Method of reducing photoelectron yield and/or secondary electron yield of a ceramic surface; corresponding apparatus and product

Abstract

A method of reducing photoelectron yield (PEY) and/or secondary electron yield (SEY) of a ceramic surface comprises applying pulsed laser radiation comprising a series of laser pulses emitted by a laser (4) to the surface of a target (10) to produce a periodic arrangement of structures on the surface of the target (10).

Claims

1. A method, comprising: applying pulsed laser radiation comprising a series of laser pulses to a ceramic surface to produce a periodic arrangement of structures on the surface, wherein a power density of the pulses is in a range 0.1 GW/cm.sup.2 to 3 GW/cm.sup.2 or in a range 0.1 TW/cm.sup.2 to 3 TW/cm.sup.2; a pulse duration of the laser pulses is in a range 300 femtoseconds (fs) to 100 nanosecond (ns); the pulsed radiation has a pulse repetition rate in a range 10 kHz to 1 MHz; an average power of the laser radiation is in a range 0.1 W to 10 W; the pulsed laser radiation comprises a pulsed laser beam that has a focal spot diameter on the surface in a range 1 μm to 100 μm.

2. A method according to claim 1, wherein a power density of the pulses is in a range 0.5 TW/cm.sup.2 to 1.5 TW/cm.sup.2.

3. A method according to claim 1, wherein a power density of the pulses is in a range 0.2 GW/cm.sup.2 to 1 GW/cm.sup.2.

4. A method according to claim 1, wherein the applying of the pulsed laser radiation comprises altering the properties of the ceramic surface such that the ceramic surface has a value of SEY less than 2.5.

5. A method according to claim 1, wherein the laser pulses have a duration less than a thermal relaxation time of a material of the ceramic surface.

6. A method according to claim 1, wherein a pulse duration of the laser pulses is in a range 300 femtoseconds (fs) to 1 nanosecond (ns).

7. A method according to claim 1, wherein the pulse duration is in a range 1 ps to 100 ps.

8. A method according to claim 1, wherein the periodic arrangement of structures on the ceramic surface comprises a periodic series of peaks and troughs substantially parallel to each other.

9. A method according to claim 1, wherein a peak to trough distance for at least some of the peaks, and/or an average or median peak to trough distance, is in a range 1 μm to 100 μm.

10. A method according to claim 1, wherein the periodic arrangement of structures comprises a cross-hatched arrangement or an arrangement of substantially parallel lines of peaks and troughs substantially without cross-hatching.

11. A method according to claim 1, wherein the periodic arrangement of structures is produced by a single pass of a laser source that provides the pulsed laser radiation.

12. A method according to claim 1, wherein the laser radiation comprises a pulsed laser beam that has a focal spot diameter on the ceramic surface in a range 5 μm to 100 μm or in a range 1 μm to 100 μm.

13. A method according to claim 1, wherein an average power of the pulsed laser radiation is in a range 3 W to 8 W or in a range 1 W to 10 W, or in a range 0.3 W to 2 W, or in a range 1 W to 5 W, or in a range 0.1 W to 1 W, or in a range 0.1 W to 2 W, or in a range 0.3 W to 5 W.

14. A method according to claim 1, wherein the applying of the pulsed laser radiation to the ceramic surface comprises scanning a pulsed laser beam over the ceramic surface, and a scan speed for the scanning is in a range 1 mm/s to 200 mm/s.

15. A method according to claim 14, wherein the scanning of the pulsed laser beam over the ceramic surface is repeated between 2 and 10 times, or is performed once.

16. A method according to claim 1, wherein an angle of incidence of the pulsed laser radiation to the ceramic surface is in a range from 0 to 30 degrees, or from 90 degrees to 60 degrees.

17. A method according to claim 1, wherein a wavelength of the radiation is in a range 100 nm to 2,000 nm.

18. A method according to claim 1, wherein the periodic arrangement of structures comprises a first series of peaks and troughs arranged in a first direction, and a second series of peaks and troughs arranged in a second direction, different from the first direction.

19. A method according to claim 18, wherein the first series of peaks and troughs and the second series of peaks and troughs intersect such that the periodic arrangement of structures comprises a cross-hatched arrangement.

20. A method according to claim 1, wherein applying the pulsed laser radiation to the ceramic surface comprises producing further structures, wherein the further structures are smaller than the structures of the periodic arrangement of structures.

21. A method according to claim 20, wherein the further structures comprise further periodic structures.

22. A method according to claim 20, wherein the further structures comprise nano-ripples or laser induced periodic surface structures (LIPPS).

