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Low temperature ZrB2 remote plasma enhanced chemical vapor deposition
Thin Solid Films 359 (2000) 68±76 www.elsevier.com/locate/tsf
Low temperature ZrB2 remote plasma enhanced chemical vapor deposition J.F. Pierson a, T. Belmonte b, T. Czerwiec b,*, D. Hertz c, H. Michel b a
b
Laboratoire de MeÂtrologie des Interfaces Techniques, PoÃle Universitaire, 4 place Tharradin, BP 427-25211 MontbeÂliard Cedex, France Laboratoire de Science et GeÂnie des Surfaces (UMR CNRS-INPL-EdF 7570), Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France c Framatome, 10 rue Juliette ReÂcamier, BP 3083-69398 LYON Cedex 03, France Received 11 June 1999; received in revised form 7 September 1999; accepted 7 September 1999
Abstract Deposition of zirconium diboride ®lms on Zircaloy-4 substrates at 733 K over 20 cm is carried out by remote plasma enhanced chemical vapor deposition (RPECVD). Different post-discharge compositions (Ar±H2, Ar±H2±BCl3 and Ar±BCl3) are tested in several process con®gurations. Experiments performed by thermal CVD and RPECVD with Ar±H2 post-discharge show that the deposition of ZrB2 ®lms on oxidized Zircaloy-4 is impossible at temperature lower than 853 K. Ar±H2±BCl3 post-discharges do not give to adherent ®lms on oxidized Zircaloy-4 at a temperature lower than 753K. It is shown that ZrB2 thin ®lms can be synthesized by using ¯owing Ar±BCl3 microwave postdischarges. Chlorine must etch the zirconia protective layer before zirconium diboride is synthesized. Therefore, the control of thickness of this zirconia layer by a previous oxidation treatment gives homogeneous deposition. The structure of the ®lms has been determined to be nanograins of ZrB2 dispersed in an amorphous solid solution of boron and zirconium oxides. The origin of the boron species incorporated in the ZrB2 ®lms is attributed to the eching of the quartz tube by chlorine. q 2000 Elsevier Science S.A. All rights reserved. Keywords: ZrB2; Chemical vapor deposition; Post-discharges; Structural characterization
1. Introduction Zirconium diboride (ZrB2) has several properties which make it a very interesting material for a large variety of industrial applications. It is a refractory compound with a melting temperature higher than 3300 K [1]. This material has good resistance against low temperature oxidation with the formation of a thin protective oxide layer [2]. ZrB2 has also the lowest bulk resistivity (9 £ 1028 V.m) among metal borides [3]. It could then be used for applications in semiconductor device fabrication as it has also excellent diffusion barrier performance [4]. The crystal structure of ZrB2, which is dominated by the two-dimensional network boron con®guration, favors high hardness (22 GPa) [5]. Its thermal expansion coef®cient (5:5 £ 1026 K 21) is very close to that of zirconium (5:8 £ 1026 K 21). The cooling of zirconium substrates coated by ZrB2 ®lms induces therefore, weak thermal stress in these ®lms. Boron containing compounds are also attractive in nuclear technology as neutron absorbers [6]. In this case, ZrB2 ®lms have to be synthesized on * Corresponding author. Tel.: 133-03-83-58-42-52, fax: 133-03-83-5347-64. E-mail address: [email protected] (T. Czerwiec)
the inner surface of Zircaloy-4 tubes. To preserve the mechanical properties of these tubes, high temperature processes can not be used (T , 733 K). For these reasons, among the different processes used to synthesize zirconium diboride coatings [7±13], a remote plasma enhanced chemical vapor deposition (RPECVD) process was chosen. In this paper, we report on the in¯uence of the postdischarge composition of the RPECVD process on low temperature zirconium diboride deposition. Four con®gurations are studied corresponding to different con®gurations of gas distribution. In each case, conclusions are drawn to clarify the advantages or disadvantages of the studied con®guration.
2. Experimental set-up The hot-wall RPECVD apparatus used in the present study was designed to enable both thermochemical [14,15] and CVD treatments [16] enhanced by remote microwave plasma. The system consists of four distinct parts: an in-situ chlorination chamber to synthesize ZrCl4 (CL in Fig. 1), a microwave generator coupled to a surfaguide wave launcher (SWL), a heating device with a deposi-
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0072 1-X
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Fig. 1. The RPECVD experimental set-up. The different con®gurations are described in the table where the investigated ranges of the main parameters are listed. Bold values are conditions referred to in the text when no details are provided.
tion chamber designed for cylindrical and ¯at substrates (HD) and a primary pumping unit (RP). The in-situ chlorination chamber is a 19-mm ID quartz tube into which high purity zirconium chips are loaded. Chlorination is performed at 623 K by introducing a chlorine and argon mixture with ¯owrates of 5 and 17 sccm (standard cubic centimetres per minute: 1 sccm 1:667 £ 1028 m 3/s), respectively. Under these conditions, the overall reaction Zr 1 2Cl2 ! ZrCl4 is complete as con®rmed by weight loss measurements on the chips. The ZrCl4 feed line to the reaction chamber is heated to prevent condensation of the halide vapor. For all experiments, the ZrCl4 ¯owrate is kept constant at 2.5 sccm. The plasma is produced in a 5-mm ID quartz tube (DT) by a 2.45-GHz microwave generator with a surfaguide surface wave launcher located upstream from the deposition chamber, approximately 1.2 m from the substrate location. By choosing such a large distance, it was expected to develop a process that could be extended to the treatment of inner surfaces of long cylindrical substrates. About 20 cm beyond the plasma gap (Fig. 1), the microwave discharge tube opens into a 15-mm ID quartz tube (ST) which enables the separation of the reactive gases before their mixing in the reaction zone. The deposition chamber is a 28-mm ID quartz tube
heated by an electric furnace with a 30-cm long isothermal zone. The gas ¯owrates are adjusted by mass-¯ow controllers. The BCl3 manifold is heated at 313 K to prevent condensation of this halide. A chemical primary pump evacuates the reactor and a throttle valve regulates the operating pressure. In all experiments, two kinds of substrates are used: Zircaloy-4 (zirconium alloy with principal alloying elements: Sn 1.4 wt.%, Fe 0.2 wt.%, Cr 0.1 wt.%) and quartz. The effect of the alloying elements will not be described here, it will be the subject of a further paper. These substrates are 9.5 outer diameter tubes, 50 mm in length. One quartz substrate is located inside the furnace at the beginning of the isothermal zone, whereas ®ve Zircaloy-4 substrates are positioned afterwards as described in Fig. 1. In all cases, the substrates are mounted on a rotating holder to ensure a radially uniform thickness of the ®lms. For the sake of simplicity, all the experiments described in the present work are carried out on the outer surface of the substrates. Before each treatment, the Zircaloy-4 substrates are oxidized at 653 K in an Ar-O2 post-discharge, leading to the synthesis of a 140-nm thick zirconia ®lm (see Section 3.4.3). The temperature increases until the deposition
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temperature is reached under argon ¯ow. It will be shown in Section 3.2. that this zirconia ®lm must be etched before any ZrB2 deposition, the necessity of this oxidation step will be discussed in Section 3.4.3. The structure of zirconium diboride ®lms is studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical composition of the coatings is determined by electron probe microanalysis (EPMA) and secondary neutral mass spectroscopy (SNMS). The plasma diagnostic is done by an optical emission spectroscopy (OES) apparatus described elsewhere [17]. In the present study, four con®gurations are studied which are presented in Fig. 1. The thermal CVD is obtained when no plasma activation is provided. The other con®gurations, called hereafter RPECVD with an Ar±H2 post-discharge, RPECVD process with an Ar±H2±BCl3 post-discharge and RPECVD with an Ar±BCl3 post-discharge correspond to different plasma compositions. Investigated experimental conditions are reported in Fig. 1 for each process. For simplicity, these conditions will not be recalled in the text. 3. Results and discussion 3.1. Thermal CVD process The lowest deposition temperature for ZrB2 CVD coatings published in the literature is 963 K on copper substrates, which seem to activate the deposition reaction [18] and 973 K on graphite substrates [19]. Although our aim is to synthesis ZrB2 ®lms at a low temperature by the RPECVD process, thermal CVD has been conducted in a ®rst step to compare with other processes. In these experiments, the total ¯owrate and the partial pressures of the reactants are compatible with those used in post-discharge conditions. Indeed RPECVD process requires life times of active species higher than their deexcitation rate. Qualitatively, this is achieved when the total pressure is low and when the argon ¯owrate is high. Thus, conditions of process 1 in Fig. 1 have been used. The growth rate of ZrB2 ®lms deposited at 1023 K reaches 0.5 mm/h. This value is close to those obtained by Berthon et al. at the same temperature [19]. A XRD pattern of a ZrB2 coating deposited at 1023 K on quartz substrates is shown in Fig. 2. By comparing the relative intensities of this pattern with those reported in the JCPDS ®le 34±423, it can be concluded that the thermal CVD ZrB2 ®lm presents a weak preferred orientation in the [100] direction. The overall deposition reaction can be written as ZrCl4 1 2BCl3 1 5H2 ! ZrB2 1 10HCl
1
For a substrate temperature of 923 K, no coating is deposited on quartz, whereas Zircaloy-4 tubes are coated by a 3mm thick ZrB2 ®lm for a 2 h treatment. This result clearly shows that the metallic substrates participate in the zirco-
Fig. 2. XRD pattern of a ZrB2 ®lm deposited at 1023 K on quartz substrate by thermal CVD (see Fig. 1 for experimental conditions).
