Surface Science 75 (1978) 538-548

D North-Holland Publishing Company

FIM OBSERVATION OF AN IRON DEPOSITED TUNGSTEN TIP

H. MORIKAWA, T. SUZUKI, T. TERAO and Y. YASHIRO Department of Coordinated Science, Nagoya Institute of‘ Technolog.y, Gokiso-cho. Showa-k-u, Nagoya 466, Japan

Received 15 December 1977; manuscript received in final form 28 March 1978

An iron-deposited tungsten tip was observed with a field ion microscope. The ohservcd

features were classified into five types correspotld~ng to the various substrate tctni)e~tures.

Epitaxial growth was observed at substrate temperatures ranging from 100°C to 500”t, though

the parallelism was not perfect. An epitaxially grown film was observed onfy on one side of a

tip cap at a low temperature. The film spread to the entire tip cap at a hi&r tcmpcraturc.

Diffusivity of the deposited iron was estimated from the observed migration of the iron atoms.

Images which implied the formation of an alloy of iron and tungsten wcrc obtained at 550

660°C. When the substrate temperature exceeded 7OO”C, iron atoms were not ohscrvcd on the

tip cap. These sequential change corresponding to the substrate tcmpcraturcs will he discussed

in relation to the surface diffusion of tbc deposited iron and Fubstrate tungsten atoms.

1. Introduction

A field ion microscope is a powerfu1 instrument to investigate a deposited thin film because of its atomic resolution. Increasing numbers of FIM investigatiorls on the vapour deposited films have been reported. These reports can be roughly classi- fied into two groups. The first group is the investigation on the texture of a thin film grown on a tip, such as W on W [I], Pt on W 121, Ni on W 131, Ir on Mo 141, Au on W, Fe on Ir [S], Pd, Pt, Rh and Iron Ir and Rh [6], MO on W, Ir and Re 171, and MO on W [S]. The second is the investigation on the behaviour of adatoms, c.g. surface diffusion of individual atoms [9] or cluster formation [lo]. The reconstruc- tion of tungsten surfaces induced by adsorbed layers of Au, Ag and Cu was also studied by FEM and FIM [11,12]. In addition to the studies on vapour-deposited films, an investigation on Pt thin films electrodeposited from aqueous solutions [ 131 and on Ga, In and Sn films deposited by a direct contact between the tip and a molten metal [ 14-171, was done.

With an electron microscope, there are many investgations on vapour deposition of metals and many aspects of the growth mechanism of the deposited film have been clarified. In many combinations of a metal deposit and a metal substrate, the growth mechanism is not the process of the nucleation and growth. but the so-

calfed monolayer overgrowth. In the latter case, much interest has been focused on pseudomorphism and misfit dislocations. In the present study, where the tip was cooled by liquid nitrogen, the resolution was not high enough to observe misfit dislocations.

Most films investigated by an electron microscope have been formed in a vacuum of about 10e4 Pa. In such a vacuum, the contamination of the substrate surface could not be neglected and might affect the growth of the deposited film. Though the cont~inants are thought to disturb the periodicity of the substrate surface, a number of epitaxial growths have been reported. In the present study, iron was deposited on a tungsten tip in a vacuum of 10S5 Pa which was a comparable vacuum in many studies by an electron microscope. The characteristic features of the deposited fitms at various substrate temperatures were studied.

2. Exper~ent~

The tungsten wires for the present study were purified by heating at 2~~0~~ in a vacuum of 1 X IO- 6 Pa for more than 100 h. The prepared tungsten tip was mounted on a tungsten loop and was flashed at 1000°C for 2 min in the field ion microscope to minimize the possible migration of contaminants from the tip shank and the loop during the deposition of iron.

