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

Abstract

The invention relates to the technical field of scanning electron microscopes, in particular to a high-signal-to-noise-ratio low-vacuum secondary electron detector for a scanning electron microscope. Comprising a circuit board; the probe and the grid mesh are both connected to the end part of the circuit board, and the grid mesh covers the outer side of the probe; the circuit board socket is arranged on one side, far away from the probe, of the circuit board, and the circuit board is inserted into the circuit board socket; the shielding plate is located on one side, far away from the circuit board, of the circuit board socket, and one side, far away from the circuit board socket, of the shielding plate is connected to the side wall of a sample chamber of the scanning electron microscope. The gap between the circuit board socket and the baffle plate can be changed by screwing the adjusting screw, the circuit board socket is connected with the probe, and the baffle plate is connected with the sample chamber of the scanning electron microscope, so that the distance between the probe and the side wall of the sample chamber can be adjusted, the distance between the probe and the objective lens can be changed, and the distance between the probe and the objective lens can be adjusted.

Description

Technical Field

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

[0002] When imaging with a low vacuum scanning electron microscope, 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 set close to the objective lens in the sample chamber. When the high voltage on the probe reaches a critical value, a discharge phenomenon will occur between the objective lens and the probe. The occurrence of the discharge scene 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. It cannot be flexibly adjusted when facing different samples or different observation effects. Summary of the invention

[0004] In view of this, the present invention provides a low-vacuum secondary electron detector with a high signal-to-noise ratio for a scanning electron microscope to solve the problem of being unable to perform flexible adjustments 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, comprising:

[0006] Circuit boards;

[0007] A probe and a coral net, wherein the probe and the coral net are both connected to the ends of the circuit board, and the coral net is covered on the outside of the probe;

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

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

[0010] There are two adjusting screws, which pass through the circuit board socket and the shielding plate;

[0011] 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;

[0012] The amplifying circuit, filtering circuit and signal processing circuit are connected in sequence;

[0013] The high voltage module is connected to the network, and the amplifying circuit is connected to the probe.

[0014] 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. The distance between the probe and the objective lens can be adjusted, and the high voltage applied to the network can be further adjusted, so that flexible adjustments can be made when facing different samples or different observation effects, such as observing samples of different sizes or for different surface observation purposes.

[0015] In an optional embodiment, the probe is rectangular, the upper edge and the lower edge are both long sides, and the middle part of the probe is connected to the end side of the circuit board, which can increase the receiving area for electrons and maximize the received electronic signals.

[0016] In an optional implementation, it also includes:

[0017] The coral net is fixed on one side of the circuit board close to the probe, and the coral net covers the outer side of the probe.

[0018] In an optional implementation, it also includes:

[0019] The adapter is arranged between the circuit board socket and the shielding plate, and the adjusting screw passes through the adapter.

[0020] In an optional implementation, it also includes:

[0021] There are two adjusting springs, both of which are abutted between the adapter and the shielding plate and are respectively mounted on the adjusting screws;

[0022] Wherein, the adjusting spring enables the adapter to abut against the circuit board socket.

[0023] The adjusting spring makes the adapter always abut against the circuit board socket. When the adjusting screw is turned, the expansion and contraction of the adjusting spring can adjust the gap between the shielding plate and the circuit board socket, thereby changing the distance between the probe and the objective lens.

[0024] In an optional implementation, it also includes:

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

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

[0027] In an optional implementation, the output voltage of the high voltage module is adjusted by an adjustable resistor;

[0028] 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.

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

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

[0031] In an optional implementation, 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 weak current into voltage.

[0033] 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, 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 to perform impedance transformation.

[0034] 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. Eight feedback resistors with different resistance values ​​can be selected, and the corresponding resistors can be connected to the feedback loop of the amplifier U4A as needed to control the gain of the second-stage amplifier circuit.

