XII INTERNATIONAL CONFERENCE AND SEMINAR EDM’2011, SECTION I, JUNE 30 - JULY 4, ERLAGOL 30

Negative Dispersion in �-Mo2N/Si(100) Films

Victor V. Atuchin1, Tokhir Khasanov1, Vasilii A. Kochubey1, Lev D. Pokrovsky1, Elena S. Senchenko2

1Laboratory of Optical Materials and Structures, A.V.Rzhanov Institute of Semiconductor Physics, SB RAS, Novosibirsk, Russia

2Technological Design Institute of Scientific Instrument Engineering, SB RAS, Novosibirsk, Russia

Abstract – Molybdenum nitride films �-Mo2N/Si have been fabricated with reactive magnetron sputtering of metal tar-get in (N2 + Ar) gas mixture. Phase composition of the films has been defined with RHEED. Refractive index and extinc-tion coefficient of �-Mo2N thin films have been evaluated with laser ellipsometry at � = 632.8 and 488.0 nm. Negative dispersion has been found for refractive index of �-Mo2N in visible spectral range. Upper limit of �-Mo2N film thickness measurable with laser ellipsometry has been estimated to be ~80 nm.

Index Terms – Molybdenum nitride, magnetron sputtering, RHEED, laser ellipsometry, refractive index, extinction coef-ficient.

I. INTRODUCTION

HIN FILMS of transition metal compounds and thin surface layers modified with diffusion, ion implanta-

tion or plasma treatment are widely used in modern mi-croelectronics, optics and nanotechnology [1]–[10]. Mo-lybdenum nitride thin films exhibit high electrical con-ductivity and chemical stability and are a promising can-didate for using as Cu diffusion barrier and electrodes in microelectronics [11]–[14]. Besides this, molybdenum nitrides show excellent hardness, catalytic ability and su-perconductivity effects. Different methods have been tested for fabrication of molybdenum nitride layers, in-cluding atomic layer deposition, ion assisted deposition, pulsed laser deposition, heavy nitrogen-ion implantation, chemical vapor deposition and nitriding Mo metal at high temperature. In comparison with other methods, reactive sputtering deposition seems be among most effective for transition metal oxide or nitride film fabrication because chemical and phase composition of the film is strongly dependent on the Ar-O2-N2 gas composition and deposi-tion conditions [11], [13]–[17]. This technique is of spe-cial interest due to applicability to large-area substrates and good compatibility with electronic industrial tech-nologies. As to reactive magnetron sputtering method, high quality films can be created even at relatively low substrate temperature because of plasma stimulation of ion interaction with substrate surface. The dependence of phase composition, mechanical and electrical properties of molybdenum nitride films on the l = p(N2)/[p(Ar) + p(N2)] ratio and substrate temperature used in reactive sputtering deposition have been observed in several stud-ies [18]–[20]. At the same time, top surface structural characteristics are less studied while crystallinity and phase composition of molybdenum nitride films is a func-tion of substrate temperature and nitrogen partial pressure

[18], [20]. There is no information reported on optical properties of molybdenum nitrides though ellipsometry was mentioned as a method used for the film thickness control [21]. Evidently, single wavelength ellipsometry is suitable and sensitive method for precise thin film thick-ness control and determination of physical parameters of nanometric films and bulk crystals [22]–[30].

This investigation presents the results of molybdenum nitride thin film fabrication by reactive magnetron sput-tering deposition. The reactive gas mixture will be opti-mized to get �-Mo2N phase as main component of the film. Refractive index and extinction coefficient of �-Mo2N/Si film will be measured with laser null ellipsome-try.

II. EXPERIMENTAL

The molybdenum nitride films were deposited on Si(100) substrates by reactive dc magnetron sputtering of molybdenum metal (99.8%) target. The sputtering cham-ber was evacuated up to ~5�10-6 Torr prior to film-deposition and the working pressure was maintained at ~1�10-3 Torr with (N2 + Ar) reactive gas mixture. The magnetron power was fixed at W = 160 W. The substrate temperature was selected to be in the range T = 375�–580�C. Higher substrate temperatures were not used be-cause partial nitrogen loss from molybdenum nitride was earlier detected at T ~ 800�C [31]. After the deposition the molybdenum nitride film was slowly cooled to room in reactive gas mixture inside the deposition chamber. Structural characteristics of the films were evaluated with reflection high energy electron diffraction (RHEED) un-der electron energy 50 keV.

