WO2002041354A1

WO2002041354A1 - Multi-beam lithography apparatus provided with a differential vacuum system

Info

Publication number: WO2002041354A1
Application number: PCT/EP2001/013099
Authority: WO
Inventors: Jan M. Krans; Mark T. Meuwese; Evert J. Van Loenen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2000-11-16
Filing date: 2001-11-09
Publication date: 2002-05-23

Abstract

In an electron-optical lithographic apparatus, objects (34-i) such as semiconductor wafers are fed through the object room (42) in which a comparatively poor vacuum prevails. The electron sources (4-i) of the apparatus must be accommodated in a source space (48) in which a much better vacuum prevails. To this end, the spaces (42, 48) are separated by pressure limiting apertures (PLA) (12-i). In accordance with the invention, the diaphragm that defines the angle of aperture of the beam also functions as the PLA. In this way only one diaphragm is needed instead of two, resulting in a simple construction of the (miniaturized) electron optical columns of the apparatus.

Description

Multi-beam lithography apparatus provided with a differential vacuum system

The invention relates to a particle-optical apparatus which includes:

(a) an object space which is provided with a first vacuum pump connection and is intended to receive an object to be irradiated by means of the apparatus,

(b) an array of particle-optical columns, each of which is provided with: - a particle source which serves to produce a particle beam of electrically charged particles and is arranged in a source space that is provided with a second vacuum pump connection, a focusing device for forming a focus of the particle beam in the vicinity of the object to be irradiated, and - a pressure limiting diaphragm that is provided between the source space and the object space.

An apparatus of the kind set forth is disclosed in the Japanese patent application bearing the publication number 6-61126. The apparatus disclosed in the cited patent application is arranged to write or inspect patterns on a substrate by irradiation by means of electron beams during the manufacture of semiconductor circuits. The substrate to be irradiated is then arranged on an object carrier which is positioned so as to face the array of particle-optical columns. In this context a particle-optical column is to be understood to mean an assembly that consists of a particle source and a focusing device. The particle source produces a beam of electrically charged particles (generally an electron beam) which is focused by the focusing device which is usually referred to as an objective. The focus of the electron beam thus formed is situated on the substrate so that a sharp electron spot is projected onto the substrate. The desired pattern is written or the desired pattern is tracked for inspection by means of the beam by displacement of said spot.

Apparatus of this kind often utilize particle sources of the field emission type (Field Emission Gun or FEG). As is generally known, it is important that such a particle source is accommodated in a source space in which a very good vacuum prevails, for example a pressure of the order of magnitude of 10^" 7 N/m 9. The objects to be irradiated in such apparatus are often formed by semiconductor wafers. In the semiconductor industry it is of essential importance to achieve an as high as possible feed-through speed for such wafers in production machines such as these particle-optical apparatus. Therefore, apparatus of this kind are provided with an input lock and an output lock for feeding the wafers to be irradiated into and out of the object space. The input lock is evacuated within a brief period of time after the introduction of the wafers from the ambient atmosphere, so that a vacuum of mediocre quality only (for example, 10^"4 N/m²) is realized. Moreover, the wafers also emit a variety of volatile constituents (notably the photoresist layers used thereon), so that these objects are always present in an object space in which a limited vacuum prevails.

In order to separate the high quality vacuum in the source space from the vacuum of lower quality in the object space, the known apparatus is provided with a differential pumping system, meaning that the source space is connected to a vacuum pump via its own pump connection and that the object space is also connected to a vacuum pump via an own pump connection. The source space and the object space cannot be hermetically separated from one another, because passage of the electron beam must be possible. Therefore, between the two spaces in the known apparatus there is provided a number of diaphragms that enable a pressure difference to be maintained between the two spaces; a first diaphragm thereof is situated directly behind the electron source and a second diaphragm is situated behind the objective and the deflection coils for the scanning motion of the electron beam. The cited Japanese patent application states that the former diaphragm has a diameter of approximately 1 micrometer and that the latter diaphragm has a diameter of from a few micrometers to some tens of micrometers. These numbers demonstrate that the former diaphragm (that is, the smallest diaphragm) constitutes the actual pressure limiting diaphragm between the source space and the object space.

