Innovative Thin Film Techniques to Eliminate Faulty Wire Bonds and Improve High Frequency Circuit Densities
Thin film interconnects are a standard circuit technology used in high frequency design components and subsystems where transmission line widths of .005" and under are critical to performance as well as high power applications where thermal conductivity is of concern and materials such as beryllium oxide and aluminum nitride must be considered. This unique technology is employed in a variety of military and homeland security platforms from radar to EW and counter-measure applications, to missile and armament receivers and sensors.
Thin film is traditionally recognized as a single plane technology, where design engineers typically use wire bonds with eutectic solder or conductive epoxies to create interconnects from thin film circuits to various passive and active components and semiconductor devices in their systems. Engineers also use air bridges at some junctions. Although these methods are mature, and their performance over time predictable, industry sources suggest they are susceptible to failure, high assembly and tuning costs, and performance inconsistency due to the fragility of wire bonds, particularly as one considers that today’s circuits have tiny bonding pads and require very short bond loops, and that it usually takes an operator with special skills to make those wire bonds. This is especially true of the center wire bonds.
Another limitation of traditional thin film approaches is that until recently, the only additional passive element that could be cost effectively integrated on thin film was a resistor. However, a novel approach to designing and manufacturing with thin film has been developed to reshape the way microwave engineers employ it, and give them much needed improvements in reliability, repeatability and density.
Modifying techniques traditionally reserved for costly MMIC fabrication, UltraSource has perfected a unique thin film multi-layering approach that has proven to provide repeatable, wire bond-less, cross-under bridges, and has allowed for further integration of not only resistors, but also inductors, capacitors and other circuit functions onto a single thin film substrate.
The key innovation from UltraSource is its UltraBridge technology. By eliminating the need for a wire bond, or an air bridge, this consistent, reliable, cost-effective solution is destined to become an important tool for circuit designers at high frequencies.
In this process, the interconnect conductor layer is applied and patterned right on the substrate surface as an under bridge. This is where this technique varies from an air bridge process, which applies the interconnect layer after the thin film conductor layer is complete. Next, silicon nitride (Si3N4) is applied over the interconnect layer to encapsulate it and to insulate it from the next metal layer. The silicon nitride features are then patterned and etched. Contact windows to the interconnect conductor are also made during the pattern and etch of the silicon nitride. Finally, the conventional thin film conductor and resistor layers are applied using standard techniques and processes. The result is a simple, cost-effective method for fabricating high frequency interconnects for Lange couplers, integrated capacitors and integrated spiral inductors.
The UltraBridge technique also reduces the number of mask steps by 30 percent over air bridges, resulting in reduced design complexity and increased reliability. The hardness of the silicon nitride creates a robust interconnect design unaffected by accidental compression during inspection or assembly. Figure 1 shows a sample of UltraBridge layers.
Improving Density with Thin Film Integration
Until now, the only element available for integration into complex RF and microwave thin film interconnects has been the thin film resistor. This is facilitated by a layer of resistive metallization of either nichrome or tantalum nitride in a predetermined sheet resistivity and etched or laser trimmed to the value and tolerance required by the design.
As the next step to complete integration of all passive elements into a single thin film circuit, the UltraInductor™ and UltraCapacitor™ have been introduced. UltraInductor technology uses the silicon nitride in the same configuration used for the UltraBridge to facilitate interconnects with the center tap co-located on the circuit with other critical components, minimizing the parasitic components and variability. Figure 2 shows an example of an UltraInductor.
The UltraCapacitor, shown in Figure 3, uses the same process steps as the UltraBridge and UltraInductor and offers the designer the ability to integrate capacitors with values of 2 to 250 pF right onto the substrate. Silicon nitride is the ideal dielectric for this application with the thickness of the silicon nitride controlling the capacitance density of the dielectric, lending to flexibility in the capacitor layout.
A Lange Coupler Application Example
UltraSource’s approach has been successfully employed in the design and fabrication of Lange couplers, offering layout of line widths and spaces as precise as 0.0004" (10 mm) and controlled to tolerances of 1.5 mm.
Lange couplers are common circuits used in microwave applications. They provide equal power division and 90° of phase shift between the coupled ports. They are widely used as power combiners and splitters in microwave amplifiers and mixers/modulators. They are based upon interdigitated lines with narrow lines and tight spaces.
Figure 4 shows an HFSS model of a Lange coupler fabricated using the UltraBridge technology. Note how there are no wire bonds. The connections between fingers of the coupler are made by the metal traces, which are isolated from the fingers. This creates a highly reliable connection. Another benefit is that the connection is very repeatable. This is important for obtaining repeatable performance from circuits such as balanced amplifiers and modulators.
The circuit was modeled in HFSS and the predicted performance is shown in Figure 5. Note how insertion loss is nearly the ideal 3 dB value. Also, the return loss and isolation are very good. The performance goal is 7 to 11 GHz with an equal power split. This was easily obtained with good electrical match and isolation. Lange couplers provide a simple example of the varied potential for employing multilayer thin film techniques in next generation high frequency designs.
UltraTechnology should be looked at for advanced interconnect designs which would benefit from combining microstrip transmission lines, filled vias, resistors, capacitors, spiral inductors, under-bridges, and selectively deposited gold-tin all on a single substrate device.
To begin the design process using UltraTechnology, a designer should talk to the factory to decide on the level of integration desired and to procure design assistance on high frequency models. Additional information may be obtained by e-mailing Robert St. Pierre at firstname.lastname@example.org.