Sensor using an actuator for self-test and method therefor

[00002] This invention relates to sensing structures, and more particularly to sensing structures having a movable mass, wherein the sensing structure has a self-test capability.

[00003] Accelerometers are a common use for sensing structures that have a movable mass. One common use of accelerometers is in airbags in cars. In such use, it is important that the accelerometer be functional. One way this is ensured is by performing a self-test regularly. One convenient time for this is when the car is started. Thus, when the car is started a self-test is performed to verify functionality of the accelerometer.

[00004] The accelerometers have had to increase in complexity due to system requirements such as having to detect acceleration in two dimensions instead of just one. This increases the complexity of the accelerometer as well as the accompanying self-test requirements. The increased complexity puts pressure on the size of the accelerometer. There is the continuing desire for the accelerometer to be small but also for it to be accurate. The smaller size makes for the mass of the movable mass to be smaller. The smaller movable mass makes it more difficult to have a sufficiently accurate accelerometer.

[00005] Thus, there is a need for a sensing structure with self-test capability with improved accuracy.

[00011] Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.

[00012] In one embodiment a self-test structure is placed in the middle of a sensing structure that utilizes a movable mass, which is also sometimes referred to as a proof mass. The sensing structure has portions aligned in two directions that are orthogonal to each other. The self-test structure is an actuator made up of one or more fixed electrodes that can apply electrostatic force to the movable mass. The self test actuator is designed to apply a force along a line that is diagonal to the two directions, thereby reducing the number of fixed electrodes required for the actuator that is used for self-test. The reduced number of fixed electrodes required for self-test results in more mass for the movable mass, thereby improving sensitivity of the sensing structure. This is better understood with reference to the drawings and the following description.

[00013] Shown in FIG. 1 is sensing structure 10 of the prior art that is helpful in understanding the invention. Sensing structure 10 comprises a substrate 11 and a movable mass 12 mounted to the substrate by springs 62, 64, 66, and 68 at corners of movable mass 12. These springs are designed to allow the movable mass to move in any plane (X-Y) direction. Movable mass 12 has a plurality openings comprising openings 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, and 36. Mounted on substrate 12 are a plurality of pairs of fixed fingers comprising fixed fingers 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, and 60 located in openings 14-36, respectively. On each side of movable mass 12 are a pair of openings with a pair fixed fingers inside. For example, on the left side of movable mass 12 are openings 14 and 16 having therein fixed finger pairs 38 and 40, respectively. For clarity in the drawing, only two openings are shown on each side, but many are preferable. For example, 48 on each side has been found to be effective. The openings 14-28 and fixed finger pairs 38-52 form capacitors between the movable mass and the fixed fingers, and these capacitors change when movable mass 12 moves in relation to substrate 11. The openings and fixed finger pairs on opposing sides provide acceleration detection in each of the two orthogonal directions. Thus openings 14, 16, 22, and 24 are used for detecting acceleration in a first direction (first axis of lateral movement) and openings 18, 20, 26, and 28 are for detecting acceleration in a second direction (second axis of lateral movement) orthogonal to the first direction. Movable mass 12 is preferably doped polysilicon but could be another material. Substrate 11 is preferably monocrystalline silicon but could also be another material that is suitable for forming fixed finger pairs thereon.

[00014] A self-test structure in the middle of movable mass 12 comprises openings 30-36 having therein fixed finger pairs 54-60, respectively. In the case of the self-test structure, the openings and fixed finger pairs are also aligned in the first and second directions. Openings 34 and 36 are for the first direction and openings 30 and 32 are for the second direction. The self-test structure is designed to move movable mass 12 in response to a bias voltage applied between the fixed fingers and the movable mass. The self-test structure in effect is an actuator, converting electrical signal to physical movement. For example, a bias applied to fixed pairs 58 and 60 relative to the movable mass will achieve movement in the first direction. This movement causes a change in capacitance in the relationship between openings 14, 16, 22, and 24 to fixed finger pairs 38, 40, 46, and 48, respectively. For example, if movable mass 12 moves upward, the edges of opening 14 move in the same way with respect to fixed finger pair 38, which comprises a lower fixed finger 70 and an upper fixed finger 70. The lower edge 74 of opening 14 moves closer to lower finger 70, and the upper edge 76 moves further from the upper fixed finger 72. This causes a change in the capacitance that can be measured to verify that movable mass 12 is still movable and that the capacitance change caused thereby can be measured for the first direction. This process is repeated for the second direction by fixed finger pairs 54 and 56 causing movement of movable mass in the second direction and such movement being detected by causing capacitance change in the relationship of openings 18, 20, 26, and 28 to fixed finger pairs 42, 44, 50, and 52, respectively. Thus there is a set of openings for self-test for each direction being tested. In this case only two openings and fixed finger pairs are shown for the self-test for each direction for clarity of the drawing. In practice, it is preferable to have more, for example, ten for each direction.

