JPS612249A - Scanning-type electron microscope - Google Patents

Abstract

PURPOSE:To enable both short-focus operation and insertion of a large sample by dividing the objective into two sections in the direction perpendicular to the lens axis to form a lens gap and enabling a sample to be placed in this gap. CONSTITUTION:The over magnetic path 10 and the excitation coil 7 of an objective are fixed to the upper surfaces of a sample chamber 14. An under magnetic path 11 fixed to a supporting stand 15 is located under the lower surface of the sample chamber 14. A sample stage 16 to which a sample 3 is fixed is installed in the gap formed between the upper and the lower magnetic paths 10 and 11. The sample stage 16 is horizontally moved by driving shafts 18a and 18b which are driven by a sample-driving part 17. Because of the above structure, it is possible to place a large sample 3 in the microscope and to operate an objective 1 with a short focal distance.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a scanning electron microscope, and particularly to a lens structure suitable for obtaining a powerful and minute electron beam.

[Background of the invention]

FIG. 1 shows some methods of detecting secondary electrons using a scanning electron microscope (SEM). (a) is the most common detection method in which a secondary electron detector 2 is provided below an objective lens 1 and obliquely above a sample 3. Here, the −order electron 4
The secondary electrons 5 generated by the irradiation are attracted to the secondary electron detector 2 on the sample 3 and detected. In this case, the distance WD between the lower surface of the objective lens and the sample 3 is 5IlI11 to 30III
It will be about 11. This method has features such as the ability to insert large samples and the ability to tilt the sample.

However, since the objective lens 1 has a long focal point, it is difficult to narrow down the electron beam.

For this reason, the method shown in (l) is to bring the sample 3 close to the bottom surface of the objective lens, place the IU and secondary electron detector 2 above the objective lens 1, and operate the objective lens 1 at a short focus. IC is here (see Japanese Patent Application No. 195814/1983).

Second group? -5 is captured by the magnetic field of the objective lens 1, climbs upward in a spiral pattern, and is attracted and detected by the secondary electron detector 2 at a place where the magnetic field disappears.

In order to achieve an even shorter focus, a method has been considered in which the sample 3 is placed within the gap of the objective lens 1 as shown in (C). This method is the method by which the electron beam can be focused most narrowly, but there are restrictions on the size of the sample, and it is limited to special applications. One of our goals, the CD-SEM, targets wafers with a diameter of 5 to 6 inches, and an IΣB tester must also be equipped with a probe card. Therefore, in these applications, the methods shown in FIGS. 1(a) and 1(b) have been used to detect secondary electrons.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a scanning electron microscope having a lens structure that allows the short focus operation of the objective lens to be compatible with the insertion of a large sample.

[Summary of the invention]

FIG. 2 shows the basic configuration of the present invention. The first example of the conventional example
The objective lens 1 shown in FIG. 3(c) is composed of an excitation coil 7 and a magnetic path 6. Due to the excitation of the excitation coil 7, the magnetic flux flows into the magnetic path 6.
flows. In the magnetic path gap 8, magnetic flux flows through the space. Here, a lens effect is applied to the primary electron beam. The sample 3 is surrounded by a magnetic path 6. For this reason, there was a limit to the size of Sample 3.

In the present invention, as shown in FIG. 2, the objective lens is composed of three parts: an excitation coil 7, an upper magnetic path 10, and a lower magnetic path 11, and a gap 11 is also provided in the outer magnetic path. By doing this,
A large sample 3 can now be inserted, and the objective lens 1 can be operated with a short focus.

[Embodiments of the invention]

FIG. 3 shows an embodiment of the present invention. Objective lens upper magnetic path 1
0 and an excitation coil 7 are fixed to the upper surface of the sample chamber 14. The lower magnetic path 11 is fixed from the lower surface of the sample chamber 14 with a support stand 15 . A sample stage 1 on which a sample 3 is fixed between the gap between the upper magnetic path 10 and the lower magnetic path 11 is placed nII Fl. This sample stage 16 is horizontally moved by drive shafts 18a and 18b from a sample drive section 17 located outside the vacuum. In the present invention, there is no restriction as long as the wafer is flat, and even 5-inch or 6-inch wafers can be inserted. 1
The secondary electron is emitted from the optical system 19, which includes an electron gun, a condenser lens, a scanning coil, a stigma corrector, etc., and scans over the sample 3. Secondary electrons 5 generated by primary electrons 4 penetrate the magnetic field of the objective lens and rise upward. A secondary electron detector 2 composed of a scintillator 22, a post high voltage 20, a light guide 21, and a photomulti 23 is installed above the lens. The secondary electrons 5 that have climbed above the lens are attracted and accelerated by the post high voltage, make the scintillator 22 glow, are led to the photomultiplier 23 by the ride guide 21, and are converted into an electrical signal. This signal creates a scanned image. Although this example does not show the structure for sample exchange, there are several methods, such as providing a preliminary chamber and attaching and detaching only the sample 3, or setting the sample chamber 14 to atmospheric pressure and pulling out the entire sample stage outside to perform sample exchange. General sample conversion methods can be applied.

