Ion milling device and processing method using the same

Abstract

The present invention comprises: a sample chamber (109); a sample stage (102) on which a sample (120) is placed via a rotation stage (103) that can be tilted about a tilt axis (T) and rotates about a rotation axis (R) and a 3-axis drive stage (104) drivable in three axial directions orthogonal to one another; an ion source (101) which emits unfocused ion beam toward the sample and which is installed in the sample chamber such that the ion beam center (B) of the ion beam is orthogonal to the tilt axis (T); and a first finder (105). An optical system of the first finder is installed on the sample stage such that an optical axis thereof matches the tilt axis (T).

Description

[Technical Field]

[0001] The present invention relates to an ion milling apparatus and a processing method using the same. [Background technology]

[0002] An ion milling device irradiates a sample (e.g., metal, semiconductor, glass, ceramic, etc.) to be observed with an electron microscope with an unfocused ion beam. By sputtering atoms off the surface of the sample, the sample surface can be polished without stress or the internal structure of the sample can be exposed. The sample surface ion-milled by ion beam irradiation or the internal structure of the sample exposed becomes the observation surface for a scanning electron microscope or a transmission electron microscope. Patent Document 1 discloses an ion source position adjustment mechanism that adjusts the position of an ion source attached to a sample chamber to align the center of the ion beam when processing the sample with the center of rotation of the stage on which the sample is placed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2019 / 167165 Summary of the Invention [Problem to be solved by the invention]

[0004] Plane milling is a method of processing a sample surface by irradiating an ion beam onto a rotating or semi-rotated sample surface using an ion milling device. When planar milling is used, for example, to remove polishing scratches on a sample surface, the center of the ion beam is usually offset from the center of the stage rotation, and the sample is rotated while being irradiated with an ion beam with a half-width of approximately 0.5 to 1 mm. This prevents the sample surface from being continuously irradiated with the most intense beam near the center, making it possible to obtain a sample surface that is smooth over a wide area.

[0005] In contrast, if the ion beam is irradiated while rotating the sample without decentering the center of the ion beam and the center of rotation of the stage, the center of the ion beam will always be located at the intersection of the sample surface and the center of rotation of the stage, and in this case, a conical hole will be formed on the sample surface. This type of processing by planar milling is effective for inspecting the internal structure of three-dimensional devices.

[0006] For example, in three-dimensional devices such as flash memory, FinFET, and GAA (Gate All Around) type FET, where memory cell arrays are stacked, minute and high aspect ratio grooves and holes are densely packed. Active elements are formed by laminating insulating films, semiconductor films, or metal films on the side walls of the grooves and holes. In order to increase the yield of mass production lines for three-dimensional devices with such internal structures, it is effective to expose the internal structure of the three-dimensional device and analyze and inspect it using SEM (Scanning Electron Microscope) images of the internal microstructure to determine whether the desired internal structure is actually formed.

[0007] When exposing the internal structure of a sample by planar milling using an ion milling device, the reproducibility of the shape of the formed conical hole becomes an issue. Sample processing using an ion milling device is performed at a high milling rate using an unfocused ion beam, making real-time control of the processed shape extremely difficult. Patent Document 1 discloses an ion source position adjustment mechanism that can align the center of the ion beam with the center of rotation of the stage to improve processing reproducibility. The ion source position adjustment mechanism can improve processing reproducibility by reducing the eccentricity between the ion beam center and the center of rotation to approximately 20% or less of the half-width of the ion beam profile.

[0008] However, Patent Document 1 does not provide a means for easily checking the eccentricity between the center of the ion beam and the center of rotation. Even if precise adjustments are made to zero the eccentricity between the center of the ion beam and the center of rotation during maintenance of the ion milling device, misalignment inevitably occurs during repeated sample replacement and processing. If the misalignment between the center of the ion beam and the center of rotation increases, it can cause significant changes in the processed shape. For this reason, it is desirable to be able to easily check that the eccentricity is zero after each sample processing.

