JP2764599B2 - Reflection electron beam diffractometer - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an apparatus for tracking and analyzing a structural change in a minute area on a sample surface with the lapse of time using reflected electron beam diffraction.

(Prior art) With the high performance of semiconductor integrated circuits, inevitably high quality of metal substrates such as polycrystalline silicon, Al, W, Mo, etc. deposited on silicon substrates and insulators when manufacturing semiconductor devices. Is desired. In this case, a high-purity thin film containing no impurities must be deposited, but if the crystallinity of the thin film is not sufficiently controlled, a highly reliable integrated circuit cannot be realized.

For example, in an integrated circuit, Al or Al alloy wiring connecting each element sometimes causes a trouble called disconnection,
It reduces the reliability of integrated circuits. It is considered that the cause is electromigration generated when a large current flows or stress migration due to stress concentrated on a portion where the base changes. In fact, the wiring metal Al or Al alloy is not formed on a flat wafer, but is formed on a surface with irregularities of about 1 μm, and locally has a current density of 10 ⁶ A / cm ^2. The current flows. In order to prevent such disconnection, it is necessary to deposit an Al or Al alloy film with a uniform crystal orientation, and to clarify the disconnection occurrence conditions by actually observing the state where the disconnection occurs.
It is necessary to determine the cause of the disconnection.

For this reason, as a method of determining the optimum thin film forming conditions of the metal thin film used in the conventional wiring, there is a method of actually flowing a current and measuring the time until the disconnection, but actually forming the wiring and evaluating it. That would take a lot of time. As another means for analyzing the state of disconnection when a current flows through a wiring metal on a semiconductor integrated circuit, there is an analysis method in which the shape of a disconnected portion after disconnection is evaluated by an electron microscope or an optical microscope. There is no evaluation method for linking the physical phenomena of crystallinity and disconnection, and there is no means for determining the optimum wiring metal formation conditions other than, for example, a statistical correlation between the wiring metal formation conditions and the time until the disconnection. However, the optimum thin film formation condition can be determined by comparing the crystallinity evaluation with the actual reliability evaluation result using another apparatus for evaluating the crystallinity of the thin film. In this case, the following points are required to evaluate the crystallinity of a thin film required in a semiconductor integrated circuit. In a polycrystalline thin film, the size of a crystal grain boundary is often on the order of microns, and it is necessary to analyze the crystal structure of a minute region on the order of microns. The processing size of a thin film used for a semiconductor integrated circuit is 1 micron or less, and the crystal structure of the thin film must also be determined with a resolution of about a micron. For example, in the Al or Al alloy thin films for wiring currently under study, the specific lattice plane of the crystal is oriented parallel to the thin film surface, but the crystal grains rotate in various directions within the thin film surface. ing. For this reason, it is necessary to evaluate how the crystal grains rotate in the plane, and how the crystal grains have an orientation distribution. However, these static evaluation means alone are not sufficient for elucidating a phenomenon that is an end point of a temporal change such as a disconnection of a wiring. For example, the deposited thin film is usually subjected to a heat treatment step to increase the density of the thin film and improve the adhesion to the base. At that time, the crystal structure changes. Further, in electromigration in which a part of a wiring is broken when a large current is applied to a wiring metal, a crystal structure changes in a minute part of about a micron leading to the breaking. Therefore, it is necessary to evaluate the heat treatment step and the change in the crystal structure of the minute region when a current is applied.

Further, when performing the above various evaluations, for example, the wafer is several mm
It is desirable that observation can be performed without applying special processing such as cutting into a square size or polishing thinly.

As a conventional crystal structure analysis means, there is a method mainly using diffraction of X-rays and electron beams. In a conventional X-ray diffraction method using X-rays having a wavelength of about 1.5 °, the crystal orientation of a plane parallel to the sample surface can be determined. However, it is extremely difficult to narrow down the X-ray beam, and the beam diameter of the incident X-ray in the conventional X-ray diffractometer is about 10 to 20 mm, so that only average information on the sample surface can be obtained.

