Methods and systems for plasma-assisted low-vacuum charged particle microscopy

Abstract

Methods and systems for plasma-assisted low-vacuum charged particle microscopy. The present disclosure provides various methods and systems for imaging a sample with a charged particle beam in low vacuum. A magnetic field is provided in a detection region of a detector. A gas and a plasma are provided in the detection region while detecting charged particles emitted from the sample. A sample image is formed based on the detected charged particles.

Description

Technical Field

[0001] The present invention generally relates to methods and systems for low-vacuum charged particle microscopy, and more particularly, to generating high-quality sample images based on secondary electrons detected in a gaseous environment. Background Art

[0002] Low-vacuum scanning electron microscopy (SEM) or environmental SEM allows non-conductive samples to be imaged in a gaseous environment using a gaseous secondary electron detector (GSED). Low-vacuum SEM provides a method for stabilizing sample charging and performing dynamic chemical experiments in an SEM setting. However, due to differences in detector design and increased electron beam scattering in a gaseous environment, the resolution and contrast of low-vacuum SEM are not as good as those of high-vacuum SEM.

[0003] A method for improving the image quality in low-vacuum SEM is introduced in US 6972412. Therein, an electric field and a magnetic field are provided in a detection space above the sample. The electromagnetic fields cause secondary electrons emitted from the sample to travel in a helical trajectory in the detection space before being captured by the anode of the detector. The long movement trajectory of the secondary electrons in the detection space increases the gas ionization degree and thus increases the detector gain. Another method for increasing the detector gain is introduced in US 7541580. Therein, the detection space extends above a pressure-limiting aperture to further increase the gas ionization degree. However, due to electron beam scattering in a gaseous environment, the signal-to-noise ratio of the SEM image is still very low. Summary of the Invention

[0004] In one embodiment, a method for imaging a sample includes providing a gas in a detection space; providing a magnetic field in the detection space; irradiating the sample with a charged particle beam while providing a plasma in the detection space; detecting charged particles emitted from the sample into the detection space in response to the irradiation; and forming an image based on the detected charged particles. By providing a plasma into the detection space, the gaseous secondary electron detector can perform imaging at a lower chamber pressure. The image quality is improved by increased detector gain and reduced beam scattering.

[0005] It should be understood that the above summary is provided to introduce in a simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is uniquely defined by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to embodiments that solve any disadvantages noted above or in any part of this disclosure. Brief Description of the Drawings

[0006] Figure 1 is a schematic diagram of a charged particle microscopy system for low-vacuum imaging.

[0007] Figure 2 shows Figure 1 a gaseous secondary electron detector (GSED) in a microscopy system of

[0008] Figure 3 is a flowchart for performing plasma-assisted low-vacuum imaging.

[0009] Figure 4A and Figure 4B show scanning electron microscopy (SEM) images acquired with and without plasma in the sample chamber, respectively.

[0010] Figure 5 is a graph showing the effect of plasma on the GSED output.

[0011] Figure 6A , Figure 6B and Figure 6C are SEM images acquired at different hydrogen ion partial pressures.

[0012] Figure 7 is a graph showing the effect of plasma on the GSED output and the specimen current.

[0013] Figure 8A and Figure 8B are SEM images acquired with and without plasma in a sample chamber having a low chamber pressure.

[0014] Figure 8C and Figure 8D are respectively Figure 8A and Figure 8B normalized SEM images of

[0015] Figure 9 is from Figure 8A and Figure 8B the power spectra obtained from the images in

[0016] Like reference numerals refer to corresponding parts throughout the drawings. DETAILED DESCRIPTION

[0017] The following description relates to systems and methods for performing charged particle microscopy in a gaseous environment using a low-vacuum microscope, such as a low-vacuum scanning electron microscope (SEM) of Figure 1 A sample located in a sample chamber is irradiated with a charged particle beam formed in a particle column coupled to the sample chamber. The vacuum in the sample chamber is lower than the vacuum in the particle column. For example, the column, particularly the electron source, is pumped to a high vacuum, such as above 10 -5Torr, for generating an electron beam. The sample chamber can be evacuated to a vacuum level between 0.01 Torr and 50 Torr. The vacuum difference from the sample chamber to the electron source inside the column can be achieved via pumping in different stages and multiple pressure limiting apertures (PLAs) positioned along the column.

[0018] The low vacuum microscopy system can operate according to Figure 3 the method for plasma-assisted low vacuum microscopy. During plasma-assisted low vacuum microscopy, a charged particle beam irradiates the sample. In response to the radiation, the charged particles emitted from the sample travel to the detection space and are detected by a detector located between the particle light column and the sample. During sample irradiation and charged particle detection, a gas and plasma are provided in the detection space. In some examples, the detector can be the Figure 2 gas secondary electron detector (GSED) shown in

[0019] for detecting secondary electrons emitted from the sample. The detection space can be above the sample surface and extend into the column. The sample in the sample chamber is immersed in a magnetic field. The magnetic field can be generated by a magnetic immersion lens located inside the particle light column. Alternatively, the magnetic field can be generated by a source different from the lens in the column. For example, the magnetic field can be generated by the detector. The emitted charged particles are amplified (i.e., cascaded amplification) as they travel in the detection space by colliding with gas molecules and generating more charged particles. The trajectory of the charged particles in the detection space is defined by the magnetic and electric fields generated by the detector. Figures 4A - 4B By providing plasma to the detection space, SEM images with improved signal-to-noise ratio (SNR) and improved contrast can be obtained, as shown in Figures 8A - 8D and Figure 9 These improvements are maximum at low pressures, and thus the present invention enables vacuum microscope operation at lower vacuum levels than conventional low vacuum SEM imaging. Conventional low vacuum SEM performs imaging at a sample chamber vacuum between 0.1 Torr and 50 Torr, and more typically between 1 Torr and 10 Torr. On the other hand, plasma-assisted low vacuum SEM can achieve operation at a sample chamber pressure as low as 0.01 Torr, as shown in Figure 5 and Figure 7As shown. The introduced plasma increases the charge mitigation effect of the low-vacuum SEM at lower gas pressures, which enables high-quality SEM imaging of non-conductive samples at higher vacuum levels. The reduced gas pressure in the sample chamber also reduces the scattering of the incident beam within the particle optical column and within the detection space, thereby increasing the SNR of the image. Additionally, with a higher sample chamber vacuum, the aperture of the PLA (the last PLA in the particle optical column) between the particle optical column and the sample chamber may be larger. In some embodiments, no PLA is required between the column and the sample chamber. The increased PLA aperture, or the absence of the last PLA in the column, can increase the field of view (FOV) of SEM imaging, particularly for low-magnification imaging.

[0020] The plasma can be introduced into the sample chamber by ionizing the gas flowing into the sample chamber or by a plasma source. For example, the gas flowing into the sample chamber can be one or more of water, atmospheric gas, and inert gases such as argon or helium. As Figures 6A - 6C shown, the plasma with small molecular weight ions can also reduce the noise in the SEM image.

[0021] Turning to Figure 1 , a schematic depiction of a charged particle microscope 100 is shown. The charged particle microscope is a non-transmission type microscope, such as an SEM. The microscope 100 includes a particle optical column 110 mounted on a sample chamber 120. The particle optical column 110 generates an electron beam along a main axis 110. Within the particle optical column 110, the electrons generated by an electron source 102 are modified by a combined lens system 103 before being focused onto a sample 124 by a lens system 104. The incident beam 106 can be scanned over the sample 124 by operating a scanning coil 105. A plurality of pumps (not shown) are coupled to the column 110 to provide differential pumping along the beam path within the column. For example, the lower the vacuum, the closer to the sample chamber, while the higher the vacuum, the closer to the electron source 102. The lens system 104 can include a magnetic immersion lens for focusing the electron beam onto the sample. The magnetic immersion lens can be the last lens of the column 110. The sample 124 is immersed in the magnetic field generated by the magnetic immersion lens. Optionally, a pressure-limiting aperture (PLA) can be integrated into the last lens (such as the magnetic immersion lens) of the optical column before the electron beam irradiates the sample. The PLA limits the amount of gas entering the column from the sample chamber, thus maintaining a higher vacuum in the column compared to the sample chamber.

