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地面
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地面打磨机的设计,地面,打磨,设计
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Experimental ResearchSTUDIES OF SURFACE GRINDING TEMPERATURE AFFECTEDBY DIFFERENT GRINDING WAYS OF SILICON WAFER*LINBin, YU Ai- bing, HU Jun, XU Yan- shen(School of M echanical Engineering,Tianjin University,Tianjin300072)Abstract:T he surface grinding temperature of the silicon wafer ground by diamond wheels is studied.Rudimentally, the properties of the surface grinding temperature generated by two grinding methods,ground by straight and cup wheels respectively,are analyzed.In addition,considering the effects of grainsize and grinding depth on surface grinding temperature during these two grinding processes, significantresults and conclusions are obtained from experimental research.Keywords:surface grinding temperature,straight wheel,cup wheel,silicon wafer?T he machining technique of silicon wafers hasgradually become one of the progressive projects in re-cent years. With the successful production of large sizesilicon wafers, researchers all over the world have paidmore attention to the machining techniques of siliconwafers 2, 3.According to the ductile grinding principle of brit-tle materials, remarkable developmentshave beenachieved to make the precision machining of large sizesilicon wafers become true by adopting the cup wheelsurface grinding technique, which is a high efficiencyprecision machining way and has wide application inhard-brittle material machining fields. T o prevent thesilicon wafer from burning, the research work of sur-face grinding temperature should not be ignored. Mostof the former research works of surface grinding tem-perature focused on the straight wheel grinding pro-cess mainly. In the difference between these two ma-chining principles, there are some distinctions in sur-face grinding temperature, so it is necessary to under-stand the difference of surface grinding temperaturebetween the cup wheel grinding and the straight wheelgrinding processes. At the same time, the relevant re-search work is meaningful to the application of the cupwheel grinding technique.1Comparison of Surface GrindingTechniques between Cup Wheel andStraight WheelFig. 1 shows the surface grinding method with astraight wheel. The contact length between the wheeland the workpiece, which is closely related to grindingdirections and grinding depth, is variable with grindingparameters and has significant influence on grindingheat. T he intensity of the heat source will change withthe contact length 1.Fig. 1Shape of grinding zone with straight wheelFig. 2 shows the surface grinding process with acup wheel. T he contact length between the wheel andthe workpiece has little relation with grinding direc-tions and grinding depth. For a given wheel, the con-tact length keeps constant within a certain range ofgrinding depths. As the cause of the moving heatTransactions of Tianjin UniversityVol. 6 No. 1 May 2000?Received date: 1998-11-05;revised date: 2000-03-17.Lin Bin, born in 1963, male, M , associate prof.* Supported by the Open Lab. Foundation of Educational Ministryof China.source is regarded as a linear source based on the tra-ditional grinding heat theory, hence, this theory doesn?t suit the cup wheel grinding process for its grindingzone is an arc instead of rectangle. Therefore, thegrinding heat model for the cup wheel should be set upto meet the needs of this technology.Fig. 2Shape of grinding area with cup wheel2Experimental Method, Apparatus andConditions2. 1Experimental method T he surface grinding temperature is measuredwith a thermocouple. T wo pieces of thin thermocoupleslices of 0. 1 mm thickness, made of standard thermo-couple wires, are clamped between two parts of theworkpiece and are insulated from the workpiece. Whenthe wheel grinds the surface of the workpiece, the twoslices? end points on the workpiece surface can bewelded together as a node. Due to the small volume ofthis node, heat capacity is small enough. If the amplify-ing and recording instruments react quickly enough,the response time is extremely short. The real grindingtemperature can be measured accurately without fur-ther deductive computing.2. 2ApparatusFig. 3 shows the experiment system. T he thermo-couple wires out of the workpiece are connected with aDC amplifier which enlarges the thermic signals 100 to1 000 times. The signals are analyzed by a digital real-time oscilloscope, and recorded by a computer. Theminute grinding depth is performed in 0. 1 ? m per stepby a micro-feed system which is manufactured by ourown laboratory. T he feed principle is shown in Fig. 3.T he computer provides accurate digital signals to theamplifier. T he amplified signal is transmitted into apiezoquartz to perform micro-feed.Fig. 3Principle of measurement system2. 3Experimental conditionsT ab. 1 shows the experimental conditions by astraight wheel, while Tab. 2 by a cup wheel.Tab. 1Equipment and parameters of surface grindingwith a straight wheelGrinding machineHZ-63 horizontal surface grinderGrinding wheelresin bonded diamondshape:straight?300mm25mm240# ,W10Workpiecesingle crystal silicon25mm10mm5mmGrinding fluidnoTab. 2Equipment and parameters of surface grindingwith a cup wheelGrinding machineCG6125A latheGrinding wheelresin bonded diamondshape:cupouter diameter: ? 100 mminternal diameter: ?90 mmabrasive size: 120# ,W28, W10Workpiecesingle crystal silicon 25 mm10 mm5 mmGrinding fluidwater-based coolant3Experimental Results3. 1 Comparison of surface grinding temperaturegenerated by straight wheel and cup wheel Fig. 4 shows the real-recorded thermic voltagesignal waveshape of the silicon wafer ground by astraight wheel ( W10) . T he grinding parameters are,grinding depth 3 ?m, tablespeed 20 m/ min, plunge anddry grinding. The lasting time of the grinding thermicvoltage wave varying from rapid ascending to desend-ing gradually only lasts 0. 02 to 0. 03 s. This result in-dicates that the range of the high grinding temperatureonly lasts a very short time on any point of the siliconwafer ground surface. T he rapid tablespeed during thestraight wheel grinding processes makes the grinding86TRANSACTIONS OF TIANJIN UNIVERSITY Vol. 6 No. 1 2000zone moving fast through the surface of the work-piece. Little burn can be found on the workpiece sur-face.Fig. 4Signal waveshape ground by a straight wheelFig. 5 shows the thermic voltage wave of the sili-con wafer ground by a cup wheel ( W28) . T he grindingparameters are as follows: grinding depth is 10 ?m, ta-blespeed is 12 mm/ min, and coolant is engaged. Com-pared with Fig. 4, the thermic voltage signal wave lastsabout 12. 5s from ascending to descending. Obviously,the lasting time of the high temperature in the cupwheel grinding process is as long as several hundredtimes compared with the straight wheel grinding pro-cess. T he longer time the high temperature lasts, themore influence the workpiece qualities will suffer byheat. A complementary test, in which the conditionsare similar to Fig. 4 but the coolant, presents furtherevidence to above opinions. Serious results appeared:extreme high temperature, cracks on the workpiecesurface, and the scorch of the adhesive between theworkpiece and its supporter. T hese experimental re-sults suggest that the coolant is essential to the cupwheel grinding process, and some suitable methods,such as decreasing the width of the cup wheel, choos-ing a proper coolant and dressing wheel in time, shouldbe adopted to reduce the generation of grinding heat.Fig. 5Signal waveshape ground by a cup wheel3. 2Experimental phenomena and analysis duringstraight wheel grinding process T he surface grinding temperature of spark-outgrinding was tested to observe the laws of surfacegrinding temperature during the elastic recovery pro-cess of the grinding system in which the actual grind-ing depth decreases gradually to zero. During thisgrinding process, the material removal mechanismmight change and there will have some influence onthe surface grinding temperature. Fig. 6 shows the relation between the surfacegrinding temperature and spark-out times. With theincrease of spark-out times, surface grinding tempera-tures degrade step by step, finally to steady state.From the experiment results, we have the followingdiscussions.Fig. 6Relation of surface grinding temperatureand spark- out times First, at the sixth spark-out grinding time, thesurface grinding temperature increases abnormally.Similar tests were repeated three times to avoid possi-ble errors. T he tests present similar abnormal resultsat the same time. There may exist several factors caus-ing the results, such as the abnormal contact of ther-mocouple node, etc. We attribute the results to thechange of material removal mechanism. When the ac-tual grinding depth decreases withthe spark-outgrinding times, the material removal mechanism turnsto ductile-mode from brittle-mode. In this process, thegrinding force and temperature will have significantchanges. T he results also express that the ductile-mode grinding has not close relationship with grain-size. Proper conditions provided, a coarse grain sizewheel can realize ductile-mode grinding, too. Becauseof the less number of grains around the coarse grainwheel periohery, the surface roughness ground by acoarse grain wheel is larger than that ground by a finegrain wheel.87TRANSACTIONS OF TIANJINUNIVERSITY Vol. 6 No. 1 2000Second, according to Fig. 6, surface grinding tem-peratures tend to have a steady value after the sixthspark-out grinding process. T hat is to say, maybethere is not any material being removed from theworkpiece?s surface. T hus there exists a limit time ofsilicon wafer spark-out grinding. It is useless beyondthe limit.Finally, after the sixth spark-out grinding pro-cess, surface temperature also can be tested even if it ismeaningless in fact. T he tested temperatures keep con-stant on the whole even after the sixtieth spark-outgrinding. T he grinding thermic voltage curve measuredis shown in Fig. 7.Fig. 7Signal waveshape after the sixth spark-out grindingIn contrast with Fig. 4, it is obvious that thesetwo kinds of thermic voltage curve differ entirely. Thefact that the thermic voltage curves change from onepeak to multi-peaks expresses that the grinding heat isgenerated by individual grains. T hough, there is no ac-tual grinding depth and no material removal, the wheeland workpiece contact anyway in the form of plough-ing or scratching. There is still some energy trans-formed into grinding heat. In order to verify the as-sumption that each peak in the multi- peak curve re-sults from the single grain grinding intercourse, the re-lationship between the number of peaks and workpiecetablespeed is observed, as shown in Fig. 8.Fig. 8Relation between grain number and table speedT he faster the tablespeed is , the fewer the peaknumber is. This is because the contact time betweenthe wheel and the workpiece becomes shorter with theincrease of tablespeed, so, the number of grain engagedinto grinding process decreases. The effective grainnumber can be measured through this method duringthe spark-out grinding process.3. 3Effect of grain size and grinding depth on sur-face grinding temperature3. 3. 1Surface grinding temperature caused bystraight wheelFig. 9 shows the experimental relations betweenthe surface grinding temperature of the silicon waferand straight wheel grinding parameters that includegrain size, grinding depth, up grinding and down grind-ing. The grinding wheel speed and the tablespeed arekept constant, where the grinding wheel speed is 22. 8m/ s, tablespeed is 8 m/ min. From Fig. 9, we knowthat, at the same grinding depth, the surface grindingtemperature ground by a fine grain wheel is higherthan that by a coarse grain wheel, in which a similarrelation in grinding force was observed by other schol-ars 4. Though the surface grinding temperature in-creases with the grinding depth increasing, the increas-ing rates are different with grain size. When the grind-ing depth is smaller than 0. 02 mm for a 240#wheel,the surface grinding temperature increases slowly.When the grinding depth is over 0. 03 mm, the surfacegrinding temperature increases abruptly and causesburn of the workpiece. As to a wheel of grain sizeW10, the surface grinding temperature grows quicklyat a grinding depth of 0. 005 mm, but, when the grind-ing depth is higher than 0. 01 mm, the grinding processis unsteady, the surface grinding temperature increasesabruptly, and the workpiece burn is found.Fig. 9Surface grinding temperature groundby straight wheel88TRANSACTIONS OF TIANJIN UNIVERSITY Vol. 6 No. 1 2000Fig. 10Surface grinding temperatureground by cup wheel3. 3. 2Surface grinding temperature caused by cupwheel T he cup wheel speed and the workpiece feedspeed are constant. Experimental conditions are listedin T ab. 2. The rotation speed of the cup wheel is 1 500r/ min, the feed rate of the workpiece is 12 mm/ minand the grinding depth ranges from 0. 001 mm to 0. 01mm. T he experimental results are shown in Fig. 10. Inthe same straight wheel grinding process, the surfacegrinding temperature ground by a fine grain wheel isobviously higher than that by a coarse grain wheel.Moreover, the coolant has critical influence on the cupwheel grinding process. Without a coolant, the grind-ing heat will remain and accumulate in the workpiece,and finally, cause the unstability of the grinding pro-cess and workpiece burn.4Conclusions1) Ground by a cup wheel, the high temperatureperiod lasts longer than that ground by a straightwheel. The coolant takes very im
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