Patent Number: 053609743
Section: summary

FIELD OF THE INVENTION The present invention relates to scanning devices delivering extremely stable, nanometer precise, two dimensional displacement of a scanning probe carriage across a target surface. The invention provides an apparatus for delivering the precise displacement for maintaining the constant tip to target surface gap while further providing flatness and thermal and drift compensation. BACKGROUND OF THE INVENTION Scanning probe microscopes (SPMs) are instruments that provide high resolution information about surface contours. Vertical movement of a sensing probe, in response to a raster scanning procedure of the sensing probe across a target surface, is used for determining the target surface contour. Implementations of SPM devices include implementations based on the interaction of attractive forces including atomic, electrical potential, magnetic, capacitive, or chemical potential to maintain a constant probe to target surface gap, or distance. One common use of these devices is imaging. Some types of SPMs have the capability of imaging individual atoms. In addition to imaging surface contours, SPMs can be used to measure a variety of physical or chemical properties with detail over the range from a few Angstroms to hundreds of microns. For these applications, SPMs can provide lateral and vertical resolution that is not obtainable from any other type of device. Examples of applications include imaging or measuring the contour properties of transistors, silicon chips, disk surface, crystals, cells, or the like. In order to provide for high resolution information about surface contours, variables for the SPM include the effective size of the scanning probe, the positioning of the scanning probe above the target surface, and the precision of the scanning device itself. The positioning of the scanning probe above the target surface is to be at a distance of one or two atoms, or an order of magnitude of tens of Angstroms. Further, a non-contact method of positioning is desirable and is the subject of the copending application Ser. No. 07/897,646. Traditionally, scanning probe microscopes have a carriage which can be displaced in x and y directions by means of a piezoelectric actuator, with facilities for fine adjustment. While the arrangement theoretically permits minute displacements of the carriage, it is more difficult to operate the smaller the desired displacement is. This is due to a certain unavoidable backlash in the mechanism and because of the natural friction of the resting stage, which is only overcome with a sudden and mostly exaggerated movement. In addition, some piezoelectric elements have some undesirable properties such as hysteresis, creep, and nonlinear motion. Further, during the scanning procedure, it is desirable to move the carriage independently in a single plane. More specifically, in measuring surface microtopography, in order to survey a surface area accurately, the carriage used to move the scanning tip across the target surface must offer flat motion (i.e. move in a single plane). Flatness is key to large area angstrom level vertical measurements, inasmuch as any vertical deviation of the carriage cannot be separated from either the measurement of the surface contour and therefore contributing to the vertical measurement, or from a component contributing to a noise level. In the first instance, an "out-of flat" carriage motion is one that leads to an anomaly in the apparent surface contour thereby degrading the accuracy of the scanning procedure and integrity of the scan result. In the second instance, the "out-of flat" carriage motion offers a significant component to the noise level of the resulting scan. Flexure devices or hinges permit motion or displacement in a member made of normally non-flexible material. Cut-outs or recesses within a flexure assembly may be separated by web-like sections that are sufficiently thin to provide a desired flexure capability. Such an embodiment is shown, for example, in U.S. Pat. No. 4,559,717. However, the embodiment is one that offers an in plane flexure that offers motion in one direction only, thereby making the device not suitable for scanning. Further, thermal creep becomes critical when making measurements at the tip to target surface gaps of attractive force measurements. Thermal creep refers to the relative motion of the sample in relation to the probe tip caused by a change in temperature. It is a time dependent function that need not be linear or monotonic, and therefore cannot be fully corrected by use of postprocessing schemes. Thermal creep is a function of many parameters, including: thermal expansion coefficients, magnitude and application of thermal gradient, shape of materials, and thermal mass of materials. Any one, or a combination, of the above parameters can effect the integrity of the scanning procedure as the tip to target surface gap varies due to thermal creep, thereby degrading the accuracy of the resulting scan. In view of the fact that the resolution of the new microscope developments and the requirements in electronic circuit manufacturing have increased over several orders of magnitude, it has become necessary to design new positioning devices which avoid the disadvantages of the prior art. OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide a scanning probe microscope scanner providing extremely flat, and highly linear orthogonal motion. It is another object of the present invention to provide a scanning probe microscope scanner having a scanning probe carriage assembly providing natural thermal stability. A further object of the invention is to provide a two axis instrument carriage which permits selective, controlled displacement along either axis independently while providing thermal and flatness compensation along a third axis. It is another object of the present invention to provide a scanning probe carriage assembly providing three-dimensional controlled flexure movement. SUMMARY OF THE INVENTION The present invention provides an apparatus for scanning a sensing probe above a target surface. The sensing probe comprises a microminiature tip integrally formed or mounted at one end which is positioned and maintained above a target surface at a desired gap. In one embodiment of the present invention, a scanning probe carriage consists of a unitary dual quad flexure carriage which comprises a base, and intermediate carriage, and an inner carriage. A set of first quad flexures are interposed between the base and the intermediate carriage so that the intermediate carriage is supported by the first quad flexures, and above the base. A set of second quad flexures are interposed between the intermediate carriage and the inner carriage so that the inner carriage is supported by the second quad flexures and suspended below the intermediate carriage. The scanning probe carriage further provides a surface upon which a scanning probe is received. Further, an embodiment of the invention includes the scanning probe carriage disposed on frame and positioned between a piezo actuators and spring assemblies in a compressed state. Each piezo actuator is interposed between one side of the scanning probe carriage and a support block. In addition, each spring assembly is disposed on a side of the scanning probe carriage opposite that of a piezo actuator at its first end, and further providing a support means to the frame at its second end. Linearity, thermal drift compensation, and flatness are critical in microscopy, since each improves the accuracy of measurements to be made while maintaining the scanning tip above the target surface.