JP5807442B2 - Secondary ion mass spectrometry method - Google Patents

Description

  The present invention relates to a secondary ion mass spectrometry method.

  Secondary ion mass spectrometry (SIMS) is an elemental analysis method in which a sample surface is irradiated with a primary ion beam to perform mass analysis of secondary ions emitted from the sample surface. The concentration is detected as secondary ion intensity. In this secondary ion mass spectrometry, the sample surface is milled by the irradiation of the primary ion beam and is deleted as the irradiation time elapses, so that the element concentration distribution in the depth direction of the sample surface can be known.

  The sample surface irradiated with the primary ion beam is milled to form a recess having a flat bottom surface. The analysis of the element concentration in the depth direction is performed by detecting secondary ions released from the bottom of the recess. However, it is difficult to converge the primary ion beam into a steep beam, and the ion intensity distribution of the beam is diffused. As a result, the peripheral edge of the recess is formed on a gentle inclined surface. Since layers with different depths are exposed on this inclined surface, secondary ions emitted from the layers with different depths are observed mixed with secondary ions emitted from the bottom of the recess, and are separated in the depth direction. An edge effect that blurs the elementary density distribution is produced. Such an edge effect deteriorates the resolution in the depth direction of the impurity distribution.

  As a method of avoiding the edge effect, the primary ion beam is scanned along the scanning line set on the sample surface, and the secondary ions emitted when the primary ion beam scans the central portion of the recess are selectively detected. How to do is known. For example, there is an electric gate method in which a pulse voltage is applied between the extraction electrode and the sample in synchronization with the time when the primary ion beam scans the center of the recess, and the secondary ions are taken into the mass analyzer only during this time. In this method, since the secondary ions are not detected while the primary ion beam irradiates the peripheral edge of the recess, mixing of secondary ions emitted from the peripheral edge of the recess is avoided, and the edge effect can be prevented.

  Further, as a method for avoiding the edge effect, a mesa method is used in which a groove is formed in advance in the periphery of the recess and the center of the recess is left in a mesa shape. In this mesa method, even when the primary ion beam protrudes from the center of the recess to the periphery of the recess and scans, secondary ions are suppressed from being released from the periphery of the recess where the deep groove is formed. Is done.

Japanese Patent Laid-Open No. 2008-232828 JP 2010-045456 A JP 2008-215847 A

  In the secondary ion mass spectrometry described above, in order to improve sensitivity, an ion beam of a highly reactive atom, for example, a beam of alkali metal ions such as cesium or lithium or a beam of oxygen ions is often used as the primary ion beam. .

  However, these highly reactive atoms easily react with the sample composition atoms to form a compound with the sample composition atoms on the sample surface. When the compound is formed, the work function of the sample changes and the intensity of secondary ions released from the sample surface changes. For this reason, the detection sensitivity of the secondary ions varies greatly, making it difficult to accurately measure the element concentration distribution. In particular, since alkali metal ions form compounds in a wide composition range, the work function varies greatly depending on the compound composition, and the fluctuation range of the detected secondary ion amount also increases. Note that when oxygen ions are used as primary ions, the oxygen composition ratio of the compound is almost constant, for example, in the case of silicon oxide, the composition ratio of SiO 2 is maintained. Changes in the detection sensitivity of ions are small and do not cause a problem in normal measurement.

  A large amount of such a compound is formed in the primary ion irradiation region, that is, the inside of the recess, and the outside of the irradiation region, particularly in the periphery of the recess. Of these, at the bottom of the recess scanned by the primary ion beam, the amount of primary ions supplied from the primary ion beam and the amount of primary ions emitted from the sample surface are balanced, and the composition of the compound is maintained at a constant composition ratio. Is done. Therefore, the formation of the compound on the bottom surface of the recess does not affect the variation in detection sensitivity.

  However, since the compound formed outside the peripheral edge of the recess is not scanned with the primary ion beam, primary ions accumulate, and the composition ratio of the primary ions of the compound increases with time. It is difficult to quantitatively grasp the production amount and composition change of this compound, and as a result, the detection sensitivity of secondary ions becomes unstable.

  In the mesa method, a groove is formed around the primary ion irradiation region by etching, so that it is possible to prevent the formation of the primary ion compound outside the peripheral portion of the irradiation region while suppressing the edge effect. This is because the primary ion atoms are prevented from diffusing beyond the groove.

  However, when the groove in the mesa method is formed using the primary ion beam, when forming the groove, the primary ions are scattered outside the groove, that is, outside the peripheral edge of the region where the recess is to be formed. An ionic compound is formed. In order to prevent the formation of such a compound, the groove is formed by an etching method not using a primary ion beam, for example, chemical etching or ion milling using an ion beam of an inert gas.

  For this reason, before carrying a sample in a secondary ion mass spectrometer, it is necessary to form a groove in the sample in advance using chemical etching or the like. Alternatively, after carrying a sample into the secondary ion mass spectrometer, it is necessary to perform etching using an inert gas ion beam. However, forming the grooves in advance by chemical etching using photolithography or the like makes the handling of the sample troublesome. Further, in etching using an inert gas ion beam, a secondary ion mass spectrometer must be further provided with a processing ion beam gun, which makes the apparatus complicated and expensive.

  The present invention relates to a secondary ion mass spectrometry method using an alkali metal as a primary ion beam, without forming a groove in a sample in advance or using an ion beam gun for groove processing outside the primary ion beam scanning region. It is an object of the present invention to provide a secondary ion mass spectrometry method capable of preventing the formation of a primary ion atom compound and analyzing the element concentration in a precise depth direction.

  According to one aspect of the present invention for solving the above problems, a primary ion beam of alkali metal ions is scanned along the scanning line set on the surface of the sample, and the sample surface is scanned. In a secondary ion mass spectrometric method for performing mass analysis of emitted secondary ions, a first scan line segment wound at least once along the spiral scan line and including the outermost periphery of the spiral Controlling the potential of the extraction electrode and the sample so that the potential of the sample with respect to the secondary ion extraction electrode becomes a positive potential when the primary ion beam is scanned, and the first scanning line segment When the primary ion beam scans the second scanning line segment that is located more inside the spiral and reaches the innermost end of the spiral, the sample with respect to the secondary ion extraction electrode As potential is a negative potential, it is provided as secondary ion mass spectrometry method characterized by having the steps of controlling the potential of the extraction electrode and the sample.

  According to the present invention, in a secondary ion mass spectrometry method using an alkali metal as a primary ion beam, a groove is formed in advance in a sample or an ion beam gun for groove processing is not used, and the outside of the scanning region of the primary ion beam. Therefore, a secondary ion mass spectrometric method capable of precisely analyzing the element concentration in the depth direction is provided.

Sectional drawing of the secondary ion mass spectrometer used in 1st Embodiment of this invention Primary ion beam current density distribution Sample cross section processed by primary ion beam The top view showing the scanning method of the primary ion beam of 1st Embodiment of this invention Sectional drawing of the sample of 1st Embodiment of this invention Diagram showing the results of mass analysis of secondary ions Diagram showing the results of mass analysis of primary ions Diagram showing the relationship between sample potential and the detected amount of primary and secondary ions Laser microscope image of the upper surface of the sample according to the first embodiment of the present invention Laser microscope image of the upper surface of the comparative sample The figure showing the Auger analysis result of the sample upper surface of 1st Embodiment of this invention The figure which shows the Auger analysis result of the sample upper surface of the comparative example Diagram showing another scanning method The figure showing the scanning method of the primary ion beam of 2nd Embodiment of this invention.

  In the first embodiment of the present invention, a positive potential is applied to the sample while the primary ion beam is wound around the outer peripheral portion of the spiral scanning line, and a negative potential is applied to the sample while the inner portion of the spiral is wound. The present invention relates to a secondary ion mass spectrometric method in which is applied.

  FIG. 1 is a cross-sectional view of a secondary ion mass spectrometer used in the first embodiment of the present invention, and shows the main equipment configuration of the secondary ion mass spectrometer.

  Referring to FIG. 1, a secondary ion mass spectrometer 100 used in the present embodiment includes a sample holder 5 for placing and holding a sample 10 in a vacuum chamber 1 evacuated from an exhaust port 1a, and a primary. An ion gun 2 for generating an ion beam 7, a mass analyzer 3 for capturing and analyzing secondary ions 9 emitted from the upper surface of the sample 10, and a controller 30 are provided.

  In the vacuum chamber 1, an extraction electrode 6 having an opening 6 a is further provided between the sample 10 and the mass analyzer 3. Given potentials V1 and V2 are respectively supplied from the power source 4 to the extraction electrode 6 and the sample holder 5. Ions emitted from the upper surface of the sample 10 are accelerated or repelled in accordance with a potential difference (voltage) applied to the extraction electrode 6 and the sample holder 5. The accelerated ions are taken into the mass analyzer 3 from the entrance 3a of the mass analyzer 3 through the opening 6a.

  The ion gun 2 ionizes an alkali metal such as cesium or lithium to generate a focused primary ion beam 7. The primary ion beam 7 is irradiated on the upper surface of the sample 10 placed on the sample holding table 5, and a deflection operation is performed so as to scan the upper surface of the sample 10.

  The controller 30 controls the deflection direction and the deflection amount of the primary ion beam 7 so as to scan the primary ion beam 7 along a preset scanning line. At the same time, the operations of the mass analyzer 3 and the power source 4 are controlled in synchronization with the scanning of the primary ion beam 7.

  FIG. 2 is a current density distribution diagram of the primary ion beam, and represents a current density distribution in the beam cross-sectional direction on the upper surface of the sample 10 of the primary ion beam 7 generated by the ion gun 2. The horizontal axis in FIG. 2 represents the distance x from the beam center axis, and the vertical axis represents the current density I of the beam.

  The primary ion beam 7 is focused on the upper surface of the sample 10. Referring to FIG. 2, the primary ion beam 7 has a Gaussian distribution-like current density distribution on the upper surface of the sample. The diameter W of the primary ion beam 7 is focused to about 50 μm to 100 μm, for example.

  Before describing the first embodiment of the present invention, problems of the conventional secondary ion mass spectrometry method will be described.

  FIG. 3 is a cross-sectional view of the sample processed by the primary ion beam, and shows the surface shape of the sample 110 milled by the primary ion beam 7 having a current density distribution like a Gaussian distribution. 3A shows a cross-section after scanning the primary ion beam 7 in the left-right direction on the sample 110 within the upper surface of the sample 110, and FIG. 3B shows the irradiation region 110a of the primary ion beam 7 using the primary ion beam 7. A cross section of a sample 110 in which a groove 103 is formed around (a region scanned by the primary ion beam 7) is shown.

  Referring to FIG. 3A, when the primary ion beam 7 is scanned on the upper surface of the sample 110 in the left-right direction on the paper surface, a concave portion 101 having a linear groove shape is formed on the upper surface of the sample 110. The left and right ends 101a of the recess 101 are formed on a gently curved surface corresponding to the current density distribution of the primary ion beam 7. Since the elements contained in layers having different depths from the upper surface of the sample 10 are released from the curved surfaces of both ends 101a of the recess 101, the resolution in the depth direction in the secondary ion mass spectrometry is deteriorated.

  Further, when an alkali metal ion beam is used as the primary ion beam 7, an irradiation region is formed on the upper surface (non-irradiation region 110 b) of the sample 110 that is in contact with the irradiation region 110 a irradiated by the primary ion beam 7 and is exposed to the outside. Primary ions that recoil or spatter from 110a adhere to form a compound of primary ions.

  On the other hand, in the mesa method, referring to FIG. 3B, the groove 103 is formed in advance around the irradiation region 110a irradiated with the primary ion beam 7 on the upper surface of the sample 10, and the irradiation region 110a is processed into the mesa 102. . In this mesa method, the primary ion beam 7 scans the upper surface of the mesa 102 formed inside the groove 103. Therefore, secondary ions are prevented from being released from the curved surfaces of the concave ends 101a described with reference to FIG. 3A, and the resolution in the depth direction is prevented from deteriorating.

  In this mesa method, even when an alkali metal ion beam is used as the primary ion beam 7, the wide groove 103 is interposed between the irradiation region 110a and the non-irradiation region 110b. Scattering is suppressed and formation of a primary ion compound on the non-irradiated region 110b is prevented.

  However, when an alkali metal ion beam is used to form the groove 103, referring to FIG. 3B, when the groove 103 is formed, the upper surface of the sample 110 outside the groove 103 (non-irradiation region 110b) is formed. Alkali metal compounds adhere and accumulate.

  The composition of these deposited alkali metal compounds varies with the lapse of irradiation time, and changes the work function of the surface of the sample 110. As a result, the emission amount (ionization rate) of secondary ions emitted from the surface (upper surface) of the sample 110 fluctuates, preventing accurate quantitative analysis. The present invention has been devised in order to prevent the deposition of such alkali metal compounds on the outside of the irradiated region and realize a precise quantitative analysis.

  FIG. 4 is a plan view showing the scanning method of the primary ion beam according to the first embodiment of the present invention, and shows the shape of the scanning line in the sample surface.

  Referring to FIG. 4, in the first embodiment of the present invention, scanning line 21 is set in advance on the upper surface of sample 10. This scanning line 21 is a line virtually assumed as a sequence of the controller 30, and is not actually drawn on the surface of the sample 10. Specifically, the setting of the scanning line 21 is performed by the controller 30 operating the deflection of the primary ion beam 7 so that the primary ion beam 7 scans the surface of the sample 10 along the scanning line 21. The The center of the primary ion beam 7 is set so as to scan along the scanning line 21. Therefore, in the scanning of the primary ion beam 7, the center of the primary ion beam 7 moves along the scanning line 21 on the scanning line 21.

  The scanning line 21 is set in a spiral shape on the upper surface of the sample 10. The scanning line 21 is set so as to be spirally wound from the outer periphery to the inner periphery between the point P1 forming one end of the outermost periphery of the spiral and the point P4 forming one end on the center side of the spiral. The shape of the spiral is not particularly limited as long as the irradiation region scanned by the primary ion beam 7 is flatly milled as will be described later. For example, as shown in FIG. 4, the scanning line 21 may be a rectangular spiral formed by being bent at a right angle. Further, the scanning line 21 may be a spiral formed in a circular shape like a mosquito coil or a polygon.

  The scanning lines 21 are desirably set so that the adjacent scanning lines 21 are parallel lines having a constant interval. For example, the scanning lines 21 extending in the vertical or horizontal direction in FIG. 4 are set so that the vertical and horizontal portions are parallel lines. By using parallel lines in this way, as will be described later, the surface of the sample 10 scanned by the primary ion beam 7 is flatly milled, and the degradation in resolution in the depth direction of secondary ion mass spectrometry is reduced. .

  Referring to FIG. 4, the scanning line 21 of the first embodiment extends from the point P1 to the right side of the drawing sheet of FIG. 4, and is refracted downward on the drawing sheet at a right angle on the right side of the spiral. Further, after being refracted at a right angle at the lower side of the spiral and changing the extending direction to the left, it is refracted at a right angle at the left side of the spiral. Then, before coming in contact with the scanning line 21 extending rightward from the point P1, the light is refracted rightward at right angles and extends rightward. Thereafter, until reaching the point P4, the same right angle refraction and linear extension are repeated to form a rectangular spiral. 4 shows the scanning line 21 wound approximately seven times, the number of turns is determined by the size of the spiral and the distance between the adjacent scanning lines 21, and if these are different, the number of turns is determined. Is also different.

  The outer shape of the spiral scanning line 21 can be set to a square having a side length of several tens to several hundreds of μm, for example. The region where the scanning line 21 is set defines an irradiation region 10 a irradiated with the primary ion beam 7.

  FIG. 5 is a cross-sectional view of the sample according to the first embodiment of the present invention, showing the A-A ′ cross section of FIG. 4.

  Referring to FIGS. 4 and 5, grooves 22-1 to 22-7 that are parallel to each other along scanning line 21 are formed on the upper surface of sample 10 irradiated and scanned with primary ion beam 7. The groove 22-1 represents a groove corresponding to the scanning line 21-1 constituting the outermost periphery (first winding line) of the spiral scanning line 21, and the groove 22-2 represents the second winding line. The groove | channel corresponding to the scanning line 21-2 to comprise is represented. Similarly, the groove 22-n (n is a positive integer) represents a groove corresponding to the scanning line 21-n constituting the n-th winding line.

  The scanning lines 21-1 to 21-7 corresponding to the first to seventh winding lines of the spiral are provided in parallel at intervals at which parts of the primary ion beam 7 overlap each other. This interval is appropriately determined according to the current density distribution and beam diameter of the primary ion beam 7 so that the bottom surface of the recess 22 formed by irradiation with the primary ion beam 7 becomes flat. For example, when the primary ion beam 7 having a half width W of 50 μm is used, the interval between the scanning lines 21-1 to 21-7 is preferably set to an interval of 30 μm to 60 μm. Thus, by scanning so that a part of primary ion beam 7 overlaps, the bottom face of the recessed part 22 formed in the irradiation area | region 10a can be made flat.

  That is, each of the grooves 22-1 to 22-7 is formed in a bowl-shaped cross-sectional shape corresponding to the current density distribution of the primary ion beam 7. However, except for the scanning line 21-1, which corresponds to the first (outermost) winding line, the grooves 22-2 to 22-7 formed at the positions of the other scanning lines 21-2 to 21-7 are mutually connected. It forms so that a part of side surface of adjacent groove | channels 22-2-22-7 may mutually overlap. For this reason, the grooves 22-2 to 22-7 are connected and the bottom surface thereof is flat. Thus, the recessed part 22 with the flat bottom face formed by the groove | channels 22-2 to 22-7 continuing in the irradiation area | region 10a is formed.

  The groove 22-1 corresponding to the outermost scanning line 21-1 defines the outer peripheral edge of the recess 22 formed in the irradiation region 10a, and is a gentle slope (due to the current density distribution of the primary ion beam 7). (Curved surface). Further, the upper surface of the original sample 10 is left as it is as the non-irradiation region 10b where the primary ion beam 7 is not irradiated outside the irradiation region 10a.

  Next, the relationship between the scanning method of the primary ion beam and the sample potential in the first embodiment will be described.

  Referring to FIG. 4 again, the primary ion beam 7 scans the upper surface of the sample 10 along the scanning line 21 extending from one end of the outermost periphery of the spiral, one end on the center side of the spiral to the point P4. At this time, scanning may be performed from the outer periphery of the spiral toward the center of the spiral with the starting point as the point P1, and conversely, with one end of the center of the spiral, the point P4, as the starting point, one end of the outer periphery of the spiral with the point P1. You may scan towards The scanning is repeated. That is, when the starting point is point P1, the end point of scanning is point P4. After the scanning is completed, the primary ion beam 7 is returned from point P4 to point P1, and the next scanning is started from point P1. When the start point is point P4 and the end point is point P1, after the end of scanning, the primary ion beam 7 is returned from point P1 to point 4, and the next scanning is started from point P4.

  In the first embodiment, the above-described scanning line 21 is divided into three line segments including first to third scanning line segments 21a to 21c. The first scanning line segment 21a is wound one or more times along the scanning line 21 with reference to a line segment drawn as a thick solid line as a part of the scanning line 21 in FIG. It consists of a line segment including the outermost periphery. That is, the line segment extends from one end on the outermost periphery of the scanning line 21, the point P 1, to the point P 2 on the scanning line 21 wound one or more times along the spiral scanning line 21.

  The second scanning line segment 21b refers to a line segment drawn with a thick broken line as a part of the scanning line 21 in FIG. 4, and is one end on the center side of the spiral, that is, an end point on the center side of the scanning line 21. This is a line segment from the point P4, which extends to the point P3 on the scanning line 21 between the points P2 and P4. This point P3 is set to a position where the primary ion beam 7 that scans on the second scanning line segment 21b can irradiate the entire observation region 10c that is set as the region to be observed on the upper surface of the sample 10 without any excess. . In other words, the observation region 10c is defined by the second scanning line segment 21b.

  The third scanning line segment 21c is a line segment extending from the point P2 to the point P3 along the scanning line 21 with reference to a thin line drawn as a part of the scanning line 21 in FIG. is there.

