Patent Number: 041707370
Section: description

A top-entry transmission electron microscope comprises a specimen stage 1, FIG. 1, with a removable cartridge 2 housing a specimen holder 3. The specimen holder 3 is mounted in the gap of pole pieces 4 of the magnetic lens. Between the pole pieces there is mounted an insertion 5 made of non-magnetic material. The stage 1 is installed on spherical bearings 6 in the microscope column 7 which serves also as a magnetic circuit for the magnetic lens. The microscope's magnetic field is formed by a magnetizing coil 8. The specimen holder 3 is mounted so that it can be tilted about the axis x perpendicular to the microscope's optical axis, it is provided with an electric drive with an actuating step motor 9 electrically connected with a unit 10 for setting the magnitude and sense of displacement of the specimen holder and with a control voltage shaper 11 for the step motor. The actuating step motor 9 is mounted on the stage 1 and comprises a rotor 12, FIG. 2, which has its axis in coincidence with the x axis and is mechanically coupled with the specimen holder 3. The moment of friction in the bearings of the rotor 12 is smaller than the moment developed by the step motor 9. The step motor 9 comprises a piezoelectrically actuated electromechanical means 13 for turning the rotor and a drive transmitting member 14 pivotally attached to the electromechanical means 13. In the removable cartridge 2 there is a pivotally secured rocker 15 to convey drive further to the rotor 12. The rocker 15 is secured so that it can move translationally with respect to its fulcrum 16. It is also in gear with the drive transmitting member 14 and has an initial offset along the electron microscope optical axis which exceeds its maximum possible travel in this direction during operation. There is another piezoelectrically actuated electromechanical means 17 for deflecting the rocker interacting with the drive transmitting member 14 via a rod 18. The control voltage shaper 11, FIG. 1, of the step motor comprises a Johnson code distributor 19 and a code-to-voltage convertor 20 connected in series, an output 21 serves as the analogue output of the control voltage shaper 11. The electromechanical means 13 for turning the rotor is connected to the output 21. The unit 10 for setting the magnitude and sense of the specimen holder displacement comprises a control unit 22 connected in series with a synchronization unit 23 having its clock pulse output 24 joined to the control voltage shaper 11, and its control output 25--to the electromechanical means 17 for rocker deflection. An input 26 of the control unit 22 serves to receive the "start" signals given by the operator. Inputs 27,28,29,30, and 31 are meant for receiving the signals of "stor," "reset," "clock pulse frequency," "sense of displecement," and "magnitude of displacement," respectively. All these signals are also given by the operator. The microscope comprises a mechanism for bringing the specimen into coincidence with the x axis constructed in the following manner. On the rotor 12 of the step motor there is mounted a bearing pulley 32, FIG. 3, rotatable about the axis x. There is a braking means 33 for braking the pulley 32 with respect to the removable cartridge body 2. Also mounter in the rotor 12 is a crank-line mechanism 34 whose crank 35 is secured on the bearing pulley 32. A link 36, FIG. 4, has a seat 37 for housing a specimen. In this case, the link 36 serves as a specimen holder. The axis y of link pin 38 lies in the specimen plane perpendicular to the x axis. The link pin 38 is mounted in the rotor 12, FIG. 3. There is also a switch 39, FIG. 1, whose input is joined to the control output 25 of synchronization unit 23, while its outputs 40 and 41 are joined, respectively, to the electromechanical means 17 for rocker deflection and to the braking means 33 of the bearing pulley 32. The microscope is also provided with a mechanism for translating the link 36 in the specimen plane which comprises a second bearing pulley 42, FIG. 3, rotatable about the x axis mounted on the rotor 12 and bearing a crank 43. There is a braking means 44 for braking the second bearing pulley 42 against the removable cartridge body 2. In the body of the link 36 there is a slot 45 cut perpendicular to its pin 38 to house the crank 43 of the second pulley 42. Correspondingly, the switch 39, FIG. 1, has an additional output 46 connected to the braking means 44 of the second bearing pulley 42. The braking means 33, FIG. 3, of the bearing pulley 32 comprises a braking rocker 47 secured in a pivot 48 in the removable cartridge 2 for interaction with the bearing pulley 32 during braking. Accordingly, the stage 1 is provided with a third electromechanical means 49 for actuating the rocker 47 during braking. Similarly, the braking means 44 of the bearing pulley 42 comprises a braking rocker 50 secured in a pivot 51 in the removable cartridge 2 for interaction with the bearing pulley 42 during braking. In the described embodiment of the invention, the electromechanical means 49 and 52 have piezoelectric elements for actuators. The rotor 12 of the step motor 9 is provided with a gear rim 53, whereas the end of the rocker 15 geared with the rotor 12 has a tooth 54 the profile of which is complementary to the profile of the tooth space of the gear rim 53. The pivot 16 bearing the rocker 15 of the step motor 9 is made in the form of an elastic cantilever element 55, one end of which is fixed in the body of removable cartridge 2 and the other serves to bear the rocker 15. This free end of the cantilever element 55, FIG. 2, is given an initial offset towards the rotor 12 along the optical axis. At the ends of the rocker 15,47,50 interacting with the electromechanical means 49,52 and with the drive transmitting member 14 of stepmotor 9, there is a means 56 arranged to increase friction. In the described embodiment, the means 56 is made of indium. The second input 57 of the switch 39 serves to receive the instructions of "sample holder coordinate selection" which is fed in by the operator. The control unit 22, FIG. 5, comprises a clock pulse generator 58 the input of which serves as the "clock pulse frequency" input 29 of the unit 10 for setting the magnitude and sense of sample holder displacement. The control unit 22 comprises also a flip-flop 59, the S-input of which serves as the "start" input 26 of the unit 10, a second lip-flop 60, the S-input of which is joined to the S-input of the flip-flop 59, while its R-input is joined to the R-input of the flip-flop 59. The unit 22 comprises also a logical OR circuit 61 whose inputs serve as the "stop" input and the "reset" input of the unit 10, a coincidence circuit 62 for the set and current value codes of the rotor displecement, the input of which serves as the "magnitude of displacement" input 31 of the unit 10, its output being connected to a third input 63 of the logical OR circuit 61 whose output is joined to the R-inputs of the flip-flops 59 and 60. The control unit 22 comprises also a bidirectional counter 64 whose output is connected to a comparison input 65 of the coincidence circuit 62, with an input 66 serving as a count input for the control unit 22. The control unit 22 comprises also a switch 67 the control input of which serves as the "sense of displacement" input 30 of the unit 10 and is joined to a control input 68 of the bidirectional counter 64, a clock input 69 of which is joined to the "reset" input of the logical OR circuit 61. The synchronization unit 23 comprises a logical AND circuit 70 whose inputs 71,72 are connected to the output of the clock pulse generator 58 and to the Q-output of the flip-flop 59, its output serving as the clock pulse output 24 of the unit 10. The unit 23 comprises also: a one-shot multivibrator 73 with its inverted output joined to an input 74 of the logical AND circuit 70, and its input--to the P-output of the flip-flop 59; a second logical AND circuit 75 with its inputs 76,77 and 78 joined to the P-output of the flip-flop 60, to the Q-output of the flip-flop 59 and to the output of the switch 67, and its output serving as the control output 25 of the unit 10 for setting the magnitude and sense of the holder displacement. The unit 23 comprises a third logical AND circuit 79 with its input 80 joined to the output of the switch 67, and its output--to the complementary C-input of the flip-flop 60; a fourth logical AND circuit 81 with its inputs 82 and 83 joined to the outputs of the logical AND circuits 70 and 75, respectively, and its output--to the count input 66 of the bidirectional counter 64. The Johnson code distributor 19 comprises an n-digit Mobius ring counter 84, the count input of which is connected to the clock pulse output 24 of the sunchro unit 23, whereas its "reset" input is joined to the "reset" input 28 of the unit 10. The distributor 19 comprises also: a logical 2AND-to-2OR circuit, whose inputs 86,87,88 and 89 are joined to the Q-output of the l.s.d., to the P-output of the m.s.d., to the P-output of the l.s.d., and to the Q-output of the m.s.d., respectively, of the ring counter 84. The output of the 2AND-to-2OR circuit 85 is joined to the input 90 of the fourth logical AND circuit 81. The Mobius ring counter 84 comprises n D-type flip-flops. In the described embodiment it has two additional D-flip-flops according to the number of two redundant digits (N+1) and (N+2) inserted between the flip-flop which corresponds to the l.s.d. (1) and the flip-flop corresponding to the m.s.d. (N). The P- and Q-inputs of the flip-flop, representing the redundant digit (N+1), are joined to the inputs 91 and 92 of the switch 67. The code-to-voltage converter 20 comprises a nonlinear Johnson-code-to-voltage converter 93, the inputs of which are joined to the Q-inputs of the flip-flops corresponding to the digits of the counter 84 from the l.s.d. (1) to the m.s.d. (N), and its output 94 being joined to the electromechanical means 13 of the step motor 9. The converter 20 comprises also a linear Johnson-code-to-voltage converter with its inputs likewise joined to the Q-inputs of the flip-flops corresponding to the digits of the counter 84 from the l.s.d. (1) to