JP3501672B2 - Surface image projection apparatus and method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface image projection apparatus and a surface image projection method for projecting an image of the surface of a plate-like object, and in particular, it is formed on the surface of a semiconductor wafer (hereinafter simply referred to as a wafer). The present invention relates to a focus adjustment technique in an optical semiconductor wafer inspection device (inspection machine) that takes a surface image of a die (chip) and inspects for defects.

[0002]

2. Description of the Related Art In recent years, semiconductor devices have tended to be more and more multi-layered as the degree of integration increases, and the number of semiconductor device production processes has increased to several hundreds. Since the final yield of semiconductor devices is determined by the accumulation of defects that occur in each process, it is important to manage so that the occurrence of defects in each process is kept low. Therefore, at the time when each layer is formed, an image on the surface of the wafer is optically captured to inspect for defects.

A microscope is used to optically capture an image of the surface of the wafer. Previously, the inspector visually observed the image of the surface of the wafer projected by the microscope to determine the presence or absence of defects, but nowadays the projected image of the microscope is composed of a one-dimensional or two-dimensional image sensor. Defects are detected automatically by image processing after capturing them with an imaging device and digitizing the image signals. A device for this purpose is a device called an inspection machine.

FIG. 1 is a diagram showing a semiconductor wafer 1 having a die 2 formed thereon. The wafer 1 has a thin plate shape, and a large number of dies 2 are formed thereon as shown in the figure. There is a space 3 between each die 2 where no circuit pattern is formed,
When the die 2 is completed, a groove is formed in the space 3 by a dicer to separate the dies, and the separated dies are attached to a package and assembled.

In recent years, the pattern formed in the semiconductor device has become finer as the degree of integration is improved, and the resolution is not sufficient when the image of one whole die is captured by one two-dimensional image sensor. Similarly, in the case of a one-dimensional image sensor, if one one-dimensional image sensor scans once to capture the image of the entire die, the resolution is insufficient.
Therefore, usually, as shown in FIG. 1, the die is divided into a plurality of stripes each having a width of one one-dimensional image sensor with sufficient resolution, and each stripe image is divided into one stripe.
It is captured by scanning once.

Defects may be detected by comparing the master image with the actually captured image. Normally, the same stripe of each die is continuously scanned along the locus 4 as shown in FIG. The same parts of adjacent or nearby dies are compared and if different, they are determined to be defective. Figure 2
It is a figure which shows the structure of the optical system of an inspection machine. The pattern of each die formed on the wafer surface is very fine and a microscope system is used to project the image of the surface. As shown in FIG. 2, the wafer 1 is placed on a stage 10 that is vertically movable (Z-axis direction). The stage 10 is provided in an XY moving mechanism 11 that can move in a horizontal plane (XY plane), and the wafer 1 can move in three XYZ directions. Actually, the stage 10 can be rotated even in a plane perpendicular to the mounting surface of the stage 10 or the Z axis, but it is omitted here. The movement of the stage 10 in the Z-axis direction is controlled by the stage Z movement control unit 12, and the movement of the stage 10 in the X-axis direction and the Y-axis direction by the XY movement mechanism 11 is controlled by the stage XY movement control unit 13.

The illumination light from the white light source 30 is reflected by the lens 29.
And the image of the light source 30 is formed at the position of the aperture stop 28. The illumination light that has passed through the aperture stop 28 is reflected by the lens 2
7, the field stop 26, the lens 25, the semi-transparent mirror (half mirror) 23, the half mirror 23, and the objective lens 21 to irradiate the surface of the wafer 1. The light emitted from one point on the wafer 1 is converged by the objective lens 21,
The image sensor 24 passes through the half mirrors 22 and 23.
Imaged above. The above is the basic configuration of the illumination system and the imaging system of the microscope system. In order to obtain high resolution, it is necessary to use an objective lens 21 having a large NA (numerical aperture), and the depth of focus is several μm or less depending on the objective lens used.

The wafer 1 has a thin plate shape, is not a perfect plane, and warps when a pattern is formed thereon. Therefore, the height of the wafer 1 placed on the stage 10 is slightly different depending on the position on the wafer. The variation in height is 100 μm or less at the most, but a good projection image cannot be obtained on the entire surface of the wafer because the depth of focus is extremely narrow as described above. Therefore, an automatic focus adjustment (autofocus) mechanism is provided. The autofocus mechanism includes astigmatism method, knife edge method, decentered auxiliary beam method (ske
w beam method) and the like, and what is provided in the inspection machine of FIG. 2 is an astigmatic autofocus mechanism.

