Patent Application
20210405350
The present disclosure relates to a micromirror device and a method of driving the micromirror device.
A micromirror device (also referred to as a micro-scanner) is known as one of the micro electro mechanical systems (MEMS) devices manufactured by using the microfabrication technology of silicon (Si). Since this micromirror device is small and has low consumed power, it is expected that the micromirror device is applied to head-up displays, retinal displays, and the like using lasers.
As an optical scanning method for displaying images, attention has been focused on a Lissajous scanning method of covering the screen by driving sinusoidally on both the horizontal and vertical axes and drawing a Lissajous waveform, as compared with the raster scanning method that has been common until now. In the Lissajous scanning method, the algorithm of the laser driver is complicated. However, the mirror can be miniaturized, and a wide angle of view can be realized while suppressing the driving consumed power.
There are various driving methods for micromirrors, but the piezoelectric driving method using deformation of a piezoelectric body is promising since the method generates a higher torque than other methods and is capable of obtaining a high scan angle.
JP6092713B and WO2016/052547A each disclose a piezoelectric mirror device in which a mirror part is connected to a pair of semi-annular piezoelectric actuators through a torsion bar and the mirror part is able to rotate and oscillate around the torsion bar as an axis. JP6092713B and WO2016/052547A each disclose a method of efficiently driving a mirror by dividing and placing piezoelectric films in accordance with stress distribution generated in the semi-annular piezoelectric actuator in a case of oscillating the mirror part and by giving each driving signal having an appropriate polarity to each of the piezoelectric films divided and placed.
However, since the mirror device disclosed in JP6092713B and WO2016/052547A is able to scan only in one axis direction, an image cannot be displayed with one chip. To realize a two-axis scan (two-dimensional scan) using the mirror device disclosed in JP6092713B and WO2016/052547A, it is necessary to combine the mirror device with another device for realizing displacement in the other one-axis direction. As a result, there is a problem that the system is large.
On the other hand, JP5151065B, JP4984117B, and JP2018-041085A propose a piezoelectric drive type optical scanner capable of two-dimensional scanning as a micromirror device.
JP5151065B discloses an optical scanner having a configuration in which the mirror part is connected to the movable frame through the first connecting part along the first axis and the movable frame is connected to the fixed frame surrounding the movable frame through the piezoelectric actuator. The movable frame and the piezoelectric actuator are connected by a second connecting part along the second axis orthogonal to the first axis, and the piezoelectric actuator is further connected to the fixed frame by a third connecting part along the first axis. A pair of movable parts are connected to each of the two third connecting parts disposed on the axis across the mirror part, and a total of four movable parts oscillate the mirror part around two axes together with the movable frame. Thereby, a two-dimensional light scanning operation is realized.
JP4984117B discloses an optical scanner comprising: a mirror part; a first actuator part that is disposed so as to surround the mirror part and connected to the mirror part through a first torsion bar extending along a first axis; an internal movable frame that is disposed exterior to the first actuator part and is connected to the first actuator on the axis of the first torsion bar; and a second actuator part that is disposed so as to surround the internal movable frame and is connected to the internal movable frame through the second torsion bar. The first actuator applies torque around the first axis to the mirror part, and the second actuator applies torque around the second axis to the mirror part, thereby realizing a two-dimensional light scanning operation.
In JP2018-41085A, the mirror part is connected to the first frame device (the movable frame) that surrounds the mirror part through the first torsion bar, and the first frame device is connected to an actuator structure that surrounds the first frame device through the second torsion bar. Further, a configuration, in which the actuator structure is connected to a second frame device that surrounds the actuator through a third torsion bar, is disclosed. The actuator structure comprises four movable parts symmetrical to the first axis and the second axis, and the mirror part is rotated around the two axes by the four movable parts. Thereby, a two-dimensional light scanning operation is realized.
According to JP5151065B, JP4984117B, and JP2018-041085A, two-dimensional scanning is possible with one chip. The optical scanners of JP5151065B, JP4984117B, and JP2018-041085A each comprise a movable frame connected to a mirror part. By comprising this movable frame, the effect of oscillation insulation can be obtained such that the oscillation energy inside the movable frame does not leak to the outside or the oscillation energy from the outside does not leak to the inside. That is, by comprising the movable frame, there is an advantage that a crosstalk between the two axes at the time of scanning can be reduced. However, since the movable frame itself is unable to generate a driving force, there is a problem that energy efficiency is poor. As a result, the advantage of low consumed power in a case of using the piezoelectric actuator is not sufficiently effective.
In order to realize a practical system for displaying high-definition images, it is necessary to achieve both a high drive frequency and a wide scan angle for both scan axes while being able to perform two-dimensional scanning with one chip. For that purpose, an actuator structure that transmits power to the mirror more efficiently is essential. As described above, the configurations of JP5151065B, JP4984117B, and JP2018-041085A were not sufficient for efficiency improvement.
The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a micromirror device capable of two-dimensional scanning and a method of driving the micromirror device having high drive efficiency.
Specific means for solving the above problems include the following aspects.
Vxy=α
1
V
1 sin(2πf1t+β1π)+α2V2 sin(2πf2t+β2π) Expression (1),
Vxy=α
1
V
1 sin(2πf1t+β1π)+α2V2 sin(2πf2t+β2π) Expression (1),
According to the present disclosure, it is possible to provide a micromirror device that is capable of two-dimensional scanning and a method of driving the micromirror device having high drive efficiency.
Hereinafter, specific embodiments of the present disclosure will be described with reference to the drawings.
The mirror part 12 has a reflecting surface 12a that reflects incident light. The reflecting surface 12a is composed of a metal thin film such as Au (gold) and Al (aluminum) provided on one surface of the mirror part 12. The material and film thickness used for the mirror coating for forming the reflecting surface 12a are not particularly limited, and various designs can be made using a known mirror material, that is, a high reflectance material.
The first connecting part 21 connects the mirror part 12 and the first actuator 14, and rotatably supports the mirror part 12 around the first axis a1. The first connecting part 21 is a pair of rod-like members, which extend outward along the first axis a1 from the outer circumference of the mirror part 12 symmetrically with the mirror part 12 interposed therebetween. One end of each of the pair of rod-like members constituting the first connecting part 21 is connected to the outer circumference of the mirror part 12, and the other end of each is connected to the first actuator 14.
