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United States Patent 5,202,564
Todokoro ,   et al. April 13, 1993
Scanning electron microscope and method and production of semi-conductor device by using the same

Abstract

An SEM can inspect a groove or hole in the surface of a specimen by irradiating the specimen with a high energy electron beam. The SEM is used in the manufacturing process of a semiconductor device to observe the configuration of the bottom of a deep hole.

Inventors: Todokoro; Hideo (Tokyo, JP); Takamoto; Kenji (Ome, JP); Otaka; Tadashi (Katsuto, JP)
Assignee: Hitachi, Ltd. (Tokyo, JP)
Appl. No.: 773729
Filed: October 9, 1991
Foreign Application Priority Data
Oct 12, 1990[JP]2-272258

Intern'l Class: H01J 037/26
Field of Search: 250/310,311,398,399,306,307,492.2 R

References Cited
U.S. Patent Documents
Re27005Dec., 1970Wingfield et al.250/307.
3614311Oct., 1971Fijiyasu et al.250/310.
3714424Jan., 1973Weber250/399.
4426577Jan., 1984Koike et al.250/310.
4658137Apr., 1987Garth et al.250/310.
5001350Mar., 1991Ohi et al.250/440.
Foreign Patent Documents
62-97246May., 1987JP.
64-48470Feb., 1989JP.
2050689Jan., 1981GB.

Primary Examiner: Berman; Jack I.
Assistant Examiner: Nguyen; Kiet T.
Attorney, Agent or Firm: Fay, Sharpe, Beall, Fagan, Minnich & McKee
Claims


What is claimed is:

1. A scanning electron microscope, comprising:

means for irradiating a primary electron beam to an object to be inspected, the primary electron beam having energy sufficient to produce reflection electrons, reflected from within a depression in the object, that penetrate through the object so as to escape from the surface of the object, or that generate secondary electrons in the surface of the object while penetrating through the object; and

means for detecting the escaped reflection electrons or the secondary electrons.

2. A scanning electron microscope according to claim 1, wherein the detection means detects the secondary electrons generated in the surface of the object by the reflection electrons.

3. A scanning electron microscope according to claim 1, wherein the detection means detects the reflection electrons.

4. A scanning electron microscope according to claim 1, wherein the irradiation means for the primary electron beam includes a final lens for focusing the beam, and further comprising an electron-ray detector above the final lens, wherein the secondary electrons pass through the final lens before being detected by the electron-ray detector.

5. A scanning electron microscope according to claim 1, further comprising means for imaging the surface of the object to be inspected according to detection signals generated by the detecting means, wherein the detection signals result from an arithmetic operation on the reflection electrons or the secondary electrons.

6. A scanning electron microscope according to claim 1, wherein the energy of the primary electron beam exceeds 50 keV, and further comprising a stage into which the object having a diameter in excess of 4 inches can be inserted and which is adjustable in inclination angle.

7. A semiconductor fabrication apparatus as claimed in claim 1, wherein the detecting means includes separate means for detecting the escaped reflection electrons and means for detecting the secondary electrons, and wherein the separate detecting means respectively and simultaneously detect the escaped reflection and secondary electrons.

8. A scanning electron microscope, comprising:

means for irradiating a primary electron beam to an object to be inspected, the primary electron beam having sufficient energy to transmit through the object to be inspected, and the transmitted beam causing secondary electrons to be emitted from the undersurface of the object; and

means for detecting the transmitted beam or the secondary electrons.

9. A method for production of a semiconductor device, comprising the steps of:

etching a semiconductor device to form a depression;

irradiating a primary electron beam to the device, the primary electron beam having energy sufficient to produce reflection electrons, reflected from within the depression in the device, that penetrate through the device so as to escape from the surface of the device, or that generate secondary electrons in the surface of the device by the reflection electrons while penetrating through the device; and

detecting the reflection electrons or the secondary electrons.

10. A semiconductor fabrication apparatus, comprising:

a first chamber in which an object has its surface manufactured;

a second chamber in which the object is inspected;

means for irradiating a primary electron beam to the object to be inspected, the primary electron beam having energy sufficient to produce reflection electrons, reflected from within a depression in the object, that penetrate through the object so as to escape from the surface of the object, or that generate secondary electrons in the surface of the object while penetrating through the object;

means for connecting the first chamber and the second chamber; and

means for transporting the object from the first chamber to the second chamber.

11. A semiconductor fabrication apparatus as claimed in claim 10, wherein the first and second chambers and the connecting means are maintained at near-vacuum pressure.

12. A semiconductor fabrication apparatus as claimed in claim 11, wherein the connecting means includes an intermediate chamber through which the object is transported, said intermediate chamber including switching valve means for selectively exposing the intermediate chamber atmosphere to the atmosphere of one of the first and second chambers.
Description


BACKGROUND OF THE INVENTION

The present invention relates to an apparatus and a method of observing surface configurations by using an electron beam, and especially provides an apparatus and a method by which observation of either configurations of the bottom of a deep hole or residues therein, used frequently in semiconductor processes, can be permitted.

The scanning electron microscope, in which an electron beam is scanned on a specimen and secondary electrons generated from the specimen are detected, has been utilized widely in the fields of biology and engineering. Especially, in the semiconductor industry, high-integration formation has been advanced and as a result, inspection based on optical microscopes has become impossible, and the utilization of the scanning electron microscope has been promoted. In a scanning electron microscope used for semiconductors, it is conventional to use an electron beam of low energy of 1 keV or less in order to avoid charging on insulators.

In the semiconductor industry, the scanning electron microscope is utilized for not only inspection of appearance of completed semiconductors but also inspection in mid-course of the manufacturing process. For example, it is used for inspection of appearance, inspection of dimension and inspection of through-holes in mid-course of the process.

As a result of the advancement of high-integration formation of semiconductor devices, it has become impossible for the method using the conventional scanning electron microscope to inspect openings of through-holes.

Referring to FIG. 2, problems encountered in observing a deep hole with the conventional scanning electron microscope will be described. FIG. 2 shows a case where a primary electron beam 1 of low energy irradiates a flat portion and a hole 3 of a specimen Thanks to the absence of any obstacles, almost all of the number of secondary electrons 2 generated at the flat portion can be detected. Similarly, reflection electrons concurrently discharged can also be detected In the case of irradiation of the hole 3, however, generated secondary electrons 2 impinge on the side wall of the hole 3 and consequently cannot escape from the hole 3 to the outside. The energy of reflection electrons is higher than that of secondary electrons but is not so high that the reflection electrons can penetrate through the side wall, and the reflection electrons are thus blocked by the side wall.

SUMMARY OF THE INVENTION

FIG. 3 shows results of calculation of the relation between the aspect ratio (depth/opening diameter) of a hole and the ratio of signals escaping from the hole. A signal at the surface (aspect ratio=0) corresponds to 1. This calculation demonstrates that observation of holes of an aspect ratio exceeding 2 is impossible with the conventional scanning electron microscope.

In the present invention, in order to solve the aforementioned problems, a primary electron beam is used which has such high energy as to allow reflection electrons generated at the bottom of a hole to penetrate through the side wall of the hole.

Referring to FIG. 1, the principle of observation of deep holes by using a primary electron beam of high energy will be described.

A case where a primary electron beam 4 of high energy irradiates a surface portion resembles a case where the surface portion is irradiated with low energy. However, in the case of irradiation on the interior of a hole, the circumstances differ greatly. Secondary electrons 2 are absorbed by the side wall but reflection electrons 6 penetrate through the side wall to run out of the surface. When the reflection electrons 6 pass through the surface, they again generate secondary electrons 5. Since the secondary electrons 5 or reflection electrons 6 have information about the bottom of the hole 3, an image of the interior of the hole can be obtained by detecting these electrons.

Thus, one of the aspects of the present invention resides in that the primary electron beam has sufficiently high energy to allow reflection electrons to penetrate through the side wall, thereby permitting observation of the bottom of high-aspect-ratio holes which has hitherto been impossible. The primary electron beam penetrates an insulating layer (.ltoreq.2 .mu.m thickness) of a semiconductor device when it has energy sufficient for the reflection electrons to penetrate through the side wall. This results in allowing electrons in the insulating layer to move, i.e. the layer becomes conductive so that the layer is not subject to charging. Further, when the primary electron beam has an energy level as defined by the present invention, the beam does not cause the insulating layer of the semiconductor device to become charged, either.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining the principle of observation according to the invention;

FIG. 2 is a diagram for explaining a conventional observation method;

FIG. 3 is a diagram showing the relation between signal intensity from a hole and aspect ratio obtained in accordance with the conventional method;

FIG. 4 shows results of actual measurement of increased signals from the bottom of a hole which are obtained by increasing accelerating voltage (energy);

FIG. 5 is a diagram showing the relation between signal intensity and aspect ratio obtained when the invention is used;

FIG. 6 is a diagram for explaining a method of detecting secondary electrons generated by reflection electrons from the bottom of a hole;

FIG. 7 is a diagram for explaining a method of observing reflection electrons from the bottom of a hole;

FIG. 8 is a diagram for explaining a method of observing both reflection electrons and secondary electrons simultaneously;

FIG. 9 is a diagram illustrative of detection of electrons transmitting through a specimen;

FIG. 10 is a diagram for explaining an embodiment of the invention which can permit observation of an object in wafer condition in accordance with the principle of the observation and detection method according to the invention;

FIG. 11 is a diagram showing a flow for determining etching conditions by using the scanning electron microscope; and

FIG. 12 is a conceptual diagram of an arrangement in which a specimen chamber is provided in common for microwave etching and for a scanning electron microscope to facilitate determination of etching conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

When a hole in SiO.sub.2 having an aspect ratio of about 3 and a depth of 1.5 .mu.m was observed, the ratio between signals from the bottom and the surface was measured by changing energy of a primary electron beam to obtain results as shown in FIG. 4. Namely, the ratio between secondary electrons generated by reflection electrons and secondary electrons generated by the primary electron beam, was measured. It will be appreciated that the ratio is maximized around 100 kV of electron beam energy. When the primary electron beam has low energy, reflection electrons are absorbed by the side wall and therefore no secondary electrons are generated by the reflection electrons. As the energy of the primary electron beam increases, the number of reflection electrons penetrating through the side wall increases and hence the number of secondary electrons thereby generated is increased gradually. However, as the energy is further increased, the primary electron beam intrudes into the specimen more deeply and the number of reflection electrons decreases, leading to a decrease in the number of secondary electrons. This is the reason for existence of the maximum value. Electron beam energy corresponding to the maximum value is related to the depth and material of the hole. The deeper the hole and the denser the material, the higher the maximum value becomes.

FIG. 5 shows the relation between the aspect ratio and the signal ratio obtained when a deep hole was observed by using 100 keV energy. The relation obtained with 1 keV is indicated for reference and it will be appreciated that with 100 keV, even when the aspect ratio exceeds 3, the signal ratio does not decrease and holes of higher aspect ratios can be observed.

In the conventional scanning electron microscope, the energy is less than 50 keV and high energy exceeding 50 keV is not used. This is because there was no concept of observation grounded on the principle described herein. Effectiveness of the high-energy primary electron beam permitting observation of deep holes is disclosed herein for the first time.

FIG. 6 shows a method of detecting secondary electrons generated by reflection electrons of high energy. This method uses a scintillator 10 and a secondary-electron multiplier 12. Supplied to the scintillator 10 is a high voltage of 10 kV from a high voltage power supply 13. By using an attraction electric field 9 formed by the high voltage, secondary electrons generated by the reflected electrons in the surface of a specimen 8 are detected. A primary electron beam 4 having energy sufficient to generate the secondary electrons from reflection electrons is focused and irradiated on the specimen 8 by means of an objective lens 7. In the Figure, circuits for scanning the primary electron beam and for displaying scanning images are omitted.

FIG. 7 shows an example of detecting not secondary electrons but reflection electrons transmitting through the side wall. A reflection electron detector 15 having a large view angle relative to a specimen 8 is interposed between an objective lens 7 and the specimen 8. The reflection electron detector 15 may be a semiconductor detector having a PN-junction or a Schottky junction or may be based on a method of causing phosphors to luminesce of and detecting luminescence (an example using a semiconductor is shown in the embodiment). Since the energy of reflection electrons is high, the surface layer of the semiconductor detector is made to be thick (1 to 10 .mu.m), thus preventing degradation of detection efficiency. In the case of phosphors, similar thickening is also employed. The thickness of the phosphor layer measures 10 to 100 .mu.m, depending on energy.

FIG. 8 shows an example where reflection electrons and secondary electrons are both detected. An attraction electrode 16 is provided which passes through the center of an objective lens 7. Secondary electrons 5 generated by the reflection electrons from a specimen 8 are drawn into a magnetic field of the objective lens 7 and pulled upwards by means of the attraction electrode 16. The secondary electrons 5 thus pulled upwards are accelerated by an attraction electric field 9 formed by a scintillator 10 so as to impinge on the scintillator 10 and cause it to luminesce. Luminescent light is guided to a light guide 11 and amplified and converted into an electrical signal by means of a secondary-electron multiplier 12. Reflection electrons generated from the specimen 8 have high energy and therefore they are hardly deflected by an electric field formed by the attraction electrode 16, keeping a substantially straight on and impinging upon a reflection electron detector 15. Through this process, the reflection electrons themselves and the secondary electrons generated from the reflection electrodes can be detected distinctively. Since scanning images formed by the two types of electrons are slightly different from each other, it is possible to select one of the images which has better contrast, or to perform addition/subtraction to improve contrast.

In the foregoing embodiments, secondary electrons and reflection electrons generated on the side upon which the primary electron beam is incident but when the specimen is thin, either secondary electrons generated by transmitted electrons or the transmitted electrons themselves may be detected.

