Silicon dual inertial sensors

[0001] The present invention relates to dual inertial sensors made with micro-machining technology, including both functions of a gyroscope (angular rate sensor) and an accelerometer (acceleration sensor), and more particularly to dual inertial sensors made with bulk-micromachining and wet etching on (110) silicon chips.

[0002] A conventional structure of dual inertial sensors made with bulk-micromachining is shown in FIG. 1. It is made of (100) silicon chips 1, comprising an outer frame 2. The outer frame 2 comprises one or more inner frame 5, and each inner frame 5 is further comprising a proof-mass 3. The proof-mass 3 is connected to the inner frame 5 by a plurality of sensing flexible beams 4, and the inner frame 5 is connected to the outer frame 2 by a plurality of driving flexible beams 6. The sensing beams 4 facilitate the proof-mass 3 to move perpendicular to the surface of the silicon chip defined as z-direction), and the driving beams 6 facilitate the proof-mass 3 to move in parallel to the surface of the silicon chip (defined as y-direction). Two glass sheets (not shown in FIG. 1) are placed on both sides of the silicon chip 1, and connected to the outer frame 2. The thin metal film electrodes 81, 82 are electro-plated on the glass sheets facing the silicon chip surface and corresponding to the two edges of the inner frame 5, respectively. The two thin metal film electrodes 81, 82, with the surface of the inner frame 5, will form edge effect electrostatic driving capacitors c8p, c8n. A thin metal film electrode 9 is electrode-plated on the glass sheets facing the silicon chip surface and the proof-mass 3. The thin metal film electrodes 9, with the surfaces of the proof-mass 3, will form two sensing capacitors c9p, c9n. The alternating driving voltage on the driving capacitors c8p, c8n will make the inner frame 5 and the proof-mass 3 vibrate along y-axis. If there is an angular velocity Ω along x-axis, there will be a Coriolis force F making the proof-mass 3 vibrate along z-axis. The angular rate can be obtained by measuring the amplitude of the z-direction vibration of the proof-mass 3. If there is an acceleration applied along the z-axis, the specific force will move the proof-mass 3 with respect to the inner frame. The acceleration can be obtained by measuring the displacement made by the movement of proof-mass with respect to the inner frame. When the proof-mass 3 moves, the capacitances of the sensing capacitor c9p, c9n will change due to the changes in the capacitor's gap. The displacement of the proof-mass can be obtained by measuring the difference of the capacitances of the capacitors c9p, c9n. As the output signal generated by the angular rate is an alternating signal, and the output signal generated by the acceleration is a low frequency or direct current signal, a signal processing method can be applied to separate the angular rate signal from the acceleration signal.

[0003] The proof-mass 3 and its sensing beams form a z-axis mass-spring vibration system. Similarly, the unit, consisting of an inner frame 5, sensing beams 4, and proof-mass 3, together with its driving beams 6, forms a y-axis mass-spring vibration system. As the amplitude of the vibration generated by the driving force of the driver is small, the resonance effect of a vibration system is used to amplify the amplitude. The amplification ratio Q is related to operating frequency and damping coefficient. The closer the operating frequency of the driver is to the resonance frequency of the vibration system, the larger the ratio Q. Similarly, the amplitude generated by the Coriolis force must rely on the resonance effect for amplification. Because the vibration frequency generated by Coriolis force is the same as that of the driving force, the resonance frequency of the sensing axis must be the same as that of the driving axis in order to generate sufficiently large output signals.

[0004] The major drawback of the aforementioned sensors is in the manufacturing process of the driving beams. As shown in FIG. 2, the etching is first applied on both sides of the silicon chip. As the speed of silicon wet etching is related to the lattice direction, the etching is slowest along the <111> direction. It is virtually impossible to etch along this direction. Hence, the initial stage of the etching would be as shown in FIG. 2(a). The slant lines represent the (111) facets. If the etching continues, it will proceed along the <110> direction from the intersection of two (111) facets, as shown in FIG. 2(b). FIGS. 2(c)–2(e) show the side views of different stages when both sides of the driving beam are etched. When the etching is perpendicular to the surface, the process should stop and the silicon chips should be removed from the etching solution. However, as there is no automatic mechanism to stop the etching as in the (111) facet, it is hard to control the width of the driving beam. The width of the driving beam affects the coefficient of elasticity, which in turn will affect the resonance frequency. If the width of the driving beam is not accurate, the resonance frequency will be different from that of the sensing beams, and deviate from the original design. For vibration systems with larger Q values, the tolerance of the deviation is smaller. This poses a major problem for the quality of the products.

