Patent Publication Number: US-2015076626-A1

Title: Electronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-190905, filed Sep. 13, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an electronic device. 
     BACKGROUND 
     A micro electro mechanical system (MEMS) element with a variable capacitor formed on a semiconductor substrate has been proposed. 
     In this system, however, if the variable capacitor is formed of an easily-oxidizable metal, such as aluminum, the surfaces of electrodes may be ununiformly oxidized to thereby produce a metal oxide, with the result that even if, for example, two electrodes are attempted to be tightly attached to each other, this cannot be realized because of the produced metal oxide. Since MEMS elements are fine elements, the capacitance of the variable capacitor is hard to accurately control if such a problem as the above occurs. The metal oxide will also involve a problem associated with reliability that an oxide film formed on the electrode may peel off. 
     There is a demand for an electronic device with a variable capacitor that includes electrodes whose surface oxidation is controllable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing part of a method of manufacturing an electronic device according to a first embodiment; 
         FIG. 2  is a schematic cross-sectional view showing part of the method of manufacturing the electronic device according to the first embodiment; 
         FIG. 3  is a schematic cross-sectional view showing part of the method of manufacturing the electronic device according to the first embodiment; 
         FIG. 4  is a schematic cross-sectional view showing part of the method of manufacturing the electronic device according to the first embodiment; 
         FIG. 5  is a schematic cross-sectional view showing part of the method of manufacturing the electronic device according to the first embodiment; 
         FIG. 6  is a schematic cross-sectional view showing part of the method of manufacturing the electronic device according to the first embodiment; 
         FIG. 7  is a schematic cross-sectional view showing part of the method of manufacturing the electronic device according to the first embodiment; 
         FIG. 8  is a schematic cross-sectional view showing part of the method of manufacturing the electronic device according to the first embodiment; 
         FIG. 9  is a schematic plan view showing the positional relationship between the structural elements of the electronic device according to the first embodiment; 
         FIG. 10  is a schematic plan view showing a modification of the positional relationship between the structural elements of the electronic device of the first embodiment; 
         FIG. 11  is a schematic plan view showing another modification of the positional relationship between the structural elements of the electronic device of the first embodiment; 
         FIG. 12  is a schematic cross-sectional view of an electronic device according to a modification of the first embodiment; 
         FIG. 13  is a schematic cross-sectional view of an electronic device according to a second embodiment; 
         FIG. 14  is a schematic cross-sectional view showing part of a method of manufacturing the electronic device according to the second embodiment; and 
         FIG. 15  is a schematic cross-sectional view of an electronic device according to a modification of the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an electronic device includes: a substrate; a first electrode provided stationary above the substrate and used for a variable capacitor; a second electrode provided movable above or below the first electrode and used for the variable capacitor; a first protective insulation film provided on a first surface of the first electrode, the first surface facing the second electrode; and a second protective insulation film provided on a second surface of the second electrode, the second surface facing the first electrode. 
     Embodiments will be described with reference to the accompanying drawings. 
     First Embodiment 
       FIGS. 1 to 8  schematically show a method of manufacturing an electronic device according to a first embodiment. 
     Firstly, as shown in  FIG. 1 , a first electrode  13   a  for a variable capacitor, a lower pad  13   b , and a lower electrode  13   c  for an MIM capacitor are formed above a semiconductor substrate  11 . More specifically, an underlying insulation film  12  formed of, for example, a silicon oxide is formed on a semiconductor substrate  11 . On the semiconductor substrate  11 , elements, such as transistors, may be formed. Subsequently, an aluminum (Al) film with a thickness of approx. several hundreds nm to several μm is formed as a metal film on the underlying insulation film  12  by sputtering. This metal film is patterned by photolithography and etching, thereby forming a stationary first electrode  13   a  for the variable capacitor. During this patterning, the lower pad  13   b , and the lower electrode  13   c  for the MIM capacitor are also formed. The etching may be reactive ion etching or wet etching. 
     Subsequently, a first protective insulation film  14  is formed on the first electrode  13   a , the lower pad  13   b , and the lower electrode  13   c  for the MIM capacitor, thereby covering them. More specifically, a silicon nitride (SiN) film with a thickness of approx. several hundreds nm to several μm is formed as the first protective insulation film  14  by chemical vapor deposition (CVD). In general, the first protective insulation film  14  is formed of a material containing silicon (Si), and at least nitrogen (N) or oxygen (O). Accordingly, a silicon oxide (SiO) film or a silicon oxynitride (SiON) film can be used as the first protective insulation film  14 . The first protective insulation film  14  can prevent an oxide of an electrode metal (e.g., a metal oxide such as alumina) from being formed on the first electrode  13   a  during a high-temperature thermal treatment performed later. 
