FR2957687A1 - PHOTOREPETEUR OR SCANNER FOR LITHOGRAPHY IN EXTREME ULTRA-VIOLET - Google Patents

Abstract

The invention relates to a photorépéteur for the exposure of lithographic resins, especially extreme ultraviolet. According to the invention, the photorépéteur comprises a projection optics comprising a thick rigid frame having at least one opening on which is stretched a thin ultraviolet transparent thin membrane of a maximum thickness of 300 nanometers carrying a pattern of etchings conferring on the membrane a diffractive optical function in transmission at the wavelength used by the photorepeater. The mask used is itself preferably made from a thick frame having one or more openings covered by a thin transparent membrane carrying a thin layer of a material opaque or partially opaque to extreme ultraviolet radiation. This thin layer is etched in a pattern defining the desired mask. The openings of the projection optics are arranged opposite those of the mask when the mask and the optics are in place in the photorépéteur.

Description

The invention relates to photolithography, and in particular photolithography at very short wavelengths. More specifically, it relates to a photorepetator for the exposure of ultra-violet extreme lithographic resins. In some aspects, however, the invention is not limited to the extreme ultraviolet but it will be described about extreme ultraviolet. Photolithography is used for the production of electronic, optical, mechanical microstructures, or microstructures combining electronic and / or optical and / or mechanical functions. It consists in irradiating with photon radiation, through a mask which defines the desired pattern on a certain surface, a layer of photosensitive resin deposited on a substrate (for example a silicon wafer); after an exposure of the surface defined by the mask, the substrate is displaced and another area of the same surface is exposed, and so on until the entire desired area has been exposed. For example, the mask corresponds to the surface of an integrated circuit chip on a substrate that will be used for the collective manufacture of several chips; photorepetition then consists in successively exposing the surfaces corresponding to the individual chips. The chemical development following the insolation reveals the desired patterns in the resin. The resin pattern thus etched can serve several purposes, the most common being the etching of an underlying layer (insulating or conductive or semiconductor) to define in this layer a pattern identical to that of the resin. Extremely small and precise patterns are sought and etched patterns are very precisely aligned in multiple superimposed layers. Typically, the critical dimension of the desired patterns is today a fraction of a micrometer and can go down to 20 nanometers and below. The wavelength of the light used in the photolithography operation limits the resolution; the shorter it is, the more patterns can be fine. Ultraviolet photolithography (currently using wavelengths down to 193 nanometers) allows for finer patterns than with visible light but not as fine as one would like. We are now looking to go down very far below these wavelengths and to work in extreme ultraviolet (EUV), at wavelengths ranging from 5 to 50 nanometers, and in particular wavelengths between 10 and 14 nanometers such as the 13.5 nanometer wavelength obtained by excitation of a xenon, tin or lithium plasma illuminated by a YAG or CO2 laser or subjected to an electric discharge. Recent work has also been done around 6.7nm.

Furthermore, it is recalled that a photorépéteur generally comprises a light source at the desired exposure wavelength, an illuminator which serves to shape the light beam emitted by the source, a mask which defines the pattern to be projected and which modulates therefore spatially the light beam to expose the resin layer according to this pattern, a projection optics that focuses this beam on a resin layer carried by a substrate, and a movable support table for the substrate, for moving the substrate relative to projection optics to successively project the image of the mask on different areas of the substrate.

But, at the wavelengths of the extreme ultraviolet, the materials forming the substrate of the mask (substrate on which is etched a masking layer) are not transparent and the photolithography operation must use masks operating in reflection and not in transmission: the mask comprises absorbing zones and reflecting zones; in the reflective zones, the mask reflects the light on the resin to be exposed, by printing its image. The path of light between the mask and the resin to be exposed passes through other reflectors whose geometries are designed to project a mask image, reduced or in real size. In addition, the projection optics placed between the mask and the substrate is not conventional optics as used, for example, with 193 nanometer ultraviolet. Indeed, in extreme ultraviolet, not only the materials have a very low transparency, but also the refractive indices are so close to 1 that we would not be able to do the convergent optics that are necessary for the formation of the image of the mask on the sensitive layer. Thus, image-forming optics operating in reflection are used, thus consisting entirely of successive mirrors of appropriate curvatures. Optics that work in reflection to project the image on the resin have a very low efficiency because it requires several reflectors and each reflector has a yield that rarely exceeds 70%; if there are 6 to 10 reflectors, the light transmitted by the reflection projection optics represents only a few percent of the light sent by the source. Moreover, irrespective of the disadvantages of the optics operating in reflection, the masks operating in reflection themselves have intrinsic disadvantages: they must be illuminated at a non-zero incidence; even if this incidence is small (5 to 7 °), the thickness of the masking layer produces a shading effect that makes the pattern reproduced is not identical to the pattern of the mask. The relative effect of this shading is all the more important as the masking layer is thicker and the patterns of the mask are smaller. Also, reflection masks use Bragg mirrors which are superimposed layers of thin dielectric layers in large numbers, with thicknesses perfectly controlled according to the expected non-zero illumination angle. However, the probability of having small defects is not negligible while one wishes a total absence of defects. To overcome these difficulties in producing a photorépéteur operating in extreme ultraviolet, the present invention proposes first to use in the photorepeater a projection optics constituted in the following manner: it comprises a thick rigid frame having at least one opening on which is tensioned a thin membrane, at least partially transparent in extreme ultraviolet, with a maximum thickness of 500 nanometers, bearing a pattern of etchings conferring on the thin membrane a diffractive projection optical function in transmission at the wavelength used by the photorépéteur. The thin membrane is preferably of silicon; it can also be zirconium. The etched pattern is preferably etched in a layer deposited on the membrane. It could also be engraved directly in part of the thickness of the membrane.

