JP4603177B2 - Scanning laser microscope - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a scanning laser microscope for measuring a film thickness on a substrate surface of a semiconductor wafer.
[0002]
[Prior art]
A transparent or semi-transparent thin film may be formed on the surface of a substrate such as a semiconductor wafer. If the film thickness of these thin films varies, a predetermined dimension may not be obtained as a product. For this reason, conventionally, a scanning laser microscope is often used as a measuring apparatus for measuring the film thickness dimension of the thin film of the sample as described above.
[0003]
According to a film thickness measuring apparatus using a scanning laser microscope, the sample formed with a thin film and the objective lens are moved with high accuracy in the optical axis direction, and the sample is irradiated with laser light through the objective lens. Luminance data corresponding to the reflected light is detected, and a measurement target surface (for example, a thin film surface and a substrate surface (a boundary surface between the thin film and the substrate)) is detected based on a change in the luminance data, and each detected measurement target The film thickness of the thin film is measured by determining the dimension between the faces.
[0004]
Some samples have a plurality of thin films formed on a substrate. In order to detect a measurement target surface (for example, a thin film surface, a boundary surface between different thin films, a substrate surface) of such a sample, for example, as described in JP-A-8-210818, Reflecting light from the sample side by moving the position of the objective lens by a predetermined distance in the direction of the optical axis (Z-axis) while scanning the laser beam with a point, line scanning in one direction, or scanning with a plane in two dimensions The luminance data corresponding to the measurement data is continuously detected, and each measurement target surface is detected from the position where the luminance data is maximum in the continuous luminance data.
[0005]
[Problems to be solved by the invention]
When film thickness measurement is performed by the apparatus described in Japanese Patent Application Laid-Open No. 8-210818, luminance data corresponding to reflected light from each measurement target surface acquired by laser light irradiation is obtained as Z. In the measurement for obtaining the peak position where the luminance data is maximized in the continuous luminance data as described above, the film thickness is calculated based on the peak position and the peak position from the acquired luminance data. Since it has to be obtained, the memory must store all the luminance data and all the Z position information corresponding to the luminance data. In particular, when laser light is scanned two-dimensionally, there is a problem that the storage capacity of the memory must be increased.
[0006]
Furthermore, in the measurement method as described above, since the film thickness is obtained by relatively moving the sample and the objective lens in the Z-axis direction, the Z-axis drive resolution directly affects the film thickness measurement accuracy. For this reason, since the Z-axis drive resolution needs to be sufficiently high and high resolution with respect to the required measurement accuracy, there is also a problem that the apparatus becomes expensive.
[0007]
The present invention has been made in order to solve the above-described problems, and requires a small memory capacity for image data. Further, the Z-axis drive resolution is not required to have high accuracy and high resolution, and an inexpensive film thickness measurement function. An object of the present invention is to provide an attached scanning laser microscope.
[0008]
[Means for Solving the Problems]
First aspect of the present invention, a laser light source, an objective lens for a laser beam emitted from the light source focused on the sample, the condensing position of the laser beam the upper sample is condensed by the objective lens Scanning means for scanning so as to move in a direction orthogonal to the optical axis of the laser beam, a light shielding member disposed at a position conjugate with the light collection position condensed by the objective lens, and the light shielding member In a scanning microscope having a light detector that receives light from the sample and outputs a luminance signal , the sample is placed so as to be orthogonal to the optical axis of the laser light, and the sample is tilted And a holder that can hold the sample in an inclined state, and light from the sample that is held on the holder is received by the photodetector and is output from the sample that is output from the detector. measuring the luminance signal and the luminance signal It was based on the tilt angle of the sample when you; and a calculation unit for performing an operation for obtaining the thickness of the sample.
[0009]
The invention according to claim 2 is characterized in that the holding table is an inclined stage.
[0010]
The invention described in claim 3 is characterized in that the scanning means uses an optical deflection member, and the laser beam is scanned with respect to the sample.
[0011]
According to a fourth aspect of the present invention, the scanning means uses an XY stage, and the sample is scanned with respect to the laser light by moving the XY stage.
