High Aspect Ratio MEMS Research Articles

Atomic Layer Deposition of Nanostructured Tunable Resistance Coatings: Growth, Characterization, and Electrical Properties

We have developed atomic layer deposition (ALD) methods to synthesize robust nanostructured composite coatings with tunable resistance comprised of conducting, metallic nanoparticles embedded in an amorphous dielectric matrix. These films are nominally composed of M:Al2O3 where M= W or Mo, and are prepared using alternating exposures to trimethyl aluminum (TMA) and H2O for Al2O3 ALD and alternating MF6/Si2H6 exposures for the metal ALD. By varying the ratio of ALD cycles for the metal and Al2O3 components in the film, we can precisely adjust the resistivity of these composite coatings over a very broad range (e.g. 1012-103 Ohm-cm). Furthermore, the self-limiting nature of ALD allows us to grow these nanocomposite films on a variety of complex substrates such as high aspect ratio porous 3D surfaces, MEMS devices, ceramics, grid structures and microchannel plates. Here we present a detailed ALD study of the nanostructured composite resistance coatings including in-situ material growth, microstructural characterization, and electrical measurements.

Investigation of the Effects of Hydrodynamic and Parasitic Electrostatic Forces on the Dynamics of a High Aspect Ratio MEMS Accelerometer

We present the results of an extensive characterization of physical and electrostatic effects influencing the dynamical behavior of a micro-electromechanical (MEMS) accelerometer based on commercial technology. A similar device has been utilized recently to demonstrate the effect of Casimir and other nano-scale interactions on the pull-in distance [Ardito et. al., Microelectron. Reliab., 52 (2012) 271]. In the present work, we focus on the influence of pressure, plate separation, and electric surface potentials on the spectral mechanical response. We finally find evidence for the presence of non-viscous damping due to compressibility of the ambient gas, and demonstrate a strong dependence of the sensitivity on the parameters of the operating point.

Analyzing and Control of Drive Mode in a Vibratory Ring Gyroscope

One of the challenges in MEMS era is dynamical modeling and control. This paper presents the analysis and control of drive mode of an 80 mm-thick, 1.1 mm in diameter high aspect-ratio (20:1) ring vibratory MEMS gyro. At first dynamic model of MEMS is derived through energy and Lagrange equations. Then state space equation is derived. Natural frequencies were derived by ABAQUS software and through equations. Frequency analysis was applied to analyze system and making comparison between two applied approaches in deriving frequencies. At last modified PID control was designed based on previous section results. We extracted results in presence of white noise to examine stability of the proposed controller. Consequently drive mode shows acceptable behavior through the applied PID controller. The aim is accomplished, though more advanced control can be applied to reduce uncertainty and compensate the noise in feedback to around zero.

Molybdenum etching using an SF6, BCl3 and Ar based recipe for high aspect ratio MEMS device fabrication

This paper presents the development of a dry etch recipe for molybdenum (Mo), based on SF6, BCl3 and Ar gases for high-topography MEMS device fabrication. The recipe was optimized for a high Mo etch rate, a low Mo by-product generation and a low Mo undercut using photoresist (PR) masks. The presented optimized recipe is very effective in Mo etching using a PR mask. The Mo by-product generated during the Mo etch can be easily removed using the ACT NE-14 etch residue remover. This optimized recipe was also used for etching Mo electrodes using oxide, PR or composite oxide-PR masks to select the better mask and selectivity. It was observed in this study that Mo has a better selectivity when using an oxide hard mask. However, it has the serious drawback of generating by-products which are difficult to clean later on. On the other hand, Mo has a poor selectivity with PR masks compared with oxide masks but there was no issue of by-product cleaning. At the same time, no extra mask layer deposition is required for Mo etching. The optimized recipe was also used for Mo electrode etching using PR and PR-oxide composite masks for the demonstration of working film bulk acoustic wave resonators and actuator microelectromechanical system devices.

Advances in Electrochemical Micromachining of Silicon: Towards MEMS Fabrication

In this work, a significant step towards the fabrication of very high aspect-ratio complex microsystems by silicon electrochemical micromachining in HF-based electrolytes (ECM) is given. High aspect-ratio MEMS structures, with different shape and dimensions, consisting of inertial free-standing masses equipped with comb-fingers and suspended by springs from the substrate were fabricated by exploiting advanced features of the ECM technology.

Self Patterned Gold Electroplating for High-Aspect Ratio MEMS Structures

not Available.

