Model 300-PMI Series Photomask Inspection Review Stations Application Note

Benefits of Visual Inspection

With the appropriate instrument and training, an inspector can detect:

• Sub-micron pinholes, chrome spots and particles in high density patterns.

• Edge defects even on 45-degree lines.

• Butting errors.

• Thin chrome.

• Particles on top of chrome.

• Transparent particles trapped beneath high standoff pellicles.

• Repeated defects.

• Glass side defects.

• Pellicle membrane defects.

There are also other benefits:

• Particles on top of a pellicle membrane can be detected while inspecting the reticle surface. This amounts to simultaneous inspection of two surfaces. On clear field masks, glass side defects can also be detected simultaneously.

• Setup time is less than one minute. If a major problem is identified, inspection can be terminated without spending resources inspecting the entire reticle.

• When an automatic system is out of service, a photomask inspection station can keep production moving.

• A single instrument for both reticle preparation and pellicle inspection.

• Precise defect detection and review of known defects.

Essential Instrument Features

Without a photomask inspection station, inspectors will miss defects because they are invisible. The spectacular improvement in the 300-PMI series performance is the result of double dark-field illumination. Double dark-field eliminates the need to optically resolve defect in order to detect them. This is because it causes the size and contrast of the defect to be exaggerated. The result in greater reliability.

Incident dark-field enables the detection of particles on top of chrome. Transmitted dark-field is used to detect particles, chemical residue, pinholes, excess chrome, edge defects and butting errors. Double dark-field enables all of these defects to look like stars in the night sky.

Once a defect is detected, the inspector can go to high magnification and bright-field illumination to classify it. Classification consists of identifying the defect (particle, chrome, pinhole, etc.) sizing it and if equipped, defining its location. Bright-field illumination is required for surface defect definition since dark-field image sizes are greatly exaggerated through mask defect detection.

The following features characterize a photomask inspection station:

• Long working distance objectives (>12mm).

• Highest magnification objective of at least 100X (200X available).

• Incident and transmitted bright-field illuminations with color contrast filters.

• Instant switching between illumination modes.

• Erect image for normal hand-eye coordination.

• Low controls (focusing, stage position, illumination).

• Stage and fixtures for handling masks with high stand off pellicles.

An instrument having the above features will enable the procedures discussed in the following sections to be fully utilized.

The same area of an IC Photomask is shown with four modes of illumination. Defect was detected utilizing Incident and Transmitted Dark-Field illumination and the procedures outlined in Table 2.

1.  The area is illuminated with Incident Bright-Field. The defect is invisible.		Fig. 1
2.  The area is illuminated with Transmitted Bright-Field. The defect is visible, but indistinguishable from other features of the photomask.		Fig. 2
3.  The area is illuminated with Incident Dark-Field. The defect is visible, but indistinguishable from other features of the photomask.		Fig. 3
4.  The area is illuminated with Transmitted Dark-Field. The defect is highly visible and shows up as a bright star in contrast to the surrounding features of the photomask.		Fig. 4

The defect in this example was determined to be a sub-micron particle on the back of the glass, and would have gone undetected utilizing normal Bright-Field illumination of most microscopes.

Inspection Procedures

Many inspectors are in a disadvantaged position to adequately inspect reticles. The reason results from the fact that the combined knowledge of reticle defects and microscopy has not been generally available. For this reason, we have developed a means by which the inspection of reticles can be accomplished quickly and reliably.

In order to help alleviate this problem, a series of tables were developed for use with the 300-PMI series Photomask Inspection Stations. These tables each contain a procedure for a part of the inspection process.  The procedures are presented in Table 1 through 7.

Definitions

The tables contain some abbreviations. The definitions are as follows:

• IBF: Incident Bright-Field Illumination. This is the illumination mode where the light shines down through the microscope objective and reflects off the specimen and back through the objective to the inspector's eyes. (Fig. 1)

• TBF: Transmitted Bright-Field Illumination. The light from an illumination system shine through the specimen from directly below the objective. (Fig. 2)

• CCBF: Color Contrast Bright-Field. Using different color filters on IBF and TBF enables both illumination systems to be used simultaneously to simplify defect identification. In this note, IBF is assumed to be yellow and TBF green.

• IDFI: Incident Dark-Field Illumination. In this illumination mode, light from above the specimen illuminates the field of view at such an angle that it does not reflect directly back into the objective lens. Consequently, the surface of a perfect mirror would appear pitch black to an operator. However, a piece of dust on the mirror would scatter light into the objective and would look like a star against the night sky. (Fig. 3)

• TDF: Transmitted Dark-Field Illumination. In this illumination mode, light from below the specimen illuminates the field of view from such an angle that it misses the objective lens. A perfect glass substrate would appear invisible to an operator. However, a defect (bubble, seed) in the substrate, chrome dot or particle on the surfaces, and pinhole in chrome will scatter light into the objective lens and enable these defects to be observed. (Fig. 4)

Scanning

We recommend a vertical scan with horizontal increments as opposed to a left to right reading type scan when performing a visual inspection. Our justification is that a person's field of view is wider than it is high. Thus, when the image moves vertically though a wide visual field, the visual image is more efficient.  The benefits of a vertical scan are indicated to be higher detection reliability, less work in eye motion within the microscope field and view, thus accommodating a faster scan rate.

