ES2221519A1 - Integrated optical silicon accelerometer, has multiple waveguides defined in rigid zones, and silicon inertial mass fastened by four bridges - Google Patents

Abstract

Integrated optical accelerometer. The present invention is based on a silicon optical accelerometer with waveguides integrated into the die itself, but with the novelty that the waveguides are only defined in the rigid areas of the accelerometer, thus avoiding negative effects at the level of mechanical stresses that They can make these guides on mobile structures. The proposed device consists of an inertial mass of silicon held by four bridges and with a centered waveguide, in such a way that cross accelerations are avoided. The operating principle of the accelerometer is based on the misalignment of facing waveguides. From a technological point of view, BESOI (Bond and Etch Back Silicon On Insulator) wafers can be used to facilitate manufacturing and allow greater control over the structures and therefore over the performance of the devices.

Description

Integrated optical accelerometer.

Technical sector

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Sensors and microelectronic devices

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Systems for acceleration and vibration detection, inclinometers.

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Applications in various areas: applications in navigation, medicine, ballistics, vibration measurement, modal analysis and fields related.

State of the art

The field of application of accelerometers is very spacious. This fact has motivated the need to do devices with high performance at a low cost, originating a great boom and increase in commercial accelerometers based in silicon micromachining technologies, at the expense of piezoelectric accelerometers with a much higher price.

The first silicon accelerometer was developed by Roylance in 1979 [L. M Roylance and J. Angel, A batch-fabricated silicon accelerometer, IEEE Trans. on Electron Devices, ED-26. pp. 1911-1917, 1979] . The main types of silicon accelerometers according to their sensor elements are: piezoresistive [L. M Roylance and J. Angel, A batch-fabricated silicon accelerometer, IEEE Trans. on Electron Devices, ED-26. pp. 1911-1917, 1979] and the capacitive [F. Rudolf, A micromechanical capacitive accelerometer with two point inertial mass suspension, Sensors and Actuators, 4, pp. 181-198, 1983.] . Piezoresistive accelerometers suffer from a great dependence on temperature which makes it necessary to implement a temperature compensation system. On the other hand, capacitive accelerometers do not depend on temperature but, due to the small values of the capacities that can be obtained in microelectronic capacitive sensors, they have many problems with electromagnetic interference.

In some applications it is necessary that the accelerometers be insensitive to the electromagnetic radiation to which they may be subject. Optical accelerometers have the advantage of being immune to electromagnetic radiation and the effects of temperature [KEBurcham, GN Branbander and JTBoyd, SPIE vol. 1793 Integrated Optics and microstructures, pp. 12-18,1992] .

The control of the damping of the structure and the crash protection systems give the accelerometers better performance. As an example, a damping control and a shock protection system was designed by NOVASENSORS using a technology based on direct silicon-silicon welding to define the shock protection system and silicon-glass welding to control damping [PW Barth, F. Pourahmadi, R. Mayer, J. Poydock, K. Petersen, A monolithic silicon accelerometer with integral air damping and overrange protection, Technical Digest IEEE, Solid-state sensor and actuator, workshop, Hiltom Heads Island, South Carolina, June 6 -9, pp. 35-38, 1988] .

Automatic testing systems give devices the ability to determine their correct operation during operation. The development of automatic testing systems is of great interest in the case of accelerometers [HV Allen, SC Terry and DW de Bruin, Accelerometer systems with selfs-testable features, Sensors and Actuators, 20, pp. 153-161, 1989] .

In some applications it is necessary to obtain the acceleration in 2 or in the 3 directions of the acceleration vector. Most biaxial or triaxial micro accelerometers are piezoresistive [Patent: 2 137 847, JA. Plaza, J. Esteve, E. Llora-Tamayo, "Triaxial Accelerometer"] [GI Andersson, A novel 3-axis monolithic silicon accelerometer, Proceedings Transducers'95-Eurosensors IX, Stockholm, Sweeden, June 25-29, 1995., 1995] or capacitive [T. Mineta, S. Kobayashi, Y. Watanabe, S. Kanauchi, I. Nakagawa, E. Suganuma and M. Esashi, Three-axis capacitive accelerometer with uniform axial sensitivities, Proceedings Transducers'95-Eurosensors IX, Stockholm, Sweeden, June 25 -29, 1995.] , which, in addition to the disadvantages of piezoresistive and capacitive systems, have very low sensitivities.

