Precise small apertures for a variety of optical applications can be formed in silicon-on-insulator (SOI) wafers by use of a photolithographic process developed specifically for the purpose. In comparison with the formation of apertures in standard silicon wafers by previously developed processes (including photolithographic ones), the present combination of SOI wafers and processing yields apertures of more precise, repeatable dimensions.
The figure illustrates a cross section of a SOI wafer at the major steps of the process. The starting material is an SOI wafer that consists of a 1-µm layer of SiO2 sandwiched between a 3-µm layer of Si and a 600-µm-thick Si substrate. Vacuum contact photolithography is used to define apertures as small as 1 µm in photoresist on the front (3-µm Si) side. With the photoresist serving as a mask, reactive-ion etching (RIE) is used to form a hole completely through the 3-µm Si layer; by suitable choice of RIE operating parameters such as the types of gases, operating pressure, and power, the hole can be formed with nearly vertical sidewalls.
Next, the wafer is heated in a furnace and exposed to an oxygen atmosphere to grow a layer of silicon dioxide (see figure). Then by use of back-side alignment photolithography (in which features on the back side are aligned with the features on the front side), the required back-side aperture pattern is formed in photoresist. In addition, the front side of the wafer is painted with photoresist to protect the front-side SiO2 during the oxide-etching step described next. The portion of the back-side SiO2 not masked by photoresist is etched by use of a buffered hydrofluoric acid solution. Then the photoresist is stripped off.
An anisotropic silicon etch such as 20-percent KOH or tetramethyl ammonium hydroxide (TMAH) is used to etch the silicon substrate preferentially along the <111> crystal plane. Then the front-side SiO2, the back-side SiO2, and the exposed portion of the buried middle SiO2 layer are etched away by use of a buffered hydrofluoric acid solution to create the desired apertures. If necessary for the application, aluminum or another suitable metal can be deposited on the front side to make the region surrounding the apertures opaque.
In a variation of the process, anisotropic chemical etching (instead of RIE) is used to form an inverted pyramidal (instead of the vertical-sided) hole in the 3-µm-thick Si layer. This variation is useful for making arrays of highly precise square or rectangular holes.
This work was done by Sridhar Manthripragada, Doug Leviton, David Brent Mott, and Christine Allen of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Manufacturing/Fabrication category.