Method and apparatus for charged particle beam inspection

A method, apparatus and computer readable medium for charged particle beam inspection of a sample comprising at least one sampling region and at least one skip region is disclosed. The method, apparatus and computer readable medium comprise receiving an imaging recipe which at least comprises information of the area of the sampling and skip regions; calculating a default stage speed according to the imaging recipe; calculating an alternative stage speed at least according to the default stage speed, the sampling region area information, and the skip region area information; calculating at least one imaging scan compensation offset at least according to the alternative stage speed; and inspecting the sample at the alternative stage speed while adjusting the motion of the charged particle beam according to the imaging scan compensation offsets, such that the charged particle beam tightly follows the motion of the stage and images only the sampling regions on the sample.

BACKGROUND OF THE INVENTION

The charged particle beam inspection system has been widely used for the inspection of physical or electrical defects in a semiconductor wafer or mask.FIG. 1is a schematic illustration of a conventional charged particle beam defect inspection system100. System100includes several portions, namely, a primary charged particle beam source portion, a secondary charged particle detection portion, an image processing portion, and a system control portion.

The primary charged particle beam source portion includes a charged particle gun10, a beam extraction electrode11, a condenser lens12, a beam blanking deflector13, an aperture14, a scanning deflector15, and an objective lens16. The secondary charged particle detection portion includes an E×B charged particle detour device17, a secondary charged particle detector21, a preamplifier22, an A/D converter23, and a high voltage electric source24.

The image processing portion includes a first image storage46, a second image storage47, an arithmetic operation device48for comparing images from storage46and47, a defect judgment device49, and a monitor display50for interfacing with the users. The system control portion includes a microprocessor computer6, a position correction and control circuit43, a stage driver34, an objective lens source45, a scan signal generator44, a sample stage30, an X-Y direction stage31, and a high voltage electric source36.

The inspection process begins as a sample9(for example, a wafer or a mask) sitting on the sample stage30is irradiated by a primary charged particle beam18. The secondary charged particles20emanating from the sample9is detoured by the E×B charged particle detour device17to the detector21, the charged particle signal is then amplified by the amplifier22and converted to digital signals by the A/D converter23for further image processing and defect judgment. In accordance with a preloaded recipe in the firmware, the microprocessor computer6guides the stage driver34and scan signal generator44to properly deliver the inspection.

The throughput of the defect inspection process is particularly critical when such inspection is part of the semiconductor manufacturing process flow. It is usually necessary to be able to measure the defect density in a shorter time for high-efficiency production flow. Referring toFIG. 2, which is a flowchart illustration of a conventional defect inspection process200. First, an imaging recipe is loaded, for example from the firmware, in step210. The imaging recipe is a group of parameters carrying inspection specifications such as the graphical data system (GDS) file of the sample being inspected, the location and area of the sampling regions (regions on the sample that are interested in and to be inspected for defects), image pixel size, scan width, image scan time, the largest available image pixel size, the number of scans for individual sampling region, etc. It is noted that the sampling regions are generally selected in a fashion which complies with the statistical process control rule. Next, in step220, a moving speed for the sample stage is calculated based on the loaded imaging recipe. This is especially important for a continuous scanning mode inspection operation. Then, the inspection is performed at the calculated stage speed and according to the specifications indicated in the imaging recipe, as shown in step230. Then, in step240, the density of inspected defects from step230is calculated. The calculation result is evaluated in step250to determine whether it meets satisfaction, so as to determine completion of the inspection. If the inspection is determined to be incomplete, the process goes back to step230and down. If the inspection is determined to be complete, the process ends at step260.

The overall defect density inspection throughput typically scales as a function of the square of the pixel size used. For example, if the linear dimension of the image pixel size is halved in order to be able to find smaller defects, the overall inspection speed decreases by a factor of four assuming the pixel data rate (sampled pixels per unit time, for example in seconds) remains constant.

The square law dependence that throughput has on the image pixel size is critical for advanced higher resolution inspection systems such as a charged particle inspection system, where the frequent need to use smaller pixels slows the inspection speed significantly. The slow speeds of high resolution defect inspection, combined with the fact that minimum sample surface area must be inspected to make a statistically significant measurement of defect density, results in inefficient use of inspection system time.

