CA1312681C - Variable temperature scanning tunneling microscope - Google Patents

Abstract

A Variable Temperature Scanning Tunneling Microsope ABSTRACT

A themally compensated tube scanner scanning tunneling microscope utilizes two concentric piezoelectric tubes, one for scanning and one for coarse translation as well as fine adjustment of sample position while in tunneling range. There are no mechanical components such as springs, levers, gears, or stepper motors which are known to result in considerable vibration sensitivity and thermal drift. Consequently, the standard mode of atomic resolution operation for the device is without vibration isolation and with a thermal drift of less than 1 angstrom per hour.

Description

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A Variable Temperature Scannin~ Tunneling Microscope This invention relates to a scanning tunneling microscope (STM) which is thermally compensated for use 5 at various temperatures and is also substantially insen-sitive to vibration and shock.

BACKGROUND
The development of atomic resolution scannng 10 tunneling microscopy (STM) and spectroscopy (STS) by Binnig and Rohrer [(G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, p~1. Phys. Lett., 40, 178 (1982); Phys.
Rev. Lett., 49 57 (1982); and G. ~innig and H. Rohrer, IBM J. Res. Dev., 30 355 (1986)] has opened a new era of 15 surface science. The first STMs were based on the original IBM*Zurich tripod design in which three orthogonal piezoelectric rods support and scan the tun-neling probe, while sample translation is accomplished by means of an electrostatic "louse". The known systems 20 of vibration isolation are of two primary types, i.e., two levels of spring suspension with associated eddy curr~nt damping, [(G. Binnig and H. Rohre, Helv. Phys.
Acta., 55, 725 (1982~] or the stacked plate arrangement with Viton spacers and springs separating a half-dozen 25 or so metal plates. [Ch. Gerber, G. Binnig, H. Fuchs, O. Marti, and H. Rohrer, Rev. Sci., 181, 92 (1987)].
More recently, the piezoelectrically driven "louse" has given way to the micrometer driven differential spring [B. Drake, R. Sonnenfeld, J. Schneir, and P.K. Hansma, 30 Surf._Sci., 181, 92 (1987)] and stepper motor gear re-duction [Sang-il Park and C. F. Quate, Rev. Sci.
Instrum., 58, 2011 (1987)] approaches for coarse sample positioning. These techniques are more reliable and the differential spring assembly is easily incorporated into 35 the overall STM design and works well at low tempera-tures. [A. P. Fein, J. R. Kirtley, and R. M. Feenstra, - Rev. Sci. Instrum., 58, 1806 (1987)]. * Trade-mark _ . _ _ _ _ _ ~3~ 2`~L

A major problem with tripod scanners is thermal drift, with millikelvin temperature stability required for low drift imaging. [Sang-il Park and C. F.
Quate, A~91 ~h~ , 48~ 112 (1986)]. This situa-5 tion has been helped by alternate designs such as thethermally compensated matrix STM of van de Walle et al., [G. F. A. van de Walle, J. W. Geritsen, Ho van Kempen, and P. Wyder, Rev~ Sci. Instrum., 56, 1573 (1985~] and the bimorph driven metal tripod design of Jericho et al.
10 [B. L. Blackford, D. C. Dahn, and M. H. Jericho, Rev.
Sci. Instrum., 58, 1343 (1987)]. The thermally compen-sated matrix design uses small cubes oE piezoelectric material arranged such that lateral and z-direction thermal drift cancel out. Although this design has 15 relatively low thermal drift, it is not low enough for variable temperature work, and the design is complex and difficult to build.
A significant step towards simplifying STM
design was the development of the tube scanner STM by 20 Binnig and Smith. [G. Binnig and D. P. E. Smith, Rev.
Sci. Instrum., 57, 1688 (1986)] In this design, a single piezoelectric tube with its outer electrode divided into four equal quadrants, parallel to the tube axis, provides lateral scanning motion by tube bending 25 when voltages are applied to two adjacent outside quadrants, and z-displacement when voltage is applied to the common inner electrode. The tunneling probe is affixed to one of the grounded outer quadrants. 8ecause of its simplicity, small size, and rigidity (with 30 associated high resonance frequencies), the tube scanner has replaced the scanning sections of many older STM
designs. Due to the tube's symmetry, a coaxially located tunneling probe would not undergo lateral thermal drift for uniform temperature changes. However, 35 the elimination of thermal drift along the z-direction would require some sort of compensation. An effective :L3~ 8~

compensation scheme is to affix the sample holder to a second, concentric piezoelectric tube which is the same length as the scanning tube. This would be the tube scanner analog of the thermally compensated matri~
5 design of van de ~alle et al. [Go F~ A. van de Walle, J.
W. Gerritsen, H. van Kempen, and P. Wyder, Rev. Sci.
Instrum., 56, 1573 (1985)~.
Although it is reasonably straightforward to thermally compensate the scanning element(s) in an STM, 10 there can still be considerable thermal drift and vibra-tion sensitivity arising from the sample holder and its associated coarse positioning system. Most STM coarse positioning systems incorporate mechanical elements such as springs, levers, gears, micrometers, and stepper 15 motors. These are coupled directly to the sample holder yet they are typically 108 times larger than the tunnel-ing gap width. Consequently, thermal drift and mechani-cal vibration of these elements directly modulates the tunneling gap.
An interesting design, which eliminates these components is the so called "Johnny Walker" STM [K.
Besocke, Surf. Sci., 181, 145 (1987)] in which a tube scanner is located symmetrically at the center of an arrangement of several additional tubes. This STM can 25 be operated inverted with a sample being placed directly on the outer legs or non-inverted, in which the STM
"walks" over a surface. Walking motion is acco~plished by slowly bending and then rapidly straightening the outer legs, resulting in inertial translation of the 30 entire STM. Inertial translation of a mass using a piezoelectric transducer configuration has been demon-strated by Pohl. [D. W. Pohl, Surf. Sci., 181, 174 (1987); and Rev. Sci. Instrum., 58, 54 (1987)]. Coarse positioning with the Johnny Walker STM is a problem 35 requiring the sample be placed on an incline, and its overall size results in the need for vibration isolation and makes variable temperature operation difficult.

SUMMARY OF THE I~ENTION
In accordance with the invention, there is provided an improved scanning tunneling microscope which 5 is thermally compensated and substantially insensitive to vibration. The microscope comprises a pair of con-centric piezoelectric tubes of the same length and composition. A tunneling probe is attached to an end of the inner tube, which is divided into equal lateral 10 quadrants for providing lateral scanning motion~
Attached to the adjacent end of the outer piezoelectric tube is an annular collar which connects to a sample holder tube. The dimensions and thermal expansion co-efficient of the annular collar are selected to compen-15 sate for thermal variations in the length of the tunnel-ing probe. The sample holder rests slidably on a platform within the holder tube, and can be adjusted to bring the sample into tunneling range by inertial move-ment of the holder.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
Figure 1 is an exploded side view in partial section, of the two subassemblies A and B which make up 25 the present invention;
Figure 2 is a cross section along the line 2-2 of Figure lA;
Figure 3 is a detail view of the probe assembly of Figure lA; and Figure 4 is a detail view of the inner pie20-electric tube of Figure lA.

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VETAILED DESCRIPTION
Shown in Figure 1 is a representation of a tunneling microscope in accordance with the invention which is compact, rigid, and fully thermally compen-5 sated. Figure lA is the microscope per se, while FigurelB shows a temperature control shroud which can be used for thermal isolation. Microscope 10 utilizes two con-centric piezoelectric tubes 11 and 12 which are indium soldered into a common base 13. Inner tube 11, 10 (typically 1.27 cm long x 0.635 cm O. D.) supports and scans tunneling probe 14 while outer~tube 12 (1.27 cm long x 0.953 cm I.D.) supports an annular collar 16 which holds a quartz sample holder tube 17. Inner scan-ning piezoelectric tube 11 is based on the design of 15 Binnig and Smith [G. Binnig and D. P. E. Smith, Rev.
Sci. Instrum., 57, 1688 (1986)]. In the design (see Figure 4) used herein, however, tunneling probe 14 is mounted along the central axis of tube 11 rather than on its edge. The advantage of this is that the radial 20 thermal expansion or contraction of the scanning tube is symmetric about the tube axis, and thus does not result in lateral displacement (in the x-y plane) of the tunneling probe. This alone, however, does not com-pensate for temperature induced length changes of the 25 scanning tube (in the z-direction). For this purpose, outer piezoelectric ube 12 is the same length and made from the same material as inner tube 11. Thus, quartz sample holder tube 17 which is attached through collar 16 to outer tube 12 moves in con~ert with inner scanning 30 tube (11) length changes in response to temperature variations, resulting in zero net change of the distance from probe 14 to sample 18 attached to sample holder 19. Collar 16 connecting outer tube 12 to quartz sample holder tube 17 is chosen with respect to its length and 35 thermal expansion coefficient such that its thermally induced length changes match that of the tunneling ~3 ~6~

probe. Collar 16 is suitably made of beryllium copper and has a length which is shorter (becuse of higher thermal expansion coefficient) than the length of probe 14, which is suitably made of tungsten. Materials other 5 than beryllium-copper and tungsten can be used for making the collar and the probe, respectively, as will be apparent to those skilled in the art, provided only that the rela ive lengths are appropriately adjusted.
Since quartz (commercially available fused silica) has 10 the lowest thermal expansion coefficient over a wide temperature range, quartz sample holder tube 17 and sample mounting block or holder 19 introduce negligible thermal drift. An OFHC (oxygen-free high purity copper) shroud 21 encloses outer piezoelectric tube 12 in order 15 to minimize temperature gradients which might defeat the desired thermal compensation.
A key feature of the present invention is the fact that outer piezoelectric tube 12 is used not only to provide thermal compensation, but also to provide a 20 means of inertially translating sample 18 under study toward or away from tunneling probe 14. For example, to translate sample lR towards probe 1~, a voltage ramp (sawtooth) is applied to outer piezoelectric tube 12, causing it to contract. At the end of the ramp, the 25 voltage is rapidly returned to its initial value, causing tube 12 to rapidly expand back to its initial position. Due to inertia, sample mounting block 19 which rests and slides on rails 22 in quartz sample holder tube 17, cannot follow the rapid motion of the 30 rails. ~hus, when the outer tube returns to its initial position, sample holder 19 block has been translated towards tunneling probe 14 by one "step". By adjusting the amplitude and timing of the ramp signal, step sizes ranging from 1 micron down to about 5 angstroms are 35 readily achieved. This stepping process can be repeated very rapidly (up to several kHz) resulting in very fast translation (up to 1 mm/sec) of sample 18 toward probe 14. By reversing the ramp symmetry, sample 18 can be readily stepped away from tunneling probe 14. In a test microscope, it was found that the sample can be trans-5 lated into tunneling range at a rate visible to thenaked eye without "crashing" into tunneling probe 14.
For this coarse translation, voltage ramps of +/-150 volts peak-to-peak are typically used. Once in tuneling range, the dc voltage level can be adjusted to fine tune 10 the sample~to-probe distance. In practice~ however, several small inertial steps (typically 5 angstroms, with +/-30 volt ramps) are taken first, while in tunnel-ing range, in order to zero the feedback voltage. This eliminates the need for large dc voltages being applied 15 to piezoelectric tubes 11 and 12, and hence, virtually eliminates the slow drift associated with piezoelectric creep. As a result of this design, no mechanical posi-tioning devices such as micrometers or stepper motors with their associated reduction gears, levers or springs 20 are necessary. Elimination of these components greatly reduces the complexity and size of the design while improving its performance.
For sample registry, quartz sample holder block 19 has two notches 23 and 24 which fit over quartz 25 rails 22 in sample holder tube 17. With the STM in its horizontal operating position, one of these quartz rails is higher than the other such that gravity forces the sample holder block to slide along the lower rail. This provides precise repositioning of the sample holder 30 block even if it is removed from the STM and subse-quently replaced.
Electrical contact to sample 18 can be achieved by sputtering or depositing chromium or other metals onto the quartz. In one embodiment, chromium is 35 sputtered onto the quartz sample holder rails 22, as well as notches 23, 24, and sides 26 of sample holder block 19. This provides two electrical contacts for a sample mounted on the face of the block; one for the tunneling current return path and one Eor auxilary sample biasing. Once sample 1~ has been mounted on 5 sample holder block 19, electrical contacts with the STM
system are automatically made by placing the block on the rails. This expedites sample turn around time and greatly simplifies oper~tion in restricted environments such as ultrahigh vacuum.
For variable temperature operation, a tempera-ture control assembly 27 (Figure lB) is placed over microscope 10 and screwed onto a threaded base 28 (Figure lA), separated from base 13 by a teflon washer 29. Thermal isolation is accomplished by fitting a 15 teflon sleeve 31 over anodized aluminum shroud 32 which then fits over a teflon plug 33 adjacent threaded base 28 when shroud 32 is screwed in place. Shroud 32 is provided with a silicon diode temperature sensor 38 and wound with a chromel heater 39 for use with a suitable 20 temperature controller (not shown)O Electrical feed-thEoughs 34, 36, and 37 through base 33 are low heat leak stainless steel coaxial cable. While the individual electrical connections to the components of the STM of the invention are not shown, those skilled in 25 the art will-appreciate that such are necessary, and will also know how to make the connections. Similarly, it will be appreciated that an appropriate control and power circuit must be supplied for use with the inven-tion.
The STM of the invention provides atomic resclution without vibration isolation, a result not previously achieved. By eliminating all of the mechan-ical positioning mechanisms used in prior designs, the STM plus sample holder block 19 move in unison in re-35 sponse to external vibrations until the static friction between sample holder block 19 and quartz ralls 22 is ~ 3~.2~

g overcome. The forces due to normal building and sound vibrations are insufficient to overcome this static friction.
The invention provides very good electrical 5 shielding for the sensitive tunneling circuit. Collar 1~ which connects sample holder tube 17 to outer piezoelectric tube 12 also covers the annular gap be-tween the tubes where the scanning and translating voltages are applied. In addition, temperature control 10 shroud 32 is maintained at ground potential to provide a shielded enclosure for the entire STM. Since vibration isolation is not required, all of the electrical con-nections are routed via coaxial cables 34, 36, 37 whose shields are soldered into base 13 of the ST~. Other 15 designs using flexible long wires to prevent vibration coupling, suffer in terms of poor electrical shield-ing.
It has also been found that teflon insulated coaxial cable must be avoided for the sensitive probe 20 circuit, since ambient vibrations generate considerable triboelectric charges. Further electrical shielding is obtained in the design ~f the invention by maintaining the inside of scanning tube 11 at ground potential, since it is in close proximity to the sensitive tunnel-25 ing circuit. As shown in Figure 3, tunneling probe 14is isolated from ground by means of alumina washer 41.
This allows tunneling probe 14 to be biased at any de-sired potential whlle minimizing spurious pickup which - could occur if the z-axis control voltage were applied 30 to the inner contact of scanning tube 11 as it is in conventional tube scanner STMs. Instead, the z-axis control voltage is electronically summed to the x- and y-axis control voltages which are then applied to the four outer quadrant contacts of scanning tube 11. Since 35 tunneling probe 14 is mounted coaxially, all four quad-rants must be used, otherwise the x- and y-axis control ~ 3 ~

voltayes would introduce an erroneous z-axis displace-ment. In other words, if one side of tube 11 is shortened to bend it in the ~x direction, tunneling probe 14 would be pulled away from sample 18 by one-half 5 of the tube shortening distance. To prevent this, the opposite side of the tube must be extended by the same amount, thus requiring the same magnitude but opposite polarity voltage. Consequently, four high voltage amplifiers (not shown) are required to provide x+z, 10 -x+z, y+z and -y+z control voltages for the scanning tube outer quadrants. Existing tube scanner STMs utilize only three high voltage amplifiers since the tunneling probe is mounted to one of two adjacent quad-rants which are at ground potential, and hencet do not 15 expand or contract. The disadvantages of this are the loss of coaxial thermal compensation and the fact that for the same lateral displacementl twice the voltage used in the present invention must be applied. Thus, for the same lateral displacement the invention exhibits 20 less piezoelectric hysteresis and creep while for the same high voltage limit it provides four times the scan area of conventional tube scanners. An additional advantage of this new operating scheme is that imper-fections in the piezoeletric tube and its machining can 25 be electronically balanced out.
Tunneling probe replacement is accomplished very easily. Tunneling probe 14 is soldered into a beryllium copper tip holder 42 (Figure 3) which is shaped for use with a conventional hexagonal wrench and 30 has a threaded tail 43, which is screwed into an internally threaded adaptor 44 on scanning tube 11. For nonsolderable metals such as tungsten, the base of tunneling probe 14 can be electroplated with nickel to enable soft soldering into the tip holder. With this 35 arrangement, minimal manipulation requirements are necessary for operation in restricted environments such as ultra-high vacuum.

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The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom as modifications will be obvious to those skilled in the 5 art.

Claims (7)

1. A scanning, tunneling microscope compris-ing:
a base;
an inner and an outer piezoelectric tube con-centrically connected to said base, said tubes having substantially the same length and the same composition;
a tunneling probe having a tip, said probe being coaxially connected to an end of said inner piezoelectric tube opposite said base;
a temperature compensating annular collar co-axially connected to an end of said outer piezoelectric tube opposite said base, said collar having a length and composition adapted to compensate for thermal changes in the length of said probe;
a sample holder tube axially connected to said collar;
a platform extending longitudinally within said holder tube and attached thereto;
a holder for holding a sample to be examined, said holder resting on and movable along the length of said platform;
conductor means for applying selected voltages to said outer piezoelectric tube to cause inertial move-ment of said sample along said platform to a distance within tunneling range from said tip of said probe; and conductor means for applying selected voltages to said inner piezoelectric tube to cause said probe to scan a portion of a surface of said sample.

2. A tunneling microscope in accordance with claim 1 wherein said platform comprises a pair of spaced rails.

3. A tunneling microscope in accordance with claim 2 wherein said sample holder is provided with spaced notches adapted to engage said spaced rails.

4. A tunneling microscope in accordance with claim 3 wherein one of said rails is at a greater eleva-tion than the other.

5. A tunneling microscope in accordance with claim 3 wherein said sample holder tube, said sample holder, and said rails are all made of fused silica or fused quartz.

6. A tunneling microscope in accordance with claim 1 wherein said base and said annular collar are made of beryllium-copper.

7. A tunneling microscope in accordance with claim 1 further including a temperature control shroud comprising a removable thermally insulated hollow cylinder adapted to envelop said microscope, said cylinder being provided with temperature control means.