JPH0334236B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a terminal electrode for a superconducting integrated circuit,
In particular, the present invention relates to a base metal film for a superconducting bump electrode used for external connection of a superconducting integrated circuit equipped with a Josephson junction element.

[Background of the invention]

A tunnel-type Josephson junction device has a sandwich structure in which an extremely thin insulating film several nanometers thick is sandwiched between two superconducting thin films, and is a switching device that applies the superconducting tunnel phenomenon at extremely low temperatures (~4K). This device has a switching speed that is about one order of magnitude faster and a power consumption that is about three orders of magnitude lower than conventional semiconductor devices, and is expected to be used as a logic operation element and a memory element for future ultra-high-speed computers. Pb alloys, Nb, NbN, etc. are mainly used for the superconducting thin films used to construct these devices. In addition, oxides of Pb and In or oxides of Nb are used for the extremely thin tunnel barrier layer. By the way, in order to use these Josephson junction devices as devices for ultra-high-speed computers, it is necessary to implement them using a large number of chips integrated at the LSI level and to develop modular logic operation circuits and memory circuits. . When mounting an LSI chip on a module board, the following points should be kept in mind: (1) When connecting a superconducting LSI chip to another superconducting LSI chip via wiring, the wiring and connection electrodes (input and output signal All of the lead-out electrodes must be made of superconducting metal.

(2) The connecting protruding electrodes (bumps) formed on the terminal electrodes of the superconducting LSI chip must have sufficient adhesive strength.

(3) Bonding of the LSI chip to the mounting board must be performed at as low a temperature as possible to prevent deterioration of the thin tunnel barrier layer. In this case, the bonding strength between the LSI chip and the module board must be strong enough to withstand thermal cycles of 300 to 4K.

etc.

Conventionally, connections between LSI chips and external electrodes were made using ultrasonic wire bonding using Al electrodes and thin Al wires, or using Au.
Thermocompression wire bonding using an electrode-Au thin wire, Al electrode-Au thin wire, reflow bonding using an Au electrode-Sn plating lead, or a solder electrode-solder electrode, etc. are used. Among these methods, the method applicable to superconducting LSI assembly is reflow bonding using solder electrodes. This is because the solder electrode itself exhibits superconducting properties, and because chip mounting allows for shorter wiring lengths than other methods, it is suitable for high-density mounting. Superconducting using reflow bonding method using solder electrodes
If this method is used to mount an LSI chip on a mounting board, the following problems will occur. That is, when forming the above-mentioned protruding electrodes on the terminal portions of a superconducting LSI, it is necessary to obtain a mechanically sufficient adhesive strength and to electrically make a superconducting connection. Generally, a metal film for connection (metal film underlying the protruding electrode) is provided between the protruding electrode and the terminal portion of the superconducting LSI. As such a metal film, an Au/Pd film (upper layer Au, lower layer Pd) is used. (C.Y. Taing et al., Journal of Electrochemical Society: Solid State Science and
Technology Volume 129, No. 4 (1982) [CY
Ting et al J.Electrochem.Soc.：Solid−State
Science and Technology Vol.129, No.4
(1982)]. The Au film is the protruding electrode and the underlying Pd film, and the Pd film is the terminal electrode (generally used) provided on the superconducting LSI chip.
Adhesion with (using Nb film) and protruding electrode material are LSI
Each plays a role in preventing diffusion into the terminal electrodes of the chip. Used in superconducting LSI
The thickness of each Pb/Au bilayer film is 100 nm. Since this two-layer film does not exhibit superconducting properties at extremely low temperatures (~4K), it exhibits a slight resistance (contact resistance) and generates heat due to Joule heat. As a result, liquid helium, a cryogenic refrigerant, vaporizes and foams.
This causes a decrease in the cooling efficiency of superconducting LSI.
In addition, such contact resistance causes distortion in the waveform of signals transmitted at high speed, resulting in problems such as deterioration of the high-speed propagation characteristics of superconductors. In order to improve such problems, it is necessary to use a connecting metal film exhibiting superconducting properties as the base film for the protruding electrodes.

[Purpose of the invention]

The present invention has been made to solve the above-mentioned problems, and its purpose is to provide a metal film underlying protruding electrodes that has a mechanically sufficient adhesive strength and does not cause contact resistance. be.

[Summary of the invention]

The present invention forms a base metal film for bonding protruding electrodes on a terminal electrode surface extending from a superconducting integrated circuit by scattering it partially, for example, in the form of islands, so that the superconducting electrode surface and Mixing the base metal film,
At the connection interface between the superconducting electrode surface and the protruding electrode, a superconducting contact is established, while the underlying metal film is intended to maintain the adhesive strength between the superconducting electrode and the protruding electrode at the same time. By the method described above, the superconducting critical current between the terminal electrode of the superconducting integrated circuit and the protruding electrode for connection is at least 600 mA.
As mentioned above, the adhesive strength of protruding electrodes is at least the shear strength.
It was found that more than 50g could be obtained.

The base metal film formed on the terminal part of the superconducting integrated circuit that forms the protruding electrodes can be dotted in the form of islands as described above, or it can be formed in the form of a lattice stripe to reduce the superconducting critical current at the adhesive interface. It was confirmed that both adhesive strength and adhesive strength were sufficient to achieve the initial objectives.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to FIGS. 1 to 4.

