This technique is currently used for fabricating ultrashallow junctions (USJs) as the heavily doped source/drain (S/D) contacts of metal–oxide–semiconductor field effect transistors (MOSFETs) as well as enabling dopant profiling of nanostructures.
This MLD technique utilizes the crystalline nature of semiconductors and its self-limiting surface reaction properties to form highly uniform, self-assembled, covalently bonded dopant-containing monolayers followed by a subsequent annealing step for the incorporation and diffusion of dopants.
[2] Compared to low-energy ion-implantation into a screening film followed by in-diffusion,[3][4] the MLD technique requires a lower thermal budget and allows conformal doping on topographic features.
[5] The MLD capping layer serves as i) preventing group V elements to desorb and ii) avoiding the dopant atoms to be lost to the ambient in order to result in the good quality junctions.
An important characteristic of the use of the substrate surface chemistry is the ability to readily control the areal dose of the dopants by forming a mixed monolayer of ‘blank’ and dopant-containing molecules.
In this regard, using trioctylphosphine oxide (TOP) as the phosphorus precursor with an approximately six-fold larger molecular footprint than DPP, the dopant dose can be modulated in the reduction of six times accordingly.
In this case, the high surface doping density with sharp spatial decay can be obtained by using this MLD method with low anneal temperatures and short times for the formation of USJs.