ch化學(xué)氣相沉積

1、CVD化學(xué)氣相沉積 要點(diǎn)：要點(diǎn)： 化學(xué)氣相淀積的基本原理化學(xué)氣相淀積的基本原理 CVD特點(diǎn)特點(diǎn) CVD裝置裝置 低壓低壓CVD 等離子體化學(xué)氣相淀積等離子體化學(xué)氣相淀積 PECVD 金屬有機(jī)物化學(xué)氣相沉淀（金屬有機(jī)物化學(xué)氣相沉淀（MOCVD） 化學(xué)氣相淀積，簡(jiǎn)稱(chēng)化學(xué)氣相淀積，簡(jiǎn)稱(chēng)CVD(Chemical Vapor Deposition)是把含有構(gòu)成薄膜元是把含有構(gòu)成薄膜元素的一種或者幾種化合物或單質(zhì)氣體供素的一種或者幾種化合物或單質(zhì)氣體供給基片，借助氣相作用或在基片上的化給基片，借助氣相作用或在基片上的化學(xué)反應(yīng)生成所需薄膜。學(xué)反應(yīng)生成所需薄膜。 gas inlet gas decomposi

2、tion gas reaction substrate adsorption gas exhaust定義定義 1) gas decomposition (1) thermal deposition (2) plasma deposition (3) photon (laser, UV) deposition2) Kindsaccording to temp., pressure, CVD chemical vapor deposition APCVD atmospheric pressure. LPCVD low-pressure. VLPCVD very low pressure PECVD

3、 plasma-enhanced. LECVD laser-enhanced. MOCVD metal-organic. ECRCVD electron-cyclotron resonance. VPE vapor-phase epitaxy3) advantages low cost dielectric ( poly-silicon, Si3N4, SiO2 ) and metal thin film high deposition rate high or low pressure control thickness, defect and resistivity high film q

4、uality LTCVD for semiconductors, ex., Si3N4, SiO2 and epilayer low radioactive damage But high deposition tempoerature4) parameters for thin film structure(1) temperature of substrate and chamber(2) growth rate(3) gas pressure These parameters affect on surface speed of the involved atoms.APCVDThin

5、FilmReaction Gas (carrier)Temperature ()Growth rate (nm/min)wafer/hrEpitaxial Si SiCl4 (H2)/H2SiHCl3 (H2)/H2SiH2Cl2 (H2)/H2SiH4 (H2)/H211251120110011501050110010001075500150050015005001000100300Poly Si SiH4 (H2)850100010040Si3N4 SiH4 /NH3 (H2)90010002040SiO2 SiH4 /O3 (H2)200500100160Table 1-1 CVD fo

6、r thin film used for semiconTable 1-1 CVD for thin film used for semiconductor processductor processLPCVDThin FilmReaction Gas (carrier)Temp. ()Growth rate (nm/min)Epitaxial Si SiH2Cl2 (H2)/H210001075100Poly Si 100% SiH4 (0.2 torr)620100Si3N4 23% SiH4 (H2) (0.1 torr)SiH2Cl2 /NH3(0.3 torr)640800194Si

7、O2 SiH2Cl2 /N2O9008SiO2 SiH4 /O3SiH4 /PH3 /O3 (0.7 torr)4504501012PECVDThin FilmReaction Gas (carrier)Temperature ()Growth rate (nm/min)Si3N4 SiH4/NH3(N2) (0.3 torr)30010SiO2 SiH2Cl2 /N2O25084 -SiSiH4 /H3(0.1 torr)3006Overviewnot all components are found in all CVD systems: Source gasReacts on subst

8、rate to deposit film2. Types of CVD reactions熱分解熱分解Pyrolysis - thermal decomposition AB(g) - A(s) + B(g) ex: Si deposition from Silane at 650oCSiH4(g) - Si(s) + 2H2(g) use to deposit: Al, Ti, Pb, Mo, Fe, Ni, B, Zr, C, Si, Ge, SiO2, Al2O3, MnO2, BN, Si3N4, GaN, Si1-xGex, . . .還原還原/置換置換Reduction/Excha

9、nge often using H2 (metal, substrate)AX(g) + H2(g) A(s) + HX(g)often lower temperature than pyrolysisreversible = can use for cleaning tooex: W deposition at 300oCWF6(g) + 3H2(g) W(s) + 6HF(g) use to deposit: Al, Ti, Sn, Ta, Nb, Cr, Mo, Fe, B, Si, Ge, TaB, TiB2, SiO2, BP, Nb3Ge, Si1-xGex, . . .氧化氧化/

10、氮化氮化Oxidation/Nitritionoften using O2 /N2AX(g) + O2(g) - AO(s) + OX(g)ex: SiO2 deposition from silane and oxygen at 450oC (lower temp than thermal oxidation)SiH4(g) + O2(g) - SiO2(s) + 2H2(g) use to deposit: Al2O3, TiO2, Ta2O5, SnO2, ZnO, . . .Compound formationoften using amonia or water vapor AX(g

11、) + NH3(g) - AN(s) + HX(g)AX(g) + H2O(g) - AO(s) + HX(g)ex: deposit wear resistant film (BN) at 1100oCBF3(g) + NH3(g) - BN(s) + 3HF(g) use to deposit: TiN, TaN, AlN, SiC, Al2O3, In2O3, SnO2, SiO2, . . .Disproportionation(歧化反應(yīng)）歧化反應(yīng)）compunds involving elements with multiple valence states（多種價(jià)態(tài)） 2AB(g)

12、 A(s) + AB2(g)ex: use to deposit: Al, C, Ge, Si, III-V compounds, . . .可逆輸運(yùn)可逆輸運(yùn)Reversible Transferex: use to deposit: GaInAs, AlGaAs, InP, FeSi2, . . .How does reaction go?Depends on parameters: Temperature Pressure reactants (purity, concentration) Thermodynamics and dynamics3. Reaction law3.1 Ther

13、modynamics of CVDidentify possible reactions ignores rate informationlDGr is the criteria. DGr0 (Textbook p119 or p111)not strictly correct in flowing system (nonequilibrium) Ellingham plots can be useful MoCl5, ReCl3 and AsCl3 are all reduced by H to metals and HCl Ni, Fe, and Co chlorides reduce a

14、t intermediate temperatures SiCl4 reduces at high temperature many other metal chlorides are too stable CrCl2 is close - but not quite Fig. 4.23.2 CVD 過(guò)程氣氣體體輸輸入入氣氣體體對(duì)對(duì)流流氣氣相相擴(kuò)擴(kuò)散散表表面面吸吸附附表表面面反反應(yīng)應(yīng)表表面面脫脫附附薄薄膜膜成成核核生生長(zhǎng)長(zhǎng)once reaction is identified, consider the process in detail: 源source: production of app

15、ropriate gas 氣相傳輸transport of gas to substrate 沉積deposition of film: 吸附吸附adsorption of gas on substrate 反應(yīng)反應(yīng)reaction on substrate phase transformation recrystallization, grain growth 廢氣排除廢氣排除transport of waste products away from substrate CVD Sources 源類(lèi)型types of sources gasses (easiest) volatile liq

16、uids sublimable solids combination materials should be 穩(wěn)定穩(wěn)定stable at room temperature 揮發(fā)性揮發(fā)性sufficiently volatile high enough partial pressure to get good growth rates reaction temperature stagnant layer (滯流層、邊界層滯流層、邊界層)diffuse gas through stagnant layer to surface mass transport depends onfundament

17、al parametersexperimental parameters反應(yīng)物濃度反應(yīng)物濃度reactant concentrationpressure擴(kuò)散擴(kuò)散diffusivitygas velocity邊界層厚度邊界層厚度boundary layer thicknesstemperature distributionreactor geometrygas properties (viscosity . . .) Simple model (Grove, 1967)AB(g) - A(s) + B(g) F1 = flux to surfaceF2 = flux consumed in fi

18、lmCG = concentration of AB in gasCS = concentration of AB at surfaceF1 = hG (CG - CS)=D/*(nG-nS)where hG = gas diffusion rate constant, or D F2 = kS CSwhere kG = surface rate constant in steady state: F1 = F2 = F growth rate of film is proportional to FTT3/23/2/pNOTE: Two rate-limiting cases傳質(zhì)限制機(jī)制傳質(zhì)

19、限制機(jī)制mass transfer limited small hG growth controlled by transfer to substrate hG is not very temperature dependent common limit at higher temperatures lower pressure, higher T Quick diffusion Quick diffusion表面反應(yīng)限制表面反應(yīng)限制surface reaction limited small kS growth controlled by processes on surface adsor

