that has undergone the Fabry-Perot interferometry will travel back through the optical fiber 501.

The Fabry-Perot sensor 503 may be used for measuring a physical quantity, such as, for example, pressure. The basic working is as follows. A physical quantity may cause a displacement of the light-reflecting membrane 109. This displacement will affect the optical path length between the light-reflecting membrane 109 and the transversal end face 504 of the optical fiber 501. Thus, the Fabry-Perot interferometry will vary as a function of the physical quantity. The Fabry-Perot interferometry, which conveys information on the physical quantity, can be measured on the basis of the light that has undergone this interferometry and that travels back through the optical fiber 501.

For example, a spectrum of the light that has undergone the Fabry-Perot interferometry may be measured. The optical path length in the Fabry-Perot sensor 503 spectrum can be derived from the spectrum. A physical quantity of interest can thus be measured on the basis of a known relationship between the optical path length and this physical quantity. WO2017077138A1 describes such a technique of measuring a physical quantity on the basis of a spectral response of a Fabry-Perot sensor 503.

The first optical sensor assembly 500 comprises an additional Fabry-Perot sensor 505. In this embodiment, the additional Fabry-Perot sensor S05 comprises two Bragg gratings 506, 507 in the optical fiber 501. The two Bragg gratings 506, 507 constitute two semi-reflective entities that provide Fabry-Perot interferometry. Here too, the Fabry-Perot interferometry depends on an optical path length between these two semi- reflective entities. Light that travels through the optical fiber 501 undergoes the Fabry- Perot interferometry of the additional Fabry-Perot sensor 505. A portion of the light that has undergone this Fabry-Perot interferometry will travel back through the optical fiber

501.

The additional Fabry-Perot sensor 505 may also be used for measuring a physical quantity, such as, for example, temperature. The basic working is the same as that described hereinbefore. The optical path length in the additional Fabry-Perot sensor 505 may vary as a function of a physical quantity to be measured, such as, for example, temperature. As explained hereinbefore, the optical path length can be derived from a spectrum of the light that has undergone the Fabry-Perot interferometry.

The additional Fabry-Perot sensor 505 may be used for measuring a physical quantity different from the physical quantity that is measured by means of the Fabry-Perot sensor 503 formed by the Fabry-Perot device 101 and the transversal end face 504 of the optical fiber 501. The latter Fabry-Perot sensor 503 will be referred to hereinafter as the primary Fabry-Perot sensor 503 for the sake of convenience. For example, the additional Fabry-Perot sensor 505 may be used for measuring temperature, whereas the primary Fabry-Perot sensor 503 may be used for measuring pressure. A temperature measurement by means of the additional Fabry-Perot sensor 505 may be used to compensate for a temperature dependency of a pressure measurement by means of the primary Fabry-Perot sensor 503.

The primary Fabry-Perot sensor 503 and the additional Fabry-Perot sensor 505 may have a difference in nominal optical path length of at least 10 pm. Such a difference allows the respective spectral responses of these respective Fabry-Perot sensors 503, 505 to be distinguished from each other relatively easily. The technique described in WO2017077138A1 can be used for making respective measurements based on respective spectral responses of respective Fabry-Perot sensors 503,505 in a single measurement system.

FIG. 6 illustrates a second exemplary optical sensor assembly 600 comprising the Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3. FIG. 6 provides a schematic cross-sectional view of this exemplary optical sensor assembly 600. The second exemplary optical sensor assembly 600 will hereinafter be referred to as second optical sensor assembly 600 for the sake of convenience.

The second optical sensor assembly 600 is similar to the first optical sensor assembly 500 in the following respects. The second optical sensor assembly 600 also comprises an optical fiber 601 to which the Fabry-Perot device 101 has been coupled. The Fabry-Perot device 101-and the optical fiber 601 jointly form a Fabry-Perot sensor 602, which may be used for measuring a physical quantity as described hereinbefore. Thesecond optical sensor assembly 600 also comprises an additional Fabry-Perot sensor 603, which may be used for measuring another physical quantity as described hereinbefore.

The additional Fabry-Perot sensor 603 in the second optical sensor assembly 600 is different from that in the first optical sensor assembly 500. The additional Fabry- Perot sensor 603 in the second optical sensor assembly 600 comprises a splice connection 604 in the vicinity of an end of the optical fiber 601 to which the Fabry-Perot device 101 is coupled.

