Louis Hennet

Silicon/silicon oxide and silicon/silicon nitride multilayers for extreme ultraviolet

Si/SiO2 and Si/Si3N4 multilayers have been fabricated using a locally made reactive diode ri-sputtering system. The layer alternation is obtained by modulating a partial pressure of oxygen or nitrogen near the sample using a silicon target with argon as sputtering gas. O2 and N2 partial pressure conditions were optimized to deposit stoichiometric SiO2 and Si3N4 films without significant reaction with the silicon target. In situ kinetic ellipsometry was used to monitor both thick film and multilayer deposition. The different interfaces appear very sharp with a little contamination of the silicon layers especially using oxygen. The multilayers were characterized by grazing x-ray reflection (Cu-K α line), and the reflectivity was measured in the soft x-ray range (120-350 A) by synchrotron radiation. Both Si/SiO2 and Si/Si3N4 multilayers exhibit well-defined Bragg peaks with very narrow bandpasses (two to three times lower than the conventional Mo/Si multilayer), and high absolute reflectivities (up to ≅22% at 130 A). Finally, thermal stability of Si/Si3N4 multilayers was evaluated. We did not find any degradation after annealing up to 800°C, which is extremely high compared to conventional Mo/Si multilayers, which are generally destroyed above 500°C.

Three materials soft x-ray mirrors: theory and application

A periodic structure alternating three different materials has been investigated and fabricated using a diode RF-sputtering system. Reflectivity was optimized using the wave propagation method. It is concluded that the amorphous character of the rhodium layers is enhanced in the W/Rh/C structures, and good reflectivities have been obtained at the carbon K-alpha line with trilayer structures including a great number of periods (28 percent of reflectivity for 40 periods stack). Some reduction of the boron interdiffusion is observed in the W/Rh/C structures, which leads to better soft X-ray performances at the boron K-alpha line (19 percent of reflectivity for a 40 periods stack).

Structural characteristics and performances of rf-sputtered Mo/Si and Co/Si multilayers for soft x-ray optics

Precise structural analysis of Mo/Si multilayers deposited by a diode rf-sputtering system has been made using in-situ kinetic ellipsometry, grazing x-ray reflection, x-ray diffraction, Auger profile analysis, Rutherford backscattering, and high-resolution electron microscopy. The main structural imperfections (interface roughness and interdiffusion) have been related to the Mo crystallization and to the molybdenum silicide formation at the interfaces. The comparison to Co/Si multilayers deposited in the same conditions was useful to deduce the influence of the intrinsic properties of these systems on their structural behavior. Silicide layers are formed in real-time during the growth of the samples. They are completely amorphous and their composition is not far from defined compounds (MoSi2 and CoSi2). In Mo/Si multilayers the Mo on Si interface is always thicker than the other interface (approximately equals 15 angstrom compared to 8 angstrom). It is not due to the deposition conditions but to the crystallization of the molybdenum layers which reduces the silicon diffusion at the Si on Mo interface. The higher reactivity of cobalt with silicon produces thicker quasi-symmetric silicide layers (approximately equals 25 angstrom). The thermal behavior of the two systems is also controlled by the interdiffusion and the crystallization of the silicide layers. Absolute soft x-ray reflection was measured on different Mo/Si x-ray mirrors by synchrotron radiation at various wavelengths above the Si L-(alpha) line and related to the structural characteristics. In spite of the occurrence of thin silicide layers at each interface, reflectivities as high as 55% in normal incidence have been obtained at 130 angstrom.

Characterization of resists and antireflective coatings by spectroscopic ellipsometry in the UV and deep-UV range

Antireflective coatings and resists are characterized precisely by spectroscopic ellipsometry from 800nm to UV and deep UV 193nm. A procedure based on the use of a polynomial dispersion law to take into account the optical indexes of the ARC in the region where it is transparent is developed and tested on samples with different thicknesses. The values obtained by this procedure are shown to be in perfect agreement with grazing x-ray reflection measurements made on the same samples. The procedure is valid even for very thin ARC. We show that using this thickness, the optical indexes can be extracted directly in all the wavelength range of the SE measurements. Practical example including a top ARC, a resist and a bottom ARC is analyzed in the same way. The physical parameters of the entire trilayer structure are deduced at the different working wavelengths. The expected reflectance performance are simulated using the same physical model and the same software.

Combined characterization of group IV heterostructures and materials by spectroscopic ellipsometry and grazing X-ray reflectance

Abstract Different kinds of group IV heterostructures have been studied by means of spectroscopic ellipsometry (SE) and grazing X-ray reflectance (GXR). GXR was used to determine the thicknesses of the SiGe and Si layers in SiGe/Si and Si/SiGe/Si structures without ambiguity. Precise optical indices can be deduced using SE on single-layer samples. The same database is used to analyze more complex double-layer structures. SiC samples obtained by excimer laser annealing of carbon-implanted silicon have also been studied by SE. We show that the amorphous silicon profile of the samples versus the energy density used for the annealing can be accurately determined.

Precise measurement of ARC optical indices in the deep-UV range by variable-angle spectroscopic ellipsometry

Antireflective coatings and photoresists are characterized precisely by spectroscopic ellipsometry from near IR to deep UV 190nm. A procedure based on the use of a polynomial dispersion law to take into account the optical indices of the films int he region where they are transparent is used. Thickness values provided by this technique are checked independently by grazing x-ray reflection technique at the cobalt K-(alpha) line. The procedure is valid for a range from very thin ARCs to very thick ones. Knowing the thickness of the film a new method to extract point to point the optical indices of the layer in the entire wavelength range has been developed; at each wavelength, we use different couples of values measured at different incidence angles, and we adjust the optical indices using a Levenberg Marquard algorithm. Compared to the standard point to point procedure, this method has tow main advantage: at the extrema of the interference fringes the better stability of the algorithm leads to more accurate extraction even for very thick film, due to the fact that the position of the extrema is slightly dependent of the angle of incidence; another interest is that the optimum angle of incidence for a given wavelength is generally always included in the interval. A final interest is that standard deviations on the n and k values are also extracted directly using this method giving an evaluation of the accuracy of the procedure at each wavelength. This method is applied first to transparent SiO2 thick film on silicon to demonstrate the better accuracy and then to antireflective coatings of various thickness down to deep UV range. Easy simulations of normal reflectance properties can then be made very easily using these optical indices.

Magnesium-silicide-based multilayers for soft x-ray optics

Using a diode rf-sputtering technique, different magnesium silicide based multilayer systems have been deposited in very thin films for optical applications in the soft x-ray range. A detailed structural analysis of the different multilayers has been made using in-situ kinetic ellipsometry, ex-situ grazing x-ray reflection at the copper K-(alpha) line and transmission electron microscopy. The multilayer performances have been measured by synchrotron radiation at the magnesium K-(alpha) and L-(alpha) lines and related to the structural characteristics. For short wavelength, the W/Mg2Si system shows characteristics very similar to those of the more common W/Si system. Non-negligible interdiffusion and limited interface roughness allow the layer thicknesses to be reduced to very low values. Well-defined Bragg peaks are observed even when the double period is as low as 44 angstrom. First Bragg peak reflectivity as high as 31 has been measured at 9.89 angstrom for a multilayer with a double period equal to 84 angstrom and a limited number of periods. This preliminary result is very promising for future applications in the field of x-ray fluorescence analysis. W/Mg2Si and Si3N4/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si system, the tungsten layers are crystallized due to their higher thickness and consequently the interface roughness is slightly higher. In spite of this, more than 20 reflectivity at the first Bragg peak has been measured at normal incidence on different W/Mg2Si samples, with a selectivity two times better than conventional Mo/Si mirrors ((lambda) /(delta) (lambda) approximately equals 20). When we replace tungsten by a thin silicon nitride layer deposited by reactive sputtering, we increase the selectivity up to (lambda) /(delta) (lambda) approximately equals 30, and the thermal stability is drastically improved ( 800 degree(s)C).

Combined Characterization of Conductive Materials by Infrared Spectroscopic Ellipsometry and Grazing X-ray Reflectance,

Abstract Non-destructive, non-contact characterization of conductive materials is an interesting challenge, especially in the field of microelectronics. One way to solve the problem is to use the optical properties of the materials that follow theoretically a Drude law in the infrared region. In this study, spectroscopic ellipsometry in the mid-infrared (up to 17 μ m) is used to extract the optical indices. To evaluate the electrical properties, the thickness of the layer is also needed. Due to the generally high absorbance of these conductive layers, it cannot be extracted directly from the ellipsometric measurements. In this study, we use a complementary technique called grazing X-ray reflectance (GXR). Eleven titanium/SiO 2 /Si samples with variable titanium thickness have been successively examined. The different thicknesses have been determined precisely by grazing X-ray reflectance at the Co K- α line with the surface roughness of the samples. Conventional X-ray diffraction was also used to determine the crystallinity of the titanium layers. Then IR spectroscopic ellipsometry was used to extract the optical indices of the different titanium layers and their resistivity using the GXR thickness and the Drude model. Values determined by this method compare well with four-point probe resistive measurements made on the same samples. The density of the titanium layers and the thickness of the bottom SiO 2 layer can be also evaluated in some cases.

Tungsten/boron nitride multilayers for XUV optical applications

W/BN multilayers are theoretically efficient x-ray mirrors at the nitrogen and boron K-(alpha) lines (31.3 angstroms and 67.6 angstroms, respectively). Their most attractive potential application is detection of light elements by x-ray fluorescence spectrometry. The performances of W/BN mirrors depend not only on the structural quality of the multilayers but also on the stoichiometry of the boron nitride layers, especially in the water window (20 - 40 angstroms). In order to get stoichiometric BN layers with low surface roughness, the deposition of thick boron nitride films has been studied in detail. In-situ kinetic ellipsometry, x-ray photoemission, grazing x-ray reflection and scanning electron microscopy show that quasi-stoichiometric BN films with low surface roughness are obtained only with a low total deposition pressure and an additional nitrogen partial pressure. This result is related to the chemical and structural properties of the BN films. W/BN multilayers with medium period value (2d approximately equals 120 angstroms) show about 80 of the maximum reflectivity at the W M4-5 line. When the period is reduced, the performances are reduced, but good quality W/BN multilayers with very low period values (2d approximately equals 50 angstroms) and a great number of periods ( 100) have been fabricated. The best structural quality is obtained when a low nitrogen partial pressure is introduced during the deposition of the BN layers. The optical indice contrast is improved and the tungsten-boron interdiffusion is reduced.

Nanostructure Study of Ti/TiN Multilayers: Effect of the Deposition Temperature

In this paper, we report on the growth and the study of Ti/TiN nanometric multilayers. The preparation of these films has been carried out by high vacuum diode r.f. sputtering. Growth was in-situ monitored by kinetic ellipsometry. Deposition temperature was kept to room and low temperature (-120°C) respectively in order to modify the interface properties and consequently to understand the effect of the interfaces on the films mechanical properties. The thickness of each layer was varied from 1 nm to 10 nm and alternately repeated in order to obtain a total thickness of 200 nm. After deposition, films were characterized by means of X-ray diffraction and grazing X-ray reflectometry for structural determination. This study shows that the TiN-Ti boundary is composed of TiN x , with x going from the nominal nitrogen concentration of the TiN layer to 0. This can be due either to a surface reaction of the incident titanium during the Ti sequence with the nitrogen present at the film surface or to a reaction of nitrogen with the target surface during the TiN sequence.