Czochralski Method The primary raw material used in modern semiconductor fabs is a silicon wafer or disk, which varies from roughly 75mm to 300mm in diameter and is less than 1mm thick. Wafers are cut from ingots of single-crystal silicon pulled from a crucible melt of pure molten silicon. This is known as the Czochralski method and is currently the most common method for producing singlecrystal material. Controlled amounts of impurities such as boron or phosphorus can be added to the molten silicon in precise amounts to dope it, thus changing it into p-type or n-type silicon with different electronic properties. A seed crystal is dipped into the melt to initiate crystal growth. The silicon ingot takes on the same crystal orientation as the seed. A graphite radiator heated by radiofrequency induction surrounds the quartz crucible holding the melt and maintains the temperature a few degrees above the melting point of silicon (1425℃) The atmosphere is typically helium or argon to prevent the silicon from oxidizing [1]. The seed is gradually withdrawn vertically from the melt while simultaneously being rotated. The molten silicon attaches itself to the seed and recrystallizes as it is withdrawn. The seed withdrawal and rotation rates determine the diameter of the ingot, and its growth rates range from 20 to 180mm/hour [1]. Ion Implantation Ion Implantation involves subjecting the silicon substrate to highly energized donor or acceptor atoms [1]. In this process, the dopant atoms are vaporized. They are accelerated by an accelerator and then bombarded on a silicon substrate. The entire wafer, or a selected part, is exposed to the vaporized, accelerated dopant atoms beam. The beam injects the dopant atoms into unmasked sections of the substrate. The dopant atoms directly enter the crystal lattice of the silicon. In the lattice, the dopant atoms start losing energy due to the collisions with silicon atoms. When the energy is lost, the dopant atoms are found at some depth within the lattice. The depth of penetrations is controlled by the acceleration energy of the incident beam and the doping concentration [2]. In order to perform ion implantation, the ion implanter is used. The ion implanter consists of an ion source, bending analyzer magnet, aperture, acceleration tube, X-Y scanner plates, and a target chamber [2]. The ion source uses dopant gas to produce charged particles through the heating filament. The bending analyzer magnet selects the desired ions based on their charge-to-mass ratio, and the ions are accelerated to high implantation energy in the acceleration tube. The aperture focuses on the beam of ions, and the X-Y scanner plates adjust the sweep of the beam over the wafer in the target chamber [2]. At elevated temperatures (> 800℃ ), diffusion occurs between silicon regions having different densities of impurities, with impurities tending to diffuse from areas of high concentration to areas of low concentration [1]. Therefore, keeping the remaining process steps at as low a temperature as possible is important once the doped areas have been implemented. However, a high-temperature annealing step is often performed after ion implantation to redistribute dopants more uniformly. Ion implantation is today's standard well and source/drain implant method. Diffusion The diffusion process in IC fabrication involves exposing a silicon wafer to a diffusant containing the required impurity at high temperatures ranging from 900°C to 1250°C. The wafer is placed in a quartz boat in a quartz diffusion tube, which holds a stack of vertically mounted wafers that are diffused simultaneously. The dopant can be in solid, liquid, or gaseous form, with preferred p-dopants being boron and n-dopants being antimony, arsenic, and phosphorus. Arsenic is also preferred for n+ type source-drain and emitter diffusions. The diffusivity of the dopants in oxide and silicon determines the imperviousness of silicon dioxide to the diffusant. In the case of solid-state dopants, the carrier gas flows over a quartz boat containing the dopant in powder form. In the case of liquid-state dopants, the gas is bubbled over a liquid source in a heated bath. However, gaseous state dopants can be problematic due to toxicity and chemical stability issues [3]. During IC fabrication, pre-deposition and drive-in diffusion are two key steps in introducing dopants into a silicon wafer [3]. Pre-deposition is the initial step in which a thin layer of dopant is deposited onto the surface of the wafer. This is typically done using a chemical vapour deposition (CVD) process, where a gas containing the dopant is passed over the surface of the wafer, causing the dopant to adhere to the surface. The thickness of the pre-deposited layer is typically less than 100 nanometers. After pre-deposition, the wafer undergoes a drive-in diffusion process. During this step, the wafer is heated to a high temperature, typically between 900°C and 1250°C, causing the dopant atoms to diffuse into the silicon lattice. The diffusion rate of the dopant atoms is a function of the temperature and time and can be controlled by adjusting these parameters. The drive-in diffusion process aims to achieve the desired depth and concentration of dopant atoms within the silicon wafer [3]. Chemical Vapor deposition Chemical vapor deposition (CVD) is a process used in the fabrication of integrated circuits (ICs) to deposit thin films of materials onto a substrate [1]. In the context of IC fabrication, CVD is used to deposit layers of material such as silicon dioxide, silicon nitride, and polysilicon onto the surface of a silicon wafer. In the CVD process, a gas mixture is introduced into a reaction chamber that contains the substrate. The gas mixture typically contains a precursor molecule that contains the element to be deposited, along with other gases such as hydrogen, oxygen, or nitrogen. The gas mixture is then heated to a high temperature, typically in the range of 700 to 1100 degrees Celsius, which causes the precursor molecule to decompose and form a solid film on the surface of the substrate. The CVD process can be carried out using different methods, including lowpressure CVD, plasma-enhanced CVD, and atmospheric-pressure CVD [5]. Each method has its own advantages and disadvantages, depending on the material being deposited and the requirements of the application. The equipment used in chemical vapor deposition (CVD) depends on the specific method being used and the materials being deposited. However, there are some common pieces of equipment used in most CVD processes such as reaction chamber, gas delivery system, heating system, vacuum system, substrate holder and exhaust system [4]. In addition to these basic components, there may be other specialized equipment depending on the specific CVD process being used. For example, plasma-enhanced CVD may require an additional power supply to generate the plasma, and atomic layer deposition may require multiple gas pulses to deposit alternating layers of material [4]. References [1] N. H. E. Weste and D. M. Harris, CMOS VLSI DESIGN: A Circuits and Systems Perspective, 3rd ed. Pearson, 2004. [2] https://www.eeeguide.com/ion-implantation-process-in-ic-fabrication/. [Accessed: 18-Mar2023]. [3] https://www.eeeguide.com/diffusion-process-in-ic-fabrication/. [Accessed: 18-Mar-2023]. [4] https://www.azom.com/article.aspx?ArticleID=1552. [Accessed: 19-Mar-2023]. [5] https://www.intechopen.com/chapters/51808. [Accessed: 19-Mar-2023].
