The fabrication of Bipolar Junction Transistors (B.J.T.) by the Diffused Planar Process is the objective of this report. The diffused planar process is self-explanatory in itself by the name. “The diffusion method for manufacturing junction transistors, pioneered by C. A. Lee at Bell Labs, uses a vapour of the impurity atoms surrounding the semiconductor material as it is heated…
The fabrication of BJT comprises of various sub-steps like RCA (Cleaning Process), Thermal Oxidation, Photolithography, Diffusion, Metallization, Alloying Process etc. The process of fabrication starts by cleaning the substrate of impurities by the RCA procedure followed by developing a layer of SiO2 on the planar surface by the process of oxidation. The wafer is then coated with an appropriate photoresist material and developed by exposure to UV through base mask (mask #1). After sufficient time of exposure, the SiO2 is removed by etching out from the region of base-diffusion followed by cleaning away the covering of the remaining photoresist coating. The P-type base i.e. Boron is diffused into this region followed by re-oxidation to develop the layer of SiO2. Next, by the process of Photolithography, the oxide layer is stripped off from the regions of collector and emitter by mask #2 and Phosphorous (N-type) is diffused on it. The entire sample is re-oxidized and once again coated with the photoresist material and developed. The substrate is then exposed to vapors of Aluminum which are allowed to condense upon it. The excess Aluminum on the substrate i.e. at non-contact regions is removed chemically by ‘lift-off’. The final step in the process is alloying of the contacts.
Dust, SiO2, oxides and metallic contaminants are removed. Consequently, the process has three chief procedures namely the Organic Clean, the Oxide Clean and the Ionic Clean.  “The RCA clean procedure should be performed immediately prior to any crucial step, especially those involving high temperatures.” The RCA clean procedure consists of the following steps: Mixing of Organic, Inorganic and Oxide Stripping Solution 1. The Organic Solution is prepared by adding 1000 ml of H2O to 200 ml H2O2 and 200 ml of NH4OH. Heat the solution for 15 min at a temperature of 80 °C.  2. The Ionic Solution is prepared by adding 1000 ml of H2O to 200 ml H2O2 and 200 ml of HCl. Heat the solution for 15 min at a temperature of 80 °C.  3. The Oxide Stripping Solution is prepared by adding 2000 ml of H2O to the polypropylene vat. Add 40 ml of HF acid into it.  Bubbler Rinse Set-Up The bubbler rinse station is filled with deionized water and nitrogen is bubbled in it.  Organic Clean This step removes dust, grease and other organic impurities from the substrate. The substrate is submerged in the Organic Solution for 15 min and then placed in the Bubbler Rinse Set-up for 5 min.  ...
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