TECHNICAL FIELD

[01] The present invention relates generally to apparatuses and methods for control or modification of surface charge, e. ., using shortwave electromagnetic radiation in nano- structuring or nano-imaging of materials, for example i ion beam and electron beam imaging and fabrication systems, including microscopy and fabrication tools where electrons and/or ions are used for surface imaging and/or fabrication (e.g., lithography, deposition, and milling), in particular at high resolutions, e.g., at sub-micrometer (μιη), and nanometre (nra) scales.

BACKGROUND

[02] Ion-beam systems and electron-beam systems are increasingly used for high resolutio imaging and fabrication (e.g., lithography, deposition, and milling), e.g., as imaging and fabrication is required at nanometre scales. Ion- and electron-beam imaging and fabrication tools include sources of electrons or positive ions, and thes sources generate a stream of the electrons or ions directed to a surface of a sample, e.g., for imaging the suriace, or for fabricatin a pattern o the surface. Example tools include electron beam lithography (EBL) tools, io beam lithography (IBL) tools, and focused ion beam (FIB) tools.

[03] A limitation and impediment to high -resolution fabricatio and imaging with ion beams and electron, beams arises from charging of the surface of the sample due to the beam of the charged particles: the electrons or the ions. Because the surface charging is caused by irradiation with the charged particles, this charging can vary unpredictably across the surface over a duration of an irradiation process (a imaging or fabrication process). The surface charging can lead to spatial errors in a plane perpendicular to the beam of the charged particles, across the surface of the sample, thus effectively reducing the resolution of the imaging tools, or distorting patterns made using the fabrication tools. This reduction in the resoluii on of imaging and fabrication is due to a spatial distributio of charge across the surface, and this spatial distribution in the surface charge can steer a beam of charged particles significantly when the beam is focused down to a sub- micrometre scale, in particular down to a few nanometres. Thus the surface charging can cause a drift in images or fabricated patterns.

[04] The surface charging effects caused by the charged particle beams may in some circumstances be reduced or ameliorated using a source of electrons (i.e.,- a second source of charged particles) known as an electron flood gun. The electron flood gun may create a more uniform charging on a patterned surface, thus reducing spatial dependencies of the surface charging caused by the beam of charged particles. The surface charging effects may also be addressed by coating the sample surface with a highly conductive layer that conducts the surface charges away from the surface as they are generated b the beam of charged particles; however, such a conductive coating requires modification of the original sample before the irradiation step. The conductive layer may be a metal coating or a polymeric coating (e.g., an -"ESPACER" coating),, and may require a conductive connection to large-volume metal tools grounded for charge removal (e.g., carbo tape connecting the sample surface to a conducting portion of the fabrication or imaging tool),

[05] Existing methods of electron flood gun illumination and conductive layer coating are in some cases macceptable e.g., because it is desirable to leave an initial patter or geometry on the sample surface unaltered, or if spatial precision of several nanometres is required. For example, when using a scanning electron microscope (SEM), a conductive coating of one to two nanometres of platinum/palladium may be required to remove the surface charge, and such a coating is expensive and may be incompatible with the sample.

[06] it is desired to address or ameliorate one or more disadvantages or limitations associated with the prior art, or to at least provide a useful alternative.

SUMMARY

[07] In accordance with the present invention there is provided an apparatus for imaging or fabrication using charged particles, the apparatus including:

a charged particle source configured to generate a charged particle beam of ions or electrons;

a sample holder mounted relative to the charged particle source to hold sample in the charged particle beam for the imaging or fabrication; and

an optical source system configured to generate a optical beam, wherei the optical source system is mounted relative to the sample holder to direct the optical beam onto the sample to modify an electric charge of the sample during the imaging or fabrication to improve spatial resolution of the imagi ng or fabrication .

[08] The present invention also provides a sample holder for an ion-beam or electron- beam imaging or fabrication tool, including an optical source system delivering light in an optical beam, wherein the optical source system is mounted to the sample holder and aligned to project the optical beam on a sample, wherein the optical beam includes a wavelength selected to modify charger carriers in the sample formed by a charged particle beam of the tool.

[0 ] The present invention also provides a method of manufacturing an apparatus for surface charge modification, the method including the step of:

mounting an optical source system in an apparatus for imaging or fabrication with a beam of charged particles,

wherei the optical source system is configured to generate an optical beam with, a wavelength selected to modify a surface charge generated in the sample by the beam of charged particles to improve spatial resolution of the imaging or fabrication.

