As technology advances, portable and handheld devices continue to decrease in size. These devices require smaller assemblies, which in turn require thinner silicon wafers. Achieving the correct wafer thinness before assembly is critical in semiconductor manufacturing. Wafer backgrinding, or wafer thinning, is a semiconductor manufacturing process designed to reduce wafer thickness. This essential manufacturing step produces ultra-thin wafers for stacking and high-density packaging in compact electronic devices. The silicon wafer backgrinding process is complex and requires advanced, customized grinding equipment. In this article, we take a look at wafer grinding machines in general and specifically introduce the advantages of aerostatic spindles and rotary tables for the wafer backgrinding process.
Wafer backgrinding, also known as wafer thinning, is a semiconductor device fabrication step during which wafer thickness is reduced to allow for stacking and high density packaging of integrated circuits (IC). Conventional grinding is an aggressive mechanical process that utilizes a diamond and resin bonded grind wheel mounted on a high speed spindle to perform the material removal. The grind recipe dictates the spindle RPM, rate of material removal, and the final target thickness of the work piece. Harder materials like sapphire typically require slower feed rates compared to more forgiving materials like silicon.
The wafer is positioned on a porous ceramic rotating vacuum chuck with the backside of the wafer facing towards the grind wheel. Both the grind wheel and wafer chuck rotate during grind. Deionized water is jetted onto the work piece to provide cooling and wash away material particles generated during the grind. A grinding tape is applied to the front side of the wafer to protect the devices from being damaged during thinning.
Aerostatic spindle is widely used in the precision equipments, for example, ultra-precision machine tool, precision turntable, due to its unique advantages of high speed, negligible friction, and excellent rotation accuracy. The aerostatic spindle is made up of the structure components and the fluid film. The basic working principle of the aerostatic spindle is that the externally pressurized air forms a thin film in the clearance between shaft and shaft sleeve after throttling action of restrictor, and the air film forces oppose the external load and maintain the separation of the surfaces of shaft and shaft sleeve. The aerostatic spindle is designed hope to with high stability and high precision, while there are too many factors affect the performance of the aerostatic spindle, such as the fluid–structure interaction effect between the structure components and the fluid film, the thermal generated in high rotation speed. All of those factors will reduce the accuracy of the aerostatic spindle.
The ultra-precise nature of our tool spindles allows for the manufacture of parts with an optical surface finish. Once warmed through aerostatic spindles don’t change their properties whereas conventional spindles, which uses roller bearings, do. This allows machining of complex parts at constant part quality, even when machined at 100.000 rpm for days or weeks.
An analysis of the interactive characteristics of the aerostatic spindle and the aerostatic rotary table cleary indicates that the relative rigidity between the spindle and the rotary table is much more important than whether the spindle is based on an orifice or a porous restrictor. The grinding force, which the aerostatic spindle and the aerostatic rotary table react to, may directly affect the accuracy, such as the total thickness variation (TTV), the thickness tolerance and the surface roughness. The stress concentrated on the neck of the spindle is a key issue solved by the high-stiffness aerostatic spindle for grinding difficult-to-cut materials.
Wafer thinning is the most critical technology in the semiconductor manufacturing process, it determines the smallest possible package size in wafer-level packaging . The mechanical process of back-grinding is able to meet the size requirements of thin wafers with a thickness of 50 micron (2 mil, 1 mil=1/1000 inch). However, during the grinding process, the grinding force will affect the processing accuracy such as total thickness variation (TTV), thickness tolerance and surface roughness.
From Figure 1. The relationship between the aerostatic spindle and the aerostatic rotary table, the gears of the grinding wheel passes through the center of rotation of the rotary table.
Fig.1 Relationship between Aerostatic spindle and Aerostatic rotary table
Considering that the teeth of the spindle grinding wheel contact the wafer, the spindle and the rotating table are subjected to 10 kgf force and reaction force. In the simulation in Fig. 2 the grinding FEM model shows that the stress is concentrated on the neck of the spindle; as Large-size aerostatic spindle bears grinding wheel deformation in FEM model (as shown in Fig.3).
Fig.2 The stress concentrated on the spindle neck in FEM model
Fig.3 Large-size aerostatic spindle bears grinding wheel deformation in FEM model
While the inclination grinding of the spindle grinding wheel in the grinding FEM model causes the deformation of the rotary table (Fig. 4), which conforms to the radiation of the grinding marks after wafer grinding, so called Spiral lines (Fig. 5).
Fig.4 The inclination of the spindle grinding wheel causes the deformation track of the rotary table in FEM model
Fig.5 After grinding the concave wafer has radial spiral grinding marks
Table 1 shows a rigidity comparison of the of the spindle and the rotary table in the grinding FEM model, it is known that under the balance of the action and reaction force system, the spindle bending rigidity (grinding wheel and grinding contact arc) and the rotary table axial rigidity (grinding wheel and grinding contact arc). Relatively close to the axis of rotation the difference is a hundred times (two orders). In practice, the grinding also reflects that the rigidity of the rotating table is better than that of the spindle.
Table 1. Rigidity Comparison of spindle and rotary table in the grinding FEM model
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