Apparatus for cathodoluminescence in transmission electron microscopy

Abstract

An apparatus for collecting upstream and downstream transmission electron microscope (TEM) cathodoluminescence (CL) emitted from a sample exposed to an electron beam is described. A first fiber optic cable carries first CL light emitted from a first TEM sample surface into a spectrograph. A second fiber optic cable carries second CL light emitted from a second TEM sample surface into a spectrograph. The first and second fiber optic cables are positioned so that the spectrograph produces a first spectrum for the first fiber optic cable and a separate spectrum for the second fiber optic cable. The described embodiments allow for the collection of TEM CL data in a manner that allows the upstream and downstream TEM CL signals to be analyzed separately and simultaneously using an imaging spectrograph.

Description

Background Art

[0001] When high-energy charged particles (such as electron or ion beams) strike a sample, photons may be emitted, depending on the sample material. This phenomenon is known as cathodoluminescence (CL). The collection and detection of these photons, which range in wavelength from ultraviolet (UV) to visible light to infrared (IR), can provide a wealth of information about the sample being studied. Samples in an electron microscope are typically examined using CL by directing the photons emitted by the CL to, for example, a light sensor, an imaging array, or a spectroscopic device, any or all of which may be located outside the electron microscope beam train. The interior of the electron microscope beam train is maintained at a low pressure so that the electrons can travel into the sample without being significantly scattered by gases in the electron beam train. Once the light is collected, it can be transmitted from the low-pressure environment through an optical window to an instrument that analyzes the CL light.

[0002] A common way to collect photons emitted by CL is via a collecting mirror located on the axis of the electron beam (e-beam) and either above the sample (typical for bulk samples examined in a scanning electron microscope (SEM)), below the sample, or above and below the sample (typical for a transmission electron microscope (TEM)). The collecting mirror typically has an aperture to allow the electron beam to pass through the mirror without obstruction.

[0003] Detailed analysis of the spectral information of the CL signal requires coupling the CL light from the collecting mirror to a spectrometer, which is typically used to produce a one-dimensional plot of the light intensity as a function of the wavelength of the light. Some spectrometers (referred to herein as "spectrograms", but may also be referred to by other terms such as "imaging spectrometers" or "imaging spectrographs") have the additional capability to produce a two-dimensional image of the light entering the entrance plane of the spectrograph, where the wavelength of the light is mapped along one dimension, and the position along the plane of incidence at which the light enters the spectrograph is mapped to another dimension.

[0004] In TEM, the electron energy (30-300 keV) is significantly higher than in scanning electron microscope (0.5-30 keV). The sample in TEM is examined at a position where the sample is thin enough so that the electron beam is transmitted substantially through the sample, and the CL light can pass through the top (electron incident surface or upstream surface) and bottom (downstream surface) of the sample. TEM CL sample holders have been developed that collect the light from collecting mirrors placed above and below the sample. Since the TEM sample environment places strict spatial requirements on the sample and the collecting mirrors, the CL light can be transmitted separately from the collecting mirrors to the outside of the TEM environment through separate optical fibers. A spectrometer can be used to analyze the combined light from the optical fibers.

[0005] The higher energy electrons used in TEM are more likely to produce braking radiation, which is also known as Cherenkov radiation, because the electrons travel through the sample faster than the phase velocity of light in the sample medium. This Cherenkov radiation is scattered in the downstream direction and will therefore be substantially collected by the collecting mirror below the sample, but less by the collecting mirror above the sample. A first order approximation of the Cherenkov signal can be achieved by subtracting a multiple of the signal from the top mirror from the signal from the bottom mirror. Analysis of the Cherenkov radiation can give information about the sample's refractive index and can be used to improve dielectric response characterization performed simultaneously by electron energy loss spectroscopy (EELS).

[0006] Some samples, such as photonic devices or layered materials, may produce light distributions that differ significantly in the downstream and upstream directions due to orientation effects unrelated to Cherenkov radiation. An estimate of these differences can be made by subtracting multiples of the signal from the top mirror from the signal from the bottom mirror. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a graph showing the emission of cathodoluminescent light (CL) in a sample generated by an excitation source;

[0008] Figure 2 is a diagram of an apparatus for collecting cathodoluminescent light from a bulk sample in an electron microscope;

[0009] Figure 3 is a plot showing CL emission from the top and bottom surfaces of a TEM sample, reflection from the upstream (top) and downstream (bottom) collection mirrors, and entry into the top and bottom optical fibers;

[0010] Figure 4 is a drawing of an embodiment of one aspect of the present invention for simultaneously imaging separated light signals from upstream and downstream CL collection mirrors;

[0011] Figure 5 is a simulated image of an embodiment according to one aspect of the present invention, showing the distribution of light from two separated optical fibers at the plane of the entrance slit of the imaging spectrograph;

[0012] Fig. 6A shows a simulated image according to an embodiment of an aspect of the present invention, showing the separated spectrum at the plane of the spectrograph camera; and

[0013] Figure 6B Intensity versus wavelength spectra and difference spectra are shown according to exemplary embodiments. DETAILED DESCRIPTION

[0014] Those skilled in the art will recognize other detailed designs and methods that can be developed using the teachings of the present invention. The examples provided here are illustrative and do not limit the scope of the present invention as defined by the appended claims. The following detailed description refers to the accompanying drawings. The same reference numerals in different drawings may identify the same or similar elements.

[0015] like Figure 1 As shown, when an excitation beam source (e.g., an electron beam 10, an ion beam, or a photon beam) transfers energy to the sample, CL light may be emitted from the sample 30. The photons 34 of the emitted CL light may have a wavelength and angle, θ (XY plane), φ (relative to the Z axis) relative to the excitation source beam, which is characteristic of the elemental, chemical, or dielectric properties of the analysis region of the sample 10. Basic information about these properties may be obtained by analyzing the distribution of the intensity (I) of the emitted photons at wavelength I (λ), angle I (θ, φ), or wavelength-angle I (λ, θ, φ). Further information may be obtained by studying the optical polarization of these distributions.

[0016] Figure 2 An electron beam (e-beam) 10 is shown emitted from a pole piece 12 of an electron microscope (not fully shown) and directed to a bulk sample 30. At the point 32 where the electron beam 10 strikes the sample 30, cathodoluminescence (CL) light 34 may be produced. A collecting mirror 20 is provided to reflect the CL light 34 to a detector (not shown), which may be located external to the electron microscope. The collecting mirror 20 will typically have a hole or opening 22 to allow the electron beam 10 to pass through, as the mirror may be made of a material that would otherwise obstruct the electron beam (e.g., diamond polished aluminum). The CL light 34 collected by the collecting mirror 20, when properly focused on the sample 30, produces a light pattern 35 that is collimated along the exit optical axis (not labeled) of the mirror 20. In a typical CL instrument, the light collected by the collecting mirror 20 is sent to one or more CL analysis instruments.

[0017] According to an embodiment of the present invention, Figure 3 A TEM electron beam 310 is shown entering an upper collector aperture 344, traveling through an electron transparent sample 331. The electron beam 310 may generate upstream CL light 332 that travels through the surface of the sample 331 where the incident electron beam 310 enters, or the electron beam 310 may generate downstream CL light 333 that travels through the surface of the sample 331 opposite to where the incident electron beam 310 enters. After traveling through the sample 331, a significant portion of the electron beam 310 continues through a lower collector aperture 345, where it may be further analyzed in a transmission electron microscope (not shown) using techniques such as TEM imaging, TEM diffraction, or electron energy loss spectroscopy (EELS).

[0018] TEM CL systems have been developed that collect CL light from collecting mirrors placed above (upstream) and below (downstream) the sample. The collecting mirrors 342 and 343 can be part of the TEM sample holder or separate from the sample holder. The TEM sample environment places strict space requirements on the sample 331 and the collecting mirrors 342 and 343. The CL light from the collecting mirrors 343 and 342 can be taken outside the TEM environment through separate optical fibers. In practice, the collecting mirrors 342 and 343 do not collect all the light emitted from the sample 331. The collecting mirror 342 will only transmit a portion 352 of the upstream CL light 332 into the upstream optical fiber 362. The collecting mirror 343 will only transmit a portion 353 of the downstream CL light 333 into the downstream optical fiber 363.

[0019] One aspect of the invention provides for simultaneous and separate spectral measurements of light 352, 353 collected from upstream 342 and downstream 343 collection mirrors in a TEM CL system. Figure 4 is a schematic diagram of an embodiment of an aspect of the invention, depicting a spectrograph creating two separate spectra 192 and 193 from light contained in two separate optical fibers 162 and 163. The signal from the upstream optical fiber 362 to the spectrograph input optical fiber 162 can be coupled via one or more fiber optic cables (not shown), which can be easily connected and reconnected. Similarly, the signal from the downstream optical fiber 363 to the spectrograph input optical fiber 163 can be coupled via one or more fiber optic cables (not shown), which can be easily connected and reconnected.

[0020] The optical fibers 162 and 163 may have numerical apertures and core sizes that are unlikely to be optimal for direct coupling into a spectrograph, so coupling optics 170 may be used to correct for the mismatch between the numerical apertures of the optical fibers 162, 163 and the numerical aperture of the spectrograph. In an alternative embodiment, coupling optics are not used, and the optical fibers 162 and 163 may be placed directly near the plane of incidence 176 of the spectrograph.

[0021] Fiber 162 is shown carrying optical signal 152, which is projected through a plane of incidence 176 of the spectrograph via coupling optics 170. The portion of optical signal 152 not blocked by the entrance slit 175 illuminates an area 182 on a diffraction grating 180. The diffraction grating 180 creates a dispersion in the wavelengths of light illuminating area 182 that is focused on area 192 of a two-dimensional camera 190.

[0022] Likewise, optical fiber 163 is shown carrying optical signal 153, which is projected through a plane of incidence 176 of the spectrograph by coupling optics 170. Coupling optics 170 may include one or more lenses or mirrors and may compensate for mismatches in numerical aperture between optical fiber 163 and the spectrograph. The portion of optical signal 153 that is not blocked by the entrance slit 175 illuminates region 183 on diffraction grating 180. Diffraction grating 180 creates dispersion in the wavelengths of light that is focused on region 193 of two-dimensional camera 190.

[0023] In an embodiment of an aspect of the invention, the diffraction grating 180 is located in the infinite space of the spectrograph, and the regions 182 and 183 may partially or completely overlap without causing signal mixing at the camera 190 .

[0024] Figure 5 The optical signal 152 ( Figure 4 ) passes through the plane of the incident slit 175 from the optical signal 152 ( Figure 4 ) is a simulated image of the light beam 172. Similarly, Figure 5 It shows that in the case of optical signal 153 ( Figure 4 ) passes through the plane of the incident slit 175 from the optical signal 153 ( Figure 4 ) is a simulated image of light beam 173. In an exemplary embodiment, light beams 172 and 173 are well separated to prevent signal mixing at camera 190.

[0025] FIG. 6a shows a video from a camera 190 ( Figure 4 ) is a simulated image 191. Image area 192 shows the light signal 152 ( Figure 4 ) after being scattered by diffraction grating 180 and focused on camera 190. Similarly, image area 193 shows light signal 153 ( Figure 4 ) after being scattered by grating 180 and focused on camera 190. Image area 192 and image area 193 are shown as being well separated. Simulated image 191 represents an image projected onto a two-dimensional pixelated device (e.g., an image sensor of camera 190), which may be organized into a plurality of horizontal rows of pixels. Spectra 192 and 193 each include a plurality of horizontal rows of a pixelated device.

[0026] The camera 190 may be a charge coupled device imaging sensor or other solid state two dimensional imaging comprising pixels arranged in an XY plane. The spectra 192, 193 may also be projected simultaneously onto more than one imaging device (eg, a CCD sensor and a photosensitive panel).

[0027] In accordance with an embodiment of an aspect of the present invention, FIG. 6b shows a one-dimensional intensity contrast wavelength spectrum 292, which can be calculated by adding the intensities of the pixels in each image row in the image area 192, which can be calculated by adding the intensities of the pixels in each image row in the image area 193. In an exemplary embodiment, a system controller (not shown) can control the output to a display (e.g., the display shown in FIG. 6a and FIG. 6b). For example, the system controller can include a processor, a microprocessor, or a processing logic unit that can interpret and execute instructions to perform various tasks, such as generating intensity contrast wavelength spectra 292 and 293, calculating the difference spectrum 294 described below, etc. The system controller can be implemented as hardware, software, or a combination of hardware and software, and can perform other tasks to control the operations described herein. In some embodiments, the system controller can be implemented in an electron microscope. In other embodiments, the system controller can be implemented externally relative to the electron microscope, such as in the optical system described herein.

[0028] According to an embodiment of an aspect of the present invention, and with reference to FIG. 6b, spectrum 292 may be obtained by reflecting light from upstream mirror 342 ( Figure 3 ) and spectrum 293 may be generated by light collected from the downstream reflector 343. An advantage of recording the upstream CL spectrum 292 and the downstream CL spectrum 293 separately and simultaneously is that their difference spectrum 294 may be calculated by, for example, a system controller / processing device and displayed in real time. In an embodiment of one aspect of the invention, the difference spectrum 294 is equal to the downstream CL spectrum 293 minus the product of the upstream CL spectrum 292 and a constant. As a first approximation, the constant may be set to 1, but may be adjusted to account for differences in collection efficiency or differences in transmission efficiency through the sample or optical fiber. In cases where Cherenkov radiation is expected to be the primary difference between the downstream CL spectrum 293 and the upstream CL spectrum 292, the constant may be set to the integral of the downstream CL spectrum 293 in the red / IR region 295 divided by the integral of the upstream CL spectrum 292 in the red / IR region 295.

[0029] In another embodiment of one aspect of the invention, there may be more than one CL collection mirror above the sample and more than one CL collection mirror below the sample. Thus, there will be more than one optical fiber coupling the upstream CL light to the spectrometer and / or more than one optical fiber coupling the downstream CL light to the spectrometer. This will result in more than one spectrum for the upstream CL light being projected onto the imaging device and / or more than one spectrum for the downstream CL light being projected onto the imaging device.

[0030] Although the present invention has been described in detail above, it should be clearly understood that it is obvious to those skilled in the relevant art that the present invention can be modified without departing from the spirit and scope of the present invention. Various changes in form, design or arrangement can be made to the present invention without departing from the spirit and scope of the present invention. Therefore, the above description is considered to be exemplary rather than restrictive, and the true scope of the present invention is defined in the following claims.

[0031] Unless explicitly described, any element, action or instruction used in the description of this application should not be interpreted as being critical or essential to the present invention. In addition, as used herein, the article "a" is intended to include one or more items. In addition, unless otherwise explicitly stated, the term phrase "based on" is intended to mean "based at least in part on".

Claims

1. A device for collecting and analyzing cathodoluminescence (CL) light generated by a transmission electron microscope (TEM) sample in an electron microscope, include: a spectrograph having an incidence plane, the spectrograph being configured to produce a spectrum comprising a two-dimensional image of light intensity, wherein one axis of the image corresponds to light wavelength and another axis corresponds to a spatial coordinate of the light entering the incidence plane of the spectrograph; a first fiber optic cable having a first receiving end and a first transmitting end, the first receiving end being configured to carry first CL light emitted from a first TEM sample surface, the first transmitting end being configured to couple the first CL light into the spectrograph; and a second fiber optic cable having a second receiving end and a second transmitting end, the second receiving end being configured to carry second CL light emitted from a second TEM sample surface from which the transmitted TEM electron beam exits, the second transmitting end being configured to couple the second CL light into the spectrograph; wherein the first transmitting end and the second transmitting end are positioned such that the spectrograph generates a first spectrum for the first optical fiber cable and a second spectrum for the second optical fiber cable, And wherein the first spectrum and the second spectrum are mathematically combined to measure a difference between the first spectrum and the second spectrum.

2. The device according to claim 1, in, Coupling optics are used to reduce mismatch in numerical aperture between the spectrograph and the first and second fiber optic cables.

3. The device according to claim 1, further comprising: include: A display, and wherein the measured difference between the first spectrum and the second spectrum is output to the display in real time.

4. The device according to claim 1, in, The difference between the spectra is configured to be used to analyze Cherenkov radiation.

5. The device according to claim 1, in, The first spectrum and the second spectrum are projected onto a single imaging device to form separated first and second spectrums in a single image.

6. A device for collecting and analyzing cathodoluminescence (CL) light generated by a transmission electron microscope (TEM) sample in an electron microscope, include: a first collecting mirror positioned to collect first CL light emitted by the sample on a surface of the sample directly exposed to the incidence of the electron beam; a second collecting mirror positioned to collect second CL light emitted by the sample at a sample surface where the electron beam exits the sample; a spectrograph having an incidence plane, the spectrograph being configured to produce a spectrum comprising a two-dimensional image of light intensity, wherein one axis of the image corresponds to light wavelength and another axis corresponds to a spatial coordinate of the light entering the incidence plane of the spectrograph; a first fiber optic cable having a first receiving end configured to carry first CL light reflected from the sample by the first collecting mirror and a first transmitting end configured to couple the first CL light into the spectrograph; and a second fiber optic cable having a second receiving end and a second transmitting end, the second receiving end being configured to carry the second CL light reflected from the sample by the second collecting mirror, the second transmitting end being configured to couple the second CL light into the spectrograph; wherein the first transmitting end and the second transmitting end are positioned such that the spectrograph generates a first spectrum for the first optical fiber cable and a second spectrum for the second optical fiber cable, And wherein the first spectrum and the second spectrum are mathematically combined to measure a difference between the first spectrum and the second spectrum.

7. The device according to claim 6, further comprising: include: a third collecting mirror positioned to collect third CL light emitted by the sample; a third fiber optic cable having a third receiving end and a third transmitting end, the third receiving end being configured to carry the third CL light reflected from the sample by the third collecting mirror, the third transmitting end being configured to couple the third CL light into the spectrograph; Wherein, the first optical fiber transmitting end, the second optical fiber transmitting end and the third optical fiber transmitting end are positioned so that the spectrometer generates a first spectrum for the first optical fiber cable, a separated spectrum for the second optical fiber cable and a separated spectrum for the third optical fiber cable.

8. The device according to claim 6, in, Coupling optics are used to reduce mismatch in numerical aperture between the spectrograph and the first and second fiber optic cables.

9. The device according to claim 6, further comprising: include: A display, and wherein the measured difference between the first spectrum and the second spectrum is output to the display in real time.

10. The device according to claim 6, in, The difference between the spectra is configured to be used to analyze Cherenkov radiation.

11. The device according to claim 6, in, The first spectrum and the second spectrum are projected onto a single imaging device to form separated first and second spectrums in a single image.

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