by Victor BYKOV, Cand. Sc. (Phys. & Math.), Director General, NT-MDT Company Ltd.
Using a regular optical microscope, we can inspect objects down to 0.25(mum 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.
Here's how this microscope works. Fixed voltage is applied to the current-carrying needle made of tungsten or platinum alloys 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. 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.
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).
Warldwide only a few companies are turning out related technology with our company, NT-MDT Ltd. (founded in 1991), as one of the leading producers. We are manufacturing scanning probe microscopes that have no peer in the world as well 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 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 modem 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 theirtum, 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 mum (accurate up to 10 0 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 leam to what extent the surface of an object is homogeneous.
The above examples 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 monofiinctional 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 drawn this way could be fitted into a slate only 3x3 cm large.
Among our customers are about 30 research centers and organizations Russia-wide. On one occasion, our company 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).
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 or 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 nonaltemative 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.
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