Virtual CVD Project
Initial Problem Statement
Chemistry
In this project you and a partner are given a task that mimics the types of problems and activities found in today’s
modern semiconductor industry. One of the central issues for this industry is developing and maintaining the ability
to accurately, reliably and efficiently deposit layers of material upon silicon wafers. Low pressure chemical vapor
deposition (LPCVD) is the primary means of depositing thin layers of materials on surfaces.
For this project you are to imagine working for a large semiconductor manufacturer. You and your partner have
been hired to help the company understand how the various operating parameters impact the deposition of silicon
nitride (Si3N4) upon silicon wafers within a virtual LPCVD manufacturing lab. Your objective is two-fold. First,
you are to determine how selected operating parameters impact the deposition of Si3N4. Secondly, you are
to minimize the cost of the testing process you use in determining the impact of these parameters.
The Chemical Engineering Department of Oregon State University has developed a virtual manufacturing lab that
closely models the actual LPCVD process used by many semiconductor manufacturers. In using this program you
will be able to adjust the chemical and physical parameters at which the LPCVD equipment operates and, in so
doing, impact the deposition of Si3N4 upon wafers. Additionally, once each test batch of wafers are made, you must
decide where and how to measure the deposition thickness. This too costs money. As you may begin to see,
understanding the effects of each operating parameters and minimizing testing costs are competing outcomes. To
fully achieve one is to compromise the other. This, in a nutshell, is much of the engineering profession – finding an
appropriate tradeoff between two opposing goals.
At this point you likely know little if anything about semiconductors, Si3N4, or the LPCVD process. This will change
over the next several days. During this time you and your partner will do the following:
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Complete the questions listed on this project description (below).
Listen to a presentation describing the specifics associated with the deposition of Si3N4 upon wafers within the
semiconductor industry, and how the Virtual LPCVD Lab works. (one half period).
Complete in-class activities intended to familiarize you with the virtual manufacturing lab program.
Use the virtual LPCVD program to investigate the parameters of temperature, chemical flow rate, and reaction
time and determine the impact of each upon Si3N4 deposition. (Two full periods).
Remember, you are now in the private sector. Cost is everything! Certainly you want to understand how the
operating parameters impact the deposition of Si3N4. However, as in the real world, EVERYTHING you do in the
virtual lab cost money. Finding the proper balance between these two things is your job!!
STOICHIOMETRY, REACTION DYNAMICS AND RE-DOX IN THE CVD
PROCESS
In general, the goal of the CVD process is to deposit a uniform layer of silicon nitride (Si3N4) onto wafer
surfaces within the furnace. Silicon nitride is useful for many reasons. For instance, because silicon
nitride prevents oxidation, it can be used to mask off specific parts of a wafer allowing the construction of
very small but complex integrated circuits. In the CVD process used when making integrated circuits,
dichlorosilane (SiH2Cl2, also abbreviated as DCS) and ammonia gas (NH3) are injected into a furnace
containing uncoated wafers. These two chemicals react and produce the desired silicon nitride (Si3N4)
that gets deposited upon the surfaces of each wafer. Unfortunately, hydrochloric acid (HCl) and
hydrogen gas (H2) are also produced in this reaction.
1. Write a balanced chemical equation for the reaction between dichlorosilane and ammonia gas that
forms silicon nitride, hydrochloric acid, and hydrogen gas.
2. If 8.342 grams of silicon nitride are required
a) How many grams of dichlorosilane are needed?
b) How many grams of ammonia gas are needed?
c) How many liters of ammonia gas at STP is this?
Unfortunately, as this first reaction takes place, a second reaction begins immediately between the
hydrochloric acid and ammonia gas. In this second reaction, ammonium chloride is produced. In other
words, some of the ammonia gas in your balanced chemical equation you determined in #1 above gets
consumed before forming the silicon nitride that you want. We will have to compensate for this.
3. Write a balanced chemical equation for this secondary reaction between all of the hydrochloric acid
produced from the first reaction and any additional ammonia gas that is needed.
4. If all of the hydrochloric acid produced from the first reaction also reacts with ammonia gas, how
many additional grams of ammonia gas will be required to still produce the 8.342 grams of silicon
nitride?
5. Combine the two reactions above to make a net balanced chemical reaction that accounts for the fact
that the HCl produced in the first reaction combines with ammonia as described in the second
reaction. (Hint: add the two reactions together and cancel like terms that show up on both the
reactants and products side of the equation).
6. Now the challenge. If 14.251 grams of dichlorosilane and 25.888 grams of ammonia gas are placed
in the furnace at the same time,
a) Which reactant runs out first?
b) How much (grams) silicon nitride can be produced?
7. In the combined reaction you found in #5 above, determine the following. (Hint: you may need to
refer to a periodic table to determine the oxidation number for Si AFTER you go through the rules for
assigning oxidation numbers.)
a) The element that is oxidized.
b) The element that is reduced.
c) The reducing agent.
d) The oxidizing agent.
Answers:
1. 3 SiH2Cl2 + 4 NH3 → Si3N4 + 6 HCl + 6 H2
2a. 18.02 g DCS
2b. 4.051 g NH3
2c. 5.328 L NH3
3. 6 HCL + 6 NH3 → 6 NH4Cl
4. 6.076 g NH3
5. 3SiH2Cl2 + 10NH3 → Si3N4 + 6NH4Cl + 6H2
6a. DCS runs out first.
6b. 6.597 g Si3N4
7a. Si is oxidized (from 0 to +4).
7b. H is reduced (from +1 to 0).
7c. Si is the reducing agent.
7d. H is the oxidizing agent.