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Submitted on 1 Jan 1972
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PREPARATION AND PROPERTIES OF THIN FILM MATERIALS WITH PEROVSKITE TYPE
STRUCTURE (MIXED OXIDES ABO3)
L. Reiber
To cite this version:
L. Reiber. PREPARATION AND PROPERTIES OF THIN FILM MATERIALS WITH PER- OVSKITE TYPE STRUCTURE (MIXED OXIDES ABO3). Journal de Physique Colloques, 1972, 33 (C2), pp.C2-265-C2-267. �10.1051/jphyscol:1972293�. �jpa-00215029�
PRÉPARATION ET PROPRIÉTÉS DE COUCHES MINCES DU TYPE PEROVSKITE C2-265
isolantes et transparentes. La constante diélectrique relative E, est de l'ordre de 80 à 100 à 1 kHz, les pertes diélectriques sont cependant assez élevées 8-30 %.
L'étude cristallographique des dépôts par diffrac- tion électronique a révélé une structure cubique. 11 semble qu'en présence d'une atmosphère de 100 %
d'oxygène on réalise des couches d'oxydes mixtes, riches en oxygène. L'étude de la composition de ces dépôts est en cours. Les propriétés électro-optiques, piézo-électriques, et élasto-optiques sont examinées dans notre laboratoire d'optique cohérente.
IV. Conclusion. - L'étude des ferroélectriques en couches minces continue a être limitée par les pro- blèmes de préparation du matériau. La technique de pulvérisation R. F. que nous avons adaptée aux condi- tions spéciales exigées par la fragilité (thermique et
mécanique) des cibles en céramique a donné des résultats encourageants. Les couches déposées pré- sentent certaines propriétés ferroélectriques (constante diélectrique, non-linéarité, etc.). L'amplitude des phénomènes est cependant encore faible et difficile à contrôler. Cependant nous pensons que la voie choisie pour la réalisation de structures complexes est bonne et qu'elle est en pleine expansion, comme l'ont montré les travaux récents chez Westinghouse [4] et chez 1. B. M. [5]. En particulier, l'utilisation récente de la technique de polarisation des dépôts pendant la décharge R. F. [6] et la commande du gaz réactif à l'aide d'un spectromètre de masse pendant la crois- sance des films, devraient permettre un meilleur contrôle de la structure et de la composition des couches minces. C'est dans cette voie que nous tra- vaillons maintenant.
PREPARATION AND PROPERTIE S
OF THIN FILM MATERIALS WITH PEROVSKITE TYPE STRUCTURE (MIXED OXIDES ABOj)
Abstract. - Barium Titanate (BaTiO3) and Lithium Niobate (LiNb03) thin films (0,2 to 1 pm) were deposited ont0 Platinum foils and Silicon wafers (M. 1. S. capacitor structure). In the present work, polycristalline films which had been prepared by R. F. cathodic sputtering of ceramic targets were studied. With a substrate temperature below 300 OC during the deposition, good insulating amorphous films with a low dielectric constant were obtained, but with a postdeposition heat tre- atment ( - 600 OC) the deposits show some ferroelectric properties (high dielectric constant, non linearity, etc...). However, the magnitude of the phenomena is still small and difficult to control, but these encouraging results justify a continuation of the study.
1. Introduction. - During the last decade, various methods have been investigated to prepare thin films of high permittivity materials for application in microelectronic circuits, as a passive element (high value capacitors), or as active elements in the M. 1. Sc (Metal-Insulator-Semiconductor) structure. More recently the need of materials with eIectro-optical pro- perties, for applications in optical display systems and optical memûries, gave a new impetus to the study of thin ferroelectric films and particularly the materials with perovskite type structura (mixed oxides ABO,).
A review of methods for depositing ferroelectric thin films has been published by Feuersanger [l]. Our efforts in this field have been concentrated on Barium Titanate (BaTiO,) [2]-[3] and Lithium Niobate (LiNbO,).
II. Sputtering techniques and equipment. - The R. F. sputtering system set up followed a conven- tional diode design. The vacuum system consisted of a Balzers high vacuum pumping unit PST 1900 A automatically controlled, including an oil diffusion
pump (2 000 11s) with multi-coolant baffle for water or nitrogel? cooling. The sputtering module was built around a multi-cathode SP 430 - Mathis module.
The main specifications are :
- 3 @ 90 mm electrodes cooled by water.
- The low pressure limit at the top of the cham- ber : 1 x torr.
- Magnetic field : 0-100 G.
- R. F. generator : 13.56 MHz : 0-1 000 W.
- Gas supply : argon and oxygen commercially available 519 are introduced into the system through stainless tubing and controlled by a needle valve ; automatically controlled or not (as a function of pressure).
- Targets : the targets are pressed and sintered in Our ceramic Laboratory, according to classical tech- niques, from very high purity powders.
- Substrates : various materials have been utilized as substrates. Among these, diamond polished pla- tinum, quartz, and silicon wafers are found. The latter has been mainly used for studying M. 1. S. capacitors structures.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972293
L. M. REIBER
Sputtering conditions
- R. F. Power during deposition : 0. S to 1.5 W/cm2 (target).
- Magnetic field : 40 G.
- Total discharge pressure : 1 to 5 millitorr.
- Oxygen concentration : O % (residual) or 100 %.
- Argon concentration : 0 % or 100 %.
- Anode-cathode spacing : 5 cm.
- Deposit bias : floating (- - 50 V).
- Deposition rate : 20 to 70 A/mn.
