Synthesis of molybdenum disilicide on molybdenum substrates

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Surface and Coatings Technology 76-77 (1995) 7-13

Synthesis of molybdenum disilicide on molybdenum substrates S. Govindarajan a, B. Mishra a, D.L. Olson a, 1.1. Moore a, J. Disam b a

Departmentof Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO-80401, USA b Schott Glaswerke, Mainz, Germany

Abstract Molybdenum electrodes are widely used in the glass-making industry for electrical resistance heating of glass melts. The electrodes are exposed to an oxidizing environment at temperatures around 1600°C, conditions which result in accelerated oxidation and eventual failure of the electrodes. One approach to protect molybdenum at elevated temperatures is to synthesize a coating system based on molybdenum disilicide (MoSi2 ) , which has excellent oxidation resistance at high temperatures due to the formation of a "self-healing" layer of silica. A potential problem is the mismatch in the coefficient of thermal expansion between molybdenum and MoSi2 , which would result in spalling of the film, on cooling from elevated temperatures. The tendency to spall can be minimized by producing multilayer films. This paper will focus on the synthesis of molybdenum disilicide on molybdenum substrates. Thin films of molybdenum and silicon were deposited (using d.c.zr.f magnetron sputtering) either as single layers or in a multilayer structure, under different processing conditions. The molybdenum layer thickness was varied between 50 nm and 1000 nm and the Si/Mo atomic ratio was maintained at 3 (i.e. a stoichiometric excess of Silo The as-deposited films were subjected to a diffusion anneal treatment at 1000-1100°C (in vacuum or an Ar ambient) for different times. Details of the phase transformations, characterized by X-ray diffraction analysis, and microstructural development (through scanning electron microscopy) will be presented. The results indicate that dense molybdenum disilicide can be synthesized on molybdenum substrates under carefully controlled processing conditions. Keywords: Synthesis; Molybdenum disilicide; Molybdenum substrates

1. Introduction

Amongstthe group of materials called the refractory metals (which comprises metals with melting points greater than or equal to that of chromium (2148 K)), molybdenum undergoes the most severe oxidation in air, at temperatures above 1200 K [1]. The catastrophic oxidation at elevated temperatures has been ascribed to the formation of volatile oxides (e.g. Mo0 3(g), (Mo0 3h(g), MoOz(g), etc.) [1,2]. While alloying of molybdenum would be the simplest process for imparting oxidation resistance, to date, a suitable alloy has not yet been developed. Consequently, the main thrust of research efforts geared towards the protection of molybdenum against oxidation has been towards the development of a suitable coating system. Molybdenum disilicide (MoSi z) has emerged as one of the most promising components of such a coating system, primarily due to excellent oxidation resistance at elevated temperatures [3-7]. However, there are two important limitations associated with the use of MoSi z on a molybdenum substrate, i.e. the coefficient of thermal expansion (CTE) mismatch, at 1600 DC, between Mo'Si, (9.7 x 10- 6 K -1) and the molybdenum substrate (7.8 x 10- 6 K -1), and Elsevier Science S.A. SSDI 0257-8972(95)02524-3

the diffusion of silicon into the substrate (which results in the formation of less oxidation resistant sub-silicides, i.e. Mo.Si, and MoSi 3 ) . One approach, which is the focus of this article, for overcoming the CTE mismatch is to develop a multilayer coating system, with a suitable layer thickness. The diffusion of silicon into the substrate can be minimized by incorporating a diffusion barrier layer. There is widespread interest in the Mo-Si system because of potential applications in a number of areas such as metallization [7-10], multilayer mirrors [11], large area pulse-heated surfaces [12], heating elements [13], and temperature sensors for turbine engines [14]. As a result, a large knowledge base on the Mo-Si system has evolved during the last two decades. In particular, Mo and Si thin films deposited by physical/chemical vapor deposition have been the subject of detailed studies with respect to their applications in the electronics and optics industries [7-11,15-20,23-29]. Stearns et al. [8] examined Mc--Si multilayers and reported that the molybdenum layers were polycrystalline, with a [110] texture in the growth direction. The silicon layers, on the other hand, were amorphous. The pure layers were separated by interfacial regions of mixed composition,

