Light element measurement

A spectrometer for detecting and quantifying elements in a sample. An exciter ionizes atoms in the sample, and the atoms thereby produce characteristic x-rays. A detector receives the x-rays and produces signals based on the x-rays. A filter system selectively blocks the x-rays from attaining the detector. The selective blocking of the x-rays is accomplished based on an energy of the x-rays. An analyzer receives the signals from the detector and detects and quantifies the elements in the sample based at least in part on the signals. In this manner, detector receives the light element x-rays, and the medium and heavy element x-rays are filtered out to avoid overwhelming the detector. This invention combines the large solid angle, high efficiency, and ability to measure the continuous background spectrum of the energy dispersive x-ray detector with the selectivity of the wavelength dispersive x-ray detector. It thus enables faster and more accurate measurement of light elements in thin films. This invention enhances the light element performance of a system by enabling higher throughput, lower e-beam and x-ray dose to the sample, and improved accuracy from the capability to measure the background radiation.

FIELD

This invention relates to the field of instrumentation for materials analysis. More particularly, this invention relates to measuring light molecular weight element concentrations in thin films using electron microprobe or x-ray fluorescence techniques.

BACKGROUND

Materials analysis is an important technology for many industries, including the integrated circuit fabrication industry, where the ability to confirm the stoichiometry of the various layers that are formed during the fabrication of an integrated circuit is essential. Analysis techniques generally distinguish elements based upon unique properties of the elements, such as molecular weight. For a variety of reasons, relatively lighter molecular weight elements, otherwise referred to herein as light elements, tend to be somewhat more difficult to detect. As used herein, light elements have a molecular weight of no more than about twenty-one atomic mass units.

One method of measuring relatively light molecular weight elements in thin films is to use a wavelength dispersive x-ray detector tuned to the characteristic x-ray line of the desired light element. Another method is to use an energy dispersive x-ray detector to detect the entire spectrum of x-rays including medium and heavy elements, as well as the continuous bremsstrahlung radiation.

The wavelength dispersive x-ray method suffers from a limited solid angle that can be collected due to the geometry necessary to satisfy the Bragg condition for the reflection of x-rays. Another disadvantage of the wavelength dispersive x-ray method is that the Bragg reflector generally has a low efficiency, usually less than about ten percent. A third disadvantage of the wavelength dispersive x-ray method is the inability to measure the continuous x-ray background at neighboring wavelengths. It is necessary to add a second wavelength dispersive x-ray detector to measure the background at a single neighboring wavelength, which does not always yield adequate information.

Energy dispersive x-ray analysis characterizes materials by exciting a sample with ionizing radiation. An energy-dispersive x-ray analyzer is a common accessory for a scanning electron microscope. The electron beam in the scanning electron microscope typically has an energy of between about five thousand and about twenty thousand electron volts, and provides the ionizing radiation. The binding energy in an atom ranges from a few electron volts up to many thousand electron volts. Atomic electrons are dislodged as the incident electrons from the scanning electron microscope beam pass through the sample, thus ionizing atoms of the sample.

After an atomic electron is ejected from the sample, another electron, such as from a nearby atom, neutralizes the ionized atom. This neutralization produces an x-ray with an energy level that is characteristic of the sample atom. Another mechanism, known as bremsstrahlung, also produces x-rays. In this case an electron from the beam is significantly deflected by the strong electric field of an atom's nucleus. As the electron curves around the nucleus, it emits an x-ray. These x-rays can be emitted over a wide, continuous energy range and are not characteristic of the atom which produced them. By using x-ray detection equipment to count the number of x-ray photons emitted at a given energy level, the energy dispersive x-ray system is able to characterize and quantify the elemental composition of the sample.

The energy dispersive x-ray method overcomes the problems of wavelength dispersive x-ray, but suffers from its sensitivity to all the x-rays, including bremsstrahlung from the light element atoms and both bremsstrahlung and characteristic x-rays from the medium and heavy elements. Generally the x-rays from medium and heavy x-rays are much more intense than those from the light elements, and can overwhelm the energy dispersive x-ray detector with too high a count rate.

