Auto-focus sensor implementation for multi-column microscopes

Abstract

A locally located autofocus sensor array provides a direct measurement of the working distance between each microscope column in the array and a substrate imaged underneath. The auto-focus sensor measures the working distance between each column and the substrate as it passes over a point to be imaged on the imaging substrate. The working distance measurements from the sensors are input into a control system, which in turn outputs the desired working distance adjustments to the microscope column. Based on auto-focus sensor input, the control system independently adjusts microscope working and / or physical distances for individual microscope columns in the multi-column microscope. The individual microscope columns in the multi-column microscope itself may also be used as the auto-focus sensor.

Description

Technical Field

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

[0002] Generally speaking, the semiconductor manufacturing industry involves highly complex techniques for making integrated circuits using semiconductor materials (e.g., silicon) that are layered and patterned onto substrates. Due to large-scale circuit integration and the decreasing size of semiconductor devices, the fabricated devices have become increasingly sensitive to defects. That is, the defects that cause failures in the devices are becoming smaller and smaller. The devices need to be substantially fault-free before being shipped to end users or customers.

[0003] The semiconductor industry requires high-resolution microscopes to inspect patterned wafers and masked reticles for defects. To increase scanning speed and inspection throughput, multiple microscope columns can be arranged for parallel imaging of desired substrates.

[0004] While the development of multi-column microscopes and their integration into functional inspection tools, particularly for scanning electron microscopes (SEMs), is progressing, a major outstanding challenge is maintaining each column in proper focus on a common inspection surface.

[0005] In order to achieve a functional multi-column inspection tool, an autofocus system is needed that provides a locally located working distance measurement between each column and the imaged substrate at the inspection point. SEMs also face another challenge in that substrate charging during imaging can cause a difference 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 correct for substrate charging. Therefore, there is a need for a system and method for implementing an autofocus sensor for use in a multi-column microscope that is suitable for maintaining each active microscope column in focus. Summary of the invention

[0006] The following presents a simplified summary of the present disclosure in order to provide a basic understanding of a specific embodiment of the present disclosure. The present disclosure is not an extended overview of the present disclosure and it does not identify the key / critical elements of the present disclosure or delimit the scope of the present disclosure. Its only purpose is to present some concepts disclosed herein in a simplified form as a prelude to a more detailed description presented later.

[0007] Aspects of the present disclosure relate to a system and apparatus. The system and apparatus include a multi-column scanning electron microscope (SEM) array. The system and apparatus also include an autofocus sensor array aligned with the multi-column scanning electron microscope array. The system and apparatus also include a controller. Finally, the system and apparatus also include a stage configured to hold an imaging sample and perform a swath scan in multiple directions.

[0008] In some embodiments, the autofocus sensor array is configured to allow simultaneous measurement of working distances or positions between multiple microscope columns and an imaging substrate. In some embodiments, the controller is configured to implement a feedback control loop that inputs an autofocus sensor signal and outputs a signal to adjust the column focus distance or column-to-sample working distance required to allow each active imaging column in a multi-column microscope to maintain proper focus on the imaging sample. In some embodiments, the controller is configured to allow each autofocus sensor in the autofocus sensor array to independently measure the column-to-sample working distance. In some embodiments, the controller is configured to allow each autofocus sensor in the autofocus sensor array to measure the column-to-sample working distance in parallel with other autofocus sensors. In some embodiments, the controller is configured to allow each column in the multi-column SEM array to independently adjust the column-to-sample working distance. In some embodiments, the controller is configured to allow each column in the multi-column SEM array to adjust the column-to-sample working distance in parallel with other columns in the multi-column SEM array. In some embodiments, the controller is configured to allow each column in the multi-column SEM array to be used as an autofocus sensor to measure the effective working distance from the imaging sample between image scans. In some embodiments, a column of autofocus sensors is located between two columns of the SEM. In some embodiments, the controller is configured to move the stage in multiple degrees of freedom.

[0009] These and other aspects of the disclosure are further described below with reference to the figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a schematic diagram showing a top view of an mxn microscope column array in a multi-column microscope according to an embodiment of the present disclosure.

[0011] Figure 2A The illustration shows a schematic diagram showing a side view of an individual SEM column in a multi-column microscope in out-of-focus according to an embodiment of the present disclosure.

[0012] Figure 2B The illustration shows a schematic diagram showing a side view of an individual SEM column in focus in a multi-column microscope according to an embodiment of the present disclosure.

[0013] Figure 3A The diagram illustrates the basic operation of a control system diagram according to an embodiment of the present disclosure.

[0014] Figure 3B An alternative control system arrangement is illustrated according to an embodiment of the present disclosure.

[0015] Figure 3C Illustrated is a control system arrangement using a combined auto-focus strategy according to an embodiment of the present disclosure.

[0016] Figure 4 An example of multi-column microscope scanning of a patterned wafer on a belt scanning stage is illustrated according to an embodiment of the present disclosure.

[0017] Figure 5 An example of a computer system configured in accordance with one or more embodiments is illustrated. DETAILED DESCRIPTION

[0018] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. The present disclosure may be practiced without the aid of some or all of these specific details. In other examples, well-known process operations are not described in detail to avoid unnecessarily obscuring the present disclosure. Although the present disclosure will be described in conjunction with specific embodiments, it will be understood that the present disclosure is not intended to be limited to the embodiments.

[0019] In a single microscope column system, there is usually a single column that looks down at the wafer or mask. A way is needed to place the inspection sample in the focal plane of the microscope plane. In a single column system, the wafer can be completely vertical because the stage can move and only a single focus needs to be focused on the 2D plane. However, for a multi-column system (such as 10 columns in a 5x2 array), the focusing problem becomes more complicated. This is because with 10 focal points, the mechanical tolerances of those focal points will not be in the same plane. Even if the system has skew and tilt control and z-axis control for the stage, if the wafer is completely flat, then it is currently impossible to fully focus 10 points in the same plane. In addition, if the wafer itself will bend, then the wafer is not flat, which creates additional challenges for focusing 10 microscope columns on the stage.

[0020] Autofocus sensors currently used in inspection microscopes rely on a non-contact method to measure the distance or displacement between the sensor and the substrate in order to prevent physical damage to the sample being imaged (usually a patterned wafer or mask). In addition, current inspection microscopes rely on remotely mounted autofocus sensors. There are two typical configurations for remote autofocus sensors.

[0021] The first configuration of a telescopic autofocus sensor implements a single distance or displacement sensor. To avoid physical interference with the microscope column, the sensor must be placed telescopically. Laser-based optical sensors are commonly used because they allow telescopic mounting over long measurement ranges. The telescopic autofocus sensor projects a laser beam onto the imaging surface at a low angle of incidence. The point at which the laser beam hits the imaging surface sets the measurement position of the sensor. The laser beam is reflected off the imaging surface and onto a position sensitive diode (PSD). Since the imaging surface is translated on the linear stage during the inspection process, height changes on the imaging surface result in changes in the laser reflection angle and are detected as shifts in the laser position on the PSD.

[0022] This technique is the operating principle behind autofocus in single-column electron microscopy. Since the sensor's measurement target can be directed at or near the electron microscope's imaging area, only a single measurement point is required; the microscope's focus or working distance is adjusted to bring the sample into focus at the single measurement point.

[0023] The second configuration relies on multiple position or displacement sensors positioned away from the imaging position of the column. The sensors measure the distance to at least three points on the imaging surface or chuck. The sensor locations can be arbitrarily far from the imaging point of the microscope. If the relative X and Y distances between the multiple sensor heads are known, the imaging plane can be calculated using the sensor measurement height in Z. Focusing the microscope can be achieved by placing the focus of the microscope on the calculated imaging plane.

[0024] Since these sensors are positioned away from the imaging column, there is less likely to be physical interference between a single imaging column and the sensor head. The sensor can be placed just above the measurement target for easier integration. And the target measurement area on the substrate can be larger than the imaging spot area of ​​the microscope without losing focus accuracy. In this case, electronic sensors such as capacitive sensors or eddy current sensors are usually implemented.

[0025] Capacitive sensors apply an electric force between the sensor head and the target substrate. The sensor measures the change in capacitance as a function of distance between the sensor head and the target.

[0026] Eddy current sensors generate an alternating current in a metal coil located in the sensor. This current generates an AC magnetic field that induces smaller AC currents (eddy currents) in the target. The eddy currents in turn generate an opposing magnetic field. The induced magnetic field is proportional to the distance between the sensor head and the target.

[0027] Previous microscope autofocus methods were designed for single-column microscopes. However, the methods do not provide an appropriate solution for multi-column microscopes. Since multi-column microscopes, especially multi-column SEMs, are still in the early stages of development, previously designed autofocus sensors have not been designed with functionality for multi-column microscopes.

[0028] A typical autofocus sensor utilizes a laser that strikes the surface of a wafer at a low angle and bounces off the surface of the wafer. The wafer mask acts like a mirror. The angle of incidence is the angle of refraction, so the laser bounces off at the same angle as it entered. The laser then strikes a position sensitive diode that relates the beam position to spatial coordinates (x, y). As the wafer moves up, the laser strikes different points on the wafer and the reflection path changes. The changed path corresponds to the shift in position on the position sensitive diode. However, one challenge is that the laser requires a very long optical path.

[0029] These typical autofocus sensors are designed for single column electron beam inspection tools. One advantage of these types of sensors is that since they are laser paths, the laser source and sensor can be mounted remotely outside the vacuum chamber. However, as mentioned above, the long optical path requires a lot of space, which is acceptable for single column systems, but impractical for multi-column systems due to interference.

[0030] Another typical sensor is a capacitive sensor. A capacitive sensor is an electrode placed on top of the target being measured (e.g., a wafer). The electrode applies a voltage and creates a capacitance across the wafer on the sensor head. The air gap acts as an insulator, and as the distance changes, the capacitance changes. However, the challenge with capacitive sensors is that for the sensor to work, the capacitor and the wafer need to be at the same bias. For optical tools, capacitive sensors work well because the bias can be ground potential. But in a scanning electron microscope, the bias on the wafer can climb to 5000 volts, which is too high for the design of capacitive sensors.

[0031] Another option is to use an interferometer. An interferometer is another laser measurement system. The light source can be mounted remote from the system. The interferometer shines straight down on the sample and the reflection path shines straight up to the sensor. However, the interferometer can be quite large. In addition, the interferometer is a relative distance measurement system, not an absolute measurement system. In other words, the interferometer gives relative changes, which works well until there are missing data points. If for some reason a data point is missed, the system loses track of what the zero point is. On a pattern wafer or mask, the wafer has pattern features on the order of ten to hundreds of nanometers, which are the same size as the wavelength of light in the laser, which can cause interference. Because the system often uses an optical position sensor to scan the wafer or mask, missing data points occur when the pattern wafer interferes with the sensor. If a data point is missed, the measurement is invalidated because there is no baseline. Therefore, an absolute measurement sensor is ideal for a column device.