The cited Japanese patent application also states that the size of said two diaphragms corresponds to the cross-section of the electron beam at the area of the relevant diaphragm. At the area of the smallest diaphragm the electron beam has a cross-over, that is, an image of the electron emitting surface of the electron source. It is a generally known fact in optics that beam limiting at the area of an image cannot define the angle of aperture of the imaging beam, which means that the smallest diaphragm cannot be the diaphragm that defines the angle of aperture. The largest diaphragm is situated underneath the deflection coils for the scanning motion of the electron beam, so that the aperture of this diaphragm must be larger than the beam cross-section at that area, because otherwise there will be no room for the scanning motion. This means that this diaphragm does not limit the beam, so that this diaphragm cannot be the diaphragm that defines the angle of aperture either. The cited Japanese patent application does not give any further indication as to how the angle of aperture of the electron beam is limited, even though it is inherent that such limiting must occur at all times.

It is an object of the invention to provide a particle-optical apparatus of the kind set forth in which the differential pump function is carried out in a simple and stable manner, without degrading the imaging quality of the particle-optical system. To this end, the apparatus in accordance with the invention is characterized in that each column includes a diaphragm that defines the angle of aperture and at the same time constitutes the pressure limiting diaphragm.

The invention is based on the recognition of the fact that the diaphragm that defines the angle of aperture is usually the smallest aperture in the particle-optical column, so that this diaphragm is excellently suitable to act as the pressure limiting diaphragm. In order to reduce disturbing (because it degrades the resolution) Coulomb interaction within the particle beam as much as possible, the diaphragm that defines the angle of aperture will be arranged as near to the particle source as possible, so that the volume of the source space (in which a very good vacuum should prevail during operation of the apparatus) is as small as possible.

The invention also offers the advantage that as a result of the combining of the two functions only one diaphragm has to be used instead of two; this is a major structural advantage in a particle-optical column which is to be constructed in highly miniaturized form so that it can form part of a larger array.

The diaphragm that defines the angle of aperture and also limits the pressure in a preferred embodiment of the invention is situated in a field-free space. This step ensures that said diaphragm does not form part of the imaging system. If it were to form part thereof, the comparatively high voltage of said diaphragm (the imaging system would then be an electrostatic lens) would give rise to a serious problem of contamination of the diaphragm by residual gases from the wafers to be irradiated. Such contamination would make it possible for electrostatic charging to occur; such charging disturbs the particle beam and hence degrades the optical quality of the array. Moreover, such contamination may cause electrical breakdowns and a virtual change of the shape of the (round) aperture; both these effects also cause degradation of the optical quality. A further advantage of said step resides in the fact that the position in the direction of the optical axis and the shape of the diaphragm are no longer very critical.

The invention will be described in detail hereinafter with reference to the Figures in which corresponding reference numerals denote the same elements. Therein: Fig. 1 is a diagrammatic view of a single electron optical column in accordance with the invention, and

Fig. 2 shows an array of electron optical columns as shown in Fig. 1.

Fig. 1 is a diagrammatic view of a single electron optical column in accordance with the invention. The column that is shown in this Figure has an optical axis 2 on which a particle source in the form of an electron source 4 of the field emission type (Field Emission Gun or FEG) is arranged so as to produce a primary electron beam 6. As is a generally known fact, it is important that such a particle source is situated in a very good vacuum, for example with a pressure of the order of magnitude of 10^"8 N/m². After emanating from the source 4 the primary beam 6 passes a condenser lens 8 that is capable of controlling the degree of convergence or divergence of the primary beam 6.

The condenser lens 8 is succeeded by a deflection unit 10 which can be used to direct the beam 6 towards the aperture 12 of the diaphragm 14 that defines the angle of aperture. This is important notably when the beam 6 has been made to converge to such an extent that the cross-section of the beam is of the same order of magnitude as the aperture 12. The cross-section of the beam 6 at the area of the aperture 12, however, may not become smaller than the aperture 12, because otherwise such a diaphragm aperture will no longer define the angle of aperture of the beam 6. The diaphragm 14 is arranged in a diaphragm holder 16. Apart from the aperture 12, the combination of the diaphragm 14 and the holder 16 constitutes a hermetic seal between the source space 18 and the object space 20 as will be described in detail hereinafter with reference to Fig. 2.

The angle of aperture of the primary beam 6 is limited by the diaphragm 14; subsequent to this diaphragm the beam continues its travel in the object space. Within this space there are accommodated, viewed from the top downwards, a detector crystal 22, an electrostatic acceleration electrode 24, a first electrical scanning electrode 26, a second electrical scanning electrode 28, a first electrostatic electrode 30 which forms part of the objective and a second electrostatic electrode 32 which also forms part of the objective. The electrons of the primary beam 6 ultimately reach the specimen 34 to be inspected or worked.

The detector crystal 22 forms part of detection means for the detection of secondary electrons that emanate from the specimen in response to the incidence of the primary beam. The detector crystal consists of a substance (for example, cerium-doped yttrium aluminum garnet or YAG) which produces a light pulse in response to the capture of an electron of adequate energy; this light pulse is conducted further by means of optical guide means (not shown) so as to be converted, in an opto-electrical converter, into an electrical signal wherefrom an image of the specimen can be derived, if desired. The latter elements also form part of said detection means. The detector crystal 22 is provided with a bore 36 for the passage of the primary beam.

The electrostatic acceleration electrode 24 is shaped as a flat plate that is provided with a bore for the primary beam and is deposited in the form of a conductive oxide, for example indium oxide and/or tin oxide, on the detection material, in particular on the detection surface of the scintillation crystal 22. The electrode 24 can be adjusted to a desired voltage, for example to 9 kV, by means of a power supply unit that is not shown.

The first electrical scanning electrode 26 and the second electrical scanning electrode 28 form part of a beam deflection system for controlling the scanning motion of the primary beam across the specimen 34. Each of these two electrodes is constructed as a tubular portion having an external shape in the form of a straight circular cylinder and an internal shape in the form of a cone that is tapered in the direction of the beam. Each of the electrodes 26 and 28 is subdivided, by way of two saw-cuts in mutually perpendicular planes through the optical axis, into four equal portions so that each of the electrodes 26 and 28 is both capable of producing electrical dipole fields in the x direction as well as the y direction by application of suitable voltage differences between the portions, so that the primary beam can be deflected across the specimen 34 and the path of the secondary electrons traveling in the direction of the detector crystal can be influenced. Instead of subdividing the electrodes 26 and 28 into four portions, they can also be subdivided into a larger number of portions, for example into eight equal portions by means of four saw-cuts in a plane through the optical axis. When the appropriate voltages are applied to the various parts of each of the electrodes, the system thus formed can be used not only for deflecting the beam but also as a stigmator. The first electrode 30 and the second electrode 32 constitute the electrode system which forms the objective of the column. Internally as well as externally the electrode 30 is shaped as a cone that is tapered downwards, so that this electrode fits within the electrode 32. Internally as well as externally the electrode 32 is also shaped as a cone that is tapered downwards; the external conical shape offers optimum space for the treatment of comparatively large specimens such as the circular wafers used for the manufacture of ICs and may have a diameter of 300 mm. Because of the external conical shape of the electrode 32, the primary beam can be made to strike the wafer at a comparatively large angle by tilting the wafer underneath the objective, without the wafer being obstructed by parts projecting from the objective. A dashed line 36 in the Figure indicates the region in which the lens effect of by the electrical objective field (so the paraxial center of the objective) can be assumed to be localized.

The objective 30, 32 focuses the primary beam in such a manner that the electron source is imaged on the (grounded) specimen with the generally very large reduction; because of this strong reduction, the distance between the surface^" of the specimen 34 and the center of the lens 36 (the focal distance) is very small; as has already been mentioned, the possibility for tilting would thus be severely limited if the external shape of the electrode 32 were not conical.