The second connecting part 22 connects the first actuator 14 and the second actuator 16 on the second axis a2 that intersects the first axis a1, and supports the first actuator 14 rotatably around the second axis a2. The second connecting part 22 is a pair of rod-like members which extend outward along the second axis a2 from the outer circumference of the first actuator 14 symmetrically with the first actuator 14 interposed therebetween. One end of each of the pair of rod-like members constituting the second connecting part 22 is connected to the outer circumference of the first actuator 14, and the other end of each is connected to the second actuator 16.
The third connecting part 23 connects the second actuator 16 and the fixing unit 20, and rotatably supports the second actuator 16. The third connecting part 23 is a pair of rod-like members extending outward along the second axis a2 from the outer circumference of the second actuator 16 symmetrically with the second actuator 16 interposed therebetween. One end of each of the pair of rod-like members constituting the third connecting part 23 is connected to the outer circumference of the second actuator, and the other end of each is connected to the fixing unit 20.
With such a configuration, the third connecting part 23 and the second connecting part 22 are provided on the same axis. Here, the first axis a1 and the second axis a2 intersect at substantially the center of the mirror part 12.
The third connecting part may be provided along the first axis a1 to connect the second actuator 16 and the fixing unit 20 on the first axis. However, it is more preferable that the second connecting part 22 and the third connecting part 23 are provided on the same second axis a2 because the non-linearity at the time of resonance can be suppressed. In such a case, since the non linearity at the time of resonance can be suppressed, the control of two-dimensional optical scanning is easy, and the angle of view (that is, a scan angle) of scanning can be sufficiently increased. As the scan angle, for example, a horizontal axis of 40° or more and a vertical axis of 30° or more are desired.
The fixing unit 20 supports the second actuator 16 through the third connecting part 23. The second actuator 16 supports the first actuator 14 through the second connecting part 22. Further, the first actuator 14 supports the mirror part 12 through the first connecting part 21. Therefore, the fixing unit 20 functions as a member for indirectly supporting the first actuator 14 and the mirror part 12 through the second actuator 16. Further, the fixing unit 20 is provided with wiring, an electronic circuit, and the like which is not shown.
In the present example, the fixing unit 20 is a frame member that surrounds the second actuator 16. The fixing unit 20 is not limited to the frame member as long as the second actuator 16 can be supported through the third connecting part 23. The fixing unit 20 may be composed of two members of a first fixing unit, which is connected to one of the third connecting parts 23, and a second fixing unit which is connected to the other thereof.
The details will be described later. However, the shape of the first actuator 14 does not matter as long as the first actuator 14 is disposed exterior to the mirror part 12 and is able to generate rotational torque around the first axis in the mirror part 12. In the present embodiment, the first actuator 14 is an annular member disposed so as to surround the mirror part 12.
Similarly, the shape of the second actuator 16 is not limited as long as the second actuator 16 is disposed on the outer circumference of the first actuator 14 and is able to generate rotational torque around the second axis in the mirror part 12 and the first actuator 14. In the present embodiment, the second actuator 16 is an annular member disposed so as to surround the first actuator 14.
Here, the exterior is used in a relative sense. Thus, the side away from the center of the mirror part as viewed from an optional position in the device is defined as the exterior. Similarly, the side facing to the center of the mirror part as viewed from an optional position is defined as the inside.
Further, the annular shape may be any shape that surrounds the inner region without interruption, and the inner and outer circumferences may not be circular, and may be any shape such as a rectangular shape or a polygonal shape. In the present example, the first and second actuators are annular. However, the shape of the actuator is not limited to annular.
The first actuator 14 and the second actuator 16 are piezoelectric actuators each including a piezoelectric element.
By driving the mirror part 12 in two-dimensional rotation, the micromirror device 1 reflects the incident light on the reflecting surface 12a of the mirror part 12. Thereby, it is possible to perform two-dimensional scanning.
In the following, for convenience of explanation, the normal direction of the reflecting surface 12a in a case where the mirror part 12 is stationary is the z-axis direction, the direction parallel to the first axis a1 is the y-axis direction, and the direction parallel to the second axis a2 is the x-axis direction.
The first actuator 14 is an annular thin plate member that surrounds the mirror part 12 in the x-y plane. In the present example, the first actuator 14 includes a pair of first movable parts 14A and 14B having semi-annular shapes. Further, as shown in
The second actuator 16 is an annular thin plate member that surrounds the first actuator 14 in the x-y plane. In the present example, the second actuator 16 includes a pair of second movable parts 16A and 16B having semi-annular shapes. Further, as shown in
In the micromirror device 1 of the present example, the mirror part 12, the first actuator 14, the second actuator 16, the fixing unit 20, and the first to third connecting parts 21 to 23 are disposed to have a line-symmetrical structure in the first axis a1 and the second axis a2. With such a symmetrical structure, rotational torque can be efficiently applied to the central mirror part 12.
The micromirror device 1 can be manufactured as a structure, in which elements such as the mirror part 12, the first actuator 14, the second actuator 16, the fixing unit 20, and the first to third connecting parts 21 to 23 are integrally formed, for example, by being processed from a silicon substrate through a semiconductor manufacturing technology.
The thicknesses of the mirror part 12, the first actuator 14, the second actuator 16, and the first to third connecting parts 21 to 23 are formed to be less than the thickness (thickness in the z direction) of the fixing unit 20. Thereby, the first actuator 14, the second actuator 16, and the first to third connecting parts 21 to 23 each have a structure which tends to cause deformation (such as bending deformation and twisting deformation).
In the first actuator 14, the pair of first movable parts 14A and 14B are provided with piezoelectric elements 34A and 34B, respectively. Further, in the second actuator 16, the pair of second movable parts 16A and 16B are provided with piezoelectric elements 36A and 36B, respectively. The first actuator 14 and the second actuator 16 causes the driving force by bending and displacing the movable parts through the deformation of the piezoelectric film due to the application of a predetermined voltage to the piezoelectric elements 34A, 34B, 36A, and 36B.
