design and performance of a programmable-temperature scanning tunneling microscope

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 69, NUMBER 5 MAY 1998

Design and performance of a programmable-temperature scanningtunneling microscope

M. S. Hoogeman, D. Glastra van Loon, R. W. M. Loos, H. G. Ficke, E. de Haas,J. J. van der Linden, H. Zeijlemaker, L. Kuipers,a) M. F. Chang, and M. A. J. KlikFOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands

J. W. M. FrenkenKamerlingh Onnes Laboratory, Leiden University, P. O. Box 9506, 2300 RA Leiden, The Netherlands andFOM-Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, TheNetherlands

~Received 29 October 1997; accepted for publication 17 February 1998!

In this article we introduce a novel scanning tunneling microscope~STM!, which operates in asample temperature range from 60 to at least 850 K. The most important new feature of this STMis that, while one selected part of the surface is kept within the microscope’s field of view, thesample temperature can be varied over a wide range of several hundreds of degrees during actualimaging. The extremely low drift of the scanner and sample was achieved by the combination of athermal-drift compensated piezoelectric scanner design with a newly developed sample stage. Thedesign of the sample stage defines a fixed center from which thermal expansions, in all threedirections, are forced outwards. The performance of the microscope is demonstrated for severalsurfaces including Au~110!, on which we follow one particular surface region over a temperaturerange of more than 270 K. ©1998 American Institute of Physics.@S0034-6748~98!02105-4#

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I. INTRODUCTION

A. Motivation

Temperature is a key parameter in many surface proties. Examples are surface phase transitions,1–5 surfacediffusion,6–11 and crystal growth phenomena.12,13 Neverthe-less, one of the most powerful real-space surface imaginstruments, the scanning tunneling microscope, is veryceptible to temperature variations of the specimen. Emodest thermal drifting distorts the images and usuamakes it impossible to follow one object on the surface oan appreciable range of temperature. Even more catastrocally, severe thermal drifting of the surface towards or awfrom the tip, ends the measurement when the change inface to tip distance exceeds the limited span of the piezoetric actuator.

In recent years scanning tunneling microscopes~STMs!have been developed which allow imaging at temperatuwell above or below room temperature. In these microsconondistorted images can be obtained after a sufficiently lthermal equilibration time. Efforts have been made to mimize the equilibration time and the thermal drift eitherlimiting the heat capacity and the total heating power,14–16orby a proper choice of materials and a uniform heating ofentire microscope.17 The company Omicron has developedvariable-temperature STM which operates down to 25 K.narrow beam-shaped Si samples a maximum temperaturebeen reached of 1500 K.15 For different temperature regimeand sample materials different sample holders have to

a!Present address: Dept. of Applied Physics, Twente University, P.O.217, 7500 AE Enschede, The Netherlands.

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used. The company JEOL has constructed a variatemperature STM which works for temperatures below aabove room temperature.16 The samples are heated eitherdirect current or by an optional indirect heating holder wwhich they reach a temperature of 1175 K.

A true optimization has been performed by Kuipeet al.18 They modeled the thermal behavior of their micrscope by means of a finite element analysis. By maximizthe temperature difference between the sample and the pelectric scanner the thermal equilibration time was mimized. The drift perpendicular to the sample surface wreduced to 0.71mm over a temperature interval of 159 KHowever, the drift in the lateral direction over the same teperature interval still exceeded the61.7 mm range of thepiezoelectric actuators. The STM in Ref. 18 was not dsigned for measurements below room temperature.

Voigtlanderet al.14 reported a STM with which the samspot on the surface is kept in view over a temperature raof 100 K. They minimized the perpendicular and lateral drby reducing the necessary heating power and the distaover which expansions occur. As in the Omicron STM, thcould only be achieved for small semiconductor sampleswere locally heated by direct current, and cannot be repduced on a bigger metal sample with indirect heating. Lding et al.17 have built a variable-temperature STM whichheated and cooled as a whole. A proper choice of materreduced the drift in the lateral plane to only 10 Å/K. Theresults were obtained for a non-ultrahigh vacuum~UHV!version of the microscope. The design has been adapteduse in UHV and for high temperature measurements of seconductor samples heated by direct current.19

In summary, many STM designs incorporate some fox

2 © 1998 American Institute of Physics

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2073Rev. Sci. Instrum., Vol. 69, No. 5, May 1998 Hoogeman et al.

of optimization of the thermal behavior to shorten the thmal equilibrium time and to lower the thermal drifting. It habeen demonstrated that during measurements the satemperature can be changed, without exceeding the spathe piezoelectric actuator. However, it is still not possibwith existing equipment to change the temperature of a nsemiconductor sample over an appreciable range, in Uwhile keeping the same surface area within the microscoview.

