Patent Publication Number: US-2020301049-A1

Title: Optical deflection element, method for manufacturing optical deflection element, and system including optical deflection element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-050485, filed on Mar. 18, 2019, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an optical deflection element, a method for manufacturing the optical deflection element, and a system including the optical deflection element. 
     Description of the Related Art 
     In recent years, microelectromechanical systems (MEMS) devices using a piezoelectric film as an actuator have been increasingly used. For example, scanner devices including a reflective surface for the purpose of optical scanning have been developed. 
     Examples of the MEMS device include an optical deflection element that includes a torsion bar including a metal alloy, an oscillation body bonded to the torsion bar, a magnet bonded to the oscillation body, and a member having a water barrier property to protect the bonding area between the torsion bar and the oscillation body with a bonding adhesive from water. In the optical deflection element, the oscillation body includes a first oscillation body and a second oscillation body between which part of the torsion bar is sandwiched. The member having a water barrier property is provided to cover the surface and its periphery of the bonding area between the torsion bar and the oscillation body with the bonding adhesive. The member having a water barrier property protects a reflective surface of the oscillation body. 
     Unfortunately, the above optical deflection element has a disadvantage such as the deterioration of the optical characteristics due to an increase in the thickness of the member that protects the reflective surface of the oscillation body. 
     SUMMARY 
     Example embodiments include an optical deflection element includes: a reflective surface; and a movable part configured to rotate the reflective surface so as to deflect light incident on the reflective surface. The movable part includes: a metal film; a high reflective layer formed on an upper surface of the metal film; and a protective film continuously covering an upper surface and a side surface of the high reflective layer and a side surface of the metal film. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages and features thereof may be readily obtained and understood from the following detailed description referring to the accompanying drawings, wherein: 
         FIG. 1  is a schematic view illustrating an example of an optical scanning system according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating an example of the hardware configuration of the optical scanning system according to the embodiment; 
         FIG. 3  is a block diagram illustrating an example of the functional configuration of a control device according to the embodiment; 
         FIG. 4  is a flowchart illustrating an example of a process performed by the optical scanning system according to the embodiment; 
         FIG. 5  is a schematic view illustrating an example of an automobile including a head-up display device according to the embodiment; 
         FIG. 6  is a schematic view illustrating an example of the head-up display device according to the embodiment; 
         FIG. 7  is a schematic view illustrating an example of an image forming apparatus including an optical writing device according to the embodiment; 
         FIG. 8  is a schematic view illustrating an example of the optical writing device according to the embodiment; 
         FIG. 9  is a schematic view illustrating an automobile including a laser imaging detection and ranging (LiDAR) device according to the embodiment; 
         FIG. 10  is a schematic view illustrating an example of the LiDAR device according to the embodiment; 
         FIG. 11  is a schematic view illustrating an example of the configuration of a laser headlamp according to the embodiment; 
         FIG. 12  is a perspective view schematically illustrating an example of the configuration of a head mount display according to the embodiment; 
         FIG. 13  is a partial view illustrating an example of the configuration of the head mount display according to the embodiment; 
         FIG. 14  is a schematic view illustrating an example of a packaged movable device according to the embodiment; 
         FIG. 15  is a plan view illustrating the movable device according to a first embodiment of the present invention; 
         FIG. 16  is a cross-sectional view taken through the line A-A in  FIG. 15 ; 
         FIG. 17  is a cross-sectional view taken through the line B-B in  FIG. 15 ; 
         FIG. 18  is a cross-sectional view illustrating an example of a mirror unit according to a comparative example; 
         FIG. 19  is a cross-sectional view illustrating an example of the mirror unit according to the first embodiment; 
         FIG. 20  is a graph illustrating a difference in the reflectance between mirror units due to the presence or absence of a protective film according to the embodiment; 
         FIGS. 21A and 21B  are schematic views illustrating the cross-section of the protective film according to the embodiment; 
         FIG. 22  is a cross-sectional view illustrating an example of a mirror unit according to a second embodiment of the present invention; and 
         FIG. 23  is a graph illustrating the difference in the reflectance of the mirror unit depending on the number of layers in the protective film according to the embodiment. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     Embodiments of the present invention are described below in detail. 
     Optical Scanning System 
     Referring to  FIGS. 1 to 4 , an optical scanning system  10  to which a movable device  13  according to the present embodiment is applied is first described in detail. 
       FIG. 1  is a schematic view illustrating an example of the optical scanning system  10 . As illustrated in  FIG. 1 , the optical scanning system  10  includes a control device  11 , a light source device  12 , and the movable device  13  including a reflective surface  14 . 
     In the optical scanning system  10 , the reflective surface  14  included in the movable device  13  deflects the light emitted from the light source device  12  under the control of the control device  11  so as to optically scan a scan surface  15 . 
     The control device  11  includes an electronic circuitry unit including, for example, a central processing unit (CPU) or a field-programmable gate array (FPGA). The movable device  13  includes, for example, an MEMS device that includes the reflective surface  14  and allows the reflective surface  14  to move. The light source device  12  includes, for example, a laser device that emits a laser. The scan surface  15  includes, for example, a screen. 
     The control device  11  generates a control command for the light source device  12  and the movable device  13  based on the acquired optical scanning information and outputs a drive signal to the light source device  12  and the movable device  13  based on the control command. 
     The light source device  12  emits light based on the input drive signal. The movable device  13  moves the reflective surface  14  based on the input drive signal in at least any of one axial direction and two axial directions. 
     Thus, for example, due to the control by the control device  11  based on image information, which is an example of the optical scanning information, the reflective surface  14  of the movable device  13  is moved back and forth in two axial directions within a predetermined range, and the light emitted from the light source device  12  and entering the reflective surface  14  is deflected around a certain axis for optical scanning, whereby any image may be projected onto the scan surface  15 . The details of the movable device  13  and the details of the control by the control device  11  according to the present embodiment are given later. 
     Referring to  FIG. 2 , an example of the hardware configuration of the optical scanning system  10  is described below.  FIG. 2  is a diagram illustrating an example of the hardware configuration of the optical scanning system  10 . As illustrated in  FIG. 2 , the optical scanning system  10  includes the control device  11 , the light source device  12 , and the movable device  13 . The control device  11 , the light source device  12 , and the movable device  13  are electrically connected to one another. The control device  11  includes a CPU  20 , a random access memory (RAM)  21 , a read only memory (ROM)  22 , an FPGA  23 , an external interface (I/F)  24 , a light source device driver  25 , and a movable device driver  26 . 
