In my last blog “Doping and Dopants” we have seen that what is doping and what are the different types of dopants and how to select which dopant to be used. More or less we all are familiar that doping is a process of adding of desired impurities in the semiconductor material to deliberately change its electrical property. As we have control on the electrical conductivity we have used this technique in making of electronic devices and components. Now the question arises that how to insert these foreign particles inside the intrinsic semiconductor so that it will damage the substrate physically and will give us desired result. So in this blog we will be discussing about the methods of adding impurities. Mainly on broad categories we have two methods of doping and those are as follows
- Diffusion Process
- Ion Implantation Process
Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of materials. To illustrate: a drop of ink in a glass of water is evenly distributed after a certain amount of time. In a silicon crystal, one finds a solid lattice of atoms through which the dopant has to move. This can be done in different ways:
- Empty Space Diffusion – the impurity atoms can fill empty places in the crystal lattice Empty Space Diffusion -which are always present, even in perfect single crystals.
- Inter Lattice Diffusion – the impurity atoms move in-between the silicon atoms in Inter Lattice Diffusion – the crystal lattice.
- Changing of Place – the impurity atoms are located in the crystal lattice and are Changing of Place – exchanged with the silicon atoms.
The speed of the diffusion process depends on several factors:
- Concentration gradient
- Crystallographic orientation of the substrate
|Diffusion with Exhaustible Source||Diffusion with Inexhaustible Source|
|Diffusion with an exhaustible source means that the dopant is available in a limited amount only. The longer the diffusion process continues, the lower the concentration at the surface, and therefore the depth of penetration into the substrate increases. The diffusion coefficient of a substance indicates how fast it moves in the crystal. Arsenic with a low diffusion coefficient penetrates slower into the substrate, as for example phosphorus or boron.||In diffusion processes with an inexhaustible source the dopants are available in unlimited amount, and therefore the concentration at the surface remains constant during the process. Particles that have penetrated into the substrate are continually replenished.|
Methods of Diffusion Process
In diffusion process wafers are heated in a quartz tube up to certain temperature and then the dopant atom is allowed inside the temperature. Dopant atom may be in gaseous phase, liquid phase, so depending on the state of the dopant we have several methods of doping and those are as follows
- Diffusion From Gas Phase – A carrier gas (nitrogen, argon,) is enriched with the Diffusion From Gas Phase – desired dopant (also in gaseous form, e.g. phosphine PH3 or diborane B2H6) and led to the silicon wafers, on which the concentration balance can take place.
- Diffusion with Solid Source – Slices which contain the dopants are placed in-between Diffusion with Solid Source -the wafers. If the temperature in the quartz tube is increased, the dopant from the source discs diffuses into the atmosphere. With a carrier gas, the dopant will be distributed uniformly, and thus reaches the surface of the wafers.
- Diffusion with Liquid Source – As liquid sources boron bromide BBr3 or phosphoryl Diffusion with Liquid Source -chloride POCl3 can be used. A carrier gas is led through the liquids and thus transporting the dopant in gaseous state. Since not the entire wafers should be doped, certain areas can be masked with silicon dioxide. The dopants cannot penetrate through the oxide, and therefore no doping takes place at these locations. To avoid tensions or even fractions of the discs, the quartz tube is gradually heated (e.g. +10 °C per minute) till 900 °C. Subsequent the dopant is led to the wafers. To set the diffusion process in motion, the temperature is then increased up to 1200 °C.
Characteristics of Diffusion Process
- Since many wafers can be processed simultaneously, this method is quite Since favourable.
- If there already are dopants in the silicon crystal, they can diffuse out in later If processes due to high process temperatures.
- Dopants can deposit in the quartz tube, and be transported to the wafers in later processes.
- Dopants in the crystal are spreading not only in perpendicular orientation but also Dopants laterally, so that the doped area is enlarged in a unwanted manner.
In the ion implantation charged dopants (ions) are accelerated in an electric field and irradiated onto the wafer. The penetration depth can be set very precisely by reducing or increasing the voltage needed to accelerate the ions. Since the process takes place at room temperature, previously added dopants cannot diffuse out. Regions that should not be doped can be covered with a masking photoresist layer.
Components of Ion Implanter
An ion implanter consists of following components
- Ion Source – the dopants in gaseous state (e.g. boron trifluoride BF3) are ionized.
- Accelerator – the ions are drawn with approximately 30 KeV out of the ion source.
- Mass Separation- the charged particles are deflected by a magnetic field by 90 degrees. Too light/heavy particles are deflected more/less than the desired ions and trapped with screens behind the separator.
- Acceleration Lane – several 100 keV accelerate the particles to their final velocity (200 keV accelerate bor ions up to 2.000.000 m/s).
- Lenses – lenses are distributed inside the entire system to focus the ion beam.
- Distraction – the ions are deflected with electrical fields to irradiate the desired location.
- Wafer Station – the wafers are placed on large rotating wheels and held into the ion beam.
Different Parameters of Ion Implantation
- Penetration depth of ions in the wafer- In contrast to diffusion processes the particles do not penetrate into the crystal due to their own movement, but because of their high velocity. Inside the crystal they are slowed down by collisions with silicon atoms. The impact causes damage to the lattice since silicon atoms are knocked from their site, the dopants themselves are mostly placed interstitial. There, they are not electrically active, because there are no bonds with other atoms which may give rise to free charge carriers. The displaced silicon atoms must be re-installed into the crystal lattice, and the electrically inactive dopants must be activated.
- Recovery the crystal lattice and activation of dopants – Right after the implantation process, only about 5 % of the dopants are bond in the lattice. In a high temperature process at about 1000 °C, the dopants move on lattice sites. The lattice damage caused by the collisions has already been cured at about 500 °C. Since the dopants move inside the crystal during high temperature processes, these steps are carried out only for a very short time.
- Channelling – The substrate is present as a single crystal, and thus the silicon atoms are regularly arranged and form “channels”. The dopant atoms injected via ion implantation can move parallel to these channels and are slowed only slightly, and therefore penetrate very deeply into the substrate. To prevent this, there are several possibilities:
- Wafer Alignment – the wafers are deflected by about 7° with respect to the ion beam. Thus the radiation is not in parallel direction to the channels and the ions are decelerated by collisions immediately.
- Scattering – on top of the wafer surface a thin oxide is applied, which deflects the ions, and therefore prevents a parallel arrival.
Characteristics of Ion Implantation
- The reproducibility of ion implantation is very high.
- The process at room temperature prevents the outward diffusion of other dopants.
- Spin coated photoresist as a mask is sufficient, an oxide layer, as it is used in diffusion processes, is not necessary.
- Ion implanters are very expensive; the costs per wafer are relatively high.
- The dopants do not spread laterally under the mask (only minimally due to collisions).
- Nearly every element can be implemented in highest purity.
- Previous used dopants can deposit on walls or screens inside the implanter and later be carried to the wafer.
- 3-D structures (e.g. trenches) cannot be doped by ion implantation.
- The implantation process takes place under high vacuum, which must be produced with several vacuum pumps