Charged particle beam device

Abstract

In an irradiation optical system according to the present invention, a sample is irradiated with ultraviolet rays during a flyback period lasting from the end of scanning of a charged particle beam in a first direction to the start of another instance of scanning in the first direction. A control unit is configured to be capable of adjusting flyback time, which is the length of the flyback period, in response to receiving an instruction to suppress electrostatic charging of the sample, and controls the irradiation optical system so as to start irradiation of the sample with ultraviolet rays in synchronization with the end of scanning of the charged particle beam in the first direction, and to end irradiation of the sample with the ultraviolet rays at a timing corresponding to the adjusted flyback period.

Description

Charged particle beam device

[0001] The present invention relates to a charged particle beam device.

[0002] When observing a non-conductive sample with a charged particle beam device such as a scanning electron microscope (SEM: Scanning Electron Microscope), it is known that abnormal observations occur due to the charging of the sample. Also, as a method for suppressing such charging of the sample, countermeasure methods such as performing pretreatment on the sample and adjusting the observation conditions are known. The former is, for example, a method of depositing metal on the surface of the sample, but a part of the surface information of the sample to be observed is lost. The latter is a method of adjusting the observation conditions, for example, by reducing the degree of vacuum in the device or setting the primary beam irradiated on the sample to a low acceleration voltage. However, although the charging of the sample can be suppressed by adjusting these observation conditions, there are problems such as a decrease in the resolution of the observation image or an observation condition that deviates from the original desired observation condition.

[0003] Patent Document 1 discloses a mask manufacturing device that discharges a shielding layer by irradiating ultraviolet rays. Patent Document 2 discloses a charged particle beam device that prevents charge-up by irradiating a charged particle beam on a sample while an electron beam operating on the sample is flyback and neutralizing the potential accumulated on the surface.

[0004] Japanese Patent Application Laid-Open No. 63-208050, Japanese Patent Application Laid-Open No. 48-95767

[0005] As disclosed in Patent Document 1, the charge of a sample can be removed by irradiating it with ultraviolet light. This method has the advantage of not requiring sample pretreatment or adjustment of observation conditions, but it has the drawback of having a low charge suppression effect because ultraviolet light cannot be irradiated onto the sample during observation with a charged particle beam apparatus. This is for the following reasons: E-T (Everhart-Thornley) detectors, which are common as detectors for detecting secondary electrons emitted from a sample, consist of a scintillator, a light guide, and a photomultiplier tube, and detect secondary electrons by converting them into light. Therefore, if ultraviolet light is irradiated onto the sample at the same time as the detection of secondary electrons, the light converted from secondary electrons will be buried in ultraviolet light, making it impossible to detect the secondary electrons.

[0006] Patent Document 2 focuses on the fact that the flyback time, the period between scan lines when the electron beam of a scanning electron microscope transitions from one scan line to the next, is not used for image formation, and proposes irradiating the sample with an electron beam during the flyback time. However, replacing the electron beam in Patent Document 2 with ultraviolet light is difficult because the flyback time is very short, making it challenging to switch the ultraviolet light ON / OFF rapidly and obtain a sufficient charge suppression effect with conventional ultraviolet light sources. On the other hand, in recent years, UV-LEDs (ultraviolet LEDs) that can rapidly switch ultraviolet light ON / OFF and light sources that can irradiate ultraviolet light in the vacuum ultraviolet (VUV) region with high charge removal capabilities have become available, and it is expected that the charging of samples can be effectively suppressed by utilizing such ultraviolet light sources.

[0007] A charged particle beam apparatus according to one embodiment of the present invention comprises a charged particle optical system for scanning a charged particle beam in two dimensions over a sample, an irradiation optical system for irradiating the sample with ultraviolet light, a detector for detecting secondary electrons generated by the irradiation of the sample with the charged particle beam, an image forming unit for forming a scanning image from the signal from the detector, and a control unit. The charged particle optical system repeatedly scans the charged particle beam in a first direction while shifting it to a second direction perpendicular to the first direction. The irradiation optical system irradiates the sample with ultraviolet light during the flyback period from the end of scanning the charged particle beam in the first direction to the start of scanning in the first direction again. The control unit is configured to adjust the flyback time, which is the length of the flyback period, in response to an instruction to suppress the charging of the sample. The control unit controls the irradiation optical system to start irradiating the sample with ultraviolet light in synchronization with the end of scanning the charged particle beam in the first direction and to end the irradiation of the sample with ultraviolet light at a timing corresponding to the adjusted flyback period.

[0008] Furthermore, another embodiment of the present invention is a charged particle beam apparatus comprising: a charged particle optical system for scanning a pulsed charged particle beam in two dimensions over a sample; an irradiation optical system for irradiating the sample with ultraviolet light; a detector for detecting secondary electrons generated by the irradiation of the sample with the charged particle beam; an image forming unit for forming a scanning image from the signal from the detector; and a control unit, wherein the charged particle optical system repeatedly scans the charged particle beam in a first direction while shifting it in a second direction perpendicular to the first direction, and the control unit ensures that ultraviolet light is irradiated onto the sample during the period in which the charged particle beam is irradiated onto the sample. The irradiation optical system is controlled to detect secondary electrons, and the detector is controlled to detect secondary electrons, and during periods when the charged particle beam is not irradiated onto the sample, the irradiation optical system is controlled to irradiate the sample with ultraviolet light, and the detector is controlled to be in a desensitized state, and the control unit causes the charged particle optical system to scan the sample in two dimensions multiple times with pulsed charged particle beams, and in each of the multiple scans, the pixel coordinates in the first direction in which the charged particle beam irradiates the sample are controlled to be different from each other, and the image forming unit integrates multiple partial frame images obtained from the multiple scans to form a scan image.

[0009] This invention suppresses the charging of samples during sample observation using a charged particle beam apparatus. Other challenges and novel features will become apparent from the description and accompanying drawings herein.

[0010] This is a diagram of the configuration of a scanning electron microscope according to Example 1. This is a first example of a time chart for acquiring a scanning image. This is a second example of a time chart for acquiring a scanning image. This is a flowchart for adjusting the charge suppression effect. This is a diagram showing an example of adjusting the flyback time. This is a diagram of the configuration of a scanning electron microscope according to Example 2. This is an example of a time chart for acquiring a frame image. This is a diagram showing how a frame image is acquired by integrating partial frame images. This is a diagram showing how a frame image is acquired by integrating partial frame images.

[0011] The following describes embodiments of the present invention. The drawings shown in these embodiments illustrate specific examples in accordance with the principles of the present invention; however, these are for the purpose of understanding the present invention and are not intended to restrict its interpretation. The following embodiments will use a scanning electron microscope using electrons as charged particles as an example, but equivalent effects can be obtained when using various ions as charged particles.

[0012] Figure 1 shows the configuration of the scanning electron microscope in Example 1.

[0013] A scanning electron microscope comprises an electron optical system for irradiating a sample with an electron beam, an irradiation optical system for irradiating the sample with ultraviolet light to suppress the charging of the sample, a detection system for detecting secondary electrons emitted from the sample due to electron beam irradiation, a stage mechanism system arranged in a vacuum chamber, a control system for controlling the components of the scanning electron microscope and processing various information, and an image processing system for performing image reconstruction and other processing on the obtained scanning image.

[0014] Specifically, the primary electrons 102 generated by the electron source 101 are deflected by the deflector 104 and focused by the objective lens 103 before being irradiated onto the sample 105 mounted on the movable stage 106. The operation of the objective lens 103 is controlled by the objective lens control unit 113, the operation of the deflector 104 is controlled by the deflector control unit 114, and the operation of the movable stage 106 is controlled by the stage control unit 107. A negative voltage may also be applied to the sample 105 via the movable stage 106.

[0015] Secondary electrons 111 generated by the irradiation of the sample 105 with primary electrons 102 are detected by the detector 112. In the configuration of Figure 1, the detector 112 is positioned closer to the electron source 101 than the deflector 104, but it may also be positioned between the deflector 104 and the objective lens 103, or between the objective lens 103 and the sample 105, as long as it can detect the secondary electrons 111. For example, an E-T detector can be used as the detector 112. The timing for acquiring the signal for forming the scanning image from the detector 112 is controlled by the detection control unit 115.

[0016] In the image forming unit 116, signals acquired by the detection control unit 115 are assigned to the pixel coordinates of the primary electrons 102 determined by the deflector control unit 114 to form a scanned image. The generated scanned image is displayed in the image display unit 117 and recorded in the recording unit 118.

[0017] The electron source 101, detector 112, deflector 104, objective lens 103, and movable stage 106 are housed in a housing 119, and the inside of the housing 119 is kept under vacuum by a vacuum pump (not shown).

[0018] To suppress the charging of the sample 105, an ultraviolet light source 120 is installed on the outside of the housing 119, and ultraviolet light 121 emitted from the ultraviolet light source 120 passes through a port 122 attached to the housing 119 and irradiates the sample 105. Since the shorter the wavelength, the greater the charging suppression effect, it is desirable to use a wavelength in the vacuum ultraviolet region as the wavelength of ultraviolet light 121. Furthermore, although there are no particular limitations on the configuration of the ultraviolet light source 120, it is desirable to use an ultraviolet light source that can be switched on and off at high speed, such as a UV-LED. The irradiation timing of the ultraviolet light 121 generated by the ultraviolet light source 120 is controlled by the ultraviolet irradiation control unit 123. Note that the irradiation optical system that irradiates the sample with ultraviolet light may include optical elements such as lenses and mirrors in addition to the ultraviolet light source 120.

[0019] The operation of the stage control unit 107, objective lens control unit 113, deflector control unit 114, detection control unit 115, image forming unit 116, image display unit 117, recording unit 118, and ultraviolet irradiation control unit 123 is controlled by a computer (control unit) 125.

[0020] Figure 2 shows a first example of a time chart for acquiring a scanning image while suppressing the charging of the sample 105 using the scanning electron microscope shown in Figure 1. The scanning electron microscope uses a deflector 104 to repeatedly scan the primary electrons 102 in the X direction while shifting their position in the Y direction, which is perpendicular to the X direction. The length of time during which the deflector 104 scans the primary electrons 102 over the sample 105 is defined as the scanning time ts, and the length of the period (flyback period) from the end of scanning in one X direction to the start of scanning in the next X direction is defined as the flyback time tf.

[0021] In the first example of the time chart, the ultraviolet irradiation control unit 123 controls the ultraviolet light source 120 to irradiate the sample 105 with ultraviolet light 121 in synchronization with the end of scanning in the X direction during scanning of primary electrons 102, and sets the pulse width tu of the ultraviolet light 121 to be less than the flyback time tf. This is because if the pulse width tu = flyback time tf, a saturation signal will appear in the scanned image due to the recovery delay of the saturated photomultiplier tube of the detector 112 (so-called overexposure occurs in the scanned image). In the time chart of Figure 2, by setting the pulse width tu < flyback time tf, a waiting time tw occurs between the end of irradiation with ultraviolet light 121 and the start of scanning of primary electrons 102, thereby avoiding the appearance of the effect of ultraviolet irradiation in the scanned image.

[0022] In the second example time chart, the ultraviolet irradiation control unit 123 controls the ultraviolet light source 120 to irradiate the sample 105 with ultraviolet light 121 in synchronization with the end of scanning in the X direction during the scanning of primary electrons 102, and also switches the detector 112 to OFF. In the time chart of Figure 3, the detector ON indicates a state in which secondary electrons are detected, and the detector OFF indicates a state in which secondary electrons are not detected. Specifically, the detector 112 is in a desensitized state. In this case, because the detector 112 is OFF, even if the pulse width tu = flyback time tf, no overexposure occurs in the scanned image. Furthermore, by making the detector 112 insensitive (for example, by applying a voltage to the photomultiplier tube that prevents electron amplification of electrons generated on the photocathode), it is possible to prevent the detector 112 from becoming saturated and to suppress its degradation.

[0023] Figure 4 is a flowchart showing how the computer 125 adjusts the charge suppression effect by ultraviolet light 121. Even if the same dose of electron beam is irradiated onto the sample 105, the degree of charge will differ. Furthermore, the longer the pulse width tu of ultraviolet light 121, the greater the charge suppression effect. However, if the flyback time tf is extended to increase the pulse width tu, the time required to acquire the scanning image increases, reducing the throughput of the device. On the other hand, the shorter the pulse width tu of ultraviolet light 121, the smaller the charge suppression effect. For this reason, the charged particle beam apparatus of Example 1 allows adjustment of the flyback time tf in order to adjust the trade-off between the charge suppression effect and the throughput of the device.

[0024] First, the observation conditions for the sample 105 are set (S01). One of the observation conditions is the scanning condition for the primary electrons 102. A scan image is acquired according to the set observation conditions (S02). The user determines whether or not the charge suppression effect needs to be adjusted (S03). Here, it is assumed that the determination is made by visual observation of the scan image, but it may also be automatically determined based on the outstanding features of the scan image obtained by image processing. Upon receiving a charge suppression instruction, the process returns to step S01, and the flyback time tf among the scanning conditions for the primary electrons 102 is adjusted. Accordingly, in the time chart of the first example (see Figure 2), the charge suppression effect is adjusted by setting, for example, the pulse width tu of ultraviolet light 121 = flyback time tf - waiting time tw (the waiting time tw is predetermined based on the time required for the detector 112, which has become saturated by detecting ultraviolet light, to recover its sensitivity to detect secondary electrons 111), and in the time chart of the second example (see Figure 3), by setting, for example, the pulse width tu of ultraviolet light 121 = flyback time tf.

[0025] Figure 5 shows an example of adjusting the flyback time tf. In image quality priority adjustment 201, the flyback time tf is extended without changing the scan waveform. In image quality priority adjustment 201, the time required to acquire a frame image increases due to the extension of the flyback time tf, thus reducing the device's throughput, but the image quality of the acquired scan image does not change. Image quality priority adjustment can be performed by the user specifying an arbitrary flyback time tf. In throughput priority adjustment 202, the flyback time tf is extended by changing the ratio of the scan time ts to the flyback time tf, without changing the time required to acquire a frame image. In throughput priority adjustment 202, the image quality deteriorates due to the reduction of the scan time ts, but the device's throughput does not decrease even if the flyback time tf is extended. Throughput priority adjustment can be performed by the user specifying an arbitrary ratio of the scan time ts to the flyback time tf. Since the image quality priority adjustment 201 and throughput priority adjustment 202 represent a trade-off between the image quality of the scanned image and the throughput of the device, it is possible to provide different levels of charge suppression modes (e.g., high / medium / low levels) that are adjusted including observation conditions other than timing adjustment, so that the user can select according to the sample and desired observation conditions.

[0026] Figure 6 shows the configuration of the scanning electron microscope in Example 2. Components common to the scanning electron microscope in Example 1 shown in Figure 1 are denoted by the same reference numerals, and redundant explanations are omitted.

[0027] The scanning electron microscope of Example 2 is equipped with a blanker that pulses primary electrons 102. The blanker comprises, for example, a blanking electrode 108 and an aperture 109. When a voltage is applied to the blanking electrode 108, the primary electrons 102 are deflected and collide with the aperture 109. On the other hand, if no voltage is applied to the blanking electrode 108, the primary electrons 102 pass through the aperture 109 and irradiate the sample 105. Therefore, the primary electrons 102 can be pulsed by controlling the voltage applied to the blanking electrode 108. The operation of the blanking electrode 108 is controlled by the blanking control unit 110. In this example, the primary electrons 102 are pulsed by the blanking electrode 108, but the method of pulsing the primary electrons 102 is not limited to this. For example, the electron source 101 may be a photocathode and a pulsed laser may be irradiated onto the electron source 101, or a voltage (not shown) for extracting primary electrons 102 from the electron source 101 may be pulsed.

[0028] Similar to Example 1, pulsed secondary electrons 111 generated by the irradiation of the sample 105 with pulsed primary electrons 102 by the blanking electrode 108 and aperture 109 are detected by the detector 112. The timing for acquiring the signal for forming a scanning image from the detector 112 is controlled by the detection control unit 115.

[0029] Figure 7 shows an example of a time chart for acquiring a scanning image while suppressing the charging of the sample 105 using the scanning electron microscope shown in Figure 6. The electron beam is blanked for each pixel coordinate, and ultraviolet light 121 is irradiated during blanking to suppress the charging of the sample 105. During the blanker ON period, primary electrons 102 are blocked by the aperture 109, and during the blanker OFF period, primary electrons 102 are irradiated onto the sample 105. During the ultraviolet ON period, ultraviolet light from the ultraviolet light source 120 is irradiated onto the sample 105, and during the ultraviolet OFF period, ultraviolet light from the ultraviolet light source 120 is not irradiated onto the sample 105. During the detector ON period, the detector 112 detects secondary electrons from the sample 105, and during the detector OFF period, the detector 112 is in a desensitized state and does not detect secondary electrons from the sample 105. The Blanca, ultraviolet light source 120, and detector 112 are synchronized and controlled, and the detector is turned OFF when the ultraviolet light is turned ON. This prevents overexposure in the scanned image and suppresses degradation of the detector 112, similar to the time chart shown in Figure 3.

[0030] Figure 8 schematically shows the scan image (frame image) obtained by the time chart in Figure 7. Time chart 301 is a time chart that generates a partial frame image 311 of even pixels, and time chart 302 is a time chart that generates a partial frame image 312 of odd pixels. The image forming unit 116 obtains a frame image 313 by integrating the partial frame image 311 and the partial frame image 312.

[0031] The time chart in Figure 7 includes a first 2D scan to obtain a partial frame image of even pixels and a second 2D scan to obtain a partial frame image of odd pixels, but the method of dividing the frame image is not limited to this. A partial frame image may be acquired for every (n-1) pixels, and the image forming unit 116 may combine n partial frame images to obtain a frame image. Figure 9 shows an example where n=3. In this case, the partial frame images 321 to 323 obtained by the three 2D scans are integrated to obtain the frame image 324. The larger the value of n, the better the charge suppression effect of the sample 105 can be.

[0032] The present invention has been described above with reference to examples and modifications. The above-described examples and modifications can be modified in various ways without changing the essence of the invention, and they can also be used in combination.

[0033] 101: Electron source, 102: Primary electrons, 103: Objective lens, 104: Deflector, 105: Sample, 106: Movable stage, 107: Stage control unit, 108: Blanking electrode, 109: Aperture, 110: Blanking control unit, 111: Secondary electrons, 112: Detector, 113: Objective lens control unit, 114: Deflector control unit, 115: Detection control unit, 116: Image forming unit, 117: Image display unit, 118: Recording unit, 119: Housing, 120: Ultraviolet light source, 121: Ultraviolet light, 122: Port, 123: Ultraviolet irradiation control unit, 125: Computer, 201: Image quality priority adjustment, 202: Throughput priority adjustment, 301, 302: Time chart, 311, 312, 321, 322, 323: Partial frame image, 313, 324: Frame image.

Claims

1. A charged particle beam apparatus comprising: a charged particle optical system for scanning a charged particle beam in two dimensions over a sample; an irradiation optical system for irradiating the sample with ultraviolet light; a detector for detecting secondary electrons generated by the irradiation of the sample with the charged particle beam; an image forming unit for forming a scanning image from the signal from the detector; and a control unit, wherein the charged particle optical system repeatedly scans the charged particle beam in a first direction while shifting it in a second direction perpendicular to the first direction; the irradiation optical system irradiates the sample with ultraviolet light during a flyback period from the end of scanning the charged particle beam in the first direction to the start of scanning in the first direction again; the control unit is configured to adjust the flyback time, which is the length of the flyback period, in response to an instruction to suppress the charging of the sample; and the control unit controls the irradiation optical system to start irradiating the sample with ultraviolet light in synchronization with the end of scanning the charged particle beam in the first direction, and to end the irradiation of the sample with ultraviolet light at a timing corresponding to the adjusted flyback period.

2. The charged particle beam apparatus according to claim 1, wherein the control unit controls the detector to an insensitive state during the period in which the irradiation optical system irradiates the sample with ultraviolet light.

3. The charged particle beam apparatus according to claim 1, wherein the control unit sets the time for which the irradiation optical system irradiates the sample with ultraviolet light to the time obtained by subtracting a predetermined waiting time from the flyback time, and the predetermined waiting time is set based on the time required for the detector, which has become saturated by detecting the ultraviolet light, to recover its sensitivity for detecting the secondary electrons.

4. The charged particle beam apparatus according to claim 1, wherein the control unit receives an instruction to suppress the charging of the sample and extends the flyback time without changing the time required for scanning the charged particle beam in the first direction.

5. The charged particle beam apparatus according to claim 1, wherein the control unit, upon receiving an instruction to suppress the charging of the sample, changes the ratio of the time required to scan the charged particle beam in the first direction to the flyback time in the total time of the time required to scan the charged particle beam in the first direction to the flyback time.

6. The charged particle beam apparatus according to claim 1, wherein the instruction to suppress the charge of the sample selects one of a plurality of charge suppression modes, and the control unit controls the charged particle optical system, the irradiation optical system, and the detector according to the selected charge suppression mode.

7. The charged particle beam apparatus according to claim 1, wherein the ultraviolet light has wavelengths in the vacuum ultraviolet region.

8. A charged particle optical system for scanning a pulsed charged particle beam in two dimensions over a sample; an irradiation optical system for irradiating the sample with ultraviolet light; a detector for detecting secondary electrons generated by the irradiation of the sample with the charged particle beam; an image forming unit for forming a scanning image from the signal from the detector; and a control unit, wherein the charged particle optical system repeatedly scans the charged particle beam in a first direction while shifting it in a second direction perpendicular to the first direction; the control unit controls the irradiation optical system so that ultraviolet light does not irradiate the sample during the period when the charged particle beam is irradiated over the sample, and controls the detector to detect the secondary electrons during the period when the charged particle beam is not irradiated over the sample, and controls the irradiation optical system so that ultraviolet light irradiates the sample, and controls the detector to be in a desensitized state; the control unit causes the charged particle optical system to scan the pulsed charged particle beam in two dimensions over the sample multiple times. In the above-mentioned multiple scans, the pixel coordinates in the first direction in which the charged particle beam irradiates the sample are controlled to be different from each other, and the image forming unit integrates a plurality of partial frame images obtained by the above-mentioned multiple scans to form the scanned image.

9. The charged particle beam apparatus according to claim 8, wherein the ultraviolet light has a wavelength in the vacuum ultraviolet region.

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