Vacuum Ultraviolet Absorption Spectroscopy Research Articles

Mechanisms of photosensitivity enhancement in OH-flooded standard germanosilicate preform plates

Ge-doped preform plates have been OH-flooded and subsequently exposed to UV pulses from ArF laser (λp=193nm) under typical conditions of Bragg grating (BG) inscription. Both Fourier Transform Infrared (FTIR) and Vacuum Ultraviolet (VUV) absorption spectroscopy have been carried out after each step of the sensitization and inscription processes. It is shown that the sensitization process not only induces the formation of a large concentration of hydroxyl species but also that of hydride species and GODC defects. The UV exposure leads to strong changes in both the VUV and FTIR absorption spectra of the sensitized plates. These changes are analyzed with a view to getting a better understanding of the OH-flooding-induced sensitization at 193nm.

Development of a low pressure microwave excited plasma and its application to the formation of microcrystalline silicon films

Microwave excited plasma source operating at a low pressure of 1.5Pa was newly developed. This plasma source was successfully applied to the formation of hydrogenated microcrystalline silicon films in a glass substrate with a mixture gas of silane (SiH4), hydrogen (H2), and xenon (Xe). It was found that the crystallinity of films was dramatically improved with decreasing pressure. The crystalline fraction was evaluated to be 82% at a substrate temperature of 400°C, a mixture gas of SiH4∕H2∕Xe: 5∕200∕30SCCM, and a total pressure of 1.5Pa by Raman spectroscopy. The absolute density of hydrogen atoms and the behavior of higher radicals and molecules in the mixture gas were evaluated using vacuum ultraviolet absorption spectroscopy and quadrupole mass spectrometer, respectively. H atom densities were of the order of 1011cm−3. The fraction of H atom density increased, while higher radicals and molecules decreased with decrease in the total pressure. The increase in H atom density and decrease in higher radical...

Nitriding of a tool steel with an electron-beam-excited plasma

Nitriding of a tool steel was carried out with an electron-beam-excited plasma (EBEP). EBEP is sustained with energetic electron beams over the pressure range of 10−3–101Pa by electron-impact ionization. Samples whose temperatures were controlled by electric radiant heater were exposed to EBEP. A nitrided layer of 100μm and a surface hardness of 1000HV(0.1) were achieved for tool steel SKD61 (JIS) at 800K and a treatment time of 3h. In order to measure the density of nitrogen atoms in EBEP, a vacuum ultraviolet absorption spectroscopy system was used. It was found that the density of nitrogen atoms increased from 1011to1012cm−3 linearly with an increase of electron beam current from 2to20A.

Fabrication of Carbon Nanowalls Using Novel Plasma Processing

Carbon nanowalls (CNWs), i.e., two-dimensional carbon nanostructures, were fabricated using fluorocarbon capacitively coupled plasma-enhanced chemical vapor deposition assisted by H radical injection. The correlation between CNW growth and the fabrication conditions was investigated. The morphologies of CNWs were dependent on the types of carbon source gases and the amount of H radicals injected. Moreover, straight and aligned CNWs with regular spacing were fabricated on the substrate set perpendicular to the electrode. In addition, H and CFx (x=1–3) radical densities in the plasma were measured using vacuum ultraviolet absorption spectroscopy and appearance mass spectrometry, respectively, to clarify the growth mechanism of CNWs. The density ratio of H radicals to CFx radicals was found to be an important factor responsible for the formation of CNWs from fluorocarbon/hydrogen systems and increased drastically as a result of H radical injection.

Progress of radical measurements in plasmas for semiconductor processing

In semiconductor processes, reactive plasma is the most important technology for etching, deposition and surface modification of thin films. Radicals have played key roles in plasma processing. In order to realize the high performance of semiconductor processing, important molecular radicals have been measured and their behaviour has been clarified using laser spectroscopic methods such as infrared diode laser absorption spectroscopy, laser induced fluorescence spectroscopy, cavity ring down spectroscopy and recently atomic radicals have also been measured using compact vacuum ultraviolet absorption spectroscopy. Quantitative understanding of kinetics of radicals in plasma will be necessary for nano-scaled semiconductor processing. Their progress is reviewed and the future prospects are presented.

Increased O(D1) metastable density in highly Ar-diluted oxygen plasmas

Enhancement of the growth rate of SiO2 with a rare gas diluted O2 plasma is of interest for application to various microelectronics fabrications. The key is the oxygen metastable atom (D1) density, which has the potential for surface activation. We used vacuum ultraviolet optical absorption spectroscopy to detect O(D1) and found a twofold increase in the density of O(D1) due to the dilution with Ar. The density increase is reasonably explained by the increase of the electron density, the oxygen dissociation fraction, and the Ar metastable density, that are experimentally obtained for low O2 fractions.

