MEMS from Additive Manufacturing
The next frontier in MEMS manufacturing
The additive manufacturing (AM) processes applied to the micrometer range are subjected to intense development motivated by the influence of the consolidated methods for the macroscale and by the attraction for digital design and freeform fabrication.
The integration of AM with the other steps of conventional micro-electro-mechanical systems (MEMS) fabrication processes is still in progress and, furthermore, the development of dedicated design methods for this field is under development. The large variety of AM processes and materials is leading to many process attempts, setup details, and case studies.
The development of a MEMS based on 3D printing and AM always needs a specific design approach. The knowledge of the available process typologies, combinations, performances, and available materials is mandatory to improve the manufacturability and sustainability of micro 3D-printed devices. The common advantage of all AM methods is the direct building from the digital geometry file or model (computer-aided design, CAD) to the real component.
Powder bed fusion (PBF)
The powder bed fusion (PBF) method was introduced the first time by Deckard and Beaman. It is based on a feedstock where the powder is exposed selectively to a descendent energy source that causes the powder to sinter or melt. Depending on the energy source (laser, electron beam, infrared, thermal) and powder pre-heating, the process gets different names. The materials compatible with PBF processes are metals, polymers, and ceramics, although other typologies can be used as composite powders, calcium carbonate, and sand. With exposure to the energetic source, the powder is locally sintered or melted and the object is created layer by layer. Vertically movable stages and powder re-coaters are needed to run the process into confined chambers with a controlled atmosphere. When the structure is completed, the un-exposed powder is removed and possibly reused. The standard manufacturing guidelines indicate a minimum feature size of 500 um for PBF of polymers and metals.
Powder bed binder jetting (PBBJ)
The PBBJ is applicable to a large variety of materials, including polymers, metals, sand, ceramics, andmixtures of them. The powder grains are deposited togetherwith a liquid binding agent working as cohesive media.
The layers are sequentially deposited upon a platform that moves progressively downwards. The result of the first phase of the process is a 3D object called the “green part”. The green part is then subjected to extensive post-processing, including debinding, sintering and, eventually, infiltration and hipping (hot isostatic pressure) to remove the binder solution and increase the mechanical strength by reducing porosity. In the case of ceramics, the resolution of PBBJ is largely variable in the range 22–500 um, while in the case of metals and polymers the typical feature size is about 100 um, with variations related to the powder grains dimension. In fact, the performances of surface roughness and mechanical resistance increase with finer powder below 20 um. The presence of the binder is responsible for accuracy issues in terms of shrinkage and deviations from the nominal dimensions.
Selective laser sintering/melting (SLS/SLM)
In the SLS, a laser beam is directed against the upper surface of the powder. The SLS is the most common AM process for polymers (nylon, polycarbonate, polymer composites, etc.), although metals, ceramics, hydroxyapatite, and glasses are also compatible. The 3D object is obtained as layer-by-layer fusion and sintering of the powder, by eventually pre-heating the material with inert atmosphere. The vertical stage is moved downward, a powder layer is deposited by the recoater arm and then exposed to the laser source. The maximum accuracy is in the range 40–100 um.
The SLM process is addressed to metals. It is very similar to SLS, but the powder particles are heated until the full melting by using laser beams with higher power. This difference provides higher mechanical strength of the processed materials than SLS. The process sequence is similar to SLS, where the layer thickness ranges between 20 and 100 um and the metal powder granulometry is about 20–50 um. The minimum feature size reported for SLM is 40–200 um.
Powder directed energy deposition (PDED)
The PDED process, also called direct laser metal deposition (DLMD), is based on injection feedstock of powder supported by robotic systems with multiple axes. The heat source is generally represented by a laser source, or by electron beam, plasma, electric arc, etc.
The injector and heat source are coupled, then the metal powder injected is immediately melted and deposited on the target surface, where the temperature rapidly decreases and causes the solidification. The physical and chemical bonding between the target and the deposited materials is obtained. The applicable strategies of injection include the lateral and off-axis orientation, and the continuous or discontinuous powder injection.
The mechanical characteristics of the process limits the resolution between 500 and 3000 um. The process variant associated with the lowest values of this range is also defined micro-PDED/DLMD process (u-PDED/DLMD). Some features with 20 um single pattern tracks were fabricated by u-PDED/DLMD.
Electron beam melting (EBM)
The EBM process is applied exclusively to pre-heated metal powders in a vacuum chamber. An electron beam heat source is used to fully melt the powder grains. The pre-heating increases the stability and compactness of powder, then suspended parts can be obtained with reduced supports density, as a general rule. The process resolution is in the range 100–200 um. For example, different versions of scaffolds with gyroid shape and composed of unit cells with a minimum feature size of 500 um were built.
Multi jet fusion (MJF)
The multi jet fusion (MJF) process, introduced by Hewlett-Packard (HP), belongs to the PBF category and operates with polymers. Generally, it is used for low volumes and fast components production with high mechanical strength as an AM alternative to the traditional injection molding. The powder feedstock is similar to the other PBF processes but the polymer material is fused by using an infrared heating source combined with chemical agents. The print surface is pre-heated to a uniform temperature and then a thin layer of powder is deposited on it. The HP thermal inkjet of the printing head is then used to deposit on the powder layer a combination of fusing and detailing agents on different selected areas. After that, the print surface is exposed to an infrared source and the powder is fused only where the fusing solution (i.e., radiation absorbing agent) is present. The available polymers for MJF are polyamide (PA11, PA12) and thermoplastic polyurethane (TPU). MJF is suitable for printing functional mechanical parts or devices, biomedical lattices structures, medical orthotics and prosthetics, mechanical tools, and fluid-tight devices.