As semiconductor, optical, sapphire, and other precision-material applications continue to demand tighter thickness and geometry control, wafer surface flatness has become an important consideration during precision lapping. A High-Precision Double-Sided Lapping Machine can process the two surfaces of a wafer simultaneously, helping reduce thickness variation while maintaining a more balanced material-removal condition. However, achieving high flatness is not simply a matter of using two grinding surfaces. Machine rigidity, pressure distribution, planetary motion, rotational speed, abrasive conditions, and process stability all influence the final wafer geometry.
Zhejiang Morinaga Optical & Electronic Equipment Co., Ltd. focuses on high-precision planarization grinding and polishing technology and manufactures both single-sided and double-sided precision grinding and polishing equipment. Its 20B/22B/24B series is designed for precision double-sided grinding and polishing, with five planetary gears and four-motor drive configurations. The series provides adjustable lower-anvil, upper-anvil, center-gear, and ring-gear speeds, while its pressure system uses a precision load cell, PLC, and electronically controlled proportional valve for closed-loop pressure feedback.
Poor wafer flatness can originate from several interacting factors rather than from a single machine setting. Uneven pressure across the workpiece, differences in abrasive contact, insufficient fixture accuracy, thermal variation, unstable rotation, and inconsistent material removal can all contribute to wafer geometry errors. If one region of the wafer experiences a higher effective removal rate than another, the final surface may develop a convex, concave, or otherwise non-uniform profile.
The problem becomes more significant when processing thin, hard, brittle, or fragile materials. Excessive localized loading can increase the risk of surface damage, while insufficient or unstable contact can reduce material-removal efficiency. For production engineers, the key question is therefore not only how much material is removed, but whether material is removed uniformly across the entire wafer.
Machine structural stability also matters. The 20B/22B/24B platform uses an integral cast base and HT300 gray cast-iron castings, together with THK cross-roller bearings and NSK bearings. Its gears are manufactured from forged gear steel and undergo surface hardening and gear grinding. These mechanical elements provide the foundation for maintaining stable relative motion between the processing components.
A major advantage of double-sided lapping is that the upper and lower surfaces are processed at the same time. Instead of removing material from only one surface and then correcting the opposite side in a separate operation, the wafer is positioned between two processing surfaces. This creates a more symmetrical material-removal process and allows thickness and flatness to be considered together.
For wafer manufacturers, this can be particularly useful when the target is not simply a smooth surface but a controlled overall geometry. If both surfaces are subjected to coordinated processing conditions, the process can help reduce the accumulation of thickness errors that may occur when the two sides are processed independently.
The planetary workpiece movement further distributes the contact path. Rather than keeping a wafer in one fixed rotational position, planetary motion continuously changes its position relative to the processing surfaces. This helps expose different areas of the wafer to changing abrasive trajectories, which can contribute to more uniform material removal.
The 20B/22B/24B models provide five planetary gears, while the planetary gear dimensions vary by model. For example, the 20B and 22B versions use M3 planetary gears, while the 24B uses M4 planetary gears. The maximum machining diameters are listed as 410 mm, 480 mm, and 530 mm respectively.
Pressure is one of the most important process variables in precision lapping. When the applied load is not sufficiently stable, the actual contact pressure can fluctuate during machining. Such fluctuations may change the local material-removal rate and make it more difficult to maintain consistent wafer geometry.
A useful approach is to treat pressure control as a closed-loop process rather than relying only on a preset mechanical load. A load cell can detect the actual processing force, while the PLC evaluates the signal and a proportional valve adjusts the pneumatic system. This creates a feedback loop between the target pressure and the actual processing condition.
The 20B/22B/24B machine uses a precision load cell, PLC, and electronically controlled proportional valve for closed-loop pressure control. Its listed operating pressure ranges from a minimum of 15 kg (150 N) to a standard maximum of 800 kg (8,000 N), with the maximum pressure expandable according to application requirements. Real-time pressure monitoring and calibration are intended to improve pressure accuracy and stability during processing.
For wafer processing, the practical objective is not necessarily to apply the highest possible pressure. Instead, the pressure should match the material, wafer thickness, abrasive system, desired removal rate, and required surface geometry. Excessive pressure can increase mechanical and thermal loading, while insufficient pressure may reduce removal efficiency or make the process less stable.
Planetary motion is another important mechanism behind double-sided lapping uniformity. In a planetary system, workpieces move through combined rotational paths generated by the center gear, ring gear, and planetary gears. As the workpiece changes position, the relative motion between the wafer and the processing surfaces also changes.
This changing trajectory can help distribute abrasive contact over a larger portion of the wafer surface. From a process-control perspective, the objective is to avoid repeatedly concentrating the same contact pattern in one location. Properly coordinated gear speeds can therefore influence the distribution of material removal.
