XI Международная студенческая научная конференция Студенческий научный форум - 2019

The fabrication of probe sensors for AFM is a rather complicated technological process, which includes the operations of photolithography, ion implantation, chemical and plasma etching. The main stages of one of the possible manufacturing techniques for probe sensors are shown in Fig13.

Fig. 13. The main stages of the process of manufacturing probe sensors

For the manufacture of probe sensors are used plates of crystalline silicon orientation (110). A thin layer of photoresist is deposited on the surface of the plate (Fig. 13, stage 2). Then the photoresist is exposed through the photomask, and part of the photoresist is removed by chemical etching. Next, the implantation of boron ions is carried out, so that the ions penetrate to a depth of the order of 10 μm in the silicon region not protected by the photoresist (step 3). After that, the photoresist is washed off in a special etchant, and then a thermal annealing of the plate is carried out, as a result of which the boron atoms are embedded in the crystal lattice of silicon. Silicon doped with boron forms the so-called stop layer, which stops the etching process for some selective etchants. Then, photolithography is again performed on the back side of the plate, as a result of which a layer of photoresist is formed exactly over the area implanted with boron. After that, the plate is covered with a thin layer of Si3N4 (step 4). Then selective etching of the photoresist is carried out, and in the process of dissolving the photoresist swells and tears the thin film of Si3N4 located directly above it (step 5). A silicon wafer is etched through to the stop layer using a selective etchant, which interacts with silicon and does not interact with doped silicon and a layer of Si3N4 (step 6). After that, Si3N4 is washed off, and islands of photoresist are formed on the back side of the plate in the doped region using photolithography (step 7, 8). Then, silicon is etched, resulting in silicon columns under the photoresist islands (step 9). Next, using plasma etching, silicon needles are formed from the silicon columns (step 10, 11). To improve the reflective properties, the cantilevers on the reverse side (with respect to the tip) are covered with a thin layer of metal (Al, Au) by vacuum deposition. As a result of these technological operations, a whole set of probe sensors on a single silicon wafer is manufactured. For electrical measurements, conductive coatings of various materials (Au, Pt, Cr, W, Mo, Ti, W2C, etc.) are applied to the probe. In magnetic AFM sensors, probes are covered with thin layers of ferromagnetic materials, such as Co, Fe, CoCr, FeCr, CoPt, etc.

Contact atomic force microscopy

Conventionally, methods for obtaining information about the relief and surface properties using AFM can be divided into two large groups - contact quasistatic and non-contact oscillatory. In contact quasistatic techniques, the tip of the probe is in direct contact with the surface, while the forces of attraction and repulsion acting from the side of the sample are balanced by the force of elasticity of the console. During the operation of AFM in such modes, cantilevers with relatively low stiffness coefficients are used, which allows for high sensitivity and avoiding undesirable excessive exposure of the probe to the sample.

In the quasistatic AFM mode, the relief image of the surface under study is formed either with a constant force of the probe's interaction with the surface (force of attraction or repulsion), or with a constant average distance between the base of the probe sensor and the sample surface. When scanning a sample in the FZ = const mode, the feedback system maintains a constant amount of cantilever bending and, therefore, the force of interaction between the probe and the sample (Fig. 14.). In this case, the control voltage in the feedback loop applied to the Z-electrode of the scanner will be proportional to the relief of the sample surface.

When examining samples with small (on the order of units of angstroms) differences in relief heights, the scanning mode is often used with a constant average distance between the base of the probe sensor and the surface (Z = const). In this case, the probe sensor moves at a certain average height Zcp above the sample (Fig. 14), and a bend of the ∆Z console proportional to the force acting on the probe from the surface is recorded at each point. The AFM image in this case characterizes the spatial distribution of the force of interaction of the probe with the surface.

Fig.14. Formation of AFM images with a constant force of interaction of the probe with the sample

F ig. 15. Formation of AFM images at a constant distance between the probe sensor and the sample.

The lack of contact AFM techniques is the direct mechanical interaction of the probe with the surface. This often leads to breakage of the probes and destruction of the sample surface during scanning. In addition, contact methods are practically not suitable for the study of samples with low mechanical rigidity, such as structures based on organic materials and biological objects.

