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DOPING - Coggle Diagram
DOPING
Definition
Process of intentionally introducing impurities into an extreme pure semiconductor i order to change its electrical properties.
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Diffusion
1. Deposition
(Pre-deposition, dep or predep)
Factors affected deposition process:
- Diffusitivity (Rate of movement of a dopant through water):
High diffusivity = Fast movement of dopant atoms
Diffusivity increases with increasing temperature.
- Maximum Solid Solubility: max. concentration of specific dopant that can be put into the wafer
- Take place in a flat zone of a tube.
- A source of dopant is placed in a source cabinet.
- Vapours are transferred into the tube (required concentration).
Diffusion system
- Wafer placed in a quart boat and positioned in the center zone of the furnace, where wafer heated to high temperature.
- Impurities transport to silicon surface and diffuse into the wafer.
- Using liquid, solid or gaseous sources.
Steps
- Preclean & etch
- Tube deposition
- Deglaze
- Evaluation
Dopant Sources
Liquid
- Chlorinated or brominated compounds of the desired element. BBr3 (4BB3 + 3O2 <--> 2B2O3 + 6Br2)
POCl3 (4POCl3 + 3O2 <--> 2P2O5 + 6Cl2)
- Placed in a temperature controlled quartz flask.
- Inert gas such as nitrogen is bubbled through the heated liquid, and the gas becomes saturated with the dopant vapours.
- Laminar flow is useful to obtain a uniform doping.
- Advantages: low-moderate cost, consistent doping.
- Disadvantage: uniformity, safety considerations, contamination.
Solid
- Common solid source of boron include trimethylborate (TMB) (2(CH3O)3B + 9O2 <--> B2O3 +6CO2 + 9H2O, at 900°C) and boron nitride wafer.
- Solid P2O5 wafers can be used as a solid sources for phosphorous. (2P2O5 + 5Si <--> 4P + 5SiO2)
- Carrier gases (usually N2 or O2) flow at a controlled rate over a sources boat placed in the furnace.
- Oxide powder of the desired dopant was placed on a quartz holder (spoon), and placed in a source tube furnace attached to the main deposition tube.
- Oxide vaporise inside the source furnace, flow into the deposition tube for diffusion to take place.
- Setup is known as remote solid source.
Advantage: economical.
Disadvantage: non-uniform doping.
Widely used to in the doping of discrete devices where less precision is required.
- Another solid source is a conformal layer spun directly on the wafer surface. (powdered oxides mixed with solvents) (slide 33)
Gas
- Hydrated forms of dopant atom. (highly toxic & explosive gas - dilute form with 99.9% argon and N2.)
Arsine (PH3) - The reaction with O2 in the furnace:
2PH3 + 4O2 <--> P2O5 + 3H2O
Diborane (B2H6) - Oxidized in either oxygen or carbon dioxide to form boron trioxide:
B2H6 + 3O2 <--> B2O3 + 3H2O (at 300°C)
B2H6 + 6CO2 <--> B2O3 + 6CO + 3H2O
- The gases are mixed in different dilutions in pressurised containers and connected directly to the gas manifold.
- Advantages: precise control through pressure regulators and favoured for deposition on larger diameter wafers, cleaner.
- Disadvantages: unwanted chemical reactions in the manifold can create silica dust that can contaminate the tube and wafers.
2. Drive in oxidation
(drive-in, diffusion, reoxidation and reox)
Purposes
i) Redistribution of the dopant in the wafer.
- Take place at a higher temperature than the deposition step.
- Heat will drive the dopant atoms deeper and wider into the surface.
- Junction depth increases.
ii) Growth of a new oxide on the exposed Si surface.
- The atmosphere in the tube is oxygen or water vapour performs oxidation while dopants are being driven deeper into the wafer.
Ion Implantation
- overcomes problems in diffusion process.
- Concept: dopants atoms are ionised, and isolated, accelerated, formed into beam and swept across the wafer.
- A beam of high-energy ions strikes selected regions of the semiconductor surface, penetrating into the exposed region.
- Dopants atoms physically bombard the wafer, enter the surface, come to rest below the surface.
Advantages
- Precise control of impurity dose, depth, profile and area uniformity.
- Excellent reproducibility.
- Wide choice masks; SiO2, Si3N4, polysilicon, photoresist.
- Low temperature processing.
- Small lateral spreading of dopant (self alignment).
- Vacuum cleanliness.
Disadvantages
- Expensive, complicated equipment.
- Junction are not automatically passivated.
Parameters
- Implanted ions: phosphorus, arsenic, antimony and boron.
- Dose: 10^11/cm2 to 10^16/cm2
- Energy: 5 keV – 2 MeV
- Depth of implant: 100Å - 1µm
- Reproducibility and uniformity: ± 5%
- Temperature: usually room temperature.
System
- Source: Gaseous source of vaporisable material e.g. BF3, AsH3at high accelerating potential. A valve controls the flow of gas to the ion source.
- Power supply: to energise the ion source and accelerate the ions into the mass separation mechanism.
- Analyser magnet: select only the ion species of interest and reject other species.
- Beam sweeping electrode: to raster the beam and give a uniform implantation across the wafer.
- Target chamber: consists of area defining aperture, Faraday cage for measuring current and a wafer feed mechanism.
Crystal Damage
- Lattice damage: when ions collide with host atoms and displace them from their lattice site.
- Damage cluster: when displaced atoms in turn displace other substrate atoms and create a cluster of displaced atoms.
- Vacancy-interstitial (most common): when an incoming ion knocks a substrate atom from a lattice site and the displaced atom comes to rest in a non-lattice position.
Channeling
- occurs when major axis of the crystal wafer is presented to the ion beam.
- Ion can travel down the channels and reach up to 10 times of the calculated depth.
- How to minimize:
i) Block amorphous surface layer
- Thin layer of grown SiO2.
- The layer randomises the direction of the ion beam so that the ions enter the wafer at different angles and not directly down the crystal channels.
ii) Mis-orientation of the wafer.
- Mis-orientation of the wafer 3-7° of the major plane may prevent the ions from entering the channels.
iii) Creating a damage layer in the wafer surface.
- Damaging the wafer surface with a heavy silicon or germanium implant creates a randomising layer in the wafer surface.