Laser Wavelength Dependent Properties of BN Thin Films Deposited by Laser Ablation
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Table I . Deposition conditions for the BN films. Laser Wavelength Pulse width Energy density Repetition rate
Pulsed Nd:YAG laser A =532 nm r =6.5 ns Ed=3.8 J/cm f=10 Hz
Target Rotating speed of the target Substrate Target-substrate distance Gas pressure Substrate temperatures RF bias power (13.56 MHz) Self-bias voltage Deposition time
KrF excimer laser A =248 nm r =25 ns 2
hBN (purity 99.5%) - 20 rpm Si (100) d=6.0 cm PN2+PAr=20.0 Pa Ts=650 0C PRF=0 - 100W Vs=0 - -200V 30 minutes
generated basically due to the higher mobility of electrons than ions in a megahertz grow discharge. After 18,000 laser pulses, the deposition process was completed. Table I shows the deposition conditions for the preparation of BN films. BN films were characterized by Scanning Electron Microscopy (SEM), Auger Electron Spectroscopy (AES), and Fourier Transform IR measurement (FT-IR). The optical emissions from the plasma plume generated by the pulsed laser irradiation were detected by an optical multichannel analyzer (OMA) with a 1024 photodiode array.
RESULTS Figure 1(a) and 1(b) show SEM pictures of typical BN films deposited with a Nd:YAG laser (532 nm) and a KrF excimer laser (248 nm), respectively. The density and size of particulates are much higher for the BN film prepared with a Nd:YAG laser (532 nm) [Fig.l(a)] as compared to the film prepared with a KrF excimer laser (248 nm) [Fig.l(b)] even though these films have been deposited using the same number of laser pulses and energy density.
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10K[
(a)
10 gim
(b)
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Figure 1. SEM pictures of BN films on Si(100) deposited with (a) a Nd:YAG laser (532 nm) and (b) a KrF excimer laser (248 nm). Other deposition conditions were nitrogen gas pressure; PN2=20 Pa, energy density; Ed=3.8 J/cm , target-substrate distance; d=6.0 cm, substrate temperature; Ts-650 "J
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These films have been investigated by AES and the results are shown in Fig. 2(a) and Fig.3(a), respectively. There are B and N peaks present in the spectrum, as well as smaller peaks of C and 0. The N/B composition ratio of the film deposited with a 532 nm laser at PN2=20 Pa, PAr=0 Pa was 0.86 [Fig.2(a)] and that of the film with a 248 nm laser at PN2=20 Pa, PAr=0 Pa was 0.81 [Fig.3(a)]. The film composition ratio (N/B=0.81 - 0.86) was 8 "- 13 % smaller than that (N/B=0.93) of the virgin BN target used here. 1 4 These films were boron rich like the films (N/B - 0.70) deposited on heated (600 °C ) Si(100) substrate with KrF excimer laser (248 nm) at PN2=4 [13 Pa. ' This nitrogen deficiency problem may be linked to the target depletion problem. 14 Ion-assisted treatments are required to fabricate stoichiometric and cubic structure BN films. ' 1 0 - 1 2 The N/B composition ratio of the film deposited with a 532 nm laser at PN2=0 Pa, PAr=20 Pa was 0.09 [Fig.2(b)] and that of the film with a 248 nm laser at PN2=0 Pa, PAr=20 Pa was 0.18 [Fig.3(b)]. These films showed an absence of nitrogen and were composed almost entirely of metallic boron. Nd:YAG laser Ed=3.8J/cm
(A 2=532nm)
KrF excimer laser ( R Ed=3o8J/c
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