Charged particle detection system and multi-beamlet inspection system

A charged particle detection system comprises plural detection elements and a multi-aperture plate in proximity of the detection elements. Charged particle beamlets can traverse the apertures of the multi-aperture plate to be incident on the detection elements. More than one multi-aperture plate can be provided to form a stack of multi-aperture plates in proximity of the detector. A suitable electric potential supplied to the multi-aperture plate can have an energy filtering property for the plural charged particle beamlets traversing the apertures of the plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to charged particle detection and inspection systems, and the invention in particular relates to such systems using multiple beamlets of charged particles.

2. Brief Description of Related Art

A conventional multi-beamlet inspection system is known from WO 2005/024881. The multi-beamlet inspection system disclosed therein is used for inspecting an object, such as a semiconductor wafer. A plurality of primary electron beamlets is focussed in parallel to each other to form an array of primary electron beam spots on the object. Secondary electrons generated from the primary electrons and emanating from respective primary electron beam spots are received by a charged particle imaging optics to form a corresponding array of secondary electron beamlets which are supplied to an electron detection system having an array of detection elements such that each secondary electron beamlet is incident on a separate detection element. Detection signals generated from the detection elements are indicative of properties of the object at those locations where the primary electron beam spots are formed.

By scanning the array of primary electron beam spots across the surface of the object, it is possible to obtain an electron microscopic image of the object. It is desirable to obtain images of a great number of objects per time such that a high throughput can be achieved. For this purpose it is desirable to obtain electron optical images of the inspected surface having a high contrast.

Conventional electron inspection systems using a single primary electron beam, such as scanning electron microscopes (SEMS) use energy filters for increasing image contrast. The energy filter allows secondary electrons exceeding a threshold energy to traverse the filter and to be incident on a detector, whereas secondary electrons having a kinetic energy below the threshold energy are rejected and not allowed to be incident on the detector. Such conventional energy filters used in scanning electron microscopes may comprise a grid electrode disposed in the secondary electron beam path between the surface of the object and an objective lens receiving the electron beam.

It is desirable to have the feature of energy filtering also available in charged particle systems using an array of multiple charged particle beamlets.

SUMMARY OF THE INVENTION

The present invention has been accomplished taking the above problems into consideration.

Embodiments of the present invention provide a charged particle detection system comprising a detector having an array of plural detection elements for detecting charged particles and having an energy filtering property.

Other embodiments of the present invention provide a charged particle detection system comprising a detector having an array of plural detection elements for detecting charged particles and a property for reducing cross talk between charged particle beamlets incident on the detection elements.

According to an exemplary embodiment of the invention, the charged particle detection system comprises a detector having an array of plural detection elements and a first aperture plate having a first array of plural apertures to be traversed by charged particle beamlets, wherein the first aperture plate is disposed at a first distance from the detector. The charged particle detection system further comprises a voltage supply for supplying electric potentials to the first detector and the first aperture plate, and the apertures of the first aperture plate and the detection elements of the first detector are aligned relative to each other such that plural beamlets of charged particles can each traverse an aperture of the first aperture plate to be incident on a detection element of the first detector. The electric potentials supplied to the first detector and the first aperture plate can be provided such that only charged particles of a beamlet having a kinetic energy greater than a threshold energy can traverse the respective aperture of the first aperture plate to be incident on the respective detection element. The other charged particles having a kinetic energy below the threshold energy are then not able to traverse the aperture, and they are not able to be incident on the detection element, accordingly.

According to a particular embodiment herein, the charged particle detection system further comprises a second aperture plate having a second array of plural apertures to be traversed by the charged particle and disposed at a second distance from the detector which is greater than the first distance. The voltage supply is configured to supply an electric potential also to the second aperture plate.

Moreover, the voltages can be supplied such that the first and second aperture plates provide a focussing effect on each of the plurality of the charged particle beamlets traversing the aperture plates such that a cross section of each of the beamlets is reducing with decreasing distance from the detection element. This will reduce a probability for existence of charged particle trajectories which traverse a given aperture of the first aperture plate and are then incident on a detection element adjacent to the one detection element associated with the given aperture which was traversed.

According to some embodiments, a value of the first distance can be within an exemplary range from 6 mm to 20 mm. According to some embodiments herein, values of the second distance can be within an exemplary range from 10 mm to 30 mm. According to some other embodiments herein, values of the second distance can be greater than the values of the first distance by an amount within an exemplary range from 2 mm to 20 mm.

According to exemplary embodiments using negative particles, such as electrons or negatively charged ions, as the charged particles, the voltage supply is configured to apply a first electric potential to the first detector which is, relative to a reference potential, greater than a second electric potential applied to the first aperture plate. With such configuration it is possible that the charged particles having a kinetic energy lower than the threshold energy do not traverse the apertures of the first aperture plate whereas the charged particles having a kinetic energy greater than the threshold energy can traverse the apertures of the first aperture plate and get accelerated towards the detector to be incident on respective detection elements. In embodiments using positive particles, such as positively charged ions, as the charged particles, the first electric potential can be smaller than the second electric potential.

In embodiments having the second aperture plate, the voltage supply can be configured to apply a third electric potential to the second aperture plate, which is, relative to the reference potential, greater than the second electric potential applied to the first aperture plate. With such arrangement it is possible to obtain a focussing of a charged particle beamlet traversing an aperture of the second aperture plate towards a corresponding aperture of the first aperture plate such that an accuracy of the energy filtering property as well as prevention of cross talk are improved.

According to further exemplary embodiments, the charged particle detection system comprises a third aperture plate having an array of plural apertures disposed in-between the first detector and the first aperture plate, wherein the voltage supply is further configured to apply a fourth electric potential to the third aperture plate. According to exemplary embodiments herein, the fourth electric potential can be in-between the electric potentials applied to the first detector and the first aperture plate. Such arrangement can be helpful in further improving the energy filtering property and in prevention of cross talk between adjacent elements.

According to further exemplary embodiments, the charged particle detection system comprises a fourth aperture plate having an array of plural apertures disposed in-between the first aperture plate and the second aperture plate, wherein the voltage supply is further configured to apply a fifth electric potential to the fourth aperture plate. According to particular embodiments herein, the fifth electric potential can have a value in-between a value of the electric potential applied to the first aperture plate and a value of the electric potential applied to the second aperture plate. Such arrangement can be advantageous with respect to focussing a charged particle beamlet towards a respective aperture of the first aperture plate in view of improving an energy filtering property.

According to further exemplary embodiments, the charged particle detection system comprises at least one charged particle lens disposed at a distance from the detector which is greater than a distance of the first aperture plate or, in embodiments having the second aperture plate, is greater than a distance of the second aperture plate from the detector. The at least one charged particle lens is configured to receive plural charged particle beamlets and to direct the plural charged particle beamlets towards the first and second, respectively, aperture plates such that each charged particle beamlet traverses a respective corresponding aperture of the aperture plate. The at least one charged particle lens may comprise an electrostatic lens providing an electrostatic field and a magnetic lens providing a magnetic field and a combination thereof providing both an electrostatic field and a magnetic field. According to embodiments therein, the at least one charged particle lens has a bore which is commonly traversed by the plurality of charged particle beamlets.

According to further embodiments, the charged particle detection system comprises a beam splitter configured to separate trajectories of the charged particle beamlets directed towards the first detector from trajectories of charged particles contained in those beamlets and rejected by the energy filtering property of the first multi-aperture plate or combination of first, second, third and fourth aperture plates in those embodiments where these are available.

In a particular embodiment herein, the charged particle detection system further comprises a second detector disposed such that charged particles rejected by the energy filtering property are incident on the second detector to generate a corresponding detection signal. Such arrangement allows to determine a number or proportion of charged particles contained in the charged particle beamlets directed to the aperture plate having a kinetic energy below the threshold energy determined by the electric potentials applied to the one or more aperture plates and the detector. According to particular embodiments herein, the second detector may comprise a plurality of detection elements. The number of detection elements of the second detector can be less than the number of detection elements of the first detector, and it is also possible that the first and second detectors have a same number of detection elements.

According to other exemplary embodiments of the present invention, a multi-beamlet inspection system for inspecting a substrate is provided, wherein the system comprises: a charged particle detection system; a charged particle source for generating a first array of charged particle beamlets; first beam shaping optics for directing the array of charged particle beamlets onto the substrate to form an array of spots illuminated with charged particles on the substrate; and second beam shaping optics for receiving charged particles emanating from the substrate and directing the received charged particles as a second array of charged particle beamlets towards the charged particle detection system; wherein the charged particle detection system comprises: a first detector having an array of plural detection elements for detecting charged particles; a first aperture plate having a first array of plural apertures to be traversed by charged particles and disposed at a first distance from the first detector; a second aperture plate having a second array of plural apertures to be traversed by charged particles and disposed at a second distance from the first detector, the second distance being greater than the first distance; and a voltage supply for supplying electric potentials to the first detector, the first aperture plate and the second aperture plate; wherein the apertures of the first aperture plate, the apertures of the second aperture plate and the detection elements of the first detector are aligned relative to each other such that plural beamlets of charged particles can each traverse an aperture of the first aperture plate and an aperture of the second aperture plate to be incident on a detection element of the first detector.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the exemplary embodiments described below, components that are alike in function and structure are designated as far as possible by alike reference numerals. Therefore, to understand the features of the individual components of a specific embodiment, the descriptions of other embodiments and of the summary of the invention should be referred to.

FIG. 1is a schematic diagram symbolically illustrating basic functions and features of a multi-beamlet inspection system. The inspection system generates a plurality of primary electron beamlets which are incident on a substrate to be inspected to produce secondary electrons emanating from the surface which are subsequently detected. While the illustrated embodiment uses electrons as primary particles incident on the substrate and as secondary particles released from the substrate, it is also possible to use other sorts of energy, such as beamlets of incident light, and beamlets of other charged particles such as protons and helium ions to produce secondary charged particles which are subsequently detected. Also the secondary charged particles can be different from electrons.

The multi-beamlet electron inspection system1is of a scanning electron microscope type (SEM) using a plurality of primary electron beamlets3for generating primary electron beam spots5on a surface of the substrate7to be inspected. The inspected substrate7can be of any type and may comprise, for example, a semiconductor wafer and a biological sample and arrangement of miniaturized features of other types. The surface of the substrate7is arranged in an object plane101of an objective lens102of an objective lens system100.

Insert I1ofFIG. 1shows an elevational view of object plane101with a regular rectangular array103of primary electron beam spots5formed thereon. InFIG. 1a number of 25 primary electron beam spots are arranged as a 5×5-array103. This number of 25 primary electron beam spots is a low number chosen for ease of illustration in the schematic diagram ofFIG. 1. In practise, the number of primary electron beam spots may be chosen substantially higher, such as 30×50, 100×100 and others.

In the illustrated embodiment, the array103of primary electron beam spots5is a substantially regular rectangular array with a substantially constant pitch p1between adjacent beam spots. Exemplary values of p1are 1 μm and 10 μm. It is however also possible that the array103is a distorted regular array having different pitches in different directions, and the array may also have other symmetries, such as a hexagonal symmetry.

A diameter of the primary electron beam spots formed in the object plane101can be small. Exemplary values of such diameter are 5 nm, 100 nm and 200 nm. The focussing of the primary electron beamlets3to form the primary electron beam spots5is performed by the objective lens system100.

The primary electrons incident on the substrate7at the beam spots5produce secondary electrons emanating from the surface of the substrate7. The secondary electrons emanating from the surface of the substrate7are received by the objective lens102to form secondary electron beamlets9. The inspection system1provides a secondary electron beam path11for supplying the plurality of secondary electron beamlets9to a charged particle detection system200. The detection system200comprises a projection lens arrangement205for directing the secondary electron beamlets9towards a detector207. The detector is a detector having plural detection elements and may comprise a CCD detector, a CMOS detector, a scintillator detector, a micro-channel plate, an array of PIN-diodes and others and suitable combinations thereof.

