Autumn99 profiling

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

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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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F i g u re 2. HRP i mage of 0.35 µm geometr y

F i g u re 3. Cross-s ecti on through cen ter s of

F i g u re 4. SEM cross-sect ion of 0.35 µm

con tacts, post-etch.

t he contacts in fig ur e 2.

g e o m e t r y contac ts, pos t-etch.

Figure 2 shows a dipping mode image of a 0.35 µm contact near the center of the wafer. The image was built up in a raster fashion with a dipping mode profile scan along the x-direction and stepping over the y-spacing between subsequent profiles. The image shows that the bottom of the contact is clearly resolved.

Figure 3 shows a cross-section through the centers of the contacts in figure 2, with the depth measured as 1.105 µm. Figure 4 shows a SEM cross-section image of a 0.35 µm contact, from approximately the same location on the wafer as the data in figure 2.

is complete, as shown in figure 1. In this way, the shear force generated during the sample-stylus interaction is minimized. This enables the use of fine styli with included angles of less than 100, and radii of curvature of 10-20 nm.

Here, we discuss a generation high-resolution profiler, the HRP-240, which provides the ability to measure the depth of high aspect ratio features up to 4:1 at 0.25 µm geometry, while maintaining the reliability and ease-of-use of a traditional profiler.

Table 1 summarizes the HRP measurements from 0.35 µm and 0.5 µm contacts at eleven sites on a wafer, approximately equally spaced along the center line and numbered from left to right. The table also summarizes results from SEM cross-section images at two locations. There is good agreement between the HRP and SEM data.

Measuring high aspect ratio contacts

Contacts are holes in the inter-level dielectric (ILD) which, when filled with metal, make contact between the transistors and the first metallization layers (M1).

Vias are similar to contacts except they connect two metallization levels. Contact or via depth is important to monitor to avoid poor electrical performance resulting from an under-etch or damage to the underlying structure resulting from an over-etch.

Depth of 0.35 µm contacts (µm) HRP-240

SEM

In conclusion, we have demonstrated the capability of the next generation high-resolution profiler, the HRP-240, as a viable Depth of 0.5 µm contacts (µm) alternative to scanning HRP-240 SEM electron microscopy for monitoring the post-etch 1.083 depth of contacts and vias. ❈ 1.113

Site 1

0.995

Site 2

0.996

Site 3

1.018

1.147

Site 4

1.058

1.198

Site 5

1.055

1.125

Site 6

1.105

1.028

Site 7

1.113

1.233

Site 8

1.106

1.250

Site 9

1.077

1.240

Site 10

1.090

1.201

Site 11

1.039

1.000

S

cir cle RS#005

Acknowledgements 1.210

1.188

1.172

The authors thank Mark Wilcoxson and Bi-ming Yen of Lam Research for wafers, SEM results and valu able insights. They also thank their colleagues, Tom McWaid and Mike Young, at KLA-Tencor.

1.140

Table 1. Summar y of HRP-240 and SEM measurem ents of 0.35 µm and 0.5 µm data at eleven sites acro s s a wafer.

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Yield Management Solutions

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