Vacuum-sealed Cavities Research Articles

Fabrication of polymeric vacuum-sealed cavities on a silicon wafer

A method to fabricate low temperature adhesively bonded vacuum-sealed insulated polymeric cavities on a silicon substrate is presented. Multi-purpose use of the same polymer (Benzocyclobutene (BCB)) as the cavity structural material and also as an adhesive bonding agent minimizes process complexity, cost, and dielectric charging. Scanning electron microscopy (SEM) based characterization results show excellent planarity of BCB surfaces and high quality bonding which is indicative of better reliability. The method paves the way to fabricate the package and the sensor in the same process run to enable higher level of integration and minimize the cost further. The technique can be used to realize capacitive micromachined ultrasonic transducers (CMUTs), MEMS pressure sensors, and 3D embedded packaging and integration of heterogeneous dies.

Room-temperature pressureless wafer-scale hermetic sealing in air and vacuum using surface activated bonding with ultrathin Au films

Wafer-scale hermetic sealing in air and vacuum was achieved by using Au–Au surface activated bonding (SAB) and sputtered ultrathin Au films (thickness: 15 nm). Because such films with small grains have smooth surfaces (root mean square surface roughness: around 0.3 nm), pressureless sealing at room temperature is feasible. For air sealing, the leakage rate in an He leak test was less than the lower limit of the tester (<1.6 × 10−10 Pa m3 s−1). For vacuum sealing, thin glass caps (thickness: 50 μm) over vacuum-sealed cavities exhibited deflection after bonding due to the pressure difference between the cavities and ambient atmosphere. The deflection was unchanged even 900 d after sealing. The maximum air leakage rate is thus estimated to be less than 1.3 × 10−14 Pa m3 s−1. These results suggest that Au–Au SAB with ultrathin Au films is useful for low-temperature wafer-level packaging of MEMS and various electronic devices.

Wafer-level hermetic thermo-compression bonding using electroplated gold sealing frame planarized by fly-cutting

In this paper, a novel wafer-level hermetic packaging technology for heterogeneous device integration is presented. Hermetic sealing is achieved by low-temperature thermo-compression bonding using electroplated Au micro-sealing frame planarized by single-point diamond fly-cutting. The proposed technology has significant advantages compared to other established processes in terms of integration of micro-structured wafer, vacuum encapsulation and electrical interconnection, which can be achieved at the same time. Furthermore, the technology is also achievable for a bonding frame width as narrow as 30 μm, giving it an advantage from a geometry perspective, and bonding temperatures as low as 300 °C, making it advantageous for temperature-sensitive devices. Outgassing in vacuum sealed cavities is studied and a cavity pressure below 500 Pa is achieved by introducing annealing steps prior to bonding. The pressure of the sealed cavity is measured by zero-balance method utilizing diaphragm-structured bonding test devices. The leak rate into the packages is determined by long-term sealed cavity pressure measurement for 1500 h to be less than Pa m3s−1. In addition, the bonding shear strength is also evaluated to be higher than 100 MPa.

Rotational Capacitive Micromachined Ultrasonic Transducers (cMUTs)

A rotational capacitive micromachined ultrasonic transducer is introduced. Two pressure-sensitive diaphragms atop vacuum-sealed cavities are mechanically coupled via a rocking beam structure. The rocking structure is designed to reduce deflection of the vacuum-sealed diaphragms under atmospheric pressure while introducing ideally zero rotational stiffness to differential diaphragm pressure. The rocking structure responds to in-plane pressure gradients and is therefore anticipated to have dipole-type directivity to ultrasound. Prototypes employing 200 μm diameter polysilicon diaphragms are presented with a fundamental resonance frequency of 250 kHz.

MEMS Wafer-level Packaging Technology Using LTCC Wafer

SUMMARY This paper describes a versatile and reliable wafer-level hermetic packaging technology using an anodically bondable low-temperature co-fired ceramic (LTCC) wafer, in which multilayer electrical feedthroughs can be embedded. The LTCC wafer allows many kinds of microelectromechanical systems (MEMS) to be more flexibly designed and more easily packaged. The hermeticity of vacuum-sealed cavities was confirmed after 3000 cycles of thermal shock (–40 30 min/+125 30 min) by the diaphragm method. To practically apply the LTCC wafer to a variety of MEMS, the electrical connection between the MEMS on a Si wafer and feedthroughs in the LTCC should be established by a simple and reliable method. We have developed a new electrical connection method: the electrical connection is established by porous Au bumps, which are part of the Au vias exposed in wet-etched cavities on the LTCC wafer. A 100% yield of both electrical connection and hermetic sealing was demonstrated. A thermal shock test up to 3000 cycles confirmed the reliability of this packaging technology.

