A low-vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope

Abstract

The present invention relates to the technical field of scanning electron microscopes, and particularly relates to a low-vacuum secondary electron detector with high signal-to-noise ratio for a scanning electron microscope. It includes: a circuit board; a probe and a grid, both the probe and the grid are connected to the end of the circuit board, and the grid covers the outside of the probe; a circuit board socket, which is arranged on the side of the circuit board away from the probe, and the circuit board is plugged into the circuit board socket; a baffle plate, which is located on the side of the circuit board socket away from the circuit board, and one side of the baffle plate away from the circuit board socket is connected to the side wall of the sample chamber of the scanning electron microscope. The gap between the circuit board socket and the baffle plate can be changed by turning the adjusting screw. The circuit board socket is connected to the probe, and the baffle plate is connected to the sample chamber of the scanning electron microscope. Therefore, the distance between the probe and the side wall of the sample chamber can be adjusted, thereby changing the distance between the probe and the objective lens, and the distance between the probe and the objective lens can be adjusted.

Description

Technical Field

[0001] The present invention relates to the technical field of scanning electron microscopes, and particularly to a low-vacuum secondary electron detector with high signal-to-noise ratio for a scanning electron microscope. Background Art

[0002] When imaging with a low-vacuum scanning electron microscope, a high voltage needs to be applied to the probe to collect the electrons generated in the sample chamber of the electron microscope. The probe is arranged at a position close to the objective lens in the sample chamber. When the high voltage on the probe reaches the critical value, a discharge phenomenon will occur between the objective lens and the probe. The occurrence of the discharge phenomenon is affected by the high voltage value and the distance between the probe and the objective lens. The greater the distance between the probe and the objective lens, the greater the high voltage critical value when the discharge phenomenon occurs.

[0003] In the existing technical solutions, the distance between the probe and the objective lens is fixed, and the high voltage critical value is also a fixed value. When facing different samples or different observation effects, flexible adjustment cannot be performed. Summary of the Invention

[0004] In view of this, the present invention provides a low-vacuum secondary electron detector with high signal-to-noise ratio for a scanning electron microscope to solve the problem that flexible adjustment cannot be performed when facing different samples or different observation effects.

[0005] The present invention provides a low-vacuum secondary electron detector with high signal-to-noise ratio for a scanning electron microscope, including:

[0006] A circuit board;

[0007] A probe and a grid, both the probe and the grid are connected to the end of the circuit board, and the grid covers the outside of the probe;

[0008] A circuit board socket, arranged on the side of the circuit board away from the probe, and the circuit board is plugged into the circuit board socket;

[0009] A baffle, located on the side of the circuit board socket away from the circuit board, and the side of the baffle away from the circuit board socket is connected to the side wall of the sample chamber of the scanning electron microscope;

[0010] Adjusting screws, two in number, passing through the circuit board socket and the baffle;

[0011] It further includes a high-voltage module, a high-voltage circuit, an amplifier circuit, a filter circuit, and a signal processing circuit. The high-voltage circuit is adapted to adjust the output voltage of the high-voltage module to the probe;

[0012] The amplifier circuit, the filter circuit, and the signal processing circuit are connected in sequence;

[0013] The high-voltage module is connected to the grid, and the amplifier circuit is connected to the probe.

[0014] The gap between the circuit board socket and the shielding plate can be changed by turning the adjusting screw. The circuit board socket is connected to the probe, and the shielding plate is connected to the sample chamber of the scanning electron microscope. Therefore, the distance between the probe and the side wall of the sample chamber can be adjusted, so as to change the distance between the probe and the objective lens. The distance between the probe and the objective lens can be adjusted, and the high voltage applied to the grid can be further adjusted, so that when facing different samples or different observation effects, such as observing samples of different sizes, or for different surface observation purposes, flexible adjustment can be carried out.

[0015] In an alternative embodiment, the probe is rectangular, the upper and lower edges are both long edges, and the middle part of the probe is connected to the end side of the circuit board. The electron receiving area can be increased to maximize the received electron signal.

[0016] In an alternative embodiment, it further includes:

[0017] A grid, fixed on one side of the circuit board close to the probe, and the grid covers the outside of the probe.

[0018] In an alternative embodiment, it further includes:

[0019] An adapter, arranged between the circuit board socket and the shielding plate, and the adjusting screw penetrates through the adapter.

[0020] In an alternative embodiment, it further includes:

[0021] Adjusting springs, two in number, both abutted between the adapter and the shielding plate, and respectively sleeved on the adjusting screw;

[0022] Wherein, the adjusting springs make the adapter abut against the circuit board socket.

[0023] The adjusting springs make the adapter always abut against the circuit board socket. When the adjusting screw is turned, the expansion and contraction of the adjusting springs can adjust the gap between the shielding plate and the circuit board socket, so as to change the distance between the probe and the objective lens.

[0024] In an alternative embodiment, it further includes:

[0025] A through-sealing plate, arranged on the outer side wall of the sample chamber of the scanning electron microscope, and one side of the shielding plate far from the circuit board socket is connected to the through-sealing plate;

[0026] A sealing ring, arranged on the side wall of the through-sealing plate close to the shielding plate, and the sealing ring is located between the through-sealing plate and the outer side wall of the sample chamber.

[0027] In an alternative embodiment, the output voltage of the high-voltage module is adjusted by a variable resistor;

[0028] Two fixed pins of the variable resistor are respectively connected to the reference voltage pin and the ground pin of the high-voltage module, and the fixed pin of the variable resistor is connected to the input pin of the high-voltage module.

[0029] In an alternative embodiment, the output voltage of the high-voltage module is adjusted by a first single-chip microcomputer;

[0030] The output end of the first single-chip microcomputer is connected to the input pin of the high-voltage module.

[0031] In an alternative embodiment, the amplifier circuit includes a preamplifier circuit and a main amplifier circuit, and the input end of the preamplifier circuit is connected to the probe;

[0032] Among them, the preamplifier circuit includes an amplifier U9 and a resistor R200. One end of the resistor R200 is connected to the feedback pin of the amplifier U9, and the other end of the resistor R200 is the input end; the amplifier U9 and the resistor R200 form a current-to-voltage circuit, which can convert a weak current into a voltage.

[0033] The preamplifier circuit further includes a buffer amplifier. The input end of the buffer amplifier is connected to the output pin of the operational amplifier U9, and the output end of the buffer amplifier is connected to the main amplifier circuit; the buffer amplifier can play the role of high-impedance input and low-impedance output for impedance transformation.

[0034] The main amplification circuit includes a first-stage amplification circuit and a second-stage amplification circuit connected thereto. Among them, the first-stage amplification circuit includes a first inverting proportional operation circuit, and the first inverting proportional operation circuit includes a resistor R58, a resistor R62, and an amplifier U17A. The resistor R58 is 70-90 times that of the resistor R62. The second-stage amplification circuit includes a second inverting proportional operation circuit, and the second inverting proportional operation circuit includes a second feedback loop, a resistor R69, and an amplifier U4A. The second feedback loop includes an analog switch chip U3, a second single-chip microcomputer, and eight feedback resistors with different resistance values. One end of each of the eight feedback resistors with different resistance values is connected to eight input pins of the analog switch chip U3 respectively, and the other ends of the eight feedback resistors with different resistance values are all connected between the resistor R69 and the inverting input terminal of the amplifier U4A. The common output pin of the analog switch chip U3 is connected to the output terminal of the amplifier U4A. The second single-chip microcomputer is connected to three address pins of the analog switch chip U3. The analog switch chip U3 selects a corresponding feedback resistor to access the second feedback loop based on the address determined by the second single-chip microcomputer. It is possible to select eight feedback resistors with different resistance values and connect the corresponding resistor to the feedback loop of the amplifier U4A according to needs, thereby controlling the gain of the second-stage amplification circuit.

[0035] In an alternative embodiment, the filter circuit includes a 50Hz notch circuit and a low-pass filter circuit, and the output terminal of the main amplification circuit is connected to the input terminal of the filter circuit;

[0036] Among them, the 50Hz notch circuit includes a twin-T network circuit, a third feedback circuit, a fourth feedback circuit, a resistor R63, and a resistor R66;

[0037] The third feedback circuit is composed of an amplifier U2A. The input terminal of the amplifier U2A is connected to the twin-T network circuit, and the output terminal of the third feedback circuit serves as the output of the entire 50Hz notch circuit;

[0038] The fourth feedback circuit is composed of an amplifier U2B. The output terminal of the fourth feedback circuit is connected to the vertical arm of the twin-T network circuit;

[0039] The resistor R63 is connected between the non-inverting input terminal of the amplifier U2B and the output terminal of the amplifier U2A. One end of the resistor R66 is grounded, and the other end is connected to the non-inverting input terminal of the amplifier U2B.

