Analysis

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The Importance of Variable Wafer Tilt for Defect Classification

by Sheldon Moll, Ph.D., Consultant

Defect review tools are available today withwafer handling stages allowing variable wafertilt, no tilt at all, or only a fixed, non-adjustable tilt. The purpose of the followinginformation is to demonstrate the imagingimprovement and consequent classificationaccuracy benefits which derive from a DRTwafer stage allowing fully variable tilting.

Three-dimensional perspectiveA natural attribute of the SEM is its abilityto form magnified images of three-dimen-sional objects which not only appear “natural”to the human eye, but also allow a direct,quantitative measure of their physical dimen-sions. An understanding of the height dimen-sion of an object can be determined directlyfrom a tilted view. A top-down, or untiltedview, does not directly reveal the magnitude

The scanning electron microscope-based Defect Review Tool (DRT) has now become an essential component of a suite ofinstruments 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 DRTsupplements the laser scan or light optical image-based inspectors (defect detection tools) by performing an operator-freeAutomatic Defect Classification (ADC) of individual defects. It is clear that the DRT must yield highly accurate defectclassifications in order to maintain high in-line yields.

of the object’s “z” — the height dimension of the posts.For example, a cylinder viewed end-on appears to be asimple circle and can falsely be indentified as such.

Figure 1 is a SEM micrograph of a group of photoresiststructures destined, after further processing, to becomethe vias of an IC. The post-like structures are readilyapparent in this image taken with the wafer tilted 45ºwith respect to the electron beam.

Figure 2 shows the same structures with the electronbeam “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 ifviewed from the direction of the scanning incidentbeam. However, from the standpoint of dark and lightareas and shadows in the image, the electron detector(or detectors) serves the eye only as the illuminationsource.

F i g u re 1. Photoresist 45º t i l t . F i g u re 2. Photores ist 0º t i l t . F i g u re 3. Photores ist 0º t i l t

shadow perspective.

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better accuracy in defect morpholo-gy identification and classification.Does this mean that a DRT withonly a single, non-variable tiltwould be appropriate? No — suchis not the case. A wafer-handlingstage with a variable tilt, allowingselection of an angle specificallysuited to the structure or processlevel to be run, is essential in manycases.

Figure 10 is a top-down (zero tilt)image of a trench exhibiting a 0.1micrometer defect, as well as thefact that the trench is not totallycleaned out.

Figures 11 and 12 show the samearea at progressively higher tilts of30º and 45º. The aspect ratio of the

trench is such that the particle ishidden at a 30º wafer tilt angle, andan inspection at 45º would hide theincomplete cleanout. In this case, aDRT operated at a low-tilt anglewould be most suited to this processlevel.

Image perspective may be enhancedsomewhat by the presence of shadowsor directional illumination. Theelectron detectors may be designedand arranged to produce a directionalillumination or shadowing effectwhich would allow a moderateunderstanding of the three-dimen-sional structure even if it is onlyimaged in a top-down orientation.

The image in figure 3 was againobtained with no wafer tilting —but with a detector configurationwhich allowed some directional illumination and shadowing.

The height (z) dimension of theposts may be suspected from theshadows. Only the tilted view offigure 1 allows a clear determinationof the post-like structure. A morepractical example, taken from anactual ADC review, is shown in figures 4 and 5. A top-down viewsuggests that the defect is a simpleflake-like structure, while the tiltedview clearly reveals its multi-layered, process-related structure.

The quality of the data from a DRTproduction line monitor dependsdirectly upon the accuracy of thedefect classification. For example,false identification of this defect as aflake — when it is actually a multi-layered particle — is clearly of littlehelp to statistical process control.The conical growth structure of thedefect imaged in figure 6, aided inits classification from a top-down

view (figure 7), provided none ofthis kind of information.

The defect in the top-down imageof figure 8 appears to be simplysome sort of staining. When tilted,however (as in figure 9), the defectis easily seen as a three-dimensionaleruption probably related to anolder, sub-surface particle.

Use of selectable tilt indefect redetectionThe prior information suggests thatsimply operating at a wafer tilt of40º or 45º is the indicated path to

F i g u re 4. ADC particle defect 0º.

F i g u re 5. ADC particle defect 45º. F i g u re 6. ADC particle defect 45º.

F i g u re 7. ADC particle defect 0º.F i g u re 8. ADC stru c t u re defect 0º.

F i g u re 9. ADC stru c t u re defect 45º.

A further example of charge reduc-tion when a surface particle wastilted is shown in figures 15 and 16.

Enhancing Electron and X-rayEmission with TiltAs discussed above, when a sampleis tilted with respect to the incidentbeam, electron penetration in termsof distance below the surface isreduced. Excited low-energy andhigh-energy electrons, as well as X-rays, travel shorter paths to thesurface. Thus, the strength of theseexiting “signals” is increased; andthe information they carry is morerepresentative of the sample surfacelayers, as opposed to the deeperbulk.

Figure 17 is a comparison of X-rayspectra obtained from a siliconwafer with a 0.5 µm surface particle(figure 18) of titanium silicidewhen the wafer was not tilted ortilted at 40º. The spectrumobtained at tilt is clearly higher inintensity, allowing better analyticalsensitivity. More importantly, the

The tilt angle appropriate for themost accurate review would seem todepend upon the wafer’s processlevel or CMP step. It seems clearthat a DRT with only a fixed tiltstage, or one with no tilt capabilityat all, could not provide the flexibil-ity to accurately review the varieddefect or process problems whichmight be encountered in-line.

Control of Sample Charging with Ti l tCharging effects in the SEM,revealed by unusual changes inimage brightness apparently

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F i g u re 10. Trench defects 0º.

unrelated to sample topology orchemistry, can cause the patternrecognition software of the ADCtool to misidentify such contrasts as defects. It is well known thattilting a sample in the SEM usuallyreduces electron charging and evenallows the use of higher incidentelectron beam energies. The reasonfor this is quite simple. The inci-dent electron beam entering thesample at an angle deposits itscharge closer to the specimen surface, allowing buried electrons to more easily escape. Specimencharge storage and its effect uponelectron emission and image bright-ness variation is thus reduced.

Figure 13, taken at 0º tilt, clearlyshows the image brightness andcontrast changes caused by thecharging of a non-conductive defectas well as charging of the oxidelayer of the wafer itself. Individualvias, not associated with the defect,are also imaged differently and maybe falsely identified as defects.Tilting to 45º eliminates the charg-ing (figure 14).

F i g u re 14. ADC charging defect 45º.

F i g u re 15. ADC charging particle 0º.

F i g u re 16. ADC charging par ticle 45º.

F i g u re 11. Trench defects 30º.

F i g u re 12. Trench defect s 45º.

F i g u re 13. ADC charging defect 0º.

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spectrum at tilt exhibits a largerTi/Si ratio which is more representa-tive of the Ti silicide particle chem-istry. When the specimen is not tilted, a greater excitation of thesubstrate yields a mixed spectrumrelatively higher in Si.

Summar yAutomatic defect location and clas-sification throughput can proceed at an appropriate rate which is independent of the tilt angle of the stage. However, the efficiencyof re-detection after inspection andthe accuracy of defect classificationcan be greatly improved for a givenprocess level by selecting the properwafer tilt. The chemical identifica-

F i g u re 17. Titanium silicide particle X-ray

spect ra 0º (yellow) vs. 45º ( re d ) .

F i g u re 18. Titanium silicide part i c l e .

tion of thin films and foreignparticulates by X-ray can also beimproved by selecting the propergeometrical configuration. ❈

There’s a very important new challenge

in the semiconductor industry today.

How to achieve superior defect imaging and the highest

accuracy in classifications. Advanced geometries, copper

interconnects and dual-damascene processes all demand it.

And now the 4300+ defect review tool meets this

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H OW YOU LOOK AT THINGS IS VERY IMPORTA N TIt can cost you time and money—or it can save you time and money.

Defect at Flat view0º Tilt

Defect at Flat view45º Tilt

Defect at 45º Tiltand 30º Rotate

c i r cle RS#031