JPS5813842B2 - thickness measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thickness measuring device, and more particularly to a device for measuring the minute thickness (depth of a concave portion or height of a convex portion) of an object having an uneven shape.

For example, when manufacturing semiconductor integrated circuits,
There is a demand for faster, more accurate, and easier measurement of the resist film thickness of a resist pattern formed on a silicon wafer for LSI, or the unevenness of the resist film thickness of the entire wafer.

Conventionally, the following method is known as a method for measuring minute thickness of an object having an uneven shape.

The first method is to adjust the focus of a microscope.

In this method, an object with a concave and convex shape is mounted on the microscope's data table, and the operator slowly moves the material while looking through the microscope, and then changes the amount of focus adjustment when focusing on the concave part and when focusing on the convex part. Thickness is measured by the difference.

This method is used to examine irregularities on machined surfaces.

The second method is to irradiate an uneven object with white light from an oblique direction through a slit, and measure the thickness by observing the reflected light from the object with a microscope.

That is, in this method, as shown in FIG. The image is then guided to the operator's eyes.

According to this method, as shown in FIG. 1b, the uneven object 7 is irradiated with light 8 cut by the slit, so that the pattern seen through the lenses 5 and 6 shown in FIG. 1a is As shown in FIG. 1C, the thickness corresponds to the uneven shape, and the thickness can be measured by the distance l.

This method is used for measuring the depth of an etched surface, measuring the thickness of a thick film IC paste, etc.

The third method utilizes movement of interference fringes.

For example, helium neon (
A collimator 12 converts the laser light emitted from the laser light source 11 into parallel light with a predetermined diameter, and the collimated light is transmitted to the first half mirror 13 and the second half mirror 14.
The light beams reflected from both the standard object 15 and the object to be measured 16 are guided to the standard object 15 with no unevenness and the other to the object to be measured 16 which has an uneven shape. The light is returned through the mirror 14 and taken out through the first half mirror 13 and lens 17.

The standard object 15 and the object to be measured 16 are imaged on the focal plane 18 of the lens 17, and this imaged pattern is observed using a microscope 19.

At this time, by tilting either the standard object 15 or the object to be measured 16 at a predetermined angle from the planes 20a, 20b perpendicular to the optical axis (in Figure 2a, the standard object 15 is tilted), the results can be obtained. A fringe pattern due to interference appears on the image plane 18, and if the object to be measured 16 has irregularities, the interference fringes at positions corresponding to the irregularities move as shown in FIG. 2b.

Here, since this amount of movement d corresponds to the thickness of the uneven object, the operator can measure the thickness of the unevenness by observing this amount of movement.

However, all of the conventional techniques described above have the following drawbacks and problems.

The first method, which involves adjusting the focus of a microscope, is relatively easy to operate, but since the operator adjusts the focus while looking through the microscope, it is difficult to use, for example, when adjusting the focus of a resist formed on an IC wafer or an LSI wafer. When a large number of objects are to be measured, as in the case of measurement, an enormous amount of time is required for the measurement, and operator fatigue is also extremely high.

Furthermore, since this method uses the amount of focusing by the operator for measurement, it cannot inherently require sufficient accuracy, and moreover, the measured object itself is not suitable for focusing, that is, what surface it has. This method can only be applied to measurements of, for example, machined items that have a rough pattern, and in which the level difference (i.e., thickness) of the unevenness is on the order of 10 μm or more.

Therefore, in the case of a resist on an IC wafer or an LSI wafer, which has a thickness of about 1 μm and both concave and convex portions are specularly reflective, not only accuracy cannot be achieved but also focusing cannot be performed.

Further, the second method uses a slit to obtain cutting light, and in order to improve accuracy, it is desirable to make the cutting light as thin as possible.

However, if the slit is made thinner for this purpose, it becomes impossible to obtain the amount of cutting light necessary for measurement.

For these reasons, the width of the slit is limited, and as a result, the measurement accuracy is also limited, making it extremely difficult to measure a thickness of about 1 μm.

More importantly, since this method involves observing a pattern like the one shown in FIG. 1C using a microscope, if the microscope is out of focus, the measurement accuracy will be greatly reduced.

Therefore, when a large number of objects to be measured are sequentially examined, such as when measuring the thickness of a resist film on an IC wafer or an LSI wafer, this focusing becomes a major problem, resulting in a decrease in measurement accuracy and measurement speed. It also poses a major obstacle when it comes to automation.

Furthermore, in the third method, since the thickness is measured from the ratio of the distance L of the interference fringes to the amount of movement d, as described above, accurate measurement cannot be made visually by the operator, and even if L and If we try to improve the accuracy by optically measuring d and converting it into an electrical signal, the hardware configuration for that purpose becomes complicated.

Furthermore, an important problem with this method, similar to the second method, is that the microscope loses focus as many objects to be measured are examined successively.

Therefore, in order to solve this problem, it is necessary to provide a mechanism for automatically adjusting the focus, which poses a major hindrance to automation.

The present invention solves various drawbacks of the prior art as described above,
The purpose of the present invention is to solve the problem and provide a device that can quickly, accurately, and easily automatically measure the thickness of an uneven object with a thickness of about 1 μm, such as a resist on an IC wafer or an LSI wafer. .

The present invention will be described in detail below with reference to the drawings.

FIGS. 3a and 3b are diagrams for explaining the measurement principle of the thickness measuring device of the present invention, and in these diagrams, 31 is an object to be measured having an uneven shape, 32 is a half mirror, 33 is a Fourier transform lens, 34 is a photoelectric converter, and 35 is a coherent beam.

FIG. 3a shows a case in which coherent light is irradiated onto a concave portion of an object to be measured, and FIG. 3b shows a case in which coherent light is irradiated half on a concave portion and half on a convex portion.

First, FIG. 3a will be explained.

The coherent parallel light 35 is guided to the concave portion of the object to be measured 31 via the half mirror 32.

The reflected light from the concave portion of the object to be measured 31 is reflected by the half mirror 32.
, through a Fourier transform lens 33 to a photoelectric converter 34 placed at the focal plane of the Fourier transform lens 33.

Here, the light receiving section of the photoelectric converter 34 is arranged on the optical axis, and is adapted to receive the 00th order light component of the light Fourier transformed by the Fourier transform lens 33.

Therefore, the photoelectric converter 34 outputs a telegraph signal corresponding to the intensity of the zero-order light component of the Fourier transformed light, and in the case of FIG. As they gather together, the strength is maximum.

Next, the object to be measured 31 moves in the direction of the arrow 36, and as shown in FIG.
Suppose you come to the position shown in .

In this case, the coherent parallel light 35 is transmitted to the half mirror 32.
Half of the light is irradiated onto the concave portion and half of the convex portion of the object 31 to be measured.

In this way, when the light is applied to both the concave portion and the convex portion, the reflected light from the concave portion and the reflected light from the convex portion do not have the same phase, but are out of phase by an amount that depends on the thickness h.

Therefore, since the zero-order light component received by the photoelectric converter 34 disposed on the optical axis decreases, the output of the photoelectric converter 34 becomes smaller.

Moreover, as shown in Figure 3b, when the areas of the concave and convex parts illuminated by coherent light become equal, 0
Although not shown in the figure in which the light intensity of the secondary light reaches its minimum, when the object to be measured 31 moves further and all of the coherent light is irradiated onto the convex portion of the object to be measured 31, the amount of received light becomes the same phase again. Since they come together on the axis, the photoelectric converter 34
The amount of zero-order light incident on is maximum.

The photoelectric converter 34 accompanies the movement of the object 31 to be measured.
Figure 4 shows how the output changes.

In FIG. 4, the horizontal axis represents the amount of movement (if a resist on an IC or LSI wafer is to be measured, it corresponds to the amount of movement of the stage on which the wafer is mounted; the vertical axis represents the output signal level of the photoelectric converter 34). , Imax represents the maximum value of the output signal, and Imin represents the minimum value.

The present invention attempts to measure the thickness h of an object to be measured using the above-mentioned Imax and Imin. Therefore, in the present invention, the beam size of the coherent light irradiated to the object to be measured is equal to the pattern size of the object to be measured. (area of concave portions and convex portions).

Now, assuming that the ratio Imin/Imax between the maximum output Imax and the minimum output Imin of the photoelectric converter 34 is IR, this R and the thickness of the object to be measured 31 (meaning the depth of the concave part or the height of the convex part) The relationship with h is determined by the following equation.

However, λ is the wavelength of the coherent light for measurement, and n is the refractive index of the object to be measured.

Here, equation (1) applies when the object to be measured is a phase object, but if the object to be measured is not a phase object but a metal film that undergoes total reflection on the surface, the thickness h is It is expressed by the following formula.

Next, an embodiment in which the measurement principle of the apparatus of the present invention described above is applied to the measurement of the thickness of a resist on a silicon wafer for LSI will be described with reference to FIG.

