Effect of oxygen on the electrical properties of thin Al films

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Effect of oxygen on the electrical properties of thin Al films

M. A. El Hiti

To cite this version:

M. A. El Hiti. Effect of oxygen on the electrical properties of thin Al films. Re- vue de Physique Appliquée, Société française de physique / EDP, 1990, 25 (7), pp.775-782.

�10.1051/rphysap:01990002507077500�. �jpa-00246238�

Effect of oxygen ^on the electrical properties of thin Al films

M. A. El Hiti

Department ^of physics, Faculty ^of Science, ^Tanta University, ^Tanta, Egypt

(Reçu ^{le 31 mars} 1989, révisé le 21 septembre ^{1989 et le 8} janvier ^1990, accepté le 6 avril 1990)

Résumé.

Des couches minces d’aluminium sont déposées ^sur des substrats de verre à 573 K sous vide

poussé. ^{On a} mesuré la résistivité électrique in ^situ pendant ^et après ^le dépôt des couches minces, pour diverses

épaisseurs, températures ^{et durées de} recuit; des couches de Al _pur ont été étudiées, ^ainsi que d’autres dopées

en présence ^{d’oxygène ou par} expositions séquentielles ^{au vide et à} l’oxygène. La théorie de Fuchs- Sondheimer pour la conduction électrique ^a été utilisée _pour expliquer les résultats expérimentaux. ^{Le libre}

parcours moyen des électrons de conduction ^a été calculé _pour les trois séries de couches.

Abstract.

Thin Al films are deposited ^onto glass substrates at 573 K in high ^vacuum. The electrical resistivity

was measured in situ during and after film deposition, ^{as a} function of film thickness, annealing temperature and annealing time, for pure Al films, ^films deposited ^{in an} oxygen atmosphere ^{and films} deposited ^under

vacuum and oxidized step by step. ^{TCR was} calculated as a function of film thickness. Fuchs-Sondheimer

theory ^for electrical conduction was applied ^{to the} experimental results and the mean free path ^{of the}

conduction electrons was calculated for the three series of the deposited ^{Al films.}

Classification

Physics ^Abstracts

73.90

1. Introduction.

Aluminium is widely ^{used in} industry ^{and modern} technology. ^{For this reason} it has been studied

extensively. ^The electrical resistivity (p ) ^{was studied}

as a function of film thickness (t) for Al films

deposited ^{onto fused} quartz ^and oxidized silicon [1],

for single crystal ^{Al films} deposited ^onto crystalline

NaCl substrates [2] and for Al films deposited ^onto glass substrates at room temperature [3]. ^{All these} experimental ^{studies indicate that the electrical} resistivity ^{decreases as the film thickness increases} [1, 2, 4-6]. ^The experimental ^{results were fitted to}

the wellknown Fuchs-Sondheimer (FS) theory ^or

electrical conduction [7, 8] ^{and to the modified}

model of Lucas [9]. The film electrical conductivity

was expressed ^{in terms of bulk} conductivity, ^reduced

thickness (film thickness/mean ^free path) ^{and the} specularity parameters (which have been assumed to

be the same for the upper and lower film surfaces in FS theory, but in the Lucas theory, the upper and lower surfaces have two different values). ^{A com-} plete survey about the effect of annealing tempera-

ture on the electrical resistivity of thin Al films has

recently ^been published [10]. ^{The effect of} annealing

temperature ^{on the} electrical resistivity of thin Al films was studied in the temperature range of 40 ^to 400 K [11, 12] ^and from 400 K to the melting point [13, 14]. The results show that the electrical resis-

tivity ^{increases as the} annealing temperature ^in-

creases.

The aim of the present ^{work was to} study ^the

effect of oxidation, ^film thickness, annealing ^tem- perature ^and annealing ^{time on} the electrical resis-

tivity ^{of thin Al films} deposited ^onto glass ^substrates

at 573 K. TCR (temperature coefficient of resistivity)

was calculated as a function of the film thickness.

imental results to calculate the value of the mean

free path ^{of the} conduction electrons.

2. Experimental.

Al of purity ^{5N from Balzers was} thermally evaporated using ^a hair-pin ^{W source under} high

vacuum onto glass substrates at 573 K, ^{with a} deposition ^{rate of 0.3 nm s-1. The} evaporation ^rate

and the film thickness were measured and monitored

using quartz crystal ^{thickness sensor. The substrate}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01990002507077500

776

temperature ^was measured and controlled using ^Ni-

CrNi thermocouples ^{sticked onto the} glass ^subs-

trates. Four platinum electrodes ^were attached to the glass substrates for electrical measurements.

Three series of Al films were deposited. ^{The first}

series are the _pure Al films deposited ^{in vacuum}

under a pressure of 10-4 Pa. The second series of films are deposited ^{in vacuum} step by step ^at

10-4 Pa (each layer ^{of 10 nm} thickness), ^{the surface}

of each layer being exposed ^to oxygen ^{at a} pressure of 10-2 Pa for 10 min. The third series were

deposited ⁱⁿ presence of oxygen ^{at a} pressure of 10-2 Pa. The electrical resistivity of the three series of the deposited ^{Al films was} measured in situ during

and after deposition ^{as a} function of film thickness,

and after deposition ^{as a} function of annealing temperature ^and annealing ^{time at 673 K.}

3. Results and discussion.

Al films of various thicknesses were studied, ^but only films of final thicknesses of 60 and 200 nm are

discussed here for the three types of Al films. The

dependence of the electrical resistivity ^{on the film} thickness, ^as measured in situ during ^film deposition

is illustrated in figure ¹ for pure Al films (curves A),

films deposited ^{in vacuum} and oxidized step by step (curves B) and for films deposited in presence of oxygen (curves C) respectively. ^It is clear that the electrical resistivity is lower for _pure films (A) ^than

for films deposited ^{in vacuum} and oxidized step by step (B), ^{while Al films} deposited in presence of oxygen (C) ^{have the} highest values. These curves of

figure 1, ⁱⁿ general ^{show a} marked decrease of the electrical resistivity ^as the film thickness increases.

The electrical resistivity ^{reaches a} nearly ^constant

saturation values for Al films of thickness higher

than 150 ^nm for all the deposited types ^{of films} A, B, C which agrees with other results [2]. ^{Pure Al films} deposited ^onto glass substrates contain a large

number of small grains ^{which are} randomly ^oriented

for small film thickness ^as indicated by ^TEM (trans-

mission electron microscope) images ^{and SAD} (selected ^area diffraction) patterns for Al films

deposited ^{at the same} conditions [15]. ^{As the films}

thickness increases, the sizes of the grains ^become larger and the films have single crystal ^{structure of} (111) orientation [1, 15]. The conduction electrons face _many scattering for small film thickness such as

grain boundaries, defects, inter-island area, the random orientational effect, ^etc. As the film thick-

ness increases, it becomes crystalline ^of preferred

orientation and with large grain sizes where the effect of grain boundary scattering and inter-island

area can be neglected, therefore, the conduction electrons face few scattering ^{centers which are} very lower than in the ^case of smaller thickness. This

explains the marked decrease of the electrical resis-

tivity of Al films with the thickness. For Al films

deposited in presence of oxygen (contaminating gas), ^the range of grain ^sizes narrowed, ^{with the}

mean shifting ^{towards a} smaller values as the _gas pressure increases [16, 17]. The existence of oxygen

during ^film growth develops ^{bunch of} growth steps decorated by pinning sites, dents, ^micro steps ^and hillocks on the (111) ^faces [17]. Contamination

layers partially ^or completely covering ^{the surfaces}

of the crystals ^{can be} developed mainly by ^the

processes accumulating the oxygen species ^on (111)

faces [18, 19], this leads to the separation ^{of Al} crystals during ^their growth. ^The codeposition ^of foreign ^{atoms, molecules or their} compounds, ^which

are not dissolved in the film material are accumu-

lated both in the form of precipitate and in the form of layers covering the surfaces during ^the growth ^of

the individual crystallites ^or polycrystalline ^films.

This local accumulation of foreign ^atoms (oxygen and/or oxide) ^can already ^{influence the} growth

mechanism of the film as well as physical ^and

chemical properties [20]. ^{As a result, the electrical} resistivity for oxidized Al films is higher ^{than for}

pure films. For Al films deposited ^{in vacuum and}

oxidized step by step, the electrical resistivity ^in-

creases after the oxidation process due ^to the

scattering ^effect of the formed oxide layers, ^then

decreases again ^{as the} deposited film thickness increases, where the oxide layers embedded and screened by ^the freshly deposited ^new Al layers, ^this

process is repeated each 5 and 10 nm for films of 60 and 200 nm thickness respectively. ^The grains ^{are of} higher sizes for films oxidized step by step ^{than for} those deposited ⁱⁿ presence of _oxygen, for this

reason they have lower values of electrical resistivity.