This problem is especially significant when the sample comprises patterns composed of both interested and uninterested regions, or simply say that the sample comprises interested and uninterested regions. For example, when a functional device has been developed, it is typically desired to have immediate report of possible defects thereof. However, these devices often coexist with other common devices which are equally inspected in the conventional inspection method.

A similar problem may happen in a scenario where a newly developed fabrication method of an existing common device is to be verified by means of defect inspection. Further, it is also possible for a sample to have complete blank regions where no pattern is formed at all. In these cases, the inspection performed on those regions other than the interested region obviously lowers the overall throughput.

Referring toFIG. 3A, which illustrates an exemplary pattern having both interested and uninterested regions. The “interested regions” are substantially equal to the sampling regions in the conventional art mentioned earlier, and will be continued to be referred to as the sampling regions hereinafter for consistency. The uninterested region on the other hand, is a region which is not interested in and preferred to be skipped for the purpose of time saving, and will be referred as a skip region hereinafter. As shown, a target pattern300A comprises an interested sampling region310A and an uninterested skip region320A. It is noted that the target pattern300A in this example is a repeating pattern, and the sampling region310A combined with the skip region320A form a pattern period330A.

FIG. 3Billustrates another exemplary pattern having both sampling regions and skip regions. As shown, an irregular target pattern300B comprises a sampling region310B and a skip region320B, and no pattern period is present in this example. It is noted that in either case inFIGS. 3A and 3B, the uninterested skip region typically has a much larger area than the interested sampling region.

For example, the sampling region may be of a few hundreds nanometers (nm) in width, while the skip region may be of a few micrometers (um) in width. As a result, much of the tool time is wasted scanning the uninterested skip regions for the conventional art inspection method as in which both regions are equally scanned and imaged.

Accordingly what is needed is an inspection method that allows for increased inspection throughput without sacrificing resolution during inspection of the sample.

SUMMARY OF THE PRESENT INVENTION

In one embodiment of the present invention, a method for imaging and inspecting a sample using a charged particle beam is disclosed. The sample comprises at least one sampling region and at least one skip region thereon. The charged particle beam scans the sample in scan lines along a first direction. The sample is secured on a stage continuously moving along a second direction. The disclosed method comprises receiving an imaging recipe; calculating a default stage speed; calculating an alternative stage speed; calculating at least one imaging scan compensation offset; and inspecting the sample at the alternative stage speed while adjusting the motion of the charged particle beam according to the calculated imaging scan compensation offsets, such that the charged particle beam tightly follows the motion of the stage and scans and images only the sampling regions on the sample. The received imaging recipe at least comprises information of the area of the sampling regions and the skip regions. The default stage speed is calculated according to the received imaging recipe. The alternative stage speed is calculated at least according to the calculated default stage speed, the sampling region area information, and the skip region area information. The imaging scan compensation offsets are calculated at least according to the calculated alternative stage speed.

In another embodiment of the present invention, a computer readable medium encoded with a program for enhancing throughput of charged particle beam inspection of a sample is disclosed. The sample comprises at least one sampling region and at least one skip region thereon. The charged particle beam scans the sample in scan lines along a first direction. The sample is secured on a stage continuously moving along a second direction. The disclosed program executes actions comprising: receiving an imaging recipe; calculating a default stage speed; calculating an alternative stage speed; calculating at least one imaging scan compensation offset; and determining activation of an alternative inspection mode, wherein if the alternative inspection mode is determined to be activated, then the program images the sample at the alternative stage speed while adjusting the motion of the charged particle beam according to the calculated imaging scan compensation offsets, such that the charged particle beam tightly follows the motion of the stage and scans and images only the sampling regions on the sample. The imaging recipe at least comprises information of the area of the sampling regions and the skip regions. The default stage speed is calculated according to the received imaging recipe. The alternative stage speed is calculated at least according to the calculated default stage speed, the sampling region area information, and the skip region area information. The imaging scan compensation offsets are calculated at least according to the calculated alternative stage speed.

In yet another embodiment of the present invention, a charged particle beam inspection system for imaging and inspecting a sample is disclosed. The sample comprises at least one sampling region and at least one skip region thereon. The disclosed charged particle beam inspection system comprises: a charged particle beam probe generator; a deflection module; a moving stage; a control module coupled with the stage and the deflection module. The charged particle beam probe generator generates a charged particle beam probe. The deflection module scans the generated charged particle beam probe across a surface of the sample, wherein the charged particle beam probe scans the sample in scan lines along a first direction. The sample is secured on the stage for imaging, wherein the stage continuously moves along a second direction. The control module functions to match the relative motion of the stage and the charged particle beam probe.

