Lightwave Logic Inc (LWLG)

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Before mass production can take place inspection systems must be updated...

Patterned wafer inspection system
There are many types of patterned wafer inspection systems, including the electron beam inspection systems, the bright-field inspection systems, and the dark-field inspection systems. Each of these has its own features, but the basic detection principles are the same.
On a semiconductor wafer, electronic devices of the same pattern are made side by side. Random defects are often caused by particles such as dust and occur in random positions, as the name suggests. The possibility that they will occur repeatedly in a specific position is extremely low.
Patterned wafer inspection system can therefore detect defects by comparing the pattern images of adjacent chips (also called dies) and obtaining the difference.


Fig.5-1 shows the principle for detecting defects on a patterned wafer.
The pattern on the wafer is captured along the die array by electron beam or light. Defects are detected by comparison between image (1) of the die to be inspected and image (2) of the adjacent die. If there are no defects, the result of the subtraction of Image 2 from Image 1 by digital processing will be zero and no defects are detected. In contrast, if there is a defect in the image of die (2), the defect will remain in the subtracted image (3) as shown in the figure. The defect is then detected and its position coordinates are registered.

Non-patterned wafer inspection system
The non-patterned wafer inspection system is used in the wafer shipping inspection by wafer manufacturers, the wafer incoming inspection by device manufacturers and the equipment condition check using dummy bare wafers to monitor the cleanliness of equipment. The equipment condition check is also performed by the equipment manufacturer at the shipping inspection and by the device manufacturer at the equipment incoming inspection.
To check the cleanliness of equipment, a bare wafer for cleanliness monitoring is loaded into the equipment and the stage inside the equipment is then moved to monitor the increase in particles.


Fig.5-2 shows the principle for detecting defects on a non-patterned wafer.
Since there is no pattern, defects are detected directly without image comparison.
A laser beam is projected to the rotating wafer and is moved in the radial direction so that the laser beam is able to irradiate entire surface of the wafer.

Fig.5-3. Principle of defect detection on a non-patterned wafer(2)
When a laser beam is projected onto a particle/defect of a rotating wafer, the light will be scattered and detected by a detector. Thus, the particle/defect is detected. From the wafer rotation angle and the radius position of the laser beam, the position coordinates of the particle/defect are calculated and registered. Defects on a mirror wafer also include crystal defects such as COP in addition to particles.

In general, the bright-field inspection system is intended for the detailed examination of pattern defects. On the other hand, the dark-field inspection system can detect at high speed and is intended for the defect inspection of a large number of wafers.

In the electron beam inspection system, electron beam is irradiated onto the surface of the wafer, and the emitted secondary electrons and backscattered electrons are detected.
Moreover, the electron beam inspection system detects the amount of the secondary electrons as an image contrast (voltage contrast) according to the conductivity of the device’s internal wiring. If the conductivity at the bottom of the contact hole of the high aspect ratio is detected, the SiO2 residue of ultra-thin thickness can be detected.

Fig.5-4. An example of detecting the residue at the bottom of a contact hole.

https://www.hitachi-hightech.com/global/en/knowledge/semiconductor/room/manufacturing/inspection.html