Article pubs.acs.org/JPCA
Comparison of Hexamethyldisiloxane Dissociation Processes in Plasma J. L. Jauberteau* and I. Jauberteau SPCTS-UMR 7315 CNRS, CEC 12 rue Atlantis, 87068 Limoges, France ABSTRACT: Different hexamethyldisiloxane (HMDSO) dissociation processes are investigated by means of absorption spectroscopy and mass spectrometry. All of these processes are expected to occur in plasma containing Ar−HMDSO gas mixture. We successively study interactions of the HMDSO molecule with electrons (energy ranges from 15 to 70 eV), with Ar(3P2) metastable species (internal energy 11.55 eV) and with VUV photon (7.3 to 10.79 eV). The studies of HMDSO interactions with Ar(3P2) and VUV photon provide new results concerning the dissociation pathways and the collision cross-sections. In the case of Ar(3P2), the dissociation mechanisms result mainly in Si−C or Si−O bond breaking, producing SiMe2,1 radicals. Less efficient mechanisms involve also Si−C and Si−O bond breaking producing Me, Si2Me5O, or SiMe3, on one hand, and, on the other hand, Si−C and C−H bond breaking producing Si2Me4OH. In the case of photon interaction, the dissociation process is more selective and mainly produces Si2OMe5 pentadisiloxane and methyl radicals due to Si−C bond breaking. Si−O bond breaking produces also SiMe3 in a lower concentration. Dissociation crosssection values of HMDSO ranging from σ = 45 × 10−20 m2 to 180 × 10−20 m2 and from σ = 0.7 × 10−22 m2 to 18.3 × 10−22 m2, correspond to a global dissociation mechanism by Ar(3P2) collision and to a selective dissociation mechanism (producing Si2OMe5 and Me) by VUV photon interaction, respectively. All results are compared and discussed.
1. INTRODUCTION Hexamethyldisiloxane is a [SiH3−nMen]2O disiloxane molecule (Me = methyl group). These compounds have been largely studied after the second world war by K. A. Andrianov, contributing to the development of heat-resistant adhesives and lubricants.1 Nowadays, they are injected in plasma to produce thin film deposition of silicon oxycarbides, which are used in applied material research2−4 as coating for scratch, corrosion protections, gas diffusion barriers, or low-k material layers. The chemical composition and the structure of plasma deposited thin films depend on the plasma parameters and the plasma chemistry. The knowledge of the different elementary reactive processes involved in the plasma is necessary for both practical and theoretical works. Various reactive mechanisms of the dissociation of organosilicon compounds have been already investigated. They are collisions with charged species (electrons or ions),5−7 photolysis processes,8 or reactive processes with neutral atoms, molecules, or radicals.9−11 In this last case, neutral species can be metastable species characterized by a long lifetime in vacuum and a large internal energy.9,10 Reactive processes between HMDSO and radicals, ions, or electrons including charge exchange transfers or electrons attachment are efficient reactive processes.5,6,11 These mechanisms have been largely studied and are now well-known. In the present work, investigations are focused on the cases of the HMDSO photolysis and of the collision between HMDSO and Ar(3P2) metastable species. Both VUV radiations and Ar(3P2) species are produced in plasma containing argon and these reactive processes can occur in such discharge. We have studied the different reaction pathways for these processes, and we have measured the reaction cross-section values, by means of mass © 2012 American Chemical Society
spectrometry and absorption spectroscopy. The results are discussed, and the dissociation processes are compared with the dissociation of HMDSO by electron collision.