Example 1 A SiO ₂ layer 2 with a thickness of about 600 nm is formed by thermal oxidation on a silicon single crystal substrate 1 that has been previously cleaned. After cleaning the substrate again, it is placed in a vacuum device and placed in a high vacuum of 5×10 ^-8 Torr or less.
Nb is deposited to a thickness of about 300 nm. Note that the Nb film may be formed by a sputtering method in addition to the above-mentioned vacuum evaporation method. Next, the Nb is processed into a desired pattern by photoetching to form a ground plane 3. In the chemical etching method, HF-
HNO ₃ mixed solution and physical etching method
A reactive ion etching method such as Ar or CF ₄ is used. This ground plane has the effect of shielding external magnetism for the Josephson junction formed in the active element part of the superconducting LSI, and the mirror effect of the magnetic flux generated from the control line. It plays the role of terminal 4. Next, a first interlayer insulating film 5 is formed on the Nb film described above except for the ground terminal portion and the protrusion electrode formation region. Next, a resistor 6 made of a MoNx film, and a wiring connection layer 7 and a lower electrode film 8 made of a Nb/NbN double layer film are then formed. Next, a second interlayer insulating film 9 made of a SiO film is formed. At this time, an opening is provided in a part of the lower electrode to expose the NbN surface. Next, plasma oxidation is used to remove NbN at the opening.
A tunnel barrier layer 10 is formed on the surface of the lower electrode, and subsequently an upper electrode 11 made of a Pb alloy is formed.
Furthermore, a third interlayer insulating film 12 made of a SiO film, followed by a control line 13 and a fourth interlayer insulating film 14 are formed. Next, a resist pattern for lift-off is formed in a desired region on the Nb surface of the terminal 4 for the protruding electrode in an island-like manner. Then Ti21 for 100n
After depositing 200 nm of Pd22 and 300 nm of Au23, lift-off is performed to form a Ti-Pd-Au film 15 at a desired portion on the protruding electrode terminal section 4 (see Figs. 2 and 3). . Thereafter, the protruding electrode terminal portion 4 is exposed to form a resist tencil pattern for forming protruding electrodes. Next, Ti
−RF in Ar atmosphere at reduced pressure to 10 ^-3 Torr to clean the Nb surface on which no Pd−Au film is formed.
I did spatuta etching. The rf discharge voltage is
250Vp-p, discharge time is 30 minutes. Thereafter, Pb, Sn, and Bi, which are materials for the protruding electrodes, are sequentially deposited. Vapor deposition is performed in a vacuum below 10 ^-7 Torr.
The diameter was 150 μmφ, and the deposited film thickness was 30 μm. Next, after performing lift-off to remove unnecessary portions of the deposited film, the substrate 1 is heated to 200°C in a nitrogen gas atmosphere containing 5% hydrogen gas, and the protruding electrode material is reflowed to form a hemisphere using surface tension. shaped protrusion 16
And so. The diameter of the spherical protruding electrode is approximately
It is 100 μm.

Connection with electrode terminals of a superconducting integrated circuit using the protruding electrode base metal film produced by the method described above exhibited a superconducting state at an extremely low temperature of 4.2K. When 5 and 4 base metal films of 40 μm ² and 30 μm ² are formed on the terminal area within 100 μm square, the obtained superconducting critical current is 700 mA, and the shear strength per protruding electrode is 60 g. Obtained.

In this example, the base metal film was formed in an island-like manner, but as shown in FIG. 4, it was confirmed that the same effect could be obtained even when the base metal film was formed in a checkered pattern. There is.

In addition, in this example, Ti is used as the underlying metal film material.
-Pd-Au is shown, but Ti-Pd-Ag, Cr
It has been confirmed that the same effect can be obtained using -Cu-Au, Cr-Cu-Ag, etc.

〔Effect of the invention〕

As explained above, according to the present invention, superconducting
It has become possible to stably form protruding electrode metals for external connections in LSIs, exhibit superconducting properties with zero contact resistance, and produce electrode structures with high adhesive strength with good reproducibility.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of a protruding electrode portion for external connection of a superconducting LSI according to the present invention, FIG. 2 is a cross-sectional view showing the structure of an improved protruding electrode base metal film, and FIGS. 3 and 4 are: FIG. 3 is a plan view showing the structure of a protruding electrode base metal film. 1...Substrate, 2...Insulating layer (SiO ₂ layer), 3...
Ground plane, 4... terminal electrode of superconducting LSI, 5... first interlayer insulating film, 6... resistor, 7...
Wiring layer, 8... lower electrode, 9... second interlayer insulating film, 10... tunnel barrier layer, 11... upper electrode, 12... third interlayer insulating film, 13... control line,
14... Fourth interlayer insulating film, 15, 15'... Protrusion electrode base metal film, 21... Ti film, 22... Pd film,
23...Au film.

Claims (1)

[Scope of Claims] 1. A superconducting integrated circuit whose main element is a Sanderch structure in which an ultra-thin tunnel barrier layer is sandwiched between two superconducting thin films, and a superconducting protruding electrode is connected to this element. A base metal film for adhering the protruding electrode is partially formed on the terminal electrode surface between the protruding electrode and the terminal electrode of the superconducting integrated circuit, and the protruding electrode and the terminal electrode are partially directly connected. A superconducting integrated circuit characterized by: 2. The superconducting integrated circuit according to claim 1, wherein the base metal film is formed in an island-like manner scattered over the terminal electrode surface. 3. The superconducting integrated circuit according to claim 1, wherein the base metal film is formed so as to be scattered in a grid pattern on the terminal electrode surface. 4. In the superconducting integrated circuit according to claim 2 or 3, the base metal film extends into an insulating opening provided to expose the terminal electrode, and surrounds the insulating opening. A superconducting integrated circuit formed using