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S. Govindarajan et al./Surfaceand Coatings Technology 76-77 (1995) 7-13

where the width of the interface corresponding to the deposition of molybdenum on silicon was typically greater than that corresponding to silicon on molybdenum. In addition, a transition from layer growth to columnar growth was observed at sputtering pressures above 6.6 x 10 -6 atm (5 mTorr). The strongly oriented growth of molybdenum was also observed by Ramos et al. [15] during a study of molybdenum deposition on cast iron substrates. The intrinsic film stress in molybdenum films deposited by magnetron sputtering has been shown to depend strongly on Ar gas pressure [12,21]. A transition from compressive to tensile stresses has been reported to occur between 3.95 x 10- 6 atm (3 mTorr) and 5.26 x 10- 6 atm (4 mTorr), depending on the geometry of the system (e.g. source-substrate distance). Ijdiyaou et al. [10] found that the nature of the interfaces in Mo-Si multilayers was strongly dependent on the order of the deposit: viz., if silicon is sputtered onto molybdenum, the interface is abrupt, whereas in the reverse order a diffuse interface is formed. This observation is explained by the facts that the cohesion energy of molybdenum (7 eV) is greater than that of amorphous silicon (3 eV) and the sputtered silicon film has a high density of voids; consequently, the implantation of molybdenum atoms in the silicon layer is easier than the reverse. Hong et al. [22] used quantum mechanical calculations to show that impurities such as carbon, niobium, oxygen, boron and sulfur decrease the adhesive energy of the Mo-McSi, interface. In the as-deposited condition, thin films of Mo are crystalline whereas Si films are amorphous, and there is no reaction between the deposited layers to form an intermetallic silicide [16-20,23-29]. Consequently, a diffusion anneal treatment is essential to effect the transformation to the intermetallic silicide. MoSi z exists in two crystalline forms: a metastable, low-temperature, hexagonal form (h-Mobij] and a stable, hightemperature, body-centered tetragonal form (t-Mo Siy). Yanagisawa et al. [23] observed that the transformation from hexagonal to tetragonal form occurred above 800°C. The effects of layer thickness, Si to Mo atomic ratio (Si/Mo], and the diffusion anneal treatment on the final phases formed, film stress, volume shrinkage, and process kinetics have been studied in detail [ 16-20,23-30]. Some of the important findings from these studies are listed below. • Above an atomic ratio of Si/Mo greater than 2, only MoSi z is formed. Below this ratio, sub-silicides such as Mo.Si, and M0 3Si are also formed [17-20,25-28]. • Following the diffusion anneal treatment, the MoSi z film is typically under a tensile stress (1-5 GPa). This could lead to cracking and spalling of the film, especially if the film thickness exceeds a critical value (around 0.1 to 0.4 urn) [17-20,26,29]. • The transformation of individual layers of Mo and Si

to MoSi z is associated with a net volume shrinkage (about 15-20%) [7,20,27]. • During diffusion anneal treatments above 1000 °C, the principal diffusing species is Si. A linear growth law is followed, which is indicative of an interfacial reaction controlled mechanism [14,20,30-31]. For co-sputtered layers of Mo and Si, intimate contact of the reactant species precludes the occurrence of excessive diffusion currents [20]. The above studies involved deposition of Mo/Si layers on silicon/silica coated substrates, which provides a large reservoir of silicon for the phase transformations during the diffusion anneal treatment. Consequently, the final product after anneal above 1000 °C essentially consisted of MoSi z. The stability of a MoSi z (deposited by CVD) coating on molybdenum was examined by Bartlett et al. [32]. The typical sequence of reactions observed at 1988 K was the conversion of Mo'Si, to MosSi 3 , followed by the growth of M0 3Si (after consumption of MoSi z). The principal diffusing species was identified as silicon. Chou et al. [33] sputter deposited MoSi z from a composite target onto Si and NaCl substrates and studied the effect of diffusion annealing at different temperatures and durations. The as-deposited, amorphous Mo'Si, was found to transform into tetragonal MoSi z and some Mo.Si, after annealing at 900°C for 2 h. The formation of t-Mo.Si, was considered to be an integral part of the phase evolution process. The synthesis of molybdenum disilicide on molybdenum substrates, using magnetron sputtering, remains a largely unexplored area of research. As the above survey indicates, a number of phenomena come into play during the deposition and diffusion annealing treatments. This paper will focus primarily on the effects of layer thickness and incorporation of some potentials silicon diffusion barrier layers.

