WO2021251190A1

WO2021251190A1 - Image generating method

Info

Publication number: WO2021251190A1
Application number: PCT/JP2021/020617
Authority: WO
Inventors: 浩太郎丸山; 伸一中澤
Original assignee: Ｔａｓｍｉｔ株式会社
Priority date: 2020-06-10
Filing date: 2021-05-31
Publication date: 2021-12-16
Also published as: JP2021197225A

Abstract

The present invention relates to a method for generating an image of a wafer using a scanning electron microscope, and in particular relates to a technique for adjusting the direction of incidence of an electron beam. Provided is a method for generating an image of a three-dimensional structural pattern (203) formed on a wafer (124), the method comprising: measuring the heights of a plurality of height measurement points (202) on the wafer (124) placed on a sample stage (121); calculating a normal vector to a surface of the wafer at the position of the three-dimensional structural pattern (203) using the heights of the plurality of height measurement points (202); irradiating the wafer (124) including the three-dimensional structural pattern (203) with an electron beam with the direction of incidence of the electron beam parallel to the normal vector; detecting electrons emitted from the wafer (124); and generating an image of the three-dimensional structural pattern (203) from an electron detection signal.

Description

Image generation method

The present invention relates to a method of generating an image of a wafer using a scanning electron microscope, and particularly to a technique of adjusting the incident direction of an electron beam.

Conventional semiconductor devices have continued to be miniaturized within the planar structure. However, especially in NAND flash memory, which has advanced the cutting edge of miniaturization, the number of electrons that can be stored in the storage element has decreased due to the influence of miniaturization, and the reliability of the device has deteriorated due to the physical limit of the device characteristics. As a solution to this problem, the physical limit of the device characteristics is solved by the three-dimensional memory cell structure in which the storage elements are arranged in the vertical direction, and the capacity of the storage elements is increased.

NAND flash memory manufacturers are shifting their direction from the development of conventional semiconductor device miniaturization technology to the development of stacking technology for 3D memory structures, aiming to further increase the capacity by increasing the number of stacks. In 2020, devices with a three-dimensional memory structure with nearly 100 layers have already been commercialized, and there is a demand for an increase in the number of layers in the future.

Japanese Unexamined Patent Publication No. 2000-348658

Warpage occurs in the wafer on which the device is formed due to thermal stress or the like applied in the device manufacturing process. The amount of change in the height direction of the wafer due to warpage is about several hundred μm at the maximum for a wafer such as a 3D NAND flash memory. Assuming that the amount of change in the height direction is 300 μm, the inclination of the contact in the vertical direction with respect to the wafer surface is
300 μm (change in height direction) / 15 mm (wafer radius) = 1/500 (rad)
Will be.

On the other hand, the contact height of the 3D NAND device is up to 10 μm. The measurement accuracy of the positional deviation (tilt) between the top and bottom of the contact is required to be 5 nm or less. In other words, the accuracy of tilt measurement is
5 nm / 10 μm = 5/10000 (rad)
The following is required. The inclination of the contact due to the warp of the wafer is larger than the required measurement accuracy, which greatly affects the measurement result.

As shown in FIG. 5, in a measuring device using an SEM or the like, a suction type wafer chuck 500 using an electrostatic chuck is generally used, but the wafer chuck 500 forcibly warps the entire surface of the wafer W. Cannot be suppressed. When the inclination of the contact is measured with the wafer W warped, the electron beam does not enter perpendicularly to the surface of the wafer W as shown in FIG. Therefore, the accuracy of the tilt measurement is lowered, for example, the electron beam does not reach the bottom of the contact 600.

Therefore, the present invention provides a technique capable of incident an electron beam perpendicular to the wafer surface.

In one aspect, it is a method of generating an image of a three-dimensional structural pattern formed on a wafer, in which the heights of a plurality of height measuring points on the wafer placed on a sample stage are measured, and the plurality of heights are measured. The height of the measurement point is used to calculate the normal vector of the wafer surface at the position of the three-dimensional structural pattern, and the three-dimensional structure is obtained in a state where the incident direction of the electron beam is parallel to the normal vector. Provided is a method of irradiating the wafer including the structural pattern with the electron beam, detecting electrons emitted from the wafer, and generating an image of the three-dimensional structural pattern from the detection signal of the electrons.