Transmission electron microscopy is a method for observing polycrystalline boundaries.
By observing the transmission electron image, the existence of the crystal grain boundaries can be confirmed. However, transmission electron microscopy cannot measure the crystal orientation of each single crystal portion surrounded by the grain boundaries, and the sample thickness is 1000 to 1000 even when an electron beam accelerated to 100 keV is used.
It has to be thinned to about 2000mm. Also, the sample size must be less than a few mm square. Since such special processing is required, it cannot be essentially a simple measuring device.

As a method for evaluating the crystallinity of the surface, there is a high-speed reflection electron beam diffraction method (RHEED method) in which the evaluation is performed using a diffraction pattern of an electron beam accelerated to 10 to 30 KeV. With the RHEED method, the surface orientation and crystallinity of the surface can be evaluated without performing special processing on the sample, but with a conventional RHEED device, the electron beam irradiation area can be 100 microns to several mm. Yes, only the average crystallinity of the surface can be evaluated as a result. As a method developed from the RHEED method, there is a microprobe RHEED method in which the beam diameter of an electron beam is reduced to about 0.1 μm and the crystallinity is evaluated in a micro area on the order of microns. The sample surface is scanned with an electron beam, and the distribution of crystal grain boundaries can be measured by the intensity change of a specific diffraction spot among the electron beam diffraction spots. However, in the conventional microprobe RHEED method, information about the crystal orientation of the plane parallel to the sample surface can be obtained, but how the crystal orientation of the plane perpendicular to the sample surface rotates within the sample surface We cannot know the distribution.

As described above, according to the conventional crystal structure analysis method, it is impossible to analyze a micro area of about a micron, or to observe the in-plane rotation distribution of crystal grains. Further, in any of the methods, there is no apparatus capable of tracking a change in the surface structure with time during energization or heating.

(Problems to be Solved by the Invention) The present invention solves the conventional problems, and in a reflection electron beam diffractometer using diffraction of an electron beam incident on a sample surface, for example, a current is applied to a thin film, heating is performed, or Another object of the present invention is to provide an apparatus capable of immediately observing a change in crystallinity in an arbitrary minute region of a thin film when the surface of the thin film is intentionally denatured.

(Means for Solving the Problems) In order to solve the above problems, in the present invention, in the reflection electron beam diffraction method, an electron beam that is nearly parallel and converges to a small diameter on the sample surface is used. Means for scanning the diffraction pattern of the electron beam incident on the sample surface at an incident angle close to parallel with the sample surface, means for applying electricity to the sample, means for heating the sample, and gas supply to the sample surface A sample processing means such as a means for performing the measurement is provided.
Here, the diffraction pattern includes not only individual diffraction spots but also the two-dimensional arrangement of individual diffraction spots and the entire background.

(Effect) It is clear that a method using X-rays for structural analysis of a microscopic region on the order of microns on the sample surface cannot be used because it is difficult to obtain appropriate convergence means. It is well known that an electron beam is suitable for observing a minute region, and the orientation of a crystal plane can be easily determined by using an electron beam diffraction method. The present invention is to observe a diffraction pattern by irradiating a sample surface with an electron beam narrowed to a small diameter. At this time, the relationship between the electron beam intensities at a plurality of points on the diffraction pattern does not change while the electron beam incident on the sample scans a single crystal region on the sample surface, but the irradiation electron beam does not change. When the crystal moves to the next crystal, the relationship between the electron beam intensities at the plurality of points changes due to a difference in the direction of the crystal plane in the crystal. Changes in this relationship are extracted by an arithmetic operation during one scan of the electron beam with respect to the detection outputs at a plurality of points, and the structure of the minute area on the sample surface and its change with time can be clearly recognized.
Coupled with the function of conducting, heating, and other processing on the sample, it is possible to follow-up the change over time of the structure under various conditions in the minute region of the sample.

(Example) An example of a scanning reflection high-speed electron beam diffractometer and a micro area structure analysis according to the present invention is described.

FIG. 1 shows a scanning reflection high-speed electron beam diffraction apparatus according to the present invention. The main device parts will be described below.

