Patent Publication Number: US-2006012836-A1

Title: Focus adjustment for imaging applications

Description:
FIELD OF THE INVENTION  
      The present invention relates to focus adjustment for imaging applications.  
     BACKGROUND OF THE INVENTION  
      As pixel arrays of imaging equipment decrease in size and focal lengths become smaller, there is need to improve focus adjustment. Conventional applications use a fixed focus, where objects close to the lens appear blurry or have an actuator to adjust the focal distance.  
      Other conventional applications perform focusing manually or automatically by moving the lens and lensmount via an actuator, but this has disadvantages for small handheld devices where the lens is exposed to the user and the actuator can be destroyed by pressure. Modern handheld devices often use smaller focal distances, and the focus needs to have less adjustment range.  
      Referring to  FIG. 1 , a conventional fixed focus lens  70  is shown in cross-sectional view. Sensor module  16 , formed over substrate  10 , comprises an image sensor  24  formed as a pixel array over an attachment layer  30 . Incoming light  40  is focused by fixed focus lens  70 .  FIG. 1  schematically shows lens  70  mounted in lens mount  72  in a fixed position over module  16 .  
      The conventional fixed focus lens system  20  shown in  FIG. 1  has a fixed position relative to sensor module  16 . There is a fixed focal length (f 0 ) from lens  70  to focal point  28 , where f 0  is the distance from L 1  to L 2 . The fixed position of lens  70  places a limit on the distance of objects that are in focus. For example, light from objects that are either nearer or farther from lens system  20  will not be in focus because of the corresponding change in focal length.  
      Referring to  FIG. 2 , a conventional manually or automatically adjustable focus lens system  120 , comprising adjustable focus lens  170 , is shown in cross-sectional view. Sensor module  116 , formed over substrate  110 , comprises image sensor  124  formed as a pixel array over attachment layer  130 . Incoming light  140  is focused by lens  170 . Focal length (f 1 ) is the distance from lens  170  to focal point  128 , or the distance from M 1  to M 2 , when lens  170  is at position B. Focal length f 1  may change, as lens  170  is adjusted to bring each image into focus. For example, as shown in  FIG. 2 , manual or automatic adjustment of lens  170  from position A to position B will change f 1 . Lens  170  may be adjusted to adjust the focus by operation of an actuated lens mount  150 . However, lens assembly  120  is subject to damage because actuated lens mount  150 , lens  170 , and other components of assembly  120  may be damaged during use.  
      Given the disadvantages of conventional image focusing techniques, it would be advantageous to improve focus techniques for imaging applications.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention provides exemplary embodiments in which automatic or manual focus of an image is performed.  
      In exemplary embodiments an image sensor is attached to an element having a changeable position for automatic or manual focus adjustment. The position of an image sensor can be selectively adjusted by, for example, changing the position of the element which changes the sensor position as needed or desired by, for example, regulating a bias voltage. The bias voltage may be regulated manually or by one or more autofocus algorithms, whereby the system can perform image focus while maintaining a fixed-position lens mount.  
      In a preferred embodiment, the changeable dimension element is a piezoelectric material which may be attached to a fabricated image sensor, or can be fabricated at the wafer level as part of the image sensor.  
      These and other features and advantages of the invention will be more apparent from the following detailed description that is provided in connection with the accompanying drawings which describe and illustrate exemplary embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  depicts a cross-sectional view of a conventional fixed focus lens assembly;  
       FIG. 2  depicts a cross-sectional view of a conventional adjustable focus lens assembly;  
       FIG. 3A  is a cross-sectional view of an adjustable focus lens assembly in accordance with one embodiment of the invention;  
       FIG. 3B  depicts an intermediate stage of processing of an image sensor and moveable element in accordance with one embodiment of the invention;  
       FIG. 4A  is a flowchart of an automatic focus operation in accordance with an exemplary embodiment of the invention;  
       FIG. 4B  is a flowchart of a manual focus operation in accordance with an exemplary embodiment of the invention;  
       FIG. 5  is a block diagram of an imaging apparatus that performs automatic focus in accordance with one embodiment of the invention; and  
       FIG. 6  is a schematic block diagram of a processing system that includes an imaging apparatus as in  FIG. 5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural and logical changes may be made without departing from the spirit or scope of the present invention.  
