WO2011030156A2

WO2011030156A2 - Collection of electromagnetic radiation emitted from particle-irradiated samples

Info

Publication number: WO2011030156A2
Application number: PCT/GB2010/051513
Authority: WO
Inventors: Thomas Walther
Original assignee: University Of Sheffield
Priority date: 2009-09-10
Filing date: 2010-09-10
Publication date: 2011-03-17
Also published as: GB0915875D0; WO2011030156A3; GB2478900A

Abstract

Apparatus for collecting electromagnetic radiation emitted from a sample irradiated with charged particles is disclosed. The apparatus comprises: a solid body of material at least substantially transparent to electromagnetic radiation having wavelengths in at least one range, the body having an external surface and being adapted to locate or enable location of a sample with respect to the body such that at least a portion of the electromagnetic radiation emitted from the sample when irradiated with charged particles enters the body and is incident on the external surface from within the body; reflecting means arranged to reflect, from the external surface and back into the body, at least emitted radiation having wavelengths within said one range incident upon the external surface from within the body; and conduit means arranged to collect electromagnetic radiation emitted into the body from a sample and reflected back into the body from the external surface, and convey the collected electromagnetic radiation away from the body. Corresponding sample holders, measurement systems, and measurement methods are also disclosed.

Description

Collection of Electromagnetic Radiation Emitted from Particle-Irradiated Samples Field of the Invention

The present invention relates to the collection of electromagnetic radiation emitted from samples irradiated with charged particles. Particular embodiments are concerned with apparatus and methods for collecting light emitted from samples irradiated with electron beams, i.e. methods and apparatus for cathodoluminescence applications.

Background to the Invention

Certain embodiments of the invention are concerned with the characterisation of samples using cathodoluminescence techniques.

Cathodoluminescence means light emission under electron irradiation and is an important tool for the optical characterisation of optoelectronic systems, such as light-emitting diodes (LEDs) and laser diodes. Existing products, systems and methods for collecting emitted light in cathodoluminescence (CL) applications mostly rely on curved mirrors or light pipes placed near the specimen to collect the light and guide it sideways into a spectrometer. This typically means there must be space enough for such a mirror or light pipe to be put into an electron microscope near the specimen. While this works well in scanning electron microscopes (SEM) whose specimen chambers are rather large (typically several 10cm), it is highly problematic in modern transmission electron microscopes (TEM) with narrow pole-pieces (typically a few mm gap between upper and lower pole-piece). In SEMs space around the specimen is thus less critical, but the spatial resolution obtainable is reduced with bulk specimens, the CL signals are very weak at typical electron energies of a few 10kV and lots of backscattered electrons make photodiodes or CCD cameras for light detection very problematic. In both types of instruments, SEMs as well as TEMs, the collection efficiency is rather low because only a certain fraction of the light emitted to one side of the specimen can be collected. Attempts using single optical fibres placed near the specimen were unsuccessful due to the combination of very low efficiency of light collection (small numerical aperture) and problems in positioning those fibres relative to the area illuminated by electrons. Commercial CL spectrometers with moving parts in the form of diffraction gratings, such as Gatan's MonoCL3 system, are very bulky and heavy and can cause mechanical vibrations of the electron column, thereby decreasing stability and spatial resolution. Hence, smaller light collection systems, better collection efficiencies and external spectrometers would be desirable for improving the instrumental performance. It should be noted that the highest spatial resolution reported for CL in a SEM so far is 20- 30nm [C E Norman, Inst. Phys. Conf. Ser. No. 169 (2001 ) pp 557-560), while CL in a TEM with an Oxford Instruments MonoCL2 system has already achieved ~15nm [M Albrecht et al., Inst. Phys. Conf. Ser. No. 169 (2001 ) pp 267-272). Most of the quantum dot structures based on compound semiconductor alloys investigated at the present are, however, even smaller, with heights of 2-8nm and lengths of 5-30nm, depending on material and chemical composition; so there is a technological need for CL system in a TEM with a few nanometre spatial resolution, which presently cannot be bought.

With regard to specific prior art documents, GB1369314A discloses a 'Scanning electron microscope having cathodoluminescence devices'. The disclosed light-collecting apparatus is intended solely for SEM applications and is far too large for incorporation into any existing TEM specimen holder. The collecting apparatus comprises a concave cavity described as 'semi-ellipsoidal or, alternatively, hemispherical', thereby allowing only collection of light from one side of the specimen. A reflective coating is applied to 'the bounding surface of the space receiving a specimen' , i.e. the inner surface of a concave piece of material. The structure defining the hollow cavity is relatively bulky.

US4900932A, entitled 'Cathodoluminescence detector utilizing a hollow tube for directing light radiation from the sample to the detector' describes an elliptical hollow mirror acting as a light pipe for SEM-CL. The mirror is concave. The drawings show the detector arrangement will not fit into any TEM specimen holder.

US6721049B, entitled 'Device for efficient light collection from a sample' uses the principle of two focal points of an ellipsoidal mirror in a light optical microscope (not SEM or TEM) for simultaneous illumination and light collection by two optical fibres, respectively, while the sample is placed somewhere in-between. The publication just mentions light, but no electrons. It will be fairly difficult if not impossible with this design to prevent the light from direct illumination contributing to the signal detected, hence, there will be a lot of stray light on top of the signal. US4479714A, entitled 'Reflection densitometer reflection surfaces', describes 'an optical assembly formed with an ellipsoidal internal reflecting surface receiving light from a specimen at a target region located at one focus of the ellipsoidal surface and directing light to a photodetector at the other focus.' The document is concerned solely with densitometers.

US4929041A, entitled 'Cathodoluminescence system for use in a scanning electron microscope including means for controlling optical fiber aperture', describes an optical fibre system for light collection in SEM-CL. The suggested way of terminating an optical fibre near the sample surface, without any coupling, will mean very low collection efficiency and problems in aligning the fibre and the electron beam relative to each other.

The hemispherical mirrors with hole and built-in annular photodetectors described in EP0598569A ('Cathodoluminescence detector', 1994) and similarly in US5468967A ('Double reflection cathodoluminescence detector with extremely high discrimination against backscattered electrons', 1995) are so much larger than the sample in its middle they cannot be incorporated into any TEM.

Summary of the Invention

Certain embodiments of the invention therefore aim to obviate, mitigate, or solve at least partly one or more of the problems associated with the prior art.

Certain embodiments are concerned with providing sample holders and light-collecting apparatus suitable for use in performing cathodoluminescence measurements on samples located between the pole pieces of TEMs.

According to a first aspect of the invention, there is provided apparatus for collecting electromagnetic radiation emitted from a sample irradiated with charged particles (e.g. irradiated with an electron beam), the apparatus comprising:

a solid body of material at least substantially transparent to electromagnetic radiation having wavelengths in at least one range, the body having an external surface and being adapted to locate or enable location of a sample with respect to the body such that at least a portion of the electromagnetic radiation emitted from the sample when irradiated with charged particles enters the body and is incident on the external surface from within the body; reflecting means arranged to reflect, from the external surface and back into the body, at least emitted radiation having wavelengths within said one range incident upon the external surface from within the body; and

conduit means arranged to collect electromagnetic radiation emitted into the body from a sample and reflected back into the body from the external surface, and convey the collected electromagnetic radiation away from the body.

Thus, in contrast to prior art arrangements in which samples were located essentially in empty cavities, and reflection of emitted radiation was via reflecting structures having surfaces forming the boundaries of those cavities, in this apparatus embodying the invention a solid body of transparent material (e.g. quartz) is arranged to receive at least some of the radiation emitted from the irradiated sample and then reflection into the appropriately arranged conduit is achieved by means of internal reflection at the external surface of the solid body. This provides a distinct advantage that a more compact EM radiation-collecting structure can be provided as the reflecting means can be supported or provided by the external surface itself (e.g. the reflecting means can simply be a suitable material or treatment applied to the external surface, in the form of a covering layer or coating). Also, collecting the emitted radiation using a solid body of transparent material provides the further advantage that the material can be selected so as to have a higher refractive index than that of air or a vacuum, thus enabling a larger proportion of the emitted radiation from the sample to be collected than was possible with prior art systems in which the sample was located in air or in a vacuum. By collecting a larger portion of the emitted radiation, a stronger cathodoluminescence signal can thus be obtained from samples in certain embodiments of the invention. In certain embodiments, the body comprises a first cavity (i.e. a first internal cavity) adapted to house a suitably sized sample or portion of a sample. In certain embodiments, this cavity is generally located about a central plane of the body, and about which the body is substantially symmetrical. This can enable EM radiation emitted from both sides of the sample to be collected, thereby further increasing the strength of the collected signal. This cavity may take the form of a slot in certain embodiments, such that a sample or a portion of a sample can be inserted into it, and optionally translated or rotated with respect to the slot to bring different portions of the sample into position for irradiation by a charged particle beam (such as a beam of electrons, or other charged particles in alternative embodiments). Alternatively, in other embodiments the body may have been moulded onto the sample or a portion of the sample. Whilst this does not permit subsequent movement of the sample with respect to the transparent body, it does provide the advantage of providing intimate contact between the body material and the sample to further improve collection of emitted EM radiation.

In certain embodiments, the body may comprise at least one hole arranged to extend from the external surface to the first cavity to enable a beam of charged particles to be directed through the hole and onto a portion of a sample located in the first cavity, and/or permit charged particles passing through a sample to exit the body. The provision of these one or more holes is advantageous in that it prevents or reduces the interaction between the charged particles and the body material, thereby eliminating or reducing the occurrence of spurious signals interfering with the EM radiation signal collected from the irradiated sample. In certain embodiments, these holes are suitably tapered, for example the holes may have substantially conical form. The holes can also be dimensioned to permit some degree of movement of the beam of charged particles with respect to the exposed portion of sample, so that a particular sample portion of interest can be targeted.

The reflecting means may take a variety of forms. For example, it may be a reflective coating, a reflecting layer, or a surface treatment applied to or formed on the external surface of the body. However, other forms of reflecting means may be employed in alternative embodiments. The conduit means in certain embodiments comprises at least one optical fibre, an end of the fibre defining an entrance aperture into which emitted light or other radiation from the sample can be directed by means of one or more reflections from the external surface of the body.

In certain embodiments, the body comprises a second cavity extending into the body from the external surface, and an end portion of the conduit means is located in the second cavity such that an end of the conduit (the end providing the entrance aperture) is positioned inside the external surface of the body.

In certain embodiments said external surface comprises a second cavity extending into the body from the external surface, and an end portion of said optical fibre is located in the second cavity such that an end of the optical fibre is positioned inside the external surface.

In such embodiments, the body may be adapted to locate or enable location of at least a portion of a sample at the first focal point or in the first focal region. Similarly, the conduit means may then be arranged to collect electromagnetic radiation directed to the second focal point or region. For example, in certain embodiments the conduit means extends into the body such that its end (i.e. its entrance aperture) is positioned substantially at or in the second focal point or region.

In certain embodiments the body is ellipsoidal. In certain embodiments the body is a rotational ellipsoid.

This shape provides the combined advantages that it defines first and second focal points or regions (at which the irradiated sample portion and collecting conduit aperture can be located respectively) and EM radiation emitted from the first focal point or region is directed to the second focal point or region by means of just a single internal reflection at the external surface. By requiring only a single internal reflection, the strength of the collected signal is kept high.

In certain other embodiments, the body is shaped such that radiation from the first focal point or region is directed to the second focal point or region by means of two or more reflections. For example, the body in certain embodiments is substantially paraboloidal, such that emitted radiation is collected from the first focal point by means of two reflections.

In certain embodiments, the body is unitary, but in alternative embodiments the body is composite, formed from a plurality of separate parts. In such embodiments, each part may be formed from the same said material, although in alternative embodiments the materials may in certain instances be different.

According to a second aspect of the invention there is provided a sample holder for use in the collection of electromagnet radiation emitted from a sample irradiated with a beam of charged particles, the sample holder comprising:

a solid body of material at least substantially transparent to electromagnetic radiation having wavelengths in at least one range, the body having an external surface and being adapted to locate or enable location of a sample with respect to the body such that at least a portion of the electromagnetic radiation emitted from the sample when irradiated with charged particles enters the body and is incident on the external surface from within the body;

reflecting means arranged to reflect, from the external surface and back into the body, at least emitted radiation having wavelengths within said one range incident upon the external surface from within the body. In certain embodiments of the first and second aspect of the invention, the external surface is flat. In certain embodiments of the first and second aspects the body is multi-faceted, that is the body comprises a plurality of faces. In such embodiments, at least one of the faces may be flat. In certain embodiments more than one face is flat, and in certain embodiments each of these faces is flat. In certain embodiments of the first and second aspects of the invention the body is substantially cuboidal. In certain alternative embodiments, the body comprises at least one pair of non-parallel opposing faces. Thus, in certain embodiments the body comprises at least one pair of opposing faces which are inclined with respect to one another, to assist in guiding internally reflected light to the conduit means.

It will be appreciated that embodiments in which the body comprises one or more substantially flat external surfaces or faces, features described in relation to embodiments with one or more curved external surfaces may also be incorporated (e.g. cavities or slots for sample location, reflective coatings, optical fibres) except where clearly incompatible.

Another aspect of the invention provides a measurement system comprising apparatus in accordance with the first aspect of the invention and a charged-particle source arranged to irradiate at least a portion of a sample located with respect to the body with a beam of charged particles.

This source may comprise an electron source arranged to provide an electron beam in certain embodiments, although in alternative embodiments the source may provide beams of other charged particles. In certain embodiments the source comprises a TEM. The system may further comprise a spectrometer (or spectrum analyser) with the conduit means being arranged to convey collected EM radiation to the spectrometer/analyser, which is in turn adapted to measure at least one property or characteristic of the collected radiation.

A further aspect of the invention provides a measurement method comprising irradiating a sample with charged particles such that the sample emits electromagnetic radiation; arranging the sample with respect to a solid body of material such that at least a portion of the emitted electromagnetic radiation enters the body and is incident on an external surface of the body from within the body; and

collecting at least a portion of the emitted electromagnetic radiation by reflecting at least a portion of the electromagnetic radiation incident on the external surface from within the body back into the body from the external surface and into a conduit (e.g. into an entrance aperture of the conduit).

In certain embodiments, the body is rotationally symmetric, the external surface defines first and second focal points or regions, and the method comprises:

irradiating a portion of the sample located at or in the first focal point or region, directing at least a portion of the electromagnetic (em) radiation emitted into the body from the first focal point or region to the second focal point or region by means of at least one internal reflection from the outer surface, and collecting a portion of the em radiation directed to the second focal point or region in the conduit.

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the accompanying figures of which:

Figs. 1 -5 are different views of a sample holder embodying the invention for use in the collection of light emitted from a sample irradiated with electrons; Fig. 6 illustrates the sample holder of Figs. 1 -5 mounted together with a sample on a sample holder rod for insertion between the pole-pieces of the objective lens of a TEM;

Fig. 7 is a schematic cross section of electromagnetic radiation collection apparatus embodying the invention and incorporating a sample holder (which may also be described as an optical cavity) of the type shown in Figs. 1 -5;

Fig. 8 is a schematic cross section of part of alternative electromagnetic radiation collection apparatus embodying the invention; Fig. 9 is a schematic cross section of yet another apparatus embodying the invention for collecting light emitted from a sample irradiated with a beam of charged particles; Fig. 10 is a schematic cross section of another sample holder embodying the invention;

Fig. 1 1 is a schematic cross section of yet another sample holder embodying the invention and in which a sample has been mounted;

Fig. 12 is a schematic representation of cathodoluminescence apparatus embodying the invention; Fig. 13 is a schematic view of another sample holder embodying the invention, and connected to an optical fibre;

Fig. 14 is a schematic view of another sample holder embodying the invention, again connected to an optical fibre; and

Fig. 15 is a schematic view of yet another sample holder embodying the invention and connected to an optical fibre for collecting emitted radiation.