2. Experimental procedure A vertical cathode sputtering unit was used to deposit thin films of molybdenum and silicon. DC magnetron sputtering, with a magnetron drive capable of supplying up to 10 kW power, was used to deposit molybdenum, and d. magnetron sputtering, with a power supply capable of delivering up to 2.5 kW, was used for silicon deposition. The silicon target (purity of 99.999%) was bonded onto a water-cooled copper backing plate whereas the molybdenum target (99.95% purity) was directly water cooled. The dimensions of both targets, supplied by Plasmaterials Inc., were 127 x 381 x 6.35 mm (5 x 15 x 0.25 inch). The target to substrate distance was kept constant during all the runs at 75 mm. The typical base pressure during each run was 2.63 x 10- 8 atm (2 x lO- s Torr). The substrates used in this study were glass

S. Govindarajan et al.ISurface and Coatings Technology 76-77 (1995) 7-13

slides (76.2 x 25.4 x 1.2 mm) for the deposition rate measurements and polished molybdenum coupons (10 x 30 x 1 mm) for microstructural/X-ray diffraction analysis. The substrates were cleaned with standard procedures, followed by glow discharge cleaning inside the sputtering chamber [34]. The deposition rates for molybdenum and silicon (under 0 bias conditions, i.e. grounded substrates) were found to vary linearly with applied power, in the range of power and inert gas pressure used [34]. Least squares regression analysis was used to develop the following relationships: dMo = 725.69 x P - 39.82

(1)

dsi=555.62 x P-82.14

(2)

where d, is the deposition rate of species i in A min- 1 and P is the d.c.zr.f power in kW. The lower deposition rate of silicon follows from the lower sputter yield for silicon as well as the use of an r.f. power supply. X-ray diffraction analysis of the as-deposited films showed that the molybdenum films were polycrystalline and exhibited a strong preferred orientation in the [110] direction. In addition, the films were in compression (determined through X-ray diffraction analysis of d-spacing data), irrespective of the substrate bias. The silicon films, however, were amorphous. Scanning electron microscopy of the molybdenum films revealed that the molybdenum films were columnar (typical of the zone 1 morphology proposed by Thornton [35]) and the density of the films (as observed in the SEM) increased with increasing substrate bias.

3. Results and discussion 3.1. Molybdenum-silicon diffusion couple Si films ranging from 1 to 5 11m thick were deposited on molybdenum substrates, using r.f magnetron sputtering. The films were then annealed in a vacuum furnace at 1000-1100 °C for 30 min. The results are presented in Table 1.

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The objectives of the above experiments were to assess the reaction between silicon and the molybdenum substrate (formation of MoSi 2 at the interface could be used to enhance adhesion) and to determine the diffusivity of silicon in molybdenum. However, XRD analysis revealed the absence of any strong peak of MoSi 2 , although the sub-silicides formed under certain conditions. The presence of the diffraction peaks for Mo and Si was indicative of incomplete reaction. Typically, the annealed films were "wrinkled", owing to the compressive thermal stresses generated by the presence of the lower CTE Si film. Analysis of the reaction mechanism and determination of the diffusivity of Si necessitates a depth profiling experiment, using auger electron microscopy (AES)/electron probe microanalysis (EPMA), which will be conducted in the near future.