      Exemplary embodiments of the invention obtain automatic or manual focus adjustment for an image by selectively changing a sensor position in accordance with changes in one or more dimensions of an element in response to an applied voltage. For example, a piezoelectric material may be attached to a sensor such that the sensor position is adjusted as needed or desired by adjusting a voltage applied continuously or incrementally to the piezoelectric material.  
      Referring to  FIG. 3A , one exemplary embodiment of the invention is shown in which lens assembly  220  (in cross-sectional view) comprises fixed focus lens  270  mounted in lens mount  272  above sensor module  216 . Sensor module  216  is formed over housing base  210 . Image sensor  224 , which has a pixel array, is mounted within sensor module  216  to piezoelectric material  226 . Piezoelectric material  226  may be mounted over, or formed upon, conductor  229  which in turn is mounted to base  210  through attachment layer  230 . Another conductor  250  is mounted to piezoelectric material  226  between material  226  and sensor  224 . Conductive element  233  couples conductor  229  to conductors provided on an upper surface of base  210 .  
      As shown in  FIG. 3A , a voltage source for applying the voltage across the piezoelectric material  226  may be obtained from the image sensor  224 . Alternatively, the control voltage applied across piezoelectric material  226  may be obtained from another source. If the piezoelectric control voltage is supplied by image sensor  224 , bonding wires  231 ,  232  may be utilized which respectively couple a voltage output from image sensor  224  to conductive layer  250  directly and  229  indirectly through the conductive trace on base  210  and conductive element  233 . Bonding wires  231 ,  232  may be adjusted to accommodate changes in thickness of the piezoelectric material  226  which occur during a manual or automatic focus adjustment. The invention may include other devices to compensate for stressing of wires  231 ,  232  during a manual or autofocus operation.  
      Automatic or manual focus adjustment of the invention is achieved by changing the thickness of piezoelectric material  226  in a continuous or non-continuous, e.g. stepped, manner under control of a continuous or stepped voltage applied to conductive layers  229 ,  250 .  
      Piezoelectric material  226  may comprise one or more types of piezoelectric material, for example piezoelectric ceramic material, which may be arranged in any number and orientation. For example, a plurality of piezoelectric materials may be arranged in a stacked manner to obtain a desired thickness or, alternatively, in a series such that each of the plurality of piezoelectric materials is aligned along a generally horizontal plane.  
      In other embodiments of the invention, sensor  224  may be operably mounted within sensor module  216  to any other material, device or mechanism, which can change a position of sensor  224  in response to an applied changeable voltage, such that an automatic or manual focus adjustment may occur. The changeable voltage may come from sensor  224  itself or a separate external circuit.  
      In the  FIG. 3A  embodiment, incoming light  240  is focused by an adjustable focus lens  270 , shown as a generally double convex lens with an upper surface  266  and a lower surface  268 . Alternatively, lens  270  may be formed of any material and shape that operates to substantially form an image by focusing rays of light, for example. Lens  270  may be formed as a substantially transparent material with upper surface  266  and lower surface  268 . Surfaces  266  and  268  may each be formed in any shape, including but not limited to curved or planar shapes. For example, surfaces  266  and  268  may each be convex, thus forming a biconvex optical lens as shown in  FIG. 3A . Alternatively, surfaces  266  and  268  may be shaped to form a generally spherical lens. In addition, lens  270  may be used in combination with at least one or more other optical instruments or lenses to form an image upon image sensor  224 .  
      In addition, although only one lens  270  is depicted in  FIG. 3A , any combination of two or more lenses may also be used in the present invention. In addition, lens  270  may be formed of any suitable material, including but not limited to glass, plastic, or any other substance, whether natural, synthetic or a combination or composite thereof. The lens or lenses of the invention may be used to focus light or other electromagnetic radiation of any wavelength, in forming an image upon image sensor  224 .  
      If lens  270  is formed having spherical surfaces, the focal distance may vary for different rays and thus produce spherical aberration (not shown). In addition, the focal length for different wavelengths may also differ and produce chromatic aberration (not shown). Nonetheless, the manual or autofocus operation of the invention, by employing, for example, a piezoelectric material biasing provides the capacity to automatically or manually correct for such aberrations and produce the desired focus of an image.  
