Difference Between Diffusion and Ion Implantation

Diffusion vs Ion Implantation
 

Difference between diffusion and ion implantation can be understood once you understand what diffusion and ion implantation is. First of all, it should be mentioned that diffusion and ion implantation are two terms related to semiconductors. They are the techniques used to introduce dopant atoms into semiconductors. This article is about the two processes, their major differences, advantages, and disadvantages.

What is Diffusion?

Diffusion is one of the main techniques used to introduce impurities into semiconductors. This method considers the motion of dopant at atomic scale and, basically, the process happens as a result of the concentration gradient. Diffusion process is carried out in systems called “diffusion furnaces”. It is fairly expensive and very accurate.

There are three main sources of dopants: gaseous, liquid, and solids and the gaseous sources are the one most widely used in this technique (Reliable and convenient sources: BF₃, PH₃, AsH₃). In this process, the source gas reacts with oxygen on the wafer surface resulting in a dopant oxide. Next, it diffuses into Silicon, forming an uniform dopant concentration across the surface. Liquid sources are available in two forms: bubblers and spin on dopant. Bubblers convert liquid into a vapour to react with oxygen and then to form a dopant oxide on the wafer surface. Spin on dopants are solutions of drying form doped SiO₂ layers. Solid sources include two forms: tablet or granular form and disc or wafer form. Boron nitride (BN) discs are most commonly used solid source that can be oxidized at 750 – 1100 ⁰C.

What is Ion Implantation?

Ion implantation is another technique of introducing impurities (dopants) to semiconductors. It is a low-temperature technique. This is considered as an alternative to high-temperature diffusion for introducing dopants. In this process, a beam of highly energetic ions is aimed at the target semiconductor. The collisions of the ions with the lattice atoms result in the distortion of the crystal structure. The next step is annealing, which is followed to rectify the distortion problem.

Some advantages of the ion implantation technique include precise control of depth profile and dosage, less sensitive to surface cleaning procedures, and it has a wide selection of mask materials such as photoresist, poly-Si, oxides, and metal.

What is the difference between Diffusion and Ion Implantation?

• In diffusion, particles are spread through random motion from higher concentration regions to regions of lower concentration. Ion implantation involves the bombardment of the substrate with ions, accelerating to higher velocities.

• Advantages: Diffusion creates no damage and batch fabrication is also possible. Ion implantation is a low-temperature process. It allows you to control the precise dose and the depth. Ion implantation is also possible through the thin layers of oxides and nitrides. It also includes short process times.

• Disadvantages: Diffusion is limited to solid solubility and it is a high-temperature process. Shallow junctions and low dosages are difficult the process of diffusion. Ion implantation involves an ad ditional cost for annealing process.

• Diffusion has an isotropic dopant profile whereas ion implantation has an anisotropic dopant profile.

Summary:

Ion Implantation vs Diffusion

Diffusion and ion implantation are two methods of introducing impurities to semiconductors (Silicon – Si) to control the majority type of the carrier and the resistivity of layers. In diffusion, dopant atoms move from surface into Silicon by means of the concentration gradient. It is via substitutional or interstitial diffusion mechanisms. In ion implantation, dopant atoms are added forcefully into Silicon by injecting an energetic ion beam. Diffusion is a high-temperature process while ion implantation is a low-temperature process. Dopant concentration and the junction depth can be controlled in ion implantation , but it cannot be controlled in the diffusion process. Diffusion has an isotropic dopant profile whereas ion implantation has an anisotropic dopant profile.

Images Courtesy:

1. Simple diffusion of a substance (blue) due to a concentration gradient across a semi-permeable membrane (pink) by Elizabeth2424 (CC BY-SA 3.0)