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The Importance of Variable Wafer Tilt for Defect Classification by Sheldon Moll, Ph.D., Consultant
The scanning electron microscope-based Defect Review Tool (DRT) has now become an essential component of a suite of instruments dedicated to Statistical Process Control in the modern fab. More recently, the tool has transitioned from an engineering-type instrument — requiring full operator attention — to a true, automated in-line monitor. The DRT supplements the laser scan or light optical image-based inspectors (defect detection tools) by performing an operator-free Automatic Defect Classification (ADC) of individual defects. It is clear that the DRT must yield highly accurate defect classifications in order to maintain high in-line yields.
F i g u r e 1. Photoresist 45 º t i l t .
F i g u re 2. Photores ist 0 º t i l t .
F i g u re 3. Photores ist 0 º t i l t shado w perspective.
Defect review tools are available today with wafer handling stages allowing variable wafer tilt, no tilt at all, or only a fixed, nonadjustable tilt. The purpose of the following information is to demonstrate the imaging improvement and consequent classification accuracy benefits which derive from a DRT wafer stage allowing fully variable tilting. Three-dimensional perspective
A natural attribute of the SEM is its ability to form magnified images of three-dimensional objects which not only appear “natural” to the human eye, but also allow a direct, quantitative measure of their physical dimensions. An understanding of the height dimension of an object can be determined directly from a tilted view. A top-down, or untilted view, does not directly reveal the magnitude 36
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of the object’s “z” — the height dimension of the posts. For example, a cylinder viewed end-on appears to be a simple circle and can falsely be indentified as such. Figure 1 is a SEM micrograph of a group of photoresist structures destined, after further processing, to become the vias of an IC. The post-like structures are readily apparent in this image taken with the wafer tilted 45º with respect to the electron beam. Figure 2 shows the same structures with the electron beam “looking” top-down — a zero tilt orientation. The posts appear only as reticulated ovals and the image yields no indication of their vertical profile. It must be understood that SEM images appear as if viewed from the direction of the scanning incident beam. However, from the standpoint of dark and light areas and shadows in the image, the electron detector (or detectors) serves the eye only as the illumination source.
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F i g u re 4. ADC particle defect 0 º.
The quality of the data from a DRT production line monitor depends directly upon the accuracy of the defect classification. For example, false identification of this defect as a flake — when it is actually a multilayered particle — is clearly of little help to statistical process control. The conical growth structure of the defect imaged in figure 6, aided in its classification from a top-down
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better accuracy in defect morphology identification and classification. Does this mean that a DRT with only a single, non-variable tilt would be appropriate? No — such is not the case. A wafer-handling stage with a variable tilt, allowing selection of an angle specifically suited to the structure or process level to be run, is essential in many cases. Figure 10 is a top-down (zero tilt) image of a trench exhibiting a 0.1 micrometer defect, as well as the fact that the trench is not totally cleaned out.
F i g u re 5. ADC particle defect 45º .
Image perspective may be enhanced somewhat by the presence of shadows or directional illumination. The electron detectors may be designed and arranged to produce a directional illumination or shadowing effect which would allow a moderate understanding of the three-dimensional structure even if it is only imaged in a top-down orientation. The image in figure 3 was again obtained with no wafer tilting — but with a detector configuration which allowed some directional illumination and shadowing. The height (z) dimension of the posts may be suspected from the shadows. Only the tilted view of figure 1 allows a clear determination of the post-like structure. A more practical example, taken from an actual ADC review, is shown in figures 4 and 5. A top-down view suggests that the defect is a simple flake-like structure, while the tilted view clearly reveals its multilayered, process-related structure.
Figures 11 and 12 show the same area at progressively higher tilts of 30º and 45º. The aspect ratio of the
F i g u re 6. ADC particle defect 45 º.
F i g u re 8. ADC stru c t u re defect 0 º .