The control module performs the following actions: receiving an imaging recipe; calculating a default stage speed; calculating an alternative stage speed; calculating at least one imaging scan compensation offset and generating a first control signal and a second control signal accordingly; and determining activation of an alternative inspection mode, wherein if the alternative inspection mode is determined to be activated, then the control module outputs the generated second control signal to the stage and deflection module to cause the stage to move at the alternative stage speed and the deflection module to guide the charged particle beam probe in accordance with the imaging scan compensation offsets, such that the charged particle beam probe tightly follows the motion of the stage and scans and images only the sampling regions on the sample. The received imaging recipe at least comprises information of the area of the sampling region and the skip region. The default stage speed is calculated according to the received imaging recipe. The alternative stage speed is calculated at least according to the calculated default stage speed, the sampling region area information, and the skip region area information. The imaging scan compensation offsets are calculated at least according to the calculated alternative stage speed. The sampling area is selected complying with the statistical process control rule.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention generally relates to defect inspection of semiconductor wafer or mask. More particularly, an apparatus and method is provided for calculating and adjusting the stage moving speed and/or corresponding scan compensation offset. By example, the invention is applied to a charged particle beam inspection system. But it would be recognized that the invention has a much broader range of applicability.

Reference will now be made in detail to specific embodiments of the present invention. Examples of these embodiments are illustrated in accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, a well known process and operations have not been described in detail, in order not to unnecessarily obscure the present invention.

In an embodiment, the stage moving direction and the charged particle beam scanning direction is designed to be at an angle. Referring toFIG. 4, which illustrates the relative motion of the scanning beam and the sample stage in accordance with an embodiment of the present invention. As shown, a sampling region410is being scanned in scan lines420by a charged particle beam scanning across a plurality of patterns430to be inspected. The scanning direction440can be different from the stage moving direction450. In this embodiment, the scanning direction440of the charged particle beam is designed to be substantially perpendicular to the moving direction450of the sample stage. In other words, the scan lines420formed during inspection will be substantially perpendicular to the track of the moving sample stage. It is also noted that as illustrated, in this embodiment the sampling region410has a high aspect ratio shape, with its longer edge substantially parallel to the scanning direction440.

Referring toFIG. 5A, which is a process flowchart illustration of a defect inspection method500in accordance with an embodiment of the present invention. This proposed method is to provide improved overall throughput for inspection of a sample having both interested and uninterested regions thereon. As, shown, the proposed method starts by receiving an imaging recipe in step510. The imaging recipe should at least comprise area information of the sampling and skip region(s). Next, in step520a default stage speed is calculated according to the received imaging recipe. Then, in step530it is determined if an alternative inspection mode is to be activated. If it is determined that the alternative mode will not be activated, the inspection job will be performed in a default inspection mode at the calculated default stage speed and according to the received imaging recipe, as shown in step540A.

It is noted that this default mode operation is substantially equal to the conventional art inspection operation, where the sampling and skip regions on the sample are equally scanned and imaged by the imaging charged particle beam. On the other hand, if the determination result indicates activation of the alternative inspection mode, the inspection job will be performed at an alternative stage speed and according to at least one imaging scan compensation offset corresponding to this alternative stage speed, as shown in step540B. The alternative mode inspection will be described in detail below.

Next, in step550, the density of inspected defects from step540A or540B is calculated. The calculation result is evaluated in step560to determine whether it meets satisfaction, so as to determine completion of the inspection process. If the inspection is determined to be incomplete, the process goes back to inspection step540A or540B and down. If the inspection is determined to be complete, the process ends at step570. It is noted that the sampling regions are generally selected in a fashion which complies with the statistical process control rule. Referring toFIG. 5B, which is a flow chart illustration of execution of an alternative inspection mode operation in accordance with an embodiment of the present invention. As shown, the execution of the alternative mode inspection can be summarized as an inspection process flow590, which comprises: receiving an imaging recipe (510); calculating a default stage speed (520); calculating an alternative stage speed (581); calculating at least one imaging scan compensation offset (582); and inspecting the sample at the alternative stage speed while adjusting the motion of the charged particle beam according to the calculated imaging scan compensation offsets (540B).

The received imaging recipe at least comprises area information of the sampling and skip region. The default stage speed is calculated at least according to the received imaging recipe. The alternative stage speed is calculated at least according to the calculated default stage speed, the sampling region area information, and the skip region area information. The imaging scan compensation offset is calculated at least according to the calculated alternative stage speed. The alternative mode is performed to guide the charged particle beam to tightly follow the motion of the stage and scan and image only the sampling regions on the sample.

In one embodiment, the imaging recipe may also comprise parameters as those in the conventional art, for example the graphical data system (GDS) file of the sample being inspected, the sampling region location information, image pixel size, scan width, image scan time, the largest available image pixel size, the number of scans for individual sampling region, etc. In another embodiment, the imaging recipe may further comprise the location and area information of the skip regions. For these embodiments, the default stage speed, alternative stage speed and imaging scan compensation offsets may be correspondingly calculated according to these more detailed parameters indicated in the imaging recipe.

Reference will now be temporarily made toFIG. 6, which is a schematic illustration of example imaging recipe parameters in accordance with an embodiment of the present invention. As shown, a sampling region610comprising for example512pixels is being scanned by a charged particle beam. Each sampling region610is scanned and imaged with a pre-selected scan width620and pixel size630. The image width620is a pre-selected linear distance from the start to the end point of a predefined imaging area along the stage moving direction, wherein the imaging area encompasses sampling region610.

The image width620typically measures in pixels. As the charged particle beam scans down and across the pixels in sampling region610, it forms scan lines640. The time required to form one scan line640is typically referred to as a line period time at a fixed vertical scanning speed of the charged particle beam. For example, inFIG. 6the line period time is the time to complete a vertical scan line640of 512 pixels at a given scanning speed. With this time length, an image scan time can be defined as the time required to complete the scan of one sampling region610. In the case shown inFIG. 6, the image scan time is three times the line period time (without averaging), as the illustrated image width620is 3 pixels.

Reference is now made back toFIG. 5. Step580illustrates the needed steps prior to performing the alternative inspection mode. First, in step581the alternative stage speed is calculated at least according to the area of the sampling and skip region indicated in the imaging recipe. For example, the alternative stage speed may be calculated according to a ratio between the area of the sampling region and the skip region, such as a ratio of the skip region area to the sampling region area. This can be better illustrated in conjunction withFIG. 7, which is a schematic illustration of determination of alternative stage speed in accordance with one embodiment of the present invention. As shown, a pattern700with a sampling region710(AB) and skip region720(AC) are being scanned and imaged. In this embodiment, the default stage speed is defined and calculated from the pixel size and line period time given in the imaging recipe according to the following formula:
Vdefault=(Pixel Size)/(Line period Time)   (1)
The alternative stage speed can be defined, for example, as
Valternative=Vdefault×AC/AB(2)
Therefore, the alternative stage speed is determined essentially based on the area of the sampling region710and the skip region720. As a result, the stage moving speed is raised by a factor of AC/AB in the alternative inspection mode. In one embodiment, the alternative stage speed can be calculated further according to other factors, such as one selected from a group consisting of the following: image pixel size, scan width, image scan time, the location of the sampling region, the location of the skip region, the largest available image pixel size, the number of scans for individual sampling region, etc., or any combination thereof.

Referring back to FIG.5A/5B, next in step582, at least one imaging scan compensation offset is calculated according to the calculated alternative stage speed. It is noted that in one embodiment, the imaging scan compensation offsets can also be calculated further according to other factors such image pixel size, scan width, image scan time, the location of the sampling region, the area of the sampling region, the location of the skip region, the area of the skip region, the largest available image pixel size, the number of scans for individual sampling region, etc., or any combination thereof. With these parameters, the alternative inspection mode is able to be performed in step540B, where the sample is inspected at the calculated alternative stage speed while the motion of the charged particle beam is adjusted according to the calculated imaging scan compensation offsets, such that the charged particle beam tightly follows the motion of the stage and scans and images only the sampling regions on the sample.

In one embodiment, the imaging scan compensation offset at least comprises tilting at least one of the scan lines to the stage moving direction and reassigning a start point for each scan line along the stage moving direction. Referring toFIG. 8, which is a schematic illustration of example compensation offsets in accordance of an embodiment of the present invention. As shown, a sampling region810is being scanned in both the default and alternative mode inspection, respectively.

