Subject: Radio wave type of non-destructive testing
Radio wave method non-destructive testing is based on recording changes in the parameters of electromagnetic waves of the radio range interacting with the test object. Usually, ultra-high frequency (microwave) waves with a length from 1 mm to 100 mm are used. They control products made from materials where radio waves are not very attenuated: dielectrics (plastics, ceramics, fiberglass), magnetodielectrics (ferrites), semiconductors, thin-walled metal objects.
By the nature of interaction with OK distinguish between methods transmitted, reflected, scattered radiation and resonant.
If the controlled quantity is directly related to the field strength (power) of reflected, transmitted or scattered radiation, the amplitude control method is used. Technical implementation The method is simple, but low noise immunity limits its use. More reliable results are obtained using phase and amplitude-phase methods, selection-based useful information, concluded in changes in the amplitude and phase of the wave.
If the thickness of the object exceeds the wavelength of the probing radiation used, it is recommended to use a geometric or time method to measure it. In the first case, the controlled parameter is associated with the deviation of the positions of the reflected beam in the recording plane relative to the selected coordinate system, in the second - with a change in the signal delay in time.
The polarization method is used to control thin-film and anisotropic materials., based on the analysis of changes in the plane or type of polarization of oscillations after the interaction of radiation with the OC. Before testing, the receiving antenna is deployed until the signal at its output from the reference OC becomes zero. Signals from the tested OCs characterize the degree of deviation of their properties from the standard one.
Holographic method gives good results when controlled internal structure OK, but due to the complexity of its hardware implementation, the method has limited application.
The most complete information is provided by the use of multi-element antennas, since in this case it is possible to reproduce the internal structure of the object.
To increase the resolution of flaw detection, the self-comparison method is used. It is implemented using two sets of emitting and receiving devices, as close to each other as possible. The resulting signal is determined by the difference in amplitudes and phases of the signals from the receivers of each channel. The presence of a defect leads to a change in the conditions of wave propagation in one channel and the appearance of a difference signal. Analysis of the dynamics of signal changes during the periodic passage of a defect through the control zone of a radio wave flaw detector makes it possible to reduce its sensitivity threshold.
Resonance method Radio wave monitoring is based on the introduction of an OC into a resonator, waveguide or long line and recording changes in the parameters of the electromagnetic system (resonant frequency, quality factor, number of excited types of oscillations, etc.). This method controls dimensions, electromagnetic properties, deformations and other parameters. The resonance method is successfully used to control the level of liquids in tanks and the movement parameters of various objects.
Radio wave monitoring is used to solve all typical tasks non-destructive testing: thickness gauging, flaw detection, structuroscopy and introscopy (control of internal structure). The equipment used in this case is, as a rule, built on the basis of standard or modernized microwave elements. A special element for solving a specific problem can be a source or receiver of radiation, as well as a device for attaching and moving an object.
Among other features of radio wave monitoring in comparison with optical and radiation monitoring, it should be noted the use of the impedance method for calculating signal parameters and the commensurability of the radiation wavelength with the dimensions of the radio wave path “radiation source - control object - radiation receiver”.
Microwave radiation belongs to the field of radio waves, which since their discovery have been used to transmit information. The use of microwave waves for NDT purposes required the creation of a theory of their interaction with the test object.
Radio wave non-destructive testing means are sensors with a sensitive element, in which the controlled quantity is converted into an informative parameter; microwave generators - sources electromagnetic vibrations; secondary converters are designed to generate registration and control signals.
Classification of devices. Radio wave monitoring devices can be classified according to various criteria.
According to the informative parameter, devices are distinguished:
– amplitude;
– phase;
– amplitude-phase;
– polarizing;
– resonant;
– radial;
– frequency;
– transformative (wave type);
– spectral.
According to the layout of the receiver and emitter of microwave energy relative to the controlled
samples can be:
– for passage (two-way access);
– for reflection (one-way access);
– combined.
The following forms of signal generation are distinguished:
– analog;
– diffraction;
– optical.
When using this type of control, the presence of defects in the products under study leads to the appearance of additional reflections of electrical energy. magnetic field, which change the interference pattern and cause additional energy losses. This method is used in flaw detection of dielectrics, as well as in studying the state of the surface of conductive bodies.
Disadvantage of the microwave method is the relatively low resolution of devices implementing this method, due to the shallow penetration depth of radio waves into metals.
The radio wave method is based on the dependence of transmitted or reflected radio radiation on the parameters and characteristics of dielectric materials (plastics, rubber, fiberglass, thermal insulation materials, plywood, grain, sand, etc. materials). The radio wave method uses a range of wavelengths called the ultra high frequency range. An electromagnetic wave is a combination of electric E and magnetic H fields propagating in a certain direction Z. In free space, electromagnetic waves are transverse, i.e. vectors E and H are perpendicular to the direction of propagation.
Vector E determines the polarization electromagnetic field(its amplitude). Based on this, the wave can be plane polarized (linearly polarized), electrically polarized, circularly polarized (right or left polarized, right - clockwise, left - counterclockwise). The magnetic field strength H is checked by its change in amplitude depending on the magnetic permeability of the material used. H can vary from zero to a maximum value, which is used in electrical paramagnetic resonance methods and in nuclear resonance methods. This makes it possible to study weak interactions within matter using these methods.
Principles of radio wave construction
non-destructive testing devices.
The radio wave method uses a wavelength range from 1 to 1 mm, which is called the ultrahigh frequency (microwave) range. When a signal passes through a controlled environment, the latter affects its characteristics. If dielectric materials are controlled, then the dielectric constant and loss tangent are used as characteristics; when monitoring semiconductor materials, the dielectric constant and magnetic permeability are assessed; When testing electrically conductive materials, conductivity is examined. Radio wave monitoring devices can be divided into phase, amplitude-phase, polarization, resonant, spectral, frequency, beam, and transformer. All these devices are based on the use of the phenomena of reflection, transmission, absorption, refraction, polarization and conversion of radio wave radiation. To measure the degree of influence of the medium on the signal, amplitude-phase devices are used. The device diagram is shown in Figure 1.
