JP3802242B2 - Sample image display method and electron beam apparatus in electron beam apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electron beam apparatus that irradiates a sample with an electron beam, detects a signal emitted from the sample by the electron beam irradiation, and displays a sample image.
[0002]
[Prior art]
Concentrations and types of impurities with almost the same composition and different amounts (for example, concentrations of impurities doped in semiconductors having gradients and distributions, and types of impurities such as pn junctions differ depending on the location) Or the difference in contrast on the image with an ordinary optical microscope or scanning electron microscope. .
[0003]
In order to obtain an image with a clear contrast with respect to such a sample, there is a technique using a scanning Auger electron spectrometer.
[0004]
In the case of semiconductors, the position of the Fermi level in the forbidden band differs depending on the type and concentration of impurities in the case of semiconductors, and accordingly, the energy of Auger electrons as seen from the vacuum level of the electron spectrometer varies accordingly. come. Therefore, for such a sample, a scanning Auger electron spectrometer is used to detect the energy position of an Auger electron peak that varies depending on the type and concentration of impurities and image it.
[0005]
[Problems to be solved by the invention]
However, such a technique cannot be used for the case where the energy of Auger electrons is not changed but only the plane orientation is different as in the case of polycrystalline metal.
[0006]
In addition, this technique has a practical disadvantage that it takes time to collect images. Image acquisition takes time because the Auger electron peak is on a high background, so the signal-to-noise ratio of the signal is poor, and the peak position is shifted because the Auger electron peak is relatively broad. The main reason is that there is not much difference in electron intensity at a specific energy. In addition, in order to obtain luminance information of the image, it is necessary to collect a spectrum in the vicinity of the peak position of Auger electrons for each pixel, and an operation for determining the peak position using an algorithm such as peak fitting. This includes things that are necessary. For this reason, at least one hour of acquisition time is required to obtain one image (number of pixels 128 × 128) having a practically practical surface resolution.
[0007]
The present invention has been made in view of the above points, and an object of the present invention is to provide a sample image display method capable of obtaining an image having a clear contrast, such as a semiconductor or a polycrystalline metal, in a shorter time than before. And providing an electron beam apparatus.
[0008]
[Means for Solving the Problems]
The sample image display method in the electron beam apparatus of the present invention that achieves this object irradiates a plurality of points on the sample with an electron beam, detects secondary electrons emitted from the sample by the electron beam irradiation, and detects each point. Energy spectrum at which a secondary electron appears in the energy spectrum is selected, and energy Eonset (L) at which a secondary electron appears in the selected spectrum L, and the energy A spectrum H having the highest energy in which secondary electrons appear is selected from the spectrum, and energy Eonset (H) in which secondary electrons appear in the selected spectrum H, Eonset (L), and spectrum L The peak position energy is as close to the peak position as possible and the spectral linearity is maintained. Determined and Energy E _0, the energy width _{Ew (= E 0 -Eonset (L} )) between the energy E ₀ and the Eonset (L), the secondary electron intensity Io in the energy E ₀ of the spectral L respectively If E ₀ is E ₀ ≧ Eonset (H), the set analysis energy of the electron spectrometer is determined to be only E ₀ , while if E ₀ ≧ Eonset (H), E ₁ is added to E _0. = E ₀ + Ew is determined as the setting analysis energy of the electron spectrometer. If E ₁ ≧ Eonset (H) is not satisfied, E ₂ = E ₀ + 2Ew is added to E ₀ and E ₁ and the setting analysis of the electron spectrometer is performed. After that, n + 1 set analysis energies En = E ₀ + n × Ew (n = 0, 1, 2,...) Are determined in the same manner, and thus the set analysis energy En of the electron spectrometer is determined in this way. Irradiate each pixel on the sample with an electron beam The analysis energy of the electron spectrometer is sequentially set to the obtained n + 1 En, and the secondary electron intensity I (n) at each energy En is detected for each pixel on the sample. When n = 0 That is, when the set analysis energy is only E ₀ , the detected secondary electron intensity signal is converted into an image signal and a sample image is displayed. On the other hand, when n ≠ 0, the following (1) and (2) Select the secondary electron intensity signal at each pixel according to the rules,
(1) Discard the secondary electron intensity signal whose intensity is greater than k × Io (k is a coefficient)
(2) When there are a plurality of secondary electron intensity signals whose intensity is equal to or smaller than k × Io, the secondary electron intensity signal of the selected secondary electron intensity signal is selected to select the secondary electron intensity signal having the highest intensity among them. A parameter S ′ = I (n) −n × Io is calculated from the value of n and its electron intensity I (n), an image signal is created based on the calculated parameter, and a sample image is displayed. And
[0009]
DETAILED DESCRIPTION OF THE INVENTION
First, the idea of the present invention will be described.
