METHOD OF MANUFACTURING A THIN FILM OF AN OXIDIC SUPERCONDUCTING MATERIAL IN ACCORDANCE WITH A PATTERN

Abstract: A method of manufacturing a thin film of an oxidic superconducting material in accordance with a pattern, in which a pattern is manufactured by means of etching using reactive ions and a mask of aluminium oxide or silicon oxide, said method enabling patterns having line widths smaller than 2 µm to be manufactured with great accuracy without influencing the composition of the superconducting thin film in such a manner that the superconducting properties deteriorate.

Description: The invention relates to a method of manufacturing a thin film of an oxidic superconducting material in accordance with a pattern. In an article by M.G. Blamire a.o., in J.Phys. D: Appl. Physics 20 , pages 1330-1335 (1987) a description is given of a method in which a thin film of YBa₂Cu₃O 7-δ is applied to a substrate of aluminium oxide by means of magnetron sputtering, an intermediate layer of yttrium oxide being interposed. Conductor tracks having a width of 100 µm are formed in the thin film by means of a mechanical milling operation. This method has been selected because the thin film is not to contact aqueous solutions and because conventional etching techniques in which use is made of a reactive plasma are not available. This known method has the disadvantage that patterns having small line widths, for example smaller than 10 µm, cannot be accurately manufactured by means of mechanical milling. It is an object of the invention to provide a method of manufacturing a thin film of an oxidic superconducting material in accordance with a pattern, in which known methods in the field of semiconductor devices are used to manufacture patterns of small line widths and great accuracy without adversely affecting the properties of the oxidic material which determine the superconducting behaviour. The problem which occurs is that oxidic superconducting materials such as YBa₂Cu₃O 7-δ are subject to degradation when they contact water, and also contact with organic compounds is to be avoided as much as possible. If the pattern manufactured is to be processed further, for example to produce an electronic circuit device, precautions have to be taken so as to avoid damage to the superconducting material, for example, when subsequent layers or electric contacts are provided. This object is achieved in accordance with the invention by means of a method which comprises the following steps: a thin film of an oxidic superconducting material is provided on an electrically insulating substrate, an inorganic masking layer is provided thereon, subsequently, a resist pattern is applied, the inorganic masking layer is etched in a plasma in a mixture of trifluoromethane and argon, the resist pattern being used as a mask, the resist pattern is removed, the thin film of oxidic superconducting material is etched in a chlorine-containing plasma, the etched inorganic masking layer being used as a mask, the inorganic masking layer is removed by means of a plasma treatment in a mixture of trifluoromethane and argon. The inorganic masking layer is preferably manufactured from aluminium oxide and/or silicon oxide. In the method in accordance with the invention the superconducting film does not contact the material of the resist pattern or any solvent. It only contacts aluminium oxide or silicon oxide at temperatures which are so low that no diffusion of aluminium or silicon in the superconducting layer can take place. At temperatures exceeding 500°C the use of aluminium oxide and/or silicon oxide in contact with YBa₂Cu₃O 7-δ leads to a substantial deterioration of the superconducting properies, for example to a decrease of the critical temperature. The composition of the gas in the plasmas used, in which reactive ion etching occurs, is selected such that the differences in etching rate between the various layers are sufficiently large to manufacture a pattern in a selective manner without using additional etch-stop layers. An oxidic superconducting material which may be used in the method in accordance with the invention is (La,Sr)₂CuO₄, wherein Sr can be replaced by Ba. In a preferred embodiment of the method in accordance with the invention the oxidic superconducting material consists of YBa₂Cu₃O 7-α-δ F α , wherein α has a value of from 0 to 1 and wherein δ has a value of from 0.1 to 0.5. YBa₂Cu₃O 7-δ (α=0) has a T c value of approximately 90 K. Oxygen can be partly replaced by fluorine, for example up to 1 atom in the compositional formula indicated, which leads to an increase of T c . Further, Y may be substituted by one or more rare earth metals and Ba may be substituted by another alkaline earth metal, for example Sr. T c is the critical temperature below which the material