Fig. 1. RHEED pattern of �-Mo2N film deposited at W = 160 W, T = 425�C and l = 100% (sample 2).

T

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ATUCHIN et al.: NEGATIVE DISPERSION IN �-Mo2N/Si(100) FILMS…

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Optical parameters of the films were defined with laser ellipsometers LEF-2 and LEF-3M (� = 632.8 nm and 488.0 nm). Ten molybdenum nitride film samples were fabricated at different T and l from the ranges 375� < T < 580�C and 31.8 < l < 100% and tested with RHEED for crystallinity and phase composition. Two most represen-tative samples fabricated at T = 375�C and l = 31.8% (sample 1) and T = 425�C and l = 100% (sample 2) were measured with laser ellipsometry.

III. RESULTS AND DISCUSSION

RHEED observation shows a superposition of polycrys-talline and amorphous phases for all deposition condi-tions. In Fig.1 the RHEED pattern is shown for the film fabricated at low T and l. As it is evident, the intensity of hallo related to amorphous phase is relatively low and electron diffraction by small polycrystalline grains is dominant. The polycrystalline phase is identified as �- Mo2N (PDF 25-1366). For the comparison, in Fig.1 the RHEED pattern is demonstrated for the film created at higher T in pure nitrogen gas. In the film the amorphous component is dominating. Residual crystalline phase is identified as �-Mo2N. As it appears, increasing of sub-strate temperature and higher nitrogen flow should be stimulating factors for molybdenum – nitrogen chemical bond formation. Under higher T and l the nucleation is so active that atomic ordering on the film surface is not fin-ished and amorphous state is conserved. Earlier, crystal-linity of �-Mo2N have been confirmed by X-ray diffrac-tion (XRD) and transmission electron microscopy (TEM) for the films deposited by reactive magnetron sputtering at T = 120�–500�C in different experiments [18], [32]. In several studies the amorphous molybdenum nitride films were produced, when the substrate was not premeditat-edly heated, and additive annealing at high T was needed for crystallization of �-Mo2N [33]. It is well-known how difficult is the task to determine real substrate surface temperature during magnetron sputtering because of addi-tive plasma plume heating.

Fig. 2. Dependence of � on � calculated with � as a parameter for �-

Mo2N/Si samples 1 and 2 (� = 632.8 nm). Points show the experimental results measured at different � with a step of 5�.

For this reason, temperature-sensitive effects, including film crystallinity, are very sensitive to such factors as dis-

tance between metal target and substrate, plasma plume geometry, heat dissipation through sample holder, magne-tron power and thermocouple position used for substrate temperature measurement. So, the temperature values reported in literature for the phase transitions in molybde-num nitride films seems to be very dependent on the ex-perimental set up and give only a rough information. To decrease plasma action, in our magnetron set up as high target-substrate distance as L = 20 mm was used that en-sure minimum plasma heating of substrate surface. Under the conditions, crystalline �-Mo2N phase is dominant at T = 375�–425�C and amorphous component is prevailing at T = 540�–580�C. In the range 425� < T < 540�C the inten-sities of crystalline and amorphous components in RHEED patterns are similar.

Fig. 3. Dependence of � on � calculated for � = 70� with �-Mo2N/Si

film thickness as a parameter (� = 632.8 nm). Optical constants of sam-ple 2 were taken for the estimations. Film thickness values are given in

nanometers.

Ellipsometric parameters � and � are defined by rela-tion:

�Rp�Rs�tan�exp ���� ��

where is complex reflection coefficient, Rp and Rs are Fresnel coefficients for p and s lightwave polarization. In the experiment the parameters � and � were measured for several values of angle of incidence �. Then, it has been found that equation (1) is fulfilled with measured � and � only in the framework of model of semi-infinite optical medium. This means that lightwave reflection is insensi-tive to the interface and substrate parameters and �-Mo2N/Si films fabricated in our experiment can be con-sidered as optically thick [34]. Ellipsometric parameters were measured for two samples. Optical constants calcu-lated for these samples using model of semi-infinite opti-cal medium are presented in Table 1. Fidelity of this re-flection model for our samples has been confirmed by calculation of �(�) and �(�) functions shown in Fig. 2. Optical parameters of �-Mo2N are evidently dependent on technological conditions of film deposition and higher n and k are found for the film fabricated at lower nitrogen partial pressure and lower substrate temperature. It is in-teresting that negative dispersion has been found for opti-

XII INTERNATIONAL CONFERENCE AND SEMINAR EDM’2011, SECTION I, JUNE 30 - JULY 4, ERLAGOL 32

cal constants of �-Mo2N over the spectral range � = 632.8–488.0 nm.

