Photodefinable Polymer Research Articles

Chemical and Morphological Modifications Induced by Argon Plasma Treatments on Fluorinated Polybenzoxazole Films.

Fluorinated photodefinable polymers are widely employed as re-distribution layers in wafer-level packaging to produce microelectronic devices because of their suitable low dielectric constant and moisture absorption, high mechanical toughness, thermal conductivity and stability, and chemical inertness. Typically, fluorinated photodefinable polybenzoxazoles (F-PBOs) are the most used in this field. In the present work, we investigated by atomic force microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy the morphological and chemical modifications induced by Ar plasma treatments on F-PBO films. This process, used to remove surface contaminant species, as well as increase the polymeric surface roughness, to improve the adhesion to the other components during electronic packaging, is a crucial step during the manufacturing of some microelectronic devices. We found that argon plasma treatments determine the wanted drastic increase of the polymer surface roughness but, in the presence of a patterned silver layer on F-PBO, needed for the fabrication of electric contacts in microelectronic devices, also induce some unwanted formation of silver fluoride species.

Scalable and selective N-type conversion for carbon nanotube transistors via patternable polyvinyl alcohol stacked with hydrophobic layers and their application to complementary logic circuits

Herein, this study reports scalable and selective n-type conversion (N/C) approach for single walled carbon nanotube (SWNT) transistors with high reproducibility by using novel control of hydroxyl groups through condensation on the surface of SWNTs, via the patternable cross-linked polyvinyl alcohol (C-PVA), followed by encapsulation of photo-definable hydrophobic polymer (~SU8). Moreover, N/C process capability is statistically evaluated in terms of selective doping, process yield, and statistical variation in electrical parameters, and as practical validation, complementary inverters, NOR and NAND logic gates are fully demonstrated. As one of key findings, it is elucidated that N/C uniformity and its underlying physics, supported by Fourier-transform infrared spectroscopy (FTIR) and Raman analysis, are highly correlated with ambient condition, C-PVA thickness, and encapsulation. More practically, reproducible field effect mobility for n-type (or p-type) SWNT TFTs after (or before) N/C are achieved at ~ 3.65 ± 1.30 (or 8.76 ± 2.16) cm2 V−1 s−1, with magnificent process yield (~100%) and reasonable mobility reduction, which is on par with the previous report. Hence, all demonstration and their analyses suggest that this scalable N/C scheme for SWNT TFTs can be one of core technologies for the next generation semiconductor-based devices and their envisioned application.

Polymer elastic bump formation through photodefinable thermal reflow process for system in foil package

In this study, hemispherical polymer elastic bumps with 200 μm diameter were formed using a highly heat-resistant photodefinable polymer to realize a system in foil package. Because polymers have a more temperature-dependent modulus than a metal, a dynamic mechanical analysis (DMA) test was performed to analyze the modulus of the polymer. The results showed that the storage modulus of the cured photodefinable polymer used for producing the polymer elastic bumps was ~3.5 GPa at room temperature, and the glass transition temperature (Tg) was 210 °C–220 °C. A single polymer elastic bump package was modeled by using the modulus values obtained from the DMA test, and the deformation under loads was analyzed using the finite element method and compared with the results for conventional copper bumps. The results showed that the bump coplanarity can be corrected easily in hemispherical polymer elastic bumps because they show large z-axis deformation under loads. The test also showed that the stress on an ultrathin chip (thickness: 20 μm) is less than half that on copper bumps. In addition, the deformation behavior depending on the metal wiring configuration on the polymer elastic bumps was analyzed under different temperature and load conditions by using a microtribometer. The analysis of the contact resistance on a single polymer elastic bump verified that the polymer can be used as a material for bumps.

Dielectric properties of ligand-modified gold nanoparticle/SU-8 photopolymer based nanocomposites

This article reports the enhanced dielectric properties of a photodefinable polymer nanocomposite material containing sub–10nm coated metal nanoparticles (NPs). The surface morphology of the synthesized dodecanethiol-functionalized gold NPs was characterized using the transmission electron microscopy (TEM). We investigated the particle agglomeration and dispersion during the various stages of the nanocomposite synthesis using TEM. Physical properties such as dielectric permittivity and dielectric loss were measured experimentally. The dependence of the dielectric permittivity and loss tangent on the particle concentration, and frequency was studied. Nanocomposite films showed an approximately three times enhancement in average dielectric constant over the polymer base value and an average dielectric loss of 0.09 at 1kHz, at a filler loading of 10% w/w.

