Implementation of an autofocus sensor for a multi-column microscope

Abstract

An array of localized autofocus sensors allows for direct measurement of the working distance between each microscope column in the array and the substrate being imaged below. The autofocus sensors measure the working distance between each column and the substrate being imaged as the column passes over a point on the substrate being imaged. The working distance measurements from the sensors are input to a control system, which then outputs any necessary working distance adjustments to the microscope columns. The control system independently adjusts the working distance and / or physical distance of each microscope column of the multi-column microscope based on the autofocus sensor input. Individual microscope columns of the multi-column microscope may also be used as autofocus sensors.

Description

[Technical Field]

[0001] The present disclosure relates generally to the field of wafer inspection systems, and more particularly to multi-column electron beam inspection systems. [Background technology]

[0002] Generally, the semiconductor manufacturing industry involves highly complex techniques for forming integrated circuits using patterned semiconductor materials deposited on a substrate, such as silicon. Due to the increasing circuit integration and miniaturization of semiconductor devices, the devices formed are becoming increasingly sensitive to defects. That is, the defects that cause device failures are becoming smaller and smaller. Devices must be substantially free of defects before they are shipped to end users or customers. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2020 / 097137 [Patent Document 2] U.S. Patent Application Publication No. 2007 / 0095805 Summary of the Invention [Problem to be solved by the invention]

[0004] The semiconductor industry requires high-resolution microscopes for defect inspection of patterned wafers and mask reticles. To increase scanning speed and inspection throughput, multiple microscope columns can be arranged in an array to perform parallel imaging of the desired substrate.

[0005] As multi-column microscopes are increasingly being developed and integrated into functional inspection tools, particularly for scanning electron microscopes (SEMs), a major challenge remains maintaining each column in proper focus on a common inspection plane.

[0006] To achieve a functional multi-column inspection tool, an autofocus system is needed that can provide localized working distance measurements at the inspection site between each column and the substrate being imaged. An additional challenge in SEMs is that substrate charging during imaging can cause a misalignment between the effective focal length and the physical working distance. An ideal autofocus system for use in a multi-column SEM would be able to compensate for substrate charging. Therefore, a system and method is needed to implement an autofocus sensor for use in a multi-column microscope that is suitable for maintaining each active microscope column in focus. [Means for solving the problem]

[0007] The following presents a simplified summary of the disclosure in order to provide a basic understanding of certain embodiments of the disclosure. This summary is not an extensive overview of the disclosure, nor is it intended to identify key / critical elements or delineate the scope of the disclosure. Its sole purpose is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

[0008] Aspects of the present disclosure relate to systems and devices. The systems and devices include a multi-column array of scanning electron microscopes (SEMs). The systems and devices also include an array of autofocus sensors aligned with the multi-column array of scanning electron microscopes. The systems and devices also include a controller. Finally, the systems and devices also include a stage configured to hold an imaging sample and move it in a strip in multiple directions.

[0009] In some embodiments, the array of autofocus sensors is configured to enable simultaneous measurement of working distances or positions between multiple microscope columns and the substrate being imaged. In some embodiments, the controller is configured to implement a feedback control loop that inputs and outputs autofocus sensor signals to adjust either the column focal length or the column-to-sample working distance necessary to enable each active imaging column of the multi-column microscope to maintain proper focus on the imaging sample. In some embodiments, the controller is configured to enable each autofocus sensor in the array of autofocus sensors to independently measure the column-to-sample working distance. In some embodiments, the controller is configured to enable each autofocus sensor in the array of autofocus sensors to measure the column-to-sample working distance in parallel with the other autofocus sensors. In some embodiments, the controller is configured to enable each column in a multi-column array of SEMs to independently adjust the column-to-sample working distance. In some embodiments, the controller is configured to enable each column in a multi-column array of SEMs to adjust the column-to-sample working distance in parallel with the other columns in the multi-column array of SEMs. In some embodiments, the controller is configured to enable each column of a multi-column array of SEMs to function as an autofocus sensor that measures an active working distance to an imaging sample between image scans. In some embodiments, one row of autofocus sensors is positioned between two rows of SEMs. In some embodiments, the controller is configured to move the stage with multiple degrees of freedom.

[0010] These and other aspects of the present disclosure are described in detail below with reference to the drawings. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic top view illustrating an m×n array of microscope columns in a multi-column microscope according to embodiments of the present disclosure. [Figure 2A] FIG. 1 is a schematic side view illustrating an individual SEM column of a defocused multi-column microscope according to an embodiment of the present disclosure. [Figure 2B] FIG. 1 is a schematic side view showing individual SEM columns of a multi-column microscope in focus according to an embodiment of the present disclosure. [Figure 3A] FIG. 1 is a block diagram of the basic operation of a control system according to several embodiments of the present disclosure. [Figure 3B] FIG. 1 is a block diagram of an alternative control system according to embodiments of the present disclosure. [Figure 3C] FIG. 1 is a block diagram of a control system using a combined autofocus scheme according to an embodiment of the present disclosure. [Figure 4] FIG. 1 is a block diagram illustrating an example of multi-column microscope scanning of a patterned wafer on a moving stage in a strip according to embodiments of the present disclosure. [Figure 5] FIG. 1 is a block diagram illustrating an example of a computer system configured in accordance with one or more embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0012] In the following description, numerous specific details are presented to provide a thorough understanding of the present disclosure. The present disclosure may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail in order to not unnecessarily obscure the present disclosure. While the present disclosure will be described in conjunction with specific embodiments, it will be understood that it is not intended to limit the disclosure to these embodiments.