FIG. 9 shows an embodiment in which secondary electrons generated on the side opposite the primary electron beam and transmitted electrons are detected. The manner of detection on the primary electron beam side is the same as that in the previously-described embodiment. Secondary electrons 20 generated from the undersurface of a specimen 8 by electrons transmitting through the specimen and secondary electrons 21 generated by impingement of transmitted electrons 19 upon a reflection plate 22 are detected by using a scintillator 10, a light guide 11 and a secondary-electron multiplier 12. Electrons of a primary electron beam of 200 keV energy have a range of 200 .mu.m and can transmit through even a Si wafer used in the semiconductor industry. Thus, by detecting transmitted electrons and secondary electrons generated from the undersurfaoe of the specimen, a hole formed in the surface of the specimen 8 can be observed.

FIG. 10 shows a scanning electron microscope using the principle of observation and the detection method described previously. The source of electrons can be a single crystal of LaB.sub.6 heated to emit electrons. Emitted electrons are controlled by means of a Whenelt 24. The emitted electrons are accelerated by accelerating electrodes 25. The accelerating voltage (energy) in the present embodiment has a maximum value of 200 keV.

An accelerating electrode 25 of the uppermost stage is applied with the accelerating voltage and divisional voltages due to dividing resistors 34 are applied to individual accelerating electrodes 25. Here, the cable and power supply for application of the accelerating voltage are omitted.

An accelerating unit including the accelerating electrodes 25 is shielded with a high voltage shield 35. An accelerated primary electron beam 4 is reduced in size by means of a first condenser lens 26, a second condenser lens 27 and an objective lens 7. When an objective lens having a focal distance of 30 mm is used, a resolution value of 3 nm can be obtained at 200 keV. The aperture of the electron beam is determined by an aperture 36 placed on the second condenser lens 27.

Scanning of the electron beam is carried out by a scanning coil 28. The scanning coil is constructed of two stages of coils so that the electron beam subject to scanning may pass through the center of the objective lens 7. Reflection electrons reflected at a specimen are detected by a reflection electron detector 15 and secondary electrons generated by the reflected electrons are led upwardly of the objective lens 7 and detected by a detector comprised of a scintillator 10, a light guide 11 and a secondary-electron multiplier 12.

The specimen is a wafer of 4 inches or more carried on an XY fine movement stage 29. The specimen can be inclined by .+-.15 degrees in desired directions by means of a specimen inclination fine movement section 30. The specimen inclination fine movement section 30 has three posts, and the length of each post can be controlled by a computer. A wafer to be observed is contained in a dedicated cassette 32 and the cassette is stored in a preparatory chamber 33. When conducting an observation, a valve 34 is opened and the specimen is brought onto the XY fine movement stage 29 by using an exchange mechanism (not shown). When the same portion of the specimen is observed by changing the inclination angle, the height can be measured (stereo-measurement).

In highly integrated semiconductor devices, the etching process for working a deep hole of high aspect ratio is important as has already been described Etching for working a deep hole of high aspect ratio is very difficult and for determination of etching conditions, observation of the bottom of the deep hole and confirmation of the progress of etching are needed.

FIG. 11 shows a flow of the confirmation wherein in accordance with the results of confirmation, feedback for urging, for example, re-etching is undertaken to ensure process integrity. The thus-determined etching conditions are relayed to the succeeding process. By repeating the confirmation at a fixed period, the process can be made to be stabler.

The scanning electron microscope is very effective for the confirmation of etching and can contribute to improvement in yield of production of highly-integrated devices. Especially, the high-energy scanning electron microscope utilizing secondary electrons generated by reflected /transmitted electrons described so far is effective.

FIG. 12 shows an arrangement contrived to simplify the aforementioned confirmation process and in which a specimen chamber is provided in common for a microwave etching apparatus 38 and a high-energy scanning electron microscope 37, and etching and inspection can be carried out alternately by merely moving a specimen from an etching apparatus specimen stage 39 to a scanning electron microscope XY fine movement stage 42. The degree of vacuum in the microwave etching apparatus is 10.sup.-4 Torr which is comparable to that in the scanning electron microscope but, because of the use of inert gas, an intermediate chamber 41 is provided in the present embodiment in order that the inert gas can be prevented from flowing into the scanning electron microscope by switching valves 40 alternately for intermediate chamber 41. In the Figure, the evacuation system is omitted

As described above, in accordance with the principle of observation according to the invention, deep holes of which observation was impossible in the past can be observed. This implies that inspection can be carried out in-line in the process of semiconductor device production, resulting in very beneficial effects.

Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the present invention as defined by the appended claims.

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