High-Aspect-Ratio Structures for MEMS
- PDF / 1,390,593 Bytes
- 2 Pages / 612 x 792 pts (letter) Page_size
- 91 Downloads / 233 Views
High-Aspect-Ratio Structures for MEMS Stella W. Pang Many microelectromechanical systems (MEMS) use the changing capacitance of movable parallel plates to drive and sense motion.1–3 An increase in this capacitance improves the performance of these micromechanical structures by means of increased electromechanical coupling for lower driving voltages and increased sensitivity of the micromechanical motion. This means that a high-aspect-ratio trench etched with large depth or small gaps is desirable because it provides increased capacitance. In addition, deep trenches allow thicker devices to be fabricated, thus increasing mass, which is advantageous for inertial sensing applications. From the mechanical point of view, having a larger thickness associated with high-aspectratio structures also increases the mechanical stability of the released features and avoids stress-related bending.4 These high-aspect-ratio structures are often formed by etching or electroplating. Dry etching has been developed for the patterning of high-aspect-ratio structures. Since directional ions are generated in the plasma, dry etching can produce anisotropic profiles by controlling the density, energy, and distribution of the reactive species in the discharge. Typically, highdensity plasma sources such as electron cyclotron resonance or inductively coupled plasma are used with separate power sources to generate the plasma and bias the wafer.5,6 These plasma sources have the advantages of providing nearly independent control of ion density and ion energy for improved etch characteristics, as well as providing dense plasmas for high etch rates and throughput. Often, fluorine-containing gases are used to dry-etch high-aspect-ratio deep trenches.7,8 Because fluorine tends to etch Si isotropically, polymer passivation of the side walls is needed to prevent undercutting and allow etching in the vertical direction.9 This technique has the advan-
MRS BULLETIN/APRIL 2001
tages of high etch rates and high selectivity to etch masks. However, roughness can form along the side walls due to cycling and passivation effects, and the polymer passivation may prevent the formation of narrow trenches with a high aspect ratio. An alternative—and simpler—technique for etching high-aspect-ratio Si structures is to use Cl2 chemistry.10–12 No passivation is needed, as Cl2 etching is an ion-assisted process and it tends to be anisotropic. The simpler process, without passivation or switching cycles, makes Cl2 etching easier to control and more reproducible. Smooth side walls can be obtained because the etch and deposition cycles create no surface roughness. It is especially useful for the etching of trenches with submicrometer dimensions, whereas the need for passivation using fluorine chemistry often limits the width of the trenches that can be
patterned. The potential drawbacks of Cl2 etching of Si are a slower etch rate and lower etch selectivity to mask. When microsensors with high-aspectratio structures are etched, it is essential to maintain a constant etch
Data Loading...