1 is an electron gun for reflection electron beam diffraction (RHEED gun).
The diameter of the electron beam 4 is set at 0.
1 μm or less is desirable, and the opening angle of the electron beam is also 1 × 10 ^−3.
Desirably less than radians. Acceleration voltage is 10 ~
50 kV, preferably about 30 kV. Reference numeral 6 denotes a fluorescent plate for observing a reflected electron beam diffraction pattern. In general, a diffraction pattern is formed on the upper side by the diffraction electron beam 5 by the electron beam 4 emitted from the RHEED gun 1. The signals from the diffraction spots are guided to photomultipliers 10, 11, and 12 via optical fibers 7, 8, and 9, and are amplified. The arithmetic circuit performs a multiplication process on the intensity of each diffraction spot by an arbitrary constant and an addition / subtraction process between the intensity of each diffraction spot subjected to the multiplication process. The signal 14 subjected to the arithmetic processing is input to the CRT 15 as a luminance signal. By a scanning signal synchronized with the scanning signal 16 of the electron beam from the RHEED gun, a diffraction intensity image from the sample surface (hereinafter, referred to as a scanning RHEED image) is displayed on the CRT.

In this embodiment, the optical fiber is installed outside the vacuum,
Any diffraction spot can be mechanically selected. The number of optical fibers is three in this embodiment, but may be four or more. In addition to determining the orientation of the crystal grain boundaries described below by providing multiple optical fibers and detecting signals at the same time, it is possible to remove the background by setting one of the optical fibers to a part without diffraction spots it can. 3 is a sample. In this embodiment, a sample up to 2 inches in diameter can be observed. Reference numeral 45 denotes a sample holder, which is provided with sample heating means in which a Ta heater is disposed on boron nitride. Reference numeral 30 denotes a sample moving mechanism which can tilt the sample with respect to the Z axis, and moves the sample in x and y directions within the tilted plane.
In addition to moving in both directions, movement and rotation in the Z-axis direction are possible. The electron beam incident position 29 is
Can be moved to any point on the entire surface of the 2-inch wafer. Further, means for detecting reflected electrons and sample absorption current in addition to secondary electrons may be provided. 25 is a vacuum exhaust system. As described later, when any gas is introduced into the apparatus, the gas is exhausted using the turbo molecular pump 25 '. However, 25 'need not be a turbo molecular pump as long as it can maintain an ultra-high vacuum even when gas is introduced. In the present embodiment, the main exhaust system 25 is composed of an ion pump and a titanium sublimation pump. However, the main exhaust system 25 can be evacuated to about 1 × 10 ⁻⁸ Pa or less, and the entire vibration of the vacuum chamber 28 is reduced to about 0.1 μm.
The configuration is not limited as long as it can be suppressed to m or less. Reference numeral 27 denotes a sample exchange spare chamber for exchanging samples without opening the vacuum chamber 28 to the atmosphere.

Reference numeral 41 denotes a metal needle for flowing an electric current to a required portion of the sample surface. The metal needle uses W, but the material may be other than W. Reference numeral 42 denotes a manipulator for arranging the metal needle at an arbitrary position on the sample.

The current is supplied to the sample from a power supply 46 located outside the vacuum.
This is performed through the electric wire 43 via the current terminal 44. The power supply can flow a DC current, an AC current, a pulse current, or the like. 50 is an ion gun for forming an inert film on the surface of the sample. The gases include N ₂ , H ₂ , O ₂ , NO ₂ , N ₂
The H ₃ used. Reference numeral 52 denotes a gas inlet for introducing a gas such as N ₂ or O ₂ to form an inert film on the sample surface.

