Sacrificial Polymer Research Articles

Free-standing porous supramolecular assemblies of nanoparticles made using a double-templating strategy

The formation of stable and ordered free-standing porous supramolecular assemblies of nanoparticles with sizes and geometries controlled at different length scales is demonstrated by a double-templating strategy. Our technique combines the directed assembly of particles, templating using nanoimprint lithography (NIL), and supramolecular layer-by-layer (LbL) assembly. First, 500-nm beta-cyclodextrin (CD)-functionalized polystyrene (PS) particles were assembled by convective assembly onto a sacrificial polymer template patterned with a predefined geometry and size using NIL, forming a 3D crystal architecture of particles. LbL assembly of alternating supramolecular host- and guest-functionalized glues of CD-functionalized Au (Au-CD) nanoparticles and adamantyl (Ad) dendrimers, sized between 3-5 nm, within the preformed PS-particle crystal effectively bound the particles together into a particle composite. These particle composites were released from the substrate together with the polymer template, and transferred onto a target substrate. The particle crystal integrity, order and functionality were preserved. Rinsing the structure with dichloromethane removed the PS core material together with the polymer template, resulting in interconnected porous capsules, the sizes and shapes of which are fully determined by the PS core size and the polymer template definition. Again, integrity and shape were preserved in the rinsing step. These capsules were capable of storing organic fluorescent molecules using specific interactions.

Air-Gaps for High-Performance On-Chip Interconnect Part I: Improvement in Thermally Decomposable Template

The incorporation of air-cavities (i.e., air-gaps) as the intralevel dielectric in integrated circuits (ICs) can provide an ultralow-k solution, especially at the 32 nm technology node and beyond. Air-gaps can be created by the templating method using norbornene (NB)-based sacrificial polymers. However, it has been found that the hardness and modulus of the templating material is critical to achieving mechanical fidelity of the structure during processing. As a result, stiffer sacrificial polymers lead to higher yield. In this study, tetracyclododecene (TD)-based sacrificial polymers were investigated and compared with norbornene-based (NB)-based polymers. Nanoindentation experiments showed that thin films of TD-based sacrificial polymers were harder than NB-based sacrificial polymers. The effect of the modulus and hardness on the process repeatability was quantitatively evaluated by comparing the straightness of 50-nm-wide lines of TD- and NB-based sacrificial polymers. It was shown that the TD-based polymer structures were straighter and had better reproducibility than those of NB-based polymers due to an increase in hardness and modulus. The thermal decomposition properties of TD-based polymers were similar to their NB-based counterparts. Both TD- and NB-based polymers were thermally stable at 300°C and the decomposition residues were less than 1% of the original weight. The thickness of the residue (solid reaction byproducts) from thin TD films was as low as 2.1 nm (depending on the atmosphere quality) and the residue was hydrophobic.

Air-Gaps for High-Performance On-Chip Interconnect Part II: Modeling, Fabrication, and Characterization

Air-gaps are the ultimate low-k material in microelectronics due to air having a low dielectric constant close to 1.0. The interconnect capacitance can further be reduced by extending the air-gaps into the interlayer dielectric region to reduce the fringing electric field. An electrostatic model (200 nm half-pitch interconnect with an aspect ratio of 2.0), was used to evaluate the dielectric properties of the air-gap structures. The incorporation of air-gaps into the intrametal dielectric region reduced the capacitance by 39% compared with SiO2. Extending the air-gap 100 nm into the top and bottom interlayer SiO2 region lowered the capacitance by 49%. The ability to fabricate air-gaps and ‹extended air-gaps’ was demonstrated, and the capacitance decrease was experimentally verified. Cu/air-gap and extended Cu/air-gap interconnect structures were fabricated using high-modulus tetracyclododecene (TD)-based sacrificial polymer. The aspect ratio of the air-gap was 1.8 and the air-gap was extended 80 nm and 100 nm into the top and bottom interlevel SiO2 region, respectively. The measured effective dielectric constant (k eff) of the Cu/air-gap and the extended Cu/air-gap structures with SiO2 interlevel dielectric was 2.42 and 2.17, respectively. The effect of moisture uptake within the extended Cu/air-gap structure was investigated. As the relative humidity increased from 4% to 92%, the k eff increased by 7%. Hexamethyldisilazane was used to remove adsorbed moisture and create a hydrophobic termination within the air-cavities, which lowered the effect of humidity on the k eff. A dual Damascene air-gap and extended air-gap fabrication processes were proposed and the challenges of using a sacrificial polymer placeholder approach to form air-cavities are compared to other integration approaches of dual Damascene air-gap.

