Paul van Gerven
19 January 2017

ASML’s exposure devices are the most expensive and complex link in the chip fabrication process. Here, we take a simplified look at both the entire process and the Veldhoven company’s specific expertise. Plus: a list of must-know terms to understand ASML’s work.

EUV beginners
Chip fabrication is an iterative process of material deposition, etching, lithography and other steps.

193 nm: Wavelength in the far UV range. Currently in standard use for the production of the most advanced chips. See also: DUV, excimer laser and immersion.

– Alignment: Lining up the wafer and the lens in order to make the overlay (see the definition) as small as possible. ASML made a name for itself in the 1980s with an alignment system based on a reference mark, a symbol still visible in the company’s logo.

Brion: American specialist in optical modelling, acquired by ASML in 2007.

CD: Critical dimension, the smallest details in an image. One of the key parameters used in describing a lithographic image and its quality.

Computational lithography: Umbrella term for computer-assisted techniques to improve lithographic performance.

Double patterning: Also known as DP/MP. A lithographic technique in which a chip layer is built up in two steps because the resolution of the scanner is not sufficient to produce the layer in a single exposure. Economically not the most attractive option for chip makers, but made unavoidable by delays in the development of EUV lithography.

DUV: Deep ultraviolet, a wavelength range in the far ultraviolet. Chip production uses 248 and 193 nanometres.

EUV: Extreme ultraviolet, the wavelength range between roughly 100 and 10 nanometres. In chip manufacture, used as an abbreviation for EUV lithography (also abbreviated EUVL), that is, lithography with light at a wavelength of 13.5 nanometres. EUV is expected to be used in production before 2020.

Excimer laser: Light source for DUV scanners.

Half-pitch: Half the distance (in nm) between two identical structures on a chip. A measurement of the size of the chip structures imaged onto a chip. See also: node.

Holistic lithography: A term coined by ASML for an approach in which the design of the chip, the mask, the lithography and the metrology are coordinated to achieve the optimal chip manufacturing process.

Immersion: Lithography ‘under water’, that is, the introduction of water between the lens and the wafer. Because water has a higher refractive index than air, the resolution is increased. Currently in standard use for the production of the most advanced chips, in combination with 193-nm light.

LER: Line edge roughness, the jagged structure of the imaged chip structures. Too rough is not good.

Mask: Also known as photomask. Plate with translucent areas that define the chip pattern. A reticle is a mask that effects a scale reduction.

Metrology: Umbrella term for various measurement techniques to monitor the lithographic process. The results can be used to make adjustments without having to take the machine offline.

Moore’s Law: Observation that the number of transistors per integrated circuit on the market doubles about every two years (you can Google more precise formulations). Chips with smaller structures are faster and, because more of them can be placed on a wafer, more economical. The latest generations of chips took longer than two years to develop, though the arrival of EUV may undo this delay.

NA: Numeric aperture, or lens opening. In part a measure of the quality of the lens. The higher the NA, the more light that contributes to the image, and the sharper the image.

Node: Designation for a chip’s generation, expressed as a round number. Used to be related to the half-pitch (see above), but currently a marketing term that every chip maker defines differently.

OPC: Optical proximity correction, a computational lithographic correction technique.

Overlay: Extent to which successive layers in a chip are displaced relative to one another. Currently on the order of a few nanometres. If the layers don’t fit together properly, the chip won’t work. For obvious reasons, very important in double patterning.

Pellicle: Membrane that protects the mask. Any nanoscopic debris that lands on this film is far enough away that it won’t be imaged.

Photo lacquer: Also known as photoresist (or just resist). Photosensitive material onto which the mask pattern is projected.

Resist: See photo lacquer.

Reticle: A mask that effects a scale reduction.

Scanner: Latest generation of exposure equipment, in which the wafer moves under the lens. In the previous generation of equipment, the steppers, the wafers were held in place and the lens column exposed areas of the wafer one step at a time.

Twinscan: A principle introduced by ASML in which two wafers are constantly being acted on by the scanner: while one is being exposed, the other is being measured.

Uptime: How long an exposure device can operate continuously without requiring maintenance or breaking down. Very important for ASML’s customers.

Yieldstar: Standalone metrology tool made by ASML. Measures overlay and focus. Is part of ASML’s holistic lithography suite.

Wafer: Round disk of very pure crystalline silicon on which chips are made.