[0022] The sample 124 is held in the sample chamber 120 by a sample stage 125. A gas source 127 is coupled to the sample chamber for providing one or more gas species into the sample chamber. A plasma source 126 is coupled to the sample chamber for providing one or more plasma species to the sample chamber. The plasma source 126 can be a plasma cleaner. The plasma supplied to the sample chamber can be generated by ionizing the same gas provided by the gas source 127 via the plasma cleaner.

[0023] The microscope 100 includes a plurality of detectors for detecting emissions from the sample 124 in response to the incidence of electrons generated from the electron source 102. The detector 123 can be a scintillator / photomultiplier or an EDS (Energy Dispersive X-ray Spectroscopy) combined module. The detector 128 is located between the column 110 and the sample 124 and is used to detect backscattered or secondary electrons emitted from the sample. The detector 128 can be a segmented electron detector having a plurality of independent detection segments (e.g., quadrants) arranged around a central hole that allows the electron beam to pass through. The detector 128 can be a GSED for detecting secondary electrons traveling into the detection space above the sample. The detailed configuration of an exemplary GSED detector is shown in Figure 2 which is shown. The signal received from the detector 128 is amplified by the amplifier 129 and transmitted to the controller 130 to form a sample image. The sample stage 125 can be grounded to the sample chamber 120 through the connection part 141. The specimen current can be measured by a picoammeter 142 connected to the sample stage via the connection part 141. When scanning the sample with a charged particle beam, the picoammeter 142 sends the measured specimen current to the controller 130. A sample image can be generated based on the specimen current.

[0024] The controller 130 includes a processor 131 and a non-transitory memory 132 for storing instructions. The controller is connected to the input / output device 107 for receiving operator instructions and / or outputting information including images, measurement values, and operation parameters. By executing the instructions stored in the memory 132 with the processor 131, the controller 130 operates various components of the microscope to image or process the sample. For example, the electron beam energy can be adjusted by adjusting the power supply to the source 102. Imaging parameters including magnification, beam position, imaging area, and beam dwell time can be adjusted by adjusting one or more of the lens systems 103, 104, and the scanning coil 105. The controller also forms an image of the sample based on the signals received from various detectors (123, 128, and 142) in the microscope. The controller can store the image in the memory 132 and / or output the image via the input / output device 107.

[0025] Although the SEM system is described by way of example, it should be understood that the microscope can be configured to image the sample in a transmission mode, such as in a Transmission Electron Microscopy (TEM) system. For example, a GSED can be used to obtain scattered electrons from a thin sample in a transmission mode in the presence of a plasma. In other examples, plasma-assisted imaging can be performed in a dual-beam microscopy system. The current discussion of SEM imaging is provided only as an example of a suitable imaging modality.

[0026] Figure 2 A microscopy system for obtaining backscattered or secondary electrons emitted from a sample is shown (such as Figure 1An example configuration of the GSED 200 in a microscope 100). A part of the column 110 connected to the sample chamber 120 is in Figure 2 shown. The electron beam 201 travels along the column 110 and irradiates the sample 124 located on the sample stage 125. The electron lens 202 is a magnetic immersion lens that provides a magnetic field in the detection space 210. The magnetic field strength can be 0.01 - 0.1 Tesla. The electron lens 202 is the last lens of the column 110 through which the electron beam passes before hitting the sample 124. The sample 124 is immersed in the magnetic field generated by the last lens. The working distance 211 is defined by the distance between the column 110 and the sample surface. In a low-vacuum SEM, the working distance is preferably small to reduce the scattering of the incident beam.

[0027] The electrode 205 is attached to the pole-piece support 204 of the support pole-piece 203. In one example, as shown by the dashed line, the electrode 205 may have an opening smaller than the opening 212 of the distal end of the column connected to the sample chamber. By having a smaller opening, the electrode 205 serves as a PLA to prevent the gas in the sample chamber from entering the column and to maintain a high vacuum inside the column. In another example, there is no PLA at the end of the column connected to the sample chamber. The distal end of the column has the same vacuum as the sample chamber. For example, the electrode 205 may have the same opening as the distal end of the column 110, as shown by the solid line. Alternatively, the electrode 205 can be omitted. Since the gas pressure in the plasma-assisted low-vacuum SEM is lower than that in the conventional low-vacuum SEM, high-quality images can be obtained even if the distal end of the column and the sample chamber have the same vacuum.

[0028] The GSED 200 is located between the column 110 and the sample 124. The GSED includes an anode 206 and an ion trap 207, which are separated from each other and separated from the electrode 205 by an insulating spacer 208. Electrons emitted from the sample in response to charged particle irradiation, such as secondary electrons, are amplified in the detection space before being detected by the anode 206. The detection space is defined by the position and structure of the GSED. The detection space is limited by the central hole of the GSED. In one example, if the PLA is located between the column and the sample chamber, the detection space extends from the surface of the sample 124 to the PLA. In another example, due to the increased opening of the PLA between the column 110 and the detector 200 in plasma-assisted low-vacuum microscopy, the detection space (shaded area 210) extends from the sample surface into the column, opposite to the direction of the electron beam 201. In this configuration, the PLA can optionally be placed upstream of the opening 212. The height (in the z direction) of the detection space 210 can be greater than the working distance 211. The height of the detection space can also be determined by the provided magnetic and electric fields. The magnetic field can be provided by a magnetic immersion lens or the GSED. The electric field is provided by the GSED. The increase in the volume of the detection space increases the cascade amplification of secondary electrons, thereby increasing the intensity of the received signal. Gas and plasma can be delivered to the detection space 210 via the gap 220 between the detector 200 and the sample stage 125. The detection region can have a vacuum degree between 0.01 Torr and 10 Torr.

[0029] In one example, the electrode 205 is electrically isolated from the pole piece 203 and maintained at a potential between -1000 V and 2000 V, where the ground potential is the typical potential. The anode 206 is biased in the range of 0 V to 2000 V, most preferably at a potential of about 400 V. The ion trap 207 is biased in the range of -1000 V to 2000 V, most preferably at a potential. The sample stage 125 is biased in the range of -1000 V to 1000 V, most preferably at a potential.

[0030] In another example, the sample image can be formed based on ions or photons generated due to gas cascade amplification in the detection region. The ions or photons can be detected by one or more detectors different from the detector 200.

[0031] Figure 3 A method 300 for plasma-assisted low-vacuum SEM imaging is shown. The SEM image is formed based on secondary electrons acquired within a detection space defined by a detector (such as Figure 2 the GSED 200). During the acquisition of secondary electrons, gas and plasma are simultaneously supplied to the detection space. Figure 3 It can be implemented by executing instructions stored in the non-transitory memory of a controller (such as Figure 1 the controller 130).

[0032] At 302, prepare the microscope for SEM imaging. The preparation may include loading a sample into the sample chamber and pumping the various parts of the microscope to an appropriate vacuum level. The step may also include setting the operating parameters of the microscope. The operating parameters may include column parameters such as beam energy, magnification, and field of view. The operating parameters may also include sample chamber parameters such as gas type, gas pressure, plasma type, and plasma pressure. The chamber parameters may be determined based on the type of sample. For example, the gas should not react with the sample. The gas may include one or more gas species. For example, the gas may be a mixture of Ar and H2. In another example, the gas may be air. The plasma may include one or more ion types. In one example, the plasma includes at least one gas molecular species with a molar mass less than 5. In another example, the molar mass of at least one gas molecular species is one.

[0033] At 304, provide gas in the detection space. In one example, the gas may be delivered into the sample chamber. The gas flows into the detection space through the gap between the GSED detector and the sample stage. In another example, the gas is directly delivered to the detection space via a nozzle connected to a gas source. The gas pressure in the detection space may be from 0.005 to 0.5 Torr. In some examples, the gas pressure is below 0.05 Torr.