[0032] The shortcomings of previous autofocus methods depend on the exact sensor configuration implementation. The specific shortcomings of each configuration are detailed below.

[0033] Single remote mounted sensors require long optical light paths that are physically incompatible with the array of microscope columns. For a single microscope column, the long optical path is advantageous in simultaneously allowing distance measurements at or near the inspection point while preventing physical interference between the light source / sensor unit and the microscope column. A single microscope column with a fairly large working distance (mm to cm range) does not usually cause physical interference with the optical light path of the sensor. However, integrating these sensors into a multi-column array becomes challenging or even impossible. For a multi-column microscope with several columns arranged in multiple array columns and rows, the columns will disturb the light path.

[0034] The optical path of the sensor is compatible with multi-column microscopes with long working distances if they are arranged in a single column array. However, to achieve multi-column microscopy, the size of the columns must be reduced to allow a meaningful number of inspection columns to fit in parallel over the same imaging substrate. For SEMs, columns of a size suitable for multi-column microscope arrays typically have a working distance of about 1 mm. This short working distance makes it impossible to integrate the sensor without the columns interfering with the light path of a laser-based optical sensor.

[0035] A second concern will be the measurement reliability of the optical sensor. Patterned wafers or masks typically have changes in topography and / or material composition on a length scale of tens to hundreds of nanometers. This length scale is similar to the wavelength of the incident laser to the sensor (typically around 200nm to 1000nm). Interference between the incident laser and the substrate pattern may result in signal loss on the PSD. In addition, non-flatness in the substrate (e.g., wafer bow) may result in larger shifts in the reflection angle than caused by topography changes alone. The sensor's reliance on angled reflections may result in erroneous sensor measurements.

[0036] Multiple remote mounted sensors are also problematic. Multiple remote sensor configurations do not allow for proper focusing of multiple microscope columns, especially on non-flat substrates (e.g., curved wafers). The remote sensor array measures the distance to points on the imaging surface (rather than directly measuring the distance to the point being imaged) and uses this information to calculate a theoretical and flat image plane. For microscopes with a single column, this system is adequate. The focus of the column can be approximated as a single point in space. By adjusting the working distance of the microscope's column, or the physical distance between the column and the imaging plane, the single focus of the microscope can be placed on the plane of the object to be imaged. Unfortunately, however, this focusing system is not suitable for multi-column microscope arrays. Each microscope column has an individual focus. Due to differences in mechanical tolerances between column manufacturing and mounting, the column foci will be spaced apart in X, Y, and Z. For arrays with more than 3 columns, the foci will not all be in the same plane. If the imaged sample is mounted on a stage with even 6 degrees of freedom, the stage can adjust the height, skew, tilt, and / or roll to match the sample plane with the foci of up to 3 columns. Furthermore, the imaged substrate will not be perfectly flat. Patterned wafers can have a bow that results in hundreds of microns of bowing in height between the center and the edge. No amount of height adjustment, skew, tilt, and / or roll adjustment of the sample stage can correct for non-flatness in the measured substrate. Global autofocus measurements, which are used to approximate the imaging surface to the wafer plane, cannot be used to focus multiple columns.

[0037] All current autofocus sensors have additional limitations for columnar, charged particle based microscopes, including but not limited to scanning electron microscopes (SEMs). Imaging in a SEM can result in charging of the sample due to exposure of the sample to incident high energy electrons from a probe beam. Charge accumulation on the surface of the sample can interact with the incoming SEM beam and result in an effective working distance for the column that is different from the physical working distance. In this case, previously implemented autofocus methods fail to account for the difference between the effective working distance and the physical working distance in the SEM.

[0038] It may be noted that in a multi-beam system, the beam is focused to a small xy region directly below a microscope column. Therefore, the field of view of a multi-beam system is approximately 250um, which is small enough to use approximation of focus using only a single autofocus sensor. However, this multi-beam approximation technique is not applicable to a multi-column system because the individual columns in a multi-beam system may have a distance of approximately 300mm between columns.

[0039] The disclosed techniques and mechanisms provide an autofocus system that functions for a multi-column microscope. The system consists of an array of local position or distance sensors. The sensors are positioned in line with the microscope column in a configuration that allows a point on a sample moving relative to an imaging column to pass under or near both the position sensor and the imaging column. The autofocus sensors are arranged so that each sensor scans at or near a location to be imaged by the microscope column to which the sensor corresponds. Multiple microscope columns can share the same sensor if the sensors measure at or near a location on a wafer to be imaged by any given microscope column in a multi-column microscope. Figure 1 1 is a schematic diagram showing a top view of an mxn microscope column array 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 array with m array columns and n array rows. Each of the n array rows will have at least one corresponding autofocus sensor 102, which is located between the m microscope columns 104 or outside the microscope array 100.

[0040] According to various embodiments, the position sensor can be any absolute position or relative distance sensor that can provide a measurement of the sample being measured. In an ideal configuration, the position sensor is a non-contact measurement sensor to avoid damaging the delicate substrate being imaged. In some embodiments, the sensor should have sufficient measurement resolution and measurement target area to allow the imaged sample (e.g., 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 on the microscope column. Examples of suitable position or distance sensors include, but are not limited to, the following sensors: optical interferometers, optical confocal sensors, capacitive sensors, and / or eddy current sensors. In some embodiments, suitable sensors can be a single type of position sensor array or a mixture of multiple sensor types. According to various embodiments, a very compact autofocus sensor array is embedded inside a microscope array. In these embodiments, when the stage is scanned with a scanning band, each autofocus sensor measures a point on the wafer that will fall in the inspection area below a microscope column. This is because it is not possible to place the sensor directly below the point where the wafer is scanned. Therefore, the autofocus sensor is placed directly next to the microscope column in order to accurately measure the distance to the point being inspected, and then wait until the time it takes for the autofocused point to fall directly under the microscope point (called the "delay time"), and the system can adjust the focus of the microscope with the delay time. In some embodiments, the idea is to embed an autofocus sensor in each row of columns, and embed at least one autofocus sensor in each column row. In other words, if the system scans in the y direction, then the system will need at least one autofocus sensor between columns spaced apart in the y direction. Therefore, there should be one autofocus sensor for each scanning direction. In some embodiments, more than one autofocus sensor can be embedded between the columns, as long as the autofocus sensors are embedded in the multi-column array itself.

