Height sensing system for electron beam metrology tool

Abstract

And the light beam is guided to the workpiece on the object placing table. The workpiece is positioned at an absolute distance from the electron beam column. The light beam reflected from the workpiece is received at a sensor. Using the beam, a nominal distance between the electron beam column and the workpiece on the stage is determined.

Description

Technical Field

[0001] The present disclosure relates to metrology systems for semiconductor wafers. Background Art

[0002] The evolution of the semiconductor manufacturing industry places increasing demands on yield management, and specifically, on metrology and inspection systems. Critical dimensions continue to shrink, yet the industry needs to reduce the time required to achieve high-yield, high-value production. Minimizing the total time from detecting a yield issue to resolving it maximizes the return on investment for semiconductor manufacturers.

[0003] The fabrication of semiconductor devices, such as logic and memory devices, typically involves processing a semiconductor wafer using a number of fabrication processes to form the various features and multiple levels of the semiconductor device. For example, photolithography is a semiconductor fabrication process that involves transferring a pattern from a mask to a photoresist disposed on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical mechanical polishing (CMP), etching, deposition, and ion implantation. The arrangement of multiple semiconductor devices fabricated on a single semiconductor wafer can be separated into individual semiconductor devices.

[0004] Metrology processes are used at various steps during semiconductor manufacturing to monitor and control the process. Metrology processes differ from inspection processes in that, unlike inspection processes, which detect defects on the wafer, metrology processes are used to measure one or more characteristics of the wafer that cannot be determined using existing inspection tools. Metrology processes can be used to measure one or more characteristics of the wafer so that the performance of the process can be determined from the one or more characteristics. For example, a metrology process can measure the dimensions (e.g., line width, thickness, etc.) of features formed on the wafer during the process. In addition, if one or more characteristics of the wafer are unacceptable (e.g., outside a predetermined range for the characteristics), the measurement of the one or more characteristics of the wafer can be used to modify one or more parameters of the process so that additional wafers manufactured by the process have acceptable characteristics. Many metrology processes are performed using electron beam tools.

[0005] The absolute distance between the electron beam column in the metrology tool and the workpiece on the stage of the metrology tool is used to provide the desired focus. Previously, a metal L-shaped target was machined onto the workpiece to achieve mechanical alignment in the X, Y, and Z directions. The L-shaped target will form a cross. A uniform cross will achieve alignment in the X and Y directions. Z-direction alignment (e.g., absolute distance) can involve finding the best focus position. The best focus can be determined by finding where the image of the workpiece has the sharpest edges and / or the highest contrast. The L-shaped target may have roughness, scratches, non-uniformity, or deposits due to its mechanical manufacture. Determining focus can be difficult due to these imperfections in the L-shaped target.

[0006] Due to limitations in processing machinery, large feature sizes do not provide sufficient pattern complexity for comprehensive measurement and / or evaluation of optical properties. Furthermore, machined metal grooves are rough, which can cause scattering and negatively impact measurement. Furthermore, the target may not be at the same height as the workpiece, which can affect alignment accuracy. These factors can impact height sensor performance.

[0007] New systems and technologies are needed. Summary of the Invention

[0008] In a first embodiment, a system is provided. The system includes a stage configured to hold a workpiece; an electron beam column configured to direct an electron beam at the workpiece on the stage; a light source configured to generate a light beam at the workpiece on the stage; a sensor configured to receive the light beam reflected from the workpiece; a mirror configured to reflect the light beam received from the workpiece toward the sensor; and a processor in electronic communication with the sensor. The processor is configured to use measurements from the sensor to determine a displacement from a nominal distance between the electron beam column and the workpiece on the stage.

[0009] The system may include a second mirror and a third mirror. The second mirror and the third mirror are positioned in the path of the light beam. The second mirror is positioned to direct the light beam from the light source to the workpiece. The third mirror is positioned to direct the light beam from the workpiece to the mirror. The second mirror and the third mirror may each be a folding mirror.

[0010] The system may further include a plano-convex lens disposed in the path of the light beam between the second mirror and the workpiece.

[0011] The system can include a beam splitter positioned in the path of the light beam between the workpiece and the light source.The beam splitter directs at least some of the light beam at the sensor.

