Final Project: Low Pressure CCP Etcher

ENGR 4390-003 Plasma Engineering

Advisor: Dr. Magesh

Submitted by: Shawn Robinson

Date: 5/14/14

Abstract

Plasma and plasma design configurations are a new frontier is science due to their unique physical and chemical properties, especially when it comes to their interaction with other material surfaces. Due to recent advances in technology such as electrical component innovations, sensor and detector designs, and better understanding in relevant physics concepts, we can better create and diagnose different plasmas, and test theories as to their application results. Plasma is an excited gas mixture, that has received enough energy, whether mechanical, thermal, or electrical, to reach gas breakdown. This process ionizes a small portion of the gas, meaning that electrons have been stripped from otherwise inert atoms of gas, and have become ions. This mixture consists of Ions, excited electrons, released photons, reactive species, and neutral atoms. For this project we use plasma for the purposes of etching, where layers of materials will be taken away by the plasma mixture. This project will consist of design, construction, diagnostics, and testing/optimization of plasma etch chamber.

Objective

Plasma etching is a relatively new method for used for the manufacturing of integrated circuits. In the 70’s plasma was used to strip resistors but in the 80’s further study into plasma resulted in the ability to add layers which gave way to better chip manufacturing and the modern day integrated circuit. . First a semiconductor material, usually Si based, is grown with perfect crystalline structure, into a column. This is then cut into disk with thicknesses ranging from 500-900um depending on the diameter which come in sizes; 150mm, 200mm, 300mm, and 450mm. This disk is then carefully transferred to a sealed container between two electrodes. These electrodes usually have a dialectic barrier on them to help the plasma form uniformly and protect the electrode. The chamber is then pumped down to < 10.Torr for processing, or < 0.1mTorr for industry grade processing. Then the gas of choice is pumped into the chamber. The gas is chosen by its electronegativity, because that the DC voltage is very dependent on this property if using DC power supply. For plasma etching Gases with low electronegativity, such as O2, N2 are used, but for more precise industrial processing gasses containing Fluorine, chlorine and bromine are used because atoms of group VII are very prone to absorb any free electron which passes nearby. This reduces the electron density an increases the reactive ion density. The reactive ions “eat” away at the disk in specified patterns. This process of etching is known as reactive ion etching or dry etching. The objective of this project is to design a plasma etch source that holds vacuum, and creates a stable plasma that contains etch species.

Initial Design

For my initial design I went with simplified version of the basic CCP etch reactor. Today capacitive coupled RF plasmas are still the most common plasmas used in dry etching. In most cases RF power (13.56MHz) is supplied to the top, bottom or both electrodes. RF is used because between 1 and 100MHz, free electrons are easily influenced in the direction of the applied electric fields. In the initial design the top electrode was going to be made of perforated graphite, but was changed to an aluminum electrode. The bottom electrode was going to be titanium alloy but was changed to graphite disc electrode instead. The chamber itself is made of steel and the window is made from Lucite.

Final Design

The final design didn’t change much from the initial other than the electrode materials due to cost. The bottom electrode is now a graphite electrode encased in Lucite housing. The top electrode is now an aluminum electrode encased in Lucite. There is no longer a stage, but a manual screw to adjust the gap distance. There is an insulated stand to raise the bottom electrode.

The final design was able to withhold a 10 tor vacuum pressure. The operation voltage was 12KV and the gas used was atmospheric air.

Construction

For the majority of this semester, we have spent our time building our plasma devices. This process has tested and developed some of my hands on skills as a mechanical engineer. I have learned the process of how to take an idea from thought, design it on paper, then in AutoCAD, and then finally fabricate. During this I’ve learned that not all designs go according to first plan, and that it’s ok to make little changes along the way, as long as it doesn’t affect the overall quality. I’ve gotten a lot of practical skills and independent problem solving practice. In addition to these experiences I’ve gotten hands on use with power tools and engineering equipment. Here is a list of some of the main equipment used

-Lathe: Used to machine parts from bulk material

Parts made on lathe: Vacuum nipple (8+hrs), Top electrode (6hrs)

