Errong Jing

Wafer-Level Vacuum Packaging of Micromachined Thermoelectric IR Sensors

In the trend towards low-cost, high-performance, and miniaturization, a wafer-level vacuum package is developed for micromachined thermoelectric infrared (IR) sensor. An IR sensor wafer and a cap wafer are bonded together in a vacuum chamber using Au-Au thermocompression bonding, where the cap wafer not only protects the floating thermopile structure but also selects IR light for the sensor. The device fabrication and Au-Au thermocompression hermetic bonding process as well as the packaged IR sensor characterization is presented in this paper. Experimental results show that the wafer-level vacuum packaged IR sensor has a four times higher responsivity and detectivity than the IR sensor with atmosphere pressure package, which confirms the IR performance improvement due to vacuum packaging. IR microscope image of the packaged device proved that the Au-Au thermocompression bonding process is compatible to the handling of fragile micromachined thermopile structure. Average leak rate and shear strength are, respectively, 3.9 × 10-9 atm cc/s and 16.709 Kgf, which shows that the Au-Au thermocompression hermetic bonding is suitable for the wafer-level vacuum packaging of micromachined thermoelectric IR sensor.

The Bond Strength of Au/Si Eutectic Bonding Studied by IR Microscope

The interface of Au/Si(100) eutectic bonding was investigated by infrared (IR) microscope and related to the bond strength. A strong relationship between the IR images and the bond strengths was found. Bond strength test showed that a strong bond has many square black spots in the IR images, whereas a poor bond has fewer or no square black spots. In order to study the nature of the relationship, the dissolution behavior of the bare Si(100) surface after bonding was investigated. During the Au/Si(100) eutectic reaction, the dissolution of the bare Si(100) surface primarily occurs by the formation of the craters which result in many square black spots in the IR images. The formation of the craters is ascribed to the anisotropic nature of Au/Si reaction that results in three-dimensional dissolution behavior on the bare Si(100) side. In order to further test the anisotropy hypothesis, Au/Si(111) bonding was also studied. Under the same bonding conditions, triangular black spots were observed in the IR images and triangular pits were found on the bare Si(111) surface. The analysis suggests that the craters on the bare Si(100) surface, in other words the square black spots in the IR images, are the indication of Au/Si(100) eutectic reaction. More craters mean a reaction between Au and Si(100), which occurs uniformly at the Au/Si(100) bonding interface compared to the case of fewer craters. No crater indicates that there is no eutectic reaction in the region. Therefore, the IR microscope may be used to evaluate and compare the different bond strengths qualitatively.

Low-temperature Au/a-Si wafer bonding

Two types of low-temperature Au–Si wafer bonding structure (i.e. Au/Si and Au/Au bonding structures) were investigated in this paper. The bond quality test showed that bond yield, bond repeatability and average shear strength are lower for the Au/Si bonding structure. The interfacial microstructure analysis of the Au/Si bonding structure suggested that the craters at the bonding interface are due to the anisotropy and non-uniformity of the Au/Si reaction, and the non-uniformity of the Au/Si reaction results in poor bond quality. The anisotropy of the Au/Si reaction is mainly because of the difference in the surface energy of each plane of the crystalline Si, and the non-uniformity of the Au/Si reaction is due to the native oxide on the bare Si surface. In the Au/Au bonding structure, a Ti/Au layer is deposited onto the bare Si surface, in which the Ti layer is used to decompose the native oxide during bonding. With this bonding structure, the bond yield, bond repeatability and average shear strength have improved significantly compared to the Au/Si bonding structure. The interfacial microstructure analysis indicated that the native oxide on the Si surface is decomposed by Ti during bonding, thereby causing the Si dissolution into Au to occur uniformly over the whole bonding interface and achieving a good bond quality. Remarkably, the measured thickness of the bonding interface has increased compared to that of the as-deposited metallic film, which is primarily attributed to the fact that Si from the substrate dissolves into the Au uniformly and then the Si precipitate forms across the bonding interface.

Wafer level vacuum packaged resonator with in-situ Au-Al eutectic Re-Distribution layer

In this paper, a wafer level vacuum packaged resonator with in-situ Au-Al eutectic Re-Distribution layer is demonstrated. A cap wafer with silicon bumps and electrical feedthroughs is bonded together with a MEMS resonator wafer using wafer level glass frit bonding technology. The silicon bumps provide close contact for the aluminum layer on the cap wafer and the gold layer on the device wafer, on which a gold-aluminum (Au-Al) eutectic is formed. The in-situ Au-Al eutectic layer achieve electrical interconnections between the cap wafer and the device wafer, which realizes the redistribution of the electrical feedthroughs of the MEMS resonator on the cap wafer. The formation mechanism of the Au-Al eutectic is illustrated. The Au-Al eutectic is observed through the FD3/SEM and IR images and is analyzed using EDX. The measured dynamic performance of the packaged MEMS resonator is presented in this paper. Experimental results show that the wafer-level vacuum packaged MEMS resonator results in over 100× higher quality factor (Q) than the resonator vibrating in atmosphere pressure. The experimental results indicate that vacuum about 3 mbar can be sealed in this approach.

Low-Temperature Wafer Bonding Based on Gold-Induced Crystallization of Amorphous Silicon

Low-temperature wafer bonding based on gold-induced crystallization of amorphous silicon has been investigated for the first time in this letter. A bonding yield of 97% and a shear strength of 10.5 MPa were achieved when bonding the oxide wafers by the bonding method at 400°C applying 0.8-MPa pressure for 30 min. The microstructure analysis indicated that the gold-induced crystallization process leads to big Si grains extending across the bonding interface and Au filling the other regions of the bonding interface, which result to a strong and void-free bonding interface. More importantly, this bonding method can be used for other substrate materials.

Analysis of air damping in micromachined resonators

An approach to the modeling and simulating the air damping effect on quality factor (Q) of a square plate Lamé mode microresonator is presented in this paper. Both the squeeze film damping and slide film damping are considered in the analysis procedure. The Reynolds equation and the Stoke-flow model are used to investigate the reactions of the resonant plate with the air flows in the transduction gaps and around the resonator plate surfaces, respectively. An electrical equivalent model has been derived for a microresonator operating in air. The model is realized with resistors equivalent for the slide film damping force and frequency-dependent resistors and capacitances connected in series equivalent for the squeeze film damping force. The simulated transmission characteristics are in good agreement with the experimental results for a 4.13 MHz Lamé mode microresonator.

Thermopile IR detector with filter cointegrated by wafer bonding technique

A novel micromachined thermopile IR detector module with filter and detector cointegrated by wafer level Au-Au vacuum bonding is presented in this paper. Filter is directly bonded on the IR detector to miniaturize the detector module, as well as reduce the packaging cost. Because the gas convection will be eliminated in vacuum, a better thermal isolation can be achieved for the vacuum packaged IR detector. Measured results show that the vacuum packaged IR detector has a 4 times larger output voltage than the atmosphere packaged IR detector. The bonding strength and hermeticity of the Au-Au bonding have also been investigated, and experiment results imply the good reliability of the IR detector with filter cointegrated.

Characterization of wafer-level XeF 2 Gas-phase Isotropic Etching For MEMS Processing

This paper reports the characteristics of wafer-level XeF2 gas-phase etching. Compared with chip-level XeF2 etching, the silicon etch rate for wafer-level XeF2 process is much smaller, which is mainly caused by the large exposed silicon area in wafer-level process. Additionally, the silicon etch rate drops off as etching time increased. The aperture size effect is apparent in wafer-level XeF2 processing. However, for etching window with large size, the aperture size effect will be minimized. The vertical aperture size effect is direct proportion to the number of etch cycle, while the lateral aperture size effect is first increase then decrease with the number of etch cycle increasing. Slight anisotropy of wafer-level XeF2 etching is also observed. Based on the characteristics of XeF2 etching, layout design rule for MEMS device with XeF2 releasing is developed and demonstrated.

3D monolithic integrated thermoelectric IR sensor

In this paper, we demonstrate a 3D monolithic integrated thermoelectric IR sensor by using CMOS-MEMS technology. Since CMOS technology is a plane process, wafer-level packaging is used to further improve the system integration density. Both the circuit interface and IR filter are integrated in the thermoelectric IR sensor. In the vertical direction, IR filter is integrated with IR sensor to miniaturize the IR system by wafer-level Au-Si bonding, as well as improve sensor performance by the vacuum bonding. In the plane direction, the thermopile microstructure was fabricated by standard CMOS process and released by XeF2 Post-CMOS technology. Thus, the sensor merges benefits of CMOS technology with the advantage of on chip signal processing.