[0040] In an alternative embodiment, at least one of the resistor R63 and the resistor R66 is a variable resistor.

[0041] Adjusting the ratio of the regulating resistor R63 and the resistor R66 can adjust the stopband width and Q value of the filter circuit, regulate the filtering characteristics of the filter circuit, and achieve a better filtering effect. The 50Hz notch circuit can filter out the 50Hz power frequency interference in the signal and improve the signal quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0043] Figure 1 Schematic diagram of the structure of the embodiment of the present invention;

[0044] Figure 2 Schematic diagram of the high-voltage circuit of the embodiment of the present invention;

[0045] Figure 3 Schematic diagram of the pre-amplifier circuit of the embodiment of the present invention;

[0046] Figure 4 Schematic diagram of the main amplifier circuit of the embodiment of the present invention;

[0047] Figure 5 Schematic diagram of the 50Hz notch circuit of the embodiment of the present invention;

[0048] Figure 6 Schematic diagram of the low-pass filter circuit of the embodiment of the present invention.

[0049] Description of the reference numerals in the drawings:

[0050] 1. Probe; 2. Circuit board; 3. Circuit board socket; 4. Baffle; 5. Adjusting screw; 6. Grid; 7. Adapter; 8. Adjusting spring; 9. Through-sealing plate. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0052] A scanning electron microscope is an electron microscope that forms an observable image by scanning the surface of the sample to be observed with a focused electron beam. When the focused electron beam is incident on the sample surface at a certain angle, the incident electrons interact with the electrons of the sample atomic nucleus, generating various information such as secondary electrons and backscattered electrons. These information contain signals such as the appearance and composition of the sample. The detector collects and amplifies the generated secondary electrons or backscattered electrons and corresponds them to the position where the electron beam scans to generate a two-dimensional image, that is, a secondary electron image or a backscattered electron image. The resolution of the scanning electron microscope can reach 1 nm and has been widely used and plays an important role in the fields of materials science, production science, physics, industrial production, national defense, and new energy, etc.

[0053] The electron gun and E-T detector of the traditional scanning electron microscope need to work in a high-vacuum environment. In a high-vacuum environment, it is required that the sample has good electrical conductivity to obtain high-quality imaging. However, some samples have poor electrical conductivity. At this time, the charging effect will occur to the sample during the imaging process. The traditional electron microscope can spray gold on the sample surface to improve the conductivity. The low-vacuum scanning electron microscope can eliminate the charging effect without spraying gold, which is convenient and easy to operate. At the same time, the sample is not processed at all to meet the needs of some customers.

[0054] Gas is input into the sample chamber of the low-vacuum scanning electron microscope to maintain a certain vacuum value. After the electron beam bombards the sample, the secondary electrons emitted from the sample surface collide with the gas molecules in the sample chamber to generate positive ions and electrons. The generated electrons collide with other gas molecules again, and so on, the electrons and ions increase geometrically. The positive ions in the sample chamber will neutralize the negative charges on the sample surface and reduce the charge effect. The detector can detect the generated electrons, collect, amplify and convert them into voltage to form a grayscale image, which is the secondary electron image in the low-vacuum mode. To collect electrons, a voltage of several hundred volts needs to be applied to the probe of the detector. Usually, this high voltage is directly applied to the probe of the low-vacuum secondary electron detector. At this time, the high voltage and the collected signal are on the same signal line, which will cause the amplifier circuit of the detector to also amplify the noise transmitted by the high voltage, which has a relatively serious impact on the signal-to-noise ratio of the signal and damages the imaging quality. The probe will be set at a position close to the objective lens. When the high voltage on the probe reaches a certain value, that is, there is a critical value (this critical value is not a fixed value and has a small fluctuation of several volts up and down), when the voltage exceeds this critical value, a discharge phenomenon will occur. The distance between the probe and the objective lens will affect the critical value of the high voltage when the discharge phenomenon occurs. The normal operating range of the high voltage is usually between 200V and 500V.

[0055] In the present invention, the high voltage is separated from the collected electronic signal. The amplifier circuit of the detector only amplifies the electronic signal, thus ensuring the signal-to-noise ratio of the signal and high-quality imaging. The distance between the probe and the objective lens in the sample chamber can also be adjusted, so that the critical high voltage value during high-voltage discharge can be adjusted, thereby increasing the intensity of the electronic signal. Usually, when the distance between the probe and the objective lens is within a certain range, the greater the value of the high voltage, the higher the intensity of its electronic signal.

[0056] The following will describe embodiments of the present invention in conjunction with Figures 1 to 6 .

[0057] According to an embodiment of the present invention, there is provided a low-vacuum secondary electron detector with high signal-to-noise ratio for a scanning electron microscope, including:

[0058] A circuit board 2;

[0059] A probe 1 and a grid 6, both the probe 1 and the grid 6 are connected to the end of the circuit board 2, and the grid 6 covers the outside of the probe 1; the probe 1 can be connected to the circuit board 2 by soldering.

[0060] A circuit board socket 3, arranged on the side of the circuit board 2 away from the probe 1, and the circuit board 2 is plugged into the circuit board socket 3;

[0061] A baffle 4, located on the side of the circuit board socket 3 away from the circuit board 2, and the side of the baffle 4 away from the circuit board socket 3 is connected to the side wall of the sample chamber of the scanning electron microscope;

[0062] Adjusting screws 5, there are two of them, passing through the circuit board socket 3 and the baffle 4; nuts are provided on the end sides of the adjusting screws 5, and both the circuit board socket 3 and the baffle 4 are arranged between the adjusting screws 5 and the nuts screwed thereto. By turning the adjusting screws 5 or the nuts, the gap between the circuit board socket 3 and the baffle 4 can be adjusted. So that there is an adjustable gap between the circuit board socket 3 and the baffle 4 in the axial direction of the adjusting screw 5.

[0063] Among them, the probe 1, the circuit board 2, the circuit board socket 3, the baffle 4, the adjusting screws 5 and the grid 6 are all arranged in the sample chamber of the scanning electron microscope.

[0064] It further includes a high-voltage module, a high-voltage circuit, an amplifier circuit, a filter circuit and a signal processing circuit. The high-voltage circuit is adapted to adjust the output voltage of the high-voltage module to the probe 1;

[0065] The amplifier circuit, the filter circuit and the signal processing circuit are connected in sequence;

[0066] The high-voltage module is connected to the grid 6, and the amplifier circuit is connected to the probe 1. It should be noted that the high-voltage module, high-voltage circuit, amplifier circuit, filter circuit, and signal processing circuit can all be arranged outside the sample chamber. Through independent wiring, the high-voltage module is connected to the grid 6, and the amplifier circuit is connected to the probe 1 respectively. Of course, the independent wiring can be respectively connected to the circuit board 2. Independent connection circuits are correspondingly arranged on the circuit board 2, and then the connections to the grid 6 and the probe 1 are completed respectively. The connection between the independent wiring and the connection circuit on the circuit board 2 can be realized through the circuit board socket 3. In this application, a high voltage is applied to the grid 6, and electron signals are collected through the probe 1, so that the electron signals are separated from the high voltage. The amplifier circuit connected to the probe 1 only amplifies the electron signals, thereby ensuring the signal-to-noise ratio of the signals and high-quality imaging.

[0067] The gap between the circuit board socket 3 and the baffle 4 can be changed by turning the adjustment screw 5. The circuit board socket 3 is connected to the probe 1, and the baffle 4 is connected to the inner side wall of the sample chamber of the scanning electron microscope. Therefore, the distance between the probe 1 and the inner side wall of the sample chamber can be adjusted, thereby changing the distance between the probe 1 and the objective lens. The distance between the probe 1 and the objective lens can be adjusted, and the high voltage applied to the probe 1 can also be further adjusted, so that when facing different samples or different observation effects, such as observing samples of different sizes, or for different surface observation purposes, flexible adjustment can be carried out.

[0068] In one embodiment, the baffle 4 has an angle of 45° to 60° with the horizontal plane downward, and the specific angle is adjusted according to the structure of the lens barrel to ensure a high secondary electron reception efficiency.

[0069] In one embodiment, the probe 1 is rectangular, and both the upper edge and the lower edge are long edges, and the middle part of the probe 1 is connected to the end side of the circuit board 2. The electron reception area can be increased, and the received electron signals can be maximized.

[0070] In one embodiment, it further includes:

[0071] An adapter 7 is arranged between the circuit board socket 3 and the baffle 4, and the adjustment screw 5 penetrates through the adapter 7.

[0072] In one embodiment, it further includes:

[0073] Adjusting springs 8, there are two of them, both are abutted between the adapter 7 and the baffle 4, and are respectively sleeved on the adjustment screw 5;

[0074] Wherein, the adjusting spring 8 makes the adapter 7 always abut against the circuit board socket 3.

[0075] Adjust the spring 8 so that the adapter 7 is always in contact with the circuit board socket 3. When turning the adjusting screw 5, the expansion and contraction of the spring 8 can adjust the gap between the baffle 4 and the circuit board socket 3, thereby changing the distance between the probe 1 and the objective lens.

[0076] In one embodiment, it further includes:

[0077] A through-sealing plate 9 is provided on the outer side wall of the sample chamber of the scanning electron microscope. One side of the baffle 4 away from the circuit board socket 3 is connected to the through-sealing plate 9; independent wiring enters the interior of the sample chamber through the through-sealing plate 9, and the through part can increase the sealing structure.

[0078] A sealing ring is provided on the side wall of the through-sealing plate 9 close to the baffle 4, and the sealing ring is located between the through-sealing plate 9 and the outer side wall of the sample chamber.

[0079] Copper can be laid around the preamplifier circuit and part of the preamplifier circuit can be enclosed by a structural member. The wiring in the preamplifier circuit is isolated by the ground wire, and dense vias are drilled on the ground wire to reduce interference; it should be noted that the signal processing circuit is a conventional circuit, and the improvement points of the present invention do not include the improvement of the signal processing circuit, so it will not be elaborated here.

[0080] In one embodiment, the output voltage of the high-voltage module is adjusted by an adjustable resistor;

[0081] Two fixed pins of the adjustable resistor are respectively connected to the reference voltage pin and the ground pin of the high-voltage module, and the fixed pin of the adjustable resistor is connected to the input pin of the high-voltage module.

[0082] In one embodiment, the output voltage of the high-voltage module is adjusted by a first single-chip microcomputer;

[0083] The output end of the first single-chip microcomputer is connected to the input pin of the high-voltage module.

[0084] As Figure 2 shown, the high-voltage circuit may include a resistor R4. Two fixed pins of the adjustable resistor R31 are respectively connected to the reference voltage pin Vref and the ground pin GND of the high-voltage module U1, and the fixed pin of the adjustable resistor R31 is connected to one end of the resistor R4;

[0085] The high-voltage circuit further includes a connector P1 and a resistor R3. One end of the resistor R3 is connected to the connector P1, and the output end of the first single-chip microcomputer is connected to the connector P1;

[0086] When the output voltage of the high-voltage module U1 is adjusted by the adjustable resistor R31, the other end of the resistor R4 is connected to the input pin Vp-in of the high-voltage module U1; when the output voltage of the high-voltage module U1 is adjusted by the first single-chip microcomputer, the other end of the resistor R3 is connected to the input pin Vp-in of the high-voltage module U1. The model of the high-voltage module U1 can be selected as wisman-mcc.

[0087] In one embodiment, the amplifying circuit includes a preamplifying circuit and a main amplifying circuit connected thereto, and the input end of the preamplifying circuit is connected to the probe 1;

[0088] Among them, the preamplifying circuit includes an amplifier U9 and a resistor R200. One end of the resistor R200 is connected to the feedback pin FB of the amplifier U9, and the other end of the resistor R200 is the input end; the amplifier U9 and the resistor R200 form a current-to-voltage circuit, which can convert a weak current into a voltage. A capacitor C200 can be connected in parallel with the resistor R200, and its main function is to adjust the frequency response, stability, and noise suppression of the circuit.

[0089] The preamplifying circuit further includes a buffer amplifier. The input end of the buffer amplifier is connected to the output pin of the operational amplifier U9, and the output end of the buffer amplifier is connected to the main amplifying circuit. As Figure 3 shown, the buffer amplifier is centered on the amplifier U17B, and also includes a resistor R68, a resistor R71, and a resistor R72. Its amplification factor is 1 times, which can play the role of high-impedance input and low-impedance output for impedance transformation. The output end of the preamplifying circuit is connected to the input end of the main amplifying circuit.

[0090] The main amplifying circuit includes a first-stage amplifying circuit and a second-stage amplifying circuit connected thereto. Among them, the first-stage amplifying circuit includes a first inverting proportional operation circuit. The first inverting proportional operation circuit includes a resistor R58, a resistor R62, and an amplifier U17A. The resistor R58 is 70-90 times that of the resistor R62, and the amplification factor of the first-stage amplifying circuit is 70 to 90 times. Specifically, the resistor R58 can be selected to have a resistance value 82 times that of the resistor R62, as Figure 4As shown, the amplification factor of the first-stage amplifier circuit is 82 times. The second-stage amplifier circuit includes a second inverting proportional operation circuit, which includes a second feedback loop, a resistor R69, and an amplifier U4A. The second feedback loop includes an analog switch chip U3, a second single-chip microcomputer, and eight feedback resistors with different resistance values. One end of each of the eight feedback resistors with different resistance values is connected to eight input pins of the analog switch chip U3, and the other ends of the eight feedback resistors with different resistance values are all connected between the resistor R69 and the inverting input terminal of the amplifier U4A. The common output pin D of the analog switch chip U3 is connected to the output terminal of the amplifier U4A. The second single-chip microcomputer is connected to three address pins A0, A1, and A2 of the analog switch chip U3. The analog switch chip U3 selects a corresponding feedback resistor to access the second feedback loop based on the address determined by the second single-chip microcomputer. Eight feedback resistors with different resistance values can be selected, and the corresponding feedback resistor can be connected to the feedback loop of the amplifier U4A as needed, thereby controlling the gain of the second-stage amplifier circuit. Among them, the eight feedback resistors with different resistance values can be resistor R1, resistor R2, resistor R27, resistor R45, resistor R50, resistor R53, resistor R56, and resistor R59, and their resistance values can be 30, 20, 10, 7, 5, 3, 1, and 0.8 times the resistance value of the resistor R69 respectively. When the corresponding feedback resistor is connected to the feedback loop of the amplifier U4A, it can be amplified by 30, 20, 10, 7, 5, 3, 1, and 0.8 times respectively.

[0091] In one embodiment, the filter circuit includes a 50Hz notch filter circuit and a low-pass filter circuit. The output terminal of the main amplifier circuit is connected to the input terminal of the filter circuit; the output terminal of the second-stage amplifier circuit is connected to the input terminal of the 50Hz notch filter circuit. Among them, the 50Hz notch filter circuit includes a twin-T network circuit, a third feedback circuit, a fourth feedback circuit, a resistor R63, and a resistor R66; among them, the twin-T network circuit includes a series-connected resistor R28 and resistor R41, a series-connected resistor R46 and resistor R47, a series-connected resistor R49 and resistor R65, a parallel-connected capacitor C125 and capacitor C130, a parallel-connected capacitor C129 and C131, and a parallel-connected capacitor C1333 and capacitor C134, which are mainly used to determine the value of the center frequency of the notch filter, and the frequency value to be filtered can be selected by changing these resistors and capacitors.

[0092] The third feedback circuit is composed of an amplifier U2A. The input terminal of the amplifier U2A is connected to the twin-T network circuit, and the output terminal of the third feedback circuit is used as the output of the entire 50Hz notch filter circuit;

[0093] The fourth feedback circuit consists of an amplifier U2B. The output terminal of the fourth feedback circuit is connected to the longitudinal arm of the twin-T network circuit. The amplifier U2B can be used as a voltage follower and forms a voltage feedback circuit with the output terminal, introducing positive feedback into the circuit.

[0094] The resistor R63 is connected between the positive input terminal of the amplifier U2B and the output terminal of the amplifier U2A. One end of the resistor R66 is grounded, and the other end is connected to the positive input terminal of the amplifier U2B.

[0095] In one embodiment, at least one of the resistor R63 and the resistor R66 is a variable resistor.

[0096] By adjusting the ratio of the resistor R63 and the resistor R66, the stopband width and Q value of the filter circuit can be adjusted, the filtering characteristics of the filter circuit can be adjusted, and a better filtering effect can be achieved. The 50Hz notch circuit can filter out the 50Hz power frequency interference in the signal and improve the signal quality.

[0097] In addition, the amplifier U10B is an impedance transformation circuit with a high input impedance and a low output impedance, which improves the signal quality. The amplifier U10A and its attached resistors and capacitors form a low-pass filter and are provided with a connector P5 to filter out high-frequency interference.