In this article we describe the design and operation oUHV-compatible, programmable-temperature STM. Wshow that during a sample temperature change of sevhundreds of degrees the same area can be kept in viewmetal sample, independent of the necessary heating pow

B. Specifications

Our main objective was to build a UHV-compatibSTM in which the sample temperature can be swept oroughly 300° during actual imaging, while one particularea on the surface is kept within the microscope’s fieldview. The temperature sweep-range of 300° guaranteeswe can follow, e.g., a complete surface phase transitionthe evolution of surface morphologies.

The ranges of our piezoelectric actuator are 1.2mm and3.533.5mm2 in the directions perpendicular and parallelthe surface. Thus the displacement in the perpendicularrection should be less than 40 Å/K and in the lateral directless than 165 Å/K. Furthermore, the drift during heatingcooling should be constant in speed and direction, to enone to keep pace with the displacements by offsettingpiezoelectric actuator voltages.

Although our STM is equipped with high-speed daacquisition,18 allowing image rates of 10 images/s, the hoping rate of diffusing adatoms on most surfaces at and abroom temperature is far beyond the imaging speed ofSTM. For metal surfaces with a typical activation energyEact

of 0.3 eV and an attempt frequencyn0 of 1012 Hz, the hop-ping rate n5n0e2Eact/kBT of adatoms is of the order o107 Hz at room temperature. The hopping rate is practicazero on the time scale of the experiment only below 100~70 K for Eact50.2 eV!. Thus, for single-atom diffusion, theSTM should operate at low sample temperatures, down toK. On many surfaces, the characteristic times at 1000 Kmost surface phase transitions and diffusion phenomenamuch shorter than the time resolution of our STM ofms.9,18 Therefore, we have strived for a maximum tempeture of 1000 K in the present design.

For growth and adatom diffusion studies we need toposit the particles during the actual measurement. Therethe sample surface should be accessible to a molecular balso after the tip is brought within the 1.2mm range of thepiezoelectric actuator. In addition, it should be possiblemonitor the tip to sample distance during the coarseproach with an optical microscope.

The STM has to work both on semiconductor and mesamples. Our metal samples are heated indirectly, e.g., wfilament from the back. To allow for a high-quality mechancal polish within 0.05° of a particular surface orientation t


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dimensions of our metal samples cannot be smaller tha3531 mm3. Due to their size and type of heating the mesamples require significantly more heating power than beshaped semiconductor samples with direct current heatNevertheless, we require the temperature range and swspecifications to apply to metal samples.

To allow for multiple experiments, the sample holdshould be an exchangeable unit. It has to be positioned inmicroscope in a reproducible way, without changing the dproperties each time the sample holder is put into the micscope. The piezoelectric scanner should also be exchaable to allowex situtip replacements.18 To measure on macroscopically different places on the sample, we requirelateral coarse positioning of the piezoelectric scanner. Twobble sticks and a transfer rod are available to transfersample holder and the scanner between the UHV chamand the loading chamber.

The remainder of this article is organized as followSection II explains how we reduced the thermal drifts of tsample and tip to a minimum. The experimental setupdescribed in Sec. III. The performance is demonstratedSec. IV. In Sec. V we give an outlook of further improvements of the present design.

II. DESIGN

A. Basic configuration

Figure 1 shows a schematic cross section of the micscope. The main body is a molybdenum block, which issample holder support. The molybdenum support blocktually consists of two separate pieces which are thermisolated from each other by three stainless steel capillarThe sample holder is mounted in the upper piece andpiezo inertial drive motors are mounted in the bottom pieOn top is the scanner with the piezoelectric actuator andtip. For the scanner we used the thermal-drift compensascanner design of Kuiperset al.18

In the scanner a radiation shield protects the piezoment against thermal radiation from the sample. Apart frits three legs the scanner is cylindrically symmetric arou

FIG. 1. Schematic cross section of the programmable-temperature SThe scannerA is cylindrically symmetric around the axis through the tipF.It rests with three legs on the support blockH. A radiation shieldE protectsthe piezo elementB against thermal radiation from the sampleC. Thesample holderD is clamped down against two supports by leaf springsG.The sample is clamped up against two ledges of the sample holder.


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2074 Rev. Sci. Instrum., Vol. 69, No. 5, May 1998 Hoogeman et al.

the axis of the tip. A finite-element analysis of the scanhas been used to match the expansion of the legs with ththe assembly of piezo element, tip holder, and tip in ordeminimize the vertical drift of the tip due to temperatuchanges of the sample. In addition, near matching has bobtained of the lateral expansion of the top plate of the scner and the block supporting the scanner’s legs~see Ref. 18!.

The sample is clamped up in a molybdenum holder wits surface against two ledges. The holder itself is clamdown with two extending arms on two supports. The ledgthe arms, and the supports all lie in the same plane coining with the tip position. This configuration ensures that epansions of the sample and the holder do not affect thetical position of the surface plane with respect to the tip.

