Thermodynamics and Transport Model of Charge Injection in Silicon Irradiated by a Pulsed Focused Laser

Focused pulsed visible laser used in laser-trimming technologies such as the laser diffused-resistor process may inject charges in the semiconductor, leading to an interaction with highly sensitive circuits. In order to evaluate the process impact on those circuits, a study of the perturbation on the free-running frequency of a ring oscillator due to a nearby laser pulse has been made. The behavior of this modification as a function of the laser-pulse power exhibits complex features. A thermodynamics and electrical model of the charge injection by a focused pulsed laser on silicon has been developed. First, the laser-induced charge diffusion is calculated by a finite-element-model coupling Boltzmann semiclassical transport and thermodynamic equations; the last one being necessary, as relatively high laser power may increase significantly the local temperature. The result is then fed into an electrical model of the ring oscillator as a perturbation injected by a current source. This model coupled to parasitic-light effects due to geometrical changes during silicon melting is able to explain the oscillator's operation-frequency modification.