在20wt%氢氧化钾中加入羟胺制备的微加工MEMS的刻蚀处理特性

Anisotropic wet etching is a most widely employed for the fabrication of MEMS/NEMS structures using silicon  bulk micromachining. The use of Si{110} in MEMS is inevitable when a microstructure with vertical sidewall is to be  fabricated using wet anisotropic etching. In most commonly employed etchants (i.e. TMAH and KOH), potassium  hydroxide (KOH) exhibits higher etch rate and provides improved anisotropy between Si{111} and Si{110} planes. In  the manufacturing company, high etch rate is demanded to increase the productivity that eventually reduces the  cost of end product. In order to modify the etching characteristics of KOH for the micromachining of Si{110}, we have  investigated the efect of hydroxylamine (NH2OH) in 20 wt% KOH solution. The concentration of NH2OH is varied from  0 to 20% and the etching is carried out at 75 °C. The etching characteristics which are studied in this work includes the  etch rates of Si{110} and silicon dioxide, etched surface morphology, and undercutting at convex corners. The etch  rate of Si{110} in 20 wt% KOH + 15% NH2OH solution is measured to be four times more than that of pure 20 wt%  KOH. Moreover, the addition of NH2OH increases the undercutting at convex corners and enhances the etch selectivity between Si and SiO2.

In all kinds of wet anisotropic etchants, Si{111} planes  exhibit minimum etch rate. If the mask edges are aligned  along the directions comprises {111} planes, wet anisotropic etching provides microstructures with smooth  sidewalls due to the emergence of {111} planes at these  directions. Te angle between sidewall and wafer surface  depends on the wafer orientation. Moreover, the number  of directions along which {111} planes appear depend  on the orientation of wafer surface. In the case of {100}  wafer, four {111} planes making an angle of 54.7° with  wafer surface expose at ⟨110⟩ directions. Hence {100}  wafer is suitable to fabricate rectangular shaped cavities  or suspended structures over rectangular shape cavity  using wet anisotropic etching [3, 6, 12, 21]. In the case  of the wafer with {110} surface, two slanted planes making an angle of 35.5° with wafer surface and four vertical  planes with respect to wafer surface appear along ⟨110⟩ and ⟨112⟩ directions, respectively. Terefore, in order to  fabricate microstructures with vertical sidewalls {110}  wafer is a most appropriate choice [10, 28–34]. It can  be used to fabricate deep channels/cavities with vertical  sidewall.

In this paper, we have studied the etching characteristics of {110}-oriented silicon wafer in 20 wt% KOH solution without and with addition of NH2OH. It is mainly  focused to investigate the efect of NH2OH in 20  wt%  KOH to achieve improved etching characteristics for  applications in silicon bulk micromachining for the formation of MEMS structures.

Etching characteristics of Si{110} including etch rate, surface roughness/morphology, and undercutting at convex  corners are studied on the samples etched in pure and  NH2OH-added 20  wt% KOH. To study etching characteristics, the concentration of NH2OH is varied from 5 to  20% in steps of 5%. Detailed descriptions of these characteristics are presented in following subsections.

Fig1

Surface morphology is one of major concerns in optical  MEMS applications and designing high-efciency solar  cell, etc. Etched surface morphology primarily depends  on the etchant type, etchant concentration, etching  temperature, additives, and agitation of etchant during  etching process. Te average etched surface roughness  decreases with the increase of etching temperature and  KOH concentration . Surface roughness  of the samples etched in 20 wt% KOH solution without  and with addition of various concentrations of NH2OH is  measured using 3D measuring laser microscope (Olympus, OLS4000) and presented in Fig.  4. Te standard  deviation indicated by error bars is calculated by taking six measurements on the same sample at diferent  locations. Figure  5 presents SEM images of the etched samples corresponding to surface roughness shown in  Fig.  4. Average surface roughness (Ra) of the samples  etched in NH2OH-added 20 wt% KOH solution is nearly  same as those are etched in pure 20 wt% KOH. It means  that the etched surface of Si{110} is not afected signifcantly when NH2OH is added into KOH solution. Main  cause of surface roughness in the wet etching process is  micromasking by the hydrogen bubbles and/or impurities on the surface during the etching process .

Etching characteristics of Si{110} surface in pure and  diferent concentration of NH2OH-added 20  wt% KOH  are studied for applications in silicon wet bulk micromachining. Te etching characteristics of KOH solution are  changed drastically when NH2OH is added. Te etch rate  and undercutting are improved signifcantly. High etch  rate is very useful to achieve larger etch depth in less time  in comparison to common etchant. Increase in undercutting at convex corner is benefcial for the fast release of the microstructures. Both these characteristics are  indispensable for reducing etch time and therefore useful for industries to increase the productivity. Moreover, NH2OH-added KOH provides high etch selectivity  between silicon and oxide (i.e. Si/SiO2) in comparison to  pure KOH. High etch selectivity can be exploited for the  fabrication of MEMS structures using silicon dioxide as  mask/structural layer. It can be concluded that the results  presented in this paper are highly useful for research and  industrial applications.