WO2010148423A1

WO2010148423A1 - An imaging detector for a scanning charged particle microscope

Info

Publication number: WO2010148423A1
Application number: PCT/AU2010/000737
Authority: WO
Inventors: Brendan Griffin
Original assignee: University of Western Australia
Current assignee: University of Western Australia
Priority date: 2009-06-22
Filing date: 2010-06-15
Publication date: 2010-12-29
Anticipated expiration: 2011-12-22

Abstract

The present disclosure provides a component for an imaging detector of a scanning charged particle microscope. The scanning changed particle microscope has a column for providing a scanning beam of the charged particles. The imaging detector is arranged for detecting off-axis electrons at a position outside the column and comprises an electron receiving element that is arranged to generate a signal in response to an intensity of received electrons. The detector component also comprises a filter that has a solid screening element and is arranged such that at least a portion of the electrons that have an initial kinetic energy within a first energy range are captured by the screening element and prevented from being received by the electron receiving element and at least a portion of the electrons that have an initial kinetic energy within a second energy range are received by the electron receiving element. The first energy range includes higher energies than the second energy range.

Description

AN IMAGING DETECTOR FOR A SCANNING CHARGED PARTICLE

MICROSCOPE

Field of the Invention

The present invention broadly relates to an imaging detector for scanning charged particle microscopy, such as a scanning electron microscope.

Background of the Invention

Scanning charged particle microscopes, such as scanning electron microscopes, are important and widely used tools in research, analysis and quality control. Instrument types range from relatively small scale manual devices to complex and automated devices used in semiconductor industry for automated quality inspection of silicon wafers.

Dependent on desired results, either backscattered or secondary electrons can be detected as well as x-rays and cathodoluminescence. Secondary electrons are usually detected if relatively high spatial resolution is required. Secondary electrons have been defined as having a kinetic energy of less than 5OeV. A number of detector types are used for detecting secondary electrons and some detectors have less advantageous properties than others.

Summary of the Invention

The present invention provides in a first aspect a component for an imaging detector of a scanning charged particle microscope, the scanning changed particle microscope having a column for providing a scanning beam of the charged particles, the imaging detector being arranged for detecting generated off-axis electrons including secondary electrons at a position outside the column and comprising an electron receiving element that is arranged to generate a signal in response to an intensity of received electrons, the detector component comprising a filter that has a screening element and is arranged such that at least a portion of the electrons that have an initial kinetic energy within a first energy range are captured by the screening element and prevented from being received by the electron receiving element and at least a portion of the electrons that have an initial kinetic energy within a second energy range are received by the electron receiving element, the first energy range including higher energies than the a second energy range.

The terms "energy", "kinetic energy" or "energy range" and derivatives thereof are used throughout the specification for a kinetic energy or energy range of electrons as generated in the sample and do not include any change in kinetic energy arising from external electric or electromagnetic fields to which a sample or emitted electrons may be exposed.

The inventor has observed that predominant detection of emitted electrons having a kinetic energy below a threshold energy, such as electrons having a kinetic energy within an energy band below the threshold energy, results in an increase in both surface sensitivity and spatial resolution of obtained images.

In one specific embodiment the filter is arranged such that at least a portion of the electrons that have an initial kinetic energy below the second energy range are prevented from being received by the electron receiving element. Consequently the filter in accordance with this specific embodiment may be regarded as a band-pass filter.

The second energy range may include "SET secondary electrons, which are secondary electrons that are generated directly by primary charged particles, and may also include Auger electrons. Further, the first energy range may include "SE2" electrons, which are secondary electrons that typically are generated by backscattered electrons. The energy filter may be arranged so that the second energy range is variable whereby the kinetic energy of electrons that predominantly reach the electron receiving element is variable, for example by varying or changing a voltage associated with a generated electrical field or by changing a relative orientation or position of a screening element of the detector component. The component may be arranged so that an extension and/or a position of the second energy range within an energy spectrum is variable.