The incidence of the primary electron beam 6 on the surface of the specimen 34 releases secondary electrons therefrom in the vicinity of the point of incidence 38. Because the voltage difference present between the objective electrodes 30 and 32 also causes an electrical field (referred to as the leakage field) in the space between the lower electrode 32 and the specimen 34, such electrons move in the direction of the objective 32, 34 under the influence of the leakage field. The secondary electrons thus form a secondary electron beam 40 whose propagation direction opposes that of the primary beam 6. The electrical field present between the objective electrodes 30 and 32 accelerates the secondary electrons to a speed that corresponds to the voltage between the objective electrodes, for example to a voltage of 10 kV. The secondary beam is also sensitive to deflection by the scanning electrodes 26 and 28; however, because of the funnel-shaped interior of these electrodes practically the entire secondary beam 40 will reach the detector crystal 22.

Fig. 2 shows an array of electron optical columns as shown in Fig. 1. Each of the columns 46-1, 46-2 (referred to in general as 46-1) shown in Fig. 2 is composed as shown in Fig. 1. Each of these columns can be used to execute inspection or production steps on a respective object 34-i; however, it is alternatively possible to make these columns co-operate, for example for the inspection of one large object such as a semiconductor wafer that has a diameter of, for example 30 cm. The wafers 43-i are accommodated in an object space 42 in which they are introduced and wherefrom they are removed via a vacuum lock 52. The FEG sources 4-i and the condenser lenses 8-i are situated in a source space 48. The source space 48 and the object space 42 communicate via a diaphragm aperture 12-i which defines the angle of aperture and also has a pressure limiting effect. Due to said method of input and output via a vacuum lock 52, the vacuum in the object space 42 cannot be maintained at the low pressure value (for example, 10^" N/m ) that is required for a FEG. Therefore, the object space 42 is separated from the source space 48 by a sealing wall 16 which also acts as a diaphragm holder for the diaphragms 14. Apart from the apertures 12-i, this sealing wall 16 constitutes a hermetic seal between the two spaces 42 and 48. The source space can now be connected to a respective vacuum pump (not shown) via a respective pump connection 50. A vacuum pump of this kind must be capable of maintaining the high-quality vacuum required for the FEG. This can be realized, for example by using an ion getter pump. The object space is also connected to a respective vacuum pump (not shown) via a respective pump connection 44. The latter vacuum pump need not create a vacuum of high quality, but must be capable of realizing the desired pressure value of, for example 10^"4 N/m² in the object space 42 and in the vacuum lock 52,if desired, within a short period of time. This can be realized, for example by using a turbo molecular pump. When the particle-optical apparatus is used as a wafer inspection apparatus, use is made of, for example 50 columns, each column having an aperture 12 with a diameter amounting to 70 micrometers. In such a case a pressure ratio of a factor of 10⁴ can be readily maintained when use is made of a customary pumping system that has a capacity of, for example 2601/s.

Claims

CLAIMS:

1. A particle-optical apparatus which includes:

(a) an object space (22) which is provided with a first vacuum pump connection (44) and is intended to receive an object (34) to be irradiated by means of the apparatus,

(b) an array of particle-optical columns (46-i), each of which is provided with: - a particle source (4) which serves to produce a particle beam (6) of electrically charged particles and is arranged in a source space (48) that is provided with a second vacuum pump connection (50), a focusing device (30, 32) for forming a focus of the particle beam in the vicinity of the object (34) to be irradiated, and - a pressure limiting diaphragm (14) that is arranged between the source space (48) and the object space (42), characterized in that each column includes a diaphragm (12) that defines an angle of aperture and at the same time constitutes the pressure limiting diaphragm (14).

2. A particle-optical apparatus as claimed in claim 1, in which the diaphragm (12, 14) that defines the angle of aperture and also acts as a pressure limiting diaphragm is situated in a field-free space.

3. A particle-optical column as defined in claim 1 or 2.