Autumn99 profiling

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Metrology

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Profiling High Aspect Ratio Features for Post-Etch Metrology by Anna Mathai, Western Regional Metrology Manager and Mustafa Oyumi, Applications Lab Manager

Plasma etching is among the most critical steps in the fabrication of microelectronics devices. It is used in a variety of processes, including the manufacture of contacts and vias, shallow trench isolation, DRAM and dual damascene. A successful etch process must manage complex trade-offs to optimize a number of parameters including critical dimension uniformity (lateral and vertical), selectivity, and a high etch rate. Failure to do so will result in poor process control and device performance. Of the challenges involved in plasma etching, the largest barrier to consistent device performance is maintaining critical dimension uniformity. Currently, there are three ways to monitor etch depth. The most reliable and hence most commonly used technique is to cross-section the wafer and use scanning electron microscopy (SEM). This technique, however, is undesirable since it requires expensive wafers to be sacrificed. It is also time consuming, with the longer time-to-results causing more wafers in process to be at risk. The second solution is to conduct electrical tests on the wafers after the interconnect level is complete. Here again all the wafers in process are at risk. Third, atomic force microscopes (AFM) are used occasionally to monitor etch depth; however, concerns about tip quality, reliability and ease-of-use have limited their effectiveness as a solution. Clearly, there is a need for a non-destructive, reliable and easy to use technique to measure etch depths with quick time-to-results.

F i g u re 1. Schematic pr ofiling of a trench in contact a s well as in dipping mode.

necessitates that one use a robust stylus characterized by a relatively large included angle. In turn, the stylus geometry limits the aspect ratios of the features that can be resolved by the stylus. A typical work-around has been to measure test structures that are scaled up from the actual geometry. However, since plasma etching depends on the size and pattern density of the features comprising the array, measurements must be performed on the structures themselves and not on test structures.

Contact-mode profiling

Fabs have traditionally used profilers to measure post-etch step heights easily and reliably. With a profiler, as shown in figure 1, a stylus is moved lightly across the surface of a sample (contact mode). The stylus-sample interaction in contact mode generates lateral or shear forces. The need to limit, as well as the ability to withstand this shear force 30

Autumn 1999

Yield Management Solutions

Dipping-mode profiling

To enable high aspect ratio measurements, a new mode of profiler operation has been developed in which a scan is comprised of a series of discrete steps. The stylus is lightly lowered into contact with the sample, a data point is obtained, and the stylus is lifted away from the sample. After the appropriate lateral motion, this sequence of events is repeated until the profile or image


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