JPH0329319A - Focus adjustment method - Google Patents

Abstract

PURPOSE:To adjust a focus more simply and easily irrespective of kinds of specimens or the like by a method wherein a change with the passage of time in an intensity of secondary electrons emitted from the surface of a plane specimen when the specimen is irradiated with a focused ion beam is measured and the focus is adjusted on the basis of this data. CONSTITUTION:A sputtering operation is caused in a part of the surface of a specimen 2 which is irradiated with a focused ion beam 1; for a short while after this irradiation, an intensity of secondary electrons is increased because the surface of the specimen is roughened by this sputtering operation and a surface area is increased. After that, when a hole 3 by the sputtering operation becomes deep, the secondary electrons cannot go out to the outside of the specimen 2 from the hole 3, and the intensity of the secondary electrons is reduced. A voltage applied to an objective arranged and installed on the surface of the specimen is changed and a change with the passage of time in the intensity of the secondary electrons is measured; a focus is adjusted on the basis of this data. Thereby, a focus of the focused ion beam can be adjusted simply and so as to be adapted to automation.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an adjustment method for focusing an ion beam on a sample surface in a focused ion beam implanter.

[Conventional technology]

Conventionally, the method used to adjust the focus of the ion beam in a focused ion beam implanter is as follows:
For example, as shown in FIG. 5, a pattern groove 12 with a width of about 10 μm and a depth of about 2 μm is formed on a GaAs substrate 11 by reactive ion etching, etc., and this pattern groove 12 is scanned with a focused ion beam (image). Based on this, methods have been taken to adjust the beam ratio and focus to the substrate surface. In Fig. 5, 13 indicates the scanning direction of the concentrated ion beam, Fig. 5 (.) is a plan view on the substrate, Fig. 5 (b) is a cross-sectional view thereof, and Fig. 5 (c) is the doublet of the groove portion with respect to the scanning distance. FIG. 3 is a diagram showing secondary electron intensity. Note that the focused ion beam implantation device is generally well known; for example, the following literature (J. MELNGAILLIS., Journal of Vacuum Science and Technology,
Vol. B5, P. 469, (1987)).

[Problem to be solved by the invention]

Although such conventional methods have the advantage of being able to adjust the shape of the beam, 1) it is necessary to create a pattern on the sample surface in advance; If you don't get it close to
I can't do 4.

2) If crystal growth, chemical etching, etc. are performed as a process before focused ion beam implantation, the shape of the pattern will be distorted.

3) Since we are essentially looking at a secondary electron image, if the beam current of the focused ion beam is less than 20 pA, the image becomes dark and focus adjustment becomes difficult.

There were other problems.

The present invention has been made in view of the above points, and its purpose is to be simpler than the above-mentioned conventional method, to be easily applicable regardless of the type of sample, and to provide a secondary electron image. It is an object of the present invention to provide a method for adjusting a focal point in a focused ion beam implantation device that can be applied even to a minute current region that is almost invisible.

[Means for Solving the Problems] In order to achieve such an object, the focus adjustment method of the present invention is based on the method of adjusting the focus of the present invention by adjusting the temporal change in the intensity of secondary electrons emitted from the surface of the sample when a focused ion beam is irradiated onto the plane sample. measure,
The main feature is that the focus is adjusted based on this data.

[Effect]

Therefore, in the present invention, there is no need to prepare a pattern on the sample in advance, and it can be applied to etched surfaces or crystal-grown surfaces as long as flatness is maintained, and can be applied to any material as long as the sample is flat. It can be applied without any problem.

In addition, instead of observing secondary electrons as images as in conventional technology, changes in secondary electron intensity can be measured using a microcurrent meter, so the focus can be precisely adjusted even for microionic currents of 1 pA or less. can. Furthermore, by applying the method of the present invention to several points on a large-area sample wafer, it is easy to control the entire surface of the wafer to be in focus.

〔Example〕

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

First, the principle of the method of the present invention will be explained using the first section. Here, the present invention basically uses a focused ion beam implantation device similar to the conventional example.
As shown in a), the focused ion beam 1 hits the sample 2.
When the surface of the surface is irradiated, sputtering occurs in this area. For a while after this irradiation, the sample surface becomes rough due to sputtering and the surface area increases, so the secondary electron intensity increases. Thereafter, as the hole 3 (FIG. 2(b)) formed by the spatter becomes deeper, the secondary electrons cannot escape from the hole 3 to the outside of the sample 2, and the secondary electron intensity decreases. Therefore, flat sample 2
When the focused ion beam 1 is irradiated to (.) '), and then decreases (the same figure (
b)). The time constant of this decrease can be expressed as the difference in time to cross the 90% and 10 width values, respectively, between the peak value and the value that stabilizes after long irradiation (also referred to as surface drilling time) tb. Further, this change is focused if the current value is constant, and as the beam becomes smaller in diameter, the current density increases and becomes faster. Therefore, as the focus increases, the above t
,, tbO value decreases. Therefore, by changing the voltage applied to an objective lens (not shown) disposed on the sample surface, these values t@! The focus can be automatically adjusted to find the voltage that minimizes tb.

