JP2015204544A - MEMS element, method for manufacturing MEMS element, and electronic device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small and highly reliable MEMS element capable of high frequency operation, in which variation of operation frequency is reduced.SOLUTION: A MEMS element 1000 has a cavity 108 formed in a substrate 100, a diaphragm 101 provided above the cavity 108, and a piezoelectric element 111 provided on the diaphragm 101. The diaphragm 101 has a lamination of at least a first thin film 102 and a second thin film 103, and the piezoelectric element 111 includes a first electrode 104, a second electrode 106, and a piezoelectric material 105 sandwiched by the first electrode 104 and second electrode 106. Consequently, a highly reliable MEMS element capable of high frequency operation, in which variation of operation frequency is reduced, can be achieved.

Description

  The present invention relates to a MEMS element, a method for manufacturing a MEMS element, and an electronic apparatus.

  In recent years, attention has been focused on MEMS devices manufactured using micromachining technology. MEMS devices are used to move obstacles, people, things, etc. in various fields such as robotics, FA, distribution / logistics, medical, nursing / welfare, and security for object detection and measurement. Widely used as a sensing system for detecting body, intruders, etc.

JP 2012-10158 A

  However, the MEMS element described in Patent Document 1 has a problem that it is difficult to operate at a high frequency. In general, the operating frequency (oscillation frequency or detection frequency) of a MEMS element such as an ultrasonic oscillator or an ultrasonic sensor is proportional to the square root of the Young's modulus of the diaphragm portion. In the MEMS element described in Patent Document 1, the diaphragm portion is composed only of the piezoelectric film and the counter electrode, and the operating frequency of the MEMS element is low because the Young's modulus of the piezoelectric film is low. Thus, the conventional MEMS device has a problem that high-frequency operation is difficult.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

(Application Example 1) A MEMS element according to this application example includes a substrate in which a cavity is formed, a diaphragm provided on the cavity, and a piezoelectric element provided on the diaphragm, The diaphragm portion includes at least a first thin film and a second thin film contacting the substrate, and the piezoelectric element portion is sandwiched between the first electrode, the second electrode, the first electrode, and the second electrode. And a piezoelectric material.
According to this configuration, by configuring the diaphragm part with at least two layers of thin films, it is possible to achieve high reliability capable of high-frequency operation. Therefore, a high-reliability MEMS element capable of high-frequency operation, having a small variation in operating frequency, and high reliability can be realized.

Application Example 2 In the MEMS element according to the application example described above, when the thickness and Young's modulus of the first thin film are h ₁ and E _1, and the thickness and Young's modulus of the second thin film are h ₂ and E _2. It is preferable that the relational expression of Formula 1 is satisfied.

  According to this configuration, the second thin film having high rigidity becomes the main element of the diaphragm portion, and thereby, a highly reliable MEMS element capable of high-frequency operation can be realized.

Application Example 3 In the MEMS element according to the application example described above, it is preferable that the thickness h ₁ of the first thin film and the thickness h ₂ of the second thin film satisfy the relational expression (2).

  According to this configuration, since the second thin film is the main element of the diaphragm portion, a stable MEMS element that operates at high frequency and has small variations in operating frequency can be realized.

Application Example 4 In the MEMS element according to the application example described above, when the film stress of the first thin film is P ₁ and the film stress of the second thin film is P ₂ , the film stress P ₁ of the first thin film and the second film stress The film stress P _{2 of the} thin film preferably satisfies the relational expression of Expression 3.

According to this configuration, the film stress P ₂ of the second thin film is smaller than the film stress P ₁ of the first thin film. As a result, generation of cracks and the like can be suppressed, and the film thickness of the diaphragm portion can be increased. That is, with this configuration, a highly reliable MEMS element can be realized with high frequency operation.

Application Example 5 In the MEMS element according to the application example described above, the second thin film is preferably a zirconium oxide film.
According to this configuration, a highly reliable MMEMES element capable of high-frequency operation by using a zirconium oxide film having high rigidity and easy to increase in thickness as the second thin film that is a main element of the diaphragm portion. Can be realized.

Application Example 6 In the MEMS element according to the application example, the first thin film is preferably a silicon nitride film.
According to this configuration, the film thickness of the diaphragm portion is stabilized by using, as the first thin film, a silicon nitride film that has high adhesion to the substrate and can be a strong etching protection layer when the substrate is etched. Therefore, a MEMS element with a stable operating frequency can be realized.

Application Example 7 A method for manufacturing a MEMS element according to this application example includes a step of laminating a first thin film and a second thin film on a substrate, a first electrode, a second electrode on the second thin film, Forming a piezoelectric element portion including a piezoelectric material sandwiched between the first electrode and the second electrode, and removing a portion of the substrate overlapping the piezoelectric element portion in plan view to form a cavity portion And a step of performing.
According to this method, the first thin film in contact with the substrate and the second thin film are laminated on the substrate. A MEMS element having a diaphragm portion formed by such a process has a high operating frequency, a small variation in the operating frequency, and can improve reliability.

