Atomic force microscopy (AFM) is one of the key methods that failure analysis engineers in data storage rely on to obtain 3D data about the topological defects on hard disk media samples. This data is critical for proper identification of defects and to eliminate defect sources.
Although optical microscopy can provide a quick image of a sample surface, the method has limited resolution due to the wavelengths of visible light. On the other hand, electron microscopy can provide higher resolution but it is essentially a destructive method and only provides a 2D image. AFM provides the highest lateral and vertical resolution and a 3D topography of the sample surface [1].
Traditional AFMs were known to have limited tip life and throughput, as imaging was performed in a destructive mode. To resolve these limitations of AFM, Park Automatic Defect Review (ADR) AFM was launched in the mid-2000s for a non-destructive high throughput 3D imaging of defects. This solution was broadly accepted in failure analysis (FA) labs worldwide.
This article looks at the latest generation of ADR AFM, which uses an improved vision technique to enable the location of defects of interest (DOI). This solution has been implemented in the Park NX-HDM AFM system.
Defect Study
The key objective of an FA lab for hard disk media is the analysis of defects and testing their potential sources to optimize yield management. For this reason, a defect study consists of two major steps: defect inspection and defect review.
In defect inspection, an inspection tool investigates the media surface using a magnetic, optical, or electron beam probe to detect the location of the defects. The result is a map of defects that specifies their distribution across the surface of the sample. The defect map can also be used to find the DOI for additional investigation.
Although the inspection tools possess high throughput and are able to analyze several samples per hour, they are restricted in resolution and are not suitable to characterize each defect effectively. Therefore, some defects are chosen for defect review and comprehensive characterization.
In defect review, DOI are characterized to obtain a high-resolution image and classified appropriately. The defect review process relies on methods that can offer high-resolution topographical data about each defect.
STEM and AFM are the examples for defect review methods. Unlike inspection tools, the review methods are comparatively slower and need to accurately locate DOI for imaging.
Locating DOI is the key challenge for defect review tools. There is a difference between the DOI’s coordinates on the stage of inspection tool and its coordinates on the stage of the review tool. This difference is called stage error, which is normally larger for in-house inspection tools (e.g. AOI, Tester) compared to commercial inspection tools (e.g. KLA-Tencor Candela).
The defect inspection and review process is displayed schematically in Figure 1a. In the past and prior to launching ADR AFM, manual AFM was used for imaging DOI. To enable locating DOI in manual AFM, FA engineers initially marked all of the defects.
As depicted in Figure 1a, an automated optical inspection (AOI) tool is used to inspect hard disk media. This tool is the most frequently used equipment in mass production and generates a map of defects.
The disk is then investigated by an optical microscope to find and mark each DOI. Marking defects was performed by making observable scratches around each defect to enable locating it through AFM optics (Figure 1b).
This process path is illustrated by green arrows in Figure 1a. This involves conducting numerous survey scans to locate DOI. In ideal circumstances, this technique had a low throughput of 10 defects per day.
Additionally, destructive scanning techniques (e.g., tapping mode) were being used by traditional AFMs. As a result, tip life was restricted because several large survey scans have to be performed to find each defect and the entire process led to high costs of ownership.
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Figure 1. a) Current process of defect inspection and review for hard disk media. The green path shows the process path for conventional AFMs. The blue path shows the process with the first generation of Park ADR-AFM. b) A defect on hard disk media sample which is marked by a surrounding scratch. The defect is hardly visible in the optical vision of an AFM.
Park ADR-AFM
In order to address the limitations of standard AFMs in the defect study process, the first generation of Park ADR-AFM was launched. Compared to standard AFMs, this solution offered two significant advancements.
First, it has a significantly enhanced throughput of up to 10 defects each hour due to the automated process. The presence of the user is not needed during the scan as the system is operated by automated software. The system can continue the process of locating and imaging an increased number of defects on its own, even overnight. This significantly enhanced the throughput of an AFM system.
Second, the imaging was carried out in non-contact mode. Imaging in this mode helped to increase the tip life, which is vital to obtain consistent data as well as a fully automated defect review process. This makes it possible to use a single tip for a significant number of images without the user present.
Figure 2 displays the ADR AFM process compared to standard process using manual AFM. In standard AFM imaging (Figure 2a), the surface of the sample was first searched using an inspection tool. Next, the DOI was marked on the surface using optical microscope, and the sample was brought to the AFM for imaging after laborious efforts of marking each DOI.
The defect in AFM was identified by visual search in vision, followed by multiple low and high magnification scans indicated as green path in Figure 1a. In ADR-AFM as indicated by the blue path, DOI coordinates from inspection tool are initially imported and this is followed by aligning the coordinates.
Following this step, the automated software shifts the tip to each defect, carries out a survey scan, images the defect, and categorizes the defect automatically. With ADR AFM, defect review throughput can increase up to 100 defects each day. This completely automated process also prevents the need for engineers to stand by and operate the system during the runs, freeing up their time, and increasing productivity.
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Figure 2. The AFM based defect study process is schematically shown for a) manual AFM, and b) ADR- AFM. In ADR-AFM, locating the defect from defect map, survey scan, final zoom-in scan are performed automatically by the system.
Based on the stage error of the inspection tool used to create defect maps, the survey scan size of the old Park ADR-AFM was ultimately selected. A survey scan of 40 µm × 40 µm is typical to cover for a stage error up to 20 µm between Candela and AFM.
Since the stage error of AOI tools are very large (≥ 50 µm), using a linkage tool such as Candela with smaller stager error was required to complete the process of defect study as highlighted by blue path in Figure 1a.
A set of 16 markers were then added to each wafer prior to the inspection run by Candela, so that the sample can be aligned in the Park ADR AFM.