This projection optical transmission could be used in combination with a mask operating in reflection, but it is much more advantageous to associate it with a mask operating in transmission, made according to a technique similar to that of optics: the mask is then a photolithography mask extreme ultraviolet, operating in transmission, comprising a thick rigid frame having at least one opening on which is stretched a transparent thin membrane ultraviolet extreme (preferably silicon or zirconium rigor) , having a thickness of up to 500 nanometers, carrying a thin layer of an opaque material or partially opaque to extreme ultraviolet radiation, this thin layer being etched in a pattern defining the desired mask in an area defined by the aperture on which is stretched the membrane. The opening in the context of the projection optics is arranged opposite that of the mask when the mask and the optics are in place in the photorépéteur. The invention therefore proposes a method of ultra-violet extreme lithography, using a photorepeater and a lithography mask, characterized in that a projection optic comprising a thick frame having at least one opening is placed in the photorepeater. on which is stretched an ultraviolet ultra thin transparent membrane having a maximum thickness of 500 nanometers bearing a pattern of etchings conferring on the thin membrane a diffractive projection optical function in transmission at the wavelength used by the photorepeater, And put in place a transmission-operated mask having a thick frame having at least one opening on which is stretched a thin ultraviolet-transparent thin membrane of a maximum thickness of 300 nanometers, carrying a thin layer of a material opaque or partially opaque to ultraviolet radiation, this thin layer being etched according to a word if defining the desired mask in an area defined by the opening on which the membrane is stretched, the opening of the projection optics being arranged opposite that of the mask when the mask and the optics are in place in the photorepeater .

The mask preferably comprises an arrangement of several regularly distributed openings, with, above each respective opening, a membrane carrying an opaque or partially opaque layer at the extreme ultraviolet end, etched in a respective pattern, and the optics projection device comprises an arrangement of several regularly distributed openings, with, above each respective opening, a membrane bearing a pattern of etchings conferring on the thin membrane a diffractive projection optical function in transmission at the wavelength used by the photorépéteur, the openings of the mask being arranged opposite corresponding openings of the projection optics when the mask is placed in the photorépéteur. The openings are preferably arranged checkerboard, that is to say arranged at the crossroads of a regular network of rows and columns.

The thin membrane is preferably monocrystalline silicon. The frame may be silicon or silica, glass, quartz, etc., both for the mask and for the diffractive projection optics. The lithography method preferably comprises the successive exposure of a photosensitive layer through different masks, each mask comprising a frame provided with several openings on each of which is formed a thin membrane carrying a thin etched layer, the shapes and / or the positions of the openings with respect to the frame being different from one mask to another to allow the complete exposure of the surface of the photosensitive layer, including in the areas masked by the frames of the different masks around their openings, and each mask is associated with a projection optic having an aperture distribution corresponding to the aperture distribution of the respective mask so that each opening of the mask is facing a respective aperture of the associated projection optics.

Thus, for the exposure of a single photosensitive layer, a set of several masks and of several associated optics will be used, in which each mask comprises an arrangement of openings (preferably in a checkerboard pattern) the shape and / or the position of which on the frame differ from one mask to another, the patterns engraved above the openings of the different masks complement each other to allow the exposure without dead zone of a global pattern on the same photosensitive layer by exposure of this layer with ultra-violet extremes successively through each of the masks of the game, and passing each time by the associated projection optics.