exhibits superconducting behaviour. The resist pattern may be provided in any suitable manner, for example by means of screen printing. In a preferred embodiment of the method in accordance with the invention, the resist pattern is provided by locally irradiating a radiation-sensitive resist, and subsequently developing it. For this purpose, both a positive and a negative resist may be used. The irradiation operation can be carried out using, for example, electrons, X-rays or light, in particular UV light. Local, patterned irradiation can be obtained, for example, by means of a mask or a moving radiation source. A preferred embodiment of the method in accordance with the invention, in which a satisfactory etching selectivity is obtained, is characterized in that the chlorine-containing plasma comprises at least one of the compounds HCl and Cl₂. United States Patent Specification US 4439294 describes a method of etching using reactive ions and a contact mask of aluminium oxide. The method is applied to ferrites, as an alternative to etching methods in which liquids are used, to preclude underetching problems. From the said United States Patent Specification it cannot be derived that such a method can suitably be used with oxidic superconducting materials, because in these materials the etching speed relative to that of aluminium oxide is important as well as the sensitivity of oxidic superconducting materials to a large number of chemical compounds, as has also been indicated in the above-mentioned article by M.G. Blamire a.o. The invention will now be described in greater detail by means of an exemplary embodiment and with reference to a drawing, in which Figure 1 a-f is a diagrammatic representation of a number of steps in the method in accordance with the invention, and Figure 2 a-d is a diagrammatic representation of a number of steps in a method of providing an electric contact on an oxidic superconducting thin film. Example . According to the present example a substrate 1 of SrTiO₃ is used having a surface area of 5 cm², see Figure 1a. Other suitable electrically insulating substrate materials are, for example, magnesium oxide and yttrium oxide, for example in the form of thin films on a mechanically stronger substrate. A layer of YBa₂Cu₃O 7-δ having a thickness of 0.3 µm is provided by means of triode sputtering, and is subsequently subjected to a treatment in oxygen at a temperature of 850°C. The layer 2 thus obtained is superconducting having a critical temperature of approximately 93 K. If desired, superconducting thin films may alternatively be provided by means of laser ablation or other suitable deposition methods. Methods of manufacturing thin films of oxidic superconducting materials such as (La,Sr)₂CuO₄ and YBa₂Cu₃O 7-δ are described by R.B. Laibowitz a.o. in Physical Review B, 35 (16), pages 8821-8823 (1987). A layer of aluminium oxide 3 having a thickness of 0.3 µm is applied by means of R.F. sputtering. Alternatively, a layer of silicon oxide may be applied, for example having a thickness of 0.5 µm, which is manufactured by sputtering or chemical deposition from the vapour phase at 300°C. The other steps of the method in which silicon oxide is used are the same as when aluminium oxide is used. Subsequently, a layer 4 of a photoresist material is provided by means of spinning. A suitable photoresist material is, for example, a positive photoresist on the basis of novolak and diazoquinone, for example AZ 1350J by Hoechst. The photoresist material is dried for 20 minutes at 80°C and subsequently exposed to UV light in accordance with a pattern by means of a mask for 45 seconds. Subsequently, the photoresist material is heated for 5 minutes to a temperature of 120°C. Subsequently, the pattern is developed, see Figure 1b, by means of an 0.1 N solution of sodium hydroxide in water, rinsed, dried and maintained at a temperature of 120°C for 15 minutes. The aluminium oxide layer is etched in a plasma in a mixture of trifluoromethane, CHF₃, and argon in a volume ratio of 1 : 4. The temperature is 50°C, the pressure is approximately 2 Pa, the power applied to the plasma is 1 W/cm². The aluminium oxide is etched in the desired pattern by means of reactive ions in 25 minutes, see Figure 1c. It has been found that the oxidic superconducting material is etched at a rate which is 8 times slower than that of aluminium oxide. The substrate and the layers it carries are heated to 180°C in a nitrogen plasma at a pressure of 100 Pa and a power of 6 W/cm² being applied, after which the remaining resist material is removed in 25 minutes by means of an oxygen plasma at a pressure of 100 Pa and a power of 2 W/cm² being applied, see Figure 1d. Subsequently, the oxidic superconducting material is etched in a plasma in HCl. The temperature is 50°C, the