205 the exhaust port 211 is generally adapted to a vacuum source and configured to pump processing gases from the inner volume 212 the substrates being processed may be disposed in a substrate boat in a vertically stacked manner and are generally rotated during the process to be acquire uniform exposure to heat and processing gases a detailed description of the processing chamber 205 may be found in a co pending u s patent application ser no 11 249 555 filed on oct 13 2005 entitled u201c reaction chamber with opposing pockets for injection and exhaust u201d which is incorporated herein as reference a heater 202 is disposed outside the processing chamber 205 and configured to heat the processing chamber 205 during process the heater 202 may be a resistive heater in one embodiment the heater 202 may have substantially the same shape as the processing chamber 205 to provide a uniform heating effect around a circumference of the processing chamber 205 the heater 202 comprises multiple independently controlled zones to achieve desired heating profile for example a uniform heating along vertical level in one embodiment the heater 202 may comprise multiple independently controlled zones 202 i where i 1 2 n that are vertically stacked together as shown in fig4 during a batch processing substrates positioned near the top and the bottom of the substrate boat are usually less heated than the substrates positioned near the center of the substrate boat resulting in different processing effects among a batch of substrates the vertically stacked configuration is particularly useful in reducing or eliminating uneven heating at different.