Glass is good for more than just windows: in recent years, the material has attracted attention for its dielectric properties and cost-effectiveness as a high-frequency-circuit substrate. It features low circuit losses over wide frequency and temperature ranges and is suitable for circuit fabrication using standard integrated-circuit (IC) planar manufacturing methods.

When evaluated for several high-frequency filter topologies, the performance of glass-based components has been shown on a par with other traditional microwave-integrated-circuit (MIC) substrate materials. Using commercial glass substrates, the authors designed and fabricated two filter topologies: a single parallel-coupled-line topology operating at C-band frequencies and a wide-bandwidth bandpass filter designed with anti-parallel coupled lines.

Although the concept of multichip module (MCM) technology makes sense for a great many systems and products because of its potential for high performance and high packaging density, the technology still has limitations. For example, single-chip solutions for RF and microwave front-ends still do not support complete system integration. External components are still required for impedance matching to the system characteristic impedance (usually 50 Ohms) as well as for power switching in phased-array antennas, and for forming resonant circuits for bandpass and other types of filters as well as for signal attenuation and processing at high signal frequencies.

Furthermore, the problems of substrate coupling (manifested as crosstalk or as a noise coupling especially in mixed-signal circuits) become more apparent and troublesome with increased integration. It is often preferable to integrate discrete components with high-quality-factor (Q) characteristics, such as waveguides and dielectric resonators (DRs), into a component package instead of on-chip passive components because of the improvements in performance possible with those discrete components. And in compact packaged multifunction components, glass and quartz substrates are viable materials for attaining high performance levels within a small package while also being extremely cost-effective.

Glass substrates have captured the imagination of high-frequency circuit designers for their low loss over wide operating frequency and temperature ranges. A number of different RF integrated passive devices (RFIPDs) have been fabricated on glass substrates for applications in wireless communications. 1 In addition, glass substrates have been used in the fabrication of multifunction integrated circuits (ICs) that combine both analog and digital functions, and for high-frequency sensors, and for microelectromechanical- systems (MEMS) designs. Glass substrates support the micromachining of structures with well-defined features for use in passive components such as filters at high frequencies. Due to its low dielectric constant, glass substrates offer ease of fabrication of high-frequency devices and circuits at RF and microwave frequencies. Glass features excellent dimensional stability with temperature compared to traditional dielectric materials along with tightly controlled sheet flatness for high circuit and device yields even at high frequencies. Signal losses associated with glass substrates are 12 dB/cm lower than those of CMOS-grade silicon substrate materials. As a result, it is possible to fabricate a 4-b phase shifter on glass substrate that exhibits average loss of only 2.7 dB at 78 GHz.2

To demonstrate the capabilities of commercially available glass substrates for high-frequency designs, two different C-band filter topologies were realized on glass substrates. The two different filters yielded better than 2.5-dB insertion loss and better than 15-dB return loss at C-band. Of the different bandpass filter topologies considered for fabrication on glass substrate, the parallel-coupled topology with half-wave- length line resonators is the simplest.3 For realizing a single-pole filter at X-band, key parameters were determined by means of the insertion-loss calculation method and standard equations. The line length for this filter was determined to be one- quarter wavelength (?g/4) at the frequency of interest. A line length optimization performed for open-ended discontinuities reduced the line length further by an amount equal to ?L as given in Eq. 1,4

where
L = the filter line length (in mm),
C0 = capacitance of the line section,
Zoo = the odd-mode impedance of the line section,
Zoe = the even-model impedance of the line section,
ee = the effective dielectric constant of the substrate material, and
e0 = the measured dielectric constant of the substrate material.

The coupled-line section shown in Fig. 1 can be represented by the impedance inverter circuit shown in Fig. 2, where
? = the phase,
J = the impedance inversion function, and
Z0 = the characteristic impedance.

The even- and odd-mode impedances can be found by solving first for Eqs. 2 and 3,

where fbw = the fractional bandwidth, Jn,n+1 = the characteristic admittance of the J-inverter, and Y0 = the characteristic admittance of the terminating lines, and then applying Eqs. 4 and 5.

The even and odd mode impedances work out to be 53 Ohms and 48 Ohms, respectively, for a single coupled section having 0.2 dB ripple. Using these values, the corresponding microstrip width (W) and gap (g) can be computed for the first bandpass filter topology on glass substrate.

