Study On Grinding-based Flattening Theory And Process Technology For Large-sized Silicon Wafer

With fast developing of the integrated circuit (IC) manufacturing technology, the devicefeature size continues reducing in order to increase the density of integration. On the otherhand, the diameter of the silicon wafer tends to be larger and larger in order to increase theyields of chips and decrease the cost per bit. While the diameter of the wafer increase, thethickness of wafer is also increased to ensure the strength of the wafer. Contrarily, the chipthickness is decreased to be thinner and thinner to meet the requirements of the advanced ICpackage technology. With the increase of the diameter and thickness of the wafer and thedecrease of the chip thickness, the current flattening process of the wafer is facing many newtechnology problems. The high machining accuracy is difficult to achieve because the largesized wafer is easy to warp, and the high machining efficiency is required because sizechanges of the wafer and the chip result in the increase of the material removal amount.Therefore, the ultra-precision grinding with fixed abrasive wheel, which is taking the place oflapping with loose abrasive, becomes the main trend of development for ultra-precisionflattening process for the large-sized silicon wafer. Although wafer rotation grinding methodis especially considered as the most promising processing technology and has been used inmanufacturing of the blank wafer, back thinning of the pattern wafer and reclaiming the rejectwafer, how to further improve surface layer quality and flatness of the ground wafer andenhance machining efficiency are still main problems to be solved. Thus, it is very necessaryto deeply and systematically research the flattening theory and key process technology basedon wafer rotation grinding.In this paper, according to relative motion between the cup grinding wheel and the siliconwafer, a kinematic model of the wafer rotation grinding is established, the kinematic equationof the grit trajectory is derived based on the concept of pitch point and pitch circle etc. Theformula of the length and number of the grit trajectories and the stable grinding period arededuced. The influence of the interval and the density of the grinding marks, formed on thewafer surface by the many grits, on surface quality of the ground wafer are analyzed.Simulation software of the grinding marks on the wafer surface is developed on operatingsystem Windows 2000/XP, and the grinding marks on the wafer ground with differentvelocity ratio of the grinding wheel to wafer are predicted and analyzed by computersimulation. The results of simulation and theoretical analyses are verified by experiments, which are conducted on a grinding machine VG401. Through theoretical and experimentalstudies, the available technique ways to improve the surface quality of the ground wafer areprovided from kinematic point of view.A theoretical model for the ground wafer surface profile is developed, in which manycritical factors, including dressing parameters of the porous ceramic vacuum chuck systemand grinding parameters of the wafer, etc., are considered. From the model, the equation ofthe ground wafer surface and the formula of the total thickness variation(TTV) that is animportant data of the wafer surface flatness, are derived. By using a computer simulationsoftware on the ground wafer surface developed with Visual C++(VC++) and Open GraphicLibrary (OpenGL), the 3D surface profile of the dressed vacuum chuck and the ground waferare predicted, and the effects of the dressed vacuum chuck surface and the wafer grindingparameters, including the rotational speed of the grinding wheel, the down feed rate of thegrinding wheel, the rotational speed of the wafer, the roll angle and pitch angle of the grindingwheel spindle, on the 3D surface profile of the ground wafer are theoretically analyzed. Theresults of theoretical analysis and computer simulation are verified through a series ofgrinding experiments on a grinding machine VG401.An innovative low damage grinding method for silicon wafers using a soft abrasivegrinding wheel (SAGW) is put forword. The SAGW with #3000 CeO₂ abrasive, which hascapability of good self-dressing and releasing active additives to provide particular machiningcondition, is developed. A detachable and interchangeable structure of the SAGW is designed.The low cost manufacturing and dressing method of the SAGW are studied, and the dressingtools for the SAGW are developed. The grinding experiments using the SAGW on thecondition of dry and wet grinding are conducted and chemical reaction between the abrasiveand additives of grinding wheel and the wafer is testified by XPS inspection of productcomponent on the ground wafer surface. The mechanism of material removal on the wafersurface can be explained as chemical and mechanical recombination action. The grindingperformances of the developed SAGW are studied in terms of surface roughness, surfacetopography and surface defect and surface/subsurface damage of the ground wafers by use ofAFM, SEM and TEM. The experiment results show that the grinding method for siliconwafers using the SAGW has not only high machining accuracy and low machining cost, butalso hardly results in such damage as scratch, crack and dislocation in the surface andsubsurface of the ground wafer.Aiming at the surface quality and material removal rate, an innovative ultra-precisionflattening process for large-sized silicon wafer, which is a procedure processing system,integrating the rough grinding with #325 diamond grinding wheel, fine grinding with #2000diamond grinding wheel, and the low damage grinding with #3000 CeO₂ SAGW on a precision grinding machine based on wafer rotating grinding method, is put forward. TheTaguchi experiment programme of four factors, including the rotational speed of the grindingwheel, the rotational speed of the wafer, the down feed rate of the grinding wheel, and theflux of cooling fluid, is designed. The optimum process parameters of the wafer rotationgrinding are obtained according to the different single machining performance characteristics.The optimum of multi performance characteristics is realized by converting the optimumproblem of multi performance characteristics to a sequencing problem of grey relational gradethrough grey system theory. And optimum process parameters for the rough grinding, finegrinding and low damage grinding are separately given.