Insert I2ofFIG. 1shows an elevational view of the detector207, wherein secondary electron beam spots213are formed on individual detection elements215which are arranged as an array217having a regular pitch p2. Exemplary values of the pitch p2are 10 μm, 100 μm and 200 μm.

The primary electron beamlets3are generated by a beamlet generation system300comprising at least one electron source301, at least one collimating lens303, a multi-aperture plate arrangement305and a field lens307.

The electron source301generates a diverging electron beam309which is collimated by collimating lens303to form a beam311illuminating the multi-aperture arrangement305.

Insert I3ofFIG. 1shows an elevational view of the multi-aperture arrangement305. The multi-aperture arrangement305comprises a multi-aperture plate313having a plurality of apertures315formed therein. Centers317of the apertures315are arranged in a pattern319corresponding to the pattern103of the primary electron beam spots5formed in the object plane101. A pitch p3of array103may have exemplary values of 5 μm, 100 μm and 200 μm. Diameters D of the apertures315are less than the pitch p3. Exemplary values of the diameters D are 0.2·p3, 0.4·p3and 0.8·p3.

Electrons of the illuminating beam311traversing the apertures315form the primary electron beamlets3. Electrons of illuminating beam311impinging on the plate313are intercepted by the plate and do not contribute to forming the primary electron beamlets3.

Moreover, the multi-aperture arrangement305focuses the individual electron beamlets3such that foci323are generated in a plane325. Insert I4ofFIG. 1shows an elevational view of plane325with foci323arranged in a pattern327. A pitch p4of pattern327may be equal to or different from the pitch p3of pattern319of the multi-aperture plate313. A diameter of foci323may have exemplary values of 10 nm, 100 nm and 1 μm.

The field lens307and the objective lens102provide an imaging system for imaging the plane325onto the object plane101to form the array103of primary electron beam spots5on the surface of the substrate7.

A beam splitter system400is provided in the primary electron beam path313in-between the beam generating system300and the objective lens system100. The beam splitter system400is also part of the secondary electron beam path11such that the beam splitter system400is located in-between the objective lens system100and the detection system200.

Background information relating to such beamlet inspection system and charged particle components used therein, such as charged particle sources, multi-aperture plates and lenses may be obtained from WO 2005/024881, WO 2007/028595, WO 2007/028596 and WO 2007/060017 by the same assignees wherein the full disclosure of these applications is incorporated herein by reference.

FIG. 2is a more detailed schematic illustration of the charged particle detection system200of the multi-beamlet inspection system1.FIG. 2shows a charged particle beamlet bundle12having an exemplary low number of three secondary electron beamlets9. This low number of secondary electron beamlets9was chosen for illustration purposes only, and this number can be significantly higher in practise as already illustrated as above.

A bundle12of secondary electron beamlets9is supplied to the detection system200from the beam splitting system400. The projection lens system205receiving the beamlets9from the beam splitter is shown in this embodiment to comprise a magnetic lens221having a coil222for generating a magnetic field, and an electrostatic lens225having two plate electrodes226and227. The plate electrodes226,227each have a circular aperture228which is commonly traversed by all beamlets9of the bundle12.

The projection lens arrangement205shapes the whole bundle12of beamlets9and the individual beamlets9such that they traverse the respective apertures251and are directed towards the detection elements215of the detector207.

A control portion231of a control system of the multi-beamlet inspection system is provided for supplying a suitable excitation current to the coil222and suitable electric potentials to the plate electrodes226and227. The control portion231may also supply suitable control signals such as currents and electric potentials to the beam splitting system400.

A plurality of multi-aperture plates241,242,243and244are disposed in the secondary electron beam path11upstream of the detector207. The multi-aperture plates241to244are spaced apart from each other and from the detector207. In particular, multi-aperture plate241has a distance d1from a surface of the detection elements213, wherein d1has exemplary values of from 6 mm to 20 mm. Multi-aperture plate242is disposed at a distance d2from the surfaces of the detection elements213, wherein d2may have exemplary values from 10 mm to 30 mm, such that d2is greater than d1by an amount of 2 mm to 20 mm.

Multi-aperture plate243is disposed in-between multi-aperture plate241and the detector207at a distance d3therefrom. Exemplary values of a difference d1-d3can be from 1 mm to 5 mm.

Each of the multi-aperture plates241to244has an array of plural apertures251which are arranged such that they are traversed by the secondary electron beamlets9on their way between the projection lens arrangement205and the detection elements213. One aperture251of a plate is traversed by one beamlet9, and different beamlets9traverse different apertures251of each plate.

In the illustration ofFIG. 2, the beamlets are shown to be orthogonally incident on the detector, and the multi-aperture plates241to244are shown to have identically aligned apertures251of equal shape. It is, however, possible that the apertures251of different multi-aperture plates are somewhat displaced relative to each other for manipulating the beamlets traversing the apertures in certain ways, and it is also possible that the beamlets are not orthogonally incident on the detector207. Moreover, it is possible that a direction of incidence on the detector of individual beamlets may vary across the beamlet bundle, and that a displacement of apertures of one plate relative to corresponding apertures of another plate varies across the array of apertures of the one plate. Still further, the multi-aperture plates shown in the illustration have surfaces which are parallel to each other. It is, however, also possible that one or more of the plates have one or two curved surfaces, and it is also possible to tilt one or more of the plates relative to the detector. Further, it is possible that the apertures of different aperture plates have different diameters. For example, an aperture plate closer to the detector may have apertures of a greater diameter than an aperture plate farther away from the detector. Still further, different aperture plates may have different thicknesses such that, for example, an aperture plate closer to the detector has a greater thickness than an aperture plate farther away from the detector. Background information on the effects of variations of aperture locations and surface curvatures of multi-aperture plates can be taken from WO 2005/024881, WO 2007/028595 and WO 2007/028596.

A voltage supply261which can be a portion of a control system of the multi-beamlet inspection system1is provided to supply electrical potentials relative to a reference potential260, which is ground potential in this embodiment, to the detector207and the multi-aperture plates241to244. The electric potentials supplied to the multi-aperture plates influence trajectories of charged particles within the individual beamlets9as well as the kinetic energies of those particles. In the embodiment shown inFIG. 2, the electric potentials are supplied to the multi-aperture plates such that electrons262having a kinetic energy above a given threshold can traverse the apertures251of multi-aperture plate241and, subsequently, the apertures251of multi-aperture plate243to be incident on the detection elements215. Electrons having a kinetic energy below the given threshold can not traverse the apertures251of multi-aperture plate241and are reflected from multi-aperture plate241. The reflected electrons can be incident on the multi-aperture plate244, and they can be directed such that they traverse the apertures251of one or both of multi-aperture plates244and, subsequently,242as illustrated by arrows263.

The multi-aperture plate241has a function of an energy filter, accordingly.

The multi-aperture plates242and244have a function of adjusting the kinetic energies and directions of the electrons within beamlets9such that the energy filter has a high performance of selecting the electrons capable of reaching the detection elements. It is of course desirable that all electrons having a kinetic energy equal to or greater than the threshold energy are allowed to reach the detection elements wherein all other electrons are rejected. In practise, however, such exact step function of transmission in dependence of kinetic energy is not achievable since the electrons within the beams travel under a plurality of angles relative to a main axis of the beamlet9such that also electrons having a kinetic energy higher than the threshold but travelling under an angle relative to the beamlet axis get rejected.

In the embodiment shown inFIG. 2, the multi-aperture plates242and244perform a function of improving directions of incidence of the electrons relative to a plane of the multi-aperture plate241and of manipulating the beamlets such that they maintain a relatively low diameter. For this purpose, the multi-aperture plates242,244and241perform a function of electrostatic lenses which can be seen in the schematic representation ofFIG. 2from the varying diameter of the beamlets9between the plates, and, in particular, the formation of a cross over within the beamlets with the multi-aperture plates244and241.

The multi-aperture plate243has a function of forming an electrostatic lens together with multi-aperture plate241to accelerate and focus the beamlets having traversed the multi-aperture plate241towards the detection elements251.

While the above illustrated embodiment comprises four multi-aperture plates disposed upstream of the detector and at a close distance from each other, it is also possible that other embodiments comprise only one or two or three multi-aperture plates or more than four aperture plates such as five or six or more aperture plates in proximity of the detector.

Distances of multi-aperture plates from a detector and electric potentials applied to these multi-aperture plates of an exemplary embodiment having four multi-aperture plates and using electrons as the charged particles are listed in table 1 below.

Distances of multi-aperture plates from a detector and electric potentials applied to these multi-aperture plates of a further exemplary embodiment having four multi-aperture plates are and using electrons as the charged particles listed in table 2 below.

In the embodiment shown inFIG. 2, only the secondary electrons having a kinetic energy above the given threshold are detected by detector207. The rejected secondary electrons having a kinetic energy below the threshold can be absorbed by multi-aperture plates244or242, or they can leave the stack of multi-aperture plates241,242,243and244to travel in a direction towards the beam splitting system400. In practise, those electrons will be incident on some mounting structure or vacuum vessel of the multi-beamlet inspection system. However, the inventor found it desirable to also detect the electrons having the kinetic energy below the threshold since also these electrons carry some information on the inspected object. The embodiment shown inFIG. 3provides a solution to this.

FIG. 3shows a detection system200aof a multi-beamlet inspection system1awhich can have a similar configuration as that shown inFIG. 1. The detection system200ahas a similar configuration as the detection system shown inFIG. 2in that a stack240aof plural multi-aperture plates241a,242aand243ais disposed in a beam path of secondary electron beamlets9abetween a projection lens arrangement225aand a detector207ahaving plural detection elements215a. The stack240aof multi-aperture plates241a,242a,243aperforms a function of an energy filter such that that electrons of the beamlets9ahaving a kinetic energy greater than a threshold are allowed to be incident on the detection elements215a, whereas electrons having a lower kinetic energy are rejected. A portion of those rejected electrons travels back towards the projection lens arrangement225a. The detection system200ashown inFIG. 3differs from the detection system shown inFIG. 2in that a beam splitter271is disposed in a beam path between the projection lens arrangement225aand the stack240aof multi-aperture plates. In the shown embodiment the beam splitter271is configured such that the secondary electron beamlets9acan traverse the beam splitter271along a substantially un-deflected straight line whereas trajectories of beamlets273of electrons having the energy lower than the threshold are deflected by a predetermined angle to be incident on detection elements277of a detector275. The beam splitter271can be embodied as a space in which orthogonal magnetic and electric fields are provided such that charged particles travelling in one direction are transmitted along a straight line whereas charged particles travelling in the opposite direction are deflected by a given angle. In other embodiments, the beam splitter can be configured such that both the secondary electron beamlets9aand the beamlets273are deflected by given angles such that none of the beamlets necessarily travel along straight lines.

For improving the kinetic energy at which the particles of beamlets273are incident on the detection elements277and to avoid cross talk between adjacent detection elements277, additional beamlet manipulating elements can be disposed in the beam path of beamlets273.FIG. 3schematically illustrates two plate electrodes281providing a global lens effect to all beamlets273. Further,FIG. 3schematically indicates a stack283of multi-aperture plates284disposed in proximity of the detector275and each having a plurality of apertures corresponding in position to positions of the detection elements277such that beamlets273of secondary electrons having energies below the threshold can be incident on corresponding detection elements277.

Summarized, embodiments of the present invention comprise a charged particle detection system with plural detection elements and a multi-aperture plate in proximity of the detection elements. Charged particle beamlets can traverse the apertures of the multi-aperture plate to be incident on the detection elements. More than one multi-aperture plate can be provided to form a stack of multi-aperture plates in proximity of the detector. A suitable electric potential supplied to the multi-aperture plate can have an energy filtering property for the plural charged particle beamlets traversing the apertures of the plate.

While the invention has been described with respect to certain exemplary embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention set forth herein are intended to be illustrative and not limiting in any way. Various changes may be made without departing from the spirit and scope of the present invention as defined in the following claims.