Defect Detection

Dark-field is superior to bright-field for defect detection. For example, some sub-micron defects are completely invisible at the highest magnification in bright-field, but are easily observed at relatively low power in dark-field. Conversely, defects cannot be adequately sized or identified in dark-field.

As a consequence of the above, we recommend a two-part, detect/classify procedure: scan for defects using simultaneous top and bottom dark-field illuminations (double dark-field) at wide viewing fields and classify them at high magnification using bright-field. A procedure for detecting defects in presented in Table 2.

Magnification

Not only is magnification important to maximize detection/classification reliability and throughput, but also the combination of optical elements that yields the magnification can affect inspection efficiency. We recommend the following rules be used to obtain desired magnification:

1.  Use 10X eyepieces whenever possible to do a large area overview because they have the widest field of view.

2.  Use the highest power objective and the lowest zoom setting in order to have the highest possible resolution and light gathering capability.

For example, to obtain a 50X or 100X inspection magnification, use 10X eyepieces, 1X zoom setting and a 10X objective. This will produce better results than either 10X eyepieces, 2X zoom or 10X objective or 20X eyepieces.

Objective Selection

Choice of the optimum objective depends upon the size of the defect, which causes rejection. Sub-micron defects are dimmer than 1 micron defects are usually surrounded by a higher pattern density. The problems facing the inspector are, therefore:

1.  If too low a magnification is selected, sub-micron defects will be very dim.  Also, the field of view will be cluttered with the bright edges of chrome lines.

2.  If too high a magnification is selected, the defects will be brighter and the clutter will be reduced.  However, inspection times will increase due to narrower field of view and the need to refocus directly over the defect.

Identification

Techniques for identifying defects depend upon the size. Defects greater than 1 micron can be identified using color contrast bright-field (CCBF). Smaller defects require "probing" with different illumination modes. Even then, an unresolved chrome spot cannot be distinguished from a particle. A defect identification procedure is presented in Table 3. It calls for a series of actions and observations. The defect is identified by correlation of the observations with items in Tables 4 and 5.

Defect Sizing

Sizing defect with a photomask inspection station is an approximate function. For defects larger than 1 micron, one significant figure accuracy is obtained. Defects less than 1 micron may be classified either as between ¼ micron and 1 micron or less than ½ micron (there is an uncertainty of about a quarter of a micron as to the size of the defect that can be observed in incident bright-field). A procedure for sizing defects is presented in Table 6.

Defect Location

The coordinates of defects may be determined by their proximity to some known features utilizing the 342-PMI. This, of course is a slow and tedious process. By utilizing the coordinate location functions of the 362-PMI and 382-PMI, exact locations of defects can be logged or recalled for further analysis. Provided that the photomask has been correctly indexed and aligned, these coordinates can easily be recalled for future analysis and relocated utilizing the field of view of the 10X objective.  This accuracy is sufficient to enable a defect to be relocated from specified coordinate or to specify the coordinates of a detected defect.

Table 1: Reticle Scan Procedure

• Determine if the reticle contains a series of parallel line segments.

• Load it on the stage with the line segments running parallel to the Y-axis (to and away from the operator).

• Start in the left-hand corner and traverse the stage away and parallel to the Y-axis.

• At the end of the scan segment, increment the stage to the left a distance of about 80% of the field of view.

• Scan the entire field in a similar pattern until the inspection is complete.

Table 2: Defect Detection Procedure

• Load the reticle.

• Switch the instrument and both bright-field illuminators on.

• Remove eyeglasses and focus (note, inspectors with astigmatism may need to wear glasses).

• Traverse to the scan starting point.

• Rotate the scanning objective into the duty position. To help narrow the choices of objectives, the following values can be used to obtain a starting point:

• Switch on both dark-field illuminators.

• Begin the scan pattern.

• Observe for an incongruous point or burst of light.

Table 3: Defect Identification Procedure

• Center the defect in the field of view.

• Rotate the 50x objective into the duty position.

• Turn on both bright-field illuminators.

• Observe the color and brightness of the defect. If the defect cannot be observed, proceed to Step 5. Otherwise, identify the defect from the description presented in Table 4.

• Note the background surrounding the defect (chrome or glass).

• If defect is invisible, go to Step 8. If visible, refer to Item 1 in Table 5 for chrome background. For glass background, refer to Items 6 and 7.

• Switch IBF off and TDT on. If defect is visible, refer to Items 2 and 4 of Table 5. If invisible, procedure to Step 9.

• Switch TDF off and IDF on. Refer to Items 3 and 5 of Table 5.

Table 4: CBF identification Guide for Defects larger than 1 micron

Table 5: Sub-micron Defect Identification Guide

Notes:

• Defects less than about ½ micron will not be observed.

• Not identifiable. A particle is visually indistinguishable from a chrome dot.

Table 6: Defect Sizing Procedure

• Go to high magnification.

• Switch to IBF illumination. Fully open the aperture diaphragm.

• Install a measuring eyepiece in one of the viewing tubes.

• Rotate the eyepieces so that the scale is parallel to the longest dimension of the defect.

• Move the stage so that the left or lower edge of the defect touches a division marker. Count the number of lines subtended by the defect. If the defect subtends less than one division, proceed to Step 6, otherwise record the size.

If the defect is less than one micron and visible, the size is between ¼ and 1 micron.  If it is invisible in IBF illumination, the size is less than ½ micron.