A facile approach toward protein-resistant biointerfaces based on photodefinable poly-p-xylylene coating

A facile and versatile tool is reported that uses a photodefinable polymer, poly(4-benzoyl-p-xylylene-co-p-xylylene) to immobilize antifouling materials, such as poly(ethylene glycol), poly(ethylene glycol) methyl ether methacrylate, dextran, and ethanolamine. This immobilization process requires the polymer's photoactivated carbonyl groups, which can facilitate light-induced molecular crosslinking and can rapidly react via insertion into CH or NH bonds upon photo-illumination at 365nm. Importantly, the process does not require additional functional groups on the antifouling materials. The immobilized fouling materials were characterized using X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS), and the resulting antifouling properties were examined through protein adsorption studies on fibrinogen and bovine serum albumin at surfaces that were spatially modified using a photomask during the photochemical process. In addition, the adsorbed fibrinogen was quantitatively analyzed using a quartz crystal microbalance (QCM), and the adsorption values were reduced to 32.8±4.9ngcm−2, 5.5±3.9ngcm−2, 21.4±4.5ngcm−2, and 16.9±3.4ngcm−2 for poly(ethylene glycol) (PEG), poly(ethylene glycol) methyl ether methacrylate (PEGMA), dextran, and ethanolamine, respectively. Finally, this antifouling modification technology was demonstrated on an unconventional substrate for a stent that was modified by PEGMA at selected areas using a microscopic patterning technique during photoimmobilization. Low levels of fibrinogen and BSA adsorption were also observed at the areas where PEGMA was attached.

Vapor-Deposited Parylene Photoresist: A Multipotent Approach toward Chemically and Topographically Defined Biointerfaces

Poly(4-benzoyl-p-xylylene-co-p-xylylene), a biologically compatible photoreactive polymer belonging to the parylene family, can be deposited using a chemical vapor deposition (CVD) polymerization process on a wide range of substrates. This study discovered that the solvent stability of poly(4-benzoyl-p-xylylene-co-p-xylylene) in acetone is significantly increased when exposed to approximately 365 nm of UV irradiation, because of the cross-linking of benzophenone side chains with adjacent molecules. This discovery makes the photodefinable polymer a powerful tool for use as a negative photoresist for surface microstructuring and biointerface engineering purposes. The polymer is extensively characterized using infrared reflection adsorption spectroscopy (IRRAS), scanning electron microscopy (SEM), and imaging ellipsometry. Furthermore, the vapor-based polymer coating process provides access to substrates with unconventional and complex three-dimensional (3D) geometries, as compared to conventional spin-coated resists that are limited to flat 2D assemblies. Moreover, this photoresist technology is seamlessly integrated with other functionalized parylenes including aldehyde-, acetylene-, and amine-functionalized parylenes to create unique surface microstructures that are chemically and topographically defined. The photopatterning and immobilization protocols described in this paper represent an approach that avoids contact between harmful substances (such as solvents and irradiations) and sensitive biomolecules. Finally, multiple biomolecules on planar substrates, as well as on unconventional 3D substrates (e.g., stents), are presented.

Facet preparation of polymer based optical wavelength filter using mechanical/chemical treatment technique

AbstractA photodefinable polymer of benzocyclobutene (BCB 4024‐40) from Dow Chemical™ has been demonstrated as a waveguiding material in a new structure of optical wavelength filter. The filter structure is based on the combination of restricted interference and general interference schemes of multimode interference technique. The filter is capable in optical wavelength filtering particularly for 1550/1310 nm. Because of the photosensitive nature of the polymer, the chemical wet‐etching fabrication technique needs to be adopted. For the purpose of laser coupling, a new facet polishing technique has been proposed and it is based on the combination of mechanical and chemical polishing. It is shown that the fabricated device works well with improved crosstalk of −15.7 dB (1310 nm) and −21.8 dB (1550 nm). © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1301–1313, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26781

Multistep liquid-phase lithography for fast prototyping of microfluidic free-flow-electrophoresis chips

We present a fast and versatile method to produce functional micro free-flow electrophoresis chips. Microfluidic structures were generated between two glass slides applying multistep liquid-phase lithography, omitting troublesome bonding steps or cost-intensive master structures. Utilizing a novel spacer-less approach with the photodefinable polymer polyethyleneglycol dimethacrylate (PEG-DA), microfluidic devices with hydrophilic channels of only 25 μm in height were generated. The microfluidic chips feature ion-permeable segregation walls between the electrode channels and the separation bed and hydrophilic surfaces. The performance of the chip is demonstrated by free-flow electrophoretic separation of fluorescent xanthene dyes and fluorescently labeled amino acids.

