Metallic Adhesion and Tunneling at the

126 Pages·2001·6.44 MB·English

Metallic Adhesion and Tunneling at the
Atomic Scale
André Sêhirrneisen
Center for the Physics of Materials
Department of Physics, McGill University
Montréal, Canada
June 1999
A thesis submitted to the
Faculty of Graduate Studies and Research
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy
@ André Schinneisen, 1999
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Abstract
The metallic adhesion and tueLing properties of an atomicaliy defined junction
were measured and analyzed. The junction consisted of a tip opposing a flat surface in
the scanning probe microscopy (SPM) con£îguration. Measurements were performed
in ultrahigh vacuum (UHV) at 150 K. Sub-nN force resolution was achieved on a
stiff cantilever beam employing an in-situ differential int erferometer. Tips were pre-
pared from W and Ir wire and imaged with atomic resolution in-situ using field ion
microscopy (FIM). Ultrasharp tips Nith an apex radius of 20-30 A were fabricated
from single crystal W(111) wire and engineered with FIM to terminate in only three
atoms. Calculations indicate that for those tips metallic adhesion forces dominate
over van der Waals and capacitive electrostatic forces. The sample was a thin (111)
oriented Au film. Metallic adhesion forces and the tunneling curent were measured
simultaneously for the W-Au system as a function of tip sample separation. In con-
trat to theoretical simulations the system featured exceptional mechanical stabiliv
with adhesive forces of up to 5 nN. In particular no indications of a sudden jumpto-
contact, which is cornmonly believed to be an inherent property of metallic contacts,
were found. Furtherrnore, the range over which the metallic adhesion forces act is
four times larger than expected. Experiments with sharp but not atomicdy dehed
W tips corroborate those results. The observed long interaction range is discussed in
the framework of various models. Some of the consequences of this new property for
force microscopy applications are pointed out.
Résumé
L'adhésion métallic et les proprietés de l'effet tunnel d'une jonction charcterisée
a l'échelle atomique on été mesurées et analysées. La jonction, configurée comme
un microscope de sonde a balayage (SPM), consiste d'une pointe opposée à une sur-
face plane. Les expériences ont été conduites sous la ultra-vide (UW) à 150 K.
L'utilisation d'un interfrometre differentiel in-situ, pour mesuré les forces sur un can-
tilevier rigide, a permis d'atteindre une résolution superieure au nN. Les pointes
furent préparées à partir de fils de W et Ir, puis mise en image in situ à l'échelle
atomique à l'aide d'un microscope a champs ionique. A partir d'un crystal W(111),
des pointes ultrapointues ont été fabriquées avec un raycn au sommet de 20-30 A-
Elles ont ensuite été taillées à l'aide d'un microscope à champs ionque, ne laissant
que 3 atomes à leur pointes. D'après les calculs effectués sur ces pointes, les forces
d'adhésion métallique dominent comparativement aux forces de van der Waals et aux
forces électrostatique. L'échantillon utilisé etait une fine couche Au(ll1). Les forces
d'adhésion métallique et le courant tunnel, pour le système W-Au, ont été mesurés
simultanément en fonction de la séparation entre l'échantillon et la pointe. A la
différence des simulations théoriques, le système a demontré une stabilité méchanique
exceptionnelle pour une force d'adhésion ne dépassant pas 5 nN. En particulier, au-
cune indication d'un soudain 'saut-au-contact', qui est généralement regardé comme
une propriété inherente des contacts métalliques, n'a été observé. De plus, la distance
sur laquelle la force d'adhésion métallique agit est quatre fois plus grande que prévue.
Des expériences faites avec des pointes de W pointues mais non définies à l'échelle
atomique, confirment ces résultats. Une discussion de cette longue interaction est
présentée en pmallèle à de nombreux modéles. Quelques conséquences de cette nou-
velle propriété concernant les applications en microscopie de force sont ainsi mises en
evidence.
Acknowledgements
First of al1 1 would like to th& my supervisor Peter Grutter for continuous
support and fnendship throughout rny entire work in his research group. He possesses
the unique ability to motivate, excite and guide his students in their research, which
makes it so very enjoyable to work in his group. Much advise and help was also
povided by Urs Dürig.