The screening element typically is positioned in front of the electron receiving element as viewed from a front portion of the detector element to enable blocking of electrons having a kinetic energy above a threshold energy. In this case the detector component typically is arranged such that a portion of lower kinetic energy electrons having a kinetic energy below the threshold energy are directed around the screening element for detection by the electron receiving element.

For example, the second energy range may range up to 2, 4, 8, 10, 15, 20 30, or even 5OeV or more. The first energy range may be an energy range above 2, 4, 8, 10, 15, 20 30, or even 5OeV or more

In one embodiment the filter is arranged to divert a path of the electrons in an energy dependent manner. In this case the filter typically is arranged to generate an electric field that diverts the electrons.

In one specific example the electron receiving element and the filter of the detector component are arranged to generate an electric field component between the electron receiving component, the filter and a sample. In this case the filter may be arranged so that the electric field component contributes to diverting the electrons in a manner such that predominantly electrons having initially a kinetic energy within the second energy range reach the electron receiving element. Further, the detector element may be arranged so that an - A -

electric field between the detector component and the sample is variable, whereby electron trajectories and the second energy range is also controllable.

The detector component may also have an aperture that reduces an intensity of electrons that are receivable by the electron receiving element. The detector component may be arranged such that an electric field, such as an electric field generated by the electron receiving element, predominantly diverts a portion of the electrons that have initially a lower kinetic energy than other electrons into the aperture for detection.

In one specific embodiment the detector component is arranged for attachment to a conventional detector, such as a conventional Everhart-Thornley secondary electron detector.

The primary charged particles typically are electrons, but may also be any other suitable charged particles such as He⁺ ions where the interaction of the charged particles and the samples causes the emission of low kinetic energy electrons.

The present invention provides in a second aspect an imaging detector for detecting electrons generated in a scanning charged particle microscope, the scanning charged particle microscope having a column for providing a scanning beam of the charged particles, the detector being arranged for positioning at a position outside the column and comprising an electron receiving element that is arranged to generate a voltage in response to an intensity of received electrons, the detector further comprising the detector component in accordance with the first aspect of the present invention.

The detector may be arranged so that the electron receiving element generates an electric signal in response to electrons generated by primary charged particles and that are directly received by the electron receiving element.

Alternatively or additionally, the detector may also be arranged so that the electron receiving element generates an electric signal in response to received electrons that were generated as a consequence of the electrons generated directly be the primary charged particles. For example, the detector may be arranged so that secondary electrons and Auger electrons generated at the sample may generate a cascade of further electrons that are directed to the electron receiving element.

The electron receiving element typically comprises a scintillator that is arranged to generate an electric field.

For example, the detector may be arranged for mounting in the proximity of a sample in a scanning charged particle microscope. In one example the detector is arranged for replacing a conventional detector, such as an Everhart-Thornley detector.

The present invention provides in a third aspect a method of taking an image using a scanning charged particle microscope, the scanning charged particle microscope having a column for providing a scanning beam of the charged particles, the method comprising the steps of: directing a scanning beam of primary charged particles to a sample to generate electrons emitted from the sample; filtering electrons emitted from sample such that at least a portion of the electrons that have an initial kinetic energy within a first energy range are captured by a screening element and being prevented from being received by the electron receiving element and at least a portion of the electrons that have an initial kinetic energy within a second energy range are received by the electron receiving element, the first energy range including higher energies than the a second energy range; detecting the filtered electrons at a position outside the column and using an electron receiving element that generates a signal in response to an intensity of received electrons; and forming an image using the generated signal.

The step of filtering typically comprises controlling a path of the electrons using an electric field. For example, the electron microscope may comprise a detector component that is arranged to generate a suitable electric field. Further, the electron receiving element may also be arranged to generate an electric field and the detector component may be arranged so that the combined electric fields control a path of the electrons.

The method typically comprises varying an electric field to control an energy band associated with electrons that predominantly reach the electron receiving element.

The method typically uses the detector component in accordance with the first aspect of the present invention or the detector in accordance with the second aspect of the present invention.

The invention will be more fully understood from the following description of specific embodiments of the invention. The description is provided with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 shows a detector for a scanning electron microscope in accordance with the prior art; and

Figures 2 to 6 show imaging detectors for scanning charged particle microscopes in accordance with specific embodiments of the present invention. Detailed Description of Specific Embodiments

Referring initially to Figure 1 , the operation of a conventional detector for a scanning electron microscope is now illustrated. Figure 1 shows a schematic outline of a conventional imaging detector 100 that is arranged for positioning within a detection chamber of a scanning electron microscope (SEM). The SEM comprises a column (not shown) with lenses and an electron source and which generates a scanning beam of primary electrons 102. The primary electrons generate secondary electrons, backscattered electrons and Auger electrons at a sample 104. The detector 100, which for example may be provided in the form of an Everhart-Thornley detector, comprises a scintillator 106 to which a bias voltage of the order of +10 to +12 kV is applied at end-face 108. Further, the detector 100 comprises a collector cage 110 which is arranged for application of a bias voltage within the range of -150V to +300V.

Dependent on the bias voltage applied to the collector cage 110, the detector 100 collects in use backscattered electrons, secondary electrons and Auger electrons generated at the sample 104 while the primary beam 102 is scanning across the sample 104. The scintillator 106 generates an electrical signal that is dependent on an intensity of received electrons and the SEM generates an image of the sample on a screen of the SEM.

Imaging detectors in accordance with embodiments of the present invention comprise filters that are arranged to allow predominant detection of SE1 electrons and/or electrons having a kinetic energy within a chosen energy range. Examples of such imaging detectors will be illustrated in the following.

Figure 2 shows a detector 200 in accordance with a specific embodiment of the present invention. For convenience, like reference numerals are used for like components. The detector 200 is positioned outside and below a column of an electron microscope. In this example, the detector 200 comprises a collector cage 202, which has a screening element 204. The bias voltage of the collector cage 202 is chosen so that higher initial kinetic energy electrons 206 that are generated in the sample and emitted in the direction of the detector 200 are largely absorbed by the screening element 204. Higher initial kinetic energy electrons that are emitted in other directions (not shown) largely bypass the detector 200. Lower initial kinetic energy electrons that are emitted in the direction of the detector 200 are also largely absorbed by the screening element 204. However, the bias voltage of the collector cage 202 is chosen so that a combined electric field (resulting form the bias voltages applied to the scintillator 106 and to the cage 202) diverts a portion of the lower initial kinetic energy electrons (the field is expected to divert most, if not all, of the lower kinetic energy electrons) through the cage 202 around the screening element 204 and towards the scintillator 106. Consequently, the detector 200 allows predominant detection of lower initial kinetic energy electrons, such as SE1 electrons or Auger electrons having relatively low kinetic energy.

Figure 3 shows an imaging detector 300 in accordance with a further specific embodiment of the present invention. The imaging detector 300 is related to the imaging detector 200 shown in Figure 2. However, the cage 202 of the imaging detector 300 typically is grounded and the detector 300 comprises a separate element 302 to which a bias voltage of approximately +150 to -300V is applied. Similar to the operation of the detector 200, the bias voltage applied to the detector 302 is selected so that predominantly lower initial kinetic energy electrons 208, such as SE1 electrons or lower kinetic energy Auger electrons, are directed through the cage 202 and toward the scintillator 106.

It should be noted that a change in bias voltage applied to the element 302 of the detector 300 and/or a change in bias voltage applied to the cage 202 of the detectors 200 or 300 results in a change in average initial kinetic energy of the electrons that are predominantly detectable by the detectors. Consequently, it is possible to control an average initial kinetic energy of electrons that are used for image formation.