Iron was evaporated from the heated tungsten filament, which was wound on an iron wire (0.56 mm diam., Merck), and deposited on one side of the field evapo- rated tip at various substrate temperatures. The deposition on one side of the tip makes us observe the surface diffusion of the deposited iron to the bare side of the tip cap. It is desirable to deposit immediately after turning off the applied voltage. However, adjustment of the tip temperature to the desired degree takes about 2 min. The deposition was started after the temperature adjustment. The substrate temperature was measured by a conventional four terminal method. The vacuum in the microscope was 2 X IOe6 Pa and the major residual gases indicated by a mass spectron~~ter were water and nitrogen. During the deposition, the vacuum in the

chamber decreased to 3 X 10M5 Pa because of the increase of the partial pressures of H, and H,O. The other species were kept at partial pressures less than 5 X 10m7 Pa by a Ti getter pump with liquid nitrogen cooling. Though the influence of IIs and Haf! contamination of the tungsten surface could not be denied perfectly, the original tungsten surface was recovered by field-evaporating the deposited iron at a low temperature. In addition, we did not observe the field-induced water etching. This might mean that there were few water molecules in the films.

The thickness of the deposited iron was estimated as follows. The iron was deposited on an eiectro-chemically polished plate at an identical deposition rate as on the tungsten tip, The deposited iron was dissolved in a definite amount of dilute hydrochloric acid. The thickness of the deposited iron layer was estimated at about 20 atomic layers from the area of the plate and the measured concentration of the

540 H. izlorikawe et al. / FIM observation of iron deposited tungsten tip

iron in the solution by an atomic-absorption spectroscopy method, neglecting the difference of the sticking coefficient on the tungsten tip and on the plate.

The FIM images of the deposited iron were observed with a ~~eli~~~n~l~ydrogen mixture at liquid nitrogen temperature. When most of the deposited iron was field desorbed and the substrate tungsten was revealed, the imaging gas was replaced by pure helium.

3. Results and discussion

The deposited iron films grown on tungster~ tips showed various types of images depending on the substrate temperatures. These features are classified into five different types as follows.

3.1. substrate at --I 98°C

The iron deposited at the substrate temperature of -198°C was field desorbed even at the best image voltage of a helium-hydrogen mixture, and no ordered structure of iron was observed, Crystalline iron was observed after heating the tip at 400°C. This implies that the randomly deposited iron atoms might loose their

kinetic energy at the first collision and be immobilized at the collision site. This result presents a remarkable contrast to the reported molybdenum film formation on tungsten at -198°C in a vacuum of lo-” Pa [S].

After field desorption of the deposited iron, many tungsten crystal planes observed before the deposition were recovered again. This indicates that the surface structure of tungsten was not changed significantly by the deposited iron at this

low temperature.

3.2. Substrate at 100-300°C

At these temperatures, epitaxiai growth of iron was observed on one side of the image facing the evaporator, as shown in fig. 1. However, no diffusion of the iron atoms to the other side was observed.

The par~lelism of the axis of the deposited crystal and that of the substrate was not perfect. The orientation relationship was nearly (1 lO)Fe//(llO)W and [OOl] Fe//[OOl]W. The observed center of the (1 IO) plane of iron was slightly misfitted sideward from that of tungsten, as shown in fig. 1. Assuming the hemispherical tip cap and radial emission of the field-ions from the tip surface, the angle between the two (110) planes can be estimated from the distance between the centers of the two planes in the image, as shown in fig. 2. The center of the (1 IO) plane of iron was indicated by A and that of tungsten by B. The angle between the two planes, 0, was ranged from 4 to 11”. It is noticed that some correlation between the shift and the deposition direction was observed and further investigation is in progress.

Fig.

1.

Iron

w

as d

epos

ited

on a

tun

gste

n tip

he

ld

at 2

40°C

. T

he

depo

sitin

g di

rect

ion

is i

ndic

ated

by

an

arr

ow

in e

very

fi

gure

. (a

) Im

age

of

fiel

d el

rapo

rate

d tu

ngst

en

befo

re

depo

sitio

n;

He

imag

e.

(b)

Iron

cr

ysta

l gr

ew e

pita

xial

ly

on

one

side

fa

cing

th

e ev

apor

ator

; th

e ob

serv

ed

cent

er

of t

he

(110

) pl

ane

of i

ron

is s

light

ly

mis

fltte

d to

war

d th

e ev

apor

ator

fr

om t

hat

of t

ungs

ten,

w

hich

is

ind

icat

ed

by t

he w

hite

cr

oss;

He-

H2

imag

e. (

c) T

he

2 la

st m

onol

ayer

co

ntac

ting

the

subs

trat

e tu

ngst

en;

the

evap

orat

ion

fiel

d of

the

m

onol

ayer

is

hig

her

than

th

at

in (

b);

He-

H2

imag

e.

