Lithography Glossary

Written by Chris Mack
Brought to you by the creators of PROLITH

Deep Ultraviolet (DUV)
A common though vague term used to describe light of a wavelength in the range of about 150nm to 300nm. Also called deep-UV.

Example: The transition of optical lithographic wavelengths from i-line to deep ultraviolet accelerated as the industry dipped below the 350nm resolution node.

Deep-UV Lithography
Lithography using light of a wavelength in the range of about 150nm to 300nm, with about 250nm being the most common.

Example: Most lithographers agree that deep-UV lithography is required for device dimensions below 0.3 microns.

Definition, Circular
see Circular Definition

Defocus
The distance, measured along the optical axis (i.e., perpendicular to the plane of best focus) between the position of a resist-coated wafer and the position if the wafer were at best focus.

Example: The amount of defocus cannot be determined without an adequate method of measuring best focus.

Degree of Coherence
see Partial Coherence

Dehydration Bake
A bake step performed on a wafer before coating with resist in order to remove water from the surface of the wafer.

Example: The dehydration bake was only partially effective in removing water from the wafer surface.

Depth of Focus (DOF)
The total range of focus that can be tolerated, that is, the range of focus that keeps the resulting printed feature within a variety of specifications (such as linewidth, sidewall angle, resist loss, and exposure latitude).

Example: Optimizing the numerical aperture by finding the value that maximized the DOF of the critical feature was found to be very effective at improving CD control.

Design Rule
A geometrical rule that defines minimum widths and/or spacings used when laying out a mask pattern.

Example: Although the designer was not sure why the design rule forbade the use of this particular pitch, he reluctantly complied.

Developer
The chemical (typically a liquid) used to selectively dissolve resist as a function its chemical composition.

Example: Control of the temperature of the developer should be better than 0.2 C.

Development
The process by which a liquid, called the developer, selectively dissolves a resist as a function of the exposure energy that the resist has received. Also called develop.

Example: A puddle development process was used to reduce developer consumption.

Development Rate
The rate (change in thickness per unit time) that the resist dissolves in developer for a given set of conditions.

Example: The development rate was plotted as a function of exposure energy on a log-log scale.

Development Rate Monitor (DRM)
An instrument used to measure the development rate of a resist by measuring the thickness of the resist in situ as the development proceeds.

Example: The development rate as a function of exposure energy was characterized using a development rate monitor.

Die
A single, complete integrated circuit as printed on a wafer, possibly sliced but before packaging. Also called a chip.

Example: The stepper could accommodate only one die in each exposure field.

Diffraction
The propagation of light in the presence of boundaries. It is the property of light that causes the wavefront to bend as it passes an edge.

Example: In an ideal imaging system, the quality of the aerial image is limited only by diffraction.

Diffraction Limited
A description of a lens such that any aberrations in the lens are small enough as to be negligible. Theoretically, no lens can be perfect so that the term diffraction limited is always an approximation and the appropriateness of its use is situational.

Example: In photographic systems and other imaging applications less stringent than lithography, lens are often described as diffraction limited when the RMS optical path deviation is less than a tenth of a wave.

Diffraction Order
For a mask pattern that repeats indefinitely, the diffraction pattern becomes discrete, made up of regularly spaced points of light called diffraction orders.

Example: In lithography, high resolution line/space patterns are imaged with only the zero and plus and minus first diffraction orders passing through the lens.

Diffraction Pattern
The pattern of light entering the objective lens due to diffraction by a mask.

Example: The diffraction pattern of a repeating pattern of lines and spaces is made up of discrete spots of light called diffraction orders.

Diffusion Coefficient
A rate constant that defines the rate at which a particle will diffuse through a given medium for a given set of process conditions.

Example: The diffusion coefficient of the acid in the chemically amplified resist was not constant during the post-exposure bake due to free volume generated by the amplification reaction.

Diffusion Length
The average distance that a particle will diffuse for a given process.

Example: The diffusion length of photoactive compound during PEB must be larger than the standing wave half period to be effective at removing standing waves from the resulting resist profile.

Diffusivity
see Diffusion Coefficient

Dill Parameters
Three parameters, named A, B, and C, that are used in the Dill exposure model for photoresists. A and B represent the bleachable and non-bleachable absorption coefficients of the resist, respectively, and C represents the first-order kinetic rate constant of the exposure reaction. (Named for Frederick Dill, the first to publish this model.) Also called the photoresist ABC parameters.

Example: The Dill parameters (A, B, and C) were measured in a single optical transmittance experiment.

Dioptric
An optical system made up of only refractive elements (lenses).

Example: Dioptric lens systems require extensive effort to correct for the chromatic aberrations that are a natural part of all-refractive lenses.

Dipole Illumination
A type of off-axis illumination where two circles of light are used as the source. These two circles are spaced evenly around the optical axis, either oriented vertically or horizontally.

Example: Dipole illumination provides the greatest possible dense line resolution, but only for one orientation of lines and spaces.

Direct Write Lithography
A lithography method whereby the pattern is written directly on the wafer without the use of a mask.

Example: Due to throughput limitations, direct write lithography may never be practical for IC mass production.

Dispersion
The variation of the index of refraction of a material as a function of wavelength.

Example: Because of the dispersion of glass, lenses invariably suffer from chromatic aberration.

Dissolution Inhibitor
A chemical which, when added to a photoresist, decreases the dissolution rate of the resist in developer. For many positive photoresists, the photoactive compound acts as a dissolution inhibitor.

Example: If the dissolution inhibitor is bound directly to the novolak resin, diffusion during PEB does not occur.

Dissolution Promoter
A chemical which, when added to a photoresist, increases the dissolution rate of the resist in developer. For many positive photoresists, the exposed photoactive compound acts as a dissolution promoter.

Example: When exposed to light, the DNQ dissolution inhibitor becomes a mild dissolution promoter.

Dissolution Rate
see Development Rate

Distortion
An optical aberration that causes a variation in pattern placement error as a function of field position.

Example: The variation of distortion from one stepper to another results in the need for lens matching when printing critical layers, or possibly even the use of a dedicated stepper.

DOF
see Depth of Focus

Dose
see Exposure Energy

Dose to Clear (Eo)
The amount of exposure energy required to just clear the resist in a large clear area for a given process. Also called the clearing dose.

Example: The dose to clear was measured once per shift and used as a process monitor.

Dose to Size
The amount of exposure energy required to produce the proper dimension of the resist feature.

Example: Changing the thickness of the photoresist resulted in a large change in the dose to size of the contact hole.

DRM
see Development Rate Monitor

DUV
see Deep Ultraviolet

DUV Lithography
see Deep-UV Lithography

Dyed Resist
A photoresist with an added non-photosensitive chemical that absorbs light at the exposing wavelength.

Example: Although the dyed resist was effective at reducing the swing curve, the resulting sidewall angle was unacceptably low.