BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method for fabricating a micro-technical structure, in particular a ferromagnetic structure for use in a MRAM (Magneto-resistive Random Access Memory), wherein a mask is structured, the mask is arranged on a substrate and the micro-technical structure is generated on a surface region of the substrate which is not covered by the mask. The invention also relates to a micro-technical component having a substrate and a micro-technical structure arranged on the substrate.
Particularly in the case of microelectronic components, i.e. electronic components whose structure dimensions reach the micron range and even the sub-micron range, special, highly developed structuring processes are employed. Structures with particularly small dimensions are desired in particular for the development of data memories. However, structuring processes of this type are also employed for microstructures for other applications, such as for example write and/or read heads for hard disks.
It is known to use lithography processes and etching processes to structure thin layers and layer systems. In particular, in this case the lithography processes generate structures in photo resist layers, and etching processes transfer these structures into the thin layers or layer systems which lie below the photo resist layers. Anisotropic plasma etching processes, such as for example RIE (Reactive Ion Etching), RSE (Reactive Sputter Etching), ECR etching (Electron Cyclotron Resonance etching), ICP etching (Inductively Coupled Plasma etching) and CAIBE (Chemically Assisted Ion Beam Etching) are known for layer structuring in the micron (μm) and sub-micron (sub-μ) range. With etching processes of this type, it is necessary for the reaction products formed from the material which is to be etched away to pass into the gas phase so that they can be removed from the reaction chamber. Numerous materials which are suitable for microstructures in terms of their physical properties cannot be satisfactorily structured using some or all of the known etching processes, since the reaction products which are formed during the etching form a passivating layer on the surface of the material which is to be etched, thus preventing further etching and removal of the material. Furthermore, etching processes in general may lead to redeposition of the material which has been etched off, for example on etching masks, the edges of the regions which are to be etched and on parts of the etching chamber. This leads, for example, to undesirable inclined etching flanks and changes to the dimensions of etching masks. However, electrical short circuits caused by electrically conductive redeposition on the flanks of multilayer systems may also occur.
Particularly for use in future MRAMs (Magneto resistive Random Access Memories), structures with ferromagnetic materials, such as Ni, Fe and Co, as well as alloys comprising these materials, are produced and tested for suitability. When structuring these materials using etching processes, the nonvolatile passivating layers described above are formed. S. J. Pearton, et al., in the publication “High Rate Etching of Metals for Magneto Electronic Applications,” Electrochemical Society Proceedings Vol. 97-21, pages 270-85 (hereinafter “Pearton”) propose using a plasma etching process with a high ion density, in order to avoid the formation of a disruptive passivating layer. According to Pearton, the high ion density leads to a high ion flux, so that normally nonvolatile reaction products are sputtered away. Pearton proposes etching ferromagnetic metal alloys, such as NiFe and NiFeCo, in the presence of Cl in the etching gas. Although this leads to higher etching rates than with pure Ar etching gas, chlorine-containing compounds that are thereby formed lead to corrosion of the metal alloys after the etching. The chlorine-containing compounds have to be removed in a further process step.
It is reported in the publication “Assessment of Dry Etching Damage in Permalloy Thin Films” by S. D. Kim et al., Journal of Applied Physics, Vol. 85, No. 8, pages 5992-5994, dated Apr. 15, 1999, that plasma dry etching processes, such as IBE (Ion Beam Etching) and RIE (Reactive Ion Etching), in the case of NiFe (Permalloy), lead to the magnetic properties being impaired, on account of the bombardment with ions.
A further drawback of plasma etching techniques is the low selectivity of the etching action both with respect to the etching mask and with respect to the substrate on which the material to be etched is arranged. This leads to etching masks being worn away and to undesirable structuring of the substrate.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method of fabricating a micro-technical structure and a micro-technical component with a micro-technical structure formed on a substrate, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and wherein the generation of the final lateral dimensions of the structure (structuring) effects and/or causes and/or has caused as far as possible no damage to the structure and the substrate. Particularly in the case of structures comprising ferromagnetic materials, the magnetic properties of these materials are not to be adversely affected by the structuring and subsequent process steps caused by the structuring.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of fabricating a micro-technical structure, in particular a ferromagnetic structure for an MRAM. The method comprises the following steps:
providing a substrate;
forming a structured shadow mask on the surface of the substrate, and defining on the surface an uncovered surface region not covered by the mask and a shadow region shadowed but not covered by the mask; and
depositing material through the mask in a directed deposition process and forming the micro-technical structure on the uncovered surface region of the substrate not covered by the mask.
