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DUV Inspection May Be Vital to Next Generation of Microchips

Photonics Spectra
Aug 1997
Lawrence Normie

YAVNE, Israel -- Scientists familiar with the semiconductor industry say deep-UV inspection systems will be an important part of next-generation microchips as feature sizes slip below the 0.25-µm mark by the turn of the century.
About every three years, a major technology shift occurs within the microelectronics industry, driven by the evolution of direct random access memory (DRAM) chips. Each advance, however, brings its own challenges. As manufacturers produce chips with larger memories and smaller features, visible inspection systems fall behind.

Meeting the challenge
During a circuit's construction, etching systems copy the circuit onto the wafer using a reticle (mask) as a master blueprint. Therefore, it is critically important for manufacturers to have defect-free reticles on their production lines.
Basically, two methods are available for improving the resolution and depth of focus of optics-based inspection systems: increasing the numerical aperture of the optical system and reducing the inspection wavelength below the feature size.
Increasing the numerical aperture of the objective lens reduces its maximum optical working distance as well as its depth of focus. A relatively large working distance is an important requirement for the reticle inspection system because most reticles have a pellicle frame that requires at least 7 mm between the reticle and the wafer. This distance can be accommodated in higher-aperture systems by increasing the diameter of the optical components. However, this approach is very expensive.
A more practical solution for improving resolution is to reduce the inspection wavelength. In addition to increasing resolution, it has a significant advantage: Reticle inspection is performed at the printing wavelength of the microlithography tool, or "stepper."
Orbot Instruments Ltd. plans to be the first to market commercial inspection systems based on the utilization of deep-ultraviolet light. Orbot's reticle inspection system uses an X-Y laser scanner with a laser interferometer positioning feedback sensor, a custom-designed optical microscope system and a transmitted light-illumination system. A charge-coupled device camera constantly images the reticle during inspection. The reticle image is compared with stored design data by means of a suite of sophisticated pattern recognition algorithms that operate in parallel. Differences between the image and the design data are regarded as defects.
Although ultrahigh-resolution x-ray, electron-beam and ion-beam techniques are available for reticle inspection, these alternatives are cost-prohibitive for many chip manufacturers. DUV optical systems, in particular, promise to provide the necessary price and performance in submicron feature resolution for reticle inspection, according to Orbot officials.
Currently, the two wavelengths of choice for steppers are the G-line (436 nm) and i-line (365 nm). To increase printing resolution for the manufacture of 256-MB and 1-GB DRAMs, for example, chip manufacturers use DUV steppers that operate at 258 nm. Industry experts expect that next-generation systems will use 193-nm wavelength steppers.

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