[0012] The light source may be a light emitting diode.

[0013] The system may further include a convex lens located between the light source and the workpiece in the path of the light beam.

[0014] The system can include a slit located in the path of the light beam between the light source and the workpiece.

[0015] The mirror may be a spherical mirror.

[0016] The workpiece may be a semiconductor wafer.

[0017] In a second embodiment, a method is provided. The method includes directing a light beam from a light source at a workpiece on a stage. Positioning the workpiece at an absolute distance from an electron beam column. Reflecting the light beam from the workpiece. Receiving the light beam reflected from the workpiece at a sensor. Using a processor, determining a displacement from a nominal distance between the electron beam column and the workpiece on the stage.

[0018] The workpiece may be a semiconductor wafer.

[0019] The method may include reflecting the light beam reflected from the workpiece using a mirror.Reflecting the light beam from the workpiece directs the light beam to the mirror.

[0020] The directing may include reflecting the light beam from a second mirror disposed in a path of the light beam between the light source and the workpiece.

[0021] Reflecting the light beam from the workpiece may include reflecting the light beam from a third mirror disposed between the workpiece and the mirror in the path of the light beam.

[0022] The method may include splitting the light beam reflected from the workpiece using a beam splitter. Some of the light beam is directed by the beam splitter to the sensor.

[0023] The directing may include focusing the light beam.

[0024] The method may include directing the light beam reflected from the workpiece a second time at the same point on the workpiece before receiving the light beam at the sensor.

[0025] The method may include adjusting a height of a storage platform based on the displacement from the nominal distance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] For a fuller understanding of the nature and objects of the present disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:

[0027] Figure 1 is a block diagram of an embodiment of a system according to the present disclosure;

[0028] Figure 2 is used for Figure 1 A block diagram of the optical components in the system;

[0029] Figure 3 Demonstration use Figure 1 Demonstration test results of the system;

[0030] Figure 4 Description Compared with single pass Figure 1 A double pass of an embodiment of the system; and

[0031] Figure 5 Display for Figure 1 System calibration techniques. DETAILED DESCRIPTION

[0032] Although the claimed subject matter will be described in terms of certain embodiments, other embodiments (including embodiments that do not provide all of the benefits and features set forth herein) are also within the scope of this disclosure. Various structural changes, logical changes, process step changes, and electrical changes may be made without departing from the scope of this disclosure. The scope of this disclosure is, therefore, to be defined solely by reference to the claims appended hereto.

[0033] The height sensing system can measure displacement from a nominal distance between the electron beam column and the workpiece on the stage of the metrology tool. For example, the absolute distance can be between the top surface of the workpiece on the stage and the lowest surface of the objective lens in the electron beam column. Certain metrology applications may require a nominal distance (which can be an absolute distance, a point along the absolute distance, or a range within the absolute distance) between the electron beam column and the workpiece on the stage of the metrology tool to provide a desired electron beam focus. The height sensing system can be used to verify that the electron beam column is positioned at this nominal height relative to the workpiece during a workpiece scan. This can occur during measurement setup or during other periods of metrology tool operation.

[0034] The absolute distance may be affected by structures on the workpiece, the shape of the workpiece (e.g., bow, thickness variation, etc.), variations in the height of the stage, and / or incorrect movement of the stage (e.g., too much or too little movement). These differences may also affect the placement of the nominal distance.

[0035] Figure 1 1 is a block diagram of a system 100. The system 100 may be part of a metrology tool. The system 100 includes a stage 102 configured to hold a workpiece 101. The workpiece 101 may be a semiconductor wafer, a calchip, or another type of substrate. The stage 102 may be moved in the X, Y, and Z directions using actuators. The system 100 also includes an electron beam column 103 configured to direct an electron beam at the workpiece 101 on the stage 102. The distance between the end of the electron beam column 103 and the top surface of the workpiece 101 is Figure 1 The absolute distance in (shown by dashed line 115).

