Adhesion Strength of CuCr Alloy Films to Polyimide
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using EDX(energy dispersive X-ray analysis), and the Cr content of the alloy layer chosen for the present analysis were respectively, 0, 2.5, 8.5, 17, 25, 34, and 100 atomic%. For the peel strength measurements, the CuCr alloy layer (50 nm thick) was thickened by depositing 55 Mat. Res. Soc. Symp. Proc. Vol. 390 ©1995 Materials Research Society
Cu by sputtering (950 nm) and electroplating (20 itm)to make the total thickness of the metal layer to be 21 jim . The thickness of the polyimide substrate was 50 Pin. The T-peel tests were conducted by holding the metal and polyimide layers separately, and extending them at the grip displacement rate of 2 mm/min.. For the durability measurements, metal/polyimide films were held in T/H environment up to 840 hours before the peel test. Fractographic studies of peeled metal surfaces were conducted using SEM and AES. Operating conditions of AES were; incident beam energy =3 keV, beam current=50-100 nA, energy of Ar ion beam =2 keV, and beam current density of Ar sputtering250 jiA/cm2. RESULTS AND DISCUSSIONS Peel strength of metal/nolvimide films In Fig. 1, the peel strength of metal/polyimide films without T/H treatments are shown as a function of the Cr content in the CuCr alloy layer (x, in atomic %). The peel strength was 1-2 g/mm for pure Cu (x=0), but increased proportionally with x up to x= 17. It is interesting to note that the peel strength saturated around 55 g/mm for x> 17. Since Cr atoms are known to react with C atoms of polyimide to form carbide-like bonds, which is primarily responsible for the enhancement of the adhesion strength, it appears that the amount of carbide-like Cr-C bonds saturates around x= 17. Other possibilities includes the formation of Cr-O bonds which is also known to enhance the adhesion strength, and the ultimate phase transition from f.c.c to b.c.c. structure with the increase of Cr. The stiffness increase of the metal layer with Cr in the CuCr alloy layer lowers the peel strength which tends to negate the beneficial effects of the Cr-C and Cr-O bond formation.
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Fig. 1. Peel strength of CuCr/polyimide films as a function of the Cr content in alloy layer(x).
Effects of 850 C/85%R.H. treatments In Fig. 2, variations of the peel strength with the T/H treatment time are presented for several Cr contents. It can be seen that applications of the T/H treatment lowered the peel strength at all Cr levels. The peel strength was lowest for the specimen with x=8.5. It dropped from 20 g/mm before the treatment to almost nil after 284 hours. Specimens with x> 17 showed substantially higher peel strength than the specimen with x=8.5, and among them the specimen with x= 17 was the most susceptible to the degradation of the adhesion strength caused by T/H treatments.
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Fig. 2. Peel strength as a function of the hold time at T/H condition for specimens with varying Cr content.
Fig. 3. SEM micrographs out of peeled metal surfa
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