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.

 

[DeeNos]

Sure, I can help you get started on solving this problem. The first step is to understand what is being asked and what information is given. The problem is asking for the flow rate of the etching solution in kilograms per hour. In order to calculate this, we need to know the amount of etching solution being used per hour, which can be determined by the number of wafers being processed per hour and the thickness of the etch.

Given that 20 μm per side is being etched and there are 6000 wafers per hour, we can calculate the total thickness of silicon being etched per hour as follows:

Total thickness = (20 μm per side) x (2 sides per wafer) x (6000 wafers per hour) = 240,000 μm per hour

Next, we need to convert this thickness into kilograms of silicon. In order to do this, we need to know the density of silicon and its molecular weight. The problem provides us with these values, so we can use them to calculate the mass of silicon being etched per hour:

Mass of silicon = (240,000 μm per hour) x (2.33 g/cm3) x (1 cm/10,000 μm) x (1 mol/28.09 g) = 19.4 kg per hour

Now, we need to determine the limiting reagent in the reaction. This is the reactant that is completely consumed and thus determines the amount of product that can be formed. In this case, we can use the stoichiometric ratio of the reactants to determine the limiting reagent. The ratio of Si to HNO3 to HF is 3:4:18, which means that for every 3 moles of Si, we need 4 moles of HNO3 and 18 moles of HF.

To determine the limiting reagent, we need to calculate the amount of each reactant needed to etch 19.4 kg of silicon per hour. The molar mass and specific gravity of each acid is provided in the problem, so we can use this information to calculate the amount of each reactant needed:

Amount of HNO3 = (19.4 kg Si/hr) x (1 mol Si/28.09 g Si) x (4 mol HNO3/3 mol Si) x (63.01 g HNO3/mol) =