Electron beam application device and time constant measurement method

Abstract

This electron beam application device includes: an electron optical system for focusing, on a sample 11, a pulse electron beam 7 emitted from a pulse electron gun 25 having a photocathode; a detector 14 for detecting signal electrons 41 emitted when the sample is irradiated with the pulse electron beam; a probe 15 capable of coming into contact with a measurement region of the sample; a voltage source 17 for applying a periodically varying sample voltage to the sample via the probe; and a timing controller 18 for synchronously controlling the voltage source and the pulse electron gun and for controlling the timing of the irradiation with the pulse electron beam in one period of the sample voltage. The electron beam application device calculates a time constant of the measurement region of the sample on the basis of a detection signal amount resulting from the detector detecting signal electrons emitted when the measurement region to which the sample voltage is applied is irradiated with the pulse electron beam.

Description

Electron beam application device and time constant measurement method

[0001] The present invention relates to an electron beam application device and a time constant measurement method using the same.

[0002] Probing systems capable of measuring electrical characteristics are used in defect and failure analysis of semiconductor devices, and the quality of the device is determined based on the measurement results of the electrical characteristics of local regions. Patent Document 1 discloses a technology in which a measurement probe is brought into contact with the wiring region of a semiconductor device sample to measure electrical characteristics and detect breaks or defects in the wiring portion to determine the quality of the device. Specifically, a sinusoidal voltage with a frequency f is applied from the backside of the semiconductor substrate, and the time variation of the voltage in the wiring region on the front side is measured. The measured voltage in the wiring region has a sinusoidal waveform with a phase shift at the same frequency f as the voltage applied from the backside, and changes in resistance value due to breaks or defects in the wiring portion are reflected in the phase shift θ. Therefore, the quality of the wiring portion can be determined based on the phase shift θ. Examples of measuring the time variation of the voltage in the wiring region on the front side using a probe and a pulsed electron beam have been disclosed. In the latter case, a scanning electron microscope (SEM) is used, which irradiates a pulsed electron beam onto the sample, scans it, detects the emitted electrons, and displays the signal intensity at each irradiation point. By detecting secondary electrons (SE) with an energy of approximately 10 eV or less emitted from the sample, information on the surface potential of the sample can be obtained.

[0003] Japanese Patent Application Laid-Open No. 2005-85806

[0004] In Patent Document 1, when measuring the time variation of voltage in a wiring region using a pulsed electron beam, the strobe waveform mode of the electron microscope is used. In strobe waveform mode, the sample is irradiated with a pulsed electron beam only at a specific phase synchronized with the frequency of the voltage applied to the sample (in Patent Document 1, the backside of the semiconductor substrate). Under optimal conditions, the diameter of the electron beam irradiation can be narrowed to approximately 1 nm, making this a potentially advantageous measurement method for measuring the electrical characteristics of device structures that are becoming increasingly miniaturized and complex, making it difficult to contact with a probe.

[0005] However, when electron microscopes use typical thermionic, field, or Schottky emission electron sources as their electron sources, the electron beam is pulsed by chopping the continuously emitted electron beam using a blanker. Therefore, the higher the frequency of the voltage applied to the sample, the faster the blanker must chop the electron beam. This requires a smaller aperture diameter through which the electron beam passes, enabling chopping with a slight deflection of the electron beam. This reduces the exposure current of the pulsed electron beam, resulting in a lower signal-to-noise ratio. This can impair quantitative measurement accuracy and measurement precision when measuring the electrical characteristics of fine structures in semiconductor devices, especially at high frequencies. Furthermore, increasing the integration time of the detection signal to improve the signal-to-noise ratio leads to a decrease in measurement throughput.

[0006] Therefore, the inventors focused on photoexcited emission electron sources (hereinafter referred to as photocathodes). Photocathodes utilize electrons emitted upon irradiation with light, and can easily generate short-pulse electron beams by pulsing the excitation light. In other words, a blanker is not required for pulsing, and pulsing does not reduce the amount of irradiation current. In particular, electrons emitted from photocathodes using gallium arsenide (GaAs), a p-type semiconductor with a negative electron affinity (NEA) on the surface, can generate pulsed electron beams with a minimum duration of approximately 10 picoseconds, making them suitable for use as high-brightness short-pulse electron sources comparable to Schottky field emission electron sources.

[0007] The present invention aims to provide a technology for measuring the time constant of a measurement area with a high spatial resolution of, for example, about 10 nm and a high temporal resolution of, for example, 1 nanosecond or less, by using a photocathode with the above-described electron emission characteristics as the electron source of an SEM and making it possible to measure high-speed temporal fluctuations in the surface potential of a sample using a high-brightness short-pulse electron beam.

[0008] an electron beam application device according to one embodiment of the present invention, comprising: a sample stage on which a sample is placed; an electron optical system including a pulsed electron gun and focusing a pulsed electron beam emitted from the pulsed electron gun on the sample; a detector for detecting signal electrons emitted when the sample is irradiated with the pulsed electron beam; a probe capable of coming into contact with a measurement region of the sample; a voltage source for applying a periodically varying sample voltage to the sample via the probe; a timing controller for synchronously controlling the voltage source and the pulsed electron gun and controlling the timing at which the pulsed electron beam is irradiated in one period of the sample voltage; and a control system, wherein the pulsed electron gun comprises: a first excitation light source for generating pulsed excitation light; a photocathode having a transparent substrate and a photoelectric film; a focusing lens disposed between the first excitation light source and the transparent substrate for focusing the pulsed excitation light transmitted through the transparent substrate on the photoelectric film; and an anode electrode for accelerating and emitting the pulsed electron beam emitted from the focusing position of the photoelectric film; The control system calculates the time constant of the measurement region of the sample based on the amount of detection signal detected by the detector from signal electrons emitted when a pulsed electron beam is irradiated onto the measurement region to which the sample voltage is applied.

