Positioning system and method

Abstract

A grid plate encoder based positioning system (1) for positioning an element is provided, the positioning system (1) comprising a grid plate (2) having a grid plate surface (21), an encoder unit (3) having one or more optical sensors (31) for sensing a grid plate surface pattern (23) on the grid plate surface (21), an input device (7) for receiving coordinates (Xd, Yd) specifying a desired position of the element, a mapping unit (8) for calculating corrective coordinate data (Xa, Ya) corresponding to estimated position data expected from the encoder unit (3) when the element is positioned at the desired position (Xd, Yd) specified by the set point coordinates, and a feedback control unit (9) for providing the corrective coordinate data (Xa, Ya) as set points (Xs, Ys) to a positioning unit (12) using feedback control based on the estimated position data obtained from the encoder unit. Further provided are a grid plate encoder based positioning method and a method for calculating the correction data.

Description

[Technical Field]

[0001] background The present invention relates to a positioning system based on a grid plate encoder.

[0002] The present invention further relates to a method for positioning an element.

[0003] The invention further relates to a method for calculating correction data for positioning.

[0004] The invention still further relates to a record carrier comprising a computer program having instructions for enabling a programmable processor to perform one or more steps of any of the methods. [Background technology]

[0005] Various production and quality inspection systems require highly accurate positioning of production or inspection system elements relative to the products being processed, such as systems for manufacturing electronic, optical, or optoelectronic products at the nanoscale.

[0006] Positioning systems are known that use a flat 2D encoder scale, also known as a grid plate, in combination with an encoder head assembly. The encoder head assembly may include multiple encoder readheads, e.g., first, second, and third 1D encoder readheads. A combination of three 1D encoder readheads (at least two of which have different decoding directions) may be used to determine the position of the encoder head assembly in three degrees of freedom (DOF), i.e., X, Y, and Rz. The grid plate typically comprises a two-dimensional periodic pattern, such as reflective grid lines on a non-reflective background, or vice versa. For example, the encoder head assembly may comprise one 1D encoder readhead for the X direction and two 1D encoder readheads for the Y direction. Examples of commercially available grid encoders include the KGM181, KGM182, KGM281, and KGM282 available from Heidenhain. Summary of the Invention [Problem to be solved by the invention]

[0007] Depending on the patterning method, the grid plate pattern may have manufacturing imperfections that lead to signal imperfections and limit the accuracy of the position signal obtained from the encoder. Therefore, a solution is needed that allows for improved accuracy. [Means for solving the problem]

[0008] overview According to a first aspect of the present invention, there is provided an improved grid plate encoder based positioning system for element positioning, the grid plate encoder based positioning system comprising: a grid plate having a grid plate surface; an encoder unit having one or more optical sensors for sensing a grid plate surface pattern on the grid plate surface; an input device for receiving coordinates specifying a desired location of the element; a mapping unit for calculating corrected coordinate data corresponding to estimated position data expected from the encoder unit when the element is positioned at a desired position specified by the setpoint coordinates; a feedback control unit that uses feedback control based on estimated position data obtained from the encoder unit to provide correction coordinate data to the positioning unit as a set value; Includes:

[0009] The combination of encoder unit and grid plate may be selected from commercially available products, for example, as specified above. Even if the grid plate has manufacturing defects, this improved position system provides accurate positioning of the element, as the mapping unit takes into account deviations in the estimated position data due to the defects.

[0010] The correction table may be used to calculate corrected coordinate data corresponding to estimated position data expected from the encoder unit when the element is positioned at a desired position specified by setpoint coordinates. Interpolation may be applied to calculate interpolated corrected coordinate data for desired positions having coordinate values ​​other than those stored in the correction table. In an alternative embodiment, a polynomial is used to calculate the corrected coordinate data from data specifying the desired position.