The sample is radiatively heated from below by a fiment. The filament is held inside the sample holder ashielded in all directions by the walls of the holder and tsample. In addition to radiative heating, the sample canheated by electron bombardment from the filament toback of the sample.

Due to the open design of the STM, the sample surfis accessible during the actual measurement from two sie.g., for a molecular beam and an optical microscope.maximum angle of incidence is 30° with the sample surfa

B. Drift reduction in lateral plane

The biggest challenge of the design is the thermal dreduction between the sample and the tip in the lateral dition. As already mentioned in Sec. I B, the sample shoulddrift out of the range of the piezoelectric actuator oversample temperature span of several hundreds of degFurthermore, residual displacements during heating or cing should be constant in speed and direction, so that weeasily adjust for the drift by offsetting the piezoelectric atuator voltages.

The cylindrical symmetry of the scanner, with respectthe tip and the near match between the lateral expansiothe top plate of the scanner and the support block, guaranthat the lateral drift of the tip is minimal. The most importanew feature in our present design is the drift reduction ofsample holder. Figure 2~a! shows a schematic top view of thholder. Four arms extend from the holder. Two of theserotated against vertical posts that form part of the Mo sport block. The other two are shaped as knife edges andpressed down against two flat supports by two leaf sprinWhen the holder is heated it expands outwards along theextensions but the center stays at its original position.make the coarse approach possible the tip is mounted 1outside this stable center position~see Sec. III B!.

The four points of contact, the two knife edges andtwo arms, seem to overdefine the position of the samholder. To avoid the overdefinition, we first let the armsthe sample holder touch the posts when the holder is pusinto the block. At this stage the sample holder still hasfreedom to rotate around the axis through these firstcontact points. The rotation freedom ensures that both kedges will make contact with the flat supports whenholder is pushed further. In this way, a completely unstrain


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four-point mechanical contact is established. Once in plathe sample holder can still rotate around the two knife edgwhich is used for the coarse approach described in Sec. I@see Fig. 2~b!#. To prevent random rotation of the sampholder due to the thermal expansion of the leaf springs,springs press on two additional knife edges. These knedges point upwards and lie on the same line as the rotaaxis such that no torque can be exerted on the holder bysprings.

The holder is made of molybdenum. This material hamodest expansion coefficient of 4.831026 K21 and a highmelting point of 2890 K Furthermore, molybdenum does ntend to alloy with most of the other metals and semicondtors used in our experiments. The size of the sample holdkept small (8310310 mm3), to minimize the length overwhich thermal expansions can occur. The sample itselpushed up by a spring against two ledges of the samholder ~see Fig. 1!.

The difference in expansion between the sample mateand the molybdenum sample holder body results in a latdisplacement of the sample with respect to the holder duheating. We have tried to control this displacement by meof a radial pattern of fine ridges, which was spark erodedthe ledges of the holder against which the sample is presThe ~extrapolated! center of this star pattern coincided witthe tip position. Our hope was that the sample would expradially along the ridges of the star pattern, keeping its cenin place. Test measurements, however, revealed thatsample always expands away from a point somewhere aone of the two ledges where it happens to be attached m

FIG. 2. ~a! Schematic top view of the sample holder body. The center cirD denotes the point from which all the lateral expansions are directedwards.~b! Perspective drawing of the sample holder. The sample holderrotate around the axisF, defined by the knife edgesE.


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strongly to the holder. To ensure that this point is withinmm of the tip position we press the sample against tbumps on the sample holder ledges and an additional modenum ‘‘hook.’’ The hook is 1 mm and the two bumps aeach 2 mm away from the tip position.

The final sample holder consists of two parts: the samholder body ~as described above! and the sample holdeframe ~which is around the sample holder body! @see Fig.3~a!#. The frame carries five gold-coated copper p~Burndy!20 for the electrical connections to the thermocoupand the filament. The five corresponding socketsmounted in the Mo support block. Also mounted on tframe are the two leaf springs, which press the sample hobody down.

III. EXPERIMENTAL SETUP AND PROCEDURES

A. Sample holder exchange

The UHV system is equipped with a wobble stick whiallows in situ exchange of the sample holder~body togetherwith frame! and the scanner. If the sample holder is beinserted into the block, the wobble stick lowers the framwhile at the same time the two leaf springs push the samholder body down with its arms along the two posts@Fig.

FIG. 3. Diagram of the assembly during insertion of the sample holder~a! the sample holder frameC and bodyA are still above the support blockG. The sample holder body first touches, with its arms, the two postsD inthe block ~b!. The leaf springsF push the body downward along the twposts, until the two knife edges touch the two supportsE. In ~c! the sampleholder is fully inserted.


