Dhananjay Bhusari

Fabrication of air-channel structures for microfluidic, microelectromechanical, and microelectronic applications

A method is presented for fabricating micro-air-channel structures encapsulated by a dielectric material using a sacrificial polymer based on polynorbornene (PNB) chemistry. A spin-coated film of PNB was patterned to define the exact geometry of the air-channels using conventional lithographic and etching techniques. The sacrificial polymer was encapsulated with a permanent dielectric material. The composite was then raised to elevated temperatures to produce gaseous products which permeate through the encapsulating material (SiO/sub 2/, SiN/sub x/ or other polymer) leaving behind minimal solid residue. Air-channels integrated with metal interconnections can be formed via a Damascene, or in-lay process. After patterning the sacrificial polymer, copper was electroplated, followed by encapsulation with the dielectric. Various issues pertaining to the processing steps have been investigated and are discussed, such as type of encapsulants, feasible air-channel sizes, and processing conditions. Such air-channel structures are believed to have potential applications in microelectronics, displays, printers, multilevel wiring boards, microscale chemical reactors on a chip, and microelectromechanical devices.

Air-gaps in 0.3 μm electrical interconnections

A copper/air-gap interconnection structure using a sacrificial polymer and SiO/sub 2/ in a damascene process has been demonstrated. The air-gap occupies the entire intralevel volume with fully densified SiO/sub 2/ as the planar interlevel dielectric. The copper was deposited by physical vapor deposition and planarized by chemical-mechanical planarization. The Ta/Cu barrier/seed layer was deposited by physical vapor deposition; the bulk copper was electrochemically deposited. The resulting structure has an effective intralevel dielectric constant of 2.19.

Plasma Treatment and Surface Analysis of Polyimide Films for Electroless Copper Buildup Process

We report here a detailed characterization of the surface chemical states and morphology of polyimide (PI) films following modifications by plasma treatment and electroless copper deposition. NH 3 and Ar plasma treatmentshave been successfully used to achieve morphological and chemical modification of the PI surface so that electroless copper plating can occur. The adhesion strength of the electroless copper to the PI surface was measured and correlated with the plasma-induced chemical and physical modifications of the PI surface. The NH 3 plasma causes primarily chemical changes to the PI surface through creation of nitrogen moieties (i.e., -N=C<) on the surface. The Ar plasma treatment brings about mainly physical changes to the surface (i.e., surface roughening). The combined-plasma treatment (Ar plasma followed by NH 3 plasma) combines the desirable chemical and physical effects of each treatment, yielding a PI surface with higher roughness for physical anchoring of the copper and surface bonding sites (nitrogen and oxygen sites). During the electroless copper surface activation step with tin chloride and palladium chloride, tin bonds mainly with the oxygen on the surface, whereas palladium reacts with tin chloride as well as with the surface nitrogen. A direct relationship has been observed between surface palladium concentration and the abundance of the -N=C< sites on the surface. This suggests that the nitrogen radicals created during the NH 3 plasma are incorporated into the surface and serve as bonding sites for the palladium. In the subsequent electroless Cu deposition, there was a direct correlation between the palladium surface concentration and Cu coverage. The adhesion strength of the electroless copper to the PI correlated well to the surface modifications and plasma treatment conditions. For the first time, a specific bonding configuration on the PI surface is shown to promote adsorption of palladium, which in turn promotes covalent bonding with Cu. The relative importance of surface roughness and chemical bonding on the adhesion strength is discussed.

Flexible pillars for displacement compensation in optical chip assembly

In chip-to-chip optical interconnect systems with surface mounted light-sources and detectors, thermal and mechanical effects can cause lateral displacements of the assembled devices. These displacements can result in optical signal losses that can critically deteriorate the bit-error-rate of the digital system. We demonstrate that, for a given loss budget of 1 dB, the use of flexible optical pillars with 150-/spl mu/m height and 50-/spl mu/m diameter can double the lateral displacement tolerance from about 15 to 30 /spl mu/m. The pillars fabricated from Avatrel polymer form an air-free path between the light source and the substrate and cause maximum optical power losses less than 0.2 dB.

Microfabricated Fuel Cell with Composite Glass∕Nafion Proton Exchange Membrane

The performance of microfabricated fuel cells with Nafion-coated glass composite membranes was investigated. The performance and durability of the fuel cells with silica glass membranes were improved by casting a thin film of Nafion on the anode side of the thin, phosphosilicate glass membrane. Nafion increased the strength of the glass membrane and provided a better chemical environment for the air-cathode. The use of a two-layer anode catalyst improved the anode performance by increasing the catalyst loading while maintaining catalyst porosity for proton transport. The performance, durability, and fuel utilization of the fuel cells using hydrogen and methanol as fuel were studied.

