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Semiconductor dopant profiling by off-axis electron holography
Ultramicroscopy 94 (2003) 149–161
Semiconductor dopant profiling by off-axis electron holography Jing Lia, M.R. McCartneyb,*, David J. Smitha,b a
Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA b Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA Received 24 October 2001; received in revised form 26 July 2002
Abstract Silicon wafers with a complex but known dopant profile were used to explore possible methods for improving the reliability of off-axis electron holography for quantitative determination of electrostatic potential profiles in doped semiconductor devices. The variability of results from nominally identical structures was attributed to local charging and associated external fields, forcing the development of a more robust approach to hologram analysis that incorporated an additional phase correction factor rather than rely on vacuum for phase flattening. Consistent results in close agreement with simulated profiles based on measured dopant distributions could then be obtained. Carbon coating was shown to be effective in reducing accumulation of charge caused by emission of secondary electrons. Overall, this work demonstrates that reliable potential profiles from unbiased samples should be obtainable on a routine basis provided that regions suitable for flattening of the phase profile can be identified. r 2002 Elsevier Science B.V. All rights reserved.
1. Introduction With the continual downsizing of device dimensions, two-dimensional (2-D) dopant profiling is an ongoing issue of great interest and importance to the semiconductor industry [1,2]. Obtaining reliable information about the dopant distribution in real structures as a function of processing parameters remains a key step in validating process and device simulations used for developing and refining prototype device designs [3]. In practice, the spatial distribution and concentration of active dopant atoms play a major role in determining the electrostatic potential within the device, which in turn determines the device *Corresponding author..
response. It would therefore be highly beneficial to develop an experimental method capable of characterizing the potential with high spatial resolution and sensitivity. Off-axis electron holography in the transmission electron microscope (TEM) provides access to the phase of the electron wavefront that has traversed a sample, so that the technique should in principle enable the electrostatic potential within semiconductor devices to be quantified. Electron holography was used by McCartney et al. to observe the potential profile across Si/Si p–n junctions [4], and the technique was later used by Rau and colleagues to map the 2-D electrostatic potential in deepsubmicron transistor structures [5,6]. Electrostatic potential variations across an AlGaN/InGaN/ AlGaN heterojunction diode have recently been
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quantified using electron holography, and also explained in terms of polarization effects and free polarization-induced interface charge [7]. In this present study, we have systematically explored the application of off-axis electron holography to a test structure of known dopant distribution. We demonstrate that accurate potential profiles can be extracted routinely from unbiased materials using a refined approach to hologram analysis and carefully controlled sample preparation conditions. Experimental factors that are liable to affect quantitative measurements are also discussed.
2. Experimental details The specimens studied here were taken from a group of test structures fabricated at IBM for a dopant metrology round-robin specifically targeted at comparing and evaluating different methods for dopant profiling [1]. The substrate was /1 0 0S p-type silicon, boron-doped at 11–25 O cm1. The test structures were fabricated using low-temperature epitaxial growth, and consisted of an abrupt p–n junction, doped with boron and phosphorus (1018 cm3). Additional features included three narrow B-doped (1020 cm3) plateaus, which were intended for evaluation of the spatial resolution for small features. A further SiO2 layer (thickness of B100 nm) was deposited on one of the test wafers because of concerns that the first p–n junction of the test structure was located too close to the sample surface and likely to be damaged during TEM sample preparation. The targeted dopant profiles, as well as the calculated electron and hole profiles are shown in Fig. 1. Secondary-ion mass spectrometry (SIMS) was used to determine the chemical concentrations and depth distributions of the dopants [1]. Fig. 2 shows SIMS depth profiles taken from different regions of the wafer. A slight difference from the targeted profile shown in Fig. 1 is apparent. Most of the specimens described here were prepared for microscope observation by low-angle wedge polishing using the MultiPrep apparatus made by Allied, followed by low-angle, lowvoltage ion milling to remove any residual surface
Fig. 1. Target dopant profiles for test structure and corresponding calculated electron and hole profiles.
Fig. 2. Experimental SIMS profiles for dopant metrology test structure as taken from different places on Si wafer verifying reasonably uniform elemental distributions [1].
debris and smoothen the surface. Carbon coating up to a thickness of B30–40 nm was occasionally used after cross-sectioning to prevent the occurrence of charging of the sample surface. Otherwise, as described later, it was often necessary to superimpose an additional voltage ramp in order to flatten out the potential profile. The off-axis electron holography was performed using a Philips CM200 field-emission electron gun
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(FEG) TEM operated primarily at 200 keV. This instrument was equipped with an electrostatic biprism in the selected-area aperture plane and a Lorentz minilens, located just below the bore of the lower objective-lens pole-piece. This additional lens enabled large fields of view (B0.3–0.5 mm) to be obtained for holographic viewing [8]. Biprism voltages in the range 120–160 V and magnifications of 20–70 kX were typically used during operation, and a Gatan 794 multiscan CCD camera was used for digital recording. Reference holograms with the sample removed were routinely recorded so that corrections could be made for non-linearities in the imaging and recording system [9]. During observation, the samples were
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usually rotated by B61 away from the [1 1 0] zone axis about the substrate surface normal and then slightly off the Kikuchi band to minimize diffraction effects. However, local thickness estimation via convergent beam electron diffraction (CBED) required zone-axis observation [10]. A 1-D Poisson simulation program was used to generate the expected electrostatic potential profile of the test structure [11]. The input file for this program requires specific specimen information such as layer doping concentration and thickness, and an output file is then generated which contains the band diagram and carrier concentrations. The corresponding profiles can then be plotted using routine graphics software. Fig. 3(a) shows the band diagram generated using the dopant profile as determined by SIMS (where step functions were used to approximate the linear doping functions). The corresponding electrostatic potential for electrons in the conduction band can be constructed from the band diagram simply by inversion, as shown by the plot in Fig 3(b), where the vertical scale is in volts and the zero has been shifted to the position of lowest potential.
3. Basis for hologram interpretation Off-axis electron holography provides a quantitative measure of the phase change experienced by the electron wave that has passed through the sample. For non-magnetic materials, and neglecting dynamical diffraction effects and external fields, the phase change is given by the expression Z Fðx; yÞ ¼ CE V ðx; y; zÞ dz; ð1Þ where x; y represent the plane of the sample, z is the incident beam direction and V is the electrostatic potential of the object. The interaction constant CE is given by CE ¼ 2peðE0 þ EÞ=lEð2E0 þ EÞ;
Fig. 3. (a) Theoretical simulation of valence and conduction bands for test structure based on experimental SIMS profile; (b) Corresponding electrostatic potential variation. Zero re-set to position of lowest potential.