[0009] The time constant of a device sample having a nanoscale fine structure or pattern can be measured over a wide range from nanoseconds to microseconds. Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

[0010] 1 is a diagram showing an outline of an electron beam application device using an SEM; 2 is a diagram showing an outline of a pulse electron gun; 3 is a diagram showing a specimen voltage V ｓ 1 is a diagram showing the waveforms of a pulsed electron beam and a pulsed electron beam; FIG. 2 is a diagram showing the energy distribution of electrons emitted by irradiation with an electron beam; FIG. 3 is a diagram showing the trajectory of secondary electrons (SE) when a positive voltage is applied to a sample; FIG. 4 is a diagram showing the energy distribution of secondary electrons (SE) detected when a positive voltage is applied to a sample; and FIG. 5 is a diagram for explaining a method for measuring the time constant of a measurement region of a sample. ｓ The detected signal amount I when the pulsed electron beam is irradiated at timing Δt to the measurement area to which the ｄｅｔ 1 is a diagram showing the change over time of the detected signal amount I ｄｅｔFIG. 1 is a diagram showing the timing Δt dependency of . FIG. 2 is a diagram showing a flowchart of the measurement procedure of Example 1. FIG. 3 is a diagram showing a sinusoidal voltage waveform applied to a sample as a periodically varying voltage. FIG. 4 is a diagram showing a square waveform voltage waveform applied to a sample as a periodically varying voltage. FIG. 5 is a diagram showing a triangular waveform voltage waveform applied to a sample as a periodically varying voltage. FIG. 6 is a diagram showing a sawtooth voltage waveform applied to a sample as a periodically varying voltage. FIG. 7 is a diagram showing a sawtooth voltage waveform applied to a sample as a periodically varying voltage. ｓ The detected signal amount I when the pulsed electron beam is irradiated at timing Δt to the measurement area to which the ｄｅｔ 1 is a diagram showing the time variation of the sample voltage V, which is a high frequency sinusoidal voltage. ｓ The detected signal amount I when the pulsed electron beam is irradiated at timing Δt to the measurement area to which the ｄｅｔ 1 is a diagram showing the change over time of the detected signal amount I ｄｅｔ Fig. 10 is a diagram showing the frequency f dependence of the amplitude of Fig. 11. Fig. 11 is a diagram showing a flowchart of the measurement procedure of Example 2. Fig. 12 is a diagram showing an example of the configuration of a voltage source using a photodetector.

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0012] FIG. 1 shows an example of the configuration of an electron beam application apparatus using a scanning electron microscope (SEM) according to this embodiment. A photocathode that emits electrons when irradiated with excitation light 5 is used as the pulsed electron gun 25 mounted on the SEM. The pulsed electron beam 7 emitted from the photocathode is focused on a sample 11 by the SEM's electron optical system. In addition to the pulsed electron gun 25, the SEM's electron optical system includes an aperture (not shown) for limiting the diameter of the pulsed electron beam 7, electron lenses such as a condenser lens and objective lens 10 for focusing the pulsed electron beam 7 on the sample 11, and a deflector 9 for scanning the focused pulsed electron beam 7 over the sample. The sample 11 is placed on a sample stage 13 that moves its measurement region within the region irradiated by the pulsed electron beam 7. A fine probe 15 is brought into contact with the sample 11, and a periodically varying sample voltage from a voltage source 17 is applied to a local region of the sample 11 via the probe 15. In order to prevent a short circuit between the sample 11 and the sample stage 13, the sample 11 is placed on the sample stage 13 via an insulator 12. When the sample 11 is irradiated with the pulsed electron beam 7, signal electrons 41 are emitted from the sample 11 and detected by the detector 14.

[0013] The timing controller 18 synchronizes the pulsed electron gun 25 with the voltage source 17, which generates a periodically varying specimen voltage, and controls the timing (phase or delay time) at which the pulsed electron beam 7 irradiates the specimen during one specimen voltage cycle. The control system 19 sets the measurement conditions for the electron beam application device and controls each component of the electron beam application device according to the measurement conditions. It also calculates the time constant of the measurement region based on the measurement results. The control system 19 includes a computer and controllers provided for the components of the electron beam application device. The controllers control the operation of the corresponding components according to commands from the computer. The computer includes an arithmetic unit, an input device, and a display device. The arithmetic unit controls each part of the electron beam application device according to the measurement conditions and performs arithmetic processing based on the detection signal from the detector 14. These controls and arithmetic processing are preprogrammed into the computer. Furthermore, although not shown, a vacuum pumping system is also provided for maintaining a vacuum inside the SEM.

[0014] An example of the configuration of the pulsed electron gun 25 is shown in Figure 2. Here, an example will be described in which a high-intensity photocathode using p-type semiconductor gallium arsenide (GaAs) formed on a transparent substrate is used as the electron source, but photocathode with a different configuration may also be used. The pulsed electron gun 25 is composed of a photocathode having a transparent substrate 1 and a photoelectric film 2 formed thereon, a condenser lens 3, an anode electrode 8 that accelerates and emits electron beams generated from the photoelectric film 2, and a vacuum pumping system (not shown) for maintaining an extremely high vacuum around the photoelectric film 2. The photoelectric film 2 is supplied with an acceleration voltage (cathode voltage) V ０ 21, and the anode electrode 8 is supplied with an extraction voltage V １ 22 is applied. １ 22 may be at ground potential. The photoelectric film 2 is placed in a vacuum chamber together with a condenser lens 3, and excitation light 5 emitted from an excitation light source 4 placed outside the vacuum chamber passes through a viewing port 6 and is condensed onto the photoelectric film 2 by the condenser lens 3 placed behind the photoelectric film 2. The electron beam emitted from this condensing position is used as the electron beam for the SEM.

[0015] Because the size of an electron source is inversely proportional to the square root of its brightness, using a low-brightness electron source makes it difficult to focus the SEM electron beam diameter to less than 10 nm due to the limitations imposed by the source diameter. Therefore, a high-brightness electron source is essential to achieve high spatial resolution on the order of nm in SEM observations. A preferred high-brightness photoexcitation electron source is a high-brightness photoelectric film made of p-type GaAs with an NEA surface. As shown in Figure 2, when a focusing lens 3 is placed near the backside of the photoelectric film 2, which serves as the electron emission surface, excitation light 5 transmitted through the transparent substrate 1 can be focused with a numerical aperture (NA) of 0.5 or greater. The focused diameter of excitation light with wavelength λ focused on the photoelectric film 2 by a focusing lens 3 with a numerical aperture NA is approximately λ / NA.

[0016] When the surface of the photoelectric film 2 is activated by NEA, the energy level of the lower end of the conduction band in the photoelectric film 2 becomes higher than the vacuum level. As a result, electrons excited from the valence band to the conduction band by irradiation with excitation light 5 are efficiently emitted from inside the photoelectric film 2 into the vacuum region. When p-type GaAs is used as the active layer of the photoelectric film 2, the effective mass of electrons emitted by irradiation with excitation light is small, 0.067 times the rest mass of electrons in vacuum, so the electron emission angle when emitted into the vacuum region is small, within approximately 10 degrees. As a result, by focusing the pulsed excitation light 5 on the photoelectric film 2 and reducing the electron emission region to approximately φ1 μm to form a point source, the peak brightness of the electron source can be made to be similar to that of a Schottky electron source (up to 10 7 A / m 2 A pulsed electron gun 25 with a pulse width of approximately 10 picoseconds can be obtained by using a short pulsed excitation light 5 as the excitation light source.

[0017] Pulse width τ of the pulsed electron beam 7 ｐ and period T ｉｎｔ (the reciprocal of frequency f) is controlled by the excitation light source 4 that irradiates excitation light 5. The relationship between the pulse width and period of the pulsed electron beam 7 is τ ｐ <T ｉｎｔ and the ratio of pulse width to period (τ ｐ / T ｉｎｔ The range of the duty ratio, defined as (1 / 2) / (2 / 3), depends on the excitation light source 4. Since the pulse waveform of the excitation light 5 becomes the pulse waveform of the pulsed electron beam 7, in order to obtain the required irradiation performance of the pulsed electron beam 7, it is necessary to select an appropriate wavelength λ and pulse width τ ｐ , period T ｉｎｔ , duty ratio, and peak intensity of the excitation light source 4 are used.

[0018] In FIG. 2, the spatial pulse length in the propagation direction of the pulsed light used as the excitation light 5 is cτ ｐ (c is the speed of light, c = 3.0 x 10 8 m / s), while the energy released into the vacuum region E ０ The velocity of the pulsed electron beam 7 is v ｅ is the rest mass of the electron m ｅ = 9.1 x 10-31 kg, which is derived from the electron energy relation ｅ =√(2E ０ / m ｅ ) can be calculated from the equation. Therefore, the spatial pulse length in the traveling direction of the pulsed electron beam 7 is v ｅ τ ｐ Assuming irradiation conditions of energy 30 keV or less used in a general SEM, v ｅ Since <<c, the spatial pulse length of the electron beam is shorter than that of the excitation light.

[0019] In the first embodiment, the periodic voltage is a pulse width τ Ｖ , period T ｉｎｔ The time constant of the sample 11 is measured by applying a rectangular pulse voltage of τ to the sample 11 via a fine probe 15. A function generator (arbitrary waveform generator), for example, can be used as the voltage source 17 that applies the rectangular pulse voltage to the sample 11. Any other voltage source that can output an equivalent rectangular pulse voltage may be used. The pulse waveform of the pulsed electron beam 7 is determined by the pulse width τ of the pulse voltage applied to the sample 11, because it is necessary to measure the potential change of the sample 11. Ｖ The pulse width τ of the pulsed electron beam 7 ｐ is set so that τ ｐ ≪τ Ｖ 3, the pulse waveform of the pulse electron beam 7 is shown as a rectangular waveform, but the magnitude relationship of the pulse width (τ ｐ <τ Ｖ ) is satisfied, the pulse waveform of the pulse electron beam 7 is not limited to a waveform having a bell-shaped peak or the like.

[0020] To enable contact with a minute region on the sample 11, the probe 15 is preferably a needle-like probe with a tip diameter of 100 nm or less. Furthermore, to enable the probe 15 to contact any position on the sample 11 while observing the measurement region with the SEM, a drive mechanism for the probe 15 is installed in the SEM sample chamber so that the tip position of the probe 15 can be controlled independently of the sample position control by the sample stage 13. The drive mechanism for the probe 15 receives power from a control power supply 16 installed in the atmospheric region. While FIG. 1 shows an SEM configuration equipped with a single probe 15, a configuration equipped with two or more probes 15 may also be used. By installing multiple probes 15, it is possible to utilize the measurement function of a conventional SEM-type probing device, and to compare or calibrate the time constant measurement result measured by the two-terminal method using the probing device with the time constant measurement result measured by the present embodiment using synchronization and timing control of the periodically varying voltage via the probe 15 and the pulsed electron beam 7. The photocathode has a similar characteristic to a Schottky electron source (up to 10 7 A / m 2 When the SEM has high brightness characteristics (e.g., 1 / sr / V), an irradiation beam diameter on the order of nm can be obtained under optimal conditions by appropriately selecting the SEM objective lens 10, the working distance of the sample 11, and the irradiation energy of the pulsed electron beam 7.

[0021] The signal electrons 41 detected by the detector 14 are low-energy secondary electrons (SEs). While the energy of the electron beam irradiated by the SEM onto the sample 11 can be set arbitrarily, setting the irradiation energy to around 1 keV increases the yield of SEs 41 emitted from the sample 11, which is preferable from the standpoint of signal-to-noise ratio. It is known that the energy spread of the electron beam emitted from the photoelectric film 2 having an NEA surface is small, at 0.2 eV or less. Therefore, when using an SEM under irradiation conditions with low irradiation energy, the configuration of this embodiment, in which the photoelectric film 2 is used as an electron source, is less affected by chromatic aberration, which limits the irradiation beam diameter, and is therefore an advantageous device configuration from the standpoint of measurement accuracy.

[0022] In this example, the objective lens 10 is a semi-in-lens type objective lens, which provides superior irradiation performance and SE discrimination detection performance in the low acceleration range. However, similar measurements can be performed using objective lenses other than semi-in-lens type, such as an out-lens type objective lens or an objective lens using a boosting method in which a positive voltage of approximately 10 kV is applied to a cylindrical electrode to allow high-speed passage through the magnetic lens region of the out-lens type objective lens. Semi-in-lens type objective lenses are advantageous in that they have a large degree of spatial freedom on the sample side, allowing sufficient space to place the measurement probe 15. Furthermore, semi-in-lens type objective lenses can shorten the focal length by leaking the lens magnetic field into the sample 11, thereby minimizing axial aberrations such as spherical aberration and chromatic aberration. Therefore, they are advantageous for obtaining an irradiation beam diameter on the order of nm, especially under low irradiation energy conditions. Low-energy SE41 generated on the sample 11 is focused by the lens magnetic field leaking onto the sample 11 and transported above the objective lens 10. SE41 can be detected efficiently by installing a deflector for deflecting SE41 off-axis above the objective lens 10. Possible deflector configurations include an electric field type, a magnetic field type, or a Wien filter type in which the electric and magnetic fields are arranged orthogonally. In order to reduce adverse effects on the illumination beam, a compensating deflector for offsetting deflection aberrations, which are a factor in degrading the illumination beam diameter, may be added closer to the electron source than the deflector for deflecting SE41 off-axis.

[0023] Energy E ０ The energy distribution of electrons emitted when a sample is irradiated with an electron beam of 100 kJ / s is shown in Figure 4. Generally, SE (signal electrons) 41 have a peak in the amount of electrons generated at an energy of 2 to 3 eV, whereas high-energy reflected electrons (backscattered electrons: BSE) 44 have a peak in the amount of electrons generated at an energy of 100 kJ / s. ０The detector 14 for the SE41, which is the main detection target, can be an Everhart & Thornley (ET) detector, which combines a scintillator, a light guide, and a photomultiplier tube (PMT), which are commonly used as detectors for SEMs. In this embodiment, a periodically varying voltage is applied to the sample 11 to measure the time variation of the detection signal, but the response speed of the detector 14 does not necessarily need to be high. Detectors other than ET detectors, such as a channeltron or a micro-channel plate (MCP), can also be used as the detector 14 as long as they have the detection sensitivity for the SE41.

[0024] Here, a static positive voltage V ｓ (V ｓ 5, an electric field 46 is formed between the sample 11 and the objective lens 10 (FIG. 5 shows a pole piece 45 of the objective lens 10), and the energy on the sample 11 is ｓ The following SE43 is pulled back onto the sample 11. Therefore, among the SE41 emitted from the sample, ｓ The following SE43 is not detected by the detector 14, and the energy is eV ｓ The SE42 exceeding the energy band of SE41 is detected by the detector 14. A schematic diagram of the energy band of SE41 is shown in FIG.

[0025] FIG. 7 shows an example of a configuration for measuring the time constant of a microscopic region resulting from the electrical resistance R and capacitance C of a sample 11 introduced into an SEM sample chamber by applying a rectangular pulse voltage via a probe 15 to the sample 11. In FIG. 7, the sample 11 is shown as a cross-section. The sample 11 has a layered structure of a substrate 53, a conductor layer 52, and an insulator layer 54. A plug 51 is embedded in the insulator layer 54 and contacts the conductor layer 52. The plug 51 exposed from the insulator layer 54 is the measurement region. From the time constant of the plug 51, it is possible to determine, for example, whether the plug 51 is properly electrically connected to the conductor layer 52. Note that the structure used as the measurement region is not limited to the example shown in FIG. 7 , and may also be a microscopic structure such as an electrode of an active element such as a transistor or a plug connected to that electrode. Calculating the time constant not only enables simple determination of whether the manufacturing process is good or bad, but also enables advanced determination of whether the electronic device meets its operating characteristics.

[0026] Period T for sample 11 ｉｎｔ The sample voltage V when measuring the time constant of the sample 11 by applying a rectangular pulse voltage of ｓ , sample surface potential φ ｓ , the detected signal amount I ｄｅｔ , the irradiation current I of the pulsed electron beam 7 ＥＢ The time variation of the sample voltage V ｓ The voltage source 17 and the excitation light source 4 are synchronously controlled, and the sample voltage V ｓ and peak intensity I ｐｅａｋ The pulsed electron beam 7 has the same period T ｉｎｔ Also, the specimen voltage V ｓ The timing Δt at which the pulse electron beam 7 is irradiated in one period is controlled by the timing controller 18 as follows: Δt=0 to T ｉｎｔ The sample surface potential φ can be changed within the range ｓ is the sample voltage V ｓ By applying a period T ｉｎｔ The sample surface potential φ at the timing Δt when the pulsed electron beam 7 is irradiated onto the sample 11 varies with ｓ will be the same value (however, the time constant τ ｄｅｃａｙ ≪Period Tｉｎｔ ).

[0027] Pulse width τ of the pulsed electron beam 7 ｐ is the sample voltage V ｓ Pulse width τ Ｖ Set it to be sufficiently small compared to τ ｐ <τ Ｖ ). Sample voltage V ｓ is a pulse voltage with a voltage amplitude ΔV (ΔV > 0 V) ​​relative to the reference voltage 0 V. Since the peak of SE generation is around 2 to 3 eV, a voltage amplitude of ΔV = 5 V is sufficient to obtain a sufficient detection signal amount I ｄｅｔ The larger the voltage amplitude ΔV, the smaller the change in the detected signal amount I ｄｅｔ However, a general function generator (arbitrary waveform generator) outputs a periodic voltage whose frequency (= 1 / T ｉｎｔ ) increases, the voltage amplitude ΔV that can be set decreases. ｉｎｔ The measurement conditions must be selected taking into consideration the set value of and the voltage amplitude ΔV that can be output under those conditions. ｓ When the voltage is close to 0 V, the electric field 46 formed on the sample 11 is weak, so most of the SE 41 is detected by the detector 14, and the detected signal amount I ｄｅｔ On the other hand, the sample surface potential φ ｓ When the voltage is close to the peak voltage ΔV, a part of SE41 is pulled back to the sample 11, so the detected signal amount I ｄｅｔ becomes smaller, resulting in a change in intensity as shown in the graph of FIG.

[0028] The sample 11 is supplied with a periodic voltage V ｓ When the voltage V is applied, an electric field lens is formed between the sample 11 and the objective lens 10, and the strength of this electric field lens is ｓThe voltage fluctuates over time with the same period as the voltage source 17. It should be noted that the focus state of the pulsed electron beam 7 on the sample 11 also fluctuates over time. When the size of the device region to be measured is sufficiently large compared to the beam diameter of the pulsed electron beam 7, changes in the focus state due to voltage fluctuations are not a major problem. On the other hand, when the device region to be measured has a nanometer-order microstructure equivalent to the beam diameter of the pulsed electron beam 7, changes in the focusing conditions of the pulsed electron beam 7 cause the pulsed electron beam 7 to irradiate regions other than the device region to be measured, making it difficult to perform the desired measurement. To avoid this problem, a focus compensation lens is installed on the path of the pulsed electron beam 7 in the SEM's electron optical system. The lens strength is synchronously controlled with the voltage source 17 and the pulsed electron beam 7, adding a mechanism to maintain the focus state on the sample 11. This enables accurate measurement of the time constant of the microstructure.

[0029] In the measurement of this embodiment, the detected signal amount I ｄｅｔ It is important to measure the change in the detected signal amount I ｄｅｔ In order to accurately measure the change in the irradiation current I of the pulsed electron beam 7 irradiating the sample 11, ＥＢ When the stability of the irradiation performance of the pulsed electron gun 25 becomes an issue in measurement, it is important that the irradiation current I to the sample 11 on the path of the pulsed electron beam 7 is controlled to be stable or constant. ＥＢ is measured, and the detected signal amount I ｄｅｔ The measured emission current I ＥＢ By normalizing the current value, the reliability of the measurement results can be improved.

[0030] The sample voltage V, which is a periodically varying voltage, is applied to the measurement area having an electrical resistance R and an electrical capacitance C. ｓ When the sample surface potential φ ｓ shows a relaxation behavior with a time constant corresponding to the product of the electrical resistance R and the electrical capacitance C. At timing Δt, the sample surface potential φ ｓ The amount of SE signal corresponding to the pulse width τ of the pulsed electron beam 7 is detected. ｐThe smaller the value of is, the smaller the amount of SE signal emitted with one pulse of electron beam irradiation. In order to obtain a sufficient amount of signal, a plurality of pulses of electron beam 7 are irradiated and the detection signals are integrated, so that the amount of detection signal I can be obtained with a sufficient signal-to-noise ratio. ｄｅｔ It is possible to measure changes over time.

[0031] Sample voltage V ｓ The timing Δt at which the pulsed electron beam 7 is irradiated to the sample 11 is set as Δt=0 to T, with the moment when the peak voltage ΔV is applied to the sample 11 as the reference (Δt=0). ｉｎｔ When the detection signal amount I ｄｅｔ The timing dependence of the sample surface potential φ ｓ The amount of detected signal I ｄｅｔ When the time variation of the sample surface potential φ is plotted against the timing Δt, a graph like that shown in FIG. ｓ The relaxation behavior of the detected signal I ｄｅｔ Therefore, the detected signal amount I ｄｅｔ The amount of signal fluctuation until saturation is defined as ΔI, and the amount of detected signal I ｄｅｔ By measuring the time it takes for the value of ΔI(1-1 / e) to increase, the time constant τ ｄｅｃａｙ It is possible to measure Δt = 0 to T ｉｎｔ Measure the signal quantity I ｄｅｔ The time constant τ is calculated by fitting the measurement points of ｄｅｃａｙ This measurement method is an application of the pump-probe method, which is often used in the field of optics. Therefore, the response speed of the detection system does not need to be high, and the time resolution of the measurement system and the accuracy of the measurement results depend on the control accuracy of the voltage source 17 and the timing controller 18, which controls the timing of irradiation of the pulsed electron beam 7, which is controlled in synchronization with the voltage source 17.

[0032] If the electrical resistance R and capacitance C of the measurement target are known, the time constant τ ｄｅｃａｙ = R × C, the detected signal amount I ｄｅｔ The period T at which ｉｎｔ On the other hand, if the electrical resistance R and capacitance C of the measurement target are unknown, the time constant τ ｄｅｃａｙWhen measuring a sample 11 with a large period T ｉｎｔ It is necessary to set the time constant τ to a sufficiently long value. ｄｅｃａｙ The sample with a large period T ｉｎｔ It is necessary to set a large value, and it is important to note that it takes a long time to obtain the time constant measurement result. ｄｅｔ The signal amount in the measurement area may be obtained by point-irradiating the pulsed electron beam 7, or the pulsed electron beam 7 may be scanned over the sample 11 to obtain a strobe SEM image, and the signal amount in the measurement area may be obtained from the strobe SEM image.

[0033] An example of the measurement procedure of Example 1 is summarized in a flowchart in Figure 10. Steps S01 to S03 are the procedure for normal SEM observation, and the electron beam irradiated onto the sample 11 does not need to be pulsed. The contact position of the probe is determined (S04), and in order to bring the probe 15 into accurate contact with the microstructure of the sample 11, the probe 15 is brought into contact with the measurement region based on the SEM image (S05). Next, measurement parameters are set that specify the measurement conditions for measuring the time constant of the microstructure (S06). The measurement parameters are as described above, and the period T ｉｎｔ , the number of measurement points, the pulse width τ of the pulse electron beam 7 ｐ , peak current I ｐｅａｋ , sample voltage V ｓ Pulse width τ ｖ , including the voltage amplitude ΔV. ｄｅｔ The timing Δt dependency of the time constant τ is measured, and the measurement results as shown in FIG. 9 are displayed on the GUI (S07). ｄｅｃａｙ is the period T ｉｎｔIf the difference is sufficiently smaller than (YES in S08), the measurement conditions are deemed appropriate. If not (NO in S08), the measurement conditions are inappropriate, so the measurement parameters are reset. If the same measurement area is to be measured under different measurement conditions (YES in S09), measurement parameters specifying different measurement conditions are set and the time constant is measured. Furthermore, if the time constant of another measurement area is to be measured (YES in S10), the probe 15 is brought into contact with the other measurement area, measurement parameters specifying the measurement conditions are set, and the time constant is measured. Once measurements of all measurement areas on the sample 11 have been completed, the sample 11 is removed from the device (S11). Using the above measurement technique, the time constant of the measurement area on the sample 11 can be measured over a wide range, from nanoseconds to microseconds.

[0034] In Example 2, a sample 11 is introduced into the sample chamber, and a sample voltage V ｓ The detected signal amount I when ｄｅｔ The procedure for measuring the time constant of a minute measurement region caused by the electrical resistance R and capacitance C of the sample 11 based on the change in the amplitude of the pulse voltage V is as follows. The configuration of the device for performing measurements according to the second embodiment is the same as that of the first embodiment shown in FIG. 1, and the pulse electron gun 25 mounted thereon is the same as that of the first embodiment shown in FIG. 2. The difference from the first embodiment is that the sample voltage V applied to the sample 11 is ｓ and its control method and analysis method, the details of which will be disclosed below.

[0035] The sample voltage V applied to the sample 11 ｓ The voltage waveform of the sample voltage V can be a pulse voltage (FIG. 3) shown in Example 1, a sine wave (FIG. 11), a square wave (FIG. 12), a triangular wave (FIG. 13), a sawtooth wave (FIGS. 14 and 15), etc. As long as the waveform fluctuates at a constant period, it is not limited to the above waveforms and any periodic fluctuating voltage can be used as the sample voltage V ｓ Here, the sample voltage V ｓ Although the case where a voltage having a sinusoidal waveform is applied will be described, the measurement principle is the same when other waveforms are used.

[0036] As mentioned above, since the peak of the SE41 generation is around 2 to 3 eV, a voltage amplitude ΔV of about 5 V is sufficient to obtain a sufficient detection signal amount Iｄｅｔ The larger the voltage amplitude ΔV, the smaller the change in the detected signal amount I ｄｅｔ However, a general function generator (arbitrary waveform generator) outputs a periodic voltage whose frequency (= 1 / T ｉｎｔ ) increases, the voltage amplitude ΔV that can be set decreases. ｉｎｔ It is necessary to select the measurement conditions taking into consideration the set value of and the voltage amplitude ΔV that can be output under that condition. In this embodiment, since it is necessary to measure the change in signal amount at a plurality of frequencies, it is necessary to measure the change in signal amount at different frequencies (1 / T ｉｎｔ Even when measuring at different frequencies, it is preferable to measure the voltage amplitude ΔV under the same conditions. For example, if the maximum voltage amplitude ΔV that can be set under high-frequency conditions is 5 V and the maximum voltage amplitude ΔV that can be set under low-frequency conditions is 10 V, it is preferable to measure with a voltage amplitude ΔV of 5 V under all measurement conditions.

[0037] When a periodic voltage of multiple frequencies is applied to a measurement area having an electrical resistance R and an electrical capacitance C, the sample surface potential φ ｓ is the applied sample voltage V ｓ However, as the frequency increases, filter characteristics due to the electrical resistance R and capacitance C are observed, and when a periodic voltage with a large frequency is applied, the sample surface potential φ ｓ Under high frequency conditions, no potential fluctuations are observed or the amplitude of the potential fluctuations is small. This situation is shown schematically in Figures 16 and 17. Figure 16 corresponds to low frequency conditions, and Figure 17 corresponds to high frequency conditions. Under high frequency conditions, the detected signal amount I ｄｅｔ Therefore, the amplitude δI of the detected signal I ｄｅｔ The amplitude δI of the periodic voltage V ｓ When the frequency f of the signal is plotted against the frequency f, a graph like that shown in Figure 18 is obtained. ｄｅｔ The amplitude δI ０ The frequency at which is half is the cutoff frequency f ｃ Then, the frequency f and the time constant τ ｄｅｃａｙFrom the relational expression, the time constant τ of this measurement region ｄｅｃａｙ is 1 / (2πf ｃ ) can be calculated as

[0038] The detected signal amount I for each measurement condition with different frequency f ｄｅｔ In order to measure the amplitude δI of the signal, the detected signal amount I ｄｅｔ In the case of the second embodiment, it is necessary to set the timing Δt so that the change in the phase can be measured along with the change in the amplitude. ｉｎｔ The timing Δt is set so that the number of measurement points is about 5 to 10 in the range of ｄｅｔ From this, the detected signal amount I for each frequency f is ｄｅｔ As in the first embodiment, in the measurement of the second embodiment, the detected signal amount I ｄｅｔ It is important to measure the change in the detected signal amount I ｄｅｔ In order to accurately measure the change in the irradiation current I of the pulsed electron beam irradiated onto the sample 11, ＥＢ When the stability of the irradiation performance of the pulsed electron gun 25 becomes an issue in measurement, it is important that the irradiation current I to the sample 11 on the path of the pulsed electron beam 7 is controlled to be stable or constant. ＥＢ is measured, and the detected signal amount I ｄｅｔ The measured emission current I ＥＢ By normalizing the signal with the current value of I, the reliability of the measurement results can be improved. ｄｅｔ The signal amount in the measurement area may be obtained by point-irradiating the pulsed electron beam 7, or the pulsed electron beam 7 may be scanned over the sample 11 to obtain a strobe SEM image, and the signal amount in the measurement area may be obtained from the strobe SEM image.

[0039] An example of the measurement procedure of Example 2 is summarized in the form of a flowchart in FIG. 19. The same steps as those in the flowchart of Example 1 (FIG. 10) are given the same reference numerals, and duplicated explanations will be omitted. In step S21, measurement parameters are set to specify the measurement conditions for measuring the time constant of the fine structure. The measurement parameters are as described above, and include the range of the frequency f to be changed (minimum value f ｍｉｎ and the maximum value f ｍａｘ ), the number of measurement points, the pulse width τ of the pulse electron beam 7 ｐ , peak current I ｐｅａｋ , sample voltage V ｓ The waveform type and voltage amplitude ΔV of the detected signal I ｄｅｔ The frequency f dependency of the amplitude δI of the frequency f is measured, and the measurement results as shown in FIG. 18 are displayed on the GUI (S22). ｃ From the time constant τ ｄｅｃａｙ By using the above measurement method, the time constant of the measurement area on the sample 11 can be measured over a wide range from nanoseconds to microseconds.

[0040] In Examples 1 and 2, a function generator (arbitrary waveform generator) is used as an example of the voltage source 17 that applies the periodically varying voltage to the sample 11, but this is not limiting. Fig. 20 shows an example of the configuration of the voltage source 17 that generates the periodically varying voltage using the voltage output of the photodetector.

[0041] An avalanche photodiode (APD) can be used as the photodetector 71 with a high response speed and time resolution of the order of nanoseconds. Ｒ By applying 72, the parasitic capacitance can be reduced, enabling high-speed operation. The excitation light 75 emitted from the excitation light source 74 for generating a periodically varying voltage is irradiated onto the photodetector 71 with an appropriate setting of intensity and period, and the current signal of the generated electron-hole pairs is converted into a voltage and amplified by the amplifier 73, thereby generating a sample voltage V ｓ Use as.

[0042] The photodetector 71 or amplifier 73 may be mounted in the sample chamber in which the sample 11 is placed, or may be configured to be installed in the atmospheric region and connected to the probe 15 via a feedthrough. The excitation light source 74 for generating the periodically varying voltage and the excitation light source 4 for the photoelectric film 2, which is the source of the pulsed electron beam 7, are connected to a timing controller 18, thereby controlling the sample voltage V applied to the sample 11. ｓ and the timing of the pulsed electron beam 7 irradiating the sample 11 are controlled in synchronization with each other. In this way, the output of the photodetector 71 shown in FIG. 20 can be used as the periodically varying voltage in the first or second embodiment.

[0043] The present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments and modifications have been described in detail to make the present invention easier to understand, and are not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment or modification with the configuration of another embodiment or modification, and it is also possible to add the configuration of another embodiment or modification to the configuration of one embodiment or modification. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment or modification with other configurations.

