PN结自致停精密腐蚀的研究

本文系统地研究了p-n结自致停精密腐蚀时的J-V特性，腐蚀自致停时电流变化的性状和实现这种精密腐蚀的关键工艺，同时也探讨了这种自致停腐蚀样品表面粗糙度、腐蚀速率与工艺参数及其他条件的关系，目的在使其能成为一种实用化的技术。p-n结自致停腐蚀克服了p^ 自致停腐蚀的缺点，且又保持了硅各向异性腐蚀的特性。     

ProTEK PSB coating as an alternative polymeric protection mask for KOH bulk etching of silicon

The utilization of a newly developed photosensitive polymeric coating, ProTEK PSB plays a significant role in realizing simple process steps in the fabrication of MEMS devices using bulk micromachining technology. The photosensitive coating which serves as an alternative to the conventional silicon nitride mask of bulk potassium hydroxide (KOH) etching in devising MEMS devices, particularly in suspended microcantilever structure, is reported in this study. Although the polymeric coating ProTEK PSB acts as an excellent outer protective layer from any pinhole issues, the coating’s lateral etching in the KOH solution is dominant, which results in an undercut problem. Therefore, few investigations have been carried out to identify the most suitable condition for the ProTEK PSB deposition on Si substrate. Initial investigation was done on the effect of Si surface modification on the stability of the ProTEK PSB in KOH etching. It was observed that the surface treatment may reduce the undercut ratio for a short period of KOH etching. However, for the extended hours, the surface treatment is not effective enough to improve the stability of the polymeric coating. Therefore, combinations of ProTEK PSB on three substrates were studied in order to obtain a minimum undercut to etch depth ratio of the polymeric coating in KOH bulk etching. The study showed that the combination of ProTEK PSB patterned on thermal oxide results in the most effective etching condition attributed by minimum undercut ratio. Further investigation was carried out on the effect of the KOH etching concentration on the stability of the ProTEK PSB coating over the long hours of bulk etching process. Three concentrations of KOH etchants, KOH 20 wt%, KOH 45 wt% and KOH with isopropyl alcohol (KOH + IPA) were investigated. The results showed that the stability of the polymeric coating was excellent in KOH 20 wt% concentration with a very minimal undercut ratio of 0.05–0.07. In conclusion, the utilization of the polymeric coating ProTEK PSB serves as an alternative etch mask in KOH wet etching which offers simpler and cheaper device fabrication in bulk micromachining technology.     

Comparison of etch characteristics of KOH, TMAH and EDP for bulk micromachining of silicon (110)

Bulk micromachining in Si (110) wafer is an essential process for fabricating vertical microstructures by wet chemical etching. We compared the anisotropic etching properties of potassium hydroxide (KOH), tetra-methyl ammonium hydroxide (TMAH) and ethylene di-amine pyro-catechol (EDP) solutions. A series of etching experiments have been carried out using different etchant concentration and temperatures. Etching at elevated temperatures was found to improve the surface quality as well as shorten the etching time in all the etchants. At 120°C, we get a smooth surface (Ra?=?21.2?nm) with an etching rate 12.2???m/min in 40wt% KOH solution. At 125°C, EDP solution (88wt%) was found to produce smoothest surface (Ra?=?9.4?nm) with an etch rate of 1.8???m/min. In TMAH solution (25wt%), the best surface roughness was found to be 35.6?nm (Ra) at 90°C with an etch rate of 1.18???m/min. The activation energy and pre-exponential factor in Arrhenius relation are also estimated from the corresponding etch rate data.     

基于金硅腐蚀自停止技术的亚微米梁制作研究

提出了利用无电极电化学腐蚀自停止技术制作亚微米梁结构的新工艺方法。根据金硅腐蚀自停止现象发生的条件,结合硅材料的各向异性腐蚀特性,设计器件结构,利用腐蚀暴露面积变化实现了硅的选择性自停止腐蚀。在(111)型硅片上利用原电池钝化效应一次性腐蚀出与衬底绝缘,由约4μm厚的金电极支撑,厚度约为235 nm的亚微米梁结构,具有制作简单、成品率高、成本低等特点,应用前景广阔。     

New 3-D Structures Fabricated on Si (hkl) Substrates by Bulk Micromachining

The paper deals with the new 3D structures fabricated by the bulk micromachining of (110), (112), and (522) silicon substrates. The structures employ a specific arrangement of {111} planes on these substrates and are entirely bounded by these slowly etching planes. Design rules and complete structures of new seismic-mass systems, suspended on two or four beams, composed of the {111} planes, are presented. The beams supporting the masses are inclined toward the substrate at different angles, which can be adjusted by an appropriate selection of crystallographic orientation of the etched substrate. The structures seem to be interesting as structural components of multiaxes accelerometers. Slanted membranes fabricated by the double-sided etching of (112) and (552) substrates have also been presented. The structures utilize the {111} planes, inclined at a low angle toward the etched substrate, both as structural elements, as well as a natural etch stop. It can be claimed that the application of Si substrates with unconventional crystallographic orientations opens new possibilities in the micromachining of 3D structures.     

