Invar Parts for Electron Microscope Stages and Columns

By Invar Machining editorial team· Published November 4, 2026 · How we write these guides

An electron microscope forms an image with a beam a fraction of a nanometer wide, and the picture is only as steady as the structure that holds the sample and the lens. At high magnification, a nanometer is a visible shift, so a frame that grows by a few nanometers per minute as the room warms spoils the image. Low-expansion alloys help, and Invar is one of the candidates. But an electron microscope is a magnetic instrument, and Invar is a magnetic metal. This guide explains where Invar fits and where it must stay away.

Drift, the problem Invar helps with

Every part in the loop between the lens and the sample contributes to thermal drift. The loop is the chain of metal from the pole piece, through the column, the chamber and the stage, to the sample. If its length changes by even a few nanometers during an acquisition, the image blurs or slides.

The size of the effect depends on the length of the loop and its CTE.

Loop material	CTE (×10⁻⁶ per K)	Change for 100 mm loop at 0.01 K per minute	Change for 100 mm loop per 0.1 K
Aluminum	about 23	about 23 nm per minute	about 230 nm
Stainless steel	about 17	about 17 nm per minute	about 170 nm
Titanium	about 8.6	about 9 nm per minute	about 86 nm
Invar 36	about 1.3	about 1.3 nm per minute	about 13 nm

Typical values for orientation only. The real loop includes several materials and a gradient.

Stage and frame designers use three methods together: a small loop, symmetric structure and low-expansion material in the parts that set the loop length. Invar is useful in the third. The room also matters. A lab held to 0.1 K per hour makes a very different drift than a room that swings by 1 K.

The magnetic problem

An electron beam is steered by magnetic and electric fields. Electromagnetic lenses shape the beam, and any extra magnetic field near the path bends it. Parts in the column, the specimen chamber and the stage are therefore chosen to be non-magnetic or very weakly magnetic, with high-permeability shielding placed on purpose where it is needed.

Invar 36 is ferromagnetic at room temperature. Its Curie point is roughly 230 °C, and below that it carries a magnetic response, which varies with composition and cold work. Placed near the beam, an Invar part can pull the beam off axis, add astigmatism or create a field that moves with the stage. The effect depends on the part’s position and mass and on the instrument’s design, and it can be small or large. Invar magnetic properties and machining covers the behavior in more detail, including how machining and stress change it.

A sensible rule of thumb for electron microscope work:

Location	Invar suitable?	Typical alternatives
Inside the column near the beam	Generally no	Non-magnetic stainless, titanium, copper, molybdenum
Specimen chamber walls and plates far from the beam	Possible, with magnetic review	Aluminum, non-magnetic stainless
Stage base, kinematic frames and metrology loop parts	Often yes, with review	Super Invar, ceramics, titanium
External frames, damping platforms and mounts	Yes	Steel, granite, Invar
Sample holders for magnetic sample work	Usually no	Titanium, non-magnetic alloys

Always check the instrument maker’s magnetic limit for each location. Some makers state a maximum field or magnetic moment for stage parts, and a shop can measure its parts against it.

Vacuum cleanliness and baking

Electron microscopes run at high or ultra-high vacuum, and the sample region is kept clean. Parts for the chamber and the stage follow vacuum hardware practice.

• Cleaning. Machining fluids are removed with a validated cleaning process, followed by rinsing, drying and clean bagging. Some users require a vacuum bake before shipment.
• Venting. Blind holes, trapped volumes and screw threads are vented so gas can escape instead of leaking slowly into the chamber. Invar cryogenic and vacuum applications lists the usual details.
• Material choices. Zinc, cadmium and other high-vapor-pressure platings are excluded. If a coating is used, nickel is the common choice and must be clean and well bonded.
• Burrs and particles. Loose particles on a stage contaminate the sample and the beam. Deburring Invar without residual stress describes clean ways to finish edges.

Bakeout, a heating cycle at 100 to 200 °C to drive off water and gas, is used for ultra-high vacuum systems. A bake in this range is below Invar’s knee and does not harm the alloy, but it can change a part that was stabilized by a hold at 95 °C if the bake runs longer or hotter. The shop should know the bake plan before choosing the stabilization cycle, and the final inspection should follow the last bake, as Invar stress relief in machining and the stabilization notes advise.

Typical Invar parts in microscope hardware

Where Invar is accepted, it appears in parts that set position or support a structure.

• Stage base plates and metrology frames that carry the interferometer or encoder mounts
• Kinematic mounts and flexure blocks between the chamber and the stage
• Reference blocks and targets for alignment and calibration
• Mounting frames for detectors and cameras that sit outside the beam path
• Support structures for cryogenic sample stages, where thermal contraction matters and magnetic limits are relaxed

Each of these follows the standard sequence of rough, stress relieve, finish, stabilize and inspect at 20 °C. Because the instrument is a research tool, the quantity is often one to a handful, which suits the approach in Invar research lab custom instrument machining.

