Microscopic Imaging Ellipsometry Gives Resolution, Throughput
Alexander E. Braun, Senior Editor -- Semiconductor International, 6/1/2000
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However, interpreting results can be complicated. This is because the conventional approach averages the signal over the light beam's entire diameter, averaging into a single number all the film structure's spatial information – including defects, scratches, dust, etc. Conventional ellipsometers also require homogeneous films, without lateral structures or defects within the beam diameter.
A start-up company, Nanofilm Technologie (Göttingen, Germany), has a real-time microscopic ellipsometry system, which shows a sample's true structure, providing immediate access to qualitative information, plus the capability to focus on a point of interest to perform a quantitative analysis. It works like a microscope that uses ellipsometry as contrast enhancement to make ultrathin films visible. It not only characterizes the film in numbers but displays an image of the sample's structure and quality, making it useful for inspecting patterned and unpatterned wafers.
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It uses a laser-based system, which in conventional ellipsometers has a spot size of 0.5 to 1 mm, and on a spectroscopic ellipsometer has a spot size of 1 to 5 mm. Microspot options are available, essentially focusing lenses on the beam and detection sides, which provide a spot size in the range of 10 to 50 µm in laser-based systems and about 100 µm or more in spectroscopic systems. Aspect ratios of up to 4:1 enlarge the spot in the x axis, making it larger in the beam's plane of incidence. While this increases lateral resolution, if multiple measurements are required the sample must be scanned using a large scanning x-y stage; this means moving and precisely realigning the sample, affecting throughput.
Microscopic imaging ellipsometry's unfocused parallel beam eliminates angle of incidence concerns. The beam achieves resolution through an imaging system on the detection side consisting of a camera with a CCD resolving detector, and optics that are basically a high-quality microscope objective to achieve an NA-determined resolution. Currently, the system has a 1 to 2 µm resolution. Traditional ellipsometric methods, such as rotating the sample, using a nulling system or using polarization modulation also can be used.
The system looks at a sample at its highest resolution, and a cursor is set to fit into a test area to do measurements. The objective images the illuminated area, with the camera positioned in the path of a directly reflected beam that goes along the detection system's axis. If film thickness differs locally, with the nulling mode the signal on one area of the sample can be minimized while simultaneously the un-nulled film-covered area provides a strong signal. The nulling method allows the use of high incident power and an increase in camera gain, making the system sensitive to ultrathin films. Film structures 1 nm thick have been imaged. Unlike with a rotating analyzer or polarizer, no Fourier transforms are necessary.
The approach enables the parallel processing of several points simultaneously, providing a micro-dimensional map of ellipsometric parameters in the x and y axes. This is useful to determine thickness distribution. The result is more data and a higher throughput, compared with conventional systems requiring sample scanning.
A version that uses a spectroscopic imaging ellipsometer is under development; it is expected to provide analysis of more complex multilayer film structures with a much higher lateral resolution. •