3.2. Mo/Si multilayerfilms Single/multilayer films of Mo/Si, with an atomic ratio of Si/Mo = 3:1 and a total coating thickness of 4 11m, were deposited on molybdenum substrates using d.c. (for Mo) and r.f. (for Si) magnetron sputtering. The number of Mo/Si layers were varied from 1 to 20, thus providing a variation in individual layer thickness. XRD analysis of the as-deposited films revealed strong preferred orientation of the Mo films in the [110] growth direction, whereas the Si films were amorphous. The results of the diffusion annealing experiments are summarized in Table 2. Visual examination of the annealed films revealed a smooth, light gray coating and no evidence of either macroscopic spalling or wrinkling of the films; since either an unreacted Mo layer or a silicide layer (with a higher CTE than the Mo substrate) formed on the substrate, the coating would not be in compression. XRD analysis of the films showed that the strongest MoSi 2 peaks occurred with the 20 and 10 layer structures, after a diffusion anneal at 1000 °C for 0.5 h. However, in no case could the formation of the subsilicides be avoided, although a stoichiometric excess of silicon was present. A possible explanation could be the

Table 1 Results of diffusion annealing experiments with Si coatings on molybdenum Coating

Diffusion anneal

Macroscopic appearance

XRD analysis: 6 strongest peaks.

1 urn Si

1000 -c, 0.5 h 1000 -c 0.5 h (Ar) 1100 -c, 0.5 h 1000 -c, 0.5 h 1000 -c, 0.5 h (Ar) 1100 -c, 0.5 h 1000 -c, 0.5 h 1000 -c, 0.5 h (Ar) 1100 -c, 0.5 h

Smooth, adherent. Wrinkled Smooth, adherent Wrinkled Spalled at edges Wrinkled, adherent Wrinkled Wrinkled, adherent

Mo, M0 3Si Mo, Si; Minor = M0 3Si M0 3Si, Mo, Si; Minor = Mo.Si, Mo, Si Mo, Si, Minor = M0 3Si Mo, Si, M0 3Si Mo, Si, Minor = MoSi 2 Mo, Si, Minor = MoSi 2 , M0 3Si

5 um Si

5 urn Si, -200 V bias

10

S. Govindarajan et al./Surface and Coatings Technology 76-77 ( 1995) 7-13

Table 2 Results of diffusion annealing experiments with Mo/Si multilayer coatings on molybdenum Coating

Diffusion anneal

Macroscopic appearance

XRD analysis: 6 strongest peaks.

Mo=0.05 urn, 20 layers;

1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC,

Smooth, Smooth, Smooth, Smooth, Smooth, Smooth, Smooth, Smooth, Smooth, Smooth, Smooth,

M0 3Si, Mo, MoSi 2 ; Minor = Mo.Si, MoSi 2 , Mo; Minor = M0 3Si Mo, MoSi 2 , M0 3Si; Minor = MosSi 3 M0 3Si, Mo, MoSi 2 ; Minor = Mo.Si, Mo, Si Minor = M0 3Si Mo, Si; Minor = Mo.Si, Mo, Si Mo, M0 3Si; Minor = MoSi 2 , MosSi 3 Mo, Si; Minor = MoSi 2 , M0 3Si

Si=0.15Ilm, 20 layers Mo = 0.1 urn, 10 layers; Si=O.3llm, 10 layers Mo=1.0 urn; Si=3.0 urn. Mo= 1.0 urn, -150 V bias; Si=3.0 urn, -150 V bias

0.5 h 0.5 h (Ar) 0.5 h 0.5 h 0.5 h (Ar) 0.5 h 0.5 h 0.5 h (Ar) 0.5 h 0.5 h 0.5 h (Ar) 0.5 h