the m.s.d. (N); a comparator 96 whose input is joined to an output 97 of the linear converter 95; an analogue storage device 98 with its input 99 joined to the output 97 of the linear converter 95 via a gate 100, and its output joined to the other input 101 of the comparator 96. The output of the comparator 96 is joined to an input 102 of the third logical AND circuit 79. A control input 103 of the gate 100 is joined to the output of the logical OR circuit 61. For better understanding the performance of the proposed device, the reference is now made to the signal diagrams in FIG. 6 where the x-axis is time, and y-axis represents voltages at the component outputs. For the sake of simplicity the Mobius ring counter 84 is shown to have 5 duty digits and 2 redundant digits the Q-outputs of which are not joined to the Johnson-code-to-voltage converters 93 and 95. FIG. 6a shows the signal at the clock pulse output 24 of the unit 10, FIG. 6b--the signal at the Q-output of the l.s.d. flip-flop (1) of the Mobius ring counter 84, FIG. 6c--the signal at the Q-output of the second-digit flip-flop of the ring counter 84, FIG. 6d--the signal at the Q-output of the third-digit flip-flop of the counter 84, FIG. 6e--the signal at the Q-output of the forth-digit flip-flop, FIG. 6f--the signal at the Q-output of the fifth-digit (N) flip-flop, FIG. 6g--the signal at the Q-output of the sixth-digit (N+1) flip-flop, and FIG. 6h--the signal at the Q-output of the seventh-digit (N+2) flip-flop in the Mobius ring counter 84. FIG. 6a shows the signal at the output 97 of the linear Johnson-code-to-voltage converter 95, and FIG. 6k--the signal at the P-output of the sixth-digit (N+1) flip-flop of the Mobius ring counter 84. The performance of the top-entry transmission electron microscope according to the invention is described in the following. Since the principle of imaging in this microscope is the same as in the conventional makes, the description of the performance will refer to the operations to be carried out after having the specimen placed in the working gap of the pole-pieces 4, FIG. 1, and the electron optics adjusted. The step drive performance will be first described for tilting the specimen holder 3 without the z-control device. It is further assumed that the investigator's object of interest coincides with the tilt axis X of the specimen holder 3 and that the control of image position on the screen is unnecessary. For driving the rocker 15, the control voltage shaper 11 generates a signal, FIG. 6a, which has a form of stepped trapezoid. These pulses cause the electromechanical means 13, FIG. 1, to perform oscillatory movements conveyed by the drive transmitting member 14 to the rocker 15 so that the rocker 15 is displaced longitudinally. The unit 10 for setting the magnitude and sence of the holder displacement generates a control signal for the rocker-position electromechanical means 17 synchronous with triangular pulses of the shaper 11 and having a square wave form, FIG. 6, in phase with either the front or the rear slopes of the triangular pulses of the shaper 11, FIG. 1. The movement of the rocker-position electromechanical means is also of oscillatory character. It is conveyed to the rocker 15 through thr rod 18 and the drive transmitting member 14 in the direction perpendicular to the microscope's optical axis. Superposition of the two oscillatory movements brings the end of rocker 15 facing the rotor 12 into periodical engagement with the rotor 12 causing the latter to turn by a set angle. Practically required are angular displacements of the specimen holder 3 not multiple to an integral number of steps of the step motor 9. The electrical circuit of the drive works as follows. A voltage pulse at the "initial setting" input 27, FIG. 5, repeats at the output of the logical OR circuit 61. As a result, the P-output of the flip-flop 60 acquires the logical ONE potential, and the Q-output of flip-flop 59--the logical ZERO potential, while the gate 100 connects the input 99 of the analogue storage 98 to the output 97 of the linear Johnson-code-to-voltage converter 95 for a period equal to the duration of the setting pulse. The potentials at both inputs of the comparator 96 are equalized and square wave oscillations appear at its output which is connected to the input 102 of the third logical AND circuit 79 of the synchronization unit. The frequency of these square wave oscillations is determined by the parameters of a generator (not shown in FIG. 5) compensating the dead zone of the comparator 96. At the same time, the Q-outputs of all digits of the Mobius ring counter 84 and the bidirectional counter 64 get the potential of logical ZERO. When a voltage pulse is fed to the "start" input 26, the Q-output of the flip-flop 59 and the inputs 72 and 76 of the logical AND circuits 70 and 75, connected with it, get the potential of logical ONE. The P-output of the flip-flop 60 gets the potential of logical ZERO. The first pulse coming from the output of comparator 96 (when the logical ONE potential is present at the input of the logical AND circuit 79) to