Surface analysis method and a device therefor

Surface analysis method capable of obtaining depth profiles of elements and chemical bonds in a nondestructive manner and with high accuracy, which comprises irradiating light to a sample surface to be analyzed with changing its energy, detecting electrons emitted from the surface of the above sample and corresponding to a certain binding energy, and subjecting the resultant detected signal to integration transform; and constitution of a device for carrying out the method.

BACKGROUND OF THE INVENTION 
This invention relates to a surface and interface analysis technique, and 
in particular, it relates to a surface analysis method suitable for 
nondestructive and high-precision depth profile analysis, and an apparatus 
therefor. 
With increasing packing density and decreasing film thickness of 
semiconductor devices, depth profile analysis (as well as small area 
analysis) has become very important. 
Chemical state of a Si/SiO.sub.2 interface determines the electric 
characteristic of an MOS transistor, and that of a polySi/SiO.sub.2 or 
polySi/Si.sub.3 N.sub.4 interface has an influence on the electric 
characteristic of a capacitor. The electric characteristics of transistors 
and capacitors can be greatly improved by these (abruption in an 
interface, and a distribution and change of elements and chemical bonds in 
the vicinity of an interface), analyzing these chemical states and feeding 
the results back to a production process. Also, in a photo-CDV process, 
the distribution of film-forming metals, such as W, Ti, etc., near the 
surface is very important. 
In the above examples, elements and chemical bonds to be analyzed exist in 
a region with a depth from a sample surface of a few to several tens nm. 
Hence, surface analysis technique necessary for analyzing them is required 
to have capability of analyzing atomic species and their chemical bonds or 
changes in their compositions in a region from the top surface of a sample 
to its several tens nm deep interior part. Also, the surface analysis 
technique is required to achieve a depth resolution of about 0.1 nm in 
case that an interface abruptly changes in its structure or composition 
and so on. It goes without saying that the technique is required to be 
nondestructive. 
Conventional depth profile analysis techniques are as follows. One of the 
well-known techniques is AES (Auger Electron Spectroscopy) or SIMS 
(Secondary Ion Mass Spectroscopy). These techniques carry out depth 
profile analysis by irradiating a sample surface with ions having a large 
kinetic energy for sputtering the surface, and analyzing the surface or 
sputtered particles. The other techniques are EDX (Energy Dispersion X-ray 
Spectroscopy) and PIXE (Particle Induced X-ray Emission), in which depth 
profile analysis is carried out by irradiating the surface with particle 
beams and measuring intensity attenuation caused by absorption of emitted 
X-ray by the sample. 
The above prior techniques have the following problems. 
One of the problems is cascade mixing found in an ions-sputtering method. 
Within a region irradiated with ions (a region from an irradiated surface 
to a 1 to 10 nm deep interior part), due to the above effect, element 
distribution tends to be uniform. Therefore, it is impossible to obtain 
any depth profiles in this region. In addition to this cascade mixing, the 
ion-sputtering method has other various factors to decrease analysis 
accuracy, such as a preferential sputtering caused by a difference in 
atomic species, crater edge effect caused by non-uniformity of ion beams, 
etc. As a result, the depth resolutions in AES and SIMS are limited to 1 
nm [C. W. Magee and R. E. Honig, Surf. Interface Anal. 4, 35 (1982)]. 
Furthermore, AES and SIMS have problems that chemical bond analysis is 
almost impossible and that the analysis is destructive. 
On the other hand, EDX and PIXE are only applicable to depth profile 
analysis of a sample with layered structures (an element distribution 
within the layer is uniform), and their depth resolution is about 5 nm. 
EDX and PIXE therefore have a low resolution, and furthermore, they do not 
make it possible to carry out depth profile analysis of elements whose 
distribution continuously changes. 
As mentioned above, the prior art methods has a low depth resolution, and 
no accurate depth profiles can be measured. Furthermore, the prior art 
methods also have a disadvantage that the chemical bond analysis is 
impossible. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a surface analysis method 
having capability of giving depth profiles of elements and chemical bonds 
in a nondestructive manner and with a high accuracy (depth resolution 
.perspectiveto. 0.1 nm). 
In order to achieve the above object, this invention uses, as a surface 
analysis technique, photoelectron spectroscopy such as XPS (X-ray 
Photoelectron Spectroscopy) and UPS (Ultraviolet Photoelectron 
Spectroscopy). In photoelectron spectroscopy, detailed information of 
atomic species and chemical bonds can be obtained by irradiating a sample 
surface with lights (e.g. soft X rays, vacuum ultraviolet rays, etc.) and 
analyzing energies of electrons emitted from the sample surface. 
By using this photoelectron spectroscopy, emitted electrons which always 
have a constant energy difference from the energy of incident light are 
detected with changing the energy of the incident light onto a sample 
surface. That is, only emitted electrons corresponding to a certain 
binding energy are detected in synchronization with a change in energy of 
the incident light. Then, signals outputted from a detector for said 
detection are subjected to mathematical transform (integration transform) 
to give an intended element depth profile. 
As discussed previously, in XPS and UPS, it is possible to analyze atomic 
species and chemical bonds by irradiating a sample surface with lights and 
analyzing energy of emitted electrons. Furthermore, this analysis method 
is essentially nondestructive since it does not use any process such as 
ion sputtering. 
The following will discuss a process for detecting only emitted electrons 
corresponding to a certain binding energy with changing the energy of 
incident lights, and subjecting detected signals to integration transform. 
In XPS and UPS, when a binding energy is taken as E.sub.B, an output signal 
from a detector after energy analysis carried out synchronizingly with a 
change in energy of incident light as I and an energy of incident light 
onto a sample surface as E, the following relationship exists: 
##EQU1## 
where x denotes the depth measured inwardly from a sample surface, .theta. 
denotes the angle for electron emission, and K, n, .sigma., f and .lambda. 
respectively denote a constant, the atomic density in the sample, the 
photoionization cross section of an element, the intensity of incident 
light and the escape depth of electrons emitted from the element (mean 
free path). In Eq. (I), it is supposed that the horizontal distribution of 
the element is uniform (this method is also effectively usable in a case 
where the horizontal distribution is not uniform, as will be discussed 
later). 
When light having an energy of not less than several tens eV is taken as an 
incident light onto a sample surface, the reciprocal .nu..sup.-1 of a 
photoabsorption coefficient .mu. is larger than 100 nm for most substances 
[B. L. Henke et al., Atomic data and nuclear data tables 27. pp. 1-144 
(1982)]. Meanwhile, if .lambda.(E-E.sub.B) is limited to several tens nm 
or less (by adjusting the energy of incident light), it can be supposed 
that .mu..sup.-1 &gt;&gt;.lambda.(E-E.sub.B). That is, in this case, attenuation 
of incident light within a sample can be neglected, and Eq. (I) can be 
rewritten as 
##EQU2## 
where 
EQU P=P(E, E.sub.B)=1/[.lambda.(E-E.sub.B) cos .theta.]. (3) 
In Eq. (3), it is known that .lambda.(E-E.sub.B) can be approximated as 
##EQU3## 
for most substances, and the constants A and B are determined [M. P. Seah 
and W. A. Dench. Surf. Interface Anal. 1, 2 (1979)]Furthermore, with 
regard to many metal elements such as Si, Au, etc., there are exactly 
measured values of .lambda. as a function of E-E.sub.B [I. Lindau and W. 
E. Spicer. J. Elect. Spectrosc. Relat. Phenon. 3, 409 (1974)]. Then, if 
the energy E of incident light onto a sample surface is given, a value of 
P can be determined on the basis of the reported values and Eqs. (3) and 
(4). 
Meanwhile, the output signal from a detector, I(E) in Eq. (2), is a 
measured value, and the intensity of incident light, f(E), is measurable 
as will be discussed in example. Furthermore, there are detailed data for 
the photoionization cross section .sigma.(E) [J. J. Yeh and I. Lindau, 
Atomic data and nuclear data tables 32. pp. 1-155 (1985)]. Therefore, 
values of G(P) can be determined for various values of P by changing the 
energy E of incident light onto a sample surface. That is, G(P) can be 
obtained as a function of P. 
The integration in Eq. (2) is a type of integration transform of Kn(x) and 
called Laplace transform. The Laplace transform has an inverse transform 
(which will be also referred to as integration transform hereinbelow) 
represented by the following equation. 