[0012]
The invention according to claim 5 is characterized in that the light shielding member is a pinhole.
[0013]
According to a sixth aspect of the invention, a monitor for displaying the luminance signal output from the optical detector as the luminance information, measured for determining the thickness of the sample based on the luminance information displayed on the monitor And an instruction means for indicating a surface.
[0014]
The invention according to claim 7 is characterized in that the calculation unit performs a calculation process based on a correction coefficient corresponding to a refractive index of the sample.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will be described with reference to FIG. 1 and FIG.
[0016]
FIG. 1 is a schematic configuration diagram showing a first embodiment of the present invention.
[0017]
Laser light emitted from the laser light source 1 is reflected by the mirror 2, passes through the half mirror 3, and enters the two-dimensional scanning mechanism 4. The two-dimensional scanning mechanism 4 performs two-dimensional scanning with laser light under the control of a control unit 5a of a computer 5 (central processing unit). The laser beam from the two-dimensional scanning mechanism 4 performs two-dimensional scanning over the entire surface of the measurement target surface of the sample 8 placed on the tilt stage 7 via the objective lens 6 that can move in the optical axis direction.
[0018]
The tilt stage 7 is placed on the XY stage 9. The XY stage 9 is connected to an XY axis drive mechanism 10 that is controlled by the control unit 5 a of the computer 5. Further, the tilt stage 7 is configured so that the microscope user enters an arbitrary angle by an observation condition input unit 11 (for example, a keyboard, a mouse, etc.) connected to the computer 5 (central processing unit). The sample 8 can be tilted by a predetermined angle by the tilt stage driving mechanism 12 by the controller 5a. Further, in addition to the tilt angle of the tilt stage 7, the magnification of the objective lens 6, the gain of the photodetector 15 described later, and the scanning range of the two-dimensional scanning mechanism 4 can be input to the observation condition input unit 11.
[0019]
Then, the reflected light from the sample 8 is condensed by the objective lens 6, reflected by the half mirror 3 through the two-dimensional scanning mechanism 4, condensed by the imaging lens 13 at the position of the pinhole 14, It is detected by the photodetector 15. Since the pinhole 14 is arranged at a position conjugate with the focal position of the objective lens 6, only the light in the focused portion on the sample 8 is detected by the photodetector 15.
[0020]
The laser beam focusing operation on the sample 8 is performed by moving the objective lens 6 in the optical axis direction by controlling the Z-axis drive unit 17 by the Z-axis drive mechanism 16 connected to the control unit 5a of the computer 5. The light detector 15 detects only the portion of the light that is in focus on the sample 8. The signal detected by the photodetector 15 is captured in synchronism with the two-dimensional scanning of the laser beam by the image input unit 5b of the computer 5 (central processing unit) and displayed on the monitor 18 as a two-dimensional image.
[0021]
Here, the objective lens 6 is moved in the direction of the optical axis to change the focal position of the laser beam. However, the present invention is not limited to this, and for example, an XY stage 9 that can be driven in the Z-axis direction. May be used.
[0022]
Further, the computer 5 has a function of measuring the size of the observation region on the microscope image (two-dimensional image) obtained by the observation condition set by the observation condition input unit 11 and displayed on the monitor 18. Image processing unit 5c. Moreover, it has the calculating part 5d which performs various calculation processes with respect to the measurement result measured by the said image process part 5c.
[0023]
2A schematically shows a state in which the sample 8 is observed using a scanning laser microscope having the configuration of FIG.
[0024]
The sample 8 shown here is an example in which a single transparent thin film is formed on a semiconductor substrate, and is placed on the XY stage 9 while being tilted by the tilt stage 7 by an angle θ. .
