Pergamon Chrmnd Engmeermq Scrence. Vol. 50, No. 22, 3615-3624, 1995 pp. Copyright 0 1995 Elsevier Scmce Ltd

Printed in Great Britam. All rights reserved

CKNW2509/95 19.50 + 0.00

0009-2509(95)00172-7

SILICON DEPOSITION FROM SILANE OR DISILANE IN A FLUID1ZE.D BED-PART I: EXPERIMENTAL STUDY

B. CAUSSAT’, M. HEMATI and J. P. COUDERC Laboratoire de GEnie Chimique, URA-CNRS 192, ENSIGC/INPT 18, Chemin de la Loge-31078 Toulouse

Cedex, France

(First received 16 November 1994; revised manuscript accepted 15 May 1995)

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 tines 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

1. INTRODUCTION

Many overall properties of powders are tightly related to the surface state of the individual particles that constitute them. This is the case of flowability, mech- anical or corrosion resistance, electrical charge, sin- terability, etc. The idea of modifying these properties by surface treatments appears then attractive. In that context, one of the interesting possibilities which has been tested only very recently, is chemical vapor de- position (CVD). Contacting the powder and the gas phase allows to coat each particle with a thin layer of a new material, developing original surface condi- tions, and hence, controlled properties of use. Of course, such a treatment must be performed in condi- tions preventing aggregation of particles by the de- posit; a possibility is to fluidize the solid particles by the gas flow.

Gas fluidization of solid particles offers two addi- tional important advantages: the first one is that fluidized beds are generally quite perfectly isothermal, which constitutes a basic interest for a chemical reac- tor; the second one is that particles are thoroughly mixed, which results in a quite perfect uniformity of surface treatment.

The negative consequences are that the particular problems linked to fluidized beds, like attrition, formation of gaseous bubbles, or instabilities, come into the process, and must be solved in connection with those related to CVD.

‘Corresponding author.

The particular case of silicon deposition from silanes at atmospheric pressures which has been ana- lyzed in this work, presents two main possibilities of industrial application. The first one considers thin silicon films deposited as able to protect either the fluidized particles themselves or surfaces immersed in the bed, from oxidation or corrosion (Sanjurjo et al., 1991 +this has been particularly tested in the case of copper substrates. In the second application, fluidized-bed silane pyrolysis constitutes an interest- ing alternative way to produce ultrapure silicon, for microelectronic and photovoltaic uses, by growth of bed silicon particles. In the conventional Siemens process, silicon is deposited on extremely hot fila- ments (T = 115O”C), by hydrogen reduction of diluted trichlorosilane. The fluidized-bed path, for which the reactive gas most often selected is monosilane, pres- ents numerous advantages, over the classical process, such as higher throughput, lower-cost technology, and the possibility of continuous operation.

A small number of experimental and theoretical studies (Rohatgi, 1986; Kojima and Morisawa, 1991; Kojima et ul., 1992; Rohatgi et al., 1982) have already been performed. They have demonstrated the feasibil- ity of such deposits from monosilane, and the high throughput of this process: more than 90% of the silicon in feed is deposited on the bed particles. How- ever, they have pointed out two significant problems: a parasitic formation of fines during experiments, and particles agglomeration phenomena, when the initial concentration of silane exceeds a critical value.

3615

3616 B. CAUSSAT et al.

For Rohatgi et al. (1982), about 10% of silicon in a basic operation for solar-cell and semiconductor the silane feed is systematically converted into fines, industries. Despite this enormous effort, the phe- for inlet silane concentrations around 40% vol. How- nomena occurring during this deposition are not en- ever, for Kojima et al. (1992), this percentage has tirely understood. Silicon deposition from silane and varied from 1 to 25%, according to the operating disilane involves both heterogeneous reactions, lead- conditions, ing directly to silicon deposition on the substrate

Whenever the inlet concentration of silane exceeded surfaces, and homogeneous decomposition reactions. 20% vol, Kojima and Morisawa (1991) and Kojima The latter produce additional chemical species, them- et al. (1992) have observed systematically at least selves able to contribute to deposition. The mecha- a partial particles agglomeration, in their 5 cm ID nisms involved in these transformations are so corn- reactor. Conversely, Rohatgi et al. (1982) have suc- plex that only the simplest homogeneous decomposi- ceeded in maintaining good fluidization conditions tions and the most elementary surface reactions are during 8 h, for an inlet silane concentration of 50% established unanimously in the literature. vol, in their 15.4 cm ID reactor, whereas they have Most authors stand that beyond 370°C, mono- encountered severe problems of partial bed clogging, silane begins to decompose in the gaseous phase fol- in a 5 cm ID reactor, lowing reaction (R1):

