Structure Analysis of Advanced Process Static Random-Access Memory SRAM

The development of generative AI (Artificial Intelligence) has brought about the fourth industrial revolution. With the numerous innovations enabled by the heterogenous integration of semiconductor 3D packaging and process node miniaturization technology, mankind is bearing witness to the age of AI. The IC (Integrated Circuit) chip employed by AI is also referred to as the AI chip and is the core of AI hardware. TSMC’s advanced logic technology has made it possible to fit more than 200 billion transistors onto a single chip, and it is anticipated that, in the future, heterogeneous integrated 3D packaging will allow a single AI chip product to reach more than 1 trillion transistors. In such a complex IC circuit, the most important embedded memory structure in the AI chip is the SRAM (Static Random-Access Memory). This type of SRAM is also referred to as embedded SRAM. The importance of embedded SRAM is twofold: (1) the SRAM is often the AI chip’s fastest reading and writing Register as well as the main Cache memory component, and (2) the SRAM process structure is often the smallest and most dense section of the AI chip. Its small size enables a chip to accommodate more memory in the same area, reducing material costs. Therefore, the importance of SRAM to the AI chip is self evident. The most advanced AI chips have already crossed into the realm of the 3nm process node, and products using the 2nm and 16A (angstrom) process nodes will be launching successively in 2025 and 2026. SRAM will continue to be the key memory component for AI chips. Therefore, the structural observation and analysis of advanced process SRAM remains an important issue.

SRAM is also known as static RAM (Random-Access Memory). Here, the term “static” indicates that, as long as the memory component has power, the data stored can be retained. When the power supply is cut off, however, the data stored in SRAM will disappear, making it a type of volatile memory. This is different from ROM (Read-Only Memory) and Flash Memory, which can store data even when the power is off. (References [1])

The structure of the SRAM circuit varies in design based on the number of transistors used, be it 4T, 6T, 8T, or 12T. etc.. The 6T is the most commonly used SRAM design. The T in 6T stands for Transistor. 6T means that a basic SRAM unit contains 6 MOS (Metal Oxide Semiconductors). The names of each part of the 6T SRAM circuit are shown in Figure 1. The six transistors include 2 PMOS and 4 NMOS. The six transistors can also be categorized according to their component functions, including PU (Pull Up), PD (Pull Down), and PG (Pass Gate), etc..

Figure 1. 6T SRAM Circuit with Labeled Parts (References [1]) WL: Word Line, 字元線 BL: Bit Line, 位元線 M1: 第一層金屬連線(訊號傳輸) M2~M6: 依M1類推 PU: Pull Up,上拉電晶體 PD: Pull Down,下拉電晶體 PG: Pass Gate, 傳送閘電晶體 VDD：D=device，表示器件的意思，即器件內部的工作電壓 VSS：S=series，表示公共連接的意思，通常指電路公共接地端電壓

There are many types of analysis commonly used on AI chips. AI chip analysis can be roughly divided into Electrical Testing, Electrical Failure Analysis (EFA), and Physical Failure Analysis (PFA). PFA includes Surface Analysis and Chemical Analysis (CA). If compared to the exams performed by medical centers, an electrical test would be equivalent to a standard health examination, as shown in Figure 2. There are a number of analytical methods commonly applied to embedded SRAM. This article will discuss SRAM observation and analysis in more depth.

The main structural observation tools for SRAM include the Optical Microscope (OM), the Scanning Electron Microscope (SEM), the Dual Beam Focused Ion Beam, microscope (DB-FIB), and the Transmission Electron Microscope (TEM). These instruments all have the ability to observe the SRAM process structures in AI chips. However, the observation range differs from instrument to instrument. Each instrument’s range of observation is shown in Figure 3. The observation method of choice is typically to begin with wide-area observation via OM to narrow down the area of the target location in the chip. The target location is then narrowed down even further through the use of SEM or FIB observation. The FIB can be used in conjunction with the SEM to conduct precise position observations. The FIB is also an important tool in the preparation of TEM samples. If the SEM/FIB is unable to provide sufficiently clear observations of the target then final observations are conducted using the TEM or the Cs-TEM. The TEM is capable of observing nanometer-scale microstructures. The Cs-TEM (spherical aberration corrected TEM) is the only instrument currently on the market that is capable of achieving a spatial resolution of 0.5A (10000000000A=1m). In other words, among the instruments currently available, Cs-TEM offers the highest magnification range. In addition, the SEM and TEM can be combined with EDS (Energy-Dispersive X-ray Spectroscopy, also referred to as EDX, EDXS or XEDS) to analyze the composition within small areas.

As the SRAM is usually the smallest and most densely populated section of the AI process structure and memory usually has a repetitive arrangement, these two characteristics are generally reliable ways to identify the SRAM area. Figure 4 shows the use of OM observation to identify the embedded SRAM area. Part of the SRAM is shown inside the blue box. Locating the general SRAM area will enable further, more in-depth analysis of its electrical and physical properties.

In addition to the OM, IR (Infrared) microscopes and SEM are also common tools used in locating the embedded SRAM within a wide area. The way they locate the embedded SRAM is also similar to that of the OM. However, the SRAM is easier to locate with the SEM because the OM’s lower magnification can make it more difficult to see clearly.