The possibility of avoiding the effects on temperature and the effects of electromagnetic interference motivated the study of optical accelerometers, passive from the electrical point of view, based on optical fibers [US Patent 5437186], [D.Uttamchandani, D.Liang, and B.Culshaw, "A micromachined silicon accelerometer with fiber optic interrogation", Proceedings of SPIE Integrated Optics and Microstructures, vol. 1793, pp. 27-33, 1992], [J.Marty, A.Malki, C. Renouf, P.Lecoy, and F. Baillieu, "Fiber optic accelerometer using silicon micromachining techniques", Sensors and Actuators A, vol. 46-47, pp. 470-473, 1995] . The use of optical fibers on the moving parts of the microsystems presents the problems of their bonding, alignment and induced mechanical stresses.

Microelectronic technology allows the integration of waveguides into microelectronic devices making the sensors much more compact. These devices can work both interferometrically due, for example, to the photoelastic effect [M. Ohkawa, M. Izutsu, and T. Sueta, "Integrated Optic accelerometer Employing a Cantilever on Silicon Substrate," Japanese Journal of Applied Physics, vol. 28, no 2, pp. 287-288, 1989.] as potentially [US Patent 5437186], [KEBurcham, GNBrabander, and JTBoyd, "Micromachined silicon cantilever beam accelerometer incorporating an integrated optical waveguides", Proceedings of SPIE Integrated Optics and Microstructures, vol. 1793, pp. 12-18, 1992] . The integrated waveguides are usually made of dielectric and / or semiconductor materials or equivalent layers. The growth or deposition of these layers on the weak parts of the accelerometers presents the problem of the introduction of intrinsic or thermal mechanical stresses, due to the different coefficients of expansion of the different materials, negative for the operation of the devices.

Description of the invention Brief Description of the Invention

The present invention relates to a device called optical accelerometer, characterized by avoiding guides wave or optical fibers on the little rigid parts of the structure like bridges, which consists of a mass held by the adequate number of bridges to the chip frame, in the mass of the chip waveguides are defined or optical fibers are joined that are will shift depending on the acceleration, causing a misalignment with waveguides or fibers in the chip frame. The main novelty of the proposed sensor is the idea of avoiding have waveguides or optical fibers on the little rigid parts of the sensor such as bridges or overhangs; avoiding in this way possible mechanical or thermo-mechanical effects of fibers or guides on the operation of the sensor. The guides of wave or fibers are placed exclusively on the rigid parts of silicon.

As a consequence of the principle of operation of the device, it is equipped with a system of intrinsic test. Since a sensor break would motivate a very important loss of power when the guides of wave. This fact does not require the implementation of no additional test system with consequent savings economic.

The reduction of certain areas where the guides go or fibers can allow to distinguish between positive accelerations or negative

The device can be manufactured on any type of micromachinable substrate, although the use of BESOI wafers (Bond and Etch back Silicon On Insulator) allows the realization of shock protection systems easily. The control of the damping can be carried out by joining the silicon accelerometer a piece with cavities of determined thickness just below the sensor. These cavities could be made in glass wafers that can be welded anodically to silicon wafers.

Integration of waveguides with curves of 180 ° allows to have the optical input and output on the same side of the chip, greatly facilitating polishing and optical connection of the devices.

The principle of operation of the sensor also It allows the principle of resonant type detection.

Placing different waveguides or fibers on the structure can be achieved biaxial accelerometers or triaxials

Detailed description of the invention

The structure that has been designed is made preferably on silicon, given its excellent properties mechanical, although it can also be manufactured with other materials. The detection principle is optical. The structure is formed by a mass (2), although they can be more, attached to the chip frame preferably by 4 bridges (1) although it may be a larger number or smaller of bridges, see figure 1.

The principle of acceleration detection is optical and is based on power loss due to a misalignment between waveguides integrated in the same chip or optical fibers. One or more waveguides (3), figure 2a, or fibers optics (3), figure 2b, are in the mass moving but rigid part of the structure (2) and the inlet and outlet guides or fibers they are in the frame (5) which is also a rigid part of the structure. Waveguides or optical fibers are never placed on the bridges (1) or weak parts of the structure. Placement of fibers or waveguides on weak parts motivates the appearance of mechanical stresses on weak parts, these efforts can be intrinsic to the materials that conform or induced by changes in temperature, since the materials used have thermal expansion coefficients different from silicon. These efforts can motivate evil operation of the devices.

In the case of designing the device with guides integrated wave on the mass (2) and frame (5) of the device, these will be connected by means of optical fibers (4) to the outside of the device, figure 2a.

The distance between waveguides or fibers optical mass (2) and frame (5) should preferably be as low as possible to reduce light losses due to divergence of the laser beam when leaving them.