  In the secondary ion mass spectrometry method of the first embodiment, while the primary ion beam 7 scans the first scanning line segment 21a, the sample 10 has a positive potential (constant constant) with the extraction electrode 6 as a reference potential. Potential) is applied. Specifically, referring to FIG. 1, the controller 30 operates the power supply 4 that supplies a voltage between the extraction electrode 6 and the sample holding base 5, so that the potential of the sample holding base 5 is greater than the potential of the extraction electrode 6. Control to be positive potential. The voltage control of the power source 4 is performed in synchronization with the scanning control of the primary ion beam 7. The positive potential applied to the sample 10 is maintained at a constant potential while the primary ion beam 7 scans the first scanning line segment 21a. The sample 10 is usually held at the same potential as the sample holder 5. For the sake of brevity, hereinafter, the description will be made assuming that the potential of the sample 10 is the same as the potential of the sample holder 5.

  Further, while the primary ion beam 7 scans the second scanning line segment 21b, a negative potential (constant potential) with the extraction electrode 6 as a reference potential is applied to the sample 10. That is, referring to FIG. 1, the voltage applied between the extraction electrode 6 and the sample holder 5 by the power source 4 is controlled so that the potential of the sample holder 5 is more negative than the potential of the extraction electrode 6. The negative potential applied to the sample 10 is maintained at a constant potential while the primary ion beam 7 scans the second scanning line segment 21b.

  Furthermore, an arbitrary potential can be applied to the sample 10 while the primary ion beam 7 is scanning on the third scanning line segment 21c. For example, the sample holder 5 can be set to the ground potential. It can also be a positive or negative potential applied during scanning on the first or second scanning line segment 21a, 21b, or a potential therebetween.

  Referring to FIG. 1 again, in the secondary ion mass spectrometry method of the first embodiment, the secondary ions 9 emitted from the upper surface of the sample 10 are incident on the mass analyzer 3 for mass analysis. The mass analyzer 3 is controlled by the controller 30 so as to continue the detection and counting operation of the secondary ions 9 at least while the primary ion beam 7 scans the second scanning line segment 21b.

  During the scanning on the first and second scanning line segments 21a and 21b, detection and counting of the secondary ions 9 can be interrupted or continued as necessary. While the primary ion beam 7 scans other than the observation region 10c, the detection and counting operation of the secondary ions 9 is interrupted, thereby avoiding the mixing of the secondary ions 9 emitted from the region other than the observation region 10c. it can. In addition, unnecessary saturation of the detector and the counter is avoided, and a decrease in detection sensitivity can be avoided. This detection and counting operation is stopped by, for example, reducing the voltage applied to the detector of the mass analyzer 3, such as a Faraday cup, an electron multiplier, or a channeltron.

  Next, the relationship between the potential of the sample 10 and the adhesion of the primary ion compound to the upper surface of the sample 10 in the secondary ion mass spectrometry method of the first embodiment will be described.

FIG. 6 is a diagram showing the result of mass analysis of secondary ions, and shows the change over time in the ion intensity of ³⁰ Si ⁻ ions detected in secondary ion mass spectrometry using the Si substrate as the sample 10. FIG. 7 is a diagram showing the result of mass analysis of primary ions. The time variation of the ion intensity of ¹³³ Cs ⁺ that is a constituent element of the primary ion beam 7 detected in the secondary ion mass spectrometry using the Si substrate as the sample 10 is shown. Represents. 6 and 7, a cesium ion beam accelerated to 4 keV as the primary ion beam 7 is incident on the upper surface of the sample 10 at an incident angle of 20 ° (angle formed with the upper surface of the sample 10), and is almost perpendicular to the upper surface of the sample 10. It is the result of measuring the emitted ³⁰ Si ⁻ ions and ¹³³ Cs ⁺ ions. 6 and 7, the potential of the sample 10 was used as a parameter.

Referring to FIG. 6, the ionic strength of ³⁰ Si ⁻ ions emitted from the upper surface of sample 10 varies with the primary ion irradiation amount, and is almost constant when the primary ion irradiation amount exceeds 4 × 10 ¹⁵ ions / cm ^2. To stabilize. Here, the primary ion irradiation dose is a value obtained by converting the irradiation time into the irradiation dose. The ionic strength of secondary ions, that is, ³⁰ Si ⁻ ions varies depending on the potential of the sample 10. Note that the measurement of the ionic strength of ³⁰ Si ⁻ ions was performed in a state where the potential of the sample 10 was maintained at a different potential between 0 V and +30 V with the extraction electrode 6 as a reference.

Referring to FIG. 7, the ion intensity of ¹³³ Cs ⁺ emitted from the upper surface of sample 10 also changes with the primary ion irradiation amount, and is stable at a constant value when the primary ion irradiation amount exceeds 4 × 10 ¹⁵ ions / cm ^2. To do. The ionic strength of ¹³³ Cs ⁺ also changes depending on the potential of the sample 10. Here, the measurement of the ionic strength of ¹³³ Cs ⁺ ions shown in FIG. 7 was performed in a state where the potential of the sample 10 was held at different potentials between +2 V and −25 V with respect to the potential of the extraction electrode 6.

FIG. 8 is a diagram showing the relationship between the sample potential and the detected amounts of primary ions and secondary ions, and was detected when the primary ion irradiation amount in FIGS. 6 and 7 was 6 × 10 ¹⁵ ions / cm ² . ^It represents the ionic strength of ³⁰ Si ⁻ ions and ¹³³ Cs ⁺ ions. The potential of the sample 10 represents the potential of the sample holder 5 when the potential of the extraction electrode 6 is 0V.

