Roland A. - PVD for microelectronics (779636), страница 23
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5.12 lllustration of the basic architecture of an applications-flexible PVD cluster tool (basedon an Applied Materials design).specific PVD cluster tools having a smaller number of process chambershave also been developed (e.g., the mb2 TM model from Varian Associates,which targeted barrier/liner applications such as preclean/Ti/TiN). However, in spite of important supplier-specific differences in tool design andconstruction, PVD cluster tools generally exhibit four basic buildingblocks:1.
Front-end for cassette-to-cassette, wafer loading/unloading into/outof the tool. Load and unload stations can be vacuum isolated from eachother, and sometimes the load station includes a wafer flat alignment aswell. Cassettes can be handled manually by an operator or robotically by118R. POWELL AND S. M. ROSSNAGELan automated guided vehicle (AGV). In some tool designs, an incoming cassette of wafers is stored at atmospheric pressure under a flow of dry, filteredair or nitrogen to prevent additional exposure to water vapor.
Individualwafers are then loaded into the tool from the cassette via a vacuum loadlock.An alternative design is to place the entire cassette in a vacuum loadlockthat is then pumped down so that subsequent removal of individual wafersfor processing occurs with the cassette under clean, vacuum conditions.2. Degas~cool station to heat-treat the wafer for subsequent processsteps or to cool the wafer sufficiently to allow placement in a plasticizedcassette. In some cases, flat alignment is done in this station as well.3. Transfer module (sometimes called a central wafer handler) in whichthe wafer is robotically moved with high positional accuracy ("pick-andplace") between vacuum-isolated process modules. In the early years ofPVD, wafer orientation during handling and processing was a topic ofmuch discussion.
Today the generally accepted practice is having the waferhorizontal and face-up during both handling and PVD.4. Process modules in which sputter deposition and other process stepssuch as a preclean before PVD or a rapid thermal anneal after PVD are carried out. The modules are vacuum-isolated from each other, which permitsparallel processing to be done at different vacuum levels or incompatiblegas chemistries to be used without cross-contamination (e.g., PVD of TiNusing Ar/N 2 in module 1, Ar + sputter etch removal of native SiO 2 in module 2, and PVD of A1-Cu using Ar in module 3 without the formation of insulating A1N or A1203).5.3 The Technology of PVD Cluster ToolsWe now consider in detail a number of interrelated hardware and processissues that are common to all PVD cluster tools, such as vacuum pumpingand gas delivery, wafer handling and holding, wafer thermal management,contamination, and particles.
Recognizing that tool productivity is today asimportant as technology, topics related to cost-of-ownership (capital andconsumables cost, throughput, maintenance, etc.) will also be discussed.5.3.1 VACUUMCONSIDERATIONSThe vacuum range encountered during PVD processing ( ~ 12 orders ofmagnitude) is arguably the largest in microelectronic production sincewafers enter the tool from a clean room at atmospheric pressure (760 Torr)SPUTTERING TOOLS119and are ultimately processed at a few mTorr in PVD modules that can haveultrahigh vacuum (UHV) base pressure of < 10 -9 Yorr.
Figure 5.13 showsone possible range of vacuum levels (both base and operating pressures)for a PVD cluster tool.A PVD cluster tool must be designed to simultaneously deliver high vacuum base pressure in the PVD process module and high wafer throughput( ~ 20-50 wafers per hour or more, depending on the overall process complexity and process time per module).
The vacuum requirement is relatedprimarily to the fact that metals such as A1 and Ti are highly reactive towater vapor and other oxidizing ambients (forming insulating A1203 andTiO2) and that film microstructure and electrical properties can be adversely affected by very small amounts of hydrocarbons, O or N [5.9]. Thethroughput requirement means that pump-down and vent-up times must beas short as possible.These requirements have led designers of PVD systems to adopt vacuumsystem design techniques previously used in UHV molecular beam epitaxy(MBE) deposition systems in which the level of vacuum seen by the waferimproves in a stepwise way from the loadlock (low vacuum) to transferFIG.
