A photomask contains the pattern of an integrated circuit. And as transistors have become smaller and smaller, photomasks have become more complex to accurately transfer the pattern to silicon wafers. The process of creating photomasks has become correspondingly more advanced—even slight defects in a photomask can impact silicon device performance. Verifying that the photomask pattern is accurate and defect-free is critical, especially in the case of high-revenue-earning chips.
A photomask is a fused silica (quartz) plate, typically 6 inches (~152mm) square, covered with a pattern of opaque, transparent, and phase-shifting areas that are projected onto wafers in the lithography process to define the layout of one layer of an integrated circuit. The size of a photomask is not tied to wafer size, and 6-inch photomasks are typically used in lithography tools that expose 300mm or 200mm wafers.
In a wafer fab, a photomask is loaded into a lithography tool that passes light through the photomask to project the pattern onto the wafer surface. These patterns guide the deposition or removal of material from the wafer in subsequent patterning steps (click here to read more about patterning). For each layer of the device, material is deposited or removed in those areas not covered by the photomask image, and a different photomask is used for each successive layer. This patterning process occurs multiple times on silicon wafers throughout chip fabrication, creating multiple layers of circuitry and interconnecting billions of transistors.
The following are the main types of photomasks used for patterning:
Binary Photomask – A photomask in which the circuit design is patterned in a light-absorbing film, such as chromium. When used in a wafer lithography tool, the light pattern transmitted through the photomask is imaged into a photoresist film on a silicon wafer.
Phase Shift Mask – A photomask similar to a binary mask, but that has an absorber film such as molybdenum silicide that transmits a small fraction of the light while changing the light’s phase. This increases the photomask complexity but improves the process window of the wafer lithography.
Photomask is an expanding segment for Applied; photomask, optics, and system design experts offer in-depth experience in the photomask production process from laser writing and resist processing to CD conformity and inspection. The same factors driving the resurgence of 200mm wafer manufacturing are responsible for an increased need to manufacture photomasks for volume production of mobile, automotive, and Internet of Things applications. To meet this growing need, Applied Materials systems address a significant part of the photomask manufacturing flow.