Comparison of etch characteristics of KOH,TMAH and EDP for bulk micromachining of silicon (110)

Bulk micromachining in Si (110) wafer is an essential process for fabricating vertical microstructures by wet chemical etching. We compared the anisotropic etching properties of potassium hydroxide (KOH), tetra-methyl ammonium hydroxide (TMAH) and ethylene di-amine pyro-catechol (EDP) solutions. A series of etching experiments have been carried out using different etchant concentration and temperatures. Etching at elevated temperatures was found to improve the surface quality as well as shorten the etching time in all the etchants. At 120°C, we get a smooth surface (Ra = 21.2 nm) with an etching rate 12.2 μm/min in 40wt% KOH solution. At 125°C, EDP solution (88wt%) was found to produce smoothest surface (Ra = 9.4 nm) with an etch rate of 1.8 μm/min. In TMAH solution (25wt%), the best surface roughness was found to be 35.6 nm (Ra) at 90°C with an etch rate of 1.18 μm/min. The activation energy and pre-exponential factor in Arrhenius relation are also estimated from the corresponding etch rate data.

     

Bulk micromachining fabrication platform using the integration of DRIE and wet anisotropic etching

This study presents a bulk micromachining fabrication platform on the (100) single crystal silicon substrate. The fabrication platform has employed the concept of vertical corner compensation structure and protecting structure to integrate the wet anisotropic etching and DRIE processes. Based on the characteristics of wet anisotropic etching and DRIE, various MEMS components are demonstrated using the bulk micromachining platform. For instance, the free suspended thin film structures and inclined structures formed by the {111} crystal planes are fabricated by the wet etching. On the other hand, the mesas and cavities with arbitrary shapes and the structures with different leve l heights (or depths) are realized by the characteristics of DRIE. Since the aforementioned structures can be fabricated and integrated using the presented fabrication platform, the applications of the bulk micromachining processes will significantly increase.This research is based on the work supported by WALSIN LIHWA Corporation and the National Science Council of Taiwan under grant of NSC-91–2218-E-007–034. The authors would like to thank the Central Regional MEMS Research Center of National Science Council, Semiconductor Research Center of National Chiao Tung University and National Nano Device Laboratory for providing the fabrication facilities.     

Bulk silicon holding structures for mounting of optical fibers inv-grooves

A set of micromachined structures for holding optical fibers in anisotropically etched v-grooves has been produced. The structures are made of bulk silicon and formed in the same etch step as the aligning v-grooves, using the photovoltaic electrochemical etch-stop technique (PHET). It is a selective etch method where n-type silicon etches and p-type are passivated by combining an illuminated pn-junction and an electrochemical cell using KOH as electrolyte. The structures were produced in a variety of shapes, based on cantilever beams and doubly clamped bridges. The structures substantially facilitated the mounting of the fibers into the v-grooves. Certain structures could even push the fibers down into position in the grooves     

Micromachining of (hhl) silicon structures: experiments and 3D simulation of etched shapes

The micromachining of various (hhl) silicon plates in a 35% KOH-water etchant is studied. Experimental shapes for membranes and mesa etched with initially circular masks are discussed. Theoretical 3D etched shapes for such microstructures are derived from a numerical simulation based on the tensorial model for the anisotropic wet etching. Experimental and theoretical shapes show a fair agreement, indicating a satisfactory adjustment of the dissolution slowness surface related to the etching of silicon in KOH etchant. The interest of the 3D simulation for designing mask patterns is outlined.     

Single mask fabrication process for movable MEMS devices

The current work reports on the realization of movable micromachining devices using self-aligned single-mask fabrication process. Only dry etching process utilizing inductively coupled plasma reactive ion etching was used to release 3D micro structures from single crystal silicon substrate. No wet etching process is required to release the structures as is the case with silicon on insulator (SOI) wafers. Also the developed process does not require an SOI substrate and accordingly dispensing with the application of a wet etching step, thus yielding uniform structures without stiction. The optimized process was applied to realize thermally actuated microgrippers. The article presents the development of the fabrication process and demonstrates the operation of the fabricated device. The optimized process provides an avenue for low cost fabrication of movable micromachining devices without the use of complicated wet etching steps typically associated with SOI substrates.     

