JP2706986B2 - Focus adjustment method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an adjustment method for focusing an ion beam on a sample surface in a focused ion beam implantation apparatus.

[Conventional technology]

Conventionally, as a method of adjusting the focus of the ion beam in a focused ion beam implanter, for example, as shown in FIG. 5, reactive ion etching or the like is applied to a GaAs substrate 11 to a width of about 10 μm and a depth of about 2 μm. Pattern groove 12
A method has been adopted in which the relative point and the focal point of the beam are adjusted to the substrate surface based on an image (image) obtained by scanning the groove 12 with a focused ion beam.
In FIG. 5, 13 indicates the scan direction of the concentrated ion beam,
2A is a plan view on the substrate, FIG. 2B is a cross-sectional view thereof, and FIG. 1C is a view showing the secondary electron intensity of the groove portion with respect to the scan distance. In addition, a focused ion beam implantation apparatus is generally well known, and for example, the following literature (J. MELNGAILLI
S., Journal of Bakiyum Science and Technology, Vol. B5, p. 469, (1987)).

[Problems to be solved by the invention]

Although such a conventional method has an advantage that the shape of the beam can be adjusted, 1) it is necessary to form a pattern on the surface of the sample beforehand, and the cross-sectional shape of the etching pattern can be adjusted vertically (θ to 90 °). If the distance is not close to), the focus cannot be adjusted properly.

2) If crystal growth and chemical etching are performed as a process before the focused ion beam implantation, the pattern shape is distorted.

3) Since a secondary electron image is essentially viewed, if the beam current of the focused ion beam is reduced to 20 pA or less, the image becomes dark and focus adjustment becomes difficult.

And other problems.

The present invention has been made in view of the above points, and its purpose is simpler than the conventional method described above, which can be easily applied irrespective of the type of the sample and the like, and furthermore, the secondary electron image is hardly visible. It is an object of the present invention to provide a focus adjustment method in a focused ion beam implantation apparatus which can be applied to a minute current region which does not exist.

[Means for solving the problem]

In order to achieve such an object, the focus adjustment method of the present invention measures the time change of the intensity of secondary electrons emitted from the surface of a sample when a focused ion beam is irradiated on a flat sample, and uses this data to determine The main feature is that the focus is adjusted.

[Action]

Therefore, in the present invention, it is not necessary to prepare a pattern on the sample in advance, and it can be applied to an etched surface or a crystal-grown table without any problem as long as flatness is maintained. Applicable regardless of material. Also, instead of observing the secondary electrons as an image as in the prior art, the change in the secondary electron intensity can be measured using a microammeter, so that the focus can be properly adjusted even for a small ion current of 1 pA or less. . Furthermore, if the method of the present invention is applied to a large area sample, that is, to several points on the wafer, it is easy to control so that the entire surface of the wafer is in focus.

〔Example〕

 Hereinafter, the present invention will be described with reference to the drawings.

First, the principle of the method of the present invention will be described with reference to FIG. Here, the present invention uses a focused ion beam implanter basically similar to the conventional example. As shown in FIG. 1 (a), the focused ion beam 1 irradiates the surface of the sample 2. And spattering occurs in this portion. For a while after this irradiation, the surface of the sample is roughened by spattering and the surface area increases, so that the secondary electron intensity increases. Thereafter, when the hole 3 (FIG. 4B) is deepened by the spatter, on the contrary, the secondary electrons cannot escape from the hole 3 to the outside of the sample 2, and the secondary electron intensity decreases. Therefore, a planar sample
When irradiated with the focused ion beam 1 to 12, the secondary electron amount is as shown in FIG. 1 (c), (also referred to as surface roughness time) the peak value is reached the time after irradiation until t _a is increased (the FIG. (A)), and it can be seen that it decreases thereafter (FIG. (B)). The time constant of the reduction can be represented by a peak value and a long (also referred to as a surface drilling time) each difference of 90% and 10% across the value time between values stabilized after irradiation t _b. Further, this change is focused when the current value is constant, and the current density increases and becomes faster as the beam diameter becomes smaller. Therefore, the above-mentioned t _a , t _b
Decreases. Therefore, the focus is automatically adjusted by changing the voltage applied to an objective lens (not shown) disposed on the sample surface and obtaining a voltage that minimizes these values t _a and t _b. can do. That is, the focused state of the ion beam 1 is evaluated from the time change of the intensity of secondary electrons emitted when the focused ion beam 1 is irradiated on the surface of the sample 2, and this data is fed back to the objective lens to apply the applied voltage to the objective lens. , The focused ion beam can be focused on the sample surface.

At this time, the secondary electron intensity may be actually measured. However, since the relationship of (sample current) = (ion current) + (secondary electron emission amount) exists, instead of measuring the secondary electron intensity, The sample current may be measured in advance. In practice, it is often easier to measure the sample current, so the following example shows an example in which measurement was performed with the sample current.

FIG. 2 shows an example in which a sample current is measured by actually irradiating a GaAs substrate with a Ga focused ion beam.
In the figure, a curve I shows a change in the sample current at a beam diameter R = 70 mmφ, and curves II and III respectively show R = 120 nm.
The change of the sample current at φ, R = 230 nmφ is shown. The beam irradiates one spot repeatedly at 1 msec intervals. In this example, the total irradiation time is 20 seconds. As the sample current, the average current during the irradiation and during the interval was measured. It can be seen that the change in the sample current expected in FIG. 1 is clearly shown, and that t _a and t _b become shorter as the beam diameter R becomes smaller. In addition, although this experiment was performed with a low current of 5 pA as the ion current, it can be seen that the application is possible up to 1 pA in consideration of noise conditions and the like. In FIG. 2, t ₀ indicates a scale of 10 seconds, and the arrow indicates the time of ion beam irradiation (“on”).

Next, t _a , t obtained by gradually changing the applied voltage (deviation from the just focus) V ₀ to the objective lens
_b is shown in FIG. However, a solid line denoted by reference numeral b in the figure represents a characteristic of t _a with respect to the objective lens applied voltage V _0, also the dotted line indicated by reference numeral B represents the characteristics of t _b. As apparent from Figure 3, Motomari is V ₀ which is approximately the focal fit from where a t _a ~0 seconds, by further finely changing the V ₀ t _b
The focus can be adjusted automatically by finding V ₀ that minimizes. All of these operations can be performed automatically by a computer, and focusing can be easily performed.

In the above embodiments, the method of focusing the focused ion beam on the sample surface has been described. However, the present invention can also focus on a wide area using this method. That is, for a flat wafer on which no pattern is formed, the method described in the above embodiment can be applied to automatically adjust the focused ion beam to focus on the entire surface of the wafer. This will be described with reference to FIG.

4 (a) and 4 (b) are a plan view and a side view of a wafer in a wafer holding mechanism to which a focused ion beam is irradiated, wherein 5 is a flat wafer, and 6 is holding the wafer 5. holder for, 7 _1, 7 ₂ and 7 ₃ are Manipiyureta for performing each a on the wafer 5, b, a height that is fine motion adjusting in the Z-direction around the point c. Here, first set the Manipiyureta 7 ₁ a point on the wafer 5 to the proper value, adjusts the voltage of the objective lens to fit the focus by using the method of Example. Then moving the wafer 5 to the point b, it is irradiated with several beam near the point b while changing the Manipiyureta 7 _2,
The Manipiyureta 7 ₂ determines the most focus of match positions from the time change. Then adjust the Manipiyureta 7 ₃ The process is similar for the wafer 5 in the vicinity of the moved point e to point c. This operation is further repeated from point a to point b to point c to obtain flatness over the entire wafer. This method can be applied to flat wafers without any pattern, all controlled by computer. It is sufficient to shift the spot by 2 μm for beam irradiation. For example, even if irradiation is performed 100 times at point a, the area required for this is only 20 μm. Thus, by using a very small area on the wafer, the wafer can be flattened to the focused ion beam. This method uses a focused ion beam, which is almost in focus, to automatically focus on the entire surface of the wafer using a computer when crystal growth and ion implantation are repeated. Useful for such things.

〔The invention's effect〕

As described above, the present invention measures the time change of the intensity of secondary electrons emitted from the surface of a sample when a focused ion beam is irradiated on a flat sample, and adjusts the focus based on the data, A focus adjustment method for a focused ion beam that is simple and suitable for automation can be provided. In addition, there is an advantage that the focus can be properly adjusted not only for a flat crystal but also for a small beam current.

[Brief description of the drawings]

FIG. 1 is a view for explaining the principle of the method of the present invention, FIG. 2 is a view showing an experimental example of a change in a sample current due to a change in a beam diameter used for explaining one embodiment of the present invention, and FIG. FIG. 4 is a diagram showing an experimental example of a change in t _a and t _b when the objective lens voltage used in the description of the embodiment is shifted from the just focus, FIG. 4 is a schematic diagram used in describing another embodiment of the present invention, FIG. 5 is an explanatory diagram showing an example of a conventional focus adjustment method. 1 Focused ion beam, 2 Sample, 3 Hole, 5
...... Wafer, 6 ...... Holder, 7 ₁ , 7 ₂ , 7 ₃ ...... Z direction fine adjustment manipulator.

Claims (1)

(57) [Claims] In a focused ion beam implanter, a focused state of an ion beam is evaluated from a temporal change of a secondary electron intensity emitted when a planar sample is irradiated with an ion beam, and the focused state is applied to an objective lens based on the result. A focus adjustment method characterized by focusing an ion beam on a sample surface by changing a voltage to be applied.