23. A method according to claim 20, wherein the further structures have a periodicity in a range 10 nm to 1 μm.

24. A method according to claim 20, wherein the further structures cover at least part of the periodic arrangement of structures and/or are formed in troughs and/or on peaks of the periodic arrangement of structures.

25. A method according to claim 1, wherein the ceramic surface forms part of a laminated structure.

26. A method according to claim 1, wherein the ceramic surface forms part of a particle accelerator, an injection kicker system, a beamline, a waveguide, a detector, a detector apparatus, a spacecraft, or a vacuum chamber.

27. A method according to claim 1, wherein the ceramic surface comprises a surface of a component of an apparatus, and the method further comprises applying the pulsed laser radiation to the surface of the component of the apparatus to produce the periodic arrangement of structures on the surface of the component of the apparatus and then installing the component in the apparatus, or the method further comprises applying the pulsed laser radiation to the surface of the component of the apparatus with the component in situ in the apparatus.

28. A method according to claim 1, further comprising forming a metal layer on at least part of the ceramic surface after the applying of the pulsed laser radiation.

29. A method according to claim 1, further comprising at least one of degreasing, cleaning or smoothing the surface after the applying of the pulsed laser radiation and/or performing a surface carbon reduction process with respect to the ceramic surface after the applying of the pulsed laser radiation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of a system used for laser treatment of a surface to reduce photoelectron emission (PEE) and/or secondary electron emission (SEE) effects, for example to reduce photoelectron yield (PEY) and/or secondary photoelectron yield (SEY);

(3) FIG. 2 is a plot of SEY as a function of primary electron energy for a sample according to an embodiment;

(4) FIG. 3 is a plot of SEY as a function of primary electron energy for the reverse side of the sample of FIG. 2;

(5) FIGS. 4 to 9 are plot of SEY as a function of primary electron energy for samples according to further embodiments;

(6) FIGS. 10a to 10c are scanning electron microscope (SEM) images of surfaces of samples according to embodiments;

(7) FIGS. 11a to 11j are SEM images of surfaces of samples according to further embodiments;

(8) FIG. 12 is a graph showing total reflectance, total transmittance and absorptance for samples;

(9) FIG. 13 is a plot of SEY as a function of primary electron energy for a sample according to a further embodiment;

(10) FIG. 14 is a plot of SEY as a function of primary electron energy for the sample of FIG. 13, including data after conditioning and exposure to air of the sample;

(11) FIGS. 15a and 15b are SEM images of surfaces of samples according to embodiments;

(12) FIGS. 16a to 16n are SEM images of surfaces of samples according to further embodiments;

(13) FIGS. 17a and 17b are schematic illustrations of photo-thermal interaction (PTI) and photo-ablation interaction (PAI) mechanisms;

(14) FIG. 18 is a plot of measurements SEY as a function of primary energy for alumina ceramic samples, both before and after coating with a layer of gold; and

(15) FIGS. 19 and 20 are SEM images of the surfaces of the samples that are the subject of FIG. 18.

DETAILED DESCRIPTION

(16) FIG. 1 shows a system used for laser treatment of a surface to reduce photoelectron emission (PEE) and/or secondary electron emission (SEE) effects, for example to reduce photoelectron yield (PEY) and/or secondary electron yield (SEY).

(17) The system 2 of FIG. 1 comprises a laser 4 connected to a laser controller 6 which is used to control operation of the laser 4 to emit a pulsed laser radiation beam of desired characteristics. The laser 4 is aligned with a target 8 such that operation of the laser 4 under control of the laser controller 6 forms periodic structures on the surface of the target.

(18) In embodiments, the laser may be one of a pulsed Nd:YVO.sub.4 or Nd:YAG or Yb:KYW or Yb:KGW solid-state bulk laser, or a pulsed fibre laser, optionally a Yb, Tm or Nd-doped pulsed solid-state fibre laser. Any other suitable laser may be used in alternative embodiments. In the embodiment of FIG. 1, the wavelength of the pulsed laser radiation is 532 nm, but any other suitable wavelength can be used in other embodiments, for example 1064 nm or 355 nm.

(19) The controller may comprise a dedicated controller, or a suitably programmed computer. The controller may be implemented in software, hardware or any suitable combination of hardware and software. In some embodiments, the controller may comprise more ASICs (application specific integrated circuits) or FPGAs (field programmable gate arrays) or other suitable circuitry.

(20) In the embodiment of FIG. 1, the target 8 and laser 4 are located in air and the laser treatment of the surface is performed in air. The target 8 and laser 4 may be positioned in a sealable and/or pumpable chamber 10 that has an associated pump and/or gas supply, and the laser processing of the surface may be performed in vacuum or in desired gaseous conditions, for example in the presence of a selected reactive gas or inert gas. The chamber 10 is omitted in some embodiments.

(21) In the embodiment of FIG. 1, the target is a ceramic target comprising alumina, for example 99.5% or greater than 99.5% pure alumina. Other ceramic targets may be used in other embodiments. For instance, the target may comprise any suitable magnetic, conductive or dielectric ceramic material.

(22) For instance, the target may comprise a ceramic material having a spinel structure, for instance a spinel structure having the formula M(Fe.sub.2O.sub.4) where M is a covalent cation. M may be a covalent cation selected from the group manganese (Mn.sup.2+), nickel (Ni.sup.2+), cobalt (Co.sup.2+), zinc (Zn.sup.2+), copper (Cu.sup.2+), magnesium (Mg.sup.2+). Alternatively M may represent a monovalent cation, for example lithium (Li.sup.+) or even a vacancy or vacancies, for instance in the case where such absences of positive charge may be compensated for by additional trivalent iron cations (Fe.sup.3+).

(23) Alternatively, in some embodiments the ceramic target may comprise a ferrite material, for example a hexagonal ferrite material, for instance a material having a structure M(Fe.sub.12O.sub.19). M may be selected from the group barium (Ba), strontium (Sr), lead (Pb).

(24) In other embodiments, the ceramic target may comprise a garnet ferrite material, for instance having the structure of the silicate mineral garnet, and for example having the chemical formula M.sub.3(Fe.sub.5O.sub.12) where M may be ytrrium or a rare earth ion.

(25) In some embodiments, the ceramic target may comprise a thick- or thin-film resistor or electrode or a material that may be suitable for use as or as part of such thick- or thin-film resistor or electrode. In embodiments, the ceramic target may comprise a metal oxide material, for instance a material selected from the group, lead oxide (PbO), ruthenium dioxide (RuO.sub.2), bismuth ruthenate (Bi.sub.2Ru.sub.2O.sub.7). The ceramic target may comprise a ceramic material having overlapping energy bands.

(26) In other embodiments, the target may comprise a ceramic conductor. For instance, the target may comprise indium oxide (In.sub.2O.sub.3) and/or tin oxide (SnO.sub.2) or indium tin oxide (ITO).

(27) In embodiments, the ceramic target may comprise a heating element or a material that may be suitable for use as or as part of such a heating element. In embodiments, the target may comprise material selected from the group silicon carbide (SiC), molybdenum disilicide (MoSi.sub.2), lanthanum chromite (LaCr.sub.2O.sub.4), zirconia (ZrO.sub.2).

(28) In embodiments, the ceramic target may comprise a thermistor or a material that may be suitable for us as or as part of such a thermistor. In embodiments, the target may comprise material selected from the group consisting of iron spinel material, cobalt spinel material and manganese spinel material.

(29) In embodiments, the ceramic target may comprise a superconductor material, for example yttrium barium copper oxide (YBa.sub.2Cu.sub.3O.sub.7), a bismuth-strontium-calcium-copper oxide material (for example Bi.sub.2Sr.sub.2CuO.sub.6, Bi.sub.2Sr.sub.2CaCu.sub.2O.sub.8, or Bi.sub.2Sr.sub.2Ca.sub.2Cu.sub.3O.sub.10), a thallium-barium-calcium-copper oxide material (for example TI.sub.2Ba.sub.2CuO.sub.6, TI.sub.2Ba.sub.2CaCu.sub.2O.sub.8, TI.sub.2Ba.sub.2Ca.sub.2Cu.sub.3O.sub.10, or TIBa.sub.2Ca.sub.3Cu.sub.4O.sub.11) or a mercury-barium-calcium-copper oxide material (for example HgBa.sub.2CuO.sub.4, HgBa.sub.2CaCu.sub.2O.sub.6, or HgBa.sub.2Ca.sub.2Cu.sub.3O.sub.8).

(30) In embodiments, the target may comprise a perovskite material. In embodiments the target may comprise barium titanate (BaTiO.sub.3) or barium titanate including non-stoichiometric lead, strontium or calcium substitutions.

(31) In operation pulsed laser radiation of desired characteristics is scanned across the surface of the target 8 by the laser 4 under control of the laser controller 6 to produce a periodic arrangement of structures on the surface. For example, in order to form peaks and troughs arranged in parallel rows, the laser beam may be scanned along parallel, spaced-apart paths across the surface to form parallel troughs separated by peaks. Any other suitable arrangements of structures can be formed by suitable scanning of the laser beam over the surface.

