Micromechanics
The micromechanics is the field of microsystem technology that deals with design, manufacture and use of mechanical devices with dimensions of a few to several 100 microns concerned. A distinction is made between simple structures (e.g. grids, holes, channels), sensors , actuators (e.g. relays , switches , valves , pumps ) and microsystems (micromotors, print heads ). Technologies are used for production that are also used in microchip production (e.g. galvanic processes, etching processes , laser technology ), but photolithography , thin-film , screen printing and LIGA technology are also used. The direct tool-free manufacture of micromechanical plastic components with the patented RMPD techniques (RMPD = Rapid Micro Product Development . See Rapid product development possible).
Silicon bulk mechanics
Free-standing mechanical structures are obtained from a silicon wafer by etching on one or both sides . They arise from the etching of silicon in alkaline solutions (usually potassium hydroxide solution, KOH solution), which is dependent on the crystal orientation; this process is often called anisotropic wet etching, which does not exactly reflect the nature of the etching process. The anisotropic character of the etching process results from the different etching rates of the different crystal directions of the silicon (see Miller's indices ). The two essential crystal planes are the {100} Si and the {111} Si planes. The etching rate RR is approximately RR {111} : RR {100} = 1: 100, depending on the process parameters , that is, the {100} Si planes are etched 100 times faster than the {111} Si planes. The reason for this difference lies in the number of atoms on the respective surface.
For the production of structures it is necessary to mask areas of the substrate surface so that the etchant cannot attack here. Typical etching masks are layers made of silicon nitride or silicon dioxide , which have significantly lower etching rates in KOH solution compared to silicon. Only the part that is not covered by an etching mask (unmasked areas) is etched.
The resulting structures depend on the substrate or the substrate orientation and the masking. Starting from a {110} Si wafer (right illustration), trenches with vertical walls, the {111} Si surfaces, are created during etching, which form a kind of natural etch stop due to the significantly lower etching rate. If, on the other hand, a {100} Si wafer is etched (left figure), trapezoidal trenches are initially created . The inclined side walls are here again the {111} Si surfaces. The tilt angle of 54.74 ° results from the diamond structure in which silicon crystallizes. If the etching time is long enough, the two side surfaces of the now V-shaped trench touch each other.
Silicon surface micromechanics
Mechanical structures are obtained through several etching and deposition processes on the wafer surface. The particular advantage of this technology is that the micromechanical structures can be combined with electrical circuits on a microchip; sometimes even joint process steps between mechanical and electrical parts are possible. This integration not only reduces manufacturing costs, but also realizes solutions that would be inconceivable with spatial separation of electrical and mechanical components, for example because of parasitic capacitances at electrical connections between the components.
The micromechanical systems already implemented include electromechanical switches for high-frequency applications, mechanically tunable capacitors and inertial sensors .
Manufacturing steps
Using the example of a capacitive acceleration sensor, the possible process steps in silicon surface micromechanics are to be illustrated:
First, a sacrificial layer made of a material that can later be removed again in a wet etching process is deposited on the top silicon layers of the electrical process . This sacrificial layer is now etched away down to the silicon layer at the points where supports for the mechanical structure are to be created later.
In the subsequent process step, silicon is deposited into the gaps that arise, so that a polycrystalline silicon layer is created, which is connected to the lower layer by supports.
After the newly created layer has been structured by a further etching process step (e.g. anisotropic dry etching ), the sacrificial layer under the polysilicon layer can be removed in a wet etching process , so that an almost arbitrarily structured layer is created.
In the case of an acceleration sensor, this layer contains a large area (reference mass) and thin webs that connect this area to anchored structures (that is, connected to the lower layer by columns). These webs act as bar springs so that the reference mass is movable under the influence of forces.
If the entire chip is now accelerated in the chip plane, a force acts on the reference mass, so that it is deflected from its rest position. This changes the distances and thus the capacities between this movable part and the adjacent immobile structures in the plane. These mostly very weak changes in capacitance can now be evaluated by the CMOS circuit on the same chip.
In order to make the changes in capacity as large as possible, the reference mass and immovable parts are usually designed as a structure of interlocking combs: the deeper the teeth of the combs dip into one another, the higher the capacity.
literature
- Ulrich Hilleringmann: Microsystem technology: process steps, technologies, applications . 1st edition. Vieweg + Teubner, 2006, ISBN 3-8351-0003-3 .
- Stephanus Büttgenbach: Micromechanics: Introduction to technology and applications . 1st edition. Teubner, 1991, ISBN 3-519-03071-3 .