In the micromirror device 1, the first actuator 14 and the second actuator 16 perform driving in a resonance mode in which the mirror part 12 tilts and oscillates around the first axis a1, and perform driving in a resonance mode in which the mirror part 12 and the first actuator 14 tilts and oscillates around the second axis a2. The micromirror device 1 is able to perform a two-dimensional scan of light by combining the drive in the first resonance mode and the drive in the second resonance mode and tilting and oscillating the mirror part 12 around the first axis and the second axis.
The piezoelectric elements 34A, 34B, 36A, and 36B each have a laminated structure in which the lower electrode 31, the piezoelectric film 32, and the upper electrode 33 are laminated in this order on the oscillation plate 30 as a movable part substrate (refer to
In the present embodiment, the piezoelectric elements 34A, 34B, 36A, and 36B is provided over substantially the entire surface of each of the movable parts 14A, 14B, 16A, and 16B, but may be provided only in a part thereof in each movable part.
The upper electrode 33 of the piezoelectric element 34A in the first movable part 14A of the first actuator 14 consists of six individual electrode parts i7, i8, i9, i10, i11, and i12. The individual electrode parts i7 to i12 are formed separately from each other. Similarly, the upper electrode 33 of the piezoelectric element 34B in the movable part 14B of the first actuator 14 consists of six individual electrode parts i1, i2, i3, i4, i5, and i6. The individual electrode parts i1 to i6 are formed separately from each other.
The upper electrode 33 of the piezoelectric element 36A in the movable part 16A of the second actuator 16 consists of four individual electrode parts o1, o2, o7, and o8. The individual electrode parts o1, o2, o7, and o8 are formed separately from each other. Similarly, the upper electrode 33 of the piezoelectric element 36B in the movable part 16B of the second actuator 16 consists of four individual electrode parts o3, o4, o5, and o6. The upper individual electrode parts o3 to o6 are formed separately from each other.
The individual electrode parts i1 to i12 and o1 to o8 each are separated by a first stress inversion region s1 and a second stress inversion region s2. In the first stress inversion region s1, positive and negative of a principal stress component having a maximum absolute value are inverted in a principal stress generated in an in-plane direction of the piezoelectric film in a case of driving in a first resonance mode in which the mirror part 12 tilts and oscillates around the first axis a1. In the second stress inversion region s2, positive and negative of a principal stress component having a maximum absolute value are inverted in a principal stress generated in the in-plane direction of the piezoelectric film in a case of driving in a second resonance mode in which the mirror part is tilted and displaced around the second axis a2. By forming the upper electrode 33 separately into a plurality of individual electrode parts i1 to i12 and o1 to o8, it is possible to independently control the voltage for each piezoelectric part Pi1 to Pi12 and Po1 to Po8 defined by each individual electrode part i1 to i12 and o1 to o8. Each the piezoelectric parts Pi1 to Pi12 and Po1 to Po8 is composed of each of the individual electrode parts i1 to i12 and o1 to o8, an opposing lower electrode 31, and a piezoelectric film 32 interposed between each of the individual electrode parts it to i12 and o1 to o8 and the lower electrode 32. The individual electrode parts i1 to i12, o1 to o8, and the lower electrode 31 are connected to the drive circuit 25 (see
In the present embodiment, the piezoelectric film 32 and the lower electrode 31 are formed as a film common to a plurality of piezoelectric parts Pi1 to Pi12 and Po1 to Po8. However, the piezoelectric film 32 or the piezoelectric film 32 and the lower electrode 31 may be separated for each individual electrode part i1 to i12 and o1 to o8 of the upper electrode 33.
Since the micromirror device of the present disclosure has a structure that does not have a movable frame leading to an increase in mass, that is, a frame contributing to driving without the piezoelectric film, the moment of inertia in the rotation in the second axis can be reduced and the resonance frequency can be increased. For example, a driving frequency of 40 kHz or more on the horizontal axis and 10 kHz or more on the vertical axis can be realized. Therefore, high speed driving can be performed on both the first axis and the second axis. That is, the high speed driving is suitable for Lissajous scanning in which driving in a sinusoidal manner on both the horizontal axis and the vertical axis is performed. Further, both the first and second actuators are piezoelectric actuators comprising piezoelectric elements and do not require an external driving mechanism. Therefore, the volume of the element can be reduced to a small size. Since a piezoelectric element is not provided and a movable frame that does not contribute to driving is not provided, drive efficiency is high, and consumed power can be reduced.
In a case where the mirror part 12 tilts and oscillates around the first axis a1 in the first resonance mode and is tilted and displaced as shown in
Whether each part is “compressive stress” or “tensile stress” is determined by the direction in which the absolute value is larger (that is, the direction of the principal stress component having the maximum absolute value) of two principal stresses in a plane substantially orthogonal to the film thickness direction of the piezoelectric film which are selected from three principal stress vectors orthogonal to each other. In a case where the film thickness direction is taken on the z-axis, the two principal stresses in the plane substantially orthogonal to the film thickness direction are the stresses generated in the x-y plane. As the notation method of the stress direction, the vector in the outward direction is defined as the tensile direction, and the vector in the inward direction is defined as the compression direction.
The reason for defining in such a manner is that, in a piezoelectric MEMS device, the dimensions of the actuator part are generally planar, and the stress in the film thickness direction can be regarded as almost 0. The phrase “dimensions are planar” means that the height is sufficiently smaller than the dimensions in the plane direction. The plane direction of the above-mentioned “x-y plane” corresponds to the “in-plane direction orthogonal to the film thickness direction of the piezoelectric film”. The stress is defined as follows. The tensile stress σt applying a force in the direction in which the member is pulled is positive, and the compressive stress σc applying a force in the direction in which the member is compressed is negative. That is, the region where the principal stress component having the maximum absolute value is positive means a region where the tensile stress is dominant, and the region where the principal stress component having the maximum absolute value is negative means a region where the compressive stress is dominant. Further, the term “stress inversion region in which the positive and negative of the principal stress component having the maximum absolute value is inverted” means a region including the boundary between the tensile stress region and the compressive stress region, and a region that the stress changes from the tensile stress to the compressive stress or from the compressive stress to the tensile stress.
