JP2003017539A - Method of observing atom diffusion within metal wiring - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that it has been difficult to microscopically identify a diffusion route of electro-migration of semiconductor device wiring, which is indispensable for improving reliability of semiconductor device, from a conventional measurement of variations in electric resistance due to current stress or measurement of the length of a missing wiring. SOLUTION: An element for observation which is different from an atom constituting the wiring is locally distributed in the wiring in advance. Variations in distribution of the element for observation caused by current stress are microscopically observed using electron beam or X-ray, thereby the diffusion route of the wiring metal being estimated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metal wiring reliability evaluation technique suitable for reliability evaluation techniques for semiconductor devices and the like.

[0002]

2. Description of the Related Art With the miniaturization of semiconductor elements, current has come to flow through wirings in semiconductor elements at an extremely high density. When an electric current flows through the metal wiring at a high density, the diffusion of the wiring constituent elements due to the flow of electrons, that is, a phenomenon called electromigration is induced, which causes the wiring to break. Therefore, in order to improve the reliability of the semiconductor device, it is extremely important to elucidate the mechanism of electromigration. At present, the metal wiring material for semiconductor devices is Al, which has been used so far.
Therefore, Cu having a lower electric resistance is being replaced, and in particular, there is an urgent need to associate the atomic diffusion mechanism of Cu in the Cu wiring with the wiring life.

Up to now, in the observation of the atomic diffusion phenomenon due to electromigration, a method in which a current is applied to a wiring to measure a change in electrical resistance due to a current stress, or a defect or lack of a wiring is generated using a microscope after the current is applied. For example, a method of observing a part and measuring the missing length is used.

[0004]

The above-mentioned conventional method observes phenomena appearing as the whole wiring, such as a change in electrical conductivity and a change in wiring missing length, and is, so to speak, a macroscopic observation method. The diffusion coefficient relating to atomic diffusion and the activation energy of diffusion obtained by these methods are the average value of the entire wiring or the value of the most dominant diffusion path. Identification of the path through which metal atoms diffuse is indispensable for constructing a process that creates wiring with excellent electromigration resistance. However, with these methods, it is difficult to identify microscopic diffusion pathways,
There was no choice but to estimate after accumulating data for many condition items in the wiring process such as film forming conditions and line widths.

[0005]

[Means for Solving the Problems] An observation element different from an element forming a metal wiring is locally distributed in the wiring,
By observing the change from the initial distribution of this element due to current stress, the migration path of the constituent metal atoms in the metal wiring is estimated.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of atomic diffusion observation in a metal wiring according to this method will be described with reference to FIG. FIG. 1 shows a top view of the wiring 1 which is formed on the wafer and is to be evaluated. The wiring 1 may be a part of the wiring in the actual device or a wiring portion of the reliability evaluation test structure, and the wiring 1 can be subjected to current stress by a prober or the like. The left side of the figure shows the state before the atomic diffusion observation by the current stress, and the right side of the figure shows the state when the state diffused by the current stress is observed.

In this method, before applying the current stress,
An observation element (for example, a Cu wiring) different from an element forming the wiring 1 is provided in a portion of the wiring 1, for example, a place where disconnection due to electromigration is likely to occur, or particularly a place where the movement of atoms is to be observed. In such a case, Ga or the like is considered) is locally distributed.
On the left side of the figure, the region 2 in which the observed elements formed are distributed
But is shown darkly.

In FIG. 1, the observation element distribution region 2 is linearly distributed across the wiring 1. However, this is an example, and the size, shape, atomic concentration, etc., depend on the line width of the wiring 1, etc. Choose appropriately. On the right side of the figure, the state of the distribution region 2 of the observing element after applying the current stress is schematically shown.

FIG. 2 shows an example of a method of forming the distribution region 2 of the observing element. One method of forming the observation element distribution region is to form the observation element distribution region 2 by leaving only the observation element distribution region 2 masked and implanting the observation element (ion). The wiring 1 is formed on the substrate 3, and the mask 4 is formed thereon. The mask 4 has holes only in the portion where the observation element is to be distributed. This mask 4
Although the ionized observing element 5 is implanted from above, the observing element 5 is distributed only to a necessary portion on the wiring 1 by the mask 4. After forming the element distribution region for observation, the mask is peeled off.

As a second method of forming the distribution region of the observing element, the observing element is scanned in a desired region to be distributed as the focused ion beam (FIB) 6, and the observing element is distributed in a local region. Is the way. In this method, the region of the wiring 1 formed on the substrate 3 in which the observation element is to be distributed (the darkened portion in the figure) is scanned with the ion beam 6 of the observation element that is finely focused. As a result, a distribution region of the observation element having a desired size and shape can be formed on the wiring 1 at a desired location.

In any of the above methods, a protective film may be further formed after forming the observing element distribution region 2 if necessary.

