Photophoretic Control of Plasma Particulate Contamination

Photophoretic Control of Plasma Particulate Contamination

      Disclosed is a method for control and removal of
particulate contamination in plasma processes using the application
of a photophoretic force to drive particles from sensitive regions of
the plasma towards the pump port or other desired region of the
plasma tool.

      Particles present in plasma, such as those used in
microelectronics fabrication, are electrostatically suspended near
the plasma/ sheath boundary [1-3].  Because of suspension, plasma
particles will generally not impact the surface of a wafer or
substrate until the end of the plasma process.  In addition to
electrostatic suspension, plasma particles are attracted towards
regions of the plasma near a topological or material discontinuity,
such as the edge of a Si wafer, or a clamp ring used for holding
the wafer against an electrode [4,6].  It is highly desirable to
remove suspended, trapped particles during the plasma process so that
they will not subsequently contaminate the surface or adversely
affect the discharge quality.

      Using a laser or other high intensity light source, it is
possible to apply photophoretic force [5] upon these particles to
direct the particles out of localized trapping regions and towards a
removal point in the tool, such as the pump port.  The photophoretic
force results from differential heating of particles in the light
field and the transfer of energy to gas molecules impacting the
illuminated side of the particle.  Gas molecules on the illuminated
(heated) side will have a greater kinetic rebound than molecules
impinging on the shadowed side of the particle.  The result is a net
directional force on the particle.  In some cases, negative
photophoresis may be observed.  This occurs for partially transparent
particles in which focussing of the light by the particle results in
greater heating on the...