(d)

Aft

er

the

W

rem

oval

of

the

mon

olay

er,

the

evap

orat

ion

fiel

d st

eepl

y in

crea

sed

and

the

tung

sten

im

age

appe

ared

; H

e im

age.

542

Fig. 2. Cross section of an iron deposited tip.

The deposited iron was field evaporated at the evaporation field of pure iron [18]. However, the last monolayer, fig. lc, contacting the substrate tungsten was evaporated at a slightly higher evaporation field. The increase of the evaporation field suggests that the monolayer of the iron at the interface strongly bound to the

substrate tungsten as in the case of Ga, In and Sn on W and MO [14-171. Although

the image of the monolayer was not clear enough to show the arrangement of iron atoms, the layer could be pseudomorph of iron. After the removal of the mono- layer, the evaporation voltage steeply increased to that of tungsten and the tungsten

image appeared as shown in fig. Id. It was expected that the structure at the interface between the substrate and the

iron deposits would be severely disordered by contaminants in the residuai gases. However, the observation of the fairly ordered pseudomorph may suggest that the contamination of solid surface by the residual gases might not be conspicuous at an elevated temperature even if the back ground vacuum was I Op5 Pa.

The volume diffusion of iron into tungsten may not be appreciable in this temperature range. Thus, a regular tungsten image was obtained after the removal of one or two (110) tungsten layers at the interface.

3.3. ~u~~trat~ at 350-450°C

The epitaxial growth of iron was observed also in this temperature range. The observed parallelism between the deposits and the substrate was the same as in the case at 240°C. A dean tungsten image was revealed by field evaporating only one or two tungsten layers from the image just beneath the deposited iron. No volume dif- fusion was noticed.

The characteristic feature of this temperature range is the surface diffusion of the iron atoms, and the area covered by iron atoms increases with the substrate temperature. The iron atoms impinged onto one side of the tip cap diffused to the other side and covered the entire cap at 37O”C, as shown in fig. 3. The interesting finding is the faceting of the {lOO> planes of the iron. The development of (100) planes was significant at the high side of this temperature range. After removal of the faceted planes, normal size (100) planes of tungsten appeared.

543

Fig. 3. Iron crystal grew epitaxiaily on the whole tip cap at substrate temperatu He- -HZ image.

re 371 3 “C;

Fig. .4. The substrate temperate is 350°C. (a) The iron deposited on one side diff itsed to the other side; He-H2 image. (b) The image of tungsten beneath the dep tilrr t; He image.

Of thl osi ted

e tip IOR

544 H. Morikawa et al. / FfM observation of iron deposited tungstett tip

The surface diffusion length (62)i120f the deposited iron can be estimated from

fig. 4. The substrate temperature was 350°C and the average tip radius was about 370 A. The diffusion length observed in fig. 4, i.e. the distance between the center of the (110) plane of tungsten and the edge of the iron crystal, was approximately - 200 A. The root mean square diffusion length (dz )‘I* in a time interval t is related to the diffusivity D [lo] as

D = &/2r .

D is also expressed by

D = De exp(-Q/M),

where De is the Arrhenius pre-exponential factor and Q is the activation energy of diffusion. Substituting the known values, Q = 59.6 kcal and De = 5 X 10’ cm2/sec [19], we obtain D = 6.2 X lo-l6 cm2/sec and (dz)l12 = 2.1 X 10T6 cm = 210 8. This value 210 A coinsides with the length obtained from fig. 4.

