The advancement of science and technology have led the market to have higher and higher expectations for electronic products. They must be smaller, operate faster, and provide more functions. These demands led to the advent of smart phones, AI servers and cloud data centers, all of which require small, high-performance IC chips. In order to increase operating speeds, increase the density of components and reduce power consumption, semiconductor components continue to shrink, and the semiconductor process has evolved from the 3μm process of the 1980s to the current 3nm fin-shaped FFET transistor and the nanosheet gate-all-around (GAA) transistor, as shown in Figure 1. The most cutting-edge technologies have already crossed into the mass production of the 3nm technology node.
To achieve the best performance, a top-notch semiconductor chip requires not only an excellent IC circuit design but also the perfect nano-component structure coupled with new generation advanced semiconductor process technology. Common materials used in the manufacturing process include Strain Si (SiGe/SiP), Gate Oxide, dielectric materials, and the ion implantation of boron (B), phosphor (P), arsenic (As) and other materials that can control component characteristics. Two capabilities are required for the analysis of these materials and components: (1) high spatial resolution (less than 1nm) and (2) the ability to detect low concentrations (less than a ppm). This article focuses mainly on the introduction of the applications of secondary ion mass spectrometry (SIMS) composition analysis in the quality control monitoring of the IC manufacturing process.
Figure 1. The Evolution of Semiconductor Processes and the Applications of Analytical Technology |
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Case 1: Strained-Si |
As Si MOSFET components evolved and shrank, it became evident that improving electron and hole mobility in channels is essential to improving component performance. Among the methods for improving mobility, Strained Engineering (Strained-Si) is one of the most effective for improving the performance of Si nano-components. SiGe materials are commonly used in strained engineering to increase the compressive stress of PMOS channels, increasing hole mobility, while SiP materials are used to increase the tensile stress of NMOS channels, reducing contact resistance and increasing electron mobility. Whether it’s the SiGe or SiP materials, small compositional changes can have drastic effects on the strain. SIMS can be used to accurately analyze the composition of SiGe and SiP while simultaneously analyzing trace amounts of doping elements. Figure 2 shows a SIMS depth profile analysis of SiGe and SiP.
Figure 2. SIMS Depth Profile Analysis of Strain Si (SiGe & SiP) |
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Case 2: Gate Oxide (SiON) Layer with High Dielectric Constant |
The gate oxide layer in the metal oxide semiconductor field effect transistor (MOSFET) is an important insulator in the metal gate that provides switch control for the channel. It is also an important structural material for controlling gate leakage. The higher the dielectric constant of this gate oxide layer, the better the performance of the entire component. When it comes to the practical application of materials with high dielectric constants in components, different high dielectric materials are selected as gate oxide layers according to the requirements of the components and process nodes in question. Oxynitride (SiON) is one commonly used high dielectric material. SiON can be used in many applications, including gate oxide layers, diffusion barrier layers, etching stop layers, Flash memory, dynamic random access memory (DRAM), etc.. The concentrations of oxygen (O), nitrogen (N) and other elements in the SiON material for each application will vary due to the different manufacturing processes. Different compositions meet different needs. Figure 3 shows an example of a SIMS analysis of high dielectric gate oxide (SiON).
Figure 3. High Dielectric Gate Oxide (SiON) is commonly used in the oxide layer of metal oxide semiconductor field effect transistors (MOSFET) in semiconductor devices. The thickness is typically tens of nanometers or less. |
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Case 3: Boro-phospho-silicate Glass (BPSG) |
In the semiconductor manufacturing process, the front end process, or the Front End of Line (FEOL), refers to the making of components, while the back end process, or the Back End of Line (BEOL), refers to the making of metal wires and insulating dielectric layers. The front and back ends are connected by a contact made of conductive material, and the non-conductive dielectric fill material is called the Inter-Layer Dielectric (ILD). The making of the contacts and ILD are referred to collectively as the Middle End of Line (MEOL). Because the surface is uneven after completion of the front end process, appropriately insulating fill materials are needed to fill the gaps. Common fill materials include BPSG and PSG. BPSG is silicon glass doped with elements such as B (boron) and P (phosphorus). The B and P contents and ratios affect the fluidity and filling capabilities of the material. They will also affect the subsequent etching parameters. As such, understanding the B and P content, distribution and ratio are essential when using it in the manufacturing process. Figure 4 shows a SIMS analysis of Boro-phospho-silicate Glass (BPSG).
