Electron spectroscopy employing controlled surface charging

Details Electron spectroscopy employing controlled surface charging Electron spectroscopy employing controlled surface charging

2001-05-03

2004-04-13

Anderson, Bruce (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

C250S307000, C250S305000, C250S310000, C250S492200, C250S492300

Reexamination Certificate

active

06720556

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the use of electron spectroscopy as a depth profiling or lateral differentiation probe useful, for example, for characterization of optoelectronic devices and other applications of mesoscopic systems, i.e. between the macroscopic solid and the atomic scale.
2. Prior Art
Progress in the development of new microelectronic technologies, leading to smaller, faster and smarter electronic and optoelectronic devices, depends on the ability to construct sophisticated interfacial structures which are well-resolved on the nanometer scale. One of the most acute problems in the development, study and application of such structures is finding characterization techniques suitable for such systems, namely having appropriate resolution in both the lateral and depth directions, the lateral direction being parallel to the surface of a sample at which such structure is disposed and the depth direction being perpendicular to the lateral direction. This is naturally of primary importance in the development stage, but also in the production stage, i.e., for on-line quality control. The continuing decrease in component dimensions, already approaching the nanometer range, poses formidable requirements on characterization methods.
X-ray photoelectron spectroscopy (XPS) is a powerful surface analytical tool, providing superior information on the chemical composition of surfaces and interfacial layers. The technique is based on illumination of the surface with X-rays and analysis of the photoelectrons ejected from the surface, thereby determining the identity and chemical state of atoms located on the surface and up to ca. 10 nm deep. In contrast to its nanometer-scale depth sensitivity, in the lateral direction XPS is essentially a macroscopic technique. Several XPS-based depth profiling methods exist, most prominently those based on ion etching or on analysis of line intensities at different detector angles (angle-resolved XPS). Both suffer from various drawbacks, including induced sample damage, distortions associated with non-planar surfaces, and others. As noted above, in the lateral direction, only macroscopic information is commonly obtained.
In addition, the quest to achieve well-defined features which are resolved on the nanometer length scale and distributed in predetermined patterns on solid surfaces is the heart of future optoelectronic devices and a major goal in science and technology. Structural analysis of such systems usually requires scanning probe methodologies, which are essentially small-area techniques. Large-area analytical tools, such as XPS, are limited in this respect. A fundamental feature of XPS is the contrast between its depth resolution and lateral resolution, typically approaching nm vs. &mgr;m length scales, respectively. This raises serious problems in the study of non-planar or heterogeneous surfaces, it especially when surface variations fall in the region between the two length scales. For various applications, however, this intermediate region is the relevant one; hence, new high resolution—large area characterization methods are crucially needed.
Electrical charging of the sample surface is commonly considered an obstacle to accurate experimental determination of binding energies in XPS measurements of poorly conducting surfaces
6, 19, 20
To compensate the extra positive charge that is a natural consequence of photoelectron emission and stabilize the energy scale on a reasonably correct value, an electron flood gun is routinely used. Such gun creates a generally uniform potential across the studied volume. This, however, is often impossible with structures comprising components which differ in electrical conductivity
9, 21-23
In such cases, chemical information may be smeared due to XPS line shifts that would follow local potential variations. On the other hand, this very effect can be used to gain structural information
10-12
. Several studies
9, 24-26
have focused on various aspects of charging in XPS, indicating that, on a macroscopic scale, differential surface potential can be analyzed using a classical approach based on charge generation vs. discharge rates.
8, 27
Application in surface analysis has been demonstrated.
8, 10, 11,-27-29
Thin layered structures a few nanometers in thickness impose demanding requirements on the depth sensitivity of analytical methods. X-ray photoelectron spectroscopy (XPS)
1
, an effective surface analytical tool providing superior chemical information, offers depth sensitivity inherently appropriate for such nanostructures. However, translation of the XPS integral line intensity into high resolution depth information is not straightforward. The commonly used XPS depth profiling methods, i.e., ion etching
2
, angular-resolved XPS (ARXPS)
2
, and Tougaard's approach
3
, are effective but limited in various aspects
2-5
. Ion etching
2
is inherently destructive and limited in application particularly with soft matter. ARXPS
2
, considered nondestructive, is hampered when applied to non-planar morphologies, it requires a large number of measurements (which may induce damage
5
), and is strongly model dependent
4
. Tougaard's approach
3
(quantitative analysis of signal-to-background correlation) requires minimal interference of neighboring lines across a wide spectral range, and is therefore less effective with small signals.
BRIEF SUMMARY OF THE INVENTION
The present invention provides improvements in the observation of such structures by the use of selected controlled surface charging (CSC) in conjunction with electron spectroscopy. CSC is basically non-destructive, allowing fast and convenient data collection. It enables differentiation of spectrally identical atoms at different locations. It is applicable to thicker structures than ARXPS, as the signal is FE not subject to increased attenuation associated with off-normal measurements. The method according to the invention offers several advantages over existing depth profiling techniques and is of general applicability to a large variety of mesoscopic heterostructures. Substrate roughness has only a minor effect on CSC depth profiling. The linear dependence on depth, which occurs in systems that have been studied, is an attractive feature of CSC. However, the invention also offers advantages for the study of other systems that may present more elaborate conduction processes, possibly causing deviations from linearity. Such deviations may contribute to the exploration of additional characteristics of the systems, such as charge distribution, conduction mechanisms, etc.
One embodiment of the invention is a method of examining a sample, comprising: performing a first spectroscopic analysis of a surface portion of the sample when the sample surface portion is in a first electrical charge state; placing the sample surface portion in a second electrical charge state that is different from the first electrical charge state and performing a second spectroscopic analysis of the surface portion of the sample when the sample surface portion is in the second electrical charge state; and comparing the first spectroscopic analysis result with the second spectroscopic analysis result to obtain at least one of structural and electrical information about the sample, wherein the first and second electrical charge states are given values that enable information to be obtained about sample structural features having dimensions of less than 50 nm.
A second embodiment of the invention is a method of examining a sample, comprising: performing a first spectroscopic analysis of a surface portion of the sample when the sample surface portion is in a first electrical charge state; placing the sample surface portion in a second electrical charge state that is different from the first electrical charge state and performing a second spectroscopic analysis of the surface portion of the sample when the sample surface portion is in the second electrical charge state; and comparing the first spectroscopic analy

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