In this figure, 51 is a helium neon (
The laser light from this laser light source is converted into parallel light having a predetermined diameter by a collimator 52 made up of a plurality of lenses, and enters a scanning mirror 53.

This scanning mirror 53 vibrates in the direction of an arrow 55 about a fulcrum 54, thereby scanning the wafer in the X direction.

Laser light reflected by the scanning mirror 53 (thousand beams)
is guided to a Fourier transform lens 57 via a half mirror 56.

The light that has passed through the Fourier transform lens 57 is irradiated onto a wafer 58 placed near the focal point of the lens 57.

Here, since the laser light becomes a thin parallel beam at the focal plane of the lens 57, the light near the wafer 58 is almost parallel.

A resist 59 is formed on the wafer 58 by a method such as photolithography.

However, in this figure, the resist 59 is drawn enlarged.

Further, the wafer 58 is mounted on the stage 60,
This stage 60 is configured to move sequentially in the Y direction.

Therefore, the scanning in the X direction by the scanning mirror 53 and the stage 6
The entire surface of the wafer 58 can be scanned with a parallel laser beam by scanning in the Y direction by zero.

Light reflected from the wafer 58 and the resist 59 is guided via the Fourier transform lens 51 and the half mirror 56 to a photoelectric converter 61 placed on the imaging plane.

Here, the photoelectric converter 61 may be an ordinary light receiving element such as a phototransistor or a photodiode, and the photoelectric converter 61 is placed on the optical axis.

Therefore, the photoelectric converter 61 connects the wafer 58 and the resist 5
The O-order light component obtained by Fourier-transforming the light reflected from 9 is received, and an electric signal corresponding to the light intensity of this O-order light component is generated.

Therefore, in accordance with the principles of the invention described above, the scanning mirror 53
and the photoelectric converter 61 by XY scanning by the stage 60.
generates a continuous signal as shown in FIG.

The output signal (analog signal) of the photoelectric converter 61 is then converted to A/
It is converted into a digital signal by a D converter 62 and supplied to an electronic computer 63 such as a microcomputer.

Here, the computer 63 sends control signals for driving or moving the scanning mirror 53 to the drive mechanism 64 and the stage 60 to the moving mechanism 65, respectively, and receives signals from the drive mechanism 64 and the moving mechanism 65.

Usually, several hundred chips are formed on one wafer 58, and when inspecting the thickness of the resist, it is sufficient to inspect each chip.

Therefore, Imax and Imin may be detected for each chip by the calculator 63, and the average resist thickness for that chip may be measured from these Imax and Imin.

In other words, if the period from time t1 to t2 in FIG. 6 corresponds to the scanning time of the entire surface of one chip, then Imax and Imin in the period from t1 to t2 are detected by the computer 63, and this chip is calculated using the above-mentioned equation (1). The average value of the resist thickness within the range can be determined.

Of course, the average value of the resist thickness within one chip can also be measured by other methods.

The first step is to measure the thickness of each resist unevenness.
The second method is to measure it over the entire surface of one chip and then calculate the average value, and the second method is to measure it over several chips (for example, 10 chips).
This is a method in which the average value of the resist thickness within this division is determined as one division, and this value is regarded as the average value of the resist thickness of each chip within this division.

The first method has an extremely high measurement accuracy, but the processing operation is somewhat complicated, and the second method, on the other hand, has a simple processing operation, but the measurement accuracy is slightly lower.

However, regardless of the method of measuring each chip or the first and second methods described above, the measurement accuracy is within the practical range, so
There is no big difference whichever method is used.

Furthermore, by measuring the resist thickness in each chip over the entire wafer surface, it is also possible to measure the thickness unevenness of the resist pattern on the wafer surface.

As explained in detail above, according to the present invention, when measuring the thickness of a resist formed on a wafer, the wafer and the resist are irradiated with a parallel laser beam whose diameter is smaller than the uneven pattern of the resist, and the reflected light is The electrical signal Imax obtained by photoelectrically converting the light intensity of the zero-order component of
Since the resist thickness is measured from Imin and Imin, the following various effects can be achieved.

(a) First, according to the present invention, focusing is completely unnecessary.

This is due to the following two reasons. First, the size of the concavo-convex pattern to be measured is generally 5 μm to 10 μm or more in many cases, and in order to measure the thickness of an object to be measured having such a pattern size, in the present invention, the beam size is narrowed down to about 5 μm to 10 μm.

When the beam size is narrowed down in this manner, the beam diameter does not substantially change even if the optical system is shifted in the vertical direction by the third place or more of the beam diameter.

