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Thin Solid Films 516 (2008) 7467 – 7474
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Platinum silicide phase transformations controlled by a nanometric interfacial oxide layer E. Conforto ⁎, P.E. Schmid Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut de Physique de la Matière Complexe, CH-1015 Lausanne, Switzerland Received 5 December 2007; received in revised form 12 March 2008; accepted 20 March 2008 Available online 30 March 2008
Abstract Nanometer-thick platinum silicide films were obtained by solid-state thermal reaction films in the presence of an interfacial native silicon oxide layer. They were studied using High-Resolution Transmission Electron Microscopy (HRTEM) and selected-area electron diffraction. Ten nm-thick sputtered Pt films reacted with the Si substrate through the oxide pinholes, which influenced the Pt–Si reaction over the whole annealing temperature range examined (165–800 °C). Silicide films grown through an interfacial oxide layer consist of two adjacent Pt2Si and PtSi layers in contrast with films obtained on oxide-free wafers, which show only PtSi grains. The continuous PtSi film transforms to an epitaxial, island-type film after annealing at 650 °C. The Pt2Si layer, to the contrary, remains unchanged up to 700 °C at least. The existence and stability of this layer at higher temperatures, together with the epitaxial relationship at the Pt2Si/PtSi interface help preserve the continuity and the good electrical conductance of silicide films obtained in presence of an interfacial oxide layer even above 700 °C. Epitaxial relationships between thin and very thin (3–5 nm) platinum silicide films and the Si substrates have also been studied directly from HRTEM images. Several orientation relationships for the PtSi/Si interface are discussed. © 2008 Elsevier B.V. All rights reserved. Keywords: HRTEM; Solid–solid thermal reaction; Platinum silicides; Pt2Si; PtSi; Epitaxy
1. Introduction Platinum silicide films are widely used in silicon devices for ohmic and Schottky contacts [1]. These technological applications require thinner contacts when the lateral dimensions of the devices are reduced. A well-defined process is necessary to produce continuous, homogeneous, well-reacted, low-resistivity, nanometer-thick silicide films. Such films are commonly obtained by solid-state thermal reaction between a very thin metallic film and a silicon substrate [2–4]. Available data in the literature about the Pt–Si reaction deal with reactants in full contact [5,6]. However, a thin oxide film may remain at the substrate surface due to an incomplete native oxide etching or to a fast native silicon oxide layer re-growth. It is generally admitted that a native oxide layer might be a cause of problems ⁎ Corresponding author. Present address: Centre Commun d'Analyses, Université de La Rochelle, F-17071 La Rochelle Cedex 9, France. Fax: +33 5 46 45 85 55. E-mail address:
[email protected] (E. Conforto). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.03.033
during contact formation [7]. We will show below that the presence of a thin oxide layer may actually stabilize the morphology of platinum silicide–silicon interfaces. In contrast with other metals (Cr, Ti and V), Pt and Pd diffuse through the oxide and form a silicide at the SiO2/Si interface [8,9]. Some authors have established that since Pt cannot diffuse through oxide layers thicker than 2.5–5 nm, transfer of Pt across thicker layers necessarily takes place through pinholes [7,9]. However, many points concerning the continuity, homogeneity, and degree of crystallographic order at the silicide/Si interface have not been discussed yet. In this work we report how Pt and Si react when an interfacial oxide layer is present, showing the silicide film morphology and structure evolution as a function of the oxide layer thickness and the annealing temperature. We compare these results with those of an oxide-free wafer annealed in the same conditions. We also show how interfacial oxide layers of the appropriate thickness can be beneficial for preserving the silicide film continuity up to temperatures higher than those observed in oxide-free wafers. Finally we analyze several
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Table 1 Platinum silicide phases, their crystallographic structure and the space group to which they belong (source: Joint Committee of Powder Diffraction Standards (JCPDS) cards: 7-251; 17-683; 34-903; 4-802) Silicide phase
