Patent Publication Number: US-2022224253-A1

Title: Electrostatic Device and Method for Manufacturing Electrostatic Device

Description:
TECHNICAL FIELD 
     The present invention relates to an electrostatic device and a method for manufacturing an electrostatic device. 
     BACKGROUND ART 
     As an electrostatic device, one that is described in Patent Literature 1 has been known, for example. The electrostatic device described in Patent Literature 1 is made of an SOI (Silicon On Insulator) substrate. The SOI (Silicon On Insulator) substrate is composed of a support layer made of silicon, a BOX (Buried Oxide) layer made of silicon oxide (SiO 2 ) formed on the support layer, and an active layer made of silicon bonded on the BOX layer. An actuator portion or sensor portion of the electrostatic device is formed from the active layer and a base material that supports the actuator portion or sensor portion is formed from the support layer. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2016-59191 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, an expensive SOI substrate is used for a substrate for device fabrication in the above described electrostatic device, and therefore the substrate cost has been one of obstructive factors for hindering cost reduction of the electrostatic device. 
     Solution to Problem 
     According to a first aspect of the present invention, an electrostatic device includes: a fixed portion; a moveable portion; an elastically-supporting portion formed integrally with the moveable portion and elastically supporting the moveable portion; and a base portion made of glass to which the fixed portion and the elastically-supporting portion are anodically bonded in a state in which the fixed portion and the elastically-supporting portion are separated from each other. 
     Preferably, according to a second aspect of the present invention, in the electrostatic device according to the first aspect, the fixed portion and the moveable portion are formed of silicon, and an electret is formed on at least one of the fixed portion and the moveable portion. 
     Preferably, according to a third aspect of the present invention, in the electrostatic device according to the second aspect, a fixed electrode is formed in the fixed portion, a moveable electrode facing the fixed electrode is formed in the moveable portion, and the moveable portion is displaced relative to the fixed portion such that capacitance changes between the fixed electrode and the moveable electrode and electricity is generated. 
     A method for manufacturing an electrostatic device according to a fourth aspect of the present invention is a method for manufacturing an electrostatic device according to any one aspect of the first to third aspects, including: forming the fixed portion, the moveable portion, and the elastically-supporting portion on a substrate in an integral manner; anodically bonding the base portion to the substrate to fix the fixed portion and the elastically-supporting portion on the base portion; and performing etching on the substrate to separate the fixed portion and the elastically-supporting portion from each other. 
     Advantageous Effect of Invention 
     According to the present invention, costs of the electrostatic device can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view of a vibration-driven energy harvesting element. 
         FIG. 2  provides views illustrating an A-A cross section and a B-B cross section of  FIG. 1 . 
         FIG. 2  is a diagram showing a cross section taken along the line A-A and a cross section taken along the line B-B in  FIG. 1 . 
         FIG. 3  is a view for explaining a first step. 
         FIG. 4  is a view for explaining a second step. 
         FIG. 5  is a view for explaining a third step. 
         FIG. 6  provides views illustrating an A-A cross section, a B-B cross section, and a C-C cross section of  FIG. 5 . 
         FIG. 7  is a view for explaining a fourth step. 
         FIG. 8  is a view for explaining a fifth step. 
         FIG. 9  is a view for explaining a sixth step. 
         FIG. 10  is a view for explaining a seventh step. 
         FIG. 11  is a view for explaining an eighth step. 
         FIG. 12  is a view for explaining a ninth step. 
         FIG. 13  is a view for explaining a tenth step. 
         FIG. 14  is a view of a Comparative Example. 
         FIG. 15  provides graphs representing results of simulation of vibration-driven energy harvesting in the Comparative Example: the view (a) represents electric current, and the view (b) represents electric power. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Modes for implementing the present invention will now be described with reference to the drawings.  FIG. 1  illustrates an example of an electrostatic device and is a plan view of an electrostatic vibration-driven energy harvesting element  1 . The vibration-driven energy harvesting element  1  includes a base portion  10 , a fixed portion  11  provided on the base portion  10 , and a moveable portion  12 . There is a right-and-left pair of the fixed portions  11 , each of which includes a plurality of comb electrodes  110  formed thereon. The moveable portion  12  located between the pair of fixed portions  11  also includes a plurality of comb electrodes  120  formed thereon. The comb electrodes  120  and the comb electrodes  110  are positioned to face each other so as to interdigitate with each other. 
