JP7783431B2 - Ion milling device and sample processing method - Google Patents

Description

The present invention relates to an ion milling apparatus and a sample processing method.

An ion milling device is a device that irradiates a sample (e.g., metal, semiconductor, glass, ceramic, etc.) to be observed with an electron microscope with an unfocused ion beam, sputtering atoms off the sample surface, polishing the sample surface and exposing the sample's internal structure without stress. The sample surface and internal structure polished by ion beam irradiation become the observation surface for a scanning electron microscope or transmission electron microscope.

Patent Document 1 relates to a processing device that uses an ion beam to remove portions that protrude from a mask, and discloses a processing device that connects a cooling mechanism to the mask that comes into contact with the sample, or that is equipped with a cooling mechanism on the sample stage, in order to prevent thermal damage when the sample is made of a material with a low melting point, such as a polymer material, which is prone to damage to the sample structure, such as deformation or shrinkage, due to heat from the energy of the ion beam.

JP 2010-257856 A

Among the samples to be processed using ion milling machines, there are some samples, such as paper and polymeric materials, that are susceptible to thermal damage caused by temperature rise due to ion beam irradiation. Examples of thermal damage include melting and deformation of the sample due to ion beam irradiation.

One way to reduce thermal damage to a sample is to reduce the amount of protrusion of the sample from the mask (shielding plate) where the ion beam is irradiated. During sample processing, an unfocused ion beam is irradiated from an ion source toward the sample. By reducing the amount of protrusion from the shielding plate, the sample is prevented from being irradiated with a low-energy ion beam that contributes little to sample processing, and sample heating can be reduced. However, with this method, if the observation surface is located deep within the sample, it is necessary to repeatedly adjust the boundary position between the sample and the shielding plate and process the sample.

The inventors have been studying ways to use a simple mechanism to adjust the boundary position between the sample and the shielding plate in an ion milling apparatus, and have arrived at the present invention. Patent Document 1 discloses cooling the mask and sample stage, but its purpose is to cool the sample, and it does not disclose or suggest using heat to adjust the boundary position between the sample and the shielding plate.

An ion milling apparatus according to one embodiment of the present invention comprises a sample stage on which a sample is placed, a sample table and a shielding plate fixing part supported by the sample stage, a shielding plate fixed to the shielding plate fixing part, an ion source that irradiates an unfocused ion beam toward the sample, and a temperature adjustment unit that is connected to the shielding plate fixing part by a heat transfer cable and adjusts the temperature of the shielding plate fixing part, wherein the sample is held by being sandwiched between the shielding plate and the sample stage, and the amount by which the sample protrudes from the shielding plate is adjusted by the set temperature of the shielding plate fixing part that is set in the temperature adjustment unit.

The present invention provides an ion milling apparatus that can control the amount of minute protrusion of a sample relative to a shielding plate using a simple mechanism. Other objects and novel features will become apparent from the description of this specification and the accompanying drawings.

1 is a schematic diagram showing an example of the configuration of an ion milling apparatus. FIG. 2 is a schematic diagram showing an ion source and a power supply circuit that applies a control voltage to the ion source. FIG. 10 is a schematic diagram showing the amount of protrusion of the sample from the shielding plate for each temperature of the shielding plate fixing portion. 1 is a flowchart showing the sample processing operation of Example 1. 10 is a flowchart showing the sample processing operation of Example 2. 10 is a schematic diagram showing another example of the configuration of an ion milling apparatus.

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

1 is a schematic side view of the main components of an ion milling apparatus 100. In FIG. 1, the vertical direction is indicated as the Y direction. The ion milling apparatus 100 includes, as its main components, an ion source 101, a sample stage 102, a sample stage 103, a sample 104, a shielding plate 105, a shielding plate fixing portion 106, a heat transfer cable 107, a temperature adjustment unit 108, a first thermocouple 109, a control portion 110, a high-voltage power supply 111, a supply gas control portion 112, an exhaust mechanism 113, and a sample chamber 114.

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 for such pretreatment devices often employs the Penning method, which is effective for miniaturizing the structure. In this embodiment, the ion source 101 also employs the Penning method, and an unfocused ion beam is irradiated from the ion source 101 toward the sample 104. The control unit 110 controls the output of the ion beam by adjusting the voltage applied to the electrodes inside the ion source 101 from the high-voltage power supply 111 and the flow rate of argon gas supplied from the supply gas control unit 112.

The sample 104 is placed on the sample stage 103. The shielding plate 105 is fixed to a shielding plate fixing portion 106 by a shielding plate fixing screw 117, and the shielding plate fixing portion 106 is supported by the sample stage 103. The sample 104 is sandwiched between the sample stage 102 supported by the sample stage 103 and the shielding plate 105, and is made to protrude from the shielding plate 105 by a predetermined protrusion amount. The sample stage 103 may be provided with a driving mechanism that can freely drive the sample 104 in five directions (X-axis direction, Y-axis direction, Z-axis direction, rotational direction around the Y-axis, and rotational direction around the X-axis) or in some of these directions.

In this embodiment, the thermal contraction of the shielding plate fixing portion 106 is utilized to adjust the amount of protrusion of the sample 104 from the shielding plate 105. For this reason, it is desirable to make the shielding plate fixing portion 106 from a material with as large a linear expansion coefficient as possible. For example, it is made of phosphor bronze, brass, or the like. The shielding plate fixing portion 106 is connected to the temperature adjustment unit 108 via a heat transfer cable 107, and the temperature adjustment unit 108 adjusts the temperature of the shielding plate fixing portion 106 to a temperature set by the control unit 110. The temperature adjustment method of the temperature adjustment unit 108 is various and is not limited to a specific temperature adjustment method. Irradiation with an ion beam heats the sample 104 and the shielding plate 105, and the temperature of the shielding plate fixing portion 106 rises due to heat transfer. For this reason, the temperature adjustment unit has at least a cooling function. For example, the shielding plate fixing portion 106 can be cooled using liquid nitrogen or dry ice. On the other hand, since the wider the controllable temperature range, the greater the degree of freedom in control, the temperature adjustment unit 108 may have both a cooling function and a heating function. For example, the above-mentioned cooling and heating by a heater may be combined, or a Peltier element may be used.

The heat transfer cable 107 is made of a material with high thermal conductivity, such as copper. Because the sample chamber 114 is maintained at a high vacuum (<1.0×10 ⁻³ Pa) by the exhaust mechanism 113 during milling processing, heat dissipation from the heat transfer cable 107 does not need to be considered. This also applies to the shield plate fixing part 106. Temperature management of the shield plate fixing part 106 is performed by a first thermocouple 109, and if a large deviation is observed between the temperature measured by the first thermocouple 109 and the set temperature, the temperature is adjusted by the temperature adjustment unit 108. Note that the thermocouple is an example of a thermometer, and any device other than a thermocouple is acceptable as long as it can measure the temperature of the shield plate fixing part 106.

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 111.

The ion source 101 includes 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, are arranged facing each other, and an anode 203 is arranged between the first cathode 201 and the second cathode 202. Electrons are generated by applying a discharge voltage Vd from a high-voltage power supply 111 between the cathodes 201, 202 and the anode 203. The electrons are retained by the permanent magnet 204 arranged in the ion source 101 and collide with the argon gas injected from the gas pipe 206 to generate argon ions. An acceleration voltage Va is applied between the anode 203 and the acceleration electrode 205 from a high-voltage power supply 111, and the generated argon ions are attracted to the acceleration electrode 205 and emitted as an ion beam.