ð2Þ
where l is the wavelength, E is the kinetic energy of the incident electron and E0 is the rest energy. The electrostatic potential of the object can be considered as having two possible components. The mean inner potential, V0 is the volume
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average of the atomic electrostatic potential of the specimen, where typical values range from about 5 to 30 V [12,13]. In addition, intrinsic electric fields accessible to electron holography may also be present. Examples of materials where electron holography has been used in visualizing such internal fields include studies of charged grain boundaries in ceramics [14,15], and polarization fields in nitrides [7,16], as well as dopant distributions in Si-based electronic materials mentioned earlier [4–6]. When the potential inside the sample does not vary in the z direction (i.e., any potential variation is normal to the beam direction only), then Eq. (1) can be expressed in the form Fðx; yÞ ¼ CE ½V0 þ Vint ðx; yÞtðx; yÞ:
ð3Þ
Consideration of this equation implies that extraction of the internal electrostatic potential, Vint ; from the measured phase shift requires prior knowledge of the mean inner potential as well as information about the local thickness. Our approach to determining the mean inner potential of Si was based on the work of Li et al. [10]. In brief, CBED patterns from a wedgepolished Si sample, with matching to computergenerated simulations, were first used to determine the local Si crystal thickness. Measurements from several areas of several Si samples gave a value for V0 of 12.070.2 V, which agrees with the value of (11.970.7) V previously reported by Rau et al. [5]. This value was then used as the reference point for the subsequent dopant profiling experiments.
4. Experimental results
Fig. 4. (a) Amplitude, and (b) phase, images reconstructed from off-axis hologram of test structure coated with SiO2 before cross-sectioning.
4.1. Dopant mapping Fig. 4 shows the amplitude (a) and phase (b) images reconstructed from one set of holograms. The band of about 100 nm width at the (left) edge of the sample is the SiO2 protection layer. The amplitude image shows no visible differences in contrast within the sample but the phase image clearly reveals stripes of contrast as a result of the presence of the dopants. From a comparison with Fig. 3(b), it can be seen that the central broad
band corresponds to the thick (B100 nm) B-doped layer, and the two thin dark stripes at the right correspond to the two narrow p+ doping regions which have nominal widths of 20 and 15 nm, respectively. Ideally, according to Eq. (3), the electrostatic potential could be calculated directly based on information about the phase and thickness retrieved from the hologram. In practice, complications occur that cause the results to vary
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considerably. As an example, Fig. 5 shows holograms taken from two regions of different thickness for the specimen with the SiO2 protecting layer. The corresponding reconstructed phase and thickness images are shown in Fig. 6, where the latter have been computed using a value of 85 nm for the inelastic mean-free-path (MFP) of silicon [17]. Phase wraps are present at the oxide-Si
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interface that could not be unwrapped due to undersampling of fringes. Line scans were then obtained from each of these images, where averaging across a region of 15 nm was used to improve the local signal-to-noise ratio [18]. Fig. 7 shows the corresponding thickness line scans, where the horizontal axis is the distance from the top of the Si layer. Artefacts at the vacuum-oxide and oxide-Si interfaces are caused by Fresnel fringes. Since the holograms were taken from small-angle wedge-polished samples, slight thickness changes can be expected. Indeed, the relative flatness of the thickness profiles within the Si regions is consistent with the specimen preparation method. (The apparent discrepancy in thickness between the Si and SiO2 regions is due to the difference in their MFP—which can be ignored in the present context.) Fig. 8 shows averaged profiles from the two phase images, where the vertical axis denotes the magnitude of the phase in radians. Clearly, the two curves show different trends in the region at the right away from the sample edge. Using these phase profiles, and the corresponding thickness profiles from Fig. 7, the potential profiles were then calculated according to Eq. (3). The results are shown plotted in Fig. 9. Apart from the deliberate offset, the trends of these two curves are in obvious disagreement: one tends to be relatively flat away from the sample edge, whereas the other tends upwards. Moreover, neither curve shows the behavior expected of the electrostatic potential as simulated and displayed in Fig. 3(b). These trends where potential profiles differed from one region to another for samples having the same nominal structure were typical of many of our initial observations. Clearly, a different and more robust method of data analysis is required in order for reliable and consistent results to be obtained that will eliminate artefacts due to charging. 4.2. Refined method for data analysis
Fig. 5. Off-axis electron holograms taken from different regions of test structure.
In addition to the electrostatic potential within the specimen, additional longer-range fields due to inadvertent or deliberate charging may surround the sample, and these fields will also cause changes in the electron phase shifts. The problem of charging and long-range electric fields has received
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Fig. 6. (a) Phase, and (b) thickness, images as reconstructed from the holograms shown in Fig. 5.
Fig. 7. Thickness profiles obtained from the images shown in Fig. 6.
Fig. 8. Phase profiles taken from the phase images shown in Fig. 6.
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Rearranging this equation to solve explicitly for Vint gives the expression Vint ðxÞ ¼ Vmea Veff ;
ð6Þ
where Vmea ðxÞ ¼ Fmea ðx; yÞ=½CE tðx; yÞ
ð7Þ
and Veff ðxÞ ¼ Fext ðx; yÞ=½CE tðx; yÞ þ V0 :
Fig. 9. Electrostatic potential profiles calculated from the thickness and phase profiles shown in Figs. 7 and 8.
considerable attention from Pozzi and colleagues who have used off-axis electron holography to map the electric field surrounding reverse-biased p–n junctions [19,20]. They demonstrated that the presence of a long-range field also affected the recording of the reference wave outside their samples [19]. In our experiments, we have not found it necessary to reverse-bias our samples, in order to obtain consistent and reproducible results that matched closely with theoretical simulations based on the measured dopant profiles [21,22]. The presence of any external fields can be accounted for by modifying Eq. (3) according to the following expression: Fðx; yÞ ¼ CE ½V0 þ Vint ðx; yÞtðx; yÞ Z þ a Vext ðx; y; zÞ dz;
ð4Þ
where Vext (x; y; z) denotes the external potential field. Let Fmea represent the total phase change as measured from the reconstructed phase image, where this will also include any phase changes due to the external electrostatic field, and let Fext represent the phase change due to the external field, which is equivalent to the integral in Eq. (4). Eq. (4) can then be re-written as Fmea ðx; yÞ ¼ CE ½V0 þ Vint ðx; yÞtðx; yÞ þ Fext ðx; yÞ: ð5Þ
ð8Þ
Thus, the intrinsic or built-in potential can be calculated provided that the other remaining contributions to the potential are pre-determined. We have developed a practical procedure that is suitable for extraction of Vint provided that there is a region where the potential should be flat, such as in the substrate well away from the junction. This process can be summarized as follows: 1. A thickness profile is obtained from the thickness image so that a fitted line can be used to approximate the sample thickness. This procedure provides tðx; yÞ while also reducing the noise which is usually introduced in the process of calculating the thickness image. 2. Calculate Vmea by dividing the measured phase profile by the product (CE :t:), according to Eq. (7). 3. Observe the general behavior of the plotted Vmea curve. Ideally, the only potential changes would result from the built-in potential, and Vmea would be flat away from the junction. In practice, however, potential profiles are obtained with different slopes, which are presumably due to the presence of an external field. In our experience, the effective external potential can usually be approximated by a straight line so that Veff can then be obtained by subtraction from the Vmea curve. Fig. 10 shows the Vmea and the fitted Veff lines for the representative holograms H1 and H2. Because insufficient substrate away from the doped regions was available for correction in H2, these lines were drawn so that the minima of the broad p+ dopant areas were on the same line. 4. Subtract Veff from V : This procedure gives Vint ; the desired internal electrostatic potential of the sample. Fig. 11 plots the resultant Vint (x) functions for the two example holograms.