[0009] Furthermore, the number of atoms displaced by the sputtering phenomenon caused by the ion beam irradiation of a sample varies depending on the ion incident angle. For efficient processing, planar milling typically involves tilting the ion beam center and the sample surface at a predetermined angle (approximately 60–70°) that results in a high sputtering yield. For this reason, the sample stage is equipped with a movable mechanism that rotates it around the tilt axis, and planar milling is performed with the sample stage tilted. Therefore, it is desirable to adjust the sample surface to be positioned on the tilt axis of the sample stage so that the ion beam irradiation position on the sample does not change with sample tilt. The position where the ion beam irradiation position does not change with sample stage tilt is called the eucentric position. In ion milling systems, the height of the sample surface is adjusted to the height of the tilt axis of the sample stage, which corresponds to the eucentric position of the sample stage.

[0010] However, even if the height of the sample surface is adjusted to the eucentric position of the sample stage during maintenance, variations in the thickness of the sample to be processed or mechanical errors can cause the height of the sample surface to deviate from the eucentric position.In planar milling, the sample stage is tilted at a relatively large angle of about 60 to 70°, so errors in adjusting the eucentric position are manifested as deviations in the irradiation position, and therefore as deviations in the amount of eccentricity, reducing the reproducibility of the processed shape.

[0011] An object of the present invention is to improve the reproducibility of processing by an ion milling apparatus by enabling the height of the sample surface to be adjusted to the eucentric position of the sample stage using a simple configuration each time the sample is processed. [Means for solving the problem]

[0012] An ion milling apparatus according to one embodiment of the present invention comprises a sample chamber, a sample stage on which a sample is placed via a rotary stage that can be tilted around a tilt axis and rotates around a rotation axis and a three-axis drive stage that can be driven in three mutually perpendicular axial directions, an ion source that irradiates an unfocused ion beam toward the sample and is attached to the sample chamber so that the center of the ion beam is perpendicular to the tilt axis, and a first finder, the optical system of which is installed on the sample stage so that its optical axis coincides with the tilt axis. [Effects of the Invention]

[0013] The present invention provides an ion milling apparatus with improved reproducibility of processed shapes. Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram showing an example of the configuration of an ion milling apparatus. [Figure 2] FIG. 2 is a schematic diagram showing an ion source and a power supply circuit that applies a control voltage to the ion source. [Figure 3A] FIG. 1 is a schematic diagram of an ion milling apparatus 100 as viewed from the X-axis direction. [Figure 3B] 3B is an example of observation through the first finder 105 in FIG. 3A. [Figure 4A] FIG. 1 is a schematic diagram of an ion milling apparatus 100 as viewed from the X-axis direction. [Figure 4B] 4B is an example of observation through the first viewfinder 105 in FIG. 4A. [Figure 5A] FIG. 1 is a schematic diagram of an ion milling apparatus 100 as viewed from the Y-axis direction. [Figure 5B] 5B is an example of observation through the second viewfinder 106 in FIG. 5A. [Figure 6] 10 is a flowchart showing a series of operations from the start of machining to the end of machining. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0016] 1 is a schematic diagram showing the main components of an ion milling apparatus 100. The ion milling apparatus 100 mainly comprises an ion source 101, a sample stage 102, a rotary stage 103, a three-axis drive stage 104, a first finder 105, a second finder 106, a control unit 107, a high-voltage power supply unit 108, a sample chamber 109, and a vacuum pumping unit 110.

[0017] The ion milling apparatus 100 is used as a pretreatment device for observing the surface or cross section of a sample using a scanning electron microscope or a transmission electron microscope. The ion source often employs a Penning-type ion source, which is effective for miniaturizing the device. The ion source 101 in this embodiment also employs the Penning-type ion source. As will be described in detail later, the Penning-type ion source 101 generates electrons by applying a high voltage from a high-voltage power supply 108 to an internal electrode to generate a Penning discharge. The generated electrons collide with argon gas supplied from an external source to generate argon ions. The ion source 101 irradiates the generated argon ions as an unfocused ion beam toward a sample 120 placed on a rotating stage 103 and a three-axis drive stage 104. The rotating stage 103 rotates the sample 120 around a rotation axis R. The three-axis drive stage 104 moves the sample 120 in the X-, Y-, and Z-axes. One of the axial directions in which the three-axis driving stage 104 is driven is parallel to the rotation axis R, and in the example of Fig. 1, the rotation axis R is parallel to the Y-axis direction in which the three-axis driving stage 104 is driven. The rotation stage 103 and the three-axis driving stage 104 are driven by a control unit 107.

[0018] The inside of the sample chamber 109 is maintained at a high vacuum by the vacuum pumping unit 110, allowing a stable ion beam to be irradiated onto the sample 120 without being affected by the gas in the sample chamber. The sample 120 is milled by the ejection of atoms through a sputtering phenomenon caused by the argon ions that make up the ion beam. The number of atoms ejected by the sputtering phenomenon varies depending on the angle of incidence of the ions with respect to the sample 120, so in order to efficiently process the sample 120, it is necessary to tilt the sample 120 with respect to the center B of the ion beam.

[0019] The sample stage 102 is equipped with a drive mechanism including a motor that rotates the sample stage 102 about a tilt axis T to tilt the sample. The sample stage 102 is positioned so that the tilt axis T is perpendicular to the center B of the ion beam from the ion source. The rotation stage 103 is positioned on the sample stage 102 so that the tilt axis T of the sample stage 102 and the rotation axis R of the rotation stage 103 are perpendicular to each other. The control unit 107 can tilt the sample 120 while maintaining a high vacuum in the sample chamber 109. However, if the sample 120 is not positioned at the eucentric position of the sample stage 102, the beam irradiation position will be eccentric from the rotation axis R when the sample stage 102 is tilted. As mentioned above, in planar milling, the tilt angle is relatively large, approximately 60 to 70°, and the amount of eccentricity is also large, reducing the reproducibility of the processed shape.

[0020] Therefore, in the ion milling apparatus 100, a first finder 105 is arranged coaxially with the tilt axis T of the sample stage 102 to confirm that the surface of the sample 120 is in the eucentric position of the sample stage 102. Specifically, as shown in FIG. 1, the optical system of the first finder is installed on the sample stage so that its optical axis coincides with the tilt axis T. While observing the sample position through the first finder 105, the three-axis drive stage 104 is driven in the height direction (corresponding to the Y axis in FIG. 1) to align the sample surface with the eucentric position. Then, the rotation stage 103 is driven to rotate the sample 120. This enables planar milling with zero eccentricity even when the sample is tilted.

[0021] Furthermore, the ion milling apparatus 100 of FIG. 1 is provided with a second finder 106 in the sample chamber 109. The optical system has an optical axis extending in the Y-axis direction (a direction perpendicular to the plane defined by the tilt axis T and the ion beam center B). The second finder 106 can be used to confirm whether the target processing position of the sample 120 is aligned with the rotation axis R of the rotating stage 103, thereby further improving the reproducibility of the processing position of the sample 120. If the target processing position of the sample 120 is not aligned with the rotation axis R of the rotating stage 103, the target processing position will rotate in accordance with the rotation of the rotating stage 103. Therefore, while rotating the rotating stage 103, the target processing position of the sample 120 is observed through the second finder 106, and the three-axis drive stage 104 is adjusted in a planar direction (either the X-axis or Z-axis, or both) until the target processing position appears stationary. At this time, the rotation axis R and the target processing position of the sample 120 are aligned. By performing the above operations, even if the sample is tilted, it is possible to perform processing at the desired processing target position with high reproducibility.