Acoustic emission technique for monitoring the pyrolysis of composites for process control

Carbonization is the first step in the heat and pressure treatment (pyrolysis) of composites in preparing carbon–carbon parts. These find many uses, including aircraft brakes, rocket nozzles and medical implants. This paper describes the acoustic emissions (AE) from various stages of the manufacturing process of carbon–carbon composites. This process involves carbonization at a high temperature and this results in both thermal expansion and volume change (due to pyrolysis in which a sacrificial polymer matrix is converted to carbon). Importantly the resultant matrix is porous and has a network of small intra-lamina cracks. The formation of these microcracks produces AE and this paper describes how this observation can be used to monitor (and eventually control) the manufacturing process. The aim is to speed up manufacture, which is currently time-consuming. The first section of the paper describes the design of unimodal waveguides to enable the AE to propagate to a cool environment where a transducer can be located. The second part of the paper describes various experimental observations of AE under a range of process conditions. In particular, this paper presents a technique based on detecting acoustic emissions and (1) uses wire waveguides to monitor parts within the autoclave to 800 °C, (2) monitors microcracking during pyrolysis, (3) uses a four-level threshold to distinguish between low- and high-amplitude cracking events, (4) recognizes the occurrence of harmful delaminations, and (5) guides the control of the heating rate for optimum efficiency of the pyrolysis process. In addition, supporting data are presented of in situ measurements of porosity, weight loss, cross-ply shrinkage, and mass spectroscopy of gases emitted. The process evolution is illustrated by the use of interrupted manufacturing cycle micrographs obtained by optical, scanning acoustic (SAM) and scanning electron (SEM) microscopy. The technique promotes in-process monitoring and control but also contributes to current understanding of pyrolysis.

Air Cavity Generation for Interconnect and High Resolution Displays

A new approach for the wafer-level fabrication of silicon-VLSI-based liquid-crystal (VLSI/LC) microdisplays has been investigated. Fabrication of the LC cell gap is based on the same processes developed for creating ultra low-k dielectrics for VLSI interconnect. A dimensionally precise planar LC cavity is created by use of a sacrificial polymer placeholder in an all build-up process. The process flow for a reflective LC display is presented along with early tests using a polyimide alignment layer.

Enhancing fluorescence of tricolor fluorescent powders by silica inverse opals

Silica inverse opal photonic crystals (PCs) were fabricated by using a sacrificial polymer template method, and were characterized by scanning electron microscopy and reflection spectra. The fluorescence of tricolor fluorescent powders has been enhanced obviously by silica inverse opals as the emission wavelengths of the tricolor fluorescent powders are in the range of stop bands of the inverse opal PCs, which demonstrates that the PCs will be efficient and selective reflection mirrors. A promising application of the strategy would be in novel effective lighting devices.

Surface micromachining of a thin film microresonator using dry decomposition of a polymer sacrificial layer

Fabrication of thin film silicon bridge microresonators on glass substrates using an unzipping sacrificial polymer has been demonstrated. Polynorbornene, which can be thermally decomposed at ∼425°C, has been used as the sacrificial layer, allowing a simple, fully dry sacrificial layer removal. The polynorbornene is spin coated onto the substrate and patterned by photolithography. A thin film silicon structural layer is deposited by rf plasma-enhanced chemical vapor deposition and patterned to form a microbridge. The microbridges are electrostatically actuated and the resulting deflection is measured optically. The fabricated microbridges show resonance frequencies of the order of ∼4.4MHz and quality factors ∼450.