2. EXPERIMENTAL SETUP 2.1. Reactor and the Mass Spectrometer. The experimental device is shown in Figure 1. It consists of a vertical stainless steel tube (internal diameter, 50 mm; length, 320 mm), equipped with fused silica windows fixed on the side of the tube and used for optical diagnostics. A mass spectrometer (QMG 421 Balzers) is fixed on the top of the reactor and is used to detect the neutral or ion species. A microwave discharge (SAIREM generator working at 2.45 GHz) is produced in a quartz tube (internal diameter 16 mm) fixed at the bottom of the stainless steel tube and is used to produce metastable argon. A deuterium lamp is fixed on the tube side and is used as a VUV photon source. The quartz tube is refreshed by means of pulsed air to keep a constant wall temperature. The total pressure in the discharge tube is held constant using a Roots blower pump (70−700 m3/hour). It ranges from 10 to 100 Pa. The gas flow velocity ranges from 10 to 30 m/s, in the stainless tube, depending on the Roots pump rotation speed. The mass spectrometer sample hole is located 22 cm above the discharge exit. Because of the large pressure in the reactor, the mass spectrometer quadrupole (QMA 400 Balzers), requiring at least the pressure of 10−3 Pa, cannot Received: May 15, 2012 Revised: July 6, 2012 Published: July 19, 2012 8840
dx.doi.org/10.1021/jp304694z | J. Phys. Chem. A 2012, 116, 8840−8850
The Journal of Physical Chemistry A
Article
Figure 1. Experimental setup.
where I0 and I are the signal intensities measured without and with discharge, respectively. The I value is corrected by taking into account the signal intensity due to the plasma emission at 811.5 nm. The relative absorption can be correlated to the argon metastable concentration using the theory of Mitchell and Zemansky.13 At a low relative absorption, this theory can be simplified, and the metastable concentration is proportional to the relative absorption value. 2.3. Photolysis Apparatus. A VUV Light Source Unit (L9026 Hamamatsu) is used for HMDSO photolysis. It consists of a 30W head-on type deuterium lamp, equipped with a MgF2 output window, allowing a large spectral distribution with vacuum UV radiation of high intensity ranging from 180 nm to 115 nm. Three main emission peaks are produced at 160.8 (the maximum), 125.4, and 121.6 nm. The VUV lamp is fixed on the reactor by means of a vacuum flange (ICF70) and is located downstream from the exit of the quartz tube (see Figure 1). The HMDSO is injected upstream of the lamp. In the following part of this work, we present results corresponding to various studies on HMDSO dissociation processes by electron, metastable collisions, and VUV interactions. For the sake of clarity, figures display an error bar for only one point. The experimental error on the other points is of the same relative value and can be easily deduced.
operate. Therefore, a two stage pumping unit is necessary. This specific setup has been detailed in a previous publication.12 The mass spectrometer ionization chamber is equipped with an electron cross-beam, and the experimental error on the electron energy values is equal to 0.2 eV. Before each experiment, the two filaments of the electron cross-beam apparatus are heated by Joule effect for 1 h using a current ranging from 1 to 3 A. The mass spectrometer signal intensity recorded is the mean value calculated over 50 consecutive data acquisitions. The two experimental devices (the microwave discharge and the deuterium lamp) presented above are used to produce metastable argon species or VUV photons, respectively. The two sources of metastable species and VUV photons are described and detailed below. 2.2. Ar(3P2) Metastable Source and Absorption Spectroscopy Device. Ar(3P2) species are produced in pure argon within the quartz tube by means of the microwave discharge and used to dissociate HMDSO. The optimal conditions to produce Ar(3P2) have been determined at a power of 100 W and a pressure of 66 Pa. The HMDSO is contained in a stainless steel vessel and is diluted with argon and injected through a calibrated leak located 2 cm downstream from the tube discharge exit. The Ar(3P2) concentration is measured by means of absorption spectroscopy, which is performed through the fused silica windows, using an argon lamp. A sketch of the optical setup is shown on the left upper side corner in Figure 1. We measure the emission line intensity corresponding to the transition [2P9(j = 3)−1S5(j = 2)] (in Paschen notation), produced between the two 4s and 4p Ar levels and detected at 811.5 nm. Observations are performed just above the HMDSO injector exit. The diagnostic is carried out by means of a THR 1000 Jobin Yvon spectrometer with a resolving power equal to 100 000 at 500 nm. It is equipped with a Hamamatsu R928 photomultiplier tube. The optical signal is introduced into the entrance slits of the spectrometer using an optical fiber (PCS 600) collecting the signal at the focal point of a lens. The relative absorption A is given by A = (I0 − I )/I0