[0013] Single-column microscope systems typically have a single column overlooking the wafer or reticle. A method for loading the inspection sample onto the microscope's focal plane is required. In a single-column system, the wafer can be perfectly vertical because the stage is movable and only one focal point needs to be focused on in a two-dimensional plane. However, focusing a multi-column system, such as 10 columns in a 5x2 array, is a more complex problem. This is because with 10 focal points, the mechanical tolerances of these focal points mean that they will never be coplanar. Even if the system can control tip and tilt as well as Z-axis control of the stage, it is currently impossible to perfectly focus 10 points on the same plane if the wafer is perfectly flat. Furthermore, if the wafer itself warps, the wafer will never be flat, making it even more difficult to focus 10 microscope columns on the stage.

[0014] Autofocus sensors currently used in inspection microscopes rely on non-contact methods to measure the distance or displacement between the sensor and the substrate to prevent physical damage to the sample being imaged, typically a patterned wafer or reticle. Additionally, current inspection microscopes rely on remote placement of the autofocus sensor. There are two typical configurations for remote autofocus sensors:

[0015] The first configuration of a remote autofocus sensor implements a single distance or displacement sensor. The sensor must be remotely mounted to avoid physical interference with the microscope column. Laser-based optical sensors are typically used because of their long measurement distance and remote mounting capabilities. A typical autofocus sensor projects a laser beam onto an imaging surface at a low angle of incidence. The point where the laser beam strikes the imaging surface defines the sensor's measurement position. The laser beam is reflected from the imaging surface and strikes a position-sensitive diode (PSD). As the imaging surface is placed over a linear stage during the inspection process, any changes in the imaging surface height cause a change in the laser reflection angle, which is detected as a shift in the laser position on the PSD.

[0016] This technique is the operating principle of autofocus in single-column electron microscopes. The sensor's measurement target can be aimed at or near the imaging area of ​​the electron microscope, so only one measurement point is needed. The microscope's focus or working distance is adjusted to bring the sample into focus at the single measurement point.

[0017] The second configuration relies on multiple position or displacement sensors located remotely from the imaging location on the column. These sensors measure the distance to at least three points on the imaging plane or chuck. The sensor locations can be any distance away from the microscope's imaging point. The relative distances between the multiple sensor heads in the X and Y axes are known, and the imaging plane can be calculated using the height measured by the sensors in the Z direction. Focusing of the microscope can be achieved by focusing the microscope on the calculated imaging plane.

[0018] Because these sensors are located remotely from the imaging column, there is little physical interference between one imaging column and the sensor head. The sensors can be placed directly above the measurement target, facilitating integration. Also, the targeted measurement area on the substrate can be larger than the microscope's imaging spot area without compromising focus accuracy. In this case, electronic sensors such as capacitance or eddy current sensors are typically implemented.

[0019] A capacitance sensor creates a charge between the sensor head and the target substrate, and the sensor measures the capacitance as a function of the distance between the sensor head and the target.

[0020] Eddy current sensors generate an alternating current in a wire coil placed inside the sensor. This current generates an alternating magnetic field that induces smaller alternating currents (eddy currents) in the target. The eddy currents then generate a demagnetizing magnetic field. The induced magnetic field is proportional to the distance between the sensor head and the target.

[0021] Conventional microscope autofocus methods have been designed for single-column microscopes. However, they do not provide a suitable solution for multi-column microscopes. Because multi-column microscopes, especially multi-column SEMs, are still in the early stages of development, previously designed autofocus sensors are not designed to work with multi-column microscopes.

[0022] A typical autofocus sensor uses a laser that strikes the wafer surface at a low angle and is reflected off the wafer surface. The reticle on the wafer acts as a mirror. Because the angle of incidence is the angle of refraction, the laser is reflected at the same angle as the angle of incidence. The laser then strikes a position-sensitive diode, which correlates the beam's position with spatial coordinates (x, y). As the wafer rises, the laser strikes different points on the wafer, changing its reflection path. This change in path corresponds to a shift in the position of the position-sensitive diode. However, one challenge is that the laser requires a fairly long optical path.

[0023] These typical autofocus sensors are designed for single-column electron beam inspection tools. One advantage of this type of sensor is that the laser light path allows the laser source and sensor to be remotely located outside the vacuum chamber. However, as mentioned above, the long light path requires a lot of space, which is fine for a single column but is impractical for multi-column systems due to interference.

[0024] Another typical sensor is the capacitance sensor. A capacitance sensor is an electrode placed above the target being measured (e.g., a wafer). The electrode applies a voltage to create a capacitance across the wafer on the sensor head. The air gap acts as an insulator, causing the capacitance to change as the distance changes. However, the challenge with capacitance sensors is that the capacitor and the wafer must be at the same bias for the sensor to function. For optical instruments, capacitance sensors work well because the bias can be at ground potential. However, for scanning electron microscopes, the bias on the wafer can be as high as 5,000 volts, a voltage too high for capacitance sensor designs.

[0025] Another option is to use an interferometer, which is another laser measurement system. The light source can be remotely located from the system. The interferometer illuminates the sample directly below, and the reflected light path illuminates the sensor directly above. However, interferometers can be quite large. Also, interferometers are relative distance measurement systems, not absolute distance measurement systems. In other words, they provide relative changes and work well until a missing data point occurs. If a data point is missing for some reason, the system loses track of where the zero point is. On patterned wafers and reticles, the wafers have pattern features on the order of tens to hundreds of nanometers, the same scale as the optical wavelength of the laser, and interference can occur. As systems scan the wafer or reticle, often using optical position sensors, missing data points occur if the patterned wafer interferes with the sensor. Missing data points result in measurements being discarded because there is no baseline. Therefore, absolute distance measurement sensors are ideal for column systems.

[0026] The drawbacks of conventional autofocus methods depend on the exact sensor configuration implemented, with the specific drawbacks of each configuration detailed below.

