Embodiments of the present disclosure relate to aberration correction apertures. Embodiments of the disclosure particularly relate to aberration correction elements introducing a phase shift correction of aberrations, particularly spherical aberration. Typically, embodiments relate to scanning charged particle beam apparatuses and methods of operating scanning charged particle beam apparatuses.
Charged particle beam systems are widely spread in the semiconductor industry. Examples of charged particle beam devices are electron microscopes such as secondary electron microscopes (SEM), electron beam pattern generators, ion microscopes as well as ion beam pattern generators. Charged particle beams, in particular electron beams, offer superior spatial resolution compared to photon beams, due to their short wavelengths at comparable particle energy.
In charged particle beam systems, for example probe forming systems like SEMs, aberrations are a limiting factor for the optical performance, particularly the achievable resolution. For example, a diameter of a probe is determined by the demagnification of the source, the source size, and axial aberrations such as chromatic aberrations and spherical aberrations.
For high resolution low-voltage SEMs, the minimum probe size is mainly determined by an optimization between diffraction and chromatic aberration. Accordingly, low energy width electron sources and/or utilization of a monochromators can improve the resolution. Utilizing at reduced energy width allows for larger aperture angles, which shifts the optimum trade-off between diffraction and chromatic aberration towards a small electron probe diameter. This results in an increasing aperture angle and increasing spherical aberrations, which may then also limit the achievable resolution.
In high current SEM systems, like systems for electron beam inspection (EBI) moderate spot diameters are advantageous. However, the need for increasing probe current inside the electron probe is a boundary condition due to electron-electron-interaction. Accordingly, EBI optics are also limited by spherical aberrations because a wider electron beam allows for a higher beam current, a reduced current density, or both.
A plurality of correctors for chromatic aberrations and/or spherical aberrations has been discussed. For example, multipole correctors or mirror correctors have been theoretically calculated. Only a few corrected SEMs have been built so far and are mainly used in an R&D (research and development) environment. SEMs, which are used in a production environment, for example for CD (critical dimensioning), DR (defect review), or EBI (electron beam inspection), beneficially have a high robustness. The above-mentioned correctors have a high complexity and sensitivity, which limits the robustness. In light of the above, it is beneficial to provide improved correctors, particularly correctors for spherical aberrations, charged particle beam systems having such improved correctors, particularly CD systems, DR systems, and EBI systems, and method of operating thereof.
According to one embodiment, a scanning charged particle beam apparatus is provided. The scanning charged particle beam apparatus includes a charged particle beam source configured for generating a primary charged particle beam; an objective lens configured for forming a probe on a specimen; a scanning deflection assembly configured for scanning the probe over a surface of the specimen; and an aberration correction aperture, wherein the aberration correction aperture includes an aperture body having a transparent aperture portion configured for having the primary charged particle beam pass through the transparent aperture portion; and a membrane portion including a solid material, wherein the membrane portion is provided at the transparent aperture portion and wherein the membrane portion is configured for having the primary charged particle beam pass through the solid material, wherein the membrane portion has a varying thickness.
According to another embodiment, a method of operating a scanning charged particle beam apparatus is provided the method includes generating a primary charged particle beam; and correcting aberrations of the primary charged particle beam with an aberration correction aperture, wherein at least a portion of the primary charged particle beam passes through a membrane portion of the aberration correction aperture for introducing a phase shift within the primary charged particle beam.
According to another embodiment, a scanning charged particle beam apparatus is provided. The scanning charged particle beam apparatus includes a charged particle beam source configured for generating a primary charged particle beam; an objective lens configured for forming a probe on a specimen; a scanning deflection assembly configured for scanning the probe over a surface of the specimen; and an aberration correction aperture, including an aperture body having a transparent aperture portion configured for having the primary charged particle beam pass through the transparent aperture portion; and a membrane portion including a solid material, wherein the membrane portion is provided at the transparent aperture portion and wherein the membrane portion is configured for having the primary charged particle beam pass through the solid material, wherein the membrane portion has a varying thickness. The scanning charged particle beam apparatus further includes a spray aperture, which is positioned downstream along the primary charged particle beam of the aberration correction aperture, wherein the spray aperture is configured for blocking charged particles scattered from the aberration correction aperture. For example, as an optional modification thereof, a spray aperture can be provided downstream of an element selected from the group consisting of: a monochromator, an electrostatic deflector, a magnetic deflector, a combined magnet electrostatic deflector, e.g. a Wien filter, and combinations thereof.
So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to embodiments of the disclosure and are described in the following:
Reference will now be made in detail to the various embodiments of the disclosure, one or more examples of which are illustrated in the figures. Within the following description of the drawings, the same reference numbers refer to same components. Generally, only the differences with respect to individual embodiments are described. Each example is provided by way of explanation of the disclosure and is not meant as a limitation of the disclosure. Further, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet a further embodiment. It is intended that the description includes such modifications and variations.
In scanning charged particle beam systems or scanning charged particle beam apparatuses, aberrations are one limiting factor for the optical performance, in particular the resolution. The size of the probe, which mainly determines the resolution, is not only defined by the magnification of the source and the size of the source, but also by axial aberrations, for example chromatic and spherical operations. Particularly for low voltage SEMs having for example the landing energy of the electrons on the specimen of 50 eV to 5 keV, for example 300 eV or below, previous improvements allow for using larger aperture angles. Accordingly, a further probe size reduction may have an increasing influence on spherical operations. In high-energy SEMs chromatic aberrations are as such of smaller relevance and spherical aberration correction is beneficial. Also high current systems, like systems for electron beam inspection (EBI), which have for example moderate spot diameters in the range of 5 nm to 100 nm, the resolution of the scanning charged particle beam apparatus is limited by spherical operations to a significant amount. Accordingly, correction of spherical aberrations is beneficial.
Embodiments described herein provide an improved aberration correction including an aberration correction aperture 200 (see
According to some embodiments, which can be combined with other embodiments described herein, a membrane of solid material, such as a thin membrane of solid material, is provided in the aberration correction aperture 200. The aberration correction aperture 200 includes an aperture body 210 having a transparent aperture portion. The transparent aperture portion is configured for having the primary charged particle beam passing through the transparent aperture portion. For example the transparent aperture portion can be an aperture opening or a thin membrane. A membrane portion 220 including a solid material is provided in the aberration correction aperture. The charged particle beam passes through the solid material, wherein a phase shift is provided to the charged particle beam. A varying thickness of the membrane portion introduces a phase shift e.g. for reducing aberrations, particularly spherical aberrations. Further details regarding the aberration correction aperture 200 are described with respect to
The extractor 112 accelerates the primary charged particle beam to a high column energy, for example 10 keV or above, 12 keV or above, such as for example 30 keV. Also column energies above 30 eV can be provided according to embodiments described herein. The primary charged particle beam can be guided at the high column energy in tube 113. According to embodiments described herein, an electrode arrangement for accelerating the primary charged particle beam can be provided. For example, the electrode arrangement for acceleration can include one or more of the extractor 112, the tube 113, and a further electrode having a high potential as compared to the emitter tip. According to some embodiments, which can be combined with other embodiments described herein, a scanning charged particle beam apparatus as described herein kay have a charged particle beam source having an energy width of 0.8 eV or below, e.g. wherein the charged particle beam source is a field emitter; and wherein the scanning charged particle beam apparatus further comprises an electrode arrangement for accelerating the primary charged particle beam to a column energy of 10 keV or above, e.g. 30 keV or above.
A deceleration electrode 138 decelerates the charged particles to low landing energy for impingement on the specimen 140. For example, the specimen 140 can be provided on the specimen support 142. The specimen support 142 can be a movable stage for positioning the specimen 140. For example, the movable stage can be configured for moving the specimen 140 in one direction (e.g. X direction) or in two directions (e.g. X-Y-directions).
In the example shown in
As shown in
According to some embodiments, which can be combined with other embodiments described herein, the aberration correction aperture can be the beam limiting aperture of the scanning charged particle beam apparatus. Charged particle beam apparatuses normally have one or more apertures, which determine the system aperture angle of the scanning charged particle beam apparatus. To reduce the spot size on the specimen, i.e. the size of the probe formed by the primary charged particle beam on the specimen, lens aberrations are corrected. According to embodiments described herein, the lens aberrations are corrected for the portion of the primary charged particle beam, which is within the system aperture angle of the scanning charged particle beam apparatus. Accordingly, large correction elements with wide correction areas, which cover also portions of the primary charged particle beam, which are not used for forming of the probe on the specimen, can be avoided. For example multipole correctors may act on all charged particles in the primary charged particle beam in a wide area. Thus, according to some embodiments, correction of the primary charged particle beam can be limited to those portions of the primary charged particle beam which ultimately impinge on the specimen.
The transparent aperture portion 212 and the membrane portion 220 is provided with a thickness of e.g. 1 nm to 100 nm. For example the transparent aperture portion can have a thickness of 25 nm, and the membrane portion can have a thickness of maximum 75 nm. Accordingly, according to some embodiments, which can be combined with other embodiments described herein, the total maximum thickness of the transfer and aperture portion together with the membrane portion can be 100 nm or below, for example 80 nm or below, such as 10 nm to 75 nm.
Particularly the membrane portion can be made of light material to minimize electron scattering. For example materials like silicon, carbon, silicon oxide, silicon nitride can be used. Optionally, also the transparent aperture portion can be provided from a similar material or the same material.
According to yet further embodiments, which can be combined with other embodiments described herein, the transparent aperture portion 212 can also be an opening in the aperture body 210. Having an opening in the aperture body 210 might increase the complexity to manufacture, attach or provide the membrane portion at or within the transparent aperture portion. Accordingly, embodiments having a transparent aperture portion 212 with a thickness larger than zero, for example a thin carrier for a membrane portion, allow for easier manufacturing of the aberration correction aperture.
As shown for example in
According to embodiments described herein, the membrane portion 220 and the transparent aperture portion 212 have a rotational symmetry, particularly around the center of the aperture opening, which may for example coincide with optical axis 102 of the penetrating charged particle beam. Yet further, the aberration correction aperture and/or the aperture body may optionally also share the same rotational symmetry. Accordingly, in light of the rotational symmetry, the varying thickness is provided in a radial direction from the center of symmetry, e.g. the optical axis 102.
The charged particle beam experiences in its radial direction different phase shifts according to the different number of membrane atoms involved when the charged particle beam passes through the solid material. The radial thickness distribution of the membrane portion is configured such that the phase shift inside the membrane compensates spherical aberrations, for example spherical aberrations of the objective lens, spherical aberrations of the condenser lens, or spherical aberrations of the objective lens and the condenser lens. Accordingly, a spherical aberration correction or a reduction of spherical aberrations can be achieved.
As for example shown in
A membrane or an area through which the charged particle beam passes can include a transparent aperture portion, i.e. a bulk area, for example in which the beam aperture diameter is defined, and a thin membrane, e.g. a membrane portion 220, whose thickness increases radially for phase shift generation which is used for aberration correction. According to some embodiments, in order to keep electron scattering small the membrane portion can be thinner at its center. The radial thickness increase, which generates the intended phase shift, is determined by the aberration coefficient of the lenses used.
In the further example of an aberration correction aperture 200, the membrane portion 220 is provided on top of the transparent portion within the aperture body 210. In light of the above, according to different examples, manufacturing of a membrane, through which the charged particle beam passes for aberration correction, can be provided by different techniques. The shaped membrane can be manufactured as one piece. For example, according to some embodiments, the membrane can be as thin as possible in the center thereof, wherein the thickness increases towards the rim, i.e. with increasing radius. This is for example shown in
According to yet further embodiments, which can be combined with other embodiments described herein, a beneficial aberration correction aperture 200 can be provided as shown in
According to other implementations (see for example
According to embodiments described herein, the membrane portion can be a part of the beam limiting aperture, i.e. the final beam aperture or the system aperture. Accordingly, the membrane portion is provided in the system aperture defining the overall system aperture. This can result in a self-centering effect of the aberration correction aperture.
According to yet further embodiments, which can be combined with other embodiments described herein, also the membrane portion and the transparent aperture portion can be made of the conductive material. Portions of the aberration correction aperture 200 being made of a conducting material enables to bias the aberration correction aperture, e.g. the portion being in direct contact with the charged particle beam, to the respective potential of the column, at which the aberration correction aperture is positioned. Accordingly, an electrostatic influence of the aberration correction aperture on the charged particle beam can beneficially be reduced or avoided. If the aperture and/or the membrane is not electrically conductive, the aperture and/or the membrane can be coated by an additional thin electrically conductive coating, e.g. by sputtering a few nanometer of carbon. According to yet further additional or alternative embodiments, one or more layers can be provided on the membrane for reduction or prevention of contamination and/or for reduction or prevention of charging on the membrane. Particularly layers or membrane materials for preventing or reduction of charging of the membrane can be beneficial for maintaining the membrane at the same potential as compared to the column of the scanning charged particle beam apparatus.
Different aspects of the thickness distribution, which can be utilized according to embodiments described herein, are described with respect to
The thickness distribution can be determined by using one or more of the aberration coefficients, wherein for example values for a spherical aberration coefficient CS and/or the chromatic aberration coefficient CC related to the sample side can be in the range of 0.2 mm to 10 mm. Further, additionally or alternatively, phase shift calculations using the material properties of the membrane portion 220 and/or the transparent aperture portion 212 can be utilized for determining the thickness distribution.
According to some embodiments, a thickness increase from the center can follow a polynomial as function of radius.
According to yet further embodiments, which can be combined with other embodiments described herein, the thickness 503 of the membrane at the center can also have a local maximum, in order to provide a defocusing of the charged particle beam. This is shown exemplarily in
An aberration correction aperture 200 is provided in the scanning charged particle beam apparatus 100. Particularly, the aberration correction aperture can be provided as a beam limiting aperture, i.e. the aperture defining the system aperture of the scanning charged particle beam apparatus. The aberration correction aperture 200 includes a membrane of solid material, such as a thin membrane of solid material. The charged particle beam passes through the solid material, wherein a phase shift is provided to the charged particle beam. A varying thickness of the membrane portion introduces a phase shift for reducing aberrations, particularly spherical aberrations. Further details regarding the aberration correction aperture 200 are described with respect to
To eliminate scattered electrons which are generated within the membrane and which may increase spot size or reduce contrast, a spray aperture 610 may be introduced according to some embodiments, which can be combined with other embodiments described herein. Since scattered electrons will have experienced energy losses, beneficial positions of the spray aperture are e.g. behind the condenser lens 120 or behind deflection systems (see e.g. deflection stages 732-734 in
As shown in
The deflection system provided by the first deflection stage 732 and the second deflection stage 734 is further configured to separate a primary charged particle beam from a signal charged particle beam. Signal charged particles, which are generated on impingement of the primary charged particle beam, i.e. the probe, on the specimen 140, are accelerated towards the objective lens 130 and through the objective lens. In light of the velocity of the signal charged particles as compared to the primary charged particles, the signal charged particles are deflected by the second deflection stage 734 towards the detector 740. According to other embodiments, additionally or alternatively, on-axis detectors may be used for detecting the signal charged particles generated on impingement of the probe on the specimen.
The scanning charged particle beam apparatus as exemplarily shown with respect to
According to some embodiments described herein, the aberration correction aperture 200 is beneficially provided as a beam limiting aperture, wherein only charged particles participating in forming the probe on the specimen pass through the correction aberration aperture and are provided with a phase shift therein. The phase shift depends on the radial position on the membrane portion, which is the distance from the optical axis.
According to other embodiments, which can be combined with the embodiments, the aberration correction aperture and the beam limiting aperture can be provided as two different components along the optical axis. This can be realized close to each other in z-direction (along the optical axis), for example at the distance of 20 mm or below, wherein a separation between the beam limiting aperture and the aberration correction aperture has less influence on the optical properties but may be rather motivated by manufacturing capabilities. Alternatively, the aberration correction aperture and the beam limiting aperture can be separated along the path of the charged particle beam, wherein for example an optimization for another function in the beam pass is provided. For example, the beam limiting aperture can be provided in correlation with a beam blanking aperture. According to yet further embodiments, which can be combined with other embodiments described herein, a spray aperture 610 as shown in
According to embodiments described herein, the charged particle beam source 110 can be provided as a field emitter, for example TFE or CFE cathodes, having a low energy width. For example, the energy width can be 0.8 eV or below. The small energy width can shift the optimum system aperture of the scanning charged particle beam apparatus to larger aperture angles due to a reduced chromatic aberration. Accordingly, spherical aberration correction, particularly with an improved aberration correction aperture as described herein, is beneficial for systems having a low energy width in the primary charged particle beam.
In light thereof, according to some embodiments, which can be combined with other embodiments described herein, the scanning charged particle beam apparatus 100 shown in
According to yet further embodiments, which can be combined with other embodiments described herein, the beam energy of the charged particle beam within the column, for example within tube 113 and/or between an extractor 112 and a deceleration electrode 138 can be 10 keV or above, e.g. 30 keV or above. The high beam energy within the column of the scanning charged particle beam apparatus reduces chromatic aberrations and further allows charged particles of the charged particle beam to more easily pass through the solid material of the membrane portion and, if existent, a solid material of the transparent aperture portion of aberration correction apertures described herein.
As shown in
According to various implementations, a monochromator can for example be provided as a filter lens, an Ω-filter, a Wien filter, or a mirror filter. According to some embodiments, for scanning charged particle beam apparatuses utilizing a CFE source, a filter lens type monochromators might be a beneficial solution in light of its simplicity. According to embodiments, which can be combined with other embodiments described herein, a filter lens-type monochromator may include a retarding lens adapted to be a high-pass energy filter for the primary charged particle beam.
For generation of different spot sizes in the scanning charged particle beam apparatus, different demagnifications can be provided in the optical system. Accordingly, different focal lengths of the lenses, for example the condenser lens and the objective lens, are provided. As a result different aperture opening sizes may be utilized, which can further result in a variation of the aberration coefficients of the one or more lenses, which are to be corrected. In light thereof, according to some embodiments, the aperture membrane, i.e. membrane portion, can be designed for two or more spot sizes. A membrane area and/or thickness distribution can be provided for two or more spot sizes in the scanning charged particle beam apparatus. Accordingly one or more, for example two, three, four, five, or six membrane portions can be provided.
In the example shown in
According to yet further embodiments, which can be combined with other embodiments described herein, the aberration correction aperture can be provided either as a single aperture having one membrane portion or as a multi-aperture having two or more membrane portions. For example, the aberration correction aperture can be fixed in location relative to the charged particle beam column or can be movable to be mechanically aligned within the column. Multi-aperture aberration correction apertures allow for a fast selection of individual charged particle beam paths, for example for different system demagnifications, which may be introduced by specific condenser lens and objective lens settings. Accordingly, different modes of operation can be addressed, wherein between two or more membrane portions can be switched for the modes of operation.
According to yet further embodiments, a multi-aperture may also be provided to generate two or more beamlets from one primary charged particle beam. That is, two or more beamlets are generated simultaneously and the plurality of beamlets has a center rotational axis or optical axis. In other words, the optical axis of the scanning charged particle beam device is not deflected in this area of the aberration correction aperture. In such a case, a rotational symmetry can be provided for the assembly of membrane portions. For example, the assembly of two or more membrane portions can have a similar thickness distribution in the regions of the individual membrane as compared to one large membrane for all beamlets, which would rotation symmetric. Each individual membrane portion which is provided off-axis from the optical axis along which the primary charged particle beam, i.e. the primary charged particle beam being divided into beamlets, travels, might in such a case not include a rotational symmetry. According to embodiments described herein, a rotational symmetry of one membrane portion or a rotational symmetry of two or more membrane portions (n-fold symmetry for an assembly of n membranes) can be provided with respect to the optical axis of the primary charged particle beam. The primary charged particle beam may be provided by one charged particle beam traveling along the optical axis or by two or more beamlets of the primary charged particle beam, wherein each beamlet may travel off-axis.
The present disclosure provides a simple solution, e.g. for spherical aberration correction. Due to the high robustness of the aberration correction apertures described herein, the lack of spherical aberration correction in SEMs, which are used in a production environment, can be overcome. According to some embodiments, which can be combined with other embodiments described herein, the scanning charged particle beam devices can particularly be SEMs for CD (critical dimensioning), DR (defect review), or EBI (electron beam inspection), and beneficially have a high robustness. In particular for EBI, spherical aberration correction not only reduces the spherical aberration itself but also reduces the influence of electron-electron interaction based upon the following effect. By enabling larger aperture angles the current inside the optical system can be spread over a wider aperture angle area which reduces electron-electron interaction. According to yet further embodiments, which can be combined with other embodiments described herein, the membrane portions and particularly the varying thickness thereof can be configured for additionally providing correction of chromatic aberrations. The thickness distribution of the membrane portions, for example the thickness distribution having a rotational symmetry, can be designed to additionally correct chromatic aberration in addition to the correction of spherical aberrations.
While the foregoing is directed to embodiments, other and further embodiments may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
The present application claims priority to U.S. Application No. 62/093,065, filed Dec. 17, 2014, the entire contents of which are incorporated by reference herein for all purposes.
Number | Date | Country | |
---|---|---|---|
62093065 | Dec 2014 | US |