Almost all authors in the literature agree on the fact Sill4 ~ Sill2 + H2. (R1) that silane pyrolysis in a fluidized bed leads simulta- neously to heterogeneous silicon chemical vapor de- According to several studies (Coltrin et al., 1989; position, and to homogeneous silane decomposition. Giunta et al., 1990; Kleijn, 1991; Azzaro, 1991), the

Rohatgi (1986) has suggested that silicon deposition extremely reactive silylene radical Sill2, then, gives on particles is due to both heterogeneous reactions, rise to a series of reactions, producing silanes of higher and catching of silicon nuclei, formed by homogene- order. These reactions can be represented by

ous silane pyrolysis. This author thinks that fines Si,H2n+2 + SiH2-mSin+tH2,+,. (R2) result from silane homogeneous nucleation, leading to silicon nuclei formation, and later, growth into fines. Most authors agree on the fact that polysilanes of It is important to remark that no more precise mecha- order higher than four are produced in very limited nism has been proposed to explain this formation, or quantities, and can reasonably be neglected in a rep- the particles agglomeration phenomenon, resentation of the gaseous phase.

The aim of this study was to develop a better insight The elementary steps leading to the formation of into silicon deposition from silanes in a fluidized bed. tetrasilane are the following:

In particular, the two problems which limit the indus- Sill , + Sill2 ~ Si2H6. (R3) trial progress of this process have been investigated. Furthermore, fines constitute a potential problem for Once formed, disilane decomposes following the all CVD processes in which silanes are employed as previous reverse reaction, at temperatures roughly reactive gases. These parasitic formations are parti- 100°C lower than monosilane pyrolysis, or for higher cularly undesirable for microelectronic applications, temperatures, by another way producing silylsilylene where contaminations of substrates must be reduced and hydrogen: to extremely low levels. So, the comprehension of their formation presents a wider interest than for Si2H6 ~ Si2H4 + H2. (R4) fluidized-bed CVD only. Yuuki et al. (1987) and Fayolle (1993) consider that

The first part of this article presents a new experi- this reaction is of minor importance, and neglect it. mental study of fluidized-bed silicon deposition on Disilane can also react with silylene, to form powders by silane and, for the first time, disilane trisilane: pyrolysis. Disilane has been employed as a silicon source, because the chemical mechanisms involved Si2H6 + Sill2 ~ Si3Hs (R5) during its pyrolysis differ strongly from those occur- and then tetrasilane: ring with silane. These mechanisms will be detailed in Si3Hs + Sill2 ~ Si,Hlo. (R6) the next paragraph. The experimental apparatus and operating procedure will then be described. After that, On solid surfaces, only a limited amount of in- the main results for silane and disilane pyrolysis will formation is available concerning silicon deposition be presented and discussed. Finally, first explanations from silanes. Thus, the direct contribution of poly- about fines formation and bed agglomeration phe- silanes to silicon deposition, or that of species formed nomena will be suggested, opening towards the theor- by their homogeneous decomposition, is not yet etical analysis which will be developed in the second clearly established. However, most authors (Coltrin part of this article, et al., 1989; Azzaro, 1991; Fayolle, 1993) think that

these compounds do not participate directly to her- 2. CHEMICAL MECHANISMS INVOLVED DURING SILANE erogeneous reactions in a significant proportion.

AND DISILANE PYROLYSIS Conversely, several studies have been performed to Numerous authors have studied silicon chemical quantify the heterogeneous decomposition of mono-

vapor deposition from silane, because it constitutes silane at atmospheric pressure. It is supposed to occur

Silicon deposition from silane or disilane in a fluidized bed--I 3617

by the following overall reaction: collect elutriated particles or fines formed during ex-

Sill4 --, Si + 2Hz. (R7) periments. Then, gases have been cooled, and their hydrogen content has been measured with a hydrogen

A few kinetic equations can be found in the litera- detector (HYDROS 100-Rosemount). It is to be noted ture about it (Azzaro, 1991). ° that the output signals delivered by this apparatus,

Lastly, concerning silylene Sill2, and more gener- and by thermocouples (1) and (2) have been continuous- ally polysilylenes Si.H2., all authors consider that ly recorded, during each run, using a PC computer. their heterogeneous decomposition rate is infinite.

3.2, Solids and gas used This assumption means that their concentration close

Monocrystalline alumina particles (99.8% A1203) to solid surfaces is approximately equal to zero.

have been employed as seed particles, instead of ultra- pure silicon powder, with the aim to facilitate accurate

3. EXPERIMENTAL ANALYSIS analysis of silicon deposits. It has been previously 3.1. Equipment verified that deposition mechanisms were not modi-

The experimental unit which has been used is sche- fled by this change in the nature of seed particles (see matically represented in Fig. 1. The reactor is entirely Caussat, 1994). Table 1 summarizes several physical constructed from a 304 1 stainless steel, and its cylin- properties of the seed particles. drical zone has dimensions of 5.3 cm ID and 63 cm Mixtures of electronic-grade monosilane or disilane height. It has an expansion zone of 10 cm ID, to allow and nitrogen have been used as fluidizing gases. entrained particles to drop back into the bed.

The gas distributor is a stainless-steel perforated 3.3. Operating procedures Initially, a weighted amount of particles is loaded

plate, with 119 holes of 0.5 mm in diameter, its overall into the reactor. The bed is then fluidized with a con- porosity is 1.2%. To keep silane temperature below

stant flow rate of pure nitrogen and heated. As soon as 450°C, and so to prevent any premature partial de-

the thermal regime is reached, the recording of tern- composition of silane at the distributor level, flanges are water cooled, as recommended by Hsu et al. peratures and hydrogen concentration is started.

After 10 min, the flow rate of nitrogen is adjusted to (1984), for whom this cooling represents one of the

the wanted value, and simultaneously, silane or di- keys to successful silicon depositions, silane is introduced into the bed.

Five chromel-alumel thermocouples, 0.5 mm in The deposition efficiency has been characterized by diameter, have been fixed to a thin rod, and situated

the following parameters: respectively at (1)2, (2)13.5, (3)33, (4)64 and (5) 73 cm above the distributor, on the central axis of - - the average deposition rate, calculated from the the bed. The reactor is externally heated by a 2.6 kW bed weight increase as measured at the end of each electrical furnace. Temperature control has been per- experiment; formed from thermocouple (2) readings, using a PID - - the average outlet percentage of hydrogen 35m, regulator, indicated by the hydrogen detector. For comparison

Monosilane or disilane and nitrogen have been purposes, a theoretical hydrogen percentage Y~2 is supplied to the bed through rotameters, Effluents calculated, assuming that all silane or disilane mol- have been treated using a cyclone and a bag filter, to ecules has been converted;

(EXTRACTION)

] ' t r D i p ~ e

/

t \ J - C F ~,~ r Sill4

rN2 C: cyclone f: in line filter CF: cooling flange R: fluidized bed reactor

by water circulation r: rotameter D: hydrogen detector S: cooling serpentine F: bag fi ter t: thcrmocouple

Fig. 1. Experimental apparatus.

3618 B. CAUSSAT et al.

Table 1. Physical properties of seed particles

Umf Umf Umf dp Pe (cm/s) (cm/s) (cm/s)

(~m) q~p (kg/m 3) 25 °C 500 °C 600 °C

82 0.73 3900 2 1.4 1.3

Table 2. Experimental results for silane runs

f Meo Ho Y0 Qtot d, ya 2 Y~z xrinc~ Rd Particles Run (°C) (g) (cm) (%) Uo/U,I (nl/h) (rain) (%) (%) (%) (pro/h) bed state

S1 596.8 1157 26.2 3.35 5.4 209.9 20 4.6 6.48 - - - - OK $2 606.8 1338.6 30.6 5 5.47 210.5 180 10.2 9.52 0.86 0.45 OK $3 608.7 1352.7 31.4 10 5.8 222.2 60 20.4 18.2 0.46 0.94 OK $5 609.3 1358.1 33.2 14 6.07 232.8 60 26.1 24.5 0.42 1.37 OK $6 596.9 1322.9 30.7 16.27 5.53 215 35 27.1 27.9 0.86 1.52 Aggregated at the

bottom $7 562.6 1351.1 31.2 20 6.17 250 17 34.5 33.3 1.24 2.13 Slight cylinder

around rod above the bed

$8 597.1 1332.7 30.9 25 6.74 266.6 14 40 40 0.27 2.88 Slight cylinder around rod

above the bed $9 546.5 1339.6 30.8 10 5.38 222.2 47 18.4 18.2 1.6 0.95 Aggregated at the

bottom S10 680.5 1335.4 30.3 25 * 8.14 266.6 6 41,4 40 2.87 2.89 Slight cylinder S12 596.1 2046 48 25 6.85 266.6 17 40.8 40 1.56 1.88 Aggregates at the top

of the bed S13 599.5 1263.3 29.8 9.28 8.56 276.5 65 17.5 17 2.41 1.16 OK S14 598.4 1315.7 30.5 9.29 17.06 551 36 18 17 1.66 2.22 OK

