JP2006179504A - Electron beam device with aberration corrector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem that an electron beam device with an aberration corrector using a multipole lens is not clear in distinguishing a failure in axial matching in the aberration corrector unit and a failure in axial matching in the portion other than the aberration corrector at the optical axis adjustment and, therefore, it takes a long time for adjustment. <P>SOLUTION: The device is provided with a scanning mode for operating the aberration corrector and a scanning mode for not operating it, and operation of the aberration corrector and capacitor lens or the like is controlled so that the object point of the objective lens may not change in both modes. When the secondary electron images of the testpiece are compared in both modes, the image magnification and focus are not changed and thereby only the effect of the aberration corrector can be distinguished and evaluated for adjustment, and axis adjustment time can be shortened. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a technique for minimizing a beam spot of an electron beam apparatus, and more particularly to a scanning electron microscope (SEM), a length measuring SEM (CD-SEM), an electron beam drawing apparatus, and the like.

  In a device such as a length measuring SEM that measures the pattern dimensions of semiconductor devices with high accuracy, a resolution of about 3 to 1 nm is achieved with an electron beam with an accelerating voltage of 1 kV or less so as not to destroy the sample by miniaturizing the device pattern in recent years. It is necessary to do. For this purpose, it is necessary to focus the electron beam on the sample surface with a diameter less than a desired resolution. In such a low acceleration, the first problem is chromatic aberration of the objective lens. In order to reduce this chromatic aberration, various attempts have already been made to reduce the chromatic aberration by devising the design of the objective lens.

While passing the objective lens with a higher energy by increasing the electron acceleration voltage by V _r in the 1990s, by applying a voltage of -V _r to the sample to decelerate the electrons, the electron beam energy at the sample entrance A retarding technique to keep it low is now used. According to this method, chromatic aberration is reduced at a rate shown in Expression (1), where V ₀ is an electron acceleration voltage in the electron gun section.

しかし、Ｖ_ｒがあまり大きいと試料が電界により破壊されるため、かけられるＶ_ｒには限界があった。そのほかに放出エネルギー幅の小さい電子源を使えばその分色収差は減るが、現在は電界放出電子銃（放出電子エネルギー幅０．３ｅＶ程度）、ショットキー電子銃（０．６ｅＶ程度）などが使われており、これらより小さい放出電子エネルギー幅と安定性をもつ電子源が探索されている段階である。

However, if V _r is too large, the sample is destroyed by the electric field, so that there is a limit to the applied V _r . In addition, if an electron source with a small emission energy width is used, the chromatic aberration will be reduced. However, field emission electron guns (emission electron energy width of about 0.3 eV), Schottky electron guns (about 0.6 eV) are currently used. At this stage, an electron source having a smaller emission electron energy width and stability is being searched.

  Since such a new electron source has not yet been put into practical use, the reduction of chromatic aberration by the above method is now at the limit.

  As a method for solving this problem, attention has been paid to two methods of using a monochromator and reducing chromatic aberration using an aberration corrector. Among them, it is proposed by Scherzer in 1947 that an aberration corrector can be constructed by combining a multipole lens to cancel out the aberration of the objective lens. Published in Reference 2. Recently, this technique has been experimentally verified using an electromagnetic field composite quadrupole / octupole aberration corrector (Non-patent Document 3).

  The outline of the operation of this type of aberration corrector will be described with reference to FIG. The aberration corrector 10 includes a multipole lens 11, a composite multipole lens 12, a composite multipole lens 13, and a multipole lens 14 in order to generate a quadrupole field and an octupole field in a superimposed manner. In the aberration corrector 10, due to the quadrupole field, a converging action and a diverging action are generated in two directions (x-axis and y-axis) perpendicular to the optical axis (z-axis) to separate the paraxial trajectory. In FIG. 4, the trajectory of the electron beam is schematically shown by a thin line. The electron beam emitted from the crossover 41 is diverged in the x-direction trajectory (x trajectory) by the first multipole lens 11 of the aberration corrector 10 (the single trajectory in the figure), and the y-direction trajectory (y trajectory) is converged. (Two orbited arrows in the figure) are separated. A trajectory in any direction can be considered as a linear combination of these x and y trajectories. The second-stage compound multipole lens 12 can generate a quadrupole electric field and a quadrupole magnetic field rotated by 45 ° with respect to the optical axis in the xy plane as compared with the left quadrupole electric field. It is possible to apply electric and magnetic fields in combination. The first stage multipole lens 11 is excited so that the y orbit crosses in the vicinity of the center of the composite multipole lens 12. At this time, the x-orbit is deviated to the maximum, and a linear crossover 42 extending in the x direction at the center of the composite multipole lens 12 is formed. The quadrupole excitation of the composite multipole lens 12 is adjusted so that the x-orbit crosses near the center of the third-stage composite multipole lens 13. The linear crossover 43 at this time has a linear shape extending in the y direction. The x trajectory and the y trajectory separated through the fourth stage multipole lens 14 coincide with each other at the crossover 44. The aberration corrector 10 is operated so that the crossover 41 is imaged on the crossover 44 in a stigmatic manner.

  At this time, in the aberration corrector 10, the intensity of the quadrupole field of the electric field and the magnetic field is controlled under the constraint that the combined multipole lens 12 does not change the resultant force acting on the incident electrons having the reference energy. It can be excited by changing the ratio. In this case, since the electron having the energy deviated from the reference is different from the electron having the reference energy, the working force changes and the orbit shifts when the ratio of the electric field and magnetic field strength changes. This deviation is large in the x direction away from the optical axis, and the y direction passing through the center of the multipole field is hardly affected. When passing through the composite multipole lens 13, this relationship is reversed x, y. In other words, by changing the ratio of the intensity of the quadrupole field of the electric field and the magnetic field in the composite type multipole lenses 12 and 13, the trajectory can be changed only in the x direction and the y direction independently of the incident energy.

  By utilizing this fact, the aberration corrector 10 shifts the high-energy electron trajectory to the outside and the low-energy electron trajectory to the inside by an amount that cancels out the chromatic aberration of the subsequent objective lens. Offset. By generating an octupole field in addition to a quadrupole field with the multipole lenses 11 to 14, it is possible to correct spherical aberration as described in Section 3.7 of Non-Patent Document 3.

  In these known examples, an example is shown in which a 12-pole is used to superimpose a 4-pole field and an octupole field. In these known examples, the electron optical system is configured from the viewpoint of reducing higher-order aberrations generated by the aberration corrector itself. However, since it is necessary to form a microprobe, an aberration corrector is installed in the electron beam apparatus. When incorporated, it is difficult to align the entire device.

JP 2000-195453 A Optik 33 (1971) 1-24 pages Optik 83 (1989) 30-40 Nuclear Instruments and Methods in Physics Research, A 363 (1995), pp. 316-325

  When such an aberration corrector is introduced into an electron beam apparatus, the excitation and axial misalignment of four stages of quadrupoles and octupoles in the aberration corrector are adjusted, and at the same time, the electron optical axes of other parts of the apparatus are aligned. Is cumbersome and time consuming. The reason for this is that even when the secondary electron image of the sample is observed, the deterioration of the image due to the poor alignment of the aberration corrector alone and the misalignment of the parts other than the aberration corrector are unifying, and the cause cannot be identified. It is.

  The present invention provides an electron beam apparatus with an aberration corrector that can easily confirm the effect of aberration correction, shorten the adjustment procedure, and is easy to use.

  In the electron beam apparatus with an aberration corrector according to the present invention, a computer that controls each lens and aberration corrector provided in the electron beam apparatus has a scan mode in which the aberration corrector is operated and a scan mode in which the aberration corrector is not operated. The aberration corrector is controlled so that the object point position of the objective lens does not change in both modes. Alternatively, the aberration corrector is controlled so that the object point position of the objective lens does not change in both modes by linking the excitation of the condenser lens and the operation of the aberration corrector. In order to enable the linked operation of the lens, deflector, aberration corrector, and the like in the above two modes, a system for uniformly controlling the power sources of the lens, deflector, aberration corrector, etc. with a computer is constructed.

  According to such a configuration, first, in the scanning mode in which the aberration corrector is not operated, normal adjustment necessary for beam focusing such as alignment of the incident axis to the objective lens, focus, astigmatism, and the like can be performed. Subsequently, the scanning mode for operating the aberration corrector is switched. In this mode, the object point of the objective lens is not moved, so the image magnification and focus are not changed. As a result, only the effect of the aberration corrector on the electron beam can be evaluated from changes in the secondary electron image and the drawing pattern before and after this procedure. In other words, the increase or deformation of the probe diameter due to various aberrations at this stage can be distinguished from the incomplete adjustment of the aberration corrector. Then, the excitation of each multipole element of the aberration corrector can be finely adjusted individually to finally cancel out the chromatic aberration and spherical aberration of the objective lens to obtain the minimum probe diameter and the best resolution. By independently adjusting the aberration corrector and the other objective lens axis alignment, the axis alignment process is simplified and the adjustment procedure of the entire apparatus is shortened.

  According to the present invention, the imaging condition of the objective lens can be kept constant regardless of ON / OFF of the aberration corrector, so that the effect of aberration correction can be easily confirmed and the adjustment operation of the aberration corrector can be easily performed. . An adjustment procedure for the entire apparatus can be shortened, and an electron beam apparatus with an aberration corrector having a high apparatus operating rate can be provided.

  FIG. 1 shows an outline of an example of the structure of an electron beam apparatus with an aberration corrector to which the present invention is applied. The Schottky electron source 1 is an electron source that diffuses oxygen and zirconium into a single crystal of tungsten and uses the Schottky effect. A suppressor electrode 2 and an extraction electrode 3 are provided in the vicinity thereof. By heating the Schottky electron source 1 and applying a voltage of about +2 kV to the extraction electrode 3, Schottky electrons can be emitted from the Schottky electron source 1. A negative voltage is applied to the suppressor electrode 2 to suppress electrons emitted from other than the tip of the Schottky electron source 1. Electrons exiting the hole of the extraction electrode 3 are accelerated and converged by the electrostatic lens formed by the first anode 4 and the second anode 5. Subsequently, the beam diameter is limited by the first condenser lens 6 and the condenser diaphragm (not shown), and passes through the second condenser lens 7, the upper deflection coil 8, and the lower deflection coil 9 to the aberration corrector 10 at a desired angle. Incident. The aberration corrector 10 includes a multipole lens 11, a composite multipole lens 12, a composite multipole lens 13, and a multipole lens 14 arranged with an optical axis as a common axis. When correcting chromatic aberration, a quadrupole electric field or quadrupole magnetic field is provided by the multipole lenses 11 and 14 in a plane perpendicular to the optical axis, and a quadrupole electric field and an xy plane are provided by the composite multipole lenses 12 and 13. A quadrupole magnetic field rotated by 45 ° with respect to the optical axis as compared with the quadrupole electric field shown on the left is formed. These fields are formed using 4-pole, 8-pole, or 12-pole electrodes (which may also serve as magnetic poles). In order to correct not only chromatic aberration but also spherical aberration, an octupole field is formed with the above electrodes and superimposed. In this case, the multipole lens needs an octupole or a 12-pole. The electron beam given chromatic aberration or spherical aberration that cancels the objective lens 17 by the aberration corrector 10 is converged and scanned on the sample 18 by the objective lens 17 via the upper scanning coil 15 and the lower scanning coil 16.

  When used as a scanning microscope, a mechanism for detecting and imaging secondary electrons and reflected electrons is attached, but is not shown here. The objective lens 17 is a rotationally symmetric lens of a magnetic field type, an electric field type, or an electric field magnetic field composite type. In order to prevent the sample from being destroyed by the electron beam and reduce the aberration, a voltage may be applied to the sample 18 by the retarding power source 29 so that the electrons are decelerated between the sample 18 and the objective lens 17. All the components of the electron beam apparatus described above are stored in the vacuum container 19 and are electrically connected to each current source and voltage source (20 to 29) outside the vacuum through connectors. A method of supporting each component in the vacuum vessel 19 is not shown for simplicity. By controlling these power supplies through the computer 30, it is possible to generate an electron beam and control scanning.

(Example 1)
FIG. 2 is a diagram for explaining a first embodiment of the present invention. Based on the configuration of the electron beam apparatus with an aberration corrector shown in FIG. 1, focusing on the electron beam trajectory, the control according to the first embodiment will be described.

  The electrons emitted from the Schottky electron source 1 are first accelerated along the trajectory of the solid line between the first anode 4 and the second anode 5, and are converged by the first condenser lens 6, the condenser aperture 39 and the second condenser lens 7. As a result, the aberration corrector 10 is reached (two condenser lenses are not necessarily required). In the scanning mode (OFF mode) in which the aberration corrector 10 is not operated, the x trajectory and the y trajectory are not separated, and the aberration corrector 10 moves straight along the broken line trajectory in the drawing to form a crossover at the position 44. . This is the object point of the objective lens 17.

  In order to obtain the maximum resolution in this state, the objective aligner 38 is adjusted so that the electron beam passes through the current center of the objective lens 17. Although this method has been established, strictly speaking, it is obliquely incident on the objective lens 17. Astigmatism and field curvature aberration generated at this stage can be corrected by the astigmatism correction coil 36, and distortion does not cause image blur. It is considered that the axes are aligned so as to minimize coma aberration and lateral chromatic aberration. Furthermore, according to the method disclosed in Patent Document 1, these aberrations caused by the oblique incidence on the objective lens 17 can be eliminated, but the spherical aberration and the longitudinal chromatic aberration remain to the end.

  After completing the axis adjustment in the OFF mode, the aberration corrector 10 is operated in the state as it is (ON mode). In this state, the electrons that are converged and incident on the aberration corrector 10 by the second condenser lens 7 are separated in the x and y directions in the aberration corrector 10 as described with reference to FIG. The intensity of each multipole lens is set so that the aberration corrector 10 forms a crossover at the position 44 at this time along the paraxial orbit indicated by the solid line. In practice, in order to do this, the intensity of the quadrupole field at each stage of the aberration corrector 10 can be determined by numerical calculation based on the following concept. Here, for the sake of simplicity, the distance between the multipole lens 11 and the composite multipole lens 12 and the distance between the composite multipole lens 13 and the multipole lens 14 are set to zero, and the electrode and magnetic pole shape of the multipole lens are high as a dipole line type. Although the next field is not considered, analytical formulas are specified, but in reality, the distance between these multipoles is made finite, electrodes and magnetic pole shapes are input, and the field is obtained by computer simulation based on the same concept as below.

  The paraxial trajectory equation of the quadrupole lens is expressed as in equations (2) and (3) with the z direction as the optical axis direction.

ここに、β^２は式（４）で表される。

Here, β ² is expressed by the formula (4).

ｋ（ｚ）はｚ方向の場の分布、φ_２は４極子磁場の強度、φ_２は４極子電場の強度、φは軸上の電位を表し、電極に加える電圧およびコイル電流により決まる。各段についてβ_ｉ（ｉ＝１〜４）を求めればよい。実際には無次元パラメータθ_ｉ＝β_ｉＬを求める。ここにＬは各電極のＺ軸方向の厚み、多極子レンズ１１にはθ_１、複合型多極子レンズ１２にはθ_２、複合型多極子レンズ１３にはθ_３、多極子レンズ１４にはθ_４が対応する。ここで、第２コンデンサーレンズ７、対物レンズ１７、収差補正器１０、クロスオーバー４４の位置は与えられているものとする。

k (z) is the distribution of the field in the z direction, φ ₂ is the intensity of the quadrupole magnetic field, φ ₂ is the intensity of the quadrupole electric field, φ is the potential on the axis, and is determined by the voltage applied to the electrode and the coil current. Β _i (i = 1 to 4) may be obtained for each stage. Actually, the dimensionless parameter θ _i = β _i L is obtained. Where L is the Z-axis direction thickness of each electrode, ₁ to multipole lens 11 _theta, composite multipole lens for 12 theta _2, composite multipole lens for 13 theta _3, the multipole lens 14 θ ₄ corresponds to this. Here, it is assumed that the positions of the second condenser lens 7, the objective lens 17, the aberration corrector 10, and the crossover 44 are given.

Under the above conditions, four variables (θ _{1 to} θ ₄ ) are calculated as in the following equations to determine the strength of the quadrupole at each stage.

  1) As a possible condition for aberration correction, as described with reference to FIG. 4, the condition of Expression (5) is obtained when the y-orbit passes through the center of the composite multipole lens 12 at the second stage.

ここにａ×Ｌは複合型多極子レンズ１２と複合型多極子レンズ１３間の距離、ｐ×Ｌは多極子レンズ１４下端面を通る光軸に垂直な面とクロスオーバー４４間の距離である。

Here, a × L is the distance between the composite multipole lens 12 and the composite multipole lens 13, and p × L is the distance between the plane perpendicular to the optical axis passing through the lower end surface of the multipole lens 14 and the crossover 44. .

  2) Since the conical converging beam is incident on the aberration corrector 10, the conditions of the equations (6) and (7) are obtained from the relationship between the coordinates and the inclination of the upper end surface of the aberration corrector 10 on the x-orbit and y-orbit. It is done.

３）収差補正器１０のＯＮ／ＯＦＦにかかわらずクロスオーバー４４の位置が一定で、その点でｘ、ｙ軌道が傾きも含めて一致するスティグマティックな結像条件から式（８）が得られる。

3) Expression (8) is obtained from a stigmatic imaging condition in which the position of the crossover 44 is constant regardless of ON / OFF of the aberration corrector 10 and the x and y trajectories coincide with each other including inclination. .

４）以上４つの式より数値的にθ_１〜θ_４を求め、近軸軌道が決定される。これらの条件では線状クロスオーバー４３は複合型多極子１３の厳密に中央で形成されるわけではない。解の存在は自明ではない。ａやｐの値によっては上記４つの式を同時に満たすθ_１〜θ_４は存在しないこともある。その場合はｘ軌道とｙ軌道のクロスオ−バー位置が一致せず、クロスオーバー４４でのスティグマティックな結像ができないので収差補正することができないことを意味する。この場合はクロスオーバー４４の位置設定をずらしてｐの値を変えたり、収差補正器の複合型多極子レンズ１２と複合型多極子レンズ１３間の距離を変えるなどθ_１〜θ_４の解が存在するように装置の設計を考慮して対処する。

4) θ ₁ to θ ₄ are obtained numerically from the above four equations, and the paraxial trajectory is determined. Under these conditions, the linear crossover 43 is not formed exactly at the center of the composite multipole element 13. The existence of a solution is not obvious. Depending on the values of a and p, θ _{1 to} θ ₄ that simultaneously satisfy the above four expressions may not exist. In this case, the crossover position of the x-orbit and the y-orbit do not coincide with each other, and stigmatic imaging at the crossover 44 cannot be performed, so that the aberration cannot be corrected. In this case, the solutions of θ _{1 to} θ ₄ are changed by changing the position of the crossover 44 to change the value of p, or changing the distance between the composite multipole lens 12 and the composite multipole lens 13 of the aberration corrector. Take into account the design of the device so that it exists.

5) Chromatic aberration at the crossover 44 for incident electron beams with different energies by changing the intensity ratio between the quadrupole electric field and the quadrupole magnetic field without changing the magnitudes of θ ₂ and θ ₃ obtained in 4) above. And the ratio of the strength of the electric field and the magnetic field is determined so as to cancel out the chromatic aberration (known) of the objective lens 17.

6) When correcting up to spherical aberration, the spherical aberration of the aberration corrector 10 is calculated under the above conditions, and the octupole field is obtained in each of the multipole lenses 11 to 14 so as to cancel the spherical aberration (known) of the objective lens 17. Correct by superimposing. At this time, since θ _{1 to} θ ₄ do not change, the paraxial trajectory does not change.

  With the above procedure, the operation conditions (excitation conditions for each quadrupole field and octupole field) of the aberration corrector 10 in the ON mode can be determined while keeping the object point of the objective lens 17 as shown in FIG. 2 constant.

  Next, if better resolution than in the OFF mode cannot be obtained in this state, the cause is insufficient adjustment of the aberration corrector 10. For example, in Non-Patent Document 3, page 319, Section 3 above. By the adjustment of the axis of the aberration corrector 10, the spherical aberration and chromatic aberration of the remaining objective lens 17 are corrected by the described method, so that the best overall resolution can be obtained.

