Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reissue Patent
1997-10-10
2002-02-26
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C250S306000
Reissue Patent
active
RE037560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the control of piezoelectric and other scanners, and more specifically to scan control for scanning probe microscopy and other fields requiring a precision scanning stage.
2. Discussion of Background
Precision scanning stages are required by disciplines including scanning probe microscopy, beam lithography, and others. A common form of scanner for these applications is the piezoelectric scanner. The piezoelectric scanner comes in a variety of forms—single crystals, bimorphs, multilayer piezoelectric stacks, and tube scanners, for example. For all of these configurations, a voltage is applied across the piezoelectric elements and the position of some part of the piezoelectric scanner moves with respect to another part that is held fixed. This motion is used to scan samples, probes, lenses, etc., for a variety of purposes. Such scanners are often used to produce motion of the probe over the sample. However, motion of the sample stage with a fixed probe is equivalent for many applications and is also often employed.
Another kind of scanner is an electrostrictive scanner made typically out of PMN (lead-magnesium-niobate).
A very important area of application for such scanners is the field of scanning probe microscopes. In a scanning probe microscope such as the scanning tunneling microscope (STM) or the atomic force microscope (AFM), a probe is scanned across the surface of a sample to determine local properties of the surface such as topography or magnetic field strength so that these properties can be displayed for viewing. Alternately, the sample can be scanned relative to a fixed probe. Some of these microscopes, for example, the STM and AFM, have been constructed with the ability to resolve individual atoms. The scanner that provides the motion is usually a piezoelectric device adapted for moving in all three dimensions, i.e., the XY-plane and in the vertical (Z-axis) direction. Such three dimensional scanners have been built from discrete piezoelectric elements or from single tubes with internal and external electrodes segmented in a way to allow translation in all three dimensions (Binnig and Smith, Review of Scientific Instruments, Vol. 57 pp. 1688-89, (1986) and U.S. Pat. No. 4,087,715).
Images of the sample are usually created by scanning the probe over the sample in a so-called “raster” pattern, in the same way that an electron beam is used to create a television picture. For example, the probe is scanned at a high rate in the X-direction, back and forth across the sample, and at a low rate in the perpendicular (Y) direction. Data about the height, magnetic field or other local properties are collected as the probe is moved over the surface. To create such a scan pattern, it is necessary to apply scan voltages to the electrodes on the piezoelectric scanners. In the simplest case, where the piezoelectric sensitivity can be assumed to be constant, triangle waves may be applied to the X and Y electrodes to produce a linear raster pattern.
To resolve movement of a probe on the atomic scale, the scanning mechanism must be stable and accurately movable in increments on the Angstrom scale. Piezoelectric scanners have been widely used as such a stable and accurate scanning stage. In addition, there is great interest in scanning stages that have such precision and stability, but also have the ability to scan very large ranges, often over 100 microns, or to do small scans at various locations over a large field, at large offsets from the rest position. A system which must remain stable to 0.1 Angstrom and scan linearly over 100 microns is operating over a dynamic range of 1:10,000,000.
For small translations, piezoelectric scanners operate in a linear and reproducible way. That is, the displacement is proportional to the applied voltage. Thus at small amplitudes and for short times a piezoelectric transducer can be accurately described by its rest position, and its sensitivity, dX/dV. Unfortunately, piezoelectric scanners acquire a number of unwanted behaviors for translations on the micron scale and larger. At large scan voltages, the scanner's response can depend nonlinearly on the scan voltage and scan frequency. Also, in multiple axis scanners, there may be a coupling between the separate scan axes. The result is that a scan voltage applied to one axis may change the resulting motion on another axis. These unwanted behaviors in the piezoelectric response will mean that at large amplitudes, a linear signal applied to the piezo electrodes no longer produces a linear scan.
In addition to non-linearities and coupling, there may be other unwanted behaviors that make the scan pattern asymmetric when driven by a symmetric scan voltage. For example, piezoelectric scanners exhibit hysteresis such that as the direction of the applied voltage changes at the end of a scan, the position of the moving part of the piezoelectric scanner does not trace out its previous path. This is because the piezoelectric response to a scan voltage (sensitivity) is a function of the previous voltage history. Additional problems are caused by slow drifts in position caused by “creep.” “Creep” is long term drift in position due to previous scan voltages applied to the scanner. There are also other drifts in the position of the scanner caused primarily by temperature variations and stresses in the scanner and its mounting hardware.
All of these effects conspire to make it difficult to create distortion-free scans for scan ranges larger than roughly 1 micron. In the field of scanning probe microscopes, it is desirable to accurately control both the scan size and pattern, but also to control the position relative to the sample where an image is being obtained. Deviations from linear scans make these accurate measurements difficult. The nonlinear scans also present similar problems in the use of scanning stages in other areas of application.
One possible solution to the hysteresis problem is to apply linear triangular voltage patterns to the electrodes, let the piezoelectric material scan in a non-linear manner and take Z data spaced evenly in time (but not space). After the data is recorded, the correct X, Y the positions of the Z data must be computed using calibration data for the scanner, and then the data is interpolated to construct Z(X,Y) for an equally spaced array in X and Y. This open loop scan control technique employing linear scan and image correction requires extensive calibration of the nonlinear and hysteretic behavior of each piezoelectric scanner during the manufacturing process. It is difficult to produce images in real time using this method, because many calculations are required after the data has been collected, This approach is described by Gehrtz et al in U.S. Pat. No. 5,107,113.
Barrett in U.S. Pat. No. 5,210,410 describes a related open loop method using position sensors which do not require extensive calibration of the scanner parameters. Instead, the position of the probe (or sample) is recorded from the sensor data at many points along the scan, along with the probe deflection data. Barrett describes several limitations of this method including the large amount of data which must be stored, inferior images resulting from the interpolation process, and the inability to measure very small position changes.
Elings et al in. U.S. Pat. No. 5,051,646 disclose another approach to solving the linearity problem by employing open loop scan control with a non-linear scan voltage. In this approach, nonlinear voltage patterns are applied to the electrodes to drive the piezoelectric scanner linearly with time by applying a nonlinear scan voltage. This open loop method preserves the inherent accuracy, low noise and high frequency response of the piezoelectric actuator for nanometer scale scans. For larger scans, in the 1 to 100 micron range, this approach has the difficulty of requiring detailed data concerning the behavior of each particular piezoelectric scanner, and therefore requires extensive calibration of
Dougherty Thomas M.
Veeco Instruments INC
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