Positional sensors have applications where a very accurate measurement of position is needed over a limited range. One example of such an application is in manipulation of a stage of a high-resolution microscope such as an electron microscope. Electron microscopes include scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), scanning transmission electron microscopes (STEMs), and various kinds of reflection and emission electron microscopes.
Such microscopes have much higher resolution than optical microscopes, and therefore require manipulation of a stage with very high precision. The location of the specimen stage is generally measured as it is moved so that when a feature is found, its location may be recorded and the microscope may return to the feature if desired. In addition to measuring the location of the stage, the angle of tilt of the stage with respect to the electron beam illumination must be measured to align the objective lens properly. Thus, the position of the stage may include location in three translational and two angular degrees of freedom of the stage.
Optical encoders may be used to measure the position of an electron microscope stage. Even though modern optical encoders overcome the light wavelength limit by interpolation and can measure down to a few nanometers displacement, the direct measurement of displacement is only possible within the periodicity of the ruler used — typically, a few um. When the displacement is larger, a counter for the traversed ruler periods is necessary. The counter can get out of synchronization with the ruler, which usually means that the experiment must be terminated and a run to a special reference mark must be made. Other mechanisms may be used to measure position; however, the environment of an electron microscope presents problems for many techniques. In particular, techniques based on magnetic fields (e.g. a linear differential transformer) may be susceptible to interference from stray magnetic fields created by an electron microscope, or may influence the performance of the microscope by its own stray field.
A simple yet powerful positional sensor was developed that can determine the position of an object to within a few nanometers. The new sensor provides information on both angular displacement and two-dimensional lateral displacement of moving objects, and can be used in any device requiring highly precise measurement of movement, such as electron microscopes and optical systems. The invention represents a breakthrough improvement over existing approaches that provide measurements within the same degree of accuracy in only one or two dimensions within one sensor.
The sensor uses opposing sets of capacitor plates (Figure, A) to measure an object's displacement by measuring changes in capacitance as overlapping areas of the capacitor plates move. One set of plates is attached to a stationary object, and another set of plates is attached to a moving object. The measurements are transmitted to a circuit that calculates angular and lateral displacement. For translational displacement, both moving capacitor plates are used for measurements (Figure, B), whereas for rotational displacement, individual measurements from the two rotational plates are compared (Figure, C). Depending on the selected electronic resolution, the system can calculate both the linear and angular displacement of one object relative to another (Figure, D) in a few tenths of a second, or faster if less precision is selected for a dynamic approach.