formation of the sub-silicides during the post-anneal furnace cooling period. The transformation of MoSi 2 to Mo.Si, occurs by the reaction 5 MoSi 2 = Mo.Si, + 7Si: the presence of free Si in the diffraction pattern would therefore provide evidence for this mechanism. The absence of Si in the diffraction pattern for the 20/10 layer structures suggests that either the amount of Si present is below the detection limits of the diffractometer or that an alternative process occurs. For example, the sub-silicides may be forming near the molybdenum substrate, or the presence of oxygen as an impurity element may cause the precipitation of the sub-silicides. Again, there is a need to carry out a depth-profiling analysis to identify the reaction mechanism. A micrograph of the as-deposited 10 layer structure, with a relatively uniform layer thickness, is shown in Fig. l(a). After annealing, the multilayer structure transformed to MoSi 2 (identified through energy dispersive spectroscopy). The typical morphology of the MoSi 2 layer is shown in Fig. 1(b). The transformation was accompanied by a volume decrease of around 20%.

adherent adherent adherent adherent adherent adherent adherent adherent adherent adherent adherent

2

8H-~-~t'8~ I) ---- - - - -

~

8f')c.lC

-

-

----- - - - -

PUDJIl

(a)

3.3. MolSi multilayers on an intermediate silicon layer

One possible explanation for the formation of subsilicides in the annealed multilayer films of Mo/Si is a deficiency of silicon in the coating. Since the exact densities of the as-deposited films are difficult to determine, the theoretical densities were used to determine the thickness ratio Mo/Si layers required to produce an Si/Mo atomic ratio of 3. There is, obviously, the potential to deviate from the required stoichiometry. Consequently, a set of experiments were conducted with an intermediate layer of Si, ranging from 0.05!-lm to 5.0 urn thick, followed by the multilayer Mo/Si structure. The presence of the intermediate layer could act as a reservoir of Si to make up for any deficiency in the multilayer structure. In addition, the possibility of improving adhesion to the substrate by diffusion bond-

(b) Fig. 1. (a) As-deposited multilayer film (10 layers each of Mo (Gl um) and Si (0.3 11m)) on Mo substrate. (b) Diffusion annealed multilayer film (10 layers each of Mo and Si) showing the morphology of the tetragonal-McSi, product.

S. Govindarajan et al.jSurface and Coatings Technology 76-77 (1995) 7-13

ing could be explored. Results of the diffusion anneal treatment are summarized in Table 3. Wrinkling or spalling of the coating could be avoided with an intermediate Si layer less than OLum thick, followed by the 20 layer structure (i.e. Mo = 0.05 urn; and Si=0.15 um). Strong peaks of MoSi 2 were obtained in the 20 layer structure, on a Si layer of 0.05-0.1 urn thick, annealed for 0.5 hat 1000 "C. However, the ubiquitous sub-silicides formed, yet again. This observation further strengthens the argument that the sub-silicides form either during the cooling period (about 2 h to reach room temperature) in the furnace or, possibly, at the Mo substrate-coating interface.

3.4. MojSi multilayers on a diffusion barrier layer By introducing a barrier layer for silicon diffusion into the molybdenum substrate, it would be possible to eliminate an interfacial reaction at the molybdenum

11

substrate-coating interface as a possible explanation for the formation of the sub-silicides. Therefore, experiments were conducted with some potential silicon diffusion barriers, viz. Ti, TiN, Cr and CrN. Refractory metal nitrides, particularly TiN, have been used in the semiconductor industry as a silicon diffusion barrier, at temperatures below 700°C. The efficacy of these materials at high temperatures has not been studied in much detail. Ti and Cr have typically been used primarily to enhance adhesion of the coating to the substrate. It is conceivable that silicon could be consumed by reaction with Ti or Cr, thus forming a "sacrificial barrier". A 0.2 urn thick barrier layer was deposited, using cathodic arc evaporation, followed by the multilayer structure. The results of a diffusion anneal treatment at 1150°C for 0.5 h are presented in Table 4. XRD analysis showed that the strongest peaks of MoSi 2 were obtained with the 50 and 10 layer structures of MojSi on the four different barrier layers. Once again,

(a)

(b) Fig. 2. (a) Spalled MoSi., coating on macroparticle formed during cathodic arc evaporation of underlying TiN diffusion barrier layer. (b) Crosssection of diffusion annealed coating (0.2 11m TiN + 50 layers each of Mo (0.01 11m) and Si (0.03 11m)), showing morphology of tetragonal-Mo'Sij.