Detailed Description of Embodiments of the Invention

Referring now to Fig. 1 , this is a perspective view of a sample holder embodying the invention. The sample holder comprises an ellipsoidal (or substantially ellipsoidal) solid body 1 formed from a material which is transparent at least to the electromagnetic wavelengths of interest. The ellipsoid (which in this example is actually a rotational ellipsoid) has a longitudinal or major axis A, and it will be appreciated that Fig. 1 is a perspective view looking at an acute angle to the axis A. Fig. 2 is a side view of the sample or specimen holder (which may also be referred to as an optical cavity), Fig. 3 is a top view, Fig. 4 is a cross section of the sample holder along line A-A from Fig. 2, and Fig. 5 illustrates how the shape of the body 1 is based on a rotation ellipsoid. In this particular example the body 1 of transparent material is formed from quartz, although in alternative embodiments the body may be formed from different materials, such as: fused silica, optical glass, transparent sintered ceramics, plastics or cured polymers. In Figs. 2- 3, and 4 a number of dimensions are shown, and the units are millimetres. The solid body 1 has an external surface 10 on which is formed a reflective coating 1 1 such that electromagnetic radiation of at least some wavelengths travelling through the transparent body and incident upon the coated surface 10 is reflected from that external surface 10 back into the body. Thus, the coating 1 1 provides reflecting means for reflecting radiation back into the body from the outer surface 10. The solid body 1 is adapted to enable location of a sample with respect to the body such that at least a portion of the electromagnetic radiation emitted from the sample when irradiated with charge particles can enter the body and be incident on the external surface from within the body. In this particular example this adaption of the solid body takes the form of a cavity 12 provided in the body and in which a suitably shaped and dimensioned sample, or part of a sample, can be located. This cavity 12 in this first embodiment is in the form of a transverse slot or slit extending part way along the ellipsoidal body (along major axis A) and extending from one side of the body to the other. Flat, opposing internal surfaces 121 of the body 1 thus define the upper and lower sides of the slot, and these surfaces 121 are parallel to one another and are equally spaced apart, above and below the longitudinal axis A. In this example, the end of the slot is straight, although it will be appreciated that in alternative embodiments the slot may have alternative shapes, for example to suit different sample geometries. The slot-shaped cavity 12 of the embodiment of Figs. 1 -5 is suitable, for example, for accommodating a variety of sample shapes and sizes, such as thin sheet or disc-shaped samples. The slot-shaped cavity 12 enables disc-shaped samples to be inserted to various depths within the holder, and rotated about an axis perpendicular to the general plane of the slot 12 to bring different portions of the sample into position for irradiation by a charged particle beam, such as an electron beam. The sample holder 100 further comprises first and second holes 14, 15 extending through the body 1 in a direction which crosses the longitudinal axis A and is perpendicular to that axis. The first hole 14 is generally conical, tapering inwardly from a mouth at the upper portion of the external surface 10 down to a smaller mouth in the upper flat wall or surface 121 of the slot 12. Similarly, hole 15 is substantially conical, and widens from a mouth in the lower flat surface or side wall 121 of the slot 12 out to a wider mouth in a lower portion of the external surface 10 of the body 1. These substantially conical holes 14, 15 are arranged to enable an electron beam to be directed through them such that it irradiates a sample (or more typically a portion of a sample) at least partially accommodated within the slot 12. The holes are conical to reduce or eliminate interaction between the electron beam and the reflective coating 1 1 or the body material, which could lead to spurious signals being generated, and to allow for tilting of the combined specimen-cavity arrangement with respect to the incident electron beam. It will be appreciated that the size and shapes of the holes 14, 15 can be adjusted to suit requirements. Generally, the larger these holes, the greater the proportion of radiation emitted from an irradiated sample that is lost (i.e. is not collected). On the other hand, larger holes 14, 15 permit more movement of an electron beam with respect to the sample located in the slot 12 such that features of interest can be identified and the electron beam targeted at them. The outer surface 10 of the ellipsoidal body 1 provided with the reflective coating 1 1 defines two focal points. A first focal point FP1 is located on the axis A, within the slot 12, and between the inner mouths of the holes 14 and 15. Thus, a sample or a portion of a sample can be located in the slot 12 such that a portion of the sample positioned on the focal point FP1 can be irradiated with an electron beam via hole 14. It will be appreciated that hole 15 provides an exit hole through which electrons travelling through the sample in the slot 12 can exit the body, without interacting with its material. The second focal point defined by the ellipsoidal body is FP2. The arrangement is such that when electromagnetic radiation is emitted from a sample or part of the sample located at the first focal point it is reflected from the inner surface 10 and directed to the second focal point FP2. In this first embodiment the solid body 1 is provided with a second cavity 13 in the form of a longitudinal hole extending from one end of the body, along the longitudinal axis A, and towards the slot 12, the end of this second cavity 13 generally coinciding with the position of the second focal point FP2. The cross section of this second cavity 13 is generally circular, and the cavity 13 is adapted to enable insertion of an optical fibre or suitable bunch of optical fibres into it to collect light from an irradiated sample reflected to focal point FP2 via the internal surface 10, and so be able to convey that collected light away from the body 1 , for example for analysis with a spectrum analyser/spectrometer. It will be appreciated that, as a result of the symmetry of the ellipsoidal body 1 of this first embodiment, the sample holder provides the advantage that it is able to collect light or other electromagnetic radiation emitted from both sides of the sample or portion of sample illuminated with the electron beam. A further advantage is that, because of the ellipsoidal shape of the body 1 , any light emitted into the body 1 from the sample is reflected generally to the second focal point (and so can be collected and conveyed away from the sample holder via a suitable conduit arranged in the cavity 13) by a single internal reflection (i.e. a single reflection at the coated external surface 10). The fact that light can be collected from both sides of the sample and only a single reflection is required to direct it to the second focal point significantly improves the strength of the collected signal, thereby offering significant advantages over prior art arrangements. Furthermore, the sample holder can be made much smaller than prior art arrangements, as, rather than reflecting surfaces being provided on the inwardly facing surfaces of a structure defining a hollow cavity, the solid body can be made small and directly supports the reflecting means in the form of a reflective coating 1 1 on its external surface 10. In effect, the solid body defines the shape of the interior of the optical cavity (albeit a cavity filled with transparent material) and a thin reflective coating does not increase the overall size of the radiation-collecting apparatus to any appreciable degree. Thus, a sample holder such as that shown in Figs. 1 -5 can be manufactured and dimensioned so as to fit within the small space between the pole pieces of a TEM for example. Returning to Fig. 4, from this cross section it can be seen that the solid body 1 in this first example has a generally circular cross section in a plane perpendicular to its longitudinal axis A, and its cross section along a plane including the longitudinal axis A is elliptical.

Referring now to Fig. 6, this shows a sample holder (which may also be described as an optical cavity) of the type shown in Figs. 1 -5 mounted together with a sample 2 on a sample holder rod for insertion between the pole pieces of a TEM. In this example the sample 2 is in the form of a thin circular disc of material, a portion of which is accommodated within the cavity or slot 12 in the sample holder 100. The arrangement is such that the TEM sample holder rod tip can be inserted between pole pieces such that an electron beam is directed through hole 14 to irradiate a portion of the sample 2 positioned beneath that hole 14. As a result of this electron radiation, light is emitted from that portion of the sample, enters the body 1 , and is then directed to the second focal point FP2 by means of an internal reflection at the external surface 10. In the apparatus shown, an optical conduit in the form of an optical fibre 3 is positioned with its end inserted in the second cavity 13 in the body 1 , that end terminating at the general location of the second focal point. Thus, light emitted from the irradiated sample is reflected from the surface of the cavity internally to the second focal point, where it can enter the optical fibre and be conveyed away from the body 1 .