F i g u re 7. ADC particle defect 0 º.
view (figure 7), provided none of this kind of information. The defect in the top-down image of figure 8 appears to be simply some sort of staining. When tilted, however (as in figure 9), the defect is easily seen as a three-dimensional eruption probably related to an older, sub-surface particle. Use of selectable tilt in defect redetection
The prior information suggests that simply operating at a wafer tilt of 40º or 45º is the indicated path to Autumn 1999
F i g u re 9. ADC stru c t u re defect 45 º.
trench is such that the particle is hidden at a 30º wafer tilt angle, and an inspection at 45º would hide the incomplete cleanout. In this case, a DRT operated at a low-tilt angle would be most suited to this process level.
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The tilt angle appropriate for the most accurate review would seem to depend upon the wafer’s process level or CMP step. It seems clear that a DRT with only a fixed tilt stage, or one with no tilt capability at all, could not provide the flexibility to accurately review the varied defect or process problems which might be encountered in-line.
A further example of charge reduction when a surface particle was tilted is shown in figures 15 and 16.
Enhancing Electron and X-ray Emission with Tilt
F i g u re 13. ADC charging defect 0 º.
Control of Sample Charging with Tilt Charging effects in the SEM, revealed by unusual changes in image brightness apparently
F i g u re 14. ADC charging defect 45 º.
F i g u re 10. Trench defects 0 º.
F i g u re 11. Trench defects 30 º .
F i g u re 12. Trench defect s 45 º.
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unrelated to sample topology or chemistry, can cause the pattern recognition software of the ADC tool to misidentify such contrasts as defects. It is well known that tilting a sample in the SEM usually reduces electron charging and even allows the use of higher incident electron beam energies. The reason for this is quite simple. The incident electron beam entering the sample at an angle deposits its charge closer to the specimen surface, allowing buried electrons to more easily escape. Specimen charge storage and its effect upon electron emission and image brightness variation is thus reduced. Figure 13, taken at 0º tilt, clearly shows the image brightness and contrast changes caused by the charging of a non-conductive defect as well as charging of the oxide layer of the wafer itself. Individual vias, not associated with the defect, are also imaged differently and may be falsely identified as defects. Tilting to 45º eliminates the charging (figure 14). Yield Management Solutions
As discussed above, when a sample is tilted with respect to the incident beam, electron penetration in terms of distance below the surface is reduced. Excited low-energy and high-energy electrons, as well as X-rays, travel shorter paths to the surface. Thus, the strength of these exiting “signals” is increased; and the information they carry is more representative of the sample surface layers, as opposed to the deeper bulk. Figure 17 is a comparison of X-ray spectra obtained from a silicon wafer with a 0.5 µm surface particle (figure 18) of titanium silicide when the wafer was not tilted or tilted at 40º. The spectrum obtained at tilt is clearly higher in intensity, allowing better analytical sensitivity. More importantly, the
F i g u re 15. ADC charging particle 0 º.
F i g u re 16. ADC charging par ticle 45 º.
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spectrum at tilt exhibits a larger Ti/Si ratio which is more representative of the Ti silicide particle chemistry. When the specimen is not tilted, a greater excitation of the substrate yields a mixed spectrum relatively higher in Si.
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tion of thin films and foreign particulates by X-ray can also be improved by selecting the proper geometrical configuration. ❈ cir cle RS#031
F i g u re 17. Titanium silicide particle X-ray spect ra 0 º (yellow) vs. 45 º ( re d ) .
F i g u re 18. Titanium silicide part i c l e .
Defect at Flat view 0º Tilt
Summar
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Automatic defect location and classification throughput can proceed at an appropriate rate which is independent of the tilt angle of the stage. However, the efficiency of re-detection after inspection and the accuracy of defect classification can be greatly improved for a given process level by selecting the proper wafer tilt. The chemical identifica-
Defect at Flat view 45º Tilt
Defect at 45º Tilt and 30º Rotate
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