SO3 Cleaning

Consulting

The Cleaning Problem
Complete removal of photoresist which has been exposed to high-dose, ion implant in excess of 1x10¹⁵ atoms/cm² is usually a problem for conventional stripping and cleaning methods such as plasma ashing.  The high-dose ion implant treatment results in the formation of a tough, carbonized crust which protects the underlying bulk photoresist from the cleaning process.

Conventional methods of cleaning require an oxygen-plasma ash, often in combination with halogen gases, to penetrate the crust and remove the photoresist.  Usually, the plasma ashing process also requires a follow-up cleaning with wet-chemicals and acids to remove the residues and non-volatile contaminants that remain after ashing.  Despite this treatment, it is not unusual to repeat the "ash plus wet-clean" cycle in order to completely remove all photoresist and residues.

Some of the problems that arise from using from these conventional processes include:

• Popping of the photoresist (and the resulting contamination) as heated, residual solvent in the bulk photoresist vaporizes under the hardened crust.
• Gate oxide erosion and line-lifting from the use of halogen gases during cleaning.
• Residual metal contamination due to the presence of non-volatile metal compounds in the photoresist which are not removed by the plasma ashing process.
• Tough residues remaining despite the use of plasma ashing and wet chemical treatments.
• Repetitive cleaning steps which increase photoresist stripping cycle times and work-in-process.

The Sulfur Trioxide Process Answer
Important features of the SO3 process distinguish it from conventional cleaning methods and offer significant improvements in cleaning capability.  For example:

• There is no resist popping, since all processing takes place below 120ºC.
• Non-volatile, residual metal compounds are flushed away with the photoresist.
• No post-ash, wet-clean is required for complete, residue-free, photoresist cleaning.
• There is no damage to gate oxides or line-lifting since no halogen gases are used.
• Multiple cleaning repeats are not required, thus substantially reducing photoresist stripping cycle times and work-in-process inventories.

This simple gas process is also effective and efficient across the usual range of typical semiconductor stripping and cleaning applications, making it possible to replace conventional cleaning tool sets (plasma asher plus a follow-up wet-clean tool) with a single SO3 tool. 
 

Capability Demonstration
The effectiveness of the process has been demonstrated on wafers covered with patterned photoresist that were first exposed to high-dose, ion implant (phosphorus, 1x10¹⁶ atoms/cm², 50 keV energy).  Figure 1 illustrates the condition of these wafers after implant, but before cleaning.  Note the hardened photoresist, heavily damaged by the ion implant process. 

Figure 1  -  BEFORE stripping high-dose, ion-implanted photoresist    (phosphorus, e16, 50 keV). 

After a short, low-temperature, partial surface removal, using a modified plasma process (with a duration measured in seconds) and followed by a sulfur trioxide exposure, all photoresist and residues are completely flushed away using only DI-water in one process cycle as illustrated in Figure 2 below.