Abstract:
A method for locating and eliminating defects on a substrate wafer includes illuminating a top surface of the substrate wafer with a first illumination source, illuminating a bottom surface of the substrate wafer with a second illumination source, forming an image of a portion of the top surface of the substrate wafer while the substrate wafer is illuminated by the first and second illumination sources, adjusting a contrast of the image to accentuate defects on the top surface of the substrate wafer, locating defects in the image, and ablating the defects on the top surface with a laser.

Description:
This is a divisional application of U.S. patent application Ser. No. 12/269,590, filed on Nov. 12, 2008, which is incorporated herein as though set forth in full. 
    
    
     GOVERNMENT SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No. FA8650-05-C-7245 awarded by DARPA. The Government has certain rights in the invention 
    
    
     BACKGROUND 
     The present disclosure relates to a method and apparatus for locating and eliminating manufacturing defects for microelectromechanical systems MEMS devices, such as quartz disc resonator used in disk resonator gyroscopes (DRGs). In particular, it relates to using an optical system to locate defects and a laser ablative system to remove defects relating to unwanted excess material on the device. A description of a DRG design can be found in U.S. patent application Ser. No. 11/458,911 filed Jul. 20, 2006 and entitled “Disc Resonator Gyroscopes,” which is hereby incorporated by reference as though set forth in full. 
     SUMMARY OF THE INVENTION 
     In a first embodiment disclosed herein, a method for locating and eliminating defects on a substrate wafer includes illuminating a top surface of the substrate wafer with a first illumination source, illuminating a bottom surface of the substrate wafer with a second illumination source, forming an image of a portion of the top surface of the substrate wafer while the substrate wafer is illuminated by the first and second illumination sources, adjusting a contrast of the image to accentuate defects on the top surface of the substrate wafer, locating defects in the image, and ablating the defects on the top surface with a laser. 
     In an aspect of this embodiment the bottom surface of the substrate wafer is affixed to an adjustable transparent base. 
     In another aspect of this embodiment the step of adjusting a contrast of the image to accentuate defects comprises adjusting the first illumination source, the second illumination source, or the image. 
     In yet another aspect of this embodiment the laser comprises an excimer laser. 
     In another aspect of this embodiment the method further comprises repeating the imaging, adjusting, locating, and ablating steps over the entire substrate wafer. 
     In yet another aspect of this embodiment the substrate wafer comprises quartz, the first illumination source comprises visible light, and the second illumination source comprises visible light. 
     In still another aspect of this embodiment the laser comprises a 193 nm wavelength ultraviolet excimer laser capable of ablating quartz. 
     In another aspect of this embodiment the substrate wafer comprises silicon, the first illumination source comprises infrared illumination, and the second illumination source comprises infrared illumination. 
     In yet another aspect of this embodiment the substrate wafer is at least partially reflective of the first illumination source and the substrate wafer is at least partially opaque to the second illumination source. 
     In another aspect of this embodiment the substrate wafer comprises a quartz disc resonator for use in a disk resonator gyroscope (DRGs). 
     In yet another aspect of this embodiment locating defects in the image of the substrate wafer further comprises aiming the laser at the defect. 
     In another embodiment disclosed herein, an apparatus for locating and eliminating defects on a substrate wafer includes an adjustable transparent base for holding a bottom surface of the substrate wafer. a first illumination source for illuminating a top surface of the substrate wafer, a second illumination source for illuminating the bottom surface of the substrate wafer, an imaging system aligned to form an image of a portion of the top surface of the substrate wafer, the imaging system adapted to find defects in the image, and a laser aligned to the top surface of the substrate wafer for ablating the defects. 
     In another aspect of this embodiment the imaging system comprises at least one adjustment for adjusting a contrast of the image. 
     In another aspect of this embodiment the substrate wafer comprises quartz, the first illumination source comprises visible light, and the second illumination source comprises visible light. 
     In yet another aspect of this embodiment the laser comprises a 193 nm wavelength ultraviolet excimer laser capable of ablating quartz. 
     In another aspect of this embodiment the substrate wafer comprises silicon, the first illumination source comprises infrared illumination, and the second illumination source comprises infrared illumination. 
     In another aspect of this embodiment the substrate wafer is at least partially reflective of the first illumination source, and the substrate wafer is at least partially opaque to the second illumination source. 
     In yet another aspect of this embodiment the substrate wafer comprises a quartz disc resonator for a disk resonator gyroscope (DRGs). 
     In another aspect of this embodiment the apparatus further comprises a first lens between the first illumination source and the substrate wafer, the first lens adjustable to adjust a contrast of the image, and a second lens between the second illumination source and the substrate wafer, the second lens adjustable to adjust the contrast of the image. 
     In yet another aspect of this embodiment the apparatus further comprises a first partially reflective mirror between the first illumination source and the substrate wafer for coupling an output of the laser to the substrate wafer, and a second partially reflective mirror in the first path for coupling the imaging system for imaging of the top surface of the substrate wafer. 
     In yet another embodiment disclosed herein, a method for locating and eliminating defects on a quartz disc resonator for use in a disk resonator gyroscope (DRGs) comprises illuminating a top surface of the quartz disc resonator with a first illumination source, illuminating a bottom surface of the quartz disc resonator with a second illumination source, forming an image of a portion of the top surface of the quartz disc resonator while the quartz disc resonator is illuminated by the first and second illumination sources, adjusting a contrast of the image to accentuate defects on the top surface of the quartz disc resonator, locating defects in the image, and ablating the defects on the top surface with a laser. 
     In an aspect of this embodiment adjusting a contrast of the image to accentuate defects further comprises adjusting the first illumination source, the second illumination source, or the image. 
     In another aspect of this embodiment the laser comprises a ultraviolet excimer laser. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an embodiment of the disclosed apparatus for locating and eliminating defects in accordance with the present disclosure. 
         FIG. 2  shows a flowchart for an embodiment of a method for locating and eliminating manufacturing defects in accordance with the present disclosure. 
         FIG. 3  depicts a DRG imaged from the top-side with top-side illumination of the DRG in accordance with the present disclosure. 
         FIG. 4  depicts a DRG imaged from the top-side with bottom-side illumination of the DRG in accordance with the present disclosure. 
         FIG. 5  depicts a DRG imaged from the top-side with a combination of top-side and bottom-side illumination of the DRG in accordance with the present disclosure. 
         FIG. 6  depicts a DRG with a located defect bridging defect prior to laser ablation in accordance with the present disclosure. 
         FIG. 7  depicts a DRG with a located defect bridging defect after laser ablation of the defect in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
       FIG. 1  shows an example of the apparatus for locating and eliminating defects on a MEMS device  100 , such as a quartz DRG. The apparatus consists of a laser micromachining workstation  102 , an XY stage with a transparent platform  104  under the device  100  to hold and position the MEMS device  100 . The MEMS device  100  can be illuminated from one side, in this example the underside, by a backlight illumination source  106 . The laser micromachining workstation  102  can consist of an illumination source  108  positioned to light the device  100  from the side opposite the side illuminated by the backlight illumination source  106 . The workstation  102  can also include a laser  110  for ablating portions of the device  100  and an imaging system  112  for viewing the MEMS device  100  under magnification. The workstation  102  may also include partially reflective mirrors  116   a  and  116   b  to allow the laser  110  and the imaging system  112  to access the device  100  without interfering with the path  101  of light from the two illumination sources  106  and  108 . Lenses  114   a - c  can be used to focus the light  101  onto the device  100  to aid in imaging and ablation. 
       FIG. 2  shows a flowchart of an example method for detecting and removing defects on a MEMS device. The apparatus shown in  FIG. 1  can be utilized to carry out the method, but other equivalent systems can be used as well. First, the device  100  is held in place in a way that allows access to both sides of the device, which is generally a flat die or wafer structure, at the same time. For example, the device  100  can be affixed in step  200  to the transparent platform  104 . The device  100  is then illuminated in step  202  on both the top and bottom surfaces by illumination sources  106  and  108  that the device material is semi-transparent to. For example, if the device is constructed from quartz, visible light can be used. If the device is silicon, then infrared light can be used. An imaging system  112 , such as a camera, is used to view the device in step  202  from one of the sides, for example the top-side. If infrared light is used for illumination, then the imaging system  112  would have to be able to translate infrared images into a visible image. The imaging system  112  can magnify the image as needed to make any defects easier to detect. An area of the device  100  is scanned for defects in etched regions of the device  100 . The image contrast is adjusted in step  204  to accentuate the point defects, such as unwanted bridges between device structures. The defects appear dark, since they tend to scatter and block the backside illumination from illumination source  106 . An example is shown in  FIG. 6 . Once a defect is detected in step  206 , an excimer laser  110  can be used to ablate the defect from the device in step  208 . For example, a 193 nm ultraviolet excimer laser can be used to ablate defects in quartz devices. The imaging system  112  and the laser  110  can be connected as to allow aiming of the laser through the imaging device. The transparent platform  204  is adjusted in step  210  to allow viewing of different areas of the device. This process is then repeated in step  212  until in step  214  all discovered defects are removed. 
       FIG. 3  depicts an example of a DRG device imaged from the top with top-side illumination only. Because the device is partially reflective to the visible light in the case of quartz devices, and IR light in the case of silicon devices, the material portions  301  of the device appear white while the etched spaces  302  between the material portions  301  appear black. 
       FIG. 4  depicts an example of a DRG device imaged from the top with bottom-side or back side illumination only. Because the device is partially opaque, the material portions  401  appear black or dark and the etched spaces  402  between the material portions  401  appear white. 
       FIG. 5  depicts an example of a DRG device imaged from the top with both bottom-side and top-side illumination. The material portions  500  appear grey, while the etched spaces  502   a  and  502   b  appear either black  502   a  or white  502   b , depending on the geographic structure and positioning of the device and the illumination sources. 
       FIG. 6  depicts an example of an image of a DRG device with a defect  600  imaged with a combination of top-side and bottom-side illumination. The defect  600  is a bridging or masking defect causing a bridge of material to extend from one portion  604   a  of the DRG device to another portion  604   b . This causes the portions  604   a  and  604   b  to be pinned together preventing them from resonating properly. In addition, when the DRG is metallized, the bridge would cause a short which would alter the electrical properties of the DRG. The combination top side and bottom side illumination allows the defect  600  to be clearly seen as a dark patch on the etched space  602 . 
       FIG. 7  depicts an example of an image of the DRG device of  FIG. 6  after ablation of the defect with the laser  110 .  FIG. 7  reference  700  shows the previous location of the defect, which in  FIG. 7  matches the tint of the rest of the etched space  602  which indicates that the defect is now removed. The surrounding DRG material  604   a  and  604   b  is unaffected by the ablation as the laser is focused to an area smaller than the width of the etched space  602 . 
     Additionally, electrical and mechanical testing can be utilized to assist in the determination of defects, followed by the localization and eliminated of these defects, by this apparatus. The same techniques can be utilized to eliminate electrical shorting defects on base wafers used subsequently to assemble finished DRG&#39;s. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise forms described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the steps of . . . ”