Graham Cross has contributed to the success of this thesis in countless mays, as a
good friend and research partner at the same the. His enthusiasrn and good humor
have made our team work a mernorable experience.
Thanks go to Phil LeBlanc for evaporating films on the cantilevers and Vincent
Tabard-Cossa for help with the scans and the translation. 1 am also grateful to
the rest of the research group, where everybody contributed to the pleasant working
atmosphere.
Furthemore, 1w ant to use this opportunity to thank my friends in Montréal who
have made this stay such a unique experience. In particular there are Pietro 'PouPou7
Pucella, Tiago 'Diablo' deJesus, Florence, May, Narly, the 'new' Wai and al1 the rest
of the gang. Many thanks go to Camille, the flying Dutchman. Special mentions go
to Sarah, Copacabana and the staff at QStix.
1a m grateful to Danny for all the love she has given to me.
Financial support from the Hydro Québec McGill Major scholarship is gratefully
acknowledged.
I am indebted to my family for their ever present help. Only the continuous
support of my parents in many ways have made my stay in Canada and this work in
particular possible.
Contents
Abstract
Résumé
Acknowledgments iii
List of Figures
1 Introduction
2 Experimental Set-up 3
3 The Probe 8
3.1 Probe Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
. . . . . . . . . . .
3.2 Macroscopic Tip Shape: Electrochemical Etching 10
3.3 Treatment in U W . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Annealing 15
. . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Field Emission 16
. . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Self Sputtering 21
. . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4 Oxygen Etchhg 22
3.4 Atomic Scale Engineering: Field Ion Microscopy . . . . . . . . . . . . 23
. . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 Working Principle 23
. . . . . . . . . . . . . . . . . . . . . . .
3.4.2 Image Interpret ation 32
. . . . . . . . . . . . . . . . . . . .
3.4.3 Electric Field of Nanotips 38
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5 Experimental Issues 40
3.5.1 Tip Cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.5.2 Gold on Tungsten: FM . . . . . * . . . . . . . . . . . . . . . 41
3.5.3 Field Emission Stability . . . . . . . . . . . . . . . . . . . . . 44
3.5.4 Background Forces . . . . . . . . . . . . . . . . . . . . . . . . 46
3.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4 The Sample 52
4.1 Preparation of Au Sample . . . . . . . . . . . . . . . . . . . . . . . . 52
4.2 Surface Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2.1 Cleaning Procedure . . . . . . . . . . . . . . . . . . . . . . . . 53
.
4.2.2 Force versus Tunneling . . . . . . . . . . . . . . . . . . . . . . 54
5 Adhesion Experiments 59
5.1 Theoretical Predictions . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.1.1 Metallic Adhesion . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.1.2 Mechanical Relaxation Effects . . . . . . . . . . . . . . . . . . 65
5.2 First Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.3 Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.4 Preliminary Studies: UndeGned Tip . . . . . . . . . . . . . . . . . . . 70
5.5 WonAuexperiments . . . . . . . . . . . . . . . . .. . . . . . . . . 81
5.5.1 Part A : Structural Stability . . . . . . . . . . . . . . . . . . . 81
5.5.2 Part B : Scaling Length and Tunneling . . . . . . . . . . . . . 87
5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
6 Conclusion and Outlook 104
List of Figures
2.1 Schematic representation for the experimental set-up. The combined
FIM/AFM/STM is housed in a UHV chamber with p=2xl0-"mbar
and cooled at 150 K. The sample stage can be quickly moved to switch
between FIM and AFM/STM mode using piezoelectric motors. . - - . 6
3.1 Experimental set-up for electrochemical etching. The W tip is dipped
into20%KOHsolution. ThecathodeismadeofWwireaswell. . . . Il
3.2 The drop-off method while etching W wire in KOH solution. Etching
products collect at the end of the wire giving rise to preferential etching
at the liquid air interface: necking occurs. The top graph shows the
current during the process, indicating the sudden change in current,
when the drop-off occurs. . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3 Etched W tip as viewed through optical microscope. Left lOOx and
right 500x magnification (using polarized light to enhance contrast) . 13
3.4 Set-up for etching iridium tips. The hole in the center of the shallow
bath, with the tip placed in it, forces the neck formation. The liquid
container is made of stainless steel and acts as the counter-electrode
at the same time. The voltage is applied in form of pulses (see text). 14
3.5 Etched iridium tip as viewed through optical microscope. Left 100 x
and right 1000x magnification. . . . . . . . . . . . . . . . . . . . . . 15
3.6 TEM micrograph of an e~ectrochemicallye tched W tip. A thick amor-
phous oxide layer covers the tip apex(image courtesy of A. Zaluska). . 16
Potential diagram governing field emission. With no field applied,
the workfunction determines the energy an electron at the Fermi edge
needs to escape fiom the material. The applied field, though, lowers
the potential. At a critical distance, the energy needed for the elec-
tron to leave the metal becomes zero and it can tunnel through the
effective potential barrier. The image potential is a correction due to
the induced charge in the metal caused by the electron. . . . . . . . .