3.4. Substrate at SO-660°C

In this temperature range, a few layers at the surface exhibited unusual structure which does not correspond to the usual iron on tungsten tip. Imaging voltage before and after the deposition did not change noticeably, indicating no appreciable varia- tion in the tip radius. However, the image obtained at the lower side of this tem- perature range exhibited significant decrease of the number of the (110) net rings between (110) plane at the center and (3 10) plane, fig. 5. These surface layers were field evaporated at the voltage slightly lower than the best image voltage of helium. The layers could be an iron-tungsten mixture similar to the next instance at

650°C. When the substrate temperature was increased to 65O”C, a very interesting image

was obtained, as shown in fig. 6. Bright spots were arranged in a three-fold sym- metry and their spacing was approximately 12 A on the average. The field evapora- tion of a single surface layer at a field a little lower than the best image field of helium removed the orderly arranged spots and the tungsten surface recovered. This resembles the observation reported by Nishikawa et al. in the systems of W ---Ga,

W-In and W-Sn [14-171. The unusual arrangement of the spots in the present study might also be the result of the formation of an iron-tungsten alloy. Accord- ing to the phase diagram [20], there are two sorts of intermetallic compounds of tungsten with iron which are thermodynamically stable. In this temperature range, tungsten atoms are also mobile. Such movement of the substrate tungsten atoms may promote the formation of mixed layer of tungsten and iron.

In this temperature range, the observed amount of iron on the tip apex area is significantly smaller than the estimated amount of deposited iron. The deposition rate of about 20 atomic layers per hour in the present study is equivalent to the

H. Morikawa et al. / FIMobservation of iron deposited tungsten tip 545

Fig. 5. The substrate temperature is 550°C. (a) The tungsten image before ima ge. (b) The image of the iron deposited tungsten tip; the number of the con siderably decreased from the number in (a); He-HZ image.

depositio: n; He (110) net rings

arrival rate of the iron atoms when the substrate was exposed to iron vapour of 5 X 10d6 Pa, which is the vapour pressure of iron at 900°C. However, the actual tip temperature was 650°C and the corresponding evaporation rate is too small to explain the disappearance of most of the deposited atoms from the tip cap by re- evaporation. It is plausible that most of the iron atoms diffused away to the shank as in blunting phenomena. The regular tungsten image was obtained after field evaporating a few tungsten layers under the unusual layers.

546 H. Morikawa et al. / FIM observation of iron deposited tungsten tip

Fig. 6. The substrate temperature is 650°C. The bright spots which arc arranged in a three-fold

symmetry may indicates an alloy formation; He-Hz image.

3.5. Substrate at 700-800°C

When the substrate temperature exceeded 7OO”C, no iron but only tungsten image was observed. Contrary to the thermal endform of bare tungsten, the tip surface deposited at 700°C exhibited extremely developed (001) and (11 l} planes and the small (1 IO} plane, as shown in fig. 7. A regular tungsten image was obtained after field evaporating 15 (110) layers and the evaporation voltage of the tungsten tip was nearly the same as that before the deposition.

Fig. 7. When the substrate temperature exceeded 7OO”C, no iron but only tungsten image was

observed; He-H2 image.

H. Morikawa et al. / FhW observation of iron deposited tungsten tip 547

It is noteworthy that even an iron-tungsten alloy could not be observed in contrast to the case at 650°C. The alloy of one monolayer might be unstable at 700QCI

4, Summary

The e~itaxiaI growth of iron on tun~ten tip was observed at substrate tempera- tures ranging from 300 to 500°C. The parallelism between the deposited iron and the substrate crystal was not perfect, and the angle between the (110) plane of iron and that of tungsten was estimated at 4 to 1 lo. The epitaxi~ly grown film was observed on one side of the tip cap at the substrate temperatures lOO-300°C. The area of an iron image increased as the temperature increased, and covered the entire tip cap at 370°C. The diffusion length of the deposited iron agreed satisfactor~y with the length calculated using Zahn’s data. At the substrate temperatures of .55Q- 65O”C, the surface diffusion of iron became ~tensive and most of the deposited iron atoms might diffuse away from the tip cap. The substrate tungsten atoms also become mobile in a surface at 650°C and then an alloy of tungsten with iron might be formed. When the substrate temperature exceeded 7oO”C, no iron image but a tungsten image with extremely developed (100) and (111) planes were obtained.

Acknowledgement

The au~ors thank Dr. 0. ~is.~kawa of Tokyo Institute of Teleology for his reviewing this report. Their thanks are also due to Dr. T. Uchida for his determina- tion of iron concentration and Mr. S. Sakakibara for his glass blowing.

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548 H. Morikawa et al. / FIM observation of iron deposited tungsten tip

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