Achieving Micron-Level Flatness on Large Aluminum Housings for Semiconductor Tools

The Flatness Requirement in Wafer Processing
In semiconductor fabrication and metrology tools,the equipment base plate or housing is more than a cover—it's the foundational plane that determines system accuracy.A typical specification might call for flatness within 10 microns over a 500mm x 800mm area on Aluminum 6061-T651.Achieving this isn't a simple milling job;it's a battle against material stress,thermal expansion,and machine tool capabilities.

Challenge 1:Material Stress Relief and Stock Selection
The first step happens before machining begins.

Stress-Relieved Plate:We insist on using pre-stress-relieved aluminum plate(e.g.,6061-T6511).Standard plate has internal stresses from the rolling process that are released during machining,causing the part to warp unpredictably.

Proper Clamping Strategy:Even with stress-relieved stock,improper clamping introduces new stress.We use a multi-point,low-pressure clamping system on a vacuum or modular fixture to hold the workpiece evenly without distorting it.

Challenge 2:Thermal Management During Machining
Cutting generates heat,and heat causes localized expansion.

Climb Milling vs.Conventional Milling:We use climb milling strategies where possible,which produces thinner chips and carries heat away from the part more efficiently.

High-Pressure Coolant:Directed high-pressure coolant isn't just for chip evacuation;it's critical for maintaining a stable part temperature throughout the operation.We monitor coolant temperature to prevent thermal drift.

Light,Consistent Passes:We avoid heavy cuts that generate excessive heat.The final passes for achieving surface finish and flatness are light skin cuts performed after the part has stabilized.

Challenge 3:The Machining Process:Roughing,Stabilizing,and Finishing
This is a multi-stage,time-intensive process.

Initial Roughing:We remove the bulk of material,leaving a uniform allowance(e.g.,1.5mm)on all surfaces.

Stress Relief and Re-Clamping:The part is unclamped,allowing any newly induced stress to relax.It is then meticulously re-clamped onto the machine bed,ensuring it's sitting on a true,clean surface.

Semi-Finishing:We take another series of passes to bring the part closer to final dimensions,again leaving a small,uniform stock(e.g.,0.2mm).

Final Finishing:Using a large-diameter,ultra-sharp face mill with a very high spindle speed and slow feed rate,we take the final pass to achieve the specified surface finish(often Ra 0.8 or better)and flatness.

Verification:Measuring What Matters
You cannot machine to a tolerance you cannot measure.For large plates,a standard height gauge is insufficient.

Laser Tracker or Large CMM:We utilize a laser tracker or a bridge-type CMM with a large envelope to map the entire surface and generate a color-contour flatness map.This data is the ultimate proof of compliance and is often included in the First Article Inspection Report.

Temperature-Controlled Environment:Final inspection is performed in a temperature-stable environment to ensure measurements are not affected by thermal expansion.

Conclusion:Flatness as a Process,Not a Feature
Achieving micron-level flatness on a large scale is a testament to process discipline.It requires the right material,a meticulous multi-stage machining strategy,rigorous thermal control,and advanced metrology.For semiconductor OEMs,partnering with a machine shop like DHT Industrial that has mastered this specific large-part,high-precision milling capability is essential.It ensures your tool's mechanical foundation is as solid and precise as the physics it is designed to exploit,directly impacting wafer yield and tool uptime.