The movement from aluminum to copper seeding and the attending challenges of copper CMP have placed tremendous responsibility on slurry delivery systems. New designs, new thinking begin to emerge.
By Mark Heffel
Copper physical vapor deposition (PVD) seeding and subsequent fill by electroplating has become the process of choice for microchip fabrication. As line widths shrink, aluminum becomes insufficiently effective as a conductor of electricity. Copper outperforms aluminum at any given current density and it's also more resistant to problems associated with electromigration.1, 2
The most effective method of patterning copper is to etch the line in the interlayer dielectric (ILD), then deposit the copper line and follow up with a Chemical Mechanical Planerization (CMP)—a process that laps the wafer flat, removing any excess copper above the etched line.
Copper CMP has proven to be a formidable hurdle for the industry, requiring complex phenomenological models and field tests to measure and gauge a whole range of variables—from compression and rebound of the pad, to intra-asperity slurry flow, wafer deformation, carrier-film contact and carrier dynamics.3
Regardless of the polishing method (rotary, orbital or linear), CMP carries with it inherent risks involving corrosion, erosion, dishing, scratching and topographical non-uniformity with damascene structures. Most recently, copper CMP has faced the challenge of integrating low-k dielectrics, which are softer and more delicate than high-k materials and, therefore, require even greater attention to the many variables at issue.4, 5
One critical set of variables concerns the slurry composition and the effectiveness of the slurry delivery system. Slurries used for copper CMP-distinctly different from those used for aluminum-are particularly susceptible to agglomeration, a state in which the abrasive particles settle down out of the suspension fluid.
This condition may result in uneven amounts of grit on the wafer, which may cause scratching. A variety of analyzers on the market will measure the slurry's particle distribution and other characteristics before application, which may assist technicians in preventing damage before it occurs.6
But the best insurance against any slurry destabilization is a delivery system carefully engineered and tested to prevent shear forces, agglomeration or contamination of the slurries.
Delivery system design
Slurries are usually pumped from a reservoir or storage area through a complex delivery system consisting of plastic tubing, fittings, valves and manifolds, to the tool and pad for wafer polishing.
Slurry delivery systems are constructed largely of fluoropolymers because stainless steel or other metals are unable to hold up under the slurry's corrosive powers, and can contribute to metal ionic contamination that can damage the circuits. Tubing and fittings are made entirely of plastics, often Teflon PFA or PTFE. Valves are constructed of mostly fluoropolymers but employ steel fasteners. Because of the corrosion risk, all-steel components should be entirely isolated from the fluid stream and the ambient air outside the valve.
A chief design objective for fluoropolymer valves is a diaphragm that resists acid permeation. The intent is to prevent acids from passing through the diaphragm and into the metal components in the valve's actuator, and then back through the diaphragm into the fluid stream. Thicker diaphragms made from a fully fluorinated material provide resistance to permeation; however, the thickness of a diaphragm must be balanced with flexibility required for long cycle life.
An ideal design will require that all metal components inside the valve's actuator are coated in fluoropolymers, so that in all locations two layers of plastic stand between slurries or acids and any metal component.
On the other hand, fittings (because they're made from fluoropolymers) are subject to “creep,” which means the plastic relaxes under strain, allowing the fitting to back off with time. The result is inevitable leakage around fittings.
Since the slurry is expensive and caustic, leakage is a significant issue. Those sections of the delivery system with concentrations of fittings are usually encased in a box to prevent evaporation of the slurry into the room's ambient air. Inside the box, a caustic fog will form.
Since properly designed valves will not leak, valves with multiple inlet and outlet ports or multiple valves formed of a single block are one means of reducing the number of fittings or leakage points in a delivery system.
Agglomeration and shear forces
Agglomeration occurs when the slurry has been allowed to sit—a situation that may be avoided through a number of commonly known methods, including regular circulation of the slurry when it is not in use.
Agglomeration may also occur through shear forces in the delivery system—a far more complicated problem. Shear forces occur in the delivery system when positive and negative velocity streams meet, causing a disturbance that destabilizes the slurry. In an ideal world, a slurry's velocity profile would remain the same throughout the delivery system. In the real world, the velocity profile will change because a delivery system, by necessity, will incorporate changes in directional flow and flow area.
The objective is to minimize these changes. Computational Fluid Dynamics (CFD) lets engineers model different geometries by recording incremental pressure drops, fluid velocity and shear forces.
A velocity profile produced by CFD is affected by a number of variables other than the force or pressure moving the slurry, including:
- Composition of the slurry itself;
- Flow area or the space through which the slurry is traveling;
- Sum of the surface or contact areas of the tubing or component;
- Flow path, formation or direction of the tubing.
Slurries do not necessarily behave as Newtonian Fluids, such as water and mineral oil. They often require special mathematical treatment as Non-Newtonian Fluids with different stress-strain relationships.7
 A screen capture of computational fluid dynamics (CFD) for two different valve designs. |
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In Figure 1, different velocity profiles are produced by fluid in two different valve designs. In each, the flow rate is 20 gpm. In these CFD images, blue represents the slowest speed, with green, yellow and red representing (in that order) an increase in speed.
The engineer concerned about shear forces will look for abrupt changes in color, indicating a sharp change in speed and/or direction, or will look for evidence of bottlenecks or dead areas—places where fluid might be allowed to sit or back up. The ideal flow path will be relatively smooth and will provide for constant movement.
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Basic points in the valve design that may affect fluid flow would include inlet and outlet ports, bowl, seat and the diaphragm (Figures 2 and 3).
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By altering the shape, size and location of the inlet and outlet ports, the width, depth and shape of bowl and seat, and the shape and functional design of the diaphragm, the engineer may create a relatively smooth fluid flow path. Of all the points along a delivery system, valves are some of the most susceptible to shear forces because it is their inherent function to redirect and control flow.
Valves are by far the most extensively engineered components in the delivery system for copper CPM. They are also, potentially, the worst performing and most torturous in terms of their treatment of the slurry.
Contingency loops
The delivery system moves slurry from the storage area or reservoir to the lapping tools and wafer, but the route is not necessarily direct.
Usually, a delivery system consists of a number of looped connections, enabling the operator to send the slurry on a number of different pathways, including those through filters or pressure transducers. Delivery systems designers build in contingency loops or pathways, so when problems occur in one section of the delivery system, that section can be drained, isolated and fixed. At the same time, via an alternative route, the slurry continues to reach the lapping tool and wafer.
Contingency loops are also a means of draining a certain section of the slurry and filtering or replacing it with new slurry. In a delivery system, therefore, you will find a series of T connections and shut-off valves, allowing for several variations in flow paths.
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In Figure 4, for instance, three shut-off valves are configured in a T formation. Slurry may flow from Valve A through Valve B, with Valve C shut off. Alternatively, with Valve A closed and Valve C open, slurry may flow back from the process tool and through Valve C to be drained.
Unfortunately, for every valve, there must be two plastic fittings, which are, as noted earlier, potential leakage points. In a valve cluster (such as the one in Figure 4), there would be six fittings or leakage points—two per valve or one for each valve inlet or outlet.
One way around the leakage problem is to employ valves with multiple ports; another option is multiple valves machined into a single block. In the first case, a single shut-off valve lets the operator move slurry in two or three directions with the turn of a knob.
A valve of this sort is either closed, preventing all slurry flow, or is open, allowing slurry to pass from the inlet port through two or more outlet ports. Alternatively, valve seats for three or four different valves may be machined into the same block. Valves in the block connect to one another (as in Figure 4), but without the fittings. In other words, one block of valves replaces three separate valves, eliminating all tubing and a number of fittings. The number of leakage points (or fittings) may be reduced from six to three or from eight to four, depending on the number of valves in the system.
For a group of valves to be useful as a single block, the individual valve designs must be simple. You must be able to disassemble any of the valves in the block and service it with relative ease; otherwise, if one valve were to go bad, all valves in the block would have to be replaced.
In fact, it's not unusual for technicians to throw away a bad-working valve rather than fix it—an unfortunate practice that results, in part, from complex valve designs consisting of many parts that don't lend themselves to easy service.
In a slurry delivery system, manifolds or distribution boxes contain the greatest cluster of valves. Located close to the slurry source or reservoir, manifolds are the means by which the slurry is distributed to several different lines with appropriate amounts of pressure. Inevitably, each line is accompanied by a cluster of valves, allowing for multiple pathways.
The challenge is to reduce the number of fittings, leakage points and the space required for the manifold. Valves with multiple ports or groups of valves machined from a single block are viable options for addressing the challenge presented by distribution boxes.
Pressure to deliver
The movement from aluminum to copper seeding and the attending challenges of copper CMP have placed tremendous responsibility on slurry delivery systems.
A copper CMP delivery system must move slurry from the storage reservoir to the polishing tool without risk of agglomeration or destabilization of the particles suspended in the slurry. Intense scrutiny of the delivery system's capacity to create sheer forces is appropriate, and, indeed, imperative to ensure that no compromise is made to product yields (especially with 300-mm wafers).
The integration of low-k dielectrics, with their sensitivity to damage, means that all variables in a copper CMP delivery system must be understood, controlled and held to very low tolerances. CFD is one means of testing the geometries of delivery systems and their capacity to produce destabilizing pressure drops or shear forces. CFD is especially useful when designing difficult components like valves, where there is the potential for rapid velocity and directional changes.
Finally, through the innovative design and consolidation of valves and fittings, the industry has the opportunity to introduce economies that may reduce leakage points, waste and risks to health and safety.
Mark Heffel is plastics product manager at Swagelok Company, Solon, Ohio. He can be reached at: heffel@swagelok
References
[1] Shon-Roy, Lita, “CMP: Market Trends and Technology,” Solid State Technology, June 2000.
[2] Reid, Jon, et. al., “Factors Influencing Damascene Feature Fill Using Copper PVD and Electroplating,” Solid State Technology, July 2000.
[3] Runnels, Scott R., and Thomas Laursen, “Establishing the Discipline of Physics-Based CMP Modeling,” Solid State Technology, March 2002.
[4] Braun, Alexander, “CMP Becomes Gentler, More Efficient,” Semiconductor International, November 2001.
[5] “Integration of Ultra Low Dielectrics for CMP,” European Semiconductors, March 2002.
[6] Nicholes, Kristi, et. al., “Measuring Particles in CMP Slurries,” Semiconductor International, July 2001.
[7] Tverberg, John C., “The Effect of Surface Roughness on Fluid Friction,” Flow Control, August 1995.