Application Example 8 In the MEMS element manufacturing method according to the application example described above, in the step of forming the cavity, the cavity is preferably formed using a solution containing potassium hydroxide.
According to this method, the cavity is formed with a solution containing potassium hydroxide, so that the first thin film is not etched and the film thickness of the diaphragm is stabilized. Furthermore, since the substrate is etched perpendicularly to the diaphragm portion, the substrate and the diaphragm portion intersect at a right angle, so that when the diaphragm portion vibrates, the end portion of the substrate that is a fulcrum may be destroyed. Can be suppressed. A MEMS element having a cavity formed by such a process has a small variation in operating frequency and can improve reliability.

(Application Example 9) An electronic apparatus according to this application example includes the MEMS element described in the above application example or the MEMS element manufactured by the method for manufacturing the MEMS element described in the above application example. And
According to this configuration, it is possible to realize a highly reliable electronic device that can operate at a high frequency and has a small variation in operating frequency.

1 is a cross-sectional view of a MEMS element according to Embodiment 1. FIG. The graph which shows the Young's modulus of a silicon oxide film, a zirconium oxide film, and a silicon nitride film. The graph which shows the film | membrane stress of a silicon oxide film, a zirconium oxide film | membrane, and a silicon nitride film. Sectional drawing explaining the manufacturing process of the MEMS element which concerns on this embodiment. The graph which shows the change of the Young's modulus by the annealing temperature at the time of zirconium oxide film formation. The graph which shows the change of the film | membrane stress by the annealing temperature at the time of zirconium oxide film formation. FIG. 6 illustrates an electronic device according to an embodiment. Sectional drawing of the MEMS element which concerns on the modification 1. FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the scale of each layer and each member is made different from the actual scale so that each layer and each member can be recognized.
In addition, in the following forms, when “on XX” is described, it is arranged so as to be in contact with the surface of XX, or is arranged on the surface of XX via another component. Or a case where a part is arranged on the surface of OO and a part is arranged via another component.

(Embodiment 1)
"Diaphragm part of MEMS element"
FIG. 1 is a cross-sectional view of the MEMS element according to the first embodiment. FIG. 2 is a graph showing the Young's modulus of the silicon oxide film, the zirconium oxide film, and the silicon nitride film. FIG. 3 is a graph showing film stresses of the silicon oxide film, the zirconium oxide film, and the silicon nitride film. First, the MEMS element 1000 according to the first embodiment will be described with reference to FIGS. 1 to 3.

  As shown in FIG. 1, the MEMS element 1000 includes a substrate 100, a diaphragm portion 101, a piezoelectric element portion 111, an extraction electrode 107, and the like. Specifically, the MEMS element 1000 includes a substrate 100 in which a cavity portion 108 is formed, a diaphragm portion 101 provided on the substrate 100 so as to cover the cavity portion 108, and a diaphragm portion 101 in a cross-sectional view. And a lead electrode 107 provided on the piezoelectric element portion 111.

In the diaphragm portion 101, a first thin film 102 and a second thin film 103 are laminated in order from the substrate 100 side. The first thin film 102 and the second thin film 103 are thin films having different characteristics.
The piezoelectric element unit 111 includes a first electrode 104, a second electrode 106, and a piezoelectric material 105 sandwiched between the first electrode 104 and the second electrode 106, and the first in order from the diaphragm unit 101 side. The electrode 104, the piezoelectric material 105, and the second electrode 106 are stacked.
The lead electrode 107 is made of a conductive material and is electrically connected to the first electrode 104 and the second electrode 106 of the piezoelectric element portion 111.

As the substrate 100, a silicon substrate containing silicon as a main component is used. Impurities such as phosphorus and boron may be added in addition to silicon. Among the silicon substrates, a single crystal silicon substrate is preferable, and it is desirable to use a single crystal silicon substrate having a plane orientation of (110). In this embodiment, the case where a single crystal silicon substrate having a plane orientation of (110) is used will be described.
As will be described later in detail, the cavity 110 is formed on the (110) surface of the single crystal silicon substrate by wet etching from the back surface (the lower side in FIG. 1) opposite to the surface on which the diaphragm 101 is formed. In this case, there is an advantage that etching can be performed vertically.
By etching vertically, the side wall of the substrate 100 forming the cavity 108 and the surface exposed to the cavity 108 of the diaphragm 101 intersect at a right angle, so that when the diaphragm 101 vibrates, the fulcrum The possibility that the end of the substrate 100 is destroyed can be suppressed.

  The diaphragm 101 vibrates in response to mechanical displacement of the piezoelectric material 105 when a voltage is applied to the first electrode 104 and the second electrode 106, and transmits a signal to the outside. Further, the diaphragm 101 mechanically displaces the piezoelectric material 105 by receiving a force generated from the outside and vibrates, and the voltage generated thereby is taken out from the first electrode 104 and the second electrode 106 to be externally supplied. A signal can be received. That is, the diaphragm unit 101 functions as a diaphragm capable of transmitting and receiving.

  By the way, generally, the operating frequency of the MEMS element is expressed by Equation 4.

