METHOD AND APPARATUS FOR DOPING A SOLID MATERIAL

Abstract: A doping method for a solid host material (2), preferably a film, using an electrochemical cell (1, 2, 5, 6) and the host material (2) as one of the electrodes, wherein the doping is confined to a selected region of the host material by using either a specially designed counterelectrode, for example a tip (1), or by covering the host material (2) by a layer with the exception of the selected region. The method is especially suitable for using oxygen as dopant and the preparation of oxide superconductors. By using control devices and (piezoelectric) transducers known from the scanning tunneling microscope, doped regions of submicron resolution can be produced.

Description: DESCRIPTION Method and Apparatus for Doping a Solid Material This invention relates to a method and apparatus for implanting dopants into a solid material, preferably a thin film of solid material. Specifically, it pertains to a, preferably reversible, method of preparing a pattern of superconducting material in an otherwise non-superconducting material or vice versa. More specifically, the invention concerns a method of implanting or extracting oxygen from solid material. It further relates to producing patterns having a resolution higher than one μm. BACKGROUND OF THE INVENTION Background of the invention is the doping of solid material, particular of thin films of solid material. Solid material as defined within the scope of this invention comprises non-organic crystal and amorphous matter. The process of doping is well known in the production of semiconducting devices. Several techniques to produce a dopant distribution of high resolution, i.e. having lateral dimensions of less than 1 /tm, are known. Most commonly, photolithographical techniques are used; photolithography is based on spinning a photosensitve polymer on top of the solid material. This polymer layer is exposed to UV-light through a mask which shows the desired pattern. Depending on the type of polymer, either the exposed area or the area of the layer which has been left unexposed can be easily removed by a solvent, leaving the selected region of the surface of the host material uncovered either for a subsequent diffusion of the dopant into the solid, for selectively etching doped material, or for extracting atoms from the solid material. The use of photolithography to extract oxygen from a superconducting material, is described, for example, in the European patent application 0 484 248. Another technical field related to the invention is known as electrochemical deposition. Though a variety of different techniques are known, in the basic apparatus, an electrochemical cell with two electrodes is applied. The material serving as substrate is connected to one of the electrodes while the electrolyte or the counterelectrode contains the material as ions to be deposited onto said substrate. With the flow of a (faradaic) current through the electrolyte, a discharge and a subsequent deposition of the ions can be achieved. As the deposition is restricted to the surface of the substrate, the electrochemical deposition method is usually not suitable for the purpose of doping, i.e. when the dopants have to be implanted within the solid material. Nevertheless, from the European patent application 0 434 480 or Physica C 173 (1991 ), pages 139-144, it is known to implant oxygen into a solid material. The current invention is partly based on this method for electrochemically oxidizing a superconducting material. However, the oxidation process described in these references is not locally confinable and, hence, can not be used for generating a pattern. High resolution patterning in an electrochemical cell is known, for example, from US patent 4 968 390. It proposes an electrochemical deposition method in which a modified version of the scanning tunneling microscope (STM) is used as one electrode. The substrate serving as the other electrode is covered by an ionic conducting polymer film as electrolyte containing the ions to be deposited. The ions move within the polymer film under the influence of the electric field towards the electrodes. However, they do not diffuse into the substrate, but form patterns at the interface between the film and the neighbouring material. Thus, diffusion takes place only in the electrolyte, itself. Further, even though the deposition of lines of 0.5 μvn width can be achieved at the polymer/air interface, the resolution of lines deposited at the polymer/substrate interface show a significantly lower resolution, as the distance between the substrate and the tip of the STM is necessarily larger than the film thickness. In view of the shortcomings of the known techniques, it is an object of the invention to provide a method and an apparatus for implanting dopants into solid materials or films thereof. It is another object of the invention to provide regions in a solid material having a lesser or higher oxygen content than the surrounding material. It is yet another object of the invention to provide a method and an apparatus for producing a pattern consisting of superconductive material within a non-superconductive host material or vice versa, wherein the patterning is preferably reversible. Another object of the invention is to provide a method and apparatus for producing a pattern with submicron resolution. SUMMARY OF THE INVENTION The new doping method uses an electrochemical cell comprising an fluid electrolyte containing the dopant and at least two electrodes to which a voltage is applied to create an electrical field wherein the electrical field is essentially concentrated at a selected region effectively restricting the doping to this region of