[0036] Light source 104 is configured to generate a light beam 111 at workpiece 101 on stage 102. The light source may be a light emitting diode (LED) or a laser. Light beam 111 may utilize visible light. For example, light beam 111 may primarily utilize red wavelengths of light. The light spot formed by light beam 111 on workpiece 101 may vary. For example, the light spot on workpiece 101 may be 2 mm in diameter or 2 mm by 2 mm square.

[0037] The light beam 111 may have a grazing angle of incidence on the workpiece 101. This avoids many mechanical components in the system 100 and provides high reflectivity. For example, the light beam 111 may have an angle of incidence of 3.5 ± 0.5 degrees relative to the flat surface of the workpiece 101.

[0038] Sensor 112 is configured to receive light beam 111 reflected from workpiece 101. Sensor 112 can be a dual-element photodiode connected to or as part of a printed circuit board (PCB). The dual-element photodiode can read whether the imaging signal is in the upper or lower half of the dual-element photodiode. The height adjustment of stage 102 can be based on the position of the imaging signal on sensor 112.

[0039] Mirror 110 can be configured to reflect a light beam received from workpiece 101 toward sensor 112. Mirror 110 can be a spherical mirror, but other mirrors can perform this function. For example, light reflected from workpiece 101 can be reflected back toward workpiece 101 using mirror 110. This reflected light can then be directed toward sensor 112 after being reflected by workpiece 101.

[0040] The processor 113 is in electronic communication with the sensor 112. The processor 113 may also be in electronic communication with the stage 102 actuator or other components in the system 100. The processor 113 may use measurements from the sensor 112 to determine a relative displacement from a nominal distance. The nominal distance for an optimal electron beam focus on the workpiece 101 may be determined. The processor 113 may also determine adjustments to the stage 102 in the Z direction to achieve a desired focus position and / or placement of the workpiece 101 within the nominal distance.

[0041] The processor 113 typically includes a programmable processor that is programmed with software and / or firmware to perform the functions described herein, and suitable digital and / or analog interfaces for connecting to other components of the system 100. Additionally or alternatively, the processor 113 includes hardwired and / or programmable hardware logic circuitry that performs at least some of the functions of the processor 113. Although for simplicity, the processor 113 is not shown in FIG. Figure 1 113 is shown as a single monolithic functional block, but in practice, the processor 113 may include multiple interconnected control units having appropriate interfaces for receiving and outputting the signals illustrated in the figures and described in the text. The program code or instructions for the processor 113 to implement the various methods and functions disclosed herein may be stored in a readable storage medium, such as a memory in the processor 113 or other storage.

[0042] The focal length of the electron beam column 103 can be fixed. Instead of changing the focus of each optical component in the electron beam column 103, an actuator associated with the stage 102 can be used to move the workpiece 101 toward or away from a fixed focal plane (i.e., movable in the Z direction). Although the Z height sensor of the stage 102 can monitor the height of the stage 102, it can be difficult to know whether the Z height sensor alone has moved the stage 102 in the Z direction to the optimal focus position. Different workpieces 101 can have different heights, and a single workpiece 101 can have height differences across the workpiece 101.

[0043] System 100 may include a second mirror 107 and a third mirror 109. Second mirror 107 and third mirror 109 may be folding mirrors, although other mirrors or optical components that perform this function may be used. For example, second mirror 107 and / or third mirror 109 may also be pentaprisms. Second mirror 107 and third mirror 109 may be positioned in the path of light beam 111 such that second mirror 107 directs light beam 111 from light source 104 toward workpiece 101, and third mirror 109 directs light beam 111 from workpiece 101 toward mirror 110. Second mirror 107 and third mirror 109 may also receive reflected light in the opposite direction of the path of light beam 111.

[0044] The system can include a composite lens 108 positioned in the path of the light beam 111 between the second mirror 107 and the workpiece 101. Two lenses are illustrated as part of the composite lens 108, but more or fewer lenses can be used. The composite lens 108 focuses the light beam 111 onto the workpiece 101. In one example, the composite lens 108 is a plano-convex lens. A set of additional composite lenses can also be positioned between the second mirror 107 and the beam splitter 106.

[0045] The beam splitter 106 can be positioned between the workpiece 101 and the light source 104 in the path of the light beam 111. The beam splitter 106 can direct at least some of the light beam 111 at the sensor 112. For example, the beam splitter 106 can be located between the light source 104 and the second mirror 107 in the path of the light beam 111.