[0041] Figure 2A and Figure 2B Both illustrate schematic diagrams showing side views of individual SEM columns in a multi-column microscope according to embodiments of the present disclosure. Figure 2A A situation is shown where the wafer is out of focus (eg, the wafer physical distance is below the post working distance). Figure 2B The situation is shown where the wafer is in focus (eg, the wafer surface is at the post working distance).

[0042] like Figure 2A to 2B, column 200 includes an electron source 202 configured to emit an electron beam 214. In some embodiments, electron source 202 is a tungsten hairpin or a tungsten hairpin with a thin metal wire, and it is used as an electron source. In some embodiments, electron source 202 can be a thermal emitter, a cold field emitter, a pure thermal emitter, or any object that emits electrons.

[0043] According to various embodiments, column 200 further comprises an extractor / buncher 204. In some embodiments, extractor / buncher 204 comprises Figure 2A and 2B 2 components in one block as depicted in . Figure 2A In the specific example shown in , the upper component is the first component and then the lower component is the second component. In some embodiments, the extractor 204 applies an electric field to pull electrons from the electron source 202. The electrons then enter the buncher 204 through an orifice in the extractor 204. In some embodiments, the extractor orifice is about one hundred microns wide. In some embodiments, the buncher 204 is another electrostatic component that includes another orifice, but has an electric potential applied across the orifice that straightens the electrons passing through. In other words, in some embodiments, the electrons are ejected from the electron source 202 at a certain angle, but are straightened when the electrons pass through the buncher 204.

[0044] According to various embodiments, the column 200 also includes a dual steering deflector 206. In some embodiments, the dual steering deflector 206 includes two octopole deflectors (placed one above the other) configured to steer the beam so that the beam hits the limiting aperture 208 as the next component. In some embodiments, the dual steering deflector 206 can be an octopole, a dodecapole, or any kind of pole as long as the pole is capable of steering the electron beam through the limiting aperture 208.

[0045] In some embodiments, the limiting aperture 208 is another element that defines the electron beam 214. In some embodiments, the limiting aperture 208 is also referred to as a "beam defining" aperture. In some embodiments, the resolution of the system is determined by the limiting aperture 208. In some embodiments, the limiting aperture 208 is unbiased, meaning that it is typically at ground potential and acts as an anode for the entire top portion of the column 200. In other words, the top component is used to extract, focus, and deflect the electron beam 214 to pass through the limiting aperture 208. Typically, the limiting aperture 208 is much smaller than any other element in the column 200. In some embodiments, the limiting aperture 208 is approximately 1 micron to 100 microns wide. According to various embodiments, the limiting aperture 208 ultimately defines the resolution and beam current in the system, so the limiting aperture is a very critical component.

[0046] According to various embodiments, column 200 also includes a detector 210. In some embodiments, detector 210 is configured to point downward toward the sample. In this way, electrons from electron source 202 pass through an aperture in detector 210 and travel down the column until the electrons strike sample 224. After the electrons strike sample 224, secondary electrons and backscattered electrons are reflected upward and strike the active side of detector 210 facing downward.

[0047] In some embodiments, the column 200 also includes a dual scanning deflector 212. In some embodiments, the dual scanning deflector 212 is similar to the dual steering deflector 206, except that the function of the dual scanning deflector 212 is to steer the beam 214 over the field of view. In some embodiments, the field of view is the area in which the electron beam 214 is raster scanned. In some embodiments, the field of view can be anywhere from 10 microns to 1 mm. Therefore, the field of view can be changed by adjusting the dual scanning deflector 212.

[0048] In some embodiments, the dual steering deflector 206 steers the beam 214, which may be a pencil electron beam (column beam), down through the column. The deflector 212 will then deflect the beam 214 and move it to one edge or the other on the wafer. When the electrons return, they are spread out in a manner that is insensitive to which part of the wafer the beam 214 hits. In some embodiments, the system is designed so that the optics flood fill the detector 210 no matter where the pencil beam spot hits on the wafer.

[0049] In some embodiments, the beam 214 passes through a magnetic objective lens 216. In some embodiments, the job of the magnetic objective lens 216 is to focus the beam. In some embodiments, the objective lens 216 is a static magnetostatic lens. In these embodiments, the lens 216 has only one focal length, which can be a fixed length, for example, 2 mm to 3 mm. Although the magnetostatic lens focuses at only one point, the lens is high quality, low aberration. In these embodiments, this element is key to achieving ultra-high resolution.

[0050] However, in some embodiments, the distance between the focal point and the wafer varies. Since the magnetostatic lens has only one focal point, and therefore requires additional components. Therefore, in some embodiments, the column 200 includes a rear lens deflector / dynamic focus device 218. The job of the rear lens deflector / dynamic focus device 218 is to make up for any object that the magnetic lens 216 cannot focus on. In other words, the rear lens deflector / dynamic focus device 218 works with the magnetic objective lens 216 to focus the beam onto the wafer. However, in order to focus the beam 214 onto the wafer 224, the system must know where the surface of the wafer is. According to various embodiments, a local autofocus sensor 228 is used to determine where the surface of the wafer is.

[0051] If the upper dual deflector bends the beam too far away from the axis of the magnetic lens, a kink will be introduced into the beam, which can in turn introduce aberrations, astigmatism, and chromatic aberrations. Therefore, the system should not overdrive the upper dual steering deflector. However, in order to still maintain a large field of view, the system needs the additional impact on the beam provided by the rear lens deflector 218. In addition, the dynamic focus aspect 218 provides an entirely separate function. In some embodiments, the dynamic focus device 218 takes the point at which the beam 214 is in focus and moves that point up and down. In some embodiments, the dynamic focus element includes an electrode with a voltage applied to it. By changing the voltage, the system can move the focus up and down.

[0052] In some embodiments, the system utilizes a confocal optical absolute position sensor. The confocal sensor takes white light, focuses it, and refracts it into several separate spectra. The confocal sensor focuses the various spectra at different distances from the lens. Whichever wavelength of light is primarily reflected back determines the distance of the wafer.

[0053] There are certain difficulties in integrating autofocus sensors into scanning electron microscopes. One difficulty is operating a scanning electron microscope inside a vacuum chamber. Therefore, the locally integrated sensors in the array need to be locally mounted close to the columns within the array. Therefore, the system requires sensors that are compact in size and fit in the array. Since the scanning electron microscope operates in a vacuum, the locally mounted sensors must be vacuum compatible. This means that typical materials used to assemble electronic sensor packages tend to outgas in vacuum systems. For example, many polymers have chemical binders that contain a range of small chains. Solvents are used to cause the small chains to crosslink, thereby forming polymers. Some of these solvents and small polymers continue to outgas when placed in a vacuum system, which can destroy the vacuum environment.

[0054] In some embodiments, all electronics in the light source on the sensor are outside the system. In some embodiments, light is brought in and out of the system using fiber optic cables that are fed through the vacuum chamber. The fiber optic cable then brings the light source in and brings it to the sensor head. In some embodiments, the sensor head includes glass optics and some mirrors that reflect the light. The light is then focused onto a surface and then bounces out. The reflected signal is carried outward to the external electronics through the fiber optic cable. In some embodiments, the fiber optic cable includes glass filaments. If the sensor is not mounted locally, then another option is to have the autofocus sensor outside the vacuum chamber with some viewports looking into the chamber. However, confocal sensors have a very short working distance, on the order of tens of millimeters, in which case the sensor needs to be mounted in a column array. In some embodiments, the glass viewport is hermetically sealed, which allows the sensor outside the chamber to shine light directly downward and reflect out.

[0055] Another difficulty is that the AF measurement distance is different from the column physical distance and the column working distance.

[0056] In some embodiments, the system measures a distance d. In an ideal situation, the sensor head would be perfectly aligned with the end of the column, so the measured distance d would be the working distance. However, this may not happen, so the working distance d of the sensor can be calibrated so that it is different from the working distance of the column. Therefore, the autofocus sensor does not need to be mounted at the same height of the column. In some embodiments, the system uses the autofocus sensor to measure the distance d, which is likely to be different from the working distance. In some embodiments, the calibration includes picking up the column and moving the wafer until it is as close to the working distance as possible, and then moving the wafer up or down until the image comes into focus. In other embodiments, the dynamic focus device on the column is adjusted until the image comes into focus. Then, the autofocus sensor will measure the distance d. Since the system knows that the column is focused on the wafer, d is equal to the focus sensor distance plus the offset. The offset is then subtracted from the measurement.

[0057] In some embodiments, the displacement or distance measurement 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 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 the control system. When the measurement point in (X, Y) passes under the microscope column to be inspected, the control system outputs an adjustment (such as Figure 2B ).

[0058] In order to control the autofocus sensor and the electronic beam column, a control system is required. Figures 3A to 3C The diagram illustrates three different examples of control systems that can be used to manage an autofocus sensor in an electron beam column. With an autofocus sensor and a small column, it can be challenging to measure the distance exactly at the measurement point or inspection point. For vertical wafers, the goal is to ensure that the system measures the distance to the inspection point on the wafer when the inspection point is directly under the autofocus sensor. Then, when the stage moves the inspection point under the inspection column, the system needs to store that height in the control system, which later tells the microscope to update the focus as the inspection point moves under the microscope.

[0059] As mentioned earlier, Figures 3A to 3C The diagrams illustrate three different examples of control systems. Figure 3A Diagram showing the basic operating control system, Figure 3B Diagram illustrating a control system using a column as an autofocus sensor, and Figure 3CThe diagram illustrates a control system utilizing a combination of dedicated sensors and columns.

[0060] Figure 3A The basic operation of the control system diagram according to an embodiment of the present disclosure is illustrated. Figure 3A 300, control system 304 may output a signal to column 306, wherein the working distance of the column is adjusted so that the column focus range overlaps with sample carrier 308. Alternatively, control system 304 may output a signal to sample carrier 308, wherein the sample carrier may adjust the height, pitch, tilt, and / or roll to place the sample in the working distance range of column 306. In some embodiments, both adjustments may be combined.

[0061] According to various embodiments, Figure 3A is a standard setting. The autofocus sensor 302 measures the height of the wafer on the sample stage 308 and stores the height as a distance d. Then, the control system 304 retrieves the stored distance and then tracks the stage motion. When the inspection point comes to be directly under the microscope column 306, then the control system 304 sends a signal to any column 306 to adjust the dynamic focus device so that the focus matches the stored distance d of the wafer. Alternatively, the control system 304 can send a signal to the sample stage 308 to move the height of the stage up or down in the z direction to bring the wafer into focus. However, moving the stage will usually only be used when all columns are closed or the distance is very large.

[0062] Figure 3B An alternative control system arrangement according to an embodiment of the present disclosure is illustrated. In some embodiments, as Figure 3B , the diagram 320 does not use an auto-focus sensor. Instead, the control system 324 uses the microscope column 326 itself as a position sensor array. In these embodiments, the SEM column 326 can scan the underlying sample 328 even if the sample is not actively imaged. In these embodiments, the SEM column 326 can be used for through-focus measurement to determine the best focus. By changing the focus from its nominal position while scanning the imaged sample 328, the correct focus can be determined by measuring the edge sharpness of the image. In this way, the microscope column 326 itself can be used as an auto-focus sensor. In some embodiments, all or a subset of the multi-column microscope columns 326 can be used as auto-focus sensors. In this configuration, the actual focus condition of the SEM will be determined even in the case of surface charging because the microscope column will measure the effective focus distance required to achieve proper microscope focus, rather than just the physical distance between the microscope column and the substrate.

[0063] Figure 3CThe diagram illustrates a control system arrangement using a combined autofocus strategy according to an embodiment of the present disclosure. Figure 3C As shown in FIG. 3 , diagram 340 illustrates that a control system 344 uses a local autofocus sensor 342 to obtain a coarse / large correction for the working distance to a sample 348 , while also controlling a stud 346 to use a through focus measurement for a fine / small adjustment of the working distance.

[0064] In some embodiments, the components required to operate the autofocus sensor include a microscope column array. In these embodiments, the microscope column array can form a minimum 1x 2 (or 2x 1) array, and there is no maximum limit on the number of columns arranged. In some embodiments, the components required to operate the autofocus sensor include a position or displacement sensor array. In these embodiments, the sensor array can form a minimum 1x 1 array, and there is no maximum limit on the number of sensors arranged. In some embodiments, the components required to operate the autofocus sensor include a sample to be imaged. In these embodiments, the sample can be but is not limited to a patterned wafer or a masked mask or any sample or combination of samples to be imaged by a microscope column array. In some embodiments, the components required to operate the autofocus sensor include a moving stage, which is used to allow relative movement between the sample and the microscope column array. In these embodiments, the stage can be a linear motion stage or a rotation stage. In addition, in these embodiments, the stage can move the sample to be imaged, the microscope column array, or a combination of the two.

[0065] In some embodiments, the array of microscope columns is fixed in position relative to each other in the X, Y, and Z dimensions. In these embodiments, the sample is mounted on a linear translation stage that moves in X and Y. The stage can move along the Z axis as well as skew, tilt, and / or roll. To roughly focus the sample under the microscope columns, the Z height (as well as skew, tilt, and / or roll) of the stage is moved to adjust the column to sample physical working distance. To precisely focus each column on the sample, the column lenses can be adjusted to adjust the column working distance.

[0066] In some embodiments, the inspection tool utilizes a stage that scans the swath back and forth. The microcolumn and stage scan very quickly from left to right underneath. In some embodiments, the column scans an entire row of the wafer from left to right. Once the column scans a complete row, the column advances a little (a predetermined amount) and then scans back along another path. This is how the column raster scans the entire wafer. The stage typically moves very quickly along the swath scan direction to achieve good throughput. For example, a single column can typically scan 50mm / s. In some embodiments, the column scans using a constant speed. If the stage moves at a constant speed, then an autofocus sensor can be placed upstream along the swath scan direction. In this way, the speed can be used to calculate the time it takes the stage to travel directly underneath the microscope. In some embodiments, the control system constantly tracks and records the height of the stage and what the current working distance is.

[0067] In some embodiments, control system 304 may send signals to columns 306 to adjust the dynamic focus devices of individual columns to individually adjust the working distance on columns 306, rather than moving the stage. In some embodiments, instead of adjusting the height of the stage, control system 304 adjusts the tilt and yaw of stage 308. In some embodiments, the stage has 6 degrees of freedom: x, y, z, as well as yaw, pitch, and pitch.

[0068] In some embodiments, the same effect can be achieved by mounting a microscope column on the stage. The microscope stage can be moved in X, Y and / or Z. In addition, the sample stage need not be a linear motion stage. The sample stage can have one or more rotational or pivot axes.

[0069] In some embodiments, the system is configured to adjust both the working distance d and the stage tilt on the dynamic focus device simultaneously. Figure 3B As shown in , the control system 324 can use the column 326 itself as a sensor (also called "overfocus"). Using overfocus, the column can be adjusted to move the dynamic focus device up and down from the focus. The system can then find the image with the sharpest focus and store that distance as the working distance. In some embodiments, using a scanning electron microscope involves firing an electron beam at a wafer multiple times. In these embodiments, the wafer has an insulating material, such as silicon dioxide and a dielectric, that charges the wafer. The charge then deflects the electron beam and effectively forms a field above the wafer that affects the actual focus. Therefore, the actual focus of the column can be slightly above the surface. In other words, the focus will be different from the actual distance measured using an optical autofocus device that always measures the physical distance from the wafer. Therefore, in these situations, it can be advantageous to use the column as a sensor.

[0070] Figure 4An example of multi-column microscope scanning of a patterned wafer on a scanning belt scanning stage according to an embodiment of the present disclosure is illustrated. The columns can be used as auto-focus sensors with over-focus measurement as they pass over the free areas between defects. Figure 4 In the example system 400, an array of microscope columns 406 is focused directly on a wafer on a stage 402 in a top view. Figure 4 In , system 400 may scan stage 402 along the x-axis in stage motion X 404. Figure 4 4, during the scan, the wafer 402 is scanned in a swath from left to right while the autofocus sensor 410 measures the distance to points on the wafer that will eventually pass under the microscope column 406. In these embodiments, the system then records the distance to these points via the controller, and as the wafer scans just below the points, the controller takes the distance and adjusts the focus distance of the column to ensure that the height measurement distance from the autofocus sensor to the wafer is based on the latest autofocus sensor distance measurement. According to various embodiments, the adjustment can be done in several different ways. First, if the wafer is mounted on a stage and the stage has z-axis control, the controller can cause the stage to move, tilt, or tilt up and down, thereby bringing the wafer into focus of the microscope. Second, the controller can also adjust the electron optics or the microscope optics via the dynamic focus assembly to change the focus of the microscope column itself so that the column is focused on the wafer at those specific distances. Finally, the system can mount the microscope column on a z-axis motor, and the controller can send signals to the motor to move the column up and down.

[0071] In some embodiments, the patterned wafer is mounted on a linear motion stage 402 with 6 degrees of freedom (6DOF). To increase imaging speed, the stage will move under the array of posts 406 in a swath scanning motion 404. The stage 402 will continuously move back and forth on the X axis. As the array of microscopes 406 images the wafer passing underneath, it can produce an imaging line scan 408 along X. Once the line scan is completed, the stage will step and advance along Y, and the stage will restart scanning until the entire area to be imaged under the array of posts has been scanned. For the example of an SEM, the posts themselves can deflect the beam to produce a raster scan. Beam deflection scans typically have a FOV of hundreds of microns. In the example system 400, as the stage 402 scans along X, the beam can scan along Y. For each stage scan band along X, the beam can cover a band along Y. Therefore, a microscope with beam deflection can produce an area scan as the wafer stage scans a swath along the X axis, such as Figure 4 As shown in .

[0072] In some embodiments, the position or distance sensor 410 is placed so that it is in line with each column 406 of the array along the X scanning direction. When the wafer is scanned along a line along X, the point to be imaged in the column scanning area 412 will pass under or near both the position sensor and the imaging point of the microscope column 406, such as Figure 4 The sensor 410 will measure the distance to the wafer passing underneath and can calculate the height difference between the current and previous height measurements.

[0073] In some embodiments, a control loop may be used to maintain proper focus for each column in the array. Based on the stage speed and the physical spacing / distance between the sensor measurement position and the microscope column imaging position on the wafer, the time between when the wafer passes under the sensor and when the wafer passes under the microscope imaging position may be determined. As the wafer passes under the microscope column, the microscope working distance may be adjusted or the physical distance between the column and the wafer may be adjusted. The former adjustment may be achieved by adjusting the column focusing elements to change the focus distance of the microscope column. The latter may be achieved by changing the Z height, skew, tilt and / or roll 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 the stage).

[0074] In some embodiments, for a microscope with a focused charged beam, such as a scanning electron microscope, charging the substrate can result in an offset between the effective focus distance and the physical working distance. In this case, an offset between the focus distance and the physical working distance can be applied to keep the column in focus.

[0075] Alternatively, in some embodiments, the microscope column itself can be used as a focus sensor. The microscope column can image while actively scanning (between defects to be imaged) or while not actively scanning. A typical patterned wafer used for inspection has an average of 100 defects, leaving a large portion of the wafer surface available for focus, such as Figure 4 406 can be used for focusing, and by changing the working distance slightly from its nominal distance and measuring the sharpness of the imaged edge, the proper working distance can be determined. In some embodiments, one advantage of this method of SEM is that the measured working distance and the effective working distance will be the same even in the case of substrate charging.

[0076] In some embodiments, these adjustments can be performed to maintain all active microscope arrays in the column in focus on the wafer in parallel. According to various embodiments, both local dedicated autofocus sensors and the column as an overfocus autofocus sensor can be used in parallel. The dedicated autofocus sensor can be used for overall working distance adjustment, such as adjusting the height, skew, tilt and / or roll of the sample stage. And the column overfocus measurement can be used to adjust the working distance of the column.

[0077] In some embodiments, one advantage of the disclosed techniques and mechanisms is that it allows all microscopic columns in a multi-column array to simultaneously maintain focus on a sample imaged below. In some embodiments, another advantage is that it allows all microscopic columns in a multi-column array to simultaneously maintain focus on a non-planar sample (e.g., a patterned wafer with a curved profile). In some embodiments, another advantage is that it allows all microscopic columns in a multi-column array to simultaneously maintain focus on a charged surface where the effective focus distance may deviate from the physical working distance, particularly for electron microscopy.

[0078] In some embodiments, a separate microscope column can be mounted on a stage and the column moved relative to the wafer (rather than the wafer moving relative to the column). Alternatively, both the column and the wafer can be moved on the stage and relative to each other.

[0079] In some embodiments, the stage may be a linear stage or may have one or more rotational axes. For example, instead of a stage having linear X, Y, Z axes of motion, the stage may have linear axes in the X and Z axes and a rotational axis in the θZ axis.