In one aspect, the step of calculating the normal vector uses the heights of the plurality of height measurement points to determine an approximate expression representing the height of the wafer surface, and the coordinates of the three-dimensional structure pattern and the said. This is a step of calculating the normal vector of the wafer surface at the position of the three-dimensional structural pattern from the approximate expression.
In one aspect, the plurality of height measurement points are at least three height measurement points located around the three-dimensional structural pattern.

In one aspect, the step of irradiating the wafer with the electron beam includes the three-dimensional structural pattern in a state where the electron beam is tilted so that the incident direction of the electron beam is parallel to the normal vector. This is a step of irradiating the wafer with the electron beam.
In one aspect, the step of irradiating the wafer with the electron beam includes the three-dimensional structural pattern in a state where the sample stage is tilted so that the incident direction of the electron beam is parallel to the normal vector. This is a step of irradiating the wafer with the electron beam.
In one aspect, the step of calculating the inclination of the three-dimensional structural pattern from the image is further included.
In one aspect, the three-dimensional structural pattern is one or more patterns having dimensions in the depth direction of the wafer.
In one aspect, the three-dimensional structure pattern comprises an upper layer pattern and a lower layer pattern, and the method further includes a step of calculating the positional deviation between the upper layer pattern and the lower layer pattern from the image.

According to the present invention, the electron beam is vertically incident on the wafer even if the wafer is warped. Therefore, it is possible to accurately measure the inclination of a three-dimensional structural pattern such as a contact formed on a wafer.

It is a schematic diagram which shows one Embodiment of an image generation apparatus. It is a schematic diagram explaining one Embodiment of the method of measuring the inclination of a wafer surface. It is a schematic diagram explaining one embodiment of the method of measuring the local inclination of a wafer surface. It is a schematic diagram which shows the stage actuator which tilts a sample stage. It is a schematic diagram which shows the wafer held in the electrostatic chuck. It is a schematic diagram which shows the electron beam incident on a wafer.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing an embodiment of an image generator. As shown in FIG. 1, the image generator includes a scanning electron microscope 100 and an arithmetic system 150. The scanning electron microscope 100 is connected to the arithmetic system 150, and the operation of the scanning electron microscope 100 is controlled by the arithmetic system 150.

The calculation system 150 includes a storage device 162 in which the program is stored and a processing device 163 that executes the calculation according to the instructions included in the program. The processing device 163 includes a CPU (central processing unit) or a GPU (graphic processing unit) that performs operations according to instructions included in a program stored in the storage device 162. The storage device 162 includes a main storage device (eg, random access memory) accessible to the processing device 163 and an auxiliary storage device (eg, hard disk drive or solid state drive) for storing data and programs.

The arithmetic system 150 includes at least one computer. For example, the arithmetic system 150 may be an edge server connected to the scanning electron microscope 100 by a communication line, or may be a cloud server connected to the scanning electron microscope 100 by a communication network such as the Internet or a local network. It may be a fog computing device (gateway, fog server, router, etc.) installed in a network connected to the scanning electron microscope 100. The arithmetic system 150 may be a combination of a plurality of servers. For example, the arithmetic system 150 may be a combination of an edge server and a cloud server connected to each other by a communication network such as the Internet or a local network. In another example, the arithmetic system 150 may include a plurality of servers (computers) that are not connected by a network.

The scanning electron microscope 100 includes an electron gun 111 that emits an electron beam composed of primary electrons (charged particles), a focusing lens 112 that focuses the electron beam emitted from the electron gun 111, and an X deflector that deflects the electron beam in the X direction. It has 113, a Y deflector 114 that deflects an electron beam in the Y direction, and an objective lens 115 that focuses an electron beam on a wafer 124 which is an example of a sample. The configuration of the electron gun 111 is not particularly limited. For example, a field emitter type electron gun, a semiconductor photocathode type electron gun, or the like can be used as the electron gun 111.