It is noted that inFIG. 8, the scan direction is substantially pointing to the right of the drawings, and the stage moving direction is substantially pointing to the bottom of the drawings. It is also noted that the scan direction in both scenarios illustrated inFIG. 8is substantially perpendicular to the stage moving direction. Moreover, the tilted scan direction in the alternative mode has been exaggerated for more explicit illustration.

Span820A and820B respectively represents one complete frame image of sampling region810in the default and alternative mode inspection. In the alternative mode, the stage is moving at a higher speed, therefore the beam deflector system needs to guide the scanning beam to tightly follow the motion of the stage so as to make useful scans. As illustrated, to achieve such goal two compensation offsets are used in this embodiment, which are tilting of scan line (830) and reassignment of scan line start point (840).

In the default inspection mode, the sample stage moves at a slower speed, and a milder slope of the scan lines is enough for sampling region810to be covered by a scan line within a given line period time. In the alternative inspection mode however, as the sample stage moves at a higher speed, assuming the line period time remains unchanged, the sampling region810could move out of the coverage of a scan line within a line period time if the scan line slope also remains unchanged.

In order to solve this problem, compensation offset830is used to tilt at least one of the scan lines to the direction of the moving stage, thereby rendering a greater slope of the scan line and thus a larger coverage of scan within the same line period time. In addition, with compensation offset840, the start point of each scan line is reassigned to jump forward at a greater interval as compared to that in the default inspection mode, allowing the scan lines a faster catch-up with the moving sample.

As a result, it can been seen inFIG. 8that over a same time length of 4 line period time, a frame image of the sampling region810is almost complete in the alternative inspection mode, while that in the default inspection mode is less than half complete.

Referring toFIG. 9A, which is a schematic illustration of a default inspection mode operation in accordance with an embodiment of the present invention. As shown, a pattern900is being inspected. The pattern900comprises a sampling region910with an area size AB and a skip region920with an area size BC. In this embodiment, the sampling region910and skip region920have identical area size (AB=BC). A pixel size940is selected to be as illustrated in the drawing.

The sample stage is selected to be moving at a speed of 1× to the right. This (default) stage speed is calculated in accordance with formula (1) described earlier, where the needed parameters such as the line period time may be, for example, obtained from the imaging recipe. As a result, the sample stage is traveling over a distance of 1 pixel size to the right per line period time.

In taking the image, one frame image is set to cover the entire sampling region910or the skip region920. As shown, it takes four scan lines (the solid/dotted arrows pointing to the bottom of the drawing) to form a frame image. The circles in sampling region910represent patterns under inspection. The dotted arrow represents a pattern being scanned; the solid arrow represents a scanned pattern.

InFIG. 9A, scenes1˜4illustrate the formation of a complete frame image, and scene5illustrates a successive scan line falling in the skip region920.

Referring toFIG. 9B, which is a schematic illustration of an alternative inspection mode operation in accordance with an embodiment of the present invention. The same pattern900as inFIG. 9Ais being inspected. The same pixel size940is used. The sample stage is selected to be moving at a speed of 2× to the right. In other words, this (alternative) stage speed is selected to be twice the default stage speed with all other imaging recipe parameters unchanged, including the line period time. As a result, the stage is traveling over a distance of 2 pixel sizes to the right per line period time.

The alternative stage speed can be selected based on varying factors. In this embodiment, the alternative stage speed is selected to be doubled because the area of sampling region910is only half of the area of the whole sample900due to the presence of skip region920which has the same area as the sampling region910. One frame image is still set to cover the entire sampling region910. As shown, it takes four scan lines to form a frame image. The dotted arrow represents a pattern being scanned; the solid arrow represents a scanned pattern. The circles in sampling region910represent patterns under inspection. An imaging scan compensation offset950is used to guide the scanning beam in forming the scan lines.

In order to keep the pixel size unchanged, a following scan line must always be formed one pixel behind the previous one. Therefore, the start point of scan lines must be set to be jumping forward a distance of 1 pixel size940over one line period time as the stage is traveling at a speed of 2 pixel sizes940per line period time. In other words, the compensation offset950is set to be 1 pixel size per line period time. It is noted that inFIG. 9Bonly the start point reassignment type of compensation offset is illustrated.