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3~b!#. After both knife edges first touch the supports, tframe is lowered another 2 mm to increase the spring loadthe sample holder body and to decouple the body fromframe@Fig. 3~c!#. The surfaces of the two supports are tiltein opposite directions, such that the vertical spring formakes the sample holder body rotate clockwise, with its tarms against the vertical posts@Figs. 2~a! and 3~a!#. Whenthe sample holder is not clamped in the block, the fracarries the sample holder body.

The support block rests with eight springs on a smcarriage. The carriage is moved through the vacuum chamover a monorail by use of a rotary feedthrough coupled tchain. The carriage can be parked under several surface cacterization and cleaning tools, e.g., a cylindrical mirror alyzer for Auger electron spectroscopy, a low-energy electdiffraction system, and a sputter ion gun. The entire chamrests on an optical table, which is supported by air-cushiolegs.

B. Coarse positioning

The coarse approach is performed by rotating the samholder body towards the tip until the sample surface is witthe 1.2mm range of the piezoelectric actuator. The samholder body rotates around the axis through the knife edgThe tip is positioned 1 mm away from this rotation axis. Wcan rotate the holder over65° which means that the tip hato be mounted in the tip holder with an accuracy of60.09mm. We rotate the sample holder body by pushing and ping the bottom with an inertial piezo motor~see Sec. III C!.The motor makes single steps of'0.5 mm which brings thesample surface each step 0.05mm closer to the tip. Thecoarse approach is fully automated and is usually complewithin 10 min.

If the end point of the tip does not coincide precisewith the reference place defined by the upper surface ofsupport block, the sample holder will end up tilted with rspect to the ideal horizontal orientation. At first sight thmight seem to result in an uncompensated vertical distain the sample holder and sample, of which the expansioncontraction would contribute directly to a vertical drift. However, since the support axis defined by the knife edges liethe surface plane of the sample, the height of this planindependent of temperature, even if this plane is tilted. Tonly uncompensated length in the case of a tip mounterror is the error in the tip length itself. Since the temperatvariation of the tip is always more than one order of magtude smaller than that of the sample, the resulting vertshift is always below 0.5 Å/K.

A second inertial piezo motor is used for lateral coapositioning of the scanner. This motor can shift the scanover a range of61 mm parallel to the rotation axis of thsample holder, thus keeping the distance between tiprotation axis constant

C. Inertial piezo motor

The coarse positioning of the sample holder and theezoelectric scanner is performed with an inertial piezo moThe motor uses the stick-slip motion of a sliding mass o

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two guiding rods.21 The motor consists of a piezoelectractuator22 and two blocks ~the slider! clamped togetheraround the two guiding rods~see Fig. 4!. The piezoelectricactuator moves the guiding rods slowly forward and retrathem fast~or vice versa!. The inertia of the two blocks makethem slide forward~or backward!.

In order to avoid high friction and excessive wear evafter extensive use under unlubricated, clean UHV contions, one has to select a suitable material combination.the blocks we use a soft material~stainless steel!, coated witha 2 mm gold film. A ‘‘hardcor’’ treatment23 was applied tothe stainless steel guiding rods to harden their surface.two blocks are held together with an adjustable push sprA V-groove in each block defines the direction in whichcan move. Four strips attached to the sides of the two bloprevent them from rotating with respect to each other.

An important property of the inertial motor is its resnance frequency. The resonance frequency limits thepart of the stick-slip cycle. To increase the resonancequency the motor is made compact and light. The piezoetric actuator is placed in a cylinder with a thin cap. To ensthat the actuator itself is not torn apart during fast contrtion, it is pushed against the cap by a screw such that thea permanent compression force of 200 N on the actuator.guiding rods are directly fixed to the cap. The whole costruction guarantees a resonance frequency above 1 kH

The motor is driven by single period wave forms withamplitude of 625 V corresponding to a 3.1mm contraction orexpansion of the piezo stack. The wave form consists ofmirrored half-parabolas, which are connected at thmaxima. The parabola shape of the wave form ensuresthe start and the end of the stick-slip cycle is smooth, whthe return point is as sharp as possible~the bandwidth of theamplifier is 50 kHz!. The piezo inertial motor makes steps'0.5 mm per wave form period.

We tested the inertial piezo motor in air and UHV. In athe motor can push and pull with a force of 2 N. In UHV wcould not measure the precise maximum push and pull fo

FIG. 4. Schematic representation of the inertial piezo motor for rotatingsample holder body. The side stripsE prevent the blocks from rotating. Thtwo pins D push against~or pull at! the bottom side of the sample holdebody, such that the translation of the blocks is converted into a rotatiothe sample holder body.~a! Side view of the setup.~b! Cross section at thelocation of the slider.


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Over a period of 5 months we observed a modest decreaaverage step size from 0.7 to 0.5mm.

D. Cooling

We reach low sample temperatures by cooling the enMo support block in which the sample holder is clampeBecause the sample holder body is pushed by twosprings against the block, the sample holder reaches almthe same temperature as the block reaches~maximum differ-ence of 6 K!.

The block is cooled by an Oxford Instruments flocryostat.24 Our setup does not allow a permanent connectbetween the cryostat and the support block of the STM.stead, we constructed a removable connection containinlarge copper claw. The jaws of the claw have an openingis 0.1 mm wider than the 80 mm width of the Mo suppoblock. If the claw is cooled from room to liquid nitrogetemperature~77 K!, the opening of the jaws shrinks 0.1mm, while the block shrinks 0.02 mm over the same teperature interval. The relatively large expansion coefficiof copper makes the claw clamp itself firmly around the mlybdenum block, resulting in an excellent thermal connetion. The claw is connected to the cryostat via a copper brWe use an untwisted copper braid in order to minimizetransmission of vibrations originating from the cooling liquid. A thin plate of aluminum nitride~0.75 mm! between thebraid and the cryostat electrically insulates the cryostat frthe support block.

IV. PERFORMANCE

A. Displacement in lateral plane

The thermal stability of the microscope in the laterplane is illustrated in Fig. 5. The figure shows a sequencedifferentiated STM images of a narrow island of monatomic height on highly oriented pyrolytic graphite~HOPG!.The time between subsequent images was 30 s. Duringaging we heated the sample from room temperature to 3above room temperature. We held the position of the sfixed during the measurement. The average displacemover this temperature interval was 40 Å/K. The displacemwas mainly unidirectional and reversible in temperatuwhen the sample was brought back to the starting tempture the sample shifted back close to its original position

The displacement is caused mainly by the differencethe expansion between the sample holder body andsample itself. The magnitude of the shift per unit of tempeture is the expansion-coefficient difference between samand holder times the distance between the tip positionthe point where the sample is fixed in the holder. A smacontribution to the shift originates from the excentric potion of the tip ~1 mm away from the rotation axis!. Theexpansion coefficients of graphite and molybdenum are31026 and 4.831026 K21, respectively. From the measured shift and the expansion coefficients we calculatethe graphite sample is fixed at a location 1.9 mm away frthe tip. The direction of the shift and the distance of 1.9 mindicate that the fixing point is located on one of the ledg

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in the sample holder body. Thus, the thermal displacemdepends on the location where the sample is fixed and onexpansion coefficient of the sample.

If the sample temperature is changed the sample moto a new position and it shifts back when the temperaturbrought back to its original value. In view of its reversibilitand unidirectional nature of the displacement, we refer todisplacements asthermal shiftsrather thanthermal drift.Long-term warming of the surroundings of the sample holincluding the scanner during heating experiments hardlyfects the drift behavior of the microscope.

The second example is a heating experiment onNi~100! surface. After a short equilibration at a sample teperature of 466 K, the heating power was suddenly increafrom 3.3 to 4.6 W. The temperature and the thermal shiftsthe x and y directions during the first 6 min following thestep in heating power are shown in Fig. 6. After 84 smaximum heating rate of 16 K/min was reached. After 32the heating rate had reduced to 3.1 K/min. The shifting ixandy directions varied roughly proportionally with the heaing rate. The near constant ratio betweendx/dt, dy/dt anddT/dt and the absence of significant noise on the shiftillustrates the unidirectionality and reproducibility of thshifting behavior. If we sum the displacements in thex andydirection over all the images, we calculate a total displament of 0.6mm and an average thermal shift of 163 Å/KNickel has an expansion coefficient of 13.431026 K21. Theobserved displacement corresponds to a distance of 1.9between the tip and the point where the Ni sample is fix

FIG. 5. Sequence of differentiated STM images of a section of a HOsurface~1840 Å32260 Å! during a temperature ramp from 294 to 324 KThe scan position was not shifted. The images show an almost unidtional drift of the surface of 40 Å/K. The numbers in the right-hand cornindicate the image number. The sudden change in the shape of the penin the last image was due to a tip instability~Vt50.32 V, I t50.3 nA!.


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within the Mo holder. Similarly, on Au~110!, which has anexpansion coefficient of 14.231026 K21, we found a driftof 180 Å/K, which corresponds to a distance between tip afixing point of 1.9 mm.

In a third example we demonstrate the reversible aunidirectional nature of the thermal shift, again on tNi~100! surface. After a short equilibration time at a samptemperature of 440 K the heating power was suddenlycreased from 3.5 to 3.8 W and after 300 s reduced bac3.5 W. Figure 7 shows the temperature and the thermal sin the x and y directions and the inset shows thex and ypositions of the tip with respect to the sample during the tiof this experiment. As previously, the shifting inx and ydirections varies proportionally with the sign and the amptude of the heating rate. The maximum displacement is 0mm and the average thermal shift is 169 Å/K, accordingAlthough we released the sample from the sample holdebetween the two Ni experiments, we find for both expements nearly the same magnitude and direction of the tmal shift. The inset of Fig. 7 shows that the sample shif

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FIG. 6. Measured thermal shifts in thexy plane between successive imagvs time on Ni~100!. Also the heating rate is indicated~right-hand axis!. Att50 s the heating power was increased from 3.3 to 4.6 W. The latdisplacements respond immediately to the temperature rise~Vt520.31 V,I t520.02 nA!.

