Lithography Glossary

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

The following is a list of common words or phrases and their corresponding definitions as used in the field of semiconductor microlithography. The definitions assume the reader has a general technical background and a basic familiarity with the semiconductor industry.

The Lithography Glossary will serve as a timeless reference for you and your colleagues. The PROLITH team of lithography experts will update the document on a regular basis and welcomes your input. Please send suggestions to FINLE.MarCom@kla-tencor.com

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 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 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.

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 180?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
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

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 180?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.