[0034] At 306, provide plasma to the detection space. In one example, the plasma may be generated by ionizing the gas supplied to the sample chamber at 304. The gas may be ionized by a plasma generator connected to the sample chamber. The power supplied to the plasma generator may depend on the gas type. For example, the power supplied to the plasma cleaner is the minimum power level to maintain the plasma. The plasma generated by the plasma generator in the sample chamber may enter the detection space through the gap between the GSED detector and the sample stage. In another example, the plasma may be supplied to the detection space via a nozzle connected to the plasma generator. The plasma may include chemical elements different from the gas.

[0035] At 308, direct a charged particle beam (such as an electron beam) to the sample by operating the charged particle source and the lenses in the charged particle column. The charged particle beam irradiates the sample. The charged particle beam may scan the region of interest on the surface of the sample according to a predetermined pattern.

[0036] At 310, charged particles emitted from the sample in response to irradiation are detected by the GSED. For example, secondary electrons emitted and traveling in the detection space under the influence of a magnetic field and an electric field are cascaded and amplified by colliding with gas molecules and generating more electrons. Electrons in the detection space are collected by the anode of the GSED. Plasma in the detection space can suppress gas breakdown and increase the gain of the GSED. While scanning the sample with an electron beam, the emitted secondary electrons are detected.

[0037] At 312, after the charged particle beam is scanned over the region of interest (ROI) on the sample surface, an SEM image of the ROI can be formed based on the detector output.

[0038] At 314, the quality of the image formed at 312 is evaluated. The image quality can include one or more of image resolution, SNR, and contrast, etc. The image quality can be evaluated by an operator. Alternatively, the image quality can be automatically evaluated by a controller. If the image quality is good, the image is stored or displayed at 318. Otherwise, the gas or plasma supply can be adjusted in 316 to further improve the image quality. For example, light ions can be introduced to reduce noise in response to a high noise level in the formed image. In another example, in response to a high noise level in the formed image, the RF power to the plasma generator can be reduced. By reducing the RF power to the plasma generator, the amount of ionized chamber gas is reduced. In yet another example, the sample chamber vacuum can be reduced to increase the SNR. The sample chamber vacuum can be reduced by reducing the partial pressure of the gas and / or the partial pressure of the plasma.

[0039] In this way, an SEM image is acquired while supplying gas and plasma to the detection space. Neither the gas nor the plasma reacts with the sample. By supplying plasma to the detection space during low vacuum SEM imaging, the sample chamber pressure can be reduced. The reduced sample chamber pressure reduces the scattering of the incident beam in the detection space. Also, due to the reduced sample chamber pressure, the pressure-limiting aperture between the column and the sample chamber can have a larger opening or be completely eliminated.

[0040] Figure 4A and Figure 4B respectively show SEM images taken at the same beam parameters in the detection space with and without plasma. The SEM images are taken with the GSED and using a column including a magnetic immersion lens. Water is supplied in the sample chamber at a pressure of 0.126 Torr. During imaging, plasma in the sample chamber is generated by ionizing the supplied water with the minimum required RF power to sustain the plasma. Figure 4A and Figure 4BThe detector voltages (voltages supplied to the anode of the detector) are 400 V and 326 V, respectively. At the lower detector voltage, the SEM images obtained in the plasma ( Figure 4A ), compared with the SEM images obtained without plasma ( Figure 4B ), have higher SNR and contrast.

[0041] It should be noted that the plasma enhances the SNR and contrast only when both a magnetic field and an electric field are provided simultaneously in the detection space of the GSED. For example, if images are acquired using the GSED in the non-magnetic immersion mode, the SEM image quality does not change in response to the introduction of plasma. In the non-magnetic immersion mode, the operating pressure must be relatively high, and gas amplification is not limited by the number of collisions between emitted electrons and gas molecules between the sample and the detector.

[0042] Figure 5 Shows the GSED output at various detector voltages. Two curves 501 and 502 are obtained in the detection space with and without plasma, respectively. The data are generated by imaging a featureless silicon sample at the same level of sample chamber vacuum of 0.4 Torr. Pure Ar is supplied into the sample chamber. The plasma in the sample chamber is generated by ionizing Ar with a plasma generator. When each detector voltage is set to the GSED detector, the detector output signal is recorded using the Figure 1 shown system. For SEM imaging without plasma, the measurement will be performed until the detector voltage causes gas breakdown. For SEM imaging with plasma, the measurement will be performed until the detector voltage causes the detector output to saturate.

[0043] For the data obtained with and without plasma, the signal amplitude increases with the increase of the detector voltage. At a specific detector voltage, the signal amplitude obtained with plasma is higher than that obtained without plasma. The difference between the signal amplitudes with and without plasma increases with the increase of the detector voltage. The increase in the signal intensity in the presence of plasma indicates that the detector gain increases significantly with the increase of plasma in the detection space. The increase in the detector gain is related to the lower ionization threshold and larger ionization cross-section of the gas molecules excited (i.e., stimulated) when the plasma is turned on.

[0044] Figures 6A - 6C Shows the effect of H2 on plasma-assisted low-vacuum SEM imaging in a gaseous environment. Imaging of a silicon sample is performed in the Figure 1 microscopy system. A mixture of Ar and H2 is supplied to the sample chamber. The plasma is generated in the sample chamber by igniting the supplied gas mixture. The partial pressure of Ar in the sample chamber is 0.4 Torr. InFigure 6A , Figure 6B and Figure 6C In Figure 6A , Figure 6B , and Figure 6C , the partial pressures of H2 in the sample chamber were 0.005 Torr, 0.01 Torr, and 0.011 Torr, respectively. Except for the amount (partial pressure) of H2 being different, SEM images were acquired with the same operating parameters. By increasing the amount of H2 in the sample chamber, the image noise decreased from Figures 6A to 6C All decreased. The image noise that decreases with the increase of H2 may be due to the contribution of light ions in the detection space. These light ions can respond faster to the fluctuations of the electromagnetic field, thereby reducing the noise by increasing the electromagnetic shielding of the detector. Instead of hydrogen ions, other types of light ions can be supplied to the sample chamber. The plasma can include a variety of ions, and at least one ion type has a molar mass less than 5. In one example, the gas supplied to the sample chamber is a mixture of multiple gas species, and at least one gas species has a molar mass less than 5. The plasma can be generated by ionizing a part of the gas in the sample chamber. In another example, the gas supplied to the sample chamber can be ionized to generate a plasma with ions having a molar mass less than 5. The plasma can contain hydrogen. The gas can include H2 and / or water. The gas can include He.

[0045] Figure 7 Shows the GSED output and the specimen current measured with and without plasma at various detector voltage values. The detector voltage increases from left to right on the x-axis. Curves 701 and 702 are the signal amplitudes of the GSED output from with and without plasma, respectively. Curves 703 and 704 are the specimen currents measured with and without plasma, respectively. A mixture of Ar and H2 was supplied to the sample chamber. The partial pressure of Ar was 0.22 Torr, while the partial pressure of H2 was 0.02 Torr.

[0046] Similar to Figure 5 , for example, the GSED signal amplitude is calculated based on the pixel values of the SEM image. The GSED signal increases with the increase of the detector voltage and the presence of plasma. In the presence of plasma, the maximum signal amplitude output from the GSED is higher. That is, the maximum value of curve 701 is higher than the maximum value of curve 702.

[0047] When scanning the sample with a charged particle beam, the specimen current can be recorded and used to generate a sample image. The specimen current is the current caused by the ions in the detection space traveling towards the sample. The specimen current is directly measured with a separate picoammeter connected to the platform. The specimen current (curves 703 and 704) increases with the increase of the detector voltage. At the same detector voltage, the specimen current with plasma (curve 703) is higher than the specimen current without plasma (curve 704). Thus, the plasma can also increase the SNR of the sample image formed by the specimen current.

[0048] Figures 8A - 8D SEM images of tin balls obtained with maximum GSED gain at a chamber pressure of 0.014 Torr. Figure 8A Obtained with plasma. The plasma is provided by ionizing H2O supplied in the sample chamber. The pressure of H2O is 0.014 Torr. Obtained without plasma Figure 8B . Figure 8C and Figure 8D are respectively Figure 8A and Figure 8B digitally normalized images. The image normalization used is histogram equalization. Figure 8A and Figure 8C have significantly higher image contrast and SNR than Figure 8B and Figure 8D .

[0049] Figure 9 is Figure 8A and Figure 8B one-dimensional power spectra in different spatial directions. The y-axis is the signal power. The x-axis is the spatial frequency. Curves 901, 902, and 903 are Figure 8A 's power spectra. Curves 904, 905, and 906 are Figure 8B 's power spectra. The power spectra show that the plasma-induced SEM imaging signal improvement exists across the entire spectrum.

[0050] Figures 8A - 8D and Figure 9 indicate that plasma can improve the quality of images obtained with GSED at very low chamber pressures (such as chamber pressures below 0.05 Torr). Plasma-assisted low-vacuum SEM can achieve high SNR and high contrast at very low pressures, which cannot be obtained by optimizing GSED alone. Plasma-assisted low-vacuum SEM can improve imaging resolution because a lower beam current can be used for SEM imaging.

[0051] The technical effect of providing a magnetic field and an electric field to the detection space is to increase the cascade amplification of secondary electrons emitted from the sample. The technical effect of providing plasma to the sample chamber during low-vacuum SEM imaging is to maintain a high detector gain at a lower gas pressure. Since a lower beam current can be used to obtain a given SNR, and since beam scattering is reduced due to the increase in sample chamber vacuum, and thus the image resolution can be improved.

[0052] In one embodiment, a method for imaging a sample includes providing a gas in a detection space; providing a magnetic field in the detection space; irradiating the sample with a charged particle beam while providing a plasma in the detection space; detecting charged particles emitted from the sample into the detection space in response to the irradiation; and forming an image based on the detected charged particles. In a first example of the method, the gas is a mixture of multiple gas species. A second example of the method optionally includes the first example and further includes that the plasma includes at least one gas molecular species having a molar mass less than 5. A third example of the method optionally includes one or more of the first and second examples and further includes that the plasma includes hydrogen. A fourth example of the method optionally includes one or more of the first and third examples and further includes that the plasma is provided by ionizing at least one species of the gas provided in the detection space. A fifth example of the method optionally includes one or more of the first and fourth examples and further includes that the sample is immersed in the magnetic field. A sixth example of the method optionally includes one or more of the first and fifth examples and further includes that the charged particles emitted from the sample are detected by a detector, and the detection space is defined by the position and structure of the detector. A seventh example of the method optionally includes one or more of the first and sixth examples and further includes that the sample is located in a sample chamber having a pressure less than 0.05 Torr. An eighth example of the method optionally includes one or more of the first and seventh examples and further includes that the charged particle beam is an electron beam and the detected charged particles are secondary electrons. A ninth example of the method optionally includes one or more of the first and eighth examples and further includes adjusting the amounts of the gas and the plasma provided to the detection space based on the formed image.

[0053] In one embodiment, a method for imaging a sample includes providing a magnetic field in a detection space; scanning the sample with an electron beam while providing a gas and a plasma in the detection space; detecting charged particles emitted from the sample; and forming a sample image based on the detected charged particles. In a first example of the method, the electron beam is guided to the sample through a light column, and the pressure in the detection space is the same as the pressure in a portion of the light column. A second example of the method optionally includes the first example and further includes recording a specimen current while scanning the sample; and forming a second sample image based on the recorded specimen current.

[0054] In one embodiment, a system for imaging a sample includes a sample chamber; a sample stage located in the sample chamber for holding the sample; a column coupled to the sample chamber for generating a charged particle beam towards the sample stage; a gas source coupled to the sample chamber; a detector for detecting charged particles in a detection space; and a controller having instructions stored in a non-transitory memory, the controller being configured to: supply gas to the detection space from the gas source; provide a magnetic field in the detection space; irradiate the sample with the charged particle beam while providing a plasma in the detection space; detect, via the detector, charged particles emitted from the sample into the detection space in response to the irradiation; and form an image based on the detected charged particles. In a first example of the system, the system further includes a plasma generator coupled to the sample chamber, and wherein providing a plasma in the detection space includes ionizing at least one gas species of the supplied gas with the plasma generator. A second example of the system optionally includes the first example and further includes, wherein the detector is located between the column and the sample stage. A third example of the system optionally includes one or more of the first and second examples and further includes, wherein the magnetic field is provided by a magnetic lens located in the column for focusing the charged particle beam onto the sample stage. A fourth example of the method optionally includes one or more of the first and third examples and further includes, wherein there is no coupling pressure limiting orifice between the column and the sample chamber. A fifth example of the method optionally includes one or more of the first and fourth examples and further includes, wherein the detection space extends from the sample surface irradiated by the charged particle beam into the column. A sixth example of the method optionally includes one or more of the first and fifth examples and further includes, wherein the detector provides an electric field in the detection space, and the trajectory of the charged particles in the detection space is controlled by the magnetic field and the electric field.

