Method and apparatus to correct for stage motion in E-beam inspection

One embodiment disclosed relates to an apparatus for inspecting a substrate. The apparatus includes at least an illumination system, a stage, a multiple-pixel detector, an imaging system, a deflector, and a deflector controller. The illumination system is configured to expose at least a portion of the substrate to an incident beam which causes said portion to emit radiation. The stage holds the substrate and moves the substrate relative to the beam during said exposure of the substrate. The imaging system images the emitted radiation onto the multi-pixel detector, which includes an array of detector elements configured to detect the emitted radiation. The deflector is configured to deflect the emitted radiation under control of the deflector controller. The deflection is controlled so as to compensate for the motion of the substrate relative to the beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to automated inspection methods and apparatus in semiconductor manufacturing and the like.

2. Description of the Background Art

It is desirable to improve the quality of image data obtained during the automated inspection of manufactured substrates such as semiconductor wafers.

SUMMARY

One embodiment of the invention pertains to an apparatus for inspecting a substrate. The apparatus includes at least an illumination system, a stage, a multiple-pixel detector, an imaging system, a deflector, and a deflector controller. The illumination system is configured to expose at least a portion of the substrate to an incident beam which causes said portion to emit radiation. The stage holds the substrate and moves the substrate relative to the beam during said exposure of the substrate. The imaging system images the emitted radiation onto the multi-pixel detector, which includes an array of pixel detector elements configured to detect the emitted radiation. The deflector is configured to deflect the emitted radiation under control of the deflector controller. The deflection is controlled so as to compensate for the motion of the substrate relative to the beam.

Another embodiment of the invention pertains to a method of inspecting a substrate. At least a portion of the substrate is exposed to an incident beam, which causes said portion to emit radiation. During the exposure, the substrate is moved relative to the incident beam. The emitted radiation is deflected and imaged onto at least one detector. The deflection compensates for the relative motion and causes emitted radiation for each image pixel to accumulate at a corresponding detector element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1is a flow chart depicting a method of inspecting a substrate in accordance with an embodiment of the invention.

FIG. 2is a flow chart depicting synchronization of the deflection of emitted radiation in accordance with an embodiment of the invention.

FIG. 3is an example of a timing diagram for deflecting the emitted radiation in accordance with an embodiment of the invention.

FIGS. 4A and 4Bare illustrations of a correspondence of image pixels to detector elements before and after a transition in accordance with an embodiment of the invention.

FIG. 5is a schematic diagram of an apparatus for inspecting a substrate including a single detector in accordance with an embodiment of the invention.

FIG. 6is a flow chart depicting a feedback loop to control the motion-compensating deflection in accordance with an embodiment of the invention.

FIG. 7is a schematic diagram of a stage including an attachment in accordance with an embodiment of the invention.

FIG. 8is a flow chart depicting the use of multiple detectors in accordance with an embodiment of the invention.

FIG. 9is a schematic diagram of an apparatus for inspecting a substrate including, multiple detectors in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Present methods of automated inspection have various disadvantages. For example, in systems using a time delay integration (TDI) sensor, the accumulated charges corresponding to image pixels are typically moved from one row to the next in discrete increments. However, the motion of the substrate under inspection may be continuous, not discrete. This results in undesirable blur of the image because the movement of the image pixels and the transfer of charge within the TDI sensor are not precisely synchronized.

FIG. 1is a flow chart depicting a method of inspecting a substrate in accordance with an embodiment of the invention. At least a portion of the substrate is exposed102to an incident beam, which causes said portion to emit radiation. The emitted radiation may comprise, for example, emitted electrons. In one example, the emitted electrons may comprise secondary electrons. In another example, the emitted radiation may comprise incident electrons that are mirrored from said portion of the substrate.

During the exposure, the substrate is moved104relative to the incident beam. In one example, the substrate may be moved upon a stage. The stage may have some vibration motion in addition to a translation motion.

The emitted radiation is deflected106and imaged108onto at least one detector. The detector may comprise an array of pixel detector elements. In one example, the detector may comprise a time delay integration (TDI) detector. In another example, the detector may comprise one of a plurality of charge coupled device (CCD)-based detectors.

The deflection106compensates for the relative motion between incident beam and substrate and causes emitted radiation for each image pixel to accumulate at a corresponding detector element. Advantageously, this increases the resolution of the resulting image by preventing unnecessary blurring between pixels. Otherwise, the relative motion would cause a portion of the emitted radiation that should be detected by a certain detector element to be detected by one or more neighboring detector elements.

FIG. 2is a flow chart depicting synchronization of the deflection of emitted radiation in accordance with an embodiment of the invention. The synchronization method shown inFIG. 2is particularly applicable when a TDI detector is utilized.