20、ption, decomposition surface migration, chemical reaction desorption of products kS =C*exp(-Er/RT) is highly temperature dependent (increases with T) common limit at lower temperatures often preferred 4. CVD 裝置Heating methods for CVDCVD 反應(yīng)室Textbook P1265. CVD 類(lèi)型 氣壓 1 mtorr - 1 torr (rather than 1 at

21、m) 低總壓、高分壓 = higher D of gas to substrate 通常是表面速率限制機(jī)制surface reaction limitingAdvantages 中等反應(yīng)速率 均勻性好 uniformity 臺(tái)階覆蓋度好 coverage over steps 缺陷濃度低, 污染少 高產(chǎn)率5.1 Low Pressure CVD (低壓低壓CVD)LPCVDJ1/p1/p5.2 Plasma Enhanced CVD (等離子體輔助等離子體輔助CVD)低壓低壓CVD中利用輝光放電等離子體的影響生長(zhǎng)薄膜。中利用輝光放電等離子體的影響生長(zhǎng)薄膜。壓強(qiáng)：壓強(qiáng)：5500Pa PECVD

22、deposition films （1）3SiH4+4NH3 Si3N4+12H2 （2）SiH4+2N2O SiO2+2N2+2H2 （3）SiH4 Si+2H2 （4）(1-x)SiH4+xPH3 Si1-xPx+H2 CPaArN350250,67,2PlasmaCPaArN350250,67,2PlasmaCAr625500,PlasmaCAr700600,Plasma Using electron energy (plasma) as the activation method to enable deposition at a low temperature and at a re

23、asonable rateDisadvantagesRequirement of a vacuum system, and a more sophisticated reactor for plasmaMore expensive than the thermally activated CVD systemDifficulty in depositing high purity filmsSubstrate damaging by strong ion bombardmentAdvantagesRelatively low temperatures on large areasHigher

24、growth rate than thermal CVDHigher adhesionGood step coverageA schematic diagram of PECVD systemTab. 5-2, pp131plasma in vicinity of substrate: Plasma breaks up gas molecules higher reactivity can use lower temperatures can use lower pressures electrons in plasma: ionize gas to keep plasma going act

25、ivate gas by dissociation to enhance CVD typically about 1% of gas is activated 降低反應(yīng)溫度，降低反應(yīng)溫度，達(dá)達(dá)600 以下，以下，典型溫度典型溫度300-350, 避免一般避免一般CVD高溫的引起高溫的引起:1.基板變形和組基板變形和組織結(jié)構(gòu)變化；織結(jié)構(gòu)變化；2.基板材料與基板材料與膜層互擴(kuò)散。膜層互擴(kuò)散。等離子體的作用等離子體的作用電子、離子密度達(dá)電子、離子密度達(dá)1091012個(gè)個(gè)/cm3(maybe more),平均電子能達(dá)平均電子能達(dá)110ev主要用于介質(zhì)膜沉積（主要用于介質(zhì)膜沉積（example:低厚度、

26、高低厚度、高、低漏電、低漏電、高絕緣的介質(zhì)薄膜）高絕緣的介質(zhì)薄膜）(1)產(chǎn)生化學(xué)活性的基團(tuán)和離子，降低反應(yīng)溫度；產(chǎn)生化學(xué)活性的基團(tuán)和離子，降低反應(yīng)溫度；(2)加速反應(yīng)物在表面的擴(kuò)散作用，提高成膜速度；加速反應(yīng)物在表面的擴(kuò)散作用，提高成膜速度；(3)濺射清洗作用，增強(qiáng)薄膜附著力；濺射清洗作用，增強(qiáng)薄膜附著力；(4)增強(qiáng)碰撞散射作用，使形成的薄膜厚度均勻。增強(qiáng)碰撞散射作用，使形成的薄膜厚度均勻。 PPECVDPSputtering（still with low pressure） ions suffer more collisions in gas phase = less energy when

27、 reach cathode = minimal sputtering effects ion energy depends on gas pressure and cathode voltage can use RF plasma discharges (especially for insulating films)process parameters substrate temperature control by external heater very little heating from PECVD process gas flow higher flow rates can i

28、ncrease deposition rate and uniformity but wastes gas pressure changes the energy of ions reaching electrodes can change deposition rate increases pressure may lead to chemical reactions in the gas effects also depend on gas concentration power 影響電子數(shù)目和電子的能量影響電子數(shù)目和電子的能量 太高引起氣相反應(yīng)太高引起氣相反應(yīng) 沉積速率隨能量上升沉積速率

29、隨能量上升 frequency 決定等離子體性質(zhì)決定等離子體性質(zhì) 改變離子轟擊特性改變離子轟擊特性 可用雙頻系統(tǒng)進(jìn)行調(diào)節(jié)可用雙頻系統(tǒng)進(jìn)行調(diào)節(jié) CVD等離子體的激勵(lì)方式：等離子體的激勵(lì)方式： 直流、射頻、微波、電子回旋共振直流、射頻、微波、電子回旋共振直流、射頻二極放電的缺點(diǎn)：直流、射頻二極放電的缺點(diǎn)：1.有電極，存在陰極濺射的污染有電極，存在陰極濺射的污染2.高功率，等離子體密度較大時(shí)，高功率，等離子體密度較大時(shí)， 出現(xiàn)弧光放電。出現(xiàn)弧光放電。3.直流二極還只能用于薄膜和電極直流二極還只能用于薄膜和電極 都是導(dǎo)體的情況。都是導(dǎo)體的情況。（1）高頻感應(yīng)）高頻感應(yīng)PECVD(電感電感)克服上述缺點(diǎn)

30、，但等離子體的克服上述缺點(diǎn)，但等離子體的均勻性較差。均勻性較差。RF source 電荷積累（on insulating surfaces） - 極性反轉(zhuǎn)（before charge saturates） low frequencies ( 1 MHz) reverses direction before ions reach substrate anode and cathode may be symmetric (identical processes at each electrode) or may be asymmetric (allowing greater use of ion

31、bombardment at one electrode)（2）微波）微波CVD微波能量的饋入：波導(dǎo)微波能量的饋入：波導(dǎo) 微波天線（圖示為微波天線（圖示為1/4波長(zhǎng)諧振腔）波長(zhǎng)諧振腔）微波波長(zhǎng)：微波波長(zhǎng)：2.45GHz，或，或915MHz。特點(diǎn)：特點(diǎn)：能在很寬的氣壓范圍能在很寬的氣壓范圍內(nèi)產(chǎn)生等離子體。內(nèi)產(chǎn)生等離子體。102103Pa，甚至甚至104Pa。(3)電子回旋共振等離子體（電子回旋共振等離子體（High Density Plasma CVD）磁場(chǎng)與微波電場(chǎng)相垂直，磁場(chǎng)與微波電場(chǎng)相垂直，電子在電磁場(chǎng)作用下作回旋共振運(yùn)動(dòng)，共振頻率為：電子在電磁場(chǎng)作用下作回旋共振運(yùn)動(dòng)，共振頻率為：mqB微

32、波頻率：微波頻率：2.45GHz，磁感應(yīng)強(qiáng)度：磁感應(yīng)強(qiáng)度：875Gs特點(diǎn)：特點(diǎn)：1.工作真空度高，工作真空度高，10-110-3Pa，以便吸收微波能量，以便吸收微波能量2.電離率幾乎為電離率幾乎為100，是一種離子束輔助沉積機(jī)制，是一種離子束輔助沉積機(jī)制 a) 臺(tái)階覆蓋性好；臺(tái)階覆蓋性好； b) 沉積離子能量為數(shù)沉積離子能量為數(shù)ev，具有濺射鍍膜的特點(diǎn)。，具有濺射鍍膜的特點(diǎn)。5.3 Metalorganic CVD (MOCVD) Uses matalorganic as precursor source Metalorganic : compounds containing metal at

33、oms bonded to organic radicalsDeposit a wide range of materials in the form of amorphous, epitaxial and polycrystalline filmsLower deposition temperatureMetalorganic precursors have lower decomposition temperatures than halides, hydrides or halohydridesAdvantagesDisadvantagesExpensive precursorsNot

34、widely available for specific coatingMost metalorganics are volatile liquids thus accurate pressure control neededLow thermal stability of precursors due to their polymerization or hydrolysisLoss of volatility and formation of residue due to ageingCompare of epitaxial methodsGrowth method timefeatur

35、eslimitLPE 1963Growth form supersaturated solution onto substrate Limited substrate areas and poor control over the growth of very thin layers VPE 1958Use metal halide as transport agents to growNo Al contained compound, thick layer MBE 19581967 Deposit epilayer at ultrahigh vacuumHard to grow mater

36、ials with high vapor pressure MOCVD 1968Use metalorganic compounds as the sourcesSome of the sources like AsH3 are very toxic.Some about the name of MOCVDIn the reference, MOCVD also have some other names. Different people prefer different name. All the names refer to the same growth method.MOCVD (M

37、etalorganic chemical vapor deposition)OMCVD(Organometallic CVD)MOVPE (MO vapor phase epitaxy)OMVPEAP-MOCVD (Atmosphere MOCVD)LP-MOCVD (Low pressure MOCVD)ex: (CH3)3Ga . . . . tri-methyl Gallium advantage: volatile at relatively low temperaturesThe MOVD growth system Vacuum andExhaust systemGas handl

38、e systemComputerControlReactorMOCVD Growth SystemGas handling systemThe function of gas handling system is mixing and metering of the gas that will enter the reactor. Timing and composition of the gas entering the reactor will determine the epilayer structure. Leak-tight of the gas panel is essentia

39、l, because the oxygen contamination will degrade the growing films properties.Fast switch of valve system is very important for thin film and abrupt interface structure growth, Accurate control of flow rate, pressure and temperature can ensure the stable and repeat. Reactor-1Reactor-2Aixtron Model-2

40、400 reactorExhaust system Pump and pressure controllerFor low pressure growth, we use mechanic pump and pressure controller to control the growth pressure. The pump should be designed to handle large gas load.waste gas treatment systemThe treatment of exhaust gas is a matter of safety concern. The M

41、OCVD system for GaAs and InP use toxic materials like AsH3 and PH3. The exhaust gases still contain some not reacted AsH3 and PH3, Normally, the toxic gas need to be removed by using chemical scrubber.For GaN system, it is not a problem.Metalorganic compound The vapor pressure of the MO source is an

42、 important consideration in MOCVD, since it determines the concentration of source material in the reactor and the deposition rate. Too low a vapor pressure makes it difficult to transport the source into the deposition zone and to achieve reasonable growth rates. Too high a vapor pressure may raise

43、 safety concerns if the compound is toxic. Further more, it is easier to control the delivery from a liquid than from a solid.Vapor pressures of Metalorganic compounds are calculated in terms of the expression Logp(torr)=B-A/T Vapor pressure of most common MO compounds Compound P at 298 K(torr)A B M

44、elt point (oC)(Al(CH3)3)2TMAl14.2278010.4815Al(C2H5)3 TEAl0.041 362510.78-52.5Ga(CH3)3 TMGa238 18258.50-15.8Ga(C2H5)3 TEGa4.79 25309.19-82.5In(CH3)3 TMIn1.7528309.7488In(C2H5)3 TEIn0.3128158.94-32Zn(C2H5)2 DEZn8.5321908.28-28Mg(C5H5)2 Cp2Mg0.05355610.56175Logp(torr)=B-A/T Calculate the mole flow rat

45、e of MO sources When we read some reference, we often see people use mol/min to indicate the flow rate. Normally, we use the formula to calculate it. F (mol/min)=p MO/p Bubbler*flow rate (ml/min)/22400 (mol/ml) We need to calculate the mole flow rate before we determine the growth condition. If we w

46、ant to grow alloys, we can use the mole flow rate to estimate the alloys composition. For example, if we grow AlGaN, we can estimate the Al concentration use the following formula if we assume the efficiency of Al and Ga sources is the same. x Al=F Al/(F Al+ F Ga) The basic reaction describe GaN gro

47、wth can simply write asGa(CH3)3+NH3 GaN+3CH4The growth procedure as follows:MO sources and hydrides inject to the reactor.The sources are mixed inside the reactor and transfer to the deposition area1. At the deposition area, high temperature result in the decomposition of sources and other gas-phase

48、 reaction, forming the film precursors which are useful for film growth and by-products.Gas phase and surface reactionThe film precursors transport to the growth surfaceThe film precursors absorb on the growth surfaceThe film precursors diffuse to the growth siteAt the surface, film atoms incorporate into the growing film through surface reactionThe by-products of the surface reactions absorb from surfaceThe by-products transport to the mai