In practice, we will use a set of three or four masks. The image that we want to project is subdivided into portions and each mask has a number of openings that depends on this subdivision; each opening corresponds to an image portion and the membrane located above this opening carries the masking pattern corresponding to this portion. The different image portions each correspond to an aperture and the corresponding masking pattern of one of the masks. The subdivision of the image depends on the maximum dimensions that are to be given to the membrane stretched over an opening so that it has sufficient flatness and solidity.

The projection optics may be a reducing optic, for example in a ratio of 4 to 1. In general, regardless of the wavelength of use, the invention relates to a photorépéteur intended to receive a mask in front of an optical projection device for projecting an image onto a surface, the projection optics being composed of several partial optics mounted on the same plane support and capable of simultaneously simultaneously projecting a respective portion of the image, each partial optics being controllable independently of the others partial optics for adjusting at least one parameter of the projection of the image portion corresponding to this partial optics. The parameters of the projection can be the focus, the magnification, the rotation in the plane, the tilt of the plane, or even the precise position of an image portion in the plane. The partial optics is preferably diffractive as indicated above, especially when the wavelength is the extreme ultraviolet. Each partial optics may itself be composed of a group of elementary optics corresponding to an image sub-portion, the elementary optics of a group being controlled simultaneously. By individually adjusting the projection parameters of the partial optics, correct focus can be achieved on larger areas than if the adjustment was done globally for the entire image. This is important when the exposed field has several centimeters of side and when the reasons to expose are very small, especially when their critical dimension goes down to 20 nm and below. Finally, we will note the following: some photolithography devices proceed by scanning, that is to say they alternate photorepetition and scanning so as to cover ever larger fields. Throughout this patent application, these devices will be designated by the name "photoreputers" even if they proceed at least partly by scanning and not by simple photorepetition.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which: FIG. 1 is a simplified representation of the general structure of a photorepeater; FIG. 2 represents the photorépéteur according to the invention; FIG. 3 represents a photorépéteur in which the mask and the image forming optics both have several openings facing each other; FIG. 4 schematically represents a mask with an opening; - Figure 5 schematically shows a diffractive optics at an aperture; Figure 6 shows a multi-aperture mask structure; - Figure 7 shows a vertical section (turned) of Figure 6; FIG. 8 represents a diffractive optics with several openings that can be used with the mask of FIGS. 6 and 7; Figure 9 shows a vertical section of Figure 8; - Figure 10 shows at 10A a complete image to be projected on a photosensitive layer and 10B to 10E a set of four masks for projecting the entire image by successive exposures through each of the masks; FIG. 11 represents at 11A a mask of FIG. 10 and at 11B through 11E a set of four diffractive projection optics each associated respectively with one of the four masks of FIG. 10; FIG. 12 represents a configuration of fixed partial optics and of adjustable partial optics, the latter being grouped into blocks of nine elementary optics; FIG. 13 represents a diagram of a projection optics grouping a plurality of partial optics on the same rigid support by means of piezoelectric wedges serving for the individual adjustment of the partial optics.

The simplified structure of a photorépéteur is shown in Figure 1. This structure is suitable for ultraviolet photolithography, but poorly adapted, as explained above, extreme ultraviolet. The photorépéteur comprises an illumination source S, for exposing a photosensitive resin deposited on the surface of a plane substrate W. Here, the substrate is for example a wafer or wafer of silicon intended for the manufacture of an integrated circuit microelectronics.

The light beam produced by the source passes into an illuminator IL used to form the beam, then into a transmission mask M which defines the desired exposure pattern for the resin, and finally in a projection or optical optics for forming the beam. OF image that forms an image of the mask in the resin, possibly with a reduction factor (often a division by four). The substrate W is placed on a table T which is movable relative to the elements of the photorépéteur itself, that is to say the source, the illuminator and the optics. The mask is generally installed removably in a drawer that can be put between the illuminator and the projection optics. The resin surface exposed by the optic OF occupies only a part of the substrate W. To expose the other parts of the resin surface, the table, and therefore the substrate, is moved step by step under the projection optics. where the name of photorépéteur given to this device. The step-by-step movements of the table are controlled with great precision. This photorépéteur structure is very complicated and has many disadvantages when working in extreme ultraviolet and especially with a wavelength of 13.5 nanometers. This complication results from the fact that we have to use reflective optics and reflection masks. According to the present invention, diffractive projection optics will be produced by a pattern engraved on a very thin thin membrane (less than 500 nanometers thick) stretched over an opening of a thick rigid frame. The membrane is preferably of silicon; it has good ultraviolet transparency due to its very small thickness. It is recalled that diffracting gratings etched on flat surfaces make it possible to provide a variety of optical functions similar to those used in conventional refractive optical systems. In particular, it is known to make projection optics with or without magnification. The Fresnel lens with etched concentric gratings, with decreasing spacings from the center to the periphery, is a typical example.