The new generation of Park ADR AFM has been launched for both 300 mm wafers [2] and hard disk media. Integrated with bright field enhanced vision, the new Park ADR AFM helps to examine the defects in the ADR AFM optical field of view (FOV).
Using precise movements of the sample via decoupled XY and Z scanners and differential frame averaging technique, the bright field improved vision observes small defects that are difficult to view through the standard optics of the system.
Park ADR AFM’s decoupled XY and Z scanner structure allows using differential frame averaging by gathering optical images of the surface of the sample at two precisely separated positions of XY scanner. The contrast of small defects is improved by the differential image of the sample surface frames collected at two different positions, making them visible in the ADR AFM optics.
As the FOV of ADR AFM is larger than the highest survey scan size, improved vision allows the ADR AFM to accommodate even larger stage errors of inspection tools as shown in Figure 3. It takes 1–30 seconds to collect a typical bright field enhanced vision image for every single defect.
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Figure 3. An example of enhanced vision image and how it facilitates linkage between the AOI tool which has larger stage errors, and Park ADR-AFM.
A new sample alignment algorithm is developed in addition to a bright field enhanced vision to enable the connection between the inspection tools and ADR AFM. The first generation of ADR AFM required a sample alignment using 16 markers to align the sample on the ADR AFM stage.
Candela generated the coordinates of the 16 markers, which were matched with the coordinates of the ADR AFM stage. The next generation of ADR AFM only required one marker for sample alignment.
There is one fine and one coarse alignment step in the new alignment process. Three points at the circular perimeter of the hard disk media sample were used in the coarse alignment to find the sample’s center. Sample rotation was detected using a zero-degree marker.
A few larger defects are used by ADR AFM in the fine alignment step to minimize the remaining positioning errors using enhanced vision. Compared to the first generation of ADR AFM, the new process is more efficient and faster. Additionally, it can also be employed as a tool to evaluate the stage error in AOI or other inspection tools (Figure 4).
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Figure 4. The schematic shows the sample alignment process in the first (left) versus the new (right) generation of Park ADR-AFM. In the latest Park ADR-AFM only one marker is needed to indicate zero-degree location of hard disk media samples.
The latest generation of Park ADR AFM has introduced dark field enhanced vision.
With this new vision, AFM can collect the light scattered as a result of surface defects in reflective surfaces and use it to observe defects. A new hardware module has been designed and implemented in the AFM system in order to achieve this.
A laser beam is emitted to the desired site on the sample, in this module. The beam angle of incidence is more than zero degrees so that that the direct beam reflection is not gathered by the on-axis ADR AFM optics. Most laser beam for a reflective sample follows the anticipated reflection path with similar angle of incidence.
If there is a topological defect on the sample surface, parts of the laser beam will be scattered in a direction different to the regular reflection path, and are gathered by the on-axis optics of ADR AFM. The gathered frame is an image of the surface with scattered light gathered from the defects.
Figure 5 shows examples of the standard optical and dark field enhanced vision images. The figure also presents schematically the upgraded ADR AFM process with dark field enhanced vision.
The ADR AFM in the new process goes to the defect site after the sample is aligned. It then uses dark field enhanced vision without AFM scanning to recognize the defect and performs the imaging, analysis, and classification. The new process allows the ADR AFM to minimize the positioning error.
Consequently, ADR AFM can accommodate defect maps from AOI systems that contain large-scale errors. With the capability to see defects with enhanced vision, ADR AFM can be employed as the reference tool to evaluate the stage errors of different inspection tools.
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Figure 5. The new ADR-AFM process with newly introduced dark field enhanced vision for hard disk media with reflective surface.
Figure 6 shows the results of a defect review on a 95 mm hard disk media sample performed by ADR AFM. An in-house inspection tool with large stage error of >50 µm provided the defect coordinates.
The defects were located using dark field enhanced vision. Consequently, the enhanced vision enabled the ADR AFM to successfully detect 33 out of 37 defects. ADR AFM classifies the defects into two groups of bumps and pits. Figure 7 shows an example of each group and also shows the AFM images and dark field enhanced vision images of defects. ADR AFM detected the defects with vertical dimension as low as <3 nm.
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Figure 6. The results of performing ADR-AFM on a 95 mm hard disk media samples for a set of defects reported by an in-house inspection tool. 33 defects out of 37 were found and imaged by ADR-AFM.
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Figure 7. Representative pit and bump type defects on a hard disk media samples which were found and imaged by ADR-AFM. The dark field enhanced vision images are shown side by side with the AFM images and selected profiles of the defects.
Conclusion
To summarize, the latest generation of Park ADR- AFM contains new dark field and bright field enhanced vision modules. Using ADR- AFM’s decoupled XY and Z scanners and frame averaging technique, the bright field enhanced vision acquires a high-resolution optical image of the sample surface to locate the DOI.
Dark field enhanced vision is enabled by collecting the scattered laser signal by the on-axis vision of ADR-AFM and helps to detect the defects on reflective surfaces. Park ADR-AFM employs non-contact mode imaging, to perform imaging in a non-destructive way.
High throughput AFM-based defect review can be conducted using Park ADR- AFM, which has updated alignment algorithms and enhanced vision modules. In addition, ADR-AFM with enhanced vision can be used as a potential reference tool to assess stage errors during in-house inspection systems that are being maintained or developed.
References
- Smith, G. T., Industrial Metrology: Surfaces and Roundness, Springer, London, 103-105 (2002).
- Ardavan Zandiatashbar et al., "Highthroughput automatic defect review for 300mm blank wafers with atomic force microscope," in Proc. SPIE 9424, Metrology, Inspection, and Process Control for Microlithography XXIX, 2015, p. 94241X.
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
For more information on this source, please visit Park Systems Inc.