[0035] In an optional implementation, the filtering circuit includes a 50 Hz trap circuit and a low-pass filtering circuit, and the output end of the main amplifier circuit is connected to the input end of the filtering circuit;

[0036] 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;

[0037] 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;

[0038] 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;

[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 optional implementation, at least one of the resistor R63 and the resistor R66 is an adjustable resistor.

[0041] By adjusting the ratio of resistor R63 and resistor R66, the stopband width and Q value of the filter circuit can be adjusted, and the filter characteristics of the filter circuit can be adjusted to 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 implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 It is a schematic diagram of the structure of an embodiment of the present invention;

[0044] Figure 2 This is a schematic diagram of a high voltage circuit according to an embodiment of the present invention;

[0045] Figure 3 Schematic diagram of a preamplifier circuit according to an embodiment of the present invention;

[0046] Figure 4 This is a schematic diagram of a main amplifier circuit according to an embodiment of the present invention;

[0047] Figure 5 This is a schematic diagram of a 50 Hz trap circuit according to an embodiment of the present invention;

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

[0049] Description of reference numerals:

[0050] 1. Probe; 2. Circuit board; 3. Circuit board socket; 4. Shielding plate; 5. Adjustment screw; 6. Coral net; 7. Adapter; 8. Adjustment spring; 9. Through-sealing plate. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are 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 observed sample 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 nuclei to generate various information such as secondary electrons and backscattered electrons. This information contains signals such as the appearance and composition of the sample. The detector collects and amplifies the generated secondary electrons or backscattered electrons, and generates a two-dimensional image corresponding to the position scanned by the electron beam, namely the secondary electron image or the backscattered electron image. The resolution of the scanning electron microscope can reach 1nm. It 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.

[0053] The electron gun and ET detector of a traditional scanning electron microscope need to work in a high vacuum environment. In a high vacuum environment, the sample must have good electrical conductivity to obtain high-quality imaging. However, some samples have poor electrical conductivity, and in this case, the sample will experience a charging effect during the imaging process. Traditional electron microscopes can spray gold on the sample surface to improve conductivity. Low-vacuum scanning electron microscopes do not need to spray gold to eliminate the charging effect, which is convenient and easy to operate. At the same time, no processing is done on the sample 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 produce positive ions and electrons. The generated electrons collide with other gas molecules again, and the electrons and ions increase geometrically. The positive ions in the sample chamber will neutralize the negative charge on the sample surface and reduce the charge effect. The detector can detect the generated electrons, collect and amplify them, 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 hundreds of volts needs to be added to the probe of the detector. This high voltage is usually applied directly 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 detector's amplifier circuit to also amplify the noise transmitted by the high voltage. At this time, the signal-to-noise ratio of the signal is seriously affected, resulting in damage to the imaging quality. The probe will be set close to the objective lens. When the high voltage on the probe reaches a certain value, there is a critical value (this critical value is not a fixed value, and there is a small change of a few volts). When the voltage exceeds this critical value, discharge will occur. The distance between the probe and the objective lens will affect the critical value of the high voltage when the discharge occurs. The normal use range of high voltage is usually between 200V and 500V.

[0055] In the present invention, the high voltage is separated from the collected electronic signal, and the amplification circuit of the detector only amplifies the electronic signal, thereby 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 strength 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 strength of the electronic signal.

[0056] Combine the following Figures 1 to 6 , describing an embodiment of the present invention.

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

[0058] Circuit board 2;

[0059] The probe 1 and the coral net 6 are both connected to the ends of the circuit board 2, and the coral net 6 covers the outside of the probe 1; the probe 1 can be connected to the circuit board 2 by welding.

[0060] 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;

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

[0062] There are two adjusting screws 5, which penetrate the circuit board socket 3 and the shielding plate 4; nuts are arranged on the end sides of the adjusting screws 5, and the circuit board socket 3 and the shielding plate 4 are arranged between the adjusting screws 5 and the nuts screwed thereto, and the gap between the circuit board socket 3 and the shielding plate 4 can be adjusted by screwing the adjusting screws 5 or the nuts, so that the circuit board socket 3 and the shielding plate 4 have an adjustable gap in the axial direction of the adjusting screws 5.