As shown in
Further, in one movable part 16A and the other movable part 16B of the second actuator 16, the right side of the paper surface is the tensile stress region t1 and the left side of the paper surface is the compressive stress region c1, with the first axis a1 interposed therebetween. In the first resonance mode, stress distribution in the second actuator 16 is axisymmetric with respect to the second axis a2.
At the boundary between the compressive stress region c1 and the tensile stress region t1, the first stress inversion region is present, which is a region where the direction of stress gradually changes, that is, a region where the positive and negative of the principal stress component having the maximum absolute value is inverted. The stress inversion line r1 shown in
In the second resonance mode, in a case where the mirror part 12 and the first actuator 14 are tilted and oscillated around the second axis a2 and are tilted and displaced as shown in
As shown in
Further, a region extending in the y direction including one end 16Aa of one movable part 16A of the second actuator 16 and a region extending in the y direction including the other end 16Ab are the tensile stress regions t2, and the central region intersecting the first axis a1 between the two tensile stress regions t2 is the compressive stress region c2. The region extending in the y direction including one end 16Ba of the other movable part 16B of the second actuator 16 and the region extending in the y direction including the other end 16Bb are the compressive stress regions c2, and the central region intersecting the first axis a1 between the two compressive stress regions c2 is the tensile stress region t2. In the second resonance mode, stress distribution in the second actuator 16 is axisymmetric with respect to the first axis a1.
At the boundary between the compressive stress region c2 and the tensile stress region t2, the second stress inversion region is present, which is a region where the direction of stress gradually changes, that is, a region where the positive and negative of the principal stress component having the maximum absolute value is inverted. The stress inversion line r2 shown in
The individual electrode parts of the upper electrodes are formed so as to correspond to division of the piezoelectric film regions t1, t2, c1 and c2 having different stress directions with respect to stress distribution shown in
The stress distribution during operation using resonance mode oscillation (resonance drive) can be analyzed by parameters such as device dimensions, Young's modulus of material, and device shape given using the known finite element method software through the mode analysis method. In design of the device, stress distribution in the piezoelectric film in a case of driving in the resonance mode is analyzed, and the upper electrodes are divided into individual electrode parts, in accordance with the division of the compressive stress region and the tensile stress region in stress distribution, based on the analysis result. Each piezoelectric part is defined by each individual electrode part.
In the first actuator 14 and the second actuator 16, by disposing the piezoelectric parts in accordance with the parts having different generated stress directions, an appropriate driving signal can be input to each of them. Therefore, the piezoelectric force can be efficiently converted into a displacement.
The driving power for driving in the first resonance mode and the second resonance mode is supplied to each piezoelectric part from the drive circuit 25. As the driving signal supplied to each of the first actuator 14 and the second actuator 16, an AC signal or a pulse waveform signal having a frequency that excites resonance can be used. Specific driving signals will be described below together with the driving method.
The drive circuit 25 inputs a driving signal, in which a first driving signal for driving the first resonance mode and a second driving signal for driving the second resonance mode are superimposed, to each of the plurality of piezoelectric parts Pi1˜Pi12 and Po1˜Po of the piezoelectric elements 34A, 34B, 36A, and 36B.
The first driving signal is a signal which includes driving voltage waveforms having opposite phases to each other. The signal is for giving the driving voltage waveform of one of the phases to the piezoelectric part located in the region, in which the principal stress component having the maximum absolute value at the time when the oscillation amplitude is maximized in a case of driving in the first resonance mode is positive, and for giving the driving voltage waveform of the other phase to the piezoelectric part located in the region in which the principal stress component having the maximum absolute value is negative at the above moment.
The second driving signal is a signal which includes driving voltage waveforms having opposite phases to each other. The signal is for giving the driving voltage waveform of one of the phases to the piezoelectric part located in the region, in which the principal stress component having the maximum absolute value at the time when the oscillation amplitude is maximized in a case of driving in the second resonance mode is positive, and for giving the driving voltage waveform of the other phase to the piezoelectric part located in the region in which the principal stress component having the maximum absolute value is negative.
Therefore, the drive circuit 25 applies, as the first driving signals, driving signal waveforms having the same phase to the piezoelectric parts located in the regions in which the principal stress component having the maximum absolute value at the time when the drive amplitude is maximized in a case of driving in the first resonance mode has the same direction (that is, the same sign). That is, a driving signal waveform having the same phase is applied to the piezoelectric parts located in the compressive stress regions, and a driving signal waveform having the same phase is applied to the piezoelectric parts located in the tensile stress regions. Then, the drive circuit 25 applies, as the first driving signals, driving signal waveforms having the opposite phases to the piezoelectric parts located in the regions in which the principal stress component having the maximum absolute value at the time when the drive amplitude is maximized in a case of driving in the first resonance mode has the different directions (that is, the different signs). That is, driving signal waveforms having opposite phases are applied to the compressive stress regions and the tensile stress regions. The amplitudes of the driving signal waveforms having the same phase may be the same between the piezoelectric parts, or may be different between the piezoelectric parts.
At the same time, the drive circuit 25 applies, as the second driving signals, driving signal waveforms having the same phase to the piezoelectric parts located in the regions in which the principal stress component having the maximum absolute value at the time when the drive amplitude is maximized in a case of driving in the second resonance mode has the same direction. That is, a driving signal waveform having the same phase is applied to the piezoelectric parts located in the compressive stress regions, and a driving signal waveform having the same phase is applied to the piezoelectric parts located in the tensile stress regions. Then, the drive circuit 25 applies, as the second driving signals, driving signal waveforms having the opposite phases to the piezoelectric parts located in the regions in which the principal stress component having the maximum absolute value at the time when the drive amplitude is maximized in a case of driving in the second resonance mode has the different directions. That is, driving signal waveforms having opposite phases are applied to the compressive stress regions and the tensile stress regions. The amplitudes of the driving signals having the same phase may be the same between the piezoelectric parts, or may be different between the piezoelectric parts.
The first resonance mode and the second resonance mode can be excited at the same time by applying a driving signal, in which the first driving signal for the first resonance mode and the second driving signal for the second resonance mode are superimposed, to each piezoelectric part. In a case where the driving signal is given to each piezoelectric part to drive the first actuator 14 and the second actuator 16, a positive voltage is applied to the piezoelectric parts located in the regions where the principal stress component having the maximum absolute value in the principal stresses generated in the respective piezoelectric films at the time of each oscillation is positive, and a negative voltage is applied to the piezoelectric parts located in the regions where the principal stress component having the maximum absolute value is negative. By applying a positive or negative voltage in accordance with the positive or negative of the strain, the piezoelectric force can be converted into the displacement very efficiently. Therefore, it is possible to significantly reduce power consumption.
An example of the driving method will be specifically described with reference to
In
The individual electrode parts i1, i2, i5, i6, i9, i10, and o5, o6, o7, o8 of the first group correspond to the compressive stress regions c1 of
The first driving signals V1a and V1b are represented as follows, respectively.
V
1
a=α
1
V
1 sin(2πf1t+π)
V
1
b=α
1
V
1 sin 2πf1t
In a case of collecting and generalizing the first driving signals V1a and V1b,
In
The individual electrode parts i2, i3, i6, i7, i10, i11, and o1, o3, o6, o8 of the third group correspond to the compressive stress regions c2 of
The second driving signals V2a and V2b are represented as follows, respectively.
V
2
a=α
2
V
2 sin(2πf2t+π)
V
2
b=α
2
V
2 sin 2πf2t
In a case of collecting and generalizing the second driving signals V2a and V2b,
The driving signal, in which the first driving signal for the first resonance mode and the second driving signal for the second resonance mode are superimposed, is applied to each piezoelectric part. For example, V1a+V2b is applied to the individual electrode part i1, V1a+V2a is applied to the individual electrode part i2, V1b+V2a is applied to the individual electrode part i3, and V1b+V2b is applied to the individual electrode part i4. Table 1 shows the combinations applied to each piezoelectric part.
The driving signal applied to each piezoelectric part xy can be represented by General Expression (1).
Vxy=α
1
V
1 sin(2πf1t+β1π)+α2V2 sin(2πf2t+β2π) (1)
Here, xy is a reference sign for specifying which part of which actuator is the individual electrode part. In the case of the placement of the individual electrode parts shown in
In Expression (1), the first term is the first driving signal for tilting and oscillating around the first axis, and the second term is the second driving signal for tilting and oscillating around the second axis.
As described above, driving thereof can be very efficiently performed by controlling so as to apply the driving signal to each piezoelectric part according to the direction of the principal stress component having the maximum absolute value in the principal stresses generated in the piezoelectric films of the first actuator and the second actuator in a case of driving the micromirror device in the first resonance mode and the second resonance mode. The drive circuit 25 is configured to input the driving signal to each piezoelectric part.
In the present embodiment, the lower electrodes of the plurality of piezoelectric parts included in each piezoelectric element are common electrodes. Therefore, the lower electrode is grounded and a predetermined driving signal (driving voltage waveform) is input to the upper electrode. However, in a case where the lower electrode is also an individual electrode, either the lower electrode or the upper electrode may be used as the earth electrode as long as a driving signal can be applied between the lower electrode and the upper electrode.
As the resonance mode, there is not only a mode accompanied by rotation (tilt oscillation) of the mirror part 12 around the axis, but also a mode accompanied by a piston motion in the vertical direction, a twisting motion in a plane, or the like. However, in the micromirror device 1 of the present embodiment, the mirror part 12 is driven by using a resonance mode accompanied by tilt oscillation.
In addition, as the first resonance mode in which the mirror part 12 tilts and oscillates around the first axis, in the resonance mode in which the mirror part 12 and the first actuator 14 oscillate in opposite phases, the Q value of the resonance oscillation is higher and the resonance frequency is higher than that in the resonance mode in which the mirror part 12 and the first actuator 14 oscillate in the same phase. As a result, it is more suitable that scanning is performed at high speed. For example, in the micromirror device of Example 1 described later, the resonance frequency of the in-phase resonance mode around the first axis was 35 kHz, and the Q value was 700. On the other hand, the resonance frequency of the resonance mode having the opposite phase around the first axis was 60 kHz, and the Q value was 1900. It is preferable to perform driving in the lowest order mode among the resonance modes in which the mirror part 12 and the first actuator 14 tilt and oscillate in opposite phases around the first axis since a high Q value can be obtained.
Further, as the second resonance mode in which the first actuator 14 and the mirror part 12 integrally tilts and oscillates around the second axis, in the resonance mode in which the first actuator 14 and the second actuator 16 oscillate in opposite phases, the Q value of the resonance oscillation is higher and the resonance frequency is higher than that in the resonance mode in which the mirror part 12 and the first actuator 14 oscillate in the same phase. As a result, it is suitable that scanning is performed at high speed. For example, in the configuration of Example 1 described later, the resonance frequency of the in-phase resonance mode around the second axis was 4.7 kHz, and the Q value was 250. On the other hand, the resonance frequency of the resonance mode having the opposite phase around the second axis was 11 kHz, and the Q value was 940. Therefore, it is also preferable to perform driving in the lowest order mode among the resonance modes in which the first actuator 14 and the second actuator 16 tilt and oscillate in opposite phases around the second axis since a high Q value can be obtained.
The stress distributions in the first actuator and the second actuator differ depending on which order of the resonance mode is used. Therefore, it is necessary to determine the resonance mode to be used in a case of driving and then dispose the individual electrode parts, based on the stress distribution in the resonance mode. In the present example, for the circumference of the first axis, the lowest order mode among the resonance modes in which the mirror part 12 and the first actuator 14 tilt and oscillate in opposite phases is set as the first resonance mode. For the circumference of the second axis, the lowest order mode among the resonance modes in which the first actuator 14 and the second actuator 16 tilt and oscillate in opposite phases is set as the second resonance mode.