Referring to curve A in FIG. 8, when the upper surface of the sample 10 is irradiated with the primary ion beam 7 made of a cesium ion beam, the ¹³³ Cs ⁺ ion intensity detected by the mass analyzer 3 is the potential of the sample 10 from + 25V. The potential increases as the potential is lowered (changes in the negative potential direction), and the potential of the sample 10 becomes maximum at + 6V. When the potential of the sample 10 is further lowered, the ¹³³ Cs ⁺ ion intensity decreases, and at +2 V, it decreases to about ¼ of the maximum value.

The sample 10 is a Si substrate and does not contain cesium. Accordingly, all ¹³³ Cs ⁺ ions detected by the mass analyzer 3 are derived from cesium atoms supplied to the upper surface of the Si substrate (sample 10) by irradiation with the primary ion beam 7. The primary ion beam 7 supplies cesium atoms to the upper surface of the Si substrate (sample 10) and simultaneously releases cesium atoms from the upper surface of the Si substrate. The detected cesium ions are primary ions recoiled on the upper surface of the Si substrate and cesium ions sputtered and emitted from the upper surface of the Si substrate.

  When the sample 10 is held at a positive potential, the mass analyzer 3 detects a large amount of cesium ions. The result shown in FIG. 8 is that the positive charge released from the upper surface of the sample 10 is obtained by setting the potential of the sample 10 to a positive potential. This means that the primary ions (the constituent ions of the primary ion beam 7) are attracted to the extraction electrode 6 side. This suggests that reattachment of primary ions released from the upper surface of the sample 10 to the upper surface of the sample 10 is suppressed. In the first embodiment, when the primary ion beam 7 irradiates the first scanning line segment 21a that forms the outer periphery of the scanning line 21, the potential of the sample 10 is held at a positive potential. For this reason, scattering and adhesion of cesium on the non-irradiated region 10b near the outside of the irradiated region 10 are suppressed.

  Referring to curve A in FIG. 8, in order to prevent cesium scattering to the non-irradiated region 10b, the potential of the sample 10 is set near the potential at which the detected cesium ion intensity is maximum, for example, + 6V ± 1V. It is preferable to do. Thereby, many of the cesium ions recoiled or released from the upper surface of the sample 10 are prevented from reattaching to the upper surface of the sample 10, and the formation of the cesium compound on the non-irradiated region 10b is effectively suppressed. Furthermore, by setting the potential of the sample 10 to a potential at which the detected ion intensity stays within approximately 1/5 of the maximum value, for example, +3 V to +10 V, a sufficient effect of preventing reattachment of the cesium compound was obtained.

On the other hand, referring to the curve B in FIG. 8, when the upper surface of the sample 10 is irradiated with the primary ion beam 7 made of a cesium ion beam, the ³⁰ Si ⁻ ion intensity detected by the mass analyzer 3 is the potential of the sample 10. It increases as the potential is increased from −30V (changes in the positive potential direction), and the potential of the sample 10 becomes maximum at + 3V. When the potential of the sample 10 is further lowered, the ³⁰ Si ^- ion intensity decreases, and at 0 V, it decreases to about 1/3 of the maximum value.

Therefore, in order to increase the sensitivity of the secondary ion mass spectrometry, the potential of the sample 10 is kept near the potential at which the intensity of the ³⁰ Si ⁻ ion of the secondary ion to be detected becomes a maximum, for example, −3V ± 1V. Is preferred. Furthermore, to the extent of the sample 10 potential -3V-10V, ³⁰ Si with high sensitivity ^- ion detection was made.

  In the field of electronic devices including semiconductor devices, the concentration distribution in the depth direction of atoms observed as secondary ions having negative charges such as Si, As, P, etc. is often measured. These atoms have a maximum detection sensitivity (detected ion intensity) in the range where the potential of the sample 10 is −3 V to −10 V. Accordingly, by setting the potential of the sample 10 when the primary ion beam 7 scans the second scanning line segment 21b to −3V to −10V, atoms (negative secondary) that require measurement in the electronic device field are required. Many of the atoms observed as ions) can be detected with high sensitivity.

  FIG. 9 is a laser microscope image of the upper surface of the sample according to the first embodiment of the present invention, and shows the distribution of the cesium ion compound attached to the upper surface of the sample after completion of secondary ion mass spectrometry. FIG. 10 is a laser microscope image of the upper surface of the sample of the comparative example, and shows the distribution of the cesium compound attached to the upper surface of the sample after the completion of secondary ion mass spectrometry in the comparative example. In the drawing, the irradiation area 10a is indicated by a white broken line, and the non-irradiation area 10b is indicated on the outside thereof. The secondary ion mass spectrometry of the comparative example is the same as that of the first embodiment except for the material of the primary ion beam 7, the scanning line 21, and the sample 10 except that the potential of the sample 10 is kept at −3V. It was made below.

  Referring to FIG. 9, in sample 10 of the first embodiment of the present invention, no cesium compound was observed on irradiation region 10a and non-irradiation region 10b extending outside irradiation region 10a. In comparison, referring to FIG. 10, in the comparative example, the cesium compound was not observed in the irradiated region 10a, but adhered to the non-irradiated region 10b so as to surround the outer periphery of the irradiated region 10a. Was observed.

  FIG. 11 is a diagram showing an Auger analysis result on the upper surface of the sample according to the first embodiment of the present invention. FIG. 11B shows an Auger image in the vicinity of the boundary between the irradiated region 10a and the non-irradiated region 10b. ) Represents the Auger electron intensity measured along the straight lines L1 to L3 perpendicular to the boundary indicated by the arrow in FIG. FIG. 12 is a diagram showing an Auger analysis result on the upper surface of the sample of the comparative example. FIG. 12B is an Auger image near the boundary between the irradiated region 10a and the non-irradiated region 10b, and FIG. b) The Auger electron intensity observed along the straight lines L1 to L3 perpendicular to the boundary indicated by the arrows in FIG.

  Referring to FIG. 11A, in the sample 10 of the first embodiment, the Auger electron intensity detected as a cesium compound has a substantially constant value in the irradiated region 10a, and abruptly in the non-irradiated region 10b. is decreasing.

  This decrease in the non-irradiated region 10b indicates that almost no cesium compound is formed on the non-irradiated region 10b in the first embodiment. On the other hand, taking a constant value in the irradiation region 10a means that in the irradiation region 10a, the cesium supplied by the primary ion beam 7 and the cesium released from the upper surface of the sample are in an equilibrium state to form a compound having a constant composition. It has been shown.

  Referring to FIG. 12A, in the sample 10 of the comparative example, the Auger electron intensity detected as the cesium compound takes a constant value similar to that in the first embodiment in the irradiation region 10a, while the non-irradiation region. It increases rapidly at 10b.

  This fact is similar to the first embodiment in that the compound having a constant composition is formed in the irradiated region 10a, whereas the non-irradiated region 10b is formed with a cesium compound containing a high concentration of cesium. It is made clear.

  As described above, in the first embodiment, while the primary ion beam 7 scans the first scanning line segment 21a that is the outer peripheral portion of the scanning line 21, the potential of the sample 10 is maintained at a positive potential. Formation of the cesium compound on the irradiation region 10b is suppressed. For this reason, since the work function variation caused by the cesium compound adhering to the upper surface of the sample 10 is small and the change in the secondary ionization rate during measurement is small, accurate secondary ion mass spectrometry is realized. Note that the cesium concentration in the irradiation region 10a is maintained at a constant value as in the conventional secondary ion mass spectrometry method. For this reason, since the work function is kept constant, the adverse effect on the analysis accuracy is small.

  Further, in the first embodiment, the potential of the sample 10 is held at a positive potential while scanning on the first scanning line segment 21a, so that detection of negative secondary ions during this period is suppressed. For this reason, detection of secondary ions emitted from the inclined surface of the periphery of the concave portion 22 formed in the irradiation region 10a is suppressed, and deterioration of resolution in the depth direction due to the edge effect is avoided. In particular, negative secondary ions emitted from within the observation region 10c can be selectively detected by setting the sample 10 potential to a negative potential only during scanning on the second scanning line segment 21b. In this case, detection of secondary ions emitted from a wide region including the observation region 10c and the irradiation region 10a in which the first scanning line segment 21a and the third scanning line segment 21c extend is suppressed. For this reason, the edge effect is more effectively suppressed.

  Note that the wider the width of the scanning region where the potential of the sample 10 is held at a positive potential, for example, the region where the first scanning line segment 21a extends, the higher the non-irradiation region 21b of the primary ions emitted from the inner region. Scattering and adhesion to the can be effectively suppressed. Therefore, from the viewpoint of preventing the formation of the primary ion compound, it is preferable to increase the number of turns of the first scanning line segment 21a. However, if the number of turns of the first scanning line segment 21a is large, a wide observation region 10c cannot be obtained. Therefore, the number of turns must be appropriately set as necessary.

  The same effect of suppressing scattering of primary ions can be achieved by holding the potential of the sample 10 at a positive potential while scanning the third scanning line segment 21c. In this case, the second embodiment will be described later.

  Referring to FIG. 4, in the first embodiment, the scanning line 21 is set in a spiral shape. It is also conceivable that the scanning line 21 is a raster scan. However, in order to make the sample 10 potential positive during raster scanning of the irradiation region 10a and scanning the periphery of the irradiation region 10a as in the first embodiment, the sample 10 is detected at both ends of the scanning line for each scan. The power supply 4 must be controlled in synchronization with the scan so that the potential becomes a positive potential, which is not preferable because the control becomes complicated.

  It is also conceivable to raster scan only the central portion in the irradiation area 10a, that is, the area where the second and third scanning line segments 21b and 21c extend.

  FIG. 13 is a diagram showing another scanning method. FIG. 13A shows a scanning line for raster scanning the central portion of the irradiation region 10, FIG. 13B shows a scanning line in FIG. 13B, and FIG. The shape of the sample 10 of AA 'cross section of is shown.

  Referring to FIG. 13A, in this other scanning method, the first scanning line segment 21a extending from the points P1 to P2 constituting the outer peripheral portion of the spiral is replaced with the first scanning line segment of the first embodiment. Same as 21a. Then, it is assumed that the area from point P2 to point P4 is raster scanned.

