Plasma Mode Selection and Electrode Installation

Process Solutions, Equipment and Services YES-G1000 Recommendations for Shelf Installation

The YES-G1000 uses capacitive plasma generation. Plasma is generated between pairs of planar electrodes that are electrically “active” or are electrically grounded. Active electrodes are at the potential of the RF power that is supplied to the tool. RF power for the YES-G1000 is 550VAC at 40kHz.

The YES-G1000 offers many different modes of plasma exposure. Plasma exposure modes are different ways of controlling substrate potential during plasma exposure, which affects the energy level of electrons and ions that come in contact with the substrate, and different ways of filtering or conditioning the plasma before it gets to the substrate. On the YES-G1000, the tool can be changed to different plasma modes by changing the sequence of electrode shelves installed in the tool.

Plasma Composition Plasma is generated in an electric field by removing an electron from a gas molecule leaving a positively charged gas ion. In an oxygen plasma, the O₂ molecule is split into monatomic oxygen and then into O+ ions. The O+ ions, over a short time period, spontaneously recombine with electrons to form neutral monatomic oxygen radicals and then recombine again to reform O₂.

An O₂ plasma is a dynamic equilibrium of O₂ molecules, O radicals, O+ ions, and free electrons. Charged ions and electrons are accelerated by electrical fields while neutral O radicals and O₂ molecules move by diffusion and bulk flow.

Plasma Surface Charging In an unfiltered plasma, ions and electrons are accelerated to high velocities by the electric field. Because electrons have a much lower mass than ions, they are accelerated to much higher velocities. Because of the higher velocities, electron collisions occur much more frequently in the plasma than collisions with other particles.

A substrate exposed to the plasma will initially experience a higher rate of electron impacts than ion impacts. Electrons can charge the substrate until the electric field created by the charge repels excess electrons and attracts positive ions and balances the collision rates. A substrate in a plasma will normally charge to a small negative potential called the plasma potential.

Ion Impact Energy Ions and electrons approaching an active or grounded electrode or substrate are accelerated by the electric field around the electrode. The electric field is proportional to the difference in potential between the plasma and the electrode. Active electrodes have a very large potential difference relative to the plasma potential; grounded electrodes have only a small potential difference. Ions and electrons that collide with the active electrode or with a substrate at active potential tend to have higher energies than charged particles that collide with a grounded electrode or substrate.

YES-G1000 Recommendations for Shelf Installation

Isotropic or Anisotropic Etch Ions accelerated to higher speeds by high electrode potential differences tend to have velocity vectors that are perpendicular to the electrode surface. If the surface is masked, the perpendicular motion of the ions will result in an uneven, or anisotropic, etch rate. Materials will etch faster in the direction perpendicular to the surface than they will parallel to the surface. When ions have low or random approach velocity vectors, the etch is more isotropic with the same etch rate in all directions.

Ablative or Reactive Plasma Cleaning Plasma can remove materials from a substrate surface by two different physical processes, ablation or chemical reaction. An ablative plasma uses a non-reactive gas like argon. Argon ions are accelerated by the electric field and strike the substrate surface at high velocities. Surface material is removed by being physically knocked off. Ablative plasma cleaning is more effective when ion concentrations and ion energies are high. Ablative etch rates are generally lower than chemically reactive etch rates. Ablative plasma cleaning is usually used when the material to be removed from the substrate surface is chemically non-reactive.

Reactive plasma uses a chemically reactive gas (oxygen is the most common) to remove material by a chemical reaction. For hydrocarbon materials like photoresist, the oxygen reacts with the material to form water vapor from the hydrogen and carbon dioxide from the carbon. Chemical reaction product gases are pumped out via the chamber vacuum exhaust. Reactive plasma cleaning is more effective when concentrations of reactive species are high. For oxygen plasma, the most reactive components of the plasma are the monatomic oxygen radicals. Particle energies need to be high enough to be above reaction threshold activation energy but do not need to be as high as in an ablative plasma. Reactive plasma etch rates can be much higher than those for ablative plasma. Reactive plasma is usually used to remove hydrocarbon contamination from substrate surfaces.

Plasma Ion Trap A plasma ion trap is a region between two electrodes at the same potential where there is no electric field. Plasma generation regions on both sides of an ion trap drive ions into the trap through the electrode perforations. Once in the trap, ions tend to remain because there are no electric fields to move them out. Ion trap plasmas have very high concentrations of ion recombination products.

www.yieldengineering.com

YES-G1000 Recommendations for Shelf Installation

Downstream Plasma Mode

In downstream plasma mode, the substrate is located on a floating tray outside of the plasma generation zone. The plasma is “filtered” by passing through the perforated ground electrode tray which greatly reduces or eliminates surface charging. Ion impact energies are low. Etch will be predominantly isotropic.

Downstream mode is usually used when the substrate is sensitive to surface charging. Downstream mode is less effective for both ablative and reactive plasma cleaning than active mode; plasma “filtering” reduces ion concentrations

Active Plasma Mode

In the active plasma mode, the substrate is located on a grounded electrode tray inside the plasma generation region. Ion concentrations are high. Surface charging will be reduced for conductive substrates that are in contact with the grounded shelf. Ion impact energies are moderate. Etch will be predominantly isotropic with etch rates a little higher perpendicular to the tray surface.

Active mode is moderately effective for both ablative and reactive plasma cleaning.

www.yieldengineering.com

YES-G1000 Recommendations for Shelf Installation

RIE Plasma Mode

RIE stands for Reactive Ion Etch. In the RIE mode the substrate is located on the active electrode tray inside the plasma generation region. Ion concentrations are high. Surface charging will be high. Ion impact energies are high. Etch will be strongly anisotropic with higher etch rates perpendicular to the tray surface.

RIE mode is very effective for ablative plasma cleaning but has low etch rates for reactive plasma cleaning.

Active Ion Trap Plasma Mode

In the active ion trap mode, the substrate is located on an active electrode shelf between two plasma generation regions. The ion trap is created by arranging the two active shelves adjacent to one another. Surface charging is probably moderate. Ion impact energies should be high for a conductive substrate. Recombination product concentrations will be high. Etch should be moderately anisotropic with higher etch rates perpendicular to the electrode surface.

Active ion trap has the highest reactive etch rates but is not very effective for ablative cleaning.

www.yieldengineering.com

YES-G1000 Recommendations for Shelf Installation

Grounded Ion Trap Plasma Mode

In the grounded ion trap mode, the substrate is located on a grounded electrode shelf between two plasma generation regions. The ion trap is created by arranging the two grounded shelves adjacent. Surface charging is low because the plasma is “filtered”. Ion impact energies should be low. Recombination product concentrations will be high. Etch should be isotropic.

Grounded ion trap has very high reactive etch rates but is not very effective for ablative cleaning.

For more information call: 1-510-954-6889 (worldwide) or 1-888-YES-3637 (US toll free)