Glossary
A
Aberration: In geometrical optics, deviation of light from traversing paths predicted by ray tracing or by the basic mathematical analysis that is used in developing ray tracing programs.
Absorption coefficient 
: It is defined per the following equation
I(x) = I0 exp(-ax)
Where I and I0 are transmitted and incident intensities, respectively on a film or substrate, and x is the material thickness. For example the absorption coefficients of polyimide at 248 nm and 308 nm wavelength are 2.8 x 105 cm-1 and 1.2 x105 cm-1, respectively.
Absorption length: 1/(absorption coefficient = )
Ablation of material: Removal of material by breaking chemical bonds using pulsed laser with short pulse width (in nseconds), wavelength that is significantly absorbed by the material, and delivered fluence that is above a certain threshold.
Ablation threshold: Fluence level at which ablation is onset.
Aerial Image: A point source on an object emits light in spherical wavefronts. The wavefronts are refracted at lens boundaries and converge (in case of positive lens) onto a point on the other side of the lens. This point of convergence is the point image of the point source. Thus, an object is a collection of point sources, which are imaged by a lens to a collection of point images. The collection of point images is referred to as the aerial image of the object.
Airy disk: A circular aperture produces a diffraction pattern with a central disk (maximum intensity) and circular fringes (alternating intensity maxima and minima) around it. The central disk is referred to as “the Airy disk”.
Alignment and Overlay accuracy: Positional errors resulting when a second level lithographic image is superimposed on a first level pattern on a wafer or substrate. Alignment accuracy is measured only at the location of alignment marks. Overlay accuracy is measured everywhere on the wafer or substrate. It includes a number of terms beyond alignment error, in particular, lens distortion, chuck-induced wafer or substrate distortion, and image placement error.
Annealing: Using short pulse laser to melt surface thin films of semiconductor. Recrystalize and dope the thin film in order to form source and drain regions for TFT.
Aperture Stop: The aperture of the system, which limits the axial cone of energy from the object.
ASIC: Application specific integrated circuit.
Aspect ratio of a feature: The ratio of width-to-height or height-to-width in a feature formed by ablation or lithography.
Aspheric Surface: A reflecting or refracting surface, which is not spherical in shape.
Average power: (pulse energy).(repetition rate) (units in W).
B
Baking of resist (Post development bake): Is often used to remove remaining casting solvent, developer, and water from within the resist. At this step baking can be performed at high temperatures, hence, improving adhesion of the resist to the substrate.
Baking of resist (Post exposure bake or PEB): Brings about chemical and physical actions similar to those of pre-expose softbake. By baking exposed resist at temperatures on the order of 5-15oC higher than what is used during pre-expose softbake, solvent content can be reduced from 4-7 % (prior to expose) to 2-5 %. The most important benefit of the PEB step is not an extended action of pre-expose bake step but instead a significant impact on standing wave reduction via thermal flow.
Baking of resist (Pre-expose bake or softbake): Resist films are coated from a polymer solution, making solvent reduction of a coated film a primary action of softbake. Other consequences of softbake include improving adhesion, lessening vulnerability to contamination by reducing tackiness, planarization, and most importantly establishing dissolution discrimination due to reducing the presence of high level of solvent.
Binary Optic: Diffractive optic where a staircase approximation to a kinoform is used for a surface profile.
Birefringent Material: Material, which displays two different refractive indices in two different directions.
C
C4: Controlled Collapse Chip Connection.
Catadioptric System: An optical system, which simultaneously has refractive and reflective elements.
Catoptric: All-reflective optical system.
Chief Ray: The ray passing through the center of the aperture stop.
Chromatic Aberration: Aberration due to different light colors (i.e. different wavelengths).
Clear Aperture: Circular area of an optical element (typically referred to by the diameter of the area) required for complete imaging over the full field size.
Coating of resist: Liquid photoresist can be dispensed by several methods including spin coating, spray coating and dip coating. Dry film resist is applied by lamination under pressure and high temperature.
Collimated light: Light in which all rays from an object are parallel. In this case, the wavefront is plane.
