Patent Number: 042232244
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a charged-particle beam optical apparatus including a specimen holder which is mounted in at least one support member of the apparatus and vibrates when the support member is subjected to shock. 2. Description of the Prior Art Charged-particle beam optical apparatus of the foregoing type are known in the art. See, for example, U.S. Pat. No. 4,058,731 which relates to an electron microscope of the foregoing type. The cause of such vibrations are, among other things, interfering soil vibrations which are first transmitted to the housing of the charged-particle beam optical apparatus and from there via the support of the specimen holder to the latter itself. At the support of the specimen holder, these interfering soil vibrations have on the average an amplitude between 1 and 10 .mu.m. The vibrations are transmitted to the housing and the support in a most pronounced manner if the frequency of the vibrations is in the neighborhood of the resonance frequency of the housing support, namely 1 to 10 Hz, and resonance peaking is then obtained. The reason a resolution of a few Anstroems is possible in electron microscopes in spite of a vibration amplitude of the specimen holder of several .mu.m is the friction coupling of the important parts of such microscopes to each other which vibrate in phase and with the same amplitude. Thus, for example, part of the specimen holder is mounted, friction-coupled, in its support which in turn can be mounted, friction-coupled, on a specimen stage. The specimen stage in turn rests, friction-coupled, on the upper pole piece of the objective lens of the particle beam apparatus. Elongated parts of the specimen holder which are not friction-coupled to adjacent parts, on the other hand, can be excited by the support vibrations to resonance vibrations which lead to a change of position of these parts of the specimen holder relative to the support of the specimen holder. If, for example, the specimen holder consists of a rod which passes through the wall of the electron microscope and is secured there in a first support and the other end of which, situated in the interior of the apparatus, engages a counter-support which is part of an adjustable specimen stage, then the portion of the rod-shaped specimen holder between the two supports can be excited to flexural vibrations in the two directions perpendicular to the rod axis. Additional vibrations can also be excited in the rod direction since at least one support must have spring action in the rod direction for adjusting the specimen holder in this direction without play. The higher the resolution or the accelerating voltage of the electron microscope, the thicker the required magnetic lenses of the microscope must be. This, however, leads to a longer rod-shaped specimen holder and, thus, to a lowering of its resonance frequency. This resonance frequency thereby approaches the resonance frequency of the microscope column, whereby the vibrations of the latter are again transmitted more strongly to the rod-shaped specimen holder. For maximum resolution (1 A), the position of the specimen must not change more than 0.1 A. Since the amplitude ratio of the interfering vibration of the specimen holder to the forcing support vibration is inversely proportional to the square of the lowest resonance frequency to the excitation frequency, it has already been attempted to make the amplitude of the interfering specimen holder vibration small by keeping the vibration frequency of the column low compared to the resonance frequency of the specimen holder vibration. For this purpose, a low-frequency microscope support, for example, an elaborate air spring system, is known. SUMMARY OF THE INVENTION It is therefore an object of the present invention to overcome the aforementioned disadvantages of heretofore known apparatus and to provide an improved charged-particle beam optical apparatus in which the interfering amplitude of the non-frictionally coupled parts of the specimen holder is limited to a degree permissible for maximum resolution in a simple manner and independently of position. These and other objects of the invention are achieved in a charged-particle beam optical apparatus including a specimen holder which is mounted in at least one support member of the apparatus and vibrates when the support member is subjected to shock. The improvement comprises at least one damped supplemental oscillator means coupled to the specimen holder at approximately the point of maximum vibration amplitude of the specimen holder. Attaching the supplemental oscillator, i.e., vibration damper, of the invention to the specimen holder approximately at the point of maximum vibration amplitude, i.e., vibration antinode, contrary to known low-frequency supports through which it has been attempted to eliminate the undesired vibration of the specimen holder by either selecting a location with extremely low soil vibrations or by not allowing the soil vibrations to be transmitted to the charged-particle beam optical apparatus itself by an elaborate air spring system, eliminates the effect of the interfering soil vibrations in the immediate vicinity of the specimen. The principle of such damped supplemental oscillators is known. The resonance frequency of the supplemental oscillator in this invention is in the vicinity of the resonance frequency of the specimen holder. With optimum damping, a flat amplitude response curve is obtained with respect to the interference frequency without pronounced resonance maxima. The interference vibrations of the specimen holder are best suppressed with a given mass ratio .rho.=m.sub.Z /m.sub.S, where m.sub.Z represents the mass of the supplemental oscillator and m.sub.S the mass of the specimen holder which is free to vibrate, when the damping .phi. of the supplemental oscillator and the frequency ratio .alpha. of the resonance frequency of the supplemental oscillator to the resonance frequency of the free-swinging part of the specimen holder are optimally set. According to "Shock and Vibration Handbook" by Harris and Crede, 2nd Ed., 1976, Section 6, pages 1 to 17, optimum damping is obtained in accordance with the equation .phi.