[0063] The probe 1, the circuit board 2, the circuit board socket 3, the shielding plate 4, the adjusting screw 5 and the mesh 6 are all arranged in the sample chamber of the scanning electron microscope.

[0064] 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;

[0065] The amplifying circuit, filtering circuit and signal processing circuit are connected in sequence;

[0066] The high-voltage module is connected to the network 6, and the amplifying circuit is connected to the probe 1. It should be noted that the high-voltage module, the high-voltage circuit, the amplifying circuit, the filtering circuit and the signal processing circuit can be arranged on the outside of the sample chamber, and the high-voltage module is connected to the network 6 and the amplifying circuit is connected to the probe 1 through independent wiring. Of course, the independent wiring can be connected to the circuit board 2 respectively, and the circuit board 2 is provided with independent connecting circuits accordingly, and then the connection with the network 6 and the probe 1 is completed respectively. The connection between the independent wiring and the connecting circuit on the circuit board 2 can be achieved through the circuit board socket 3. In the present application, high voltage is applied to the network 6, and the electronic signal is collected through the probe 1, so that the electronic signal is separated and isolated from the high voltage, and the amplifying circuit connected to the probe 1 only amplifies the electronic signal, thereby ensuring the signal-to-noise ratio of the signal and high-quality imaging.

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

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

[0069] In one embodiment, the probe 1 is rectangular, with the upper and lower edges being long sides, and the middle portion of the probe 1 is connected to the end side of the circuit board 2. This can increase the receiving area for electrons and maximize the received electronic signals.

[0070] In one embodiment, it further includes:

[0071] 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 .

[0072] In one embodiment, it further includes:

[0073] 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 screws 5;

[0074] The adjusting spring 8 enables the adapter 7 to always abut against the circuit board socket 3 .

[0075] The adjusting spring 8 makes the adapter 7 always abut against the circuit board socket 3. When the adjusting screw 5 is turned, the expansion and contraction of the adjusting spring 8 can adjust the gap between the shielding plate 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 arranged on the outer wall of the sample chamber of the scanning electron microscope, and the side of the shielding plate 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-portion can increase the sealing structure.

[0078] The sealing ring is arranged on the 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.

[0079] Copper can be laid around the preamplifier circuit and structural parts can be used to enclose part of the preamplifier circuit, and the wiring within the preamplifier circuit is isolated by ground wires, and dense vias are drilled on the ground wires to reduce interference; it should be noted that the signal processing circuit is a conventional circuit, and the improvements of the present invention do not include improvements to the signal processing circuit, so they will not be repeated.

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

[0081] 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.

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

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

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

[0085] The high-voltage circuit also 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 amplifier circuit includes a preamplifier circuit and a main amplifier circuit connected thereto, and the input end of the preamplifier circuit is connected to the probe 1;

[0088] The preamplifier 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 weak current into voltage. A capacitor C200 can be connected in parallel to the resistor R200, which mainly adjusts the frequency response, stability and noise suppression of the circuit.

[0089] 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, and the output end of the buffer amplifier is connected to the main amplifier circuit. Figure 3 As shown, the buffer amplifier is based on the amplifier U17B, and also includes resistors R68, R71 and R72. Its amplification factor is 1, which can play the role of high impedance input and low impedance output to perform impedance transformation. The output end of the preamplifier circuit is connected to the input end of the main amplifier circuit.