Coma: An off axis aberration where the outer periphery of a lens has a higher or lower magnification than the central portion of the lens. The resulting image of a point object looks like a small comet.
Conjugate: A location, which is at an image of another location.
Cumulative energy: (energy/pulse).number of pulses (units in mJ).
Curvature of a surface: the reciprocal of the radius of curvature of the surface.
D
Depth of field: maximum change in axial position of an object producing acceptable image quality.
Depth of focus: Focus shift corresponding to plus or minus one-quarter wave error. By Rayleigh criterion, it is defined as:
DOF = k2.l/(NA)2, where k2 is a process-dependent factor which can range from 0.5 to 0.8.
Development: Development of conventional resist materials is based on image-wise dissolution discrimination. To understand and optimize photoresist development it is necessary to characterize its exposure-dependent dissolution properties.
Development methods: Development methods used for conventionally coated substrates include static immersion, continuous spray with slow rotation (~500 RPM) and stationary or slow rotation puddle development.
Dioptric: All-refractive optical system.
Diffraction: Spreading of light after a wavefront passes by an opaque edge.
Diffractive Optics: Optics in which holographic, kinoform, and binary surfaces are used to control diffraction of wavefronts.
Dipsersion: The change of refractive index with color (i.e. wavelength).
Direct writing: Laser beam is focused to one spot at the substrate plane. A pattern is then generated by moving a substrate relative to the focused beam or by moving the focused beam relative to the substrate.
Distortion: Mapping error due to change of magnification as function of field location.
Divergence: Using a knife-edge technique, angle, in each axis, within which half the laser energy is confined (for excimer typically 2 mrad x 5-6 mrad). Using pinhole technique, solid angle within which half the laser energy is enclosed.
Dosage: Cumulative energy/unit area (units in mJ/cm2)
E
ECLAT: Edge cleaning laser ablation tool.
Energy/shot or pulse energy: Energy released in one pulse (100-1000 mJ)
Entrance pupil: In one definition, the location and size of the aperture stop image when looking into the front of the system.
Excimer: Excited Dimer, e.g. KrF*, XeCl* (* indicates the molecule exists only in an excited state)
Exit pupil: In one definition, the location and size of the aperture stop image when looking into the rear of the system.
Exposure of photoresist: Involves the absorption of radiation and subsequent photochemical change, generally resulting in modification of its dissolution properties. The absorption characteristics of the photoresist largely influence its resolution and process capabilities.
Exposure system: A complex piece of machinery which projects the image of a desired photomask pattern onto the surface of a substrate (semiconductor wafer, PC board, etc.) being fabricated.
F
f-number or fl#: The ratio of focal length to the clear aperture diameter.
Field size: The longest dimension of an image produced by an imaging system before being limited by stops.
Fluence: (Energy/pulse) per unit area (mJ/cm2).
Focal length: The distance along the optical axis from the image to the plane where the backwards-extending axial imaging cone of light intersects the extended incoming light cone.
Fringes: Dark and light bands on an interferogram formed by interference of two coherent light beams.
G
g-line : A mercury arc lamp emission line with significant intensity peak centered around 435.8 nm.
Gas lifetime: Number of laser pulses after which laser energy output degrades to a level below ablation system spec typically 106-107 pulses.
Global mapping alignment: Intrafield and stepping error terms can be simultaneously derived in a constrained fit to a large number of measured alignment mark positions across the substrate.
H
h-line: A mercury arc lamp emission line with significant intensity peak centered around 404.65 nm.
I
i-line: A mercury arc lamp emission line with significant intensity peak centered around 365.48 nm
Illumination source: Typically in photolithography, it is a high-pressure mercury arc lamp, which emits a UV broadband of wavelengths. An elliptical mirror is used to collect the light and the undesired wavelengths are removed with a multi-layer dielectric filter. The remaining light is sent through a series of relay optics and uniformizing optics and is then projected through the photomask.
Inkjet nozzles: Nozzles formed by excimer laser to be used in inkjet printer heads.
ITO: Indium Tin Oxide conductive coating that is significantly transparent in the visible spectral range. It is typically patterned by photolithography and lately by newly developed excimer technique for display applications.
