Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
1999-04-02
2002-03-12
Evans, F L. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
C356S237200, C250S225000
Reexamination Certificate
active
06356347
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface inspection device for inspecting the rough surface of a sample, and more particularly to a surface inspection device that inspects the microscopic surface contamination of a sample such as an IC (Integrated Circuit) chip or a processed Silicon wafer or a contact device.
2. Description of the Related Art
In a conventional fabrication process of a semiconductor device, wiring or bonding metal pads have been formed as fine patterns on the surface of a semiconductor chip, and the metal pads are connected to bonding wires or the connection terminals of chip components. Masking materials are applied to the surface of metal pads in the fabrication process, and because this material may hinder electrical connections, cleaning is carried out to remove all traces of the material.
Still, the microscopically rough surface of metal pads complicates the total cleaning of minute contamination from the surface. When the surface of metal pads are contaminated during the fabrication process, this contamination impedes electrical connections and causes insufficient bonding. It is therefore vital in fabricating semiconductor devices to inspect and analyze contamination on the surface of metal pad, investigate the sources of contamination, and then take measures to improve the yield rate of the semiconductor devices.
However, the analysis of minute contamination on the surface of metal pads are made difficult by the microscopically rough surface, as well as their small dimensions. Satisfactory analysis is particularly difficult in cases in which the contaminant is a mixture of organic and inorganic substances.
At present, surface inspection on the nano-scale is presently being carried out using STM (Scanning Tunneling Microscope) or AFM (Atomic Force Microscope) as the basic research in surface inspection of fine structure as described above. Unfortunately, this approach is impractical because it entails considerable time and expense.
Surface inspection is also carried out by a FTIR (Fourier Transform Infrared) spectroscope, which employs the absorption of infrared rays. However, microanalysis by this method is difficult and operation is also complex , and the considerable time and expense thus required for analysis render this method impractical.
One surface inspection method that solves the above-described problems involves analysis by polarized light (ellipsometry), in which surface inspection is realized by irradiating rays onto the surface of a sample to cause reflection and then analyzing the polarized light components of the reflected rays. This method allows microanalysis and simplifies operation.
Ellipsometry allows easy inspection of the surface of a sample but presupposes that the surface of the sample to be inspected is a mirror surface. If the sample is an integrated circuit during fabrication, however, fine metal pads are formed on the mirror surface of a semiconductor wafer by means of sputtering or electroplating, and the surface of the sample becames therefore microscopically rough.
If ellipsometry of the prior art is applied to the surface of this type of sample, accurate inspection is complicated by the diffused reflection of the rays caused by the microscopic bumps and depressions of the surface of the sample. In other words, the surface contamination of the sample, i.e., an integrated circuit during fabrication, cannot be inspected, and the method cannot provide an improvement in the yield rate of integrated circuits.
In addition, defects that occur in the surface of an integrated circuit during fabrication include contamination by inorganic substances, contamination by organic substances, contamination by mixtures of organic and inorganic substances, and the adhesion of extraneous matter. Inspection that can distinguish between these various defects was difficult in the ellipsometry of the prior art.
SUMMARY OF THE INVENTION
It is an object according to the present invention to provide a surface inspection method and device that allow satisfactory inspection and analysis of the surface of a sample such as a semiconductor wafer on which metal pads have been formed without requiring considerable time and expense.
According to one surface inspection method according to the present invention, two-dimensional scanning is effected by irradiating a focused laser beam onto the surface of a sample and then individually detecting the intensities of each of the s-polarized light component and p-polarized light component of the laser beam reflected by each location of the two-dimensionally scanned surface of the sample. The RR (Reflectance Ratio), which is the ratio of the reflected intensities of the detected s-polarized light component and p-polarized light component, is observed for each location of the sample surface, and the distribution of the observed RR on the sample surface is measured. This measured RR distribution width is then compared with the natural width of a clean sample, and the sample surface is determined to be contaminated when, as the comparison results, the RR distribution width diverges from the natural width. In the surface inspection method of this invention, the presence or absence of contamination on the microscopically rough surface of the sample can therefore be accurately and easily determined based on the RR of the reflected intensities of s- and p-polarized light.
The basic principles of the above-described invention are explained hereinbelow. First, in a case in which the sample is the metal pad of a mass-produced circuit component, the sample surface is not microscopically smooth, and the reflection of a laser beam irradiating this surface is generally diffuse. The reflected intensities R., and R
0
p of the s- and p-polarized light can be approximately assumed in this invention as shown below:
R
os
=R
ou
×R
s
(1a)
R
op
=R
ou
×R
p
(1b)
where R
ou
is a specular reflective power on a rough surface, and R
s
and R
p
are the amplitude reflectances of s- and p- polarized light on an ideally smooth surface observed by means of a Drude reflection equation or a Fresnel reflection equation.
If the sample surface is unevenly contaminated, reflectances R
s
and R
p
of s- and p- polarized light are subject to complex modification. The ratio of reflective intensities R
os
and R
op
by the above-described equations (1a) and (1b) becomes the ratio RR of reflective intensities of s- and p-polarized light as shown in the following equation (2):
RR=R
os
/R
op
=(R
ou
×R
s
)/(R
ou
×R
p
)=R
s
/R
p
(2)
This ratio RR is independent of the roughness of the sample surface and the characteristic of the device cancel each other out. However, the s-polarized light and p-polarized light differ from each other in their interaction with the physical surface of the electric vector of light, and the proportion of change in reflective intensity of the s-polarized and p-polarized light due to contaminants is therefore not identical.
In this invention, the state of the sample surface is analyzed using the above-described equation (2) because the use of RR allows the state of contamination to be detected while excluding the effect of the roughness of the sample surface.
The numerical value of the ratio Rs/Rp for the case of a clean sample surface is calculated as a theoretical value from the dielectric constant of the material or the angle of incidence of the beam using a Fresnel reflection equation. If, for example, the angle of incidence is 60°, the theoretical value of RR for gold is 1.09 and 1.95 for rhodium. The state of contamination of the sample surface can be determined by comparison with actually measured values for ratio RR.
The natural width of a clean sample referred to in this invention indicates the distribution width for a case in which RR is determined by detecting the reflective intensities of the s-polarized light component and p-polarized light component for a clean sample surf
Okubo Akiko
Watanabe Masao
Advantest Corporation
Evans F L.
Knobbe Martens Olson & Bear LLP
Smith Zandra V.
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