JPH01264152A - Automatic focusing device for electron microscope - Google Patents

Abstract

PURPOSE:To perform focusing at a high speed by calculating the position of a pattern edge based on the pattern data corresponding to a sample pattern on the visual field and extracting only the local region containing the pattern edge for automatic focusing. CONSTITUTION:A means outputting the pattern data corresponding to the pattern of a sample 5 on the visual field of an electron microscope, a means calculating the position of a pattern edge based on the pattern data, and means 12, 13, 3 deflection-scanning in the fine region of the position of the pattern edge according to the calculation value are provided. Only the local image at the position of the pattern edge is detected prior to the pattern detection of the visual field of the microscope, the exciting current of an objective lens is set so that the peak value of the differential wave-form of the pattern edge is made maximum for automatic focusing. Automatic focusing can be surely performed at a high speed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic focusing device for an electron microscope, and in particular to an automatic focusing device for an electron microscope, and in particular for an automatic focusing device for an electron microscope (SIM) or a scanning transmission electron microscope (STEM). The present invention relates to an automatic focusing device suitable for application to an exposure mask or a wafer appearance inspection device.

[Conventional technology]

Conventionally, in automatic focusing in a scanning electron microscope (8BM), etc., the spot diameter of the electron beam becomes smaller as the state of the focused point approaches, and the amplitude value of the AC component signal of the image signal increases. Take advantage of the maximum.

After manually setting a pattern on the sample that has edge portions in the direction intersecting the raster scanning direction of the electron beam within the field of view of the electron microscope, the detected image signal of the pattern edge portion is filtered. The AC signal component is extracted, and the excitation current of the objective lens is changed stepwise while determining its amplitude value, and is set so that the amplitude value becomes maximum.

Note that pages 2-5 to 2-8 of the Hitachi H-8010 Scanning Electron Microscope Observation Equipment Service Manual are related to focusing in conventional electron microscopes.

[Problem to be solved by the invention]

In the conventional technology described above, automatic focusing does not function effectively if there is no sample pattern image that has an edge in the detection field of the microscope in a direction that intersects with the raster scanning direction of the electron beam. Since this method performs two-star scanning over the entire surface of the camera and performs focusing while processing the detected image, there are problems in that image detection takes a long time and high-speed automatic focusing is difficult.

An object of the present invention is to provide an automatic focusing device for an electron microscope that can perform automatic focusing at high speed and reliably without affecting the pattern shape within the field of view.

[Means to solve the problem]

The above purpose is to provide a means for outputting pattern data corresponding to a sample pattern in the field of view of an electron microscope, a means for calculating a position where a pattern edge exists from this pattern data, and a means for calculating a position where a pattern edge exists according to the calculated value. This can be achieved by providing a means for deflecting and scanning within a minute area.

[Effect]

Design data used when drawing a pattern of an X-ray mask or the like to be inspected is used as a comparison standard during pattern inspection. Then, for each field of the electron microscope, a corresponding reference image signal is generated based on the data and compared with the detected image signal of the test pattern. When performing a defect inspection, in parallel with the operation of generating a reference image signal for one field in advance and storing it in the image memory,
Detect the position of pattern edges from design data. Then, prior to detecting the pattern in the field using an electron microscope, only the local image where the pattern edge exists is detected, and while obtaining the differential waveform of the pattern edge, the objective lens is adjusted so that the peak value of the differential waveform is maximized. Set the excitation current and perform automatic focusing.

Further, when detecting the local image, the scanning direction of the electron beam is controlled to be orthogonal to the direction of the pattern edge so that an edge image is detected.

In this way, the present invention eliminates the need for pattern detection over the entire field, regardless of the density or sparseness of the pattern to be inspected, and enables automatic focusing by detecting only a local image at the position where a pattern edge exists. , faster focusing can be achieved. Furthermore, by controlling the scanning direction of the electron beam in accordance with the direction of the pattern edge, reliable automatic focusing can be performed without depending on the pattern shape.

〔Example〕

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

FIG. 2(a) shows a cross-sectional structure of an X-ray mask to be inspected by an automatic focusing device according to an embodiment of the present invention. An XM mask is made of a material that easily transmits XI radiation, such as boron nitride (BN) or silicon nitride (SiN).
The X-rays are transmitted through the tanmple 31 before and after μffIt.
The circuit pattern 32 is formed of heavy metal such as metal or tantalum, and the surface may be further covered with a polyimide thin film 33 to protect the circuit pattern. The outer periphery of the mask is held by a support 34 made of silicon or the like.