Devices of this type contain a transmitting antenna 4 and a receiving antenna 6, a microwave generation source 1, a valve 2, an attenuator 3.7, with which the radiation can be weakened, a detector 8 and a processing and information output unit 9. After the radiation passes through the control object 5, the signal power will be assessed using the formula:
The power of radio emission passing through the test object;
Radiating antenna area 4;
Power of the radiating antenna 4;
Radio wave transmission coefficients at the interface between two media of the material under study and the environment in which it is located; , Where
Length of the radiating antenna in cross section;
Distance from the edge of the radiating antenna to the surface of the test product 5;
Distance to the edge of the receiving antenna from the surface of the product being tested after the passage of radio radiation;
Thickness of the product being tested;
Reflection coefficients when radio radiation falls on the surface of a product and when it exits from the surface of the product; , Where
Wave number;
Wavelength of radio emission.
From expression 1 it is clear that at a given power it is possible to determine the thickness of the controlled object or physical parameters. To eliminate reflections, it is necessary to coordinate the boundaries with the receiving and emitting antennas, i.e. distances Radio wave devices can be built on the principle of receiving a signal reflected from a defect. The device diagram is shown in Fig. 2.
The principle of operation of such devices is as follows: the signal from microwave generator 1 is fed through valve 2 and separation unit 3 to the radiating antenna 4, the signal reflected from object 6 enters antenna 5, is detected in element 7 and identified in system 8. A feature of devices based on reception of reflected signals is the presence of communication (the strength of the electromagnetic field of radio emission) between the emitting and receiving antennas. This connection is realized due to part of the radiation of antenna 4 and is the reference signal with which the reflected signals are summed. The totality of all signal components is of an interference nature, depending on the relationship between the amplitude and phase of the reflected signal and the communication signal. The type of interference pattern depends on the reflected signal, which carries information about the internal structure of the controlled object, i.e. depends on . Radio wave polarization devices are based on the polarization dependence electromagnetic wave, i.e. on the orientation of vector E in space as it spreads in a controlled environment. By the type of polarization (plane, circular, electric) one can judge the internal structure of the material. Typically, the device is configured so that if there are no internal defects in the object, the signal in the receiving antenna is zero. If there is a defect or structural inhomogeneity, the plane or type of polarization of the emitted signal changes, and a signal carrying information about the defects appears in the receiving antenna.
In radio wave resonant devices, the state of the controlled object is determined by the effect of the medium on the quality factor, the resonant frequency shift, or the field distribution in the resonator. Figure 1 shows a cylindrical resonator in the form of a diagram:
|
Typically, a resonator 1 of cyclic shape with a diameter is excited by a wave. The test sample of 2 diameters is placed inside the resonator. In this case, there is a shift in the resonant frequency. The magnitude of the displacement determines the homogeneity of this sample and its continuity. If there is a discontinuity or any defect inside the test object, the resonant frequency shift increases. This determines the control of the test sample.
In the case (Fig. 1 b), differently polarized radio waves arise. Some are right polarized, others are left polarized. If such a resonator is placed on a sample, then in the presence of defects in the sample, a change in the polarization of the radio wave will occur, and some components of the value of this polarization will appear (this is shown in the figure as). By measuring the position of this value, you can find the location of this defect and its extent.
Scheme of operation of beam devices
Figure 2a) shows the passage of a radio beam through the sample. Typically a millimeter-wave beam is used, and its transmission is subject to the laws of geometric optics. As a result, the refractive index is determined by the magnitude of the deviation and this is how the characteristic of the medium is found. If the medium is homogeneous, then the beam refracts out from the opposite side of the product, but if the medium is inhomogeneous, then in addition to refraction, reflection of the radio beam also occurs, as shown in Figure 2b). In devices of this type, a radio image of internal defects is recorded.
Radio wave thickness gauges.
Radio wave methods make it possible to control the thickness of dielectric materials, dielectric layers on metal and metal sheets. Thickness information may be contained in amplitude, phase, resonance line offset, and resonance curve. The most important object parameters that affect the transmitted or reflected signal are the thickness and dielectric constant of the material. The more homogeneous the material, the more accurately the thickness is measured. The reflection and transmission coefficients of a radio wave for a flat homogeneous layer at normal incidence are oscillating functions that decrease with increasing thickness and the ratio , where is the wavelength of the radio beam.
The period of these functions is determined by the wavelength and refractive index of the medium. And the degree of decrease is the wave attenuation coefficient. Figure 3 shows graphs of reflection coefficients for two dielectrics.
Row 1 – gypsum concrete (); row 2 – plexiglass ( 
)
Fig.4
Row 1 – medium decay 
; row 2 – low attenuation; row 3 – large attenuation; - loss angle.
It can be seen that the period of oscillation of the reflection coefficient is inversely proportional to the dielectric constant. An unambiguous relationship between transmission coefficient and thickness occurs at high attenuation. The appearance of ambiguity at low attenuation makes it difficult to use thickness gauges based on wave transmission. As an example, consider a thickness gauge for measuring the thickness of a rolled metal sheet.
Thickness gauge for measuring thickness
rolled metal sheet.
1- node for processing signals and issuing them for indication and control
2 - microwave generator 10 - lens
3- tee 11- measured object
4- valve 12- lens
7 - adjusted short-circuiting plunger 15 - short-circuiting plunger
9 – radiating antenna (horn) 17 – matching load
18 – valve
In devices for this purpose, there is a specular reflection of an electromagnetic wave from the surface of the controlled object, while a current antinode and a voltage node are installed on the surface itself. When measuring the thickness of an object, the constructed field pattern changes, which is noted by the device. The generated microwave signals through the tee 3 and valves 4 and 18 are supplied to branches 8 and 14, and then to horn antennas 9 and 13 with lenses 10 and 12. The signals, reflected from the surface of the measured object 11, form standing waves. Resonators of reflected waves are adjusted to resonance using short-circuited plungers 7 and 15.
|
Radio wave moisture meters.
