多晶硅表面改性技术以减少微观结构的粘附

A new and simple surface-modification technique is proposed to reduce sticking of microstructures fabricated by surface micromachining.  This technique realizes a very rugged surface at the polysilicon substrate, resulting in reduced sticking through a decrease of real contact area.  The surface, which consists of honeycomb-shaped grain holes at the polysilicon substrate layer, is defined by a two-step dry etch without an  additional masking step for photolithography or deposition of thin films. By varying the time for etching the grain holes of the polysilicon  substrate, controlled surface roughness can be obtained. Test structures, including polysilicon cantilever beams of various lengths, fabricated  by surface micromachining with the proposed surface modification show a doubled detachment length without sticking to the substrate.  

The production yield and the reliability of microelectromechanical systems (MEMS) fabricated by surface micromachining are reduced by the irreversible sticking of freestanding microstructures to the substrate. The capillary force  of rinsing liquid between released microstructures and the  underlying substrate causes mechanical contact with each  other during the post-release dry. After the liquid is completely dried, the surface tension of the two solids at the  contact interface area brings about permanent sticking of the  microstructures.  

Various anti-sticking techniques have been proposed by  many researchers and these techniques can be classified into  two categories. One is an effort to prevent the sticking by  eliminating the capillary force of the rinsing liquid with supercritical CO2 drying  or by sublimation of the frozen rinsing  liquid. The other is to alleviate the surface energy of the  interface between the contacted solids by reducing the real  surface contact area. This can be done by introducing antisticking dimple (s) or mesas, and by increasing the surface roughness of the substrate plane with a texturization  technique .  

In this paper, a new technique is proposed to increase the  surface roughness of the polysilicon substrate without any  additional photolithographic masking step. No change in conventional silicon processing for IC fabrication is needed for  this technique. This technique was initially proposed to  enlarge the capacitance of a node capacitor for high-density  dynamic random access memory (DRAM) cells. Polysilicon grain-sized trenches are formed at the polysilicon substrate by a two-step dry etch. These trenches were named  'grain holes'. After the formation of grain holes at the polysilicon substrate, the area density of surface energy of this  layer beneath the released microstructures decreases significantly through the reduction of the real contact area.  

Fig1

From the previous equations, it is known that the detachment length of cantilever beams can be increased by reducing  the surface energy density of the solid (Ys) with the proposed  surface-modification technique by grain-hole formation at the  polysilicon substrate, by rinsing with a liquid of low surface  tension, and by drying at elevated temperature. The surface  roughness of the grain-holed polysilicon can be controlled  through three important parameters of the grain holes: the  average depth of the grain holes; the average width of the  remaining polysilicon side walls; and the average grain size.  But in this research, only the average depth of the grain holes  was varied with the other parameters remaining fixed.  

Cantilever beams were fabricated by surface micromachining as test vehicles. 10 μm wide cantilever beams of  various lengths from 20 μm to 1 mm with 20 μm step  increases were designed. The beams were fabricated over an  ordinary polysilicon substrate without modification and over  a grain-holed polysilicon substrate of the same batch for comparison.