(32) Operating parameters of the laser, and certain equations linking such parameters, can be represented as follows,

(33) Wavelength (λ) [m]

(34) Repetition rate of the laser (γ) [Hz]

(35) Pulse length of the laser (r) [s]

(36) Average power of the laser (P.sub.avg) [W]—represents the energy flow over one period t

(37) Energy per pulse (E.sub.p) [J]

(38) Fluence of the laser (F) [J/cm.sup.2]

(39) Beam spot radius on the target (r) [m]

(40) Beam spot area on the target (A=πr.sup.2)[m.sup.2]

(41) Number of times surface of the target was scanned by the laser beam (N) dimensionless

(42) Speed at which surface of the target was scanned by the laser beam (V) [m/s]

(43) Number of pulses fired per each spot on the surface of the target (n) [dimensionless]

(44) Time interval between the pulses—one period (t) [s]

(45) Peak Power (P.sub.peak) [W]—defines the energy flow within a single pulse

(46) Power density or Intensity (I) [W/cm.sup.2]

(47) Key Equations

(48) $t=\frac{1}{\gamma }$$n=\frac{\left(2r\right)\gamma }{V}$$E_p=\frac{P_{\mathrm{avg}}}{\gamma }$$F=\frac{E_{\mathrm{pulse}}}{A}$$P_{\mathrm{peak}}=\frac{E_{\mathrm{pulse}}}{\tau }$$I=\frac{P_{\mathrm{peak}}}{A}$

(49) Suitable operating parameters can be selected, for example based on the equations and representations above, to obtain pulsed laser radiation of desired properties, for example a desired power density of the pulses.

(50) Table 1 provides operating parameters of the laser of the apparatus of FIG. 1 to produce a desired periodic arrangement of structures on the surface of various samples of alumina. The laser processing of the surface was performed in an argon atmosphere for the PSCA sample and in air for all of the other samples of Table 1.

(51) TABLE-US-00001 TABLE 1 Sample name PSCA PS2L1R PS2C1R PS2L2R PS2C2R PS4L1R PS1C1R Wavelength 532 532 532 532 532 532 532 (nm) τ 10 ps 10 ps 10 ps 10 ps 10 ps 10 ps 10 ps Repetition rate 200 200 200 200 200 200 200 (kHz) Power (W) 2 2 2 2 2 4 1 Laser focal spot 12 12 12 12 12 12 12 diameter (μm) Pulse energy (μJ) 10.00 10.00 10.00 10.00 10.00 20.00 5.00 Fluence (J/cm.sup.2) 8.84 8.84 8.84 8.84 8.84 17.68 4.42 Structure Cross lines cross lines cross lines cross (cross/line) Hatch distance 24 24 24 24 24 24 24 (μm) Scan speed 6 6 6 6 6 6 6 (mm/s) Passes 1 1 1 2 2 1 1 Pulses per spot 400 400 400 400 400 400 400 per pass Width of laser- 14 15.2 15.2 15.2 15.2 15.2 15.2 processed area (mm) Length of laser- 14 15.2 15.2 15.2 15.2 15.2 15.2 processed area (mm) Process time 45.37 26.74 53.48 53.48 106.96 26.74 53.48 (minutes)

(52) The samples of Table 1 had a thickness of 3 mm, and the laser processed areas of the samples were 14 mm by 14 mm (for sample PSCA) or 15.2 mm by 15.2 mm (for the other samples). Each of the samples were aluminium oxide (alumina) of greater than 99.7% purity, which is the grade used for certain particle accelerator applications.

(53) The secondary electron yield (SEY) of each of the samples of Table 1 was measured for different primary electron energies at either three or four different spots on the samples. To avoid charging effects the surfaces were bombarded with low energy electrons (36 eV) between each measurement point. The maximum applied dose to measure one data point was about 1×10.sup.−12 C. The total dose to measure one spectrum was therefore about 1×10.sup.−11 C.

(54) FIG. 2 is a plot of SEY as a function of primary electron energy for sample PSCA at three different spots on the laser treated surface. FIG. 3 is a plot of SEY as a function of primary electron energy for sample PSCA at three different spots on the reverse side of the sample which was not laser treated, for comparison purposes.

(55) It can be seen that the laser treatment of the surface of the PSCA sample resulted in a reduction of SEY from around 8 to 9 (for the untreated reverse surface) to around 2.2 for the laser treated surface.

(56) FIGS. 4 to 9 are plot of SEY as a function of primary electron energy for samples PS1C1R, PS2C1R, PS2C2R, PS2L1R, PS2L2R and PS4L1R respectively.

(57) The approximate maximum SEY values for the various samples of Table 1 are summarised in Table 2 below.

(58) TABLE-US-00002 TABLE 2 Maximum SEY value obtained Sample for laser-treated surface PSCA 2.2 PS1C1R 2.2 PS2C1R 1.6 PS2C2R 1.7 (outlier 1.9) PS2L1R 1.9 PS2L2R 2.2 PS4L1R 2.5

(59) Scanning electron microscope (SEM) images of the samples of Table 1 were obtained, and show structures formed by the laser treatment of the surfaces of the samples.

(60) FIG. 10a is an SEM image of the laser treated surface of sample PSCA.

(61) 10b shows SEM images of the laser treated surfaces of the PS2L1R sample (top left image), the PS2L2R sample (bottom left image), the PS2C1R sample (top right image) and the PS2C2R sample (bottom right image).

(62) FIG. 10c shows SEM images of the laser treated surfaces of the PS4L1R sample (left hand image) and the PS1C1R sample (right hand image).

(63) FIGS. 11a to 11e are SEM images of laser treated surfaces for which the power density of the laser beam was 0.74 TW/cm.sup.2, 0.88 TW/cm.sup.2, 0.95 TW/cm.sup.2, 2 TW/cm.sup.2, 1.3 TW/cm.sup.2 respectively and the sample was laser treated in an argon atmosphere. FIGS. 11f to 11j are SEM images of laser treated surfaces for which the power density of the laser beam was 0.3 TW/cm.sup.2, 0.4 TW/cm.sup.2, 2 TW/cm.sup.2, 0.6 TW/cm.sup.2 and 1 TW/cm.sup.2 respectively and the sample was laser treated in air. Other operating parameters were substantially the same as for sample PSCA of Table 1.

(64) It is noted that in a normal, unmagnified view of sample PSCA (which was processed under argon atmosphere) the surface appeared black, whilst in normal unmagnified views of the other samples of Table 1 the surfaces of the samples appeared white, or at least much lighter than the surface of sample PSCA.

(65) Measurements of the spectral properties of samples of aluminium oxide were also performed. FIG. 12 is a graph showing total reflectance, total transmittance and absorptance of a sample of aluminium oxide (referred to as C Cera, having a purity of around 99.7%) used by CERN in a kicker magnet and of another sample of aluminium oxide (referred to as White Cera) before any laser processing. The spectra show some absorptance at 532 nm and very low absorptance at 1064 nm. Reflectance is different for the samples, which is in accordance with the different colours of the samples that are apparent to the human eye. The thickness of both samples was 3 mm. It was determined from these measurements that laser processing at 532 nm, or 515 nm—another available laser wavelength, may be suitable.

(66) It can be seen from the results outlined above that the maximum SEY value measured for the laser treated surfaces of the samples of Table 1 varies between 1.6 and 2.2. The sample with the lowest SEY is PS2C1R with a maximum value of 1.6. There seems to be substantially no link between reflectivity and the SEY of the samples. Some samples have outliers at 200 eV electron energy, which could be due to inhomogeneities at the surface.

(67) In some other embodiments where the sample is alumina, operating parameters may be selected from Table 3 as follows to produce a desired periodic arrangement of structures on the surface of the sample. The values of the operating parameters may also be selected from Table 3 in the case of other ceramic materials of interest.

(68) TABLE-US-00003 TABLE 3 Pulse Focal Repetition Average Scan Hatch Power Wavelength width spot rate power speed Repetition distance density (nm) range diameter (KHz) (W) (mm/s) number (mcm) (intensity) 355 nm, 500fs-1 ns 1 μm-100 μm 10 kHz-200 kHz 1-10 1-100 1-10 10-100 0.1 TW/cm.sup.2 532 nm or to 1064 nm 3 TW/cm.sup.2

(69) Good results may be achieved with a power density range of 0.1 TW/cm.sup.2 to 3 TW/cm.sup.2, with particularly good results for wavelength of 532 nm in the power density range 0.5 TW/cm.sup.2 to 1.5 TW/cm.sup.2 for processing in argon. However, highly organised structuring is achieved for laser processing in either air or argon.

(70) The results discussed above in relation to Tables 1 to 3 and FIGS. 2 to 12 were obtained using laser beam power densities in the TW/cm.sup.2 range. In alternative embodiments, laser patterning of ceramic surfaces is obtained using laser beam power densities in the GW/cm.sup.2 range.

(71) Table 4 provides operating parameters of the laser of the apparatus of FIG. 1 to produce a desired periodic arrangement of structures on the surface of a sample (referred to as the NSCA sample) of alumina, using laser beam power densities in the GW/cm.sup.2 range. The laser processing of the surface was performed in an argon atmosphere for the NSCA sample.

(72) TABLE-US-00004 TABLE 4 Sample name NSCA Wavelength 532 (nm) T 10 ns Repetition rate 20 (kHz) Power (W) 3.46 Laser focal spot 60 diameter (μm) Pulse energy 173.0 (μJ) Fluence (J/cm.sup.2) 6.12 Structure cross (cross/line) Hatch distance 70 (μm) Scan speed 20 (mm/s) Passes 10 Pulses per spot 60 per pass Width of laser- 14 processed area (mm) Length of laser- 14 processed area (mm) Process time 46.67 (minutes)

(73) Table 4 is in the same format as Table 1 above, which provided operating parameters for certain samples treated with laser power densities in the TW/cm.sup.2 range.

(74) The NSCA sample of Table 1 had a thickness of 3 mm, and the laser processed area of the samples was 14 mm by 14 mm. The sample was aluminium oxide (alumina) of greater than 99.7% purity, which is the grade used for certain particle accelerator applications.

(75) The secondary electron yield (SEY) of the NSCA sample of Table 4 was measured for different primary electron energies at three different spots on the sample. To avoid charging effects the surfaces were bombarded with low energy electrons (36 eV) between each measurement point. The maximum applied dose to measure one data point was about 1×10.sup.−12 C. The total dose to measure one spectrum was therefore about 1×10.sup.−11C.

(76) FIG. 13 is a plot of SEY as a function of primary electron energy for sample NSCA at three different spots on the laser treated surface.

(77) After the performance of the SEY measurements, the NSCA sample was conditioned in an attempt to obtain even lower SEY values by bombarding the sample with 500 eV electrons up to a total dose of 1×10.sup.−2 C/mm.sup.2 and then exposing the sample to air during one night.

(78) FIG. 14 is a plot of SEY as a function of primary electron energy for sample NSCA at three different spots on the laser treated surface following the conditioning using 500 eV electrons and exposure to air. The SEY results of FIG. 13 are also included on FIG. 14 for comparison.

(79) It was found that the SEY results of FIG. 14 obtained after the conditioning and air exposure showed an increased maximum SEY, suggesting a reactive surface. The measurements of SEY spectra after the conditioning and air exposure were not well reproducible, for which a possible reason may be implantation of charges into the bulk of the ceramic.

(80) To verify the charging of the NSCA sample surface, X-ray photoelectron spectroscopy measurements to obtain an XPS spectrum were performed following the conditioning and air exposure. The XPS spectrum indicated that aluminium (15 atomic %) and oxygen (79 atomic %) were the main elements present, that carbon contamination was low (1 atomic %) and that some fluorine was present (5 atomic %).

(81) FIGS. 15a and 15b are SEM images, at different levels of magnification, of the laser treated surface of sample NSCA.

(82) FIGS. 16a to 16d are SEM images of laser treated surfaces for which the power density of the laser beam was 0.25 GW/cm.sup.2, 0.35 GW/cm.sup.2, 0.45 GW/cm.sup.2, and 0.55 GW/cm.sup.2 respectively and the sample was laser treated in air. FIGS. 16e to 16n are SEM images of laser treated surfaces for which the power density of the laser beam was 0.6 GW/cm.sup.2, 0.65 GW/cm.sup.2, 0.7 GW/cm.sup.2, 0.75 GW/cm.sup.2, 0.8 GW/cm.sup.2, 0.85 GW/cm.sup.2, 0.9 GW/cm.sup.2, 0.95 GW/cm.sup.2, 1 GW/cm.sup.2 and 1.5 GW/cm.sup.2 respectively and the sample was laser treated in an argon atmosphere. Other operating parameters were substantially the same as for sample NSCA of Table 4.

(83) In some other embodiments where the sample is alumina, operating parameters may be selected from Table 5 as follows to produce a desired periodic arrangement of structures on the surface of the sample. The values of the operating parameters may also be selected from Table 5 in the case of other ceramic materials of interest.

(84) TABLE-US-00005 TABLE 5 Pulse Focal Repetition Average Scan Hatch Power Wavelength width spot rate power speed Repetition distance density (nm) range diameter (KHz) (W) (mm/s) number (mcm) (intensity) 355 nm, 1 ns-100 ns 20 μm-100 μm 10 kHz-200 kHz 3-8 10-50 1-10 10-100 0.002 GW/cm.sup.2 532 nm or to 1064 nm 3 GW/cm.sup.2

(85) Good results may be achieved with a power density range of 0.1 GW/cm.sup.2 to 3 GW/cm.sup.2, with particularly good results for wavelength of 532 nm in the power density range 0.2 GW/cm.sup.2 to 1 GW/cm.sup.2 for processing in argon. However, highly organised structuring is achieved for laser processing in either air or argon.

(86) It is a feature of embodiments that periodic structures can be formed on ceramic surfaces by applying to the surfaces laser radiation having a power density in the TW/cm.sup.2 range or in the GW/cm.sup.2 range. Without wishing to be bound by theory, and without limitation to the scope of protection, the following comments are provided which relate to processes which may occur in relation to at least some embodiments.

(87) Laser engineering provides an overarching methodology that provides for the formation of periodic structures according to embodiments. Precision laser engineering is expected to excite free electrons within metals, vibrations within insulators, and indeed both types of excitations within semiconductors. The mechanisms by which lasers can engineer materials include the following:

(88) (i) Photo-thermal interaction (PTI)—commonly achieved using laser beams providing short dwell time (e.g. lasers with nanosecond pulsewidth);

(89) (ii) Photo-ablation interaction (PAI)—envisaged using laser beams providing ultra-short dwell time (e.g. lasers with picosecond or femtosecond pulsewidth).

(90) The laser processing in respect of the embodiments described in relation to Tables 4 and 5 and FIGS. 13 to 16 may be in the PTI regime. The laser processing in respect of the embodiments described in relation to Tables 1 to 3 and FIGS. 2 to 11 may be in the PAI regime.

(91) In the PTI regime the focused laser beam acts as a spatially confined, intense heat source. Targeted material is heated up rapidly, eventually causing it to be vaporized. Without wishing to imply any limitation to the scope of protection, the targeted material could be referred to as being boiled away. An advantage of this approach is that it may enable rapid removal of relatively large amount of target material. However, the peripheral heat affected zone (HAZ) damage and the presence of some recast material after processing present limitations in terms of heat confinement for precision laser materials engineering.

(92) In the PAI regime, the laser drives multi-photon absorption of light inside the material. This strips electrons from the material, which then explode away due to Coulomb repulsion.

(93) Because PAI involves directly breaking the molecular or atomic bonds that hold the material together, rather than simply heating it, it is intrinsically not a ‘hot’ process. Since the material is removed in a very short timeframe, the ablated material carries away most of the energy before heat can spread into the surrounding material. These effects may result in a significantly reduced HAZ. Furthermore, this is a clean process and may leave minimal recast material, thereby eliminating the need for elaborate post-processing. The PAI mechanism is compatible with a very broad range of materials, including high band-gap materials that have low linear optical absorption and therefore are difficult to engineer with existing techniques. The PAI mechanism can be considered ‘wavelength neutral’; that is, nonlinear absorption can be reduced even if the material is normally transmissive at the laser wavelength.

(94) The PAI mechanism should fundamentally allow for custom design of electron work function of ceramic surfaces. Ceramics usually consist of metallic and non-metallic atoms joined by bonds that are partly ionic and partly covalent, giving them such properties such as hardness, brittleness and resistance to heat. Therefore, it may be of importance to correctly identify parameters that may play a significant role in the light-matter interaction mechanisms in these materials and ultimately contribute to challenges of the laser precision structuring processes and the design of the surface potential of ceramic surfaces.

(95) The PTI and PAI mechanism are illustrated schematically in FIGS. 17a and 17b respectively.

(96) It is a further feature of embodiments that the characteristics of the pulsed radiation that is applied to the surface, for example the use of pulse durations in the picosecond range or less, may be such that the periodic structures that are formed may be of shallower depth and/or more gently sloped than features formed using pulsed radiation of higher energy and/or longer duration for example pulse durations in the nano-second range.

(97) Again without wishing to be bound by theory, and without limitation to the scope of protection, the following further comments are provided which relate to processes which may occur in relation to at least some embodiments.

(98) In irradiation at very high intensities (or high irradiance) one is confronted with the issue of a dense, strongly absorbing material, in the first few tens of nm of which energy at a rate of some 10.sup.20 W/cm.sup.3 is liberated. Part of this energy, once randomised, is conducted into the bulk of the material, while part is converted into directed kinetic energy by thermal expansion of the heated layer. Two regimes are distinguished in this respect.

(99) 1. Nanosecond pulsed laser interaction which is dominated by the expansion and ablation of material. Here the thermal pressure of the heated layer is sufficient to cause significant compression of the underlying target material.

(100) 2. Picosecond pulsed laser interaction (which is heat conduction dominated since hydrodynamic motion during the pulse duration is negligible (laser pulses here may be 1000 times or more shorter than nanosecond ones). In the picosecond regime the strong heating of the dense material may occur before hydrodynamic expansion of the processed layer has even started. The plasmas produced in this regime may have essentially the same density as the solid target itself. This—upon cooling—leads to the formation of fine structures—in the range from 1 micrometres to 50 micrometres depending on the irradiation parameters—covered with nano-structures.

