Study On Electroplated Diamond Wire Saw Slicing Single Crystal Silicon Technology And Mechanism

Nowadays,semiconductors are widely used in microelectronic applications,such as computer systems,automobiles,industrial automation,telecommunications equipment,consumer electronics and industry control systems.The majority of semiconductors are built on silicon wafers.Wafer slicing process is a key technology to machine the silicon rod to wafer,and the machining quality of wafer can directly affect the workloads and costs of next working procedures.The fixed-abrasive diamond wire saw machining technology,which has some advantages of high machining efficiency, low kerf loss,clean working environment and high wafer surface quality for slicing,is supposed to be used for slicing silicon crystal widely in the future.The electroplated diamond wire saw(EDWS) machining single crystal silicon technology was studied by experimental and theoretical analysis in this paper,in order to supply the experimental and theoretical basis for further application of EDWS machining technology.The effects of single crystal silicon anisotropy on the wafer quality sliced by EDWS were studied theoretically.Then the recommendatory wire approach directions (WAD) were made by analyzing the influences of machining single crystal silicon along different crystal planes and orientations on wafer quality.The study results show that the wafer can get a better surface finish and reduce deviation from the desired surface normal by maintaining symmetry of the elastic modulus of silicon on the two sides of the wire.And the tendency of wafer breakage can be reduced by maintaining the same of the WAD and a cleavage direction in crystal planes.Synthesizing the research results of the effects of WADs on wafer surface shape quality and wafer breakage,when slicing (100) crystals,the preferred WAD are[001],[010],[001]and[010];when slicing (110) crystals,the preferred WADs are[110]and[110];And there are exactly preferred six WADs for slicing(111) crystals:[1(?)0],[(?)10],[01(?)],[0(?)1],[(?)01] and[10T].Through the reciprocating electroplated diamond wire saw slicing single crystal silicon experiments,the influences of wire saw speeds,ingot feed speeds and cutting fluids on the silicon wafers surface topography,surface roughness(SR),warp,total thickness variation(TTV) and subsurface damage(SSD) depth were studied,and the wear forms and wear mechanism of wire saw were studies.In the range of the experimental parameters adopted in this paper,the SR and SSD values of silicon wafers decrease along with the increase of wire saw speed and the decrease of feed speed;the wafers warp values decrease along with the decrease of wire saw speed and feed speed; and the wafer TTV values are closely related with the matching relations between wire saw speed and ingot feed speed.The experimental results show that the synthetic cutting fluid has a good effect to reduce the silicon wafer surface roughness,warp and TTV,and get a better wafer surface.The wear forms of EDWS include grain-abrasion and coating wear,which was observed by using the SEM.But the pulled-out of grits is the main wear form,therefore,the new approaches should be studied to improve the grits retaining strength,which can improve the service life of wire saws.Ignoring the lateral vibration of the wire,the geometrical model of wire saw machining process was founded to calculate the average cut depth of single grit on wire surface theoretically.Based on the geometrical mode and experimental observation of the machined surface topography and chip shapes,the material removal mechanism and wafer surface formation were studied.And the relationship between the material removal mode and process parameters was analyzed.The study results show that the single grit cut depth has a close relation with the grit position on the wire surface.The grits on the bottom of wire saw have deeper cut depths,contributing to main material removal to form the kerf;the grits near to silicon machined surface have lower cut depths,mainly contributing to wafer surface formation.When using the same wire to machining the single crystal silicon,the cut depth of any grit on wire surface has a monotone increasing non-linear correlation with the ratio r value of ingot feed speed and wire speed,is g∝D·(V_W/V_S)^4/9,and D is an invariable value.The material removal mode can be changed through controlling and adjusting the r value.According to the experimental condition in this paper,when r≤1.0μm/mm,the material removal on wafer surface can be in a near-ductile removal mode.A theoretical model for predicting the silicon wafer SSD depth was founded.This model considered the abrasives machining in wire sawing process as a moving indention subjected to both normal force and tangential force.Based on indentation fracture mechanics theory,the propagation length of median crack was analyzed by synthetically considering the contributions of elastic stress field and residual stress field beneath the abrasive.Then the depth of median crack propagation layer was postulated as equal as the subsurface damage layer thickness(SSD),and the lateral crack depth was equal to the peak-to-valley surface roughness(SR),thereby a theoretical model of relationship between SSD and SR was founded for predicting the SSD depth.There exists monotone increasing non-linear correlation between SSD depth and SR(R_z) in wire saw slicing single crystal silicon process,that is,SSD - SR+χSR^3/4.The results indicate that the errors between the experimental measure values and the theoretical prediction values are less than 12.78%,which shows the experimental measure and theoretical prediction values coinciding comparatively.So the theoretical model can be used for predicting SSD rapidly,expediently and accurately.The high performance diamond impregnated wire manufacture is the key of application for this technology,so the development of electroplated diamond wire was investigated briefly.The development process and electroplating parameters were studied and optimized.The piano wire with a 0.2mm diameter was selected as the saw matrix and the diamond grits with a 20um average size was selected to develop the EDWS.Some experimental studies were done to analyze the effects of cathode current density of pre-plating and electro-embedding of diamond grits(tack-on) and time at tack-on stage on the wire saw appearance,diamond grits density and adhesion between saw matrix and plating coating,then the optimum electroplating parameters for developing EDWS were obtained.Electro-embedding of diamond grits on the wire saw surface used the inter-sand method.And the optimum process parameters are as following:the temperature of the plating bath is 35～40℃,and the pH value is 3.8～4.2. The cathode current densities of pre-plating,tack-on and buildup process are 1.8,1.5 and 2.0A/dm² in turn,and the corresponding times at various stages are 6,8 and 18min. The wire saw is heat-treated for 1 hour at 200℃to remove hydrogen at last.The diameter of wire saw developed is about 0.25mm,which will increase the utilization rate of materials.