When the structure is subjected to acceleration, depending on the direction of it, the structure deforms in a certain way. The mass (2) of the device moves causing misalignment between waveguides or fibers optical (3) on the mass and those on the frame. For accelerations perpendicular to the plane of the structure in the case of structures of 4 bridges the mass goes up or down flat depending on of the direction of acceleration, see figure 3a. This displacement produces a loss of power due to misalignment of optical guidance structures that form the accelerometer.

Decentration of waveguides or fibers in the device on the mass and frame by whatever means necessary to discriminate between positive accelerations and negative If you want to discern between positive accelerations and negative guides can be offset that move (in the mass 2) of the fixed ones (in frame 5), serve as an example to perform a small recess in the dough (2) where the guide or fiber will be placed that moves. Properly choosing the recess can be Offset the guides to make them work in the curve area shown in figure 3b as desired. Will be placed preferably the guides centered on the plane of the dough for prevent structures from being sensitive to accelerations in The device plane.

Biaxial optical accelerometer in which they are defined at least two additional waveguides at the ends of the mass to detect accelerations in the device plane. For the additional detection of accelerations in the direction and, see figure 4a and 4b, fibers or waveguides will be placed at the ends of the moving mass (2). The central guide (6) can detect accelerations in the z-direction and the lateral guides (7) can detect accelerations in the direction and. When an acceleration is applied in the direction and, two of the bridges bend down and the two opposite up. Due to the symmetry of the structure in the xz plane, the center guide or fiber is not misaligned while than those on the sides if, giving information on the value of the acceleration in the direction and. The lateral guides will be located preferably as far as possible from the center of the structure to maximize the sensitivity of the structure.

Triaxial optical accelerometer in which they are defined at least 3 additional waveguides perpendicular to the defined above, which allows the detection of all three Vector acceleration components. For additional detection of accelerations in the x direction perpendicular fibers will be placed as shown in figure 4c. Operating principle for accelerations in the z direction and in the y direction is like in the previous case. When the structure is submitted a acceleration in the x direction, two opposite bridges move down and the remaining two up so that the power through the fibers (9) decreases while maintaining that of the guide (8). The reduction of the areas where the fibers go or guides in mass 2 for off-centering of them (to be able to discriminate between up and down displacements) could facilitate signal processing to calculate each  of the components of the acceleration vector.

The operating principle based on loss of power gives the device a functional test intrinsic, since a rupture of the sensor would produce the total misalignment of waveguides or fibers (3) in the mass (2) and under the chip (5). In this case the output power it would fall almost to zero indicating the sensor break.

Waveguides integrated in the device with curves to have the optical input and output in the directions that are required. Integration of waveguides with curves allows to have the sensor's optical input and output on the side of the chip that is convenient, see figure 5. Waveguides integrated into the device with curves to have the input and the optical output on the same side of the chip. In particular the preferably integration of waveguides with 180 ° curves it allows to have the exit and the entrance in the same side of the chip, Figure 5b, which simplifies the polishing of the chip, since there is only Than polish one side of it. It also facilitates optical connection of the encapsulation.

Detailed explanation of the drawings

Figure 1. Drawing of the mechanical structure for Acceleration detection. The structure consists of a mass (2) fastened to the chip frame by 4 jumpers (1), see figure la, or fastened by 2 bridges (1), see figure 1b. The chip frame is not shown in the figure.

Figure 2. Drawing of the mechanical structure with the waveguides (3), figure 2a, or optical fibers (3), figure 2b, on rigid parts of the structure such as mass (2) or frame of the chip (5). The connection of the device with the outside is made through optical fibers (4).

Figure 3. Figure 3a shows a section of figure 2 through the guides or fibers. By submitting to accelerometer to acceleration the mass and thus the fiber or guide on it moves up or down in such a way that the produced displacement causes the output power to decrease, figure 3b.

Figure 4. Figure 4a shows the deformation of the structure for accelerations perpendicular to chip plane, z direction. The guide (6) on the dough moves thus decreasing the intensity of light at the exit. In figure 4b the deformation of the structure for accelerations in the address and. The guides (7) suffer a decentralized and a loss of power while the guide (6) does not vary. In figure 4c it is shows the deformation of the structure for accelerations in the x direction. The guides (9) are decentralized and suffer a loss of power while the guide (8) does not vary.

Figure 5. Figure 5a shows a guide configuration that allows light output and input on two adjacent sides of the chip. And Figure 5b shows a configuration that allows the entry and exit of light through it chip side

Figure 6. Schematic drawing of a section and section of the accelerometer welded to a glass (10). He glass has a cavity to allow the displacement of the mass (2). The depth of the cavity (a) serves to control the accelerometer damping.

