Design and Characterization of a Quasi-Optical Mixer

Fabricated using Silicon Micromachining

D.Neculoiu, I.Petrini, C. Buiculescu,

M.Dragoman, A.Muller IMT-Bucharest

32 B, Erou Iancu Nicolae str 077190 Bucharest Romania

neculoiu@imt.ro

R.Marcelli CNR–M2T,

00133 Rome, Italy

G.Bartolucci “Tor Vergata” University

Via del Politecnico 1 00133 Rome Italy

F.Giacomozzi ITC-IRST

Via Sommarive 18 38050 Povo Trento, Italy

Abstract—A silicon micromachined uniplanar quasi-optical mixer is presented for the first time. The circuit is based on a membrane-supported double folded slot antenna and a GaAs flip chip Schottky diode, mounted on the bulk area of the structure. The 1.5 μm thin dielectric membrane was obtained by micromachining of high resistivity <100> oriented silicon. The circuit is designed for the 38 GHz frequency range using coplanar waveguide technology and full-wave electromagnetic simulations. The mixer is characterized using the isotropic conversion losses parameter and a value of 10 dB was measured at 38 GHz.

I. INTRODUCTION

In recent years, the micromachining technology has been proposed for the fabrication of millimeter wave circuits on very thin dielectric membrane [1]. This kind of realization offers many advantages: the reduction of dielectric losses, reduction of dispersion effects, the suppression of substrate modes and the possibility of using higher transmission line characteristic impedance in the design.

Integrated antenna-mixer circuits, consisting of a planar antenna integrated along with the mixing element, offer a very practical solution for building receivers for the millimeter- and sub-millimeter- wave frequency range. They are easier to manufacture, smaller and much less expensive than the waveguide mixers. Some quasi-optical mixers designed at various frequencies have been reported [2]-[4].

This paper presents a uniplanar quasi-optical mixer fabricated using silicon micromachining. The circuit is based on coplanar waveguide (CPW) technology and is manufactured on a large thin dielectric membrane. The

structure was designed for 38 GHz, a frequency devoted to mobile communications.

II. DESIGN AND FABRICATION

The receiving module is a single Schottky diode mixer with quasi-optical feed for both radio frequency (RF) and local oscillator (LO) signals. The circuit schematic diagram is shown in Fig. 1. The main block of the receiver is the membrane supported double folded slot antenna. This type of antenna was intensively investigated both on silicon bulk [5] and micromachined silicon [6]. In the present work, the antenna was designed and fabricated on a 1.5 μm thick three-layer dielectric membrane. The mixer diode is a GaAs flip chip Schottky diode (DMK 2790, Alpha Ind.). The data sheet electrical specifications are: series resistance RS = 4 Ω, total capacitance CT = 0.05 pF (the overlay capacitance Cp = 0.02 pF), ideality factor n = 1.05, saturation current IS = 0.5 pA. The Schottky diode is placed in series between the matching network connected to the antenna and the low-pass filter and it is mounted using conductive epoxy.

The membrane supported receiver structure involves a micromachined area, a bulk area and the transition between those two. This transition region is determined by the chemical etching behaviour of silicon. The membrane to bulk transition is a non-uniform CPW line (the substrate thickness varies from membrane thickness to 525 μm) and it is modeled as a cascade of very short transmission lines with constant effective permittivity and characteristic impedance [7]. The transmission line parameters are found by means of electromagnetic simulations. The matching network is

IEEE MELECON 2006, May 16-19, Benalmádena (Málaga), Spain

1-4244-0088-0/06/$20.00 ©2006 IEEE 210

fabricated on bulk material and consists from a 1000 μmlong 50/50/50 μm CPW line followed by a 450 μm long 175/350/175 μm CPW line (350 μm is the width and 175 μmis the gap of the CPW line).

The low-pass filter presents a short circuit at the diode terminal at the frequency of 38 GHz. The filter consists of 3 sections: two low-impedance sections (25/350/25 μm CPW line) and one high-impedance section (175/50/175 μm CPW line). All the sections are 775 μm long.

