Microwave Applications Using Photo-imageable Technology

Microwave Applications Using Photo-imageable Technology

The need to reduce manufacturing costs requires technologies that allow the integration of a microwave circuit and its associated driver on the same substrate. FODEL® photo-imageable technology, an extension of a traditional thick-film process providing a higher line resolution, has been considered in the field of microwave applications requiring a resolution better than 50 micron for operations above C-band. The developed double-pass process has produced 40 micron-wide lines and 30 micron spaces. The compatibility of a FODEL gold conductor with different gold-wire bonding has been evaluated. The wire bond reliability has been demonstrated after 1000 hours of storage at 125°C. The electrical characterization has been performed from 4 to 18 GHz on test circuits, which include DC blocks, 50 ohm loaded lines, attenuators and coupled lines designed with the assistance of microwave circuit analysis software. All results comply with the specifications on S-parameters. The good performance of FODEL technology in microwave applications has enabled the production of circuits with a dielectric resonator oscillator (DRO) and its associated driver and power supplies on the same substrate.

B. Guillaume, J. Mabire, P. Maurin and D. Reffet
Thomson-CSF Microelectronique
Massy, France
F. Chirol and E.K. Polzer
Du Pont de Nemours France
Les Ulis, France

Thick-film hybrid circuits for microwave applications require a resolution better than 50 micron for operation above C-band. FODEL photo-imageable technology, an extension of traditional thick-film technology, can reach this level of resolution.¹ Furthermore, the technology's compatibility with standard thick-film resistive compositions and bonding techniques permits the integration of a microwave circuit and its associated driver on the same substrate, providing an efficient solution that combines high resolution and reduced cost. The evaluation of process performances and the characterization of test circuits in microwave fields from 4 to 18 GHz are presented.

Technology Description

FODEL photo-imageable technology is based upon a screen-printing process followed by ultraviolet (UV) exposure and development steps. The FODEL gold conductive material uses the inorganic constituents of a conventional thick-film composition -- metal powders, metal oxides and refractory powders -- and the organic materials of printed wiring board (PWB) photoresists -- polymers, monomers, photoinitiators and stabilizers. The similarities with standard thick-film technology concern process (screen-printing and firing) and the final fired material composition. The main differences deal with the ink's Newtonian rheology to improve leveling, the low temperature drying (80°C), and the pattern formation using UV exposure and development.² To be performed, the process requires standard thick-film equipment plus conventional PWB equipment for UV exposure and development. Since the process involves a photosensitive material, it must be performed under inactinic light.

Process Conditions

Specific conditions must be determined to process FODEL conductive material. Screen-printing conditions have to be optimized because of the ink's Newtonian rheology, and leveling is necessary to eliminate the screen mesh marks. The drying time at 80°C must be adjusted to allow proper solvent removal. However, polymerization of photoinitiators can occur when the drying time is too long. UV exposure associated with development determines the process resolution. As the material is formulated from a negative photoresist, it must be reticulated enough to become insoluble in the developer. The development step is critical. The results are impacted by the chosen process (dip or spray), the developer concentration (an Na₂ CO₃ aqueous solution) and its temperature, and the developing time. Development is followed by immediate water rinsing and drying. A lateral attack of the conductors during development is unavoidable due to the gold particles that stop UV light through the material, making photoresist polymerization possible only at the surface.

The process must be optimized to minimize the lateral attack and to make it reproducible. The firing condition is the same as for conventional thick-film materials: a 60-minute-long profile with a 10-minute peak firing temperature of 850°C on a belt furnace in an air atmosphere. As a result, the photosensitive organic part is burned out. The fired conductors show properties similar to those of conventional thick-film gold conductors. A shrinkage in width with edge curling appears after firing, revealing the lateral attack of development, as shown in Figure 1 .

Fig. 1: Lateral attack and width shrinkage of conductors; (a) after development and (b) after firing.

Process Evaluation

A single-pass process has been experimented with using FODEL composition 5956 in which the conductors are obtained after the process cycle described previously. The final thickness after firing must be at least 8 micron, thus a minimum of 15 micron dried thickness is necessary. The process window has been determined from the analysis of results obtained with a definition pattern. The real line and space sizes have been measured and correlated with the pattern dimensions. An average 20 micron line width shrinkage, due essentially to the firing step, has been observed between the pattern and the final line width on substrates. The process limits experienced are 40 micron-wide lines and 60 micron line spaces as final values on substrate.

Additional experiments have demonstrated that line quality varies substantially with this process. Material development is critical. Line width shrinkage is sensitive to the development parameters. When development is extended for only a few seconds to remove a material residue, final line width shrinkage increases and is affected by a large dispersion.

The conductors' definition and width reproducibility have been improved using FODEL composition 5956L and a double-pass process. The complete process (screen-printing, drying, UV exposure, development and firing) is performed twice, each time with a reduced thickness of dry material (11 micron instead of 15 micron in the single-pass process). This double-pass process allows a minimized lateral attack at development and better control of final width shrinkage: 15 ±5 micron for the two stacked layers. The final thickness (two layers) is between 9 and 10 micron, and the material density is improved by the double-pass process. The limits of this process are 40 ±5 micron-wide lines and 30 ±5 micron spaces obtained on separate designs, as shown in Figure 2 .

Fig. 2: Examples of (a) 40 micron lines, and (b) 60 micron lines and a 30 micron space.

Wire-bonding Evaluation

The compatibility of FODEL gold conductors with different gold-wire bonding techniques has been evaluated. The tests involved manual and automatic ultrasonic 25 micron Au wire wedge bonding (tests one and two), manual thermosonic 25 and 38 micron Au wire ball bonding (tests three and four), automatic thermosonic 25 micron Au wire ball bonding (test five) and semi-automatic thermocompression 25 micron Au wire wedge bonding (test six). The bonding parameters were optimized for each technique and wires were bonded on samples metallized with a 9 micron-thick gold layer. Pull tests were performed initially and after 500 and 1000 hours of storage at 125°C in a nitrogen atmosphere (50 wires/step for each bonding condition), as shown in Figure 3 .

Fig. 3: Average pull strength of gold wire bonds vs. storage time at 125° C.

All failures were heel breaks and no lift off was observed. The recorded pull strengths meet the requirements of MIL.STD.883-D -- Method 2011-7, even for the minimum values obtained: 6 gf with the 25 micron wire and 13 gf with the 38 micron wire. In all cases, the wire connection reliability improved slightly with 1000 hours of storage at 125°C due to gold diffusion.

The Test Circuits

The test circuits include elements found commonly in microwave applications: DC blocks, 50 ohm loaded lines, attenuators and coupled microstrip lines, as shown in Figure 4 . The test objectives for S-parameters are listed in Table 1 . The microstrip line size parameters (lengths, widths and spaces) have been optimized with the assistance of microwave circuit analysis software. The substrate thickness (microwave-grade alumina) is 0.635 micron for the 4 GHz frequency and 0.381 micron for the 12 and 18 GHz frequencies to minimize transmission losses. Consequently, the W value is 0.6 and 0.4 micron, respectively. The DC blocks require the optimization of four parameters (L1, L2, S1 and W1) and the coupled lines require the optimization of three parameters (Sc, Xc and X1). The optimized values are listed in Table 2 . For the 50 ohm loaded lines and the attenuators, the main parameters are the geometry of printed resistors and the ink resistivity. The selection of a trapezoidal resistor for the 50 ohm loaded lines relates from experience in thin-film technology. The test circuits have been characterized electrically on a test bench equipped with a vector network analyzer and a UTF 26-type test cell. All results comply with the required performance specifications and are listed in Table 3 .

Fig. 4: Test circuit layouts; (a) DC block, (b) 20 dB coupled lines, (c) 50 ohm-loaded lines and (d) a 6 dB T attenuator.

Industrial Applications

The industrial applications of FODEL technology concern an emitter/ transmitter for a missile. From the study of the basic elements described previously, a complete DRO function has been developed for this application. FODEL technology allows the integration of the DRO and its associated low frequency functions (power supplies and driver) on the same substrate. The advantage is both technical and economical as the DRO requires a resolution of 60 mm-wide lines and 30 mm spaces, and the driver includes resistors from 10 W to 100 kW not manufacturable with thin-film technology because of their integration area.

Conclusion

FODEL photo-imageable technology is an appropriate solution for microwave applications above C-band. A resolution of 40 ±5 micron line widths and 30 ±5 micron line spaces can be reached with a double-pass process. The conductors then are fully compatible with standard resistive compositions and bonding techniques. The good performance of FODEL technology in microwave applications has been demonstrated and has enabled the production of circuits with a DRO and its associated driver and power supplies on the same substrate for a military application. The technology can be used where it is necessary to combine low frequency and microwave patterns (up to 20 GHz) on the same substrate.

Acknowledgment

The authors gratefully acknowledge the DRET Authority (French Ministry of Defense) for its financial support to perform this work. The vector network analyzer used was from Anritsu Wiltron Co., Morgan Hill, CA. The UTF 26 test cell is a product of Cascade Microtech, Beaverton, OR. The microwave circuit analysis software is a product of HP EEsof, Westlake Village, CA. FODEL is a Du Pont product.

References

1. V. Manier and O. Sockeel, "FODEL Photoprintable Thick Film Technology in Microwave Applications," Microwave Conference, 1994, Rennes.
2. T.R. Suess and M.A. Skurski, "FODEL Photoprintable Thick Film: Materials and Processing," Local ISHM, 1994, US.