Thin Film Packaging for Vacuum MEMS Encapsulation: A Study on Outgassing Phenomenon

Most of the time, MEMS devices require hermetic encapsulation for protection against atmosphere, moisture, particles and standard back-end manufacturing technologies. In the last few years, Wafer Level Packaging (WLP) is moving toward developments on Thin Film Packaging (TFP) in order to save footprint, to reduce chip thickness and packaging costs. In the specific case of high-vacuum MEMS encapsulation (gyro, compass), long term pressure stability is required. As the final performances of these kinds of devices are strongly dependent on the working pressure, using TFP for MEMS encapsulation with controlled vacuum becomes more challenging due to very small cavity volumes. It is then necessary to understand the outgassing phenomenon that take place during TFP encapsulation in order to reduce it. In this paper, our latest developments on thin film packaging technology are presented. Outgassing from materials used in TFP and MEMS devices become key parameters to decrease the pressure inside the package and to improve the reliability. In a first part, pressure and Residual Gas Analysis (RGA) of typical TFP and typical MEMS materials are measured under different time / temperature baking processes. Measurements show that material outgassing mainly comes from H2 and maximum pick appears in the beginning of the thermal process. Thanks to these characterizations, an optimized outgassing baking process in term of time and thermal budget is presented. By minimizing the internal outgassing, materials deposited by PVD sputtering can now be implemented as sealing materials for low pressure MEMS devices. In a second part, specific low temperature Al based materials which has been developed on equipment fully compatible with front-end fab is presented. Multi-layer materials like Ti / Al based materials are compared to our single Al based material to decrease the microstructure size and to improve the sealing performances. Scanning Electronic Microscopy (SEM) and Focused Ion Beam (FIB) cross section characterizations confirm that the grain sizes are highly impacted by sputtering process parameters and a compromise has to be done with MEMS outgassing. Finally, the most suitable outgassing baking process for the inside cavity materials and various Al-based sealing materials and stacks are performed for a MEMS compass device on 200 mm wafers. Pressure inside the cavity less than 10 mbar is obtained and the TFP yield is presented on each process conditions. These results are very promising and showed the capabilities of TFP for vacuum MEMS encapsulation applications despite very small volume cavity. Development of such technology is still under way in order to reach high vacuum level in the range of 10-1 to 10-3 mbar.

Download Full-text

Integration Technology for Wafer-Level LiNbO3 Single-Crystal Thin Film on Silicon by Polyimide Adhesive Bonding and Chemical Mechanical Polishing

Nanomaterials ◽

10.3390/nano11102554 ◽

2021 ◽

Vol 11 (10) ◽

pp. 2554

Author(s):

Wenping Geng ◽

Xiangyu Yang ◽

Gang Xue ◽

Wenhao Xu ◽

Kaixi Bi ◽

...

Keyword(s):

Thin Film ◽

Single Crystal ◽

Chemical Mechanical Polishing ◽

Adhesive Bonding ◽

Mechanical Polishing ◽

Bonding Interface ◽

Infrared Analysis ◽

Mems Devices ◽

Wafer Level ◽

Integration Technology

An integration technology for wafer-level LiNbO3 single-crystal thin film on Si has been achieved. The optimized spin-coating speed of PI (polyimide) adhesive is 3500 rad/min. According to Fourier infrared analysis of the chemical state of the film baked under different conditions, a high-quality PI film that can be used for wafer-level bonding is obtained. A high bonding strength of 11.38 MPa is obtained by a tensile machine. The bonding interface is uniform, completed and non-porous. After the PI adhesive bonding process, the LiNbO3 single-crystal was lapped by chemical mechanical polishing. The thickness of the 100 mm diameter LiNbO3 can be decreased from 500 to 10 μm without generating serious cracks. A defect-free and tight bonding interface was confirmed by scanning electron microscopy. X-ray diffraction results show that the prepared LiNbO3 single-crystal thin film has a highly crystalline quality. Heterogeneous integration of LiNbO3 single-crystal thin film on Si is of great significance to the fabrication of MEMS devices for in-situ measurement of space-sensing signals.

Download Full-text

Ion Beam Deposition for Novel Thin Film Materials and Coatings

Materials Science Forum ◽

10.4028/www.scientific.net/msf.674.195 ◽

2011 ◽

Vol 674 ◽

pp. 195-200 ◽

Cited By ~ 2

Author(s):

A.Y. Goikhman ◽

S.A. Sheludyakov ◽

E. A. Bogdanov

Keyword(s):

Thin Film ◽

Thin Films ◽

Ion Beam ◽

High Vacuum ◽

Growth Conditions ◽

High Energy ◽

Present Contribution ◽

Ion Beam Deposition ◽

Thin Film Preparation ◽

Beam Deposition

The Ion Beam Deposition (IBD) technique is not very widespread, but simple and very powerful methodic of thin film preparation, allowing to obtain high quality, smooth and very uniform films on big substrate areas (until 40 cm diameter), by target ablation with high energy particles in high vacuum. For the bombarding of the target is convenient to use the charged particles – ions of Ar, because they are easy to disperse in the electric field. Also, including neutralizing system, allow to obtain high-energy neutrals, irradiating the target, producing thin films from any kind of solid targets: from simple metals to complex conducting and non-conducting stoichiometric alloys. Thus, energy of condensing target particles is an average from several units to tens of eV. In the present contribution, we discuss the possibilities and advantages of IBD technology on application examples, including results of functional properties research of Ti, TiO2, SiO2 and Ag thin films for medicine applications, Ni, NiOx, Co and CoO single layers and structures for spintronics applications, and TiO2-SiO2, Ti-Zr-O-SiO2 multilayer structures for laser mirrors applications, produced by IBD system. Good structural, morphological quality (with roughness ~ 0.3 nm) and high uniformity on big areas along with right phase and stoichiometric state is demonstrated by convenient standard techniques for the structures prepared under the optimized growth conditions.