Referring now to Fig. 7, this is a schematic cross section of radiation-collecting apparatus embodying the invention incorporating a sample holder generally of the type described above with reference to Figs. 1 -6, and being used to collect light emitted from a sample 2 irradiated with an electron beam. The apparatus comprises a sample holder 100 comprising a solid body 1 of transparent material having a generally ellipsoidal outer surface 10 on which a reflective coating 1 1 is provided. It will be appreciated that a wide variety of reflective coatings and reflective materials may be employed in embodiments of the invention to provide suitable reflection for wavelengths of interest. For example, suitable reflective coatings include coatings of platinum, sliver or aluminium. These coatings may be formed on the surface using a variety of techniques, including dip coating, sputter deposition, evaporation, liquid or chemical vapour phase epitaxy. Also, although in certain embodiments reflection at the external surface 10 may be achieved by suitably coating the outer surface, in alternative embodiments other techniques may be employed to achieve this reflection, for example by encapsulating the solid body 1 in a different material having a suitable refractive index. Returning to the embodiments shown in Fig. 7, a sample 2 of material for investigation has been partially inserted into the slot-shaped cavity 12 such that a portion of that sample 2 is located at the first focal point FP1 of the ellipsoidal reflecting structure. An electron beam is directed at that focal point FP1 (this beam being denoted generally by arrow A1 in the figure and so irradiates and interacts with the portion of the sample 2 under entrance hole 14). Electrons passing through the sample 2 are able to exit the body via exit hole 15. The figure shows two ray-paths of electromagnetic radiation emitted from the sample portion at focal point FP1 . A first ray R1 is emitted from a first, upper side of the sample 2, enters the upper half of the rotationally-symmetric body, undergoes a single reflection at the external surface 10, and is directed to the second focal point FP2, where it enters an optical conduit in the form of optical fibre 3 which can convey the collected radiation away from the body (indicated generally by arrow A2). A second ray R2 is emitted from the second, lower side of the sample 2, enters the lower half of the body 1 , and also undergoes a single internal reflection at the surface 10, by means of which it is also directed to the second focal point FP2 and into the conduit 3.

Referring now to Fig. 8, this shows another sample holder embodying the invention. In this embodiment, a solid body of transparent material 1 has been moulded around a sample 2. As such, the sample 2 can still be regarded as being located in a cavity within the body 1 , although the sample 2 completely fills this cavity. In this example, rather than the body 1 being ellipsoidal, it comprises left and right halves 102, 101 respectively, each of which is substantially parabaloidal. The left half 102 has a parabolic cross section, and defines a second focal point FP2. Light travelling parallel to the axis A of this second half 102 will always be reflected from the outer surface 10 of this left half 102 towards the second focal point FP2. Similarly, the right half 101 of the body has a parabolic cross section, and defines a first focal point FP1 which in this example lies within the encapsulated sample 2. This right half 101 is arranged such that light emitted from the portion of the sample at focal point FP1 is reflected from the external surface 10 in a direction parallel to the common longitudinal axis A of the two halves. Although not shown in the figures, a beam of charged particles may be directed at the sample 2, and in particular at the portion of the sample at or close to focal point FP1 , such that substantially all electromagnetic radiation emitted from the sample 2 is directed by means of two internal reflections (as shown for first and second rays, R1 and R2) to the second focal point FP2. It will be appreciated that suitable holes may be formed in the body through to the encapsulated sample for the incident and exiting beams. These holes may be formed at the same time the body is moulded around the sample, or subsequent to that operation. Referring now to Fig. 9, this shows alternative apparatus embodying the invention. Here, the solid body 1 of transparent material and its reflective outer surface 10 is arranged to collect radiation emitted just from a single side of a sample 2 irradiated by beam B of charged particles. Rather than being rotationally symmetric, the body 1 has a curved or domed upper outer surface 10 on which a reflective coating or layer 1 1 is formed, but has a substantially flat, non-coated lower surface 120 on which the sample 2 can be located/mounted. The outer surface upper portion 10 is generally shaped so as to define a first focal region FR1 and a second focal region FR2. Thus, generally electromagnetic radiation emitted into the body 1 from a position within the first focal region FR1 will be directed to the second focal region FR2 by means of a single reflection at the external surface 10. A beam exit hole 15 is provided through the body 1 such that charged particles passing through the sample 2 can exit the body 1 without interacting with its material, and hence without generating spurious electromagnetic radiation which would interfere with the signal collected from the second focal region FR2. In this example, a conduit in the form of a light pipe having reflective side walls 31 is arranged to collect light reflected into the second focal region FR2 and convey that reflected light away from the body 1 . Although this arrangement is only able to collect radiation emitted from one side of the sample, it again provides the advantage that it can be manufactured in a very compact form, for example for location between the pole-pieces of the objective lens of a TEM, as the reflecting surface is simply defined by the external surface of the solid body 1 .

Referring now to Fig. 10, this shows an alternative sample holder embodying the invention. The sample holder 100 again comprises a solid body of transparent material, but rather than being formed as a single piece of material, the body is formed from a plurality of pieces, namely an upper body portion 1A and a lower body portion 1 B. In this example these two body portions substantially form respective halves of the composite structure, and so they are substantially identical to one another. When they are arranged together, as shown in the figure, with the boundary between them located generally along the centreline A, they together define a first cavity 12 inside which a sample can be located, and a second, open-ended cavity 13 for insertion of a suitable conduit for collecting and conveying the emitted electromagnetic radiation from the sample. As will be seen in the figure, apart from the beam entrance and exit holes 14, 15 respectively provided in the first and second halves 1A, 1 B, the cavity defined by the two halves is closed, i.e. it does not extend to the end of the composite body (i.e. the right hand end of the composite body in the figure). This arrangement provides the advantage that a larger proportion of the radiation emitted from the irradiated sample can be collected and directed to the second focal point FP2. Again, the external surface 10 of the composite body is arranged to define a first focal point FP1 inside the cavity 12, and a second focal point FP2, generally at the end of the second cavity 13.

Referring now to Fig. 1 1 , this shows yet another sample holder embodying the invention. Again, the sample holder 100 comprises a composite body of transparent material, formed by fitting together a plurality of parts. These parts in this example comprise a main part 1A and a minor part 1 B which represents approximately one quarter of the volume of the assembled composite body. With the minor portion 1 B removed, the main portion 1A provides a flat surface 120 on which a sample 2 can be positioned/mounted. The minor portion 1 B can then be arranged in place to complete the body, with the sample 2 thus sandwiched between the two components 1A, 1 B. Again the body is provided with a reflective coating 1 1 on both of its parts 1A, 1 B, and the external surface 10 of the composite body again defines first and second focal points FP1 , FP2.