[0005] The major features of the present invention are: (1) dual inertial sensors made by etching (110) silicon chip whose width can be accurately controlled during the etching process; (2) the design to reduce the air damping of the (110) silicon proof-mass; (3) preventing the proof-mass from sticking to glass sheets; and (4) built-in temperature sensing capacitors placed in the chip area unaffected by the inertial force, and compensating the temperature error in the dual inertial sensors by direct measurement of the temperature changes in the chip.

[0016] FIG. 3 shows the top view of a dual inertial sensor in accordance with the present invention. Its structure is made by wet etching a (110) silicon chip 11. It is shaped as a parallelogram, instead of a rectangular shown in FIG. 1. The sides of all the components are in parallel with the intersection line of the (110) silicon chip surface and the silicon lattice {1-1-1} facet or {1-11} facet. Any two sides form an angle of 109.48° or 70.52°; hence most of the components are shaped as parallelograms. Because the two {111} facets, namely {1-1-1} and {1-11}, of the (110) silicon chip are perpendicular to the {110} facet, and conventional KOH and EDP etching solutions etch {111} facet much slower than {100} or {110} facets, a vertical surface can be obtained by keeping the protective mask aligned with the intersection line of the (110) silicon chip surface and the {1-1-1} facet or the {1-11} facet during the etching process. In this embodiment, the structure comprises an outer frame 2. Inside the outer frame 2 there is an inner frame 5, and the inner frame 5 further comprises a proof-mass 3. The proof-mass 3 is connected to the inner frame 5 by a plurality of sensing resilient beams 4, and the inner frame 5 is connected to the outer frame 2 by a plurality of driving resilient beams 6. The sensing resilient beams 4 facilitate the proof-mass 3 to move perpendicular to the surface of the silicon chip (defined as z-direction), and the driving resilient beams 6 facilitate the inner frame 5 and the proof-mass 3 to move in parallel to the surface of the silicon chip (defined as y-direction). Two glass sheets 71, 72 (not shown in FIG. 3) are placed on both sides of the silicon chip 11, and connected to the outer frame 2. The thin metal film electrodes 81, 82 are electro-plated on the two glass sheets 71, 72 facing the silicon chip surface and the inner frame 5. The two thin metal film electrodes 81, 82 of each glass sheet, with the corresponding surface of inner frame 5, will form edge effect electrostatic driving capacitors c8p, c8n. A thin metal film electrode 9 is electrode-plated on the glass sheets 71, 72 facing the proof-mass 3. The thin metal film electrode 9, with the surfaces of the proof-mass 3, will form two sensing capacitors c9p, c9n. The alternating driving voltage on the driving capacitors c8p, c8n will make the inner frame 5 and the proof-mass 3 vibrate along y-axis. If there is an angular velocity Ω along x-axis, there will be a Coriolis force F making the proof-mass 3 vibrate along z-axis. The angular rate can be obtained by measuring the amplitude of the z-direction vibration of the proof-mass 3. If there is an acceleration applied along the z-axis, the specific force will move the proof-mass 3 with respect to the inner frame. The acceleration can be obtained by measuring the displacement made by the movement of the proof-mass with respect to the inner frame. When the proof-mass 3 moves, the capacitances of the sensing capacitors c9p, c9n will change due to the changes in the distance. The displacement of the proof-mass can be obtained by measuring the difference of the capacitances of the sensing capacitors c9p, c9n. As the output signal generated by the angular rate is an alternating signal, and the output signal generated by the acceleration is a low frequency or direct current signal, a signal processing method can be applied to separate the angular rate signal from the acceleration signal.

[0017] In this embodiment, the thickness of the sensing beam 4 can be controlled by the automatic etching stop of the p+ doped layer. On the other hand, the driving beam 6 is in parallel with the intersection line of the (110) silicon chip surface and {1-1-1} facet or the {1-11} facet, hence the etching can be automatically stopped to provide the accurate width of the driving beam 6. Therefore, the difference between the resonance frequency of the driving vibration system and that of the sensing vibration system can be accurately controlled.

[0018] The driving force of the capacitor edge effect electrostatic driver is proportional to the length of the capacitor. To increase the driving force, it must increase the effective edge length. The driver can be designed in an integrated style, as shown in FIG. 4a. By widening the two sides 51, 52 of the inner frame 5, and etching a plurality of long trenches or slits 5t, two driving blocks 51, 52 are formed, and each trench or slit 5t can provide two edge effect drivers. The surfaces of glass sheet 71, 72 facing the surface of the corresponding driving block 51 have two sets of long stripe electrodes 81, 82, which interpose each other, and are parallel to the long trenches or slits 5t and connected to bond-pads 81p and 81n, respectively, as shown in FIG. 4.b. The relative position between long trenches or slits 5t on the driving block 51, and its corresponding stripe electrodes 81, 82 is shown in the cross-sectional view in FIG. 4.c. Two driving capacitors c81p, c81n are formed. Similarly, the surfaces of glass sheet 71, 72 facing the corresponding driving blocks 52 have two sets of long strip electrodes 81, 82 which interpose each other, and are parallel to the long trenches or slits 5t and connected to bond pad 82p and 82n, respectively. The surface of the driving block 52 and its corresponding long electrodes 81, 82 also form two driving capacitors c82p, c82n. By adjusting the phase of the external alternating voltage of the driving capacitors c81p, c81n, c82p, c82n to make the drivers exerting the force in the same direction, the driving force is increased for several times. The driving capacitors can also be used in measuring driving amplitude and to feedback for controlling driving amplitude.

[0019] FIG. 5a shows a preferred embodiment of a silicon dual inertial sensor of the present invention. The silicon dual inertial sensor comprises two structures shown in FIG. 3 and is also made by wet etching on a (110) silicon chip 11. This embodiment comprises an outer frame 2, which further comprises two inner frames 5a, 5b, a central anchor 21, and a plurality of connecting blocks 22. The inner frames 5a, 5b have proof masses 3a, 3b, respectively, which are connected to the corresponding inner frame by at least a sensing resilient beam 4. Each inner frame is connected to two common connecting beams 61 by at least a driving resilient beam 6, and then connected to the central anchor 21 by the common resilient supporting beam 60. Each surface of the silicon chip, with the exception of the areas of the outer frame 2, the central anchor 21 and the connecting blocks 22, is etched for about 3 mm. The sensing beams make it easier for the proof-masses 3a, 3b to move perpendicularly to the surface of the silicon chip (the z-axis), and the driving beams make it easier for the inner frames 5a, 5b to move along one of the directions of the surface of the silicon chip (the y-axis). The two sides 51, 52, perpendicular to the y-axis, of the inner frames are the driving bodies, and each surface comprises a plurality of long trenches or slits 5t, perpendicular to the y-axis.

[0020] Two glass sheets 71, 72 are placed on the front side and the back side of the silicon chip 11, respectively. The glass sheets 71, 72 are combined with the outer frame 2, the central anchor 21 and the connecting mass 22. The surface of each glass sheet facing the corresponding driving body 51 has two sets of long electrodes 81, 82, which interpose each other, and are parallel to the long trenches or slits 54 and connected to bond pads 81p and 81n, respectively, as shown in FIG. 4.b. The relative position between long trenches or slits 5t on the driving body 51, and its corresponding long electrodes 81, 82 is shown in the cross-sectional view in FIG. 4.c. Two driving capacitors c81p, c81n are formed. Similarly, the surface of each glass sheet facing the corresponding driving body 52 has two sets of long electrodes 81, 82, which interpose each other, and are parallel to the long trenches or slits 5t and connected to bond pads 82p and 82n, respectively. Each surface of each driving body 52 and its corresponding long electrodes 81, 82 also form two driving capacitors c82p, c82n.

