Abstract:
A varifocal mirror and a camera module comprising the same are disclosed. The varifocal mirror comprises a substrate, a thin film having a circumference supported by the substrate and a variable radius of curvature, and a reflective plane attached to the thin film to reflect light. The reflective plane has a radius of curvature varying in response to variation in the radius of curvature of the thin film. The camera module embodies focus and optical zoom functions via actuation of the varifocal mirror instead of using an electromagnetic actuator, a step motor and the like when adjusting positions of lens units, significantly reducing a volume of the overall system. With this structure, the camera module can be applied to various portable digital assistances requiring a minimized camera module, such as PDAs, mobile phones and the like.

Description:
[0001]     Pursuant to 35 U.S.C. § 119(a), this application claims the benefit of earlier filing date and right of priority to Korean Patent Application No. 10-2005-0008235 filed on Jan. 28, 2005, which is hereby incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a varifocal mirror, and a camera module comprising the same. More particularly, the present invention relates to a varifocal mirror, and a camera module comprising the same, which embodies focus and optical zoom functions via actuation of the varifocal mirror instead of using an electromagnetic actuator, a step motor and the like for adjustment of lens units, thereby significantly reducing a volume of the overall system.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, new technology for personal digital assistances integrating various functions such as information processing, computation, communication, input and output of image information, etc. therein has been developed with advances in communication technology and digital information processing technology. For example, there are PDAs incorporating a digital camera and a communication function therein, mobile phones incorporating the digital camera and functions of the PDAs therein, and the like. In this regard, it is common in market to incorporate a high performance digital camera module to the PDAs due to advances in digital camera technology and information storage capability.  
         [0006]     As an image sensor of a mega pixel level has been employed for the digital camera module mounted on the PDAs and the like with advances of related technology, functions such as auto-focus and optical zoom have become more important. In order to realize the auto-focus and optical zoom functions in a minimized digital camera module, it is necessary to provide an actuator which can satisfy requirements such as rapid moving speed, low power consumption, large displacement, and the like while occupying a relatively small space in the camera module. Specifically, it is necessary to provide an actuator which can satisfy requirements corresponding to an increase in displacement resulting from advances in optical zoom function. In this regard, for a magnetic actuator such as a conventional voice coil motor (VCM), there are disadvantages of a limit in an increase of the displacement, and of great power consumption when operating the zoom function. In addition, for a rotatable actuator such as a stepping motor, which linearly moves a movable part by rotating a lead screw using, there are disadvantages such as a complicated actuating mechanism, friction and noise from a gear box, and the like. In addition, for an actuator employing both VCM and step motor, since it has a complicated structure, there are problems of high manufacturing costs, and of a limit in reduction of an actuator&#39;s size.  
         [0007]      FIG. 1  is a diagram illustrating a conventional camera module which has optical zoom and auto focusing functions realized by two lens units. According to a conventional method, the lens units are shifted on an optical axis in order to realize such functions. If a zoom lens unit  21  is moved to a predetermined position to realize the optical zoom function, an auto-focus lens unit  22  is moved to a predetermined position determined according to the position of the zoom lens unit  21 , and then forms an image on an image sensor  23 . Generally, in such a two-lens unit type optical zoom camera module, it is necessary for the zoom lens unit to be moved in a relatively large range, and for the auto-focus lens unit to move rapidly. In addition, in order to improve the optical zoom function together with image quality, the actuator must satisfy more strict requirements.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made to solve the above problems, and it is an object of the present invention to provide a camera module which has a varifocal mirror incorporated to an optical system thereof in order to realize an ultra-minimized optical zoom function.  
         [0009]     It is another object of the present invention to provide the camera module which realizes an auto-focus function using the varifocal mirror.  
         [0010]     It is yet another object of the present invention to provide the camera module which is minimized in size, and reduced in weight by integrating the varifocal mirror therein.  
         [0011]     In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a varifocal mirror, comprising: a substrate; a thin film having a circumference supported by the substrate, and a variable curvature; and a reflective plane attached to the thin film to reflect light, and having a curvature varying in response to variation in curvature of the thin film.  
         [0012]     Preferably, either the thin film or the reflective plane is formed of a circular plate.  
         [0013]     Preferably, the curvature of the thin film or the reflective plane varies asymmetrically with respect to a center thereof.  
         [0014]     Preferably, the thin film is made of at least one material selected from the group consisting of silicon, silicon nitride, silicon oxide, dielectrics, ceramic, polymer, and metal.  
         [0015]     Preferably, the reflective plane is made of at least one material selected from the group consisting of metal, dielectrics, and lamination of metal and dielectrics.  
         [0016]     Preferably, the circumference of the thin film is partially connected to the substrate in a bridge structure such that the circumference of the thin film is supported by the substrate.  
         [0017]     Preferably, the varifocal mirror further comprises: a metallic member attached to the thin film in an insulated state from an outside, and to which electric potential is applied thereto; and an electrode positioned below the thin film, wherein the curvature of the thin film varies via electrostatic force caused by potential difference between the metallic member and the electrode.  
         [0018]     Preferably, the varifocal mirror further comprises: a magnetic member attached to the thin film; a coil wound around the thin film; and a power supplier to apply electric current to the coil, wherein the curvature of the thin film varies via movement of the magnetic member within a magnetic field generated by the electric current applied to the coil. More preferably, the magnetic field is generated perpendicular to the thin film by the electric current applied to the coil.  
         [0019]     Preferably, the varifocal mirror further comprises: a magnetic field supplier to supply a magnetic field from an outside to the thin film; a coil attached to the thin film; and a power supplier to apply electric current to the coil, wherein the curvature of the thin film varies via electrostatic force generated by application of the electric current to the coil within a magnetic field generated by the magnetic field supplier. The magnetic field supplier may comprise at least one permanent magnet. More preferably, the magnetic field supplier supplies the magnetic field perpendicular to the thin film.  
         [0020]     Preferably, the varifocal mirror further comprises: a piezoelectric member attached to the thin film; and a power supplier to apply voltage to the piezoelectric member, the curvature of the thin film varying via a piezoelectric actuating force generated by the voltage applied to the piezoelectric member.  
         [0021]     Preferably, the varifocal mirror further comprises: hetero-thermal expansion material lamination formed by stacking two materials having different thermal expansion coefficients in a vertical direction, and attached to the thin film, the curvature of the thin film varying via thermal expansion of the materials in the hetero-thermal expansion material lamination. More preferably, the hetero-thermal expansion material lamination is attached to the thin film so as to be symmetrical with respect to a center of the thin film.  
         [0022]     Preferably, the thin film and the reflective plane have shapes being flat or curved in an initial state not applying force to vary the curvature thereof.  
         [0023]     In accordance with another aspect of the present invention, a camera module is provided, comprising: a varifocal mirror according to the present invention. Preferably, the camera module performs zoom adjustment and focus adjustment by changing curvatures of a thin film and a reflective plane in the varifocal mirror. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]     The foregoing and other objects and features of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0025]      FIG. 1  is a diagram illustrating a two-lens unit type optical zoom camera module;  
         [0026]      FIGS. 2   a  to  2   c  are perspective views illustrating a varifocal mirror in accordance with one embodiment of the present invention;  
         [0027]      FIGS. 3   a  to  3   c  are cross-sectional views illustrating a varifocal mirror actuated using electrostatic force in accordance with one embodiment of the present invention;  
         [0028]      FIGS. 4   a  to  4   c  are cross-sectional views illustrating a varifocal mirror actuated by various methods in accordance with one embodiment of the present invention;  
         [0029]      FIGS. 5   a  to  5   c  are a plan view and cross-sectional views illustrating the varifocal mirror supported by a connecting member and actuated by electrostatic force in accordance with one embodiment of the present invention;  
         [0030]      FIG. 6  is a diagram illustrating a camera module incorporating varifocal mirrors in accordance with one embodiment of the present invention; and  
         [0031]      FIGS. 7   a  to  7   c  are cross-sectional views illustrating a camera module incorporating a varifocal mirror in accordance with another embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings in which like components are denoted by the same reference numerals, and detailed description of well-known function and configuration making the subject matter of the present invention unclear will be omitted.  
         [0033]      FIGS. 2   a  to  2   c  are perspective views illustrating a varifocal mirror in accordance with one embodiment of the present invention.  
         [0034]     According to this embodiment, the varifocal mirror comprises a thin film, and a reflective plane.  
         [0035]      FIGS. 2   a  to  2   c  schematically show a method of stacking the thin film and the reflective plane, and a method of operating the varifocal mirror according to this embodiment.  
         [0036]     Referring to  FIG. 2   a , the varifocal mirror has the structure in which a reflective plane  2  is positioned on a circular thin film  1 , and changes a focal length of reflected light  12  via variation in curvatures of the thin film  1  and the reflective plane  2 . As shown in  FIGS. 2   b  and  2   c , if the center of the reflective plane  2  is changed in position by d 1  or d 2  below a horizontal line, the radius of curvature thereof is varied to r 1  or r 2 , so that the focal length of reflected light is changed. If the center of the reflective plane  2  is further lowered, the radius of curvature thereof is further increased, and the focal length of the reflected light is shortened in inverse proportion to an increase amount of the radius of curvature.  
         [0037]     The varifocal mirror of the invention can be manufactured in various configurations using various materials by micromachining or fine machining, and can be actuated by various methods using electrostatic force, electromagnetic force, heat, piezoelectric force, and the like.  
