Pyroelectric detection device with stressed suspended membrane

Abstract

Pyroelectric detection device, comprising at least: a suspended membrane; a pyroelectric detection element located on the suspended membrane and comprising at least one portion of pyroelectric material located between first and second electrodes, the first electrode being located between said at least one portion of pyroelectric material and the suspended membrane; and in which the membrane and the pyroelectric detection element are subjected to a higher compression stress than a limiting buckling stress of the suspended membrane and the pyroelectric detection element and together form a bistable structure.

Claims

1. Pyroelectric detection device, comprising at least: a suspended membrane; a pyroelectric detection element located on the suspended membrane and comprising at least one portion of pyroelectric material located between first and second electrodes, the first electrode being located between said at least one portion of pyroelectric material and the suspended membrane; and in which the suspended membrane and the pyroelectric detection element are subjected to a higher compression stress than a limiting buckling stress of the suspended membrane and the pyroelectric detection element and together form a bistable structure.

2. The pyroelectric detection device according to claim 1, wherein the suspended membrane comprises at least one of the following materials: SiO.sub.2, Si, SiN.

3. The pyroelectric detection device according to claim 1, also comprising a substrate in which at least one cavity is formed, the suspended membrane comprising edges fixed to the substrate and at least one suspended part located facing said at least one cavity.

4. The pyroelectric detection device according to claim 1, wherein the pyroelectric detection element comprises a black body comprising at least one of the second electrode and a portion of material absorbing infrared radiation located on the second electrode.

5. The pyroelectric detection device according to claim 4, wherein the material absorbing infrared radiation comprises at least one of the following materials: TiN, Ni—Cr, Ni, black metal such that platinum black or black gold.

6. The pyroelectric detection device according to claim 1, wherein the pyroelectric material corresponds to at least one of the following materials: PZT, AlN, KNN, NBT-BT, PMN-PT, LTO, LNO, PVDF.

7. The pyroelectric detection device according to claim 1, wherein the first electrode comprises platinum.

8. The pyroelectric detection device according to claim 1, wherein the second electrode comprises at least one of the following materials: Pt, Ru, Ir, TiW, Au, Ni, Ni—Cr, TiN.

9. Method of fabricating a pyroelectric detection device, comprising at least: fabrication of a suspended membrane; fabrication of a pyroelectric detection element located on the suspended membrane and comprising at least one portion of pyroelectric material located between first and second electrodes, the first electrode being located between said at least one portion of pyroelectric material and the suspended membrane; and in which the suspended membrane and the pyroelectric detection element are subjected to a higher compression stress than a limiting buckling stress of the suspended membrane and the pyroelectric detection element and together form a bistable structure.

10. The method according to claim 9, wherein the suspended membrane is obtained by making at least one layer of material stressed in compression that will form the suspended membrane on a substrate, then after making the pyroelectric detection element on said at least one layer of material, making at least one cavity in the substrate, releasing at least part of the suspended membrane that is suspended facing said at least one cavity.

11. The method according to claim 10, wherein said at least one layer of material stressed in compression is made by thermal oxidation of the substrate that comprises at least one semiconductor, and/or by deposition of SiO.sub.2 on the substrate.

12. The method according to claim 10, wherein fabrication of the pyroelectric detection element includes the following steps: fabrication of at least one first electrode layer on said at least one layer of material stressed in compression; fabrication of at least one layer of pyroelectric material on said at least one first electrode layer; fabrication of at least one second electrode layer on said at least one layer of pyroelectric material; structuring of each of said at least one first electrode layer and said at least one second electrode layer and of said at least one layer of pyroelectric material such that remaining portions of these layers form the pyroelectric detection element.

13. The method according to claim 12, also comprising a step to deposit at least one layer of material absorbing infrared radiation on said at least one second electrode layer between the step to deposit said at least one second electrode layer and the structuring step, and wherein the structuring step is also carried out for said at least one layer of material absorbing infrared radiation such that a remaining portion of said at least one layer of material absorbing infrared radiation located on the second electrode forms part of a black body of the pyroelectric detection element.

14. The method according to claim 12, wherein said at least one second electrode layer comprises a thickness and a material such that the second electrode forms part of the black body of the pyroelectric detection element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) This invention will be better understood after reading the description of example embodiments given purely for information and that is in no way limitative with reference to the appended drawings on which:

(2) FIG. 1 shows a pyroelectric detection device according to one particular embodiment,

(3) FIGS. 2 and 3 show measurements made by a profilometer of the surface of the pyroelectric detection device when the suspended membrane is in one of the two stable positions,

(4) FIGS. 4 and 5 show steps in a method of making a pyroelectric detection device according to one particular embodiment.

(5) Identical, similar or equivalent parts of the different figures described below have the same numeric references to facilitate comparison between the different figures.

(6) The different parts shown on the figures are not necessarily all at the same scale, to make the figures more easily understandable.

