Nanomaterials provide the basis for flexible embedded components that meet the demands of new low-cost, large-area, flexible and lightweight devices.

There has been increasing interest in the development of electronic circuits on flexible substrates to meet the growing demand for low-cost, large-area, flexible and lightweight devices, such as roll-up displays, e-papers and keyboards, etc. Flexible circuits are often used as connectors in various applications where flexibility, space savings or production constraints limit the serviceability of rigid circuit boards. Organic materials have attracted much attention for building large-area, mechanically flexible electronic devices. These materials are widely pursued since they offer numerous advantages in terms of ease of processing, good compatibility with a variety of substrates and great opportunity for structural modifications. Recently, much attention has been paid to Q-switched Nd:YAG laser-micromachining for MEMS/microsystems applications due to a number of advantages.¹ It is a single-step process with high flexibility. It does not contaminate the material being processed, and it allows highly localized treatment of materials with a spatial resolution of tens of microns. The present study describes a novel process that uses a computer controlled Nd:YAG laser system to create complex 3D micromachined embedded capacitors.

Nanocomposite-based embedded capacitors deserve special attention as they provide the greatest potential benefit for high-density, high-speed and low-voltage IC chip packaging. Capacitors can be embedded into the interconnect substrates to provide decoupling, bypass, termination and frequency-determining functions.^2,3 For embedded capacitors to be useful, the capacitor must be flexible and show high capacitive densities to make layout areas reasonable. Available commercial polymer composite technology is not adequate for flexible, high capacitance density, thin-film embedded passives. Several polymer nanocomposite studies have focused on processing high capacitance density thin films within small substrates/wafers.^4-7 One of the important processing issues in thin-film polymer nanocomposite-based capacitors is to achieve flexibility and high-capacitance density with large-area coatings.

There are novel barium titanate (BaTiO₃)-epoxy-based polymer nanocomposites that have the potential to surpass conventional composites in producing thin-film capacitors over large surface areas with high capacitance density and low loss. New flexible composite dielectric materials are available that can be integrated into boards, laminate chip carriers (LCCs) and roll-to-roll manufacturing processes.

A variety of nanocrystalline BaTiO₃ powders were utilized with the objective of manufacturing low-cost, high-performance, flexible nanocomposites with subsequent laser micromachining of the nanocomposites to produce novel micromachined 3D capacitors. Micromachining technologies can produce variable thickness and discrete capacitors from a single sheet (layer) of capacitors and could be integrated in the same layer (Figure 1). Moreover, an improved micromachining technique was developed to control the surface morphology of sol-gel thin films. Experimental data shows micromachining is highly efficient for controlling film thickness. With this method, new structures can be designed without losing dielectric properties. It can generate multifunctional capacitors.

Experimental Procedure

A variety of BaTiO₃ nanoparticles and their dispersion into epoxy resin were investigated to achieve a thin uniform film. In a typical procedure, BaTiO₃ epoxy nanocomposites were prepared by mixing appropriate amounts of the BaTiO₃ nanopowders and epoxy resin in organic solvents. A thin film of this nanocomposite was then deposited onto a copper substrate and cured. BaTiO₃-epoxy polymer nanocomposites modified with nanomaterials were also prepared. Various nanomaterials, including PZT (lead zirconate titanate), PLZT (lead lanthanum zirconate titanate), ZnO (zinc oxide), PMN (lead magnesium niobate), PMN-PT (lead magnesium niobate-lead titanate) and several other single- and multicomponent systems with an oxidation state from zero to five were used. BaTiO₃ fluoropolymer nanocomposites were also prepared by mixing appropriate amounts of the BaTiO₃ nanopowders and fluoropolymer in organic solvents.

In the case of laminates, two thin films were prepared, dried, and then laminated together. BaTiO₃-sol-gel thin films were prepared from a 0.5 molar aqueous acetate solution of Ba (CH₃COO)₂ and Ti (OC₂H₅)₄. The film was deposited on Si or glass substrates by spin coating, and dried successively at 150? and 450?C to remove all the organics. The film was then annealed at ~600?C in air to generate a crystalline phase.

Laser Parameters and Micromachining Results

Laser drilling was performed on an ESI 5210 Laser Microvia Drill System. A frequency-tripled Nd:YAG laser operating at a wavelength of 355 nm was used. The pulse width of the laser was on the order of 50 ns. The beam was positioned relative to the surface of the work piece by coordinated motion of the stage on which the sample is mounted (Y axis), the optics (X axis), and galvo mirrors (X and Y axes), as shown in Figure 1. The position of the sample with respect to the focal plane of the laser beam (along the Z axis) can also be adjusted. The spatial distribution of energy in the circular laser spot is homogenized by use of ESI-supplied optics. In this instance, beam diameter at the surface of the work piece was varied by adjusting the relative position of the imaged beam with respect to the surface of the film. Other salient parameters are listed in Table 1 [PDF format]. Parameters are defined as follows.

• Rep Rate is the number of laser pulses delivered to the work piece per unit time.
• Power is the average power of the laser.
• Z-Offset is the relative position of the imaged beam with respect to surface of the substrate; a negative number indicates that the beam is imaged below the surface.
• Velocity is the speed at which the beam is traced along the programmed beam path.
• Repetition is the number of times the system traces the programmed beam path.
• Bite Size is the distance the system moves the beam relative to the work piece, along the programmed beam path.
  

Direct laser ablation of nanocrystalline BaTiO₃-filled epoxy was performed using the parameters given in Table 1. The BaTiO₃ filled epoxy nanocomposite and sol-gel thin film was micromachined to form various surface morphology and capacitors arrays.

Electrical properties (capacitance, Dk, loss) of the nanocomposite thin films were measured at room temperature using an impedance/gain-phase analyzer. The dielectric constant/capacitance as a function of temperature was determined using a precision LCZ meter at 1 MHz. Surface morphology and particle distributions of nanocomposite films were characterized by an LEO 1550 scanning electron microscope. The thickness of the film was determined by optical microscope and SEM.

Results and Discussion

A real challenge in the development of large-area thin-film nanocomposites is the incompatibility that exists between the typically hydrophilic nanoparticles and hydrophobic polymer matrix, which leads to nanoparticle agglomeration. As a result, inferior coatings with poor performance are obtained. Surface treatments that result in excellent dispersability of the nanoparticles, and good quality monolithic coatings have been identified. Surface treatment of a ceramic depends on its processing routes. Ramesh et al⁷ reported silane treatment of hydrothermally prepared BaTiO₃ nanoparticles.

The finer details of the particles and their surface morphologies were investigated using SEM. Figure 2 shows SEM micrographs of nanocomposite thin films. Nanoparticles formed a uniform dispersion in the epoxy matrix (Figures 2a and 2b). The particles in the epoxy matrix are so intimately compacted that analysis of individual particles is difficult. However, closer observation of the micrographs clearly reveals a uniform distribution of closely packed, well-connected particles. The uniform dispersion of nanoparticles into the epoxy matrix enables the production of a very thin film.

Figures 2c and 2d represent SEM microstructure BaTiO₃ nanoparticles dispersed into a flexible fluoropolymer. Figure 3 shows large-area, thin flexible capacitors. Capacitors can be bent or rolled without damaging the structure. For conventional composites, rolling or bending will cause structural damage such as cracking. The remarkably increased flexibility of the nanocomposite is due to uniform mixing of nanoparticles in the polymer matrix and the resulting improved polymer-ceramic interface. Integration of this flexible capacitance layer into a flexible structure could be useful for developing high-performance multilayer flexible electronics, such as roll-up displays, e-papers and keyboards, etc.

Measurement of electrical properties of ~2 to 100 mm² capacitors fabricated from nanocomposite thin films showed high capacitance density ranging from 10 nF/in.² to 100 nF/in.², depending on composition, particle size and thickness of the coatings. Thin-film capacitors fabricated from 40% to 60% v/v BaTiO₃ epoxy nanocomposites showed a stable capacitance density in the range of 40 to 80 nF/in.². Measurement of electrical properties of capacitors fabricated from ~70% v/v nanocomposite showed capacitance density of about 100 nF/in.². Capacitance density of BaTiO₃-epoxy polymer nanocomposites modified with nanomaterial was also investigated. The final film was assumed to have nanomaterials incorporated homogeneously into a BaTiO₃-epoxy matrix. Capacitance density of nanomaterial modified films was increased up to 500 nF/in.², about five to 10 times higher than obtained with BaTiO₃-epoxy nanocomposites.

