Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 30
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As a result,an elaborate magnetically-coupled rotation mechanism is used (to preservevacuum integrity) to accomplish this. The major disadvantage of such elec-166Chemical Vapor Deposition for MicroelectronicsLOADMATCHINGNETWORKRF POWER SUPPLYOUTERZONEATCCENTERZONEATCINNERZONEATCTHREE ZONETEMPERATURECONTROLSYSTEMGAS PANELFigure 18: Applied Materials AMP-3300 plasma CVD system.trode rotation is that it is inconvenient to position the electrode heater on theunderside of the electrode. Therefore, the three-zone heater heats the electrode by radiation, being about %-inch from it.Gas enters at the center of the electrode along the axis of rotation fromthe gas panel.
It was found that if the gas were allowed to flow in directlyand impinge normally to the upper electrode, that excessive deposits builtup rapidly at the center of this upper electrode, and poor deposition uniformities were observed. Therefore, a gas injection shield was placed at the centerof the lower electrode to provide some radial momentum to the incominggas flow. This shield is illustrated in Figure 19.The system operates at about 400 mTorr and with flow rates of about1 slm, so a roots blower is used to pump the system to base pressure ("1 mTorr)and then handle the gas flow at these low pressures.The top of the reaction chamber is hinged to permit access to the lowerelectrode (platen) that holds the wafers.
This arrangement is illustrated inFigure 20. The platen is anodized, while the upper electrode is bare aluminum.The upper electrode housing is flame spray coated with aluminum oxide toel iminate electrical discharges above the upper electrode. The platen has pocketsto accept wafers and maintain them in fixed positions as it rotates.As noted earlier, wafer throughput is a key concern. The more wafers thatcan be placed on the platen, the higher the throughput. For 3" wafers, a loadof 42 wafers could be handled.
As wafer sizes have increased, however, thecapacity of this system has been severely impacted. For 5" wafers, only 16can be loaded at one time. Obviously, the problem is more difficult for 6"and eventuallywafers. This difficulty eventually led to the developmentof the hot tube PECVD system to be discussed next.a"Production CVD Reactor SystemsANODIZEDPLATENFigure 19: AMP-3300 gas inject shield. Applied Materials, Inc.ELECTRODEBASEPLATEFigure 20: AMP-3300 process chamber. Applied Materials, Inc.167168Chemical Vapor Deposition for MicroelectronicsIt should also be noted that this reactor has to occupy space in the cleanroom, and is not operated under computer control.
For application to modernfab Iines, these are two disadvantages of this system.The hot wall approach to the plasma-enhanced CVD system has been described in Chapter 3. A schematic of a typical system is shown in Figure 21.The elements of this system are similar to that of the cold-wall system justdescribed. There is a gas panel, vacuum system, and an RF power supply tocreate the discharge.
The RF frequency typically used is 400 kHz. The reaction chamber of such a system is shown in Figure 22. The electrodes are a setof several long narrow rectangular slabs of graphite with pockets cut into them.The graphite electrodes lead to some problems with particulate contamination,but attempts to use aluminum have not been successful.PRESSURE SENSORGRAPHITE BOATGAS INAIROPERATING/VACUUMVALVEVIEWGLASS0000oGASIN00PRESSURECONTROLVALVE0FURNACE CONTROLAMANIFOLDAFC'SSIH4N,NH3DOPBMANIFOLDAFC'SC,F.0,N,OBALFigure 21: Schematic of hot wall plasma deposition system, PWS-450. PacificWestern Systems, Inc.Figure 22: Plasma-enhanced CVD reaction chamber, PWS-450.
Pacific WesternSystems, Inc.Production CVD Reactor Systems169The major advantage of the hot tube approach is in wafer throughput. Atypical wafer load would be seventy 3" wafers, as contrasted to the 42-waferload in the AMP-3300. The advantage is not quite 2: 1, as the hot tube waferboat cools off when it is withdrawn to replace finished wafers. Time to reheatthis structure lengthens processing time.As in the LPCVD reactor discussed earlier, allowing the electrode structure to touch the tube wall as it is inserted leads to considerable particle contamination.
Therefore, cantilever loaders are available here also, and a typicalunit on an ASM reactor is shown in Figure 23. Again, in contrast to the AMP3300, this system is operated under computer control. Automated handlingof wafers is more difficult to achieve, and is not generally available.Figure 23: Cantilever loading system for plasma-enhanced CVD system. ASM,Inc.6.6 NEW CONCEPTSAll of the reactor systems described so far have shortcomings of one sortor another.
They may have excellent throughput, but introduce too manyparticles into the process. They may be awkward to disassemble and clean, leading to substantial downti:71e. Uniformity on individual wafers has historicallybeen adequate at ± 5%. Today, uniformities of ± 1 to 2% are being requiredbecause of more stringent process requirements, and this is hard to achieveover all the wafers in a large batch load. Uniformity of dopant on a wafer isalso a concern.As a result of the ever tighter specifications being imposed on CVD systems, a number of new generation systems have been introduced recently.We will review several of these, although none has yet achieved any substantialcommercial acceptance.170Chemical Vapor Deposition for Microelectronics6.6.1 Hot Wall Cross-Flow ReactorOne of the difficulties with the traditional LPCVD hot tube reactor isdepletion of reactant gases as they flow down the length of the tube.
To overcome this problem, designers either ramp up the temperature at the tube backend to increase deposition rates, or introduce the gases in a distributed fashionalong the tube length. Neither solution is ideal, so several years ago Aniconintroduced a new hot wall configuration. In this system, a hot quartz bell jaris used to provide the uniform temperature ambient for deposition.The reaction chamber is also quartz, and is contained within the largerbell jar. Since the reaction chamber is operated at low pressures, a ballast gasoccupies the space between the bell jar and the reaction chamber.Reactant gases enter the reaction chamber along its vertical axis, and areintroduced at the top. The wafers are placed vertically in cassettes with typical close spacing. The gas then flows parallel to the wafer surfaces and leavesthe system. Therefore, the only reacting gases a wafer sees are fresh ones, anddepletion is not an issue.Another feature of this approach is a claimed lower particulate count thatthe wafers are exposed to.
If the longer flow path in an LPCVD furnace promotes gas phase nucleation and stirs up particles, then this system should minimize such effects.In the traditional LPCVD hot tube reactor, cleaning the tube required removing it from the furnace, which can be quite a job. In the Anicon system,the claim is that the quartz reaction chamber can be replaced in 15 minutes.Anicon claims this reactor can process up to one hundred 5" wafers atone time.
Temperatures up to 740°C are available, and pressures of 250 mTorrto 10 Torr can be run. All LPCVD processes that were described earlier inChapter 3 can be executed on this system.6.6.2 Cold-Wall Thermal SystemsAll of the ren1aining new CVD reactor systems are cold wall reactors carrying out thermal or plasma-enhanced CVD processes. They are being developedto deposit a variety of films, but each system is initially targeting a particularmaterial.The first of these was developed several years ago to deposit tungsten andtungsten silicide films.
A diagram of the reactor chamber is shown in Figure24. It is based on the axisymmetric cold-wall hexode reactor heated fromwithin, described in Chapter 1. Since there are eight wafers inside the reactorat one time, it is a batch reactor, even though there are eight gas injectionports. If the susceptor did not rotate during deposition, one could characterizeit as a single chamber with eight single-wafer stations.The obvious question to ask is why not run this process in a hot tube andtreat 50 to 100 wafers at a time rather than eight? The explanation is that theWSi 2 process (WF6 + SiH 4 ~ WSi 2 + ..... ) proceeds very rapidly so thatwhen reasonable deposition rates are achieved, the process is diffusion controlled . As we recall, the hot tube batch reactor only works when the process can be operated in a surface-controlled regime.Water CooledTop Plate &SIdewallsProcessChamber-B WaferTurret AssemblyiRobotiCWafer HandlerCircumferentialGa'S DistributIon ~Manifold'S1B DistributedEllhaust Header'>i(NF)ILDGasControlSystem(sccm)0-500Water CooledDome & SkirtN:;:OB Gas MillingChambers0-1000Argon 0-200oIRFIsolationr~--ORecirculationFilterallDN 2 Ca.e/PurgeI~L_~--oN 2 Ballast! .-- PreyentlonI1300 CFMRoot.
BlowerL-DCapacitanceManometer~----oTurretSpindle HousingTurret RotatingMechanismT.C. VacuumGauge-0OedicatedOlfferentlalSeal Pumpac..IR LampA •• emblyri-c(")o:lTemperatureControl_D--D21S00W13.58MHzRF Generator()<o:0C'l)R~ MatchlniNetworkOJ(")ri-~CJ')-<enElectronics RackriC'l)3enFigure 24: Genus tungsten CVD reactor system.'-J~172Chemical Vapor Deposition for MicroelectronicsA second approach to a cold wall system is the single-wafer CVD reactordeveloped by Varian-Torrex.
A schematic of the reaction chamber is shownin Figure 25. Again, tungsten silicide is deposited in this cold-wall reactor.Other conducting films such as blanket and selective tungsten can also be deposited.l.R. HeaterGasIn.MassFlowControllerReactionChamberGateValveThrottleValveWaferLoaderr::-lL.:::..JFigure 25: Varian-Torrex 5101 single-wafer CVD reactor system.In comparison to the Genus reactor, this system holds the wafer upsidedown to minimize any particulate on the wafer.
Also, since this is a singlewafer machine, a loadlock is provided to ensure that the reaction chamberis never opened to the atmosphere. Attempts to provide this feature on a batchreactor are difficult and expensive, due to the size of the chamber needed.Heating is done in a way similar to the Genus system. High-intensity lampsshine on the back of a chuck to heat it to processing temperature.A final point should be made concerning the single-wafer CVD reactorconcept.
This approach only makes sense if each wafer can be processed in1 to 2 minutes, so reasonable throughput can be achieved. In many applications, conducting films can be thin, ---2000 A, so deposition rates of 1000 to1500 A/min would be suitable. Such rates are not unreasonable, for example,for WSi 2 films.The remaining system is a plasma-enhanced CVD system for the lowtemperature deposition of low hydrogen content silicon nitride. The systemis shown in Figure 26, and a schematic of the reaction chamber in Figure 27.As can be seen, this reactor is a batch system where the wafers are placed ina square array.In this reactor, N2 is introduced into a number of small glow dischargechambers.
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