CMP Grows in Sophistication

Ruth Dejule, Associate Editor -- Semiconductor International, 11/1/1998

Improvements in uniformity and edge exclusion are equally attributable to advances in consumables and the polishing tool.

Polishing head designs

Polishing techniques typically use rotary, orbital or linear mechanical motion coupled with an inert abrasive in a chemical solution. The different methods attempt to strike a balance among removal rate, planarization and within wafer uniformity through a combination of pressure, speed and pad hardness. Generally, a harder pad improves local (within die) planarization; however within wafer uniformity declines when the pad fails to conform to the surface. A softer pad promotes better global planarization. Similarly, a combination of high pressure and speed will increase removal rates, and when pressure is reduced, planarization improves, but uniformity declines.

Most CMP technologies use rotary motion, and one, IPEC Planar, uses a combination of orbital and rotational motion. The wafer head rotates in a circular path, while the polish head is orbital. The net effect is that each point on the pad traces a spirographic trajectory. This configuration facilitates direct slurry delivery to the wafer surface while maintaining high relative velocities and a small overall tool footprint, according to Dr. Thomas Bibby, manager of strategic technology at IPEC Planar.

Lead Photo: New CMP tools incorporate fully automated process control to address next generation design rules. Source: Strasbaugh)

In the past two years, the considerable improvement seen in uniformity and edge exclusion is equally attributable to advances in consumables and the polishing tool, according to Scott Meikle, sector manager from Micron (Boise, Ida.). The most significant tool contribution was in carrier design.

Conventional carrier design applied pressure to the wafer with a steel plate carrier covered with a compliant carrier film that was intended to buffer variations in the wafer's backside. The softer film conformed to wafer irregularities and produced adequate polish uniformity, but planarity suffered, said Applied Materials' Gregory Amico, 200 mm global product manager, CMP Division. To compensate, higher pressures were required. One major drawback was that the carrier film added another variable, and some films needed replacement every 500 wafers. Major design changes replaced this film. Applied Materials, for example, designed a flexible membrane that conforms to the wafer's backside, so pressure is applied uniformly (Fig. 1). Now a harder pad and lower pressure (as low as 2 psi down from 7 psi) can be used to promote good global uniformities, Amico said. Average non-uniformity for oxide films within the wafer could be as high as 6% or 7% but was improved to less than 3% with the redesign, he added.

The retaining ring that holds the wafer within the pocket and contains it during CMP also was redesigned. Since the retaining ring is not conventionally pressure controlled, pad deflection at the wafer's perimeter was not contained well, leading to a narrowing of usable material close to the edge. Edge-exclusion specifications for hard carrier heads could range from 5 to 7 mm. By using a variable pressurized retaining ring, suppliers like Applied Materials and Strasbaugh have been able to control the pad profile at the wafer's edge for improved removal-rate performance. This design allows independent control from the main head pressure. According to Amico, edge-exclusion as good as 3 mm has been achieved.

Belt polishing

Fig. 1. A wafer is loaded onto a polishing head that is newly designed to provide improved removal uniformity and repeatability. (Source: Applied Materials)

As device design rules shrink, within die planarization becomes a greater concern. To compensate for the lack of local planarization in STI, dummy structures are used to help hold up the pad and improve local planarization, said Michael Leach, senior member of technical staff at Aplex (Sunnyvale, Calif.). Predominantly a stop-gap measure, these dummy structures pose electrical, design and space constraints. The main issue, according to one end-user, is that 5X more structures within the die must be added over previous designs, significantly raising defect levels. In Damascene, Leach said, a process for manufacturing has not been developed easily because of the severe dishing problem associated with within die nonuniformity. Ultimately, the best process to provide local planarity performance without limitations of cross-wafer uniformity introduced by rotary motion may be linear tools, Meikle said. Because of a harder polishing surface, higher linear speeds and potentially lower pressures, belt CMP may be in a better position to address this issue.

Companies that use some form of linear or belt polishing are Aplex, Doosan (Korea) and Lam (Fremont, Calif.). Lam implements a belt polish with a fluid-bearing platen positioned below (Fig. 2) to decouple the effects of removal rate, planarization and within wafer uniformity. Thus, each parameter can be optimized independently. The pressure distribution across the wafer's surface is profiled with the fluid-bearing platen. At a given pressure and speed, adjustments to the pressure distribution profile can be made to eliminate non-uniformities.

The Aplex belt polisher is oriented vertically to limit contamination and enhance air-flow. In this configuration, the slurry adheres to the surface through capillary action and proprietary grooving of the pad, said Rick LaFrance, vice president of customer support and marketing. Particles generated as a result of the mechanical motion potentially can be reduced.

Fig. 2. This CMP technology uses fluid bearings to profile pressure across the wafer surface and eliminate non-uniformities. (Source: Lam)

Because of higher effective linear speeds, belt CMP also may be effective for dielectric CMP to achieve improved planarization efficiencies, said Milind Weling, interconnects and CMP manager at VLSI Technology. According to the SIA roadmap, planarity requirements will decrease from 0.3 µm in 1997 to 0.25 µm in 1999, with some processes requiring planarity in the 1000-1500 Å range. Oxides have been polished conventionally at relatively lower linear velocities with higher downforce. However, higher velocities and lower pressure can provide better planarization in a shorter time.

End-point detection

Some end-point detection systems are optically-based, others motor current. Using in-situ monitoring when polishing tungsten and stopping on the bottom layer is fairly straightforward. However, when polishing copper, stopping with 1000 Å of copper remaining poses particular challenges. Currently, end-pointing is restricted to averaging across the wafer, and full wafer mapping is provided by integrated measurement tools like a Nova (Rehovoth, Israel) or Luxtron. All tool manufacturers have some form of end-pointing, even with oxide layers.

Aplex is developing motor current technology that detects when one film is polished and a second is encountered, for example, during STI when going through an oxide and stopping on nitride or in damascene when polishing through a metal and stopping on an oxide. In this technique, the noticeable change in the electrical current that drives the head or motor when polishing through different barrier levels (Fig. 3) is monitored.

Fig. 3. Output of motor current in-situ motoring indicates Cu polishing breaking through a Ta barrier to an oxide layer. (Source: Aplex)

A new approach to in-situ endpoint detection is multiwavelength reflectance spectroscopy used in Lam's CMP system. Unlike single-wavelength laser technology, which detects changes in film thickness and motor-current technology, which detects changes in film type, multiple channels of spectroscopic information provide absolute film thickness measurement, said Rahul Jairath, Lam's director of CMP technology. The multiwavelength approach can detect and delineate different film types present on the surface of patterned wafers and reduce wafer-to-wafer dielectric and metal film nonuniformity (WTWNU) by a factor of five.

Dry-in/dry-out

The dry-in/dry-out concept is a manufacturing issue. The industry is moving toward 'a one-stop solution' said Steven Strausser, SEMATECH program manager for advanced CMP technology. The goal is to put the wafer into the process and have confidence that it is going to be clean enough to go to the next step. Instead of delaying the clean until an entire cassette of wafers is polished, an integrated dry-in/dry-out system effectively reduces the gap. Suppliers of integrated systems include Applied Materials, Peter Wolters Group, Ebara, IPEC Planar, Lam, SpeedFam and Strasbaugh.

One particular challenge is Cu CMP. Copper is a relatively soft material that forms an oxide surface requiring special attention and is susceptible to corrosion. From a polishing perspective, this means greater difficultly in the control of dishing, scratching and erosion. A higher polishing rate further adds to the mix. From a cleaning perspective, copper is much less inert than either oxide or tungsten, said Dr. John deLarios, vice president at OnTrak (San Jose, Calif.). Contaminants are therefore more likely to react with Cu structures if not removed immediately, making integrated cleans more critical in Cu CMP. Essentially, until the slurry is removed from the Cu surface, corrosion is an issue.

Moreover, studies indicate that defectivity may be time-dependent, said Kathy Leaf, product marketing manager at Speedfam (Chandler, Ariz.). Internal evaluations have shown a 50% reduction in particles when an integrated clean was used compared to a polisher with stand-alone cleaner. This indicates that the longer the slurry is on the wafer, the higher the defectivity, Leaf said. Dry-in/dry-out systems also have been shown to reduce consumables such as DI water by as much as 30%.

Multi-step processing

Cu and low-k CMP require polishing through all layers in the damascene process, leaving the wafer's surface ready for the next level. This approach is significantly different from oxide and tungsten CMP, where single layers are polished. With these new materials, polishing three to four films including liners and barriers is expected. Without a universal pad and slurry that have the selectivity for each film polished, different recipes for multi-step processing are being developed. Initial results indicate that multi-step processing delivers better results than a single-step polish, Strausser said.

Fig. 4. A three-step Cu/Ta polish using multiple slurries, one pad and 10% overpolish minimizes Cu recess and oxide erosion. (Source: IPEC Planar)

Virtually all tool manufacturers have some form of multi-step processing capabilities that implement combinations of multiple platens, slurries and heads. In particular, Applied Materials, IPEC Planar, Lam and Strasbaugh have systems specifically designed to accommodate three-step processing for Cu polishing. The Symphony-CMP, for example, has three platens each with a slurry dispense system and two spindles each with two wafer carriers and can polish up to 55 wph, said Susan Price, marketing specialist. Because the turret heads move independently, each of the three tables can have different polish times without penalizing throughput. The first platen could be used for Cu removal and stop on the Ta or TaN barrier; the second for barrier removal and the third for passivation, dielectric touch-up or buff.

Another approach is one developed by IPEC Planar (Fig. 4). Simultaneous through-the-pad delivery of multiple slurries to each of four autonomous polishing modules provides process optimization. The result is three or more polish steps with no additional wafer handling that can compromise throughput and yield. Each polish module can be accessed independently for full parallel processing. According to Bibby, this multi-step process approach has demonstrated a reduction in dishing and erosion to less than 800A for worse case structures.

Slurry-less CMP

Slurry-less CMP is the only divergence from conventional CMP that may offer significant improvement, Meikle said. From the standpoint of cost of ownership and process control, slurry-less CMP has considerable potential. Some companies are polishing with just DI water, and some are using 3M's slurry-less pad that provides an abrasive once water is added. There is however, at least one tool manufacturer that offers a complete slurry-less CMP system, Obsidian (Fremont, Calif.). Essentially, a roll of abrasive pad material is spooled across a platen and incremented after each wafer. No slurry or conditioning is required.

Over the past three years, fabs such as Micron have developed CMP processes that are more uniform and stable and require less operator intervention. As the industry goes to 0.18 µm design rules, sensitivity to manufacturability concerns such as defects increases with every process in the fab. The fact that defects also are becoming a primary CMP issue ahead of stability and control issues is another indication of its growing maturity.