An observation example according to the present embodiment is shown below. First, the basic observation method is described. After loading the sample 3, the diffraction pattern by the reflected electron beam 5 by the electron beam 4 from the RHEED gun 1 is measured, and a scanning PHED image is observed based on the intensity from a specific diffraction spot on the diffraction pattern. FIG. 2 shows an example of a diffraction pattern and diffraction spots. The numbers are the same as those in FIG. The incident electron beam 4 from the RHEED gun 1 is incident on the surface of the sample 3 at an incident angle θ. The incident angle θ is 1 ° to 3 °. The incident electron beam 4 produces a diffracted electron beam 5 depending on the crystallinity of the sample surface. The diffracted electron beam is located at the positions of black points (points A, B, C, M, etc.) shown in the diffraction pattern 32 in FIG.
Has a strong strength on the primary rawling. The diffraction pattern 32 shown in FIG. 2 is visually displayed on the fluorescent screen 6 shown in FIG. The point M in the diffraction pattern 32 is called a specular reflection point, and is generated by an electron beam specularly reflected on the sample surface. Other diffraction spots (A, B, C, etc.) occur depending on the orientation of the crystal plane on the sample surface. Diffraction spots generated on a line perpendicular to the sagittal surface 31 on which the electron beam is incident and the detection surface 6 (for example,
A or B) is a diffraction point from a crystal lattice plane parallel to the sample surface. If the crystal lattice planes parallel to the sample surface are different,
The distance between the diffraction spots A and M changes. Therefore, the sagittal surface
From the position of the diffraction spot generated on a line perpendicular to the detection surface 6 and 31, it is possible to determine what crystal plane is parallel to the sample surface. A diffraction spot (for example, C) generated on a line parallel to a line orthogonal to the sagittal plane 31 and the detection plane 6 is a diffraction spot from a grating plane parallel to the sagittal plane. Therefore, even if the lattice plane parallel to the sample surface is the same, if the lattice plane parallel to the sagittal plane 31 is rotated, the intensity of the diffraction spot C changes. That is, in the scanning RHEED image of the diffraction spot A or B, even if the grating surface is rotated within the sample surface even in a portion where the intensity is strong, the intensity changes in the scanning RHEED image by the diffraction spot C. This will be specifically described with reference to FIG. In FIG. 3, sample 3 is composed of two crystal grains (34 and 35).
Description will be made assuming that the lattice plane parallel to the surface is the (001) plane.
The plane parallel to the sample surface in both the crystal grains 34 and 35 is the (001) plane. The (110) plane orthogonal to the (001) plane is parallel to the sagittal plane in the crystal grain 34, but is rotated by φ in the crystal grain 35. When the incident electron beam 4 enters the region of the crystal grain 34, the diffraction pattern 32 shown in FIG. 2 is generated. On the other hand, when the incident electron beam enters the region of the crystal grains 35, the lattice plane parallel to the sample surface of the crystal grains 35 is the (001) plane, so that the positions and the intensities of A and B of the diffraction pattern 32 in FIG. Is not changed, but the lattice plane (110) plane perpendicular to the sample surface is rotated by φ, so that the rotation spot of the point C in the diffraction pattern 32 is generated at a position different from the crystal grain. That is, the crystal grains 34
In crystal grains 35, the intensity of diffraction spots A and B on diffraction pattern 32 does not change, and the intensity of diffraction spot C changes.

When the entire sample 3 is rotated by φ, the crystal grains 35 (11
0) Since the plane is parallel to the sagittal plane, the intensity of the diffraction spot from the crystal grain 35 becomes stronger for both A, B, and C, while the intensity of the diffraction spot from the crystal 34 is the same for A and B, and for C Strength decreases. Therefore, by determining φ, it is possible to determine how many times the crystal lattice rotates in the sample plane in the crystal grains 34 and 35.

As described above, the crystal orientation of the plane parallel to the sample surface and the crystal orientation perpendicular to the sample surface can be determined by simultaneously measuring the scanning RHEED images using a plurality of diffraction spots on the diffraction pattern.

A specific sample observation example using the scanning type reflection high-speed electron beam diffractometer according to the present invention will be described.

Observation examples are observation of the growth process of crystal grains by heat treatment of polycrystalline silicon and the surface modification process by oxygen gas treatment,
This is an observation of the process leading to disconnection of the Al wiring.

(Observation Example of Polycrystalline Silicon) The sample is polycrystalline silicon deposited by a chemical vapor deposition method on a silicon wafer on which silicon oxide is formed. First, an initial observation of the surface structure of the sample is performed in the above-described place.

Next, the sample 3 is heated to observe how the scanning RHEED image changes. In this embodiment, the observation of the scanning RHEED image is as follows.
Since it is completed in a few seconds, it is possible to sufficiently observe a change in crystallinity which usually changes in the order of minutes or time.

The scanning RHEED image measurement speed can be further increased by using a means for storing the scanning RHEED image in a high-speed memory instead of displaying the scanning RHEED image on the CRT 15. An example of observation will be described with reference to FIG.

