A generalized surface morphology model is presented and validated to more effectively simulate the effects of surface variations (e.g., surface roughness and local thickness variations) on integrated Si photonic waveguides. To demonstrate the validity of this approach, a variety of surface roughness morphologies generated using this model are incorporated within a commercial finite-difference-time domain (FDTD) modeling package to simulate waveguide loss in Si strip optical waveguide geometries typically encountered in Si-based photonic integrated circuits. The FDTD simulation results are in very good agreement with the Payne and Lacey theoretical model for radiation-mode scattering loss based on surface correlation functions for RMS roughness values exceeding 2 to 3 nm where such scattering dominates. This agreement validates the efficacy of the generalized surface morphology model and its use for investigating the effects of irregular surface variations on photonic device performance.
Due to the increasing impact of smaller particles, mask cleaning continues to become more and more challenging in EUV lithography. To improve mask cleaning efficiency, advances in the fundamental understanding of the interaction between defect particles and mask surfaces are necessary. For this reason, surface force measurements were performed with an atomic force microscope on various mask surfaces relevant to EUV lithography. Experiments in air were carried out to illustrate particle interaction during mask transport and storage, while measurements in deionized ultrapure water were undertaken to investigate the influence of a basic cleaning chemistry. The effects of particle size were studied using SiNx tips with a nominal radius of 10 nm and spherical SiO2 probes with a radius of 500 nm. Particle interactions with mask surfaces in air were characterized by adhesion. Due to comparable surface roughness and surface chemistry, adhesion forces of a quartz mask substrate and a mask blank were similar. However, for a SiO2 sphere, the absolute values of the measured adhesive forces were greater than for a conventionally fabricated SiNx tip consistent with the probes' relative radii. Using a quartz mask substrate and deionized water as the intervening medium, the probe-substrate interaction observed was no longer characterized by attraction, but dominated by repulsive forces and hence potentially advantageous for cleaning purposes.
Although conventional optical lever technology typically used for scanning probe microscope applications has proven high sensitivity, accuracy, and is cost effective for most applications involving micromachined cantilever deflection measurements, the frequency limitations and space needs limit its applicability to emerging ultrasonic-based scanning probe microscopy (SPM) applications. Recently, the fabrication of cantilevers integrated with actuation and sensing components has opened avenues for feedback-based driving of micromachined cantilevers at higher order resonance frequencies, while sensing average deflection without the need of an optical deflection pathway for average deflection sensing. The work presented here reviews recent efforts by our group in fabricating micromachined cantilevers with integrated ZnO actuation layers to induce cantilever deflection. These cantilevers are being fabricated for use in a heterodyne force microscopy system (HFM) to enable SPM imaging contrast based on viscoelastic response of a surface in contact with a micromachined tip, wherein active-feedback technology is being applied to maintain ultrasonic tip excitation at higher order cantilever resonances. The first- and second-pass fabrication results are presented and reviewed regarding cantilever release and ZnO actuator (and electrode) fabrication. Dynamic response data from these structures, measured via laser Doppler vibrometry, reveal the expected resonance structure for a cantilever of these dimensions.
Differential ultrasonic force microscopy (d-UFM) has been used to investigate the nanoscale mechanical response of multi-walled carbon nanotubes (MWNTs) synthesized via chemical vapor deposition (CVD). In contrast to earlier studies, MWNTs with relatively high numbers (~ 60) of concentric walls were investigated. Quantitative analysis of the MWNT d-UFM data utilized Si3N4 cantilevered tips and Si substrates as a calibration standard. Initial investigations of the CVD MWNTs through d-UFM revealed a surprisingly large radial indentation modulus compared to Si. Frictional force imaging (FFM) was also carried out on the MWNTs in the presence and absence of an ultrasonic vibration. A FFM contrast reversal was observed between the MWNT and the Si substrate as the static set point force of the AFM cantilever was increased. This is attributed to an increase in the local indentation of the MWNT by the Si3N4 tip.
