JP3154827B2 - Backscattered electron detectors such as scanning electron microscopes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backscattered electron detector such as a scanning electron microscope, and more particularly to a backscattered electron detector of a PIN type semiconductor.

[0002]

2. Description of the Related Art Backscattered electron detectors used in scanning electron microscopes and the like are broadly classified into scintillator type and semiconductor type. 1A and 1B show a scintillator-type backscattered electron detector. In FIG. 1A, reference numeral 1 denotes an objective lens of a scanning electron microscope, 2 denotes a sample stage, and 3 denotes a sample stage. This is the sample placed. A scintillator-type backscattered electron detector 4 is arranged at a portion of the space between the objective lens 1 and the sample 3 which is separated from the optical axis. The detector 4 includes a scintillator 5 in which a phosphor such as P47 is applied on an acrylic chip, and a metal such as aluminum is deposited thereon, and a light guide 6 for guiding light emitted by the scintillator. I have. When the scintillator 5 is used as a secondary electron detector, a voltage of about 10 kV is applied. When the scintillator 5 is used as a backscattered electron detector, the applied voltage is cut off. The backscattered electron detector shown in FIG. 1B is called a Robinson-type detector, and the portion of the light guide 6 is extended between the objective lens 1 and the sample 3 so that the incident electron beam EB can pass therethrough. An opening 7 is formed in the light guide 6. A film-shaped scintillator 8 is provided on the surface of the light guide 6 facing the sample 3. In the detector shown in FIGS. 1A and 1B, reflected electrons generated by the irradiation of the sample 3 with the electron beam EB are incident on the scintillator 5 to emit light.
This light is guided to a photomultiplier tube (not shown) by the light guide 6, and is converted into an electric signal.

FIG. 1C shows a scanning electron microscope using a semiconductor type backscattered electron detector. A hole for passing an incident electron beam EB is provided in a space between an objective lens 1 and a sample 3. Drilled donut-shaped PIN semiconductor detector 9
Is arranged. FIG. 2 shows the details thereof. The semiconductor detector 9 includes a P-type semiconductor (P ⁺ ) layer 10 serving as a light-receiving surface of reflected electrons, an I layer 11 in an intrinsic region, and an N-type semiconductor (N ⁺ ) layer 12.
And constitutes a PIN photodiode.
That is, in the P ⁺ layer 10, impurities such as boron (B) are diffused in silicon, and in the N ⁺ layer 12, impurities such as phosphorus (P) are diffused. With this configuration, N
^{The +} layer 12 is grounded, and a signal having an intensity corresponding to incident electrons is extracted from the P ⁺ layer 10.

[0004]

Generally, the sensitivity of a backscattered electron detector depends on the distance from the sample surface to the light receiving surface of the detector, and it is known that the shorter the distance (interval), the higher the sensitivity. ing. However, FIGS. 1 (a) and 1 (b)
In the scintillator-type detector shown in (1), there is a risk that the sample may come into contact with the detector 4 when the large sample is tilted. Therefore, there is a limit to shortening the distance between the sample 3 and the detector 4. On the other hand, the semiconductor detector 9 shown in FIG.
Since the sample 3 has a thin plate structure, the sample 3 can be brought as close to the detector 9 as possible, which is advantageous in terms of sensitivity as compared with the scintillator type.

However, a conventional detector having a light receiving surface of a P-type semiconductor, for example, when the accelerated voltage of the incident electron beam EB is in the range of 0.5 kV to 35 kV, when the accelerated electrons enter the sample 3. The primary incident electrons are reflected from the sample surface and the inside of the sample by the interaction with the sample, and are incident on the P ⁺ layer of the detector. At this time, the energy of the reflected electrons incident on the P ⁺ layer varies depending on the constituent elements and shape of the sample, the distance between the detector and the sample, the degree of vacuum in the sample chamber, the gas atmosphere, and the like. have. As described above, when the sample is irradiated with the electrons accelerated at 0.5 kV to 35 kV, the space charge formed between the PIN layers against the reflected electrons generated from the sample surface (the donor ion shown in FIG. And acceptor ion)
Is acting in the opposite direction. For this reason, the reflected electrons when the acceleration voltage is lower are combined with the holes which are the majority carriers in the P ⁺ layer, so that the reflected electrons cannot cross the wall of the electric field E.
Very few backscattered electrons that have passed over the wall of the electric field E excite the lattice (electron-hole pairs) of the I layer, which is the sensitive layer, and the electron and hole pairs of the I layer are free electrons and free holes, respectively. Is separated into

[0006] The separated free electrons move to the N ⁺ layer under the action of the electric field E. The free holes move to the P ⁺ layer under the action of the electric field E. As described above, the current i flows from the N ⁺ layer to the P ⁺ layer due to the movement of free electrons and free holes. Since this current is extremely weak, even if it is amplified by an electric circuit, the amount of signal never becomes sufficient when observing an image at a high magnification with a scanning electron microscope having an acceleration voltage of 0.5 kV to 35 kV. .