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Fig. 10. Potential profiles from Fig. 9 with additional line fitting used to determine phase correction.
Fig. 12. Comparison of potential profiles from different regions of several samples after phase correction.
Fig. 11. Potential profiles after application of phase correction.
SiO2 protection layer and the samples denoted by S2 and S3 were made from the original wafer. It is significant that our procedure does not use the standard condition of a flat vacuum as an adjacent criterion, unlike previous work [12]. Not only is there insufficient area in the phase image, but it is also our experience that the sense of the fringing field outside the sample may differ from that inside the sample. When the thickness of the sample has relatively large local variations, the procedure outlined here is no longer usable. Moreover, this method will obviously fail when the effective potential profile outside the junction region(s) cannot be approximated with a linear function, as for example, might occur when these regions are charged. 4.3. Effect of surface charging
Except for some small discrepancies near the Si surface, the curves match each other closely. Reasons for these discrepancies will be discussed later. We have successfully applied this phase-correction procedure to measurements from different regions of several TEM samples, and we find that the final results are in close agreement. As an example, Fig. 12 shows results from three TEM samples, where the sample denoted by S1 has the
From our hologram analysis, it appeared that sample charging could be a problem, since the phase images showed downward slope with increased sample thickness. Carbon coating was therefore used to minimize this effect. Fig. 13 shows phase images of the sample edge, displayed in the pseudo-contour mode, before and after C coating. The vacuum region before the sample was coated shows phase variations of greater than p radians apparently originating from the region of
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2 H4 With C coating H3 With C coating H2 No C coating
1.5
Potential (V)
1
0.5
0
-0.5
-1 0
100
200
300 400 Distance (nm)
500
600
Fig. 14. Comparison of simulated potential profiles with experimental results before and after C coating (thickness of B40 nm). Note drop in potential close to top surface of sample after C coating.
of the downward slope from the uncoated profiles. The profiles largely agree except near the oxide interface. Note that the profiles of the coated samples bend downward near the surface. 4.4. Comparison with SIMS measurements
Fig. 13. (a) Reconstructed phase image showing effect of specimen charging originating from region of surface oxide and extending into vacuum. Phase change in vacuum in excess of p radians; (b) Same region after C coating showing flat phase in vacuum.
the surface oxide, whereas the phase of the vacuum is flat after C coating. Fig. 14 shows the potential profiles before and after C coating, after removal
Secondary-ion mass spectrometry (SIMS) had already been used to determine the chemical concentrations and depth distributions of the dopants, as plotted in Fig. 2 which shows the results obtained from different regions of the original wafer [1]. Because of some discrepancies between experimental and simulated potential profiles, these SIMS measurements were repeated locally. The results of this analysis are shown in Fig. 15. Note that the local SIMS measurement suggests that the surface region should be n-type whereas a p-type region would be expected from the Sematech data. Based on these dopant profiles, the band diagrams were calculated using a 1-D Poisson simulation program [11]. The corresponding electrostatic potentials, constructed from the band diagrams by inversion, are shown in Fig. 16(a), where the vertical scale is in volts and the zero has been shifted to the position of lowest potential. The two simulations show important
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1021 B P
20
-3 Concentration (cm )
10
1019 1018 1017 1016 1015
0
100
200
300
400
500
600
Distance (nm) Fig. 15. Experimental SIMS profile for B and P dopants recorded at ASU.
differences between the length scales and the predicted potential at the surface. The profile labelled Sematech shows a potential drop of B0.8 V near the surface, whereas that labelled ASU is flat. By shrinking the x-scale of the Sematech profile by 5%, the two curves can be made to match except near the surface, as shown in Fig. 16(b). This figure also shows the profile simulated for a neutral surface region, i.e., the concentration for the electrons and holes are taken to be the same over the first 60 nm. The experimental potential profiles from the electron holography measurements are compared with the simulation results in Fig. 17. Close agreement is achieved for the region between 200 and 500 nm. For the area closest to the surface, the electron holography measurement with C coating agrees closely with the neutral surface profile. The minor discrepancy between the simulations and the experimental data at about 120–180 nm is not understood. Similarly, the reasons for the small differences at the onset of the epilayer growth at B550 nm are not clear but the presence of B2% Ge during the epilayer growth may be a contributing factor.
4.5. Effect of thickness Our results demonstrate that variations in sample thickness and also the effects of external fields and sample charging can usually be taken into account so that close agreement with theoretical simulations can be achieved. However, for accurate and reliable measurement of real devices, when simulations might not be available, any further effect(s) that might cause spread of the results need to be identified. In our experience, information about the local specimen thickness is one of the more important factors that is likely to introduce error into the calculation of the electrostatic potential. The MFP of Si was taken to have a value of 85 nm, based on our previous experience with holographic analysis of Si-based structures, and this value used for the initial thickness determinations via the amplitude image. The CBED technique was used in our later experiments to determine the sample thickness more accurately. By comparing CBED simulations with experimental [1 1 0] zone-axis CBED patterns obtained from different positions in the sample, it was found that the thickness of the sample could be routinely
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2
1.2 V_ASU V_SEMATECH
1
H3 V_int (S2) H7 V_int (S3 C-coated) V_Neutral
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0.6
1
Potential (V)
Potential (V)
0.8
0.4 0.2 0 -0.2
0
0
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200
(a)
300 400 Distance (nm)
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600 -0.5
1.2
-1 0
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V_ASU V_Neutral V_Sematech
0.8
0.4
0 0
100
200
300
400
200
300
400
500
600
700
Fig. 17. Comparison of simulated potential profile with experimental profiles before, and after, C coating.
