Electroplating of Gold-Tin Solder for Optoelectronic Applications
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Mat. Res. Soc. Symp. Proc. Vol. 515 ©1998 Materials Research Society
EXPERIMENT Bath Stability The plating bath in this work consisted of KAuC14 , SnC12 , ammonium citrate, stabilizers and a leveler. Stabilizer-I was chosen to stabilize gold ions and stabilizer-2, a weak acid, was used to minimize the hydrolysis of bivalent tin ions in the bath. The solutions listed in Table 1 were prepared to test and compare the bath stability. Since AuCI 4 - ions are hydrolyzed to some extent in aqueous solution, citrate was dissolved first in deionized water during the preparation of solutions. Other chemicals were then added in the following sequence: gold salt, stabilizers (if used), tin salt, and leveler (if used). # S1
S2 S3 S4 S5 S6 S7
Ammonium citrate (g/l) 200 400 200 200 200 200 200
Table 1. Solutions utilized for bath stabilitv tests. SnC12.2H20 Stabilizer-2 KAuCI 4 Stabilizer-I
( 5 5 5 5 5 7 10
(/1)
(9/1)
-----
-----
--60 60 60 60
15 --15 15 15
(9/1) 5 5 5 5 5 7 10
Bath stability (days) 0 0 0 4 >15 9 7
Electroplating of Au-Sn alloy (1) Direct current (DC) plating and pulse current (PC) plating The following experiments were designed to compare DC and PC plating. DC plating was done with Solution S8, which contained 1 g/1l of leveler in Solution S5. Solutions S8, S9 and S10 had the same formula except for the content of KAuCI4, which was 5 g/l, 7 g/l and 10 g/l, respectively. The substrates were InP wafers, which were coated with a Ti(25nm)/Au(250nm) metallization. A specific exposed plating area of about lcmxlcm was defined by a stop-off lacquer. The current density (average current density in PC) was varied from 1.6 to 3.6 mA/cm 2 . PC plating was carried out at 2 ms of ON time and 8 ms of OFF time. The total plating time for each sample was about 1 hr. Deposit composition, thickness and microstructure were examined by scanning electron microscopy (SEM), using a Hitachi S-2700 with a Link eXL energy dispersive x-ray (EDX) system. (2) Effect of ON time on PC plating Solution S8 was used to perform PC plating at different ON times from 0.2 to 5 ms. The substrates were InP wafers with the same Ti/Au metallization. The cycle period was maintained at 10 ms. The average current density was 2.4 mA/cm 2, and the plating time was 1 hr. Deposit composition was determined by SEM and surface roughness was checked by atomic force microscopy (AFM). RESULTS AND DISCUSSION Bath Stability The results for bath stability tests are shown in Table 1. Solution S1, S2 and S3 became turbid when the tin salt was added. The concentration of ammonium citrate had no influence on solution stability for these solutions. The instability of the solutions may be due to the strong oxidizing ability of gold ions and chemical reactions between gold ions and tin ions. Stabilizer-2 154
was unable to prevent the reactions. Gradual gold precipitation was observed four days after preparation of S4, which contained stabilizer-1. Solution S5 contained both stabilizer-I and stabilizer-2 and remained stable for at least 15 days after preparation
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