nium diboride synthesis. A diffusion step necessarily occurs because no ZrB2 is deposited on quartz substrates or on the reactor walls. As reaction (1) is not allowed thermodynamically at 733 K, no ZrB2 ®lms can be deposited at this temperature on quartz. Moreover, the synthesis of ZrB2 by a diffusion mechanism on Zircaloy-4 could not be achieved experimentally at 733 K. To synthesize zirconium diboride ®lms on long Zircaloy-4 substrates at this temperature, an activated process should be used instead of thermal CVD. 3.2. RPECVD process with Ar±H2 post-discharges It is expected that Ar±H2 post-discharges would supply atomic hydrogen to react with boron and zirconium chlorides and then, will lower the deposition temperature of ZrB2. Boron trichloride diluted in argon is then introduced into the reactor through the annular section to avoid gas phase reaction with the post-discharge (see process 2 in Fig. 1). For a substrate temperature of 923 K no ®lm is deposited on quartz whereas Zircaloy-4 substrates are coated by ZrB2 as con®rmed by electron diffraction analysis (Fig. 3). These coatings have the same characteristics (growth rate, crystal size, microstructure¼) than those synthesized by thermal CVD (see Section 3.1). It can be concluded that the Ar±H2 post-discharge does not improve the deposition of ZrB2 ®lms at a low temperature. This is probably due to the rapid recombination of atomic hydrogen into molecular hydrogen, which depletes the ¯owing post-discharge in H species so that no signi®cant effect is noticed [20]. For processes 1 and 2, which exhibit a similar behavior, a minimal temperature was determined at which ZrB2 synthesis on Zircaloy-4 occurs. This temperature is 853 K. All the metallic substrates coated by ZrB2 have lost weight after treatment. Together with the diffusion step required to synthesize ZrB2 on Zircaloy-4, this result shows that the growth mechanism includes the etching of the substrate. We have studied the stability of the zirconia
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layer grown by post-discharge oxidation when boron trichloride is introduced without microwave excitation. Zirconia is thermodynamically unstable when it is in contact with BCl3 whatever the temperature [21]. The overall reaction is 3ZrO2 1 4BCl3 ! 3ZrCl4 1 2B2 O3
2
We have checked in our experimental conditions that reaction (2) does not occur at a temperature lower than 853 K. As this temperature is the same than that required for ZrB2 synthesis, it can be concluded that zirconium diboride synthesis is only possible when zirconia is etched by BCl3. As reaction (2) cannot occur at a temperature lower than 853 K by using process 1 or 2, we have used Ar±H2± BCl3 post-discharges to synthesis ZrB2 on oxidized Zircaloy-4 at 733 K. It was expected that the addition of BCl3 in the plasma would make it react with hydrogen before the substrates. 3.3. RPECVD process with Ar±H2±BCl3 post-discharges In this con®guration, argon, hydrogen and boron trichloride are introduced in the reactor upstream the gap (see process 3 in Fig. 1). A second hydrogen ¯ow may be added in the reactor through the additional line. The ®rst discussion focuses on the reactions occurring between hydrogen and boron trichloride in the discharge tube. Then, the results on ZrB2 synthesis on Zircaloy-4 or quartz substrates are discussed. Finally, the in¯uence of the amount of hydrogen in Ar±H2±BCl3 post-discharges is described. Two kinds of plasma are distinguished herein depending on the ratio of hydrogen ¯owrate to that of boron trichloride: ² post-discharges where Q(H2)/Q(BCl3) . 1.5; ² post-discharges where Q(H2)/Q(BCl3) , 1.5. 3.3.1. Reactions in Ar±H2±BCl3 plasmas An adherent coating is deposited on the walls of the
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discharge tube when Ar±H2±BCl3 discharges are used whatever the hydrogen ¯owrate. XRD analysis clearly shows that this coating is a two-phase layer: amorphous and b -boron. EPMA analysis indicates that chlorine and oxygen contents are too low to be detected. The nature of the boron coating does not seem to be in¯uenced by the composition of the discharge. This coating is essentially located around the gap (about 3±4 cm on both sides). As for Ar±BCl3 plasmas, boron trichloride dissociation is high in this zone of the discharge [22]. For experimental conditions where Q(H2)/ Q(BCl3) . 1.5 (Fig. 1), the amount of boron deposited is estimated to be at least 25% of the total BCl3 amount introduced in the reactor. As the discharge tube diameter is 5 mm, deposition of boron induces a signi®cant decrease of the tube section. Typically, in the previous conditions, experiments are limited to within 3 h. It must be noted that deposition of boron increases the atomic ratio of chlorine to boron in the post-discharge. This increase probably induces the synthesis of atomic and/or molecular chlorine and hydrogen chloride in the post-discharge. These species are known to be used as an etching agent in many processes and it can be expected that they react with Zircaloy-4 substrates in the post-discharge. As soon as the discharge is on, a yellow-orange deposit starts appearing in the near post-discharge. This deposit sublimates at a temperature much lower than 733 K because it is not detected close to the furnace. As the only compounds that can be solid at room temperature in the ternary system B±Cl±H are boranes with high molecular weight (B10H14, B20H16¼), we assume that this yellow orange deposit is a borane or a mixture of boranes. If boranes with high molecular weight are synthesized in Ar±H2±BCl3 discharges, they are obviously by-products of reactions between atomic boron and hydrogen. Therefore, it can be stated that boranes with a low molecular weight are also synthesized. Indeed, we have evidenced the A 1P ±X 1S transition of the BH molecule by OES analysis of Ar±H2± BCl3 discharges (Fig. 4). Similarly to the case of boron
Fig. 3. Electron diffraction pattern and dark ®eld micrograph of a ZrB2 ®lm synthesized at 923 K on Zircaloy-4 in an Ar±H2 post-discharge.
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Fig. 4. Optical emission spectroscopy observation of the A 1P ±X 1S transition for BH molecules in an Ar±H2±BCl3 plasma (990-7.5-15 sccm) with a total pressure of 700 Pa and a microwave power of 140 W.
deposition, the synthesis of boron compounds (boranes here) also increases the relative amount of atomic and/or molecular chlorine in the post-discharge. 3.3.2. Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) , 1.5. Two temperatures are investigated for Ar±H2±BCl3 postdischarges where Q(H2)/Q(BCl3) , 1.5: 573 and 753 K. In this con®guration, a second ¯owrate of hydrogen (100 sccm) is added to the reactive mixture via the additional line to increase the atomic ratio H/Cl near the substrates. When the temperature for ZrB2 deposition is 573 K, Zircaloy-4 substrates are initially oxidized at this temperature rather than at 653 K as in other experiments. After a 2 h treatment, no coating is detected on reactor walls and neither on the quartz substrate or on Zircaloy-4. However, metallic substrates have lost 0.07% of their weight. As the weight loss due to the etching of the zirconia layer synthesized at 573 K only corresponds to 0.006%, the Zircaloy-4 substrates have also been etched. This result shows that at 573 K the zirconia layer is completely removed. Anyway, this temperature is too low to activate the zirconium diboride synthesis. The weight loss of Zircaloy-4 substrates submitted to an Ar±H2±BCl3 post-discharge is 0.78% at 753 K. By comparison to this value, the total etching of the zirconia layer synthesized at 653 K induces a weight loss of 0.03%. At this temperature, zirconia is completely removed. However, at 753 K, a ®lm is synthesized with low adhesion on Zircaloy-4 substrate. As there is no coating on the quartz tubes or on the reactor walls, this ®lm results from a diffusion mechanism. EPMA analysis shows that this ®lm contains zirconium, boron, oxygen and chlorine. Considering the low adhesion of this ®lm, Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3),1.5 are not suitable to the synthesis of ZrB2 on Zircaloy-4 substrates.
3.3.3. Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) . 1.5 For Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) .1.5, no additional hydrogen is introduced in the additional line (see process 3 in Fig. 1). For a substrate temperature of 753 K, SEM analyses show a ®lm with low adhesion on the Zircaloy-4 substrate (Fig. 5). Contrary to Ar±H2 postdischarge, there is no substrate weight loss in this case. After SNMS analyses, this coating is proved to be boron. Furthermore, this technique reveals that the zirconia layer is still present between the boron ®lm and the Zircaloy-4 substrate. This point shows that in these experimental conditions a metallic substrate is not requested. As a consequence, Ar±H2±BCl3 discharges where Q(H2)/Q(BCl3) .1.5 are not worth being used to synthesis ZrB2 on Zircaloy-4 substrates at a low temperature. 3.3.4. Discussion on the in¯uence of the hydrogen percentage in Ar±H2±BCl3 post-discharges In paragraph 3.3.1., we have stated that hydrogen reacts with boron trichloride to form boranes. These reactions also induce the formation of atomic and/or molecular chlorine. These species are also by-products of boron deposition on the discharge tube walls. As zirconia is not removed by a non-excited Ar±H2±BCl3±ZrCl4 mixture at 753 K, its etching in Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) , 1.5 probably results from species as Cl or Cl2. Thus, the hydrogen ¯owrate is certainly too low to convert these species into by-products like HCl that etches zirconia with a so much lower rate, that no signi®cant effect can be noticed within 2 h. In Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) .1.5, the hydrogen ¯owrate is high enough to ensure complete conversion and zirconia is not removed. As boranes in post-discharge do not react with zirconium tetrachloride to synthesis ZrB2 at 753 K the coating deposited is only boron. Its low adhesion is attributed to chlorine incorporation during the ®lm growth. In summary, hydrogen participates in boranes synthesis and reacts with chlorine after the BCl3 dissociation to synthesis by-products like HCl that etch zirconia with a much
Fig. 5. SEM micrograph of a boron coating deposited at 753 K with an Ar± H2±BCl3 post-discharge (900-50-15 sccm), treatment duration: 2 h.
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lower rate. As these post-discharges give ZrB2 with low adhesion at a low temperature, we have chosen to study Ar±BCl3 post-discharges. As no borane can be synthesized in this case, one main advantage expected is the decrease of chlorine concentration in the post-discharge.
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chlorine content in Ar±BCl3 post-discharges is probably valid.
3.4.1. Ar±BCl3 post-discharges advantages to synthesis ZrB2 at 733 K The ®lm thickness varies linearly versus time between 1 and 6 h. The ZrB2 growth rate is around 1 mm/h and radial dispersion is low enough to be neglected. This growth rate is nearly independent of parameters like total pressure or ¯owrates (ZrCl4, BCl3, H2). By using Ar±BCl3 post-discharges, it is possible to coat Zircaloy-4 tubes with a constant thickness over a distance of about 18 cm (Fig. 6). The growth rate decrease after 18 cm is due to the thermal gradient at the end of the furnace. Despite the thickness increases linearly versus time, the coated substrates lose weight during the treatment. The weight loss variation is also linear versus time. For a 2 h treatment with a ZrCl4 ¯owrate of 2.5 sccm, the weight loss is 0.2%. The zirconia layer is totally etched. This weight loss is four times lower than that measured for Ar±H2±BCl3 discharges where Q(H2)/ Q(BCl3) , 1.5 (0.78%, see paragraph 3.3.2.). This result shows that the previous assumption we made on the lower
3.4.2. Characterization of RPECVD ZrB2 ®lms. A SNMS pro®le analysis of a zirconium diboride ®lm synthesized at 733 K in an Ar±BCl3 post-discharge is represented in Fig. 7. One has successively from the outer surface to the core of the substrate: a zirconia layer of 100-nm thickness (which does not contain boron), a zirconium diboride ®lm (which also contains oxygen) and ®nally the Zircaloy-4 substrate. It is important to note that the super®cial zirconia layer is not the oxidation layer due to the postdischarge pre-treatment. Indeed zirconia has to be totally removed to synthesize ZrB2. This super®cial layer is synthesized after the growth of the ZrB2 ®lm. Its formation mechanism is not well established. It appears clearly in Fig. 7 that no zirconia layer is found between the ZrB2 ®lm and the Zircaloy-4 substrate. Moreover, it is worth noticing that chlorine has accumulated at this interface. Therefore, it can be concluded that the mechanism of ZrB2 deposition consists of a diffusion step of zirconium from the substrate to the gas-®lm interface. This point described in detail in reference [23] will not be developed here. EPMA characterization of ZrB2 ®lms requires the removal of the 100-nm thick super®cial oxide layer observed in Fig. 7. Atomic concentrations of zirconium, boron, oxygen and chlorine are found to be 21, 43, 35 and 1%, respectively. As literature does not mention any ternary compound in the Zr±B±O system, several phases are mixed. However, the atomic concentrations mentioned above are not consistent with the following binary mixtures: ZrB2 1 B2 O3 , ZrB2 1 ZrO2 or ZrO2 1 B2 O3 . The only possible solution is to consider that ZrB2 ®lms are composed of a ternary mixture of ZrB2, B2O3 and ZrO2 with molar concentrations of 50, 27 and 23%, respectively. The ZrB2 ®lms observed by SEM are dense (Fig. 8). They do not grow with a columnar structure as it is commonly
Fig. 6. ZrB2 growth rate versus position z along the Zircaloy-4 substrate (see process 4 in Fig. 1 for experimental conditions).
Fig. 7. SNMS pro®le analysis of a ZrB2 ®lm synthesized on Zircaloy-4 in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
3.4. RPECVD process with Ar-BCl3 post-discharges When Ar±BCl3 post-discharges are used, a second hydrogen ¯owrate is added to the reactive mixture via the additional line (see process 4 in Fig. 1). Boranes do not condense in the post-discharge. However, a boron ®lm is deposited on discharge tube walls. Its characteristics are the same than those presented in Section 3.3.1. The amount of boron deposited in this case is estimated to be between 5 and 10% of the total BCl3 amount introduced in the reactor whereas it is 25% in Ar±H2±BCl3 discharges.
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Fig. 8. Cross-section SEM micrograph of a ZrB2 ®lm synthesized on Zircaloy-4 in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
seen in coatings with small crystal sizes [24]. Indeed, the crystal size of ZrB2 grains has been estimated at 10 nm from TEM observations. X-ray diffraction analysis of ZrB2 ®lms allows us to identify only two crystalline compounds: ZrB2 and a -ZrO2 (Fig. 9). However, electron diffraction shows only one crystalline phase: ZrB2 (Fig. 10). In fact, the a ZrO2 phase identi®ed by XRD corresponds to the super®cial zirconia layer detected by SNMS (Fig. 7). XPS measurements show two chemical shifts of Zr 3d photoelectrons which are characteristic of ZrB2 and ZrO2 and two chemical shifts of B 1s photoelectrons which are characteristic of ZrB2 and B2O3. As zirconium and boron oxides are expected to be in the ®lm, they are very likely amorphous. We think that these oxides are an amorphous solid solution as described in the structural model of Lebugle [25,26]. The ®lms synthesized at 733 K in Ar±BCl3 post-discharges are a mixture of nanocrystalline grains of zirconium diboride dispersed in an amorphous solid solution of boron and zirconium oxides.
Fig. 9. X-ray diffraction pattern of a ZrB2 ®lm synthesized on Zircaloy-4 in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
3.4.3. In¯uence of the zirconia layer. BCl3 dissociation in the discharge part of the reactor produces great quantities of atomic chlorine and BClx (x 1 or 2) species that are known to be very ef®cient etching agent. As a matter of fact, Si emission line and BO emission bands have been identi®ed by OES diagnostic of Ar±BCl3 plasma [22,27]. To obtain ZrB2 ®lms that grow from zirconium diffusion mechanism, it is necessary to control the etching of the Zircaloy-4 substrates during the treatment. In our process, this function is ful®lled by the zirconia protective layer. As the zirconia layer is completely removed during the zirconium diboride synthesis, we have studied the in¯uence of this layer on ZrB2 synthesis. Thus, we have treated in a single experiment, four Zircaloy-4 substrates coated by different zirconia layers synthesized by:
Fig. 10. Electron diffraction pattern and bright ®eld micrograph of a ZrB2 ®lm synthesized in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
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² oxidation in Ar±O2 post-discharge at 653 K (140 nm thickness) [14]; ² oxidation in Ar±O2 post-discharge at 753 K (770 nm thickness) [14]; ² oxidation in Ar±O2 post-discharge at 873 K (4 mm thickness) [14]; ² RPECVD deposition at 733 K with an Ar±H2±O2 postdischarge (16 mm thickness) [16].
