JPS59152647A - Multilayer interconnection method - Google Patents

Abstract

PURPOSE:To improve the yield of an air bridge by covering lower layer wirings with an insulating material which is softened at high temperature of 500 deg.C or lower, forming upper layer wirings, and removing by gas plasma the insulating material, thereby eliminating the restriction in the steps due to the stepwise disconnection and weakness in mechanical strength. CONSTITUTION:Upper layer wirings 22 cross lower layer wirings 21 through an insulating film 23 having a smooth section. The film 23 is etched by gas plasma which does not affect the influence to a substrate or wiring metal, thereby foring an air bridge in noncontact. After lower layer wiring metal 31 is formed by normal photolithographic and lift-off methods, a photoresist (OFPR-800) is formed on the entire substrate. A resist film 33 is allowed to remain only on the crossover of the upper and lower layer wirings forming the air bridge by the lithography and the peripheral regions, and baked at 200 deg.C for 20min. Then, upper layer wirings 32 are formed by the method equivalent to the lower layer wirings. The film 33 of air bridge structure is removed by plasma etching with oxygen gas an air bridge.

Description

[Detailed Description of the Invention] Technical Field The present invention provides transistors, resistors, coils, etc. on a semiconductor substrate.
This invention relates to multilayer wiring technology for so-called integrated circuits in which electronic components such as capacitors are formed and interconnected.

For the sake of simplicity, we will use an example of an integrated circuit that includes a field-effect transistor (hereinafter referred to as FET) formed on a semi-insulating potassium arsenide (hereinafter referred to as GaAs) substrate as a case where the effects of the present invention will be noticeable. Let me explain.

BACKGROUND ART In order to increase the operating speed of integrated circuits and reduce power consumption, it is essential to miniaturize elements and increase the degree of integration of circuits. As a result, each component and the wires for interconnecting these components are brought closer to each other, and the number of intersections between the wires further increases.

As a result, the wiring capacitance in particular increases, leading to a reduction in operating speed. To prevent this, an effective insulation method between wires is required.

The air bridge method is an effective means for achieving this purpose. As shown in Figure 1, within the integrated circuit,
This is a multilayer wiring technology in which when the lower metal wiring 11 and the upper metal wiring 12 intersect, the upper wiring metal intersects three-dimensionally without interposing an insulating film between them. The feature of this method is that since the two types of wiring are insulated by air having the lowest dielectric constant, the capacitance between the plane wirings is the smallest compared to the case where any other insulating film is used. This reduces the wiring capacitance within the circuit and improves the operating speed of the integrated circuit. However, due to the structure of this air bridge, the upper layer wiring is suspended in the air, and after the air bridge is formed, it cannot come into contact with the surface. For this reason, various technical problems and restrictions such as those described below arise in subsequent steps.

In other words, ■) Chemical etching is essential when thinning the substrate from the back side to improve high frequency characteristics! It is necessary to form a protective film. On the other hand, when thinning the substrate by mechanical polishing,
Must be in contact with the substrate surface.

2) Separating semiconductor devices repeatedly formed on a substrate and making them into chips, a seven-part process j4
-;J,,l! In order to prevent debris and dirt from adhering to or scratching the surface, processing is often applied from the back side, which inevitably comes into contact with the front surface.

3) In the bonding process in which chipped semiconductor devices are attached to a package one by one, the surface side of the chip is attracted using a collet or vacuum tweezers.

4) When a protective film is finally formed on the integrated circuit, an opening is provided in the protective film in order to take out electrical connections to the outside.

If photolithography using normal contact exposure is applied here, the substrate surface will be tightly adhered to the photomask.

The above steps or handling cannot be carried out from the viewpoint of protecting the air bridge, and in order to carry out the same steps as those described above, complicated steps will be added.

In this case, a problem arises in that the productivity and yield of semiconductor devices are significantly deteriorated.

On the other hand, the air bridge itself is structurally weak, and there is a problem in that even a slight external force causes the upper layer wiring to break or come into contact with the lower layer wiring, resulting in a low yield. Also,
In order to prevent contact between wires, the upper layer wires are often formed as high as possible, and there is a risk of step breakage.

DISCLOSURE OF THE INVENTION The present invention provides a new multilayer wiring method that solves the problems of the prior art described above.

The following description will be made based on examples.

FIG. 2 is for showing an example of an embodiment of the present invention,
The upper layer wiring 22 intersects with the lower layer wiring 21 via an insulating film 23 having a smooth cross section. By etching the insulating film 23 with gas plasma that does not affect the substrate or metal wiring, an air bridge can be formed without contact. An example of how an air bridge is manufactured will be described below along the steps shown in FIG.

After forming the lower layer wiring metal 31 by ordinary photolithography and lift-off method, photoresist (OFPR-800) is applied to the entire surface of the substrate to a thickness of 2.5 μm by spin coating method.
Form to a thickness of . The resist film 33 is left only at the intersection of the upper layer and lower layer wiring and the surrounding area where the air bridge is formed by lithography, (E 3-(A)), 2
Bake for 20 minutes at a temperature of 00°C. The temperature at this time is sufficiently higher than the temperature at which the resist softens, so the resist flows and takes on a hemispherical shape due to surface tension. Figure a-
(s) Next, the upper layer wiring 8 is
form 2.

(Figures a-(C)) Since the cross section of the insulating film 38 is softened and almost circular, it is possible to eliminate so-called step breakage in which the upper layer wiring is broken at the step portion of the insulating film.

Subsequently, the photoresist film 33 having the air bridge structure was removed by blast etching using oxygen gas, and the photoresist film 33 having the air bridge structure was removed.
■) Create an air bridge as shown in ().

Thereafter, a semiconductor device can be realized by applying conventional techniques to assembly processes such as back polishing, scribing, separating, bonding, etc., and protective film forming processes. Here, the step of removing the photoresist film 33 with the air bridge structure is etching using gas plasma, and the insulating film 3 is removed until it no longer needs to contact the substrate surface during the assembly process.
You can also leave 3. This solves the problem that the air bridge is damaged and the yield is reduced.

In the example, photoresist was used for the insulating film 38, but 5
Any material that can be softened at temperatures as low as 00° C. or lower and can be removed by gas plasma satisfies the requirements of the present invention, and is not limited to photoresist in any way.

Effects of the Invention According to the present invention, the disadvantages of air bridges, such as step breakage and process constraints due to weak mechanical strength, can be solved at once, and the yield of air bridges can be greatly improved. □

[Brief explanation of drawings]

FIG. 1 shows an example of an air bridge structure, and FIG. 2 shows an example of an embodiment of the present invention. Figures 3 (A), (B), (C), and ■) are manufacturing process diagrams. 11.21.31...Lower wiring 12.22.32...Upper wiring 23.33...Insulating film

Claims (1)

[Claims] (1) In the air bridge method, which is a multilayer wiring technology for integrated circuits in which lower and upper wiring metals intersect three-dimensionally without an insulating film, the lower wiring is covered with an insulating material that softens at a high temperature of 500°C or less. A multilayer wiring method characterized in that an upper layer wiring is then formed and the insulating material is removed by gas plasma.