FIG. 2(b) shows the external appearance of the X-ray mask. In this embodiment, the entire area 59 where the pattern is formed is inspected, and various defects shown in FIG. White dots 35, black dots 66 outside the pattern 32, recesses 57 in the pattern 32. The convex portion 38 and the like are detected.

FIG. 1 is a block diagram of an automatic focusing device according to an embodiment of the present invention. In the figure, an electron gun 1, a focusing lens 2, a deflection coil 3, an objective lens 4, a sample stage 6, a scintillator 7, a light guide 8, a photomultiplier tube 9
, X deflection driver 12, X deflection driver 13, and excitation driver 14 are
M). The electron beam generated by the electron gun 1 is focused onto a sample 5 placed on a stage 6 by an electron lens consisting of a focusing lens (condenser lens) 2 and an objective lens 4.
The sample 5 is deflected and scanned within the microscope field by the deflection coil 3 driven by the X and Y deflection drivers 12 and 13 according to control signals from the sample 6 and the synthesis circuit 15. Then, the electrons transmitted through the sample 5 are converted into light by a scintillator 7, and then converted into an electric signal by a photomultiplier tube 9, amplified by an amplifier 11, and synchronized with a scanning signal.
An i converter 28 performs quantization to obtain a scanning transmission electron image (STEM image) for one field.

Meanwhile, in parallel with this detection operation, the readout circuit 26 reads out the design data 19 that describes the vertex coordinates of the rectangular pattern used when drawing the pattern of the sample 5, and the image signal generator 201C reads out the design data 19 that describes the vertex coordinates of the rectangular pattern, and the image signal generator 201C reads out the design data 19 that describes the vertex coordinates of the rectangular pattern used when drawing the pattern of the sample 5. A reference image of a pattern in the field is released and stored in the field image memory 22 or 23, and in parallel with the STEM image detection described above, the defect is inspected by pattern comparison while being input to the defect determiner 24. Then, the calculator 25
Then, the stage control circuit 10 sequentially repeats the pattern inspection in each field while moving the sample stage 6 step by step at intervals of the field 40 as shown in FIG. Conduct inspections.

In this configuration, two field image memories 22 and 23 are provided to enable alternate storage and output of reference images, and the readout circuit 26 and image signal generator 20 have S
By creating a reference image signal while reading design data at a position one field ahead of the detection field of the Tl3M image and storing it in the field image memory 22 or 23, pattern inspection can be performed continuously at high speed. ing.

In this embodiment, after the step feeding of the sample stage 6 is completed,
The purpose is to quickly and reliably perform automatic focusing of the image detection system before each field inspection. However, although this embodiment shows an example in which a transmitted electron (STEM) image is detected, the following explanation also holds true when a secondary electron or reflected electron image is obtained.

In the automatic focusing procedure, prior to inspecting each field, the vertex coordinate data of the rectangular pattern read from the design data 19 is input into the computer 25 to calculate the position of the pattern book edge.

Immediately before each field inspection, the vector scan signal generator 17 is given the inclination angle θ of the pattern edge 41 and the position (XO+76) of the edge on the X and Y coordinates provided for the field 40, and two composite circuits 15 and
, the Y deflection driver 12° 15 is controlled and driven to form a predetermined width 2w including the pattern edge 41, as shown in FIG.
A local area 42 having a length of 2t is deflected and scanned in a direction perpendicular to the edge. In FIG. 4, the horizontal scanning direction (H) and the parallel direction are shown as the vertical scanning direction (V). While obtaining the differential waveform, the excitation control circuit 27 and the excitation driver 14 are controlled by the computer 25 so that the peak value of the differential waveform is maximized to achieve the best focusing.

Data giving the local area scanning position' - (X o , 7
a) is the apex coordinate (X+*
y+ )(x2ey2 ) to W, Figure 6 shows a specific example of the configuration of the vector scan signal generator 17, and the GGa51n signal output circuit 51
．． It is composed of a 5ins signal output circuit 52, a polarity inversion circuit 55, multiplication circuits 53a to 53d, and addition circuits 54a to 54d.
*70) and an H counter 56 that counts the clock signal.
and the output value of the V counter 57 to DAi converters 58a and 58b.
Using the value converted to an analog value by , the result of X according to the following calculation formula.

