440 Zirong Peng et al.


whereas with the pulse repetition rate of 1MHz, most carbon ions were detected as singles. The much lower apparent multiple fraction and the much higher carbon singles strongly indicate that a certain number of multiple events was not properly recorded. This is assumed to be the eason why the resulting chemical composition is seriously biased for 1MHz pulse repetition rate. To solve or mitigate this problem, an attempt to reduce the multiple hits was proposed (Thuvander et al., 2013). Also, Thuvander et al. (2011) suggested a method based on the abundance


Figure 8. The fraction of carbon ions detected in single and mul- tiple events plotted using APT data of test series 5000PRRA and 5000PRRD. The detailed experimental conditions are listed in Table 2.


strength of the local electric field. Therefore, more than one ion may evaporate within the same pulse and multiple hits occur. Accordingly, the co-evaporated ions are closely correlated in time and space, enhancing the detector pile-up effect (De Geuser et al., 2007). The second one is molecular ion dissociation. There is evidence that under the effect of a strong electric field, metastable molecules or clusters may decompose into two or more small fragments which will also lead to multiple events (Tsong, 1985; Saxey, 2011; La Fontaine et al., 2015; Santhanagopalan et al., 2015; Gault


et al., 2016). Intrinsically, fraction of the multiple events is field-dependent. With the increase of the evaporation field, co-evaporation events will take place more frequently (De Geuser et al., 2007), and the dissociation of molecular ions is more likely to happen. Previous experimental results have confirmed this trend (Tang et al., 2010; Müller et al., 2011). In Figure 7, the proportions of multiple detection events


were plotted against the tungsten charge-state ratios [W4+/ (W2+ +W3+)] for various APT test series: (a) 3000PE and 3000BT acquired using the LEAP 3000X HR instrument, (b) 5000PRR and 5000PEA acquired using the LEAP 5000 XS instrument. The detailed analysis conditions are listed in Table 2. As mentioned above, the charge-state ratios of tungsten ions can be used to evaluate the intensity of the


of the 13C isotope to correct the results. Compared with 12C, 13C suffered less from the detector pile-up effect. Thus, by using the measured 13C content and natural isotope abundance ratio, a more accurate 12C content can be acquired. The changes in electric field induced by varying the


analysis condition can also be inferred by combining Figures 5 and 7. As expected, the laser pulse energy shows the largest impact on the evaporation field. Figure 5b reveals that within the tested range, increasing the laser pulse energy can reduce the field by ~6%. Increasing the specimen base tem- perature can also decrease the evaporation field, but the extent is much smaller, <1%, as explained previously. There is no distinct change in evaporation field caused by varying the laser repetition rate, implying that the temperature of the specimen is not affected. This observation is also consistent with what we inferred previously.


CONCLUSIONS


effective evaporation field at the tip apex. With a stronger field, the probability of postionization is greater, resulting in larger fraction of tungsten ions in 4+ charge state. As expected, the main trend is the fraction of multiple events increasing with the effective evaporation field. However, the data obtained with 1MHz pulse repetition rate show a very large deviation, where the apparent multiple fractions are much lower than expected. Figure 8 shows the percentages of carbon detected in single and multiple events for the test series 5000PRRA and 5000PRRD. It can be clearly seen that for laser pulse repetition rates from 100 to 500 kHz, the majority of carbon ions were detected as multiple events,


The impact of the laser pulse energy, pulse repetition rate, specimen base temperature and specimen geometry on the mass resolution, NSR, chemical composition and multiple hit fraction have been investigated using cemented tungsten carbide sample in two differentAPT instruments. The results show that increasing the laser pulse energy or specimen base temperature both improved mass resolution, varying the laser pulse energy was more effective. The pulse repetition rate had no impact on mass resolution, but affected the NSR. Low pulse repetition rate will lead to high NSR. Increasing the specimen radius was good for mass resolution, but led to an increase in NSR. Increasing the specimen shank angle also had positive effects on mass resolution. Satisfactory results for chemical compositions were obtained with all the applied analysis conditions except the 1MHz pulse repetition rate, where a severe detector saturation effect was observed.


ACKNOWLEDGMENTS


Z.P. acknowledges financial support from the InitialWear project, funded by the Max-Planck-Gesellschaft (MPG) and the Fraunhofer-Gesellschaft. The authors are thankful to Daniel Kurz,who conducted the chemical analyses, and toUwe Tezins and Andreas Sturm for their support of the APT and FIB facilities at MPIE. Yifeng Lu’s (RWTHAachen University) vital help in Matlab coding is greatly acknowledged.


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