FIG. 4 is a diagram schematically showing a scanning RHEED image using different diffraction spots shown in FIG. 2 in which different crystal grains are observed in a gray scale by performing an acceleration multiplication / division operation. In each of FIGS. 4a and 4b, five types of crystal grains having different crystal orientations were observed. The crystal orientation of each crystal grain is determined by fixing the electron beam from the RHEED gun 1 to each crystal grain part and observing the diffraction pattern, and confirming that the crystal grains are (100), (110), (311) and amorphous, respectively. confirmed. FIG. 4a shows the crystal grain distribution before the heat treatment, in which small crystal grains of 1 μm or less and amorphous portions can be observed. Next, by heating the sample and measuring the scanning RHEED image every moment, it was possible to observe how small crystal grains were expanding. In this measurement example, as shown in FIG. 4, the heat treatment at 950 ° C. for 2 hours increased the crystal grains to several μm. It was also found that the amorphous portion had disappeared and all had been crystallized.

In the above measurement examples, the crystal grain change of the polycrystalline silicon thin film was measured.

Grain change by scanning RHEED image observation function and heating function 41 using different diffraction spots characterized by the present invention,
It was possible to measure not only polycrystalline silicon but also metal and magnetic material samples. (Observation of crystal grain change and disconnection when current is passed through Al wiring) The sample is an Al-Si wiring on a silicon wafer on which silicon oxide is formed.

FIG. 5 is an example of a sample provided with Al wiring. An Al thin film is formed on a Si wafer on which a thermal oxide film is formed by, for example, a sputtering method, and an Al wiring having a line width of about 1 μm is formed by using a photolithography technique. A pad portion for contacting a sharp metal needle (FIG. 41) with a sharp tip is formed to allow current to flow. Since a large number of patterns as shown in FIG. 5 are formed on the surface of the sample, markers can be provided so that positioning can be easily performed when forming the Al wiring, and the sample can be distinguished. An observation example according to the present embodiment is shown below.

After loading the sample 3, the diffraction pattern of the reflected electron beam 5 by the electron beam 4 from the RHEED gun 1 is measured, and a scanning RHEED image is observed based on the intensity from a specific diffraction spot on the diffraction pattern.

Next, the current introduction probe 41 was brought into contact with the pad portion of the Al wiring to be observed, and a current was applied to observe a change in crystal grains.

With reference to FIG. 6, a description will be given of a measurement example of a crystal grain change when a current of an Al wiring flows.

FIG. 6 is an observation example in which only the Al wiring portion is taken out.
A crystal grain interface exists perpendicular to the length direction of the wiring. Each crystal grain was classified into about four types. From the diffraction spot pattern, it was found that all the crystal grains had the (111) plane as the surface, but were crystal grains rotated on the sample surface.
When the sample was heated to approximately 150 ° C. and a current having a current density of approximately 5 × 10 ⁶ A / cm ² was passed, it was observed that the crystal grains 1 in FIG. 6b expanded. Also, approximately 10 hours later, a break occurred at the interface between the crystal grains 2 and 3.

In the conventional measuring method, the state of the disconnection cannot be observed except for observing the shape with an electron microscope after the disconnection is made. Scanning reflection high-speed electron beam diffractometer (Micro RHEE
By performing the above-described analysis on the diffraction pattern using D), the distribution of crystal grains as shown in FIG. 6b can be measured after the disconnection, but it is possible to simply measure the disconnection process when a current is applied. Can not. The device of the present invention is characterized in that the crystal grains can be enlarged while the current is flowing, and the crystal grains can be observed in-situ at the disconnection point.

(Other Measurement Examples) The scanning reflection high-speed electron diffraction apparatus according to the present invention is provided with an ion gun 50 and a gas introduction function 52 for forming an inert film on the surface as described in detail in the embodiments. I have.

For example, when a metal is left in the air, it reacts with moisture in the air to form an oxide film.

Oxide films formed in air are often not uniform in composition and not stable as a protective film.

When the inert film forming function is used in the present apparatus, the conditions for forming a stable inert film can be found.

 FIG. 7 shows an example of observation and will be described.

