JPH02144840A - Automatic focusing method for charged particle beam device - Google Patents

Abstract

PURPOSE:To make automatic focusing with high precision possible by evaluating a detection signal with the ratio of a peak height to a peak width. CONSTITUTION:For example, reflected electrons generated along with radiation of electron beam EB to a sample 2, are detected by a reflected electron detector 7, and the detection signal of the detector 7 is supplied to a peak height detection unit 9 and a peak width detection unit 10 through an amplifier 8. And, the signal corresponding to the peak height detected by the peak height detection unit 9 and the signal corresponding to the peak width detected by the peak width detection unit 10 are supplied to an arithmetic circuit 11, and the ratio of the peak height to the peak width calculated by the arithmetic circuit 11 is supplied to a computer 6. In this way, high precision focusing can be performed by evaluating the degree of the focusing of beams based on the ratio of the peak height to the peak width.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an automatic focusing device used in an electron beam writing device, an electron beam device such as a scanning T7-microscope, or an ion beam device such as an ion beam writing device. .

(Prior Art) In an electron beam lithography apparatus or the like, the electron beam is focused before starting lithography, and this focusing is performed automatically. A three-point focusing method is used for this automatic focusing. This three-point driving method will be explained with reference to FIGS. 3 and 4. When the beam is scanned across a mark on a sample and the backscattered electron signals generated along with the scanning are detected, a sharp peak appears at the edge of the mark. Figure 3 shows the backscattered electron detection signal, where the vertical axis is the signal strength and the horizontal axis is the scanning value! It is. The peak-to-peak value between the maximum value D+ and the minimum value D2 of this detection signal becomes larger as the electron beam becomes more focused, and the width of the peak becomes narrower as the electron beam becomes more focused.

Here, when the electron beam is scanned across the mark while changing the excitation current of the focusing lens for focusing the electron beam, the peak-to-peak value of the detection signal such as reflection t'-f will be as shown in Figure 4. , it changes in a parabolic shape. The peak position of this parabola is the just focus position. In the three-point focusing method, the excitation current of the focusing lens is
, I2. I, and detect the peak-to-peak values P+, P-, and Ps of the mark signals obtained as the electron beam scans at each excitation current.

Among these peak-to-peak values, the lowest intensity value P
1 is discarded, and the current I2.1 corresponding to the peak-to-peak value F'x, Ps is calculated. Leave I3.

Next, gIJ magnetic current ■ between Is and Is.

is selected, the peak-to-peak value P4 at this time is detected, the three peak-to-peak values Px, Ps, and P* are compared, the lowest value P) is discarded, and the remaining two peak-to-peak values P2. Similarly, the peak-to-peak value P2, leaving P4
, P4, the focusing lens is scanned across the mark at fiEIJ61 with a current I intermediate between the t currents I2, ■, corresponding to P4, and a peak-to-peak value P is obtained. In this way, the peak-to-peak values of the excitation current at three points are always detected, the lowest of the peak-to-peak values is discarded, and the excitation current corresponding to the remaining two peak-to-peak values is detected. By setting a new intermediate l1iJ7J magnetic current and repeating this operation, at the peak P0 of the parabola (
Excitation current value of the focusing lens (at the time of just focus)■
. is required. After this focusing lens is set to an excitation current value I0, writing is performed using a predetermined electron beam.

(Problem to be solved by the onset stage) In the above-mentioned focusing method, a peak-to-peak value is used to evaluate the degree of focusing of the electron beam, but this peak-to-peak value is Near the peak of the parabola shown in the figure, the rate of change is small and is easily affected by disturbance factors, making it difficult to accurately find the peak of the parabola and perform focusing with high precision.

The present invention has been made in view of these points, and an object thereof is to realize an automatic focusing method for a charged particle beam device that can automatically perform focusing with high precision.

(Means for Solving Task U) The automatic focusing method in an electron beam device based on the present invention scans a charged particle beam over a material multiple times, and the In addition to detecting the signal, the intensity of the focusing lens for the charged particle beam is changed every time multiple scans are performed, the detected signal at each lens intensity is evaluated, and the intensity of the focusing lens is set based on this evaluation. The automatic focusing method is characterized in that the detection signal is evaluated based on the ratio of the peak height and width of the detection signal.

(Function) The signal peak obtained as a result of irradiating a sample with a charged particle beam becomes sharper and the peak width becomes narrower as the beam is focused, so the detected signal is evaluated based on the ratio of peak height to width. A sample is irradiated with a pencil-shaped beam, and the peak waveform of a signal obtained from the sample can be approximated by a normal distribution curve shown in FIG. That is, the peak waveform f(X) can be expressed as follows.

f(x = (1/ff-) = Y/σ)
e + 飄-sl /12″ 1・・・・・・ (1
) At this time, m is the value of X at the peak time, and when x=m, f(X) becomes the maximum value. Therefore, by substituting X=m into equation (1), the peak height H is: H=f (m)=1/ (r tama・σ)...
(2), and the distribution width W of one curve f (x> is generally σ
(k is a positive real number), and as a result, the peak waveform f (x) can be approximated by the peak height H and width W.

Here, the ratio between the peak height H and 6w is as follows.

H/W= 1/ (k −rI1・σ2)・・・・・・
(3) By the way, considering the standard surface difference σ of the normal distribution curve f (x>), this standard surface difference σ becomes smaller as the beam is focused, and this value is used to evaluate the degree to which the beam is focused. Therefore, rather than evaluating the peak waveform using the peak height H in equation (2), the ratio of peak height to width (H/W) in equation (3), where σ is expressed as the square of If the evaluation is performed by , the value changes greatly, so it is possible to more accurately know the degree to which the beam is focused.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an example of a device for implementing an automatic focusing method in an electron beam device based on the present invention.
The electron beam EB is narrowly focused onto a sample 2 by a focusing lens 1 and deflected by a deflector 3. 4
5 is a focusing lens power supply, 5 is a deflector control unit, and both the power supply 4 and the unit 5 are controlled by a computer 6. For example, backscattered electrons generated when the sample 2 is irradiated with the electron beam EB are detected by a backscattered electron detector 7. A detection signal from the detector 7 is supplied via an amplifier 8 to a peak height detection unit 9 and a peak width detection unit IO. A signal corresponding to the peak height determined by the peak height detection unit 9 and a signal corresponding to the peak width determined by the peak width detection unit 10 are supplied to an arithmetic circuit 11. The peak height to width ratio determined by the arithmetic circuit 11 is supplied to the computer 6.

The operation in the above configuration is as follows.

The computer 6 controls the focusing lens power supply 4 and changes its lens strength. At this time, the computer 6 controls the deflector control unit 5 so that the electron beam is linearly scanned across the mark on the sample 2 by the deflector 3 when the excitation current is applied to the focusing lens. do. Reflected electrons generated by irradiating the sample 2 with the electron beam are detected by the detector 7 , amplified by the amplifier 8 , and then supplied to the peak height detection unit 9 and the peak width detection unit 10 .

The peak height detection unit 9 detects a peak-to-peak value (corresponding to H) from the maximum value D2 and the minimum value D2 of the detection signal shown in FIG. 2(a). The peak width detection unit 10 inverts the absolute value of the signal shown in FIG. 2(a) and outputs the signal shown in FIG.
), then check the signal in Fig. 2(b) at an arbitrary level value, perform square wave processing, and obtain the signal in Fig. 2(c).
) signal. The high level signal @ (w, +W2) of the signal shown in FIG. 2(C) is detected and outputted as a signal of width W.

The signal corresponding to H and the signal corresponding to W obtained in this way are supplied to the arithmetic circuit 11, and the value of H/W is determined. The H/W signal is supplied to the computer 6, and in the computer 6, based on the supplied signal, the focusing lens 4 is determined to be in the most focused state, for example, by the three-point focusing method described above. Excitation current L
I'm looking for 0. This value of Lo is supplied to the focusing lens t'a4 via the computer 6, and the focusing lens 1 is excited by the excitation current, so that the sample 2 is irradiated with a focused electron beam. become. After this,
In an electron beam drawing device, material is drawn using a focused electron beam.

Although one embodiment of the present invention has been described above, the present invention is not limited to this embodiment. For example, the peak-to-peak values of positive and negative peak signals among the detection signals are determined
The height and width of a specific peak in the detection signal may be determined. Also, although the three-point focusing method was used as the automatic focusing method, the present invention may also be applied to Ike's method. Can be done. Furthermore, although the electron beam lithography apparatus has been described as an example, the present invention can also be applied to other electron beam apparatuses and ion beam apparatuses. Furthermore, although reflected electrons are detected, secondary electrons may also be detected.

(Effects of the Invention) As explained above, in the present invention, the degree of beam focusing is evaluated based on the ratio of the peak height and width of the signal obtained as a result of irradiating the beam onto the sample. So,
Compared to evaluation based only on height information, the rate of change in the vicinity of just focus increases, the influence of disturbance factors can be reduced, and the just focus point can be found accurately, allowing for highly accurate focusing. ze can be done.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of an electron beam lithography apparatus for implementing the present invention, and FIG. 2 is a diagram showing signal waveforms used to explain the operation of the peak height and width detection unit.
FIG. 3 is a diagram showing a mark detection signal, FIG. 4 is a diagram for explaining the three-point tracking method, and FIG. 5 is a diagram showing a normal distribution curve. 1...Focusing)/lens 2...Sample 3...
Deflector 4... Focusing lens power supply 5...
Deflector control unit I-6...Computer 7...Detector 8...Amplifier 9...Peak height detection unit 10...Peak width detection unit 11...? ? ! Arithmetic angle etc. 2 Figure I word exclamation 3 Figure 4J! Intensity scanning position 1 Figure angle etc. 5 mouth 5 3rd pull

Claims (1)

[Claims] The charged particle beam is scanned over the material multiple times, the signal obtained based on the irradiation of the charged particle beam onto the material is detected, and the intensity of the focusing lens of the charged particle beam is changed for each multiple scan. In an automatic focusing method that evaluates the detection signal at the lens intensity and sets the intensity of the focusing lens based on this evaluation, the detection signal is evaluated by the ratio of the peak height and width of the detection signal. An automatic focusing method in a charged particle beam device, characterized in that: