JPH04226090A - Silicon substrate having epitaxial superconductive layer, and its manufacture - Google Patents

Abstract

PURPOSE: To provide a silicon substrate which has characteristics required for actual use and has a high temperature super-conductive layer which in made epitaxial growth, and its manufacturing method. CONSTITUTION: A surface of a silicon substrate 10 is purified by using a spin- etching method, in order to generate a surface which is automatically purified and ends in an atomic layer of elements like hydrogen which does not act on silicon. This surface is contaminated and can be carried to an accumulation chamber. Hydrogen is evaporated in the accumulation chamber, and next a YSZ layer 12 is made epitaxial growth by laser ablation. Thereafter, a HTSC material 14 like YBCO is made epitaxial growth by laser ablation. This structure is next cooled by the oxygen atmosphere.

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION This invention relates generally to semiconductor and superconducting technology, and specifically to silicon substrates, such as integrated circuits, having superconducting layers. Furthermore, the invention relates to a method for epitaxially growing superconducting layers on silicon that have good technical properties for electronic devices and circuits.

0002

BACKGROUND OF THE INVENTION Many studies have focused on a new class of copper oxides combined with lanthanum, yttrium, barium, thallium, lead, bismuth, calcium, praseodymium, and strontium, etc., which have relatively high superconducting transition temperatures (Tc). This research is being carried out in the field of superconductivity related to ceramic materials. Yttrium, barium, and copper oxide materials (Y1B
a2Cu3O7) is called a 1-2-3 compound,
Here, it is expressed as YBCO.

Recently, research at Stanford University has been conducted to maximize the critical current density of superconducting materials. The material is a perovskite compound of yttrium, barium, and copper oxide. This material has good superconducting properties at 77°K and will eventually be used in conjunction with liquid nitrogen cooling. Thin film structures of this material can be fabricated by vacuum evaporation, ie electron beam evaporation or magnetron sputtering. Many of these planar films also have critical current density constraints due to random or inappropriate crystal orientation and grain boundary effects, as well as position relative to the surroundings. If the film is deposited on a single crystal titanium strontium oxide with an optimally selected crystal orientation, the deposited film will have an orientation and, after heat treatment, will be epitaxially grown on the titanium strontium oxide substrate. This film exhibits an unusually reasonable critical current density of 1.0 to 1.5 x 10 6 amps per square centimeter at 4.2K.

The growth of high temperature superconductors on silicon substrates is
This has been a goal acknowledged by researchers for several years. Silicon integrated circuits are capable of operating at superconducting temperatures (currently on the order of 125°K). A constraint in realizing an ultra-high-speed semiconductor IC is the interconnection of various circuits on a semiconductor substrate. Generally, this is accomplished with a metal interconnect layer on the surface of the semiconductor substrate. By reducing the resistance of the interconnect metallization, the operating speed of the circuit is increased.

[0005]

[Problem to be Solved by the Invention] Superconducting materials such as YBCO cannot be used in silicon because the chemical reaction between silicon and YBCO either completely destroys the superconductivity of the material or significantly lowers the superconducting transition temperature. It cannot be deposited directly. Many attempts have been made to use barrier layers between the silicon and the superconducting material to prevent chemical reactions and yet enable the growth of highly oriented high temperature semiconductor films at high superconducting temperatures. Barrier layers conventionally used consist of oxides such as silicon, zirconium, magnesium, strontium and titanium, magnesium and aluminum, aluminum, lanthanum and aluminum. Zirconium oxide doped with yttrium oxide (YSZ) has been proposed as a barrier layer. However, the superconducting materials used in this barrier lacked the technical properties necessary for practical use. Two of the problems encountered when using this barrier are contamination by the silicon substrate and oxidation of the silicon substrate that occurs during processing.

The present invention provides a silicon substrate with an epitaxially grown high temperature superconducting layer having the properties necessary for practical use.

[0007]

OBJECTS OF THE INVENTION In view of the above, the object of the present invention is a structure consisting of a silicon substrate and a high temperature superconducting layer grown thereon which has the properties necessary for practical use. Another object of the invention is a method of fabricating high temperature superconducting layers on silicon substrates using barriers that facilitate epitaxial growth of superconducting materials.

Another object of the present invention is to
exhibits a zero-resistance transition temperature of at least 85°K, with a superconducting transition width (10-90%) of less than or equal to 300°K.
with a resistivity of less than 300 micro-ohm-centimeter and a resistance ratio of 3.0 ± 0.2 (300 °K /1
High temperature superconducting interconnections of silicon integrated circuits with 00°K). Other objects of the invention include electronic integrated circuits such as infrared detector arrays and signal/control electronics using silicon integrated circuits for signal processing and control of high temperature superconducting element arrays such as infrared detector arrays in bolometers. It is.

Still another object of the present invention is to provide high temperature superconducting interconnections and/or superconducting microwave devices/
It is a silicon integrated circuit that uses a resonator.

0010

SUMMARY OF THE INVENTION Briefly, in fabricating structures in accordance with the present invention, a single crystal silicon substrate is provided having a major surface. The main surface is cleaned using a spin-etching method and finished with an atomic layer of an element that does not react with the silicon to destroy the crystallization of the surface and is easily removed by heating the silicon. An atomically clean surface is formed.

[0011] The substrate is then sent to a deposition chamber and heated to evaporate the elements from the surface of the silicon substrate, and then a buffer layer of YSZ is formed on the main surface and the substrate is left in the deposition chamber. YSZ grows epitaxially from the main surface. Then, a high temperature superconducting material is deposited on the buffer layer, and the substrate is left in the deposition chamber to epitaxially grow the high temperature superconducting material on the YSZ buffer layer.

In a preferred embodiment, the high temperature superconducting material is YBC.
It is composed of O. In addition to Y-Ba-Cu with an elemental composition ratio of 1-2-3 or 2-4-8, other oxide superconductors can be created. All rare earth elements are substituted for Y in the above compounds. Further, part or all of the constituent Ba can be replaced with Ca or Sr. Furthermore, some of the Cu is replaced by transition metals and by Zn and Ga. Other oxide superconducting materials deposited include many Bi-Ca-Sr-Cu and Tl
-Ba-Ca-Cu and its variations. Ba(Pb,B
i) O3, (Ba,K)BiO3 and n-type superconductors such as Nd2-x Cex CuO4-y can also be produced. YBCO and YSZ materials can be deposited by laser ablation, sputtering, electron beam evaporation, vacuum evaporation, molecular beam epitaxy, or chemical vapor deposition.

According to one embodiment of the present invention, a silicon substrate with a 1-0-0 crystallographic orientation is used to
A buffer layer of Z forms a cubic crystal structure thereon. The YBCO film grown thereon is in excellent crystal epitaxy with the underlying silicon crystal, as required for electronic devices and their circuits. This process is easy to perform and the resulting structure is consistent with silicon integration technology. The invention, its objects and characteristics, will become more readily apparent from the following detailed description and accompanying drawings taken in conjunction with the drawings.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, FIG. 1A is a perspective view of a monolithic silicon substrate 10 on which an epitaxially heterogeneously bonded YS
Z barrier layer 12 and YBCO epitaxially deposited layer 14
is formed. By employing the process according to the invention, the superconducting layer of YBCO produced has at least 8
It exhibits a zero-resistance transition temperature of 5°K and a transition width (10-90%) of 1.0°K or less. The resistivity of superconducting material at 300°K is 300 micro-ohms.
less than a centimeter, and the resistance ratio (300°K/10
0°K) is 3.0±0.2. The superconducting layer is therefore suitable for forming interconnect patterns in integrated circuits in silicon substrates, as shown in the isometric view of FIG. 1B. To form the interconnect pattern, YBCO is selectively etched by photolithographic masking and etching using nitric acid, hydrochloric acid, phosphoric acid, bromine and alcohol, or using laser ablation as the etching reagent. . The underlying YSZ is selectively removed by ion milling, plasma etching, or laser ablation.