Wafer-Level Packaging of Micromechanical Resonators

An approach to low-cost, wafer-level packaging of microelectromechanical systems (MEMS), e.g., microresonators, is reported. The process does not require wafer-to-wafer bonding and can be applied to a wide range of MEMS devices. A sacrificial polymer-placeholder is first patterned on top of the MEMS component of interest, followed by overcoating with a low dielectric constant polymer overcoat. The sacrificial polymer decomposes at elevated temperature, and the volatile products from the sacrificial material permeate through the overcoat polymer leaving an embedded air-cavity around the MEMS structure. Thus, the device is released from the sacrificial polymeric material, housed in a protective overcoat. The protected MEMS device can then be handled and packaged like an integrated circuit. The electrical characteristics of the microresonators before and after packaging were essentially the same, showing the packaging scheme does not alter the device performance. This approach is applicable to both surface and bulk micromachined devices

UV-induced porosity using photogenerated acids to catalyze the decomposition of sacrificial polymers templated in dielectric films

A UV-initiated process for selective incorporation of porosity and tuning of refractive index in a dielectric film is presented. The formation of porosity via radiation offers the advantages of lower processing temperatures and shorter processing times than most processes that rely on thermally-induced porosity. The ability to pattern porosity via the use of a photomask allows structures such as porous direct-write waveguides to be made. The direct-write process presented here used UV-generated acids to catalyze the decomposition of sacrificial polymers templated in a crosslinked matrix, resulting in the formation of porosity and decrease of refractive index and dielectric constant after subsequent removal of the generated acid. In unexposed areas, no acid molecules were generated, thus these areas remained non-porous and their refractive index remained relatively high compared to porous regions. Base-catalyzed gelation was used to crosslink the matrix at temperatures well below the glass transition temperature of the matrix in order to avoid thermal activation of the photoacid generators (PAGs) in the direct-write formulation. After crosslinking the matrix, deep ultraviolet (DUV) radiation was used to initiate the direct-write reactions thus creating a refractive index profile within the film. Neutralization or volatilization of the photogenerated acid was used to deactivate the system. The lowest interline dielectric constant achieved by direct-written films was 2.36 after the volatilization step compared to a value of 2.29 for porous films formed via the traditional template process. At optimum conditions, the average amount of UV-induced porosity was estimated to be as high as 21%. Transmission electron microscopy (TEM) imaging showed that elongated mesopores (10 nm by about 20–30 nm) and elongated macropores (100 nm by about 200 nm in size) were produced within the internal structure of direct-written films. The porous films formed via the traditional template technique had internal structures with large collections of spherical mesopores (5–10 nm) located in close proximity to one another due to phase segregation of the polymer at high temperatures.

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.

Molding ceramic microstructures on flat and curved surfaces with and without embedded carbon nanotubes

This paper explores micromolding fabrication of alumina ceramic microstructures on flat and curved surfaces, the transfer of carbon nanotube (CNT) micropatterns into the ceramic and oxidation inhibition of these CNTs through ceramic encapsulation. Microstructured master mold templates were fabricated from etched silicon, thermally embossed sacrificial polymer and flexible polydimethylsiloxane (PDMS). The polymer templates were themselves made from silicon masters. Thus, once the master is produced, no further access to a microfabrication facility is required. Using the flexible PDMS molds, ceramic structures with mm scale curvature having microstructures on either the inside or the outside of the curved macrostructure were fabricated. It was possible to embed CNTs into the ceramic microstructures. To do this, micropatterned CNTs on silicon were transferred to ceramic via vacuum molding. Multilayered micropatterned CNT–ceramic devices were fabricated, and CNT electrical traces were encapsulated with ceramic to inhibit oxidation. During oxidation trials, encapsulated CNT traces showed an increase in resistance that was 62% less than those that were not encapsulated. The processes described here could allow fabrication of inexpensive 3D ceramic microstructures suitable for high temperature and harsh chemical environments.