Although the opening 24 was provided in the lower magnetic path 11 in this embodiment,
Since the structure is such that the electron beam 4 does not reach this opening 24, this opening 24 can be omitted. In this case, there is an advantage that the alignment precision between the upper magnetic path 10 and the lower magnetic path 1 can be relaxed.

Further, in the present invention, the excitation coil 7 is provided on the sample 3, but
A similar effect can be obtained even if it is provided below the sample 3.

Since the objective lens of the present invention has a gap on the outside,
The current flowing through the excitation coil increases. On the other hand, if the excitation current increases by more than twice, the excitation coil 7 generates heat and the coil is damaged. Alternatively, problems such as the inability to downsize the objective lens arise.

This problem can be addressed by making the reluctance created by the outer gap smaller than the reluctance created by the inner gap. In order to maintain this relationship, the thickness of the magnetic path at the gap outside the objective lens must be t, as shown in Figure 4.
, and the inner gap is fine. Here it is. is the diameter of the outer gap, and D is the diameter of the inner gap. For example, tl is 3nwn, D+ is 16mn, D is 120
If nvn is set and to is set to 0.4nwn, the above relationship is satisfied, and the excitation current may be within twice. If to is 4n
If it is set to wn, the excitation current will increase by only 10%.

FIG. 5 shows another embodiment of the invention. Here, lower magnetic path 2
5 also serves as a sample stage, and the sample 3 is mounted on the lower magnetic path 25. Since this lower magnetic path is not provided with an opening (FIG. 3-24), the magnetic flux generated between the upper magnetic path 10 and the lower magnetic path 25 hardly changes even if the lower magnetic path 25 is moved horizontally. The method of moving the lower magnetic path 25, the method of detecting secondary electrons, etc. are the same as in the embodiment shown in FIG.

In this example, by setting the relationship between the outer diameter D2 of the lower magnetic path 25 and the outer diameter of the upper magnetic path to satisfy D2≧D, +d (d is the movement range of the sample stage), The effect on lens performance caused by moving the lower magnetic path 25 can be almost completely eliminated.

FIG. 6 shows another embodiment of the invention. In this embodiment, a sample 3 and a probe card 26 are mounted on the lower magnetic path 25. Sample 3 of this example is a wafer that has undergone a process and has not been assembled into a package. This embodiment is an EB tester that operates integrated circuits fabricated within a wafer in a sample chamber and measures electrical characteristics with an electron beam.6 This is an EB tester that operates integrated circuits fabricated within a wafer and uses probes to supply power supply voltage and signal voltage to the circuits within the wafer. The needle 27 of the card 26 is brought into contact. To ensure accurate contact, moving mechanisms 28a and 28b for moving the sample 3 on the lower magnetic path 25 and vertical mechanisms 29a and 29b for the probe card 26 are provided. A lead wire 30 is brought into contact with the probe card 26 and guided to the vacuum tube via a hermetic wire 31. Lead wire 32 is connected to a power source that operates the integrated circuit. The XY moving mechanisms 36a and 36b connected to the lower magnetic path 25 are connected to the sample drive unit 17 outside the vacuum via drive shafts 18a and 18b.
You can move it with . This moving table is mounted on a moving plate 35 fixed to the front wall 34 of the sample chamber, and is pulled out of the sample chamber when changing the sample. This embodiment also includes a suction grid 3
4 and a reflection grid 33 are provided to more clearly show potential chains within the sample as signal differences. The suction grid 34 has a positive potential (for example, 10
0V) is applied, which serves to efficiently guide the secondary electrons generated in the sample to the reflection grid 33, and a negative potential (for example, -5V) is applied to the reflection grid 33, forming a potential barrier against the secondary electrons. However, only high-energy secondary electrons are detected. Note that the measurement of potential using the reflective grid 33 is detailed in Japanese Patent Application No. 112729/1982.