     Thereafter, the first protective insulation film  14  is patterned using photolithography and RIE to form an opening reaching the lower pad  13   b.    
     After that, as shown in  FIG. 2 , a first sacrifice film  15  is formed on the first protective insulation film  14 . An organic material (such as polyimide) film with a thickness of approx. several hundreds nm to several μm can be used as the first sacrifice film  15 . Subsequently, the first sacrifice film  15  is patterned to form, for example, openings. More specifically, the first sacrifice film  15  can be patterned by coating the film  15  with an organic material film having photosensitivity, and then exposing the resultant structure to light and developing the exposed structure. Alternatively, the first sacrifice film  15  may be patterned by etching the same using a patterned photoresist formed thereon as a mask. Yet alternatively, the first sacrifice film  15  may be patterned using a predetermined insulation film as a hard mask. 
     After that, as shown in  FIG. 3 , a second protective insulation film  16  is formed on the first sacrifice film  15 . More specifically, a silicon nitride (SiN) film with a thickness of approx. several nm to several hundreds nm is formed as the second protective insulation film  16  by CVD. In general, the second protective insulation film  16  is formed of a material containing silicon (Si), and at least nitrogen (N) or oxygen (O). Accordingly, a silicon oxide (SiO) film or a silicon oxynitride (SiON) film can be used as the second protective insulation film  16 . Subsequently, the second protective insulation film  16  is patterned to form, for example, openings by photolithography and RIE. 
     The second protective insulation film  16  is formed sufficiently thinner than the first protective insulation film  14 . For instance, the thickness of the second protective insulation film  16  is set to approx. 1/10 or less of that of the first protective insulation film  14 . Further, the second protective insulation film  16  is formed sufficiently thinner than a second electrode  17   a , described later. For instance, the thickness of the second protective insulation film  16  is set to approx. 1/10 or less of that of the second electrode  17   a.    
     Subsequently, as shown in  FIG. 4 , the second electrode  17   a  for the variable capacitor is formed on the second protective insulation film  16 . At this time, an upper pad  17   b , and an upper electrode  17   c  for the MIM capacitor, are also formed. More specifically, an aluminum (Al) film with a thickness of approx. several hundreds nm to several μm is formed on the second protective insulation film  16 . This metal film is patterned using photolithography and etching, thereby forming the second electrode  17   a  (which is movable) for the variable capacitor, the upper pad  17   b , and the upper electrode  17   c  for the MIM capacitor. As the etching, reactive ion etching (RIE) or wet etching may be used. During this etching, the second protective insulation film  16  may be etched continuously. If a silicon nitride film is used as the second protective insulation film  16 , this film can be removed by RIE using CF 4  or chemical dry etching (CDE). 
     Since the second protective insulation film  16  is formed under the second electrode  17   a , the surface of the second electrode  17   a  can be prevented from being coated with an oxide (e.g., a metal oxide such as alumina) of the electrode metal when a high-temperature thermal treatment is performed later. 
     After that, as shown in  FIG. 5 , a connecting section  18  is formed to connect the second electrode  17   a  to the upper pad  17   b . More specifically, the connecting section  18  is provided by forming a silicon nitride film with a thickness of approx. several hundreds nm to several μm by CVD, and then patterning the film. The connecting section  18  functions as part of a spring for the second electrode (movable electrode)  17   a . The connecting section  18  may be formed of an insulator or a conductor of, for example, a metal. 
     Thereafter, as shown in  FIG. 6 , a second sacrifice film  19  is formed to cover a structure including the second electrode  17   a  and other elements. More specifically, the second sacrifice film  19  may be formed of an organic material, such as polyimide. The second sacrifice film  19  is then patterned. Specifically, the second sacrifice film  19  can be patterned by etching using, as a mask, a photoresist pattern formed on the second sacrifice film  19 . Alternatively, the second sacrifice film  19  may be patterned by coating this film with an organic material having photosensitivity, then exposing the resultant structure to light, and developing the exposed structure. 
     Subsequently, as shown in  FIG. 7 , a cover insulation film  20  for covering the second sacrifice film  19  is formed. Specifically, an insulation film, such as a silicon oxide film, is formed as the cover insulation film  20  by plasma CVD. On the cover insulation film  20 , a patterned photoresist  21  is formed by photolithography. Using the patterned photoresist  21  as a mask, the cover insulation film  20  is etched to form therein a plurality of openings  22 . 