[00015] Note that a portion 75 of movable mass 12 is between fixed finger pairs 14 and 16. Between each pair of fixed finger pairs has the same type of structure between them as portion 75. Portion 75 is continuous, isthmus-like in being connected to two sections of the movable mass 12. As an alternative, this portion 75 could instead be connected to only one side of movable mass 12 and thereby be peninsula-like and perhaps called a finger. In practice, with perhaps 48 fixed finger pairs on a side, most of the fixed finger pairs will have a portion such as portion 75 on both sides. It is the change in location of these portions that is measured by measuring the capacitance change due to the change in location.

[00016] Shown in FIG. 2 is a sensing structure 100 according to an embodiment of the invention in which common structures to that of FIG. 1 retain common numerals. Sensing structure 100 comprises a different movable mass 102 because of the replacement of the self-test structure of FIG. 1 being replaced by self-test structure 104. Self-test structure 104 comprises openings 106, 108, and 110 having therein fixed finger pairs 112, 114, and 16 on substrate 11. Self-test structure 104 is aligned diagonally with the first and second direction. The self-test operates in substantially the same way as described for FIG. 1 except that both the first direction and the second direction are achieved in a single application because of the diagonal alignment. Thus, application of a bias to fixed finger pairs 112, 114, and 116 results in movement of movable mass 102 in the diagonal direction. The diagonal direction in this case is at 45 degrees to both the first and second direction but could be selected to be a different angles with respect to the first and second directions. This diagonal movement causes movement of movable mass 102 in both the first and second directions that can be measured as a result of the capacitance change caused by the change in location of movable mass 102.

[00017] The distance in the diagonal direction, at 45 degrees, to achieve the desired movement in the first and second direction is the square root of two (approximately 1.4) times the desired movement in the first and second direction. This increases the force required by the same multiplier, the square root of two. The movement is spring held so the force required increases linearly with distance. Thus with an increase in distance of 1.4, the force applied to movable mass 102 must be increased by 1.4. This is achieved by increasing the number of openings for self-test structure 104 by 1.4, but only for the diagonal direction, not two directions. Thus, the ratio is 1.4 to 2 for the total number of openings required for the self-test structure for the diagonal case of FIG. 2 as compared to the self-test structure of FIG. 1 that has separate openings for the two directions. Thus, for the same movement in the two directions there is a 30 percent reduction (100)(2-1.4)/2 in the space required for the self-test structure. The total number of openings for the self-test structure would thus go from 40 (20 for each direction) to 28 (20×1.4). The result is that for a given dimension of movable mass 102 there is a reduction in the total amount of opening in movable mass 102, thereby increasing the mass of movable mass 102. The calculated improvement is a 7 percent increase in mass with a corresponding 7 percent increase in resolution for a given set of dimensions of movable mass 102.

[00018] Of course all of the openings do not have to the same size so the benefit in the reduction in needed openings as described above can instead be achieved using different size openings suitable for the space available. The space saving will be achieved in total opening space not necessarily in the count of openings.

[00019] The method of performing the self-test using this structure of FIG. 2 is shown in method 300 of FIG. 3. The first step shown, step 302, is to apply a DC bias to the fixed fingers of the self-test structure to move movable mass 102 diagonally. The next step shown, step 304, is to sense the change in location of movable mass 102 along the X-axis, which can be considered the first direction. The next step, step 306, is to sense the change in location of movable mass 102 along the Y-axis, which can be considered the second direction. The sensing of steps 304 and 305 can be done simultaneously or reversed in order. After the change of movable mass 102 in both the X and Y axes has been measured, the DC bias is removed as shown in step 308. This completes the self-test. The outcome of the self-test will be either a pass or a fail based on the change in location measurements.

[00020] Shown in FIG. 4 is a block diagram of circuitry 400 for performing the self-test using the self-test structure of FIG. 2 comprising sensing structure 100, self-test circuit 404 coupled to sensing structure 100 and sensing circuit 406 coupled to sensing structure 100. The self-test circuit 404 is coupled to the fixed finger pairs of self-test structure 104, and sensing circuit 406 is coupled to fixed finger pairs 38-52. This is substantially the same as in the prior art except for the presence of sensing structure 100 in place of prior art structures. The application of DC bias to the fixed finger pairs of the self-test structure 104 is achieved by applying a bias to one finger, the shield finger, of the pair at a voltage that is the same as that applied to movable mass 102. This is preferably about 2.5 volts, but another voltage can be used. A different voltage is applied to the other finger, the active finger, of the pair. This is preferably at about 1.5 volt but could be a different voltage as well. The shield finger, which receives the same bias as the movable mass 102, operates as a shield to avoid acting to counter the force being asserted by the active finger.

[00021] In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention.

[00022] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not including only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.