Benefits of Waterless Photo Resist Stripping
Kevin Schumacher - Essex Products International
Bruce Fleischhauer - Medtronic
Vincent G. Leon - Olin Micro Electronics Material

Application
A new process station has been specifically designed to remove both Novolak resin based photo resist and side wall polymer residue created by dry etch processes used for Via and Metal layers. The process ensures complete removal of post plasma etch polymers including Halogenated sidewall deposits created by Chlorine based metal etch, without causing corrosion. In addition to reducing the strip process to three steps, the equipment significantly reduces the burden of equipment operating cost and foot print requirements which are usually associated with a photo resist strip process.

Figure 1 The All Solvent Strip Ô System has two PRS baths and IPA Rinse/Dry.

Introduction
Today’s wet stations and spray processors require several steps to remove photo resist and sidewall polymers from metallized wafers. The standard wet bench configuration typically includes two photo resist strip (PRS) rinses, two IPA rinses, a DIW dump rinse, and a final spin rinse/dry. Often, an additional post-strip dip in an inorganic polymer etchant is needed to complete the process. The necessity for multiple rinses creates a high cost of ownership and a large equipment footprint in valuable cleanroom space. By identifying the main drawbacks of the current processing methods, a new equipment design has evolved which provides state of the art processing by eliminating the need for DIW, reducing multiple rinses, and shortening the number of process steps. (fig 1)

New Technology
Incorporating vapor phase drying, and solvent reprocessing offers significant advantages to the burdens associated with the current stripping methods. The use of a new vertically integrated IPA rinse process can replace the need for an intermediate IPA rinse and final DIW rinse. Vapor phase drying eliminates the need for spin dryers, which tend to add particles, and can leave water spots on the wafers. IPA solvent reprocessing can significantly reduce the difficulties associated with facilities waste disposal and TOC emissions.

Cost Comparison
The historical technologies used to complete the stripping process after metalization are batch wet processing stations or spray processors, utilizing an organic solvent. These two methods deviate on chemical consumption when compared (fig. 2) but both demonstrate higher operating cost than the ALL SOLVENT STRIP Ô process station. The majority of cost drivers can be eliminated by removing DIW from the process and recycling the IPA solvent.

A key factor for success of any photo resist and sidewall polymer stripping process, is the combination of a stripper chemistry and equipment. Olin Microelectronic Materials has recently introduced Microstrip 5001, a new photo resist and sidewall polymer stripper chemistry. Microstrip 5001 allows the benefits and cost reduction of the EPI "All Solvent StripÔ System to be realized with no additional tool requirements.

Equipment Process Design
The wafers are processed through three tanks. The first two tanks incorporate a Photo Resist Stripper. The third tank provides rinsing and drying (Fig 1). Field testing was completed at Medtronic with the following configuration.

The first two baths incorporated Microstrip 5001 from Olin with ultrasonics and triple guard filtration. The utilization of ultrasonics ensures a quick process time of 15 minutes through the first and second stripper baths. The wafers were then processed through tank 3 resulting in complete rinsing and drying of the product.

The new patent pending Olin Micro strip 5001 and EPI PRS-400 system are designed to effectively remove novolak resin resist and dry plasma etch resides.

Elimination of Corrosion
One of the key elements in preventing galvanic corrosion of the metal surfaces is the elimination of water from the process. Current processing moves the wafers from the stripper baths to a static IPA rinse, in order to dilute the stripper chemical, without dissociation, before the wafers are placed in a DIW dump rinser. When mixed with water, Amine containing photo resist strippers will form Hydroxyl ions that can attack Aluminum and aluminum-Copper alloys. (Figure 3c) Shows aluminum bond pads that macroscopically appear ‘stained’. SEM analysis (fig 3d) shows that the top surface of the metal has been pitted by Hydroxyl ions during the DIW rinse process. The pits tend to scatter light, giving the pads a brown appearance. This reaction is also influenced by the presence of light. Copper nucleation sites in Aluminum Copper alloys can also be attacked, giving the metal a ‘peppered’ appearance (fig. 3e). Hydroxyl ions act as the electrolyte for galvanic reactions when dissimilar metals are present, such Si-Chrome resistors contacted by Aluminum. In this case, Aluminum near the dissimilar metal appears to be simply dissolved away, usually in a semicircular pattern. These issues can be avoided by replacing the DIW dump rinser with an Alcohol rinse/dry process.

The ALL SOLVENT STRIPÔ process Incorporates a patent pending process which consists of three major rinsing features. Tank 3 incorporates a unique vertically integrated process bath. (fig. 3) The bath allows thorough rinsing of the (PRS) solvent with IPA. The mechanism for removal is achieved by solubilization of the IPA and (PRS) solutions.

The volume of liquid IPA in the lower section of the final rinse tank is approximately five to seven gallons depending upon the size of the wafers processed. The IPA is heated to ensure rapid removal of the (PRS) and sent to an insitu reprocessor and filtration loop where particles and stripper are removed.

Once the (PRS) has been separated from the IPA through distillation, it is automatically returned to the process tank and reused. The heated IPA is continually recirculated through the rinse bath to ensure a consistent stable process.

The final rinse is provided by a thin film of clean distilled IPA which condenses on the wafer surface from the vapors located above the IPA liquid. The amount of IPA which condenses on the surface is minimal due to the temperature of the wafers. The thin layer of IPA liquid prepares the wafers for drying and ensures that the final IPA on the wafer surface is clean. The wafers are now ready for drying and have completed the entire rinse step in an IPA liquid/vapor phase environment absent from the presence of air.

Particle Free Drying
The wafers remain at rest in the vapor enriched section of the process tank. As the wafer temperature reaches a state of equilibrium and IPA vapor, condensation on the surface ceases, the wafers are dry. Once this condition exists the wafers can be robotically transferred to the upper region of the process tank and cooled down for transfer to the next process. The vapor drying step is stable and is particle neutral as shown in (fig 6).

BIOGRAPHIES
At the time this article was published Kevin Schumacher was the Vice President of Operation and Engineering at S&K Products International Inc. He is now President and owner of Essex Products International. He has developed several Patents for isopropyl alcohol (Solvent) vapor drying, isopropyl alcohol (Solvent) Reprocessing HFU Megasonics and patents pending on the PRS-400 system. He holds a BS degree in Engineering from Thomas Edison University.

Bruce Fleischhauer graduated from the University of Arizona with a BA in Chemistry in 1979. He then joined Burr-Brown Corporation where he worked as a Photolithography and Etch Development Engineer. In 1992 he joined Medtronic/ Micro-Rel Division where he has worked as a Principle Etch Engineer.

Vincent Olin works for Olin Micro Electronics Material as an Applications & Development Engineer and he has 12 years experience in the Semiconductor Industry. He holds a BS degree in Physics from Arizona State University, where he is pursuing advanced studies in Process Technology.