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The Role of E-Beam Inspection in a Production Environment by Jeff Hamilton, Senior Product Marketing Manager
The process and yield challenges that face today’s device manufacturers are increasing dramatically. The combination of new materials such as copper, low-k and DUV resists, new processes such as dual damascene, and rapidly shrinking design rules necessitate solutions that detect and identify all defect mechanisms. With manufacturers moving from traditional alu minum processing to dual damascene, defect mechanisms will switch from surface-related defects to defects within metal fills and high aspect ratio structures. In addition, design rule shrinks below 0.18 µm will require the detection of defects down to below 50 percent of the minimum design rule. Further, the accelerated development cycles of today plus the requirements for high yields at process transfer and higher yield ramps significantly compound the challenge. The nature of defects is changing as customers develop processes with design rules less than 0.18 µm. As a result, new materials and fabrication techniques are being introduced at an unprecedented rate. Defects associated with high aspect ratio lithography and etch, as well as high aspect ratio fills will predominate in these new processing schemes (such as copper dual damascene). With each progression in device generation, the increased aspect ratios of damascene structures and the variety of new materials place a larger burden on etch and fill processes. Specific detection needs include:
conventional optical inspections. Defects such as voids within metals (copper, tungsten, aluminum), residues at the bottom of contact/via holes and poor silicide formation cause electrical signatures that can be easily detected with this method. 3. High depth of focus imaging: A high depth of focus is needed that allows for the detection of physical defects (residues, particles, etc) within vias and trenches that cause failures and/or reliability issues. Electron-beam inspection effectively addresses these needs. In order to reliably detect all yield limiting defects, customers must now develop strategies that utilize both optical and e-beam inspection solutions.
1. High resolution imaging: Sensitivities below 100 nm are needed that can detect very small residues and stringers at the bottom of high aspect ratio structures (vias and trenches), as well as very small surface defects. 2. Voltage contrast imaging: Due to the nature of “hidden” defects, voltage contrast imaging offers a quick method to identify these defect types. The interaction between electrons and specific defect mechanisms enable this imaging capability to detect the “hidden” defects that cause electrical yield failures and/or reliability issues. Detection of these defects within the process is critical since these defects are undetectable through 6
Autumn 1999
Yield Management Solutions
F i g u re 1. “Hig h aspect ratio” a nd “hig h resol ution, hig h depth of f ield ” via/trench i nspecti on technolo gy is re q u i r ed (i.e., e-beam) to identify defects in the ppb level in today ’s new fabr ication pro c e s s e s .
Process Metal Etch
Physical bridging, stringers, particles, corrosion, extra/missing pattern
Voltage Contrast opens, shorts, voids, resistance issues, organic residues, extra/missing pattern
Contact/Vias
polymer residues, CD variations, particles,
missing, under-etch, residues, resistance issues
Tungsten Plugs
coring, residual W, particles, CMP scores, filled scores, slurry
voids, stringers, coring missing plugs, stress breaks
Damascene Trench/Fill/CMP
bridging, stringers, particles, µscratches, CMP scores, slurry
missing, under-etch, residues, resistance issues, voids, stress breaks, liner voids
Polys
polymer residues, stringers, extra/missing pattern
opens/breaks, shorts, resistance issues, organic residues
Capacitors
polymer residues, bridging, stringers
capacitor shorts, cell contact opens
F i g u re 2. Yield issues detected using e-beam technol ogy.
Early adopters of e-beam inspection technology were motivated by the need to rapidly increase learning cycles in process development and get their product to market, so that they could obtain initial high margins and capture market share. As these processes were introduced into high volume production, the need to monitor certain critical layers became apparent. Knowing that the narrow process windows were inherent in some of these process layers and that the conventional methods to identify these defects (electrical tests and end of line yield) created a large window of product at risk, these early adopters implemented a “line auditing” scheme — sampling a few wafers per day on key critical layers. Although not true line monitoring, the “auditing” scheme served to minimize risk of critical layer yield excursions on their most advanced processes.
F i g u r e 3. Impact of comb ined e-beam a nd optical line monit oring strategy on yi eld learn i n g .
Examples
With new defect types and shrinking design rules, a line monitoring solution is needed for these critical layers in advanced production fabs. To meet these needs, KLA-Tencor has introduced an e-beam line monitoring solution, the eS20. With this technology, significant increases in inspection speed have been obtained, up to 75 times over current inspections on many critical layers. As a result, this new e-beam inspection capability provides the vital link that IC manufacturers need to develop comprehensive in-line monitoring strategies that leverage both e-beam and optical inspection technologies— enabling the detection of all critical defect types at all critical layers at the high speeds needed for true production monitoring. Optical systems alone are not enough, nor are e-beam systems alone. However, together they represent a powerful and unique combination for meeting the development and production challenges of today’s IC manufacturing. Through effective sample planning and judicious use of e-beam inspection with optical methods, risk due to yield excursions of these new process technologies can be minimized and an optimum use of inspection tools can be developed. A high return on investment can then be realized by minimizing the effects of yield excursions in these advanced high volume production fabs. ❈ cir cle RS#029
F i g u re 4. Optimized sa mpling stra teg ies provide high ROI.
Autumn 1999
Yield Management Solutions
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