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Advanced Inspection Strategies: The requirement for a multiple-technology solution

by Kern Beare, Senior Marketing Communications Manager

As the processes that produce semiconductor devices become increasingly complex, so do the metrology and inspection technologies required to monitor and control those processes. Nowhere is this more true than in wafer defect detection and inspection.

Today, new materials and processes for the deposition, patterning, etching and planarization of complex, multi-layer devices have led not only to new defect mechanisms and types, but also to new process noise sources that interfere with defect detection (figure 1).

These noise sources include multiple layers of non-uniform transparent dielectrics that create extreme color variation, high aspect ratio surface features and, at the backend, grainy etched metal lines. Since different inspection technologies have different sensitivities to defect types and noise sources, some are more suitable than others for defect detection at different points in the process flow.

Engineering Analysis: Cutting-edge technologies speed product development and transfer

Engineering Analysis — a concentrated effort to identify and track yield problems back to their sources, identify the causes and then verify the fix — is the primary focus at the product or technology development stage, although it continues through product/technology transfer and yield ramp. With fabs facing continuous pressure to accelerate the development of new technologies to speed time-to-market and achieve profitability goals, selecting the right engineering analysis inspection technologies and methodologies is of paramount importance.

Ultra-Broadband Brightfield Inspection Technology Inspection requirements also vary with the overall application for which an inspection technology will be used: engineering analysis, line monitoring, equipment monitoring, or some combination of the three. IC manufacturers who understand these requirements are reaping the rewards of faster development, transfer and ramp of new products and technologies, faster time-to-market, and increased ROI. 14

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In engineering analysis, the primary inspection requirement is sensitivity. Today, ultra-broadband (UBB) brightfield illumination combined with high-speed digital image processing is the industry’s leading engineering analysis inspection technology, offering the highest sensitivity of all optical inspection systems.

UBB brightfield technology offers other advantages, particularly reducing noise resulting from non-uniform film thickness. Non-uniform film thickness causes inter-

Defect Signal

Process Noise Pattern

Grain

Film Thickness

Automated E-Beam Inspection Technology

Pattern

Underlying Noise

Surface Embedded Underlying

As the industry begins development and production of sub-0.25 µm devices, greater levels of defect detection sensitivity will be required. One of the most difficult detection challenges is inspection of high aspect ratio contacts, vias and trenches. As the aspect ratio of these features grow, it becomes more difficult to propagate light down and back out of such features for defect detection. Additionally, as new materials and fabrication techniques are introduced, defects associated with filling those structures (such as voids, blocked etches and residues) will predominate, creating further challenges for detection.

Planar HAR

Figure 1. New materials and processes have led to new defect types and noise sources.

ference with the reflected light, resulting in a change in color. This variation in background color creates a contrast variation from die to die that can be misinterpreted as a defect by the wafer inspection tool. UBB technology — with its broad wavelength spectrum and uni-

Because of the difficulty in detecting these defect types optically, UBB brightfield inspection technology NB 0.8 must be complemented by automatUBB ed electron beam (e-beam) inspec0.6 tion. This technology offers unrivaled sensitivity, easily detecting 0.4 NarrowBand defects as small as 0.1 µm on pat(NB) UBB 0.2 terned wafers, even in densely packed, high aspect ratio, multi0.0 layer geometries. It also offers the 400 450 500 550 600 650 700 750 ability to detect electrical defects TDI Digitized Images Wavelength (nm) using voltage contrast techniques Figure 2. UBB technology offers a wider range of wavelengths than a narrow band source, enabling (figure 4). This capability allows it to optically average out the effect of color variation caused by non-uniform film thickness. automated e-beam inspection to “see” the hidden defects associated with fill at current form xenon illumination source intensity — is the most and previous layers. Previously, these defect types could successful at optically averaging out this intensity variaonly be detected using electrical test methodologies tion and increasing signal-to-noise for high sensitivity much later in the process, potentially exposing many to all types of defects, while reducing or eliminating wafers to yield losses occurring from similar defects. nuisance defect counts (figure 2).

Automated e-beam inspection technology is orders of magnitude faster than electrical test, reducing time-toDefect sensitivity and capture can be further improved information from weeks to a matter of hours or days. in a brightfield tool with Segmented Auto Threshold (SAT) technology, an image processing technique that Line Monitoring: Optimizing inspection increases sensitivity on wafers with grainy metal and to accelerate yield learning, minimize color variation (figure 3).

(NB and UBB normalized separately)

Die A

Die B

Die C

Relative Spectra

1.0

Background Noise (gray level difference)

excursions Fixed Threshold Defect Signal

Local Segment Auto Threshold Defect Signal

Segments Previous Layer

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Figure 3. Without SAT, a fixed threshold level would be required to minimize nuisance defect counts caused by the grain. With SAT, lower thresholds are automatically applied to each segment, maximizing defect capture and reducing nuisance defect counts.

Once a new product or technology is developed, the next step is to transfer it into production. Part of ensuring a successful transfer and start-up is developing in-line inspection strategies that (1) enable excursions to be detected at the earliest possible point and (2) effectively identify the top defect types that, if eliminated, will reduce baseline defect density for higher yields. Line monitoring methodologies — looking for defects caused by the integration of multiple process steps and correlating them to yield — is a critical part Figure 4. E-beam inspection technology can also detect electrical defects of this strategy. using voltage contrast techniques.

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Line monitoring methodologies typically consist of inspecting several wafers, often from the same lot, throughout the entire manufacturing cycle. Defects are identified and classified, sourced through layer-to-layer analysis, and then correlated to electrical test. Corrective action can then be implemented to reduce the occurrence of the defect and increase overall yield.

For line monitoring, defect detection sensitivity is paramount, but throughput now plays a larger role. To maintain production, inspection must be performed quickly, with the sensitivity and defect capture required to provide statistically valid results. Another common requirement for line monitoring is the ability to see previous layer defects. Because the inspection will often occur at the end of a process zone, detecting previous layer defects is frequently necessary to trace a yield-killing defect to the exact process step that created it.

Ultra-Broadband Brightfield Inspection Technology The sensitivity of image-processing based UBB brightfield inspection technology that makes it ideal for engineering analysis also makes it the technology of choice for many line monitoring applications, including etch/final inspect and photolithography. By utilizing this technology for both engineering analysis and line monitoring, fabs can have a singleplatform solution to facilitate recipe transfer and system matching.

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Oblique Darkfield Laser Imaging Inspection Technology

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For CMP line monitoring applications, a new inspection technology — oblique darkfield laser imaging — has demonstrated superior performance over brightfield inspection techniques. While CMP has become critical for the development and production of sub-0.25 µm devices — enabling both increased levels of interconnect and tighter packing densities — it also introduces many new yield-limiting defect types and noise sources, including color from oxide thickness variation, underlying and top surface pattern variations, and grain noise from metal and polysilicon.

Directional darkfield inspection Objective technology is extremely effective both at minimizing these noise sources and providing exceptional capture of CMP-related defect types. These results are achieved by integrating the benefits of oblique (low angle) laser illumination for Figure 5. Oblique darkfield laser imaging technology combines oblique improved surface selectivity, grain noise angle laser illumination with high-speed, suppression and high throughput, with high-resolution digital image processing. the benefits of small pixel, high-speed digital image processing for high resolution and capture rate (figure 5).

Equipment Monitoring: Fast defect sourcing for improved overall equipment efficiency

In contrast to line monitoring, equipment monitoring inspects wafers from a given process tool. By monitoring the total defect count (or a specific type of defect) from a process tool, equipment monitors are able to verify whether it is operating within specifications or if some sort of maintenance is required.

While sensitivity is again extremely important, throughput is paramount since inspection should never “gate” production. Another requirement is

sensitivity to only the current layer, since, unlike line monitoring applications, previous layer defects are not relevant to the current process step being inspected and would only add to the nuisance defect count.

Line monitoring and equipment monitoring are complementary, and line monitoring results can drive equipment monitoring strategies: When the line monitor tracks a defect to a particular process zone, the equipment monitors will inspect wafers from the zone to track the source of the defect to a particular process tool. This is best accomplished using patterned wafers, since this allows the detection of defects created by process or process integration issues.

Utilizing product wafers to monitor tool performance is an increasingly common approach. This is because it maximizes throughput of product wafers, increasing overall equipment efficiency (OEE). This will become even more important as the industry moves to more costly 300 mm wafers.

Double-Darkfield Laser Scanning Technology for Patterned Wafers

go/no-go decisions to be made quickly and with a high degree of confidence. Double darkfield also offers the lowest cost-per-inspection, making it economically feasible to perform equipment monitoring on every critical piece of process equipment, greatly reducing risk to current yields.

Stationary Beam Laser Scattering Technology for Unpatterned Wafers For equipment qualification using unpatterned wafers, laser scattering systems using stationary beam technology (SBT) offer the highest sensitivity and throughput. With this technology, a fixed laser beam illuminates the wafer surface, while the wafer spins and moves beneath the beam to trace out a spiral scanning pattern (figure 7). A primary benefit of this design is that it allows the use of uniform, axi-symmetric collection optics that surround the illuminating beam. Light scattered by defects is collected regardless of the direction of scatter; defects are detected uniformly and repeatably across the entire wafer surface — even on wafers with low defect densities.

Double-darkfield laser scanning For applications technology — which combines lowwhere exceptional angle (darkfield) illumination with sensitivity on rough films is low-angle collection optics — is required, SBT can be configured uniquely suited to provide the senwith up to three illumination paths. sitivity, throughput and layer selecThese paths include normal illumitivity required for most patterned nation for high sensitivity to surface wafer equipment monitoring applications (figure 6).

Able to detect Scattered Light particulate con(darkfield) Collection Optics tamination, microscratches Incident Optics and planar defects at extremely high throughput, double-darkfield Specular Beam Incident beam technology (brightfield) enables comprehensive equipment monitoring Figure 6. Double-darkfield inspection technology suppresses background inspection strate- noise for high defect capture. gies for critical Summer 1998

defects and oblique illumination for better performance on high scattering surfaces. A brightfield channel based on Nomarski differential interference contrast technology is used for the detection of low or non-scattering defects that cannot be easily detected in darkfield. This triple beam illumination technology provides unprecedented defect information for the development and production of advanced semiconductor devices.

Multiple technologies allow comprehensive wafer inspection

Clearly, no single inspection technology can be optimized for all process challenges and all applications. However, a combination of inspection technologies can be integrated to provide a comprehensive and complementary wafer inspection methodology to capture all critical defect types across

all inspection levels Technology combines a short, at high fixed illumination path with throughuniform, axi-symmetric collecput. tion optics and a rotating Leading wafer design — providing IC manuexceptional sensitivity and facturers measurement uniformity. are realizing the value of this integrated, multiple technology solution, and are developing new best practices for engineering analysis, line monitoring and equipment monitoring.

This, in turn, is enabling accelerated R&D and product transfer, and faster, higher yield ramps for maximum ROI. Figure 7. Stationar y Beam

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