Time: Tuesday and Thursday 2:00 - 3:30 pm

Place: ECJ 1.214, University of Texas at Austin

Instructor: Rui Huang, WRW 117D, (512) 471-7558, ruihuang@mail.utexas.edu

Lecture notes (coming soon)

Homewrok sets (coming soon)

Brief Outline of Topics

• Introduction to thin film processes: deposition methods, growth modes, microstructures

• Stress in thin films: growth stress, epitaxial stress, Stoney formula, wafer curvature

• Fracture and delamination: channeling cracks, interface debonding
  

• Film buckling: buckle-delamination, buckling without delamination (wrinkling), pressurized bulging

• Plasticity and dislocations: strength and hardening, dislocation formation and interactions

• Creep and mass transport: grain-boundary diffusion, interface diffusion, deformation mechanisms, stress-induced voiding, electromigration

• Morphological evolution: chemical potential, surface roughening, self-assembly of quantum dots

  
  

Auxiliary notes

• Mechanical Properties of Thin Films, William D. Nix, Stanford University
• Fracture Mechanics of Thin Films and Multilayers, John W. Hutchinson, Harvard University
• Fracture Mechanics of Thin Films and Composite Materials, J. W. Hutchinson and Z. Suo

Other references

• Thin Film Materials: Stress, Defect Formation and Surface Evolution, by L.B. Freund and S. Suresh. Cambridge University Press, 2003.
• Materials Science of Thin Films (2nd ed.), by M. Ohring. Academic Press, 2002.
• W.D. Nix, Mechanical properties of thin films. Metall. Trans. 20A, 2217-2245 (1989).
• J. W. Hutchinson and Z. Suo, Mixed-Mode Cracking in Layered Materials. Advances in Applied Mechanics 29, 63-191 (1991).
• L.B. Freund, The mechanics of dislocations in strained-layer semiconductor materials. Advances in Applied Mechanics 30, 1-66 (1994).
• Z. Suo, Reliability of interconnect structures.

Remarks

• Prerequisite: a graduate course on solid mechanics.
• No textbook is required. Notes will be posted periodically.

Stress-induced voiding (SIV) is investigated in Cu-based, deep-submicron, dual damascene technology. Two failure modes are revealed by TEM failure analysis. For one mode, voids are formed under the via when the via connects a wide metal lead below it. For the via which is instead under a wide metal line, voids are formed right above the via bottom. The void source results from the supersaturated vacancies which develop when Cu is not properly annealed after electroplating and before being constrained by dielectrics. The driving force comes from the stress built up due to grain growth and the thermal expansion mismatch (CTE) between Cu interconnect and dielectrics. A diffusion model is introduced to investigate the voiding mechanism primarily for the vias connected to wide metal leads.

References:

1. E. T. Ogawa, J. W. MePherson, J. A. Rosal, K. J. Dickerson, T. C. Chiu, L. Y. Tsung, M. K. Jain, T. D. Bonifield, J. C. Ondrusek, and W. R. McKee, “Stress-Induced Voiding Under Vias Connected To Wide Cu Metal Leads”, 2002 IEEE International Reliability Physics Symposium Proceedings, 2002, pp. 312-321.

2. M. Kawano, T. Fukase, Y. Yamamoto, T. Ito, S. Yokogawa, H. Tsuda, Y. Kunimune, T. Saitoh, K. Ueno, and M. Sekine, “Stress Relaxation in Dual-damascene Cu Interconnects to Suppress Stress-Induced Voiding”, Proceedings of the 2003 International Interconnect Technology Conference, 2003.

3. J. M. Paik, J. K. Jung, and Y. C. Joo, “The Dielectric Material Dependence of Stress And Stress Relaxation on The Mechanics of Stress-Voiding of Cu Interconnects”, 2005 IEEE International Reliability Physics Symposium Proceeding, pp. 195-202.

Dislocations are common in epitaxial systems. For a thin film epitaxially grown on a substrate with coherent interface, it may have spontaneously-formed dislocations when its thickness is larger than certain value, i.e. critical thickness. The presence of dislocations can have an adverse effect on electrical performance of semiconductor materials, providing easy diffusion paths for dopants to lead to short circuits, or recombination centers to reduce carrier density. And, formation of dislocations is one of the most observed mechanisms of relaxation of mismatch strain. However, in optoelectric applications, strain alters the electronic bandgap and band edge alignment, and should be maintained. So, controlling formation of dislocations is very important in the manufacture of microelectronic and optoelectronic devices.

This term paper will review some basic concepts and try to produce some understanding about the control dislocation formation.

References:

1. Freund, L.B. (2000) The mechanics of electronic materials, International Journal of Solids and Structures 37, 185-196.

2. Freund, L.B. and Suresh, S. (2003) Thin film materials: stress, defect formation and surface evolution, Cambridge University Press, Cambridge.

Today low-k dielectric materials are integrated into computer chips to improve the operation speed and reduce the cross-talk noise. Due to weak mechanical properties of low-k dielectric materials, cohesive failure is subjected to occur. Channel cracking is one common mode of cohesive failure. In this term paper, several potential issues relevant to channel cracking of low-k dielectric thin films are reviewed. These issues include the well known substrate constrain effect; the concentration of crack driving force due to patterned structure, and the degrading of fracture toughness as scaling down the dielectric constant of the films. Some design rules of applying the low-k dielectric thin films are also discussed in this report.

References:
1. J.L. Beuth Jr., “Cracking of thin bonded films in residual tension”, International Journal of
Solids and Structures, 29, 1657-75 (1992)
2. Ting Tsui et al.,“Constraint Effects on Cohesive Failures in Low-k Dielectric Thin
Films”, Proceedings of the Materials Research Society 2005 spring meeting.
3. X. H. Liu, “Channel cracking in low-k films on patterned multi-layers”, Proceedings of the
IEEE 2004 International Interconnect Technology Conference, page 93-95.

For films or coatings deposited on substrate at high temperature, residual compressive stresses are often induced in the surface layers because of the mismatch in the thermal expansion coefficients. Under such compressive residual stresses, the surface thin film is susceptible to buckling-driven delamination. Various shapes of buckled region are observed, including long straight-sided blisters, circular and the ‘telephone cord’ blister.

Many studies have been done on systems with rigid substrates. Recent studies have shown that substrate compliance has an importance influence on both the film buckling stress and the energy release rate of the interface delamination crack when the substrate is very compliant compared with the film. This report will focus on the effects of substrate compliance on a straight –sided delamination buckle.

References:

1. Yu, H.H. and J.W. Hutchinson(2002). Influence of substrate compliance on buckling delaminaton of thin films. International Journal of Fracture 113, 39-55.
2. Cotterell, B. and Chen, Z. (2000). Buckling and crackling of thin films on compliant substrates under compression. International Journal of Fracture 104, 169-179.

Each student completes a term paper of selected topics that (a) addresses a phenomenon in thin film materials, and (b) involves analyses using mechanics. The project contributes 25% of the grade, distributed as follows:

• 5%: November 30 (Thursday). Post your title and abstract in iMechanica, formated as below

1. Title (EM 397 Term Paper: e.g., Dislocations in Epitaxial Thin Films).
2. Tags (EM 397, Fall 2006, University of Texas at Austin, thin films, term paper)
3. Body: (i) Describe the phenomenon. (ii) Explain how mechanics is relevant. (iii) Cite at least 1 journal article.

• 10%: December 12 Tuesday (2:00-4:00 pm). 30 minute presentation. Use power point slides.
• 10%: December 18 Monday. Upload a pdf file of your final term paper as an attachment to the same blog entry as your abstract.