Abstract:
Methods and systems for creating microelectromechanical system (MEMS) gyros. The methods and systems include generating a map of motor bias and creating MEMS gyros based on the map of motor bias to achieve a higher yield of usable MEMS gyros per wafer. The systems include a processor with components configured to determine paths of optimal motor bias for a given deep reactive ion etcher on a wafer, a stepper for imprinting a pattern for each gyro in an orientation that corresponds to the path of optimal motor bias each gyro is calculated to be most near on the wafer, and a deep reactive ion etcher to etch the gyros in the wafer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Current methods for producing Micro Electro Mechanical System (MEMS) gyros using Deep Reactive Ion Etching (DRIE) technology result in subtle differences in the angle of some structural components of MEMS gyros. For example, if the angle of a flexure is not completely perpendicular to the plane of a MEMS gyro device, it will deviate out of place creating an error source when it is driven to oscillation. These differences are distributed across each wafer in a reproducible pattern for particular DRIE devices. The subtle differences are referred to in this application as motor bias. If this motor bias exceeds specified limits, it results in a loss of yield of the number of usable MEMS gyros per wafer.  
         [0002]     Thus, there is a need for reducing the effect of motor bias in the creation of MEMS gyros and creating a higher yield per wafer.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003]     The present invention comprises methods and systems for creating microelectromechanical system (MEMS) gyros by generating a map of motor bias and creating MEMS gyros based on the map of motor bias to achieve a higher yield of usable MEMS gyros per wafer. The systems include a processor with components configured to determine paths of optimal motor bias for a given deep reactive ion etcher on a wafer, a stepper for imprinting a pattern for each gyro in an orientation that corresponds to the path of optimal motor bias each gyro is calculated to be most near on the wafer, and a deep reactive ion etcher to etch the gyros in the wafer. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0004]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0005]      FIG. 1  illustrates a schematic view of a system in accordance with an embodiment of the present invention;  
         [0006]      FIG. 2  illustrates a schematic view showing additional components of the system shown in  FIG. 1  in accordance with an embodiment of the present invention;  
         [0007]      FIGS. 3-5  are flowcharts of a method of creating MEMS gyros in accordance with another embodiment of the present invention;  
         [0008]      FIG. 6  shows an example of a map of motor bias for one wafer;  
         [0009]      FIG. 7  is an example of a motor bias map showing paths of optimal motor bias in accordance with one embodiment of the present invention;  
         [0010]      FIG. 8  is an example showing the rotational distribution of gyros based on the optimal paths shown in  FIG. 7 ;  
         [0011]      FIG. 9  shows an example of MEMS devices produced on a wafer for dicing release; and  
         [0012]      FIG. 10  shows an example of MEMS gyros produced on a wafer in preparation for release by etching. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  illustrates a schematic view of an example system  18 . In one embodiment, the system  18  includes a testing system  20  and a manufacturing system  30 . The testing system  20  is configured to produce a map of motor bias for a given deep reactive ion etching (DRIE) device. The manufacturing system  30  uses the map of motor bias to produce an optimal path of motor bias for the DRIE device across a wafer. This optimal path is then rotated by one or more angles to produce additional optimal paths of motor bias that are related to the angle by which the initial optimal path of motor bias was rotated. The manufacturing system  30  is also configured to determine the best orientation for each gyro to be created on a wafer by determining the closest optimal path of motor bias in relation to each gyro on the wafer. The manufacturing system  30  is further configured to create MEMS gyros based on the determined orientation. The system  30  may include a gyro generating device that in some embodiments includes a DRIE device.  
         [0014]      FIG. 2  illustrates a more detailed schematic view of the testing system  20  and the manufacturing system  30  in accordance with another embodiment of the present invention. The testing system  20  includes a Deep Reactive Ion Etch (DRIE) device  22 , a testing device  24 , and a storage medium  26 . In some embodiments, the testing device  24  includes a probe stand and a probe mounted on the probe stand, both not shown. The manufacturing system  30  includes a computer  32  having a processor  33  and a memory  34  in data communication with a stepper device  35 . The manufacturing system  30  also includes a DRIE device  37 , a dicer  38 , and a packaging device  39 .  