Mechanical properties of sacrificial polymers used in RF-MEMS applications

This paper presents the analysis, fabrication and measurement of bent-beam strain sensors for mechanical characterization of typical RF-MEMS sacrificial layers such as photoresist and PMMA. The analysis demonstrates that a simple kinematic model of the bent-beam strain sensor can be used to accurately calculate strain from the observed deflections. A simple fabrication process using a dry isotropic silicon etch to release the beams is detailed. This process is then used to measure the elastic modulus and coefficient of thermal expansion for AZ-4110 resist as well as PMMA. The measured results for PMMA are in excellent agreement with previously reported values.

Effects of the photoacid generator type on the imaging and thermal decomposition properties of photodefinable, thermally sacrificial poly(propylene carbonate) materials

AbstractPhotosensitive, thermally sacrificial polymer systems consisting of poly(propylene carbonate) (PPC) and a photoacid generator (PAG) were studied for use in the fabrication of buried microchannel systems such as microfluidic devices. Because the fabrication of microchannels was affected by the performance of these materials, changes in the imaging properties were monitored upon the variation of the PAG used in the system. Thin films of PPC loaded with a PAG were spin‐cast onto silicon wafers, exposed to UV light to selectively generate acid, and placed on a hotplate to decompose the exposed regions. Properties such as the contrast, sensitivity, and amount of residue remaining after decomposition varied significantly, depending on the PAG used. The use of a metal‐free, ionic methide PAG gave the best performance for formulating a chemically amplified, photosensitive, sacrificial polymer system based on PPC. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 266–271, 2006

AirGaps for Ultra Low-k Interconnects Fabricated with Hardened Sacrificial Polymer

Polymers based on norbornene have been used for low-k dielectric materials, advanced photoresists, and as sacrificial materials in the formation buried air-gaps. In this study, the hardness of thin films of butylpolynorbornene - triethoxysilylpolynorbornene copolymer was enhanced by exposure to electron beam radiation. The effect of electron energy, dose, and temperature on the thermal and mechanical properties of the irradiated copolymer films was investigated using dynamic thermal gravimetric analysis and nanoindentation. The hardness of the copolymer thin films was increased by electron beam irradiation; however, the thermal stability was degraded.

Three-dimensional nanochannels formed by fast etching of polymer

Nanochannels are widely used in biomedical applications such as DNA analysis and biomolecule detection. We report a study using sacrificial polymer and oxide to form three-dimensional (3D) nanochannels. Polymer nanostructures were patterned on Si substrates using optical lithography or nanoimprint lithography, followed by oxide deposition to form the sealed channels. A high-speed dry etching technique for removing the sacrificial polymer was developed using an O2 plasma at high power, high pressure, and elevated temperature. This dry etching technique provides a fast lateral etch rate of 3.91μm∕min for the polymer inside nanochannels, which is an order of magnitude higher than conventional reactive ion etching. High selectivity of 1200 was obtained between the lateral etch rate of polymer inside the nanochannels and the vertical etch rate of oxide. Etch rate dependence on pressure, temperature, and channel width were studied. It was found that the etch rate increases with pressure and temperature. To form multiple levels of nanochannels, the oxide covering the channels was planarized by a photoresist coating and etch-back process. After oxide planarization, the channel formation process is repeated and multiple levels of nanochannels can be stacked to build 3D nanostructures. A two-level channel structure was demonstrated. Interconnect openings between channels of adjacent levels were also demonstrated. With such technique, complex 3D system can be fabricated. Since oxide is transparent to visible light and the channels have hydrophilic surfaces, therefore such a 3D nanofluidic system is suitable for various biomedical studies.

Nanoporous organosilicate films as antireflection coatings

Nanoporous poly(methylsilsesquioxane) (PMSSQ) films prepared by the sacrificial-porogen (pore generator) approach have been demonstrated as potential antireflection coatings. The simplicity of film preparation, tunability of the refractive index and excellent control of thickness make the nanoporous film a good candidate material. The nanoporous structures were generated by removing organic sacrificial polymer from hybrid films of PMSSQ and organic polymer. In the hybrid films, the organic polymer is phase separated from and entrapped in the crosslinked PMSSQ matrix. The optical thickness (refractive index × film thickness) was controlled simply by varying the amount of organic polymer (% loading) that controls porosity (thus refractive index), and the solid content of the solution and spin casting speed that determine the final film thickness. Film thicknesses ranging from about 10 nm to 1 μm have been achieved with refractive indices ranging from 1.18 to 1.44. The reflectivity of glass slides coated with the porous films is in good agreement with theoretical calculations using basic optical equations. Light transmissions of greater than 99% were observed at certain wavelengths.

Wafer-level MEMS packaging via thermally released metal-organic membranes

This paper reports on the design, implementation and characterization of wafer-level packaging technology for a wide range of microelectromechanical system (MEMS) devices. The encapsulation technique is based on thermal decomposition of a sacrificial polymer through a polymer overcoat to form a released thin-film organic membrane with scalable height on top of the active part of the MEMS. Hermiticity and vacuum operation are obtained by thin-film deposition of a metal such as chromium, aluminum or gold. The thickness of the overcoat can be optimized according to the size of the device and differential pressure to package a wide variety of MEMS such as resonators, accelerometers and gyroscopes. The key performance metrics of several batches of packaged devices do not degrade as a result of residues from the sacrificial polymer. A Q factor of 5000 at a resonant frequency of 2.5 MHz for the packaged resonator, and a static sensitivity of 2 pF g−1 for the packaged accelerometer were obtained. Cavities as small as 0.000 15 mm3 for the resonator and as large as 1 mm3 for the accelerometer have been made by this method.

Air-Gap Fabrication Using a Sacrificial Polymeric Thin Film Synthesized via Initiated Chemical Vapor Deposition

Thin films of a sacrificial polymer based on poly(cyclohexyl methacrylate) (PCHMA) were synthesized using initiated chemical vapor deposition (iCVD). iCVD is able to make cross-linked PCHMA in situ on a substrate in one step without using any solvents. Although cross-linked, the polymer was found to decompose cleanly, leaving behind a maximum of 0.3% of residue by thickness upon completion of thermal annealing. Cross-linking renders the polymer stable in practically all solvents, so photoresist used for patterning can be removed by dissolution instead of ashing. The high etch rate in oxygen reactive-ion etching in addition to the stability in solvents eliminates the need of a hard mask during etching. This represents an improvement over previously reported spin-on sacrificial materials. Infrared spectroscopy and solubility tests confirm the identity of the iCVD polymer and its stability in solvents. The onset of thermal decomposition is 270°C, allowing the polymer to survive high-temperature processing steps. Fabrication using conventional lithographic, etching, and deposition techniques resulted in void structures having feature sizes of a minimum of . This work represents a novel approach for air-gap fabrication using a sacrificial polymer, combining the rationale behind cross-linking and the technique required for thin-film deposition.

Air-Gaps for Ultra Low-k Interconnects Fabricated with Hardened Sacrificial Polymer

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Sealed three-dimensional nanochannels

A technique to create sealed three-dimensional nanochannel networks is developed using sequentially stacked thermal nanoimprint lithography on planarized self-supporting dielectric sealing material over polymer sacrificial layers. Void-free plasma enhanced chemical vapor deposited SiO2 encloses and seals nanochannels that are formed upon the removal of the sacrificial polymer. Planarization of the SiO2 surface allows the utilization of the vertical dimension to sequentially apply nanoimprint lithography for the formation of multiple-level nanochannel networks. Removal of the sacrificial polymer is performed with a high-power and high-pressure O2 plasma. Wet chemical processes using common solvents are found to be ineffective in removing the sacrificial polymer. Two level nanochannels with cross-sectional dimensions of 300nm×200nm and lengths of 65μm that are aligned offset from one another and aligned on top of one another are demonstrated.