Apertureless-based, near-field Raman imaging holds the potential for nanoscale stress metrology in emerging Si devices. Preliminary application of near-field Raman imaging on Si device structures has demonstrated the potential for stress measurements. However, detailed investigations have not been published regarding the effect of tip radius on observed near-field enhancement. Such investigations are important to understand the fundamental limits regarding the signal-to-noise ratio of the measurement and the spatial resolution that can potentially be achieved before wide application to semiconductor metrology can be considered. Investigations are presented into near-field enhancement of Raman scattering from Si device structures using a modified near-field optical microscope (NSOM). The nano-Raman system utilizes an off-axis (45°) backscattering NSOM geometry with free-space collection optics. The spectroscopic configuration utilizes a single-bounce spectrometer incorporating a holographic notch filter assembly utilized as a secondary beam-splitter for an apertureless backscattering collection geometry. Near-field enhancement is observed for both Al- and Ag-coated probes. An inverse square power-law relationship is observed between near-field enhancement factor and tip radius.
Although the conventional optical lever technology typically used for scanning probe microscope applications has proven highly sensitive, accurate, and cost effective for most applications involving micromachined cantilever deflection measurements, frequency limitations and space needs limit its applicability to emerging ultrasonic-based SPM applications. Recently, the fabrication of cantilevers integrated with actuation and sensing components has opened avenues for feedback-based driving of micromachined cantilevers at higher-order resonance frequencies while sensing average deflection without the need for an optical deflection pathway for average deflection sensing. The work presented here will review recent efforts by our group in fabricating micromachined cantilevers with integrated piezoresistive deflection-sensing components combined with integrated ZnO actuation layers to induce cantilever deflection. These cantilevers are being fabricated for use in a heterodyne force microscopy system (HFM) to enable SPM imaging contrast based on viscoelastic response of a surface in contact with a micromachined tip wherein active-feedback technology is being applied to maintain ultrasonic tip excitation at higher order cantilever resonances. The first and second-pass fabrication results will be presented and reviewed regarding cantilever release and ZnO actuator (and electrode) fabrication. Dynamic response data from these structures, measured via laser Doppler vibrometery reveal the expected resonance structure for a cantilever of these dimensions.
Micromachined cantilevers used as force probes in atomic force microscopy are extremely sensitive to a variety of environment factors such as acoustic noise, temperature and humidity. This unwanted interference can be exploited to produce highly sensitive systems with proper design and under precise conditions. In this paper, we report the development of a new generic process for the fabrication of a microprobe with integrated piezoresistive read-out and built-in piezoelectric actuators. The mechanical performance of cantilever probes of various dimensions was studied. The result from the Finite Element Analysis (FEA) was compared to the experimental results. Application of this probe in a nondestructive, general-purpose, near-field nanomechanical imaging system will be discussed.
The relative surface contact stiffness of SnO2 nanobelts has been investigated via nondestructive ultrasonic force microscopy (UFM). The nanobelt crystal structure, as determined via transmission electron microscopy, was indexed to the tetragonal rutile structure (with lattice constants identical to those of bulk SnO2) as reported previously. The atomic Sn:O composition of the nanobelts studied was at or near 1:2. Topographic imaging studies revealed the nanobelt surface to be atomically flat with the exception of surface nano-dots, assumed to be local SnO2 crystallites. Preliminary local (10nm x 10nm) reduced modulus measurements were carried out via differential UFM on both the flat and nanodot regions of the nanobelt. Using the underlying Si substrate as a calibration standard the SnO2 modulus was estimated at 157±12 GPa, significantly lower than corresponding bulk values for any of the observed crystal orientations. We speculate this discrepancy is due in part to a combination of the aspherical probe tip and unknown adhesive properties of nanobelt, although an intrinsic reduction of the SnO2 nanobelt modulus cannot be ruled out.
A novel cross-sectional characterization technique for nanomechanical profiling of low-k dielectrics has been developed based on ultrasonic force microscopy. So-called CS-UFM has been demonstrated on silicon-based, spin-on dielectrics (SOD) used for gap-fill in 0.15 μm trenches in an SiO2 integrated circuit test structure. The SiO2 trench walls were coated with a thin (~ 24 nm) plasma-enhanced, chemical vapor deposited (PECVD) silicon nitride layer. CS-UFM imaging clearly differentiated the SOD, SiO2 and silicon nitride on the basis of elastic modulus. Variations in the elastic uniformity of the SOD and silicon nitride were observed. In addition, mechanical defects were identified within the SOD-filled trenches.