S. Govindarajan et al./Surface and Coatings Technology 76-77 (1995) 7-13

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Table 3 Results of diffusion annealing experiments with Mo/Si multilayer coatings on a Si layer Coating

Diffusion anneal

Macroscopic appearance

XRD analysis: 6 strongest peaks.

0.05 urn Si: 20 layers each of Mo (0.05 urn) and Si (0.15 urn) 0.05 urn Si; 10 layers each of Mo (0.1 um) and Si (0.3J.lm). 0.05 urn Si; Mo = 1.0 um; Si=3.0 urn

1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC,

0.5 h 0.5 h (Ar) 0.5 h 0.5 h 0.5 h (Ar) 0.5 h 0.5 h 0.5 h (Ar) 0.5 h

Smooth, adherent Smooth, adherent Smooth, adherent Spalled at edges Wrinkled, colored

MoSi z, Mo, Mo 3Si; Minor = Mo.Si, Mo, MoSi z; Minor = Mo 3Si Mo 3Si, MoSi z, Mo; Minor = Mo.Si, Mo, MoSi z,; Minor = Mo 3Si, Mo.Si, Mo.MoSiz; Minor = Si, Mo.Si,

Spalled at edges Dark grey; spalled at edges Dark grey, wrinkled

Mo: Minor = Mo 3Si, Si, Mo, Si; Minor = Mo 3Si Mo, Si, Mo 3Si

0.1 urn Si; 20 layers each of Mo (0.05 urn) and Si (0.15 urn)

1000 DC, 0.5 h 1000 DC, 0.5 h (Ar) 1100 DC, 0.5 h

Smooth, adherent Smooth, adherent Smooth, adherent

MoSi z, Mo, Mo 3Si; Minor = Mo.Si, Mo, MoSi z; Minor = Mo 3Si Mo 3Si, MoSi z, Mo; Minor = MosSi 3

1.0 urn Si: 20 layers each of Mo (0.05 urn) and Si (0.15 urn) 1.0 urn Si, Mo=O.l um, 10 layers; Si=OJ urn, 10 layers 1.0 urn Si, Mo = 1.0 urn; Si=3.0 J.lm

1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC,

0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h 0.5 h

Smooth, adherent. Spalled at edges

Mo, MoSi z; Minor = Mo 3Si Mo, MoSi z; Minor = Mo 3Si

Spalled at edges Spalled at edges Spalled at edges Spalled at edges Spalled, wrinkled Spalled, wrinkled Spalled at edges Adherent, wrinkled

Mo, Mo, Mo, Mo, Mo,

1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC, 1000 DC, 1000 DC, 1100 DC,

0.5 h 0.5 h (Ar) 0.5 h 0.5 h 0.5 h (Ar) 0.5 h 0.5 h 0.5 h (Ar) 0.5 h

Wrinkled Wrinkled, spalled

Mo, MoSi z; Minor = Mo 3Si, Si Mo, MoSi z; Minor = Mo 3Si, Si

Wrinkled, spalled Spalled

Mo, Si; Minor = MoSi z, MosSi 3 , Mo 3Si Mo, MoSiz; Minor = Si, Mo 3Si, MosSi 3

Wrinkled, spalled Spall ed, dark grey

Mo, MoSiz; Minor = Mo 3Si

1.0 urn Si, Mo = 1.0 urn; Si = 3.0 urn, Both at -150 V bias 5.0 urn Si; 20 layers each of Mo (0.05 urn) and Si (0.15 um). 5.0 urn Si, Mo = 0.1 um, 10 layers; Si=0.3 um, 10 layers 5.0 urn Si, Mo = 1.0 urn; Si=3.0 J.lm

(Ar)

(Ar)

(Ar)

(Ar)

MoSi z; Minor = Mo 3Si MoSiz; Minor = Mo 3Si MoSi z; Minor = Mo 3Si Mo 3Si Si: Minor = Mo 3Si, MoSi z

Mo, Mo 3Si; Minor = MnSi, Mo, Si; Minor = Mo 3Si

Table 4 Results of diffusion anneal experiments with Mo/Si multilayers on various diffusion barrier layers Barrier layer

Coating

Macroscopic appearance

XRD analysis: 6 strongest peaks

0.2J.lm Ti

50 layers of Mo, Si (0.01, 0.03 urn, respectively). 10 layers of Mo, Si (0.05, 0.15 urn, respectively).

Smooth, adherent

MoSi z, MosSi 3 , Mo

Smooth, adherent

MoSi z, Mo, MosSi 3

50 layers of Mo, Si (0.01, 0.03 urn, respectively). 10 layers of Mo, Si (0.05, 0.15 urn, respectively).

Smooth, adherent

MoSi z, MosSi 3 , Mo, Mo 3Si

Smooth, adherent; some spalling near edges

MoSi z, Mo, MosSi 3 , Mo 3Si

Smooth, adherent, minor spalling

MoSi z, MosSi 3 , Mo

Coating spalled

MoSi z, Mo, MosSi3 ; Minor = Mo 3Si

Smooth, adherent

MoSi z, Mo, MosSi 3 ; Minor = Si, Mo 3Si Mo, MoSi z, MosSi 3 ; Minor = Mo 3Si

0.2J.lm Ti

0.2 urn TiN 0.2J.lm TiN

0.2 urn Cr 0.2 urn Cr

0.2J.lm CrN 0.2J.lm CrN

50 layers of Mo, Si (0.01, 0.03 urn, respectively). 10 layers of Mo, Si (0.05, 0.15 um, respectively). 50 layers of Mo, Si (0.01, 0.03 urn, respectively). 10 layers of Mo, Si (0.05, 0.15 urn, respectively).

Coating spalled

S. Govindarajan et aljSurface and Coatings Technology 76-77 (1995) 7-13

the formation of the sub-silicides could not be avoided. As the thickness of the individual Mo layers was increased above 0.05 11m (i.e. number of Mo layers decreased below 10), there was an increased tendency to form the sub-silicides. A significant problem with the use of the cathodic arc evaporation process to deposit the barrier layers was the generation of "macroparticles" . Fig. 2(a) shows the effect of the macroparticles on the annealed films: the MoSi 2 film (analyzed using EDS) breaks away from the adjacent layers and exhibits a layered growth structure. Fig. 2(b) shows a cross-section of the MoSi 2 film formed in the TiN + 50 layer structure of Mo/Si. The presence of the macro particles in the barrier will be avoided by switching over to magnetron sputtering. The efficacy of the "diffusion barrier" layers will be assessed by diffusion annealing of Si coated barrier layers, followed by compositional depth profiling.

4. Conclusions The following conclusions may be drawn from this study: 1. Sputter deposited molybdenum films were polycrystalline in nature and exhibited a strong preferred orientation in the [110] direction. The Si films were amorphous, dense and uniform. 2. The columnar structure of the molybdenum films is characteristic of the zone 1 morphology proposed by Thornton. The films were under compression, irrespective of power or bias (up to -150 V). 3. X-ray diffraction analysis of diffusion annealed (1000-1100°C for 0.5 h) single layer coatings of Si on molybdenum substrates did not reveal strong peaks of tetragonal-MoSi-. 4. Diffusion anneal treatment, at 1000 °C for 0.5 h, of multilayer films of Mo/Si, with a molybdenum layer thickness of 0.01 to 0.05 urn, resulted in the strongest tetragonal-MoSi, diffraction peak intensities and the coatings were smooth and adherent. As the Mo layer thickness was increased, the extent of MoSi 2 formation decreased. 5. The presence of an intermediate Si layer, to provide a reservoir of Si for any potential deficiency in the multilayer structure, did not eliminate the occurrence of the sub-silicides. As the thickness of the intermediate layer was increased above O.lllm, the tendency of the coating to spall increased. 6. Similarly, the use of silicon diffusion barrier layers such as Ti, TiN, Cr and CrN did not prevent the formation of the sub-silicides. The presence of macroparticles in the cathodic arc evaporated barrier layers points to the need to sputter the barrier layer. 7. In order to determine the site where the sub-silicides

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form, as well as to determine the cause(s) and mechanism of formation, compositional depth profiling of the films must be carried out.

