by Viktor BYKOV, Cand. Sc. (Phys. & Math.), Director General, NT-MDT Company Ltd.
The origin of the microscope dates back to the end of the sixteenth century when Dutch and Italian glass grinders thought of combining two lenses to examine smallish articles. In 1610 the famous Galileo Galilei used a spyglass he had improved for this purpose; and a few decades after, in 1673, the Dutch natural scientist and microscopist Antony van Leeuwenhoek was the first to see protists-bacteria and erythrocytes-under the microscope. Since then the microscope has been improved all along-in its design, performance and resolution.
Using a regular optical microscope, we can inspect objects down to 0.25 um in size, while its electronic counterpart allows us to make out details equal to 0.1 nanometers (nm), with nanometer being a billionth part of a meter. Hence a new trend in science- nanotechnology, which caters to a range of disciplines from molecular technology and gene engineering to solid-state physics, electrochemistry and microelectronics. Since here one deals with magnitudes on the scale of molecules and atoms, microscopes with much higher resolving power become necessary. Orthodox models are not satisfactory to this end.
In 1981 two Swiss scientists, T. Bining and G. Rohrer, designed the world's first scanning tunnel microscope, an achievement that won them a Nobel prize in 1986. With it we can observe atoms singly, and in assigned points at that. The main sounding element, or probe, of this microscope is an electric conductor stylus (needle) made of tungsten or platinum alloys.
Here's how this microscope works. Fixed voltage is applied to the needle that scans the surface of an object and to the object itself; after the needle and the object have approached each other to a distance of decimal fractions of an angstrom (A), a tunnel current starts flowing between them-hence the name of the microscope, a tunnel microscope. This current is sustained at a constant value with the aid of a servo system which either lifts or lowers the scanner depending on the relief of the surface. A computer keeps a tab on these movements and processes the data thus obtained; thereupon one can inspect the object at required resolution.
Yet such tunnel microscopes have certain constraints on their employment. By and large, they are used in high (fine) vacuum. Otherwise, say, in the air or in water only particular varieties of graphite and some lamellar semiconductors can be scanned at atomic resolution. The main constraint: the examined surface should be an electric current conductor.
In 1986 a second generation of sounding microscopes-atom-power ones-entered the stage. Their scanning device is similar to a gramophone in design. A pointed small needle, say, from a broken sapphire monocrystal is attached to one end of a flat taut plate made of thin platinum foil, the cantilever, while the other end is fixed in the holder. In the process of scanning, this needle, while rerunning the relief features of the examined
surface, causes the cantilever to oscillate vertically, and also turn around the longitudinal axis. The different positions of the sound (probe) are measured in a variety of ways-with interferometers, strain gauges and so on. But today the optical scheme of registration is more common.
Here two techniques are best. If a specimen is small (its linear dimensions within 40-50 mm and thickness up to 10 mm), it is mounted on a piezoelectric three-axis scanner, with the cantilever immobilized in a fixed holder. A laser beam is directed at the free end of the probe. Reflected, it falls on a sensitive element, a photodiode composed of four sections which makes it easier to register both the inclination of the cantilever (normal transition) and its axial bends, that is the lateral forces.
If a specimen is large, it should be immobilized. But in this case an extra servo system should be mounted to ensure interaction of the laser, cantilever and photodiode.
Atom-power microscopy is employed in two modes, contact and noncontact. In the contact mode the needle is always in touch with the examined surface, and so the microscope ought to form a good image to satisfy even most rigid standards. And yet there are certain snags. Moisture, for one: it oozes into the gap between the needle's tip and the object. In fact, moisture is always present on the object's surface if scanning is performed in the air. As a result, the so-called capillary effect mars the resolving power. This effect is absent in three cases: if hydrophobic needles are used on non-moist surfaces; in fine vacuum; and in the solid body-liquid interface.
Yet another consequence of the capillary effect: the cantilever's needle should be pressed tight to the object with much force (the pressure may be as high as 30 atm with the needle's curvature radius equal to 20 nm), and this can damage the surface and even destroy it.
All these drawbacks are absent in the noncontact method, though it is not perfect either. Say, the needle and the object come to attract each other if the distance between them is 10 A. Such mutual attraction interferes with the scanning procedure on account of the frequent sticking of the cantilever.
And so yet another, third method gained recognition in 1993. This is a resonance, semicontact technique otherwise known as tapping. In it a vibrating cantilever is used (not immobile as previously); an external piezogenerator excites vibrations. As the cantilever approaches the surface of an investigated object, the pattern of vibrations changes, with the amplitude depending on the relief features, and the phase sensitive to the physical characteristics of the surface (elasticity, viscosity and the like).
Worldwide only a few companies are turning out related technology, with our company, NT-MDT Ltd. (founded in 1991), as one of the leading producers. It is located on the grounds of the Lukin R&D Center (involved with physics and its problems) at Zelenograd, a satellite town near Moscow. This is not a random location: it was here, at Zelenograd, that a new research discipline, molecular electronics, had its baptism (*). NT-MDT products, and nanotechnology at a later date, found a good market there. We are manufacturing scanning probe microscopes that have no peer in the world as well as accessories to them, and lots of other things.
The scanning microscope unit has a scanner, a measuring head and a cantilever as essential components. A computer, too, is important: it processes the data and flashes the results on the display.
Depending on what kind of operation is carried out in particular and on the size of an object, the scanner either moves this object at
* See also: "Focus on Zelenograd" in the present issue of our magazine.-Ed.
a desired pace or controls the cantilever's movements. The latest models are equipped with a pitch engine to move the object under the microscope back and forth. This is a high- precision manipulation taking account of decimal fractions of a micron. It thus becomes possible to scrutinize one and the same site of the surface for days on end, which is a necessary procedure when dealing with sluggish processes. The measuring heads allow to vary the operational modes and obtain high-resolution images (even at atomic resolution); besides, we can measure more than 20 different characteristics of examined samples and modify their surface (in what we term the lithography modes).
Yet it is the cantilever needles that are the most essential part of a modern high- performance scanning microscope. They had their second birth in 1990 when methods of silicon micromechanics were suggested for their production. That was a modified classical procedure of microelectronic technology with the use of doping, oxide layer formation and photolythographic processes. Selective etching is of particular importance for the making of cantilever needles, for it becomes possible to manufacture actually identical needles to a tolerance of several units on the nanometer scale. Such needles are fastened on beams which, in their turn, are made to preassigned thickness either by doping silicon with boron or phosphorus to required depth or by sputtering adequate film structures.
The basic parameters of the cantilever and its application domain are these: stiffness and resonance characteristics; the radius of the curvature of the needle, its form and type of coating (magneto-sensitive and current-conducting layers, dielectric characteristics and hardness). Besides, the needle angle is very important for studying the surface topology: an object under study may have minute relief features (say, narrow and deep "holes") to baffle needles with a fantastically small radius of the tip, not above 1.5-2 nm (the usual radius is 5 to 15 nm): the needles will just pinpoint the indentations but will not determine their depth. This is a skip, or dead zone. To minimize it superfine hairs, the so-called whiskers, are built up on the needle's tip.
For this purpose we at our company use a strongly focused electronic beam in a vacuum unit to make the whisker material similar in its hydrophobic characteristics to amorphous carbon. The tiny hairs are 50 to 100 nm thick, the radius of the curvature of their tips is 2 to 3 nm, while their length can be preassigned to 3 um (accurate up to 10 nm). Depending on the mode of growth, whiskers may be in the shape of a cone, a "sharpened pencil" or a "multitier tower". Since this or that form is preassigned, it is taken into account in the interpretation of measurement results.
So: scanning cantilever microscopes give an insight into many characteristics of materials. However, the end result directly depends on a modification of needles. For instance, those with a current-conducting surface are used for measuring the relative distribution of surface resistance and capacity as well as the electric characteristics of subsurface structures. Conductor probes supplied with dielectric coating are employed for determining the distribution of subsurface magnetic fields and capacity. Needles coated with high- strength materials (boron nitride, diamond-like coats, etc.) are good for determining the surface hardness. And probes with a chemically modified structure identify and interpret the distributions of adhesive forces; using such probes, we learn to what extent the surface of an object is homogeneous.
The above examples do not cover all the possibilities of probing (sounding) microscopy. But they
are enough to show that many cantilever modifications are needed to get to know all the various characteristics of objects. It takes time to replace cantilevers and find an appropriate one among many; indeed, it is hardly possible to choose the right cantilever and fix it at the right time and place. That is why we are designing multiprobe cartridges: each cartridge is supplied with dozens of needles with different coatings and different characteristics. Today our company is turning out third-generation microscopes (of the Solver series). Since cantilever microscopes are in demand on the market (they are needed for research in narrow fields), we can manufacture mono-functional apparatuses from base models, and numerous complete sets besides.
New-generation microscopes possess superhigh resolution enabling them to scan not only atomic lattices but individual atoms as well. Furthermore, they are capable of modifying various surfaces and changing their structure on the nanometer scale. A subtle, miniature piece of work! Say, the portraits and biographies of all Russians (150 million) drawn this way could be fitted into a slate only 3x3 cm large.
Our firm, as we have said, is one of the world's leading designers of sounding microscopy. We have obtained something like 25 patents and authorship certificates for new microscopes and fixings (scanners, measuring heads, cantilevers) as well as for our research methods.
Among our customers are about 30 research centers and organizations Russia-wide. Our company is getting great assistance from the Ministry of Science and Technology of the Russian Federation. On one occasion, it has helped commission an atom-power microscope at the State Research Center of Virology and Biotechnology in the town of Koltsovo, Novosibirsk Region, Western Siberia (this center has the word Vector on its logo). There are plans to set up a regional center of sounding microscopy to cater to a range of customers.
NT-MDT products are purchased by many countries, including the United States, Canada, Germany, Japan and China. Small wonder: such high-power microscopes are a must in submicron electronics, microbiology, in polymer production (quality inspection and identification of materials obtained) for the optical industry. The application range of these unique apparatuses keeps expanding, and they are quite indispensable in many areas. For instance, in testing the quality of eye lenses, which is a rather sophisticated procedure: being transparent, such lenses should be placed into a water solution for observation. The only nondestructive method available today is through sounding microscopy, for it allows to keep the lens surface intact. The manufacture of digital video disks is yet another nonalternative application domain of such scanning microscopy. Today these disks are made by die-stamping. And since the dies used in such stamping are of magnetic material, nickel in particular, no other methods but sounding microscopy are good for checking their surface.
Thus the new generation of scanning microscopes supplied with probes (cantilevers) has a good future in physical and metrological research alike.
Prepared by Arkady MALTSEV
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