0.2
(b)
100
Distance (nm)
0.6
-0.2
0.5
500
600
Distance (nm)
Fig. 16. (a) Simulated electrostatic potential profiles based on SIMS measurements. Note differences in length scales and different potentials near the surface; (b) simulated potential profiles after adjustment of length scale for Sematech dopant profile, also including profile for ‘neutral’ surface region between 0 and 60 nm.
measured with an accuracy of about 710 nm. As an example to demonstrate this sensitivity, Fig. 18 compares an experimental CBED pattern from sample S3 with three simulated patterns for thicknesses of (b) 330 nm; (c) 310 nm; and (d) 350 nm. In this case, the experimental thickness estimated visually would be about 330720 nm. Table 1 compares the thicknesses as measured from CBED patterns and reconstructed thickness images. The two sets of data were obtained from sample S3 before and after C coating. It is apparent that for those sample regions without C coating, the thicknesses estimated from the amplitude images using the MFP of 85 nm have underestimated the real thickness. After C coating, the real thickness is overestimated, which is perhaps
not surprising considering that the thickness estimated from the holograms also includes a contribution from the amorphous C layer. The CBED technique is more appropriate for thicker samples, when dynamical diffraction increases contrast levels and the detail visible within the diffraction disks. Using the various thicknesses as determined by CBED, the potential profiles were re-calculated, and Fig. 19 compares the simulated potential plot with experimental profiles before and after thickness adjustment. By adding an additional 30 nm to the original 180 nm thickness, in accord with the CBED estimate, the potential profile is closer to the simulated profile, especially for the first 100 nm from the top of sample.
5. Discussion Our observations have shown that charging can have a serious effect on electron holographic measurements of electrostatic potential, especially in specimen regions close to the vacuum edge which tend to become positively charged, due to the emission of secondary electrons. The number of secondary electrons ejected is related to the nature of the surface as well as the overall thickness of the sample. A greater fraction of secondary electrons is more likely to be emitted at
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Fig. 18. Comparison of experimental CBED pattern (a) with simulated patterns for thicknesses of: (b) 330 nm; (c) 310 nm; and (d) 350 nm, respectively.
Table 1 Comparison of thickness measurements estimated by CBED and electron holography before and after C coating Position
1 2 3 4 5 6
Thickness before C coating (nm)
Thickness after C coating (nm)
CBED
Holography
Holography
200 225 250 280 330 390
170 200 210 — 260 320
— 260 270 300 330 380
the thinnest edge thus making this region more positively charged, and giving rise to external fringing fields. Insulating surface layers are more likely to retain high levels of charge. Coating the samples with a conducting C layer of about 40 nm provides a better contact to ground, and the likelihood of secondary electron emission is greatly diminished, which therefore also reduces the amount of charging. Conversely, our experience also implies that any C coating layer should not be too thin. Our recommendation is that in order to reduce uncertainties about the quality of the recorded data, samples intended for electron holography should be routinely coated with carbon before microscope observations are started.
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acknowledge use of facilities in the Center for High Resolution Electron Microscopy at Arizona State University.
2
H2 (S1) H2 (180+30) V_Neutral
1.5
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Potential (V)
1
References 0.5
0
-0.5
-1 0
100
200
300
400
500
600
Distance (nm)
Fig. 19. Comparison of simulated profile with experimental profiles before and after adjustment of thickness as obtained from CBED.
6. Conclusion Electron holography has recently attracted a great deal of attention in the semiconductor industry because it appears to provide a possible solution to the critical but unresolved issue of dopant profiling in deep-submicron devices [1]. The results reported here offer real promise that the problems of surface charging can be overcome and that accurate potential profiles should be obtainable on a routine basis provided that there is some prior knowledge of the likely junction depth so that a region suitable for flattening the potential profile can be identified. Indeed, we have recently completed characterization of 1-D [21] and 2-D [22] device structures. Based on composition profiles, and comparisons with process simulations, we have demonstrated in both studies that spatial resolutions closely approaching 5 nm and sensitivities close to 0.1 V can be achieved.
Acknowledgements The authors thank Professor Jian-Min Zuo for use of his CBED simulation program, and Dr. Rick Hervig for assistance with SIMS analysis. We
[1] A.C. Diebold, M.R. Kump, J.J. Kopanski, D.G. Seller, J. Vac. Sci. Technol. B 14 (1996) 196. [2] ITRS Metrology 2000 Update, Table 84a: 2000 Front End Processes Technology Requirements—Near Term, Semiconductor Industry Association, San Jose. [3] R. Subrahmanyan, J. Vac. Sci. Technol. B 10 (1992) 358. [4] M.R. McCartney, D.J. Smith, R. Hull, J.C. Bean, . E. Volkl, B. Frost, Appl. Phys. Lett. 65 (1994) 2603. . [5] W.D. Rau, P. Schwander, F.H. Baumann, W. Hoppner, A. Ourmazd, Phys. Rev. Lett. 82 (1999) 2614. [6] W.D. Rau, P. Schwander, A. Ourmazd, Phys. Stat. Sol. B 222 (2000) 213. [7] M.R. McCartney, F.A. Ponce, J. Cai, D.P. Bour, Appl. Phys. Lett. 76 (2000) 3055. [8] M.R. McCartney, D.J. Smith, R.F.C. Farrow, R.F. Marks, J. Appl. Phys. 82 (1997) 2461. [9] W.J. de Ruijter, J.K. Weiss, Ultramicroscopy 50 (1993) 269. [10] J. Li, M.R. McCartney, R.E. Dunin-Borkowski, D.J. Smith, Acta Crystallogr. A 65 (1999) 652. [11] I.-H. Tan, G.L. Snider, L.D. Chang, E.L. Hu, J. Appl. Phys. 68 (1990) 4071. [12] M. Gajdardziska-Josifovska, M.R. McCartney, W.J. de Ruijter, D.J. Smith, J.K. Weiss, J.M. Zuo, Ultramicroscopy 50 (1993) 285. . [13] M. Gajdardziska-Josifovska,A.H. Carim, in: E. Volkl, L. Allard, D.C. Joy (Eds.), Introduction to Electron Holography, Plenum, New York, 1999 (Chapter 12). [14] V. Ravikumar, R.P. Rodrigues, V.P. Dravid, Phys. Rev. Lett. 75 (1995) 4063. [15] K.D. Johnson, V.P. Dravid, Interface Sci. 8 (2000) 189. [16] J.S. Barnard, D. Cherns, J. Electron Microscopy 49 (2000) 281. [17] M.R. McCartney, M. Gajdardziska-Josifovska, Ultramicroscopy 53 (1994) 283. [18] J.K. Weiss, W.J. de Ruijter, M.R. McCartney, M. Gajdardziska-Josifovska, D.J. Smith, Ultramicroscopy 50 (1993) 301. [19] S. Frabboni, G. Matteucci, G. Pozzi, Ultramicroscopy 23 (1987) 29. [20] G. Matteucci, G.F. Missiroli, G. Pozzi, J. Electron Microsc. 45 (1996) 27. [21] M.R. McCartney, M.A. Gribelyuk, J. Li, P. Ronsheim, J.S. McMurray, D.J. Smith, Appl. Phys. Lett. 80 (2002) 3213. [22] M.A. Gribelyuk, M.R. McCartney, J. Li, C.S. Murthy, P. Ronsheim, B. Doris, S. Hedge, J.S. McMurray, D.J. Smith, Phys. Rev. Lett. 89 (2002) 025502.