      In the embodiment illustrated in  FIG. 3A , a piezoelectric material  226  is formed over conductor  229  which in turn is mounted to base  210  through attachment layer  230 . Another conductor  250  is formed over piezoelectric material  226 , and a die containing a pixel array, forming image sensor  224 , is mounted to conductor  250 .  
      In accordance with another embodiment of the invention, as shown at an intermediate stage of processing depicted in  FIG. 3B , a conductor  250  is formed over a backside of an image sensor  224  die, wherein surface  223  of sensor  224  functions as the upper surface of the sensor. Piezoelectric material  226  is formed at the wafer level on the backside  251  of conductor  250 . Another conductor  229  is then formed over piezoelectric material  226 . The image sensing unit  252  is then positioned by inverting the unit  252 , as shown in  FIG. 3B , to an upright position with sensor  224  positioned over piezoelectric material  226 . The unit  252  may then be subject to further processing steps as needed or desired.  
      According to one exemplary embodiment, the thickness of piezoelectric material  226  may be changed by regulating the bias voltage to change the thickness of piezoelectric material  226  by approximately 0.3 microns per applied volt, though the amount of thickness change per applied volt is dependent on the size and shape and other physical properties of the piezoelectric material  226 .  
      The bias voltage may be regulated manually or based on one or more autofocus algorithms, for example based on regional entropy maximization, whereby the system can perform image focus. For manual focus, an operator may select a bias voltage using either a continuous or stepped bias voltage value.  
      Autofocus algorithms for use in the invention may operate by any known autofocus technique, for example according to regional entropy maximization or other entropy focus criteria. In some circumstances,, algorithms based on minimizing the Renyi entropy of an image may be used, utilizing one or more contrast-enhancement criteria. In other circumstances, autofocus algorithms for the automatic correction of motion artifacts or blurring in images may be employed. In yet other circumstances, autofocus algorithms may be utilized that adjust the focus of an image according to the degree of contrast detected between adjacent pixels. For example, a greater degree of contrast may correspond to, and thus be interpreted as, a greater degree of focus of the image. By regulating the bias voltage across a piezoelectric material, based on one or more autofocus algorithms, the system can perform autofocus while maintaining a fixed-position lens mount. In addition, the autofocus algorithms used in the invention may be used in conjunction with signal-processor-based acquisition systems for further image correction, focus and enhancement.  
      Image sensor  224  may be, for example, a CMOS image sensor comprising a focal plane array of pixel cells, each one of the cells including either a photogate, photoconductor, or photodiode overlying a charge accumulation region for accumulating photo-generated charge. The active elements of a pixel cell may perform photon to charge conversion; accumulation of image charge; transfer of charge to a floating diffusion node accompanied by charge amplification; resetting the floating diffusion node to a known state before the transfer of charge to it; selection of a pixel for readout; and output and amplification of a signal representing pixel charge.  
      Although one exemplary embodiment of the invention is shown in  FIG. 3 , and described above, those skilled in the art will recognize that the invention encompasses any type of image sensor having a pixel array, and that substitutions, additions, deletions, modifications and/or other changes may be made to the exemplary embodiment without departing from the spirit or scope of the invention. For example, image sensor  224  may be employed in any number of different types of semiconductor-based imagers, including for example charge coupled devices (CCDs), photo diode arrays, charge injection devices and hybrid focal plane arrays.  
      The invention may be employed in many digital applications such as, for example, cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, star trackers, motion detection systems, and image stabilization systems.  
       FIG. 4A  is a flowchart of an automatic focus operation which may be used with the  FIG. 3  exemplary embodiment of the invention. As shown in  FIG. 4A , an image is first detected by an image sensor ( 300 ). The image sensor, for example image sensor  224 , may be a component of any imaging device, such as a CMOS, CCD or other imaging device. The position of the image sensor is detected and stored in memory ( 304 ). The obtained image is processed using an image processor ( 308 ). Image processing may be performed according to any known image processing techniques. For example, image processing may comprise sampling of pixels in an image array according to one or more criteria, such as color processing, white balancing, or other criteria. In the embodiment shown in  FIG. 4A , information on the position of the image sensor may be used with other information, such as the values of the pixels in the sample (not shown), in calculating whether the image is in focus and, thus, whether adjustment of the position of the image sensor is required for autofocus ( 312 ).  
