Katsuhiro Sasada

Characterizing cross-sectional profile variations by using multiple parameters extracted from top-down SEM images

This paper describes a new approach towards monitoring the semiconductor lithography process using critical dimension scanning electron microscopy (CD-SEM). In the lithography process, there are two important process parameters, exposure dose E and focus F. To monitor both the E and F variation, a new method for characterizing the cross-sectional profile of the photoresist pattern from the secondary electron (SE) waveform has been developed. An innovative feature of this method is that it can quantify the degree of top rounding (TR) and bottom rounding (BR) of the cross-sectional profile separately.

Evaluation of the long-term stability of critical-dimension measurement scanning electron microscopes using a calibration standard

In critical-dimension measurement using critical-dimension measurement scanning electron microscopes (CD-SEMs), measurement variation due to a long-term instability of CD-SEM cannot be disregarded to achieve a measurement reproducibility of 3 nm, which is required for next-generation subquarter-micron large scale integrations. We have evaluated the long-term stability of the Hitachi S-8820 CD-SEM by using a calibration standard, which has 0.24 μm pitch structures. The results of our evaluation have demonstrated that the use of the calibration standard for CD-SEMs was effective for checking the instability of instruments, which might produce inaccuracy and imprecision of CD measurements, and measurement error due to instrumentation of 1 nm can be achieved by precise control of instrumental parameters using the calibration standard.

Novel CD-SEM calibration reference patterned by EB cell projection lithography

A silicon grating pattern with a 100-nm pitch size for calibration of electron-beam (EB) metrology systems was formed by EB cell projection writing using a grating stencil mask and dry etching. According to the evaluation results from a critical-dimension scanning electron microscope (CD-SEM), the uniformity of the pitch size in a 1.8 x 1.8-mm chip was smaller than 3 nm within 3 sigma in the x and y directions. The obtained 100-nm pitch size was calibrated by DUV laser diffraction. The difference between designed 100-nm pitch size and the calibrated pitch size by DUV laser diffraction was smaller than 0.1 nm. It is thus concluded that more precise calibration of the CD-SEM using this 100-nm pitch grating is expected compared with conventional calibration using 240-nm pitch reference grating fabricated by laser-interferometer lithography and anisotropic chemical etching.

Effect of various ArF resist shrinkage amplitudes on CD bias

The beam parameters of CD SEM, accelerating voltage, beam current, measurement time, frame number, and magnification are evaluated to get the optimal setting for reducing the shrinkage of ArF resist. We check image resolution, resist shrinkage amplitude, CD bias between resist line and etched pattern to valuate the impact of beam parameters. On image resolution, the poly film is better resolved with the 800 V accelerating voltage. On the other hand, 300 V is more suitable for resist image. It also produces much lower resist shrinkage compared with 800 V. Beam current, measurement time, frame number, and magnification produce much less impact on resist shrinkage than the accelerating voltage. On CD bias, we also found that 300 V produces better accuracy and stability compared to 800 V. This is attributed to the lower resist shrinkage. Finally, we suggest an important concept that the optimal beam condition cannot be judged only by precision and resolution but also by the resist shrinkage and CD bias stability.

Methodology for establishing CD-SEM robust metrology algorithm for development cycles applications

ArF lithography is still the main technology in the most advanced processes of semiconductor fabrication. Being able to reliably measure and characterize these lithographic processes in-depth is becoming more and more critical. Critical Dimension-Scanning Electron Microscope (CD-SEM) continues to be the work horse tool for both in-line critical dimension (CD) metrology and characterization of ArF photoresist pattern. CD shrink of ArF photoresist has been one of the major challenges for CD-SEM metrology, and it becomes more difficult to measure shrinkage accurately for smaller feature size than ~50nm. The authors have developed a new measurement technique of photoresist shrinkage which measures CD difference between shrunk and non-shrunk sites after etching. There are many imaging and image processing parameters in CD-SEM which need to be optimized to obtain small shrinkage and good precision. There is a trade-off relationship between shrinkage and precision, and a comprehensive and systematic methodology is required for optimization of parameters. The authors have developed an optimization method that uses Taguchi method, where only 18 experiments are required. We can predict shrinkage, precision and relative CD offset for any combination of measurement parameter settings used in the 18 experiments by Taguchi method, and these predicted data can be used for optimization. A new concept of secondary reference metrology is also introduced in this paper to reduce the number of measurement by a reference metrology tool.

Evaluation of total uncertainty in the dimension measurements using critical-dimension measurement scanning electron microscopes

Total uncertainty can be discussed in terms of bias and reproducibility. We have studied bias and reproducibility in the critical dimension measurement scanning electron microscope and measurements have derived expressions of them, and discussed the parameters which determine them by making a comparison with measurement data.

Highly accurate CD measurement with a micro standard

Accurate measurement with CD-SEMs requires the use of a calibrated standard. A new standard, micro-scale was developed using laser interferometer lithography and anisotropic chemical etching on Si-material and was reported previously. In this paper, we report on a method to control measurement accuracy of CD-SEMs using the micro-scale. We have studied various factors for measurement errors and have estimated the 95 percent confidence level. We have carried out 3-pitch measurement of the micro-scale in a fully automated mode and estimated the 95 percent confidence level. Then, we compared two 95 percent confidence levels and concluded that the estimation expected from the measurement errors was reasonable.