Uniformity control for low flow process and chamber to chamber matching

A gas box for distributing gases into a processing chamber, comprising:

• a gas distribution plate having a plurality of apertures disposed therethrough;
• and
• a high pressure drop blocker plate having a first plurality of apertures and a second plurality of apertures disposed therethrough and opening into a space between the high pressure drop blocker plate and the gas distribution plate;
• wherein a first gas pathway is configured to deliver a first gas sequentially through the first plurality of apertures in the blocker plate and the plurality of apertures in the gas distribution plate, wherein the first plurality of apertures in the blocker plate is selected to create a first pressure drop of at least approximately 0.8 torr for a gas flow rate of less than 2000 sccm therethrough,
• wherein a second gas pathway is configured to deliver a second gas around the blocker plate, through the second plurality of apertures in the blocker plate, and through the plurality of apertures in the gas distribution plate, wherein the first and second gas pathways pass through the space between the high pressure drop blocker plate and the gas distribution plate,
• and wherein the first plurality of apertures in the blocker plate are configured such that the first pressure drop thereacross is greater than a second pressure drop across the second plurality of apertures in the blocker plate for a given flow rate.