Characterization of epitaxial layers using scanning microwave microscopy.
Nowadays, with the development of electronics and microelectronics the complexity of electronic devices increased approximately four times every three years. For this reason, leading world producers invest into the development of electronic structures and networks in nano-scale. In this area, ultra-thin layers, nanocomposites and nanoparticles, among others, play an important role.
Ultra-thin layers are very interesting area of physics, with properties located between microphysical structures, where classic physics is used, and nanophysical structures, where quantum physics takes place. These ultra-thin layers have great potential in the industry, e.g. as diffusion barrier in electrical components fabrication . These layers are also commonly used in semiconductor industry. Moreover, these layers can be used in photonics , optoelectronics  and spintronics . Nanoparticles can be used for modification of the mechanical properties of materials  and nanocomposites, like ultra-thin layers, can be used to enhance the electrical properties of materials .
Measuring methods and capabilities for the quality control and diagnostics of these structures are significantly limited due to their structure size which is often approaching atomic resolution. There are only few possible measuring methods--diffraction techniques and microscopy--scanning electron microscopy and scanning probe microscopy. 
In the present work, we studied epitaxial layers with different dopant doping levels deposited on a silicon substrate by the vapour phase epitaxy [8, 9, 10, 11]. Scanning probe microscopy techniques--Atomic force microscopy  and scanning microwave microscopy were used for the diagnostics of dopant doping changes among the growth layers, to confirm the manufacturing process. This was essential for following experiments. Scanning microwave microscopy is a derived method of atomic force microscopy that combines electromagnetic measurement capabilities of vector network analyzer with nanometer resolution of atomic force microscopy.  This allows simultaneous measuring of both topographical and electromagnetic properties of analyzed sample, which is a great advantage comparing to other measuring techniques.
2. Materials and methods
Several epitaxial layers with different dopant doping levels were deposited on a silicon substrate by the vapour phase epitaxy. The levels of doping were specified by the manufacturing data (Fig. 1.) from lowest dopant concentration approximately C=[10.sup.16] [cm.sup.-3] in depth of 1 [micro]m up to depth of 10 [micro]m with dopant concentration approximately C=[10.sup.19] [cm.sup.3].
Scanning microwave microscopy and atomic force microscopy were used for characterization of the manufactured sample.
Atomic force microscopy is based on bending of cantilever by interatomic forces between atoms at the tip of the scanning cantilever and surface atoms of measured sample. These forces are attractive or repulsive depending on the distance between the tip and the sample. Bending of the cantilever due to tip -sample force is measured by laser beam which is reflecting from the upper part of the cantilever to four-segment photodiode. Voltage changes in each segment determine position of the tip.
Scanning microwave microscopy can be easily understood using optical analogy. As shown in Fig. 3., when light strikes an object, some of the light is reflected from the object and some passes through depending on optical properties of the object. By measuring amount of light reflected and/or transmitted, optical properties of the object can be determined. Applying the same principle for a microwave spectrum, measurement of the incident, reflected and/or transmitted waves can provide detailed information about the properties of the material. [15, 16, 17]
To measure the properties of the sample and its individual layers, the cross section of the sample was needed. The cut was performed using diamond pen. No other special preparation was necessary and all measurements were taken under common laboratory conditions.
The atomic force microscope Agilent 5420 SPM/AFM, vector network analyzer PNA N5230A and metal-coated (Pt-Ir) conductive AFM probe were used--size 400 x 60 [micro]m, spring constant of 0.3 N x m-1, diameter less than 7 nm, resonant frequency of 4.5 kHz. The vector network analyzer provided the microwave signal of frequency 2.468808 GHz.
3. Results and discussion
Scanning area of 15 x 15 [micro]m was chosen next to the upper edge of the manufactured sample to measure all growth layers at the cross section (Fig. 4.). AFM topography, PNA amplitude and dC/dV signal were measured, visualized and analyzed using Gwyddion  and MatLab software. Figure 5. shows topography of the measured sample. Topography was almost flat as expected.
The PNA amplitude has not provided any relevant information. However, the most useful results provided dC/dV signal as can be seen in Fig. 6., epitaxial layers with different doping concentration are visible in depths corresponding to manufacturing data.
Figure 7. shows the extracted profile of the measured data. The horizontal axis represents a depth of the sample while the vertical axis represents dC/dV signal.
For the purpose of the comparison with the manufacturing data, the obtained data were approximated and integrated in MatLab software. Figure 8. presents the integrated profile of the measured signal. The vertical axis is relative. Calibration standard would be needed to get absolute values of the dopant concentration; however, when compared with the manufacturing data (Fig. 1.), it is obvious that both figures correspond and each concentration is in the correct depth of the sample.
The experiment has shown that scanning microwave microscopy can be powerful tool to probe electromagnetic properties on scales of micro or nanometer. The informative character obtained by scanning microwave microscopy reveals it is one of the most suitable techniques for the quality control in semiconductor industry and for control of the ultra-thin layers and nanostructures. A limitation of the study was approximation of the data to calculate the integration. Secondly, the results are only relative. To get absolute values calibration standard would be needed. In the future research, the measured sample will be modified using nanolithography.
Submicron characterization of several epitaxial layers with different dopant doping concentration deposited on a silicon substrate by the vapour phase epitaxy was presented using atomic force microscopy and scanning microwave microscopy. Scanning microwave microscopy showed the potential for submicron characterization of epitaxial layers with different properties and for the control of a deposition processes.
This work was performed with financial support by the Ministry of Education, Youth and Sports of the Czech Republic within the National Sustainability Programme project No. LO1303 (MSMT -7778/2014) and with support by Internal Grant Agency of Tomas Bata University under project IGA/CebiaTech/2015/045 and IGA/FAI/2015/048.
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Tomas Martinek, Josef Kudelka, Milan Navratil, Vojtech Kresalek
Tomas Bata University in Zlin, Nad Stranemi 4511, Zlin 76005, Czech Republic
Caption: Fig. 1. Manufacturing data of the sample.
Caption: Fig. 2. Atomic force microscopy detection principle .
Caption: Fig. 3. Optical analogy to high-frequency characterization .
Caption: Fig. 4. Manufactured sample.
Caption: Fig. 5. Topography image.
Caption: Fig. 6. dC/dV signal.
Caption: Fig. 7. Extracted profile from Fig. 6.
Caption: Fig. 8. Integrated profile extracted from Fig. 6.
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|Author:||Martinek, Tomas; Kudelka, Josef; Navratil, Milan; Kresalek, Vojtech|
|Publication:||Annals of DAAAM & Proceedings|
|Article Type:||Technical report|
|Date:||Jan 1, 2015|
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