Fuchs-Sondheimer theory ^{for the} electrical con-

duction expressed the film electrical resistivity

p f in terms of the bulk resistivity p o, the specularity parameter ^{P and the reduced thickness K =} t / A o where 03BB0 ^{is the mean free} path (mfp) ^{of the}

conduction electrons, ^the expression ^{of FS can be}

written in the form :

Drawing ^{the relation} between the film electrical

resistivity p f and the reciprocal of the film thickness

1 / t ^{using the} experimental data, according ^to equation (1) ^a straight line with slope (3/8) 03C1003BB0(1-P) ^and intercept part from the vertical axis equals po will result. The calculated values of the electrical resistivity of the bulk material (or ^of

films of infinite thickness) ^{and the mean free} path ^of

the conduction electron A _{o are} listed in table 1 for

the three types ^{of the} deposited Al films of thickness

60 and 200 _nm, using ^P

0. The results in table 1

indicate that the mean free path of the conduction

electron increases while the bulk electrical resistivity

Fig. 1.

^The dependence of the electrical resistivity of thin Al films of thickness 60 and 200 nm on film thickness, ^where

curves A, B, ^{C are} for pure films, films deposited ^{under vacuum} and oxidized step by step ^{and films} deposited ⁱⁿ

presence of _oxygen respectively.

decreases as the film thickness increases. The films

deposited ^{in presence of oxygen have the} highest

values of _{p o} and the lowest values of A o, pure Al films have the lowest values of _po and the highest

values of À _o and Al films deposited ^{in vacuum and}

oxidized step by step ^{have a} moderate values of

p o and A _o. The results in table 1 are far from those of

the bulk materials, this may be related ^to the

778

1

L

Fig. ^2.

^The relationship between the electrical resistivity ^{and the} reciprocal of the film thickness according ^{to FS} theory for Al films of thickness 60 and 200 nm deposited ^onto glass substrates at 573 K. Curves A, B, C have the same

meaning ^{as in} figure ^1.

presence of defects, grain boundaries and oxide film

layers ^{which are not} present (or ^{of lower} order) ^for

bulk materials.

The relation between the electrical resistivity ^and

the annealing temperature which is lower than the

deposition temperature ^is presented ⁱⁿ figure ^{3 for}

Table 1.

The variations of the calculated values of ^{the mean} free path of the conduction electron

03BB0 and the bulk electrical resistivity po with the film ^thickness.

Fig. ^3.

The variations of the electrical resistivity ^{with the} annealing temperature for Al films of thickness 60 and

200 _nm, curves A, B, C have the same meaning ^{as in} figure ^1.

780

Al films of thickness 60 nm and 200 nm. It is clear that the electrical resistivity increases with the an-

nealing temperature ^{for all} deposited types ^{of Al} films. The variations in electrical resistivity ^{with the} annealing temperature ^are mainly ^{related to one of}

the two competing processes. The first process is the

recrystallization ^and rearrangement of the film small

crystallites ^{and this causes a} marked decrease in the number of the scattering ^{centers, this} process de-

creases the film resistivity. The second _process is the

physical adsorption ^{and chemical} absorption ^{of the}

residual gas molecules (oxygen) and the formation of amorphous ^oxide film, accumulation of the im-

purity ^atoms and the thermal generation ^of defects,

this process increases the film electrical resistivity.

According ^{to this} fact, the increase in the electrical

resistivity ^{with the} annealing temperature (which ^is

lower than the deposition temperature) ^{can be}

related to the second process, i.e. ^to the thermal

generation ^{of defects and} bulk character for pure Al

films, ^{beside these it can be related to the} absorption and/or absorption of oxygen and the formation of oxide films for the two other deposited ^films.

The dependence of the electrical resistivity ^{of Al}

thin films on the annealing ^{time at 673 K} is shown in

figure ⁴ for films of thickness 60 nm. The electrical

resistivity decreases with the annealing ^{time for pure}

films and films deposited ^{under vacuum and oxidized} step by step, the decrease for the later films is faster than for pure films. This decrease in the electrical

resistivity ^{with the} annealing time may be related ^to the annealing ^{at 673 K which is} higher ^{than the} deposition temperature, ^this annealing process leads to the rearrangement ^and recrystallization ^{of the}

film small crystallites. ^{Al films} deposited ^under

vacuum and oxidized step by step ^{contain a} large

number of smaller grains ^{than for} pure films, ^the annealing ^{at 673 K leads to the} migration ^and

coalescence of neighbouring ^small grains, ^{as a result}

the size of the grains ^increases and the reorientation of the film crystallites proceeds. ^For this the electri- cal resistivity of Al films deposited ^{in vacuum and}

oxidized step by step ^{decreases more} rapidely ^than

for _pure films. Al films deposited ⁱⁿ presence of oxygen and annealed in presence of oxygen ^atmos-

phere ^{too show an} increase of the electrical resistivity

with the annealing time, ^{this can} be related to the continuous process of oxide formation beside all the other scattering ^centers.