After treatment, only the substrates oxidized at 653 and 753 K are coated by ZrB2. As the substrate oxidized at 753 K is covered by a zirconia layer thicker than that oxidized at 653K, its ZrB2 ®lm is thinner. In spite of ZrB2 synthesis, Zircaloy-4 substrates lose weight again during the treatment. The two other samples are not coated by ZrB2. However, their weights have decreased after treatment. XRD analysis con®rms that zirconia layer is still present on Zircaloy-4 substrates. To synthesize ZrB2 on Zircaloy-4, the ®rst step consists of totally removing the zirconia layer. As long as zirconia is still present on Zircaloy-4, the diffusion mechanism for ZrB2 synthesis cannot occur because zirconia is a diffusion barrier. When Zircaloy-4 substrates are not oxidized in Ar± O2 post-discharge before the treatment, a non-uniform thickness distribution along the substrate is obtained. This phenomenon is probably due to the non-uniformity of the native oxide layer. To avoid this problem, all metallic substrates were oxidized before treatment. 3.4.4. Origin of boron incorporated in ZrB2 ®lms. The density of B atoms produced by an Ar-1.5% BCl3 microwave plasma have been determined in post-discharge by a titration technique involving the B 1 O2 chimiluminescent reaction [28]. It was found that the B-atom density nearly reaches 10 15 cm 23 at the plasma end (,10 cm after the gap). This corresponds to a BCl3 dissociation degree of about 30%, for a BCl3 density of 3 £ 10 15 cm 23. However, the B-atom density [B] at a distance d from the plasma end is quickly decreasing in post-discharge to follow a ®rst order kinetic law given by d B 3 2kw ln v B 0 where [B]0 is the B-atom concentration at the plasma end (d 0), v is the gas velocity (180 m/s in our experimental conditions) and kw is the destruction frequency of B atoms on the tube wall. From the results reported in reference [28] it was determined that kw (1.3±1.7) £ 10 4 s 21. Therefore, boron atoms quickly disappeared on the surface wall of the quartz tube wall before reaching the Zircaloy-4 substrate located at d < 110 cm. Boron incorporation in ZrB2 ®lms could come from BClx (x 1 or 2) species created in the discharge and transported in post-discharge. Oxygen in the ®lms is very likely due to the quartz discharge tube, which is etched by Ar±BCl3 discharges.
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Another possibility is that boron comes from BOx (x 1 or 2) species produced directly at the surface wall or in the gas phase. As a matter of fact, an increase in BO emission signal was observed at the end of the ¯owing discharge [22]. 4. Conclusion In this work, the in¯uence of the post-discharge composition on the ZrB2 synthesis at 733 K by RPECVD has been investigated. Experiments performed by thermal CVD and RPECVD with Ar±H2 post-discharge show that the deposition of ZrB2 ®lms on oxidized Zircaloy-4 is impossible at temperature lower than 853 K. Ar±H2±BCl3 post-discharges do not give to adherent ®lms on oxidized Zircaloy-4 at temperature lower than 753 K. Among the different tested con®gurations, only Ar±BCl3 post-discharges allow us to synthesize ZrB2 coatings. By using these post-discharges, it is possible to coat Zircaloy-4 tubes with a constant thickness over 20 cm long. ZrB2 coatings are dense but contain oxygen that mainly comes from the etching of the quartz discharge tube by the Ar±BCl3 plasma. As ZrB2 is the only crystallized compound, zirconium and boron oxides are amorphous. The coating is a mixture of nanograins of ZrB2 (10 nm crystal size) dispersed in an amorphous solid solution of boron and zirconium oxides. As the ZrB2 coating can only be synthesized on Zircaloy4 substrates at 733 K, the deposition mechanism consists of a diffusion step of zirconium from the substrate to the gas®lm interface where zirconium atoms react with boron chlorides to synthesis ZrB2. For this reason and also because the Ar±BCl3 postdischarge produces very ef®cient etching agents, it is necessary to perform a preoxidation treatment, leading to a ZrO2 layer. However, this protective layer must be removed during the treatment to produce a ZrB2 ®lm. We have also demonstrated that boron incorporated in the ®lms could not come from boron atoms produced in the ¯owing discharge.
Acknowledgements The authors wish to express their thanks to J.P. PreÂlot for his technical support. The company Fragema is gratefully acknowledged for its ®nancial support of this work.
References [1] P. Rogl, P.E. Potter, Calphad 12 (1988) 191. [2] A. Lebugle, G. Montel, Rev. Int. Hautes Temp. Refrac. 11 (1974) 231. [3] C.C. Wang, S.A. Akbar, W. Chen, V.D. Patton, J. Mater. Sci. 30 (1995) 1627. [4] M.P. Grimshaw, A.E. Staton-Bevan, Mater. Sci. Eng. B 5 (1989) 21. [5] C. Mroz, Am. Ceram. Soc. Bull. 72 (1993) 126.
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[6] D. Hertz, H. Michel, T. Belmonte, J.F. Pierson, French Patent, FR 2743-088 (1997). [7] J.F. Pierson, A. Billard, T. Belmonte, H. Michel, C. Frantz, Thin Solid Films 347 (1999) 79. [8] D.L. Chambers, C.T. Wan, K.A. Taylor, G.T. Susi, J. Vac. Sci. Technol. A 8 (1990) 2113. [9] A. Wang, G. MaleÂ, J. Euro. Ceram. Soc. 11 (1993) 241. [10] S. Motojima, K. Funahashi, K. Kurosawa, Thin Solid Films 189 (1990) 73. [11] A.L. Wayda, L.F. Schneemeyer, R.L. Opila, Inorg. Mater. 31 (1995) 965. [12] S. Reich, H. Suhr, K. Hanko, L. Szepes, Adv. Mater. 4 (1992) 650. [13] G.W. Rice, R.L. Woodin, Comm. Am. Ceram. Soc. 4 (1988) C181. [14] X. Iltis, M. Viennot, D. David, D. Hertz, H. Michel, J. Nucl. Mater. 209 (1994) 180. [15] H. Malvos, A. Ricard, J. Szekely, H. Michel, M. Gantois, D. Ablitzer, Surf. Coat. Technol. 59 (1993) 59. [16] J. Gavillet, T. Belmonte, D. Hertz, H. Michel, Thin Solid Films 301 (1997) 35. [17] J.F. Pierson, T. Czerwiec, T. Belmonte, H. Michel, A. Ricard, Plasma Sources Sci. Technol. 7 (1998) 54.
[18] S. Motojima, K. Funahashi, K. Kurosawa, Thin Solid Films 189 (1990) 73. [19] S. Berthon, G. MaleÂ, Ann. Chim. Fr. 20 (1995) 13. [20] A. Rousseau, A. Granier, G. Gousset, P. Leprince, J. Phys. D: Appl. Phys. 27 (1994) 1412. [21] JANAF Thermochemical Tables (1985). [22] J.F. Pierson, T. Czerwiec, T. Belmonte, H. Michel, Surf. Coat. Technol. 97 (1997) 749. [23] J.F. Pierson, T. Belmonte, H. Michel, Surf. Coat. Technol., 1999 (in press). [24] H.O. Pierson, Handbook of Chemical Vapor Deposition (CVD): Principles, technology and applications, Noyes Publication, New Jersey, 1992. [25] J. Livage, K. Doi, C. MazieÂres, J. Am. Ceram. Soc. 51 (1968) 349. [26] A. Lebugle, Ph.D. Thesis, Institut National Polytechnique de Toulouse, France, 1975. [27] T. Czerwiec, J.F. Pierson, T. Belmonte, H. Michel, A. Ricard, in: M. Bordage, A. Glaizes (Eds.), Proc. 23rd Int. Conf. on Phenomena in Ionized Gases, IGPIG, 1997, p. 198. [28] A. Ricard, T. Czerwiec, T. Belmonte, S. Bockel, H. Michel, Thin Solid Films 341 (1999) 1.