Integrated polysilicon and DRIE bulk silicon micromachining for anelectrostatic torsional actuator

This paper presents a fabrication process that integrates polysilicon surface micromachining and deep reactive ion etching (DRIE) bulk silicon micromachining. The process takes advantage of the design flexibility of polysilicon surface micromachining and the deep silicon structures possible with DRIE. As a demonstration, a torsional actuator driven by a combdrive moving in the out-of-plane direction, consisting of polysilicon fingers and bulk silicon fingers, has been fabricated. The integrated process allows the combdrive to be integrated with any structure made by polysilicon surface micromachining     

Fabrication and characterization of truly 3-D diffuser/nozzlemicrostructures in silicon

We present microfabrication and characterization of truly three-dimensional (3-D) diffuser/nozzle structures in silicon. Chemical vapor deposition (CVD), reactive ion etching (RIE), and laser-assisted etching are used to etch flow chambers and diffuser/nozzle elements. The flow behavior of the fabricated elements and the dependence of diffuser/nozzle efficiency on structure geometry has been investigated. The large freedom of 3-D micromachining combined with rapid prototyping allows one to characterize and optimize diffuser/nozzle structures     

Manufacturing costs for microsystems/MEMS using high aspect ratio microfabrication techniques

The emphasis on high aspect ratio micromachining techniques for microsystems/MEMS has been mainly to achieve novel devices with, for example, high sensing or actuation performance. Often these utilize deep structures (100–1,000 μm) with vertical wall layers but with relatively modest spatial resolution (1–10 μm). As these techniques move from research to industrial manufacture, the capital cost of the equipment and the cost of device manufacture become important, particularly where more than one micromachining technique can meet the performance requirements. This paper investigates the layer-processing costs associated with the principal high aspect ratio micromachining techniques used in microsystems/MEMS fabrication, particularly silicon surface micromachining, wet bulk etching, wafer bonding, Deep Reactive Ion Etching, excimer laser micromachining, UV LIGA and X-ray LIGA. A cost model (MEMSCOST) has been developed which takes the financial, operational and machine-dependent parameters of the different manufacturing techniques as inputs and calculates the layer-processing costs at the wafer and chip level as a function of demand volume. The associated operational and investment costs are also calculated. Cost reductions through increases in the wafer size and decreases in chip area are investigated, and the importance of packaging costs demonstrated.     

A novel <Emphasis Type="Italic">x-</Emphasis>axis tuning fork gyroscope with “8 vertical springs-proofmass” structure on (111) silicon

A novel x-axis tuning fork gyroscope with “8 vertical springs-proofmass” structure is presented. Wafer-thick proofmasses are made out of (111) silicon with bulk micromachining processes to achieve lower thermo-mechanical noise. Each proofmass is supported by 8 vertical springs, which are symmetrically distributed around the proofmass. The dimensions of 8 vertical springs are precisely confined by thermal oxide protected sidewalls and the extreme slowly etched (111)-planes in KOH etching. A mode mismatch of less than 30 Hz is achieved before tuning. Initial test shows a sensitivity of 0.15 mV/(deg/s) and a rate resolution around 0.1 deg/s under atmosphere pressure.     

Dry release for surface micromachining with HF vapor-phase etching

A new method for dry etching of silicon dioxide for surface micromachining is presented to obtain very compliant polysilicon microstructures with negligible stiction problem and to greatly simplify the overall releasing procedure as well. By etching the sacrificial silicon dioxide with hydrofluoric acid (HF) vapor instead of conventional aqueous HF solution, the need for subsequent rinsing and an elaborate drying procedure is eliminated. Condensation of water on the etch surface is first identified as the cause that prevented the success of HF vapor release in the past. Use of an anhydrous HF/CH₃OH mixture under low pressure solves the problem of water condensation and enables us to take advantage of vapor-phase etching (VPE) for surface micromachining. The mechanism of oxide etching with the HF/CH₃OH mixture is explained, and the developed VPE system is described and characterized. Polysilicon cantilevers up to 1200 μm in length are successfully released with this HF VPE technique. The beams tested are 2 μm thick with a 2-μm gap from the substrate, and no antistiction dimples are used. The fabricated structures are observed using both scanning electron microscopy (SEM) and an optical profilometer. The reported VPE technique provides a robust releasing method for polysilicon microstructures and is compatible with integrated circuit (IC) fabrication, even including cluster processors     

Comparative study of spin coated and sputtered PMMA as an etch mask material for silicon micromachining

SiO₂ and Si₃N₄, are usually used to mask the selected portions during etching of silicon in anisotropic etchants like KOH but polymers are expected to be very good alternative to SiO₂ and Si₃N₄ as masking materials for MEMS applications. An adherent spin coated PMMA layer is reported to work as a mask material. It is a low temperature process, cheaper and films can be easily deposited and removed. One of the problems in its use is its adhesion to the substrate. Our previous experience in the field made us feel that sputtered PMMA will act as better mask because of its better adhesion to silicon. In the present article, a comparative study of spin coated PMMA with sputtered PMMA as an etch mask for silicon micromachining is reported. Structural and adhesive characteristics of the films are determined and compared with those available in the literature. These films deposited on silicon wafer were exposed to anisotropic etchant, KOH, to estimate the masking behavior. The maximum masking time of 32 min in 20 wt.% KOH at 80 °C was obtained for spin coated PMMA samples, which were prebaked at 90 °C. Masking time of sputter deposited PMMA films was found to be 300 min under similar conditions such as 20 wt.% KOH at 80 °C. This masking time is sufficient for fabrication of various MEMS structures, thus indicating candidature of sputtered PMMA as masking material. Various properties of the films are discussed and compared with the ones obtained through literature.     

A novel method to fabricate single crystal nano beams with (111)-oriented Si micromachining

Single crystal beams made by (111)-oriented Si micromachining are usually anchored to the substrate directly. It is difficult to use the beams as resonators, since they are electrically connected to the substrate. This paper presents a modified process to fabricate single crystal nano beams which are electrically isolated from the substrate. In this process, the single crystal nano beams are fully released from the substrate and mechanically supported by metal wires, which also serve as electrical connections. The metal wires are much stiffer than the beams and do not degrade the mechanical properties of the beam according to simulations. The length and width of the beams are determined by photolithography. The thickness of the beam and the gap between the beam and the substrate are determined by the dry etching and KOH etching processes. The influence of KOH etching on the beam thickness is documented through ongoing experiments. At present, the double clamped beams and the cantilever beams have been fabricated. The thinnest beam to date was measured to be 47 nm. The resistance between the beam and the substrate was measured to be 214GΩ, while the resistance of a 147 nm-thick beam was measured to be 816 Ω. The surface roughness of the (111) plane is also discussed. The RMS surface roughness of the nano beam was measured to be 1.08 nm in an area of 5 μm × 5 μm, which was etched with 45%wt. KOH at 50°C.     

A new process for releasing micromechanical structures in surface micromachining with polysilicon support and LPCVD Si3N4 embedded mask

This paper presents a new process for releasing micromechanical structures in surface micromachining with polysilicon support and LPCVD (low pressure chemical vapor deposition) Si₃N₄ embedded mask for one polysilicon layer process, which can be adjusted to be suitable for the structure stiffness by changing the distance between two supports. The results of test structures show that this process may be a good technology to eliminate the sticking of microstructures to the substrate during the wafer drying after the sacrificial etching process.     

Surface micromachining of polycrystalline SiC films usingmicrofabricated molds of SiO2 and polysilicon

As an alternative to conventional SiC reactive ion etching (RIE), polycrystalline (poly-SiC) films were patterned into micron-sized structures using sacrificial SiO₂ and polycrystalline silicon (polysilicon) molds in conjunction with mechanical polishing. The molds were made from thermally grown SiO₂ and LPCVD polysilicon films and were fabricated using conventional patterning techniques. The poly-SiC micromolding process combines film deposition, polishing, and selective wet chemical etching of the molds to achieve the desired pattern. The process is simple and does not suffer from the difficulties associated with RIE of SiC. Micrometer sized lines, spaces, and complex device structures have been patterned using this technique. The micromolding technique has been used in a SiC surface micromachining process to fabricate fully released lateral resonant structures     

Copper-encapsulated silicon micromachined structures

Selective copper encapsulation on silicon has been used to fabricate micromachined devices such as inductors with quality factors over 30 at frequencies above 5 GHz. The devices are fabricated using either polysilicon surface micromachining, or integrated polysilicon and deep reactive ion etching bulk silicon micromachining. Their exposed silicon surfaces are selectively activated by palladium activation, which allows the subsequent copper deposition on the activated silicon surfaces only. This silicon encapsulated-with-copper technique takes advantage of both the excellent mechanical properties of silicon (to maintain structural integrity), and the high conductivity of copper (for electrical signal transmission). Furthermore, the process not only minimizes interfacial forces typical of physical metal deposition on silicon, but also balances the forces by metal encapsulation on all sides of the silicon structures