[10] The present invention also provide a method of modifying an electronic charge of a sample irradiated by a beam of charged particles, the method including the step of:

illuminating a surface of the sample with an optical beam including one or more wavelengths selected to modify a surface charge generated by the beam of charged particles irradiating the surface to improve spatial resolution of imaging or fabricatio with the beam of charged particles.

BRIEF DESCRIPTION OF THE DRAWINGS

[11] Preferred embodiments of the present invention are hereinafter described, b way of example only, with reference to the accompanying drawings, in which:

[12] Figures I A to IE are schematic diagrams of different configurations of an apparatus for charged-beam imaging or fabrication;

[13] Figure 2 is a schematic diagram of a sample holder for the apparatus;

[14] Figure 3 is a schematic diagram of electron excitation to a free vacuum level from defects in a sample in the apparatus;

[15] Figure 4 is a schematic diagram of an example sample and holder in the apparatus; [16] Figure 5 A is an im ge of a pattern milled into titanium dioxide (Ti0 ₂ ) using ion-beam lithography without surface charge control;

[17] Figure 5B is an image of a pattern milled into TiO?. with surface charge control;

[18] Figure 6 is a precision map of mi l led holes in Ti0 for different intensities

( I.,) of a light source used for surface charge control ;

[19] Figure 7 is a graph of averaged spatial error for a normalised use of the different intensities;

[20] Figure 8A is an image of a circle patter design for milling on a TiG ₂ surface;

[21] Figure 8B is a scanning electron microscope (SEM) imag of the circle pattern from Figure 8A milled using illumination with a wavelength of 250 nm at 80% current (includi a scale bar of 1 micrometer);

[22] Figure SC is an SEM image of the circle pattern trom Figure 8 A milled

Using illumination with a wavelength of 270 nm at 80% current (including a scale bar of 1 micrometer),

[23] Figure 8D is an SEM image of the circle pattern from Figure 8 A milled using ilhrminati on light with a wavelength of 280 nm at 80% current (including a scale bar of 1 micrometer);

[24] Figure 8E is an SEM image of the circle pattern from Figure 8A milled using illumination light with a wavelength of 290 nm at 80% current (Including a scale bar of 1 micrometer);

[25] Figure 8F is a SEM image of the circle pattern from Figure 8A milled without illumination (including a scale bar of 1 micrometer); and

[26] Figure 9 is an image of SEM circle patterns milled into aluminium foil under illuminatio from a optical source with a wavelength of 290 nanometres.

DETAILED DESCRIPTION

Apparatus

[27] Described herein is an apparatus 100 configured: (i) to irradiate a sample with a charged particle beam (e.g., for imaging or fabrication using electrons and/or ions); and (ii) to illuminate the sample with light ( .<?., photons) to control electronic charges in the sample— and in particular at or on the surface of the sampl— generated by the charged beam, The control of the surface charges may be referred to as "surface charge compensation" or "modification": the effects of the surface charges, which are strongly influenced by the charged beam, on the spatial resolution of the charged beam may simply be ameliorated or completely removed, rather than being completel controlled. The apparatus 1 GO may totally remove all excess charge created by charged particle beam, e.g., so na o-fabrication can happen without distortions. The control of surface charge can depend ( 1) on the wavelength of the light and (2) on the light's intensity, thus in some circumstances a rate of charge removal, can be controlled by changing the intensity of the incident light (e.g>, see Figure 7). By modifying the surface charge with an optical beam of the light, spatial resolution of imaging and fabrication with the charged beams may be improved. The apparatus 100 may allow performance of lithography at under 22 am resolution, or fabrication of photonic crystals with sub.-10.0-nm precision and an aspect ratio of 4 to 5 on dielectrics. The apparatus 100 may be used for fabrication of sub-20-nm features in the semiconductor industry. The apparatus 100 may be used for fabrication of photonic crystals with a full photonic bandgap in the visible range, which may be used to couple sunlight into solar calls more strongly. The apparatus 100 may be used for high- resolution ion fabrication in micro nano-fluidic application, including maskless writing of gratings, hole array or other more complex patterns. The apparatus may be used to add material to the sample surface. The materials can be conductive (e.g., platinum, used by tools from FEI Company, or tungsten used by tools from Raith GmbH) or isolating (e.g., Si02, or carbon (C) used by tools from Raith GmbH). The irradiation and illumination can be performed simultaneously or in succession with each other (e.g., repeatedly and in rapid succession) for some applications (e.g., switching the charged particle beam and the light source on and off in sequence at a high frequency.