[0027] A single remotely mounted sensor requires a long optical path that is not physically aligned with the array of microscope columns. In the case of a single microscope column, a long optical path is advantageous to prevent physical interference between the light source / sensor unit and the microscope column, while still allowing distance measurements at or near the inspection site. A single microscope column with a reasonably long working distance (in the mm-cm range) typically does not physically interfere with the sensor's optical path. However, incorporating such a sensor into a multi-column array becomes difficult or impossible. In the case of a multi-column microscope where the columns are arranged in multiple array rows and columns, the columns obstruct the optical path.

[0028] The optical path of the sensor could potentially be matched with a multi-column microscope with a long working distance if it were configured as a single column array. However, to achieve multi-column microscopy, the column dimensions must be small enough to allow a meaningful number of inspection columns to be placed side-by-side on the same imaging substrate. For SEMs, columns with dimensions suitable for multi-column microscope arrays typically have a working distance of approximately 1 mm. The short working distance makes it impossible to incorporate laser-based optical sensors without the columns interfering with the sensor's optical path.

[0029] A second concern may be the reliability of optical sensor measurements. Patterned wafers or reticles typically exhibit topography and / or material composition variations on length scales of tens to hundreds of nanometers, a length scale similar to the wavelength of the sensor's incident laser (typically on the order of 200–1000 nm). Interference between the incident laser light and the substrate pattern can result in loss of signal to the PSD. Furthermore, substrate non-planarity, such as wafer warpage, can cause deviations in the reflection angle that are larger than those caused by topography variations alone. The sensor's reliance on oblique reflections can introduce errors into the sensor's measurements.

[0030] Multiple remotely mounted sensors are also problematic. A multiple remote sensor configuration cannot adequately focus multiple microscope columns, especially on non-planar substrates (such as warped wafers). A remote sensor array does not directly measure the distance to the point being imaged, but rather to a point on the imaging plane, and uses this information to calculate a theoretical, flat image plane. This system is sufficient for single-column microscopes. The focal point of the column can be approximated as a single point in space. By adjusting the working distance of the microscope column or the physical distance between the column and the imaging plane, a single focal point of the microscope can be placed at the plane of the object being imaged. Unfortunately, this focusing system does not work for multi-column microscope arrays. Each microscope column has its own focal point. Due to differences in mechanical tolerances in column manufacturing and installation, the column focal points are spaced apart in the X, Y, and Z directions. In arrays with more than three columns, the focal points may not all be coplanar. If the imaging sample is mounted on a stage with six degrees of freedom, the height, tip, tilt, and / or yaw of the stage can be adjusted to align the sample plane with the focal points of at most three columns. Furthermore, imaging substrates are never perfectly flat. Patterned wafers can have hundreds of microns of warp in height between the center and edge. No amount of sample stage height, tip, tilt, and / or yaw adjustment can correct for non-planarity of the measurement substrate. Multiple columns cannot be brought into focus using global autofocus measurements that simply approximate the imaging plane as the wafer plane.

[0031] All current autofocus sensors have an additional limitation in column-based charged particle microscopes, including but not limited to scanning electron microscopes (SEMs). When imaged in an SEM, samples can become charged due to their exposure to incident high-energy electrons from the probe beam. Charges accumulated on the sample's surface can interact with the incident SEM beam, resulting in the active working distance of the column differing from its physical working distance. In this case, conventionally implemented autofocus methods cannot address the mismatch between the active working distance and the physical working distance in an SEM.

[0032] It is worth noting that in a multi-beam system, the beams are focused to a narrow x-y region directly beneath the microscope column. Therefore, the field of view of a multi-beam system is on the order of 250 μm, which is small enough to approximate the focus using a single autofocus sensor. However, this multi-beam approximation technique does not work for multi-column systems, because the inter-column distances between the individual columns of a multi-beam system can be on the order of 300 mm.

[0033] The disclosed techniques and mechanisms provide an autofocus system that functions as a multi-column microscope. The system includes an array of local position or distance sensors. These sensors are positioned to align with the constituent microscope columns, allowing a point on a sample moving relative to the imaging columns to pass under or near both the position sensors and the imaging columns. The autofocus sensors are positioned so that each sensor scans at or near a target location to be imaged by its corresponding microscope column. Multiple microscope columns may share the same sensor, provided that the sensor measures at or near a location on a wafer to be imaged by any given microscope column of the multi-column microscope. FIG. 1 is a schematic top view of an m×n array of microscope columns in a multi-column microscope according to an embodiment of the present disclosure. The microscope array 100 shows microscope columns 104 arranged in a regular m array columns and n array rows. Each of the n array rows may have at least one corresponding autofocus sensor 102 located either among the m microscope columns 104 or outside the microscope array 100.

[0034] According to various embodiments, the position sensor may be any absolute position sensor or relative distance sensor capable of measuring the sample being measured. In an ideal configuration, the position sensor is a non-contact measurement sensor to avoid damaging delicate substrates during imaging. In some embodiments, the sensor should have a measurement resolution and measurement target area sufficient to allow the imaging sample (such as a patterned wafer) to remain within the working distance of the microscope column. In some embodiments, the measurement resolution of the sensor should be equal to or less than the depth of focus of the microscope column. Examples of suitable position or distance sensors include, but are not limited to, optical interferometers, optical confocal sensors, capacitance sensors, and / or eddy current sensors. In some embodiments, a suitable sensor may be an array of one type of position sensor or a combination of multiple types of sensors. According to various embodiments, an array of extremely small autofocus sensors is embedded within the microscope array. In such embodiments, as the stage moves and scans in a strip, each autofocus sensor measures a point on the wafer that falls under the microscope column within the inspection field. This is because it is not possible to place a sensor directly below the point where the wafer is scanning. Therefore, an autofocus sensor is placed adjacent to the microscope column to accurately measure the distance to the point under inspection, and the system can adjust the microscope focus during the delay time until the autofocused point is directly below the microscope point (called the "delay time"). In some embodiments, the idea is to embed an autofocus sensor in each row of the column, and at least one autofocus sensor in each column row. In other words, if the system is scanning in the y direction, the system needs at least one autofocus sensor between columns spaced apart in the y direction. Thus, each scan direction should have an autofocus sensor. In some embodiments, multiple autofocus sensors can be embedded between columns, as long as the autofocus sensors are embedded in the multi-column array itself.

[0035] 2A and 2B are schematic side views illustrating individual SEM columns of a multi-column microscope according to embodiments of the present disclosure. Figure 2A illustrates a wafer out of focus, i.e., the physical distance of the wafer is less than the column working distance. Figure 2B illustrates a wafer in focus, i.e., the wafer surface is located at the column working distance.

[0036] 2A-2B, column 200 includes an electron source 202 configured to emit electron beam 214. In some embodiments, electron source 202 is a tungsten hairpin or a tungsten hairpin using a thin wire to function as the electron source. In some embodiments, electron source 202 can be a thermal emitter, a cold field emitter, a pure thermal emitter, or any device that emits electrons.

[0037] According to various embodiments, the column 200 also includes an extractor / condenser 204. In some embodiments, the extractor / condenser 204 includes two elements in the same block shown in FIGS. 2A and 2B. In the particular example shown in FIG. 2A, the upper element is the first element, and the lower element is the second element. In some embodiments, the extractor 204 applies an electric field to extract electrons from the electron source 202. The electrons then pass through an opening in the extractor 204 and enter the condenser 204. In some embodiments, the width of the extractor opening is on the order of 100 microns. In some embodiments, the condenser 204 is another electrostatic element that includes another opening, but a potential is applied across the opening to direct the electrons passing through. In other words, in some embodiments, the electrons emerge from the electron source 202 at an angle but become more linear as they pass through the condenser 204.

[0038] According to various embodiments, column 200 includes a dual steering deflector 206. In some embodiments, dual steering deflector 206 includes two stacked octopole deflectors configured to steer the beam so that it strikes the next element, aperture stop 208. In some embodiments, dual steering deflector 208 can be an octopole, a dodecopole, or any type of pole, as long as the pole is capable of steering the electron beam through aperture stop 208.

[0039] In some embodiments, the aperture stop 208 is yet another element that defines the electron beam 214. In some embodiments, the aperture stop 208 is also referred to as a "beam-defining" aperture. In some embodiments, the resolution of the system is determined by the aperture stop 208. In some embodiments, the aperture stop 208 is unbiased, i.e., typically at ground potential, and acts as an anode for the entire top of the column 200. In other words, the top element serves to extract, concentrate, and direct the electron beam 214 through the aperture stop 208. Typically, the aperture stop 208 is much smaller than any other element in the column 200. In some embodiments, the width of the aperture stop 208 is on the order of 1-100 microns. According to various embodiments, the aperture stop 208 is a critical element because it ultimately determines the system's resolution and beam current.

[0040] According to various embodiments, column 200 also includes a detector 210. In some embodiments, detector 210 is configured to face downward toward the sample, such that electrons emitted from electron source 202 pass through an aperture in detector 210 and travel down the column until striking sample 224. After the electrons strike sample 224, secondary and backscattered electrons are reflected upward and strike the active side of detector 210, which faces downward.

[0041] In some embodiments, column 200 also includes a dual scanning deflector 212. In some embodiments, dual scanning deflector 212 is similar to dual steering deflector 206, except that the function of dual scanning deflector 212 is to steer beam 214 across a field of view. In some embodiments, the field of view is the area across which electron beam 214 is raster scanned. In some embodiments, the field of view can be anywhere from 10 microns to 1 mm. Thus, the field of view can be changed by adjusting dual scanning deflector 212.

[0042] In some embodiments, dual steering deflector 206 directs beam 214, which may be a pencil beam (columned beam) of electrons, down through the column. Deflector 212 then deflects beam 214 to move past one edge or the other onto the wafer. As the electrons return, they spread out regardless of which part of the wafer beam 214 strikes. In some embodiments, the system is designed so that the optics completely cover detector 210 regardless of where on the wafer the pencil beam point strikes.

[0043] In some embodiments, beam 214 passes through magnetic objective lens 216. In some embodiments, the role of magnetic objective lens 216 is to focus the beam. In some embodiments, objective lens 216 is a fixed magnetic field lens. In such embodiments, lens 216 has only one focal length, which may be a fixed length of, for example, 2-3 mm. Although magnetic field lenses only focus to a single point, the lens is of high quality with low aberrations. In such embodiments, this is essential for achieving ultra-high resolution.

[0044] However, in some embodiments, the distance between the focal point and the wafer varies. Because static magnetic lenses only have one focal point, additional components are required. Therefore, in some embodiments, column 200 includes a post-lens deflector / dynamic focus 218. The role of post-lens deflector / dynamic focus 218 is to compensate for objects that magnetic lens 216 cannot focus. In other words, post-lens deflector / dynamic focus 218 works in conjunction with magnetic objective lens 216 to focus the beam onto the wafer. However, to focus beam 214 on wafer 224, the system needs to know the location of the wafer's surface. According to various embodiments, local autofocus sensor 228 functions to determine the location of the wafer's surface.

