1、Project TitleSiGe Strained-Layer Epitaxy SiGe Alloys Pseudomorphic Growth and Film Relaxation Putting Strained SiGe into SiGe HBTs The challenge of SiGe Epitaxy SiGe Growth Surface Preparation Growth Techniques Stability Constrains TheoryProject TitleSiGe Alloys Si and Ge-Group elemental semiconduct
2、ors-Diamond lattice structure Vegards rule-a(Si1-XGeX)aSix(a Gea Si)a：lattice constant x：Ge fraction Diffraction measurement-a(Si1-XGeX)0.002733x20.01992x0.5431Project TitleSiGe AlloysUnit cell of the diamond latticeTheoretical and experimental lattice constant of a Si1-xGex alloy as a function of G
3、e fractionProject TitlePseudomorphic Growth and Film Relaxation Lattice mismatch between Si(a5.431A)and Ge(a5.658A)-4.17%at 300K SiGe film on thick Si substrate-Initial growth Pseudomorphic-SiGe film is forced to adopt Si smaller lattice constant-Desired result for most device application-Reach“crit
4、ical thithiness”Relax-Stain energy too large to maintain local equilibrium-SiGe film relaxes via misfit dislocation formation-Defected film unsuitable for high-yielding device applications Generationrecombination trapping center High diffusivity pipes for dopantsProject TitlePseudomorphic Growth and
5、 Film RelaxationSchematic 2-D representation of both strained and relaxed SiGe on a Si substrateSchematic representation of misfit dislocation formed at the SiSiGe growth interfaceProject TitlePseudomorphic Growth and Film Relaxation Metastable-Film remain pseudomorphic，may have exceeded the critica
6、l thickness-Will relax during subsequent thermal processing step that add energy to the systemPlan-view TEM(top down image)of an unstable SiGe film that has been annealed and undergone relaxation.The visible linear structures are misfit dislocationProject TitlePutting Strained SiGe into SiGe HBTs Th
7、ree-layer composite structure-A thin，undoped Si buffer layer-The actual boron-doped SiGe active layer-A thin，undoped Si cap layerSchematic epitaxial SiGe film for use in a SiGe HBT.The film consists of a thin Si buffer layer,the compositionally graded SiGe layer of thickness(h),and a Si cap layer of
8、 thickness(H).The boron base doping is contained within the SiGe layer.Project TitlePutting Strained SiGe into SiGe HBTsCross sectional TEM showing the active device region of a fabricated SiGe HBT.The(table)strained SiGe base layer has a peak Ge content of 10%and is defect free,and cannot be deline
9、ated from the Si matrixProject TitlePutting Strained SiGe into SiGe HBTs Thin Si buffer layer-Help ensure pristine SiGe epitaxial growth interface is preserved between the original Si substrate High-temperature Si epitaxy process，coming SiGe strained layer by difficult low-temperature epitaxy proces
10、s-Device design Allow the incorporation of intrinsic layers（i-layers）to be easily embedded in the collector-base junction Decrease the junction field and aid in breakdown voltage tailoringProject TitlePutting Strained SiGe into SiGe HBTs Active SiGe layer-Thickness h，position-varying Ge composition-
11、Embedded boron doping spike，10nm by 241019cm-3，for an integrated base charge of roughly 241019cm-2-Form the active region of the band-engineered device-The specific shape，thickness，placement of the Ge profile with respect to the boron base profile determine the resultant performance of the transisto
12、r Project TitlePutting Strained SiGe into SiGe HBTs Si cap layer-Thickness H-Provide a Si termination to the SiGe composite Most SiGe HBT fabrication involve oxidation step to form the emitter-base spacer used in self-alignment，and SiGe does not oxidize-Provide additional space to allow the modest o
13、ut-diffusion of the boron base profile，provide room for the emmitter out-diffusion-As with Si buffer layer introduce an active i-layer into the emitter-base junction to lower the junction electric field Thereby reduce the parasitic EB tunneling current-Help improve the overall stability of the filmP
14、roject TitleThe challenge of SiGe Epitaxy Si epitaxy in device fabrication enable one to overcome the fundamental limitations by ion implantation-Implantation energy-dependent Gaaussian distribution of dopants as a function of depth-Ion channeling of the implanted dopant species-The need for high te
15、mperature annealing to remove implant damage and activate the dopants Both cleaning and growth temperatures for conventional Si epitaxy are in the range of 1,000-At such temperatures，any advantage obtained from precise device layer formation by epitaxy is lost in the subsequent diffusion of dopants
16、away from their intended locations The key to the successful use of Si（or SiGe）epitaxy to make advanced devices is thus to affect high-quality film growth at very low temperature（600）Project TitleSurface PreparationTwo distinct phases-Initial growth interface-Film growthConsider the means of growth
17、surface preparation-Must identify the nature of the surfaceIn classical high temperature Si epitaxy-Surface being prepared was that of an unpatterned，bulk-grown Si waferIf patterned and implanted regions were present during the thermal cycles employed in classical Si epitaxy-Where temperatures in ex
18、cess of 1,000 for 10 minutes are typicalProject TitleGrowth Techniques Passivate a Si surface with hydrogen-A 1015 seconds etch in a dilute 10：1 H2OHF solutionthe hydrogen adlayer create-Reduce the reactivity of the growth interface approximately 13 orders of magnitude from that of a bare Si surface
19、 with respect to its oxidation rate in ambient air Boron dose in excess of 1010 cm-2 at the initial growth interface，even in the UHV conditions employed in MBE-Reduce the magnitude and impact of this contamination Deposition of a buffer layer of material to bury the contamination well below the acti
20、ve device region Depositing layers on patterned substrate，this is not a viable approachProject TitleGrowth Techniques High temperature for conventional Si epitaxy-Provide for adatom mobility-Suppress the inclusion of undesirable dopant species in the films being deposited Achieve adequate film purit
21、y during low temperature epitaxy-Best known are the UHV techniques associated with MBE，vacuum range 10-11 torr-To reduce the complexity and expense Chemically selective form of the UHV technique Simplified UHV technique-Employ O-ring seals and quarts reaction tube-“soft”levels of UHV，range of 10-9 t
22、orr-The preponderance of residual gas is hydrogen-Oxygen and water levels are reduced to the range of 10-11 rorr partial pressure-Carbon-bearing species are not detectable owing to the use of turbo-molecular pumpingProject TitleStability Constrains“critical thickness”(hcrit)-The maximum thickness for obtaining pseudomorphic growth post-fabricatiom“energy minimization”SiGe strained-layer thermodynamics stability diagram comparing UHVCVD Experimental data to Matthews and Blakeslees theoretical result
