Ion milling device

Abstract

Provided is an ion milling device capable of forming a processed surface in which an amorphous layer is suppressed. This ion milling device comprises: a sample chamber provided with a gas introduction port; a sample stage installed in the sample chamber; an ion gun that is attached to the sample chamber and emits an unfocused ion beam; an electrode that can be disposed so as to face a sample placement surface of the sample stage; and a power supply that applies a voltage between the electrode and the sample stage. The sample stage is provided with a magnetic circuit.

Description

Ion milling apparatus

[0001] The present invention relates to an ion milling apparatus.

[0002] Patent Document 1 relates to a method for preparing a sample for a transmission electron microscope (TEM) using a focused ion beam (FIB). Patent Document 1 discloses that an amorphous layer is formed on the side surface of a thin film portion to be observed by TEM due to irradiation with FIB (gallium ions), which becomes an obstacle to TEM observation, and thus the amorphous layer is etched and removed. Further, as an example of the process of etching and removing, sputtering using argon gas is mentioned.

[0003] Japanese Patent Application Laid-Open No. 2002-228562

[0004] An ion milling apparatus is a device that irradiates a sample to be observed with an unfocused ion beam (argon (Ar) ions) to process the sample surface. When Ar ions penetrate into the sample due to the irradiation of the ion beam, an amorphous layer is formed according to the beam energy and the ion beam irradiation angle. When the observation surface of a semiconductor exposed by an ion milling apparatus is observed with a scanning electron microscope (SEM), the formed amorphous layer has come to affect the contrast of the observation image. This is because as the miniaturization of the semiconductor structure has advanced, the thickness of the amorphous layer formed with respect to the size of the structure has become non-negligible. Therefore, it is necessary to remove or reduce the amorphous layer formed by the penetration of Ar ions into the semiconductor.

[0005] An ion milling apparatus according to an embodiment of the present invention includes a sample chamber having a gas introduction port, a sample stage installed in the sample chamber, an ion gun attached to the sample chamber and emitting an unfocused ion beam, an electrode that can be arranged to face the sample placement surface of the sample stage, and a power source that applies a voltage between the electrode and the sample stage. The sample stage includes a magnetic circuit.

[0006] An ion milling apparatus capable of forming a processed surface with suppressed amorphous layer formation can be provided. Other challenges and novel features will become apparent from the description herein and the accompanying drawings.

[0007] This is a schematic diagram showing the main parts of an ion milling apparatus. This is an example of the internal structure of an ion gun. This is a schematic diagram showing the planar milling process by an ion milling apparatus. This is a schematic diagram showing how the ion beam is irradiated onto the sample. This is an exploded perspective view of the sample stage for magnetron etching. This is a cross-sectional view of the sample stage for magnetron etching. This is a schematic diagram showing the magnetron etching process by an ion milling apparatus. This is a schematic diagram showing the magnetron etching process. This is an example of a series of operations from the start to the end of sample processing.

[0008] Figure 1 is a schematic diagram showing the main parts of the ion milling apparatus 100. The ion milling apparatus 100 is used as a pretreatment device to expose the observation surface for observing the internal structure of a sample such as a semiconductor device using a SEM or TEM.

[0009] Since ion beam milling is performed in a vacuum environment, a rotary pump 102 and a turbomolecular pump 103 are connected to the sample chamber 101 as an exhaust mechanism for evacuating the interior. In addition, a gas introduction port 104 is provided in the sample chamber 101 to introduce Ar gas into the sample chamber 101 during magnetron etching, which will be described later. A first mass flow controller (MFC) 105 is connected to the gas introduction port 104, and Ar gas whose flow rate is adjusted by the first MFC 105 is introduced into the sample chamber 101 via the gas introduction port 104. The sample chamber 101 is also provided with an observation window 106 for checking the status of the milling process of the sample 110 to be processed, and a shutter 107 for protecting the observation window 106. The shutter 107 is movable at least in the XZ plane. During ion beam milling, sputtered particles are scattered from the sample 110. By covering the observation window 106 with the shutter 107, the adhesion of sputtered particles to the observation window 106 can be suppressed. The shutter 107 may be provided with a drive mechanism and moved by the control unit 130, or it may be moved manually by the user.

[0010] An ion gun 111, which irradiates the sample 110 with an unfocused ion beam, is mounted on the side of the sample chamber 101. In Figure 1, the central axis B of the ion beam ideally emitted by the ion gun 111 is shown to coincide with the Z axis. The ion gun 111 is connected to a second mass flow controller 112 that controls the flow rate of Ar gas supplied from an external source, and a first high-voltage power supply 113 that supplies the high voltage necessary to ionize the Ar gas. An example of the structure of the ion gun 111 will be described later. An ion beam shutter 114 is provided between the ion gun 111 and the sample 110, allowing control of the irradiation / non-irradiation of the sample 110 with the ion beam emitted from the ion gun 111.

[0011] The sample 110 is placed on the stage mechanism. The stage mechanism has a configuration in which a sample stage 123 is mounted on a rotary tilt stage 121. Multiple types of sample stages can be attached to the sample stage 123, and when performing magnetron etching, which will be described later, a magnetron etching sample stage equipped with a magnetic circuit inside is attached via an insulating material 122. Figure 1 shows this state. It is also possible to perform milling by attaching a normal sample stage (without a magnetic circuit) to the rotary tilt stage 121. The rotary tilt stage 121 is a stage that has the function of tilting the sample 110 with respect to the ion beam about the tilt axis T and the function of rotating the sample about the rotation axis R. The rotary tilt stage 121 can be tilted about the tilt axis T (extending in the X-axis direction in Figure 1) in a range of, for example, 0 to 90 degrees. The rotary tilt stage 121 is adjusted so that the tilt axis T is located on the surface of the sample 110, and the ion beam central axis B intersects the tilt axis T on the surface of the sample 110, thereby preventing the position to which the ion beam is irradiated from shifting due to the tilt of the sample 110.

[0012] To perform magnetron etching, as described later, it is necessary to generate a magnetic field near the sample using a magnetron etching sample stage 123 as the sample stage, and to generate a strong electric field to ionize the Ar gas. To generate this electric field, an electrode 125 is attached to the surface of a shutter 107 facing the sample stage 123, and a second high-voltage power supply 124 that applies a high voltage is connected between the electrode 125 and the sample stage 123. Both the shutter 107 and the electrode 125 are conductors and are electrically connected.

[0013] The control unit 130 is a computer that controls the processing of a sample by the ion milling apparatus 100. An interface 131 is connected to the control unit 130 to receive input such as processing conditions from the user.

[0014] Figure 2 shows the internal structure of the ion gun 111 employing the Penning method. The power supply circuit that applies the control voltage to the electrode components of the ion gun 111 is part of the first high-voltage power supply 113.

[0015] The ion gun 111 has as its main components a first cathode 201, a second cathode 202, an anode 203, a permanent magnet 204, an accelerating electrode 205, and a gas pipe 206. To generate an ion beam, Ar gas is introduced into the ion gun 111 through the gas pipe 206. The flow rate of the Ar gas introduced into the ion gun 111 is regulated by the second MFC 112. Depending on the processing conditions, the maximum flow rate is approximately 1 SCCM (Standard Cubic Centimeter per Minute).

[0016] Inside the ion gun 111, a first cathode 201 and a second cathode 202 are positioned opposite each other via a permanent magnet 204, and an anode 203 is positioned between the first cathode 201 and the second cathode 202. When a discharge voltage Vd is applied from a first high-voltage power supply 113 between the cathodes 201, 202 and the anode 203, electrons are generated by a discharge inside the ion gun 111. Due to the Lorentz force acting on the electrons generated by the permanent magnet 204 placed inside the ion gun 111, the electrons undergo helical motion. The electrons collide with Ar gas injected from the gas pipe 206, forming a plasma and generating Ar ions. An acceleration voltage Va is applied from the first high-voltage power supply 113 between the anode 203 and the accelerating electrode 205, and the generated Ar ions are drawn out by the accelerating electrode 205 and emitted as an ion beam 200.

[0017] Figure 3A is a schematic diagram showing the planar milling process performed by the ion milling apparatus 100. The control unit 130 sets the tilt angle θ of the rotary tilt stage 121 (defined as the angle between the normal to the sample surface and the central axis B (or Z axis) of the ion beam) to, for example, 85 degrees, and moves the shutter 107 below the observation window 106 to protect it. The control unit 130 also supplies Ar gas from the second MFC 112 and a control voltage from the first high-voltage power supply 113 to the ion gun 111 to start the emission of the ion beam and rotate the rotary tilt stage 121 around the rotation axis R. In Figure 3A, the rotation axis R and the central axis B of the ion beam are depicted as intersecting, but the point where the sample surface 110 intersects the rotation axis R and the point where it intersects the central axis B of the ion beam do not necessarily have to coincide. After the ion beam emitted from the ion gun 111 stabilizes, the control unit 130 opens the ion beam shutter 114 to irradiate the sample 110 with the ion beam 200 and perform the milling process. During the planar milling process, the sample chamber 101 is kept under high vacuum, and no Ar gas is supplied from the gas introduction port 104.

[0018] Figure 3B is a schematic diagram showing how the ion beam 200 irradiates the sample 110. The intensity of the unfocused ion beam 200 in a plane perpendicular to the central axis B of the ion beam is thought to follow a Gaussian distribution, being strongest at the central axis B of the ion beam and decreasing as it moves away from the central axis B. Therefore, the depth D of the amorphous layer formed by the penetration of Ar ions can be estimated as D = Lcosθ. Here, L is the penetration depth of the Ar ions that penetrated the sample 110 along the central axis B of the ion beam, and θ is the incident angle of the ion beam. The depth D depends on the processing conditions, but under general conditions it is about 10 nm to 30 nm thick.

[0019] An example of the configuration of a sample stage for magnetron etching will be described using Figures 4A and 4B. Figure 4A shows an exploded perspective view of the sample stage 123 for magnetron etching, and Figure 4B shows a cross-sectional view of the sample stage 123 for magnetron etching. The sample stage 123 for magnetron etching is equipped with a magnetic circuit. The magnetic circuit includes, for example, a cylindrical permanent magnet 301, a magnetic path member 303 made of ferromagnetic material having a circular base and cylindrical side walls, and a support member 302 made of non-magnetic material that supports the permanent magnet 301 so that its base contacts the base of the magnetic path member 303 and the permanent magnet 301 is coaxial with respect to the magnetic path member 303. A cover 304 is placed so as to cover the upper surface of the magnetic circuit assembly composed of the permanent magnet 301, the support member 302, and the magnetic path member 303, and the cover 304 is fixed to the magnetic circuit assembly by a cover retainer 305. Pure iron, which is a ferromagnetic material, can be used for the magnetic path member 303. The support member 302, cover 304, and cover retainer 305 are all made of non-magnetic materials, but it is particularly desirable that the support member 302 and cover 304 be made of materials that allow the magnetic field lines of the permanent magnet 301 to pass through easily. Furthermore, it is desirable that the upper surface of the cover 304 be made of a material with a low sputtering yield because Ar ions collide with it during magnetron etching. For this reason, for example, aluminum may be used for the support member 302 and titanium for the cover 304.

[0020] Figure 5 is a schematic diagram showing the magnetron etching process performed by the ion milling apparatus 100. The sample stage 123 is the magnetron etching sample stage described in Figures 4A and 4B, and the second high-voltage power supply 124 is connected to the magnetic path member 303. The shutter 107 and the positive electrode of the second high-voltage power supply 124 are connected to an external reference potential, and the negative electrode of the second high-voltage power supply 124 is connected to the sample stage 123, thereby applying a negative potential to the sample stage 123 by the second high-voltage power supply 124. Even when a negative potential is applied to the sample stage 123, it remains electrically insulated from the rotating tilt stage 121 by the insulating material 122.

[0021] The control unit 130 sets the tilt angle θ of the rotary tilt stage 121 to 90 degrees and moves the shutter 107 below the observation window 106. This positions the upper surface of the cover 304, which is the sample mounting surface of the sample stage 123, and the electrode 125 attached to the shutter 107 facing each other. In this way, the ion milling apparatus 100 of this embodiment is configured to use the shutter 107 to generate an electric field near the sample. The electrode 125 has the effect of shortening the distance between the electrode that generates the electric field and the sample stage 123. In addition, if it deteriorates due to sputtered particles scattered by etching, the original condition can be restored by replacing only the electrode 125. Alternatively, instead of using the shutter 107, an electrode with a movable mechanism that moves to face the sample stage 123 during the magnetron etching process may be installed separately.

[0022] The control unit 130 supplies Ar gas into the sample chamber 101 from the gas introduction port 104. The flow rate of Ar gas introduced into the sample chamber 101 is adjusted by the first MFC 105. Depending on the processing conditions, the maximum is approximately 5 SCCM. The first MFC 105, which adjusts the Ar gas flow rate for the magnetron etching process, is adjusted to supply a larger flow rate of Ar gas than the second MFC 112, which adjusts the Ar gas flow rate for the milling process. Subsequently, the control unit 130 rotates the rotary tilt stage 121 around the rotation axis R and applies a voltage between the electrode 125 and the sample stage 123 using the second high-voltage power supply 124.

[0023] Figure 6 schematically shows the magnetron etching process. When a high voltage is applied between the sample stage 123 and the electrode 125, a discharge occurs and electrons are generated. When the cover 304 and support member 302 are made of a material that easily allows magnetic field lines 401 to pass through, the magnetic field lines 401 formed by the permanent magnet 301 penetrate the cover 304 and support member 302 and are shielded by the magnetic path member 303, as shown in Figure 6. The electrons generated by the discharge move in a helical motion due to the Lorentz force and accumulate near the sample 110. The accumulated electrons collide with Ar to form Ar ions 402, and the Ar ions 402 sputter the surface of the sample 110 (Figure 6 shows sputtered particles 403 scattered by the collision of Ar ions 402 with the surface of the sample 110). Unlike in the case of an ion gun, no accelerating voltage is applied to the Ar ions, so their energy is very small, and only the very surface of the sample 110, including the amorphous layer 400, can be sputtered. This allows the amorphous layer formed on the sample 110 by the ion beam to be removed by magnetron etching while minimizing damage to the sample 110. This maintains the advantages of ion milling, such as high-speed and deep processing, while obtaining a processed surface in which the formation of an amorphous layer is suppressed.

[0024] Furthermore, it is preferable that the second high-voltage power supply 124 be a DC power supply as in this embodiment. This is because by keeping the direction of the voltage between the electrode 125 and the sample stage 123 constant, the collision direction of Ar ions with the sample 110 can be aligned.

[0025] Figure 7 shows an example of a series of operations from the start to the end of sample processing for the ion milling apparatus 100 shown in Figure 1. The user sets the sample 110 on the sample stage 123 (S01) and sets the sample stage 123 on the rotary tilt stage 121 (S02). In step S02, the position of the tilt axis T is also adjusted. In the following, an example using a sample stage for magnetron etching as the sample stage 123 will be described, but if it is desired to control the irradiation position of the ion beam more precisely in planar milling processing with an ion beam, it is advisable to use a normal sample stage (without a magnetic circuit) and then switch to a sample stage for magnetron etching during magnetron etching processing. This eliminates the influence of the magnetic circuit on the irradiation position of the ion beam during planar milling.

[0026] When the sample 110 is placed in the sample chamber 101, the control unit 130 drives the rotary pump 102 to start exhausting the sample chamber 101 (S03).

[0027] Here, the control unit 130 determines whether to perform milling or magnetron etching based on the processing depth in the processing conditions received from the user via the interface 131 (S04). If the processing depth is minute, for example 10 nm or less, magnetron etching is performed from the beginning in order to avoid damage (formation of an amorphous layer) caused by planar milling.