the solid material. Any conventional electric cell can be utilized for employing the new method. Apart from a simple cell with just two electrodes, the cell may contain two compartments separated for example by a frit. Further, the voltage between the electrodes can be controlled more accurately by using a third (reference) electrode, e.g. a calomel electrode or a hydrogen electrode. Between both electrodes, a voltage is applied the amount of which strongly depends on the used materials and the distance between both electrodes. However, a voltage ranging from 100 mV to 5V, preferably from 200mV to 1V, has been found to be suitable for a wide range of applications. To define the optimum value of the voltage for specific materials, a person skilled in the art usually relies on a measurement of a (cyclic) voltammogram, i.e. the measurement of the current as a function of the applied voltage. Changes in the slope of this current function indicate the onset of an electrochemical reaction. Thus, a person skilled in the art is able to establish an optimum value for the voltage by analyzing the voltammogram. To control the doping reaction, either a constant voltage or a sequence of short pulses can be applied. Applying a voltage to both electrodes gives rise to an electrical field, which exerts a force upon the ions in the electrolyte. The moving ions form the faradaic current. The described method is preferably applicable at room temperature (20-25°C). This forms a major advantage over some of the known techniques. It is, however, feasible to increase the diffusion of the dopants into the solid material by using elevated temperatures. Confining the doping to selected regions of the solid material is essential for producing defined patterns. The invention discloses several methods for confining the doping. These methods are based on concentrating the electrical field between both electrodes up to a large extend to the selected regions, i.e. the strength of the electric field is significantly higher near to the selected regions then it is near to other areas of the solid material. In one embodiment of the invention, the confinement of the doping to the selected regions is achieved by adapting the geometry of the counterelectrode and the distance between both electrodes in accordance with the desired pattern. Depending on the required resolution, the distance between both electrodes may vary between 0.1 nm and 100 m. Either the counterelectrode consists of a single, ultrafine tip, which is scanned over the selected regions, or the utmost part of the counterelectrode is patterned in conformity with the selected regions or at least a part of them. Such a patterned electrode is manufactured by using conventional photolithograpy or, if submicron resolution is desired, by electron beam lithography. A combination of the above arrangements is the use of an electrode to which several tips have been attached, and which, hence, allows doping in parallel. With a single tip a very high resolution is achievable as the single tip can be sharped within atomic dimensions by using known techniques, such as etching, sputtering or field evaporation. The movement of the tip in vertical (z-) direction is controlled by means of a known scanning tunneling microscope feedback control keeping the current flow through the tip constant. In contrast to conventional scanning tunneling microscopy, the tunnel current is blotted by the larger faradaic current. By keeping the faradaic current constant, the amount of dopant at the surface of the host material is simultaneously controlled, resulting in a pattern with uniform line width. Beside the constant-current mode, it is further possible to keep the distance between the two electrodes constant. A desired pattern is written by either moving the tip over the surface of the other electrode or by moving this electrode in lateral (x-,y-) directions. In all cases the fine movement of the electrodes can best be performed by using a piezoelectric transducer submitted to an appropriate voltage. By using a patterned counterelectrode, as described above, the scanning movement can be reduced to a relatively coarse positioning. The movement in z-direction, however, still requires a fine positioning and is, thus, done by piezoelectric transducers attached to the tip or patterned electrode. To produce patterns of submicron resolution, the electrode - counterelectrode distance is kept below 1 m. To produce patterns having a resolution in the range of 1 nm to 100 nm, the electrodes are preferably spaced in tunneling distance (0.1 nm to 10nm), i.e. the distance in which a tunnel current can be measured. A careful choice of this distance allows to vary the resolution of the pattern considerably, as the lateral spread of the faradaic current varies with said distance. Changes of a linewidth in the range of an order of magnitude, for example from 1 nm to 100 nm or even larger, can be achieved. According to the new method not only extremely small lines but also wider lines can be written by enlarging the distance between the electrodes. With the distance, the area at which the doping takes place increases roughly linear. The resolution is further improved by coating the counterelectrode, with the exception of its ultimate tip, i.e. its utmost part by a dielectric material. The ionic current is, thus, restricted to the uncovered part of the counterelectrode. The counterelectrode can be insulated or covered to a large extend by using apiezon wax, teflon, epoxy resins or similar materials. Only the utmost tip or edge of the electrode is freed or kept free from this insulating layer. The coatings produced by these known methods have typically a thickness of more than 1 m