3 shows a schematic diagram of states 301 to 303 showing the protrusion amount ΔH of the sample 104 relative to the shielding plate 105 for each temperature of the shielding plate fixing portion 106. The temperatures of the shielding plate fixing portion 106 in states 301 to 303 are 25°C, 0°C, and -25°C, respectively. Since the shielding plate 105 and the sample 104 are simply in close contact with each other, cooling or heating the shielding plate fixing portion 106 causes the shielding plate fixing portion 106 and the shielding plate 105 to expand and contract, thereby moving the boundary (edge) between the shielding plate 105 and the sample 104. This changes the distance (protrusion amount) ΔH from the edge to the end face of the sample 104.

When the temperature of the shielding plate fixing portion 106 is 25° C. (state 301), the protrusion amount ΔH of the sample 104 relative to the shielding plate 105 is denoted as Δh, the total length of the shielding plate fixing portion 106 is denoted as L1, the distance from the bottom surface of the shielding plate fixing portion 106 to the shielding plate fixing screw 117 is denoted as L3, and the linear expansion coefficient of the shielding plate fixing portion 106 is denoted as α1. Furthermore, the total length of the shielding plate 105 is denoted as L2, the distance from the shielding plate fixing screw 117 to the end surface of the shielding plate 105 is denoted as L4, and the linear expansion coefficient of the shielding plate 105 is denoted as α2.

When the temperature of the shielding plate fixing portion 106 becomes 0°C (state 302), the temperature difference from state 301 becomes 25°C, so the shielding plate fixing portion 106 and the shielding plate 105 contract by 25×α1×L1 and 25×α2×L2, respectively. The position of the shielding plate 105 is pulled down by the thermal contraction of the shielding plate fixing portion 106, and the shielding plate 105 itself also contracts due to the thermal contraction, so that the protrusion amount ΔH in state 302 becomes (Equation 1).

ΔH=Δh+25×α1×L3+25×α2×L4 (Formula 1)
On the right side of (Equation 1), the first term represents the protrusion amount in state 301, the second term represents the amount of change in the position of the shielding plate fixing screw 117 due to thermal contraction of the shielding plate fixing portion 106, and the third term represents the amount of contraction of the shielding plate 105 with the shielding plate fixing screw 117 as the fulcrum due to thermal contraction of the shielding plate 105. In this way, the protrusion amount ΔH also depends on the support structure of the shielding plate fixing portion 106 and the shielding plate 105. The support structure of the shielding plate fixing portion 106 and the shielding plate 105 shown in the schematic diagram of FIG. 3 is one example, and this embodiment is not limited to a specific support structure.

Similarly, when the temperature of the shielding plate fixing portion 106 becomes −25° C. (state 303), the temperature difference from state 301 becomes 50° C., so the shielding plate fixing portion 106 and the shielding plate 105 contract by 50×α1×L1 and 50×α2×L2, respectively. The protrusion amount ΔH in state 303 can be expressed by (Equation 2).

ΔH=Δh+50×α1×L3+50×α2×L4...(Formula 2)
In this way, the amount of protrusion ΔH of the sample 104 from the shielding plate 105 changes according to a linear function with the temperature difference as a variable.

4 shows a series of operations from the start to the end of sample processing in the ion milling apparatus of Example 1. The series of operations in this flowchart are executed by the control unit 110. Each operation will be described in detail below.

S01: Set the sample processing conditions (acceleration voltage of the ion source 101, discharge voltage, argon gas supply rate, etc.) for the ion milling apparatus 100. The sample processing conditions include the sample protrusion amount Δh of the sample 104 relative to the shielding plate 105 at the start of the milling process, and it is desirable to set the sample protrusion amount Δh to as small a value as possible.

S02: The control unit 110 sets a temperature (referred to as a set temperature) at which the sample protrusion amount Δh set in step S01 is achieved, and the temperature adjustment unit 108 adjusts the temperature of the shielding plate fixing part 106 to the set temperature. When continuing from step S01, the sample 104 is sandwiched between the shielding plate 105 and the sample stage 102 so that the protrusion amount from the shielding plate is Δh at room temperature. This makes it unnecessary to adjust the temperature of the shielding plate fixing part 106 when starting milling.

S03: The sample 104 is irradiated with an ion beam from the ion source 101, thereby processing the sample 104 (milling process).

S04: During the milling process, the temperature of the shielding plate fixing part 106 is monitored by the first thermocouple 109. If the temperature of the shielding plate fixing part 106 is outside the allowable range of the set temperature due to heating caused by the long-term milling process, the processing is temporarily stopped and the process returns to S02 to cool the shielding plate fixing part 106. As soon as the temperature of the shielding plate fixing part 106 returns to the set temperature set in step S02, the processing is resumed (S03).

S05: It is confirmed whether the entire portion of the sample 104 protruding from the shielding plate 105 has been scraped off. If not, processing continues (S03). There are no limitations on the method of confirmation. The simplest method is to determine that the protruding portion has been scraped off when a predetermined processing time has elapsed. A transparent window may be provided in the sample chamber, and direct confirmation may be made through the window using monitoring means such as an optical microscope or electron microscope. Alternatively, the amount of processing may be estimated from the amount of sputtered particles generated during sample processing.

S06: It is confirmed whether the sample 104 has been removed down to the target portion. The target portion is, for example, the cross section to be observed if this processing is a pretreatment for observing the cross section of the sample 104 using a scanning electron microscope or a transmission electron microscope. If the target portion has not been removed, the process returns to step S02, and the temperature of the shielding plate fixing part 106 is controlled so that the amount of protrusion of the sample from the shielding plate becomes Δh. Specifically, the set temperature is reset to a value ΔT lower than the set temperature used during the previous temperature control, and the temperature adjustment unit 108 cools the shielding plate fixing part 106 to the reset set temperature. For example, in the example of FIG. 3, ΔT = Δh/(α1L3 + α2L4).

S07: If the target portion has been cut, the processing is terminated.

As described above, in the first embodiment, by adjusting the temperature of the shielding plate fixing portion 106, it is possible to adjust the minute protrusion amount of the sample from the shielding plate using a simple mechanism. As a result, by performing the cross-section milling process with the protrusion amount kept as small as possible, it is possible to minimize the thermal load on the sample 104 associated with the cross-section milling process. This makes it possible to suppress thermal damage to the sample 104 associated with ion beam irradiation.

As Example 2, an ion milling apparatus capable of reducing the influence of temperature drift due to ion beam irradiation on the shielding plate 105 will be described. The hardware configuration of the ion milling apparatus of Example 2 is the same as that of the ion milling apparatus 100 shown in FIG. 1 , so a duplicated description will be omitted. In Example 2, the temperature of the shielding plate fixing portion 106 when no ion beam irradiation is performed before processing begins is stored as a reference temperature. If the temperature of the shielding plate fixing portion 106 deviates from the reference temperature due to ion beam irradiation after processing begins, the temperature adjustment unit 108 adjusts the temperature of the shielding plate fixing portion 106 to the reference temperature. This reduces the influence of temperature drift on the shielding plate 105 during cross-section milling, and allows processing to be performed while maintaining the position of the boundary (edge) between the shielding plate 105 and the sample 104 at the position at the start of processing.

5 shows a series of operations from the start to the end of sample processing in the ion milling apparatus of Example 2. The series of operations in this flowchart are executed by the control unit 110. Each operation will be described in detail below.

S11: Set the sample processing conditions (acceleration voltage of the ion source 101, discharge voltage, argon gas supply rate, etc.) for the ion milling apparatus 100. If this processing is a pretreatment for observing the cross section of the sample 104 with a scanning electron microscope or a transmission electron microscope, the sample 104 is sandwiched between the shielding plate 105 and the sample stage 102 so that the cross section to be observed is located at the boundary (edge) between the shielding plate 105 and the sample 104. The sample processing conditions set include the sample processing time required to mill the area of the sample 104 placed in this manner that protrudes from the shielding plate 105.

S12: The temperature of the shielding plate fixing portion 106 measured by the first thermocouple 109 is stored as a reference temperature.

S13: The sample 104 is irradiated with an ion beam from the ion source 101, thereby processing (milling) the sample 104.

S14: During the milling process, the processing time is monitored.

S15: During the milling process, the temperature of the shield plate fixing portion 106 is monitored by the first thermocouple 109.

S16: If there is a difference between the temperature of the shielding plate fixing portion 106 and the reference temperature, the temperature adjustment unit 108 adjusts the temperature of the shielding plate fixing portion 106 to the reference temperature, and continues processing.

S17: When the set sample processing time is reached, processing ends.

As described above, the ion milling apparatus 100 includes the thermocouple 109, which is a first thermometer, that measures the temperature of the shielding plate fixing portion 106, and the control portion 110, which sets the sample processing time, and the control portion 110 stores the temperature of the shielding plate fixing portion 106 measured by the thermocouple 109, which is a first thermometer, as a reference temperature before the ion beam is irradiated from the ion source 101, and maintains the temperature of the shielding plate fixing portion 106 measured by the thermocouple 109, which is a first thermometer, at the reference temperature using the temperature adjustment unit 108 during the ion beam irradiation from the ion source 101. This makes it possible to provide an ion milling apparatus that can control the amount of minute protrusion of the sample relative to the shielding plate using a simple mechanism.

FIG. 6 shows a modified example of an ion milling apparatus. Components common to the ion milling apparatus shown in FIG. 1 are designated by the same reference numerals, and redundant explanations will be omitted. The ion milling apparatus 200 further includes a second thermocouple 116 for checking the temperature of the sample 104. A thermometer other than a thermocouple may be used for the second thermocouple 116. This configuration allows adjustment of the protrusion amount to be calculated taking into account the thermal contraction or thermal expansion of the sample 104 as well as the thermal contraction of the shielding plate fixing portion 106 and the shielding plate 105, thereby enabling more precise adjustment of the amount of protrusion of the sample from the shielding plate. This configuration is effective when the material of the sample 104 has a relatively large linear expansion coefficient.

The invention made by the present inventor has been specifically described above based on the embodiments, but 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, although an example in which the amount of sample protrusion is adjusted by adjusting the temperature of the shielding plate fixing portion 106 has been described, it is also possible to adjust the temperature of the shielding plate 105 directly.

The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to make the present invention easier to understand, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

100, 200: ion milling apparatus, 101: ion source, 102: sample table, 103: sample stage, 104: sample, 105: shielding plate, 106: shielding plate fixing part, 107: heat transfer cable, 108: temperature adjustment unit, 109: first thermocouple, 110: control part, 111: high voltage power supply, 112: supply gas control part, 113: exhaust mechanism, 114: sample chamber, 116: second thermocouple, 117: shielding plate fixing screw, 201: first cathode, 202: second cathode, 203: anode, 204: permanent magnet, 205: accelerating electrode, 206: gas piping.

Claims (12)

