Abstract:
A biaxial scanning mirror is disclosed in the present invention. The mirror includes: a first wafer having several cavities forming a first row and a second row, several permanent magnets each installed in one of the cavities, a spacer and a second wafer. The second wafer includes: a mirror unit, rotating around a first axis, for reflecting light beams; and a rotating unit, formed around the mirror unit, for rotating the mirror unit around a second axis which is perpendicular to the first axis. At least one coil substrate having a planar coil is assembled in the rotating unit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a biaxial scanning mirror for an image forming apparatus and a method for operating the same. More particularly, the present invention relates to a biaxial scanning mirror magnetically rotating around two axes or magnetically rotating around one axis and electrically rotating around the other axis. 
     BACKGROUND OF THE INVENTION 
     Micro mirrors made by micro-electro-mechanical system (MEMS) process are wildly used for light beam scanning devices, such as a scanning mirror in a mini projector. Conventionally, it is driven by electrostatic forces at high rotating speed. 
     Please refer to  FIG. 1A . U.S. Pat. No. 6,817,725 discloses a micro mirror unit  100  for incorporation in a device, such as an optical switch. The micro mirror unit  100  includes a mirror-formed portion  110  having an upper surface provided with a mirror surface (not illustrated), an inner frame  120  and an outer frame  120  (partly un-illustrated), each formed with comb-like electrodes integrally therewith. Specifically, the mirror-formed portion  110  has ends facing away from each other, and a pair of comb-like electrodes  110   a  and  110   b  are formed respectively on these ends. In the inner frame  120 , a pair of comb-like electrodes  120   a  and  120   b  extend inwardly, corresponding to the comb-like electrodes  110   a  and  110   b . Also, a pair of comb-like electrodes  120   c  and  120   d  extend outwardly. In the outer frame  120 , a pair of comb-like electrodes  120   a  and  120   b  extend inwardly, corresponding to the comb-like electrodes  120   c  and  120   d . The mirror-formed portion  110  and the inner frame  120  are connected with each other by a pair of torsion bars  140 . The inner frame  120  and the outer frame  120  are connected with each other by a pair of torsion bars  150 . The pair of torsion bars  140  provides a rotation axis for the mirror-formed portion  110  to rotate with respect to the inner frame  120 . The pair of torsion bars  150  provides a rotation axis for the inner frame  120 , as well as for the associating mirror-formed portion  110 , to rotate with respect to the outer frame  120 . 
     With the above arrangement, in the micro mirror unit  100 , a pair of comb-like electrodes, such as the comb-like electrode  110   a  and the comb-like electrode  120   a , are opposed closely to each other for generation of static electric force, and take positions as shown in  FIG. 1B , i.e. one of the electrode assuming a lower position and the other assuming an upper position, when there is no voltage applied. When an electric voltage is applied, as shown in  FIG. 1C , the comb-like electrode  110   a  is drawn toward the comb-like electrode  120   a , thereby rotating the mirror-formed portion  110 . More specifically, in  FIG. 1A , when the comb-like electrode  110   a  is given a positive charge whereas the comb-like electrode  120   a  is given a negative charge, the mirror-formed portion  110  is rotated in a direction M 1  while twisting the pair of torsion bars  140 . On the other hand, when the comb-like electrode  120   c  is given a positive charge whereas the comb-like electrode  120   a  is given a negative charge, the inner frame  120  is rotated in a direction M 2  while twisting the pair of torsion bars  150 . 
     As a conventional method, the micro mirror unit  100  can be made from an SOI (Silicon on Insulator) wafer which sandwiches an insulating layer between silicon layers. However, according to the conventional method of manufacture as described above, the thickness of the wafer is directly dependent on the thickness of the micro mirror unit  100 . Specifically, the thickness of the micro mirror unit  100  is identical with the thickness of the wafer which is used for the formation of the micro mirror unit. For this reason, according to the conventional method, the material wafer must have the same thickness as the thickness of the micro mirror unit  100  to be manufactured. This means that if the micro mirror unit  100  is to be thin, the wafer of the same thinness must be used. For example, take a case of manufacturing a micro mirror unit  100  having a mirror surface having a size of about 100 through 725 μm. In view of a mass of the entire moving part including the mirror-formed portion  110  and the inner frame  120 , the amount of movement of the moving part, the size of the comb-like electrodes necessary for achieving the amount of movement, etc considered comprehensively, a desirable thickness of the moving part or the micro mirror unit  100  is determined. In this particular case, the desirable thickness is 100 through 200 μm. As a result, in order to manufacture the micro mirror unit  100  having such a thickness, a wafer having the thickness of 100 through 200 μm is used. 
     According to the conventional method, in order to manufacture a thin micro mirror unit  100 , a correspondingly thin wafer must be used. This means that the greater diameter the wafer has, the more difficult to handle the wafer. For instance, take a case in which a micro mirror unit  100  is to be manufactured from an SOI wafer having a thickness of 200 μm and a diameter of 6 inches. Often, the wafer is broken in a midway of the manufacturing process. Further, the limitation on the size of the flat surface of the wafer places a limitation on the manufacture of micro mirror array chips. Specifically, when the micro mirror array chips are manufactured by forming a plurality of micro mirror units in an array pattern on a single substrate, the size of the array is limited. 
     Precise lateral alignment between two sets of comb-like electrodes, e.g., electrodes  120   c  and  120   a , are inherently difficult to achieve since they are not coplanar and are fabricated in two different layers of the substrate. This can further result in non-linear and unstable behavior. Furthermore, driving force provided by the comb-like electrodes is limited and power needed by the electrodes to drive the mirror is large. 
     Please refer to  FIG. 2A . In order to overcome the aforementioned disadvantages, magnets  210  are used to replace the comb-like electrodes for providing driving force to rotate a biaxial mirror assembly  200 . Two side magnets  210   a  with the same magnetization direction are positioned on both sides of the biaxial mirror assembly  200  above a bottom magnet  210   b  with an opposite magnetization direction. However, these two side magnets  210   a  occupy a large space, and thus, such a structure is too big. Furthermore, it is hard to increase the magnetic field without drastically increasing the magnet volume. 
     To minimize the total size another structure having two magnets  230   a  placed on top of a biaxial mirror assembly  220 , and another two magnets  230   b  placed below the biaxial mirror assembly  220  is shown in  FIG. 2B . However, the total size of such structure is still too large due to the fact that the magnets  230   a  and  230   b  need to allow enough space for the biaxial mirror assembly  220  to rotate. 
     Therefore, a biaxial mirror assembly having a small size with large driving force is desperately desired. 
     SUMMARY OF THE INVENTION 
     This paragraph extracts and compiles some features of the present invention; other features will be disclosed in the follow-up paragraphs. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. 
     In accordance with an aspect of the present invention, a biaxial scanning mirror for an image forming apparatus, comprises: a first wafer having a plurality of cavities forming a first row and a second row; a plurality of permanent magnets each installed in one of the plurality of cavities, two adjacent permanent magnets of the same row having an air gap formed therebetween; a second wafer, comprises: a mirror unit, rotating around a first axis, for reflecting light beams; and a rotating unit, formed around the mirror unit, for rotating the mirror unit around a second axis which is perpendicular to the first axis; at least one coil substrate having a planar coil, assembled in the rotating unit; and a spacer, formed between the first wafer and the second wafer, for separating the first wafer and the second wafer. 
     Preferably, the mirror unit is driven by a comb drive actuator. 
     Preferably, the mirror unit is actuated by rotation of the rotating unit around the second axis. 
     Preferably, resonant frequency of the mirror unit around the first axis is higher than that of the rotating unit around the second axis. 
     Preferably, the coil substrate has a thickness smaller than 150 μm. 
     Preferably, the rotating unit has at least one slot for vertically receiving the coil substrate. 
     Preferably, the air gap has a width smaller than 250 μm. 
