Measurement Of Carbon And Oxygen In Silicon Wafers By MTR-IR Method And Polymer Brushes Patterning On Silicon Surface

During the Czochralski(CZ)procedure in growing silicon single crystals for photovoltaics,oxygen and carbon are incorporated into the molten silicon to different extents from the quartz crucibles and the graphite heaters.As oxygen atoms can find sites within the lattice structure among silicon atoms,interstitial oxygen(Oi)appears.Moreover,carbon atoms occupy positions generally taken by silicon atoms in the lattice structure,and this kind of impurity is defined as substitutional carbon(Cs).Interstitial oxygen and substitutional carbon are the main forms of oxygen and carbon impurities existing in silicon.Varying levels of interstitial oxygen and substitutional carbon in silicon are not only involved in intrinsic absorption,but also in physical and electrical properties of silicon crystals.The advantage of interstitial oxygen is to enhance silicon strength because of its nailing for stretching of dislocation in silicon lattice.Furthermore,interstitial oxygen intrinsic gettering technique avoids defects emerging during the growth process.High content of substitutional carbon affects the way interstitial oxygen nucleates and precipitates,as well as resulting in softening and breakdown of electronic components.Consequently,to control the silicon quality for high performance devices,it is absolutely necessary to monitor the content of interstitial oxygen and substitutional carbon in silicon wafers more accurately and sensitively.The diblock copolymers,polystyrene-block-poly-(4-vinyl pyridine(PS-b-P4VP)and polystyrene-block-poly-(2-vinyl pyridine)(PS-b-P2VP),have been used as various soft nanopatterning templates,such as dots,lines,and fingerprints after spin-coating their micelle solution on substrates and post-treatments such as heating and solvent vapor annealing,due to phase separation.Then the soft masks can be etched by hydrofluoric acid(HF),O2 plasma,reactive ion etching(RIE)etc,to generate cancave nanopatterns composing of Si-Hx or SiOx terminations alternatively.Further polymer brushes can be grafted on these patterns to attain different functionalities of integrated circuits,biological chips,and chemical micro-reactors etc.In this thesis,first,measurements of substitutional carbon and interstitial oxygen in single-crystal and poly-crystal silicon wafers for solar cells were carried out quantitatively by MTR-IR method and its relative principles were discussed.Secondly,polymethacrylic acid(PMAA),2-hydroxyethyl methacrylate(HEMA),polymathylme thacrylate(PMMA)and poly-N-isopropyl acrylamide(PNIPAM)brushes were grafted on silicon surfaces following the cancave nanopatterns with Si-Hx termini.The main contents and results in this thesis were summarized as follows:1.A comparison study on measurement of interstitial oxygen and substitutional carbon in silicon wafers for solar cells by MTR-IR and standard IR methodsA new infrared spectroscopic measurement of interstitial oxygen and substitutional carbon in silicon wafers by Multiple Transmission-Reflection Spectroscopy(MTR-IR)has been established.First,the superiority of MTR-IR to conventional IR has been analyzed in principle and verified by theoretical calculation.Then samples were measured using MTR-IR and IR methods respectively.The advantage of MTR over IR is signal enhancement of the absorption band of interstitial oxygen by 9?10 times at 1107 cm-1 and that of substitutional carbon by 7?8 times at 605 cm-1,which consequently extends the limit of detection an order of magnitude lower.In addition,MTR-IR can eliminate the interference fringes at p-polarization and decrease the interference fringes greatly at s-polarization for thin silicon wafers less than 0.3 mm thick.It has been demonstrated that MTR-IR showed good signal to noise ratio,credibility and reproducibility by experimental measurements.The principle for elimination and reduction of interference fringes for thin silicon wafers was explored.First,when the Brewster angle incidence was adopted,p-polarized light transmitted completely and no reflection occurred,which eliminated the interference fringes completely.Secondly,at s-polarization,integrated sphere(destructive interference)effect works,i.e.peaks from reflection light and valleies from transmission light meet together,and they offset each other,so the interference fringes are reduced greatly.Calculation of substitutional carbon and interstitial oxygen contents in thin silicon wafers of 0.1 mm and 0.2 mm thick was performed by the transmission formula at p-polarization.The IR spectra of thin silicon wafers using s-and p-polarized light were simulated,and the principle for the higher peak height from p-polarization than from s-polarization was explained.Carbon and oxygen distribution in polycrystalline silicon wafers were analyzed,and measurement of substitutional carbon and interstitial oxygen in 1 mm polycrystalline silicon wafers was performed.Comparing both MTR-IR and IR methods,the former was superior to the latter both in accuracy and reproducibility.2.Dot array patterns of polymer brushes from PS-b-P4VP templateThe PS-b-P4VP diblock copolymer mice 11s self-assembled in toluene was spin-coated on silicon surface,forming a pseudo-hexagonal dot array composing of P4VP with diameters of 50?80 nm in the PS matrix.Then the chip coated with the PS-b-P4VP film was immersed in a dilute HF solution for a given time to get arrayed Si-Hx pits with diameters of 50?80 nm.The etching occurred exclusively beneath the hydrophilic vertical P4VP columns via protonization of pyridine rings.To graft polymer brushes,the suspended Si-Hx bonds on silicon pits were hydrosilylated to introduce the tertiary alkyl bromide initiator,then via Surface-Induced Atom Transfer Radical Polymerization(SI-ATRP),polymer brushes of PMAA/HEMA/PMMA/PNIP AM were grafted from their respective monomer solutions,and a convex array of polymer brushes was obtained on silicon surface.The whole experiment was monitored by MTR-IR,AFM and SEM,that recorded the infrared and morphological evolutions of silicon surfaces step by step.3.Nano-fingerprint patterns from PS-b-P2VP templateThe amphiphilic PS-b-P2VP diblock copolymer in toluene self-assembled into micells and the micelle solution was spin-coated on silicon surface,forming a pseudo-hexagonal dot array composing of P2VP with diameters of 20?30 nm in the PS matrix.The slice coated with the PS-b-P2VP array was solvent-annealed by vapor from a 10:1 THF/H2O mixture for 30?40 h at room temperature to form line or fingerprint patterns composing of horizontal P2VP columns.Then the chip had been immersed in a solution of Na2PtCl4/HCl for 3?24 h,during the time electrostatic attraction of cationic ion[P2VP]+ and anionic ion[PtCl4]2-happened.Followed by exposure to O2 plasma,PS-b-P2VP diblock copolymers were removed and Pt nano-dotted wires were generated.Finally soaking the chip in a solution of 1:1:4 HF/H2O2/EtOH within an appropriate time window fabricated concave Si-Hx nano-lines or fingerprints with spacings of 50?80 nm,where PMAA polymer brushes were grafted to generate opposite convex nano-lines or nano-fingerprints.