Patent Number: 054830648
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION The present invention consists of two main concepts that differentiate it from the prior art. The first one relates to a mode of operation that is based on the permanent alignment of the probe of the instrument with the main axis of the piezoelectric element providing the scanning motion (the instrument may be any type of scanning microscope having the sample stage mounted on the scanning device). The second concept relates to a sample positioning mechanism that allows the precise placement of the target areas of interest in the sample under the tip of the probe between scanning operations without interfering with the sample during scanning. Both concepts are necessarily related because the permanent alignment of the probe and scanning element by definition prevents the use of translational mechanisms to direct the probe to a particular target area on the sample or the target area on the sample to a particular position under the probe, as done heretofore for coarse targeting. Therefore, the sample itself must be moved to place the target area under the probe and a suitable mechanism to effect that movement in efficient fashion between scanning operations must be provided. As is detailed below, a problem of implementation for any such mechanism is its potential interference with the sample as a result of the sample motion during scanning. This problem is solved by providing a stand-off approach for placing the sample stage to the proper position relative to the probe and by incorporating a known amount of take-up gap between the stand-off and the positioning mechanism. It is noted that the embodiments of the present invention are described with reference to x, y and z orthogonal coordinates wherein x and y define a horizontal plane and z defines a vertical direction, but it is obvious that the structure and operation of the features detailed herein could be rotated in any direction with equivalent results. Similarly, reference to translational motion is at times described as coarse in contrast to the very fine movement (in the order of several microns, with control within 0.1.ANG.) provided by the scanning piezoelectric means. In fact, though, as those skilled in the art understand, the positioning of the target area on the surface of a sample directly under the tip of the probe is a very exact operation that requires precision instrumentation and mechanisms. Finally, it is understood that the sizes of the various components shown in the drawings are not drawn to relative scale because of the microscopic dimensions of some features (such as the probe 10 and the x-y motion of the piezoelectric tube 18) in comparison to others illustrated in the FIGURES. Rather, they are represented only schematically for illustration. Referring to the drawings, wherein like parts are designated throughout with like numerals and symbols, FIG. 4 illustrates in schematic view the basic mode of operation and structure of a scanning microscope 30 according to this invention. It comprises a probe 10 having its main axis permanently disposed coaxially with the vertical piezoelectric tube 18 utilized for providing scanning to the system (note that the vertical motion required for making measurements during scanning may be provided by other means). A flat stand-off 32 is slidably mounted on the flat top 34 of the piezoelectric tube 18, thereby being positioned on a horizontal plane substantially subject to x-y movements (and tilt) as a result of voltage gradients applied to the tube 18. A sample 16 is mounted on the stand-off 32 (either directly or through a sample stage, as illustrated below) and positioned with the target area for investigation directly under the tip 36 of the probe 10. Such precise placement is accomplished by a sample-positioning mechanism 40 that is capable of pushing the stand-off 32 in the x-y directions and cause it to slide over the top of the piezoelectric tube 18, thereby also moving the sample 16 to the desired location. It is important to stress the fact that the structural members described schematically in FIG. 4 consist of high-precision, small components that cannot be properly handled or adjusted without the use of precision instrumentation, such as a focused light source and an optical viewfinder, that is also housed within the support structure of the scanning microscope. In addition, depending on the type of scanning microscope with which the present invention is combined, other hardware essential to the functioning of the microscope is also housed within the same support structure, such as means for sensing the movement of the probe and for controlling the vertical distance between the probe and the sample. Therefore, any mechanism 40 used to translate the stand-off and place the target area of the sample under the probe must be operable within minute tolerances in the crowded environment surrounding the probe/sample area. Manual translation would not be acceptable because it is not sufficiently precise to place the sample in the desired spot, even with the aid of a viewfinder, because of the relatively very coarse spatial control of human motion. On the other hand, the use of mechanical devices, either manually or electronically operated, allows fine adjustments to the position of the sample by pushing the stand-off precisely to the point of interest, but then must be removed to avoid continued contact with the standoff which would interfere with its motion during scanning. Accordingly, a sample positioner is needed that is compatible with the overall general configuration of scanning microscopes and is implemented with hardware