Structure
Space group
Lattice parameter (nm)
PtSi [10]
Orthorhombic; MnP type
Pbnm (62)
α-Pt2Si [11]
Tetragonal; H2Th type Tetragonal; Ni12P5 type FCC, Cu type
I4/mmm (139)
a = 0.5932; b = 0.5595; c = 0.3603 a = b = 0.3933; c = 0.5910 a = b = 0.9607; c = 0.5542 a = b = c = 0.39231
Pt12Si5 [12] Pt [13]
I4/m (87) Fm3m (225)
epitaxial relationships between silicon and silicide films annealed at high temperatures. The silicide phases found in our samples are listed in Table 1. 2. Experimental procedure Pt films (2–14 nm thick) were deposited by sputtering on Si substrates heated to 165 °C. The substrates used were n-type, phosphorous-doped, [001]Si Czochralski-grown four-inch wafers with a resistivity in the 3–5 Ω cm range. Prior to film deposition, the substrates were submitted to different surface cleaning procedures, so as to vary the thickness of the remaining native silicon oxide, from zero to 2.2 nm. We obtained 3 to 5 nm-thick and 20 nm-thick platinum silicides without interface oxide and similar 20 nm-thick films with interfacial oxide layers of different thicknesses. Details on the procedures can be found in [14]. Transmission Electron Microscopy (TEM) observations were performed on planar-views and cross-sections. For planar-view images and Selected Area Electron Diffraction (SAED) observations, specimens were prepared in the [001]Si orientation. Crosssectional HRTEM specimens were prepared in the [110] orientation of the Si substrate. The samples were obtained by standard preparation techniques: mechanical polishing followed by ion milling. We used a Gatan DuoMill with a cooling accessory, a plasma beam of 5 keV in energy, at an incidence angle of 14°. Planar-views were bombarded with a single beam, on the substrate side; cross-sections were prepared using a double beam. The microscope was a Philips EM 430 ST instrument, with a point-topoint resolution better than 0.20 nm. The identification of crystalline phases in silicide films was performed by SAED and confirmed by the atomic plane spacings measured in the HRTEM images. 3. Results 3.1. The formation of platinum silicide films The mechanism of platinum silicide formation with and without an interfacial silicon oxide layer and the succession of phase transformations as a function of the annealing temperature, have been described by Conforto and Schmid [14]. According to this study, the silicide phases formed during the Pt–Si solid–solid reaction, and their existence as a function of the annealing temperature, depend on the size and on the density of the oxide pinholes.
Fig. 1 shows the reaction progress and the crystallinity of silicide films with and without an interface silicon oxide layer after dc-sputtering deposition of platinum and annealing at 165 °C. The reaction stage reached by each type of film depends on the surface contact between platinum and silicon. In Fig. 1a the substrate is an oxide-free wafer (called hereafter “wafer X”). An unreacted Pt film covers the platinum silicide layer which in turn consists of two parts: the upper sublayer is partially crystallized while the lower sublayer is amorphous. The latter, close to the Si interface, corresponds to a mixture of Pt and Si atoms. The creation of non-crystalline mixtures by solid-state amorphization has been previously reported by Ogawa et al. [15]. The Si substrate also shows a certain degree of crystallographic disorder over a few monolayers close to the reaction front. The crystallized layer is formed by metal-rich silicides such as Pt2Si and Pt12Si5, which are both of tetragonal symmetry. In Fig. 1b the Pt–Si reaction on wafer Y proceeds through a 1.3 nm-thick interfacial oxide layer and the silicide layer grows below it. It is well known that silicon oxide layers exhibit a pore density which varies with the type of growth (wet, dry, native) [16]. These oxide pores (or pinholes, noted as “P” in Fig. 1b) can be identified thanks to the contrast produced by the Pt diffusing through them. In wafer Y, the mean pinhole separation, measured on several images, is about 2.0 nm. The platinum silicide layer is quite similar to that obtained without an interfacial oxide layer, except for the absence of the amorphous silicide layer. The only crystalline phase present in such films is Pt2Si which can also be identified inside the pinholes, showing atomic planes in the [110] direction. The silicide layer is continuous. The interface with silicon is flat between atomic-size steps. Fig. 1c shows wafer Z, on which a metallic Pt layer sputterdeposited over a 2.2 ± 0.3 nm-thick native oxide film. The oxide layer completely covers the substrate surface and no regions of direct contact between Pt and Si are found. We can see the diffusion through the pinholes of the oxide, however. Their mean spacing is 8.0 nm, four times larger than in the 1.3 nmthick oxide of wafer Y. In this sample we identify the first step of the Pt–Si reaction, in which the silicide does not form a continuous layer yet, but develops amorphous Pt + Si agglomerates in the vicinity of the oxide pinholes. The large differences observed between as-deposited samples with and without an oxide barrier with respect to pinhole density are even more patent after annealing at a slightly higher temperature. The three types of silicide films formed at 250 °C are continuous and constituted of polyhedral grains with sizes between 10 and 20 nm. The SAED pattern in planar-view specimens and the HRTEM images in cross-sectional samples, reveal different sets of silicide phases: a) Pt2Si and PtSi in oxide-free wafers; b) Pt2Si and unreacted Pt in samples with 1.3 nm-thick interfacial oxide; c) Pt12Si5, PtSi and unreacted Pt in samples with 2.2 nm-thick interfacial oxide. The most striking morphology difference with respect to the other two types of silicide films at the same annealing temperature is found in wafer Z. The cross-sectional image (Fig. 2a) shows the formation of silicide agglomerates below each oxide pinhole. The reaction front, the oxide layer and the silicide/SiO2 interface are not flat and some roughness is
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Fig. 1. HRTEM cross-sectional images of the 165 °C-annealed (as-deposited) samples: a) Wafer X (oxide-free), constituted of an unreacted Pt film, a partially crystallized Pt-rich silicide layer and an amorphous Pt + Si layer; b) Wafer Y (with 1.3 nm-thick interfacial oxide), with reactants diffusing through the oxide pinholes (noted P) to form a well crystallized silicide layer; c) Wafer Z (with 2.2 nm-thick interfacial oxide) shows amorphous Pt + Si agglomerates below each oxide pinhole.
observed at the top of the silicide agglomerates. Grains of the Pt12Si5, a tetragonal Pt-rich phase, are found just below the oxide layer, close to the pinholes. The PtSi phase, less rich in platinum, can be found at the silicide/silicon interface and at the center of the horizontal bridges connecting two agglomerates. Each agglomerate causes an increase in volume of about 15% under the wafer surface, which is no longer flat.
After annealing at 550 °C for 30 min, silicide films formed with and without an interfacial oxide layer are continuous and polycrystalline. However, the film obtained without an interfacial oxide layer is the only film consisting of a unique silicide phase (PtSi), the other films being biphasic (Pt2Si at the surface and PtSi at the interface with Si, as in wafer Y in Fig. 3).
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Fig. 2. HRTEM cross-sectional images of wafer Z annealed at 250 °C. a) It shows non-overlapping silicide agglomerates, each one corresponding to an oxide pinhole, formed by metal-rich phases (at the top) or PtSi (at the bottom); b) Detail of an agglomerate from panel a showing a Pt12Si5 grain in the [1,0,− 1] zone axis.
After annealing at 650 °C the PtSi phase re-orders, becoming epitaxial to the Si substrate. Facets first appear in PtSi grains, almost parallel to the (111)Si direction. This process starts at the bottom of the grains, progresses towards the top, and eventually separating them. In these conditions, films formed exclusively
by PtSi become discontinuous, island-type films. We will see later in detail different possibilities for these orientations. In wafer Y, continuity is preserved up to at least 700 °C (Fig. 4a). In spite of the growing facets at the PtSi/Si interface which tend to isolate the PtSi grains, the Pt2Si layer remains
Fig. 3. HRTEM cross-sectional images of sample Y annealed at 550 °C showing a silicide film formed by polycrystalline PtSi grains and with a continuous Pt2Si layer at the top. It is in contact with the remaining unreacted Pt (a discontinuous layer) through the oxide pinholes.
