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EBB 323 Semiconductor Fabrication Technology Epitaxy Dr Khairunisak Abdul Razak Room 2.16 School of Material and Mineral Resources Engineering Universiti Sains Malaysia [email protected] Topic outcomes At the end of this topic, students should be able to: • Name 2 types of epitaxy • Describe the applications of epitaxial layers • Explain techniques to produce epitaxial layers • Describe structure and defects in epitaxial layers Introduction • Epitaxy comes from Greek words: – – • Epi: upon Taxis: ordered Epitaxial growth: single crystal growth of a material in which a substrate serve as a seed 2 types of epitaxy: • 1. 2. • Homoepitaxy – material is grown epitaxially on a substrate of the same material. E.g. grow of Si on Si substrate Heteroepitaxy – a layer grown on a chemically different substrate. E.g. Si growth on sapphire Similar crystal structures of the layer and the substrate, BUT – – The shift of composition causes difference in lattice parameters Limit the ability to produce epitaxial layers of dissimilar materials Film deposited on a <111> oriented wafer <111> orientation The presence of SiO2 Layer cause depositing atoms have no structurepolysilicon Epitaxial and polysilicon film growth Applications of epitaxial layers 1. Discrete and power devices 2. Integrated circuits 3. Epitaxy for MOS devices 1. Discrete and power devices • Technology change: junction transistors diffused planar structure – Requires a material structure that are not achieved by diffusion of dopants from the surface • Si epitaxy was developed to enhance the electrical performance of discrete bipolar transistors • Breakdown voltage of the discrete transistor was limited by the field avalanche breakdown of the substrate material – Use higher resistivity substrates produced higher breakdown voltages but increased collector series resistance • Structure needed: thin, lightly doped and single crystal layer of high perfection upon more heavily doped Si substrate – But, the use of a more heavily doped substrate reduces the collector series resistance while the base-collector breakdown voltage is governed by the lighter doping in the near surface region • Epitaxial deposition of a lightly doped P+ epitaxial layer on a N+ substrate make the desired properties are achievable • Epitaxial grows also allows accurate control of doping levels and advantages which arises from a generally low oxygen and carbon levels in epitaxial layer • Epitaxial technique was developed to 2 and 3 layers epitaxial structure – For lightly doped area of collector – Based region was also grown epitaxially • E.g. of multilayer structures: Si-Controlled Rectifier (SCR), Triac, high voltage or high power discrete products Mesa discrete transistor fabricated in an epitaxial layer on a heavily doped N+ substrate Transistors Diodes 2. Integrated circuit (IC) • Development of planar bipolar IC caused the requirement for devices built on the same substrate to be electrically isolated – The use of opposite typed substrate and epitaxial layer met part of the requirement – Device isolation was completed by the diffusion of “isolation” region through the epitaxial layer to contact the substrate between active areas • In planar bipolar circuits, common to employ a heavily doped diffused (or implanted) region under the transistor – Usually called ‘buried layer’ or ‘DUF’ for diffusion under film – The buried layer • serves to lower the lateral series resistance between collector area below the emitter and the collector contact • produce uniform planar operation of the emitter, avoiding current crowding which leads to hot spots near edges of the emitter Integrated circuits (a) A junction isolated bipolar device fabricated as part of an integrated circuit using a buried layer subcollector and a lightly doped n-epitaxial layer (b) An N-Well CMOS structure fabricated in a lightly doped pepitaxial layer 3. Epitaxy for MOS devices • Unipolar devices such as junction fieldeffect transistors (JFETs), VMOS, DRAMs technology also use epitaxial structures • VLSI CMOS (complimentary metal-oxidesemiconductor) devices have been built in thin (3-8 micron) lightly doped epitaxial layers on heavily doped substrates of the same type (N or P) – That epitaxial structure reduces the “latch up” of high density CMOS IC by reducing the unwanted interaction of closely spaced devices Advantages of epitaxy • Ability to place a lightly oppositely doped region over a heavily doped region • Ability to contour and tailor the doping profile in ways not possible using diffusion or implantation alone • Provide a layer of oxygen free material that is also contained low carbon Techniques for silicon epitaxy 1. 2. 3. 