Download Full-text

A Robust Thin-Film Wafer-Level Packaging Approach for MEMS Devices

Journal of Microelectronics and Electronic Packaging ◽

10.4071/imaps.270 ◽

2010 ◽

Vol 7 (3) ◽

pp. 175-180 ◽

Cited By ~ 1

Author(s):

Krishnan Seetharaman ◽

Bart van Velzen ◽

Johannes van Wingerden ◽

Hans van Zadelhoff ◽

Cadmus Yuan ◽

...

Keyword(s):

Thin Film ◽

Silicon Nitride ◽

Wafer Bonding ◽

Sacrificial Layer ◽

Early Stage ◽

Bulk Acoustic Wave ◽

Mems Devices ◽

Wafer Level ◽

Front End ◽

Mems Device

Micro-electromechanical systems (MEMS) devices are extremely sensitive to their environment, especially at the wafer level, until they are packaged in final form. The harsh back-end (BE) operations that the MEMS devices have to endure include dicing, pick-and-place, wire bonding, and molding. During these processing steps, the MEMS device is exposed to particles and contaminants. Therefore, protection at an early stage is a fundamental requirement. We describe a silicon nitride thin-film capping, which is processed using a sacrificial layer technique only with front-end technology. This approach is suitable for mass production of MEMS devices, owing to the fact that it is more cost-effective when compared to other approaches such as wafer-to-wafer bonding and die-to-wafer bonding. A bulk acoustic wave (BAW) resonator that finds application in the radio frequency (RF) front end, for example, in cell phones, is taken as a MEMS vehicle for our work. It is an example of an extremely sensitive MEMS device, because the resonance frequency shifts significantly when additional mass is accidentally deposited on its surface. The thickness of the silicon nitride capping that is required to withstand all the BE steps, in particular transfer molding, is estimated using simple analytical calculations and finite element model (FEM) simulations. The pressure acting on the thin film capping and the thermal load during molding are included in the FEM model. Using this, the minimum thickness required for the capping is determined. We prove that a BAW resonator capped with silicon nitride at the wafer level can be wafer-thinned, diced, wire bonded, and molded without major degradation in performance.

Download Full-text

A micro-thin-film getter–heater unit for high vacuum capsulation of MEMS devices

AIP Advances ◽

10.1063/6.0001580 ◽

2022 ◽

Vol 12 (1) ◽

pp. 015105

Author(s):

Yucheng Ji ◽

Liuhaodong Feng ◽

Song Guo ◽

Xinlin Peng ◽

Shuo Chen ◽

...

Keyword(s):

Thin Film ◽

High Vacuum ◽

Mems Devices

Download Full-text

Wafer-level High Vacuum Packaging using Titanium Thin Film as Bonding and Gettering Material

IEEJ Transactions on Sensors and Micromachines ◽

10.1541/ieejsmas.138.387 ◽

2018 ◽

Vol 138 (8) ◽

pp. 387-391 ◽

Cited By ~ 1

Author(s):

Toshihiko Takahata ◽

Hideki Hirano ◽

Joerg Froemel ◽

Shuji Tanaka ◽

Yuji Takakuwa

Keyword(s):

Thin Film ◽

High Vacuum ◽

Vacuum Packaging ◽

Wafer Level ◽

Titanium Thin Film

Download Full-text

A Robust Wafer-Level Capping Approach for MEMS Devices

Additional Conferences (Device Packaging HiTEC HiTEN & CICMT) ◽

10.4071/2010dpc-tp21 ◽

2010 ◽

Vol 2010 (DPC) ◽

pp. 000891-000924

Author(s):

Krishnan Seetharaman ◽

Bart van Velzen ◽

Hans van Zadelhoff ◽

Cadmus Yuan ◽

Frank Rietveld ◽

...

Keyword(s):

Thin Film ◽

Silicon Nitride ◽

Wafer Bonding ◽

Sacrificial Layer ◽

Early Stage ◽

Bulk Acoustic Wave ◽

Mems Devices ◽

Wafer Level ◽

Front End ◽

Mems Device

Micro-electromechanical systems (MEMS) devices are extremely sensitive to their environment, especially at wafer-level, until they are packaged in final form. The harsh back-end (BE) operations that the MEMS devices have to endure include dicing, pick-and-place, wire bonding and molding. During these processing steps, the MEMS device is exposed to particles and contaminants. Therefore, protection at an early stage is a fundamental requirement. In this work, we describe a silicon nitride thin-film capping, which is processed using a sacrificial layer technique only with front-end technology. This approach is suitable for mass production of MEMS devices, owing to the fact that, it is more cost-effective when compared to other approaches such as wafer-to-wafer bonding and die-to-wafer bonding. A Bulk Acoustic Wave (BAW) resonator, that finds application in the Radio Frequency (RF) front end, e.g., in cell phones, is taken as a MEMS vehicle for our work. It is an example of an extremely sensitive MEMS device, because the resonance frequency shifts significantly when additional mass is accidentally deposited on its surface. The thickness of the silicon nitride capping that is required to withstand all the BE steps, in particular transfer molding, is estimated using simple analytical calculations and finite element model (FEM) simulations. The pressure acting on the thin film capping and the thermal load during molding are included in the FEM model. Using this, the minimum thickness required for the capping is determined. We prove that, a BAW resonator capped with silicon nitride at wafer-level can be wafer-thinned, diced, wire bonded and molded without major degradation in its performance.

Download Full-text