New X-ray measuring system with microfocus sources for the diagnosis of solid micro- and nanostructures
It should be emphasized the fact that in the framework of the working community multilateral feedback, through the combined efforts of fundamental and applied science, higher educational institutions, industrial enterprises will be organized. Such synergies should be actively used to obtain competitive advantages over foreign companies under the import substitution policy.
X-ray analysis methods for Russian microelectronics industry were tested in Mikron, the MIET cluster and in other enterprises of Zelenograd scientific-industrial complex. Based on the obtained experience, the programs of development of X-ray analysis methods on the basis of domestic equipment were developed, which meets not only the import substitution challenges, but also provides new opportunities for X-ray diagnostics in the field of precision research, in particular, in solid state nanotechnology. Preliminary works in these areas and their results also gave the opportunity to formulate new approaches to metrological support of micro- and nanoelectronics, to mass use of X-ray control methods in other high-tech areas such as reactor material science, space engineering and mining.
The principles of a new X-ray analysis system
Interaction with leading production companies allowed to develop an integrated approach to the process control in microelectronic production. It should be noted that these developments have received a positive response from the specialists of Mikron and other the enterprises. The choice of an integrated approach to the organization of control measurements is caused by that in practice nanoelectronics faces many challenges at the stage of development of new processes: unaccounted additional sublayers are formed in manufactured structures, the parameters of the underlying functional layers change during the multi-stage process.
To overcome these difficulties analytical X-ray system CompleXRay  is developed. The concept of this system initially provides the ability to upgrade to meet the requirements of the customer. First of all, it should be noted radiation safety. The system comes with a low-power X-ray tube and does not require registration and accounting. The equipment is modular and is designed for a broad class of complex research facilities, including crystals, polycrystalline and amorphous environment, nano-scale structures. The innovative X-ray optical scheme provides a unique opportunity for the study of thin films, multilayer nanostructures and interfaces. The modularity and versatility of the design allows to measure remotely that can be used for distance learning and the study of hazardous materials .
X-ray measuring system implements the most complete combination of analytical methods on a single platform. Thus, through the use of fine-focus source and interchangeable monochromators record characteristics of the angular resolution and accuracy of the low-angle measurements are achieved. Patented metrological scheme based on parallel data logging at multiple wavelengths, allows to increase significantly the accuracy of the X-ray measurements [4,5].
On the metrology platform of X-ray complex was first implemented relative measurement mode by determining the ratio of signals on two or more selected spectral lines (fig.1). This eliminates the hardware error and enables correct measurement at close to zero scattering angle. Various types of filters and monochromators can be fitted, including monocrystalline diamond monochromator, which allows to work with the polarized radiation.
In the scheme of selection of spectral lines the sample is irradiated with characteristic and bremsstrahlung radiation with a maximum energy of X-rays up to 40 keV, which enables the determination of elemental composition by X-ray fluorescence spectra. Thus spectrometric measurements can be taken both in a static position and during angular or linear scanning for additional information on the distribution of the composition in the area and depth of a sample.
The most important source parameter for X-ray measurements is the size of the focus. The system is equipped with fine-focusing X-ray source with a focus projection of 20 µm (optional – 10 µm). This provides a record high angular resolution and allows for the first time to use in practice a method of X-ray refractometry in the study of layered nanostructures. To increase the intensity of X-ray beams can be effectively applied refractive and mirror focusing optics. Furthermore, the capacities of the equipment allows to precisely measure the quality of the optics used. Studies of the characteristics of periodic multilayer X-ray mirrors were performed in collaboration with the Institute for Physics of Microstructures RAS.
Measurement of multilayer thin-film nanostructures
The developed comprehensive approach to the measurement of multilayer thin-film nanostructures primarily provides unambiguous and reliable results through the use of complementary research methods, which are based on different physical principles and allow to resolve any ambiguity in the solution of inverse problems. Currently a full technological cycle of production of nanoelectronic devices, in particular in the transition to a design rules of about 90 nm or less, requires solving problems of measurement of manufactured structures. Difficulties are caused by insufficient informational capabilities of standard techniques and ambiguity of models, and also by incorrect assumptions about the structure and composition of the created objects.
The X-ray reflectometry used in the measuring system is a standard for the study of multilayer thin-film structures, however, the analysis only of the specular component of the X-ray reflection doesn’t allow to separate the contributions to the waveform from the density gradient of material of layers and from the roughness and irregularities of the interfaces. In this regard, along with the relative reflectometry in a complex of methods were included X-ray refractometry and diffuse X-ray scattering (fig.2).
Using of fine-focusing X-ray source allows to investigate the X-ray refraction in thin-film structures, to calculate the index of refraction and, therefore, to obtain information about the density distribution of the material. To clarify the roughness parameters of interfaces the diffuse X-ray scattering is used. All this helps to resolve ambiguities of “density-roughness” in the solution of inverse problems and to obtain unambiguous results of measurements of dimensions of multi-layer solid-state nanostructures.
