This paper presents the different processing steps of a new generic surface micromachining module for MEMS hermetic packaging at temperatures around 180 degrees C based on nickel plating and photoresist sacrificial layers. The advantages of thin film caps are the reduced thickness and area consumption and the promise of being a low-cost batch process. Moreover, sealing happens by a reflow technique, giving the freedom of choosing the pressure and atmosphere inside the cavity. Sacrificial etch holes are situated above the device allowing shorter release times compared to the state-of-the-art. With the so-called over-plating process, small etch holes can be created in the membrane without the need of expensive lithography tools. The etch holes in the membrane have been shown to be sufficiently small to block the sealing material to pass through, but still large enough to enable an efficient release.
Unlike CMOS chips, chips containing MEMS (microelectro-mechanical-system) cannot be directly packaged in a plastic or ceramic package (the so-called first level package) as MEMS are often composed of fragile and/or mobile free-standing parts that can easily be harmed during dicing and assembly. To avoid this damage, a MEMS device should be protected on the wafer level, before dicing. This is possible with the so-called zero-level package [1]. Zero-level packaging is typically done by bonding a capping wafer or die to the MEMS wafer [2]. One route is to bond a micromachined capping wafer (usually glass or silicon) directly to the device substrate. However in that case, high process temperature techniques are required [3]. Other common ways are by using anodic or glass frit bonding. These are preferred because they require lower processing temperature (300-500 °C) than fusion bonding (1000°C). In addition, there has been a considerable fast development in packaging technologies using low temperature wafer bonding [4,5,6]. However, all the above mentioned techniques increase die area, and thus the cost substantially.
Thin film encapsulation is an alternative wafer-level packaging technique with a minimum amount of wasted area [7,8]. By this technique hermetic encapsulation can be achieved by the fabrication and sealing of surface micromachined membranes covering the MEMS. By making the membrane thick enough or by using supports, plastic packaging can still be used as first-level packaging technique [9,10].
In this work, a new generic surface micromachining module for MEMS thin film packaging with a metallic membrane at temperatures below 200 °C based on nickel electroplating is presented. With this technology the MEMS devices can be hermetically sealed and enclosed in a controlled atmosphere and pressure as required for proper operation and ensured lifetime of the MEMS device. The advantages of this integrated packaging technique are the reduced thickness and area consumption, the promise of being a low cost process and the low thermal budget.
State-of-the-art sealing is in general done using horizontal sacrificial etch channels. For example, Stark and Najafi reported surface micromachined caps with horizontal etch channels using nickel electroplating and solder sealing. Release times of several hours were needed [11]. In order to have a high-speed sacrificial release, sacrificial etch holes in the membrane are favourably situated above the device [12] (see Fig. 1). Recently, Rusu et al. reported a versatile sealing method for sealing vertical access holes by using a two layer thin film reflow process. Also in this work sealing is accomplished by using a reflow technique, but at much lower temperatures (180 °C) compared to Rusu et al. (600 °C) [7].
The membrane layer needs to be rigid, strong and it preferably has a low tensile stress in order to prevent any bending or breaking. A high deposition speed is preferred to be able to deposit thick layers in case plastic moulding is required afterwards. We choose a nickel electroplated membrane on top of a sacrificial photoresist layer because of its high young’s modulus (182GPa) [13]. The membrane layer will be structured in order to allow the sacrificial etching of the underlying material. Sacrificial layer access holes are situated above the device (see Fig. 1).
The basic idea of the proposed sealing technique is to deposit a material with low melting temperature on top of openings in the membrane layer (see Fig. 2, step 7). After sacrificial layer etching, this material is then reflowed in a furnace with controlled atmosphere and pressure to close the final openings (see Fig. 2, steps 8 and 9
). An optimization of the thickness of both membrane and sealing layer as well as an optimization of the size and shape of the etch holes is required. These holes should be large enough to enable efficient sacrificial etching but also small enough such that no or only negligible deposition inside the MEMS cavity takes place during sealing. The chosen sealing layer is indium with its low melting temperature (156.61 °C) because it allows a low thermal budget sealing process. Indium has also an excellent ductility which allows joining materials of different thermal expansion coefficients.
The process flow in this work starts with unreleased MEMS devices (Fig. 2, step 2), followed by the deposition and patterning of the photoresist sacrificial layer (sacrificial spacer) (Fig. 2, step 3). Subsequently, the seedlayer for plating is sputtered on (Fig. 2, step 4) and the non-conductive plating mould (photoresist) for the membrane is spun on. In order to reduce release times, etch holes are defined above the device by means of photolithography (Fig. 2, step 5). Then nickel is electroplated until a certain thickness, which is either smaller or larger than the mould resist height (Fig. 2, step 6). In the latter case, with the so-called over-plating process [14], smaller etch ho
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