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Evaluation of the KLA-Tencor AIT II by Brian Metteer, Chris Meier, Ben VanZante; Texas Instruments Jon Button, Steven Short, Ph.D., Mike Rodriguez, Rebecca Howland Pinto, Ph.D., Pete Muraco, Arlisa Labrie-Miller KLA-Tencor Corporation
As integrated circuit feature sizes continue to shrink, and new process technologies such as Cu dual damascene are introduced, defect inspection technology must continue to improve for device manufacturers to manage their yield. Inspection equipment requires increased sensitivity to smaller defects and new defect types, while maintaining cost of ownership (CoO), primarily via throughput improvements.
To increase throughput, sensitivity and defect capture on its AIT laser scattering inspector product line, KLA-Tencor introduced the AIT II inspection system. Primary sensitivity improvements are achieved by increasing the photon density in the vicinity of the defect by employing a higher-powered laser and by focusing the laser beam to a smaller spot size. Increasing the size of the collection optics provides additional improvements in sensitivity and also enhances capture of defects such as microscratches and defects in trenches. Speeding up movement of the stage and increasing the data processing rate improve throughput. Texas Instrument’s KFAB development facility evaluated an AIT II beta system from September 1998 to January 1999 for sensitivity, throughput, and reliability. The primary purpose of the evaluation was to compare the capabilities of the AIT II system to those of the AIT I and the KLA-Tencor 2138 image comparison inspector. Eight process levels were studied, all from Texas Instrument’s next-generation Digital Signal Processing (DSP) technology. Generally, two wafers from 6 to 7 lots were studied for each layer on each tool. On each layer, the AIT I’s fixed 10 µm spot size was compared to the AIT II’s 10 µm, 7 µm and 5 µm spot sizes. Additionally, a 2138 (typically 0.39 µm pixel size) inspection was run on each layer. 24
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AIT II technology overview
The AIT II operates in similar fashion to the AIT I, by performing a die-to-die comparison of light scattering signals to detect defects. The wafer surface is illuminated using an oblique-incidence laser beam, and the scattered light is collected by two independent photo-multiplier tubes (PMTs). Particles, scratches, circuit patterns, pattern anomalies and surface contamination all scatter the incident laser light. To preferentially detect the defect signal, unwanted pattern scatter may be filtered out optically or mechanically prior to being collected by the PMTs. Fourier filters minimize scatter from periodic structures such as dense array pattern. Other, customizable spatial filters allow exclusion of certain solid angles of collection, for optimization of signal-to-noise under specific conditions and for the capture of specific defect types. Polarization of the oblique incident beam and oblique collected beam can be leveraged to suppress grainy surface noise, often seen with metal layers, and color variation on the surface of the wafer due to varying layer thickness. The oblique-incidence, oblique-collection design also minimizes scatter from rough surfaces. Typically, Yield Enhancement engineers prefer detection of current-layer defects to previous-layer defects. The AIT’s design tends to result in the capture of more current-layer defects than normal-incidence, bright-field inspection. The collection optics direct the scattered light into separate PMTs that independently convert the collected light energy into an equivalent electronic signal. As the wafer is scanned, signal processing algorithms compare
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AIT I’s fixed 10 µm spot size test. All 2138 tests were 0.39 µm Segmented Auto-Threshold (SAT) tests, with the exception of a 1.25 µm Random Auto-Threshold (RAT) test on Metal 2 Sputter.
F i g u re 1. Defect detection in a l aser sc attering inspection s ystem.
the signals from neighboring die, subtracting out all the common information between them (Figure 1). Each non-defective die should have the same electronic signature as its neighbor, but defects on the wafer will scatter differently from the equivalent area in the neighboring die. Any areas of the die whose scattering intensity difference exceeds a user-defined threshold are considered defective. The AIT II offers improved sensitivity and throughput compared to the AIT I. Sensitivity improvement was achieved using larger collection optics, increased laser power, smaller spot sizes (5 µm and 7 µm), and software improvements contained in version 3.95 and higher versions. AIT II collection optics area increased with a wider azimuth angle for improved scratch detection, and a higher elevation angle for increased trench defect capture. Software improvements include a “multi-threshold” capability that allows the user to use different contrast and threshold values in different geographical areas of the die, e.g., in logic and array areas. Throughput was improved by a faster wafer stage and faster data processing. Experimental methodology
Table 1 is a summary of the tests completed on the AIT I, AIT II, and 2138 during this beta evaluation. All process levels were from the same device, an advanced DSP chip under development at the time by Texas Instruments. This “head to head” comparison provided a good picture of the tools’ respective capabilities. On each process level, all three AIT II spot size tests were compared to results from the
To prevent any possible bias towards the AIT I or AIT II during the evaluation, control was exercised over their 10 µm spot size tests. Initially, after the AIT II 10 µm recipe optimization was complete on a certain process level, every effort was made to ensure that the corresponding AIT I recipe polarization and sensitivity parameters were identical. Then, the AIT I recipe was sometimes reduced in sensitivity where necessary to eliminate nuisance defects. This was generally necessary due to the improved optics and overall system capability improvements of the AIT II and was most notable on the edge dice. Additionally, the multithreshold capability of the AIT II was leveraged for the purposes of this evaluation, thereby improving the sensitivity of AIT II recipes. To ensure consistency across inspections and defect classifications, and also to maintain recipe quality and data integrity during the evaluation: • KLA-Tencor applications engineers created and optimized all 2138 and AIT recipes. • Texas Instruments desired to inspect the entire area of the wafers under evaluation. During recipe setup, masking (excluding) some areas of the die was allowed where the overall sensitivity of the test could be improved. However, masking in the same areas was not required on other tests/tools. In other words, if masking was found necessary on the AIT I test for some reason, it was not required that the same area be masked on the 2138 test. • One Texas Instruments person was assigned to each specific level for defect classification to ensure consistency.
Table 1. AIT II evaluat ion comparison t ests.
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• Approximately 125 defects were reviewed and classified from every test during this evaluation. • Texas Instruments personnel classified all defects and created all results files. Texas Instruments had complete control over all data through final reporting for all wafers studied. To ensure a fair comparison of the sensitivity capabilities of each inspection tool, the raw inspection data from each tool was clustered consistently, using pre-set clustering parameters in KLA-Tencor’s defect data management system. C h a rt 2. N et c ount summary for all evaluated levels.
Results
Two major parameters were of primary concern to the team evaluating the AIT II at Texas Instruments. First, throughput tests are important due to obvious semiconductor manufacturing loading issues. Next, overall sensitivity of the technology is critical to ensure that semiconductor manufacturers have the tools required to meet their next generation yield goals. To gauge sensitivity, the Texas Instruments team looked to two major categories: capture of unique defects, and net defect counts. While the overall net count of defects is a good indicator of tool sensitivity, it is also important to credit a technology for finding specific defects of interest. This is especially true if one tool can find strategic, yield-limiting defects that another cannot. Throughput results for the tools compared in this evaluation are summarized in Chart 1. The AIT II 10 µm spot size test is approximately 34% faster than the AIT I 10 µm spot size. The AIT II smaller spot size tests are substantially slower than the AIT I 10 µm spot size test, the price paid for the enhanced sensitivity of these tests. Of course, the 2138 0.39 µm test is significantly
Chart 1. Throughput of defect inspection tools measured during evaluation.
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slower than all AIT tests. All throughput tests assume that 24 20 mm wafers per lot are run. Sensitivity of each tool, summarized in Net Counts, is defined as: Net Count = True Counts (Random Defects + Clustered Defects = 1) minus Previous Level Defects minus Nuisance Defects minus Surface (Fall-On) Defects. This methodology for comparing tool sensitivity focuses purely on current level, real defects. In other words, the tools do not get credit for finding previous-level or nuisance defects, which are of little interest to TI Yield Enhancement (YE) Engineers and dilute the focus on the defects of interest. Additionally, since the wafers were not always inspected on all of the inspection tools as the same time, surface (fall-on) contamination defects were eliminated from the sensitivity scoring. Net Counts for each process level are summarized in Chart 2. As expected, the AIT I and AIT II performed better than the 2138 on the two CMP levels studied. Especially notable is the “stair-step” function in sensitivity between the AIT I and AIT II laser spot size tests. As expected, the smaller spot size tests were found to be more effective than the larger spot size tests. Also, the 2138 was found to be more effective than the AIT II on Metal Etch and Gate Etch. Finally, the 2138 outperformed the AIT I and AIT II on all other levels, although the AIT II typically found the same defect types as did the 2138. One surprise in the data was at Metal 2 Etch, were the AIT technology did not follow the expected “stair-step” sensitivity relationship. At this level, it was found that the smaller AIT II spot size test was more efficient at finding the underlying nuisance source, in this case metal graininess. Therefore, the inspection recipe required desensitization to control nuisance capture rate.
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C h a rt 3. Tungst en CMP defect ty pe summar y.
Additionally, it was found that the combination of the improved sensitivity of the AIT II 5 Âľm spot size test and multi-thresholding narrows the gap on most process levels where the 2138 is considered preeminent between the 2138 and the AIT technology. The combination of the smaller laser spot size and the ability to focus on specific defect types and regions of interest may allow Yield Enhancement engineers to implement the speed of the AIT II platform on some traditional 2138 levels. Finally, the team was interested in specific defect type capture by the platforms. For example, an overall Net Count score could be offset somewhat by the ability of a certain tool to find strategic defects of interest more efficiently than another tool. For example, Chart 3 illustrates a case where the AIT II is much more efficient at finding microscratches, small particles, and CMP slurry at Tungsten CMP. Additionally, Chart 4 illustrates a case where the 2138 finds a pitting defect, while the laser scatter tools miss this defect type completely.
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Reviewing the results of this evaluation, it becomes clear that the inspection tools still follow traditional patterns. The AIT technology, especially the smaller AIT II spot size, was very effective at the Oxide CMP and Tungsten CMP levels in finding microscratches, small particles, and CMP slurry. The combination of smaller spot size capability and geographical multi-thresholding was found to provide increased sensitivity, approximately 10 percent to 20 percent improvement in sensitivity than the AIT I using the 10 Âľm spot. The 2138 was more effective than the AIT technology in detecting defects in patterned areas and in capturing residue and other high contrast, low profile defects. The penalty in using the 2138 is that the tool does tend to capture substantially more previous level defects than the AIT technology. Additionally, the AIT II smaller spot size tests do tend to find more previous level defects than the larger spot size tests. The throughput results illustrated in the previous section clearly show the speed advantage of running the AIT I or AIT II in production. However, it is clear from the results on most levels that the 2138 is substantially more sensitive than the AIT technology. On the other hand, the combination of the AIT II smaller spot size tests and multi-thresholding clearly narrows this performance gap.
Conclusions
Traditionally, optical imaging inspection systems have been proven to be very effective when inspecting in areas of dense pattern and on defects with high contrast. On the other hand, laser scattering tools have been proven very effective on film and especially on CMP levels.
C h a rt 4. Metal 2 sputter defect type summar y.
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