Here, f is the operating frequency, a is the length of the short side of the diaphragm 101 in plan view, h is the film thickness, ρ is the density, λ is a constant, and D is the bending rigidity.
From Equation 4, it can be seen that increasing the bending rigidity D or shortening the short side length a is an effective means for increasing the operating frequency f. However, if the length a of the short side is shortened, it becomes more sensitive to the processing variation of the MEMS element, so that the variation of the oscillation frequency of each MEMS element becomes large. That is, increasing the bending rigidity D is a suitable means for high-frequency operation. This bending rigidity D is expressed by Equation 5.

  Here, E is Young's modulus and ν is Poisson's ratio. In order to increase the bending rigidity D, it is effective to use a material having a large Young's modulus E and to increase the film thickness h.

  FIG. 2 shows the results of measuring the Young's modulus of the silicon oxide film, the zirconium oxide film, and the silicon nitride film. Accordingly, it can be seen that the zirconium oxide film or the silicon nitride film has a Young's modulus four times or more higher than the silicon oxide film. Since the operating frequency f is proportional to the square root of the Young's modulus, when the diaphragm portion 101 has the same film thickness, the operating frequency is higher when the diaphragm portion 101 is made of a zirconium oxide film or a silicon nitride film than a silicon oxide film. f will be approximately twice as high. Therefore, for high frequency operation, it is desirable to configure the diaphragm portion 101 using a zirconium oxide film or a silicon nitride film.

  FIG. 3 shows the results of measuring the film stress of the silicon oxide film, the zirconium oxide film, and the silicon nitride film. The Y-axis scale of the graph indicates the film stress (Gpa), the positive direction indicates tensile stress, and the negative direction indicates compressive stress. This shows that the silicon nitride film has an extremely large compressive stress. The film stress is closely related to the crack of the film and the warp of the substrate in both the tensile stress and the compressive stress. When the stress is large, cracks are likely to occur when the film thickness is increased. In addition, when the warpage of the substrate becomes large, problems such as difficulty in transporting the substrate in the manufacturing apparatus at the time of manufacturing the MEMS element are caused. That is, when the silicon nitride film is used as the main element of the diaphragm portion 101, the film thickness must be reduced. From Equations 4 and 5, since the operating frequency f is proportional to the film thickness h of the diaphragm 101, the operating frequency is low when the film thickness h of the diaphragm 101 is thin. On the other hand, the zirconium oxide film has less film stress than the silicon nitride film, so that cracks are hardly generated and the substrate 100 is not easily warped. Therefore, it is easy to increase the film thickness h.

  As described above, when the zirconium oxide film is configured as the main element of the diaphragm portion 101 rather than the silicon nitride film as the main component of the diaphragm portion 101, the thick diaphragm portion 101 can be formed. High frequency operation is possible.

  In the present embodiment, the diaphragm 101 has a configuration in which a first thin film 102 and a second thin film 103 having different characteristics are laminated. Specifically, the diaphragm portion 101 is configured by using a first thin film 102 that plays a role of improving adhesion to the substrate 100 and a second thin film 103 that has high rigidity and can be easily thickened. Yes. By doing so, a highly reliable MEMS element 1000 capable of high-frequency operation can be realized.

The second thin film 103 is a main element of the diaphragm portion 101. By doing so, a highly reliable MEMS element 1000 having a higher frequency operation can be realized. In the present embodiment, when the thickness of the first thin film 102 is h ₁ , the Young's modulus is E ₁ , the thickness of the second thin film 103 is h ₂ , and the Young's modulus is E ₂ , the relational expression of Equation 6 is satisfied. It has a simple structure.

Referring to Equation 5, the bending stiffness D is proportional to the product Eh ³ of the Young's modulus and the cube of the film thickness. By taking the configuration of Equation 6, the second thin film 103 is made to be the main element of the diaphragm portion 101. It can be seen from Equation 4 that the MEMS element 1000 capable of high-frequency operation is realized.

In the present embodiment, a silicon nitride film is used for the first thin film 102 that is in contact with the substrate 100 among the two thin films constituting the diaphragm portion 101. Since the silicon nitride film is a material containing the same kind of material as the material of the substrate 100, adhesion between the diaphragm portion 101 and the substrate 100 can be improved.
The film thickness of the first thin film 102 is in the range of 50 nm to 200 nm. Thereby, compared with the case where the second thin film 103 is used as the diaphragm portion 101 in a single layer, the adhesion between the substrate 100 and the diaphragm portion 101 is improved. Furthermore, even if the MEMS element 1000 is operated at a high frequency, the risk of cracks occurring at the interface between the substrate 100 and the diaphragm portion 101 can be suppressed. That is, the MEMS element 1000 operates stably at a high frequency.

  Further, the silicon nitride film is not the same material as the substrate 100, but includes a material that partially includes the same material as the substrate 100 and is not included in the material of the substrate 100. Thus, the first thin film 102 can be used as an etching stopper layer when forming the cavity 108. As will be described later, when forming the cavity 108 in the substrate 100, the silicon nitride film as the first thin film 102 functions as an etching stopper when the substrate 100 is etched from the back surface. In particular, in the etching using a solution containing potassium hydroxide, the silicon nitride film (first thin film 102) is hardly etched, so that variations in the film thickness of the diaphragm portion 101 can be suppressed. From Equations 4 and 5, since the operating frequency f is proportional to the film thickness h of the diaphragm portion 101, suppressing the variation in the film thickness h leads to suppressing the variation in the operating frequency f.