Figure 4a shows a top view of laser micromachined capacitor arrays. The basic structure consists of two layers: the micromachined capacitor and the glass substrate. Using this method, fully micromachined capacitors can be consistently generated. To understand the capacitance density of the various laser micromachined spots at different laser repetitions, a series of samples were tested, and a representative SEM micrograph is shown in Figures 4b, 4c and 4d. The depth of micro-fabricated capacitors varies from four microns to 70 microns depending on the number of laser pulses. The surface smoothness of the ablated areas that define the laser micromachined capacitors will depend upon the uniformity of the spatial energy distribution of the laser beam. Although the smoothness of the ablated regions shown in Figure 4 is not optimum, adjustment of the optics used in the present study could produce a more homogeneous energy profile for the beam. Alternatively, Excimer laser micromachining systems have been shown to produce very uniform ablation over reasonably large areas. Yet another potential alternative is to micromachine a B-staged dielectric material, and smooth the micromachined surface during a final composite lamination.

Figure 5 shows variation of the depth of the micromachined areas with the number of laser pulses. Figure 5 also shows possible capacitance density (nF/in.²) for a 15-micron capacitor film assuming a dielectric constant of 30. The thickness of the capacitors decreases with an increasing number of laser pulses. It is clear from Figure 5 that as the depth of the micromachined area is increased (thickness of the remaining dielectric reduced), the corresponding capacitance increases. In other words, micromachining can generate capacitor libraries with variable capacitance density. Increasing the number of pulses eventually results in complete removal of the dielectric. This would be useful for making discrete capacitors from a capacitance layer. Figure 6 shows a representative example of discrete capacitors formed from a larger capacitance layer. Figure 6b represents a 3D optical micrograph where the capacitor dielectric was to the point where the bottom electrode was exposed.

The concept of laser processing is based on the interaction between laser radiation and materials. Different materials have a different response at a given laser wave length. In the present case, BaTiO₃/PZT based sol-gel thin film was used, and the response of the material to laser irradiation was observed to change with the number of laser pulses. For example, when films are exposed to the third harmonic of a Nd:YAG laser (355 nm) with high energy density, materials absorption favors ablation. On the other hand, when sol-gel thin film is exposed to the same laser operating at 355 nm with low energy density, materials absorption favors annealing for producing a new channel-like structure that might reflect/interact with light to produce an optically active surface.

The surface topography of the BaTiO₃ thin-film surfaces was examined by performing AFM, SEM and optical imaging. Figure 7 shows different surface morphology generated by variation of laser fluence. Sufficiently strong laser pulses induce local melting and rapid cooling, leading to the formation of an amorphous particle. One can use laser energy to raise the temperature of the film above crystallization but below melting temperature, thus converting an amorphous region to crystalline phase. Figures 7a and 7b represent typical examples of melting and solidification of amorphous particles. Ablation was observed at a very high energy density. With a gradual decrease in energy, but still at high enough fluence values, the central part of the area traced by the beam path completely melts, whereas the adjacent surrounding area is partially melted.

Laser processing of the as-deposited BaTiO₃ film leads to a noticeable migration from a smooth surface to wavy cavity structure as can be seen in the SEM and optical height image (Figures 7c, 7d and 7e). This is apparently due to localized melting of the top surface induced by the transient laser heating. The annealing substantially reduces the surface grain structure and generates a new porous channel-like network reminiscent of a labyrinth structure. Each channel has an average width of about two to five microns and a depth of about 0.1 to 0.5 microns. It is interesting to note that all the laser-annealed/micromachining spots, irrespective of their location on the film (center or periphery), maintain almost the same kind of channel structure. Moreover, it can be created selectively wherever it is needed. This kind of structure was shown to be critical for achieving random laser action as it favors efficient coupling of the pump light into the film material⁸. Excess laser energy favors ablation. It can be seen that at high laser energy, the barium titanate film is ablated and the underlying silicon surface exposed (Figure 7f).