FIG. 7. Measured thermal shifts in thexy plane between successive imagtogether with the heating rate on Ni~100!. The measured relative position othe sample is shown in the inset. Att50 s the heating power was increasefrom 3.5 to 3.8 W and att5300 s the heating power was reduced to 3.5 WThe inset shows that after a maximum displacement of 2200 Å at 453 Ksample shifts back to within 215 Å from its starting position att50 s ~Vt

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back during the cooling, returning almost precisely alongsame unidirectional trajectory as during the heating. Tsample moved back to within 215 Å of its original positioafter a maximum displacement of 2200 Å at 453 K.

Although the precise fixing point varies from samplesample, the thermal shift is reproducible for each samplelong as it is not released from the sample holder. The pdictable thermal shift of the sample makes it easy to antpate during temperature ramps. Even in case the particarea of interest is lost, this place can always be found simby going back to the temperature where the area wasserved for the last time. Figure 8 shows a sequence of imaof the Au~110! surface during a temperature ramp of 271We coarsely adjusted the center of each scan by meanoffset voltages on the piezoelectric scanner. Over the whtemperature range one particular surface feature couldkept in view. The sum of the applied offset voltages corsponded to a total displacement of'4 mm yielding an aver-age drift of 150 Å/K. This drift is slightly lower than thevalue mentioned above. We attribute the difference to loterm warming of the surroundings and the scanner resulin a small compensating drift. At elevated temperaturesmonoatomic steps appear rugged. This is a consequencthe fast diffusion of the steps.8,9 Also, modest local morphol-ogy changes are visible during the temperature ramp.

B. Drift in vertical direction

In all experiments the vertical thermal drift in the micrscope, measured in the range between 300 and 850 K,

FIG. 8. Sequence of gray scale STM images of one particular surface re~2490 Å31245 Å! on Au~110! which is kept in view during a temperaturramp from 294 to 565 K. The duration of the sequence was 120 min andtotal drift of the surface was'4 mm. The contrast originates from monoatomic steps~1.4 Å!. The vague lines in some of the images are a Mo´pattern between the scan lattice and the missing-row structure ofAu~110! terraces~Vt520.69 V, I t520.03 nA!. The arrow points to thelocation that we succeeded to keep in view.


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extremely low, namely 5 Å/K or less. Figure 9 shows tvertical position of the tip measured on a Ni~100! surface. Att50 s we switched on the heating at a power of 1.8Initially, the surface and the tip drift away from each othwith 110 Å/s. After a short time of 12 s the drift directioreversed and after 135 s the net shift had reduced to 330a sample temperature of 367 K. If we switch off the heatiwe observe the opposite behavior.

Modest delays in heating or cooling are responsiblethe transient behavior. First, the legs of the scanner heatcausing the tip to drift away from the sample. Somewhlater, the piezoelectric actuator also warms up and compsates for the expansion of the scanner legs. The net driftÅ/K. The overshootcan be reduced by stepwise increasithe heating power and waiting in between the steps untildrift has reversed and reduced.

The extremely low vertical drift makes this STM idefor investigating the evolution of local surface morphologiinduced by temperature changes, as the temperature cachanged over a wide range without making it necessarymechanically adjust the vertical distance. On Ni~100!, forexample, we have been able to follow the entire anneaprocess of a sputtered surface, from room temperature alway up to 730 K, without making any mechanical adjuments of tip or sample holder.25

Although we use the same piezoelectric scanner deas Kuiperset al., we find a significantly lower vertical driftthan was reported in Ref. 18. Kuiperset al. already sug-gested that most of their vertical drift resulted from the leral drifting of the sample holder with respect to its tiltesupport. The tilt was necessary in that design for the coaapproach, which consisted of pushing the sample holdeand down a ramp of 5° with the horizontal plane. The ndesign of our sample holder and coarse approach mechahas completely eliminated this problem, which has resulin much more idealistic behavior of the microscope.

Until now the STM has been tested up to a sample teperature of 850 K. For this temperature the required hea

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FIG. 9. Measured thermal drift in thez direction between the tip and aNi~100! surface due to an increase of the sample temperature. Att50 s weapplied 1.8 W heating power, which corresponds to a temperature rise osample from room temperature to 370 K. Initially, the surface and thedrift away from each other with a speed 110 Å/s to a maximum shift of 11Å. After 12 s the drift has reversed and after 135 s the net shift hascreased to 330 Å at a sample temperature of 370 K~Vt520.46 V, I t520.03 nA!.