and leave several μm 2 of a tip uncovered. In another embodiment of the invention, the electrochemical doping is confined to selected regions by covering the solid material with a photoresist which is insoluble in the electrolyte, and create the selected regions by using conventional photolithography. The solid material with the remaining parts of the photoresist is then exposed to the electrochemical doping. This method is insensitive to the shape of the counterelectrode and the interelectrode distance. The methods described above are applicable to semiconducting and metallic materials. Preferably, they are applied to semiconducting or superconducting metal oxides, such as perovskites (SrFe0 3 , La 2 Ni0 4 ). Examples of suitable superconducting materials are listed in the European patent application 0 426 570, page 3, lines 36-55. Other suitable superconductors belong to the ' infinite layers" category, such as Ca,_ x Sr x CuO y , with Ln standing for an element selected from the lanthanides, including yttrium and lanthanum, Ba,_ x Sr x CuO y , and compounds of the above-mentioned category, including dopants selected from the group comprising C, N, F, and CI, or electron-doped superconductors. The invention is preferably applicable to films of the above mentioned materials. However, by using thin films even electrically insulating solid materials can be doped in accordance with the new method. Films of solids are characterized by their compounds, their thickness, and by crystallographic parameters. For this purpose, in general, the orientation of a crystal axis of the film material is used with respect to the substrate onto which it is deposited. For example, an "a-axis orientated film" is a thin film the a-axis of which is oriented perpendicular to the surface of the substrate. The new method is found to be applicable to thin films without regard to their crystallographic orientation, i.e a-, b-, or c-axis films. Within the scope of the invention, a thin film is defined by having a thickness in the range of 1 nm to 5/rnι, preferably in the range of 10nm to 0.9 m. It is produced by using conventional techniques, e.g. sputtering, molecular beam epitaxy, or metaiorganic chemical vapor deposition, and the like. In addition, the new method can not only be applied for simple isolating, semiconducting, metallic, or superconducting thin films, but also to a variety of heterostructures wherein the film is either on top of a structure such as a metallic layer, a bilayer system (metallic layer / thin insulator) and any other combination of this kind, and/or is covered by such a structure. The new method can be applied stepwise, in the sense that first one layer of a film is doped, and after deposition of the next film layer on top of the existing structure, the method is applied again. Stacks of patterned layers on top of each other can thus be produced. A metallic layer beneath the film can by used, for example, as a contact area for applying the voltage to the film. It is further possible to use a thin covering layer to protect the underlying film in cases in which this film is unstable in the electrolyte solution or under other ambient conditions. In a preferred embodiment, a film of potentially superconducting material is doped with oxygen. For this purpose, liquid electrolytes containing KOH, NaOH, or H 2 SO., in an aqueous or alcoholic solution can be used. The counterelectrode is brought close to the surface of the film using the standard STM techniques described above. Then the voltage between counterelectrode and film is increased up to the voltage region required for the doping reaction. Locally - as defined by the uncovered tip region - oxygen is introduced into the film. By scanning the counterelectrode according to any chosen pattern or by applying a patterned counterelectrode, oxidized regions can be defined. By controlling the time during which the counterelectrode is kept over the same area of the solid material, i.e. the scanning speed, or the faradaic current, it is possible to vary the amount of oxygen within the solid. Hence, the critical temperature of the superconductor can be varied locally and continuously in the range from 0°K up to the optimum value of said superconducting material. The improved control over the oxygen content of the film forms a major advantage of the new method over the existing techniques. Further, focussed beam techniques also used for patterning superconductors damage, to a certain extent, the surface of the film, deteriorating its superconducting properties. The edges and boundaries formed by applying the new method are found to have less detrimental influence on the superconductivity. In another preferred embodiment of the invention, the voltage between electrode and counterelectrode is shifted into a voltage region in which a reduction reaction takes place. A person skilled in the art can identify this region in the voltammogram. This method not only allows to transform selected regions of a superconducting material into non-superconducting material, but also enables a reversal of the writing step by the use of the same apparatus. Thus, it is suitable for producing barriers of non-superconducting material in otherwise superconducting material. If these barriers are produced with a thickness of less than 10 nm, they can be used for the manufacturing of Josephson contacts, SQUIDs, or superconducting FETs. Another preferred dopant is Cu, which is, for example, applicable as a solution of CuS0 4 . With regard to superconducting material, the use of copper as dopant offers similar advantages as the use of oxygen. The preferred embodiments of the invention are also specified in the claims. LIST OF THE DRAWINGS The invention