[0025]
The laser light from the laser light source 1 is spotted in the X and Y directions by the two-dimensional scanning mechanism 4 through the objective lens 6 and the thin film surface 8a and the substrate surface 8b as the measurement target surface of the sample 8 as indicated by arrows. Scan with light. Then, the reflected light from the sample 8 is detected by the photodetector 15. The reflected light detected by the photodetector 15 is reflected light from the thin film surface 8a and the substrate surface 8b where the spot light is focused, compared to the reflected light from the part where the spot light is not focused. Therefore, substantially only the reflected light from the portion where the spot light is focused on the thin film surface 8a and the substrate surface 8b is detected by the photodetector 15.
[0026]
Therefore, as shown in FIG. 2B, the reflected light detected by the photodetector 15 includes light reflected by the focusing surface of the thin film surface 8a and light reflected by the focusing surface of the substrate surface 8b. It is displayed as a line on the observation screen on the monitor 18 and observed. FIG. 2C shows the luminance distribution of the signal of the photodetector 15 in the section AA in FIG. 2A, where the horizontal axis indicates the position in the X direction, and the vertical axis indicates the luminance of the signal. Yes.
[0027]
FIG. 3 is an example of an observation screen measured when the film thickness of the thin film formed on the substrate surface 8b is uneven. On the substrate surface 8b represented by a straight line, FIG. On the other hand, the thin film surface 8a with uneven film thickness is displayed on the observation screen by meandering lines.
[0028]
2A, 2 </ b> B, 2 </ b> C, and FIG. 3, W represents a position where the spot light is focused on the thin film surface 8 a and the substrate surface 8 b, and the line width of W is the computer 5. Is measured by the image processing unit 5c.
[0029]
Therefore, when the thickness of the thin film of the sample 8 is t, it is expressed by the following formula.
[0030]
t = W · sinθ
(W represents the line width and θ represents the tilt angle of the sample)
Here, θ is the tilt angle of the tilt stage 7 instructed by the observation input unit 11 and is a known value. Therefore, if the calculation unit 5c of the computer 5 performs the above calculation, the film thickness t of the thin film is obtained. Can be sought.
[0031]
A modification of the first embodiment will be described.
[0032]
In the first embodiment, two lines by reflected light on the thin film surface and the substrate surface can be observed on the same observation screen on the monitor 18, and the distance between the two lines (by the image processing unit 5c of the computer 5 ( However, the present invention is not limited to this. For example, as shown in FIG. 4, the optical axis of the laser beam with respect to the sample is fixed and XY is not scanned as shown in FIG. It is also possible to obtain the value of W from the moving distance of the XY stage 9 by driving the stage to XY so that the laser beam is scanned on the sample.
[0033]
In the first embodiment, the sample 8 is formed with one thin film. However, the thin film layer is not limited to one layer, and is observed when there are two thin film layers. When there are 3 lines or 3 layers representing the measurement target surface, the number of lines representing the measurement target surface is 4 lines, and if the line width between the lines is measured, the film thickness of each thin film can be measured. Can do.
[0034]
According to the first embodiment, the following effects can be obtained.
[0035]
Since the film thickness of the thin film on the sample surface can be easily measured without scanning the objective lens in the optical axis direction (Z-axis direction), the memory capacity for storing the luminance information can be small. Further, it is sufficient for the Z-axis drive to be focused, and the Z-axis drive resolution does not affect the measurement accuracy, so that it can be configured with an inexpensive structure. Furthermore, film thickness measurements of various dimensions can be performed by appropriately setting the magnification of the objective lens and the tilt angle θ of the tilt stage.
[0036]
Note that the tilt angle θ of the tilt stage is preferably changed according to the sample or the measurement time. For example, when measuring the film thickness of a very thin film, the tilt angle θ of the tilt stage is reduced. As a result, the inter-line distance W between the surface of the thin film and the substrate surface can be increased, so that accurate film thickness measurement can be performed.
[0037]
In the above-described embodiment, the sample in which one thin film is formed on the substrate has been described. However, even a sample in which a plurality of thin films are formed on the substrate, for example, the surface of the thin film and the boundary surface between the thin films Film thickness and the film thickness between the boundary surface of the thin film and the substrate surface (boundary surface) can be measured. By increasing the inclination angle θ, it is possible to shorten the measurement time by shortening the inter-line distance W between the thin film surface and the substrate surface.