     The moveable portion  12  is supported by 4 sets of elastically-supporting portions  13 , and the moveable portion  12  vibrates in a right-left direction in the figure (x-direction) when the vibration-driven energy harvesting element  1  is subjected to an external force. Each elastically-supporting portion  13  includes a fixed area  13   a  fixed on the base portion  10 , and an elastic portion  13   b  that joins the fixed area  13   a  with the moveable portion  12 . At least one of either comb electrodes  110  or comb electrodes  120  includes electrets formed thereon, and electricity is generated in response to a change in the amount of interdigitation between the comb electrodes  110  and the comb electrodes  120  when the moveable portion  12  vibrates in the right-left direction in the figure. The fixed portion  11  includes an electrode pad  111  formed thereon, and similarly an electrode pad  131  is formed on the fixed area  13   a  of the elastically-supporting portion  13 . Generated electricity is to be output from the electrode pads  111 ,  131 . 
       FIG. 2  illustrates cross sections of  FIG. 1 : the view (a) in  FIG. 2  illustrates an A-A cross section, and the view (b) in  FIG. 2  illustrates a B-B cross section. The fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  are formed of an Si substrate, and SiO 2  films  202  that contain alkali metal ions such as potassium are formed on surfaces of the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13 . The electret is formed in the SiO 2  film  202 . The fixed portion  11  and the fixed area  13   a  of the elastically-supporting portion  13  are anodically bonded on the base portion  10  formed of a glass substrate. A recess  101  is formed in the base portion  10 . 
     The fixed portion  11  and the fixed area  13   a  are separated from each other by a separating groove g 1 , and the fixed portion  11  is electrically isolated from the elastically-supporting portion  13  and the moveable portion  12 . A separating groove g 2  illustrated in the view (b) in  FIG. 2  is one for electrically separating the right-and-left pair of fixed portions  11 . The moveable portion  12  is elastically supported above the recess  101  by the elastically-supporting portion  13 . A metal layer  102  is formed on a back face of the base portion  10 . The comb electrodes  110  of the fixed portion  11  are also located above the recess  101  so as to interdigitate with the comb electrodes  120  of the moveable portion  12 . 
     (Method for Manufacturing Vibration-Driven Energy Harvesting Element  1 ) 
       FIGS. 3 to 16  illustrate an example procedure of manufacturing the vibration-driven energy harvesting element  1 . In a first step illustrated in  FIG. 3 , SiN films  201  are formed by means of LP-CVD on both front and back faces of the Si substrate  200 .  FIG. 4  provides views for explanation of a second step: the view (a) in  FIG. 4  is a plan view, and the view (b) in  FIG. 4  is an A-A cross-sectional view. In the second step, the SiN film  201  on the front face side is subjected to dry etching to form patterns P 1 , P 2  for forming electrode pads  111 ,  113  and patterns P 3 , P 4  for forming separating grooves g 1 , g 2 . 
       FIGS. 5 and 6  illustrate a third step for explanation:  FIG. 5  illustrates a plan view, the view (a) in  FIG. 6  illustrates an A-A cross-sectional view, the view (b) in  FIG. 6  illustrates a C-C cross-sectional view, and the view (c) in  FIG. 6  illustrates a B-B cross-sectional view. In the third step, Al (aluminum) mask patterns (not illustrated) are formed on the front face side of the Si substrate  200  for forming the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13 , and the Al mask patterns are used to perform etching through the Si substrate  200  and the SiN film  201  by means of Deep-RIE. In this etching, structures of the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  that are included in an area D illustrated in  FIG. 5  are formed. Specifically, portions of the fixed portion  11  and the moveable portion  12  where the comb electrodes  110 ,  210  are formed and the elastically-supporting portion  13  are formed. The area D in  FIG. 5  represents an area above the recess  101  in  FIG. 2 . 