[0046] The system can include a convex lens 105 positioned between the light source 104 and the workpiece 101 in the path of the light beam 111. For example, the convex lens 105 can be positioned between the light source 104 and the beam splitter 106 in the path of the light beam 111. The convex lens 105 can collimate the light beam 111. Although a single convex lens 105 is illustrated, a group of lenses can also be used. The convex lens 105 can be part of a Kohler illumination system.

[0047] The slit 114 can be positioned between the light source 104 and the workpiece 101 in the path of the light beam 111. For example, the slit 114 can be positioned between the light source 104 and the beam splitter 106 in the path of the light beam 111, such as downstream of the convex lens 105. In one example, the slit 114 has dimensions of 2*0.12 mm. The slit 114 can be fixed or adjustable.

[0048] The pattern effect in system 100 can be controlled by adjusting the illumination uniformity of light source 104, the slit focus of slit 114, the spherical mirror focus of mirror 110, or the eccentricity of the primary and secondary beams. Figure 5 In the instructions shown in . Figure 5 Shows an L-shaped target on a workpiece. Figure 5 The beam profiler in the can collect alignment images, e.g. Figure 3 and Figure 5 Alignment images as illustrated in .

[0049] Figure 2 is used for Figure 1 The stage controller may move the stage in the Z direction. The stage controller may include an actuator that may move the stage in at least the Z direction. The stage controller may be in electronic communication with a processor ("stage PC") that may communicate with the stage controller. Figure 1 The processor 113 in the embodiment of the present invention may be the same or different. In an example, the storage station controller is part of the processor 113. In another example, the storage station controller is an additional processor in electronic communication with the processor 113.

[0050] exist Figure 2 During operation of the embodiment of the present invention, a beam of light (ie, a light beam) from a light source is directed at a workpiece on a stage. Figure 1 As shown in , the workpiece is positioned at a certain absolute distance from the electron beam column. Figure 2 , the light beam is reflected from the workpiece and received at the sensor. If the position of the workpiece is non-nominal, the dual-element photodiode generates a voltage difference for feedback control of the stage 102.

[0051] Using a processor, the relative displacement from the nominal distance can be determined. The nominal distance for the optimal electron beam focus on the workpiece can be determined. The system can then be aligned to this specific height.

[0052] The absolute distance to the electron beam column and the workpiece on the stage can also be determined. A poor signal can mean that the absolute distance is out of specification.

[0053] For example, if Figure 2As shown in Figure 2, imaging the top or bottom of a dual-cell photodiode in a PCB by light from a beam means that the absolute distance or displacement from the nominal distance is out of specification. The stage height can be adjusted in the Z direction to bring the absolute distance or displacement into specification. The stage height can be adjusted to accommodate different workpiece heights, different wafer curvatures, or different chuck designs.

[0054] The light beam reflected from the workpiece can be further reflected using a mirror. Reflecting the light beam from the workpiece can direct the light beam to the mirror.

[0055] The light beam may be reflected from a second mirror positioned between the light source and the workpiece in the path of the light beam.The light beam reflected from the workpiece may be reflected from a third mirror positioned between the workpiece and the mirror in the path of the light beam.

[0056] The light beam reflected from the workpiece can be split using a beam splitter.At least some of the light beam is directed by the beam splitter to a sensor.

[0057] The light beam may be focused as it travels along its path.

[0058] Height sensor tooling camera images of patterns on the wafer can be used to perform positional and focus alignment. For example, the streets between dies on the workpiece can be used for alignment in the X and Y directions. Geometric patterns can be used to adjust focus or field tilt.

[0059] Figure 3 Exemplary test results using system 100 are shown. Figure 3 Used to determine the height sensor conjugate reference during the alignment process. The slit height variation for Tool #4, which may include an embodiment of system 100, is shown. This is compared to a benchtop tool and two other tools (Tool #1 and Tool #2) of an embodiment of system 100. The image on the left is the L target. The images on the right are the corresponding wafer images at different conjugate positions for each tool.

