Lead base alloys and their application for reducing electromigration activity

Abstract: The solder composition of matter consists essentially of a leadbase alloy including a substantially uniform distribution of at least one intermetallic compound formed from said alloy and a solute element, the concentration of said solute element being at least about 0.5% by weigth of the amount of said alloy. The composition is applied for reducing electromigration activ­ ity and extending life time of terminations in microelectronic devices.

Description: This invention relates to reducing electromigration activity and extending life time of terminations in microelectronic devices wherein said terminations consisting of lead/tin alloys. Metal conductors carrying a high current density such as those necessary for practical solid state microelectronic configurations are susceptible to a current-induced mass transport failure mechanism, in which removal of material from one or more locations in the current path and build-up of material at one or more other locations in the current path occurs. Such mass transport phenomenon is termed "electromigration", and results in early failure of devices, either due to an open circuit caused in the current-carrying path as a consequence of the removal of material or to a short circuit formed as a consequence of build-up of material between the current-carrying path and an adjacent conductor. For example, in the fabrication of C-4 terminals (C-4 is a designation for controlled collapse chip connection), a wettable metal area (typically a circle about 0.1mm (0.005 inch) in diameter, although other sizes and geometries may be employed) is evaporated onto a dielectric through holes in a previously-prepared metal mask The wettable metal area is called the BLM (ball limiting metallurgy) and is formed of layers of Cr-Cu-Au, with a total thickness of about 1 micrometer. After the BLM is formed, Pb-Sn solder (typically about 5% Sn) is evaporated through the metal mask and the solder is then reflowed (i.e., melted) to form the ball. During reflow, tin from the solder and copper from the BLM react to form Cu 3 Sn plus a small amount of Cu 6 Sn 5 . During the initial reflow and subsequent reflows to join to a substrate, some of the Cu 3 Sn and Cu 6 Sn 5 spalls (i.e., flakes off) into the liquid solder where it is trapped upon solidification. The resulting Cu 3 Sn + Cu 6 Sn 5 particle distribution in the solder due to spalling is non-uniform. As a consequence, such a non-uniform distribution of the copper/tin intermetallic compounds in the lead/tin solder can increase electromigration damage in the C 4 content. The combination of lead/tin solder alloys and copper is described in the prior art. In patent GB-A-721 881 a solder alloy is disclosed containing lead and tin in the ratio of about 50/50 and copper in the range of 0.5 to 5 percent of the total weight. The alloy is dedicated to reduce the wear of the soldering tools by providing a tin/copper ratio close to the eutectic of Sn-Cu. Another tin-rich alloy containing a small amount of copper is specified in JP-A-58 038 693. Intermetallic compounds of tin, lead and copper are formed during heating to about 400°C. These compounds are described to float in the solder melt at standard soldering temperatures of 250 to 260°C. As a result a better soldering performance is reported. In these documents the problem of electromigration is not discussed or even realized. Moreover the alloys given in the cited documents are containing tin at an rather high amount of about 50 percent of weight. As described above in modern techniques solder alloys typically contain only about 5% Sn. The problem of electromigration occurring within the use of such solder alloys therefore remains unresolved with respect to the prior art. It is therefore an object of the invention to reduce the elctromigration or thermomigration activity in a lead/tin solder employed in solder stripe terminations. It is another object of the invention to provide a substantially uniform distribution of particles of an intermetallic compound in a lead/tin solder so as to reduce electromigration activity therein. SUMMARY OF THE INVENTION In accordance with the invention as given in claim 1, a method is provided for reducing electromigration activity or thermomigration activity and extending the lifetime of terminations in microelectronic devices consisting of a lead/tin alloy by forming a substnatially uniform distribution of at least one intermetallic compound formed from said alloy and a solute element by coevaporating the solute element with lead and tin and reflowing the resulting solder. Solute elements are those elements which form an intermetallic alloy with tin, and include copper, silver, gold and the rare earth elements. Preferred embodiments are given in the dependent claims. In particular, the method comprises forming in a lead/tin solder an intermetallic alloy of copper with tin. The intermetallic alloy comprises primarily Cu 3 Sn (plus small amounts of Cu 6 Sn 5 ). The copper concentration in the solder is less than about three times the tin concentration and ranges from an amount effective to reduce the electromigration activity - at least about 0.5% - to less than about 10% of the amount of the solder. The foregoing method yields a novel composition of matter having the composition and characteristics noted above. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Electromigration activity resulting from electron flow and the occurrence of sites of flux divergence in a lead/tin solder causes open circuits or shorts as material is moved under the influence of an electrical current. Since the lead/tin solder contains about 5% tin, most of the material transported comes about by diffusion of lead. At the device operating temperature in the range of 50° to 90°C, the mass transport is dominated by diffusion along grain boundaries. In accordance with the invention, the grain boundary diffusion and electromigration activity in a lead/tin solder terminal has been found to be substantially reduced by forming an intermetallic alloy of copper with tin in the lead/tin solder. Addition of 3.5 wt. % copper to the Pb-95 wt Sn solder has been found to be effective in reducing the grain-boundary diffusion of Pb by a factor of ten (see Example 2). The intermetallic alloy is primarily Cu 3 Sn, although it appears that an indefinitely small amount of Cu 6 Sn 5 detectable by x-rays is also formed at the same time. The Cu 6 Sn 5 forms during cooling from the reflow temperature. The amount is dependent on the cooling rate but the amount of Cu 6 Sn 5 should be less than 10% of the total amount of intermetallic, i.e. (Cu 3 Sn + Cu 6 Sn 5 ). The copper/tin alloy is formed under conditions such that the copper concentration in the solder is less than about three times the tin concentration and ranges from an amount effective to reduce the electromigration activity - at least 0.5% - to less than about 10% of the amount of the solder. As a consequence of the inventive approach, the lifetime of solder stripes is accordingly extended. Thus, by adding copper to the solder itself, the formation and non-uniform distribution of the copper/tin intermetallics arising from the spalling of the copper from the BLM is prevented. Cu 3 Sn is introduced into the solder by co-depositing copper with the lead/tin. This may be done, for example, by co-evaporating copper with lead/tin (or lead) from a second evaporation source. This is conveniently done either by evaporating pure copper along with lead/tin or, since copper and tin have very similar vapor pressures, by evaporating lead from one source and, substantially simultaneously, evaporating copper/tin from another source. Composition control is achieved by carefully measuring the amount of the material to be evaporated and operating both sources to completion, with the initial copper content in the lead/tin alloy being set at about 0.5%, this being the minimum amount that apparently reduces the electromigration activity. Some upward adjustment of this value might be required, however, depending upon the amount of tin in the lead/tin solder and the structure of the solder following subsequent processing. A substantial advantage of the co-evaporation method is that a very large number of very small Cu 3 Sn intermetallic particles are formed. As is well-known, in evaporating a lead/tin composite, the tin evaporates after the lead has evaporated and forms a cap on the lead; this is a consequence of the much lower vapor pressure of tin compared to lead. Employing a second source of copper during the deposition of lead/tin permits co-deposition of the copper with almost pure lead during most of the deposition. Since, as is known from the Cu-Pb binary phase diagram, lead and copper are mutually insoluble in solid state and do not react, then copper forms many extremely fine particles in the lead matrix. The particle size depends on temperature and evaporation rate, and ranges from several hundred Ångstroms to several micrometers. Upon reflow, the fine copper particles act as heterogeneous sites for the formation of Cu 3 Sn (and Cu 6 Sn 5 ), which solidifies before the lead/tin. Without subscribing to any particular mechanism, it appears that, at least in systems having a relatively low concentration of copper, most of the intermetallic particles tend to be formed at the grain boundaries in the final solidified structure. The diffusion (electromigration) that is being reduced in accordance with the invention generally occurs along the grain boundary at the temperature of use. It is thus considered important to have at least the grain boundaries uniformly filled. On the other hand, it appears that at higher concentrations of copper, on the order of a few percent, the distribution of the copper/tin intermetallic compounds is substantially uniform throughout the solder. Whether the copper/tin intermetallic particles are substantially uniformly distributed along the grain boundaries or throughout the grains themselves or some combination thereof is not material, other than that the distribution of particles fall into one of the foregoing patterns. As a consequence of the method of the invention, the Cu 3 Sn content in the solder is increased without affecting the BLM structure, and permits use of a thin copper BLM without decreasing the electromigration resistance. Improvement of electromigration resistance manifests itself in that for a constant copper thickness in the BLM, the electromigration resistance increases as more reflows are performed. During each reflow cycle, Cu 3 Sn spalls from the BLM and decreases the Cu 3 Sn thickness at the BLM while increasing the amount of Cu 3 Sn in the solder near the BLM. Spalling occurs because the grain boundaries of Cu and/or Cu 3 Sn dissolve preferentially into the liquid solder, thus releasing a particle into the melt. Thus, increased electromigration resistance is obtained with (essentially) thinner copper in the BLM. The copper concentration in the solder is maintained at less than about three times the tin concentration in order to avoid using up all the tin in forming the Cu/Sn alloy. Further, the amount of copper is at least that effective in reducing the electromigration activity; at least 0.5% copper in a 95 Pb/5 Sn solder has been found to be sufficient in this regard. The maximum amount of copper is less than about 10% of the amount of the solder in order to avoid adversely affecting the mechanical properties of the solder. Preferably, the amount of copper ranges from about 3 to 4% of of the total amount of the solder, and most preferably about 3.5%. In this range, the grain boundary diffusivity of lead is reduced by a factor of ten at about 126°C. The substantially uniform distribution of the copper/tin intermetallic particles in the solder is essential, since an uneven distribution results in an excessive electromigration at those locations that do not have the copper/tin intermetallic particles. Accordingly, a greater current density and temperature (due to Joule heating) occurs at those locations that do not have such intermetallic particles, after some electromigration has taken place effectively reducing the area available for current conduction. Consequently, uneven particle distribution results in early failures. Without subscribing to any particular theory, it appears that the presence of Cu 3 Sn (and possibly Cu 6 Sn 5 ) fine particles in lead/tin solder acts to slow any grain boundary diffusion, thereby increasing the resistance to electromigration. The composition suitably employed in the practice of the invention is a lead/tin solder. Such compositions are typically 95% Pb, 5% Sn; however, other lead/tin compositions are expected to be suitably employed in the method of the invention. While the use of copper has been shown to reduce electromigration in lead/tin solders, other elements having properties similar to those of copper, such as silver, gold, and the rare earth metals. While this disclosure is primarily directed to the improvement in electromigration activity in C-4 terminals, the method of the invention is also suitably employed to enhance Josephson electrodes and reduce Soret effects (diffusion in a temperature gradient). The invention acts to slow diffusion in grain boundaries; such effect slobs electromigration and also thermomigration. EXAMPLES Example 1: Electromigration data was accumulated for C-4 terminals as a function of copper thickness. These data are shown below. The failure rate was measured at 1.1 amps/C-4 terminal at 150°C. Cu Thickness in BLM, nm Failure Rate, %/Khrs @ 100 Khrs 1,000 4.4 x 10 -10 500 2.1 x 10 -6 250 1.3 x 10 -4 As can be seen, the failure rate increases by a factor of 10 6 as the copper thickness decreases from 1,000 to 250 nm. Example 2: Grain boundary diffusion studies of the Pb 203 radioactive tracer were carried out in a lead/tin alloy, with the tin concentration maintained at 5% and copper substituted for lead. The temperature was 126°C. The results are shown below: Copper Addition, wt% Diffusivity, (126°C) cm 2 /sec 0 5.2(±0.8) x 10 -15 0.3 2.8(±0.8) x 10 -15 1.4 1.4(±2.0) x 10 -15 3.5 5.2(±0.8) x 10 -16 As can be seen, the grain boundary diffusion of lead in a lead/tin alloy containing 5% tin is reduced by a factor of 10 at 126°C when 3.5% copper is added.

Claims: A method for reducing electromigration activity and extending life time of terminations in microelectronic devices, said terminations consisting of a lead/tin alloy including a substantially uniform distribution of at least one intermetallic compound formed from said alloy and a solute element selected from the group consisting of copper, silver, gold and the rare earth elements, the concentration of said solute element being at least 0.5% by weight of the amount of said alloy, wherein said substantially uniform distribution is achieved by co-evaporating the solute element with lead and tin and reflowing the resulting solder. Method according to claim 1, wherein the concentration of said solute element ranges from 0.5 to 10% and preferably from 3 to 4% by weight of the amount of said alloy. Method according to claim 1 or 2, wherein the concentration of said solute element being less than about three times the tin concentration. Method according to any one of claims 1 to 3, wherein said lead/tin alloy includes bismuth in a concentration similar to that of tin. Method according to any one of claims 1 to 4, wherein said intermetallic compound consists of Cu 3 Sn plus a trace amount of Cu 6 Sn 5 .

Inventors: FOUTS DAVID PERRY, GUPTA DEVANDRA, HO PAUL SIU-CHUNG, JASPAL JASVIR SINGH, LLOYD JAMES ROBERT JR, OBERSCHMIDT JAMES MICHAEL, SKIKRISHNAN KRIS VENKATRAMAN, SULLIVAN MICHAEL JAMES, FOUTS, DAVID PERRY, GUPTA, DEVANDRA, HO, PAUL SIU-CHUNG, JASPAL, JASVIR SINGH, LLOYD, JAMES ROBERT, JR., OBERSCHMIDT, SKIKRISHNAN, KRIS VENKATRAMAN, SULLIVAN, MICHAEL JAMES, OBERSCHMIDT, JAMES MICHAEL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: B23K35
IPC codes: H01B13, C22C1, C22C11, B23K35, H01B1, B23K1