Surface Micromachining Method Research Articles

Fabrication of Monolithic Integrated Bimaterial Resonant Uncooled IR Sensor

In this work we studied the fabrication of a monolithic bimaterial micro-cantilever resonant IR sensor with on-chip drive circuits. The effects of high temperature process and stress induced performance degradation were investigated. The post-CMOS MEMS (micro electro mechanical system) fabrication process of this IR sensor is the focus of this paper, starting from theoretical analysis and simulation, and then moving to experimental verification. The capacitive cantilever structure was fabricated by surface micromachining method, and drive circuits were prepared by standard CMOS process. While the stress introduced by MEMS films, such as the tensile silicon nitride which works as a contact etch stopper layer for MOSFETs and releasing stop layer for the MEMS structure, increases the electron mobility of NMOS, PMOS hole mobility decreases. Moreover, the NMOS threshold voltage (Vth) shifts, and transconductance (Gm) degrades. An additional step of selective removing silicon nitride capping layer and polysilicon layer upon IC area were inserted into the standard CMOS process to lower the stress in MOSFET channel regions. Selective removing silicon nitride and polysilicon before annealing can void 77% Vth shift and 86% Gm loss.

Process-induced stress and hydrogen effects on monolithic integrated BiCMOS-MEMS resonators

In this article, the simultaneous fabrication of MEMS and Bi-CMOS components on a monolithic integrated micro-cantilever resonator is investigated. Surface micromachining method for cantilever fabrication has been merged with conventional CMOS process, and release of MEMS structure is conducted after CMOS process. We for the first time propose the additive effects of hydrogen and MEMS steps induced mechanical stress on the IC performance, and at the same time provide a re-adjusting method for integrated process design, in which the NMOS/PMOS threshold voltage is re-adjusted to symmetry by utilizing the MEMS-process-induced IC performance degradation. On-chip digital/analog Bi-CMOS circuits function properly as demonstrated. The proposed re-adjustment process, suitable for fabrications with a variety of design CMOS-MEMS integrated process flexibilities, has the potential to fabricate integrated devices (high quality factor variable capacitors, micromirror arrays and multi-axis gyroscopes etc.).

Photolithographic surface micromachining of polydimethylsiloxane (PDMS)

A major technical hurdle in microfluidics is the difficulty in achieving high fidelity lithographic patterning on polydimethylsiloxane (PDMS). Here, we report a simple yet highly precise and repeatable PDMS surface micromachining method using direct photolithography followed by reactive ion etching (RIE). Our method to achieve surface patterning of PDMS applied an O(2) plasma treatment to PDMS to activate its surface to overcome the challenge of poor photoresist adhesion on PDMS for photolithography. Our photolithographic PDMS surface micromachining technique is compatible with conventional soft lithography techniques and other silicon-based surface and bulk micromachining methods. To illustrate the general application of our method, we demonstrated fabrication of large microfiltration membranes and free-standing beam structures in PDMS.

Process-Induced Stress and Hydrogen Effects on Monolithic Integrated CMOS-MEMS Micro-Bimaterial Cantilever Sensor Array

Abstract A monolithic integrated bimaterial micro-cantilever resonant infrared sensor with on-chip auxiliary circuits was fabricated and tested. Surface micromachining method for cantilever fabrication has been merged with conventional CMOS process, and release of MEMS structure is conducted after CMOS process. Both the Cr/Au felling off the polysilicon cantilever during releasing process and the performance degradation of MOSFETs manufactured by standard CMOS or post-CMOS process are ascribed to the additive effects of hydrogen and mechanical stress. A novel post-CMOS process was raised to decrease the mechanical stress and to retain the metal layer of Cr/Au. On-chip digital/analog Bi-CMOS circuits function properly as demonstrated.

Integrated optical and MEMS based design process for a variable optical attenuator

Abstract This paper presents the design, fabrication and characterization of a Variable Optical Attenuator (VOA). The VOA is based on a device known as the Mechanical Anti-reflection Switch (MARS) that has been well studied for modulation applications. Therefore, the requirements of the MEMS device need to be freshly ascertained and are done so in this paper taking the optical and MEMS parameters into account. In addition, the effect of the MEMS parameters on the optical ones and vise versa were also exploited in the design process. A good VOA should have a linear variation of attenuation with voltage and a large change in attenuation with a small change in voltage. In addition, the device should have as small an insertion loss as possible. Keeping this in mind, the dimensions of the device were chosen to achieve maximum performance. Four design variables were used to design MARS for the VOA application. This paper will discuss how they are related. A surface micro-machining fabrication method was developed using thermal evaporation and Plasma Enhanced Chemical Vapor Deposition (PECVD) techniques. It includes a fluoroform (CHF3) based reactive ion etching for silicon nitride. The devices were released using low surface tension liquids and Rapid Thermal Annealing (RTA). The achieved process yield was over 90%. Several devices were tested and attenuation up to 5.6 dB was observed experimentally.