Referring now to Fig. 12, this illustrates a cathodoluminescence spectroscopy apparatus embodying the invention. The apparatus comprises a source of electrons 4, arranged to emit an electron beam B as a portion of a sample 2 located between entrance and exit holes 14, 15 in a sample holder 100 also embodying the invention. The portion of the sample 2 irradiated by the beam B is generally located at a first focal point of the sample holder 100. The incident electrons cause that portion of the sample to emit electromagnetic radiation and that emitted radiation is directed into an optical fibre or optical fibre bundle 3 having an inlet (aperture) arranged at a second focal point of the structure. Just a single reflection is required to achieve this. The optical conduit 3 then conveys the collected radiation emitted from the sample to a spectrometer 5.

It will be appreciated from the above description that certain embodiments provide a specimen holder for cathodoluminescence applications/measurements. Certain embodiments provide a new system for improved collection of cathodoluminescence signals in a transmission electron microscope. Certain embodiments provide a mechanism to collect light simultaneously from both sides of a thin film sample that is surrounded or encapsulated by an optically transparent rotational ellipsoid or paraboloid that is covered by a highly reflective metal coating on the outside, has an opening to illuminate a part of the sample by an electron beam and is small enough to fit into the objective lens pole piece of a transmission electron microscope. Certain embodiments of the invention consist of a newly conceived optical cavity small enough to fit into the restricted space of the specimen stage in a transmission electron microscope . The solid body forming the "optical cavity" has a convex outer surface coated to render it reflective, thereby defining an inwardly facing concave reflective surface. The cavity couples the light collected from the specimen in its centre out to a spectrometer or other detector by means of an optical fibre or a bundle thereof. The cavity is made of a material transparent to light (UV, visible or infrared, or any combination thereof) and coated by a thin metal layer which reflects light. Light emitted in both forward and backscattering geometry can thus be collected by internal reflection and fed into an external detector by integrated optical fibre(s). Certain embodiments of the invention relate to the field of physical/optical characterisation of optoelectronic materials by means of electron microscopy. The stimulation in cathodoluminescence is performed by a small focused electron beam.

One embodiment of the invention consists of a newly conceived optical cavity that is small enough to fit into the restricted space of a transmission electron microscope's pole-piece, almost completely surrounds the specimen, lets the electron beam pass through a tiny hole, is coated by a metal layer and guides the light collected to a spectrometer or other detector by means of an optical fibre or a bundle thereof. One novel aspect is the design of the cavity which is ellipsoidal or paraboloidal in form, with the specimen region of interest and the optical fibre (bundle) near the two optical focal points to yield an effective coupling and with a reflective outer coating to minimise radiation losses. The design can be made as small as a few millimetres, making it suitable for incorporation into very small spaces such as specimen holders in transmission electron microscopes with narrow pole-piece gaps. At the same time it enhances the optical efficiency by detecting light emitted both below and above the specimen's mid-plane, i.e. in forward and backscattering geometry, simultaneously.

Embodiments of the invention have substantial commercial relevance. CL is a method useful to study the optical quality of small structures such as nanoparticles or quantum dots made of semiconductors and therefore plays a key role in the development and the quality control of such nanostructures. The excitation is performed by an electron beam generated either in an SEM or a (S)TEM. The SEM route is fairly straight-forward and some commercial systems exist. The TEM/STEM route is technologically much more complicated but potentially more rewarding due to the higher spatial resolution obtainable. There are only a handful CL-TEM systems worldwide, mostly in university laboratories, and none is commercialised. The systems embodying the invention are of significant commercial importance for a number of reasons, including:

1 . because of their miniature size, certain sample holders embodying the invention it can be used in high-resolution TEM instruments that have a narrow pole-piece gap and often field-emission electron guns with higher brightness than standard thermal emitters

2. due to their capability to collect light from upper and lower specimen sides, certain specimen holders embodying the invention are able to deliver higher (stronger) signals, with improved signal-to-noise ratios

3. the ability to incorporate sample/specimen holders embodying the invention into a specimen holder for a TEM, this improves instrument flexibility (transferring the functionality from the main microscope column to an exchangeable add-on detector) and reduces costs (there is no need for external mechanics).

Sample holders embodying the invention may be combined with cooling systems (such as liquid nitrogen or liquid helium sample cooling holders or cooling stages) to enable cathodoluminescence spectroscopy in high-resolution field-emission transmission electron microscopes.

Specimen holders embodying the invention find applications in various fields, for example in the testing/measurement/characterisation of semiconductor materials used in optoelectronics.

Sample holders embodying the invention may be manufactured by a variety of techniques, including (but not limited to) the following: i) production from clear resin (e.g. ABS-like resin) using rapid prototyping, followed by machining to produce one or more features (e.g. slot, second cavity, first and second holes). The resin is selected to have appropriate optical properties for the intended application. For example, certain resins absorb light strongly below 350nm, so they can only be used for the visible-IR range (suitable for e.g. GaAs based systems), not for the UV-visible range (i.e. unsuitable for GaN-based systems).

ii) production from moulded plastics.

iii) production from solid transparent materials by appropriate machining (e.g. CNC cutting).

Suitable materials include quartz, and Spectrosil, a form of fused silica with the required transmission properties over the whole range of wavelengths of interest for cathodoluminescence measurements. Other suitable materials include: optical glass or cured polymer materials. Referring now to Fig. 13, this shows an alternative sample holder embodying the invention, which does not possess any curved external surfaces, but instead is substantially a simple rectangular solid with six faces (i.e. the body is cuboidal). Thus, the body 1 is multi-faceted, and this rectangular solid provides the advantages that it is relatively simple to manufacture. The slot 12 for location of a sample may, for example, be cut in one end of the rectangular solid, and the holes 14 and 15 for the electron beam may be formed by drilling completely through the rectangular solid body 1 . In this example, external surfaces 10 of the body 1 are provided with a reflective coating 1 1 , but the internal surfaces of the slot 12 are not so-coated. Thus, radiation emitted from a sample located within the slot as a result of bombardment of the sample by charged particles can enter the body 1 , and then are internally reflected from the coated external surfaces 10, at least some of this internally reflected radiation being collected and conveyed away from the sample holder 100 by means of optical fibre 3. In use, the sample holder 100 may be rotated about the tilt axis TA, which is generally perpendicular to the longitudinal axis A of the body.