III. Films properties. - The thickness of the thin films is of the order of 0.2 to 1 Pm, they are transpa- rent, and adhere very well to the substrates. Their properties depend on the deposition parameters, the main factors determining the properties of the perov- skite films are : substrate temperature, oxygen partial pressure, deposition rate. For the moment, we cannot control oxygen partial pressure, during sputtering, it is equal to, either the residual oxygen pressure in the chamber (noted O %) or 100 % of the total discharge pressure. With O % of oxygen, the BaTiO, target remains unchanged (no reduction) after several hours of ionic bombardment and the films present some beautiful interference colors. However, with the same conditions, the LiNbO, films have a dark brown color, almost metallic in appearence. And in order to have a transparent film we have worked up to now at 100 %
oxygen.
The cristallographic studies as well as the leakage current at high field have shown that the BaTiO, films had a slight oxygen deficiency, and LiNbO, films an excess. In the two cases, the films have some semi- conductive properties.
In the future, we shall use a mass spectrometer which will monitor oxygen introduction during sputtering.
Electrical studies of the M. 1. S. capacitance versus bias and measurement frequency yield informations about the evolution of the charges in the films, and the coupling of the ferroelectric and semiconductor properties.
BaTiO, FILMS CHARACTERISTICS. - Immediately after deposition of the films on cooled substrates (surface temperature - 300 OC) there are amorphous and do not show ferroelectric properties (er - 20).
A post-deposition heat treatment at 6000C to 7000C (1-2 hours) in an oxygen atmosphere was necessary to obtain the following characteristics dielectric constant e, on platinum is of the order of 100 to 350, on silicon, the measured values are lower (50 to 150) - due to a thin parasitic SiO, film between BaTiO, and the silicon wafer. The dissipation factor tg 6 at 1 KHz varied from 0.62 to 0.08. The breakdown field is of the order of 5 x IO5 V/cm.
The current-field dependence is ohmic, p - 109 to
IO1: Q.cm for low electric fields, and non-ohmic
(- exponential) and asymmetric in the Pt-BaTi0,-Al structure for high field > 1 x 105 V/cm. Differences of several orders of magnitude in DC conductivity were encountered among virious samples. Our best samples gave a high dependence of the dielectric constant on field strength, a variation of AC/C = 45 %
for a variation of the electric field from O to 6 x IO5 V/cm (Fig. 1).
- 6 - 4 - 2 O 2 4 6
E ( 1 0 ~ ~ l c r n ) FIG. 1. - Dependence of the dielectric constant on field strength at room temperature. Structure : Pt-BaTiO3-Al ; Reference : Pt 4 BaTiOs thickness : 5 700 A ; Heat treatment : T = 600 OC
(1 hour).
F ~ G . 2. - Dependence of the M. 1. S. capacitance with a ferro- electric insulator versus electric field at room temperature.
Structure : Si (N : 0,05 il. cm)-BaTiO3-Al ; Reference : SF Al 8 C-BaTiOs thickness = 5 400 A ; Heat treatment : T = 660 OC
(1 hour).
Deposits on silicon show coupling of non-linearity properties of the dielectric constant versus electric field in the positive voltage area on figure 2, and the silicon surface properties in the negative voltage area,
PREPARATION AND PROPERTIES OF THIN FILM MATERIAL WITH PEROVSKITE C2-267
since silicon is of N type conductivity. X rays analyses show that ferroelectric and paraelectric states coexist in the films at room temperature, with, however, a predominancy of the cubic phase.
It was not possible to change this structure by means of heat treatments (T < 900 O C ) .
LiNbO, FILMS CHARACTERISTICS. - Our first films deposited in pure oxygen (100 %) have given encou- raging results ; the films are insulating and transparent.
The dielectric constant is of the order of 80 to 100, the dielectric losses are however rather high 8-30 %.
The crystallographic studies by electron diffraction showed a cubic structure. The 100 % oxygen atmo- sphere seems to have led to a deposit of mixed oxides rich in oxygen whose composition is in process of analysis.
The optical, electro-optical, piezoelectric and elasto-
optical properties of these films are studied in Our coherent optics laboratory.
IV. Conclusion. - The deposits show some ferro- electric characteristics (high dielectric constant, non linearity, etc.). The magnitude of the phenomena are still small and difficult to control. However, we feel that the way chosen for the realization of these complex structures is adequate and is continually improving, as has been shown by Westinghouse [4]
and 1. B. M. [5] recent work.
In particular, the recent use of substrate-tuning methods [6] in a R. F. discharge, and the monitoring of the reactive gas with a mass spectrometer during film growth, should allow a much better control of structure and composition of thin films.
Our current efforts are aimed in this direction.
References
[l] FEUERSANGER (A. E.), « Thin Film Dielectrics )> (ed. [4] TAKEI (W. J.), FORMIGONI @J. P.) and FRANCOMBE F. Vratny) p. 209, The electrochem. Soc. Inc. (M. H.), J. of Vacuum science and technologie,
New York, 1969. 1970, 7,442.
[2] HUY DAT (R. V.) and BAUMBERGER (C.), Phys. Statw [ 5 ] ATKIN (R. B.), 1. E. E. E. Symposium on applications Solidi, 1967, 22, K 67. of ferroelectrics, Yorktown N. Y., 1971.
[3] REIBER (L. M.), ler Symposium européen sur la [6] LOGAN (J. S.), MAKKOCKS (F. S.), DAVIDSE (P. D.), pulvérisation, Toulouse, Octobre 1969. 1. B. M. Journal of Res. and Dev., 1970,14,2-182.
