We have investiagted the Formation behavior of grown-in defects in silicon crystals. The void defects are formed by the aggregation of the super-saturated vacancies during the growth process and the V/G parameter (V, growth rate: G, axial temperature gradient at the crystal-inch: interface) controls the vacancy concentration. We investigated how the void defects Were influenced by V/G and the cooling rate. The density of void defects (N-v) is proportional to the cooling-rate (C-r) to the power of 1.5 that is, N-v = beta C-r(1.5). This relation holds tip to a cooling rate of 40 K/min. On the other hand, the coefficient beta increases with a decrease in V/G. The relation of N-v = C-r(1.5) does not hold For cooling rates > 40 K/min, and oxygen precipitation increases instead of void formation. This behavior Supports the void formation model of Voronkov. We also discuss this phenomenon in this paper. (C) 2002 Elsevier Science B.V. All rights reserved.