On N-Type silicon wafer, i am depositing Phosphorous silicate glass of thickness 28-32nm and then depositing Silicon nitride layer of 105nm. Later i need to etch SiN layer by 20nm in HF solution but i am finding Pin holes after HF Dip as shown in attached image. How i can avoid mini-pop out of SiN layer.
The chemical quality of your deposited nitride film is very important. If you have used a deposition method that allows even tiny but localized oxidation of your film, then you may expect problems when etching the same. I would suggest using LPCVD (thermal deposition at low pressure, providing the best chemical quality) if possible, or PECVD (Plasma Enhanced CVD) as a second choice.
Moreover, the choice of HF to etch the nitride is not the best one. HF etches SiO2 much easier than nitride, and if you have local SiOx point defects on your deposited nitride film, you will find pits and nearly no nitride removal.
I think you should use concentrated hot orthophosphoric acid to etch Si3N4. The selectivity with SiO2 is around 10:1 and the nitride etch rate may be as low as 5 nm/min (the lower the acid temperature, the lower the etch rate, but temperature shoud be at least 150 °C).
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