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     

High-resolution long-array thermal ink jet printhead fabricated by anisotropic wet etching and deep Si RIE

This paper describes the fabrication and characterization of a thermal ink jet (TIJ) printhead suitable for high speed and high-quality printing. The printhead has been fabricated by dicing the bonded wafer, which consists of a bubble generating heater plate and a Si channel plate. The Si channel plate consists of an ink chamber and an ink inlet formed by KOH etching, and a nozzle formed by inductively couple plasma reactive ion etching (ICP RIE). The nozzle formed by RIE has squeezed structures, which contribute to high-energy efficiency of drop ejector and, therefore, successful ejection of small ink drop. The nozzle also has a dome-like structure called channel pit, which contributes to high jetting frequency and high-energy efficiency. These two wafers are directly bonded using electrostatic bonding of full-cured polyimide to Si. The adhesive-less bonding provided an ideal shaped small nozzle orifice. Use of the same material (Si substrate) in heater plate and channel plate enables the fabrication of high precision long printhead because no displacement and delamination occur, which are caused by the difference in thermal expansion coefficient between the plates. With these technologies, we have fabricated a 1" long printhead with 832 nozzles having 800 dots per inch (dpi) resolution and a 4 pl. ink drop volume.     

Thick glass film technology for polysilicon surface micromachining

This paper explores the use of thick glass films as suitable alternatives to CVD oxide films for use as sacrificial, planarization, and passivation layers in polysilicon surface micro-machining processes. Such glasses can be spin-coated to produce films up to 20 μm thick in one step and to globally planarize the wafer surface, extending the overall mechanical design capability by enabling additional device structural complexity. Glass optical constants were determined, and the film quality was evaluated using SEM, EDS, XPS, and XRD. The films were found to have low intrinsic stresses and other characteristics desirable for sacrificial layer applications. A glass chemical-mechanical polishing process with 5300~Å/min removal rate and acceptable selectivity to polysilicon was developed, along with a wet etch chemistry that preferentially etches the film at 3.24 μm/min without affecting the silicon substrate or the structural polysilicon. The film was used to planarize up to 10-μm-tall topographies associated with surface micromachined features through spin-on and polish-back steps, and was in addition demonstrated to be a viable protective layer for silicon wafers during extended KOH etching in silicon bulk micro-machining processes. The glass has stable constituents that do not diffuse or contaminate either the substrate or the device features during the application and firing procedures     

Process yields for laser repair of aged, stiction-failed, MEMSdevices

Stiction, the adhesion of micromachined components to each other or the substrate, decreases production yields and operational lifetimes for MEMS devices. A recent study demonstrated the feasibility of using a Nd:YAG, 1063 nm laser to repair 2 μm thick polycrystalline silicon microcantilevers with lengths up to 1000 μm which had adhered to the substrate. This investigation examines the influence of sample age at the time of laser irradiation on the repair of stiction-failed MEMS structures. The operating conditions for the 1064 nm, Nd:YAG laser included a fluence of 70 mJ/cm², a repetition rate of 20 Hz, a pulse duration of 20 ns, and an exposure time of 60 s. For samples irradiated on the same day of release, repair yields were 100% for beams up to 500 μm in length. For 1000 μm long beams, the 10- and 30-μm-wide cantilevers had same-day repair yields of 56% and 71%, respectively. The experimental results show that increasing the amount of time the microstructures are adhered to the substrate before laser irradiation decreases the ability to repair stiction-failed devices with delays of longer than 1 month resulting in a decreased repair yield     

Bent-beam electrothermal actuators-Part I: Single beam and cascadeddevices

This paper describes electrothermal microactuators that generate rectilinear displacements and forces by leveraging deformations caused by localized thermal stresses. In one manifestation, an electric current is passed through a V-shaped beam anchored at both ends, and thermal expansion caused by joule heating pushes the apex outward. Analytical and finite element models of device performance are presented along with measured results of devices fabricated using electroplated Ni and p⁺⁺ Si as structural materials. A maskless process extension for incorporating thermal and electrical isolation is described. Nickel devices with 410-μm-long, 6-μm-wide, and 3-μm-thick beams demonstrate 10 μm static displacements at 79 mW input power; silicon devices with 800-μm-long, 13.9-μm-wide, and 3.7-μm-thick beams demonstrate 5 μm displacement at 180 mW input power. Cascaded silicon devices using three beams of similar dimensions offer comparable displacement with 50-60% savings in power consumption. The peak output forces generated are estimated to be in the range from 1 to 10 mN for the single beam devices and from 0.1 to 1 mN for the cascaded devices. Measured bandwidths are ≈700 Hz for both. The typical drive voltages used are ⩽12 V, permitting the use of standard electronic interfaces that are generally inadequate for electrostatic actuators     

A micro active probe device compatible with SOI-CMOS technologies

A surface-micromachined active probe device with built-in electrostatic actuator and on-chip CMOS circuits is described. The device has been fabricated on a silicon-on-insulator (SOI) substrate using a 0.6-μm CMOS-based process containing four polysilicon layers and one metal layer, and its basic functionality has been verified experimentally. A 0.135-μm-thick surface silicon layer of an SOI substrate was used to form cantilever beams. The very thin structures enable a probe to be turned on at a voltage as low as several volts. A stopper structure, used to avoid contact between a deflector electrode and its paired stator electrode, was formed with a small overlap area of approximately 0.05 μm². The small overlap area results in a small adhesion force, approximately 70 nN. An n-p-n junction was exploited as an isolator in the probe. A p-n junction in a released beam had only a 5-pA leakage current at a 9-V reverse bias. In addition, it has been found that electrostatic charging is a major source causing unrestorable postrelease stiction     

Released Si microstructures fabricated by deep etching and shallowdiffusion

A novel etch-diffusion process is developed for fabricating high-aspect-ratio Si structures for microsensors. This is accomplished by first dry etching narrow gap Si microstructures using an electron cyclotron resonance (ECR) source, followed by a shallow B diffusion to fully convert the etched microstructures to p⁺⁺ layer. Microstructures up to 40 μm deep with 2-μm-wide gaps were etched with a Cl₂ plasma generated using the ECR source. Vertical profile and smooth morphology were obtained at low pressure. A shallow B diffusion at 1175°C for 5.5 h. was then carried out to convert the 40-μm-thick resonant elements to p⁺⁺ layer. A second dry etching step was used to remove the thin p⁺⁺ layer around the bottom of the resonant elements, followed by bonding to glass and selective wet etch. Released high-aspect-ratio Si microsensors with thicknesses of 35 μm have been demonstrated. At atmospheric pressure, only 5 V_dc driving voltage is needed for 2.5 μm vibration amplitude, which is less than the 10 V_dc required to drive 12-μm-thick resonators fabricated by conventional dissolved wafer process     

Photo-electroforming: 3-D geometry and materials flexibility in aMEMS fabrication process

Photo-electroforming is a new manufacturing process for making microelectromechanical systems (MEMS). Photo-electroforming builds parts by an additive process which defines geometry by depositing powder in layers and creating regions of selective conductivity by laser-enhanced electroless plating. The conductive region is then joined by a second plating to form an integral part. The unmetallized portion is removed by selective etching in one step after all layers are defined and joined. Single-layer and two-layer stand-alone parts made of nickel/silicon carbide composites of overall size 75-100 μm and feature size 25 μm were created. Writing speeds of up to 24 cm/s and in-plane resolution of 15 μm were demonstrated. The high laser-induced plating rates were found to be due to elevated substrate temperatures under the laser spot and enhanced mass transfer due to pumping by the hydrogen bubbles resulting from the plating reaction. In the regimes studied, mass transfer defined the rate limit. The in-plane resolution was found to be limited by a combination of laser spot size, thermal conduction in the substrate, and laser divergence due to gas bubbles     

Electrothermally activated paraffin microactuators

A new family of electrothermally activated microactuators that can provide both large displacements and forces, are simple to fabricate, and are easily integrated with a large variety of microelectronic and microfluidic components are presented. The actuators use the high volumetric expansion of a sealed, surface micromachined patch of paraffin heated near its melting point to deform a sealing diaphragm. Two types of actuators have been fabricated using a simple three mask fabrication process. The first device structure consists of a 9 μm thick circularly patterned paraffin layer ranging in diameter from 400 to 800 μm all covered with a 4-μm-thick metallized p-xylylene sealing diaphragm. All fabricated devices produced a 2.7-μm-peak center deflection, consistent with a simple first order theory. The second actuator structure uses a constrained volume reservoir that magnifies the diaphragm deflection producing consistently 3.2 μm center diaphragm deflection with a 3-μm-thick paraffin actuation layer. Microactuators were constructed on both glass and silicon substrates. The actuators fabricated on glass substrates used between 50-200 mW of electrical power with response times ranging between 30-50 ms. The response time for silicon devices was much faster (3-5 ms) at the expense of a larger electrical power (500-2000 mW)     