VUV and IR absorption spectra induced in H2-loaded and UV hyper-sensitized standard germanosilicate preform plates through exposure to ArF laser light

H2-loaded Ge-doped preform plates have been UV hyper-sensitized and subsequently post-exposed by means of UV ArF laser pulses at 193nm. Both Fourier Transform Infrared (FTIR) and Vacuum Ultraviolet (VUV) absorption spectroscopy has been carried out at each step of the sensitization process with a view to get a better understanding of the UV hyper-sensitization process at 193nm. Exposing the H2-loaded samples firstly triggers a partial bleaching of the 5eV absorption band, followed by a new growth of the band for longer exposure time. As it has already been reported for exposure at 248nm, the exposure at 193nm firstly yields an increase in the absorption ascribed to hydroxyl (≈3600cm−1), hydride species (≈2140–2185cm−1) and GeE′ defects (≈6.3eV). The evolution of the absorption related to the GeH2 species is not monotonous but rather the absorption reaches a peak (Npre=2000) and then slightly decreases for further exposure time. Post-exposing the hyper-sensitized samples (Npre=2000) lead to a partial bleaching of the absorption ascribed to hydride species and to increases in the intensity of the GeE′ defect-related absorption and in the absorption ascribed to hydroxyl species. The accurate determination of the species concentration from the absorption spectra proved to be rather tricky due to the problem of accurately measuring the depth of the UV beam penetration at each time of the exposure. Nevertheless, a correlation could be established between the growth of the UV-induced UV excess loss ascribed to GeE′ species and the UV-induced decrease in the intensity of the IR bands related to hydride species. These observations are discussed within the frame-work of the two step model.

Oxygen atom density in rare gas diluted O 2 radio frequency plasma

The influence of rare gas dilution on absolute oxygen atom density in O 2 radio frequency (RF) capacitively coupled plasma (CCP) was investigated by vacuum ultraviolet absorption spectroscopy (VUVAS) with pulse modulation of main plasma. Helium dilution is effective for raising O atom density at high total pressure (5 Torr) due to the Penning ionization by metastable He. Argon and Krypton dilution keeps O atom density same to pure O 2 case at 0.5 Torr while helium dilution lowers the O atom density by the enhanced diffusion flux of O atoms toward walls. The trend is supported by a diffusion model of O atoms.

Current trends in 157 nm dry lithography

Lithography at 157 nm using F 2 laser is the next step after 193 nm for ULSI fabrication with dimensions below 50 nm. However, there are certain issues to be addressed before the acceptance of the technology by the industry. The most important are: (1) design of resists with low absorption and outgassing at 157 nm, (2) improved line edge roughness and resolution and (3) defect-free resist surface following illumination at 157 nm. In this communication it has been confirmed theoretically and experimentally that outgassing of resists at 157 nm is an intrinsic molecular property and does not depend on the laser parameters. Based on a theoretical model, a new method for measuring outgassing of resists has been developed by measuring the thickness loss, following illumination at 157 nm, using vacuum ultraviolet absorption spectroscopy and atomic force microscopy.

Vertical growth of carbon nanowalls using rf plasma-enhanced chemical vapor deposition

Two-dimensional carbon nanostructures (carbon nanowalls) were fabricated by capacitively coupled, radio-frequency (rf) plasma-enhanced chemical vapor deposition (PECVD) employing fluorocarbon/hydrogen (H) mixtures. Correlation between carbon nanowall growth and fabrication conditions, such as the carbon source gases, was investigated. In addition, H atom density in the plasma was measured using vacuum ultraviolet absorption spectroscopy (VUVAS), in order to discuss the growth mechanism of carbon nanowalls.

Influence of driving frequency on oxygen atom density in O2 radio frequency capacitively coupled plasma

The influence of the driving frequency on the absolute oxygen atom density in an O2 radio frequency (RF) capacitively coupled plasma (CCP) was investigated using vacuum ultraviolet absorption spectroscopy with pulse modulation of the main plasma. A low-power operation of a compact inductively coupled plasma light source was enabled to avoid the significant measurement errors caused by self-absorption in the light source. The pulse modulation of the main plasma enabled accurate absorption measurement for high plasma density conditions by eliminating background signals due to light emission from the main plasma. As for the effects of the driving frequency, the effect of VHF (100 MHz) drive on oxygen atom production was small because of the modest increase in plasma density of electronegative O2 in contrast to the significant increase in electron density previously observed for electropositive Ar. The recombination coefficient of oxygen atoms on the electrode surface was obtained from a decay rate in the afterglow by comparison with a diffusion model, and it showed agreement with previously reported values for several electrode materials.

