Understanding Electrostatics Impacts on photomasks for SEMI Manufacturing

copper_hou

Definition:
A photomask (also called reticle) is a high precision tool used in lithography process to manfuacture microelectronics (typically semiconductors) which contains the microscopic images of microelectronics.
Note: referred to Photronics incorporations (USA).

Figure 1: Photomask for SEMI lithography process (Source: DNP)
The Electrostatic Susceptibilities of a Photomask
The images on a photomask (herein refers to transimission type, EUV type not included) are mainly consisted by chrome which exhibit significant susceptibilities (Electric Field Sensitive mode) to the electrostatics  common found in photomask fabrications and SEMI fabrication production lines.

Figure 2: Device model of a photomask
The ways of electrostatics to impact photmask consists of EFM (Electric Field induced ion Migrations) and ESD (Electro-Static Discharge) occurences.

Figure 3: EFM impacts on a photomask

Figure 4: ESD impacts on a photomask
Electrostatic Issues in Fabrication production lines
The handlings and transferrings of photomasks in fabrication production lines (including photomask and SEMI manufacturers) would induce electrostatics due to the insulating property of quartz substrate.

Figure 5: Typical handlings and transferrings of photomasks in fabrication automations (image source: EU photomask fabrication automation manufacturer)
It is highly precautioned to take conductive or static dissipative materials and grounding for photomask contact parts (mask supporters or retainers in automations), tools (mask hand grippers) and packages (SMIF pod, mask box). Basically, any grounded conductive or dissipative materials making contacts with an electrostatic charged photomask would Enhance EFM and ESD Risks to the chrome patterns within it.


Figure 6: common mistake of handling tools for photomask electrostatic protections
So far, the technical know-hows of Appropriate and Effective conceptions to protect photomasks (inclduing SEMI and FPD applications) against electrostatic risks have been found still very limited in China including photomask manufacturers and photomask users (SEMI Fab and FPD fab) .Therefore, Electrostatics induced photomask defects issues caused by ESD have been still common seen while EFM issues would many more.