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power is 24 W. Despite this high power the thermal drbehavior of the STM, both in the lateral and in the verticplane, is not affected.

C. Cooling and low temperature performance

Although the present setup is not optimized for coolinwe reach a sample temperature of 60 K with liquid heliuIn the present configuration the time needed to reachtemperature is quite long. The time to cool down from rootemperature to 150 K is already 30 min. The reason isinefficient method of cooling. We cool the sample indirecby cooling the entire upper piece of the Mo support bloThe large heat capacity of the block causes the slow coorate. The high thermal radiation input of 3 W from the sur-roundings at room temperature limits the final temperaturethe block to 55 K. The sample holder is pushed againstblock with leaf springs. The thermal contact is good enouto keep the temperature difference between sample hoand block within 6 K. In a future design the sample holdwill be mounted in a subsection of the Mo support blockwhich the braid is connected directly. This will reduce bothe heat capacity and the thermal radiation input~see Sec.V!.

The mechanical vibrations in the STM during coolinwith liquid helium have an amplitude below 0.15 Å. Figu10 shows a perspective representation of a STM imageAu~110! measured at 60 K. The measured corrugation ofmissing-row structure is 0.7 Å. The monoatomic step haheight of 1.4 Å. With liquid nitrogen we reach an end temperature of 130 K. The vibration level is somewhat larg~,0.4 Å!, but the missing-row structure and monoatomsteps on Au~110! can still be resolved with the STM.

V. DISCUSSION AND OUTLOOK

We have constructed a new programmable-temperaSTM. The STM has been tested for temperatures betweeand 850 K. The novel STM has demonstrated its capab

FIG. 10. Perspective representation of a STM image of a Au~110! surface,measured at 60 K. The dark lines are the missing rows of the~132! recon-struction. The measured corrugation of the missing row structure is 0.At the left we see three kinks of a higher terrace. The small dots onlower terrace are adsorbates observed afterin situ evaporation. The imagewas obtained in 5.4 s. We measured a vibration amplitude introduced bcooling of ,0.15 Å ~82 Å344 Å, Vt520.69 V, I t520.02 nA!.


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of keeping the same area in view over a wide temperarange of at least 270 K. Over a much wider interval ttemperature can be changed during the measurement witthe necessity for coarse vertical adjustments. The STMsign is not optimized for cooling, but nevertheless we reacsample temperature of 60 K, using liquid helium.

The main future improvement of the STM design cocerns the cooling system. The sample holder is cooled viaMo support block, which has a large heat capacity and a hthermal radiation input. To minimize the temperature diffeence between the support block and the sample holdegood thermal contact is required between both objectsstrong thermal coupling, however, is a disadvantage duoperation at elevated sample temperatures. The heat osample holder leaks away to the block, thereby increasthe required heating power, and heating up the Mo suppblock. To improve the sample cooling we are adaptingdesign in collaboration with Oxford Instruments.26

The support block will be divided into an inner and aouter section. Figure 11 shows a schematic top view ofnew block. The sample holder will be clamped in the innsection of the block and the scanner stands on the outertion. The inner section is separated from the outer sectionfour thin bridges (5.032.030.5 mm3). The cryostat will bedirectly connected to the inner section via a copper braid.cool the sample in the new design, only the inner sectionto be cooled instead of the whole block. The heat capacitythe inner section is approximately five times lower than tof the whole Mo block. The radiation input will be reducefrom 3.2 to 0.6 W. At a sample temperature of 50 K, the hconduction through the bridges will amount to 1.6 W. In tpresent setup the heat input through conduction is negligiThus, the total heat input at a sample temperature of 5will be reduced from 3.2 to 2.2 W. All the numbers reportwere calculated assumingInvar as block material.Invar is aniron–nickel alloy and is known for its extremely low expasion coefficient and low thermal conductance.27 To ensurethat the outer section stays within the temperature window

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FIG. 11. Schematic top view of the new support block design. The blocsplit in an outer section and an inner section. Four bridges connect thesections. The sample holder sits in the inner section. If the inner secexpands or shrinks with respect to the outer section, the center doeshift, but the inner section rotates around its center. The scanner is stanon the outer section.


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which the expansion coefficient is extremely low, we anchthe outer section via braids to the room temperatureroundings.

When the sample is heated above room temperatureinner section will be heated up by the sample holder. In tsituation the thin bridges again thermally isolate the insection from the outer one. Therefore, the outer sectiongether with the scanner will remain closer to room tempeture during both heating and cooling.

If the sample is cooled or heated the inner section wcontract or expand. Assuming that the spring constants obridges are equal and that the temperature profile is symric around the central axis, the inner section will not shbut rotate around its center. We expect a rotation of 0.1 mfor a temperature difference of 200 K between the innerouter section. The middle line of the bridges is aligned wthe upper plane of the inner and outer section to prevunwanted vertical drift of the inner section due to the strotemperature gradients over the bridges.