Therefore, according to the invention, conventional refractive optics OF, unusable in the case of the extreme ultraviolet, are replaced by a diffractive optical transmission formed by a grating engraved on an extremely thin membrane and transparent to the extreme ultraviolet. The mask will preferably also be made in the same manner, that is to say on a membrane stretched over an opening formed in a rigid frame. The opening of the frame of the optics is then placed exactly opposite the opening of the frame of the mask when the optics and the mask are in place in the photorépéteur. Figure 2 shows in a very simplified way the general structure of photorepeater which results. It comprises the same elements as in FIG. 1 but the conventional mask M which could not operate in extreme ultraviolet light is replaced by a very thin diaphragm MSK mask, and the optical refractive lens OF is replaced by a diffractive optic DO very thin diaphragm whose function, such as refractive optics, the formation of an image of the mask inside the resin to be insolated. To ensure the readability of Figure 2, the relative dimensions are not respected. For example, in practice, the distance between the MSK mask and the diffractive optic DO may be a few millimeters and the distance between the optical DO and the substrate W may be a few microns. In FIG. 2, the mask and the optics are shown as each having a reinforcing frame with a single opening above which a very thin membrane is stretched, and the opening of the mask is exactly opposite the opening optics. But in practice such thin membranes are very difficult to achieve when the opening exceeds a few millimeters or at most one centimeter side, while one may need masks having dimensions greater than one centimeter side. This is why we will rather adopt a configuration of masks and optics each having a frame with several openings of limited dimensions, as shown in Figure 3, and the openings of the mask are exactly opposite openings of the optics. FIG. 4 represents an MSK transmission mask used in the present invention, and FIG. 5 represents a diffractive projection optic DO, also operating in transmission. The ultraviolet extreme photolithography mask comprises a rigid holding frame 10 having an opening 12 completely surrounded by the frame, and an extremely thin silicon membrane having a thickness of less than 500 nanometers which is stretched over the opening and completely covering the opening. For convenience, there is shown the mask seen from below, that is to say that the membrane stretched above the opening is placed below in Figure 4. The membrane is secured to the frame on all the around the opening; she is flat. The degree of flatness of the membrane is determined by the tensile forces exerted in the plane of the membrane, and these forces are related to the mask manufacturing process. The membrane could also be zirconium, but silicon is preferred, both for its better transparency and for better control of the behavior with respect to the rigid frame.

The thickness of the membrane should be in practice at least 30 nanometers to be manufactured industrially with sufficient strength. The preferred thickness is between 50 and 150 nanometers, which offers a good compromise between the resistance of the membrane and the transparency to the EUV radiation. The silicon of the membrane is preferentially monocrystalline silicon. The frame is preferably essentially silicon (monocrystalline), in particular to minimize the thermal stresses that may be exerted between the frame and the membrane (other than the prestresses voluntarily introduced to manufacture). The silicon membrane is transparent to the extreme ultraviolet because of its very small thickness. The light transmission coefficient can be between 30 and 90% depending on the thickness chosen and a compromise can be found to have sufficient transmission, so a not too thick, and sufficient stress resistance (therefore a thickness sufficient). A thin layer of silicon oxide, not shown, can be interposed between the silicon frame and the membrane. This layer is used during the manufacture of the mask as discussed below.

The silicon membrane 20 carries a masking layer etched in a chosen masking pattern. If it is considered that the silicon membrane has an inner face 20a (located inside the opening 12) and an outer face 20b which is the opposite face, the masking layer is preferably formed on the outer face 20b of the membrane. FIG. 4 shows symbolically a masking pattern MM which is deposited on the outer face 20b and which is therefore seen in transparency through the membrane 20. The masking layer may be metallic, in particular made of tungsten. It can also be made of other materials, for example titanium nitride, provided that it is opaque or partially opaque to ultra-violet radiation even at very low thicknesses (generally less than 1 micrometer). If it is partially opaque, it is preferably also out of phase; Molybdenum is a material that is partially opaque and out of phase.

The masking pattern MM has in principle a larger scale than the pattern that will be projected on a photosensitive layer when using the mask but it is not mandatory. For example, the mask is made on a scale four times larger and the masking pattern is etched on the membrane 20 with an ultraviolet photolithography process or an electron beam. These patterns, when projected through a reduction optics by four, will produce very small patterns illuminated in extreme ultraviolet light. The desired pattern size can go down to about 20 nanometers and even below.

The diffractive projection optics is produced according to the same principle as the mask with a rigid frame 10 ', an opening 12', and a thin membrane (thickness less than 500 nanometers) of silicon 20 'stretched over the opening. An example has been schematized in FIG. 5 in the form of a Fresnel lens, with a diffraction pattern MD in the form of concentric circles of decreasing spacings from the center to the edges. Other diffraction patterns could be used to accomplish the desired function which is essentially a function of forming the mask image on the substrate to be exposed, preferably with scaling (eg 1/4). The MD motif is etched on a layer deposited on the membrane or even on the membrane itself (provided that it does not weaken excessively). The pattern may be a pattern of index variations in a transparent layer or an opaque or partially opaque pattern made in a layer which may be of the same nature as the mask masking layer, i.e. tungsten or molybdenum or tantalum nitride. The membrane carrying the diffractive projection optics can be flat (membrane tight on the frame), but it can also be curved. A curvature can be obtained by limiting the tension of the membrane on the frame and this is determined at the time of manufacture. The etching pattern takes into account the curvature of the membrane to achieve the desired imaging function. The etching pattern can be calculated so that the diffractive optics perform not only its image forming function within the resin but also a possible aberration correction function. It is also possible that the diffractive projection optics comprise not a single membrane but two superimposed membranes (contiguous or not), one carrying a pattern allowing the formation of images (for example concentric circles of decreasing spacings towards periphery), the other a motive for correcting aberrations.

Given the constraints of flatness and resistance of the membrane that must be respected to achieve a mask and diffractive optics usable, it may be necessary to limit the lateral dimensions of the membrane bearing the etched pattern. By way of example, all the lateral dimensions of the membrane are limited to a maximum of 1 centimeter whereas the masking pattern may be larger. In the latter case, it is preferable to use a silicon frame provided with several openings and not a single aperture, both for the mask and for the diffractive projection optics. A membrane portion will be stretched over each aperture of the mask and will carry a corresponding portion of a masking pattern to be projected onto a photosensitive layer. Similarly, a membrane portion will be stretched over each opening of the diffractive optics and will have an individual diffraction pattern (all diffraction patterns are in principle identical unless aberrations related to the position of the diffraction pattern are to be corrected. 'opening).

FIG. 6 shows such an embodiment, in which the mask comprises four openings 121, 122, 123, 124, and four corresponding masking patterns, a priori different from each other, MM1, MM2, MM3, MM4. FIG. 7 represents a vertical section of the mask of FIG. 6. The view of FIG. 7 is reversed with respect to FIG. 6, the membrane 20 being above the openings of the frame (external face 20b above, internal face 20a below). The openings are entirely surrounded by the silicon frame so that the membrane fixed on this frame is well held taut on the opening; at least one lateral dimension of the membrane (and preferably two lateral dimensions) is limited to a maximum width (for example 1 cm). FIG. 8 represents the diffractive optics associated with the mask of FIG. 6. The optic comprises four openings 121 ', 122', 123 ', 124' arranged in such a way that they are opposite the openings of the mask when the mask and the optics are placed in the photorépéteur. Each opening is covered by a respective membrane carrying a diffraction pattern MD1 to MD4. Indeed, each mask portion (MM1 to MM4) spatially modulates a light beam portion and each opening of the projection optics receives the corresponding beam portion to focus the respective image on a photosensitive layer area to be photolithographed carried by the substrate. The diffraction patterns MD1 to MD4 are in principle all identical. The openings shown in FIG. 8 are rectangular or square, and of identical dimensions to those of the mask, but they could be circular or have another shape or other dimensions, provided that they are distributed like those of the mask. that is to say that each opening of the optics is opposite a respective opening of the associated mask.

