Proceedings of the 43rd European Microwave Conference
Modeling and Characterization of Copper Tape Microstrips on Paper Substrate and Application to
24 GHz Branch-Line Couplers C. Mariotti, F. Alimenti, P. Mezzanotte, M. Dionigi, Marco Virili, S. Giacomucci, L. Roselli,
University of Perugia, Dept. of Electronic and Information Engineering via G. Duranti 93, 06 125 Perugia, Italy (e-mail:
[email protected]).
Abstract-In this work, a new technique to fabricate mi
crowave circuits in paper substrates is adopted. This technique relies on a copper adhesive tape that is shaped by a photo lithographic process and then transferred to the hosting substrate by means of a sacrificial layer. Microstrip lines in paper substrates have been electromagnetically characterized accounting also for the adhesive layers. Then a simple CAD model is proposed
and the possibility to use conventional eutectic alloy instead of epoxy to mount lumped components. The Cu tape method is applied, for the first time, to a microstrip line which has been fabricated and tested up to 30 GHz. An accurate electromagnetic modeling of this structure is proposed accounting also for the adhesive layers.
and experimentally validated in the microwave frequency range.
Measured results show an insertion loss of about 1.8 dB/cm at
II.
30 GHz. Finally, a branch-line coupler, working at 24 GHz has
been designed using the previous characterization, fabricated and
measured. The experiment shows a 4.1 dB insertion loss in good agreement with the simulations.
Keywords-flexible electronics; paper technology; microwave
According to [ 10] the fabrication process can be summa rized as follows: 1)
circuits; transmission lines; green electronics; Wide Area Elec tronics (WAE), 24 GHz, Branch-line coupling.
I.
2) INTRODUCTION
In last years paper has been proposed as a substrate for planar circuits up to microwave frequencies [ 1], [2]. Paper is organic, eco-compatible, flexible and cheap, thus suitable for green and on-wide-area distributed electronic applications, such as [3]-[7], potentially reaching the 24 GHz ISM frequency band and beyond [8]. In the above applications the nano-particle silver ink jet printing technology has been used to fabricate circuits and antennas. The Ag ink used has a thickness of 3/Lm, reached by overlapping several prints, and a conductivity of l. 1 x 107 Slm, obtained after a post-print curing process [9]. The advantages of the printing method are a high feature resolution and, in principle, the possibility to fabricate resistive and semiconductor layers by a mere selection of the ink. In other words, the vision behind such an approach is that of an electronic that can be printed on-demand. However, the price of the ink is still quite high ($ 450 for a 25 ml quantity) and a two hours curing process is needed. In this work an innovative fabrication process for the connecting network is demonstrated starting from a copper (Cu) adhesive tape etched by means of a photo-lithographic technology. It is worth noticing that a direct layout shaping is possible by using laser cutting machine, if more repeatability and precision are required. The main advantages of the Cu tape are a superior conductivity with respect to cured Ag ink
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PRO C E S S D E S CRIPTION
3)
4)
A photo-resist film is deposed on the copper surface. Then the circuit layout is transferred to the photo resist exploiting a photo-mask, an UV light source and a NaOH solution to remove the unimpressed film. The copper is wet etched. Such a step exposes the adhesive layer where the copper has been removed while, on the opposite side, it remains covered by the protection layer of the original adhesive copper tape. In summary, the first two steps are actually similar to those used in standard photo-lithographic technology. Moreover, different techniques to remove the unused copper, for example by means of numerical control pattern cutting plotters, can be adopted in substitution of these steps without lack of generality, [ 1 1]. A sacrificial layer is attached on the copper side and then the protection layer is removed. The sacrificial layer has a very important function to keep the rel ative distances among the layout features especially when these are not physically connected. Such a layer can be a simple paper sheet or a plastic adhesive tape. In the first case, the exposed adhesive (where the copper has been removed) is sufficient to stick the sacrificial paper sheet. In the second case, instead, the adhesion of the sacrificial layer has been selected so as to be less than that of the copper tape itself. The etched metal is transferred to the hosting paper substrate and, finally, the sacrificial layer is removed. This last step also removes most of the exposed adhesive material. Such an effect is stronger if a plastic adhesive tape is used as sacrificial layer.
As an example, Fig. 1 shows a 24 GHz branch-line coupler fabricated with the described technology. The coupler is a
7 - 10 Oct 20 13, Nuremberg, Germany
90°, 4-ports hybrid junction, has a diameter of about 3 mm and is attached on a 230 f.Lm paper substrate from Mitshubishi Electric. 20mm 1
4
the geometry dimensions and material parameters (Mitsubishi photo-paper) quoted in Table I. Afterwards, the simulation results have been compared with the measured ones and finally an equivalent uniform substrate has been defined, using a high frequency CAD tool, matching the beforehand obtained results for both the microstrip lines, see Fig. 2(b). In particular two microstrip lines of 50 D and 70 D characteristic impedances, have been considered in order to cover the range typically used in microwave circuits (i.e. in hybrid junctions and power dividers). TABLE 1.
e e
Parameter
= ('f")
'W50
Fig. 1 . 24 GHz branch-linc couplcr on papcr substrate, fabricated with the described method. The junction has a diameter of about 3 mm
III.
CAD M O D EL ING
It has been shown how, with the above described method, complex metal patterns can be easily transferred from the original tape to the hosting substrate by preserving the relative distances among the various circuit features. At the end of such a process, however, a thin adhesive layer will be interposed between each metal and the substrate.
1------"'---1
to
30
tTri
flm flm
35
Mm
Er
3.2
h
230
Parameter
W 70
Value 377
C'r.a
Mm
1.3 5.8 x 10
CITri tan