[0010]
When the surface of a solid sample is irradiated with an electron beam, electrons having an energy distribution as shown in FIG. 1 are generated. Of these, the portion having the highest intensity on the lowest energy side is called “true secondary electron” or simply “secondary electron”.
[0011]
FIG. 2 is an enlarged view of the lowest energy side portion of the secondary electrons in FIG. Here, when the energy at which the secondary electrons appear is Eonset, the secondary electron intensity rapidly increases from the point where it exceeds Eonset. This Eonset is determined by the work function Φs of the solid sample and the work function Φsm of the electron spectrometer that detects secondary electrons, and Eb, and is given by Eonset = Eb + Φs−Φsm. Eb is Eb = eVb, e is the elementary charge (1.6 × 10 ⁻¹⁹ C), Vb is the absolute value of the negative bias voltage applied to the sample, and the electron spectrometer has a vacuum level ( It is assumed that the potential is adjusted based on the potential of the outermost surface of the spectrometer. Note that FIG. 2 is displayed with the vacuum level of the electron spectrometer as the coordinate origin.
[0012]
Now, the spectrum near the appearance of secondary electrons is as shown in FIG. 3, for example, in the case where the impurity concentration differs depending on the location of a p-type semiconductor. In FIG. 3, H represents a spectrum of a portion having a relatively high impurity concentration, L represents a spectrum of a portion having a low impurity concentration, M represents an intermediate one thereof, and the appearance position of secondary electrons is It depends on the work function of the sample. In any spectrum, there is a sharp rise from the secondary electron appearance portion, and the secondary electron intensity decreases after passing the peak. Unless there are circumstances such as severe irregularities on the surface of the sample, these spectra have the same shape in a normal sample, with only the rising portion shifted.
[0013]
In the present invention, attention is paid to the spectral shift caused by the difference in the work function of the sample, and an attempt is made to obtain a contrast that does not appear clearly in a normal secondary electron image. That is, in the present invention, the analysis energy of the electron spectrometer is set to E ₀ in FIG. 3, for example, and the detected value of the electron spectrometer is converted into luminance information while synchronizing with scanning of the electron beam on the sample surface. Replace and collect images.
[0014]
When Φs ≧ Φsm holds, in principle, Vb = 0 may be used. In this case, a bias voltage applying device is not required, but the work function of the spectrometer is usually compared with the work function of the sample. The bias voltage application device is practically indispensable because it is not a very small value but may be reversed. In addition, the same result can be obtained by applying a bias of reverse polarity to the entire spectroscope instead of applying a bias voltage to the sample. However, in practice, the former is often easier to implement.
[0015]
Although the idea of the present invention has been described above, embodiments of the present invention will be described below with reference to the drawings.
[0016]
FIG. 4 is a view showing an example of the electron beam apparatus of the present invention.
[0017]
In FIG. 4, reference numeral 1 denotes a lens barrel. Inside the lens barrel 1, an electron gun 2, a focusing lens 3, and a deflector 4 are arranged. The electron beam emitted from the electron gun 2 is focused by the focusing lens 3 and then deflected by the deflector 4. As a result, the finely focused electron beam is scanned on the sample 6 disposed in the sample chamber 5. Is done. The sample 6 has almost the same composition and the concentration of impurities contained in a trace amount varies depending on the location. The sample 6 is placed on the sample stage 7, and a negative bias voltage is applied to the sample 6 by a bias voltage applying device 8.