Referring now to Fig. 14, this shows another sample 100 which, like the embodiment of Fig. 13, possesses no curved external surfaces. Instead, it is multi-faceted, and each face is substantially flat. This solid body 1 has been formed with cut edges to remove corners which could otherwise protrude when the sample holder is tilted. The body 100 in this example is slightly tapered such that is narrower towards the side at which the optical fibre 3 is connected. This helps in avoiding light bouncing back and forth infinitely between the main opposing faces of the body. If those main faces were parallel to one another, then collection of light by the optical fibre 3 would be less efficient, and hence signals would be weaker. Thus, in Fig. 14 the main opposing faces (which are the faces on top of the body and beneath the body in the figure) are non-parallel, that is they are slightly inclined with respect to each other. Similarly, the opposing minor faces (the edge faces in which the ends of the slot 12 can be seen) may also be inclined with respect to one another, again so that they narrow towards the optical fibre end. It will be appreciated that in alternative embodiments the inclination of pairs of opposing faces may be arranged differently, such that, for example, those faces widen towards the position of the conduit arranged to collect radiation. Again, in the embodiment of Fig. 14, the external surfaces 10 are rendered reflective by suitable coatings and/or treatment. The slightly tapered nature of the body 1 of Fig. 14 assists in guiding the internally reflected light towards the entry of the optical fibre 3. Referring to Fig. 15, this shows another embodiment in which the body 1 is a solid of trapezoid shape, incorporating a plurality of pairs of opposing faces which are inclined with respect to one another so as to further improve light collection efficiency compared with the embodiments shown in Fig. 13.

It will be appreciated that the faceted bodies of the embodiments shown in Figs. 13, 14, and 15 provide the advantage that they are easier to fabricate than some of the bodies incorporating curved external surfaces, but embodiments incorporating elliptical or parabaloid designs can provide the following advantages over the multi-faceted arrangements; the light collection efficiency of the elliptical and parabaloid designs may be much higher than the faceted bodies because those faceted bodies lack precise focal points. The elliptical or parabaloid designs provide a greater degree of freedom for tilting compared with the faceted designs. When tilted (as will be necessary to align crystals and layer structures edge-on between the pole pieces of a charged particle source) the edges and corners of the faceted designs may protrude from the holder and thus will restrict the tilt range compared with curved embodiments, particularly in narrow high-resolution pole piece designs. The curved (e.g. elliptical or parabaloid) sample holders thus provide the advantage that they are able to be tilted without potentially damaging pole pieces by scratching against them. It will be appreciated that a wide range of materials may be used for faceted embodiments such as those shown in Figs. 13-15, as described above in relation to the other embodiments.

Claims

1 . Apparatus for collecting electromagnetic radiation emitted from a sample irradiated with charged particles, the apparatus comprising:

reflecting means arranged to reflect, from the external surface and back into the body, at least emitted radiation having wavelengths within said one range incident upon the external surface from within the body; and

2. Apparatus in accordance with claim 1 , wherein the external surface is curved.

3. Apparatus in accordance with any preceding claim, wherein the body comprises at least one substantially flat surface against which a sample is or can be located.

4. Apparatus in accordance with any preceding claim, wherein the body comprises a first cavity adapted to house a suitably sized sample or portion of a sample.

5. Apparatus in accordance with claim 4, wherein said first cavity comprises a slot extending into the body from said external surface.

6. Apparatus in accordance with claim 5, wherein said body is substantially symmetrical about a plane, and the slot extends along said plane.

7. Apparatus in accordance with claim 4, wherein the first cavity comprises a chamber inside the external surface.

8. Apparatus in accordance with any one of claims 4 to 7, wherein the body has been moulded onto the sample such that the sample or part of the sample fills the first cavity.

9. Apparatus in accordance with any one of claims 4 to 8, wherein the cavity, external surface, and reflecting means are arranged such that electromagnetic radiation emitted from opposite sides of a sample located in the cavity can be collected.

10. Apparatus in accordance with any one of claims 4 to 9, wherein the body comprises a first hole extending from the external surface to the first cavity to enable a beam of charged particles to be directed through the first hole and onto a sample or portion of a sample located in the first cavity.

1 1 . Apparatus in accordance with claim 10, wherein the body comprises a second hole aligned with the first hole and extending from the first cavity to the external surface and arranged such that charged particles passing through the sample can exit the body via the second hole.

12. Apparatus in accordance with 1 1 , wherein the body is rotationally symmetric about a rotational axis, and the first and second holes extend along a line through and substantially perpendicular to, the rotational axis.

13. Apparatus in accordance with any preceding claim, wherein said reflecting means comprises at least one of a reflective coating, reflective layer, or surface treatment applied to the external surface.

14. Apparatus in accordance with any preceding claim, wherein the conduit means comprises at least one optical fibre.

15. Apparatus in accordance with claim 14, wherein the body comprises a second cavity extending into the body from the external surface, and an end portion of said optical fibre is located in the second cavity such that an end of the optical fibre is positioned inside the external surface.

16. Apparatus in accordance with any preceding claim, wherein said external surface is shaped so as to define a first focal point or region and a second focal point or region, such that electromagnetic radiation originating from the first focal point or region is directed, by at least one reflection at the external surface, to the second focal point or region.

17. Apparatus in accordance with claim 16, wherein the body is adapted to locate or enable location of at least a portion of a sample at the first focal point or in the first focal region.

18. Apparatus in accordance with claim 16 or claim 17, wherein the conduit means is arranged to collect electromagnetic radiation directed to the second focal point or region.