Low temperature ZrB2 remote plasma enhanced chemical vapor deposition J.F. Pierson a, T. Belmonte b, T. Czerwiec b,*, D. Hertz c, H. Michel b a
b
Laboratoire de MeÂtrologie des Interfaces Techniques, PoÃle Universitaire, 4 place Tharradin, BP 427-25211 MontbeÂliard Cedex, France Laboratoire de Science et GeÂnie des Surfaces (UMR CNRS-INPL-EdF 7570), Ecole des Mines, Parc de Saurupt, 54042 Nancy Cedex, France c Framatome, 10 rue Juliette ReÂcamier, BP 3083-69398 LYON Cedex 03, France Received 11 June 1999; received in revised form 7 September 1999; accepted 7 September 1999
Abstract Deposition of zirconium diboride ®lms on Zircaloy-4 substrates at 733 K over 20 cm is carried out by remote plasma enhanced chemical vapor deposition (RPECVD). Different post-discharge compositions (Ar±H2, Ar±H2±BCl3 and Ar±BCl3) are tested in several process con®gurations. Experiments performed by thermal CVD and RPECVD with Ar±H2 post-discharge show that the deposition of ZrB2 ®lms on oxidized Zircaloy-4 is impossible at temperature lower than 853 K. Ar±H2±BCl3 post-discharges do not give to adherent ®lms on oxidized Zircaloy-4 at a temperature lower than 753K. It is shown that ZrB2 thin ®lms can be synthesized by using ¯owing Ar±BCl3 microwave postdischarges. Chlorine must etch the zirconia protective layer before zirconium diboride is synthesized. Therefore, the control of thickness of this zirconia layer by a previous oxidation treatment gives homogeneous deposition. The structure of the ®lms has been determined to be nanograins of ZrB2 dispersed in an amorphous solid solution of boron and zirconium oxides. The origin of the boron species incorporated in the ZrB2 ®lms is attributed to the eching of the quartz tube by chlorine. q 2000 Elsevier Science S.A. All rights reserved. Keywords: ZrB2; Chemical vapor deposition; Post-discharges; Structural characterization
1. Introduction Zirconium diboride (ZrB2) has several properties which make it a very interesting material for a large variety of industrial applications. It is a refractory compound with a melting temperature higher than 3300 K [1]. This material has good resistance against low temperature oxidation with the formation of a thin protective oxide layer [2]. ZrB2 has also the lowest bulk resistivity (9 £ 1028 V.m) among metal borides [3]. It could then be used for applications in semiconductor device fabrication as it has also excellent diffusion barrier performance [4]. The crystal structure of ZrB2, which is dominated by the two-dimensional network boron con®guration, favors high hardness (22 GPa) [5]. Its thermal expansion coef®cient (5:5 £ 1026 K 21) is very close to that of zirconium (5:8 £ 1026 K 21). The cooling of zirconium substrates coated by ZrB2 ®lms induces therefore, weak thermal stress in these ®lms. Boron containing compounds are also attractive in nuclear technology as neutron absorbers [6]. In this case, ZrB2 ®lms have to be synthesized on * Corresponding author. Tel.: 133-03-83-58-42-52, fax: 133-03-83-5347-64. E-mail address: [email protected] (T. Czerwiec)
the inner surface of Zircaloy-4 tubes. To preserve the mechanical properties of these tubes, high temperature processes can not be used (T , 733 K). For these reasons, among the different processes used to synthesize zirconium diboride coatings [7±13], a remote plasma enhanced chemical vapor deposition (RPECVD) process was chosen. In this paper, we report on the in¯uence of the postdischarge composition of the RPECVD process on low temperature zirconium diboride deposition. Four con®gurations are studied corresponding to different con®gurations of gas distribution. In each case, conclusions are drawn to clarify the advantages or disadvantages of the studied con®guration.
2. Experimental set-up The hot-wall RPECVD apparatus used in the present study was designed to enable both thermochemical [14,15] and CVD treatments [16] enhanced by remote microwave plasma. The system consists of four distinct parts: an in-situ chlorination chamber to synthesize ZrCl4 (CL in Fig. 1), a microwave generator coupled to a surfaguide wave launcher (SWL), a heating device with a deposi-
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0072 1-X
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69
Fig. 1. The RPECVD experimental set-up. The different con®gurations are described in the table where the investigated ranges of the main parameters are listed. Bold values are conditions referred to in the text when no details are provided.
tion chamber designed for cylindrical and ¯at substrates (HD) and a primary pumping unit (RP). The in-situ chlorination chamber is a 19-mm ID quartz tube into which high purity zirconium chips are loaded. Chlorination is performed at 623 K by introducing a chlorine and argon mixture with ¯owrates of 5 and 17 sccm (standard cubic centimetres per minute: 1 sccm 1:667 £ 1028 m 3/s), respectively. Under these conditions, the overall reaction Zr 1 2Cl2 ! ZrCl4 is complete as con®rmed by weight loss measurements on the chips. The ZrCl4 feed line to the reaction chamber is heated to prevent condensation of the halide vapor. For all experiments, the ZrCl4 ¯owrate is kept constant at 2.5 sccm. The plasma is produced in a 5-mm ID quartz tube (DT) by a 2.45-GHz microwave generator with a surfaguide surface wave launcher located upstream from the deposition chamber, approximately 1.2 m from the substrate location. By choosing such a large distance, it was expected to develop a process that could be extended to the treatment of inner surfaces of long cylindrical substrates. About 20 cm beyond the plasma gap (Fig. 1), the microwave discharge tube opens into a 15-mm ID quartz tube (ST) which enables the separation of the reactive gases before their mixing in the reaction zone. The deposition chamber is a 28-mm ID quartz tube
heated by an electric furnace with a 30-cm long isothermal zone. The gas ¯owrates are adjusted by mass-¯ow controllers. The BCl3 manifold is heated at 313 K to prevent condensation of this halide. A chemical primary pump evacuates the reactor and a throttle valve regulates the operating pressure. In all experiments, two kinds of substrates are used: Zircaloy-4 (zirconium alloy with principal alloying elements: Sn 1.4 wt.%, Fe 0.2 wt.%, Cr 0.1 wt.%) and quartz. The effect of the alloying elements will not be described here, it will be the subject of a further paper. These substrates are 9.5 outer diameter tubes, 50 mm in length. One quartz substrate is located inside the furnace at the beginning of the isothermal zone, whereas ®ve Zircaloy-4 substrates are positioned afterwards as described in Fig. 1. In all cases, the substrates are mounted on a rotating holder to ensure a radially uniform thickness of the ®lms. For the sake of simplicity, all the experiments described in the present work are carried out on the outer surface of the substrates. Before each treatment, the Zircaloy-4 substrates are oxidized at 653 K in an Ar-O2 post-discharge, leading to the synthesis of a 140-nm thick zirconia ®lm (see Section 3.4.3). The temperature increases until the deposition
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temperature is reached under argon ¯ow. It will be shown in Section 3.2. that this zirconia ®lm must be etched before any ZrB2 deposition, the necessity of this oxidation step will be discussed in Section 3.4.3. The structure of zirconium diboride ®lms is studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical composition of the coatings is determined by electron probe microanalysis (EPMA) and secondary neutral mass spectroscopy (SNMS). The plasma diagnostic is done by an optical emission spectroscopy (OES) apparatus described elsewhere [17]. In the present study, four con®gurations are studied which are presented in Fig. 1. The thermal CVD is obtained when no plasma activation is provided. The other con®gurations, called hereafter RPECVD with an Ar±H2 post-discharge, RPECVD process with an Ar±H2±BCl3 post-discharge and RPECVD with an Ar±BCl3 post-discharge correspond to different plasma compositions. Investigated experimental conditions are reported in Fig. 1 for each process. For simplicity, these conditions will not be recalled in the text. 3. Results and discussion 3.1. Thermal CVD process The lowest deposition temperature for ZrB2 CVD coatings published in the literature is 963 K on copper substrates, which seem to activate the deposition reaction [18] and 973 K on graphite substrates [19]. Although our aim is to synthesis ZrB2 ®lms at a low temperature by the RPECVD process, thermal CVD has been conducted in a ®rst step to compare with other processes. In these experiments, the total ¯owrate and the partial pressures of the reactants are compatible with those used in post-discharge conditions. Indeed RPECVD process requires life times of active species higher than their deexcitation rate. Qualitatively, this is achieved when the total pressure is low and when the argon ¯owrate is high. Thus, conditions of process 1 in Fig. 1 have been used. The growth rate of ZrB2 ®lms deposited at 1023 K reaches 0.5 mm/h. This value is close to those obtained by Berthon et al. at the same temperature [19]. A XRD pattern of a ZrB2 coating deposited at 1023 K on quartz substrates is shown in Fig. 2. By comparing the relative intensities of this pattern with those reported in the JCPDS ®le 34±423, it can be concluded that the thermal CVD ZrB2 ®lm presents a weak preferred orientation in the [100] direction. The overall deposition reaction can be written as ZrCl4 1 2BCl3 1 5H2 ! ZrB2 1 10HCl
1
For a substrate temperature of 923 K, no coating is deposited on quartz, whereas Zircaloy-4 tubes are coated by a 3mm thick ZrB2 ®lm for a 2 h treatment. This result clearly shows that the metallic substrates participate in the zirco-
Fig. 2. XRD pattern of a ZrB2 ®lm deposited at 1023 K on quartz substrate by thermal CVD (see Fig. 1 for experimental conditions).