Magnetic resonance spectroscopy

Until recently, the basis of our ideas about the structure of atoms and molecules were studies using optical spectroscopy. In connection with the improvement of spectral methods that advanced the spectroscopic region to ultrahigh (about 103–106 MHz; microradiowave) and high frequencies (about 10–2–102 MHz; radio waves), new sources of information on the structure of matter appeared. When radiation is absorbed and emitted in this frequency range, the same basic process occurs as in other ranges of the electromagnetic spectrum, namely, when switching from one energy level to another, the system · absorbs or emits a quantum of energy.

The energy difference between the levels and the energy of the quanta involved in these processes is about 10–7 eV for the radio frequency range and about 10–4 eV for ultrahigh frequencies.

The existence of nuclear moments was first discovered when studying the hyperfine structure of the electronic spectra of some atoms with the help of high-resolution optical spectrometers.

The hyperfine structure of the atomic spectra brought Pauli in 1924 to the idea that some nuclei have an angular momentum, and, consequently, a magnetic moment that interacts with atomic orbital electrons. Subsequently, this hypothesis was confirmed by spectroscopic measurements, which made it possible to determine the values ​​of angular and magnetic moments for many nuclei.

Under the influence of an external magnetic field, the magnetic moments of the nuclei are oriented in a certain way, and it becomes possible to observe transitions between nuclear energy levels associated with these different orientations: transitions occurring under the action of radiation of a certain frequency. The quantization of the energy levels of the nucleus is a direct consequence of the quantum nature of the angular momentum of the nucleus, which takes 2I + 1 values. The spin quantum number (spin) I can take any value multiple of; The highest of the known values ​​of I (7) has 17671Lu. The measurable largest value of the angular momentum (the greatest value of the projection of the moment on the selected direction) is, where, and h is the Planck constant. The values ​​of I for specific nuclei cannot be predicted, however, it was noted that isotopes, in which both the mass number and atomic number are even, have I = 0, and isotopes with odd mass numbers have half-integer spin values.

Such a situation, when the numbers of protons and neutrons in the nucleus are even and equal (I = 0), can be considered as a state with “full pairing”, similar to the full pairing of electrons in a diamagnetic molecule.

In 1921, Stern and Gerlach using the atomic beam method showed that the measurable values ​​of the magnetic moment of an atom are discrete according to spatial quantization of an atom in a non-uniform magnetic field. In subsequent experiments, passing a beam of hydrogen molecules through a constant magnetic field, we managed to measure a small magnetic moment of the hydrogen nucleus. A further development of the method was that the beam was affected by an additional magnetic field oscillating with the frequency at which the transitions between nuclear energy levels, corresponding to the quantum values ​​of the nuclear magnetic moment, are induced.

If the nuclear spin number is I, then the nucleus has (2I + 1) equally spaced energy levels; in a constant magnetic field with an intensity H, the distance between the highest and the lowest of these levels is 2μH, where μ is the maximum measurable value of the magnetic moment of the nucleus. Hence, the distance between adjacent levels is equal, and the frequency of the oscillating magnetic field, which can cause transitions between these levels, is.

In the experiment with a molecular beam, those molecules whose energy does not change reach the detector. The frequency at which resonant transitions occur between levels, is determined by successive changes (sweep) of the frequency in a certain range. At a certain frequency, there is a sudden decrease in the number of molecules reaching the detector.

The first successful observations of nuclear magnetic resonance (NMR) of this kind were performed with the main magnetic fields of the order of several kiloersted, which corresponds to the frequencies of the oscillating magnetic field in the range 105–108 Hz. Resonant energy exchange can occur not only in molecular beams; it can be observed in all aggregative states of matter.

In 1936, Horner tried to detect the resonance of Li7 nuclei in lithium fluoride and H1 nuclei in aluminum potassium alum. Another unsuccessful attempt was made by Gortner and Bruhr in 1942. It was supposed to record the absorption of high-frequency energy at resonance in these experiments using the calorimetric method and the anomalous dispersion, respectively. The main reason for the failure of these experiments was the selection of unsuitable objects. Only at the end of 1945 by two groups of American physicists under the leadership of F. Bloch and Z.M. Parcella was the first to receive nuclear magnetic resonance signals. Bloch observed resonant absorption on protons in water, and Purcell succeeded in detecting nuclear resonance on protons in paraffin. For this discovery they were awarded the Nobel Prize in 1952.