Methods for measuring the moisture content of materials are based on the absorption and scattering of radio waves by water molecules in the microwave region. Informative parameters are the amplitude, phase and angle of rotation of the plane of polarization of the electromagnetic wave. It is known that resonant absorption occurs in the microwave region. In addition, the dielectric constant of water in the specified frequency range varies from 80 to 20, while this value for other materials lies in the range of 2-9. This circumstance makes it possible to use the radio wave method for constructing moisture meters for various purposes. Figure 6 shows the dependences of dielectric constants on frequency.
Row 1 – permeability, row 2 – permeability.
To measure the moisture compound, an amplitude moisture meter is used, which is based on attenuation of the power of the signal transmitted through the object; its diagram is shown in Figure 2. In the region of weakly bound moisture, the signal transmission coefficient is proportional to the water content.
Amplitude moisture meter.
1- microwave generator 9 – transformation control device
2- valve 10 – indicating device
3 - waveguide tee 11 - detector
4 - radiating antenna 12 - shorted plunger
5 – receiving antenna 13 – amplifier
6- converter
7- plunger shorted
8- detector
Amplitude-phase moisture meter.
1- Microwave generator 5 – receiving antenna
2- Variable converters 6 – load matching device
3- Tee 7 – waveguide tee
4- Radiating antenna 8 – indicator
9 – amplifier 10 – detector
The device operates on the principle of comparing the signal passed through a wet object and the signal passed along the waveguide path. In the waveguide tee 7, the signals are compared in amplitude and phase. The difference signal after amplification is displayed in device 8.
Radio wave flaw detectors.
These instruments are used to inspect cracks, air inclusions, foreign inclusions, inhomogeneities, bonding defects, etc. in dielectric materials. Radio wave flaw detectors are built on the principle of transmission or reflection of a wave, which carries information about the thickness of the layers and the refractive index, i.e. about the physical parameters of the layers (density, porosity, humidity, composition, etc.) in Figure 9, as an example, a diagram of a flaw detector with mechanical scanning is shown.
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Introduction
One of the most important problems of pipeline transport is maintaining the normal condition of the linear part of field and main pipelines. Underground pipelines operating under normal conditions last for at least several decades. For example, in the USA, some pipelines that have been in operation for about twenty years are completely preserved and do not require repair. This was facilitated by great attention, which is devoted to systematic monitoring of the condition of underground and above-ground pipelines and timely elimination of emerging defects.
As a rule, most defects in pipelines appear as a result of corrosion and mechanical damage, determining the location and nature of which is associated with a number of difficulties and high material costs. It is quite obvious that opening the pipeline for direct visual inspection is not economically justified. In addition, only the outer surface of the pipeline can be examined. Therefore, within recent years in our country and abroad, the efforts of specialized research and design organizations are aimed at solving the problem of determining the condition of underground and above-ground field and main oil product pipelines without opening them. This problem is associated with great technical difficulties, but with the use of modern methods and measuring equipment it is successfully solved.
In this work we will consider one of the methods that ensures the identification of defects.
1. Peculiaritiesradio wavemethod
Radio wave non-destructive testing is based on recording changes in the parameters of microwave electromagnetic oscillations interacting with the object of study. The wavelength range predominantly used in radio wave monitoring is limited to 1 - 100 mm. The 3 cm and 8 mm subranges are more mastered and provided with measuring equipment.
Radio wave testing is used to solve all typical problems of non-destructive testing: thickness gauging, flaw detection, structuroscopy and introscopy (control of internal structure). The equipment used in this case is, as a rule, built on the basis of standard or modernized microwave elements. A special element for solving a specific problem can be a source or receiver of radiation, as well as a device for attaching and moving an object.
Among other features of radio wave monitoring in comparison with optical and radiation monitoring, it should be noted the use of the impedance method for calculating signal parameters and the commensurability of the radiation wavelength with the dimensions of the radio wave path “radiation source - control object - radiation receiver”.
Microwave radiation belongs to the field of radio waves, which since their discovery have been used to transmit information. The use of microwave waves for NDT purposes required the creation of a theory of their interaction with the test object. It is quite natural that the developed theory took into account the results obtained in radio communications for wave systems with distributed parameters (long lines, waveguides, etc.) using the impedance method, in which the radio wave path “radiation source - control object - radiation receiver” is replaced by a model in the form long line. In this case, the channel for propagation of microwave oscillations (two-wire lines, waveguides, free space) is characterized by wave impedance. For an ideal dielectric it is real when e r =1 is equal to z 0 =377 Ohm.
Attitude g/(yet a)=tgд is called the dielectric loss tangent and is considered one of the most important parameters of dielectrics. Here r is specific electrical conductivity; u - angular frequency. At one frequency (tgд< 0,01) материал может считаться диэлектриком, на другой (tgд >100) - conductor. In calculations, ideal dielectrics include materials for which tgd< 0,01. На частотах, меньших 9x10 6 Гц, морскую воду относят к классу диэлектриков; на частотах, больших 9x10 10 Гц, - к классу проводников. В промежуточной области 0,001 < tgд < 100 материал называют несовершенным диэлектриком, характеризующимся комплексной диэлектрической проницаемостью и комплексным волновым сопротивлением.
For conductors imaginary part complex dielectric constant is large compared to the real part: e ">>e a and the wave impedance is determined by the expression z c will be equal to the square root of the ratio (mm a) / g. With increasing frequency, z c increases and the waves cannot penetrate deeply into the conductor The phenomenon of shielding of deep layers of material from field penetration by outer layers of material is called the skin effect. It is characterized by the depth of penetration plane wave, at which the field strength E and H decreases by e times.