     Thereafter, as shown in  FIG. 8 , the first sacrifice film  15  and the second sacrifice film  19  are removed. Specifically, these films are removed by ashing using oxygen (O 2 ). Through the openings  22  formed in the process step of  FIG. 7 , oxygen is introduced into the cover insulation film  20  to thereby remove the first sacrifice film  15  and the second sacrifice film  19 . By this ashing, the patterned photoresist  21  is simultaneously removed, whereby a cavity  23  is formed within the cover insulation film  20 . 
     Subsequently, an organic insulation film  24  is formed to cover the cover insulation film  20 . An inorganic insulation film  25  is formed on the organic insulation film  24 . As the organic insulation film  24 , a UV-curable epoxy resin film, for example, can be used. As the inorganic insulation film  25 , a silicon nitride film, for example, can be used. By thus forming the organic insulation film  24  and the inorganic insulation film  25 , the openings  22  are sealed. The organic insulation film  24  can pass therethrough harmful gases in the cavity  23  to exhaust them. Thus, the organic insulation film  24  has a function of adjusting the atmosphere in the cavity  23 . The inorganic insulation film  25  suppresses entering of harmful gasses, such as water vapor, into the cavity  23  through the organic insulation film  24 . 
     An MEMS element having a variable capacitor is formed as described above. Namely, an electronic device is formed which comprises the first electrode  13   a  provided stationary above the semiconductor substrate  11  and used for a variable capacitor, the second electrode  17   a  provided movable above or below the first electrode  13   a  and used for the variable capacitor, the first protective insulation film  14  provided on the first surface of the first electrode  13   a , the first surface facing the second electrode  17   a , and the second protective insulation film  16  provided on the second surface of the second electrode  17   a , the second surface facing the first electrode  13   a.    
     As shown in  FIG. 8 , the first electrode (stationary electrode)  13   a  and the second electrode (movable electrode)  17   a  oppose each other, and provide a variable capacitor. The second electrode  17   a  is connected to the upper pad  17   b  via the connecting section  18  and supported by the upper pad  17   b  via the connecting section  18 . When a desired voltage has been applied to the second electrode  17   a , an electrostatic force is exerted between the first electrode  13   a  and the second electrode  17   a  to vary the position of the second electrode  17   a . As a result, the distance between the first and second electrodes  13   a  and  17   a  varies to vary the capacitance of the variable capacitor. 
       FIG. 9  is a schematic plan view showing the positional relationship between the structural elements of the electronic device according to the first embodiment. The plan view of  FIG. 9  merely schematically shows the positional relationship between the structural elements, and hence does not correspond to the cross-sectional views of  FIGS. 1 to 8 . 
     As shown in  FIG. 9 , the upper pad  17   b , the upper electrode  17   c  for the MIM capacitor, a dummy electrode (dummy pad)  17   d  and a dummy electrode (dummy pad)  17   e  are provided outside the second electrode (movable electrode)  17   a . The second electrode (movable electrode)  17   a  is connected to the upper pad  17   b  by a bias line  17   f.    
     The second protective insulation film  16  protects the second electrode  17   a . Accordingly, the pattern of the second protective insulation film  16  is substantially identical to or includes that of the second electrode  17   a . In the example of  FIG. 9 , the pattern of the second protective insulation film  16  includes that of the second electrode  17   a.    
       FIG. 10  is a schematic plan view showing a modification of the positional relationship between the structural elements of the electronic device of the first embodiment. In the example of  FIG. 10 , the pattern of the second protective insulation film  16  includes that of the upper electrode  17   c  for the MIM capacitor, and that of the dummy electrode (dummy pad)  17   d . The other basic structure is similar to that shown in  FIG. 9 , and therefore explanation thereof is omitted. 
       FIG. 11  is a schematic plan view showing another modification of the positional relationship between the structural elements of the electronic device of the first embodiment.  FIG. 11  also shows the positional relationship between connecting sections  18   b ,  18   c ,  18   d  and  18   e . The other basic structure is similar to that shown in  FIG. 9 , and therefore explanation thereof is omitted. 
     As described above, in the first embodiment, the first protective insulation film  14  is provided on the first electrode  13   a  of the variable capacitor, and the second protective insulation film  16  is provided on the second electrode  17   a . In other words, the first electrode  13   a  is covered with the first protective insulation film  14 , and the second electrode  17   a  is covered with the second protective insulation film  16 . Thus, the first and second electrodes  13   a  and  17   a  are protected by the first and second protective insulation films  14  and  16 , respectively. As a result, oxidation (e.g., ununiform oxidation) of the surfaces of the first and second electrodes  13   a  and  17   a  can be suppressed. 
