Topographic and Material Contrast in Low-Voltage Scanning Electron Microscopy

Transcription

1 SCANNING Vol. 17, (1995) Received August 15, 1994 FAMS, Inc. Accepted October 5, 1994 Topographic and Material Contrast in Low-Voltage Scanning Electron Microscopy J. HEJNA Institute of Electron Technology, Technical University of Wrocl aw, Wrocl aw, Poland Summary: Two scintillation backscattered electron (BSE) detectors with a high voltage applied to scintillators were built and tested in a field emission scanning electron microscope (SEM) at low primary beam energies. One detector collects BSE emitted at low take-off angles, the second at high takeoff angles. The low take-off detector gives good topographic tilt contrast, stronger than in the case of the secondary electron (SE) detection and less sensitive to the presence of contamination layers on the surface. The high take-off detector is less sensitive to the topography and can be used for detection of material contrast, but the contrast becomes equivocal at the beam energy of 1 kev or lower. Key words: low-voltage scanning electron microscope, backscattered electrons, detectors, material contrast, topographic contrast Introduction Operation of a scanning electron microscope (SEM) at low primary beam energies offers several advantages. As the energy of the primary beam is lowered, the range of primary electrons decreases and the secondary electron (SE) yield increases. As a result, the beam-specimen interaction is more localized, charging artefacts decrease, and the depth of specimen radiation damage decreases (Pawley 1984, Reimer 1993). Observations at low primary beam voltages can be routinely conducted in instruments equipped with high-brightness field-emission guns. Contrast formation in the low-voltage SEM (LVSEM) differs from that in the standard SEM (operating at 5 30 kev), however, and it is not yet fully understood. The present paper deals with an analysis of topographic and material contrast in LVSEM. Address for reprints: J. Hejna Institute of Electron Technology Technical University of Wrocl aw Wybrzez e Wyspiańskiego Wrocl aw, Poland The topographic contrast can be divided into three categories (Hejna 1994): high resolution contrast, when dimensions of surface features are comparable with the transport length of the detected electrons enlarged by the delocalization of their generation process; diffusion contrast, when dimensions of surface features are smaller than the interaction volume of primaries; and tilt contrast, when surface features are larger than the interaction volume. When the primary beam energy is decreased, the ratio of feature size to the interaction volume increases; as a result, diffusion contrast of small features increases and a range of an appearance of tilt contrast expands towards the range of smaller features. The tilt contrast originates from the dependence of an electron emission on the local inclination angle of the surface. The dependence of the SE yield on the tilt angle decreases as the beam voltage is lowered (Böngeler et al. 1993, Joy 1987), and as a result the tilt contrast, obtained with SE, decreases when the beam voltage is reduced. In addition, as the electron range decreases, the SE signal becomes more sensitive to a presence of any contamination layers on the surface (Katnani et al. 1991). Material contrast originates from the monotonically increasing dependence of the backscattering coefficient on the atomic number of the specimen at accelerating voltages > 5 kv. At beam energies 1 kv, however, the contrast mechanism fails for high Z materials (Böngeler et al. 1993, Sternglass 1954). This fact, and the change of the angular distribution of BSE at lower voltages, leads to a different appearance of material contrast in LVSEM. BSE detectors used in the standard SEM (scintillators, semiconductor diodes) have very poor efficiency at low energies for BSE. Consequently, detectors with multichannel plates (Helbig et al. 1987, Russel and Mancuso 1985) or detectors which accelerate the BSE before they are detected by scintillators or semiconductor diodes (Autrata and Hejna 1991, Lee and Ward 1991) are used. Autrata and Hejna (1991) designed scintillation detectors (low and high takeoff) with a high voltage applied to the scintillators and with grids screening the primary beam from the influence of the scintillator voltage. The present paper reports further studies of the use of such detectors. Atomic number contrast is quantitatively studied with these BSE detectors, and topographic images taken with a standard SE detector are compared with images obtained with the special BSE detectors. Experiments were conducted in the Hitachi S-4000 fieldemission SEM. The discussion concerns low and medium resolution studies of untilted specimens.

2 388 Scanning Vol. 17, 6 (1995) Origins of Topographic and Material Contrast in the SEM Topographic tilt contrast occurs when surface features in a homogeneous material are larger than the interaction volume of the primary beam in the specimen. It is caused by the dependence of the SE yield, δ, and the BSE coefficient, η, on the surface inclination angle. In the range 5 to 30 kev of a standard SEM, δ and η increase with increasing tilt angle (Arnal et al. 1969, Reimer and Seidel 1968); however, in LVSEM such behaviour is valid only for η (Böngeler et al. 1993, Darlington and Cosslett 1972, Koshikawa and Shimizu 1973). The dependence of δ on the tilt angle becomes weaker with lowering of the primary energy and at 1 kev δ is nearly independent of the inclination angle (Böngeler et al. 1993, Joy 1987). In the case of BSE, tilt contrast is also influenced by the change of the angular distribution of BSE with the tilt angle. Above 5 kev the angular distribution of BSE follows a cosine law at normal incidence and is elongated in the reflection direction at tilted incidence (Darlinski 1981, Kanter 1957, Reimer and Pfefferkorn 1973). As the primary energy is lowered below 5 kev, the distribution becomes elongated in the direction of the primary beam (Alekseev and Borisov 1962; Bronshtein et al. 1972a, 1972b; Jahrreiss and Oppel 1972; Kadlec and Eckertova 1970). This effect is caused by the change of the scattering cross section of primary electrons when the beam energy is reduced and it is more pronounced for elements with higher atomic numbers (Czyzewski et al. 1990, Ichimura et al. 1980, Jablonski and Gergely 1989, Kessler and Lindner 1965, Lödding and Reimer 1981, Reimer and Lödding 1984). The diffusion contrast dominates when surface features are smaller than the interaction volume of primaries. It is caused by the change of the shape of the interaction volume when the beam scans the feature, resulting in a change in the number and energy of BS (diffused) electrons reaching the surface. These BS electrons generate SE2, so the total number of SE (SE1 + SE2) changes. In a standard SEM, when the feature is much smaller than the interaction volume, the diffusion contrast obtained with BSE is weak. As the beam energy is lowered, the ratio of the feature size to the interaction volume becomes larger, resulting in an increase of diffusion contrast for BSE. Figure 1 shows the approximate ranges over which tilt and diffusion contrast predominate in LVSEM. The relation of Fitting (1974), R = 90*ρ 0.8 *E 1.3, was used to calculate the electron range, R, in nm (ρ is the material density in g/cm 3 and E o is the incident electron energy in kev). The material contrast in the SEM originates from the dependence of η and δ on the atomic number of the specimen. The BSE signal originates in an upper half of the interaction volume and contains information about this whole part, the SE signal which is partially dependent on BSE comes from a thin surface layer and may be strongly dependent on the composition of this layer. Typical SEM specimens do not have physically clean surfaces and the SE signal will be influenced by contamination layers present on the surfaces. At higher beam energies, η is a reproducible and monotonic function of the atomic number and BSE are commonly used for detection of material contrast. The monotonic dependence fails at lower beam energies (Böngeler et al. 1993, Reimer and Tollkamp 1980, Sternglass 1954); at 1 kev, η is independent of Z for Z > 30, and below 1 kev there is a maximum of η at Z 30 with η decreasing with increasing Z > 30 (Böngeler et al. 1993). Pure material contrast occurs when the whole volume of BSE generation is of uniform composition. When there are inhomogeneities in this volume, the material contrast is modified and, depending on the size of the inhomogeneity, can range from the contrast characteristic for the matrix to the contrast characteristic for the inhomogeneity. As the beam energy is lowered, the range of the pure material contrast increases but the BSE signal becomes sensitive to the presence of contamination layers. Experimental Figure 2 shows detectors used in the work: the ring detector is used for the detection of BSE emitted at low takeoff angles and the annular top detector is used for detection of BSE emitted at high takeoff angles. A high voltage, in the R 60 nm Tilt contrast Diffusion contrast Al Cu Au E 0 /kev FIG. 1 Dependence of the electron range R on the primary beam energy E o for three elements and approximate ranges of dominating tilt and diffusion contrast. 5 HV 2 0 V V FIG. 2 Arrangement of detectors. 1 primary beam, 2 microscope lens, 3 specimen, 4 ring scintillator, 5 light guide, 6 disc scintillator, 7 light guide, 8 grid, 9 metal tube, 10 teflon tube. Full lines = trajectories of BSE, dashed lines = trajectories of SE

