JPH09306411A - Focused ion beam working device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a focusing ion beam working device capable of working a vertical section by decreasing a tapered angle. SOLUTION: Two-dimensional scanning of an ion beam is raster scanned, but at this time, a scanning interval d in a work section vertical direction can be arbitrarily set to an input device 11, so that an optimum scanning interval in accordance with a work purpose can be assigned. When a desired probe current is assigned to the input device 11, a control device 9, from a relation between the stored probe current and the scanning interval, reads an optimum scanning interval to the assigned probe current, to be supplied to a scanning circuit 10, a scanning interval of a scanning signal is optimized. Further, in the scanning signal from the scanning circuit 10, a scanning interval in a vertical direction of a work section of an end part of the scanning region is decreased in a region distant from the work section, the interval can be increased in a region adjacent to the work section.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a focused ion beam processing apparatus which is optimal for use in preparing a sample for a scanning electron microscope or a transmission electron microscope.

[0002]

2. Description of the Related Art In recent years, a focused ion beam apparatus has been used to prepare a sample for a scanning electron microscope or a sample for a transmission electron microscope. For example, when a defective portion or a foreign substance exists on a semiconductor wafer (chip) or when structural evaluation is performed in each step of semiconductor manufacturing, the cross section of the defective portion or the structural portion to be evaluated is observed with a scanning electron microscope. There is a request.

In this case, a specific region of a semiconductor sample is two-dimensionally scanned with an ion beam, and the sample surface is sputtered by the ion beam to be processed to expose a desired cross section. Then, the scanning electron microscope image is observed with respect to the exposed cross section. In some cases, the focused ion beam apparatus may be provided with an observing function of a scanning electron microscope, and processing of a sample for observing a cross section and observation of a scanning electron microscope image may be performed by the same apparatus.

Further, in the case of observing with a transmission electron microscope, it is necessary to process the sample thin enough to allow the electron beam to pass therethrough. In that case, a thin observation region of the sample is left and both sides of the ion beam are used. It is processed by.

[0005]

The processing of a sample by a focused ion beam device means that the accelerated ion beam is focused by an optical system suitable for the ion beam and the scanning range is limited according to the purpose. This is to make a hole in the sample by utilizing the spattering phenomenon caused by irradiation.

Therefore, the surface accuracy of the cross section of the sample processed by the focused ion beam device is greatly influenced by the diameter of the ion beam and its profile (current distribution of the ion beam). As a result, this impedes the complete vertical machining of the cross section, leaving a taper in the cross section.

FIG. 1 shows an example of a processed sample cross section. FIG. 1A shows an example of processing a sample for a scanning electron microscope, and FIG. 1B shows processing of a sample for a transmission electron microscope. An example is shown. As shown in the figure, it is usually impossible to perform vertical cross-section processing, and there is a taper angle θ.

The existence of such a taper causes the following problems. First of all, the presence of the taper angle θ makes it impossible to observe the vertical cross section in the original sense. That is, the upper part of the cross section is observed at a distant position, and the lower part of the cross section is observed at a close position. This is an important problem to be solved because the miniaturization and densification of semiconductor patterns have been advanced in recent years.

Secondly, since the taper angle is influenced by the machining depth in addition to the diameter and profile of the ion beam, it is difficult to specify the machining position. Therefore, if the processing time is set incorrectly, there is a risk of digging up the desired observation site.

In order to avoid the above first and second problems, at present, the first cross section is processed in front of the desired observation point,
After that, a method is adopted in which processing is gradually advanced into a slice shape so that all cross sections of a desired observation location can be observed. This method has the effect of reducing the taper angle θ, and is also effective in knowing the three-dimensional structure of the cross section.
It has the drawback of being time consuming. Especially when only one cross-sectional observation is required, it leads to extra time.

Thirdly, it is impossible to form a thin film for observing a transmission electron microscope image with a uniform thickness. At present, in order to obtain a uniform thickness, a method of tilting and processing the sample at the end to remove the taper part is taken as a finish, but this method is not suitable for thin film There is a risk that most of the thin film will be broken if a hole is made or if it is extremely severe.

Fourthly, in the case of processing a sample for a transmission electron microscope, it is necessary to limit the thickness of the thin film in order to leave the whole portion without dropping the upper part of the thin film. However, this not only spoils the single information of the observation point,
Limit the observation conditions of the transmission electron microscope.

In order to solve the above problems,
In processing with an ion beam, it is important to make the taper angle as small as possible. The present invention has been made in view of the above circumstances, and an object thereof is to realize a focused ion beam processing apparatus capable of processing a vertical cross section with a small taper angle.

[0014]

[Means for Solving the Problems] Here, the factors that determine the taper angle in ion beam processing will be discussed numerically. A well-tuned ion beam can be represented by a polar coordinate function a.PHI. (R) that is symmetric about the central axis of a normal ion beam. When r is 0, Φ (r) is 1.0, and when r is ∞, Φ (r) is 0.0.

Here, r (≧ 0) is the distance from the central axis of the ion beam, and a is a factor. Here, in order to simplify the subsequent calculation, if a two-dimensional Cartesian coordinate system is used and the central axis of the ion beam is x = 0, then Φ
Is as follows.

That is, when x is 0, Φ (x) is 1.
When it becomes 0 and x is ∞, Φ (x) becomes 0.0. Further, Φ (+ x) = Φ (−x). Further, when the half width Δ is defined, the following relationship is satisfied.

When Δ = 2 · x ₀ , Φ (± x ₀ ) = 1/2 In the case of rectangular processing with a focused ion beam apparatus, a general processing method is an ion beam scanning method as shown in FIG. , Or a similar scanning method. In FIG. 2, S is a processing range, s
Is a scanning line, and d is an ion beam scanning interval in the direction perpendicular to the cross section. Here, assuming that the processing depth is proportional to the current dose amount of the ion beam, the processing depth distribution D (x) in the direction perpendicular to the processing cross section is expressed by the following equation.

[0018]

[Equation 1]

In the above equation (1), f (x) is a factor function that collectively includes the milling rate ε of the material to be processed, the processing time t, the factor a, and the variable factors of ε due to the difference in the processing environment. . Further, n is the number of times the ion beam is scanned in the direction perpendicular to the cross section, which determines the processing range in the direction perpendicular to the cross section. It should be noted that ε can be represented by ε = V / (t · Ip), where V is the processing volume and Ip is the ion beam current. From this equation (1), it becomes clear that the taper angle is affected not only by the current distribution function of the ion beam but also by the scanning interval d.

In the case where the machining depth is not extremely larger than the machining range in a certain machining current range, the following equation generally holds in good approximation.

[0021]

[Equation 2]

From this (2), it can be understood that f (x) = N = (constant) can be expressed by a good approximation, if a certain specific material is processed for a specific time. At this time, N can be used as a depth normalizing factor.

Furthermore, it is well known that the current distribution function can be represented by a Gaussian function with a relatively good approximation. From this, the taper angle can be actually calculated using the following formula.

[0024]

(Equation 3)

FIG. 3 shows the depth distribution D (x) calculated using the above equation (3), and FIG. 3 (a) shows Δ = 1.
0, d = 0.25, and FIG. 3B shows Δ = 1.
0, d = 1.0. It can be seen from FIG. 3 that the taper angle depends on the scanning interval in the direction perpendicular to the processed cross section. Further, it can be seen from FIG. 3 that the accuracy of the machined bottom surface deteriorates when the scanning interval is set too large compared to the half-value width of the current distribution (d ≧ Δ).

However, in the general processing, the processing accuracy of the bottom surface is rarely a problem. It is also understood from equation (3) that the taper angle depends on the depth. Therefore, it is understood that the taper angle is not an essentially essential factor, but the taper length L, which does not depend on the working depth, is essential. The taper length L is as shown in FIG.
As shown in (a), it is the length of the tapered portion in the direction perpendicular to the processed cross section.

