Interface contribution to the capacitance of thin"'film A( ... ~U203..AI trnayer structures A. F. Hebard, S. A. Ajuria,a) and R. H. Eick AT&T Bell Laboratories, Murray Hill, New Jersey 07974
(Received 6 August 1987; accepted for pUblication 28 August 1987) A dual-gun reactive ion beam sputtering technique has been used to reproducibly fabricate A1 2 0 3 dielectrics with low electrical loss for controlled thickness ranging from approximately 10 to 360 A.. The linear dependence of the reciprocal capacitance on dielectric thickness of AI-AI 20 J -AI triIayer structures incorporating this dielectric reveals a significant contribution from an interfacial capacitance in serres with the geometric capacitance. Room-temperature measurements of both the de resistance and the frequency-dependent complex impedance demonstrate that, with respect to bulk, there is an enhanced frequency-dependent dielectric tosS associated with this interfacial capacitance.
The constantly shrinking size of circuit elements in microelectronic applications necessitates a thorough understanding of physical processes occurring on length scales ap~ proaching atomic dimensions. This is particularly true for thin-film capacitors where attributes such as frequency-dependent loss, electric field breakdown strength, and charge storage capability become strongly modified as the electrode separation is decreased. The nature of the Si/Si02 interface in very large scale integrated technology is just one example of the importance of increased understanding of these issues. I An indication that interface processes begin to become significant in the limit of smail electrode separation can be found in the work of Mead z on Ta-Ta 2 0 s -Au and Ta-Ta 2 0 S -Bi tunneling structures in which it was shown that for dielectrics thin enough to allow direct electron tunneling it is necessary to model the total capacitance as a series combination of an interfacial capacitance and a geometrical capacitance. This interfacial capacitance was shown to vary in proportion to the electronic density of states of the Au and Bi counterelectrodes. Such a dependence is based on the notion that an electrode with a low density of states has a larger charge penetration depth with a correspondingly smaner capacitance. 2 There are, however, serious theoretical objections 3 to this interpretation which are based on the question of which electric field boundary conditions to use at the metal-dielectric interface. The physics of the situation is further complicated by polarization and space-charge effects which have the effect of creating a "blocking" capacitance at the interface. 4 •5 Although a complete theory is lacking, the microscopic origin of these effects is suspected to be influenced by the presence of electrons trapped with a finite lifetime in localized states near the interface. 6 The research reported here is motivated by this same question concerning the behavior of the capacitance of thinfilm metal-insulating-meta! trilayer structures when the electrode separation d is reduced towards zero. We have chosen for simplicity to study the symmetric AI-Al z0 3 -AI system in which there is only one metallic element. The 3)
1986 recipient of all AT&T Bell Laboratories Cooperative Research Fellowship award. presently at MIT. Cambridge, MA.
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Appl. Phys. Lett. 51 (17). 26 October 1987
A1 2 0 3 dielectric with predominantly ionic bonding and large band gap is wen known in tunnel junction and artificial barrier applications 7 for its low leakage and pinhole~free coverage. In contrast to the work by Mead,2 our AIzO} dielectric is grown with known thicknesses so that the dependence of capacitance on absolute rather than relative thickness can be determined. Interpretation of electrical impedance data on a sequence of AI-A12 0 3 -Al thin-film capacitors with varying thickness is based on a model in which dc conduction processes are ascribed to afrequency-independent shunt resistor in parallel with a capacitor having a frequency-dependent dielectric constant. We find that the total measured capacitance em arises from a geometric capacitance Cb with "bulk" dielectric constant in series with an interfacial capaci.tance C j • Interestingly, the magnitUde of C j is an appreciable fraction of C b over the entire thickness range (d,;;; 360 A). As a consequence, the frequency-dependent loss is COIlsiderably enhanced over that of the bulk, an enhancement which becomes especially pronounced at low frequencies. A dual-gun reactive ion beam sputter deposition technique is used for the fabrication of the AI z0 3 dielectric. A beam of xenon ions from the first gun, the deposition gun, is directed onto an Al target at an incident angle of approximately 45°. Simultaneously, a beam of oxygen ions from the second gun, the etching gun, impinges directly on the rotating substrate which is in a location outsi.de of the first ion beam. Oxide growth on the substrate thus occurs under dynamic nonequilibrium conditions in the presence of a plasma of incoming aluminum atoms and oxygen ions. By careful tuning of ion beam intensities the rate of oxide accumulation can be adjusted to be slightly greater than the rate of oxide erosion. The net oxide growth rate of approximately 4 A. per minute is thus slow enough to allow a homogeneous wel1compacted film to grow. Transmission electron microscopy examination confirms a homogeneous void-free amorphous filmS which is consistent with increased packing densities commonly found in films prepared using ion-assisted deposition processes. ".10 Eight capacitor samples, each in a cross configuration with an area of 4X 10- 4 cm 2 , are prepared simultaneously. The Al electrodes are deposited through photoresist masks
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© i 987 American Institute of Physics
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