FIG. 9 represents a vertical section of the diffractive optics of FIG. 8, in one example where the etched diffraction patterns MD1, MD2 are formed in a layer deposited on the outer face 20'b of the membrane 20 '. To produce the mask and the optics, it is possible to start from a silicon substrate covered with a silicon oxide etch stop layer, itself covered with a monocrystalline silicon layer which will subsequently constitute the membrane of the silicon. mask. Silicon is deposited and then etched, a layer of tungsten or tantalum nitride or other material, to define the desired masking or diffraction patterns, taking into account, of course, the future position of the openings that will be required. later form in the silicon frame. A layer of masking resin is then deposited on both sides of the substrate to protect the silicon and this resin is photograved on the rear face of the substrate (face opposite to that which carries the masking or diffraction patterns) to define openings according to a pattern corresponding to the openings that must be formed in the frame of the mask or optics. Finally, the substrate silicon is etched all over its thickness where it is no longer protected by the resin. The attack ends on the etch stop layer which is then removed. Other methods of manufacturing masks may be envisaged, for example a method by assembling two substrates, one comprising the frame of the mask with its openings, and the other (for example a SOI substrate, silicon on insulator) comprising the material of the membrane. The assembly is made by gluing the face which carries the material of the membrane on the silicon frame; during assembly, the temperatures of the two substrates are preferably different to generate controlled stress (if desired); after assembly, the second substrate and its etching stop layer are removed by keeping only the silicon membrane welded to the first substrate. Alternatively, the deposition and etching of the mask can be performed after the formation of the membrane rather than before. With respect to diffraction optics, etching can be done electronically, rather than by photolithography through a mask, to achieve a higher resolution. To implement the photorepetition process when the mask and the associated projection optics have several apertures each, the apertures being each smaller than what would be necessary to expose a complete pattern, the procedure is as follows: successively several masks (in practice 3 or 4 masks) each having several openings, the openings of the different masks having different shapes and / or positions, such as the openings of the whole set of masks make it possible to expose the entire surface the photosensitive layer (for example the entire surface of a microelectronic chip of 2 cm x 2 cm), including the areas hidden by the silicon frame surrounding the openings of the different masks.

Each mask is then associated with a respective diffractive projection optic whose openings are designed each time to face the openings of the associated mask. The choice of shapes and positions of openings of the different masks depends on the maximum dimensions of the openings and the minimum width of the frame that we want to keep around each opening. It takes at least three masks to expose the entire surface without a dead zone, regardless of the shape of the openings. Four of them are needed if we want the openings to be square or rectangular and distributed in a checkerboard, which is the simplest configuration.

FIG. 10 represents at 10A a (symbolic) image to be projected, which has been subdivided by way of example into portions so that each portion can be projected through a masking membrane whose dimension is industrially feasible. By way of example, it is considered that the industrially feasible dimension is 1 cm on all sides, that the reduction factor in the projection is equal to 4 (therefore a projection on a surface of 2.5 millimeters of side ), and that the total surface to be exposed is 2cm by 2cm, or 8x8 times the exposed surface through a single membrane. The image is subdivided into N x N portions, where N is here equal to 8. To print this pattern on a photosensitive layer, we will use a set of four masks each having a checkerboard of (N x N) / 4 openings which all have the same shape and dimensions but are arranged in different places on the different masks. The outer dimensions of the masks are all identical.

The four masks are represented at 10B, 10C, 10D, and 10E and are designated MSK1 through MSK4. The masks are shown with their gray frame 10, and the apertures show the mask portion they must each receive and which corresponds to a respective one of the N x N image portions of FIG. 10A. It can be seen that if the image of FIG. 10A is projected onto a photosensitive layer through the first mask, a quarter of the N x N image portions will be projected onto the layer, the layer not being illuminated where it is masked by the silicon frame surrounding the openings. But if we expose the layer successively through each of the masks, the entire image will finally be projected on the layer. This will be chemically developed after the four exposures. Exposure is by photorepetition, that is to say that for each mask is carried out several steps of offset of the table carrying the substrate to be exposed. In the example shown in FIG. 10, it has been considered that the frame width which surrounds each opening is equal to the width of the opening, so that the openings of the four masks are all identical, but this solution is not identical. not mandatory. Also, it was considered that the openings were square, but they could be rectangular.

The solution of using a set of four masks is the simplest. The masks then comprise P = N2 / 4 identical to each other and identical openings from one mask to another but at different positions. But we can reduce the number of masks to three by adopting other configurations of openings that can cover in three stages the entire surface to expose. The superposition of the three or four masks covers the entire image to be projected; it can be envisaged that there is a slight overlap of the openings and a very slight superimposition of the corresponding masking patterns, to avoid border defects. A mask designed to form an image of 2cm by 2cm with a reduction factor of 4 can be formed of a frame of about 10cm side, machined in a silicon wafer at least 6 inches in diameter (about 160 millimeters).

FIG. 11 represents the respective diffractive optics DO1 to D04 which will be associated respectively with the masks MSK1 to MSK4, that is to say which will have openings respectively opposite the openings of their associated mask. The mask MSK1 was again shown in FIG. 11 to show that the diffractive optic DO1 has openings facing the openings of the mask MSK1. The other diffractive optics DO2, DO3, DO4, have apertures offset with respect to the apertures of the optical DO1 as well as the associated masks MSK2, MSK3, MSK4 respectively have apertures offset with respect to the mask MSK1.