     As aforementioned, if the electrodes of the variable capacitor are formed of an easily-oxidizable metal, such as aluminum, the surfaces of the electrodes may well be ununiformly oxidized. For instance, in a high-temperature process, such as a curing step of the sacrifice films  15  and  19 , the surfaces of the first and second electrodes  13   a  and  17   a  may be oxidized. When an ununiform oxide film has been formed on the first electrode  13   a  or the second electrode  17   a , even if, for example, the two electrodes are attempted to be tightly attached to each other, this cannot be realized. As a result, it becomes difficult to accurately control the capacitance of the variable capacitor. Further, a problem in reliability that an oxide film formed on an electrode peels off may also occur. 
     In the first embodiment, the first protective insulation film  14  and the second protective insulation film  16  can suppress oxidation of the surfaces of the first and second electrodes  13   a  and  17   a . Consequently, the embodiment can provide a highly reliable electronic device. 
       FIG. 12  is a cross-sectional view of a modification of the first embodiment. 
     In the modification, the second protective insulation film  16  is removed after the process step of  FIG. 8 . In general, no special high-temperature treatment is performed after removing the sacrifice films  15  and  19  in the process step of  FIG. 8 . Therefore, a problem that the surface of the second electrode  17   a  is oxidized little occurs even if the second protective insulation film  16  is removed. Because of this, the second protective insulation film  16  may be removed as shown in  FIG. 12 . 
     Further, note that when the second protective insulation film  16  is removed by etching, the first protective insulation film  14  is simultaneously etched. Since, however, the first protective insulation film  14  is sufficiently thicker than the second protective insulation film  16 , it is not completely removed but part of the same remains. 
     Yet further, the second protective insulation film  16  may be removed when removing the sacrifice films  15  and  19  in the process step of  FIG. 8 . 
     Second Embodiment 
     A second embodiment will now be described. Since the second embodiment is similar to the first embodiment in basic structure and basic manufacturing method, only different matters will be described. 
       FIG. 13  is a schematic cross-sectional view of an electronic device according to the second embodiment. In the second embodiment, elements similar to those shown in  FIGS. 1 to 8  (first embodiment) denoted by corresponding reference numbers, and no detailed description will be given thereof. 
     As shown in  FIG. 13 , the second embodiment incorporates a third protective insulation film  30  provided on the upper surface (third surface) of the second electrode  17   a  opposite to the lower surface (second surface) thereof, in addition to the structure shown in  FIG. 8 . 
     Referring to  FIG. 14 , a method of manufacturing the third protective insulation film  30  will be described. 
     In the second embodiment, the process step of  FIG. 14  is performed after the process step of  FIG. 3 . In the process step of  FIG. 14 , after a metal film (aluminum film) for forming, for example, the second electrode  17   a  is formed, the third protective insulation film  30  is formed on the metal film before the metal film is patterned. More specifically, on the metal film, a silicon nitride (SiN) film with a thickness of approx. several nm to several hundreds nm is formed as the third protective insulation film  30  by CVD. In general, the third protective insulation film  30  is formed of a material containing silicon (Si), and at least one of nitrogen (N) and oxygen (O). Accordingly, the third protective insulation film  30  may be formed of silicon oxide (SiO) or silicon oxynitride (SiON). 
     Subsequently, the third protective insulation film  30 , the metal film and the second protective insulation film  16  are patterned by photolithography and etching to form, for example, openings. As the etching, RIE, CDE, wet etching, etc., can be used. 
     Thus, such a structure as shown in  FIG. 14  is formed. After that, the same steps as those shown in  FIGS. 5 to 8  (first embodiment) are performed to obtain an electronic device having the variable capacitor shown in  FIG. 13 . 
     As in the modification of the first embodiment shown in  FIG. 12 , the second protective insulation film  16  may be removed after the process step of  FIG. 13 . When the protective insulation film  16  is removed, the third protective insulation film  30  may be simultaneously removed. Further, when the sacrifice films  15  and  19  are removed in the process step of  FIG. 13 , the second and third protective insulation films  16  and  30  may also be removed. 
     In the second embodiment, oxidation of the surfaces of the first and second electrodes  13   a  and  17   a  can be suppressed by providing the first and second protective insulation films  14  and  16 , as in the first embodiment. Further, since the second embodiment also employs the third protective insulation film  30 , oxidation of the opposite surface of the second electrode  17   a  can also be suppressed. 
     Further, although in the above-described first and second embodiments, the second electrode  17   a , the upper pad  17   b  and the upper electrode  17   c  are flat, they may have their ends downwardly bent as shown in  FIG. 15 . If the ends of the second electrode  17   a  are bent outside the first electrode  13   a , there is no problem unless the ends of the second electrode  17   a  are in contact with the ground layer (e.g., the protective insulation film  14 ), when the second electrode  17   a  is in the down state position. Thus, the second electrode  17   a  may have a shape that will be engaged with the first electrode  13   a.    
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.