Patent Number: 041707370
Section: summary

This invention relates to scientific instrumentation, particularly, to a top-entry transmission electron microscope used, mainly, in high resolution transmission electron microscopy. The present invention may be utilized for obtaining information in the studies of spatial organization of biological microobjects at the molecular level. A prerequisite for the solution of problems dealing with the three-dimensional reconstruction of biological macro molecules is the high resolution--better than 10 A--of an electron microscope. No less important parameters for the solution of these problems are the range of specimen holder tilt angles and the stability of tiltable specimen holder construction badly affecting the quality of micrographs. Another important parameter of an electron microscope is the speed the micrographs can be taken with since the lifetime of irradiated biological microobjects under study happens to be a few seconds. The situation required then that the object be brought under the beam only for the period of photosensitive material's exposition. A success in obtaining stereo pairs of the investigated microobject is possible only when the tiltable specimen holder has a mechanism for bringing the microobject into coincidence with the tilt axis. The procedure is known as Z-control. Z-control eliminates translation and defocusing of the image when the specimen holder is tilted thereby minimizing the time spent on preparatory operations when photographing stereo pairs. Tiltable specimen holders in electron microscopes with the axial loading of specimens have a limited range of tilt angles (e.g., see Review of Scientific Instruments, 44, No. 4, pp. 511-512, 1973, "A High-Angle Tilting Stage for 650 kV Hitachi Electron Microscope," M. Bouchard, etc.). Besides, all conventional makes of electron microscopes of the kind specified have no devices for Z-control. A conventional top-entry transmission electron microscope comprises a specimen stage with a removable cartridge housing a specimen holder positioned in the lens field and tiltable about the axis (x) which is perpendicular to the microscope's optical axis, and an electric drive of the holder having a step servomotor electrically connected with the unit for setting the magnitude and direction of the holder displacement and with the unit for settling the magnitude and direction of the holder displacement and with the control voltage shaper (e.g., see FRG Pat. No. 1614528 of 1971). The specimen holder in a microscope of the kind specified above is actuated by the servomotor through a special vacuum-tight sealed shaft furnished with an electromagnetic clutch to disengage the shaft from the motor during the manual control. The electric step motor is located on the external surface of the electron microscope column, its pulse setting device being designed so as to make the number of control pulses determine the magnitude of the desired specimen displacement. The presence of lengthy mechanical links between the actuating step motor and the specimen holder brings about the high level of drift. The level of drift of the tiltable specimen holder depends on the value of torque on the motor's shaft influenced in its own turn by the condition of the vacuum-tight sealing. Thus, the image drift becomes a factor which limits the maximum possibilities of the electron microscope. The absence of facilities for Z-control results in inevitable translation of image and its defocusing when the specimen holder is tilted, which precludes maintaining the set degree of magnification throughout the range of tilt angles. One object of the present invention is to provide a top-entry transmission electron microscope possessing a reduced level of the tiltable specimen holder drift. Another object of the invention is to increase the range of the specimen holder tilt angles. Yet another object of the invention is to eliminate image wanderings caused by changes in the size of projection of the tilted specimen. Still another object of the invention is to maintain the set degree of magnification throughout the range of tilt angles. And another object of the invention is to provide angular displacements for the step motor's rotor not multiple to the integral number of steps. The present invention is basically intended to solve the problem of providing a top-entry transmission electron microscope in which the location and construction of the actuating step motor of the specimen holder's electric drive together with the realization of the electric control circuitry would make it possible to reduce the drift of the tiltable specimen holder and increase its range of tilt angles, eliminate wandering off of the specimen image caused by the changes in the size of projection, maintain the set degree of magnification throughout the full range of tilt angles and ensure that the rotor of the step motor could be displaced by the value not multiple to the integral number of steps. The problem is solved by providing a top-entry transmission electron microscope comprising a specimen stage with removable cartridge housing a specimen holder mounted in the field of magnetic lens so that it can be tilted about the axis "x" perpendicular to the optical axis of the microscope, and an electric drive for the specimen holder having an actuating step motor electrically connected with a unit for setting the magnitude and sense of the specimen holder displacement and with its control voltage shape circuit, in which, according to the invention, the actuating step motor is mounted on the specimen stage and has a rotor the axis of which is made coincident with the "x" axis and which is mechanically coupled with the specimen holder to ensure synchronous turning of the rotor and specimen holder, a piezoelectrically actuated electromechanical means for turning said rotor, a drive transmitting member secured on said electromechanical means, a rocker geared to said drive transmitting member to convey drive to said rotor and pivotally mounted within the removable cartridge so that it can be displaced longitudinally with respect to its pivot fulcrum, a second piezoelectrically actuated electromechanical means for moving said rocker and mechanically coupled with said drive transmitting member, said control voltage shaper circuit of the step motor representing a Johnson code distributor and a code-to-voltage converter connected in series, its output serving as an analogue output of said control voltage shaper circuit connected to said first electromechanical means for turning the rotor, while the unit for setting the magnitude and sense of the specimen holder displacement comprises a control unit and a synchronization unit connected in series, its clock-pulse output being connected to said control voltage shaper, its control output to said second electromechanical means for displacing the rocker, while the inputs of said control unit are fed with the signals of "start," "stop," "reset," "clock-pulse frequency," "sense of displacement" and "magnitude of displacement." Preferably, a transmission electron microscope should be provided with a positioning mechanism for bringing the specimen into coincidence with the "x" axis which would comprise a bearing pulley mounted on a rotor so that it could turn about the "x" axis, a means for braking said bearing pulley against the removable cartridge, and a crank-link mechanism the crank of which would be on said bearing pulley, the link of which would have a seat for the specimen, while the axis of said link would be fixed in said rotor perpendicularly to the "x" axis and lie in the specimen plane, there should also be provided a switch with its input joined to the control output of the synchronization unit and its outputs joined to the electromechanical means of rocker displacement of the step motor and to the electrical input of said bearing pulley braking means, respectively. It will be expedient to provide a transmission electron microscope with a mechanism for translating the link in the specimen plane which would comprise a second bearing pulley with a crank mounted on the rotor so that it could turn about the "x" axis, a means for braking said second bearing pulley against the removable cartridge, there being a slot made in the link arm perpendicular to its axis geared with said crank of said second bearing pulley the switch then should be provided with an additional output joined to the electrical input of said braking means of said second bearing pulley. It will be useful to provide the braking means of each bearing pulley with a brake rocker pivotally mounted in the removable cartridge to be engaged with its bearing pulley during braking, and to have on the specimen stage additional electromechanical means to actuate the rockers during braking. It is advantageous to use piezoelectric elements for actuation of said electromechanical means. Preferably, the rotor of step motor should have a gear rim, and the end of rocker engaged with the rotor should bear a tooth the profile of which would be complementary to the tooth space profile of said gear rim. It will be expedient to have the pilot securing the rocker of step motor made in the form of an elastic cantilever with one end fixed in the body of removable cartridge and the other, free, end bearing the rocker rigidly secured to it. It will be useful to ensure that the ends of said rockers interacting with said electromechanical means and with the drive transmitting member of said step motor be provided with a means for increasing friction. Conveniently said control unit should comprise a clock-pulse generator whose input would be the "clock-pulse frequency" input of the unit for setting the magnitude and sense of specimen holder displacement; a first flip-flop the S-input of which would serve as the "start" input of said setting unit; a second flip-flop the S-input of which would be connected to the S-input of said first flip-flop its R-input being connected to the R-input of said first flip-flop; a logical OR circuit whose inputs would serve as the "stop" and "reset" inputs of said setting unit, respectively; a coincidence circuit for the codes of the set and current values of rotor displacement the input of which would serve as the "magnitude of displacement" input of said setting unit, and its output being connected to one of the inputs of said logical OR circuit, the output of the latter being connected to said R-inputs of flip-flops; a bidirectional counter with its output joined to the comparison input of said concidence circuit and its input serving as the count input of said control unit; a switch whose control input would serve as the "sense of displacement" input of said setting unit and would be joined to the control input of said bidirectional counter, while its clock input would be connected to the "reset" input of said logical OR circuit. Preferably said synchronization unit should comprise a first logical AND circuit the inputs of which would be joined to the output of said clock-pulse generator and to the Q-output of said first flip-flop of said control unit, the output of said AND circuit serving as the output of clock pulses of said setting unit; a single-shot multivibrator with its inverted output joined to the input of said logical AND circuit while its input would be joined to the P-output of said second flip-flop of said control unit; a second logical AND circuit having its inputs joined to the P-output of said second flip-flop, to the Q-output of said first flip-flop, and to the output of said switch of said control unit, while its output would serve as the control output of said setting unit; a third logical AND circuit with its input joined to the output of said switch of said control unit and its input joined to the complementing input of said second flip-flop of said control unit; a fourth logical AND circuit with its inputs joined to the output of said first AND circuit and to the output of said second AND circuit, while its output would be joined to the count input of said bidirectional counter. It will be expedient to provide the Johnson code distributor with an n-digit Mobius ring counter whose count input would be joined to the clock-pulse output of said setting unit, its "reset" input being joined to the "reset" input of said setting unit; a logical 2AND-to-2OR circuit with its inputs joined to the Q-output of the 1st digit, to the P-output of the m.s. digit, to the P-output of the 1-st digit, and to the Q-output of the m.d. digit, respectively, of said Mobius ring counter, while its output would be joined to the input of said fourth logicsl AND circuit of said synchronization unit; said Mobius ring counter should comprise at least one redundant digit inserted between said l.s. and m.s. digits, the P- and Q-outputs of one of said redundant digits being connected to the inputs of said switch of the control unit. It will also be expedient to provide said code-to-voltage converter with a nonlinear Johnson-code-to-voltage converter whose inputs would be joined to the outputs of said Mobius ring counter, and whose output would be joined to said electromechanical means of step motor; with a linear Johnson-code-to-voltage converter also having its inputs joined to the outputs of said Mobius ring counter; with a comparator the input of which would be joined to the output of said linear converter; an analogue storage having its input joined to the output of said linear converter through a gate and its output joined to the other input of said comparator the output of which would be joined to the input of said third logical AND circuit of said synchronization circuit, while the control input of said gate would be joined to the output of said logical OR circuit of said control unit. The disclosed top-entry transmission electron microscope is characteristic of a reduced level of drift of the tiltable specimen holder achieved by the rational positioning of the actuating step motor in the immediate vicinity to the specimen, on the goniometer stage. It is exclusively this implementation of the drive that has made it feasible to place the actuating motor on the same axis with the specimen holder. This permitted the number of external mechanical links of the tiltable specimen holder to be reduced to only two pivoting supports with predetermined frictional moment. The proposed microscope is also characteristic of a wider range of specimen holder tilt angles, i.e. up to 360.degree., which allows obtaining "mirror" image of specimens and determining the position of the stereopair axis with them in a simple and straightforward manner. The bearing pullies and the crank-link mechanism will preclude the specimen image from wandering away which would otherwise happen owing to changes in the size of tilted specimen and will allow the set degree of magnification to be maintained throughout the entire range of tilt angles. The specimen holder drive control circuitry in the microscope according to the invention permits the angular displacements of the step motor's rotor to be executed both by integral steps and by fractions of a step, which enables one to view the investigated specimen from any desired angle.