MEMS element, method for manufacturing MEMS element, and method for controlling rigidity of MEMS element

Abstract

To provide a method for controlling rigidity of an MEMS element by using a material which has a high compression strength and can control initial stress in a thin film state, and the MEMS element whose rigidity is controlled in such a manner, and a method for manufacturing the same.SOLUTION: An MEMS element comprises: a first thin film comprising a nitride or a carbonitride which contains at least one kind of transition metal M1; a second thin film comprising a nitride or a carbonitride which contains at least one kind of transition metal M2; and a suspended structure pinched therebetween. A method for manufacturing the MEMS element includes steps of: preparing a substrate for the MEMS element comprising the suspended structure; forming the first thin film on one surface of the suspended structure; and forming the second thin film on the other surface of the suspended structure. The first thin film and the second thin film may have the same or different composition.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to a MEMS element, a manufacturing method for a MEMS element, and a method for controlling the rigidity of a MEMS element, and in particular to a MEMS element whose rigidity is controlled by the initial stress of a thin film made of a nitride or carbonitride containing at least one type of transition metal, a manufacturing method for such a MEMS element, and a method for controlling the rigidity of a MEMS element by the initial stress of the thin film. [Background technology]

[0002] In microelectromechanical systems (MEMS), the rigidity of a structure is one of the main factors that affect the performance of the device. Here, when referring to the rigidity of a MEMS element (also called a MEMS device), it can mean the rigidity of the entire element (device) in a broad sense, but in this specification, it is understood that the rigidity of the structure constituting the element (device) is intended depending on the context, and more specifically, the rigidity of the structure constituting the part (main part) necessary for the element (device) to exhibit the main function is intended.

[0003] The stiffness of a MEMS device can be controlled by introducing local stresses: tensile stress stiffens the structure, while compressive stress softens it. Such local stresses can be introduced by active or passive methods, including:

[0004] Active methods include methods that generate internal stress in a structure through various physical mechanisms such as electrostatic, electrothermal, and piezoelectric (see Non-Patent Document 1). These methods require sophisticated circuit and structural design to control the stress, so their scope of application is narrow and the costs of design and manufacturing are high.

[0005] Passive methods include a method in which a structure is made of a thin film or a thin film is applied to the structure, and the prestress of the thin film is used to generate local stress in the structure. For example, thin film materials such as silicon dioxide to which compressive stress is applied are used to fabricate buckled beams and membranes (see Non-Patent Documents 2 and 3). Such a method using a thin film prestress is a relatively inexpensive and widely applicable stiffness control method. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] MLC de Laat et al., A review on in situ stiffness adjustment methods in MEMS, J. Micromech. Microeng., 26(2016) 063001. [Non-Patent Document 2] Ruize Xu et al., Buckled MEMS Beams for Energy Harvesting from Low Frequency Vibrations, Research, Vol. 2019, Article ID: 1087946. [Non-Patent Document 3] B. Haelg, On a nonvolatile memory cell based on micro-electro-mechanics, IEEE Proc. 1990, 172. Summary of the Invention [Problem to be solved by the invention]

[0007] However, the conventional approach using the thin film prestress mentioned above has a major weakness in that it is difficult to adjust the stiffness of the target structure. First, conventionally used thin film materials have low mechanical strength and cannot withstand the high stress required for efficient stiffness control. Second, there is no effective protocol for controlling the prestress of conventionally used thin film materials in established deposition processes.

[0008] An object of the present invention is to provide a method for controlling the stiffness of a MEMS element by using a material that has high compressive strength and whose initial stress can be controlled in a thin film state, as well as a MEMS element having stiffness controlled in this manner and a method for manufacturing the same. [Means for solving the problem]

[0009] The features of the present invention for achieving the above object are as follows.

[0010] [1] A MEMS element comprising a first thin film made of a nitride or carbonitride containing at least one type of transition metal M1, a second thin film made of a nitride or carbonitride containing at least one type of transition metal M2, and a suspended structure sandwiched between the first thin film and the second thin film, wherein the composition of the nitride or carbonitride constituting the first thin film is the same as or different from the composition of the nitride or carbonitride constituting the second thin film. [2] The MEMS element according to [1], wherein the initial stress of the first thin film is the same as that of the second thin film. [3] The MEMS element according to [1], wherein the initial stress of the first thin film is smaller than that of the second thin film. [4] The MEMS element according to [1], wherein the initial stress of the first thin film is greater than that of the second thin film. [5] The MEMS element according to any one of [1] to [4], characterized in that the first thin film and the second thin film are each a thin film made of a nitride or carbonitride containing one type of transition metal, a nitride containing an alloy of one type of transition metal and a group II element or a group III element, or a nitride containing an alloy of two or more types of transition metals, or a laminated film made by laminating two or more types of thin films thereof. [6] The MEMS element according to any one of [1] to [5], characterized in that the first thin film and the second thin film are each made of a nitride or carbonitride containing one type of transition metal selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), copper (Cu), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), erbium (Er), uranium (U), and plutonium (Pu). [7] The MEMS element described in [6], wherein the first thin film is made of titanium nitride (TiN), the second thin film is made of titanium nitride (TiN), and the stiffness of the MEMS element is within the range of -1000 N / m to 1000 N / m. [8] The MEMS element according to [7], wherein the stiffness of the MEMS element is substantially zero. [9] The MEMS element according to [7], wherein the suspended structure is in a buckled state.

[10] The MEMS element according to any one of [1] to [9], further comprising a sensitive film on the first thin film.

[11] The MEMS element according to

[10] , further comprising a sensitive film on the second thin film.

[12] A method for controlling the rigidity of a MEMS element, comprising: preparing a substrate for a MEMS element having a suspended structure; measuring the rigidity of the suspended structure; forming a first thin film made of a nitride or carbonitride containing at least one type of transition metal M1 on one side of the suspended structure; forming a second thin film made of a nitride or carbonitride containing at least one type of transition metal M2 on the other side of the suspended structure; measuring the initial stresses of the first thin film and the second thin film; and measuring the rigidity of a suspended structure comprising the first thin film and the second thin film, wherein the composition of the nitride or carbonitride constituting the first thin film and the composition of the nitride or carbonitride constituting the second thin film are the same or different.

[13] The method according to

[12] , wherein the first thin film and the second thin film are formed by magnetron sputtering.

[14] The method according to

[12] or

[13] , further comprising performing a pre-processing step before or after preparing the substrate for the MEMS element, the pre-processing step comprising: preparing a substrate made of the same material as the suspended structure; forming the first thin film or the second thin film on the substrate under one or more film formation conditions; measuring the initial stresses of the first thin film and the second thin film; and expressing the relationship between the rigidity of the suspended structure and the initial stresses of the first thin film and the second thin film in a fixed form from the obtained results.

[15] The method according to any one of

[12] to

[14] , characterized in that the first thin film and the second thin film are each a thin film made of a nitride or carbonitride containing one type of transition metal, a nitride containing an alloy of one type of transition metal and a group II element or a group III element, or a nitride containing an alloy of two or more types of transition metals, or a laminated film made of two or more types of thin films stacked together.

[16] The method according to any one of

[12] to

[15] , characterized in that the first thin film and the second thin film each comprise a nitride or carbonitride containing one type of transition metal selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), copper (Cu), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), erbium (Er), uranium (U), and plutonium (Pu).

[17] A method according to any of

[12] to

[16] , further comprising performing an additional step after forming at least the first thin film and the second thin film, characterized in that the additional step comprises forming a sensitive film containing an arbitrary sensitive film material on the first thin film and / or the second thin film when the additional component is a sensitive film. Effect of the Invention

[0011] According to the present invention, there is provided a method for controlling the stiffness of a MEMS element by using a material that has high compressive strength and whose initial stress can be controlled in a thin film state, as well as a MEMS element having stiffness controlled in this manner and a method for manufacturing the same. [Brief description of the drawings]

