Sacrificial Polymer Layer Research Articles

Planarization of photoresist sacrificial layer for MEMS fabrication

We describe a process to improve the planarization and the thermal stability of a polymeric sacrificial layer, which is used for the release of MEMS structures. This process involves a novolak matrix polymer. We show that the sacrificial layer topography critically depends on the process conditions, and we determine optimized methods based on a bi-layer patterned polymer process. Our results demonstrate that the technological difficulties of sacrificial layer integration in MEMS can be overcome.

Design and control of porosity in oxide thin films grown by PECVD

This work presents a series of results about the synthesis and characterization of porous oxide thin films (thicknesses ∼ ∼300 nm) prepared at low temperature by PECVD. Two different experimental strategies are described. A first one, used for the preparation of porous SiO2 thin films, consists of the use of a polymeric sacrificial layer that is removed during deposition of the oxide thin film. A second one, used for the preparation of TiO2 thin films, relies on the modification of some critical deposition parameters (i.e., temperature, Ar/O2 ratio in the plasma mixture, etc.). Thin films with a large variation of pore structures as evidenced by Scanning Electron Microscopy (SEM) have been prepared by the two methodologies. The thin films have been characterised by different techniques and some of their properties related with their microstructure assessed by ellipsometry (optical behaviour) or water contact angle measurements (hydrophobic/hydrophilic character). A quartz crystal monitor has been used to measure water vapour adsorption/desorption isotherms in the films. From the shape of these isotherms it is possible to estimate the type of pores existing in the different thin films. Examples of the potential use of these porous thin films as humidity sensors or hydrophilic surfaces are reported.

Perforated polymer membranes fabricated by nanoimprint

A bi-layer process was developed based on a polymeric sacrificial layer to generate free-standing polymer membranes. Regular hole patterns were generated in the top layer via nanoimprint and etched down to the sacrificial layer using reactive ion etching. The underlying layer was partly dissolved in a wet etching process through the pores, thus creating an air gap. Large arrays with interconnected pores could be fabricated. This process can be applied to a wide selection of polymeric materials and was successfully applied to pore sizes of 200 nm.

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.

A Novel PECVD Procedure for the Room‐Temperature Synthesis of SiO2 Thin Films with Controlled Porosity

Room‐temperature synthesis of porous SiO2 is achieved using a novel PECVD procedure. Porosity of the films is found to directly correlate with the thickness of a sacrificial polymer layer which is removed during the deposition of the oxide thin film. Films prepared with this method (see Figure) are transparent with low refractive index and are very well suited for antireflective coatings and other related optical applications. Membrane‐like layers can also be prepared by this method.

Polymeric Sacrificial Layers for the Control of Microstructure and Porosity of Oxide Thin Films Deposited by Plasma-Enhanced Chemical Vapor Deposition

Scanning electron microscopy images of two porous thin films of similar thickness deposited by PECVD at room temperature and low pressure. The microstructure and porosity of the films were controlled using intermediate plasma polymer sacrificial layers during the deposition.

Potential for micromachined actuation of ultra-wide continuously tunable optoelectronic devices

Tailored scaling represents a principle of success that, both in nature and in technology, allows the effectiveness of physical effects to be enhanced. Mutation and selection in nature are imitated in technology, e.g. by model calculation and design. Proper scaling of dimensions in natural photonic crystals and our fabricated artificial 1D photonic crystals (DBRs, distributed Bragg reflectors) enable efficient diffractive interaction in a specific spectral range. For our optical microsystems we illustrate that tailored miniaturization may also increase the mechanical stability and the effectiveness of spectral tuning by thermal and electrostatic actuation, since the relative significance of the fundamental physical forces involved considerably changes with scaling. These basic physical principles are rigorously applied in micromachined 1.55-μm vertical-resonator-based devices. We modeled, implemented and characterized 1.55-μm micromachined optical filters and vertical-cavity surface-emitting laser devices capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation of one or several air-gaps that are part of the vertical resonator including two ultra-highly reflective DBR mirrors of strong refractive index contrast: (i) Δn=2.17 for InP/air-gap DBRs (3.5 periods) using GaInAs sacrificial layers and (ii) Δn=0.5 for Si3N4/SiO2 DBRs (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters, a continuous tuning of >8% of the absolute wavelength is obtained. Varying the reverse voltage (U=0–5 V) between the membranes (electrostatic actuation), a tuning range of >110 nm was obtained for a large number of devices. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. In two filters, tuning of 127 and 130 nm was observed for about ΔU=7 V. The extremely wide tuning range and the very small voltage required are record values to the best of our knowledge. For thermally actuated dielectric filters based on polymer sacrificial layers, Δλ/ΔU=-7 nm/V is found.

Fabrication of micromechanical switches for routing radio frequency signals

Micromechanical switches have several advantages over other switch technologies for the routing of microwave and mm-wave signals. They offer low loss, low switching power, very low standby power, and are extremely linear. The switching speeds are very slow compared to solid state switches; however, for a number of applications, their specifications appear attractive. For instance, electronically steerable antenna arrays operating at 10, 20, and 30 GHz need phase shifters for each antenna element. Micromechanical RF switches that offered lower losses could find significant applications as phase shifters for telecommunications applications. We describe the fabrication of membrane micromechanical RF switches that switch signals of 10 GHz and higher. Dry etching plays a critical role in fabrication. In particular the isotropic removal of a sacrificial polymer layer between the bottom electrode and the membrane is a critical process. Reasonable rates must be obtained at moderate temperatures and there must be as little residue as possible. A high degree of selectivity over that of the capacitor dielectric is also a requirement. Our process should be compatible with microwave monolithic integrated circuit (MMIC) processing technology allowing integration with RF amplifiers and other components.