[28] As shown in Figure I A, the apparatus 100 includes a charged particle source 102 (or a "gun") for generating a charged particle beam 104.

[29] The charged particle source 1.02 can be a source of electrons, thus generating beam of electrons in the charged particle beam 104. Alternatively, the charged particle source 102 can be a ion source, thus generating a beam of ions in the charged particle beam 104. Example ions are gallium (Ga ⁺ ) ions, helium (He ^" ) ions, neon (Ne ^" ) ions, xenon ions (Xe+), gold ions (Au+), silicon ions (Si+), and other ion sources.

[30] The apparatus 100 includes a sample holder 106 configured to hold a sample 108 in position relative to the charged particle source 102 and the charged particle beam 104 so that the charged particle beam 104 can be used to fabricate or mill the surface of the sample 108, or image the surface of the sample 108, using commercially available fabrication or imaging components in the apparatus 100, and in accordance with existing fabrication or imaging procedures. The sample holder 106 can include a plurality of mechanically connected components, for example; a sample mount for securing the sample 108 in position; and a stage (e.g., with actuators) for moving the sample 108 relative to the charged particle beam 104. The sample 108 may be a dielectric slab with a thickness of 50 nm (e.g., for a silicon nitride membrane) to 2 millimetre (mm) (e.g., for soda lime glass), but the sample 108 can be any height as long as it fits under the charged particle source 102 and the sample surface can be illuminated by an optical beam 1 14 described hereinafter to modify the surface charge. A bulk material of the sample 108 can be TiQ ₂ , soda-lime glass, borosilicate (BK7) glass, diamond, sapphire, or aluminium oxide (AI ₂ O3). The sample material can be a metal.

[31] The charged particle source 102 and the sample holder 106 are mounted i a casing 1 10 of the apparatus 100. The sample holder 106 can include a kapton tape spacer, which may electrically isolate of the surface of the sample 108 from other portions of the apparatus 100, e.g., the casing 1 1 .

[32] The casing 110, the charged particle source 102 and the sample holder 106 ca be components of commercially available imaging and fabrication tools. Example tools include electron beam lithography (EBL) tools, ion beam lithography (ML) tools, and focused ion beam (FIB) tools. A particular" example is the "IonLiNE" apparatus from Raith GmbH. The apparatus 100 may include a vacuum chamber around the sample 108 and sample holder 106, and an optical source system may be mounted or installed i the vacuum chamber. The apparatus 100 can be configured for nano-scale operatio through the inclusion o actuators to control a spot (referred to as the "ion beam spot") of the charged particle beam 104 on the surface of the sample 108 with nanometre precision .

[33] The apparatus 100 includes, mounted i or to the casing ! 10, the optical source system including an optical source 1 12 which generates (i.e., provides) the light for the optical beam 1 14 including optical wavelengths (e.g., ultraviolet (UV)). The optical source 1 12 can be a lamp, a laser or a light-emitting diode (LED), The optica! source 1 12 can be a semiconductor diode-based source, e.g., an LED or diode laser. The optical source .112 ca be a coherent light source (e.g., a laser) or an incoherent light source (e.g., an LED). An example optical source can be a commercially available deep-UV LED operating at short electromagnetic radiation wavelengths of about 240 to 280 nanometres. Example 240nm LEDs are available in the market at the moment; however, 200nm or 150nm LEDs may be preferable, e.g., for particular sample materials. The optical source system may be referred to as an optical "anti -charging gun", e.g. , a short- wavelength electromagnetic-radiation anti-charging gun.

[34] The optical source 1 12 may be sufficiently small to fit inside the casing 1 10 of the apparatus 100, requiring only a power source, e.g., for electrical power, or a connection to a power source. The optical source 1 12 can be mounted o a gun nozzle of the charged- partide source 102. The optical source 112 can be controlled to operate simultaneously with the charged particle source 102: i.e., the apparatus is configured (b relative mounting and control of the optical source system and the charged particle source 102) such that the optical source system can direct the optical beam 1 14 onto the sample at the same time as the charge particle source 102 generates the charged particle beam 104 and directs the charged particle beam 104 onto the sample 108; alternatively or additionally, the optical source 1 12 can be. controlled to be operate successively or in sequence with the charged particle source 102. The optical beam 114 may be a focussed or directed beam provided (i.e., directed) by micro-optical / optical guiding components, e.g., (optical filters, mirrors, lenses, waveguides and optical fibres in the optical source system), and/or a pluralit of focussed or directed different wavelength beams coincident on the sample 1 8, and/or a diffuse area of light directed onto the sample 108. The optical guiding components may deliver the light to form the optical beam 1 14, e.g., through or i the casing 1 10. For example, optical fibres can guide light from LEDs to form the optical beam 1 14. The optical guiding components may include collimators to improve angular control of the optical beam 114. The guiding optics for the optical beam 114 may be inside or exterior to the casing 1 10, The fibres may be UV fibres for guiding wavelengths, The final emitting optical guiding component (e.g., emitting end of a fibre) is optically connected to the optical source 1 2 (which can be outside the casing 1 10) and delivers the light to provide and form the optical beam 1 14 (i.e., directs the optical beam 1 14).

[35] The optical source system (include the optical source 112 and any optical guiding components) is mounted relative to the sample holder 106 i the casing 110 to direct the optical beam 114 onto the sample to create an optical spot that overlaps the ion beam spot formed where the charged particle beam: 104 strikes the sample surface. A distance between the emitting end of the optical source system and the sample 108, and an angle of incidence of the optical beam 1 14 on the sample 108, can be selected using mounting positions of the optical source system and the sample holder 106 in the casing 1 10; a shorter distance between the optical source 1 12 and the sample 108 may b more efficient for charge modification. The optical source 112 and the sample 108 can be mounted relative to each other such that the incidence angle of the optical beam 114 on the sample 108 is equal to or close to Brewster's angle to minimise reflection of the optical beam 1 14 from the surface of the sample 108. The charged particle source 102 and the optical system are mounted / arranged so as to not interfere with, occlude or block each other's beams 1 4, 1 14. Furthermore, the photon in the optical beam 1 14 do not substantially deflect the particles in the charged particle beam 104.

[36] The optical source 1.12, or the final component of the guiding optics, is mounted in the casi g 110 relative to the sample holder 106 and the charged particle source 102 so as to direct the optical beam 1 14 onto the surface of the sample .108 during irradi atio of the sample 108 with the charged particle beam 104 so that the optical beam 1 1.4 modifies charges on and in the sample 108 as they are generated by the charged particle beam 104 (e.g., photons in the optical beam 1 14 may act to eject electrons from the sample surface).

[37] The optical beam 1 14 may include light (i.e., photons), generated by the optical source 1 12, at wavelengths that are suitable to eject charged particles from the surface of the sample 108, thus causing removal of electrons from the sample 108 to a free state (e.g., a tree vacuum state) in the apparatus 1 0. The wavelengths of the light .in. the optical beam 114 may be selected or controiled based o determined material properties of the sample 108. The light, wavelength can be short enough to cause electron transfer from the surface to the free vacuum state. Ultraviolet (UV) wavelengths can be used, including deep UV wavelengths with energies of about 5 electron Volts (eV) or more. For example, for a glass sample or a diamond sample, illumination with 250-nm or 260-nm wavelength light may release electrons trapped in defect states created during irradiation with a G beam. A short illumination wavelength may in general be more efficient for charge modification.

[38] As shown in Figures IB to IE, the apparatus 100 can have different configurations with different mounting locations of the optical source 1 12 relative to the casing 110. In a gun-mounted configuration 100B, as shown in Figure IB, the optical source 1 12 is mounted adjacent or on the charged particle source 102 so that the optical beam 1 14 is incident onto the sample 108 at an incidence angle of about 90 degrees to the sample surface. In a holder-mounted configuration lOQC, as shown in Figure 1C, the optical source 112 is mounted in or on the holder 106 (which can include a two-part holder 106A and a stage 106B) so that the optical beam 1 14 is incident onto the sample 108 at an incidence angle of about 30 to 60 degrees to the sample surface. In a guide-on-gun configuration 100D, as shown in Figure ID, a optical guide 1 18 (e.g., including an optical fibre and coupling components) is connected to the optical source 112 (which may be outside the casing 110), and the optical guide 1.18 is mounted adjacent or o the charged particle source 102 so that the optical beam 1 14 is incident onto the sample 108 at an incidence angle of about 90 degrees to the sample surface. In a guide-on-holder configuratio 100E, as shown in Figure IE, the optical guide 1 18 is connected to the optical source 1 12 (which may be outside the casing 1 10), and the optical guide 1 18 is mounted in or on the holder 106 (which can include a two-part holder 106A and a stage 1.06B) so that the optical beam 1 14 is incident onto the sample 108 at an incidence angle of about 30 to 60 degrees to the sample surface,

[39] As shown in Figure 2, the end of the optical source system— for example the optical source 1 12 in the form of an array of LEDs— can be mounted to the sample holder 106, e.g., using a two-part sample holder 202 having a first face 204 with a sample holder 206 and a second face 208, visible from the first face 204, with a location mount 210 for the optical source 1 1.2. The second face 208 can be on tilted plane overlooking the sample location 206, e.g., at an angle of about 60 degrees. For example sample holder, with an incidence angle of about 60 degrees for the optical beam 1 14, a distance between the optical source 1 12 and the location of the sample 108 can be about 9 mm.

[40] The optical source system is mounted and controlled in the apparatus 1.00 so that it can be used simultaneously with the charged particle source 102. The locations and orientations of the optical source 1 12 and the optical source system in the easing 110 may differ for different commercially-available imaging and fabrication tools. For example, in Raith GmbH's EBL and IBL tools, the optical source 1 12 can be above the sample 108. In another example, in a dual-beam FIB tool, e.g., from Hitachi, JEOL USA, Inc. or FEI Company, the optical source 1 12 can be mounted to deliver the optical beam 1 14 at a slanted angle to the sample 108.

[41] The apparatus 100 includes a controller 116 connected to the optical source system (in particular to the optical source 1 12, and to any active components of the guiding components, e.g., active mirrors, active filters, etc.) to control the optical power (and thus th intensity) of the optical beam 114, and/or to control the optical wavelength(s) in the optical beam 1 14 (and in some cases to control the location of the optical spot relative to the sample 108). The controller 116 can include one or more commerciall available electronic controllers for light sources and active optical components. The controller 1 16 can control or steer the location of the optical spot from the optical beam 1 14 on the sample surface, e.g., for spatial control or modification of the surface charge e.g., if the surface charge has been delivered to different locations on the surface by the charged particle beam 104, The beam steering can be performed using the guiding components, e.g., mirrors, or by moving the optical source 1 12, e.g., a LED. In general, compared to the size and location of the ion beam spot, the optical spot is wide-spread and stationary.

[42] The optical source 1 12 may generate a. plurality of different optical wavelengths in one or more sub-beams of the optical beam 1 14. The optical source 112 may include a plurality of different sources controlled by the controller 1 16 (which may include a plurality of sub-controllers). The plurality of different sources may be arranged in different locations in the casing 1 10, may operate at respective different, optical waveiengths, and may generate the different optical sub-beams (which may be colliitear or may be non -col linear) in the optical beam 1. 14. Having a plurality of different optical wavelengths in the optical beam 1 14 can allow the apparatus 100 to discharge electrons trapped at different energy levels in the sample 108, e.g., arising due to different materials in the sample, and/or different charged particles in the charged particle beam 104, and/or different defects and trapping effects caused by the charged particle beam 104.

[43] The apparatus 100 may include a plurality of electrodes 1 18 mounted in the casing 1 10, e.g., close to and above the surface of the sample 108, and not occluding the charged particle beam 104 or the optical beam 1 14, to gather or collect the electrons freed by the optica! beam 1 14. The electrodes 1 18 may be positivel electrically biased by an direct- current electronic controller.

Methods

[44] A method of manufacturing the apparatus 100 includes at least the following steps:

[45] mounting the optical system (including the optical source 112 and any of the optical guiding components that guide the optical beam 114) in or to the casing 1 10 of the fabrication tool or the imaging tool with the charged particle source 102 and the sample holder 106 such that the optical source system does not occlude or block the charged particle beam 104, and to direct the optical beam 1 14 onto the sample 108 in an area (referred to as the "optical spot") that at least covers the incident area of the particle beam 104 (and may be significantly larger), and optionally at or close to the Brewster's angle (to make the charge modification effect more efficient; the angle may also be chosen to lie between 89.9 degrees (when it is mounted close to gun) and 0.1 degrees (when it is mounted on sample holder), which may include mounting and aligning the optical guiding components {e.g., mirrors, lenses and optical fibres) in or to the casing 110 to direct the optical beam 1 14— so that the optical spot can illuminate a sufficient area of the sample surface to modify electronic charges that would adversely affect the location of the ion beam spot;

