Indium-Copper Multilayer Composite Solder for Fluxless Bonding
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ornic Percentage Indium
Fig. 1. In-Cu equilibrium phase diagram 3 . There are several stable compounds and phases which are Cu 4In (46 wt. % In, 3 phase), y (43.6 wt. % In), Cu 9In 4 (32 wt. % In), Cu 2In (S phase), and ý phase (55.66-56.68 wt. % In).
4 89 wt.% In I 11 wt.% Cu t
-
4-
Chromium (Cr) Indium (In) Copper (Cu)
0.03 um 5.0 lm / 0.5 gm 70 wt.% In 30 wt.% Cu
S
----
Gold (Au)
SCopper (Cu) S-Chromium (Cr)
0.1 gm 1.2 gim 0.03 lam
( Figure not to scale )
Fig 2. In-Cu multilayer composite for fluxless bonding.
226
Cu 9In 4 (32 wt.% In), Cu 2In (. phase), and ý phase (55.66-56.68 wt. % In). For indium composition of 56.7 to 99.3 wt. %, the alloy is a mixture of ý phase and indium with a solidus temperature of 153°C. However, studies found that copper reacts with indium at room temperature to form CuIn intermetallic6,7 which is not shown in the phase diagram. Fig. 2 displays In-Cu multilayer composite. Chromium, indium, and copper layers are deposited on GaAs dice in one high vacuum cycle to inhibit indium oxidation. Chromium improves the adhesion to the dice. On the glass substrate, chromium and copper layers are deposited with a thin gold coating to prevent copper from tarnish. After the copper and indium layers are deposited on GaAs, CuIn intermetallic will form almost immediately as shown in Fig. 3(a) because of high diffusion rate of Cu in In6. The diffusion coefficient D at 373K is about 1.7 x 10-g m 2 /s. This outer CuIn layer is expected to protect the inner indium from oxidation.
Cr
(a)
In Culn Compound
4--
*--
As Deposited
Die 0c Cr (b)
'i"': : 2;•7
--i
It
t i
_' t r
Liquid In
Culn
Cu Cr
S•
tX
+
Au
-
At 200°C
4--
(c)
----
S4--
Cr In
+
Culn
grains
Cr
Solidification at Room Temperature Fig. 3. Bonding principle description of the In-Cu multilayer composite. (a) Right after deposition, the In-Cu composite converts into In-CuIn which prevents indium from oxidation. (b) Melting of indium and solid-liquid interdiffusion. (C) Solidification at room temperature.
227
Fig. 3 exhibits the reaction in the bonding process. GaAs die and glass substrate are held together and heated in hydrogen atmosphere above the indium melting point of 157°C. The indium layer in the In-CuIn composite melts to break up the CuIn intermetallic layer, forming a mixture of solid CuIn grains and liquid. Since the mixture now has contact with the Au and Cu layers on the glass substrate, it would wet and dissolve the Au and Cu layers through liquid-solid interdiffusion to produce more Culn. After cooling down to room temperature, a In-Cu joint is formed. The gold on the copper is too thin to have a significant effect in the bonding process. In this process, we eliminate the use of flux due to the prevention of indium oxidation. FABRICATION PROCESS Fig. 2 shows the In-Cu multilayer design used in this study. In fabrication, chromium, indium, and copper are deposited on the polished side of a GaAs wafer in high vacuum with the
thickness of 300 A, 5.0 rtm, 0.5 .tm, respectively. After deposition,
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