Investigation of the Ti‐Pt Diffusion Barrier for Gold Beam Leads on Aluminum

Abstract

In order to avoid Ai-Au interactions and to ensure the electrical and mechanical stability of the Au beam leaded structures using Al metallization, a Ti-Pt diffusion barrier between Al and Au has been investigated. Ti and Pt films of different thicknesses were employed as diffusion barriers. The resulting AI-Ti-Pt-Au thin film sandwich structures were aged at temperatures in the range of 300~176 to evaluate the effectiveness of the barrier in preventing Ai-Au interaction. The interaction was followed by use of optical and electron microscopy, x-ray diffraction, and sheet resistance measurements. It was found that metallurgical interactions in these structures occurred at preferential sites which were shown to be hillocks in A1. On continued aging, reaction spread around these sites and finally covered the entire surface. The number of such reaction sites increased with decreasing barrier metal thickness and with increasing time and temperature of anneal. The reaction was considerably inhibited (i) by annealing A1 at 450~ for 1/z hr prior to barrier metallization and (if) by using at least 2000A each of Ti and Pt in the barrier. An understanding of the effectiveness and need of a 2000A Ti-2000A Pt diffusion barrier between A1 and Au was obtained by additional studies of the metallurgical interactions induced in A1-Pt, A1-Ti, and A1-Ti-Au thin film sandwich structures by aging at teml~eratures in the range of 200 ~ 500~ Ti is required to cover A1 well so as to prevent A1-Pt interactions and Pt is required to prevent Ti-Au interactions caused by diffusion of Au through Pt to Ti. Aluminum is most widely used as a practical metal-lization material in the IC technology. In order to extend the use of aluminum metallization to multi-chip assemblies it is advantageous to form gold beam leaded sealed structures using such metallization. To avoid gold-aluminum (1) interactions, a suitable diffusion barrier between A1 and Au has to be provided. This barrier metallization must prevent A1-Au interactions and must not lead to a loss of electrical conduction between the outside world and the circuit during its operation. Several barrier metallization schemes-such as Ti-Pd, Ti-Pt, Ti-Rh, Ti-Niphave been considered in the past (2-9) in one form or the other. Such two metal schemes have been chosen to satisfy both the adherence and barrier criteria and to provide sufficient corrosion resistance. The results of both Melliar-Smith and Polito (7) and DeBonte et al: (8) indicate that a Ti-Pt diffusion barrier between A1 and Au would be a good choice. Speight and Bill (2), who studied the effects of aging in dry air at 150 ~ and 250~ and in wet and dry air with 10 ppm HC1 at 150~ on the contact resistance of Ti-Pt-Au conductor film metalliza-tion on Ta2N, reported such metallization to exhibit by far the greatest stability. We have investigated the feasibility of using a Ti-Pt diffusion barrier between A1 metallization and the Au beams. In order to understand the barrier charac-terestics and its effectiveness, one must learn about the metallurgical interactions in two (e.g., A1-Pt, A1-Ti, etc.), three (e.g., A1-Ti-Au, A1-Pt-Au, etc.), and four metal (the A1-Ti-Pt-Au) systems. In the temperature range of 200~176 there is considerable interdiffusion in Ti-Au (8-9) and Pt-Au (10-11) films although no intermetallics are reported for the latter. AI-Au films are known (1) to react catastrophically. A1-Ti films have been reported (12) to react at bigher temperatures, leading to the formation of AlaTi which retarded further interaction between A1 and Ti. On the other hand, there is very little inter-diffusion reported in Ti-Pt (8) films. In a recent * Electrochemical Society Active Member. Present address: Technion, Haifa, Israel, Key words; aluminum metallization, beam leads, diffusion barrier. paper (13) we have reported the results of our investigations of the metallurgical interactions in thin films of A1 and Pt. A1 and Pt films were found to interact very rapidly, leading to the formation of several intermetallics. In this paper we report and discuss the results of our studies of the A1-Ti-Pt-Au metallization scheme together with the results of the relevant studies of the metallurgical interactions in A1-Pt, A1-Ti, and A1-Ti-Au thin film structures. It will be shown that Ti-Pt diffusion barrier provides excellent barrier characteristics between A1 and Au. Experimental The metal films were always deposited on oxidized silicon substrates. Throughout this paper the metal deposited first is written first. For example, A1-Ti-Au would mean that A1 was deposited on the substrate, followed by Ti deposition on this A1, and Au (the outermost layer) deposition on Ti. A~-Ti-Pt-Au structures.-Three different test structures were used in this investigation as shown in Fig. 1. The first [Fig. l(a)] was the simple planar structure formed by depositing various films one after another. 1.5 ~m thick A1 was evaporated onto hot substrates at 320~ Ti (or Ti/TiN) and Pt were sputter deposited on chemically cleaned (in 30:1 am-monium fluoride:HF for 30 sec) A1 in the" desired thicknesses. 5000A thick Au was then electroplated on top of Pt. (In a typical beam lead about 10-15 ~m of Au is electroplated on Pt. Such a thick layer of Au is, however, not suitable for the metallurgical interaction studies.) Most of the initial studies were made by measuring the sheet resistance of the composite film and by following the reaction visually under an optical microscope. A few x-ray diffraction patterns were obtained for the reacted specimens to learn about the possible intermetallics formed. Based on the results of the studies on the above-mentioned test structures, new test structures of the type shown in Fig. lb were made using a meta]liza-tion scheme 2000A Ti-2000A Pt-5000A Au on hot-deposited (320~ A1. The A1 was annealed at 450~ for 0.5 hr in hydrogen to simulate the actual device-fabricating conditions, and to reduce the hillock growth 156) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 150.29.115.139 Downloaded on 2018-12-17 to IP