Previously, photomasks used to be produced manually by using rubylith and BoPET. As complexity continued to grow, manual processing of any sort became difficult. This was solved with the introduction of the optical pattern generator which automated the process of producing the initial large-scale pattern, and the step-and-repeat cameras that automated the copying of the pattern into a multiple-IC mask. The intermediate masks are known as reticles, and were initially copied to production masks using the same photographic process. The initial stages produced by the generators have since been replaced by electron beam lithography and laser-driven systems. In these systems there may be no reticle, the masks can be generated directly from the original computerized design.
Mask materials have also changed over time. Initially, the rubylith was directly used as the mask. As feature size shrank the only way to properly focus the image was to place it in direct contact with the wafer. These contact aligners often lifted some of the photoresist off the wafer and the mask had to be discarded. This helped drive the adoption of reticles, which were used to produce thousands of masks. As the power of the lamps exposing the masks increased, film became subject to distortion due to heat, and was replaced by silver halide on soda glass. This same process led to the use of borosilicate and then quartz to control expansion, and from silver halide to chromium which has better opacity to the ultraviolet light used in the lithography process.
A Photomask set, each defining a pattern layer in integrated circuit fabrication, is fed into a photolithography stepper or scanner, and individually selected for exposure. In multi-patterning techniques, a photomask would correspond to a subset of the layer pattern.
In photolithography for the mass production of integrated circuit devices, the more correct term is usually photoreticle or simply reticle. In the case of a photomask, there is a one-to-one correspondence between the mask pattern and the wafer pattern. This was the standard for the 1:1 mask aligners that were succeeded by and scanners with reduction optics.
The pattern is projected and shrunk by four or five times onto the wafer surface. Lithography experts back higher magnification in photomasks to ease challenges // EETimes 2000 To achieve complete wafer coverage, the wafer is repeatedly "Stepper" from position to position under the optical column until full exposure is achieved.
Features 150 nm or below in size generally require phase-shift mask to enhance the image quality to acceptable values. This can be achieved in many ways. The two most common methods are to use an attenuated phase-shifting background film on the mask to increase the contrast of small intensity peaks, or to etch the exposed quartz so that the edge between the etched and unetched areas can be used to image nearly zero intensity. In the second case, unwanted edges would need to be trimmed out with another exposure. The former method is attenuated phase-shifting, and is often considered a weak enhancement, requiring special illumination for the most enhancement, while the latter method is known as alternating-aperture phase-shifting, and is the most popular strong enhancement technique.
As leading-edge semiconductor features die shrink, photomask features that are 4× larger must inevitably shrink as well. This could pose challenges since the absorber film will need to become thinner, and hence less opaque.Y. Sato et al., Proc. SPIE, vol. 4889, pp. 50-58 (2002). A 2005 study by Imec found that thinner absorbers degrade image contrast and therefore contribute to line-edge roughness, using state-of-the-art photolithography tools.M. Yoshizawa et al., Proc. SPIE, vol. 5853, pp. 243-251 (2005) One possibility is to eliminate absorbers altogether and use "chromeless" masks, relying solely on phase-shifting for imaging.
The emergence of immersion lithography has a strong impact on photomask requirements. The commonly used attenuated phase-shifting mask is more sensitive to the higher incidence angles applied in "hyper-NA" lithography, due to the longer optical path through the patterned film.C. A. Mack et al., Proc. SPIE, vol. 5992, pp. 306-316 (2005)
Photomasks are made by applying photoresist to a quartz substrate with chrome plating on one side and exposing it using a laser or an electron beam in a process called maskless lithography. The photoresist is then developed and the unprotected areas with chrome are etched, and the remaining photoresist is removed resulting in stencil.
Particle contamination can be a significant problem in semiconductor manufacturing. A photomask is protected from particles by a pelliclea thin transparent film stretched over a frame that is glued over one side of the photomask. The pellicle is far enough away from the mask patterns so that moderate-to-small sized particles that land on the pellicle will be too far out of focus to print. Although they are designed to keep particles away, pellicles become a part of the imaging system and their optical properties need to be taken into account. Pellicles material are Nitrocellulose and made for various Transmission Wavelengths.
The costs of creating new mask shop for 180 nm processes were estimated in 2005 as $40 million, and for 130 nm - more than $100 million. An Analysis of the Economics of Photomask Manufacturing Part – 1: The Economic Environment, Weber, February 9, 2005. Slide 6 "The Mask Shop's Perspective"
The purchase price of a photomask, in 2006, could range from $250 to $100,000 ; page 23 table 1 for a single high-end phase-shift mask. As many as 30 masks (of varying price) may be required to form a complete mask set.