Semiconductor dopant profiling by off-axis electron holography Jing Lia, M.R. McCartneyb,*, David J. Smitha,b a
Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA b Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA Received 24 October 2001; received in revised form 26 July 2002
Abstract Silicon wafers with a complex but known dopant profile were used to explore possible methods for improving the reliability of off-axis electron holography for quantitative determination of electrostatic potential profiles in doped semiconductor devices. The variability of results from nominally identical structures was attributed to local charging and associated external fields, forcing the development of a more robust approach to hologram analysis that incorporated an additional phase correction factor rather than rely on vacuum for phase flattening. Consistent results in close agreement with simulated profiles based on measured dopant distributions could then be obtained. Carbon coating was shown to be effective in reducing accumulation of charge caused by emission of secondary electrons. Overall, this work demonstrates that reliable potential profiles from unbiased samples should be obtainable on a routine basis provided that regions suitable for flattening of the phase profile can be identified. r 2002 Elsevier Science B.V. All rights reserved.
1. Introduction With the continual downsizing of device dimensions, two-dimensional (2-D) dopant profiling is an ongoing issue of great interest and importance to the semiconductor industry [1,2]. Obtaining reliable information about the dopant distribution in real structures as a function of processing parameters remains a key step in validating process and device simulations used for developing and refining prototype device designs [3]. In practice, the spatial distribution and concentration of active dopant atoms play a major role in determining the electrostatic potential within the device, which in turn determines the device *Corresponding author..
response. It would therefore be highly beneficial to develop an experimental method capable of characterizing the potential with high spatial resolution and sensitivity. Off-axis electron holography in the transmission electron microscope (TEM) provides access to the phase of the electron wavefront that has traversed a sample, so that the technique should in principle enable the electrostatic potential within semiconductor devices to be quantified. Electron holography was used by McCartney et al. to observe the potential profile across Si/Si p–n junctions [4], and the technique was later used by Rau and colleagues to map the 2-D electrostatic potential in deepsubmicron transistor structures [5,6]. Electrostatic potential variations across an AlGaN/InGaN/ AlGaN heterojunction diode have recently been
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quantified using electron holography, and also explained in terms of polarization effects and free polarization-induced interface charge [7]. In this present study, we have systematically explored the application of off-axis electron holography to a test structure of known dopant distribution. We demonstrate that accurate potential profiles can be extracted routinely from unbiased materials using a refined approach to hologram analysis and carefully controlled sample preparation conditions. Experimental factors that are liable to affect quantitative measurements are also discussed.
2. Experimental details The specimens studied here were taken from a group of test structures fabricated at IBM for a dopant metrology round-robin specifically targeted at comparing and evaluating different methods for dopant profiling [1]. The substrate was /1 0 0S p-type silicon, boron-doped at 11–25 O cm1. The test structures were fabricated using low-temperature epitaxial growth, and consisted of an abrupt p–n junction, doped with boron and phosphorus (1018 cm3). Additional features included three narrow B-doped (1020 cm3) plateaus, which were intended for evaluation of the spatial resolution for small features. A further SiO2 layer (thickness of B100 nm) was deposited on one of the test wafers because of concerns that the first p–n junction of the test structure was located too close to the sample surface and likely to be damaged during TEM sample preparation. The targeted dopant profiles, as well as the calculated electron and hole profiles are shown in Fig. 1. Secondary-ion mass spectrometry (SIMS) was used to determine the chemical concentrations and depth distributions of the dopants [1]. Fig. 2 shows SIMS depth profiles taken from different regions of the wafer. A slight difference from the targeted profile shown in Fig. 1 is apparent. Most of the specimens described here were prepared for microscope observation by low-angle wedge polishing using the MultiPrep apparatus made by Allied, followed by low-angle, lowvoltage ion milling to remove any residual surface
Fig. 1. Target dopant profiles for test structure and corresponding calculated electron and hole profiles.
Fig. 2. Experimental SIMS profiles for dopant metrology test structure as taken from different places on Si wafer verifying reasonably uniform elemental distributions [1].
debris and smoothen the surface. Carbon coating up to a thickness of B30–40 nm was occasionally used after cross-sectioning to prevent the occurrence of charging of the sample surface. Otherwise, as described later, it was often necessary to superimpose an additional voltage ramp in order to flatten out the potential profile. The off-axis electron holography was performed using a Philips CM200 field-emission electron gun
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(FEG) TEM operated primarily at 200 keV. This instrument was equipped with an electrostatic biprism in the selected-area aperture plane and a Lorentz minilens, located just below the bore of the lower objective-lens pole-piece. This additional lens enabled large fields of view (B0.3–0.5 mm) to be obtained for holographic viewing [8]. Biprism voltages in the range 120–160 V and magnifications of 20–70 kX were typically used during operation, and a Gatan 794 multiscan CCD camera was used for digital recording. Reference holograms with the sample removed were routinely recorded so that corrections could be made for non-linearities in the imaging and recording system [9]. During observation, the samples were
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usually rotated by B61 away from the [1 1 0] zone axis about the substrate surface normal and then slightly off the Kikuchi band to minimize diffraction effects. However, local thickness estimation via convergent beam electron diffraction (CBED) required zone-axis observation [10]. A 1-D Poisson simulation program was used to generate the expected electrostatic potential profile of the test structure [11]. The input file for this program requires specific specimen information such as layer doping concentration and thickness, and an output file is then generated which contains the band diagram and carrier concentrations. The corresponding profiles can then be plotted using routine graphics software. Fig. 3(a) shows the band diagram generated using the dopant profile as determined by SIMS (where step functions were used to approximate the linear doping functions). The corresponding electrostatic potential for electrons in the conduction band can be constructed from the band diagram simply by inversion, as shown by the plot in Fig 3(b), where the vertical scale is in volts and the zero has been shifted to the position of lowest potential.
3. Basis for hologram interpretation Off-axis electron holography provides a quantitative measure of the phase change experienced by the electron wave that has passed through the sample. For non-magnetic materials, and neglecting dynamical diffraction effects and external fields, the phase change is given by the expression Z Fðx; yÞ ¼ CE V ðx; y; zÞ dz; ð1Þ where x; y represent the plane of the sample, z is the incident beam direction and V is the electrostatic potential of the object. The interaction constant CE is given by CE ¼ 2peðE0 þ EÞ=lEð2E0 þ EÞ;
Fig. 3. (a) Theoretical simulation of valence and conduction bands for test structure based on experimental SIMS profile; (b) Corresponding electrostatic potential variation. Zero re-set to position of lowest potential.
