The "Micro-Electro-Mechanical Systems (MEMS)" device manufacturing frequently uses in the most of the cases complex methods, where it is necessary to iplement specialized tools that could solve the micro-machining needs. Within these processes are generally found process, photolithography, thin films coating through thermal evaporation and sputtering, silicon wafer oxidation, different material substrates union and deep etching using reactive ions "Deep Reactive-Ion Etching (DRIE)". In this note we will focus on the DRIE system available in the facilities of the CIDESI Microtechnology Management.

The video shows the MEMS manufacturing process using the DRIE system "Deep Reactive Ion Etching" located at CIDESI Queretaro clean room.

What is the operation principle of the DRIE system?

The DRIE system, (Figure 1) is characterized by performing dry etching (plasma based) over silicon wafers using the "Bosch" method named after Robert Bosch.

Fig. 1. DRIE system "Deep Reactive-Ion Etching" of the CIDESI Queretaro clean room.

The process is characterized for being a highly anisotropic recording process that is used to create deep penetration perforations (from the point of view of micro-fabrication) for the creation of complex structures during the MEMS manufacturing where the etching via humid processes can not solve, it is characterized by implementing 3 fundamental steps: protecting layer coating, protecting layer pattern etching and silicon etching, these are of short span which represent an etching cycle (19 seconds approximately) and the quantity of material removed is controlled by the cycles quantity. However, the complex procees consists of 5 steps shown in figure 2, which are next described:

  1. Pattern definition: In this step it is defined by means of photolithography processes the pattern to be portrayed on the surface of a silicon wafer, the adventage is that any 2D geometry can be implemented to subequently penetrate into the silicon bulk, unlike humid etching that depend totally of the crystallographic orientation of Silicon. It is very important to consider the type of material that is going to be used to define the pattern due it will allow to reach some desired depths depending on the selectivity respect to silicon, generally photoresin, SiO2, Si3N4, Cr, can be used, among others.
  2. First etching (SF6):  This process is characterized by performing the first Silicon etching and defining the beginning of the cavity, where the protecting layer is also damaged, but in a lower level, for this process a plasma is used based on SF with a small part of O2 without ion acceleration, it is important to mention that the time is relatively very short 1-2 seconds due the most it lasts the more damage the walls suffer.
  3. Pasivación (C4F8): In this process a teflon protecting layer is deposited using plasma based on C4F covering in a conformal way surfaces and cavities, with the goal of protecting the walls.
  4. Second etching (SF6) with “bias”: This step is used to remove the protecting teflon previously deposited using a plasma based on SFwith O2 but with an acceleration towards the surface achieving the removal of the protecting layer located on the silicon surface and on the cavity base leaving only the protection on the walls.
  5. Second etching (SF6) without “bias”: This step is immediate after the previous one with the difference of no using the ions acceleration and a bigger pressure inside of the chamber to generate the next etching in the silicon cavity, in a short time, to avoid the wall damage.
  6. Etching cycle: This process consists in repeating steps 3, 4 and 5 the times it is necessary, a complete cycle may take around 19 seconds depending on the recipe tu use, that at the same time depends on the structures to define, the speed of the etching depends also on the exposed area of the cavity so it is important to always characterize the etching speed for each particular sample with the availability of performing small adjusts in the recipes.

Fig. 2. General description of the DRIE process.

What kind of devices can we manufacture with the DRIE system?

The applying area is very wide for this kind of systems, it goes from electronic devices such as capacitors to systems with micro-mechanisms and even micro chambers for flow or gas sensors. In figure 3 different kinds of applications can be observed that the CIDESI microtechnologies working team has performed using the DRIE system.

Fig. 3. Applications obtained through the DRIE system, (A) contact angle increase (hydrophobicity) (B) and (C) Surface area increase for supercapacitors with a high dielectric constant.

  1. Super-hydrophobicityThe figure 3 (A y B) and figure 4(B) show an arrengement of trenches that limit the contact area of the surface with water, additionally a coating was added with a surface treatment which avoids water adhesion on the surface, where mixing mechanical and chemical combinations on the surface applications can be achieved where super-hydrophobic surfaces are required.
  2. Gas and flow sensors and filtered: The figure 3 (C and D) show cavities with different configurations which allow the creation of micro-chambers and micro-channels that provide the posiblities of manufacture flow sensors and particle filters and micro-channels.
  3. Membranes:  Figure 3 (D and E) show Si3N membranes manufacured through the creation of a circular cavity using the DRIE system, these membranes are used as Windows for x-Ray applications.
  4. Súper-capacitors Figures 4 (A and C) show images obtained through Scanning Electronic Microscopy (SEM) of high aspect relation trenches with the goal of increasing the surface area of a small bidimensional area in comparison, and to be able to obtain super capacitors with a high dielectric constant.
  5. Pressure sensors:  The figure 3 (F) shows the back part of a silicon wafer where the cavity is created for pressure sensors manufacture based on piezoresistive efects on membranes.

Fig. 4. Scanning Electronic Microscopy of manufactured structures by DRIE system, (A) cross-section of high aspect relation trenches, (B) Cross-section in a micro-pillars angle, (C) angle sight of high surface area trenches and (D) cross-section of a mixture of structures.

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