Jean-Yves Degorce

Three-dimensional crystallization inside photosensitive glasses by focused femtosecond laser

Using scanning electron microscopy, we analyze the laser fluence dependence of three-dimensional crystallized areas induced in bulk photosensitive glass (Foturan) by focused femtosecond laser pulses exposition and subsequent heat treatment. For low fluences (F 2J∕cm2), the crystallization length is higher than predicted from the energy distribution due to the filamentation which occurs above a critical fluence Fcrit=1.4±0.3J∕cm2. The filamentation length is found to follow the square root dependence F−Fcrit.

General model and segregation coefficient measurement for ultrashallow doping by excimer laser annealing

A general model of ultrashallow doping by excimer laser annealing is derived from only one diffusion-segregation equation. In our model, the relative dopant profile after some laser shots reaches a stationary distribution, which only depends on the segregation and liquid-phase diffusion coefficients of the dopant but not on the laser-process parameters. From this result, a one-point method is proposed to experimentally determine the out-of-equilibrium segregation coefficient k. Only the relative dopant concentration at the material surface has to be measured prior to determine the k value. Experimental dopant profiles are compared to simulations generated with experimental k values.

Three-dimensional transient temperature field model for laser annealing

A three-dimensional transient temperature field model (TTFM) is proposed for the general problem of laser-induced out-of-equilibrium annealing of a bilayer device, which is made up of a bulk material covered by a transparent layer. The TTFM solves the moving-boundary problem with a deterministic relation between the interface velocity and temperature in contrast to preceding problem-dependent models, which use an interface-tracking heuristic algorithm. The TTFM is the first step to model many temperature-driven phenomena such as diffusion and segregation in laser annealing. Both computed transient temperature field and melted-zone dimensions of a SiO2∕Si example device, which is irradiated by a focused visible (532nm) laser, are in very good agreement with experimental measurements.

2D Dopant Determination in Laser-Diffused Si Resistors Using Dopant-Selective Etching

Two-dimensional (2D) dopant profiles, in the range of 9 X 10 1 6 to 3.6 X 10 1 8 atoms/cm 3 , in laser-diffused silicon resistors were obtained using dopant selective etching (DSE) in combination with cross-sectional transmission electron microscopy (TEM) and focused ion beam technique. Compared with conventional DSE/TEM dopant evaluation, the properties of this technique, related to the reliability, reproducibility, and accuracy of quantification of dopant concentration from 9 X 10 1 6 to 6 × 10 1 9 atoms/cm 3 , have been improved by considering a vector instead of a scalar etching rate, as determined by an etching model and by a novel calibration method. Those evaluated profiles were accurately compared with a numerical simulation based on heat-transfer and diffusion equations.

Surface modifications during femtosecond laser ablation in vacuum, air, and water

Femtosecond laser ablation technique has been used to process Si and Au targets in vacuum, air and water environment. The threshold of ablation was found to be much lower for Si compared to Au and that was related to much better radiation absorption of Si. The values of the threshold were almost identical for vacuum, air and water in the case of Si (0.4 J/cm2 0.2 J/cm2 in the single and multi-pulse irradiation regime, respectively) and Au (0.9 J/cm2 and 0.3 J/cm2). Craters on the surface of Si and Au were essentially similar for low fluences, suggesting an involvement of the same radiation-related mechanism of material removal, whereas for high fluences significant differences could take place. In particular, quite different crater morphologies were observed during the laser ablation in water, including ones with nanoporous layers for Si and ones with concentric spheres for Au. The differences of morphologies for high laser fluences were explained by the involvement of plasma-related effects under the processing in relatively dense media.

Laser-induced local modification of silicon microdevices: a new technique for tuning analog microelectronics

Highly accurate resistances can be made by iteratively laser inducing local diffusion of dopants from the drain and source of a gateless field effect transistor into the channel, thereby forming an electrical link between two adjacent p-n junction diodes. Using transmission electron microscopy, we showed that the laser induced diffusible resistance can be performed without any structural modification to the microdevices. Current-voltage (I-V) characteristics of these new microdevices are shown to be linear at low voltages and sublinear at higher voltages where carrier mobility is affected by the presence of high fields. A process model involving an approximate calculation of the laser melted region in which the dopant diffusion occurs has been developed. Experimental results are well described by the proposed model.

Modeling the laser-induced diffusible resistance process

Highly accurate resistances can be made by iterative laser-induced local diffusion of dopants from the drain and source of a gateless field effect transistor into its channel, thereby forming an electrical link between two adjacent p-n junction diodes. In this paper we present a complete modeling, which permits to obtain the device characteristics from process parameters. Three-dimensional (3D) temperature calculations are performed from heat diffusion equation using an apparent heat capacity formulation. Melted region determinations are satisfactory compared with in-situ real-time optical measurements of the melted region behavior. Then 3D dopant diffusion profiles are calculated using Fick’s diffusion equation. Finally electronic characteristics are obtained from the new tube multiplexing algorithm for computing the I-V characteristic and the device differential resistance. Numerical simulations using our software are satisfactory compared with experimental I-V measurements.

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 oscillators operation-frequency modification.

ANALYTICAL SOLUTIONS OF A GROWTH MODEL FOR A MELT REGION INDUCED BY A FOCUSED LASER BEAM

We consider processes in which a focused laser beam is used to induce the melting of silicium. The first goal of this paper is to propose a simple three-dimensional (3D) model of this melting process. Our model is partly based on an energy balance equation. This model leads to a nontrivial ODE describing the evolution in time of the dimension of the melt region. The second goal of this paper is to obtain approximate analytical solutions of this ODE. After using basic solution methods, we propose an original geometrical method to derive asymptotic solutions for

Modeling of three-dimensional diffusible resistors with the one-dimensional tube multiplexing method

\textrm{time} \rightarrow \infty