DE69904881T2 - Projection exposure apparatus - Google Patents

Description

The present invention relates to a lithography apparatus comprising:

a radiation system having a radiation source and an exposure system for providing a projection beam;

a first movable stage with a mask holder for holding a mask;

a second movable stage with a substrate holder for holding a substrate; and

a projection system for imaging an illuminated portion of the mask onto a target portion of the substrate.

The invention particularly relates to such a device in which an image-bearing electromagnetic beam is used to generate an electron beam which exposes a radiation-sensitive layer of a substrate.

For simplicity, the projection system will be referred to as a "lens" hereinafter; however, this term is intended to be broadly understood and to include different types of projection systems, including refractive optics, reflective optics, catadioptric systems, and charge carrier optics, to name a few examples. The exposure system may also include elements that operate according to any of these principles to align, shape, or control the projection beam, and such elements may also be referred to, collectively or individually, as a "lens." In addition, the first and second object stages may be referred to as the "mask stage" and the "substrate stage," respectively. In addition, the lithography device may be of a type that includes two or more masks and/or two or more substrate stages. In such "multi-stage" devices, the additional stages may be used in parallel, or in one or more stages, preparatory steps may be performed while one or more other stages are used for exposure. Twin stage lithography devices are described, for example, in international patent applications WO 98/28665 and WO 98/40791.

Lithography projection devices can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) can project a circuit pattern corresponding to a single layer of the IC, and this pattern can be imaged onto a target area (die) on a substrate (silicon wafer) coated with a layer of photosensitive material (resist). Generally, a single wafer will comprise an entire network of adjacent dies which are successively exposed by the reticle, one after the other. In one type of lithographic projection apparatus, each die is exposed by imaging the entire reticle pattern onto the die in one operation; such an apparatus is generally referred to as a wafer stepper. In an alternative apparatus - generally referred to as a step-and-scan apparatus - each die is exposed by progressively scanning the reticle pattern under the projection beam in a reference direction (the "scan" direction) while synchronously scanning the wafer stage parallel or antiparallel to that direction. Since the projection system will generally have a magnification factor M (generally < 1), the speed v at which the wafer table is scanned will be a factor of M times that at which the reticle table is scanned. Further information regarding lithography devices of the type described here can be found in the international patent application WO 97/33205.

In a lithography apparatus, the size of the features that can be imaged on the wafers is limited by the wavelength of the projection radiation. In order to produce integrated circuits of high assembly density and consequently high operating speeds, it is desirable to be able to image small features. While most current lithography projection apparatus use ultraviolet light generated by mercury lamps or excimer lasers, it has also been proposed to use higher frequency (energy) rays as exposure radiation in a lithographic apparatus, e.g. X-rays or EUV, or even particle beams, e.g. electrons or ions. However, electron and ion beam projection lithography apparatuses that have been proposed have limited throughput because the total beam current must be limited in order to limit stochastic (random) scattering effects. These effects are proportional to the field area or (field area)3/4, and in one known system the beam current is effectively limited to 35 uA.

In the known electron beam projection lithography devices, the size of the exposure field is further limited by the necessary mask structure and system aberrations (e.g. The application is limited by aberrations (e.g., field curvature and distortion) that cannot be corrected by electron optics. The two known methods for correcting aberrations in electron optics systems use foil lenses or line charges or currents along the axis of the system. In known lithography devices, foil lenses cause unacceptable scattering and beam attenuation, while the electron beam used is too close to the axis to allow the provision of a line charge or current.

A hybrid optical/electron beam lithography apparatus was proposed in US 5,156,942 and US 5,294,801. In this apparatus, an ultraviolet (UV) beam was used to illuminate a mask, and an image of the mask was projected onto a photoemissive plate. The photoemissive plate was thereby excited to emit electrons in a pattern corresponding to that of the mask. The photoelectrons were accelerated and projected onto the substrate wafers to expose the resist layer thereon.

According to the present invention, there is provided a lithography projection apparatus for imaging a mask pattern of a mask onto a substrate having a radiation-sensitive layer, the apparatus comprising:

a radiation system having a radiation source and an exposure system for generating an exposure beam;

a first movable stage with a mask holder for holding a mask;

a second movable stage having a substrate holder for holding a substrate;

a projection system for imaging irradiated portions of the mask onto target portions of the substrate, the projection system comprising:

a photocathode;

a first projection device for projecting an electromagnetic ray image of the mask onto the photocathode to allow the emission of photoelectrons in a pattern corresponding to that of the mask; and

a second projection device for projecting the photoelectrons onto a substrate;

characterized in that:

the photocathode is curved to compensate for aberrations.

The present invention can therefore provide a hybrid optical/electron beam lithography device that has a higher throughput than conventional electron beam devices. It can also have a larger field while reducing field aberrations of known hybrid devices.

