Fundamental Research On Electrochemical Multi-wire Sawing Technique For Silicon Wafers

Recently,multi wire sawing has emerged as a leading technology of wafer preparation in the photovoltaic industry.In this method,a single wire web is fed to a silicon ingot with slurry consisting abrasives and cutting fluid to slice the silicon ingot.The multi wire sawing can be divided into free abrasive and fixed abrasive wire saw according to the ways that abrasives attached to the wire.At present,the mono-crystalline silicon wafers are mostly manufactured by fixed abrasive slicing.Then the textured structure with random and uniform pyramid is fabricated on the surface of wafers by anisotropic etching.The wafers can be produced in high efficiency and low cost with this mature technology.However,the polycrystalline silicon wafers that sliced by fixed abrasive cannot be textured effectively by acid wet texturing method.Recent progress in nanostructure textured black silicon has attracted intensive attention due to its great potential for applications in polycrystalline solar cells.But the cost of this textured method is higher.The manufacturing of polycrystalline silicon wafers is still low efficiency free abrasive wire saw.What's more,there are more difficults when this method is used for slicing polycrystalline silicon ingot with impurities.Due to the deficiency and shortage of polycrystalline silicon wafers manufacturing,an electrochemical multi wire sawing(EMWS)method that based on current multi wire sawing system is proposed.During the EMWS process,the metal slicing wire(cathode)and the silicon ingot(anode)are connected through a continuous or pulsed low-voltage D.C.power supply and the cutting fluid is sprayed into the slicing zone and used as electrolyte.The removal of material is achieved by the interaction of grinding and anodic oxidation(erosion).The thesis shows the research results of the material removal mechanism and key techniques,etc.The major contents of the dissertation are as follows:(1)The electrochemical reaction of silicon has been investigated by cyclic voltammetry(CV)method.The CV curves of P type polycrystalline silicon electrode are tested in ethylene glycol solution with potassium chloride(0.5 mol/L).A current peak with the current density of 0.07A/dm2 occurred at a positive scan potential of 5 V.Then CV curves of silicon electrode are tested in aqueous solution with potassium chloride(0.5 mol/L).The current peaks occurred at a potential of 1.5 V and 4V respectively,which are similar to the CV curves that tested in HF.The current densities are about 0.03 and 0.08 A/dm2 respectively.The results illustrate that an anodic oxidation with weak anodic corrosion occurs in aqueous solution under the anodic electric field.The oxide layer becomes denser and denser with the accumulation of oxidation time,which prevents further oxidation.If the oxide layer can be removed in a timely manner,the anodic oxidation reaction can be continued.The properties of the anodic oxide are also analyzed.The oxide layer could reach a depth of as much as 100 nm after 100 seconds at 20 V DC supply.Some localized porous features with a diameter of about 2-3?m are generated on the surface of the anodic oxide layer.What's more,the oxygen is found in the anodized oxide,which confirms the reaction of anodic oxidation for silicon further.(2)Nano-indentation simulations for silicon and silicon oxide that based on molecular dynamics are carried out.The simulation results manifest that the indentation depth and the residual depth of silicon oxide after unloading are all deeper than that in silicon under the same load.Based on the simulation results,the nano-indentation experiments are conducted on original and anodized silicon wafer.The experimental results indicate that the maximum depth of indentation and residual depth on original wafer are about 300 nm and 100 nm respectively under a load of 4 mN,while the values on anodized wafer are 550 nm and 200 nm respectively,which are obviously higher.The results of hardness test also show that the hardness of the wafer with oxide layer lower than that of the original wafer,and the indenter is easier to be pressed into surface.So the mechanism of material removal of EMWS is illustrated.A complete description of the process with regard to slicing has to be taken into account the compound action of the electrochemical reaction and the mechanical grinding.The oxidation layer with a loose and porous structure is easily removed,which is beneficial for reducing the cutting load and improving slicing efficiency.Mechanical grinding is good for the continuous anodic oxidation(erosion).The removal of material is achieved by the interaction of grinding and anodic oxidation.(3)In order to solve the problems that existing in engineering application,the key technologies of EMWS are investigated.The end face power transmission method is designed for anode.The power supplied for cathode could be transferred by carbon brush,roller,graphite block or the double workpieces to be sliced.The power supply for EMWS is developed.It could be set as unipolar output or bipolar output models.The frequency,pulse width and output energy can also be controlled.A remote monitoring system for EMWS is designed based on the power supply.It is demonstrated that the working status and machining parameters can be steady and reliably remote monitored with the system.(4)The EMWS sawing experiments and comparison experiments are carried out on NTC 442 DW.The results show that the total thickness variation(TTV)and BOW of wafers that cut by EMWS are 11.37?m and 7.38?m respectively,while that of comparison experiments are 12.06?m and 10.12?m respectively.The average value and distribution range for EMWS wafers are both better than that of conventional sliced wafers.What's more,fewer saw marks and particle shedding appeared on EMWS wafers.The method is intended to decrease the thickness of sub-surface damage layer and improve the pass rate.Meanwhile less damage would occur on the steel wire under smaller cutting load,the rate of wire broken is also lower.Then the EMWS are used for the manufacturing of polycrystalline silicon ingot with impurities and structure wire slicing method based on the system of HCT B5.And some satisfactory results are achieved,and the pass rates are increased by 2.54 % and 2.08%,respectively.(5)The polycrystalline silicon ingots are sliced by electrochemical wire saw system using single wire saw.The surface of the cut wafer by conventional method is very smooth and shining while the wafer cut by electrochemical diamond wire saw is dark and has electrochemical corrosion on its surface.What's more,a uniform and dense textured structure is attained on surface of the wafer by standard acid wet texturing process.The reflectivity of textured wafer is lower than that of wafer cut by conventional diamond wire saw,especially in the range of short wavelength(300-600nm).However,the polycrystalline wafers that cut by multi wire saw have a very bright surface,while the wafers that cut by electrochemical diamond multi wire saw still have a very bright surface.The bright surface results in an undesirable texture.During the process of electrochemical multi wire saw,the current density is only 0.18 A/dm2,which is lower than the value of 3.4 A/dm2 in the process of single wire saw.So the defect centers that caused by electrochemical corrosion are obviously decreased,which is unfavorable to acid texturing.Further research can be conducted to overcome the related engineering issues.If the current density of the single wire for multi-wire machining can be improved,the reaction of electrochemical will generate more defect centers on the surface of wafers,this method will get more expectations on providing a new possible method to future polycrystalline silicon wafers slicing by diamond wire saw.