Autumn 1999 Yield Management Solutions 15

The semiconductor industry is shrinking the area den-sity of devices by 40 percent per year. The challenge forcompanies developing inspection systems is to maintainimage acquisition time and CoO constant while mov-ing to higher and higher image resolution. We considerhow image acquisition, image processing, and defectclassification might meet this challenge.

Obtaining the ImageImage acquisition is the first step in the inspectionprocess. It consists of illuminating the wafer with asource (lamp or laser), imaging or collecting the scat-tered light, and detecting this light with a photo-detector (PMT, TDI, or CCD). The source has to bebright enough to provide sufficient photo-electronsfrom the detector to obtain a reasonable signal-to-noise

System considerationsAn inspection system obtains an image(electron or photon), then processes it todetermine if a defect is present, classifies it according to some criteria, and finallypasses the information on to a yield manage-ment system. Each of these steps may havecertain limitations and we briefly describesome of the system considerations necessaryto optimize the inspection strategy.

Ideally an inspection system should havehigh sensitivity, high throughput, and lowcost of ownership (CoO). However, all thesedesired system characteristics are coupledand one must do trade-offs to achieve theoptimum system.

Wafer Inspection Technology Challenges forULSI Manufacturing — Part II

by Stan Stokowski, Ph.D., Chief Scientist; Mehdi Vaez-Irvani, Ph.D., Principal Research Scientist

Continued pressure to increase the return-on investment for the semiconductor fabricator has made it critical for inspection systems to evolve from stand-alone “tools” that just find defects to being part of a more complete solution wheredetecting defects, classifying them, analyzing these results and recommending corrective actions are their functions. Part Iof this article, published in the Spring issue of this magazine, discussed the challenges of detecting defects with differingscattering characteristics and the need for multiple technology wafer inspection solutions. Part II addresses system consid -erations to meet the design shrink challenge and future needs and developments in wafer inspection technology.

InspectionF E A T U R E S

convolutions done after digitization. In addition, noiselimits sensitivity; noise sources include photo-electronshot-noise, detector noise, electronic noise, noise fromthe analog-to-digital converter, aliasing noise, and spatialquantitization noise. These last two noise sourcesdepend on the spatial sampling frequency relative tothe system spot size. Generally one increases sensitivityby decreasing the system spot size.

Note that system spot size is a governing factor in sensi-t i v i t y, n o t pixel size.Throughput of an inspection system,on the other hand, is inversely related to the s q u a r e of thepixel size. Thus, the time for actually inspecting the waferis determined by the pixel rate of an inspection system,given its pixel size. Additional factors affecting through-put are operations such as wafer loading and unloading,alignment and registration, and data processing.

Figure 1 shows the relationship between pixel size andinspection time for different pixel rates. Clearly, onetries to use as large a pixel as one can while achieving agiven sensitivity. Here is where darkfield systems have agreat advantage over brightfield systems; the ratio ofsystem spot size to defect size is considerably greaterthan 1 in darkfield systems, whereas brightfield systemshave a ratio closer to 1. For example, albeit, a particu-larly advantageous situation, a darkfield system existsthat can detect small PSL spheres on bare silicon with adefect-to-spot area ratio of 3 x 105.

The detector or scanner is limited in speed. For imagingsystems, the fastest ones employ TDI detectors with400-600 Mpps. The fastest scanners use AOD technolo-g y, currently running at an equivalent pixel rate ofabout 50 Mpps. However, the slower pixel rate in ascanning system is more than compensated by the larg e rdefect-to-pixel size ratio in a darkfield config u r a t i o n .

Processing the ImageAfter obtaining the image, the image processor has todetermine the presence of a defect and accomplish thisfunction at a rate almost as large as that for the front-end detection. In a simple unpatterned wafer inspec-tion system a simple threshold scheme works well.However, die-to-die and/or cell-to-cell comparisons arerequired for patterned wafers.

For DRAM chips with their highly periodic structures,some inspection systems use optical spatial filtering toeliminate the light scattered from the periodic structurebefore it reaches the detector. Thus, only light scatteredby the non-periodic defects is detected. This technique

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F E A T U R E S

ratio (S/N). In the case of unpatterned wafers, S/Nshould be about 8 to 10 for 95 percent capture proba-bility and one false count per 200 mm wafer.

While brightfield systems usually use a high-pressuremercury (mercury-argon) arc lamp, darkfield systemsuse lasers. The recent development of reliable solid-state, diode-pumped lasers with greater than 1-wattpower has provided inspection systems with sufficientpower for most inspection tasks.

Image acquisition by existing inspection systems fallinto one of two main categories: imaging systems orscanner systems. In imaging systems the source opticsilluminate the area to be inspected, which is thenimaged by microscope optics on to a TDI or CCD camera. In a scanning system a focused laser beam“paints” the inspected area and a single element detector(usually a PMT) detects the collected scattered light.

These two types of systems have their own advantages.An imager is basically a fast optical microscope; thus, theoptical system design is straightforward. A TDI or CCDcamera obtains the image elements in a parallel fashion.A scanner-type system has no constraints on the anglesover which one collects the scattered light because it is anon-imaging system. It obtains the image in a serialfashion. An imaging system is useable in a brightfield orsingle darkfield configuration, but not with double dark-field. A scanner can have all three configurations; howev-er its disadvantage is the high speed required for thes c a n n e r, the detector and its electronics.

It may be relevant to describe the relationship betweenvarious terms commonly used in inspection systems,such as pixel size, spot size, and system spot size. In acamera-based system, pixel size, as referenced to thewafer surface, is the detector element size divided bythe magnification of the collection optics from thewafer to the detector. Note that this definition hasnothing to do with the resolution of the objective lens.In a scanner system, the focused Gaussian spot size isthe full width between the e-2 points. In this case, theresolution of the focusing optics determines the spotsize. For these systems the pixel size is the spot sizedivided by the number of electronic samples per e-2

width.

The sensitivity of an inspection system is related to the system spot size, which includes the resolution (ormodulation transfer function) of the optics, the detectorelement size, the front-end electronic bandwidth, and

The speed and cost of the image processor for patternedwafer inspection is critical. Fortunately, inspection systems can leverage off the improvements in themicroprocessor industry. In a sense, benefits can bederived from developments in the industry that isbeing supported. Computer speeds have improved byapproximately 30 percent per year over the last threedecades1 and the cost per MIP has fallen by approxi-mately 65 percent per year. However, the semiconduc-tor manufacturing industry is increasing the area densi-ty of IC devices by 40 percent per year. Thus, the timeit takes for doing the image processing of a wafershould remain approximately constant, even as therequired pixel rate needs to increase by 40 percent peryear to maintain throughput. Processing cost shouldalso fall, except for the fact that processing is becomingmore complex (more MIPS!).

Classifying DefectsIn the early days of wafer inspection systems, classifica-tion consisted of simply classifying and reporting adefect size. High resolution, brightfield systems could

only works with coherent laser illumination. Opticalfiltering typically lowers the background scatteringfrom the array by 100 times or greater.

F E A T U R E S

M i n i mum scan time (minutes for 200 mm wafer)

F i g u re 1. Actual inspection time (no overhead) fo r a 200 mm wafer

as a function of pixel size, wi th pixel rate (Mpps ) as a parameter.

Points indica te range of some existing systems.

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resolve the defect and determine itsarea. Because darkfield systemsdetected defects much smaller thanthe system spot size, they measuredonly the scattering light signal in asingle channel. Defect sizing camefrom comparing this signal againsta calibration curve for PSL sphereson the substrate.

For extended defects, post-process-ing algorithms in current systemscan classify clusters, scratches, andrandom defects. For defects smallerthan the scanner spot size or theimager optical resolution, however,real time classification requires mul-tiple views or channels. As describedin Part I of this article, multipleangles of incidence or multiple col-lection channels can provide superi-or classification capability. However,all this comes at a price becauseeach channel needs support, particu-larly in image processing.

Current gap in inspectionInspecting contacts and vias or highaspect ratio structures represents agap in the performance of currentwafer inspection systems.

Inspecting contacts and vias: B o t hoptical and SEM inspectors are effective in helping to develop andcontrol IC manufacturing processes.H o w e v e r, there is one major gap in the performance of currents y s t e ms— the ability to see smalldefects or residue at the bottom ofhigh aspect ratio structures.

Optically one can detect partiallyfilled or missing contacts in high-resolution systems. However, if aresidue of 5 nm is at the bottom ofa 250 nm diameter by 1000 nmdeep via, we are requiring a capa-bility that is difficult for opticalsystems (for example, ability todetect a volume difference equiva-

lent to a 75 nm diameter sphere atthe bottom of the hole2). Thus, ifcontact/vias must be checked indi-vidually, we are not going to do itoptically on real wafers. However, ifall the contact/vias within a localarea are incompletely etched, thenoptical means can detect it.

In a SEM system a voltage contrastmode can detect a residue at thebottom of a via or contact. However,SEM inspectors are not fast; thus, toinspect contacts/vias in a reasonabletime, we must resort to samplingsmall areas. Therefore, as with opti-cal techniques, here we can observeincomplete etching if this fraction ison the order of roughly 10-4, butfinding 5 nm of residue in one contact/via out of 1010 of them isbeyond practical consideration.

Future needs and developmentsSmaller critical dimensions, largerwafers and more integrated inspec-tion systems are part of our future.Inspection systems will follow thelead of lithography and migrate toultraviolet wavelengths. We willalso see an even closer coupling ofinspection with process equipment,review stations, and yield manage-ment systems.

Using UV in inspection systems: F o rdetecting smaller defects, bright-field systems need the higher resolution of shorter wavelengths.H o w e v e r, in darkfield systems the system spot sizes currentlyemployed are not limited by thevisible wavelength. Thus, it is notimperative that these systems useUV immediately.

In darkfield systems, the shorterwavelength of a UV laser leads to a greater scattering cross-sectionfrom particles on bare silicon

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surfaces. That is clear from the Rayleigh “blue sky”factor of λ- 4. Therefore, UV systems will be able todetect particles in the range of 20 nm diameter onsmooth surfaces.

In terms of patterned wafers, however, using UV hasthe following issues. In darkfield scattering mode opera-tions, one ultimately relies on the phase associated withthe interaction of light with the structures. Patternedand unpatterned wafers with films on them will bothsee a more rapid thin-film effect fluctuation. Thus,process variations across the wafer will have a greatereffect with UV illumination. It is therefore not obviousthat one necessarily gains from detecting defects ondense structures where the amount of scattered power isnot an issue. The shorter wavelength will result in thegeneration of more diffraction orders in the Fourierspace to filter out. For larger cell sizes, this also meansthat the orders are closer together, causing difficulty inremoving them. UV optics and lasers, of course, mustbe developed and available. For non-PMT detection,UV necessitates back-thinning of TDI/CCD detectorarrays or coating them with a fluorescence. UV lightalso can cause photochemical deposition of air-bornecontaminants on the optical surfaces, thus necessitatinge.g. a constant nitrogen purge.

These issues can be resolved, so UV systems will beavailable in the not too distant future.

Integrated inspection systems: Time-to-results is always animportant driver in the industry. Thus, we will seemore and more integration of inspection hardware unitsinto an overall system that can find the defects, reviewthem, and determine the source of the problem.

The industry has a great incentive to “shorten the loop”.As a result there is considerable investigation intobringing metrology and inspection within the processchamber (“in-situ”) or into a port on the process equip-ment. However, both technical and economic barriersexist that make it difficult to accomplish this. Highperformance (sensitivity and throughput) inspection has engineering constraints that make compatibilitywith process equipment difficult. In addition, the costof a metrology/inspection module has to be relativelylow compared to present-day systems to make it cost-effective. On the other hand, we will see somedevelopment of integrated inspection units that aretuned to the specific defects generated by process tools and are sensitive to relatively large defects.

Wafer inspection system performances have kept upwith semiconductor manufacturing industry require-ments. Both darkfield and brightfield systems continueto increase in sensitivity and throughput. To meetfuture needs these systems will go to higher resolutionwith faster image acquisition and processing. Real timeclassification will improve, with better coupling toreview, data management, yield learning, and yieldmanagement. Ultraviolet wavelength systems will pro-vide an additional increase in capability. ❈

For Part I of this article, visit our website at: www.kla-tencor.com/corpmag.

1. Brenner, A., Physics Today 49, 25 (1996).

2. Socha, Robert J., Neureuther, Andrew R., J. Vac. Sci Technol. B 15, 2718-2724 (1997)

This article is an adaptation of a paper presented at the 1998 Intern a t i o n a lC o n f e rence on Characterization and Metrology for ULSI Te c h n o l o g y, NationalInstitute of Standards and Te c h n o l o g y, Gaithersburg, MD. March 23-27, 1 9 9 8 .