[0080] In some embodiments, the sensor array may offset the sensors relative to each row in the microscope column array. In these embodiments, the sensor array may have less than or more than one sensor per column row. In these embodiments, the main requirement for the number and positioning of the sensors is that the sensors be able to obtain a sufficient number of measurement points to map the wafer in X, Y, and Z, allowing each active column of the microscope array to maintain its focus. For example, if each individual column has a depth of focus of + / - 50nm and there is only 100nm of wafer curvature from the center to the edge of the wafer, then the sensors can be spaced a greater distance apart. In these embodiments, the sensors will only need sufficient spatial resolution to capture the height change of the wafer before it falls out of focus on the microscope column.

[0081] In some embodiments, the autofocus measurement may be performed in series with the microscope imaging / scanning. In these embodiments, the autofocus sensor need not be used in parallel with the microscope imaging / scanning, although this will result in a slower overall process time. In these embodiments, the sensor may first scan and record the height map of the wafer, followed by the microscope imaging / scanning. The pre-recorded height map may be used to adjust the focus during a later imaging / scanning. Furthermore, the autofocus sensor may be in a tool separate from the multi-column imaging / scanning microscope used to record the height map.

[0082] The examples described above present various features utilizing a computer system or an auto-focus sensor system including a computer. However, embodiments of the present disclosure may include all or various combinations of each of the features described above. Figure 5 An example of a computer system according to an embodiment of the present disclosure is illustrated. According to a specific embodiment, a system 500 suitable for implementing a specific embodiment of the present disclosure includes a processor 501, a memory 503, an interface 511, and a bus 515 (e.g., a PCI bus or other interconnection structure). When acting under the control of appropriate software or firmware, the processor 501 is responsible for implementing applications such as an operating system kernel, a containerized storage driver, and one or more applications. Various specifically configured devices may also 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.

[0083] Specific examples of supported interfaces include Ethernet interfaces, frame interrupt interfaces, cable interfaces, DSL interfaces, token ring interfaces, and the like. In addition, various very high-speed interfaces may be provided, such as Fast Ethernet interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces, POS interfaces, FDDI interfaces, and the like. Generally speaking, these interfaces may include ports suitable for communicating with appropriate media. In some cases, the interfaces may also include independent processors, and in some examples, volatile RAM. Independent processors may control communication-intensive tasks, such as packet switching, media control, and management.

[0084] According to various embodiments, as shown herein, system 500 is a computer system configured to manage an auto-focus sensor system. In some implementations, one or more of the computer components may be virtualized. For example, a physical server may be configured in a local or cloud environment. A physical server may implement one or more virtual server environments in which an auto-focus sensor system is executed. Although a particular computer system is described, it should be recognized that a variety of alternative configurations are possible. For example, a module may be implemented on another device connected to the computer system.

[0085] The specific embodiments of the disclosure presented herein generally address problems in the field of electron beam columns and are not limited to the hardware, algorithm / software implementations and architectures and use cases outlined above.

[0086] Although the foregoing disclosure has been described in considerable detail for the purpose of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and devices of the present disclosure. Therefore, the present embodiments should be considered illustrative rather than restrictive, and the present disclosure should not be limited to the details given herein.

Claims

1. A system comprising: Multi-column scanning electron microscope (SEM) array; an autofocus sensor array in line with the multi-column scanning electron microscope array; Controller; and A stage is configured to hold an imaging sample and perform swath scanning in multiple directions.

2. The system of claim 1, wherein the autofocus sensor array is configured to allow simultaneous measurement of working distances or positions between multiple microscope columns and an imaging substrate.

3. 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 a column focus distance or a column-to-sample working distance required to allow each active imaging column in a multi-column microscope to maintain proper focus on the imaging sample.

4. The system of claim 1, wherein the controller is configured to allow each auto-focus sensor in the auto-focus sensor array to independently measure a post-to-sample working distance.

5. The system of claim 1, wherein the controller is configured to allow each auto-focus sensor in the auto-focus sensor array to measure a column-to-sample working distance in parallel with other auto-focus sensors.

6. The system of claim 1, wherein the controller is configured to allow each column in the multi-column SEM array to independently adjust a column-to-sample working distance.

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

8. The system of claim 1, wherein the controller is configured to allow each column in the multi-column SEM array to function as an auto-focus sensor to measure an effective working distance from the imaged sample between image scans.

9. The system of claim 1, wherein a column of the auto-focus sensor is located between two columns of the SEM.

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

11. An apparatus comprising: Multi-column scanning electron microscope (SEM) array; an autofocus sensor array in line with the multi-column scanning electron microscope array; Controller; and A stage is configured to hold an imaging sample and perform swath scanning in multiple directions.

12. The apparatus of claim 11, wherein the autofocus sensor array is configured to allow simultaneous measurement of working distances or positions between multiple microscope columns and an imaging substrate.

13. The device 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 a column focus distance or a column-to-sample working distance required to allow each active imaging column in a multi-column microscope to maintain proper focus on the imaging sample.

14. The device of claim 11, wherein the controller is configured to allow each auto-focus sensor in the auto-focus sensor array to independently measure a post-to-sample working distance.

15. The device of claim 11, wherein the controller is configured to allow each auto-focus sensor in the auto-focus sensor array to measure a column-to-sample working distance in parallel with other auto-focus sensors.

16. The apparatus of claim 11, wherein the controller is configured to allow each column in the multi-column SEM array to independently adjust a column-to-sample working distance.

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

18. The device of claim 11, wherein the controller is configured to allow each column in the multi-column SEM array to function as an auto-focus sensor to measure an effective working distance from the imaged sample between image scans.

19. The apparatus of claim 11, wherein a column of the auto-focus sensor is located between two columns of the SEM.

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

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