ð2Þ
where l is the wavelength, E is the kinetic energy of the incident electron and E0 is the rest energy. The electrostatic potential of the object can be considered as having two possible components. The mean inner potential, V0 is the volume
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average of the atomic electrostatic potential of the specimen, where typical values range from about 5 to 30 V [12,13]. In addition, intrinsic electric fields accessible to electron holography may also be present. Examples of materials where electron holography has been used in visualizing such internal fields include studies of charged grain boundaries in ceramics [14,15], and polarization fields in nitrides [7,16], as well as dopant distributions in Si-based electronic materials mentioned earlier [4–6]. When the potential inside the sample does not vary in the z direction (i.e., any potential variation is normal to the beam direction only), then Eq. (1) can be expressed in the form Fðx; yÞ ¼ CE ½V0 þ Vint ðx; yÞtðx; yÞ:
ð3Þ
Consideration of this equation implies that extraction of the internal electrostatic potential, Vint ; from the measured phase shift requires prior knowledge of the mean inner potential as well as information about the local thickness. Our approach to determining the mean inner potential of Si was based on the work of Li et al. [10]. In brief, CBED patterns from a wedgepolished Si sample, with matching to computergenerated simulations, were first used to determine the local Si crystal thickness. Measurements from several areas of several Si samples gave a value for V0 of 12.070.2 V, which agrees with the value of (11.970.7) V previously reported by Rau et al. [5]. This value was then used as the reference point for the subsequent dopant profiling experiments.
4. Experimental results
Fig. 4. (a) Amplitude, and (b) phase, images reconstructed from off-axis hologram of test structure coated with SiO2 before cross-sectioning.
4.1. Dopant mapping Fig. 4 shows the amplitude (a) and phase (b) images reconstructed from one set of holograms. The band of about 100 nm width at the (left) edge of the sample is the SiO2 protection layer. The amplitude image shows no visible differences in contrast within the sample but the phase image clearly reveals stripes of contrast as a result of the presence of the dopants. From a comparison with Fig. 3(b), it can be seen that the central broad
band corresponds to the thick (B100 nm) B-doped layer, and the two thin dark stripes at the right correspond to the two narrow p+ doping regions which have nominal widths of 20 and 15 nm, respectively. Ideally, according to Eq. (3), the electrostatic potential could be calculated directly based on information about the phase and thickness retrieved from the hologram. In practice, complications occur that cause the results to vary
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considerably. As an example, Fig. 5 shows holograms taken from two regions of different thickness for the specimen with the SiO2 protecting layer. The corresponding reconstructed phase and thickness images are shown in Fig. 6, where the latter have been computed using a value of 85 nm for the inelastic mean-free-path (MFP) of silicon [17]. Phase wraps are present at the oxide-Si
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interface that could not be unwrapped due to undersampling of fringes. Line scans were then obtained from each of these images, where averaging across a region of 15 nm was used to improve the local signal-to-noise ratio [18]. Fig. 7 shows the corresponding thickness line scans, where the horizontal axis is the distance from the top of the Si layer. Artefacts at the vacuum-oxide and oxide-Si interfaces are caused by Fresnel fringes. Since the holograms were taken from small-angle wedge-polished samples, slight thickness changes can be expected. Indeed, the relative flatness of the thickness profiles within the Si regions is consistent with the specimen preparation method. (The apparent discrepancy in thickness between the Si and SiO2 regions is due to the difference in their MFP—which can be ignored in the present context.) Fig. 8 shows averaged profiles from the two phase images, where the vertical axis denotes the magnitude of the phase in radians. Clearly, the two curves show different trends in the region at the right away from the sample edge. Using these phase profiles, and the corresponding thickness profiles from Fig. 7, the potential profiles were then calculated according to Eq. (3). The results are shown plotted in Fig. 9. Apart from the deliberate offset, the trends of these two curves are in obvious disagreement: one tends to be relatively flat away from the sample edge, whereas the other tends upwards. Moreover, neither curve shows the behavior expected of the electrostatic potential as simulated and displayed in Fig. 3(b). These trends where potential profiles differed from one region to another for samples having the same nominal structure were typical of many of our initial observations. Clearly, a different and more robust method of data analysis is required in order for reliable and consistent results to be obtained that will eliminate artefacts due to charging. 4.2. Refined method for data analysis
Fig. 5. Off-axis electron holograms taken from different regions of test structure.
In addition to the electrostatic potential within the specimen, additional longer-range fields due to inadvertent or deliberate charging may surround the sample, and these fields will also cause changes in the electron phase shifts. The problem of charging and long-range electric fields has received
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Fig. 6. (a) Phase, and (b) thickness, images as reconstructed from the holograms shown in Fig. 5.
Fig. 7. Thickness profiles obtained from the images shown in Fig. 6.
Fig. 8. Phase profiles taken from the phase images shown in Fig. 6.
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Rearranging this equation to solve explicitly for Vint gives the expression Vint ðxÞ ¼ Vmea Veff ;
ð6Þ
where Vmea ðxÞ ¼ Fmea ðx; yÞ=½CE tðx; yÞ
ð7Þ
and Veff ðxÞ ¼ Fext ðx; yÞ=½CE tðx; yÞ þ V0 :
Fig. 9. Electrostatic potential profiles calculated from the thickness and phase profiles shown in Figs. 7 and 8.
considerable attention from Pozzi and colleagues who have used off-axis electron holography to map the electric field surrounding reverse-biased p–n junctions [19,20]. They demonstrated that the presence of a long-range field also affected the recording of the reference wave outside their samples [19]. In our experiments, we have not found it necessary to reverse-bias our samples, in order to obtain consistent and reproducible results that matched closely with theoretical simulations based on the measured dopant profiles [21,22]. The presence of any external fields can be accounted for by modifying Eq. (3) according to the following expression: Fðx; yÞ ¼ CE ½V0 þ Vint ðx; yÞtðx; yÞ Z þ a Vext ðx; y; zÞ dz;
ð4Þ
where Vext (x; y; z) denotes the external potential field. Let Fmea represent the total phase change as measured from the reconstructed phase image, where this will also include any phase changes due to the external electrostatic field, and let Fext represent the phase change due to the external field, which is equivalent to the integral in Eq. (4). Eq. (4) can then be re-written as Fmea ðx; yÞ ¼ CE ½V0 þ Vint ðx; yÞtðx; yÞ þ Fext ðx; yÞ: ð5Þ
ð8Þ
Thus, the intrinsic or built-in potential can be calculated provided that the other remaining contributions to the potential are pre-determined. We have developed a practical procedure that is suitable for extraction of Vint provided that there is a region where the potential should be flat, such as in the substrate well away from the junction. This process can be summarized as follows: 1. A thickness profile is obtained from the thickness image so that a fitted line can be used to approximate the sample thickness. This procedure provides tðx; yÞ while also reducing the noise which is usually introduced in the process of calculating the thickness image. 2. Calculate Vmea by dividing the measured phase profile by the product (CE :t:), according to Eq. (7). 3. Observe the general behavior of the plotted Vmea curve. Ideally, the only potential changes would result from the built-in potential, and Vmea would be flat away from the junction. In practice, however, potential profiles are obtained with different slopes, which are presumably due to the presence of an external field. In our experience, the effective external potential can usually be approximated by a straight line so that Veff can then be obtained by subtraction from the Vmea curve. Fig. 10 shows the Vmea and the fitted Veff lines for the representative holograms H1 and H2. Because insufficient substrate away from the doped regions was available for correction in H2, these lines were drawn so that the minima of the broad p+ dopant areas were on the same line. 4. Subtract Veff from V : This procedure gives Vint ; the desired internal electrostatic potential of the sample. Fig. 11 plots the resultant Vint (x) functions for the two example holograms.