[0018]
A secondary electron detector 9 and an electron spectrometer 10 such as an electrostatic hemispherical energy analyzer are disposed inside the sample chamber 5, and their detection signals are sent to the display control means 11. The analysis energy of the electron spectrometer 10 is set by analysis energy setting means 12, and this analysis energy setting means 12 is controlled by the display control means 11.
[0019]
In the figure, 13 is a deflection signal generating means for supplying a deflection signal to the deflector 4, and this deflection signal generating means 13 is controlled by the display control means 11. Reference numeral 14 denotes input means, and reference numeral 15 denotes display means, both of which are connected to the display control means 11.
[0020]
Reference numeral 16 denotes an exhaust device for exhausting the sample chamber 5 to an ultra-high vacuum. Reference numeral 17 denotes an ion etching apparatus for cleaning the sample surface.
Although the configuration of the electron beam apparatus of FIG. 4 has been described above, the operation of this apparatus will be described below.
[0021]
First, the operator inputs an instruction for acquiring a secondary electron image using the secondary electron detector 9 to the input unit 14. When this input is performed, the display control means 11 controls the deflection signal generating means 13, and the deflection signal generating means 13 sends a deflection signal for scanning the electron beam two-dimensionally on the sample to the deflector 4. . As a result, the electron beam is scanned two-dimensionally on the sample.
[0022]
Secondary electrons are emitted from the sample 6 by this electron beam irradiation. The secondary electrons are detected by the secondary electron detector 9, and the detected signal is sent to the display control means 11. The display control means 11 causes the display means 15 to display a secondary electron image of the sample 6 based on the detection signal sent.
[0023]
As described above, since the sample 6 has almost the same composition and the concentration of impurities contained in a trace amount varies depending on the location, the secondary electron image displayed on the display means 15 has a difference in concentration. It does not appear clearly as a difference in contrast.
Therefore, the operator selects some representative points within the field of view using the input means 14 while viewing the secondary electron image. In this selection, the operator selects representative points so as to include points where secondary electrons are expected to appear on the lowest energy side and the highest energy side.
[0024]
When the representative point on the sample is selected in this way, the display control unit 11 controls the deflection signal generating unit 13, and the deflection signal generating unit 13 deflects for sequentially irradiating the representative point with the electron beam. A signal is sent to the deflector 4. The display control means 11 controls the analysis energy setting means 12, and the analysis energy setting means 12 continuously applies the analysis energy of the electron spectrometer 10 within a certain range every time the electron beam is irradiated to each representative point. Change (sweep) to. As described above, the detection signal of the electron spectrometer 10 is sent to the display control means 11, and the display control means 11 obtains a spectrum at each representative point based on the sent signal. FIG. 5 shows the spectrum. In this case, four representative points are selected, so that four spectra are obtained.
[0025]
Then, the display control means 11 obtains a spectral energy value at which the electron intensity Ia predetermined by the operator is obtained for each spectrum, and selects a spectrum L having the minimum energy value and a spectrum H having the maximum energy value.
When the display control means 11 selects the spectrum L and the spectrum H in this way, the display control means 11 then enlarges the rising portion of the secondary electrons of the spectra L and H as shown in FIG. It is displayed on the display means 15. The operator designates the energy values Eonset (L) and Eonset (H) at which the rise of the secondary electrons shifts from the curve to the straight line using the input means 14 from the spectra.
[0026]
When such designation is performed, the display control unit 11 causes the display unit 15 to display the spectrum L and the spectrum H as shown in FIG. Then, the operator inputs, from the spectrum, the energy E ₀ that is as close as possible to the peak position between the Eonset (L) and the peak position of the spectrum L and that maintains the linearity of the spectrum. Use to select. The electron intensity at this time is Io.
