The use of plasma is an effective way to clean without using hazardous solvents. Plasma is an ionized gas capable of conducting electricity and absorbing energy from an electrical supply. Manmade plasma is generally created in a low-pressure environment. (Lightning and the Aurora Borealis are naturally occurring examples of plasma.) When a gas absorbs electrical energy, its temperature increases causing the ions to vibrate faster and “scrub” a surface.
In semiconductor processing, plasma cleaning is commonly used to prepare a wafer surface prior to wire bonding. Removing contamination (flux) strengthens the bond adhesion, which helps extend device reliability and longevity.
In biomedical applications, plasma cleaning is useful for achieving compatibility between synthetic biomaterials and natural tissues. Surface modification minimizes adverse reactions such as inflammation, infection, and thrombosis formation.
- Wire bond surface preparation
- Removing contaminants (flux) or sterilizing a surface
- Promoting adhesion between two surfaces
- Controlling surface tension to achieve either a hydrophobic or hydrophilic surface
- Increasing biocompatibility
- Improving polymer performance through cross-linking to decrease friction that wears out devices
How Plasma Cleaning Works - Ion Excitation
When a gas absorbs electrical energy, its temperature increases causing the ions to vibrate faster. In an inert gas, such as argon, the excited ions can bombard a surface ("sandblast") and remove a small amount of material. In the case of an active gas, such as oxygen, ion bombardment as well as chemical reactions occur. As a result, organic compounds and residues volatilize and are removed.
Radio frequency (RF), microwaves, and alternating or direct current can energize gas plasma. Energetic species in gas plasma include ions, electrons, radicals, metastables, and photons in short-wave ultraviolet (UV) range. The energetic species bombard substrates resulting in an energy transfer from the plasma to the surface. Energy transfers are dissipated throughout the substrate through chemical and physical processes to attain a desirable surface modification – one that reacts with surface depths from several hundred angstroms to 10µm without changing the material's bulk properties.
History of Using 13.56 MHz
In the 1940s, coroners used diffusion tubes, also known as ashers, as forensics tools. Samples from a deceased body would be placed inside a quartz diffusion tube and brought to temperatures exceeding 1000 °C, and a spectrophotometer would be used to view the burning samples in a stream of oxygen. Viewing the samples at varying frequencies enabled coroners to measure light at the electron level and perform chemical analysis to determine whether poisoning had occurred. However, early diffusion tubes had a slow rise in temperature that allowed heavy metals to escape before they could be read correctly.
In these early years, ashers were made by industrial medical equipment manufacturers. Since the allowable frequency standard for building medical equipment was 13 – 14 MHz, this became the target range by default. Original ashers fell in one of two frequency ranges: 13.54 or 13.46 MHz. For two decades, ashers were sold solely to the medical industry, and their demand remained small and isolated.
In the 1960s, process engineers became interested in ashers. They needed an alternative, safe way to remove photoresist from 1- and 2-inch wafers. Until then, they'd been using a dangerous mix of sulfuric acid and hydrogen peroxide (known as "piranha") to eat away thick layers of resist. The problem was this exothermal mixture boiled and ate through anything organic, destroying processing equipment such as wooden etch benches and posing immediate harm to process engineers who came in contact with it.
For this reason, ashers were readily adopted for use in semiconductor fabrication. The tool, sometimes called a barrel resist stripper, inherited the legacy frequency standard for medical equipment established in the 1940s. However, there was no underlying physical reason to prefer 13.56 MHz to 12 MHz to 14.5 MHz (or even a lower 40 kHz frequency). Eventually, barrel resist strippers lost favor due to issues with slow heat processes and electron damage to wafers.
As research on plasma energy became more advanced, the optimum frequency was found to be more a function of plasma chamber design and the shape of the object to be cleaned. Today's commercial plasma equipment may run DC, 40 kHz, 13.54 MHz, or 2.54 GHz. As long as the plasma generator is designed to correlate with the appropriate frequency, there is no discernable difference in power level between any frequencies.
As a rule of thumb:
- DC to below 100 Hz is used for sputtering plasma, with electron guns providing the plasma energy
- 30 to 100 kHz is used for capacitive plasma generators, typically used for flat pieces
- 10 to 100 MHz is used for inductive plasma generators where a circular or tube is needed with the plasma being generated at the outside of the tube
- 2.54 GHz and above is microwave range and used for small chambers with power beamed in, typically single wafer resist strippers where a large amount of energy is needed in a small space, i.e. one circular wafer up to 300mm diameter
Benefits of Low Frequency
Plasma processing equipment commonly uses RF to generate gas plasma. A variety of parameters can affect the physical characteristics of plasma and subsequently affect the surface chemistry obtained by plasma modification. In order to achieve uniform, superior results, Yield Engineering Systems recommends low frequency plasma (40-50 kHz) over high frequency plasma (13.56 MHz or 2.54 GHz) for the following reasons:
- Higher Ion Density. Low frequency plasma provides more energy per square inch than high frequency cleaning. While this may seem counterintuitive, high frequency plasma cleaning systems actually lose considerable energy through heat loss. Energy loss with a 13.56 MHz system is up to 850 times greater than with a 40 kHz system.
- Increased Efficiency.The efficiency of a plasma system is the ratio of the energy used in producing the plasma vs. the energy dissipated in losses such as heat. A low frequency plasma system acts like a perfect capacitor with infinite capacitive impedance, or zero current drain when in standby mode. Current applied across the capacitive pair (electrodes) causes the gas to ionize, and the impedance is bridged causing current flow (plasma) between the electrodes.
- Better Uniformity. Low frequency systems have no shadowing, which occurs when samples on upper shelves form a mask that prevents plasma from reaching samples on the lower shelves.