ABSTRACT
Current-voltage characteristics of the electrically conductive silver-filled epoxy Ablefilm ECF-563 preform switches to a high-resistive state under low bias voltage. The observed phenomena is argued to be an intrinsic property of electrically conducting composite materials caused by strong localized centers that introduce space charge.
INTRODUCTION
Electrically conductive silver-filled epoxy preform, ECF-563 Ablefilm [1], is used in an Ultra High Frequency (UHF) power amplifier circuitry as shorting pads for very small (0.055 in. diameter) cross-sectional circuit elements. The circuit functions under a pulse condition in which multiple pin diodes switch to on/off positions. The UHF power amplifier developed some intermittent behavior which was traced to switching in the epoxies under the pin diodes. This paper describes a set of tests which were performed on ECF-563 preform samples for the purpose of understanding the switching phenomena and to propose a relevant transport model. We observed intermittent switching to a high-resistive state in silver-filled epoxy preform Ablefilm ECF-563 under an applied voltage. For a 0.003-in. thick sample of ECF-563 sandwiched between two gold contacts, a threshold voltage of 0.4-1.9 V exists for switching to a high-resistive state. This observation raises a concern regarding the use of ECF-563 in hybrid microelectronics.
Additional switching to a high-current-carrying state was also observed in the same material under higher applied voltage. These phenomena appear not to involve damage to the material (although they can be accompanied by some incidental damage). Understanding of the switching mechanisms will enable reliability enhancement of hybrid circuits and application of these materials to control detrimental effects of ESD and electrical surges.
We have searched the literature on switching instability and intermittent behavior of materials to identify the contributing transport mechanism. The models and their features are briefly introduced.
A thin layer of an insulator sandwiched between metal electrodes frequently possesses special electrical switching properties that at first can be dismissed as dielectric breakdown. The phenomena of "forming" (a reproducible change in electrical conductivity induced by a high electric field) is different from arcing or destructive dielectric breakdown phenomena.
There are numerous articles in the literature on the phenomena of switching in metal-insulator-metal (MIM) junctions (with thin insulators). This subject was considered mostly during the 60s and 70s and is covered in review articles [2,3]. "Forming" governs the behavior of as-manufactured MIM junctions exhibiting switching, which do not require electrical pretreatment ("electroforming"). The current-voltage characteristics of these MIM junctions exhibit S-type or N-type nonlinearity with negative differential resistance (NDR) behavior. The most popular theory, explaining the above mentioned phenomena, is carbonaceous filamentation and its rupture indicative of S-type instability. There is no simplified theory that can describe N-type instability. Although some physical evidence for the filament formation has been reported in the literature, e.g., via chemical vapor decoration [9], a comprehensive microscopic theory governing both S-type and N-type phenomena in MIM junctions is desired. Both instabilities have been observed in a variety of MIM junctions and composites of metal particles in an insulator background; metals vary over a wide range (Ag, Al, Au, Pt, Si, Nb, Be, Mg, Cu, Zn, Ti, Cr, Mn, Fe, Co, Ni, In, Zr, Sn, Pb, Bi, W) and insulators vary from polymers (styrene, acetylene, analine) to oxides (SiOx, AlOx, NbOx, TiOx, CrOx, VOx, TaOx, CuOx, MgO) and others (AlNx,..). Common among all these systems is metal entities separated by a thin dielectric film. The insulator film is undoubtedly far from an ideal pure dielectric. One can easily envision the presence of defects, traps, and localized centers in the dielectric. Polymers contain dangling bonds, broken chains, free radicals, spin and charge defects, dopants, etc. Oxides fabricated via fast and cheap industrial processes possess a well-exhibited space charge [10] and are far from single crystalline oxides used in some of the MIM junction studies mentioned previously. In a typical system, electrons can transfer from one metal entity to another through a variety of mechanisms. These mechanisms involve inelastic interaction of electrons with the defects present in the dielectric material and, therefore, lead to excess heating, runaway phenomena, and dielectric breakdown.
A microscopic picture of the conduction mechanism in thin disordered materials is developed in [11]. The authors emphasize the role played by deep localized trap centers in capturing transit-free carriers and the importance of boundary conditions in determining carrier injection and ejection.
To understand switching to a high-current-carrying state, one needs to distinguish between this reproducible switching phenomena and a destructive mechanism that may finally lead to dielectric breakdown, arcing, and carbonization.
A microscopic picture of switching to a high-current-carrying state can be envisioned with the injection of free carriers from metal particles and their free flight through the thin dielectric material in between. Deep localized traps can capture mobile carriers, but this will have a minimum effect if the time of flight through the dielectric is much less than the time required by mobile charges to equilibrate with trap centers [12].
We argue that switching to a high-resistive state is an intrinsic property of a particulate composite where metal particles are embedded in an insulating matrix with a concentration close to the percolation threshold. We further claim that strongly localized defects in the insulator, surrounding the individual metal particles, form space charges that generate high electric fields in a direction opposing the current and inhibit charge flow. Injection and ejection of charge between the contact pads and the bulk of the epoxy are due to the presence of defects in the matrix layer on the ECF-563 epoxy preform surfaces.
REFERENCES
[1] Ablestik Laboratories, Rancho Dominguez, CA 90221.
[2] H. Pagnia, N. Sotnik, "Bistable switching in electroformed metal-insulator-metal
devices," Phys. Stat. Sol. (a), 108, 11-65 (1988); D.P. Oxley, "Memory effects
in oxide films," Oxides and Oxide Films, vol 6, Ed. A.K. Vijh, chapter 4.
[3] Dearnaley, A. M. Stoneham, D. V. Morgan, "Electrical phenomena in amorphous
oxide films," Rep. Prog. Phys., 33, 1129-1191 (1970).
[4] J. F. Gibbons, W. E. Beadle, Solid State Electronics, 7, 785 (1964).
[5] S. K. Srivastava, R. Bhattacharga, Thin Solid Films, 9, 357 (1972).
[6] K. L. Chopra, J. Appl. Phys., 36, 184 (1965).
[7] T. W. Hickmott, W. R. Hiatt, Solid State Electronics, 13, 1033 (1970).
[8] G. Taylor, B. Lalevic: "RF relaxation oscillations in polycrystalline TiO2
thin films," Solid State Electronics, 19, 669 (1976).
[9] H.-E. Knaus, Wissenschaftliche Hausarbeit, Technical University Darmstadt,
(1982).
[10] D. V. Morgan, A. E. Guile, Y. Bektore, "Stored charge in anodic aluminum
oxide films," J. Phys. D: Appl. Phys., 13, 307 (1980).
[11] V. P. Parkhutik, V. I. Shershulskii, "The modelling of dc conductivity
of thin disordered solids," J. Phys. D: Appl. Phys., 19, 623 (1986).
[12] A. I. Rudenko, V. I. Arkhipov, "Trap-controlled transient current injection
in amorphous materials," J. Non-Crystalline Solids, 30, 163 (1978).