Introduction
Micro-electromechanical Systems (MEMS) is a technique used to manufacture miniaturized mechanical and electro-mechanical components using semiconductor manufacturing processes such as ion implantation, diffusion, oxidation, sputtering, etc., in combination with specialized micromachining techniques. Both the mechanical parts and the electronics that control them are built in the same piece of silicon. The resulting devices are also called MEMS.
MEMS devices layout varies from a simple structure without moving parts to complex electro-mechanical components having several moving parts controlled by integrated circuits. The size of the devices also varies from less than one micron to several millimeters. The functional elements of the devices are few, consisting of transducers (micro sensors and micro actuators), some other micro structure, and microelectronics used to control the MEMS. In the last two decades, several types of MEMS sensor have been developed covering all sensing modes, including temperature, pressure, humidity, inertial forces, magnetic fields, radiation, chemical species, etc. These sensors normally exceed the performance of their macroscale similar, and because their fabrication process uses the same batch fabrication techniques used in the semiconductor production cost per device is much lower that corresponding macrodevice. The combination of low production cost and better efficiency has provided numerous commercial market opportunities.
MEMS fabrication techniques
There are several fabrication techniques used nowadays. Normally to fabricate a MEMS device more than one of these techniques is used. There are three features that must be considered in MEMS fabrication technologies: (a) miniaturization allows the fabrication of compact and quick response devices, (b) multiplicity refers to the capability of the fabrication techniques, inherent in semiconductor processes, that produces thousands or millions of units concurrently, and (c) microelectronics to provide the merger of sensors, actuators and logic in a unit, so that feedback (intelligent) mechanisms can be implements. To fabricate MEMS combine the first two features, both inherited from the semiconductor IC fabrication technology, with a sophisticated micromachining processes. Let’s look at some of these techniques.
IC fabrication
This process is the normal technique used to manufacture typical integrated circuits. The fabrication consists in the application of the following steps, normally several times during the manufacturing. The process starts with a polished silicon – the substrate - wafer that undergoes these steps.
· Thin film growth. In order to build active and passive components, a thin film should be deposited on the wafer. The type of film includes epitaxial silicon, silicon dioxide (SiO2), silicon nitride (Si3N4) polysilicon, a metal and others.
· Doping. In order to control the conductivity of the wafer at different locations a low level of impurities (boron, phosphorous, etc.) should be added by thermal deposition or ion implantation.
· Lithography and etching. Using masks designed to produce certain patterns on the wafer and using a photosensitive chemical called photoresist a pattern is generated and transferred to the wafer. This process, called photolithography, is used to either add impurities or to etch the wafer in selected locations. Etching is a process used to selectively remove unwanted regions of the thin film or substrate in order to delineate and shape the components. There are two modalities of etching: wet and dry etching.
Figure 1: shows the main steps of the IC fabrication process.
To fabricate MEMS, we mention this before; it is needed to add a micromachining process. There are several available.
Bulk micromachining
This is the oldest micromachining technology; it is used to selectively remove substrate to shape mechanical components. It can be accomplished using physical (dry etch) of chemical means (wet etch). Chemical etching is widely used because it can provide high selectivity and high etch rate. These two parameters can be altered by simply changing the chemical composition of the etching liquid.
Wet etching involves the immersion of the substrate (wafer) into a chemical reactive that will remove exposed regions of the substrate at measurable rates. There are two types of wet etching in bulk micromachining: isotropic and anisotropic etching. In isotropic etching, the etch rate is equal in all directions, including under the masking area and laterally as well. Lateral etching, however is much slower than vertical etching if the process is performed without stirring the system. This is the reason isotropic wet etching is always performed with vigorous stirring to avoid deformation of the etching profile. Figure 2 illustrates the profiles using an isotropic etchant with and without stirring the system.
Figure 2: Illustration of the etch profile, with and without stirring, using an isotropic wet chemical etchant. Credit: https://www.memsnet.org/
In bulk silicon micromachining anisotropic etching is more widely used. In this process, the chemical solution used has an etch rate dependent on the crystallographic orientation of the substrate. Figure 4 illustrates the profiles of <100> oriented silicon wafer.
Figure 3: Shape of etch profiles of <100> oriented silicon substrate using anisotropic etchant. Credit: https://www.memsnet.org/
Surface micromachining
This is also a very popular technique for the fabrication of MEMS devices. It consists of a series of steps starting with the deposition of a thin film material that acts as a temporary layer over which the permanent mechanical structural (device) layers will be built. The initial thin film deposition is followed by the deposition and patterning of the material that will become the permanent structure. Then, the removal of the temporary layer releases the mechanical structure.
An illustration of this technique is shown in Figure 4 where it is depicted the manufacturing of a cantilever. The deposition and patterning of silicon oxide as the first step. This oxide layer is temporary – also called sacrificial layer. Subsequently a polysilicon layer is deposited and patterned. This layer forms the permanent structure. Finally the sacrificial layer is removed leaving the permanent structure in place. In this example, we created a cantilever with an anchor point.
Figure 4: Fabrication of a cantilever bean using surface micromachining
High-aspect ratio techniques
The techniques described so far are not suitable if the structure to be fabricated has a high-aspect ratio (HAR). Aspect-ratio is the ratio of depth to width of an etched feature. For structures with a HAR there are some techniques available. Two of the most widely used are introduced here.
Deep reactive ion etching
Deep reactive ion etching (DRIE) is a relatively new technique used to fabricate MEMS. With it, we can fabricate HAR structures where the etched holes (or vias) are nearly vertical and their depth can be thousands of microns. This technology uses dry plasma etching. The substrate is positioned inside a plasma reactor and it is bombarded by heavy ions to remove the exposed substrate. The following figure shows the cross section of a high density plasma reactor. The substrate is placed at the bottom on an RF-powered chuck where it is bombarded with accelerating etching species (ions).
Figure 5:High-density plasma reactor used in DRIE etching. Courtesy of https://www.corial.com/
Figure 6 illustrates how deep reactive ion etching is performed. The figure shows the etching of long walls and the deposit of an etch resistant polymner layer on the sidewalls. The etching is performed using SF6 ions.
Figure 6: DRIE etching. Credit: https://www.corial.com/
Figure 7 shows an actual Scanning Electron Microscope (SEM) image of a MEMS device using the DRI*E method.
Figure 7: SEM image of a MEMS device fabricated using DRIE technology. Credit: https://www.corial.com/
LIGA
The LIGA is a very popular method to create HAR structures for the fabrication of MEMS devices. The name represents the German description of the process: Lithographie, Galvanoformung, Abformung. An English translation is: Lithography, Electroplating, Molding. This is a non-silicon-based process that requires X-ray radiation. Normally LIGA uses a type of photoresist called polymethyl methacrylate (PMMA) which is a polymer that is well suited for imaging processes like LIGA. The fundamental process is described in Figure 8.
Figure 8: Steps in a basic LIGA process. Credit: https://www.memsnet.org
The process, as seen in the diagram starts by depositing PMMA on a substrate. For the X-ray exposure of the PMMA a special X-ray mask is used. After development a very well defined structure with perfect vertical sidewalls is created. It is important to notice that the X-ray exposure penetrates deeply into any thick PMMA layer. After this step, the patterned PMMA becomes a polymer mold that can be electroplated by immersing it into a electroplating bath of nickel that covers all open areas of the PMMA. Finally, the PMMA is removed leaving a solid metallic structure. Figure 9 shows an SEM image of a tall gear with a high aspect ratio.
Figure 9: A SEM image of a HAR of a gear. Credit: https://www.memsnet.org
Other techniques
Besides the techniques introduced in this article, there are other processes the MEMS industry uses. Some of these include: XeF2 Dry Phase Etching, laser micromachining, electro-discharge micromachining, focused ion-beam micromachining, hot embossing, wafer bonding, and others.
