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Process Parametrics

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Oxide Etch Process Control Using a High Resolution Profiler

by Kelly Barry, KLA-Tencor Corporation

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 tradeoffs 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 these etch process parameters, 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 crosssection the wafer and use scanning electron microscopy (SEM). This technique is undesirable since it requires the destruction of product wafers. In addition, it is time-consuming; a longer time to results means that more wafers will have gone through process prior to depth monitoring feedback, increasing the risk to these wafers. A high resolution profiler (HRP) such as the KLA-Tencor HRP-240ETCH has the capability of monitoring etch depths

in a repeatable, non-destructive manner with quick time-to-results. In this article, we present the results of a comparative study of oxide etch depth monitoring via SEM and HRP, in which the HRP demonstrates strong correlation to the SEM. Etch depth challenges

Etch depth monitoring is important in order to avoid poor electrical performance (under-etch) or underlying structural damage (over-etch). 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.

F i g u re 1. The etched wa fer is broken in order to revea l t he contacts via SEM im aging. The depth measur ement of these contacts b y SEM was 1112. 5 nm.

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F i g u r e 2. The HRP-240 E T C H takes a 3-D sca n of t he wafer’s etched contacts , then provides a n a utomated de pth mea suremen t ba sed on thi s data. The depth measurement of these contacts by HRP was 1111.8 nm.

In addition to the destructive (expensive) and timeconsuming method of measuring etch depth via crosssectional SEM, other process monitoring techniques include electrical testing and atomic force microscopy (AFM). Electrical testing of the wafers can be performed at the interconnect level, but it is also time-consuming, leaving untested wafers in process for longer periods of time. AFM analysis has been used to monitor etch depth, but speed, ease-of-use and tip quality issues have prevented it being used in a production environment.

measurements by HRP correlate very well to existing etch depth control by SEM. These depth measurements via high-aspect ratio profiling also yield results much faster than the SEM. The HRP platform is a process control method that is well-accepted for its proven reliability and ease-of-use. This study demonstrates the capability of the HRP-240 ETCH as a viable alternative to scanning electron microscopy for monitoring the post-oxide etch depth of contacts and vias.

An HRP addresses these issues by providing reliable, repeatable, and non-destructive high-aspect ratio depth measurements quickly and easily.

SEM-HRP depth measurement comparison

For cross-sectional depth measurement via SEM, the wafer is broken to reveal the exposed contacts, as shown in Figure 1. The contact depth is then measured from the SEM image. In the HRP-240ETCH depth measurement, a 3-D scan of the contacts is taken, in approximately the same region of the wafer. An automated depth measurement is made based on the scan data, as shown in Figure 2. For a series of local interconnects (0.27 µm dia.) and vias (0.6 µm dia.), the correlation results are shown in Figure 3. There is excellent agreement between the HRP and SEM depth measurements.

Etch process control solution

The HRP allows process control of etch depth without costly breakage of wafers, which will become increasingly important as fabs migrate to 300 mm. The depth

F i g u r e 3. Correla tion between the HR P- 240 E T C H and the SEM f or etch depth measurements of loca l interconnects and vias.

Acknowledgments

The author thanks Chuan-Cheng Cheng and Warren Uesato of LSI Logic for HRP and SEM results. She also thanks her colleagues, Anna Mathai, Mustafa Oyumi and Mike Young, at KLA-Tencor.

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