Claims

1. A method for imaging a sample, comprising: Providing a gas in a detection space; Providing a magnetic field in the detection space; Irradiating the sample with a charged particle beam while providing a plasma in the detection space, wherein the sample is located in a sample chamber in the detection space with a pressure less than 0.05 Torr; Detecting charged particles emitted from the sample into the detection space in response to the irradiation; And Forming an image of the sample based on the detected charged particles.

2. The method according to claim 1, wherein the gas is a mixture of multiple gas species.

3. The method according to claim 1, wherein the plasma comprises at least one gas molecular species with a molar mass less than 5.

4. The method according to claim 1, wherein the plasma comprises hydrogen.

5. The method according to any one of claims 1 - 4, wherein the plasma is provided by ionizing at least one species of the gas provided in the detection space.

6. The method according to any one of claims 1 - 4, wherein the sample is immersed in the magnetic field.

7. The method according to any one of claims 1 - 4, wherein the charged particles emitted from the sample are detected by a detector, and the detection space is defined by the position and structure of the detector.

8. The method according to any one of claims 1 - 4, wherein the charged particle beam is an electron beam, and the detected charged particles are secondary electrons.

9. The method according to any one of claims 1 - 4, further comprising adjusting the amount of the gas and the amount of the plasma provided to the detection space based on the formed image.

10. A method for imaging a sample, comprising: Providing a magnetic field in a detection space; Scanning the sample with an electron beam while providing a gas and a plasma in the detection space, wherein the sample is located in a sample chamber in the detection space with a pressure less than 0.05 Torr; Detecting charged particles emitted from the sample; and Forming a sample image based on the detected charged particles.

11. The method according to claim 10, wherein the electron beam is guided to the sample through a light column, and the pressure in the detection space is the same as the pressure in a part of the light column.

12. The method according to any one of claims 10 - 11, further comprising forming a second sample image by recording a specimen current.

13. An imaging system for a sample, comprising: A sample chamber; A sample stage located in the sample chamber for holding the sample; A column connected to the sample chamber for generating a charged particle beam towards the sample stage; A gas source connected to the sample chamber; A detector for detecting charged particles in a detection space; And A controller having instructions stored in a non - transitory memory, the controller being configured to: Provide gas to the detection space with the gas source; Provide a magnetic field in the detection space; Irradiate the sample with the charged particle beam while providing a plasma in the detection space, wherein the sample is located in a sample chamber in the detection space with a pressure less than 0.05 Torr; Detect, via the detector, charged particles emitted from the sample into the detection space in response to the irradiation; and Form an image of the sample based on the detected charged particles.

14. The system according to claim 13, further comprising a plasma generator coupled to the sample chamber, and wherein providing the plasma in the detection space includes ionizing at least one gas species of the provided gas with the plasma generator.

15. The system according to any one of claims 13-14, wherein the detector is located between the column and the sample stage.

16. The system according to any one of claims 13-14, wherein the magnetic field is provided by a magnetic lens located in the column for focusing the charged particle beam towards the sample stage.

17. The system according to claim 13, wherein there is no connection of a pressure-limiting orifice between the column and the sample chamber.

18. The system according to claim 17, wherein the detection space extends from the sample surface irradiated by the charged particle beam into the column.

19. The system according to claim 13, wherein the detector provides an electric field in the detection space, and the trajectory of the charged particles in the detection space is controlled by the magnetic field and the electric field.

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