Prediction of a threshold density of atomic hydrogen for nanocrystalline diamond growth at low pressures

A lower-threshold density of atomic hydrogen (H) for nanocrystalline diamond growth in inductively coupled plasma has been examined. Vacuum-ultraviolet absorption spectroscopy was employed to measure the absolute H density in low-pressure conditions below 10.6 Pa, where nanocrystalline diamond films with grain sizes of ∼20 nm showing a distinct diamond peak in the Raman spectra were deposited. A mechanistic explanation for the growth limitation was made by comparing kinetic rates of incorporation and desorption of adsorbed methyl radicals as principal growth precursors. The results predict a threshold H density for net diamond growth of the order of 10 12 cm −3.

Etching organic low dielectric film in ultrahigh frequency plasma using N2/H2 and N2/NH3 gases

The behavior of species in the gas phase and their effects on the etching characteristics of organic film with the lower dielectric constant (low-k) were investigated in 500 MHz ultrahigh frequency (UHF) plasma using N2/H2 and N2/NH3 gases. The absolute H and N radical densities and NH3 molecule density in etching plasmas were evaluated by the vacuum ultraviolet absorption spectroscopy and the infrared diode laser absorption spectroscopy technique, respectively. The effects of frequency that excited the plasma in the production of H and N radicals were clarified by comparing the behavior of radicals in the UHF plasma with that in the inductively coupled plasma (ICP) exciting at the frequency of 13.56 MHz reported in the previous study. H radicals were produced more efficiently from NH3 gas and N radicals more efficiently from N2 gas in the UHF plasma than in the ICP. H radicals were generated not only through the dissociation of the H2 molecule but also from NH3 molecules produced in the N2/H2 plasma. On the other hand, the N radical density was lower by one order of magnitude than the H radical density, and was seldom produced from the NH3 molecule. It was found that the etch rate and etched profile could be controlled with an internal plasma parameter of the ratio of the H and N radical densities under the constant electron density in the plasma gas chemistry of both N2/H2 and N2/NH3. The optimal conditions for the anisotropic profile and high etch rate were well determined by the ratio of the H and N radical densities and the substrate temperature. The results of this study are very useful not only for understanding the fundamental process of organic low-k film etching but also for precisely controlling the etching process.

Measurement of oxygen atom density employing vacuum ultraviolet absorption spectroscopy with microdischarge hollow cathode lamp

The compact measurement system for absolute density of oxygen (O) atom has been developed, which employs a vacuum ultraviolet absorption spectroscopy (VUVAS) technique with a high-pressure microdischarge hollow cathode lamp (MHCL) as a light source. The influences of self-absorption, emission line profile of the MHCL, and background absorption of oxygen molecule (O2) on the determination of absolute O atom density were taken into consideration. This system has been applied for measuring absolute O atom densities in an inductively coupled O2 plasma. O atom densities were estimated to be on the order of 1×1012–1×1013 cm−3 at an input power of 100 W and an O2 pressure ranging from 1.3 to 26.7 Pa. The behavior of O atom density measured using VUVAS technique was consistent with that obtained by actinometry technique using O emission intensities of 844.6 nm and 777.4 nm lines. Moreover, the lifetime of O atom in the afterglow plasma has been measured. The decay curves of the O atom density were fitted with exponential functions. The extinction process of O atom in the inductively coupled O2 plasma has been discussed.

Measurements of atomic carbon density in processing plasmas by vacuum ultraviolet laser absorption spectroscopy

Measurements of the absolute C atom density in an inductively coupled plasma (ICP) source were carried out by using vacuum ultraviolet (VUV) laser absorption spectroscopy with the resonance lines of C atoms at wavelengths around 94.5 and 165.7 nm. A tunable VUV laser covering these wavelength ranges was generated by a two-photon resonance/four-wave mixing technique in Xe gas. No absorption at around 94.5 nm could be observed, but from the absorption spectra around 165.7 nm we successfully derived the absolute density of C atoms in the ICP source. The obtained values varied from 1×1010 to 1×1011 cm−3, depending on the source gas and operating conditions of the plasma source. The relatively small density values compared to other atomic species are attributed to the large loss rates, which mostly occur on the surface.

Investigation of Nitrogen Atoms in Low-Pressure Nitrogen Plasmas Using a Compact Electron-Beam-Excited Plasma Source

We developed a new compact electron-beam-excited plasma (EBEP) source with a multihole grid. The source can be reliably operated under low-pressure conditions by controlling the electron-beam current and electron-beam energy independently. In this study, we applied the compact EBEP source to the generation of nitrogen plasma. The absolute nitrogen (N) atom densities in the plasma were evaluated using vacuum ultraviolet absorption spectroscopy (VUVAS) with a high-pressure microdischarge hollow cathode (MHCL) as a light source. The electron density and electron temperature were measured using a Langmuir probe. The behavior of N atom densities and electron densities in the plasma are discussed.