(Example 2)
FIG. 3 shows a second embodiment of the present invention. In this example, when the aberration corrector 10 is in the OFF mode, the condenser lens 7 is used with weak excitation (broken line) as in the first embodiment, and the crossover 44 that is the object point of the objective lens 17 is used after the aberration corrector 10. Tie to the side. The switching of strong excitation / weak excitation of the second condenser lens 7 and ON / OFF of the aberration corrector 10 are linked by the computer 30. In the ON mode, the second condenser lens 7 is used with strong excitation, and the aberration corrector 10 A crossover 41 is formed on the front side (solid line). This is stigmatically imaged at the position of the crossover 44 by the aberration corrector 10 to be an object point of the objective lens 17. In the ON mode of the aberration corrector 10, the total magnification of the spots up to the crossover 44 needs to be smaller than that in the OFF mode of the aberration corrector 10. Otherwise, even if the aberration corrector 10 can adjust the axis well, the resolution is not improved because the diameter of the convergence spot 45 is originally large.

  In such use of the aberration corrector 10, the set of the multipole lens 11, the multipole lens 14, and the composite multipole lens 12 and the composite multipole lens 13 are excited with good symmetry, so that the aberration corrector 10 itself is used. Since the spherical aberration is smaller than that in the first embodiment, there is an advantage that the intensity of the octupole field for correcting the spherical aberration can be reduced.

It is a figure which shows the outline | summary of a structure of an example of the electron beam apparatus with an aberration corrector with which this invention is applied. It is a figure explaining the electron optical system of the electron beam apparatus with an aberration corrector of Example 1. FIG. It is a figure explaining the electron optical system of the electron beam apparatus with an aberration corrector of Example 2. FIG. It is a figure explaining the electron optical system in the conventional aberration corrector.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Schottky electron source, 2 ... Suppressor electrode, 3 ... Extraction electrode, 4 ... 1st anode, 5 ... 2nd anode, 6 ... 1st condenser lens, 7 ... 2nd condenser lens, 8 ... Upper deflection coil, 9 DESCRIPTION OF SYMBOLS ... Lower deflection coil, 10 ... Aberration corrector, 11 ... Multipole lens, 12 ... Composite type multipole lens, 13 ... Composite type multipole lens, 14 ... Multipole lens, 15 ... Upper scanning coil, 16 ... Lower scanning coil , 17 ... objective lens, 18 ... sample, 19 ... vacuum vessel, 20 ... electron gun power source, 21 ... control voltage source, 22 ... acceleration voltage source, 23 ... first condenser lens power source, 24 ... second condenser lens power source, 25 ... deflection coil power supply, 26 ... aberration corrector power supply, 27 ... scanning coil power supply, 28 ... objective lens power supply, 29 ... retarding power supply, 30 ... computer, 35 ... astigmatism correction coil power supply, 36 Astigmatism correction coil, 37 ... objective aligner power supply, 38 ... objective aligner, 39 ... condenser aperture, 40 ... crossover, 41 ... crossover, 42 ... linear crossover, 43 ... linear crossover, 44 ... crossover, 45: Converging spot.

Claims (2)

  An aberration corrector that functions to simultaneously cancel chromatic aberration and / or spherical aberration of the objective lens by combining an electron gun, a condenser lens, and a plurality of multipole lenses, and an objective lens that converges the electron beam on the sample surface An electron beam apparatus with an aberration corrector comprising: a scanning coil for scanning a focused electron beam on a sample surface; and a computer for controlling the lenses and the aberration corrector. The computer includes an aberration corrector. It has a scanning mode for operating and a scanning mode for not operating the aberration corrector, and the lens and the aberration corrector are controlled so that the object point position of the objective lens does not substantially change in any mode. An electron beam apparatus with an aberration corrector.   An aberration corrector that functions to simultaneously cancel chromatic aberration and / or spherical aberration of the objective lens by combining an electron gun, a condenser lens, and a plurality of multipole lenses, and an objective lens that converges the electron beam on the sample surface An electron beam apparatus with an aberration corrector comprising: a scanning coil for scanning a focused electron beam on a sample surface; and a computer for controlling the lenses and the aberration corrector. The computer includes an aberration corrector. There is a scanning mode to be operated and a scanning mode in which the aberration corrector is not operated. By switching the excitation intensity of the condenser lens and ON / OFF of the aberration corrector, the objective lens can be operated in any mode. The lens and the aberration corrector are controlled so that the object position does not substantially change. Electron beam apparatus with the aberration corrector.

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