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Fig. 10. Potential profiles from Fig. 9 with additional line fitting used to determine phase correction.
Fig. 12. Comparison of potential profiles from different regions of several samples after phase correction.
Fig. 11. Potential profiles after application of phase correction.
SiO2 protection layer and the samples denoted by S2 and S3 were made from the original wafer. It is significant that our procedure does not use the standard condition of a flat vacuum as an adjacent criterion, unlike previous work [12]. Not only is there insufficient area in the phase image, but it is also our experience that the sense of the fringing field outside the sample may differ from that inside the sample. When the thickness of the sample has relatively large local variations, the procedure outlined here is no longer usable. Moreover, this method will obviously fail when the effective potential profile outside the junction region(s) cannot be approximated with a linear function, as for example, might occur when these regions are charged. 4.3. Effect of surface charging
Except for some small discrepancies near the Si surface, the curves match each other closely. Reasons for these discrepancies will be discussed later. We have successfully applied this phase-correction procedure to measurements from different regions of several TEM samples, and we find that the final results are in close agreement. As an example, Fig. 12 shows results from three TEM samples, where the sample denoted by S1 has the
From our hologram analysis, it appeared that sample charging could be a problem, since the phase images showed downward slope with increased sample thickness. Carbon coating was therefore used to minimize this effect. Fig. 13 shows phase images of the sample edge, displayed in the pseudo-contour mode, before and after C coating. The vacuum region before the sample was coated shows phase variations of greater than p radians apparently originating from the region of
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2 H4 With C coating H3 With C coating H2 No C coating
1.5
Potential (V)
1
0.5
0
-0.5
-1 0
100
200
300 400 Distance (nm)
500
600
Fig. 14. Comparison of simulated potential profiles with experimental results before and after C coating (thickness of B40 nm). Note drop in potential close to top surface of sample after C coating.
of the downward slope from the uncoated profiles. The profiles largely agree except near the oxide interface. Note that the profiles of the coated samples bend downward near the surface. 4.4. Comparison with SIMS measurements
Fig. 13. (a) Reconstructed phase image showing effect of specimen charging originating from region of surface oxide and extending into vacuum. Phase change in vacuum in excess of p radians; (b) Same region after C coating showing flat phase in vacuum.
the surface oxide, whereas the phase of the vacuum is flat after C coating. Fig. 14 shows the potential profiles before and after C coating, after removal
Secondary-ion mass spectrometry (SIMS) had already been used to determine the chemical concentrations and depth distributions of the dopants, as plotted in Fig. 2 which shows the results obtained from different regions of the original wafer [1]. Because of some discrepancies between experimental and simulated potential profiles, these SIMS measurements were repeated locally. The results of this analysis are shown in Fig. 15. Note that the local SIMS measurement suggests that the surface region should be n-type whereas a p-type region would be expected from the Sematech data. Based on these dopant profiles, the band diagrams were calculated using a 1-D Poisson simulation program [11]. The corresponding electrostatic potentials, constructed from the band diagrams by inversion, are shown in Fig. 16(a), where the vertical scale is in volts and the zero has been shifted to the position of lowest potential. The two simulations show important
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1021 B P
20
-3 Concentration (cm )
10
1019 1018 1017 1016 1015
0
100
200
300
400
500
600
Distance (nm) Fig. 15. Experimental SIMS profile for B and P dopants recorded at ASU.
differences between the length scales and the predicted potential at the surface. The profile labelled Sematech shows a potential drop of B0.8 V near the surface, whereas that labelled ASU is flat. By shrinking the x-scale of the Sematech profile by 5%, the two curves can be made to match except near the surface, as shown in Fig. 16(b). This figure also shows the profile simulated for a neutral surface region, i.e., the concentration for the electrons and holes are taken to be the same over the first 60 nm. The experimental potential profiles from the electron holography measurements are compared with the simulation results in Fig. 17. Close agreement is achieved for the region between 200 and 500 nm. For the area closest to the surface, the electron holography measurement with C coating agrees closely with the neutral surface profile. The minor discrepancy between the simulations and the experimental data at about 120–180 nm is not understood. Similarly, the reasons for the small differences at the onset of the epilayer growth at B550 nm are not clear but the presence of B2% Ge during the epilayer growth may be a contributing factor.
4.5. Effect of thickness Our results demonstrate that variations in sample thickness and also the effects of external fields and sample charging can usually be taken into account so that close agreement with theoretical simulations can be achieved. However, for accurate and reliable measurement of real devices, when simulations might not be available, any further effect(s) that might cause spread of the results need to be identified. In our experience, information about the local specimen thickness is one of the more important factors that is likely to introduce error into the calculation of the electrostatic potential. The MFP of Si was taken to have a value of 85 nm, based on our previous experience with holographic analysis of Si-based structures, and this value used for the initial thickness determinations via the amplitude image. The CBED technique was used in our later experiments to determine the sample thickness more accurately. By comparing CBED simulations with experimental [1 1 0] zone-axis CBED patterns obtained from different positions in the sample, it was found that the thickness of the sample could be routinely
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0 0
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Fig. 17. Comparison of simulated potential profile with experimental profiles before, and after, C coating.