The laser beam emitted from the semiconductor laser 33 passes through the lens 32, is reflected by the half mirror 31, and enters the half mirror 22 . Half mirror 22
Is a dichroic mirror that reflects only light near the wavelength of the semiconductor laser, and the light of the semiconductor laser reflected by the half mirror 22 is converged by the objective lens 21 so as to form a small spot on the surface of the wafer 1. The light of the semiconductor laser reflected on the surface of the wafer 1 passes through the objective lens 21, is reflected by the half mirror 22, and is reflected by the half mirror 31.
Through the lens 34 and the cylindrical lens 35, and enters the four-division element 36.

FIG. 3 is a diagram for explaining an autofocus mechanism based on the astigmatism method. As shown in (1) of FIG.
The light converged to form a small spot on the surface of the wafer 1 is reflected by the surface of the wafer 1 and emits a light beam so that a point light source exists on the wafer 1. This light beam passes through the objective lens 21 and is converged by the lens 34 at the position indicated by P. This light beam is further converged by the cylindrical lens 35 at a position where only one component is indicated by Q.
Since the other component is converged at the position indicated by P, the light beam has a different convergent position depending on the direction as shown in the figure, and is converged in a linear shape extending in one direction at the position of Q and perpendicular to it at the position of P. It is converged into a linear shape extending in the direction, and has a circular cross section at an intermediate position. Here, the four-divided element 36 is arranged at a position where the intermediate cross section is circular so that the dead zone between the four elements is 45 degrees with respect to the lines at the positions P and Q. When the convergence position of the light of the semiconductor laser on the wafer 1 shifts in the direction perpendicular to the wafer 1, the position of the point light source shifts in the optical axis direction, and the light beam on the four-division element 36 changes as shown by the dotted line. To do. That is, when converged in front of the surface of the wafer 1, it becomes an ellipse extending in the directions of the elements A and C,
When converged on the tip of the surface of the wafer 1, it becomes an ellipse extending in the directions of the elements B and D. Therefore, the differential amplifier 38
Then, when the difference signal of the sum of the outputs of the elements A and C and the sum of the elements of B and D is calculated, the output AF signal changes as shown in (2), so that the AF signal becomes zero. The state shows a state in which the spot is focused on the surface of the wafer 1, that is, a focused state. Within the range indicated by R, A
Since the F signal changes according to the focus shift, AF
If feedback control is performed so as to move the stage 10 up and down according to a signal, the focus is always maintained. For example, if the AF signal is positive, the stage 10 is moved upward, if the AF signal is negative, the stage 10 is moved downward, and if the AF signal is zero, the stage 10 is not moved. In other words, the AF signal indicates a deviation from the focus position.

Although the astigmatism method has been described here, it is possible to control the state in which the focus is always maintained and to detect the deviation from the focus position even with other methods. In any case, a light source such as a semiconductor laser other than the illumination is used for the autofocus mechanism,
The light source spot is focused on the wafer. In the above example, the stage was moved up and down to adjust the focus,
It is also possible to adjust the focus by moving the entire microscope or objective lens up and down. In any case, by using the autofocus mechanism, it is possible to always project a focused and clear image regardless of the position (surface shape) of the wafer surface.

[0012]

As described above, no matter which autofocus mechanism is used, it is necessary to use a light source such as a semiconductor laser other than the illumination and to focus the spot of the light source on the wafer. is there. Recent semiconductor devices are
The line width is about 0.2 μm, but the light of the light source used for the autofocus mechanism has a relatively long wavelength, and the diameter of the spot converged on the wafer can be several times to several tens of times the line width. is there. FIG. 4 is a diagram showing a state of a spot 20 of a light beam for autofocus focused on a wafer.

As shown in FIG. 4, the surface of the die formed on the wafer has irregularities depending on the pattern, and these irregularities change at a pitch smaller than the spot 20 of the light beam. Therefore, focusing by the autofocus mechanism is performed by averaging the surface heights in the range where the light beam spot 20 is irradiated.
For example, if the surface is a transparent layer, the light will be focused on the position determined by the reflection intensity of the light beam, which is the average of all the components reflected by the underlying opaque metal wiring layer. There was a problem that the layers other than the layers were in focus. In particular, as will be described later, a confocal microscope has recently been used as an optical system of an inspection machine. However, when a confocal microscope is used, a desired pattern can be formed when many layers are formed on the surface of a wafer. Although it is possible to capture the image of only the layer with the height of, the focus is on the position where the light beams reflected on each layer are averaged, so there was a problem that the layer to be inspected could not be focused. .