nium diboride synthesis. A diffusion step necessarily occurs because no ZrB2 is deposited on quartz substrates or on the reactor walls. As reaction (1) is not allowed thermodynamically at 733 K, no ZrB2 ®lms can be deposited at this temperature on quartz. Moreover, the synthesis of ZrB2 by a diffusion mechanism on Zircaloy-4 could not be achieved experimentally at 733 K. To synthesize zirconium diboride ®lms on long Zircaloy-4 substrates at this temperature, an activated process should be used instead of thermal CVD. 3.2. RPECVD process with Ar±H2 post-discharges It is expected that Ar±H2 post-discharges would supply atomic hydrogen to react with boron and zirconium chlorides and then, will lower the deposition temperature of ZrB2. Boron trichloride diluted in argon is then introduced into the reactor through the annular section to avoid gas phase reaction with the post-discharge (see process 2 in Fig. 1). For a substrate temperature of 923 K no ®lm is deposited on quartz whereas Zircaloy-4 substrates are coated by ZrB2 as con®rmed by electron diffraction analysis (Fig. 3). These coatings have the same characteristics (growth rate, crystal size, microstructure¼) than those synthesized by thermal CVD (see Section 3.1). It can be concluded that the Ar±H2 post-discharge does not improve the deposition of ZrB2 ®lms at a low temperature. This is probably due to the rapid recombination of atomic hydrogen into molecular hydrogen, which depletes the ¯owing post-discharge in H species so that no signi®cant effect is noticed [20]. For processes 1 and 2, which exhibit a similar behavior, a minimal temperature was determined at which ZrB2 synthesis on Zircaloy-4 occurs. This temperature is 853 K. All the metallic substrates coated by ZrB2 have lost weight after treatment. Together with the diffusion step required to synthesize ZrB2 on Zircaloy-4, this result shows that the growth mechanism includes the etching of the substrate. We have studied the stability of the zirconia
J.F. Pierson et al. / Thin Solid Films 359 (2000) 68±76
layer grown by post-discharge oxidation when boron trichloride is introduced without microwave excitation. Zirconia is thermodynamically unstable when it is in contact with BCl3 whatever the temperature [21]. The overall reaction is 3ZrO2 1 4BCl3 ! 3ZrCl4 1 2B2 O3
2
We have checked in our experimental conditions that reaction (2) does not occur at a temperature lower than 853 K. As this temperature is the same than that required for ZrB2 synthesis, it can be concluded that zirconium diboride synthesis is only possible when zirconia is etched by BCl3. As reaction (2) cannot occur at a temperature lower than 853 K by using process 1 or 2, we have used Ar±H2± BCl3 post-discharges to synthesis ZrB2 on oxidized Zircaloy-4 at 733 K. It was expected that the addition of BCl3 in the plasma would make it react with hydrogen before the substrates. 3.3. RPECVD process with Ar±H2±BCl3 post-discharges In this con®guration, argon, hydrogen and boron trichloride are introduced in the reactor upstream the gap (see process 3 in Fig. 1). A second hydrogen ¯ow may be added in the reactor through the additional line. The ®rst discussion focuses on the reactions occurring between hydrogen and boron trichloride in the discharge tube. Then, the results on ZrB2 synthesis on Zircaloy-4 or quartz substrates are discussed. Finally, the in¯uence of the amount of hydrogen in Ar±H2±BCl3 post-discharges is described. Two kinds of plasma are distinguished herein depending on the ratio of hydrogen ¯owrate to that of boron trichloride: ² post-discharges where Q(H2)/Q(BCl3) . 1.5; ² post-discharges where Q(H2)/Q(BCl3) , 1.5. 3.3.1. Reactions in Ar±H2±BCl3 plasmas An adherent coating is deposited on the walls of the
71
discharge tube when Ar±H2±BCl3 discharges are used whatever the hydrogen ¯owrate. XRD analysis clearly shows that this coating is a two-phase layer: amorphous and b -boron. EPMA analysis indicates that chlorine and oxygen contents are too low to be detected. The nature of the boron coating does not seem to be in¯uenced by the composition of the discharge. This coating is essentially located around the gap (about 3±4 cm on both sides). As for Ar±BCl3 plasmas, boron trichloride dissociation is high in this zone of the discharge [22]. For experimental conditions where Q(H2)/ Q(BCl3) . 1.5 (Fig. 1), the amount of boron deposited is estimated to be at least 25% of the total BCl3 amount introduced in the reactor. As the discharge tube diameter is 5 mm, deposition of boron induces a signi®cant decrease of the tube section. Typically, in the previous conditions, experiments are limited to within 3 h. It must be noted that deposition of boron increases the atomic ratio of chlorine to boron in the post-discharge. This increase probably induces the synthesis of atomic and/or molecular chlorine and hydrogen chloride in the post-discharge. These species are known to be used as an etching agent in many processes and it can be expected that they react with Zircaloy-4 substrates in the post-discharge. As soon as the discharge is on, a yellow-orange deposit starts appearing in the near post-discharge. This deposit sublimates at a temperature much lower than 733 K because it is not detected close to the furnace. As the only compounds that can be solid at room temperature in the ternary system B±Cl±H are boranes with high molecular weight (B10H14, B20H16¼), we assume that this yellow orange deposit is a borane or a mixture of boranes. If boranes with high molecular weight are synthesized in Ar±H2±BCl3 discharges, they are obviously by-products of reactions between atomic boron and hydrogen. Therefore, it can be stated that boranes with a low molecular weight are also synthesized. Indeed, we have evidenced the A 1P ±X 1S transition of the BH molecule by OES analysis of Ar±H2± BCl3 discharges (Fig. 4). Similarly to the case of boron
Fig. 3. Electron diffraction pattern and dark ®eld micrograph of a ZrB2 ®lm synthesized at 923 K on Zircaloy-4 in an Ar±H2 post-discharge.
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Fig. 4. Optical emission spectroscopy observation of the A 1P ±X 1S transition for BH molecules in an Ar±H2±BCl3 plasma (990-7.5-15 sccm) with a total pressure of 700 Pa and a microwave power of 140 W.
deposition, the synthesis of boron compounds (boranes here) also increases the relative amount of atomic and/or molecular chlorine in the post-discharge. 3.3.2. Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) , 1.5. Two temperatures are investigated for Ar±H2±BCl3 postdischarges where Q(H2)/Q(BCl3) , 1.5: 573 and 753 K. In this con®guration, a second ¯owrate of hydrogen (100 sccm) is added to the reactive mixture via the additional line to increase the atomic ratio H/Cl near the substrates. When the temperature for ZrB2 deposition is 573 K, Zircaloy-4 substrates are initially oxidized at this temperature rather than at 653 K as in other experiments. After a 2 h treatment, no coating is detected on reactor walls and neither on the quartz substrate or on Zircaloy-4. However, metallic substrates have lost 0.07% of their weight. As the weight loss due to the etching of the zirconia layer synthesized at 573 K only corresponds to 0.006%, the Zircaloy-4 substrates have also been etched. This result shows that at 573 K the zirconia layer is completely removed. Anyway, this temperature is too low to activate the zirconium diboride synthesis. The weight loss of Zircaloy-4 substrates submitted to an Ar±H2±BCl3 post-discharge is 0.78% at 753 K. By comparison to this value, the total etching of the zirconia layer synthesized at 653 K induces a weight loss of 0.03%. At this temperature, zirconia is completely removed. However, at 753 K, a ®lm is synthesized with low adhesion on Zircaloy-4 substrate. As there is no coating on the quartz tubes or on the reactor walls, this ®lm results from a diffusion mechanism. EPMA analysis shows that this ®lm contains zirconium, boron, oxygen and chlorine. Considering the low adhesion of this ®lm, Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3),1.5 are not suitable to the synthesis of ZrB2 on Zircaloy-4 substrates.
