Laser-Pulsed Atom Probe Analysis Condition 437
Figure 5. a: The charge-state distribution of tungsten ions as a function of the applied laser pulse energy for data collected in the LEAP 3000X HR (numbers in italic font and green color) and LEAP 5000 XS systems (numbers in normal font and purple color). b: A section of Kingham curve of tungsten (Kingham, 1982). The marked range indicates the estimated electric field range induced by the applied various laser pulse energies in this study.
fraction of ions detected in multiple events (blue dotted line) as a function of laser pulse energy (Fig. 4a), laser pulse repetition rate (Fig. 4b), and specimen base tempera- ture (Fig. 4c). With the increase of laser pulse energy, smaller amounts of ions were detected as multiple events. As for the pulse repetition rate, in the range of 200 kHz–1MHz, increasing the pulse repetition rate led to a drop in the multiple fraction. Unlike laser pulse energy and repetition rate, there is no distinct impact on the multiple amount induced by varying specimen base temperature. As the absolute values obtained from the ascending serial tests and the corresponding descending serial tests are not exactly equal to each other, specimen geometry also appears to be an influencing factor. Errors in the measurements will also lead to deviations. Even so, the trends acquired from both series are consistent. Comparing the trend line of multiple fraction with that of chemical composition (solid black line), no apparent relationship can be directly found. It is worth to mention that due to the limited time and space resolution of the detector, some multiple hit events may have been interpreted as single impacts or the multiplicities may have been recorded incorrectly. As a result, the multiple fraction may have been underestimated (Jagutzki et al., 2002).
Comparison Between Laser Energy Settings In 3000X HR and 5000 XS The two APT instruments utilized in this work have different laser systems. In the 3000X HR system, a 532-nm- wavelength green laser, which has a spot size of about 10 µm is used, whereas in the 5000 XS system, a 355-nm- wavelength UV laser with a spot size smaller than 5 µmis employed. Both laser systems produce ~10 ps pulse width
(Bunton et al., 2007). To generate comparable evaporation fields in both instruments, the 3000X HR system requires a higher laser pulse energy due to the larger laser spot size. This laser energy difference is also material dependent as different materials have different absorption efficiency to green and UV laser. In a previous work, Santhanagopalan
et al. (2015) have conducted a comparative study for Si pre- sharpened tips, using the green and UV laser systems (LEAP 3000X Si and LEAP 4000X HR) similar to the ones applied in this study, and found out that ~15 times higher energy is required for the 3000X Si system. The baseline for compar- ison between different experiments is to compare the actual electric field condition at the specimen apex during eva- poration. According to the postionization theory developed by Kingham (1982), the electric field can be estimated by the charge-state distributions of the detected ions. Even though the adequacy of applying the postionization theory to pulsed laser experiments of poorly-conducting materials is under debate (Shariq et al., 2009; Schreiber et al., 2014), the pro- portion of the charge states of the field-evaporated ions can
provide some insights into the strength of the evaporation field. In this study, the test series 3000PEA and 5000PEA were employed to compare the laser energy settings from both laser systems. As the specimen geometry will also affect the electric field (Gipson & Eaton, 1980; Hyde et al., 1994; Loi et al., 2013), specimens of similar shapes were used. Table 2 lists the detailed analysis parameters: equivalent laser repetition rate (250 kHz) and base temperature (60 K). As stated above, to account for the difference in the detection efficiency (~37% for 3000 HR and ~80% for 5000 XS), dif- ferent target detection rates (0.002 ion/pulse for 3000 HR and 0.005 ion/pulse for 5000 XS) were chosen. In Figure 5a, the measured percentages of different charge stated tungsten ions were plotted against various laser energies for both data collected in the 3000X HR (numbers in italic font and green color) and 5000 XS systems (numbers in normal font and purple color). As the evaporation field required to evaporate ions from the specimen surface is temperature dependent, a higher laser pulse energy means higher heat input and thereby the specimen apex temperature is higher and cor- respondingly the base field will be lower. As a result, the amount of tungsten ions detected in the 4+ charge state is smaller, which is consistent with the postionization theory. Interestingly, 40 and 60 pJ energies in the UV laser system are estimated to yield similarWion charge-state ratios to 500
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255 |
Page 256 |
Page 257 |
Page 258 |
Page 259 |
Page 260 |
Page 261 |
Page 262 |
Page 263 |
Page 264 |
Page 265 |
Page 266 |
Page 267 |
Page 268 |
Page 269 |
Page 270 |
Page 271 |
Page 272 |
Page 273 |
Page 274 |
Page 275 |
Page 276 |
Page 277 |
Page 278 |
Page 279 |
Page 280 |
Page 281 |
Page 282 |
Page 283 |
Page 284
