EP0584650A1 - Coplanar waveguide with low wave impedance - Google Patents

Abstract

The invention relates to a coplanar waveguide which is suitable for MMIC. In order to achieve characteristic impedances which are as low as possible, two identical coplanar lines are connected in parallel. Air-bridge interfaces are incorporated at the branches of the parallel-connected coplanar lines. The electrical behaviour of a 30 OMEGA line is achieved by connecting, for example, two identical 60 OMEGA lines in parallel, using CPW technology.

Description

The invention relates to a coplanar waveguide according to the preamble of patent claim 1.

Such waveguides are found e.g. Use in monolithically integrated microwave circuits (MMIC), which are manufactured in coplanar waveguide technology, so-called CPW technology.

Compared to the microstrip line technology (see, inter alia, T. Hiraoka et al., IEEE Trans. Microwave Theory Tech., Vol. MTT-37 (1989), pp. 1569 to 1575), the CPW technology has the advantage that the line structure is applied on the front.

Therefore, the thinning of the substrate, the via-hole technique and the backside metallization of the substrate are not necessary. In connection with the implementation of MMIC'S in CPW technology, it sometimes proves necessary to design coplanar lines with very low characteristic impedances. For example, transmission lines for MMIC'S with characteristic impedances of less than Z _L = 30Ω are required. It is known from field theoretical considerations that the impedances of coplanar waveguides (CPW) are mainly determined by the ratio of the strip width w to the slot width s.

In order to implement characteristic impedances of Z _L = 30Ω for MMIC'S on GaAs, whose line structures typically have a metallization thickness of 3µ, these CPW's must have a ratio w / s ≈ 10 (e.g. stripe width w = 50µ and slot width s = 5µ).

. Diese Grenze für den minimalen inneren Radius der Koplanarleitung ist notwendig, um Kopplungseffekte zu vermeiden. Die Schwierigkeit koplanare Leitungen mit charakteristischen Impedanzen unterhalb von Z_L = 30Ωherzustellen, liegt darin, daß für immer kleiner werdende Impedanzen das Verhältnis w/s drastisch ansteigt.In addition to the physically unfavorable ratio of w / s = 10, such a line structure has the disadvantage that a large chip is required if lines with a length of 1 = 1 mm are to be produced, taking into account minimal radii of curvature ${\text{r}}_{\text{min}}\text{= w + 2s}$

. This limit for the minimum inner radius of the coplanar line is necessary in order to avoid coupling effects. The difficulty in producing coplanar lines with characteristic impedances below Z _L = 30Ω is that the ratio w / s increases drastically for ever smaller impedances.

The invention is therefore based on the object of specifying a coplanar waveguide with low wave resistance, which is technically simple and reliable to manufacture and which requires little space.

This object is achieved by the features specified in the characterizing part of patent claim 1. Advantageous refinements and / or further developments can be found in the subclaims.

The invention is described using an exemplary embodiment using drawings.

In FIG. 1 shows a test structure on a GaAs substrate with two 60Ω lines 1 and 2 and a 30Ω line 4 connected in parallel.

FIG. 2 shows a four-stage monolithically integrated 30 GHz amplifier on GaAs in coplanar technology, in which four coplanar lines P connected in parallel are used.

With the coplanar lines according to the invention, waveguides with low wave resistances are realized, which at the same time have a low ratio of w / s. Two identical transmission lines are used, each with the desired length but with twice the impedance than required. Airlift T junctions, which are found in Koster et al., Proc. EMC 1989, London, pp. 666 to 671 are known, are attached to the junction points of the two coplanar waveguides in order to suppress higher modes. The two coplanar lines have, for example, a slot width of s = 12µm and a stripe width of w = 10µm with an impedance of Z _L = 60Ω. The length of the transmission lines is, for example, 1 = 2mm. By connecting two 60Ω lines in parallel using CPW technology, the electrical behavior of a 30Ω line (s = 5µm, w = 50µm, 1 = 2mm) can be realized. A test structure which contains a 30Ω coplanar line 4 and a parallel connection P of two 60Ω lines 1 and 2 and an air bridge T transition 3 is shown in FIG. 1 shown.

In filter networks, the parallel connection of coplanar lines can be extended to airlift cross branches or general multiple branches. FIG. 2 shows an example of a Ka-band GaAs amplifier circuit which contains four coplanar lines P connected in parallel. This four-stage monolithically integrated amplifier is suitable for frequencies of 30 GHz, whereby only very low wave resistances occur.

However, the invention is not limited to the coplanar lines specified in the exemplary embodiment, but it can e.g. the behavior of a 20Ω line can be realized by connecting two identical 40Ω lines in parallel (on GaAs, for example, slot width s = 10µm, stripe width w = 35µm, metallization height t = 3µm). Such a 20Ω line can no longer be realized in conventional CPW technology on GaAs.

Furthermore, the use of other substrate materials such as e.g. Si or InP possible for the production of MMIC'S in CPW technology.

Claims (5)

Coplanar waveguide, in particular for microwave circuits, characterized in that the coplanar waveguide contains two identical coplanar lines (1, 2) which are connected in parallel, and that by connecting two identical coplanar lines in parallel with the characteristic impedance Z _L, a coplanar waveguide with a characteristic impedance of Z _{L / 2 can be} produced. Coplanar waveguide according to Claim 1, characterized in that air bridge-T transitions (3) are provided in the region of the splitting of the waveguide. Coplanar waveguide according to Claim 1, characterized in that air bridge cross branches are provided in filter networks at the branching points of the waveguide. Coplanar waveguide according to one of the preceding claims, characterized in that a coplanar waveguide with a characteristic impedance of 30Ω can be produced by connecting two identical 60Ω coplanar lines in parallel. Coplanar waveguide according to Claim 4, characterized in that the parallel 60Ω coplanar lines have a length of 1 = 2mm, a slot width s = 12µm and a stripe width w = 10µm.

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