The speed of propagation of an electromagnetic wave in an imperfect dielectric depends on the frequency since e "= r / n. The value v characterizes the speed of movement of points that maintain the same phase of the wave. The dependence v = f ( n) is called dispersion. Through the speed, the wavelength l is found =vT v .
When an electromagnetic wave passes from one medium to another along the normal to the boundary surface, a reflected wave is formed. When both waves are superimposed, a standing wave is formed, characterized by a standing wave voltage coefficient k stU = E max / E min or a traveling wave voltage coefficient k du = l / k stU. The maxima of a standing wave are obtained where the effective values of the intensity of the incident and reflected waves are added, and the minima are obtained where they are subtracted.
Parameters of conductive materials at a frequency of 10 10 Hz
The given formulas indicate the possibility of obtaining the required result based on the laws of geometric optics or the theory of long lines. When applying the second approach to calculate the parameters of microwave signals, the real system “radiation source - test object - receiver” is replaced by a model in the form of a long line with the same wave impedances and dimensions as in the real system. A variant of constructing such a model is shown below. The electromagnetic parameters of the layers of the product (e i, m i, g i) are taken into account through the complex wave impedances Z i of the long line segments. The input impedance of the receiver and the output impedance of the radiation source (generator) are taken into account by the wave impedances Z p and Z g.
The defect in the form of delamination is replaced in the model by a plane-parallel layer of the same thickness as the defect. The amplitude of the signal from a defect decreases in proportion to the area occupied by the defect relative to the area of the controlled zone.
The commensurability of the wavelength of microwave radiation with the dimensions of the elements of the radio wave path determines the complex nature of the electromagnetic field in the control system. For this reason, the technique for assessing signals in the system has a characteristic feature. If the distance between the boundaries of the various homogeneous media that make up the object under study exceeds the wavelength in the material, the components of the electromagnetic wave are estimated based on the laws of geometric optics.
Otherwise, the impedance method is preferable. In both cases, the resulting estimates of signals in the system are approximate and large errors cannot be ruled out. Therefore, it is recommended to use the calculation method to determine relative values quantities - changes in signal amplitudes with small changes in the parameters of the object under study or control conditions. As for the absolute values of the signals, they should be assessed experimentally.
Let us briefly discuss the methods and means of radio wave monitoring. If the controlled quantity is directly related to the field strength (power) of reflected, transmitted or scattered radiation, the amplitude control method is used. The technical implementation of the method is simple, but low noise immunity limits its use. More reliable results are obtained using phase and amplitude-phase methods, based on the extraction of useful information contained in changes in the amplitude and phase of the wave. To isolate this information, a reference arm “radiation source - radiation receiver” and a circuit for comparing signals from the test object with the reference one are introduced into the monitoring equipment.
If the thickness of the object exceeds the wavelength of the probing radiation used, it is recommended to use a geometric or time method to measure it. In the first case, the controlled parameter is associated with the deviation of the positions of the reflected beam in the recording plane relative to the selected coordinate system, in the second - with a change in the signal delay in time.
To control thin-film and anisotropic materials, a polarization method is used, based on the analysis of changes in the plane or type of polarization of vibrations after the interaction of radiation with OC. Before testing, the receiving antenna is deployed until the signal at its output from the reference OC becomes zero. Signals from the tested OCs characterize the degree of deviation of their properties from the standard one.
The holographic method gives good results in monitoring the internal structure of the OC, however, due to the complexity of its hardware implementation, the method has limited application.
Radio wave testing using transmitted radiation makes it possible to detect product defects if their parameters m a and e a differ significantly from similar parameters of the base material, and their dimensions are comparable or exceed the wavelength of the probing radiation. In the simplest version of such control, a traveling wave mode is maintained in the receiving path. The most complete information is provided by the use of multi-element antennas, since in this case it is possible to reproduce the internal structure of the object. To increase the resolution of flaw detection, the self-comparison method is used. It is implemented using two sets of emitting and receiving devices, as close to each other as possible. The resulting signal is determined by the difference in amplitudes and phases of the signals from the receivers of each channel. The presence of a defect leads to a change in the conditions of wave propagation in one channel and the appearance of a difference signal. Analysis of the dynamics of signal changes during the periodic passage of a defect through the control zone of a radio wave flaw detector makes it possible to reduce its sensitivity threshold.
The resonant method of radio wave monitoring is based on introducing an OC into a resonator, waveguide or long line and recording changes in the parameters of the electromagnetic system (resonant frequency, quality factor, number of excited types of oscillations, etc.). This method controls dimensions, electromagnetic properties, deformations and other parameters. The resonance method is successfully used to control the level of liquids in tanks and the movement parameters of various objects.
Radio wave non-destructive testing means are sensors with a sensitive element in which the controlled quantity is converted into an informative parameter; microwave generators - sources of electromagnetic oscillations; secondary converters are designed to generate registration and control signals.
radio wave testing non-destructive flaw detection
2. Sources and receivers of microwave radio wave radiation
Microwave oscillations can be obtained using magnetron-type generators, backward wave tubes, reflective klystrons, quantum mechanical generators and semiconductor devices. Klystrons are most commonly used, followed by magnetrons, backward wave tubes, and semiconductor oscillators.
Reflective klystrons are widely used as master oscillators in radar stations, in amplification chains of low-power transmitters, in radio relay communication lines, low-power microwave generators of continuous or pulsed radiation in short-range transmitting devices (radio range finders, radio beacons, transponders), as well as low-power generators in measuring and small-sized equipment due to a number of advantages over other low-power microwave generators. This is, in particular, low level fluctuation noise, ease of operation and high reliability when operating conditions vary widely. The low-power reflective klystrons produced (up to 100 mW) cover a wide range of wavelengths, down to submillimeter wavelengths. Some types of klystrons require forced air cooling, especially those designed to operate in the short-wave part of the millimeter range, when it is fundamentally difficult to increase their efficiency. Unfortunately, thermal frequency drifts prevail over all others and are inherent in any type of microwave generator.
