A U.S. research team has demonstrated a reflection microscope with a tabletop-sized laser for illumination.
Courtney Brewer (left) and Fernando Brizuela examine images obtained with the EUVL microscope. The microscope chamber can be seen in the back.
Semiconductor manufacturers envision using extreme ultraviolet lithography (EUVL) to achieve the next level of integrated-circuit miniaturization. For the necessary EUVL mask inspection, a U.S. research team has demonstrated a reflection microscope with a tabletop-sized laser for illumination (Opt. Lett. 34, 271).
This new technology, which employs 13.4-nm-wavelength illumination and all-reflective optics in the projection systems, will require a suite of new metrology tools to assess performance of the lithographic process. Critical to EUVL is the capability to detect and characterize defects on the reflective extreme ultraviolet masks that will be used for printing. Such examination uses light near 13.4 nm to find defects that lie on the surface of the mask or are buried inside the reflective Mo/Si multilayer coating.
However, there are only a couple of synchrotron sources for such soft X-rays, one in the United States and one in Japan. Fabricators want to have on-site microscopes to inspect masks before using them to manufacture circuits.
Colorado State University (CSU) graduate student Fernando Brizuela, OSA Fellow Carmen Menoni and their colleagues built the 13.2-nm-wavelength microscope at the National Science Foundation’s Engineering Research Center for Extreme Ultraviolet Science and Technology (Fort Collins, Colo., U.S.A.). The microscope uses as illumination the directed beam from a plasma-based Ni-like Cd laser developed at CSU by a team led by Jorge Rocca.
The EUV laser generates a pencil-like beam of 13.2-nm-wavelength light through ablation of a cadmium target with an intense visible laser that creates a plasma hot enough for lasing to occur at such short wavelengths (OPN, November 2006, p. 30). The microscope uses innovative condenser and objective zone plates, developed at Lawrence Berkeley National Laboratory’s Center for X-Ray Optics, that focus the light output from the EUV laser onto the mask and form the image of the illuminated mask on an array detector.
Both optical elements are small: The condenser is 5 mm in diameter, and the objective zone plate has a pupil diameter of 120 µm. The EUVL mask pattern features are on the order of 22 nm. Zone plates have the advantage of creating less aberration than multilayer coated mirrors, Brizuela said.
The microscope can acquire images with a field of view of 15 x 15 µm2 in 20-s exposures, and the instrument’s spatial resolution is 55 nm. What is unique to this microscope is that it provides the same illumination conditions that the EUVL masks find in a lithographic system and thus images defects as they would print on the wafer.
Next, the EUVL microscopy team will work on increasing the uniformity of the illumination and boosting the imaging system’s throughput to cut exposure times further. The researchers also want to study defects that are buried within the coatings of the masks.