The 20B/22B/24B series allows independent speed adjustment ranges for the major rotating components: the lower anvil is specified at 1–60 RPM, the upper anvil at 1–20 RPM, while both the center gear and ring gear can operate from 1–30 RPM. These adjustable parameters give engineers additional process variables for matching the motion pattern to different workpiece materials and machining requirements.
Planetary gear design must also be considered together with gear accuracy, bearing performance, machine rigidity, and loading conditions. Even when nominal speeds are correctly set, mechanical instability or excessive vibration can influence the consistency of the actual contact conditions.
Lapping speed and pressure should normally be considered as a combined process window. Increasing pressure can increase the mechanical interaction between the abrasive and wafer, while increasing relative speed can change the frequency and distribution of abrasive contact. The resulting material-removal rate therefore depends on more than either parameter individually.
For high-precision wafer processing, simply increasing speed to achieve higher productivity may not produce the desired geometry. A higher relative speed can change friction, heat generation, slurry or abrasive distribution, and contact behavior. Similarly, increasing pressure may accelerate removal but can also increase mechanical stress or create greater sensitivity to local surface differences.
This is why adjustable multi-axis speed control is valuable. On the Morinaga 20B/22B/24B platform, the upper and lower anvils and the center and ring gears have specified adjustable rotational-speed ranges rather than being restricted to a single fixed speed. Engineers can therefore establish process recipes around material type, workpiece dimensions, abrasive characteristics, removal requirements, and flatness targets.
Temperature should also be considered when optimizing the process. Precision lapping involves mechanical contact and friction, and changes in temperature can affect both the workpiece and machine structure. Stable operating conditions can help reduce process drift when tight geometric tolerances are required.
TTV, or total thickness variation, describes thickness differences across a wafer, while parallelism describes the relationship between the two major surfaces. These parameters are closely related to double-sided lapping because the process simultaneously influences both surfaces.
To control TTV, engineers need to maintain consistent material removal across the wafer rather than focusing only on average removal rate. Pressure distribution, planetary motion, upper and lower plate conditions, abrasive distribution, and process time can all affect the final thickness profile. For parallelism, the relative alignment and stability of the two processing surfaces become particularly important.
Process control should therefore begin with accurate incoming-wafer measurement and continue with controlled machine parameters throughout the cycle. Measuring thickness at multiple positions can reveal whether the process is producing center-to-edge variation, edge effects, or other geometry deviations. The process recipe can then be adjusted through pressure, rotational speed, abrasive conditions, or machining time.
Morinaga states that its precision grinding and polishing equipment is used for materials including silicon wafers, germanium wafers, sapphire substrates, ceramic substrates, optical glass, and quartz crystals. Its application examples include 8-inch wafer polishing, 12-inch sapphire polishing, gallium arsenide wafer polishing, and 12-inch glass polishing.
Even a well-designed process recipe can be difficult to reproduce if the mechanical platform is unstable. In high-precision lapping, vibration, bearing movement, gear backlash, structural deformation, and thermal changes can potentially appear as variations in surface geometry.
For this reason, the machine's mechanical construction should be evaluated alongside its control system. The 20B/22B/24B series combines an integral cast base with precision bearings, forged and ground gears, a high-precision worm gear reducer, PLC control, variable-frequency drives, and an HMI system. The machine is also specified with a separate electrical cabinet and an operating environment of 15–35°C with humidity below 85%.
These specifications illustrate an important principle in wafer lapping: surface flatness is a system-level result. Pressure control alone cannot compensate for poor mechanical stability, just as precise gear motion cannot compensate for an unsuitable abrasive or process recipe.
Improving wafer surface flatness with a High-Precision Double-Sided Lapping Machine involves coordinating material removal, pressure, planetary motion, rotational speed, machine rigidity, and process conditions. Simultaneous two-sided processing helps establish a balanced material-removal process, while planetary motion distributes the machining path across the workpiece. Closed-loop pressure control adds another layer of process stability by continuously monitoring and regulating the applied load.
For manufacturers evaluating double-sided lapping equipment, specifications such as maximum pressure or machine size should therefore be considered together with the actual control architecture, gear system, bearing configuration, speed ranges, workpiece capacity, and application experience. Morinaga's 20B/22B/24B series is one example of this integrated approach, offering five planetary gears, four-motor drive, adjustable multi-axis rotational speeds, and closed-loop pressure control for precision grinding and polishing applications.
For companies processing semiconductor wafers, sapphire, optical materials, ceramics, or other precision components, selecting the appropriate double-sided lapping configuration ultimately requires matching machine capability with material properties, wafer dimensions, target TTV, parallelism, flatness, surface roughness, and required production throughput. A process-oriented evaluation can provide a more useful basis for equipment selection than considering machine specifications in isolation.
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