- - the percentage of fines collected XFin,s, corres- tor; without silane, the maximum thermal gradient ponding to the ratio between the weight of fines col- remained always lower than 15°C. This important letted in the filters, and the total weight of silicon phenomenon has not been reported by previous introduced into the reactor during the experiment, authors. In more details, Fig. 2 shows the time vari-

ations of temperatures into the bed, and of the outlet 4. SILICON DEPOSITION FROM SILANE hydrogen concentration, for run $5. It appears clearly

4.1. Experimental results that silane injection produces very rapidly a marked Table 2 lists the experimental conditions of the 12 change in the organization of bed temperatures: lower

runs performed, and the corresponding results. It is regions become colder and higher regions hotter (T3 important to specify that the average bed temperature has reached 660°C for run $5). Moreover, local data has" been assumed equal to the value measured by fluctuate then markedly around their mean value. thermocouple (2). Moreover, whenever bed agglomer- These characteristic trends disappear rapidly after ation occurred, only values before clogging have been silane flow has been stopped. These features will be retained, discussed in more detail afterwards.

Several important observations deserve to be men- Furthermore, in complete agreement with previous tioned, authors, two important problems have been observed.

First of all, deposition has been possible without Firstly, fines have been found systematically adher- any probleca during 5 runs, with a quite total silane ing to the reactor wall, above a constant level in the consumption and average deposition rates around column, during all the runs performed. This particular 2 t~m/h. This constitutes a very encouraging result, level corresponds to the maximum height for which which demonstrates that CVD on powders in the external furnace radiates directly on the walls. It is fluidized beds, works conveniently, and confirms all to be noted, that under this level, a metallic and dense the conclusions obtained by previous authors: the silicon deposition was visible on the reactor walls. equipment and operation are cheap and simple; silane Fines were also present in the lines and apparatuses conversion is high, and the deposit uniformity on disposed downstream, with, of course, a greater particles satisfactory, as proved by microscopic obser- amount in the bag filter. Their appearance was that of vations (see Section 6). a brown residue, both pulverulent and oily. More

However, an important point to note is that a ther- details will be provided on tlae~r morphology in another mal gradient of about 80°C appeared systematically paragraph. Their percentage remained, in all cases, between the top and the bottom of the bed of par- lower than 3% of the total amount of silicon injected, ticles, as soon as silane was introduced into the reac- which is quite low in comparison with literature re-

Silicon deposition from silane or disilane in a fluidized bed--I 3619

O T 13,5 cm above distributor - - % H2 - - -o - T 2 cm above distributor

30 t t I 680

2 5 - . ~ 635

2 0 - . ~ '

15 590 - !

5 - "f,/" !

o / I I ~ 5oo 0 / ~ 19.5 39 58.5 ~ 78

Time (min)

Fig. 2. Time variations of temperatures and outlet hydrogen concentration for run $5.

Suits. Moreover, for all experiments, fluidized-bed - - o - T 13.5 cm above distributor particles were hardly found in the cyclone or the filter, - - % H2 ~ T 2 cm above distributor which means that no significant elutriation happened. 30 I - - - q - - f t---~- ~ 7oo

Secondly, for inlet percentages of silane higher than - - - - ~ ( ~ , k ', •' 651) 20%, bed agglomeration took place systematically. 25 These particles aggregations caused a sudden marked 20 ~ ~i,~,,,.: ~ / ~ ! 6o0 , i change in temperatures distributions, as shown in i 550 Fig. 3, for run $6. When such perturbations occurred, ~ 15 ~ ' , 500

