592 IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS, AND MANUFACTURING TECHNOLOGY, VOL. 13, NO. 3, SEPTEMBER 1990 Correlation Between Electrical Resistance and Microstructure in Gold Wirebonds on Aluminum Films LISA MAIOCCO, DONNA SMYERS, PAUL R. MUNROE, AND IAN BAKER Abstract-Gold ball bonds attached to either pure AI films or Ai films with Cu and Si additions were annealed at temperatures in the range 77-277°C for periods of up to 3000 h. Electrical resistance of the bonds was measured to within +/A1 mR using a manual four probe arrangement with an applied current of up to 100 mA. Nonlinear multiple regression analysis of the data produced an empirical model for the resistance increase up to 8 mR. The resistance increases are related to the intermetallic phases and void configurations observed. 1. INTRODUCTION OLD wire connections to aluminum layers are common G in semiconductor components. At the elevated tempera- tures arising from device processing or in-service resistance heating, interdiffusion of the gold and aluminum leads to inter- metallic formation and void formation at the gold-aluminum interface [ 11. Voids and high resistivity intermetallic phases at the bond base increase the electrical resistance of the connec- tion [2], [3]. This paper reports on correlations between changes in re- sistance and the presence of intermetallic phases and voids after various annealing treatments. 11. EXPERIMENTAL METHOD A. Fabrication of Test Structure Silicon wafers were thermally oxidized to form a 0.84-pm layer of SiOz. Without breaking vacuum, a 0.1-pm diffusion barrier layer of titanium and a 1.2-pm aluminum-based met- allization layer were sequentially evaporated onto the Si02. Three different compositions of the aluminum-based film were produced: 99.999% pure aluminum, Al- 1.7 wt % Cu- 1.8 wt %, Si, and A1-6.5 wt % Cu- 1.9 wt % Si. (Copper addi- tions are typically made to A1 films in semiconductor devices to increase their electromigration resistance; silicon additions are made to prevent spiking [4] .) Thermosonic gold ball bonds were formed on each of the metallizations, with the use of a Kulicke and Soffa Dawn I 1419 wirebonder equipped with PROBE 2 At TI >I" \ SILICON SUBSTRATE Fig. 1. Electrical test configuration: currents of 25-100 mA were applied through probes 1 (I,") and 2 (Iout). Standard resistance measurements were taken using the voltage drop across probes 3 (V,) and 4 ( VL), with probe 3 (VH) at point A as shown. a model 4322 Ultrasonic generator. The same bonding pa- rameters were used for each metallization (40-g compressive force, 40-ms bonding time, stage temperature 180°C, ultra- sonic generator set at 3.25 corresponding to 0.6 mW). The 33-pm stabilized gold bond wire used resulted in bond diam- eters of approximately 100 pm for the pure A1 film and 80 pm for the Al-Cu-Si films. B. Heat Treatments Annealing was performed by placing samples in Pyrex tubes which were evacuated, partially back-filled with argon, and sealed to prevent oxidation. Isothermal annealing was per- formed at 77°C for times ranging from 750 to 3000 h; at 107°C and 150°C for times ranging from 2 to 1730 h; and from 187°C to 277°C for times ranging from 2 to 500 h. C. Electrical Testing Electrical resistance of the bonds was measured to within +/-1 mR using a manual four probe arrangement, as shown in Fig. 1. Twelve bonds on each sample were measured, with an applied current of 25-100 mA. For all bonds exhibiting a low resistance, a current of 100 mA was used. Bonds exhibit- ing resistances in Currents in order to limit the Voltage drop across the bond to 2 mV in all cases. In an attempt to isolate resistance increases due to phase and void growth, the resistance data of the annealed samples were analyzed on the basis of the in- crease from the mean of the as-received samples of the same metallization type. This approach was chosen to correct for the different sheet resistances and bonding characteristics of Manuscript received August 11, 1989; revised May 1, 1990. This work was supported by IBM under the Shared University Research Program. L. Maiocco was with the Thayer School of Engineering, Dartmouth col- lege, Hanover, NH. She is now with the Philips Elmet Corporation, Lewiston, ME 04240. D. Smyers was with the Thayer School of Engineering, Dartmouth College, Hanover, NH. She is now with the Department of Health, Physical Education and Recreation, University of Oklahoma, Norman, OK 73069. Baker are with the ThaYer School of Engineering, Dartmouth College, Hanover, NH 03755. Of l5 were tested with p. R. Munroe and IEEE Log Number 9037160. 0148-641 1/90/0900-0592$01 .00 O 1990 IEEE