What is a Photomask?
A photomask is a quartz or glass substrate, coated with an opaque film into which is etched the design of the device being manufactured.
The photomask plays a critical role in the microlithography process used by our customers for the manufacture of integrated circuits (ICs), photonic devices, and micro-electro-mechanical systems (MEMS). A photomask consists of a fused silica (QZ) or glass (SL) substrate coated with an opaque film, into which an accurate replication of the device designer’s pattern is etched. In addition to a high degree of pattern fidelity, the photomask also has to meet our customers’ expectations for line width, critical dimension (CD) control, pattern placement (registration), and defectivity control. The terms "masters", "sub-masters", and "copy masks" are usually used to refer to 1X photomasks, while the term "reticles" refers to 1.8X, 2X, 2.5X, 4X, 5X, and 10X stepper or scanner photomasks. A functioning device can require between 5 to 40+ individual photomasks, one mask for each step used in the fabrication process.
How is a Photomask Used?
While photomasks have a variety of applications, the majority are used to support the microlithography process that our customers use.
The photomask image consists of a binary pattern accurately replicating the original design. While photomasks have a variety of applications, the majority are used to support our customers' microlithography process as an intermediate step between design and manufacture of IC, photonics, and MEMS devices.
This binary pattern modulates the intensity of light in the lithography tool being used to "print" the design onto the silicon wafer or other substrate being used.
How is a Photomask Made?
A photomask is made by exposing, or writing, the designer’s pattern onto a resist-coated chrome mask blank. The latent image in the resist is developed to form the required pattern. This resist image acts as a mask during the etching process. The pattern is transferred into the chrome film when the resist layer is removed. CDs and pattern placement are measured to ensure they meet our customers' specifications. The pattern is inspected for any defects that may affect device functionality, which are repaired if necessary. A final clean and, if required, a protective pellicle is attached to complete the photomask manufacturing process.
Our customers' processes and lithography tools vary significantly, and all influence the physical characteristics of the photomasks, including:
- The scale factor (1X to 10X reduction ratios)
- 1X Masters to substrate printing proximity (full-contact to close-proximity aligners, projection steppers, or scanners)
- The wavelength of light used (from "broadband" to deep-UV)
When our customers use hard-contact printing to transfer the design to their substrates, the photomask can quickly deteriorate due to mechanical damage. When the feature size and specifications allow, the most cost-effective solution can be to use a copy photomask made from a "master" or "sub-master", which we retain for making additional copies as required. Copy masks are usually made on soda-lime (SL) glass substrates.
To learn more about copy masks, click here.
When our customers use hard-contact, close-proximity printing or projection aligners to transfer the design to their substrates, little or no attrition of the photomask occurs. These systems typically use broadband or near-UV light (300-450nm) to expose the wafer or substrates. While still at the same scale factor (1X) as the final device, higher-pattern fidelity and tighter specifications can be met using photomasks made directly by our lithography tools. 1X masters can be made on either SL glass or QZ substrates.
To learn more about 1X masters, click here.
When our customers use an optical projection stepper or scanner with a reduction ratio of 1:1, 2.5:1, 4:1, or 5:1, the photomasks used in these systems are usually called "reticles". These systems use single wavelengths from i-line (365nm) to deep-UV (248nm or 193nm). Reduction stepper or scanner reticles typically support the most stringent lithography requirements. In some of the advanced fabs, there are imaging features smaller than the wavelength of light. Reticles are typically made on QZ substrates.
To learn more about reticles, click here.