What is needed, therefore, is a system for light element material analysis that at least reduces some of the problems with the currently used techniques.

SUMMARY

The above and other needs are met by a spectrometer for detecting and quantifying elements in a sample. An exciter ionizes atoms in the sample, and the atoms thereby produce x-rays. A detector receives the x-rays and produces signals based on the x-rays. A filter system selectively blocks some of the x-rays from attaining the detector. The selective blocking of the x-rays is accomplished based on an energy of the x-rays. An analyzer receives the signals from the detector and detects and quantifies the elements in the sample based at least in part on the signals.

In this manner, the detector receives the light element x-rays, and most of the medium and heavy element x-rays are filtered out to avoid overwhelming the detector. This invention combines the large solid angle, high efficiency, and ability to measure the continuous background spectrum of the energy dispersive x-ray detector with the selectivity of the wavelength dispersive x-ray detector. It thus enables faster and more accurate measurement of light elements in thin films. This invention enhances the light element performance of a system by enabling higher throughput, lower e-beam and x-ray dose to the sample, and improved accuracy from the capability to measure the background radiation.

In various preferred embodiments, the filter system includes a collimator that collects the x-rays emitted from the sample. The collimator absorbs, or alternately does not deflect, x-rays having an energy greater than a first desired energy, and directs x-rays having an energy less than the first desired energy along a collimated beam. A reflector receives the collimated beam from the collimator at an angle of incidence that is below a critical angle for energies of the x-rays in the collimated beam that are below a desired cutoff energy. Thus, x-rays in the collimated beam with an energy less than the desired cutoff energy are reflected, and x-rays in the collimated beam with an energy greater than the desired cutoff energy are either absorbed or transmitted through the reflector.

In a most preferred embodiment, the first desired energy is about four thousand electron volts, and the desired cutoff energy is about five hundred electron volts. Most preferably, the collimator comprises a series of concentric parabolic collimators having smooth metallic surfaces with angles of reflection that are arranged such that the collimator absorbs or does not deflect x-rays having an energy greater than the first desired energy, and directs x-rays having an energy of less than the first desired energy along the collimated beam. Preferably, a second collimator receives the collimated beam from the reflector and refocuses the collimated beam onto the detector. The first desired energy and the desired cutoff energy are preferably adjustable by means of changes to the geometry and materials of the collimator and reflector, respectively.

According to another aspect of the invention, there is described a scanning electron microscope of the type having an electron beam that ionizes atoms in a sample, where the atoms thereby produce x-rays. A detector receives the x-rays and produces signals based on the x-rays. A filter system selectively blocks the x-rays from attaining the detector, where the selective blocking of the x-rays is accomplished based on an energy of the x-rays. An analyzer receives the signals from the detector and detects and quantifies elements in the sample based at least in part on the signals.

According to yet another aspect of the invention, there is described an energy dispersive x-ray spectrometer having a filter system adapted to selectively block x-rays ejected from a sample. The selective blocking of the x-rays is accomplished based on an energy of the x-rays.

DETAILED DESCRIPTION

With reference now to the figure, there is depicted a functional block diagram of a system10according to the present invention. The system10may be scanning electron microscope in which the elements as described are incorporated, or a standalone spectrometer, such as an energy dispersive x-ray fluorescent spectrometer, in which various elements have been added. The major parts of the system10are an exciter12, filter system14, detector16, and analyzer18. The filter system14preferably includes at least a collimator24and reflector28, and optionally includes a second collimator32. One function of the system10is to detect and quantify elements, and more especially light elements, in a sample20.

An electron beam or x-ray beam22stimulates the emission of x-rays from a small spot on a sample20that contains one or more light elements, such as nitrogen, carbon, and oxygen. X-rays emitted from the sample20are collected by a collimator24, which is preferably a parabolic collimator, and most preferably a series of concentric parabolic collimators that are disposed around the sample20as shown in the figure. The collimator24reflects the x-rays diverging from the emission spot into a collimated beam26. The collimator24preferably has smooth metallic surfaces. The angles of reflection of the surfaces are preferably arranged such that only x-rays with energies less than a first desired energy, preferably about four thousand electron volts, are completely reflected outside of the collimator24and along the collimated beam26. Higher energy x-rays are preferably absorbed by the collimator24.

Without being bound by theory, it is understood that typically only atoms with an atomic number of less than about twenty-one have K shell x-rays with energies less than about four thousand electron volts, so the collimator24is preferably adapted to filter out K x-rays from elements that are heavier than calcium. Medium and heavy atoms also emit L shell x-rays that are preferably filtered. In particular, copper emits L x-rays with an energy of about nine hundred and thirty electron volts. In addition, heavy elements emit low energy M shell x-rays. As an example, tantalum emits M x-rays at eighteen hundred electron volts.

The L and heavy element M x-rays are preferably filtered by reflecting the collimated beam26from a reflector28, which is most preferably a flat metal surface. The reflector28is configured with an angle of incidence to the collimated beam26that is below the critical angle for x-rays in the collimated beam26that have an energy that is below a desired cut off energy, such as about five hundred electron volts. In this manner, x-rays in the collimated beam26that have an energy that is less than the desired cutoff energy are almost completely reflected by the reflector28. However, those x-rays in the collimated beam26that have an energy that is above the desired cutoff energy, or in other words have a critical angle that is less than the angle of incidence of the collimated beam26against the reflector28, are absorbed by or are transmitted through the reflector28.

Thus, higher energy x-rays are absorbed by the reflector28, and lower energy x-rays are reflected by the reflector28. Thus, the reflector28helps to further filter the x-rays emitted by the sample20. The x-rays30reflected from the reflector28correspond mostly to characteristic x-rays emitted from light elements. Other x-rays that may be reflected are: low-energy bremsstrahlung from all elements, medium element M x-rays, and heavy element N x-rays. These other x-rays are generally very weak. The reflected x-rays30are detected by a detector16, which produces signals based upon the x-rays, such as the energy with which they are received, and the amount of x-rays that are received at a given energy. Thus, the detector16detects characteristic x-rays that indicate both the materials of which the sample20is comprised, and the relative amounts of those materials. In a most preferred embodiment, the detector16is one such as is found in a standard energy dispersive x-ray system.

The signals produced by the detector16are preferably sent to an analyzer18, which determines the materials in the sample20, and the relative amounts of such materials, such as by comparing the signals that are received to a database of information that correlates such signals to known elements. The analyzer18is preferably one such as is found in a standard energy dispersive x-ray system.

In a preferred embodiment, the collimated beam30reflected from the reflector28is refocused with a second collimator32as depicted in the figure. This configuration allows a smaller detector16to be used, which tends to reduce the cost and improve the energy resolution of the system10. The material and angle of the reflector28is preferably variable, to selectively adjust the desired cutoff energy. The system10can optionally include other detectors, such as prior art energy dispersive x-ray detectors and wavelength dispersive x-ray detectors, to simultaneously measure medium and heavy elements that may be present in the sample20.

As a specific example, the system10can be used to measure a copper/TaN barrier seed film on a semiconductor wafer20. The copper is twelve hundred angstroms thick on top of a one hundred angstrom TaN barrier film. Two wavelength dispersive x-ray detectors tuned to the copper K (eight thousand electron volts) and tantalum L (eighty-four hundred electron volts) x-rays are used to measure the quantity of copper and tantalum. A parabolic collimator24followed by a cutoff mirror reflector28are used with an energy dispersive x-ray detector16to measure the nitrogen and its neighboring background. The cutoff of the mirror28is set at four hundred and fifty electron volts to filter out the nine hundred and thirty electron volt copper L x-rays, as well as the eighteen hundred electron volt silicon K x-rays and the eighteen hundred electron volt tantalum M x-rays.

Thus, in the system10as described herein, the detector16receives the light element x-rays, and the medium and heavy element x-rays are filtered out to avoid overwhelming the detector16. Therefore, this invention enables faster and more accurate measurement of light elements in thin films, and enhances the light element performance of a system by enabling higher throughput, lower e-beam and x-ray dose to the sample20, and improved accuracy from the capability to measure the background radiation.