The present invention relates to a defect review method and a defect review apparatus which is used to review a semiconductor wafer.
For improvement of yield in semiconductor manufacturing process, it is important that the source of defect on a semiconductor wafer be immediately investigated. At semiconductor manufacturing sites, defects are currently analyzed with a defect inspection apparatus and a defect review apparatus.
The defect inspection apparatus observes wafers with optical means or electron beam and outputs detected defect positional coordinates.
It is important that the defect inspection apparatus rapidly process a board area. Therefore, the defect inspection apparatus reduces the amount of image date by maximizing (that is, by lowering the resolution)the image area of be acquired per one pixel (hereinafter “pixel size”).
In most cases, a detected low-resolution image may indicate the presence of any defect, but it is difficult to discriminate a type of the defect (defect type) in exact detail.
Under such circumstances, the defect review apparatus is used to discriminate the defect type detected by the defect inspection apparatus in exact detail.
The defect review apparatus images a defect at a defect coordinate of a wafer by high-resolution using an output information generated by the defect inspection apparatus and outputs an image.
As the defect size is now on the order of tens of nanometers due to an increased degree of miniaturization, the semiconductor manufacturing process requires a resolution on the order of several nanometers in order to review the details of defects.
As such being the case, the defect review apparatus (hereinafter “review SEM”) using a scanning electron microscope has been widely used in recent years.
An automatic review operation is desired in a semiconductor high-volume production line The review SEM mounts an ADR (Automatic Defect Review) function, which automatically acquires an image at defect coordinates of a sample.
ADR is a function which automatically acquires a high-resolution image at defect area using the defect coordinates obtained by the defect inspection apparatus.
There is an issue that a margin of error between defect coordinates outputted by the defect inspection apparatus and actual defect coordinates.
In general, there is about ±4 [μm] variation as a margin of error of the defect coordinates outputted by the defect inspection apparatus.
So if the area at a defect coordinates outputted by the defect inspection apparatus is imaged by high-resolution, a field of view is about 2.5 [μm] (for example, magnification is 50000), it has a potential that there is no defect in the field of view.
Therefore, an image is taken by a first magnification (for example magnification is 15000), which field of view is about 9 [μm], then a defect is detected based on the low-resolution image, and an area at detected defect is taken by a second magnification (for example magnification is 50000).
The specific realization method of ADR is described in JP-P-3893825. In JP-P-3893825, first, a defect image imaged at a defect area by a low magnification (a first magnification) and a perfect image imaged at the area patterned a same pattern as the defect area are respectively obtained.
Then these two images are compared and the differenceis decided as a defect and a defect coordinate is detected.
Regarding the detected defect coordinate, a high-resolution defect image is imaged by a high-resolution (a second magnification).
The semiconductor wafer is arranged a plurality of identical chips.
So it is possible to use the image of the chip next to the defect existing chip as a perfect image.
In recent years, the number of defects to be reviewed per wafer has increased due to an increase in the diameters of semiconductor wafers. In addition, the review apparatus exhibits a lower throughput than the inspection apparatus. Consequently, the speed of ADR needs to be increased.
Regarding a general flow of ADR disclosed in JP-P-3893825, a considerable amount of time is spent on moving a stage between an initial position and target position and imaging a perfect image and a defect image.
Therefore, regarding a speeding up of ADR, increasing the speed of these operations and eliminating some imaging procedures are efficiently.
In JP-A-2007-40910, the technology is described that a perfect image is made from a defect image and comparison the perfect image and the defect image as a technology about imaging a perfect image which technology is often eliminated generally.
The technology described in JP-A-2007-40910 is such that a perfect image is made from a defect image using a repeat pitch of circuit pattern of the defect image and a defect is detected by comparison detection between the defect image and the made perfect image.
In a general ADR processing described in JP-P-3893825, a considerable amount of time is spent on moving a stage between an initial position and a target position and imaging a perfect image and a defect image. Therefore, the shortening of the image obtaining time including imaging an image can't be achieved.
The technology described in JP-A-2007-40910 is such that the method to detect defects without using a perfect image, but shortening the time required for defect images obtaining is not sufficiently studied.
Image obtaining time relies on the combination of the image obtaining conditions such as amount of frames of adding and averaging obtained images (hereinafter “frame addition amount”) and amount of imaging pixels (hereinafter “image size”).
In general, imaging image of SEM has low S/N, the same area is imaged F frames (F≧1) and an adding and averaging image of F images is outputted as a obtaining image.
If frame addition amount increases, S/N of obtained image also increases, but the image obtaining time also increases.
Also, if image size increases, much amount of information could be obtained, but the image obtaining time also increases.
As well, image size, imaging field of view and area per pixel (pixel size) have a relation each other. For example, if an image size increases, the pixel size decreases, so it become possible to detect smaller defect.
If the imaging field of view increases, it is possible to detect defect even though a defect position coordinates accuracy (hereinafter “defect coordinates accuracy”) of a defect inspection apparatus is low.
It may not obtain the enough pixel size to defect detection without increasing the pixel size.
In case of defect detection operation about low-magnification image obtained by low-magnification in ADR, an accuracy of defect coordinates by defect detection (hereinafter “defect detection accuracy”) is important.
Image obtaining condition basically should be set as a condition which obtains a enough defect detection accuracy (frame addition amount is large and image size is large) regardless of the kind and largeness of the defect and low-magnification defect image is obtained.
In general, images (for example high S/N and so on) obtained by the image obtaining condition which needs more time to obtain images are apt to obtain high defect detection accuracy.
But some images which obtained by the image obtaining condition which is shorter time to obtain an image may have high defect detection accuracy (that is the case is easy to detect a defect) depending on a kind of defect (for example a big defect) or defect coordinates accuracy.
In particular, in case of large defects, it is possible to obtain a high defect detection accuracy even though the images are imaged by some low S/N imaging obtaining condition or a imaging obtaining condition of large pixel size (that is image size is small).
Additionally, in case of high defect coordinates accuracy, an image detected by the image obtaining condition of small imaging field of view (that is the image size is small) is possible to obtain high defect detection accuracy.
The technology described in JP-A-2007-40910 is such that skip of imaging a perfect image increases the ADR throughput.
But in this method, obtaining a defect image, using the fixed image obtaining condition for the case that defect detection is not easy, is condition regardless of in case of that the defect detection is easy as discussed previously.
The way to shorten a defect image obtaining time isn't studied.