-Mill: Used mill to machine high tolerance features on parts

Parts made on mill: Bottom electrode assembly and housing (5hrs)

-Drill press: Used drill press to machine high tolerance holes

Parts: Bottom electrode assembly, top electrode housing, tap threads

-Soldering Iron: Used to connect electrical circuit parts to High voltage wire

Parts: bottom electrode, top electrode

-Band saw: used to cut bulk material to precise measurements for machining

Parts: glass window, top electrode housing, bottom electrode housing

General skills

Vacuum checking

Weld inspection

Correct caliper use

Troubleshooting

Parts list Steel chamber 12”x 9”

1- 3” aluminum disk electrode

2- 3” graphite electrode

9.5” Lucite window

6- 1” flange bolts

9.5” rubber gasket

3- 1” rubber plugs

Diagnostics

OES diagnostics techniques will be used to diagnose the plasma. If the chamber is a stainless steel chamber, then OES can be done through a quartz window using a laser or fiber optics. If it is Lucite container then sensor wafers or probes are available to perform OES. Things to look for in etch plasma are low electron density, low gas temperature, high gas density, and low high electron temperature. To the right is what is an expected spectrum of an oxygen-nitrogen etch plasma.

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Species Wavelength Lower transition Upper transitionO IV 588.245  2p2(3P)3p  2D°  5/2 2p2(3P)3d  2P  3/2

O II 765.6779  2s22p2(3P)3d  4D  1/2  2s22p2(3P)4p  4D°  1/2

O II 398.27141  2s22p2(3P)3s  2P  3/2 2s22p2(3P)3p  2P°O II 404.82163  2s22p2(3P)3d  4F  7/2 2s22p2(3P)4f F  2[3]°O IV 426.047  2s2p(1P°)3p  2D  3/2  2s2p(1P°)3d  2F°  5/2

Ar II 532.96992  3s23p4(3P2)4f  2[5]°  11/2 3s23p4(3P2)7g  2[6]  13/2

Plasma Spectrum

Wavelength (nm)

Spectrum 300-400nm Spectrum 400-500nm

Optimization/ Troubleshooting:

During the first demonstration, the plasma was unstable. This was due to leaks in the vacuum. The gasket had to be resized and the ports needed to be resealed with threaded nylon fittings. This improved the quality of the vacuum and was able to pump down to 10tor instead of the previous 30tor. Also the plasma would arc to the walls of the chamber, so to avoid this extra insulator was added to the top and bottom electrodes and the gap distance was adjusted. This kept the chamber grounded and the circuitry electrically isolated and safe.

Summary/ Future work

During the course, I learned the basics of non-thermal plasma, plasma generation techniques, and got a good handle on Optical emission spectroscopy. The final design worked well and it’s something I’m proud of. I got a lot of other diagnostic and trouble shooting skills that will be able to transfer over into other experiences. Future work would include creating a bolt on flange window, experimenting with different gasses, and playing with electromagnets to change plume shape and size. I would also like to try changing targets to find other species in the plasma.

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Spectrum 700-800nm

Spectrum 500-600nm

References

http://www.nist.gov/pml/data/asd.cfm

Wafer Net Inc. Worldwide Silicon & Semiconductor provider

Sillicon Valley Microelectronics, Inc

Parallel Synthesis Technologies, Inc.

The National Institute of Standards and Technology (NIST)

Verdonck, “Dry Etching for Integrated Circuit Fabrication”, in Processos de Microeletrônica, editorBaranauskas, 1990

DC PLASMA POTENTIAL PATTERN IN LOW-PRESSURE RF DISCHARGE; Lbvaky V.A., KraranilPov O.V. Kharkov University, Department of Physics and Techndogy, Scientific Center of Ph* and Technology

M. Kogelschatz, G. Cunge, O. Joubert, L. Vallier, and N. Sadeghi, Contrib.Plasma Phys. 44, 413 _2004_.

GaN etch enhancement in inductively coupled BCl3 plasma with the addition of N2 and SF6 gas Chang Seok Oh, Tai Hong Kim, Kee Young Lim and Jeon Wook Yang

Impact of Etcher Chamber Design on Plasma Induced Device Damage for Advanced Oxide Etching