SCANNING ELECTRON MICROSCOPE FOR OBSERVING AND MEASURING MINUTE PATTERN OF SAMPLES

Abstract: A scanning electron microscope for observing and measuring a minute pattern of a sample (2) comprising an electron lens (1) for focusing an electron beam (4) from an electron source onto the sample (2), wherein the electron beam (4) scans on the sample (2), an inclining means for inclining the sample (2) at an arbitrary angle α, and two secondary electron detectors (3a, 3b) disposed at positions which are respectively turned at the same angle ϑ around the central axis of the electron beam (4) from a plane which contains the axis of inclination of the sample (2) and the central axis of the electron beam (4). As it is possible to measure the length of the minute pattern by detecting the secondary electron signal (5a, 5b) having a symmetrical wave-form while observing the minute pat­ tern at the arbitrary angle, high measuring accuracy is at­ tained.

Description: The present invention relates to a scanning electron microscope which is suitable for observing a minute pattern of a sample and at the same time measuring the length of the minute pattern of the sample such as a semiconductor device. In the general apparatus of the scanning electron microscope, the sample is not inclined and an electron beam irradiated from an electron source to the sample is inclined at a previously determined angle in order to observe the sample. When measuring the minute pattern of the sample, the electron beam is perpendicularly irradiated from the electron source which is disposed in vertical direction over the sample. Because, when the electron beam is inclined, the wave-form of secondary electron signal detected by the detector becomes non-symmetrical, and it becomes difficult to accurately measure the length of the pattern of the sample. Therefore, when measuring, the electron beam is perpendicularly irradiated. U.S.P. 4,670,652, U.S.P. 4,767,926, U.S.P. 4,751,384 are cited as examples of such device. In these conventional system, there arise the following drawbacks. When observing the sample, sometimes the electron beam is needed to be inclined, and when measuring the portion of the sample which is needed to be observed in some inclined angle, the sample is perpendicularly irradiated by moving the irradiation angle of the electron beam. And, the portion which is needed to be observed does not correspond to the portion which is needed to be measured, because the electron beam is irradiated with different angle in observing from that in measuring. Therefore, it is difficult to measure the portion of the sample which is needed to be observed with high accuracy. The present invention has been accomplished to overcome the above mentioned problem of the conventional apparatus. An object of present invention is to provide a scanning electron microscope which makes it possible to measure the length of the portion of the sample in high accuracy while observing the portion of the sample by inclining the sample at an arbitrary angle. Another object of the present invention is to provide a scanning electron microscope having two secondary electron detectors which makes it possible to detect a symmetrical wave-form of the secondary electron signal generated from a portion of the sample without losing high efficiency of detection of the detectors even when the sample is inclined at a large angle. These objects are met by the invention as defined in claim 1. Figure 1 shows a side view of one embodiment of an electron microscope having two secondary electron detectors in the present invention. Fig. 2 shows a plane view of the embodiment shown in Fig. 1. Figure 3 shows a sectional view of the secondary electron detectors viewed from a point P in Fig. 2. Figure 4 is a diagram showing the relationship between an angle α of inclination of a sample and an efficiency µ of detection of secondary electrons obtained when the secondary electron detectors are respectively turned at the same angle ϑ. Figure 5 is a diagram showing the relationship between the ratio B/A which means a symmetrical factor of the wave-form of the secondary electron signal and the angle ϑ. Figure 6 is a schematic diagram of a wave-form when the electron beam scanned the photoresist pattern 6 on the sample. Figure 7 is a schematic diagram showing the positional relation of detectors 3a, 3b, scanning direction 7 and photoresist 6. The present invention will be described in detail relative to the embodiment shown in the figures. In Figs. 1 and 2, numeral 1 denotes an objective lens. A sample 2 such as a silicon wafer is inclined at an arbitrary barge angle α under the objective lens 1, and electron beam 4 is converged so as to scan on the surface of the sample 2. On this occasion, the secondary electrons 5a, 5b are emitted from the sample 2, as shown Fig. 3. Fig. 3 shows a sectional view of the secondary electron detectors (3a, 3b) viewed from a point P in Fig. 2. In Fig. 3, 10 denotes an earth electrode, 11 a scintillator, 12 a light guide, and 13 a post electrode. The angle α is defined relative to a plane 2' (shown in Figure 1) which is perpendicular to a central axis of