Design and Application of a Quadrupole Detector for Low-Voltage Scanning Electron Mcroscopy

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1 SCANNING Vol. 8, (1986) 0 FACM. Inc. Received: August 29, 1986 Original Paper Design and Application of a Quadrupole Detector for Low-Voltage Scanning Electron Mcroscopy R. Schmid and M. Brunner" lnstitut fur Angewandte Physik, Universitat Tubingen. D-7400 Tubingen, FRG * Research Laboratories of Siemens AG. ZT ZFE FKE 44, Otto-Hahn-Ring 6, D-8000 Munich 83, FRG Abstract Scanning electron microscopes operated at low voltages are applied in the inspection and metrology of microstructures as well as in the testing of integrated circuits. The efficiency of a conventional Everhart-Thornley-detector is poor in this application, especially if large samples, e.g. wafers, are inspected or tested at small working distances. In addition. the primary beam is deflected and aberrations are added by the extraction field of this detector. A new type of detector for secondary electron recording in low-voltage scanning electron microscopy was. therefore. designed. built and tested. It utilizes crossed electric and magnetic quadrupoles to compensate for each other in their effect on the primary beam. On the other hand, both fields support the extraction and collection of the secondary electrons. During the test application, the detector showed high efficiency resulting in low-noise images without any negative effect on the resolution. Introduction Scanning electron microscopes are progressively being used at low accelerating voltages to serve as inspection and measurement tools in semi-conductor manufacturing. The convcntional Everhart-Thornleydetector (ETD) is commonly used for secondaryelectron detection in these instruments (Everhart and Thornle)! 1960). In most cases. the SEM has to be operated at small working distances in order to obtain high resolution. This impairs the efficiency of the ETD. especially if large samples like wafers are inspected using an SEM with a flat pole piece. In this case, the extraction field of the ETD cannot reach the scanned area where the secondary electrons are generated. The efficiency of a small ETD near the scanned area is higher, but it causes beam deflection and astigmatism because of the strong electric field in the scanned area. Another approach to improving the collection efficiency in wafer inspection is to position the detector above the lens. An ETD at this location also causes a displacement of the primary beam with the result that the beam does not pass the lens along the optical axis. In the following paper a new type of detector is described and tested which avoids these disadvantages. Principles of the Detector The theoretical basis of the detector proposed by Rose is discussed in the preceding article (Zuch and Rose 1986). Here we will just give a brief description of its operation. The basic idea is to compensate the influence of electric fields on the primary beam by magnetic fields. and to maintain extraction of secondary electrons towards a scintillator. This is achieved by crossed magnetic and electric multipoles whose positive electrodes are formed by entry nets of secondary electron detectors. The simplest type of electric magnetic multipoles is the original Wien filter (Wien 1902 and Hawkes 1972). It consists of a homogeneous electric field and a perpendicular homogeneous magnetic field. It can be operated so as not to deflect primary electrons. Secondary electrons, however, are extracted by the electric field towards the positive detector entry net.

2 R. Schmid, M. Brunner: Quadrupole detector application 295 The magnetic field supports this effect because secondary electrons have opposite velocity with respect to the primary electrons. According to calculations made by Zuch and Rose (1986), this type of detector causes both astigmatism and (predominantly) dispersion, and, if used in an SEM, limits the resolution to 0.1 pm at 1 kvacceleration voltage. The practical application of this type of detector has, therefore, not been considered. The next higher order of electric magnetic multipoles is a system of electric and magnetic quadrupoles which offers less aberrations (Zuch and Rose 1986). Electric and magnetic fields vanish along the optical axis, so that no deflection of the beam is caused. Offaxis compensation in the effect on the primary beam can be achieved by arranging the magnetic and electric quadrupoles to be rotated by 45" with respect to each other if suitable currents and voltages are applied (Wien condition). Secondary electrons are attracted towards the two positive grid electrodes, which suggests the use of two scintillators and photomultipliers. The quadrupole detector has been investigated experimentally because calculations by Zuch and Rose (1986) predicted promising characteristics. '\ n U Fig. 1 Electric magnetic quadrupole detector. 1 = light pipe with detector electrode, 2 = ring. 3 = pole piece, 4 = electrode. Design of the Detector Figures 1 and 2 show the design of the electric magnetic quadrupole detector. The pole pieces and the electrodes are mounted in a ferromagnetic ring, which also guides the magnetic field. In the present detector, only one of the two positive electrodes is equipped with a scintillator and attached to a photomultiplier. The whole detector is supported by the light pipe and can be moved mechanically. It is positioned directly underneath the objective lens of the SEM (Fig. 3). This allows working distances of approximately 5-10 mm. In future designs, the height of the detector can be reduced to allow smaller working distances. It can also be placed inside or above the lens. The voltages applied to the electrodes of the electric quadrupole (+U and -U) can be adjusted in relation to each other by small values of AU. The coil currents of the magnetic quadrupole can be adjusted in a similar way. Alignment Procedure of the Detector Centering To achieve a coarse centering, the whole detector is moved mechanically until the center of the image does not move while symmetrically wobbling the voltages Fig. 2 Detector electrode. 1 = entry net, 2 = ring, 3 = scintillator, 4 = insulator, 5 = light pipe. I I Lens T Fig. 3 Position of the quadrupole detector in the SEM

3 296 R. Schmid, M. Brunner: Quadrupole detector application applied to the electrodes of the electric quadrupole. Fine corrections can be made electronically by adjusting AU. The magnetic quadrupole is centered electronically in the same way. Compensation for distortions The scanning field is distorted if only the electric quadrupole is used. It is compressed in the direction of the negative electrodes and stretched in the direction of the positive electrodes (Fig. 4). The distortion is compensated for if the Wien condition is fulfilled (Fig. 5). Determination of the correct coil current The coarse determination of the coil current to be used with a fixed electric quadrupole potential can be done by minimizing the distortions of known, possibly rectangular structures on the sample. The remaining deviation from the Wien condition, which is not noticeable in distortion patterns, causes astigmatism in the focussed beam. This effect can be used to further adjust the coil current. The correct current is attained when the astigmatism is minimized to yield best resolution. This requires proper adjustment of the instrument astigmatism beforehand. Fig. 4 Distortion produced by the electric quadrupole field. The quadrupole position is shown schematically. SCANNING Voi (1986) Fig. 5 Compensation for the distortion by the quadrupole field. The quadrupole position is shown schematically.

