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The Challenges of 300 mm CMP Brian Stephenson, Ebara Technologies Inc.
The transition from 200 mm to 300 mm presents many challenges for semiconductor manufacturers throughout the entire processing flow. From raw silicon to packaging operations, processes and processing equipment must meet the strictest speci fications at ever increasing chip complexity and shrinking design rules. Throw in rapid moves to advanced technologies such as low- dielectric and copper metallization converging at the same time, and the 300 mm transition is even more challenging.
Past transitions from 125 mm to 150 mm, and 150 mm to 200 mm, were considered to be incremental changes, the transition from 150 mm to 200 mm being the most significant as far as equipment goes. For most of these transitions, the industry was not near the processing limits for the linewidths being transitioned. Only recently has IC manufacturing began to push the limits of optical lithography as gate lengths are rapidly dropping below the 0.18 µm level, to 0.13 µm and now sub-0.1 µm levels. Also, front-end and back-end processes and materials have been, up to now, virtually unchanged or experienced relatively slow migration into advancing technologies as the industry transitioned to larger wafer sizes. With the advent of 300 mm, this past model does not appear to be holding any longer. The manufacturers taking the risk now to introduce 300 mm lines are not simply putting in proven, stable technology to ramp up a fab. There appears to be a push to put in the most advanced processes (low-κ, copper, sub-0.13 µm linewidths) at the initial stages to allow faster return on investment: faster chips with higher ASP, and smaller chips on twice the silicon real estate. However, if device and line yields are poor, this strategy is all for naught. Therefore, chipmakers are putting large 16
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pressures on the capital equipment makers to produce equipment with unprecedented levels of integration, process flexibility, automation, and reliability. So, with all of these advanced technologies converging at the 300 mm node, where does that leave CMP? Is it drastically different than the 200 mm processes your familiar with? Well, as it turns out, not really. 300 mm CMP—Is it ready?
The short answer is “yes” with regards to process capabilities. The major CMP players in the market today have demonstrated capability at 300 mm across the board from STI to copper polishing. Companies such as Ebara even have second-generation equipment out in the field being started up in pilot and production lines around the world. So from the process standpoint, there doesn’t appear to be any major hurdles in the transition. However, increasingly tight specifications on uniformity and planarization efficiency will drive further design improvements in 300 mm tools moving forward. This reality, plus a host of new requirements for 300 mm manufacturing equipment, provides the greatest challenges to CMP. Transitioning CMP—The challenges
CMP is already a major process module for most 200 mm manufacturers, with advanced logic manufacturers utilizing CMP for 12 to 15 layers (see Figure 1). For 200 mm, the bulk of CMP is utilized for three process steps: shallow trench isolation, intermetal dielectric
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polish (IMD—usually PETEOS, HDP Oxide, or doped oxide such as PSG or BPSG), and tungsten plug polish. Other processes may include polysilicon polish (usually used in memory manufacturing), tungsten local interconnect polish, and more recently, damascene copper polishing. For 300 mm, all of these processes will be needed, with copper polishing becoming more prevalent.
F i g u re 1. Illustra tive cross-section of 5-level copper metal logic devi ce. Up to 11 diff e ren t CMP p rocess steps wo ul d b e utilized in the c o n s t ruct ion of a devi ce such as this.
Key issues
For 300 mm CMP, the challenges are many. Most of the issues faced in the transition from 200 mm to 300 mm are many of the same issues that CMP continues to face today. Across-wafer uniformity control is one of the primary difficulties with respect to hardware design that doesn’t scale well. Simple scale-up of polishing head design doesn’t guarantee the same performance seen at 200 mm. Typically modification or redesign is required.
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Following are some of the key issues facing 300 mm CMP today. Note that many of these issues are not unique to CMP, but apply to most 300 mm capital equipment in the fab.
Reliability As with previous fab equipment, 300 mm equipment will require improved levels of reliability, especially for CMP. CMP equipment for years has been much maligned for reliability issues. Equipment downtime creates costly disruptions in the production line. With the focus on reducing CoO and increasing ROI in 300 mm, these disruptions become even more undesirable. Other issues, such as wafer breakage, long an issue with most CMP equipment, become unacceptable at 300 mm with not only the value of the lost die on the wafer, but the raw material costs as well. Ever increasing pressure is being applied to CMP equipment makers with respect to reducing wafer scrap and mean time to repair (MTTR), and increasing mean time between failures (MTBF). The main factor for 300 mm reliability right now is simply maturity. Many of the tools in fabs today are still in the alpha or beta phase of development. While some equipment is a scale-up of their 200 mm counterparts, many pieces have been redesigned from the ground up. Larger motors, heavier infrastructure, and beefed-up power distribution mean a lot of new untested parts. Newer, increasingly complicated software designed to meet today’s ergonomic, user-friendly requirements, in addition to expanded host control, means a lot of new software bugs and tool crashes. For the more complex equipment, it will simply take time and effort to work the bugs out of most of these systems. Those companies that kept on the 300 mm course during the pause in the late 90’s are well ahead of the curve with respect to equipment reliability and have worked many of these issues out through extended marathon testing at consortiums such as SC300 and International Sematech.
Polish uniformity control Other issues facing CMP in 300 mm have to do with the increased level of control and automation required. Due to the high costs associated with 300 mm, manufacturers can ill afford the costs of wafer breakage or re-work. Also, yield loss due to process variation, such as post-CMP thickness control, becomes an even more critical issue at larger wafer sizes. If die yields don’t scale, then the advantages of moving to a larger wafer size are lost. The financial toll would be too great to make a successful transition.
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At 300 mm there are two major concerns regarding polish uniformity. One is the polishing head design itself. It is the most critical component in the tool when it comes to polishing uniformity control. In the short term, fabs will require CMP tools that can provide process flexibility with respect to across-wafer profile control. Wafer carriers must be designed to allow polishing profile control across the wafer to compensate for incoming non-uniformity from immature electroplating and dielectric deposition equipment and
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processes. As upstream tools and processes mature, providing better incoming uniformity, the equipment must allow easy process modification to compensate for the changing incoming profiles. Ideally, through the use of in-situ measurement equipment that can determine removal rates across a wafer during polish, the wafer profile can be closed-loop controlled on the fly, mitigating the effects of incoming non-uniformity and polish consumables degradation and variation. Advanced floating head designs, such as floating head or membrane based technologies that have worked extremely well at 200 mm, are having some difficulty transitioning to 300 mm. Machining tolerances are more critical. Doubling the wafer area doubles the frictional forces between the pad and wafer surfaces. The resultant torque produced by these frictional forces can induce unwanted vibrations and uneven pressure distribution. Carriers with configurable pressure profiles will be a necessity for long-term, stable processes at 300 mm. Another early concern was that slurry transport would be an issue at the larger wafer size, starving the center of the wafer for slurry, creating large center-to-edge uniformity issues. However, this has turned out to be not much of an issue as a result of advanced pad grooving techniques and newer wafer carrier designs. Uniformity on oxide <3 percent (1 sigma) @ 3 mm edge exclusion can be achieved using conventional slurries and pads used for 200 mm processes. For conventional slurries, slurry transport does not seem to be a major issue with regards to uniformity.
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when they do occur will become more important, driving the industry to on-board inline defect inspection equipment to detect major defect events such as scratching or incomplete metal removal. In-line defect inspection allows real-time, wafer-bywafer characterization of defect performance of a tool. Given shutdown criteria, the equipment can shut itself down if defects go out of control. This reduces equipment downtime for process monitoring and reduces the fabâ&#x20AC;&#x2122;s scrap exposure in the event of a defect occurrence that could have affected several lots of wafers. Again, these items tie into the theme of aggressive cost reductions for 300 mm by eliminating scrap and enforcing strict process controls which translate into increased line and die yields.
Endpoint, In-line Thickness Metrology (ITM), and Closed Loop Process Control (CLC) Process control for all CMP processes becomes more critical at 300 mm. Losses due to overpolish, underpolish, or missed endpoints at metal polish cannot be tolerated at 300 mm. Figure 2 shows a schematic of an advanced polisher with in-situ sensors and measurement systems. Process sensors that can measure thickness and reflectance across the wafer during polishing are becoming a necessity.
Defectivity Not only does slurry have to be well distributed underneath the wafer for consistent polishing performance, the waste products from the polishing have to get out from underneath the wafer surface. If these products are trapped due to waste transport issues, defectivity problems can surface. Also, if the equipment is not designed to keep pad and wafer carrier well cleaned between wafers, buildup can occur that can lead to scratching issues. With the larger platen sizes at 300 mm, this is made more difficult. Again, with modern pad grooving designs and proper design of pad cleansing/ conditioning components, this does not appear to pose a major issue. For 300 mm, there does not appear to be any major concerns with defectivity above and beyond what is already faced at 200 mm. However, the ability to quickly and accurately detect and react to defect excursions 20
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F i g u re 2. 300 mm CMP tool wi th integra ted cleaning and vari ous o n - b o a rd process sensor s a nd metro l o g y.
For STI and oxide processes, timed polishes are no longer acceptable. Accurate endpointing systems based
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Accelerating Yield
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host software control. Because of the strict yield requirements, reduction of human interaction, ergo human error, is paramount to increasing line yields. Recipe control, data collection, error reporting, remote diagnostics, and interfacing with yield-management tools are all examples of the significantly increased requirements out of the gate for the 300 mm equipment set.
Cost Of Ownership F i g u re 3. Potential gain in dishing and erosio n p er f o rman ce b y re d u c ing amount of over polish re q u i red with an accurate endpoint system .
on film thickness measurements are currently being developed and tested. For metal polishing, accurate endpointing, when the wafer is fully clear of bulk metal, is critical in reducing required overpolish. Figure 3 shows the impact of overpolish on dishing and erosion in copper polishing. An accurate endpoint system, allowing endpoint to be called as close to true endpoint as possible, is critical to reducing dishing and erosion to acceptable levels in metal and shallow trench CMP applications.
The cost of 300 mm capital equipment is very high. Therefore another key issue facing equipment manufacturers and fab managers alike is reducing cost of ownership. For CMP, this means pushing to reduce power and water usage, extending consumable lifetimes, and implementing advanced technologies such as fixed or bonded abrasives and slurriless CMP. Other areas of focus are increasing throughput and reliability while at the same time reducing footprint. Most first-generation 300 mm equipment has unoptimized (large) footprints, mainly due to pressures from shortened engineering schedules due to the rapid turn-on of 300 mm activity after a â&#x20AC;&#x153;breakâ&#x20AC;? in the late 1990â&#x20AC;&#x2122;s. These items, while not new to CMP, become requirements (as opposed to options in the past). Summary
In-situ thickness metrology (ITM) provides the capability to pre- and post-read dielectric thickness for closed-loop process control within the CMP process, as well as providing feed-forward capabilities to downstream processes. For example, as we push the limits of photolithography and dielectric etching, film thickness control becomes more critical for the stability of these downstream processes. Knowing film thickness on a per-wafer basis prior to processing can allow on-the-fly process adjustments to improve the process window of these steps. Since CMP requires much of the same information for its own process control, ITM makes it convenient for providing this data.
Automation At 300 mm, automation hits a supercritical stage. Most fabs that make the switch to 300 mm are requiring 100 percent of their equipment set to be enabled for
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The news is not that bad for CMP at 300 mm with regards to process. Most of the process learning from 200 mm will transfer with relative ease to 300 mm. Concerns of slurry transport issues and major non-uniformity problems have not been realized. Uniformity on oxide <3 percent (1 sigma) @ 3 mm edge exclusion can be achieved using conventional slurries and pads used for 200 mm processes. So, for the fab beginning the process toward 300 mm, consumable sets established for 200 mm CMP should transfer with only minor process optimization, reducing risk for their pilot and production lines. The main issues for CMP, which will be more challenging to resolve, are muchimproved reliability through reliable hardware design, more advanced process controls utilizing in-situ sensors, metrology and automation, and further reduction of CoO through reduced utilities usage and advanced CMP technologies such as fixed abrasive or slurriless CMP.