##EQU4## 
As is shown in Eq. (5), after numerical differentiation of G(P) is carried 
out many times up to convergence, Kn(x) can be obtained, i.e. a depth 
profile n(x) of the element corresponding certain binding energy E.sub.B 
is obtained (a value for K can be determined by usual photoelectron 
spectroscopy). The depth resolution in this method is about a changing 
interval of .lambda., i.e. about 0.1 nm, as is clear from Eqs. (5) and 
(3). 
As discussed above, the present invention makes it possible to obtain the 
depth profiles of elements and chemical bonds with a resolution of 
.perspectiveto.0.1 nm by changing the energy of incident light onto a 
sample surface, detecting only electrons corresponding a certain binding 
energy in synchronization with the change, and subjecting output signals 
from a detector to integration transform. Furthermore, the method of the 
present invention uses light, and hence it is nondestructive. 
When a distribution of elements, etc., is expected to change in the 
horizontal direction as well as in the lateral direction, the above method 
is also usable with focusing the light. 
The above explanation uses the Laplace transform, but it is of course 
possible to use other integration transforms. Furthermore, to be more 
precise, correction terms, e.g. angular dependence, etc., will be taken 
into account in Eq. (1) to (5). However, these correction terms do not 
have any essential influence on the above discussion (and it should be 
understood that these correction terms can be considered as required).

DESCRIPTION OF PREFERRED EMBODIMENTS 
Embodiments of the present invention will be explained by referring to the 
drawings and the following Examples. 
EXAMPLE 1 
In FIG. 1, sample 2 supported by a sample stage 4 is irradiated with light 
from a monochromator 1. Electrons, which are emitted from the surface of 
the sample 2, are energy-analyzed and detected by a detector 3. The 
monochromator 1 is under the control of a controller 9, whereby wavelength 
of incident light onto the sample 2 can be scanned. On the other hand, the 
detector 3 is under the control of a controller 5 synchronizing with the 
controller 9. As a result, with changing the wavelength of incident light 
onto the sample 2 (i.e. energy), it is possible to detect only emitted 
electrons corresponding to a certain binding energy in synchronization 
with the change in energy of the incident light. A signal outputted from 
the detector 3 is inputted into a processor 6. And a photoionization cross 
section .sigma.(E) of an element is also inputted from an input device 7 
into the processor 6. (In this instance, it is supposed that the intensity 
f(E) of incident light described in the previous section is nearly 
constant in the entire range of wavelength scanned. A case where f(E) is 
changed will be discussed in Example 2.) On the basis of the above data, 
integration transform is carried out in the processor 6, and the resultant 
processed data, i.e. n(x), is outputted to an output device 8. The 
processor 6 may be one whose hardware is devised to be suitable for 
integration transform, or a high speed computer under the control of a 
software. 
According to this embodiment, it is possible to measure depth profiles of 
certain elements and chemical bonds in a nondestructive manner and with a 
high accuracy (high depth resolution) by changing the wavelength of the 
incident light onto the sample 2, simultaneously detecting only electrons 
corresponding to a certain binding energy in synchronization with the 
change and subjecting the detected signals to integration transform. 
EXAMPLE 2 
In Example 1, it has been supposed that f(E) is nearly constant in the 
entire range of wavelength. This Example 2 describes a case where f(E) is 
changed in the range of wavelength scanned. 
FIG. 2 shows a constitution of a device therefor. The different point of 
the device in this Example 2 from that of Example 1 is that a monitor 10 
for measuring the intensity of incident light is provided between a 
monochromator 1 and a sample 2. Light from the monochromator 1 is 
transmitted through the monitor 10 and then to the sample 2. A light 
intensity signal from the monitor 10 (e.g. an electric current signal by 
secondary electron emission resulting from light irradiation) is inputted 
into a processor 11. In the processor 11, changes in intensity of incident 
light onto the sample 2 and photoionization cross section of the elements 
are taken into account in integration transform according to the 
processing step discussed in the previous section. The resultant processed 
data is outputted into an output device 8. 
According to this embodiment, it is possible to determine an exact depth 
profile even if the intensity of incident light onto the sample 2 is 
changed, since the change is corrected. 
EXAMPLE 3 
In the foregoing Examples, it has been supposed that the element 
distribution in the horizontal direction within the sample 2 is uniform. 
However, some samples have a non-uniform element distribution in the 
horizontal direction. In such a case, in order to determine the depth 
profile of the element, it is necessary to focus an incident light within 
a small area on the sample surface. 
FIG. 3 shows an embodiment for such a case. Light from a monochromator 1 is 
focused, with an optical system 12, within a small area on the surface of 
a sample 2. Electrons emitted from the surface by light irradiation are 
energy-analyzed and detected by a detector 3. The other portions are the 
same as those in Example 1. 
In this case, the optical system 12 is desirably a combination of mirrors 
using total reflection. XPS and UPS use the light ranging from soft X-ray 
to vacuum ultraviolet regions. The lights in the above region can be 
focused by using a system of transmission.multidot.diffraction or mirror 
optics. In the mirror optical system, optical characteristics such as 
focal distance do not change even if the wavelength is changed. Therefore, 
mirror optical systems are suitable for the present invention. The system 
of transmission.multidot.diffraction optics is also usable if a 
displacement mechanism for the sample 2, corresponding to changes in focal 
distance, and a fine adjustment mechanism for the optical system 12 are 
provided, although these mechanisms are not shown in FIG. 3. 
According to this embodiment, an accurate depth profiles of elements can be 
obtained even if the sample has element distributions in the horizontal 
direction, since light is focused within small area. 
EXAMPLE 4 
FIG. 4 shows one embodiment for a device to obtain three-dimensional 
distributions of elements and chemical bonds. The largest different point 
of the device of this Example from that of Example 3 is that a controller 
13 permits fine displacement of a sample stage 4. A signal on the 
displacement of a sample is inputted from the controller 13 to a processor 
14. 
In this embodiment, a depth profile can be obtained by a change in 
wavelength of incident light and integration transform, and a distribution 
in the horizontal direction by fine displacement of a sample. These two 
obtained data are combined to give a three-dimensional distribution of an 
element. 
EXAMPLE 5 
In Example 4, a distribution in the horizontal direction is obtained by 
fine displacement of the sample stage 4 (i.e. a sample). However, a 
distribution in the horizontal direction can be also obtained by some 
other methods. FIG. 5 shows one embodiment therefor. 
In FIG. 5, light is reflected on a reflection mirror 15. In this case, 
scanning the light beams on the surface of a sample 2 is possible by 
changing the angle and position of a control stage 16, on which the 
reflection mirror 15 is placed, by a controller 17 with a high accuracy. A 
signal on the changes of angle and position of the control stage 16 is 
inputted from the controller 17 to a processor 14 as a signal of scanning 
the light beam. 
According to this embodiment, it is possible to obtain the same 
three-dimensional distribution as that of Example 4 since scanning the 
light beam is carried out with the reflection mirror 15. 
The essence of Examples 4 and 5 is a change of the light irradiated 
position on the surface of the sample 2. The light irradiated position can 
be also changed by some other methods than those shown in these Examples. 
The present invention also includes a method to obtain a three-dimentional 
distribution by using such other methods. 
In the foregoing Examples, adjustment of the incident angle of the light 
onto a sample and the angle of electron detection (an angle between the 
axis of the detector 3 and the surface normal of a sample) makes it 
possible to carry out the analysis with a higher depth resolution. In 
order to make clear the essence of the present invention, however, 
mechanisms for the adjustments of these angles have not shown in FIGS. 1 
to 5. It should therefore be understood that these angle adjustment 
mechanisms can be supplied as required. 
The light source for the monochromator 1 is required to have a wide 
wavelength range. For example, synchrotron radiation and bremsstrahlung in 
a X-ray tube can be considered as such. 
As discussed in detail above, in the present invention, while the energy of 
incident light onto a sample surface is changed, only electrons 
corresponding to a certain binding energy are detected in synchronization 
with the change, and the detected signals are subjected to integration 
transform. It is therefore possible to obtain depth profiles of elements 
and chemical bonds in a non-destructive manner and with a high accuracy. 
Furthermore, when incident light is focused on the sample surface and 
allowed to be scanned it is possible to obtain three-dimensional 
distributions of elements and chemical bonds.