What determines step coverage in sputtering?

1. What determines step coverage in sputtering? Petteri Kilpinen S-69.4114 Postgraduate Course in Electron Physics II 7.5.2010

2. Contents • sputtering (Argon ion bomardment) PVD process • 3 different simulation methods for sputtering process • What good or badstep coverage mean? • step coverage of sputtering process 1 - 11 • Conlusion & why sputtering process is good? • References

3. Sputtering (Argon ion bomardment) PVD process • Most important Physical Vapour Depositon process • Dominat method for thin film deposition of various materials in IC prosessing • Low substrate temperature = ideal method to deposit contact metals for thin transistors • Sputtering is also used to metalize plastics such as potato chip bags. http://en.wikipedia.org/wiki/Sputter_deposition.

4. 3 different simulation methods for sputtering process • Topogaphy or string simulators, 2D (SAMPLE 2D, EVOLVE) • Atomistic Monte Carlo model with ballistic trajectories, 2D or 3D (SIMBAD, SIMSPUD) • Atomistic Monte Carlo simulator with the depositionevent and surface diffusion, 3D (ADEPT) S. Franssila, Introduction to Micro Fabrication. Wiley, 2004.

5. step coverage of sputtering process What good or bad step coverage mean? • Gap filling is possible with conformal step coverage. • Voids and cusps are fromed with poor step coverage. • Goog step coverage in metallization is essential for reliability. S. Franssila, Introduction to Micro Fabrication. Wiley, 2004.

6. 270° 180° 90° Arrival angles of depositing specie at different positions step coverage of sputtering process – 1. Angle of the arriving atoms: • on horizontal free surfaces = 180° • in convex corners = 270° • in the bottom concave corners = 90° ”Visible” area: • The growth rate of the film at each point of the interfaceis determined by the ‘‘visible’’ area of the sputter target • ”Visible” area is determined at a given point on the feature bymarching radially outwards along each sector and checkingfor tangency of the ray emanating from that point. S. Franssila, Introduction to Micro Fabrication. Wiley, 2004. P. L. O’Sullivan Et al, 2000, Journal of Applied Physics, Vol 88, No 7, pp 4062 – 4068.

7. step coverage of sputtering process – 2. Aspect ratio: • Step coverage is usually no major problem for low aspect ratio stuctures (<0.5:1), but at 1:1 and higher-aspect ratios, the step coverage rapidly deteriorates. 1:1 2:1 P. L. O’Sullivan Et al, 2000, Journal of Applied Physics, Vol 88, No 7, pp 4062 – 4068. E. Bär, J. Et al., 2002, Microelectric Engineering, 64, pp 321 – 328.

8. step coverage of sputtering process – 3. 3D geometries : • In real microdevices, there are always structures of various shapes and variable spacing and the film deposition over all these spaces needs to be considered. • a), b) and c) are different dual damascene topographies • VIA figure d) is a baseline as it is a worse case scenario with regard to film coverage of the side walls and bottom of the via. T. Smy, Et al.,2002, Thin Solid Films, 415, pp 32 – 45.

9. step coverage of sputtering process – 4. Wall taper : • The tapering of the contact or via walls has a very considerable effect on the coverage and quality of the film deposited in down the feature walls. • The thickness of the film on the side wall is much more uniform from top to bottom and has increased from 10% to 25% as the wall is altered from vertical to 82.58°. • An obvious drawback of this method of altering the side wall coverage is the increase in effective size of the feature. T. Smy, Et al.,2002, Thin Solid Films, 415, pp 32 – 45.

10. step coverage of sputtering process – 5. Overhangs: • An undesirable feature of direct sputtering is the formation of an overhang at the top corners of vias and trenches. • The presence of an overhang can cause poor side wall and bottom coverage and discontinuous film immediately below the overhang. • Rounding of the corner clearly removes the overhang and results in much better side wall coverage. T. Smy, Et al.,2002, Thin Solid Films, 415, pp 32 – 45.

11. step coverage of sputtering process – 6. Resputtering: • The introduction of a energetic flux by increasing the substrate bias during sputtering cause some degree of film resputtering. • This is effective because (a) it reducing overhangs; and (b) resputtered material is preferentially trapped within topography, especially along side walls. T. Smy, Et al.,2002, Thin Solid Films, 415, pp 32 – 45.

12. step coverage of sputtering process – 7. Flux distribution: • Another source of asymmetry in step coverage is an non-cylindrically symmetric incident flux distribution. • This will occur at the edge of a wafer. • The flux distribution will be shifted off normal resulting in a non-isotropic flux impingement. U. H. Kwon, W. J. Lee, 2006, Japanese Journal of Applied Physics, Vol 45, No 11, pp 8629 – 8638.

13. step coverage of sputtering process – 8. Surface diffusivity: • It determines how much lateral movement the impinging specie is allowed before it is ’frozen’ in the growing film. • As deposition rate decreases at a given nominal deposition temperature, the step coverage increases. • The increase in step coverage with decreasing deposition rates especially at high temperatures is due to higher surface diffusion rates The filmsshown were deposited at 448 K at rates of 20 (Fig. 1a), 220 (Fig. 1b), and 140 (Fig. 1c)Å/s, and at 523 K at rates of20 (Fig. 1d)and 220 (Fig. 1e)Å/s. D. S. Taylor, Et. al., 1998, J. Vac. Sci. Technol. A 16 (5), pp 3123 – 3126.

14. step coverage of sputtering process – 9. Distance between target and wafer: • When increasing the distance between target and wafer a point will be reached where the reduction of solid angle impacts not only the flux in the field regions but also the flux at the inner portions of the feature. • This is the point where the maximum step coverage is achieved. • Further increase of the distance target–wafer will reduce step coverage because of the increasing directionality of the flux which leads to smaller angles of metal atom impingement at the sidewalls than at the top surface. P. L. O’Sullivan Et al, 2000, Journal of Applied Physics, Vol 88, No 7, pp 4062 – 4068.

15. step coverage of sputtering process – 10. Material to be deposited: • The probability of suffering a collision with the background gas varies like exp(2x/l), where x is distance traveled and l is the mean free path. • For instance Cugas-phase scattering has the effect of collimating the arriving flux • Since the atomic mass of Ti is less than that of Cu, this collimation effect is more pronounced at Ti • And for instance Ta has also higher order effects such as reflection and re-sputtering from the substrate. P. L. O’Sullivan Et al, 2000, Journal of Applied Physics, Vol 88, No 7, pp 4062 – 4068.

16. step coverage of sputtering process – 11. Surface topography already made: • Without planarization between sputtering, patterning and sputterning the step coverage will be poor. S. Franssila, Introduction to Micro Fabrication. Wiley, 2004. P.B. Zantye et al. / Materials Science and Engineering R 45 (2004) p. 95

17. Conclusion • Step coverage in sputtering is determined by: • Angle of the arriving atoms & ”Visible” area • Aspect ratio • 3D geometries • Wall taper • Overhangs • Resputtering • Flux distribution • Surface diffusivity • Distance between target and wafer • Material to be deposited • Surface topography already made why sputtering process is good? • an important advantage of sputter deposition is that even the highest melting point materials are easily sputtered while evaporation of these materials in a resistance evaporator is problematic or impossible • deposited films have a composition close to that of the source material • is a conceptually simple technique http://en.wikipedia.org/wiki/Sputter_deposition. http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/backbone/r6_4_1.html

18. References • http://en.wikipedia.org/wiki/Sputter_deposition. • S. Franssila, Introduction to Micro Fabrication. Wiley, 2004. • H. Huang, G.H. Gilmer, T. Díaz de la Rubia, An atomistic simulator for thin film deposition in three dimensions, 1998, Journal of Applid Physics, Vol 84, No 7, pp 3636 – 3649. • D. S. Taylor, M.K. Jain, T.S. Cale, Deposition rate dependence of step coverage of sputter depositedaluminum-.1.5%. copper films, 1998, J. Vac. Sci. Technol. A 16 (5), pp 3123 – 3126. • T. Smy, S.K. Dew, R.V. Joshic, Modeling 3D effects of substrate topography on step coverage and filmmorphology of thin metal films, 2002, Thin Solid Films, 415, pp 32 – 45. • D.G. Coronell, E.W. Egan, G. Hamilton, A. Jain, R. Venkatraman, B. Weitzman, Monte Carlo simulations of sputter deposition and step coverageof thin films, 1998, Thin Solid Films, 333, pp 77 – 81. • U. H. Kwon, W. J. Lee, Multiscale Monte Carlo Simulation of Circular DC Magnetron Sputtering:Influence of Magnetron Design on Target Erosion and Film Deposition, 2006, Japanese Journal of Applied Physics, Vol 45, No 11, pp 8629 – 8638. • P. L. O’Sullivan, F. H. Baumann, G. H. Gilmer, Simulation of physical vapor deposition into trenches and vias:Validation and comparison with experiment, 2000, Journal of Applied Physics, Vol 88, No 7, pp 4062 – 4068. • E. Bär, J. Lorenz, H. Ryssel, Simulation of the influence of via sidewall tapering on stepcoverage of sputter-deposited barrier layers, 2002, Microelectric Engineering, 64, pp 321 – 328. • P.B. Zantye et al. Chemical mechanical planarization for microelectronicsapplications, Materials Science and Engineering R 45 (2004) pp 89–220