Franz Laermer

Deep Reactive Ion Etching

This chapter discusses the deep reactive ion etching in detail. Reactive Ion Etching (RIE), also known as plasma etching or dry etching, and its extension deep reactive ion etching (DRIE) are processes that combine physical and chemicals effect to remove material from the wafer surface. Etchant phase, gas or liquid, has been used as a dividing factor: wet etching in liquids vs. dry etching in gaseous environment, usually in vacuum. Silicon and its compounds can be etched in fluorine, chlorine or bromine plasmas. Oxygen is an active ingredient in many etching processes. Ion beam etching relies on energetic argon ions and differs thus from RIE. The division between RIE and DRIE can be made according to etch rate, selectivity, aspect ratio capability or reactor type. The main etching mechanism of the masking material is typically physical sputtering. Therefore, the masking material is etched relatively fast in this kind of RIE equipment. The parameters available for DRIE process optimization include etchant gases, flow rate of the chosen gases, RF-power, bias voltage, process pressure and temperature. Both Bosch and cryo-processes use purely fluorine-based plasma chemistry, since fluorine-based etching processes for silicon offer superior etch rates and high mask selectivities. The fastest and the simplest way to create patterns is to use photoresist as an etch mask. Silica and glass also overcome many limitations of polymers because of their low auto-fluorescence, mechanical wear resistance, reusability, and smooth surfaces. Clamping can be realized with two different methods, mechanically or electrostatically. Micromasking in glass etching is similar to silicon etching: small particles of mask material sputter off and land on areas to be etched. Polymers can be etched by oxygen plasmas using silicon, metal and oxide masks. The notching effect is seen when high-density plasma etching reaches an insulator surface.

A new capacitive type MEMS microphone

A new capacitive type of MEMS microphone is presented. In contrast to existing technologies which are highly specialized for this particular type of application, our approach is based on a standard process and layer system which has been in use for more than a decade now for the manufacturing of inertial sensors. For signal conversion, a mixed-signal ASIC with digital sampling of the microphone capacitance is used. The MEMS microphone yields high signal-to-noise performance (58 dB) after mounting it in a standard LGA-type package. It is well-suited for a wide range of potential applications and demonstrates the universal scope of the used process technology.

Monocrystalline Si membranes for pressure sensors fabricated by a novel surface micromachining process using porous silicon

We developed a novel surface micromachining process to fabricate monocrystalline silicon membranes covering a vacuum cavity without any additional sealing steps. Heart of the process is anodic etching of porous silicon, annealing and epitaxial growth. The porous silicon layer consists of two parts, a starting mesoporous silicon layer with low surface porosity and a nanoporous silicon layer with a high porosity. The following annealing step removes native oxide within the later cavity, and the surface is sealed for the subsequent epitaxial layer deposition. The observed stacking fault density in the epitaxial layer about 1E5 cm-2. The temperature budget of the following ASIC-process leads to a complete transformation of the nanoporous silicon layer into a large cavity. The whole structure can be used as a pressure sensor. The estimated pressure in the cavity is smaller than 1 mbar. First integrated pressure sensors have been fabricated using this process. The sensors show a good linearity over the whole pressure range of 200 mbar to 1000 mbar. This novel process has several advantages compared to already published processes. It is a “MEMS first” process, which means that after the epitaxial growth the surface of the wafer is close to a standard wafer surface. Due to full IC compatibility, standard ASIC processes are possible after the fabrication of the membrane. The use of porous silicon enables a high degree of geometrical freedom in the design of membranes compared to standard bulk micromachining (KOH, TMAH). The monocrystalline membranes can be fabricated with surface micromachining without any additional sealing or backside processing steps.

Chapter Twenty Three – Deep Reactive Ion Etching

Publisher Summary This chapter discusses the deep reactive ion etching in detail. Reactive Ion Etching (RIE), also known as plasma etching or dry etching, and its extension deep reactive ion etching (DRIE) are processes that combine physical and chemicals effect to remove material from the wafer surface. Etchant phase, gas or liquid, has been used as a dividing factor: wet etching in liquids vs. dry etching in gaseous environment, usually in vacuum. Silicon and its compounds can be etched in fluorine, chlorine or bromine plasmas. Oxygen is an active ingredient in many etching processes. Ion beam etching relies on energetic argon ions and differs thus from RIE. The division between RIE and DRIE can be made according to etch rate, selectivity, aspect ratio capability or reactor type. The main etching mechanism of the masking material is typically physical sputtering. Therefore, the masking material is etched relatively fast in this kind of RIE equipment. The parameters available for DRIE process optimization include etchant gases, flow rate of the chosen gases, RF-power, bias voltage, process pressure and temperature. Both Bosch and cryo-processes use purely fluorine-based plasma chemistry, since fluorine-based etching processes for silicon offer superior etch rates and high mask selectivities. The fastest and the simplest way to create patterns is to use photoresist as an etch mask. Silica and glass also overcome many limitations of polymers because of their low auto-fluorescence, mechanical wear resistance, reusability, and smooth surfaces. Clamping can be realized with two different methods, mechanically or electrostatically. Micromasking in glass etching is similar to silicon etching: small particles of mask material sputter off and land on areas to be etched. Polymers can be etched by oxygen plasmas using silicon, metal and oxide masks. The notching effect is seen when high-density plasma etching reaches an insulator surface.

1.08 – Dry Etching

Deep reactive ion etching (DRIE) of silicon, notably the Bosch DRIE process, has virtually changed microsystem technology over the past decade. Since then, a wide variety of silicon-based microsystems have been implemented in high-volume industrial applications. An important example is inertial sensors for measuring acceleration and yaw rate. This chapter starts with a historic overview of the origin of this micromachining technology. The background of different microstructuring technologies, including classical wet etching, LIGA technology, and early plasma processing, is given. Special emphasis has been given for comparing different solutions for silicon DRIE, both from the process side and from the equipment side. The Bosch process and its most important features are described in detail. Key issues related to DRIE, for example, high-speed etching, mask selectivity, and micromasking, reactive ion etching (RIE) lag effect, sidewall scalloping, and notching at dielectric etch stop layers are analyzed and corrective measures have been suggested.

10 – Mechanical Microsensors

Publisher Summary This chapter discusses the applications of mechanical microsensors. Micromachining technologies have enabled a reduction in the size of mechanical sensors and an increase in their functionality to unprecedented levels of miniaturization. The sensor applications that so closely link the microworld and the macroworld involve the largest complexity of all the applications discussed in the chapter, which is, in many cases, a true obstacle preventing the successful replacement of classical sensing elements by microsensing devices. Another difficulty is that the microsensing cell is fixed to a rotating part, with additional issues raised by the supply of energy from and the transfer of information to the environment. Telemetric systems based on electromagnetic fields for both wireless energy transfer and information exchange are solutions to sensing on moving objects and are attracting growing attention from microelectromechanical systems (MEMS) developers. The complexity of combined microsystems and macrosystems has put effective barriers against a stronger penetration of MEMS into the application fields.