Lithography with high depth of focus by an ion projection system

Ion projection lithography is developed to generate structures with minimum feature sizes in the 100-nm range with a high pixel transfer rate. The high depth of focus (DOF) resulting from the telecentric beam path concept is also noteworthy. A silicon wafer exhibiting 200-μm-deep cavities, which are fabricated by anisotropic etching, is patterned with a grating of 0.6 μm periodicity running with identical spacings from the bottom to the top. SiO₂ serves as an inorganic ion sensitive resist. Exposed to 73 keV helium ions, SiO₂ shows an enhanced etching rate in hydrofluoric acid, the structure developing agent. The patterning techniques considered are promising for the fabrication of two-dimensional reflecting mirrors or sensoric elements distributed on spherical surfaces     

Fabrication and operation of polyimide bimorph actuators for aciliary motion system

In order to extract macroscopic mechanical work out of microelectromechanical systems, we have proposed the concept of distributed micromotion systems (DMMS). The key idea of DMMS is to coordinate simple motions of many microactuators in order to perform a task. Design, fabrication, and operation of a type of DMMS, called a ciliary motion system, are presented. A bimorph thermal actuator using two types of polyimides with different thermal expansion coefficients and a metallic microheater in between them was fabricated. The cantilever-shaped actuator curled up from the substrate owing to the residual stress in polyimides which built up during the cooling process after they were cured at 350°C. It flattened and moved downward by flowing current in the heater. The dimensions of the cantilever were 500 μm in length, 100 μm in width, and 6 μm in thickness. The tip of the cantilever moved 150 μm in the direction vertical to the substrate and 80 μm in the horizontal direction; these were the maximum displacements obtained with 33 mW dissipated in the heater. The cut-off frequency was 10 Hz. On a 1-cm-square substrate, 512 cantilevers were fabricated to form an array. Two sets of cantilevers were placed opposing to each other. We operated them in coordination to mimic the motion and function of cilia and carried a small piece of a silicon wafer (2.4 mg) at 27-500 μm/s with 4-mW input power to each actuator     

Post-CMOS processing for high-aspect-ratio integrated siliconmicrostructures

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-μm-wide 25-μm-thick silicon released plate was 0.15 μm, 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-μm-thick single-crystal silicon proof mass. The measured noise floor is 1 mG/√Hz, limited by electronic noise. A vertical electrostatic spring "hardening" effect was theoretically predicted and experimentally verified     

Fabrication of the monolithic polymer–metal microstructure by the backside exposure and electroforming technology

Monolithic polymer–metal microstructures can be fabricated on the silicon or glass substrate using two kinds of photoresists and electroforming technologies for the inkjet and microfluidic application. However, it suffers from the high shrinkage problem of first SU8 resist after exposure and post exposure baking. This paper reports a novel approach to solve the shrinkage problem by introducing backside exposure of first SU8 resist for the fabrication of the monolithic polymer–metal microstructure. In combination with the light absorption layer coating on the unexposed SU8 resist, metal seed layer deposition, frontside exposure for second JSR resist on the seed layer and the nickel (Ni) electroforming together with release process, we have demonstrated a high physical resolution of 1,200 dpi monolithic Ni nozzle plate with negligible shrinkage. It also has the advantages of low cost and high resolution for the improvement of the traditional bonding of polymer and metal nozzle plate, which is generally in need of a complex alignment to stick the metal nozzle plate and dry film polymer on the heating chip together.     

Fabrication of silicon and oxide membranes over cavities usingion-cut layer transfer

Silicon and oxide membranes were fabricated using an ion-cut layer transfer process, which is suitable for sub-micron-thick membrane fabrication with good thickness uniformity and surface micro-roughness. After hydrogen ions were implanted into a silicon wafer, the implanted wafer was bonded to another wafer that has patterned cavities of various shapes and sizes. The bonded pair was then heated until hydrogen-induced silicon layer cleavage occurred along the implanted hydrogen peak concentration, resulting in the transfer of the silicon layer from one wafer to the other. Using this technique, we have been able to form sealed cavities and channels of various shapes and sizes up to 50-μm wide, with a 1.6-μm-thick silicon membrane. As a process variation, we have also fabricated silicon dioxide membranes for optically transparent applications     

Fabrication of metallic microstructures by electroplating usingdeep-etched silicon molds

A new technology is presented here to fabricate three-dimensional micromachined metal structures. The microstructures are manufactured by electroplating in deep-etched silicon structures followed by a separation from their mold. Up to 140-μm-deep silicon structures with vertical sidewalls are realized by an anisotropic plasma etching process producing the mold for electroplating. An etching gas mixture of SF_6s and CBrF₃ is used to achieve both an anisotropic etching behavior by protective film formation of CF₂ -radicals and high etching rates. The anisotropy is due to photoresist masking, which enhances the polymer formation. The vertical trenches are electroplated from the trench base filling the structures uniformly to the substrate surface. By avoiding overplating across the whole substrate the resulting structures are suitable for micromechanical devices. If needed, released microstructures from the silicon mold can be obtained by direct lift-off     