[46] configuring the optical source 1. 12 to include one or more wavelengths in the optical beam 1 14 to modify the surface charge (e.g., by treeing electrons from the material of the sample surface);

[47] configuring the optical source 1 12 to provide an intensity or a range of intensities in the optical beam 1 14 to provide a sufficient rate of surface charge modification for the particular fabrication or imaging tool (e.g., taking into account the rate of charged particles in the particle beam 104, the spot size of the particle beam 104 on the surface of the sample 1 8, etc.}; and

[48] electronicall connecting the controller 116 to the optical source 1 12 to allow electronic communication between the controller 1 16 and the optical source 1 12.

[49] A method of modifying an electronic charge of a sample irradiated by a beam of charged particles using the apparatus 300 includes at least the following steps: [50] irradiating the ion beam spot (an area) on the surface of the sample 108 mounted on the sample holder 106 with the charged particle beam 104 to image the surface or to mill / fabricate the surface; and

[51] illuminating the optical spot (an area) on the surface of the sample 108 with the optical beam 1 14 to modify the charge distribution in and/or on the sample (e.g., by removing electrons from the surface) at least withi the optical spot, wherein the optical spot overlaps with the ion beam spot (e.g., the optical spot can completely surround and overlap with the ion beam spot), to improve spatial resolutio of the imaging or fabrication with the beam of charged particles.

[52] The irradiating step and the illuminating step can be performed simultaneously, or can be performed in successi on.

[53] The charge modification method can include the steps of controlling the optical intensity, optical wavelength, and/o location of the optical spot on the sample 108 to modify surface charges generated by the charged particle beam 104, e.g., by sweeping the optica] spot across the surface in a patter that follows the ion beam spot. In. other eases, the source 1 12 is stationary, and the optical beam 1 14 covers a much larger area than the area covered during fabrication or imaging with the particle beam 104 on the sample 108.

Examples

[54] In experimental examples, focused io fabrication was carried out using Raith's lonLiNE to nano-pattern surfaces of different materials, A nano-hole pattern arranged into a cross-shape, with separation between the fabrication sites of about 1 μ ^η ι, was used to test charging-mduced distortions. The typical ion fabrication current was about 20 pA for a 40μ.ΐη aperture, and an ion beam was focused to a 20-nm spot on the sample's surface at a 35 kV voltage.

[55] The experimental samples were dielectric slabs of about 50nm to 2 mm thickness. The materials tested in the discharging experiments exhibited strong charging effects, including: TiO?, soda-lime and borosilicate (B ?) glasses, chemical vapour deposition (CVD) diamond, Al ₂ i¾, S13N4, and LiNb ⁽ ¾. A kapton spacer 402 was used between an example stage 404 and example samples 406 to maximise charge modification by the illumination compared to other effects (e.g., charge escaping through the sample-sample holder interface) during the ion fabrication, as shown in Figure 4. All previously mentioned materials were tested for charging on kapton isolation pads with and without U V illumination.

[56] Example deep-UV LEDs emitting at about 250-nm to 290-nm wavelengths were used in an example LED anti-charging gun. The example LEDs were mounted on a tilted plane overlooking the sample at about 60° angle of incidence with a LED-to-irradiatio spot distance of about 9 mm. The angle was chosen to be close to Brewster's angle in order to minimize reflection. The emission power of the LED was proportional to the driving current, which was from 0 to 20 raA (100%)..

[57] The electrons may be treed from the example sample surface as shown in Figure 3: an electron is excited to a free vacuum level from traps and defects induced by the ion fabrication. The bandgap is E _g and the Fermi level is E _F , The energy of vacuum (free) level for the electron is 0 eV, Possible UV-ltght induced transitions are shown by vertical arrows in Figure 3.

[58] Figures 5A and SB show IBL images of a pattern on TiOi without (5 A) and with (5B) compensation of charging. Ion beam imaging was carried out by the Ratth IonLiNE.

[59] Figure 6 shows a position map of milled nano-holes on the surface of Ti0 ₂ . at different intensities of LED anti-charging gun illumination. The patterns were numerically overl ayed at the top-left corner O the central square feature. The departure of the most faraway corner points was compared with the designed positions by calculating the departure parameter &K - Δχ ² + Ay ² . The first writing point was the one in the most bottom-left corner of the pattern. The charging effects can be evaluated by overlaying the patterns at the staring fabrication point; however, then the errors for the first fabricated holes are the largest and are not representative for the entire pattern. The fidelity of surface patterning is quantified by plotting AR a I _a as a line 702 where the normalized intensity of LED is = II ILED _HKIX with iLEDmax being the intensity of LED at the maximum 20 mA current, as shown in Fig. 7. Here, the average value of AR was normalized to the length of the pattern L (see, Fig. 6) for the 8 comer points of the pattern. Almost perfect patter geometry (as designed) wa retrieved when the LED anti-charging gun was working above 80 of its intensity. After fabrication, the anti-charging gun was switched off, and imaging was carried out by IBL at low current (without the LED anti-charging gun) and some charging and pattern distortions were still present (shown as a horizontal line in Fig. 7). Scanning electro microscopy (SEM) was used to characterize the quality of the fabrication and modification of the charging. When the charging was strong during ion fabrication, the distortions were large, unpredictable, and took place all over the entire region where fabrication was carried out (as shown in Figure 5 A).

[60] The anti-charging actio of the deep UV photons (wit energies of about 5 eV) may be explai ed by reference to the photo effect of electrons trapped in traps and defects induced by heav Ga-ions during fabrication. The positive charge ions may have created avalanches of secondary electrons and defects in pre-surface regions of the sample (see Figure 3), The acceleration voltage of the Ga-ions was up to 35 ke and a strong generation of defects was expected. Wavelengths of 250 and 260 nm showed similar performances (see Figure ?), and had similar emitting surface geometries and powers oft the surface of the Ti0 ₂ and the diamond. The example LEDs did not have focusing optics. As illumination flux was increased, the error of the fabrication decreased (see Figure 7). At 80% of the maximum, .current, full compensatio of the distortions was achieved.

[61] Several substrates were used with strong charging: BK7 glass, diamond, S13N4, TiQ ₂ arid AI2O3. The electron work functions for the materials were: AI ₂ O ₃ 4.3-5.5 eV, S1O2 4.4-5.5 eV, S13N4 2.6 eV, Ti<½ 4.9-5.2 eV. The effect of charge modification was present for the shortest tested 250 nm wavelength illumination. The known electron work functions of different materials became less relevant in quantifying the discharging effect from ion-structured surfaces since the defects ifttroduced by io damage could have different energy locations close to the valence or conduction bands (see Figure 3). Consequently, a different UV wavelength may be required for electron removal from the surface (i.e., not from the Fermi level as would be the case for an untreated material). Table 1 shows a qualitative summary of the charge modification of the sample at several experimental LED wavelengths and for several materials. Marker (+) corresponds to the case where start and end points are separated less than the width of the milled groove, and (-) when the separation is larger. A O-circle was milled on the surface- under illuminatio corresponding to a 80% current. If chargin was present, the start and end points did not match. At the longest wavelength of 290 11m (4.27 eV), only partial charge modification was observed in T1O ₂ (work function about 5.0 eV). Figure 8 shows data for T1O2 (Table I). As the wavelength was increased, separation betwee the start and end points increased, and the UV illumination may have become less effective in terms of charge modification whe λ > 290 nm. [62] TABLE 1 :

[63] Ga-ions beams can be capable of milling through different materials, dielectrics and metals, and can work on complex 3D nano-landscapes, e.g,, it may be desirable to ion- structure a metal on a dielectric, in which case charging of metals may become an issue. Figure 9 shows Ga-fabrication of A! foil placed on an isolating kapton film under a 290 nm LED illumination, which is close to the work function of Al (about 4, 1 eV). Strong distortions were observed for the G-shape, indicating a poor charge modification by a longer wavelength UV illumination. Under the shorter wavelengths, there were no discemable shape distortions (Table I). For Ni (5.01 eV), there was no discemable charge modification for the wavelengths l _gs >250 nm (4,96 eV)„ indicating a possible influence of the photo-effect in excess charge removal from electrically isolated metals.

INTERPRETATION

[64] Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

[65] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms ^' part of the common general knowledge in the field of endeavour to which this specificati on relates.

RELATED APPLICATIONS

[66] The present application is related to Australian provisional patent application no. 2013903073 filed on 15 August 2013, the original specification of which is hereby incorporated in its entirety by reference herein.