MICHELLE supports both secondary electron emission involving both true secondary electrons (SSE) and back-scattered primary electrons (BSE). The release of SSE are derived from electronic excitation states resulting from collisions incurred with the passage of primary electrons into a material. BSE result from the rediffusion of the incident electrons back toward the entered surface through collision processes within a substrate material.
Configuring secondary emission is done either in the simulation properties dialog box using the MICHELLE Solver/Solver Setup/Basic/Additional Physics/Secondary Emission Defaults/Distribution By Generation parameter or the using the Distribution By Generation parameter of a Secondary Emitter excitation attribute. The former is used to specify the default secondary emission for all Secondary Emitter excitations while the latter can be used to override the defaults for a specific Secondary Emitter excitation.
Secondary emission is controlled via a set of triplets for both SSE and BSE. The purpose of using a series or "schedule" of secondary distributions is to gradually increase the total number of particles in the simulation as a function of cycle number, in order to more efficiently converge to a final solution. For optimal convergence, this schedule should increase the particle count vs. cycle number, and decrease the particle count vs. generation number.
In each triplet, the first entry corresponds to the number of energy bins in the distribution. The second entry is the number of launch planes in the distribution, and the third entry is the number of angles. "Angles" here is defined as polar angles. "Launch planes" are equally spaced segments of a hemisphere sliced in the direction normal to the plane of the polar angles.
Entries can be constant scalar values, or variables. Entries cannot be expressions.
The product of the three numbers gives the total number of secondaries per incident particle. The triplets are arranged in a table with each column referring to a distribution as a function of secondary generation, where each row refers to a particular generation. Each distribution (column) is applied over a range of cycles as defined by the first row, Cycle to Begin. The code will switch to the next column in the table at these cycle numbers.
The number of secondary generations is implied by the number of rows in the previously mentioned table. This can be overridden using the simulation parameter Solver Setup/Additional Physics/Secondary Emission Defaults/Number of Secondary Generations.
A recommended distribution by generation for 3 generations of secondaries (assuming 30 cycles) is shown is shown in the following figure.