Abstract:
A rotating stage assembly performs high precision rotational angle and position error correction by continuous sensing and correcting motor stage assembly errors. It performs these corrections, to adjust for motor environmental and operational errors by sensing and correcting using five sensors placed to measure the adjustments of five corresponding actuators, which adjust the entire motor rotating stage and rotary motor assembly relative to a reference frame.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The need for accurate movement is evident in all aspects of the miniaturization of electronics, from nanometer precision x-y stage movement in semiconductor lithography to accurate linear motion with regard to rotating media in optical and magnetic disk arms, but these do not need to deal with three dimensional correction of rotating stages. 
         [0002]    One application that requires high three dimensional positional precision when rotating an object is three dimensional (3D) x-ray imaging with computed tomography (CT), where the rotation axis must be known accurately in three dimensions with precision well below the imaging resolution. At least one such x-ray inspection tool, as described in U.S. Pat. No. 7,215,736, granted May 8, 2007, requires the rotation of a sample to be accurate to within tens of nanometers in all three dimensions. This allows a sample to be rotated in the x-ray beam thereby enabling tomographic data acquisition by accurately generating multiple projections of the sample for later tomographic reconstruction possibly without additional alignment procedures. 
         [0003]    Such precision is difficult to achieve in rotating stages due to random errors from bearings and spindle wobble and play, as well as manufacturing variations in the motor housing and the dimension and smoothness of the stage assembly attached to the motor. Furthermore, no matter how accurate the components can be made, some portion or all of it must be constructed out of normal engineering materials, which in general have significant thermal expansion characteristics. 
       SUMMARY OF THE INVENTION 
       [0004]    This invention pertains to a rotating stage assembly, which can be used to perform high precision position error correction by continuously sensing and correcting motor stage assembly errors. It performs these corrections, to adjust for motor environmental and operational errors by sensing and correcting using five sensors, in one embodiment, placed to measure the adjustments of five corresponding actuators, which adjust the entire motor rotating stage and rotary motor assembly relative to a reference frame, maintaining the position accuracy of the rotation axis of the stage. 
         [0005]    In general, according to one aspect, the invention features five axis correction of the whole rotating motor and stage assembly using five piezo actuators; one for translation in x, one for translation in y and three for both translational and angular motion of the z corrections, with five corresponding capacitive sensors for measuring the corrected positions. 
         [0006]    In the present embodiment, actuators adjust the position of a rotation stage relative to a reference frame. The rotation stage houses a motor for rotating a metrology disk, the center of which contains a sample stage, where a sample is placed. The metrology disk is made of a material with low or well-characterized thermal expansion characteristics. The disk is preferably uniformly coated with a conductive material allowing the sensors, such as capacitive sensors for example, which are mounted to the reference frame. These sensors correspond to five actuators mounted on an actuator stage, which is also attached to the reference frame, to measure the position and angle of the metrology disk and thus the corresponding sample stage. 
         [0007]    In a preferred embodiment, control logic measures the sensors and adjusts the actuators on a continuous basis, taking into account the current rotational angle (wobble) of the metrology disk relative to the reference frame, and adjusting accordingly for all anomalies due to the motor&#39;s mechanical tolerances, and temperature variations. 
         [0008]    In another embodiment a sixth reference sensor may be used to collect the form errors of the circular metrology disk during normal rotation. The reference measurements along with other residual error measurements of all angular rotations of the metrology disk may be taken and used to pre-compute form corrections for each rotation angle of the metrology disk, which when stored in a form corrections data file, may read back and applied at each angle the disk is subsequently moved to during normal operation. 
         [0009]    The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
           [0011]      FIG. 1  is a schematic perspective view of the rotating stage assembly according to the present invention, 
           [0012]      FIG. 2  is a simplified schematic view showing the placement of the actuators within the actuator stage, 
           [0013]      FIG. 3  is a diagram of an unaligned metrology disk with its coordinate system and the coordinate system of the reference stage, 
           [0014]      FIG. 4  is a simplified schematic of a vertical view of the rotating stage assembly showing the placement of actuators and sensors, and 
           [0015]      FIG. 5  is a schematic of the connections between the host computer, the sensors and the actuators of the rotary disk assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    Many other products utilize high precision linear motion control, but rotary motion control is less straight forward. 