[0012] [Figure 1] 1 is a schematic cross-sectional view showing a configuration of a MEMS element according to an exemplary embodiment of the present invention. [Figure 1A] FIG. 2 is a schematic cross-sectional view showing one aspect of the embodiment shown in FIG. [Figure 1B] FIG. 2 is a schematic cross-sectional view showing another aspect of the embodiment shown in FIG. [Diagram 2] FIG. 11 is a schematic cross-sectional view showing the configuration of a MEMS element according to another exemplary embodiment of the present invention. [Figure 2A] FIG. 3 is a schematic cross-sectional view showing one aspect of the embodiment shown in FIG. 2. [Figure 2B] FIG. 3 is a schematic cross-sectional view showing another aspect of the embodiment shown in FIG. 2. [Diagram 3] FIG. 11 is a schematic cross-sectional view showing the configuration of a MEMS element according to another exemplary embodiment of the present invention. [Figure 3A] FIG. 4 is a schematic cross-sectional view showing one aspect of the embodiment shown in FIG. [Figure 3B] FIG. 4 is a schematic cross-sectional view showing another aspect of the embodiment shown in FIG. [Figure 3C] FIG. 4 is a schematic cross-sectional view showing another aspect of the embodiment shown in FIG. [Figure 4] FIG. 11 is a schematic cross-sectional view showing the configuration of a MEMS element according to another exemplary embodiment of the present invention. [Figure 4A] FIG. 5 is a schematic cross-sectional view showing one aspect of the embodiment shown in FIG. [Diagram 5] 4 is a flow chart illustrating a method for controlling the stiffness of a MEMS element in accordance with an embodiment of the present invention. [Figure 6] 6 is a flowchart showing a step of a pre-process in the flowchart of the stiffness control method shown in FIG. 5 . [Figure 7] 6 is a flowchart showing a step of forming a sensitive film in the flowchart of the stiffness control method shown in FIG. 5 . [Figure 8] FIG. 1 is a schematic diagram showing a control scheme for the stiffness of an MSS element in an embodiment. [Figure 9] FIG. 2 is a schematic diagram of a method for forming a TiN thin film by DC reactive magnetron sputtering in an embodiment. [Figure 10] A schematic diagram showing how to calculate the residual stress σf of a thin film (TiN) formed on a substrate (Wafer). R, ts, and tf are the radius of curvature of the substrate, the thickness of the substrate, and the thickness of the thin film, respectively. [Figure 11] FIG. 1 shows the initial stress σp of TiN thin films formed with various substrate biases and thin film thicknesses in the examples, (a) film thickness: 20 nm, (b) substrate bias: −120 V. [Figure 12] The graph shows the initial stress σp of a TiN thin film (sputtered thin film) with the horizontal axis representing the substrate bias (V) and the vertical axis representing the thin film thickness tf (nm). [Figure 13]Schematic diagram showing measurement of the stiffness of the MSS element body by nanoindentation method. [Figure 14] FIG. 13 is a graph showing load-displacement curves obtained for the MSS element body before the TiN thin film was formed (Before) and on which the TiN thin film was formed under five different film formation conditions (Batch 1-5). [Figure 15] FIG. 13 shows the effect of the total initial stress σp,total of the TiN thin film on the stiffness of the MSS element body. [Figure 16] Schematic of a hinged beam in compression for the nonlinear extension of the Euler-Bernoulli beam theory. [Figure 17] Figure 1 shows the load-displacement curves of the beam at different compression levels calculated from equation (S6) obtained in the nonlinear beam theory. [Figure 18] Conceptual diagram of humidity measurement (water vapor detection) using an MSS element with controlled stiffness. [Figure 19] FIG. 1 shows micrographs of a comparative MSS element after deposition of a PMMA layer (left side), and an MSS element after deposition of a PMMA layer on a TiN thin film (right side). [Figure 20] FIG. 2 shows the relative humidity profile (measurement sequence) of the humid nitrogen flow used in a humidity measurement test using an MSS element. [Figure 21] (a) Graph showing the output of the MSS element obtained in a humidity measurement test. (b) Humidity response curve obtained by plotting the saturated output at each humidity step from the results of (a) against the corresponding humidity. (c) Graph showing the output of the MSS element obtained in a humidity measurement test performed using an MSS element with a buckled sensing film and an MSS element with an unbuckled sensing film. (d) Humidity response curve obtained by plotting the saturated output at each humidity step from the results of (c) against the corresponding humidity. [Figure 22] FIG. 1 is a diagram for explaining model settings in simulation software. [Figure 23](a) Simulation results of the relationship between the film center displacement and the polymer swelling strain when the total initial stress σp,total of the TiN thin film is changed. (b) Simulation results of the humidity response curve of the MSS element under the same total initial stress as in (a). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail. The following description of the components may be based on a representative embodiment of the present invention, but the present invention is not limited to such an embodiment.

[0014] [MEMS elements] The MEMS element of the present invention comprises a suspended structure, which is sandwiched between a first thin film (hereinafter simply referred to as the "first thin film") made of a nitride or carbonitride containing at least one type of transition metal M1, and a second thin film (hereinafter simply referred to as the "second thin film") made of a nitride or carbonitride containing at least one type of transition metal M2. Here, the composition of the nitride or carbonitride constituting the first thin film is the same as or different from the composition of the nitride or carbonitride constituting the second thin film.

[0015] In this specification, the term "suspended structure" refers to a structure having a predetermined shape, at least a portion of which is connected to (or fixed to) a base, and which has a certain space on at least the side where the first thin film is located and the side where the second thin film is located. Here, the "certain space" may be an open space or a closed space. As described later, a sensitive film or the like may be formed on the first thin film and / or the second thin film as desired, and when such a component is included, it will be interpreted as meaning that a certain space exists on the side where the component is located, as appropriate.

[0016] In an exemplary embodiment of the present invention, the suspended structure in the MEMS element is a diaphragm. Figure 1 is a schematic cross-sectional view showing the configuration of a MEMS element having a diaphragm structure.

[0017] 1 includes a diaphragm 102, both ends of which are connected to a base 104. Such a diaphragm structure is typically formed by processing a substrate material such as a silicon wafer used in the semiconductor device technology field. In this case, the material of the diaphragm 102 may be the same as the material of the base 104 (e.g., silicon).

[0018] The diaphragm 102 is sandwiched between a first thin film 106 and a second thin film 108. In other words, in the MEMS element 100, the first thin film 106 is formed on the upper surface of the diaphragm 102, and the second thin film 108 is formed on the lower surface of the diaphragm 102.

[0019] Here, in the MEMS element 100 shown in FIG. 1, a first thin film 106 is formed on the upper surfaces of the diaphragm 102 and the base 104, and a second thin film 108 is formed on the lower surface of the diaphragm 102 and the side and lower surface of the base 104. However, in this embodiment, it is not an essential requirement that the first thin film 106 and the second thin film 108 are formed on the base 104.

[0020] 1A, the first thin film 106 and the second thin film 108 may be formed only on the upper and lower surfaces, respectively, of the diaphragm 102. Such a structure may be formed, for example, by applying a mask to the surface of the base 104 by any method when forming the first thin film 106 and the second thin film 108.

[0021] In this embodiment, the diaphragm structure of the MEMS element is not limited to the embodiment shown in FIG. 1 and FIG. 1A. For example, as in the MEMS element 140 shown in FIG. 1B, the diaphragm 102 may be provided in the vicinity of the approximate center of the base 104 in a cross-sectional view. Naturally, the diaphragm 102 may be formed above or below the position shown in FIG. 1B. In addition, in the MEMS element 140 shown in FIG. 1B, the first thin film 106 may be formed on the upper surface and side surface of the base 104, and the second thin film 108 may be formed on the lower surface and side surface of the base 104.

[0022] In the MEMS element of the present invention, the configuration of the suspended structure is not limited to the embodiment described with reference to Figures 1, 1A and 1B, and the suspended structure may have at least a part fixed to a base.

[0023] FIG. 2 is a schematic cross-sectional view showing the configuration of a MEMS element according to another exemplary embodiment of the present invention.

[0024] 2 includes a suspended structure 202, both ends of which are fixed to the upper surface of a base 204. In a MEMS element 200 having such a configuration, the suspended structure 202 may be referred to as a diaphragm, a doubly supported beam, or some other term, depending on its shape, dimensions, etc.

[0025] Suspended structure 202 is sandwiched between first thin film 206 and second thin film 208. In other words, in MEMS element 200, first thin film 206 is formed on the upper surface of suspended structure 202, and second thin film 208 is formed on the lower surface of suspended structure 202 (excluding the portion in contact with the upper surface of base 204).

[0026] Here, in this embodiment as well, as explained in the above embodiment, it is not an essential requirement that the second thin film 208 be formed on the side and bottom surfaces of the base portion 204.

[0027] For example, as in MEMS element 220 shown in Figure 2A, second thin film 208 may be formed only on the lower surface of suspended structure 202 (excluding the portion in contact with the upper surface of base 204). Here, first thin film 206 may be formed on the upper surface of suspended structure 202 in an area that generally corresponds to the area in which second thin film 208 is formed on the lower surface of suspended structure 202. Alternatively, if desired, first thin film 206 may be formed over the entire upper surface of suspended structure 202, as in MEMS element 240 shown in Figure 2B.

[0028] FIG. 3 is a schematic cross-sectional view showing the configuration of a MEMS element according to still another exemplary embodiment of the present invention.

[0029] 3 includes a suspended structure 302, one end (the left end) of which is fixed to a side surface of a base 304. In a MEMS element 300 having such a configuration, the suspended structure 302 may be referred to as a cantilever beam depending on its shape, dimensions, etc., or may be referred to by other terms (for example, a movable part, a vibrating part, etc.).

[0030] Suspended structure 302 is sandwiched between first thin film 306 and second thin film 308. In other words, in MEMS element 300, first thin film 306 is formed on the upper surface of suspended structure 302, and second thin film 308 is formed on the lower surface of suspended structure 302. Here, first thin film 306 or second thin film 308 may be formed on the upper surface, lower surface, and / or side surface of base 304 as desired or due to manufacturing process considerations when forming first thin film 306 and second thin film 308.

[0031] Here, in the MEMS element 300 shown in Fig. 3, the upper surface of the first thin film 306 formed on the upper surface of the suspended structure 302 is substantially flush with the upper surface of the base 304, but this is not an essential requirement in this embodiment. For example, as in the MEMS element 320 shown in Fig. 3A, the fixing position of the suspended structure 302 to the side surface of the base 304 may be higher than in the embodiment shown in Fig. 3, the upper surfaces of the suspended structure 302 and the base 304 are substantially flush with each other, and the upper surface of the first thin film 306 formed on the upper surface of the suspended structure 302 may be located higher than the upper surface of the base 304 by the thickness of the first thin film 306. Alternatively, in the MEMS element 300 shown in Fig. 3, the thickness of the first thin film 306 may be thin, so that the upper surface of the first thin film 306 may be located lower than the upper surface of the base 304 (not shown). In addition, the fixing position of the suspended structure 302 relative to the side of the base 304 may be lower than in the embodiment shown in Figure 3, and the upper surface of the first thin film 306 may be located lower than the upper surface of the base 304 (not shown).

