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New Inspection Technology for 45nm WafersBecky Pinto, Jason Saito, William Shen, Lisa Cheung, Albert Wang – KLA-Tencor Corporation

by re-cleaning or re-polishing the wafer. The ability to know which wafers have to be scrapped, and which can be re-worked, is of tremendous economic and technical advantage to both wafer and IC manufacturers.

A new evolution of the Surfscan® SP2 system, the Surfscan SP2XP, supports surface and near-surface defectivity require-ments for wafers used for 45nm device manufacturing. For prime wafers, a previously undifferentiated category of defects known as large light point defects (LLPDs) can now be sepa-rated into reworkable defects and fatal process-induced defects. Scratch detection has also improved for prime wafers. For silicon-on-insulator (SOI) wafers, the new technology improves the ability to distinguish cleanable particles from detrimental voids. Finally, device-killing epi stacking faults (ESFs) can now be separated from more benign particles and flakes on epitaxial silicon (“epi”) wafers. Altogether, these advancements lead to fewer scrapped wafers for wafer manufacturers, and improved ability for device manufacturers to systematically build high performance devices at high yield.

New Wafer Inspection Technology

A reliable way of detecting crystallographic defects on epi, prime and SOI wafers and separating them from fall-on defects is now available on the Surfscan SP2XP. These capabilities are driven by new subsystem technology, including an extended dynamic range, an integrated differential interference contrast (DIC) channel, and a new ability to scan each wafer using both oblique- and normal-incidence scans without reloading the wafer. New algorithms compare scattering intensities from dif-ferent available channels of data to deliver defect binning with substantially higher accuracy and purity.

Surfscan Optical Design

Surfscan SPx wafer inspection systems work by rapidly scan-ning a laser spot in a spiral pattern across the surface of the wafer. Scattered light is collected in large solid-angle collec-tors, which integrate the signal to detect even small defects (Figure 1). Because the shape, size and material of the defect

In the past, the role of the substrate in integrated circuit manufacturing was primarily one of physical support for the devices built upon it. The substrate had to be flat and relatively free of particles, flakes and residues in active areas so that these defects were not incorporated into the structure of the transistor. The substrate’s crystallographic properties, or equivalently its electronic structure, can be engineered to play an active role in enhancing carrier mobility or decreasing leakage current. Substrate quality and uniformity have become critical to ensuring the best possible device performance.

Not only are wafer flatness, microroughness and particle count critical, but crystallographic defects also need to be detected and distinguished from particles and other fall-on defects. IC manufacturers typically have zero tolerance for intrinsic (grown-in) defects because they can impede or kill the device. On the other hand, particles or residues can often be removed

Defect ManageMent

Wide

BrightFieldDIC

Narrow

ObliqueIlluminationScan

RotatingWafer

NormalIllumination

Figure 1: Optical design of Surfscan SP2XP system. While the wafer spins below, the darkfield subsystem uses a 355nm laser to illuminate a spot on the wafer from a normal or oblique angle of incidence. Darkfield collectors span either narrow or wide solid angles. A separate, normal-incidence brightfield DIC channel operates simultaneously with either of the darkfield channels, and uses a 632nm laser.

A new unpatterned inspection system captures all intrinsic, polishing and fall-on defects, then separates them into

re-workable versus scrap defect types.

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and the wafer substrate affect the way the defect scatters light, normal and oblique incidence angles, narrow and wide collec-tion channels, and selectable polarizations provide flexibility to capture all defect types.

Brightfield Differential Interference Contrast (DIC) Channel

While all Surfscan SPx systems provide darkfield inspection, as described above, the new system incorporates a brightfield channel as well. (Darkfield detection relies on light scattered out of the direct beam, while brightfield detection makes use of the direct beam.) The brightfield channel not only provides another means of detecting challenging defect types, it also includes a DIC capability.1 This technique makes use of the phase of the laser beam to distinguish concave from convex defects. The DIC signal polarity is useful as a descriptor for defect classification.

Dual-Incidence Scans and Throughput Increase

One lot of wafers may contain a number of different defect types: some are captured more readily using oblique-incidence darkfield, some using normal-incidence darkfield, and some using brightfield. For this reason, the new Surfscan SP2XP can perform two successive scans on the same wafer, covering both oblique and normal incidence, without removing the wafer from the system. The brightfield channel operates simultane-ously with either or both scans. As a result, data are col-lected for five optical configurations: Oblique-Narrow (ON), Oblique-Wide (OW), Normal-Narrow (NN), Normal-Wide (NW), and Brightfield (BF). New algorithms allow sizing comparisons among the available collection channels to aid in distinguishing defect types of interest.

When the range of defect types on the wafer necessitates dual-incidence scanning, throughput is a consideration. The Surfscan SP2XP delivers a 20% throughput increase for single scans, and a 40% throughput increase for dual scans, compared with the Surfscan SP2 (Figure 2).

Extended Dynamic Range

Wafer inspection systems such as Surfscan SP1 and SP2 detect defects by collecting light scattered by the defect as a laser spot traverses it. The amount of light scattered by the defect is related to its physical size and shape and its reflectance.

Wafer inspection systems typically report the amount of light scattered in units of latex sphere equivalent (LSE) size. The reference is the amount of light scattered by polystyrene latex spheres of a given physical diameter. LSE size is also a func-tion of the angles of incidence and collection; thus, sizing of a given defect differs from one channel to another. Some defects scatter so strongly that they saturate the detection channel, and no specific size can be assigned.

The Surfscan SP2XP system increases the dynamic range of the darkfield collection channels by raising the saturation limit by a factor of sixteen (Figure 3). This allows more defects to be given an LSE size (Figure 4). LSE size in each channel is an important comparative descriptor for defect classification; therefore, extending the dynamic range of the collection chan-nels enables classification of more defect types.

Defect ManageMent

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Figure 2: Throughput improvement for Surfscan SP2XP relative to Surfscan SP2. HT = high throughput; ST = standard throughput; HS = high sensitivity.

Figure 3: Improved PMT signal processing methods underlie a sixteen-fold increase in the dynamic range of the Surfscan SP2XP, compared with Surfscan SP2.

Figure 4: Sizing in the Oblique-Wide channel versus the Oblique-Narrow channel shows that for this wafer, many defects fall into the ‘saturated’ category for one or both channels on the Surfscan SP2. The extended dynamic range of the Surfscan SP2XP allows all defects on the wafer to be assigned a size in both channels.

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reworked for prime wafers in some cases, either through an additional cleaning process or by re-polishing the wafer. A small number of polishing defects can be tolerated for 45nm device manufacturing.

The Surfscan SP2XP uses two aspects of its new technology to separate previously undifferentiated LLPDs into faceted pits and polishing defects. First, the brightfield channel is neces-sary for its differential interference contrast capability, because faceted pits are always captured in both brightfield and darkfield channels and always produce a negative DIC polarity. While polishing defects are also captured in both brightfield and darkfield channels, their DIC polarity can be either posi-tive or negative. Second, the extended dynamic range clearly differentiates between the two categories: faceted pits and polishing. As a result of this advance in detection and method of binning, prime wafers that were previously scrapped because their LLPD count was too high, can now be re-worked for cases in which polishing defects predominate in the LLPD count.

Detecting Scratches and Emerging Defects in Prime Wafers

Certain defect types have been difficult to detect using the standard light point defect mode on wafer inspection systems. Besides looking for localized scattering events, the Surfscan SP2XP can also generate a high-resolution haze map, called SURFimage.2 First introduced on the Surfscan SP2, SURF-image has been improved further on the new system. With a pixel size one-third as large as that of the original SURFim-age, the Surfscan SP2XP can capture even more shallow and faint CMP scratches (Figure 7) — which have been shown to affect yield for flash applications.3

The new high-resolution SURFimage has also uncovered previously unnoticed defect types such as orange peel, water-marks, slurry residue, and surface roughness changes. These ‘emerging defects’ have low scattering intensity and a full- wafer signature. With tighter focus on surface quality, emerging defect types may prove important to high k gate performance.

Distinguishing Voids from Particles for SOI Wafers

Silicon-on-insulator wafers can provide speed and power con-sumption advantages over polished silicon wafers. SOI wafers, like prime wafers, contain defect types which can be addressed

by the new technology. A void is an intrinsic or crystallographic defect at the surface of the SOI wafer—effectively a material rip-out—which could arise in either the bonded-wafer process when the top silicon is sepa-rated from the bottom or when particles are present on the substrate before BOX implant for the SIMOX process. Like faceted pits in prime wafers, voids are fatal to device manu-facturing and are not tolerated by IC manufacturers.

Cross-Channel Rules-Based Binning

With two darkfield angles of incidence and two independent darkfield collectors, plus the brightfield (DIC) subsystem, the new inspector has a total of five distinct data channels (Figure 5). The dual-scan capability means that multiple-channel data can readily be collected on every wafer. Ratios of defect sizing by various channels can be used as attributes for rules-based binning (RBB), providing new capability to distinguish among defect types.

Actionable LLPD Classification for Prime Wafers

Large light point defect (LLPD) is an empirical defect classifica-tion referring to any defect captured by both brightfield and darkfield channels by an unpatterned wafer inspection system such as a Surfscan. Defect review shows that LLPDs fall into two broad categories (Figure 6): intrinsic crystalline defects best described as ‘faceted pits,’ but also called air pockets or air bubbles; and ‘polishing defects,’ including polishing divots and chatter marks.