[0060] like Figure 3 Tool #1 has a sharp outer edge compared to a standard wafer result with good patterning effect (PE) as shown in Figure 4. There is a difference in image quality between the L target and the wafer.

[0061] As the slit height on tool #4 changes, the dataray image becomes clearer. Finally, the RS pattern becomes visible. The pattern effect decreases by approximately 450 nm as the slit height changes from 0.4 mm to -1.2 mm. This change results in a pattern effect of 240 nm.

[0062] Figure 4 Description Compared with single pass Figure 1A double pass of an embodiment of the system of FIG. If the workpiece is a high contrast wafer, then a single pass may not be sufficient. The system may read this as a height difference, which is incorrect because the wafer is planar. A high contrast wafer contains a brighter side on the right side, which reflects more height. A method similar to that used in Figure 1 The double pass performed in the embodiment of avoids this problem. Using a spherical mirror and two passes of the light beam means that the images neutralize each other, which is shown in the lower right example.

[0063] Although the present disclosure has been described with respect to one or more specific embodiments, it should be understood that other embodiments of the present disclosure can be made without departing from the scope of the present disclosure. Therefore, the present disclosure is considered to be limited only by the appended claims and a reasonable interpretation thereof.

Claims

1. A system comprising: a storage table configured to hold a workpiece; an electron beam column configured to direct an electron beam toward the workpiece on the stage; a light source configured to generate a light beam at the workpiece on the stage; a sensor configured to receive the light beam reflected from the workpiece; a mirror configured to reflect the light beam received from the workpiece toward the sensor; and A processor is in electronic communication with the sensor, wherein the processor is configured to determine a displacement from a nominal distance between the electron beam column and the workpiece on the stage using measurements from the sensor.

2. The system of claim 1 , further comprising a second mirror and a third mirror, wherein the second mirror and the third mirror are positioned in a path of the light beam, wherein the second mirror is positioned to direct the light beam from the light source at the workpiece, and wherein the third mirror is positioned to direct the light beam from the workpiece to the mirror. 3 . The system of claim 2 , further comprising a plano-convex lens disposed in the path of the light beam between the second mirror and the workpiece.

4. The system of claim 2, wherein the second mirror and the third mirror are each folding mirrors.

5. The system of claim 1, further comprising a beam splitter positioned in a path of the light beam between the workpiece and the light source, wherein the beam splitter directs at least some of the light beam at the sensor. The system of claim 1 , wherein the light source is a light emitting diode.

7. The system of claim 1, further comprising a convex lens located between the light source and the workpiece in the path of the light beam.

8. The system of claim 1, further comprising a slit located in the path of the light beam between the light source and the workpiece.

9. The system of claim 1, wherein the mirror is a spherical mirror.

10. The system of claim 1, wherein the workpiece is a semiconductor wafer.

11. A method comprising: directing a light beam from a light source to a workpiece on a stage, wherein the workpiece is positioned at a certain absolute distance from the electron beam column; causing the light beam to reflect from the workpiece; receiving, at a sensor, the light beam reflected from the workpiece; and A processor is used to determine a displacement from a nominal distance between the electron beam column and the workpiece on the stage.

12. The method of claim 11, wherein the workpiece is a semiconductor wafer.

13. The method of claim 11, further comprising reflecting the light beam reflected from the workpiece using a mirror, and wherein reflecting the light beam from the workpiece directs the light beam to the mirror.

14. The method of claim 13, wherein the directing comprises reflecting the light beam from a second mirror disposed in a path of the light beam between the light source and the workpiece.

15. The method of claim 14, wherein the reflecting the light beam from the workpiece comprises reflecting the light beam from a third mirror disposed between the workpiece and the mirror in the path of the light beam.

16. The method of claim 11, further comprising splitting the light beam reflected from the workpiece using a beam splitter, wherein some of the light beam is directed by the beam splitter to the sensor.

17. The method of claim 11, wherein the directing further comprises focusing the light beam.

18. The method of claim 11, further comprising directing the light beam reflected from the workpiece a second time at the same point on the workpiece before receiving the light beam at the sensor.

19. The method of claim 11, further comprising adjusting a height of a storage platform based on the displacement from the nominal distance.

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