[0045] If the upper dual deflector bends the beam too far off the axis of the magnetic lens, the beam will twist, resulting in aberrations, astigmatism, and chromatic aberrations. Therefore, the system should not overdrive the upper dual steering deflector. However, to still maintain a large field of view, the system needs to further stiffen the beam, which is accomplished by the post-lens deflector 218. Additionally, the dynamic focus aspect 218 provides an entirely separate function. In some embodiments, the dynamic focus 218 selects the point at which the beam 214 is focused and moves it up or down. In some embodiments, the dynamic focus element includes electrodes to which voltage is applied. By varying the voltage, the system can move the focus up or down.

[0046] In some embodiments, the system utilizes a confocal optical absolute position sensor. The confocal sensor selects white light, focuses the white light, and refracts the white light to separate it into spectra. The confocal sensor focuses the different spectra at different distances from the lens. The wavelength of the light that is primarily reflected back determines the distance of the wafer.

[0047] Integrating an autofocus sensor into a scanning electron microscope poses certain challenges. One of the challenges is that scanning electron microscopes operate within a vacuum chamber. Therefore, sensors integrated locally into an array must be placed locally adjacent to a column within the array. Therefore, the system requires sensors that are small and fit snugly into the array. Because scanning electron microscopes operate in a vacuum, locally placed sensors must be vacuum compatible. This is because typical materials used in the assembly of electronic sensor packaging tend to outgas in vacuum systems. For example, many polymers have chemical binders that contain a range of small chains. These small chains are crosslinked using a solvent to form a polymer. Some of these solvents and small polymers will continually outgas if placed in a vacuum system, thereby contaminating the vacuum environment.

[0048] In some embodiments, all electronics within the light source on the sensor are external to the system. In some embodiments, light enters and exits the system through a fiber optic cable inserted through the vacuum chamber. The fiber optic cable then passes through the light source and leads to the sensor head. In some embodiments, the sensor head includes glass optics and a mirror that reflects the light. The light is then focused onto a surface and reflected. The reflected signal is sent through the fiber optic cable to the external electronics. In some embodiments, the fiber optic cable includes glass fibers. If the sensor is not located locally, another option is to install an autofocus sensor outside the vacuum chamber and provide several viewports looking into the chamber. However, the working distance of a confocal sensor is quite short, on the order of a few tens of millimeters, which requires the sensor to be located within the column array. In some embodiments, the glass viewports are sealed, allowing the sensor outside the chamber to shine light directly downward and reflect it outward.

[0049] Another difficulty is that the AF measurement distance is different from the physical distance of the column and the working distance of the column.

[0050] In some embodiments, the system measures distance d. In an ideal case, the sensor head would be precisely aligned with the end of the column, so the measured distance d would be the working distance. However, this may not occur, so the sensor's working distance d may be calibrated to be different from the column's working distance. Therefore, the autofocus sensor does not need to be installed at the same height as the column. In some embodiments, the system measures the autofocus sensor's distance d, which will most likely be different from the working distance. In some embodiments, calibration involves selecting a column, moving the wafer as close to the working distance as possible, and then moving the wafer up and down until the image is in focus. In other embodiments, the dynamic focus of the column is adjusted until the image is in focus. The autofocus sensor then measures distance d. Because the system knows the column is focused on the wafer, d is equal to the focus sensor distance plus an offset. The offset is then subtracted from the measurement.

[0051] In some embodiments, the displacement or distance measurements from the sensor can be used to calculate the physical distance between the column and the substrate. Even if the autofocus sensor is not perfectly aligned in the Z-axis, the offset value can be used to determine the physical distance between the microscope column and the substrate. The measured physical distance (Z) for a given measurement location (X,Y) on the substrate is input into a control system. When the measurement point represented by (X,Y) passes under the microscope column under inspection, the control system outputs an adjustment to bring the sample within the column working distance (as shown in Figure 2B).

[0052] A control system is required to control the autofocus sensor and electron beam column. Figures 3A-3C show three different examples of control systems that can be used to manage the autofocus sensor in the electron beam column. With an autofocus sensor and a small column, it can be difficult to measure distance precisely at the measurement or inspection point. For a standing wafer, the goal is to ensure that the system measures the distance to the inspection point on the wafer when the inspection point is directly below the autofocus sensor. Then, as the stage moves the inspection point under the inspection column, the system needs to store its height in the control system, which will later instruct the microscope to update the focus if the inspection point moves under the microscope.

[0053] As mentioned above, Figures 3A-3C show three different examples of control systems: Figure 3A shows a basic motion control system, Figure 3B shows a control system that utilizes a column as an autofocus sensor, and Figure 3C shows a control system that utilizes a combination of a dedicated sensor and a column.

[0054] 3A illustrates the basic operation of a control system diagram according to an embodiment of the present disclosure. As shown in FIG. 3A, in block diagram 300, control system 304 can output a signal to column 306 to adjust the working distance of the column so that the column focal range overlaps with sample stage 308. Alternatively, control system 304 can output a signal to sample stage 308, which can adjust its height, pitch, tilt, and / or yaw to bring the sample into the working distance range of column 306. In some embodiments, both adjustments may be combined.

[0055] According to various embodiments, Figure 3A shows a standard configuration. Autofocus sensor 302 measures the height of the wafer above sample stage 308 and stores the height as distance d. Control system 304 then selects the measured distance to track stage movement. When the inspection point is directly under microscope column 306, control system 304 sends a signal to one of columns 306 to adjust the dynamic focus to match the stored distance d of the wafer. Alternatively, control system 304 can send a signal to sample stage 308 to move the stage height up or down in the z-direction to focus on the wafer. However, stage movement is typically only used when all columns are disabled or the distance is extremely large.

