JP2001099795A - Element mapping device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain mapped image with necessary and sufficient precision in a short period of time. SOLUTION: Counting is started after confirming that an electron beam strikes an analysis point properly. When one of the following conditions (a) a count value of a temporary counter exceeds a maximum count value Nmax, (b) the count value of the temporary counter after passing a zero determination time is not more than Nmin, and (c) a count time exceeds a maximum count time Tmax is satisfied, the counting is finished and a scanning position moves to the next point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam microanalyzer (EPMA) and a scanning electron microscope (SEM) equipped with a semiconductor detector, which are excited while scanning over a sample with an electron beam, and the sample is excited from the sample. The present invention relates to a mapping apparatus that measures generated characteristic X-rays and obtains an element map image of a sample surface. Further, the present invention relates to an element mapping apparatus for measuring characteristic X-rays generated from a sample using an X-ray or an ion beam as an excitation beam.

[0002]

2. Description of the Related Art For example, an EPMA or SEM, which is a kind of element mapping apparatus, irradiates an electron beam to one point of a sample and converts X-rays generated from the irradiated point into energy (wavelength).
Qualitative and quantitative analysis of the elements contained in the sample is performed by separately measuring each time. The electron beam is electromagnetically scanned two-dimensionally on the sample surface, or the electron beam is fixed and the sample placed on the sample stage is two-dimensionally scanned to obtain a two-dimensional image on the sample surface. It is possible to analyze a three-dimensional element distribution state. In the following description of the present specification, beam scanning includes both electromagnetically scanning an electron beam on a fixed sample surface and scanning a sample stage with the electron beam fixed. To separate and measure X-rays for each wavelength,
In the case of EPMA, a wavelength dispersive spectroscope using a spectral crystal for dispersing X-rays and a proportional counter type X-ray detector is often provided. In the case of SEM, an energy dispersive X-ray detector that uses a semiconductor detector with good energy resolution and processes the output with a multi-channel analyzer is widely used.

[0003] An element mapping image is an intensity which is a count value of characteristic X-rays of a display target element or a value standardized per unit time by associating a beam irradiation position on a sample with a position of a display device such as a CRT. It can be obtained by expressing the value, and furthermore, the element concentration value calculated from the value by the brightness or color of a CRT as a display device.

FIG. 5 shows an example of an X-ray measurement portion of a conventional element mapping apparatus. Here, this is an example using an energy dispersive X-ray detector. X-rays generated from a sample are detected by a semiconductor X-ray detector 51 having good energy resolution, and then amplified and waveform-shaped by a linear amplifier 52 before entering a multi-channel analyzer (MCA) 53. MC
From A53, the X-ray count number of the energy corresponding to the element to be mapped is stored in the element mapping data memory 5.
4 is output. The mapping memory 54 is also supplied with the irradiation position information of the excitation beam from the beam scanning control unit 55, and an element mapping image is obtained based on these two pieces of information. At this time, the time for counting the X-rays is a fixed time set by the operator, and after the counting circuit measures only the set fixed time, the beam scanning control unit 55 sets each measurement point to the next measurement time. The beam position is moved to the point.

[0005]

Since the element mapping device measures the X-ray intensity at a number of points on the sample surface,
There is a problem that one measurement takes a considerably long time. In particular, when an energy dispersive type using a semiconductor detector is used as an X-ray detector, X-rays containing various energies must be detected by one detector at the same time. The count rate of X-ray pulses cannot be made too high. In particular, when the energy resolution must be increased, the time constant of the waveform shaping needs to be increased to several to several tens of μs. In this case, the number of pulses that can be counted is about several to 10 kcps.

For example, if X-ray measurement is performed at 10 kcps and measurement is performed under the condition that an average of 1000 counts of X-rays are accumulated per measurement point (pixel) in the entire energy region, the measurement time per pixel is 100 ms. . The number of pixels is, for example, 512 ×
In the case of 512, the required measurement time requires a long time of 2500 seconds or more. As in this example, there is a problem that an extremely long measurement time is required to perform element mapping with a large number of pixels and with high accuracy.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an element mapping apparatus that can shorten a mapping image measurement time as a whole while maintaining necessary and sufficient measurement accuracy. I do.

[0008]

According to the first aspect of the present invention, in order to solve the above-mentioned problem, an excitation beam is sequentially irradiated on each point on a sample, and the excitation beam is generated from the excitation beam irradiation position of the sample. In an apparatus for performing elemental mapping analysis of a sample by measuring the intensity of characteristic X-rays, each counting means for counting each characteristic X-ray for one measurement point, and the maximum count number for setting the maximum count number in each counting means Setting means, maximum count time setting means for setting a maximum time for counting X-rays, and first comparison for comparing each element count number counted by each count means with the preset maximum count number Means, when it is determined by the first comparing means that each element count exceeds the maximum count, or when the count time is equal to the maximum count. And providing a next advancing irradiation position changing means the irradiation position of the excitation beam when a count time.

Further, the present invention described in claim 2 provides:
In addition to the configuration according to claim 1, the zero determination time setting means and the minimum count number setting means for setting a zero determination time and a minimum count number for determining the presence / absence of characteristic X-rays are counted by the respective count means. Second comparing means for comparing each element count number obtained with the minimum count number set in advance, and when the count time reaches the zero determination time, each element count number is set to the minimum value by the second comparison means. An irradiation position changing unit for advancing the irradiation position of the excitation beam to the next position when it is determined that the number is equal to or smaller than the count number is provided.

The element mapping apparatus according to the first aspect of the present invention is provided with a memory for storing a pulse count number for detecting a characteristic X-ray of each element to be mapped, and stores the stored count number and a preset maximum number. The irradiation position changing means compares the count number with the count number and when the count number stored in the memory satisfies the comparison condition, advances the irradiation position of the excitation beam to the next. Normally, when the counting time reaches the preset maximum counting time, the counting of the X-ray pulse is stopped and the irradiation position of the excitation beam is advanced to the next, but even if the time does not reach the preset counting time for each pixel. If the condition of the count number satisfies a preset condition, the irradiation position changing means moves the beam irradiation position to the next point, so that the entire measurement time for performing the mapping analysis of the sample surface can be reduced.

The accuracy of the X-ray intensity measured at each measurement point (also referred to as a pixel from the viewpoint of displaying the measured result) has a statistically fixed relationship with the number of X-rays counted at that position. Therefore, if the number of X-rays to be counted exceeds a predetermined value, required accuracy can be secured.

Further, the element mapping apparatus according to the second aspect of the present invention sets the minimum count time and the minimum count number to determine whether or not the target element is present at the analysis point in a short time, or to determine whether the target element is extremely small. Judge whether or not the amount is included, and if the element is not included, the irradiation position changing means moves the beam irradiation position to the next point, shortening the entire measurement time for mapping analysis of the sample surface can do.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of an element mapping apparatus in which a semiconductor X-ray detector is combined with an SEM. An electron beam 5 emitted from an electron gun 1 is narrowed down by a condenser lens 2 and an objective lens 3 and is irradiated on a surface of a sample 6 placed on a sample stage 4 having three axes of XYZ. X from the sample surface irradiated with the electron beam 5
A line 7 is generated which is detected by a semiconductor X-ray detector 8. The X-rays 7 include X-rays of various energies (wavelengths) such as characteristic X-rays and continuous X-rays corresponding to the component elements contained in the sample. The semiconductor X-ray detector 8 has good energy resolution and outputs an electric pulse having a height proportional to the detected X-ray energy. The X-ray pulse is amplified and shaped by a linear amplifier 9 and then counted by a multi-channel analyzer (MCA) 10 for each X-ray energy. The count result of the MCA 10 is sent to the controller 11 and stored in the memory.

On the other hand, the sample stage 4 on which the sample 6 is placed is configured as an XYZ stage that moves in a horizontal plane (XY plane) and in the height direction (Z direction), and is controlled by a controller 11 via a stage controller 12. Controlled. The Z-axis, which is the height direction, is used for aligning the height of the sample surface, and the X-axis and Y-axis, which are drive axes in the horizontal plane, are used for observation and observation of the sample.
Electron beam 5 for determination of analysis position and mapping analysis
It moves the analysis position on the sample surface to the irradiation position of. When performing the mapping analysis, for example, a rectangular area on the sample surface is specified in advance, and the electron beam 5 is fixed so as to irradiate a single point, and the electron beam 5 is applied throughout the specified rectangular area. The sample stage 4 is sequentially scanned in the X and Y directions so as to be irradiated. In the meantime, the detector 8 counts the X-rays 7 and can obtain mapping data indicating how many characteristic X-rays of the target element have been counted for each analysis point (pixel) in the specified rectangular range.

FIG. 2 shows the electrical configuration of the X-ray counting section.
Each X-ray photon included in the X-ray 7 incident on the detector is converted by the X-ray detector 8 into an electric pulse signal having a height proportional to the energy of the X-ray photon. The X-ray pulse signal is amplified and waveform-shaped by a linear amplifier 9 and sent to a multi-channel analyzer (MCA) 10 where the number of pulses is counted for each pulse height, that is, for each X-ray energy. The count number for each line energy is stored in a memory in the MCA 10. The MCA 10 outputs an X-ray corresponding to the element to be subjected to the mapping analysis, that is, a count value of a characteristic X-ray of the element, and uses this output and position information from the electron beam scanning unit 25. A mapping image of each element is obtained.

In the apparatus of the present invention, a temporary memory 21 is provided for each analysis element, and the pulse count number corresponding to the X-ray corresponding to each element in the MCA 10 is stored. In the temporary memory 21, memories 1 to n are prepared for a predetermined number of elements. MCA1
The data transfer from 0 to this memory may be performed every time an X-ray pulse is detected, or M may be transferred at regular intervals.
Data may be read from the CA. The count number of the temporary memory 21 is sent to the count comparator 22, where it is compared with a preset count condition, that is, the maximum count number. When the count value of each memory satisfies a preset condition, a measurement end signal is sent to the beam scanning controller 25, and the beam scanning controller 25 moves the beam to the next analysis point. Further, the count value corrector 23 converts the count number actually counted into a predetermined time, that is, the count number of the maximum count time, and based on the time when the count value is measured by the count value corrector 23. The corrected values are stored in the element mapping memory 24. The element mapping memory 24 is a memory that stores the distribution state on the sample surface for each element, and is prepared for the number of elements.

Next, the procedure of mapping measurement will be described with reference to FIG. When performing element mapping, first, an element to be analyzed is set. Thereby, an energy region of the X-ray corresponding to the element to be analyzed is set in the MCA 10. The number of X-ray pulses in this energy region is transferred to the temporary memory 21. In addition, the analysis position and the number of the mapping and the measurement time Tmax at each analysis point are set. Tmax is the count time for the longest measurement at each analysis point, and is the maximum count time. Further, the parameter of the comparison condition of the count comparator 22 is also set. Each parameter has the following meaning. Maximum count (Nmax)
Is a count number indicating that data collection ends when the value of the temporary counter exceeds this value. Minimum count (Nm
in) is the count number for terminating data collection by regarding that this element is not included if the count value of the temporary counter is equal to or less than this value after the lapse of the zero determination time T0. The zero determination time (T0) is a minimum time for determining whether or not an element is present at the analysis point.

The count comparator 22 makes the following comparison to determine the end of X-ray counting at one measurement point. As shown in FIG. 3, counting is started after confirming that the electron beam is correctly applied to one analysis point, and for all the elements to be mapped, (a) the value of the temporary counter exceeds the maximum count number Nmax. (B) the count value of the temporary counter is Nmin or less when the zero determination time has elapsed,
(c) The counting is terminated when one of the conditions of the counting time exceeds the maximum counting time Tmax is satisfied. Then, the scanning position is moved to the next point. (a) is a case where the count number for obtaining the required accuracy is obtained in a time shorter than Tmax.
(b) means that counting is stopped when it is determined that the element is not at the analysis position. (c) is a case where a predetermined count time has been reached. The determination within the dotted line in FIG. 3 is performed independently for each element to be mapped.

Nmax, Nmin, and T0 are set to statistically significant numerical values. That is, the X-ray count number
Since the statistical error of N can be expressed as sqr (N) (sqr (N) is the square root of N, the same applies hereinafter), Nmin and Nmax are set according to the accuracy required by the operator. be able to. For example, when it is desired to set sqr (N) / N to 10%, Nmax may be set to 100 counts. As a result, the scanning position can be shifted to the next scanning point when the value of the temporary counter has the necessary accuracy for the measurer before the preset measurement time Tmax is reached. Therefore, a mapping profile having a required accuracy can be obtained in a shorter time than in the past. T0 and
Nmin can be estimated from a statistical error calculated from the background continuous X-ray intensity. For example, T0
Tmax is set to one fifth of the value of Tmax, and the statistical error amount calculated from the continuous X-ray count number counted at that time is 2
About twice the value may be set as Nmin.

Next, correction of the count number in the count value corrector 23 will be described. FIG. 4 shows how the count of X-rays of two different intensities (the number of photons per unit time) increases with time. The pulse signal written on the lower side is a clock signal that determines the counting time. Tmax is the maximum count time, T1 is the time for counting X-rays of intensity K1, and T2 is the time of counting X-rays of intensity K2. In this example, since the count number of the X-rays of intensity K2 did not exceed the maximum count number Nmax within the time of Tmax, T1
At time (= Tmax), N1 count was obtained. Since the count number of the X-ray having the intensity K2 exceeded the maximum count number Nmax immediately before the time T2, the count was stopped when the time T2 passed, and the count number N2 was obtained.

In this case, the element mapping data memory 2
Assuming that the data stored in 4 is normalized to the count number measured at the time of Tmax, the corrected count values N1 ′ and N2 ′ of the X-rays of the intensity K1 and the intensity K2 are N1 ′ = N1,
N2 '= N2 (Tmax / T2). Generally, Nn '= Nn (Tmax / Tn), and when Tn = Tmax, Nn' = Nn. The corrected count number Nn 'is stored in the element mapping data memory 24. It is needless to say that the stored data may be stored not as the count number but as an intensity value. That is, the value obtained by converting the count number to the intensity, K1 = N1 /
T1, K2 = N2 / T2,..., Kn = Nn / Tn may be stored in the memory 24 as element mapping data.

In the above embodiment, the MCA 10 corresponds to each counting means for counting each characteristic X-ray, the controller 11 includes a maximum counting time setting means and a maximum counting number setting means as functions, and the count comparator 22 Correspond to the first and second comparing means, and the beam scanning control unit 2
5 corresponds to irradiation position changing means. The controller 11 is mainly composed of a computer, and has a temporary memory 2.
1, a count comparator 22, a count value corrector 23, and an element mapping data memory 24. Further, the beam scanning control unit 25 is not limited to either a mechanism for scanning the electron beam on the sample surface or a mechanism for scanning the sample with the stage while fixing the electron beam. It is possible.

Further, in the above-described embodiment, the apparatus in which the semiconductor X-ray detector is combined with the SEM has been described as an example. However, the excitation beam is not limited to the electron beam, and the element mapping apparatus of the present invention uses the ion beam or the X-ray detector. Needless to say, an apparatus using a line beam as an excitation beam is included. Further, like EPMA, it also includes a device provided with a plurality of spectroscopes using an analysis crystal and a proportional counter around an electron beam.

[0024]

The mapping apparatus of the present invention not only advances the irradiation position when the count time reaches the preset maximum count time, but also provides a beam if the count number condition satisfies the preset condition. Since the irradiation position is moved to the next point, a mapping image with necessary and sufficient accuracy can be acquired in a short time.

[Brief description of the drawings]

FIG. 1 is a diagram showing an overall configuration of a mapping device of the present invention.

FIG. 2 shows an electrical configuration of an X-ray counting section according to the present invention.

FIG. 3 shows a procedure of mapping measurement in the present invention.

FIG. 4 is a diagram illustrating correction of a count number in the present invention.

FIG. 5 shows an electrical configuration of an X-ray counting section in a conventional mapping apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Electron gun 2 ... Converging lens 3 ... Objective lens 4 ... Sample stage 5 ... Electron beam 6 ... Sample 7 ... X-ray 8 ... X-ray detector 9 ... Linear amplifier 10 ... MCA 11 ... Controller 12 ... Stage controller 21 ... Temporary memory 22: Count comparator 23: Count value corrector 24: Element mapping data memory 25: Beam scanning unit

Claims (2)

[Claims] In an apparatus for sequentially irradiating each point on a sample with an excitation beam and measuring the intensity of characteristic X-rays generated from the excitation beam irradiation position of the sample to perform element mapping analysis of the sample, one measurement point is provided. Each counting means for counting each characteristic X-ray, maximum counting number setting means for setting a maximum counting number in each counting means, maximum counting time setting means for setting a maximum time for counting X-rays, First comparing means for comparing each element count number counted by each counting means with the preset maximum count number, and the first comparing means determines that each element count number has exceeded the maximum count number. Or when the count time reaches the maximum count time, the irradiation position of the excitation beam is advanced to the next position. An element mapping device characterized by further comprising additional means. 2. A zero judgment time setting means and a minimum count number setting means for setting a zero judgment time and a minimum count number for judging the presence or absence of characteristic X-rays, and a count number of each element counted by each count means. And a second comparing the minimum count number with the preset minimum count number.
Comparing means, when the count time is determined to be zero determination time, when the element number of each element is determined to be equal to or less than the minimum count number by the second comparison means, the irradiation position of the excitation beam is next 2. The element mapping apparatus according to claim 1, further comprising an irradiation position changing unit that advances the irradiation position.