SURFACE ENGINEERING IN ELECTRONICS INDUSTRY

SEMICONDUCTOR DOPING

Abhishek AnandGaole DaiHarmeet SinghIshank ChopraYunzhe He

Introduction

• Semiconductor plays key role in the applications in area of thermostat, IR sensors, diodes, transistors etc.

• The gradient of concentration of a dopant in a substrate provides different properties like variable conductivity, light emission etc.

• Most popular material used in substrate are silicon and germanium.

Type of semiconductors

Two kind of semiconductors:-• Intrinsic semiconductor- Intrinsic semiconductor is pure .

It has poor electric conduction.• Extrinsic semiconductor- Extrinsic semiconductor is also

known as impurity semiconductor.

• It is lightly or moderately doped and it has great capacity of electric conduction.

• A semiconductor doped to high levels such as it acts like conductor called degenerate.

Impurity semiconductor is classified in two types• N-type • P-type

• For N-type semiconductor , impurities are chosen from range of pentads, mostly phosphorus

• For P-type semiconductors , the selected impurities should be trivalent elements, mostly boron.

• The following table shows the various materials used in semiconductor industry

Techniques of Semiconductor doping

There are numerous techniques being followed in the industry for doping, most widely used techniques are

• Ion implantation• Diffusion• OxidationWe are going to focus on two major techniques; Diffusion and Ion implantation

Diffusion What is Diffusion?

The movement of impurity atoms (dopant) at high temperature into a semiconductor material due to concentration gradient is known as diffusion. Diffusion of impurities in the silicon lattice takes place at temperatures in the range of 900-1100o C.

There are two major ways in which diffusiondoping process can be carried out:• Predeposition: The impurities diffuse into the

parent material with a constant concentration gradient.

• Drive-in: A layer of the dopant is deposited on the surface. In this case, the impurity gradient at the surface of the substrate decreases with time.

Predeposition Drive-In

Diffusion at microscopic level

Substitution diffusion

Interstitial diffusion

Diffusion process parameters

• Temperature• Type of impurity• Diffusion time• Defects in silicon crystalChallenges:-• Crystal structure: • Junction depth control • Doping concentration control:

Advantages:-

• No damage to surface• Batch fabrication is possible• An isotropic process• Cost associated with process is lowDisadvantages:-• Low dose doping is difficult to carry out• Shallow junctions are difficult to fabricate• Cant be carried out at room temperature

Ion implantation

• What is ion implantation? Ions of the desired dopant are first accelerated

using an electric field resulting in formation of a beam of ions. The beam is then projected upon the parent lattice material causing a bombardment of the ions on the substrate resulting in a uniform deposition of dopant on the parent lattice.

The Process• In order to form a beam of ions, the first step is to

generate ions. • The dopants are heated on a hot filament causing

generation of ions. • The ions generated are accelerated away from the

source by electric field.• The ions then pass through a magnetic field which

diverts the ions and separates them according to their size or according to the requirement using a predesigned aperture.

The separated ions are brought to the desired energy by accelerating them again using an electric field and are bombarded on the substrate after passing through a focusing lens.

The focused accelerated ions strike the substrate and get implanted in the area exposed to the beam.

Advantages:-

• It is a low temperature process and fast process.

• The dose of ion can be controlled• Precise depth control possible• It can be used to implant ions through thin

layers of oxide• The method can be used to obtain extremely

low as well as extremely high dope.

Parameters effecting ion implantation process:-

• The energy of the incoming impurity• Intensity• Project rangeChallenges:-• Damage to the surface during implantation• Formation of amorphous regions• Annealing for a longer time causes the

implanted ion undergo diffusion.• Channeling effect

Disadvantages:-• It causes physical damage to the surface• Annealing is required to relive the stresses and

remove physical damage to the material• Amorphous regions are formed in the crystal

lattice• Channeling occurs, causing irregular

distribution of ions.• It is expensive and one of the most hazardous

process.

Comparative study:-

Conclusion:-• The driving force in the diffusion process is the

difference between the concentrations of the materials involved, is carried out at high temperature. It is a non-destructive process and causes no damage to the material surface. Batch formation is possible with diffusion process, increasing the overall output

• The Ion Implantation process offers better doping concentration control, precise junction depth control, and easy reproducibility and doesn’t require high temperature for being carried out. The ion implanted product has to undergo annealing process to repair the damage which makes this process relatively expensive.

Feedback• An improvement to the ion implantation process is the plasma immersion

ion implantation (PIII). In PIII process, the parent material is directly immersed in a plasma chamber. The chamber has plasma of ions of the impurity to be implanted. The chamber is then impulse with a very high negative potential.

• This potential drives the electron and lead to formation of a positive

cloud around the parent material. The positive ion round the parent material is attracted towards the material due to its negative potential and it gets implanted on the surface uniformly.

• PIII is a non-line of sight technique unlike the conventional process and it doesn’t require all the complexities and sophisticated mechanisms of the conventional Ion Implantation Method.

• It is a simpler, cheaper, less time consuming, less hazardous and flexible process than the conventional Ion Implantation.

References1. J D Plummer, M D Deal and P B Griffin, “Silicon VLSI Technology: fundamentals,

practice and modelling”, Pearson Edu. Inc., 20012. Razeghi and Manijeh, “Technology of Quantum Devices”, Springer, 20103. Bose D.N., “Semiconductor Material and Devices”, New Age Publishers, 20124. F.G, Tseng, “IC Fabrication Process 2: Diffusion, Ion Implantation, Film

Deposition,Interconnection and contacts”. Lecture conducted from National Tsing Hua University,

Taiwan. 5. John (2010, June 1), “Diffusion of impurities for IC fabrication”6. R.C. Jaeger (vol.5), Introduction to Microelectronic fabrication, Pearson Education

Inc., New Jersey,USA ,20027. Advancements in ion implantation Modelling for Doping of Semiconductors, Sivaco,

Inc. Available [Online] http://www.silvaco.com/content/kbase/ion_implantation.pdf

8. Plummer D. James, Deal Michael , Griffin D. Peter, “Silicon VLSI Technology: Fundamentals, Practise and Modelling”, Prentice Hall , 2000.

9. Gupta Dushyant (2011) , “Plasma Immersion Ion Implantation Process Physics and Technology", International Journal of Advancements in Technology.

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