JPH0440825B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to an automatic focus adjustment method for an electron microscope, etc., and more particularly, to control of the amount of detected signals accumulated by multiple scans during automatic focus adjustment. .

(Prior art) Conventionally, when performing automatic focus adjustment (autofocus) of an electron microscope,
Alternatively, scan the electron beam in the Y direction, read the signal amount from the sample every scan or a certain number of scans, and change the current flowing through the objective lens coil each time to obtain the maximum signal amount. things are being done. This method utilizes the phenomenon that the amount of signal obtained from a sample is greatest when it is in focus.

FIG. 2 is a block diagram of the main parts of such an automatic focus adjustment circuit. In the figure, 1 is an arithmetic control unit (hereinafter abbreviated as CPU) that performs various arithmetic controls.
From the CPU1 to the interface (hereinafter abbreviated as ITF)
2 via digital scan generator 3
Scanning direction instruction data is added to the integration number setting circuit 4, integration number data, that is, scanning number data is added to the integration number setting circuit 4, and initial value data of the current flowing through the objective lens coil 7 is added to the objective lens controller 6 via the ITF 5. , a gain control signal is applied to the gain controller 9 via the ITF 8. 10 is a marker cursor control circuit, ITF
2, a control signal is applied from the CPU 1, and a control signal is also applied from the digital scan generator 3, and based on these control signals, a predetermined control signal is output to the integration number setting circuit 4 and the gain controller 9. 11 is the electron beam Bi
12 is a detector that detects a reflected signal from the sample 11. A detection signal output from the detector 12 is applied to a data integration circuit 13 via a gain controller 9. A data sampling control signal is applied to the data integration circuit 13 from the integration number setting circuit 4. The detection signal integrated by the data integration circuit 13 is
The signal is converted into an analog signal by the A converter 14 and applied to the peak value detection circuit 15, where the peak value is detected. The peak value data detected by the peak value detection circuit 15 is converted into a digital signal by the A/D converter 16, and then taken into the CPU 1 via the ITF 2.

FIG. 3 is a flowchart showing the conventional operation flow of a circuit configured as described above.

First, the CPU 1 supplies data to the digital scan generator 3 to set the scanning direction (step), and then the CPU 1 supplies data to the digital scan generator 3 to set the scanning direction (step).
The CPU 1 then sends data to the objective lens controller 6 to set the number of scans (step).
data is given to set the initial value of the current flowing through the objective lens coil 7 (step). Then, the sample 11 is scanned according to each of these setting conditions, and the reflected signal from the sample 11 at that time is detected by the detector 12, amplified by the gain controller 9, and then provided to the data integration circuit 13. This process is repeated a predetermined number of times, and the data integration circuit 1
3, the data for a predetermined number of scans are integrated, and the integrated detection signal is converted into an analog signal by the D/A converter 14, and then the output of the D/A converter 14 is input to the peak value detection circuit 15. to detect the peak value (step).

Next, it is determined whether or not the step operation is the first time (step), and if it is the first time, the current flowing through the objective lens coil 7 is changed and the step and step operations are executed again. Then, the process moves to a step of comparing signal amounts. In step, the previous peak value data and the current peak value data are compared to determine whether to increase or decrease the current flowing through the objective lens coil 7. In this way,
The CPU 1 adjusts the value of the current flowing through the objective lens coil 7 so that the peak value of the detection signal is maximized.
Find the current value that is in focus.

(Problems to be Solved by the Invention) However, according to such a conventional procedure, the number of scans is fixed to an initial value. For this reason, when the amount of integrated detection signals is small, the S/N ratio deteriorates and the focus adjustment accuracy decreases, and conversely, when the amount of integrated detection signals is too large, the circuit becomes saturated and the accuracy increases. will get worse.

The present invention has been made in view of these points, and an object thereof is to realize an automatic focus adjustment method for an electron microscope, etc., which can perform automatic focus adjustment with high precision.

(Means for Solving the Problems) The present invention, which solves the above-mentioned problems, reads and integrates signals from a sample obtained by scanning a charged particle beam multiple times, and detects an object using an object according to the amount of the integrated detection signals. An automatic focus adjustment method for an electron microscope, etc., which adjusts the focus so that the amount of detected signal is maximized by changing the current flowing through the lens coil,
The present invention is characterized in that the optimum number of scans is calculated based on the detection signal amount obtained by the initially set number of scans, and automatic focus adjustment is performed in accordance with the optimum number of scans.

(Function) The optimum number of scans is determined so that the S/N ratio is good and the circuit is not saturated from the detection signal integrated value obtained by scanning only the initially set number of scans.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a flowchart showing one embodiment of the method of the present invention. The present invention differs from the conventional one in that the optimum number of scans is calculated based on the detection signal amount obtained by integrating the initial set number of scans, and automatic focus adjustment is performed according to the optimum number of scans. be. The method of the present invention will be explained using the circuit shown in FIG. That is, as in the conventional case, first, the CPU 1 gives data to the digital scan generator 3 to set the scanning direction (step), and then the CPU 1 gives data to the integration number setting circuit 4 to set the initial number of scanning ( Furthermore, the CPU 1 gives data to the objective lens controller 6 to set the initial value of the current flowing through the objective lens coil 7 (step). Then, according to each of these setting conditions, sample 1
1 is scanned, and the reflected signal from the sample 11 at that time is detected by the detector 12, amplified by the gain controller 9, and then provided to the data integration circuit 13. This process is repeated a predetermined number of times,
The data integration circuit 13 integrates data for a predetermined number of scans, the integrated detection signal is converted into an analog signal by the D/A converter 14, and then the output of the D/A converter 14 is sent to the peak value detection circuit. 15 to detect the peak value (step).

Next, it is determined whether or not the step operation is the first time (step), and if it is the first time, the CPU 1 calculates the appropriate number of scans based on the peak value and updates the number of scans according to the result (step). ). As a result, from the second time onwards, the adjustment operation will be performed using this updated number of scans. If it is not the first time, the process moves to a step of comparing the accumulated detection signal amounts. In the step, compare the previous peak value data and the current peak value data,
Decide whether to increase or decrease the current flowing through the objective lens coil 7. In this way, the value of the current flowing through the objective lens coil 7 is adjusted so that the peak value is maximized, as in the conventional case, and the current value at which the object is in focus is found.

Here, the appropriate number of scans N is calculated as follows. That is, if the maximum information amount of the circuit is A, the number of scans is n ₁ , the signal value is B, and the coefficient is C, it can be calculated as N=n ₁ ×(A/B)×C. For example, A=4096, n ₁
= 20, B = 1024, C = 0.5, N = 20 x (4096/1024) x 0.5 = 40, and by scanning 40 times, a signal amount suitable for automatic focus adjustment can be obtained. This allows for highly accurate automatic focus adjustment. Further, if necessary, an appropriate input signal may be calculated based on the first measurement result, and the calculated result may be fed back to the gain controller 9 in FIG. 2. In the above description, the automatic focus adjustment method was explained using an electron microscope as an example, but the present invention is not limited to this, and can be applied to any structure in which a charged particle beam is scanned and irradiated onto a sample. Can be applied. For example, it can be applied to a focused ion beam device, etc.

(Effects of the Invention) As explained above, according to the present invention, an automatic focus adjustment method for electron microscopes, etc., that can perform highly accurate automatic focus adjustment by calculating the optimal number of scans from the accumulated detection signal amount is realized. can.

[Brief explanation of drawings]

FIG. 1 is a flowchart showing an embodiment of the method of the present invention, FIG. 2 is a block diagram of a main part of an automatic focus adjustment circuit, and FIG. 3 is a flowchart showing a conventional flow of operation. 1...CPU (calculation control unit), 2, 5, 8...
ITF (interface), 3...Digital scan generator, 4...Integration number setting circuit, 6...
...Objective lens controller, 7...Objective lens coil, 9...Gain controller, 10...Marker/cursor control circuit, 11...Sample, 12...
Detector, 13...Data integration circuit, 14...D/
A converter, 15...Peak value detection circuit, 16...
A/D converter.

Claims (1)

[Claims] 1. Read and integrate the signals from the sample caused by multiple scans of the charged particle beam, and adjust the focus so that the detected signal amount is maximized by changing the current flowing through the objective lens coil according to the integrated detected signal amount. An automatic focus adjustment method for an electron microscope, etc., which calculates the optimal number of scans based on the amount of detection signal obtained by the initially set number of scans, and performs automatic focus adjustment according to the optimal number of scans. An automatic focus adjustment method for an electron microscope, etc., characterized in that: