Optics and Lasers in Engineering 83 (2016) 116–125

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Optics and Lasers in Engineering

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Femtosecond laser ablation of tungstate for arrays

S. Richards a,b,n, M.A. Baker b, M.D. Wilson a, A. Lohstroh b, P. Seller a a STFC-Rutherford Appleton Laboratory, Harwell, Oxfordshire OX11 0QX, United Kingdom b Faculty of Engineering and Physical Sciences, University of Surrey, Guildford, Surrey GU2 7XH, United Kingdom article info abstract

Article history: Ultrafast pulsed laser ablation has been investigated as a technique to machine CdWO4 single crystal Received 31 July 2015 scintillator and segment it into small blocks with the aim of fabricating a 2D high energy X-ray imaging Received in revised form array. Cadmium tungstate (CdWO4) is a brittle transparent scintillator used for the detection of high 29 January 2016 energy X-rays and γ-rays. A 6 W Yb:KGW Pharos-SP pulsed laser of wavelength 1028 nm was used with a Accepted 5 March 2016 tuneable pulse duration of 10 ps to 190 fs, repetition rate of up to 600 kHz and pulse energies of up to Available online 29 March 2016 1 mJ was employed. The effect of varying the pulse duration, pulse energy, pulse overlap and scan pattern Keywords: on the laser induced damage to the crystals was investigated. A pulse duration of Z500 fs was found to Micro-machining induce substantial cracking in the material. The laser induced damage was minimised using the following Femtosecond laser operating parameters: a pulse duration of 190 fs, fluence of 15.3 J cm 2 and employing a serpentine scan Sub-surface damage pattern with a normalised pulse overlap of 0.8. The surface of the ablated surfaces was studied using Brittle materials Structured scanning electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy. Ablation products were found to contain cadmium tungstate together with different cadmium and tungsten oxides. These laser ablation products could be removed using an ammonium hydroxide treatment. & 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

1. Introduction potentially segment cadmium tungstate and other scintillator materials with minimal damage and in geometries which minimise Cadmium tungstate is a commonly used material for the the need for the handling of the small fragile segments. For the detection of X-rays and γ-rays, due to its high density of 7.9 g cm3 array to function efficiently, the laser machining should not induce and average high atomic number [1]. It converts incident ionising any changes in the material which could negatively affect the light radiation into visible photons which are isotropically emitted from transporting properties of the array. Such changes include the for- the point of radiation absorption. These can be detected by an mation of cracks and a heat affected zone including recast material optical sensor coupled to the scintillator. Thick 2D arrays of optically which could absorb the scintillation light. Laser machining is one of isolated small pitch segments can be used for the detection of high only a few viable cutting methods and is cost effective, due to the energy X-rays whilst also giving positional information for imaging fast cutting times. [2]. Any decrease in spatial resolution, typically associated with an Several groups have reported the use of lasers to structure or segment scintillators using either ablation or sub-surface laser increase in thickness, can be prevented using a material which is engraving. These ablation studies have typically focussed on the opticallyhighlyreflective between each segment. The manufacture use of Nd:YAG nanosecond pulsed lasers or nanosecond pulsed of these arrays is typically undertaken by hand where scintillator Excimer lasers, since many scintillators are transparent to UV crystals are mechanically cut, polished and assembled into large wavelengths [4,5]. The researchers have encountered difficulties arrays. This approach is costly, time consuming and becomes diffi- with crystal heating and segment melting [4] or have been forced cult for higher resolution arrays containing segments with a width to employ a lower energy laser beam to pre-score the crystal, m fi below 500 m. In addition to handling dif culties, cadmium tung- enabling containment of any fractures [5]. Due to problems asso- fi state is dif cult to machine into small segments due to its brittle- ciated with the brittleness of many scintillator materials, attempts 1/2 ness; with a reported fracture toughness of 0.2 MPa m and a at segmenting scintillators using ablation were abandoned in cleavage along the (010) plane [3]. Laser micromachining can favour of sub-surface laser engraving which had previously shown promise [6–9]. The sub-surface laser engraving technique relies on

n modifying regions of the scintillator crystal to either induce Corresponding author at: STFC-Rutherford Appleton Laboratory, Harwell, Oxfordshire OX11 0QX, United Kingdom. changes in refractive index or micro-cracks which act to reduce E-mail address: [email protected] (S. Richards). the spread of scintillation light from one segment to another via http://dx.doi.org/10.1016/j.optlaseng.2016.03.004 0143-8166/& 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125 117

Fig. 1. Schematic of the cutting geometry (left) and the measured crystallographic orientation of the ablated sheets measured. The laser was incident on the (010) plane. The a- and c-axes of the single crystal did not align to the edges of the CdWO4 sheets. absorption and total internal reflection [6]. Compared to optical regions, atomic force microscopy (AFM) was used to measure isolation in the segmentation method, the sub-surface laser surface topography, energy dispersive X-ray spectroscopy was engraving technique has a potentially worse spatial resolution used to analyse the elemental composition of the bulk of the laser since light can still pass through the low refractive index region ablated regions and X-ray photoelectron spectroscopy (XPS) was between each segment. employed to investigate the chemical composition of the laser The absorption mechanisms of ultrashort pulse laser ablation ablated surfaces. make it a promising method to segment scintillators such as cadmium tungstate. The authors are not aware of any published research on the effectiveness of ultrashort pulsed laser ablation for 2. Method segmenting brittle transparent scintillators. Due to the high intensities achievable in ultrashort laser ablation, the absorption The Yb:KGW pulsed laser was a Light Conversion Pharos-SP process is due to a multiphoton non-linear process. As a result of (supplied by Photonic Solutions Ltd. Edinburgh, UK) employing a this, matching of the optical absorption properties of the ablated wavelength of 1028 nm. A pulse compressor system enabled the material with the wavelength of the laser light is not as crucial as pulse duration to be adjusted from 190 fs to 10 ps with a max- for ns pulse laser ablation or continuous wave laser ablation [10]. imum power of 6 W and an adjustable repetition rate from single Ultrashort pulse laser ablation is also capable of ablating with pulses up to 600 kHz giving pulse energies as high as 1 mJ. The minimal heating of the material due to the ablation process laser was integrated into a CAM system which controlled a Scanlab occurring faster than heat can diffuse into the neighbouring SCANcube 10 galvanometer scanner coupled to a telecentric fθ material [10]. Whilst ultrashort pulse laser ablation could enable scanning lens. A slightly elliptical spot was formed due to astig- rapid micromachining that does not melt or crack the scintillators, matism in the optics. The spot size was determined to be 30 μmby previous studies have not paid particular attention to the effect the plotting the squared diameter of a series of single pulse holes ablation process might have on the surface of the material, and against the logarithm of the pulse energy [12]. how this might influence the performance of a scintillator array. Cadmium tungstate was supplied by Hilger crystals in the form This work aims: (i) to establish the optimum laser operating of sheets of 15 mm 25 mm with the thickness varying from 340 conditions to efficiently cut the material but induce minimum to 470 mm. The laser was incident on the (010) plane to cut damage; (ii) to investigate the nature of the ablated material and through the thickness of the sheets (Fig. 1). X-ray diffraction establish a methodology to clean the ablated surfaces to allow a showed that the long edge of the sheet was 7.44° from the (101) scintillator array to be assembled. plane and the short edge was 8.67° off the (101) plane. The A 6 W Yb:KGW 1028 nm laser was used in air with a Scanlab sheets were supported at both ends giving a small air gap of 1 mm SCANcube 10 galvanometer scanner and telecentric fθ lens system. above the sample stage. This arrangement was employed follow- The laser was capable of pulse durations in the range of 190 fs to ing initial tests which showed that debris from the sample stage 10 ps with pulse energies as high as 1 mJ. A series of experiments damaged the crystal as the laser cut, penetrated through the sheet were conducted to determine the optimum laser parameters for and into the aluminium plate on the stage. the application. The following parameters were investigated: Initially, a series of holes were ablated at increasing pulse (i) the effect of pulse energy on the ablation rate; (ii) the scan energy using 1, 2, 5 and 10 pulses to assess the ablation rate as a speed with different repetition rates – to investigate the effect of function of energy and number of pulses. The pulse energies were the pulse overlap on definition of the cut; (iii) the degree of controlled by changing the laser power and repetition rate. At a cracking induced by different cutting directions, pulse energy, frequency of 25 kHz and pulse duration of 190 fs, ten power set- pulse duration and scan pattern. The normalised pulse overlap tings were employed. The power was initially set at 550 mW, (NPO) is the degree of overlap for subsequent pulses as they are which due to losses in the optics, delivered 450 mW to the sample. scanned across the sample. It can be defined by the spot diameter The power was then progressively increased up to a laser power of

