Arthur Sherman - Chemical Vapor Deposition for Microelectronics (779637), страница 29
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1..... <~:~~",..... ",;""~I,.'"ff'f#l"Ul~,; ... t·OlFigure 8: Tylan-Tytan LPCVD reactor details.158Chemical Vapor Depos.ition for MicroelectronicsFigure 9: Vacuum and gas flow module for LPCVD furnace, Tylan.6.4 EPITAXIAL SILICON REACTORSCommercial epitaxial silicon reactors are difficult to design and hard tooperate rei iably because of the very high temperatures they must operate at(~1 000° to 1200°C).
One of the more successful systems has been the radiantly heated reactor developed by Applied Materials, Inc. and shown in Figure10. The system has the same elements as the LPCVD system just described.A schematic showing how these elements are arranged is shown in Figure 11.This is a cold-wall barrel reactor system with a 3-zone bank of high intensitylamps to provide the heat to the wafers on the susceptor. The system can be runas either an atmospheric pressure or reduced pressure reactor (about 100 Torr),and can operate with either SiCI 4 or SiCI 2 H2 as the silicon source.
When SiCI 4is used, a bubbler to vaporize it must be provided. A more detailed sketch ofthe reactor module is shown in Figure 12. The susceptor is hung vertically andProduction CV D Reactor Systems159Figure 10: AMC 7800 radiantly-heated epitaxial reactor system-Applied Materials, Inc.ATUOSPHERtC VENTTO SCRUBBERr------'r-------------- ---,AEACTOA CAIlINfTCONTROl CONSOl£III::IIIIIIIIPUMP EXHAUSTTO SCRull8fRIl,.;:::;~:.t-----1r--1 : "'::.~r;::===~-t--r~~;-lIr - - - IIIPRESSURE~~- ---,IIIIIIII1 1 - - -......1~;;;~==~~~~*~PARTICULATEri"TRHI PUtoIPII A PIIIIIIL..b~--lllVALVE"5'"•LI.oo-_..I_.....L_COMMUNICATIONIIII -TOIf-HO~~~NTRAlIICOMPuHR,IL..JVACUUt.ISl..c:Ni pu"pVAlVEI,III~~u,:-Figure 11; Schematic of AMC 7800 series epitaxial reactor system-Appl iedMaterials, Inc.:.J160Chemical Vapor Deposition for Microelectronics(,/\')INPl,r( .1IN!'I)Figure 12: Reaction chamber of AMC 7800 epi reactor system-Applied Materials, Inc.rotated during processing.
Heat is provided by radiant heating modules whoseradiation passes through the quartz bell jar and is absorbed by the silicon carbide-coated graphite susceptor. Air flow is used to cool the bell jar to minimize silicon deposition there. The radiant heating modules are gold coated toreflect thermal radiation returning from the hot susceptor. This is a workablearrangement, as this gold coating is transparent to the high-intensity opticalradiation of the lamp assembly, but reflects the thermal radiation of the susceptor.Production CVD Reactor Systems161The effect of heating this axisymmetric arrangement from the outside isto create a very uniform temperature environment for each wafer.
By thismeans, thermally-induced slip is virtually eliminated.At the pressures at which this system is run, deposition is diffusion controlled. Therefore, the flow patterns set up and boundary layer thicknessesare important to the goal of uniform deposition on all wafers. As noted inChapter 3, the narrowing flow passage in the flow direction compensates forreactant depletion.As wafers get larger, fewer wafers per load can be handled. For example,the susceptor can have 10 faces with 4 wafers per face for 3" wafers, for atotal of 40 wafers per load. For 5" wafers, the susceptor will have 5 faces and2 wafers per face, for a total of 10. This loss of capacity can only be compensated for by building a larger reactor or by purchasing two systems.The other configuration of silicon epi reactor that has been successfulcommercially is the so-called pancake reactor. The basic concept behind thesystem was shown in Chapter 1, Figure 22.
As a production reactor, it is simple,rugged and inexpensive compared to the radiantly-heated barrel just described.The typical graphite wafer holder inside a quartz bell jar is shown in Figure13.Figure 13: Vertical pancake epi silicon reactor chamber-Gemini Research.162Chemical Vapor Deposition for Microelectronics"These systems have been the work horses for the less demanding epi applications. In g8neral, epi thickness uniformity has not been as good as theradiative systems, but the biggest difficulty has been that they often producewafers with significant slip defects. As noted earlier in Chapter 3, recent design modifications have tended to minimize these problems.A new epi reactor, the "Precision Epi 7010," has recently been introducedby Appl ied Materials.
The unusual layout of this new system is shown in Figure14. The central concept in this new reactor system is lower processing costper wafer, while hopefully maintaining the same quality of epi films. The design features that lower the per wafer cost are the dual susceptor design andthe use of low-frequency induction for heating. With two susceptors, one canbe loaded or unloaded while the other is being processed.Figure 14: Schematic of Appl ied Materials Precision Epi 7010 reactor system.The high-intensity lamp array in the earlier design is expensive, so theyhave been replaced by a 10kHz RF induction coil placed around the entirebell jar. The bell jar and coil are shown in Figure 15.
By using this arrangement and an electrically-conducting silicon carbide-coated graphite susceptor,the coil heats the susceptor by inducing currents to flow in it. The bell jaris then gold coated to reflect thermal radiation back onto the wafers for a moreuniform thermal environment than is usually achieved with induction heating. Water cooling on the outside of the bell jar is used to minimize depositsand subsequent cleaning operations.
This concept of dual direction heatingis illustrated in Figure 16.Production CVD Reactor Systems163Figure 15: Appl ied Materials Precision Epi 7010 bell jar and induction coil.In addition to the design goal of reducing the processing cost per wafer,this system has been developed to reduce the particulate contamination to aminimum. To accomplish this, the entire system is enclosed within a laminarflow hood, so that the wafers are handled in a class 10 environment. Secondly,wafers are loaded and unloaded by an automated robot, so that human intervention is not necessary to handle the two susceptors.
Both of these featuresare illustrated in Figure 15.164Chemical Vapor Deposition for MicroelectronicsBE('1IIr-- -- -\IIrI'ErIII)Figure 16: Dual-direction heating in Applied Materials Epi 7010 reactor system.Finally, clean room floor space is expensive, so this system has been designed so that the entire reactor can be outside the clean room, with one faceoccupying wall space only.Another new epi reactor has also been introduced recently by GeminiResearch. This system uses a cold quartz bell jar much the same as the earlierpancake reactor. The susceptor configuration is new, however, and is shownin Figure 17.Production CV D Reactor Systems165Figure 17: Reactor chamber of Gemini-Tetron , epi system.Just as in the new Applied Materials reactor, the problem being addressedis the reduced throughput with conventional epi reactors as wafers get larger.In this reactor configuration, 50 wafers can be placed on the 25 tapered cavities placed radially within the bell jar.
There are resistance heaters above andbelow the silicon carbide-coated graphite wafer holders, and heat loss at theouter periphery is compensated for with external heat lamps.Other features of this system are comparable to the Appl ied MaterialsEpi 7010, in that robotic wafer loading is included, and the system is fullycomputerized as well as designed for through-the-clean-room-wall operation.6.5 PLASMA-ENHANCED SYSTEMSThere have been a wide variety of plasma-enhanced CVD reactors developedover the years, but only a few have had significant success as production systems.
3 The first commercially successful plasma-enhanced reactor system wasa parallel-plate, capacitively-coupled, cold-wall system based on the conceptdescribed in Chapter 2, Figure 13. A block diagram of the complete systemis shown in Figure 18. In this application, a 50-kHz RF power supply is usedto power the upper electrode, while the lower electrode and the balance ofthe chamber is grounded. For this system, it was felt that it was necessaryto rotate the lower electrode to smooth out the non uniformities.
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