     The CPU  20  is an arithmetic device that loads a program or data from a storage device, such as the ROM  22 , onto the RAM  21  and executes processing so as to perform the overall control on the control device  11  and perform a function of the control device  11 . 
     The RAM  21  is a volatile storage device that temporarily stores programs and data. 
     The ROM  22  is a non-volatile storage device that may retain programs and data even when the power is off. The ROM  22  stores processing programs and data that are executed by the CPU  20  to control each function of the optical scanning system  10 . 
     The FPGA  23  is a circuitry that outputs a control signal appropriate for the light source device driver  25  and the movable device driver  26  in accordance with processing of the CPU  20 . 
     The external I/F  24  is an interface with, for example, an external device or a network. Examples of the external device include a higher-level device such as a personal computer (PC) and a storage device such as a universal serial bus (USB) memory, a secure digital (SD) Card, a compact disc (CD), a digital versatile disk (DVD), a hard disk drive (HDD), or a solid state drive (SSD). Examples of the network include a controller area network (CAN) for automobiles, a local area network (LAN), or the Internet. The external I/F  24  may be configured to enable the connection or communication with an external device. The external I/F  24  may be provided for each external device. 
     The light source device driver  25  is an electric circuitry that outputs a drive signal such as a drive voltage to the light source device  12  in accordance with an input control signal. 
     The movable device driver  26  is an electric circuitry that outputs a drive signal such as a drive voltage to the movable device  13  in accordance with an input control signal. 
     In the control device  11 , the CPU  20  acquires optical scanning information from an external device or a network via the external I/F  24 . As long as the CPU  20  is configured to acquire optical scanning information, the optical scanning information may be stored in the ROM  22  or the FPGA  23  in the control device  11 , or the optical scanning information may be stored in a storage device, such as an SSD, which is newly provided in the control device  11 . 
     The optical scanning information is the information indicating how the scan surface  15  is optically scanned. For example, the optical scanning information is image data when an image is displayed due to optical scanning. Furthermore, for example, the optical scanning information is the writing data indicating a writing order or a writing location when optical writing is executed due to optical scanning. Moreover, for example, the optical scanning information is the emission data indicating the timing and the emission range for emitting light for object recognition when the object recognition is executed due to optical scanning. 
     The control device  11  may provide the functional configuration described below by using the hardware configuration illustrated in  FIG. 2  in accordance with a command from the CPU  20 . 
     Referring to  FIG. 3 , an example of the functional configuration of the control device  11  in the optical scanning system  10  is described below.  FIG. 3  is a block diagram illustrating an example of the functional configuration of the control device  11  in the optical scanning system  10 . 
     As illustrated in  FIG. 3 , the control device  11  includes a controller  30  and a drive signal output unit  31  as functions. 
     The controller  30  is implemented by using, for example, the CPU  20  or the FPGA  23 . The controller  30  acquires optical scanning information from an external device, converts the optical scanning information into a control signal, and outputs the control signal to the drive signal output unit  31 . For example, the controller  30  acquires image data as the optical scanning information from an external device, or the like, generates a control signal from the image data through predetermined processing, and outputs the control signal to the drive signal output unit  31 . 
     The drive signal output unit  31  is implemented by using, for example, the light source device driver  25  or the movable device driver  26 . The drive signal output unit  31  outputs a drive signal to the light source device  12  or the movable device  13  based on an input control signal. The drive signal is a signal for controlling the driving of the light source device  12  or the movable device  13 . For example, for the light source device  12 , the drive signal is the drive voltage for controlling the emission timing and the emission intensity of the light source. For example, for the movable device  13 , the drive signal is the drive voltage for controlling the moving timing and the movable range of the reflective surface  14  included in the movable device  13 . 
     Referring to  FIG. 4 , the process performed by the optical scanning system  10  to optically scan the scan surface  15  is described below.  FIG. 4  is a flowchart illustrating an example of the process performed by the optical scanning system  10 . 
     In step S 11 , the controller  30  acquires optical scanning information from an external device, etc. 
     In step S 12 , the controller  30  generates a control signal, which is a control command, from the acquired optical scanning information and outputs the control signal to the drive signal output unit  31 . 
     In step S 13 , the drive signal output unit  31  outputs a drive signal to the light source device  12  and the movable device  13  based on the input control signal. 
     In step S 14 , the light source device  12  emits light based on the input drive signal. The movable device  13  moves the reflective surface  14  based on the input drive signal. Driving of the light source device  12  and the movable device  13  allows the light to be deflected in any direction for the optical scanning. 
     In the optical scanning system  10 , the device for controlling the light source device  12  and the device for controlling the movable device  13  are incorporated into the single control device  11 , that is to say, the single control device  11  has the function to control the light source device  12  and the function to control the movable device  13 ; however, a control device that controls the light source device  12  and a control device that controls the movable device  13  may be provided separately. 
     In the optical scanning system  10 , the single control device  11  has the function of the controller  30  for the light source device  12  and the movable device  13  and the function of the drive signal output unit  31 ; however, these functions may be provided separately. For example, a drive signal output device including the drive signal output unit  31  may be provided separately from the control device  11  including the controller  30 . In the optical scanning system  10 , the movable device  13  including the reflective surface  14  and the control device  11  may constitute an optical deflection system that executes optical deflection. 
     Image Projection Device 
     Referring to  FIGS. 5 and 6 , an image projection device using the movable device  13  according to the present embodiment is described below in detail. 
       FIG. 5  is a schematic view illustrating an example of an automobile  400  including a head-up display device  500 , which is an example of the image projection device, according to an embodiment of the present invention.  FIG. 6  is a schematic view illustrating an example of the head-up display device  500 . 
     The image projection device projects an image due to optical scanning. The image projection device is, for example, a head-up display device. 
     As illustrated in  FIG. 5 , the head-up display device  500  is provided, for example, near the windshield (e.g., a front glass  401 ) of the automobile  400 . A projection light L emitted from the head-up display device  500  is reflected by the front glass  401  and is headed to the viewer (a driver  402 ) who is a user. This allows the driver  402  to visually recognize the image, or the like, projected by the head-up display device  500  as a virtual image. A combiner may be provided on the inner wall surface of the windshield so that the projection light reflected by the combiner allows the user to visually recognize a virtual image. 
