Scanning electron microscope and image forming method

Abstract: A method for forming an image of a specimen, comprises scanning a particle beam on a scanning field including the bottom of a deep hole or groove in the surface of the specimen, said deep hole or groove having an aspect ratio of at least 3, the particle beam scanned on the bottom of the deep hole or groove thereby generating charged particles from the specimen surface and from the bottom of the deep hole or groove; detecting the charged particles generated or reflected from the specimen surface and the charged particles reflected from the bottom of the deep hole or groove or generated from the specimen surface by said charged particles reflected from the bottom, in the scanning step; and displaying the scanning field including the specimen surface and the bottom of the deep hole or groove, based on the detected charged particles.

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 field 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. general 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 process. For example, it is used for inspection of appearance, inspection of dimension and inspection of through holes in mid course of 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 on 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. 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 also blocked by the side wall. JP 01048470 discloses an inspection apparatus constituted such that a sample absorbing current generated in a sample by absorbing secondary electrons and reflected electrons is used for outputting a detection signal. Another detection signal is combined with said signal to form a picture signal. Thus, the bottom configuration can be observed. US 4426577 discloses an electron microscope of scanning type. It comprises first and second detectors for detecting secondary electrons admitted from a specimen irradiated by a scanning electron beam, said detectors being disposed across the magnetic field of an objective lens of an electron optical system of the microscope. The detection signals obtain from the output of both detectors are subjected to simultaneous signal processing. 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 that allows reflection electrons generated at the bottom of a hole to have energy by which the reflection electrons can 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. Here, these secondary electrons are called tertiary electrons 5. Since the tertiary 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. Namely, one of the aspects of the present invention resides in that the primary electron beam has high energy to allow reflection electrons to have energy sufficient for the 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 a insulating layer (≤ 2 µm thickness) of a semiconductor device when it has energy sufficient for the reflection electrons to penetrate through the side wall. It results in allowing electrons in the insulating layer to move, i.e. the layer turns to have conductivity so that the layer is not subjected to charging on. Namely, 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 charge on, 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 tertiary 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 of reflection electrons and tertiary 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 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 scanning electron microscope to facilitate determination of etching conditions. DESCRIPTION OF THE PREFERRED EMBODIMENTS When a hole in SiO 2 having an aspect ratio of about 3 and a depth of 1.5 µm 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 tertiary electrons and secondary electrons 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 tertiary electrons are generated. As energy of the primary electron beam increases, the number of reflection electrons penetrating through the side wall increases and hence the number of tertiary electrons is increased gradually. However, as the energy is further increased, the primary electron beam intrude into a specimen more deeply and the number of reflection electrons decreases, leading to a decrease in the number of tertiary 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 density of material, the higher the necessary energy 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 tertiary 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, tertiary electrons generated in the surface of a specimen 8 are detected. A primary electron beam 4 having energy sufficient to generate the tertiary 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 tertiary 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 luminescence and detecting luminescence (an example using a semiconductor is shown in the embodiment). Since energy of reflection electrons is high, the surface layer of the semiconductor detector is made to be thick (1 to 10 µm), 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 µm, though depending on energy. Fig. 8 shows an example where reflection electrons and tertiary electrons are both detected. An attraction electrode 16 is provided which passes through the center of an objective lens 7. Tertiary electrons generated 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 tertiary electrons 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 multifier 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 substantially straight on and impinging upon a reflection electron detector 15. Through this process, the reflection electrons themselves and the tertiary electrons can be detected distinctively. Since scanning images formed by the two types of electrons are delicately different from each other, it is possible to take such a process as selecting one of the images which has better contrast or performing addition/subtraction to improve contrast. In the foregoing embodiments, tertiary electrons and reflection electrons