FIGS. 2A and 2B are perspective and cross-sectional views of another illustrative embodiment of a bolometer infrared detector comprising an HTSC material 20 formed in accordance with the present invention above a recessed portion 22 of a silicon substrate 24. be. The integrated circuit 26 is
It is formed at the end of the substrate 24. The excellent properties of superconducting materials formed on silicon substrates are due to the unique process of forming barrier layers and superconducting layers on silicon substrates. Figure 3
1 is a flowchart of process steps for practicing the invention using YSZ as a buffer layer and YBCO as a superconducting layer on top of YSZ.

First, a single crystal silicon substrate having a crystallographic orientation of 1-0-0 is fabricated. The silicon substrate is
For example, it is composed of an integrated circuit. The main surface of the board is
All oxides, intentionally formed or native oxides, are removed in the next step. A standard degreasing operation is followed by a spin etching process in flowing nitrogen. In this process, a silicon wafer is spun, cleaned with a few drops of high-purity alcohol, and then etched with a few drops of a mixture of hydrogen fluoride, ethanol, and water (1:10:1), all of high purity. Ru. By spin etching method,
An atomically clean surface finished with a single atomic layer of hydrogen is formed. The surface is therefore highly inert to contamination or re-oxidation even in air, and the silicon wafer becomes a stage for film deposition with a highly ideal surface.

Spin cleaning to clean and finish the surface of the substrate
The etching method is described in Fuener et al., "Passivation of Silicon Surfaces by Hydrogen Finishing: A Comparative Study of Pretreatment Methods," Journal of Applied Physics 66(1), July 1989, pp. 419-424. As described therein, the oxide on the wafer surface is etched using a hydrogen fluoride-alcohol reagent in an atmosphere of flowing nitrogen at room temperature while the wafer is rotating.

The substrate is then moved to a deposition chamber and placed under a moderate vacuum (eg 10-6 Torr) where hydrogen evaporates from the surface.
) is heated to about 800°C. Preferably, the substrate is first elevated to about 250°C to remove weakly bound and adsorbed impurities before heating the substrate to about 800°C. A YSZ layer is then epitaxially deposited on the substrate surface by a form of laser ablation, sputtering, electron beam evaporation, thermal evaporation, molecular beam epitaxy, or chemical vapor deposition. The laser melting method is described in Falk et al., “Applied Physics Letters”,
53, 337 (1988). In summary, the laser beam focuses on the pressed YSZ target. A commonly used laser is an excimer laser, which generates intense pulses in the ultraviolet region. 3 Neodymium:
Other UV lasers such as YAG and pulsed C
Lasers that emit pulses of light in other regions of the electromagnetic spectrum can also be used, such as O2 lasers. The target used is prepared by grinding a mixture of ZrO2 and Y2O3 into a powder in an agate mortar and pestle and pressing it into a 0.5 inch diameter disk at a pressure of 50,000 psi. The disk was then heated to 1,000°C in flowing oxygen.
Sinter for 36 hours. YSZ films are fabricated from similarly extruded mixed ZrO2 and Y2O3 targets, which are pulsed at the desired ratio. Y
Membranes made from extruded pellets of SZ result in smooth featureless membranes compared to membranes made from mixed ZrO2 and Y2O3 using current methods. When the surface of this target is heated with a laser, feather-like Y, Zr, and O atoms are formed that adhere to nearby substrates. The film grows with every pulse applied. Hundreds to thousands of pulses are required to grow a 250 nm thick film. After depositing the first few layers equivalent to a buffer film of YSZ, oxygen gas is delivered to the chamber at a low pressure on the order of 5 x 10-4 Torr. A YSZ film composed of highly ordered (100) cubic crystal fluorite (CaF2) is shown in Figure 4A.
and as shown in FIG. 4B, it grows epitaxially on the surface of single crystal silicon.

Immediately after the buffer film is deposited, the silicon is cooled to about 750° C. and the oxygen pressure is increased to about 200 mm.
The pressure is increased to Torr, and the YBCO film is deposited by laser melting. As the temperature and oxygen pressure of the silicon substrate changes, YBCO is deposited by sputtering, electron beam evaporation, vacuum evaporation, molecular beam epitaxy, or chemical vapor deposition. Laser ablation of YBCO is described in Koren et al., “Vapour-deposited superconducting laser ablated thin films of highly oriented SrTiO3 zirconia and Y1Ba2Cu3O3 on silicon substrates,” Applied Physics Letters 53 (23);
Written on December 5, 1988. The YBCO compound can be 1-2-3, 2-4-8, or other combinations with pulse variations of the ablation process.

[0020] Combination ratio is 1-2-3 or 2-4-8
Superconductors of oxides other than Y-Ba-Cu can also be produced. All rare earth elements are substituted for Y in the above compounds. Further, part or all of Ba is replaced with Ca or Sr. In addition, some of the Cu is replaced by transition metals and Zn and Ga. Other oxide superconductors deposited include many Bi-Ca-Sr
-Cu and Tl-Ca-Cu systems and their variations. Ba(Pb,Bi)O3, (Ba,K)BiO3
, and n-type superconductors such as Nd2-xCeXCuO4 can also be fabricated.

After the YBCO film is deposited to the desired thickness, the substrate is cooled at a controlled rate in an oxygen atmosphere to prevent film reduction. YBCO film is a high quality superconductor with excellent electrical resistance properties. Figure 4A
indicates the coplanar epitaxial orientation of YBCO on YSZ, where the coplanar direction of YBCO (
100) is parallel to the coplanar (100) direction of the YSZ, which is associated with the high quality of the aforementioned YBCO crystals on the YSZ. Figure 4A also shows the coplanar epitaxial orientation of YSZ on silicon, where the coplanar (100) orientation of YSZ is similar to that of the aforementioned YSZ on silicon.
Coplanarity of silicon (100
) is parallel to the direction of FIG. 4B shows the crystallographic orientation of the metal. Therefore, the YSZ buffer film and the YBCO film are
Both have excellent crystal epitaxy with silicon crystal.

Transmission electron micrographs revealed the congruent growth of highly textured (100) cubic crystal fluorite-structured YSZ on silicon and highly C-axis oriented YBCO on YSZ. . X-ray diffraction of the film shows that YSZ has only a cubic crystal fluorite structure and YBCO has a C-axis orientation.
and shows a highly coplanar epitaxial orientation. The zero-resistance transition temperature and transition width, along with the resistivity and resistivity ratio at 300°K, demonstrated that this superconducting material had good technical properties for applications in integrated circuits and other superconducting environments. ing.

Although the invention has been described with respect to specific embodiments, this description is illustrative of the invention and is not to be construed as limiting the invention. For example, Y.S.
Z films are grown by laser ablation on silicon surfaces having other crystallographic orientations (eg, 111). Additionally, other superconducting materials may be deposited using the process according to the present invention.

Other oxide superconducting materials other than Y-Ba-Cu having a composition ratio of 1-2-3 or 2-4-8 can also be produced. All rare earth elements can be substituted for Y in the above compounds. Also, some or all of Ba may be Ca or S.
Replaced by r. Besides, some of the Cu is replaced by transition metals and Zn and Ga. Other oxide superconductors deposited include many Bi-Ca-Sr
-Cu system, Tl-Ba-Ca-Cu system, and variations. N-type superconductors such as Ba(Pb,Bi)O3, (Ba,K)BaO3 and Nd2-xCeXCuO4-Y can also be fabricated.

Accordingly, various modifications and uses thereof may occur to those skilled in the art without departing from the original spirit and scope of the invention as defined by the appended claims.

[Brief explanation of the drawing]

FIG. 1A is a perspective view of a structure according to the invention consisting of a silicon substrate, a buffer layer, and a high temperature superconductor layer formed thereon. FIG. 1B is a perspective view of the structure of FIG. 1 in which the silicon substrate constitutes an integrated circuit, with the high temperature conductor layer selectively etched to form the interconnection pattern of the circuit.

FIG. 2A is a perspective view of an integrated electronic circuit using silicon circuitry for signal processing and control of a high temperature device array. FIG. 2B is a cross-sectional view of the circuit.

FIG. 3 is a flow diagram illustrating the steps of assembling a structure according to the present invention.

FIG. 4A is a crystal lattice model of the structure of crystalline silicon and YSZ at the interface of a preferred embodiment. Figure 4B
is a top view showing the crystallographic orientation of the material.

Claims (14)

[Claims] Claim 1: A method for forming a high temperature superconducting layer on a silicon substrate, comprising: (1) forming a single crystal silicon substrate having a main surface; (3) cleaning said major surface using a spin etching method to produce an atomically clean surface finished with an atomic layer of the element that has not reacted with the element; and (3) evaporating said element. (4) depositing a buffer layer of YSZ on the main surface;
(5) the substrate remains in the deposition chamber, and the YSZ is epitaxially formed on the main surface of the single crystal;
depositing a high temperature superconducting material on the buffer layer, the substrate remaining in the deposition chamber, and forming the high temperature superconducting material epitaxially on the buffer layer of YSZ. The method described above. 2. The YSZ is epitaxially formed, and the coplanar (100) direction of the YSZ layer is parallel to the coplanar (100) direction of the single crystal silicon. Method described. 3. The method according to claim 1, wherein the element is hydrogen. 4. The method of claim 3, wherein step (3) of claim 1 comprises heating the substrate to about 800° C. in a vacuum of about 10 −6 Torr. 5. Step (3) of claim 1 further comprises heating the substrate to about 250° C. to remove weakly bound adsorbed impurities before heating the substrate to about 800° C. 5. The method according to claim 4, characterized in that the method comprises: 6. Step (4) of claim 1 comprising first depositing YSZ in said vacuum at about 10 −6 Torr and then introducing oxygen into said chamber. The method according to claim 5. Claim 7: The introduction of oxygen is approximately 5×10-4 Torr.
"Claim 6" characterized in that the process is carried out at a pressure of
The method described in. 8. Claim 7, wherein the main surface of silicon has a crystallographic orientation of 1-0-0 and the step of depositing a buffer layer forms a YSZ with a cubic structure. The method described in. 9. Step 5 of claim 1 includes lowering the temperature of the silicon substrate to about 750° C. and lowering the oxygen pressure to about 20° C.
9. The method of claim 8, further comprising increasing the temperature to 0 mTorr and then depositing a high temperature superconducting material of YBCO. 10. A method according to claim 8, characterized in that the coplanar YBCO direction (100) is parallel to the coplanar silicon direction (110). 11. A single crystal silicon substrate having a main surface, a layer of zirconium oxide added with yttrium oxide epitaxially formed on the main surface, and a layer of zirconium oxide added with yttrium oxide epitaxially formed on the layer of zirconium oxide added with yttrium oxide. a layer of high-temperature superconducting material, said layer of high-temperature superconducting material having a zero-resistance transition temperature of at least 85° K and resistant to superconducting transition from 10% of its initial resistance value to Transition to 90% is 1.0°K
A structure characterized by occurring within the following transition temperature range. 12. The layer of high temperature superconducting material further has a temperature of 300°K of less than 300 microohm centimeters.
with a resistivity of 3.0±0.4 at 300°K
12. The structure of claim 11, having a ratio of resistivity at 100°K to resistivity at 100°K. 13. The high temperature superconducting material is yttrium.
13. The method according to claim 12, wherein the oxide is barium-copper oxide. 14. The coplanar (100) direction of the yttrium oxide-doped zirconium oxide is parallel to the coplanar (100) direction of the single crystal silicon substrate. structure.