Precise linewidth measurement using the electron beam metrology system

Microelectronic Engineering 6 (1987) 6 4 5 - 6 5 1 North-Holland

645

PRECISE LINEWIDTH MEASUREMENT USING THE ELECTRON BEAM METROLOGY SYSTEM

Genya Matsuoka, Hisaya Murakoshi, Kenichi Yamamoto and Mikio Ichihashi Central Research Laboratory, Hitachi, Ltd. Kokubunji, Tokyo 185, Japan

I.

INTRODUCTION

Feature sizes of recent semiconductor devices have reached the submicron level. Such small dimensions, especially below the half-micron, require many new technologies. Linewidth measurement to inspect the manufactured devices is one of the problems arising below the I um level. The conventional method using an optical microscope encounters resolution difficulty. Thus it is difficult to inspect device features using this method. Electron beam metrology uses a focused electron beam rather than an optical beam. Thus it can measure the submicron devices because of its very fine spot capability(1),(2),(3). Because this technology is newly developed, there are several uncertainties in this application(4). The main uncertainties are repeatability of measured length, or precision and absoluteness of the obtained value, accuracy(5). Accuracy means the obtained value is absolutely correct and that systems with good accuracy have equal results. It is very difficult to evaluate accuracy. Many elements must be considered such as operating conditions, specimens, and methods of calculating size. The generating mechanism of the secondary electron signal from the sample must also be understood(6),(7). Precision is very important in the field of practical application. Precision means that under the same conditions with the same specimen, the same results are obtained. The process or lithographic conditions can be evaluated using these reliable data. Many factors affect precision. However, to evaluate an instrL~nent's precision, the standard sample used must be stable within the experimental period. There is also the positional factor that the measured results differ depending on the measuring point in the deflection field. To eliminate these factors, the experiments must be done with as consistent sample position ms possible, and the techniques reduce positional error. This paper discusses the repeatability of the newly developed electron beam metrological system(8). The system's features are explained, elements affecting repeatability are considered, and the experimental results are shown.

2.

ELECTRON BEAM METROLOGY SYSTEM

A block diagram of the electron beam metrology system is shown in Fig.1. The system's main features are: (I) Low accelerating voltage ( 0.5-2.0 kV ) electron beam optics with field emission gun

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© 1987, Elsevier Science Publishers B.V. (North-Holland)

646

G. Matsuoka et aL /Precise linewidth measurement

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(2) A precision specimen stage with laser interferometer (3) Four secondary electron detectors for good signal-to-noise ratio (4) Automatic linewidth calculation function using digitized detected signals The top of specimen stage is shown in Fig.2. The stage holds two standard samples. These samples are dry-etched Si patterns. They are fabricated using electron beam lithography to ensure line pitch. The standard samples are at different heights, as Fig.2 shows. Their lower position is adjusted to that of the specimen to be measured. The relation between objective lens current and specimen height is calibrated using these samples and 2.15 um/mA is obtained. These standard samples are also used in deflection gain calibration, automatic focus correction, and astigmatism correction. Standard samples ~ Faraday ////"~/CUpSCUpS L -shaped mirror

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FACTORS AFFECTING REPEATABILITY

Several factors affect measurement repeatability.

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G. Matsuoka et al. / Precise linewidth measurement

deflection gain, the change of raw signal profile caused effect, and the algorithm by which linewidth is calculated.

647

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Deflection gain change directly affects the measured linewidth. The factors affecting deflection gain change are: (I) Control circuit instability (2) Changing environmental conditions where the system is installed (3) Deflection distortion (4) Change of sample height Many components such as the gun, lens, and aligners are used in electron optics. The circuits control these components. These circuits are carefully designed and adjusted to achieve good performance. Room temperature, vibration, and stray magnetic field are the environmental factors affecting repeatability. The system is placed in a temperature-controlled clean room. The area's conditions are inspected in advance. The resulting data for the area are less than I milligauss of stray magnetic field and 0.03 um p-p vibration is obtained. These are sufficiently small to have a negligible effect on measurement. Deflection distortion affect measurement results because the measured values depend on the position in the deflection field where the measurement is performed. To eliminate this effect the specimen must be measured in a fixed place in the field, or the results must be corrected according to the position where the measurement is performed. Deflection gain also changes with sample height. If the specimen is set higher, the measured length decreases. The specimen height changes according to specimen or to specimen position. Thus specimen height must be checked before measurement and the results must be corrected according to height. In the present system, specimen height is evaluated from the current of the objective lens. Before linewidth measurement, the focus is automatically corrected. After automatic focus correction, the obtained optim~ current of the lens depends on specimen height. The specimen height is then calculated from the obtained optimum lens current, and the deflection gain is calibrated according to height. The charging effect of the specimen is difficult to evaluate and control quantitatively. To avoid this effect, the appropriate measuring conditions, accelerating voltage, and scanning method must be considered. In the present system, the accelerating voltage can be adjusted between 2.0 kV to 0.5 kV to obtain a constant signal. Repeatability also depends on the procedure by which linewidth is calculated. The most widely used alogorithms are: (I) peak-to-peak detection (2) threshold comparison (3) maxim~ slope detection. The peak-to-peak method chooses two peaks of the signal as the edges of a pattern. Threshold comparison sets an appropriate slice level and compares the wave profile with the level. The linewidth is obtained from the crosspoints between the level and the profile. These two methods show relatively good repeatability because of their simple algoritb~ns. The maxim~ slope detection determines the linewidth from the two points which reach the plus and minus maximum values of the first differential of the line profile. This procedure requires signal differentiation. Thus the signal-to-noise ratio degrades, causing poor repeatability.

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G. Matsuoka et al, / Precise linewidth measurement

EXPERIMENTAL RESULTS

Control-circuit stability and the effect of environment can be checked by measuring the sample repeatedly. As described above, the standard samples are made with an etched Si pattern. They show no charging effect and are fixed on the specimen stage. Thus instrument repeatability can be checked by measuring the sample repeatedly over time and control-circuit stability and environmental fluctuation can be evaluated. Deflection gain stability over 5 hours is shown in Fig.3. It is clearly unnecessary to calibrate over several hours to achieve ±0.005 um precision. Stability over 30 days is shown in Fig.4. The deflection gain changes about ±0.2 % / day and can be calibrated within the accuracy ±0.2 %.

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The effect of deflection distortion on linewidth variation is checked by measuring the same pattern at 9 points in the field. The results are shown in Fig.5. X linewidth variation in the field is shown in Fig.5(a) and and Y linewidth variation in Fig.5(b). Near the edge of the field, the measured linewidth tends to increase. However, the change is about ±0.15 %, sufficiently small to measure widths less than I um. Fietd I

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G. Matsuoka et al. /Precise linewidth measurement

649

The relation of repeatability to the various calculation algorithms at several signal stm~nation times is shown in Fig.6. Summing more signals improves repeatability, but the degree depends on the algorithm. The threshold comparison reaches the limit first. The maxim~n slope detection has poorer repeatability because it uses differentiation. The limit of the repeatability is discussed later. The measured linewidth is also affected by the defocusing of the electron beam and by the specimen height. Edge sharpness is degraded in line profiles obtained with defocused beams, causing measurement error. The automatic focus correction procedure adjusts the beam size and measures sample height, using the band-passed power spectr~n of the detected signal. The effect of objective lens current on measured linewidth, obtained from different calculations, is shown in Fig.7. The width changes with lens current, i.e. beam size. ±2mA correction accuracy is necessary to achieve less than ±0.01um measurement error. '