The receiver antenna is an array of two folded slot antennas placed approximately one-half wavelength apart for good radiation pattern [4, 6]. The two-folded slot antennas are fed in phase by a 150/50/150 μm CPW transmission line. The central conductor of the CPW line is short-circuited to the ground plane by means of a quarter wavelength 50/50/50 μm CPW stub and this provides the DC return for biasing the diode. The stub’s input impedance is open circuit at the antenna folded-slot reference plane.

Because the effective permittivity of the central CPW line is close to 1, there is no need to fold this line to assure the correct feed phase of the two antennas. The symmetry of the layout results in no need for air bridges to equalize the ground planes and the fabrication process is simplified.

In the design the circuit is divided into the membrane supported circuit block and the silicon bulk supported block. Every block is modeled and designed using full-wave electromagnetic simulation software IE3D from Zeland Inc. [8]. The whole circuit is analyzed by means of linear/nonlinear techniques using the Microwave Office design environment from Applied Wave Research Inc.

The simulated 3D radiation pattern of the double folded antenna at 38 GHz is presented in Fig.2. The radiation pattern is bi-directional and almost symmetric, with a moderate gain of about 6 dBi in each direction normal to the circuit surface. One lobe of the antenna is used to receive the RF signal while the opposite lobe is used for the quasi-optical coupling with the LO signal.

The electromagnetic simulations were integrated with the nonlinear harmonic balance engine of Microwave Office software. In general the harmonic balance method requires the electromagnetic simulation of the planar structures at several harmonics of the LO frequency to obtain accurate results. In the present work the diode behavior is only weak nonlinear because the input power level from the LO is low. In order to compensate this low level the diode is DC biased using a current generator connected to the circuit by means of an external bias tee inserted into the intermediate frequency (IF) chain.

The membrane was manufactured using conventional technological processes. Thermal oxidation process, of high resistivity silicon substrate, performed at 1150ºC was used to grow the first 7500Å thin silicon dioxide layer. The nitride

membrane-supported double folded slot

antenna

Schottky diodes

low-pass filter

membrane to bulk transition

IF output

Figure 1. The schematic diagram of the quasi-optical receiver.

Figure 2. Simulated 3D radiation pattern for the membrane supported double folded slot antenna.

211

layer (3000Å thin) was deposed by a LPCVD process at 750ºC using ammonia (NH4) and dichlorosilane (SiH2Cl2)(ratio 4:1). These two layers were deposited on both sides of the wafer. The third silicon dioxide layer (4500Å thin) was deposited by CVD at 400ºC, only on the top of the wafer. The total thickness of the three layer membrane is 1.5 μm.

The receiver structure was patterned on a thin chromium/gold layer deposited on the top of the wafer, while the opening of the etching windows at the rear was obtained by using a double-sided alignment. Subsequently, for the final receiver structure, well-defined lines of 2 μm thickness were obtained by a selective gold electroplating process on the top of the wafer, using SU-8 negative resist. The final step for the structures manufacturing is the anisotropic etching of the <100> oriented silicon wafer in TMAH solution. The photo of the fabricated quasi-optical mixer with mounted diodes is presented in Fig.3.

III. MEASUREMENTS RESULTS

The quasi-optical mixer structure was mounted on a printed circuit board (PCB) above an opening that allow the coupling with the radiation from the LO source (Fig. 3). The IF signal is picked out from the chip through the bond wires that connect the pads on the chip as well as the ground plane to the PCB circuit and to the output connector. A coaxial cable is used to guide the IF signal to a bias tee and then to the spectrum analyzer that measure the IF power. The bandwidth of the bias tee is 50 – 1000 MHz. The characteristic impedance of the IF chain is 50 Ω. The home made current generator supplies an adjustable bias current between 1 μA and 1 mA.

The experimental setup used for the quasi-optical mixer characterization is presented in Fig. 4. The horn antennas, the quasi-optical mixer and the mechanical system for positioning were shielded inside a wooden box covered inside with suitable absorber material for 38 GHz frequency

range.

Since the circuit combines the function of antenna and mixer, the isotropic conversion loss Liso parameter is used to characterize the efficiency of the receiver [2]. Liso is defined as the ratio between the power level at IF and the incident isotropic power at RF.