       FIG. 7  provides views for explanation of a fourth step: the view (a) in  FIG. 7  illustrates an A-A cross-sectional view, the view (b) in  FIG. 7  illustrates a C-C cross-sectional view, and the view (c) in  FIG. 7  illustrates a B-B cross-sectional view. In the fourth step, etching is performed to form the separating grooves g 1 , g 2  by means of Deep-RIE. The separating grooves g 1 , g 2  are formed in positions of the patterns P 3 , P 4  (see  FIG. 4 ). In the fourth step, however, etching is performed to a certain depth in such a way that the separating grooves g 1 , g 2  are not completely separated and the entire Si substrate  200  from the substrate back face side is kept integral (so called half etching). 
       FIG. 8  provides views for explanation of a fifth step: the view (a) in  FIG. 8  illustrates a plan view, and the view (b) in  FIG. 8  illustrates an A-A cross-sectional view. In the fifth step, the SiO 2  films  202  that contain alkali metal ions such as potassium are formed on exposed surfaces of the Si substrate  200 . 
       FIG. 9  provides views for explanation of a sixth step: the view (a) in  FIG. 9  illustrates a plan view, and the view (b) in  FIG. 9  illustrates an A-A cross-sectional view. In the sixth step, the SiN film  201  on the back face of the substrate is first removed by RIE using CF 4  gas. Similarly, the SiN film  201  on the front face side of the substrate is removed. 
       FIG. 10  provides views for explanation of a seventh step: the view (a) in  FIG. 10  illustrates a plan view, and the view (b) in  FIG. 10  illustrates a cross-sectional view. In the seventh step, the recess  101  is formed in a glass substrate  300  for forming the base portion  10 . A step height H between a bottom surface of the recess  101  and an end face of the frame portion  103  is set to such a dimension that interference of the moveable portion  12  is avoided when it is vibrating (for example, tens of micrometres). A glass substrate used for anodic bonding (for example, a sodium-containing glass substrate) is used for the glass substrate  300 . 
       FIG. 11  provides views for explanation of an eighth step: the view (a) in  FIG. 11  illustrates a plan view, and the view (b) in  FIG. 11  illustrates a sectional view. In the eighth step, the metal layer  102  such as aluminum deposited film is formed on the back face side of the base portion  10 . The metal layer  102  on the back face side is formed for dispersing an electric field to the entire surface of the glass substrate  300  during an anodic bonding process. However, the anodic bonding is achievable without the metal layer  102 , and therefore the metal layer  102  is not essential. 
     In a ninth step illustrated in  FIG. 12 , the base portion  10  composed of the glass substrate illustrated in  FIG. 11  is anodically bonded to the back face side of the Si substrate  200  in which the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  are formed (see  FIG. 9 ). The base portion  10  is placed on a heater  40 , and the Si substrate  200  in which the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  are formed is stacked on the base portion  10 . Temperature of the heater  40  is set to a temperature at which thermal diffusion of sodium ions in the glass substrate is sufficiently active (for example, 500° C. or higher). Voltage V 1  of the Si substrate  200  with reference to the heater  40  is set to, for example, 400 V or higher. 
     In anodically bonding the silicon substrate (Si substrate  200 ) and the glass substrate (base portion  10 ), while the stack of the silicon substrate and the glass substrate is heated, a DC voltage of hundreds of volts is applied to the stack with the silicon substrate side being an anode. Sodium ions in the glass substrate move to the negative potential side, and an SiO −  space charge layer (a layer depleted of sodium ions) is formed in an interface on the glass substrate side between the glass substrate and the silicon substrate. The resultant electrostatic attraction causes the glass substrate and the silicon substrate to be bonded. 
       FIG. 13  provides views for explanation of a tenth step: the view (a) in  FIG. 13  illustrates an A-A cross-sectional view, the view (b) in  FIG. 13  illustrates a C-C cross-sectional view, and the view (c) in  FIG. 13  illustrates a B-B cross-sectional view. In the tenth step, the Si substrate  200  anodically bonded to the base portion  10  is subjected to etching by means of Deep-RIE partway to open the separating grooves g 1 , g 2  illustrated in  FIG. 7 , which are left unpenetrated, through from front to back of the Si substrate  200 . This completely separates the fixed portion  11  from the elastically-supporting portion  13  that elastically supports the moveable portion  12 . Note that in this etching, hole-shaped electrode pads  111 ,  131  are also formed, in addition to the fact that the separating grooves g 1 , g 2  are opened through. 
     Thereafter, electrets are formed on at least one of either comb electrodes  110  or comb electrodes  120  according to a known method for forming electrets, for example, the Bias-Temperature method described in Japanese Patent Laid-Open No. 2013-13256 to complete the vibration-driven energy harvesting element  1  in  FIG. 1 . 
     The vibration-driven energy harvesting element  1  of the embodiment is configured such that the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  are formed in a silicon substrate, and the fixed portion  11  and the elastically-supporting portion  13  are fixed to the base portion  10  formed of a glass substrate. Accordingly, cost reduction can be achieved because an expensive SOI substrate as in the electrostatic device described in Patent Literature 1 is not used. 
     COMPARATIVE EXAMPLE 
       FIG. 14  illustrates a Comparative Example. A vibration-driven energy harvesting element  50  of the Comparative Example is formed by using an SOI substrate. A fixed portion  51 , a moveable portion  52 , and an elastically-supporting portion  13  that is not illustrated of the vibration-driven energy harvesting element  50  are formed in an active layer  61 , which is an upper silicon layer of the SOI substrate, and a base portion  53  is formed in a support layer  63 , which is a lower silicon layer. Electrets  520  are formed on comb electrodes of the moveable portion  52 . Since an active layer  61  and a support layer  63  are disposed with a BOX layer  62  composed of SiO 2  interposed therebetween, stray capacitances Cs 1 , Cs 2  generated between the active layer  61  and the support layer  63  may adversely affect electric power generated by the vibration-driven energy harvesting element  50 . 
     When the moveable portion  52  vibrates in the right-left direction in the figure relative to the fixed portion  51 , capacitances C 1 , C 2  between comb electrodes of the fixed portion  51  and the moveable portion  52  are changed and an AC current due to a change in capacitances C 1 , C 2  is output as a terminal current I 1 . In the output terminal current I 1 , a part of currents I 3  flows through the stray capacitances Cs 1 , Cs 2 , and the rest of currents I 2  flows through a load resistance R connected to the vibration-driven energy harvesting element  50 . 
       FIG. 15  represents results of simulation of power generation by the vibration-driven energy harvesting element  50 : the view (a) in  FIG. 15  represents currents I 2 , I 3 , and the view (b) in  FIG. 15  represents power W 2  consumed in the load resistance R and power W 3  moving in and out of the stray capacitance Cs 1 . The phase of the current through the stray capacitance Cs 1  leads a terminal voltage by 90 degrees. The power W 3  moving in and out of the stray capacitance Cs 1  is a reactive power that is not drawn out to the outside. The same applies to the power moving in and out of the stray capacitance Cs 2 . As the stray capacitances Cs 1 , Cs 2  increase, the reactive power W 3  increases and the active power W 2 , which is a power consumed in the load resistance R, decreases. 
     On the other hand, in the vibration-driven energy harvesting element  1  of the embodiment, since the fixed portion  11  and the moveable portion  12  that are formed of silicon are bonded to the base portion  10  formed of the glass substrate, it is possible to prevent generation of the stray capacitance. As a result, it is possible to prevent generation of reactive power caused by the stray capacitance and allow generated electric power to be consumed in the load resistance R without waste. 
     Note that even in a case of the vibration-driven energy harvesting element  50  formed from the SOI substrate, it is possible to reduce the reactive power as in the case in which the base portion  10  made of the glass substrate is used by making a thickness of the BOX layer smaller than that of a prior art to reduce the stray capacitance. 
     Advantageous effects of the embodiment described above may be summarized as follows. 
     (1) The vibration-driven energy harvesting element  1 , which is an electrostatic device, includes as illustrated in  FIG. 1 : a fixed portion  11 ; a moveable portion  12 ; an elastically-supporting portion  13  formed integrally with the moveable portion  12  and elastically supporting the moveable portion  12 ; and a base portion  10  made of glass to which the fixed portion  11  and the elastically-supporting portion  13  are anodically bonded in a state in which the fixed portion  11  and the elastically-supporting portion  13  are separated from each other. Accordingly, cost reduction can be achieved comparing to the vibration-driven energy harvesting element  50  fabricated by using the SOI substrate. 
     In the embodiment described above, although the vibration-driven energy harvesting element  1 , which is an electrostatic device, has been taken as an example for explanation, the present invention is not limited to the vibration-driven energy harvesting element  1  and may be applied to an actuator or a sensor as those described in Patent Literature 1. Specifically, such an actuator or a sensor is to be configured such that it is made from a silicon substrate and supported by a base portion made of glass. In this way, in addition to cost reduction, it is possible to reduce the stray capacitance. Instead of a silicon substrate, any other glass substrate or a glass substrate on which a silicon thin film is formed may be used to form an actuator or a sensor, provided that the substrate is electrically conductive and has a coefficient of linear expansion that sufficiently matches with that of the glass substrate. 
     (2) Further, the fixed portion  11  and the moveable portion  12  may be formed of silicon, and an electret may be formed on at least one of the fixed portion  11  and the moveable portion  12 . 
     (3) In the vibration-driven energy harvesting element  1 , which is an electrostatic device, illustrated in  FIG. 1 , a comb electrode  110 , which is a fixed electrode, is formed in the fixed portion  11 , a comb electrode  120 , which is a moveable electrode, facing the comb electrode  110  is formed in the moveable portion  12 , an electret is formed on at least one of the fixed portion  11  and the moveable portion  12 , and the moveable portion  12  is displaced relative to the fixed portion  11  such that capacitance changes between the comb electrodes  110  and the comb electrodes  120  and electricity is generated. Since the base portion  10  is made of glass, in addition to cost reduction as described above, it is possible to prevent generation of the stray capacitances Cs 1 , Cs 2  in the vibration-driven energy harvesting element  50  illustrated in  FIG. 14  for which the SOI substrate is used, and prevent generation of the reactive power W 3  caused by the stray capacitance. 
     (4) In a method for manufacturing the electrostatic device described above, the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  are formed on a substrate, for example, the Si substrate  200 , in an integral manner, the base portion  10  made of glass are anodically bonded to the Si substrate  200  to fix the fixed portion  11  and the elastically-supporting portion  13  on the base portion  10  made of glass, and etching is performed on the Si substrate  200  to separate the fixed portion  11  and the elastically-supporting portion  13  from each other to electrically separate the fixed portion  11  from the moveable portion  12 . 
     As described above, before the fixed portion  11  and the elastically-supporting portion  13  are separated, the Si substrate  200  on which the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  are integrated is anodically bonded to the base portion  10  and separation is performed after the anodic bonding. Accordingly, the fixed portion  11 , the moveable portion  12 , and the elastically-supporting portion  13  can be bonded to the base portion  10  while their positional relation is maintained on a wafer level. 
     The present invention is not limited to the content of the embodiment described above and any other aspects conceivable within the scope of technical ideas of the present invention are also within the scope of the present invention. 
     The disclosed contents of the following priority basic applications and patent publications are incorporated herein by reference. 
     Japanese Patent Application No. 2019-106230 (filed on Jun. 6, 2019) 
     Japanese Patent Laid-Open No. 2013-13256 
     REFERENCE SIGNS LIST 
       1 ,  50  . . . vibration-driven energy harvesting element,  10 ,  53  . . . base portion,  11 ,  51  . . . fixed portion,  12 ,  52  . . . moveable portion,  13  . . . elastically-supporting portion,  13   a  . . . fixed area,  13   b  . . . elastic portion,  40  . . . heater,  110 ,  120  . . . comb electrodes,  200  . . . Si substrate,  300  . . . glass substrate, Cs 1 , Cs 2  . . . stray capacitance