19. Apparatus in accordance with either claim 17 or claim 18 as depending from claim 4, wherein said first focal point or at least a portion of said first focal region is located in said first cavity.

20. Apparatus in accordance with any one of claims 17 to 19 as depending from claim 15, wherein an internal end of the second cavity is arranged to coincide with the second focal point or region.

21 . Apparatus in accordance with any preceding claim, wherein the body is at least substantially ellipsoidal.

22. Apparatus in accordance with any one of claims 1 to 20, wherein the body is at least substantially paraboloidal.

23. Apparatus in accordance with any preceding claim, wherein the body is unitary.

24. Apparatus in accordance with any one of claims 1 to 23, wherein the body is composite, formed from a plurality of separate body parts.

25. Apparatus in accordance with claim 24, wherein each body part is formed of said material.

26. A sample holder for use in the collection of electromagnet radiation emitted from a sample irradiated with a beam of charged particles, the sample holder comprising:

a solid body of material at least substantially transparent to electromagnetic radiation having wavelengths in at least one range, the body having an external surface and being adapted to locate or enable location of a sample with respect to the body such that at least a portion of the electromagnetic radiation emitted from the sample when irradiated with charged particles enters the body and is incident on the external surface from within the body; reflecting means arranged to reflect, from the external surface and back into the body, at least emitted radiation having wavelengths within said one range incident upon the external surface from within the body.

27. A sample holder in accordance with claim 26, wherein the body comprises a first cavity adapted to house a suitably sized sample or portion of a sample.

28. A sample holder in accordance with claim 26 or claim 27, wherein the body comprises a first hole extending from the external surface to the first cavity to enable a beam of charged particles to be directed through the first hole and onto a sample or portion of a sample located in the first cavity.

29. A sample holder in accordance with claim 28, wherein the body comprises a second hole aligned with the first hole and extending from the first cavity to the external surface and arranged such that charged particles passing through the sample can exit the body via the second hole.

30. A sample holder in accordance with claim 29, wherein the body is rotationally symmetric about a rotational axis, and the first and second holes extend along a line through and substantially perpendicular to, the rotational axis.

31 . A sample holder in accordance with any one of claims 26 to 29, wherein the body comprises a second cavity extending into the body from the external surface, the second cavity being adapted to receive an end portion of a conduit such that an end of the end portion can collect electromagnetic radiation emitted into the body from a sample and reflected at least once from the external surface, into the end, and convey the collected electromagnetic radiation away from the body.

32. A sample holder in accordance with claim 31 , wherein the body is rotationally symmetric about a rotational axis, and the second cavity extends into the body from an end of the body, along the rotational axis.

33. A sample holder in accordance with any one of claims 26 to 32, wherein said external surface is shaped so as to define a first focal point or region and a second focal point or region, such that electromagnetic radiation originating from the first focal point or region is directed, by at least one reflection at the external surface, to the second focal point or region.

34. A sample holder in accordance with claim 33, wherein the body is rotationally symmetric about a rotational axis, and the first and second focal points or regions are located on said rotational axis.

35. A sample holder in accordance with claim 33 or claim 34, wherein the body is adapted to locate or enable location of at least a portion of a sample at the first focal point or in the first focal region.

36. A sample holder in accordance with any one of claims 33 to 35 as depending from claim 27, wherein said first focal point or at least a portion of said first focal region is located in said first cavity.

37. A sample holder in accordance with any one of claims 33 to 36 as depending from claim 31 , wherein an internal end of the second cavity is arranged to coincide with the second focal point or region.

38. A sample holder in accordance with any one of claims 26 to 37, wherein the body is at least one of: substantially ellipsoidal; substantially paraboloidal; unitary; composite.

39. Apparatus in accordance with claim 1 or a sample holder in accordance with claim 26, wherein said external surface is flat.

40. Apparatus or a sample holder in accordance with any one of claims 1 , 26 or 39, wherein said body is multi-faceted.

41 . Apparatus or a sample holder in accordance with any one of claims 1 , 26, 39 or 40, wherein said body comprises a plurality of faces.

42. Apparatus or a sample holder in accordance with claim 41 , wherein at least one of said faces is flat.

43. Apparatus or a sample holder in accordance with any one of claims 39 to 42, wherein said body is substantially cuboidal.

44. Apparatus or a sample holder in accordance with any one of claims 39 to 42, wherein said body comprises at least one pair of non-parallel opposing faces.

45. A measurement system comprising:

apparatus in accordance with any one of claims 1 to 25 or claims 39 to 44;

a charged-particle source arranged to irradiate at least a portion of a sample located with respect to the body with a beam of charged particles.

46. A system in accordance with claim 45, wherein the charged-particle source comprises an electron source arranged to provide an electron beam.

47. A system in accordance with claim 45 or claim 46, wherein the charged-particle source comprises a TEM.

48. A system in accordance with any one of claims 45 to 47, further comprising a spectrometer, wherein the conduit means is arranged to convey collected electromagnetic radiation to the spectrometer, and the spectrometer is adapted to measure at least one property or characteristic of the collected electromagnetic radiation.

49. A measurement method comprising:

irradiating a sample with charged particles such that the sample emits electromagnetic radiation;

arranging the sample with respect to a solid body of material such that at least a portion of the emitted electromagnetic radiation enters the body and is incident on an external surface of the body from within the body; and

50. A method in accordance with claim 49, further comprising conveying the collected electromagnetic radiation away from the body in the conduit and to a spectrum analyser or spectrometer, and using the spectrum analyser or spectrometer to measure at least one property or characteristic of the collected electromagnetic radiation.

51 . A method in accordance with claim 49 or claim 50, wherein the body is rotationally symmetric, the external surface defines first and second focal points or regions, and the method comprises:

irradiating a portion of the sample located at or in the first focal point or region, directing at least a portion of the em radiation emitted into the body from the first focal point or region to the second focal point or region by means of at least one internal reflection from the outer surface, and collecting a portion of the em radiation directed to the second focal point or region in the conduit.

52. Apparatus, a sample holder, a measurement system, or a measurement method substantially as hereinbefore described with reference to the accompanying drawings.