Figure 7. Graph in which the voltage applied to a piezoelectric pickup to apply a sinusoidal acceleration to the accelerometer (upper graph). And the voltage output proportional to the optical output power of the accelerometer

Figure 8. Schematic drawing of a section and section of the accelerometer with the offset of the guides of wave or fibers located in the mass (2) with respect to the guides of wave or fibers in the frame (5). Through a recess in the dough (2), this and the frame (5) are no longer in the same plane with which when placing the guides or fibers (3) on the dough they have a small initial misalignment that makes the accelerometer work in the lower upper area of figure 3.b. This way you get  differentiate between positive and negative accelerations.

Example of embodiment of the invention

An accelerometer device has been made optical characterized by avoiding waveguides or optical fibers over the little rigid parts of the structure like bridges, consisting of a mass attached by four bridges to the frame of the chip, waveguides are defined in the mass of the same will shift depending on the acceleration, causing a misalignment with waveguides or fibers under the chip.

Optical accelerometer characterized by combining in its manufacturing micromachining technologies in silicon volume and anodic welding techniques with glass wafers, in combination with definition techniques of integrated ARROW guides. Following the specifications set forth for the accelerometer uniaxial silicon micro accelerometers have been manufactured using a technology that combines the manufacture of waveguides integrated ARROW type and micromachining of silicon for manufacture the mobile structures.

Optical accelerometer characterized by combining in its manufacturing micromachining technologies in silicon volume with surface micromachining technologies using wafers BESOI (Bond and Etch Back Silicon on Insulator) and techniques anodic welding with glass wafers, in combination with techniques of definition of integrated ARROW guides. Technology Micromachining is based on BESOI wafers. This technology combines anisotropic silicon attack with a stop technique in the buried oxide layer of the BESOI wafer and a subsequent sacrificial attack of this oxide. Attack that allows definition of crash protection structures. Finally the wafers of Silicon are welded anodically to a glass wafer. The wafer of glass (10) apart from stiffening the structure allows the accelerometer damping control by small ones cavities (a) in the glass (10) under the moving parts of the accelerometer with mass (2), figure 6.

Accelerometers have been measured so dynamic. The results can be seen in Figure 7. In the upper curve shows the sinusoidal acceleration at which the accelerometer is subjected and the output of the  sensor. The period of the sensor output curve is double than the input since both for positive accelerations as negative the output power decreases, figure 3.b, and the excitation signal is sinusoidal with positive part and part negative. This phenomenon is easily solvable if you work only on one side of the graph in figure 3.b, offsetting the waveguides (3) on the mass (2) of those of the frame (5) a certain  distance (b), figure 8.

Claims (10)

1. Device called optical accelerometer, characterized by avoiding waveguides or optical fibers on the less rigid parts of the structure such as bridges, which consists of a mass held by the appropriate number of bridges to the frame of the chip, where in the mass of the Waveguides are defined or optical fibers are joined that will shift depending on the acceleration, causing misalignment with the waveguides or fibers in the frame of the chip. 2. Optical accelerometer device based on claim 1, characterized by avoiding waveguides or optical fibers on the less rigid parts of the structure such as bridges, consisting of a mass fastened by four bridges to the chip frame, in the mass of the same waveguides are defined that will move according to the acceleration, causing a misalignment with the waveguides or fibers in the frame of the chip. 3. Waveguides characterized by a curved shape that allows to have the optical input and output in the required directions. 4. Waveguides characterized by a curved shape that allows the optical input and output on the same side of the chip. 5. Optical accelerometer according to claims 1 and 2, characterized in that the wave chiases are offset on the mass and frame by the means necessary to discriminate between positive and negative accelerations. 6. Optical accelerometer according to claims 1 and 2, characterized in that the wave chiases are offset on the mass and frame by means of recesses of the areas where they are to be placed, to allow discriminating between positive or negative accelerations. 7. Biaxial accelerometer based on claim 1 wherein at least two waveguides are defined additional at the ends of the mass to detect accelerations in the plane of the device. 8. Triaxial accelerometer based on claim 7 wherein at least 3 waveguides are defined additional perpendicular to those defined in claim 7, which allows the detection of the three components of the vector acceleration. 9. Optical accelerometer based on claim 1 characterized by combining in its manufacture silicon volume micromachining technologies and anodic welding techniques with glass wafers, in combination with integrated ARROW guide definition techniques. 10. Optical accelerometer based on claim 9 characterized by combining in its manufacture silicon volume micromachining technologies with surface micromachining technologies using BESOI (Bond and Etch Back Silicon on Insulator) wafers and anodic welding techniques with glass wafers, in combination with definition techniques of integrated ARROW guides.