[0032] In this embodiment, the manner of fixing suspended structure 302 to base 304 is not limited to the embodiment shown in Fig. 3. For example, as in MEMS element 340 shown in Fig. 3B, the lower surface of one end of suspended structure 302 may be fixed to a part of the upper surface of base 304. Alternatively, as in MEMS element 360 shown in Fig. 3C, the lower surface of one end of suspended structure 302 may be fixed over the entire upper surface of base 304. In the embodiment shown in Figs. 3B and 3C, second thin film 308 may be formed on a region of the lower surface of suspended structure 302 excluding the portion in contact with the upper surface of base 304.

[0033] It should be noted that, although suspended structure 302 and base 304 are depicted as separate structures in each of MEMS elements 300, 320, 340 and 360 shown in Figures 3, 3A, 3B and 3C, it is also possible to form suspended structure 302 and base 304 integrally by processing a substrate material such as the silicon wafer mentioned above, and that even such an integrated configuration can be included in the concept of this embodiment.

[0034] FIG. 4 is a schematic cross-sectional view showing the configuration of a MEMS element according to still another exemplary embodiment of the present invention.

[0035] 4 has a generally flat plate-like appearance and includes a suspended structure 402, both ends of which are fixed to the side surfaces of a base 404. A MEMS element 400 having such a configuration may be called a sensor chip (or simply a chip) depending on its shape, dimensions, etc., or may be called by other terms.

[0036] The suspended structure 402 is sandwiched between a first thin film 406 and a second thin film 408. In other words, in the MEMS element 400, the first thin film 406 is formed on the upper surface of the suspended structure 402, and the second thin film 408 is formed on the lower surface of the suspended structure 402. Here, in the MEMS element 400 shown in FIG. 4, the surface (upper surface) of the first thin film 406 and the upper surface of the base 404 are substantially aligned, and the surface (lower surface) of the second thin film 408 and the lower surface of the base 404 are substantially aligned, but this is not an essential requirement in this embodiment. Furthermore, if desired, or due to circumstances regarding the manufacturing method when the first thin film 406 and the second thin film 408 are formed, the first thin film 406 or the second thin film 408 may be formed on the upper surface, lower surface, and / or side surface of the base 404.

[0037] In addition, in this embodiment, the manner of fixing suspended structure 402 and base 404 is not limited to the embodiment shown in Fig. 4. For example, both ends of suspended structure 402 may be fixed to the side surfaces of base 404 via connecting portions (fixing portions) 405, as in MEMS element 420 shown in Fig. 4A. Here, in MEMS element 420 shown in Fig. 4A, first thin film 406 is formed on the upper surfaces of suspended structure 402, base 404, and connecting portion 405, and second thin film 408 is formed on the lower surfaces of suspended structure 402, base 404, and connecting portion 405. Of course, in MEMS element 420, first thin film 406 may be formed only on the upper surface of suspended structure 402, and second thin film 408 may be formed only on the lower surface of suspended structure 402.

[0038] In the cross-sectional view of MEMS element 400 shown in Figure 4, the thicknesses of suspended structure 402 and base 404 are exemplary. Similarly, in the cross-sectional view of MEMS element 420 shown in Figure 4A, the thicknesses of suspended structure 402, connecting portion 405, and base 404 are exemplary, and for example, suspended structure 402 and connecting portion 405 may have substantially the same thickness.

[0039] Next, features that may be common to the various embodiments described above will be described.

[0040] In the MEMS element of the present invention, the suspended structure is preferably made of a material that is unlikely to undergo volumetric shrinkage under vacuum conditions. This is related to the fact that the formation of the first and second thin films on the suspended structure is typically performed under vacuum conditions. In other words, if the first and second thin films are formed in a state in which the suspended structure has undergone volumetric shrinkage, the initial stress of the first and second thin films may be relaxed when the suspended structure returns to atmospheric pressure from the vacuum condition, and the desired effect on the rigidity of the suspended structure may not be obtained. Therefore, more preferably, the suspended structure is made of a material that does not substantially undergo volumetric shrinkage under vacuum conditions, or, even if volumetric shrinkage may occur under vacuum conditions, the effect of the volumetric shrinkage on the initial stress of the first and second thin films is substantially negligible. In an exemplary embodiment, the suspended structure is made of a material that is metal, silicon, or a combination of these. In contrast, materials generally referred to as polymeric materials are likely to undergo (rapid) volumetric shrinkage under vacuum conditions, and the relaxation phenomenon of the initial stress of the first and second thin films described above is unavoidable, and therefore are considered to be unsuitable as materials for constituting the suspended structure.

[0041] In addition, the suspended structure is preferably made of a material that is softer than the first and second thin films. In this context, it is intended that the elastic modulus of the material constituting the suspended structure is smaller than the elastic modulus of the material (nitride or carbonitride) constituting the first and second thin films. This allows the initial stress of the first and second thin films to act on the suspended structure more efficiently. However, even if the suspended structure is made of a material that is harder than the first and second thin films, it is possible for the initial stress of the first and second thin films to act on the suspended structure.

[0042] In one embodiment, the initial stress of the first thin film may be the same as the initial stress of the second thin film. In this case, the effect of the initial stress of the first thin film and the effect of the initial stress of the second thin film on the rigidity of the suspended structure are considered to be the same (balanced), so that the suspended structure can maintain the state (shape) before the first thin film and the second thin film are formed. For example, when the suspended structure is originally approximately flat (flat shape) and it is desired to maintain the state (shape) in light of the use of the MEMS element, etc., it is preferable that the initial stress of the first thin film is the same as the initial stress of the second thin film. This makes it possible to avoid unintended excessive bending and fracture failure in the suspended structure.

[0043] In the manufacture of the MEMS element of the present invention, it may be difficult to completely match the initial stress of the first thin film with the initial stress of the second thin film due to the specifications of the device used to form the thin film. In other words, an unavoidable manufacturing error may occur with respect to the preset target value of the initial stress. Even if the initial stress of the first thin film and the initial stress of the second thin film do not completely match, it may be possible to obtain the above-mentioned effect. In fact, when the inventors of the present application conducted thin film manufacture experiments under various conditions using the device used in the examples described below, it was confirmed that the reproducibility of the target value of the initial stress was within a range of about 10%, and that within this range, the thin films can be treated as having substantially the same initial stress. Therefore, in the MEMS element of the present invention, not only when the value of the initial stress of the first thin film is completely the same as the value of the initial stress of the second thin film, but also when the value of the initial stress of the first thin film is within a range of ±10% of the value of the initial stress of the second thin film, the initial stresses of both thin films are treated as being the same. On the other hand, if the value of the initial stress of the first thin film is outside the range of ±10% of the value of the initial stress of the second thin film, the two initial stresses are treated as being different.

[0044] In this specification, the "initial stress" of the first thin film and the second thin film means an in-plane force generated by (within) the thin film. p is the residual stress σ f and the film thickness t f Product of: σ p = σ f t f and its unit is N / m.

[0045] In another embodiment, the initial stress of the first thin film may be smaller than the initial stress of the second thin film. In this case, since the effect of the initial stress of the first thin film on the rigidity of the suspended structure is considered to be smaller than the effect of the initial stress of the second thin film, when the initial stress is compressive stress, the suspended structure is curved (deformed) downward to a certain extent (for convenience of explanation, the side where the first thin film is located is referred to as the upper side, and the same applies below). For example, when it is intended to further form a sensitive film (also called a receptor layer) on the suspended structure having the first thin film and the second thin film, if the suspended structure has a shape that draws an arch downward, it may be possible to efficiently deposit a sensitive film material on its surface (on the first thin film). In addition, if the suspended structure has a shape that draws an arch downward in a state where the sensitive film is formed, it compensates for the stress generated in the sensitive film, and such an area may be able to respond with higher sensitivity to an external stimulus, so that the performance of the MEMS element as a sensor element can be expected to be improved.

[0046] In yet another embodiment, the initial stress of the first thin film may be greater than the initial stress of the second thin film. In this case, the effect of the initial stress of the first thin film on the rigidity of the suspended structure is considered to be greater than the effect of the initial stress of the second thin film, so that if the initial stress is compressive, the suspended structure will bend (deform) upward to a certain extent. For example, in the case where the above-mentioned sensitive film is intended to be formed, if the stress generated in the sensitive film and the response to an external stimulus in the state where the sensitive film is formed on the first thin film tend to be opposite to those in the above-mentioned case, the performance of the MEMS element as a sensor element can be expected to be improved by the suspended structure having an upwardly arched shape.

[0047] In the above-mentioned embodiment in which there is a substantial difference between the initial stress of the first thin film and the initial stress of the second thin film that affects the state (shape) of the suspended structure, if the difference is large, or if the initial stress of the first thin film and the initial stress of the second thin film are both relatively large values, the action of the initial stress of the first thin film and the initial stress of the second thin film exceeds the buckling load, and the suspended structure is in a buckled state and can be stable in a deformed state. A MEMS element having such a suspended structure in a buckled state can be expected to be applied, for example, to the field of cellular materials as a material having a bistable structure. In this way, the MEMS element of the present invention can also be applied to the field of meta-materials.

[0048] In the MEMS element of the present invention, the composition of the nitride or carbonitride constituting the first thin film and the composition of the nitride or carbonitride constituting the second thin film may be the same or different. In other words, the transition metal M1 contained in the first thin film and the transition metal M2 contained in the second thin film may be the same or different. In this specification, the term "transition metal" broadly refers to an element present between Group 3 and Group 12 in the periodic table. However, typically, it is preferable to select a transition metal from elements present between Group 3 and Group 11 according to the IUPAC classification. Note that each of the transition metal M1 and the transition metal M2 may be one type of transition metal or a combination of two or more types of transition metals.

