# Presentation: 232701

## “Volcano Eruptions” from Electromigration  G. S. Cargill III, Lehigh Univ., and A. M. Maniatty, RPI – MET DMR-0312189

- Pores

- Cathode

**Hillocks**

**Voids**

**Anode End                                        Cathode End**

**Volcano**
**Eruption**

- Electromigration is  atom movement caused by flow of an electrical current, a reliability problem in metal conductor lines of integrated circuits since the 1960's.  Atoms moving  with the electron flow cause voids and swelling (hillocks).

- We have discovered and analyzed another, unusual mode of electromigration damage in copper conductor lines, causing failure by volcano-like eruptions of molten copper in some cases, as shown, and reversible near-failure / recovery phenomena, involving local melting and resolidification in other cases.

**Notes:**
Electromigration is mass transport caused by flow of an electrical current.  Since the 1960's it has been recognized as a reliability problem in metal conductor lines of integrated circuits.   Stresses from thermal cycling of integrated circuits can also affect reliability.  Better understanding of electromigration and thermal stresses in thin, narrow, passivated conductor lines is important for continued technological advances in integrated circuit size reduction, performance enhancement, and reliability improvement. 

The main focus of our research has been application of x-ray microdiffraction to measure strains, and finite element calculations to model strains, induced by electromigration and by thermal cycling in conductor lines.   These studies require selection of suitable currents and temperatures for accelerated testing, for which electromigration will occur during hours and days, rather than months and years.  Resistance measurements are convenient and widely used to monitor electromigration, which increases electrical resistance by thinning, narrowing, or introducing voids in conductor lines.  Using resistance measurements, optical and scanning electron microscopy, and focused ion beam sectioning and imaging, we have discovered an unusual mode of electromigration damage in copper conductor lines, resulting in line failure by volcano-like eruptions of molten copper in some cases, and in reversible near-failure/recovery phenomena involving local melting and resolidification in other cases.  Although related features have been reported for aluminum-based conductor lines, we find that these phenomena also occur in copper conductor lines under accelerated testing conditions.  Further work is needed to determine whether similar phenomena could occur under normal operating conditions for integrated circuits.  

We have carried these studies of electromigration in Cu conductor lines using in-situ resistance measurement, and post-electromigration ex-situ microscopy.   Results for 0.2 micron, 0.8 micron, and 2 micron wide lines have revealed unexpected, nearly reversible, spike-like resistance changes.  The microscopy studies and Joule heating calculations suggest that local melting and resolidification occur from time to time under the temperature and current density conditions of the experiments, and that they are responsible for the observed resistance changes.  These observations and analyses were presented in a contributed paper at the Spring 2006 MRS meeting and have been submitted in longer version for journal publication. 

The optical micrograph a the top of the first slide shows the three conductor lines, contact pads, and attached wires for one of the samples used in this study.  Electrons flow from the cathode end to the anode end, from left to right.  The three scanning electron microscope (SEM) images show the ends of the three conductor lines after electromigration testing, with voids at the cathode ends and hillocks at the anode ends, as expected.  However the third line also has a volcano-like eruption at its cathode end, which is unexpected from usual models of electromigration.  Our experiments and calculations indicate that this eruption results from local melting and extrusion of the copper, when growth of a void causes local increase in electrical resistivity, resulting in local heating to temperatures above the melting point of copper.  When the copper melts, it expands, and in this case it erupted through the top surface insulating layer, like magma erupting through the earth’s outer crust.  The bottom left SEM micrograph shows a side view of the eruption, and the bottom right SEM micrograph shows the porous interior structure of the eruption, obtained by focused ion beam sectioning.  Porosity occurs because the molten copper shrinks as it solidifies from its outer surface following the eruption. 

Publications appearing in 2005-2006 on research supported by this grant:
G. S. Cargill III, L. E. Moyer, G. Wang, H. Zhang, C.-K. Hu, W. Yang, B. C. Larson, and G. E. Ice, “Thermal and Electromigration-Induced Strains in Polycrystalline Films and Conductor Lines: X-ray Microbeam Measurements and Analysis,” 8th International Workshop on Stress-Induced Phenomena in Metallization, AIP Conf. Proceedings 817, 303 (2006).
G. Wang, H. Zhang, G. S. Cargill III, C.-K. Hu, L. Ge, and A. Maniatty, “Thermal and Electromigration-Induced Strains in Copper Conductor Lines: X-ray Microbeam Measurements and Analysis,” MRS Symp. Proc. 914 (2006), to be published..
H. Zhang, G. Wang, and G. S. Cargill III, “Electrical Resistance Anomalies During Electromigration Testing of Cu Conductor Lines: Examples of Local Melting?”  MRS Symp. Proc. 914 (2006), to be published.

Invited talks given in 2005-2006 on research supported by this grant:
G. S. Cargill III, “Thermal and Electromigration-Induced Strains in Polycrystalline Films and Conductor Lines: X-ray Microbeam Measurements and Analysis,” Workshop on Microdiffraction, Advanced Light Source Users Meeting, Berkeley, CA, October 22, 2005 (oral).
G. S. Cargill III, “Thermal and Electromigration-Induced Strains in Polycrystalline Films and Conductor Lines,” Materials Science and Engineering Seminar, Georgia Institute of Technology, January 31, 2006.

## “Volcano Eruptions” from Electromigration G. S. Cargill III, Lehigh Univ., and A. M. Maniatty, RPI – MET DMR-0312189

- Experiments were carried out by Lehigh graduate students Hongqing Zhang and Gan Wang, who presented papers at the Spring MRS meeting.  Laura Moyer completed her PhD and is now employed by Alcoa Inc. During summer and fall 2006, Mr. Zhang has gained industrial research experience as an intern at the IBM Watson Research Center.  Jen Deng continued as a part-time undergraduate work study student.
- During summer 2006, two other undergraduate students, Kylan McQuaig and Tom Nizolek, and a graduate student visitor from Japan, Toshikazu Irisawa, also worked in our research group.

- Dr. Cargill (far right) with Lehigh University students (from left) Hongquing Zhang, Jen Deng, Laura Moyer, and Gan Wang.

**Broader Impacts:**

- Top, Linda Ge and Dr. Maniatty.
- Bottom, Chia-Ju Yang and Nicole Bieber.

- Simulation work has been carried out at Rensselaer by graduate students Chia-Ju Yang and Linda Ge.  Dr. Yang finished his PhD in August 2004 and is currently employed at CCI America, Inc.  Nicole Bieber, a high school student, visited Maniatty’s group in Summer 2005.