[0056] FIG. 3B illustrates an alternative control system configuration according to an embodiment of the present disclosure. In some embodiments, as shown in FIG. 3B, the block diagram 320 does not use an autofocus sensor. Instead, the control system 324 uses the microscope column 326 itself as a position sensor array. In such embodiments, the SEM column 326 can scan the underlying sample 328 even when not actively imaging the sample. In such embodiments, the SEM column 326 can be used for through-focus measurements to determine optimal focus. By varying the focus from a nominal position as the imaging sample 328 is scanned, the edge definition of the image can be measured to determine proper focus. In this manner, the microscope column 326 itself can be used as the autofocus sensor. In some embodiments, all or part of the multi-column microscope column 326 can be used as the autofocus sensor. In this configuration, the microscope column measures the effective focal length required to achieve proper microscope focus, rather than simply the physical distance between the microscope column and the substrate, thereby determining the actual focus state of the SEM even in the presence of surface charging.

[0057] 3C illustrates a control system configuration using a combined autofocus scheme according to an embodiment of the present disclosure. In some embodiments, as shown in FIG. 3C, block diagram 340 illustrates a control system 344 that uses a local autofocus sensor 342 to obtain coarse / large corrections of the working distance to a sample 348, and also controls a column head 346 that uses through-focus measurements to make fine / small adjustments of the working distance.

[0058] In some embodiments, the components necessary for the operation of the autofocus sensor include an array of microscope columns. In such embodiments, the array of microscope columns may include at least a 1x2 (or 2x1) array, with no upper limit on the number of columns. In some embodiments, the components necessary for the operation of the autofocus sensor include an array of position or displacement sensors. In such embodiments, the array of sensors may include at least a 1x1 array, with no upper limit on the number of sensors. In some embodiments, the components necessary for the operation of the autofocus sensor include a sample to be imaged. In such embodiments, the sample may be, but is not limited to, a patterned wafer or a masked reticle, or any sample or combination of samples to be imaged by the microscope column array. In some embodiments, the components necessary for the operation of the autofocus sensor include a translation stage that allows relative motion between the sample and the microscope column array. In such embodiments, the stage may be a linear or rotary stage. Also in such embodiments, the stage may translate the sample to be imaged, the microscope column array, or a combination of both.

[0059] In some embodiments, the array of microscope columns is fixed in position relative to one another in the X, Y, and Z dimensions. In such embodiments, the sample is mounted on a linear stage that moves in the X and Y axes. The stage is capable of tip, tilt, and / or yaw movement in addition to Z movement. To coarsely focus the sample under the microscope columns, the physical working distance from the columns to the sample is adjusted by moving the stage's Z height (and tip, tilt, and / or yaw). To finely focus each column on the sample, the column working distance is adjusted by adjusting the column lenses.

[0060] In some embodiments, the inspection tool utilizes a stage that moves back and forth in a strip. The microcolumn and stage scan very quickly from left to right underneath. In some embodiments, the column scans the entire length of the wafer from left to right. Once the column has scanned the entire length, it increments a small amount (a predetermined amount) and then scans in the opposite direction. In this way, the column raster scans the entire wafer. In the strip direction, the stage typically moves very quickly to ensure good throughput. For example, a single column can typically scan at 50 mm / s. In some embodiments, the column scans at a constant velocity. If the stage is moving at a constant velocity, an autofocus sensor can be placed upstream in the strip direction. This allows the speed to be used to calculate the time it takes for the stage to move directly underneath the microscope. In some embodiments, the control system constantly tracks and records the stage height and current working distance.

[0061] In some embodiments, the control system 304 can individually adjust the working distance of the columns 308 by sending signals to the columns 308 to adjust the dynamic focus of each individual column, rather than moving the stage. In some embodiments, the control system 304 adjusts the tilt and tip of the stage 306, rather than adjusting the stage height. In some embodiments, the stage has six degrees of freedom: x, y, z, tip, tilt, and pitch.

[0062] In some embodiments, a similar effect can be achieved by mounting the microscope column on a stage. The microscope stage may be movable in the X, Y, and Z directions. Also, the sample stage need not be a linear stage, but may have one or more rotational or pivot axes.

[0063] In some embodiments, the system is configured to simultaneously adjust both the working distance (d) of the dynamic focus and the tip-tilt of the stage. As shown in FIG. 3B, the control system 324 can use the column 326 itself as a sensor (also known as a "through-focus"). Through-focus allows the column to be adjusted to move the dynamic focus above or below the focal point. The system can then search for the sharpest focused image and store that distance as the working distance. In some embodiments, using a scanning electron microscope involves multiple exposures of the wafer with an electron beam. In such embodiments, the wafer contains an insulating material, such as silicon dioxide or a dielectric, that charges the wafer. This charge then deflects the electron beam, effectively creating an electric field above the wafer, which affects the actual focus. Therefore, the actual focus of the column may be slightly above the surface. In other words, the focus may differ from the actual distance measured using an optical autofocus, which always measures the physical distance to the wafer. Therefore, using the column as a sensor in such cases may be advantageous.

[0064] FIG. 4 illustrates an example of a multi-column microscope scan of a patterned wafer on a stage that moves in a swath, according to several embodiments of the present disclosure. As the columns pass over the free areas between defects, they can be used as autofocus sensors to perform through-focus measurements. In FIG. 4, an exemplary system 400 includes an array of microscope columns 406 that, in a top view, focus directly on a wafer on a stage 402. In FIG. 4, the system 400 can scan the stage 402 with stage motion X 404 along the x-axis. In FIG. 4, the wafer 402 moves in a swath from left to right during the scan, while the autofocus sensor 410 measures the distance to points on the wafer that eventually pass under the microscope column 406. In such an embodiment, the system then records the distances to these points via a controller, and as the wafer scans directly under these points, the controller selects the distances and adjusts the focal distances of the columns to ensure that the height measurement distance from the autofocus sensor to the wafer is based on the most recent autofocus sensor distance measurement. According to various embodiments, the adjustment can be accomplished in several different ways. First, if the wafer is mounted on a stage and the stage is z-axis controlled, the controller can cause the stage to perform a pitch, tip, or tilt motion to focus the microscope on the wafer. Second, the controller can also change the focus of the microscope column by adjusting the electron optics or microscope optics via a dynamic focus element to focus the column on the wafer at these specific distances. Finally, the system can mount the microscope column on a z-axis motor, and the controller can send a signal to the motor to move the column up or down.

