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Inspection

 

Because of the pellicle's structure, only film can be inspected automatically. Other components have to be visual inspected. Defects should be categorized, i.e. killing defect, functional defect, or cosmetic defect, and appropriate actions should be taken. Functional defects should be well-defined between vendor and user, while cosmetic defects should be compared with a sample standard. To properly inspect a pellicle, attention must be given to several components - transmission and uniformity of the pellicle film, particle contamination on the film, frame, and adhesive; and overall integrity of the pellicle.

Inspection of Film Transmission and its Uniformity
To inspect transmission, a spectrophotometer is used to measure pellicle film transmission and its uniformity. Consideration must be given to the background stray light noise, resolution, i.e. bandwidth of the slit, and the angle of the measuring incident light from the spectrophotometer to the pellicle. Although some pellicles might occasionally have been out of specification from some pellicle suppliers, transmission generally presents no problem because each pellicle's transmission is individually measured.

Transmission uniformity is typically inspected with a monochromatic light, usually green mercury light or a helium-neon light. With the aid of a monochromatic light, detection of non-uniform spots on the film can easily be found, such as those from the spin-coating process. Similarly, a laser reflection inspection machine has been used by MLI to inspect the coating uniformity of uncoated wafer stepper film. Uncoated film thickness can easily be measured by calculations from the transmission or reflection spectra of a spectrophotometer or any commercial film thickness measuring machine.

Inspection of Film Particles
Particle standards should be used for inspection calibration. Specifically, a 1 µm, 0.5 µm and 0.3 µm standard on pellicle surface should be used to calibrate any inspection tool. Even for visual inspection, these standards should regularly be shown to operators to insure accurate inspection.

Currently, there are three methods of inspecting film contamination - human eye, laser scan, and video camera inspection. The human eye is quite sensitive. MLI has found that with proper lighting a particle as small as 0.3 µm can be detected (calibrated with a standard polystyrene bead on pellicle film and verified by a laser scan machine). However, human-eye inspection has a non-quantitative nature. There are several factors involved in this inspection process, such as operator eye sensitivity, ability to focus, inspection angle and position, film distance from the eye, incident and background light intensity, and the eye's pupil response to background light. The ability to detect a particle below 1 µm can be very different between operators.

Even inspection with machines can give irreproducible results. With laser inspection of film particles, the best reproducibility is approximately +/- 20%. This limitation is primarily controlled by the scan line overlap and scan line stability, which is affected by the vibration limit of the scanning optics. Laser scan can detect particles smaller than 0.3 µm.

The video camera can easily detect particles smaller than 0.3µm with proper illumination. With current high-speed computer video capture and processing, this is the best method to detect particles on pellicle film.

All three inspection methods have their limitations. Human-eye, laser scan, and video camera detection is a detection of scattered light, not the real particle size. The real particle size can be substantially larger than the size its scattering light appears, sometimes as large as 10 times that of the detected particle size. In addition, laser detection is limited to about 2-3 mms from the frame's edge, depending on the laser scanning angle, intensity distribution of the laser spot, height of the frame, and light scattering of the frame edge.

Comparing human-eye, laser scan, and video inspections, the advantage of the laser and video inspection is that they are both a mechanical process that generates more reproducible results, whereas human-eye inspection under strong light scattering presents an unpleasant condition to work. However, human-eye inspection does have the desired sensitivity, fast inspection speed, ability to inspect up to the frame's edge, and even the ability to determine which side of the film the particle lies on, at least for larger particles.

Inspection of Frame
A frame is usually machined, sandblasted, and black anodized. Currently, human-eye inspection is used under a projector light to inspect such pellicle frames. However, an operator can not distinguish particles of 3 µm or even larger from frame irregularities which came from machining marks, a rough surface from sandblasting, and a porous anodized surface with etch pits. Therefore, it is very difficult to differentiate between a frame surface irregularity and particle contamination. In addition, the strong background scattered light from the frame makes the task even more difficult. Inspection of frame particles under a microscope has been attempted but has not been successful. Seen under a microscope, the irregularity of the frame will become even more obvious and more difficult to distinguish it from particles. Therefore, surface coating of the frame is necessary to give the frame a smooth surface and make automated and human-eye inspection reasonably possible.

Inspection of Adhesive
Foam adhesives previously used had holes larger than 50 µm on the side wall. The irregularity of the foam material made it almost impossible to detect small particles on such an adhesive, only a rigorous cleaning procedure was performed to ensure cleanliness. As the illustration shows in Figure 6, cast-in-place, non-carrier adhesives allow the surface to be much smoother, making it easier to differentiate between particles and adhesive irregularities. However, there is still difficulty detecting particles on the edge of the frame and adhesive.