Fabrication of a high aspect ratio thick silicon wafer mold and electroplating using flipchip bonding for MEMS applications

We have developed a microfabrication process for high aspect ratio thick silicon wafer molds and electroplating using flipchip bonding with THB 151N negative photoresist (JSR micro). This fabrication technique includes large area and high thickness silicon wafer mold electroplating. The process consists of silicon deep reactive ion etching (RIE) of the silicon wafer mold, photoresist bonding between the silicon mold and the substrate, nickel electroplating and a silicon removal process. High thickness silicon wafer molds were made by deep RIE and flipchip bonding. In addition, nickel electroplating was developed. Dry film resist (ORDYL MP112, TOK) and thick negative-tone photoresist (THB 151N, JSR micro) were used as bonding materials. In order to measure the bonding strength, the surface energy was calculated using a blade test. The surface energy of the bonding wafers was found to be 0.36–25.49 J m−2 at 60–180 °C for the dry film resist and 0.4–1.9 J m−2 for THB 151N in the same temperature range. Even though ORDYL MP112 has a better value of surface energy than THB 151N, it has a critical disadvantage when it comes to removing residue after electroplating. The proposed process can be applied to high aspect ratio MEMS structures, such as air gap inductors or vertical MEMS probe tips.

Ultra high aspect-ratio MEMS and NEMS on basis of fibrous composite technology

Indisputably, microelectronics is the mother of the MEMS/NEMS technologies. Unfortunately, the majority of developed MEMS and NEMS devices inherited from microelectronics technologies not only of merit but also deficiencies one of which is their planarity. Recently developed devices on the flexible base and also devices with the moving elements on the hinges, in principle, remain geometrically flat. With this micro- and nano-mechanisms (motors, actuators, sensors, etc.) they are not divided with the base (from silicon or another material) large part of which (volume and mass) functionally is not used and has usual of macro sizes (dimensions of a chip). As a result, these ultra modern planar technologies are helpless with the creation of powerful autonomous 3D-devices with the overall sizes ∼(1–10) mm3. The author makes the attempt to estimate the prospects for development and fabrication of topology complex 3D MEMS/NEMS, for example, flaying microrobot and elastic micro motor on the basis of fibrous composites—ultra high aspect-ratio glass structures with predetermined 3D micro- and nano-topologies and embedded wires.

High aspect ratio MEMS and CMOS: Collaborators or competitors?

As commercialisation of MEMS continues, integrated MEMS devices are under development, whereby the MEMS and CMOS or BICMOS technologies are brought together on the same chip. This technique is an alternative to hybrid packaging, where the MEMS and CMOS components are manufactured separately and bonded to complete the device. The cost of processing integrated MEMS devices has been analysed, using the MEMSCOST spreadsheet, showing that the MEMS component is only a few percent of the overall cost of manufacturing, even when a High-Aspect-Ratio (HAR) process is employed and even less when test and packaging costs are taken into account.

Design, modeling, fabrication and testing of a high aspect ratio electrostatic torsional MEMS micromirror

As an essential part of an optical imager project, there was the need for a very high aspect ratio MEMS optical scanning mirror (5 mm × 150 µm clear aperture), capable of a ±2° sweep at 1 kHz with an applied voltage less than 200 V. This paper reports on the design, fabrication, modeling and testing of such an electrostatically actuated MEMS mirror. Fabrication involves using a ⟨1 0 0⟩ n-type double side polished silicon wafer, along with surface and bulk micromachining techniques, to produce a mirror with nickel torsion springs and nickel electrostatic actuators. There were 8 masks and 20 processing steps required. The performance of these devices was measured and found to be within the required specifications. Analysis involved developing models to predict the dynamic behavior of these MEMS micromirrors. A basic parallel plate capacitor model was adjusted with finite element analysis to account for fringing fields. Young's modulus of the electroplated nickel was determined to be 110 GPa from a comparison of the model with the results of dynamic testing, and the residual strain test structures led to a value of 0.0029 for the residual strain. Each of these values is within the rather wide range of published values. Once these values were determined, the model agreed very well with the measurements of the dynamic angular response of the mirror.

Simulation of deep UV lithography with SU-8 resist by using 365 nm light source

The deep lithography of thick resist layer is the primary step of LIGA technology. UV Proximity lithography, which is used to fabricate high aspect ratio MEMS normally, is investigated in this paper. The light intensity distribution in the thick photoresist layer mainly depends on the diffraction produced by the gap between mask and wafer in proximity lithography. Fresnel diffraction model is used to simulate lithography process normally, which the thin photoresist layer is used in the lithography process. But it is not accurate in deep lithography process. A correction to the Fresnel diffraction theory is used to simulate the lithography process depending on the scalar diffraction theory of light propagation in this paper. The difference of this two models is given in this paper. The simulation results show that the corrected model can obtain more accurate simulation results than the Fresnel diffraction model. The experiment results and the theory analysis both indicate that: the structure contrast decreases with the increase of the film thickness. The smaller the structure linewidth is, the faster the structure contrast decreases. The linewidth of microstructure is not equivalent from the top to the bottom, broaden in the middle part normally. The theory simulation gives the quantitative analysis.