Further, the autofocus mechanism always detects the focus state and performs feedback control, and there is a problem in that the image is projected in a deteriorated state due to a delay in the feedback system, which causes a slight vibration. .
Furthermore, the spot of the light beam is larger than the pattern of the die, but the diameter is still several μm or less, so if there is a defect in the irradiated position or if the height is different from other parts, the There was a problem that the focus of the part was largely out of focus.

The present invention is intended to solve such a problem, and an object of the present invention is to realize a surface image projection apparatus and method which can stably focus on a desired layer without vibration of focus.

[0016]

In order to achieve the above object, the surface image projection apparatus and method of the present invention measure and store the surface shape of a plate-like object in advance, and focus according to this surface shape. To control. That is, the surface image projection device of the present invention, the projection means for projecting a part of the surface of the plate-like object, is relatively moved along the surface of the plate-like object,
A surface image projection device that obtains an image of the entire surface of a plate-like object by combining projection images at each position, and a surface height detection unit that measures heights at a plurality of positions on the surface of the plate-like object, Surface shape data storage means for calculating and storing the surface shape of the plate-shaped object from the heights of a plurality of points, focus adjustment means for changing the focus state of the projection means with respect to the surface of the plate-shaped object, and projection means for the plate-shaped object. And a focus control unit that controls the focus adjustment unit according to the surface shape of the plate-shaped object stored in the surface shape data storage unit when the relative movement is performed along the surface.

Further, according to the surface image projection method of the present invention, while projecting a part of the surface of the plate-like object, the plate-like object is relatively moved along the surface of the plate-like object, and the projection images at each position are combined to form the plate-like object. A surface image projection method for obtaining an image of the entire surface of an object, comprising a surface height detecting step for measuring heights of a plurality of positions on the surface of a plate-like object, and a surface of the plate-like object from the measured heights of the plurality of places. A surface shape data storage step of calculating and storing the shape; and a scanning step of projecting a part of the surface of the plate-like object while relatively moving along the surface of the plate-like object,
At the scanning step, the focus state is changed according to the stored surface shape of the plate-like object.

The inventors of the present invention have found that the thickness of each layer formed on the wafer has a small error, the height variation mainly occurs due to the height variation of the surface of the wafer, and the height variation of the surface of the wafer is gentle. We paid attention to the fact that the height can be accurately measured by measuring the height of a portion having no pattern other than the die. According to the surface image projecting device and method of the present invention, the surface shape of the plate-like object calculated from the heights of the measured surface of the plate-like object is stored in advance, and the surface shape along the surface shape during scanning is stored. Since the focus is controlled by the focus control, it is possible to adjust the focus to a desired layer without being affected by the minute unevenness of the surface, and since it is not the feedback control, vibration does not occur.

When a confocal microscope is used as the projection means and an optical sensor for detecting a projected image is provided, a desired layer can be projected. However, if the present invention is applied, after focusing on the layer, Since the focus is adjusted only in accordance with the height variation of the plate-like object, the layer can be focused and scanned. In this case, the operator specifies the layer to be projected by focusing on the layer to be projected.

The present invention can also be applied to the case where the one-dimensional and two-dimensional image sensors are normally used without using the confocal microscope. When projecting a semiconductor wafer, the height of the wafer can be measured without being affected by the pattern by measuring the height of a portion around the die formed on the semiconductor wafer in which the pattern is not formed.

[0021]

FIG. 5 is a diagram showing the configuration of an optical system of an inspection machine according to an embodiment of the present invention. The inspection machine of the embodiment uses a confocal microscope and uses the height measurement mechanism by the astigmatism method described in FIGS. 2 and 3 as a mechanism for measuring the height of the wafer 1. Therefore, the description of the height measuring mechanism composed of the elements 31 to 37 is omitted here.

The light from the white light source 57 passes through the condenser lens 56 and is reflected by the beam splitter 53 to illuminate the spinning disk 52. Spinning disc 5
Reference numeral 2 denotes a rotating disc, which is provided with a large number of apertures 58. Image of aperture 58, ie aperture 58
The light beam that has passed through is reflected by the lens 51 and the half mirror 22.
Then, it is converged as a spot on the wafer 1 by the objective lens 21, and an image of the aperture 58 is formed on the wafer 1. The light beam irradiated on the wafer is reflected and converges on the corresponding aperture 58 through the optical path opposite to the above.
The light that has passed through the aperture 58 is reflected by the beam splitter 53.
And is converged on the optical sensor 55 by the lens 54. The spinning disk 52 rotates, and the image of the aperture 58 on the wafer 1 sequentially scans within the field of view. If the intensity distribution is obtained by synthesizing the signals from the optical sensor 57 at each position, 2
A three-dimensional image is obtained.