     Preferably, the air gap has a magnetic flux larger than 0.82 Tesla. 
     Preferably, the coil substrate is formed by micro-electro-mechanical systems (MEMS) process. 
     In accordance with another aspect of the present invention, a method of operating the biaxial scanning mirror, includes the steps of: a) generating a first magnetic field by the permanent magnets in the first row; b) generating a second magnetic field of which direction is opposite to that of the first magnetic field by the permanent magnets in the second row; c) providing a first signal to the planar coil for triggering the mirror unit to rotate around the first axis; and d) providing a second signal to the planar coil for triggering the rotating unit to rotate around the second axis. 
     Preferably, the first signal is sinusoidal with a frequency larger than 18 KHz and the second signal has a waveform of saw-tooth with a frequency of 60 Hz. 
     In accordance with yet another aspect of the present invention, a method of operating the biaxial scanning mirror, includes the steps of: a) generating a first magnetic field by the permanent magnets in the first row; b) generating a second magnetic field of which direction is opposite to that of the first magnetic field by the permanent magnets in the second row; c) providing a first signal to the mirror unit for triggering the mirror unit to rotate around the first axis; and d) providing a second signal to the planar coil for triggering the rotating unit to rotate around the second axis. 
     Preferably, the first signal is sinusoidal with a frequency larger than 18 KHz and the second signal has a waveform of saw-tooth with a frequency of 60 Hz. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a micro mirror of a prior art. 
         FIGS. 1B and 1C  show mechanism of detailed structures of the prior art in  FIG. 1A . 
         FIGS. 2A and 2B  show biaxial scanning mirrors according to other prior arts. 
         FIG. 3  shows a biaxial scanning mirror according to the present invention. 
         FIG. 4  illustrates a cross-sectional view of the biaxial scanning mirror of the present invention. 
         FIG. 5  illustrates a first wafer of the biaxial scanning mirror of the present invention. 
         FIG. 6  illustrates a second wafer of the biaxial scanning mirror of the present invention. 
         FIG. 7  shows a coil substrate of the second wafer of the biaxial scanning mirror of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described more specifically with reference to the following embodiment. It is to be noted that the following descriptions of preferred embodiment of this invention are presented herein for purpose of illumination and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed. 
     Please refer to  FIG. 3 . An embodiment is described. A biaxial scanning mirror  300  has a first wafer  302 , a spacer  304  and a second wafer  306 . The first wafer  302  and the second wafer  306  are both silicon wafers. In order to have detailed description for each component, a perspective view cut along lines AA′ and BB″ in  FIG. 3  is shown in  FIG. 4 . It is obvious that the spacer  304  is fabricated between the first wafer  302  and the second wafer  306  for separating the first wafer  302  and the second wafer  306 . Meanwhile, distance between the first wafer  302  and the second wafer  306  can be fixed as well. 
     Please refer to  FIG. 5 . The first wafer  302  has a number of cavities (not shown) which can be formed by deposition, photolithography, or etching There are permanent magnets  3022 , having the same number as that of the cavities, installed in the cavities, respectively. The permanent magnets  3022  form a first row  3022   a  and a second row  3022   b . An air gap  3024  is formed between two adjacent permanent magnets  3022  of the same row  3022   a  or  3022   b . Due to the fact that the cavities are precisely positioned by micro-electro-mechanical systems (MEMS) technology, the air gap  3024  can be relatively narrow, thereby providing a large magnetic field by high density of magnetic lines in the air gap  3024 . In the present invention, the air gap  3024  has a width smaller than 250 μm, allowing the permanent magnets  3022  to generate a magnetic flux of approximately 0.82 Tesla therebetween. Hence, a high Lorenz force can be produced by the large magnetic flux in the narrow air gap  3024 . 
     As shown in  FIG. 5 , the permanent magnets  3022  in the first row  3022   a  provide a first magnetic field in a direction, and the permanent magnets  3022  in the second row  3022   b  provide a second magnetic field in an opposite direction. 