The variation of TCR with the film thickness is

presented ^{in table II.} This table shows that TCR increases with the film thickness, ^this result is in accordance with those of figure ¹ for the electrical

resistivity ^{and can be} explained ^{in the} same manner.

4. Conclusion.

The experimental results of thin Al films deposited

in high ^{vacuum onto} glass substrates at 573 K have the following ^{features :}

1) the electrical resistivity ^{decreases as the film}

thickness increases ;

2) the electrical resistivity increases with the ^an-

nealing temperatures ^{which are lower than the} deposition temperature ;

3) ^the electrical resistivity decreases with the

annealing time when the annealing process takes

place ^at temperature higher ^{than the} deposition

temperature, for Al films deposited ^{under vacuum}

and oxidized step by step ^{as well as} for pure films,

the decrease is lower for the later films ;

4) ^{Al films} deposited and annealed in presence of oxygen have higher values of electrical resistivity

which increases with the annealing time ;

5) TCR increases as the film thickness increases ; 6) the calculated values of the mean free path ^of

the conduction electrons was found to increase while the bulk electrical resistivity ^{decreases as the film}

thickness increases, pure films have the lowest values of po ^and highest ^{values of} A o, ^films deposited

in _presence of _oxygen have the highest ^{values of}

p o and lowest values of A o while films deposited

under vacuum and oxidized step by step ^have moderate values.

Acknowledgments.

The author gratefully acknowledges ^{Prof. P. B.}

Barna and Dr. L. Toth, Research Institute for Technical Physics, Budapest, Hungary, ^{for their} help during ^{some of the} experiments.

Table II.

The dependence of ^{TCR on} the thickness of ^Al films deposited ^onto glass substrates at 573 K.

Fig. ^4.

The effect of annealing ^{time on} the electrical resistivity ^{of thin Al} films of thickness 60 nm deposited ^{at 573 K}

onto glass substrates and annealed ^at 673 K, A, B, C have the same meaning ^{as in} figure ^1.

782

References

[1] ^MAYADAS A., ^{FEDER F. and ROSENBERG} R., ^{J. Vac.}

Sci. Techn. 6 (1969) ^690.

[2] JAYADEVAIAH T. and KIRBY R., Appl. Phys. ^{Lett. 15} (1960) 150.

[3] ^MAYADAS A., ^J. Appl. Phys. ³⁹ (1968) ^4241.

[4] TOCHISKII E. and BELYAVSKII N., Phys. Status Solidi

(a) ⁶¹ (1980) ^K21.

[5] ^{TELLIER C. and TOSSER} A., Thin Solid Films 37

(1976) ^207.

[6] DOBIERZEWSKA-MORZRZYMAS E., ^BIEGANSKI P.,

RADOSZ A. and BOCHENEK A., Thin Solid Films 102 (1983) ^77.

[7] ^FUCHS K., ^Proc. Cambridge ^{Philos. Soc. 34} (1938)

100.

[8] SONDHEIMER E., Adv. Phys. ¹ (1952) ^1.

[9] ^LUCAS M., ^J. Appl. Phys. ³⁶ (1965) ^1362.

[10] ^DESAI P., ^{JAMES H.} and Ho C., ^J. Phys. ^Chem. Ref.

Data 13 (1984) ^1131.

[11] SETH R. and WOODS S., Phys. ^{Rev. B 2} (1970) ^2961.

[12] ^{SIMMONS R. and BALLUFFI} R., Phys. ^{Rev. 117} (1960)

62.

[13] ^KEDVES F., ^{GERGELY L. and HORDOS} M., ^Acta Phys. Chem. Debrecina 18 (1973) ^67.

[14] GRIGOROVICI R., DÉVÉNYI A. and BOTILA T., ^J.

Phys. Chem. Solids 23 (1962) ^428.

[15] ^{REICHA F. M., Ph. D.} Dissertation, Budapest, ^Hun-

gary (1982).

[16] ^REIMER J., J. Vac. Sci. Techn. A 2 (1984) ^242.

[17] ^BARNA P., ^REICHA F., ^BARCZA G., ^{GOSZTOLA L.}

and KOLTAI F., ^{Vacuum 33} (1983) ^25.

[18] ^{BARNA P.} and REICHA F., ^{Proc. VIII} Int. Vacuum

Congress, ^22-26 September 1980, Cannes, France, Vol. 1, ^P. 165, Supple Vide, ^Les

Couches Minces N 201 (1980).

[19] ^{REICHA F. and BARNA} P., ^Acta Phys. Hung. ⁴⁹ (1980) ^237.

[20] ^BARNA Á., ^BARNA P., ^RADNOCZI G., ^{REICHA F.}

and TOTH L., Phys. Status Solidi (a) ⁵⁵ (1979)

427.