Wafer Level Cameras – Novel Fabrication and Packaging Technologies

The increasing demand for more functions and features coming along with cost reduction plays a significant role in today’s product design and manufacturing technologies of mobile devices such as PDAs, laptop computer and mobile phones. Besides the main function of the device, imaging is considered as core feature by the user and major mobile phone manufacturers. Therefore the industry puts a lot of effort onto performance improvements and the optimization of the manufacturing method of mobile phone cameras. Wafer Level Camera (WLC) is supposed to be the technology of choice to address these requirements. In recent years Wafer Level Packaging of CMOS image sensors has become a well established technology in the industry. This technology provides a cost efficient packaging method for shrinking devices sizes coupled with higher I/O density. In addition to this Wafer-level optics is a novel technology that is designed to meet the demand for smaller form factors of the optical system and cost reduction in the next generation of camera phones. The optical components are fabricated by replicating the optics through a stamp material into a polymer layer, coated on a glass wafer. Another key challenge is the wafer alignment. Replicated lens wafers are aligned and adhesively bonded at the wafer level using a UV curing process in order to achieve excellent alignment results. Finally the bonded Opto Wafers are subsequently diced to form individual camera modules [1]. This paper explores the latest fabrication techniques as used in the Wafer Level Cameras (WLC) where Opto Wafers and CMOS-Wafers are mounted by Wafer Level Packaging (WLP) and describes all the challenges and available solutions. The processing issues encountered in those techniques are discussed with a focus on each WLC process step. A typical Wafer Level Camera layout (Fig. 1) is described, the replication of microlenses (Fig. 2) and the packaging of such microlens wafers (Opto Wafers) via UV curing is depicted as well. Also wafer level packaging of the CMOS wafer using bonding techniques is part of this paper. UV curable materials for microlens replication and for Wafer Level Packaging of Opto Wafers (lens stacking) is presented as well. Optical measurement technology for quality assurance of micro-lenses finally concludes the paper. Typical Wafer Level Camera Design Typically a wafer level camera consist of two main pieces, the image sensor and the optics. Figure 1 shows such a schematic cross section of a classical wafer level camera. Figure 1. Schematic of Typical Wafer Level Camera Design Figure 2. 8” Lens Master (left) and replicated 8” Wafer (right) [2] CMOS Sensor Packaging (Glass encapsulation) In a camera device the CMOS image sensor needs to be covered by a glass layer to protect the active area. This is typically done on Wafer Level using bonding techniques. The challenges in this process are high alignment accuracy and excellent temperature and pressure uniformity to achieve best yield. Void-free bond interfaces are also required. The most popular process is adhesive bonding, chosen because of low bonding temperature (below 200°C). Typically the adhesive is dispensed or rolled on frames located at one of the wafers. The next step is alignment of both wafers. As one of the wafers is transparent (glass) a live alignment is feasible. Due to the sensor dimensions typically the required post bond alignment accuracy is below 10μm. Followed that thermal curing takes place in the bonding chamber, SUSS SB8e. The given pressure uniformity of ±1,5% leads to excellent bonding results. In return the optics is manufactured using other machine types, SUSS MA8 Gen3 Maskaligner equipped with dedicated tooling. Wafer Level Packaging of Microlens Wafer (Opto Wafers) CMOS Sensor Wafer Level Packaging Replication of Microlenses UV cureable materials for microlense replication and for Wafer Level Packaging of Opto Wafers