Figure 4. Boro-phospho-silicate Glass (BPSG) SIMS Analysis |
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Case 4. Fluorinated Silicon Glass (FSG) |
As mentioned earlier, the Back End of Line (BEOL) refers to the process of making metal wires and insulating dielectric layer materials. The process requires metal wires to be made of low resistance materials, while the insulating dielectric layer requires materials with a low dielectric constant (Low-k). These two materials are used to reduce the resistance-capacitance delay (RC-Delay) of the entire IC. Low-k materials are usually made by adding elements such as C (carbon) or F (fluorine) to silicon oxide to reduce its dielectric constant. Therefore, the amount of C or F added is an important factor in determining the dielectric constant of the material. FSG is one commonly used Low-k material. FSG is the result of adding F (fluorine) to silicon oxide to reduce the material’s dielectric constant. Figure 5 is an example of using SIMS to accurately determine the concentration of F in FSG.
Figure 5. SIMS Depth Profile Analysis of FSG |
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Case 5: New Ternary Thin Film Material SiCN |
The new ternary thin film material SiCN has many advantages, including high hardness, a wide optical band gap, good oxidation resistance at high temperatures and corrosion resistance. It can be applied in ICs, liquid crystal displays (LCD), flat panel displays (FPD), optoelectric components and other products. The ratio and uniformity of C/N in this material has a great impact on its properties. Figure 6 is a SIMS analysis of SiCN.
Figure 6. SIMS Depth Profile Analysis of SiCN |
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Case 6: Backside SIMS |
Backside SIMS analysis can help ameliorate the knock-in of an element from the high-concentration layer to the low concentration layer. This is particularly helpful when analyzing downward diffusion of metal into the subsequent layer. When analyzing from the surface, it is easy for small amounts of metal to be pushed into the lower layers during the analysis process, creating an illusion. This problem can be avoided through the use of Backside SIMS, enabling the accurate assessment of an element’s depth distribution. Excellent sample preparation technology is required to obtain the flat surface needed for the best Backside SIMS results. Figure 7 shows the results of front and back analysis of copper diffusion.
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Figure 7-1. Typically, when conducting SIMS analysis from the surface, copper will be pushed into the lower layer by the SIMS ion beam |
Figure 7-2. Backside SIMS can present the true distribution of copper and has excellent vertical depth resolution |
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Case 7: Organic Contamination Analysis |
Generally, FT-IR is the first choice when conducting analysis on organic materials. However, when there are only trace amounts of organic contamination on the very surface, the signals may be too weak to be measured via FT-IR. The Time-of-Flight Secondary Ion Mass Spectrometer (TOF-SIMS) can, through mass spectrometry, analyze and compare normal and abnormal products (or suspected contaminants). It can thus be used to determine the possible sources of organic contamination. Figure 8 is an example of a TOF-SIMS analysis of organic contamination. Here, it was determined through mass spectrometry comparisons that silicon oil contamination during the packaging process caused the Epoxy to be unable to adhere properly to the chip.
Figure 8-1. During the packaging process, residual organic contaminants prevented the Epoxy from adhering properly to the chip
Figure 8-2. The contaminant was determined to be silicon oil via TOF-SIMS surface analysis and comparison |
Secondary Ion Mass Spectroscopy (SIMS) analysis is conducted by bombarding a sample’s surface with an ion beam to excite secondary ions. After being accelerated, the secondary ions enter the mass spectrometry analysis system, and the deflection of electric and magnetic fields is used to separate the ions according to their mass-to-charge ratios (m/e), enabling component analysis. The intensity of the secondary ions can be converted into the concentration of the element, and the ion bombardment time can be converted into the depth distribution. The secondary ion mass spectrometer has excellent detection limits and can measure the concentration of elements in solid materials to one part per million or less (ppm-ppb). There are three types of secondary ion mass spectrometers: Magnetic-Sector SIMS, Quadrupole-SIMS and TOF-SIMS. Magnetic-Sector SIMS has the best detection limit and is suitable for micro-contamination analysis. Quadrupole-SIMS has good depth resolution and is suitable for thin film and ultra-shallow junction analysis. In addition to organic material analysis, the latest models of ToF-SIMS also have depth-analysis capabilities comparable to those of the Quadrupole SIMS. Table 1 is a comparison of the characteristics of these three SIMS.
MA-tek’s comprehensive set of SIMS models and wealth of practical experience enable us to provide customers with a full range of surface analysis services.
Table 1. Comparison of the Three Types of SIMS
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