• ,450 ~ the flow of silane was stopped immediately, for secur- I0 ~ ity reasons (bed agglomeration induces progressively 5° i ~ "' 4 41)0 ~ a rise in pressure at the reactor entrance). It is impor- "'. ~, ~ 3511 tant to note that, in this case, temperatures did not P I - - + - - - - + - ' , ~ ' " f~ 30o recover their original values after silane flow had been 0 10 20 30 40 50 ~ 60 70 stopped, as a consequence of the irreversibility of the Time (min) aggregation phenomena. For these runs, quite always, Fig. 3. Time variations of temperatures and outlet hydrogen a small cylindrical agglomerate appeared on the ther- concentration for run $6. mocouples rod, at least 10 cm above the level of the initial fixed bed. This suggests that agglomeration starts first on fixed internals, when deposition rate is high (Yo and/or T high). In these cases, the ratio of ponding to pressure vs time signals have been sam- fines collected increased only slightly with regard to pled, at a frequency of 30 Hz. During each sampling the previous results, probably because the flow of run, 2048 pressure data have been recorded, and ac- silane has been rapidly stopped, after the bed agglom- quisitions have been repeated several times, to in- eration began. The previously presented observations, crease accuracy. concerning fines morphology and localization, remain The power spectral density function, and the auto- valid for these experiments too, correlation function of the pressure drop fluctuations

have been determined, using a Fast Fourier Trans- 4.2. Characterizat ion o f f luidization quality form algorithm. durin9 runs Two specific experiments have been realized;

It is well known that fluctuations of the pressure Table 3 lists the mean pressure drop values, and their drop across a fluidized bed are directly related to the mean deviations. As an example, Fig. 4 shows the power motion of bubbles and of solids inside the bed (Clark spectral density function for run S13, before, during et al., 1991). So, with the aim of analyzing more and after injection of silane into the reactor• accurately the bed fluidization state of quality during Table 3 presents the pressure drop values and their CVD experiments, a fast response differential pressure mean deviations; a very small increase has been ob- transducer has been used. The low-pressure tap has served for these two values, when silane is injected been located between the reactor exit and the cyclone, into the bed, but which seems not to be significant. and the high-pressure tap has been situated just before It is interesting to observe that there is a correct the reactor entrance. Voltage vs time signals, cortes- agreement between the measured and theoretical

CES 50-2E-I

3620 B. CAUSSAX et al.

Table 3. Mean pressure differences and mean deviations for silane runs

Run S13 Run S14

N2 alone N2 + Sill4 N2 alone N2 alone N2 + Sill4 N 2 alone before 30 min after 10 min after before 7 min after 14 min after

injection injection injection stop injection injection injection stop

AP (mbar) 53.4 56.9 55.1 57.6 63.1 59.6 APth (mbar) 55.9 - - 57.6 58.5 - - 59.8

Mean deviation 6 6.7 5.8 15 16.5 14.9

1 . 2 [ I I values (weight of solid particles per unit area of cross section); the slight differences observed can come from

N2 alone several facts: firstly pressure taps have been inserted 0.8-~- before inieetion before the entrance and after the exit of the bed, and

secondly, a nonnegligible part of the solid particles 0.6 - can be defluidized at the distributor level, between the

o holes. 0 .4- Much more important are the results of the Fourier

analysis. The power spectral density function proves 0.2- ~ _ _ that the behavior of the bed has been notably modi-

fied by silane. Effectively, before its injection, domi- 0 . . . . . . . L, nant frequencies of the signals are between 3 and 5 Hz,

0 5 10 15 which indicates that a pre-slugging regime has been Frequency (Hz) reached. With silane, the spectrum becomes larger,

and shifts toward lower frequencies. No dominant frequencies are visible, which means that the signals become less regular. After silane injection has been

1.2 I [ stopped, the signals do not recover exactly their orig- inal behavior, but become more periodic.

1 N2.+SiH4 6min These results strongly suggest that the bubble 0.8 after iniection movements inside the bed become much more irregu-

lar with than without silane. It is the first time, to our 0.6 , t knowledge, that such a strong correlation between

~- ~ fluidization quality and the fluidizing gas nature or (I.4 reactivity has been shown.

(1.2 5. SILICON DEPOSITION FROM DISILANE

0 ~ , Disilane is an expansive product, difficult to handle; 0 5 10 15 for these reasons, only two experiments have been

Frequency (Hz) performed, with a very bad precision. Table 4 lists the experimental conditions of these

two runs, and the corresponding results. The great inaccuracy of these experiments appears clearly in this

1,2 [ [ table, because the operating conditions selected for the two runs were similar, and the corresponding

1 N2 alone 15rain after results are quite different. Despite this, these experi- iniection stop

(I.8 ments have been exploited, because they have pro- duced important qualitative information.