The splice connection 604 constitutes a semi-reflective entity. A transversal end face 605 of the optical fiber 601 constitutes another semi-reflective entity. These two semi-reflective entities provide Fabry-Perot interferometry. The Fabry-Perot interferometry depends on an optical path length between the splice connection 604 and the transversal end face 605 of the optical fiber 601. Light that travels through the optical fiber 601 undergoes the Fabry-Perot interferometry of the additional Fabry-Perot sensor 603. A portion of the light that has undergone this Fabry-Perot interferometry will travel back through the optical fiber 601.

FIG. 7 illustrates a third exemplary optical sensor assembly 700 comprising the Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3. FIG. 5 provides a schematic cross-sectional view of this exemplary optical sensor assembly 700. The third exemplary optical sensor assembly 700 will hereinafter be referred to as third optical sensor assembly 700 for the sake of convenience.

The third optical sensor assembly 700 is also similar to the first optical sensor assembly 500 in the following respects. The third optical sensor assembly 700 also comprises an optical fiber 701 to which the Fabry-Perot device 101 has been coupled. The Fabry-Perot device 101-and the optical fiber 701 jointly form a Fabry-Perot sensor 702, which may be used for measuring a physical quantity as described hereinbefore. The third optical sensor assembly 700 also comprises an additional Fabry-Perot sensor 703, which may be used for measuring another physical quantity as described hereinbefore.

The additional Fabry-Perot sensor 703 in the third optical sensor assembly 700 is different from that in the first optical sensor assembly 500 and from that in the second optical sensor assembly 600. The additional Fabry-Perot sensor 703 in the third optical sensor assembly 700 comprises a local modification 704 of a core 705 of the optical fiber 701 in the vicinity of an end of the optical fiber 701 to which the Fabry-Perot device 101 1s coupled. The local modification 704 may have been made by means of, for example, laser ablation.

The local modification 704 of the core 705 of the optical fiber 701 constitutes a semi-reflective entity. A transversal end face 706 of the optical fiber 701 constitutes another semi-reflective entity. These two semi-reflective entities provide Fabry- Perot interferometry. The Fabry-Perot interferometry depends on an optical path length between the local modification 704 of the core 705 of the optical fiber 701 and the transversal end face 706 of the optical fiber 701. Light that travels through the optical fiber 701 undergoes the Fabry-Perot interferometry of the additional Fabry-Perot sensor 703. A portion of the light that has undergone this Fabry-Perot interferometry will travel back through the optical fiber 701.

The optical fiber 501, 601, 701 in the optical sensor assemblies 500, 600, 700 described hereinbefore may be of the single mode type. A single mode fiber typically provides stable and large signal-to-noise ratio. This is primarily because there is essentially no mode mixing noise in a single mode optical fiber, contrary to multi mode optical fibers. That is, a single mode fiber is essentially interference free. Accordingly, in case the optical fiber 501, 601, 701 in the optical sensor assemblies 500, 600, 700 described hereinbefore is of the single mode type, relatively high resolution and relatively high sensitivity measurements can be achieved by means of the primary Fabry-Perot sensor 503, 602, 702, as well as by means of the additional Fabry-Perot sensor 505, 603, 703.

In the optical sensor assemblies 500, 600, 700 described hereinbefore an end portion of the optical fiber 501, 601, 701 is received in a back portion the elementary Fabry-Perot cavity 108 in the Fabry-Perot device 101. That is, the end portion of the optical fiber 501, 601, 701 penetrates the back portion of the Fabry-Perot device 101. However, there are other manners in which the Fabry-Perot device 101 may be coupled to an optical fiber so as to form a Fabry-Perot sensor. Several alternative manners are presented in what follows.

FIG. 8 schematically illustrates a first alternative optical sensor assembly

800. FIG. 8 provides a schematic cross-sectional diagram of the first alternative optical sensor assembly 800. In the first alternative optical sensor assembly 800, an optical fiber 801 essentially entirely penetrates the elementary Fabry-Perot cavity 108 in the Fabry- Perot device 101 presented hereinbefore with reference to FIGS. 1-3. The optical fiber 801 has an end portion 802 that comprises a cavity 803. The cavity 803 may define a nominal optical path length of a Fabry-Perot sensor 804 formed by the Fabry-Perot device 101 and the optical fiber 801 received therein.