Therefore, even if the wafer surface shifts in the vertical direction due to movement of the stage on which the wafer is mounted, the shift will not affect the output of the photoelectric converter.

Furthermore, even if the optical system shifts vertically to the extent that the beam diameter changes, the present invention measures the ratio of the maximum value and minimum value of the light amount change, so no matter what happens in the optical system, it will not be the maximum value. The ratio of the minimum values is constant, so deviations in the optical system will not enter the thickness measurement value as an error.

(b) Next, since the present invention does not involve visual inspection using a microscope, operator fatigue is eliminated.

(c) Furthermore, since the present invention can sufficiently use the amount of laser light from the laser light source for measurement, it is possible to obtain higher accuracy than the conventional method of measuring thickness using cutting light obtained by a slit.

(d) Furthermore, the present invention can be applied to He-Ne, which is usually easily available.
Using a laser light source, the thickness is 1 μm, such as a resist, etc.
It is possible to accurately measure even objects with a diameter of about m.

As described above, according to the present invention, automation especially in the semiconductor manufacturing process can be achieved easily and with high precision, and extremely large practical effects can be expected.

As a result of measuring the thickness of a resist pattern formed on a silicon wafer for LSI using the example shown in FIG. 5, it was possible to measure a resist having a thickness of about 1 μm with an accuracy of about 0.1 μm.

FIG. 7 is a diagram showing the configuration of another embodiment of the present invention.

This figure shows an example in which the object to be measured is a light-transmitting object. For example, it is used to measure the thickness of a resist formed on a sapphire substrate, or to measure the color on an optical glass substrate installed in a color camera. It can be applied to measuring the thickness of filters, etc.

In FIG. 7, 51 is a He-Ne laser light source, 52 is a collimator, 53 is a scanning mirror, 61 is a photoelectric converter, and 6
2 is an A/D converter, 63 is an electronic computer, 64 is a drive mechanism for the scanning mirror 53, and 65 is a moving mechanism for the stage 60, which are the same parts as shown in FIG.

Further, 66a and 66b are first and second Fourier transform lenses, 67 is a light-transmitting substrate, 68 is an uneven object formed on the light-transmitting substrate 67, and 69 is a stage.

As is clear from the comparison with the reflective type shown in Figure 5,
In the case of the transmission type shown in Fig. 7, a half mirror is not required, but one extra Fourier transform lens is provided.
The laser beam focused by the first Fourier transform lens 66a passes through the light-transmitting substrate 67, and then is guided to the second Fourier transform lens 66b.

However, the measurement principle is exactly the same as that of the reflective type shown in FIG. 5, and the thickness of the uneven object formed on the light-transmitting substrate 67 can be measured with high precision.

However, as shown in this figure, when used as a transmission type, the thickness h of the uneven object and Ima obtained by the photoelectric converter 61
The ratio IR of x and Imin (IR = Imin/Ima
x) is expressed by the following equation.

Therefore, the thickness h can be measured by causing the calculator 63 to calculate the equation (3).

FIG. 8 is a diagram showing the configuration of still another embodiment of the present invention.

This figure shows the case of a reflective type and does not use a half mirror, and the method shown in this figure can be used when it is desired to prevent a decrease in the amount of light due to the half mirror.

That is, the laser beam reflected by the scanning mirror 53 is irradiated onto the wafer 58 and the resist 59 via the first Fourier transform lens 70a, and the reflected light is guided to the photoelectric converter 61 via the second Fourier transform lens 70b. It is like that.

Note that in this figure, θ indicates the incident angle of the laser beam.

In this embodiment, the measurement principle is exactly the same as that in FIGS. 5 and 7, but the formula for determining the thickness of the resist 59 is as follows.

The uneven objects to be measured in Figures 5, 7, and 8 are all phase objects, but by using a reflective type as shown in Figure 5 or 8, It is also possible to measure the thickness of uneven objects where laser light is totally reflected.

For thickness measurement in this case, the above-mentioned equation (2) may be applied.

As described above, the present invention can be modified in various ways based on its principles, and the thickness of any object to be measured can be measured with high precision.

[Brief explanation of the drawing]

FIG. 1a is a configuration diagram showing one example of a conventional thickness measuring device, FIG. 1b is a diagram showing the state of cutting light obtained by the configuration of FIG. 1a, and FIG. Figure 2a is a configuration diagram showing another example of a conventional thickness measuring device; Figure 2b is a diagram showing a concavo-convex pattern observed with a microscope in the configuration of
A diagram showing the movement of interference fringes observed in the configuration of Figure a,
Figures 3a and b are diagrams for explaining the measurement principle of the thickness measuring device of the present invention, Figure 4 is a diagram showing the output signals of the photoelectric converters in Figures 3a and b, and Figure 5 is a diagram for explaining the measurement principle of the thickness measuring device of the present invention. A diagram showing an embodiment in which the invented device is applied to measuring the thickness of a resist on a silicon wafer for LSI, and FIG. 6 shows how the output signal of a photoelectric converter changes according to scanning in the embodiment shown in FIG. 5. , FIG. 7, and FIG. 8 are diagrams showing other embodiments of the present invention. 51...Laser light source, 52...Collimator, 53...Scanning mirror, 56...
・Half mirror, 57... Fourier transform lens, 58... Ueno-, 59... Resist, 60... Stage, 61...・Photoelectric converter, 62...A/D converter, 63...
...Electronic computer, 64...Scanning mirror drive mechanism, 65...Stage moving mechanism, 6 6 a
, 6 6 b, 7 0 a, 70 b...
...Fourier transform lens.

Claims (1)

[Scope of Claims] 1: a coherent light source, and means for focusing the coherent light from the coherent light source into a parallel beam having a diameter smaller than the size of the uneven pattern of the object to be measured and irradiating the uneven surface of the object to be measured; A Fourier transform lens for Fourier transforming the light reflected from the uneven surface of the object to be measured or the light transmitted from the object to be measured irradiated by this means, and the 0th order of the light Fourier transformed by the Fourier transform lens. means for receiving a light component to obtain an electrical signal spread over the light intensity; means for obtaining the maximum and minimum values of the electrical signals obtained by this means; and the maximum and minimum values obtained by this means. A thickness measuring device comprising: means for calculating the thickness of the unevenness of the object to be measured using the minimum value. 2. The thickness measuring device according to claim 1, wherein the coherent light source is a helium-neon laser. 3. In the device described in claim 1, the means for irradiating the uneven surface of the object to be measured as a parallel beam having a diameter smaller than the size of the uneven pattern is a collimator for collimating the coherent light from the coherent light source. and an optical lens for focusing the parallel light obtained by the collimator to a diameter smaller than the size of the uneven pattern on the uneven surface of the object to be measured. 4. In the device described in claim 1, the means for obtaining the maximum value and the minimum value includes means for A/D converting an electric signal corresponding to the light intensity of the zero-order light, and a means for obtaining the maximum value and the minimum value by this means. 1. A thickness measuring device comprising: means for obtaining a maximum value and a minimum value from the digital quantities obtained. 5. In the item described in claim 1, the object to be measured is a phase object, and the thickness h of the unevenness of the phase object is
When measuring using reflected light, the calculation means for determining the thickness h is as follows: (IR=λ: wavelength of coherent light for measurement, n: refractive index of phase object, θ: angle of incidence) A thickness measuring device characterized by: 6. In the item described in claim 1, when the object to be measured is a total reflection object and the thickness h of the unevenness of the total reflection object is measured using reflected light, the calculation for determining the thickness h. The means are (however, beads = -, λ: wavelength of coherent light for measurement, θ:
A thickness measuring device characterized by executing an arithmetic expression (incidence angle). 7. In the item described in claim 1, the object to be measured is a phase object, and the thickness h of the unevenness of the phase object is
When measuring the thickness h using transmitted light, the calculation means for determining the thickness h is: ? ．． ．． 8 = Shino, 2. Yo, Oyo33-2, wavelength of light,
A thickness measuring device characterized by executing an arithmetic expression where n is a refractive index of a two-phase object and θ is an incident angle. 8 A coherent light source, means for narrowing the coherent light from the coherent light source into a parallel beam having a diameter smaller than the size of the uneven pattern of the object to be measured, and scanning the uneven surface of the object to be measured with the parallel beam obtained by this means. means for Fourier transforming the light reflected from the uneven surface of the object to be measured scanned by the means or the light transmitted from the object to be measured; The method is characterized by comprising means for receiving the next light component and obtaining an electrical signal according to the light intensity, and means for measuring the thickness of the unevenness of the object to be measured using the electrical signal obtained by this means. Thickness measuring device. 9. In the device described in claim 8, the means for scanning the uneven surface of the object to be measured includes a scanning mirror for deflecting the irradiated light onto the uneven surface of the object to be measured in the X-axis direction; A thickness measuring device comprising: a moving mechanism for moving the stage on which the object to be measured is mounted in the y-axis direction.