Inductively coupled plasma (ICP) sources are widely used in the semiconductor industry for plasma proces­sing. They can easily produce high density plasma and provide good plasma control. For photoresist strip (dry clean) processes, direct plasma interaction with a wafer is undesirable and plasma is used as an intermediate for modification of a gas composition and creating chemi­cal­ly active radicals for processing the wafers. As strip rate directly relates to the flux of radicals to the surface of the wafer this process usually uses high gas flows (5-20slm), high RF power (3-5kW) and high gas pressure (~1000mTorr). Obviously, when the gas and energy consumption is that high, the efficiency of the source becomes very important, since it affects both capital and oper­­a­tional costs, and with newer and more restrictive regulations, efficiency may become determining characteristic of the source.

There are a few factors that strongly affect source efficiency, as well as the process, the most important of which are the gas flow and electron heating efficiency. If only small fraction of gas molecules participate in collisions with hot electrons then most of the gas simply flows through the system without dissociation. In this case most of the gas is simply wasted. So it is important to make the gas flow in such a way that most (if not all) gas molecules have a high probability of collision with hot electrons. To increase efficiency of generating hot electrons one has to improve electron confine­ment in the region where the induced electric field is high.

We accomplished both goals by modifying Mattson Technology Inc.’s production-proven ICP strip source. By modifying the gas injection system into the plasma generating volume and optimizing the vessel, we simultaneously achieved significantly increased photoresist removal rate at reduced input of process gas and a much larger operational window for the source, with concomitant process benefits.

Our implementation is scalable for processing 450mm wafer sizes and the underlying concepts are generalizable for optimizing other types of ICP sources, as will be discussed in the presentation.