The focusing lens 112 and the objective lens 115 are connected to the lens control device 116, and the operation of the focusing lens 112 and the objective lens 115 is controlled by the lens control device 116. The lens control device 116 is connected to the arithmetic system 150. The X deflector 113 and the Y deflector 114 are connected to the deflection control device 117, and the deflection operation of the X deflector 113 and the Y deflector 114 is controlled by the deflection control device 117. The deflection control device 117 is also connected to the arithmetic system 150 in the same manner. The secondary electron detector 130 and the backscattered electron detector 131 are connected to the image acquisition device 118. The image acquisition device 118 is configured to convert the output signal of the secondary electron detector 130 and the output signal of the backscattered electron detector 131 into an image, respectively. The image acquisition device 118 is also connected to the arithmetic system 150 in the same manner.

The sample stage 121 arranged in the sample chamber 120 is connected to the stage control device 122, and the position of the sample stage 121 is controlled by the stage control device 122. The stage control device 122 is connected to the arithmetic system 150. A transfer device 140 for mounting the wafer 124 on the sample stage 121 in the sample chamber 120 is also connected to the arithmetic system 150.

The electron beam emitted from the electron gun 111 is focused by the focusing lens 112, then focused by the objective lens 115 while being deflected by the X deflector 113 and the Y deflector 114, and is irradiated on the surface of the wafer 124. When the wafer 124 is irradiated with the primary electrons of the electron beam, the secondary electrons and backscattered electrons are emitted from the wafer 124. Secondary electrons are detected by the secondary electron detector 130, and backscattered electrons are detected by the backscattered electron detector 131. The secondary electron detection signal output from the secondary electron detector 130 and the backscattered electron detection signal output from the backscattered electron detector 131 are input to the image acquisition device 118, and the secondary electron image and the backscattered electron image are input. Each is converted to.

Above the sample stage 121, a displacement sensor 170 as a height measuring device for measuring the height of the surface of the wafer 124 on the sample stage 121, and a light source 171 that emits light toward the surface of the wafer 124 are arranged. Has been done. The light source 171 and the displacement sensor 170 are arranged so that the light from the light source 171 is reflected on the surface of the wafer 124 and reaches the displacement sensor 170. The positions of the light source 171 and the displacement sensor 170 are not particularly limited as long as the surface height of the wafer 124 can be measured, but in the present embodiment, the displacement sensor 170 is located on the upper surface of the sample chamber 120. The configuration of the displacement sensor 170 is not particularly limited, but in the present embodiment, the displacement sensor 170 is an optical displacement sensor.

Next, a method of measuring the inclination of the wafer around the pattern to be measured will be described.
FIG. 2 is a schematic view of the wafer 124 held on the sample stage 121 by the electrostatic chuck as viewed from above. The arithmetic system 150 sets a plurality of height measurement points 202 on the entire surface of the wafer 124 having the contact 203, which is an example of the three-dimensional structural pattern to be measured. The number of height measurement points 202 is not particularly limited, but in one example, four or more height measurement points 202 are set.

The displacement sensor 170 measures the height of each of the plurality of height measuring points 202. The height measurement is sent to the arithmetic system 150. The height of each height measurement point 202 is the height from a predefined reference plane. The reference plane is a virtual plane. For example, the reference plane may be the upper surface of the sample stage 121. The height z at the coordinates (x, y) on the wafer surface can be expressed by the following approximate expression.
z = a ₁ x ² + a ₂ y ² + a ₃ xy + a ₄ x + a ₅ y + a ₆ = f (x, y) ... (1)

Let the coordinates of the height measurement point 202 be (Xn, Yn) and the measured height be Zn. The calculation system 150 applies the least squares method to the approximate expression (1), and the height z of the plurality of height measurement points 202 calculated by using the approximate expression (1), and each height measurement. Coordinates (x) on the wafer surface by determining the _{coefficients a 1} , a ₂ , a ₃ , a ₄ , a ₅ , a ₆ with the smallest error from the height Zn measured at the point (Xn, Yn). , Y), an approximate expression f (x, y) indicating the height z is obtained. The arithmetic system 150 stores the approximate expression (1) determined as described above in the storage device 162.