0 .6- The comparison of the measured percentages of o

hydrogen at the exit of the bed 17H2, and the calculated (1.4- I l l one Y~2 suggests that disilane conversion in the bed

always remained incomplete. This must certainly be Ik 0.2- linked to the low value of the operating temperature. ,4 . . . . . . On the contrary, the fines percentage has been

0 , b clearly greater than during silane experiments. In ef- 0 5 10 15

Frequency (Hz) fect, for the two runs, the amount of fines was so important that dust obstructed the exit lines of the

Fig. 4. Power spectral density functions for run S13 . reactor, which induced a rise in pressure. However,

Silicon deposition from silane or disilane in a fluidized bed--I 3621

Table 4. Experimental results for disilane runs

M~o Ho Yo Qtot d, 37H 2 37* 2 XFi,e~ Rd Particles Run (°C) (g) (cm) (%) Uo/U.y (nlfh) (rain) (%) (%) (%) (~m/h) bed state

Dl 480 .2 1294.3 29.4 9 8.7 364 15 19.5 24.8 23.9 1.67 OK D2 485 .7 1331.4 30.9 8.2 7.9 327 20 13 22.7 8.4 1.19 OK

the zone of apparition of these fines remained exactly $5. Figure 8 shows SEM pictures of fines collected in the same as for the silane experiments, above the the bag filter after run S15, and Fig. 9 those of fines furnace, present on a small plate, fixed horizontally 10 cm

It is important to note that no significant thermal under the reactor top, during run $9. gradients appeared in the bed during disilane runs, as Silicon coatings on bed particles look massive and proved by Fig. 5. This suggests that changes in the nodulous. This granular appearance becomes more fluidization quality due to the injection of disilane was marked, if the inlet concentration of silane is high, like much more attenuated than for silane, during run $5, performed with an inlet concentration

of silane three times higher than during run $2. 6. SEM ANALYSIS OF SILICON DEPOSITS AND FINES The fines morphology differs depending on the lo- Figures 6 and 7 show several scanning electron cation where they have been collected: in the bag

microscope (SEM) pictures of silicoh coatings on bed filter, their appearance is similar to that of silicon particles, as obtained, respectively, during runs $2 and coatings, with however, several differences, such as

smaller nodulous, and less massive appearance. Con- versely, fines collected on the reactor walls have a particular morphology: they appear very light and

o T 13,5 cm above distributor formed of micronic entities of matter, connected by - - %H2

• o- T 2 cm above distributor thin tentacular filaments. The micronic parts of mat- 16 I- /--~ ~ ~ - - - ~ ~ 520 ter have a morphology similar to that of fines col- 14 ' ~ - l ~ " , , ~ ' x t lected in the bag filter. 12 ~ " ~ - ~ 477.5 ~ The appearance of silicon deposits on bed particles 10 ~- I - - t ''~':1 ~ and of fines collected after run D2 with disilane, is 8 ~- I t ~ 435 ~ completely similar to that observed for silane experi- 6 ~ ments. This is why these SEM photographs are not

~ I I I I presented here. 392.5 ~ So, the morphologies observed here are similar to those previously presented in the literature (Hsu et al.,

0 350 1984; Kojima and Morisawa, 199l), excepted for the 1 ~ 6 ~ 3 fines collected directly into the reactor, which have an

Time (min) appearance completely new. This important result is Fig. 5. Time variations of temperatures and outlet hydrogen not yet completely understood; it could be essential to

concentration for run D2. explain the fines formation.

Fig. 6. SEM pictures of silicon coatings after run $2.

3622 B. CAUSSAT et aL

l ~ t a ~5, O1"30

XlO,OOO

Fig. 7. SEM pictures of silicon coatings after run $5.

Fig. 8. SEM pictures of fines collected in the bag filter after run S15.

I ~ XIO.OO~

Fig. 9. SEM pictures of fines collected in the reactor after run $9.

