1. 58th IEEE Holm Conference on Electrical Contacts
Quantitative evolution of electrical contact resistance between aluminum thin films
David Mercier1,*, Vincent Mandrillon1, Anthony Holtz1,
Fabien Volpi2, Marc Verdier2, Yves Bréchet2
1CEA, LETI, MINATEC Campus, 17 rue des Martyrs, F Grenoble, France
2 Université de Grenoble, Lab. SIMaP-CNRS, BP 75, St Martin d’Hères, F Cedex, France *
58th IEEE Holm Conference on Electrical Contacts
23-26 September, 2012
Portland, Oregon, USA

2. Electrical connection
Context
3D Chip stacking [1]
Flip-chip process
Wire bonding
20µm
200µm
Electrical connection
Al connection pad
Chip
Preload
Load
Microinsertion
technology
Temperature (°C)
Load/insert (mN)
Miro-insert array
Glue
Silicone substrate
Time (min)
30 mN/micro-insert
Plateau at 120°C = glue reticulation
[1] Boutry H. et al., « Reliability characterization of Ni-based microinsert interconnections for flip chip die on wafer attachment and their evaluation in multichip simcard prototype. », in: Proc. 10th IEEE EPTC, Singapore, 2008, 1334–1339.

3. Crossed rods experiments [2]
Issue
Chip
Substrate
1µm
Ni Microinsert Al
Al2O3
Electrical
current
3-D silicon chip stacked package using flip chip micro-insert technology
 2 main goals :
Electrical connection
Mechanical maintain F V i
Mechanisms of the electrical contact establishment between Al thin film and Ni microinsert ?
Role of the native Al2O3 layer ?
Goal of this work
How ?
Crossed rods experiments [2]
(Hertzian contact)
[2] Holm R., “Electric contacts: theory and applications.”, Springer, 4th Ed., 1999.

4. Outline Samples description & Experimental details
1st result and scenario
Electrical contact at low loads
Transient electrical contact
Electrical contact at high loads

5. Samples description SiO2 Ti 11nm Al 942nm Native Al2O3 3-4nm R V F
Cross-section of the silica (BK7) lens with deposited metallic thin films by PVD
(Radii (R) of 3 and 6mm) V F
Contact zone i
Voltmeter
1cm
Young’s modulus (E) (GPa)1
Hardness (H) (GPa)1
Poisson’s coefficient (n)
Electrical resistivity
(µW.cm)2
BK7
81 ± 1
7.5 ± 0.5
0.21 [3] -
Al thin film
68 ± 1
0.8 ± 0.1
0.34 [4]
4.6 Al
Al2O3
Glue
AFM observation of
the Al thin film surface
(RMS roughness : 1.80nm)
TEM cross section of Al film with native Al2O3
1Measured by instrumented nanoindentation
2Measured by 4 probes aligned method
[3] T. Chudoba, N. Schwarzer, F. Richter and U. Beck, “ Determination of mechanical film properties of a bilayer system due to elastic indentation measurements with a spherical indenter.” Thin Solid Films, 2000, 377,
[4] A. C. Fischer-Cripps, “Nanoindentation – 2nd Edition.” (Springer, 2004).

6. Load in function of time
Experimental details V F
Contact zone i
Voltmeter
Mechanical parameters :
Maximal load : 50N
Loading rate : 0.1, 0.2 and 1 N/s
Load resolution : 50mN
Electrical parameters :
4 wires measurement
Constant applied voltage (±5mV)
Compliance current : 500mA
Data acquisition frequency : 10Hz
Contact detection
Load in function of time V
Load (F)
Spring 1
(Stiffness K1)
Spring 2
(Stiffness K2)
Guidance system
4 wires measurement
Lenses with same radii R

7. Evolution of Electrical Contact Resistance
Structure contact 1 e- Al
Al2O3
Hypothesis : Electrical contact is formed through cracks into native alumina [5] 1
Tunnel effect
Loading 2 e- 2
Transient metallic contact 3 e-
Adhesion effect or plastic ductile extension [6] ? 3
Permanent ohmic contact
Unloading
3 different main stages observed during the electrical contact formation, revealing 3 different regimes for the oxide rupture.
Different behavior between loading and unloading.
[5] R. S. Timsit, “Electrical contact resistance: fundamental principles”, in Electrical Contacts: Principle and Applications (ed. By P. G. Slade - Marcel Dekker, pp , 1999).
[6] J. Pethica et D. Tabor, “Contact of characterised metal surfaces at very low loads: Deformation and adhesion,” Surface Science, vol. 89, n°. 1, p , 1979.

8. Electrical contact at low loads
Tunnel Effect [7]
Electrical contact at low loads (until 5N) matches to tunnel effect.
[7] [J. G. Simmons, “Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film.” Journal of Applied Physics, 1963, 34(6),

9. Transient electrical contact (1/3)
Ballistic conduction mode [8]
Quasi-ballistic to diffusive conduction mode ?
Tunnel effect
One contact spot
Ballistic conduction mode during the transient electrical contact.
[8] Y. V. Sharvin, Zh. Exp. Teor. Fiz., 1965, 48.

10. Transient electrical contact (2/3)
 Influence of lenses radii
Loading rate : 0.2N/s
Fracture strain
Contact stabilization strain
Hertz theory [9]
No major influence of lenses radii on ECR evolution.
Calculation of the fracture strain of alumina.
 With E and u of silica (lens)
[9] K. L. Johnson, “Contact mechanics.” (Cambridge University Press, 1987).

11. Transient electrical contact (3/3)
 Influence of loading rate
Lenses radii : 6mm
Major influence of loading rate.
Al creep or Al through cracks oxidation ?
Hertz theory
Loading rate
0.1N/s
0.2N/s
1N/s
Frupt
erupt (r=ae)
Exp.
10.7 N
2.5 N
1.2 N
Same order of magnitude
with values found in
literature
Litt.
erupt (r=0) [10]
N. B. : loading rate not given
erupt [11]
[10] R. S. Timsit, “Some fundamental properties of aluminum-aluminum electrical contacts.” Ieee Transactions on Components Hybrids and Manufacturing Technology, 1980, 3(1),
[11] J. C. Grosskreutz, “Mechanical properties of metal oxide films.” Journal of the Electrochemical Society, 1969, 116(9),

12. Electrical contact at high loads
Loading rate : 0.2N/s
Lenses radii : 6mm
Very low change of the ECR value between 25 and 50N.
Final value controlled by geometric effects (spreading, crowding…).

13. Electrical contact at high loads
Estimation of the spreading resistance by FEM
Schematic lens plane view
Probing wire
location
Contact zone i V F
Voltmeter
k1 and k2 are constants
for b/l >1.

14. Electrical contact at high loads
Rcontact
A real contact resistance of Rcontact≈13mW is extracted at high loads from our results.
Corresponding to an equivalent circular constriction with a diameter of 1.8µm (only 1% of elastic contact).

15. Conclusion and Outlook
3 ≠ steps observed during the evolution of ECR in function of load :
Low load : Tunneling effect (MOhm),
Transient : Metallic contacts formation through cracks in Al2O3 with a ballistic conduction mode,
High load : Ohmic contact (<100 mOhm), controlled by geometry and resistivity of the thin film.
Mechanical characterization of native alumina 
Extraction of real electrical contact resistance corrected with FEM simulations.
Outlook
Crossed rods compression between different lenses (thin film of Ni and Al).
Intermetallic compounds formation.
Study of the effect of roughness.
Measurement in temperature ( 200°C).
Thanks for your attention !
Questions ?