  The first thin film 102 is not limited to a silicon nitride film as long as it has good adhesion to the substrate 100 and functions as an etching stopper when the cavity 108 is processed from the back surface. A silicon oxide film, a silicon film containing oxygen and nitrogen (SiON), or the like can also be used.

A second thin film 103 formed on the first thin film 102 when viewed from the substrate 100 is a main element of the diaphragm portion 101. As described above, in order to operate at a high frequency, it is required to increase the bending rigidity D, and it is necessary to increase the product Eh ³ of the Young's modulus and the cube of the film thickness. In this embodiment, a zirconium oxide film is used for the second thin film 103. The zirconium oxide film has a higher bending rigidity than the silicon nitride film of the first thin film 102, and the MEMS element 1000 can be operated at a high frequency by using the zirconium oxide film as a main element of the diaphragm portion 101.
Since the second thin film 103 is a main element of the diaphragm portion 101, it is preferable that the second thin film 103 be formed as thick as possible to be 400 nm or more. By setting the thickness of the second thin film 103 to 400 nm or more, the second thin film 103 becomes the main element of the diaphragm portion 101, and the MEMS element 1000 capable of high-frequency operation can be realized.

  As the second thin film 103, for example, an aluminum oxide film, a titanium oxide film, a hafnium oxide film, or a thin film obtained by synthesizing these materials can be used as long as it is easy to increase the thickness and has a high Young's modulus. May be.

The film thickness of the second thin film 103 is preferably larger than that of the first thin film 102. Specifically, the thickness h ₁ of the first thin film 102 and the thickness h ₂ of the second thin film 103, it is preferable to satisfy the relational expression of Equation 7.

  Accordingly, the second thin film 103 can be used as the main element of the diaphragm portion 101 by satisfying Expression 6.

Moreover, it is preferable to make the film thickness of the second thin film 103 which is the main element of the diaphragm portion 101 as large as possible. As described above, it is desirable that the film stress is close to 0 in order to increase the film thickness. Specifically, the first thin film stress and P _1, the second thin film stress upon the P _2, the film stress P ₁ of the first thin film 102 and the film stress P ₂ of the second membrane 103 However, it is preferable that the relational expression of Expression 8 is satisfied.

  According to this configuration, the film stress of the second thin film 103 is smaller than the film stress of the first thin film 102. Accordingly, it is easy to increase the thickness of the second thin film 103, and it is possible to increase the thickness of the diaphragm portion 101 while suppressing defects such as cracks. That is, with this configuration, the MEMS element 1000 with high frequency operation and high reliability can be realized.

  In addition, it is preferable that the thickness of the 2nd thin film 103 shall be 2000 nm or less. By setting the thickness of the second thin film 103 to 2000 nm or less, the probability of occurrence of cracks due to film stress can be reduced, and the substrate can be easily transported in the manufacturing apparatus when the MEMS element 1000 is manufactured. Therefore, the MEMS element 1000 capable of high-frequency operation can be stably manufactured with a high yield.

  With the configuration of the diaphragm unit 101 described above, a high-reliability MEMS element 1000 capable of high-frequency operation, small variation in operating frequency, and high reliability can be realized. This configuration is merely an example, and for example, the functions described in the thin film of each layer may be configured by a combination of multiple layers.

"Piezoelectric element part of MEMS element"
Next, the piezoelectric element unit 111 will be described. The piezoelectric element unit 111 includes a first electrode 104, a second electrode 106, and a piezoelectric material 105 sandwiched between the first electrode 104 and the second electrode 106.

  For the first electrode 104, for example, a conductive film selected from nickel, iridium, gold, platinum, tungsten, titanium, palladium, silver, tantalum, molybdenum, chromium, a composite film of strontium and ruthenium, a composite film of lanthanum and nickel, and the like. Sex materials are used. The film thickness of the first electrode 104 is preferably in the range of 20 nm to 400 nm.

The piezoelectric material 105 is made of a piezoelectric material made of a perovskite oxide represented by the general formula ABO ₃ . In this embodiment, lead zirconate titanate (PZT) is used as an example. However, for example, a relaxor ferroelectric material in which a metal such as niobium, nickel, magnesium, bismuth or yttrium is added to a ferroelectric piezoelectric material of lead zirconate titanate (PZT) may be used.
The composition may be appropriately selected in consideration of the characteristics and application of the piezoelectric element portion. For example, PbTiO ₃ (PT), PbZrO ₃ (PZ), Pb (Zr _x Ti _1-x ) O ₃ (PZT) ), Pb (Mg _1/3 Nb _2/3 ) O ₃ —PbTiO ₃ (PMN-PT), Pb (Zn _1/3 Nb _2/3 ) O ₃ —PbTiO ₃ (PZN—PT), Pb (Ni ₁ ) _{/ 3} Nb _2/3 ) O ₃ —PbTiO ₃ (PNN-PT), Pb (In _1/2 Nb _1/2 ) O ₃ —PbTiO ₃ (PIN-PT), Pb (Sc _1/3 Ta _2/3 ) O ₃ -PbTiO ₃ (PST-PT), Pb (Sc _1/3 Nb _2/3 ) O ₃ -PbTiO ₃ (PSN-PT), BiScO ₃ -PbTiO ₃ (BS-PT), BiYbO ₃ -PbTiO ₃ (BY-PT) and the like.
The film thickness of the piezoelectric material 105 is preferably in the range of 1000 nm to 5000 nm.

  Similar to the first electrode 104, the second electrode 106 is, for example, nickel, iridium, gold, platinum, tungsten, titanium, palladium, silver, tantalum, molybdenum, chromium, a composite film of strontium and ruthenium, lanthanum and nickel. A conductive material selected from a composite film or the like is used. The film thickness of the second electrode 106 is preferably in the range of 50 nm to 400 nm.

  The extraction electrode 107 is similar to the first electrode 104 and the second electrode 106, for example, nickel, iridium, gold, platinum, tungsten, titanium, palladium, silver, tantalum, molybdenum, chromium, a composite film of strontium and ruthenium, lanthanum A conductive material selected from a composite film of nickel and nickel is used. The film thickness of the extraction electrode 107 is preferably in the range of 50 nm to 400 nm. With the extraction electrode 107, the first electrode 104 and the second electrode 106 can be connected to an external terminal and an external element.

As described above, according to the MEMS element 1000 according to the present embodiment, the following effects can be obtained. Diaphragm 101 is composed of at least two thin films of first thin film 102 and second thin film 103. Thereby, the adhesiveness with the board | substrate 100 is strengthened by the 1st thin film 102, The possibility that the MEMS element 1000 may be damaged by the vibration of the diaphragm part 101 is suppressed, and the high reliability of the MEMS element 1000 is ensured. Furthermore, since the first thin film 102 functions as an etching stopper layer when the substrate 100 is etched to form the cavity 108, the entire film thickness of the diaphragm portion 101 is stabilized and the frequency variation of the MEMS element 1000 is suppressed. Can do.
By using a material with a high Young's modulus and a small film stress for the second thin film 103, the probability of occurrence of cracks and the like can be greatly suppressed, and the second thin film 103 can be formed thicker. A possible MEMS element 1000 can be stably manufactured.
In this manner, by configuring the diaphragm portion 101 with at least two layers of thin films, a high-reliable MEMS element 1000 capable of high-frequency operation, small variation in operating frequency, and high reliability is realized.

"Production method"
FIG. 4 is a cross-sectional view illustrating the manufacturing process of the MEMS element according to this embodiment. FIG. 5 is a graph showing the change in Young's modulus depending on the annealing temperature when forming the zirconium oxide film. FIG. 6 is a graph showing changes in film stress depending on the annealing temperature when forming a zirconium oxide film. Next, with reference to FIG. 4 thru | or FIG. 6, the manufacturing method of the MEMS element 1000 which concerns on this embodiment is demonstrated.

The manufacturing method of the MEMS element 1000 includes a step of laminating the first thin film 102 and the second thin film 103 on the substrate 100 (first step), and a step of forming the piezoelectric element portion 111 after the first step (second step). And a step (third step) of forming a cavity 108 by removing a portion of the substrate 100 that overlaps the piezoelectric element portion 111 in plan view after the second step.
The piezoelectric element portion 111 formed in the second step includes a first electrode 104, a second electrode 106, a piezoelectric material 105 sandwiched between the first electrode 104 and the second electrode 106 on the second thin film 103, Formed from.

In the first step, first, a first thin film 102 is formed on a substrate 100 as shown in FIG. The first thin film 102 serves as an etching stopper layer for securing adhesion between the substrate 100 and the diaphragm portion 101 and forming a cavity portion 108 by removing a part of the substrate 100.
In order to have these functions, a material containing the same element as the substrate 100 is desired. In the present embodiment, a silicon nitride film is formed using a plasma CVD method, particularly an inductively coupled plasma CVD method (ICP-CVD method), and the first thin film 102 is formed.

When a general parallel plate type plasma CVD method is used, the film stress can be controlled and the film thickness can be increased. However, since a film containing a large amount of hydrogen is formed by using silane (SiH ₄ ), ammonia (NH ₃ ), and nitrogen (N ₂ ), the oxide material of the piezoelectric material 105 described later is reduced. Cause malfunctions, leading to degradation of piezoelectric characteristics.
Further, since it is difficult to form a dense film, the substrate 100 is etched when it is used as an etching stopper layer when the cavity portion 108 is formed by etching from the back surface, and the film of the diaphragm portion 101 is formed. The thickness may vary.

From Equations 4 and 5, since the film thickness h of the diaphragm portion 101 is proportional to the operating frequency f, if the film thickness of the diaphragm portion 101 varies, the operating frequency f varies. The silicon nitride film formed by the ICP-CVD method used in this embodiment is a very dense film and is hardly etched even when the cavity 108 is formed, so that the thickness of the diaphragm 101 is stabilized.
That is, according to the present embodiment, a MEMS element with extremely small variation in the operating frequency f can be manufactured. Further, since the film can be formed only with silane (SiH ₄ ) and nitrogen (N ₂ ), a film having a very low hydrogen content can be formed, and the piezoelectric characteristics are not deteriorated. ing.