With the above and other objects in view there is also provided a micro-technical component, comprising:
a substrate; and
a micro-technical structure formed on the substrate and extending along a common interface with the substrate, the micro-technical structure having been fabricated with the above-outlined method, and a shaping of a surface or at least one layer of the structure at an edge of the structure resulting only from a deposition of the structure material in the directed deposition process.
An important idea of the present invention is to arrange the material of the structure on the substrate in the same process step as at least part of the structuring of the structure which is to be fabricated. The structure is laterally delimited in at least one location of the substrate purely by the fact that the structure material is arranged on the substrate. This eliminates the need for a following etching process, which could cause damage to the structure and/or the substrate.
In one embodiment, a mask is structured and arranged on the substrate in such a manner that the mask shadows but does not cover a surface region of the substrate. In the direction of a surface normal, the mask is situated at a distance from the surface in this surface region. Then, material of the structure which is to be fabricated is deposited on the substrate in a directed deposition process. The term “directed deposition process” is understood as meaning a deposition process wherein the material which is to be deposited generally moves in a directed manner, namely in a straight line, toward the deposition location. This does not rule out the possibility of the direction of movement of the material which is to be deposited being changed, for example by the interaction of a plurality of particles of the material being deposited and/or deflection from structure edges and/or by scattering on fixed structures. However, most of the material which is to be deposited will move substantially in a straight line from a deposition source or a source region of the deposition to the deposition location.
A significant advantage of this embodiment consists in the fact that significantly less material is deposited on the shadowed surface region of the substrate than on uncovered and unshadowed surface regions of the substrate. Furthermore, even within the shadowed region the deposition rate is dependent on the distance between the deposition location and the edge of the shadowed region. Generally, the deposition rate will fall continuously at increasing distance from the edge of the shadowed region. The dimensioning of the mask and setting of the distance between the mask and the shadowed surface region of the substrate, as well as the nature and process management of the directed deposition process, therefore allows the local distribution of the thickness of the deposited material in the shadowed region to be controlled. It is therefore possible, for example, to produce a more or less steep flank of the deposited material at the edge of the shadowed region.
In a special embodiment of the invention, wherein, by deposition of a plurality of layers on top of one another on the substrate, the in this case multilayer micro-technical structure is generated, the control options referred to above can be utilized to good effect. For example, it is possible for a layer which is deposited at a later stage to completely cover a layer which was deposited at an earlier stage all the way over the edge of the layer which was deposited earlier. The layer deposited later is, for example, a tunnel barrier, a diffusion barrier and/or a separating layer which prevents the material of the layer deposited earlier from coming into contact with material of a layer which was deposited even later than the separating layer. In particular, a TMR (Tunnel Magneto resistance) element or a GMR (Giant Magneto resistance) element can be produced in this way. TMR and GMR elements can be used, for example, as memory elements for future MRAMs.
In particular, the layer which was applied later and the layer which was applied earlier are produced, in deposition processes, with a different angular distribution of the material which is to be deposited. In this context, the term angular distribution is understood as meaning the angular distribution of the material to be deposited from the viewpoint of a source of the deposition process or a source region of the deposition source. Sources of the deposition process may, for example, be a sputtering target, an electrically heatable evaporation source and/or the target of an electron beam evaporator. The angular distribution can be set in particular by ionization of the material to be deposited and by means of electrical fields. Pure PVD (Physical Vapor Deposition) processes have a more uniform angular distribution than PVD processes with ionized deposition material in the presence of electric field forces. In the case of electron processes, the flux of deposition particles in the direction of the electric field forces is greater.