[0038]
A second embodiment will be described with reference to FIG.
[0039]
Since the configuration of the scanning laser microscope is the same as that of the first embodiment, the same reference numerals are given and description thereof is omitted. In this embodiment, only the calculation method in the calculation unit 5d of the computer 5 is different in order to measure the film thickness of the thin film more accurately than in the first embodiment.
[0040]
In FIG. 5, the sample 8 has a transparent thin film having a relative refractive index n formed on the substrate surface. Further, the sample 8 is placed on the XY stage 9 while being inclined by θ by the inclined stage 7. The reflected light from the point S on the thin film surface 8a is collected by the objective lens 6 to form a line Sm on the right side of the observation screen of the monitor 18. The reflected light from the point Q on the substrate surface 8b crosses the optical path from the inside of the thin film into the observation atmosphere, so that the light beam is refracted, and on the observation screen of the monitor 18, as if reflected from the point P in the figure. Will be observed. Therefore, this reflected light forms a line Pm on the left side of the observation screen of the monitor 18.
[0041]
At this time, due to the law of refraction,
sin α = nsin β (1)
Here, if the distance between OP is T and the distance between OQ is t,
t · tanβ = T · tanα… ▲ 2 ▼
(2) By transforming the equation,
t = (tan α / tan β) · T (3)
Furthermore, if the measured value of the dimension (line width) between the lines on the observation screen is W, the distance T between the OPs is expressed by the following equation.
[0042]
T = W · sinθ… ▲ 4 ▼
Therefore, the actual film thickness t to be measured can be calculated from Equations (3) and (4).
t = (tan α / tan β) · W · sin θ (5)
Can be expressed as
[0043]
Since α in the above equation varies depending on the tilt angle θ of the tilt stage 7 and the numerical aperture (NA) of the objective lens 6, W is measured in advance for a known film thickness t, The value of (tan α / tan β) is obtained experimentally. And this value is memorize | stored in the memory part of the calculating part 5d of the computer 5, and the measurement of an unknown film thickness can be measured more correctly by performing calculation of (5) Formula in the calculating part 5d. .
[0044]
According to the second embodiment, the following effects can be obtained.
[0045]
When measuring the film thickness of the thin film, correction can be made according to the refractive index of the thin film, so that accurate film thickness measurement can be performed.
[0046]
The present invention is not limited to the film thickness measurement described in the above embodiment, but can be applied to the step measurement on the substrate surface in addition to the measurement of the thin film. Further, the present invention is not limited to a laser scanning microscope and can be applied to any microscope provided with a confocal optical system. Further, as a two-dimensional scanning mechanism, in addition to scanning a sample by vibrating two mirrors, a disk having a large number of spiral holes is used, and by rotating such a disk, the sample is rotated. You can also scan.
[0047]
Further, as the diaphragm member, the pinhole is used in the above-described embodiment. However, the diaphragm member is not limited to this. For example, a slit can be used, and an acoustooptic device is used as a scanning unit. Even if the pinhole cannot receive light from the sample, by combining the slit and the line sensor, not only the same effect as the above-described embodiment, but also by using an acousto-optic element, The time required for scanning can be further shortened.
[0048]
In the above-described embodiment, the sample is scanned with a laser beam using, for example, a galvanometer mirror, and the sample is scanned with a laser beam by moving the sample with respect to the laser beam on an XY stage. However, the present invention is not limited to this, and a scanning unit that scans a laser beam and a scanning unit that moves a sample are combined to perform scanning of the laser beam in the X direction using, for example, a galvanometer mirror. At the same time, the laser beam may be scanned in the Y direction on the sample by moving the sample in the Y direction on the XY stage.
[0049]
In the above-described embodiment, the measurement value W can be manually designated with respect to the two-dimensional image displayed on the monitor. However, the threshold value is set in advance, and the luminance signal output from the photodetector is obtained. It may be determined that the portion exceeding the threshold is the substrate surface or the thin film surface, and the film thickness is automatically measured.
