The relationship between materials, process structures, and models in semiconductor devices

Since the development of semiconductors, there have been many improvements in materials, structures, processing technology models, and other aspects. The size of semiconductor devices is constantly shrinking, and the integration level is also constantly improving. The manufacturing process ranges from 90nm, 65nm, 45nm, 32nm, 22nm, 14nm, 10nm, to the current 7nm, 5nm, 3nm. With the requirement of integration, device size is decreasing, and various problems will arise accordingly, such as short channel effect (DIBL, mobility degradation, etc.), increased gate leakage, decreased gate control capability, and many other issues. Therefore, people are constantly searching for methods from the three aspects of materials, structure, and process to provide solutions for the next generation of process technology. Every change in material and process structure is a challenge for device models, and device model development engineers need to develop new models to characterize the device characteristics under new materials, structures, and processes.

Figure 1: Schematic diagram of materials, structure, and process

In terms of materials:

The first generation of semiconductors was "elemental semiconductors". Since the 1950s, the emergence of the first generation of semiconductor materials represented by silicon (Si) and germanium (Ge) has replaced bulky electronic tubes, leading to the development of the microelectronics industry centered on integrated circuits and a leap forward in the entire IT industry.
However, due to the narrow bandgap, low electron mobility, and low breakdown electric field of silicon materials, their application in the field of optoelectronics and high-frequency high-power devices is subject to many limitations. Therefore, the second-generation semiconductor materials represented by gallium arsenide (GaAs) have begun to emerge, bringing the application of semiconductor materials into the field of optoelectronics, especially in infrared lasers and high brightness red diodes, applied in millimeter wave devices, satellite communication, mobile communication, GPS navigation and other fields.
On the basis of second-generation semiconductor materials, people hope that semiconductor components can have high voltage resistance, high temperature resistance, high power, radiation resistance, stronger conductivity, faster working speed, and lower working losses. The third-generation semiconductor materials are also born based on these characteristics. The third-generation semiconductor materials, represented by silicon carbide, gallium nitride, etc., are suitable for producing high-temperature, high-frequency, radiation resistant, and high-power devices due to their excellent properties such as high breakdown electric field, high thermal conductivity, high electron saturation rate, and radiation resistance. They can significantly improve energy conversion efficiency and reduce system costs.

In terms of process structure:

In order to extend the lifespan of traditional planar transistor manufacturing technology and compensate for the negative effects of key size reduction on traditional planar transistors, the industry has developed many technologies that can improve the performance of traditional planar transistors. Here are some commonly used technologies in the employment industry:

Strain Silicon Technology：The reduction in transistor size has led to mobility degradation caused by vertical electric fields. There are many methods to enhance the performance and mobility of transistors. As shown in Figure 1 (Strain Silicon Technology), strain silicon technology is the use of various means to physically stretch or compress silicon crystals, thereby increasing the mobility of charge carriers (electrons/holes) and enhancing the performance of transistors.

HKMG Technology：The thickness of SiO2 dielectric should be proportional to its channel length. If the oxide thickness decreases further with size, gate leakage will increase to an unacceptable level. Therefore, choosing dielectric materials with high dielectric constant (K) to increase oxide capacitance is a good solution. HKMG is a technology that replaces the traditional SiO2 oxide layer with a High-K insulation layer and replaces the old silicon gate with a metal gate material. This technology mainly helps to improve the switching speed of transistors and reduce gate leakage current.

SOI (silicon on insulator) technology：Compared with traditional MOS structures, the main difference of SOI MOS structures (Figure 1 SOI technology, PDSOI, FDSOI) is that SOI devices have a buried oxide layer (BOX layer) that isolates the substrate from the substrate. The basic idea of burying the oxide layer is to reduce parasitic junction capacitance. The smaller the parasitic capacitance, the faster the transistor operates. Professor Hu Zhengming and his team proposed UTB-SOI (FD SOI) in 2000. Due to the presence of the BOX layer, there is no leakage path far from the gate, which leads to lower power consumption. Therefore, it is particularly suitable for mobile and consumer grade multimedia applications. Disadvantages: The floating body voltage of PDSOI leads to unstable threshold voltage; Isolation leads to self heating issues.

FinFET technology：As the device size shrinks, the short channel effect becomes more pronounced at lower technology nodes, such as 22nm, reducing the performance of the device. To overcome this problem, Professor Hu Zhengming and his team proposed the concept of FinFET in 1999. FinFET is a three-dimensional structure (Figure 1 structure: FinFET), also known as a three gate transistor. FinFET can be implemented on Si or SOI chips. The FinFET structure consists of thin (vertical) silicon fins on the substrate. Because its Si body resembles the posterior fin of a fish, it is also known as a fin transistor.

GAA (Gate Al Around) Nanotechnology：Gate Al Around refers to the surrounding gate (Figure 1 structure: GAAFET, MBCFET). Compared to the current FinFET Tri Gate triple gate design, the transistor's underlying structure will be redesigned to overcome the physical and performance limitations of current technology, enhance gate control, and greatly improve performance. Samsung's GAA technology is called MBCFET (Multi Bridge Channel Field Effect Transistor). The characteristic of this technology is to achieve the four sided wrapping of the gate to the channel, and the source and drain are no longer in contact with the substrate. Instead, multiple sources and drains are distributed horizontally perpendicular to the gate using linear (which can be understood as stick shaped), flat plate shaped, sheet shaped, etc., to achieve the basic structure and function of MOSFET.

Model aspect：Every change in material or process structure presents new challenges for device models. From the very beginning, the Schichman Hodges (Level 1) model was very simple and did not include carrier mobility degradation or carrier saturation effects. It is commonly used to simulate large digital circuits without the need for detailed simulation models. Moving on to the Grove Frohman (Level 2) model, which is a geometric analysis model that includes short channel, narrow channel effects, mobility degradation, carrier saturation, and other effects. The device model is also improving with the improvement of process structure. It is worth mentioning that BSIM Group, also known as Berkeley Short channel IGFET Model Group, is a group led by Hu Zhengming at the University of California, Berkeley. They developed a series of precise MOSFET Spice Models, known as BSIM, which have now become a standard in the industry. The MOSFET model provides a series of Level numbers in the simulator. Taking Synopsys as an example, it provides Level 6 IDS: either the MOSFET model or one of the BSIM models (Level 13, 28, 39, 47, 49 (BSIM3V3), 53, 54 (BSIM4), 57, 59, 60, 70 (BSIMOI4.0)).

So when you model or simulate, you need to choose the appropriate MOSFET model version to achieve the best model fitting effect and simulation results. Figure 2 lists the commonly used device classifications and corresponding models in the industrial sector.

Figure 2: Commonly used device models in the industrial sector

reference:

1.Understanding the Development History of Semiconductor Materials in One Article - Zhihu (zhihu. com)

2.Semiconductor processes: Bulk Si, SOI, FinFET, GAA, etc

3.hspice_mosmod.pdf