Special attention is also paid to the correctness of the solution of inverse problems. Regularization (smoothing, etc.) of experimental data in order to reduce an error when working with the ratio of the signals at the two wavelengths. Then to find the global extremum of the residual functional when fitting the theoretical and experimental curves are used well-established stochastic algorithms: genetic algorithm and bees algorithm .
To improve performance, parallel processing of experimental data on GPUs  is used. This technology allows to reduce the computation time by two orders of magnitude and, thus, to use X-ray methods directly to control the growth of multilayer structures during the manufacturing process.
Fig.3–6 shows the results of a comprehensive analysis of thin-film structures obtained by the process with a design rule of 180 nm: a diffusion-barrier layers TiN (5 nm) / Ti (10 nm) / SiO2 (15 nm) / Si; porous low-k dielectrics films SiCN (90 nm) / SiOC (180 nm) / Si . Problems of identification and calculation of parameters of unaccounted layers formed during technological processes were solved, corrections of measurements (spectral ellipsometry, etc.) were made. To confirm the results of the reflectometry transmission electron microscopy were used.
Two-wavelength X-ray reflectometry method is highly effective in the study of weakly perturbed (by density) layered structures, in particular produced by ion implantation . Fig.7-8 presents the reflectograms of silicon wafers implanted with fluorine ions. Instead of the exponential drop-down standard reflectometry curve (fig.7) in the relative reflectogram (fig.8) obtained convenient for mathematical processing function with pronounced extreme in the area of specular reflection angles 2θ<1° for which it is possible to observe a radical manifestation of the contrast of the intensity of desired signal.
The presented concept provides a competitive advantage in the market of scientific research equipment. It is distinguished, in the first place, by the versatility and simplicity of design ensuring high safety, maintainability, ease of upgrading, low prime cost and operating cost. An integrated set of methods for specific measurement tasks of solid state technology determines the accuracy and certainty of the results. In addition, the possibility of rapid processing of experimental data to monitor the production process is provided.
Of particular note is the use in these systems of a new generation of portable microfocus X-ray sources. Specialists of LPI RAS involving MELZ and Angstrem developed microfocus X-ray generator of new generation XRS COMPÅCT (fig.9), which brings together a number of innovative solutions that provide record-breaking performance, reliability and wide scope of application. Microfocus X-ray generator is a standalone device, containing a miniature X-ray tube and high-voltage module. Thanks to the unique two-stage electrostatic focusing system and built-in high voltage control circuit, the focus size less than 30 μm is achieved. The patented design of the anode block with a transparent substrate of single-crystal diamond provides simultaneous generation of X-ray and optical radiation. It allows visualization of the X-ray beam, which greatly increases the safety of the source and facilitates the adjustment of the measuring circuit. The use of transparent diamond anode substrate also provides the maximum brightness of the X-ray focus.
The high position stability and small focal spot size allow effectively to use the focusing X-ray optics, including curved X-ray mirrors and polycapillary lenses. The use of microfocus source with a parabolic mirror optics allows to create a paraxial monochromatic X-ray beams with a divergence of ~1 mrad and intensity more than 107 photon/s.
In the design of a compact source (fig.10) is used air cooling system and transmission type anode. The anode is an optically activated diamond substrate coated with a metal film, in which X-rays are excited. The system allows X-ray diffraction measurements of crystals and polycrystals and creation of desktop and mobile models of diffractometers, reflectometers and elemental analyzers. These devices can be embedded, including, in production equipment to control the growth of the structures in real time. High brightness of X-ray source may be used in deep lithography and electroforming (LIGA) to produce MEMS.
Prospects for import substitution
Instead of a conclusion, we will note that the solution to the problem of import substitution as applied to X-ray equipment is considered in several ways.
Supplying the hi-tech industries with effective systems of nondestructive X-ray diagnostics demands expansion of interaction between the scientific, production and educational organizations for the solution of technical tasks, introduction of new developments and training of highly qualified specialists. Only the combination of these factors will allow in the short term to provide the necessary level of integration to achieve the stated goals.
On certain important areas already obtained significant results. These include the creation and development of new X-ray systems based on two-wave measurement and compact microfocus X-ray sources of new generation with diamond anode substrates. To meet the requirements of real high-tech production a set of complementary research methods is developed, as well as hardware and software for remote measurements and rapid processing of the obtained results, which is crucial for the analysis of hazardous materials, including fuel elements of nuclear power plants.
Presented in this article developments and proposals were discussed at the roundtable “The new generation of microfocus X-ray sources for non-destructive testing and multifunctional X-ray diagnostics systems”, organized by the LPI RAS in the framework of the XIV International exhibition NDT Russia (17-19 February 2015, Moscow). Special response was received by proposals for the organization of production and development of these X-ray systems through various forms of cooperation within the BRICS and the SCO.