the C-input of the flip-flop 60 puts the latter into the state of its P-output having the logical ONE potential. Since the D-input of the flip-flop 60 is constantly at the logical ONE potential, all succeeding pulses coming to its C-input do not affect its state. The flip-flop 60 is switched over by the C-input when the voltages at the inputs of comparator 96 are equal. It occurs only when the logical ONE potential is present at the output of switch 67 which is a prerequisite for the actuation of the electromechanical means 17 for rocker positioning; FIG. 1. When the signals at all the three inputs 76,77,78, FIG. 5, of the logical AND circuit 75 coincide, the rocker-positioning electromechanical means 17, FIG. 3, is actuated, its working cycles being coincident with the signal cycles at the Q-output (or P-output, see FIGS. 6g and 6k) of the (N+1)-digit flip-flop of the Mobius ring counter 84, FIG. 5. Switching of the flip-flop 60 by its synchronization C-input triggers the one-shot multivibrator 73. During the time the one-shot multivibratot 73 is ON, the pulses from the clock-pulse generator 58 cannot pass through the logical AND circuit 70 to the input of the Johnson code distributor 19. The pulse duration of the multivibrator 73 is somewhat longer than the working-process transient of the rocker-positioning electromechanical means 17, FIG. 1. This ensures maintaining the magnitude of the first step after restart, in the case when the logical ONE potential is present at the output of the switch 67, FIG. 5, at the starting moments. The voltage clock pulses fed to the input of the Johnson code distributor 19, are, at the same time, fed to the input 82 of the logical AND circuit 81. But only those clock pulses pass to the output of the logical AND circuit 81 (i.e. to the input of the counter 64) which come at the actuation of the rocker-positioning electromechanical means 17, FIG. 1, which is ensured by the presence of the rocker-position control signal at the input 83 of the logical AND circuit 81, FIG. 5. When the redundant digits in the Mobius ring counter 84 in the Johnson code distributor are actuated, the logical 2AND-to-2OR circuit 85 forbids passing the said pulses even if the rocker-positioning electromechanical means 17 is actuated, FIG. 1. In this way, only those clock pulses are being stored in the bidirectional counter 64 whose passage has been followed by the displacement of rotor 12, FIG. 1, corresponding to one step of the control voltage at the electromechanical means 17 for turning the rotor. The drive stops when the code of setting and the code stored in the bidirectional counter 64, FIG. 5, coincide. This causes a voltage pulse to appear at the output of the coincidence circuit 62 which is then fed to the input 63 of the logical OR circuit 61, and from the output of the latter--to the R-inputs of the flip-flops 59 and 60. The flip-flop 59 is switched to the state of having the logical ZERO potential at its Q-output, which inhibits the transmission of the clock pulses from the output of the clock pulse generator 58 to the input of the Johnson code distributor 19 through the logical AND circuit 70. The Q-outputs of the Mobius ring counter 84 register the state corresponding to a current voltage value at the output 94 of the non-linear converter 93 for the moment the drive stopped at. A lofical ZERO appears also at the output of the logical AND circuit 75, an output 77 of which is joined to the Q-output of the flip-flop 59, which causes the rocker 15, FIG. 3, to retract from the rotor 12 with the fixed voltage at the rotor-turning electromechanical means 13. The gate 100, FIG. 5, joins the input 99 of the analogue storing device 98 to the output 97 of the linear converter 95 for a period equal to the duration of the pulse generated by the comparison circuit 62 at the coincidence of codes. At the output of the analogue storing device 98 and at the first input of the comparator 96, there appears a voltage corresponding to the output voltage of the linear converter 95 at the moment the drive stopped. Manual stoppage may be performed at any moment by sending a voltage pulse to the "stop" input 27, which is exactly identical to the coincidence of codes in the circuit 62. To reverse the motion of the rotor 12, one should send the "sense of displacement" instruction to the input 30 of the control unit 22, set a new displacement code at the input 31 of the control unit 22, and send a pulse to the input 26 of the control unit 22. This results in setting the logical ONE potential at the Q-output of the flip-flop 59, which causes the clock pulses to be fed to the input 24 of the Johnson code distributor 19, whereas the logical ZERO potential is established at the P-output of the flip-flop 60, which causes the output of the logical AND circuit 75 to keep the logical ZERO potential. The rocker-positioning electromechanical means 17 is not actuated, and the logical AND circuit does not let the clock pulses pass to the input of bidirectional counter 64. Thus, the Johnson code distributor 19 starts working when the rocker 15 is out of gear. When the (N+1)-th digit of the Mobius ring counter stores, its flip-flop outputs swap the states, which causes the logical ONE potential to appear at the output of the switch 67 and, consequently, at the output of the logical AND circuit 75. The voltage at the outputs 94 and 97 of the Johnson-code-to-voltage converters 93 and 95 after reaching its maximum starts diminishing with each new pulse arriving at the input of the Johnson code distributor 19. At the moment when the current value of voltage at the output of the linear Johnson code converter 95 becomes equal to the voltage at the output of the analogue storage 98, the comparator 96 generates a signal (square waveform) fed to the input 102 of the logical AND circuit 79, and, consequently, to the synchro C-input of the flip-flop 60, since a logical ONE signal is present at the input 80 of the logical AND circuit 79. The flip-flop 60 is switched into the state of its P-output having the logical ONE potential. The output of the logical AND circuit 75 becomes the logical ONE, whereas the one-shot multivibrator 73 inhibits the passage of clock pulses from the generator 58 to the input of the Johnson code distributor 19 via the logical AND circuit for the time sufficient for the actuation of the rocker-positioning electromechanical means 17, FIG. 3. After this, the Johnson code distributor 19, FIG. 5, goes on working, whereas the bidirectional counter 64 starts a reversed counting (substraction) till the set code coincides with the code of its current value at the coincidence circuit 62 in the same way as with the direct movement. After the set magnitude of displacement is achieved, thr rocker 15, FIG. 3, is disengaged once again, while the analogue storage 98 stores the value of voltage at the output 97 of the linear converter 95 at the moment of this new stop of the drive. The described sequence of operations of blocks and units corresponds to the displacement of piezoelectric elements in the rotor-turning electromechanical means 13 which perform over one and the same partial hysteresis loop while executing the instruction set for any value and sense of displacement. This makes it possible to compensate for the non-linearity of working characteristics of the piezoelectric elements by simple conventional means. A microobject under observation is registered with the tilt axis x of the specimen holder 3, FIG. 3, in the following way. The rotor 12 of the step motor is set at a "step forward-step backward" oscillatory movement. To do this, the input 57 of the switch 39 is fed with the instruction, according to which the controlling output 25 of the unit 10 forsetting the magnitude and sense of the specimen holder's displacement is joined to the input 46 of the switch 39, and the output 40 of the switch 39--to a d.c. voltage source (not shown in FIGS.), the polarity and amplitude of which correspond to those of the signal at the controlling output 25 of the unit 10. In this mode of operation the brake pulley 4, FIG. 3, is periodically braked, synchronously with the "step forward, step backward" turns, by the rocker 50, the position of which is controlled by the electromechanical means 52 joined to the controlling output 25 of the unit 10 by the switch 39, FIG. 5. In accordance with the set sense-of-displacement signal, the crank 43, FIG. 3, cyclically displaces the link 36 along the pin 38 in the direction of the y-coordinate, FIG. 4. The specimen placed in the seat 37 moves together with the link 36. This mode of the drive operation is retained till the micro-object gets registered with the x-axis projection on the electron microscope screen. Keeping the "step forward, step backward" mode, one should increase the magnification of the electron microscope until the amplitude of image oscillations becomes not less than 1/3 of the screen size. After that, the electromechanical means 49 should be joined to the controlling output 25 of the unit 10 by means of the switch 39, FIG. 5. During this operation, the brake pulley 32, FIG. 3, is being periodically and synchronously with the "step forward, step backward" turns of the rotor 12 braked by the rocker 47, the position of which is controlled by the electromechanical means 49. In the meantime, the rocker 50 is kept in the out-of-gear state by the electromechanical means 52. In accordance with the sense-of-displacement signal set at the input 30 of the control unit 22, FIG. 5, the crank 35, FIG. 3, cyclically displaces the link 36 in the direction of the optical axis z. With the correct selection of the sense of motion along the z-axis, the amplitude of image oscillations on the electron microscope screen starts diminishing. The least possible amplitude of oscillations of the image corresonds to the optimum coincidence of the microobject under observation with the tilt axis x of the specimen holder 2. The required value of magnification should be set, the switch 39, FIG. 1,--be put to the state which makes it feasible to perform the x-tilts (this state corresponds to the electromechanical means 17 being joined to the controlling output 25 of the unit 10).