Metrology of high aspect ratio MEMS

Lack of metrology tools for inspecting high aspect ratio MEMS severely limits the degree to which tolerances of a given part can be examined. Tools such as SEMs, AFMs, vision-based systems, and profilometers are good at examining two-dimensional entities of a part or for calculating surface roughness characteristics. None of these tools, however, can extract full three-dimensional data sets of high aspect ratio MEMS for part inspection. The hardware is either limited by the steep sidewalls of the part or by the simple fact that the acquisition method only collects two-dimensional data. This research proposes a method for extracting three-dimensional information from a part using multiple two-dimensional pointclouds. A fiducial setup is proposed which would allow for the registration of multiple pointclouds. A computer-aided inspection (CAI) software platform has been developed to handle the multiple data sets. The software platform implements a least-squares localization routine to compute the best-fit deviations from the nominal CAD geometry, as well as algorithms to determine the correct alignment between two pointclouds. With this software platform, both form errors of a single pointcloud and geometric errors with respect to multiple pointclouds can be calculated.

LATERALLY ISOLATED POLYSILICON BEAM PROCESS

This paper describes a process technology to make MEMS devices with laterally integrated polysilicon beam as the structural material instead of single crystal silicon for sensors/ actuators. The process consists of fabricating deep silicon trenches, which are then seamlessly filled with the layers of thermal oxide and polysilicon. The polysilicon on the surface is etched back, leaving behind the polysilicon only inside the trenches. The beams are then released by isotropically etching away the silicon surrounding the polysilicon/ silicon oxide structure. This results in free standing polysilicon beams, which form the MEMS structures. The thermal oxide surrounding the polysilicon can be removed by wet HF etch or by isotropic plasma etch. This process can serve as a general platform for realising various types of high aspect ratio MEMS devices. The various stages of the process development, in realising a low stress polysilicon beams and electrical isolation structures are described in the paper.

Post-CMOS processing for high-aspect-ratio integrated silicon microstructures

Presents a new fabrication sequence for integrated-silicon microstructures designed and manufactured in a conventional complementary metal-oxide-semiconductor (CMOS) process. The sequence employs a post-CMOS deep silicon backside etch, which allows fabrication of high aspect ratio (25:1) and flat (greater than 10 mm radius of curvature) MEMS devices with integrated circuitry. A comb-drive resonator, a cantilever beam array and a z-axis accelerometer were fabricated using this process sequence. Electrical isolation of single-crystal silicon was realized by using the undercut of the reactive ion etch (RIE) process. Measured out-of-plane curling across a 120-/spl mu/m-wide 25-/spl mu/m-thick silicon released plate was 0.15 /spl mu/m, which is about ten times smaller than curl of the identical design as a thin-film CMOS microstructure. The z-axis DRIE accelerometer structure is 0.4 mm by 0.5 mm in size and has a 25-/spl mu/m-thick single-crystal silicon proof mass. The measured noise floor is 1 mG//spl radic/Hz, limited by electronic noise. A vertical electrostatic spring hardening effect was theoretically predicted and experimentally verified.

A single crystal silicon 3 dimensional processing technique with applications in large displacement electrostatic actuation

A new process has been developed to produce mechanical structures of multiple levels on a silicon substrate. The process lends itself to high aspect ratio bulk micromachining of single crystal silicon. Structures can be fabricated at multiple levels with a single lithography step and without alignment requirements. Any required difference in height between levels is defined precisely at intermediate etch steps. One application of this process utilizes the out of plane asymmetries produced by the multiple levels to create a large displacement, large force and highly linear out of plane actuator stage. This technique is easily combined with single level high aspect ratio MEMS processing methods.

Confocal optoelectronic holography microscope for materials and structural characterization of MEMS

ABSTRACTIn this paper, we describe confocal optoelectronic holography microscopy (COEHM) technique specifically being developed for characterizing the shape of MEMS and microelectronics. This is particularly important because shape is directly related to the functionality, performance, and integrity of the microstructures of interest. A specific feature of COEHM is that it allows characterization of high aspect ratio MEMS and microelectronics. Representative applications demonstrating the capabilities of COEHM are presented. It is shown that measurement resolution is highly dependent on the numerical aperture (NA) of the optical components comprising COEHM and on the quality of image digitization. By utilizing optical components characterized by a magnification factor of 50x and a NA of 0.45, measurement resolution of 25 nm is achieved. The resolution is increased to 8 nm when utilizing optical components characterized by a magnification factor of 100x and a NA of 0.73.

Study on fabrication of high aspect ratio MEMS microparts using a compact SR beamline

A compact beamline dedicated to hard x-ray deep lithography for fabrication of high aspect ratio MEMS microparts has been developed. The exposure stage was only 3 meters away from the synchrotron radiation (SR) source so that a relatively high photon flux could be achieved with a compact super-conducting SR source. The deep lithography using PMMA resist could be as deep as 1000 μm and the maximum aspect ratio achieved was about 50. The throughout for the 200 μm-deep lithography was found to be on the order of 5 cm2/h using the membrane-free mask under the routine SR conditions. Templates with the high aspect ratio microstructures have been made of the PMMA resist based on conducting substrates and applied further to electroforming to create metallic microstructures. In order to fabricate microparts for the MEMS applications, we have concentrated on development of masks for the hard x-ray deep lithography. The masks now can be made to have the 8 μm-thick gold absorber on the 2 μm-thick SiC membrane.