Systems and methods of hysteresis compensation

#The present disclosure relates to systems and methods of controlling actuators, such as piezoelectric actuators, to compensate for hysteresis.

#Piezoelectric actuators are well-suited to applications requiring high stiffness, low drift, fast response times, and the ability to produce relatively large forces. However, piezoelectric materials are known to exhibit a significant amount of hysteresis. This can pose a significant barrier to the use of piezoelectric actuators in applications such as semiconductor processing and electron microscopy in which samples and/or tools must be moved and positioned with a high degree of accuracy and a minimum of disturbance. Accordingly, a need exists for improvements to systems and methods of controlling piezoelectric actuators.

#The present disclosure concerns control systems and methods for controlling actuators, such as piezoelectric actuators, to compensate for hysteresis. In a representative embodiment, a method comprises selecting a path between a first position and a second position, and applying a drive signal that includes a hysteresis-compensated portion to an actuator element to move an object along the selected path.

#FIG. 1 illustrates a representative embodiment of a multi-beam system.

#FIG. 2A illustrates a schematic cross-sectional side view of a representative embodiment of a positioning system mounted to the side of an electron microscope system.

#FIG. 2B illustrates the system of FIG. 2A with the workpiece displaced along the z-axis.

#The present disclosure concerns positioning systems for precisely positioning a workpiece relative to an instrument, and control methodologies for such systems. The systems described herein can allow a workpiece (a semiconductor wafer or biological sample) to be precisely positioned with respect to one or more tools (e.g., a scanning electron microscope (SEM), transmission electron microscope (TEM), ion column, laser beam, etc.) in a process chamber where, according to the constraints of the system, the tools and/or the workpiece may be positioned at various angles. More particularly, systems and methods are provided herein that allow for compensation of hysteresis in actuator elements, such as piezo actuators, or other actuators with components that move along an axis. By approximating the lag in position due to hysteresis of the actuator element, the drive signal applied to the actuator element can be modified to compensate for the hysteresis. For example, in certain embodiments, the drive signal of the actuator element can be modified based at least in part on the inverse of the path traced by the actuator element when no hysteresis compensation is applied. As used herein, the “inverse” of an equation or function refers to the mathematical inverse ƒ−1(ƒ(x)) of a function ƒ(x), where the inverse function ƒ−1(ƒ(x)) mirrors or inverts the graph of the function ƒ(x) about an axis. Using such processes, it is possible to repetitively operate a piezo actuator in a stepping or walking drive unit to position a mover element driven by the drive unit with precision on the order of 10−9 m. It is also possible to reduce or eliminate perturbations in the position and velocity of the mover element during operation.

#In the following, systems and methods are described in the context of semiconductor processing or the preparation and analysis of biological samples. However, the position systems, methods, actuators, and actuator controls described herein can be used in other fields where precise positioning and/or speed control are needed.

#Referring to FIG. 1, in a representative embodiment, a multi-beam system can be configured as a dual-beam system 100 comprising a scanning electron microscope (SEM) generally indicated at 102 and an ion beam column generally indicated at 104. The SEM 102 can comprise one or more charged particle beam (CPB) lenses such as a condenser lens 116 and an objective lens 106. In some embodiments, one or more CPB lenses can be magnetic lenses, and particularly, the objective lens 106 can be a magnetic objective lens. The ion beam column is arranged to provide a focused ion beam (FIB) to a workpiece W, and the SEM 102 is situated for production of an image of the workpiece W.