Method of Producing Superconducting Ceramic Patterns by Etching

This invention relates to a method of manufacturing superconducting oxide patterns by eching.

Recently, superconducting ceramic materials have been attracting a great deal of attention. These materials were first reported by IBM's Zurich Laboratories in the form of Ba-La-Cu-O type high temperature superconducting oxides. In addition, YBCO (YBa.sub.2 Cu.sub.3 O.sub.6-8) type materials are also known. The applicant has proposed the method for depositing superconducting materials of these types in thin film form.

It has been known to produce desired patterns in ceramic films by wet etching using an acid etchant. Contrary to this, no dry etching method for ceramis films in gaseous phase has not been proposed, whereas NF.sub.3 and CF.sub.4 can not used as an etchant gas for etching of ceramics. In case of patterning of superconducting ceramic films, it is necessary to carry out dry etching rather than wet etching because the oxide ceramics are deteriorated by water.

It is therefore an object of the invention to provide a etching method which can be applied for superconducting ceramic patterns.

In order to accomplish the above and other objects, the etching of a superconducting ceramic film is performed by use of active bromine atoms. The active atoms are generated by, for example, the electron cyclotron resonance, the reactive ion etching, photo-excitation method utilizing ultraviolet light rays of 185 nm wavelength and so forth. By use of such a superconducting pattern for interconnection between devices formed within a semiconductor substrate, high frequency characteristics (high speed operations) are obtained in the light of high mobilities of electrons and holes in the superconducting material at the liquid nitrogen temperature.

FIGS. 1(A) to 1(D) are cross sectional views showing manufacturing process of a semiconducrtor device in accordance with the present invention.

FIG. 2 is a schematic view showing a microwave-enhanced plasma etching apparatus utilizing magnetic fileds.

A substrate to be treated by the method in accordance with the present invention is coated with a blocking film made of such as ZrO.sub.2, SiTiO.sub.3, YSZ (yttirium stabilized zircon), yttrium oxide and strontium titanate or a platinum film (for covering electrodes), which shall not cause any interaction with oxide ceramics to be deposited at 500.degree.-700.degree. C. The substrate may be a silicon semiconductor substrate within which are formed a plurality of devices such as insulated gate field effect transistors, bopolar transistors, statistic induction type transistors, resistors and capacitances. Then, a superconducting ceramic film is deposited to a thickness of 0.1-10 microns. The substrate is preferably heat-resistant. The superconducting ceramic film is treated by anisotropic etching utilizing an etchant comprising bromine to produce superconducting patterns.

The melting points (MP) and the boiling points (BP) of the products of the etching reaction between etchants (comprising bromine, fluorine or chlorine) and superconducting ceramics are as follows.

As seen from the above figures, all the bromine compounds of the constituent elements of the YBCO type superconducting ceramic have melting points not higher than 904.degree. C. (the melting point of YBr.sub.3). The fluorine compounds require 1200.degree. C. (the melting point of YF.sub.3) or higher temperatures to melt all the products. Generally, the lower the melting point is, the easier the vaporization is done under negative pressures or vacuum pressures. The use of chlorine gas is dangerous because it tends to react with stainless steel constituting the reaction chamber. The use of iodine is expensive and not suitable for commercialization.

Referring to FIGS. 1(A) to 1(D), an exemplary method of the present invention will be described. A substrate 1 is a single crystalline silicon semiconducor substrate within which are formed, in advance, active devices such as insulated gate field effect transistors and bopolar transistors, and passive devices such as resistors and capacitances. In the surface of the substrate, impurity areas are provided for making electrical contact with the inner devices by interconnecting means which is to be formed on the substrate. A dual insulating film 2 consisting of a 0.3 microns thick SiO.sub.2 film 2--1 and a 0.4 microns thick zirconium film 2--2 is deposited by sputtering. Contact holes 8 are opened in the insulating film 2 corresponding to the impurity areas. Conductive films 9 of 0.1 to 10 microns thickness are formed at the holes in order to prevent acid-base reaction between the silicon semiconductor and the superconducting ceramic material to be deposited thereon. The conductive film 9 is made of, for example, platinum, gold, silver, tungsten and tinanuim, and patterned by photolithography.

A superconducting ceramic film 3 is deposited to a thickness of 0.2-2.0 microns, e.g. 1 micron on the structure as shown in FIG. 1(B) by sputtering, screen press printing technique, vacuum evaporation, chemical vapor deposition or the like. The ceramic is preferably comprised of elements belonging to alkali earth metals (Group IIa), lanthanides (Group IIIa) and cupper. In case of sputtering deposition, the deposition conditions are, for example, the substrate temperature, 450.degree. C.; the frequency, 50 Hz; and the output power, 100 W. After the sputtering deposition, the film is annealed at 700.degree. C. for ten hours in an oxidizing atmosphere. The interaction between the ceramic film and the substrate is prevented from by virtue of the blocking conductive films 9. For reference, when annealing was effected to a ceramic film making direct contact with the underlying silicon semiconductor, it was confirmed by auger spectrometory that silicon oxide was formed at the contacting surface only after about 20 minutes annealing.

Next, the etching process in accordance with the present invention is described in details. In FIG. 2, a plasma processing apparatus is comprised of a vaccum chamber 12 including a plasma resonating space 31, a microwave generator 14 connected to the vaccum chamber 12 through a microwave introduction window 39 made of an artificial quartz, Helmhortz coils 15 and 15' surrounding the plasma resonating space 31, a power source 35 for supplying electric energy to the coils, an evacuating system connected to the vaccum chamber 12 through a pressure control valve 19 and consisting of a turbo molecular pump 26 and a rotary pump 24 connected through and a valve 25, gas introduction systems 16 and 17 and a water cooling system 28 and 28' provided for the system.

First, substrates 10-1 and 10-2 are mounted on holders 10'-1 and 10'-2 made of stainless steel in the chamber 12. The substrates have (100) planes and are made of, for example, MgO, SrTiO.sub.3, YZT or a silicon wafer coated with a ZrO.sub.2 insulating film, on each of which a 2 microns thick oxide superconducting cramic film is deposited.

After evacuating the chamber 12 to 1.times.10.sup.-6 Torr or lower, an etchant gas comprising bromine, for example, Br.sub.2 (MP=-7.3.degree. C., BP=58.78.degree. C.), is introduced from the system 17. The bubbler 33 contains a liquid mixture including bromine. Bromine is carried by nitrogen gas passing therethrough at 200 SCCM. Eventually, the pressure of the etchant gas is 0.003 Torr in the chamber. Then, microwaves at 500 MHz or higher frequencies, e.g. 2.45 GHz are inputted from the microwave generator 14' at 0.1-1.0 KW, e.g. at 0.5 KW under a magnetic field of 875 Gauss induced by the Helmhortz coils 15 and 15'. Active bromine ions are generated in a high density plasma formed by virtue of the electron cyclotron resonance. The active ions attack the surface of the substrates 10-1 and 10-2 and produce patterns thereon in accordance with photomasks covering the surface in advance. The etching speed is 0.1 to 1.0 micron/min at room temperature. The ceramic material constituting the surface to be etched is removed in the form of gaseous compounds such as YBr.sub. 3, BaBr.sub.2, CuBr or the like. The Helmholtz coils 15 and 15' are cooled by the cooling system 28 and 28'. The positions of the substrates to be treated in the chamber are determined by the purpose. When mounted on the holder 10'-2, the substrate 10-2 is subjected to anisotropic etching in accordance with a divergent magnetic field.

Thermal annealing is effected to the superconducting film at 500.degree.-1000.degree. C. for 1-20 hours before or after the etching. The annealing is effective for improving the superconducting property of the film. The deposition step, the etching step and/or the annealing step may be repeated several times in order to construct a complex interconnection of zero resistance within a single or multi-layered structure. In accordance with experiments, the Tc onset of the superconducting pattern was 93.degree. K. The resistance started to drop at 93.degree. K. and substantially vanished at 83.degree. K.

By use of this etching method, superconducting ceramic lines of a width of 10 microns to 1 micron can be produced as well as interval spaces therebetween of a width on the same order.

Superconducting ceramics for use in accordance with the present invention also may be prepared in consistence with the stoichiometric formulae (A.sub.1-x B.sub.x).sub.y Cu.sub.z O.sub.w X.sub.v, where A is one or more elements of Group IIIa of the Priodic Table, e.g., the rare earth elements, B is one or more elements of Group IIa of the Periodic Table, e.g., the alkaline earth metals including beryllium and magnesium, X is one or more elements of a group consisting of Ge, Sn, Pb, F and Cl, and x=0-1; y=2.0-4.0, preferably 2.5-3.5; z=1.0-4.0, preferably 1.5-3.5; w=4.0-10.0, preferably 6.0-8.0; and v=0-3. Also, superconducting ceramics for use in accordance with the present invention may be prepared consistent with the stoichiometric formulae (A.sub.1-x B.sub.x).sub.y Cu.sub.z O.sub.w, where A is one or more elements of Group Vb of the Priodic Table such as Bi, Sb and As, B is one or more elements of Group IIa of the Periodic Table, e.g., the alkaline earth metals including beryllium and magnesium, and x=0.3-1; y=2.0-4.0, preferably 2.5 -3.5; z=1.0-4.0, preferably 1.5-3.5; and w=4.0-10.0, preferably 6.0-8.0. Examples of this general formula are BiSrCaCuCu.sub.2 O.sub.x and Bi.sub.4 Sr.sub.3 Ca.sub.3 Cu.sub.4 O.sub.x. Tc onset and Tco samples confirmed consistent with the formula Bi.sub.4 Sr.sub.y Ca.sub.3 Cu.sub.4 O.sub.x (y is around 1.5) were measured to be 40.degree.-60.degree. K., which is not so high. Relatively high critical temperatures were obtained with samples conforming to the stoichiometric formulae Bi.sub.4 Sr.sub.4 Ca.sub.2 Cu.sub.4 O.sub.x and Bi.sub.2 Sr.sub.3.sub.2 Ca.sub.2 Cu.sub.2 O.sub.x. The number x denoting the oxygen proportion is 6-10, e.g. around 8.1. Such superconducting materials can be formed by screen press printing, vacuum evaporation or CVD.

While a description has been made for several embodiments, the present invention should be limited only by the appended claims and should not be limited by the particular examles. What follow are some examples of modifications and variation according to the invention.

CBr.sub.4 and HBr can be used as the etchant. HBr is not suitable depending on the case since hydrogen tends to react with oxygen constituting the ceramic and degrades the superconductivity thereof.

The application field of the present invention includes ULSIs such as 14M to 1 G bits memories and compound semiconductor devices such as III-V compound semiconductors, e.g. GaAs epitaxially grown on a silicon substrate.