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 main types of photomasks are used for patterning:
Binary Photomask – A photomask in which the circuit design is patterned in a light-absorbing film, such as chromium (Cr). 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 Shifting Mask (PSM) – A photomask similar to binary mask, but it 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.
Producing a photomask is the first point in the semiconductor manufacturing process that a chip design actually becomes a physical object. The following steps summarize the photomask production process, highlighting which steps the blank manufacturer completes and which steps the mask maker completes.
Completed by Blank Manufacturer
Create Quartz Substrate. The blank substrate is typically 6 inches square and 0.25 inches thick for advanced photomasks. It is made of pure fused silica and is usually referred to simply as quartz. The surface of the substrate must be extremely flat and defect-free.
Deposit Absorber Layer. On the substrate is a thin absorber layer that can block the exposure light in a wafer lithography tool. For binary photomasks, Cr compounds are the most common absorbers. For PSMs, a special shifting material, such as molybdenum silicide, is used as the absorber, which is then coated with a pattern transfer film made of chromium.
Completed by Blank Manufacturer
or Mask Maker
Deposit Photoresist Layer. On top of the absorber is a thin layer of photoresist that a mask writer, such as the ALTA®, exposes. For binary photomasks, either the blank manufacturer or mask maker applies the photoresist. For the second layer of PSMs, the mask maker applies the coating after completing the first layer of the mask process.
Completed by Mask Maker
Write. A coated blank, along with the circuit pattern data, is loaded into a mask writer. The mask writer exposes the pattern data in the photoresist, using either laser beams or electron beams.
Bake. The exposed photoresist is then baked in a very controlled way with highly uniform temperature at every point across the photomask for a precise amount of time.
Develop. To form the required pattern, the image in the photoresist is then developed using a water-based developer, rinsed and dried without leaving any residue. Because the photoresist image acts as a mask during the etching process, this development step is critical because any non-uniformity in development will lead to non-uniformity of the final pattern dimensions.
Etch. The developed photomask is then loaded into an etch tool, which uses a plasma to precisely etch away the absorber material revealed by the write and develop steps. With a dry process, very straight sidewalls are achieved in the final image on the photomask surfaces.
Strip and Clean. The etched photomask is then loaded into a cleaning tool, which strips the photoresist using a dry plasma or a wet chemistry. Then the photomask goes through several cleaning steps to remove any residual material or particles from the photomask.
Measure. To verify feature uniformity and placement, the critical dimension (CD) and pattern placement accuracy are measured to ensure they meet customer specification.
Inspect. To verify pattern accuracy, the photomask is loaded into an inspection tool. If defects are found, they are classified and the photomask may need to go through a repair step. Depending on process flow, the photomask may go through additional final clean and inspection steps before being boxed and shipped to the wafer fab.
Throughout the photomask development process, the cleanliness requirements are very stringent because the pattern will be replicated many thousands of times. Any particle or defect could kill or compromise the printed chips. In addition, any non-uniformity in baking, developing or etching will cause variation in the photomask pattern dimensions, impacting chip performance.
What Role Does Applied Materials Play?
With three decades of experience in the photomask production process—from laser writing and resist processing all the way down to CD conformity and inspection—our photomask, optics, and system design experts provide:
The ability to design and integrate highly complex process equipment
The understanding of how tool performance influences quality and yield
The knowledge needed to optimize system performance in a production environment
Photomask is a growing business area at Applied Materials, driven both by greater demand for our existing products and additions to our current product offerings. The same factors behind the resurgence of 200mm equipment sales are responsible for an increased need for capacity to manufacture photomasks for volume applications such as mobile, automotive and Internet of Things. Also, an increased use of PSM photomask patterning exists at advanced technology nodes, for which the ALTA mask writer is especially well-suited.
To meet this increased need, Applied Materials provides solutions for a significant part of the photomask manufacturing flow:
Write – ALTA® 4700 Plus Mask Writer
for cost-effective patterning of binary masks and phase shifting masks (PSM)
Coat – Applied Sigmameltec™ CTS Mask Coat Series
for high quality resist layers with repeatable characteristics and ultra-low defect levels
Clean – Applied Sigmameltec™ MRC Mask Clean Series
for the resist strip and cleaning requirements unique to 193i and extreme ultraviolet (EUV) masks
Bake – Applied Sigmameltec™ SFB Mask Bake Series
for repeatable thermal exposure at every point on the mask, taking into account not only the steady state temperature uniformity but also the consistency of the temperature-time profiles across the mask
Develop – Applied Sigmameltec™ SFD Mask Develop Series
for the accurate transfer of exposed photomask patterns into resist profiles
Etch – Centura® Tetra™ EUV Advanced Reticle Etch & Tetra™ Z Photomask Etch
for etching new materials and complex film stacks used in extreme ultraviolet (EUV) photomask, as well as optical lithography photomasks for logic and memory devices at 10nm and beyond
Measure – Holon Mask CD-SEM and Defect Review DR-SEM
for sharp, accurate images—even on insulating substrates—and CD measurement repeatability consistent with 10nm mask production and 7nm mask development
Inspect – Aera4™ Mask Inspection
for performing highly sensitive photomask inspections as required for double- and quadruple-patterning lithography technologies
The Applied Materials’ ALTA mask writers have long been the industry standard for writing aligned layers on PSMs. The ALTA is also used to write most of the binary photomasks for consumer devices including chips for automobiles, mobile devices, sensors and the Internet of Things. It is the only deep ultraviolet (DUV) laser writer on the market, providing rapid turnaround with high yield. To avoid the need for conductive topcoats, the ALTA exposes with laser light, reducing process complexity and improving yield.
Resist Processing & Cleaning
Applied Materials’ full line of Sigmameltec mask resist processing and cleaning products are used for leading edge photomask production to meet exacting requirements for feature size control, as well as for mature photomasks where cycle time and cost of ownership are key factors. The modular design of the Sigmameltec tools facilitates adaptation to our customers’ requirements, drawing from a wide array of process modules and materials.
Defect Review & Measurement
In 2016, Applied Materials became the sole distributor of Holon’s mask CD-SEM and defect review DR-SEM products in North America and Europe. Holon’s newest CD-SEM tool provides sharp, accurate images—even on insulating substrates—and delivers CD measurement repeatability consistent with 10nm mask production and 7nm mask development.
Our photomask portfolio also includes the Centura Tetra dry etch systems for optical masks and EUV masks. The Tetra Z system delivers state-of-the-art performance required to etch optical lithography photomasks for logic and memory devices, extending immersion lithography for quadruple patterning with unprecedented CD performance. The Tetra EUV system etches new materials and complex film stacks used in EUV photomasks, meeting the stringent pattern accuracy, surface finish and defectivity specifications required to achieve high lithography yields when operating in reflected mode.
The Aera4 mask inspection system operates in both standard high-resolution applications and aerial inspection, making it the tool of choice for 1x nm technology nodes using optical lithography and for early-production EUV mask inspection. The system performs highly sensitive mask inspection, as required for double- and quadruple-patterning lithography technologies, while maintaining a very low false alarm rate.