In FIG. 4, the scanning interval d and the taper length L (a = 0.1, b =) when the half-value width Δ is 1.0 and the maximum machining depth D is −1.0. 0.9) and the height h indicating the bottom surface processing accuracy are shown. Note that h
Is the distance between the upper and lower parts of the unevenness due to the processing accuracy, and D, L, h, a, b as shown in FIG.
The values of etc. are all normalized with Δ.

From FIG. 4, it is understood that, when the same depth is dug, the taper angle decreases as the scanning interval d of the processing ion beam in the direction perpendicular to the processing cross section increases, but the surface accuracy of the bottom surface deteriorates. To be done. However, as mentioned above,
When observing a cross-section that occupies a large specific gravity by using a focused ion beam device or preparing a sample for a transmission electron microscope, the surface accuracy of the bottom surface hardly poses a problem. Therefore, by utilizing the relationship between the taper angle and the scanning interval of the ion beam, it is possible to improve the cross-sectional surface accuracy in the focused ion beam processing.

By the way, all the units of the above equation (3) and each length in FIG. 4 are normalized by the half width Δ of the ion beam. This means that the result of FIG. 4 can be applied to all beam diameters (half-width Δ).

Here, from FIG. 4, it is assumed that the optimum normalized beam scanning interval d _b [1 / Δ] for processing with good taper angle and bottom surface accuracy is 1.1. Then, the relationship of the non-normalized optimum scanning interval D _b [nm] when the beam half width Δ [nm] is as follows.

D _b = d _b · Δ = 1.1 · Δ Further, the optimum taper length L _b [nm] and the optimum bottom surface accuracy h _b at this time are L when d = 1.1 in FIG. , H. That is, it becomes as follows.

L _b = LΔ h _b = hΔ The optimum beam scanning interval d _b = 1.1 [nm] described above.
Is a value of an example selected so that both the taper angle and the bottom surface accuracy are improved. However, if the processing purpose is to reduce only the taper angle θ, set d> 1.1, and the processing purpose is If the bottom surface accuracy is better, d <1.1 may be selected.

In the above example, the relationship between the beam half width Δ and the optimum scanning interval d _b is described, but this can be replaced with the relationship between the probe current Ip and the optimum scanning interval d _b . In a usual focused ion beam apparatus, the value to be controlled is not the beam half width, but the probe current, so it is convenient to use this relationship.

Usually, in any focused ion beam apparatus, the relationship between the beam half width Δ [nm] and the probe current Ip [pA] is well measured. Now, this relationship is Δ = f (I
If it is represented by p), f is on average a simple increasing function. From this, the optimum scanning interval D _b in the case of the probe current Ip (pA) is given by the following formula.

D _b = d _b · f (Ip) [nm] From the above consideration, the focused ion beam processing apparatus according to the invention of claim 1 places the ion beam source and the ion beam from the ion beam source on the sample. A focusing lens for narrowly focusing, a deflector for two-dimensionally scanning an ion beam on a sample, and a scanning circuit for supplying a two-dimensional scanning signal to the deflector are provided. It is characterized in that the scanning interval of the scanning signal supplied to the deflector in the processing section scanning direction can be arbitrarily adjusted.

Further, in the focused ion beam processing apparatus according to the second aspect of the invention, the ion beam source, the focusing lens for focusing the ion beam from the ion beam source finely on the sample, and the ion beam on the sample. The scanning circuit includes a deflector for two-dimensional scanning, a scanning circuit for supplying a two-dimensional scanning signal to the deflector, and means for setting a probe current of an ion beam with which a sample is irradiated. It is characterized in that the scanning interval of the scanning signal supplied from the to the deflector in the processing section scanning direction is changed according to the probe current.