The word "curved" as used in reference to the photocathode is intended to indicate that it is non-planar, i.e. concave and/or convex with respect to the "optical axis" of the projection system.

Preferably, the photocathode has the shape of a partially spherical surface.

According to the present invention, there is also provided a method for manufacturing a device comprising the steps:

Providing a substrate at least partially coated with a layer of an energy-sensitive material;

Providing a mask with a pattern;

using a beam to project at least a portion of the mask pattern onto a target surface of the layer of energy sensitive material;

characterized,

that the step of projecting comprises the following steps;

irradiating the mask pattern with a beam of electromagnetic radiation to form an image-bearing electromagnetic beam carrying the image of at least a portion of the mask pattern;

directing the image-bearing electromagnetic beam onto a photocathode plate having a curved surface so that photoelectrons are generated; and

directing the photoelectrons onto the target surface of the substrate.

In a manufacturing process with a lithographic production device according to the invention, a pattern of a mask is imaged onto a substrate which is at least partially coated with a layer of energy-sensitive material (resist). Before this imaging step, the substrate can be subjected to several steps, e.g. priming, resist coating and soft baking. After exposure, the substrate can be subjected to other steps, e.g. post-exposure baking (PEB), developing, Hardbaking and measuring/inspecting the imaged features. This group of operations is used as a basis for designing a single layer of the assembly, e.g. an IC. A layer designed in this way can then be subjected to various operations such as etching, ion implantation (doping), metallization, oxidation, chemi-mechanical polishing, etc., all of which serve to refurbish a single layer. If multiple layers are required, the entire procedure, or a variation thereof, must be repeated for each layer. Eventually an array of assemblies will be present on the substrate (wafer). These assemblies are then separated from one another by a process such as dicing or sawing, and subsequently the individual assemblies can be mounted on a carrier, connected to pins, etc. Further information regarding such operations can be found e.g. in the documentation of the respective assemblies. The information can be found in the book "Microchip Fabrication: A Practical Guide to Semiconductor Processing", third edition, by Peter von Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.

Although this text specifically refers to the use of the device according to the invention in the manufacture of ICs, it is intended to be made explicitly clear that such a device has many possible applications. For example, it can be used to produce integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid crystal displays, thin film magnetic heads, etc. Those skilled in the art will recognize that in the context of such alternative applications, the terms "reticle", "wafer" or "die" in this text are to be thought of as being replaced by the more general terms "mask", "substrate" and "target area".

The present invention is described below with reference to exemplary embodiments and the accompanying schematic drawings, in which:

Fig. 1 illustrates a lithographic projection apparatus according to a first embodiment of the invention;

Fig. 2 is a diagram of the projection system of the first embodiment of the invention; and

Fig. 3 shows electron tracks in a pseudomonopole magnetic field.

In the different figures, identical parts are labeled the same.

Fig. 1 shows schematically a lithography projection device according to the invention. The device comprises:

- a radiation system LA, Ex, IN, CO for providing a projection beam PB (e.g. UV radiation);

- a first object table (mask table) MT with a mask holder for holding a mask MA (e.g. a reticle), which is connected to a positioning device for precisely positioning the mask with respect to the object PL;

- a second object table (substrate table) WT with a substrate holder for holding a substrate W (e.g. a resist-coated silicon wafer) connected to a second positioning device for precisely positioning the substrate with respect to the feature PL;

- a projection system ("lens") PL for imaging an illuminated section of the mask MA onto a target section C (die) of the substrate W.

As shown here, the entire device comprises transmissive components; however, it may alternatively comprise one or more reflective components.

The radiation system comprises a source LA (e.g. a Hg lamp or an excimer laser) which generates a beam. This beam passes through various optical components - e.g. the beam shaping optics Ex, an integrator IN and a capacitor CO - so that the resulting beam PB is essentially parallel and of uniform intensity throughout its cross-section.

The beam PB then strikes the mask MA, which is held in a mask holder on a mask table MT. After passing the mask MA, the beam passes the lens PL, which focuses the beam PB on the target area C of the substrate W. With the aid of an interferometric displacement and measuring means IF, the substrate table WT can be moved precisely, e.g. to bring different target areas C into the beam path of the beam PB. In a similar way, the first positioning means can be used to precisely position the position of the mask MA with respect to the beam path of the beam PB, e.g. after mechanically scanning the mask MA from a mask library. In general, the movement of the object tables MT and WT is controlled with the aid of a long-step motion sensor. module (coarse positioning) and a short-step module (fine positioning), which are not explicitly shown in Fig. 1.

The device shown can be used in two different operating modes:

- in step mode, the mask table MT is generally held stationary, and a complete mask pattern is projected onto the target area C in one step (i.e. a single "flash"). The substrate table WT is then translated in the x and/or y directions so that different target areas C can be illuminated with the beam PB;

- in scanning mode, essentially the same thing happens, except that a given target area C is not exposed in a single "flash". Instead, the mask table MT is movable in a given direction (the so-called "scanning direction", e.g. the x-direction) at speed v, so that the projection beam PB is caused to scan a mask pattern; at the same time, the substrate table W is simultaneously moved in the same or opposite direction at a speed V = Mv, where M is the magnification of the lens PL (typically M = ¹/₼ or 1/5). In this way, a relatively large target area C can be exposed without compromising resolution.

Fig. 2 is a schematic representation of the projection system PL of Fig. 1. In this system, a reticle 1 is illuminated by a light source (e.g. ultraviolet light of wavelength 248 nm) and an exposure system LA, Ex, IN, CO. Light that has passed through the transmissive areas of the reticle 1 is collected into an image-bearing beam 2 by the optical system 3 and projected onto a photocathode 4.

The optical system 3 is shown as a single lens for simplicity, but in practice will comprise several lenses and/or other components as necessary for focusing, reducing (if desired) and correcting aberrations.

The photocathode 4 comprises a partially spherical surface (e.g. made of glass or quartz) which is coated with gold, gallium arsenide or carbide, for example. The special coating used here determines the working function and efficiency of the photocathode 4. The used The coating(s) may therefore vary according to the wavelength of the exposure light used in Beam 2.

Photoelectrons 5 are emitted from the photocathode 4 in a pattern corresponding to the image projected thereon and are then accelerated by the acceleration plate 6 to approximately 100 keV and projected onto the wafers 8 by the electron-optical system 7. (In the case of a pseudo-monopole field (as discussed below), an acceleration voltage as low as 2 kV may be sufficient.) Again, the electron-optical system 7 is shown as a single lens, but, as discussed below, in practice it will comprise suitable electric and/or magnetic field generators. The electron-optical system 7 may also reduce the image carried by the photoelectron beam 5 (e.g., to a magnification of ¹/4) when projecting it onto the wafers 8.

The curvature of the photocathode 4 is calculated to correct the curvature of the field of the electron-optical system. Other distortions can be corrected using the light-optical system in front of the photocathode 4.

When a photon from the light source hits the photocathode, an electron is emitted in a random direction. The emitted electrons are accelerated by the electric field generated by the accelerator plate 6 and follow a parabolic path, asymptotically approaching the field lines. If the paths of the electrons leaving the accelerating field are extrapolated backwards, it appears as if the electrons came from a virtual source behind the photocathode.

Calculations show that the blurring of the wafer caused by the random emission angles of the photoelectrons will be negligible if the half-opening angle is limited to 80 mrad for an acceleration energy of 10 keV and to 800 mrad for 100 keV electrons with an energy width of 1 eV. In both cases, an acceleration gap of 10 mm is assumed. In practice, an energy width of 0.2 eV is expected in the entire system, so that the effect of the random emission angles can be neglected.

In the present embodiment of the invention, the accelerator had the form of a plate with a central hole for the electrons to pass through. To reduce aberrations it may be necessary to use an accelerator grid instead of the accelerator plate with a hole. Such a grid would, however, cast shadows on the substrate (wafer). An alternative would be to use a "grid" consisting of wires extending in only one direction. In this case, the effect of a shadow can be avoided by scanning the mask and substrate relative to the grid in a direction perpendicular to the wires of the grid. The shadows cast by the grid will then move across the substrate and have a negligible reduction in dose over the entire field of the sequence, rather than representing local obstructions.

The photoelectron current generated by the incident light beam 2 depends on the quantum efficiency of wavelength conversion according to the following formula:

where S(λ) is in units of mA/Watt and is the electron beam divided by the incident light power, Y(λ) is the quantum efficiency in % and λ is the wavelength in nm. For a wavelength of 248 nm, the quantum efficiency can be as high as 20%, so that an incident light power of 150 mW per cm² produces a beam current of 60 uA/mm². This is sufficient current to achieve a reasonable throughput even if the quantum efficiency is reduced to 2%.

The present invention can be used in a conventional electron optical system with a telecentric system of two lenses. In this case the photocathode is arranged in the front focal plane of the first lens. The back focal plane of the first lens coincides with the front focal plane of the second lens. The substrate (wafer) is then arranged at the back focal plane of the second lens. In this arrangement the magnification M = f2/f1, where f1 and f2 are the focal lengths of the first and second lenses respectively. It is also possible to cancel most aberrations by giving the lenses the same shape and excitation (ampere-turns in the lens) but scaling the geometric dimensions according to M. The most important aberration that remains is the field curvature, which can be corrected by the curved photocathode of the invention. An existing photolithography device has Due to field curvature and chromatic aberration, there is a 23.5 nm blur in the wafer. This can be essentially eliminated with the invention, so that the field area of the wafer can be quadrupled and the maximum current can be increased by a factor of typically 2.5 to 4. This is a significant increase in throughput.