  Referring to FIG. 13B, in this case, grooves 22x and 22-2 formed by scanning the primary ion beam 7 on the first scanning line segment 21a are formed into grooves 22x formed by raster scanning. Abut vertically. Therefore, when the groove 22x terminates in front of the grooves 22-1 and 22-2, a protrusion 23 is formed on the bottom surface of the recess 22. When the grooves 22-1 and 22-2 are overlapped and terminated, a recess is formed on the bottom surface of the recess 22. Such protrusions 23 and depressions are not preferable because they impair the flatness of the bottom surface of the recesses 22 and cause deterioration in analysis accuracy. In order to prevent the formation of the protrusions 23 and the depressions, it is necessary to precisely control the positions of both ends of the groove 22x formed by the raster scan, and it is difficult to control the scanning of the primary ion beam 7. Therefore, the scanning line 21 is desirably set in a spiral shape.

  The second embodiment of the present invention relates to a secondary ion mass spectrometry method in which a scanning line includes a first scanning line and a second scanning line.

  FIG. 14 is a diagram showing a scanning method of a primary ion beam according to the second embodiment of the present invention, and shows the shape of the scanning line in the sample surface.

  Referring to FIG. 14, the scanning line 21 of the second embodiment has the same shape as the scanning line 21 of the first embodiment shown in FIG. However, the first embodiment differs from the first embodiment in that the scanning line 21 includes a first scanning line segment 21a and a second scanning line segment 21b.

  In the second embodiment, the first scanning line segment 21a is configured as the scanning line 21 extending from the point P1 to the outermost periphery of the spiral, that is, the point P2 obtained by winding the outer periphery of the spiral once. The first scanning line segment 21a may be the first scanning line segment 21a wound a plurality of times as in the first embodiment shown in FIG.

  In the second embodiment, the second scanning line segment 21b is directly connected to the first scanning line segment 21a. That is, the third scanning line segment 21c in the first embodiment is excluded. That is, the end point P2 of the first scanning line segment 21a coincides with the end point P3 of the second scanning line segment 21b. As a result, between the outer periphery of the irradiation region 10a and the outer periphery of the observation region 10c (hereinafter referred to as “observation region outer edge portion 10d”), the first scanning line segment 21a makes a round around the outer periphery of the observation region 10c. Set to .

  Similar to the first embodiment, the potential of the sample 10 is maintained at a positive potential with respect to the extraction electrode 6 while the primary ion beam 7 scans the first scanning line segment 21a, and the primary ion beam 7 is While scanning the second scanning line segment 21b, the negative potential is maintained.

  Therefore, the secondary ion mass spectrometry is performed in the observation region 10c where the second scanning line segment 21b extends. On the other hand, in the observation region outer edge 10d where the first scanning line segment 21a extends, secondary ion mass spectrometry is not performed, and scattering and adhesion of primary ions to the upper surface of the sample 10 are suppressed.

  In the present embodiment, it is sufficient to control the potential of the sample 10 to two potentials, so that the setting of the controller 30 is simpler than in the first embodiment. Further, since the third scanning line segment 21c is not provided, the observation region 10c can be set wide.

  By applying the present invention to a secondary ion mass spectrometry method for detecting negative secondary ions, the analysis of a precise element concentration with less adhesion of primary ion compounds to the sample surface can be performed, and the resolution in the depth direction can be improved. A high secondary ion mass spectrometry method is realized.

DESCRIPTION OF SYMBOLS 1 Vacuum chamber 2 Ion gun 3 Mass analyzer 4 Power supply 5 Sample holding stand 6 Extraction electrode 6a Opening 7 Primary ion beam 8 Primary ion 9 Secondary ion 10, 110 Sample 10a, 110a Irradiation area 10b, 110b Non-irradiation area 10c Observation area 10d Observation region outer edge 21 Scan line 21a First scan line segment 21b Second scan line segment 21c Third scan line segment 22, 101 Recess 22-1 to 22-7, 22x Groove 23 Projection 30 Controller 100 Secondary ion Mass spectrometer 102 Mesa 103 Groove

Claims (5)

A secondary ion that scans the surface of the sample with a primary ion beam of an alkali metal ion along a scanning line set on the surface of the sample, and performs mass analysis of secondary ions having a negative charge emitted from the surface of the sample. In the mass spectrometry method,
The sample with respect to a secondary ion extraction electrode when the primary ion beam scans a first scanning line segment that is wound at least once along the spiral scanning line and includes the outermost periphery of the spiral. Controlling the potential of the extraction electrode and the sample so that the potential of
When the primary ion beam scans on the second scanning line segment that is located inside the spiral from the first scanning line segment and reaches the innermost end of the spiral, the secondary ion extraction electrode Controlling the extraction electrode and the potential of the sample such that the potential of the sample becomes a negative potential;
A secondary ion mass spectrometry method comprising:   The secondary ion mass spectrometry method according to claim 1, wherein the first scanning line segment is an outermost circumference of the spiral. The secondary ion according to claim 1 or 2, wherein the detection or counting operation of secondary ions emitted from the sample surface is stopped while the primary ion beam scans the first scanning line segment. Mass spectrometry method.   4. The secondary ion mass spectrometry method according to claim 1, wherein the primary ion beam is a Cs + ion beam. The negative potential is −3 V to −10 V;
The secondary ion mass spectrometry method according to any one of claims 1 to 4, wherein the positive potential is + 3V to + 10V.