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Glossary of Lithography Terms - A


ABC Parameters see Dill Parameters


Aberrations, Lens Any deviation of the real performance of an optical system (lens) from its ideal (Fourier optics) performance. Examples of lens aberrations include coma, spherical aberration, field curvature, astigmatism, distortion, and chromatic aberration. One way to describe lens aberrations is through a Zernike polynomial fit to the wavefront error at the exit pupil of the lens for each field point.

Example: The aberrations of the objective lens caused a noticeable degradation in image quality.


Absorption Coefficient The fractional decrease in the intensity of light traveling through a material per unit distance traveled. See also Extinction Coefficient.

Example: The absorption coefficient of the ArF resist was so high that only a thin film of the resist could be used.


Acid-Catalyzed Resist A type of chemically amplified resist where an acid is the product of exposure and this acid serves as the catalyst for a thermal reaction which changes the solubility of the resist. See also Chemically Amplified Resist.

Example: Acid-catalyzed resists are the most common type of resist for deep-UV lithography.


Activation Energy Defined by its role in the Arrhenius equation, the activation energy determines the temperature dependence of chemical reaction rate constants, diffusivities, and other temperature-dependent rate terms. High activation energies produce a large temperature dependence.

Example: This low activation energy resist will begin its acid-catalyzed deblocking reaction at room temperature.


Actinic Wavelength The wavelength used to expose the photoresist in a lithographic system.

Example: Measurement of the refractive index of the substrate at the actinic wavelength is necessary in order to design an optimal BARC.


Additive Patterning A process by which material is added in the places where the pattern is to be formed. Examples include lift-off and electroplating processes.

Example: Due to our inability to plasma etch copper, an additive patterning approach was chosen instead.


Adhesion Promoter A chemical that is applied to the surface of a wafer in order to improve the adhesion of resist to the wafer, often by eliminating water from the wafer surface.

Example: The HMDS, used as an adhesion promoter, was applied using the vapor prime unit.


Advanced Process Control (APC) The use of automated feedback and feed-forward loops to control a lithographic process.

Example: Both rework rates and end-of-line yield were improved after turning on the overlay APC system.


Aerial Image An image of a mask pattern that is projected onto the photoresist-coated wafer by an optical system.

Example: The aerial image of the isolated line was found to differ significantly from that of the dense line.


Aligner see Mask Aligner


Alignment The act of positioning the image of a specific point on a photomask (the alignment key) to a specific point on the wafer (the alignment target) to be printed. Alignment accuracy is the overlay measured at this alignment target.

Example: The alignment system of the stepper used an advanced image recognition algorithm.


Alignment Key The pattern on a photomask used to perform alignment.

Example: This mask had several alignment keys for use on several different steppers.


Alignment Mark see Alignment Key or Alignment Target


Alignment Target The pattern on a wafer used to perform alignment.

Example: A mesa structure is used as the alignment target for the gate level of this device.


Alternating PSM A type of phase-shifting mask where the clear region to one side of a small chrome line is shifted in phase by 180º compared to the clear region to the other side of that same line. Also called alternating aperture PSM or Levenson PSM.

Example: Although alternating PSM promises extreme resolution and good depth of focus, phase conflicts limit their use for general circuit patterns.


Annular Illumination A type of off-axis illumination where a doughnut-shaped (annular) ring of light is used as the source.

Example: The use of annular illumination was found to give a noticeable improvement in depth of focus for these features.


Antireflective Coating (ARC) A coating that is placed on top or below the layer of resist to reduce the reflection of light, and hence, reduce the detrimental effects of standing waves or thin-film interference. See also Top Antireflective Coating and Bottom Antireflective Coating.

Example: By optimizing the thickness of the antireflective coating, the swing curve amplitude was reduced to almost zero.


APC see Advanced Process Control


Aperture, Numerical see Numerical Aperture


Aperture Stop see Pupil, Lens


ARC see Antireflective Coating


ArF Argon Fluoride, a type of excimer laser used in optical lithography that emits light at about 193nm.

Example: Due to the difficulty in producing calcium fluoride lens components, ArF exposure tools are considerably more costly to manufacture.


Arrhenius Coefficient Defined by its role in the Arrhenius equation, the Arrhenius coefficient is the pre-exponential term in the equation that defines the temperature dependence of chemical reaction rate constants, diffusivities, and other temperature-dependent rate terms.

Example: The Arrhenius coefficient is often thought of as the extrapolation of the temperature-dependent rate constant to an infinitely high temperature.


Arrhenius Equation The temperature dependence of chemical reaction rate constants, diffusivities, and other temperature-dependent rate terms as an exponential relationship with the inverse of absolute temperature.

Example: The Arrhenius equation is used to determine how reaction rates and diffusion change with PEB temperature.


Aspect Ratio The ratio of a resist feature’s height to its width.

Example: The resist images suffered from pattern collapse whenever the aspect ratio exceeded about 3:1.


Astigmatism An aberration that results in a shift in best focus for radially oriented line patterns compared to tangentially oriented patterns.

Example: Typically, a variation of H - V bias with focus is a sign of astigmatism.


Attenuated PSM A type of phase-shifting mask where the nominally dark region of the mask is allowed to transmit a fraction of the light (e.g., 6%) with a 180º phase shift from light transmitted through the clear regions of the mask.

Example: Although alternating PSM provides better performance, attenuated PSMs have become very popular due to their ease of design and manufacture.


Autofocus System A part of a projection imaging tool that automatically places the top surface of the wafer a set distance from the focal plane.

Example: Despite the sophistication of the scanner’s autofocus system, the lithographer must still determine best focus manually by shooting a focus-exposure matrix.