FIG. 9 schematically illustrates a second alternative optical sensor assembly

900. FIG. 9 provides a schematic cross-sectional diagram of the second alternative optical sensor assembly 900. In the second alternative optical sensor assembly 900, an optical fiber 901 does essentially not penetrate the elementary Fabry-Perot cavity 108 of the Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3 Rather, a transversal back face 902 of the Fabry-Perot device 101 contacts a transversal end face 903 of the optical fiber 903.

FIG. 10 schematically illustrates a third alternative optical sensor assembly

1000. FIG. 10 provides a schematic cross-sectional diagram of the third alternative optical sensor assembly 1000. The third alternative optical sensor assembly 1000 comprises an intermediate coupling device 1001, In this alternative assembly, the Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3 is coupled to one side of the intermediate coupling device 1001. An end of an optical fiber 1002 is coupled to an opposite side of the intermediate coupling device 1001. The third alternative optical sensor assembly 1000 allows forming a Fabry-Perot sensor that has a relatively long optical path length.

The various embodiments of an optical sensor assembly that have been presented hereinbefore have the following in common. The optical sensor assembly comprises a cavity that extends between, on the one hand, an end of an optical fiber to which the Fabry-Perot device 100 presented hereinbefore with reference to FIGS. 1-3 is coupled and, on the other hand, the light-reflecting membrane 109 of the Fabry-Perot device 100. This cavity constitutes an effective Fabry-Perot cavity of a Fabry-Perot sensor that is formed by the Fabry-Perot device 100 being coupled to the optical fiber.

The effective Fabry-Perot cavity at least partially includes the elementary Fabry-Perot cavity 108 of the Fabry-Perot device. In the first, second, and third optical sensor assemblies 500, 600, 700 illustrated in FIGS. 5, 6, and 7, respectively, the effective Fabry-Perot cavity corresponds with a portion of the elementary Fabry-Perot cavity 108. The same applies to the first alternative optical sensor assembly 800 illustrated in FIG. 8. In the second alternative optical sensor assembly 900, the effective Fabry-Perot cavity corresponds with the elementary Fabry-Perot cavity 108. In the third alternative optical sensor assembly 1000, the effective Fabry-Perot cavity includes the elementary Fabry- Perot cavity 108 in its entirety.

The effective Fabry-Perot cavity may comprise ambient air that was present when the optical sensor assembly was assembled. Alternatively, the effective Fabry-Perotcavity may comprise a particular gas, which may have a composition different from ambient air or may have a pressure different from that of ambient air, or both.

As another alternative, the effective Fabry-Perot cavity may essentially be void, which implies there is a vacuum inside the Fabry-Perot cavity.

The Fabry-Perot sensor that incorporates the effective Fabry-Perot cavity may exhibit a temperature dependency that depends on what is contained in the effective Fabry-Perot cavity.

For example, temperature dependency may be reduced when there is a vacuum inside the effective Fabry-Perot cavity.

In order to make that the effective Fabry-Perot cavity contains a particular gas, a step of coupling the Fabry-Perot device 100 to the optical fiber may be carried out in an environment that comprises that particular gas.

Likewise, a vacuum in the effective Fabry-Perot cavity can be achieved if the step of coupling the Fabry-Perot device 100 to the optical fiber is carried out in a vacuum environment.

Alternatively, a thermal process involving a getter causing chemisorption may be used.

In another technique, the optical fiber may be used as a piston to withdraw air from the elementary Fabry-Perot cavity 108 before coupling the Fabry-Perot device 100 to the optical fiber.

FIG. 11 illustrates optical sensor system 1100. FIG. 11 provides a schematic block diagram of the optical sensor system 1100. The optical sensor system 1100 comprises a plurality of Fabry-Perot sensors 1101-1109 that are coupled to an optical read- out arrangement 1110 through an optical fiber network 1111. The Fabry-Perot sensors 1101-1109 may have respective nominal optical path lengths that are different from each other.

For example, there may be a difference of at least of at least 10 pm between the respective nominal optical pop lengths.

Any one of the Fabry-Perot sensors 1101-1109 in the optical sensor system 1100 may be formed by a Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3 that is coupled to an end of an optical fiber in the optical fiber network 1111. Moreover, any one of the Fabry-Perot sensors 1101-1109 may form part of an optical sensor assembly, which may correspond to any one of the optical sensor assemblies 500, 600, 700 presented hereinbefore with reference to FIGS. 5 to 7. In that case, an additional Fabry-Perot sensor is associated with a primary Fabry-Perot sensor as described hereinbefore.

The optical read-out arrangement 1110 may be arranged and operate as described in WO2017077138A1. The optical read-out arrangement 1110 then comprises a spectral acquisition arrangement 1112 and a spectral analysis arrangement 1113 asillustrated in FIG. 11. In this embodiment, a measurement processor 1114 is coupled to the spectral analysis arrangement 1113.

In a basic mode of operation, the spectral acquisition arrangement 1112 may acquire a spectral response from any of the Fabry-Perot sensors 1101-1109 in the optical sensor system 1100. The spectral analysis arrangement 1113 may detect a periodicity in the spectral response that has been acquired. The periodicity in a spectral response is representative of an optical path length in the Fabry-Perot sensor concerned. The spectral analysis arrangement 1113 may thus derive the optical path length from the periodicity in the spectral response.

In an advanced mode of operation, the spectral acquisition arrangement 1112 may acquire successive spectral responses from a Fabry-Perot sensor during successive time intervals. The spectral analysis arrangement 1113 may then detect a phase evolution of the periodicity throughout the successive spectral responses. The phase evolution is representative of a variation in the optical path length in the Fabry-Perot sensor concerned. The spectral analysis arrangement 1113 may thus derive the variations in optical path length from the phase evolution of the periodicity in the spectral response.

In more detail, the spectral analysis arrangement 1113 may apply a transform of the time-to-frequency type to the successive spectral responses that have been acquired. Accordingly, an amplitude representation and a phase representation of the transform of a spectral response is obtained, The spectral analysis arrangement 1113 may then identify a location where a peak occurs in an amplitude representation, whereby the location is characteristic of the Fabry-Perot sensor concerned. The spectral analysis arrangement 1113 then extracts from successive phase representations, local phase data at a location corresponding to the location where the peak occurs. Accordingly, a series of successive local phase data is obtained that represents a local phase evolution in the successive phase representations.

By way of illustration, let it be assumed that the first optical sensor assembly 500 presented hereinbefore with reference to FIG. 5 forms part of the optical sensor system 1100 illustrated in FIG. 11. Any one of the Fabry-Perot sensors 1101-1109 may then correspond with the primary Fabry-Perot sensor 503 of the first optical sensor assembly 500. The additional Fabry-Perot sensor 505 is then extremely close to this one of the Fabry-Perot sensors 1101-1109. The additional Fabry-Perot sensor 505 may be considered to be present at a physical location in the sensor system 1100 that corresponds with that of the primary Fabry-Perot sensor 503. This location will hereinafter be referredto as measurement location for the sake of convenience. A pressure and a temperature exist at the measurement location.

In this example, the optical read-out arrangement 1110 may then provide a primary read-out result and an additional read-out result for the measurement location. The primary read-out result may correspond with the optical path length in the primary Fabry- Perot sensor 503. The additional read-out result may correspond with the optical path length in the additional Fabry-Perot sensor 505. The primary read-out result may be indicative of the pressure at the measurement location. The additional read-out result may be indicative of the temperature at the measurement location.

The measurement processor 1114 may provide a pressure measurement result for the measurement location on the basis of the primary read-out result and the additional read-out result. The primary read-out result may not solely depend on the pressure at the measurement location, but may also depend on the temperature at that location. The measurement processor 1114 may then compensate for this temperature dependency of the primary Fabry-Perot sensor 503 on the basis of the additional read-out result from the additional Fabry-Perot sensor 505, which is indicative of the temperature at the measurement location. Accordingly, the pressure measurement result may be essentially independent of the temperature at the measurement location and may thus accurately indicate the pressure at this location.

There are various manners in which the measurement processor 1114 may implement a temperature compensation as described hereinbefore. For example, the measurement processor 1114 may comprise compensation data that specifies respective corrections to be applied to the primary read-out result for respective values of the additional read-out result. The respective values of the additional read-out result correspond with respective temperatures. The measurement processor 1114 may then apply a correction to the primary read-out result that is derived from the aforementioned compensation data on the basis of the additional read-out result. To that end, the measurement processor 1114 may make an interpolation between two respective corrections specified for two respective values of the additional read-out result in case the read-out result provided by the read-out arrangement 1110 is in between these two aforementioned respective values.

FIGS. 12-18 schematically illustrate a semiconductor substrate at various successive stages in a method of manufacturing a Fabry-Perot device 101 as presented hereinbefore with reference to FIGS. 1-3. FIGS. 12-18 each provide a schematic cross-

sectional diagram of the semiconductor substrate at a particular stage in the method of manufacturing.

