Dry Substrate Surface Cleaning

"Dry" substrate cleaning refers to processes in which reactive, unstable ions and/or radicals are created in a plasma, directed to a substrate surface and there allowed to react with contaminants on the surface to produce volatile products that can be pumped away. In this way, contaminants such as organic material can be removed from the substrate surface.

The most common application for dry substrate cleaning is photoresist stripping and "descum" steps. Photoresist stripping involves the removal of photoresist residues by reaction with atomic oxygen radicals or some other highly reactive species to produce a volatile product that can be pumped away by the vacuum system. Photoresist descum is a similar but milder process carried out after photoresist patterning and development to remove residual photoresist left in a developed area. Such residues can negatively affect the uniformity in subsequent dry or wet etching steps. A descum step can also improve sidewall profiles.

Components in a photoresist strip tool
Figure 1. Components in a photoresist strip tool.

A representative chemical reaction sequence for the generation of oxygen radicals in a photoresist stripping process is shown in Figure 1 and in the chemical equation below. In this process, molecular oxygen gas is fed into a plasma chamber in which a microwave plasma is present. The plasma is generated under high vacuum conditions to ensure that the electrons, ions and other reactive species within it have a relatively long mean free path and thus long lifetimes. Highly energetic free electrons, present in the plasma, collide with the incoming, low energy oxygen molecules. If enough energy is transferred in a collision or if enough additive transfers occur, the internal energy of the oxygen molecule is raised to the point where the molecule is no longer stable and splits apart into two atomic oxygen radicals:

O2 + e- → O2 + e- → O2•• + e- → O•↗ O

Oxygen radicals are oxygen atoms with an unpaired electron. These extremely reactive species are long-lived under the material and vacuum conditions present in the system. They are pumped into the process chamber where they impinge on the heated surface of the substrate. There they react with organic hydrocarbon residues such as photoresists to generate carbon oxides and water. These volatile oxides are then pumped away by the vacuum system. Other chemistries may also be employed in cleaning processes, depending on the nature of the contaminants that need to be removed from the substrate surface.

The increase in etch and deposition steps, new materials, and new structures used in 2.5D and 3D packaging have made cleaning processes like photoresist strip and descum increasingly important for high device yield. The varying levels of cleanliness requirements and the different materials employed in the manufacturing process have made the availability of multiple cleaning options in a product line increasingly important to high yield.

Surface activation, an important process tied to cleaning, prepares the surface for the next process step ensuring good quality adhesion resulting in high quality die. This can also be accomplished using plasma techniques. Acceptable cleaning processes for semiconductor applications must achieve the following challenging objectives:

  • Contamination free surfaces for highly sensitive devices
  • Surface preparation and activation for better adhesion
  • High throughput, ensuring minimal cycle time for higher overall productivity
  • Ecologically friendly alternatives to concentrated sulfuric acid wet clean or others

Dry Substrate Surface Cleaning Products

MKS offers RF and microwave plasma alternatives to wet substrate surface cleaning. These alternatives are compatible with multiple process gases, ensuring the best clean based on material chemistries. Our RF and Microwave Plasma products are an economical and environmentally sustainable alternative to wet cleans, avoiding the use of acids and solvents that require special storage and disposal.

The R*evolution® is an integrated remote plasma source that provides extremely clean reactive gas species for surface cleaning applications. It integrates a quartz vacuum chamber, an RF power supply and all necessary controls into a compact, self-contained unit easily installed on a tool's process chamber. The extreme cleanliness of the reactive gas supplied by the R*evolution® is ensured by its advanced design that employs a low-field toroidal RF plasma source. RF power is inductively coupled through ferrite cores into plasma that is confined within a toroidal quartz chamber. The source operates efficiently over an extremely wide range of gas flows and pressures, and generates reactive species in O2, N2, H2, H2/N2, and H2/He gasses. The use of quartz as the material of construction of the plasma chamber significantly reduces losses of atomic gases such as O, H, and N through wall recombination. Surface recombination rates of atomic gases on quartz are 100-1000 times lower than on most metals and dielectrics resulting in higher reactive gas output for the R*evolution.

Toroidal RF sources are uniquely suited for high throughput photoresist stripping in advanced device fabrication. Their design produces high plasma densities and high concentrations of reactive species while the low electric fields result in minimal ion bombardment of the chamber walls and excellent gas stream purity. ICP mass spectrometry (ICP-MS) and x-ray photoelectron spectroscopy (XPS) analysis of wafers processed in a photoresist strip tool using a remote toroidal RF source under conditions producing strip rates 2-3 times greater than in the same system equipped with a microwave source show extremely low (~1 x 1010 cm-2) particulate, quartz and metal contamination.

The AX7610 Downstream Plasma Source is a general duty microwave plasma source for use in remote plasma applications such as photoresist strip and passivation, surface modification, chamber cleaning and reactive gas generation. It can be configured with either a quartz tube for cleaning applications requiring atomic oxygen, hydrogen and nitrogen or a sapphire tube compatible with more corrosive reactive gas generation from species such as CF4 and NF3. The AX7610 is designed to be integrated into a microwave plasma subsystem comprised of power supplies, microwave magnetron heads, matching systems, etc.

Related Topics

Wafer Cleaning

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