In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

We develop a theory for laser-induced periodic surface structure by associating each Fourier component of induced structure with the corresponding Fourier component of inhomogeneous energy deposition just beneath the surface. We assume that surface roughness, confined to a region of height much less than the wavelength of light, is responsible for the symmetry breaking leading to this inhomogeneous deposition; we find strong peaks in this deposition in Fourier space, which leads to predictions of induced fringe patterns with spacing and orientation dependent on the angle of incidence and polarization of the damaging beam. The nature of the generated electromagnetic field structures and their relation to the simple "surface-scattered wave" model for periodic surface damage are discussed. Our calculation, which is for arbitrary angle of incidence and polarization, applies a new approach to the electrodynamics of randomly rough surfaces, introducing a variational principle to deal with the longitudinal fields responsible for local field, or "depolarization," corrections. For a $p$-polarized damaging beam our results depend on shape and filling factors of the surface roughness, but for $s$-polarized light they are essentially independent of these generally unknown parameters; thus an unambiguous comparison of our theory with experiment is possible.

Laser-induced periodic surface structure. I. Theory

Laser induced periodic surface structure can be understood as a universal phenomenon which occurs when high intensity pulses are absorbed near the surface of solids or liquids. The phenomenon occurs on metals, semiconductors and insulators because of the interference between the incident pulse and an induced “radiation remnant”. This scattered field may be enhanced by the existence of true surface modes such as surface plasmons or phonon-polaritons but this is not essential. The universality characteristics include beam polarization, since we show that circularly polarized light can induce surface ripples, with the damage structure showing a dependence on the sense of rotation. We also present time resolved results of the formation of the ripples to illustrate the essential dynamical processes that occur.

Laser Induced Periodic Surface Structure

The transition to single crystal of ion‐implanted amorphous Si and Ge layers is described in terms of a liquid‐phase epitaxy occurring during pulsing‐laser irradiation. A standard heat equations including laser light absorption was solved numerically to give the time evolution of temperature and melting as a function of the pulse energy density and its duration. The structure dependence of the absorption coefficient and the temperature dependence of the thermal conductivity were accounted for in the calculations. In this model the transition to single crystal occurs above a well‐defined threshold energy density at which the liquid layer wets the underlying single‐crystal substrate. Experiments were performed in ion‐implanted amorphous layers of thicknesses ranging between 500 and 9000 A. The energy densities of the Q‐switched ruby laser ranged between 0.2 and 3.5 J/cm2; time durations of 20 and 50 ns were used. The experimental data are in good agreement with the calculated values for the amorphous thickness–energy−density threshold. The model deals mainly with plausibility arguments and does not account for processes occuring in the near‐threshold region or below the melting temperature.

A melting model for pulsing‐laser annealing of implanted semiconductors

Spontaneous, highly periodic, often permanent surface gratings or "ripples" can develop on the surface of almost any solid or liquid material illuminated by a single laser beam of sufficient intensity, under either pulsed or CW conditions. The grating periods are such that the incident laser beam is diffracted into a tangential wave which skims just along or under the illuminated surface. These spontaneously appearing surface ripples are generated by a runaway growth process analogous to stimulated Brillouin or Raman scattering or small-scale self focusing, but having many of the same properties as Wood's anomalies in diffraction gratings. Hence, it seems appropriate to refer to these spontaneous surface structures as "stimulated Wood's anomalies."

Stimulated Wood's anomalies on laser-illuminated surfaces

We have discovered that interference effects extant during pulsed laser irradiation annealing of ion‐implanted silicon produce periodic property variations in the annealed material that mimic the interference pattern. These are manifest at near‐annealing threshold power densities as surface ripple and at higher power densities may be revealed by etching. The surface ripple observed at low power densities is correlated with the occurrence of polycrystalline silicon regions in the annealed material. Our observations suggest that surface melting and epitaxial regrowth are responsible for the annealing effect.

Periodic regrowth phenomena produced by laser annealing of ion‐implanted silicon

Arsenic‐ and antimony‐implanted silicon wafers have been annealed by a cw Ar ion laser. Glancing‐angle Rutherford backscattering and transmission electron and optical microscopy measurements indicate that the mechanism for recrystallization is one of thermal solid‐phase regrowth from the underlying crystalline‐amorphous interface. No implant redistribution is observed.

Solid-phase epitaxy of implanted silicon by cw Ar ion laser irradiation

UHV‐deposited amorphous silicon was recrystallized on (100) single‐crystal substrates using pulsed Nd : YAG laser irradiation Prior to deposition, substrates were prepared with either atomically clean surfaces, residual argon ion sputter damage, or from one to three monolayers of residual oxide Epitaxial regrowth of bulk crystalline quality occurred regardless of substrate preparation Annealing energy densities were significantly higher, however, for substrates retaining residual impurities Certain layers were doped with Ga At moderate doping levels recrystallization behavior was unaffected At a 2% doping level, the annealing energy threshold dropped by one‐half and Ga atoms were displaced toward the epitaxial surface It is shown that the decrease in energy threshold is due to an impurity‐induced increase in light absorption within the amorphous layer Layers were crystallized with substitutional Ga concentrations of up to 10 times the equilibrium solid solubility limit

Substrate and doping effects upon laser‐induced epitaxy of amorphous silicon

Solid phase epitaxial regrowth of ion‐implanted Si was obtained with cw infrared radiation of CO2 (10.6 μm) and Nd : YAG lasers. Arsenic‐implanted samples were analyzed by optical and electron transmission microscopy and by Rutherford backscattering with channeling. Dislocation‐free monocrystalline structures were obtained on substrates which were preheated to 200–400 °C and then scanned with a laser beam of ∼25 W. There was no redistribution of the implanted As and the sample surface was free of any features which might indicate melting. A channeling angular scan showed that the As atoms were located exactly on the lattice sites. However, the relatively high values of sheet resistance suggest the presence of residual point defects.

cw infrared laser annealing of ion‐implanted silicon

Rutherford backscattering and channeling, TEM, SEM (Scanning Electron Microscopy), and optical microscopy have been employed to characterize annealing of ion‐implanted silicon with overlapping spots of pulsed Nd:YAG (yttrium aluminum gamet) laser light. The arsenic implant dose, substrate orientation, laser power, and laser pulse length have been varied to provide a comprehensive picture of significant annealing trends. We have confirmed the earlier observations that melt regime annealing can produce essentially perfect recrystallization of implanted amorphous silicon, regardless of prior implant or substrate conditions. Our study has also confirmed differences between the melting threshold of amorphous and crystalline silicon. We observe substitutional As concentrations in the bulk that far exceed quoted maximum equilibrium values, but substitutional solubility variations and segregation effects in the near surface suggest that ’’surface’’ solubility limit may differ from bulk values. Additional data ind...

G. A. Rozgonyi

Papers

Periodic regrowth phenomena produced by laser annealing of ion‐implanted silicon

Solid-phase epitaxy of implanted silicon by cw Ar ion laser irradiation

Substrate and doping effects upon laser‐induced epitaxy of amorphous silicon

cw infrared laser annealing of ion‐implanted silicon

Characterization of pulsed Nd:YAG laser‐annealed, arsenic‐ion‐implanted silicon