After the sample is formed, the sample is loaded into the apparatus without being exposed to the atmosphere. The sample is Al on a Si wafer. After loading the sample, the orientation of the crystal grains on the metal surface is measured in the same procedure as in the above-described embodiment. The result was as shown in FIG. 7a. This sample was composed of four types of crystal grains. As for the crystal grains, the crystal plane parallel to the sample surface is (111) (10
0), (110), and (311). In this measurement example, when oxygen ions were irradiated from the ion gun, it was observed that the diffraction intensity from the interface of the crystal grain boundaries weakened.
The crystal interface became unclear like b and c. this is,
This is because an oxide film thicker than the penetration depth of the electron beam was formed on the surface. In this measurement example, it was measured that the oxidation proceeded from the crystal grain interface and that there was a crystal plane where the oxidation easily occurred. It was found that the similar oxidation proceeds when oxygen gas is introduced without using ions.

(Effect of the Invention) By using the scanning reflection electron beam diffractometer according to the present invention,
It is possible to observe the progress of the change of the crystal grain boundaries when heating the sample having the crystallinity or applying the current. In the conventional observation method, only information before and after processing such as heating can be obtained. In this apparatus, it is possible to observe, for example, which crystal grain boundary increases in size due to the heat treatment and whether there is movement of the grain boundary interface.

In addition, the ability to allow current to flow through the metal on the surface of a sample, for example, to determine where Al wiring metal formed in a semiconductor integrated circuit is broken and at what crystallinity it is likely to break Can be. In addition, when a current is applied after heating the sample, the time required for disconnection, which requires 1000 to 10,000 hours at room temperature, can be reduced to approximately 1 to 10 hours.

[Brief description of the drawings]

FIG. 1 is a longitudinal side view of an apparatus according to an embodiment of the present invention, FIG. 2 is a view of an electron beam diffraction pattern of a single crystal, FIG. 4a and b are display images showing the change of the sample surface crystal of the Si sample obtained by the above embodiment of the present invention due to heating, FIG. 5 is a perspective view of another sample, and FIG. FIG. 7 is a model image of the change, and FIG. 7 is a display image showing a change in the process of forming the surface film of another sample. 1 ... reflection electron beam diffraction electron gun (RHEED gun) 3 ... sample 4
... electron beam from RHEED gun, 5 ... reflected electron beam diffraction line, 6 ...
Fluorescent plate (detection surface) for reflected electron beam diffraction spot observation, 7 ... optical fiber 1, 8 ... optical fiber 2, 9 ... optical fiber 3, 10 ... photomultiplier tube 1, 11 ... photomultiplier tube 2, 12 ... photomultiplier tube 3, 13 arithmetic circuit, 14 electric signal obtained from reflected electron beam diffraction spot intensity, 15 CRT1, 16 scanning signal for scanning electron beam from RHEED gun, 17 electron beam from SEM gun , 18 ... secondary electrons generated from the sample surface by the incident electron beam, 19 ... secondary electron detector, 20 ... secondary electron signal, 21
... CRT2, 22 ... Scanning signal for scanning electron beam from RHEED gun, 24 ... Sample observation window, 25 ... Vacuum exhaust equipment, 26 ... Gate valve, 27 ... Sample loading spare room, 28 ... Vacuum chamber, 29 ... Electron beam incidence point, 30 ... Sample moving mechanism, 41 ... Current introduction probe, 42 ... Probe manipulator, 45 ... Sample holder (with heater), 46 ... Power supply, 50 ... Ion gun for surface inert film formation, 52 ... gas inlet.

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Tadahiro Omi 2-1-1-17-301 Yonegabukuro, Sendai City, Miyagi Prefecture (72) Inventor Kazuo Tsubouchi 2-30-38, Hitokita, Sendai City, Miyagi Prefecture (72) Inventor Profit Kazuya 1-3-1-106 Mikamine, Sendai City, Miyagi Prefecture (56) References JP-A-53-59486 (JP, A) JP-A-61-151449 (JP, A) (58) Fields investigated (Int. Cl) . ^6, DB name) G01N 23/00 - 23/227

Claims (1)

(57) [Claims] 1. A means for irradiating a sample surface at an incident angle near parallel to a sample surface with an electron beam flux converged on a sample surface to a very small diameter at a small solid angle, and scanning the sample beam on the sample surface. Means, and means for performing an analysis operation between a plurality of diffraction spots on the diffraction pattern of the electron beam incident on the sample,
A reflection electron beam diffractometer having means for supplying electricity to a sample, means for heating the sample, and means for supplying gas to the surface of the sample or performing ion irradiation.