1063-7397/03/3203- $25.00 © 2003 MAIK “Nauka /Interperiodica”
Russian Microelectronics, Vol. 32, No. 3, 2003, pp. 165–171. Translated from Mikroelektronika, Vol. 32, No. 3, 2003, pp. 210–218.
Original Russian Text Copyright © 2003 by S. Sen’ko, A. Sen’ko, Zelenin, Puglachenko.
With the general tendency toward higher circuit
complexity and smaller feature sizes, increasingly
stringent requirements are placed on wafer surface
quality. For this reason, surface inspection is an active
area of research in electronic materials technology.
Recent electronic technology has made increasing
use of optical topography for the inspection of mirror-
like surfaces . This approach originated from the
schlieren and the magic-mirror method. It consists in
illuminating the surface with a collimated beam from a
point source and viewing a grayscale image on a special
screen. Surface defects are recognized by characteristic
brightness patterns on the screen. Like x-ray topogra-
phy, the approach is nondestructive and rapid; there-
fore, it can be useful for total inspection in industrial-
scale production. Other advantages are simplicity,
applicability to any type of wafer-topography defect,
and high resolution. Speciﬁcally, the vertical resolution
was estimated at 6 nm for defects of lateral size about
1 mm .
By direct examination of an optical topograph, one
can identify surface defects and determine their total
number, total area, and lateral sizes. Nevertheless, these
data do not allow one to accept or reject the wafer. To
make a decision, it is more important to know the ver-
tical sizes of the defects. These can be calculated from
measured levels of brightness on the image.
This work addresses major stages of the optical
characterization of topographical defects on mirror-like
wafer surfaces, including the generation and computer-
aided analysis of topographs. Methods for the calcula-
tion of defect dimensions are presented. An image-
analysis software system developed in this study is
described in outline.
Figure 1 gives a sketch of an experimental apparatus
producing optical topographs. The apparatus includes a
point light source with a power-supply and control unit,
a wafer holder, and a screen. A grayscale image of the
wafer surface is produced on the screen by reﬂection of
light from the surface; the dimensions of the image are
proportional to those of the wafer and depend on the
distances from the wafer to the source and the screen.
Any uneven area on the wafer perturbs the reﬂection
angle, so that a characteristic brightness pattern appears
at the corresponding site on the screen. The lateral and
the vertical size of a defect determine the size and the
brightness of the pattern, respectively.
In this study, we assume that any defect is either a
pit or a hillock, since any real defect is a combination
of these elemental types with different shapes and sizes.
A pit focuses light and therefore shows up as a lighter
spot whose shape and size are generally related to those
of the pit in plan. A hillock defocuses light, so that a
darker spot appears on the screen. If a pit or a hillock is
large enough in the vertical direction, it can be recog-
nized with the naked eye in luminescent light. Since
any pit has a convex edge and any hillock has a concave
one, their images are marked by a darker or a brighter
Figure 2 shows example optical topographs of sili-
con wafers: a wafer with surface defects resulting from
the cutting of the boule, as well as low hillocks and
Quantitative Optical Inspection of Mirror-Like Wafer Surfaces
S. F. Sen’ko, A. S. Sen’ko, V. A. Zelenin, and E. G. Puglachenko
Institute of Physics and Technology, National Academy of Sciences of Belarus, Belarus
Received September 9, 2002
—A conceptual framework for the quantitative optical inspection of mirror-like wafer surfaces is pre-
sented. Methods for the computer-aided analysis of surface defects on the basis of optical topographs are
described. Defect imaging by optical topography is addressed. Relations between geometric parameters of an
individual defect and the brightness of its image on the screen are established. Some measures of wafer topo-
graphical quality are proposed. The concepts are illustrated in the context of silicon wafers.
Sketch of an experimental apparatus producing opti-