Faceted pits are generated during ingot pulling and are ex-posed after the ingot is sliced and polished. These large defects — 20 to 200µm — are not re-workable, and if they align with an active area of the device, they typically cause device failure. IC manufacturers reject all wafers having fac-eted-pit defects. On the other hand, polishing defects can be

Defect ManageMent

NormalOblique BF DIC

Wid

eN

arro

w

Figure 5: Dual scan capability of the Surfscan SP2XP collects inspection data from all five channels. These data are used as defect attributes for advanced binning, using logical and comparison rule-based algorithms.

Figure 6: LLPDs fall into two broad categories: intrinsic crystalline defects best described as ‘faceted pits,’ but also called air pockets or air bubbles (red); and ‘polishing defects,’ including polishing divots and chatter marks (blue).

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The Surfscan SP2XP system’s new technology enables the separation of voids from the relatively innocuous particles and other fall-on defects, which may be cleanable or otherwise re-workable. When the dual-incidence capability of the new

system is used, LSE sizing from the oblique and normal dark-field scans can be compared. Figure 8 shows a plot of defect sizing from the Normal-Narrow and Oblique-Wide channels. For both types of SOI wafers (SIMOX and bonded), voids are

Defect ManageMent

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Figure 7: New high resolution SURFimage with a smaller pixel size demonstrates better defect detection on shallow scratch defects.

Figure 8: Comparison of defect sizing from the Normal-Narrow and Oblique-Wide channels. For SOI wafers, voids are clearly distinguishable from particles and other fall-ons. The impact of this capability is that wafers having large particles only would not be scrapped along with wafers having voids.

Figure 9: Comparison between Normal-Narrow and Oblique-Wide sizing distinguishes ESFs from other defect types. Wafers having ESFs will be scrapped, while wafers having small numbers of relatively innocuous defect types may pass IC manufacturers’ requirements for 45nm.

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Wide sizing, as shown in figure 9, demonstrates that ESFs are clearly distinguishable from other defect types. Wafers having ESFs will be scrapped, while wafers having small numbers of other defect types may pass IC manufacturers’ requirements for 45nm.

Summary

The Surfscan SP2XP has been introduced with new and enhanced capability to distinguish crystallographic defects from particles, flakes and other fall-ons. This ability to discern which wafers are free of intrinsic defects means that fewer wafers may be scrapped. As wafers become more complex, with epitaxial layers and strained layers of various materials, their increased cost makes the identification of intrinsic defects of tremendous economic advantage to both wafer manufacturers and IC makers.

References

1. C. Dennis; R. Stanley; S. Cui, “Detection of a New Surface Killer Defect on Starting Si Material using Nomarski Principle of Differential Interference Contrast,” to be presented at ASMC 2007. See also “SP1 Brightfield Defect Detection,” Wilson Cheh, KLA-Tencor Application Note, March, 2003.

2. A. Belyaev; A. Steinbach; Hamlyn Yeh, and B. Pinto; N. Microdevices, “New Technology for Generating High-Speed, Full-Wafer Maps of Micror-oughness and Grain Size,” July, 2006 (Japanese). Also printed in English in Yield Management Solutions, Summer 2006, pp. 64-70.

3. J. Park, “Memory Wafer Trend for 45 nm and Beyond,” Department of Electronics and Computer Engineering, Hanyang University, Semi STP, December, 2005.

4. Gartner/Dataquest, December 2005.

clearly distinguishable from particles and other fall-ons using this method. The result is that wafers having large particles only would not be scrapped along with wafers having voids.

Classification of Stacking Faults on Epi Wafers

Epitaxial silicon wafers are designed to provide advantages in device speed, breakdown voltage, and latchup resistance. Demand for epitaxial wafers is expected to grow 45% between 2006 and 2010.4 As with SOI or prime wafers, epi wafers are subject to crystallographic defects as well as particles and other fall-ons. The most common kind of crystallographic defect is the epi stacking fault (ESF). These represent a misalignment in the otherwise perfect crystal lattice, originating when a par-ticle or other defect lies on the surface of the bulk silicon upon which the epi layer is grown. The interface defect initiates the crystal misalignment, and as the epitaxial film grows, the ESF propagates through the crystal to the surface of the epi layer. Epi stacking faults – including related crystal defects such as hillocks, mounds and spikes – are deadly to the transistor built upon them. IC manufacturers typically specify that epi wafers must be ESF-free.

Although epi wafers cannot be reworked, the Surfscan SP2XP enables improved separation of stacking faults from other common epi defects, such as particles and flakes. While previ-ous Surfscan models were able to distinguish stacking faults to a certain extent, the dual-incidence scan capability of the system improves accuracy and purity by as much as 50%. The new system has demonstrated that ESF defects scatter more strongly under normal incidence, while particles and other less damaging defects scatter more strongly under oblique inci-dence. A comparison between Normal-Narrow and Oblique

Defect ManageMent

2007

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