Figure 4 shows an imaging detector 500 in accordance with a further specific 5 embodiment of the present invention. The detector 500 comprises a scintillator 502 which has an end-face 504 to which a positive voltage of 10 - 12 kV is applied. The scintillator 502 is positioned in a rounded cylindrical shield 506. Further, the detector 500 also comprises a screening element 508, which functions as a screening plate. A positive voltage typically is applied to theo screening element 508. Electrons that are directed towards the detector 500 in a substantially straight line from the sample 104 are largely absorbed by the screening element 508. The scintillator 502 has an end-face 504 to which a voltage applied. Consequently, an electric field is formed between the end-face 504 of the scintillator 502 and the screening element 508. Dependent on5 applied voltage, this field will divert predominantly lower initial kinetic energy electrons, that are emitted from the sample 104 in directions not directly towards the detector 500, towards the scintillator 502 as schematically indicated by a path 510. Consequently, the detector 500 is arranged for predominant detection of lower initial kinetic energy electrons, such as SE1 electrons and o low kinetic energy Auger electrons.

Figure 5 shows an imaging detector 700 in accordance with a further specific embodiment of the present invention. The imaging detector 700 comprises a scintillator 702 to which a positive voltage of approximately 10-12 kV is applied5 to an end-face 704. The detector 700 comprises a shield 706 having openings 707 for receiving electrons and to which a bias voltage of +150 V is applied. These conditions may for example be used for a working distance of 7.2mm and a primary voltage of 15kV. The shield 706 is in this example provided in the form of a mesh and consequently an electric field associated with the scintilator o 702 projects through openings 707 of the mesh. In another variation of the described embodiment the shield 706 may have closed wall portions. Further, the detector 700 comprises a screening element 708 which in this example is at ground potential. The shield 706 has in this embodiment an outer diameter of 40mm at a [position at which the shield 706 is attached to a housing of the scintilator 702. The openings of the shield 706 are squares of 6mm² and the shield 706 is formed from a metallic wire having a diameter of 0.5mm. The shield 706 has a cone that has a length of approximately 35mm. The screening element has a diameter of 12mm. The distance from the screening element 708 to the sample is variable depending on the 'working distance'; the vertical position of the sample in the sample chamber. At a typical working distance of 7mm the screen element 708 is positioned at approximately 15mm from a midpoint (= charged particle beam axis) of the sample.

Alternatively, a voltage of for example + 300 V may be applied to the shield 706 and the screening element 708 may be at ground potential.

A person skilled in the art will appreciate that many different voltages may be applied to the shield 706 and the screening element 708. Further, a voltage applied to the sample may be used to influence an electrical field (and consequently electron trajectories) between the sample, the shield 706 and the screening element 708.

The operation of the detector 700 is generally similar to that of detector 500 illustrated in Figure 4. Electrons that are emitted in a direction directed towards the detector 700 are typically absorbed by the element 708 or the grounded shield 706. However, a portion of electrons that are emitted in other directions are diverted by the electric field generated by the element 708 and the end-face 704 of the scintillator 702. Dependent on the voltage potential at the element 708, especially low initial kinetic energy electrons are diverted around the element 708 into the opening of the grounded shield 706 and towards the scintillator 702. Consequently, the detector 700 allows predominant detection of low initial kinetic energy electrons, such as SE1 electrons. However, electrons of very low kinetic energy are predominantly captured by the shield 706 and consequently the detector 700 comprises a band pass filter. A person skilled in the art will appreciate that a range of voltages may be applied to the shield 706, the screening element 708 and the sample. Control of the applied voltages enables control of an energy range of electrons that pass between the shield 706 and the screening element 708.

Figure 6 shows an imaging detector 800 that is a variation of the detector 700 shown in Figure 5. The detector 800 comprises an additional biased deflector plate 802 that is positioned within the grounded shield 706. In this embodiment the shield 706 has solid walls and an aperture at a front portion through which electrons are received. Further, the shield 706 comprises an internal aperture 804. The bias deflector plate 802 together with the internal aperture 804 allow further fine tuning of an average initial kinetic energy of electrons that is allowed to pass through the aperture 804 towards the scintillator 702. The detector 800 consequently is particularly well suited for carefully fine tuning an average initial kinetic energy of detectable electrons.

It is to be appreciated that in variations of the described embodiments electric fields may also be varied for example by varying voltages applied to end-faces of the scintillators. Further, it is to be appreciated that voltage ranges that are given are only examples of many possible voltage ranges.