In Figure 11, there is shown circular openings for diffractive optics while the openings of the mask are square. The important thing is the position of the openings and not their shape. The photorépéteur may comprise a fixed optics, or a mobile optics, or a mobile mask / optical block. at. fixed optics: There is a need for three or four different optics each corresponding to a respective mask aperture configuration. The fixed optical block can then include all these optics juxtaposed, with an individual mask loader for each of them. A deflector makes it possible to distribute the source on the different masks if its power is sufficient, or to direct the totality of the energy of the source successively on each mask; but one can also provide several sources each associated with one of the individual optics juxtaposed. The substrate to be insolated is moved under each mask, the alignment being done by the usual systems provided on the photoreputers.

b. mobile optics: The source is fixed. The photoreceptor includes a mask feeder and an optical charger.

These two chargers may be similar since the optics are similar to the mask; but one can alternatively provide that the optical charger is replaced by a barrel carrying three or four optics that can be brought selectively in front of the mask for an insolation operation.

The substrate is each time set up under the mask / optical assembly and then is finely aligned, after which the next mask and the corresponding optics are loaded and aligned and the substrate is aligned below. vs. mobile mask / optical block: Each mask can be associated with a corresponding configuration optics, the optics can be easily duplicated. The integral + optical mask can be treated as a mask and set up in the photorépéteur by a mask charger.

The source is fixed. The alignment of the substrate under the optics is done in a conventional manner.

More generally, a photorépéteur is proposed for receiving a mask in front of a projection optics for projecting an image onto a surface, the projection optics being composed of several partial optics mounted on the same plane support and capable of simultaneously projecting a portion respective of the image. Each partial optic can be controlled independently of the other optics to adjust at least one parameter of the projection of the image portion corresponding to this optic. The parameters of the projection can be the focus, the magnification, the rotation in the plane, the tilt of the plane, or even the precise position of an image portion in the plane. The optic is preferably diffractive as indicated above, especially when the wavelength is the extreme ultraviolet. Each partial optics may itself be composed of a group of elementary optics corresponding to an image sub-portion, the elementary optics of a group being controlled simultaneously. The image is formed by several partial images whose juxtaposition reconstructs the overall image, but the partial images or groups of partial images can be adjusted individually by action on the partial optics (which can be groups of elementary optics) . Each elementary optics is associated with a corresponding portion of the mask. This method is particularly well suited for thin-wall etched optics, but it can also be extended to any type of optics. It can be provided that the different optical or groups of elementary optics are assembled on shims with piezoelectric control controlled independently of each other. The assembly is mounted on a rigid support. The rigid support, of thickness adapted to the desired flatness, is preferably made of material with a low coefficient of expansion (typically: materials known as zerodur or cervit), but the flatness defects can be corrected by the action on the holds. For example, N individual blocks may be provided for controlling a partial optics or a group of elementary optics forming partial optics. The simultaneous control of the N blocks of a group makes it possible to act on the position of the optics with respect to the projection plane of the image, thus on the focusing. The individual control of each of the N cleats makes it possible to act on the inclination, the rotation, the magnification of the individual image. This is of course in this case fine corrections of the parameters of the optics, the displacement by piezoelectric effect being of very limited amplitude. Non-piezoelectric systems (electromagnetic coils, capacitors, etc.) allow larger displacements for larger corrections.

In one example, it is possible to provide a matrix of individual partial optics arranged in N rows and P columns, with rows and columns of fixed elementary optics (not controllable individually) and blocks of several elementary optics joined together to form an optic Partially ordered individually. By way of example, FIG. 12 shows a matrix of elementary optical 7 × 7 which may have a central line of fixed optics OL, a central column of fixed optics OC, and four blocks OBI to OB4 each having nine optics elementary elements secured to a rigid support associated with the block; the blocks of nine optics are individually controllable by piezoelectric shims. The control of the focus is first made globally by a conventional adjustment system which acts on the entire support carrying the optics. It can be done through the optics themselves. The fine adjustment of the parameters of the optics or optical blocks can be done retrospectively after examination of test chips, to correct image defects by action on each of the blocks. The defects of the image can come from either defects of the projection system or flatness defects of the projection surface. The action on individually adjustable optics makes it possible to correct these defects. FIG. 13 gives an example of mounting of individual optics, adjustable separately from each other, on a rigid support 200 provided with recesses 210 for passing light; the optics 220, constituted here by thin membranes and their peripheral frame, are placed in front of the recesses and mounted on wedges 230 placed on the rigid support at the edge of the recesses. The shims can have any shape and be made in one or more parts according to the desired movements. In the case of extreme ultraviolet, the distances between the optics and the projection surface are of the order of a millimeter while the distances between the mask and the optics is of the order of a centimeter. The adjustment movements of individual optics or optical blocks are of the order of a micrometer, which is possible with piezoelectric shims.