0.2
(b)
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Distance (nm)
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-0.2
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Distance (nm)
Fig. 16. (a) Simulated electrostatic potential profiles based on SIMS measurements. Note differences in length scales and different potentials near the surface; (b) simulated potential profiles after adjustment of length scale for Sematech dopant profile, also including profile for ‘neutral’ surface region between 0 and 60 nm.
measured with an accuracy of about 710 nm. As an example to demonstrate this sensitivity, Fig. 18 compares an experimental CBED pattern from sample S3 with three simulated patterns for thicknesses of (b) 330 nm; (c) 310 nm; and (d) 350 nm. In this case, the experimental thickness estimated visually would be about 330720 nm. Table 1 compares the thicknesses as measured from CBED patterns and reconstructed thickness images. The two sets of data were obtained from sample S3 before and after C coating. It is apparent that for those sample regions without C coating, the thicknesses estimated from the amplitude images using the MFP of 85 nm have underestimated the real thickness. After C coating, the real thickness is overestimated, which is perhaps
not surprising considering that the thickness estimated from the holograms also includes a contribution from the amorphous C layer. The CBED technique is more appropriate for thicker samples, when dynamical diffraction increases contrast levels and the detail visible within the diffraction disks. Using the various thicknesses as determined by CBED, the potential profiles were re-calculated, and Fig. 19 compares the simulated potential plot with experimental profiles before and after thickness adjustment. By adding an additional 30 nm to the original 180 nm thickness, in accord with the CBED estimate, the potential profile is closer to the simulated profile, especially for the first 100 nm from the top of sample.
5. Discussion Our observations have shown that charging can have a serious effect on electron holographic measurements of electrostatic potential, especially in specimen regions close to the vacuum edge which tend to become positively charged, due to the emission of secondary electrons. The number of secondary electrons ejected is related to the nature of the surface as well as the overall thickness of the sample. A greater fraction of secondary electrons is more likely to be emitted at
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Fig. 18. Comparison of experimental CBED pattern (a) with simulated patterns for thicknesses of: (b) 330 nm; (c) 310 nm; and (d) 350 nm, respectively.
Table 1 Comparison of thickness measurements estimated by CBED and electron holography before and after C coating Position
1 2 3 4 5 6
Thickness before C coating (nm)
Thickness after C coating (nm)
CBED
Holography
Holography
200 225 250 280 330 390
170 200 210 — 260 320
— 260 270 300 330 380
the thinnest edge thus making this region more positively charged, and giving rise to external fringing fields. Insulating surface layers are more likely to retain high levels of charge. Coating the samples with a conducting C layer of about 40 nm provides a better contact to ground, and the likelihood of secondary electron emission is greatly diminished, which therefore also reduces the amount of charging. Conversely, our experience also implies that any C coating layer should not be too thin. Our recommendation is that in order to reduce uncertainties about the quality of the recorded data, samples intended for electron holography should be routinely coated with carbon before microscope observations are started.
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acknowledge use of facilities in the Center for High Resolution Electron Microscopy at Arizona State University.
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References 0.5
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Fig. 19. Comparison of simulated profile with experimental profiles before and after adjustment of thickness as obtained from CBED.
6. Conclusion Electron holography has recently attracted a great deal of attention in the semiconductor industry because it appears to provide a possible solution to the critical but unresolved issue of dopant profiling in deep-submicron devices [1]. The results reported here offer real promise that the problems of surface charging can be overcome and that accurate potential profiles should be obtainable on a routine basis provided that there is some prior knowledge of the likely junction depth so that a region suitable for flattening the potential profile can be identified. Indeed, we have recently completed characterization of 1-D [21] and 2-D [22] device structures. Based on composition profiles, and comparisons with process simulations, we have demonstrated in both studies that spatial resolutions closely approaching 5 nm and sensitivities close to 0.1 V can be achieved.
Acknowledgements The authors thank Professor Jian-Min Zuo for use of his CBED simulation program, and Dr. Rick Hervig for assistance with SIMS analysis. We
[1] A.C. Diebold, M.R. Kump, J.J. Kopanski, D.G. Seller, J. Vac. Sci. Technol. B 14 (1996) 196. [2] ITRS Metrology 2000 Update, Table 84a: 2000 Front End Processes Technology Requirements—Near Term, Semiconductor Industry Association, San Jose. [3] R. Subrahmanyan, J. Vac. Sci. Technol. B 10 (1992) 358. [4] M.R. McCartney, D.J. Smith, R. Hull, J.C. Bean, . E. Volkl, B. Frost, Appl. Phys. Lett. 65 (1994) 2603. . [5] W.D. Rau, P. Schwander, F.H. Baumann, W. Hoppner, A. Ourmazd, Phys. Rev. Lett. 82 (1999) 2614. [6] W.D. Rau, P. Schwander, A. Ourmazd, Phys. Stat. Sol. B 222 (2000) 213. [7] M.R. McCartney, F.A. Ponce, J. Cai, D.P. Bour, Appl. Phys. Lett. 76 (2000) 3055. [8] M.R. McCartney, D.J. Smith, R.F.C. Farrow, R.F. Marks, J. Appl. Phys. 82 (1997) 2461. [9] W.J. de Ruijter, J.K. Weiss, Ultramicroscopy 50 (1993) 269. [10] J. Li, M.R. McCartney, R.E. Dunin-Borkowski, D.J. Smith, Acta Crystallogr. A 65 (1999) 652. [11] I.-H. Tan, G.L. Snider, L.D. Chang, E.L. Hu, J. Appl. Phys. 68 (1990) 4071. [12] M. Gajdardziska-Josifovska, M.R. McCartney, W.J. de Ruijter, D.J. Smith, J.K. Weiss, J.M. Zuo, Ultramicroscopy 50 (1993) 285. . [13] M. Gajdardziska-Josifovska,A.H. Carim, in: E. Volkl, L. Allard, D.C. Joy (Eds.), Introduction to Electron Holography, Plenum, New York, 1999 (Chapter 12). [14] V. Ravikumar, R.P. Rodrigues, V.P. Dravid, Phys. Rev. Lett. 75 (1995) 4063. [15] K.D. Johnson, V.P. Dravid, Interface Sci. 8 (2000) 189. [16] J.S. Barnard, D. Cherns, J. Electron Microscopy 49 (2000) 281. [17] M.R. McCartney, M. Gajdardziska-Josifovska, Ultramicroscopy 53 (1994) 283. [18] J.K. Weiss, W.J. de Ruijter, M.R. McCartney, M. Gajdardziska-Josifovska, D.J. Smith, Ultramicroscopy 50 (1993) 301. [19] S. Frabboni, G. Matteucci, G. Pozzi, Ultramicroscopy 23 (1987) 29. [20] G. Matteucci, G.F. Missiroli, G. Pozzi, J. Electron Microsc. 45 (1996) 27. [21] M.R. McCartney, M.A. Gribelyuk, J. Li, P. Ronsheim, J.S. McMurray, D.J. Smith, Appl. Phys. Lett. 80 (2002) 3213. [22] M.A. Gribelyuk, M.R. McCartney, J. Li, C.S. Murthy, P. Ronsheim, B. Doris, S. Hedge, J.S. McMurray, D.J. Smith, Phys. Rev. Lett. 89 (2002) 025502.