US 3757117 A
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ilnited States Patent [191 Miiller et al. Sept. 4, 1973 ARRANGEMENT FOR THE Primary Examiner-James W. Lawrence MAGNIFICATION ADJUSTMENT OF AN ELECTRON MICROSCOPE Inventors: Karl-Heinz Miiller; Volker Rindfleisch, both of Berlin, Germany Siemens Aktiengesellschaft, Munich, Germany Filed: Sept. 15, 1972 Appl. No.: 289,531
Foreign Application Priority Data Sept. 28, 1971 Germany P 21 49 108.7
References Cited UNITED STATES PATENTS 8/1971 Gutter 250/495 A Akahori et al 250/495 A Assistant Examiner-B. C. Anderson Attorney- Hugh A. Chapin, Richard L. Mayer et al.
[5 7 ABSTRACT An improved setting device for providing accurately reproducible, incrementally variable control voltages to the input of a control and regulating circuit for adjusting electromagnetic coil current in magnifying lenses of electron microscopes. The setting device includes a digital counter, preferably a forwardbackward counter, connected to the input of the control circuit through a digital-to-analog converter. A digital-to-digital converter, programmed with a series of predetermined control settings, is connected between the output of the digital counter and the input to the digital-to-analgo converter to serve as a read-only memory for supplying a programmed magnification setting in response to address signals from the counter. A pulse source, preferably pushbutton-controlled, provides stepping pulses for adjusting the output voltage of the setting device to achieve desired values of magnification as indicated by a numeric display device connected to the output of the digital counter.
7 Claims, 1 Drawing Figure ARRANGEMENT FOR THE MAGNIFICATION ADJUSTMENT OF AN ELECTRON MICROSCOPE BACKGROUND OF THE INVENTION This invention relates to systems for controlling electric current, and more particularly to systems for stepwise adjustment of current flowing through electromagnetic coils for influencing beams of charges particles.
There is a large class of apparatus, including electron microscopes, ion microscopes, X-ray microscopes, diffraction equipment, devices for machining by streams of charged particles, cathode-ray Oscilloscopes, and so forth, in which the magnetic fields of electromagnetic coils are used to influence beams of charged particles.
With particular reference to electron microscopes, several electromagnetic coils, forming so-called electromagnetic lenses, may be used to provide the desired magnification and focusing. In a typical electron microscope, for example, there may be a condenser lens, a diffraction lens, an intermediate lens, and an objective lens. The first three lenses primarily influence the magnification of the microscope, while the objective lens controls the focus.
Because it is usually necessary to operate the microscope over a range of magnification values and focus lengths, depending on the specimen being investigated, each electromagnetic lens is provided with a control circuit for adjusting the coil current. A conventional setting device for providing variable control voltages to the input of such a control consists of merely a potentiometer, either continuously variable or preferably variable in steps by means of a multi-position selector switch.
It is customary also to provide regulating circuits for each lens coil in the electron microscope. A proposed circuit that combines the above described control and regulating functions includes a power amplifier with its input connected to the output of a differential amplifier having two inputs, one for a control voltage and the other for a stable reference voltage. The output of the power amplifier is connected in series with the electromagnetic lens coil and a measuring resistor across a current source. Current regulation is obtained by inverse feedback from the measuring resistor to the control input of the differential amplifier. Control voltages related to different coil currents are obtained from a setting device, such as the above mentioned multiposition potentiometer, which is also connected to the input of the difi'erential amplifier.
It is possible to change the magnification of an image generated in an electron microscope having more than one lens influencing the magnification by adjusting the current in each lens individually and successively. This can be done by varying the setting voltage applied to each control and regulating circuit.
Experience shows it is undesirable to adjust the magnification by operating each lens from the lowest to the highest excitation current before addressing the next lens in the same manner. This leads to distortion even at low magnification values. It is better to operate and control simultaneously all the lenses that are provided for the adjustment of the magnification.
SUMMARY OF THE INVENTION An object of the invention is to provide a setting device capable of simultaneously adjusting the coil currents of a plurality of electromagnetic lenses according to individually programmed values for each lens.
Another object of the invention is to provide a setting device having a numeric display, preferably independently resettable, for indicating the value of the function being controlled.
According to the invention, a setting device for a control and regulating circuit for the electromagnetic coil current of electron microscope lenses comprises a digital counter having an output connected to a digitalto-analog converter for supplying a broad range of incremental control voltages to the control input of the control and regulating circuit in response to stepping pulses applied to the input of the counter. Preferably the digital counter is a forward-backward counter to facilitate either positive or negative adjustment of the lens current.
A digital-to-digital converter is connected between the output of the digital counter and the input to the digital-to-analog converter. This device is particularly useful with a control and regulating circuit for adjusting coil current in lenses that determine the magnification of an electron microscope, such as the condenser, diffraction, and intermediate lenses described earlier.
The digital-to-digital converter acts as a read-only memory and can be programmed with predetermined setting values for a desired number of magnification powers. The output of the digital counter corresponds to an address for one of the setting values, which is then fed to the digital-to-analog converter. In this way, setting values for the lens control circuit, which may vary substantially from one magnification power to the next, can be easily and rapidly selected by using only a relatively small number of steps from the digital counter.
Another advantage of using the digital-to-digital converter between the counter and the digital-toanalog converter is that more than one lens current can be set simultaneously by operation of a single digital counter, even if the adjustment value for one of the lenses is different than for another. For example, if the electron microscope has three lenses that influence the magnification, adjusting one lens alone causes distortion, even at low magnification powers. It is much better to adjust all three lenses simultaneously.
For such an arrangement, the output of the digital counter is connected to a separate digital-to-digital converter connected to a digital-toanalog converter for each lens. The individual digital-to-digital converters are programmed with the setting values for their respective lenses that correspond to each magnification value called for by the digital counter.
The digital-counter used to adjust the magnification of the electron microscope preferably is also connected, through an additional digital-to-digital converter to a setting device for the control circuit of the objective lens of the microscope so as simultaneously to adjust the magnification and the focus. Altematively, the additional digital-to-digital converter may be connected to means for driving the object stage or other specimen adjustment means to change the focus by moving the specimen instead of changing the objective lens current.
In addition, the output digital counter used to adjust the magnification preferably is connected through still another digital-to-digital converter to a numeric display device in order to identify the magnification steps selected in each case. The other additional digital-todigital converter is programmed to convert the counter output to signals causing the numeric display to read out a number indicating the magnification or diffraction lengths.
BRIEF DESCRIPTION OF THE DRAWING The FIGURE shows a block diagram of an arrangement for the magnification adjustment of an electron microscope according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the FIGURE, the magnification system of a typical electron microscope includes three electromagnetic lenses, Ll, L2, and L3. For example, Lil may be a condenser lens, L2 a diffraction lens, and L3 an intermediate lens. The current through each of the lenses is adjusted by means of identical control circuits S1, S2, and S3. Each control circuit comprises a power amplifier L preceded by a differential amplifier D. Each lens L1 through L3 is connected in the output circuit of its corresponding power amplifier L in series with a measuring resistor M. An inverse feedback loop from each measuring resistor M transmits a regulating voltage proportional to the currents through lenses L1, L2, and L3 to the inputs Ell, E2, and E3 of the control circuits S1, S2, and S3, respectively. Also connected to the inputs E1 through E3 is a setting device according to the present invention.
The setting device includes a digital counter 1, which is designed as a forward-backward counter. Stepping pulses are fed to the forward counting input 2 and backward counting input 3 of digital counter 1 via lines 4 and 5. The stepping pulses are controlled by means of keys 6, 7 of a control panel 8. In the embodiment example shown, the counter 1 has five stages and can therefore be switched in 2 32 steps.
The outputs 9 to 13 of the digital counter l are connected to digital-to-digital converters M, 15, 16, which act as read-only memories. In these digital-to-digital converters, individual current control programs associated with the lenses L1 to L3 are set. At the outputs of the digital-to-cligital converters 14 to 16, digital signals according to the correspondingly set step of the counter are available. These are converted into analog voltage values via the succeeding digital-to-analog converters 17, 18 and 19. The output 20 of the digital-toanalog converter 17 is connected to the input E1, the output 21 of the digital-to-analog converter 18 to the input E2, and the output 22 of'the digital-to-analog converter 19 to the input E3.
If the counter 1 is driven by individual pulses, a defnite current value, specific for each step of the counter l, is thus fed to each lens L1 through L3 which can be matched optimally to the characteristics of all lenses with a view to most distortion-free imaging.
A digital-to-digital converter 23 is also connected with the digital counter I. It serves as a read-only memory and controls a numerical indicating device 24. Digital-to-digital converter 23 contains in digital form the summed program of the digital-to-digital converters 14 to 16 with regard to the magnification of the microscope and controls the indicating device 24, which preferably shows numerals which indicate the magnification or diffraction lengths.
Digital-to-digital converter 23, or alternatively an additional read-only memory (not shown) also provides digital signals at its output for controlling the adjustment sensitivity of those parts of the microscope which determine the focusing current or adjust the position of the object. In the first case the output signal affects the focusing current of the objective lens, while in the second case the output signal acts on the drive of an object stage or of other specimen-adjustment means.
Through the application of the invention it is possible to simplify the operation of an electron microscope, while at the same time increasing the reproducibility of the adjustable magnification. Furthermore, an indicating device calibrated in difi'raction lengths can be provided, which opens up the possibility, advantageous in may cases, of carrying out measurements on the specimen placed in the electron microscope.
1. An arrangement for adjusting the magnification of an electron microscope having electromagnetic lenses which influence the electron beam, the excitation current through said lenses being controlled by a control and regulating circuit with an input connected to the output of a setting device for supplying control voltages for adjusting the lens current, wherein the setting device comprises:
a. a digital counter having an input and an output for supplying a digital signal equal to the count of stepping pulses fed to the input;
b. a digital-to-digital converter acting as a read-only memory having an input connected to the output of the digital counter; and
c. a digital-to-analog converter having an input connected to the output of the digital-to-digital converter and an output connected to the input of the control and regulating circuit for one of said lenses.
2. The arrangement of claim I wherein the digital counter comprises a forward-backward counter.
3. The arrangement of claim I further comprising key-operated means for controlling the stepping pulses fed to the input of the digital counter.
4. The arrangement of claim 3 wherein the key means controls the stepping pulses one-by-one.
5. The arrangement of claim 1 further comprising:
a. second digital-to-digital converter acting as a readonly memory and connected to the output of the digital counter; and b.an indicating device connected to the output of the second digital-to-dig'ital converter.
6. The arrangement of claim 5 wherein the indicating device comprises numerals indicating the magnification or diffraction lengths.
7. The arrangement of claim 1 further comprising:
a. at least one additional digital-to-digital converter acting as a read-only memory connected to the output of the digital counter; and
b. a digital-to-analog converter connected between each additional digital-to-digital converter and the input to the control and regulating circuit for at least one other of said lenses.