Although the invention has been described with reference to particular examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. For example, the filter may comprise a magnetic or electric sector field filter element or spectrometer through which the electrons are directed in a curved path. Further, the detector or detector component may be arranged to influence a kinetic energy of detectable electrons by changing an angular orientation of the detector or the detector component relative to the sample.

Claims

The Claims:

1. A component for an imaging detector of a scanning charged particle microscope, the scanning changed particle microscope having a column for providing a scanning beam of the charged particles, the imaging detector being arranged for detecting generated off-axis electrons including secondary electrons at a position outside the column and comprising an electron receiving element that is arranged to generate a signal in response to an intensity of received electrons, the detector component comprising a filter that has a screening element and is arranged such that at least a portion of the electrons that have an initial kinetic energy within a first energy range are captured by the screening element and prevented from being received by the electron receiving element and at least a portion of the electrons that have an initial kinetic energy within a second energy range are received by the electron receiving element, the first energy range including higher energies than the a second energy range.

2. The component of claim 1 wherein the component is arranged such that at least a portion of the electrons that have an initial kinetic energy below the second energy range are prevented from being received by the electron receiving element.

3. The component of claim 1 or 2 wherein the energy filter is arranged so that the second energy range is variable whereby the kinetic energy of electrons that predominantly reach the electron receiving element is variable.

4. The component of claim 3 wherein the component is arranged so that an extension of the second energy range within an energy spectrum is variable.

5. The component of claim 3 or 4 wherein the component is arranged so that a position of the second energy range within an energy spectrum is variable.

6. The component of any one of the preceding claims wherein the screening element is positioned in front of the electron receiving element as viewed from a front portion of the detector element.

7. The component of any one of the preceding claims wherein second energy range includes "SE1" secondary electrons.

8. The component of any one of the preceding claims wherein the filter is arranged to divert a path of the electrons in an energy dependent manner.

9. The component of claim 8 wherein the filter is arranged to generate an electric field that diverts the electrons.

10. The component of any one of the preceding claims wherein the electron receiving element and the filter of the detector component are arranged to generate an electric field component between the electron receiving component, the filter and a sample.

11. The component of claim 10 wherein the detector element is arranged so that the electric field component is variable.

12. The component of any one of the preceding claims wherein the detector component is arranged for attachment to a conventional detector.

13. The component of any one of the preceding claims wherein the detector component is arranged for attachment to an Everhart-Thornley detector.

14. The component of any one of the preceding claims wherein the primary charged particles are electrons.

15. An imaging detector for detecting electrons generated in a scanning

5 charged particle microscope, the scanning charged particle microscope having a column for providing a scanning beam of the charged particles, the detector being arranged for positioning at a position outside the column and comprising an electron receiving element that is arranged to generate a voltage in response to an intensity of received electrons, the detector further comprisingo the detector component in accordance with any one of claims 1 to 14.

16. The detector of claim 15 wherein the detector is arranged for replacing an Everhart-Thornley detector. 5

17. A method of taking an image using a scanning charged particle microscope, the scanning charged particle microscope having a column for providing a scanning beam of the charged particles, the method comprising the steps of: directing a scanning beam of primary charged particles to a sample to o generate electrons emitted from the sample; filtering electrons emitted from sample such that at least a portion of the electrons that have an initial kinetic energy within a first energy range are captured by a screening element and being prevented from being received by the electron receiving element and at least a portion of the electrons that have5 an initial kinetic energy within a second energy range are received by the electron receiving element, the first energy range including higher energies than the a second energy range; detecting the filtered electrons at a position outside the column and using an electron receiving element that generates a signal in response to an0 intensity of received electrons; and forming an image using the generated signal.

18. The method of claim 17 wherein the step of filtering comprises controlling a path of the emitted electrons using an electric field.

19. The method of claim 18 comprising varying an electric field to control an energy band associated with electrons that predominantly reach the electron receiving element.

20. The method of any one of claims 17 to 19 using the component in accordance with any one of claims 1 to 14.