Calculate the flow rate of the etching solution

[Pr0x1mo]

Homework Statement

In the semiconductor industry, integrated circuit (IC) production begins with the mechanical
slicing of silicon rods into wafers. Once the wafers are sliced, the surfaces are lapped and
polished to uniform flat surfaces. Contaminants and microscopic defects (work damage) are then removed chemically by etching. A tradition etching solution consists of a 4:1:3 volumetric ratio of 49% hydrofluoric (HF), 70% nitric (HNO3), and 100% acetic (CH3CO2H) acids, respectively.

Although work damage is usually 10 μm deep, overetching to 20 μm per side is common. The
reaction for dissolving the silicon surface is:

3 Si + 4 HNO3 + 18 HF -> 3 H2SiF6 + 4 NO + 8 H2O

Calculate the flow rate of the etching solution in kilograms per hour if 20μm per side is to be
etched for 6000 wafers per hour of 150 mm diameter. What is the limiting reagent?

Si – 2.33 g/cm3 M.W. 28.09g/mol
Sp gr 49% HF is 1.198 M.W. 20.01g/mol
Sp gr 70% HNO3 is 1.4134 M.W. 63.01g/mol
Sp gr 100% CH3CO2H is 1.0492 M.W. 60.05g/mol




Ok, this is a question that i have for my chemical engineering class, its one out of 10 questions (its not due, or has to be done). But looking at it, i want to know how to solve this. I've tried looking up in my textbook for a way to calculate this but i couldn't find anything on it. Maybe my teacher pulled this question from another book.

I don't even know the first step into solving this. Can someone get me on the right track?

 

[Borek]

I think it is a simple stoichiometry. How much silicon is to be etched? In what time? Make sure stoichiometric amount of etching solution is introduced in this time and you should be done.

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[Pr0x1mo]

Well, Si is the limiting reagent I'm assuming.

and If CH3CO2H is 100% i can set that to CH3CO2H 100 mols and using the stoichemetric ratios, i get:

CH3CO2H 100 mols
Si 52.5 mols
HNO3 70 mols
HF 49 mols
H2SiF6 52.5 mol
NO 70 mol
H2O 140 mol

From here, where does the specific gravity's come into play? Do i have to add each one of their sp's and multiply it by 20μm*20μm times 150mm diameter times 6000 wafers for one hour?

 

[Borek]

What volume of silicon is going to be removed? What is its mass? Number of moles?

Actually I am not sure what is a limiting reagent - but as after the reaction ends a lot of silicon should stay in, silicon is not.

There is a stoichiometric ratio of acids and introduced ratio of acids - compare them.