Presentation on theme: "FABRICATION PROCESSES"— Presentation transcript:
1 FABRICATION PROCESSES Presentation on ‘FABRICATION PROCESSES’Course no : EEE 453Course by : Mohiuddin MunnaPresented byShahadat Hussain Parvez ( )Jubair Hossain Jitu ( )Rezwan Matin ( )Gurucharan Mahato ( )
2 Before Starting the Fabrication It starts with making silicon
3 Before Starting the Fabrication Ingots get cut into wafers, which are 1-2mm thick, and upto 12” in diameterEntire wafers are processed, and then cut into chips when needed
4 Fabrication Processes Six main process stepsOxidationDiffusionIon implantationLithographyThin film depositionEpitaxy
5 Oxidationthe first step in semiconductor device fabrication involves the oxidation of the wafer surface in order to grow a thin layer of silicon dioxide (SiO2). This oxide is used to provide insulating and passivation layers.The most common method of oxidation is thermal, and can be classified as either "dry" or "wet" oxidation. Wafers are loaded into quartz boats and slid into a furnace heated to approximately 1200ºC.
6 OxidationIn dry oxidation, thin oxide layers are grown in an environment containing oxygen and hydrogen chloride near atmospheric pressure
7 OxidationThicker oxide layers require higher pressures and the use of steam (wet oxidation). Wet oxidation is performed by exposing the wafer to a mixture of oxygen and hydrogen in the furnace chamber. Water vapor is formed when the hydrogen and oxygen react
9 Oxidation Process Standard oxidation temperature 800-1200 C Heat is produced by resistance heatingCoil like heating elements are arranged in 3 controlled zoneOuter zones operates at higher power to compensate heat lossSimply Oxygen is fed for dry oxidationCarrier gas like Ar on N2 is used in wet oxidation along with heated water or burning O2 and H2 at input of tubeRequired time in furnace depends on temperature and desired thicknessWhole system is automated
10 Oxidation ProcessFig: Typical Thickness Vs Oxidation time for 100 crystal compared for dry and wet oxidation
11 Diffusion process Process of doping Si wafer is exposed to solid, liquid or gaseous source containing desired impurityA reaction at wafer surface establishes a supply of dopant atoms immediately adjacent to Si crystalAt elevated temperature atoms difuse in the region Si is not protected by oxideSurface doping concentration is up to 1021 / cm3Diffusion in SiO2 is relatively lowSiO2 protects Si for a limited time depending on oxide thickness ,temperature and background droping.
14 Ion Implantation It’s an alternative process of introducing dopants Dopant ion is accelerated in high energy range from 5 keV to 1MeV then shooted into semiconductorIons displace Si atoms along their path into crystalfollow-up heating binds ions with crystelBut before that automatic scanning is performed automatically to determine total number of ions / cm3Si wafer can be masked using thin flims of SiO2 ,Si3N4 and photoresist .
16 Advantage of Ion Implantation Lower temperature processImplantation is performed in room temperatureFollow-up heating is done in 600 CGives precise control over impurityIdeally suited for a number of modern device structures requiring extremely shallow junctionsdamage from implantation can be annealed by heating the wafer in a furnace to T > 900 C.
17 Doping by Ion Implantation Dose = ion beam ﬂux (# cm-2 s-1) x time for implant ... units # cm-2
18 Doping by Ion Implantation SiO2 ﬁlm masks the implant by preventing ions from reaching the underlying silicon (assuming it’s thick enough)after implantation, the phosphorus ions are conﬁned to a damaged region near the silicon surface
19 Doping by Ion Implantation Annealing heals damage and also redistributes the ions (they diffuse further into the silicon crystal)
20 Doping by Ion Implantation Fig: Computed phosphorus Implantation profile assuming a constant dose of /cm
21 LithographyProcess of selectively removing SiO2 and other masking material covering wafer surface.Transfers circuit diagram on wafer
22 Lithography ProcessAt first Si wafer is coated with UV light sensitive photoresist in a thin uniform coating.Wafer is pre baked at CExposing wafer to UV light through a maskMask here is carefully prepared with glass or quartz photo pale containing a copy of pattern to be transferred to SiO2Exposed photoresist parts undergo chemical changes depending on photo resistIn negative photo resist exposed parts form polymer like structures and unexposed parts dissolves after developing
23 Lithography Fig: majos steps in lithography Apply resist Expose resist through maskAfter developingAfter oxide etching and resist removal
24 Thin Film deposition Three different Techniques Evaporation Sputtering Chemical Vapor deposition