The NMR method, although it is called the method of nuclear magnetic resonance, has nothing to do with nuclear physics, which, as we know, studies the processes of the transformation of nuclei, i.e. radioactive processes. At the same time, the magnetic energy (and the NMR phenomenon takes place when a sample is placed in a constant magnetic field) does not affect the thermodynamic properties of the substance, since it is many times (or rather, several orders of magnitude) less than the thermal energy typical of normal processes, including biological.

The main advantages of the NMR method:

- high resolution - ten orders of magnitude greater than that of optical spectroscopy;

- the ability to conduct quantitative accounting (counting) of resonating nuclei. This opens up possibilities for the quantitative analysis of a substance;

- NMR spectra depend on the nature of the processes occurring in the test substance. Therefore, these processes can be studied by this method. Moreover, the time scale is available in a very wide range - from many hours to small fractions of a second;

- modern electronic equipment and computers allow to obtain the parameters characterizing the phenomenon in a form convenient for researchers and consumers of NMR. This circumstance is especially important when it comes to the practical use of experimental data.

The main advantage of NMR in comparison with other types of spectroscopy is the possibility of transforming and modifying the nuclear spin Hamiltonian at the will of the experimenter with virtually no restrictions and fitting it to the special requirements of the problem being solved. Due to the large complexity of the picture of not fully resolved lines, many infrared and ultraviolet spectra can not be decoded. However, in NMR, the transformation of the Hamiltonian so that the spectrum can be analyzed in detail, in many cases, allows simplifying complex spectra.

The ease with which the nuclear spin Hamiltonian can be transformed is due to certain reasons. Due to the fact that nuclear interactions are weak, it is possible to introduce strong disturbances sufficient to suppress unwanted interactions. In optical spectroscopy, the corresponding interactions have much more energy and such transformations are virtually impossible.

The modification of the spin Hamiltonian plays an essential role in many applications of one-dimensional NMR spectroscopy. At present, the simplification of spectra or the enhancement of their informativeness with the help of spin decoupling, coherent averaging by multi-pulse sequences, sample rotation, or partial orientation in liquid-crystal solvents has become widespread.

Speaking about the advantages of NMR instruments, it is necessary to proceed from real possibilities in the acquisition and operation of NMR spectrometers. In this regard, the following should be noted.

Conducting experiments on NMR is as follows. The sample is placed in a constant magnetic field, which is created by a permanent magnet or, more often, an electromagnet. In this case, radiofrequency radiation is applied to the sample, usually in the meter range. Resonance is detected by the corresponding radio-electronic devices, processed by them and given out in the form of a spectrogram, which can be sent to an oscillograph or chart recorder, in the form of a series of numbers and tables obtained using a printing device.

The output resonant signal can also be introduced into one or another technological process to control this process or cycle.

Usually, if we are talking about the study in stationary conditions of monomeric compounds on hydrogen nuclei with a molecular weight of several hundred units (and most of these substances in the study), the mass of the sample should be from a few milligrams to a hundred milligrams. The sample is usually dissolved in a particular solvent, with a solution volume of 0.7 1 mm3. When detecting NMR signals from other (in addition to H1) nuclei, the sample mass can reach two grams. If the test substance is a liquid, then, naturally, it is not necessary to prepare a solution in this case - it all depends on the objectives of the experiment.

Using spectrometers operating in pulsed mode, it is possible to detect NMR signals from any arbitrarily small amount of a substance. Of course, in this case, it just takes more time to get reliable enough experimental results.

Many substances are not known to dissolve or dissolve in a limited way. In this case, the NMR signal can be registered from the solid phase. The required sample of the sample under study is up to three grams. It is appropriate to note here that during the experiment the sample is not destroyed and can be subsequently used for other purposes.

The high specificity and efficiency of the NMR method, the absence of a chemical effect on the sample, the possibility of continuous measurement of parameters open up many ways for its application in industry.

The implementation of the NMR method was hindered by the complexity of the equipment and its operation, the high cost of the spectrometers, and the research character of the method itself.

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