The following formula is used to calculate the isotropic power at the membrane supported antenna plane:

2

Horngeniso R4GPP ⎟⎟⎠

⎞⎜⎜⎝

⎛πλ=

(1) where: λ is the free-space wavelength; R is the distance

between horn antenna and double folded slot antenna (defined in Fig. 4); GHORN is the gain of the horn antenna and Pgen is the power level at the output of the cables connecting the horn antennas and the signal sources.

The isotropic conversion losses of upper sideband versus IF frequency at 38 GHz RF frequency is presented in Fig. 5 (almost identical results were obtained for the lower sideband). The isotropic conversion losses have a constant value of about 10 dB for a wide band of the IF frequency range. The increase of Liso as the IF frequency increases over 1 GHz is due to the mismatch between the quasi-optical mixer and the IF chain.

The mixer was also measured for RF frequencies between 35 GHz and 40 GHz. The experimental results for Liso at 100 MHz intermediate frequency value are shown in Fig. 6.

Figure 3. Photo of the 38 GHz receiver structure with Schottky diode mounted

Figure 4. The experimental setup used for the characterization of the micromachined quasi-optical mixer.

212

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6IF Frequency (GHz)

0

5

10

15

20

Con

vers

ion

Loss

es (

dB)

Figure 5. The isotropic conversion losses of upper sideband versus IF frequency.for 38 GHz RF frequency

35 36 37 38 39 40RF Frequency (GHz)

0

5

10

15

20

25

Con

vers

ion

Loss

es (

dB)

Figure 6. The isotropic conversion losses of upper sideband versus RF frequency (100 MHz IF frequency).

IV. CONCLUSIONS

This paper presented the design, fabrication and characterization of a micromachined quasi-optical mixer operating in the 38 GHz frequency band. The measured values for the isotropic conversion losses demonstrate state of art results.

This approach opens a new window of opportunity for the development of new architectures for microwave and millimeter wave microsystems.

ACKNOWLEDGMENT

The authors acknowledge the support of European Commission through the European Project 507352 AMICOM and the support of the Romanian Ministry of Education and Research through the projects ACOMEMS (CEEX) and MINASIST Plus.

REFERENCES

[1] H.J. De Los Santos, RF MEMS – Theory, Design and Technology, Artech House, Boston-London, 2002

[2] V.D.Hwang, T.Uwano and T.Itoh, “Quasi-optical integrated

antenna and receiver front end”, IEEE Trans. Microwave Theory and Tech, vol.36, no.1, pp.80-85, January 1988

[3] S.V.Robertson, L.P.B.Katehi and G.M.Rebeiz, “A planar

quasi-optical mixer using a folded-slot antenna”, IEEE Trans. Microwave Theory and Tech, vol.43, no.4, pp.896-898, April

1995

[4] G.P.Gauthier, W.Y.Ali-Ahmad, T.P.Budka, D.F.Filipovic and

G.M.Rebeiz, “A uniplanar 90 GHz Schottky diode millimeter-

wave receiver”, IEEE Trans. Microwave Theory and Tech,

vol.43, no.7, pp.1669-1672, July 1995

[5] G.P.Gauthier, S.Raman and G.M.Rebeiz, “A 90-100 GHz

double-folded slot antenna”, IEEE Trans. Ant. And Prop.,vol.47, no.6, pp.1120-1122, June 1999

[6] D.Neculoiu, A.Muller, P.Pons, L.Bary, M.Saadaoui, C.Buiculescu, I.Petrini, and R.Plana, “The design of membrane-supported millimeter-wave antennas”, 2003 Int. Semiconductor Conf. Proc., pp.65-68

[7] D. Neculoiu, P.Pons, D.Vasilache, L. Bary, A. Muller R. Plana, “Membrane-supported Yagi-Uda Antennas for Millimeter-Wave Applications”, Proc of the 3rd ESA Workshop on Millimeter Wave Technology and Applications, 21-23 May 2003, Millilab Espoo,Finland, p 603-609

[8] Zeland IE3D ver. 11, www.zeland.com

213