Silicon dioxide is widely used in MEMS to selectively protect certain areas of the device and open the unprotected areas for chemical action with silicon, like for doping or metal contacts. After these processes, the oxide layer may have to be removed by etching and Hydrofluoric acid (HF) is widely used for this purpose. But 49% HF etches oxide rapidly which makes it uncontrollable for most purposes. Buffered Oxide Etchants (BOE) are blends of 40% ammonium fluoride (NH4F) and 49% hydrofluoric acid mixed together in various proportions. BOE gives much controllable etch rates and are often used for etching silicon dioxide.
The etch rate of BOE mainly depends on the etchant temperature and the percentage concentration of HF in the solution. BOE is prepared by mixing 40% NH4F and 49% HF in various volume ratios like 5:1 or 12:1 etc. To convert the mixing ratio to the percentage concentration of HF(wt), a rough but quick method will be to assume 50ml of HF dissolved in total volume of the solution. So for 7:1 ratio this will be 50/800 or 6.25 % HF.
This design interface can be used to determine the etch rate of undoped thermally grown silicon dioxide for a given etchant temperature and HF concentration. The etch rate increase with temperature and also concentration of HF, and needs to be controlled to ensure uniform etch rate. The temperature of operation is limited between 20°C and 40°C. The percentage concentration of HF should be between 1% and 8%.
The plot shows the variation of etch rate with etchant temperature for the given concentration. The etch rate is found to increase rapidly with temperature. Using the crosshair, the etch rate corresponding to the given temperature can be read out.