[0049] Here, non-limiting examples of nitrides containing one type of transition metal are roughly as follows. Titanium nitride (TiN), zirconium nitride (ZrN), hafnium nitride (HfN), vanadium nitride (VN), niobium nitride (NbN), tantalum nitride (TaN), chromium nitride (CrN), molybdenum nitride (MoN), tungsten nitride (WN or W 2 N), manganese nitride (MnN), iron nitride (FeN), copper nitride (Cu 3 N), scandium nitride (ScN), yttrium nitride (YN), lanthanum nitride (LaN), cerium nitride (CeN), praseodymium nitride (PrN), neodymium nitride (NdN), samarium nitride (SmN), erbium nitride (ErN), uranium nitride (UN), and plutonium nitride (PuN). Here, the chemical formulas in parentheses for the above nitrides are examples, and other chemical formulas may be used as long as they indicate the initial stress. For example, iron nitride is not limited to FeN, and can be Fe 3 N, Fe 4 N, Fe 16 N 2 or may be represented by a chemical formula other than these.

[0050] Among these, thin films of titanium nitride, zirconium nitride, tantalum nitride, chromium nitride, molybdenum nitride, tungsten nitride, iron nitride, copper nitride, etc. have relatively high compressive strength, and the initial stress of the thin film can be controlled by appropriately adjusting the parameters of the film formation method, including the sputtering method described below, and are therefore preferably used as the nitrides that constitute the first thin film and the second thin film.

[0051] In addition, non-limiting examples of carbonitrides containing one kind of transition metal include carbonitrides containing one kind of transition metal selected from the group consisting of the constituent elements of the above-mentioned nitrides: titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), copper (Cu), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), erbium (Er), uranium (U), and plutonium (Pu). For example, when the transition metal is titanium (Ti), it is titanium carbonitride (TiCN).

[0052] In an embodiment in which the transition metal M1 contained in the first thin film and the transition metal M2 contained in the second thin film are each one type of transition metal, the first thin film and the second thin film may each be a nitride containing an alloy of one type of transition metal and a group II element (group 2 element and group 12 element) or a group III element (group 13 element). Non-limiting examples of such nitrides include TiGaN, TiAlN, NbAlN, TiCaN, and TiMgN.

[0053] In the above-mentioned embodiment in which the transition metal M1 and the transition metal M2 are each a combination of two or more transition metals, the first thin film and the second thin film may each be a nitride containing an alloy of two or more transition metals, non-limiting examples of such nitrides include TiScN, TiYN, TiZrN, TiTaN, TaZrN, and TiLaN.

[0054] Alternatively, in another embodiment, the first thin film and the second thin film may each be a laminated film formed by laminating two or more thin films selected from the above-mentioned nitride or carbonitride containing one kind of transition metal, nitride containing an alloy of one kind of transition metal and a group II element or group III element, or nitride containing an alloy of two or more kinds of transition metals. For example, the first thin film may be a laminated film of a nitride thin film of a transition metal M1a and a nitride thin film of a transition metal M1b (wherein the transition metal M1a and the transition metal M1b are different transition metals), and / or the second thin film may be a laminated film of a nitride thin film of a transition metal M2c and a nitride thin film of a transition metal M2d (wherein the transition metal M2c and the transition metal M2d are different transition metals).

[0055] In one embodiment, both the first thin film and the second thin film may be TiN thin films. Here, the stiffness of the suspended structure including such first thin film and second thin film may be substantially zero. In this context, substantially zero stiffness is intended to be within the range of -10 N / m to 10 N / m. Alternatively, the stiffness of the suspended structure including such first thin film and second thin film may be within the range of -1000 N / m to 1000 N / m. In the MEMS element of the present invention, the initial stress of the first thin film and the second thin film can be adjusted over a wide range from positive to negative values ​​(i.e., tensile stress to compressive stress), and therefore the stiffness of the suspended structure can be controlled over a wide range accordingly. As a result, the MEMS element of the present invention is useful as a device with controlled stiffness according to various applications, such as a cantilever structure device, an energy harvester, and a clamped beam resonator.

[0056] There is no particular limitation on the method for measuring the rigidity of a MEMS element (the suspended structure that constitutes the MEMS element). For example, static measurement methods include methods using nanoindentation or an atomic force microscope (AFM), and dynamic measurement methods include a method of measuring the resonant frequency of the suspended structure.

[0057] In a representative embodiment, the MEMS element of the present invention is a nanomechanical sensor element. A nanomechanical sensor has a sensitive film (receptor layer) that selectively adsorbs and absorbs a substance (or a group of substances) to be measured that exists in a gas phase or liquid phase, and performs analysis from changes in various physical parameters caused in the sensor by this adsorption or the like. A nanomechanical sensor has the advantages of being relatively sensitive despite its small size and simple structure, and being able to handle a wide range of substances to be measured by appropriately selecting the sensitive film. The present inventors have focused on a surface stress sensor that detects changes in the surface stress of a sensitive film caused by the adsorption or the like of a substance to be measured to the sensitive film, among other nanomechanical sensors, and have conducted a search for a sensitive film material that shows high selectivity for various substances to be measured. In addition, the inventors of the present application have developed a membrane-type surface stress sensor (MSS), which is different from the classical piezoresistance cantilever structure adopted in conventional surface stress sensors, and have succeeded in improving its characteristics, such as high sensitivity and improved operational stability, making it easy to use in various applications. For the principle and structure of the MSS, please refer to International Publication No. 2011 / 148774 as necessary. In the following, in order to facilitate understanding of the present invention, a surface stress sensor element will be taken up as a representative example of a nanomechanical sensor element, and the MSS element may be mentioned in particular, but please note that this does not cause any loss of generality.

[0058] As will be described in the Examples section with reference to Fig. 8, MSS elements (MSS chips) are usually highly structurally symmetrical, and therefore are suitable as structures to be used for carrying out demonstration experiments and / or obtaining basic knowledge about the control effect of the stiffness of a suspended structure by the initial stresses of the first and second thin films described above. Experimental results and / or knowledge obtained using such MSS elements can be utilized as guidelines for applying the present invention to surface stress sensor elements, nanomechanical sensor elements, and more general MEMS elements that have principles and structures different from those of MSS elements.

[0059] In one embodiment, the MEMS element of the present invention may further include an additional component on the first thin film and / or the second thin film. Such an additional component may be, for example, the above-mentioned sensitive film. In other words, the MEMS element of the present invention has a structure in which the suspended structure is sandwiched between the first thin film and the second thin film, but this does not limit the use of the MEMS element, and an additional component such as a sensitive film can be provided in the same manner as in the case where the suspended structure does not have the first thin film and the second thin film, and the suspended structure can exhibit a desired function. Rather, the MEMS element of the present invention has a structure in which the suspended structure is sandwiched between the first thin film and the second thin film, and thus the rigidity of the suspended structure is controlled, thereby improving the performance of the device. Furthermore, in the present invention, a target value (or target range) of the rigidity of the suspended structure is set in consideration of the performance required for the target device, and then the initial stress of the first thin film and the second thin film is adjusted to provide a MEMS element in which the rigidity of the suspended structure is controlled to the value (or range).

[0060] [Method of manufacturing MEMS elements] Next, a method for manufacturing the above-mentioned MEMS element will be described.

[0061] A method for manufacturing a MEMS element according to an embodiment of the present invention includes the steps of: Providing a substrate for a MEMS device having a suspended structure; forming a first thin film made of a nitride or carbonitride containing at least one transition metal M1 on one surface of the suspended structure; forming a second thin film of a nitride or carbonitride containing at least one transition metal M2 on the other surface of the suspended structure; Includes:

[0062] In one embodiment, the substrate for the MEMS device is a structure including a diaphragm 102 and a base 104 of a MEMS device 100 shown in FIG. In another embodiment, the substrate for the MEMS device is the structure of MEMS device 200 shown in FIG. 2, including suspended structure 202 and base 204 . In yet another embodiment, the substrate for the MEMS device is the structure of MEMS device 300 shown in FIG. In yet another embodiment, the MEMS device substrate is a structure including suspended structures 402 and base 404 of MEMS device 400 shown in Figure 4. Note that in this embodiment, the MEMS device substrate may be a structure including suspended structures 402, base 404, and connecting portion 405 of MEMS device 420 shown in Figure 4A.

[0063] The method for forming the first thin film on one side of the suspended structure of the MEMS element substrate and the method for forming the second thin film on the other side of the suspended structure are not particularly limited, but may be, for example, a sputtering method. The sputtering method is a type of physical vapor deposition (PVD) method generally used for coating and thin film formation, and is a technique for forming a film using plasma in a vacuum. Note that another method classified as a dry plating method similar to the above-mentioned PVD method is the chemical vapor deposition (CVD) method, which includes thermal CVD and plasma CVD.

[0064] In principle, the sputtering method has the feature that the kinetic energy of particles (atoms and molecules) ejected from the target (film-forming material) is large (5 to 10 eV) compared to the vacuum deposition method, so that the adhesion of the formed thin film is high and the range of target material selection is wide. In addition, various devices for carrying out the sputtering method have been developed in recent years, and the operation and film-forming conditions can be adjusted relatively easily. Therefore, the sputtering method can be preferably used for forming the first thin film and the second thin film in the MEMS element of the present invention. In the present invention, since the formation of a thin film made of a nitride or carbonitride containing a transition metal is intended, the sputtering method is reactive sputtering in which an inert gas and a reactive gas are mixed and introduced to perform sputtering. Representative types of sputtering include a two-pole method and a magnetron method, and in the present invention, the magnetron sputtering method is preferable because it has a faster film-forming speed than the two-pole method.

[0065] The characteristics of thin films formed by sputtering (hereinafter also referred to as "sputtered thin films") have been studied in various ways from the viewpoints of the growth process and structure formation of the thin film. For example, the stress of a sputtered thin film can be controlled by the bias voltage applied to the substrate and the film thickness. In the formation of a sputtered thin film, atoms and molecules ejected from the target by sputtering are incident on the growth surface of the thin film, and inert gas ions required for sputtering are reflected by the target and incident on the growth surface of the thin film. It is said that such incident atoms, molecules, and ions themselves or they push the atoms that make up the target thin film into the interstitial atomic positions, resulting in a kind of atomic peening effect. The stress of a sputtered thin film can be compressive or tensile stress depending on the sputtering conditions, and linear (or quasi-linear) control is also possible.

[0066] In this way, by forming the first thin film and the second thin film as sputtered thin films, it is possible to control the rigidity of the suspended structure within the above-mentioned range of -1000 N / m to 1000 N / m, or to make the rigidity of the suspended structure substantially zero within the range of -10 N / m to 10 N / m. Of course, it should be understood that even if a film formation method other than sputtering is used, the above-mentioned MEMS element (MEMS element with controlled rigidity) can be obtained by performing preliminary tests under a plurality of film formation conditions and expressing (modeling) the relationship between the rigidity of the suspended structure and the initial stresses of the first thin film and the second thin film in a certain format.

[0067] As will be specifically shown in the examples described later, one of the major features of titanium nitride (TiN) is that the residual stress of a sputtered thin film can be linearly controlled by the above-mentioned atomic level peening effect. Conventionally, the residual stress controllability of thin films such as TiN has been well studied, but to the best of the inventors' knowledge, there have been no reports of the application of thin films of transition metal nitrides such as TiN, or thin films made of nitrides or carbonitrides containing transition metals such as titanium, for the purpose of controlling the rigidity of micro / nano-scale MEMS devices.

[0068] In the case where the MEMS element of the present invention further comprises the additional component described above, the method of forming the additional component on the first thin film and / or the second thin film is not particularly limited. For example, in the case where the additional component is the above-mentioned sensitive film, the sensitive film material may be applied (deposited) on the thin film using an application means such as an inkjet device, a dispenser, or a spray coater.

[0069] [Method of controlling the stiffness of MEMS elements] Next, a method for controlling the stiffness of the above-mentioned MEMS element will be described with reference to FIGS.

[0070] A method for controlling the stiffness of a MEMS element according to an embodiment of the present invention (hereinafter, also simply referred to as a "stiffness control method") includes the following steps, as shown in the flowchart of FIG. Preparing a substrate for a MEMS device having a suspended structure (S510) and measuring the stiffness of the suspended structure (S512); forming a first thin film made of a nitride or carbonitride containing at least one transition metal M1 on one surface of the suspended structure (S520); forming a second thin film of a nitride or carbonitride containing at least one transition metal M2 on the other surface of the suspended structure (S530); Measuring initial stresses of the first thin film and the second thin film (S540); Measuring the stiffness of a suspended structure including the first membrane and the second membrane (S550). Includes:

[0071] In the above-mentioned stiffness control method, the MEMS element substrate is the same as that described with reference to Figures 1, 2, 3, 4, and 4A for the above-mentioned manufacturing method of the MEMS element. Methods for measuring the stiffness of the suspended structure provided on the MEMS element substrate include the above-mentioned static measurement methods (such as nanoindentation) and dynamic measurement methods. Note that, for the MEMS element substrate prepared in S510, if the stiffness of the suspended structure can be estimated or known in advance based on theory, simulation, or experiment, it is also possible to omit the step of S512 and use the estimated value or experimental value.

[0072] Here, if desired, it is determined whether or not to perform a pre-process (S502), and the pre-process may be performed as necessary (S600). The pre-process may be performed for the purpose of measuring in advance the initial stresses of the first thin film and the second thin film formed by a predetermined film forming method. Figure 6 is a flow chart showing the steps of the pre-process.

[0073] The front end process is Preparing a substrate (S610); forming a thin film made of a nitride or carbonitride containing a transition metal on the surface of the substrate (S620); Measuring the initial stress of the thin film (S630) and Determine whether to perform steps S610 to S630 under different film forming conditions (S640), and perform steps S610 to S630 as necessary. Includes:

[0074] In the above-mentioned pre-processing, it is preferable that the surface of the substrate has at least the same material as the suspended structure of the MEMS element substrate prepared in S510. In addition, it is preferable that the substrate has a substantially flat (flat) appearance, so that the initial stress of the thin film formed on the surface can be calculated (estimated) by measuring the curvature of the substrate. For example, if the MEMS element substrate prepared in S510 is fabricated by processing a silicon wafer, it is preferable to use a silicon wafer as the substrate prepared in S610.

[0075] In S620, the transition metal is intended to be a transition metal M1 or a transition metal M2.

[0076] In S630, the initial stress of the thin film formed on the substrate surface is typically calculated by measuring the curvature of the substrate, calculating the residual stress using the following Stoney formula, and then calculating the residual stress from the value and the film thickness based on the above definition.

number

[0077] In S640, it is determined whether or not the steps S610 to S630 need to be repeated. Here, it is expected that by setting different conditions for one or more specific parameters according to the film formation method and repeating the steps S610 to S630, the relationship between the parameter conditions and the initial stress of the thin film can be expressed (modeled) in a certain format.

[0078] In this way, by carrying out the pre-processing, it is possible to more appropriately set the conditions for forming the first thin film and the second thin film on the MEMS element substrate actually prepared in S510, and it is believed that the stiffness of the MEMS element can be controlled more efficiently by the stiffness control method of this embodiment. Note that the determination of whether to carry out the above-mentioned pre-processing (S502) and the execution of the pre-processing (S600) do not necessarily have to be before S510 and S512 as illustrated in Fig. 5, but may be after S512.

[0079] Returning to the flowchart of Fig. 5, after measuring the rigidity of the suspended structure provided on the MEMS element substrate in S512, it is determined whether or not to set a target value (or target range) for the rigidity of the suspended structure after forming the first thin film and the second thin film (S514), and the value (or range) may be set as necessary (S516). For example, as described above, when the requirements for the rigidity of the suspended structure are determined in consideration of the performance required for the target device, it may be preferable to set a target value (or target range) in advance. On the other hand, when the fabricated MEMS element (with controlled rigidity) is to be temporarily (trially) applied to the target device, it is not necessarily necessary to set such a target value (or target range).

[0080] Next, in S520, the first thin film is formed on one surface of the suspended structure. The method for forming the first thin film is the same as that described above, so the description is omitted here. The method for forming the second thin film, which will be described later, is also the same.

[0081] Here, after forming the first thin film, if desired, it is determined whether to measure the initial stress of the first thin film before forming the second thin film (S522), and the measurement may be performed as necessary (S524). For example, if the above-mentioned stiffness control method has been applied to the target MEMS element substrate and the initial stress of the first thin film can be estimated or known in advance based on any of theory, simulation, and experiment, the step of S524 can be omitted and the estimated value or experimental value can be used.

[0082] Next, in S530, the second thin film is formed on the other surface of the suspended structure.

[0083] Next, in S540, the initial stresses of the first thin film and the second thin film are measured. Here, if the initial stress of the first thin film is measured in S524, the measurement of the initial stress of the first thin film may be omitted as appropriate. Also, as in the above-mentioned S524, if the above-mentioned stiffness control method has been applied to the target MEMS element substrate and the initial stress of the second thin film can be estimated or known in advance based on any of theory, simulation, and experiment, it is also possible to omit the step of S540 and use the estimated value or experimental value.

[0084] Next, in S550, the stiffness of the suspended structure including the first thin film and the second thin film is measured. As for the stiffness measurement method in S550, a static measurement method (such as nanoindentation) and a dynamic measurement method can be used, as described above. Typically, the step of S550 is essential, but it is also possible to omit the stiffness measurement in S550, for example, when it is determined in advance to perform an additional process described later. Alternatively, when the stiffness control method described above has been applied to the target MEMS element substrate, and the stiffness of the suspended structure including the first thin film and the second thin film can be estimated or known in advance based on any of theory, simulation, and experiment, it may be possible to omit the step of S550 and use the estimated value or experimental value.

[0085] Here, if a target value (or target range) of the stiffness is set in S516, the measurement result of the stiffness in S550 (or the estimated value or experimental value described above) is compared with the value (or range) to perform verification. For example, if the deposition conditions for the first thin film and the second thin film are appropriately set based on the result obtained by performing the pre-process S600 described above, the verification result is expected to be good. On the other hand, even if the substrate used in the pre-process is the same as the material of the suspended structure of the substrate for MEMS element prepared in S510, the change in stiffness of the suspended structure in the actually fabricated MEMS element may differ from the initial assumption. In such a case, the deposition conditions for the first thin film and the second thin film can be reset more appropriately, taking into account the results of the verification.

[0086] After S550, the obtained MEMS element can be used for a desired purpose as it is. Alternatively, if desired, it is determined whether to perform an additional process (whether to provide additional components) (S552), and the additional components can be provided as necessary. In FIG. 5, as an example of a specific embodiment, a step of forming a sensitive film (S700) in the case where the additional components are a sensitive film is shown.

[0087] FIG. 7 is a flow chart showing the steps of forming the sensitive film described above.

[0088] The formation of the sensitive film shown in Figure 7 is Preparing a substrate (S710); Determining whether to form a sensitive film on the first thin film (S720), and forming the sensitive film if necessary (S730); When a sensitive film is formed on the first thin film, determining whether or not to form a sensitive film on the second thin film (S740), and forming a sensitive film as necessary (S750), or when a sensitive film is not formed on the first thin film, forming a sensitive film on the second thin film (S750); Measuring the stiffness of the suspended structure on which the sensitive film is formed (S760); Decide whether to re-form the sensitive film (S770), remove the sensitive film if necessary (S780), and perform the steps from S720 onwards. Includes:

[0089] Regarding S710, the substrate here refers to a substrate for an MEMS element in which a first thin film and a second thin film are formed on a suspended structure.

