WO2004064098A2

WO2004064098A2 - Detector for environmental scanning electron microscope

Info

Publication number: WO2004064098A2
Application number: PCT/GB2004/000080
Authority: WO
Inventors: Frank Baker; Milos Toth
Original assignee: Cambridge University Technical Services Ltd CUTS
Current assignee: Cambridge University Technical Services Ltd CUTS
Priority date: 2003-01-09
Filing date: 2004-01-09
Publication date: 2004-07-29
Anticipated expiration: 2005-07-09
Also published as: GB0300474D0; WO2004064098A3

Abstract

A detector system for use in a scanning electron microscope. The detector system comprises means for supplying a primary electron beam to the surface of a sample positioned in the detector in use. A specimen is support positioned in the path of the primary electron beam; and first and second electrodes arranged to create an asymmetric field such that secondary electrons are attracted in use to the first electrode and excess ions are removed in use by the second electrode from the region of the sample which is irradiated by the primary electron beam. There is also means for amplifying the output of one of the electrodes to produce a detection signal.

Description

Detector for Environmental Scanning Electron Microscope

The present invention relates to a detector for use with an environmental scanning electron microscope (ESEM), which is also sometimes known as a variable pressure scanning electron microscope (VPSEM), or a low vacuum scanning electron microscope (LVSEM). Such scanning electron microscopes are used to image materials in a low vacuum environment. ESEM type instruments are useful in that they have certain benefits not available to standard high vacuum scanning electron microscopes. Such standard high vacuum scanning electron microscopes require the materials that are to be imaged to be vacuum tolerant and electrically conductive. In view of this, poorly conductive and non-conductive specimens traditionally needed to be coated with a metallic layer which must then be electrically grounded inside the microscope specimen chamber. In addition, wet and liquid specimens must be dried or frozen before they are coated. The preparation procedures, and the vacuum tolerance requirements that they require, therefore often result in significant modification of the specimen prior to characterisation in the microscope. In addition, dynamic processes that would fracture the metallic coating generally cannot be imaged without great difficulty. ESEM devices overcome such problems as they can tolerate a low vacuum environment in their specimen chamber and do not require specimens to be coated prior to imaging. This means that non-conductive, wet and liquid specimens can be imaged, and dynamic experiments, as well as gas sample reactions can be performed inside the microscope specimen chamber and studied in real time.

However, such ESEM devices have, themselves, certain drawbacks. All currently use secondary electron (SE) detectors which depend upon gas ionisation events to either preserve and amplify electron signals or convert electron currents to light (by employing gas luminescence). Whilst many ESEM type instruments can tolerate high gas pressures in the specimen chamber (in excess of 15 torr) none can generate good quality images at such high pressures. This is primarily a consequence of the fact that, at such high gas pressures and under the conditions of primary beam gas path length generally employed in ESEM, the fraction of the primary beam scattered by gas molecules is too high for the generation of good quality images. In order to minimize the scatter of the primary beam by gas molecules the distance between the final pressure limiting aperture and the sample surface must be minimized. At gas pressures greater than 15 torr, the primary beam gas path length generally needs to be smaller than 1 mm in order to reduce beam scatter to an acceptable level. This may be achieved by the employment of a short working distance (of less than 1 mm). In order to relax this restriction on the working distance, the beam may optionally pass through a pressure limiting aperture which may be contained within a conducting piece suspended from the pole piece (see for example, XL3O ESEM FEG SEM Operating Instructions, FEI Company, Boston, 2000). If such a conducting piece is employed, beam scatter may be minimized by the employment of a short piece-sample separation, typically of less than 1 mm.

However, if the primary beam gas path length is reduced so as to reduce beam scatter to an acceptable level, current use ESEM detectors cannot generate good quality images, for several reasons. Firstly, gas amplification in the region between the sample surface and the electrode located above the specimen (i.e., the pole piece or the abovementioned conducting piece) is very inefficient because the distance between the sample and the overhead electrode is typically too short for the production of an intense gas cascade without the employment of electrode biases that would cause the gas to break down. Secondly, a large fraction of gaseous ions generated in the volume between the sample surface and the overhead electrode drift to the sample surface, thereby reducing the gas amplification efficiency through secondary electron-ion recombination, and ion-induced reduction of the detector field. Thirdly, current use off-axis detectors such as the "large field secondary electron detector (LFGSED)" (XL3O ESEM FEG SEM Operating Instructions, FEI Company, Boston, 2000) do not produce good quality images under conditions of short gas path length because most of the electrons emitted from the sample do not reach the LFGSED anode, and are not amplified in the gas cascade, but are instead collected by the overhead electrode.

Some prior art ESEMs have used the placement of an electrode above the sample surface to make ions drift to this electrode rather than the sample surface (J

P Craven et al, J Microscopy, Vol 205, pp 96-1 05, 2002, US-A-5396067 and US-A- 5466936). However, these designs cannot be employed in conjunction with the short gas path lengths (of less than 1mm) required for high pressure operation. The use of needle-shaped electrodes has been proposed in WO-A02/1 5224. However, this document is primarily concerned with the detection of photons generated in the gas cascade and attempts to use a needle shaped electrode in order to localize the gas cascade to the vicinity of a specific point within the specimen chamber in order to maximize the collection efficiency of detectors that detect light generated in the gas cascade. This has the problem that the size of the volume within which the gas cascade occurs is important. It also attempts to use needle-shaped electrodes in order to preferentially detect secondary or backscattered electrons emitted from the specimen, which is based on arguments that are physically flawed and no evidence is provided for the effectiveness of the proposed approach for electron signal filtering.

Additionally this document does not address the problems of electron imaging under conditions of very short primary beam gas path length or very high gas pressure, it does not provide a way of directing the flow of gaseous ions away from the sample surface and it does not provide a way of manipulating of electrostatic boundary conditions at and around the imaged region of the sample surface.

A consequence of the inability of current devices to generate good quality images at high gas pressures is that it is usually not possible to image wet and liquid samples unless they are cooled to approximately 1 to 3 °C. The present invention seeks to overcome some of the above problems with

ESEMs through the employment of detector geometries designed to facilitate high quality electron imaging under the conditions of short primary beam gas path length required for high pressures operation.

According to the present invention there is provided a detector system for use in a scanning electron microscope, the detector system comprising: means for supplying a primary electron beam to the surface of a sample positioned in the detector in use; first and second electrodes arranged to create an asymmetric field such that electrons are attracted, in use, to the first electrode and excess ions are removed, in use, by the second electrode from the region of the sample which is irradiated by the primary electron beam; and means for amplifying the output of one of the electrodes to produce a detection signal.

According to. the present invention there is also provided a detector system for use in a scanning electron microscope, the detector system comprising: means for supplying a primary electron beam to the surface of a sample positioned in the detector in use; an electrode arranged to create an asymmetric field such that electrons are attracted, in use, to the electrode; means for amplifying the output of one of the electrodes to produce a detection signal, and a specimen support positioned in the path of the primary electron beam and made from a material that does not terminate electric fields.

This arrangement not only gives enhanced image quality; but also due to the geometry and shape of the electrodes produces an electric field such that the gaseous cascade electron amplification process occurs essentially independent of the path length of the primary electron beam within the chamber before it strikes the sample. It is possible therefore to use path lengths smaller than 1mm to reduce the scatter in the electron beam and hence produce good images of the sample at pressures higher than 15 torr (19.95 mbar). Another result of the electrode arrangement is that for the case of non-conductive samples, deliberately placing an insulator between the sample and the conducting stage can improve further the image quality.

An example of the present invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 is a side schematic diagram view of a detector according to the present invention.

Figure 2 is a detailed schematic diagram of an example conducting piece 4.

Referring to figure 1, a specimen 5 receives an electron beam 2 from a source (not shown). The beam 2 is focussed through an objective lens 3 and enters the specimen chamber 1 through a tubular or frusto-conical conducting piece 4, onto a specimen 5 to be imaged. The conducting piece 4 contains an optional pressure limiting aperture 11 at the tip thereof. In this case, the long internal bore 13 of the typical frusto-conical conducting piece 4 shown in Figure 2 acts as the pressure limiting aperture 11. The surface of the specimen 5 is located 0.5 mm below the base of the conductive piece 4. An insulating specimen 5 is placed on an insulating support 6 made of PTFE of thickness selected such that the total distance between the base of the conductive piece 4 and the base of the insulating support 6 is equal to 5 mm. Adjacent to the specimen 5 is needle-shaped anode electrode 10 (tip radius of curvature = 30 microns), which may be positioned such that the tip of the needle is located 0.5 mm above the sample surface and the shortest straight-line distance between the tip of the needle and the electrode beam impact point on the sample surface is 10 mm. The anode 10 is connected, in use, to a positive voltage of 800 V. Adjacent to the specimen 5 is a needle-shaped cathode electrode 7 (tip radius of curvature = 30 microns), positioned such that the tip of the needle is located at the same height as the sample surface and the shortest straight-line distances between the tip of the needle and both the electron beam impact point on the sample surface and the anode electrode 10 are equal to 10 mm. The cathode electrode 7 may be connected to ground via an amplifier 8 to an image acquisition system 9. The cathode electrode 7 also has benefits in that, because it is at ground potential, electronic amplification of the heavily gas-amplified imaging signal can be performed easily using inexpensive electronic amplifiers.