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The second bandpass filter topology studied uses the concept of anti-parallel coupled lines to increase the filter bandwidth. Anti-coupled lines with short circuits at one end exhibit the characteristics of multiple transmission zeros and can be visualized as a frequency- selective coupling structure (FSCS). The short circuit can be achieved by a single low-impedance resonator, implemented using a quarter-wavelength open stub.5,6 Unlike traditional filter topologies, the input and output lines are in the same plane and perpendicular to the circuit, providing an additional degree of freedom in the implementation of this filter circuit. The even- and odd-mode impedances and microstrip line dimensions can be calculated as described above.

Armed with these two different filter topologies and the required microstrip line dimensions for C-band operation, the different bandpass filters were fabricated on Pyrex 7440 glass substrates having a thickness of 550 m. After being subjected to standard thin-film substrate cleaning cycles, the Pyrex 7440 glass substrates were sputtered with a thin layer (200 to 300 Angstroms) of chromium (Cr) followed by 7000 Angstroms of gold film on both sides of the substrates. The sputtered metallization is electroplated with gold to the required thickness of 4.5 m 3 percent and circuits were patterned using standard optical lithography and a subtractive etching process.

The patterned substrates were attached to gold-metallized Kovar carrier plates using silver-based conductive epoxy. The carrier plate was then mounted on a test jig and RF connectors were attached by means of gold ribbon with 20-mil width and 1-mil thickness using parallel gap welding. The two types of fabricated bandpass filters, the standard single-pole filter bandpass filter and the anti-parallel coupled wideband bandpass filter, are shown in Figs. 3 and 4, respectively.

The filters with optimized structure (L = 3.28 mm, w = 0.206 mm, and g = 0.08 mm) were fabricated on a commercial glass wafer (er = 4.82, tand= 0.0054) and tested using a PNA series model E8363B vector network analyzer from Agilent Technologies with frequency range of 10 MHz to 40 GHz. The parallel coupled topology (Fig. 3) exhibits a resonance at around 10.76 GHz as shown in Fig. 5. The deviation from the simulated value of 10.4 GHz is due to parasitics in the measurement setup (test fixture, connecting cables, etc.) and variations in the permittivity of the glass substrate.

Figure 6 shows the measured performance of the anti-parallel topology. As can be seen from the measurements of the two different filter topologies on glass, the insertion loss is better than 2.5 dB while the return loss was better than 10 dB for both filter topologies.

This experiment shows the potential for glass substrate materials as a cost-effective alternative to traditional dielectric materials for high-frequency circuit designs. Two different filter topologies were fabricated on low-cost Pyrex 7440 glass substrates, a standard single-pole bandpass filter and an anti-parallel coupled wideband bandpass filter. After determining the physical line dimensions of the filters for implemenation on glass substrates by means of straightforward calculations, the filters were fabricated and evaluated with a commercial VNA. The measured results validate the use of glass substrates for narrowband and wideband circuit applications. Furthermore, the slight deviations noticed between simulated/calculated performance and actual measurements can be attributed to measurement-induced parasitics and variations in the relative permittivity of the glass substrates. Given its low cost and tremendous potential for high-performance, compact RF/microwave circuits, glass substrates should find increasing interest as a building block for a wide range of high-frequency applications

References

1. Inho Jeong, T. Y. Kim, B. J. Lee, J. J. Choi, and Y. S. Kwon, "Comparison of RF integrated passive devices on smart silicon and glass substrates," Microwave & Optical Technology Letters, Vol. 45, No. 5, June 2005, pp. 441-444.

2. Juo-Jung Hung, L. Dussopt, and G. M. Rebeiz, "Distributed 2-and 3-bit W-band MEMS phase shifter on Glass substrates," IEEE Transactions on Microwave Theory & Techniques, Vol. MTT-52, No. 2, February 2004, pp. 600-606.

3. David M Pozar, Microwave Engineering, 3rd ed. Wiley, New York, 2005.

4. Jia-Shong Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, 2001.

5. Kamaljeet Singh and K. Nagachenchaiah, "Compact ultrawideband microstrip bandpass filter using step-impedance resonator (SIR) approach," IETE Mid-Term Symposium, April 2008, Jaipur, India.

6. Kamaljeet Singh and K. Nagachenchaiah, "Coupled Stubs Support Microstrip Bandpass Filter," Microwaves and RF, June 2008, pp. 76-83.