Invar and Super Invar parts for wafer stages and reference frames

By Invar Machining editorial team· Published September 26, 2026 · How we write these guides

Invar 36 and Super Invar appear in lithography and wafer handling tools wherever a structure has to hold position to well under a micron while the tool runs: stage bodies and their mirror blocks, metrology frames that carry the interferometers and encoders, reticle stage and reticle handling parts and the substructures under wafer chucks. The alloy is chosen because a cleanroom that holds 22 ± 0.1 °C is still not constant, and at 1.2 to 1.6 ppm/°C Invar turns that small swing into tens of nanometres instead of the microns that aluminum or steel would give. This guide covers the stage and frame parts. For the broader picture across the tool, including gauge fixtures and traceability, see machining Invar for semiconductor equipment.

Where the alloy goes in the tool

Not every part in a lithography tool is Invar. Most of the structure is granite, ceramic, aluminum or steel, chosen for stiffness, damping or cost. Invar and Super Invar take the places where a metal part sits inside the metrology loop, the chain of components between the measuring system and the point being measured.

Part	Alloy, typical	Why	Critical feature, typical
Stage mirror block or mirror mount	Super Invar or Invar 36	Carries interferometer mirrors; any growth reads as stage motion	Mirror seat flatness 1 to 3 µm, position ±0.005 mm
Metrology frame or reference frame	Invar 36, Super Invar for the tightest tools	Holds encoder heads and sensors relative to each other	Sensor seat positions ±0.005 to ±0.010 mm over 300 to 600 mm
Reticle stage chuck substructure	Invar 36	Reticle position sets overlay	Flatness 2 to 5 µm, position ±0.005 mm
Reticle handling end effector and library fixtures	Invar 36	Repeatable pick up across temperature	Position ±0.010 mm, flatness 5 µm
Wafer chuck substructure or carrier	Invar 36 or ceramic	Matches or isolates chuck expansion	Flatness 2 to 5 µm
Sensor and encoder brackets	Invar 36	Keeps sensor gap constant	Gap ±0.010 mm

Typical placements and tolerances. The tool maker’s drawing governs, and each program allocates its budget differently. Both alloys are covered on the Invar 36 machining and Super Invar machining pages.

Why sub-micron drift matters

Overlay in a modern lithography tool, the alignment of one layer to the layer below, is budgeted in single digit nanometres. Every source of error takes a slice of that budget, and thermal drift of the metrology loop is one of them. The arithmetic is short. Growth equals length times CTE times temperature change.

Material	CTE, typical, ppm/°C	Growth of a 300 mm reference length for 0.1 °C
6061 aluminum	23 to 24	0.70 µm
Carbon steel	11 to 12	0.35 µm
316 stainless	16 to 17	0.50 µm
Invar 36, annealed	1.2 to 1.6	0.04 µm
Super Invar	0.3 to 0.6	0.014 µm
Zerodur	about 0 ± 0.1	0.003 µm or less

Typical room temperature values from handbook data, computed at the midpoint of each range. The mill certificate governs for a specific heat lot.

A 0.1 °C swing is realistic across a tool during operation, because motors, lasers and electronics dump heat and the water cooling loop has its own control band. At 0.7 µm per 0.1 °C, an aluminum frame would consume the whole overlay budget many times over. Invar 36 brings that to 40 nm. Super Invar brings it to 14 nm. Active thermal compensation and temperature sensing take care of part of what remains, but only if the structure moves in a predictable, repeatable way, which is where the machining sequence comes in. A frame with residual stress moves on its own, unrelated to temperature, and no sensor can compensate for that. The long term dimensional stability guide covers that failure mode.

Invar 36 or Super Invar

Super Invar earns its place in the parts closest to the measurement: mirror blocks and the frame sections between the encoder heads. Its low expansion holds only over a narrow band, roughly 20 to 90 °C at the low value and rising faster than Invar 36 above that, and it costs more per kilogram because of the cobalt. In a cleanroom at 22 °C the narrow band is not a limitation. The extra cost and the smaller number of mills that produce it are. Many tool makers use Invar 36 for the larger frames and brackets and reserve Super Invar for a handful of parts. The guide Super Invar: when Invar 36 is not enough sets out the decision.

One caution on both alloys. The handbook CTE assumes an annealed, stress free part. Machining cold work raises the local expansion, and a part finished without a stress relief can sit above 2 ppm/°C at the surface. The sequence of rough, stress relieve, finish and stabilize is what brings the finished part back to the certificate value, and on a lithography frame the stabilize step is not optional. The Invar 36 CTE versus temperature guide shows how far the curve moves with condition.

Cleanliness and outgassing

Stage and frame parts sit in a controlled environment and, in EUV tools, in vacuum. Invar 36 itself is a clean material: it is a solid metal with no volatile components and it outgasses as little as stainless steel once the surface is clean. The contamination risk comes from the process, not the alloy.

