The present invention relates to a method of machining of flat parts, such as for example silicon wafers.
Machining of silicon wafers are known in the art. For producing silicon wafers, silicon ingots with a cylindrical shape are sliced into the thin wafers by cut-off grinding. The wafer circumference is ground with a profiled diamond wheel, so-called edge rounding. Then the wafers have to be lapped to ensure precise thickness, flatness, and parallelism. After this process, a chemical etch treatment is used. The wafer is then polished by chemical-mechanical polishing on one side, with surface roughness in the vicinity of 5 nm.
During edge rounding of silicon wafer distinct scratches are produced occasionally by silicon particles which break away from the wafer periphery. In order to avoid this edge-chipping, the wafer circumference is ground with a profiled diamond wheel in accordance with a traditional approach. During the cut-off grinding, edge-chipping of the wafer circumference takes place. This is precisely the etch chipping which is a source of breakage of wafers during the subsequent lapping. There is no other reason why the edge-rounding is used, in particular to avoid the edge chipping of the wafer circumference and to prevent breakages of wafers during lapping.
Double-side lapping process has several disadvantages. First of all, in order to perform the processing of cutting, rigid kinematic connection is used between a workpiece, abrasive grains and lapping plates. This rigid kinematic connection leads to necessity of utilization of rigid, massive, precise power tools with a rigid frame essential for vibration-free operation, and the lapping plates are machined together with the part to be machined, so that they change their shape and as a result, accuracy of machining is lost. In particular, it results in a spherical flatness of the upper and lower plate surfaces which affects thickness variations, parallelism and flatness of the workpiece. As a result, the lapping plates are continuously machined to obtain required shape. Also, brittle and hard workpieces (for example silicon wafers) are broken since they are integrated in the rigid kinematic system of the power tool.
For cutting a slurry is utilized. The slurry contains abrasive grains of various sizes in water or oil base. The presence of slurry leads to the fact that it is no longer possible to use liquid for removal of cutting products from a machining zone, since liquid washes off oil or water utilized for retention of grains in the machining zone. Swarf remains in the machining zone and is not removed, while abrasive grains engage into it. It is not possible to increase the speed of cutting, and the number of revolutions does not exceed 120 per minute, since oil and water can not retain abrasive grains in the machining zone in condition of high rotary speeds.
Provision of the rigid kinematic connection and the slurry leads to the use during machining of abrasive grains having different sizes, in particular large sizes in order to increase a material removal rate and small sizes in order to obtain a high quality surface of workpiece. The use of the large grains increases a depth of damage which results to unavoidable warping of workpieces and difficulties in obtaining parallelism and flatness. The use of small grains leads to a loss of efficiency of machining. Also, with the rigid kinematic connection and the slurry the requirements for a precise grading of abrasive gains are necessary. A tolerance of the grain size does not exceed several .mu.m. Such particles can scratch or fracture the workpiece and do not contribute to improvement of surface finish. Finally, it is necessary to provide several types of lapping plates in order to obtain the desired quality of surfaces to be machined.
It is known to provide machining of wafers with the use of magnetic-abrasive process, as disclosed for example in U.S. Pat. No. 5,239,172. The disadvantage of this method is that an electromagnet with a yoke, coil and poles is utilized in this process. It is well known that in electromagnets with the increase of surface of pole faces, the field diminishes directly proportionally. For example,
______________________________________ S.sub.1 MM.sup.2 .phi., MM Field, T ______________________________________ 314 .phi.20 1.756 1'256 .phi.40 1.333 2'025 Square 45 0.916 3'300 .phi.65 0.202 ______________________________________
whereas an 5 is an area and 6 a diameter of pole faces. These values of the magnetic field are obtained with the pole gaps 5 mm, magnet current maximum 5.0 amps with water cooling (Operation Manual, Laboratory Electromagnet, Model 347 GMW).
Thus, for this invention, with the pole face formed as rings 25a and 25b, a field gradient will be insufficient for the magnetic abrasive machining with S pole equal to or more than 2000 mm.sup.2, since the powder will not be retained in a gap during machining. Moreover, the presence of yoke increases the size of the power tool, and the connection of the disks by a rotary shaft extending through the center of the disks makes impossible machining of a workpiece with a whole surface of the pole face. Therefore, the center of the workpiece can not be machined with this device if its diameter is greater than the diameter of disks. All above mentioned disadvantages result in a limitation to the diameters of the workpieces to be machined. On the other hand, currently the diameters of silicon wafers reach 400 mm, and the diameter of machining with setting of 10 wafers reaches 2000 mm.
Magnetic abrasive machining disclosed in U.S. Pat. No. 4,211,041 has the disadvantages of a weak magnetic field and gradient between poles, due to the fact that poles of two electromagnet systems are not connected by yoke, but instead are connected by direct and feedback electrical connection. Also, when the pole of the rotor is located in a gap between the poles of the conductor. The machining is not performed at all. Between this extreme conditions, the workpieces are machined only partially. Finally, the electric circuitry is connected with changing of polarity of the rotor electromagnetic poles so that each counter opposed pair of inductors and rotor poles will have a different polarity which is very complicated.
The existing chemical-mechanical polishing has its own disadvantages. In particular, the wafer is hard to clean, it has a relatively high cost, it is of limited productivity, and single-pass system and end-point detection are unavailable. In this process, a polishing pad, slurry and special wafer clamping technique are utilized. Silicon wafer requires polishing in several stages, in particular initially the wafer surface has to be polished for removing surface defects, then oxide surface after each lithography step has to be removed and than action process must be formed to achieve planarization. The polishing fluid is an alkaline solution which contains chemical reactive particles with a size of approximately 100 nm. The material removal rate in this process involves chemical and mechanical process. A rise of temperature significantly increases the material removal rate. A significant part of the relatively high cost of this treatment is the cost of reactive particles with the size of 100 nm.