(65ap) Deep Trenches of Varying Depths Etched in Si in a Single Process Step

Patterned Si substrates with trenches etched to more than one depth have applications in photonics and micro-electro-mechanical systems (MEMS). For example, it may be desirable to form pedestals of different heights for discrete photonic devices so that they may be aligned and integrated on a Si base. A technique has been developed that uses only one lithography step to form features like the ones in this example. Deep Si etching is accomplished in a plasma etcher using the Bosch process [1]. A Si substrate is coated with a protective photoresist film which is then patterned, and the substrate is selectively etched in the exposed regions. Patterns with narrow open spaces typically etch slower than wide open areas. This effect is exploited to form trenches of different depths by varying the printed photoresist openings. The complete process is as follows: a) lithography is used to create gratings consisting of 4 ?Ým wide photoresist lines with varying spaces between them, followed by b) deep Si etching which forms trenches with vertical sidewalls; c) the photoresist is then stripped, and d) after a second, isotropic etch, the thin 4 ?Ým lines are removed, leaving large wells in the substrate. The depth of each well is controlled by the pattern density of the gratings. Photomasks were designed to measure the etch depth variation. After deep Si etching, substrates were cross-sectioned so that the depths could be determined, as for example the microscope images in Fig. 1 a. After resist strip and the isotropic etch, the resulting deep wells were measured by the same technique (Fig. 1 b). Figure 2 shows a plot of the effect of pattern spacing on depth. The results of this plot were then used to design unique patterns such as "staircases" and cylindrical lens elements.