Optimization of Plasma Barrel Etching for Microprocessors

By Acacia Caraballo and Paul Filichkin

Basic Processes of Microprocessor ProductionOne step in the microprocessor production process is the etching away of layers of silicon dioxide from the surface of a silicon wafer.

The surface of the wafer is first coated with photo resist and then exposed to ultra-violet light with a mask covering certain areas.

The ultra-violet rays take off the photo resist and the photo resist that is left works as a protective barrier against plasma etching.

The wafer is then placed inside of a plasma barrel etcher, which carves away the silicon dioxide.

Etching silicon dioxide requires CF4 gas, but due to safety reasons these experiments were conducted with O2 and polyethylene coupons instead of silicone wafers.

What is a plasma barrel etcher? A plasma barrel etcher is a device used in the making of microprocessors. It is used for taking away very small and precise layers in a specific area of the microprocessor.

What is plasma? Plasma is a chemically reactive gas. It is made up of ions, electrons, free radicals, excited gas molecules, and gas molecules.

Why does plasma glow? Plasma glows because electrons are moving from a higher energy level to a lower energy level, releasing an energy wave in the form of light.

Background

ObjectivesThe purpose of this lab was to become familiar with plasma and to observe the effects of pressure and power on the removal of a polymer substrate in a plasma reactor. To optimize the process of plasma etching to minimize cost and waste generation.

Equation

PV = nRT Concentration CO2 = n/V = P/RT

P (Pressure), V (Volume), R (Gas Constant), T (Temperature), n (Number of Moles of O2)

Process Conditions Sample Weights Process Results

Power (watts

)Pressure (torr)

Initial Weight (grams)

Final Weight (grams)

Weight Loss

(grams)

Average Temperature

(Celsius)

200 0.207 2.0677 2.062 0.0057 31

200 0.219 2.0114 2.003 0.0084 47.5

200 0.502 2.1005 2.085 0.0155 31

200 0.516 2.0553 2.0404 0.0149 50

200 0.797 1.8506 1.8386 0.012 48

200 0.820 1.9065 1.8985 0.008 51

400 0.215 2.1114 2.1023 0.0091 38.5

400 0.237 2.0047 1.9919 0.0128 39.25

400 0.506 2.0065 1.9924 0.0141 36.25

400 0.516 1.9775 1.9585 0.019 44.5

400 0.805 1.935 1.9226 0.0124 40.75

400 0.801 1.9985 1.9886 0.0099 42.5

Pressure vs. Weight Loss at 200 and 400 Watts in the Plasma Etcher

0

0.005

0.01

0.015

0.02

0.2 0.4 0.6 0.8

Pressure (Torr)

Wei

gh

t L

oss

(G

ram

s)

200 Watts

400 Watts

Analysis Our tests found that the optimum removal rate is around 0.5 Torr (0.025 mol O2/m3) at 400 Watts.

The reason the reaction rate is smaller at 0.2 Torr (0.01 mol O2 /m3) is because there is less O2 in the

Plasma Etcher therefore there are less free radicals to react with the polyethylene.

The reason the reaction rate is smaller at 0.8 Torr (.04 mol O2 /m3) is because there are too many

molecules which collide and reduce the mean free path (the average molecular travel distance before a collision) thus reducing the number of free radicals

that react with the polyethylene.

This experiment also uncovered that doubling the power from 200 -400 Watts did not double the

removal of polyethylene.

After finding the results of the three pressure settings we can see the general trend but for future work it is important to find the values of the points in between. That way we can find a true peak and be able fully optimize this process.

Also on two of our test the polyethylene melted and gave us faulty data. It is important to investigate the effects of temperature on the reaction in the plasma barrel etcher.

Future Work

This controls the gas flow rate, thus allowing us to control the reactor pressure.

Gas Rotometer:

This is the box that controls the valves into the etcher.

Pneumatic Gas Control Box:

This controls the energy that flows into the reactor.

Power Source:

This is where the reaction takes place. It goes down to extremely low pressures and has electrodes that enable the creation of the plasma.

Reactor Vessel: