Au-Al引线键合的可靠性(英文)

Evolution of IMCs in Au/Al & Al/Au
2.Experiment results(Au/Al)
Fig.3 interface evolution of Au/Al joints under 200 ℃[1]
Evolution of IMCs in Au/Al & Al/Au
Background
Wire Bonding(WB) takes more than 90% in chip interconnecting. And about 25% failure occurs in WB joints，especially in Au-Al joints. Au-Al system: Au/Al、Al/Au、Au-Al diffusion couples.
The reliability of Au-Al system in wire bonding
Content
1.Background 2.Principle 3.Evolution of IMCs in Au/Al & Al/Au 4. The IMC formation in Au-Al diffusion couples
Forming a much thicker purple phase with the former disappearing
4.The IMC formation in Au-Al diffusion couples
2.Experiment results
2.3 Diffusion kinetics
Etch rate：Au2Al is much faster than Au5Al2， AuAl2（purple）、 Au4Al（matte yellow）、 AuAl （white or grey） are unaffected
4.The IMC formation in Au-Al diffusion couples
1.Experiment conditions
Materials：Pure Au and Al wire、95Al-5Si wire Temperature：200-460℃ Atmosphere：5%H2-95%N2 Etchant：commercial cyanide solution added by acetic acid
(2)Chromium followed by silver metallization with gold wire bond (3)Aluminum metallization with aluminum wire bond
Disadvantages：relatively low current carrying capacity compared to Au wire
4.The IMC formation in Au-Al diffusion couples
2.Experiment results
2.4 Strength of couples
Diffusion rate difference Kirkendall voids CTE difference microcracks
2.Experiment results(Al/Au)
Fig.5 interface evolution of Au/Al joints under 200 ℃ Each line from left to right:240/480/720(h) [1]
4.The IMC formation in Au-Al diffusion couples
[6]Ji H, Li M, Wang C, et al. Comparison of interface evolution of ultrasonic aluminum and gold wire wedge bonds during thermal aging[J]. Materials Science and Engineering: A, 2007, 447(1): 118.
Fig.9 Total intermetallic layer thickness vs. the square root of time at six temperatures.[5]
Fig.10Arrhenius plot for the rate constant, k. [5]
[5] Philofsky E. Purple plague revisited[C]//Reliability Physics Symposium, 1970. 8th Annual. IEEE, 1970: 184.
WB
Fig.1 sketch map of wire bonding
Principle
Why especially in Au-Al joints ? IMCs are expected to be formed in this system.
brittle、bad conductivity
2.Experiment results
2.1 Observations
（1）AuAl2 is observed；above 400℃ small voids appear （2）AuAl only appears after long time （3）Au2Al nucleates slowly but grows at second fastest rate （4）Au5Al2 is the predominant phase； voids appear at Au rich side above 300℃ （diffusion rate） and cracks nucleate above 400℃ （5） Au4Al nucleates late and grows slowly
Au5Al2 Au
95Al-5Si
AuSiAl4
Au2Al
These protuberances coalesce
Al Si
Fig.7Reproduction of an electron microprobe trace across Au-(Al-5%Si) diffusion couple aged at 400℃ for 100 minutes.[3] [3] Philofsky E. Purple plague revisited[C]//Reliability Physics Symposium, 1970. 8th Annual. IEEE, 1970: 183.
Decreasing strength
Kirkendall voids generated severely at the interface of gold side and IMCs because of bigger diffusion speed of elemental gold; the cracks mainly resulted from the overgrowth of the AuAl2 IMC.[6]
4.The IMC formation in Au-Al diffusion couples
2.Experiment results
2.2 Effect of Silicon
A thin purple layer (AuAl2)
Au
Another purple phase in protuberances (AuSiAl4)
Fig.2 Au-Al compounds diagram
Principle
Different IMC shows different properties.
Tab.1 properties of Au-Al system
Evolution of IMCs in Au/Al & Al/Au
1.Experiment conditions
4.The IMC formation in Au-Al diffusion couples
2.Experiment results
2.3 Diffusion kinetics
k = 5.2 x 10 -4 exp(-15900/RT)
Once Au or Al is depleted, the equation is invalid.
Fig.6 Au-Al diffusion couple aged at 460℃ for 100 minutes，polished and etched.[2]
[2] Philofsky E. Purple plague revisited[C]//Reliability Physics Symposium, 1970. 8th Annual. IEEE, 1970: 182.
4.The IMC formation in Au-Al diffusion couples
3.Methods to eliminate purple plague
(1)Aluminum followed by chromium metallization with gold wire bond
Disadvantages：Cr-Al IMC formation，black plague forms around Au wire Disadvantages：Si-Cr IMC formation；Ag migration
Assuming volume diffusion predominates: x=k t 1/2
Why the growth of AuAl2 phase is anomalous?
Fig.8 Layer thickness of the five intermetallic phases vs. the square root of time at 400℃.[4] [4] Philofsky E. Purple plague revisited[C]//Reliability Physics Symposium, 1970. 8th Annual. IEEE, 1970: 183.
(4)Gold metallization with gold wire bond
Disadvantages：residual oxide on the silicon.[7]
[] Selikson B, Longo T A. A study of purple plague and its role in integrated circuits[J]. Proceedings of the IEEE, 1964, 52(12): 1638-1641.
2.Experiment results(Au/Al)
Fig.4 interface evolution of Au/Al joints under 200 ℃ From top to bottom:240/480/720/960(h) [1]
Evolution of IMCs in Au/Al & Al/Au