[0098] The signal amplification circuit provides a coarse adjustment of the electronic signal gain, can transmit the signal to the subsequent signal processing circuit to complete the processing of brightness, contrast and input signal, and can be adapted to most scanning electron microscope imaging systems.

[0099] The control of the analog switch chip and the high-voltage circuit, the communication with the upper computer, etc. can be realized by a single-chip microcomputer.

[0100] Working principle:

[0101] In the low-vacuum mode of 10Pa - 500Pa of the scanning electron microscope, the traditional ET type secondary electron detector needs to apply a high voltage of 10 - 12KV to the scintillator and the photomultiplier tube, which will cause a discharge phenomenon in the sample chamber. Therefore, a secondary electron detector that can work normally in a low-vacuum environment is required.

[0102] In the present invention, the method of directly detecting secondary electrons is adopted. The high-voltage and electronic signal lines are separated and isolated. A voltage of 200 to 500V is applied to the grid 6. The high voltage will attract the electrons generated by the collision of the sample secondary electrons and gas molecules to the probe 1. The absorbed electrons form a very weak current, and this current flows to the subsequent amplification circuit through the circuit board 2 and independent wiring, and is amplified by the subsequent circuit and converted into a voltage and transmitted to the signal processing circuit of the electron microscope.

[0103] The preamplifier circuit amplifies the current signal of the absorbed electrons and converts it into a voltage signal. The gain of the preamplifier circuit can be adjusted according to the working conditions of the circuit, thereby providing the frequency response of the circuit. The buffer circuit plays the role of impedance transformation between the preamplifier circuit and the main amplifier circuit. This circuit has a large input impedance and a low output impedance, which can effectively improve the signal quality. The main amplifier circuit includes two-stage amplification. The first stage amplifies by 82 times; the second stage amplifier circuit provides gains of 0.8, 1, 3, 5, 7, 10, 20, and 30 times, which can be selected according to needs. The filter circuit filters out high-frequency and 50Hz power frequency interference.

[0104] Working process:

[0105] Gas is input into the sample chamber of the low-vacuum scanning electron microscope to maintain a certain vacuum value. After the electron beam bombards the sample, secondary electrons emitted from the sample surface collide with gas molecules in the sample chamber to generate positive ions and electrons. The generated electrons collide with other gas molecules again, and in this way, the electrons and ions increase geometrically. The positive ions in the sample chamber will neutralize the negative charges on the sample surface, reducing the charge effect. The electrons are attracted by the bias voltage of +200 to +500V on the grid 6 and fall onto the probe 1. The attracted secondary electrons form a weak current, which successively flows through the probe 1 and the wire and reaches the charge amplifier.

[0106] The present invention is a secondary electron detector used in the low-vacuum 10Pa - 500Pa mode of a scanning electron microscope, including the following aspects:

[0107] 1. The separation and isolation of the electron signal and the high voltage of the low-vacuum secondary electron detector improve the signal-to-noise ratio;

[0108] 2. The probe 1 has the ability to adjust the distance, and the distance between the probe 1 and the objective lens can be adjusted;

[0109] 3. The amplifier circuit uses an analog switch chip to select different gain circuits, enabling the circuit to have a variety of amplification multiples. The resistance on the feedback circuit can be adjusted according to needs, and the amplification multiple can be set arbitrarily;

[0110] 4. A notch filter is used to filter out 50Hz power frequency interference to improve the imaging quality.

