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Production QC and Tool Monitoring Using an Automated Macro ADI Defect Inspection System

by Iain Rutherford, Brian Haile and Tony DiBiase, KLA-Tencor Corporation This paper was presented at KLA-Tencor’s Yield Management Solutions Seminar during SEMICON/Europa in April 2000.

The major weakness of traditional, manual after-develop inspection (ADI) lies in the variability of the results. Defect capture rates are variable due to differences in the ability and experience of the inspection operators. Subsequent analysis of defects can also be inconsistent as some operators might flag a defect while others might pass it thinking it unimportant.

Manual inspection is also one of the most tedious and unpopular jobs in the fab among the operators.

The ability to drive yield improvement from manual inspection results can be very poor. Data from manual inspections can be vague and subjective, making it difficult to archive or correlate with yield and parametric results. The bottom line is that manual ADI misses macro defects and costs money.

Manual vs. automated ADI

A solution to these issues is an automated, optical macro defect inspection system such as KLA-Tencor’s 2401. This system was installed for evaluation at NEC in Livingston, Scotland, and this paper reports the results from the evaluation at that site.

The study was conducted in two parts: First, a comparison of manual versus automated inspection using the 2401 for ADI, and second, an investigation of the potential of the 2401 for evaluating and monitoring certain aspects of stepper performance.

Eight layers from one particular product were chosen.

Lots were then randomly sampled from these layers.

The layers represented a typical mix of front-end and back-end layers, both critical and non-critical.

The 2401 works by simultaneously scanning and capturing darkfield and brightfield images of a wafer. The inspected field is compared with two fields adjacent to it and any discrepancies between the two fields are flagged as a potential defect. The system has an 80 wafer-per-hour throughput and can capture defects greater than 50 µm. If needed, wafers can be reviewed using a variety of software tools on the machine.

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Over 5500 wafers were involved in the first part of the evaluation, and most of these (5019) were randomly selected for after-develop inspection. Some wafers with known problems were chosen (225), and some originated from engineering (322). The wafers were inspected manually, then taken to the 2401 for automated inspection.

Figure 1 demonstrates the overall sensitivity of the 2401 compared with manual inspection. While manual inspection found that 3 percent of lots had issues worth Visual Inspection Results

2401 Inspection Results

3%: Failed

97%: Passed

46%: Failed

54%: Passed

Figure 1. With 213 lots inspected at random, 6 lots failed visual inspection while 97 lots failed the 2401 inspection, demonstrating a 10X difference in capture rate when replacing manual with automated macro inspection.

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Figure 2. A breakdown of the detected defect types shows that stepper problems such as leveling, hot spots and acceleration errors dominated. This chart also emphasizes the difference in capture rate between the 2401 and manual inspection.

investigating further, the 2401 found that nearly 50 percent of lots had defect issues of interest to the NEC engineers. These results correspond to a 10-fold difference in capture rate between the automated inspector and the manual inspection procedure at NEC. Examining these results as a function of defect type, Figure 2 illuminates the important fact that the top three defect types — leveling, hot spots, and acceleration errors — are related to the stepper (acceleration errors are caused by movement or vibration of the stepper stage during exposure). Figure 2 also reinforces the difference in capture rate between the 2401 and manual inspection.

Of the 13 ADI defect types that were part of this evaluation, the 2401 was able to capture 10 of them with a “good” rating, and two of the remaining three with a “fair” rating. These qualitative results are tallied in Figure 3. The 2401 failed to capture whole-wafer focus defects, but the next version of software is expected to remedy this omission. The two defect types receiving “fair” ratings are also whole-wafer defect types: no coat and no exposure/develop. The ability of the 2401 to capture these defect types is also expected to improve with the version 2.2 software release.

Defect Type Hot Spot Leveling Acceleration Scratch Contamination Comet Particle Lifting Resist Poor Coat No Coat* No Exposure/Develop* Film Thickness Variation Whole Wafer Focus*

VI Capability Fair Poor Fair Fair Good Fair Poor Fair Good Unknown Unknown Good Good

*WWD software version 2.2 will implement capture Figure 3. Qualitative ratings of defect capture by type for the 2401 and manual inspection further demonstrates the benefits of automated macro inspection.

the lots were only flagged by the 2401 and fell into the “rework/save” category as yield loss would have resulted and the 2401 enabled them to be reworked and rescued.

The most common defect type seen during the evaluation was leveling problems. In particular, nearly 60 percent of metal 2 lots were found with leveling issues. Any problems with the stepper focus or leveling at metal layers can cause catastrophic electromigration failures out in the field. In the case of metal 2, these defects were not caught by manual inspection but the 2401 was able to capture them reliably. Further investigation suggested a solution to NEC for the problem.

7%: Rework/Save 5%:

Rework/ No Save

In contrast, only 4 of the 13 defect types received a “good” capture rating using manual inspection. Five received a “fair” rating, two received a “poor,” and two received an “unknown.”

The heart of the yield benefits for the 2401 over manual inspection is given in Figure 4. Of the lots that were chosen at random for inspection, 12 percent were sent for rework. Five percent of the lots were flagged by both manual inspection and the 2401. These lots fell into the “rework/no save” category. However, another 7 percent of

2401 Capability Good Good Good Good Good Good Good Good Good Fair Fair Good n/a

88%: No Rework Figure 4. While 12 percent of randomly inspected wafers were reworked, only 5 percent were detected by visual inspection. The 2401 detected these wafers but also detected a further 7 percent that could be reworked successfully.

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Figure 5. Four focus-offset fields were created per wafer, and demonstrated that the 2401 could detect a process window of 0.7 µm, making it suitable for monitoring the stepper for focus-offset excursions.

Of second highest importance — on 21 percent of the lots — were hot spots. A particularly challenging layer for manual inspection was a high-aspect ratio contact layer called “hole.” NEC found it impossible to see any hot spots or other focus-related defects at this layer using manual inspection. It was proven that the 2401 was able to capture these defect types at “hole”.

The third most important defect type was acceleration errors. For one of the poly layers, ten lots were reported to have acceleration errors. Four of them were found by manual inspection, while nine out of the ten were found by the 2401.

Two other defect types of note included no coat/exposure/develop defects and CVD stripe defects. The no coat/exposure/develop defects were created intentionally to test the 2401, because NEC did not see any natural examples during the evaluation though this defect type was known to occur occasionally. The CVD stripe defect was found on several lots that had been put on hold by the operators and had been categorized as lifting resist. NEC found that adjusting the recipe on the 2401 enabled them to either flag the defect or screen it out while still capturing photo related defects.

Stepper monitoring

The second part of the 2401 evaluation investigated applying the automated system for stepper monitoring.

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The goal was to set up a quick, sensitive process for looking at focus-exposure matrices, focus problems and leveling problems. A diffraction grating reticle was created with 0.3 µm lines and gaps, and then printed on unpatterned, resist-coated wafers using a KrF, deep UV stepper. This diffraction grating was used to reduce the process window of the photo process and bring it close to the process window of the stepper itself.

A focus-exposure matrix wafer was created (Exposure: centre 31mJ/cm2, step 2mJ/cm2 Focus: centre 0.0 µm, step 0.1 µm). The focus-exposure matrix produces a histogram that indicates the optimum focus and optimum exposure for the process. Changes in the focus or exposure conditions would produce a change in this histogram. Refinement of the field size used and focus and exposure steps should produce an effective and sensitive check for the stepper.

For the focus offset check, wafers were created with four offset fields, and each field had increasing positive and negative focus offsets (Figure 5). The wafers were then inspected and the focus process window estimated. The 2401 was able to detect a 0.4 µm positive offset and a 0.3 µm negative offset. This equates to a detectable process window of 0.7 µm. KrF steppers typically have inherent process windows of 0.6 to 1.0 µm. This would indicate that the 2401 can be used to detect any focusoffset excursions outside the stepper’s optimum process window.

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Figure 6. Four tilt-offset fields were created per wafer, and demonstrated that the 2401 could detect a process window of 20 - 35 µrads, depending on the direction of the grating. This sensitivity suggests that the 2401 would be suitable for monitoring the stepper for tilt excursions.

The investigation of leveling offsets was similar. Wafers were created with four offset fields, this time with increasing tilts on both the “X” and “Y” axes (Figure 6). Again the offsets showed up clearly using the 2401.

In the “X” axis, the 2401 detected tilt offsets of 20 µrad and greater, while in the “Y” the 2401 detected offsets of 35 µrad and greater. By calculating the effective focus offset at the edge of the titled fields the detectable process window is in the order of ±0.35 µm offset at the edge of a 22 mm field. This is comparable to the focus offset detectable process window and again suggests that the 2401 can be used to detect tilt excursions outside of the stepper’s own process window.

Return on investment

At the conclusion of the evaluation, the return on investment was calculated by comparing manual versus automated after-develop macro inspection. The calculations were based on data from the eight layers evaluated, which were designed to represent the whole process. It should be noted that the product studied was a stable main runner. The yield kill rate of each defect type was considered using historical rework data and compared with the rework rate that resulted from using the 2401. The model did not include any of the savings that would be gained through using fewer operators or by taking into account the shorter time to detection of defect incidents. Furthermore, the opportunities for

savings are expected to be greater on shrinking technologies having higher wafer and die costs. Even using these conservative assumptions and a low average selling price product, potential savings of over $66,000 per month were calculated from using the 2401.

Summary

The KLA-Tencor 2401 was installed in less than 10 days. Twelve operators and three engineers were trained on the system and provided positive feedback about its ease of use and production integration. The 2401 proved to be 10 times more sensitive than manual inspection, with 12 out of 13 critical defect types captured reliably.

The thirteenth type is expected to be captured as soon as the next software version is installed. This system demonstrated potential for a very simple focus-exposure matrix utility and promising capability for monitoring steppers for focus and tilt excursions. The return on investment is quite aggressive even for a low average selling price product. Finally, NEC was able to reduce scrap and reduce excursion detection time, which translates to better yield and profitability. circle RS#034

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