3 J. Hejna: Topographic and material contrast in LVSEM 389 range from 6 to 7 kv, is applied to scintillators. The primary beam is screened from the influence of the scintillator high voltage by the metal tube and the hemispherical metal grid, both held at the earth potential. The specimen is biased with a potential of +100 V which, coupled with the electric field between the specimen and the grid, retards SE emitted from the specimen. The specimen bias also increases the accelerating voltage and therefore changes the magnification, which is different than the reading on the display of the microscope. However, biasing of the specimen causes less shift of the primary beam than biasing the grid (Autrata and Hejna 1991). The tilt contrast in the SEM can be tested by recording an image of a sphere; a specimen which provides all tilt and azimuth angles (Blaschke and Schur 1974, Lange et al. 1984). Figure 3 shows images of a metal sphere with linescans across it. SE images were taken at 1 kev with the standard Everhart- Thornley detector and the BSE with the detectors shown in Figure 2. For comparison an SE image was taken at 5 kev. At 1 kev the tilt contrast obtained with SE is weak, much weaker than at higher energies. A weak topographic tilt contrast is obtained also with the high takeoff BSE detector. Good tilt contrast at low beam energies can be obtained with the low takeoff BSE detector. The atomic number (material) contrast can be tested experimentally by preparing a calibration curve (BSE signal vs. atomic number Z) for a given detector and experimental conditions (Ball and McCartney 1981). Figure 4 shows the dependence of the signal on the atomic number of the specimen for the two BSE detectors shown in Figure 2. A polished Be disc and thin layers of different materials evaporated on a surface of a polished Si wafer were used as specimens. The thickness of each layer was larger than half of the electron range at 3 kev, so they could be treated as bulk materials. Microroughness and oxide layers present on surfaces influence BSE signals, especially in the case of the low takeoff detector. In Figure 4 only results for elements whose experimental points lie not far from the extrapolating curve are shown. The shape of the curve obtained with the low takeoff detector changes only a little with the beam energy, while that obtained with the high takeoff detector changes much more. The signal of the high takeoff detector is less sensitive to the topographic contrast and this detector is more suitable for detection of the material contrast. The dependence of the signal of this detector on the atomic number is monotonic for the beam energy of 1.5 kev. At 1 kev the signal is nearly constant for Z > 50, at 0.7 kev there is the maximum of the signal at Z = 50, and next the signal decreases with increasing Z. Comparing this dependence with the dependence of η on Z (Böngeler et al. 1993), we see that the maximum is shifted to the range of heavier elements. This effect probably is due to the change of the angular distribution of BSE with Z. For heavier elements more electrons are scattered to high takeoff angles and this increase compensates to some extent for the decrease of the total number of BSE. The factor which influences the material contrast at higher beam energies, that is, the change of the energy distribution of BSE with Z, does not have a large influence in this case because BSE are acceler- (c) (d) FIG. 3 Images of metal spheres with linescans across centers. Taken with SE detector, low takeoff BSE detector, (c) high takeoff BSE detector, all at E o = 1 kev, and (d) SE detector at 5 kev. Horizontal field width = 120 µm.