         [0038]      FIGS. 3   a  to  3   c  are cross-sectional views illustrating the varifocal mirror actuated by electrostatic force in accordance with one embodiment of the present invention  
         [0039]     In this embodiment, the varifocal mirror actuated by the electrostatic force comprises a thin film, a reflective plane, an upper substrate, an electrode, and a lower substrate.  
         [0040]      FIGS. 3   a  to  3   c  schematically show a device for adjusting a focus of reflected light using electrostatic force generated between the thin film and the electrode or between the reflective plane and the electrode.  
         [0041]      FIGS. 3   a  to  3   c  show one example of the varifocal mirror which employs the electrostatic force for actuation thereof. As shown in  FIG. 3   a , the varifocal mirror is prepared by coupling an upper substrate  41  having a thin film  1  and a reflective plane  2  positioned thereon to a lower substrate  43  having an electrode  42  positioned thereon. The thin film  1  may be composed of various materials including ceramic materials such as silicon, silicon nitrides, and silicon oxides, and other materials such as dielectrics, ceramics, polymers, metals, etc. The reflective plane  2  may comprise a single or several layers of metal or dielectrics, or a lamination of metal and dielectrics. When manufacturing the varifocal mirror, it is possible to adjust an initial state of the reflective plane  2  by adjusting stress on the thin film  1  and the reflective plane  2 . Thus, it is possible to manufacture the varifocal mirror so as to be free of an initial deflection as shown in  FIG. 3   a , or to manufacture the varifocal mirror so as to have a predetermined initial deflection d 1  as shown in  FIG. 3   b.    
         [0042]     For the electrostatic force actuating type varifocal mirror, voltage is applied between the reflective plane  2  and the electrode  42 . If the reflective plane  2  is made of any of the materials described above other than metal, it is necessary to form a metal layer on the thin film  1  for actuation. As potential difference between the reflective plane  2  and the electrode  42  is increased, the center of the reflective plane  2  is lowered as shown in  FIGS. 3   b  and  3   c , and the radius of curvature thereof is increased.  
         [0043]      FIGS. 4   a  to  4   c  are cross-sectional views illustrating a varifocal mirror actuated by various methods in accordance with one embodiment of the present invention.  
         [0044]     In  FIG. 4   a , the varifocal mirror is actuated using electromagnetic force, and comprises a thin film, a reflective plane, a substrate, a coil, and a magnetic member.  
         [0045]      FIG. 4   a  schematically shows the varifocal mirror actuated by the electromagnetic force.  
         [0046]     Referring to  FIG. 4   a , the varifocal mirror comprises a thin film  1  supported by a substrate  41 , a magnetic member  45  stacked on the thin film  1 , and a reflective plane  2  stacked on the magnetic member  45 . The varifocal mirror further comprises a coil  44  wound around the substrate  41 . When electric current flows through the coil  44 , a magnetic field is generated around the coil  44 . Here, the intensity of the magnetic field is proportional to the intensity of the electric current. The magnetic field applies force to, and moves the magnetic member  45 . When the thin film  1  under the magnetic member  45  is lowered due to movement of the magnetic member  45 , the reflective plane  2  is thus moved, and changes a focal length of light.  
         [0047]     In  FIG. 4   b , the varifocal mirror is actuated by means of a piezoelectric member, and comprises a thin film, a reflective plane, a substrate, the piezoelectric member, and an electrode.  
         [0048]      FIG. 4   b  schematically shows the varifocal mirror actuated by the piezoelectric member.  
         [0049]     Referring to  FIG. 4   b , the varifocal mirror comprises a thin film  1 , a reflective plane  2  stacked at a central region on the thin film, and a piezoelectric member  46  stacked on the thin film at a portion other than the reflective plane  2 . The piezoelectric member is symmetrical with respect to a rotational axis  40 . The varifocal mirror further comprises a pair of electrodes  47 - 1  and  47 - 2  to supply electric charges to the piezoelectric member  46 . The piezoelectric member  46  can convert electric force to mechanical force, or vice versa. When potential difference is applied to the electrodes on the piezoelectric member  46 , the piezoelectric member  46  is deformed, thereby enabling the curvature of the reflective plane  2  to be adjusted.  
         [0050]     In  FIG. 4   c , the varifocal mirror is actuated by means of a thermal expansion member, and comprises a thin film, a reflective plane, a substrate and a thermal expansion member.  
         [0051]      FIG. 4   c  schematically shows the varifocal mirror actuated by the thermal expansion member.  
         [0052]     Referring to,  FIG. 4   c , the varifocal mirror comprises a thin film  1  supported by a substrate  41 , a reflective plane  2  stacked at a central region on the substrate  41 , and thermal expansion members  48 - 1  and  48 - 2  stacked on the thin film  1  outside the reflective plane  2 . The thermal expansion members  48 - 1  and  48 - 2  are formed by stacking two materials having different thermal expansion coefficients. Preferably, the thermal expansion members  48 - 1  and  48 - 2  are symmetrical with respect to a rotational axis  40 . The thermal expansion members  48 - 1  and  48 - 2  are subjected to volume change according to surrounding temperature. In this regard, since the materials constituting each of the thermal expansion members  48 - 1  and  48 - 2  have different thermal expansion coefficients, the thermal expansion members  48 - 1  and  48 - 2  extend in different lengths from each other. Thus, one of the thermal expansion members  48 - 1  and  48 - 2  is deflected towards the other having a lower thermal expansion coefficient, thereby enabling the curvature of the reflective plane  2  to be adjusted.  
         [0053]      FIGS. 5   a  to  5   c  are a plan view and cross-sectional views illustrating a varifocal mirror supported by a connecting member and actuated by electrostatic force in accordance with one embodiment of the present invention.  
         [0054]     According to this embodiment, the varifocal mirror comprises a bridge type thin film.  
         [0055]     Referring to  FIG. 5   a , the thin film is constituted to have a bridge structure instead of a simple sheet shape. The varifocal mirror comprises a circular thin film  1  positioned at the center of a peripheral thin film, and a circular reflective plane  2  on the circular thin film  1  in which the thin film  1  is connected with the peripheral thin film by the connecting bridges  3 , and supported thereby. Although the varifocal mirror is described as comprising the circular thin film supported at the center thereof by the connecting bridges in this embodiment, it should be understood that various modifications of this embodiment can be apparent to those skilled in the art.  
         [0056]      FIG. 5   b  is a cross-sectional view taken along line A-A′ of  FIG. 5   a , in which the connecting bridges are shown.  FIG. 5   c  is a cross-sectional view taken along line B-B′ of  FIG. 5   a , in which the connecting bridges are not shown.  
         [0057]      FIG. 6  is a diagram illustrating a camera module incorporating varifocal mirrors in accordance with one embodiment of the present invention.  
         [0058]     According to this embodiment, the camera module comprises the varifocal mirrors, an optical system, and an image sensor.  
         [0059]      FIG. 6  schematically shows a path of light focused on the image sensor by means of the varifocal mirror in the camera module according to this embodiment.  
         [0060]     Referring to  FIG. 6 , the camera module employs an optical path  13  along which light is refracted twice until it reaches an image sensor  23  vertically positioned in the camera module. The camera module comprises two varifocal mirrors  24  and  26  positioned at refraction parts in the camera module to perform a focus adjustment function and an optical zoom function. An optical system  25  is positioned between the varifocal mirrors  24  and  26 , and is prepared by employing a lens, a reflective plane, a filter, a mechanical shutter, a diaphragm, and the like. In addition, a suitable optical system may be optionally provided in front of the varifocal mirror  24  or between the varifocal mirror  26  and the image sensor  23 . It is possible to selectively apply the focus adjustment and optical zoom functions using the varifocal mirrors to a camera module employing a conventional method for moving lens units. It is also possible to provide various optical paths in the camera module using the varifocal mirrors of the invention.  
         [0061]      FIGS. 7   a  to  7   c  are cross-sectional views illustrating a camera module incorporating varifocal mirrors in accordance with another embodiment of the present invention.  
         [0062]      FIG. 7   a  shows an optical path  13  in a camera module, along which light is refracted twice by means of two varifocal mirrors as in the case shown in  FIG. 6 . In this embodiment, suitable optical systems  25 - 1 ,  25 - 2  and  25 - 3  are positioned at all suitable positions between an input end and an image sensor  23 .  
         [0063]      FIG. 7   b  shows an optical path  14  through which light is refracted once in a camera module, which comprises one varifocal mirror  24  for realizing one of the focus adjustment function or the optical zoom function, and an additional optical system  27 - 1  for realizing another function (for example, if the varifocal mirror  24  serves to provide the focus adjustment function, it serves to provide the optical zoom function). In this case, the optical system  27 - 1  performs the focus adjustment function or the optical zoom function in the manner of shifting the lens units by an actuator such as a step motor.  
         [0064]     As apparent from the above description, since the camera module using the varifocal mirror according to the invention can realize the optical zoom function or the focus adjustment function without an additional actuator or a driving mechanism unlike a conventional camera module realizing these functions using the actuator or the driving mechanism such as a step motor, it can be easily reduced in size and weight.  
         [0065]     In particular, since the camera module of the invention can satisfy requirement functions while occupying a minimum space, it can be suitably applied to various portable digital assistants, such as PDAs, mobile phones, and the like.  
         [0066]     It should be understood that the embodiments and the accompanying drawings have been described for illustrative purposes and the present invention is limited by the following claims. Further, those skilled in the art will appreciate that various modifications, additions and substitutions are allowed without departing from the scope and spirit of the invention as set forth in the accompanying claims.