Electrochemical Micro Technologies for Polymeric MEMS and Biochip Applications

Flexible electrode chips were fabricated on polymer substrates, either SU-8 (epoxy-based photo-definable polymer) or PDMS (Polydimethylsiloxane, elastomer-base polymer), using a photolithography process. In one case, Ppy (Polypyrrole, a conductive polymer) was deposited on the electrodes' gold seed layer. In another case, Ppy was deposited on the micro actuator's copper seed layer, both cases by electropolymerization using cyclic voltammetry. Thus, we developed a process for a flexible electrode that integrates a polymer substrate and a conductive polymer. The polypyrrole is reinforcing the Gold and Copper wires and electrically shunting the interconnects, increasing thereby the device durability and reliability.

Dynamic thermal mechanical characterization of Epoclad negative photoresist for micro mechanical structures

Dynamic thermal mechanical properties of Epoclad negative photoresist have been studied. These experiments are part of the mechanical characterization of several photo definable polymers which can be used as a permanent, active and, or structural layer in a MEMS. This paper reports on the visco-elastic properties such as the loss modulus, the storage modulus and the glass transition temperature.

Neodymium-complex-doped photodefined polymer channel waveguide amplifiers

Channel waveguides based on a polymer, 6-fluorinated-dianhydride/epoxy, which is actively doped with a Nd complex, Nd(thenoyltrifluoroacetone)(3) 1,10-phenanthroline, are fabricated by a simple and reproducible procedure, spin coating a photodefinable cladding polymer onto a thermally oxidized silicon wafer, photopatterning, backfilling with the active core polymer, and spin coating with an upper cladding layer. Photoluminescence at 1060 nm from the Nd(3+) ions with a lifetime of 130 mus is observed. Optical gain at 1060 nm is demonstrated in channel waveguides with different Nd(3+) concentrations. By accounting for the waveguide loss of 0.1 dB/cm, an internal net gain of 8 dB is demonstrated for a 5.6-cm-long channel waveguide amplifier. Owing to the nature of the Nd(3+) complex, energy-transfer upconversion affects the gain only at Nd(3+) concentrations above 1 x 10(20) cm(-3).

Photo-definable microchannels made with spin-on polymers and short sacrificial etch times

Hollow tubular microfluidic channels were fabricated from two spin-on, photodefinable polymers.

Metal-organic thin-film encapsulation for MEMS

This paper demonstrates a metal-organic thin-film encapsulation technique based on the photolithography of multiple layers of positive-tone photo-definable polymer. This encapsulation process, performed at low temperatures (<110 °C) in a standard basic developer, leaves no residue. Piezoelectrically-actuated length-extensional bulk acoustic wave resonators were successfully encapsulated for a proof-of-concept demonstration. Blanket metallization can lead to complete sealing, which helps exclude moisture and particles from sensitive MEMS devices such as high frequency resonators, RF filters and inertial transducers.

An ultrathin (∼100μm thick) flexible light plate fabricated using self-alignment and lift-off techniques

An ultrathin (∼100μm thick) flexible light plate was designed and fabricated on a parylene template using a combination of self-alignment and lift-off techniques. The solid-state InGaN light-emitting diodes (LEDs) (λp=465nm) was used as the light source to overcome the problem of conventional organic light-emitting devices which require perfect encapsulation against the permeation of water and oxygen. After the sidewalls of LEDs were passivated by the photodefinable polymer, the LED chip array can be further sandwiched by the indium-tin oxide (ITO) and Al electrodes to form a thin-film package with all the processing temperatures below 250°C. The ITO-coated transparent parylene template can be peeled off from the glass carrier after forming the ultrathin LED light plate. The flexible light plates present no damage even after they were flexed 1000 times around a 3-cm-diameter cylinder. The present self-alignment or mask-less process is a very promising approach to flexible backlight applications.

Colloids with high-definition surface structures.

Compared with the well equipped arsenal of surface modification methods for flat surfaces, techniques that are applicable to curved, colloidal surfaces are still in their infancy. This technological gap exists because spin-coating techniques used in traditional photolithographic processes are not applicable to the curved surfaces of spherical objects. By replacing spin-coated photoresist with a vapor-deposited, photodefinable polymer coating, we have now fabricated microstructured colloids with a wide range of surface patterns, including asymmetric and chiral surface structures, that so far were typically reserved for flat substrates. This high-throughput method can yield surface-structured colloidal particles at a rate of approximately 10(7) to 10(8) particles per operator per day. Equipped with spatially defined binding pockets, microstructured colloids can engage in programmable interactions, which can lead to directed self-assembly. The ability to create a wide range of colloids with both simple and complex surface patterns may contribute to the genesis of previously unknown colloidal structures and may have important technological implications in a range of different applications, including photonic and phononic materials or chemical sensors.

