The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Controlling Iron and Copper Precipitation in Silicon Wafers

Hele Savin

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Large Seminar Hall of Micronova at Helsinki University of Technology (Espoo, Finland) on the 21st of October, 2005, at 12 noon.

Overview in PDF format (ISBN 951-22-7873-1)   [1508 KB]
Dissertation is also available in print (ISBN 951-22-7872-3)


This thesis studies the two most common transition metals in silicon – copper and iron. The purpose of the experiments and theoretical calculations presented in this thesis is to increase the current knowledge of the precipitation behavior of these metals under different processing conditions. The study also includes the development of recombination lifetime methods for impurity characterization in silicon.

A method to measure trace copper contamination in silicon using the microwave photoconductivity decay is proposed. The method is based on the observation that copper precipitates can be created using light activation. It is shown that external charge on wafer surfaces can reduce the copper out-diffusion, which extends the applicability of the method. Moreover, oxide precipitates are found to increase the sensitivity of the method.

An analytical solution to the current continuity equation for excess carriers in an epitaxial structure under time dependent optical excitation is derived. In addition, some analytical approximations are developed and numerical calculations are made to check their accuracy. The built-in potential between the epitaxial layer and the substrate is taken into account and also the light induced barrier lowering is included in the model. The barrier lowering is found to be noticeable and can affect the effective lifetime by up to two orders of magnitude.

Several annealing profiles and contamination levels are used to study the iron gettering behavior at varying supersaturation levels. It was found that the gettering efficiency depends strongly on the initial iron concentration and considerably high supersaturation is needed to initiate the gettering. The results are discussed from the perspective of thermodynamics.

The thermal stability of internally gettered iron is studied experimentally by deep level transient spectroscopy. The experiments reveal that the dissolution is a reaction limited process. The simulations of iron re-dissolution show that instead of simulating only the final cooling, it is important to simulate the whole thermal cycle. This is important since low thermal budgets are becoming more and more common in the IC technology.

This thesis consists of an overview and of the following 7 publications:

  1. H. Väinölä, M. Yli-Koski, A. Haarahiltunen, and J. Sinkkonen, Sensitive copper detection in p-type CZ-silicon using μPCD, Journal of The Electrochemical Society, Vol. 150, G790-G794 (2003). © 2003 The Electrochemical Society. By permission.
  2. H. Väinölä, E. Saarnilehto, M. Yli-Koski, A. Haarahiltunen, J. Sinkkonen, G. Berenyi, and T. Pavelka, Quantitative copper measurement in oxidized p-type silicon wafers using microwave photoconductivity decay, Applied Physics Letters, Vol. 87, 032109 (2005). © 2005 American Institute of Physics. By permission.
  3. H. Väinölä, J. Storgårds, M. Yli-Koski, and J. Sinkkonen, Evaluation of effective carrier lifetime in epitaxial silicon layers, Solid State Phenomena, Vols. 82-84, 771-776 (2002).
  4. H. Väinölä, J. Storgårds, M. Yli-Koski, and J. Sinkkonen, Light induced change on the built-in potential of p/p+ structures and its effect on carrier lifetime measurements, Materials Science and Engineering B, Vols. 91-92, 421-424 (2002).
  5. H. Väinölä, A. Haarahiltunen, E. Saarnilehto, M. Yli-Koski, J. Sinkkonen, and O. Anttila, Enhancement of internal gettering efficiency of iron by low temperature nucleation, The Electrochemical Society Proceedings, Vol. 05, High Purity Silicon VIII, Editors: C. L. Claeys, M. Watanabe, R. Falster, P. Stallhofer, p. 160-164 (2004). © 2004 The Electrochemical Society. By permission.
  6. P. Zhang, H. Väinölä, A. Istratov, and E. R. Weber, Thermal stability of internal gettering of iron in silicon and its impact on optimization of gettering, Applied Physics Letters, Vol. 83, 4324-4326 (2003). © 2003 American Institute of Physics. By permission.
  7. H. Väinölä, P. Zhang, A. Haarahiltunen, A. Istratov, and E. R. Weber, Simulations of iron re-dissolution from oxygen precipitates in CZ-silicon and its impact on gettering efficiency, Solid State Phenomena, Vols. 95-96, 581-586 (2004).

Errata of publication 7

Keywords: photoconductivity decay, recombination lifetime, epitaxial, internal gettering, oxide precipitates

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© 2005 Helsinki University of Technology

Last update 2011-05-26