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Low Pressure CVD of Silicon Nitride from a Silane-Ammonia Mixture: Analysis of Preliminary Experimental and Simulation Results

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Low Pressure CVD of Silicon Nitride from a Silane-Ammonia Mixture: Analysis of Preliminary Experimental and Simulation Results Low Pressure CVD of Silicon Nitride from a Silane-Ammonia Mixture : Analysis of Preliminary Experimental and Simulation Results K. Yacoubi, C. Azzaro, J. Couderc To cite this version: K. Yacoubi, C. Azzaro, J. Couderc. Low Pressure CVD of Silicon Nitride from a Silane-Ammonia Mixture : Analysis of Preliminary Experimental and Simulation Results. Journal de Physique IV Pro- ceedings, EDP Sciences, 1995, 05 (C5), pp.C5-291-C5-298. 10.1051/jphyscol:1995534. jpa-00253860 HAL Id: jpa-00253860 https://hal.archives-ouvertes.fr/jpa-00253860 Submitted on 1 Jan 1995 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE IV Colloque C5, suppltment au Journal de Physique 11, Volume 5, juin 1995 Low Pressure CVD of Silicon Nitride from a Silane-Ammonia Mixture: Analysis of Preliminary Experimental and Simulation Results K. Yacoubi, C. Azzaro and J.P. Couderc ENSIGC INPT, Laboratorre de Gknie Chimique, URA 192 du CNRS, I8 Chernin de la Lage, 31078 Toulouse cedex, France Abstract :Results of a study dealing with the deposition of silicon nitride from a silane-ammonia mixture and combining experimental approach and deposition modelling are presented. First, a thorough QRKK analysis has compensated for the lack of kinetic information in the gas phase. A reduced reaction set reproducing the essential features of the full mechanism and involving 6 species including two silylamine intermediates SiH3NH2 and SiHNH2 has been identified. A local two-dimensional model previously developed in our laboratory has then been adapted to the treatment of silicon nitride deposition. The identification of the kinetic parameters of the heterogeneous mechanism has been achieved through a combined approach of experimental and simulation results. 1. INTRODUCTION LPCVD has become a classical production method for silicon nitride films, commonly used as insulation or passivatioh materials within the MOS technology. When this kind of deposition is carried out from a silane-ammonia mixture, the following processing difficulties may be encountered: (9-Growth rate decreases markedly relative to pure polysilicon deposition; (i)-A significant radial nonuniformity is reported, both in deposition rate and film composition; (%-A longitudinal decrease in deposition rate is generally observed In spite of its importance and difficulties, the case of silicon nitride has surprisingly received little attention. Only the deposition from a mixture of dichlorosilane and ammonia, presenting the same general trends as those reported from a silane-ammonia source, has been treated in the literame [I]. Results of this study indicate that the in-wafer film thickness nonuniformities may be explained by the effect of diffusion-limited film growth from highly reactive gas-phase intermediates, with simultaneous deposition hmless reactive dichIorosilane. The purpose of this work is to combine both modelliig and experimental analysis in order to elucidate the complex physicochernical phenomena involved in the deposition process from a silane- ammonia mixture. Due to the Iack of kinetic information avaiIabIe in the Literature, a major part of this work has been focused on the investigation of gas phase reactions. In the first part of this paper, the results of a 5orough QRRK (Quantum Rice Ramsberger Kassel) analysis compensating for the lack of kinetic information will be presented. Then, first experimental results of silicon nitride deposition will be Presented and analysed. They will be compared with the predictions of the CVD2 model previopsly dfveloped in our laboratory 121. The extension principles of the CVD2 model to the case of sillcon Nmde will be discussed. It must be pointed out that the results presented in this paper are prelimnary , but that a promising agreement between experimental and simulation results has been obtained. Perspectives of this work will finally be suggested. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1995534 C5-292 JOURNAL DE PHYSIQUE IV 2. THERMAL DECOMPOSITION OF A SILANE-AMMONIA MIXTURE To our knowledge, the kinetics of thermal decomposition of a mixture of silane and ammonia has received very little attention in the literature. In this study, the following reactions have been taken into account [4]: Type of reactions Chemical mechanism / Remarks - [References] Si& = SiH2 +Hz - Extensively studied Silane pyrolysis [3] -The formation of higher Sia s SiHj + H order silanes is negligible [3] Si2J& = SW+ SiH2 Ammonia ~~ol~sis[51 NH3 = NH2 + H Ammonia-silicon Sfi + NH3 =G SiH3NH2 + H2 SiH2 + NH3 = SiH3NH2 hydrides reactions [4] P~lysisof Sa3NH2 [41 SiH3NH2 c SiHNH2 + H2 SiH3NH2 = SiH2NH2 + H SiH3NH2 = SiH2NH + H2 SiH3NH2 = SiH3+ NH2 - Molecule-radical and NH3+Ht? NH2+H2 NH2+ Sm=z SiH3+ NH3 radical-radical reactions [6l NH2 + NH2 =G N2H4 Sfi + H =z Sag+ H2 NH3+ NH2 =z N2H4+ H SS& + SiHj = Si2Q + H NH2+ SiH4= SiH3NH2 + H SiHNH2+ H = SiH2NH2 NH3+ SiH3 =G SiH3NH2 + H SiH2NH + H = SiH2NH2 Table 1 :Homogeneous gas phase reactions Different methods have been used to evaluate unknown rate constants (carbon hydrides analogies, collision frequency limits, Benson's method [7])at P=l atm and T = 298OK. Using these evaluations, or measured constants in other conditions (note that they are only available for silane pyrolysis), rate constants of unirnolecular or bimolecular reactions deduced from the reactions presented in Table 1, corresponding to LPCVD conditions, i.e. to the pressure fall-off region, have been calculated with QRRK th&ry 181. Note that this theory has been successful in predicting the reduction of the unimolecular rate constants with decreasing pressure for silane or SIPOS decomposition [8] and the estimation of bimolecular rate constants. A detailed presentation of the QRRK model used can be found in [8]. By lack of place, kinetic rate constants of the whole mechanism will not be presented Using that mechanism, the time evolution of all the species concentrations which could be observed in a hypothetical gas volume, uniform in its properties and without any contact with solid surfaces, starting from the mixture of silane and ammonia has been computed using numerical integration based on Gear's method [9]. Typical results obtained are presented in Figure 1 for a temperature of 750°C a pressure set at 0.4 Torr and a silanelammonia ratio equal to 0.4/0.6 A distinction must be made between radical and stable species : On the one hand, disilane and hydrazine, which are stable molecules, have very low mncentrations relative to the concenmtions of the other stable species, i. e. silane and hydrogen. A series of simulation runs was performed in which secondary reactions of these species were neglegted. The results obtained showed that the evolution of the remaining species of the system is not affected by this modification. On the other hand, even in very low concentrations, silylamine radicals (sm2,SiH2NH and SiH2NH2) may have a non negligible impact on the whole mechanism. Due to the similar atomic structure of these species, it could be assumed that their contribution to deposition rate was directly dependent on the molar fraction of the species. As SiHNH2 molar fraction is approximately 104 times greater than the other ones, its contribution will mask the effect of the other species. A similar analysis has been applied to the cases of SiH2 and SiH3. It has led to the elimination of SiH3 in the chemical mechanism (see Figure 2). This approach has been repeated for a wide range of temperatures, pressures and silane/ammonia ratios. In all cases, the same species have been candidates for and have been finally removed from the complete scheme. MYIY .I "lr I....L.-. ....*. * : : - - 4 .,..o . ) . I'.. 1 0 'Y, - " I ,.., AN . .L.IPT. .a*..- r-- -O '-1 -"""I .."I .I,_.. ii .".* .>'... 0 * .-1 . ._Y ., 'L.. A .w- ..,... ol ,. .I._.. .li_.. .,.." . ...a 1 ...a ' ....a ....I ..I.. UU%* .Y L1 .U Figure 1 : Time evoIuuon of the species cohcmt&on~ Figure 2 :Time evolution of the species (Complete mechanism) concenttations (Reduced mechanism) Finally, a reduced reaction set (see Table 2) which reproduces the essential features of the full mechanism and involves only 6 species including two silylamine intermediates, i.e., SiH3NH2 (stable) and SiHNJ32 (reactive) has been adopted. Chemical reaction Kinetic constants (results of QRRK analysis) (R = 8.314 J mole-1 K-1, P in pa]) SiH2 + I32 Sa kl = 0.074 P exp (6627 lo3/RT) [m3 mole-l s- k1= 1,3810~ P exp (-209.65 1o3KI') [ s- l] SiH2 + NH3 --, SiH3NH2 = 4,082 lo4 exp (34.97 103 /RT) [m3 mole-l s- ] 3 k2= 921 lo4 exp (16.18 10 /Rp [ s ] Sa2+ NH3 =C Sm2+H2 kj = 17.4 P exp (6453 103~[m3 mole-I s- '1 k-3 = 3.23 lo8 P exp (-217.51 I~/RT)[m3 mole-l s- ] kq = 1,781 P~,~~exp (121.4 103/R?4 [m3 mole-ls ] SiHNH2+ Hz SiH3NH2 = 9,68 lo8 P~'~~exp (-178.09 103~ [ s- ] Table 2 :Homogeneous gas phase mechanism adopted in this study 3. EXPERIMENTAL STUDY 3.1 Operating conditions Experiments were carried out in a conventional tubular horizintal hot-wall LPCVD reactor, in which 82 P-type <loo> Si wafers were stacked vertically inside the reactor. The main characteristics of the equipment used are presented in Table 3. Four test wafers were placed at selected locations along the length of reactor ( 57, 62, 67, 72 h). The position of each wafer in the batch is numbered from the first wafer reached by the gases.
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