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Figure 1. The post-etch and strip stack measured in this study. Note the etch process removes some of the oxide layer. |
John R. (Bob) McNeil
Center for High Technology Materials
Univ. of New Mexico
Albuquerque, NM 87131
In the manufacture of semiconductor devices, storage media, and in other areas, the creation of structures manufactured to pre-defined dimensional tolerances is required. It is therefore essential to have the ability to obtain a precise as well as accurate measurement of the dimensions of these manufactured structures. Precision refers to the ability to repeatedly obtain the same dimensional measurement while accuracy is concerned with the traceability of the measurement to a fundamental dimensional standard. Dimensional metrology of semiconductor structures has, to date, primarily been accomplished through the formation of an image of the structure. The image may be recorded, printed or displayed using a plethora of devices currently available for this purpose. The formation of an image requires the recording of the interaction of an incident radiation or probe with the object whose image is required.
This past decade has also seen the development of a non-imaging optical dimensional metrology technique known as scatterometry. Scatterometry is a non-destructive optical technique that records and analyzes changes in the intensity of light reflected from a periodic scattering surface. By measuring and analyzing the light diffracted from a patterned periodic sample, the dimensions of the sample itself can be measured. Scatterometry exploits the sensitivity of diffraction from a sample to changes in the line-shape of the sample. The use of an elementary sample illumination system, combined with a rigorous scattered signal analysis procedure presents a significant advantage over conventional metrology techniques. These will be discussed in detail in this paper.
Applications of scatterometry technique have included characterization of photomasks1; monitoring stepper focus2 and dose3 ; and the photoresist post exposure bake process4 ; and even the characterization of three dimensional features such as contact hole and DRAM arrays5. The most widely reported scatterometry data have been for measurements of developed photoresist and etched poly-Si gratings6 . Like other optical metrologies, scatterometry measurements are rapid, non-destructive and highly repeatable.
Although the types of features mentioned above represent a considerable portion of a typical process, other types of materials, such as metal layers, are also commonly present, and are important to characterize. Recent scatterometry measurement data from metal layers will be presented; see Figure 1 below. The scatterometer used in this investigation was the so-called 2-Q variety, where the scatter signature is obtained by measuring the diffraction efficiency of a particular order as the incident angle is varied. The analysis involved comparing the measured data to a library of theoretical scatter signatures (generated a priori from rigorous coupled wave theory) to find the best match. The scatterometer measurements agreed well with AFM measurements performed on the same samples. Details will be presented.
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Figure 1. The post-etch and strip stack measured in this study. Note the etch process removes some of the oxide layer. |
Another primary consideration of a metrology technique is the measurement repeatability. The scatterometer apparatus is simple, consisting of a laser or white light illumination arrangement and a simple detection arrangement. Because of this simplicity, the technique is inherently repeatable. This is illustrated in Table 1 for the AlCu sample discussed.
Table 1. Measurement precision data for the etched metal samples. |
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| Parameter name | Repeatability (6-sigma) | Reproducibility (6-sigma) | Precision (6-sigma) |
| Linewidth | 0.13 nm | 0.45 nm | 0.47 nm |
| AlCu Thk | 2.0 A | 2.36 A | 3.09 A |
| Etched Ox Thk | 1.61 A | 1.70 | A 2.34 A |
| Unif Ox Thk | 2.0 A | 2.02 A | 2.84 A |
Measurement precision will also be discussed. Precision is dependent upon two issues: sensitivity of the measured quantity to changes in sample features, such as how the measured intensity changes with changes in linewidth, and the measurement precision of the quantity being measured. The latter is dependent on the particular scatterometer technique employed, and this will be discussed.
1. S. M. Gaspar-Wilson, S. Naqvi, J. R. McNeil, H. Marchman, B. Johs, R. French, F. Kalk, “Metrology of etched quartz and chrome embedded phase shift gratings using scatterometry,” Integrated Circuit Metrology, Inspection and Process Control IX, Proc. SPIE 2439, 1995.
2. L. M. Milner, K. P. Bishop, S. S. H. Naqvi, J. R. McNeil, “Stepper focus characterization using diffraction from latent images,” Journal of Vacuum Science and Technology B 11(4), pp. 1258-1266, 1993.
3. K. C. Hickman, et al., “Use of diffraction from latent images to improve lithography control,” Journal of Vacuum Science and Technology B 10(5), pp. 2259-2266, 1992.
4. J. Sturtevant et al, “Post exposure bake as a process-control parameter for chemically-amplified photoresist,” Proc. SPIE 1926, pp. 106-114, 1993.
5. Z. R. Hatab, J. R. McNeil, S. S. H. Naqvi, “Sixteen-megabit dynamic random access memory trench depth characterization using two-dimensional diffraction analysis,” Journal of Vacuum Science and Technology B 13(2), pp. 174-182, 1995.
6. C. J. Raymond, “Milestones and future directions in applications of optical scatterometry,” Proc SPIE CR72, pp. 147-177, 1999.