      If adjustment of the image sensor is required for an autofocus operation ( 312 ), the image sensor may be elevated or lowered as needed according to one or more autofocus algorithms ( 316 ). As determined by one or more autofocus algorithms, a control voltage may be applied across a piezoelectric material from a voltage source. For example, the control voltage for regulating the piezoelectric material may be generated on the chip contacting the image sensor  224 , as depicted schematically in  FIG. 3A . The one or more autofocus algorithms regulate the bias voltage across the piezoelectric material ( 320 ) according to one or more autofocus criteria, such as image entropy. Regulation of the bias voltage across the piezoelectric material can be used to change the thickness of the piezoelectric material. As a result, the position of the image sensor  224  may be adjusted to bring an image into focus ( 324 ). After image processing ( 308 ), the same autofocus inquiry is made ( 312 ) and steps  316  through  324  may be repeated until autofocus is obtained. At this stage, the entire process may begin anew as a new image is obtained for autofocus.  
       FIG. 4B  is a flowchart of a manual focus operation which may be used with the  FIG. 3A  exemplary embodiment of the invention. For a manual focus operation, manual focus occurs before image capture. An operator manually views an image through a viewfinder ( 375 ). To manually focus the image, the operator manually selects a bias voltage ( 376 ) which is applied to a piezoelectric material  226  ( 380 ). The operator then adjusts the bias voltage to change the thickness of piezoelectric material  226  and, accordingly, adjust the position of the image sensor  224  until the image is in focus ( 384 ). The image is then captured ( 385 ) for subsequent image processing.  
      An exemplary embodiment of an imaging apparatus  400  incorporating features discussed above is shown in  FIG. 5 .  FIG. 5  depicts imaging apparatus  400  that performs a focus operation, e.g. automatic focus, in accordance with an exemplary embodiment of the invention. Apparatus  400  includes a lens system  402  for directing light from an object to be imaged to image sensing unit  404 . Image sensing unit  404  may comprise an image sensor further comprising a pixel array, wherein the image sensor is mounted over a piezoelectric material. Analog-to-digital converter  406  converts the analog image signals from image sensing unit  404  into digital signals. Image processor  408  performs image correction processes on the digital signals, including a focus operation, e.g. an autofocus operation ( 508 ), and other processes such as data correction for defective pixels, color interpolation, sharpness filtering, etc., in producing digital image data. Output format converter/compression unit  410  converts the digital image data into an appropriate file format for output or display to the user. Controller  412  controls the operations of the apparatus  400 .  
      In one embodiment, an image sensor in the image sensing unit  404  is constructed as an integrated circuit (IC) that includes pixels having respective photosensors. The IC can include, as part of lens system  402 , an array of microlenses over the pixels. The image sensor in unit  404  is mounted over a piezoelectric material, or any other device or mechanism that may be adjusted, and may be a complementary metal oxide semiconductor (CMOS) sensor, a charge coupled device (CCD) sensor, or other pixel imaging sensor. The IC can include AID converter  406 , image processor  408 , such as a CPU, digital signal processor or microprocessor, output format converter  410  and timing controller  412 .  
      Without being limiting, such an imaging apparatus  400  could be part of a computer system, camera system, scanner, machine vision system, vehicle navigation system, video telephone, surveillance system, and other image processing systems.  
       FIG. 6  shows an exemplary embodiment in which processor system  500 , such as used, for example, in a digital camera system, includes an imaging apparatus  400  as in  FIG. 5 . System  500  includes a central processing unit (CPU)  544  that communicates with an input/output (I/O) device  546  over a bus  552 . Apparatus  400  communicates with CPU  544  and other components of the system over bus  552  or a ported connection. System  500  also includes random access memory (RAM)  548  and may include peripheral devices such as a removable FLASH memory  554  which also communicates with CPU  544  over the bus  552 . FLASH memory  554  may provide information storage in any type of imaging application, for example in digital cameras. Examples of FLASH memory  554  that may be used in the invention include, for example, removable solid-state storage devices such as memory cards.  
      The above description and drawings illustrate embodiments which achieve the objects of the present invention. Although certain advantages and embodiments have been described above, those skilled in the art will recognize that substitutions, additions, deletions, modifications and/or other changes may be made without departing from the spirit or scope of the invention. Accordingly, the invention is not limited by the foregoing description but is only limited by the scope of the appended claims.