3.3.3. Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) . 1.5 For Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) .1.5, no additional hydrogen is introduced in the additional line (see process 3 in Fig. 1). For a substrate temperature of 753 K, SEM analyses show a ®lm with low adhesion on the Zircaloy-4 substrate (Fig. 5). Contrary to Ar±H2 postdischarge, there is no substrate weight loss in this case. After SNMS analyses, this coating is proved to be boron. Furthermore, this technique reveals that the zirconia layer is still present between the boron ®lm and the Zircaloy-4 substrate. This point shows that in these experimental conditions a metallic substrate is not requested. As a consequence, Ar±H2±BCl3 discharges where Q(H2)/Q(BCl3) .1.5 are not worth being used to synthesis ZrB2 on Zircaloy-4 substrates at a low temperature. 3.3.4. Discussion on the in¯uence of the hydrogen percentage in Ar±H2±BCl3 post-discharges In paragraph 3.3.1., we have stated that hydrogen reacts with boron trichloride to form boranes. These reactions also induce the formation of atomic and/or molecular chlorine. These species are also by-products of boron deposition on the discharge tube walls. As zirconia is not removed by a non-excited Ar±H2±BCl3±ZrCl4 mixture at 753 K, its etching in Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) , 1.5 probably results from species as Cl or Cl2. Thus, the hydrogen ¯owrate is certainly too low to convert these species into by-products like HCl that etches zirconia with a so much lower rate, that no signi®cant effect can be noticed within 2 h. In Ar±H2±BCl3 post-discharges where Q(H2)/Q(BCl3) .1.5, the hydrogen ¯owrate is high enough to ensure complete conversion and zirconia is not removed. As boranes in post-discharge do not react with zirconium tetrachloride to synthesis ZrB2 at 753 K the coating deposited is only boron. Its low adhesion is attributed to chlorine incorporation during the ®lm growth. In summary, hydrogen participates in boranes synthesis and reacts with chlorine after the BCl3 dissociation to synthesis by-products like HCl that etch zirconia with a much
Fig. 5. SEM micrograph of a boron coating deposited at 753 K with an Ar± H2±BCl3 post-discharge (900-50-15 sccm), treatment duration: 2 h.
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lower rate. As these post-discharges give ZrB2 with low adhesion at a low temperature, we have chosen to study Ar±BCl3 post-discharges. As no borane can be synthesized in this case, one main advantage expected is the decrease of chlorine concentration in the post-discharge.
73
chlorine content in Ar±BCl3 post-discharges is probably valid.
3.4.1. Ar±BCl3 post-discharges advantages to synthesis ZrB2 at 733 K The ®lm thickness varies linearly versus time between 1 and 6 h. The ZrB2 growth rate is around 1 mm/h and radial dispersion is low enough to be neglected. This growth rate is nearly independent of parameters like total pressure or ¯owrates (ZrCl4, BCl3, H2). By using Ar±BCl3 post-discharges, it is possible to coat Zircaloy-4 tubes with a constant thickness over a distance of about 18 cm (Fig. 6). The growth rate decrease after 18 cm is due to the thermal gradient at the end of the furnace. Despite the thickness increases linearly versus time, the coated substrates lose weight during the treatment. The weight loss variation is also linear versus time. For a 2 h treatment with a ZrCl4 ¯owrate of 2.5 sccm, the weight loss is 0.2%. The zirconia layer is totally etched. This weight loss is four times lower than that measured for Ar±H2±BCl3 discharges where Q(H2)/ Q(BCl3) , 1.5 (0.78%, see paragraph 3.3.2.). This result shows that the previous assumption we made on the lower
3.4.2. Characterization of RPECVD ZrB2 ®lms. A SNMS pro®le analysis of a zirconium diboride ®lm synthesized at 733 K in an Ar±BCl3 post-discharge is represented in Fig. 7. One has successively from the outer surface to the core of the substrate: a zirconia layer of 100-nm thickness (which does not contain boron), a zirconium diboride ®lm (which also contains oxygen) and ®nally the Zircaloy-4 substrate. It is important to note that the super®cial zirconia layer is not the oxidation layer due to the postdischarge pre-treatment. Indeed zirconia has to be totally removed to synthesize ZrB2. This super®cial layer is synthesized after the growth of the ZrB2 ®lm. Its formation mechanism is not well established. It appears clearly in Fig. 7 that no zirconia layer is found between the ZrB2 ®lm and the Zircaloy-4 substrate. Moreover, it is worth noticing that chlorine has accumulated at this interface. Therefore, it can be concluded that the mechanism of ZrB2 deposition consists of a diffusion step of zirconium from the substrate to the gas-®lm interface. This point described in detail in reference [23] will not be developed here. EPMA characterization of ZrB2 ®lms requires the removal of the 100-nm thick super®cial oxide layer observed in Fig. 7. Atomic concentrations of zirconium, boron, oxygen and chlorine are found to be 21, 43, 35 and 1%, respectively. As literature does not mention any ternary compound in the Zr±B±O system, several phases are mixed. However, the atomic concentrations mentioned above are not consistent with the following binary mixtures: ZrB2 1 B2 O3 , ZrB2 1 ZrO2 or ZrO2 1 B2 O3 . The only possible solution is to consider that ZrB2 ®lms are composed of a ternary mixture of ZrB2, B2O3 and ZrO2 with molar concentrations of 50, 27 and 23%, respectively. The ZrB2 ®lms observed by SEM are dense (Fig. 8). They do not grow with a columnar structure as it is commonly
Fig. 6. ZrB2 growth rate versus position z along the Zircaloy-4 substrate (see process 4 in Fig. 1 for experimental conditions).
Fig. 7. SNMS pro®le analysis of a ZrB2 ®lm synthesized on Zircaloy-4 in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
3.4. RPECVD process with Ar-BCl3 post-discharges When Ar±BCl3 post-discharges are used, a second hydrogen ¯owrate is added to the reactive mixture via the additional line (see process 4 in Fig. 1). Boranes do not condense in the post-discharge. However, a boron ®lm is deposited on discharge tube walls. Its characteristics are the same than those presented in Section 3.3.1. The amount of boron deposited in this case is estimated to be between 5 and 10% of the total BCl3 amount introduced in the reactor whereas it is 25% in Ar±H2±BCl3 discharges.
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Fig. 8. Cross-section SEM micrograph of a ZrB2 ®lm synthesized on Zircaloy-4 in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
seen in coatings with small crystal sizes [24]. Indeed, the crystal size of ZrB2 grains has been estimated at 10 nm from TEM observations. X-ray diffraction analysis of ZrB2 ®lms allows us to identify only two crystalline compounds: ZrB2 and a -ZrO2 (Fig. 9). However, electron diffraction shows only one crystalline phase: ZrB2 (Fig. 10). In fact, the a ZrO2 phase identi®ed by XRD corresponds to the super®cial zirconia layer detected by SNMS (Fig. 7). XPS measurements show two chemical shifts of Zr 3d photoelectrons which are characteristic of ZrB2 and ZrO2 and two chemical shifts of B 1s photoelectrons which are characteristic of ZrB2 and B2O3. As zirconium and boron oxides are expected to be in the ®lm, they are very likely amorphous. We think that these oxides are an amorphous solid solution as described in the structural model of Lebugle [25,26]. The ®lms synthesized at 733 K in Ar±BCl3 post-discharges are a mixture of nanocrystalline grains of zirconium diboride dispersed in an amorphous solid solution of boron and zirconium oxides.
Fig. 9. X-ray diffraction pattern of a ZrB2 ®lm synthesized on Zircaloy-4 in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
3.4.3. In¯uence of the zirconia layer. BCl3 dissociation in the discharge part of the reactor produces great quantities of atomic chlorine and BClx (x 1 or 2) species that are known to be very ef®cient etching agent. As a matter of fact, Si emission line and BO emission bands have been identi®ed by OES diagnostic of Ar±BCl3 plasma [22,27]. To obtain ZrB2 ®lms that grow from zirconium diffusion mechanism, it is necessary to control the etching of the Zircaloy-4 substrates during the treatment. In our process, this function is ful®lled by the zirconia protective layer. As the zirconia layer is completely removed during the zirconium diboride synthesis, we have studied the in¯uence of this layer on ZrB2 synthesis. Thus, we have treated in a single experiment, four Zircaloy-4 substrates coated by different zirconia layers synthesized by:
Fig. 10. Electron diffraction pattern and bright ®eld micrograph of a ZrB2 ®lm synthesized in an Ar±BCl3 post-discharge (see process 4 in Fig. 1 for experimental conditions).
