Additive manufacturing (AM), or 3D printing, is finding increasing applications in virtually all fields including electronics and sensors—starting with plastic prototype housings and custom printed circuit boards (PCBs) . The wide variety of materials that can be used, reduced system parts count, and faster prototyping times are the prime incentives for employing AM. 3D printing of individual industrial and automotive metallic sensors and packages [2,3] is still a relatively expensive option for metal working, and in many cases only costcompetitive with computer numerical control (CNC) casting and welded assembly for low-volume prototyping. To overcome this cost disadvantage, multiple sensors can be fabricated during the same print operation. This is the same commercialization path that was taken for microelectromechanical systems (MEMS) sensors decades ago. The first MEMS pressure sensors used the entire 1” silicon wafer, and now more than 10,000 MEMS sensors are made on 150mm and 200mm wafers. AM allows the engineer to move from a computer-aided design (CAD) file to a structured multi-sensor wafer without the cost and time of photomask fabrication. Figure 1 shows how a 3D-printed metal MEMS 100mm diameter wafer can produce multiple small sensors and leverage wafer fab lithography tooling for building up surface circuitry layers. This titanium wafer was printed using the direct metal laser sintering (DMLS) method.
Figure 1: 3D-printed 100mm titanium wafer.
Most metal additive manufacturing does not stop after the 3D printing step. Challenges associated with AM processes, such as cracks and warpage due to stress, have to be overcome to enable the successful 3D printing of micromachined wafers. For many applications, including micromachined wafers, post-processing will be required. Postprocessing of printed metal parts can start immediately after the printing step. Laser and e-beam fabrication metal products are in a stressed state very similar to a welded metal part. As in the case of welding, an anneal step can reduce some of this built-in stress, which reduces the likelihood of warpage and cracking. Polishing is another post-print step needed for most AM MEMS wafers to produce a smooth, flat surface suitable for subsequent photolithography processing. The use of this patent-pending AM process for making wafer-level packaging (WLP) top cap chips is shown in Figure 2. The same types of cavities and bond pad openings made in silicon wafers (left image in Figure 2) as a cap over MEMS devices using wafer bonding [4,5] can be duplicated with various 3D-printed metals and optically clear plastics. Thick metal wafers for better electromagnetic compatibility (EMC) or rad-hard performance can be printed. Ink jet and 3D-printed coatings of circuit boards have already been explored to provide radiation shielding for space applications . Graded-Z, or graded atomic number, AM layers can enhance the reliability of components, and shielding can start with a chip-scale package (CSP), or as a postsurface-mount cap over the electronic component for space applications.
Figure 2: WLP top cap chips, conventional MEMS silicon (left) and 3D-printed DMLS titanium and SLA clear plastic.
AM capping wafers, or device wafers using a chemically-active metal like titanium, enables the fabrication of a gas gettering surface for vacuum packaging without the need for a thin-film getter [7,8] deposition and patterning steps. The rough as-printed titanium surface shown in the cavity recess in Figure 2 is well suited for a higher surface area impurity-absorbing CSP. The printed wafer material itself can act as this absorbent getter material. Printing a clear plastic, shown on the right of Figure 2, or glass, also can be used in optical sensor applications. As Figure 3 illustrates, wafer-to-wafer (W2W) bonding is used in conventional silicon WLP to partially or fully enclose the fragile MEMS cantilevers’ and resonators’ device elements. 3D printing can combine the capping cavity and moving device element into one wafer. One big advantage to AM fabrication of complex MEMS and microfluidic structures is that it can reduce the number of wafer fab processing steps that would have been required with conventional.
Figure 3: a) (left) Conventional silicon WLP vs. b) (right) 3D-printed WLP.