[0050]
Further, by correcting the curvature of field caused by the change of the focus position of the laser beam between the center position of the objective lens 6 and the peripheral position of the objective lens 6, more accurate film thickness measurement can be performed. More specifically, the film thickness data including the field curvature data obtained by measuring the reference sample in advance in a state where the reference sample is held at a predetermined inclination angle θ in addition to the measurement sample and including the curvature of field data obtained from the reference sample is shown in FIG. It is possible to correct the field curvature data from the film thickness data obtained by measuring the measurement sample and storing it in the memory that is not to be measured, thereby realizing accurate film thickness measurement.
[0051]
In addition, the above-described film thickness measuring apparatus further stores a storage means for storing normal thin film thickness data including errors (hereinafter referred to as comparison data), and comparison data and measurement stored in the storage means. By comparing with the film thickness data, and provided with a comparison means for determining that the film thickness of the thin film of the sample is abnormal when the film thickness data greatly exceeds or falls below the comparison data, for example, Therefore, it is possible to realize a system for efficiently determining whether a semiconductor wafer substrate is good or defective.
[0052]
In addition, since the film thickness can be measured only by two-dimensional scanning (XY scanning) with the two-dimensional scanning mechanism without driving the Z axis, the detection speed can be improved as compared with the film thickness measurement performed by driving the Z axis. Can do.
[0053]
【The invention's effect】
According to the present invention, it is possible to provide an inexpensive scanning laser microscope with a film thickness measurement function that requires a small memory capacity for image data and that does not require high accuracy and high resolution in the Z-axis drive resolution. .
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a scanning laser microscope according to a first embodiment of the present invention.
FIGS. 2A and 2B are diagrams showing an observation state of a thin film in the first embodiment of the present invention. FIG. 2A is a diagram schematically showing an observation state of the thin film, and FIG. The figure showing the image, (C) is a figure showing the luminance distribution of the image.
FIG. 3 is a diagram showing an image on a monitor when there is unevenness in the film thickness of the thin film in the first embodiment of the present invention.
FIG. 4 is a diagram for explaining a modification in the first embodiment of the present invention.
FIG. 5 is a diagram schematically showing a thin film thickness measuring method according to a second embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Laser light source 4 Two-dimensional scanning mechanism 5c Image processing part 5d Calculation part 6 Objective lens 7 Inclination stage 8 Sample 9 XY stage 15 Photo detector 18 Monitor

Claims (7)

A laser light source, an objective lens for condensing the laser light emitted from the light source on the sample, and a condensing position of the laser light condensed on the sample by the objective lens on the optical axis of the laser light Scanning means for scanning so as to move in a direction orthogonal to the light; a light shielding member disposed at a position conjugate with the light collecting position condensed by the objective lens; and light from the sample via the light shielding member In a scanning microscope having a photodetector that receives light and outputs a luminance signal,
Is mounted so as to be orthogonal to the optical axis of the laser beam, it is possible to tilt the sample, a holder that can be held in an inclined state of the sample,
Receiving light from the sample held on the holding table by the light detector, a tilt angle of the sample when the luminance signal and the luminance signal from the sample output from the detector was measured A calculation unit for calculating the thickness of the sample based on
A scanning laser microscope characterized by comprising:   The scanning laser microscope according to claim 1, wherein the holding table is an inclined stage.   2. The scanning laser microscope according to claim 1, wherein the scanning means uses an optical deflecting member, and the laser beam is scanned with respect to the sample.   The scanning laser microscope according to claim 1, wherein the scanning unit uses an XY stage, and the sample is scanned with respect to the laser light by moving the XY stage.   The scanning laser microscope according to claim 1, wherein the light shielding member is a pinhole. A monitor for displaying the luminance signal output from the optical detector as the luminance information, and instruction means for instructing the object surface for determining the thickness of the sample based on the luminance information displayed on the monitor The scanning laser microscope according to claim 1, wherein the scanning laser microscope is provided.   The scanning laser microscope according to claim 1, wherein the calculation unit performs calculation processing based on a correction coefficient corresponding to a refractive index of the sample.

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