Furnaces: The oxidation is conducted in high temperature furnaces. The furnaces may be either vertical or horizontal tube furnaces. In one batch many wafers (~100) will be processed. The duration of the process, the supply of gas and the heating are precisely controlled using computer. In horizontal furnaces, contamination can be an issue and vertical tube furnaces were introduced to minimize this issue.  At present, wafers of 300 mm diameter are commonly used in the industry. In the vertical tube furnaces, the wafers are loaded from the top, in cassettes (batch of 10 or 13 wafers). In horizontal furnaces, the movement of the cassette against the furnace wall will generate particles, and this is avoided in the vertical furnaces.  Also, since the gas moves vertically, there is no separation of gas due to density difference. In horizontal tube furnace, the gas will flow in the horizontal direction, which leads to lighter gases moving up. This results in uneven distribution of gases and hence lack of uniformity in the oxide grown. In vertical furnaces, better uniformity can be achieved.