The mask which shadows the surface region of the substrate may be structured before, during and/or after its application to the substrate. It is advantageously arranged and structured in such a manner that a first part of the mask covers a surface region of the substrate and a second part of the mask forms an overhang which is supported against the substrate by the first part and which defines the shadowed surface region. The length of the overhang is preferably adapted to the abovementioned factors of influence for controlling the deposition of the structure material in such a manner that the edge of the structure generated does not abut the mask. In this case, in a following method step, filler material may be arranged between the mask and the edge, serving, for example, as electrical insulation or a diffusion barrier.
In accordance with a particularly preferred feature of the invention, when arranging the mask on the substrate initially a first mask layer is deposited on the substrate. Then, a second mask layer is deposited on the first mask layer. Therefore, in the region wherein the micro-technical structure is subsequently to be generated, the substrate is covered by two mask layers which lie on top of one another. To structure the mask, the second mask layer and the first mask layer are removed again in defined regions. The result is a surface region of the substrate which is neither covered nor shadowed and is therefore available for the deposition of the structure material. The first and second mask layers preferably comprise different materials. The process for removal of the first and second mask layers may be a single-stage or multistage process. Then, in an isotropic etching process, additional material of the first mask layer is removed between the second mask layer and the substrate. This results in the overhang which is formed by the second mask layer.
Often, there is a need for micro-technical structures which are intended to have a continuous encircling edge with a defined local arrangement. By way of example, microelectronic structures at a defined location are deposited directly on a material which is used for electrical connection to the microelectronic structure. If the structure is not arranged at precisely the correct position, the electrical contact is insufficient and short circuits may occur at a later stage. The above-described arrangement and structuring of the mask offers a reliable solution to this problem. The first and second mask layers can be opened up at precisely the correct location, for example by lithographic methods. The edges of the regions which have been opened up in this way substantially also define the edges of the structure which is subsequently to be fabricated. In particular, the shadowed surface region forms a continuous encircling edge around an unshadowed surface region of the substrate, which is to be provided with the micro-technical structure.
In order, for example, to electrically insulate the micro-technical structure and/or mechanically stabilize the structure, the shadowed surface region of the substrate and/or the surface of the micro-technical structure is preferably covered with a filler material after the micro-technical structure has been generated. Particularly with the above-described type of arrangement and structuring of the mask, it is possible for the entire region which has been opened up in the first and second mask layers to be filled with the filler material. A planar surface is then formed by removing part of the mask and part of the filler material. The planar surface then, for example, allows the deposition of further planar layers and therefore allows the generation of further structure elements in a manner known per se.
The micro-technical component according to the invention is wherein the structure, which extends along the common interface with the substrate, has been fabricated using the method according to the invention, and the shaping of the structure surface or of at least one layer of the structure at the edge of the structure is exclusively the result of the deposition of the structure material in the directed deposition process. Therefore, the surface of the edge of the structure or of the layer has no traces whatsoever of a subsequent structuring measure carried out after the end of the deposition of the structure material. In particular, it has no residues of chemicals and/or ions which are used in an etching process, and, as is typical for a deposition process, the surface profile is slightly irregular, i.e. the surface has the typical roughness. This can be established by X-ray structural analysis tests in a corresponding way to that described in the abovementioned publication by S. D. Kim et al.
Particularly if the micro-technical structure has been fabricated with a sufficiently long overhang of the shadow mask, the structure at its edge has a characteristic S-shaped surface. In this area, the thickness of the structure increases in an S-shape from approximately 0 to a mean value for the structure.
The structure is preferably enclosed by an adjoining filler material which is applied to the substrate.
A significant advantage of the invention is that there is no need for complex plasma etching processes to structure the micro-technical structure. To structure the shadow mask it is possible, for example, to use much simpler etching installations with proven etching processes which are used for damascene processes. Particularly in the case of microelectronic structures, these installations are already in use for fabrication of the electrical connection lines. On account of the in-situ structuring of the structure by targeted, locally restricted deposition of the structure material, damage to the structure and/or the substrate caused by structuring etching processes is avoided.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating a micro-technical structure, and micro-technical component, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.