capable of slidably moving the sample on a horizontal plane and retracting away from any structure connected to the sample to eliminate the possibility of contact during the scanning procedure. FIGS. 5 and 6 illustrate the basic components of the preferred embodiment of a sample positioner according to the invention. FIG. 5 shows an isolated top view of the positioner 40, which consists of a horizontal plate 42 anchored to portions of a support structure 44 that is integral with or rigidly connected to the horizontal base 46 (FIG. 6) that supports the bottom end 48 of the vertically-disposed piezoelectric tube 18. The plate 42 is adapted for horizontal movement in any direction. As illustrated in the FIGURES, this feature may be accomplished by means of orthogonal, horizontally disposed plungers 50 slidably engaged by conforming guides 52 in the support structure 44 together with cooperatively-aligned push-pull mechanisms for sliding the plungers 50 in and out of the guides 52, as necessary in order to effect the desired translation of the plate 42 to target x-y coordinates. The plungers 50 or the plate 42 may be spring-loaded by means of springs 51 pushing against the plungers in the guides 52 (as illustrated in the-drawings) or against the plate 42 to enhance the plate's rapid response in the pull mode. The push-pull mechanisms may consist, for example, of screw gears 54 attached to the plate 42 (or simply pushing against a springloaded plate) and engaged by drive-gear devices 56 mounted on the support structure 44. As is clearly understood in the mechanical arts, each screw gear 54 so disposed is capable of bidirectional linear motion, thus providing, in combination, the push-pull function required to effect translation of the plate 42 to any x-y coordinates within the range of the screw gears 54. Obviously, a simple screw or set of screws rotatably mounted on the structure 44 and adapted for pushing on the plate 42 would provide equivalent function, but such an arrangement would not be suitable for automatic control. The drive-gear devices 56 may be actuated manually, such as by means of a rotating knob, or mechanically, such as by high-precision stepper motors, possibly through a system of reducing gears that ensure microscopic motion of the plate 42 for each turn of a drive gear. Any alternative mechanism that would permit the precise motion of the plate 42 in the x-y plane would be equivalently suitable to practice the invention. The plate 42 contains an opening 58 sufficiently large to surround a coplanarly-disposed stand-off 32 adapted for slidable coupling with the top end 60 of the piezoelectric tube 18, preferably through an intermediate scanning stage 62 made of low-friction material that is fixedly attached to the piezoelectric tube (FIG. 6). The relative sizes of the opening 58 and the stand-off 32 are chosen so that a sufficient horizontal take-up gap h is present in all directions when the two are coaxially aligned to ensure the unobstructed horizontal movement of the stand-off 32 as the top end 60 of the piezoelectric tube moves in the x-y plane as a result of scanning voltages applied to it. (The gap h is seen in FIG. 5 in the partially cut-away portion of the lip 64, which is described in detail below.) Although any shapes for the opening 58 and stand-off 32 that allow this condition to be met are acceptable to practice the invention, a circular stand-off and a substantially square opening are preferred. To the extent that the stand-off would never come into contact with the corners of the square (it can only come to within a distance equal to its radius), the precise shape of the opening 58 in the vicinity of the corners (illustrated by rounded corners in the FIGURES) is irrelevant to the functioning of the apparatus. If applicable for the particular type of scanning microscope combined with the invention, a vertical gap v must also be left between the top surface of the scanning stage 62 (or of the piezoelectric tube 18, if a scanning stage is not used) and the bottom surface of the plate 42 in order to ensure the unobstructed vertical movement of the scanning stage (and correspondingly of the stand-off and sample) as the top end 60 of the piezoelectric tube 18 moves in the z direction as a result of vertical positioning voltages applied to it. Inasmuch as the horizontal range of typical piezoelectric elements is about 100 microns, a horizontal gap h larger than about 50 microns is sufficient for these purposes. Similarly, since the vertical measurements, and therefore the vertical range required for the piezoelectric element, are in the order of about 15 microns, a vertical gap v of a fraction of a millimeter is suitable to practice the invention. Thus, a sample 16 mounted on top of a sample stage 38 may be moved in the x-y plane as needed to place the specific target area to be scanned directly under the tip of the probe 10. This is accomplished by pushing or pulling, as necessary, the stand-off 32 in the x and y directions by the amount required to meet the target position. The plate 42 is then retracted by an amount sufficient to disengage the stand-off 32 and ensure a horizontal gap h around the entire edge of the stand-off. Since the take-up gap h is predetermined by the dimensions and geometry of the assembly and corresponds to a fixed motion of the devices 56 (such as, for example, a certain number of turns of the screw gears 54) this step may be easily implemented automatically so as to avoid reliance on a user for fine adjustments of the positioner plate 42. Obviously, during the translational movement of the stand-off, the horizontal motion of the plate 42 must also be unobstructed in