Glass Micromachining Process: 7 Steps for MEMS Parts

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  Key takeaways
• 2
  Why glass behaves differently at the micro scale
• 3
  Pick the glass grade before you pick the cutter
• 4
  What CNC can and cannot hold on glass
• 5
  Clamping and workholding decide the outcome
• 6
  Step by step: running the glass micromachining process
• 7
  Glass micromachining process comparison

Glass removes material by brittle fracture, not plastic flow. A carbide cutter that works well on aluminium will chip borosilicate at the same feed. The crack propagates ahead of the cutting edge, so the damage zone is wider than the chip you see. That is the first thing to accept before choosing a glass micromachining process.

At features under 1 mm, the edge condition dominates everything. A 50 μm chip on a 300 μm wall is a 17 percent material loss. In practice this means you plan the tool path so the cutter exits into open space, never into a finished wall.

Thermal expansion is low but not zero. Borosilicate sits near 3.3 × 10⁻⁶ /°C and fused silica near 0.55 × 10⁻⁶ /°C. Local heating from dry cutting can still open a crack at a sharp corner. Flood coolant keeps the cut near room temperature and stabilizes the measured dimension.

Thickness changes the rules. A 1 mm wafer behaves like a membrane and will flex under clamping force. A 6 mm plate behaves like a rigid body. Clamp pressure, support spacing, and feed all shift with that difference.

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  Brittle fractureMaterial leaves as chips and micro-cracks, not continuous swarf.
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  Edge-first thinkingDesign the path so the tool exits off the part, not into a wall.
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  Thickness is a variableThin wafers need lower clamp force and finer passes.

Borosilicate (Borofloat 33, Pyrex 7740) is the workhorse for microfluidics and MEMS packaging. It bonds well by anodic bonding, machines predictably, and costs less than fused silica. For most channels and vias in the 100–800 μm range, it is the right starting point.

Fused silica gives better optical transmission and a much lower thermal expansion coefficient. It is harder and more abrasive, so tool life drops and you should budget more passes. Use it when the part sees high-temperature steps or deep UV light.

Soda-lime glass is cheap and easy to source, but it contains mobile ions. That rules it out for many electrical and biological MEMS devices. It is fine for structural spacers and covers where ion migration does not matter.

Tooling comes down to three options. PCD (polycrystalline diamond) end mills hold an edge longest on long runs. Diamond-coated carbide is a lower-cost middle ground for prototypes and small batches. Solid carbide works only for very short runs or roughing where edge quality is not critical.

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  BorosilicateDefault for microfluidic channels and anodic bonding.
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  Fused silicaChoose for optical and high-temperature parts; expect faster tool wear.
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  Soda-limeCost-effective for spacers and covers, not for ion-sensitive devices.

Aspect ratio is the first limit. CNC milling handles channels and slots up to roughly 3:1 depth-to-width well. Past 5:1, tool deflection and chip evacuation both get worse, and the wall starts to chip. If your design calls for 10:1, plan for etching or laser instead.

Internal corners cannot be sharper than the tool radius. A 200 μm end mill leaves a 100 μm corner radius. Engineers often draw a sharp 90 degree corner in CAD and then wonder why the part fails inspection. Add a corner relief or accept the radius at the design stage.

Through-holes follow the same rule. A 300 μm hole in a 1 mm plate is a 3.3:1 ratio and is machinable with a diamond drill. A 100 μm hole in the same plate is 10:1 and will break drills. For those, use ultrasonic machining or laser drilling.

Surface finish is achievable but not free. Ra 0.8–1.6 μm is a normal milled result. Ra 0.2–0.8 μm needs a finishing pass with a sharp diamond tool and reduced feed. Below Ra 0.2 μm, polishing or etching is usually the better route.

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  Keep aspect ratio under 3:1Up to 5:1 is possible with careful setup; beyond that, switch processes.
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  Corner radius equals tool radiusDesign reliefs instead of fighting physics.
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  Know your finish targetRa 0.8–1.6 μm is standard; finer needs a dedicated finishing pass.

Glass does not tolerate point loads. A single hard clamp screw will start a crack that shows up three operations later. Use a soft backing plate such as POM or a wax mount, and spread the clamp force over a wide area with rubber or PTFE shims.

For thin wafers under 1 mm, vacuum chucking on a flat porous plate is the cleanest option. It holds the part flat without edge stress. Check the chuck face for flatness before every run; a 10 μm bump becomes a crack initiation site.

Wax mounting works well for parts with irregular shapes. The wax must be applied evenly and cooled slowly. Trapped air pockets create high spots that the cutter will hit first. A 30-minute controlled cooling cycle avoids most of this.

Support the cut zone, not just the part perimeter. When a channel runs across a thin web, add a sacrificial backing strip under that area. The strip absorbs the exit force and stops the chip-out on the bottom face.

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  No point loadsSpread clamp force with soft shims and a compliant backing.
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  Vacuum for thin wafersFlat porous chuck under 1 mm thickness.
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  Support the cut zoneSacrificial backing under thin webs prevents bottom chip-out.