A novel approach of mechanical chemical grinding

In this paper, the silicon substrates machined by nanogrinding and chemo-mechanical-grinding (CMG) were characterized using atomic force microscopy, transmission electron microscopy and instrumented nanomechanical tests. It was found that the nanogrinding-generated silicon subsurfaces consisted of an amorphous layer and a damaged crystalline layer underneath, but the CMG-generated subsurface was defect-free. The formation of the amorphous and damaged crystalline layers was dependent on the maximum undeformed chip thickness involved in the nanogrinding process. Nanoindentation and nanoscratch tests revealed that the amorphous silicon exhibited to be more plastically deformed than the damaged crystalline layer under the same loading condition.

Characteristics of silicon substrates fabricated using nanogrinding and chemo-mechanical-grinding

Ultra-precision machining technologies are the essential methods, to obtain the highest form accuracy and surface quality. As more research findings are published, such technologies now involve complicated systems engineering and been widely used in the production of components in various aerospace, national defense, optics, mechanics, electronics, and other high-tech applications. The conception, applications and history of ultra-precision machining are introduced in this article, and the developments of ultra-precision machining technologies, especially ultra-precision grinding, ultra-precision cutting and polishing are also reviewed. The current state and problems of this field in China are analyzed. Finally, the development trends of this field and the coping strategies employed in China to keep up with the trends are discussed.

Review on the progress of ultra-precision machining technologies

A novel approach of high-performance grinding is proposed using developed diamond wheels, to obtain minimally damaged surface layer in silicon wafers. For this reason, resin bond diamond wheels are specifically developed with lanthanum oxide (La2O3), magnesium oxide (MgO), and ceria (CeO2) as additives, respectively. The wheels contain grains with a mesh number of 20,000 and a volume fraction of diamond grains of 37.5%. The diamond wheel with ceria additives demonstrates the best grinding performance in terms of surface integrity and roughness. It allows to generate an amorphous surface layer of 43 nm in thickness, without grinding damage beneath in a silicon wafer. This is different from previous reports, in which an amorphous layer is at the top, followed by a damaged crystalline layer underneath induced by a diamond wheel. Below the amorphous layer is the pristine crystalline lattice, which is confirmed using the high-resolution transmission electron microscopy (HRTEM). The ceria wheel results in a surface roughness R
 a of 0.88 nm and a peak-to-valley (PV) value of 8.3 nm over an area of 70 × 50 μm2 on a Si wafer at a feed rate of 15 μm/min.

A novel approach of high-performance grinding using developed diamond wheels

A comprehensive surface roughness model is established to predict the average surface roughness achieved by shear-thickening polishing (STP) based on calculation of Brinell hardness number (BHN), shear-thickening mechanism and plastic indentation on abrasive wear theory. The model takes the effects of material hardness and plastic wear into account in addition to the calculation of the surface roughness factors concerning the normal pressure, slurry performance, et al. The “size effect” as the ratio of abrasive size to solid colloidal particle size, has also been successfully integrated into the model. STP experiments validate that the maximal difference of surface roughness between theoretical and experimental results is no greater than 12.02%. The surface evolution process can be theoretically and experimentally described with certain feasibility and veracity. An inflection point appears in the trend line of predicted surface roughness when the ratio approaches to 0.5. An equivalent control law of surface roughness in STP exists when the ratio is greater than 0.5 and is revealed by the model and experiments due to the competing effects of kinematics, indentation depth, cutting width, plastic plowing of materials on surface formation. The freedom control for an invariable surface roughness can be achieved by STP according to the equivalent control law. The prediction model for brittle materials shall be revised based on material properties and more removal forms.

Evolution and equivalent control law of surface roughness in shear-thickening polishing

Defect-free fabrication for single crystal silicon substrate by chemo-mechanical grinding

Grinding process on the Si wafer develops subsurface damage, which remarkably degrades deflective strength of the wafer and constitutes a barrier against producing a thin wafer for low-profile packaging. In this paper, the authors propose a new index for evaluation of the Degree of Subsurface Damage (DSD). Requiring no costly instrument, the new index is easily calculated via the external geometry of the ground wafer. With the new index, it is able to quantitatively evaluate the subsurface damage introduced by different processes (or wheel) and to estimate the minimally achievable thickness of the wafer by each process. Also, a novel fixed abrasive process of Chemo-Mechanical Grinding (CMG) has been proposed for stress relief. All results indicate that the subsurface damage after CMG is nearly zero.

Fabrication and evaluation for extremely thin Si wafer

A precision machining apparatus is structured such that a feed-screw mechanism and an actuator are mounted on a second mount which supports a rotating device that rotates a wheel, and grinding is performed while appropriately adjusting the amount of movement of the second mount in a rough grinding stage through an ultra-precision grinding stage. A posture control device is interposed between a first mount and a rotating device that rotates a grinding target body. The thickness and evenness of the grinding target body are measured by an optical probe and the measurement results are sent to a computer. A feedback command is then sent to the posture control device to reduce the difference between target values and the measurement values and posture control is performed accordingly.

Precision machining apparatus and precision machining method

A precision machining method capable of accurately performing grinding. The method comprises a first step for manufacturing an intermediate ground body by roughly grinding a ground body (a) with a diamond grinding wheel (b) and a second step for manufacturing a final ground body by grinding the intermediate ground body with a CMG grinding wheel. In the first step, by a control based on a moved amount, a rotary device (6b) and a base (3) are controllably fed by multi-stage feeding with different feed rates. In the second step, by a control based on a pressure, the rotary device (6b) and the base (3) are controllably moved by a fixed pressures or by multi-stage fixed pressures with different pressures.

Precision machining method

Disclosed is a synthetic grinding stone used for polishing of silicon wafer, which is composed of a structure comprising cerium oxide fine particles as abrasive grain, a resin as a binder, a salt as a filler and a nano diamond as an additive. This synthetic grinding stone is characterized in that the purity of the cerium oxide is not less than 60% by weight, the content of the salt as a filler is not less than 1% but not more than 20%, the volume content of the nano diamond as an additive is not less than 0.1% but less than 20% relative to the total volume of the structure, and the porosity as the volume fraction relative to the total volume of the structure is less than 30%.

Hisao Iwase

Papers

Defect-free fabrication for single crystal silicon substrate by chemo-mechanical grinding

Fabrication and evaluation for extremely thin Si wafer

Precision machining apparatus and precision machining method

Precision machining method

Synthetic grinding stone