AR-concepts for hermetic wafer level packaging of uncooled FIR bolometer arrays

Abstract

Uncooled FIR-bolometer image sensors are established in many applications like building inspections, cold bridge analyses and predictive maintenance. New fields of application are discovered, like automotive night vision, advanced presence detection, gesture recognition etc. but these require a lower cost, small form factor packaging of the μ-bolometer sensors. Wafer level packaging (WLP) is seen as the enabling housing technology compared to ceramic packages for high volume production. Monolithic integrated μ-bolometer image sensors require a vacuum packaging with vacuum level in the range of mbar or less. The growing demand for reliability especially in automotive applications has also a large impact on the package construction. The overall challenge for high sensitive μbolometer sensors is to create a small package that allows for a maximum IR transmission at minimum cost. The work describes a wafer level technology on 200mm wafers with a hermetic sealing for large evacuated cavity dimensions with the process integration of different antireflective surface treatments. Cavities are created with 90μm thick poly-silicon frames in an additive deposition technology. The IR window region in the caps features different customer specific anti-reflective concepts. One approach is a double side moth eye pattern that can be designed to suppress short wavelength by destructive interference. It is possible to use different geometries of moth eyes inand outside of the cavities to create a low cost filter. To reduce sunlight transmission a combination of moth eyes inside the cavities and a multi-layer filter coating outside can be achieved. The moth eyes patterns are realized in silicon wafers by reactive ion etching. To generate a high vacuum up to mbar a getter with large exposed surface is required. A 3D structured getter solution is presented that generates a maximum getter surface in a small area in the cap. First wafers with a good optical resolution and thermoelectric sensitivity have been achieved by a eutectic wafer bonding process Vacuum WLP construction The hermetic capping of monolithic integrated μbolometer image sensors enables a small package form factor but increases the required development efforts as a seal frame on active CMOS circuits is formed. Beside the fact that special care is needed to prevent CMOS damages due to mechanical overstress, the exposed CMOS area inside the cavity may cause outgassing problems during the high temperature step of the wafer bonding. Considering all pros and cons of alternative hermetic wafer sealing technologies a eutectic AuSn wafer bond process was selected [1,2]. For using μ-bolometer it is necessary to use a cap construction that provides an access for 8-12μm infrared (FIR) radiation but blocks the sunlight. Without an antireflective coating or surface treatment, about 45% of the signal will not pass a silicon cap. Depending on the choice of Anti-reflective coating (ARC) of the silicon IRwindow up to 90% transmission in the range of 8-12 μm can be achieved. Process integration demands for an ARC technology that is fully compatible with all WLP-process steps and vacuum compatible. The choice may be further complicated by the high cost of multi-layer thin film ARC and possible yield loss due to local film defects. The texturing of the surface with special geometries, called moth-eyes, is an alternative approach increase the IRtransmission. The texturing avoids high costs, the need for wafer travelling, and reduces yield loss. Although the demand for a full sun blocking filter cannot be fulfilled, advanced moth-exe pattern can block at least a part of the short wavelength range up to 7 μm. These moth-eyes patterns are realized by reactive ion etching in the silicon surface. So the refractive index n can be changed from 3.4 down to 1.8 by limiting Fresnel reflection [3]. This innovative process is realized in combination with a thick deposited polysilicon distance frame that generates a cavity. The cap wafer construction integrates a double side wafer processing with a high resolved moth eye pattern inside a deep cavity (Fig. 1). Fig.1: Schematic of WLP construction with two different moth eye textures (A), (B) and TIGER3D-Getter (C). The cavity is formed by an additively grown ultra-thick polysilicon distance frame (D), metallized with AuSn for eutectic bonding [4].