JPH0644937A - Electron beam device - Google Patents

Abstract

PURPOSE:To keep an optical system in optimum state at all times, independent of an operator, by providing a scanning type electron beam device with an alternate execution control means having the capability of automatically executing image acquisition mode and optical adjustment mode alternately. CONSTITUTION:A scanning type electron beam device scans a sample via an electron beam emitted from an electron gun, and detects secondary electrons emitted from the surface of the sample in image acquisition mode, thereby acquiring an image corresponding to the surface configuration of the sample. In optical adjustment mode, the device executes the adjustment mode of an electron beam optical system, for example, the mode of focal point adjustment, flying spot adjustment and optical axis adjustment. In addition, an alternate execution control means automatically performs an optical system adjustment operation at the predetermined intervals during an image acquisition process. Thus, the optical system can be always maintained in optimum state, without requiring an operator to be aware of necessity of optical system adjustment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam apparatus such as an electron beam tester used for LSI testing and failure analysis, and in particular, an electron beam optical system is always in an optimum state without requiring special work by an operator. The present invention relates to an electron beam device that can be maintained.

In the conventional scanning electron beam apparatus, skill and experience are required to adjust the electron beam optical system. Therefore, there is an urgent need to develop an electron beam device having an automatic adjustment function of an optical system, and various electron beam devices with an automatic adjustment function have been proposed. However, in any of the devices, the activation of the automatic adjustment function itself is left to the judgment of the operator, so that the electronic system can always maintain the optimum optical system state without annoying the operator's hand during operation. Beam devices are desired.

[0003]

2. Description of the Related Art In this type of electron beam apparatus, the focal point and astigmatism of the electron beam optical system are often displaced due to movement of the measuring point. Therefore, when the measurement point moves, it is usually necessary to readjust the electron beam optical system.

Conventionally, the adjustment of the optical system in the electron beam apparatus is performed manually by an operator's experience and intuition, or automatically by a servo system based on a deviation between a preset optimum value and a current value. Has been done.

However, in any method,
If you always want to keep the optical system in an optimal state,
The operator is always aware of whether or not the optical system needs to be adjusted,
If necessary, the image acquisition mode must be interrupted and the optical system adjustment mode must be activated, resulting in poor operability.

[0006]

As described above, as described above,
The adjustment of the optical system in the electron beam device is performed by a manual method based on the experience and intuition of an operator, or an automatic method using a servo system based on a deviation between a preset optimum value and a current value. In any of the methods, when always trying to maintain the optical system in an optimal state, the operator should always be aware of the necessity of adjusting the optical system, and if necessary, suspend the image acquisition mode. Then, the optical system adjustment mode has to be started, and there is a problem that operability is poor.

The present invention has been made in view of the above problems, and an object thereof is to always optimize the state of the optical system without making the operator aware of the necessity of adjusting the optical system. An object is to provide an electron beam device that can be maintained.

[0008]

In order to achieve the above object, the present invention scans a sample with an electron beam emitted from an electron gun and detects secondary electrons emitted from the surface of the sample. In the scanning electron beam apparatus having an image acquisition mode for acquiring an image corresponding to the surface shape of the above and an optical system adjustment mode for adjusting the electron beam optical system which is a prerequisite for this image acquisition, the image acquisition mode and the optical system It is characterized in that an alternate execution control means for automatically performing alternate operation with the adjustment mode is provided.

[0009]

As shown in FIG. 1, according to the present invention, the image acquisition mode (step 101) and the optical system adjustment mode (step 102) are automatically alternated. Therefore, the optical system is always maintained in an optimum state while performing image observation.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described in detail below with reference to the drawings. The electron beam apparatus according to the present invention scans a sample with an electron beam emitted from an electron gun, detects secondary electrons emitted from the sample surface, and thereby acquires an image corresponding to the surface shape of the sample. In a scanning electron beam apparatus having a mode and an optical system adjustment mode for adjusting an electron beam optical system which is a prerequisite for image acquisition, alternate execution of automatically performing the image acquisition mode and the optical system adjustment mode It is characterized in that a control means is provided (see FIG. 1).

The search for the optimum value in a normal optical system is performed within a relatively wide search range while narrowing the range. However, in the present invention, it is equivalent to constantly adjusting the optical system while acquiring an image, and it is usually not considered that the optical system is extremely shifted. However, when the observation position changes due to movement of the stage or the like, the optical system may shift due to changes in the height of the sample, changes in the environment around the observation position, or the like. Moreover, it is unclear how much it deviates. Therefore, the search range in the adjustment mode activated between the image acquisition modes is determined by how much the stage has moved immediately before.

FIG. 2 shows an example of the processing when the stage is not moved or the moving distance is short when the image acquisition and the focus adjustment are automatically performed alternately. In this case, the movement distance of the stage is small, and thus the fluctuation of the focus is small, so that the search range may be narrow. Therefore, when the focus adjustment mode is started once, the exciting current values I _o and I _{o of the} OL are generated.
The electron beam is line-scanned with three values of ± Δi (steps 201 to 208), and an evaluation amount is obtained from the sampling secondary electron signal amount (steps 209 and 210).
Here, as the evaluation amount, it is preferable to use the sharpness between adjacent data disclosed in JP-A-60-147117. This evaluation amount can be used for both focus and astigmatism adjustment. The exciting current that gives the maximum value among the obtained evaluation values is fed back to the next image acquisition mode.

FIG. 3 shows an example of the processing when the stage movement amount is large when the image acquisition and the focus adjustment are automatically alternately performed. In this case, the movement distance of the stage is large, and thus the fluctuation of the focus is large. Therefore, it is necessary to widen the search range. Therefore, the entire search range is divided into n (step 303), and only one of the divided ranges is searched in one adjustment mode (step 30).
5). Then, when the next adjustment mode is activated (step 310), the search (step 305) and the image acquisition (step 306) are alternately performed as in the next divided search range (step 305). After the search of the entire search range is completed (YES in step 307), the exciting current value that maximizes the evaluation amount is fed back (step 308).

Although the above description has been made with respect to the focus adjustment, the astigmatism adjustment can be similarly performed.
Next, some examples in the case where the image acquisition mode and the optical system adjustment mode (focus adjustment mode, astigmatism adjustment mode, optical axis adjustment mode) are automatically alternated are shown in FIGS.

FIG. 4 shows a case where the image acquisition mode and the focus adjustment mode are automatically alternated (steps 401 to 40).
3), in FIG. 5, when the image acquisition mode and the astigmatism adjustment mode are automatically alternated (steps 501 to 505),
FIG. 6 shows a case where the image acquisition mode, the focus adjustment mode, and the astigmatism adjustment mode are automatically alternated (steps 601 to 6).
07), FIG. 7 shows a case where the image acquisition mode and the optical axis adjustment mode are automatically alternated (steps 701 to 70).
3), FIG. 8 shows a case where the image acquisition mode, the focus adjustment mode, and the optical axis adjustment mode are automatically alternated (step 8).
01 to 805), FIG. 9 is a case where the image acquisition mode, the astigmatism adjustment mode, and the optical axis adjustment mode are automatically alternated (steps 901 to 907), and FIG. 10 is the image acquisition mode, the focus adjustment mode, and the astigmatism. When automatically performing the adjustment mode and the optical axis adjustment mode alternately (steps 1001 to 1)
009) processing.

Next, FIG. 11 shows an embodiment in which the process of carrying out the optical system adjustment mode is alternately repeated after carrying out the image acquisition mode a plurality of times. In this example, every time the image acquisition mode is repeated n times, the adjustment mode (for example, the focus adjustment mode, the astigmatism adjustment mode, the optical axis adjustment mode) is activated (steps 1101 to 1107).

Next, FIG. 12 shows a start-up time schedule in the case where the image acquisition mode and the optical system adjustment mode are alternately executed by the scheduling. In the figure, the black-filled time is the time when the mode is activated, and the image acquisition mode is activated at equal intervals. In the meantime, each mode is activated between the image acquisition modes according to an arbitrary set activation interval. If no mode is scheduled to be activated, nothing may be done at that time or the image acquisition mode may be activated.