T FIG. 12 schematically illustrates the semiconductor substrate 1200 that will undergo the method of manufacturing the Fabry-Perot device 101. The semiconductor substrate 1200 is thus at an initial stage in the method.

The semiconductor substrate 1200 may essentially be comprised of silicon and will therefore hereinafter be referred to as silicon substrate 1200 by way of illustration.

The silicon substrate 1200 may have a thickness of, for example, 300 um, which extends between two main faces 1201, 1202. One main face 1201 of the silicon substrate 1200 will hereinafter be referred to as upper main face 1201 for the sake of convenience.

The other main face will hereinafter be referred to as lower main face 1202. FIG. 13 schematically illustrates a one step processed silicon substrate 1300 that is obtained by applying an oxidation step to the silicon substrate 1200 illustrated in FIG. 12. In the oxidation step, two silicon oxide layers 1301, 1302 are formed on the silicon substrate 1200. A silicon oxide layer 1301 is formed on the upper main face 1201 of the silicon substrate 1200. A silicon oxide layer 1302 is also formed on the lower main face 1202. The two silicon oxide layers 1301, 1302 on the upper main face 1201 and on the lower main face 1202 may each have a thickness of, for example, 2 um.

FIG. 14 schematically illustrates a two step processed silicon substrate 1400 that is obtained by applying a silicon nitride deposition step to the one step processed silicon substrate 1300 illustrated in FIG. 13. In the silicon nitride deposition step, two silicon nitride layers 1401, 1402 are formed on the one step processed silicon substrate 1300 illustrated in FIG. 13. A silicon nitride layer 1401 is formed on top of the silicon oxide layer 1301 on the upper main face 1201 of the silicon substrate 1200. A silicon nitride layer 1402 is also formed on top of the silicon oxide layer 1302 on the lower main face 1202 of the silicon substrate 1200. The two silicon nitride layers 1401, 1402 on the upper main face 1201 and on the lower main face 1202 may each have a thickness of, for example, 1100 nm.

FIG. 15 schematically illustrates a three step processed silicon substrate 1500 that is obtained by applying a photolithographic mask deposition step to the two step processed silicon substrate 1400 illustrated in FIG. 14. In the photolithographic mask deposition step, a photolithographic mask 1501 is formed on top of the silicon nitride layer 1401 on the upper main face 1201 of the silicon substrate 1200. The photolithographic mask 1501 may be formed by means of, for example, a conventional technique insemiconductor manufacturing. The photolithographic mask 1501 comprises openings 1502, 1503 that define exposed areas on the silicon nitride layer 1401 on the upper main face 1201 of the silicon substrate 1200. An exposed area is thus an area that is not covered by lithographic mask material.

FIG. 16 schematically illustrates a four step processed silicon substrate 1600 that is obtained by applying an etch step to the three step processed silicon substrate 1500 illustrated in FIG. 15. In the etch step, two cavities 1601, 1602 are formed in the three step processed silicon substrate 1500 illustrated in FIG. 15: a cylindrical center cavity 1601 and a circular trench cavity 1602. These cavities 1601, 1602 are locally formed in the exposed areas where the openings 1502, 1503 in the lithographic mask 1501 are present. The cavities 1601, 1602 may be formed by means of, for example, a dry etching technique. The cavities 1601, 1602 extend from the upper main face 1201 of the silicon substrate 1200 to the silicon oxide layer 1302 on the lower main face 1202 as illustrated in FIG. 16. At this stage, the cavities 1601, 1602 therefore each have a bottom that is formed by a portion of the silicon oxide layer 1302 and a portion of the silicon nitride layer 1402 on the lower main face 1202 of the silicon substrate 1200.

FIG. 17 schematically illustrates a five step processed silicon substrate 1700 that is obtained by applying a photoresist removal step to the four step processed silicon substrate 1600 illustrated in FIG. 16. In the photoresist removal step, the photolithographic mask 1501 is removed by means of, for example, a chemical composition that constitutes a dissolvent for the photolithographic mask material.

FIG. 18 schematically illustrates a six step processed silicon substrate 1800 that is obtained by applying a silicon oxide removal step to the five step processed silicon substrate 1700 illustrated in FIG. 17. In the silicon oxide removal step, the portion of the silicon oxide layer 1302 that is comprised in the bottom of the cylindrical center cavity 1601 is removed. The portion of the silicon oxide layer 1302 that is comprised in the bottom of the circular trench cavity 1602 is removed too. These portions of the silicon oxide layer 1302 are exposed so that, for example, a wet isotropic process may remove these portions. The portions of the silicon nitride layer 1402 that are comprised in the bottom of the cylindrical center cavity 1601 and in the circular trench cavity 1602 remain, although these portions may become slightly thinner, such as, for example, 100 nm thinner. Accordingly, at this stage, the bottom of the cylindrical center cavity 1601 and the bottom of the circular trench cavity 1602 are essentially comprised of silicon nitride.

The six step processed silicon substrate 1800 illustrated in FIG. 18 may correspond with the monolithic semiconductor chip 100 presented hereinbefore with reference to FIGS. 1-3, although proportions in size are represented differently. The circular trench cavity 1602 in the six step processed silicon substrate 1800 illustrated in FIG. 18 may correspond with the device-containing cavity 104 in the bulk of substrate of the monolithic semiconductor chip 100 presented hereinbefore with reference to FIGS. 1-3. A portion of the six step processed silicon substrate 1800 illustrated in FIG. 18 that is comprised within the circular trench cavity 1602 may constitute thus a Fabry-Perot device corresponding with the Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3. The cylindrical center cavity 1601 in the six step processed silicon substrate 1800 illustrated in FIG. 18 may correspond with the elementary Fabry-Perot cavity 108 in the Fabry-Perot device 101 presented hereinbefore with reference to FIGS. 1-3.

The method described hereinbefore may be applied to a silicon wafer so as to manufacture a plurality of monolithic semiconductor chips on the silicon wafer, as illustrated in FIG. 4, which comprise a plurality of Fabry-Perot devices. The method may comprise a wafer cutting step subsequent to the various steps described hereinbefore. In the wafer cutting step, the silicon wafer is cut so to separate the monolithic semiconductor devices from each other.

The method described hereinbefore may comprise an additional coating step, which may be subsequent to the silicon oxide removal step that provides the six step processed silicon substrate 1800 illustrated in FIG. 18. In the additional coating step, a light reflecting coating may be deposited on an inner side of the bottom of the cylindrical center cavity 1601. The light reflecting coating may be deposited by means of, for example, sputtering in a vacuum bell jar.

NOTES The embodiments described hereinbefore with reference to the drawings are presented by way of illustration. The invention may be implemented in numerous different ways. In order to illustrate this, some alternatives are briefly indicated.

The invention may be applied in numerous types of products or methods related to optical sensing. For example, the invention may be applied in any type of optical sensing that involves a Fabry-Perot sensor based on a monolithic semiconductor device coupled to an optical fiber. The invention may be applied in any type of domain, such as, for example, structure monitoring, energy industry, surveillance, seismic applications, oiland gas industry, and metrology.

Acoustic transducer applications may include fiber optic geophones or hydrophones.

There are numerous different ways of implementing an optical sensor system in accordance with the invention.

For example, an optical sensor system may comprise a single Fabry-Perot sensor only.

An optical sensor system may equally comprise a relatively large number of Fabry-Perot sensors, such as, for example, tens of Fabry-Perot sensors, which can be practically feasible, in particular when a measurement technique as described in WO2017077138A1 is used.

In general, there are numerous different ways of implementing the invention, whereby different implementations may have different topologies.

In any given topology, a single entity may carry out several functions, or several entities may jointly carry out a single function.

In this respect, the drawings are very diagrammatic.

The remarks made hereinbefore demonstrate that the embodiments described with reference to the drawings illustrate the invention, rather than limit the invention.

The invention can be implemented in numerous alternative ways that are within the scope of the appended claims.

All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Any reference sign in a claim should not be construed as limiting the claim.

The verb “comprise” in a claim does not exclude the presence of other elements or other steps than those listed in the claim.

The same applies to similar verbs such as “include” and “contain”. The mention of an element in singular in a claim pertaining to a product, does not exclude that the product may comprise a plurality of such elements.

Likewise, the mention of a step in singular in a claim pertaining to a method does not exclude that the method may comprise a plurality of such steps.

The mere fact that respective dependent claims define respective additional features, does not exclude combinations of additional features other than those reflected in the claims.

Claims

CONCLUSIONS:

A monolithic semiconductor device (101) adapted to form a Fabry-Perot sensor (503) when coupled to an optical fiber (501), the monolithic semiconductor device comprising: - a cavity (108) extending from a back side ( 105) to a front side (106) of the monolith semiconductor device, the back side of the monolith semiconductor device being adapted to be coupled to an end (504) of the optical fiber so that the end of the optical fiber meets the cavity in the monolith semiconductor device. the back and seal; and - a light reflecting membrane (109) sealing the cavity in the front monolithic semiconductor device, - the light reflecting membrane comprising silicon nitride.

A monolithic semiconductor device according to claim 1, wherein the light reflective membrane (109) consisting of silicone nitride has a thickness of between 0.1 and 10 µm.

A monolithic semiconductor device according to any one of claims 1 and 2, wherein the monolithic semiconductor device has a length that extends between the back (105) and the front (106) of between 0.1 and 1 mm.

A monolith semiconductor device according to any one of claims 1 to 3, wherein the cavity (109) in the monolith semiconductor device is cylindrical and has a diameter ranging from 0.05 to 0.5 mm.

An optical sensor assembly (500, 600, 700) comprising an optical fiber (501, 601, 701) and a monolithic semiconductor device (101) according to any one of claims 1 to 4 coupled the optical fiber to a Fabry-Perot sensor (503, 602, 702).

An optical sensor assembly according to any one of claim 5, wherein the optical fiber (501, 601, 701) comprises an additional Fabry-Perot sensor (505, 603, 703) formed by at least one reflective structure (506, 604, 704) in the optical fiber (501, 601, 701) to which the monolithic semiconductor device (101) is coupled.

An optical sensor assembly according to claim 6, wherein the additional Fabry-Perot sensor (505) comprises two Bragg grids (506, 507) in the optical fiber (501).

An optical sensor assembly according to claim 6, wherein the additional Fabry-Perot sensor (603) is formed by a splicing junction (604) and a transverse end face (605) of the optical fiber (601) to which the monolithic semiconductor device (101) is coupled.

An optical sensor unit according to claim 6, wherein the additional Fabry-Perot sensor (703) is formed by a local modification (704) of a core (705) of the optical fiber (701) and a transverse end face (706) of the optical fiber (701) to which the monolithic semiconductor device (101) is coupled.

An optical sensor assembly according to any one of claims 5 to 9, wherein the Fabry-Perot sensor formed by the monolithic semiconductor device (101) coupled to the optical fiber (501, 601, 701), on the one hand, and the additional Fabry-Perot sensor (505, 603, 703) formed by the at least one reflective structure (506, 604, 704) in the optical fiber (501, 601, 701), on the other hand, have a nominal optical length difference of at least 10 um.

An optical sensor assembly according to any one of claims 5 or 10, wherein the optical fiber (501, 601, 701) is of the single mode type.

An optical sensor system (1100) comprising at least one Fabry-Perot sensor (1101-1109) formed by a monolithic semiconductor device (101) according to any one of claims 1 to 4 coupled to an optical fiber (1111) and an optical readout device ( 1110) adapted to measure an optical length in the Fabry-Perot sensor.

An optical sensor system according to claim 12, wherein the optical readout device (1110) comprises: - a spectral acquisition device (1112) adapted to acquire consecutive spectral responses from the Fabry-Perot sensor at consecutive time intervals; and - a spectral analyzer (1113) adapted to detect a periodicity in at least one of the consecutive spectral responses obtained and to detect a phase evolution of the periodicity during the consecutive spectral responses.

A method of manufacturing a monolithic semiconductor device (101) according to any one of claims 1 to 4, the method comprising: - a silicon nitride deposition step in which a silicon nitride layer (1401, 1402) is deposited on at least a portion of at least one major face (1201, 1202) of a semiconductor substrate; (1200); and - a cavity forming step in which a major face (1201) of the semiconductor substrate is exposed to at least one etching substance to form a cavity (1601) extending from the surface of the semiconductor substrate exposed to a silicon nitride layer (1302) which is deposited on an opposite major face (1202) of the semiconductor substrate, the silicon nitride layer forming the light reflecting membrane (109).

The method of manufacturing a monolithic semiconductor device according to claim 14, wherein the semiconductor substrate (1200) is a silicon substrate comprising two major faces (1201, 1202) that are at least partially covered by a silicon oxide layer (1301, 1302) and wherein, in the cavity forming step, the silicon oxide layer (1302) covering the silicon nitride layer (1402) within the cavity (1601) is removed.