[0011] In one embodiment, a grid plate encoder based positioning system includes: a wafer stage for carrying a wafer having optically detectable marks on a surface of the wafer facing the grid plate surface; a calibration head movable between the wafer stage and the grid plate, the calibration head including an encoder unit having one or more optical sensors for detecting a grid plate surface pattern on the grid plate surface, and a mark sensor for detecting optically detectable marks on a reference wafer carried by the wafer stage; a calibration head positioning unit for positioning the calibration head at a plurality of different lateral positions, and a calibration control unit for controlling the calibration head positioning unit and calculating correction data for use by the mapping unit from measurement data obtained from the further encoder unit and the mark sensor at the plurality of different lateral positions. Further includes:

[0012] During the calibration phase, the calibration head positioning unit positions the calibration head at multiple different lateral positions distributed within an X, Y plane defined parallel to the grid plate and defined by marks on the surface of the reference wafer. At each of these positions, sensed data is acquired from the mark sensor and encoder unit. The sensed data acquired from the mark sensor is considered to represent the true position of the calibration head. Thus, each pair of mark sensor data and encoder sensor data provides an indication of the position estimated by the encoder at the position indicated by the mark sensor using a possibly defective grid plate. The data acquired in this way can be used to provide, for each different lateral position, a correction table having table entries containing data specifying the corresponding (possibly deviating) position addressable at the position indicated by the mark sensor and estimated by the encoder. Additional further table entries can be provided by interpolation, e.g., bilinear interpolation, of the table entries acquired for a particular mark position. Additionally or alternatively, interpolation can be applied to the correction table data following the calibration phase. In another embodiment, the mark sensor data and encoder sensor data acquired during the calibration phase are used to calculate a polynomial that defines the relationship between positions expected as estimates for each real position within the spatial range defined by the grid.

[0013] The reference wafer used for detection by the mark sensor may be one specially designed for this purpose. However, alternatively, a customer wafer may be used as the reference wafer if it has optically detectable marks, such as general alignment markers, on its surface. The reference wafer may further include a fiducial for obtaining a reference position. Alternatively or additionally, the wafer stage may be provided with the fiducial.

[0014] The mark sensor may include any commercially available camera available for this purpose, including an objective lens, a tube lens, a camera, an objective lens actuation mechanism for focusing on the wafer, wafer illumination, etc. As an example, a Basler acA4024-29um USB 3.0 camera can be used for this purpose.

[0015] It should be noted that once calibration is complete, the elements specifically involved in the calibration may be removed and reused for other purposes. Thus, the calibration head, calibration head positioning unit, and calibration control unit are not required components in a calibrated positioning system. A wafer stage may be absent unless the positioning system is included in a wafer handling system.

[0016] For example, in one embodiment, the grid plate encoder-based positioning system is included in an atomic force microscope (AFM) system for wafer inspection. The AFM further includes an AFM head having a probe with a tip positioned by the positioning system, the AFM head including an encoder unit. In operation, the positioning unit positions the AFM head at a position where the encoder unit indicates coordinates that correspond to corrected coordinates from the correction unit.

[0017] According to a second aspect of the present invention there is provided an improved method of positioning an element, comprising the steps of: providing a grid plate having a grid plate surface; providing an encoder unit having one or more optical sensors for sensing a grid plate surface pattern on a grid plate surface; receiving coordinates specifying the desired location of the element; calculating corrected coordinate data corresponding to estimated position data expected from the encoder unit when the element is positioned at a desired position specified by the setpoint coordinates; Using feedback control, positioning the element at a position where the estimated coordinate data of the encoder unit corresponds to the calculated corrected coordinate data. An improved method is provided, which includes:

[0018] According to a third aspect, there is provided a method of calculating correction parameters for use with each grid plate in a grid plate encoder based positioning system that uses a grid plate having a grid plate surface with a surface pattern, the method comprising: providing a reference wafer having optically detectable marks on a surface of the wafer facing the grid plate surface; providing a calibration head between a grid plate and a reference wafer, the calibration head having a mark sensor facing the wafer surface and at least one or more optical sensors of a grid encoder facing the grid plate surface; positioning the calibration head at a plurality of laterally distinct positions of the calibration head between the grid plate and the reference wafer to obtain, from the mark sensor, respective position indications indicative of its sensed position relative to the optically detectable mark, and from the encoder, respective encoder position indications indicative of its position relative to the grid plate surface; calculating correction parameters based on the obtained position indications; Includes: BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects are described in more detail with reference to the drawings. [Brief explanation of the drawings]