That is, when the focused ion beam 1 is irradiated onto the surface of the sample 2, the focused state of the ion beam 1 is evaluated from the temporal change in the intensity of the secondary electrons emitted, and this data is fed back to the objective lens to determine the applied voltage. By changing , the focused ion beam can be focused onto the sample surface.

At this time, it is actually possible to measure the secondary electron intensity, but since the following relationship exists: (sample current) = (ion current) + (secondary electron emission amount), it is better to measure the secondary electron intensity. The sample current may also be measured separately. In reality, it is often easier to measure the sample current, so the following examples will show examples in which measurements were made using the sample current.

FIG. 2 shows an example in which a GaAa substrate is actually irradiated with a Ga focused ion beam and a sample current is measured. In the same figure, curve I shows the change in sample current when the beam diameter R = 70 nmφ, and curves ■ and ■ indicate R
= 12011rflφ. It shows the change in sample current when R=230 nmφ. 1, the beam irradiates one spot with repetitions at 1 msec intervals. In this example, the total irradiation time is 20 seconds. As the sample current, the average current during irradiation and during intervals is measured. The expected change in the sample current in Figure 1 is clearly visible, and it can be seen that the trench t, +tb becomes shorter as the beam diameter R becomes smaller. In addition, in this experiment, the ion current was 5p.
It was carried out at a low current of A, but considering the noise situation etc.
It can be seen that application is possible in At. In addition, t in Figure 2
o indicates a scale of 10 seconds, and the arrow indicates the time of ion beam irradiation (“on”).

Next, t is determined by gradually changing the voltage Vo applied to the objective lens (deviation from just focus).
@． tb is shown in FIG. However, in the figure, the solid line indicated by the symbol A represents the characteristic of t1 with respect to the voltage Vo applied to the objective lens, and the dotted line also indicated by the symbol A represents the characteristic of tb. As is clear from FIG. 3, t. The focus can be automatically adjusted by first finding vo that is in focus from a point where the distance is ~0 seconds, and then by finely changing Vo to find vo that minimizes tb.

All of these tasks can be performed automatically by a computer, making focusing easy.

Although the above embodiments have described a method for focusing a focused ion beam on a sample surface, the present invention can also be used to focus a focused ion beam over a large area using this method. In other words, the method described in the above example is applied to a flat wafer on which no pattern is formed.
Automatic adjustment can be made so that the focused ion beam is focused over the entire surface, and the outline thereof will be explained with reference to FIG. 4.

Figures 4 (.) and (b) are a plan view and a side view of the wafer in the wafer holding mechanism section to which the focused ion beam is irradiated. Holders 71172 and T3 hold the height trenches near each 11 + b r e point on the wafer 5.
This is a manipulator for making fine adjustments in direction. Here, the manipulator T1 is set to an appropriate value at the point a on the S5, and the voltage of the objective lens is adjusted using the method of the above embodiment so that the object is in focus. Next, move the Ueno 5 to point b, irradiate the beam several times near point b while changing the manipulator 72, and from the time change, the manipulator 7! Determine.

Next, the wafer 5 is moved to point C, and the same operation is performed near point C to align the manipulator T3. This movement is from point a → b
The process from point to point C is repeated to obtain flatness of the entire Ueno 1. This method can be applied to flat wafers without any patterns and can be completely controlled by a computer. In addition, it is sufficient for beam irradiation to shift the spot by 2 μm. For example, even if irradiation is performed 100 times at point a, the area required for this is at most 20 μm.
It is the mouth. Therefore, by using an extremely small area on the wafer, the wafer can be flatly aligned with the focused ion beam. This method uses a focused ion beam that is almost in focus, and when performing crystal growth, repeated ion implantation, etc., a computer automatically focuses the entire surface of the wafer on the surface after crystal growth. It is useful for matching purposes.

〔Effect of the invention〕

As explained above, the present invention measures the temporal change in the intensity of secondary electrons emitted from the sample surface when a focused ion beam is irradiated onto a flat sample, and adjusts the focus based on the data. b1 A method for adjusting the focus of a focused ion beam that is simple and suitable for automation can be provided. It not only can be applied to flat crystals, but also has the advantage of being able to precisely adjust the focus even when using a minute beam current.

[Brief explanation of drawings]

Fig. 1 is a diagram explaining the principle of the method of the present invention, Fig. 2 is a diagram showing an example of sample current change due to a change in beam diameter, which is used to explain an embodiment of the present invention, and Fig. 3 is a diagram illustrating an example of the change in sample current due to a change in beam diameter. FIG. 4 is a diagram showing an experimental example of changes in t and tb when the objective lens voltage is shifted from just focus to explain one embodiment. FIG. 4 is a schematic diagram to explain another embodiment of the present invention. 5
The figure is an explanatory diagram showing an example of a conventional focus adjustment method. 1 ● Focused ion beam, 2 ・Sample,

Claims (1)

[Claims] In a focused ion beam implanter, the focused state of the ion beam is evaluated from the temporal change in the intensity of the secondary electrons emitted when the ion beam is irradiated onto a flat sample, and the voltage applied to the objective lens is changed based on this result. A focusing method characterized by focusing an ion beam on a sample surface by: