Thin-Film Vacuum Packaging Based on Porous Anodic Alumina (PAA) for Infrared (IR) Detection

Gwang-Jae Jeon*, Woo Young Kim, and Hee Chul Lee Department of Electrical Engineering

Korea Advanced Institute of Science and Technology (KAIST) Daejeon, Republic of Korea E-mail: gjjeon@kaist.ac.kr

Abstract— In this paper, we present a thin-film vacuum packaging method for infrared (IR) detection. It is based on the formation of porous anodic alumina (PAA) with vertical nanopores as a supporting layer and an etch hole for the removal of the sacrificial layer. To etch away the sacrificial material, the etchant penetrates into the cavity underneath the PAA film through the nanopores in the PAA. In addition, the PAA is capable of high transmittance in the IR wavelength range. To seal the packaging, an IR-transparent material is post-deposited over the PAA film in a vacuum ambient environment. The vacuum level of the packaging is verified by measuring the center deflection of the sealing layer and the long-term hermeticity is monitored for 60 days. The IR transmittance of the PAA membrane and a sealing layer on the PAA was also measured for IR applications.

I. INTRODUCTION

Low-cost and mass-production packaging for IR sensors has been recognized as one of the most important key technologies for commercial applications. Thus far, the preferred forms of vacuum packaging in the industry for IR sensors have been metallic and ceramic types of packaging. These techniques are based on one-by-one pumping through a pinch-off tube [1]. However, it is difficult to improve the productivity and reduce the cost when using these packaging technologies [2]. To satisfy market demand in this field, advanced wafer-level packaging (WLP) technologies such as wafer-bonding and monolithic thin-film packaging have been developed [3-6]. Among the WLP approaches, thin-film packaging has been developed. This type involves the formation of a sacrificial layer on an unreleased IR sensor to organize the cavity, after which a supporting layer is deposited over the cavity. It is considered to be more compact in size and less costly, as the wafer bonding method requires an extra window wafer and additional processes such as the formation of solder material and alignment with the chip-window wafer. However, conventional photolithography-based etch holes or channel formation techniques used in conjunction with thin-film packaging have a few drawbacks, such as the requirement of highly complex processing steps, the internal deposition of

the sealing layer, and a prolonged release time of the sacrificial layer [5, 7].

To overcome these issues, we suggest thin-film vacuum packaging using a porous anodic alumina (PAA) membrane. PAA has numerous vertical nanopores and can be easily formed through the anodization of aluminum at a low temperature. The diameter of the nanopores and the thickness of the PAA membrane can be controlled by changing the anodization parameters. Also, PAA has a transparent characteristic in the visible/infrared wavelength range. Following the release of the sacrificial layer through nanopores in the PAA membrane, the cavity is sealed with an IR-transparent material in vacuum ambient environment. This method satisfies the critical requirements of IR sensor packaging of hermetic vacuum sealing and an IR-transparent window.

II. FORMATION OF FREESTANDING POROUS ANODIC

ALUMINA (PAA) MEMBRANE

A Porous Anodic Alumina (PAA) membrane with vertical nanopores can usually be formed by an electrochemical treatment known as anodization of the aluminum (Al) at low temperature [8]. The size of the vertical nanopore diameters can be controlled by changing the anodization parameters of the electrolyte, driving voltage (or current), and temperature [9]. However, a barrier layer forms at the bottom of the PAA after the anodization process, indicating that a gas or liquid etchant cannot penetrate into the sacrificial layer underneath the PAA membrane through the vertical nanopores. To apply the PAA membrane as a supporting layer with release holes for the etching of the sacrificial layer in thin-film packaging, a removal process for the barrier layer is required.

The process flow for the formation of a freestanding PAA membrane is shown in Fig. 1. Silicon oxide (SiO2) 100 nm thick was thermally grown on the silicon (Si) substrate as a means of isolation, after which a 500 nm thick aluminum (Al) was deposited by DC-magnetron sputtering at room temperature (Fig. 1(a)). The Al anodization process was

Supported by the Center for Integrated Smart Sensors funded by the Ministry of Education, Science and Technology as Global Frontier Project (CISS-N01110724).

978-1-4577-1767-3/12/$26.00 ©2012 IEEE

Figure 1. Process flow and FE-SEM images of the fabrication of thefreestanding PAA membrane.

Figure 2. CoventoWareTM finite element analysis of the deflection in the thin-film vacuum packaging. The total capping layer thickness of 2.65 umincludes the 2.2 um thick sealing layer and the 0.45 um thick PAAmembrane.

Figure 3. Process flow for thin-film vacuum packaging using the PAAmembrane.

conducted under a DC-40V constant driving voltage in 0.3M oxalic acid as an electrolyte to offer an electrical path between the anode and the cathode at room temperature. In the anodization process, a Pt-sputtered Si substrate was used as a counter electrode (cathode) and the anodic current was monitored by a serially connected current meter. The anodization process was conducted until the anodic current level roughly approached zero, meaning that the Al was completely converted to porous alumina (Fig. 1(b)). A 2.5 um thick photoresist was spin-coated and patterned to define the freestanding membrane areas, and thinning of the barrier layer and pore widening were performed by means of 5 wt% H3PO4 isotropic wet etching at room temperature. To remove the barrier layer completely without reducing the thickness of the sidewall of vertical nanopores, anisotropic directional reactive ion etching (DRIE) with mixture of the CF4 and O2 gas was performed (Figs. 1(c), (e)) [10]. After the removal of the barrier layer, the SiO2/Si substrate underneath the PAA membrane was released through the vertical nanopores by the isotropic plasma gas mixture of the CF4 and O2 (Fig. 1(d)).

FE-SEM images of the fabricated freestanding PAA membrane are shown in Fig. 1(f). The inset shows a magnified view of the edge of the membrane, in which an air-gap exists. This indicates that the vertical nanopores of the PAA membrane functioned successfully as release holes without any cracks.

III. THIN-FILM VACUUM PACKAGING USING A PAA

MEMBRANE

A. Design

Due to the external loads on the packaging caused by the pressure difference between the atmosphere and the inside vacuum cavity of the packaging, an appropriate thickness of the sealing layer is required to avoid a collapse of the

packaging. To analyze the effect of the pressure difference on the capping layer (stacked sealing layer over the PAA membrane) of the packaging, a simulation was performed using the commercial finite element analysis tool, CoventoWareTM. During the modeling process, the size of the packaging was 200 um × 170um with a 2.2 um thick stacked silicon nitride (SiNx) sealing layer over the 450 nm thick PAA membrane. An atmospheric pressure load of about 105 Pa (760 Torr – 1 Torr) was then applied to the capping layer. Figure 2 shows the simulation result, showing a maximum capping layer deflection amount of 0.54 um at the center of the packaging. This indicates that the minimum height margin of the cavity in the packaging should be designed so that it is higher than 0.54 um so as to prevent the capping layer from contacting on the substrate.

Figure 4. FE-SEM images of fabricated thin-film packaging: (a) 200 um ×170 um dual-cavity connected by a narrow channel; (b) intentionally cleavedcross-sectional view of the formation of a residue-free 2.5 um height cavity;(c) the cavity encapsulated with a sealing layer over the PAA membrane; (d)a close-up image of the rectangular mark with the white dotted-line in (c). theinset shows the PAA with diameters in rage of 40 to 50 nm.

Figure 5. Surface morphology microscope images of dual-cavity packagingto measure the center deflection of the capping layer of the packaging: (a)before and (b) after breaking the vacuum-state inside of the packaging.

Figure 6. Long-term hermeticity of the thin-film vacuum packaging using thePAA membrane after 60 days.

B. Fabrication

The overall process flow of the PAA based thin-film vacuum packaging is detailed in Fig. 3. The process started with a thermally grown 1 um thick SiO2 on a Si substrate. For the formation of the cavity shape of the packaging, a 2.5 um thick polyimide sacrificial layer was spin-coated, cured, and patterned. The thickness of the sacrificial layer, at 2.5 um, is much larger than the deflection value from the simulation result. A 40 nm thick Ti layer and a 350 nm thick Al layer were then sputtered at room temperature for the anodization (Fig. 3(a)). Then, selective anodization, involving a two-step anodizing process using only a photoresist (PR), was conducted (Figs. 3(b), (c)) [11]. Anodization was performed in oxalic acid at a power level of DC-40V at room temperature. The conductive Ti layer under the Al layer was used for thinning of the barrier layer at the bottom of the PAA membrane [12]. The barrier layer at the bottom of the PAA membrane was removed in-situ by the widening of the pore diameter in 5 wt% H3PO4 and by etching the Ti layer successively in the mixture of NH4OH, H2O2, and H2O. A polyimide sacrificial layer was released by O2 plasma ashing through the nanopores in the PAA while the PAA membrane maintained its cavity shape (Fig. 3(d)). Finally, the cavity was vacuum-sealed in a post-deposition process involving the use of plasma-enhanced chemical vapor deposition (PECVD) SiNx as a sealing layer over the PAA membrane (Fig. 3(e)).

