Laser patterning of amorphous silicon thin films deposited on flexible and rigid substrates

Abstract

The possibility of direct writing thin semiconductive channels and structures on insulating substrates in a clean room-free process is attractive for its simplicity, cost effectiveness, and possibility of a wide choice of substrates. A broad range of applications, such as large-area electronic devices (touch screens, flexible displays), sensors, or optical wave guides could benefit from such a process. In this work, we directly write on doped hydrogenated amorphous silicon (a-Si:H), with thickness in the range 10 nm-1 mm, using a Nd-YAG laser operating at 532 nm that is part of a Witec Raman confocal system. The contrast in conductivity between the exposed and unexposed areas is so high that the a-Si:H matrix needs not to be removed after exposure. B-and P-doped films were deposited on plastic, glass, and oxidized silicon wafers. The laser power threshold for crystallization was studied. The highest conductivity (886 V À1 cm À1) was obtained on wafer. On hard substrates, it is possible to tune the mesoscopic electrical conductivity in a very broad range of values ($10 À4 -10 3 ) by design of the pattern to be transferred. Patterned films are piezoresistive with gauge factors as high as þ18 and À29 for p-and n-type patterns, respectively. SEM image of laser written lines on a 10 nm thick a-Si:H film deposited on a Si/SiO 2 substrate. Four regions are clearly distinguishable: the metal contact on the top area; the laser eroded area (lines); the crystallized areas adjacent to lines; the amorphous region at the bottom right.

Analysis of nascent silicon phase-change gratings induced by femtosecond laser irradiation in vacuum

“…The formation of phase-change grating modifies the reflectivity of the silicon surface also in absence of significant topographic features resulting in an external halo characterized by reflectivity fringes departing from several localized defects as well as from the crater edge (see Optical and 2D confocal measurements in the Supplementary Information). A comparison of 2D confocal micrographs with the corresponding SEM and Raman images indicates that a -Si corresponds to the brighter areas in the confocal image and to the darker ones in the SEM image, in agreement with an increase of the a -Si reflectivity and a reduction of its electrical conductivity with respect to the pristine c -Si 25 , 26 .…”

“…In fact, the formation of a thin a -Si layer is typically accompanied by a reflectivity increase at visible illumination wavelength 17 , 25 . Moreover, amorphous and crystalline phases of silicon laser processed surfaces present a significant difference in electrical conductivity 26 . To verify that the modulation observed in the SEM and optical images is due to localized phase changes resulting in an alternate disposition of amorphous ( a -Si) and crystalline Si ( c -Si) stripes, we carried out Raman spectroscopy and imaging of the irradiated sample surface.…”

Fabrication of amorphous micro-ring arrays in crystalline silicon using ultrashort laser pulses

“…Laser interference patterning of a-Si to form parallel crystalline lines and dots with sub-wavelength separation was reported in 14 . Recently, direct writing of crystalline lines in hydrogenated amorphous Si films was reported, which can be considered as semiconductive channels in an insulating matrix, due to the enormous difference in electrical conductivity of both phases 15 . The inverse process, direct laser writing of amorphous structures in crystalline Si by high repetition rate femtosecond laser irradiation was reported by Kiani et al 16,17 .…”

Kinetics of the laser-induced solid phase crystallization of amorphous silicon—Time-resolved Raman spectroscopy and computer simulations

“…On the contrary, for the laser focus and powers applied in this study, AFM or SEM microscopy cannot reveal any changes related to the laser irradiation [9], indicating that the laserinduced temperatures are below the melting point. Note that clear marks (SEM detectable) are left on the sample surface when the Si melting temperature (1414°C) is reached [11,12]. The bright area corresponds to a decreased hydrogen content, the inner darker zone marks the crystallized volume.…”