When measuring the inclination of the contact 203, the calculation system 150 calculates the normal vector of the wafer surface at the position of the contact 203 from the coordinates of the contact 203 and the above approximate expression f (x, y). Further, the arithmetic system 150 tilts the electron beam so that the incident direction of the electron beam is parallel to the normal vector. More specifically, the arithmetic system 150 issues a command to the deflection control device 117 to control the voltage applied to the X deflector 113 and the Y deflector 114, thereby tilting the electron beam and the incident direction of the electron beam. Is parallel to the normal vector. The electron beam irradiates the wafer 124 including the contact 203 with the incident direction of the electron beam parallel to the normal vector.

When the wafer 124 is irradiated with an electron beam, secondary electrons and backscattered electrons are emitted from the wafer 124. Secondary electrons are detected by the secondary electron detector 130, and backscattered electrons are detected by the backscattered electron detector 131. The image acquisition device 118 generates a secondary electron image and a backscattered electron image from the secondary electron detection signal and the backscattered electron detection signal, respectively.

Contact 203 is a pattern with a high aspect ratio. The inclination of the contact 203 can be obtained from the reflected electron image of the contact 203. That is, the scanning electron microscope 100 generates a backscattered electron image in which the top and bottom of the contact 203 appear, and the arithmetic system 150 acquires the backscattered electron image of the contact 203 from the scanning electron microscope 100 and puts it on the backscattered electron image. The inclination and taper angle of the contact 203 are calculated from the positions of the top and bottom of the contact 203. The inclination of the contact 203 is the inclination of the entire contact 203, and the taper angle of the contact 203 is the inclination angle of the side wall of the contact 203. According to the present embodiment, since the electron beam is vertically incident on the wafer surface, the arithmetic system 150 can accurately measure the inclination of the contact 203.

In the above-described embodiment, the contact 203 is used as an example of the three-dimensional structure pattern, but the three-dimensional structure pattern may be one or more patterns having dimensions in the depth direction of the wafer. For example, the tertiary structure pattern may include an upper layer pattern and a lower layer pattern instead of the contact 203. In this case, the calculation system 150 may calculate the positional deviation between the upper layer pattern and the lower layer pattern from the reflected electron images of the upper layer pattern and the lower layer pattern.

In the above-described embodiment, the optical displacement sensor 170 is used as the height measuring device for measuring the height of the height measuring point 202, but the present invention is not limited to this embodiment. In one embodiment, a laser interferometer or a capacitive displacement sensor may be used as the height measuring device.

In one embodiment, the height of the height measuring point 202 may be measured from the exciting current of the objective lens 115 instead of the height measuring device such as the displacement sensor 170. Specifically, the scanning electron microscope 100 generates an image of each height measurement point 202 in a state where the electron beam is focused on each height measurement point 202 by the objective lens 115, and the arithmetic system 150 generates an image of each height measurement point 202. The height of each height measurement point 202 may be determined from the exciting current to the lens 115. The exciting current to the objective lens 115 when in focus depends on the height of the height measuring point 202. The arithmetic system 150 stores in advance the relational expression between the exciting current to the objective lens 115 and the height of the height measuring point 202 in the storage device 162, and excites the objective lens 115 when the lens is in focus. The height of the height measuring point 202 corresponding to the current can be determined. The height measured in this way may be applied to the above formula (1).

In the above-described embodiment, a quadratic polynomial is used as an approximate expression for obtaining the height of the wafer, but a polynomial of degree 3 or higher can also be used according to the shape of the warp of the wafer. It is also possible to use interval approximation such as bi-tertiary splines.

