Page 38 - Sami Franssila Introduction to Microfabrication

2

Micrometrology and Materials

Characterization

When micrometre lines are patterned and nanometre not enough for thickness determination of, for example,
films are grown, measurement tools have to be available CMOS gate oxides.
to characterize those processes. In addition to seeing Transmission electron microscope (TEM) provides
and measuring those structures, we sometimes have to ultimate image resolution, down to atomic imaging
see details of the structures, and sometimes atomic level (Figure 2.2). High-resolution TEM (HRTEM) has a
analysis is required, for example, to understand thin- special advantage in calibration: lattice spacing of atoms
film nucleation and interface quality. This is possible can be used as accurate internal calibration standards.
but time consuming, and it should not be mixed up with
quick and simple methods that are used in everyday 2.2 LATERAL AND VERTICAL DIMENSIONS
process monitoring.
For device lateral dimensions, 10% deviation is usually
accepted as fabrication tolerance. Measurement preci-
2.1 MICROSCOPY AND VISUALIZATION sion should be 10% of that variation, that is, 10 nm for
1 µm structures. For 100 nm structures, this translates to
Optical microscopy resolution is similar to wavelength, 1 nm, which is very difficult indeed.
that is, in the micrometre range. This is useful in many Linewidth is often known as critical dimension(CD).
applications because we can always include test struc- All major CD measurements rely on scanning: an
tures of any dimensions, irrespective of actual device optical slit or aperture, a laser or electron beam
dimensions. Dark field microscopes have illumination spot or a mechanical stylus is scanned over the line.
from the side, which gives an enhanced detection of Linewidth measurement depends on edge detection
steps and edges that reflect light up, and in confocal in all these methods. This has both inherent and
microscopy, light from focus depth alone is collected microstructure-related limitations. A signal from the
by the optical system. Fluorescence microscopy can be edge is not a delta function even in the case of perfectly
used to see organic residues on the wafer and Nomarski vertical sidewall. Beam spot and mechanical stylus
interference contrast images provide enhanced informa- alike have dimensions that are similar to microstructure
tion about surface-height differences. dimensions and these lead to systematic errors in
Scanning electron microscopy (SEM) has minimum linewidth measurement. Needle radius of curvature
resolution down to 5 nm, which makes it applicable determines the minimum line/space (pitch) that can be
to almost all microfabricated structures. In top view resolved. Both electromechanical stylus systems (known
imaging, SEM is like optical microscope, except for the as surface profilers) and atomic force microscopes
higher resolution. Its real power comes into play in tilted (AFM) can be used, but as can be seen from Figure 2.3,
and cross-sectional views (Figure 2.1). Cross-sectional they seldom provide information about profile. The
images can be used to obtain topographic information former have needle radius of curvature 1 to 10 µm, and
(photoresist sidewall angle, deposition step coverage) the latter 1 to 10 nm.
but at the expense of sample destruction and associated Film thicknesses range from one atomic layer to
increase in analysis time. SEM resolution is, however, hundreds of micrometres, and no single method can

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)