         [0017]      FIG. 1  shows a projection of the five axis compensated rotary stage assembly  10  comprised of a reference frame  11 , with an actuator stage  20  attached to it, a rotation stage  40  containing a rotary motor  45 , whose shaft is connected to a metrology disk  50 . A sample  60 , to be rotated, is placed upon the sample platform  55 , attached to the center of the metrology disk  50 . The rotary motor rotates the metrology disk  50 , the sample platform  55  and the sample  60 , relative to the reference frame  11 . The actuator stage  20  contains five actuators to adjust the placement of the rotation stage  40  in five axes. The spindle shaft  46  of the rotary motor  45  extends through a hole in the top plate  12  of the reference frame  11 . Six sensors  71 ,  72 ,  73 ,  75 , and  76  ( 74  not shown) are mounted on fixed mechanical arms, which are attached to the top plate  12  of the reference frame  11  to position the sensors near the side and bottom of the metrology disk  50 . This allows five axis measurements between the metrology disk and the platform, which can then be corrected by the actuators. The reference frame, sensor arms, sample platform and metrology disk are made out of low thermal expansion materials to minimize errors due to thermal expansion. The metrology disk  50  is centered above the reference frame  11  and the sensor arms are made of the same material as the reference frame to minimize uncorrectable horizontal errors due to thermal expansion. In this fashion the sensors can detect all other variations between the sample  60  and the reference frame  11 . 
         [0018]    In one embodiment, the reference frame  11  and sensor arms are constructed out of Invar, while the metrology disk is constructed out of Zerodur. While both materials have very low coefficients of thermal expansion, Zerodur, which is used in reflector telescope construction, is better suited for extremely accurate polishing and reflective coatings, which are required in the construction of the metrology disk, to get the flatness of end surface down to &lt;10 nanometers. 
         [0019]      FIG. 2  shows a schematic of the placement of actuators used to compensate for errors during rotation of the sample  60 . There are five actuators  31 ,  32 ,  33 ,  34  and  35 , which adjust the position of the rotation stage  40 , relative to the actuator stage  20 . Two actuators  31  and  32 , adjust the lateral position of the rotation stage  40  in planes mostly perpendicular to the axis of rotation of the metrology disk  50 . The other three actuators  33 ,  34  and  35 , extend between the reference frame  11  and the rotation stage to adjust the tilt angle of the rotation stage  40 . The three actuators  33 ,  34 ,  35 , are placed preferably equidistant from the center axis of the metrology disk projected onto the rotation stage and located at 120 degree angles from each other about the center axis of rotation of the motor&#39;s spindle shaft. Together these three actuators  33 ,  34  and  35  correct for small angular variations of the metrology disk&#39;s axis of rotation off of the plane defined by the top plate  12  of the reference frame. Each of the actuators, their housings and their placement between the actuator stage and the rotation stage are designed to minimize crosstalk between the other actuators during correction. 
         [0020]    For example,  FIG. 3  shows the metrology disk  50  displaced and tilted at an angle with respect to a coordinate system related to the top plate  12  of the reference frame (not shown), consisting of X  121 , Y  122 , and Z  123  axes. A coordinate system of the metrology disk also contains X  151 , Y  152 , and Z  153  axes. To align the metrology disk to the coordinate system of the reference frame, the origins  120  and  150  of the two coordinate systems must be aligned by adjusting actuators  31  and  32  to move the metrology disk in the direction of their arrows, and the Z  121 ,  151  axes must be aligned by aligning the plane determined by the X  121  and Y  122  axes to the plane of the metrology disk  50  determined by the X  151  and Y  152  axes. This is done by adjusting the actuators  33 ,  34 , and  35  to move the metrology disk in the direction of their arrows. 
         [0021]      FIG. 4  is a schematic top view of the sensor and actuator placement within the rotary stage assembly. The two sensors  71  and  72  are placed to measure perpendicular distances to the edge of the metrology disk on the plane perpendicular to the metrology disk&#39;s axis of rotation. They correspond to the two actuators  31  and  32 , which adjust the rotation stage in a plane parallel to the plane of the two sensors  71  and  72 . The three sensors  73 ,  74  and  75  are between the flat surfaces of the top plate  12  and the metrology disk  50 , and all measure the distance between the top plate  12  and the metrology disk  50  in a direction parallel to the disk&#39;s axis of rotation. Each actuator is paired with a sensor. All five actuators  31 ,  32 ,  33 ,  34  and  35  are placed equidistant from their corresponding sensors  71 ,  72 ,  73 ,  74 , and  75  on axes parallel to the disk&#39;s axis of rotation, and all five sensors  71 ,  72 ,  73 ,  74 , and  75  sense distances to the metrology disk, in directions which are parallel to their corresponding actuators  31 ,  32 ,  33 ,  34  and  35 . This placement minimizes the calculations necessary to correct for errors, particularly in vertical direction and tip-tilt due to variations induced by the rotation stage, motor, spindle and assembly to the metrology disk. 
         [0022]    For very high precision rotational placement, the actuators are piezo-electric devices with minimum adjustment in the nanometers but full travel is limited to hundreds of micrometers. Actuators based on other technologies such as voice coils, linear motors, or electrostatic actuators are used in other embodiments. 
         [0023]    The metrology disk has a thin metallic coating, of preferably gold, and the sensors sense capacitance. The capacitive sensors are of sufficient size to average the surface variations of the metallic coating and still have the same level of sensitivity as the actuators. Other equally position sensitive, fine variation insensitive sensor technologies such as large spot laser interferometers or grating-based optical sensors are used in other examples. 
         [0024]    Alternatively, to get larger error correction with less precision, other less accurate actuators such as lead screw assemblies may be used along with less precise sensors, such as ultrasound, which can handle correspondingly larger ranges of error correction. 
         [0025]      FIG. 5  is a schematic of the control electronics  100 , which controls the correction of the rotation stage to compensate for the variations in spindle wobble, thermal expansion and bearing errors in the motor and its housing. The control electronics  100  has sensor amplifiers  70  to read signals from the six sensors  71  through  76 , drivers  30  to control the five actuators  31  through  35 , both of which are continuously controlled by the controller  80 , which repeatedly reads the sensors and adjusts the actuators, and a computer  90 , which controls the rotary motor, rotating the metrology disk, provides a set of preprocessed offsets to the controller, and receives the final adjusted position error measurements from the controller. 
         [0026]    The corresponding placement of the sensor actuator pairs, along with the design of the actuators which minimizes cross talk between actuators, generally minimizes the calculations needed to correct the position errors. The result of each sensor is primarily just fed back to its actuator, with at most small corrections due to the other sensor measurements. The independent nature of these corrections simplifies the controller and minimizes the time needed to do the position corrections. 
         [0027]    If the metrology disk had a perfectly circular edge there would be no need for the reference sensor  76 , since the X sensor  71  and the Y sensor  72  would detect the placement error of the metrology disk in the plane perpendicular to its axis of rotation, but while flat surfaces such as are constructed on the top and bottom of the metrology disk may be polished flat within 10 nanometers, it is not currently possible to achieve that level of precision for the curvature of the disk. As a result, measurements from the reference sensor  76 , which are previously collected after corrections using the other five sensors, may later be used to correct for the disk&#39;s edge distortions (form errors). 
         [0028]    In one embodiment, a disk calibration procedure is performed to generate a form corrections data file, containing the disk&#39;s form corrections for each rotation angle of the metrology disk. During this disk calibration, at each rotation angle of the metrology disk, the actuators  31 ,  32 ,  33 ,  34 , and  35 , and sensors  71 ,  72 ,  73 ,  74 , and  75 , are in closed loop correction through controller  80 , and following the correction, the output of the reference sensor  76  stored in the computer  90  in a look up table (LUT) or algorithmically. By applying the closed loop correction prior to obtaining the output of the reference sensor, the effects of all other errors except the form error of the disk are eliminated from the reference sensor measurement The form corrections may then be calculated from the stored measurements of the reference sensor according to the geometry relationship between the sensor  71 ,  72  and  76 , and outputted along with the rotation angle to the form corrections data file. Later, during normal operation, the form corrections for the current rotation angle of the metrology disk may be read from the form corrections data file and added to the closed loop correction of actuators  31  and  32  to correct the disk&#39;s form errors, thereby simplifying the control logic in the controller  80  and eliminating the need to use the reference sensor  76  during normal operation. 
         [0029]    In this fashion, all positional errors of the metrology disk, relative to the reference frame, may be corrected for any rotation of the sample stage and sample. Corrections to center the sample stage or sample on the axis of rotation of the metrology disk, may be done prior or during the gathering of tomography data by the x-ray imaging equipment. 
         [0030]    It is further contemplated that the high precision positional correction capability of this rotary stage assembly may be used along with a high precision external measuring device to accurately measure the circular characteristics of a sample, regardless of centering, providing the sample is within the range of the external sensing device throughout the measured rotation. 
         [0031]    It is also contemplated that metrology disks and top plates of reference frames without high precision flat surfaces may be used by creating a planarity corrections data file prior to creating the form corrections data file. The planarity corrections data is gathered by first aligning a reference sample to external measurement equipment, and then for each rotation angle, outputting sensors  73 , 74  and  75 , along with an external measurement of the reference sample after zeroing actuators  33 , 34 , and  35 , and only running sensors  31  and  32  with actuators  71  and  72  in closed loop correction through the computer  80 . Then in a fashion similar to the creation of the form correction data file, the external measurements and the output from sensors  73 ,  74  and  75  may be used to create a planarity corrections data file, which may then be used in the calibration process to create the form corrections data file, by initializing the actuators before each closed loop correction. Thereafter, during normal operation, the reference sensor and external measurement equipment is not needed, and again in a manner similar to the use of the form corrections data, the planarity corrections for the current rotation angle of the disk are read from the planarity corrections data file and added to the closed loop correction of actuators  33 ,  34  and  35  to correct the disk&#39;s planarity errors, along with reading the form corrections data file and adding the form corrections to the closed loop correction of actuators  31 , and  32  to correct the disk&#39;s form errors, thereby simplifying both the creation of the mechanical and electrical subsystems. 
         [0032]    While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.