As you know, semiconductor technology is going to reach 7nm lithographic precision soon. Which means that the smallest segment length on a semiconductor die can be as short as 7nm. After making some calculations, I wondered how can someone dope such a small silicon block uniformly.
Density of silicon is \$2.328 \;\text{g}/\text{cm}^3\$.
Number of atoms in silicon is \$2.144217404 \times 10^{22} \;\text{atoms}/\text{g}\$.
Then we have number of atoms in unit volume as
\$(2.328 \;\text{g}/\text{cm}^3) * (2.144217404 \times 10^{22} \;\text{atoms}/\text{g}) ≈ 5 \times 10^{22} \;\text{atoms}/\text{cm}^3\$.
I read some papers online and deduced that a typical doping density is
\$10^{18} \;\text{doping-atoms}/\text{cm}^3\$.
Which means that, the doping ratio is about \$50000\$ silicon atoms per each doping material atom.
Now, consider a \$7nm \times 7nm \times 7nm\$ cubic silicon block and we wish to dope it with atoms of some other material. Van Der Vaals radius of silicon atom is \$219pm\$ (found this on Google). After doing some elementary geometry calculation, we can find that \$4082\$ atoms can fit in this block.
We need
\$(4082 \;\text{silicon-atoms}) / (50000 \dfrac{\;\text{silicon-atoms}}{\text{doping-atoms}}) = 0.08164 \;\text{doping-atoms}\$
to dope this tiny 7nm x 7nm x 7nm cubic silicon block, which is less than 1. Which means a tiny block like this have a 8% change of receiving a single atom from the doping material. Which means there is a 92% change that the tiny block will remain pure silicon!
Seriously, what?!
I don't understand this and I have some questions on this.
- What happens if a supposedly P-type or N-type substrate (e.g.; emitter of a BJT or channel of a MOSFET) remains purely silicon?
- What happens if the block was lucky and it received a single atom of a doping material. Does it matter the location of that atom? What if the atom stays in the farthest corner of of the block?
- What happens if the block was super luck and it received more than 1 doping atoms?
In a chip which will contain more than 1 thousand transistors, occurrence of these kind of situations are statistically inevitable. How do they dope very small semiconductor materials? How do they overcome problems like this?