S parameters for magnetostatic wave transducers on silicon microstructures

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Microelectronic Engineering 51–52 (2000) 479–483 www.elsevier.nl / locate / mee

S parameters for magnetostatic wave transducers on silicon microstructures Elena Matei*, George Sajin National Research and Development Institute for Microtechnologies, Str. Erou Iancu Nicolae 32 B, Bucharest, Romania Abstract The paper presents the design, realisation and measurements on MSW transducers on silicon wafers and silicon membranes. S parameter values of these structures, with and without magnetised YIG thin film demonstrates the feasibility of such transducers. Using silicon membranes to support these devices offers the advantage of miniaturisation, with important openings toward microwave circuit integration on semiconductor substrates.  2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetostatic waves; Transducers; Membranes; Microwaves

1. Introduction Launching and detecting the magnetostatic waves (MSW) are realised with a dedicated microstrip structure. These transducers, in the meander or grid configurations, are excited with a high frequency current. This current induces a radiofrequency magnetic field that generates a wide variety of propagation modes through the ferrite monocrystalline YIG depending on the external polarising magnetic field. The usual frequency domain ranges between 2 GHz and 10 GHz and the power handling possibility is some milliwatts. One demonstrates [1] that the narrower are these microstrip lines, the better are the results in MSW handling. To make these lines narrow it is necessary that the substrate supporting this microstrip structure be thin. The authors present their experiments to realise MSW transducer structures on wafers and on silicon membranes. The thickness of the silicon membrane (50 mm) allows design of microstrip lines of about 40 mm width, expected to permit a better fitting between MSW transducers theory and practice and to facilitate the integration of ferrite components in microwave integrated circuits.

*Corresponding author. E-mail addresses: [email protected] (E. Matei), [email protected] (G. Sajin) 0167-9317 / 00 / $ – see front matter PII: S0167-9317( 99 )00501-8

 2000 Elsevier Science B.V. All rights reserved.

E. Matei, G. Sajin / Microelectronic Engineering 51 – 52 (2000) 479 – 483

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2. Design of MSW transducers Theoretically, the transducers consist of a number of N conducting strips, each of width a, carrying a current I0 . They may form a grid with N parallel conducting strips (IT 5 NI0 ), or a microstrip line with N meanders (IT 5 I0 ), where IT is total current. In our experiments N51 and the radiating resistance is given by:

F

(s) sin(ak s / 2) ]]]] R (s) m 5 1R l (ak s / 2)

G

2

where l is the effective length of the launcher and detector and the subscript s defines the sign of the wavenumber k s in a magnetic polarized ferrite loaded structure. The radiating resistance of the MSW launcher and detector is computed for enough distant transducers to neglect the interaction. The current distribution on the microstrip lines is assumed to be uniform. The width of microstrip lines forming the launcher and detector structures were designed for 50 V characteristic impedance using the well-known relations [2].

3. Technology and measurements In order to obtain the structure under test the following technological steps (cf. [3,4]) were performed: • k100l p silicon wafers with a resistivity of 2–3 V cm were oxidized in order to obtain 1-mm-thick SiO 2 . This layer prevents silicon surface damage during the subsequent technological steps; • deposition of 0.1 mm SiNC that act as a mask for the anizotropic silicon etching; • for the structures having membranes, an anizotroping etching of silicon was performed and finally 50 mm thickness membrane was obtained; • metallisation is carried out with gold due to its good characteristics in the high frequency domain; the thickness of the gold layer was 1 mm having in view the penetration depth at the frequencies up to 10 GHz. A thin layer of Ti (50 nm) was deposited prior to the gold in order to ensure a better adhesion to the substrate; • the microstrip transducers pattern was obtained by a standard photolithographic process. The S parameters of these structures, with and without monocrystalline magnetized ferrite load, were drawn using microwave standard measurement equipment and procedures.

3.1. MSW transducers on silicon wafers The geometry of these transducers is shown in Fig. 1a. In Fig. 2a there is a plot of S11 for a frequency sweep between 0.5 GHz and 8 GHz, in the absence of ferritic monocrystalline YIG film. One can see a minimum of this parameter (indicating a maximum matching) in a frequency band between 3 GHz and 4 GHz. The maximum matching value, obtained at f 53.22 GHz is uS11 u518.24 dB that means a standing wave ratio SWR51.28.

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Fig. 1. (a) Magnetostatic wave transducers on silicon wafers. (b) Magnetostatic wave transducers on silicon membranes.

Fig. 2. (a) S11 for MSW transducers on silicon wafer in the absence of YIG monocrystalline film. (b) S11 for MSW transducers on silicon wafer in the presence of magnetised YIG monocrystalline film. (c) S21 for MSW transducers on silicon wafer in the presence of magnetised YIG monocrystalline film.

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E. Matei, G. Sajin / Microelectronic Engineering 51 – 52 (2000) 479 – 483

If the structure is loaded with a magnetized YIG monocrystalline film, the value of S11 changes. The plot for this new case is shown in Fig. 2b for a frequency sweep between 0.5 GHz and 6 GHz. One can see a decrease of the reflexion parameter, uS11 u530.3 dB, that means an improvement of standing wave ratio value to SWR51.06, at the same frequency as in the unloaded situation. In Fig. 2c is presented a plot of S21 transmission parameter in the presence of ferrite film for a frequency sweep between 1 GHz and 13 GHz. One can observe two maximum values of transmission parameter. One value S21 5 229 dB at the frequency f(3 GHz and another value S21 5 225 dB at the frequency f 59.4 GHz.

3.2. MSW transducers on silicon membranes The geometry of these transducers is shown in Fig. 1b. The frequency sweep for characterization of these launching and detecting structures was limited between 0.5 GHz and 5 GHz.

Fig. 3. (a) S11 for MSW transducers on silicon membranes in the absence of YIG monocrystalline film. (b) S11 for MSW transducers on silicon membranes in the presence of magnetised YIG monocrystalline film. (c) S21 for MSW transducers on silicon membranes in the presence of magnetised YIG monocrystalline film.

E. Matei, G. Sajin / Microelectronic Engineering 51 – 52 (2000) 479 – 483

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In Fig. 3a there is a plot of reflexion parameter uS11 u for a frequency sweep in the domain shown above, in the absence of ferrite loading. One may see a maximum matching value uS11 u519.2 dB for the frequency f 51.5 GHz that means a standing wave ratio SWR51.25. If the transducers structure is loaded with magnetised ferritic film the reflexion parameter has a lower value and the maximum matching frequency moves from 1.5 GHz to 1.69 GHz. The value of reflexion parameter in this situation is uS11 u530.6 dB (see Fig. 3b). The corresponding standing wave value is SWR51.065. The signal attenuation between launcher and detector in the presence of ferrite loading is shown in Fig. 3c. The value of transmission parameter is S21 5 228.5 dB. One can see that the maximum transmission and the minimum reflexion values are at the same frequency f 51.69 GHz. In all experiments presented above the polarising magnetic field was applied normal at the ferrite film (and silicon substrate) surface and had the value Hapl 52100 Oe. The ferrite was a monocrystalline YIG layer with a thickness of 20 mm grown by liquid phase epitaxy on a gallium– gadolinium garnet substrate with length L513 mm, width w54 mm and thickness t5200 mm. 4. Conclusions The experiments with the MSW transducers on silicon wafers and silicon membranes show the possibility to implement such devices in an integrated complex microwave circuit. Due to the low resistivity of the silicon wafer, the signal losses were rather high so that the other S parameters are difficult to interpret. For a more effective MSW handling (signal delaying, pulse reshaping, soliton inducing, magnetic field tunable filtering, etc.) it is necessary that the semiconductor substrate have a higher resistivity in order to minimize the microwave signal losses. References [1] A.K. Ganguly, D.C. Webb, Microstrip excitation of magneto-static surface waves, IEEE Trans. MTT 23 (1975) 998–1006. [2] G. Sajin et al., Models of nonreciprocal microwave components on magnetic polarised ferrite, in: IMT Research Report, 1997. [3] G. Sajin et al., Soliton propagation in thin magnetic films, in: IMT Research Report, 1997. [4] E. Matei et al., Magnetostatic wave transducers on silicon wafers and on silicon membranes, CAS’98 Proceedings Vol. 2 (1998) 635–638.

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