The piezoelectric element provided in the first actuator 14 and the second actuator 16 will be described. As described above, the piezoelectric element has a laminated structure of a lower electrode 31, a piezoelectric film 32, and an upper electrode 33.
The thickness of the lower electrode and the upper electrode is not particularly limited, and is, for example, about 200 nm. The thickness of the piezoelectric film is not particularly limited as long as it is 10 μm or less, and is usually 1 μm or more, for example, 1 to 5 The method for forming the lower electrode, the upper electrode and the piezoelectric film is not particularly limited, but the vapor deposition method is preferable, and the sputtering method is particularly preferable.
The main components of the lower electrode are not particularly limited, and are metals or metal oxides such as Au, Pt, Ir, IrO2, RuO2, LaNiO3, and SrRuO3, and combinations thereof.
The main component of the upper electrode is not particularly limited, and examples thereof include materials exemplified for the lower electrode, electrode materials generally used in semiconductor processes such as Al, Ti, Ta, Cr, and Cu, and combinations thereof.
Examples of the piezoelectric film include those containing one or more types of perovskite-type oxides (P) represented by the following formula.
General formula ABO3 (P)
Examples of the perovskite-type oxide represented by the above-mentioned general formula include lead-containing compounds such as lead titanate, lead zirconate titanate (PZT), lead zirconate tit, lead zirconate tit lanthanate, lead zirconate titanate lanthanate, lead zirconate titanate magnesium niobate, lead zirconate titanate titanate, and lead zirconate titanate zinc niobate, and mixed crystal systems thereof; lead-free compounds such as barium titanate, strontium barium titanate, bismas sodium niobate, potassium niobate potassium, niobate, sodium, potassium niobate, lithium niobate, and bismas ferrite, and mixed crystal systems thereof.
Further, the piezoelectric film of the present embodiment preferably contains one kind or two or more kinds of perovskite-type oxides (PX) represented by the following formula.
Aa(Zrx,Tiy,Mb-x-y)bOc (PX)
The piezoelectric film consisting of the perovskite-type oxide represented by the above general formulas (P) and (PX) has a high piezoelectric strain constant (d31 constant). Therefore, the piezoelectric actuator comprising such a piezoelectric film is excellent in displacement characteristics. The perovskite-type oxide represented by the general formula (PX) has a higher piezoelectric constant than that represented by the general formula (P).
Further, the piezoelectric actuator comprising the piezoelectric film which consists of the perovskite-type oxide represented by the general formulas (P) and (PX) has a voltage-displacement characteristic with excellent linearity in the driving voltage range. These piezoelectric materials exhibit favorable piezoelectric properties in carrying out the present disclosure.
The micromirror device 2 of the present embodiment has a different shape of the second actuator from the micromirror device 1 of the first embodiment. In the first embodiment, the second actuator 16 is an annular member disposed so as to surround the first actuator 14. However, the second actuator 50 of the present embodiment comprises second movable parts 52A and 52B and third movable parts 53A and 53B disposed exterior to the first actuator 14 which are line-symmetric around the first axis a1.
As shown in
Piezoelectric elements 62A, 62B, 63A, and 63B are comprised in each of the second movable parts 52A and 52B and the third movable parts 53A and 53B of the second actuator 50, and function as piezoelectric actuators. The configuration of the piezoelectric actuator is the same as that of the first embodiment, and has a laminated structure in which the lower electrode 31, the piezoelectric film 32, and the upper electrode 33 are laminated in this order on the oscillation plate 30 which is the base material of the movable part.
As shown in
On the other hand, as shown in
The individual electrode parts it to i12 and o1 to o8 each are separated by a first stress inversion region s1 and a second stress inversion region s2. In the first stress inversion region s1, positive and negative of a principal stress component having a maximum absolute value among principal stresses are inverted in a principal stress generated in an in-plane direction of the piezoelectric film in a case of driving in a first resonance mode in which the mirror part 12 tilts and oscillates around the first axis a1. In the second stress inversion region, positive and negative of a principal stress component having a maximum absolute value among principal stresses are inverted in a principal stress generated in the in-plane direction of the piezoelectric film in a case of driving in a second resonance mode in which the mirror part is tilted and displaced around the second axis a2.
In a case where the mirror part 12 tilts and oscillates around the first axis a1 in the first resonance mode and is tilted and displaced as shown in
As shown in
Further, in one of the second movable parts 52A and the other second movable part 52B of the second actuator 50, the region including one ends 52Aa and 52Ba connected on the second axis a2 is the tensile stress region t1, and the region including the other end 52Ab and the region including the other end 52Bb are the compressive stress regions c1. In one third movable part 53A and the other third movable part 53B of the second actuator 50, the region including one ends 53Aa and 53Ba connected on the second axis a2 is the compressive stress region c1, and the region including the other end 53Ab and the region including the other end 53Bb are the tensile stress regions t1. In the first resonance mode, stress distribution in the second actuator 50 is axisymmetric with respect to the second axis a2.
At the boundary between the compressive stress region c1 and the tensile stress region t1, the first stress inversion region is present, which is a region where the direction of stress gradually changes, that is, a region where the positive and negative of the principal stress component having the maximum absolute value is inverted. The stress inversion line r1 shown in
In the second resonance mode, in a case where the mirror part 12 and the first actuator 14 are tilted and oscillated around the second axis a2 and are tilted and displaced as shown in
As shown in
Further, the second movable part 52A of the second actuator 50 has a compressive stress region c2, the second movable part 52B has a tensile stress region t2, the third movable part 53A has a compressive stress region c2, and the third movable part 53B has a tensile stress region t2. In the second resonance mode, stress distribution in the second actuator 50 is also line-symmetric with respect to the first axis a1.
There is a second change region, which is a region where the direction of stress gradually changes at the boundary between the compressive stress region c2 and the tensile stress region t2, that is, the principal stress component having the maximum absolute value changes from positive to negative. In
The individual electrode parts of the upper electrodes are formed so as to correspond to stress distributions shown in
As described above, in a case where the shape of the actuator is different, stress distribution during operation due to the resonance mode oscillation is different. However, individual electrode parts may be formed in accordance with each stress distribution. In the first actuator 14 and the second actuator 16, by disposing the piezoelectric parts in accordance with the parts having different generated stress directions, an appropriate driving signal can be input to each of them. Therefore, the piezoelectric force can be efficiently converted into a displacement.