ACKNOWLEDGMENTS

The authors gratefully thank B. Ambrose and T. Palmof Oxford Instruments26 ~former WA Technology! for theircollaboration during the design and testing of tprogrammable-temperature STM.28 They also acknowledgeW. J. Barsingerhorn, J. ter Beek, R. Boddenberg, W.Brouwer, R. van Gastel, J. ter Horst, P. T. Keaˆ, E. deKuyper, H. Neerings, M. Rost, and H. Voort for their asstance during the building of the microscope. R.J.I.M. Kopis also acknowledged for the preparation of the samples. Twork is part of the research program of the FoundationFundamental Research on Matter~FOM! and was made possible by financial support from the Netherlands Organizatfor the Advancement of Research~NWO!.

1J. Lapujoulade, Surf. Sci. Rep.20, 191 ~1994!, and references therein.2J. G. Dash, Contemp. Phys.30, 89 ~1989!.3J. W. M. Frenken, R. J. Hamers, and J. E. Demuth, J. Vac. Sci. TechA 8, 293 ~1990!.

4M. S. Hoogeman, D. C. Schlo¨ßer, J. B. Sanders, L. Kuipers, M. F. Chan


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M. A. J. Klik, D. Glastra van Loon, R. W. M. Loos, J. J. van der Lindeand J. W. M. Frenken, inSurface Diffusion: Atomistic and CollectiveProcesses, NATO ASI Series B, Physics~Plenum, New York, 1997!.

5J. W. M. Frenken, P. M. J. Mare´e, and J. F. van der Veen, Phys. Rev.34, 7506~1986!.

6G. L. Kellog, Surf. Sci. Rep.21, 1 ~1994!, and references therein.7R. Gomer, Rep. Prog. Phys.53, 917 ~1990!.8L. Kuipers, M. S. Hoogeman, and J. W. M. Frenken, Phys. Rev. Lett.71,3517 ~1993!.

9L. Kuipers, M. S. Hoogeman, J. W. M. Frenken, and H. van BeijerPhys. Rev. B52, 11387~1995!.

10M. Giesen-Seibert, R. Jentjens, M. Poensgen, and H. Ibach, Phys.Lett. 71, 3521~1993!; 73, 911~E! ~1994!.

11T. R. Linderoth, S. Horch, E. Lægsgaard, I. Stensgaard, and F. Bebacher, Phys. Rev. Lett.78, 4978~1997!.

12Kinetics of Ordering and Growth at Surfaces, NATO ASI, edited by M.G. Lagally ~Plenum, New York, 1990!, Vol. 239.

13M. Zinke-Allmang, L. C. Feldman, and M. H. Grabow, Surf. Sci. Rep.16,377 ~1992!, and references therein.

14B. Voigtlander, A. Zinner, and T. Weber, Rev. Sci. Instrum.67, 2568~1996!.

15General product information from Omicron Vakuumphysik GMBH, Isteiner Straße 78, D-65232 Taunusstein, Germany.

16General product information from Jeol Ltd., 1-2 Musashino 3-chome, Aishima, Tokyo 196, Japan.

17J. W. Lyding, S. Skala, J. S. Hubacek, R. Brockenbrough, and G. Gmie, Rev. Sci. Instrum.59, 1897~1988!.

18L. Kuipers, R. W. M. Loos, H. Neerings, J. ter Horst, G. J. Ruwiel, A.de Jongh, and J. W. M. Frenken, Rev. Sci. Instrum.66, 4557~1995!.

19J.W. Lyding ~private communication!.20Framatome Connectors Nederland bv, Essebaan 7, 2908 LJ Cappel

IJssel, The Netherlands.21Ch. Renner, Ph. Niedermann, A. D. Kent, and O” . Fischer, Rev. Sci. In-

strum.61, 965 ~1990!.22We used a PIC 151, UHV compatable, piezostack of Physik Instrume

~PI! GmbH & Co., Polytec-Platz 5-7, 76337 Waldbronn, Germany.23The ‘‘hardcor’’ treatment was carried out by Hardiff bv, P.O. Box 6

7360 AB, The Netherlands.24Oxford Instruments, Old Station Way, Eynsham, Witney Oxon OX8 1T

United Kingdom.25M. S. Hoogeman, M. A. J. Klik, R. van Gastel, and J. W. M. Frenk

~unpublished!.26Oxford Instruments, SPM-group, Chesterton Mills, French’s Road, Ca

bridge CB4 3NP, United Kingdom.27General product information from Goodfellow Cambride Ltd., Cambrid

CB4 4DJ, United Kingdom.28An improved version of the programmable temperature STM describe

this article is produced by Oxford Instruments, in Ref. 26.


design and performance of a programmable-temperature scanning tunneling microscope

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