3. DISSOCIATION BY ELECTRON COLLISION HMDSO dissociation processes by electron collisions have been largely studied in the past,4,6,14−16 and the various dissociation pathways and cross-section values have been determined and measured. In the present work, investigations are performed by means of mass spectrometer to determine the part of the parent molecule HMDSO or of neutral residual species produced in the reactor in the total signal measured for different species detected in the experiments when the discharge or the deuterium lamp are switched on. These neutral residual species are sticking on the reactor wall and can be desorbed in the reactor a long time after the end of the experiments, when the microwave discharge or the deuterium lamp is switched off. In the present experiments, HMDSO is
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dx.doi.org/10.1021/jp304694z | J. Phys. Chem. A 2012, 116, 8840−8850
The Journal of Physical Chemistry A
Article
errors (typically 10%). Theses species are probably due to desorbed species. According to Ceatore et al.,7 the low ionization threshold value observed for m/q = 58 is probably due to the dissociation ionization of Me3SiH or Me2SiH2 species desorbed by the wall. In our case, these processes can also contribute to produce the two radicals observed at m/q = 58 and m/q = 43. In a previous article, we have pointed out the effect of wall desorbed species on the plasma chemistry in the case of the tetramethylsilane under the same experimental conditions.17 Peaks have been observed for m/q = 44, 45, and 58 and have been ascribed to dissociation ionization processes of HSiMe2 or HSiMe, probably desorbed from the reactor wall. This effect increases with the diameter of the reactor decreasing. In the present work, a peak is observed for m/q = 38 and has not been observed by Basner et al.6 It could be ascribed to C3H2+, which has an ionization threshold value close to the one observed by Benedikt et al.4 for C2H+ (17.22 eV) in a remote Ar/C2H2 expanding thermal plasma. As previously for m/q = 58 and 43, this peak could be also due to the dissociation ionization of wall desorbed species. Figure 2 displays the mass spectrometer signal intensity of the main species versus electron energy ranging from 15 to 30 eV, detected when the discharge is off.
injected and mixed with Ar upstream of the quartz tube, 30 cm below the mass spectrometer sample hole. The total pressure in the vessel is equal to 10 Pa, and the discharge or the lamp is switched off. In a first qualitative study, using an electron energy equal to 70 eV in the ionization chamber, the mass spectrum displays peaks at m/q = 162, 147, 146, 133, 132, 131, 102, 88, 87, 73, 66, 58, 43, and 15, corresponding to Si2OMe6+, Si 2 OMe 5 + , Si 2 OMe 4 (CH 2 ) + , Si 2 OMe 4 H + , Si 2 OMe 4 + , Si2OMe3(CH2)+, Si2OMe2+, SiMe4+, SiMe3(CH2)+, SiMe3+ and Si2OMe42+, SiMe2+, SiMe+, and Me+, respectively. The various mechanisms producing these ions are given in the literature and are reported in Table 1. They correspond to Table 1. Main Ions Detected by Electron Collision at 70 eV on HMDSO and the Different Dissociation Mechanisms m/q 162 147 146 133 132 131 102 88 87 73 66 58 43
products Si2OMe6+ Si2OMe5+
+e + Me + e Si2OMe4(CH2)+ + H + Me + e Si2OMe4H+ + Me + CH2 + e Si2OMe4+ + 2Me + e Si2OMe3(CH2)+ + 2Me + H + e Si2OMe2+ + 4Me + e SiMe4+ + SiMe2O + e SiMe3(CH2)+ + SiMe2O + H + e SiMe3+ + SiMe3O + e Si2OMe42+ + 2Me + 2e SiMe2+ + SiOMe4 + e SiMe+ + SiOMe5 + e
dissociation mechanisms6,14,15 with methyl group abstraction and Si−C bond breaking (for m/q = 147, 132, 102, and 66). These mechanisms can be associated to H abstraction (for m/q = 146 and 131). Other mechanisms produce CH2 abstraction (for m/q = 133) or Si−O bond breaking (for m/q = 88, 87, and 73). The ion detected at m/q = 66 (Si2OM42+) is doubly ionized and results from a two methyl groups abstraction process, produced at low energy (27 eV to 30 eV).8 Its two first electronic states, which are very close to the energy balance of the dissociation, make it stable. The loss of two methyl groups leads to a doubly charged ion stable in planar geometry with a small internal energy.8 Table 2 compares the ionization threshold values measured in the present work for the main ion detected at low electron energy (