Scenes1˜4illustrate the formation of a complete frame image. As shown, for each sequential scene, the stage moves forward 2 pixel sizes940, and the scan line start point moves forward 1 pixel size940as instructed by the compensation offset950. As a result, the scanning beam constantly keeps up with the moving stage and forms a scan line right on the next pattern column. For example, in scene1the first pattern column is scanned. Here, a base line960will be used as a reference for illustrating the compensation offset950.

In scene2, the first scanned pattern column moves forward 2 pixel sizes940with respect to the base line960. At the same time, the start point of the next scan line moves forward 1 pixel size940, whereby the second pattern column in sampling region910is scanned. Without compensation offset950, this scan would still be made at the location of the base line960, forming the second scan line on the third pattern column of sampling region910, and the second pattern column is thus missed.

Next, in scene3the first scanned pattern column moves forward 2 pixel sizes940again. At the same time, the scan line start point also moves forward 1 pixel size940, causing a third scan line to be formed on the third pattern column of the same sampling region910. The same process proceeds till all the four pattern columns of sampling region910are scanned in scene4, completing a frame image.

In scene5, the start point of the next scan line is guided to jump backward, skipping the coming up skip region920, to base line960. At the same time, the first pattern column of the next sampling region910also arrives at base line960, thus initiating the above cycle process again. In one embodiment, this jumping-back action of the scan line start point is incorporated in compensation offset950. The compensation offset-applied scan of the next sampling region910then proceeds, as shown in scene6.

Referring toFIG. 9C, which is a schematic illustration of an alternative inspection mode operation in accordance with an embodiment of the present invention. As mentioned earlier, the sample (or say its pattern) being inspected could have sampling and skip regions of varying area sizes.

InFIG. 9C, a pattern901comprises a sampling region911, a first skip region921and a second skip region931. Sampling region911has an area size of AB. The first skip region921has an area size of BC which equals to AB. The second skip region931however, has an area size of DE which is three times of AB.

In this embodiment, the sample stage moves to the right at a varying speed. When region BD is being inspected, the stage moves at a speed of 2×. When the inspection is to be performed on region DF, the stage is set to move at a speed of 3×. At the same time, the compensation offset is set to vary accordingly, so as to deliver proper scan for both regions BD and DF.

It is noted that in this embodiment, information of the location of skip regions921and931on the sample is critical for calculating the varying alternative stage speeds and associate compensation offsets. This skip region location information may be obtained from the imaging recipe.

Referring toFIG. 9D, which is a schematic illustration of an alternative inspection mode operation in accordance with an embodiment of the present invention. The same pattern901as inFIG. 9Cis being inspected. The sample stage is selected to be moving at a speed of 3× i.e. the stage moves three pixel sizes to the right per line period time. The compensation offset is set to be951which is 2 pixel sizes to the right per line period time, as shown. The start point of scan lines thus tightly follows the moving stage, rendering formation of the successive scan lines on the corresponding successive pattern columns in sampling region911.

In this embodiment, one frame image is set to cover one sampling region911. Scenes1˜4illustrate the formation of a first frame image. Scenes5˜8illustrate the formation of a second frame image. As shown, in scene5, while the stage is fast moving to the right, the scan line start point is guided to jump back (to the left) a certain distance to catch the next sampling region911. And in scene9, the scan line start point is guided to jump back again to base line961. In one embodiment, these back-jumping actions are incorporated in compensation offset951.

Referring toFIG. 9E, which is a schematic illustration of an alternative inspection mode operation in accordance with an embodiment of the present invention. The same pattern901as inFIG. 9Cis being inspected. The sample stage is selected to be moving at a speed of 2× i.e. the stage moves 2 pixel sizes to the right per line period time. The compensation offset is set to be952which is 1 pixel size to the right per line period time, as shown. A base line961is used for illustrating the compensation offset952.

Scenes1˜4illustrate the formation of a first frame image. Scenes5˜8illustrate the formation of a second frame image. Scene13illustrates the beginning of the formation of a third frame image. Scenes9˜12illustrate another type of the imaging scan compensation offset, which is a delay of the scanning action. Different fromFIG. 9D, in this embodiment the stage is moving at a slower speed.

