Abstract:
The present disclosure provides a system and method of optical inspection of substrates that have relative large variations in topography. The present disclosure provides a system wherein optical components of the optical inspection system can be automatically moved vertically towards or away from the substrate during optical inspection of the substrate. The system moves the optics in a controlled and precise manner, thereby enabling accurate on-the-fly inspection of substrates having a large variation in topography.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is being filed on 18 Dec. 2014, as a US National Stage of PCT International patent application No. PCT/US2013/048256, filed 27 Jun. 2013 which claims benefit to U.S. Provisional Application Ser. No. 61/666,362 filed Jun. 29, 2012, and which applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     In many optical systems, the lenses, objectives or other optical elements that direct light onto a surface and/or receive light back from a surface are essentially fixed with respect to the object that is being imaged or analyzed. The only movement that is typically provided for is that which is required to properly focus the optical system on the object. Given the high resolution of most modern optical systems, this amount of movement is quite small. One skilled in the art will appreciate that the amount of travel of an optical system that is required for focusing is dependent on the resolution of the optical system. 
     In some applications the variation in topography of the substrate that is being analyzed is sufficiently large so that some of the features are out of the focus range of the optics at any set distance between the optics and the substrate. Some optical systems have been developed to accommodate this scenario by raising and lowering the substrate relative to the optics during the optical inspection of the substrate. Repeatedly raising and lowering the substrate can cause damage to the substrate and/or result in slow and inaccurate and inefficient inspection thereof, and can also be very difficult if not impossible to accomplish depending on the size and nature of the substrates. There is a need for improved optical systems and methods that are able to inspect substrates that have relative large variations in topography. 
     SUMMARY 
     The present disclosure provides a system and method of optical inspection of substrates that have relative large variations in topography. The present disclosure provides a system wherein optical components of the optical inspection system can be automatically moved vertically towards or away from the substrate during optical inspection of the substrate. The system moves the optics in a controlled and precise manner, thereby enabling accurate on-the-fly inspection of substrates having a large variation in topography. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front elevation view of one embodiment of an inspection system according to the present disclosure; 
         FIG. 2  is a perspective view of a flying sensor head of the inspection system of  FIG. 1 ; 
         FIG. 3  is a front elevation view of the flying sensor head of  FIG. 2 ; 
         FIG. 4  is a first side elevation view of the flying sensor head of  FIG. 2 ; 
         FIG. 5  is a second side elevation view of the flying sensor head of  FIG. 2 ; 
         FIG. 6  is a schematic view of an alternative embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-5 , an embodiment of an inspection system and method according to the present disclosure is described in further detail. In the depicted embodiment the inspection system  10  includes a substrate stage  12  that is configured to support a substrate (i.e., the object of inspection such as a patterned semiconductor wafer, an LCD screen, etc.) and move the substrate in the X-Y plane (horizontal plane) relative to an optics assembly  14  during inspection thereof. 
     In the depicted embodiment the optics assembly  14  includes an optics sub-assembly that extends downward towards the substrate stage  12 . The optics sub-assembly is also referred to herein as a flying sensor head assembly  16 . In the depicted embodiment the flying sensor head assembly  16  is configured to move optical components in a vertical direction (Z-direction) towards and away from the substrate stage  12  during inspection. The upwards and downward motion of the sensor head assembly  16  can be based on the topography of the substrate. For example, when certain optical components of the flying head sensor are oriented above a relatively raised feature on the substrate (a high point), the flying head sensor automatically moves optical components upwardly so that the optics can focus on the raised feature. Likewise, when certain optical components of the flying head sensor are above a relatively depressed (a low point) feature on the substrate, the flying head sensor automatically moves optical components downwardly so that the optics can focus on the relatively low feature. This ability of the flying head assembly  16  to move optical components towards and away from the substrates during inspection (terrain following) enables fast and accurate inspection of substrates that include features that are located in spaced-apart vertical planes, especially wherein the spaces between the vertical planes exceed the range of focus of standard high precision inspection optics. For example, the flying head sensor assembly  16  is particularly advantageous for the inspection of delicate substrates that include features thereon that are in vertical planes spaced more than 8 mm apart (e.g., LCD screens). 
     As described above,  FIG. 1  is a depiction of a particular embodiment of the present disclosure. It should be appreciated that many alternative embodiments of the present disclosure are possible. For example, in an alternative embodiment a sensor head sub-assembly could be configured to move in all three directions during inspection (X, Y and Z) or in two directions such as the Z direction and the Y direction during inspection. In embodiments where the flying head sensor moves in all three directions, the substrate could remain stationary during inspection, and in embodiments where the flying sensor head can move in both the Z and Y directions, the substrate need only be moved in the X direction. It should be appreciated that in alternative embodiments the inspection system can be of a variety of different types and may include a different arrangement and configuration (different chassis structure, different structure to hold and move the substrate during inspection, different optics configuration, etc.). The depicted embodiment is an example configuration and application of the present disclosure. 
     Referring to  FIGS. 2-5 , the flying sensor head assembly  16  is described in further detail. In the depicted embodiment the flying sensor head assembly  16  can raise and lower the optics thereby shifting a focal plane of the optical system by as much as an inch (+/−0.5 inches). In another embodiment the focal plane of the sensor head  16  may be moved by as much as two inches (+/−1.0 inches) about a nominal position. In alternative embodiments of the flying sensor head assembly  16 , even larger vertical travel distances may be achieved. 
     In the depicted embodiment optical elements  18 ,  20  for directing and focusing a beam of light  24  onto a surface  28  of a substrate, and optical elements  19 ,  21  for collecting and collimating light from the beam of light  25  reflected from the surface, as well as other optical elements  22  are attached to a movable plate  26  by an alignment assembly  82  (towards and away from the movable plate  26  adjustment),  28 ,  30 ,  32  (left and right adjustment),  34  (up and down adjustment),  36 ,  38  (see  FIG. 3 ). While in the depicted embodiments the above-identified optical elements located at a distal end of the flying sensor head  16  are arranged in an oblique incidence input/output arrangement, it is specifically intended that normal incidence arrangements may also be useable. 
     In the depicted embodiment, mirrors or other coupling elements couple an optical or electromagnetic beam to sensors, filters, and other conditioning and/or relay elements of a typical metrology system are located in the optics assembly  14  above the flying sensor head  16 . In one embodiment the relay mirrors optically couple the sensor head optics  18 ,  19 ,  20 ,  21 ,  22  to an opto-acoustic metrology system such as the type of system sold by Rudolph Technologies, Inc. under the mark MetaPulse®. In another embodiment, the sensor head optics are coupled to a metrology system such as an ellipsometer. One example of an ellipsometer that may be used with the present invention is the S3000™ ellipsometry system sold by Rudolph Technologies, Inc. In another embodiment, the sensor head forms part of an optical inspection system such as that sold under the trademark AXI™, again from Rudolph Technologies. Additional metrology and inspection systems and other sensors of the type commonly used in semiconductor manufacturing processes may be adapted to utilize the flying sensor head  16  of the present disclosure. Examples include confocal optical systems, spectroscopes of various types, Shack-Hartman sensors, interferometers, and the like. As discussed above, the optics assembly  14  can include a variety of different alternative configurations. 
     In the depicted embodiment, the movable plate  26  is connected to a chassis/frame of the optics assembly  14  via an optics stage assembly  40 , also referred to herein as a bracket assembly. The optics stage assembly  40  provides a degree of vertical travel to the flying sensor head assembly that greatly expands the utility of the flying sensor head assembly  16 . In the depicted embodiment, vertical movement of the flying sensor head assembly  16  is accomplished by an electronically controlled actuator such as a stepper  42  coupled between the movable plate  26  and a base plate  44 . In the depicted embodiment the base plate  44  is connected to a leveling plate  46 , also referred to herein as an alignment adjustment plate (see  FIGS. 4-5 ). In the depicted embodiment the alignment adjustment plate  46  includes three points of contact with the base plate  44 , thereby allowing the movable base plate  44  to be adjusted for rotation both about a vertical and a horizontal axis. The adjustment of the base plate  44  results in the adjustment of the optics mounted to the movable plate  26  given the secure connection between the movable plate  26  and the base plate  44  via the stepper  42 . 
     In the depicted embodiment a single very high resolution actuator (e.g., a stepper)  42  capable of moving the stage in increments approaching a nanometer in magnitude is coupled between the portions of the optics stage assembly  40 . In other embodiments, multiple steppers/actuators working in parallel or in series are included to accomplish both high and coarse adjustment of the stage portions relative to one another. 
     In the depicted embodiment the stage assembly  40  includes a component that supports the mass (mass support device) of a portion of movable plate  26  and the optical elements attached thereto. The mass support device, also referred to herein as an inductor, lessens the weight load on the stepper  42 . In the depicted embodiment the mass support device is a pneumatic cylinder (gas cylinder)  48  that is coupled between the movable plate  26  and the base plate  44  in parallel with the stepper  42 . In the depicted embodiment the pneumatic cylinder has no external moving parts (except for the piston that extends therefrom), and therefore is particularly suited for clean room type inspection applications. It should be appreciated that other types of mass support devices can also be used including, for example, a spring-based configuration, a hydraulic cylinder configuration, or a secondary electronic controlled actuator such as a stepper. In the depicted embodiment the pneumatic cylinder  48  is preloaded to carry approximately the weight of the lower movable portion of the flying sensor head  16  (the movable plate  26  and the optical elements attached thereto). The preloading of the pneumatic cylinder  48  removes a great deal of stress and resistance from the stepper  42  of the stage, and allows for faster and more precise adjustment of the stage and hence the optical components on the movable plate  26 . 
     In the depicted embodiment the pneumatic cylinder  48  is provided with a mechanism for increasing, decreasing or maintaining the pressure within the cylinder to accommodate positional adjustment of the optics stage assembly  40  and to damp out vibrations or elastic movements of the lower movable portion of the flying sensor head  16  (the movable plate  26  and the optical elements attached thereto) relative to the upper portion (e.g., base plate  44 ), which in the depicted embodiment is fixed. In the depicted embodiment the mechanism is an adjustable aperture that leaks air at a known rate, used in combination with a pressure gauge to replenish air lost through the aperture. In other embodiments the adjustment mechanism is dynamically controlled in conjunction with the stepper to increase, decrease or maintain pressure in the cylinder to allow or restrict movement of the stage, as needed. For example, when a movement of the stepper  42  is undertaken, the volume within the pneumatic cylinder is modified. In some instances air is emitted from the pneumatic cylinder to reduce the amount of force required for the stepper to counteract the preloading force of the pneumatic cylinder (e.g., when the movement of the stepper would act to compress air within the pneumatic cylinder). Conversely, where the stepper is actuated to move the stage in a manner that would tend to reduce pressure within the cylinder, air is added to the cylinder to maintain the preloading force exerted by the cylinder. In the depicted embodiment electrical power as well as control signals are transmitted to the optics stage assembly  40  via wires (e.g., pin connector  56 ). 
     In the depicted embodiment the effective travel of the stage is controlled by the pneumatic cylinder  48  and the stepper  42 . Using longer cylinders and steppers increases the travel of the stage and, hence, the flying sensor head assembly  16 . In the depicted embodiment a bumper stop assembly  50  is connected to the leveling plate  46 . The bumper stop assembly  50  includes a rubber stopper  52  that would engage a stop bar  54  that extends away from the back side of the movable plate  26 , if the stepper  42  and/or the pneumatic cylinder  48  malfunctions. Therefore, the stop assembly  50  is configured to prevent damages to the optics as well as to the substrate under inspection due to contact between the two. 
     As described above, the flying sensor head of the depicted embodiment is configured to move in the Z direction during inspection. In the depicted embodiment during inspection the substrate is automatically moved by the stage in an X-Y plane to pass the substrate under an optical sensor head. The flying head assembly determines whether the features under the sensor are too close or too far from the lenses  20 ,  21  for proper inspection. This determination can be accomplished by sending a particular type light through mirror  18 , lens  20 , lens  21  and back up via mirror  19 . Alternatively, the camera  22  can be used to make the determination as well. The camera could otherwise be used to visually locate the flying head assembly  16  in the horizontal plane relative to the substrate. The system is figured to determine whether movement of the flying head sensor in the Z direction is warranted and if so move the optics accordingly. As discussed above, the movement is accomplished by optics stage  40 . In the depicted embodiment the precise controlled movement of the optics is accomplished via the coordinated control of the cylinder  48  and the stepper  42 . Once the optics reach their target high, the features below the optics can be inspected and the process can be repeated until inspection of the substrate is complete. 
     As discussed above, it should be appreciated that the flying sensor head assembly  16  could also be configured to move in either or both of the X and Y directions as well. For example the leveling plate  46  could be mounted to a second stepper that move the flying head sensor in either or both of the X and Y directions instead of being fixedly mounted to the optics assembly  14 .  FIG. 6  depicts an example embodiment wherein the flying head sensor moves in the Z direction as well as the X and Y directions. In the depicted embodiment the flying head assembly  74  is mounted on a cross member  76  and is driven in the Y direction along tracks  78  that are mounted to the cross member  76 . The cross member  76  is itself driven in the X direction along a pair of tracks  80  that are fixed to the optic assembly base frame  64 . In the depicted embodiment a light beam  60  is reflected off an angled mirror  62  mounted on the optics head chassis base frame  64  to mirror  66  mounted to the cross member  76 , which directs the light beam  60  to the flying head assembly  82 . A return light beam  68  from the flying head assembly is reflected off mirror  70  on the cross member to mirror  72  on the base frame  64  back to the main compartment of the optics assembly. Alternatively, the optics assembly  14  could be configured to move as a whole in the X and Y directions and the flying sensor head could be connected fixedly to the optics assembly. It should be appreciated that the coupling mirrors may be provided with an adaptive adjustment mechanism that would allow the flying sensor head  16  to be gimbaled or moved along complex XYZ paths relative to the chassis to which it is coupled. These additional degrees of freedom would allow the flying sensor head to be used to inspect highly variable topographies of selected substrates without moving the substrates, which may be advantageous in certain applications. 
     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.