Microbulk

Microbulk refers to a technology used to fabricate amplification structures for micropattern gaseous detectors. It denotes a family of ultra-thin foils produced by chemically etching a copper-clad polyimide (commonly kapton) sheet to create a microstructured amplification layer. The resulting foils are characterized by very smooth surfaces and a regular microscopic pattern that acts as the gas multiplication region when placed under an electric field in a gas mixture.

In production, a thin polyimide sheet with copper cladding is used as the starting material. Through selective chemical etching, a precise microscopic pattern—typically a regular array of channels or holes—forms in the copper and/or the polyimide to yield a self-supporting amplification element. The microbulk approach can be used to create structures that function similarly to traditional gas electron multipliers (GEMs) but with advantages in surface quality, mechanical flatness, and radiopurity.

Applications and use cases include gas detectors in particle and nuclear physics, X-ray imaging, and low-background experiments where large-area coverage and low material budgets are desirable. Microbulk foils are used to provide gain in time projection chambers and other MPGD-based readouts, sometimes in cryogenic environments.

Advantages of microbulk technology include smooth surfaces leading to uniform gain, reduced outgassing, favorable radiopurity, and compatibility with flexible, large-area readouts. Limitations involve manufacturing complexity, cost, and the handling required for ultra-thin foils. Related technologies in the field include GEMs and Micromegas, which employ different fabrication approaches within micropattern gaseous detectors.