基于金硅原电池保护电容加速度计的制作

提出并实现了一种利用SoI结合金硅原电池保护和反熔丝制作电容式加速度计的新工艺方法。该工艺用SoI顶层硅制作梁和上电极,用衬底制作质量块。采用DRIE从正面刻蚀形成释放孔,TMAH腐蚀实现质量块的释放,在TMAH腐蚀过程中利用金硅原电池保护实现梁和表面极板的保护。在TMAH腐蚀完成前,反镕丝保持断开状态,腐蚀完成后,击穿反镕丝形成导通状态。通过测量金和硅的极化曲线得到60℃25%TMAH中实现原电池保护的金硅面积比不小于5∶1。成功制作成电容式加速度计结构,释放前后梁宽度均在9.4～10μm范围内,表明原电池保护有效。击穿后反熔丝并联导通电阻为5～25 kΩ之间。     

Deflection and maximum load of microfiltration membrane sieves madewith silicon micromachining

With the use of silicon micromachining, an inorganic membrane sieve for microfiltration has been constructed having a silicon nitride membrane layer with thickness typically 1 μm and perforations typically between 0.5 μm and 10 μm in diameter. As a support a 〈100〉-silicon wafer with openings of 1000 μm in diameter has been used. The thin silicon nitride layer is deposited on an initially dense support by means of a suitable chemical vapor deposition method (LPCVD). Perforations in the membrane layer are obtained with use of standard photo lithography and reactive ion etching (RIE). The deflection and maximum load of the membrane sieves are calculated in a first approximation. Experiments to measure the maximum load of silicon-rich silicon nitride membranes have confirmed this approximation     

Geometrical modulation-based interferometry for displacementsensing using optically coupled suspended waveguides

A geometrical modulation-based interferometry (GMI) for a displacement sensor is presented. The implementation of the GMI is based on the suspended optical waveguide displacement sensors (SOWDSs) technology. The interferometry effect of the GMI results from light propagating in geometrically modulated and mutually coupled suspended waveguides with an in-plane degree of freedom. The building block of the suspended waveguides is a single-crystal silicon (SCS) beam with superficial layers comprising a 0.6-μm-thick SiO₂, 0.4-μm-thick Si₃N₄, and 0.6-μm-thick SiO₂. The SCS beam is fabricated with a cross section of 1.6 μm×10 μm and may guide light with wavelength in the 1.3-1.5-μm range. The first SiO₂ layer serves as a buffer layer that allows light with wavelength in the 0.6-0.9-μm range to be guided in the Si₃N₄ layer. This paper discusses the theoretical consideration and the characterizations of a GMI displacement sensor     

太阳电池超细栅电极喷印控制系统设计与实现

针对传统丝网印刷技术制造太阳电池金属化栅极容易造成基材破损,且制造的栅极精度和高宽比很难提高的问题,采用喷墨技术直接将纳米银墨水打印到太阳电池基材上实现栅极金属化,并设计实现了太阳电池栅极喷印样机系统．样机系统通过USB模块实现高速数据通信;两组动态随机存储器（SDRAM）内存模块构成乒乓操作,实现不间断打印;喷印控制模块采用现场可编程门阵列（FPGA）产生喷头复杂时序波形和压电驱动波形．基于流体体积法建立了微液滴喷射模型,研究了微滴喷射机制及压电驱动波形幅值对微液滴尺寸、速度一致性的影响,并进行实验验证．通过优化喷印分辨率和在线固化温度进行系统测试,结果表明,多晶硅硅片固化温度80 ℃条件下,随着喷印层数的增加,栅极高度线性增加,喷印一层大约增高0.5 μm,栅极宽度基本维持在35～40 μm,打印层数60～80层时,形成三维形貌均匀的具有高“高宽比”的栅极．     

Silicon-processed microneedles

A combination of surface- and bulk-micromachining techniques is used to demonstrate the feasibility of fabricating microhypodermic needles. These microneedles, which may be built with on-board fluid pumps, have potential applications in the chemical and biomedical fields for localized chemical analysis, programmable drug-delivery systems, and very small, precise sampling of fluids. The microneedles are fabricated in 1, 3, and 6 mm lengths with fully enclosed channels formed of silicon nitride. The channels are 9 μm in height and have one of two widths, 30 or 50 μm. Access to the channels is provided at their shank and distal ends through 40-μm square apertures in the overlying silicon nitride layer. The microneedles are found to be intact and undamaged following repetitive insertion into and removal from animal-muscle tissue (porterhouse steak)