=(.mu./2(1+.mu.)).sup.1/2 and the optimum frequency ratio from the equation .alpha. .sub.opt =1/(1+.mu.). Examples of applications of damped supplemental oscillators are bridges and foundations of buildings. Supplemental oscillators are also known in connection with open-air power lines in which case a rod spring is fastened at one end to the open-air line; at the other end the line has a plate as a swinging mass. The amplitudes of the vibrations of this plate are damped by the air resistance exerted on the plate. It is also known to provide for tubular bus bar lines a tri-axial undamped or also damped supplemental oscillator in the interior of the tubular line. See German Pat. No. 20 56 164. In all known examples, the amplitudes of the interference vibrations to be reduced are in the centimeter, or at least in the millimeter range. The purpose of a vibration damper on the specimen holder of a charged-particle beam optical apparatus, on the other hand, is to limit vibration amplitude to a value of a few tenths of an Angstroem. If the interfering specimen holder vibrations occur only in one direction, then only a uniaxially damped supplemental oscillator is necessary to reduce them. Suitable for this purpose is, for example, a leaf spring which can be considered rigid for all practical purposes in the plane of the spring leaf and is resilient only perpendicular to this plane. If interference vibrations along several axes must be suppressed, or at least if their amplitudes are to be reduced, then a separate uniaxial supplemental oscillator can be provided for each axis. It is more advantageous in such a case, however, to use a single, multi-axial supplemental oscillator. For a rod-shaped unilaterally supported specimen holder, it is advantageous if the supplemental oscillator is bi-axial and comprises a spring rod which is unilaterally fastened to such a specimen holder and is surrounded by energy-consuming material, and the resonance frequencies of which are approximately equal in both axes to the corresponding resonance frequencies of the specimen holder. It is assumed in this case that the rod-shaped specimen holder does not vibrate with an objectionable amplitude in the rod direction. The two possible directions of vibration of this specimen holder are in the plane perpendicular to the axis of the rod. The possible directions of vibration of the supplemental oscillator must also lie in the same plane, which is achieved by disposing the supplemental oscillator parallel to the rod-shaped specimen holder. To the extent possible, the supplemental oscillator should be fastened in the vicinity of the free end of the specimen holder, since in the case of a cantilevered rod, a vibration antinode is located at this point and, thus, a maximum amount of energy is transmitted to the supplemental oscillator. If the rod-shaped specimen holder has a rectangular-shaped cross-section, then different resonance frequencies are obtained due to the different spring stiffness in the two principal directions parallel to the sides of the cross-section. For optimum effectiveness of the supplemental oscillator in the two axes, i.e., in this case, in the two principal directions, it has been found to be most advantageous if the supplemental oscillator also has a rectangular-shaped cross-section with approximately the same sides ratio as the specimen holder. With appropriate length and appropriate choice of material, i.e., a specific modul of elasticity and specific weight, the resonance frequencies of the supplemental oscillator in the two principal directions will again be close to the corresponding resonance frequencies of the specimen holder. However, this also means that, with all other parameters entering into the equation for the resonance frequency being constant, the ratio of the area moments of inertia in the two axes of the supplemental oscillator must be approximately equal to the corresponding ratio of the area moments of inertia of the specimen holder. Material suitable for the supplemental oscillator includes all non-magnetic, resilient materials such as alloy steel, bronze, tungsten or vanadium. The material directly surrounding the supplemental oscillator must consume energy when deformed in order to have a damping effect. This material may comprise, for example, natural or synthetic rubber or fluorine-containing polymers such as, for example, polytetrafluoroethylene. A material which develops little gas in a high vacuum is preferred. Besides the mentioned and similar materials, it is also possible to apply to the supplemental oscillator a thick layer of varnish or lead. If the vibrating part of the specimen holder has a circular cross-section, it is then advantageous if the rod forming the supplemental oscillator also has circular cross-section and a tube of elastomer material is shrunk onto the rod. With a correspondingly small mass ratio .mu. of the mass of the supplemental oscillator m.sub.Z and the vibrating mass m.sub.S of the specimen holder, the supplemental oscillator can consist in the simplest case of a spring wire. This wire may, for example, be soldered at one end to a holder which is screwed to the rod. In one embodiment of the invention, a weight is movably disposed on the rod. This weight may comprise, for example, nuts which are screwed with a tight fit on a section of the rod provided with a thread. By means of this additional movable weight, the resonance frequency of the supplemental oscillator can be varied and can therefore be adapted more easily to the optimal conditions for reducing the amplitude of the interference vibrations. If the interfering vibrations of the specimen holder are to be reduced in all three spatial directions, an advantageous further embodiment of the invention is obtained if the supplemental oscillator is tri-axial and consists of a rigid mass which is located in the center of a mass which is resilient in all three axes and consists of energy-consuming material, and if the resonance frequency of the supplemental oscillator in the individual axes is approximately equal to the corresponding resonance frequencies of the specimen holder. Again, a single supplemental oscillator is sufficient. This tri-axial supplemental oscillator can be attached to the specimen holder in a particularly simple manner if the latter is at least partially tubular shaped. The supplemental oscillator is then located inside this tube, the outer rim of the resilient and energy-consuming mass being rigidly connected to the inside wall of the tube. This supplemental oscillator may comprise, for example, a sphere of elastomer material in the center of which a piece of metal is located as the rigid vibrating mass. Likewise, the shape of the supplemental oscillator may be that of a disc which is also constructed of an elastomer and contains in the center a rigid mass comprising, for example, metal. The dimensions of this supplemental oscillator depend on the required resonance frequencies in the different directions. These and other novel features and advantages of the invention will be discussed in greater detail in the following detailed description.