a sample stage on which a sample is placed;
a sample stage and a shielding plate fixing portion supported by the sample stage;
a shielding plate fixed to the shielding plate fixing portion;
an ion source for irradiating an unfocused ion beam toward the sample;
a temperature adjustment unit connected to the shielding plate fixing portion by a heat transfer cable and adjusting the temperature of the shielding plate fixing portion,
the sample is held by being sandwiched between the shielding plate and the sample stage;
An ion milling apparatus in which the amount of protrusion of the sample from the shielding plate is adjusted by a set temperature of the shielding plate fixing portion set in the temperature adjustment unit. In claim 1,
The temperature adjustment unit is an ion milling apparatus that cools the shielding plate fixing portion. In claim 1,
The temperature adjustment unit is an ion milling apparatus that cools or heats the shielding plate fixing portion. In claim 1,
a control unit for setting the sample protrusion amount;
the control unit irradiates the sample with an ion beam from the ion source after the temperature adjustment unit adjusts the temperature of the shielding plate fixing portion to the set temperature;
The set temperature is set as a temperature at which the amount of protrusion of the sample from the shielding plate becomes the sample protrusion amount. In claim 4,
The control unit stops the irradiation of the ion beam from the ion source after the portion of the sample protruding from the shielding plate has been milled off, resets the set temperature so that the amount of protrusion of the sample from the shielding plate becomes the sample protrusion amount again, and after the temperature adjustment unit adjusts the temperature of the shielding plate fixing portion to the reset set temperature, resumes the irradiation of the ion beam from the ion source. In claim 4,
a first thermometer for measuring the temperature of the shielding plate fixing portion;
The control unit of the ion milling apparatus stops the irradiation of the ion beam from the ion source when the temperature of the shielding plate fixing portion measured by the first thermometer is outside the allowable range of the set temperature, and adjusts the temperature of the shielding plate fixing portion to the set temperature using the temperature adjustment unit. In claim 1,
a first thermometer for measuring the temperature of the shielding plate fixing portion;
A control unit for setting a sample processing time,
The control unit stores the temperature of the shielding plate fixing portion measured by the first thermometer as a reference temperature before ion beam irradiation from the ion source, and maintains the temperature of the shielding plate fixing portion measured by the first thermometer at the reference temperature using the temperature adjustment unit during ion beam irradiation from the ion source. In claim 1,
an ion milling apparatus having a second thermometer for measuring the temperature of the sample; A sample processing method using an ion milling apparatus, comprising:
the ion milling apparatus includes a sample stage on which a sample is placed, a sample table and a shielding plate fixing part supported by the sample stage, a shielding plate fixed to the shielding plate fixing part, an ion source that irradiates an unfocused ion beam toward the sample, a temperature adjustment unit that is connected to the shielding plate fixing part by a heat transfer cable and that adjusts the temperature of the shielding plate fixing part, and a control part that sets a sample protrusion amount;
the temperature adjustment unit adjusts the temperature of the shielding plate fixing portion to a set temperature;
the ion source irradiates an ion beam toward the sample held between the shielding plate and the sample stage;
A sample processing method, wherein the set temperature is set as a temperature at which the amount of protrusion of the sample from the shielding plate becomes the sample protrusion amount. In claim 9,
the ion source stops irradiating the ion beam after the portion of the sample protruding from the shielding plate is scraped off;
the control unit resets the set temperature at which the protrusion amount of the sample from the shielding plate becomes the sample protrusion amount again;
the temperature adjustment unit adjusts the temperature of the shielding plate fixing portion to the reset set temperature;
The ion source restarts irradiation of the ion beam. In claim 9,
the ion milling apparatus includes a thermometer for measuring a temperature of the shielding plate fixing portion,
the ion source stops irradiating the ion beam when the temperature of the shielding plate fixing portion measured by the thermometer is outside the allowable range of the set temperature;
The temperature adjustment unit adjusts the temperature of the shielding plate fixing portion to the set temperature. A sample processing method using an ion milling apparatus, comprising:
the ion milling apparatus comprises: a sample stage on which a sample is placed; a sample table and a shielding plate fixing part supported by the sample stage; a shielding plate fixed to the shielding plate fixing part; an ion source that irradiates an unfocused ion beam toward the sample; a temperature adjustment unit that is connected to the shielding plate fixing part by a heat transfer cable and adjusts the temperature of the shielding plate fixing part; a thermometer that measures the temperature of the shielding plate fixing part; and a control part that sets a sample processing time;
the control unit stores the temperature of the shielding plate fixing portion measured by the thermometer before the ion beam irradiation from the ion source as a reference temperature;
the ion source irradiates an ion beam toward the sample held between the shielding plate and the sample stage;
The temperature adjustment unit maintains the temperature of the shielding plate fixing portion measured by the thermometer at the reference temperature during irradiation of the ion beam from the ion source.