[0022] In this embodiment, the first finder 105 and the second finder 106 are configured as optical microscopes that confirm the sample position using optical images. The optical microscope may be one that uses an eyepiece for observation, or one that displays an image formed on an image sensor (such as a CCD or CMOS image sensor) on a monitor. Similarly, a magnifying glass may be used as a means for confirming the sample position using optical images. To enable more precise adjustment, an electron microscope that confirms the sample position using an electron optical image or a white light interference microscope that confirms positional deviation using an interference image may be used. The finders can be selected from these examples or similar confirmation means to enable alignment with the desired accuracy. Different optical systems may be used for the first finder 105 and the second finder 106.

[0023] The position adjustment may be performed by a user visually observing the position of the sample 120 through the viewfinder while adjusting the three-axis drive stage 104, or by automatically adjusting the three-axis drive stage 104 by image processing of an image captured by an image sensor. To accurately confirm the eucentric position, it is desirable to equip the optical system of the first viewfinder 105 with a reticle. The reticle displays crosshairs indicating the position of the optical axis. In contrast, the second viewfinder 106 does not require a reticle because it is sufficient to confirm that the processing target position on the sample 120 does not rotate. To accurately adjust the sample surface to the eucentric position of the three-axis drive stage 104, it is desirable to employ a decelerated linear helicoid structure with high adjustment accuracy for the height direction drive structure.

[0024] 2 is a schematic diagram showing an ion source 101 employing the Penning method and a power supply circuit that applies a control voltage to the electrode components of the ion source 101. The power supply circuit is a part of a high-voltage power supply unit .

[0025] The ion source 101 mainly comprises a first cathode 201, a second cathode 202, an anode 203, a permanent magnet 204, an acceleration electrode 205, and a gas pipe 206. To generate an ion beam, argon gas is injected into the ion source 101 through the gas pipe 206. Inside the ion source 101, the first cathode 201 and the second cathode 202, which are at the same potential via the permanent magnet 204, are arranged facing each other, and an anode 203 is arranged between the first cathode 201 and the second cathode 202. A discharge voltage Vd is applied between the cathodes 201 and 202 and the anode 203 from a high-voltage power supply 108, generating electrons. The Lorentz force acts on the electrons generated by the permanent magnet 204 arranged in the ion source 101, causing the electrons to move in a spiral motion. The electrons collide with the argon gas injected from the gas pipe 206, generating plasma and generating argon ions. An acceleration voltage Va is applied between the anode 203 and the acceleration electrode 205 from the high-voltage power supply 108, and the generated argon ions are extracted by the acceleration electrode 205 and emitted as an ion beam.

[0026] FIG. 3A is a schematic diagram of the ion milling apparatus 100 as viewed from the X-axis direction. FIG. 3A shows the state in which the tilt angle of the sample stage 102 is set to 0°. In this state, the sample set on the three-axis drive stage 104 is observed through the first finder 105. FIG. 3B shows an example of observation through the first finder 105 at this time. Because the optical axis of the optical system of the first finder 105 coincides with the tilt axis T of the sample stage 102, the center of the crosshairs of the reticle is at the eucentric position E. The three-axis drive stage 104 is adjusted so that the top surface of the sample 120 is aligned with the crosshairs passing through the eucentric position E.

[0027] Figure 4A shows the sample stage 102 when the tilt angle is set to 45°. Figure 4B shows an example of observation through the first finder 105 at this time. If the sample top surface is at eucentric position E, the sample top surface will not move from eucentric position E, even if the sample stage 102 is tilted, as shown in Figure 4B. If the sample top surface can be aligned with eucentric position E, tilting the sample stage 102 will prevent the machining position from becoming eccentric, thereby improving the accuracy of the desired machined shape and the precision of repeatable machining.

[0028] Figure 5A is a schematic diagram of the ion milling system 100 as viewed from the Y-axis direction. The tilt angle of the sample stage 102 is set to 0°. Rotating the rotary stage 103 rotates the sample 120 mounted on the three-axis drive stage 104. The target processing position is marked on the sample 120 in advance. However, this step is not necessary if the target processing position is clear. Observing through the second viewfinder 106, as shown in Figure 5B, allows the offset Δr between the rotation axis R of the rotary stage 103 and the marking position M on the sample 120 to be confirmed. By moving the three-axis drive stage 104 in the planar direction (X-axis and Z-axis directions) to align the rotation axis R of the rotary stage 103 with the marking position M, the target processing position is positioned on the rotation axis R. If the top surface of the sample is aligned with the eucentric position E, tilting the sample stage 102 will prevent the processing position from becoming eccentric, allowing the desired processing shape to be accurately formed at the marking position M.

[0029] 6 is a flowchart showing a series of operations from the start to the end of sample processing by the ion milling apparatus 100. Details of each operation are as follows.

[0030] S401: Mark the processing target position on the sample 120. If the processing target position is visible, this operation can be skipped.

[0031] S402: The sample 120 is set on the three-axis driving stage 104. After the setting is completed, the sample chamber 109 is evacuated by the evacuation unit 110 until a high vacuum is reached.

[0032] S403 to S405: The height of the sample set on the three-axis drive stage 104 is confirmed using the first finder 105 (S403). It is confirmed that the top surface of the sample is in the eucentric position (S404). Specifically, it is confirmed that the sample surface coincides with the crosshairs of the reticle in the image in the first finder 105. If the sample surface is not in the eucentric position, the height of the three-axis drive stage 104 is adjusted (S405), and the height of the sample is confirmed again using the first finder 105 (S403). On the other hand, if the sample surface is in the eucentric position, the process proceeds to step S406.

[0033] S406: The sample stage 102 is tilted to the tilt angle for processing. The tilt angle is set so that the sample 120 is processed efficiently.

[0034] S407: The rotary stage 103 is driven.

[0035] S408-S410: The sample 120 set on the three-axis drive stage 104 is checked using the second finder 106 (S408). It is checked that the target processing position of the sample 120 marked in step S401 has not moved, i.e., that the rotation axis R of the rotation stage 103 and the target processing position of the sample 120 are aligned (S409). If they are not aligned, the three-axis drive stage 104 is moved and the plane coordinates are adjusted so that the rotation axis R of the rotation stage 103 and the target processing position of the sample 120 are aligned. After the adjustment, the sample 120 is checked again using the second finder 106 (S408). On the other hand, if they are aligned, the process proceeds to step S411.

[0036] S411: Via the control unit 107, the acceleration voltage and discharge voltage of the ion source 101 applied from the high voltage power supply unit 108, and the amount of gas introduced from the gas pipe 206 are set.

[0037] S412: Sample processing begins.

[0038] S413: The sample processing is completed, and the sample chamber 109 is opened to the atmosphere. After opening to the atmosphere, the sample 120 is removed from the sample stage 102.

[0039] If there is a tolerance for the deviation of the processing target position of the sample 120 so that it is not necessary to check for each sample, the checking step using the second finder 106 (S407 to S410) may be omitted.

[0040] Although the invention made by the present inventor has been specifically described above based on the embodiments, the present invention is not limited to the described embodiments and can be modified in various ways without departing from the spirit of the invention. For example, by providing an alignment mechanism (ion source position adjustment mechanism) that enables position adjustment in three mutually orthogonal directions with respect to the ion source, the accuracy and precision of processing can be further improved. [Explanation of symbols]

[0041] 100: ion milling device, 101: ion source, 102: sample stage, 103: rotation stage, 104: three-axis drive stage, 105: first finder, 106: second finder, 107: control unit, 108: high-voltage power supply unit, 109: sample chamber, 110: vacuum pumping unit, 120: sample, 201: first cathode, 202: second cathode, 203: anode, 204: permanent magnet, 205: accelerating electrode, 206: gas piping.