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Figure 7. Effect of objective lens condition on measured linewidth The accuracy of automatic focus correction is shown in Fig.8. "S" means the number of signal summation times. This automatic focus correction with appropriate s~mnation and smoothing procedures yields signals of ±2 mA accuracy. These are accurate enough to achieve the target repeatability. The effect of specimen height is experimentally investigated by measuring the line pitches of a deliberately inclined wafer. The distribution of measured line pitches is shown in Fig.9. A relatively large pitch is chosen to exaggerate the height effect. The measured line pitches spread ±I um because of the wafer height change. The relation between these line pitches and lens currents where the pitches are measured is shown in Fig.10. There is clearly a simple relation between line pitch and current. The relation between objective lens current and height is obtained using standard samples located at 195 um height differences. It is 2.15 um/mA. In Fig.10, 0 mA indicates the wafer located just at exactly the same level as the lower standard sample. The wafer is set at about -200 um to -950 um lower from the standard sample. Distribution after the height correction is shown in Fig.11. The ±I um distribution is reduced to ±0.05 t~n. This accuracy with 18 um line pitch means the error due to height change can be corrected ±0.3 %.

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DISCUSSION

The factors such as gain fluctuation, deflection distortion, and height change cause the error of measurement. The amount of each error is ±0.2 %, ±0.15 %, and ±0.3 % respectively. Because of these errors occur independently, the overall error can be estimated by the square scrnmation of each error. In the case of I um pattern measurement, the error due to each factor is ±0.002 urn, ±O.O015 um, and ±0.003 urn respectively and the combined error is ±0.004 um. This is the sufficiently negligible error to measure submicron patterns. The step pitch of the electron beam scanning (LSB) is 0.005 um in these experiments. Thus, if the results have W or W + 0.005 um ( W + ILSB ) equally, the repeatability(3o) becomes 1.5 times of LSB, that is 0.006 um. The experimentally obtained result( Fig.6 ) shows that the repeatability has a limit about 0.006 urn. Moreover, previously reported result with 0.0025 um step pitch shows the repeatability of 0.004 um(8). These results indicate the repeatability of present system is mainly determined by the step pitch of the electron beam scanning.

G. Matsuoka et al. / Precise linewMth measurement

6.

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CONCLUSIONS

Several factors affecting repeatabilityhave been discussed. These factors are deflection gain change, quality of signal profile, and the algorithm used to calculate linewidth. Deflection gain stability has been investigated. Gain can be calibrated to achieve +0.2 % precision. The deflection area is limited within 40 x 40 um to reduced the effects of deflection distortion less than +0.15 %. Automatic focus correction adjusts the beam size and corrects the linewidth error within +0.3 % of the measured length which is caused by specimen height change. The resulting overall repeatability is mainly determined by the step pitch of the electron beam scanning and is less than +0.01 um. This is sufficient for measuring submicron patterns.

ACKNOWLEDGMENTS

The authers wish to express their thanks to Yoshinori Nakayama and Naoki Matsuo for their extensive, thoughtful discussions of the experimental results.

REFERENCES

[I] T.Ohtaka et al.: "Hitachi S-6000 Field Emission CD-Measurement SEM"; SPIE Voi.565 p205 (1985) [2] T.North: "Precise measurement in a scanning electron microscope"; SPIE Voi.565 p187 (1985) [3] R.Norville: "SCANLINE - A Dedicated Fab Line SEM LW Measurement System"; SPIE Voi.565 p209 (1985) [4] F.Robb: "In-process linewidth measurement of polysilicon gates using a scanning electron microscope"; SPIE Voi.775 p89 (1987) [5] M.T.Postek: "Submicrometer dimensional metrology in the scanning electron microscope"; SPIE Vol.775 p166 (1987) [6] M.G.Rosenfield: "Analysis of linewidth measurement techniques using the low voltage SEM"; SPIE Voi.755 p70 (1987) [7] D.K.Atwood et al.: "Improved accuracy for SEM linewidth measurements"; SPIE Voi.755 p159 (1987) [8] G.Matsuoka et al.: "Automatic electron beam metrology system for development of very large-scale integrated devices"; J. Vac. Sci. & Technol. B; P79 Voi.5(I) 1987