the electron beam 4. As shown in Figures 1 and 3, the electron beam 4 intersects the axis of rotation X-X' of the sample 2, and thus the central axis of the electron beam 4 and the axis of rotation X-X' lie in a single plane, which may be referred to as a reference plane. On the surface of the sample 2, various minute patterns are formed in semiconductor processes, and the electron beam 4 is accelerated by a low acceleration voltage such as 1 to 1.5 kV so as to observe the patterns on the sample directly without evaporating them. The secondary electrons 5a and 5b emitted from the sample 2 are detected by secondary electron detectors 3a and 3b. Said secondary electron detectors 3a and 3b are disposed in a first position and a second position, respectively, as shown in Figure 2, which do not coincide with one another and are on the same side of the reference plane containing the central axis of the electron beam 4 and the axis of rotation X-X'. Each of the first and second positions is at an angle ϑ relative to the reference plane, as defined by rotation about the electron beam central axis, as shown in Figure 2. An experiment conducted in such a disposition as described above with the angle ϑ varied showed that the range of the angle ϑ was 5° to 30°. Figure 4 is a diagram showing the relationship between an arbitrary angle α of inclination of the sample and an efficiency µ of detection of secondary electrons obtained when the rotational angle ϑ of the secondary electron detector is varied. It is seen from the result of this experiment that such an excellent efficiency of detection of secondary electrons as 90% or above can be obtained even when the sample 2 is inclined from a horizontal state to 60° around the axis of inclination, on condition that ϑ is 5° or above. When the value of the angle ϑ is increased, on the other hand, the right-to-left symmetry of a wave-form of the electron beam signal from the detectors 3a, 3b is varied. Figure 5 is a diagram showing the relationship between the rotational angle ϑ and a ratio B/A of signals generated in the right and the left edge part of the wave-form of the secondary electron signal obtained when an electron beam is made to scan in the direction 7 which is perpendicular to a resist pattern 6 on the surface of a sample such as a silicon wafer. Figure 6 is a diagram showing a signal intensity on the left side of the wave-form 8, denoted by A, and a signal intensity on the right side of the wave-form 8, denoted by B. Excellent symmetry of the wave-form of the secondary electron signals is requisite for measuring the length of the pattern with high accuracy, and B/A of 80% or above is requisite experimentally. In view of this, the rotational angle ϑ of the secondary electron detectors 3a and 3b is limited to 30°. Therefore, an experiment for the angle α of inclination of the sample 2 ranging 0° to 60° showed that an excellent efficiency of detection of the secondary electrons is obtained with an excellent symmetry of the wave-form of the secondary electron signal from the detectors 3a and 3b when the rotational angle ϑ was in the range from 5° to 30°. Patterns formed on a silicon wafer generally consist of resist patterns intersecting each other perpendicularly, and both the efficiency of detection and the symmetry of the wave-form are important for the detection of the intersecting patterns and can be met by the present embodiment. According to the present invention, as described above, secondary electrons are detected without any reduction in the efficiency of detection and with excellent symmetry of the wave-form of the secondary electron signal so as to observe the minute pattern with high accuracy while observing the minute patterns on the sample, which is inclined with an arbitrary angle.

Claims: A scanning electron microscope for observing and measuring a minute pattern of a sample, comprising: an electron lens (1) for focusing an electron beam (4) from an electron source onto the sample (2), wherein the electron beam (4) scans on the sample (2), said electron beam (4) having a central axis; and an inclining means for inclining the sample (2) around an axis of inclination at an arbitrary angle α with respect to a plane perpendicular to the electron beam central axis; where the electron beam central axis and the axis of inclination intersect at right angles and thus define a reference plane containing said axes; the scanning electron microscope further including two secondary electron detectors (3a, 3b) which are positioned, respectively, in a first position and a second position which are on the same side of the reference plane and which are not coincident with one another, where each of the first and second positions is at the same angle ϑ from the reference plane as defined by rotation about the electron beam central axis. A scanning electron microscope as defined in claim 1, wherein said angle ϑ is between 5° and 30°. A scanning electron microscope as defined in claim 1, wherein said angle α is between 0° and 60°.

Inventors: OTAKA, TADASHI
Applicants: HITACHI, LTD.
Application year: 1989
Main classification: H01J37
IPC codes: H01J37, G01B15