generated on the side upon which the primary electron beam is incident but when the specimen is thin, either tertiary electrons generated by transmitted electrons or the transmitted electrons may be detected. Fig. 9 shows an embodiment in which tertiary 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. Tertiary electrons 20 generated from the under surface of a specimen 8 by electrons transmitting through the specimen and tertiary 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 µm and can transmit through even a Si wafer used in the semiconductor industry. Namely, by detecting transmitted electrons and tertiary electrons generated from the under surface 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. A source of electrons is a single crystal of LaB 6 and the single crystal is heated to emit electrons to be used. Emitted electrons are controlled by means of a Whenelt 24. The emitted electrons are accelerated by accelerating electrodes 25. 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, 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 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 is detected by a reflection electron detector 15 and tertiary 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 and 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 cassetted is stored in a preparatory chamber 33. When conducting 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 condition 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 integrity of process. 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 tertiary 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 -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 for intermediate chamber alternately. 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, substitution and alternations can be made hereto without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims: A scanning electron microscope comprising: means for irradiating a primary electron beam (4) to an object (8) to be inspected, wherein the primary electron beam (4) having energy sufficient for reflection electrons (6), reflected from the side wall or bottom of a groove or hole in the object (8), said groove or hole having an aspect ratio of three or higher, can penetrate from a side wall of said groove or hole through the object (8) and escape from the surface of the object (8) or generate tertiary electrons (5) in the surface of the object (8) ; and means (10, 11, 12, 15) for detecting the reflection electrons (6) or the tertiary electrons (5) . A scanning electron microscope according to claim 1, wherein the detection means (10, 11, 12) detects the tertiary electrons (5) generated in the surface of the object (8) by the reflection electrons (6) which are generated from the side wall or bottom of the groove or hole and which penetrate through the object (8). A scanning electron microscope according to claim 1, wherein the detection means (15) detects the reflection electrons (6) generated from the side wall or bottom of the groove or hole and which penetrate through the object (8) . A scanning electron microscope according to claim 1, wherein the irradiation means for the primary electron beam (4) includes a final lens (7) for focusing the beam (4), and further comprising an electron-ray detector (10, 11, 12, 15, 16, 17, 18) above the final lens, and that the tertiary electrons (5) caused to pass through the lens (7) are detected. A scanning electron microscope according to claim 1, further comprising means (100) for imaging of the surface of the object (8) to be inspected according to detection signals generated by the detecting means (10, 11, 12, 15), wherein the detection signals result from arithmetic operation of the reflection electrons (6) and the tertiary electrons (5). A scanning electron microscope comprising: means for irradiating a primary electron beam (4) to an object (8) to be inspected, said object having grooves or holes of an aspect ratio of three or higher, wherein the primary electron beam (4) having sufficient energy to transmit the object (8) to be inspected, and the transmitted beam (4) causes tertiary electrons (20) in the under surface of the object (8); and means (11, 12, 13, 22) for detecting the transmitted beam (19) or the tertiary electrons (20). A scanning electron microscope according to claim 1, wherein energy of the primary electron beam (4) exceeds 50 keV and a stage (29) is provided into which the object (8) having a diameter in excess of 4 inches can be inserted and which is adjustable in inclination angle. A method for forming an image of a specimen, comprising the following steps; scanning a primary electron beam across a scanning field including the bottom of a deep hole or groove in the surface of the specimen, said deep hole or groove having an aspect ratio of at least 3, the electron beam having energy sufficient for electrons, reflected from the bottom of the deep hole or groove, penetrating from a side wall of said groove or hole into the specimen and escaping from its surface or generating in the specimen surface tertiary electrons, detecting the escaped reflected electrons or the tertiary electrons, and displaying the specimen surface and the bottom of the deep hole or groove, based on the detected electrons. A method according to claim 8, wherein the electron beam has an energy level of at least 10 keV. A method according to claim 8, wherein the electron beam has an energy level of at least 30 keV. A method according to claim 8, wherein the particle beam has an energy level of at least 50 keV. A method according to claim 8, wherein the particle beam has an energy level of 50-100 keV. A method according to claim 8, wherein the particle beam has an energy level of 100-200 keV.

Inventors: OTAKA TADASHI, TAKAMOTO KENJI, TODOKORO HIDEO, OTAKA, TADASHI, TAKAMOTO, KENJI, TODOKORO, HIDEO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01J37
IPC codes: G01B15