4 R. Schmid, M. Brunner: Quadrupole detector application 297 Performance of the Detector Compensation for the scanning-field deflection Operating a single conventional ETD close to the scanned surface area causes deflection of the scanning beam, which entails a shift of the whole image. This deflection depends on the primary energy (Fig. 6) and is nonuniform over the scan field. The curve in Fig. 6 was obtained by switching off the magnetic quadrupole and grounding all electrodes other than the detector extraction grid. The image shift vanishes if the quadrupole detector is operated under the Wien condition. Influence on resolution According to calculations by Rose and Zach, the voltages applied to the electrodes of the electric quadrupole limit the resolution by chromatic and spherical aberrations. The axial chromatic aberration of first order predominates under normal operating conditions. When operating the detector in a field emission SEM with a small energy spread of the primary energy, the chromatic error associated with electric quadrupole voltages of up to k SOOVturns out to be negligible. The resolution of the SEM, specified as being 25 nm at 1000 V acceleration voltage, is theoretically not impaired by the chromatic aberration of the detector. These calculations are based on ideal quadrupole fields. However, the present detector only approximates these ideal conditions because stray fields of lens and sample introduce deviations from the ideal conditions. The effect of the flat shape of the electrodes and pole pieces should only add astigmatism of higher orders with no measurable effect on the resolution. The practical impact of the aberrations introduced by the detector on the resolution of a Hitachi S 800 field emission SEM was investigated. The sample was an uncoated resist pattern on GaAs with edge structures produced by standing waves during optical exposure. These structures can be resolved at an acceleration voltage of 1000 V using the quadrupole detector (Fig. 7). Figure 8 shows a reference image obtained with the conventional detector, which is mounted on the vacuum chamber wall and therefore has insufficient collection efficiency if large samples are inspected at 1 kv. Only a small sample of 5 mm diameter was mounted during this comparison. The resolution obtained when employing the quadrupole detector is not noticeably affected, although the detector is much closer to the beam and thus yields a higher electric extraction field in the inspected area 3001 \ Everhart-Thornley-Detector close to primary beam / Quadrupole detector close to primary beam kv 20 Up Fig. 6 Comparison for the beam deflections Fig. 7 Low-voltage image of standing waves on GaAs obtained with the electric magnetic quadrupole detector at 1 kv Fig. 8 Low-voltage image of standing waves on GaAs obtained with the conventional ETD at 1 kv

5 298 R. Schmid, M. Brunner: Quadrupole detector application Fig. 9a Fig. 9b Fig. 9 Comparison between conventional (a) and quadrupole detector (b) than the conventional detector. The detection efficiency is therefore improved, which reduces the noise in the recorded images. Efficiency The ideal position of an ETD for best collection efficiency is dircctly adjacent to the scanned surface area of the sample. The efficiency of the quadrupole detector was, therefore. compared with an ETD in this position. The practical realization of this comparison was straightforward; it was accomplished by switching off the magnetic quadrupole and grounding all electrodes other than the detector grid. Figure 9 shows an image obtained with the ETD close to the scanned sample area in comparison with an image obtained by switching on the quadrupole currents and voltages. The signal-to-noise ratio is about the same in both images, but the whole image is substantially shifted in Fig. 9a. Although distortions are not directly visible in Fig. 9a, there may be nonlinearities due to field gradients, impairing the measurement accuracy in quantitative applications. The image of Fig. 9b appears darker in the lower left corner, which is due to the operation with only one detector. The brightness would be uniform if both detectors of the electrostatic quadrupole arrangement were activated. Due to the shielding effect of large inspected samples, a conventional ETD assembly as originally implemented in the SEM has much less collection efficiency. In the present investigations, a similar shielding effect was caused by the support structure of the quadrupole detector. Figure 10 shows a comparison of images obtained by the quadrupole detector (Fig. lob) and the conventional ETD mounted on the chamber wall (Fig. 10a). The signal-to-noise ratio is much better when imaging with the quadrupole detector than when using the conventional detector on the chamber wall. Conclusion It has been demonstrated that the electric magnetic quadrupole detector improves the collection efficiency in high-resolution low-voltage scanning electron microscopy without causing distortions or image displacement. The high extraction field does not cause significant aberrations that would impair the resolution of the SEM. The new detector allows small working distances and avoids shielding effects as caused by large samples.

6 R. Schmid, M. Brunner: Quadrupole detector application 299 Fig. 10a Fig. 10b Fig. 10 Shielding effect: Conventional detector (a) and quadrupole detector (b). References Everhart T E, Thornley R F M: Wideband detector for micro-micro-ampere low electron currents. J Sci Instr 37, 246 (1960) Hawkes P J: Electron Optics and Electron Microscopy. Taylor and Francis Ltd, London 1972, pp Wien W Untersuchung uber die elektrische Entladung in verdunnten Gasen. Ann der Physik 8, (1902) Zach J, Rose H: Efficient detection of secondary electrons in low-voltage scanning electron microscopy, Scanning, (1986)