Improved Process Adhesion with Sharkskin

Advancements in Process Residue Adhesion Decrease Chip Defects and Increase Yields
In this era of the incredible shrinking chip, improving process residue adhesion is one of the thorniest challenges facing chip fabricators. Since adhesion is improved by roughening surfaces, new materials and technologies are being sought to replace or complement such traditional ceramic roughening methods as grit blasting or twin-wire arc spray (TWAS). With marching orders to improve process uptime, drive down manufacturing costs and improve or maintain yield, while decreasing process-related defects, the semiconductor industry is turning to new materials that help meet their needs. The emerging solar technology industry is also using the materials to drive down their costs and increase their competitiveness in the marketplace.

Today's Thinner Integrated Circuits Demand Better Film Adhesion
The chip fabrication process uses various processes to layer films on wafers, including physical vapor deposition (PVD) and chemical vapor deposition (CVD). As integrated circuit linewidths get thinner, insulation between the layers that make up the chips is becoming even more difficult to achieve. In just a few short years, internal linewidths have been reduced from 65 nanometers (nm) to 45 nm and now to 32 nm, making insulator performance more significant.

This is particularly important for barrier layers, which require denser films that contain more compressive stress, with the potential for spalling failure, blisters and film "firing" off the chip surface. Deposited barrier layers composed of titanium, tantalum, or tungsten nitrides tend to be more stressed than other materials, because the layers have to be very dense to prevent elements within the integrated circuit from migrating.

During the process of layering film on the wafer, there is a build up of residual film on the tops and sides of ceramic reactor chamber components, including domes, shields and cover rings. This residual film is also highly stressed, so the deposits that build up on the chamber components may break off and fall onto wafers, ruining the chip or device.

Improving Productivity and Yield
Semiconductor wafer productivity is a combination of tool throughput (number of wafers per hour per machine), device yield per wafer and servicing time, which includes both preventive and corrective servicing. Fabricators want to get as many wafers as possible from their process, while conducting as little maintenance as possible. The need to shut down process tools to strip off residual material before it breaks off is extremely disruptive and can be costly. The more residue that can be safely accumulated, the higher the yield and the longer the tool can run between services.

Yield and productivity improvements have become more urgent as linewidths on chips get thinner and the insulator geometry becomes more significant. As the chip manufacturers are continuing to reduce the linewidth to get more and more devices, processes have become more sensitive to defects.

The Good Old Days are Gone - Prior Methods are Failing
In the early days of chip fabrication, film stresses were not high and did not limit high wafer yields. As barrier film requirements evolved, engineers began looking for ways to increase film adhesion on tool components to increase productivity.
Just a decade ago, grit blasting was the method of choice for providing a rough surface on ceramic chamber components to improve residue adhesion. Like using sand paper, grit blasting uses abrasive particles to give a rough surface finish. Time marches on, however, and grit blasting doesn't roughen surfaces enough for today's high-stress films. It also can damage the underlying surface, causing it to be flaky or dusty. This dust and loose material finds its way into reactor chambers, contaminating wafers and reducing yield.

A few years ago another method was developed, called twin wire arc spraying (TWAS). With TWAS, aluminum wires are shorted out together, creating melted aluminum droplets that are transported to the part's surface by a nitrogen jet, creating a rough "foresty" texture. Because it is made of aluminum, it sticks easily, conforming to the underlying surface. Considered state of the art until recently, TWAS works well for metal deposition processes. It has a few drawbacks. For instance, TWAS cannot be used in the presence of fluorine gas because the fluoride gas will attack the aluminum and generate aluminum fluoride, causing process drift, which means that the process is less repeatable. Also, aluminum fluoride causes particulation onto a chip. Further, it cannot be cleaned in situ; pulling of the chamber and wet cleaning would be required.

There is also a major concern over TWAS's adhesion to alumina when used to make the latest generation of chips. In the last year or so, fabricators have found that the TWAS layer was starting to peel off the ceramic chamber parts when depositing high stress films. Engineers suggested that this might be caused by the preparation steps used prior to TWAS application; the common practice has been to bead-blast the ceramic parts near the area to be arc-sprayed to provide texture for TWAS adhesion. Because the ceramic material is so hard, it is impossible to abrasively roughen such substrates to greater than about 50 µ-inches Ra without creating significant sub-surface damage. Even at a roughness of 50 µ-inches Ra, some surface defects and subsurface damage is created in the alumina. These defects promote failures of the TWAS/alumina surface as film processing residues build up, resulting in the TWAS layer delaminating from the alumina.

Improved Process Adhesion with Sharkskin
An alternative method to improve process residue adhesion for next generation chips is Sharkskin™, a textured, high-purity alumina material manufactured by Morgan Technical Ceramics (MTC). MTC's Sharkskin™ provides a controlled, textured surface on a high purity alumina material already widely used in semiconductor manufacturing tools. It has the same acid and plasma-resistant qualities as the parent aluminum material, and can be cleaned and recycled using current technology.

Used primarily for reactor domes, shields, and cover rings, Sharkskin™ was originally envisioned as a foundation to help the TWAS layer adhere better to the base ceramic. Recent work with a leading industry supplier demonstrates that Sharkskin™ can actually replace TWAS film, reducing fabrication, cleaning, and refurbishment costs. Sharkskin™ eliminates the need to re-grit blast parts after each cleaning, because the roughness is permanent and doesn't degrade with multiple uses and cleanings. A further advantage is that Sharkskin™ is acid resistant and can therefore be used in environments where use of TWAS is precluded.

The Sharkskin™ material is mainly used for barrier layer film processes, primarily for PVD or plasma-enhanced PVD. It can be used for tungsten, aluminum and copper PVD applications. Deposited films include titanium nitride, tantalum nitride, tungsten nitride and any other diffusion barriers. These brittle films usually exhibit a great deal of stress and so benefit most from Sharkskin™.

The Sharkskin™ texture can be placed exactly and precisely where it is most needed. Existing components that are now coated with TWAS can be textured with the Sharkskin™ material, minimizing disruption to existing processes and speeding the new component qualification process.

Although primarily used for PVD and CVD processes, the Sharkskin™ material may also be suitable for etch processes, where photoresist materials can deposit within chambers and flake off. Sharkskin™ can also be used in solar applications to change reflectivity of surfaces for improved efficiency of solar panels. Medical and laser applications that require high adhesion properties may also benefit from use of the new material.

Figure 1 shows a micrograph and surface roughness profile of Sharkskin™. Figure 2 provides a graph of residue adhesion at various thickness levels, showing improvements over conventional methods. At roughnesses of up to 1200 µ-inches Ra, a two-fold adhesion improvement over twin wire arc spray and plasma spray ceramic films has been demonstrated.

Adhesion Strength Future
Increasing device integration and reductions in chip size mean that chip fabricators must be more concerned with film adhesion strength on chamber components. Alumina and zirconia-coated reactor components, such as domes, shields and cover rings, could potentially hold thicker process residues than components having surfaces coated with TWAS or plasma spray films alone. The adhesion strength of such intermediate films would no longer be a factor in component life, thus eliminating another process variable and increasing productivity and yield.

To meet the need for materials suitable for next generation chips, application engineers from Morgan Technical Ceramics, a leader in engineered ceramics, have introduced Sharkskin™, a new, innovative technology that can significantly improve process residue adhesion, decrease process-related defects, and increase yields during the manufacturing of integrated circuits.