all directions by any of the components constituting the hardware of the invention and a sufficient range must be provided to reach each end of the sample 16 being analyzed. Typically, sample dimensions have been limited to several millimeters because of the tilt-distortion problems that this invention is addressing, but a sample of any size could be analy in equivalent fashion by the arrangement illustrated in FIGS. 5 and 6. Therefore, the range of horizontal movement of the plate 42 need only be commensurate with the expected size of the samples analy by the scanning microscope. FIG. 7 illustrates in more detail the stand-off portion of the preferred embodiment shown schematically in FIGS. 5 and 6. Since a very small horizontal take-up gap h (in the order of 150.mu.) is sufficient to ensure the free scanning motion of currently available piezoelectric elements 18, the stand-off can be incorporated as a free-moving but coupled component of the positioner 40 by providing lips 64 along the outer edge of the stand-off extending over a conforming inner edge 66 of the opening 58. By carefully sizing the inner diameter of the stand-off 32 (at the side edge 65) in relation to the dimensions of the opening 58, the desired gap h may be maintained around the stand-off 32 even though the outer diameter of the stand-off (at the lips) is larger than the opening 58, thus ensuring that the stand-off is loosely engaged by the plate 42, as illustrated in the plan view of FIG. 8. In practice, a gap h of about 500.mu. is preferred because of standard machining tolerances that would render prohibitively expensive the manufacture of components with a smaller, more precise gap. Thus, a gap of approximately 500.mu. is suitable for manufacturing purposes and is well outside the scanning range on conventional piezoelectric scanners. Obviously, the precise width of the lips 64 is not critical so long as sufficient to overlap the opening 58 and provide interlocking connection with the plate 42. In operation, the plate 42 is actuated to position the portion of the sample to be scanned directly under the tip of the probe 10, which is aligned with the axis A of the scanning element 18. Then the plate is moved back from the stand-off by an amount sufficient to leave at least a gap h in all directions. Since this distance is predetermined, this step is easily accomplished either manually or automatically, such as by knowing the corresponding turns of the actuating gears or screws necessary to effect a desired translation both in the x and y directions after the sample has reached its intended position. In FIG. 7, for example, the sample is shown as being positioned for scanning of its left edge after a corresponding translation of the stand-off 32 and sample stage 38 toward the right side of the FIGURE. As understood from the FIGURE, scanning may now occur according to conventional processes with the probe 10 coaxially aligned with the scanning tube 18 within the tolerances of the gap h in all directions, so that minimal distortions are produced by the tilt of the top surface 34 of the tube. Note that the concepts of the invention may be easily integrated by those skilled in the art with automated systems normally used to operate scanning microscopes. For example, as illustrated schematically in FIG. 9, the operation of the sample positioner 40 may be controlled manually by a directional lever 72, such as a joy stick, in response to visual input received through a viewfinder 74 focused on the tip of the probe 10 or through a video screen 76 connected to such viewfinder. A microprocessor 78 would normally be utilized to actuate the mechanisms effecting the motion of the sample positioner 40 in response to movements of the lever 72 and the system would preferably be programmed to cause the plate 42 to retract a predetermined, fixed distance h in response to a signal that the sample had reached a desired target location. In this mode of operation, the take-up gap h is set automatically and independently of any control action by the user, which enables rapid response and uniformity of results. In the best mode of the invention, a scanning stage 62 made of low-friction ferromagnetic material is fixedly mounted on the top 34 of the scanning element: 18 and a magnet 68 is embedded in the lower portion of the stand-off 32, which is also made with low-friction material, to provide firmer coupling between the two. Thus, the stand-off is readily slidable over the scanning stage during the placement operation but is securely connected to it during the scanning stage. Similarly, the sample stage 38 may also be made of ferromagnetic material and a magnet 70 (which may be combined with magnet 6 in a single magnetic unit) may be provided to improve mounting of the sample stage over the stand-off 32. Obviously, in both cases the stand-off may equivalently be made of ferromagnetic material and the magnets incorporated into the top of the scanning element or of the sample stage, as applicable. Various changes in the details, steps and materials that have been described may be made by those skilled in the art within the principles and scope of the invention herein illustrated and defined in the appended claims. For example, it is clear that the usefulness of the concept of coaxial alignment between the probe and the scanning means is not limited to piezoelectric tubes; rather it may be used advantageously with any scanning means that produces a tilt of the sample as a result of the scanning motion. Therefore, while the present invention has been shown and described herein in what is believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent processes and products.