Behavior of atomic radicals and their effects on organic low dielectric constant film etching in high density N2/H2 and N2/NH3 plasmas

An organic film, FLARE™, is one of the most prospective candidates for interlayer insulating films with low dielectric constants (low k). This organic low k film was etched in inductively coupled high-density plasmas employing N2/H2 and N2/NH3 gases. By changing the mixing ratio of these gases, the anisotropic etching profile was obtained. The etching plasmas were evaluated by quadruple mass spectroscopy and the vacuum ultraviolet absorption spectroscopy employing microplasma as a light source. N and H radical densities were estimated on the order of 1011–1012 cm−3 and 1012–1013 cm−3, respectively. The behavior of etch rate corresponded well to that of H radical density. H radicals were found to be important species for organic low k film etching, while N radicals could not etch without ion bombardments. On the other hand, N radicals were found to be effective for the formation of protection layer on the sidewall against the etching by the H radicals. The ratio of H and N radical densities would be important for the etching of organic low k film employing N–H plasmas.

Vacuum ultraviolet emission bands of LiLuF4:Tb3+ crystals in the spectral range from 157 to 200 nm

Abstract The absorbance of most carbonated polymeric materials just has the right value of ∼0.1–0.4 μm −1 for lithographic imaging in the spectral region from 170 to 193 nm. Also, the resist out-gassing is less in this spectral region in comparison to 157 nm. As a consequence the development of vacuum ultraviolet (VUV) light sources for lithographic applications at wavelengths between 157 and 193 nm should be seriously taken into consideration. The experimental results of this work suggest that the LiLuF 4 :Tb 3+ crystal can be considered as a candidate towards the development of new VUV light sources for lithographic applications at wavelengths between 157 and 193 nm. The 4f 8 ↔4f 7 5d dipole allowed transitions of the Tb 3+ ions in LiLuF 4 crystal hosts, were investigated with VUV absorption spectroscopy and laser-induced fluorescence using as the excitation source the molecular fluorine laser at 157 nm.

Characterization of ion-implanted silica glass by vacuum ultraviolet absorption spectroscopy

We investigate the mechanisms of defect formation and optical absorption induced by ion implantation, for fabrication of optical devices by radiation effects. High-purity silica implanted by H + or He 2+ was characterized using vacuum-ultraviolet spectroscopy and electron-spin-resonance measurement. Defect formation is suppressed by OH groups, possibly by the release of atomic hydrogen. The E ′ center and nonbridging oxygen hole center were created through pair generation from the normal SiOSi bond. The peroxy radical was generated through the reaction of the E ′ center with interstitial oxygen, which is a Frenkel-defect pair with an oxygen vacancy. By the Kramers–Kronig analysis on the MeV-ion implantation-induced defects, a refractive index increase of the order of 10 −4 was estimated.

Absolute concentration and loss kinetics of hydrogen atom in methane and hydrogen plasmas

A measurement technique of the absolute concentration of hydrogen (H) atoms in methane (CH4) and/or hydrogen molecule (H2) plasmas has been established. The H-atom concentration was evaluated by vacuum ultraviolet absorption spectroscopy (VUVAS) using a high-pressure H2 microdischarge hollow cathode lamp (H2-MHCL) as the Lyman α (Lα 121.6 nm) light emission source. A measurement technique of the background absorption caused by species other than H atoms at the Lα line was developed by using the VUVAS technique with the MHCL employing nitrogen molecules (N2-MHCL). The lines around Lα used for the background absorption measurements are 2p23s P5/24–2p3 S3/204 at 119.955 nm, 2p23s P3/24–2p3 S3/204 at 120.022 nm, and 2p23s P1/24–2p3 S3/204 at 120.071 nm of the N atom. By using the VUVAS technique with the MHCLs and subtracting the background absorption from the absorption of H atoms at Lα, we have achieved the measurement of the H-atom concentration in an inductively coupled plasma operated in CH4 and/or H2. The H-atom concentration increased from 2×1011 to 3×1012 cm−3 when increasing the CH4 flow rate ratios from 0% to 50% in the CH4–H2 mixture and was almost constant in its range between 50% and 100% at a pressure of 1.33 Pa, a radio frequency power of 200 W, and a total flow rate of 100 sccm. The behavior of the H-atom concentration was compared with that of the Balmer α emission intensity. The decay of the H-atom concentration in the H2 plasma afterglow was investigated to clarify the loss kinetics of H atoms. The dependence of the decay time constant on the pressure showed that H atoms were dominantly lost through diffusion to the wall surface. The diffusion constant of H atoms in H2 plasmas was determined to be 3.0×105 cm2 Pa s−1 at 400 K. The surface loss probability of H atoms on the stainless-steel and the hydrocarbon walls were estimated to be 0.15 and 0.07, respectively.