Photoresist ashing, stripping, and descum. Organic Contamination Removal for Silicon Wafer

Typical applications

  • Dry ashing of photoresist
  • Descum after patterning
  • Clean organic contamination on wafer
  • Increase surface hydrophilicity for MEMS device
  • Improve bonding strength for wire bonding step

Dry photoresist ashing, stripping, and descum use oxygen plasma to generate radical oxygen species to chemically remove the photoresist layer on the silicon wafer. The byproducts of oxygen plasma ashing are not toxic. It’s more environmentally friendly than the wet etching process. Energetic electrons inside the plasma can break down oxygen molecules and generate reactive oxygen atoms. Oxygen radicals can then oxidize the photoresist and generate high vapor pressure by-products CO, CO2, and H2O. Adding a small amount of CF4 or SF6 gas can significantly increase the photoresist etching rate because highly reactive fluorine atoms can boost the rate of extracting hydrogen from the photoresist polymer.

Oxygen plasma ashing of photoresist

Photoresist ashing using oxygen plasma in the Tergeo-plus plasma system.

Photoresist descum is an additional gentle photoresist etching process after photoresist patterning and development. It is used to remove the residual photoresist scum left on the developed area. Such residual can prevent subsequent dry or wet etching steps and affect the uniformity of the etching rate across the wafer. Descum step can also improve the sidewall profile of the photoresist mask and improve the process uniformity.

Photoresist descum using oxygen plasma
Resist residual after patterning and development

For MEMS devices fabricated on the silicon or quartz wafer, organic contaminations on the wafer surface can increase the water contact angle and make the surface hydrophobic. Oxygen plasma treatment can easily remove the organic contamination and make the surface hydrophilic. It’s also important to remove the organic contaminant on the bonding pads before wire bonding steps.

Silicon wafer surface before plasma cleaning
Silicon wafer surface after plasma cleaning

Oxygen plasma cleaning can remove organic contamination on the wafer surface and increase surface wettability

In traditional direct mode plasma etching, sample surfaces are subject to both chemical reactions with neutral radicals and physical bombardment by energetic ions. For devices with thin coatings or highly sensitive to surface charge, UV radiation, or ion bombardment, direct mode plasma cleaning may cause irreversible damage to the device. For such a device, gentle downstream mode plasma cleaning is recommended.

Related products

Oxygen plasma system for photoresist descum
Oxygen plasma asher for photoresist

Tergeo and Tergeo-plus tabletop plasma asher

Advantage of Tergeo plasma cleaner over competitors

  • Better plasma uniformity. PIE Scientific has carried out extensive research to improve the uniformity of the plasma generator. It’s extremely difficult to achieve a uniform rf electric field with an external inductor coil type antenna that is used by some cheap plasma cleaner. If a metal rf electrode is placed inside the plasma chamber, plasma will be stronger close to the high voltage rf electrode, especially at the edge of the electrode. Tergeo plasma cleaner uses two symmetric external electrode designs to achieve much better plasma uniformity over other types of plasma cleaner designs.
  • Immersion and ion-free downstream cleaning in one system Tergeo plasma cleaner integrate two plasma sources in one system. The direct plasma source is used for traditional immersion mode plasma cleaning, e.g. samples are immersed in the plasma. In this processing mode, samples are subject to chemical reaction with oxygen or fluorine radicals and physical ion sputtering bombardment. If the device has a very thin coating, ion sputtering may damage the sensitive coatings. If the device is very sensitive to surface charge, UV photons, the direct mode may damage the sensitive IC device. In this case, downstream plasma cleaning is recommended. In this mode, plasma is generated in a separate remote plasma source, only neutral oxygen and fluorine radicals can diffuse into the sample chamber and react with photoresist and organic contaminants.
  • Advanced process control technology. Plasma uniformity and strength can change with chamber pressure. Tergeo plasma cleaner uses advanced corrosion-resistant pressure sensor technology for pressure monitoring. In addition, PIE Scientific developed a unique plasma sensor technology that can quantitatively monitor the plasma strength in real-time. Quantitative data is the key to achieve repeatable and consistent results from day to day.
  • Fully automatic operation. In some basic plasma systems, the user needs to manually adjust the needle valve to introduce gas into the plasma chamber. It’s almost impossible to achieve a repeatable gas flow rate with manual coarse needle valves without any feedback. Tergeo plasma cleaner uses fully automatic mass flow controllers (MFCs) to precisely regulate gas flow rate from 0 to 100sccm with accuracy better than 0.2 sccm. Fully automatic operation with multiple recipe support also ensures repeatable performance from day to day because the execution of the cleaning process is controlled by embedded microcomputers.