D, the scan speed V, the pulse duration τp and the laser repetition 5500 mW, delivering 4230 mW to the sample. The power was rate R as follows: (D(V/R)V*τp)/(D)¼NPO [11]. Hence, a higher measured using a Coherent FieldMate power metre. A Zeiss LSM- NPO corresponds to a greater overlap of pulses. 400 CLSM was employed to image the holes generated and their Following ablation, Confocal laser scanning microscopy (CLSM) volume and depth were measured with a Bruker Dimension Edge was used to study sub-surface damage, scanning electron micro- AFM in the non-contact tapping mode. The Gwyddion SPM ana- scopy (SEM) was used to study the morphology of the ablated lysis software was employed to analyse the AFM data [13]. 118 S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125

The pulse overlap tests were performed by changing the scan with a radius of 400 μm for the reference sample and 200 μm for speed and keeping the repetition rate fixed at 25 kHz using the ablated samples. The hemispherical analyser was operated powers of 4400, 4950 and 5500 mW. NPO values of 0.3–0.8 in with a pass energy of 50 eV and a step of 0.2 eV for the core level 0.1 increments were employed. spectra. An electron flood gun was used to compensate for surface The damage induced by different laser parameters including charging. The binding energies of the photoelectron peaks were cutting direction, pulse duration, pulse energy and cut width to referenced to the adventitious hydrocarbon C 1 s peak at 285.0 eV. depth ratio was investigated as follows. The differences in cutting Curve fitting was undertaken using a mixed Gaussian–Lorentzian direction were tested by cutting 3 trenches, one parallel to the lineshape and quantification of atomic concentrations was per- long edge of the sheet, one parallel to the short edge of the sheet formed from the peak areas using the Thermo Scientific Avantage and one at 45° to the edges of the sheet. The sample was then software which employs instrument modified Wagner sensitivity rotated by 90° and another 3 trenches were ablated in the same factors after a Shirley background subtraction. configuration to study possible effects due to polarisation of the laser. To study the damage induced by the different pulse energies and pulse durations as a function of ablated depth three different 3. Results and discussion pulse durations were tested, 1 ps, 500 fs and 190 fs. Pulse dura- tions up to 10 ps were tested, but the significant damage induced 3.1. Material removal rate at these pulse durations prevented detailed studies of the samples. The depth was controlled by the number of passes employed for The ablated volume of the 190 fs holes was measured by AFM. each trench. The number of passes was controlled by the CAM Two measurements were performed for each hole, with the system and 9 trenches were ablated for each of the pulse energies scanning direction being reversed, minimising any effect of the with a progressive increase in the number of passes from 5 passes steep sided walls of the holes on the results recorded. for the first trench to 800 for the final trench. The NPOs were kept The volume was determined for each ablated hole using the at a value of 0.8 and the pulse energies varied by changing the AFM datasystem and an average taken of both measurements. The pulse repetition rate. The selected repetition rates were 100, 84.6, volume of ablated material was plotted against the total incident 73.3, 64.7 and 57.9 kHz which corresponded to pulse energies of energy (Fig. 2). The 3 highest pulse energy holes ablated with 10 55, 65, 75, 85 and 95 mJ and fluences of 15.6, 18.4, 21.2, 24.1 and pulses were too deep to be measured by the AFM. 26.9 J cm 2 respectively. The samples were cleaned by immersion The volume of ablated material was found to be greater using in a solution of 30 vol% NH3 in H2Oat45°C for 15 min. This multiple lower energy pulses than fewer higher energy pulses solution was used for cleaning as it has been shown to dissolve delivering the same total incident energy. The holes ablated using cadmium tungstate and tungsten oxides [14]. 10 of the lowest energy pulses at 18 μJ, delivering a total incident The morphology of the ablated surfaces was studied using a energy of 180 μJ resulted in significantly more material being Hitachi S3200N SEM. An incident electron beam energy of 20 keV removed than those ablated using 5 pulses of the highest pulse was employed in the low pressure mode (15 Pa) to prevent char- energy of 169 μJ delivering a total incident energy of 846 μJ, ging of the samples. This limited the SEM to the use of backscatter despite 5 pulses at the highest energy being equivalent to imaging only. A Bruker Dimension Edge AFM in the PeakForce 4.7 times the total incident energy of the lowest energy pulses. tapping mode was employed to study the morphology of the This increase in the material removal rate is consistent with trench walls before and after cleaning. XPS was performed on incubation effects in the crystal where the initial pulses induce surfaces of a reference sample, an ablated sample and cleaned defects in the material which then increase absorption of the sample. The XPS instrument employed was a Thermo Scientific incident laser light in subsequent pulses [10]. Theta Probe with a monochromated Al k-alpha X-ray source Fractionating the total incident energy into more pulses was also (hν¼1486.6 eV) operated at 15 kV and 20 mA using an X-ray spot found to influence the morphology of the ablated regions. Holes

Fig. 2. The ablated volume as a function of total incident laser energy. S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125 119

Fig. 3. 3D AFM topographic images of two ablated holes ablated using 5 169 μJ pulses (A) and 10 18 μJ pulses (B).

Fig. 4. SEM micrographs of: (A) a trench ablated with an NPO of 0.3; (B) a trench ablated with an NPO of 0.8 and (C) fine ripples at the edge of a trench ablated with an NPO of 0.3.

ablated using few high energy pulses were found to have a smooth This was due to low NPOs leaving a ‘neck’ of CdWO4 in between crater that was surrounded by a raised lip (Fig. 3(A)). Holes ablated each pulse giving rise to an undulating trench edge (Fig. 4(A)). The using many low energy pulses were not surrounded by a raised lip morphology of the ablated regions has a periodic structure which and the bottom of the holes exhibited a coarse ripple structure corresponds to the pulse overlap. The regions where the laser (Fig. 3(b)). The surrounding regions for both low and high energy pulses strongly overlap (Fig. 4(B)) exhibits fewer pores than the holes were found to contain evidence of liquid phase material regions which have only been exposed to only one or two pulses ejected from the ablation volume. The ripple formation in the holes due to the low overlap (Fig. 4(A)). As a result of the reduction in ablated using low pulse energy is consistent with the evolution of porosity of the ablated regions and better definition of the trench nanoplasma bubbles [15]. The raised lip surrounding the holes edges an NPO of 0.8 was selected for this application. ablated using high energy pulses was attributed to re-deposited The material adjacent to the trenches was also found to be ablated material. As the removal of material at higher pulse ener- modified by the lower intensity tails of the beam profile. The gies is less influenced by incubation effects than that at lower modified regions were found to have two types of fine surface energies, it is thought that the material is removed before the ripple ripples. The finer ripples had a spacing of 337 nm and the coarser structure can form. Hence, the formation of ripples by nanoplasma ripples had a spacing of 884 nm (Fig. 4(C)). The spacing of the bubbles is suppressed at higher pulse energies. This observation is ripples was less than the laser wavelength of 1028 nm and is contrary to what has been previously reported about the formation consistent with the initial stages of the formation of nanoplasma of coarse surface ripples [16]. The discrepancy is attributed to the bubbles [15]. The widths of the trenches increased with NPO and fluences used in this study being significantly higher than those were different for the two cutting directions, due to the elliptical typically used for the study of surface ripples [16]. laser spot. In the vertical cutting direction, the trenches ablated The formation of surface ripples is undesirable in the machined with an NPO of 0.3 were on average 11 mm narrower than those surfaces as they would lead to an increase in light leakage between ablated with an NPO of 0.8, and due to the slightly elliptical spot each machined segment and poorer spatial resolution [17]. 12 mm narrower in the horizontal direction. The increase in trench Therefore, despite the lower material removal rate, the higher width was due to the higher overlap leading to more laser energy pulse energies were considered to be more desirable for the seg- being deposited at the tails of the beam profile which ablated mentation of scintillators. more material.

3.2. Pulse overlap 3.3. Damage and cracking

The trenches ablated using NPOs of 0.6 and above show much Nine parallel trenches were ablated at a constant power of straighter edges than those with lower NPO values (see Fig. 4). 5500 mW with an increasing number of passes from 5 to 800 120 S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125 passes at 1 ps, 500 fs and 190 fs pulse duration for 5 different due to the serpentine scan pattern was also attributed to the repetition rates of 100, 84.6, 73.3, 64.7 and 57.9 kHz. This corre- reduction in the build-up of heat in the trench. sponded to pulse energies at the sample of 43, 50, 58, 66 and 73 mJ While the edges of the sheet were not aligned to any specific and fluences of 12.1, 14.1, 16.4, 18.7 and 20.7 J cm 2 respectively. crystal plane, the angle of the planes to the edge of the sheet is CLSM was used to non-destructively evaluate sub-surface damage. small and the extent of the cracking obscures the exact relation- The sub-surface damage was seen as bright regions in the images, ship between the cracking and the crystal planes. The increase in where the laser light was reflected off the crack interfaces as the cracking seen with an increased pulse duration and energy toge- microscope was focused into the crystal. ther with a reduction in cracking observed when using a serpen- The anisotropic thermal and mechanical properties of cadmium tine scan pattern would suggest that the edges of the ablation tungstate make it highly susceptible to thermal shock [18]. This volume experience a rapid increase in temperature during the resulted in pulse durations of 500 fs and longer inducing more machining leading to a thermal up-shock. Sabharwal et al. mea- damage for the same pulse energy than 190 fs. The sub-surface sured a 60% difference in the coefficient of thermal expansion of cracks were observed closer to the illuminated surface as the pulse CdWO4 along the (100) plane compared to the (010) plane but the energy and pulse duration were increased. Both 1 ps and 500 fs coefficient of thermal expansion along the (001) plane was not fi pulses were found to induce signi cant cracking at the lowest measured [18]. Macavei et al. showed that there was a significant tested pulse energy of 43 mJ. The cracking was found to begin difference in the mechanical properties of CdWO4 between the a-, m when the trench extended to a depth of 20 m for the 500 fs b- and c-axes with the c-axis showing the highest stiffness and the m fi pulses and a depth of 10 m for the 1 ps pulses. Signi cant sub- b-axis showing the lowest stiffness [20]. This suggests that the m surface cracking could be observed to a depth of 120 m. anisotropic cracking is due to the differences in the mechanical For trenches ablated using 190 fs pulses significant sub-surface and thermal properties of CdWO4, but detailed characterisation of damage was only observed for pulse energies of 66 mJ and greater. the thermal and mechanical properties of CdWO4 would be nee- Smaller scale damage was observed in some cases at pulse ener- ded to investigate this phenomenon further, which is beyond the gies above 50 μJ. The degree of sub-surface damage was found to scope of this study. be a function of the orientation of the cut with respect to the dimensions of the CdWO4 sheet (Fig. 5). Thedamagewasfoundtobemoresevereforcutorientation2 3.4. Analysis of the trench wall composition (Fig. 1) which was parallel to the 25 mm edge of the CdWO sheet 4 fi fi (Fig. 5(A)). Significantly less damage was observed for cut orienta- The ablation process deposited a signi cant amount of ne powder debris on the surface of the cadmium tungstate (Fig. 7). As tion 1 (Fig. 1) which was parallel to the 15 mm edge of the CdWO4 sheets (Fig. 5(B)). Cutting at a 45° angle to either edge produced less the laser cut deeper into the crystal, layers of debris were deposited damage than cutting along the 25 mm edge but significantly more on the walls of the ablated trench. Immersion in an ammonium than cutting along the 15 mm edge. It was possible to machine along the short edge direction without causing observable damage by reducing the scan speed from 707 mm s1 to 175 mm s1.The sheets were cut into a series of strips for assembly into a scintillator array. Sequentially machining the strips was found to induce a large amount of cracking (Fig. 6(A)). The cracking could be avoided through the use of a serpentine scan pattern giving each trench one pass of the laser at a time (Fig. 6(B)). The time delay between each pass was found to influence the amount of cracking. If the delay was too short, then large cracks would begin from the ends of the cuts and span to other cuts. This was attributed to the build-up of heat in the trench [19]. Arranging the laser cuts in the form of a serpentine scan pattern also enabled a pulse energy of 54 mJ to be used without inducing cracking which Fig. 6. A schematic diagram of the different scan patterns showing: (A) the corresponded to a fluence of 15.3 J cm2, as opposed to 12.1 J cm2 sequential pattern and (B) the serpentine scan pattern. The dotted lines show the when not using a serpentine scan pattern. The reduction in damage path of the scanner between each cut.

Fig. 5. CLSM images of the sub-surface damage spanning from the trenches ablated at a power of 5500 mW using 190 fs pulses and a repetition rate of 25 kHz: (A) parallel to the 25 mm edge; (B) parallel to the 15 mm edge. S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125 121

Fig. 7. SEM micrographs of the machined CdWO4. The upper images show the trench wall: (A) before cleaning and (B) after cleaning. The lower images show the CdWO4 surface and a view down an ablated trench: (C) before cleaning and (D) after cleaning.

Fig. 8. 3D AFM topographic images of the wall of an ablated trench before (A) and after (B) cleaning. hydroxide solution was used to remove the debris from the ablated although the ridges were still evident after cleaning (Fig. 7(B)). surfaces. Trench walls ablated using 190 fs pulses at a pulse energy This suggests that the ablation debris was depositing on the sur- of 54 μJ and an NPO of 0.8 were imaged using SEM before (Fig. 7(A)) face in a manner which enhanced the ridged morphology. The and after cleaning (Fig. 7(B)). The micrographs show that the sur- ridges were not present in the bottom half of the trench which face after cleaning is smoother and there are fewer traces of parti- corresponded to the depth at which the trench width became cles on the surface (Fig. 7). The micrographs also showed a series of constant. Therefore, the ridges correspond to points at which the light and dark bands on the top half of the trench. The banding power delivered at the edges of the laser spot becomes lower than appeared to be ridges that were smoothed out by the cleaning the threshold required for ablation. process. SEM micrographs looking down an ablated trench showed Energy dispersive X-ray spectroscopy was performed on the the extent of the debris and the effectiveness of the cleaning pro- ablated wall of a trench which had not been cleaned and on one cess (Fig. 7(C) and (D)). The width of the trench before (Fig. 7(C)) that had been cleaned in ammonium hydroxide. Before cleaning and after (Fig. 7(D)) cleaning was measured and the width after the ratio of cadmium to tungsten was found to vary significantly cleaning had increased by an average of 2 μm, indicating that the debris was up to 1 μm thick on each side of the trench. from the top (Fig. 7(A) and (B) analysis point 1) to the bottom of An AFM was used to measure the topography of an the trench (Fig. 7(A) and (B) analysis points 10 and 13 respec- 80 μm 80 μm region at the top of a trench before (Fig. 8(A)) and tively). The results show an increase in the cadmium concentration after cleaning (Fig. 8(B)). These images confirmed that the banding of at the top of the trench and a large depletion in the cadmium observed in the SEM micrographs were ridges formed during the concentration at the bottom of the trench. In agreement with the ablation process. The height of the ridges was not constant and the SEM images in Fig. 6, the ammonium hydroxide cleaning process cleaning process had a smoothing effect on the ridge morphology, removes the surface debris, exposing the underlying CdWO4 bulk 122 S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125

Fig. 9. EDX determined Cd:W ratio for different positions on the trench wall of the laser ablated cadmium tungstate.

Fig. 10. High resolution XPS Cd 3d peak of the ablated surface with fitted components.

and giving rise to a more consistent cadmium to tungsten ratio as CdO2 leads to a progressive shift to lower binding energies [19,20]. a function of depth (Fig. 9). Consequently, the Cd 3d peaks for the ablated sample (Fig. 10)

XPS was performed on three samples; a sheet of cadmium were fitted with: (i) Cd 3d5/2 and 3d3/2 peaks at binding energies of tungstate before ablation, a segment of cadmium tungstate that 405.2 and 412.0 eV respectively, associated with the Cd binding had been ablated using 190 fs pulses at a pulse energy of 54 μJ and energies measured in the reference CdWO4 sample (however, an NPO of 0.8 and a similarly ablated segment of cadmium tung- these binding energies also correspond to the compounds CdO and state that was cleaned using ammonium hydroxide. The reference CdCO3 [21]); (ii) Cd 3d5/2 and 3d3/2 peaks at binding energies of sample was a cadmium tungstate sheet in an as-received state. For 403.6 eV and 410.6 eV corresponding to reported binding energies this sample, the XPS O 1 s peak exhibited two components, one at for CdO [21]. 530.6 eV corresponding to oxygen bonded to cadmium and tung- 2 The W 4f peaks (Fig. 11) showed a convolution of 2 sets of sten (CdWO4) and a second component at 532.2 eV attributed to overlapping peaks. The higher energy pair of W 4f7/2 and W 4f5/2 absorbed hydroxide species. The Cd 3d5/2 and 3d3/2 peaks had peaks have binding energies of 35.5 and 37.7 eV respectively, cor- binding energies of 405.2 and 412.9 eV respectively. The W 4f7/2 responding to tungsten in the hexavalent W6þ state such as in and 4f5/2 peaks exhibited binding energies of 35.6 and 37.7 eV respectively. CdWO4 and WO3. The lower energy pair of W 4f7/2 and W 4f5/2 The XPS spectra from the ablated surface showed a signficant peaks have binding energies of 33.5 and 35.8 eV respectively peaks 5þ change in the surface composition compared to the native cad- associated with the W state which is found in non-stoichiometric mium tungstate. The Cd 3d peaks do not typically show a strong tungsten binary oxides such as WnO3n1 or a tungsten bronze such chemical shift due to their filled 4d shell, but oxidation to CdO or as Cd0–0.18WO3 [14,22–25]. S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125 123

Fig. 11. High resolution XPS W 4f peaks in the ablated surface with fitted components.

Fig. 12. High resolution XPS O 1s peak of the ablated surface with fitted components.

The O 1s peak was fitted with three components (Fig. 12), a low Tanaka et al. published several papers on pulsed laser deposi- energy peak at 528.7 eV, corresponding to the reported binding tion of cadmium tungstate from cadmium tungstate targets and energy of CdO [19], a peak at 530.5 eV corresponding to the targets composed of a mix of CdO and WO3, in both vacuum and in 2 oxygen [26–30]. They were unable to grow cadmium tungstate reference CdWO4 sample (but also reported for O compounds films in vacuum but were successful in depositing cadmium which include CdO2 and WO3 [14,20]), and a high energy peak attributed to adsorbed hydroxides at 532.3 eV. With regard to this tungstate in an oxygen atmosphere. An increased cadmium to tungsten ratio in the deposited films was observed at higher latter peak, a high energy O 1 s component has also been attrib- oxygen pressures and target to substrate distance [28]. This result uted to a defective oxide state that is present in tungsten bronzes would agree with our EDX findings where high cadmium to and tungsten sub-oxides [24,25]. tungsten ratios are observed at the top of the trench and low From the XPS peak fits, the concentrations for each of the cadmium to tungsten ratios occur deep within the trench. Within elements in the different chemical states were compared to the the ablated plume there is a higher concentration of Cd species at expected stoichiometry for the compound(s) identified and rea- angles close to the surface normal which leads to an increase in sonable agreement was found in each case. The relative elemental the cadmium to tungsten ratio in the debris deposited at the top of fi fraction for the identi ed compounds (%) at the surface was then the trench. In the middle of the trench the high number of colli- estimated and is given in Table 1. The XPS results in Table 1 sions between the ablated species and oxygen leads to the for- indicate that the cadmium and the tungsten species in the ablated mation of CdWO4. At the base of the trench, the cadmium to plume form cadmium tungstate, cadmium oxides and tungsten tungsten ratio is much lower due to lighter species being more oxides. forward peaked than the heavier species [28,30], yielding a higher 124 S. Richards et al. / Optics and Lasers in Engineering 83 (2016) 116–125

Table 1

XPS Cd 3d, O 1s and W 4f components identified from the peak fits, associated compounds and relative surface fractions for the laser ablated CdWO4 surfaces.

Name Peak binding energy Peak FWHM (eV) Atomic % Compounds identified Relative elemental fraction for identified com- (eV) pounds (%)

2þ Cd 3d3/2 (Cd ) 412.0 1.6 14.3 CdWO4 7.4 2þ Cd 3d5/2 (Cd ) 405.2 1.6 CdO/Cd(OH)2 6.9 þ Cd 3d3/2 (Cd ) 410.6 2.0 3.1 CdO2 3.1 þ Cd 3d5/2 (Cd ) 403.6 2.0 2 O1s(O ) 530.5 1.7 54.2 CdWO4 25.3

WO3 9.6

CdO2 6.1

Cd0–0.18WO3/WnO3n1 13.2 O 1s (Cd0–0.18WO3,WnO3n1) 532.3 1.7 9.3 OH /Cd0–0.18WO3/WnO3n1 9.3 O1sO2(CdO) 528.7 1.7 7.3 CdO 7.3 6þ W4f5/2 (W ) 37.5 1.4 8.0 CdWO4 5.3 6þ W4f7/2 (W ) 35.3 1.4 WO3 2.7 5þ W4f5/2 (W ) 35.8 1.4 3.7 WnO3n1 and/or Cd0–0.18WO3 3.7 5þ W4f7/2 (W ) 33.6 1.4 concentration of W containing products. Combining the EDX and at pulse energies of r50 mJ. Scanning the beam in a serpentine XPS results, it can be concluded that the debris deposited on the pattern rather than a normal sequential raster mode delocalised trench wall is predominantly CdWO4. However, cadmium oxides the laser energy enabling pulse energies of r54 mJ to be used and tungsten oxides are also formed and deposited as debris, with without inducing cracking in the crystal. The laser induced the former being primarily located near the top of the trench and damage was minimised using the following operating parameters: the latter mostly deposited near the base of the trench. a pulse duration of 190 fs, fluence of 15.3 J cm2 and employing a Due to their optical absorption bands the observed cadmium serpentine scan pattern with a normalised pulse overlap of 0.8. and tungsten oxides found on the surface would be detrimental to XPS analysis showed that following ablation, ablation products the efficiency of a high aspect scintillator array, as any scintillation of cadmium tungstate, cadmium oxides and tungsten oxides were light incident on these compounds would be absorbed. Use of a deposited onto the trench walls and on the surface of the CdWO4 boiling ammonium hydroxide solution has been shown to suc- crystal, close to the trench. Immersing the ablated CdWO4 seg- cessfully remove these ablation products. The cleaned sample ments in boiling ammonium hydroxide was found to be a useful showed only Cd and W XPS peaks attributable to CdWO4 or other method of removing the ablation products from the CdWO4 sur- Cd or W compounds with the same peak binding energies. How- face. The ammonium hydroxide treatment led to the formation a ever, the O 1s peak for the cleaned sample was significantly thin layer of white cadmium hydroxide on the surface which could broader with a FWHM of 2.3 eV compared to 1.4 eV for that of the be beneficial to the optical efficiency of the cadmium tungstate reference sample. Furthermore, the cadmium to tungsten ratio scintillator array. was found to be 4.1 in the cleaned sample, much higher than found for the reference sample The broadening of the CdWO4 O1s peak and increase in the cadmium to tungsten ratio can be Acknowledgements explained by the formation of Cd(OH)2 on the CdWO4 surface following the ammonium hydroxide immersion treatment. One The authors would like to thank Dr Patricia Kidd (PANalytical possible reaction path for this is ammonium hydroxide reacting Research Centre United Kingdom) for performing the XRD work 2 þ with the cadmium compounds to form [Cd(NH3)4] in solution and contributions to Fig. 1, Dr Steven Hinder (Surface Analysis Lab, and then precipitating cadmium hydroxide (Cd(OH)2) on the sur- University of Surrey) for recording the XPS results and Neil Sykes/ face [31]. The formation of a layer of cadmium hydroxide on the Pete Sykes at Micronanics Laser solutions centre for performing fi surface could be bene cial to the scintillator array as it has a white the laser ablation. This work was funded by the Science and fi colour which would not absorb signi cant amounts of the scin- Technology Facilities Council – Centre for Instrumentation and the fl tillation light and could re ect it back into the array. EPSRC via the MiNMaT Centre for Doctoral Training at the Uni- versity of Surrey (Grant no. 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