Device for mounting wideband microwave monolithic integrated circuits.

0001The present invention relates to a device for mounting very broadband monolithic microwave integrated circuits, in particular AsGa circuits.

0002Broadband monolithic microwave integrated circuits, such as AsGa circuits, are widely used in equipment claiming to operate on very wide bands, for example from 2 to 20 GHz or more. AsGa circuits improve performance due in particular to the uniformity of the gain obtained and they allow a reduction in volume and a reduction in serial prices. These advantages are further accentuated, in use at very high bandwidth, by the use of the technique of distributed amplifiers, simultaneously allowing a high gain, a very wide band, low coefficients of reflection at input and output, and good input-output insulation.

0003However, these integrated circuits must be provided with circuits for decoupling the supply or bias circuits. However, these decoupling circuits use capacities to decouple the power supplies serving to polarize the amplifiers and these capacities must be all the more important (therefore larger) that one wants to work at lower frequencies. These capacities therefore take up a lot of semiconductor surface area, thus preventing a reduction in cost and bulk as large as might be hoped. In practice, we do not seek to decouple on the chips frequencies below 5 GHz so as not to penalize too much the size thereof and the manufacturing yield, the capacities produced in thin layers not having very good yields.

0004If one wishes to operate at frequencies lower than 5 GHz, in the case of very broadband applications, a known solution then consists in completing the decoupling with high-value external capacities carried out on an attached support carrying the integrated circuits. . This support may be a metal support with added capacities or a conductive silicon support on which MOS (Metal-Oxide-Semiconductor) type capacities have been produced.

0005A disadvantage of this solution is however the appearance of a resonance due to the capacity system on integrated circuit / connecting wire / external capacity. The resonant frequency cannot practically be reduced because that would suppose a value of capacity on integrated circuit too high and an inductance of the connecting wire very high, therefore a too important obstruction and problems of parasitic modes for a long connecting wire.

0006The object of the invention is to remedy this drawback by preventing this resonance phenomenon from occurring.

0007Another object of the invention is a mounting device eliminating the drawback of the known solution through the use of a ferrite support.

0008According to the invention, there is therefore provided a device for mounting very wide band monolithic microwave integrated circuits comprising a support on which are fixed the integrated circuit chips which include capacities for decoupling the DC power supplies of said circuits, said support also carrying means for connecting said capacities to said power supplies, said device being characterized in that said support is made of ferrite and in that said connection means comprise metallized areas of connection on the support, connected by connection wires to said decoupling capacities, said wires being arranged opposite bare parts of said ferrite support.

0009The invention will be better understood and other characteristics and advantages will appear with the aid of the description below and of the accompanying drawings in which:<ul id="ul0001" list-style="dash"><li>Figures 1 and 2 are top views and in section of a mounting device in a hybrid circuit according to the prior art;</li><li>Figures 3 and 4 are top views and in section of the mounting device according to the invention;</li><li>Figure 5 is an enlarged sectional view of a detail of the device according to the invention; and</li><li>Figure 6 is a diagram illustrating the performance of the device according to the invention.</li></ul>

0010As illustrated in FIGS. 1 and 2, in known use on a support added in a hybrid circuit, the monolithic integrated circuit chips of AsGa 3, 4, 5 are mounted on a support 2, for example made of conductive silicon. The support is arranged between an input line 1 and an microstrip output line 2 with an impedance conventionally equal to 50 Ω, produced on ceramic CER substrate. Each chip has decoupling capabilities C_p₁, C_p₂ ... C_pn connected by connecting wires, such as 7 (thermocompression wire), to external capacities C_m₁, C_m2 ... VS_mp themselves connected to continuous power supplies (not shown) by connections, such as wire 8. As can be seen better in FIG. 2 which is a partial section of the device of FIG. 1 along line II-II, the chip d 'AsGa 5 is mounted via its metallized underside 14 on the conductive silicon support 2. Capacity C_pn produced on the chip 5 comprises a dielectric layer 11 comprised between two metal layers 10 and 12, all of these layers being produced in thin film technology. The conductive layer 12 is connected to the layer 14 serving as ground by a metallized hole 13. The external capacity C_min is a capacity of the MOS type comprising a metal layer 15, an oxide layer SiO₂ 16 and a semiconductor layer constituted by the support 2 serving as a mass. Layer 10 and layer 15 are connected by a connecting wire 7 (thermocompression wire).

0011The capacity C system_pn/ connecting wire 7 with inductance L / capacity C_min has a resonance at frequency f_r :<maths id="math0001"><img file="EP0545809A1_D0001.tif" /></maths>

0012In practice, for example:<dl id="dl0001"><dt>L</dt><dd>= 0.1 nH</dd><dt>VS_pn</dt><dd>= 20 pF</dd><dt>VS_min</dt><dd>= 200 pF</dd></dl> from where :    f_r = 3.5 GHz

0013This value is very difficult to decrease because it would imply a capacity C_pn very high, solution eliminated in practice for the reasons already mentioned, and a very high inductance L which would cause problems of space and parasitic modes for a long thermocompression wire.

0014The invention proposes to replace the silicon support 2 by a ferrite support 20 as shown in FIG. 3 seen from above and FIG. 4 in section along the line IV-IV of FIG. 3. The same reference numbers refer to same elements as in FIGS. 1 and 2. The support 20 has the form of a plate with a thickness of a few hundred micrometers and is for example made of a pure nickel ferrite, having a low permeability and a maximum resonance frequency (without magnetic field) (2 GHz). The plate 20 has a uniform metallization 26 serving as a general ground plane on one of its faces (the lower face in the drawing) and several metallization areas on its other face: some of these areas, such as the area 24 serve as the ground for the chips 3, 4, 5 which are mounted therein and the others 21, 22, 23 (one or more as the case may be) serve as connection pads for the supply voltages to be applied. Thus range 21 is connected to capacity C_pn by the 7 'connecting wire and to the power supply by an 8' wire.

0015Each upper mass range, such as 24, to which an AsGa chip is attached is connected to the general ground plane 26 by one (or more) metallized hole 25.

0016Figure 5 shows an important aspect of the invention. In this FIG. 5 is shown enlarged a part of the section of FIG. 4. As can be seen, the wire 7 is placed facing a part of the ferrite 20 which appears bare. This is important because it allows the coupling between the magnetic field H created around the wire 7 '(as materialized by a line of force 27) and the ferrite material. Thanks to this coupling and to the absorption which results therefrom, any resonance is prevented from occurring in the wire 7 '.

0017Figure 6 illustrates the performance obtained. Curve 30 shows the variation in gain as a function of frequency in an equipment according to the prior art and curve 31 shows the variation in gain for the same equipment but with a ferrite support according to the invention.

0018It can be seen that annoying parasitic resonances have thus been removed over a very wide band descending well below 3 GHz.

0019Of course, the example of embodiment described is in no way limitative of the invention.