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Inline SEM Electron Beam Review Accelerates LTPS Display Yield Ramps

Kerry Cunningham

Inline inspection-based analytical techniques adapted from high-volume semiconductor manufacturing can help manufacturers of low-temperature polysilicon (LTPS) LCD and flexible OLED displays. During production ramp-up, the use of a new technology under development at Applied Materials—electron beam review (EBR)—will enable display manufacturers to achieve optimum yields faster than traditional methods, allowing them to capture millions of dollars in revenue and avoid costly yield excursions.

Combining state-of-the-art scanning electron microscopy (SEM) with large-scale display vacuum platforms, EBR is a new approach that can shave several months from the average LTPS display fab ramp time of 15 months (see figure 1). Assuming a factory output of 30,000 displays/month, a 3-month savings for example, would be equivalent to $250 million in added revenue.

Figure 1. Excursion management improves fab ramp time and overall yield.

Applied Materials is developing inline EBR technology (see figure 2) as a better, faster way to discover and address the root causes of killer defects in mobile display production, which is characterized by an increasing number of process steps that can generate more—and smaller—particles and new types of defects.

Figure 2: Applied Materials EBR platform.

Killer defects, or electrically active faults, destroy pixels and reduce yield. It can be difficult to determine their origin because while they may result from a straightforward one-time issue, they also may be symptomatic of a problem with a tool or process that negatively impacts yields on an ongoing basis.

Consider the example of a cluster defect that affects 10 displays out of 250 (a 4% yield loss). It would be wrong to say that the cluster is a systematic defect, because the cluster is just the location and not the root cause. The root cause could be a mask defect, a defective sputter target, etc. Thus, the goal is to determine the systematic root cause, which will fix not only the 4% yield loss issue but also other issues that may manifest themselves in different locations but share the same root cause.

While inline automated optical defect inspection tools can detect the location of defects on a sample substrate, they simply report all visually abnormal regions, both killer defects and also nuisance or non-killer defects. Prior to EBR, in order to do SEM analysis, the substrate had to be broken into pieces and each piece examined separately under microscope (see figure 3). Although such offline SEM analysis does enable root-cause analysis, a crucial linkage is lost by having to break the substrate into pieces: it’s impossible to compare the inline defect to an end-of-line array test, because the defect location on the full panel can no longer be determined.

Figure 3. In conventional LTPS display manufacturing, LTPS panels have become so large that in order to inspect one using a SEM tool it must be broken into pieces and each piece examined separately. Crucial information that may give insight into the source(s) of the defects is therefore lost.

As a result, insight into the source(s) and kill ratio of the defects is lost, making discovery a problematic and time-consuming process. Using EBR, however, it is possible to determine which defects are killer defects without breaking the substrate for analysis.

Moreover, because production continues while random samples are inspected, any lots that have been processed during that time may suffer from the same problem and have to be scrapped.

Minimize Killer Defects in Real Time

To address these issues, Applied Materials has introduced an analytical approach based on EBR technology that will help LTPS display manufacturers minimize killer defects in real time when a process or tool is not performing to specifications (see figure 4).

Figure 4. Yield excursion identification and correction cycle. (Source: AKT)

EBR makes use of both optical imaging analysis and SEM/energy-dispersive X-ray (EDX) tools. It can handle LTPS panels up to Generation 6 (1850mm x 1500mm) with no need to break them into pieces. The overall approach is to isolate the most probable source of a defect (e.g., a CVD, PVD, etch, photolithography, or other process step), select the best sampling plan to isolate and monitor the defect, and then quantify the number of lots impacted.

Sample substrates are inspected after each process step. Automated optical inspection is used to generate a map of defects on these wafers. The wafers are then reviewed with the EBR SEM tool, and the results of that inspection are overlaid with the results of the optical inspection so that the EBR results can be analyzed to determine the composition and size of the defects (see figures 5 and 6). These tests can and should be repeated after each process step.

Figure 5. Array panel identification and mapping for “killer defect” root-cause analysis.

Figure 6. Characterization of defects by the SEM is performed nondestructively by means of EDX analysis.

A focused electron beam stimulates atoms in the sample with uniform energy. The atoms instantaneously send out X-ray radiation in characteristic signatures unique to each element, as shown in the graph. These give information about the elemental composition of the sample. Chemical elements starting with the atomic number 4 (Be) can be identified this way.

This information will enable users to “work backward” from the defect to determine where it is coming from so that the problem can be fixed quickly. In this way, the tool or process step that is most likely to be the source of the problem can be more readily identified, along with the best set of lots to measure. The resulting information is then fed into the fab’s yield-management system (see figure 7).

Figure 7. Elements of defect kill-ratio analysis.

Not all defects are killer defects, and the EBR technology under development at Applied Materials will make kill ratio analysis possible. The kill ratio is the percentage of defects from a group of different types of defect that destroy a pixel. From left to right above, the process begins with an optical defect map. Next, EDX analysis classifies the defects as to type. Then, a focused ion beam is used to perform an array test. The “dead” pixels identified by this test are compared with the classified defects and a kill ratio is determined for use as a metric to improve yields.

It’s important to note that particles aren’t the only source of killer defects. The new Applied Materials EBR technology can help identify other sources as well, such as ineffective laser annealing, by monitoring grain size uniformity after excimer laser annealing, effectively enabling process control to monitor the performance of the laser.

Designed for Productivity

Applied Materials’ new inline EBR technology will be integrated into a system using SEM technology with the Applied AKT Electron Beam Array test platform, the fastest, highest resolution and least-damaging e-beam technology commercially available (see figure 8). The AKT product line is used for dynamic pixel and TFT characterization.

Figure 8. Applied Materials EBR system layout design.

Applied Materials is developing the inline EBR LTPS display inspection technology with features like a precision stage used to locate defects based on coordinates supplied by an optical map. The stage can move the SEM/EDX tool over the entire surface of a sixth generation display panel, and can be tilted for precise electron beam exposure.

Looking Forward

With some 20 years of innovation and experience with inline SEM-based inspection techniques in the semiconductor industry, the company is now extending those capabilities to the manufacturing of LTPS displays for better and faster root-cause analysis of defects, and for greater control of LTPS grain sizes. For additional information, contact kerry_cunningham@ amat.com.