A new focused ion beam (FIB) miore method is proposed to measure the in-plane deformation of object in a micrometer scale. The FIB moire is generated by the interference bewteen a prepared specimen grating and FIB raster scan lines. The principle of the FIB moire is described. Several specimen gratings with 0.14 and 0.20 micron spacing are used to generate FIB miore patterns. The FIB moire method is successfully used to measure the residual deformation in a MEMS structure after removing the SiO2 sacrificial layer with a 5000 lines/mm grating. The results demonstrate the feasibility of this method.
We present measurements of the nanoscale elastic properties of hinge structures supporting micro-mirror arrays using a new characterization technique called Ultrasonic Force Microscopy (UFM). This technique is based on Atomic Force Microscopy with ultrasonic excitation which provides a means of testing the elastic response at MHz frequencies. The simultaneous recording of topography with elastic imaging allows the elimination of any artifacts. In this report, we demonstrate that UFM can achieve nano-scale elastic resolution to reveal mechanical stress induced changes as well as process induced material fatigue in the micro-mirror devices. The main aim of this study is polysilicon-based hinge structures that support the micro-mirror because they show the highest stress during mirror switching. Our results indicate that no significant structural and mechanical change of the polysilicon-based hinge support structure occurs even after more than 1,000,000,000 switching cycles. This method offers a non-destructive way to perform reliability characterization on MEMS devices. This technique developed will offer new opportunities for the evaluation of structural and mechanical integrity of MEMS devices.
Commercial application of organic light-emitting materials currently employs organic thin film deposition techniques including spin-coating, dip-coating, and evaporation. Broader technologies will also benefit from such organic optoelectronic materials. Emerging applications include on- chip light sources for integrated circuit (IC) optical signal transduction, on-chip microdisplays, and on-chip visible and infrared sources for integrated microsystems (MEMS). However, to fully adapt to the so-called cluster tool platform used for closed-loop manufacturing of IC's a remote vapor phase deposition scheme is required for low molecular weight organic light-emitting materials as opposed to conventional evaporation based techniques. We report the development of a remote organic vapor phase deposition process for growing high-quality aluminum tris-hydroxyquinoline (Alq3) films. This process utilizes a remote reservoir for organic vaporization coupled with mass-flow controllers for delivery to the condensation zone. A load-lock equipped chamber, capable of 5' wafer transfer, is utilized to eliminate ambient contamination without restricting wafer loading/unloading or Alq3 material loading. Data will be presented for a design-of-experiment (DOE) statistical process optimization investigation of Alq3 thin film growth as a function of the system deposition parameters. Electrical characterization of the Alq3 films and their applicability for optoelectronic device structures are presented and discussed.
MEMS promise to revolutionize nearly every product category by bringing together silicon-based microelectronics fabrication with silicon micromachining technology, thereby, making possible the realizing of complete systems-on-a-chip.
The backplanes of Liquid-Crystal-on-Silicon microdisplays are derived from a VLSI silicon chip that includes the active matrix as well as row and column drivers. One away to convert this silicon chip into a functional backplane is to planarize the silicon chip, then etch vias through the planarization layer and finally to pattern an array of flat, highly reflective electrodes, each of which is electrically connected to a corresponding cell of the active matrix underneath. Such a post-processing sequence can be carried out in different ways, using either Chemical-Mechanical Polishing or spin-on planarization. We have chosen spin-on planarization with Dow Chemical's Cyclotene resin followed by reactive ion etching of the vias. Finally, electrodes are patterned by aluminum sputtering and lift-off. This step also establishing the electrical connection to the underlying metalization. To demonstrate this sequence we have fabricated a two-level passive silicon backplane with aluminum stripe electrodes. We describe in detail the processing steps involved and report on the achieved degree of planarization, polymer and aluminum roughness.
The pyroelectric properties of three chiral smectic liquid crystalline materials have been investigated. The magnitude of the pyroelectric coefficient for in two of these compounds is very high exceeding 200 nC/cm2K. A mixture containing one of these compounds exhibits a high pyroelectric coefficient over a temperature range that is attractive for thermal (or IR) sensing applications. The relevant figure of merit of these materials as elements for IR sensors is discussed in comparison to those that are currently used in sensing devices.
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