(101) Using picosecond duration pulsed radiation according to some embodiments can in some cases also cause formation of nano-ripples or other small scale structures on the surface in addition to the larger scale peaks and troughs obtained by scanning the laser beam in an appropriate pattern over the surface. It is possible that such nano-ripples or other small scale structures may in some cases decrease the PEY or SEY further, in addition to the reduction obtained by larger periodic peak and trough structures. Furthermore, in some cases the nano-ripples or other small scale structures and/or the shallower peaks and troughs associate with picosecond rather than nanosecond pulses may also provide improved or alternative electrical properties of the surface, for example reduced induction, and/or can provide the surface with an increased area at the nano- or micro-scale.

(102) Again, without wishing to be bound by theory, and without limitation to the scope of protection, further comments are as follows.

(103) With increasing surface roughness the maximum SEY may decrease for a surface characterized by (for example statistically containing) more valleys, while it may increase significantly at a surface spread with (for example statistically containing) more hills. The observation indicates that hill and valley structures may be very effective in increasing and decreasing the SEY, respectively, due to their different morphologic features and surface electron work functions (EWF).

(104) The total SEY may denote the ratio of both emitted true secondary electrons (SEs) and backscattered electrons (BSEs) to primary electrons (PEs) incident to the surface.

(105) Example: Sample surfaces exposed to air can be easily contaminated by adsorbed gases and hydrocarbons, and their SEY may increase.

(106) High SEY caused by contaminations should be likely to give rise to the electron multiplication and eventually degrades the performance of microwave devices and the destructive electron-cloud instability in large particle accelerators.

(107) Increase in SEY indicates the reduction in electron work function (EWF).

(108) With increasing roughness SEY.sub.max has a significant increase for surfaces that are spared by hills. This phenomenon implies that hill structures should play a positive role in the SE emission. Moreover, SEY.sub.max decreases rapidly by surfaces that are mainly characterized by valleys, which should be the dominant factor of the reduction in SEY.sub.max.

(109) In the case of valley structures, SEs can be trapped effectively through collisions with sidewalls, thus the SEY declines. Nevertheless, for hill structures, apart from the negative effect of sidewalls, there are some positive effects on SE emission. For example, some PEs strike the local surfaces with hills obliquely, which will induce more SEs than normal incidence. Additionally, SEs are likely to re-enter sidewalls of the hills, resulting in further SEs generations. Most re-entered SEs should be the BSEs those with high energies to overcome the surface potential barrier and generate plenty of true SEs with low energies escaping to the vacuum.

(110) Changes in the electron work function (EWF) induced by different surface morphologies may also be responsible for the SEY variations. The work function may decrease at surface peaks and increases at surface valleys with increasing the surface roughness. Hills and valleys in our structuring work may be considered as surface peaks and valleys. A rougher surface introduced by hill structures often has a lower electron work function (EWF), thus the SEY naturally increases. However, the EWF will be enhanced by roughing a surface with valley structures, and finally the SEY decreases.

(111) It has been found that forming of a metal layer on the ceramic surface after the laser treatment that forms the periodic structures can result in a significant reduction of SEY. FIG. 18 is a plot of measurements SEY as a function of primary energy for the alumina ceramic samples with sample names PS2C1R and PS2C1R mentioned above, both before and after coating with a layer of gold of 10 nm thickness. It can be seen that there is a significant reduction of SEY for both samples.

(112) FIGS. 19 and 20 show SEM images of the surfaces of the PS2C1R and PS2C1R alumina sample after coating with the gold layer.

(113) In alternative embodiments any other suitable metal of any other suitable thickness may be used for the metal layer on the ceramic surface.

(114) The layer of gold or other metal may be formed using any suitable deposition process, for example any suitable chemical or physical vapour deposition process, for instance a sputtering process, an evaporative deposition process or a laser deposition process. By way of example, an Edwards® 308 coating unit may be used to form the coating. Any other suitable deposition apparatus may be used.

(115) In alternative embodiments, the ceramic surface, or a metal layer deposited on the ceramic surface may be subject to a degreasing, cleaning or smoothing process and/or a surface carbon reduction process after the applying of the laser radiation, which may result in a decrease in SEY. Cleaning using an NGL® degreasing product may be used. Any suitable degreasing, cleaning, smoothing or surface carbon reduction process may be used in alternative embodiments. The degreaser may, in some embodiments, be such as to not change substantially the morphology of the sample but may remove a layer of carbon or carbon-containing compounds, mixtures or other materials or other undesired and/or extraneous compounds, mixtures or materials from the surface, for example metal oxides, grease or dirt. For example, in some embodiments 99.7% glacial acetic acid (any other suitable concentration may be used) may be used for example at room temperature to remove surface materials, for example copper (I) and copper (II) oxides and/or other materials without substantially changing the surface morphology.

(116) It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. Each feature in the description, and (where appropriate) the drawings may be provided independently or in any appropriate combination with any other such feature.