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Fig. 4. HRTEM cross-sectional images of wafer Y annealed at 700 °C showing partially isolated, nearly disrupted PtSi grains capped by a continuous Pt2Si layer. b) Detail of the polycrystalline and continuous Pt2Si layer and its interface to PtSi, c) fast Fourier transform of b) where the two spots corresponding to the dense planes are connected by a Δg vector perpendicular to the interface.
unchanged and guarantees the continuity of the silicide film. At this annealing temperature all Pt has reacted. Fig. 4b, a detail of Fig. 4 at higher magnification, shows the continuity of the Pt2Si layer and the flatness of the Pt2Si/PtSi interface. The fast Fourier transform in Fig. 4c will be discussed in section 4.3. The Pt–Si phase diagram [17] predicts the disappearance of the α-Pt2Si phase above 700 °C, and in fact at 750 °C the Pt2Si layer is no longer observed. The transformation to a PtSi island-type film is thus complete at 750 °C.
Fig. 5. HRTEM image of a 5 nm-thick PtSi film on a [001]Si substrate. (220) atomic planes of PtSi correspond to (220) planes of Si.
3.2. Orientation relationships between PtSi and [001]Si Epitaxial relationships can be observed by HRTEM between PtSi and [001]Si after re-ordering of the PtSi layer. The reordering in very thin PtSi films (3–5 nm thick) is complete at lower temperatures (b 600 °C) than in 20 nm-thick films (650 °C).
Fig. 6. HRTEM image of another grain in the same structures as in Fig. 5. (002) planes of PtSi correspond to (220) planes of Si. Circles on the grazing view (b) underline misfit dislocations.
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Fig. 7. HRTEM cross-sectional images of wafer X annealed at 700 °C: an isolated PtSi grain shows (200) planes, in epitaxial relationship with (220) planes of the substrate.
Fig. 5 shows an interface between a 5 nm-thick PtSi film and a [001]Si substrate without an interfacial silicon oxide layer and annealed at 600 °C during 30 min. Their orientation relationship was first described by Ben Ghozlene et al. [18] on the basis of diffraction patterns. It concerns the following zone axes: [110] Si//[001]PtSi. The (220) planes of Si correspond to (220) of PtSi, across the interface. We can verify that the angle between (110) and (1–10) of orthorhombic PtSi is larger than 90°. We can see the same structures in Fig. 6a, where the (220) planes of Si match the (002) planes of PtSi. It is the same epitaxial relationship as in the previous case but rotated by 90° with respect to the growth axis: [001]Si//[1–10]PtSi. As the (110) and (1–10) planes of PtSi are not perpendicular to each other, atomic [110] columns are not visible after rotation of 90°. This is the reason why only the (001) planes are observable. In Fig. 6a and b some edge dislocations are present every 13 atomic planes, which corresponds to the compensation of a misfit of the order of 1/13 = 7.7%. As the distance between (002) planes of PtSi is: 0.3587 nm ÷ 2 = 0.1793 nm, and that between (220) planes of Si is: 0.1920 nm, the misfit to be relaxed is: (0.1920–0.1793)/0.185 = 0.013/0.185 = 6.85%. Thus, the misfit can completely relax with the help of the edge dislocations. Fig. 7 shows an isolated and epitaxial PtSi grain of sample X, 20-nm thick, without an interfacial oxide layer and annealed at 700 °C. The (220) planes of Si match with (200) planes of PtSi, which are close to the [001] zone axis. Here, two planes of PtSi correspond to three planes of Si. This orientation is different from those reported by Ben Ghozlene et al. 4. Discussion and conclusions
the PtSi grains has not started yet. This re-ordering is induced by the substrate recrystallization and identified by the facet formation in the PtSi grains. This is the only mechanism acting in silicide films grown without an interfacial oxide layer above 550 °C. It leads to film discontinuities at temperatures below 650 °C. The silicide formation follows other paths when a barrier hindering the diffusion of the reactants is present. The oxide pinhole density, the influence of which has been observed on the flatness of the silicide/Si interface after low-temperature annealing, controls the Pt supply to the Pt–Si reaction front. Then, the silicide/Si interface flatness mainly depends on the distance between the pinholes in