Capping layer to impede atom ejection

A method of reducing atom ejection from a sample during electron beam bombardment. An electron beam is directed through a low pressure environment toward a surface of the sample. The electron beam thereby impinges on the sample at a target location, and thereby causes characteristic x-ray emission from the target location of the sample. A capping precursor is introduced into the low pressure environment, where the capping precursor forms a capping layer on the surface of the sample at the target location when contacted by the electron beam. The capping layer thereby reduces atom ejection from the sample at the target location, while not appreciably impeding and confounding the characteristic x-ray emission from the target location of the sample.

FIELD

This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to electron beam stimulated x-ray analysis of integrated circuit layers.

BACKGROUND

Integrated circuit fabrication is typically accomplished by forming many different layers on a substrate. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III–V compounds like gallium arsenide, InP, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices. Because the design tolerances of an integrated circuit are so strict, it is desirable to monitor the properties, such as thickness and elemental composition, of the various layers as they are formed. One way to measure the properties of film layers is to use a technique called electron stimulated x-ray metrology.

In very general terms, electron stimulated x-ray metrology works by directing a beam of electrons through a low pressure environment toward a sample surface. The electrons excite the atoms of the sample as they impinge against it. The excited atoms produce x-rays having properties that are characteristic of the properties of the sample, such as layer composition and layer thickness. Electron stimulated x-ray metrology is a highly favored technique, because in principle, it can be performed on production integrated circuits without damaging them.

Unfortunately, there are some problems with the technique. For example, deterioration of the x-ray signal over time, due to atom ejection, generally referred to as trending herein, requires that relatively short electron beam exposure times be used in order to get a reading that is closest to the initial value. However, a short acquisition time tends to result in a relatively large error bar spread. Thus, while repeated measurements at different points tend to result in a reduced precision, trending causes repeated measurements at the same point to result in a precision that is even further reduced.

What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.

SUMMARY

The above and other needs are met by a method of reducing atom ejection from a sample during electron beam bombardment. An electron beam is directed through a low pressure environment toward a surface of the sample. The electron beam thereby impinges on the sample at a target location, and thereby causes characteristic x-ray emission from the target location of the sample. A capping precursor is introduced into the low pressure environment, where the capping precursor forms a capping layer on the surface of the sample at the target location when contacted by the electron beam. The capping layer thereby reduces atom ejection from the sample at the target location, while not appreciably impeding and confounding the characteristic x-ray emission from the target location of the sample.

In this manner, a capping layer is formed on the surface of the sample wherever the electron beam impinges on the sample. The capping layer reduces, and preferably eliminates the incidence of atom ejection from the sample, and thereby reduces and preferably eliminates the trending that is displayed when the electron beam impinges a given target location for an appreciable length of time. Because the trending is reduced, the readings taken using the method as described tend to be more exact. Thus, longer readings can be taken, and a higher precision in the results is attained.

In various embodiments according to this aspect of the invention, the method is implemented in an electron stimulated x-ray metrology system. Preferably, the capping layer substantially eliminates atom ejection from the sample at the target location. The capping precursor preferably includes at least one of alcohol, ketone, aldehyde, ether, ester, organic acid, organic halide, unsaturated organic molecule, organic molecule with a dipole that can form a weak bond, hydrogen bond, or van-der-Waals bond to the surface of the sample, silicon based halide, inorganic molecule capable of bonding to the surface of the sample, molecules with polar bonds such a carbon-halogen bond and silicon-halogen bond, dipoles such as unsaturated organic molecules, silicon based molecules such as SiCl4. Most preferably the capping precursor includes a short chain hydrocarbon with an oxygen functional group, such as 2-propanol.

According to another aspect of the invention there is described an apparatus that directs electrons at a sample while reducing atom ejection from the sample. An electron beam source generates an electron beam and directs the electron beam toward the sample. A low pressure environment receives the electron beam from the electron beam source and encloses a target location on the sample. An injector receives a capping precursor and injects the capping precursor into the low pressure environment.

In various embodiments according to this aspect of the invention, the apparatus is implemented in an electron stimulated x-ray metrology system. A capping precursor source is preferably connected to the injector, and holds a store of the capping precursor. The injector may be a plurality of injectors. A flow rate adjustment is preferably connected to the injector, and adjusts a flow rate of the capping precursor. In one embodiment, the injector is disposed to inject the capping precursor at the target location. The capping layer preferably substantially eliminates atom ejection from the sample at the target location. The capping precursor preferably includes a short chain hydrocarbon and an oxygen functional group.

According to yet another aspect of the invention there is described an electron stimulated x-ray metrology system that directs an electron beam at a sample while reducing atom ejection from the sample. An electron beam source generates an electron beam and directs the electron beam toward the sample. A low pressure environment receives the electron beam from the electron beam source and encloses a target location on the sample. A capping precursor source holds a store of a capping precursor, and an injector disposed between the capping precursor source and the low pressure environment injects the capping precursor into the low pressure environment.

In various embodiments according to this aspect of the invention, the injector is a plurality of injectors. A flow rate adjustment is preferably connected to the capping precursor source, and adjusts a flow rate of the capping precursor into the low pressure environment. Preferably, the injector is disposed to inject the capping precursor at the target location. The capping layer preferably substantially eliminates atom ejection from the sample at the target location. The capping precursor most preferably includes a short chain hydrocarbon with an oxygen functional group.

DETAILED DESCRIPTION

With reference now to the FIGURE, there is depicted a functional block diagram of a system10according to a preferred embodiment of the present invention. The system10is preferably implanted as an electron stimulated x-ray metrology system, and thus includes the components and subsystems as are traditionally included with such a system, some of which are not depicted in the FIGURE so as to more completely focus attention on the more novel aspects of the system10. The system10preferably includes an electron beam source16which directs an electron beam18toward a target location26on a substrate12. Preferably, at least the electron beam18and the target location26of the substrate12are disposed within a low pressure environment14.

A capping precursor source20preferably holds a store of a capping precursor, which has properties as described more completely below. The capping precursor is preferably introduced to the low pressure environment14through one or more injectors24. As depicted in the FIGURE, injector24ais disposed so as to introduce the capping precursor at a location within the low pressure environment14that is as close to the target location26on the substrate12as practical. Alternately, injector24bis disposed so as to introduce the capping precursor at a more generalized location within the low pressure environment14. The capping precursor is most preferably introduced as a gas into the low pressure environment14.

While both or additional injectors24could be used, most preferably the injector24ais used, so that a relatively smaller amount of the capping precursor is used, and the other components within the low pressure environment14are not exposed to the capping precursor to such a great extent. The amount of the capping precursor delivered to the low pressure environment14is preferably metered to some extent, such as through a valve22.

The capping precursor is preferably a short chain hydrocarbon containing an oxygen functional group, such as 2-propanol. The capping precursor preferably contains a molecule that is broken up under the electron beam18, and carbon atoms bond to the surface layer of the substrate12, creating a capping layer that tends to impede the ejection of the atoms near the surface of the substrate12. Because carbon is a relatively light element, thin layers of carbon do not significantly adversely affect the energy of the incoming electrons, nor do they absorb a significant amount of the emitted x-ray signal. Thus, the measurement capabilities of the system10are not significantly impeded by the capping layer that is formed on the substrate12.

In this manner, the measurement duration can be greatly extended. In addition, signal precision is greatly enhanced. Thus, the measured value is still very close to the initial value, even after many minutes of measurement.

Other organic or inorganic molecules could also be used as the capping precursor, instead of 2-propanol or other carbon containing materials. The molecules that are most effective are preferably capable of van-der-Waals bonding or hydrogen bonding to the surface of the substrate12. 2-propanol is a good example of a short chain hydrocarbon with a hydroxide group, and thus has the desired properties. Other molecules that contain an oxygen functional group such as a ketone (C═O), aldehyde (C═OH), organic acid (COOH), ester (COOR), or ether (COC), or possess other types of polar bonds such as carbon-halogen or silicon-halogen, or dipoles such as unsaturated organic molecules, that have a high sticking coefficient to the substrate12surface and acceptable volatility under the high vacuum conditions of the low pressure environment14may also serve well.

Silicon based molecules such as SiCl4or similar materials may have the added advantage of forming a silicon cap on the target location26, which may be desirable for in-line gate process applications. In an alternate procedure, a pre-treatment of the substrate12with the capping precursor, in either a liquid or a gaseous state, prior to bringing the substrate12into the low pressure environment, is followed by an electron beam18exposure on the target location26to form the capping layer.

The method and system10as described herein has proven to be a most effective method for obtaining stable, low-noise, x-ray signals for nitrogen and oxygen, such as in thin silicon oxynitride gate layers. The method and system10may also function for measurements of other films, such as aluminum oxide, or other layers that exhibit a signal that deteriorates, or trends, with exposure to an electron beam18. The material of the capping precursor may be selected so as to be most compatible with the layer being measured, in addition to preferably exhibiting the other properties as described herein.