As a result, in scene9when the next scan (i.e. for the formation of the third frame image) is due, the third sampling region911has not been able to make it to base line961. A possible method to help in such situation is to set compensation offset952to allow a larger jump-back distance for the scan line start point. However, the increase in this distance is limited by the deflector's physical design as the scanning beam is guided by the deflector to scan the designated position on the sample.

Therefore, another method is used in this embodiment, which is to delay the scanning action for a certain period of time. This is illustrated in scenes9˜12where no new scan line is formed at all. For example, the scanning beam can be idled or blanked, waiting the next sampling region911to come closer to base line961. Then, the scanning action is resumed and the next scan line is formed in scene13, initiating the formation of the third frame image. In one embodiment, this delayed scanning action is incorporated in compensation offset952.

It is noted that although the above embodiments described in conjunction withFIG. 9A˜9Eare designed to handle a pattern having a constant sampling region area along with a varying skip region area, it would be obvious to those skilled in the art that alternatives of the above designs can also be applied to scenarios where the sampling region area or both the sampling and skip region areas are changing as well.

Referring toFIG. 10, which is a schematic illustration of throughput of the default and alternative inspection mode operation in accordance with an embodiment of the present invention. As shown, the alternative mode inspection is performed in a scan-skip-scan fashion whereby only the sampling regions are scanned and imaged. The default mode on the other hand, equally scans and images both the sampling and skip region.

It is noted that in one embodiment of the present invention, the default mode inspection is substantially equivalent to the conventional art inspection operation. A processed image1001and1002from the alternative and default mode, respectively, are illustrated for comparison. It can be seen that the time needed to form 3 frames of image is shorter in the alternative mode than in the default mode.

The proposed method can be practiced in the form of an executing code or a computing program. The code or program can be implemented in pure hardware, firmware, pure software, or combination thereof. For example, the method may be coded in an embedded computing device or as part of a control system. Or the method can be written in a software application and run on a compatible computing device such as a mainframe host, terminals, personal computers, any kind of mobile computing devices or combination thereof.

In another embodiment, a control module of a charged particle beam inspection system is disclosed, which contains a computer readable medium encoded with the proposed method. This control module is at least coupled to a beam probe deflector module and a sample stage of the charged particle beam inspection system. The beam probe deflector module is used to scan a primary charged particle beam probe across the surface of a sample to be imaged and inspected. The sample sits on the sample stage, and during the charged particle beam inspection the sample stage moves at a certain designated speed while the charged particle beam probe scans the sample.

The control module is implemented such that it is able to perform the proposed method (as described earlier in conjunction withFIG. 4˜FIG10) of the present invention. In one embodiment, the control module is coupled to the beam probe deflector module and the sample stage through a medium selected from a group consisting of the following, or any combination thereof: cable wire, optical fiber cable, portable storage media, IR, Bluetooth, intranet, internet, wireless network, wireless radio, etc.

The control module first receives an imaging recipe loaded into the inspection system. The imaging recipe at least comprises information of the area of a sampling and skip region on the sample. A default stage speed can be calculated in advanced and be included in the imaging recipe received by the control module, or it can be calculated by the control module. The control module then calculates an alternative stage speed and corresponding imaging scan compensation offsets in accordance with the default stage speed and the received imaging recipe. The generation of the alternative stage speed and imaging scan compensation offsets can be based on the proposed method of the present invention as described earlier and will not be repeated here.

The control module then determines whether an alternative inspection mode will be activated. For example, an activation signal can be inputted by the user of the charged particle beam inspection system through a user input means or interface to indicate activation of the alternative mode operation. If the alternative inspection mode is determined not to be activated, the control module generates for example a first control signal to the beam probe deflector module and the sample stage to cause the stage to move at the default stage speed and the deflector module to guide the motion of the primary charged particle beam probe in accordance with the imaging recipe, such that the charged particle beam probe equally scans the and images the sampling and skip regions on the sample.

On the contrary, if the alternative mode is determined to be activated, the control module generates a second control signal to the beam probe deflector module and the sample stage to cause the stage to move at the alternative stage speed and the deflector module to guide the motion of the primary charged particle beam probe in accordance with the imaging scan compensation offsets, such that the charged particle beam probe tightly follows the motion of the sample stage and scans only the sampling regions on the sample. It is noted that the detailed example variations, equivalents and alternatives of the alternative stage speed and compensation offsets that can be used by the control module to carry out the alternative inspection mode have been described in previous embodiments and will not be repeated here.