• Cutting fluids. Sulfur and chlorine bearing cutting oils leave residues that are hard to remove and that outgas. Shops serving semiconductor customers use fluids chosen for cleanability and say so on request.
• Cleaning. Multi stage aqueous cleaning, ultrasonic rinse and a final rinse in deionised water are typical. Some tool makers specify their own cleaning procedure and a residual particle or hydrocarbon check.
• Surface condition. Bare Invar 36 will rust slowly in humid air. Parts are often left bare and kept dry, or given a thin electroless nickel coating. The coating adds a layer with its own expansion and has to be specified thin and uniform. The surface finish and coating guide covers the trade.
• Packaging. Cleaned parts are bagged in cleanroom compatible film, often double bagged, with desiccant and without paper or cardboard touching the part.
• Blind holes and trapped volumes. Vacuum drawings vent blind tapped holes or use vented screws. Say which on the drawing.

The cryogenic and vacuum applications guide goes further on vacuum practice.

Magnetic considerations near linear motors

Invar 36 is ferromagnetic at room temperature, with a Curie temperature around 230 °C. Super Invar is similar. Lithography stages are driven by linear motors and planar motors with strong permanent magnet arrays, and a ferromagnetic part near an array feels a force, distorts the field the motor was designed for and adds cogging. Three approaches are common:

1. Keep the distance. Design the Invar structure so that it sits outside the motor’s magnetic envelope. Often the frame is Invar and the moving coil carrier is not.
2. Shield. A soft magnetic shield between the array and the Invar part carries the stray field. The shield itself is a designed part.
3. Change material locally. Where a part must sit in the field, a non-magnetic material such as titanium or a ceramic takes its place, at a CTE penalty that the error budget has to absorb.

There is also the shop side. A part ground on a magnetic chuck carries residual magnetism and must be demagnetized before shipping. Chips cling to magnetic parts and to fixtures. The Invar magnetic properties guide covers both, and the RFQ should state a residual field limit when it matters.

Tolerances and inspection

The numbers on lithography stage parts are at the tight end of what machining reaches. Typical drawings show position tolerances of ±0.005 to ±0.010 mm on sensor seats and mirror seats, flatness of 1 to 5 µm on mounting faces and parallelism of a few microns across 300 mm. Reaching them takes the full sequence, then finish grinding or lapping on the reference faces, then a soak and a CMM measurement at 20 °C.

Inspection follows ISO 1 at 20 °C, with soak times of 12 to 24 h for a frame. The CMM must have a calibrated uncertainty small next to the tolerance, which usually means a temperature controlled metrology room and a machine with a stated uncertainty of about 1 µm or better over the part. Many tool makers add a thermal cycling acceptance test between two temperatures, measured before and after, to prove that the part has stopped moving, which the thermal cycling test guide explains. Reports include the measured points, the temperature at measurement and the material certificate with heat lot.

Worked example, hypothetical

Take a hypothetical metrology frame for a wafer stage: Invar 36, 400 × 300 × 40 mm, machined from 45 mm plate, with six encoder head seats at position ±0.008 mm and flat to 3 µm each, plus two reference faces parallel to 5 µm across the length. The tool runs at 22 ± 0.1 °C, and the tool maker allocates 30 nm of the overlay budget to thermal drift of this frame.

Check the alloy first. Over 400 mm at 0.1 °C, Invar 36 at 1.4 ppm/°C grows 56 nm, over the 30 nm allocation. Super Invar at 0.45 ppm/°C grows 18 nm. So this frame, in this budget, is Super Invar, or the frame is redesigned so that the encoder heads span 200 mm and Invar 36 gives 28 nm. That second option is cheaper and is the kind of conversation worth having before the RFQ goes out.

Suppose the design keeps Super Invar. The sequence would be: rough all faces and seats with 1 mm stock, stress relieve, semi finish to 0.2 mm, a full anneal and stabilize cycle to the tool maker’s specification, finish mill, grind the reference faces and encoder seats, clean, then a 24 h soak and CMM inspection at 20 °C with a five cycle 15 to 30 °C thermal test and re-measurement. A hypothetical quote structure would show material as a large line because of the cobalt and the plate size, machining time in the tens of hours, two furnace cycles, grinding and about a day of inspection. On a batch of two frames the inspection and heat treatment lines rival the machining line.

What to include on the RFQ

State the alloy and spec, Invar 36 to ASTM F1684 or Super Invar, plus the condition. Give the heat treatment sequence or write “shop to recommend” and say whether the tool maker’s own stabilize cycle applies. Mark the metrology loop features, the ones whose position sets overlay, and loosen everything else. Give the operating temperature and its control band, the cleanliness or outgassing specification, any residual magnetism limit and the packaging requirement. List the reports: CMM data at 20 °C, thermal cycling results if required and the material certificate. When you send the RFQ with the drawing and a 3D model, we match you with up to three shops that machine these alloys to this level and can run the sequence, the cleaning and the inspection. Spec references are on the Invar specifications page.

Frequently asked questions

Reviewed against

• Wikipedia, Invar
• ASTM F1684, Standard Specification for Iron-Nickel and Iron-Nickel-Cobalt Alloys for Low Thermal Expansion Applications

Values in this guide are typical handbook figures for planning. The mill certificate for your heat lot and the shop's quote for your geometry govern.

Keep reading

• Machining Super Invar and Invar 36 for Semiconductor Equipment · Materials
• Super Invar: The Lower CTE Option, Its Limits and How Shops Machine It · Materials
• Specifying a dimensional stability test for Invar parts · Choosing a Supplier
• Invar is ferromagnetic: what that changes in the shop and in service · Materials

All materials guides