Acknowledgement The authors gratefully acknowledge the sponsorship of this project by Schott Glaswerke, Mainz, Germany.

References [1] C.A. Krier, DMIC Report No. 162, November 24, 1961. [2] S. Govindarajan et aI., CSM Internal Report No. Mt-Ext-092-038, Dec. 1992, pp. 5-30. [3] l Schlichting, High Temp. High Press; 10 (1978) 241. [4] A.K. Vasudevan and I I Petrovic, Mater. Sci. Eng., A155 (1992) 1. [5] Ll., Jeng and EJ. Lavernia, J. Mater. Sci., 29 (1994) 2557. [6] PJ. Meschter and D.S. Schwartz, J. Organomet. Chem., 41 (1989) 52. [7] S.P. Murarka, J. Vac. Sci. Tech., 17 (1980) 775. [8] S.P. Murarka, SUicides for VLSI Applications, Academic Press, New York, 1983. [9] S.P. Murarka, Metallization - Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, Stoneham, MA, 1993. [10] Y. Ijdiyaou et aI., Appl. Surf. Sci; 55 (1992) 165. [11] D.G. Stearns, R.S. Rosen and S.P. Vernon, J. Vac. Sci. Technol. A, 9 (1991) 2662. [12] D.M. Mattox et al., Surf. Coat. Technol., 36 (1988) 117. [13] Kanthal, Swedish Patent 155836, 1953. [14] en. Ho et aI., Surf. Coat. Technol., 39/40 (1989) 79. [15] M.M.D. Ramos et aI., Vacuum, 39 (1989) 735. [16] LY, Cheng, H.e. Cheng and LJ. Chen, J. Appl. Phys., 61 (1987) 2218. [17] R.S. Nowicki and l.F. Moulder, J. Electrochem. Soc; 128 (1981) 562. [18] K Sakiyani, Y. Yamauchi and K. Matsuda, J. Vac. Sci. Tech., B3 (1985) 1685. [19] TL. Martin and r.a Mahan, J. Mater. Res., 1 (1986) 493. [20] S.P. Murarka et aI., J. Appl. Phys., 51 (1980) 5380. [21] l.A. Thornton and D.W. Hoffman, J. Vac. Sci. Technol., 14 (1977) 164. [22] THong, lR. Smith and DJ. Srolovitz, Phys. Rev. B, 47 (1993) 13615. [23] S. Yanagisawa and T Fukuyama, J. Electrochem. Soc., 127 (1980) 1150. [24] A.H. van Ommen, A.H. Reader and lW.e. de Vries, J. Appl. Phys., 64 (1986) 3574. [25] TP. Chow et aI., J. Electrochem. Soc., 130 (1983) 952. [26] O.B. Loopstra et al., J. Apply. Phys., 63 (1988) 4960. [27] F. Neppl, G. Menzel and U. Schwabe, J. Electrochem. Soc., 130 (1988) 4960. [28] S. Inoue et aI., J. Electrochem. Soc; 128 (1981) 2402. [29] T. Adler and en, Houska, J. Appl. Phys., 50 (1979) 3288. [30] G. Ottaviani, Thin Solid Films, 16 (1979) 112. [31] G. Ottaviani, Thin Solid Films, 140 (1986) 3. [32] R.W. Bartlett, P.R. Gage and P.A. Larssen, Trans. AIME, 230 (1964) 1528. [33] r.c Chou and TG. Nieh, Thin Solid Films, 214 (1992) 48. [34] S. Govindarajan et al., Surf. Coat. Technol., 68/69 (1994) 45. [35] lA. Thornton, Annu. Rev. Mater. Sci., 7 (1977) 239.