Two-photon excited LIF determination of H-atom concentrations near a heated filament in a low pressure H_2 environment

Meier, Ulrich; Kohse‐Höinghaus, Katharina; Schäfer, Lothar; Klages, Claus‐Peter

doi:10.1364/ao.29.004993

Cited by 91 publications

(35 citation statements)

References 16 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In this approach often a ''flow-tube reactor'' is used as a source of ground-state atomic radicals, in which the concentration of these radicals is determined by a chemical titration. This method is based on well-established flow reactor and titration techniques in chemistry 24 and has been applied for the calibration of LIF density measurements of several atomic radicals in plasmas, including atomic hydrogen 17,25 and atomic oxygen. 26 Another approach is to also use an independent quantitative measurement technique to determine the density of the probed species.…”

Section: Introductionmentioning

confidence: 99%

Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

Boogaarts

Mazouffre

Brinkman

et al. 2002

Review of Scientific Instruments

View full text Add to dashboard Cite

We report on quantitative, spatially resolved density, temperature, and velocity measurements on ground-state atomic hydrogen in an expanding thermal Ar-H plasma using two-photon excitation laser-induced fluorescence ͑LIF͒. The method's diagnostic value for application in this plasma is assessed by identifying and evaluating the possibly disturbing factors on the interpretation of the LIF signal in terms of density, temperature, and velocity. In order to obtain quantitative density numbers, the LIF setup is calibrated for H measurements using two different methods. A commonly applied calibration method, in which the LIF signal from a, by titration, known amount of H generated by a flow-tube reactor is used as a reference, is compared to a rather new calibration method, in which the H density in the plasma jet is derived from a measurement of the two-photon LIF signal generated from krypton at a well-known pressure, using a known Kr to H detection sensitivity ratio. The two methods yield nearly the same result, which validates the new H density calibration. Gauging the new ''rare gas method'' by the ''flow-tube reactor method,'' we find a krypton to hydrogen two-photon excitation cross section ratio Kr ͑2͒ / H (2) of 0.56, close to the reported value of 0.62. Since the H density calibration via two-photon LIF of krypton is experimentally far more easy than the one using a flow-tube reactor, it is foreseen that the ''rare gas method'' will become the method of choice in two-photon LIF experiments. The current two-photon LIF detection limit for H in the Ar-H plasma jet is 10 15 m Ϫ3 . The accuracy of the density measurements depends on the accuracy of the calibration, which is currently limited to 33%. The reproducibility depends on the signal-to-noise ͑S/N͒ ratio in the LIF measurements and is orders of magnitude better. The accuracy in the temperature determination also depends on the S/N ratio of the LIF signal and on the ratio between the Doppler-width of the transition and the linewidth of the excitation laser. Due to the small H mass, the current linewidth of the UV laser radiation is never the accuracy limiting factor in the H temperature determination, even not at room temperature. Quantitative velocity numbers are obtained by measuring the Doppler shift in the H two-photon excitation spectrum. Both the radial and axial velocity components are obtained by applying a perpendicular and an antiparallel excitation configuration, respectively. The required laser frequency calibration is accomplished by simultaneously recording the I 2 absorption spectrum with the fundamental frequency component of the laser system. This method, which is well-established in spectroscopic applications, enables us to achieve a relative accuracy in the transition frequency measurement below 10 Ϫ6 , corresponding to an accuracy in the velocity of approximately 200 m/s. This accuracy is nearly laser linewidth limited.

show abstract

Section: Introductionmentioning

confidence: 99%

Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

Boogaarts

Mazouffre

Brinkman

et al. 2002

Review of Scientific Instruments

View full text Add to dashboard Cite

show abstract

“…25 Experimental investigations on the surface role of H have been carried out by Ohl et al 26 and Butler and co-workers, 27 among others. Studies on atomic hydrogen in the gas phase of CVD systems have been carried out in a hot filament reactor by resonance enhanced multi-photon ionization 28 and two-photon laser induced fluorescence ͑LIF͒, 29,30 which technique has also been used in a rf plasma reactor 31,32 and a microwave reactor. 33 In the present work the two-dimensional laser induced fluorescence ͑2D-LIF͒ technique is applied to measure the distributions of atomic hydrogen and C 2 in the oxyacetylene flame during diamond growth.…”

Section: Introductionmentioning

confidence: 99%

“…In low pressure environments the collisional quenching of the fluorescence signal can be determined 37,40 and the H LIF signal can be quantified by comparison to measurements in a calibration reactor where the H concentration is known. 29,31,32 At atmospheric pressure, however, quantification of the signal is hampered by collisional quenching of the fluorescence, which not only depends on temperature and pressure, but also on the nature of the collision partners. Since at atmospheric pressure many collisions will take place during the fluorescence lifetime and a multitude of different species is available to collide with, quantitative measurements are not attempted and the results therefore will be interpreted in a more qualitative way.…”