the oxide layer. The phase distribution is also different from that observed in oxide-free wafers. Due to the reduced Pt supply, the Pt–Si reaction progresses more slowly than in oxide-free samples. Depending on the oxide pinhole density and diameter, the silicide film obtained after annealing at 165 °C can be either in the first stage of the Pt–Si reaction, or in an advanced state, similar to that obtained with an oxide-free wafer. In the later case however, the amorphous layer at the Si interface is absent, due to the low reaction speed at the Si interface, and the entire layer is able to crystallize. Around 250 °C, silicide films with an interfacial oxide show a complex phase distribution corresponding to Pt/SiO2/Pt-rich silicide/PtSi/Si. The type of Pt-rich silicide phase formed at the oxide interface (Pt2Si in wafer Y, and Pt12Si5 in wafer Z) depends on the local concentration of Pt atoms, which itself depends once again on the oxide pinhole density and diameter. At higher annealing temperatures (N 350 °C), however, the only Pt-rich phase found was Pt2Si, even for large distances between pinholes, which transforms to PtSi at the Si interface. Pt2Si, PtSi and unreacted Pt can exist simultaneously as long as Pt atoms remain available in the unreacted metal film. As in oxide-free samples, silicide films with an interfacial oxide seem to reach a relative crystallographic stability between 350 and 550 °C. This apparent stability reflects in fact a dynamic equilibrium: the Pt2Si phase appears to form and to dissociate at the same rate, extracting atoms from the unreacted Pt layer through the oxide pinhole and providing Pt atoms for the formation of PtSi. The thickness of the Pt2Si layer and the PtSi grain size remain almost unchanged over the 350–550 °C temperature range, but the total thickness of the silicide film increases slightly. At variance with the oxide-free wafers, two transformation mechanisms are in competition in silicide films with an interfacial oxide layer above 550 °C. The first process is the
4.1. Phase distribution and reaction mechanism The phase distribution in silicide films grown without an interfacial oxide layer, and annealed for 30 min, is described in Table 2. This phase distribution has been discussed in the literature and reaction mechanisms have been proposed [14,19]. Stable polycrystalline PtSi films with a flat silicide/Si interface are obtained after annealing in the 350–550 °C temperature range because the Pt–Si reaction is complete and the re-ordering of
Table 2 Phase distribution as a function of the annealing temperature in samples without an interfacial oxide layer Temperature
165 °C
250 °C
N350 °C
Phase distribution
Pt/Pt2Si/ a-Pt+Si/Si
Pt2Si/PtSi/Si, where all Pt has been consumed
PtSi/Si All Pt consumed
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continuing Pt2Si formation during the course of the Pt–Si reaction, and its later transformation into PtSi. This process helps maintain film continuity as the Pt2Si film does not recrystallize and remains flat. The second process is the recrystallization of PtSi and facet creation (as observed in oxide-free wafers), leading to a discontinuous, island-type film. If the oxide is thick, the oxide pinhole density is low and the Pt supply to the Pt–Si reaction is slow, even with a non-negligible unreacted Pt film available up to 650 °C. In this case PtSi recrystallization becomes faster than Pt2Si formation, leading to film discontinuity below 650 °C. In other words: no more available Pt (as in oxide-free wafers) or not enough Pt to react with Si (as with thick interfacial oxide) leads to film discontinuity in the same temperature range. With a thin oxide layer, both mechanisms mentioned above remain in equilibrium until the complete consumption of the unreacted, slowly diffusing platinum. The Pt2Si layer is continuous and homogeneous in thickness up to 700 °C (Fig. 4). As Pt2Si is no longer formed, it progressively disappears above this temperature, and PtSi recrystallization becomes the preponderant mechanism. Overlayer discontinuity sets in around 750 °C, i.e., about 100 °C higher than for thick interfacial oxide or oxide-free samples. 