Direct-Writing of High-Aspect-Ratio Trenches in Silicon
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DIRECT-WRITING OF HIGH-ASPECT-RATIO TRENCHES IN SILICON
G.V. Treyz, R. Beach, and R.M. Osgood Jr. Microelectronics Sciences Laboratories, Columbia University, New York, New York 10027
ABSTRACT Deep trenches have been etched in crystalline silicon with polarization-controlled, variable-curvature walls. Scan speeds of up to 10mm/s have been demonstrated. A model of the etching process has been developed which is based on a local, melt-enhanced etch rate. Comparisons of model predictions and experimental data are presented.
INTRODUCTION Many applications in microelectronics and integrated circuits require techniques for forming high-aspect-ratio surface-relief structures. Direct writing of etch grooves has previously been reported to be an effective technique for fabricating such structures.[l,21 Very little is known, however, about the process physics and the factors influencing the etch-structure profiles. In this work, we report that the shape of the etched trench features is greatly influenced by the beam polarization. Further, the etching process is now understood to be a thermal, meltenhanced reaction, and a model of the etching process has been developed.
EXPERIMENTAL Silicon samples of varying resistivities and dopant types were used. The silicon was first cleaned with acetone and methanol. The native oxide was then removed with a 10% HF solution, and the samples placed directly into a stainless-steel etching chamber. Within the vacuum chamber, the samples were located on a gold- and nickel-plated copper pallet 0.125" below a 5"-diameter, fused silica window. The trenches were laser-etched in a chlorine ambient, typically at a pressure of 300 Torr. The 514-nm line of an Ar+ laser was focused normal to the surface to a FWHM spot size of 1.5 im. The beam was scanned at velocities of 0.5 to 10 mm/s by an optical system mounted on an x-y translation stage, accurate to 0.1 ýim. The system preserved the linear polarization of the beam. A half-wave plate and rotatable polarizer were used in the study of polarization effects. A lOx focusing objective was mounted on a linear-motion z-stage for real-time focusing. A focusing scheme was implemented to provide maximum power density during trench formation by maintaining the minimum achievable spot size at the substrate surface.
RESULTS AND DISCUSSION Figure 1 shows a scanning-electron-microscope (SEM) photograph of two typical trench cross-sections obtained by cleaving a sample perpendicular to the direction of the trench formation. By performing a series of similar experiments, we have parameterized trench depth as a function of laser power for various scan velocities (Fig. 2). A simple model has been developed which is in agreement with this data. Figure 3 shows the simplified geometry used in our model. We assume the laser spot has a square cross section with sides of length w at the Mat. Res. Soc. Symp. Proc. Vol. 75. 1987 Materials Research Society
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