Patent Number: 053609743
Section: description

DETAILED DESCRIPTION OF THE INVENTION The assembly of FIG. 1 is used in a scanning probe microscope. It includes a frame 10 which provides a reference surface with respect to which a dual quad flexure carriage 12 is mounted. The dual quad flexure carriage 12 provides a surface upon which a scanning probe tip (not shown) is received and offers movement of the scanning probe tip with respect to the frame 10 in either an X or Y direction. A pair of piezo actuators 14 (available as the DPTC actuator and manufactured by Queensgate Instruments Inc.) are attached to adjacent sides of the dual quad flexure carriage 12 such that the one end of the piezo actuator 14 bears against a side of the dual quad flexure carriage 12. The second end of each piezo actuator 14 is attached to a holder block 16, which in turn is fastened to the frame 10. The actuators are commercial units combining precision piezo stack actuators with capacitive position feedback sensors to produce motion linearities to +/-0.15% of full range. In the preferred embodiment, a one dimensional (1D) flexure 17 is interposed between a first end of the piezo actuator 14 and the respective side of the dual quad flexure carriage 12, and between a second end of the piezo actuator 14 and the respective holder block 16. The nature and mechanics of one dimensional flexures are known, as illustrated for example, in U.S. Pat. No. 4,667,415. The preferred embodiment further provides for a bearing 18 that is interposed between the second end of the piezo actuator 14 and the 1D flexure 17. In this embodiment, the 1D flexure elements allow a very small rotation about the flexure line without friction. A pair of spring assemblies 20 are attached to adjacent sides, and opposite piezo actuators 14, of the dual quad flexure carriage 12 such that a first end of each spring assembly 20 bears against a side of the dual quad flexure carriage 12 and a second end is fastened to the frame 10 by a block 16 or similar support means. Each spring assembly 20 thus urging the dual quad flexure carriage 12 against the piezo actuator and maintaining the entire assembly in a compressed state. The preferred embodiment of compression of the invention is one in which the piezo actuator 14, carriage 12 and spring assembly 20 combination with support means are maintained substantially at 20 lbs. pressure. Referring now to FIG. 2, it can be seen that the dual quad flexure carriage 12 consists of a unitary flexure assembly which comprises a base 24, an intermediate carriage 22, and an inner carriage 26. The dual quad flexure carriage 12 further comprises four outer flexures 28 and four inner flexures 29. The intermediate carriage 22 is supported off of the base 24 by the four outer flexures 28. The intermediate carriage 22 and base 24 are each quadrilaterals that, along with the four outer flexures 28, form a first parallelogram. Similarly, the inner carriage 26 is suspended from the intermediate carriage 22 by the four inner flexures 29. The inner carriage 26 and intermediate carriage 22 likewise are quadrilaterals that, along with the four inner flexures 29, form a second parallelogram. As further illustrated in FIG. 2, the second parallelogram is smaller than the first parallelogram and is disposed within the first parallelogram such that the intermediate carriage 22 offers a common plane to each parallelogram. In turn, the inner carriage 26 provides a surface for receiving a scanning probe assembly (not shown). The unitary construction of the dual quad flexure carriage 12 provides inherent geometric integrity of matched flexure pairs. The unitary construction of the dual quad flexure carriage 12 along with the matched flexure pairs further offer extremely flat horizontal motion and natural thermal stability. That is, if otherwise constructed with discrete components, the dual quad flexure carriage 12 could result in a structure with unmatched flexure pairs, and a structure lacking the benefits the flatness and thermal stability. The feature of the dual quad flexure carriage 12 providing the property of flat motion is illustrated in FIG. 3. Each inner flexure 29 is double cantilevered between the inner carriage 26 and the intermediate carriage 22. Further, each outer flexure 28 is double cantilevered between the base 24 and the intermediate carriage 22. FIG. 3 illustrates a partial view of the dual quad flexure carriage 12 in a typical deflected condition. As shown in FIG. 3, a lateral force tends to urge the inner carriage 26 in the direction of the force and upward, toward the intermediate carriage 22. Likewise the same lateral force, tends to urge the intermediate carriage 22 downward toward the base 24. Provided the dimensions of both flexures 28, 29 and all cantilever and end conditions are the same, a lateral force applied to the dual quad flexure carriage 12 deflects each flexure 28, 29 the same amount. The upward motion of the inner carriage 26 toward the intermediate carriage 22 is then exactly canceled by the downward motion of the intermediate carriage 22 toward the base 24, thereby resulting in a flat horizontal motion of the inner carriage 26, as well as the entire dual quad flexure carriage 12, Further, the horizontal motion of the inner carriage 26, as well as the deflection of the flexures 28, 29 is dependent on the direction of the force applied. That is, the resulting displacement of the inner carriage 26 is limited to the direction of the force on the dual quad flexure carriage 12. Still however, one significant attribute of the system is that the dual quad flexure carriage 12 moves along one planar axis with minimum displacement along a second planar axis. The dual quad flexure carriage 12 rotates about the fixed 1D flexures 17 of each piezo actuator 14. Because of the large dimension of the piezo actuators 14 versus the scanning motion, displacement off a linear axis is small: less than 5 nm for the full 75 micron range, and less than 1 Angstrom for a useful 10 micron scan. Thermal stability is also a property of the matched flexure 28, 29 pairs. Assuming both flexures 28, 29 in one corner of the dual quad flexure carriage 12 are subjected to the same ambient temperature, each will grow at the same rate (up and down) thereby canceling any net vertical motion of the inner carriage 26. Because of this differential property of the beam pair configuration, each of the four flexure pairs may be subjected to different ambient temperatures (within a few degrees) without theoretically affecting the inner carriage vertical position. This provides a measure of immunity to thermal gradients. To further minimize thermal effects, in the preferred embodiment, the dual quad flexure carriage 12 is made from annealed super invar which has a thermal coefficient of expansion better than two orders of magnitude below that of steel. To further minimize thermal effects, the matched flexures 28, 29 are set in close proximity to one another. This increases the likelihood of both flexures 28, 29 maintaining a same temperature. For thermal stability it is sufficient that each matched flexure 28, 29 pair be maintained at equal temperature; different pairs may be at different temperatures. Although the preferred embodiment of the invention is one in which the dual quad flexure carriage 12 comprises two nested parallelograms, alternate embodiments may include any means whereby two geometrically similar structures are nested with a common reciprocating surface. The two similar structures further being displaced, one from the other, by a flexure means; the combination providing a single, unitary structure. In so doing, the arrangement provides for flat motion as well as natural thermal stability as described in the preferred embodiment of the present invention. The scanner subsystem electronics are shown in FIG. 4. Each piezo actuator 14 is driven by a servo controller to commanded positions. This controller implements a predominantly integral control law and, combined with high precision feedback sensors, provide piezo actuator 14 linearity. In the preferred embodiment, analog command signals are delivered to the servo controllers over an interconnect plane from a multichannel digital to analog conversion board. This high resolution converter/driver receives the position command information in digital form over a high speed serial channel from the system controller. The system controller originates XY position coordinates as part of its task of overall motion coordination and data collection. High resolution, 18 bit conversions are necessary to achieve small command voltage differences that result in the high resolution motion required of the scanner. Noise on the analog side directly contributes to the overall noise floor that becomes the lower limit of the motion resolution. This design is implemented to minimize that noise. All analog components are packaged in close proximity, and signals are distributed over backplanes instead of cables. The analog package is uniformly shielded, and all analog cable runs have been eliminated. While the invention has been described above in connection with a preferred embodiment therefore as illustrated by the drawings, those of skill in the art will readily recognize alternative embodiments of the invention can be easily produced which do not depart from the spirit and scope of the invention as defined in the following claims.