Mechanical lapping and polishing process of polycrystalline diamond wafers

Abstract

ObjectivesDiamond is a critical material potentially or already applied in infrared windows, electronic components and acoustic devices for its excellent optical transmittance, high eletron mobility and high breakdown voltage. Mechanical lapping is one of the mainstream methods for diamond planarization. However, it is more difficult to mechanically planarize polycrystalline diamond due to the grains and the boundaries which may lead to defects and internal stress release. Variable-parameter mechanical lapping are conducted on polycrystalline diamond to investigate the effects of abrasive grain size, lapping pressure and abrasive concentration on material removal rate (RMRR) and surface roughness Ra.MethodsA group of {100} polycrystalline diamond wafers are attached to a load plate with lapping fluid speed of 8 mL/min, rotational speed of 30 r/min and orbital speed of 45 r/min. The grain size (W7～W50), the concentration of fluid (3%～6%) and the loading pressure (0.1～0.4 MPa) are tested for a reasonable process. A surface profiler is used to observe the morphology of three equal division points (800 μm×800 μm) on diamond surface along the diagonal of the 5 mm×5 mm×1 mm wafer. The average roughness are used to characterize the lapping effect.ResultsIt is found that the RMRR increases with the increase of grain size, reaching its maximum of 25.210 μm/h with grain size of W50 and Ra of 240 nm. But there appears micro cracks on diamond surface. The RMRR slightly increases as the grain concentration increases from 3% to 5% but decreases at concentration of 6%, varying around 11 μm/h. The MRR also increases from ～8 μm/h to ～17 μm/h as the lapping pressure increases from 0.1 MPa to 0.4 MPa. Conversely, the surface roughness decrease from 240 nm to 100 nm with finer abrasive, which is a dominant factor affecting the surafce quality. The surface roughness also decreases first but increases then with higher lapping pressure and abrasive concentration.ConclusionThe optimal process parameters for polycrystalline diamond wafer are determined, namely the lapping pressure of 0.3 MPa, the abrasive grain size of W10 and the lapping fluid concentration of 4%, where the processed polycrytalline diamond wafer achieves the best surface quality of Ra ～96 nm and a corresponding RMRR of 7.097 μm/h.

Keywords

• diamond
• lapping fluid
• mechanical lapping
• lapping pressure
• abrasive grain