[0090] 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, and the amplification factor of the first-stage amplifier circuit is 70 to 90 times. Specifically, the resistor R58 can be selected to be 82 times the resistance value of the resistor R62. 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, 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, and 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 D 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 A0, A1 and A2 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. The eight feedback resistors with different resistance values ​​can be selected, and the corresponding feedback resistors can be connected to the feedback loop of the amplifier U4A as needed, so as to control 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 of the resistance value of resistor R69 respectively. When the corresponding feedback resistors are selected to be connected to the feedback loop of amplifier U4A, the resistance can be amplified by 30, 20, 10, 7, 5, 3, 1 and 0.8 times respectively.

[0091] In one embodiment, the filtering circuit includes a 50Hz trap circuit and a low-pass filtering circuit, the output end of the main amplifier circuit is connected to the input end of the filtering circuit; the output end of the second-stage amplifier circuit is connected to the input end of the 50Hz trap 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; wherein the double-T network circuit includes a resistor R28 and a resistor R41 connected in series, a resistor R46 and a resistor R47 connected in series, a resistor R49 and a resistor R65 connected in series, a capacitor C125 and a capacitor C130 connected in parallel, a capacitor C129 and a capacitor C131 connected in parallel, and a capacitor C1333 and a capacitor C134 connected in parallel, which are mainly used to determine the value of the center frequency of the trap, and the frequency value to be filtered out can be selected by changing these resistors and capacitors.

[0092] 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;

[0093] 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 amplifier U2B can be used as a voltage follower to form a voltage feedback circuit with the output end, introducing positive feedback in the circuit.

[0094] 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.

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

[0096] By adjusting the ratio of resistor R63 and resistor R66, the stopband width and Q value of the filter circuit can be adjusted, and the filter characteristics of the filter circuit can be adjusted to 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.

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

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

[0099] The control of analog switch chips and high-voltage circuits, communication with the host computer, etc. can be achieved through the microcontroller.

[0100] Working principle:

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

[0102] The present invention adopts a method of directly detecting secondary electrons. The high voltage and electronic signal lines are separated and isolated. A voltage of 200 to 500V is applied to the network 6. The high voltage will attract the electrons generated by the collision of the sample secondary electrons with the gas molecules to the probe 1. The absorbed electrons form a very weak current, which flows to the subsequent amplifier circuit through the circuit board 2 and independent wiring, and is amplified by the subsequent circuit and converted into voltage for transmission 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 in the preamplifier circuit and the main amplifier circuit. The circuit has a large input impedance and a low output impedance, which can effectively improve the signal quality. The main amplifier circuit includes two stages of amplification, the first stage amplifies 82 times; the second stage amplifier circuit provides 0.8, 1, 3, 5, 7, 10, 20 and 30 times of gain, which can be selected as needed. 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, the secondary electrons emitted from the sample surface collide with the gas molecules in the sample chamber to produce positive ions and electrons. The generated electrons collide with other gas molecules again, and the electrons and ions increase geometrically in this way. The positive ions in the sample chamber will neutralize the negative charge on the sample surface and reduce the charge effect. The electrons are attracted by the bias voltage of +200 to +500V on the grid 6 and fall on the probe 1. The attracted secondary electrons form a weak current, which flows through the probe 1 and the wiring, and then flows to the charge amplifier.

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

[0107] 1. The electronic signal of the low vacuum secondary electron detector is separated from the high voltage to improve the signal-to-noise ratio;

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

[0109] 3. The amplifier circuit uses an analog switch chip to select different gain circuits, so that the circuit has a variety of amplification factors. The resistance on the feedback circuit can be adjusted as needed, and the amplification factor can be set arbitrarily;

[0110] 4. Use a notch filter to filter out 50Hz power frequency interference and improve imaging quality.

[0111] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations are all 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 coral net (6), wherein the probe (1) and the coral net (6) are both connected to the end of the circuit board (2), and the coral net (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 coral network (6), and the amplifying circuit is connected to the probe (1).

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 scanning electron microscope according to claim 1, characterized in that: 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.

9. The low vacuum secondary electron detector with high signal-to-noise ratio for scanning electron microscope according to claim 8, 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.

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

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