distribution of holes of two types of gas delivering panel according to this invention first as shown in fig3 a besides the symmetrically distributed holes 202 a around the center of the gas delivering panel 200 a an arc of holes 204 a is formed at one corner sector hence an asymmetrical distribution of holes is created the holes 204 a are positioned according to the actual flow of gas with the reaction chamber for example the holes are placed near areas where the gas flow rate is slow or gas pressure is low consequently those u201c weak u201d areas can be compensated for by the extra gas flow from the extra holes 204 a and hence a more homogeneous distribution of gas is achieved in another embodiment as shown in fig3 b besides the symmetrically distributed hexagonal pattern of holes 202 b around the center of the gas delivering panel 200 b a bank of holes 204 b is formed not far from one side of the polygon hence an asymmetrical distribution of holes is created the holes 204 b are positioned according to the actual flow of gas within the reaction chamber for example the holes are placed near areas where the gas flow rate is slow or gas pressure is low consequently those u201c weak u201d areas.