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What is a Mask?
A mask, or photomask, is a glass or quartz plate coated on one side with chrome. Mask pattern generation systems use precision lasers or electron beams to image the design of just one layer of an integrated circuit (IC), or chip, onto the mask. After the design has been exposed on the mask, the pattern is etched into the chrome, and the mask is inspected. The mask is then used much like a photographic negative to transfer circuit patterns onto a semiconductor wafer. The patterns translate into the tiny transistors and electrical circuits that make up each final chip. The pattern can be transferred over and over onto wafers, through photolithography. A set of 15 to 30 masks is usually used to construct a wafer.
How is a Mask Made?
1. Circuit Design
In the circuit design process, the patterns that represent the layers in a chip are created by the chip designer. These patterns are then sent via magnetic media or electronically to the mask shop where the pattern data is prepared for mask manufacturing.
2. Data Preparation
At the mask shop, the data is processed and formatted to the specific writing system and tools needed to produce the mask. To optimize the mask manufacturing process, the data is often sized or rotated. A jobdeck is then created that contains the commands for the patterning system to image the required patterns onto the unetched mask blank.
3. Photomask Materials and Sizes
The primary material used to make a mask is a quartz substrate that has a layer of chrome on one side. The chrome layer is covered with an anti-reflective coating and a photosensitive resist. Mask sizes range from three to nine inches square, but most masks produced today are five or six inches square.
4. Patterning
During the patterning process the circuit design is written on the mask. The ALTA laser-beam or MEBES electron-beam (e-beam) pattern generation systems are used to write the pattern, or geometry, onto the mask by exposing the resist. The laser or e-beam changes the molecular composition of the resist, making it soluble in a developing solution. After the resist is removed, the chrome layer is ready for etching.
5. Etch
The etch process, wet or dry, removes the chrome and anti-reflective layers from the mask at points where the resist was removed. In wet etching, the mask is immersed in a chemical bath which removes the coating from the exposed areas of the mask. In dry etching, highly reactive ions are used to produce a chemical reaction on the exposed area of the mask, removing the desired layers.
6. Defect Inspection
For the wafer's circuits to function, the mask must meet certain quality standards. To determine if a mask meets those standards, it is inspected for defects after etching. Defects include chrome extensions, spots and bridging between the pattern, as well as unwanted clear area extensions, pin holes or breaks. All defects larger than those specified by the customer must be repaired, or the mask is rewritten.
7. Repair
The repair process adds missing chrome or removes extra chrome based on the findings during the inspection step. Repairing a mask means avoiding the costly rewrite step.
8. Critical Dimension Inspection
Customers have very specific requirements for pattern sizes or critical dimensions (CDs). As part of the maskmaking process, customers indicate target sizes, specific CDs and acceptable variance from those targets, which is known as the tolerance. CDs are measured to ensure an acceptable CD size.
9. Registration Inspection
During registration inspection, mask pattern positions are measured in relation to each other and other mask layers.
10. Cleaning
Prior to shipment, the mask is carefully cleaned and then inspected for contamination. Contamination on a mask can have the same negative effect as a chrome defect when it is projected onto a wafer. Often pellicles (metal frames with protective membranes) are attached to the mask to help keep it clean and protect it during production.
11. Final Inspection
During final inspection, the mask is inspected for defects in areas that could not be properly inspected by automated equipment. This inspection also ensures that no particles were trapped under the pellicle. Technicians verify that the mask is without cosmetic defects and has the correct title, field tone, array rotation, data parity, and fiducials that are key in enabling the mask to create the customer's specific design on the wafer. |
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