J.P. Couderc

Silicon deposition from silane or disilane in a fluidized bed—Part I: Experimental study

Abstract Fluidized-bed silicon deposition from silane constitutes an attractive alternative way to produce ultrapure silicon for solar-cell and microelectronic industries. Moreover, it allows the protection of bed particles from oxidation and corrosion. Studies available in the literature have proved the feasibility of this process, and pointed out its numerous advantages (high throughput, low-cost technology,…). However, two significant limiting problems exist: the parasitic formation of fines during experiments, and particles agglomeration when the initial concentration of silane exceeds a critical value. The first part of this article presents an experimental study about silicon deposition from monosilane and, for the first time, from disilane, in a fluidized-bed reactor. During experiments with monosilane, for inlet concentrations lower than 20% in nitrogen, silane conversion has always been quite complete, and fines formation limited. For higher concentrations, the fluidized bed has agglomerated systematically. Furthermore, for the first time, thermal perturbations of the fluidized bed have been put in evidence during all the runs, as soon as silane was introduced into the reactor. Such disturbances did not appear during disilane experiments, but fines were formed in larger amounts. Their zone of apparition was restricted to the coldest regions of the reactor. This has led us to think, in complete disagreement with literature opinions, that fines are formed from heterogeneous chemical reactions, on cold surfaces of the reactor. Moreover, thermal disturbances and agglomeration phenomena have been explained by an increase in particles cohesiveness, due to the presence of silane in the bed.

Two‐Dimensional Modeling of Low Pressure Chemical Vapor Deposition Hot Wall Tubular Reactors I . Hypotheses, Methods, and First Results

A new two-dimensional model taking into account hydrodynamics and mass transport with chemical reactions has been developed. The system of partial differential equations has been solved by finite differences procedures and a Gauss-Seidel algorithm. The case of pure polysilicon deposition is first described, taking into account the chemical species produced by silane pyrolysis, i.e., disilane and silylene

Silicon deposition from silane or disilane in a fluidized bed—Part II: Theoretical analysis and modeling

Abstract To strengthen the results obtained during the experimental study presented in Part I of this article, a detailed theoretical analysis of silicon deposition from silanes in a fluidized bed has been performed. In a first step, four classical models of fluidized-bed reactors have been modified, to treat silicon deposition from monosilane, considering only reactions on surfaces. A convenient agreement has been obtained between the observed and the calculated data, particularly for the Kato and Wen model. But, this approach has been completely unable to treat the case of disilane. Consequently, for the first time, to our knowledge, chemical reactions in the gaseous phase have been included in the Kato and Wen model. This original approach, applied both to the cases of silane and disilane, has improved our understanding of deposition phenomena. More precisely, concerning fines formation, this work has confirmed the explanation given in Part I: it has shown the presence of sufficient active species at the top of the bed, so that these compounds could react on the cold walls of the reactor to give fines. It has also reinforced our rejection of the mechanism of gas nucleation inside the bed proposed by several authors. Secondly, this study has allowed us to strengthen our explanation of thermal disturbances and bed agglomeration, by the fact that the increase in particles cohesiveness seems to be due specifically to deposition mechanisms from monosilane, and not from silylene. However, investigations are still in progress on these subjects.

Mouvement des particules solides en fluidisation liquide solide

Abstract Solid motion in liquid fluidized beds has been analysed using a radioactive particle and two detection devices partly superimposed, dividing the be Particle velocity was determined by two different estimations, using the net flux across a zone frontier and the residence time between two successive Particle dispersion coefficients, determined from residence time distribution, were also strongly increasing with increasing liquid velocity. It was co

Analysis and Modeling of Low Pressure CVD of Silicon Nitride from a Silane‐Ammonia Mixture: II. Deposition Modeling

This paper, Part II of this series of two articles, is devoted to two-dimensional modeling of silicon nitride low pressure chemical vapor deposition from a silane-ammonia mixture. In that aim, the CVD2 model previously developed in our laboratory, which takes into account hydrodynamics, mass transport, and chemical reactions, is adapted to this special kind of deposition. The gas phase mechanism described in Part I is integrated in the model. The concentration profiles of NH 3 , SiH 4 , SiH 2 , SiHNH 2 , SiH 3 NH 2 , and H 2 have been computed and the contribution of the different species to deposition rate have been evaluated. The results obtained confirm the importance of the entrance zone length of the reactor, which conditions the formation of the stable species SiH 3 NH 2 , generating SiH 2 and SiHNH 2 , which are responsible for the strong overthicknesses observed at the wafer periphery.

Two‐Dimensional Modeling of Low Pressure Chemical Vapor Deposition Hot Wall Tubular Reactors II . Systematic Analysis of Pure and Phosphorus In Situ Doped Polycrystalline Silicon Depositions

Using the new two-dimensional model, described in the preceding article, which takes into account hydrodynamics, mass transport, and chemical reactions, the special case of phosphorus in situ doped polysilicon deposition are simulated and analyzed. The main contribution to the deposition of in situ phosphorus-doped polysilicon must now be attributed to silylene

Thermal and kinetic modelling of low-pressure chemical vapour deposition hot-wall tubular reactors

A mathematical model, describing the complete behaviour of low-pressure chemical vapour deposition (LPCVD) reactors has been developed. The improvement of this model over existing ones reported in the literature consists in that it gives a more detailed account of the temperature profile on the solid surfaces in an LPCVD reactor. On the other hand, the proposed model is considerably simpler than most others already proposed, since it utilizes a continuously stirred tank reactor approach for mass balances. This approximation makes the model only relevant but, nevertheless, particularly useful to the case of depositions naturally leading to uniform layers across each wafer and, therefore, to the case of pure polycrystalline deposition from silane. A systematic use of this model, combined with a comparison of its predictions with experimental data, has served as an incentive for further investigations of other chemical vapour depositions systems (in situ boron-doped polysilicon and semi-insulating polyoxide silicon depositions) and is of interest for the choice of new industrial processes.

Analysis and Modeling of Low Pressure CVD of Silicon Nitride from a Silane‐Ammonia Mixture: I. Experimental Study and Determination of a Gaseous Phase Mechanism

This paper, Part I of two, presents the results of a study combining experimental and modeling approaches of low pressure chemical vapor deposition (LPCVD) of silicon nitride from a silane‐ammonia mixture. The experimental study consists in a reduced number of runs, chosen in order to identify the main features of the deposition process, i.e., marked nonuniformities at the wafer edge both in thickness and in Si/N composition. It is then shown that a complex gas‐phase mechanism may be responsible for the observed physicochemical phenomena. A gaseous reaction model is thus proposed for a silane‐ammonia mixture under typical low pressure CVD conditions. A complete reaction scheme is first studied. A thorough quantum Rice Ramsberger Kassel (QRRK) analysis compensated for the lack of kinetic information in the gas phase and allowed the identification of kinetic constants for uniand bimolecular reactions. Its appropriateness is examined with one‐dimensional nonsteady computations. A combined analysis of these calculations and of the QRRK results shows that the reaction model could be simplified, thus leading to a reduced reaction set reproducing the essential features of the full mechanism experimentally observed, which involves six species with two silylamine intermediates and . In Part II of this article series, this mechanism is integrated in a 2‐D model of LPCVD reactors, previously developed in the laboratory (called CVD2) and adapted to this kind of deposition, taking into account hydrodynamics, mass transport, and chemical reactions.

Analysis and Modeling of In Situ Boron‐Doped Polysilicon Deposition by LPCVD

The low pressure chemical vapor deposition of in situ boron-doped polysilicon films from a mixture of SiH 4 and BCl 3 was investigated in a tubular horizontal hot-wall reactor. Only when isothermal conditions and no distributed injection scheme are used, this process presents the following features: (i) the presence of the boron doping gas enhances the deposition rate of polysilicon; and (ii) a sharp drop in the polysilicon growth rate and boron concentration along the load is generally observed which may be detrimental to the reactor yield. Based on a combined approach of modeling and experimental results, an understanding of the complex physicochemical phenomena involved is presented. In particular, the accelerating effect of boron and the slowing down effect of hydrochloric acid, one of the products of chemical reactions, are demonstrated. The global model developed, based on a continuously stirred tank reactor approach for mass balances, has proven to be a valuable tool for determining the effects of process parameters, and specially the influence of the X BCl3 /X SiH4 ratio on deposition rate. An adequate solution for solving the practical problem of depositing boron-doped polysilicon on a large number of wafers with a satisfying uniformity is proposed, which requires a reactant injection scheme and temperature ramping

Comments on the influence of the entrance zone in LPCVD reactors for in situ phosphoros-doped polysilicon deposition

The objective of the article is to present several original results, obtained through the combined approach of experiments and two-dimensional modeling, which could suggest solutions in order to improve the choice of the operating conditions in an LPCVD (low-pressure chemical vapor deposition) reactor for this kind of deposition