As the substrate moves relative to the incident beam, motion-compensating deflection202of emitted radiation occurs for a period of time. In other words, the emitted radiation is deflected for a period of time to compensate for the relative motion between the incident beam and the substrate. The motion-compensating deflection202causes emitted radiation for the image pixels to accumulate at corresponding detector elements. The duration for the period of time depends on the speed of the relative motion and on the deflection capabilities and other parameters of the apparatus. The duration should be less than the maximum amount of time that the apparatus can deflect the emitted radiation so as to maintain a static correspondence between image pixels and detector elements.

After the period of time ends, a transition occurs. During the transition two things occur. The accumulated charges are shifted204from one row of detector elements to a next row of detector elements. This effectively shifts the image by one row on the array of detector elements. In addition, the deflection is shifted206by one row in synchronization with the charge shift. With both charge and deflection shifted by one row, the process loops back and the next period of time for the motion-compensating deflection202begins.

FIG. 3is an example of a timing diagram for deflecting the emitted radiation in accordance with an embodiment of the invention. The timing diagram ofFIG. 3is particularly applicable when a time delay integration (TDI) detector is utilized. The timing diagram plots a magnitude of the deflection of the emitted radiation versus time. A deflector control signal may follow, or be similar to, the sawtooth-shaped plot in the timing diagram.

The motion-compensating deflection202is shown in linear form in the example of FIG.3. However, the motion-compensating deflection202may also have non-linear components or characteristics, depending on the particular configuration of the apparatus. At the end of each period of time, the deflection undergoes a transition during which the deflection is shifted206by the aforementioned one row of detector elements. After the shift206by one row, a following time period for the motion-compensating deflection202begins.

FIGS. 4A and 4Bare illustrations of a correspondence of image pixels to detector elements before and after a transition in accordance with an embodiment of the invention. The illustrations ofFIGS. 4A and 4Bare particularly applicable to a TDI detector. As an image is scanned and charge is integrated with a TDI detector, accumulated charges are shifted from one row to the next in discrete increments. In other words, as the image pixels move from one row to the next, the accumulated charges are also transferred to the next row.

For reasons of simplification, the illustrations only depict a 5×5 array of detector elements. Of course, a practical detector would generally include a much larger array. In these figures, the detector elements are represented by the 5×5 grid of squares. The corresponding image pixels are represented by the symbols in the squares.

Prior to the transition (i.e. during one period of the motion compensating deflection202), the correspondence is shown by FIG.4A. Image pixels labeled D1through D5are shown to correspond to the first row (actually column in the illustration) of detector elements. Similarly, image pixels E1through E5correspond to the second row of detector elements, image pixels F1through F5correspond to the third row of detector elements, image pixels G1through G5correspond to the fourth row of detector elements, and image pixels H1through H5correspond to the fifth row of detector elements. In other words, the charge accumulating in the first row of detector elements correspond to image pixels D1through D5. The charge accumulating in the second row of detector elements correspond to image pixels E1through E5. And so on.

After the transition (i.e. during a next following period of the motion-compensating deflection202), the correspondence is shown by FIG.4B. As shown, the correspondence has been shifted204to the left by one row. As shown inFIG. 4B, image pixels labeled E1through E5are now shown to correspond to the first row (actually column in the illustration) of detector elements. Similarly, image pixels F1through F5correspond to the second row of detector elements, image pixels G1through G5correspond to the third row of detector elements, image pixels H1through H5correspond to the fourth row of detector elements, and image pixels11through15correspond to the fifth row of detector elements. In other words, the charge accumulations corresponding to the image pixels have been shifted204to the left by one row.

The accumulated charge for the image pixels D1through D5are shifted to a row of detector elements (not illustrated) to the left of the first row. The accumulated charge for the image pixels11through15came from a row of detector elements (not illustrated) to the right of the fifth row. Hence, in order for an array of detector elements to be practical, it should include a sufficient number of extra rows to allow the charge accumulations to be shifted until the integration for an image is complete.

FIG. 5is a schematic diagram of an apparatus for inspecting a substrate including a single detector in accordance with an embodiment of the invention. The apparatus depicted comprises a direct (rather than scanned) electron beam instrument. The elements depicted include an electron gun502, a beam separator504, an objective lens506, a surface of a sample substrate508, a deflector510, a deflector controller512, and a detector514.

The electron gun502supplies electrons that illuminate the sample or regions thereof. Advantageously, use of incident electrons allows observation of detail much smaller than a light optical microscope can resolve.

The beam separator504may comprise, for example, a magnetic prism. The beam separator504directs the incident electrons towards the substrate508, and the objective lens506(which for example may comprise a magnetic lens) focuses the incident electrons onto a portion of the substrate508. The incident electrons causes radiation in the form of secondary electrons or mirrored electrons to be emitted from the portion illuminated. The magnetic prism or other beam separator504deflects the emitted radiation in a direction that separates the emitted radiation from the incident beam.

A series of substrates508may be continuously moved under the incident beam in an inspection system or in similar such systems. In such systems, the substrate508under inspection may move relative to the incident beam. To compensate for the relative motion, the deflector510deflects the emitted radiation by a controlled amount prior to the emitted radiation being detected. The deflector510may comprise, for example, a magnetic or an electrostatic deflector.

A deflector controller512is coupled to the deflector510. The deflector controller512transmits one or more control signals to the deflector510so as to control the magnitude (and, in certain embodiments, the direction) of the deflection. The deflector controller512may receive a movement data signal513that provides information on the motion of the substrate508relative to the incident beam. The movement data signal513may be derived, for example, using an interferometer to track the movement of the substrate.

The detector514receives the emitted radiation for imaging and generates image data therefrom. The detector514may comprise, for example, a TDI detector.

FIG. 6is a flow chart depicting a feedback loop to control the motion-compensating deflection in accordance with an embodiment of the invention. The positional movement of the stage on which the substrate travels is tracked. For example, the tracking may be performed using an interferometer to detect602the positional movement of the stage. Data from the tracking of the positional movement is fed back604to the deflector controller. The emitted radiation is deflected606using the feedback control data to determine the magnitude (and in some embodiments, the direction) of the deflection. In embodiments where the incident beam also moves, data on the movement of the incident beam may also be fed into the deflector controller. In other embodiments, the incident beam may be stationary.

In some embodiments, there is vibration in the stage as it moves. The tracking data may include such vibrational movement such that the deflection606may also compensate for the stage vibration.

FIG. 7is a schematic diagram of a stage including an attachment in accordance with an embodiment of the invention. The wafer702or other substrate is held by the stage704. In order to inspect a series of substrates, such a stage may move laterally under the incident beam. An interferometry device706may be attached to the stage704. One or more such interferometry attachments706may be utilized to track the stage motion. The attachment706may comprise, for example, a laser for the interferometry. In other examples, the attachment706may comprise a mirror or prism to redirect the laser beam used for the interferometry.

FIG. 8is a flow chart depicting the use of multiple detectors in accordance with an embodiment of the invention. The method ofFIG. 8is particularly applicable when a charge coupled device (CCD) detector is being utilized because there is typically a delay when data from the CCD array is being read out. The method should also be advantageous when utilized with other types of detectors with similar read-out delay characteristics.

As the substrate moves relative to the incident beam, motion-compensating deflection802of emitted radiation to an active detector occurs for a period of time. In other words, the emitted radiation is deflected802to the active detector for a period of time to compensate for the relative motion between the incident beam and the substrate. The motion-compensating deflection802causes emitted radiation for the image pixels to accumulate at corresponding detector elements. The duration for the period of time depends on the speed of the relative motion and on the deflection capabilities and other parameters of the apparatus. The duration should be less than the maximum amount of time that the apparatus can deflect the emitted radiation so as to maintain a static correspondence between image pixels and detector elements.

While the motion-compensating deflection802is occurring to the active detector, data is being read out804from a currently inactive detector (that previously was the active detector). This advantageously circumvents the read-out delay characteristic of the CCD array.

After the period of time ends, a transition occurs. Here, the transition to involves making the active detector to be inactive, and making an inactive detector (perhaps, but not necessarily the one from which data is being read out) to be the active detector. In other words, the active detector is changed806by shifting the deflection of the emitted radiation to a different one of the multiple detectors. Thereafter, the process loops back and the next period of time for the motion-compensating deflection802and data read-out804begins.

FIG. 9is a schematic diagram of an apparatus for inspecting a substrate including multiple detectors in accordance with an embodiment of the invention. The apparatus depicted comprises a direct (rather than scanned) electron beam instrument. The elements depicted include an electron gun502, a beam separator504, an objective lens506, a surface of a sample substrate508, a deflector910, a deflector controller912, and two or more detectors914. The primary differences between the apparatus of FIG.5and the apparatus ofFIG. 9comprises the deflector910, the deflector controller912, and the two or more detectors914. The illustration ofFIG. 9shows two detectors914A and914B, but additional detectors may be utilized in other embodiments. These detectors may comprise CCD-based detectors or detectors with similar read-out delay characteristics.

Similar to the apparatus ofFIG. 5, the electron gun502supplies electrons that illuminate the sample or regions thereof. The beam separator504may comprise, for example, a magnetic prism. The beam separator504directs the incident electrons towards the substrate508, and the objective lens506(which for example may comprise a magnetic lens) focuses the incident electrons onto a portion of the substrate508. The incident electrons causes radiation in the form of secondary electrons or mirrored electrons to be emitted from the portion illuminated. The magnetic prism or other beam separator504deflects the emitted radiation in a direction that separates the emitted radiation from the incident beam. A series of substrates508may be continuously moved under the incident beam in an inspection system or in similar such systems. In such systems, the substrate508under inspection may move relative to the incident beam.

The apparatus ofFIG. 9utilizes multiple detectors914to speed up the process, and so requires the imaging system to be configured differently than in FIG.5. In particular, the deflector910should have a wider range of deflection capability so as to be able to deflect the emitted radiation from one detector to another detector. The deflector controller912should be configured so as to implement the method described above in relation to FIG.8.