RF Plasma Sources and How to Use Helicons Francis F. Chen Professor Emeritus, UCLA Semes Co., Ltd., Chungnam, Korea, February 15, 2012

Plasma is necessary for etching UCLA SHEATH

Three kinds of RF discharges UCLA Capacitively Coupled Plasmas (CCPs), formerly called Reactive Ion Etchers (RIEs) Inductively Coupled Plasmas (ICPs) Helicon Wave Sources (HWS)

UCLA Capacitive Discharges (CCPs)

UCLA Schematic of a capacitive discharge

UCLA Sheaths keep a plasma neutral UCLA

Sheaths are very thin UCLA numerically, Let Then Debye sheaths are approximately 5 D thick

UCLA The Child-Langmuir Law UCLA The Debye (normal) sheath has a voltage drop of about 5KT e. If additional voltage is applied, a CL sheath forms with only ions. ions and electrons ions only quasi-neutral +  +++  d  V 3/4

Most of the volume is sheath UCLA Electrons are emitted by secondary emission (  and  modes) Ionization mean free path is shorter than sheath thickness (  ) Ionization occurs in sheath, and electrons are accelerated into the plasma (gamma mode) Why there is less oxide damage is not yet known

Effect of frequency on plasma density profiles MHz 27 MHz 40 MHz 60 MHz

Problems with the original CCP discharges UCLA The electrodes have to be inside the vacuum Changing the power changes both the density and the sheath drop Particulates tend to form and be trapped Densities are low relative to the power used In general, too few knobs to turn to control the ion and electron distributions and the plasma uniformity

Ion velocity distribution can be adjusted by applying low frequencies to substrate UCLA Thin gap. Unequal areas to increase sheath drop on wafer High frequency controls plasma density Low frequency controls ion motions and sheath drop

A LAM Exelan oxide etcher

CCPs are great for large gas feed UCLA Fast and uniform gas feed for depositing amorphous silicon on very large glass substrates for displays (Applied Komatsu)

UCLA Inductively Coupled Plasmas (ICPs)

The antenna can be on top or on the side TCP (Transformer Coupled Plasma) PlasmaTherm etcher

Or it can be a combination UCLA Applied Materials patent

UCLA RF B-field pattern comparison Lam type AMAT type

Simulation of the plasma in an ICP UCLA

Early AMAT system UCLA

The electrostatic chuck is an essential part UCLA

How can the RF energy get inside? The Plasma-Therm etcher

The density is high where RF is low! UCLA

Explanation 1: electron path with Lorentz force UCLA

Electrons spend more time near center UCLA

Explanation 2: Sheaths at endplates UCLA The Simon short-circuit effect at endplates causes an electric field to develop that drives the ions inwards, and the electrons can “follow” even across magnetic fields.

UCLA Disadvantages of stove-top antennas Skin depth limits RF field penetration. Density falls rapidly away from antenna If wafer is close to antenna, its coil structure is seen Large coils have transmission line effects Capacitive coupling at high-voltage ends of antenna Less than optimal use of RF energy

UCLA Proposed enhancements of ICPs

UCLA Coupling can be improved with a magnetic cover UCLA   H = J B =  H

UCLA The ferrite is inside the vacuum Meziani, Colpo, and Rossi, Plasma Sources Science and Technology 10, 276 (2001)

Fluxtrol F improves both RF field and uniformity (Meziani et al.)

Magnets are used in Korea (G.Y. Yeom) SungKyunKwan Univ. Korea

Both RF field and density are increased SungKyunKwan Univ. Korea

Serpentine antennas (suggested by Lieberman) Magnets

Density uniformity in two directions G.Y. Yeom, SKK Univ., Korea

Effect of wire spacing on density (calc.) Park, Cho, Lee, Lee, and Yeom, IEEE Trans. Plasma Sci. 31, 628 (2003)

UCLA Helicon Wave Sources (HWS)

In helicon sources, an antenna launches waves in a dc magnetic field The RF field of these helical waves ionizes the gas. The ionization efficiency is much higher than in ICPs.

Why are helicon discharges such efficient ionizers? The helicon wave couples to an edge cyclotron mode, which is rapidly absorbed.

A helicon discharge at Wisconsin UCLA

A commercial helicon etcher (PMT MØRI) It required two heavy electromagnets with opposite currents.

UCLA Replace heavy electromagnet with small permanent magnet and Design of an array source with small tubes

A long antenna requires a long tube, and plasma goes to wall before it gets out. An m = 0 loop antenna can generate plasma near the exit aperture. Note the “skirt” that minimizes eddy currents in the flange. Antenna must be short so that a short tube can be used UCLA

The low-field peak: constructive interference R is the plasma resistance, which determines the RF power absorbed by the plasma, The tube length is designed to maximize rf energy absorption

Final design of the discharge tube UCLA

Use the far-field of the magnet NdFeB material, 3”x 5”x1” thick B max = 12 kG

The magnet position sets the B-field magnitude UCLA

A quartz tube with 3-turn antenna UCLA

An 8-tube staggered array in operation UCLA

The magnet tray UCLA The magnets are dangerous!

Array fed by a 50  water-cooled transmission line UCLA

Density profiles along the chamber Staggered configuration, 2kW Bottom probe array

UCLA Density profiles along the chamber Compact configuration, 3kW Bottom probe array Data by Humberto Torreblanca, Ph.D. thesis, UCLA, 2008.

Conclusion UCLA Helicon sources can produce high densities over large areas. Permanent-magnet helicon arrays are cheap and compact. Their design was made possible by extensive theory. To penetrate the industry, other gases must be tried.

UCLA THE END Thank you