Photoresist Removal after High Dose Implant (HDIS)

High Dose Implant Strip (HDIS) – Challengeable process in semiconductor manufacturing

Photoresist strip after high dose implant strip remains one of the most demanding processes in
semiconductor industry. The challenge is to remove both, the crust (or hardened layer formed during implant) and the soft bulk resist without burning the crust and leave carbonized residues which are very difficult to clean off. Controlled removal of hardened organic material and untouched resist at low temperature is key to cleanliness.

Microwave Plasma – Isotropic resist removal with least thermal load

Due to the nature of the RF-plasma, it exposes the substrate to a relative high intake of charged species (ions). These ions transfer kinetic energy when hitting the surface and cause the substrate to heat up. High temperature is very unfavorable to the post high dose implant strip as it will participate to further baking and hardening of the resist crust. Cooling of the substrate helps to prevent volume heating-up of the substrate but it cannot avoid local heating of the surface where the actual reaction takes place. Hence, even if the average temperature is kept low, the local temperature remains high
since the underlying reaction (ion-sputtering) is not affected by the cooling. Using Muegge’s microwave assisted plasma systems, the energy transfer is extremely low, no charged particles are formed which could be released into the process chamber. As a result the substrate remains cold, the only heat-source it is the chemical reaction of the exothermic resist
removal. As the reaction heat is a direct result of the resist-strip process, it can be easily controlled by the process conditions.

Muegge’s solution – accurate, reliable and repeatable

Microwave assisted plasma efficiently uses the energy to form highly reactive, neutral particles called radicals. No ions are formed which are the carriers of kinetic energy, causing the substrate to heat up. Therefore the MW-assisted remote

plasma system is ideal for post high dose implant strip where

temperature control is critical – no additional polymerization or carbonization occurs and the implanted resist can be removed smoothly.

  • The radicals released from the MW assisted plasma guarantee a smooth, pure chemical removal of the damaged resist. As a consequence, the thermal load is extremely low which guarantees isotropic resist removal without further carbonization of the damaged resist.

MW assisted plasma for flexible process control at low temperature

Figure 1: Applicator surface temperature heating and cooling over time at a microwave power of PMW=2000W.
Figure 2: Average relative etch rates of standard PR at constant MW power of PMW=3000W in presence or absence of CF4 for a sample temperature of T=150°C as well as switched off heating plate

Radical-only formation results in little to no interaction with the surrounding surface of the plasma chamber, the temperature remains low (Figure 1). The pure chemical HDIS-process can be fine-tuned
for the three steps of the removal process:

  • crust removal
  • bulk ash
  • Over-ash

No ions mean no need for additional cooling of the substrate.

Advantages of radical based post high dose implant strip

The three steps of high dose implant strip require appropriate flexibility to meet the different demands of each step (Figure 2). Microwave-assisted plasma allows to cut through the crust, remove the bulk resist without blistering or resist burning and removed residues softly and with high selectivity during over-ash. Due to the physical properties, MW assisted plasma is specially designed for fast but smooth ashing without attacking sensitive areas. The radicals generated in the plasma initiate the chemical reaction at the surface only. Without measurable electrical fields and ions present at substrate level, no plasma induced charging damage of critical structures occurs. MW assisted plasma is highly environmentally compliant due to nearly complete dissociation of process gases like CF4.


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