[0065] In some embodiments, the patterned wafer is mounted on a linear motion stage 402 with six degrees of freedom (6 DOF). To increase imaging speed, the stage moves 404 in a strip beneath the array of columns 406. The stage 402 continuously reciprocates along the X axis. As the microscope array 406 images the wafer passing underneath, an imaging line scan image 408 can be generated along the X axis. Once the line scan is complete, the stage increments along the Y axis and continues scanning until the entire area to be imaged beneath the column array is completed. In an SEM example, the column itself can deflect the beam to generate a raster scan image. The FOV of a beam deflection scan image is typically several hundred microns. In the exemplary system 400, as the stage 402 scans along the X axis, the beam can scan along the Y axis. Each strip of the stage movement along the X axis allows the beam to cover a narrow area along the Y axis. The beam-deflected microscope can therefore generate an area-scan image as the wafer stage moves in a strip along the X axis, as shown in FIG.

[0066] In some embodiments, a position or distance sensor 410 is positioned to align with each column 406 of the array in the X scan direction. As the wafer is scanned along a line in the X axis direction, an imaged point in the column scan region 412 passes under or near both position sensors and the imaging point of the microscope column 406, as shown in Figure 4. The sensor 410 measures the distance to the wafer as it passes under it and can calculate the height difference between the current and previous height measurements.

[0067] In some embodiments, a control loop can be used to maintain proper focus at each column in the array. Based on the stage speed and the physical separation / distance between the sensor measurement location on the wafer and the microscope column imaging location, the time difference between when the wafer passes under the sensor and when the wafer passes under the microscope imaging location can be determined. As the wafer passes under the microscope column, either an adjustment can be made to the working distance of the microscope or the physical distance between the column and the wafer. The former adjustment can be achieved by adjusting the focusing elements of the microscope column to change the focal length of the column. The latter adjustment can be achieved by changing the Z-height, tip, tilt, and / or yaw of the wafer stage relative to the microscope column (or the Z-height of the column relative to the sample, if the column is mounted on a stage).

[0068] In some embodiments, for microscopes that emit a focused beam of charged particles, such as scanning electron microscopes, charging the substrate can cause a shift in the effective focal length and physical working distance, in which case the shift in focal length and physical working distance can be applied to keep the column in focus.

[0069] Alternatively, in some embodiments, the microscope column itself can be used as the focus sensor. The microscope column can image either while actively scanning (between defects to be imaged) or when not actively scanning. A typical patterned wafer for inspection has an average of 100 defects, and as shown in FIG. 4, most of the wafer surface is free space available for focusing. The column 402 can be used for focusing using a through-focus technique, and the appropriate working distance can be determined by varying the working distance slightly from the nominal distance and measuring the sharpness of the imaged edges. In some embodiments, one advantage of this method for SEM is that both the measured working distance and the active working distance are the same even if the substrate is charged.

[0070] In some embodiments, these adjustments can be performed in parallel to maintain on-wafer focus for all active microscope arrays in the column. According to various embodiments, both a local dedicated autofocus sensor and the column as a through-focus autofocus sensor can be used in parallel. The dedicated autofocus sensor can be used for global working distance adjustments, such as adjusting sample stage height, tip, tilt, and / or yaw, while the column's through-focus measurements can be used to adjust the column's working distance.

[0071] In some embodiments, one advantage of the techniques and mechanisms of the present disclosure is that all microscope columns in a multi-column array can simultaneously maintain focus on a sample being imaged underneath. In some embodiments, another advantage is that all microscope columns in a multi-column array can simultaneously maintain focus on a non-planar sample, such as a patterned wafer having an arcuate shape. In some embodiments, another advantage is that all microscope columns in a multi-column array can simultaneously maintain focus on charged surfaces, which can cause the effective focal length to deviate from the physical working distance, particularly in the case of electron microscopes.

[0072] In some embodiments, individual microscope columns may be mounted on stages that allow the columns to move relative to the wafer (rather than the wafer moving relative to the column). Alternatively, both the column and wafer may be on stages and move relative to each other.

[0073] In some embodiments, the stage may be a linear stage or may have one or more rotational axes, for example, rather than a stage with linear X, Y, and Z axes of motion, the stage may have linear X and Z axes and a rotational axis in the theta Z axis.

[0074] In some embodiments, the sensor array may deviate from the sensor by one row in the array of microscope columns. In such embodiments, there may be fewer or more than one sensor per row of columns. In such embodiments, the primary requirement for the number and positioning of sensors is that each active column of the microscope array can acquire a sufficient number of measurement points to map the wafer in the X, Y, and Z axes so that it can remain in focus. For example, if each column has a depth of focus of ±50 nm and the wafer has only 100 nm of curvature from the center to the edge of the wafer, the sensors can be spaced more widely apart. In such embodiments, the spatial resolution only needs to be sufficient to capture the change in wafer height before the microscope columns move out of focus.

[0075] In some embodiments, the autofocus measurement can be performed sequentially with the microscope imaging / scanning. In such embodiments, the overall process time is slower, but the autofocus sensor does not need to be used in parallel with the microscope imaging / scanning. In such embodiments, the sensor may first scan and record a height map of the wafer, followed by the microscope imaging / scanning. The pre-recorded height map may be used to adjust the focus during the subsequent imaging / scanning. Furthermore, the autofocus sensor may be a separate tool from the multi-column imaging / scanning microscope that records the height map.

[0076] The above-described embodiments present various features utilizing an autofocus sensor system including a computer system or computer. However, embodiments of the present disclosure may include all or various combinations of the above-described features. FIG. 5 illustrates an example computer system according to embodiments of the present disclosure. According to certain embodiments, a system 500 suitable for implementing certain embodiments of the present disclosure includes a processor 501, memory 503, an interface 511, and a bus 515 (e.g., a PCI bus or other interconnect fabric). When operating under control of appropriate software or firmware, the processor 501 is responsible for implementing applications such as an operating system kernel, containerized storage drivers, and one or more applications. Various specially configured devices may be used instead of or in addition to the processor 501. The interface 511 is typically configured to send and receive data packets or data segments over a network.

[0077] Specific examples of supported interfaces include Ethernet interfaces, Frame Relay interfaces, Cable interfaces, DSL interfaces, Token Ring interfaces, etc. Also, various super-high speed interfaces may be provided, such as Fast Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces, POS interfaces, FDDI interfaces, etc. Generally, these interfaces may include ports suitable for communication with the appropriate media. In some cases, they may include a separate processor, and in some instances, volatile RAM. The separate processor may control communication-intensive tasks such as packet switching, media control and management, etc.

[0078] According to various embodiments, system 500 is a computer system configured to manage an autofocus sensor system as described herein. In some implementations, one or more computer elements may be virtualized. For example, a physical server may be configured locally or in a cloud environment. The physical server may implement one or more virtual server environments in which the autofocus sensor system executes. While a specific computer system has been described, it should be recognized that various alternative configurations are possible. For example, modules may be implemented on separate devices connected to the computer system.

[0079] The particular embodiments of the present disclosure presented herein are generally directed to the field of electron beam columns and are not limited to the hardware, algorithmic / software implementations and architectures and use cases outlined above.

[0080] Although the foregoing disclosure has been described in some detail for purposes of ease of understanding, it will be apparent that certain changes and modifications may be made within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatuses of the present disclosure. Therefore, the present embodiments should be considered illustrative and not restrictive, and the disclosure is not limited to the details presented herein.

Claims

1. a multi-column array of scanning electron microscopes (SEMs); an array of autofocus sensors aligned with the multi-column array of the scanning electron microscope; A controller; A system including a platform configured to hold and move an imaging sample in a strip in multiple directions.

2. The system of claim 1 , wherein the array of autofocus sensors is configured to enable simultaneous measurement of working distances or positions between multiple microscope columns and substrates to be imaged.

3. 2. The system of claim 1, wherein the controller is configured to implement a feedback control loop that inputs an autofocus sensor signal and outputs a signal to adjust either a column focal length or a column-to-sample working distance required to enable each active imaging column of the multi-column microscope to maintain proper focus on the imaging sample.

4. 10. The system of claim 1, wherein the controller is configured to enable each autofocus sensor in the array of autofocus sensors to independently measure a working distance from a column to a sample.

5. 2. The system of claim 1, wherein the controller is configured to enable each autofocus sensor in the array of autofocus sensors to measure a working distance from a column to a sample in parallel with other autofocus sensors.

6. The system of claim 1 , wherein the controller is configured to enable each column of the multi-column array of the SEM to have an independently adjustable column-to-sample working distance.

7. 10. The system of claim 1, wherein the controller is configured to enable each column of the multi-column array of the SEM to adjust a column-to-sample working distance in parallel with other columns of the multi-column array of the SEM.

8. 10. The system of claim 1, wherein the controller is configured to enable each column of the multi-column array of the SEM to function as an autofocus sensor to measure an active working distance to the imaging sample between image scans.

9. The system of claim 1 , wherein one row of autofocus sensors is positioned between two rows of SEMs.

10. The system of claim 1 , wherein the controller is configured to move the platform in multiple degrees of freedom.

11. a multi-column array of scanning electron microscopes (SEMs); an array of autofocus sensors aligned with the multi-column array of the scanning electron microscope; A controller; An apparatus including a stage configured to hold and move an imaging sample in a strip in multiple directions.

12. 12. The apparatus of claim 11, wherein the array of autofocus sensors is configured to allow simultaneous measurement of working distances or positions of multiple microscope columns and substrates to be imaged.

13. 12. The apparatus of claim 11, wherein the controller is configured to implement a feedback control loop that inputs an autofocus sensor signal and outputs a signal to adjust either a column focal length or a column-to-sample working distance required to enable each active imaging column of the multi-column microscope to maintain proper focus on the imaging sample.

14. 12. The apparatus of claim 11, wherein the controller is configured to enable each autofocus sensor in the array of autofocus sensors to independently measure a working distance from a column to a sample.

15. 12. The apparatus of claim 11, wherein the controller is configured to enable each autofocus sensor in the array of autofocus sensors to measure a working distance from a column to a sample in parallel with other autofocus sensors.

16. 12. The apparatus of claim 11, wherein the controller is configured to enable each column of a multi-column array of the SEM to have an independently adjustable column-to-sample working distance.

17. 12. The apparatus of claim 11, wherein the controller is configured to enable each column of the multi-column array of the SEM to adjust a column-to-sample working distance in parallel with other columns of the multi-column array of the SEM.

18. 12. The apparatus of claim 11, wherein the controller is configured to enable each column of the multi-column array of the SEM to function as an autofocus sensor to measure an active working distance to an imaging sample between image scans.

19. The apparatus of claim 11 , wherein one row of autofocus sensors is positioned between two rows of SEMs.

20. The apparatus of claim 11 , wherein the controller is configured to move the platform in multiple degrees of freedom.

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