A significant part of a process engineer’s job is to develop simple and effective monitors that employ minimal non-productive equipment time - Dose To Clear or DTC is one such test.

ASML 5500 Yield Improvement: Full Field Dose to Clear Technique

A significant part of a process engineer’s job is to develop simple and effective monitors that employ minimal non-productive equipment time. In state of the art modern wafer fabrication facilities, engineers rarely actually look at the wafers to fully understand the failure cause and rely on the automated metrology equipment to evaluate and define the corrective action. This is typically effective provided the metrology analysis algorithm has been pre-programed to provide the answer to a previously observed issue/solution. Occasionally a process problem occurs that has not been observed before, which the automated system can output inadequate solutions. This is where the process engineer needs to go into the fab and physically visually inspect the wafers and equipment. The human eye is an excellent detector, typically can out perform most civilian defect detection equipment, can execute extremely fast analysis, and is very inexpensive .

An old monitor technique that goes by many names: Dose to Clear, E_o, Openframe, and threshold exposure has been eliminated in most wafer fabs, because many process engineers see little value in this visual monitor. The problem with this monitor is it requires a trained eye to interpret the complex photoresist patterns to determine the correct solution. A CD SEM monitor cannot provide full field coverage of illumination and photoresist process issues. A full field dose to clear process monitor can rapidly detect process limitations resulting from optical degradation, optical mis-alignment, defects in optical path, stray light, lamp or laser degradation, and the photoresist coat-develop process. The dose to clear monitor will save time and result in a rapid solution to many illumination and photoresist process problems. Simax has developed innovative methodological techniques using the Dose to Clear technique to monitor and solve process problems.
 

Source: Simax

Full field dose to clear is a simple and effective monitor of CD process issues . Example shows stray light and misaligned optics .
          

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Steve Brainerd is a Senior Applications Engineer with Simax Lithography. With over 30 years “hands-on” wafer fabrication experience concentrated in semiconductor wafer, MEMs, thick photoresist, solar, and LCD fabrication manufacturing processes, Steve uses his strong analytical skills to develop robust processes for customers. His knowledge and experience include process-equipment startup, process development, process modeling, anti-reflection coating design/modeling, cost reduction, yield improvement, and capacity increases.