[0090] In S730 and S750, the methods for forming the sensitive films on the first thin film and the second thin film are the same as those described above, and therefore the description thereof will be omitted here.

[0091] The method of measuring the rigidity in S760 is the same as that described above for S550. Also, the "verification" for S760 in Fig. 7 is the same as that described above for S550, so a description thereof will be omitted here. However, with regard to the formation of the sensitive film as S700, it is determined whether or not the sensitive film needs to be re-formed (S770), and if necessary, the sensitive film is removed (S780), and the steps from S720 onwards are carried out again, whereby a MEMS element having a sensitive film formed in a more desirable manner can be fabricated using the same substrate.

[0092] The method for removing the sensitive film is not particularly limited, but for example, the part where the sensitive film is formed can be washed with an appropriate solvent such as water or an organic solvent depending on the sensitive film material. Note that, although such a washing process is basically not considered to impair the properties of the first thin film and the second thin film, care should be taken not to cause unintended defects in the first thin film and the second thin film during the sensitive film removal process. EXAMPLES

[0093] The present invention will be described in more detail below with reference to examples. Note that the following examples are not intended to limit the present invention, but are intended to aid in understanding the present invention. In this embodiment, an MSS element is used as the above-mentioned surface stress sensor element, and a titanium nitride (TiN) thin film is used as the above-mentioned first thin film and second thin film. Fig. 8 is a schematic diagram showing a control scheme of the stiffness of the MSS element in this embodiment.

[0094] The left side of FIG. 8 is a schematic perspective view showing the appearance of a sensor chip (MSS chip) including an MSS element. The MSS chip is formed from a silicon wafer cut out from a silicon single crystal. The MSS element has a structure in which a circular part (or other shape such as a square) with the letters "MSS" in the figure is connected and fixed to a surrounding frame-shaped part at four points on the top, bottom, left and right of the circular part. Furthermore, piezoresistors are provided in the four fixing parts (fixing areas). In a typical use mode of such an MSS element, a fluid component given to the MSS element is adsorbed and desorbed by a sensitive film applied to the surface of the circular part, and the surface stress applied to the MSS element is concentrated on the four fixing parts, resulting in a change in the electrical resistance of the piezoresistor. These piezoresistors are interconnected by a conductive area provided in the frame-shaped part to form a Wheatstone bridge. A voltage is applied between two opposing nodes of this Wheatstone bridge, and the voltage appearing between the remaining two nodes is taken out of the MSS element as a signal output from the MSS element, and the required analysis is performed. That is, in the MSS element, the circular portion corresponds to the above-mentioned suspended structure. In addition, since the circular portion is a structure that constitutes the essential part of the MSS element, it is often referred to as the "main body" or "element main body". The structure and operation of such an MSS element are described in detail in, for example, the above-mentioned International Publication No. 2011 / 148774, so please refer to it as appropriate.

[0095] In this embodiment, the basic scheme is to prepare an MSS element without a sensitive film formed on the surface of the circular portion (main body), and to apply TiN thin films as the first thin film and the second thin film, thereby obtaining an MSS element with controlled rigidity, as shown on the right side of Figure 8.

[0096] [Measurement of initial stress in TiN thin film] First, as the pre-processing step described above, a silicon wafer was used as the substrate to measure the initial stress of the TiN thin film.

[0097] The TiN thin film was formed on a 3-inch silicon wafer at room temperature using a DC reactive magnetron sputtering system (CFS-4EP-LL (i-miller), Shibaura Mechatronics).

[0098] FIG. 9 is a schematic diagram of a method for forming a TiN thin film by DC reactive magnetron sputtering. A silicon wafer (Wafer) as a film-forming object and a metallic titanium (Ti) target are placed in a vacuum chamber (not shown), and the chamber is evacuated to a vacuum state by using a vacuum pump. A reactive gas is introduced into the chamber together with an inert gas. In the figure, from the left, the inert gas argon (Ar) and the reactive gas nitrogen (N 2 ) is introduced to the target, and the gas is exhausted to the right (Outlet). A DC source is connected to the target, and an RF power is connected to the substrate on which the silicon wafer is placed, allowing a negative bias voltage (Neg. bias) to be applied to the substrate. When a certain voltage is applied, the inert gas argon discharges and produces positive ions (Ar + ), which collides with the target surface, releasing titanium atoms that react with the nitrogen reactive gas to form titanium nitride (TiN), which is deposited on the silicon wafer facing the target. At this time, argon ions (or argon atoms that have been neutralized when reflected by the target, etc.) are incident on the growing surface of the thin film. In this way, a thin TiN film is formed on the surface of the object to be filmed.

[0099] In this example, the DC sputtering power was 200 W, and the deposition rate was 0.044 nm / s, estimated from film thickness measurements with a field emission scanning electron microscope (S-4800, Hitachi High-Tech). Prior to deposition, a 3-inch titanium target was sputtered for 60 seconds to remove oxide impurities. During deposition, the silicon wafer was rotated at 20 rpm. The nitrogen and argon flow rates were 1.6 sccm and 11.4 sccm, respectively, and the chamber pressure was 0.22 Pa.

[0100] The curvature of the silicon wafer on which the TiN thin film was formed was measured using a thin film stress measurement device (FLX-2000-A, Toho Technology Co., Ltd.), and the residual stress σ of the thin film was calculated based on the above-mentioned Stoney equation. f In the case of the 3-inch silicon wafer used in this embodiment, t in the Stoney formula is s , E / (1-ν) are respectively, t s =380[μm], E / (1-ν)=1.805×10 11 [N / m 2 ].

[0101] In this example, the initial stress σ of the TiN thin film was calculated based on the above definition by changing two deposition parameters, the substrate bias and the film thickness. p The results are shown in Figures 11 and 12.

[0102] Fig. 11(a) shows the initial stress σ of TiN thin films formed with a fixed thickness of 20 nm and various substrate bias voltages. p Fig. 11(b) shows the initial stress σ of TiN thin films formed with various thicknesses at a fixed substrate bias of -120 V. p The graph shows the plot of the above. Note that the number of trials for each condition was three or more.

[0103] As shown in Fig. 11(a), when the film thickness is constant, the initial stress σ p increased quasi-linearly. This is believed to be due in part to the increase in the compressive residual stress of the TiN thin film caused by the increase in the defect concentration in the thin film due to enhanced irradiation with high-energy ions, known as the atomic-level peening effect in sputtered thin films. In addition, as shown in Figure 11(b), when the substrate bias is constant, the initial stress σ of the TiN thin film increases with increasing film thickness. p increased asymptotically. This is probably due in part to the fact that as the film thickness increases, the compressive residual stress in the thin film decreases due to the tensile stress caused by the anisotropic growth of TiN. In other words, the residual stress σ of the sputtered thin filmf and the film thickness t f It can be said that there is a negative correlation. Note that, following the convention that tensile stress is positive and compressive stress is negative, the initial stress is defined as negative. The minimum initial stress (-13.7 N / m) and the maximum initial stress (-309.2 N / m) obtained in this example occurred in TiN thin films with film thicknesses of 10 nm and 88 nm formed with substrate biases of -45 V and -180 V, respectively.

[0104] FIG. 12 shows the substrate bias (V) on the horizontal axis and the film thickness t f (nm) of the TiN thin film, and shows the initial stress σ p of the TiN thin film. Here, the plotted black circles are the measured data points, and the contour lines drawn based on these are shown by dotted lines. As shown in FIG. 12, by the above-described previous process, the relationship between the film formation parameters in sputtering and the initial stress σ p of the sputtered thin film (TiN thin film) could be modeled.

[0105] [Control of the stiffness of the MSS element] An experiment was conducted to control the stiffness of the MSS element by utilizing the controllability of the initial stress of the TiN thin film described above. The sensor chip (MSS chip) including the MSS element was purchased from NanoWorld AG. In order to make the initial stiffnesses of the MSS elements used in this experiment equivalent, 20 MSS elements with an average stiffness of 125.3 ± 10.0 N / m were selected and divided into five batches described later.

[0106] [Formation of the TiN thin film] Using the same apparatus and method as the above-described previous process, a TiN thin film was formed on the MSS element (more specifically, the circular portion of the element body) by DC reactive magnetron sputtering. The formation of the TiN thin film was performed on both the upper and lower surfaces of the element body. In the MSS element in which TiN thin films were formed on both surfaces of the element body, the bending moment due to the initial stress of the TiN thin film can be balanced. Thereby, it is possible to suppress the element body from bending excessively, and it is possible to maintain a state close to the initial (flat shape before the formation of the TiN thin film) in which the stiffness of the element body is minimized.

[0107] In addition, in this embodiment, the initial stress of the TiN thin film on the upper surface of the element body was designed to be larger than the initial stress of the TiN thin film on the lower surface. Due to this slight imbalance, the element body always curves (deforms) upward. This was confirmed by the height profile measurement using the microscope described above. Because the element body is curved upward in this way, in the nanoindentation measurement described later, the element body (circular film-like body) is pressed through a neutral planar configuration, so that the measurement result can obtain perfect rigidity characteristics.

[0108] Considering the above-mentioned measurement results of the initial stress of the TiN thin film (see Fig. 11 and Fig. 12), five sets of deposition conditions were selected as shown in Table 1. The deposition conditions were designed to satisfy the above requirements regarding the bending direction of the element body. Furthermore, under the conditions of batch numbers 1 to 5, the total initial stress σ p,total (Initial stress σ of TiN thin film applied from the top and bottom p The total of the two was designed to increase.

