Effect of laser intensity on the properties of carbon plasmas and deposited films

Abstract

The effect of laser intensity on the deposition of diamondlike carbon ͑DLC͒ films has been studied using an ArF ͑193 nm͒ pulsed excimer laser. Our results are found to be distinct from other studies using Nd-YAG infrared or excimer 248-nm lasers. Two issues concerning the growth mechanism of the films are discussed: ͑1͒ the dynamics of the laser-induced plasma and ͑2͒ the dependence of the nature of the deposited films on laser intensity. To address the first issue, time-integrated optical emission spectroscopy has been carried out to investigate the carbon plasma induced by the ArF ͑193 nm͒ laser. Instead of molecular carbon bands ͑C 2 ͒, monoatomic neutral ͑C I ͒ and ionic ͑C II ͒ emission lines are found to dominate the spectra. The emissions of ͑C I ͒ and ͑C II ͒ have been studied as a function of laser intensity. For low laser intensity, the laser irradiation removes the target surface material primarily through thermal evaporation. When the laser intensity is above a threshold of ͑3.7-4͒ϫ10 8 W/cm 2 , the evaporated species are also ionized. The observed phenomenon can be attributed to higher multiphoton ionization and inverse bremsstrahlung rate as the laser intensity is increased. For the second issue, films deposited at various intensities have been characterized by ellipsometry. Results show that films deposited at low intensity are found to have excellent optical transparency ͑E g ϭ2.3 eV͒, which implies a considerable amount of sp 3 bonds. However, films deposited at higher intensities are found to be more graphitic. The damage threshold has also been located at 3.7ϫ10 8 W/cm 2 . A qualitative structural analysis based on the effective-medium approximation has been performed on the deposited films to investigate the influence of laser intensity on their microstructures. ͓S0163-1829͑97͒01407-0͔

Particle formation from pulsed laser irradiation of soot aggregates studied with a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope

“…The main products are C, C 2 , and C 3 and their cationic counterparts. [43][44][45][46][47][48][49][50][51][52] Small clusters initially produced are thought to form larger clusters via termolecular reactions, [53][54][55][56] and at higher pressures larger clusters have been observed. 57 The fine structure of the graphite substrate also appears to have an impact on the size of clusters produced.…”

Dynamics of confined plumes during short and ultrashort pulsed laser ablation of graphite

“…3,4 In vacuum, PLA of graphite is typically used to produce smooth, hard, high density amorphous carbon ͑a-C͒ films with a high sp 3 bonded C content ͑up to 90%͒. 5,6 The high sp 3 content has been attributed to local densification, caused by the impact of high energy C atoms/ions. This subplantation model, has been shown to provide a suitable representation of the growth mechanism for a-C films grown by a variety of techniques.…”

Metal incorporation into nanoporous carbon

“…Nanosecond pulsed laser ablation (PLA) of graphite results in the deposition of quite different carbonaceous structures depending on the pressure of ambient gas in the chamber [2,3]. PLA of graphite in vacuum is typically used to produce smooth, hard, high-density amorphous carbon (a-C) films with high sp 3 bonded C content (up to 90%) [4,5]. The introduction of a low pressure of inert gas into the growth chamber reduces the kinetic energy of the growth species due to the increased collision frequency.…”