     Please refer to  FIG. 6 . The second wafer  306  comprises a mirror unit  3061  and a rotating unit  3062 . The mirror unit  3061  rotates around a first shaft  3065  and can reflect light beams. The rotating unit  3062  is formed around the mirror unit  3061  for rotating the mirror unit  3061  around a second shaft  3066 . The first shaft  3065  is perpendicular to the second shaft  3066 . In the present invention, the mirror unit  3061  is driven by a comb drive actuator  3064 . The comb drive actuator  3064  can also be replaced by other driving devices. 
     Alternatively, the mirror unit  3061  can also be actuated by rotation of the rotating unit  3062  around the second shaft  3066  due to the fact that the first shaft  3065  is connected to the rotating unit  3062 . By this way, the mirror unit  3061  does not need the comb drive actuator  3064  for actuation. 
     In this embodiment, the rotating unit  3062  has two slots (not shown) each for vertically assembling a coil substrate  3063 . Each of the two coil substrates  3063  has a planar coil  3067 . As shown in  FIG. 7 , the planar coil  3067  is formed on one side of the coil substrates  3063 . Line CC′ is across the middle part of the coil substrates  3063 . The coil substrate  3063  has a thickness smaller than 150 μm. The planar coil  3067  is symmetrically arranged on two sides of the line CC′. As long as the planar coil  3067  is applied with an alternatively changed signal, current direction in the planar coil  3067  on two sides of the line CC′ will change accordingly. When the first wafer  302 , the spacer  304  and the second wafer  306  are configured, the lower part of planar coil  3067  is inserted into the air gap  3024  between two adjacent permanent magnets  3022  for interacting with the magnetic field formed by the permanent magnets  3022  to produce a Lorenz force which pushes the rotating unit  3062  to rotate around the second shaft  3066 , thereby allowing the mirror unit  3061  to rotate around the second shaft  3066 . 
     In the present embodiment, the coil substrates  3063  are positioned perpendicular to the second shaft  3066  rather than parallel, such that moment of inertia of the rotating unit  3062  does not increase too much by the Lorenz force. However, the coil substrates  3063  are not limited to such positions. 
     A saw-tooth signal with a frequency of 60 Hz is used as the alternatively changed signal applied to the planar coil  3067 . In the present invention, a sinusoidal signal with a frequency larger than 18 KHz is provided to the mirror unit  3061  via the comb drive actuator  3064  for triggering the mirror unit  3061  to rotate around the first shaft  3065 . Alternatively, a sinusoidal signal with a frequency larger than 18 KHz can also be provided to the mirror unit  3061  via the planar coil  3067  while a comb drive actuator is not used. 
     Usually, resonant frequency of the mirror unit  3061  around the first shaft  3065  is higher than that of the rotating unit  3062  around the second shaft  3066 . In practice, the number of coil substrates  3063  is not limited to two. One coil substrate  3063  is enough for rotating the mirror unit  3061  around the second shaft  3066 . Two are better to keep stability when rotating. The coil substrate  3063  is formed by micro-electro-mechanical systems (MEMS) process. 
     Hence, the biaxial scanning mirror  300  operates by generating a first magnetic field by the permanent magnets  3022  in the first row  3022   a , generating a second magnetic field of which direction is opposite to that of the first magnetic field by the permanent magnets  3022  in the second row  3022   b , providing the sinusoidal signal to the planar coil  3067  for triggering the mirror unit  3061  to rotate around the first shaft  3065 , and providing a saw-tooth signal to the planar coil  3067  for triggering the rotating unit  3062  to rotate around the second shaft  3066 . 
     Alternatively, the sinusoidal signal can be provided to the comb drive actuator  3064 , for triggering the mirror unit  3061  to rotate around the first shaft  3065 . 
     Due to the fact that the cavities are precisely positioned by micro-electro-mechanical systems (MEMS) technology, the first wafer  302 , the second wafer  306 , the permanent magnets  3022 , and the coil substrates  3063  can be precisely assembled without dislocation. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.