Application of photoacoustic technology in electronic devices

As early as the end of the 19th century, AGBell observed the photoacoustic effect, that is, when the intensity-modulated beam enters the closed medium space, it will produce an acoustic wave effect. But because of the time. Considering that the incident light only starts to be absorbed at the surface of the sample and begins to liner | sample attenuation, the thermal diffusion equation is solved in this space, and the expression of the photoacoustic signal excited when periodic light with a modulation frequency of w is incident: Pg Where: / 0 is the light intensity of the incident light, T0 is the ambient temperature, the absorption coefficient of the US sample to the incident light, Z is the efficiency of the conversion of light energy at the wavelength X into heat energy during the sample without radiation, P0 is the cavity The ambient pressure of the internal gas, V is the heat capacity ratio, b = kbe / ke, g = kg% / ke, r = U / es (subscript bsg stands for backing, sample and gas, respectively).

The above formula can be used to estimate the amplitude and phase of the photoacoustic signal of the condensed sample. When the size of the cavity is much smaller than the wavelength of the acoustic wave, the strength of the photoacoustic signal is proportional to the optical power absorbed, inversely proportional to the volume of the cavity, and is related to the thermal, optical properties and modulation frequency of the sample and medium. In this way, using the above formula You can directly determine the thermal optical properties of the medium.

The whole detection system usually consists of 3 parts: radiation light source, photoacoustic cell and signal processing and display system. Excitation sources include incandescent lamps, lasers, electron beams, ion beams, molecular beams, proton flux neutron flux, meson flux, x-rays, etc.

2 Application of photoacoustic technology in electronic devices 21 Measuring the thermal diffusivity of thin film materials such as metal-coated semiconductor functional films, diamond films, etc. With the development of space technology, optoelectronic technology, microwave acoustic technology, and the need for wear-resistant high-temperature thermal materials , People are more and more important and urgent to study the thermal properties of materials (especially thin film materials). The use of photoacoustic phase method is a relatively feasible method to study the thermal diffusivity of thin film materials.

Adams and Kirkbght of the United Kingdom used a tungsten halogen lamp with a power of 20w as the excitation source, and studied the thermal diffusivity of a 20nm diameter 17mm thick copper sheet. The results were satisfactory. The experiment showed the light for the first time to people The back-illumination method of acoustic phase method is an attractive prospect for studying the properties of thin-layer materials. However, in the measurement of thermal diffusivity by back-illumination method, overcoming the aftershock effect plays an important role in improving the accuracy of the data. Lateral illumination method, frontal incidence method, etc. The frontal incidence method is used to study the thermal diffusivity of multi-layer thin-film materials, which has great application prospects in the research of the thermal properties of semiconductor heterojunction devices. However, these methods use photoacoustic photothermal The "mirror effect" in the technology measures the performance of the material, and its data processing is more complicated. Existing people use photoacoustic phase fitting technology to measure the thermal diffusivity of nano-alumina, and the effect is remarkable. At present, the use of laser piezoelectric technology Thermal diffusivity is gaining popularity because of its simple and compact experiment and wide frequency response.

AC photothermal method is another very effective method for measuring the thermal diffusivity of thin film materials a. It can be used to measure samples that span 4 orders of magnitude. It is suitable for polymer films, metal foil electronic substrate materials and artificial diamond films. The testing of various material samples shows that the results are satisfactory, and how to eliminate systematic errors in this method is the key.

22 When the photoacoustic signal of the surface defect of the integrated circuit passes through the material to be tested, it carries the characteristic information of the material itself. After receiving and detecting this photoacoustic information, the properties of the material will be obtained. If the standard spectrum of this material is input into a computer and compared with the detected unknown spectrum, unknown defects can be identified, so that different defects can be identified qualitatively or semi-qualitatively. For micron-level defects, the most researched is laser ultrasonic inspection, which has become a research hotspot in the international academic community, but its theory and experiments need to be further deepened. The commonly used materials are: aluminum alloy, steel, single crystal silicon polycrystalline silicon , Glass, piezoelectric ceramics, etc. The electron-acoustic microscope has achieved very meaningful results in this respect. The electron-acoustic microscope is used to detect the subsurface defects of silicon integrated circuits, and the image is intuitive and clear.

2.3 Study the microscopic electrical properties of electronic device materials 2.3.1 Study the surface electrical properties It is very important to study the microscopic electrical properties of microelectronic components, ultra-large integrated circuits, microelectronic mechanical systems and nanomaterials, such as: Main factors affecting the unstable performance of semiconductor components and IC systems. During the photoacoustic spectroscopy test, since the phase of the photoacoustic signal changes with the change of the modulation frequency, the state of the material in the depth direction can be analyzed by changing the modulation frequency. When incident with high-frequency incident light, the sample surface information can be obtained according to the light absorption of the sample, so it can be used to determine the surface electrical properties of electronic device materials. Among them, the more mature technology is surface photovoltaic technology (SPS), which is increasingly used to study the surface properties of semiconductor materials and the charge transfer process between phases. Infrared photoacoustic spectroscopy also has excellent functions for surface electrical properties research. F. McClland et al. Used PAS to analyze the surface state of Ge crystals, defects due to laser irradiation, changes in thermal properties, etc. The results of how to control the semiconductor annealing process There's important meaning. In addition, the PAS method can also be used to study the ultraviolet light-induced aging of semiconductor surfaces in different gas environments.

2.3.2 Measuring electrical conductivity, electron mobility and carrier concentration SC has been considered as a potential high-temperature and high-power radiation-resistant electronic material due to its excellent electrical characteristics, but it is worth noting that although in recent years The material preparation of SC and the process technology for growing low-defect oxides have developed rapidly, but the measurement of the electron mobility of its inversion layer has been unsatisfactory. The measurement of electron mobility with the PAS method is expected to improve its measurement accuracy and bring new research mechanisms to SiC for better application. PAS method can also measure the conductivity, electron mobility and carrier concentration of heavily doped polysilicon.

2.3.3 Measuring the three-doped distribution of doping concentration in semiconductors and the forbidden band width ion implantation is a common technique for doping semiconductor materials. Generally, the thermal conductivity of doped regions and their substrates are very different. In terms of semiconductor materials, electroacoustic microscopy is used for ion implantation imaging and doping imaging, which can easily measure the lateral distribution of impurities in the matrix, so it is a very useful non-destructive ion implantation distribution range The conventional analysis method has the advantage that it can detect the speed of change of the damaged layer structure.

Lecturer of the Department of Mechatronics, the research direction is laser and electronics.

23.4 Determining the dislocation distribution of crystals The electron-acoustic microscope can also clearly show the distribution of crystal dislocations. The resistance of the crystal sheet is related to the dislocation density, and the dislocation network plays an important role in the formation of semiconductor mechanisms, so it is very valuable to show the dislocation density distribution and it is a non-destructive method.

24 Measuring the thickness of semiconductor thin layers or metal coatings In the photoacoustic spectroscopy test, when the low-frequency incident beam is used as the modulation frequency, the information of the deeper part of the sample can be obtained, and then the sample thickness can be determined. The PAS interference technique is now commonly used to measure the thickness of materials deposited on the base material, such as the thickness of the Si2 layer on an opaque Si substrate with poor emission properties. In addition, Jiang Tao et al. Used laser-induced elastic waves in the photoacoustic method to measure the thickness of aluminum and copper plates with a thickness of about Q 0 mm. The theory and practice are consistent, and the thinner the thickness, the higher the accuracy. Tongji University is engaged in the detection of the thickness of thin film materials by laser ultrasound, and the introduction of digital signal processing methods into the study of photoacoustic effects. 25 Research on nanomaterials In 1993, Qian Menglu of Tongji University applied laser ultrasound technology to the analysis of nanomaterials, and obtained the relationship between the speed of sound and the pressure and temperature when preparing nanomaterials; in 1994, Zheng Yibing of Nanjing University and others used Photoacoustic spectroscopy analyzed the characteristics of the nano-films and concluded that the nano-materials have high disorder. Since then, there have been many reports on the characteristics of nanomaterials using laser ultrasound, (such as X.Rzhang et al. Using laser ultrasound technology to study the sound velocity and sound attenuation of nano-metals Zn and Ag), but the current research is still limited to a limited number of substances Therefore, studying the acoustic properties of different nanomaterials and revealing their relationship with the microstructure of nanomaterials are the main tasks in the application of photoacoustic technology and nanomaterials research for a period of time in the future.

Wang Peiji et al. Used laser piezoelectric photoacoustic technology to measure the thermal diffusivity of nano-titanium dioxide. The results show that this method is a very effective detection method. Because it is based on the measured piezoelectric photoacoustic signal to directly nonlinearly fit the frequency, it improves the accuracy of the experimental results. At the same time, because it only needs one laser during the experiment, the adjustment is very convenient, so the laser pressure Electro-optic acoustic technology is a very effective method in the detection of new nanomaterials. 26. Making micro-silicon beam resonant sensors In the 1980s, scientists used the etching technology in the production of microelectronic devices to manufacture micro-mechanical components. Micro-silicon beams were one of the earliest micro-mechanical components. When the modulated laser is incident on the surface of the micro-silicon beam, the light pressure effect and the photothermal effect are simultaneously generated at the incident point. The photothermal effect of the modulated light at the incident point of the micro-silicon beam forms periodic thermal strain due to thermal penetration, and the thermal strain transforms It is the driving force for the lateral movement of the micro-silicon beam, thereby causing oscillation, and can be made into a light-excited resonant sensor. This sensor has extremely high sensitivity and is inherently safe and explosion-proof. In addition, photoacoustic technology will also be used to study ferroelectric thin films, and photoacoustic detectors will also be used to study high-Tc superconducting materials.

3 Conclusion With the improvement and development of photoacoustic technology, people will gradually deepen their understanding, and the application is increasingly widespread. In the future, the application of photoacoustic (photothermal) technology at low temperatures will become another subject worthy of study, because the signal-to-noise ratio is high at low temperatures, which brings great convenience to signal processing. For electronic devices, physical properties of integrated circuits, semiconductor functional films and coatings, superconducting films, etc., especially non-destructive subsurface non-destructive testing will be the most promising applications in photoacoustic technology.