JP2007095576A - Charged particle beam device and its focus control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately perform focus control by making the variable voltage of an electrostatic lens extremely small, in regard to a charged particle beam device requiring accurate and speedy focus control, especially one performing observation and inspection of the surfaces of a semiconductor wafer and a photo mask, and its focus control method. <P>SOLUTION: The charged particle beam device is equipped with the electrostatic lens disposed in the lens magnetic field of a magnetic field type objective lens, and a focus control means to perform focus control by applying a voltage on an electrode forming the electrostatic lens, weakening or strengthening lens action by the magnetic field type lens by accelerating or decelerating the charged particle beam passing through the magnetic field of the magnetic field type objective lens, and focusing the charged particle beam by the electrostatic lens. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a charged particle beam apparatus that requires accurate and rapid focus control, and more particularly to a charged particle beam apparatus and a charged particle beam focus control method for observing and inspecting the surface of a semiconductor wafer, a photomask or the like.

  Conventionally, a field emission scanning electron microscope (FE-SEM) has been used for observing semiconductor wafers and photomask surfaces and measuring patterns. In this apparatus, an electron beam emitted from a ZrO / W emitter or the like is focused on a sample by an objective lens, and two-dimensionally scanned on the sample surface by a deflector. At this time, secondary electrons emitted from the sample by irradiation with the primary electron beam are detected by a secondary electron detector and displayed as an SEM image.

  As an objective lens such as an FE-SEM, a magnetic field type lens that can obtain a higher resolution image than an electrostatic lens is usually used. A magnetic lens is composed of a coil and a magnetic yoke that surrounds the coil. In order to obtain a focused image, the intensity of the magnetic field type objective lens, that is, the value of the coil current is adjusted, and the electron beam is focused on the sample. For example, in the length measuring SEM, the magnitude of the coil current is automatically changed minutely at each length measuring point on the wafer or mask surface so that an image with the best resolution is automatically obtained.

  More specifically, the focus current value is gradually increased from a small value to a large value, and secondary electron signals are collected and evaluated at each stage, and set to a coil current value that provides an image with the best resolution. To do. The problem in this case is the well-known hysteresis of the magnetic material (for example, iron). Even if the current value of the coil is the same, the strength of the magnetic lens differs depending on the history when setting the current value, and the focus is adjusted. In addition, there is a problem that if the coil current is controlled so as to reduce the influence of hysteresis, the measurement time for each point becomes longer and the throughput decreases.

  However, in order to solve these problems, it is conceivable that the intensity of the magnetic field type objective lens is made constant and the electrostatic lens that does not generate hysteresis is varied to perform focus control. For example, an electrostatic lens is provided independently of the magnetic field type objective lens at a position closer to the electron source than the magnetic field type objective lens, thereby performing focus control. When such a configuration is adopted, there are problems as described below.

  FIG. 7 is an explanatory diagram (part 1) of the prior art. FIG. 7 shows an arrangement example of a conventional electrostatic lens. The electrostatic lens 73 is separated from the lens magnetic field 75 of the magnetic field type objective lens 74 and is positioned so that there is almost no overlap between the lens magnetic field and the lens electric field. For example, the electrostatic lens 73 is disposed at a position 10 mm from the upper pole of the magnetic field type objective lens 74 on the electron source side. The electron beam emitted from the electron source is focused by the lens magnetic field 75 of the objective lens 74 and scanned on the sample by a deflector (not shown). Since the positive voltage Ve is applied to the electrode of the electrostatic lens 73, the secondary electrons 79 emitted from the sample 71 can pass through the electrostatic lens 73 and are detected by the secondary electron detector 72.

  FIG. 9 is an explanatory diagram (part 3) of the prior art. FIG. 9 shows a relationship between the minute displacement ΔZ of the height of the sample surface and the focus voltage Ve of the electrostatic lens 73 when the acceleration voltage U = 1.5 KV in the structural example of FIG. 7 (solid line). For comparison, the case where Ve is negative is also shown (dotted line). However, the reference position in the Z direction of the sample is set to ΔZ = 0, and ΔZ <0 indicates that the focus surface approaches the electrostatic lens, and ΔZ> 0 indicates that the focus surface moves away from the electrostatic lens.

In FIG. 9, first, the electron beam is focused by the magnetic field type objective lens 74 at a position of ΔZ = 15 μm. From this state, if the electrostatic lens voltage | Ve | is increased without changing the intensity of the magnetic field type objective lens, the focus position approaches the electrostatic lens side. From FIG. 9, the maximum value of Ve required for focus control by the electrostatic lens from ΔZ = 15 μm to ΔZ = −15 μm is about 230 V when the applied voltage Ve is positive, and the applied voltage Ve is negative. In this case, it is understood that the voltage is about 200V.

  The problem in this case is that the focus voltage Ve is not linear with respect to the sample displacement ΔZ, so that the control becomes complicated. If | Ve | is increased to about 100 V in order to make it more linear, the necessary | Ve | The maximum value is larger. Furthermore, the electron beam is scanned by the deflector so as to pass through the center of the magnetic lens. However, since the electron beam is subjected to the deflection action of the electrostatic lens away from the lens center of the magnetic lens, the applied voltage of the electrostatic lens When Ve is changed, there arises a problem that the magnification is changed.

  In the configuration of FIG. 7, it is considered that the electrostatic lens may be moved to substantially the same position as the magnetic lens in order to solve the above-described problem of changing the magnification.

  FIG. 8 is an explanatory diagram (part 2) of the prior art. FIG. 8 shows the height of the sample surface when the electrostatic lens is moved to substantially the same position as the magnetic lens, the strength of the magnetic lens is the same, the acceleration voltage U = 1.5 KV, and Ve is positive. The relationship between the minute displacement ΔZ and the focus voltage Ve of the electrostatic lens is shown. As will be described later, there is no focusing action of the electrostatic lens until Ve is 0 to + 1400V, and it appears that the focusing action is generated from around 1400V. As can be seen from FIG. 8, in order to perform focus control with an electrostatic lens from ΔZ = 15 μm to ΔZ = −15 μm, the applied voltage Ve requires an extremely large value of about +1400 to +1500 V. Absent.

  Further, by applying a voltage to the sample itself and making the voltage variable, the effective acceleration voltage of the primary electron beam incident on the sample is adjusted to be small, and as a result, focus control is performed. Things are also done. However, when the sample is an insulator such as a mask or a resist on a wafer or close to the insulator, there is a problem that the focus accuracy is deteriorated.

  In order to solve these problems, the present invention reduces the lens action of the objective lens by accelerating or decelerating the charged particle beam of the electrostatic lens by placing the electrostatic lens in the magnetic field of the objective lens. At the same time, the lens action is also performed by the electrostatic lens, and the variable voltage of the electrostatic lens is made extremely small so that the focus is controlled accurately.

The present invention has the following effects.
(1) Since there is no hysteresis, an image with high focus and high resolution can be obtained.

  (2) Since there is no hysteresis, quick focusing is possible and throughput is improved.

  (3) When secondary electrons emitted from the sample are detected after passing through the hole of the magnetic pole of the objective lens, focus control is possible with a relatively small applied voltage to the electrode of the electrostatic lens, and this often occurs in the electrostatic lens. There is no problem of discharge. Even when the vacuum near the sample is poor, discharge in the electrostatic lens can be suppressed.

  (4) Since the electrostatic lens and the magnetic lens are disposed at substantially the same position, a change in magnification when the electrostatic lens is changed and focused can be suppressed to a small value.

  In the present invention, an electrostatic lens is arranged in the magnetic field of the objective lens to weaken or strengthen the lens action of the objective lens due to acceleration or deceleration of the electrostatic lens with respect to the charged particle beam. By performing the lens action of the lens, the variable voltage of the electrostatic lens is made extremely small to perform accurate focus control, and the length measurement accuracy and throughput in the length measurement SEM are greatly improved.

FIG. 1 shows a structural diagram of one embodiment of the present invention.
In FIG. 1, a magnetic field type objective lens 1 is composed of a coil 2 and a magnetic yoke that surrounds the coil 2 symmetrically. Both ends of the magnetic yoke are composed of an upper pole 3 and a lower pole 4. When a current is passed through the coil 2, a lens magnetic field is generated between the upper pole 3 and the lower pole 4, and a magnetic flux density distribution (Bz) 6 in the optical axis direction on the optical axis 5 is formed. For example, a sample 7 made of a photomask is installed in a magnetic flux density distribution (referred to as a magnetic field type immersion lens). Furthermore, an electrostatic lens (electrostatic eye lens lens) 8 composed of three electrodes is incorporated in the objective lens magnetic field with the optical axis 5 as an axis.

  The electron beam from the direction of the optical axis 5 is focused mainly by the action of a magnetic lens and is scanned on the surface of the sample 7 by a scanning deflector (not shown). The secondary electrons 9 generated thereby are focused on the optical axis by the action of the objective lens magnetic field, and after passing through the objective lens, the secondary electron detector 10 (here, MCP) provided on the electron source side from the electrostatic lens. : Microchannel plate) (referred to as TTL detection).

  In the electrostatic lens 8, for example, a positive voltage of several tens of volts is applied to the center electrode, and the electrodes on both sides are set to the ground potential. Since the energy of secondary electrons from the sample 7 is around −5V, when a negative voltage of several tens of volts is applied to the electrostatic lens 8, the secondary electrons cannot pass through the lens, and the secondary electron detector. 10, normally, a positive voltage is applied, and the secondary electron travels toward the upper secondary electron detector 10 while rotating around the optical axis by the action of the magnetic lens, and is detected.

  In a state where the electron beam is focused at the height reference position of the sample by the action of the magnetic field type objective lens, if the sample surface is slightly displaced, the strength of the magnetic lens is constant, and the electrostatic lens 8 The focus can be finely controlled by controlling the applied voltage.

  FIG. 2 is an explanatory diagram (part 1) of the present invention. FIG. 2 shows an example of the relationship between the minute displacement of the sample surface height and the focus voltage of the electrostatic lens when the acceleration voltage U = 1.5 KV. The relationship between the two is almost linear. The solid line in FIG. 2 shows an example when the applied voltage of the electrode is positive at the center. That is, the focus voltage is set to +32 V at the sample height reference position ΔZ = 0. For example, when the sample position displacement ΔZ = −15 μm, that is, when the sample surface approaches the electrostatic lens 8 from the reference position by 15 μm, the electron beam can be focused on the sample 7 by setting the focus voltage to 0V.

  Further, when ΔZ = + 15 μm, that is, when the surface of the sample 7 is moved away from the electrostatic lens 8 by 15 μm from the reference position, the focus voltage is set to + 67V. Thereby, when the sample position is displaced ± 15 μm before and after the reference position, the focus can be controlled by controlling the applied voltage of the electrostatic lens. When the displacement range of the sample position to be dealt with is made larger than ± 15 μm, it is possible to set the control range of the electrostatic lens applied voltage appropriately to be large.

FIG. 3 shows a structural diagram of another embodiment of the present invention.
As shown in FIG. 3, the magnetic field type objective lens 31 includes a coil 32 and a magnetic yoke that surrounds the coil 32 in an axially symmetrical manner. Both ends of the magnetic yoke are composed of an upper pole 33 and a lower pole 34. When a current is passed through the coil 32, a lens magnetic field is generated between the upper pole 33 and the lower pole 34, and a magnetic flux density distribution (Bz) 36 in the optical axis direction on the optical axis 35 is formed. For example, the sample 37 made of a photomask is installed outside the magnetic flux density distribution. Furthermore, an electrostatic lens (electrostatic Einzel lens) 38 composed of three electrodes is incorporated in the objective lens magnetic field with the optical axis 35 as an axis.

  The electron beam from the direction of the optical axis 35 is focused mainly by the action of the magnetic lens 31, and is scanned on the surface of the sample 37 by a scanning deflector (not shown). The secondary electrons 39 generated thereby are attracted and detected by a high voltage of about +10 kV applied to the scintillator surface of the secondary electron detector 40 (here, the scintillator and the photomultiplier tube).

  In the electrostatic lens 38, for example, a negative voltage of several tens of volts is applied to the center electrode, and the electrodes on both sides are set to the ground potential. Since the secondary electrons from the sample do not need to pass through the electrostatic lens 38 as in the first embodiment (FIG. 1), a negative voltage can be applied to the electrostatic lens 38. The secondary electrons 39 are turned back by this negative voltage and are efficiently detected by the secondary electron detector 40. In the embodiment of FIG. 3, when a positive voltage is applied to the electrostatic lens 38, some or most of the secondary electrons are secondary electron detectors 41 (for example, provided on the electron source side from the electrostatic lens). MCP).

  When there is a minute displacement in the height of the sample surface, the intensity of the magnetic field type objective lens 31 is constant, and the focus control can be performed by controlling the voltage applied to the electrostatic lens 38. The dotted line in FIG. 2 shows an example of the relationship between the minute displacement of the height of the sample surface and the focus voltage of the electrostatic lens when the applied voltage of the center electrode is negative at the acceleration voltage U = 1.5 kV. ing. That is, the focus voltage is set to −30 V at the height reference position ΔZ = 0 of the sample 37. For example, when the sample position displacement ΔZ = −15 μm, that is, the sample surface approaches 15 μm from the reference position to the electrostatic lens 38, the electron beam can be focused on the sample 37 by setting the focus voltage to −57V.

  Further, when ΔZ = + 15 μm, that is, when the sample surface is 15 μm away from the electrostatic lens 38 from the reference position, the focus voltage is set to 0V. Accordingly, when the sample position is displaced ± 15 μm before and after the reference position, the focus can be controlled by controlling the voltage applied to the electrostatic lens. When the displacement range of the sample position to be dealt with is larger than ± 15 μm, it is possible to increase the control range of the electrostatic lens applied voltage. In the embodiment of FIG. 3, the relationship between the minute displacement ΔZ of the height of the sample surface and the focus voltage Ve when the applied voltage of the electrostatic lens is positive is the same as the solid line of FIG.

Explanation of focus voltage value for sample height displacement

  The reason why the inclination of the focus voltage intensity | Ve | with respect to the sample displacement is reversed depending on the polarity of the electrostatic lens applied voltage as described in Examples 1 and 2 and FIG. 2 will be described below.

(1) When the applied voltage of the electrostatic lens is negative: When the electrostatic lens voltage | Ve | is increased as described with the dotted line in FIG. 2 under the structure of the second embodiment (FIG. 3), the focus position Approaches the electrostatic lens 38. The main reason for this is that when a negative potential is applied to the electrode of the electrostatic lens 38, the electrons (charged particles) are decelerated in this vicinity, and the focusing effect of the magnetic field lens becomes relatively large. It is considered that the focus point moves to a position close to the electric lens 38. This situation is shown in FIG.

  FIG. 4A shows an example in which a negative voltage is applied to the electrostatic lens. Here, when the electrostatic lens voltage | Ve | shown in the figure is increased, that is, when the electrostatic lens strength is increased, the focus position is close to the electrostatic lens 38, so that it is easy to understand intuitively.

  (2) When the applied voltage of the electrostatic lens is positive: In contrast to the above (1), as described with the solid line in FIG. 2 under the structure of the first embodiment (FIG. 1), the electrostatic lens applied voltage | When Ve | is increased, the focus position moves away from the electrostatic lens 8. This situation is shown in FIG. When the electrostatic lens voltage Ve is increased, that is, when the strength of the electrostatic lens is increased, the focus position is far from the electrostatic lens 8, so that it is difficult to understand intuitively.

These situations will be described in more detail.
FIG. 5 shows an explanatory diagram (part 3) of the present invention. FIG. 5 shows the electrostatic lens strength Ve / U focused on a certain sample position Z0 and the magnetic lens strength J / √U (AT / V ^1/2 ) in Example 1 (solid line in FIGS. 1 and 2). Indicates a combination. Here, Ve is an applied voltage of the electrostatic lens, U is an acceleration voltage, and J is a magnetomotive force AT of the magnetic lens.

  At point A, the intensity of the magnetic lens is 0, and focusing on Z0 is performed only by the electrostatic lens to which a negative voltage is applied.

At point C, the strength of the electrostatic lens is zero, and focusing on Z0 is performed only by the magnetic lens.
At point B, the intensity of the magnetic lens is 0, and focusing on Z0 is performed only by the electrostatic lens applied to the positive voltage.

  In the range from the point C to the vicinity of the point D having the maximum value of J / √U, that is, in the range where the slope of J / √U is positive, the magnetic lens strength at the point C is maintained and the positive potential applied to the electrostatic lens is maintained. When the value is increased, the electrons are accelerated in the vicinity of the electrostatic lens, and the focusing effect of the magnetic field lens becomes relatively small. This effect is larger than the focusing action as the electrostatic lens, so that the electrostatic lens is more than the point Z0. It is interpreted that the focus point moves in a direction away from 8.

  Accordingly, it is considered that the focus characteristics as shown by the solid line in FIG. 2 and FIG. 4B are obtained.

In FIG. 5, the movement of the focus point will be described in further detail. When the magnetic lens strength J / √U is kept at the point C and the electrostatic lens strength Ve / U is increased from the position C to the position D, the focus point moves away from the electrostatic lens 8 from the point Z0. Moving. When the magnetic lens strength J / √U is further increased from the point D to the point E, the focus point returns to the direction of the point Z0, and at the point E, the focus point becomes Z0. That is, when the intensity of the electrostatic lens 8 coincides with the focus position Z0 (point C) where the intensity is zero, and when Ve / U is increased from the point E, the focus point is increased from the point Z0 to electrostatic with the increase of Ve. Move toward the lens. At point E, when the strength Ve / U of the electrostatic lens is increased, the focus point moves in a direction approaching the electrostatic lens, so that it is easy to understand. However, as described in FIG. A lens applied voltage is required. As described above, in order to perform focus control with a relatively small positive applied voltage to the electrostatic lens, the value of Ve / U is in the range from point C to point D, that is, Ve / U is 0.4. It is necessary to be.

  FIG. 6 is a structural diagram of another embodiment of the present invention. In FIG. 6, instead of the electrostatic einzel lens shown in FIG. 1, an electrostatic immersion lens having two electrodes is arranged, and the upper electrode 108a is applied with the ground potential and the lower electrode 108b is applied with a positive voltage. As in the first embodiment, focus control is possible by applying a positive voltage to the lower electrode 108b and varying it. The focus voltage dependency is the same as the solid line in FIG. The difference from the first embodiment is that the sample surface is placed in the electric field generated by the lower electrode 108b. However, the voltage applied to the lower electrode 108b for focus control is relatively small at 100 V or less, so 2 Secondary electrons can also be detected as in FIG.

  Further, it is possible to improve the resolution by applying a constant high voltage of about +1 KV to the upper electrode 108a.

  In the present invention, an electrostatic lens is arranged in the magnetic field of the objective lens to weaken or strengthen the lens action of the objective lens due to acceleration or deceleration of the electrostatic lens with respect to the charged particle beam. The present invention relates to a charged particle beam apparatus and a charged particle beam focus control method, in which a lens action is also performed, and a variable voltage of an electrostatic lens is extremely reduced to perform accurate focus control.

1 is a structural diagram of an embodiment of the present invention. It is explanatory drawing (the 1) of this invention. It is another Example structure figure of this invention. It is explanatory drawing (the 2) of this invention. It is explanatory drawing (the 3) of this invention. It is another Example structure figure of this invention. It is explanatory drawing (the 1) of a prior art. It is explanatory drawing (the 2) of a prior art. It is explanatory drawing (the 3) of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 31: Magnetic field type | mold objective lens 2, 32: Coil 3, 33: Upper pole 4, 34: Lower pole 5, 35: Optical axis 6, 36: Magnetic flux actual distribution (BZ)
7, 37: Sample 8, 38: Electrostatic lens 9, 39: Secondary electron 10, 40, 41: 2 electron detector

Claims (10)

In a charged particle beam apparatus that focuses a charged particle beam using a magnetic field type objective lens and irradiates the sample,
An electrostatic lens arranged in the lens magnetic field of the magnetic field type objective lens;
A voltage is applied to the electrodes constituting the electrostatic lens, and the charged particle beam beam passing through the magnetic field of the magnetic objective lens is accelerated or decelerated to weaken or strengthen the focusing action by the magnetic lens, and A charged particle beam apparatus comprising: a focus control unit that focuses and controls a charged particle beam with an electrostatic lens.   The charged particle beam apparatus according to claim 1, wherein focus control is performed by adjusting a voltage applied to the electrostatic lens in a direction in which the charged particle beam beam is accelerated.   3. The charged particle beam apparatus according to claim 1, wherein the positive applied voltage Ve applied to the electrostatic lens is Ve / U <0.4, where U is an acceleration voltage.   2. The secondary electrons that pass through the magnetic field objective lens and the electrostatic lens are detected for secondary electrons emitted by irradiating the focused charged particle beam to a sample. The charged particle beam apparatus according to claim 3.   The charged particle beam apparatus according to claim 1, wherein focus control is performed by adjusting a charged particle beam beam in a decelerating direction as a voltage applied to the electrostatic lens.   The secondary electrons emitted by irradiating the focused charged particle beam to the sample are blocked by the voltage applied to the electrode of the electrostatic lens, and the sample and the magnetic field type objective lens or the electrostatic lens The charged particle beam device according to claim 5, wherein the secondary electrons are detected between the charged particle beam devices.   The charged particle beam apparatus according to claim 1, wherein the magnetic field type lens is a magnetic field type immersion lens.   The charged particle beam apparatus according to claim 1, wherein the electrostatic lens is an electrostatic Einzel lens.   The charged particle beam device according to claim 1, wherein the electrostatic lens is an electrostatic immersion lens. In a charged particle beam focus control method in which a charged particle beam is focused using a magnetic field type objective lens and irradiated onto a sample,
An electrostatic lens is provided in the lens magnetic field of the magnetic field type objective lens,
A voltage is applied to the electrodes constituting the electrostatic lens, and the charged particle beam beam passing through the magnetic field of the magnetic field type objective lens is accelerated or decelerated to weaken or strengthen the lens action by the magnetic field type lens, and A charged particle beam focus control method comprising: focusing a charged particle beam with an electrostatic lens and performing focus control.

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