[0019] [Figure 1] 1 shows a schematic representation of a positioning system based on a grid plate encoder. [Figure 2]2 shows further details of the grid plate encoder based positioning system of FIG. 1; [Figure 3] 1 illustrates an aspect of a method for calculating a correction table for use in a grid plate encoder based positioning system. DETAILED DESCRIPTION OF THE INVENTION

[0020] Detailed Description of the Embodiments 1 shows a schematic diagram of a grid plate encoder-based positioning system 1 for positioning an element. In the illustrated embodiment, the positioning system 1 is part of an atomic force microscope system AFM100. Here, the element to be positioned is the tip 13b of a probe 13a of an AFM head 13. In another example, the element to be positioned is a production system element. The positioning system 1 includes a grid plate 2, an encoder unit 3, a specified coordinate input device 7, a compensation table 6, a mapping unit 8, a feedback control unit 9, and a positioning unit 12.

[0021] The input device 7 is provided for receiving coordinates (Xd, Yd) specifying the desired position of the element 13b relative to the surface 42 of the customer wafer 4 carried by the wafer stage 14. The encoder unit 3 is part of the AFM head 13 and includes one or more optical sensors 31 and a signal processor 32 that enables the encoder unit 3 to estimate the position of the encoder unit 3 relative to the grid plate surface 21 of the grid and provide its estimated coordinates (Xe, Ye). Alternatively, processing of the signals from the encoder sensor 31 can be performed remotely from the AFM head. The grid plate 2 has a grid plate surface 21 with a grid plate surface pattern 23 indicated by a thick dashed line. Imperfections in the grid plate surface pattern 23 may cause the estimated coordinates to deviate from the true coordinates of the encoder unit 3.

[0022] The mapping unit 8 uses the correction table 6 to calculate corrected coordinate data (Xa, Ya) corresponding to the estimated position data expected from the encoder unit 3 when the element is positioned at the desired position (Xd, Yd) specified by the setpoint coordinates. If the encoder unit 3 in the AFM head does not include a signal processor 32, the mapping unit 8 may have a signal processing module that provides the estimated coordinates from the detection signal of the optical sensor 31.

[0023] The feedback control unit 9 provides the corrected coordinate data (Xa, Ya) to the positioning unit 12 as setpoints (Xs, Ys), and uses feedback control to achieve a position indicated by the encoder unit 3 having estimated coordinates corresponding to the corrected coordinate data (Xa, Ya). As a result, the actual probe tip 13b is positioned by the positioning unit 12 at the position indicated by the desired position (Xd, Yd), despite imperfections in the grid plate surface pattern 23.

[0024] As shown in FIG. 1, the grid plate encoder based positioning system incorporated in the atomic force microscope system AFM 100 further includes a calibration head 10 equipped with another encoder unit 3 a and a mark sensor 5 .

[0025] 2, the grid plate encoder based positioning system includes a calibration head positioning unit 12a and a calibration control unit 11. For clarity, the elements involved in positioning the AFM head 13, apart from the correction table 6, are not shown.

[0026] Figure 2, in combination with Figure 3, also illustrates generally a method for calculating a correction table 6 for use with a grid plate 2 in a grid plate encoder based positioning system 1. The method comprises the following steps:

[0027] A reference wafer 4r is provided having a wafer surface 42 bearing optically detectable marks 4-1, 4-2, . . . , 4-n facing the grid plate surface 21.

[0028] A calibration head 10 is provided between the grid plate 2 and the reference wafer 4r. The calibration head 10 has a mark sensor 5 facing the wafer surface and having at least one or more optical sensors 31 of a grid encoder facing the grid plate surface 21.

[0029] 2, with the configuration thus obtained, the calibration head 10 is then positioned at a plurality of laterally different positions (j=1, ..., n) between the grid plate 2 and the reference wafer 4. At each of the laterally different positions, a respective mark sensor position indication Bj of the mark sensor 5 and a respective encoder position indication Dj of the encoder are obtained. The mark sensor position indication Bj indicates the sensing position of the mark sensor relative to the optically detectable mark 41 on the wafer surface 42, and the encoder position indication Dj indicates the sensing position of the encoder relative to the grid plate surface 21. A respective calibration parameter Nj of the correction table 6 is calculated for each pair of mark sensor position indication Bj and encoder position indication Dj.