Also in the configuration of the present embodiment, in a case of driving, in the placement form of the piezoelectric parts divided as described above, a driving voltage having the same phase is applied to the piezoelectric parts corresponding to the regions having the same stress direction of the stresses generated in the first resonance mode in which the mirror part tilts and oscillates around the first axis. That is, a driving voltage having the same phase is applied to the piezoelectric parts located in the compressive stress region, and a driving voltage having the same phase is applied to the piezoelectric parts located in the tensile stress region. Then, different driving signals are applied between the piezoelectric parts corresponding to the regions having the different stress directions.
Thereby, a positive voltage is applied to the region where the principal stress component having the maximum absolute value is positive, and a negative voltage is applied to the region where the principal stress component having the maximum absolute value is negative. In such a manner, piezoelectric force can be very efficiently converted into displacement. Consequently, it is possible to significantly reduce power consumption.
An example of the driving method will be specifically described with reference to
In
The individual electrode parts i1, i2, i5, i6, i9, i10, and o1, o4, o6, o7 of the first group correspond to the compressive stress regions c1 of
In
The individual electrode parts i1, i4, i5, i8, i9, i12, and o1, o2, o7, o8 of the third group correspond to the compressive stress regions c2 of
Table 2 shows the combinations applied to each piezoelectric part.
Also in the micromirror device 2 of the present embodiment, the driving signal applied to each piezoelectric part xy can be represented by the general Expression (1). Also In such a case, α1 and α2 may be different for each piezoelectric part. By increasing the voltage amplitude as the magnitude of the stress generated in the individual electrode part increases, it is possible to further increase the drive efficiency and further reduce the power consumption.
[Example of Design Change]
For example,
As shown in
The second movable parts 152A and 152B and the third movable parts 153A and 153B of the second actuator 150 each have an L-shape consisting of a part including one end connected to the second connecting part 122 and extending in the y direction and a part extending in the x direction from the other end in the y direction. The part extending in the y direction is thinner than the part extending in the x direction, and this region does not comprise a piezoelectric element. The part extending in the x direction is wider than the part extending in the y direction, and only this region includes the piezoelectric elements 162A, 162B, 163A, and 163B.
Further, in the third connecting part 123 that connects the fixing unit 120 and the second actuator 150, two pairs of rod-like members disposed in parallel with the second axis a2 interposed therebetween are disposed to face each other with the second actuator interposed therebetween.
As in the case of each of the above embodiments, the upper electrode of each piezoelectric element may be composed of a plurality of individual electrode parts, each of which is separated by a first stress inversion region and a second stress inversion. In the first stress inversion region, a principal stress component having a maximum absolute value changes from positive to negative in a principal stress generated in an in-plane direction of the piezoelectric film in a maximum displacement state in a case of driving in a first resonance mode in which the mirror part is tilted and displaced around the first axis. In the second stress inversion region, a principal stress component having a maximum absolute value changes from positive to negative in a principal stress generated in the in-plane direction of the piezoelectric film in a case of driving in a second resonance mode in which the mirror part is tilted and displaced around the second axis.
As described above, in the present disclosure, the shapes of the first actuator provided exterior to the mirror part and the second actuator provided exterior to the first actuator are not particularly limited. The individual electrode parts are provided which correspond to the direction of the principal stress component having the maximum absolute value in the principal stresses generated in a case where driving the first resonance mode and the second resonance mode, depending on the configuration of the mirror part, the first actuator, the second actuator and their connecting parts. In such a case, it is possible to input a driving signal having high conversion efficiency, and it is possible to effectively suppress power consumption.
Hereinafter, the micromirror devices of examples of the present disclosure will be described.
—Manufacturing Method—
(Step 1) A Ti layer with 30 nm was formed and an Ir layer with 150 nm was formed, at a substrate temperature of 350° C., by a sputtering method, on an silicon on insulator (SOI) substrate having a laminated structure of a Si handle layer 350 μm, a silicon oxide (SiO2) box layer 1 μm, and a Si device layer 100 μm. The laminated structures of the Ti layer and the Ir layer correspond to the lower electrode 31 in
(Step 2) A piezoelectric film with 3 μm was formed on the substrate on which the Ti/Ir lower electrodes obtained above were laminated and formed using a radio frequency (RF) sputtering device. As the target material for sputtering film formation for the piezoelectric film, a material having a composition of Pb1.3((Zr0.52Ti0.48)0.88Nb0.12) O3 was used. The film forming pressure was 2.2 mTorr, and the film forming temperature was 450° C. The obtained piezoelectric film was an Nb-doped PZT thin film to which Nb was added at an atomic composition ratio of 12%.
(Step 3) An upper electrode including the plurality of individual electrode parts using a Pt/Ti laminated structure was patterned by a lift-off method, on the substrate on which the piezoelectric film obtained above was formed.
(Step 4) After that, the piezoelectric film and the lower electrode were pattern-etched by inductively coupled plasma (ICP) dry etching.
(Step 5) Further, an insulating layer consisting of SiO2 was formed on the entire surface by a chemical vapor deposition method (TEOS-CVD: tetraethoxysilane-chemical vapor deposition) using tetraethoxysilane as a raw material, and then the insulating layer was patterned by the ICP dry etching.
(Step 6) A laminated structure of Au/Ti was formed into a pattern, and a reflecting surface of a mirror part, an electrode pad, and a wiring layer were formed, by the lift-off method.
(Step 7) The device layer was pattern-etched by a silicon dry etching process to process the shapes of the actuator, the mirror part, and the fixing member.
(Step 8) Next, the handle layer was subject to deep-drilled reactive ion etching from the back surface of the substrate. Basically, the handle layer was removed such that a part to be a fixing member remains.