The Fowler-Nordheim plot as measured for a polycrystalline W tip.
The slope of 7760 V gives k R = 11952.1 , from which one can calculate
the tip radius (knowing the field reduction factor k). . . . . . . . . . .
FIM images of a W tip. Graph a) shows a typical (111) orïented W tip
imaged at 5.0 kV. Subsequently the tip was treated with field emission,
extracting currents of 5pA for 1 min. Graph b) shows the tip apex
right after, imaging at 4.5 kV, indicating rearrangement of the atoms.
In c) and d) the original apex is recovered. . . . . . . . . . . . . . . .
Field ion microscopy images of a (111) oriented W tip treated with
oxygen etching. Image a) shows the tip before, and image b) after the
treatment. The larger FIM mapification in b) indicates an overall
sharpening of the apex. . . . . . . . . . . . . . . . . . . . . . . . . . .
3.11 The imaging process in field ion microscopy. The image gas is attracted
towards the tip, ionized and then repelled along the field lines. A screen
5-10 cm away from the tip visuahes the He atoms. . . . . . . . . . .
3.12 Potential diagram for the electron of an image gas atom close to a
metal surface at high fields typical for FIM. Only at a critical distance
the electron can tunnel fiom the atomic level of the image gas into the
metal surface, into an empty level just above the Fermi energy. If the
atom is too close, the atomic Ievel is actually beiow the Fermi energy,
and there is no fiee state for the tunneling electron. If the atom is too
far kom the surface, the tunneling probability is negligibly small. . .
vii
3.13 FIA4 image of a polycrystRlline W tip indicating the different crystal-
lographic planes. Imaging was done with He at 5.6 kV and 140 K. . .
3.14 FIM images of a (111) oriented W tip taken at room temperature
using He at 5.1 kV. In between each image the voltage was increased
to 6.0 kV for the field evaporation of single atoms. . . . . . . . . . . .
3-15 Potential diagram for the process of field evaporation. As described in
the text, the ionic (field evaporated) state is at higher potential than
the atomic state, unless a strong external field is applied. Then, at a
certain distance x,;t;,r, the ionic state has a Lotver potential energy
and field evaporation can occur. . . . . . . . . . . . . . . . . . . . . .
3.16 W(111) tip imaged at different voltages, demonstrating atomic resolu-
tion FIM at 2 kV,w here the regular image forms only at 4 kV. . . . .
3.17 W(111) tip terminating in one single atom. FIM image taken with He
at 2.3 kV. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.18 FIM image (left) of a (110) oriented W tip and the corresponding
projection map (right), indicating the different crystallographic poles
(from reference [30]). The grey circle in the map outlines the area
imaged in the FIM graph. . . . . . . . . . . . . . . . . . . . . . . . .
3.19 FIM image of a polycrystalline W tip (left) and the reduced facet
projection map (right) of a (110) oriented bcc crystal. The grey circle
shows the area seen in the FIM image. . . . . . . . . . . . . . . . . .
3.20 FIM image of W(111) tip (left) and the reduced facet projection map
(right) of a (111) orïented bcc crystal. . . . . . . . . . . . . . . . . . .
3.21 FIM image of polycrystalline Ir tip (left) and the reduced facet projec-
tion map (right) of a 100 oriented fcc crystai. The grey circle outlines
the area image in the FIM graph. . . . . . . . . . . . . . . . . . . . .
V ..mlll