[0030] FIG. 3 schematically illustrates the calibration head 10 at the first position as 10(1) and at the nth position as 10(n). Furthermore, FIG. 3 illustrates the known distance Q between the first mark 4-1 and the last mark 4-n. The value C represents the position of the mark sensor relative to the encoder. The grid map calibration parameters N1 through Nm are determined from the n measurements. The calibration parameters N1 through Nm are, for example, respective entries in a correction table, where each table entry is addressable as a desired position and contains data specifying the corresponding (possibly deviating) position estimated by the encoder. The table entries N1 through Nm may include n entries obtained directly from the n measurements, i.e., each position indicated by the mark sensor is used as the address of a table entry, and the position indicated by the encoder is stored in the entry having that address. Furthermore, m through n additional entries in the table may be calculated by interpolation. If the resolution at which the desired position is specified exceeds the resolution of the table 6, correction coordinate data may be calculated by interpolating the table data for desired positions that do not have entries in the table. In an alternative embodiment, the grid map calibration parameters N1 through Nm are parameters that specify a polynomial that specifies the corrected coordinate data as a function of the desired coordinate data.

[0031] It will be understood that the method may include computer-implemented steps. All steps described above may be computer-implemented steps performed, for example, by a special purpose signal processor, a configurable signal processor, a general-purpose processor, or a combination of processors. Various embodiments may be implemented using hardware elements, software elements, or a combination of both. Examples of hardware elements may include processors, microprocessors, circuits, application-specific integrated circuits (ASICs), programmable logic devices (PLDs), digital signal processors (DSPs), field-programmable gate arrays (FPGAs), logic gates, registers, semiconductor devices, microchips, chipsets, etc. Examples of software may include software components, programs, applications, computer programs, application programs, system programs, machine programs, operating system software, mobile apps, middleware, firmware, software modules, routines, subroutines, functions, computer-implemented methods, procedures, software interfaces, application program interfaces (APIs), methods, instruction sets, computing code, computer code, etc.

[0032] The invention also extends to computer programs, in particular computer programs on or in a carrier adapted for carrying out the invention. The program may be in the form of source or object code or in any other form suitable for use in the implementation of the processes according to the invention. The carrier may be any entity or device capable of carrying a program. For example, the carrier may comprise a storage medium such as a ROM, e.g. a semiconductor ROM, or a hard disk. Furthermore, the carrier may be a transmissible carrier, such as an electric or optical signal, which may be conveyed via an electric or optical cable, or by radio or other means, e.g. via the Internet or the cloud.

[0033] Some embodiments may be implemented, for example, using a machine or tangible computer-readable medium or article capable of storing instructions or sets of instructions that, when executed by a machine, cause the machine to perform methods and / or operations in accordance with the embodiments.

[0034] The present invention has been described herein with reference to specific exemplary embodiments thereof. However, it will be apparent that various modifications, variations, substitutions, and alterations can be made thereto without departing from the essence of the invention. While features are described herein as part of the same or separate embodiments for purposes of clarity and conciseness, it is understood that alternative embodiments having all or any combination of the features described in those separate embodiments are also contemplated and fall within the scope of the invention as outlined by the claims. Accordingly, the specification, figures, and examples are to be regarded in an illustrative, and not a restrictive, sense. The invention is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims. Furthermore, many of the described elements are functional entities that can be implemented as discrete or distributed components, or in conjunction with other components, in any suitable combination and location.

[0035] In the claims, reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words "a" and "an" shall not be construed as limiting to "one only" but are instead used to mean "at least one" and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A grid-plate encoder-based positioning system (1) for element positioning, comprising: a grid plate (2) having a grid plate surface (21); an encoder unit (3) having one or more optical sensors (31) for detecting a grid plate surface pattern (23) on the grid plate surface (21); an input device (7) for receiving coordinates (Xd, Yd) specifying the desired position of said element; a mapping unit (8) for calculating corrected coordinate data (Xa, Ya) corresponding to estimated position data expected from the encoder unit (3) when the element is positioned at a desired position (Xd, Yd) specified by the setpoint coordinates; a feedback control unit (9) that provides the correction coordinate data (Xa, Ya) to a positioning unit (12) as set values ​​(Xs, Ys) using feedback control based on the estimated position data acquired from the encoder unit; A grid plate encoder based positioning system (1) comprising:

2. 2. The grid plate encoder based positioning system (1) according to claim 1, wherein the mapping unit (8) calculates the corrected coordinate data (Xa, Ya) using a correction table (6).

3. a wafer stage (14) for carrying a wafer (4, 4r) having an optically detectable mark (41) on a wafer surface (42) facing said grid plate surface (21); a calibration head (10) movable between the wafer stage (14) and the grid plate (2), the calibration head (10) including an encoder unit (3a) having one or more optical sensors (31a) for detecting the grid plate surface pattern (23) on the grid plate surface (21), and a mark sensor (5) for detecting optically detectable marks (41) on a reference wafer (4r) carried by the wafer stage (14); a calibration head positioning unit (12a) for positioning the calibration head (10) at a plurality of different lateral positions, and a calibration control unit (11) for controlling the calibration head positioning unit (12a) and for calculating correction data for use by the mapping unit (8) from measurement data (Di, Bi) acquired from the further encoder unit (3a) and the mark sensor (5) at the plurality of different lateral positions. Grid plate encoder based positioning system (1) according to claim 1, further comprising:

4. 3. An atomic force microscope (AFM) system (100) including a grid plate encoder-based positioning system (1) according to claim 1 or 2, further including an AFM head (13) having a probe (13a) with a tip (13b) positioned by the positioning system (1), the AFM head (13) including the encoder unit (3).

5. A method for positioning an element (13b), comprising the steps of: providing a grid plate (2) having a grid plate surface (21); providing an encoder unit (3) having one or more optical sensors (31) for detecting a grid plate surface pattern (23) on said grid plate surface (21); receiving coordinates (Xd, Yd) specifying a desired location of said element; calculating corrected coordinate data (Xa, Ya) corresponding to estimated position data (Xe, Ye) predicted from the encoder unit (3) when the element is positioned at the desired position (Xd, Yd) specified by the setpoint coordinates; Using feedback control, positioning the element at a position where the estimated coordinate data (Xe, Ye) of the encoder unit corresponds to the calculated corrected coordinate data (Xa, Ya). A method comprising:

6. A method for positioning an element (13b) using a grid plate encoder based positioning system (1) according to claim 1, comprising the steps of: The setpoint coordinates (X ｄ , Y ｄ ) to provide calculating corrected coordinate data (Xa, Ya) indicative of estimated position data expected from the encoder unit (3) when the element is positioned at the desired position; The corrected coordinate data (X ａ , Y ａ ) as a setpoint to the positioning unit. A method comprising:

7. A method of calculating correction parameters for use with a respective grid plate in a grid plate encoder-based positioning system (1), the grid plate (2) having a grid plate surface (21) with a surface pattern, the grid plate encoder-based positioning system (1) including an encoder unit (3) having one or more optical sensors (31) for optically detecting the surface pattern on the grid plate surface (21), and a signal processor (32) for calculating estimated position data from detection signals obtained from the one or more optical sensors (31), the method comprising: providing a reference wafer (4) having optically detectable marks (41) on a wafer surface (42) facing said grid plate surface (21); providing a calibration head (10) between the grid plate (2) and the reference wafer (4), the calibration head (10) having a mark sensor (5) facing the wafer surface and at least the one or more optical sensors (31) of a grid encoder facing the grid plate surface (21); positioning the calibration head (10) at a plurality of laterally different positions (j=1,...,n) between the grid plate (2) and the reference wafer (4) to obtain from the mark sensor (5) respective position indications (Bj) indicative of its sensed position relative to the optically detectable mark (41), and from the encoder (3) respective encoder position indications (Dj) indicative of its position relative to the grid plate surface (21); calculating correction parameters (Nj) based on the acquired position indications (Bj, Dj); A method comprising:

8. A record carrier containing a computer program having instructions for enabling a programmable processor to perform one or more steps of the method according to any one of claims 5 to 7.

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