(Step 9) Finally, the micromirror device 1 described with reference to
In the above manufacturing step, the reflecting surface of the mirror part is formed in Step 6, but the reflecting surface may be formed by using a reflective material different from the material of the electrode pad and the wiring layer. In that case, for example, subsequently to Step 6, the reflecting surface may be formed by a lift-off method or the like.
In carrying out the present disclosure, the present disclosure is not limited to the configuration and manufacturing method of Example 1, and the substrate material, electrode material, piezoelectric material, film thickness, film forming conditions, and the like may be appropriately selected in accordance with the purpose.
Various dimensions of the micromirror device of the present example are specified as shown in
Various dimensions in Example 1 were as follows.
The thicknesses of the mirror part 12, the first actuator 14, the second actuator 16, and the first connecting part 21, the second connecting part 22, and the third connecting part 23 are equal to the thickness of the device layer.
In the micromirror device having this configuration, the dimensions of the first connecting part 21 and the third connecting part 23 were set such that the frequency of the first resonance mode was about 60 kHz and the frequency of the second resonance mode was about 10 kHz.
The stress distribution during operation using resonance mode oscillation (resonance drive) was analyzed by parameters such as the device dimensions, Young's modulus of material, and device shape given using the known finite element method software through the mode analysis method. The stress distributions in each of the first resonance mode and the second resonance mode were obtained (refer to
The driving signal, in which a second driving signal having a frequency coinciding with that in the second resonance mode is superimposed on a first driving signal having a frequency coinciding with that in the first resonance mode, was applied to each piezoelectric part of the micromirror device according to Example 1 mentioned above.
Vxy=α
1
V
1 sin(2πf1t+β1π)+α2V2 sin(2πf2t+β2π) Expression (1),
The correction coefficients of the voltage applied to each piezoelectric part (each individual electrode part) are as shown in Table 3.
In Example 1-1, the correction coefficients α1 and α2 are all set to 1 in common. That is, the voltage amplitude of the first driving signal is common to all the piezoelectric parts, and the voltage amplitude of the second driving signal is common to all the piezoelectric parts.
Table 5 shows a standardized average in-plane stress for each region corresponding to each individual electrode part for the principal stress component having the maximum absolute value generated in the first and second actuators in a case where the mirror part is maximally displaced during each driving in the first resonance mode and the second resonance mode. In Example 1-2, the correction coefficients α1, β1 and α2 and β2 were set in each individual electrode part in accordance with the magnitude of the average in-plane stress generated in each individual electrode part region shown in Table 5 (refer to Table 4).
In
On the other hand, in
That is, in Reference Example 1, only the first driving signal was input to the upper electrode of the piezoelectric element of the first actuator, and only the second driving signal was input to the upper electrode of the piezoelectric element of the second actuator. In the present example, each parameter of Expression (1) was set as shown in Table 6. Thereby, the first and second resonance modes are excited to rotate the mirror part 12 around the first axis a1, and at the same time, the first actuator 14 and the mirror part 12 are substantially integrated and rotate around the second axis a2.
For driving the micromirror devices of Examples 1-1 and 1-2 and Reference Example 1, driving in the first resonance mode and the second resonance mode was performed, and examination was performed on the driving voltage amplitude, which is necessary to perform scanning of 45° around the first axis a1, and the basic voltage amplitude value V1 of the first driving signal, the basic voltage amplitude value V2 of the second driving signal, and the power consumption, which are necessary to perform scanning of 30° around the second axis a2. Table 7 shows the results. The optical scan angle was measured in the following method. Laser is vertically incident onto the reflecting surface of the mirror part of the micromirror device, the length of the scanning line is measured using a ruler, and the like, and the total angle of scanning is calculated from the geometrical relationship.
The power consumption can be calculated based on the following method.
As shown in Table 7, in Example 1-1 and Example 1-2, the voltage amplitude can be significantly reduced as compared with Reference Example 1, and as a result, it is apparent that power consumption can be suppressed. Particularly, as in Example 1-2, the power consumption can be suppressed more remarkably by changing the voltage amplitude applied to the piezoelectric part in accordance with the magnitude of the principal stress component having the maximum absolute value.
A driving signal represented by Expression (1) was applied to each piezoelectric part of the micromirror device of Example 2 as in the case of Example 1. The correction coefficients in the driving signal applied to each piezoelectric part are as shown in Table 8.
In Example 2, the correction coefficients α1 and α2 are all set to 1 in common. That is, the voltage amplitude of the first driving signal is common to all the piezoelectric parts, and the voltage amplitude of the second driving signal is common to all the piezoelectric parts.
In
On the other hand, in
That is, in Reference Example 2, only the first driving signal was input to the upper electrode of the piezoelectric element of the first actuator 14, and only the second driving signal was input to the upper electrode of the piezoelectric element of the second actuator 50. In the present example, each parameter of Expression (1) was set as shown in Table 9. Thereby, the first and second resonance modes are excited to rotate the mirror part 12 around the first axis a1, and at the same time, the first actuator 14 and the mirror part 12 are substantially integrated and rotate around the second axis a2.
For driving the micromirror devices of Example 2 and Reference Example 2, driving in the first resonance mode and the second resonance mode was performed as in the case of Example 1 and Reference Example 1, and examination was performed on the driving voltage amplitude, which is necessary to perform scanning of 45° around the first axis a1, and the basic voltage amplitude value V1 of the first driving signal, the basic voltage amplitude value V2 of the second driving signal, and the power consumption, which are necessary to perform scanning of 30° around the second axis a2. Table 10 shows the results.
As shown in Table 10, in Example 2, the voltage amplitude can be significantly reduced as compared with Reference Example 2, and as a result, it is apparent that power consumption can be suppressed.
The present disclosure of JP2019-063659 filed on Mar. 28, 2019 is incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-063659 | Mar 2019 | JP | national |
This application is a continuation application of International Application No. PCT/JP2020/006638, filed Feb. 19, 2020, the disclosure of which is incorporated herein by reference in its entirety. Further, this application claims priority from Japanese Patent Application No. 2019-063659, filed Mar. 28, 2019, the disclosure of which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2020/006638 | Feb 2020 | US |
| Child | 17468631 | US |