is described below by way of example with reference to the drawings, which are schematic and not drawn to scale. They only show the basic devices whereas standard peripheral components are being left out for the sake of simplicity. FIG. 1 shows a schematic drawing of an apparatus as used for the new method. FIGs. 2-4 show various superconducting films patterned in accordance with embodiments of the invention. FIG. 5 shows a two-layer superconducting film patterned in accordance with another embodiment of the new method. EXAMPLES FIG.1 shows the basic components of an apparatus utilized for the new method. The apparatus is largely designed after the scanning tunneling microscope (STM). The means (not shown) for moving the tip 1 in x,y,z-directions relatively to the solid film 2 comprises piezoelectric transducers controlled by a voltage source, a feedback position controller, and a comparator to minimize the difference between the current flowing through the tip 1 and a constant reference current. Further, the apparatus comprises a source to apply a voltage V 0 between the film 2 and the tip 1 . The tip 1 itself consists of a platinum wire 3 trimmed by electrochemical etching and an insulating cover 4 of epoxy resin. Only the utmost part of the wire 3 is left uncovered. The thin film 2 of La 2 Cu0 4 is grown in a layer-by-layer deposition mode using molecular beam epitaxy on a substrate 6. The deposition conditions include a substrate temperature of 700°C, an atomic oxygen flux of approximately 4 x 10' 4 and an oxygen background pressure of 1.5 x 10- 7 Torr. The growth is observed to ensure that the surface of the film is not contaminated with impurity phases. After a thickness t of 260 nm is reached, deposition is stopped and the sample is cooled to room temperature with the atomic oxygen flux shut off. The prepared film 2 has a room temperature resistance - measured with a two point contact technique - of 2 MΩ. X-ray diffraction indicate that the film is single phase c-axis oriented with a value of the c-axis of 1.30984 nm. Atomic force microscopy pictures taken from the surface reveal a relatively smooth surface with a roughness of + /- 5 nm. No large precipitates on the surface are detectable. The film 2 and the tip 1 are immersed in an electrolyte solution 5 of 1 N KOH. Electrical contacts to the film are made using a platinum wire and silverpaint. Afterwards, the contact area as well as the platinum wire are insulated from the electrolyte 5 using silicone rubber material. The tip 1 then is brought into a distance d of approximately 0.5 m to the film 2. A voltage V 0 of 550 mV between film and tip is applied. The voltage is periodically reduced from its peak value V 0 of 550 mV to 200 mV to allow a restoration of the original concentration of the electrolyte 5 in the spacing between tip and film and a measurement of the potential between both. The potential between both electrodes at an external voltage of zero allows to check the doping. The tip 1 is then moved relatively to the film 2 in accordance with a predetermined pattern. In FIG.2, a line of superconducting La 2 CuO 4+(i 21 in otherwise non superconducting La 2 CuO 4 22 is shown. Taking the interruption of the movement during the time in which the voltage V 0 is reduced into account, the line 21 is produced with a scan speed of 20 nm/s. Its width is approximately 0.3//m. After the writing step, a new STM surface scan (at a lower voltage) is performed to check the surface conductivity. To simplify the subsequent measurements proving that the oxygen is not solely deposited at the surface of the film 22, but penetrates the film through its entire thickness, a larger sample is prepared by using a broader electrode and a larger distance d (2mm) between electrode and film. X-ray diffraction on said larger sample reveals that the c-axis lattice parameter has increased to 1.31464 nm in agreement with the incorporation of additional oxygen. The difference in lattice parameter between the oxidized and not oxidized sample is of the order of 0.005 nm. Atomic force microscope images show locally that the surface morphology did not change drastically. The features observed previously are slightly smeared out. Resistivity measurements indicate a superconducting onset of 30°K and a zero resistance state at 26°K. Finally kinetic inductance measurements prove that superconductivity in this system in not a surface but a bulk effect. In another example, the scanning speed is varied from 0 to 20 nm/s. FIG.3 shows a patterned superconducting region 31 produced by this method. In another example, shown in FIG.4, a superconducting line 41 is initially generated in the film material 42 in accordance with the first example. For the second writing step, V 0 is shifted to a value (250 mV), in which a reduction of the superconducting line 41 takes place. The accurate voltage for the reduction reaction is taken from a voltammogram. As FIG.4 indicates, a non-superconductive barrier 43 is then produced by scanning the tip perpendicular to line 41. Again a sample of larger scale is prepared to prove that the barrier 43 penetrates the line 41 and the film 42 through its entire thickness. In another example a 500 nm thick film 52 of La 2 Cu0 is grown on a film 54 of YBa 2 Cu 3 O 7 _ Λ with an equal thickness. Again 550 mV are applied between the films 52, 54 and the counterelectrode. The electrolyte is 1 N KOH. In contact to this electrolyte, the film 54 of YBa 2 Cu 3 O 7 _ t 5 tends to be unstable. The film 52 of La 2 CuO thus serves as protecting layer. A scan speed of 100 nm/s is used. Under these conditions, in both layers 52, 54, a zone 51 , 53 of superconducting La 2 Cu0 4+ ,. and YBa 2 Cu 3 0 7 is produced, respectively.