4. Chemical Vapour Deposition (CVD) Molecular Beam Epitaxy (MEB) Liquid Phase Epitaxy (LPE) Solid phase regrowth 1. Chemical Vapour Deposition (CVD) • The most common technique in Si epitaxy • In the CVD technique – Si substrate is heated in a chamber: sufficient heat to allow the depositing Si atoms to move into position to – Reactive Si containing gaseous compounds are introduced – Gaseous react on the hot surface of the substrate and deposit a Si layer – The deposit will take on Si substrate structure if the substrate is atomically clean and the temperature is sufficient for atoms to have surface mobility Schematic drawing of a simple horizontal flow, cold wall, CVD reactor Schematic CVD reactor geometries for (a) True vertical reactor (b) Classic horizontal flow reactor (c) Modified vertical (or pancake) reactor (d) Downflow cylinder reactor CVD processes and products CVD for silicon devices CVD reactions 1. Pyrolysis: chemical reaction is driven by heat alone, e.g. silane decomposes with heating SiH4 Si + 2H2 2. Reduction: chemical reaction by reacting a molecule with hydrogen, e.g. silicon tetrachloride- reduction in hydrogen ambient to form solid silicon SiCl4 + 2H2 Si + 4HCl 3. Oxidation: chemical reaction of an atom or molecule with oxygen, e.g. SiH4 decomposes at lower temperature SiH4 + O2 SiO2 + 2H2 4. Nitridation: chemical process of forming silicon nitride by exposing Si wafer to nitrogen at high temperature e.g. SiH2Cl2 readily decomposes at 1050C 3SiH2Cl2 + 4NH3 Si3N4 + pH + 6H2 CVD CVD film growth steps 1. Nucleation • Dependent on substrate quality • Occurs at first few atoms or molecules deposit on a surface 2. Nuclei growth • Atoms or molecules form islands that grow into larger islands 3. Island coalescence • The islands spread , and coalescing into a continuous film • This is the transition stage of the film growth, thickness several hundreds Angstroms • Transition region film possesses different chemical and physical properties for thicker bulk film 4. Bulk growth • Bulk growth begins after transition film is formed CVD film growth steps Types of film structure Amorphous Polycrystalline Single crystal Basic CVD subsystem • CVD Process steps: Pre-clean Deposition Evaluation • Pre-clean: remove particulates and mobile ionic contaminants • Deposition: Load wafer into chamber, inert atmosphere Heat Introduce chemical vapour • Evaluation: thickness, step coverage, purity, cleanliness and composition Remove vapour Flush excess chemical vapour source 2. Molecular Bean Epitaxy (MBE) • Uses an evaporation method • MBE is carried out at a lower temperature than 1000-1200C (typical CVD temperature) – Reduces outdiffusion of local areas of dopant diffused into substrates and reduce autodoping which is unintentionally transfer of dopant into epitaxial layer • MBE is favourable – preparation of sub-micron thickness epitaxial layers or – high frequency devices requiring hyper-abrupt transition in the doping concentration between the epitaxial layer and the substrate • In MBE, – Si and dopant(s) are evaporated in an ultra high vacuum (UHV) chamber – The evaporated atoms are transported at relatively high velocity in a straight line from the source to the substrate – They condense on the low temperature substrate – The condensed atoms of Si or dopant will diffuse on the surface until they reach a low energy site that they fit well the atomic structure of the surface – The “adatom” then bonds in that low energy site, extending the underlying crystal by a vapour to solid phase crystal growth – Usual temperature range of the substrate is 400-800C. Higher than 800C is possible but it will increase outdiffusion or lateral diffusion of dopants in the substrate Schematic drawing of a molecular beam epitaxial system • Insitu cleaning of the substrate – Can be done by high temperature bake at 1000-1250C for several minutes under high vacuum to decompose the native surface oxide and to remove other surface contaminants – Other technique is by using a low energy beam of inert gas to sputter clean the substrate – Difficult to remove carbon but will decrease at the surface by diffusion into the substrate during short anneal at 800-900C • Wider range of dopants for MBE than CVD epitaxy: – Typical dopants: Antimony, Sb (N-type), aluminum, Al or gallium (Ga) for P-type – N-type dopant: As and P, evaporate rapidly even at 200C. Difficult to control – P-type dopant: Boron, evaporate slowly even at 1300C Schematic drawing of a multiple chamber MBE system MBE Equipment Liquid Phase Epitaxy (LPE) • LPE technique is widely used for preparation of epitaxial layers on compound semiconductors and for magnetic bubble memory films on garnet substrate • In films growth by LPE from solution melts, low cooling rates, when the surface reaction (growth) • Kinetics are rapid compare to the mass transport of Si to the seed, epitaxial layer thickness will vary in proportion to the temperature drop • Increase cooling rates, mass transport rate will increase and the growth rate will increase with cooling rate until growth rate becomes limited by surface reaction kinetics • Growth rate increases with cooling rate up to about 1 degree/min while growth rate above 2 degree/min occurred under kinetically limited conditions LPE growth rate increasing with cooling rate up to about 1 micron per minute Schematic drawing of a typical silicon liquid phase epitaxy (LPE) •Fabrication sequence for a vertical channel field effect transistor •N and N+ epitaxial structure can be built using liquid or vapour phase epitaxial growth •Preferential etching can be used to open areas part way through the N type epitaxial layer • In this figure, LPE is used to fill the etched out gate areas which control current flow vertically from the top side source to the N+ substrate drain region Schematic of fabrication steps in the fabrication vertical field effect transistors by etch and LPE refill techniques 4. Solid Phase Re-growth i. • • • Re-growth of amorphous layers Surface layers subjected to high dose ion implants are in amorphous structure due to the heavy damage inflicted on the lattice as the energetic ions are absorbed Annealing above 600C amorphous layer recrystallize Re-crystallisation occurs from interface moves toward the surface and results in solid phase epitaxial re-growth ii. • • • • • Re-crystallisation of thin films Involves re-crystallisation of a deposited amorphous or polysilicon film Si film is deposited on a Si substrate or more commonly SiO2 heated using a strip heater passed over the surface or by a scanned pulsed laser to crystallise the film to single crystal or large grain polysilicon This fabrication technique is used to produce a stacked n-channel device in re-crystallised polysilicon on a thermally grown or deposited oxide Oriented epitaxial growth can be obtained by making series of holes in the oxide to allow points of contact between the underlying substrate and the deposited polysilicon The contact points become “seeds” areas for establishing re-growth orientation Re-crystallisation solid phase epitaxy using a moving strip heater A stacked MOS structure over an insulating oxide fabricated in a recrystallised polysilicon layer Structure and defects in epitaxial layer • Surface morphology of Silicon epitaxial deposits is affected by growth and substrate parameters Growth parameters: • – – – – • Temperature Pressure Concentration of Si containing gas Cl : H2 ratio Substrates parameters – – – Substrate orientation Defects in the substrate Contaminants on the surface of the substrate Typical defects in epitaxial layers 1. 2. 3. 4. 5. Substrate orientation effects Spikes and epitaxial stacking faults Hillocks and pyramids in epitaxial layers Dislocations and slip Microprecipitates (S-pits) 1. Substrate orientation effect • Growth of smooth epitaxial films can be obtained on (100) and (110) oriented Si substrates Epitaxial growth on substrate surface on oriented on (111) plane results in facetted “alligator skin” surface • – – • (111) surfaces contain no atomic steps to provide a density of growth sites Without atomic steps, the growth produces pyramids and terraces Misorientation of the surface by 0.5 degree introduces a sufficient density of steps for growth of smooth planar films 2. Spikes and epitaxial stacking faults i. • • • • • Growth spike Originate from Si particle on the surface not removed by the pre-epitaxial cleaning process Si Chips may expose faster growing crystal planes than the plane of the substrate Chips nucleate and produce polysilicon nodule. The chips then protrude above the substrates surface into a region of richer supply of gaseous reactants Results in nodule grows at 2-10 times the rate of epitaxial film on the substrate. May be removed mechanically before the next step but will leave a region unusable for functional materials ii. • • Epitaxial stacking faults Crystallographic in nature and arise from defects in atomic arrangement during film growth Could result from an extra atomic layer (extrinsic fault) or a missing atomic layer (intrinsic fault) along {111} type plane Epitaxial growth spike Stacking fault on <111> Si 3. Hillocks and pyramids in epitaxial layers • • • Hillocks: Small oval mounds on the surface of the epitaxial Pyramids: Faceted regions on the epitaxial surface Density of hillocks and pyramids is dependent on growth parameters such as type and concentration of Si source and deposition temperature 4. Dislocations and slip • • • • Non-uniform heating of a substrate results in non-uniform thermal expansion of the substrate which produces elastic stresses The thermal stress can cause bowing which may lift the edge of the substrate away from the substrate in response to the thermal stress At lower temperature (< 900C) the yield point of the Si lattice is sufficiently high that the substrate behaves elastically. During cooling, the thermal stress is removed and the substrate returns to its original shape If the stress exceeds a critical values, the substrate will yield plastically occurs due to generation and motion of dislocations which are atomic level line defects which glide along slip planes of the crystal • • • • • The passage of one dislocation offsets the material above and below the slip plane by a unit known as “Berger’s vector” of the dislocations Dislocations normally propagate from near the edge of the substrate (highest stress), and glide towards the centre of the substrate and produce plastic deformation of the substrate which relieves the thermal stress Dislocation motions is slow because dislocation moves to a region of lower shear stress The continuous slow motion of the dislocations produces “creep” deformation of the crystal Device impact from slip normally comes from rapid “pipe diffusion” of dopant along the core of the dislocations Crystal slip Typical wafer edge slip as a result of excessive within wafer temperature gradients during heating or during epitaxial film growth 5. Microprecipitates (S-pits) • • • Microprecipitates may come from metallic elements such as copper, nickel, iron and chromium This is due to their solubility in Si at high temperatures and fast diffusion rates through the Si The metal contaminants may exist in the starting substrates or being pick up during handling in the loading operation or from metal parts or susceptors within the epitaxial reactor itself