Flexible polymer pillars for optical chip assembly: materials, structures, and characterization

In an effort to address the need for robust optical chip I/O interconnects, we describe the fabrication and testing of microscopic polymer pillars for use as a flexible optical bridge between the chip and the substrate. The polymer pillars are photoimaged using the polymer Avatrel to a height of up to 350 &mgr;m. The photodefinable polymer Avatrel was used for the fabrication of the optical pillars due to its ease of processing and its unique material properties that include high Tg and low modulus. To evaluate the performance of the polymer pillars, the optical coupling efficiency from a light source to an optical aperture with and without an optical pillar is measured. For a light source with 12o beam divergence, a 30x150 &mgr;m polymer pillar improves the coupling efficiency by 3 to 4.5 dB compared to pillar-free (free-space) optical coupling. Due to the high mechanical compliance of the optical pillars, we also demonstrate that polymer pillars enhance the optical coupling efficiency between the chip and the substrate when they are misaligned in the lateral direction and that the displacement tolerance can be doubled from 15 to 30 &mgr;m for a 1dB power loss budget.

Development of P-Doped SiO 2 as Proton Exchange Membrane for Microfuel Cells

Phosphorus-doped silicon dioxide PSG thin films with improved ionic conductivity were deposited via plasma-enhanced chemical vapor deposition for application as a thin-film proton exchange membrane PEM in microfabricated fuel cells. More than three orders of magnitude improvement in the ionic conductivity is obtained by P doping of low-temperature deposited SiO2. The area resistance of 3 m thick film of PSG is comparable to a 200 m thick film of Nafion. Application of these PSG films as PEM in microfuel cells yielded more than one order of magnitude improvement in power density compared to low-temperature, undoped SiO2 membranes.

Growth of hydrogenated microcrystalline silicon (μc-Si:H) films by HWCVD using a Graphite catalyzer

A graphite catalyzer has been used to grow μc-Si:H films using the thermo-catalytic (HW) chemical vapor deposition (CVD) technique. The films grown in the amorphous-microcrystalline ‘transition regime’ have been found to exhibit a high photosensitivity of the order of 102-103 at a crystalline volume fraction of 0.2-0.4. The effects of deposition parameters such as silane concentration, pressure and substrate temperature on the microstructure and electrical properties of the films have been studied. It has been found that the graphite catalyzer offers a wider window of the deposition parameters for the growth of the ‘transition regime’ films as compared to the conventional W and Ta catalyzers. In addition, the graphite wires also exhibit significantly greater chemical as well as mechanical stability than the W and Ta wires, which results in improvement of the reproducibility of the technique. However, at the same filament temperature and other conditions, the deposition rates are about 10 times lower than for W or Ta filament. Increasing of the filament temperature, on the contrary, lead to radiative heating and carbon contamination of the growing film.

Fabrication of Air-Gaps Between Cu Interconnects for Low Intralevel k.

We present here a method for fabrication of air-gaps between Cu-interconnects to achieve low intralevel dielectric constant, using a sacrificial polymer as a ‘place holder’. IC compatible metallization and CMP processes were used in a single damascene process. The air-gap occupies the entire intralevel volume between the copper lines with fully densified SiO 2 as the planer interlevel dielectric. The width of the air-gaps was 286 nm and the width of the copper lines was 650 nm. The effective intralevel dielectric constant was calculated to be 2.19. The thickness of the interlevel SiO 2 and copper lines were 1100 nm and 700 nm, respectively. Further reduction in the value of intralevel dielectric constant is possible by optimization of the geometry of the metal/air-gap structure, and by use of a low k interlevel dielectric material. In this method of forming air-gaps, the layer of sacrificial polymer was spin-coated onto the substrate and formed into the desired pattern using an oxide or metal mask and reactive-ion-etching. The intralevel Cu trench is then inlaid using a damascene process. After the CMP of copper, interlevel SiO 2 is deposited by plasma-CVD. Finally, the polymer place-holder is thermally decomposed with the decomposition products permeating through the interlevel dielectric material. The major advantages of this method over other reported methods of formation of air-gaps are excellent control over the geometry of the air-gaps; no protrusion of air-gaps into the interlevel dielectric; no deposition of SiO 2 over the side-walls, and no degradation of the interlevel dielectric during the formation of air-gap.