FE-SEM images of the fabricated thin-film vacuum packaging with the PAA membrane and the sealing layer are shown in Fig. 4. No polyimide sacrificial material residue and no internal deposition of the sealing material were found in the inside of the packaging. These facts are attributed to the high aspect ratio of the PAA membrane [13].

IV. RESULTS AND DISCUSSION

A. Hermeticity of the Packaging

Due to the pressure difference between the atmosphere and the inside of the packaging, the capping layer is deflected. This indicates that the packaging is hermetically vacuum-sealed at the pressure of the PECVD process. To verify the hermeticity of the fabricated thin-film vacuum packaging, the center deflection of the capping layer encapsulating the packaging, designed with a dual-cavity structure connected by a narrow channel, was measured in terms of its surface morphology by an optical microscope (Keyence VK-7500) before and after one cavity was broken (Fig. 5). After one cavity was broken, the deflected capping layer in the other cavity was assumed a flat state because the vacuum ambient environment of the packaging was cleared. A measured deflection of 0.525 um was noted, showing good agreement with the results from the CoventoWareTM simulation. This indicates that the thin-film packaging was successfully sealed at 1 Torr during the deposition of the sealing layer. The long-term hermeticity was also monitored to confirm the reliability of the fabricated packaging for 60 days, as shown in Fig. 6.

Figure 7. Transmittance spectra of the PAA membrane and the capping layerof the packaging (stacked sealing layer over the PAA membrane).

No significant deflection change was observed, indicating that the vacuum-sealed packaging using the PAA membrane has good durability.

B. IR Transmittance

In order to be used in IR applications, the IR transmittance levels of the PAA membrane and the capping layer were measured in a wavelength range of 3 to 16.5 um by FT-IR microspectroscopy. To measure the transmittance of the thin-film, the PAA membrane and capping layer, which were separated from the substrate, were transferred to a carrier-holder for the measurement of the transmittance mode. Plots in Fig. 7 is the measured transmittance from the PAA membrane (black) and that of the capping layer (red). The average transmittance level of the PAA membrane was about 94% in the measured wavelength range. After the deposition of the sealing layer, the average transmittance level was about 88% in a wavelength range below 9 um. However, it was drastically decreased in a wavelength range beyond 10 um, especially near a wavelength of 12 um.

V. CONCLUSION

In this paper, the thin-film vacuum packaging for IR detection using a PAA membrane with vertical nanopores as a supporting layer and an etching hole for the release of the sacrificial layer was demonstrated. Combining the removal process of the barrier layer at the bottom of the PAA membrane in situ, a freestanding PAA membrane was successfully fabricated by etching away the sacrificial material through the nanopores in the PAA membrane. To form the thin-film vacuum packaging, an IR-transparent material (SiNx) was post-deposited over the PAA film in a vacuum ambient environment. The vacuum level (1 Torr) and hermeticity of the packaging were verified by measuring the center deflection of the capping layer, and the long-term hermeticity was observed for 60 days. The average IR transmittance of the sealing layer on the PAA was determined to be more than 88% in a wavelength range of 3 to 9 um.

For the first time, we confirm the feasibility of the use of a PAA membrane and a sealing material for thin-film vacuum

packaging for IR sensor applications. Further study related to the optimization of the IR transmittance in the wavelength range beyond 10 um and the formation of a vacuum level below 10 mTorr may lead to the development of the low-cost, advanced, powerful packaging technology for the uncooled-type IR sensors.

ACKNOWLEDGMENT

This work was supported by the Center for Integrated Smart Sensors funded by the Ministry of Education, Science and Technology as Global Frontier Project (CISS-N01110724). The authors would like to thank S.-D. Ko, H.-H. Yang, and Prof. J.-B. Yoon at Korea Advanced Institute of Science and Technology for their helpful comments in the fabrication process.

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