The present invention has been made in view of the above circumstances, and has as its object to provide a backscattered electron detector such as a scanning electron microscope which is excellent in the detection sensitivity of backscattered electrons and can obtain a sufficient amount of detected current. It is to realize.

[0008]

A backscattered electron detector such as a scanning electron microscope according to the present invention is a PIN type semiconductor detector for irradiating a sample with an electron beam and detecting backscattered electrons from the sample. A plurality of backscattered electron detectors having an N-type semiconductor as the light receiving surface of the backscattered electrons facing the
It is characterized in that child detectors are connected in series . Other books
A backscattered electron detector, such as a scanning electron microscope according to the invention,
Reflection provided symmetrically to the electron beam optical axis and facing the sample
First and second reflected electrons having an electron receiving surface as an N-type semiconductor
The detector and the reflected electrons reflected from the sample are detected at a low angle.
And a third backscattered electron detector for emitting light.
It is a sign.

[0009]

According to the backscattered electron detector such as a scanning electron microscope of the present invention, the light receiving surface of the backscattered electrons facing the sample of the PIN type semiconductor detector for detecting the backscattered electrons from the sample is made of an N-type semiconductor, and the detection sensitivity is improved. Improve.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 3 shows an embodiment of the present invention.
The same parts as those of the conventional apparatus of FIG. 2 are denoted by the same reference numerals. In this embodiment, the light-receiving surface of the semiconductor detector 9 facing the sample 3 is formed by an N-type semiconductor (N ⁺ ) layer 12, and the other surface is formed by a P-type semiconductor (P ⁺ ) layer 10. That is, attention is paid to the direction of the electric field due to the space charge formed between the PIN layers of the silicon (Si) semiconductor, and the light receiving surface of the reflected electrons is set to N so that the electric field acts on the movement of electrons and holes in the forward direction. ⁺ Layer. The operation principle of such a configuration will be described below.

When an electron beam EB from a source of an electron beam (not shown) enters the sample 3 placed on the sample stage 2 through the electron beam passage hole 13 of the reflected electron detector 9, reflected electrons are generated, and the reflected electrons are generated. Is N ⁺ of the Si semiconductor detector 9
Light is incident on the layer 12. N ⁺ layer 1 in which electrons are majority carriers
The surface layer portion of No. 2 is filled with reflected electrons having negative charges, the electron density increases, and the electrons flow into the I layer 11. At this time, an electric field E due to space charges (donor ions and acceptor ions shown in FIG. 7B) formed between the PIN layers with respect to the reflected electrons accelerates the electrons flowing from the N ⁺ layer 12 into the I layer 11. Works. For this reason, even when the acceleration voltage is low, even the reflected electrons can easily reach the I layer, which is the sensitive layer, and excite the lattice (electron-hole pairs) of the I layer. As a result, more free electrons and free holes can be obtained than in the conventional case (the light receiving surface is a P ⁺ layer). The free electrons move to the N ⁺ layer under the action of the electric field E, and the free holes move to the P ⁺ layer under the action of the electric field E. The current i obtained by moving a large number of free electrons and free holes in this way is larger than in the conventional case (the light receiving surface is a P ⁺ layer). That is, the amount of current depends on the number of free electrons and free holes separated in the I layer. In other words, the above-mentioned phenomenon is that the energy loss of the reflected electrons causes P
^This is probably because the number of N ⁺ layers was smaller than that of N ⁺ layers.

FIG. 4 shows a more specific embodiment of the present invention. FIG. 4 (a) is a plan view of a Si semiconductor detector 15,
FIG. 4B is a schematic diagram when the detector is regarded as a photodiode. The detector used in this example is
The light receiving surface of the reflected electrons is an N ⁺ layer cathode, and the back P ⁺ layer is an anode. In the figure, H is an aperture through which the electron beam passes. The reason for dividing into two is to enable the separation of the composition signal of the sample and the uneven signal by calculating the output signals of the two detectors. That is, when the output signals of the detectors 15a and 15b are added, only a signal based on the composition of the sample is obtained. If the added signal is supplied as a luminance signal to a cathode ray tube to which a scanning signal of a primary electron beam is supplied, a composition image is obtained. Can be displayed. on the other hand,
When the output signals of the detectors 15a and 15b are subtracted, only a signal based on the unevenness of the sample is obtained. By supplying the subtracted signal to the cathode ray tube, an uneven image of the sample can be displayed.

FIG. 5 shows another embodiment of the present invention.
FIG. 5A is a plan view of the Si semiconductor detector 16, and FIG.
(B) is a schematic diagram when the detector is regarded as a photodiode. The light receiving surface of the reflected electrons is an N ⁺ layer cathode, and the back side P ⁺ layer is an anode. The four detectors 16a to 16d are electrically connected by a lead wire so as to be electrically connected in series to form a substantially single detector. The other four detectors 16e to 16h are also connected by lead wires so as to be electrically connected in series to form a single detector. When a large number of detectors (N) are connected in series as in this embodiment, the parasitic capacitance (also called stray capacitance) between the PIN layers becomes 1 /
N, which has the advantage of increasing the electrical response speed.

FIG. 6 shows another embodiment of the present invention.
In this embodiment, a pair of two-part detectors 17a and 17b are used.
And an independent detector 17c. Each detector 1
The detection signals of 7a to 17c are amplified by the preamplifier and guided to the operational amplifier. If the signal of the detector 17c is turned off electrically and both signals of the detectors 17a and 17b are added, the composition image can be obtained. If a difference signal between the two signals is obtained, a signal for a concavo-convex image is obtained. Up to this point, it is as described in the embodiment of FIG. In this embodiment, if the output signal of the detector 17c is further added to the addition signal of the detectors 17a and 17b, a rich three-dimensional image in which the composition signal and the concavo-convex signal are mixed can be formed. In this embodiment, the detector 17c is moved from the electron beam optical axis so that the observation region of the sample is obliquely viewed by the detector 17c, that is, so that the reflected electrons reflected from the sample can be received at a low angle. By arranging them at a certain distance or more, a three-dimensional effect can be enhanced.

[0015]

As described above, a backscattered electron detector such as a scanning electron microscope according to the present invention uses a PIN-type semiconductor detector that detects backscattered electrons from a sample to detect the reflected light receiving surface facing the sample. Since an N-type semiconductor is used, the detection sensitivity of reflected electrons can be improved, and a sufficient amount of detected current can be obtained. In addition, according to the first aspect of the present invention, a plurality of detectors are provided.
(N) are connected in series, so the parasitic capacitance between PIN layers
(Stray capacitance) can be reduced to 1 / N, and the electric response speed is fast.
Wear. Further, according to the second aspect of the present invention, the light receiving surface is N-type.
From the first and second backscattered electron detectors as semiconductors and the sample
A third method for detecting reflected backscattered electrons at a low angle
A backscattered electron detector so that the first and second reflections
The output of the third backscattered electron detector is added to the sum signal of the electron detector.
If the signals are added, a three-dimensional mixture of the composition signal and the uneven signal
An image with a rich feeling can be formed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a conventional backscattered electron detector.

FIG. 2 is a diagram showing a conventional semiconductor backscattered electron detector.

FIG. 3 is a diagram showing a backscattered electron detector according to one embodiment of the present invention.

FIG. 4 is a diagram showing a specific example of the present invention.

FIG. 5 is a diagram showing a specific example of the present invention.

FIG. 6 is a diagram showing a specific example of the present invention.

FIG. 7 is a diagram for explaining a comparison between a conventional type and the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Objective lens 2 Sample stand 3 Sample 9 Semiconductor type detector 10 P ⁺ layer 11 I layer 12 N ⁺ layer

Continuation of the front page (56) References Tokiko 1-27549 (JP, B2) Jikken 44-44,474 (JP, Y1) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01J 37 / 244 G01T 1/24 G01T 1/28 H01J 37/28

Claims (2)

(57) [Claims] 1. A irradiated with an electron beam to a sample, a PIN type semiconductor detector for detecting the reflected electrons from the sample, reflecting the light receiving surface of the reflected electrons facing the sample is N-type semiconductor
A plurality of electron detectors are provided, and these reflected electron detectors are connected in series.
Backscattered electron detectors such as a scanning electron microscope, characterized in that connection was. 2. A irradiated with an electron beam to a sample, a PIN type semiconductor detector for detecting the reflected electrons from the sample, electrostatic
Reflection currents are provided symmetrically with respect to the optical axis of the
1st and 2nd backscattered electron detection using an N-type semiconductor
Detector and reflected electrons reflected from the sample at a low angle
And a third backscattered electron detector for performing
A backscattered electron detector such as a scanning electron microscope.