The SEM is an electron microscope that produces images of a sample’s surface by scanning it with a focused electron beam. It is usually used to observe structures of over 100nm in size. The SEM has a wide range of uses. Its main purpose is the observation of a sample’s surface structures. Additionally, when paired with nano-probing, the SEM can be used to perform electrical measurements, though this topic will not be covered in this article. The use of the SEM in conjunction with an FIB (Focused Ion Beam) or DB-FIB will be discussed in the next chapter. Here, we will be introducing the Voltage Contrast (VC). VC positioning uses the primary electron beam or ion beam of the SEM or the FIB to scan the sample’s surface. Different sections of the chip’s surface will have different electrical potentials. After SEM scanning, those different areas will exhibit different levels of brightness. The resulting contrasts are referred to as the VC. The principles behind the VC can be found in the reference materials [2].

The SRAM includes the PMOS region and the NMOS region. There will usually be a VC effect when the SEM scans the Via or Contact areas of the IC while it is operating at a specific voltage. When SEM observation is conducted under low voltage operation, the greatest via/contact brightness will be in the PMOS region. The via/contact brightness of the NMOS will be second, and, if there is a structure in the gate area via/contact, that area will appear darkest. Figure 5 shows the different brightness contrasts caused by SEM scanning in the SRAM contact area.

The Focused Ion Beam (FIB) microscope uses gallium (Ga) metal as its ion source. Galium has a melting point of 29.76°C, at which point the vapor pressure is «10-13 Torr, making it suitable for operating under a vacuum. When in use, the liquid gallium flows along the filament to the tip of the needle. When the external electric field is strong enough to pull the liquid gallium at the tip into a Taylor cone (a cone with a curvature radius smaller than the critical radius), the gallium will disassociate and be ejected in the form of a gallium ion beam. These ions measure less than 10nm, and the energy dispersion is approximately 4.5 eV, with a brightness of approximately 106A/cm2.sr. These characteristics make this an excellent tool for precision nanostructure processing. It can considered a nano-carving knife. A focused ion beam microscope can also be equipped with an electron beam system to form a Dual Beam FIB—in other words, a system that combines a scanning electron microscope (SEM) with a focused ion beam microscope (FIB). This system is able to use electron beams to locate target areas and make observations as well as to use ion beams for precision cutting of the target area without damaging other structures in the sample. As such, this system is able to achieve precise positioning and cutting at the nanometer level in addition to enabling the thin slicing needed for TEM sample preparation. See Figure 3 for the DB-FIB machine. Figure 6 is a schematic diagram showing the relative positions of the SEM (E-beam) and the FIB (i-beam) in the DB-FIB. Figure 7 is a diagram of the cross-sectional structure of the SRAM where the DB-FIB was used to provide precise positioning and cutting. In the figure, you can see both the Front End Of Line (FEOL) and the Back End Of Line (BEOL) of the process at the same time.

Figure 6. Schematic Diagram of the Relative positions of the SEM (E-beam), the FIB (i-beam) and the Sample in the DB-FIB

Figure 7. Use the DB-FIB’s precision positioning and cutting to observe the cross-sectional structure of the SRAM

In order to clearly observe the component structure and fine process levels of the advanced process 6T SRAM, you must use either the TEM (Transmission Electron Microscope) or the STEM (Scanning Transmission Electron Microscope). The TEM uses a 200KV accelerated electron beam to penetrate a thin test piece (100nm thick or less) and project it onto a detector. The results are presented in the form of a photograph. Whether it’s for today’s 3nm and mature processes or the 2nm and 16A (angstrom) process nodes of the future, the TEM continues to be the most important tool for observing and analyzing the SRAM structure because it has the best spatial resolution and image presentation method.

The true appearance of the circuit structure and labeled parts of the 6T SRAM in Figure 1 can be observed clearly using the TEM. Figure 8 shows a plane-view STEM observation of an advanced process 6T SRAM. You can observe the planar structure and relative arrangement of the six transistors, including the planar arrangement of the Contact, Gate, Fin, STI and other front-end process structures. The yellow box in Figure 8 shows one unit Cell of 6T SRAM. Figure 9 shows the Cross Section (XS) TEM observation of part of the transistor structure of an advanced process 6T SRAM. You can see the vertical shape and arrangement of the transistor structure, including the cross-sectional arrangement of the Contact, Gate, Fin, STI and other front-end process structures.

Figure 8. Plane-View Transmission Electron Microscope (TEM) Observation of Advanced Process 6T SRAM; The Yellow Box Indicates 1 unit Cell of 6T SRAM

Figure 9. Cross Section (XS) Transmission Electron Microscope (TEM) Observation of Part of the Transistor Structure of an Advanced Process 6T SRAM

SRAM structural observation is an essential part of AI chip inspection and analysis. This article covered the step by step process for SRAM observation, from the application of the OM to the use of the SEM and the TEM, etc.. In essence, we begin with the wide-area, low magnification OM then work our way up to the small area, high magnification TEM and use graphics, text and photos to illustrate the important structures and related terms of the SRAM, thus realizing an effective and accurate method for SRAM structure observation.

MA-Tek is the world’s number one semiconductor analysis laboratory. It has the largest, most comprehensive collection of materials analysis, failure analysis, and reliability analysis equipment in the world. As such, MA-Tek has the technical capability to analyze not only the most advanced IC processes of the present but also the more advanced processes and components expected to emerge over the next five to ten years and beyond.

Reference: 

[1] https:// zh.wikipedia.org/zh-tw/静态随机存储器

[2] V.G. Dyukov, S. A. Nepijko, Gerd Schoenhense, Voltage Contrast Modes in a Scanning Electron Microscope and Their Application, August 2016, Advances in Imaging and Electron Physics.