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² oxidation in Ar±O2 post-discharge at 653 K (140 nm thickness) [14]; ² oxidation in Ar±O2 post-discharge at 753 K (770 nm thickness) [14]; ² oxidation in Ar±O2 post-discharge at 873 K (4 mm thickness) [14]; ² RPECVD deposition at 733 K with an Ar±H2±O2 postdischarge (16 mm thickness) [16].
After treatment, only the substrates oxidized at 653 and 753 K are coated by ZrB2. As the substrate oxidized at 753 K is covered by a zirconia layer thicker than that oxidized at 653K, its ZrB2 ®lm is thinner. In spite of ZrB2 synthesis, Zircaloy-4 substrates lose weight again during the treatment. The two other samples are not coated by ZrB2. However, their weights have decreased after treatment. XRD analysis con®rms that zirconia layer is still present on Zircaloy-4 substrates. To synthesize ZrB2 on Zircaloy-4, the ®rst step consists of totally removing the zirconia layer. As long as zirconia is still present on Zircaloy-4, the diffusion mechanism for ZrB2 synthesis cannot occur because zirconia is a diffusion barrier. When Zircaloy-4 substrates are not oxidized in Ar± O2 post-discharge before the treatment, a non-uniform thickness distribution along the substrate is obtained. This phenomenon is probably due to the non-uniformity of the native oxide layer. To avoid this problem, all metallic substrates were oxidized before treatment. 3.4.4. Origin of boron incorporated in ZrB2 ®lms. The density of B atoms produced by an Ar-1.5% BCl3 microwave plasma have been determined in post-discharge by a titration technique involving the B 1 O2 chimiluminescent reaction [28]. It was found that the B-atom density nearly reaches 10 15 cm 23 at the plasma end (,10 cm after the gap). This corresponds to a BCl3 dissociation degree of about 30%, for a BCl3 density of 3 £ 10 15 cm 23. However, the B-atom density [B] at a distance d from the plasma end is quickly decreasing in post-discharge to follow a ®rst order kinetic law given by d B 3 2kw ln v B 0 where [B]0 is the B-atom concentration at the plasma end (d 0), v is the gas velocity (180 m/s in our experimental conditions) and kw is the destruction frequency of B atoms on the tube wall. From the results reported in reference [28] it was determined that kw (1.3±1.7) £ 10 4 s 21. Therefore, boron atoms quickly disappeared on the surface wall of the quartz tube wall before reaching the Zircaloy-4 substrate located at d < 110 cm. Boron incorporation in ZrB2 ®lms could come from BClx (x 1 or 2) species created in the discharge and transported in post-discharge. Oxygen in the ®lms is very likely due to the quartz discharge tube, which is etched by Ar±BCl3 discharges.
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Another possibility is that boron comes from BOx (x 1 or 2) species produced directly at the surface wall or in the gas phase. As a matter of fact, an increase in BO emission signal was observed at the end of the ¯owing discharge [22]. 4. Conclusion In this work, the in¯uence of the post-discharge composition on the ZrB2 synthesis at 733 K by RPECVD has been investigated. Experiments performed by thermal CVD and RPECVD with Ar±H2 post-discharge show that the deposition of ZrB2 ®lms on oxidized Zircaloy-4 is impossible at temperature lower than 853 K. Ar±H2±BCl3 post-discharges do not give to adherent ®lms on oxidized Zircaloy-4 at temperature lower than 753 K. Among the different tested con®gurations, only Ar±BCl3 post-discharges allow us to synthesize ZrB2 coatings. By using these post-discharges, it is possible to coat Zircaloy-4 tubes with a constant thickness over 20 cm long. ZrB2 coatings are dense but contain oxygen that mainly comes from the etching of the quartz discharge tube by the Ar±BCl3 plasma. As ZrB2 is the only crystallized compound, zirconium and boron oxides are amorphous. The coating is a mixture of nanograins of ZrB2 (10 nm crystal size) dispersed in an amorphous solid solution of boron and zirconium oxides. As the ZrB2 coating can only be synthesized on Zircaloy4 substrates at 733 K, the deposition mechanism consists of a diffusion step of zirconium from the substrate to the gas®lm interface where zirconium atoms react with boron chlorides to synthesis ZrB2. For this reason and also because the Ar±BCl3 postdischarge produces very ef®cient etching agents, it is necessary to perform a preoxidation treatment, leading to a ZrO2 layer. However, this protective layer must be removed during the treatment to produce a ZrB2 ®lm. We have also demonstrated that boron incorporated in the ®lms could not come from boron atoms produced in the ¯owing discharge.
Acknowledgements The authors wish to express their thanks to J.P. PreÂlot for his technical support. The company Fragema is gratefully acknowledged for its ®nancial support of this work.
References [1] P. Rogl, P.E. Potter, Calphad 12 (1988) 191. [2] A. Lebugle, G. Montel, Rev. Int. Hautes Temp. Refrac. 11 (1974) 231. [3] C.C. Wang, S.A. Akbar, W. Chen, V.D. Patton, J. Mater. Sci. 30 (1995) 1627. [4] M.P. Grimshaw, A.E. Staton-Bevan, Mater. Sci. Eng. B 5 (1989) 21. [5] C. Mroz, Am. Ceram. Soc. Bull. 72 (1993) 126.
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[6] D. Hertz, H. Michel, T. Belmonte, J.F. Pierson, French Patent, FR 2743-088 (1997). [7] J.F. Pierson, A. Billard, T. Belmonte, H. Michel, C. Frantz, Thin Solid Films 347 (1999) 79. [8] D.L. Chambers, C.T. Wan, K.A. Taylor, G.T. Susi, J. Vac. Sci. Technol. A 8 (1990) 2113. [9] A. Wang, G. MaleÂ, J. Euro. Ceram. Soc. 11 (1993) 241. [10] S. Motojima, K. Funahashi, K. Kurosawa, Thin Solid Films 189 (1990) 73. [11] A.L. Wayda, L.F. Schneemeyer, R.L. Opila, Inorg. Mater. 31 (1995) 965. [12] S. Reich, H. Suhr, K. Hanko, L. Szepes, Adv. Mater. 4 (1992) 650. [13] G.W. Rice, R.L. Woodin, Comm. Am. Ceram. Soc. 4 (1988) C181. [14] X. Iltis, M. Viennot, D. David, D. Hertz, H. Michel, J. Nucl. Mater. 209 (1994) 180. [15] H. Malvos, A. Ricard, J. Szekely, H. Michel, M. Gantois, D. Ablitzer, Surf. Coat. Technol. 59 (1993) 59. [16] J. Gavillet, T. Belmonte, D. Hertz, H. Michel, Thin Solid Films 301 (1997) 35. [17] J.F. Pierson, T. Czerwiec, T. Belmonte, H. Michel, A. Ricard, Plasma Sources Sci. Technol. 7 (1998) 54.
[18] S. Motojima, K. Funahashi, K. Kurosawa, Thin Solid Films 189 (1990) 73. [19] S. Berthon, G. MaleÂ, Ann. Chim. Fr. 20 (1995) 13. [20] A. Rousseau, A. Granier, G. Gousset, P. Leprince, J. Phys. D: Appl. Phys. 27 (1994) 1412. [21] JANAF Thermochemical Tables (1985). [22] J.F. Pierson, T. Czerwiec, T. Belmonte, H. Michel, Surf. Coat. Technol. 97 (1997) 749. [23] J.F. Pierson, T. Belmonte, H. Michel, Surf. Coat. Technol., 1999 (in press). [24] H.O. Pierson, Handbook of Chemical Vapor Deposition (CVD): Principles, technology and applications, Noyes Publication, New Jersey, 1992. [25] J. Livage, K. Doi, C. MazieÂres, J. Am. Ceram. Soc. 51 (1968) 349. [26] A. Lebugle, Ph.D. Thesis, Institut National Polytechnique de Toulouse, France, 1975. [27] T. Czerwiec, J.F. Pierson, T. Belmonte, H. Michel, A. Ricard, in: M. Bordage, A. Glaizes (Eds.), Proc. 23rd Int. Conf. on Phenomena in Ionized Gases, IGPIG, 1997, p. 198. [28] A. Ricard, T. Czerwiec, T. Belmonte, S. Bockel, H. Michel, Thin Solid Films 341 (1999) 1.