Flexible Hybrid Electronics

Figure 1. Integrated passives on flex fabricated at HiDEC include (clockwise from upper left) spiral inductors, multilayer capacitors, resistors, filter networks.The University of Arkansas High Density Electronics Center (HiDEC, http://hidec.uark.edu) has a longstanding reputation for expertise in electronic and system packaging since its inception in 1991. Started by an 11-year DARPA grant for superconducting multiple chip module packaging, the Center fabricated the world's first YBCO superconducting multi-chip module. HiDEC capabilities include a unique combination of high-density substrate fabrication, packaging, assembly, reliability, and test expertise/capabilities housed in one location.

HiDEC has been engaging in flexible hybrid electronic research since the early 1990s. Much of that work has centered on integrated passives, sensor/electronic integration and material synthesis.

Integrated Passives on Flex

Integrated passive devices are attracting an increasing interest due to the popularity of handheld electronic devices and the need to further decrease their size and cost while also adding functionality. Functional blocks, such as impedance matching circuits, couplers, baluns, harmonic filters, and power combiners/dividers can be implemented using integrated passive device technology. These integrated passive devices improve electronic system reliability by removing solder joints and the reliability concerns associated with them. HiDEC has a long history of integrating various passive devices on flexible substrates. Two HiDEC faculty members edited a book on "Integrated Passives Component Technology" published by IEEE Press in 2003. Figure 1 shows some of the integrated passive devices fabricated at HiDEC, such as a spiral inductor, multilayer capacitors, a filter network, and various resistors/capacitors on flexible substrates.

Figure 2. An example of Surface applied passives containing resistors and capacitors.HiDEC also developed and patented surface applied passives (SAPs), a technology used to seamlessly place passive components embedded in flex on circuit boards. An example of these devices in a flexible substrate is shown in Figure 2. HiDEC's patented SAPs are flexible patches of integrated passive devices such as resistors, capacitors, inductors, and other passive circuits with solder ball attachment on one side. SAPSs can be placed anywhere on an electronic circuit board, including underneath peripherally attached plastic integrated circuit packages.

HiDEC has worked with a number of capacitive dielectric materials such as Al2O3, Ta2O5, TiO2, and barium titanate. Ferroelectrics such as BaTiO3, and BaxSr1-X/TiO3 can exhibit dielectric constants up to three orders of magnitude higher than those of paraelectric materials like SiO2, Al2O3, Ta2O5, and BCB. However, the dielectric properties of ferroelectrics are typically a stronger function of temperature, frequency, film thickness, and bias, resulting in significant nonlinearities in their performance. Also, the dielectric constant of some ferroelectrics degrades with time. These factors must be considered when determining which dielectric material is the best choice for a specific application.

For integrated resistor materials, HiDEC has worked on NiCr, TaNx, and CrSi. TaNx may be the best of the group due to its ease of processing, low TCR, and stability, but all three have been demonstrated as integrated resistors for organic substrates. HiDEC's data shows that a TCR of 40 ppm/C has been measured for sputtered CrSi films on polyimide. Post-deposition stabilization for 250 hours at 125C resulted in the loss of only about 0.6% of its initial resistance value, after which it was stable at that temperature. Ni0.8Cr0.2 has a resistivity of around 100 ohm-cm, depending on Figure 3. Interconnected Mesh Power System (IMPS) on flex.its microstructure, and a TCR of -55 to +100. As such, for a 50 ohm/square sheet resistance, a thickness of 200 angstroms would be required. Various annealing treatments at hundreds of degrees are used to modify and stabilize NiCr resistor properties; 250C for 5 hrs is typical. NiCr is known to be sensitive to moisture conditions, and passivation layers may be used to stabilize its value against time and humidity. SiO2 and polymer coatings have been used for this purpose but, for integrated applications, burial beneath layers of substrate material may suffice. TaNx, formed by reactive sputtering of Ta in a nitrogen-containing atmosphere, is another practical integrated resistor material. Typical deposition conditions are 10-20 mTorr of 10-20% N2, resulting in 100-1000 ohm-cm, 2000 angstrom films, and < 200 ppm/C. However, TaNx is more immune to recrystallization with temperature and time than NiCr or CrSi. Also, it can be surface anodized in a controlled fashion for trimming and passivation.

The Interconnected Mesh Power System (IMPS), an offshoot the Center's many efforts related to optimization of multichip module interconnect, is a patented design topology that allows power planes to be absorbed into signal layers with minimal impact on routing density and power distribution system performance. This technology has also be implemented in flex substrates, as can be seen in the example shown in Figure 3. Reduction of metal layer count improves the flexibility of the substrate and cuts processing steps during manufacture by one-half.

Active Devices Integration on Flexible Substrates

Figure 4. Flip chip die attach on flex. Shown is a switching converter fabricated at HiDEC.HiDECFigure 5. Flexible ball-grid array multichip module. has also worked on thin silicon dies on flexible substrates. The degree of miniaturization and reliability can be enhanced by removing part of the assembled packages from the outer layers of the substrates and incorporating both passive components and active circuitry (integrated circuits) on and inside the flexible substrates. Possible hurdles for this technology include the testing of the dies before embedding (Known Good Die problem), accurate placement of bare die, and the need for a very fine pitch flexible substrate compatible with the pad pitch of the embedded IC chip. Silicon devices are usually thinned to a thickness of about 20-30 microns. Die thickness is one of many crucial aspects for developing thinner and smaller packages: thin chips allow more functionality per unit volume in a stacked die package, reduce the weight, and even become flexible. The most effective way for silicon back thinning is the commercial back grinding of the entire wafer, followed by polishing to the desired thickness. Figure 4 shows a switching converter fabricated on a flexible substrate with flip chip attachment.

Figure 6. A power converter on flex, wrapped around an electrolytic capacitor.HiDEC has also developed a ball-grid array multiple chip package on flexible substrates, as shown in Figure 5. In one version, the IC chips are wire bonded while in another version solder ball attach is used. Vias are created in the flexible substrates to provide interconnect between the top and bottom layers of the substrates. After assembly, the package is encapsulated using an insulating epoxy. Solder balls are then populated onto the bottom side of the package to create the BGA package on flexible substrate, an example of which is also shown in in Figure 5.

HiDEC successfully integrated an entire power converter using the flexible substrate as shown in Figure 6. As can be seen, the substrate and circuitry of the entire converter was wrapped around an electrolytic capacitor.

Integrated Sensors on Flexible Substrates

Figure 7. An example of flexible polymer organic electronics and sensors fabricated at HiDEC.Figure 8. Polymer-based glucose sensors with functionalized nanowires and nanotubes.

HiDEC researchers and staff work on many forms of flexible substrate sensors for biomedical, chemical, physical, and photonic sensing. Figure 7 and Figure 8 show some of the polymer-based nano and glucose sensors. Strain sensors are shown in Figure 9.Figure 9. Strain sensors on flex fabricated at HiDEC.

Figure 10. UV photodetectors using ZnO nanorods fabricated at HiDEC.HiDEC researchers have worked on a number of efforts that leverage the properties of zinc oxide (ZnO). Some examples of these include UV photo detectors and gas sensors using ZnO nanorods. In the ZnO photodetectors, oxygen is adsorbed on the ZnO surface and extracts free electrons from the surface to form oxygen ions. Upon exposure to UV, and electron-hole pair is created and holes combine with oxygen, releasing the oxygen from the surface. Remaining electrons contribute to the conduction band and increase the photoconductivity of the nanorods, enabling detection by the circuit. Examples of these sensors can be seen in Figure 10.

In the ZnO cancer detectors shown in Figure 11, the nanorods are functionalized to detect alpa-fetoprotein (AFP). Here, a Wheatstone bridge structure is used to enhance detection and allow real-time analysis, a desirable feature that offers diagnostic advantages. Figure 11. A ZnO-based cancer detector fabricated at HiDEC that uses a Wheatstone bridge structure to enhance detection.Figure 12. Various nano ZnO structures grown by HiDEC researchers.A real-time detection limit of 1 ng/mL of AFP antigen was demonstrated. HiDEC researchers have grown various nano ZnO structures (as shown in Figure 12) for various applications.

In other work, HiDEC researchers embedded sensors in flexible microprobes, as seen in Figure 13. Polymer-based and wearable multi-electrode array membranes allow precise placement of sensors electrodes to monitor patients as they can adhere to non-planar human body surfaces better than rigid multi-electrode arrays. Both stimulating and recording with redox capability multi-electrode arrays are fabricated. Furthermore, these flexible microprobes, with appropriate antibody attachment, can be used as bacteria detectors for food safety monitoring.Figure 13. Flexible microprobe sensors.

Organic TFT on Flexible Substrate

Figure 14. Organic TFTs on a flexible substrate.HiDEC researchers have been working on pentacene based thin film transistor and sensors as shown in Figure 14. The reported current-voltage characteristics are shown in Figure 15. By using a physical vapor deposition method, the deposited pantacene can avoid the degradation due to organic solvent during the patterning process. For typical gate width and length of 20 micron and 200 micron, respectively, a field effect mobility of 0.98 cm2/Vs can be obtained in the saturation region at a VDS of 30 V with an on/off current ratio of greater than 10^6.Figure 15. Typical current-voltage characteristics of the pantacene thin film transistors

Flexible Batteries

HiDEC researchers have also investigated the integration of thin-film flexible batteries as power sources for flexible electronics (Figure 16). While HiDEC houses all necessary equipment to achieve this goal, the following two issues need to be addressed. First, highly flexible thin-film batteries with excellent cycling performance need to be mass-produced at low cost. In order to do so, the core components, including electrodes, current collectors, separators, electrolytes, and packaging materials, need to be not only highly functional and mechanically flexible but also compatible with each other. As a result, good contacts and seals can be achieved while cracking problems can be minimized and avoided even under stretched or compressed conditions. Second, thin-film flexible batteries can be efficiently integrated into flexible electronics at low cost.Figure 16. Flexible batteries. (left) An electrode on stainless steel foil prepared through roll-to-roll printing; (right) Scheme of a flexible battery with the electrode encapsulated in polymer packaging.

Roll-to-roll (R2R) processing is a very promising process technology to address the above-mentioned issues. First, this process is fast and low-cost per unit area, and thus well suited for mass-production and wide commercialization of flexible batteries. Second, this process allows for the incorporation of multiple functions into one component (e.g., using nanocomposites) so that components in batteries can be both flexible, functional, and compatible with each other. Third, flexible batteries can be integrated into flexible electronics through printing. Figure 16 shows one example of a flexible battery: on the left is a flexible electrode in which a thin-layer of cathode electrode materials was printed onto a thin and flexible stainless steel (SS) foil by a R2R coating process. Mechanically strong but flexible current collectors such as conducting polymers can replace the SS foil; on the right side of the figure, a scheme for a flexible battery obtained through a R2R process is shown. All components can be assembled through multiple R2R coating processes and the polymer packaging can be realized through a R2R lamination process.