In the above embodiment, the height of each of the plurality of height measurement points 202 distributed over the entire surface of the wafer 124 is measured, and the inclination of the wafer surface is applied to the entire surface of the wafer to approximate the warpage of the wafer. can. However, this method may not be accurate enough for local wafer tilt. Further, when measuring only a part of the contacts of the wafer, if the height is measured in advance over the entire surface of the wafer, the loss of measurement time becomes large. Therefore, as described below, in one embodiment, the height may be measured in advance only around the contact to be measured, and the local inclination may be calculated.

FIG. 3 is a schematic diagram illustrating an embodiment of a method for measuring a local inclination of a wafer surface. Since the operation of the present embodiment, which is not particularly described, is the same as that of the above-described embodiment described with reference to FIGS. 1 and 2, the duplicate description thereof will be omitted.

The arithmetic system 150 sets three height measurement points 210, 211, and 12 around the contact 214, which is an example of the three-dimensional structural pattern to be measured. The displacement sensor 170 (see FIG. 1) measures the heights of the height measuring points 210 and 211,212, respectively. The height measurements of the height measurement points 210, 211,212 are sent to the arithmetic system 150.

The coordinates (x, y) of the height measurement points 210, 211,212 and the measured height z are (x1, y1, z1), (x2, y2, z2), (x3, y3, z3), respectively. do. At this time, the normal vector 213 of the plane having the height measurement points 210, 211, and 122 as vertices can be expressed by the following equation.

As shown in FIG. 3, when measuring the inclination of the contact 214, the arithmetic system 150 calculates the normal vector 213 by the above equation (2) and makes the incident direction of the electron beam 215 parallel to the normal vector 213. .. More specifically, the arithmetic system 150 issues a command to the deflection control device 117 to control the voltage applied to the X deflector 113 and the Y deflector 114, thereby tilting the electron beam 215 and tilting the electron beam 215. Make the incident direction parallel to the normal vector 213. The electron beam 215 irradiates the wafer 124 including the contact 214 with the incident direction of the electron beam 215 parallel to the normal vector 213.

In the above-described embodiment, the normal vector of the plane is obtained using three height measurement points, but the height of four or more height measurement points can be measured to improve the accuracy of the normal vector. It is possible. Specifically, the following polynomial representing a plane is used.
z = a ₁ x + a ₂ y + a ₃ = f (x, y) ... (3)
Let the coordinates of each height measurement point be (Xn, Yn) and the measured height be Zn. The arithmetic system 150 applies the least squares method to the polynomial (3), and has a plurality of height measurement points height z calculated using the polynomial (3), and each height measurement point (Xn, _{By determining the coefficients a 1} , a ₂ , and a ₃ having the smallest error from the height Zn measured in Yn), a polynomial representing a plane and a normal vector of the plane can be obtained.

In the above-described embodiment, the optical displacement sensor 170 is used as the height measuring device for measuring the height of the height measuring points 210, 211,212, but the present invention is not limited to this embodiment. In one embodiment, a laser interferometer or a capacitive displacement sensor may be used as the height measuring device.

In one embodiment, instead of the height measuring device such as the displacement sensor 170, the height of the height measuring points 210, 211, and 12 may be measured from the exciting current of the objective lens 115. Specifically, the scanning electron microscope 100 generates an image of each height measurement point in a state where the electron beam is focused on each height measurement point by the objective lens 115, and the arithmetic system 150 generates an image of each height measurement point. The height of each height measuring point may be determined from the exciting current to. The height thus measured may be applied to the calculation of the above equation (2) or equation (3).

By the above method, the incident direction of the electron beam at the

contacts

203 and 214 is controlled, and the electron beam is irradiated to the

contacts

203 and 214 to acquire an image. The method of controlling the incident direction of the electron beam in the embodiments described so far is a method of tilting the electron beam by controlling the voltage applied to the X deflector 113 and the Y deflector 114, but the present invention is limited to this. Not done. In one embodiment, the sample stage 121 on which the wafer 124 is placed may be mechanically tilted to tilt the entire wafer 124 so that the incident direction of the electron beam is parallel to the normal vector of the wafer 124.

FIG. 4 is a schematic diagram showing a stage actuator 250 that tilts the sample stage 121. The scanning electron microscope 100 includes a plurality of stage actuators 250 that tilt the sample stage 121. These stage actuators 250 are arranged around the center of the sample stage 121 and support the sample stage 121. At least three stage actuators 250 are provided. The specific configuration of each stage actuator 250 is not particularly limited, but is composed of, for example, a piezoelectric element or a combination of a ball screw mechanism and a servomotor.

The stage actuator 250 is electrically connected to the arithmetic system 150, and the operation of the stage actuator 250 is controlled by the arithmetic system 150. Specifically, the arithmetic system 150 tilts the sample stage 121 so that the incident direction of the electron beam is parallel to the normal vector. The electron beam is irradiated to the wafer 124 including the contact, which is an example of the three-dimensional structural pattern, in a state where the incident direction of the electron beam is parallel to the normal vector.

The above-described embodiment is described for the purpose of allowing a person having ordinary knowledge in the technical field to which the present invention belongs to carry out the present invention. Various modifications of the above embodiment can be naturally made by those skilled in the art, and the technical idea of the present invention can be applied to other embodiments. Accordingly, the invention is not limited to the described embodiments, but is to be construed in the broadest range in accordance with the technical ideas defined by the claims.

The present invention relates to a method for generating an image of a wafer using a scanning electron microscope, and can be used particularly for a technique for adjusting the incident direction of an electron beam.

100 Scanning electron microscope 111 Electron gun 112 Condensing lens 113 X deflector 114 Y deflector 115 Objective lens 116 Lens control device 117 Deflection control device 118 Image acquisition device 120 Sample chamber 121 Sample stage 122 Stage control device 124 Wafer 130 Secondary electron detection Instrument 131 Reflected electron detector 140 Conveyor device 150 Computational system 170 Displacement sensor 171 Light source 202 Height measurement point 203 Contact (three-dimensional structure pattern)
210, 211,212 Height measurement point 213 Normal vector 214 Contact (tertiary structure pattern)
215 electron beam 250 stage actuator

Claims

A method of generating images of the three-dimensional structural pattern formed on a wafer.
The heights of a plurality of height measuring points on the wafer placed on the sample stage are measured, and the heights are measured.
Using the heights of the plurality of height measurement points, the normal vector of the wafer surface at the position of the three-dimensional structural pattern is calculated.
The electron beam is irradiated to the wafer including the three-dimensional structural pattern in a state where the incident direction of the electron beam is parallel to the normal vector.
The electrons emitted from the wafer are detected and
A method of generating an image of the three-dimensional structural pattern from the electron detection signal.
In the step of calculating the normal vector, an approximate expression representing the height of the wafer surface is determined using the heights of the plurality of height measurement points, and the coordinates of the three-dimensional structure pattern and the approximate expression are used. The method according to claim 1, which is a step of calculating a normal vector of a wafer surface at a position of the three-dimensional structural pattern.
The method according to claim 1, wherein the plurality of height measurement points are at least three height measurement points located around the three-dimensional structural pattern.
In the step of irradiating the wafer with the electron beam, the electron beam is tilted so that the incident direction of the electron beam is parallel to the normal vector, and the electron beam is applied to the wafer including the three-dimensional structural pattern. The method according to any one of claims 1 to 3, which is a step of irradiating a beam.
In the step of irradiating the wafer with the electron beam, the electrons are applied to the wafer including the three-dimensional structural pattern in a state where the sample stage is tilted so that the incident direction of the electron beam is parallel to the normal vector. The method according to any one of claims 1 to 3, which is a step of irradiating a beam.
The method according to any one of claims 1 to 5, further comprising a step of calculating the inclination of the three-dimensional structure pattern from the image.
The method according to any one of claims 1 to 5, wherein the three-dimensional structural pattern is one or more patterns having dimensions in the depth direction of the wafer.
The three-dimensional structure pattern includes an upper layer pattern and a lower layer pattern.
The method according to claim 7, further comprising the step of calculating the positional deviation between the upper layer pattern and the lower layer pattern from the image.