The light reflected by the spot portion of the light beam irradiated on the wafer 1 is focused on the portion of the corresponding aperture 58, so that the reflected light from other portions is blocked and a clear contrast is obtained. Higher images are obtained. Also,
When the position of the spot shifts in the direction of the optical axis, most of the light reflected by the spot where the spot spreads and spreads out
Since it does not pass through, the image of only the layer focused on the spot can be captured.

As described above, if the height measuring mechanism based on the astigmatism method, which is composed of the elements 31 to 37, is used, the height change can be detected by the AF signal. In this embodiment, an analog / digital (A / D) converter 3 for converting the AF signal output from the signal processing circuit 37 into a digital signal.
8, a surface shape data storage unit 39 that stores height distribution data output from the A / D converter 38 during surface shape measurement, and stores surface shape data calculated based on the stored data; A focus instructing unit 40 is newly provided to instruct the layer that captures an image to adjust the focus and perform scanning while maintaining the focus position. The stage Z movement control unit 12 that controls the movement of the stage 10 in the vertical direction (Z direction) and the stage XY movement control unit 13 that controls the movement of the stage 10 in the horizontal direction (XY direction) are the same as conventional ones. It is realized by software of the control computer together with the data storage unit 39.

FIG. 6 is a flow chart showing the processing for obtaining the surface image of the die on the wafer in the embodiment. The processing will be described below according to this flowchart. In step 101, the wafer 1 is placed on the stage 10,
Alignment for aligning the die arrangement direction and the like is performed using a TV camera (not shown).

In step 102, the stage 10 is moved to XY.
The height is measured at a plurality of points on the wafer 1 by moving in the plane and stored in the surface shape data storage unit 39. This height measurement is performed, for example, on the part marked with X shown by reference numeral 6 in (1) of FIG. This portion is a portion to be grooved by a dicer between the dies 2 arranged on the wafer 1, and since a circuit pattern is not formed, it is a flat surface with no unevenness on the surface of the wafer 1, and the height measuring mechanism is used. It is possible to measure the height accurately. The measurement is performed by scanning in the X direction such that the space between the dies 2 is below the optical axis of the objective lens, that is, the spot of the laser beam of the height measuring mechanism is irradiated onto the space between the dies 2. , The digital value of the AF signal at the measurement position is stored. This scanning is performed by changing the position in the Y direction. As described above, the height of the portion marked with X in (1) of FIG. 7 can be measured.

In step 103, the surface shape data storage unit 39 calculates and stores the height distribution of the entire surface of the wafer 1 from the heights of the plurality of positions on the wafer measured in step 102. For example, as shown in (2) of FIG. 7, since the heights of nine points are measured for a certain Y coordinate, the measured data is interpolated using a spline function or the like to obtain a continuous height change. . Such calculation is performed on the entire surface of the wafer 1 to calculate the surface shape data of the wafer 1.

In step 104, the operator looks at the image of a certain portion and focuses on the layer from which the image is to be obtained. This focus adjustment is performed by adjusting the stage Z movement control unit 1 even if the Z axis direction moving mechanism of the separately provided stage is adjusted.
The data value given from the outside to 2 may be adjusted.
When the adjustment is completed, the focus operation unit 40 is operated by a key to indicate that the adjustment is completed.

In step 105, the XY coordinates of the stage are changed so that the upper left position of the wafer 1 in FIG. 7 becomes the scanning start position. At this time, from the surface shape data stored in the surface shape data storage unit 39, the position of the stage 10 in the Z-axis direction is changed by the height difference between the position at which the instruction in step 104 is given and the upper left position of the wafer 1. Let In step 106, the stage XY movement control unit 13 outputs a signal for performing scanning by sequentially changing the X coordinate, and when the scanning from the left end to the right end is completed, the Y coordinate is changed and scanning is performed in the opposite direction. The movement of the stage 10 is controlled so as to repeat such an operation. At this time, the height data at each scanning position is output from the surface shape data stored in the surface shape data storage unit 39 to the stage Z movement control unit 12, and the height of the stage 10 is changed by the height data. Thereby, the focus of the optical system is always focused on the layer designated by the operator. By the above processing, a surface image of the entire surface of the wafer can be obtained.