4.2. Diffusion-induced disorder Strongly disordered silicon and amorphous mixtures of Pt and Si atoms have been observed at the interface front of the low-temperature Si–Pt2Si reaction. Similar disordering induced by the diffusion of Mo atoms into a TiCN crystal has been observed at the interface between the core and the rim of a cermet grain [20]. Silicon amorphization can also be produced by ion implantation, according to [21]. It is not clear yet whether these effects result from a complete amorphization, or to an amount of disordering sufficient to blur out high resolution images. In any case, the present observation shows that the Si– Pt2Si solid–solid state transformation cannot be described by simple and homogeneous atomic displacements. 4.3. Orientation relationships and interfaces Figs. 5 and 6 show that the orientation relationship between Si and PtSi already observed by Ben Ghozlene et al. [18], who called it (1–10) as the interface is parallel to (1–10)PtSi and to (001)Si. The orientation relationship in Fig. 7 has not been reported before. The corresponding interface is parallel to (010) PtSi and (001)Si. Both orientation relationships are described in the reciprocal space, along a zone axis perpendicular to the original (001) Si surface, in Fig. 8a and b. The second (1–21) orientation relationship mentioned by Ben Ghozlene et al. has not been observed. A fast method to determine the best orientation relationship and the corresponding interface has been recently proposed [22]. One rule states that if the superposition of the two reciprocal lattices involves two nearly coinciding diffraction vectors at right angle, then the plane containing these two directions define a favorable interface plane. This is obviously the case in Fig. 8a where (220)Si ≈ (002)PtSi, and (2–20)Si ≈ (220)PtSi, as well as
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Fig. 8. a) Orientation relationship for which the interface is parallel to (1–10) PtSi and (001)Si observed by Ben Ghozlene et al [18]; b) Orientation relationship for which the interface is parallel to (010)PtSi and (001)Si.
in Fig. 8b where (220)Si ≈ (300)PtSi, and (2–20)Si ≈ (002)PtSi. The interfaces observed by Ben Ghozlene et al. and by us are thus likely to have a fairly low energy. The rules developed in [22] are based on the continuity of dense planes across the interface (edge-to-edge matching) [23,24], which is equivalent to the so-called Δg concept in the reciprocal lattice [25,26]. Dense planes are actually continuous in Figs. 5 and 6, as well as in Fig. 7. In the latter case, however, only a fraction of the dense planes are continuous, in agreement with the fact that the coinciding vectors are (220)Si and (300)PtSi whereas the corresponding dense planes are (220)Si and (200)PtSi. The absence of the second (1–21) orientation relationship reported by Ben Ghozlene et al. can be explained by the sequence of the transformations observed in our experiments, namely Si–Pt2Si–PtSi. As a matter of fact, Ben Ghozlene et al. proposed that the (1–10) orientation relationship of PtSi may result from the transformation of an initially formed Pt2Si phase, whereas the (1–21) orientation relationship of PtSi may form directly on the Si substrate. One can note that dense planes are also continuous across the PtSi/Pt2Si interface shown in Fig. 4b. This zone is situated between two original oxide pinholes, i.e., it was a “bridge” between two ancient silicide agglomerates, formed by lateral diffusion of Pt. It is not situated in the straight diffusion path of Pt atoms towards the PtSi layer. Thus, it is a more stable interface zone than that under pinholes. Fig. 4c shows the fast Fourier transform of the Pt2Si/PtSi interface of the same figure, where the two spots corresponding to the upper and lower dense planes are connected by a Δg vector perpendicular to the horizontal interface. This is in agreement with the continuity of the corresponding dense planes, according to [22–25]. It confirms that it is a stable, low-energy interface. This stability requires a higher energy to be transformed and, indeed, it contributes to the continuity of the silicide until 750 °C, instead of 650 °C. Acknowledgements We thank Dr. Daniel Caillard for useful discussions. The financial support of the Brazilian Conselho Nacional de Pesquisa e Desenvolvimento (CNPq) and of the Swiss National Science Foundation is gratefully acknowledged.
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