4 390 Scanning Vol. 17, 6 (1995) ated before they are detected and relative differences of their energies decrease. One of the main advantages of LVSEM is a possibility of an observation of nonconducting specimens without conductive coating. Figure 5 shows images of the specimen containing zirconium oxide in a glass and corundum matrix. Stable images can be obtained when the sum of the SE and BSE coefficients is larger than unity. In this case a positive charge builds up on the surface but it is partially compensated for by secondary electrons attracted to the surface by the charge. As a result, the surface has a potential of only a few electronvolts (Reimer et al. 1992). This occurs at 1 kev in the sample described above (Fig. 5a, b, and c). The SE image (Fig. 5a) shows a relatively flat surface with sharp edges of features S/% S/% kev 1.0 kev 2.0 kev 3.0 kev Z kev 1.0 kev 1.5 kev 2.0 kev (edge contrast). Good topographic tilt contrast is seen in the micrograph taken with the low takeoff detector for BSE (Fig. 5b). The high takeoff detector for BSE gives mainly material contrast with little topography (Fig. 5c). When the accelerating voltage is increased, the total yield of electron emission (η + δ) becomes less than unity and a negative charge builds up on the surface. The negative potential of the surface reduces the accelerating voltage to the value at which η + δ = 1 (Reimer et al. 1992). In this case there is a local enhancement of the SE yield and a resulting increase of the SE signal. This effect is seen in Figure 5d at the primary beam energy of 1.6 kev. During BSE detection SE are stopped by the electric field between the specimen and the grid. When the value of the negative potential of the surface is higher than the specimen bias (100 V in the present work), SE can pass the grid and contribute to the BSE signal (white areas in Fig. 5e and f). An improvement of the topographic contrast when the primary beam energy is lowered is demonstrated in Figure 6. The specimen is a silicon wafer with a gold line and some titaniumtungsten residues on the surface. The wafer was coated with a thin Ti-W layer and then with a 0.3 µm thick Au layer. Au lines were made by a photolithographic process and then the specimen was plasma-etched, but the Ti-W layer was not completely removed. Images at 20 kev (Fig. 6e and f) show mainly material contrast and superimposed diffusion topographic contrast. When the beam energy is lowered, topographic tilt contrast dominates (Fig. 6a d). The topography observed at low-beam energy differs much from that observed at 20 kev. Micrographs taken with the BSE low takeoff detector at 2 and 3 kev look similar and show a rough surface of the Au line and Ti-W residues. Micrographs taken with SE differ a little. The surface probably is covered with a layer of light organic contamination and it influences the image at 2 kev: dimensions of Ti-W particles are enlarged, edges of Au grains and gaps between them are worse visible than at 3 kev. Figure 7 shows images of an etched silicon wafer. The shape of pyramids etched on the surface is clearly seen in the BSE low takeoff image (Fig. 7b). The tilt contrast in the SE image (Fig. 7a) is nearly absent and, in addition, there is strong material contrast of etching residues covering the bottom of pyramids. This contamination layer is also seen in Figure 7b but it does not mask the topography. The BSE high takeoff image (Fig. 7c) shows weak topographic contrast (but stronger than in the case of SE) and material contrast of residues. The contamination is not seen in micrographs taken at 20 kev, but edges of pyramids are not sharp because of large interaction volume of primary electrons kev Conclusions Z FIG. 4 Dependence of the BSE signal on atomic number Z. Low takeoff BSE detector, high takeoff BSE detector. Signals are normalized to the maximum value. The angle-selected detection of BSE is a valuable tool in LVSEM. BSE detected at low takeoff angles are recommended for detection of topographic tilt contrast, because the contrast is stronger than that obtained with SE and less sensitive to the presence of any contamination layer on the surface. Detection of material contrast is possible with a high takeoff

5 J. Hejna: Topographic and material contrast in LVSEM (c) (d) (e) (f) 391 FIG. 5 Micrographs of the specimen containing zirconium oxide in glass and corundum matrix. (a,b) SE images, (c,d) low takeoff BSE images, (e,f) high takeoff BSE images. Eo = 1 kev (a,c,e) and 1.6 kev (b,d,f). SE and BSE images are taken at different locations. Horizontal field width = 85 µm.

6 392 Scanning Vol. 17, 6 (1995) (c) (d) (e) (f) FIG. 6 Micrographs of silicone wafer with Au line and Ti-W particles. (a,c,e) SE images, (b,d,f) low takeoff BSE images. Eo = 2 kev (a,b), 3 kev (c,d), and 20 kev (e,f). Micrographs taken at different locations. Horizontal field width = 8.5 µm.

7 J. Hejna: Topographic and material contrast in LVSEM (c) (d) (e) (f) 393 FIG. 7 Micrographs of etched silicone wafer. (a,b) SE images, (c,d) low takeoff BSE images, (e,f) high takeoff BSE images. Eo = 1 kev (a,c,e) and 20 kev (b,d,f). SE and BSE micrographs taken at different locations. Horizontal field width = 40 µm.

8 394 Scanning Vol. 17, 6 (1995) detector, but we should be very careful when interpreting images obtained at low voltages. Material contrast is influenced by a microroughness of the surface and contamination and oxide layers present on the surface, and it is equivocal at the energy of 1 kev. Acknowledgments Experiments were done during the author s stay at the Institute of Physics of the University of Münster, Germany. The author is indebted to Professors L. Reimer and H. Kohl for providing laboratory means needed for experiments. The support of the stay by the Katholischer Akademischer Ausländer Dienst is gratefully acknowledged. References Alekseev VA, Borisov VL: Angular distribution of secondary electrons for an effective emitter consisting of an MgO film on a CuAlMg alloy base. Sov Phys Sol State 4, (1962) Arnal F, Verdier P, Vincensini PD: Coefficient de retrodiffusion dans le cas d électrons monocinétiques arrivant sur la cible sous une incidence oblique. 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