It outputs a Y signal.

X=x+V-House θ-H-Ground θ y=y0+vφ ratio θ+H part θ Here, H knee W-W, V=-L-1, and the H counter repeats from the minimum value -W to the maximum W. Count back and increment the V counter by +1 every time one round of counting is completed, and output the X and Y deflections every time the clock is input during the period when the V counter counts from the minimum value -t to the maximum. do,
Deflection scanning is performed on the local detection area of interest.

The seventh factor is a specific configuration example of the focus detector 18 that calculates the differential value of the detection signal waveform in real time in synchronization with image detection.

The register 61 registers A in synchronization with the scanning of local areas H and V.
The detected image f(H
. is made possible. The difference circuit 63 includes both registers 61 .
Difference between output values of 62 F(H, V)=f(H-1, V)-
f(H, V) is output as a signal differential value.

On the other hand, the shift register 65 has a data area of 2w pieces of detected data in one horizontal scanning range of the local area, and has a data area of F(H,
V) in the V direction () (H, n) = Σ F (H,
V).

--corresponding data G of H direction address every tb clocks
(H, n) is read out and the adder 64 calculates the above difference value!
'(H, V) and the addition value G(H, n+1)=G(H, n
)+F'(H, V) and stores it again in the same data area on the shift register 65. This process is repeated every clock. ) is then converted into absolute value data by the absolute value circuit 66 and input to the calculator 25 (FIG. 1).

FIG. 8 shows a differential waveform F(I(,V):f(H-1,V)-f(H
, V) and F'(H, V) cumulatively added in the V direction (] (H1), and the cumulative addition waveform G in the ■ direction.
(By determining Hl, randomly mixed noise components can be reduced and a stable differential waveform with improved S/H can be obtained.If the detection signal is a falling waveform, the differential waveform is negative. To show the polarity, the absolute value is calculated and input into the calculator.

〔Effect of the invention〕

According to the present invention, automatic focusing is performed by extracting only a local area including a pattern edge, so that focusing can be performed at high speed. Furthermore, since scanning is performed in a direction perpendicular to the pattern edge, reliable focusing is possible without depending on the pattern shape.

[Brief explanation of the drawing]

FIG. 1 is a block configuration diagram of an automatic focusing device according to an embodiment of the present invention, FIG. The figure is an explanatory diagram of the scanning method for the entire pattern area, Figure 4 is an explanatory diagram of the scanning method of the local area, Figure 5 is an explanatory diagram of the local area scanning position data calculation method, and the sixth factor is the generation of the pectascan signal shown in Figure 1. The specific configuration diagram of the vessel, Figure 7 is the first
A specific configuration diagram of the focus detector shown in the figure, and FIG. 8 is a signal waveform diagram illustrating the focus detection method. 1... Electron gun 3... Deflection coil 4...
・Objective lens 5...Sample 6...Sample stage 7...Scintillator 8...Light guide
9...Photomultiplier tube 11...Amplifier
12... Deflection driver 13... Y-direction scanning driver 14... Excitation driver 16... Mask scan signal generator 17... Pectascan signal generator 18... Focus detector 19... Design data Storage MT 20...Reference image signal generator 22.23...Field image memory 24...Defect determiner 25...Computer. Fig. 2 Akira 3 (21 in No. 40 No. 50 ?16 Fig. No. 7 Kuchimi 8 Kuchi Ao@sk Exit book! J (>fH) = L r tM, vnogJ

Claims (1)

[Claims] 1. Deflect and scan the electron beam on the sample through an electron lens,
In an electron microscope that detects reflected electrons, secondary electrons, or transmitted electrons to obtain an electron image, means for outputting pattern data corresponding to a sample pattern on the field of view of the electron microscope, and the existence of pattern edges from this pattern data. A means for calculating the position where the pattern edge exists, and a means for deflecting and scanning a minute area at the position where the pattern edge exists according to this calculated value, and the focus of the electron microscope is automatically adjusted based on the detected image of this minute area. An automatic focusing device for an electron microscope characterized by: 2. In the automatic focusing device for an electron microscope according to claim 1, reading the design data used to create the sample pattern corresponding to the sample pattern on the microscope field of view, and calculating the position where the pattern edge exists, An automatic focusing device for an electron microscope characterized by determining the deflection position of an electron beam during focus detection.