ICMCTF2003 Session H5: Multilayer Thin Films: Fabrication and Applications

Friday, May 2, 2003 8:30 AM in Room Royal Palm 4-6
Friday Morning

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Start Invited? Item
8:30 AM Invited H5-1 Multilayer Coatings for EUV Lithography
E. Spiller (Spiller X-Ray Optics and Lawrence Livermore National Laboratory)

The next generation lithography for the fabrication of computer chips with critical dimensions in the 25 to 50 nm range will use extreme ultraviolet radiation with wavelengths around 13.4 nm. No transparent materials for wavelengths (λ) below 110 nm that could be used for lenses exist, and reflecting mirrors have to be used for all optical components and the masks. However, the reflectivity of all materials is below 0.5% around λ = 13.4 nm, and multilayer coatings are needed to enhance the reflectivity. The first commercial cameras will contain six near normal incidence mirrors and additional mirrors in the illuminator. The specification for all components of an EUV camera are stringent. Figure errors of all mirrors should be in the 1 Angstrom range, and the coatings should not add any noticeable Figure errors. Multilayers of Mo/Si with 50 periods have achieved reflectivities close to 70% near λ = 13.4 nm. Tight control of the layer thickness, interface quality, and thickness profile has been obtained with added figure errors around 0.5 Angstrom.

Masks have to be free of defects. Ideally the multilayer coating on a mask should reduce the effect of substrate defects and smoothen substrate roughness. Many deposition methods have successfully been used to produce the multilayer coatings, and we will compare their relative strengths and weaknesses. We will also discuss the metrology that has been developed to test the performance of all components.

9:10 AM H5-3 Single Crystal CrN/ScN Soft X-ray Superlattice Mirrors
J. Birch, F. Eriksson, T. Joelsson, N. Ghafoor (Linköping University, Sweden); G.A. Johansson (Biomedical and X-ray Physics, Royal Institute of Technology, Sweden); F. Schäfers (BESSY, Germany); L. Hultman (Linköping University, Sweden); H.M. Hertz (Biomedical and X-ray Physics, Royal Institute of Technology, Sweden)

Due to their extremely thin layers (<1 nm), normal-incidence X-ray multilayer mirrors for space born solar telescopes, free-electron lasers, lithography tools, and microscopes are extremely sensitive to thermally activated interdiffusion or mechanical wear. We explore CrN/ScN superlattices as stable mirrors for X-ray wavelengths of λ=3.1-3.4 nm.

Single crystal CrN/ScN superlattices, with modulation periods of 1.7 nm were grown on MgO(001) substrates by alternating growth of sub-nm layers of CrN and ScN by low energy ion-assisted reactive magnetron sputtering. The ion-to-nitride arrival flux ratios at the substrate were measured to 45 for CrN and 144 for ScN. The effects of ion energies in the range [12-58 eV], and growth temperatures in the range [535-853°C], on the interface widths, crystal quality, and microstructure evolution were investigated using hard x-ray reflectivity (HXR), soft x-ray reflectivity (SXR), x-ray diffraction (XRD), and transmission electron microscopy (TEM).

Ion energies of 24 and 28 eV for CrN and ScN, respectively, gave a minimal interface width of ~0.35 nm and an optimum crystal quality at a growth temperature of 735°C. Lower ion energies or growth temperatures gave rougher interfaces and poor crystal quality. Higher ion energies or growth temperatures yielded interdiffused interfaces and decreased crystal quality. TEM showed a highly ordered single-crystal superlattice with some nodular domains evolving from surface defects. A 61 periods superlattice showed 7.2% reflectivity at 63°C for an X-ray wavelength λ=3.115 nm indicating an interface width <0.4 nm in accordance with HXR. Thermal and mechanical stability was compared to state-of-the-art metallic Cr/Sc multilayer mirrors. .

9:30 AM H5-4 Deposition of Multilayered and Multicomponent Coatings using Hybrid Filtered Cathodic Arc and Magnetron Sputtering Technique
S.J. Dixit, A.K. Rai, R.S. Bhattacharya (UES, Inc.)
A novel hybrid technique employing Filtered Cathodic Arc and Magnetron sputtering technique is utilized for the deposition of multi-layered and multi-component coatings. Nano-layered coating architecture involving alternate layers of TiN and TiB2 is studied. Similarly, simultaneous deposition of mixed TiN and TiB2 multi-component coatings is also carried out to understand the complexity of the process. The structure and composition of the films is characterized by X-ray diffraction, Transmission Electron Microscopy (TEM) and Auger Electron Spectroscopy (AES) respectively.
10:10 AM H5-6 Effect of N2/Ar Ratio on the Crystal Structure and Texture Evolution of the CrN/NbN Superlattice Coating Prepared by r. f. Magnetron Sputtering
H.-J. Park, J.-K. Park, Y.-J. Baik (Korea Institute of Science and Technology, South Korea)
Recently, nanomultilayer metal nitride films with cubic structure have attracted a large amount of attention because of the enhanced hardness value over 5000 kg/mm2, which is larger than that of the constituent individual nitride film. Besides the hardness, other property such as corrosion resistance has also been reported to be better in the nanomultilayer system such as CrN/NbN. In the single CrN, NbN films, however, the optimum deposition conditions such as bias, temperature and gas atmosphere to obtain cubic phase are quite different from each other. Therefore, such processing parameters should be controlled carefully to obtain CrN/NbN superlattice films with cubic structure. In this study, we investigate the effect of N2/Ar gas ratio on the crystal structure and texture evolution in CrN/NbN superlattice film. The film was deposited on Si substrate by r.f. magnetron sputtering. As preliminary study, the deposition conditions for the single CrN and NbN film with cubic structure were determined. The NbN film showed cubic structure when the N2/Ar ratio was less than 0.03, whereas CrN film had cubic structure with the N2/Ar ratio of 0.25. The CrN/NbN nanomultilayer film were deposited under the condition of N2/Ar ratio between 0.03 ~ 0.3. Compared to the constituent single nitride film, the N2/Ar ratio limit to show cubic structure for the CrN/NbN superlattice film was widened and critically dependent on the first layer deposited. The relation between crystal structure and N2/Ar ratio of the CrN/NbN superlattice film will be discussed.
10:30 AM H5-7 Structural Optical and Electrical Properties of BaTiO3/SrTiO3 Multilayer Thin Films
S.C. Roy, M.C. Bhatnagar, G.L. Sharma (Indian Institute of Technology Delhi, India)
Multilayers of barium titanate and strontium titanate (BT/ST) are currently under intense study for their remarkable property of dielectric enhancement along with low leakage currents and low loss factor. These multilayer films can form artificial superlattices, which have the potential to drastically improve the material properties. In the present work, we have prepared (BaTiO3)m/(SrTiO3)m multilayer thin films with periodicity factor m varying from 1 to 4, with spin-on sol-gel technique. Fused quartz, stainless steel and platinized silicon wafer substrates were used for film deposition. The samples were pre-sintered at 4000C for 15 minutes after the deposition of individual layers and then finally sintered at 7000C for two hours after deposition of all the layers. The total thickness of all the layers was kept approximately same at 300nm for all samples by suitably adjusting the stack periodicities. X ray diffraction technique was used to study the crystallinty of samples, which showed the presence of satellite peaks along with the main peaks. The optical behaviour of these films was examined using a Perkin-Elmer spectrophotometer, in the range of 200nm to 900nm. The optical band gap energy of each sample was deduced from the spectral dependence of the absorption coefficient by applying the Tauc relation. A direct allowed transition was observed in these films with the band gap value in the range of 3.9 eV to 4.1 eV, which is higher than the single crystal value of 3.6 eV. This variation from the bulk value has been correlated with the presence of large stress at the BT/ST interface due to inherent mismatch between BT and ST lattices. The dielectric and electrical measurements reveal the presence of non-Debye type relaxation mechanism in these multilayer films. The results of electrical studies on stainless steel and platinized silicon substrates have been compared and discussed.
10:50 AM H5-8 Si/SiGe Superlattices for Thermoelectric Applications
L.C. Olsen, P.M. Martin (Pacific Northwest National Laboratory)
In the last decade, new materials with increased values of the material figure of merit, ZT, have been developed for thermoelectric energy conversion. Whereas the ZT values for thermoelectric materials were apparently pegged at 1.0 from 1970 to 1990, values greater than 2.0 were reported in the 1990s. The increased values for ZT indicate that efficiencies greater than 20% and coefficient of performance values greater than two are possible for power production and refrigeration, respectively. Some of the most encouraging results have been achieved with thin film superlattice structures. We report on development of and recent progress in scale up of Si/SiGe superlattices for thermoelectric applications. Films were deposited onto single crystal silicon substrates by magnetron sputtering. Initial studies determined the optimum temperatures for obtaining epitaxial Si films on single crystal silicon. Both n-type and p-type superlattices comprised of 100 - 1000 alternating 100 Å -thick layers of Si and Si0.8Ge0.2 were grown and characterized. The electrical conductivity of the superlattices was a factor of ten larger than values measured for as-deposited Si or Si0.8Ge0.2 films, while the Seebeck coefficient values were similar. Typical values of electrical conductivity and Seebeck coefficient for a p-type superlattice structure were 1000 ohm-1cm-1 and 800 V/°K, respectively. These results indicate that ZT of these materials should be significantly greater than 1.0. A direct measurement of ZT is underway and will be reported. Results of depth composition profiles by XPS and XRD studies will also be discussed and related to electrical and thermoelectric properties.
11:10 AM H5-9 Comparative Studies of Mg Doped ZnO and Multilayer ZnO/MgO Thin Films
P. Bhattacharya, R. Das, K. Upireddy, R.S. Katiyar (University of Puerto Rico)