Claims

1. a sample chamber; a sample stage on which a sample is placed via a rotation stage that is tiltable around a tilt axis and rotates around a rotation axis and a three-axis drive stage that is driveable in three mutually orthogonal axial directions; an ion source that irradiates an unfocused ion beam toward the sample and is attached to the sample chamber so that the center of the ion beam is perpendicular to the tilt axis; a first viewfinder; an optical system of the first finder mounted on the sample stage so that its optical axis coincides with the tilt axis;

2. In claim 1, In the ion milling apparatus, the optical system of the first finder is equipped with a reticle that displays crosshairs that indicate the position of the optical axis.

3. In claim 2, an ion milling apparatus having a control unit that drives the rotary stage and the three-axis drive stage;

4. In claim 3, the rotation axis is perpendicular to the tilt axis and parallel to a first axis direction, which is one of the three axis directions in which the three-axis drive stage is driven; The control unit drives the three-axis drive stage in the first axial direction so that the surface of the sample coincides with the crosshairs of the reticle in the image in the first finder.

5. In claim 3, It has a second viewfinder, The ion milling apparatus is configured such that the optical axis of the second finder optical system is perpendicular to the plane defined by the tilt axis and the center of the ion beam.

6. In claim 5, the rotation axis is perpendicular to the tilt axis and parallel to a first axis direction, which is one of the three axis directions in which the three-axis drive stage is driven; The control unit drives the three-axis drive stage in either or both of two axial directions perpendicular to the first axial direction so that the target processing position of the sample is positioned on the rotation axis in the image of the second finder.

7. In claim 5, the rotation axis is perpendicular to the tilt axis and parallel to a first axis direction, which is one of the three axis directions in which the three-axis drive stage is driven; The control unit rotates the sample by rotating the rotating stage in the image of the second viewfinder, and drives the three-axis drive stage in either or both of two axial directions perpendicular to the first axial direction so that the target processing position of the sample appears stationary.

8. In claim 1, The ion milling apparatus is configured such that the ion source is attached to the sample chamber via an ion source position adjustment mechanism that can adjust the position in three mutually perpendicular directions.

9. A processing method for processing a sample using an ion milling apparatus including: a sample chamber; a sample stage on which a sample is placed via a rotary stage tiltable about a tilt axis and rotating about a rotation axis and a three-axis drive stage that is driveable in three mutually perpendicular axial directions; an ion source that irradiates an unfocused ion beam toward the sample and is attached to the sample chamber so that the ion beam center of the ion beam is perpendicular to the tilt axis; and a first finder, the optical system of the first finder is installed on the sample stage so that its optical axis coincides with the tilt axis; After the sample is placed on the sample stage, the sample chamber is evacuated to a vacuum; driving the three-axis drive stage so that the surface of the sample coincides with the optical axis of the optical system of the finder in the image of the first finder; The processing method includes tilting the sample stage at a predetermined angle, and then irradiating the sample with the ion beam from the ion source.

10. In claim 9, the optical system of the first finder is equipped with a reticle that displays crosshairs indicating the position of its optical axis; the rotation axis is perpendicular to the tilt axis and parallel to a first axis direction, which is one of the three axis directions in which the three-axis drive stage is driven; A processing method for aligning the surface of the sample with the optical axis of the optical system of the finder by driving the three-axis drive stage in the first axial direction so that the surface of the sample coincides with the crosshairs of the reticle in the image of the first finder.

11. In claim 9, the ion milling apparatus has a second finder; the optical system of the second finder is installed in the sample chamber so that its optical axis is perpendicular to a plane defined by the tilt axis and the center of the ion beam; a processing method in which the sample stage is tilted to the predetermined angle, and then the three-axis drive stage is driven so that the target processing position of the sample is positioned on the rotation axis in the image in the second finder.

12. In claim 11, the rotation axis is perpendicular to the tilt axis and parallel to a first axis direction, which is one of the three axis directions in which the three-axis drive stage is driven; A processing method in which the sample is rotated by rotating the rotary stage in the image in the second finder, and the target processing position of the sample is positioned on the rotation axis by driving the three-axis drive stage in either or both of two axial directions perpendicular to the first axial direction so that the target processing position of the sample appears stationary.

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