[0027]
When the selection of the energy E ₀ is performed, the display control unit 11 obtains the energy width between the energy E ₀ and Eonset (L) Ew (= E 0 -Eonset (L)). Then, the display control unit 11 obtains an analysis energy set in the electron spectrometer 10 when acquiring a secondary electron image using the electron spectrometer 10 described later. Explaining how to obtain it, the display control means 11 sets the set analysis energy to only one E ₀ if the condition of Eonset (H) ≦ E ₀ is satisfied. If this condition is not satisfied, the display control means 11 newly determines E ₁ on the higher energy side by Ew than E ₀ in addition to E ₀ . That is, E ₁ = E ₀ + Ew.
[0028]
If the condition of Eonset (H) ≦ E ₁ is not satisfied, the display control means 11 determines E ₂ at a position further increased by Ew. Similarly, n + 1 En = E ₀ + n × Ew (n = 0, 1,...) Are determined. The number of n is determined by the difference between the gradient of the rising portion of the secondary electrons and the maximum value and the minimum value of the rising position at each representative point in the field of view. The number of n is the concentration of impurities contained in the sample, Depends on the type. In most cases, n is 0 to 1. That is, the analysis energy to be set is usually one or two. In the case of FIG. 7, the analysis energy to be set is only one E ₀ and n = 0. In FIG. 8, there are two analysis energies to be set, E ₀ and E ₁ , and n = 1.
[0029]
First, the case where the secondary electron image is acquired by setting the analysis energy of the electron spectrometer to E ₀ as shown in FIG. 7 (when n = 0) will be described.
[0030]
When the display control unit 11 determines the set analysis energy E ₀ , the display control unit 11 controls the acquisition of the secondary electron image using the electron spectrometer 10. That is, the display controller 11 controls the deflection signal generator 13 so that the electron beam is scanned two-dimensionally on the sample, and the analysis energy of the electron spectrometer 10 is set to E _0. The analysis energy setting means 12 is controlled. By this control, the electron beam is scanned two-dimensionally on the sample 6, and only the secondary electrons having energy E ₀ are detected by the electron spectrometer 10 among the secondary electrons emitted from the sample. Then, the detection signal (electron intensity data) of the electron spectrometer 10 is sent to the display control means 11.
[0031]
In this way, when one piece of electron intensity data I is obtained for each pixel, the display control means 11 calculates a parameter S ′ from the electron intensity I for each pixel. The calculation formula is S ′ = I.
[0032]
Next, the display control means 11 obtains the maximum value S′max and the minimum value S′min from the calculated parameter S ′, and uses one of the following equations (1) and (2) for each pixel. S is obtained. Which expression is used is determined by the operator.
[0033]
S = S′max−S ′ (1)
S = S'-S'min (2)
When equation (1) is used, the data with the highest electron intensity is converted to zero, while when equation (2) is used, the entire data value is such that the data with the lowest electron intensity is zero. Is shifted.
[0034]
When the display control means 11 obtains S for each pixel in this way, the display control means 11 assigns S to a displayable gradation, for example, 128 gradations, and causes the display means 15 to display the secondary electron image of the sample 6. When the above equation (1) is used, the magnitude of the work function is reflected as it is in the luminance of the image, and the higher the work function value, the higher the luminance. On the other hand, when equation (2) is used, this is reversed.
[0035]
For example, in the case of an n-type semiconductor, the higher the impurity concentration, the smaller the work function. In the case of a p-type, the opposite is true. An image reflecting the density intuitively can be obtained.
The case where the secondary electron image is acquired with the analysis energy of the electron spectrometer 10 set to only E ₀ has been described above. Next, since the difference in impurity concentration in the sample is large and the difference in work function of the sample is large, the analysis energy of the electron spectrometer 10 is set to two, E ₀ and E ₁ , as shown in FIG. The case where a secondary electron image is acquired (when n = 1) will be described.
[0036]
When the display control means 11 determines the set analysis energies E ₀ and E ₁ in this way, the display control means 11 sets the deflection signal generation means 13 so that the electron beam is scanned on one horizontal (or vertical) line on the sample. First, the analysis energy setting means 12 is controlled so that the analysis energy of the electron spectrometer 10 is set to E ₀ . By this control, the electron beam is scanned on the sample 6 and only the secondary electrons having the energy E ₀ are detected by the electron spectrometer 10 among the secondary electrons emitted from the sample. Then, the electron intensity data which is a detection signal of the electron spectrometer 10 is sent to the display control means 11.
[0037]
When the acquisition of the electron intensity data at the analysis energy E ₀ is completed, the display control unit 11 controls the deflection signal generation unit 13 so that the electron beam is scanned on the above-described one line on the sample. At the same time, the analysis energy setting means 12 is controlled so that the analysis energy of the electron spectrometer 10 is set to E ₁ . By this control, the electron beam is scanned on the sample 6, and only the secondary electrons having the energy E ₁ among the secondary electrons emitted from the sample are detected by the electron spectrometer 10. Then, the electron intensity data which is a detection signal of the electron spectrometer 10 is sent to the display control means 11. Next, the vertical (or horizontal) position is moved to perform the same operation.
[0038]
When two pieces of electron intensity data are obtained for each pixel in this way, the display control means 11 selects the electron intensity data to be adopted for each pixel according to the following rules. That is,
(1) Discard the electron intensity data whose intensity is larger than k × Io. Note that k is a coefficient determined in consideration of the noise level of the signal, and is about 1.1, for example.
{Circle around (2)} When there are a plurality of electron intensity data whose intensity is equal to or smaller than k × Io, the electron intensity data having the highest intensity is selected.
[0039]
For example, when an electron beam is irradiated to a pixel that obtains a spectrum as shown in S of FIG. 8, a signal of electron intensity I (o) is obtained at analysis energy E ₀ , and electron intensity I at analysis energy E ₁ . The signal of (1) is obtained. In this case, the signal of the electron intensity I (1) is selected according to (2). In FIG. 8, the signal having the electron intensity I ′ (1) is discarded according to (1).
[0040]
Next, the display control unit 11 calculates a parameter S ′ for each pixel from the value n of the selected electron intensity data (in this case, n is 0 or 1) and the electron intensity I (n). The calculation formula is S ′ = I (n) −n × Io. This means that the maximum value of S ′ is I ₀ and all S ′ are arranged corresponding to the appearance positions of secondary electrons. As secondary electrons appear from the higher energy side, the value of S ′ becomes smaller and may be a negative value.
[0041]
Next, the display control means 11 obtains the maximum value S′max and the minimum value S′min from the calculated parameter S ′, and uses one of the above formulas (1) and (2) for each pixel. S is obtained. When the display control means 11 obtains S for each pixel in this way, the display control means 11 assigns S to a displayable gradation, for example, 128 gradations, and causes the display means 15 to display the secondary electron image of the sample 6.
[0042]
As described above, even when the difference in impurity concentration in the sample is large, a secondary electron image having a clear contrast reflecting the difference in concentration can be obtained in a short time.
[0043]
As mentioned above, the sample has almost the same composition and the concentration of impurities contained in a trace amount is explained as an example. However, it is different for impurities of different types or samples with the same composition but different plane orientation depending on the location. On the other hand, even when the present invention is applied, it is possible to obtain a clear contrast image reflecting the difference in the type of impurities and the difference in the plane orientation.
[0044]
In addition, a line profile can be obtained by measuring S in the same procedure as described above along a specific line in the field of view instead of a two-dimensional image and displaying the intensity corresponding to each point of the line. .
[0045]
Also, based on the responsiveness of the control power supply of the energy analyzer, intensity data is collected for one energy in one scan while scanning an electron beam on one horizontal or vertical line of the image. Generally, the intensity at each energy can be measured for each pixel. At this time, it is necessary to wait for a response delay of the power supply system when the energy value is changed.
[0046]
If a focused ion beam apparatus is provided, the cross section of the IC can be created on the spot, and the pn junction portion can be observed, so that the presence or absence of defects can be easily confirmed.
[Brief description of the drawings]
FIG. 1 is a diagram showing an energy distribution of electrons generated from a sample when the sample is irradiated with an electron beam.
FIG. 2 is an enlarged view of the lowest energy side portion of the secondary electrons in FIG.
FIG. 3 is a diagram showing a spectrum in the vicinity of where secondary electrons appear when the impurity concentration varies depending on the location of a p-type semiconductor.
FIG. 4 is a diagram showing an example of an electron beam apparatus of the present invention.
FIG. 5 is a diagram showing a spectrum at each representative point selected.
6 is an enlarged view of rising portions of L and H secondary electrons in the spectrum of FIG. 5;
FIG. 7 is a view for explaining setting analysis energy of an electron spectrometer.
FIG. 8 is a diagram for explaining setting analysis energy of an electron spectrometer.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Barrel, 2 ... Electron gun, 3 ... Condensing lens, 4 ... Deflector, 5 ... Sample chamber, 6 ... Sample, 7 ... Sample stage, 8 ... Bias voltage application apparatus, 9 ... Secondary electron detector, 10 DESCRIPTION OF SYMBOLS ... Electron spectrometer, 11 ... Display control means, 12 ... Analysis energy setting means, 13 ... Deflection signal generation means, 14 ... Input means, 15 ... Display means, 16 ... Exhaust device, 17 ... Ion etching apparatus

Claims (1)

Irradiate multiple points on the sample with an electron beam,
By detecting secondary electrons emitted from the sample by the electron beam irradiation, an energy spectrum at each point is obtained,
A spectrum L having the lowest energy at which secondary electrons appear from the energy spectrum is selected, and energy Eonset (L) at which secondary electrons appear in the selected spectrum L;
A spectrum H having the highest energy at which secondary electrons appear from the energy spectrum is selected, and energy Eonset (H) at which secondary electrons appear in the selected spectrum H;
Energy E ₀ where the linearity of the spectrum is maintained as close as possible to the peak position between the Eonset (L) and the peak position energy of the spectrum L;
Energy width Ew between the energy _{E 0} and the _{Eonset (L) (= E 0} -Eonset (L)) and,
The secondary electron intensity Io at the energy E ₀ of the spectrum L is obtained,
If E ₀ is E ₀ ≧ Eonset (H), the set analysis energy of the electron spectrometer is determined as E ₀ only,
On the other hand, if not _{E 0} ≧ Eonset (H), determine the _E 1 _{= E} 0 + Ew as set analysis energy electron spectrometer in addition to the _{E 0,}
Further, if E ₁ ≧ Eonset (H) is not satisfied, E ₂ = E ₀ + 2Ew is determined as the set analysis energy of the electron spectrometer in addition to E ₀ and E ₁ .
In the same manner, n + 1 set analysis energies En = E ₀ + n × Ew (n = 0, 1, 2,...) Are determined,
When the set analysis energy En of the electron spectrometer is determined in this way, each pixel on the sample is irradiated with an electron beam, and the analysis energy of the electron spectrometer is sequentially set to the obtained n + 1 En to obtain the sample. Detect secondary electron intensity I (n) at each energy En for each pixel above,
When n = 0, that is, when the set analysis energy is only E ₀ , the detected secondary electron intensity signal is converted into an image signal to display a sample image,
When n ≠ 0, a secondary electron intensity signal in each pixel is selected according to the following rules (1) and (2) :
(1) Discard the secondary electron intensity signal whose intensity is greater than k × Io (k is a coefficient)
(2) When there are a plurality of secondary electron intensity signals whose intensity is equal to or smaller than k × Io, the secondary electron intensity signal of the selected secondary electron intensity signal is selected to select the secondary electron intensity signal having the highest intensity among them. The parameter S ′ = I (n) −n × Io is calculated from the value of n and its electron intensity I (n),
A sample image display method in an electron beam apparatus, wherein an image signal is generated based on a calculated parameter to display a sample image.

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