Detailed introduction of the mask plate

I. Definition and Function

Definition: A mask plate is a structure in which various functional patterns are fabricated and precisely positioned on a film, plastic or glass substrate material for selective exposure of photoresist coatings.

Function: The mask plate is a master plate for graphic transfer in the microelectronics manufacturing process. Its function is similar to the "negative" of a traditional camera, used for transferring high-precision circuit designs and carrying intellectual property information such as graphic design and process technology. The graphics are transferred to the product substrate through exposure to achieve batch production.

Ii. Structure and Composition

The substrate: The mask plate is mainly composed of a substrate and a light-blocking film. Substrates are divided into resin substrates and glass substrates. Glass substrates mainly include quartz substrates and soda substrates. Among them, quartz substrates have high chemical stability, high hardness, low expansion coefficient and strong light transmittance, and are suitable for the production of products with higher precision requirements, but the cost is relatively high.

Light-blocking film: The main materials of light-blocking film include metal chromium, silicon, iron oxide, molybdenum silicide, etc. Among various hard light-blocking films, due to the high mechanical strength of chromium material and its ability to form fine patterns, chromium films have become the mainstream of hard light-blocking films.

Protective film: An optical film (Pellicle) made of polyester material, attached to the surface of the mask plate, serves to protect the surface of the mask plate from contamination by dust, dirt, particles, etc.

Iii. Classification and Application

Category:

According to the base material: It can be divided into quartz masks, soda masks, etc.

By application field, they can be classified into flat panel display masks, semiconductor masks, touch masks and circuit board masks, etc.

According to the light source of the photolithography process, it can be classified into binary masks, phase-shift masks, EUV masks, etc.

Application:

In the field of flat panel display: By taking advantage of the exposure masking effect of the mask plate, the designed TFT array and color filter graphics are successively exposed and transferred to the glass substrate in the order of the thin-film transistor's film layer structure, ultimately forming a display device with multiple film layers superimposed. The flat panel display field is the largest downstream application market of mask plates, accounting for approximately 80%, and is applied to panels such as LCD, AMOLED/LTPS, and Micro-LED.

In the semiconductor field: During the wafer manufacturing process, multiple exposure procedures are required. By taking advantage of the exposure masking effect of the mask plate, gate, source and drain, doping Windows, electrode contact holes, etc. are formed on the surface of the semiconductor wafer. The requirements for important parameters such as the minimum line width, CD accuracy, and position accuracy of semiconductor masks are significantly higher than those of mask products in fields such as flat panel displays and PCBS. Semiconductor chip mask manufacturing enterprises can be divided into two major categories: in-house wafer fabrication plants and independent third-party mask producers. At present, the proportion of self-supplied wafer fabrication plants is 52.7%, but the market share of independent third parties is gradually expanding.

Other fields: Mask plates are also widely used in touch screens, printed circuit boards (PCBS), micro-electromechanical systems (MEMS), and other fields.

Iv. Production Process Flow

The production process of mask plates mainly includes steps such as graphic design, graphic conversion, graphic lithography, development, etching, demolding, cleaning, dimensional measurement, defect inspection, defect repair, film application, inspection, and shipment. The corresponding equipment includes photolithography machines, developing machines, etching machines, cleaning machines, measuring instruments, LCVD repair equipment, CD measuring machines, barrier repair machines, panel repair equipment, TFT inspection equipment, film laminating machines, etc. Among them, photolithography is the core processing technology.

V. Technological Development and Challenges

Technological development: With the development of the integrated circuit industry, the Critical Dimension (CD) of chips is constantly shrinking, which puts forward higher requirements for the accuracy and quality of masks. To address this challenge, mask manufacturers have adopted a variety of technical measures, such as optical proximity correction (OPC) and phase shift masks (PSM), to enhance the resolution of masks and the contrast of graphics.

Challenge: When the key dimensions of the chip reach below the wavelength of the illumination light source, Optical Proximity Effects such as optical diffraction will occur when the light wave passes through the mask, resulting in distortion of the optical image of the mask. Therefore, the mask needs to be redesigned according to the target graphic. Furthermore, with the development of advanced manufacturing processes, the process issues of EUV masks have become more difficult to detect and fatal.