Claims: CLAIMS 1. Method for doping a semiconductive or conductive solid material by using an electrochemical cell with at least two electrodes, one being the solid material, and an fluid electrolyte which contains the dopant, and applying a voltage between the two electrodes to create an electric field between them, wherein the electric field is essentially concentrated onto a selected region (21 ,31 ,41 ,51 ,53) effectively confining the doping to said selected region of the solid material (2,22,32,42,52,54). 2. Method in accordance with claim 1 , wherein the doping is confined to the selected region (21 ,31 ,41 ,51 ,53) of the solid material (2,22,32,42,52,54) by bringing a second electrode (1 ) into proximity of the solid material, which second electrode (1) is at least partly patterned in conformity with the selected region (21 ,31 ,41 ,51 ,53) or is formed as or comprises a tip. 3. Method in accordance with claim 2, wherein the second electrode (1) is scanned over at least a part of the selected region (21 ,31 ,41 ,51 ,53). 4. Method in accordance with claim 2, wherein the distance (d) between the second electrode (1 ) and the solid material (2,22,32,42,52,54) is varied during the doping. 5. Method in accordance with claim 3, wherein the scanning speed is varied during the doping. 6. Method in accordance with any of the claims 3, 4, or 5, wherein the movement of at least one of the electrodes (1 ,2,6) is controlled by using a current or voltage controlled feedback loop and at least one transducer, in particular a piezoelectric transducer. 7. Method in accordance with claim 1 , wherein the solid material (2,22,32,42,52,54) is covered by a layer of material insoluble in the electrolyte (5) with the exception of the selected region (21 ,31 ,41 ,51 ,53). 8. Method in accordance to claim 1 , wherein the solid material is a film (2,22,32,42) or a stack of several films (52,54) with a thickness (t, preferably in the range of 1 nm to 5/ιm. 9. Method in accordance with claim 1 , wherein the solid material (2,22,32,42,52,54) is an oxide semiconductor, a precursor of an oxide superconductor, or an oxide superconductor. 10. Method in accordance with claim 1 , wherein the dopant is oxygen. 11. Use of a method according to claim 1 to transform a selected region of non-superconductive solid material into superconductive solid material and vice versa. 12. Use of a method according to claim 1 to confine the doping into regions having at least one lateral dimension of less than 1 13. Apparatus for implanting dopants into a solid material at a selected region, comprising • a cell containing a fluid electrolyte (5), • one electrode which consists of or comprises the solid material (2), • a counterelectrode (1) which is at least partly patterned in conformity with the selected region (21 ,31 ,41 ,51 ,53), and which is insulated against the electrolyte (5) with the exception of at least one utmost tip or edge to concentrate an electrical field essentially onto said region effectively confining the dopant to said region. • means for applying a voltage (V 0 ) between both electrodes, and • means for varying the distance (d) between both electrodes. 14. Apparatus in accordance with claim 14, wherein the cell comprises two compartments separated by a diaphragm of porous material. 15. Apparatus in accordance with claim 14, comprising a third electrode, in particular as a reference electrode. 16. Apparatus in accordance with claim 14, comprising means for moving one electrodes relatively to the other in the x,y-plane. 17. Apparatus in accordance with claim 14 or 15, wherein the means for moving the electrode comprises a transducer, in particular a piezoelectric transducer, and means for controlling it. 18. Apparatus in accordance with claim 13, wherein the counterelectrode is formed as or comprises a tip (1). 19. Apparatus in accordance with claim 13, wherein the counterelectrode (1) is at least partly insulated against the electrolyte (5) with an epoxy resin.

Inventors: GERBER CHRISTOPH, LOCQUET JEAN-PIERRE, GERBER, CHRISTOPH, LOCQUET, JEAN-PIERRE
Applicants: GERBER CHRISTOPH, IBM, LOCQUET JEAN PIERRE, GERBER, CHRISTOPH, INTERNATIONAL BUSINESS MACHINES CORPORATION, LOCQUET, JEAN-PIERRE
Application year: 1993
Main classification: H01L39
IPC codes: C25D15, C30B31, H01L21, H01L39