Patent Number: 047629967
Section: description

DESCRIPTION OF THE EMBODIMENT Referring to FIG. 1, the object or sample 1 to be positioned is attached to a sample holder 2 which in turn is fixed to a block 3. Resting on a tilting knife 4 is one edge of the bottom side of block 3, while the other edge thereof is supported by a first spring 5. Tilting knife 4 and spring 5 rest on a common frame 6 of which one arm 7 supports an adjustment screw 8 which acts on a second spring 9 arranged between said screw 8 and the upper surface of block 3. Since a scanning tunneling microscope was chosen for the explanation of positioning device in accordance with the invention, a tunnel tip 10 is mounted to a tripod 11 having three legs 12, 13 and 14 (leg 14 extending vertically into the plane of the drawing and not visible in FIG. 1), each leg being associated with one coordinate of an xyz coordinate system and having one end fixed to frame 6. The other ends of legs 12, 13 and 14 are connected in a common joint 15. The legs 12 . . . 14 of tripod 11 may, for example, consist of piezoelectric elements which can be caused to expand or contract by way of applying electric potentials to the electrodes they carry (not shown). Tripods of this type are known in the art, e.g., from IBM Technical Disclosure Bulletin Vol. 27, No.10B (1984) p.5976. With tunnel tip 10 mounted on an arm 16 of tripod 11, even fine-adjustment of the distance between tunnel tip 10 and sample 1 can be performed. In FIG. 1, sample 1 is supported between two springs, 5 and 9. Depending on the ratio of the spring constants, i.e., with spring 5 being harder than spring 9, the resulting movement of sample 1 will be proportionally smaller than the movement of screw 8 when turned. It is to be understood that the tilting action performed by block 3, as screw 8 is lowering and compressing spring 5, is so small that for the purposes of this invention the motion of sample 1 attached to block 3 can be considered linear, as opposed to occurring on a circle, as is mathematically correct. One embodiment of the coarse-approach positioning device in accordance with FIG. 1 had an eigenfrequency against the tripod that was considered too low, leading to a tendency of propagating vibrations. This disadvantage can be overcome with a positioning device in accordance with FIG. 2. Referring to FIG. 2, there is shown sample 1 held by a sample holder 2 which is attached to a block 17 that rests, along one edge, on a tilting knife 4. Block 17 is balanced between springs 5 and 9 which may have different spring constants as explained above. So far, this arrangement is similar to the embodiment in accordance with FIG. 1. The approach of sample 1 towards tunnel tip 10 down to a distance of about 2 micrometers may be controlled with an optical microscope, or by field-emission under ultra-high vacuum. When this distance is reached, spring 5 is complemented with a considerably harder spring which may take the form of a wedge 18 resting on frame 6. By turning screw 8 and compressing springs 5 and 9, block 17, with its shoulder 19, will eventually touch down upon wedge 18 so that block 17 will become balanced between spring 9 and wedge 18, with spring 5 only contributing negligibly. Wedge 18 may, e.g., consist of rubber, or better viton, but in any case must have a spring constant at least one order of magnitude greater than spring 5. As screw 8 is now turned further, block 17 will be moving at a considerably slower pace than before. Still another embodiment of the positioning device in accordance with the invention is shown in FIGS. 3 and 4 being, respectively, a perspective view and a cross section of the same embodiment (though not drawn to scale). In the embodiment of FIGS. 3 and 4, tilting knife (4) and spring (5) on which the block (3, 17) carrying the sample (1) rests in FIGS. 1 and 2, have been combined into one single element, viz. a leaf spring (20). Referring to FIGS. 3 and 4, leaf spring 20 is fixed to a frame 21 (equivalent of frame 6 in FIGS. 1 and 2) and extends beyond the upper surface 22 thereof. Attached to leaf spring 20 is a block 23 in such a way that a gap 24 is formed between frame 21 and block 23. Gap 24, of course, extends parallel with the surface 22 of frame 21. An arm 25 is screwed to frame 21 on the back of leaf spring 20 such that block 23 is free to move without interfering with arm 25, as shown in FIG. 4. Arm 25 in its horizontal section 26 has a thread 27 with very low pitch, which accommodates a pressure screw 28. On the top surface 29 of block 23 there rests a pad 30 consisting of elastic material. The lower end 31 of screw 28 is aligned to engage pad 30 when screw 28 is threaded through. The pressure thus exerted on pad 30 will cause block 23 to be tilted to the left in FIG. 4 as leaf spring 20 bends. The amount of tilting of block 23 is determined by the geometric parameters of the embodiment including, of course, the movement of screw 28, as well as by the relationship of the spring constants of elastic pad 30 and leaf spring 20, respectively. As a general rule, the spring constant of leaf spring 20 will be chosen much higher than that of pad 30. Extending from block 23 at the side opposite leaf spring 20 is a sample holder 32. Held on the lower surface of sample holder 32 is the sample 33 to be inspected. The inspection is made with the aid of a sharply pointed tunnel tip 34 which is attached to a fine-approach and scanning device 35. Device 35 may, e.g., comprise the tripod 11 of FIGS. 1 and 2. In the embodiment of FIGS. 3 and 4 it is realized in the form of a piezoceramic tube 36 which, together with frame 21, rests on a common base plate 37. As is known in the art, tube 36 carries pairs of electrodes attached to diagonally opposite surface areas thereof which, when energized with voltages of appropriate waveform and phase, enable tube 36 to perform linear or circular motion. Attached to the upper end of tube 36 is tunnel tip 34. As tube 36 moves, tunnel tip 34 scans the surface of sample 33 along such tracks as determined by the voltages applied. Either the stem 38 of tunnel tip 34 or the fine-approach and scanning device 35 may be designed to be controllable by a feedback signal derived from the tunneling current flowing across the tunnel gap between tunnel tip 34 and sample 33, and being a measure for the width of that gap, as is well known in the scanning tunneling microscope art. The tilting movement of block 23 and, hence, the (essentially) vertical movement of sample 33 are derived from the movement of screw 28 as modified by the relationship of the spring constants of leaf spring 20 and elastic pad 30. With the arrangement of FIGS. 3 and 4 so far described it will be possible to approach sample 33 towards tunnel tip 34 to within a fraction of one micrometer. When that distance has been reached,--and this can be determined through inspection with an optical microscope,--the relationship of the spring constants is changed by inserting a wedge 39 into gap 24 so that block 23 with one edge now rests on said wedge. The movement of block 23, as screw 28 is turned, now depends on the relationship of the spring constants of pad 30 and wedge 39. In this case, the contribution of leaf spring 20 is negligible. With wedge 39 inserted in gap 24, and wedge 39 assumed to consist of a material with low elasticity, it is easily possible to position sample 33 within the tunneling region, i.e., with a gap between sample 33 and tunnel tip 34 on the order of one nanometer. As mentioned before, fine adjustment during scanning operation is made by energizing fine-approach device 35.