Hypothetical worked example

A research group is building a custom sample stage for a scanning electron microscope. The stage metrology frame is a 120 mm span between two kinematic seats. The room is controlled to 0.1 K per hour. Numbers are hypothetical.

• With an aluminum frame, a 0.1 K per hour drift gives about 0.28 µm per hour, or about 4.6 nm per minute, over 120 mm.
• With an Invar frame, the same drift gives about 0.016 µm per hour, or about 0.26 nm per minute.
• The group measures the magnetic response of a test piece of the same Invar with a gaussmeter at the planned position and finds it within the maker’s limit. They place the frame 80 mm from the beam axis.
• The frame is machined, stress relieved, stabilized and baked at 150 °C, then final inspected.
• The first scan after installation shows drift under 1 nm per minute, which meets the acquisition requirement.

If the magnetic reading had been high, the group would have moved the Invar part farther from the beam or changed the frame to a non-magnetic low-expansion material such as a ceramic.

Low-expansion options when magnetism is a problem

Because Invar and its relatives, including Super Invar and Kovar, are all ferromagnetic at room temperature, a design that cannot accept any magnetic response needs another route. The usual choices are:

Option	Strength	Limit
Glass-ceramic (Zerodur, ULE)	Near zero CTE, non-magnetic	Brittle, cannot be machined like metal, costly
Silicon carbide and other technical ceramics	Stiff, stable, non-magnetic	Brittle, long lead time, see Invar vs silicon carbide
Titanium	Non-magnetic, light, available	CTE of about 8.6, moves more
Non-magnetic stainless	Common, easy to source	CTE of about 16 to 17, moves most
Carbon fibre composite	Light, near-zero CTE possible	Moisture and vacuum effects, see Invar vs carbon fibre
Invar placed far from the beam	Keeps the low CTE	Needs a magnetic review and distance

Many designs combine them: a ceramic or titanium core near the beam and an Invar frame in the outer loop. The combined drift is then set by the part of the loop with the largest CTE, so the designer asks where in the loop the length is concentrated and puts the stable material there.

Cryogenic and variable-temperature stages

Some microscope stages cool the sample to low temperature or heat it. A cryogenic holder pulls heat out of a small region, and the surrounding structure sees a gradient. Differences in contraction between the cold holder and the warm frame cause drift and stress. Invar contracts little on cooling, which makes it useful for parts that tie a cold element to a warm frame, as long as the magnetic limit is met. Above and below room temperature the CTE of Invar changes, with a minimum near room temperature and a rise as the alloy warms toward its knee, so designers check the mean CTE over the actual range rather than relying on the room temperature value. Invar 36 CTE versus temperature shows the curve.

Demagnetizing and measuring

A part with a magnetic response near the limit can sometimes be improved. A demagnetizing cycle after machining reduces the remanent field. A magnetic measurement with a gaussmeter or magnetometer on the finished part confirms the result. Typical practice:

• Keep steel tools and magnetic chucks away from the finished part, or demagnetize afterward
• Measure remanence at a stated distance with a calibrated gaussmeter
• Record the value in the inspection report and ship with a clear label

Invar machined on a magnetic chuck can pick up a field that persists until demagnetization. A drawing for a magnetically sensitive part should say that, and name the limit.

Planning the order of operations

A part for a microscope stage has more constraints than most, and the order of operations matters. Rough machining and the first stress relief come first. Finish machining follows, with the magnetic limit in mind: avoid magnetic chucks for the last cuts, or demagnetize after them. The stabilization hold comes next, then final lapping or grinding of the interface faces. Cleaning and the bake come last, followed by inspection at 20 °C and magnetic measurement. If the bake can shift dimensions by a micrometer, the final inspection must follow it, which means the shop needs access to the bake oven or a partner who runs it. Settling this at the quote stage avoids a part that passes inspection and then changes in the customer’s chamber.

What to put in the RFQ

• The location of the part relative to the beam and the maker’s magnetic limit
• Drift budget and loop length
• Cleaning, venting and packing requirements
• Bake temperature and time, and the stabilization plan
• Demagnetization and magnetic measurement requirements
• Flatness, position and surface requirements on the interfaces

Send the drawing through the RFQ form with the magnetic limit and the bake plan stated, and we match you with shops that machine vacuum-clean Invar parts for instruments. For handbook values see the Invar reference data, the Invar 36 machining page and Invar beamline vacuum chamber machining for related vacuum work.

Frequently asked questions

Reviewed against

• Wikipedia, Electron microscope
• Wikipedia, Invar

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

• Invar is ferromagnetic: what that changes in the shop and in service · Materials
• Invar Vacuum Chamber and Beamline Component Machining · Process
• Machining Invar for Cryogenic and Vacuum Applications · Materials
• Sourcing Custom Invar Parts for Research Lab Instruments · Choosing a Supplier

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