EPMA (Electron Pr) is used for observing the observation element distribution region 2 after the current stress is applied.
an X-ray Microanalyzer) or a fluorescent X-ray microscope using an X-ray microbeam is used. However, in the case of EPMA, the observation element distribution region 2
Since it is necessary to directly irradiate the electron beam on the surface, it is necessary to remove the protective film if it is attached. On the other hand, a fluorescent X-ray microscope using an X-ray microbeam is a method of detecting fluorescent X-rays from a sample while irradiating a narrowed X-ray microbeam to perform imaging of element distribution. Since X-rays have a higher penetrating power than electron beams, the distribution of the observing element can be observed even if the protective film is attached. Also, when the electron beam irradiates the sample, it spreads to about 1 micron inside the sample,
The spatial resolution obtained is up to 1 micron, but in the method using the X-ray microbeam, if the X-ray microbeam is made thin, observation with a resolution of 1 micron or less is possible, which is more suitable for this method. Can be said.

FIG. 3 shows a schematic view of a fluorescent X-ray microscope using an X-ray microbeam as an example. The X-rays 7 are generated from a light source (for example, synchrotron radiation light), and are made monochromatic or the beam divergence angle is shaped as necessary. The X-ray 7 is connected to the wiring 1 by the X-ray optical element 8.
The light is focused on the sample 9 on which is formed. The X-ray optical element 8 may be an oblique incidence reflecting mirror as shown in this figure, or an X-ray focusing optical element other than a reflecting mirror such as a zone plate.

Sample 9 irradiated with condensed X-rays
The size of the upper region is determined by the performance of the X-ray optical element 8, but becomes the spatial resolution when observing the distribution region 2 of the observing element. The fluorescent X-rays generated from the minute region irradiated with the X-rays 7 are detected and energy analyzed by the detector 10, and only the X-rays from the observing element are discriminated and output as a fluorescent X-ray intensity signal. By scanning the sample 9 two-dimensionally as shown by the arrow in the figure, the fluorescent X-ray intensity distribution from the observation element in the sample 9 can be obtained.

In any case, the measured fluorescent X-ray intensity is proportional to the concentration of the element, and the concentration distribution of the observing element can be known from the intensity distribution of the fluorescent X-ray. It is possible to know how the element diffused.

It is considered that the diffusion path of the observation element due to the current stress traces the diffusion path of the metal element forming the wiring 1. Therefore, it is possible to know from the observation result how much the observing element is distributed in the crystal grain boundary of the metal wiring, the interface with the barrier metal, etc., and the route along which the wiring constituent element causes the electromigration. Can be estimated.

Further, the fluorescent X-ray intensity from the observing element reflects the concentration of the observing element, and the diffusion coefficient due to the current stress of the observing element can be estimated by comparison with the initially distributed concentration. it can. It is possible to estimate the electromigration diffusion coefficient of the wiring-constituting metal element or the activation energy of electromigration from the obtained diffusion coefficient of the observing element by the following method.

That is, the diffusion coefficient of the observing element is obtained by this method for the wiring having the same structure formed by the same process as the wiring whose electromigration diffusion coefficient is measured, and the observing element diffusion coefficient is obtained. Diffusion coefficient,
It suffices to create a calibration curve with the diffusion coefficient of wiring metal atoms. From this calibration curve, the activation energy for electromigration of wiring metal atoms can be obtained from the diffusion coefficient of the observing element obtained by this method.

[0019]

According to the present invention, the electromigration diffusion path of wiring constituent atoms due to current stress in semiconductor metal wiring can be estimated microscopically, and the process for forming a long-life wiring can be performed in a shorter period of time. Can be developed to.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of the principle of the present invention.

FIG. 2 is a perspective view showing an example of a method of forming a distribution region of an observation element.

FIG. 3 is a perspective view showing an example of a method of observing a distribution region of an observation element.

[Explanation of symbols]

1 ... Wiring, 2 ... Observation element distribution region, 3 ... Substrate, 4 ... Mask, 5 ... Observation element, 6 ... Observation element focused ion beam, 7 ... X-ray, 8 ... X-ray focusing optical element, 9 … Sample, 10
…Detector.

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Claims (5)

[Claims] 1. A method of observing the movement of a metal element constituting a semiconductor element metal wiring in the wiring, wherein an observing element different from the metal element is distributed in the metal wiring with a specific concentration distribution. A method for observing atomic diffusion in metal wiring, which comprises estimating a diffusion path of the metal element by observing a change in the distribution of the observation element due to current stress. 2. The method according to claim 1, wherein the observation element is implanted after masking a region other than a certain local region in the wiring, and then the mask is peeled off to distribute the observation element. A method for observing atomic diffusion in metal wiring, characterized by. 3. The method according to claim 1, wherein the observation element is a focused ion beam, and the observation element is distributed in a local region in the wiring by scanning the beam. Method for observing atomic diffusion in metal wiring. 4. The method according to any one of claims 1 to 3, wherein the X-rays are locally irradiated and the fluorescent X-rays emitted from the observation elements are thereby detected to detect the observation elements. A method for observing atomic diffusion in metal wiring, characterized by observing distribution. 5. The method according to claim 1, wherein the electron beam is locally irradiated to detect the fluorescent X-ray emitted from the observation element.
A method for observing atomic diffusion in metal wiring, which comprises observing a distribution of an element different from the above-mentioned observation metal element.