As described above, the thickness of the silicon nitride film is preferably in the range of 50 nm to 200 nm. As a result, adhesion between the substrate 100 and the diaphragm portion 101 is ensured, and a stable operation can be ensured without causing cracks at the interface between the substrate 100 and the diaphragm portion 101 even in high-frequency operation.
In addition, when forming the cavity 108 in the substrate 100, when the substrate 100 is etched from the back surface, the silicon nitride film that is the first thin film 102 functions as an etching stopper layer, so that the variation in operating frequency is extremely small. An element can be manufactured.

Note that the silicon nitride film can be formed not only by the ICP-CVD method but also by using a low pressure CVD method (LPCVD method). Dichlorosilane (SiH ₂ Cl ₂ ) and ammonia (NH ₃ ) are used as the reaction gas, and the film is formed at about 700 ° C. to 800 ° C.
The silicon nitride film formed by the LPCVD method is a very dense film, like the silicon nitride film formed by the ICP-CVD method, and has an extremely small hydrogen content and is suitable as the first thin film 102. The film stress is a compressive stress as shown in FIG. 3 for the silicon nitride film formed by the ICP-CVD method, whereas the silicon nitride film formed by the LPCVD method has a tensile stress. Therefore, it is difficult to increase the film thickness, and the thickness formed as the first thin film 102 is in the range of 50 nm to 200 nm, similar to the silicon nitride film formed by the ICP-CVD method. In addition, the formation method of the 1st thin film 102 can be used suitably, such as CVD method.

  Subsequently, as shown in FIG. 4B, a second thin film 103 is formed. Zirconium (Zr) is deposited and annealed in an oxygen atmosphere to form a zirconium oxide film. As a method for forming a zirconium (Zr) film, a CVD method or a sputtering method can be used. In the present embodiment, deposition is performed using a sputtering method. The annealing is first performed at 900 ° C. in an oxygen atmosphere by a rapid annealing apparatus (RTA; Rapid Thermal Anneal). As a result, zirconium is almost oxidized to form a zirconium oxide film (ZrOx). Subsequently, annealing is performed in an oxygen atmosphere in an annealing furnace for 60 minutes. Thus, a zirconium oxide film (ZrOx) as the second thin film 103 can be formed.

As a result of intensive studies by the present inventors, it has been found that the film stress can be controlled by the annealing temperature in an oxygen atmosphere in an annealing furnace. FIG. 5 shows a graph showing the change in Young's modulus depending on the annealing temperature during the formation of the zirconium oxide film. FIG. 6 is a graph showing changes in film stress depending on the annealing temperature when forming a zirconium oxide film.
From these figures, it was found that the film stress greatly changes depending on the annealing temperature, but the change in Young's modulus is small. From Equations 4 and 5, it can be seen that the operating frequency f is proportional to the square root of the Young's modulus and proportional to the film thickness. In other words, the Young's modulus is slightly reduced by lowering the annealing temperature, but the effect is minor because the operating frequency is proportional to the square root. On the other hand, since the film stress changes greatly by lowering the annealing temperature, it is easy to increase the film thickness, and the effect is proportional to the operating frequency.

That is, it is understood that it is preferable to lower the annealing temperature for high frequency operation. Even at an annealing temperature of 900 ° C., it can be said that the film stress is sufficiently small (close to zero) as compared to the silicon nitride film of FIG. 3, but by suppressing the annealing temperature to 800 ° C. or less, generation of cracks due to the stress is suppressed. A thicker film can be formed.
However, it is necessary to anneal at a temperature higher than the firing temperature when forming the piezoelectric material 105 described later. This is because if the annealing temperature is lower than the firing temperature at the time of forming the piezoelectric material 105, the film stress of the zirconium oxide film below the piezoelectric material 105 changes during the firing, and the warpage changes. Stress is applied, leading to degradation of piezoelectric characteristics.
Therefore, the annealing temperature is desirably 700 ° C. or higher and 900 ° C. or lower, and more preferably 700 ° C. or higher and 800 ° C. or lower.

  From the above points, the zirconium oxide film used as the second thin film 103 for enabling high-frequency operation is desirably 400 nm or more and 2000 nm or less, and more preferably 800 nm or more and 2000 nm or less. By suppressing to 2000 nm or less, the probability of occurrence of cracks can be greatly suppressed.

  In the present embodiment, the second thin film 103 is deposited using the sputtering method, but a method of forming the second thin film 103 using, for example, an ALCVD method or a reactive sputtering method may be used.

Next, the process proceeds to a second step for forming the piezoelectric element portion 111.
In the second step, first, as shown in FIG. 4C, a first conductive film 104C that will later become the first electrode 104 is formed. As the first conductive film 104C, platinum and iridium were deposited on the second thin film 103 to a thickness of about 20 to 400 nm by a sputtering method. As an electrode material, for example, a conductive material selected from nickel, gold, tungsten, titanium, palladium, silver, tantalum, molybdenum, chromium, a composite film of strontium and ruthenium, a composite film of lanthanum and nickel, or the like is used. Also good. As a film forming method, not only a sputtering method but also a sol-gel method or a thermal CVD method may be used.

  Next, as shown in FIG. 4D, the first conductive film 104C is subjected to photolithography, anisotropic etching, and resist stripping in this order, and is patterned into a predetermined shape to form the first electrode 104.

Next, as shown in FIG. 4E, for example, a piezoelectric material 105 made of lead zirconate titanate (PZT) is formed. Here, in this embodiment, a so-called sol-gel method is used in which a so-called sol in which a metal organic substance is dissolved and dispersed in a catalyst is applied, dried, gelled, and further baked at a high temperature to obtain a piezoelectric material 105 made of a metal oxide. Thus, a piezoelectric material 105 made of lead zirconate titanate (PZT) is formed.
The manufacturing method of the piezoelectric material 105 is not limited to the sol-gel method, and for example, a MOD (Metal-Organic Decomposition) method or the like may be used. In addition, when the piezoelectric material 105 is formed in this way, the lead component of the piezoelectric material 105 may diffuse into the diaphragm portion 101 during firing. However, a second thin film 103 made of a zirconium oxide film is provided below the piezoelectric material 105. Therefore, the lead component of the piezoelectric material 105 can be prevented from diffusing into the diaphragm portion 101.
As described above, high output characteristics can be obtained by forming the piezoelectric material 105 without losing the optimum composition ratio.

  Subsequently, as shown in FIG. 4E, a second conductive film 106C to be the second electrode 106 later is formed. The second conductive film 106C was deposited on the piezoelectric material 105 to a thickness of about 20 nm to 400 nm by sputtering. As the second conductive film 106C, for example, a conductive material selected from nickel, gold, tungsten, titanium, palladium, silver, tantalum, molybdenum, chromium, a composite film of strontium and ruthenium, a composite film of lanthanum and nickel, or the like. It may be used. As a film forming method, not only a sputtering method but also a sol-gel method or a thermal CVD method may be used.

  Next, as shown in FIG. 4F, the second conductive film 106C is sequentially subjected to photolithography, etching, and resist peeling to form the second electrode 106. At this time, the piezoelectric material 105 is simultaneously processed into a predetermined shape.

  Next, as shown in FIG. 4G, the extraction electrode 107 is formed. First, a conductive material to be the extraction electrode 107 was deposited by sputtering to a thickness of about 20 nm to 400 nm. As the conductive material, for example, a conductive material selected from nickel, gold, tungsten, titanium, palladium, silver, tantalum, molybdenum, chromium, a composite film of strontium and ruthenium, a composite film of lanthanum and nickel, or the like is used. Can do. Subsequently, photolithography, etching, and resist stripping are sequentially performed on the conductive material thus deposited to form the extraction electrode 107. Thereby, the first electrode 104 and the second electrode 106 can be connected to the external terminal and the external element.

  Next, a protective film (not shown) is applied to the entire upper surface of the substrate 100 so as to cover the diaphragm portion 101, the first electrode 104, the piezoelectric material 105, the second electrode 106, and the like. For example, a photoresist can be used as an example of the protective film.

Next, the process proceeds to a third step for forming the cavity 108.
In the third step, first, as shown in FIG. 4H, a cavity 108 is formed on one surface (back surface) of the substrate 100 on which the piezoelectric element portion 111 is not formed. First, a silicon oxide film (not shown) is formed on one surface (back surface) of the substrate 100. Subsequently, photolithography, etching, and resist removal are sequentially performed, and the silicon oxide film is processed so as to have a shape other than the portion where the cavity 108 is formed in a plan view.
Using this silicon oxide film as a hard mask, the substrate 100 is etched to form the cavity 108. Although a silicon oxide film is used in this embodiment, a resist may be used as long as it can withstand etching of the substrate 100, for example.

Furthermore, when a silicon nitride film is formed by LPCVD at the time of forming the first thin film 102, a silicon nitride film of the same degree is formed on the back surface, and therefore it can be used as a hard mask.
For etching the substrate 100, wet etching using a solution containing potassium hydroxide is preferable. The solution containing potassium hydroxide is a highly selective solution that can etch the substrate 100 and hardly etch the first thin film 102.

  Since the first thin film 102 is not etched, the film thickness of the diaphragm portion 101 is stabilized, so that variation in operating frequency can be suppressed. In addition, when a single crystal silicon substrate having a plane orientation of (110) is used as the substrate 100, the substrate 100 can be etched vertically. By etching vertically, the side wall of the substrate 100 forming the cavity 108 and the diaphragm portion 101 intersect at a right angle. Therefore, when the diaphragm portion 101 vibrates, the end portion of the substrate 100 that is a fulcrum is The possibility of destruction can be suppressed, and a highly reliable MEMS element 1000 can be manufactured.

  As a method of forming the cavity 108 by etching the substrate 100, it is sufficient to satisfy the requirements that the first thin film 102 can be an etching stopper layer and that the first thin film 102 can be etched perpendicular to the diaphragm 101, for example, dry etching. The method can also be used.

  Finally, the protective film formed on the entire upper surface of the substrate 100 is removed. In this way, the cavity 108 is formed in the substrate 100. The portion where the cavity 108 is formed in the substrate 100 and the first thin film 102 is exposed on the back surface side becomes the diaphragm portion 101. The MEMS element 1000 is completed through the above steps.

  As described above, according to this embodiment, it is possible to realize a MEMS element 1000 that can operate at high frequency, has a small variation in operating frequency, and has high reliability.

"Electronics"
FIG. 7 is a diagram illustrating the electronic apparatus according to the present embodiment. Next, the electronic apparatus according to the present embodiment will be described with reference to FIG.

As shown in FIG. 7, the ultrasound diagnostic apparatus S transmits an ultrasound signal to a subject H such as a living body (not shown) and receives a reflected ultrasound signal of the ultrasound reflected by the subject H. It has an ultrasonic probe 2 and an ultrasonic diagnostic apparatus main body 1.
The ultrasonic probe 2 and the ultrasonic diagnostic apparatus main body 1 are connected via a cable 50. In the ultrasonic diagnostic apparatus S, the ultrasonic probe 2 is caused to transmit an ultrasonic signal to the subject H by transmitting an electrical signal from the ultrasonic diagnostic apparatus body 1 to the ultrasonic probe 2. Further, a reception signal is generated according to the reflected ultrasonic signal from the subject H received by the ultrasonic probe 2.
Next, based on the received signal of the electrical signal (reflected ultrasound signal) generated by the ultrasound probe 2, the internal state in the subject H is imaged as a medical image as an elastic image (and tomographic image). .
The MEMS element 1000 described in this embodiment or the MEMS element 1000 manufactured by the manufacturing method described in this embodiment is used in the ultrasonic probe 2.

  As described above, the electronic apparatus according to the present embodiment includes the ultrasonic probe 2 that is capable of high-frequency operation, has a small variation in operating frequency, and has high reliability. Therefore, a clear ultrasonic image with high resolution can be obtained. Can be obtained.

  Note that the present invention is not limited to the above-described embodiment, and various modifications and improvements can be added to the above-described embodiment. A modification will be described below.

(Modification 1)
FIG. 8 is a cross-sectional view of a MEMS element 1100 according to Modification 1. Hereinafter, the MEMS element 1100 according to the first modification will be described with reference to FIG. In addition, about the component same as Embodiment 1, the same number is attached | subjected and the overlapping description is abbreviate | omitted.

  In this modification, an adhesion film 109 is formed between the first thin film 102 and the second thin film 103 of the diaphragm portion 101a. When the adhesion between the first thin film 102 and the second thin film 103 is F1, the adhesion between the first thin film 102 and the adhesion film 109 is F2, and the adhesion between the adhesion film 109 and the second thin film 103 is F3, Equation 9 Are in a relationship.

  That is, by forming the adhesion film 109, the adhesion force of the diaphragm portion 101a is further improved. As a result, even in high-frequency operation or long-term operation, it is possible to greatly suppress the probability that the interface of each film of the diaphragm portion 101a peels off. Thereby, the reliability of the MEMS element can be further improved.

  DESCRIPTION OF SYMBOLS 1 ... Ultrasonic diagnostic apparatus main body, 2 ... Ultrasonic probe, 50 ... Cable, 100 ... Board | substrate, 101 ... Diaphragm part, 102 ... 1st thin film, 103 ... 2nd thin film, 104 ... 1st electrode, 104C ... 1st One conductive film, 105 ... piezoelectric material, 106 ... second electrode, 106C ... second conductive film, 107 ... extraction electrode, 108 ... cavity, 109 ... adhesion film, 111 ... piezoelectric element part, 1000 ... MEMS element.

Claims (9)

A substrate having a cavity formed thereon;
A diaphragm provided on the cavity,
A piezoelectric element portion provided on the diaphragm portion,
The diaphragm portion is formed by laminating at least a first thin film and a second thin film in contact with the substrate,
The piezoelectric element portion includes a first electrode, a second electrode, and a piezoelectric material sandwiched between the first electrode and the second electrode. When the thickness of the first thin film is h ₁ , the Young's modulus is E ₁ , the thickness of the second thin film is h ₂ , and the Young's modulus is E ₂ , the relational expression of Formula 1 is satisfied. The MEMS device according to claim 1.

3. The MEMS element according to claim 2, wherein a thickness h ₁ of the first thin film and a thickness h ₂ of the second thin film satisfy a relational expression of Formula 2. 4.

When the film stress of the first thin film is P ₁ and the film stress of the second thin film is P ₂ , the film stress P ₁ of the first thin film and the film stress P ₂ of the second thin film are expressed by Equation 3. The MEMS element according to claim 1, wherein the relational expression is satisfied.

  5. The MEMS element according to claim 1, wherein the second thin film is a zirconium oxide film.   The MEMS device according to claim 1, wherein the first thin film is a silicon nitride film. Laminating a first thin film and a second thin film on a substrate;
Forming on the second thin film a piezoelectric element portion comprising a first electrode, a second electrode, and a piezoelectric material sandwiched between the first electrode and the second electrode;
Removing a portion of the substrate overlapping the piezoelectric element portion in plan view to form a cavity portion;
A method for manufacturing a MEMS device, comprising:   The method for manufacturing a MEMS device according to claim 7, wherein in the step of forming the cavity, the cavity is formed using a solution containing potassium hydroxide.   An electronic device comprising the MEMS element according to claim 1 or the MEMS element manufactured by the method for manufacturing a MEMS element according to claim 7 or 8.

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