ISSN 10637850, Technical Physics Letters, 2013, Vol. 39, No. 7, pp. 640–643. © Pleiades Publishing, Ltd., 2013. Original Russian Text © D.I. Smirnov, R.M. Giniyatyllin, I.Yu. Zyul’kov, N.A. Medetov, N.N. Gerasimenko, 2013, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2013, Vol. 39, No. 14, pp. 34–42.
Problems in Measurements of Parameters of Elements and Structures in Modern Micro and Nanoelectronics Considering TiN/Ti Diffusion Barrier Structures As an Example D. I. Smirnov, R. M. Giniyatyllin, I. Yu. Zyul’kov, N. A. Medetov, and N. N. Gerasimenko National Research University of Electronic Technology (MIET), Zelenograd, Moscow, Russia Lebedev Physical Institute, Russian Academy of Sciences, Moscow, Russia Molecular Electronics Research Institute and Mikron Plant, Zelenograd, Moscow, Russia Moscow Institute of Physics and Technology (State University), Dolgoprudny, Moscow oblast, Russia email:
[email protected] Received March 11, 2013
Abstract—Results of a comprehensive analysis of process variables of TiN/Ti diffuse barrier structures used in modern microelectronics are presented. To provide reliable spectral ellipsometry results, which is a com mon technique for postprocess control of these structures during the testing stage of the manufacturing pro cess, a comprehensive approach is suggested that consists in consistent processing of transmission electron microscopy and Xray reflectometry data. The resulting data were used to calculate the variances of optical coefficients of spectral ellipsometry, which depend on features of the manufacturing process and are required for analysis of the parameters of manufactured diffuse barrier structures. DOI: 10.1134/S1063785013070249
The complete manufacturing cycle for nanoelec tronic devices is currently connected with difficulties in measurements of parameters of manufactured structures, especially when a change is made to design standards of about 90 nm or less, because of the insuf ficient informational content of certain standard tech niques, ambiguity of models used, and the use of incorrect assumptions regarding the structure and composition of the manufactured objects. In this work, several mutually complementary analysis tech niques are used for diagnostics of process variables of TiN/Ti diffuse barrier structures (DBSs) with the aim of eliminating the above difficulties. Multilayer TiN/Ti DBS are used in modern micro electronics to prevent parasitic diffusion in layers of multilevel metallization. In this work, when testing the manufacturing process and organization of necessary metrological control, samples of TiN/Ti/SiO2 struc tures on silicon were used manufactured on the 180 nm production line of Micron Corp. The technology specified thicknesses of layers in a structure were 5 nm for TiN, 10 nm for Ti, and 15 nm for SiO2. A prepared oxidecoated Si wafer was coated with Ti and TiN in one technological procedure in a highvacuum (10–8 Torr) cluster setup. The magne tron physical vapor deposition (PVD) technique was used for the Ti coating. To obtain more a uni
form coating, the process was carried out in a cham ber with a socalled ionized metal plasma (IMP) source [1]. TiN coating was carried out by the chemi cal vapor deposition technique. During this process, TDMAT compound (Tetrakis–demethylamino–tita nium–TiNxCyHz) was thermally decomposed and then the deposited film was sealed in the N2/H2 gas plasma to eliminate carbon admixtures and stabilize its physical properties [2]. According to the accepted technological proce dure, the postprocess metrological control of parame ters of coating layers is carried out using an industrial spectral ellipsometry (SE) setup KLATencor Aset F5x intended for the nondestructive express control of thickness of multilayer structures. To process the SE experimental data, the spectral dispersion of the refractive index and absorption coefficient should be specified (hereinafter, dispersion model of optical coefficients) [3]. Determination of optical parameters is a difficult problem for thin films (100 nm and thin ner) due to their complex dependences on such parameters as the thickness of applied layers and den sity and optical flatness of the interface. In the case considered, the use of standard TiN and Ti dispersion models did not provide satisfactory results and required refinement of the parameters of the manufac tured structures to develop proper models.
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We consider this to be the main disadvantage of spectral ellipsometry, since it means that implementa tion of industrial measurements with an ellipsometer is possible only for wellstudied structures and pilot manufacturing processes, relative to which the main parameters of the manufactured structures are assumed to be repeatable and predictable. However, deviations from the specified parameters are possible in practice. This is especially important in cases in which a sequence of technological processes can influ ence the parameters and properties of preliminary produced structure elements—in particular, individ ual layers in a multilayer structure when coating in dif ferent ways. For scientific applications and beginning a new technological process, the need for refinement of the dispersion models for each new structure makes metrological control significantly more complicated and requires specialists with high levels of skills who are able to improve existing models and work out new ones rapidly on the basis of data obtained by other methods. To solve the above problems, a complex of consis tent metrological investigations were suggested. Xray reflectometry (XRR) was used as a basic technique; it is an accurate instrument for thinfilm structure diag nostics with high informational content. Reflecto grams were recorded on a CompleXRay setup in the θ–2θ scanning mode at two radiation wavelengths λ(CuKα) = 0.154 nm and λ(CuKβ) = 0.139 nm simul taneously. Processing of the experimental data has shown [4, 5] that the use of the ratio of reflection intensities of two wavelengths in the calculations allows an increase in the parameter accuracy of struc tures under study and a decrease in the processing time of dependences measured. Figure 1 shows the measured reflectometry depen dences. A complicated structure of interference max ima was observed at small sliding angles; i.e., there were two close maxima in the region of a critical angle of total external reflection and the third angle was deformed, which was evidence of at least one addi tionally formed layer in the structure under study and/or an inhomogeneous distribution of optical den sity throughout the sample thickness. To obtain the required data on the number and structure of unconsidered layers in the DBS under study, a TEM study was carried out using a Jeol JEM 2100 microscope at the Common Use Center of the Moscow Institute of Physics and Technology (MIPT). To protect the sample, it was additionally coated with an Cr layer of ~20 nm in thickness; then, the sample was cross cut using the ultrasonic cutting technique. Figure 2 shows a TEM image of the cut sample. It was ascertained that the manufactured structure differed noticeably from the specified TiN/Ti/SiO2/Si struc ture: the TiN film was represented by three separate layers—amorphous external layers and a polycrystal line intermediate layer. TECHNICAL PHYSICS LETTERS
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Fig. 1. Angular dependence of (a) the intensities of reflec tion I(λ1) and I(λ2) from a TiN/Ti DBS for the radiation wavelength (1) λ1 = 0.154 nm and (2) λ2 = 0.139 nm and (b) the ratio of reflection intensities I(λ1)/I(λ2); points show experimental data, and the solid curve, calculation results.
