Makoto Ezumi

Improved CD-SEM optics with retarding and boosting electric fields

Because of rapidly decreasing line-width of integrated circuits, it is necessary to measure and control their critical dimensions with high accuracy. Hitachi has developed a new critical-dimension-measurement scanning electron microscope S-9000 series, which has a new electron optics with retarding and boosting electric fields. The upper pole piece of the objective lens is kept at a high positive voltage with respect to the ground so as to reduce aberration of the objective lens drastically. To optimize the boosting voltage we have developed optics simulators that is capable of computing aberration coefficients in electric and magnetic mixed fields. At the optimized boosting voltage of around 5kV, 3nm resolution is achieved for a final accelerating voltage of 800V. The high boosting voltage is effective in imaging bottoms of contact holes having high aspect ratios.

Long-term critical dimension measurement performance for a new mask CD-SEM, S-9380M

To realize good repeatability in CD measurements, many issues have to be addressed including system stability, sample charging and contamination, measurement conditions, and image processing. The S-9380M is a mask CD-SEM (Critical Dimension Scanning Electron Microscope) system developed for measurement and inspection of 45 nm node photomask. The S-9380M has several innovations like a newly designed optical system to minimize sample charge-up and drift effects, ultra-high resolution (3nm) imaging to enable measurements at high magnification, and an integrated ultra-violet (UV) unit for pre-treatment of the mask to rid the surface of organic contaminants. This paper presents measurements carried out using the S-9380M system, and showed that superior performance is achieved with short-term dynamic repeatability of 3σ less than 0.6 nm (for line patterns), and long-term dynamic repeatability of 3σ less than 1.0 nm without trend modification.

Correction method for high-precision CD measurements on electrostatically charged wafers

A correction method for automatic, high-precision CD-measurements on electrostatically charged wafers has been developed and installed in the Hitachi CD-SEM S-9260 to evaluate its performance. There are two types of charging: global and local. Global charging is stable and spreads all over a wafer while the local charging area is limited within the beam scanning area. A conventional CD-SEM has two weak points with respect to those charged wafers: one is failure at the positioning and autofocusing procedure which interferes with the fully automatic measurement sequence, and the other is disturbance of optical magnification which degrades the precision of CD-measurement values. By probing the global charging voltage with an electrostatic voltmeter prior to the CD-measurements, we subtract the voltage from a retarding voltage and then apply it to the wafer holder. The beam-focusing condition can stay within the fully automatically tunable range. And by generating numerical functions to represent the relationship between the global charging voltage, wafer height, excitation current of the objective lens and optical magnification, with the help of electron optical simulations, we can calculate the true optical magnification and the correct CD-measurement values. The local charging voltage is derived from the voltage shift of S-curves of secondary electron yield between conductive and insulated wafers measured with an energy filter. We correct the CD-measurement values using the simulated proportional relationship between magnifications of the electrostatic micro-lens and the local charging voltage. The coefficient is almost constant when the charging area is smaller than an equivalent circle of 100mm radius. We demonstrate that the CD-measurement values are successfully corrected within 0.1 percent in deviation for both charging types.

Nanometer-level metrology with a low-voltage CD SEM

This paper describes the application of a low-voltage scanning electron microscope (SEM) with nanometer-level accuracy for measurement in ultra-large-scale integration (ULSI). Minimum feature sizes of integrated circuits are expected to reach the 100-nm level and below (the nanometer region) in the near future. For the lithography process under that regime, precise critical dimension (CD) control and high resolution of resist patterns will be quite important for device fabrication, because variations in pattern sizes will degrade circuit performance. Therefore, metrology with nanometer-level accuracy is required for device fabrication under the regime. Here, we report on a CD-SEM that operates at 500 V to measure patterns at the 1 Gbit level. We used the S-8840 (Hitachi) to measure holes, lines/spaces, and the calibration standard (Micro-Scale). Several voltages from 500 V to 1000 V were used for the measurements. Static variation of less than 3 nm (3(sigma) ) was obtained in the pitch measurement of the Micro- Scale regardless of the acceleration voltages. For the holes, a lower voltage provided higher accuracy in static measurements. In the nanometer region, resist-pattern sizes microscopically fluctuate to the level of 10 nm due to the polymer characteristics of the resists (nano edge roughness). We could also characterize resist-pattern fluctuations with high accuracy. We compared our measurements with those from an atomic force microscope (AFM) for nanometer-level metrology, and conclude that at present CD-SEMs are more advantageous because of their higher accuracy and throughput.

Damage-free metrology of porous low-k dielectrics using CD-SEM

Copper damascene process and interlayer dielectrics with ultra-low permittivity have been introduced for manufacturing future devices with higher function speed. As materials with permittivity values lower than 2.2 are required, several kinds of porous materials have been proposed as candidates. However, these porous materials have been observed to shrink easily during CD (critical dimension) measurements with a CD-SEM. To solve this problem, the mechanism of shrinkage and the solution for damage-free SEM observation condition was studied. The shrinkage caused by different electron beam irradiation conditions in a CD-SEM (S-9260, Hitachi High-Technologies Corporation) was investigated with an atomic force microscope (AFM). The result shows that the shrinkage depends on the energy and the dose of electron irradiation. In addition, the change of chemical states and composition caused by electron beam irradiation was analyzed and the shrinkage mechanism was studied. The optimum electron beam irradiation conditions for damage-free measurement are proposed based on experimental results.

High resolution CD-SEM system

Because of rapidly decreasing line-width of integrated circuits, it is necessary to measure and control their critical dimensions with high accuracy. We have developed a new critical-dimension-measurement scanning electron microscope (CD-SEM) S-9000 series, which has a new electron optics with retarding and boosting electric fields. To optimize the boosting voltage we have developed optics simulators that are capable of computing aberration coefficients and secondary electron detection efficieny in electric and magnetic mixed fields. At the optimized boosting voltage of around 5 kV for a final accelerating voltage of 800 V, not only 3 nm resolution but also highlighted bottom imaging of high aspect ratio contact holes is obtained.