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A compact two-dimensional micro scanner with small volume, large deflection angles and high frequency is presented and the two-dimensional laser scanning is achieved by specular reflection. To achieve large deflection angles, the micro scanner excited by a piezoelectric actuator operates in the resonance mode. The scanning frequencies and the maximum scanning angles of the two degrees of freedom are analyzed by modeling and simulation of the structure. For the deflection angle measurement, piezoresistors are integrated in the micro scanner. The appropriate directions and crystal orientations of the piezoresistors are designed to obtain the large piezoresistive coefficients for the high sensitivities. Wheatstone bridges are used to measure the deflection angles of each direction independently and precisely. The scanner is fabricated and packaged with the piezoelectric actuator and the piezoresistors detection circuits in a size of 28 mm×20 mm×18 mm. The experiment shows that the two scanning frequencies are 216.8 Hz and 464.8 Hz, respectively. By an actuation displacement of 10 μm, the scanning range of the two-dimensional micro scanner is above 26° × 23°. The deflection angle measurement sensitivities for two directions are 59 mV/deg and 30 mV/deg, respectively.

With the development of the micro-electronics technology, micro-optical-electro mechanical systems (MOEMS) have provided new features to spacecraft and the micromation of satellites has become a general trend. For optical scanning and space detection, the laser scanning technique is an active way to detect objects and measure both range and orientation [

Aiming at fixing these deficiencies, a compact two-dimensional micro scanner with a piezoelectric actuator and piezoresistors is presented in this paper. It has a small volume with high integration and a decoupling measurement method of deflection angles with high sensitivities is presented for the two-dimensional micro scanner. The details concerning the structure, operation principle, simulation, deflection angles measurement, fabrication process and experimental results are described.

A two-dimensional micro scanner with a piezoelectric actuator and piezoresistors is designed as shown in

The piezoelectric actuator deforms along z-axis by pulsant driving voltage and the excited part vibrates in the z-axis. Because the center of gravity of the reflector and inertia generator is away from each rotational axis (x and y), the scanner has two resonance vibration modes: twisting around the y-axis and bending along the x-axis, as shown in _{T}_{B}

In the two-dimensional micro scanner structure, the reflector and inertia generator can be considered as a single mass at the end of the flexible beam. Accordingly, the deformations in two directions are equivalent to the twisting and bending of the flexible beam. The two-dimensional micro scanner is driven by the actuation of the piezoelectric actuator in z-axis and the flexible beam is twisted and bent by the inertial force. Therefore, the movements of the micro scanner can be equivalent to the forced vibration of the system based movement. Under micro deformation circumstances, the movements of the micro scanner can be described as a “mass-spring-dashpot” system. For the twisting and bending of the flexible beam in the two different modes of the micro scanner, each movement is independent from the other. Therefore, the movement equations of the 2-DOF model can be represented by _{n}_{n}_{n}_{n}_{n}_{n}

When one resonance frequency is far from the other, the steady-state response of the 2-DOF system can be approximated as follows:
_{n}_{y}_{x}_{1}_{2}

Accordingly, if the difference between the resonant frequencies of the scanner is large, the effect of the imposed external actuation which results in the resonance in one direction can be ignored in the other direction. When the actuations include both resonant frequencies, the micro scanner can achieve resonance simultaneously in both directions with large amplitudes. From the theoretical inference, the maximal deflection angles in two directions are proportional to the actuation displacement of the piezoelectric actuator.

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The micro scanner is then analyzed by the harmonic response simulation. Since the piezoelectric actuator deforms along the z-axis at a certain frequency, the reflector is driven by a harmonic inertial force along the z-axis. The movement of the reflector with the changed driving frequency is shown in

The simulation results indicate that the first mode will not appear when the micro scanner is actuated along the z-axis, while twisting around the y-axis and bending by the x-axis with large deflection angles could be achieved at the respective resonant frequencies. The difference between the two resonant frequencies is large, which meets the design requirement. Furthermore, the maximal deflection angles are linear to the inertial force which is proportional to the actuation displacement of the piezoelectric actuator [

Deflection angle sensing is based on the piezoresistive effect which has the advantages of favorable dynamic characteristics and high sensitivities. The stresses in the longitudinal, transverse and tangential directions of the piezoresistor cause the change of the resistance. The piezoresistive effect in plane is described as follows [_{l}_{t}_{τ}_{l}_{t}_{τ}

Piezoresistors are laid on the flexible beam for the measurement of the deflection angles for two vibration modes. The micro scanner twisting around y-axis and bending of the x-axis are equivalent to the torsion of a rod and the bend of a cantilever beam with a mass at the end, respectively. The surface stresses generated on the flexible beam are shown in

When the scanner is scanning two-dimensionally, the motions of the flexible beam are coupled with both the torsion and bend. The surface stress states of a cell A at distance

Therefore, the change of the resistance in the piezoresistor is influenced by both motions and not proportional to either deflection angle. The piezoresistor design of appropriate directions and crystal orientations can realize the decoupling measurement for two deflection angles, which will be presented below with detailed information.

The change of the resistance in piezoresistor is related to not only the longitudinal, transverse and tangential stresses, but also the piezoresistive coefficients in longitudinal, transverse and tangential directions. These piezoresistive coefficients depend strongly on the crystal orientation and doping type, which are described as follows [_{ij}_{1}, _{1}, _{1}) and (_{2}, _{2}, _{2}) are the sets of direction cosines of the longitudinal piezoresistor orientation and the transverse piezoresistor orientation in the crystal axis.

According to the general expressions for the piezoresistive coefficients, an n-type silicon substrate in (110) wafer is selected. In order to extract the shear stress _{T}_{1} and _{T}_{2} are oriented along ±45 degrees off y-axis and a p-type silicon piezoresistor _{B}

With the _{T}_{T}_{T}_{1} and _{T}_{2} while _{B}_{B}_{B}_{T}_{B}_{i}_{T}_{T}_{B}_{B}

The fabrication process flows are shown in _{0} = 1.1×l0^{-2} Ω·cm, the dimensions of the piezoresistors are set to 100 μm × 10 μm with 0.5 μm depth and the boron ion implantation density is 8.0×10^{18} ions/cm^{2} at the temperature of 1,100°C with 11 minutes duration. The sputter and lift-off process was adopted and golden thin film lines with width of 10 μm are connected and laid on the flexible beam. The micrographs of the flexible beam and piezoresistors are shown in

The two-dimensional micro scanner is excited by the piezoelectric actuator which is connected to the excited part with epoxy resin. There are two circuit boards linked from top to bottom. The piezoelectric actuator is connected to the bottom board and the piezoresistors are connected to the top board with golden wire bonding. The detection signal of the deflection angles are output from the bottom board on which the detection circuits are operated.

The micro scanner is packaged and protected by a stainless steel case. The top of the package is open for the reflector and closed with a translucent optical glass. The actuation and detection signal are imported and exported through a window in the side. The micro scanner, the piezoelectric actuator and the piezoresistors detection circuits are integrated in the unit module with the size of 28 mm × 20 mm × 18 mm, as shown in

The two resonance frequencies of the two-dimensional micro scanner are measured by the frequency sweeping method and the scanning amplitude is detected by a laser interferometer measurement system. With an actuation displacement of 3.6 μm, the scanning amplitude-frequency responses in two directions are shown in

The relationships of each deflection angle and the actuation displacement in the resonance modes are shown in

The experimental results indicate that there are linear relationships between each deflection angle and the actuation displacement when the actuation displacement is less than 4 μm, which is in agreement with the theoretical

The deflection angles of the two-dimensional micro scanner are measured by the piezoresistors detection circuits and the piezoresistor characteristics are indicated by the output voltages of the two Wheatstone bridges. The relationships of the corresponding piezoresistor output voltage and each deflection angle in the resonance modes are shown in

The piezoresistor output voltage in twisting mode is less than the theoretical calculation while the one in bending mode approaches the theoretical value. This is because that the length of the piezoresistor is close to the width of the flexible beam. The average stress on piezoresistors _{T1} and _{T2} in practice are less than the maximal shear stress on the surface of the flexible beam in the theoretical calculation. By comparison, the length of the piezoresistor is far less than the flexible beam length. The average stress on piezoresistor _{B}

To achieve two-dimensional laser scanning, a compact two-dimensional micro scanner has been developed in this paper. The micro scanner is excited by a piezoelectric actuator and the piezoresistors are integrated for the deflection angle measurement in two directions. The design, modeling, simulation and fabrication are described and the experimental results are characterized. The unit module including the micro scanner, the piezoelectric actuator and the piezoresistors detection circuits are of a size of 28 mm × 20 mm × 18 mm. The two scanning frequencies are 216.8 and 464.8 Hz for each direction of the micro scanner. With an actuation displacement of about 10 μm, a scanning range above 26° × 23° is obtained. The piezoresistor design has realized decoupling measurements for the bending and twisting deflection angles of the two-dimensional micro scanner. The piezoresistor output sensitivities for the two directions are 59 and 30 mV/deg, respectively. The total power consumption of the piezoelectric actuation and piezoresistors measurement is below 1 W. The two-dimensional micro scanner has the great advantages of low power consumption, small volume, high frequency, large deflection angles and high measurement sensitivities. It is suitable for the micro laser scanning system and has a broad range of potential applications in the field of space target detection and position measurement in micro-satellites.

The two-dimensional micro scanner was processed by Hebei Semiconductor Research Institute. We are grateful to Mr. Xu Yongqing and Ms. Luo Rong for their help.

Structure of two-dimensional micro scanner.

Two resonance vibration modes.

Dimensions of two-dimensional micro scanner.

Three modes by FEM modal simulation.

FEM Harmonic Response Simulation.

Surface stresses on flexible beam.

Stress states of coupled motions.

Directions and crystal orientations of piezoresistors.

Piezoresistors connection of two Wheatstone bridges.

Fabrication process flows.

Micrographs of flexible beam and piezoresistors.

Package of two-dimensional micro scanner.

Scanning amplitude frequency responses in two directions.

Deflection Angles Characteristics in Two Directions.

Scan Patterns of Two-Dimensional Micro Scanner.

Piezoresistor Output Characteristics in Two Directions.

The value of the piezoresistive coeffients (10^{-2}/GPa).

_{lT} |
_{tT} |
_{τT} |
_{lB} |
_{tB} |
_{τB} |
---|---|---|---|---|---|

88.1 | -50 | -32.6 | 71.8 | -1.1 | 0 |