In this chapter we examine long mean free path transport. This topic will involve reconsidering some of the chemistry issues from Chapter 5 and will also introduce material relevant to trench profile evolution, which is handled primarily in Chapter 7. We begin with long mean free path transport and its effect on the chemistry of neutrals in a cylindrical discharge. Ion transport and etching by ions are introduced; this is followed by a discussion of long mean free path transport in a trench that is being etched.

Chemistry at Long Mean Free Path

When the mean free path λ is large compared to the reactor dimension L (which we take to be a cylinder radius a) the particles have collisions every time they travel a distance equal to about 2a, the cylinder diameter. If λ > L, the particles predominantly have collisions with the walls. If they are moving mainly in the z direction they may go further than this, but if they are moving nearly along the surface of the wall with little motion in the z direction they will travel far less.