In use, electrons are provided to the specimen 5 from the output of an electron gun (not shown), passing through the pressure limiting aperture 11. The anode electrode 10 has a positive voltage on it which generates an electric field that gives rise to a gas ionisation avalanche and controls the path of low energy electrons traversing the gas in the chamber of the detector 1. The cathode electrode 7 may be grounded directly or through an amplifier such that it removes excess ions from the chamber of the detector 1 and also ensures that excess ions drift towards it rather than the specimen 5.

If the primary beam gas path length is shorterthan 1 mm, efficient amplification of secondary electrons emitted from the sample 5 requires extraction of the electrons from the region between the sample surface and the overhead electrode (this may be the pole piece, or as is the case in figure 1 , the conductive piece 4) into a region of the specimen chamber where (i) the emitted signal can be heavily amplified in a gas ionization avalanche, and (ii) excess ions generated in the gas can be made to drift away from the specimen using an applied electric field. In the present invention, this is achieved using a highly asymmetric electric field generated using purpose-made multiple electrodes, and a detector geometry that gives rise to electrostatic boundary conditions that allow for efficient manipulation of the electric field in the region between the sample surface and the overhead electrode 4. The highly asymmetric electric field can be produced by the employment of electrodes containing abrupt features such as the edges and sharp points on the ends of razor blades and needles. The required length scale of such features can be described in terms of the radii of curvature, or apex radii, which should be approximately two to three orders of magnitude smaller than the characteristic length scale of the detector volume. The latter can be defined in terms of inter-electrode distances which are generally in the range of 1 to 10 mm. Electrodes containing such features may be electrically biased and positioned inside the specimen chamber so as to generate the required electric field inside the specimen chamber.

The arrangement shown in figure 1 uses two electrodes to generate the required asymmetric electric field. The first electrode 10 is needle shaped and biased positive (typical bias: 800 V) in order to make some of the secondary electrons emitted from the sample drift to the electrode 10. It may be positioned 0.5 mm above the sample surface, sufficiently far from the beam 2 axis (e.g., 10 mm) so as not to physically restrict the minimum primary beam gas path length (the distance between the sample surface and the conductive piece 4, which typically needs to be smaller than 1 mm to facilitate ESEM operation at pressures greater than 15 torr).

There are various measures that may be taken in order to ensure that the fraction of emitted electrons drifting to the first electrode is sufficiently high for the generation of a (gas amplified) imaging signal capable of producing good quality images at high gas pressures. The second electrode 7 modifies the electric field inside the specimen chamber so as to prevent excess gaseous ions from drifting to the sample surface, (instead the ions drift towards the second electrode 7) and it may be shaped and biased such that it may be positioned so as not to physically restrict the minimum primary beam gas path length. If the base of the pole piece is too wide or too close to the roof of the specimen chamberto accommodate the first and second electrodes, the final pressure limiting aperture may be contained in a conductive piece 4 suspended from the pole piece. The length (along the beam axis) and width (normal to the beam axis) of the conductive piece 4 may be selected so as to accommodate the first 10 and second 7 electrodes inside the specimen chamber. The width of the base of the conductive 4 piece may be minimized, in order to facilitate efficient modification of the electric field at the beam impact point. The conductive piece 4 or the specimen stage 12 may be biased in order to minimize the number of emitted electrons collected by the piece 4 or the surface of the sample 5. The second electrode 7 of the present invention overcomes the problem of preventing excess gaseous ions from drifting to the sample surface while being able to use short path lengths, being located off to one side of the beam impact point on the sample surface and being used in conjunction of with the first electrode 10 that extracts emitted electrons into a region of the specimen chamber from which excess ions can be directed to the second electrode 7. The second electrode 7 may be electrically grounded and positioned at the same height as the sample surface, sufficiently far from the beam axis (e.g., 10 mm) so as not to physically restrict the minimum primary beam gas path length, and sufficiently far from the first electrode 10 (e.g., 10 mm) so as not to cause the gas to arc between the two electrodes 10 and 7. To enhance the drift of excess ions to the second electrode 7, it may be shaped so as to comply with the abovementioned requirements for the formation of highly asymmetric fields. For example, the needle tip of electrode 7 may have a radius of curvature of 30 microns.

To enhance the drift of excess ions to the second electrode 7, it may optionally be biased negative (typical bias: -200 V).

The first electrode 10 and the second electrode 7 will from here. on be collectively referred to as "off-axis" electrodes, to distinguish them from the "overhead" and "underlying" electrodes. The "overhead" electrode is either the conductive piece 4, or (if such a piece is not employed) the pole piece located above the sample. The "underlying" electrode is the electrically conducting specimen stage 12 located below the specimen support 6.

The effectiveness of the off-axis electrode geometry of the present invention may be improved by the employment of an insulating specimen support 6 designed to enhance the ability of off-axis electrodes to manipulate the electric field within the volume between the sample 5 and the overhead electrode. The insulating support 6, positioned between the (typically non-conductive) sample and the underlying electrode provides control over the distance, D, between the overhead and underlying electrodes which define the electrostatic boundary conditions for the electric field between the overhead and underlying electrodes. If the specimen 5 and the support 6 are both insulating, electric fields generated by off-axis electrodes do not terminate in the region between the overhead and underlying electrodes (the employment of an insulating specimen support will not yield any benefits if the sample is a grounded conductor). The insulating support 6 can be used to increase the ability of off-axis electrodes to control the electric field at the beam impact point and, hence, to increase fraction of emitted secondary electrons that are made to drift to the first electrode 10. Critically, the control over D facilitated by the insulating support 6 is independent of the primary electron gas path length, thereby allowing for the generation of good quality images at high gas pressures. The support material can be any insulator that does not modify significantly electric fields applied across it (typically, the relative dielectric constant should be smaller than 10_λ Obviously, the insulator must also be thermodynamically stable in the gaseous environments encountered in ESEM. Potential materials include most solid polymers, (e.g. PTFE), quartz and Al₂0₃. Attainment of the best imaging conditions requires the simultaneous optimisation of D and the placement of off-axis electrode 10 (and electrode 7 is used). Both E_ia_, the lateral component of the electric field at the beam impact point and E_ax, the axial component of the electric field at the beam impact point must be accounted for when performing such an optimisation. E_iat causes emitted secondary electrons to drift away from the primary beam

2, towards the off-axis anode 10. The greater the magnitude of E_iat, the greater the electrostatic force that causes electron drift towards the off-axis anode 10. If D = 0, E_iat is equal to zero. In general, as D is increased (e.g., by increasing the combined thickness of an insulating sample 5 and the support 6), the magnitude of E,_at initially increases, goes through a maximum, and decreases as D is increased further. As such, if an insulating specimen is unusually thick (typically, thicker than about 10 mm), the specimen may need to be thinned in order to achieve optimum imaging conditions. The magnitude of D also depends on the shape and lateral extent of the overhead electrode 4. In general, the greater the lateral extent of the base of the overhead electrode 4, the lower the magnitude of E_iat

E_ax, determines the polarity and magnitude of the electrostatic force in the direction parallel to the electron beam 2. Both the polarity and magnitude of E_ax can be altered by changing the position(s) of the off-axis electrode(s) and D. The significance of E_ax lies in that it affects the trajectories of emitted secondary electrons and, ultimately, whether or not a given electron trajectory intersects the overhead electrode 4 or the sample surface. The greater the fraction of emitted electrons collected by the overhead electrode 4 or the sample 5, the lower the magnitude of the imaging signal. Since the trajectories of emitted electrons are functions of initial energy and emission angle, E_ax and E_iat must be optimised so as to minimise the fraction of emitted electrons that are collected by the overhead electrode 4 or the specimen 5. Additional control over E_ax can be attained by biasing of the overhead electrode 4 or the stage.

The geometry of the support and the electrodes may be optimised by the measurement or simulation of the imaging signal intensity as a function of D, bias applied to off-axis, overhead and underlying electrodes, the shape and placement of off-axis electrode(s), and the shape of the overhead electrode. The net result of such an optimisation is that the detector of the present invention can operate under conditions of shorter gas path length and higher gas pressures than those of prior art, whilst retaining high image quality. Consequently, high quality images of wetand liquid specimens containing secondary electron contrast can be attained at room temperature.

In some circumstances the optimisation may be provided by using the anode 10, overhead electrode 4 and support 6. In other circumstances cathode 7 may be added to assist with the optimisation and to provide capability to extract excess ions.