FIG. 7 shows another embodiment of the present invention, in which a sample exchange chamber 41 and a sample exchange mechanism 42 are provided so that the sample exchange of the embodiment shown in FIG. 6 can be carried out quickly. In this embodiment, the lower magnetic path 25 not only moves horizontally in XY but also moves vertically (Z)
It rides on the XvZ movement mechanism with. In the XY movement, the probe card 26 and the lower magnetic path 25 move together, but in the Z movement, only the lower magnetic path 25 moves up and down. The XY movement can be operated from outside the vacuum using the drive shafts 18a, 18b, and the Z axis is the drive shaft 18c. Lead wires 39 of the probe card 26 are guided outside the vacuum by a hermetic 38 and connected to a power source for operating the circuit.

Mount sample 3 using the following procedure = (i) Lower magnetic path 25
Lower the valve 40 by 2 to 3 sides from the standard position (+i)
(iii) Place the sample 3 on the lower magnetic path 25 using the sample exchange mechanism 42. (1v) Close the valve 40. (V) While looking at the scanned image, place the sample 3 on the pad (place where the needle is placed) inside the wafer. Check the degree of coincidence with the needle of the probe card 26 and make them match. (1) Gradually raise the lower magnetic path 25 on the Z axis. (vii) Using the voltage contrast in the scanned image (activating the reflection grid 33). (vm) Stop the rise of the lower magnetic path 25 by the Z-axis, confirming that the potential has been introduced into the circuit.

The sample is attached and detached using the following procedure = (1) Lower magnetic path 25
(11) Open valve 4o, (ii)
i) Return the sample 3 to the sample exchange chamber 41 using the sample exchange mechanism 42, (iv) Close the valve 4°, (V) Make the sample exchange chamber 41 atmospheric, (vl) Open the exchange window (omitted in the figure) ) and take out sample 3.

[Effects of the Invention] 5.1 As described in detail above, according to the present invention, a human-sized sample such as a wafer can be captured using an objective lens with a short focal length, and images can be observed with high resolution. It is effective for measuring electrical potential.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view of an objective lens and sample section showing a conventional secondary electron detection method, FIG. 2 is a vertical cross-sectional view of main parts showing the basic configuration of the scanning electron microscope of the present invention, and Figure 7 shows a scanning electron microscope 'f3r' which is an embodiment of the present invention.

Claims (1)

[Claims] 1. A lens system including an electron source that emits electrons and an objective lens that focuses the electrons emitted from the electron source onto a sample;
In a scanning electron microscope consisting of a scanning power source that scans focused electrons onto a sample and an image display system that includes a secondary electron detector,
A scanning electron microscope characterized by using a magnetic field type electron lens in which an objective lens part is divided into two parts in a direction perpendicular to the lens axis to form a lens gap, and a sample can be inserted between the gaps. 2. Scanning according to claim 1, characterized in that one of the divided magnetic paths is a disk having an opening at the center, and the center and outer circumference of the disk form a lens gap. Shape electron microscope. 3. A scanning electron microscope according to claim 1, which uses a disk-shaped magnetic path without openings. 4. A scanning electron microscope according to claim 3, characterized in that a sample is placed on a disk magnetic path, and the disk-shaped magnetic path can be moved. 5. The scanning electron microscope according to claim 5, wherein the diameter of the magnetic path of the disk is greater than or equal to the sum of the amount of movement of the disk-shaped magnetic path and the diameter of the magnetic field lens. 6. A scanning electron microscope according to claim 6, characterized in that a sample and a probe card for introducing a voltage to the sample are installed on the disc-shaped magnetic path. 7. A scanning electron microscope according to claim 6, characterized in that the probe card can be moved horizontally and vertically with respect to the disk-shaped magnetic path. 8. The probe card and the disc-shaped magnetic path are integrally movable horizontally, and only the height of the disc-shaped magnetic path is movable independently. scanning electron microscope. 9. An attraction grid that attracts secondary electrons into the electron path of the magnetic path provided opposite to the disk-shaped magnetic path, and a reflection grid that separates the energy of the secondary electrons are provided outside the attraction grid. Characterizing claims 6 and 7,
The scanning electron microscope according to item 8.