Further, in the focused ion beam processing apparatus according to the invention of claim 3, an ion beam source, a focusing lens for narrowly focusing the ion beam from the ion beam source on the sample, and the ion beam on the sample. A deflector for two-dimensionally scanning and a scanning circuit for supplying a two-dimensional scanning signal to the deflector are provided, and the scanning interval of the scanning signal supplied from the scanning circuit to the deflector in the processing cross section scanning direction. Is configured to be relatively small in a region far from the machining cross section position and relatively large in a region close to the machining cross section position.

[0038]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 5 shows a focused ion beam processing apparatus according to the present invention, and 1 is an ion beam source. The ion beam IB generated from the ion beam source 1 and accelerated is an electrostatic condenser lens 2 and an objective lens 3.
Thus, it is finely focused on the sample 4. Further, the ion beam IB is two-dimensionally scanned on the specific region of the sample 4 by the deflector 5.

A lens power source 6 applies a lens voltage to the condenser lens 2, and a lens power source 7 applies a lens voltage to the objective lens 3. Ion beam IB
A diaphragm 8 is inserted on the optical axis of, and the probe current of the ion beam with which the sample 4 is irradiated can be changed by changing the voltage of each lens. The lens power supplies 6 and 7 are controlled by signals from a control device 9 such as a computer.

The electrostatic deflector 5 is supplied with a two-dimensional scanning signal from the scanning circuit 10. The scanning circuit 10 is controlled by the control device 9 and can change the scanning interval in the scanning signal. . An input device 11 is connected to the control device 9, and a desired probe current of the ion beam, a scanning interval of a scanning signal, and the like can be designated by the input device 11, and the control device 9 controls each circuit by this designation. To do.
The operation of such a configuration will now be described.

For example, when a sample for scanning electron microscope observation is set as the sample 4, two-dimensional scanning of the ion beam IB is performed near the observation cross-section position of the sample. That IB
The scanning region of is sputtered by the ion beam, and the cross section is exposed at the desired position of the sample. Then, this sample is taken out and introduced into the sample chamber of the scanning electron microscope, whereby a scanning electron microscope image of a desired cross section can be obtained. When a transmission electron microscope sample is set as the sample 4, the ion beam is two-dimensionally scanned on both sides of the linear observation region, and a thin film transmission electron microscope sample is prepared.

Here, the first processing method using the apparatus of FIG. 5 will be described. The two-dimensional scanning of the ion beam is raster scanning as shown in FIG. 3, and the scanning interval d in the direction perpendicular to the processing cross section at this time can be arbitrarily set in the input device 11. The input device 11 supplies the input scanning interval d to the control device 9. The control device controls the scanning circuit 10 based on the given scanning interval d, and as a result,
The scanning interval of the ion beam IB on the sample is set to a desired value.

As described above, since the scanning interval d in the direction perpendicular to the processing cross section of the IB can be arbitrarily changed, it is possible to specify the optimal scanning interval according to the processing purpose. For example, when it is desired to reduce the taper angle, the scanning interval may be set relatively large. If it is desired to improve the surface accuracy of the processed bottom surface, the scanning interval may be set to be relatively small.

Next, a second processing method using the apparatus shown in FIG. 5 will be described. In the apparatus of FIG. 5, the probe current of the ion beam IB irradiated on the sample 4 can be arbitrarily changed by adjusting the condenser lens 2 and the objective lens 3 using the lens power supplies 6 and 7. It should be noted that this probe current corresponds to the ion beam current distribution (actually, beam half-value width Δ) in a ratio of 1: 1.

In this case, since the optimum scanning interval d corresponding to the probe current distribution exists, the optimum beam scanning interval d (Φ) for each current distribution (probe current) is stored in advance in the memory of the controller 9. Remember. The relation between this current distribution and the optimum scanning interval is obtained experimentally or by calculation.

When a desired probe current is designated to the input device 11, the control device 9 controls the condenser lens 2 and the objective lens 3 via the lens power sources 6 and 7 so that the lens strength is to realize the probe current. To do. Further, the control device 9 reads the optimum scanning interval for the specified probe current from the stored relationship between the probe current and the scanning interval, and supplies it to the scanning circuit 10 to optimize the scanning interval of the scanning signal. It should be As a result, the sample is processed with a small taper angle with any probe current.

Next, a third processing method using the apparatus shown in FIG. 5 will be described. The scanning signal from the scanning circuit 10 is shown in FIG.
As shown in, the scanning interval in the vertical direction of the machining section Q, which is the end of the scanning area S, is small in the area far from the machining section Q, and is large in the area close to the machining section Q. In the figure, s is a scanning line. As a result, the surface accuracy of the machined bottom surface can be improved in the distant region where the scanning interval is small, while the taper angle of the machined cross section can be reduced in the region close to the machined section Q.

[0048]

As described above, in the focused ion beam processing apparatus according to the first aspect of the present invention, it is possible to arbitrarily adjust the scanning interval in the processing section scanning direction of the scanning signal supplied from the scanning circuit to the deflector. Since it is configured, when the sample for the scanning electron microscope or the sample for the transmission electron microscope is prepared, the taper angle of the processed cross section can be remarkably reduced.

Therefore, in the case of processing a sample for a scanning electron microscope, a sample capable of observing a vertical cross section in the original sense can be prepared in a short time. Further, in processing a sample for a transmission electron microscope, it is possible to prepare a thinner film-like sample having a uniform thickness without causing breakage of the sample.

Further, in the focused ion beam processing apparatus according to the second aspect of the present invention, the scanning interval of the scanning signal supplied from the scanning circuit to the deflector in the processing section scanning direction is changed according to the probe current. The same effect as the invention of claim 1 can be achieved.

Further, in the focused ion beam processing apparatus according to the third aspect of the invention, the scanning interval of the scanning signal supplied from the scanning circuit to the deflector in the processing section scanning direction is made relatively small in the area far from the processing section position. Since it is configured to be relatively large in the region close to the processed cross-section position, the same effect as the invention of claim 1 can be achieved.

[Brief description of drawings]

FIG. 1 is a view showing a cross section of a processed sample.

FIG. 2 is a diagram showing an ion beam scanning method during processing.

FIG. 3 is a diagram showing a depth distribution D (x).

FIG. 4 is a diagram showing a relationship between a taper length L and a height h indicating bottom surface processing accuracy.

FIG. 5 is a diagram showing a focused ion beam processing apparatus according to the present invention.

6 is a diagram showing an ion beam scanning method when a processing method using the apparatus of FIG. 5 is executed.

[Explanation of symbols]

 1 Ion Beam Source 2 Condenser Lens 3 Objective Lens 4 Sample 5 Deflector 6, 7 Lens Power Supply 8 Aperture 9 Control Device 10 Scanning Circuit 11 Input Device

Claims (3)

[Claims] 1. An ion beam source, a focusing lens for finely focusing an ion beam from the ion beam source on a sample, a deflector for two-dimensionally scanning the ion beam on the sample, and a deflector. And a scanning circuit for supplying a two-dimensional scanning signal to the deflector, and the focused ion beam processing apparatus is configured so that the scanning interval of the scanning signal supplied from the scanning circuit to the deflector in the processing section scanning direction can be arbitrarily adjusted. . 2. An ion beam source, a focusing lens for narrowly focusing the ion beam from the ion beam source on the sample, a deflector for two-dimensionally scanning the ion beam on the sample, and a deflector. A scanning circuit for supplying a two-dimensional scanning signal to the sample, and means for setting a probe current of the ion beam with which the sample is irradiated. A focused ion beam processing apparatus configured to change the scanning interval according to the probe current. 3. An ion beam source, a focusing lens for narrowly focusing the ion beam from the ion beam source on the sample, a deflector for two-dimensionally scanning the ion beam on the sample, and a deflector. A scanning circuit for supplying a two-dimensional scanning signal to the deflector, and the scanning interval of the scanning signal supplied from the scanning circuit to the deflector in the machining section scanning direction is made relatively small in the region far from the machining section position, A focused ion beam processing device configured to be relatively large in a region near the processing cross-section position.