Micropyramidal hillocks on KOH etched {100} silicon surfaces: formation, prevention and removal

Abstract

The formation, prevention and removal of micropyramids at the {100} bottom of anisotropically etched cavities are investigated and discussed. In the case of pure KOH solutions the base of micropyramids has been found to be always rectangular or octagonal shaped. The formation is independent of the KOH supplier and the etchmask opening process. The arrangement of the rectangular based micropyramids on the {100} etch bottom depends on the etching time, etching position of the chip (vertical or horizontal) and on the oxygen content in connection with the thermal history of the wafer material. For both types of micropyramid the surface density and size increase with decreasing KOH concentration and etching temperature. Moreover the proportion of octagonal micropyramids rises in those conditions. The origin is discussed in terms of micromasking by H 2 -bubbles in connection with plateau generation and layer by layer peeling of {111} planes. Other mechanisms which are suggested in the literature are discussed. Furthermore it was exhibited that already arisen micropyramids can be removed by a short re-etching in the same etch bath in the same conditions.

Development of etch hillocks on different Si(hkl) planes in silicon anisotropic etching

“…7f). Similar observations, concerning natural hillocks have been already presented in the literature [1,2,4,6].…”

“…The problem of hillocks formation has been widely discussed in the literature concerning silicon etching in KOH and TMAH, especially in KOH solution saturated with isopropanol or other additives [1][2][3][4][5]. Among the reasons of hillocks formation, the following are most often mentioned in the literature: incomplete dissolution of reaction products remaining on the etched surface [1,3,[6][7][8], hydrogen bubbles created in the dissolution process [2,[8][9][10], oxygen precipitates in bulk crystal and stress following the thermal treatment of the substrates [2,4,11,12], metal impurities [7,[13][14][15]. It is evident that all heterogeneities coming from the bulk of crystal or encountered on its surface can disturb the homogeneity of etching process and cause its temporary stopping.…”

“…Similar effects can be achieved by adding of surfactants, which reduce the surface tension and facilitate removal of gaseous hydrogen, gathering on the etched surface [3,5,8,17,18]. Suppression of micromasks can be also achieved by taking out the substrate from the etchant, washing it up and re-etching [1,2,4,6].…”

“…In the literature there are many descriptions of hillocks morphologies and evolution during the etching process [1][2][3][4][5][6][7][19][20][21]. The appearance of the hillock resembling a pyramid composed of stepped {1 1 1} faces is not simple to explain.…”

Surface termination and hydrogen bubble adhesion on Si(100) surfaces during anisotropic dissolution in aqueous KOH

“…The overall reaction is [13,14] Si þ 2H 2 Etching defects and surface roughness can result if the hydrogen bubbles produced remain long enough on the surface to mask it from the etching solution [15][16][17][18][19]. The purpose of this work was to investigate the physicochemical aspects of bubble adhesion from measurements of bubble size, lifetime and IR spectroscopy of Si(1 0 0) surfaces in contact with aqueous KOH.…”