The curved photocathode of the invention enables an alternative embodiment of the present invention in which the electron image on the substrate (wafer) is reduced by means of a pseudomonopole magnetic field. A magnetic monopole would produce a magnetic field in which the field lines are straight and come from a point, i.e. a magnetic field in a form similar to the electric field of a point charge. While magnetic monopoles are not known, it is possible to produce a magnetic field that approximates a monopole field in a volume large enough to accommodate and reduce the electron beam.

Because of the symmetry of the pseudomonopole field, each field line can be considered as an axis. In a sufficiently strong, rotationally symmetric field, an electron will always return to the axis from which it started. In a pseudomonopole field, the electrons will therefore be bound to the field line they cross in their first gyration. Fig. 3 illustrates this effect by showing electron tracks from different initial (emission) angles bound to different field lines. In Fig. 3, the horizontal axis represents the distance along the optical axis and the projection system, and the vertical axis shows the distance from the optical axis, both in arbitrary units.

The pseudomonopole field can therefore be arranged so that the electrons are guided to the wafer with the necessary reduction and without an increase in spherical or chromatic aberration at each crossing. The absence of a single global crossing point, as is present in conventional reduction systems, means that the beam current is not limited by stochastic effects due to the space charge at the crossing point. This therefore allows a further increase in the total beam current by at least an order of magnitude compared to conventional systems. For example, a 3 x 3 mm² emission area can be divided into nine 1 x 1 mm² subfields. The pseudomonopole field provides each subfield with its own Reduction lens, so that a current nine times higher can be used without increasing the stochastic blur. The monopole field can be generated, for example, as described in US 5,268,579 (Bleeker) and with the device disclosed therein.

A further advantage of the system of the present invention is that it allows the use of line charges or line currents when magnetic lenses are used to correct aberrations in the electron optics. In embodiments of the invention, if the portion of the photocathode that is illuminated is off-axis, then the relevant electron beam will also be off-axis, thus providing space for the line charge or line current.

While we have described above a specific embodiment of the invention, it is to be understood that the invention may be practiced otherwise than as described. The description is not intended to limit the invention, which is defined by the scope of the appended claims.

Claims (12)

1. Lithographic projection device for imaging a mask pattern of a mask onto a substrate with a radiation-sensitive layer, the device comprising: a radiation system having a radiation source and an exposure system for generating an exposure beam; a first movable stage having a mask holder for holding a mask; a second movable stage with a substrate holder for holding a substrate; and a projection system for imaging illuminated portions of the mask onto target portions of the substrate, the projection system comprising: a photocathode; a first projection means for projecting an electromagnetic radiation image of the mask onto the photocathode to stimulate the emission of photoelectrons in a pattern corresponding to that of the mask; and a second projection means for projecting the photoelectrons onto a substrate; characterized in that the photocathode is curved to compensate for aberrations. 2. Device according to claim 1, wherein the photocathode has a curved shape that is suitable for compensating field aberrations of the second projection means. 3. Device according to claim 1 or 2, wherein the photocathode substantially corresponds to a part-spherical surface. 4. Apparatus according to claim 1, 2 or 3, wherein the second projection means comprises an electron accelerating means and an electron focusing means. 5. Apparatus according to claim 4, wherein the electron focusing means comprises means for generating a pseudo monopole magnetic field. 6. Apparatus according to claim 4 or 5, wherein the electron accelerating means comprises a grid of wires extending substantially in one direction only. 7. The apparatus of claim 6, further comprising means for scanning the mask and the substrate relative to the grid. 8. Device according to one of the preceding claims, wherein the first projection means is suitable for pre-compensating distortions of the second projection means by generating counteracting distortions in the electromagnetic radiation image. 9. Device according to one of the preceding claims, wherein the electromagnetic radiation comprises ultraviolet radiation. 10. Device according to one of the preceding claims, wherein the second projection means is suitable for projecting a reduced electron image onto the substrate. 11. A method for producing a device comprising the steps: Providing a substrate at least partially coated with a layer of energy-sensitive material; Providing a mask with a pattern; Using a beam to project at least a portion of the mask pattern onto a target area of the layer of energy sensitive material; characterized in that the step of projecting comprises the following steps; illuminating the mask pattern with a beam of electromagnetic radiation to produce an image-bearing electromagnetic beam having an image of at least a portion of the mask pattern; directing the image-bearing electromagnetic beam onto a photocathode plate having a curved surface to generate photoelectrons; and directing the photoelectrons to the target area of the substrate. 12. A device produced by the method of claim 11.