Silicon deposition from silane or disilane in a fluidized bed -I 3623

7. RESULTS AND DISCUSSION Furthermore, some authors (Baron et al., 1992) 7.1. Thermal perturbations and bed agglomeration stand that electrostatic forces in a high temperature

It is important to remember that thermal distur- fluidized bed cannot exist. bances occurred only for experiments with silane, and So, in our opinion, in the present state of know- not disilane. By analyzing the pressure fluctuations ledge, the more plausible explanation for the increase across the fluidized bed, it has been clearly seen that in particle cohesiveness is certainly that related to the the injection of silane induces modifications of the bed adsorption of reactive species on particle surfaces, behavior, and more precisely, a kind of partial deflu- during the deposition process. In all cases, it is clear to idization, us that phenomena relative to thermal disruptions,

We think that this phenomenon is due to an in- modification of the bed fluidization and agglomer- crease in the surface cohesiveness of the particles, ation are inter-connected, and are directly related directly induced by the presence of silane; this leads to with a change in the surface state of the particles, due a rise in interparticle forces, and hence, in a decrease to the presence of silane. in particle mobility and in the mixing intensity in- The fact that for disilane runs these phenomena side the bed. Consequently, silane injection into the were limited can be due to the lower temperatures reactor leads to local and fleeting interparticle ag- employed, and to the differences existing in the chem- glomerations, responsible for the observed thermal ical mechanisms involved. This point will be con- perturbations, sidered more completely in Part 1I.

However, these phenomena remain reversible as long as the interparticle forces, related to the surface state of the particles, are smaller than the disintegra- tion ones, which depend on the intensity of the par- ticle movements in the bed. In the opposite case, 7.2. Fines formation which takes place when the deposition rate is high, The important new experimental evidence pro- and/or when the particle circulation in the bed is not duced by this work is that fines are present only intense enough, the agglomerate formation speeds above a precise level in the reactor, located above up, and the corresponding aggregates tend to solidify, the fluidized-bed surface and the furnace. It demonstra-

This increase in particle surface cohesiveness can tes then, in complete contradiction with previous have several origins. The following hypotheses have authors, that no fines form inside the bed, either in the been proposed: emulsion phase or inside bubbles.

What seems to happen is a fines formation on cold - - the presence of silane in the bed may induce the surfaces of the reactor, and then a transport by the gas

apparition of chemisorbed gas layers on particle sur- flow towards downstream regions, thanks to their faces, which can act as a glue for the solids. These pulverulent nature. In these conditions, the main adsorbed layers could lead to the existence of hanging mechanism we can suggest for fines formation is chemical bindings, during silicon deposition; chemical reactions, or maybe condensation, of active

- - polarized compounds may cover the particle species existing in the gas phase leaving the fluidized surface during deposition, leading to the apparition of bed. These reactions would take place on surfaces too repulsive electrostatic forces between particles; the cold to produce metallic and dense silicon, and for this volume allocated to the solids being limited, this phe- reason, they would give rise to a brown pulverulent nomenon would bring about agglomerates; and oily product we call fines.

finally, parasitic formation of fines inside the Of course, it is possible that limited amounts of bed, which however, has never been demonstrated, these fines were formed in other ways. In particular, may cause the apparition of interparticle bridges, particle attrition could play a part, because it is

a common phenomenon in high temperature fluidized As reported in the Introduction, Hsu et al. (1984) beds, but, it cannot be a preponderant mechanism.

think that silicon deposition is due to both heteroge- In all cases, according to the results obtained, it is neous reactions and catching of silicon nuclei formed obvious that the theory of polymerization or nuclea- by homogeneous silane pyrolysis. They suppose then, tion inside the fluidized bed, stated by several authors, implicitly, that fines appear inside the bed. But, first, must be rejected. the results previously obtained are in complete con- Finally, the percentages of fines collected in the tradiction with this theory (see next section). Second- cyclone and the bag filter during this work have been ly, this explanation does not seem very realistic: the clearly smaller than those in the literature. Such a dif- heterogeneous deposition kinetics and that of the fines ference can be explained by the fact that the particles formation and of their capture by the bed particles fluidized here are much smaller than those used in cannot be in a ratio such that the spaces between previous works. The surface area free for deposition, nuclei were entirely filled into the deposit. Moreover. per unit volume of bed, is therefore more important in according to Kojima et al. (1989), the overall activa- our case, which implies a better heterogeneous con- tion energy of particles growth in the bed corresponds version of silane in the bed. Consequently, the residual to that obtained in a fixed bed, essentially for hetero- amount of reactive gas available to be converted into geneous phenomena, fines is lower.