Comparison of residual stress measurement in thin films using surface micromachining method

Conductive, dielectric, semiconducting, piezoelectric and ferroelectric thin films are extensively used for MEMS/NEMS applications. One of the important parameters of thin films is residual stress. The residual stress can seriously affect the properties, performance and long-term stability of the films. Excessive compressive or tensile stress results in buckling, cracking, splintering and sticking problems. Stress measurement techniques are therefore essential for both process development and process monitoring. Many suggestions for stress measurement in thin films have been made over the past several decades. This paper is concentrated on the in situ stress measurement using surface micromachining techniques to determine the residual stress. The authors review and compare several types of stress measurement methods including buckling technique, rotating technique, micro strain gauge and long–short beam strain sensor.

A novel infrared absorbing structure for uncooled infrared detector

Abstract In this paper, we proposed a novel infrared absorbing structure for uncooled infrared detectors. The infrared absorber makes use of a quarter-wavelength structure composed of a dielectric layer, a protecting layer, an active layer, a supporting layer and a reflecting layer. Sputtered amorphous silicon is used as a dielectric layer because of its high refractive index. We fabricated the uncooled microbolometer with the proposed infrared absorbing structure by surface micromachining method. Then we characterized various bolometric properties such as thermal conductance, thermal time constant, responsivity and infrared absorptance. The fabricated bolometer showed the thermal conductance of 6.72 × 10 −7 W/K, the thermal mass of 4.43 × 10 −9 J/K, the thermal time constant of 6.6 ms and the responsivity of 7.76 × 10 3 V/W at 10 Hz chopper frequency and 9.22 μA bias current. From the results, the estimated absorptance is about 80%. We expect that the proposed absorbing structure shows high infrared absorption and high performance of uncooled microbolometer.

Measurement of the mechanical properties of electroplated gold thin films using micromachined beam structures

We have measured the Young’s modulus, residual stress, and stress gradient of electroplated gold thin films using surface micromachined beam structures. Cantilever and bridge beam structures of different lengths were fabricated using UV-LIGA surface micromachining and dry-release methods. The Young’s modulus and residual stress of the fabricated beams were determined from the resonance frequencies of electrostatically excited beams, and the stress gradient was evaluated from the self-deformation of released cantilevers. The observed Young’s modulus was smaller than the bulk Young’s modulus, and showed small changes depending on the deposition current density. The average residual stress was found to be tensile in nature, and the observed residual stresses showed no differences, regardless of the current density. However, the stress gradient increased with increasing current density. The deformation of the cantilever beam after release was dependent on the plasma ashing time used. This result implies that additional thermal effects from the post-deposition process may have an influence on the final performance of fabricated micro electro mechanical systems (MEMS) devices.

Calibration of AFM cantilever stiffness: a microfabricated array of reflective springs

Calibration of the spring constant of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules. We have developed a compact and easy-to-use reference standard for this calibration. The new artifact consists of an array of 12 dual spiral-cantilever springs, each supporting a mirrored polycrystalline silicon disc of 160 μm in diameter. These devices were fabricated by a three-layer polysilicon surface micromachining method, including a reflective layer of gold on chromium. We call such an array a Microfabricated Array of Reference Springs (MARS). These devices have a number of advantages. Cantilever calibration using this device is straightforward and rapid. The devices have very small inertia, and are therefore resistant to shock and vibration. This means they need no careful treatment except reasonably clean laboratory conditions. The array spans the range of spring constant from around 0.16 to 11 N/m important in AFM, allowing almost all contact-mode AFM cantilevers to be calibrated easily and rapidly. Each device incorporates its own discrete gold mirror to improve reflectivity. The incorporation of a gold mirror both simplifies calibration of the devices themselves (via Doppler velocimetry) and allows interferometric calibration of the AFM z-axis using the apparent periodicity in the force–distance curve before contact. Therefore, from a single force–distance curve, taking about one second to acquire, one can calibrate the cantilever spring constant and, optionally, the z-axis scale. These are all the data one needs to make accurate and reliable force measurements.