K
Kinoform: The surface profile of a diffractive optic, which resembles a saw tooth to manipulate diffraction of a wavefront.
L
Lab-on-a-chip: Micro-channels and reservoirs made by excimer laser in thin polymeric films for conducting accurate, low cost, fast turn-around, genetic and medical tests.
Laser: light amplification by stimulated emission of radiation.
LCD: Liquid crystal display.
Lithographic projection lens: A lens made out of multiple lens elements to minimize aberrations. It is designed to produce an optical image of a photomask pattern. A photoresist applied on a substrate surface captures the image.
M
Magnification, lateral: The ratio of the image size to the object size.
Magnification, longitudinal: The magnification of the longitudinal thickness of the object. It is equal to the square of the lateral magnification.
Mask: A plate of material that is transparent at a certain wavelength (e.g. SiO2 at UV wavelengths) coated according to a certain pattern with material that does not transmit (mostly reflects) at the same wavelength (e.g. chrome, aluminum, dielectric, etc.).
Mask plane: Coated mask side, which coincides with the object plane of an imaging system.
Mask pattern projection: Imaging of a mask pattern using a projection (lithographic) lens.
MEMS: Micro Electro Mechanical System.
1 Microlithography: The science and art of building a mask pattern on top of a substrate (e.g. quartz glass) and then transferring this pattern into another substrate. This process is repeated many times to form an integrated circuit, MEMS, etc.
2 Microlithography: Is a manufacturing process for producing highly accurate, microscopic, two-dimensional patterns in a photosensitive resist material. These patterns are replicas of a master pattern on a durable photomask, typically made out of a thin patterned layer of chromium on a transparent glass plate.
Modulation transfer function (MTF): The ratio of the modulation in the image to the modulation in the object as a function of spatial frequency.
N
Numerical Aperture (NA): It provides a measure of the light gathering power of the system (acceptance angle). Relates to the speed of the system. In one definition, NA = 1/ (2.fl#)
O
Objective (projection) lens: The primary lens, which refracts light from an object (mask pattern) and forms an image. Generally, it consists of multiple elements in order to minimize aberrations.
Off-axis alignment: Alignment by use of external microscope objectives to detect alignment marks on a substrate surface.
Optical path difference (OPD): Traversing an optical system, the difference between a reference or perfect wavefront and a real wavefornt. If the OPD is one-quarter of the wavelength, then the system just meets the Rayleigh criteria and is essentially diffraction limited.
P
Paraxial: The region where the angles between rays and the optical axis are very small. Hence, the approximation that the sines and tangents of the angles are equal to their values in radians is valid.
Peak power: (pulse energy)/pulse width (units in W).
Photolytic (photochemical): Laser light absorbed by a substrate breaks chemical bonds directly within the substrate surface.
Photoresist: A layer of material that will react when exposed to actinic energy, UV light (or x-ray, e-beam etc.). The resist layer exposed to UV light, in certain portions, undergoes a change in its solubility. It is then developed by washing it with a basic developer solution, thereby removing the non-irradiated (in a negative-tone resist) or irradiated (in a positive tone resist) portions.
Photolithography: A type of lithographic process used in the manufacturing of semiconductor devices, integrated circuits, and photomasks. It comprises applying a layer of material known as photoresist or simply resist, exposing the resist to UV light, developing the exposed resist (see photoresist for detail)
Proximity and contact printing systems: Exposure systems in which a chrome-on-glass mask, or silver halide-on-mylar artwork is held in close proximity or in actual contact with a photoresist covered substrate. The resist is exposed through the backside of the mask or artwork by a flood exposure source. The mask pattern covers the entire substrate and is necessarily designed with a magnification of 1x.
Polymer: Small molecular unit (monomer), containing 6-40 atoms is repeated over and over to form a polymer; polymer molecule may consist of 100-1000 monomer units; polymer molecules consist of 103-105 atoms, mainly carbon, hydrogen, oxygen, and nitrogen.
Power density: fluence/pulse width ( units in W/cm2).
Projection Lens: Consists of many lens elements. It projects an object from the mask plane as an image in the substrate plane. Lens can be 1x, 2x-reduction, 2.5x-reduction, 5x-reduction, etc.
Pulse Duration: Time during which light is on during a pulse (for excimers it is typically 10-40 nsec FWHM (full width at half maximum).
Pulsed Laser: Lasers can be continuos wave (cw) or pulsed. Pulsed lasers with very short pulse duration (on the order of nsec) are critical for achieving ablation since the laser energy has to be delivered in a very short amount of time. Hence, the laser energy will be confined to a very thin layer of material inducing intense photochemical or photothermal reaction, which results in ablation.
Pyrolytic (photothermal): Laser beam incident on an absorbing substrate induces a local temperature rise and thereby activates a chemical reaction.
R
Rayleigh criterion: The rule developed by lord Rayleigh that if the difference between the longest and the shortest paths leading to a selected focus is less than or equal to one-quarter of the wavelength, then the imaging quality is nearly indistinguishable from perfect.
Reduction Steppers: Allow high NA designs, hence improved resolution over 1x systems. A projection lens is used with a field size just large enough to expose one or two chips or integrated circuits. The fields are exposed sequentially, with the substrate being repositioned by an accurate x-y stage between exposures.
Refractive Index of a certain material: The ratio of the velocity of the radiation (e.g. light) in vacuum to the velocity of radiation (e.g. light) in the material.
Repetition rate (rep. Rate): Number of occurrences of a pulse within a second (i.e. rep.rate unit = Hz)
Resolution or resolving power: In one definition, the ability of an optical system to separate two closely spaced point sources at the nominal object distance. By Rayleigh criterion, resolution R is defined as:
R= k1.l/NA,
where k1 is a process factor depending on the coherence of the source. K1 is 0.25 and 0.5 for incoherent and coherent illumination, respectively.
S
Sag: The height of an optic curved surface from the cord.
1x scanners: They project the image of a mask through a lens onto the resist-coated surface of a substrate. The mask is typically a 1x chrome-on-glass mask that is large enough to cover the entire substrate. Substrates are printed simultaneously by scanning both the mask and the substrate, both held on a x-y stage, as one unit relative to the light beam.
Site-by-site Alignment: In this strategy, alignment marks at each exposure site are measured and corrections specific to that site are calculated. The site is exposed after the alignment measurement is done, and the process is repeated at the next exposure site.
Solder bump process: A method based on photolithography for placing solder balls to provide electrical interconnects in a microelectronic package.
Spherical aberration: The axial aberration where rays from the outer periphery of the lens focus closer (or further) from the lens than the rays closer to the axis.
Step-and-scan systems: A 1x or reduction lens is used to scan the image of a large exposure field onto a portion of the substrate. The substrate is then moved to a new position where the scanning process is repeated. The lens field is required only to be a small area or narrow slit as in the full-wafer scanners. This allows a scanned exposure that is limited only by the size of the mask and the travel of the mask positioning stage.
Strehl ratio: The ratio of the peak intensity in the diffraction pattern of a point image with aberration to the peak intensity in the diffraction pattern of the same point image, but without aberration.
Sub-micron vias: Via holes formed in thin polyimide film with minimum diameter of 0.5 microns used for medical and pharmaceutical applications.
Substrate: Material sample to be ablated (e.g. film, wafer, printed circuit (PC) board, etc.)
Substrate plane: Typically the top surface of the substrate at which the laser beam is focused. It is also the image plane of an imaging system.
T
TFT: Thin film transistor.
Through the lens alignment (TTL): Capturing the image of the alignment marks on a substrate surface directly through the lithographic lens.
U
UBM: Under bump metallurgy.
UV laser ablation mechanism: Depends on absorptivity of substrate material, pulse length, wavelength. Photochemical mechanisms are dominant at the short UV wavelength (e.g. 157 nm, 193 nm) and photothermal mechanisms become active towards the long UV wavelength (e.g. 308 nm,351 nm).
V
Via drilling: Use of excimer laser to form through or blind holes in substrate layers.
Via wall angle: The angle a via wall makes with a direction normal to the substrate (usually along the via axis).