[0044] 1...transparent substrate, 2...photoelectric film, 3...condenser lens, 4...excitation light source, 5...excitation light, 6...viewing port, 7...pulsed electron beam, 8...anode electrode, 9...deflector, 10...objective lens, 11...sample, 12...insulator, 13...sample stage, 14...detector, 15...probe, 16...control power supply, 17...voltage source, 18...timing controller, 19...control system, 21...accelerating voltage (cathode voltage), 22...extraction voltage, 25...pulsed electron gun, 35...sample voltage, 41, 42, 43...secondary electrons (SE), 44...backscattered electrons (BSE), 45...pole piece, 46...electric field, 51...plug, 52...conductor layer, 53...substrate, 54...insulator layer, 71...photodetector, 72...reverse bias, 73...amplifier, 74...excitation light source, 75...excitation light.

Claims

a detector for detecting signal electrons emitted when the sample is irradiated with the pulsed electron beam; a probe capable of coming into contact with a measurement region of the sample; a voltage source for applying a periodically varying sample voltage to the sample via the probe; a timing controller for synchronously controlling the voltage source and the pulsed electron gun and controlling the timing at which the pulsed electron beam is irradiated in one period of the sample voltage; and a control system, wherein the pulsed electron gun comprises: a first excitation light source for generating pulsed excitation light; a photocathode having a transparent substrate and a photoelectric film; a focusing lens disposed between the first excitation light source and the transparent substrate for focusing the pulsed excitation light that has passed through the transparent substrate on the photoelectric film; and an anode electrode for accelerating and emitting the pulsed electron beam emitted from the focusing position of the photoelectric film, The control system is an electron beam application device that calculates a time constant of the measurement area of ​​the sample based on the detection signal amount detected by the detector of the signal electrons emitted when the pulsed electron beam is irradiated onto the measurement area to which the sample voltage is applied.

2. An electron beam application device as claimed in claim 1, wherein the voltage source applies the sample voltage having a pulse waveform to the sample, and the control system applies the sample voltage of a predetermined period to the measurement region of the sample using the voltage source, varies the timing at which the pulsed electron beam is irradiated in one period of the sample voltage using the timing controller to measure the timing dependency of the detection signal amount, and calculates the time constant of the measurement region of the sample from the measured timing dependency of the detection signal amount.

3. An electron beam application device as claimed in claim 1, wherein the control system applies the sample voltages of multiple frequencies to the measurement region of the sample, measures the frequency dependence of the amplitude of the detection signal amount which fluctuates due to periodic fluctuations of the sample voltage, and calculates the time constant of the measurement region of the sample from the frequency dependence of the amplitude of the measured detection signal amount.

4. An electron beam application device according to claim 3, wherein the sample voltage has any one of a pulse waveform, a sine waveform, a square waveform, a triangular waveform and a sawtooth waveform.

5. An electron beam application device according to claim 1, wherein the photoelectric film is a semiconductor having a negative surface electron affinity.

6. An electron beam application device according to claim 1, wherein the electron optical system includes a focus compensation lens for compensating for time fluctuations in the focus state of the pulsed electron beam caused by application of the specimen voltage to the specimen, and the lens strength of the focus compensation lens is controlled to be synchronized with the specimen voltage.

7. An electron beam application device according to claim 1, wherein the voltage source comprises a second excitation light source which generates excitation light whose intensity varies periodically, a photodetector which is irradiated with the excitation light from the second excitation light source, and an amplifier which converts a current signal from the photodetector into a voltage and amplifies it, and the timing controller synchronously controls the second excitation light source of the voltage source and the first excitation light source of the pulse electron gun.

8. A method for measuring a time constant using an electron beam apparatus, the electron beam apparatus comprising: a sample stage on which a sample is placed; an electron optical system including a pulsed electron gun and focusing a pulsed electron beam emitted from the pulsed electron gun on the sample; a detector for detecting signal electrons emitted as a result of the sample being irradiated with the pulsed electron beam; a probe capable of coming into contact with a measurement region of the sample; a voltage source for applying a periodically varying sample voltage to the sample via the probe; a timing controller for synchronously controlling the voltage source and the pulsed electron gun and controlling the timing at which the pulsed electron beam is irradiated in one period of the sample voltage; and a control system, the pulsed electron gun comprising: a first excitation light source for generating pulsed excitation light; a photocathode having a transparent substrate and a photoconductive film; a focusing lens disposed between the first excitation light source and the transparent substrate and focusing the pulsed excitation light transmitted through the transparent substrate on the photoconductive film; and an anode electrode for accelerating and emitting the pulsed electron beam emitted from the focusing position of the photoconductive film; A time constant measurement method in which the detector detects the signal electrons emitted when the pulsed electron beam is irradiated onto the measurement area to which the sample voltage is applied, and the control system calculates a time constant of the measurement area of ​​the sample based on the amount of detection signal detected by the detector.

9. A time constant measurement method as claimed in claim 8, wherein the voltage source applies the sample voltage having a pulse waveform to the sample, and the control system applies the sample voltage of a predetermined period to the measurement region of the sample using the voltage source, changes the timing at which the pulsed electron beam is irradiated in one period of the sample voltage using the timing controller to measure the timing dependency of the detection signal amount, and calculates the time constant of the measurement region of the sample from the measured timing dependency of the detection signal amount.

10. A time constant measurement method as claimed in claim 8, wherein the control system applies sample voltages of a plurality of different frequencies to the measurement region of the sample, measures the frequency dependence of the amplitude of the detection signal amount which fluctuates due to periodic fluctuations of the sample voltage, and calculates the time constant of the measurement region of the sample from the frequency dependence of the amplitude of the measured detection signal amount.

11. A time constant measuring method according to claim 10, wherein the sample voltage has any one of a pulse waveform, a sine waveform, a square waveform, a triangular waveform and a sawtooth waveform.

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