pressure is approximately 1 Pa and the power applied to the plasma is 2 W/cm². The superconducting material is etched in the desired pattern in 10 to 12 minutes, see Figure 1e. A suitable value of the power applied to the plasma is of from 0.5 to 2.5 W/cm². A suitable etching gas which may alternatively be used is Cl₂. The etching rate of the superconducting material is approximately 1.8 µm/h, the etching rate of the aluminium oxide is approximately 0.6 µm/h. The remaining quantity of aluminium oxide is removed in about 6 minutes, see Figure 1f, in a plasma in trifluoromethane and argon, under the same conditions as described hereinbefore for forming a pattern in the aluminium oxide. By means of the method described herein, patterns having line widths smaller than 2 µm were manufactured. Contacts for measuring the electrical properties are provided in the manner described hereinafter. The substrate and the pattern of oxidic superconducting material formed thereon are heated to 120°C in a nitrogen plasma and are subsequently treated in an oxygen plasma at a pressure of 100 Pa for 20 minutes. This treatment enhances the bonding of the metal layers to be applied. Subsequently, the surface is purified by means of sputter etching for 1 minute. By means of vapour deposition, a layer of molybdenum 5 having a thickness of 60 nm, a layer of gold 6 having a thickness of 300 nm and a layer of molybdenum 7 having a thickness of 60 nm are provided in succession, see Figure 2a. By means of spinning a layer 8 of a photoresist material is provided thereon, for example a layer of the material in layer 4 as described hereinbefore. A resist pattern is formed by means of exposing and developing, as described hereinbefore, see Figure 2b. At the areas which are not protected by photoresist the molybdenum in layer 7 is etched by means of an etching solution comprising 70 parts by weight of phosphoric acid, 16 parts by weight of acetic acid, 3 parts by weight of nitric acid and 5 parts by weight of water, which solution does not attack gold. After rinsing and drying the gold in layer 6 is etched by means of an etching solution comprising 80 g of KI and 30 g of I₂ in 200 cm³ of water, the layer of molybdenum 5 acting as a stop layer, see Figure 2c. After rinsing and drying the remainder of the photoresist layer 8 is removed using acetone, after which the rinsing and drying operations are repeated. The remainder of molybdenum, in particular of layer 5, is removed by means of etching for 4 minutes in a plasma in a mixture of tetrafluoromethane and oxygen having a volume ratio of 1 : 25, see Figure 2d. The temperature is 70°C, the pressure is 100 Pa, the power applied to the plasma is 0.5 W/cm². The contacts are provided in such a manner that the superconducting material 2 does not directly contact water or organic compounds. In the case of a circuit manufactured in accordance with the present example it has been observed that superconductivity occurs at a critical temperature of 93 K.

Claims: A method of manufacturing a thin film of an oxidic superconducting material in accordance with a pattern, characterized in that the method comprises the following steps: a thin film of an oxidic superconducting material is provided on an electrically insulating substrate, an inorganic masking layer is provided thereon, subsequently, a resist pattern is provided, the inorganic masking layer is etched in a plasma in a mixture of trifluoromethane and argon the resist pattern being used as a mask, the resist pattern is removed, the thin film of oxidic superconducting material is etched in a chlorine-containing plasma, the etched inorganic masking layer being used as a mask, the inorganic masking layer is removed by means of a plasma treatment in a mixture of trifluoromethane and argon. A method as claimed in Claim 1, characterized in that the inorganic masking layer is manufactured from aluminium oxide and/or silicon oxide. A method as claimed in Claim 1 or 2, characterized in that the oxidic superconducting material comprises YBa₂Cu₃O 7-α-δ F α , wherein α has a value of from 0 to 1 and δ has a value of from 0.1 to 0.5. A method as claimed in any one of the Claims 1 up to and including 3, characterized in that the resist pattern is provided by locally irradiating a radiation-sensitive resist and subsequently developing it. A method as claimed in Claim 4, characterized in that the resist pattern is removed by means of a plasma treatment in oxygen. A method as claimed in any one of the Claims 1 up to and including 5, characterized in that the chlorine-containing plasma contains at least one of the compounds HCl and Cl₂.

Inventors: HEIJMAN, MARITZA GERARDA JOSEPHA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01L39
IPC codes: G03F7, C23F4, H01B12, H01B13, H01L39, H01L21