Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Including heating
Reexamination Certificate
1999-03-17
2001-04-17
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Including heating
C216S052000, C369S043000
Reexamination Certificate
active
06218086
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method of forming ultrasmall structures which have dimensions in the order of nanometers in a thin film; the invention is particularly applicable to nano-lithography production methods, in which a thin polymer layer is used. Preferably, these structures are formed by means of devices the basic construction of which is similiar to that of an atomic force microscope (AFM). Furthermore, the invention relates to an apparatus for carrying out the method.
BACKGROUND OF THE INVENTION
In the prior art, there are known several methods for forming structures that are of very small dimension (nanostructures); many methods of this kind make use of AFM-like devices, i.e. devices which mainly consist of a tip which is mounted on a cantilever, the latter being relatively movable with respect to a substrate. Particularly, nano-lithography by AFM has a high potential due to the ability of this technology to create structures in the order of nanometers, which dimension is even smaller than the wavelength of conventional laser light.
However, a problem that usually occurs in these known methods is the wear of the tip of the AFM; this is because the nanostructures are formed by applying current to the tip in order to modify the structure of a photoresist layer, this current destroying the apex of the tip after relatively short time. A further disadvantage of these conventional methods has to be seen in that the writing into a layer of polymer by an AFM device does not give satisfactory results with respect to the resulting pit size.
Furtheron, in most of these prior-art devices, the application of force writing has been used in order to form the structures. However, the process speed is relatively slow in this approach because the relatively soft cantilevers of the AFM devices have to be deformed to a large extent in order to obtain a force of sufficient strength. Thus, the formation of the nanolithographic structures takes a very long time.
Accordingly, it is the object of the present invention to provide a method of forming ultrasmall structures, i.e. structures having dimensions in the order of nanometers, which both allows to form extremly small pit sizes and to avoid wear of the tip. When in the following it is referred to avoidance of tip wear, also a reduced tip wear with respect to known methods shall be implied.
SUMMARY OF THE INVENTION
This object is solved by the characterizing features of the claims. Advantageous further developments of the invention are subject-matter of the subclaims.
The basic idea of the present invention which is linking the subject-matter of the afore-mentioned independent claims is a measure to provide a limitation of the penetration depth of the tip, the limited penetration depth being selected such that the apex (distal end) of the tip is hindered from touching the comparatively hard surface of the substrate. The tip, therefore, is only contacting the film, the hardness of which, usually, is much smaller than the hardness of the tip; thus, even after a long process time, the tip will not show any wear at all.
Acordiling to the first embidoment, the present invention proposes a method for forming ultrrasmall structures which is characterized by the usage of a thin film which is formed in a thickness which is in the same order as the smallest dimension of the ultrasmall structures to be formed therein. That is to say, the structures formed in the thin film are substantially of a similar size as the thickness of the film. Experiments made by applicants have proven that that such a thin layer effectively avoids wear of the tip, if the structures or pits (recesses) are formed by means of a tip which is heated to a temperature which is sufficient to locally deform the thin film below the tip, i.e. by means of a thermal treatment of the thin layer; the thin layer, due to its small thickness, is effectively cooled by the underlying substrate which, normally, has a much higher heat conductance than the thin layer; hence, the heat deformation of the thin layer is stopped before the apex of the tip may contact the substrate and the tip does not show wear because the thin layer is much softer than the tip.
The tip used in the afore-mentioned method preferably is provided with an integral heater which is able to heat substantially the tip only; the surrounding portions of the cantilever, however, should be heated as little as possible, i.e. these parts should be isolated with respect to the tip. An integrally heated tip of that type has the advantage of a very fast process speed; furthermore, the reduced energy consumption of such a tip allows the usage of a plurality of tips in parallel so that the processing speed can be dramatically increased. This is an important aspect if very complex lithographic structures have to been formed in a reasonably short time.
According to the second embodiment, another approach of the present invention to solve the afore-mentioned problem is the use of a tip which, near the distal end thereof, is broadening so as to form a step which is limiting the penetration depth of the tip into the thin film. By suitably selecting the dimensions of this step it is possible to limit the penetration depth of the apex of the tip; i.e., normally, the tip will penetrate into the thin film up to that depth only at which the step is contacting the surface of the film, thereby abruptly enlarging the contact face. Due to this measure, the apex will not contact the underlying substrate and, thus, will not be subjected to wear.
According to the third embodiment, a still further approach of the present invention to solve the afore-mentioned problem is the use of a double-layer film, wherein the lower layer, i.e. the layer which is formed on the substrate, is made of a material which is harder than the material of the upper layer but softer than the material of the tip. If the tip is penetrating into such a film the further movement of the tip will be stopped at that moment at which the apex of the tip is beginning to penetrate into the surface of the harder lower layer. As the hardness of this lower layer nevertheless is much smaller than that of the tip, wear of the tip can be avoided.
In the afore-mentioned two further approaches of the present invention, i.e. when using a stepped tip or a double-layer film, the structures (recesses) in the film preferably are formed by means of heat treatment, preferably by means of a heater which is integrated in the tip, as mentioned above. Moreover, in all three of the afore-mentioned approaches of the invention, the additional application of a predetermined force which is acting substantially perpendicularly to the thin film may be useful.
A combination of the thin-film approach with the stepped-tip approach and/or the double-layer film approach may be useful as well.
The tip used in the present invention, in general, is similiar to those tips which are used in AFM devices, i.e. this tip is movable relative to the surface of the thin film and preferably mounted on a cantilever; the tip preferably being made of silicon.
The thin film preferably consists of a material which should be either deformable by a heat treatment through the tip or deformable by the force applied through the tip. Further, the material of the film should be of such a consistency that it can be formed as a homogenous, smooth and thin film. A suitable material having such properties is e.g. a polymer such as PMMA. A thin metal film having a low melting temperature could be used as well.
By the afore-mentioned advantageous measures of the present invention, no significant wear of the tip is caused, neither by deforming the film by a heat treatment nor by deforming the film by a mechanical penetration of the (heated) tip. Additionally, according to the present invention, no (electro-) chemical processes are involved so that wear of the tip by a chemical reaction can be avoided, too.
Another advantage of the afore-mentioned methods has to be seen in that the penetration depth of the tip can be controlled relati
Binnig Gerd K.
Despont Michel
Haeberle Walter
Vettiger Peter
Barreca Nicole
Drumheller Ronald L.
Huff Mark F.
International Business Machines - Corporation
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