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.
Lithography using light of a wavelength in the range of about 150nm to 300nm, with about 250nm being the most common.
see Circular Definition
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
A bake step performed on a wafer before coating with resist in order to remove water from the surface of the wafer.
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.
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.
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.
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.
The rate (change in thickness per unit time) that the resist dissolves in developer for a given set of conditions.
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.
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.
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.
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.
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.
The pattern of light entering the objective lens due to diffraction by a mask.
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.
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.
see Diffusion Coefficient
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.
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.
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.
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.
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.
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.
see Development Rate
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.
see Depth of Focus
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.
see Development Rate Monitor
see Deep Ultraviolet
see Deep-UV Lithography
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.