         [0015]     In this embodiment, a wafer (not shown) having MEMS gyro devices in a single orientation prepared for the etching step, is placed in the DRIE device  22  for etching. After the MEMS gyros have been etched into the wafer, the testing device  24  is used to probe the gyros on the wafer. The testing results are recorded as a map of motor bias, and in some embodiments it may be stored on the storage medium  26  which is in data communication with the processor  33  at least a portion of the time. In this embodiment, the testing device  24  is in data communication with the processor  33  and as such, the computer  32  is able to store the testing results directly without use of the storage medium  26 . The testing results may reside in the memory  34  or the computer  32  may store these results on a hard drive or other secondary storage medium (not shown). The computer  32  contains a program residing in the memory  34  which instructs the processor  33  to determine a path of optimal motor bias based on the map of motor bias produced by the testing device  24 . The path of optimal motor bias is then rotated through one or more angles with respect to a coordinate system in the plane of the wafer and having its origin in the center of the wafer by a program residing in memory  34  which instructs the processor  33  to carry out the rotations and store the results. These one or more angles are determined based on the capabilities of a stepper to be used in imprinting patterns for the MEMS gyros on a wafer.  
         [0016]     After all desired paths of optimal motor bias have been created, the computer  32  determines the best orientation for each MEMS gyro to be created on a new wafer (not shown). The processor  33  determines which path of optimal motor bias is closest to each MEMS gyro to be produced on the wafer. These results are stored in the memory  34  or alternative storage. The computer  32  is then used to instruct the stepper device  35  to expose a wafer with each gyro in its proper orientation. In some embodiments, the stepper device  35  may expose blocks of gyros in a given direction rather than exposing each gyro individually. After the stepper device  35  has exposed the entire wafer of MEMS devices, by a photo lithography process for example, the wafer is placed in the DRIE device  37  for etching.  
         [0017]     After the wafer has been etched, the dicer  38  is used to separate the gyros from the wafer in this embodiment. In other embodiments, the gyros may be released from the wafer using other processes such as ultrasonic machining or etching through the entire wafer. After the gyros have been released from the wafer, the packaging device  39  packages the gyros in preparation for sale or use.  
         [0018]      FIG. 3  is a flowchart of a method for producing MEMS gyros. The method of this embodiment includes producing MEMS gyros on one or more wafers using a particular DRIE device at a block  100 . Next, at a block  200 , a map of motor bias is created for the one or more wafers, including a path of optimal motor bias. At a block  300 , MEMS gyros are produced based on the map created.  
         [0019]      FIG. 4  shows additional detail for the block  200  of  FIG. 3  in accordance with one embodiment of the invention. The step of creating a map of motor bias including a path of optimal bias is shown to include three more detailed blocks in  FIG. 4 . First, at a block  210 , a wafer is tested for values of the magnitude and direction of motor bias. This may be performed by placing the wafer in a position where it can be electrically probed to activate each MEMS gyro while the bias values are measured and stored. Next, at a block  220 , a map or relationship of spatial values in the plane of the wafer along with the motor bias values is produced. Then, at a block  230 , a path of optimal motor bias in the produced map is determined.  
         [0020]      FIG. 5  shows additional detail for the block  300  of  FIG. 3  in accordance with one embodiment of the present invention. First, at a block  310 , rotation angles are chosen. Next, at a block  320 , the path of optimal motor bias is rotated for each angle to produce a combined map of optimal motor bias paths. Then, at a block  330 , it is determined which optimal motor bias path is closest to each MEMS gyro to be produced on the wafer and the resulting values are stored along with their corresponding angles. This is followed by a block  340 , where a wafer with each MEMS gyro part rotated to the correct angle determined in the preceding block is exposed by a stepper. Then, at a block  350 , the parts are etched in the wafer and at a block  360  are released from the wafer. This is followed by a block  370  where the parts are packaged.  
         [0021]      FIG. 6  illustrates an example of a motor bias map  40  of a wafer produced in accordance with one embodiment of the present invention. A wafer that has been subjected to the etching process and undergone testing is illustrated along with a variety of regions showing different motor bias values. In the example shown, a region  60  illustrates the region with motor bias values closest to zero. Regions  62 ,  64 ,  66 , and  68  show regions with increasing positive values of motor bias in this example. As can be seen in the figure, the regions are not necessarily contiguous. The region  64  which indicates a region of MEMS gyros having motor bias values greater than those in the region  62  but less than those in the region  66  is shown to exist mostly in a contiguous band, but also includes two areas not within or adjacent to this band and which are surrounded by other regions. Regions  70 ,  72 ,  74 , and  76  show regions with increasingly negative values of motor bias in this example.  
         [0022]      FIG. 7  illustrates an example of a motor bias map including three determined optimal motor bias paths  44 ,  46 , and  48 . Two of the optimal motor bias paths  46  and  48  have been rotated 45° and 90°, respectively, from an initial value corresponding to the angle of the optimal motor bias path  44 . The icons  42  show MEMS gyro orientation consistent with the corresponding optimal motor bias paths  44 ,  46 , and  48 .  
         [0023]      FIG. 8  illustrates an example distribution of MEMS gyro orientations based on their proximity to a given optimal motor bias path. These values correspond to those determined in the block  330  of  FIG. 5 . Area  50  indicates a region of a first angle for MEMS gyros. For example, the first angle could be defined as 0° in one embodiment relative to a coordinate system established in the plane of a wafer with its origin at the center of each MEMS gyro when the gyros are oriented in a direction corresponding to the orientation that minimizes motor bias along a non-rotated determined optimal motor bias path. Areas  52  show another region where the MEMS gyros will be rotated at a second angle. For example, this could be at 45° in one embodiment, indicating a counterclockwise rotation of each MEMS gyro about an axis located at the center of each gyro. Similarly, areas  54  illustrate a third region of rotation for the MEMS gyros. In one embodiment, this could be 90°, indicating a counterclockwise rotation of each MEMS gyro from the originally defined 0° orientation about an axis located at the center of each gyro. Other embodiments could include different angles of rotation. For example, the second and third angles could be 120° and 240° rather than 45° and 90°. It is also possible that fewer or more angles of rotation could be used, resulting in more gyro orientations on the wafer. The angles are determined based on the capabilities of a stepper to be used in imprinting patterns for the MEMS gyros on a wafer. Additionally, it is possible that the original gyro orientation corresponding to a non-rotated optimal motor bias path might not even be used, instead using only rotations other than those corresponding to a 0° rotation.  
         [0024]      FIG. 9  illustrates an example of a wafer  80  that has been exposed to produce a plurality of gyros  82  in accordance with the process described above. In this example, the gyros  82  have been prepared for dicing. This is why the lines between the MEMS gyro are shown as being straight. By positioning the gyros  82  in this manner, the wafer  80  can be easily diced.  
         [0025]      FIG. 10  illustrates a wafer that has been exposed in accordance with an additional embodiment of the invention. In  FIG. 10 , wafer  90  has MEMS gyro devices  92 , each of which are surrounded by a circle. These circles represent regions that would be either etched or ultrasonically machined in this example. Ultrasonic machining would be preferably used in the case where a pyrex substrate was employed. Etching all the way through the substrate could be used in an embodiment where the substrate is silicon.  
         [0026]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, computations may be performed in the same or in different computing devices. Also, the stepper may be controlled by a controller other than the computer used to calculate optimal paths and placements of the MEMS gyros to be produced. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.