3D-feed horn antenna-coupled microbolometer

Abstract In this paper, we proposed a microbolometer coupled with 3D feed horn antenna. It is a new device that enhances the performance of the microbolometer by coupling 3D horn shape antenna. We designed the optimum horn antenna size that has 20.08 dB directivity. And we designed the microbolometer as a circular shape in order to reduce the coupling loss between the horn antenna. We confirmed that the detectivity of the designed microbolometer would be improved as the noise characteristics of the microbolometer are enhanced by coupling feed horn antenna which acts as a cold shield. The detectivity of the designed 3D antenna-coupled microbolometer was improved about seven times more than that of the conventional microbolometer in state of background limited infrared performance. Fabrications of the microbolometer are carried out by a surface micromachining method. We achieved the thermally good isolated floating structure. And the 3D feed horn antenna was constructed by using a mirror-reflected parallel beam illuminator (MRPBI) system which is invented for rotating and tilted illumination. Using this method, we acquired the feed horn shape antenna mold. And we also acquired antenna plate by using PDMS injection method. Finally, to couple the antenna and the microbolometer, we proposed the PDMS injection bonding method.

MEMS 공정을 이용한 전도성 고분자 액추에이터용 마이크로 구조물의 제작

일반적인 표면 마이크로머시닝 공정과 고분자의 중합공정을 결합하여 전도성 고분자인 폴리피롤 액추에이터를 제작하였다. 폴리피롤 액추에이터의 제작 공정을 검증하기 위한 가장 기본적인 구조물은 폴리피롤 캔틸레버이며 이를 이용하여 세포 조작에 응용 가능한 폴리피롤 그리퍼 및 밸브의 기본 구조물들을 제작하였다. 그리퍼는 손가락과 유사한 형태로 뼈에 해당하는 단단한 고분자와 근육에 해당하는 폴리피롤 등으로 구성된다. 밸브는 폴리피롤 캔틸레버에 유로가 결합된 형태로 제작되었다. 제안한 폴리피롤 액추에이터의 제작 공정 및 기본 구조물들은 세포 조작기구와 같은 바이오 관련 응용에 이용될 수 있을 것이다. Polypyrrole microactuators have been fabricated by the standard surface micromachining method combined with the electropolymerization of polypyrrole. The fundamental structure to verify the feasibility of the fabrication process is polypyrrole cantilever. Based on these process, polypyrrole grippers and valves for the manipulation of the cell have been fabricated. Grippers have the structure of bone and muscle which are rigid polymers and polypyrrole, respectively. Valves have the assembled structure of channels with polypyrrole cantilevers. The proposed fabrication process and structures are expected to be used for bio-related applications, for example, the cell manipulation.

Micromachining of mesoporous oxide films for microelectromechanical system structures

The high porosity and uniform pore size of mesoporous oxide films offer unique opportunities for microelectromechanical system (MEMS) devices that require low density and low thermal conductivity. This paper provides the first report in which mesoporous films were adapted for MEMS applications. Mesoporous SiO2 and Al2O3 films were prepared by spin coating using block copolymers as the structure-directing agents. The resulting films were over 50% porous with uniform pores of 8-nm average diameter and an extremely smooth surface. The photopatterning and etching characteristics of the mesoporous films were investigated and processing protocols were established which enabled the films to serve as the sacrificial layer or the structure layer in MEMS devices. The unique mesoporous morphology leads to novel behavior including extremely high etching rates and the ability to etch underlying layers. Surface micromachining methods were used to fabricate three basic MEMS structures, microbridges, cantilevers, and membranes, from the mesoporous oxides.

Micromachining applications of porous silicon

Besides its exciting electro-optical properties, porous silicon shows some interesting features for micromechanical applications. Owing to the doping sensitivity of the porous silicon formation, a wide range of pore and crystallite sizes can be formed easily. Because of their high aspect ratio, macropores from n-type silicon can be used for the etching of trenches or vias into the wafer. Mesoporous silicon created from heavily doped silicon has a smooth surface suitable for further thin-film processes, such as silicon nitride and carbide deposition, to be performed on it. In the smallest size range, nanostructures are formed with structure sizes beyond 10 nm without electron-beam lithography. Another important point is the high surface-to-volume ratio. After impregnation of the porous layer with a catalyst, it can be applied in gas and humidity sensors. For the most common micromachining applications, free-standing membranes, cantilevers and bridges are needed. In contrast to the surface micromachining method, where a thin film of silicon dioxide (< 10 μm) is dissolved as a sacrificial layer, by means of porous silicon technology, thick sacrificial layers can be produced (up to 100 μm). This results — after their dissolution — in a high distance of membranes and bridges to the bulk silicon, which is an important aspect for the sensitivity of thermal transducers. The capabilities of porous silicon application in micromachining will be demonstrated by some examples, e.g. thin-film bolometers and polysilicon hot-wire anemometers.