Answers to new wet-process challenges

Silicon chip fabrication is a complex sequence of different process steps. For highly integrated, advanced processor or memory chips often more than 500 process steps are required. Some 25-30% of these steps involves wet processing. Although put under a ‘death sentence’ several times in the past, wet processing is still very important for chip production – either in a single wafer or batch process, depending on requirements. Especially for batch processing, immersion-style processing is used on a large scale. Wet processing starts with photo resist strip, continues through etching of various layers to the cleaning of complete structured devices. With the increasing level of integration and decreasing dimensions, cleaning becomes more and more important. At the end of the wet processing, a drying step is necessary to get wafers out of the equipment and ready for the next process step. As the last step of the wet processing chain, drying has an important contribution to the process result of the wet processing application. Indeed, ironically drying is one of the most important ‘wet’ process steps.

With new integration levels and shrinking dimensions for 90nm and beyond processing, often in combination with large wafer sizes and high aspect ratios up to 50:1, much smaller particle-debris sizes and moisture residues can affect yields. Today’s monitored particle sizes go down to 0.1µm and even smaller for the most advanced applications. All these lead to new requirements for drying technology and performance.

From the mid-1990s on, all leading companies have been using drying systems based on the Marangoni principle [1]. This effect, discovered 80 years ago, involves an alcohol layer measuring of the order of microns thick put on top of a water surface to create a surface tension gradient. The alcohol (most times IPA is used) is introduced within a nitrogen-IPA-mixture into the process chamber. The IPA dissolves into the water introducing the surface tension gradient into the wetting film on the substrate. This gradient causes a very good withdrawal of the liquid from a substrate surface.

The first application of the effect in the semiconductor industry occurred in mid-1980s. The biggest disadvantage of today’s tools is the small process window that limits opportunities for process control. It becomes more and more difficult to adapt existing equipment to the broad range of new process demands.

Supersonic

After years of development, followed by internal lab testing and evaluation on production sites a new drying technology is being introduced to the industry – the patented AeroSonic drying technology. The first development work for this technology was carried out by the Californian company L-Tech. This company was acquired by Austrian company SEZ in 2002 in a deal that included the patents for AeroSonic. AP&S together with SEZ further developed the principle into a tool for industrial applications. Testing was first carried out at SEZ's process lab in Austria and then tests/evaluations were made at several customer sites.

AeroSonic uses, in its final stages, the industry-accepted Marangoni principle. The new, core element of the technology is an improved and unique alcohol introduction into the process chamber. Here the alcohol is introduced in its original liquid phase. Using an ultrasonic oscillator on top of the process chamber, an aerosol is created with an average droplet size of about 10-20µm. The new injection technique allows for higher concentrations compared to the saturation of alcohol within nitrogen. Supported by a controlled separate nitrogen flow, a homogenous aerosol distribution is guaranteed across the process chamber and on the whole liquid surface.

The aerosol generation using an ultrasonic oscillator offers improved process control compared to the current tools based on the Marangoni principle. One of the most important process parameters – the alcohol concentration - can now be controlled in-situ and therefore becomes a recipe parameter (Figure 1). The alcohol flow rate to the ultrasonic oscillator and the sonic energy allows control of the process concentration over a wide range. Nevertheless, alcohol utilisation stays very low, depending on the application, within the range 5-10ml per run.

The alcohol introduction method allows a higher IPA-concentration during the whole drying step, especially within the boundary layer on top of the water surface, right down to the end of the wafer drying process – one of the most important disadvantages with currently used tools. This leads to high performance uniformity across the whole wafer surface within the complete batch at all wafer sizes, especially for larger wafer sizes and smaller critical dimensions (CDs).

Another important process parameter is the speed of the boundary layer moving across the wafer surface during the drying step (Figure 1). For the drying of wafers and cassettes, it is set by constant draining of the liquid from the process bath. With the help of an electrical driven pump, it also can be controlled in-situ and is now a recipe parameter. The different drain speeds across a wafer surface match the different conditions during the drying step (Figure 2). The hardware configuration allows a wide range of adjustable speeds of typically 0.3-2.5mm/s and, if required, up to 3.5mm/s. A typical process window is around 1mm/s. An additional advantage is the absence of any active mechanism in the process area, i.e. no lifting is required. This makes this technology attractive for thin wafer applications, too.

A low number of approved and simple components guarantee a robust and reliable configuration. The low alcohol utilisation in combination with an inert nitrogen atmosphere reduces safety risks. A built-in off-line alcohol-circulation loop offers permanent filtration for cleaning. This increases process stability and performance. So a watermark free drying that is almost particle neutral down to a particle size of 0.12µm is possible (Figure 2).

The wide process window of the AeroSonic dryer offers the possibility of tailored recipes for various substrate materials such as Si, SiO2, quartz and GaAs, regardless of hydrophilic or hydrophobic surfaces. There is no limitation in wafer size - it is applicable to all wafer diameters used today. It’s even possible to dry different wafers or substrates sizes simultaneously within one process bath.

The new drying technology can be used as a stand-alone tool or can be integrated into automated wet processing equipment. Because of the low number of required components the technology offers a compact design. The availability of integrated rinsing makes these systems flexible enough to have the potential for integrated processing within one process chamber. Chemical injection of HF or HCl is possible for special drying applications or for one-bath immersion processing.

With the lack of large-scale installations following the recent deep downturn, the industry has focused on additions to existing capacity and the need for equipment to be capable of special applications or process developments. Institutes and research centres need small, highly flexible tools with a wide range of process options at low expense. The result has been a generation of new downsized automated tools with tailored equipment solutions, flexible configurations, combined with a small footprint and an easy installation. This equipment type closes the gap between manual stations with flexibility on one side and the highly automated systems with accuracy for large-scale production on the other.

Two-step

Typical of such systems is TwinStep - a combination of two wet process stations within a flow optimised immersion bath for circulated processing and overflow. The AeroSonic drying technology - with chemical treatment, rinsing and drying within one bath - is at the core of the new tool.

Overflow refers to the technique of spiking the rinse to achieve certain process outcomes. This can be accomplished simply on the AeroSonic base. However, not all processes can be done by overflow - either in principle or economically. So a second route is to use wet bench techniques with chemical modules where the chemistry is circulated while processing the wafers. The chemistry is used for processing several batches before being refreshed. To accommodate the wet bench type of process, the AeroSonic-dryer base is enlarged with circulated chemical process stations. With two process modules, one gets TwinStep.

By contrast with manual benches, TwinStep offers controlled processing from automated chemical mixing through processing to service functions. Various recipes can be stored within the tool controller and edited on- or off-line. Process automation with integrated material handling finally leads to stable, reliable, repeatable processing, with dry-in/dry-out wafers.

The flexible tool architecture offers a broad range of tailored process configurations such as photo resist strip, metal and oxide etch, or even cleaning. By avoiding active handling of cassettes or wafers, a simultaneous processing of different wafer/substrate sizes within one process bath is possible and increases the flexibility. Tailored solutions are applicable not only to silicon IC-manufacture - automated wet processing is also needed in micro-electro-mechanical system (MEMS), GaAs, bumping or substrate processing.

Fig.1: Typical process window for common STG-drying technology vs. new AeroSonic DST-drying.

Fig.2: Typical process window for common STG-drying technology vs. new AeroSonic DST-Drying.

Fig.3: Flexibility in process application of todays wet equipment.

Authors:

Jürgen Funkhänel, Dr Joachim Pethe, AP&S. Jörg Leberzammer, SEZ. Dr Michael Dalmer, Product Steag Hamatech.

References:

1. Solid State Technology, August 1996 p87ff or AFM Leenaars et al., Langmuir 1990, 6, 1701 ff.