[0111] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A low vacuum secondary electron detector with high signal-to-noise ratio for a scanning electron microscope, characterized in that: include: Circuit board (2); A probe (1) and a grid (6), wherein the probe (1) and the grid (6) are both connected to ends of a circuit board (2), and the grid (6) covers the outside of the probe (1); A circuit board socket (3) is arranged on a side of the circuit board (2) away from the probe (1), and the circuit board (2) is plugged into the circuit board socket (3); A shielding plate (4) is located on a side of the circuit board socket (3) away from the circuit board (2), and the side of the shielding plate (4) away from the circuit board socket (3) is connected to a side wall of a sample chamber of a scanning electron microscope; Two adjusting screws (5) pass through the circuit board socket (3) and the shielding plate (4); It also includes a high-voltage module, a high-voltage circuit, an amplifying circuit, a filtering circuit and a signal processing circuit, wherein the high-voltage circuit is suitable for adjusting the output voltage of the high-voltage module to the probe (1); The amplifying circuit, filtering circuit and signal processing circuit are connected in sequence; The high-voltage module is connected to the grid (6), and the amplifying circuit is connected to the probe (1); The amplifier circuit comprises a preamplifier circuit and a main amplifier circuit, and the input end of the preamplifier circuit is connected to the probe (1); Wherein, the preamplifier circuit includes an amplifier U9 and a resistor R200, one end of the resistor R200 is connected to the feedback pin of the amplifier U9, and the other end of the resistor R200 is an input end; The preamplifier circuit also includes a buffer amplifier, the input end of the buffer amplifier is connected to the output pin of the operational amplifier U9; The main amplifier circuit includes a first-stage amplifier circuit and a second-stage amplifier circuit connected thereto, wherein the first-stage amplifier circuit includes a first inverting proportional operation circuit, the first inverting proportional operation circuit includes a resistor R58, a resistor R62 and an amplifier U17A, the resistor R58 is 70-90 times the resistor R62, the second-stage amplifier circuit includes a second inverting proportional operation circuit, the second inverting proportional operation circuit includes a second feedback loop, a resistor R69 and an amplifier U4A, the second feedback loop includes an analog switch chip U3, a second single-chip microcomputer and eight feedback resistors with different resistance values, one end of the eight feedback resistors with different resistance values ​​are respectively connected to the eight input pins of the analog switch chip U3, the other ends of the eight feedback resistors with different resistance values ​​are all connected between the resistor R69 and the inverting input end of the amplifier U4A, the common output pin of the analog switch chip U3 is connected to the output end of the amplifier U4A, the second single-chip microcomputer is connected to the three address pins of the analog switch chip U3, and the analog switch chip U3 selects the corresponding feedback resistor to connect to the second feedback loop based on the address determined by the second single-chip microcomputer.

2. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 1, characterized in that: The probe (1) is rectangular, with the upper edge and the lower edge both being long sides, and the middle portion of the probe (1) is connected to the end side of the circuit board (2).

3. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 1, characterized in that: Also includes: The adapter (7) is arranged between the circuit board socket (3) and the shielding plate (4), and the adjusting screw (5) passes through the adapter (7).

4. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 3, characterized in that: Also includes: There are two adjusting springs (8), both of which are abutted between the adapter (7) and the shielding plate (4) and are respectively mounted on the adjusting screw (5); Wherein, the adjusting spring (8) enables the adapter (7) to abut against the circuit board socket (3).

5. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 1, characterized in that: Also includes: A through-sealing plate (9) is arranged on the outer side wall of the sample chamber of the scanning electron microscope, and a side of the shielding plate (4) away from the circuit board socket (3) is connected to the through-sealing plate (9); A sealing ring is arranged on a side wall of the through-sealing plate (9) close to the shielding plate (4), and the sealing ring is located between the through-sealing plate (9) and the outer wall of the sample chamber.

6. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 1, characterized in that: The output voltage of the high voltage module is adjusted through an adjustable resistor; The two fixed pins of the adjustable resistor are respectively connected to the reference voltage pin and the ground pin of the high voltage module, and the fixed pin of the adjustable resistor is connected to the input pin of the high voltage module.

7. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 1, characterized in that: The output voltage of the high voltage module is adjusted by the first single chip microcomputer; The output terminal of the first single-chip microcomputer is connected to the input pin of the high-voltage module.

8. The low vacuum secondary electron detector with high signal-to-noise ratio for a scanning electron microscope according to claim 1, characterized in that: The filter circuit includes a 50Hz trap circuit and a low-pass filter circuit, and the output end of the main amplifier circuit is connected to the input end of the filter circuit; Wherein, the 50Hz trap circuit includes a double-T network circuit, a third feedback circuit, a fourth feedback circuit, a resistor R63 and a resistor R66; The third feedback circuit is composed of an amplifier U2A, the input end of the amplifier U2A is connected to a double-T network circuit and the output end of the third feedback circuit serves as the output of the entire 50Hz trap circuit; The fourth feedback circuit is composed of an amplifier U2B, and the output end of the fourth feedback circuit is connected to the longitudinal arm of the double-T network circuit; The resistor R63 is connected between the non-inverting input terminal of the amplifier U2B and the output terminal of the amplifier U2A. One end of the resistor R66 is grounded, and the other end is connected to the non-inverting input terminal of the amplifier U2B.

9. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 8, characterized in that: At least one of the resistor R63 and the resistor R66 is an adjustable resistor.

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