Effect of Resist Thickness Resists usually do not have uniform thickness on the wafer Edge bead: The build-up of resist along the circumference of the wafer - There are edge bead removal systems Step coverage Centrifugal Force

Effect of Resist Thickness The resist can be underexposed where it is thicker and overexposed where it is thinner This can lead to linewidth variations Light intensity varies with depth below the surface due to absorption where  is the optical absorption coefficient Thus, the resist near the surface is exposed first We have good fortune. There is a process called bleaching in which the exposed material becomes almost transparent i.e.,  decreases after exposure to light - Therefore, more light goes to deeper layers

aexposed = B and aunexposed = A+B C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. 27(23), Dec. 1, 1988, pp. 4913-4919.

Photoresist Absorption If the photoresist becomes transparent, and if the underlying surface is reflective, reflected light from the wafer will expose the photoresist in areas we do not want it to. However, this leads to the possibility of standing waves (due to interference), with resultant waviness of the developed resist We can solve this by putting an antireflective coating on the surface before spinning the photoresist  increases process complexity

Standing Waves due to Reflections

Standing Waves Due to Reflections http://www.lithoguru.com/scientist/lithobasics.html

Removal of Standing Wave Pattern                                                                                                                         (a)                                     (b)                                (c) Diffusion during a post-exposure bake (PEB) is often used to reduce standing waves. Photoresist profile simulations as a function of the PEB diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm.   http://www.lithoguru.com/scientist/lithobasics.html

Mask Engineering There are two ways to improve the quality of the image transferred to the photoresist Optical Proximity Correction (OPC) Phase Shift Masks (PSM) We note that the lenses in projections systems are both finite and circular Most features on the mask are square We lose the high frequency components of the pattern We thus lose information about the “squareness” of the corners

Mask Engineering The effects are quite predictable We can correct them by adjusting feature dimensions and shapes in the masks

Mask Engineering

Phase Shift Masks In a projection system, the amplitudes of the diffracted light at the wafer add Closely spaced lines interact; the intensity at the wafer is smeared If we put a material of proper index of refraction on part of the mask, we can retard some of the light and change its phase by 180 degrees Properly done, the amplitudes interfere The thickness of the PS layer is n is the index of refraction of the phase shift material

Phase Shift Masks (PSM) Intensity pattern is barely sufficient to resolve the two patterns.

Scanning Projection Aligners The wafer is simultaneously scanned, thus printing the mask on the wafer Systems of the type shown on the next page are cost effective, but they must use 1:1 masks The concept is that it is easier to correct for aberrations in small regions than a large area Integration of a focusing laser and vertical positioner allowed adjustment of imaging plane to maximize resolution Technology became obsolete as wafer size increased and linewidths became smaller

Scanning Projection Printer

Step and Repeat Projection Systems Steppers expose a limited portion of the wafer at a time Features on the masks (reticules) are 4-5X the size of the features exposed on the wafer Steppers also allow better alignment because they align on the exposure field rather than for the entire wafer Integration of a focusing laser and vertical positioner Laser is also used to read information that is scribed on wafer prior to processing

Projections Systems Key features of steppers include Kohler illumination Off-axis illumination Kohler illumination focuses the light at the entrance pupil of a projection lens, rather than on the photoresist This setup allows the projection lens to capture the diffracted light from any features on the mask

Kohler Illumination

Off-Axis Illumination By changing the angle of incidence of the light on the mask, we also change the angle of the diffracted light Although some of the diffracted light is lost in this scheme, much of the higher order diffraction is captured

Off-Axis Illumination

Step and Scan Aligners https://www.chiphistory.org/product_content/lm_asml_pas5500-400_step&scan_system_1990_intro.htm

Step and Scan

DNQ/Novolac Resist Process Hexamethyldisilane (HMDS) is often used as an adhesion promoter As a liquid, drops are deposited on the wafer and then spread by spinning at 3000 – 6000 rpm for 30 s Sometime HMDS is applied from the vapor The surface chemistry is that the silane end of the molecule bonds with the Si while the other end bonds with the resist Resist is then spun on immediately following HMDS http://bmrc.berkeley.edu/courseware/ICMfg92/images/gif/spin-on.gif

DNQ/Novolac Resist Process Prebake is usually done on a hot plate at 90—100oC Infrared or microwave heating can also be used This step: Evaporates the last of the solvent Solvent content in the photoresist film decreases from 25% to 5% Adhesion is improved because heat strengthens the bonds between the resist and HMDS Stresses in the resist caused by spinning are thermally relieved The resist flows slightly

DNQ/Novolac Resist Process Exposure times and source intensity are reciprocal—one can reduce exposure times with more intense sources Exposure time is increase by increasing the bake temperature (due to decomposition of the PAC and thus decreased sensitivity) The postexposure bake is often done before development because the PAC can diffuse and this will eliminate the standing wave pattern The developer is a basic solution such as TMAH, NaOH, or KOH and is applied by immersion, or spraying

DNQ/Novolac Resist Process Rinsing in H2O stops the development process The rate for developing depends strongly on temperature, developer concentration, and the exposure and bake procedures The chemistry is the dissolution of the carbolic acid The final step is postbake (typically 10—30 min at 100—140 C) This hardens the resist and improves etch resistance The resist flows a bit during the process, and all remaining solvents are driven off Many of the steps described above are done in a single system called a wafer track system

DNQ/Novolac Resist Process A SITE system ESVG 86 track and coat system

Measurement Methods Measurements are made to determine Mask Features and defects Resist Patterns Etched Features Alignment Accuracy (x-y-theta)

Measurement of Mask Features and Defects Because almost all production now uses reducing steppers, a defect in a mask will produce a defect in every die Therefore the mask must be “perfect” Because of the complexity of the masks, the inspection must be fully automated manual observation under a microscope is not possible The process is illustrated in the next slide

Mask Inspection System

Measurement of Mask Features and Defects Light is passed through the mask and collected by an image recognition system Solid state detectors are used to collect the light The information is compared against the database of the mask design or with an identical mask The inspection process is more difficult if the mask contains OPC (optical proximity correction) or is a PSM (phase shift mask) Often, defects found in this process can be corrected Lasers can burn off excess Cr Adding Cr to clear areas is harder Done using chemical vapor deposition

SEM Measurement

State-of-the-Art Capable of exposing down to ~ 10nm E-beam lithography X-ray lithography Extreme UV lithography E-beam and EUV are performed under vacuum Throughput is very slow New resist families are required Most are very difficult to remove after use Research needed on mask material for x-ray and EUV Glass absorbs Thickness of metal needed to block x-rays is very thick (20-50mm)