TABLE I OPTICAL CONSTANTS OF �-Mo2N FILMS

Sample n k �, nm 3.287 2.690 632.8 1 2.680 2.481 488.0 3.020 1.970 632.8 2 2.550 1.713 488.0

Laser ellipsometry is a nondestructive method of thin

film thickness control [34]. As to �-Mo2N/Si system, the range of film thickness h measurable by ellipsometry with reasonable possible error is limited by high optical absorp-tion. The dependence of � on � calculated for different h values is shown in Fig.3. It is evident that �-Mo2N film thickness can be measured at � = 632.8 nm within the range 0 < h < 85 nm. This range is in good relation with the limit h ~ 80 nm critical for ellipsometric control as reported for molybdenum nitride films created with atomic layer deposition [21].

VI. CONCLUSION

The �-Mo2N films are fabricated with reactive magne-tron sputtering of molybdenum metal target. Micromor-phology and crystallographic properties of deposited layer were observed with SEM and RHEED methods. These give a basis for selection of adequate ellipsometric model and define the optical constants of �-Mo2N. Negative dis-persion of refractive index and extinction coefficient of �-Mo2N has been found in visible spectral range.

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Victor V. Atuchin was born in Pro-kopyevsk / Russia on August, 1957. He received the diploma degree in radiophysics from the Tomsk State University, Tomsk/Russia in 1979 and the Ph.D. degree in solid state physics from the Institute of Semi-conductor Physics of SB of RAS, Novosibirsk/Russia, in 1993. In 1980, he joined the Institute of Semiconduc-tor Physics as an Engineer of Elec-tronics, and as Head of Laboratory of Optical Materials and Structures in July 2002. Since 1980, his principal research interests have been in the field of solid state physics and mate-

rials science. His interests include fabrication and characterization of thin films, surface science and chemistry of oxide crystals. He is the author or coauthor of 189 publications in refereed journals. V.V. Atuchin is a member of IUCr.

Tokhir Khasanov was born in Samarqand, Republic Uzbekistan on Mach, 1950. He received the diploma degree in optics and spectroscopy from the Samarqand State University, Samarqand/Uzbekistan in 1973 and the Ph.D. degree in optics from the Insti-tute of Automation and Electrometry of SB of RAS, Novosibirsk/Russia, in 1988. His principal research interests are in the field of ellipsometry and physical optics. He is the author or coauthor of 44 publications in refereed journals including 6 patents of RF (RUSSIYA).

Vasilii A. Kochubey was born in Kha-barovsk / Russia on May, 1951. He received the diploma in physics from the Novosibirsk State University, Novosi-birsk / Russia in 1973 and Ph.D. degree in experimental physics from the Insti-tute of Space Researches of the Academy of Sciences of the USSR. Now he is with the Laboratory of Optical Materials and Structures of the Institute of Semiconductor Physics, SB RAS. Since 2001, his principal research interests have been in the field of solid state physics and materials science.

Lev D. Pokrovsky received the diploma degree in physics from the Saratov State University, Sara-tov/Russia in 1957. During a period of 1957-1962 he worked as Engineer in Laboratory of Radioelectronics, Institute of Mechanics and Physics, Saratov State University. In 1962-1972 he worked in the Institute of Mathematics, SB RAS, Novosi-birsk/Russia. In 1972, he joined the Institute of Semiconductor Physics, SB RAS, as Researcher. His interests include electron microscopy, struc-tural properties of films and crystals.

He is the author or coauthor of 59 publications in refereed journals.

Elena S. Senchenko received the

Master of engineering and technology in the field "Optotechnik" from No-vosibirsk State Technical University, Novosibirsk, Russia, in 2010. Now she is a Ph.D. student in the Technological Design Institute of Scientific Instru-ment Engineering, SB RAS, Novosi-birsk, Russia. Her research interests include development of industrial optoelectronic systems for dimen-sional inspection of 3D objects. Her work includes theoretical research and practical creation of prototypes of measuring systems.