Magnetron generators cover a wide range of frequencies and provide a wide range of pulse powers: from a few watts to tens of megawatts. They are widely used in electronic equipment as master oscillators, microwave power sources, etc. However, in Lately There is a plan to abandon their widespread use due to the high instability of the generated frequency and thermal frequency drift. In addition, the presence of permanent magnets increases the mass of magnetrons; high voltage and intensive cooling (by blowing) of the resonator are required for power supply.
Backward wave lamps (BWVs) belong to the class of wide-range microwave oscillation generators with electronic frequency tuning. Available big number types of VOCs covering the wave range from 60 cm to tenths of a millimeter. To focus the electron beam in a BWO, permanent tubular magnets are mainly used. Such VOCs are produced in the form of a packaged design, which combines the body of the VOC, a permanent magnet and an adjusting device. Therefore, the normal operation of the BWO may be disrupted in the presence of external magnetic fields or ferromagnetic materials located near the BWO. As a rule, the distance between VOCs and similar materials should be at least 400 mm. The operating mode of the VOC strongly depends on external conditions (temperature, humidity), as well as coordination with the load.
Back wave lamps are especially critical to changes in ambient temperature. When the lamps are exposed to a back wave of mechanical shocks and vibrations, periodic changes in the distance between the individual electrodes of the electron gun or their transverse displacements relative to each other occur, which is accompanied by amplitude and frequency modulation of the generated oscillations. The deviation of the BWO frequency during vibration is usually somewhat greater than that of klystrons. The disadvantages of lamps of this type also include the fact that these lamps, which were in storage and long time(more than two months) that do not turn on must be subjected to training, which takes at least 1.5 hours. Generators based on VWOs, like all microwave generators with a wide range of electronic frequency tuning, do not have high frequency stability when operating in any range point.
An effective self-oscillator of centimeter and millimeter waves can be created on the semiconductor equivalent of a reflective klystron - an avalanche-transit diode (ALD), which serves as the basis for a number of microwave devices (generators, amplifiers, frequency converters).
The operation of the LPD is based on the effect of generating coherent oscillations during avalanche breakdown of microwave semiconductor diodes. The resulting oscillation power in continuous mode ranges from tens of microwatts to several milliwatts for various diodes at a wavelength of 0.8-10 cm. The generator consists of an avalanche diode and a hollow resonator associated with the payload. Feature LPD - increased noise level at high (>10 4 GHz) frequencies. Even in germanium diffusion LPDs with a uniform breakdown, this level is 25-30 dB higher than the shot noise of a vacuum diode with the same current. In silicon LPDs, where breakdown is accompanied by microplasma phenomena, the noise level can exceed shot noise by 60-70 dB.
Small-sized generators in the centimeter range (3-15 GHz) provide continuous output power from 5 to 50 mW at a supply current of 10-20 mA and a voltage of 20-70 V with an efficiency of 3-7%. The significant level of higher harmonics in the spectrum of the avalanche current allows the use of centimeter-wave LPDs to create millimeter-wave generators. It is advisable to make the resonator of such a generator two- or three-circuit, so that one of the circuits, not associated with the payload, is tuned to the fundamental frequency in the short-wave part of the centimeter range (10-15 GHz), and the rest - to higher harmonics. Generators of this type have an output power (in continuous mode) of the order of several milliwatts in the upper part of the millimeter range. However, the spectral density of fluctuations in the amplitude and frequency of the LPD is 15-20 dB higher than that of reflective klystrons. So, LPD-based microwave devices have such advantages as small dimensions, weight, power efficiency, etc. Their main disadvantage is high level noise
Created and also received practical use semiconductor microwave generators based on Gunn diodes. They operate at low supply voltages (4-8.5 V), while consuming current from 0.4 to 1.5 A.
Comparative characteristics of some types of microwave generators
Literature
1. Non-destructive testing. Volume 6. Handbook. Under general ed. V.V. Klyueva, Moscow, 2006
2. Milman I.I. “Radio wave, thermal and optical control”, part 1, tutorial. manual, Ekaterinburg, 2001.
3. Ermolov I.N., Ostanin Yu.A. “Methods and means of non-destructive testing”, 1988, Higher. school.
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Ministry of Education of the Republic of Belarus
Belorussian State University computer science and
radio electronics
Department of RES
“Radio wave and radiation methods for monitoring RESI. Electron microscopy methods"
MINSK, 2008
Radio wave method
Radio wave methods are based on the interaction of an electromagnetic field in the wavelength range from 1 to 100 mm with the test object, conversion of field parameters into electrical signal parameters and transmission to a recording device or information processing means.
Based on the primary informative parameter, the following microwave methods are distinguished: amplitude, phase, amplitude-phase, geometric, time, spectral, polarization, holographic. The scope of application of microwave methods of radio wave non-destructive testing is given in Table 1 and in GOST 23480-79.
Radio wave methods of non-destructive testing
| Method name | Application area | Factors limiting the scope | Controlled parameters | Sensitivity | Error | ||||||||
| Amplitude tight | Thickness gauging of semi-finished products and products made from radiotransparent materials | Complex configuration. Changing the gap between the transducer antenna and the control surface. | Thickness up to 100 mm | 1 – 3 mm | 5% | ||||||||
| Flaw detection of semi-finished products, products and structures made of dielectric | Defects: cracks, delaminations, underpressure | Cracks more than 0.1 - 1 mm | |||||||||||
| Phase | Thickness measurement of sheet materials and semi-finished products, layered products and dielectric structures. | Waviness of the profile or surface of the test object at a step of less than 10L. Detuning from the influence of signal amplitude | Thickness up to 0.5 mm | 5 – 3 mm | 1% | ||||||||
| Control of “electrical” (phase) thickness | Thickness up to 0.5 mm | 0.1 mm | |||||||||||
| Amp-tud-phase | Thickness gauging of materials, semi-finished products, products and structures made of dielectrics, control of thickness changes. | Ambiguity in the reading when the thickness changes by more than 0.5 A, E Change in the dielectric properties of the material of the test objects by more than 2%. Thickness more than 50 mm. | Thickness 0 – | 0.05 mm | ±0.1 mm | ||||||||
| Amp-tud-phase | Flaw detection of layered materials and dielectric and semiconductor products up to 50 mm thick | Changing the gap between the transducer antenna and the surface of the test object. | Delaminations, inclusions, cracks, changes in density, uneven distribution of constituent components | Inclusions of the order of 0.05 A, E. Cracks with an opening of about 0.05 mm. Density difference of about 0.05 g/cm3 | |||||||||
| Geometric | Thickness gauging of products and structures made of dielectrics: control of absolute thickness values, residual thickness | Complex configuration of control objects; non-parallel surfaces. Thickness more than 500 mm | Thickness 0 -500 mm | 1.0 mm | |||||||||
| Flaw detection of semi-finished products and products: control of cavities, delaminations, foreign inclusions in products made of dielectric materials | Complex configuration of control objects | 1.0 mm | 1 –3% | ||||||||||
| Time- | Thickness measurement of structures and media that are dielectrics | Presence of a “dead” zone. Nanosecond technology. At- | Thickness more than 500 mm | 5-10 mm | 5% | ||||||||
| Noah | Flaw detection of dielectric media | Change of generators with a power of more than 100 mW | Determination of defect depth up to 500 mm | 5 - 10 mm | 5% | ||||||||
| Spectral | Flaw detection of semi-finished products and products made from radiotransparent materials | Generator frequency stability is more than 10 -6. Presence of a magnetic field source. The difficulty of creating a sensitive path in the frequency tuning range of more than 10% | Changes in the structure and physicochemical properties of materials of control objects, inclusions | Microdefects and microinhomogeneities are significantly smaller than the operating wavelength. | - | ||||||||
| 1 | 2 | 3 | 4 | 5 | 6 | ||||||||
| Polarizing | Flaw detection of semi-finished products, products and structures made of dielectric materials. | Complex configuration. Thickness more than 100 mm. | Structural and technological defects causing anisotropy in the properties of materials (anisotropy, mechanical and thermal stresses, technological disturbances in the order of the structure) | Defects with an area of more than 0.5 - 1.0 cm2. | |||||||||
| Holographic | Flaw detection of semi-finished products, products and structures made of dielectric and semiconductor materials with the creation of a visible (volumetric) image | Generator frequency stability is more than 10 -6. The difficulty of creating a reference beam or field with uniform amplitude-phase characteristics. Complexity and high cost of equipment. | Inclusions, delaminations, variations in thickness. Changes in the shape of objects. | Cracks with an opening of 0.05 mm | |||||||||
Note: λ – wavelength in the controlled object; L – size of the antenna opening in the direction of waviness.
A necessary condition for the use of microwave methods is compliance with the following requirements:
The ratio of the smallest size (except thickness) of the controlled object to the largest size of the transducer antenna aperture must be at least unity;
The smallest size of minimally detectable defects must be at least three times the value of the surface roughness of the controlled objects;
The resonant frequencies of the spectrum of reflected (scattered) radiation or the magnetic field strengths of the materials of the object and the defect must differ, determined by the choice of specific types of recording devices.
Options for the location of the transducer antennas in relation to the test object are given in Table 1.
Methods of this type of control make it possible to determine the thickness and detect internal and surface defects in products mainly made of non-metallic materials. Radio wave flaw detection makes it possible to measure the thickness of dielectric coatings on a metal substrate with high accuracy and productivity. In this case, the amplitude of the probing signal is the main information parameter. The amplitude of radiation passing through a material decreases due to many reasons, including the presence of defects. In addition, the wavelength and its phase change.
There are three groups of radio wave flaw detection methods: transmission, reflection and scattering.
The equipment of the radio wave method usually contains a generator operating in continuous or pulsed mode, horn antennas designed to input energy into the product and receive transmitted or reflected waves, an amplifier of received signals and devices for generating command signals that control various types of mechanisms.
When testing foil dielectrics, the surface of the sample being tested is scanned with a directed beam of microwaves with a wavelength of 2 mm.
Depending on the information used parameter of microwaves, flaw detectors are divided into phase, amplitude-phase, geometric, and polarization.
The change relative to the wave amplitude is measured on a reference product. Amplitude flaw detectors are the simplest from the point of view of setup and operation, but they are used only for detecting sufficiently large defects that significantly affect the level of the received signal.
Amplitude-phase flaw detectors make it possible to detect defects that change both the amplitude of the wave and its phase. Such flaw detectors are capable of providing fairly complete information, for example, about the quality of foil dielectric blanks intended for the manufacture of individual layers of multilayer printed circuit boards.
Polarization flaw detectors record changes in the polarization plane of a wave during its interaction with various inhomogeneities. These flaw detectors can be used to detect hidden defects in various materials themselves, for example, to study dielectric anisotropy and internal stresses in dielectric materials.
Radiation methods
Radiation methods of non-destructive testing mean a type of non-destructive testing based on the registration and analysis of penetrating ionizing radiation after interaction with the controlled object. Radiation methods are based on obtaining flaw detection information about an object using ionizing radiation, the passage of which through a substance is accompanied by the ionization of atoms and molecules of the medium. The inspection results are determined by the nature and properties of the ionizing radiation used, the physical and chemical characteristics of the products being inspected, the type and properties of the detector (recorder), the inspection technology and the qualifications of flaw detectors.
Radiation methods of non-destructive testing are designed to detect microscopic discontinuities in the material of controlled objects that arise during their manufacture (cracks, ovals, inclusions, cavities, etc.)
The classification of radiation MNCs is presented in Fig. 1.
Electron microscopy (EM) methods
Electron microscopy is based on the interaction of electrons with energies of 0.5 - 50 keV with matter, while they undergo elastic and inelastic collisions.
Let's consider the main ways of using electrons in monitoring thin-film structures (see Fig. 2)
Table 1 -
Layout of transducer antennas in relation to the test object.
| Converter antenna layout | Possible control method | Note |
| 1 | 2 | 3 |
| Amplitude, spectral, polarization | - | |
|
| Phase, amplitude-phase, time, spectral | - |
|
| Amplitude, geometric, spectral, polarization | - |
|
| Phase, amplitude-phase, geometric, temporal, spectral | - |
|
| Amplitude, spectral, polarization. | - |
|
| Amplitude, polarization, holographic. | A monoelement antenna is used as a receiving antenna. |
|
| Amplitude, holographic. | A multi-element antenna is used as a receiving antenna. |
|
| Amplitude, amplitude-phase, time, polarization | - |
|
| Amplitude, phase, amplitude-phase, spectral. | Transmitting (emitting) and receiving functions Multiple antennas are combined in one antenna. |
Designations: - transducer antenna;
Load. 
1 – microwave generator; 2 – object of control; 3 – microwave receiver; 4 – lens for creating a (quasi) flat wave front; 5 – lens for forming a radio image; 6 – reference (reference) arm of bridge circuits.
Note: it is allowed to use combinations of transducer antenna layouts in relation to the test object.
Scanning electron microscopy (SEM). A focused beam of electrons 1 (Fig. 2) with a diameter of 2-10 nm, using a deflecting system 2, moves along the surface of the sample (either dielectric film Z1 or semiconductor Z-11.) Synchronously with this beam, the electron beam moves along the screen of the cathode ray tube . The intensity of the electron beam is modeled by the signal coming from the sample. Line and frame scanning of the electron beam allows one to observe a certain area of the sample under study on the CRT screen. Secondary and reflective electrons can be used as a modulating signal.
Figure 1 – Classification of radiation methods
Figure 2 – Operating modes of scanning electron microscopy
a) contrast in transmitted electrons; b) contrast in secondary and reflected electrons; c) contrast in the induced current (Z11 - conditionally taken outside the device). 1 – focused beam; 2 – deflection system; 3 – object of study - dielectric film; 4 - detector of secondary and reflected electrons; 5 - amplifier; 6 - scan generator; 7 - CRT; 8 - detector grid; 9 - reflected electrons; 10 - secondary electrons.
Transmission electron microscopy (TEM) is based on the absorption and diffraction of electrons interacting with atoms of a substance. In this case, the signal transmitted through the film is removed from a resistor connected in series with sample Z1. To obtain an image on the screen, powerful lenses are placed behind the sample. The sides of the sample must be plane-parallel and clean. The thickness of the sample should be much less than the electron mean free path and should be 10.. 100 nm.
TEM makes it possible to determine: the shapes and sizes of dislocations, the thickness of samples and the profile of films. Currently, there are PE microscopes up to 3 MeV.
Scanning electron microscopy (SEM).
The image is formed both due to secondary electrons and due to reflected electrons (Fig. 2). Secondary electrons make it possible to determine chemical composition sample, and the reflected ones – the morphology of its surface. When a negative potential of -50 V is applied, low-energy secondary electrons are blocked and the image on the screen becomes contrasting, since the faces located under negative angle to the detector, are not visible at all. If a positive potential (+250 V) is applied to the detector grid, secondary electrons are collected from the surface of the entire sample, which softens the image contrast. The method allows you to obtain information about:
Topology of the surface under study;
Geometric relief;
The structure of the surface under study;
Secondary emission factor;
About changes in conductivity;
About the location and height of potential barriers;
On the distribution of potential over the surface and in the surface (due to the charge on the surface when irradiated with electrons) when a scanning beam hits the surface of semiconductor devices, currents and voltages are induced in it, which change the trajectories of secondary electrons. IC elements with a positive potential appear dark compared to areas with a lower potential. This is due to the presence of retarding fields above areas of the sample with a positive potential, which lead to a decrease in the signal of secondary electrons. Potential contrast measurements provide only qualitative results due to the fact that the retarding fields depend not only on the geometry and voltage of the spot, but also on the voltage distribution over the entire surface of the sample;
Large spread of velocities of secondary electrons;
The potential contrast is superimposed on the topographic contrast and the contrast associated with the heterogeneity of the composition of the sample material.
Induced mode (induced electron beam current).
An electron beam with high energy is focused on a small area of the microcircuit and penetrates several layers of its structure, as a result of which electron-hole pairs are generated in the semiconductor. The connection diagram for the sample is shown in (Fig. 2, c). At appropriate external voltages applied to the IC, currents caused by newly born charge carriers are measured. This method allows you to:
Determine the perimeter р-n junction. The shape of the perimeter affects breakdown voltages and leakage currents. The primary electron beam (2) (Fig. 3 and 4) moves along the surface of the sample (1) in the x directions, and depending on the direction of movement, the value of the induced current in the pn junction changes. From photographs of the p-n junction you can determine distortions perimeter р-n transition (Fig. 5).
Determine local locations breakdown p-n transition. When a local breakdown of a p-n junction occurs at the breakdown site, an avalanche multiplication of current carriers is formed (Fig. 6). If the primary beam of electrons (1) falls into this region (3), then the electron-hole pairs generated by the primary electrons are also multiplied in the p-n transition, as a result of which an increase in the signal will be recorded at this point and, accordingly, the appearance of a light spot in the image. By changing the reverse bias at the pn junction, you can identify the moment of breakdown formation, and by identifying structural defects, for example, using selective etching or TEM, you can compare the breakdown region with a particular defect.
Figure 3 – Electron beam transmission diagram
Figure 4 – Image of an end pn junction with a target
determining its perimeter
1 – end p-n junction; 2 – electron beam;
3 – region of generation of electron-hole pairs.
Figure 4 – Image of a planar pn junction with a target
determining its perimeter
1 - planar p-n junction; 2 - electron beam;
3 - region of generation of electron-hole pairs.
Figure 5 – Distortion of the perimeter of a planar p-n junction from above
Observe defects. If in area р-n transition there is a defect (4) (Fig. 6), then when the primary beam of electrons hits the defect region, some of the generated pairs recombine on the defect, and accordingly until p-n boundaries Fewer carriers will reach the junction, which will reduce the current in the external circuit. On photos р-n transition, this area will appear darker than the rest of the background. By changing the relationship between depth bedding р-n transition and the penetration of primary electrons, it is possible to probe the electrical activity of defects located at different depths. Observation of defects can be carried out using reverse and direct displacements р-n transition.
Auger electron spectroscopy (EOS).
It consists of obtaining and analyzing the spectrum of electrons emitted by surface atoms when exposed to an electron beam. Such spectra carry information:
On the chemical (elemental) composition and state of atoms of surface layers;
On the crystal structure of matter;
On the distribution of impurities over the surface and diffusion layers; The installation for Auger spectroscopy consists of an electron gun, an Auger electron energy analyzer, recording equipment and a vacuum system.
Figure 6 – Image of a planar p-n junction for the purpose of determining breakdown and identifying a defect.
1 – electron beam; 2 – planar pn junction; 3 – metal impurity; 4 – defect.
The electron gun focuses the electric beam on the sample and scans it. The beam diameter in installations with local Auger analysis is 0.07... 1 µm. The energy of primary electrons varies within the range of 0.5... 30 keV. In Auger spectroscopy installations, a cylindrical mirror type analyzer is usually used as an energy analyzer.
The recording device, using a two-coordinate recorder, records the dependence , where: N is the number of electrons hitting the collector;
E k – kinetic energy of Auger electrons.
The vacuum system of the EOS installation must provide a pressure of no more than 10 7 – 10 8 Pa. In worse vacuums, residual gases interact with the sample surface and distort the analysis.
Among the domestic EOS installations, the raster Auger spectrometer 09 IOS - 10 - 005 should be noted. Its Auger locality in the raster mode is 10 μm.
(Fig. 7) shows the Auger spectrum of a contaminated GaAs surface, from which it is clear that, along with the main spectra of GaAs, impurity atoms S, O and C are present in the film. By recording the energies of Auger electrons emitted by atoms during their excitation and comparing these the tabulated values determine the chemical nature of the atoms from which these electrons were emitted.
Figure 7 – Auger spectrum of a contaminated GaAs surface
Note: the method got its name from the French physicist Pierre Auger, who in 1925 discovered the effect of the emission of electrons by atoms of a substance as a result of excitation of their internal level by X-ray quanta. These electrons are called Auger electrons.
Emission electron microscopy (EEM).
At special conditions the surface of the sample can emit electrons, i.e. be a cathode: when applying a strong electric field to the surface (field emission) or under the influence of bombardment of the surface with particles.
In the emission microscope shown in Fig. 8, the surface of the sample is an electrode of a system that forms an electron lens with the anode.
The use of EEM is possible for materials that have a low work function. The product under study is, as it were, integral part electron-optical system EEM, and this is its fundamental difference from SEM.
EEM is used to visualize microfields. If the pn junction (1) (Fig. 9) is placed in a uniform electric field (2) and a blocking voltage is applied to it, then the field created by the pn junction (3) (at high leakage currents) will bend main field lines.
The curvature of the lines makes it possible to determine the potential distribution over the surface of the sample.
Electron reflectance spectroscopy (ERS).
In EOS, the surface of the observed sample is maintained at such a potential that all or most of the irradiating electrons do not reach the surface of the sample.
The principle of its operation is shown in Fig. 10. The collimated electron beam is directed at the surface of the sample perpendicular to it. Electrons,
Figure 8 – Operating principle of an emission microscope
Figure 9 – Visualization of the p-n junction using EEM
P-n junction connected in reverse direction; - electronic
trajectories of the pn junction field.
Lenses flying through the last aperture quickly slow down and turn back at a point determined by the potential of the sample surface relative to the cathode and the electric field strength on the sample surface. After turning, the electrons are accelerated again, flying back through the lenses, and the enlarged image is projected onto a cathodoluminescent screen. Additional magnification can be obtained by separating the outgoing beam from the incoming beam in a weak magnetic field and using additional magnifying lenses in the path of the outgoing beam.
The contrast in the output beam is determined by the topology of the surface and changes in the electric potential and magnetic fields on it.
Sample voltage
Figure 10 – Operating principle of an electron reflectance microscope
LITERATURE
1. Gludkin O.P. Methods and devices for testing RES and EVS. – M.: Higher. school., 2001 – 335 p.
2. Testing of radio-electronic, electronic computer equipment and testing equipment / ed. A.I.Korobova M.: Radio and communications, 2002 - 272 p.
3. Mlitsky V.D., Beglaria V.Kh., Dubitsky L.G. Testing of equipment and measuring instruments for exposure to external factors. M.: Mechanical Engineering, 2003 – 567 pp.
4. National system certification of the Republic of Belarus. Mn.: Gosstandart, 2007
5. Fedorov V., Sergeev N., Kondrashin A. Control and testing in the design and production of radio-electronic equipment - Technosphere, 2005. - 504 p.