[0021] The surfaces of the glass sheets 71, 72 facing the surfaces of each proof-mass are electroplated with metal thin film electrodes 9, which are connected to bond pads 9p, 9n, respectively, and form sensing capacitors c9p, c9n with the surfaces of each proof-mass.

[0022] By adjusting the phase of the external alternating voltages on the driving capacitors, it is possible to make the proof-masses 3a, 3b move in the opposite directions of y-axis. If there is a rotating angular speed Ω in the x-axis direction, there will be a Coriolis force to move the proof-masses 3a, 3b in the opposite direction of z-axis. If an acceleration is input along the z-axis, the specific force will move the proof-masses 3a, 3b along the z-axis. When the proof-masses 3a, 3b move, the capacitance of the sensing capacitors c9p, c9n formed by the proof-masses and the metal thin film electrodes 9 on the glass sheets 71, 72 will change due to the distance change. The displacement (distance) of the proof-masses 3a, 3b can be calculated by measuring the capacitance difference between the sensing capacitors c9p, c9n. The output signal from the rotation rate is an alternating signal, and the output signal from the acceleration is a direct signal. These two signals can be separated by signal processing technology. The electrodes 9 of the sensing capacitors c9p, c9n can also be partitioned, as shown in FIG. 5b, into a feedback electrode 9f, and its bond pad 9fb for the gyroscope to rebalance the Coriolis force.

[0023] To reduce the air damping due to squeeze film caused by the vibration of the proof-mass 3, 3a or 3b along the z-axis and increase the resonance amplification ratio Q of the z-axis vibration system, the surface of the proof-mass 3 can be etched to form a plurality of long trenches or slits 3t parallel to the side of the proof-mass, as shown in FIG. 6. Because the present invention uses (110) silicon chip, which is easy for vertical deep etching, the etching of long trenches or slits will not use a large area for sensing capacitors, and can increase the sensitivity of the output signal, as compared with the (100) silicon case. If the air resistance is still too large, it must be partially vacuumized.

[0024] To avoid the stickiness problem in the process of bonding the silicon chip and the glass sheets, a plurality of small bumps 3s and its insulation layer 3i can be provided on the surface of the proof-mass 3 for separation, as shown in FIG. 6.

[0025] To enhance the bonding force between the silicon chip and the glass sheets, the part of the silicon chip which does not interfere with the movement of components should be kept as the connecting block 22 in FIG. 5.a to attach to the glass sheets.

[0026] To eliminate the impact of temperature on the output signals, and to improve the performance of the gyroscope and accelerometers, a small recessed area TS1 can be etched on the surfaces of the outer frame 2, the central anchor 21, or the connecting blocks 22. A metal thin film electrode TS2 is electroplated on the corresponding surface of the glass sheet, and connected to the bond pad TS to form a capacitor cTS for sensing temperature. Because it is affected by neither the rotation rate nor the acceleration, and only affected the temperature, it can be used to compensate for the input of the temperature on the capacitors c9p, c9n, which are for sensing the inertial force.

[0027] As shown in FIG. 5.a, because the silicon chip is electrically conductive, a small recessed slit st must be etched on the surfaces of the outer frame 2, where the thin film metal wire passes, in order to avoid short circuit.

[0028] Based on the design of the two embodiments, there can be many different designs and combinations. For example, the capacitor gap between the proof-mass and the two glass sheets can be replaced by recessed areas etched on the surface of the glass sheets. The layout of the resilient beams can also be varied, for example, the driving resilient beams 6 of the inner frame can also be connected to the outer frame 2.

[0029] In summary, the present invention discloses a dual inertial sensor made of a (110) silicon chip with vertical deep etching. The width of driving beam and the driving resonance frequency can be precisely controlled to improve the yield rate and the performance of the gyroscope. It also provides the other features: a design to reduce air resistance to improve the resonance amplification ratio of the sensing axis; a design for preventing the stickiness problem between the proof-mass and the glass sheets; and at least a built-in temperature sensing capacitor for real time measurement of temperature change in the chip to improve the temperature effect of the dual inertial sensors.

[0030] While we have shown and described the embodiment in accordance with the present invention, it should be clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention.