When a nano-level advanced process component fails, what kind of electrical measurement should be used to accurately locate the defect at a micro scale?
In recent years, major manufacturers from all over the world are committed to challenging the 3nm and 2nm miniaturization competition, and the process technology is a never-ending competition. When R&D encounters a bottleneck, whether it is IC design, wafer manufacturing, testing, packaging, etc., fault analysis assistance is needed to clarify the problem.
Fault analysis in advanced manufacturing processes is even more important for R&D and production capacity. However, as the size of components is getting smaller and smaller, how to measure the characteristics of transistors and locate defects at a tiny scale of only a few nanometers has become a major problem.
The nano-probing electrical measurement (Nano-Probing) of iST Technology’s fault analysis experiment is very helpful to the above problems. The nano-probing system is a combination of nano-probing and scanning electron microscopy (SEM). The resulting tool, SEM’s nanometer-level high resolution combined with extremely small nanometer probes, enables the measurement of transistor characteristics at the nanometer level during the manufacturing process, or precisely locates a transistor or even a contact point (Contact) positioning analysis can be realized. This issue of iST Primary School will share with you “How to use the nano-probe system to locate the application of the defect position”.
what isNano-probe system (Nano-Prober)
Through the very small radius of curvature probe of the SEM microscope, the internal circuit or contact layer (Contact Layer) of the IC is lapped, so that it is connected to the electrical measurement equipment externally, so as to input the signal and measure the electrical characteristic curve; in addition, SEM electronics can also be used Beam characteristics, related application analysis, including Electron Beam Induced Current (EBIC for short), Electron Beam Absorbed Current (EBAC for short), Electron Beam Induced Resistance Change (Electron Beam Induced Resistance Change, Abbreviated as EBIRCH).
1. Electron Beam Absorption Current (EBAC)
Figure 1 is a schematic diagram of the electron beam absorption current (EBAC). When the electron beam (E-beam) in the SEM is irradiated on the sample and scanned to a certain point, if there is a metal wire here and there is conduction, the electrons will be absorbed by the component. The absorption of the metal wire is then exported to the amplifier (Amp) through the nano-probe (Nano-Probe), and then the absorption current at the position is obtained, and the current image can be formed through signal processing, which we call EBAC image.
▲Figure 1: Schematic diagram of EBAC (Source: iST)
As mentioned above, the position where the current is derived will form a bright area on the current image. Conversely, if there is “not a metal wire” or an abnormal phenomenon of “open circuit (Open)” occurs, causing the probe to fail to obtain current, it will be a dark area on the current image. Usually, the intersection of bright and dark areas is the position where the wire opens.
The secondary electron image (Secondary Electron, referred to as SE) in the SEM can reflect the surface topography (Topography) of the component. After combining the EBAC image and the secondary electron image, the current distribution on the surface of the component can be obtained. This measurement method can detect the direction distribution of the entire conductor. If there is an open in the conductor, the precise location where the abnormality occurs can be located by comparing good and bad products or comparing with the layout.
The following figure is the actual case, Figure 2 (left) is the circuit layout (Layout), Figure 2 (right) is the superimposed image made by superimposing the EBAC image on the SEM secondary electron image, the lighted area indicates the absorption of electrons The electrons in the beam are exported through the nanoprobes to the current flow of the parameter analyzer. Comparing with Figure 2 (left), we can clearly see the distribution of the wires, and iST’s fault analysis laboratory uses this method to judge whether there are abnormalities such as faults and open circuits in the wires.
In this case, there is an Open at the end of the wire, and the junction of the light and dark in the EBAC image is where the Open occurs. In addition to being commonly used in the measurement and condition judgment of metal wires, EBAC can also be used to judge the abnormal leakage detection of the gate oxide layer (Gate Oxide).
▲Figure 2 (left): Circuit layout (Layout); Figure 2 (right): EBAC image overlapping SEM secondary electron image (Source: iST)
2. Electron Beam Induced Current (EBIC for short)
EBIC is the current induced by the electron beam, especially the measurement of the positive and negative junction (PN Junction) (see Figure 3). Because a “Depletion Region” (yellow area in the middle of Figure 3) will be formed in the middle of the PN Junction, a “Build-in Electric Field” will be formed in the depletion region.
▲Figure 3: Schematic diagram of EBIC (Source: iST)
When the electron beam is irradiated on the surface of the PN Junction sample, an electron-hole pair (Electron-Hole Pair) will be excited. Taking the common material silicon (Si) as an example, the energy required to excite a pair of electron-hole pair is 3.6eV. Then the excited electron-hole pairs are separated by the built-in electric field in the depletion region, and the current is formed, which is then exported to the amplifier (Amp) through the nanoprobe, and finally the current image is formed.
EBIC can analyze the characteristics in PN Junction, such as the built-in electric field intensity distribution image, or more advanced use the intensity distribution to calculate the diffusion length of carriers (Diffusion Length). This information is very helpful for the analysis of material properties .
3. Electron Beam Induced Resistance Change (EBIRCH for short)
The principle of EBIRCH is the same as that of laser beam resistance anomaly detection (OBIRCH), the difference is that the excitation source used in OBIRCH is “laser”, which is limited by the relationship of laser wavelength, and its spatial resolution is about micron (µm) level ; while the excitation source of EBIRCH is “electron beam”, its wavelength is much smaller than that of laser, so it can improve the spatial analysis to the nanometer (nm) level, and then accurately locate the defect position, greatly increasing the subsequent physical failure analysis (PFA) ) or the success rate of transmission electron microscopy (TEM) analysis at sample failures.
▲Figure 4: Schematic diagram of EBIRCH (Source: Science and Technology News)
When the electron beam is irradiated on the sample, due to the dual influence of the temperature rise coefficient of different substances and the thermoelectric effect (Seebeck Effect), the impedance of the abnormal part of the defect is different from that of the normal part. Through the comparison of good and bad products, the defect can be located unusual location.
During EBIRCH analysis, more precise location of anomalies can be performed by reciprocating and selecting different bias conditions. EBIRCH has a wide range of uses, and can usually be used to detect and locate abnormalities such as Open, Short, Leakage, and High Resistance.
The actual case in Figure 5 is that leakage was found in the IV measurement, so the contact layer (Contact Layer) was first pointed at the component with a nanoprobe system to obtain an EBIRCH image (Figure 5 (left)), and then EBIRCH and After the secondary electron image is superimposed, the contact point of the fault can be accurately found (Figure 5 (right)), providing a precise location for subsequent PFA or TEM analysis.
▲Figure 5: After superimposing EBIRCH and the secondary electron image, the fault contact point can be accurately found (Source: iST)
On the road of continuous development and continuous improvement of semiconductor nano-scale process, iST can make good use of nano-probes and SEM for precise measurement according to customer needs, whether it is the measurement of transistor characteristics or failure analysis, even Defects are hidden in unknown corners, and electrical characteristics testing combined with electron microscopy can help you locate them accurately.
This article is to share with you who have supported iST for a long time. If you have relevant needs or want to know more about relevant knowledge, please contact +886-3-579-9909 extension 1067 Ms. Zhang / Email: marketing_tw@ istgroup.com
(Source of the first picture: iST; Data source: iST)