Rajiv R. Shah

Laser heating of semiconductors—effect of carrier diffusion in nonlinear dynamic heat transport process

An analytic theory is presented to describe the nonlinear dynamic heat transport process in a semiconductor irradiated by a pulsed laser beam. The input rate of laser energy to the lattice is sensitively influenced by the ambipolar diffusion of the dense, laser‐produced excess charge carriers. Additionally, the high heating rate of the laser beam significantly changes the material transport coefficients during the pulse. This nonlinear laser beam‐solid interaction is examined from the viewpoint of the transport process, using a parametrized perturbation technique in Green′s function formulation. An explicit, analytical solution of the lattice temperature rise is presented and the threshold pulse energy for the onset of surface melting for the case of amorphous Si is calculated as a function of laser beam intensity as well as the carrier diffusion length. Our results are compared with both the melting and nonthermal models for laser annealing.

Erratum: A general analytic technique for nonlinear dynamic transport processes during laser annealing

We present an approximate analytic technique capable of describing the dynamics of nonlinear heat and mass diffusion. The technique is based on using adiabatic approximation in the Green’s‐function formulation and is illustrated for the case of heat transport during pulsed and cw laser annealing of Si. In particular, the heating efficiency is discussed explicitly as a function of pulse intensity for a fixed pulse energy.

Pulsed Laser Recrystallization of Polysilicon: Analysis Via A Novel Sem Technique

Large crystals of silicon were obtained via pulsed laser annealing of thinfilms of fine grain CVD polycrystalline silicon. These films were analyzed using a novel technique that provides rapid feedback of crystallographic and defect information. The technique uses very shallow angle metallurgical sections in conjunction with chemical decoration and scanning electron microscopy and results in excellent depth resolution ( 100 μm). The technique was used to reveal conversion of very large areas (≈ 1 mm 2 ) of polysilicon films deposited on silicon into single crystal film, albeit with point defects, without requiring the melt depth to reach the polysilicon/silicon interface. A gradual transition from single crystal to increasingly polycrystalline material was observed going from top to bottom of the initially uniform polycrystalline film depending on the pulse energy used. Polysilicon on top of an oxide layer, on the other hand, transforms into large (≈ 10 μm) single crystals with grain boundaries penetrating the entire polysilicon film thickness. These experimental results shed new light on two very important questions: (1) Is laser annealing entirely an epitaxial process or is it strongly influenced by the thermal properties of the underlying substrate, and (2) Whether substantial regrowth of polysilicon requires equilibrium thermodynamics.

Effects of Pulsed Laser Irradiation on Thermal Oxides of Silicon

The electrical effects associated with pulsed laser irradiation of thermal oxides of silicon were investigated. Three of the functions performed by thermal oxides are control of field effects, junction passivation and electrical isolation. Representative oxide thicknesses suitable for these applications were studied: ≈ 100 nm; ≈ 500 nm and ≥, 1.0 μm respectively. The oxides were irradiated by a repetitively Q-switched Nd 3+ :YAG laser operated at the fundamental frequency. The key laser parameters of energy and power density were varied over a wide range, from no change to the onset of discernible change and irreversible damage. The range of energy densities over which the oxide was not electrically degraded was established for each thickness of the oxide. This range of energy densities is compared to the useful range of energy densities for annealing polysilicon and for activating an implant through an oxide. The electrical data was obtained using a mercury probe and through the analysis of a large number of MOS capacitors formed on the irradiated oxides. Parameters measured include: breakdown voltages, surface state charge densities and flat band voltages.

Laser Applications To Semiconductor Device Processing

Laser applications to the thermal processing of integrated circuits may be divided into two broad categories: one, applications to the improvement of existing process steps and, two, applications to processes that will allow fabrication of novel device structures. This paper will briefly review these applications, discuss potential problems in the use of lasers for device processing and possible solutions to these problems.

Characterization of Thermal Oxides of Laser Annealed Polysilicon

We report an investigation of the effects of laser processing on the thermal oxides of polysilicon. LPCVD polysilicon, 500 nm thick, deposited on 500 nm thermal oxide of single crystal silicon was laser processed at various stages in the process sequence for device fabrication. Effects of CW Ar + and pulsed 1.06 and 0.53 μm laser processing were investigated. Laser annealed polysilicon was oxidized in a steam ambient. Using a second level of polysilicon, guard ring diode and capacitors were fabricated. Electrical characterization revealed an improvement in breakdown field strengths of these oxides without deleterious effects on any of the associated interfaces.

ADIABATIC APPROACH TO TRANSPORT PROCESSES: A GENERAL ANALYTIC TECHNIQUE FOR NONLINEAR DYNAMIC HEAT AND MASS DIFFUSION

We present a general technique capable of analytically describing the dynamics of nonlinear heat and mass diffusion. The technique is based on using adiabatic approximation in the Greens function formulation and is illustrated for the case of heat transport during pulsed and cw laser annealing of Si.