This article is a continuation of last month’s article. As we discussed last month, sometimes failure analysts can best understand the FA procedure for a component by thinking about the process in terms of the type of failure. This is a flowchart that describes how to analyze an open circuit failure at either the wafer or package level.
Figure 1 shows a flowchart that describes how to analyze an open circuit. We begin by examining the device with a curve tracer to check for the open. The pin or bump exhibiting the open should be identifiable from the automatic test equipment (ATE) datalog for the device. If the device has not been tested it should be tested using ATE to confirm the failure. We can also do this directly with the curve tracer, but it can be very time-consuming for a device with many pins, bumps, or bond pads if we’re doing this at the wafer level. The flow now branches if we have a packaged device. If we have a packaged device, we can perform x-ray radiography and scanning acoustic microscopy to look for a void or crack. There are other advanced techniques one can try like time domain reflectometry or Superconducting Quantum Interference Device (SQUID) microscopy as well to help localize this type of problem. If we are able to locate a broken wire, lifted ball, or other obvious anomaly, we can document it and proceed to the report generation. If there is no obvious defect or problem, then the problem most likely lies on the die itself. It makes sense to decapsulate the de- vice to expose the die surface. After decapsulation, one should re-check the pin in question to make sure the open is still present. If the failure is no longer present, then an optical or SEM inspection might be in order at this point to look for anything that might have been missed. If the failure is still present, then we can begin to isolate it on the die. If the package structure is still present, we could use a laser-based technique like Seebeck Effect Imaging to localize the open. However, if the package structure is no longer present, then we will need to use a more traditional technique like passive voltage contrast. We then enter a loop where we remove overlying layers to expose the layer of interest, and perform voltage contrast. We can continue in the loop until we identify the failure site or reach the silicon substrate. Analysts use passive voltage contrast to identify opens and shorts. If we don’t see incorrect contrast, then we can remove the chip layers down to the next metal layer in the node, or down to the upper-most metal layer in the next candidate. We would continue this process down through the backend of the process, or through the interconnect and dielectric layers. Once we see incorrect contrast or the defect itself, we can determine if we need further analysis. This might involve a cross-section or liftout of material. We can then examine the defect with the SEM or TEM as appropriate. If we do not see the defect after removing all of the interconnect and dielectric layers, we can decorate the substrate to highlight potential defects in the silicon. At this point, we can write up our findings in a failure analysis report.
In this technical tidbit, we will discuss rapid thermal processing hardware. Engineers normally perform rapid thermal processing (RTP) one wafer at a time, so RTP hardware is designed to process one wafer at a time. The system rotates the wafer while in the system to minimize the effects of heating non-uniformities and gas flow non-uniformities. RTP hardware operates either at atmospheric pressure or reduced pressure, and is compatible with both dry and chlorine oxidations. Wet oxidations can be performed using in situ steam generation, where one introduces hydrogen and oxygen gas onto a hot wafer surface where they react and form steam. Obviously, the introduction of hydrogen gas in the presence of oxygen is a safety issue, and must be performed carefully with the appropriate mitigation protocols. The manufacturers of rapid thermal processing equipment have created various designs and configurations for use in the semiconductor industry. The drawings in Figure 1 show the more common configurations. They range from quartz tube and lamps, like we show in figures A and B, to chambers with IR bulbs, rods, and hot plates, like we show in figures C, D, and E respectively. Rapid thermal annealing is fast becoming the first choice for thermal processing steps. Engineers use rapid thermal annealing for post-implant damage annealing and dopant activation, where the heat drives the dopant atoms to the lattice sites to become electrically active. They form metal silicides for contacts with the technique. Materials like titanium, cobalt, and nickel silicide lend themselves well to rapid thermal processing. Process engineers also perform rapid thermal oxidation to create thin oxide and oxynitride layers for transistor gates, capacitors, pad oxides, side wall spacers, and shallow trench isolation liners.
Rapid Thermal Processing employs either cold wall or hall wall technology. Cold wall technology is the traditional method. Here, an array of tungsten or halogen lamps heats the wafer through radiation. This process brings the wafer up to the requires temperature within a few seconds. However, the wafer is not in thermal equilibrium with the chamber itself or the lamp, so this can lead to non-uniformities in the anneal process. Hot wall technology is the newer method. Here, heating elements like silicon carbide radiate blackbody heat to the wafer through convection and conduction. This method leads to a more uniform temperature profile, which means a more uniform anneal process across the wafer.
Figure 2 shows examples of rapid thermal processing hardware. The image on the left shows a rapid thermal processing system from the exterior. The image on the right shows the RTP chamber, with a wafer in the system. While RTP is a relatively new technique, process engineers now use it for a wide variety of processing steps in modern semiconductor processing. The precise control makes it invaluable for placement of dopant atoms, alloying of materials, curing of low-k dielectrics, and annealing steps.
This time, I thought we might answer a question posted to the Failure Analysis Group on LinkedIn:
Q: Recently we received a PCBA (printed circuit board assembly) with a detached BGA (ball grid array). The pads look black in an optical microscope, so we refer to this as the "Black Pad" issue. The analysis results revealed mud crack symptoms and severe corrosion activities on the Nickel layer of the PCB pads. What might be causing this?
A: You might want to see if you can determine the phosphorus content present on the surface of the surface of the pads. The black discoloration definitely sounds like corrosion, but it would be good to know what the other element in the corrosion product is. Excess phosphorus on the pads can cause this type of problem.
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