[0109] [Table 1]

[0110] When the MSS elements after the TiN thin film was formed were observed with a microscope (laser scanning confocal microscope (VK-X1000), manufactured by KEYENCE), it was confirmed that the element bodies obtained under all film formation conditions had a slight yellowish tinge characteristic of TiN.

[0111] <Nanoindentation measurement> The rigidity of the element body was measured using the nanoindentation method for the MSS element before the TiN thin film was formed and for the MSS element obtained under the conditions of each batch number, and the effect of the initial stress of the TiN thin film on the rigidity (initial rigidity) of the element body was investigated. Specifically, a nanoindentation tester (ENT-5, Elionix) was used to displace the center of the element body from the top side of the upwardly curved element body to the bottom side using a diamond probe as an indenter, as shown in Figure 13, while recording the reaction force, and a load-displacement curve that characterizes the flexibility of the element body was generated.

[0112] Figure 14 shows a subset of the load-displacement curve data obtained in this experiment, which shows the average characteristics of each batch condition. As shown in Figure 14, the total initial stress σ of the TiN thin film is p,total It was confirmed that as increases, it gradually becomes nonlinear. Such a change in nonlinearity is associated with a decrease in the stiffness of the element body and can be evaluated as the slope at the inflection point where the second derivative of the curve is equal to zero and the element body exhibits the minimum stiffness. In fact, as shown in Figure 15, the total initial stress σ p,total It can be seen that the stiffness of the MSS element body decreases linearly with increasing stiffness. Furthermore, this linearity is maintained up to the negative stiffness (-42N / m), which means the buckling state of the element body. This linear relationship can be qualitatively understood by the Euler-Bernoulli beam theory, which is well known in the fields of structural mechanics and material mechanics.

[0113] <Nonlinear beam theory> In order to better understand the relationship between the initial stress of the TiN thin film and the rigidity of the MSS element body, as obtained in the above experiment, a concentrated load F 0 Consider the case of a beam compressed along the x-axis under a force of . The deformation profile u(x) can be solved using the Euler–Bernoulli equation with the addition of an axial load term, and by assuming the beam is initially flat, we obtain:

number

number

number

number

number

number

number

number

[0114] The stiffness of the beam is determined by the inflection point d 2 f / dw 2 = 0. For an initially flat beam, the point of minimum stiffness coincides with the origin, i.e., w = 0. The stiffness is calculated by taking the first derivative of equation (S6) with respect to the normalized displacement w, setting w = 0, which gives:

number

[0115] Thus, the relationship between the initial stress of the TiN thin film and the stiffness of the MSS element body obtained in the above experiments was found to be in good agreement with the nonlinear load-displacement curve predicted based on the well-known nonlinear extension of the Euler-Bernoulli beam theory.

[0116] [Analysis of the characteristics of stiffness-controlled MSS elements] Next, the MSS element with the rigidity controlled as described above was used as a humidity sensor to examine its mechanical behavior. Here, the MSS element was functionalized by coating the TiN thin film on the MSS element body with polymethyl methacrylate (PMMA). For comparison, an MSS element body without a TiN thin film was also prepared with PMMA coated on it. PMMA is a polymer that is sensitive to moisture (water vapor) contained in the sample gas (i.e., a moisture-sensitive polymer), and is a useful material for the sensitive film in humidity sensors.

[0117] Inkjet spotting was used to functionalize the MSS element. Specifically, PMMA was dissolved in DMF at a concentration of 10 g / L and loaded into the piezo nozzle of an inkjet spotter (LaboJet-500SP, MICROJET). The inkjet spotter sends a voltage pulse to a piezoelectric actuator to generate an acoustic wave in the nozzle and extrudes the polymer solution in the form of droplets. The droplet speed was kept at 6 m / s by using a single rectangular pulse of 27 V for 95 μs. After a predetermined number of droplets were dropped onto the TiN thin film of the MSS element body (or onto the MSS element body), the MSS element was dried while heating it to 80 °C to form a PMMA sensitive film. For convenience of explanation, the MSS element body with the TiN thin film is referred to as the sensing film below.

[0118] For the experiment of the MSS element as a humidity sensor, the following gas supply system was used. Two mass flow controllers (MFCs) (SEC-N112MGM, HORIBA) were used. One MFC fed dry nitrogen at a specified flow rate into a glass vial containing a certain amount of water, thereby pumping out the saturated water vapor in the headspace of the glass vial and introducing it into one inlet of the mixing chamber, and the other MFC fed dry nitrogen at a specified flow rate into the other inlet of the mixing chamber to dilute the nitrogen gas containing the above-mentioned saturated water vapor, generating wet nitrogen flows with various relative humidities. The total gas flow rate given to the MSS element was fixed at 100 sccm, and the humidity of the wet flow was measured with a humidity sensor (SHT21-D2CE3, SENSIRION). All measurements began with a 240-second dry nitrogen purge to set the baseline, and the wet flow was fed into the MSS element placed in a sealed detection chamber while increasing and decreasing the relative humidity in 42 steps from 0% to 81%. Each humidity step (humidity flow supply period) was set for 90 seconds to allow the output of the MSS element to reach equilibrium at each step. The output of the MSS element was measured at a bias voltage of -0.5V.

[0119] Figure 18 shows a conceptual diagram of humidity measurement (water vapor detection) in this experiment. 2 " corresponds to the dry nitrogen purge described above, and "Humidified N 2 " corresponds to the supply section of the wet flow described above. As shown on the right side of Figure 18, when water molecules are adsorbed to PMMA (Polymer), the PMMA layer swells, the sensing membrane underneath it deforms, and the output of the piezoresistive elements embedded in the four fixing parts supporting the sensing membrane (%ΔR / R 0 ) changes. Because a more flexible sensing membrane makes the MSS element more sensitive to water vapor, the sensitivity to humidity is a second characterization indicator of the stiffness of the sensing membrane.

[0120] In this experiment, PMMA functionalization was performed on three MSS elements with sensing membrane stiffnesses of 30.1, 3.0, and -7.8 N / m, and a comparative MSS element with an element body stiffness of 95.5 N / m. The comparative MSS elements serve as a reference for considering the experimental results, and the three MSS elements with TiN thin films were fabricated under the conditions of batches 2 to 4 in Table 1 above. For reference, Figure 19 shows micrographs of the comparative MSS element after forming a PMMA layer (left side) and the MSS element after forming a PMMA layer on the TiN thin film (right side).

[0121] Figure 20 shows the relative humidity profile (measurement sequence) of the wet nitrogen flow used in this experiment. When the relative humidity of the wet flow applied to the MSS element increases or decreases, the PMMA on the sensing film expands or contracts, and the piezoresistance output increases or decreases. Specifically, as shown in Figure 21(a), the piezoresistance output changed in response to the change in the relative humidity of the wet flow. Note that the stiffness of each MSS element is listed in the figure, and the order in which they are listed corresponds to the order of the graphs showing the piezoresistance output. In addition, to enable comparison in a single figure, the four graphs are each shown after baseline subtraction processing.

[0122] Now, by plotting the saturated output at each humidity step (the value when the piezoresistor output becomes nearly constant) against the corresponding humidity, the increasing and decreasing piezoresistor output sequence can be shown as a humidity response curve, as shown in Figure 21(b). The circles in the figure represent the results obtained with the profile where the relative humidity increases (Increasing, see Figure 20), and the crosses represent the results obtained with the profile where the relative humidity decreases (Decreasing, see Figure 20).

[0123] These results show that in the MSS element with the above-mentioned sensing film, the stiffness of the element body is controlled to be lower than that of the comparative MSS element due to the effect of the initial stress of the TiN thin film formed on both sides of the element body, thereby improving the sensitivity as a humidity sensor. Therefore, the effectiveness of stiffness control of the suspended structure by applying a transition metal nitride thin film to the suspended structure of the MEMS element was confirmed.

[0124] In general, when the stiffness of a structure becomes zero or less, the structure enters a buckling state, which is accompanied by the appearance of clear mechanical behavior. In the above experiment, the MSS element with a sensing membrane with a negative stiffness of -7.8 N / m did not exhibit any abnormal behavior (see Figure 21(a)). This is believed to be because the sensing membrane had a PMMA layer on it, which returned the stiffness of the sensing membrane to a positive value. Therefore, to investigate abnormal mechanical behavior due to buckling, it is necessary to use a sensing membrane with a larger negative stiffness than that used above. Therefore, an MSS element with an initial stiffness (68.7 N / m) that was almost half that of the one selected for the above experiment was selected for the next experiment.

[0125] A 20 nm TiN thin film was deposited on the top and bottom surfaces of the body of the selected MSS element using the same equipment and method described above at substrate biases of −150 V and −70 V, respectively, and the total initial stress of the TiN thin film, σ p,totalAfter the TiN thin film was deposited, the sensing film buckled, and a nanoindentation test confirmed that it had a large negative stiffness of -105 N / m.

[0126] The buckled sensing membrane was functionalized with PMMA along with the sensing membrane of another MSS element with positive stiffness (108.0 N / m), and an experiment was conducted in which dry and wet nitrogen flows were alternately applied using the same gas supply system as described above, and the output of each MSS element was measured. The results are shown in Figure 21(c). As in Figure 21(a) above, the stiffness of each MSS element is listed in the figure, and the order of listing corresponds to the order of the graphs showing the piezoresistance output. In addition, both graphs have been subjected to baseline subtraction processing so that they can be compared in a single figure.

[0127] From the results shown in Fig. 21(c), it was confirmed that the MSS element with a buckled sensing membrane exhibits a sudden change in the piezoresistive output compared to the non-buckled MSS element. Furthermore, it was found that the sudden change occurs at different relative humidity depending on whether the humidity is rising or falling. This hysteresis can be better confirmed by the humidity response curve shown in Fig. 21(d). Here, the humidity response curve was obtained in the same manner as in Fig. 21(b) above, and the arrows in the figure indicate the occurrence of a sudden change. The sudden jumps indicated by these arrows correspond to the snap-through motion of the bistable buckled membrane, which is a well-known phenomenon in buckling structures. This is due to the instability of the negative stiffness under dead load. Also, near the critical threshold, the buckled membrane becomes extremely sensitive to changes in humidity. Therefore, due to these features, it can be said that the MSS element with a buckled sensing membrane can be used as a bistable threshold sensor with a particularly high signal-to-noise ratio at the critical threshold.

[0128] <Numerical Simulation> In order to reproduce the nonlinear humidity response curve of the buckled sensing membrane described above, the deformation of the sensing membrane of the MSS element was modeled by finite element analysis (FEA) using the Structural Mechanics Module of COMSOL Multiphysics 6.0. Here, in order to accurately model the deformation of the sensing membrane, the thickness of the sensing membrane was measured using an optical film thickness meter (OPTM-A1, manufactured by Otsuka Electronics Co., Ltd.). Specifically, for a sensing membrane with a stiffness of 68.7 N / m, the film thickness was measured to be 1.92 μm. Using this result, a model was constructed with a 3 μm-thick polymer (PMMA), a 1.92 μm-thick silicon membrane coated with a 20 nm-thick TiN thin film, and four fixed piezoresistive bridges. The detailed shape of the MSS element other than the film thickness is as previously reported, such as in the above-mentioned International Publication No. 2011 / 148774.

[0129] The polymer and the sensing film were explicitly modeled as solid elements to reduce errors near the fixed end, and the entire shape was discretized with more than 8,000 rectangular serendipity elements, with three elements assigned in the thickness direction of all layers. The Young's moduli of the polymer, TiN, and silicon were set to 2.28 GPa, 400 GPa, and 170 GPa, respectively, and the Poisson's ratios were set to 0.40, 0.325, and 0.28, respectively. In this study, taking into account the symmetry of the MSS element structure, only one-quarter of the entire shape was modeled, as shown in Figure 22.

[0130] The swelling of the polymer is induced by the isotropic strain ε polymer The initial stress of TiN was implemented as the equiaxial stress of the TiN thin film, and the sum of the initial stresses of the two thin films (top and bottom of the silicon film) was the total initial stress σ p,total Note that the initial stress is directly related to the compressive axial load from the substrate, similar to the nonlinear beam theory above. The piezoresistive output is obtained by averaging the differential stress on the top surface of the piezoresistive bridge according to the following equation:

number

[0131] The deformation of the initially stressed membrane upon polymer swelling was solved by the Newton-Raphson method, resulting in the differential stress concentrated at the piezoresistive bridge (data not shown). The displacement of the membrane center as the polymer swelling progresses is shown in Fig. 23(a) and the corresponding piezoresistive output, i.e., humidity response curve, in Fig. 23(b). The humidity response curves were found to become increasingly nonlinear, consistent with the above-mentioned experiments. However, under a total initial stress of -120 N / m, the modeled membrane does not buckle, whereas the actual experiment shows that the membrane buckles and exhibits an abrupt jump in the humidity response curve, which differs from the above-mentioned experiments. The cause of the overestimation of the membrane stiffness in this simulation is unclear and may be due to thickness variations within the membrane, simplification of the polymer geometry, or a thickness-dependent stress gradient within the TiN thin film. [Industrial Applicability]

[0132] The stiffness of a structure is a key design parameter that determines the performance of various micro- and nano-scale mechanical devices. For example, a structure with near-zero (or practically zero) stiffness can significantly improve the sensitivity of nanomechanical sensors. Since negative stiffness leads to bistability, such structures have broad applications in auxeticity in mechanical metamaterials, broadband energy harvesters in bistable MEMS, large deformation actuation of microrobots, nanomechanical memory storage and switches, etc.

[0133] The present invention provides a low-cost and effective alternative to conventional stiffness control methods for micro- and nano-scale devices. The method of the present invention can be applied to a variety of MEMS devices that require low or negative stiffness for improved performance. For example, nanomechanical sensors require low stiffness for increased sensitivity in gas sensing and biosensing applications, diaphragm pressure sensors require low stiffness to detect small air pressure changes, and MEMS-based energy harvesters require negative stiffness for wide operating bandwidth. Thus, the present invention provides a simple alternative fabrication technique for controlling the stiffness of micro / nanoscale devices.

[0134] In the above-mentioned embodiment, it was demonstrated that the stiffness of the MEMS sensor element can be precisely controlled over a wide range from positive to zero and negative values ​​by using the MSS element and evaporating TiN. When the stiffness is reduced, functionality such as high sensitivity and bistability is manifested, and the mechanical behavior of the device can be significantly changed. The stiffness control method according to the present invention can also be applied to various MEMS devices including microcantilevers. Therefore, the present invention can be applied to the fabrication of structures with near-zero stiffness or negative stiffness, and is expected to significantly improve the performance of various micro / nanoscale mechanical devices and contribute to the realization of the fabrication of devices with excellent detection and actuation functions. [Explanation of symbols]

[0135] 100, 120, 140 MEMS elements 102 Suspended structure (diaphragm) 104 Base 106 First Thin Film 108 Second Thin Film 200, 220, 240 MEMS elements 202 Suspended Structure 204 Base 206 First Thin Film 208 Second Thin Film 300, 320, 340, 360 MEMS elements 302 Suspended Structure 304 Base 306 First Thin Film 308 Second Thin Film 400, 420 MEMS elements 402 Suspended Structure 404 Base 405 Connecting part (fixed part) 406 First Thin Film 408 Second Thin Film

Claims

1. A MEMS element comprising a first thin film made of a nitride or carbonitride containing at least one transition metal M1, a second thin film made of a nitride or carbonitride containing at least one transition metal M2, and a suspended structure sandwiched between them, wherein the composition of the nitride or carbonitride constituting the first thin film and the composition of the nitride or carbonitride constituting the second thin film are the same or different.

2. The MEMS element according to claim 1, characterized in that the initial stress of the first thin film is the same as that of the second thin film.

3. The MEMS element according to claim 1, characterized in that the initial stress of the first thin film is smaller than that of the second thin film.

4. The MEMS element according to claim 1, characterized in that the initial stress of the first thin film is greater than that of the second thin film.

5. The MEMS element according to claim 1, characterized in that the first thin film and the second thin film are each thin films made of a nitride or carbonitride containing one type of transition metal, a nitride containing an alloy of one type of transition metal and a group II or group III element, or a nitride containing an alloy of two or more types of transition metals, or a laminated film formed by stacking two or more of these thin films.

6. The MEMS element according to claim 1, characterized in that the first thin film and the second thin film each consist of a nitride or carbonitride containing one transition metal selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), copper (Cu), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), erbium (Er), uranium (U), and plutonium (Pu).

7. The MEMS element according to claim 6, characterized in that the first thin film is made of titanium nitride (TiN), the second thin film is made of titanium nitride (TiN), and the rigidity of the MEMS element is in the range of -1000 N / m to 1000 N / m.

8. The MEMS element according to claim 7, characterized in that the rigidity of the MEMS element is substantially zero.

9. The MEMS element according to claim 7, characterized in that the suspended structure is in a buckled state.

10. The MEMS element according to any one of claims 1 to 9, further comprising a sensitive film on the first thin film.

11. The MEMS element according to claim 10, further comprising a sensitive film on the second thin film.

12. A substrate for a MEMS element equipped with a suspended structure is prepared, and the rigidity of the suspended structure is measured. A first thin film made of a nitride or carbonitride containing at least one type of transition metal M1 is formed on one surface of the suspended structure. A second thin film made of a nitride or carbonitride containing at least one type of transition metal M2 is formed on the other side of the suspended structure. The initial stresses of the first thin film and the second thin film are measured, To measure the rigidity of a suspended structure comprising the first thin film and the second thin film. It includes, A method for controlling the rigidity of a MEMS element, characterized in that the composition of the nitride or carbonitride constituting the first thin film and the composition of the nitride or carbonitride constituting the second thin film are the same or different.

13. The method according to 12, characterized in that the first thin film and the second thin film are formed by a magnetron sputtering method.

14. The process further includes performing a pre-process before or after preparing the substrate for the MEMS element, and the pre-process is: A substrate made of the same material as the suspended structure is prepared. The first thin film or the second thin film is formed on the substrate under one or more film formation conditions. The initial stresses of the first thin film and the second thin film are measured. From the obtained results, the relationship between the rigidity of the suspended structure and the initial stresses of the first and second thin films can be expressed in a certain form. The method according to 12, characterized by encompassing the above.

15. The method according to 12, characterized in that the first thin film and the second thin film are each thin films made of a nitride or carbonitride containing one type of transition metal, a nitride containing an alloy of one type of transition metal and a group II or group III element, or a nitride containing an alloy of two or more types of transition metals, or a laminated film formed by laminating two or more of these thin films.

16. The method according to any one of claims 12 to 15, characterized in that the first thin film and the second thin film each consist of a nitride or carbonitride containing one transition metal selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), iron (Fe), copper (Cu), scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), erbium (Er), uranium (U), and plutonium (Pu).

17. The method according to 12, further comprising performing an additional step after forming at least the first thin film and the second thin film, wherein the additional step comprises forming a sensitive film on the first thin film and / or the second thin film, the additional component being a sensitive film, the sensitive film comprising any sensitive film material.

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