In this embodiment, the inspection machine is used. The read image data is compared with the master data and the image data of the adjacent die to detect the defect position, and the image data is further analyzed for the defect position. Various kinds of processing such as classification of defect types are performed, but since they are not directly related to the present invention, detailed description thereof is omitted here. Although the embodiment using the optical system of the confocal microscope has been described above, the present invention is not limited to the configuration using the confocal microscope, and a normal one-dimensional image sensor or a two-dimensional image sensor is used. It is also applicable to the configuration.

[0031]

As described above, according to the present invention,
It is possible to focus on a desired layer without being affected by minute irregularities on the surface, and because feedback control is not performed, a good image without vibration can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram illustrating scanning for obtaining a surface image of a die (semiconductor chip) formed on a semiconductor wafer.

FIG. 2 is a diagram showing a configuration of an optical system of a conventional inspection machine for obtaining a surface image of a die formed on a semiconductor wafer.

FIG. 3 is a diagram illustrating an autofocus mechanism by an astigmatism method in a conventional example.

FIG. 4 is a diagram showing a state of a wafer surface and a spot of a light beam of an autofocus mechanism.

FIG. 5 is a diagram showing a configuration of an optical system of an inspection machine according to an embodiment of the present invention.

FIG. 6 is a flowchart showing a process for obtaining a surface image of a die on a wafer in an example.

FIG. 7 is a diagram showing an example of a portion where surface height of a wafer is measured and surface shape data in the embodiment.

[Explanation of symbols]

1. Plate-shaped object (semiconductor wafer) 10 ... Stage 12 ... Stage Z movement controller 13 ... Stage XY movement control unit 21 ... Objective lens 33 ... Semiconductor laser 34 ... Lens 35 ... Cylindrical lens 36 ... 4-division element 39 ... Surface shape data storage means 40 ... Focus instruction section

Claims (10)

(57) [Claims] 1. A projection means for projecting a part of the surface of a plate-like object is relatively moved along the surface of the plate-like object, and the projection images at each position are combined so that the entire surface of the plate-like object is covered. A surface image projection device for obtaining an image, wherein a surface height detecting means for measuring heights of a plurality of positions on the surface of the plate-like object, and a surface shape of the plate-like object from the measured heights of the plurality of positions. Surface shape data storage means for calculating and storing, focus adjustment means for changing the focus state of the projection means with respect to the surface of the plate-like object, and when the projection means is relatively moved along the surface of the plate-like object And a focus control unit that controls the focus adjustment unit according to the surface shape of the plate-shaped object stored in the surface shape data storage unit. 2. The surface image projection device according to claim 1, further comprising a one-dimensional image sensor that detects the projection image.
A surface image projection apparatus that obtains a two-dimensional image by synthesizing a one-dimensional image when the projection means is relatively moved along the surface of the plate-like object. 3. The surface image projection device according to claim 1, wherein the projection means is a confocal microscope. 4. The surface image projection device according to claim 1, further comprising a two-dimensional image sensor that detects a projected image. 5. The surface image projection device according to claim 1, wherein the plate-shaped object is a semiconductor wafer. 6. The surface image projecting device according to claim 5, wherein the surface height detecting means determines a height of a portion around the die formed on the semiconductor wafer in which a pattern is not formed. Surface image projection device to measure. 7. The surface image projector according to claim 1, wherein the plate-shaped object is a semiconductor wafer, the projection means is a confocal microscope, an optical sensor for detecting a projected image, and the projection. A surface image projection apparatus comprising a focus instruction means for instructing a layer to be focused on the semiconductor wafer when the means is relatively moved along the surface of the plate-like object. 8. While projecting a part of the surface of the plate-like object,
A surface image projection method of relatively moving along the surface of the plate-like object to obtain an image of the entire surface of the plate-like object by combining projection images at respective positions, and a plurality of positions on the surface of the plate-like object. A surface height detecting step for measuring the height of the plate-like object, a surface shape data storing step for calculating and storing the surface shape of the plate-like object from the measured heights of the plurality of points, and along the surface of the plate-like object. And a scanning step of projecting a part of the surface of the plate-like object while relatively moving, and changing the focus state in accordance with the stored surface shape of the plate-like object during the scanning step. Surface image projection method. 9. The surface image projection method according to claim 8, wherein the plate-like object is a semiconductor wafer, and in the surface height detecting step, a pattern around a die formed on the semiconductor wafer. Surface image projection method for measuring the height of an unformed part of a surface. 10. The surface image projection method according to claim 8, wherein the plate-shaped object is a semiconductor wafer, and the projection optical system includes a confocal microscope and an optical sensor for detecting a projected image. A front surface image projection method comprising a focus position designating step of designating a focusing layer of the semiconductor wafer before the scanning step.