Next, the structure of the adjustment end judging means in the focus adjustment mode and the astigmatism adjustment mode will be described. First, the current value is the origin. When the adjustment result is shifted from the origin in the + direction, +1 is added to the adjustment end flag. On the contrary, when the shift is made in one direction, -1 is added. The end determination is determined to be end when the value of the end determination flag is within a certain range, for example, the range of -1 to +1 and the adjustment result of the specified number of times is within. If the shift continues to the +1 side or the -1 side, the value of the end determination flag is reset when the shift is made in the reverse direction. Then, the counting is started again from 0. When the measurement end determination function determines that the adjustment operation has ended, the subsequent adjustment operation is skipped and the operator is notified of the adjustment result. As means for notifying the operator, for example, visual means or auditory means can be appropriately selected.

Conventionally, the optical axis is adjusted by using a Faraday cup or the like, and the optical axis is adjusted so that the beam current is maximized. However, the optical axis adjustment in the present invention is performed by searching for the maximum value of the secondary electron signal amount obtained by line scanning the electron beam. However, since the secondary electron signal amount changes depending on the sample surface shape and the like, one line scan is performed for one parameter, and the average value of the secondary electron signal amount for one line is used to obtain the sample. Reduce the influence of surface shape, etc.
Also, since the optical axis rarely changes greatly,
The search range can be narrowed and the best optical axis can always be obtained.

The above-described adjustment of the optical system by switching between the image acquisition mode and the adjustment mode is effective only when the stage is stationary, and when the stage is moved, the image acquisition mode is adjusted. Preferably only one is activated.

Further, in the image acquisition mode, it is preferable to display the image by frame addition averaging. By adopting this method, it is possible to apparently absorb the unnaturalness that causes a delay in image update due to the activation of the adjustment mode between the image acquisition modes. In the present invention, the effect can be expected as the acquisition time per frame of image acquisition is shorter.

[0022]

As described above, according to the present invention, the state of the optical system can always be optimally maintained without making the operator aware of the necessity of adjusting the optical system.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram showing focus adjustment processing when stage movement is small.

FIG. 3 is a diagram showing focus adjustment processing when stage movement is large.

FIG. 4 is a diagram showing a process in a case where an image acquisition mode and a focus adjustment mode are automatically alternately executed.

FIG. 5 is a diagram showing a process in the case where an image acquisition mode and an astigmatism adjustment mode are automatically alternated.

FIG. 6 is a diagram showing a process in a case where an image acquisition mode, a focus adjustment mode, and an astigmatism adjustment mode are automatically alternately executed.

FIG. 7 is a diagram showing processing in a case where image acquisition mode and optical axis adjustment mode are automatically alternated.

FIG. 8 is a diagram showing a process when an image acquisition mode, a focus adjustment mode, and an optical axis adjustment mode are automatically alternated.

FIG. 9 is a diagram showing a process in a case where an image acquisition mode, an astigmatism adjustment mode, and an optical axis adjustment mode are automatically alternately executed.

FIG. 10 is a diagram showing a process in a case where an image acquisition mode, a focus adjustment mode, an astigmatism adjustment mode, and an optical axis adjustment mode are automatically alternately executed.

FIG. 11 is a diagram illustrating an example in which the process of performing the optical system adjustment mode is alternately repeated after performing the image acquisition mode a plurality of times.

FIG. 12 is a diagram showing a startup time schedule in the case where the image acquisition mode and the optical system adjustment mode are alternately performed by scheduling.

[Explanation of symbols]

 101 ... Image acquisition mode 102 ... Adjustment mode

Claims (1)

[Claims] 1. An image acquisition mode in which a sample is scanned with an electron beam emitted from an electron gun to detect secondary electrons emitted from the surface of the sample and thereby an image corresponding to the surface shape of the sample is acquired, and this image. In a scanning electron beam apparatus having an optical system adjustment mode for adjusting an electron beam optical system which is a prerequisite for acquisition, the image acquisition mode (101) and the optical system adjustment mode (1
02) is provided with an alternate execution control means that automatically alternates between the above and.