In the field of ESEM, the general advice for optimising imaging conditions involves the placement of a grounded conductor into the vicinity of the imaged region of the sample, in order to terminate the detector field near the imaged region of the sample (see, for example, D.E. Newbury, Scanning 18, 474 - 482 (1996)). In contrast, the insulating support 6 of the present invention can increase image quality by performing the opposite task (i.e., the support 6 prevents the termination of the detector field in the vicinity of the imaged region of the sample surface). That is, the geometry of the present invention is sufficiently different from prior art detectors to invert some of the requirements for attainment of best imaging conditions. In summary, with the detector of the present invention it is possible to illuminate a sample with an electron beam which has a very small gas path length, typically 1 mm or less, that is independent from the gas amplification path length, and decoupled from the distance D that governs the intensity of the electric field generated at the beam impact point by off-axis electrodes. Because of this and due to the geometry of the electric field produced by the aforementioned electrode arrangement and the insulating specimen support, the gas amplification path length can be optimised in order to maximise image quality and also ensures that a heavily amplified imaging signal can be obtained from either the anode/anodes or preferentially from the cathode/cathodes. Furthermore, the electric field geometry causes excess gaseous ions to drift preferentially to the second electrode, thereby minimising gas amplification self-damping and secondary electron signal reduction caused by gaseous ions. The configuration of the present invention also enables maximisation of the fraction of emitted electrons that are swept by the asymmetric field towards off- axis electrode(s) without intersecting other objects in the microscope, hence information carried by the secondary electrons is retained in the images.

Claims

1. A detector system for use in a scanning electron microscope, the detector system comprising: means for supplying a primary electron beam to the surface of a sample positioned in the detector in use; first and second electrodes arranged to create an asymmetric field such that electrons are attracted, in use, to the first electrode and excess ions are removed, in use, by the second electrode from the region of the sample which is irradiated by the primary electron beam; and means for amplifying the output of one of the electrodes to produce a detection signal.

2. A system according to claim 1_. further comprising means for suspending the sample in the path of the primary electron beam.

3. A system according to claim 1 , further comprising a specimen support positioned in the path of the primary electron beam.

4. A system according to any preceding claim, wherein at least one of the electrodes is shaped so as to intensify the electric field at specific regions in the specimen chamber.

5. A system according to any preceding claim, wherein at least one of the electrodes is shaped so as to contain geometric features characterised by an apex diameter or radius of curvature of between 10^"6 mm and 2mm.

6. A system according to claim 5, wherein at least one of the electrodes is needle shaped.

7. A system according to claim 6, wherein the tip of the needle shaped electrode has a radius of curvature of between 1 μm and 1 mm and wherein the tip of the needle is positioned at a height of between 50 mm below the sample surface and 50 mm above the sample surface and so that the shortest straight-line distance between the tip of the needle and the electron beam impact point on the sample surface is between 0.1 mm and 50mm.

8. A system according to any preceding claim, wherein at least one of the electrodes is positively biassed in use.

9 A system according to claim 8, wherein the other electrode is grounded or negatively biassed in use and connected to an amplifier which provides an imaging signal.

10. A detector system for use in a scanning electron microscope, the detector system comprising: means for supplying a primary electron beam to the surface of a sample positioned in the detector in use; an electrode arranged to create an asymmetric field such that electrons are attracted, in use, to the electrode; means for amplifying the output of one of the electrodes to produce a detection signal, and a specimen support positioned in the path of the primary electron beam and made from a material that does not terminate electric fields.

11. A system according to claim 10, wherein the support is an insulator.

12. A system according to claim 11 , wherein the geometry of the support is chosen so as to allow control of the electric field at and around the electron emission region of a specimen.

13. A system according to claim 12, wherein the control is independent of the primary beam gas path length.

14. A system according to claim 12 or claim 13, wherein the thickness of the support is chosen so as to optimise the extraction of emitted secondary electrons.

15. A system according to any of claims 3 to 14, wherein the specimen support has a relative dielectric constant of less than 10.

16. A system according to any of claims 3 to 15, wherein the thickness of the specimen support is between 0.1 mm and 10mm.

17. A system according to any of claims 3 to 9, further comprising a stage positioned, in use, underneath the specimen support.

18. A system according to claim 17, wherein the stage is positively or negatively biassed in use.

19. A system according to any of claims 3 to 9, claim 17 or claim 18, wherein the electrodes and specimen support are arranged such that the gas amplification path of the system is optimised whilst providing a minimised primary electron beam path, thereby providing secondary electron contrast at chamber pressures greater than 15 torr.

20. A system according to any of claims 3 to 9 or claims 17 to 19, wherein the primary electron beam passes through a pressure limiting aperture.

21. A system according to claim 20, wherein a conducting piece acts as the pressure limiting aperture.

22. A system according to claim 21 , wherein the conducting piece is tubular or frusto conical.

23. A system according to claim 20 or 21 , wherein a bias is applied to the conducting piece in use.