Advances in Multifunctional I/O

A copper-to-copper bonding process was developed for an all- copper, chip-to-substrate interconnect technology. High aspect ratio, polymer molds for electroplating were formed using a photodefinable polymer on both the chip and the substrate surfaces. Copper pillars were fabricated by electroplating metal in the polymer molds. The chip-to-substrate all-copper connections were formed by joining the two pillars with an electroless copper plating and anneal process. The copper-to- copper bonding of the high aspect ratio pillars does not require the use of solder or other non-copper metals. Mechanical shear force measurements were used to characterize the bonding process as a function of annealing conditions. Excellent bond strength of the electrolessly joined pillars was achieved with a 250{degree sign}C anneal. The bond strength of the copper pillar interconnects exceeded 148 MPa. High aspect ratio of the pillars can provide mechanical compliance and the electroless fabrication method compensates for pillar misalignment and non-planarity of the bonded surfaces.

Multimode interference wavelength multi/demultiplexer for 1310 and 1550 nm operation based on BCB 4024-40 photodefinable polymer

A 1310 and 1550 nm coarse wavelength multi/demultiplexer based on benzocyclobutene (BCB 4024-40) polymer is demonstrated for the first time. The device is designed based on a combination of general interference and paired interference mechanisms of multimode interference (MMI). It is fabricated on BK7 glass substrate with a thin layer of SiO 2 as cover. A cost effective chemical etching technique is used in the fabrication process to take advantage of the photosensitive nature of the polymer. The device length was significantly reduced by adopting the restricted multimode interference scheme, lower beat length ratio and cascaded MMI couplers. The measured crosstalk at 1310 nm was 14.4 dB and at 1550 nm was 20.6 dB. The measured insertion loss is around 3.2–3.5 dB for both ports.

Photodefinable Thermally Sacrificial Polycarbonate Materials & Methods for MEMS & Microfluidic Device Fabrication

In this work, a cross-linkable polycarbonate sacrificial polymer and examples of microfabrication processes utilizing it are presented. The cross-linkable polycarbonate was made by end-capping a hydroxyl terminated polycarbonate oligomer with methacryloyl chloride, thus allowing direct polymer patterning using both positive and negative tone lithographic processes. Cross-linking the polymer significantly increases its resistance to thermal deformation and flow even at low cross- link densities. Thermogravimetric analysis of low cross-link density polycarbonates shows that decomposition of the cross- linked polymer exhibits the same behavior as the pure polycarbonate oligomer except for a low weight fraction tail that requires slightly higher temperatures to fully decompose. As a result of the increased resistance to thermal deformation and flow in the cross-linked polycarbonate, this new photodefinable sacrificial polymer has been shown to be compatible with a variety of microfabrication processes including metal evaporation, sputtering, and electroplating which are typically incompatible with non-crosslinked polycarbonate sacrificial materials.

Photodefinable electro-optic polymer for high-speed modulators

We present a series of polymeric electro-optic (EO) materials based on the tricyanovinylidenediphenylaminobenzene (TCVDPA) chromophore which exhibits an exceptionally high photostability combined with a high EO activity. The EO properties of these derivatives are given and the best result was obtained with the di-tBut derivative 2, which showed a nearly doubling effect of $r_{33}$ (14 pm/V to 25 pm/V), while increasing the concentration from 25 wt% to 37.5 wt%. A microring resonator design was made based on the PC-TCVDPA system. Waveguides were fabricated by photodefinition of inverted ridges in VSC, a negative epoxy based photoresist with low refractive index.

Low-Temperature Bonding of Copper Pillars for All-Copper Chip-to-Substrate Interconnections

A copper-to-copper bonding process was developed for an all-copper, chip-to-substrate interconnect technology. High aspect ratio polymer molds for electroplating were formed using a photodefinable polymer on both the chip and the substrate surfaces. Copper pillars were fabricated by electroplating metal in the polymer molds. The chip-to-substrate all-copper connections were formed by joining the two pillars with electroless copper plating followed by an anneal process. The copper-to-copper bonding of the high aspect ratio pillars does not require the use of solder or other noncopper metals. Mechanical shear force measurements were used to characterize the bonding process as a function of annealing conditions. Excellent bond strength of the electrolessly joined pillars was achieved with a 250°C anneal, with the bond strength of the copper pillar interconnects exceeding 148 MPa. High aspect ratio pillars can provide mechanical compliance, and the electroless fabrication method compensates for pillar misalignment and nonplanarity of the bonded surfaces.