Patent Description:
It is generally appreciated that combining Raman Spectroscopy or Cathodoluminescence detection with electron microscopy has a number of advantages, particularly when used with what are generally referred to as Environmental Scanning Electron Microscope (ESEM) techniques. For example, Raman microscopy is useful for material identification and when combined with scanning electron microscopy one can specifically identify features of a sample that may have differing characteristics in composition and/or structure. ESEM techniques can operate in modes that enable imaging in low vacuum, relatively humid environments that reduces sample desiccation that produces movement of the sample reducing image resolution. However, it is also generally appreciated that low vacuum, high humidity environments also have negative effects on the imaging performance of the electron microscope. In the presently described example, what is referred to as a Pressure Limiting Aperture (PLA) has been used to separate high vacuum environments, beneficial for electron microscopy, from low vacuum environments, beneficial for ESEM techniques.

The combination of electron microscopes with optical elements for Raman spectroscopy or Cathodoluminescence detection and PLA's is well known, an example of which is described in <CIT>, titled "Particle-Optical Apparatus for Simultaneous Observing a Sample with Particles and Photons". For example, the '<NUM> patent describes a mirror with a hole in the center positioned between an electron column pole piece and a sample. An electron beam travels from a source, passes through the hole in the mirror, and interacts with the sample. The mirror also has a surface that reflects and redirects light from a light source to the sample and is positioned to collect light from the sample and direct it to a light detector.

However, previously described embodiments also had some serious disadvantages. First, the mirror is limited to use in high vacuum environments and because the mirror used to reflect light is positioned between the sample and the electron column pole piece, the mirror significantly reduces the ability of the electrons to reach the electron column pole piece from the sample and thus significantly reduces the efficiency of the electron microscope detectors. Further, it is very desirable to keep the working distance of the electron beam path as short as possible. However, previously described PLA configurations increased the working distance thus degrading the imaging performance of the electron microscope.

<CIT> discloses an apparatus for simultaneous detection of backscattered electrons and photons from a sample. The device includes a direct detection backscattered electron detector and a photon detector.

<CIT> discloses an electron beam excitation fluorescence imaging apparatus and method. The imaging apparatus comprises a scanning electron microscope system, a scanning signal generator, a fluorescence collection coupling system, a semiconductor photodetector, a scanning synchronization signal collector and a synergic control and data processing output system.

<CIT> discloses a collector mirror for carrying out satisfactory light collection, as well as being used to detect charged particles. The collector mirror is arranged between an electron lens system and a sample stage.

<CIT> discloses a sample measuring device that adjusts a position of an energy beam to irradiate and a position of a focal point of a light condensing mirror part, and to prevent displacement of the light condensing part due to vibration with a simple arrangement.

<CIT> discloses a light guide assembly for an electron microscope system, comprising a mirror that includes an aperture through which an electron beam from an electron source passes, wherein the mirror is configured to reflect light from the sample to a light detector and to deflect secondary electrons from the sample to a detector configured to collect said secondary electrons.

<CIT> discloses a system for obtaining layered cathodoluminescence images of a sample wherein the light collecting equipment is highly efficient and wherein the microtoming or Focused Ion Beam equipment does not interfere with the efficiency of the light collecting equipment and wherein the position of the sample with respect to the light collecting equipment is not disturbed in the microtoming or ion beam milling process.

Therefore, a design of an electron microscope configured to enable Raman Spectroscopy in a low vacuum environment without the negative impacts from signal blockage and increased working distances would provide a significant advantage over previous embodiments.

A light guide assembly according to the present invention is defined in claim <NUM>. An electron microscope system comprising such a light guide assembly is defined in claim <NUM>.

The above and further features will be more clearly appreciated from the following detailed description when taken in conjunction with the accompanying drawings. In the drawings, like reference numerals indicate like structures, elements, or method steps and the leftmost digit of a reference numeral indicates the number of the figure in which the references element first appears (for example, element <NUM> appears first in <FIG>). All of these conventions, however, are intended to be typical or illustrative, rather than limiting.

As will be described in greater detail below, embodiments of the described invention include a scanning electron microscope enabled for Raman Spectroscopy or Cathodoluminescence detection. More specifically, the scanning electron microscope is configured with a light guide assembly that detects back scattered and secondary electrons at short working distances.

<FIG> provides a simplified illustrative example of user <NUM> capable of interacting with computer <NUM> and scanning electron microscope <NUM>. Embodiments of scanning electron microscope <NUM> may include a variety of commercially available scanning electron microscopes. For example, scanning electron microscope <NUM> may include the Quattro or Prisma scanning electron microscopes both available from Thermo Fisher Scientific. <FIG> also illustrates a network connection between computer <NUM> and scanning electron microscope <NUM>, however it will be appreciated that <FIG> is intended to be exemplary and additional or fewer network connections may be included. Further, the network connection between the elements may include "direct" wired or wireless data transmission (e.g. as represented by the lightning bolt) as well as "indirect" communication via other devices (e.g. switches, routers, controllers, computers, etc.) and therefore the example of <FIG> should not be considered as limiting.

Computer <NUM> may include any type of computing platform such as a workstation, a personal computer, a tablet, a "smart phone", one or more servers, compute cluster (local or remote), or any other present or future computer or cluster of computers. Computers typically include known components such as one or more processors, an operating system, system memory, memory storage devices, input-output controllers, input-output devices, and display devices. It will also be appreciated that more than one implementation of computer <NUM> may be used to carry out various operations in different embodiments, and thus the representation of computer <NUM> in <FIG> should not be considered as limiting.

In some embodiments, computer <NUM> may employ a computer program product comprising a computer usable medium having control logic (e.g. computer software program, including program code) stored therein. The control logic, when executed by a processor, causes the processor to perform some or all of the functions described herein. In other embodiments, some functions are implemented primarily in hardware using, for example, a hardware state machine. Implementation of the hardware state machine so as to perform the functions described herein will be apparent to those skilled in the relevant arts. Also in the same or other embodiments, computer <NUM> may employ an internet client that may include specialized software applications enabled to access remote information via a network. A network may include one or more of the many types of networks well known to those of ordinary skill in the art. For example, a network may include a local or wide area network that may employ what is commonly referred to as a TCP/IP protocol suite to communicate. A network may include a worldwide system of interconnected computer networks that is commonly referred to as the internet, or could also include various intranet architectures. Those of ordinary skill in the related art will also appreciate that some users in networked environments may prefer to employ what are generally referred to as "firewalls" (also sometimes referred to as Packet Filters, or Border Protection Devices) to control information traffic to and from hardware and/or software systems. For example, firewalls may comprise hardware or software elements or some combination thereof and are typically designed to enforce security policies put in place by users, such as for instance network administrators, etc..

As described herein, embodiments of the described invention include a scanning electron microscope configured with a light guide assembly comprising a mirror configured with a PLA, and that collects back scattered and secondary electrons at short working distances.

<FIG> provides an illustrative example that shows an embodiment of scanning electron microscope <NUM> that includes chamber <NUM> and electron column pole piece <NUM>. Embodiments of electron column pole piece <NUM> include any type of electron column pole piece (sometime referred to as an electron column or a pole piece) typically found with an embodiment of scanning electron microscope <NUM> and generally include elements such as one or more coils and/or one or more electromagnetic lenses such as final lens <NUM> for focusing electron beam <NUM> from electron source <NUM>. It will also be appreciated that <FIG> is for illustration purposes and should not be considered as limiting. For example, <FIG> illustrates final lens <NUM> as an ellipse, however electromagnetic lenses include various configurations and shapes.

The environment within chamber <NUM> may include a high vacuum environment, however as described above it may be desirable to operate chamber <NUM> with a low vacuum or ESEM environment. For example, chamber <NUM> may include a pressure of about <NUM> mbar which is sufficient to remove charge from the surface of non-conductive samples irradiated by electron beam. In some cases, chamber <NUM> may have a pressure substantially equivalent to water vapor of about <NUM> mbar that may be used in combination with an embodiment of sample holder <NUM> with cooling features to achieve equilibrium pressure (<NUM>% relative humidity) of water. Chamber <NUM> may include pressures up to about 40mbar to achieve water vapor equilibrium pressure at a room temperature of about <NUM> or higher. It will, however, be appreciated that the equilibrium pressure of water depends on the temperature of the environment and thus different pressures may be used.

Sample holder <NUM> is typically employed for positioning sample <NUM> in the path of electron beam <NUM> as well as within the field of view of mirror <NUM>. It will be appreciated by those of ordinary skill in the art that sample <NUM> may include any type of sample such as, for example, a biological sample. <FIG> also provides an illustrative example of light guide assembly <NUM> that includes mirror <NUM> with pressure limiting aperture <NUM> positioned to allow electron beam <NUM> to pass through. Those of ordinary skill in the related art appreciate that ESEM microscopes typically use two pressure limiting apertures. For example, one pressure limiting aperture may be located inside an objective lens (e.g. final lens <NUM>) which may have a small diameter as the influence on the field view is weaker, and a second pressure limiting aperture in position close to sample, such as pressure limiting aperture <NUM>, which then limits the field of view. The diameter of pressure limiting aperture <NUM> is therefore optimized for good orientation of the field of view in relation to sample <NUM> at lower magnifications. In the embodiments described herein, the diameter of pressure limiting aperture <NUM> is sufficiently small so that a pressure differential can be maintained between chamber <NUM> and the environment that includes electron source <NUM>. For example, the diameter of pressure limiting aperture <NUM> may be in the range of <NUM>-<NUM>, but typically more than about <NUM>. As described above, the pressure in chamber <NUM> may include a low vacuum pressure of about 30mbar separated by pressure limiting aperture <NUM> from the environment that includes electron source <NUM> that may include a high vacuum pressure below about <NUM>. 1mbar (e.g. to limit the degree of electron scattering that can occur at lower vacuum pressures). In the presently described example, vacuum pressures can be maintained using well known techniques (e.g. vacuum pumps, etc.).

In the embodiments described herein, mirror <NUM> of light guide assembly <NUM> is coupled to electron column pole piece <NUM> with a pressure tight seal. In some cases, light guide assembly <NUM> is coupled to final lens <NUM> of electron column pole piece <NUM> with a pressure tight seal. Also, in some embodiments formation of the pressure tight seal can be improved by using intermediate element <NUM> constructed of a desirable material and configured to interface with electron column pole piece <NUM> or final lens <NUM> and mirror <NUM> without gaps (e.g. there is a pressure tight seal between mirror <NUM> and intermediate element <NUM> as well as a pressure tight seal between intermediate element <NUM> and electron column pole piece <NUM> or final lens <NUM>). It will also be appreciated that in some embodiments pressure limiting aperture <NUM> may be associated with intermediate element <NUM> rather than mirror <NUM>, however it is desirable to position pressure limiting aperture <NUM> as close to sample <NUM> as possible to shorten the path of electron beam <NUM> in the environment of chamber <NUM>. For example, intermediate element <NUM> may be constructed from non-magnetic material so that electron beam <NUM> does not deteriorate. When mirror <NUM> is biased with an electric charge, intermediate element <NUM> should provide galvanic separation (e.g. isolation to prevent flow of electric current) of mirror <NUM> and the objective lens in pole piece <NUM>. In some cases light guide assembly <NUM> may be configured to provide the galvanic separation. In the presently described example, intermediate element <NUM> is not irradiated by electron beam <NUM> that could charge intermediate element <NUM> and deterriorate quality of electron beam <NUM>.

Also, as described above it is an important aspect of the presently described invention to keep the working distance between electron column pole piece <NUM> and sample <NUM> as short as possible to limit beam spreading, but with sufficient distance to a detection element to allow for what is referred to as "cascade amplification" of secondary electrons to form. For example, "cascade amplification" may occur in a mode of operation where water vapor is present. Secondary electrons interact with the water molecules to produce additional secondary electrons, which in turn interact with adjacent water molecules producing more secondary electrons, thereby "amplifying" the number of secondary electrons. It will, however, be appreciated that water vapor is not required for cascade amplification to occur. As described above, it is highly desirable to optimally position sample holder <NUM> close to light guide assembly <NUM>. For example, in a low vacuum environment a desirable working distance between pressure limiting aperture <NUM> and sample <NUM> may include a distance in the range of about <NUM>-<NUM>.

<FIG> also illustrates light source <NUM> that may include any type of light source known to those of ordinary skill in the art for Raman spectroscopy (e.g. a laser, LED, or other type of source). Similarly, <FIG> illustrates detector <NUM> that may include any type of detector known to those of ordinary skill in the art for Raman spectroscopy (e.g. a CCD, photomultiplier, or other type of detector). It will be appreciated that various optical elements known to those in the art may typically be employed to direct light (not shown, e.g. mirrors, beam conditioning elements, and/or lenses) along optical path <NUM> between light source <NUM>/detector <NUM> and sample <NUM> via mirror <NUM>, as well as to condition the characteristics of the light for desirable Raman spectroscopy performance.

<FIG> provides an illustrative example showing a side view of light guide assembly <NUM> with a mirror <NUM> that acts as a Gaseous Secondary Electron Detector (GSED) and collects secondary electrons <NUM> shed from the surface of sample <NUM> in response to electron beam <NUM> when scanning electron microscope <NUM> operates in a mode where the pressure in chamber <NUM> is greater than about <NUM>-<NUM> mbar. For example, electron beam <NUM> comprises primary electrons that interact with the surface of sample <NUM> creating secondary electrons <NUM> that leave sample <NUM> and travel towards mirror <NUM> that has a positive electric charge (e.g. may include about a <NUM> V positive bias) in the mode of operation. In the present example, electrode <NUM> and/or front plate <NUM> may include a substantially neutral charge so that secondary electrons efficiently travel to mirror <NUM> (front plate <NUM> may include a very small positive bias).

<FIG> provides an illustrative example showing a side view of light guide assembly <NUM> when scanning electron microscope <NUM> operates in a mode where chamber <NUM> includes a pressure lower than 1mbar. For example, chamber <NUM> may include a pressure of about <NUM>. 5mbar, where it may be desirable to extend the distance between sample <NUM> and a detector to produce a larger signal through cascade amplification. In such a mode, secondary electrons <NUM> are not collected by mirror <NUM>, rather secondary electrons <NUM> are collected and detected by electrode <NUM> which is positively biased instead of mirror <NUM> that includes a substantially neutral bias (may include a very small negative bias). In the presently described example, electrode <NUM> may be spaced some distance away from mirror <NUM> that provides additional time and space for cascade amplification to occur. Further, light guide assembly <NUM> may include front plate <NUM> that includes a substantially neutral bias (may include a very small negative bias) with aperture <NUM> through which electron beam <NUM> and secondary electrons <NUM> from sample <NUM> pass.

<FIG> provides an illustrative example showing the side view of the light guide assembly <NUM> when scanning electron microscope <NUM> operates in a mode where the pressure in chamber <NUM> is greater than about <NUM> mbar where mirror <NUM> acts as a conversion electrode for back scattered electrons <NUM>. For example, back scattered electrons <NUM> are collected by mirror <NUM> that comprises a negative bias and produces converted secondary electrons <NUM>' by a known process which are released from mirror <NUM>. Converted secondary electrons <NUM>' are then collected and detected by front plate <NUM> that acts as a Gaseous Back-Scattered Detector (GBSD) collection electrode (e.g. with a substantially neutral bias but may include a very small negative bias). In the example presented in <FIG>, electrode <NUM> may include a negative bias.

<FIG> provides an illustrative example showing a side view of light guide assembly <NUM> when scanning electron microscope <NUM> operates in a mode where chamber <NUM> includes a pressure lower than about 1mbar. Similar to the embodiment of <FIG>, chamber <NUM> may include a pressure where it may be desirable to produce a larger signal using cascade amplification, and thus converted secondary electrons <NUM>' are collected and detected by electrode <NUM> which may include a substantially positive bias (e.g. mirror <NUM> may include a substantially negative bias, and front plate <NUM> may include a substantially neutral bias with a very small negative bias).

For the embodiments of both 4A and 4B, converted secondary electrons <NUM>' may be further amplified by cascade amplification within the internal environment of light guide assembly <NUM>. For example, the internal environment of light guide assembly <NUM> may be substantially the same as the environment within chamber <NUM>, that when operated in ESEM mode may include a high degree of relative humidity (e.g. a RH of about <NUM>%). Therefore, converted secondary electrons <NUM>' traveling from mirror <NUM> to front plate <NUM> (as in <FIG>) or to electrode <NUM> (as in <FIG>) increase in number by cascade amplification within the internal environment of light guide assembly <NUM>.

In some embodiments, as illustrated in the example of <FIG>, back scattered electrons <NUM> can be detected by detector <NUM> that may include any type of detector known in the art such as, for example, what are referred to as solid state detectors. This may be desirable in situations when the pressure in chamber <NUM> and in lightguide assembly <NUM> do not provide sufficient amplification between mirror <NUM> and front plate <NUM> and/or in high vacuum conditions. It will also be appreciated that there may two or more embodiments of detector <NUM> positioned in light guide assembly <NUM>, such as the two embodiments illustrated in the example of <FIG> that provides a view from the bottom of the light guide assembly <NUM> (e.g. a view looking towards electron column pole piece <NUM> without front plate <NUM>). Notably, the implementations of detector <NUM> may be positioned laterally relative to mirror <NUM> so that the path of secondary electrons <NUM> and/or back scattered electrons <NUM> from sample <NUM> to mirror <NUM> is not blocked by detector <NUM>. Also, it may be desirable to position detector <NUM> some distance above front plate <NUM> to enable better collection efficiency of back scattered electrons <NUM>.

It will also be appreciated that the pressure in chamber <NUM> may vary in some embodiments, such as a pressure that may be higher or lower for the mode of operation as described for <FIG>, <FIG>, and <FIG>. In some cases, the variation in pressure may have an impact on performance but may also provide other advantages and thus is considered within the scope of the described invention. Further, mirror <NUM> is illustrated if the example of <FIG> as having a substantially ellipsoidal shape, however mirror <NUM> may include any shape that effectively directs light from light source <NUM> to sample <NUM>. It is also important that the shape of mirror <NUM> is effective for collecting light from sample <NUM> (e.g. Raman emissions or cathodoluminescence) and directing the light to detector <NUM>. It is further desirable that the working distances within light guide assembly <NUM> are as short as possible. Therefore, it will be appreciated that the embodiments of <FIG>, <FIG>, <FIG>, and <FIG> are for illustrative purposes and should not be considered as limiting.

Claim 1:
A light guide assembly (<NUM>) for an electron microscope system, comprising a mirror (<NUM>) that includes a pressure limiting aperture (<NUM>) through which an electron beam (<NUM>) from an electron source (<NUM>) passes, wherein the mirror (<NUM>) is configured to reflect light from the sample (<NUM>) to a light detector (<NUM>), to collect back scattered electrons (<NUM>) and secondary electrons (<NUM>) from the sample (<NUM>), and to convert the back scattered electrons (<NUM>) to secondary electrons (<NUM>'), and one or more detectors (<NUM>,<NUM>) configured to collect the converted secondary electrons (<NUM>').