Section: Introductionmentioning

confidence: 99%

Spatial distributions of atomic hydrogen and C2 in an oxyacetylene flame in relation to diamond growth

Klein-Douwel

Meulen

1998

Journal of Applied Physics

View full text Add to dashboard Cite

Two-dimensional laser induced fluorescence measurements are applied to the chemical vapour deposition of diamond by an oxyacetylene flame to visualize the distributions of atomic hydrogen and C 2 in the gas phase during diamond growth. Experiments are carried out in both laminar and turbulent flames and reveal that atomic hydrogen is ubiquitous at and beyond the flame front. Its presence extends to well outside the diamond deposition region, whereas the C 2 distribution is limited to the flame front and the acetylene feather. The diamond layers obtained are characterized by optical as well as scanning electron microscopy and Raman spectroscopy. Clear relations are observed between the local variations in growth rate and quality of the diamond layer and the distribution of H and C 2 in the boundary layer just above the substrate. These relations agree with theoretical models describing their importance in ͑flame͒ deposition processes of diamond. Three separate regions can be discerned in the flame and the diamond layer, where the gas phase and diamond growth are predominantly governed by the flame source gases, the ambient atmosphere, and the interaction of both, respectively.

show abstract

“…Moreover, it is also possible to increase [H] by means of various filament configurations. The results obtained by Meier et al [10] showed that the concentration of atomic hydrogen increases with the surface area of the filament. The surface area can be greatly increased by winding the filament into a helix.…”

mentioning

confidence: 95%

Diamond Films Synthesized at Ultralow Filament Temperatures

Zhang¹,

Buck

2002

Adv. Eng. Mater.

View full text Add to dashboard Cite

In chemical vapor deposition of diamond films, a recent research challenge is to push the limits of boundary conditions. [1,2] These attempts may lead to the development of new deposition techniques and applications, and can also be expected to give insight into diamond growth mechanisms. In the hot-filament CVD method, it is known that filament and substrate temperature, chamber pressure, and gas concentration are the main quality control variables for diamond films. Typically, these parameters are 2000 C, 800 C, 40 mbar, and 1 % CH 4 in H 2 , respectively. Limiting deposition conditions have been investigated by many researchers. For example, the deposition pressure can be varied in either direction: higher up to nearly one atmosphere [3] to improve technical application, or lower to about 0.2 mbar [4] for basic research to reach conditions at which standard plasma deposition can be applied. The lowest substrate temperature is of special interest for many industrial applications: it is about 100 C. A high filament temperature of about 2890 C has been used to increase growth rate by using higher methane concentration. [5] Similarly, a question of common interest is the possibility of depositing diamond films at lower filament temperatures, because the growth of diamond films under these conditions might give a new insight into the deposition process. A lower filament temperature would have several advantages. Filaments would hold their shape better, resulting in stable deposition conditions. Moreover, filaments operating at a lower temperature would be better able to withstand mechanical disturbance and therefore have longer lifetimes.In the present study, diamond films were deposited in the low filament temperature range of 1300±1700 C. The synthesized diamond crystals were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM).The experiments were conducted in a typical hot-filament CVD system. The hot filament was made of a 0.5 mm diameter tantalum wire wound into 10 circles of inner diameter 2 mm. Si substrates were abraded with 0.1 lm diamond paste, then ultrasonically cleaned in acetone for 10 min before fixing to a boron nitride holder. During the deposition process, the total ambient pressure was kept at 40 mbar, while the flow rates of hydrogen and methane were controlled by mass flow controllers with 100 sccm total flow rate. The concentration of methane was fixed at 0.5 %. The temperatures of the filament and the substrate were 1300±1700 C and 650±950 C measured by an optical pyrometer and a K-type thermocouple, respectively. The distance between the filament and the substrate was 2±5 mm. Atomic hydrogen concentration, [H], as a function of filament temperature was measured by using the calorimetric method. [6] [H] was determined after calibration of the temperature difference DT measured by two thermocouples, one with a silver-coated tip and the other with a non-catalyzing quartz tip. The films produced under different filament temperature were characterized by SEM and by XRD usi...

show abstract

Two-photon excited LIF determination of H-atom concentrations near a heated filament in a low pressure H_2 environment

Cited by 91 publications

References 16 publications

Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

Quantitative two-photon laser-induced fluorescence measurements of atomic hydrogen densities, temperatures, and velocities in an expanding thermal plasma

Spatial distributions of atomic hydrogen and C2 in an oxyacetylene flame in relation to diamond growth

Diamond Films Synthesized at Ultralow Filament Temperatures

Contact Info

Product

Resources

About