ZnO with direct bandgap of 3.3 eV and excitonic energy of 60 meV, has drawn much attention for possible applications in wide bandgap optoelectronic devices, in the ultraviolet region. The solid solution with MgO can produce wide bandgap MgxZn1-xO (MZO) thin films for application in quantum well related devices. The solid solubility of MgO in ZnO is merely 4% in the bulk form. However, the realization of >50% MgO in ZnO thin films has been demonstrated by the pulsed laser deposition (PLD) technique explicitly in metastable state with a bandgap of >5 eV. In this work, we have studied MZO thin films grown from single MZO (x= 0.20, 0.5) targets and sequential deposition of ZnO/MgO multilayers. The films were fabricated using PLD technique from their respective targets on Al2O3(0001) substrates. The total thickness of the films used in this study was ~300 nm. For multilayer deposition, the thickness of single ZnO layer was varied in the range of 0.75-2.5 nm at a constant MgO layer thickness of 1 nm. As-grown films were post annealed for 30 min at 750°C in O2ambient. The structural transition from hexagonal to cubic phase was observed with the increase of Mg concentration for x> 50%. Similar structural change was also observed with the decrease in thickness of single ZnO layer from 2.5 to 0.75 nm for multilayer deposition. The bandgap was increased from 3.5 to 6.2 eV with the change of ZnO sublayer thickness. The post annealing of MZO films grown from single MZO target was segregated into both hexagonal and cubic phase. However, in case of multilayer deposition annealing did not show any significant change in the crystal structure and the optical properties. Detailed optical and electrical properties of MZO thin films will be presented in close correlation with their crystal structure and method of fabrications.

This work is supported in parts by DOD-F49620-01-1-1004 and NASA-NCC5-518 grants.

11:30 AM H5-10 Studies on Agglomeration and Stress Migration Characterization of Barriers for Cu Interconnects
T.C. Wang, T.E. Hsieh (National Chiao-Tung University, Taiwan, ROC); Y.L. Wang (National Chiao-Tung Univ. and Taiwan Semiconductor Manufac. Co., Ltd, Taiwan, ROC); J.C. Tsao (Taiwan Semiconductor Manufacturing Co., Ltd, Taiwan, ROC); K.W. Chen (National Chiao-Tung Univ. and Taiwan Semiconductor Manufac. Co., Ltd, Taiwan, ROC)
The stress-migration of Cu becomes a very important issue as integrated circuit manufacture moving to copper interconnection for more fast speed requirement. Cu film properties depends on the barrier film characteristic for that agglomeration during electroplating and void free after electroplating and annealing. In this paper, TaN was the barrier for Cu diffusion. The Cu film texture for different stoichiometry of TaN and agglomeration of Cu during electro-plating were studied. Stress migration was the index for the different stoichiometry and film types of TaN. AFM is also use to investigate the film surface characteristic.
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