[0028] If it is determined in step S04 that milling should be performed, the sample chamber 101 is evacuated to a high vacuum. When the rotary pump 102 has evacuated the sample chamber 101 to a single-digit Pa, the turbomolecular pump 103 is activated (S05), and the sample chamber 101 is evacuated to a high vacuum. －２ When the pressure is reduced to below Pa (S06), the milling process becomes possible.

[0029] The control unit 130 sets a processing time corresponding to the processing depth, which is a processing condition received from the user via the interface 131 (S07), and starts processing the sample 110 (S08). When the set processing time has elapsed (S09), the control unit 130 closes the ion beam shutter 114 and moves the shutter 107, and the user checks the sample 110 through the observation window 106 to confirm whether the desired processing has been completed (S10). The user can determine whether the desired processing has been completed from the interference color of the processed surface of the sample 110. If the processing is deemed insufficient (No in S10), the user sets an additional processing time and performs additional processing (S07-S10). Although an example of determining processing completion by the user's visual inspection has been shown, the determination may also be made based on processing time alone, or the amount of processing may be determined by estimating the amount of sputtered particles emitted from the sample 110.

[0030] If the amorphous layer is to be removed (Yes in S11), the turbomolecular pump 103 is turned off (S12), and the process proceeds to magnetron etching. If the amorphous layer is not to be removed (No in S11), the process is terminated. When the turbomolecular pump 103 is stopped, the sample chamber 101 becomes a vacuum of a single order of magnitude Pa, which is suitable for magnetron discharge.

[0031] If the amorphous layer is to be removed after milling (Yes in S11), or if it is determined in step S04 that magnetron etching will be performed, the control unit 130 starts introducing Ar gas, whose flow rate has been adjusted by the first MFC 105, into the sample chamber 101 (S13). The control unit 130 determines whether the vacuum level in the sample chamber 101 is at an appropriate value (for example, 6 to 12 Pa) (S14), and if it is at an appropriate value, proceeds to the next step. If the pressure is too high, vacuuming may be performed again.

[0032] The control unit 130 sets a processing time corresponding to the processing depth, which is a processing condition received from the user via the interface 131 (S15), starts processing the sample 110 (S16), and ends the processing when the set processing time has elapsed (S17). Since it is assumed that the processing depth by magnetron etching is small, the processing amount is intended to be controlled by time, but the processing amount may also be determined by estimating the amount of sputtered particles emitted from the sample 110.

[0033] The present invention is not limited to the embodiments described above, and various modifications are included. For example, the embodiments described above are explained in detail to make the present invention easier to understand, and are not necessarily limited to those having all the configurations described. Furthermore, it is possible to add, delete, or replace some of the configurations in the embodiments with other configurations.

[0034] 100: Ion milling apparatus, 101: Sample chamber, 102: Rotary pump, 103: Turbomolecular pump, 104: Gas introduction port, 106: Observation window, 107: Shutter, 110: Sample, 111: Ion gun, 112: Second mass flow controller, 113: First high-voltage power supply, 114: Ion beam shutter, 121: Rotating tilt stage, 122: Insulating material, 123: Sample stage, 124: Second high-voltage power supply, 1 25: Electrode, 130: Control unit, 131: Interface, 200: Ion beam, 201: First cathode, 202: Second cathode, 203: Anode, 204: Permanent magnet, 205: Accelerating electrode, 206: Gas piping, 301: Permanent magnet, 302: Support member, 303: Magnetic path member, 304: Cover, 305: Cover retainer, 400: Amorphous layer, 401: Magnetic field lines, 402: Ar ions, 403: Sputtered particles.

Claims

1. An ion milling apparatus comprising: a sample chamber equipped with a gas introduction port; a sample stage installed in the sample chamber; an ion gun attached to the sample chamber for emitting an unfocused ion beam; an electrode that can be positioned to face the sample placement surface of the sample stage; and a power supply for applying a voltage between the electrode and the sample stage, wherein the sample stage is equipped with a magnetic circuit.

2. An ion milling apparatus according to claim 1, comprising a control unit for controlling the processing of a sample placed on the sample placement surface of the sample stage, wherein the control unit introduces argon gas into the sample chamber from the gas introduction port and processes the sample with argon ions generated by ionizing the argon gas by generating an electric field between the electrode, which is arranged to face the sample placement surface of the sample stage, and the sample stage using the power supply.

3. The ion milling apparatus according to claim 2, wherein the sample is a sample processed by irradiation with an unfocused ion beam emitted from the ion gun, and an amorphous layer is formed on the surface of the sample.

4. An ion milling apparatus according to claim 1, wherein the power supply is a DC power supply whose positive electrode is connected to the electrode and whose negative electrode is connected to the sample stage.

5. An ion milling apparatus according to claim 1, wherein a sample is placed on the sample placement surface of the sample stage, and the sample is placed so as to face the magnetic circuit with the sample placement surface in between.

6. The ion milling apparatus according to claim 5, wherein the magnetic circuit includes a cylindrical magnet, a magnetic path member made of a ferromagnetic material having a circular bottom surface and cylindrical side walls, and a support member made of a non-magnetic material that supports the magnet so that the bottom surface of the magnet is in contact with the bottom surface of the magnetic path member and the magnet is positioned coaxially with respect to the magnetic path member, and the magnetic circuit is provided such that the upper surface of the magnet and the upper surface of the magnetic path member face the sample placement surface.

7. An ion milling apparatus according to claim 1, comprising: an observation window provided in the sample chamber so as to be able to observe the sample placement surface of the sample stage; and a shutter that protects the observation window during processing of the sample with an unfocused ion beam emitted from the ion gun, wherein the electrode is attached to the shutter.

8. An ion milling apparatus according to claim 7, comprising a rotary tilt stage to which the sample stage is attached, wherein the sample stage is attached to the rotary tilt stage via an insulating material.

9. An ion milling apparatus comprising: a sample chamber equipped with a gas introduction port; a first mass flow controller for adjusting the flow rate of argon gas introduced into the sample chamber via the gas introduction port; a sample stage installed in the sample chamber; an ion gun attached to the sample chamber; a second mass flow controller for adjusting the flow rate of argon gas introduced into the ion gun; and a control unit for processing a sample placed on the sample placement surface of the sample stage in a first and a second step, wherein in the first step, argon gas whose flow rate is adjusted by the second mass flow controller is introduced into the ion gun, and the sample is milled by an ion beam emitted from the ion gun; and in the second step, argon gas whose flow rate is adjusted to a larger flow rate than in the first step by the first mass flow controller is introduced into the sample chamber, and the sample is magnetron etched.

10. An ion milling apparatus according to claim 9, comprising: an electrode that can be positioned to face the sample mounting surface of the sample stage; and a power supply for applying a voltage between the electrode and the sample stage, wherein the sample stage used in at least the second step comprises a magnetic circuit.

11. An ion milling apparatus according to claim 10, wherein the power supply is a DC power supply whose positive electrode is connected to the electrode and whose negative electrode is connected to the sample stage.

12. The ion milling apparatus according to claim 10, wherein the sample is placed on the apparatus so as to face the magnetic circuit with the sample placement surface in between.

13. The ion milling apparatus according to claim 12, wherein the magnetic circuit includes a cylindrical magnet, a magnetic path member made of a ferromagnetic material having a circular bottom surface and cylindrical side walls, and a support member made of a non-magnetic material that supports the magnet such that the bottom surface of the magnet is in contact with the magnetic path member and the magnet is positioned coaxially with respect to the magnetic path member, and the magnetic circuit is provided such that the upper surface of the magnet and the upper surface of the magnetic path member face the sample placement surface.

14. An ion milling apparatus according to claim 10, comprising: an observation window provided in the sample chamber so as to be able to observe the sample placement surface of the sample stage; and a shutter for protecting the observation window during the milling process in the first step, wherein the electrode is attached to the shutter.

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