Diamond-based capacitive micromachined ultrasonic transducers

Capacitive micromachined ultrasonic transducers (CMUTs) employing diamond membranes are demonstrated. The design, finite element modeling, microfabrication, and experimental characterization of diamond-based CMUTs are reported. Ultrananocrystalline diamond having a chemical mechanical polished silicon dioxide interlayer deposited via high temperature oxide (HTO) process at 850°C in a low pressure chemical vapor deposition (LPCVD) furnace is employed as the membrane to form vacuum sealed cavities using plasma-activated direct bonding technology. Electrical impedance, deflection, and transmission measurements of a fabricated CMUT are performed in air using an impedance analyzer, a white light interferometer and a hydrophone, respectively. Experimental results verify the accuracy of finite element modeling. Diamond-based CMUTs possess 3-dB fractional bandwidth of 3% at a center frequency of 1.74MHz in air. Our experimental results demonstrate diamond as an alternate membrane material for CMUTs, and that diamond can be employed in novel microelectromechanical devices.

Microfabrication of vacuum-sealed cavities with nanocrystalline and ultrananocrystalline diamond membranes and their characteristics

Vacuum-sealed cavities featuring diamond membranes are fabricated using plasma-activated direct bonding technology. A chemical mechanical polished (CMP) silicon dioxide interlayer, deposited on diamond with a high temperature oxide (HTO) process at 850 °C in a low pressure chemical vapor deposition (LPCVD) furnace, is employed for successful direct bonding and vacuum cavity formation. The circular cavities are defined on the thermally grown oxide of the phosphorus-doped Si wafer (4-in, < 100>, 1.2 Ω/sq) using reactive ion etching (RIE). The same microfabrication steps are applied for low residual stress (i.e. < 50 MPa) nanocrystalline (NCD) and ultrananocrystalline (UNCD) diamonds to determine and compare membrane characteristics. For both diamond types, successful microfabrication of membranes is demonstrated using the optimized process flow. Profilometer measurements of membrane deflection are compared with finite element modeling (FEM), and indicate a Young's modulus of 1000 GPa for NCD and 850 GPa for UNCD. Furthermore, FEM analysis suggests the residual stress of UNCD membrane is approximately 100 MPa tensile, whereas NCD one does not show any significant residual stress (< 50 MPa). Our results show that NCD is a more promising choice than UNCD as a membrane material for electromechanical transducers.

A wireless microsystem for the remote sensing of pressure, temperature, and relative humidity

This work presents a microsystem that utilizes inductive power and data transfer through a backscatter-modulated carrier and a transducer interface that monitors its environment through embedded capacitive transducers. Formed on a single chip, transducers for temperature, pressure, and relative humidity are realized using a silicon-on-glass process that combines anodic bonding and a silicon-gold eutectic to realize vacuum-sealed cavities with low-impedance (6 /spl Omega/) electrical feedthroughs. Temperature is sensed capacitively using a row of Si/Au bimorph beams that produce a sensitivity of 15 fF//spl deg/C from 20 to 100/spl deg/C. The absolute pressure sensors have a sensitivity of 15 fF/torr and a range from 500 to 1200 torr, while the relative humidity sensor responds with 39 fF/%RH from 20 to 95%RH. A relaxation oscillator implements low-power capacitance-to-frequency conversion on a second chip with a sensitivity of 750 Hz/pF at 10 kHz, forming a 341 /spl mu/W transducer interface. The system is remotely powered by a 3-MHz carrier and has a volume of 32 mm/sup 3/, including the hybrid antenna wound around the perimeter of the system.

An independent, temperature-controllable microelectrode array.

Rapid, localized temperature control and negligible power consumption are key requisites for realizing effective parallel and sequential processing in the miniaturized, integrated biomedical microdevices where temperature-dependent biochemical reactions and fluid flow occur. In this study, an independent, temperature-controllable microelectrode array, with excellent temperature control rates and minimal power consumption, has been developed using microelectromechanical systems technology. The microfabricated array consists of Pt microelectrodes (100-microm diameter), with n-doped polysilicon microheaters (1.4-k Omega resistance), and vacuum-sealed cavities of depth 6.2 microm and diameter 200 microm. The thermal characteristics of each microelectrode were evaluated electrochemically through surface temperature measurements. The large heater power coefficient (2.1 +/- 0.1 degrees C mW(-1)) and the short heating and cooling times (less than 0.2 s for T(0.95)) are consequences of the vacuum-sealed cavities, which facilitate good thermal isolation and low thermal mass. The temperature of each microelectrode is independently controlled by a dedicated microheater, without thermally influencing the adjacent microelectrodes significantly.

A surface micromachined capacitive absolute pressure sensor array on a glass substrate

Abstract The fabrication process and the characteristics of a new surface micromachined pressure sensor array on a glass substrate are presented. The array consists of electrically parallel individual sensors with composite SiO 2 -Cr-SiO 2 diaphragms and vacuum-sealed cavities underneath. The cavities are created by lateral etching of an Al sacrificial layer. Capacitive sensing of pressure is accomplished via chromic electrodes. The performance of the sensor array is described. In a pressure range of 800 kPa a terminal-based nonlinearity less than 4% was measured for the arrays. The pressure sensitivity of the array is about 0.28 fF hPa −1 .