Laser micromachining is important for generating high-resolution patterns. In contrast to many patterning techniques, this approach does not necessarily require special laboratory conditions and toxic chemicals. Figure 8 shows a simple micromachined pattern composed of letters, lines and a square grid. The vertical lines were positioned on a 200 µm pitch (left side) and a 100 µm pitch (right side). The length and width of these sides of the squares in the grid is 200 µm and 50 µm, respectively. The laser can be used to machine a small gap for micro-devices. For instance, one can machine small planar energy storage devices for remote sensors in security or defense applications. The AFM image shown in Figure 9 represents a sharp micromachined BaTiO₃ edge at the glass surface. This kind of sharp edge will be useful for microfabricating membranes for MEMS applications.

Conclusions

A thin-film technology based on BaTiO₃-epoxy polymer nanocomposites was developed to manufacture high-performance, large-area, flexible/rollable, thin-film capacitors. It is possible to produce very thin films in the range of two microns using the nanocomposites. The remarkable flexibility and thickness of the nanocomposite is due to uniform mixing of nanoparticles in the polymer matrix that results in improved polymer-ceramic interfaces. A frequency-tripled Nd:YAG laser operating at a wavelength of 355 nm was used for the micromachining study. We have shown a variety of micromachined surfaces suitable for multifunctional capacitor applications. Lasers can provide various complex micromachined patterns. This technique can be used to prepare capacitors of various thicknesses from the same capacitance layer, and ultimately, can produce variable capacitance density, or a library of capacitors. The process is also capable of making discrete capacitors from a single sheet of capacitance layer. As the demand grows for complex multifunctional embedded components for advanced organic packaging, laser micromachining will continue to provide unique opportunities. PCD&M

Ed. note: This paper was previously presented at the 57th Annual ECTC Conference and Exhibit in Reno, Nevada, May 2007.

Rabindra Das, Frank Egitto and John Lauffer are all senior advisory technologist at Endicott Interconnect. Voya Markovich is the CTO and Senior VP of R&D at Endicott Interconnect. Rabindra Das can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

REFERENCES

1. J. M. Bustillo, R. T. Howe, and R. S. Muller, “Surface Micromachining for Microelectromechanical Systems,” Proceedings IEEE, vol. 86, pp. 1552–1574, 1998.

2. “Passive Integration: Easier Said Than Done,” Prismark Partners LLC, August 1997

3. Post J.E., “Microwave performance of MCM-D embedded capacitors with interconnects,” Microwave and Optical Technology Letters Vol. 46, No5, (2005), pp. 487-492.

4. Rao Y., Ogitani S., Kohl P., Wong C. P., “Novel polymer-ceramic nanocomposite based on high dielectric constant epoxy formula for embedded capacitor application,” Journal of Applied Polymer Science Vol. 83, No. 5, (2002), pp.1084-1090.

5. Rao Y., Wong C. P., “ Material characterization of a high-dielectric-constant polymer-ceramic composite for embedded capacitor for RF applications,” Journal of Applied Polymer Science Vol. 92, No. 4, (2004), pp. 2228-2231.

6. Windlass H., Raj P. M., Balaraman D., Bhattacharya S. K., and Tummala R. R., “ Colloidal processing of polymer-ceramic nanocomposites for integral capacitors,” IMAPS International Symposium on Advanced Packaging Materials, Braselton, 2001, pp. 393-398

7. Ramesh S., Shutzberg B. A., Haung C., Gao J., Giannelis E. P., “Dielectric nanocomposites for integral thin film capacitors: Materials design, fabrication, and integration issues,” IEEE Transactions on Advanced Packaging Vol. 26, No. 1, (2003), pp. 17-24.

8. R.N. Das, A. Pathak and P. Pramanik, “Low-Temperature Preparation of nanocrystalline Lead Zirconate Titanate and Lead Lanthanum Zirconate Titanate Powders Using Triethanolamine,” Journal of American Ceramic Society, 81[12] 3357-60(1998).

9. Hai Ni, Hoo-Jeong Lee, Ainissa G. Ramirez, “A robust two-step etching process for large-scale microfabricated SiO2 and Si3N4 MEMS membranes,” Sensors and Actuators A 119 (2005) 553–558.

• Prev
• Next