     As illustrated in  FIG. 6 , in the head-up display device  500 , red, green, and blue laser light sources  501 R,  501 G, and  501 B emit laser lights. The emitted laser lights pass through an incident optical system including collimator lenses  502 ,  503 , and  504 , which are provided for the respective laser light sources, two dichroic mirrors  505  and  506 , and a light intensity adjuster  507 . Then, the light is deflected by the movable device  13  including the reflective surface  14 . The deflected laser light passes through a projection optical system including a free-form surface mirror  509 , an intermediate screen  510 , and a projection mirror  511  and is then projected onto a screen. In the head-up display device  500 , the laser light sources  501 R,  501 G, and  501 B, the collimator lenses  502 ,  503 , and  504 , and the dichroic mirrors  505  and  506  are housed as a light source unit  530  in an optical housing. 
     The head-up display device  500  projects the intermediate image displayed on the intermediate screen  510  onto the front glass  401  of the automobile  400  so as to allow the driver  402  to visually recognize the intermediate image as a virtual image. 
     The respective color laser lights emitted from the laser light sources  501 R,  501 G, and  501 B are converted into substantially parallel lights by the collimator lenses  502 ,  503 , and  504 , respectively, and are then combined by the two dichroic mirrors  505  and  506 . After the light intensity of the combined laser light is adjusted by the light intensity adjuster  507 , the laser light is two-dimensionally swept by the movable device  13  including the reflective surface  14 . The projection light L, which is two-dimensionally swept by the movable device  13 , is reflected by the free-form surface mirror  509  to correct distortion and is then focused on the intermediate screen  510  so as to display an intermediate image. The intermediate screen  510  includes a microlens array in which microlenses are arranged in two dimensions. The intermediate screen  510  enlarges the projection light L incident on the intermediate screen  510  by microlenses. 
     The movable device  13  moves the reflective surface  14  back and forth in two axial directions to two-dimensionally sweep the projection light L entering the reflective surface  14 . The driving control of the movable device  13  is executed in synchronization with the emission timing of the laser light sources  501 R,  501 G, and  501 B. 
     The head-up display device  500 , which is an example of the image projection device, has been described above. The image projection device may be any device that executes optical scanning by using the movable device  13  including the reflective surface  14  to project an image. The image projection device is also applicable as, for example, a projector that is placed on a desk, or the like, to project an image on a display screen, or a head mount display device that is provided in a mounting member attached to the viewer&#39;s head, etc., to project an image onto a reflection-transmission screen included in the mounting member or project an image onto an eyeball as a screen. 
     The image projection device may be mounted in not only a vehicle or a mounting member but also a movable body such as an aircraft, a ship, or a movable robot, or an immovable body such as a work robot that operates a driving target such as a manipulator without moving from the spot. 
     The head-up display device  500  is an example of a “head-up display” described in claims. The automobile  400  is an example of a “vehicle” described in claims. 
     Optical Writing Device 
     Referring to  FIG. 7  and  FIG. 8 , an optical writing device using the movable device  13  according to the present embodiment is described below in detail. 
       FIG. 7  is a schematic view illustrating an example of an image forming apparatus including an optical writing device  600 .  FIG. 8  is a schematic view illustrating an example of the optical writing device  600 . 
     As illustrated in  FIG. 7 , the optical writing device  600  is used as a component of the image forming apparatus, typically a laser printer  650  having a printer function using laser light. In the image forming apparatus, the optical writing device  600  optically scans a photosensitive drum, which is the scan surface  15 , with one or more laser beams so as to execute optical writing on the photosensitive drum. 
     As illustrated in  FIG. 8 , in the optical writing device  600 , the laser light from the light source device  12 , such as a laser element, passes through an imaging optical system  601  such as a collimator lens and is then deflected by the movable device  13  including the reflective surface  14  in one axial direction or two axial directions. The laser light deflected by the movable device  13  passes through a scanning optical system  602  including a first lens  602   a , a second lens  602   b , and a reflection mirror unit  602   c  and is then emitted to the scan surface  15  (e.g., a photosensitive drum or photosensitive paper) for optical writing. The scanning optical system  602  focuses the optical beam in the form of a spot on the scan surface  15 . The light source device  12  and the movable device  13  including the reflective surface  14  are driven based on the control of the control device  11 . 
     As described above, the optical writing device  600  may be used as a component of an image forming apparatus having a printer function using laser light. With a different scanning optical system, optical scanning may be executed in two axial directions as well as in one axial direction so that the optical writing device  600  may be used as a component of an image forming apparatus such as a laser label apparatus that deflects laser light to a thermal medium for optical scanning and heats the thermal medium to execute printing. 
     The movable device  13  including the reflective surface  14  applied to the optical writing device  600  is advantageous in power saving of the optical writing device  600  because of low power consumption for driving as compared with a rotary polygon mirror using a polygon mirror, etc. Furthermore, the movable device  13  is advantageous in the improvement of quietness of the optical writing device  600  because of a small wind noise during the oscillation of the movable device  13  as compared with a rotary polygon mirror. The installation space for the movable device  13  in the optical writing device  600  is smaller than that for a rotary polygon mirror and the movable device  13  generates a small amount of heat; therefore, a size reduction is easy, and the optical writing device  600  is advantageous in a size reduction of the image forming apparatus. 
     Object Recognition Device 
     Referring to  FIGS. 9 and 10 , an object recognition device using the movable device  13  according to the present embodiment is described below in detail. 
       FIG. 9  is a schematic view illustrating an automobile  701  including a LiDAR device  700  that is an example of the object recognition device.  FIG. 10  is a schematic view illustrating an example of the LiDAR device  700 . 
     The object recognition device recognizes an object in the target direction. The object recognition device is, for example, a LiDAR device. 
     As illustrated in  FIG. 9 , the LiDAR device  700  is mounted in, for example, the automobile  701  to execute optical scanning in the target direction and receive the reflected light from a target object  702  existing in the target direction so as to recognize the target object  702 . 
     As illustrated in  FIG. 10 , the laser light emitted from the light source device  12  passes through an incident optical system including a collimator lens  703 , which is an optical system that converts diffuse light into substantially parallel light, and a planar mirror  704 . Then, the laser light is swept by the movable device  13  including the reflective surface  14  in one or two axial directions. Then, the laser light passes through for example a projection lens  705 , which is a projection optical system, to be emitted to the target object  702  in front of the apparatus. The driving of the light source device  12  and the movable device  13  is controlled by the control device  11 . The light reflected by the target object  702  is optically detected by a photodetector  709 . Specifically, the reflected light enters a condensing lens  706 , or the like, which is an incident-light detection/reception optical system, and is received by an imaging element  707 . The imaging element  707  outputs a detection signal to a signal processing circuitry  708 . The signal processing circuitry  708  performs predetermined processing, such as binarization or noise processing, on the input detection signal and output the result to a distance measuring circuitry  710 . 
     The distance measuring circuitry  710  determines the presence or absence of the target object  702  based on the difference between the timing at which the light source device  12  emits laser light and the timing at which the photodetector  709  receives the laser light or the difference in phase between the pixels of the imaging element  707  that have received the light. Furthermore, the distance measuring circuitry  710  calculates the distance information on the target object  702 . 
     As the movable device  13  including the reflective surface  14  is less likely to be damaged as compared with a polygonal mirror and is small in size, the movable device  13  may provide a highly-durable small-sized radar device. The above-described LiDAR device is installed in, for example, a vehicle to execute optical scanning over a predetermined range so as to determine the presence or absence of an obstacle or the distance to an obstacle. 
     The LiDAR device may be mounted in not only a vehicle but also a movable body such as an aircraft, a ship, or a movable robot, or an immovable body such as a work robot that operates a driving target such as a manipulator without moving from the spot. 
     With regard to the above-described object recognition device, the LiDAR device  700  has been described as an example. However, the object recognition device is not limited to the above-described embodiment and may be any device as long as the control device  11  controls the movable device  13  including the reflective surface  14  so as to execute optical scanning and the photodetector receives the reflected light to recognize the target object  702 . 
     The object recognition device is also applicable to, for example, the biometric authentication in which the object information, such as a shape, is calculated from the distance information obtained due to the optical scanning on a hand or a face and the record is checked to recognize the object, a security sensor that recognizes an intruder object due to the optical scanning in the target range, or a component of a three-dimensional scanner that calculates and recognizes the object information, such as a shape, from the distance information obtained due to optical scanning and output three-dimensional data. 
     Laser Headlamp 
     Referring to  FIG. 11 , a laser headlamp  50  using the movable device  13  according to the present embodiment as an automobile headlight is described.  FIG. 11  is a schematic view illustrating an example of the configuration of the laser headlamp  50 . 
     The laser headlamp  50  includes the control device  11 , a light source device  12   b , the movable device  13  including the reflective surface  14 , a mirror  51 , and a transparent plate  52 . 
     The light source device  12   b  emits blue laser light. The light emitted from the light source device  12   b  enters the movable device  13  and is reflected by the reflective surface  14 . The movable device  13  moves the reflective surface  14  in the X-direction and the Y-direction based on a signal transmitted from the control device  11  so that the blue laser light emitted from the light source device  12   b  is swept two-dimensionally in the X-direction and the Y-direction. 
     The scanning light from the movable device  13  is reflected by the mirror  51  to enter the transparent plate  52 . At least one of the front surface and the back surface of the transparent plate  52  is covered with a yellow phosphor. When the blue laser light from the mirror  51  passes through the yellow phosphor coating of the transparent plate  52 , the blue laser light is changed into white light in the range that is officially defined as the color of a headlight. This allows the front of the automobile to be illuminated with white light coming from the transparent plate  52 . 
     The scanning light from the movable device  13  scatters by a predetermined degree as the scanning light passes through the phosphor of the transparent plate  52 . This reduces the glare for the illumination target in front of the automobile. 
     When the movable device  13  is applied to the headlight of an automobile, the colors of the light source device  12   b  and the phosphor are not limited to blue and yellow. For example, the light source device  12   b  may emit near-ultraviolet radiation and the transparent plate  52  may be coated with the uniform mixture of phosphors in blue, green, and red, which are the three primary colors of light. Even in this case, the light passing through the transparent plate  52  may be converted into white light, and the front of the automobile may be illuminated with white light. 
     The laser headlamp  50  may be mounted in not only a vehicle but also a movable body such as an aircraft, a ship, or a movable robot, or an immovable body such as a work robot that operates a driving target such as a manipulator without moving from the spot. 
     Head Mount Display 
     Referring to  FIGS. 12 and 13 , a head mount display  60  using the movable device  13  according to the present embodiment is described below. The head mount display  60  is attachable to the human head. For example, the head mount display  60  may be shaped like glasses. Hereinafter, the head mount display is abbreviated as HMD. 
       FIG. 12  is a perspective view illustrating an example of the external appearance of the HMD  60 . As illustrated in  FIG. 12 , the HMD  60  includes pairs of a front  60   a  and a temple  60   b  that are substantially symmetric. The front  60   a  may include, for example, a light guide plate  61 . An optical system, a control device, and the like, may be incorporated in the temple  60   b.    
       FIG. 13  is a partial view illustrating an example of the configuration of the HMD  60 . Although  FIG. 13  illustrates the configuration for the left eye, the HMD  60  has the same configuration for the right eye. 
     The HMD  60  includes the control device  11 , the light source unit  530 , the light intensity adjuster  507 , the movable device  13  including the reflective surface  14 , the light guide plate  61 , and a half mirror  62 . 
     As described above, the light source unit  530  includes the laser light sources  501 R,  501 G, and  501 B, the collimator lenses  502 ,  503 , and  504 , and the dichroic mirrors  505  and  506 , which are housed as a unit in the optical housing. In the light source unit  530 , the dichroic mirrors  505  and  506  combine the laser lights in the three colors from the laser light sources  501 R,  501 G, and  501 B. The light source unit  530  emits the combined parallel light. 
     After the light intensity adjuster  507  adjusts the intensity of the light from the light source unit  530 , the light enters the movable device  13 . The movable device  13  moves the reflective surface  14  in the X-direction and Y-direction based on a signal from the control device  11  to two-dimensionally sweep the light from the light source unit  530 . The driving control of the movable device  13  is performed in synchronization with the emission timings of the laser light sources  501 R,  501 G, and  501 B so that a color image is formed with the scanning light. 
     The scanning light from the movable device  13  enters the light guide plate  61 . The light guide plate  61  guides the scanning light to the half mirror  62  while the inner wall surface reflects the scanning light. The light guide plate  61  includes a resin, or the like, having a permeability for the wavelength of the scanning light. 
     The half mirror  62  reflects the light from the light guide plate  61  to the back side of the HMD  60  to emit the light toward the eyes of a wearer  63  of the HMD  60 . The half mirror  62  has, for example, a free-form surface shape. The image with the scanning light is formed on the retina of the wearer  63  due to the reflection at the half mirror  62 . Alternatively, the image is formed on the retina of the wearer  63  due to the reflection at the half mirror  62  and the lens effect of the crystalline lens in the eyeball. The reflection at the half mirror  62  corrects the spatial distortion of the image. The wearer  63  may observe the image formed with the light swept in the X-direction and Y-direction. 
     Due to the half mirror  62 , the wearer  63  observes the image with the light from outside and the image with the scanning light in a superimposed manner. With a mirror instead of the half mirror  62 , it is also possible to eliminate the light from outside and observe the image with the scanning light. 
     Packaging 
     Referring to  FIG. 14 , the packaging of the movable device  13  according to the present embodiment is described below. 
       FIG. 14  is a schematic view illustrating an example of the packaged movable device  13 . 
     As illustrated in  FIG. 14 , the movable device  13  is packaged such that the movable device  13  is attached to a mounting member  802  provided inside a package member  801  and part of the package member  801  is covered with a transmissive member  803  to be sealed. The inside of the package is hermetically filled with inert gas such as nitrogen. This prevents the deterioration of the movable device  13  due to oxidation and improves the durability against changes in the environment such as a temperature. 
     Referring to the drawings, the movable device  13  according to the present embodiment used in the optical deflection system, the optical scanning system, the image projection device, the optical writing device, the object recognition device, the laser headlamp, and the head mount display are described below in detail. In each drawing, the same component is denoted by the reference numeral, and duplicate descriptions are sometimes omitted. 
     In the description according to the embodiment, the optical scanning due to the rotation around a first axis is the sub-scanning, and the optical scanning due to the rotation around a second axis is the main scanning. The terms such as rotation, oscillation, and movement in the embodiment are synonymous. With regard to the directions indicated by arrows, the X-direction is parallel to the second axis, the Y-direction is parallel to the first axis, and the Z-direction is perpendicular to the XY plane. The Z-direction is an example of a “laminating direction”. 
     First Embodiment 
     Structure of the Movable Device 
       FIG. 15  is a plan view illustrating the movable device  13  according to a first embodiment of the present invention.  FIG. 16  is a cross-sectional view taken through the line A-A in  FIG. 15 .  FIG. 17  is a cross-sectional view taken through the line B-B in  FIG. 15 . 
     The movable device  13  illustrated in  FIG. 15  is a cantilever optical deflection element that rotates a movable part including a reflective surface due to resonance oscillation to deflect the light incident on the reflective surface in two axial directions (around the first axis and the second axis). 
     The movable device  13  is configured to enable the rotation of a mirror unit  101  around the first axis corresponding to the main scanning direction and the rotation of the mirror unit  101  around the second axis corresponding to the sub-scanning direction. That is, the movable device  13  may deflect the incident light while sweeping the light in two axial directions due to the rotation of the mirror unit  101  in two axial directions. The structure of the movable device  13  is described below in detail. 
     The movable device  13  includes the mirror unit  101 , first drives  110   a  and  110   b , a first support  120 , second drives  130   a  and  130   b , a second support  140 , and an electrode connection  150 . The mirror unit  101  reflects incident light. The first drives  110   a  and  110   b  are coupled to the mirror unit  101 , which is a movable part, to drive the mirror unit  101  around the first axis parallel to the Y-axis. The first support  120  supports the mirror unit  101  and the first drives  110   a  and  110   b . The second drives  130   a  and  130   b  are coupled to the first support  120  to drive the mirror unit  101  and the first support  120  around the second axis parallel to the X-axis (perpendicular to the first axis). The second support  140  supports the second drives  130   a  and  130   b . The electrode connection  150  is electrically connected to the first drives  110   a  and  110   b  and the second drives  130   a  and  130   b.    
     For example, a single silicon on insulator (SOI) substrate is formed by etching processing, or the like, and the reflective surface  14 , first piezoelectric drives  112   a  and  112   b , second piezoelectric drives  131   a  to  131   f  and  132   a  to  132   f , the electrode connection  150 , and the like, are formed on the substrate so that various components are integrally formed in the movable device  13 . Each of the above-described components may be formed after the SOI substrate is formed or while the SOI substrate is being formed. 
     The SOI substrate includes a silicon oxide layer  162  provided on a first silicon layer including monocrystal silicon (Si) and a second silicon layer that includes monocrystal silicon and is provided on the silicon oxide layer  162 . Hereinafter, the first silicon layer is referred to as a silicon support layer  161 , and the second silicon layer as a silicon active layer  163 . 
     As the silicon active layer  163  is thin in the Z-axis direction as compared with the X-axis direction and the Y-axis direction, a member including the silicon active layer  163  functions as an elastic member having elasticity. The SOI substrate does not need to be flat and may have a curvature, etc. The member used to form the movable device  13  is not limited to the SOI substrate and may be any substrate as long as the substrate enables the integral formation by etching processing, etc., and partial elasticity. 
     The mirror unit  101  is a movable part including, for example, a circular mirror unit base  102  and the reflective surface  14  formed on the +Z side surface of the mirror unit base  102 . The mirror unit base  102  includes, for example, the silicon active layer  163 . The reflective surface  14  includes a metal thin film including, for example, aluminum, gold, or silver. 
     The mirror unit  101  may include a rib on the −Z side surface of the mirror unit base  102  to reinforce the mirror unit  101 . The rib includes, for example, the silicon support layer  161  and the silicon oxide layer  162  to suppress the distortion of the reflective surface  14  caused due to the movement. 
     The first drives  110   a  and  110   b  include two torsion bars  111   a  and  111   b  and first piezoelectric drives  112   a  and  112   b . The torsion bars  111   a  and  111   b  have one end coupled to the mirror unit base  102  and extend in the direction of the first axis to movably support the mirror unit  101 . The first piezoelectric drives  112   a  and  112   b  have one end coupled to the torsion bars  111   a  and  111   b  and have the other end coupled to the inner circumferential portion of the first support  120 . 
     As illustrated in  FIG. 16 , the torsion bars  111   a  and  111   b  include the silicon active layer  163 . In the first piezoelectric drives  112   a  and  112   b , a lower electrode  201 , a piezoelectric part  202 , and an upper electrode  203  are formed in this order on the +Z side surface of the silicon active layer  163  which is an elastic part. The upper electrode  203  and the lower electrode  201  include, for example, gold (Au) or platinum (Pt). The piezoelectric part  202  includes, for example, PZT (lead zirconate titanate), which is a piezoelectric material. 
     Returning back to  FIG. 15 , the first support  120  is a rectangular support that includes, for example, the silicon support layer  161 , the silicon oxide layer  162 , and the silicon active layer  163  so as to surround the mirror unit  101 . 
     The second drives  130   a  and  130   b  include, for example, a plurality of second piezoelectric drives  131   a  to  131   f  and  132   a  to  132   f  that are coupled so as to be folded. One end of each of the second drives  130   a  and  130   b  is coupled to the outer circumferential portion of the first support  120 , and the other end thereof is coupled to the inner circumferential portion of the second support  140 . 
