Fabrication Of Micromechanical Structures Research Articles

Deposition of Low Stress Silicon Nitride Thin Film and Its Application in Surface Micromachining Device Structures

Surface machining processes are responsible for creating microstructures that reside near the surfaces of a substrate and are characterized by the fabrication of micromechanical structures from deposited thin films. These films can be selectively removed to build three-dimensional structures whose functionality typically requires that they should be freed from the planar substrate. Silicon nitride thin film is one of these important materials. In this paper, by adjusting the SiH2Cl2/NH3gaseous ratio, low stress silicon nitride (LS SiN) is deposited by the low pressure chemical vapor deposition (LPCVD) process. The internal stress generally in 135 MPa has been detected using an FLX-2320 film stress tester. Based on the wide application in surface micromachining devices, the mechanical properties of LS SiN are measured by nanoindentation, giving the value of Young’s modulus of 224 GPa and the hardness of 22.5 GPa, respectively. Dry etching and wet etching are utilized to fabricate the LS SiN thin film for structural layers. The etching rate compared with normal Si3N4film by LPCVD is demonstrated for silicon chip manufacture.

Fabrication of micromechanical structures on substrates selectively etched using a micropatterned ion-implantation method

An advanced micromachining technique using ion implantation to modify materials was studied. Gold ion implantation into silicon decreased the etching rate when the silicon was etched in potassium hydroxide solution after the ion implantation; the implanted region remained, thus forming the microstructure. Observation of the cross-section of the resulting etched structure by transmission electron microscopy showed that the structure was made only from the ion-implanted region, and that gold was precipitated on the surface. To clarify the mechanism involved in the decrease in the etching rate, we varied the etching conditions. Our results show that precipitation of implanted gold on the surface decreased the etching rate, because solubility of gold is lower.

Adhesion and Friction Issues Associated With Reliable Operation of MEMS

A growing interest exists in developing technologies that use silicon and other electronic materials as mechanical materials. Using standard processes of the integrated-circuit industry, researchers have successfully fabricated miniature mechanical components (micromachines) such as membranes, gears, motors, pumps, and valves. The integration of miniaturized mechanical components with microelectronic components has spawned a new technology known as microelectromechanical systems (MEMS). It promises to extend the benefits of microelectronic fabrication to sensing and actuating functions. Early applications of this technology include the digital mirror display, which has of the order of 106 aluminum thin-film micromirrors fabricated on top of a complementary-metal-oxide-semiconductor static random-access-memory integrated circuit. Other applications include integrated accelerometers for tasks such as air-bag deployment.A number of fabrication techniques have been developed for this technology and have been reviewed elsewhere. In this review, I focus on surface-micromachining technology and adhesion and friction problems in surface-micromachined polycrystalline silicon (polysilicon) structures, though many of the principles discussed will also apply both to single-crystalline silicon and nonsilicon-based structures. Surface micromachining, defined as the fabrication of micromechanical structures from deposited thin films, is one of the core technological processes underlying MEMS. Surface microstructures have lateral dimensions of 50-500 μm with thicknesses of 0.1–2.5 μm and are offset 0.1–2 μm from the substrate. The basic steps in a surface-micromachining process appear in Figure 1. First the substrate is typically coated with an isolation layer (Figure la) that protects it during subsequent etching steps. A sacrificial layer is then deposited on the substrate and patterned. For simplicity, Figure 1b shows that the opening of the sacrificial layer is terminated on the isolation layer. The microstructural thin film is then deposited and etched (Figure 1c). Finally selective etching of the sacrificial layer creates the freestanding micromechanical structures such as the cantilever beam shown in cross section in Figure 1d. The technique can be extended to make multiple-layer microstructures.

A new contactless electrochemical etch-stop based on a gold/silicon/TMAH galvanic cell

Fabrication of micromechanical structures with a reproducible thickness is usually accomplished by electrochemically controlled etching. This method requires an external contact to the n-type silicon to be passivated. The contact is usually accomplished by a wafer holder, which may introduce stress into the wafer and complicate batch fabrication. A new technique based on a gold/silicon/TMAH galvanic cell is presented. The passivation potential and current are generated internally, eliminating the need for external contacts and making the technique more suitable for batch fabrication. The theory of operation, experimental results and an application of the technique are presented in this paper.

Surface micromachining for microelectromechanical systems

Surface micromachining is characterized by the fabrication of micromechanical structures from deposited thin films. Originally employed for integrated circuits, films composed of materials such as low-pressure chemical-vapor-deposition polycrystalline silicon, silicon nitride, and silicon dioxides can be sequentially deposited and selectively removed to build or machine three-dimensional structures whose functionality typically requires that they be freed from the planar substrate. Although the process to accomplish this fabrication dates from the 1960's, its rapid extension over the past few years and its application to batch fabrication of micromechanisms and of monolithic microelectromechanical systems (MEMS) make a thorough review of surface micromachining appropriate at this time. Four central issues of consequence to the MEMS technologist are: (i) the understanding and control of the material properties of microstructural films, such as polycrystalline silicon, (ii) the release of the microstructure, for example, by wet etching silicon dioxide sacrificial films, followed by its drying and surface passivation, (iii) the constraints defined by the combination of micromachining and integrated-circuit technologies when fabricating monolithic sensor devices, and (iv) the methods, materials, and practices used when packaging the completed device. Last, recent developments of hinged structures for postrelease assembly, high-aspect-ratio fabrication of molded parts from deposited thin films, and the advent of deep anisotropic silicon etching hold promise to extend markedly the capabilities of surface-micromachining technologies.

Recent Advances in Surface Micromachining

Surface micromachining is the fabrication of micromechanical structures from deposited thin films. Although the process dates to the 1960s, this paper will survey its rapid extension over the past few years and its application to batch-fabrication of micromechanisms and of monolithic micro electromechanical systems (MEMS). Two central issues in surface micromachining are: (i) the understanding and control of the material properties of microstructural films, such as polycrystalline silicon, and (ii) the release of the microstructure, for example by wet-etching silicon dioxide sacrificial films, followed by the drying and surface passivation of the microstructure. Recent developments in hinged structures for post-release assembly and high-aspect ratio fabrication of molded parts from deposited thin films promise to extend greatly the capabilities of surface micromachining technology.

Micromechanics compatible with an 0.8 μm CMOS process

Results on a new approach for the monolithic integration of micromechanics and electronics are presented. The same process steps and layers can be used for both the mechanical and the electronic elements. This is a promising process concept for integrated microelectromechanical systems with a minimum of additional costs compared to the standard electronics process. Results on the fabrication of micromechanical structures using the process steps of a 0.8 μm CMOS process are given. The sublimation technique is used to dry the released surface-micromachined structures and different sublimating chemicals are compared. The influences of polycrystalline and amorphous deposition as well as diffusion doping and ion implantation on the strain gradients of the structural layers are presented. A fabricated electrostatically deflectable and capacitively detectable cantilever is presented and the dependence of the capacitance on the drive voltage is shown. By using the presented theory and fitting calculations to the measured non-linear characteristic, a value of 170 GPa for Young's modulus of the polysilicon film is extracted.

Reactive ion etching (RIE) techniques for micromachining applications

Abstract Plasma etching is important for the fabrication of micromechanical structures in silicon with fine feature size. In this paper two reactive ion etching (RIE) processes for given micromachining applications are described. The first is a multi-step RIE process for patterning of polysilicon/silicon nitride/polysilicon sandwiched structural layers, while stopping on the lower polysilicon layer. The first step of the process involves etching the top polysilicon layer using a gas mixture of 70 sccm CF4 + 10 sccm SF6 + 10 sccm O2. Then the gas mixture is changed, in situ, to 5 sccm CHF3+45 sccm N2 as the second step in order to etch the nitride layer, with an etching selectivity of 15 for nitride over the polysilicon and improved uniformity in the nitride etching. Therefore, the top two layers of the sandwich structure can be etched with good uniformity with the two-step process, while the integrity of the lower polysilicon layer is maintained. The second is a silicon trench etching process for applications involving refill with selective epitaxial growth (SEG) silicon, using 210 sccm CF4 with 10 sccm Cl2 addition. Crystal silicon can be etched anisotropically with vertical sidewall using this chemistry. It was found that by adding 10 sccm Cl2, both the etch rate and sidewall profile of the silicon trench are improved. Photoresist is shown to be essential to provide sufficient polymer-forming reaction product for sidewall protection. However, when the Cl2 addition exceeds 50 sccm, bowing and crevices appear in the sidewall profile.

Buried selectively etchable microstructures in GaAs using deep nitrogen implantation

Abstract Results are presented for deep implantation of nitrogen into n-type GaAs. The main purpose of these investigations is to clarify whether deep implantation can be a suitable process to create buried, selectively etchable layers for the fabrication of micromechanical structures in GaAs. The implanted layers have been characterized by XPS sputter profiling and electrical measurements using test Schottky diodes as well as selected etching techniques. XPS depth profiling shows the formation of a GaAs1-x N y layer with y 5%. The current/voltage characteristics show that implantation damage recovers to a certain extend after annealing at a temperature of 750°C.

Ion Beam Machining of Micromechanical Structures

ABSTRACTThe needs of high precision engineering currently call for dimensional tolerances of the order of 0.1μm and surface roughness of order 0.01μm, often in hard, brittle materials or materials of relatively poor machinability. The development of argon ion beam machining for milling, drilling and turning or lathing is reported as a means of satisfying these requirements. Highly controllable material removal is achieved for the fabrication of micromechanical structures, profiling of surfaces and fine surface finishing. Surface evolution at submicron precision is demonstrated, with particular reference to the production of both circular and non-circular sections in an “ion beam lathe”.