3624 B. CAUSSAT et al.

8. CONCLUSION Qtot overall gaseous flow, nl/h This experimental study has provided a lot of im- Rd deposition rate, #m/h

portant information. Firstly, for the experiments with T characteristic temperature of the fluidized monosilane, as long as the inlet concentration of the bed, °C reactive gas remained smaller than 20%, no agglom- Uo inlet superficial gas velocity, cm/s eration took place, and the conversion was maximum. U,,y superficial gas velocity at the minimum flui- In all cases, the fines ratio never exceeded 3%, which dization state, cm/s is quite small, in comparison with the literature data. Xri~es ratio of collected fines, % These very encouraging results demonstrate that sill- YH2 experimental molar fraction of hydrogen, di- con CVD deposition on powders in a fluidized bed is mensionless possible and efficient, y*2 theoretical molar fraction of hydrogen, di-

We have also shown variations in the fluidization mensionless quality, due to the injection of silane, and to the Yo inlet molar fraction of reactive gas, dimen- subsequent chemical reactions between this gas and sionless the particles. This phenomenon has been connected to the thermal disturbances, and also to the bed agglom- Greek letters eration, by the fact that the presence of silane in the AP mean pressure drop undergone by the reactor seems to modify particle cohesiveness. The fluidizing gas during its crossing of the par- more plausible explanation for this modification is the ticles bed, mbar adsorption of reactive species on particle surfaces, A P t h theoretical pressure drop calculated from which could act as a glue for solids, the weight of solid per unit area of cross

It is to be noted that such observations are corn- section, mbar pletely new in the fluidization field, and are of great eml void fraction of the bed at the minimum interest, fluidization state, dimensionless

The feasibility of silicon coatings by CVD from ~be mean shape factor of particles, dimension- disilane in a fluidized bed has also been demonstrated, less For these disilane experiments, no thermal distur- PP density of particles, g/cm a bances appeared, but larger amounts of fines were formed. REFERENCES

An important point to note is that fines were pres- Azzaro, C., 1991, Ph.D. thesis, I.N.P. Toulouse. ent at the end of all experiments, on the reactor walls, Baron, T., Briens, C. L., Hazlett, J. D., Bergougnou, M. A.

and Galtier, P., 1992, Can. J. Chem. Engng 70, 631. above the bed surface and the furnace. Therefore, the Caussat, B,, 1994, Ph.D, thesis, I.N.P. Toulouse. hypothesis of chemical reactions of gaseous species on Clark, N. N., Mc Kenzie, Jr, E. A. and Gautam, M., 1991, cold surfaces appears to be the most realistic mecha- Powd. Technol. 67, 187. nism for fines formation. In all cases, our results are in Coltrin, M. E., Kee, R. J. and Evans, G. H., 1989, J. Elec-

trochem. Soc. 136(3),819. complete contradiction with previous works, for Fayolle, F., 1993, Ph.D. thesis, I.N.P. Toulouse. which fines were formed homogeneously in the Giunta, C. J., Mac Curdy, R. J., Chapple-Sokol, J. D. and fluidized bed. Gordon, R. G., 1990, J. Appl. Phys. 67(2), 1062.

However, for this problem, as for the agglomeration Hsu, G., Hogle, R., Rohatgi, N. and Morrison, A., 1984, one, a lot of elements remain unknown, and experi- J. Electrochem. Soc.: Solid State Sci. Technol. 131(3), 660.

Kleijn, C. R., 1991, J. Electrochem. Soc. 138(7), 2190. ments are still in progress. The theoretical analysis Kojima, T., Hiroha, H., Iwata, K. and Furusawa, T., 1992, and modeling of silane and disilane pyrolysis, which Int. Chem. Engng 32(4), 739. will be presented in the second part of this article, will Kojima, T. and Morisawa, O., 1991, Proceedings of the 8th provide new information on silicon deposition pro- European Conference on CVD, Glasgow, Scotland (Edited cesses by CVD from silanes in fluidized beds. It will by Hitchman and Archer), C2-475.

Kojima, T., Iwata, K. and Furusawa, T., 1989. J. Chem. help to analyze more accurately all the phenomena Engng Jpn 22(6), 677. observed during the experimental study which has Rohatgi, N., 1986, Silicon production in a fluidized bed just been described, reactor: Final Report, J.P.L.D.E.O. 1012-1123.

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D. K., Mc Kube, M. C. H., and Song, H. K., 1991, Surf Ho height of the fixed bed, cm Coatings Technol. 49, 103. Mpo weight of particles present in the fluidized Yuuki, A., Matsui, Y. and Tachibana, K., 1987, Jpn J. Appl.

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