     The connection point between the second drive  130   a  and the first support  120  and the connection point between the second drive  130   b  and the first support  120  are point-symmetric with respect to the center of the reflective surface  14 . Furthermore, the connection point between the second drive  130   a  and the second support  140  and the connection point between the second drive  130   b  and the second support  140  are point-symmetric with respect to the center of the reflective surface  14 . 
     The second piezoelectric drives  131   b ,  131   d , and  131   f  constitute a piezoelectric drive group  170 A. The second piezoelectric drives  132   a ,  132   c , and  132   e  also constitute the piezoelectric drive group  170 A. The piezoelectric drive group  170 A has bending and deformation in the same direction when a drive voltage is simultaneously applied to each piezoelectric part. This deformation, as a rotational force, causes the mirror unit  101  to rotate around the first axis. 
     The second piezoelectric drives  131   a ,  131   c , and  131   e  constitute a piezoelectric drive group  170 B. The second piezoelectric drives  132   b ,  132   d , and  132   f  also constitute the piezoelectric drive group  170 B. The piezoelectric drive group  170 B has bending and deformation in the same direction when a drive voltage is simultaneously applied to each piezoelectric part. This deformation, as a rotational force, causes the mirror unit  101  to rotate around the first axis in the direction opposite to the direction due to the rotation by the piezoelectric drive group  170 A. 
     As illustrated in  FIG. 17 , in the second drives  130   a  and  130   b , the lower electrode  201 , the piezoelectric part  202 , and the upper electrode  203  are formed in this order on the +Z side surface of the silicon active layer  163 , which is an elastic part. The upper electrode  203  and the lower electrode  201  include, for example, gold (Au) or platinum (Pt). The piezoelectric part  202  includes, for example, Pb[ZrxTi1-x]O3 0&lt;x&lt;1 (PZT) (lead zirconate titanate) which is a piezoelectric material. 
     Returning back to  FIG. 15 , the second support  140  is a rectangular support including, for example, the silicon support layer  161 , the silicon oxide layer  162 , and the silicon active layer  163  to surround the mirror unit  101 , the first drives  110   a  and  110   b , the first support  120 , and the second drives  130   a  and  130   b.    
     The electrode connection  150  is formed on, for example, the +Z side surface of the second support  140  to electrically connect the control device  11  to the upper electrodes  203  and the lower electrodes  201  of the first piezoelectric drives  112   a  and  112   b  and the second piezoelectric drives  131   a  to  131   f  via an electrode wire including aluminum (Al), etc. Each of the upper electrode  203  and the lower electrode  201  may be directly coupled to the electrode connection  150  or may be indirectly coupled due to, for example, the connection of the electrodes. 
     In the example of the case described according to the present embodiment, the piezoelectric part  202  is formed on one surface (the +Z side surface) of the silicon active layer  163  that is an elastic part. However, the piezoelectric part  202  may be provided on a different surface (e.g., the −Z side surface) of the elastic part, or the piezoelectric part  202  may be provided on both the surfaces of the elastic part. 
     The shape of each component is not limited to the shape according to the embodiment as long as the mirror unit  101  may be driven around at least one of the first axis and the second axis. For example, the torsion bars  111   a  and  11   b  and the first piezoelectric drives  112   a  and  112   b  may be shaped to have a curvature. 
     An insulating layer including a silicon oxide layer further may be formed on at least any of the +Z side surface of the upper electrode  203  of the first drives  110   a  and  110   b , the +Z side surface of the first support  120 , the +Z side surface of the upper electrode  203  of the second drives  130   a  and  130   b , and the +Z side surface of the second support  140 . 
     An electrode wire is provided on the insulating layer, and the insulating layer is partially removed or no insulating layer is provided to form an opening at the connection spot where at least one of the upper electrode  203  and the lower electrode  201  is coupled to the electrode wire, whereby the design freedom of the first drives  110   a  and  110   b , the second drives  130   a  and  130   b , and the electrode wire may be increased, and short-circuiting due to the contact between the electrodes may be prevented. The silicon oxide layer also functions as an antireflective member. 
     Next, the control of the control device  11  that drives the first drives  110   a  and  110   b  and the second drives  130   a  and  130   b  of the movable device  13  is described in detail. 
     The application of a positive or negative voltage in a polarization direction causes the piezoelectric parts  202  included in the first drives  110   a  and  110   b  and the second drives  130   a  and  130   b  to deform (for example, expand and contract) in proportion to the potential of the applied voltage so as to produce what is called an inverse piezoelectric effect. The first drives  110   a  and  110   b  and the second drives  130   a  and  130   b  use the inverse piezoelectric effect to move the mirror unit  101 . 
     The angle formed between the XY plane and the reflective surface  14  of the mirror unit  101  when the reflective surface  14  is tilted in the +Z direction or the −Z direction with respect to the XY plane is referred to as a deflection angle. The deflection angle in the +Z direction is referred to as a positive deflection angle, and the deflection angle in the −Z direction as a negative deflection angle. 
     In the first drives  110   a  and  110   b , when a drive voltage is applied in parallel to the piezoelectric parts  202  included in the first piezoelectric drives  112   a  and  112   b  via the upper electrodes  203  and the lower electrodes  201 , each of the piezoelectric parts  202  deform. The effect of the deformation of the piezoelectric part  202  causes the first piezoelectric drives  112   a  and  112   b  to be bent and deformed. Accordingly, the torsion of the two torsion bars  111   a  and  111   b  causes the driving force acting on the mirror unit  101  around the first axis so that the mirror unit  101  rotates around the first axis. The drive voltage applied to the first drives  110   a  and  110   b  is controlled by the control device  11 . 
     Therefore, when the control device  11  causes the drive voltage having a predetermined sinusoidal waveform to be applied in parallel to the first piezoelectric drives  112   a  and  112   b  included in the first drives  110   a  and  110   b , the mirror unit  101  may be moved around the first axis in the period of the drive voltage having a predetermined sinusoidal waveform. 
     In particular, for example, when the frequency of the sinusoidal voltage is set to approximately 20 kHz that is nearly equal to the resonance frequency of the torsion bars  111   a  and  111   b , the mirror unit  101  may resonate at approximately 20 kHz by the use of the mechanical resonance occurring due to the torsion of the torsion bars  111   a  and  111   b.    
       FIG. 18  is a cross-sectional view illustrating an example of the mirror unit  101  according to a comparative example. A mirror unit  101 X illustrated in  FIG. 18  includes the silicon support layer  161 , the silicon oxide layer  162 , the silicon active layer  163 , an interlayer film  240 , a metal film  250 , and a high reflective layer  260 . The material of the interlayer film  240  is, for example, SiO 2 , SiN X , or Al 2 O 3 . The material of the metal film  250  is, for example, Al, AlCu, AlSiCu, Ag, Ag alloy, or Au. The upper surface of the metal film  250  is a reflective surface that reflects light. 
     The high reflective layer  260  is provided so as not to lower the reflectance of the metal film  250 . In this example, as the high reflective layer  260  is provided for the metal film  250  on which light is incident after passing through a protective film, the reflectance of light may be increased as compared with a case where no high reflective layer is provided. Thus, the use efficiency of light may be increased. 
     The high reflective layer  260  is a layer including a dielectric multi-layer film and is formed by alternately laminating a low refractive index material layer and a high refractive index material layer. Examples of the low refractive index material include SiO 2  or MgF 2 . Examples of the high refractive index material include TiO 2 , Nb 2 O 5 , ZrO 2 , or Ta 2 O 5 . The high reflective layer  260  may include a layer including an intermediate refractive index material such as Al 2 O 3 . The high reflective layer  260  may be formed by using, for example, a vapor deposition method, an atomic layer deposition (ALD) method, a CVD method, or a sputtering method. 
       FIG. 19  is a cross-sectional view illustrating an example of the mirror unit  101  according to the first embodiment. In the mirror unit  101  illustrated in  FIG. 19 , a protective film  270  protects the metal film  250  and the high reflective layer  260 . The mirror unit  101  according to the present embodiment includes the metal film  250 , the high reflective layer  260  laminated on the upper surface of the metal film  250 , and the protective film  270  that covers the metal film  250  and the high reflective layer  260 . 
     The protective film  270  continuously covers the upper surface and the side surface of the high reflective layer  260  and the side surface of the metal film  250 . As the high reflective layer  260  is formed on the upper surface of the metal film  250 , the protective film  270  is not directly formed on the upper surface of the metal film  250 . 
     That is, the mirror unit  101  is different from the mirror unit  101 X according to the comparative example in that the mirror unit  101  includes the protective film  270 . The protective film  270  may continuously cover the upper surface of the high reflective layer  260 , the side surface of the high reflective layer  260 , and the side surface of the metal film  250  so that the lower surface of the protective film  270  is in contact with the upper surface of the interlayer film  240 . This is to ensure that the metal film  250  and the high reflective layer  260  are protected. 
     The protective film  270  may have a material to be a film that allows the passage of light entering the reflective surface of the mirror unit  101  and that is suitable for film formation by using the ALD method. Examples of such a film include an oxide film or a nitride film, which is an inorganic film. Specifically, examples of the material of the protective film  270  include Al 2 O 3 , Ta 2 O 5 , SiO 2 , or SiN X . A metal film or an organic film may be used as the material of the protective film  270 . Selecting an oxide film or a nitride film as the material of the protective film  270  is advantageous in suppressing the deterioration of the optical characteristics. It is, in particular, advantageous as compared with a metal film or an organic film. 
       FIG. 20  is a graph illustrating a difference in the reflectance between the mirror units due to the presence or absence of a protective film. The Y-axis represents the reflectance, and the X-axis represents the incidence angle of a light beam when the incidence angle of a light bean entering the mirror unit at right angle is zero degrees. Specifically,  FIG. 20  illustrates the reflectance of the mirror unit when the incident light has a wavelength λ of 905 nm and the protective film  270  having a refractive index n of 1.76 is formed as a single layer having a physical film thickness d.  FIG. 20  also illustrates the reflectance of the mirror unit (the comparative example) in which the protective film  270  is not formed. Consideration is given to five types of the physical film thickness d, i.e., 5 nm, 10 nm, 30 nm, 50 nm, and 100 nm. 
     It is understood from  FIG. 20  that, when the physical film thickness d of the protective film  270  exceeds 50 nm, the reflectance is 99% or less in the range of the incident angle from 0 degrees to 30 degrees. Therefore, the physical film thickness d of the protective film  270  may be 5 nm or more and 50 nm or less. 
     A decrease in the reflectance in the range of a small incident angle is a disadvantage peculiar to a mirror including a movable part. As the incident angle increases, the optical path length passing through the protective film  270  becomes longer; therefore, the apparent film thickness changes, and the dependence of the reflectance on the incident angle increases. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Physical film 
                 Optical film 
                 QWOT 
                 QWOT 
               
               
                 thickness d 
                 thickness nd 
                 [Reference 
                 [Reference 
               
               
                 [nm] 
                 [nm] 
                 wavelength 550 nm] 
                 wavelength 905 nm] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 5 
                 8.8 
                 0.0589 
                 0.0389 
               
               
                 10 
                 17.6 
                 0.1178 
                 0.0779 
               
               
                 30 
                 52.8 
                 0.3535 
                 0.2336 
               
               
                 50 
                 88.0 
                 0.5891 
                 0.3894 
               
               
                 100 
                 176.0 
                 1.1782 
                 0.7788 
               
               
                   
               
            
           
         
       
     
     Table 1 illustrates the values of the physical film thickness d, an optical film thickness nd, and QWOT (the film thickness corresponding to ¼ of the wavelength in terms of the optical film thickness) of the protective film  270  under the conditions illustrated in  FIG. 20 . In Table 1, the optical film thickness nd=n (refractive index)×d (physical film thickness), and QWOT is the value of k in nd=k×λ/4. 
     When the physical film thickness d of the protective film  270  is increased, the optical film thickness nd is also increased by the multiplication of the refractive index. To form the high reflective layer  260 , a high refractive index layer and a low refractive index layer having the film thickness of QWOT are deposited alternately so that the incident light is reflected most efficiently. As the value of QWOT of the protective film  270  approaches the value of QWOT of the high reflective layer  260 , the effect on the reflectance increases. 
     Therefore, as illustrated in  FIG. 20 , as the physical film thickness increases, the reflectance in the range of the incident angle from 0 degrees to 30 degrees decreases. In this design, the reflectance of 99% is satisfied when the incident angle is 0 degrees to 40 degrees if QWOT=0.5891 or less in the case of the reference wavelength of 550 nm and if QWOT=0.3894 or less in the case of the reference wavelength of 905 nm. In this description, it is assumed that the deterioration of the optical characteristics of the protective film  270  may be suppressed when the reflectance of 99% is satisfied in the case of the incident angle of 0 degrees to 40 degrees. 
     That is, it is possible to suppress the deterioration of the optical characteristics of the protective film  270  when QWOT=0.5891 or less in the case of the reference wavelength of 550 nm and when QWOT=0.3894 or less in the case of the reference wavelength of 905 nm. 
     The method for manufacturing the movable device  13  which is an optical deflection element may include a step of forming the protective film  270  by using ALD. In other words, the protective film  270  may be formed by using the ALD. The ALD is one of the vacuum film formation techniques. According to the ALD, a thin film is formed on the deposition target surface for each atomic layer by utilizing the self-regulating characteristics of atoms. 
     Compared with the chemical vapor deposition (CVD), which is a vapor growth method by a chemical reaction using the assist by plasma, the ALD has characteristics such as the capability to form a thin film, the capability to form a film with few defects, and the desirable coverage. Referring to  FIG. 21 , an explanation is given of the fact that the ALD is superior to the CVD in forming the protective film  270 . 
       FIGS. 21A and 21B  are schematic views illustrating the cross-section of the protective film. In  FIGS. 21A and 21B , there are film defects, such as a crack  310 , a grain boundary  320 , and a particle  330 . Typically, the film formation by the CVD results in many film defects as illustrated in  FIG. 21A . Furthermore, as the CVD has a high film formation speed (approximately 10 to 15 nm/s), the film thickness is increased so as to affect the optical characteristics of the high reflective layer. Thus, it is difficult to form a film having a thickness of several tens of nanometers. 
     On the other hand, as the ALD forms a film for each atomic layer, the film formation speed is lower than that of the CVD, and therefore the occurrence of cracks and particles is less likely to occur. As illustrated in  FIG. 21B , the ALD has few film defects as compared with the CVD. Furthermore, as the ALD forms a film at high temperatures, the film density in the ALD is desirable as compared with the CVD. 
     The film formation by the ALD has the desirable coverage as compared with the CVD. In a case where multiple layers of dissimilar materials are included (in a case where the metal film  250  and the high reflective layer  260  are covered) as in this example, seams are likely to occur on a side wall surface. Therefore, it is difficult to form a film having a thickness of several tens of nanometers by the CVD. As the ALD is a film formation using a surface reaction, the film formation by even 1 nm may theoretically result in the desirable coverage. By taking advantage of these features, it is possible to achieve both the protection of the reflective surface and the optical characteristics from the viewpoint of the reliability and the optical characteristics. 
     Thus, the use of the ALD allows the protective film  270 , which improves the environmental resistance performance (e.g., moisture-proof performance) of the reflective surface, to be formed as a highly-dense ultrathin film (a film having a thickness of several tens of nanometers) that does not affect the optical characteristics. As a result, it is possible to achieve both the protection of the reflective surface and the prevention in the deterioration of the optical characteristics. 
     In order to suppress the deterioration of the optical characteristics, the protective films for the upper surface (hereinafter referred to as the upper surface) of the high reflective layer and for the side surface (hereinafter referred to as the side surface) of the high reflective layer and the metal film may have a small difference in thickness and may be uniform. A deposition method and a sputtering method have poor particle adhering performance as compared with the ALD. In the deposition method and the sputtering method, if the protective film  270  is formed to have the physical film thickness d of 5 nm or more and 50 nm or less, there is a possibility that the side surface is not sufficiently covered when the thickness of the protective film for the upper surface reaches the target thickness. To sufficiently cover the side surface, the thickness of the upper surface becomes more than the target thickness, which results in a disadvantage such as the deterioration of the optical characteristics. 
     For example, in the CVD, when the thickness of the protective film formed on the upper surface is 100%, the side surface formed is approximately 50 to 60%. On the other hand, in the ALD, when the thickness of the protective film formed on the upper surface is 100%, the side surface formed may be approximately 70 to 100%. Thus, the ALD is effective in solving the above disadvantages. 
     Second Embodiment 
     In an example described according to a second embodiment of the present invention, the protective film has a two-layer structure. In the second embodiment, the descriptions of the same components as those in the above-described embodiment may be omitted. 
       FIG. 22  is a cross-sectional view illustrating an example of a mirror unit  101 A according to the second embodiment. The mirror unit  101 A illustrated in  FIG. 22  is different from the mirror unit  101  (see  FIG. 19 ) in that the protective film  270  is replaced with a protective film  270 A. The protective film  270 A has a two-layer structure of a first protective film  271  and a second protective film  272 . 
     In the same manner as in the first embodiment, the metal film  250  is formed on the upper surface of the interlayer film  240 , and the high reflective layer  260  is formed on the upper surface of the metal film  250 . Unlike the first embodiment, the first protective film  271  is formed to continuously cover the upper surface and the side surfaces of the high reflective layer  260  and the side surface of the metal film  250 , and the second protective film  272  is formed to cover the upper surface and the side surfaces of the first protective film  271 . 
     The first protective film  271  and the second protective film  272  are formed of dissimilar materials. For example, a low refractive index material is used as the first protective film  271  and a high refractive index material is used as the second protective film  272 . Alternatively, a high refractive index material may be used as the first protective film  271  and a low refractive index material may be used as the second protective film  272 . Examples of the low refractive index material include SiO 2  or MgF 2 . Examples of the high refractive index material include TiO 2 , Nb 2 O 5 , ZrO 2 , or Ta 2 O 5 . 
     An intermediate refractive index material such as Al 2 O 3  may be used as at least one of the first protective film  271  and the second protective film  272 . The above is an example, and the magnitude relationship of the refractive indexes of the first protective film  271  and the second protective film  272  are not specified as long the first protective film  271  and the second protective film  272  include dissimilar materials. 
       FIG. 23  is a graph illustrating the difference in the reflectance of the mirror unit depending on the number of layers in the protective film.  FIG. 23  illustrates the difference in the reflectance in a case where the protective film includes two layers (the first protective film  271  is an Al 2 O 3  film having a thickness of 25 nm, and the second protective film  272  is a SiO 2  film having a thickness of 25 nm) and in a case where the protective film includes a single layer (an Al 2 O 3  film having a thickness of 50 nm). As illustrated  FIG. 23 , the protective film including two layers with an appropriate combination of dissimilar materials may improve the reflectance in the range of small incident angles (e.g., the range of incident angles of 0 degrees to 30 degrees) as compared with the protective film including a single layer. 
     Thus, the protective film including two separate layers may reduce the optical loss due to the protective film as understood from the tendency of the reflectance illustrated in  FIG. 23  while dispersing film defects such as the above-described particles. 
     The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention. 
     Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above. 
     Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuitry also includes devices such as an application specific integrated circuitry (ASIC), digital signal processor (DSP), field programmable gate array (FPGA), and conventional circuitry components arranged to perform the recited functions.