Claims (14)

REVENDICATIONS1. Photorépéteur for extreme ultraviolet lithography characterized in that it comprises a projection optics comprising a thick rigid frame having at least one opening on which is stretched a thin ultra-violet transparent membrane with a maximum thickness of 500 nanometers bearing a pattern etchings conferring on the thin membrane a diffractive projection optical function in transmission at the wavelength used by the photorépéteur. 2. The photorépéteur according to claim 1, characterized in that the thin membrane is monocrystalline silicon. 3. photorépéteur according to one of claims 1 and 2, characterized in that the thick rigid frame is silicon or silica, or glass, or quartz. 4. photorépéteur according to one of claims 1 to 3, characterized in that the etching pattern is formed in an opaque layer or partially opaque ultraviolet radiation extreme, deposited on the thin membrane. 5. photorépéteur according to claim 4, characterized in that the layer is tungsten or molybdenum or tantalum nitride. 6. Photorépéteur according to one of claims 1 to 5, characterized in that the diffractive optics comprises patterns allowing not only the formation of an image of a mask in a resin to be exposed but also the correction of aberrations . 7. Photorépéteur according to claim 6, characterized in that the diffractive optics comprises two superimposed membranes each comprising a pattern of engravings, one of the elements allowing the formation of an image of a mask, the other allowing the correction of aberrations. 15 20 8. Photorépéteur according to one of claims 1 to 7, characterized in that the frame of the projection optics comprises an arrangement of several regularly distributed openings, with, above each respective opening, a membrane bearing a pattern of etchings imparting to the thin membrane a diffractive optical function in transmission at the wavelength used by the photorépéteur. 9. Photorépéteur according to one of claims 1 to 8, characterized in that the projection optics is removably mounted to be able to change optics at the same time as changing masks, to set up an optical adapted to the mask. 10. Photorépéteur according to one of claims 1 to 9, for receiving a mask in front of a projection optics for projecting an image onto a surface, the projection optics being composed of several partial optics mounted on the same plane support and capable of simultaneously projecting a respective portion of the image, each partial optic being controllable independently of the other partial optics to adjust at least one parameter of the projection of the image portion corresponding to this optic, such as focusing, magnification , the rotation in the plane, the tilt of the plane, or the precise position of an image portion in the plane. 11. photorépéteur according to claim 10, characterized in that each partial optics is composed of a group of elementary optics corresponding to an image sub-portion, the elementary optics of a group being controlled simultaneously. 12. Ultra-violet extreme lithography process, using a photorepeater and a lithography mask, characterized in that a projection optics comprising a thick frame having at least one aperture on which a tension is stretched is placed in the photorepeater. Ultra thin ultraviolet transparent membrane with a maximum thickness of 500 nanometers carrying a pattern of etchings conferring on the membrane membrane a diffractive projection optical function in transmission at the wavelength used by the photorépéteur, and setting up a transmission-operated mask having a thick frame having at least one opening on which is stretched a thin ultraviolet transparent thin membrane of a maximum thickness of 500 nanometers, carrying a thin layer of opaque material or partially opaque to radiation extreme ultraviolet, this thin layer being etched in a pattern defining the desired mask in an area defined by the opening on which is stretched the membrane, the aperture of the projection optic being arranged opposite that of the mask when the mask and optics are positioned in the stepper. 13. lithography method according to claim 12, characterized in that the mask comprises an arrangement of several regularly distributed openings, with, above each respective aperture, a membrane carrying an opaque or partially opaque layer at the extreme ultraviolet. violet, etched in a respective pattern, and the projection optics comprises an arrangement of several regularly distributed openings, with, above each respective aperture, a membrane carrying a pattern of etchings conferring on the thin membrane an optical function diffractive projection transmission at the wavelength used by the photorépéteur, the openings of the mask being disposed opposite corresponding openings of the projection optics when the mask is placed in the photorepéteur. 14. lithography method according to claim 13, characterized in that it comprises the successive exposure of a photosensitive layer through different masks, each mask comprising a frame provided with several openings on each of which is formed a thin membrane bearing a thin layer etched, the shapes and / or positions of the openings relative to the frame being different from one mask to another to allow the complete exposure of the surface of the photosensitive layer, including in the areas masked by the frames of the different masks around their openings, and each mask is associated a projection optics having an aperture distribution corresponding to the aperture distribution of the respective mask so that each opening of the mask is opposite a respective aperture of the mask. associated projection optics.