(7) The different possibilities (variants and embodiments) must be understood as not being mutually exclusive and can be combined with each other.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

(8) Refer to FIG. 1 that shows a pyroelectric detection device according to one particular embodiment.

(9) The device 100 comprises a substrate 102. The substrate 102 advantageously comprises a semiconductor, for example silicon.

(10) The device 100 also comprises a suspended membrane 104. In the particular embodiment described herein, the membrane 104 comprises one or several layers, comprising at least one of the following materials: SiO.sub.2, Si, SiN. The membrane 104 is qualified as suspended because it comprises edges 106 or ends, fixed to the substrate 102, and a free central part 108, in other words that is not in contact with the substrate 102, located facing a cavity 110 formed through the substrate 102.

(11) As a variant, it is possible that the membrane 104 is suspended from the substrate 102 through arms, for example comprising portions of material extending between specific parts of the membrane 104 and the fixed part of the substrate 102.

(12) The device 100 also comprises a pyroelectric detection element 112 placed on the membrane 104. This element 112 comprises: a lower electrode 114; a portion 116 of pyroelectric material; an upper electrode 118; a portion 120 of infrared radiation absorption material.

(13) The lower electrode 114 is located on the membrane 104. The portion 116 of pyroelectric material is located between the lower and upper electrodes 114, 118. The portion 120 of infrared radiation absorption material is located on the upper electrode 118.

(14) The lower electrode 114 advantageously comprises platinum, which facilitates growth of the pyroelectric material in portion 116. The upper electrode 118, 110 comprises for example at least one of the following materials: Pt, Ru, Ir, TiW, Au. Each of the lower 114 and upper 118 electrodes may for example be between 10 nm and 200 nm thick. Although it cannot be seen, an adhesion layer may be located between the membrane 104 and the lower electrode 114. This adhesion layer comprises for example TiO.sub.2 or any other material adapted such that the lower electrode 114 adheres well to the membrane 104, and for example is between about 2 nm and 40 nm thick.

(15) The portion 116 of pyroelectric material advantageously comprises PZT, but more generally may comprise at least one of the following materials: PZT, AlN, KNN, NBT-BT, PMN-PT, LTO, LNO, PVDF. The thickness of the portion 116 of pyroelectric material may for example be between about 50 nm and 2 μm.

(16) The portion 120 of absorbing material comprises at least one of the following materials: TiN and/or Ni—Cr and/or Ni and/or black metal (platinum black, black gold, etc.). The thickness of the portion 120 may for example be between about 1 nm and 5 μm.

(17) A compressive stress, or compression stress, is applied to the membrane 104, such that the assembly formed by the membrane 104 and the element 112 is subjected to a higher compressive stress than the limiting buckling stress of this assembly, that causes buckling of this assembly that forms a bistable structure, in other words it can be in one of two stable positions.

(18) A first stable position of this assembly 104+112 is a position in which this assembly 104+112 is curved, such that the central part 108 of the membrane 104 is outside the plane in which the edges 106 of the membrane 104 that are in contact with the substrate 102 are located, and projects from the side of the pyroelectric detection element 112. This first stable position is shown on FIG. 2 that corresponds to a measurement made by a profilometer of the surface of the device 100 located in this first stable position.

(19) A second stable position of the assembly 104+112 is a position in which this assembly 104+112 is curved, such that the central part 108 of the membrane 104 is outside the plane in which the edges 106 of the membrane 104 that are in contact with the substrate 102 are located, and projects from the side of the cavity 110. This second stable position is shown on FIG. 3 that corresponds to a measurement made by a profilometer of the surface of the device 100 located in this second stable position.

(20) To obtain buckling of the assembly formed by the membrane 104 and the pyroelectric detection element 112 and for it to form a bistable structure, the mechanical parameters of the layers forming this assembly are chosen in an appropriate manner to obtain this result. The following parameters are considered and are judiciously chosen: the dimensions and particularly the thickness of the different layers of the membrane 104 and the pyroelectric detection element 112; the value of the stress in the different layers of the membrane 104 and the pyroelectric detection element 112; Young's Modulus and Poisson's ratio of the materials in the different layers of the membrane 104 and the pyroelectric detection element 112;

(21) The limiting buckling stress σ.sub.buckling_limit for a circular single-layer membrane structure is given by the following formula:

(22) ${\sigma }_{\mathrm{limite}\_\mathrm{flambage}}=14.68.Math.\frac{D}{r^2.Math.t.Math.{10}^6}$

(23) In which

(24) $D=E.Math.\frac{t^3}{12.Math.\left(1-{\vartheta }^2\right)}$

(25) Where D is the bending stiffness, r is the radius of the membrane, t is the thickness of the membrane, E is Young's Modulus and ϑ is Poisson's ratio. For a multilayer structure, as is the case herein with the assembly formed from the membrane 104 and the pyroelectric detection element 112, its limiting buckling stress is calculated as a first approximation by considering this assembly as an equivalent single-layer structure with mechanical parameters (Young's modulus, Poisson's ratio, stress) calculated by the average weighted by the ratio of the thickness of each layer to the total thickness, namely as follows for n layers:

(26) $\mathrm{Eeq}=\frac{\underset{i=1}{\overset{n}{.Math.}}\mathrm{tiEi}}{\underset{i=1}{\overset{n}{.Math.}}\mathrm{ti}},$
the equivalent Young's modulus,

(27) $\mathrm{veq}=\frac{\underset{i=1}{\overset{n}{.Math.}}\mathrm{tivi}}{\underset{i=1}{\overset{n}{.Math.}}\mathrm{ti}},$
the equivalent Poisson's ratio,

(28) $\sigma \mathrm{eq}=\frac{\underset{i=1}{\overset{n}{.Math.}}\mathrm{ti}\sigma i}{\underset{i=1}{\overset{n}{.Math.}}\mathrm{ti}},$
the equivalent stress.

(29) For example, for a device 100 comprising: a suspended membrane 104 that is circular in shape, with a radius equal to 400 μm and formed by an elastic layer of SiO.sub.2 with thickness equal to 1 μm (E=70 GPa, vν=0.18, σ=−200 MPa), a lower electrode 114 comprising platinum with a thickness equal to 0.1 μm (E=180 GPa, ν=0.3, σ=600 MPa), a portion 116 made of PZT with a thickness equal to 0.5 μm (E=80 GPa, ν=0.39, σ=50 MPa), an upper electrode 118 comprising platinum with a thickness equal to 0.05 μm (E=180 GPa, ν=0.3, σ=100 MPa), and a portion 120 comprising TiN with a thickness equal to 0.05 μm (E=360 GPa, ν=0.3, σ=1000 MPa),

(30) the calculated limiting buckling stress (in compression) is equal to −2 MPa. However, since the equivalent stress of the stack is −35 MPa, therefore with an absolute value higher than the limiting buckling stress, the membrane 104 and the element 112 are buckled.

(31) The change from one of the two stable states of the membrane 104 to the other may be made by applying a relatively high voltage to the terminals of the first and second electrodes 114, 118, for example between about 10 V and 20 V and/or by applying a sufficiently high mechanical stress (therefore corresponding to a force higher than the force holding the membrane 104 in one of its two stable positions) on one of the main faces of the membrane 104 depending on the stable position in which the membrane 104 is to be positioned.

(32) An example of a method of fabricating the device 100 is described below with reference to FIGS. 4 and 5.

(33) As shown on FIG. 4, one (or several) layers 104 of material stressed in compression and that will form the suspended membrane is/are made on a front face 103 of the substrate 102. In the example embodiment described herein, the substrate 102 comprises silicon and the layer 104 comprises SiO.sub.2. According to a first example, the layer 104 may be made by thermal oxidation from the front face 103 of the substrate 102. According to a second example, the layer 104 may be formed by a deposition, for example a PECVD (plasma enhanced chemical vapour deposition), of SiO.sub.2, advantageously followed by a densification corresponding for example to annealing in an oven under oxygen, at a temperature equal for example to about 800° C. and for a duration equal to about 3 hours. The layer 104 thus made is in compression due to the state of compression stress inherent to SiO.sub.2.

(34) At least one first electrode layer 122 that will form the lower electrode 114 is then deposited on the layer 104. In the example embodiment described herein, the first electrode layer 122 comprises platinum. Advantageously, the deposition of this first electrode layer 122 is preceded by a deposition of a bond layer (not visible on FIG. 4) corresponding for example to a layer of TiO.sub.2 deposited on the layer 104, the first electrode layer 122 then being deposited on this bond layer.

(35) At least one layer 124 of pyroelectric material that will form the portion 116 of pyroelectric material is then deposited on the first electrode layer 122. This layer 124 is formed for example by a sol-gel type method or by cathodic sputtering or by pulsed laser ablation.

(36) At least one second electrode layer 126, for example comprising platinum, that will form the upper electrode 118 is then deposited on the layer 124.

(37) A layer 128 of material absorbing infrared radiation that will form the portion 120 is then deposited on the second electrode layer 126.

(38) Each of the layers 122, 124, 126 and 128 is then structured, for example by lithography, etching and stripping, such that the remaining portions 114, 116, 118 and 120 of these layers form the pyroelectric detection element 112 (see FIG. 5).

(39) The device 100 is completed by forming the cavity 110 from a back face 105 of the substrate 102, to release the central part 108 of the membrane 104. For example, this etching may correspond to deep reactive ion etching (DRIE). The device 100 obtained corresponds to that shown on FIG. 1.

(40) As an alternative, the layer(s) of the suspended membrane 104 may be discontinuous, i.e. holey and/or pierced.