5.13Representative vacuum levels encountered in PVD cluster tool processing.120R. POWELL AND S. M. ROSSNAGELchambers and precleaning chambers (medium vacuum) to process chambers (high or ultrahigh vacuum). For example, a wafer might be taken froma cassette under atmospheric pressure (high purity N 2 gas at ~ 760 Torr),into a vacuum loadlocked degas station ( ~ 10 -4 Tort), into the centraltransfer module ( ~ 10 -7 Torr), and finally into a PVD process module( ~ 10 -8 Torr). In some PVD tools, the volume containing the wafer cassettes is pumped down and maintained under low mTorr-level vacuum[5.8, 5.10], thereby reducing the pressure change to the next vacuumisolated stage.
In other designs (see Fig. 5.12 and ref. 5.8), an intermediate level of vacuum can be added to further control interstage pressure gradients and/or to achieve true UHV PVD conditions ( ~ 10 -9 Torr). Theso-called vacuum buffering [5.10] inherent in all these designs reduces thechance of process chamber contamination from atmospheric gases. Itshould be remarked that obtaining ultralow pressure in a PVD Ti moduleis facilitated by the fact that Ti is an excellent getter of oxygen and watervapor, and the Ti that invariably coats the relatively large surface area ofshields, collimator, etc.
provides an intrinsic vacuum-pumping capability.Also, even though a module achieves ultrahigh vacuum base pressure, thepressure during PVD is about 106 times greater so that even trace levels ofimpurities in the process ambient can compromise the expected benefits ofhigher quality vacuum.With regard to the vacuum levels in a PVD cluster tool, it is importantto note that operating pressure and base pressure are sometimes confusedand that these pressures can be very different in practice. Operating pressure is primarily determined by process conditions and how long one iswilling to pump on the chamber before it is opened to another part of thetool.
Base pressure is the ultimate, lowest pressure that can be achieved ina particular vacuum chamber after, in theory, an infinite pump-down time(see Eq. 5.1). Base pressure depends on such things as outgassing from thechamber walls, real and virtual leak rates of gas into the chamber, and thetype of vacuum pumping being used. For example, an unbaked PVD chamber pumped down from atmosphere with a mechanical roughing pumpmight "base out" at 10 -4 Torr. The same chamber after vacuum baking andpumping with a cryopump might base out at 10 -9 Torr. During PVD thechamber is backfilled with Ar so that its operating pressure at this point inthe wafer process sequence might be ~ 1 mTorr.
After deposition, but before the vacuum valve between chamber and transfer module were opened,the PVD chamber might be quickly pumped down to a pressure of, say,10 -6 T o r r - which is much higher than the base pressure of the modulebut sufficiently low to prevent an unacceptable gas load from getting intothe transfer module.SPUTTERING TOOLS121In other words, even though the base pressure of the module is 10 -9Torr, it might actually vacuum cycle between 10 -3 Torr and 10 -6 Torr during operation. In a similar vein, the wafer degas station at the front end ofa PVD cluster tool may in fact base out in the high vacuum range, butwould effectively operate in the medium vacuum range as a result of thewater vapor outgassing of wafers being continually processed and thethroughput-driven need to keep pump-down times as short as possible.Finally, while base and operating pressure are certainly relevant parameters in PVD cluster tool design, the residual gas composition of the vacuum ambient m e.g., the partial pressure of water vapor and oxygencan be equally relevant with regard to film properties.The scope of this book does not permit a tutorial on vacuum science andtechnology; the reader is referred to several excellent monographs on thesubject [5.11-5.13] and articles with a focus on vacuum technology forsemiconductor processing [5.14, 5.15].
On the other hand, state-of-the-artPVD cluster tools share the same vacuum considerations, leading to identifiable trends in the types of pumps, seals, and materials of construction[5.16].Vacuum PumpingGiven the great variation in vacuum level and chamber volume in a PVDcluster tool (5 x 10 -6 Torr in a 10-liter degas/cool module, 5 x 10 -8 Torrin a 40-liter transfer module, etc.), it is not surprising that a wide variety ofvacuum pumps must be used.
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