Acoustic signals from laser-annealed amorphous silicon

Abstract

Hydrogen loss from laserannealed amorphous hydrogenated carbon films studied by secondaryion mass spectrometry Appl.

Photoacoustic study of KrF laser heating of Si: Implications for laser particle removal

“…In region III, a simultaneous decrease of P comp and a sharp increase of P rare appearing as the strongly asymmetric wave forms ͓Fig. 1͑b͒, curve 3͔ seems to result, as shown elsewhere, [10][11]13 from Si surface melting and consequent Si contraction, m Ϸ10%, at fluences of 0.5-0.75 J/cm 2 for the 248 nm wavelength and a laser pulse width of about 20 ns. 16 The interface vibrational velocity during the acceleration stage, which is approximately proportional to P comp , remains nearly constant over the range of 0.5-1.4 J/cm 2 , while that at the deceleration stage is significantly enhanced by contraction of the material during melting.…”

“…The bipolar shape of the Si surface vibrational velocity response at low fluences, F loc Ͻ0.5 J/cm 2 , is apparent, as is the additional small minimum of z vib ͑t͒ and maximum of a vib ͑t͒, resulting from the significant concentration-deformation contribution in this fluence range. This behavior differs from the unipolar vib ͑t͒ and the corresponding step-like z vib ͑t͒ and bipolar a vib ͑t͒ profiles obtained from photodeflection studies measured on the same time scale with a visible laser pump pulse, 3 but are very typical for the Si vibrational velocity response with expansion/contraction and contraction/ expansion cycles in UV and IR spectral ranges, [9][10][11][12] respectively, illustrating the need for introduction of the concentration-deformation contraction effect into the onedimensional thermal expansion model previously used to describe DLC of Si. 3,4 Considering the poor DLC efficiency in region I, the relatively low vibrational response of Si in this fluence range does not provide, apparently, optimal condi- tions for cleaning applications, but further PA experiments with higher temporal resolution during the pump laser pulse would be desirable.…”

“…In region I, there is a quasilinear growth of P comp and P rare with increasing F loc consistent with literature data for both IR and ultraviolet ͑UV͒ nanosecond laser heating experiments. [9][10][11][12][13] The relatively low vibrational response in this region results predominantly from the nonlinear dependence of thermal and deformation stresses on temperature and EHP density, resulting in high L dep values at moderate temperatures ͑due to the thermal conductivity of Si scaling as ϳT Ϫ1 ) and at moderate EHP densities ͑due to longer lifetimes and diffusion times for EHP at lower plasma den-sities͒ and thus effectively decreasing these temperature-and plasma-dependent weighted stresses ͑see the numerical estimates of these parameters below͒. The contributions of thermoacoustic and concentration-deformation mechanisms seem to be comparable, producing the asymmetrical bipolar wave form of PA transients with lower expansion velocities and higher contraction velocities ͓cf.…”

“…Within the operating time window of the acoustic transducer ͑the first 300 ns͒ these transients exhibit prominent bipolar wave forms where the first, compression ͑positive͒ pulse corresponds to expansion of a laser-excited near-surface region of the Si wafer and the second, rarefaction ͑negative͒ pulse corresponds to contraction in the region resulting from cooling at the end of the laser pulse. 8 The bipolar profile of the PA wave forms recorded is characteristic for the thermoacoustic regime of acoustic generation at a free air/Si boundary, 8 but its asymmetry over the whole fluence range shows pronounced influence of other acoustic generation mechanisms known for Si, i.e., the nonthermal concentration-deformation mechanism ͑contraction of Si due to a laser-induced EHP͒, [8][9][10][11][12] melting ͓contraction m ϭ(⌬V/V 0 ) m ͔, [10][11]13 and laser ablation at higher fluences when Si damage occurs. 15 Broadening of the wave forms by 3-4 times and diminution of their amplitudes occur because of dissipative losses of ϳ20 dB in the 3 mm protective brass disk ͑specific acoustic absorption coefficient of brass at a frequency of about 10 MHz equal to ␣ ac / f Ϸ7 ϫ10 Ϫ6 Hz Ϫ1 m Ϫ1 ) 14 and reflections at all interfaces in the acoustic delay line.…”

“…7,8 In the particular case of DLC, PD techniques have been used to study the vibrational response of a Si surface upon visible laser irradiation, exhibiting the unipolar temporal vibrational velocity profile characteristic of thermal expansion. 3 Nevertheless, depending on the experimental conditions of laser-matter interaction ͑laser intensity, wavelength, and pulse width, material properties such as optical absorption, ambipolar diffusion, and the thermal diffu-sivity͒, more complicated expansion/contraction cycles of a laser-excited semiconductor surface layer, e.g., Si contraction preceding its expansion at 1064 nm laser wavelength excitation due to electron-hole plasma ͑EHP͒ excitation, [9][10][11][12] may be observed as bipolar or multipolar vibrational velocity transients, [9][10][11][12][13] all of which can strongly affect the DLC efficiency and can make particle removal dynamics much more complicated. 4 Although a general analysis of the vibrational surface response for laser-heated materials ͑including semi-conductors͒ is available, 8 its application to the optimization of experimental conditions of DLC has not been reported, especially for DLC with excimer lasers, which are widely used for this application.…”