TEM data on the number of separate layers, their approximate thickness, and the interface sharpness were used for analysis of XRR curves. To process rela tive experimental XRR dependences (Fig. 1b), a genetic algorithm was used in the solution of the inverse reflectometry problem [6]. The layer thickness and density and the interface roughness were opti mized parameters. The Xray reflection from a multi layer structure was computersimulated on the basis of recurrent equations [7]. The diffuse blurring of the interface was taken into account by the method sug gested in [8]. Exact values of thicknesses of separate layers calcu lated by XRR were used as the known parameters for the solution of the inverse SE problem, which con sisted in our case in determination of optimal models
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TiN Si
5.9 nm SiO2
Ti
12.6 nm 13.7 nm Cr 50 nm Fig. 2. TEM image of a TiN/Ti DBS sample cut.
of optical coefficients for TiN/Ti structures analyzed (Fig. 3). Models from an openaccess SOPRA N&K Database [9] were used as an initial approximation. To simulate the spectral dispersion of refractive index n and absorption coefficient k, a method for a harmonic oscillator was used [3]. As follows from the data in Fig. 3, the dispersion models for the TiN film were transformed the most, due to the complicated k, a.u. 3.5
structure of this layer. The socalculated optical parameters were successfully used later for reliable express diagnostics of similar manufactured structures during this technological procedure. The table presents the final analysis results of the parameters of TiN/Ti DBS. During generalization of data on the phase composition, density, and features of the coating process of the films, sublayers were identi fied in the TiN film. The thin amorphous layer between crystalline Ti and TiN layers of 2.10 g/cm3 in density was a crumbly film, which was not completely sealed and crystallized during treatment in a dense plasma medium. A thin amorphous layer of 3.04 g/cm3 in density on the surface was TiN oxidized in air due to the absence of a top protective layer and a residual charge in the nearsurface region after treatment in plasma. The studies performed show that a comprehensive approach is required in this case for accurate metro logical control. The approach consists in consistent processing of results of the independent techniques, which do not provide reliable information when applied individually. Thus, in the case of postprocess control of process variables of TiN/Ti DBS by spectral ellipsometry, complementary XRR and TEM studies were carried out. The results of this work are, in our opinion, a char acteristic example of the need to improve the method ology of metrological control of complex nanostruc tures and objects. This conclusion points to a need for
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Fig. 3. Spectral dispersion of absorption coefficient k and refractive index n for (a) Ti and (b) TiN layers. Solid curves correspond to standard dispersion models, and dashed curves, to calculated models. TECHNICAL PHYSICS LETTERS
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Generalized measurement results of process variables of TiN/Ti DBS Layer thickness, nm Layer XRR* TiN amorphous, oxidized
2.0
TiN crystalline
2.4
TiN, TDMAT amorphous
1.4
TEM
SE
5.9
5.8
Roughness, nm (XRR)
Density, g/cm3 (XRR)
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Ti
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SiO2
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* Acronyms of the techniques used for measurements of the parameters are given in parentheses.
developing standards for not only individual diagnos tic techniques for micro and nanostructures and materials, but also for consistent use of the techniques in general. ACKNOWLEDGMENTS This work was performed under financial support of Ministry of Science and Education of the Russian Federation with the use of equipment of the MIPT Common Use Center. REFERENCES 1. S. R. Wilson, C. J. Tracy, and J. L. Freeman, Jr., Hand book of Multilevel Metallization for IC (Noyes Publica tions, 1994).
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2. R. Doering and Y. Nishi, Handbook of Semiconductor Manufacturing Technology (Taylor&Francis Group, 2008). 3. H. G. Tompkins and E.A. Irene, Ellipsometry and Polarized Light (William Andrew, Inc., 2005). 4. R. M. Feshchenko, I. V. Pirshin, A. G. Touryanski, and A. V. Vinogradov, J. Russian Laser Res. 20 (2), 136 (1999). 5. S. A. Aprelov, V. M. Senkov, N. N. Gerasimenko, et al., Izv. Vuzov. Electron., No. 5, 27 (2006). 6. A. Ulyanenkov and S. Sobolewski, J. Phys. D: Appl. Phys. 38, A235 (2005). 7. L.G. Parratt, Phys. Rev. 95, 359 (1954). 8. L. Nevot and P. Croce, Rev. Phys. Appl. 15, 761 (1980). 9. http://www.sspectra.com/sopra.html
Translated by O. Ponomareva
