Patent Application
20120062870
This application claims the benefit of Japanese Patent Application No. 2010-205402, filed on Sep. 14, 2010, in the Japan Patent Office, and U.S. Patent Application No. 61/386,132, filed on Sep. 24, 2010, in the United States Patent and Trademark Office, the disclosures of which are incorporated herein in their entireties by reference.
1. Field of the Invention
The present invention relates to a physical state measuring apparatus and a physical state measuring method which can measure a physical state of an object to be measured in a non-contact manner.
2. Description of the Related Art
Accurately measuring a physical state (for example, an internal structure or a temperature) of a substrate (for example, a semiconductor wafer) to be processed by using a substrate processing apparatus is very important in order to accurately control shapes, properties, and so on of films or holes formed on or in the semiconductor wafer based on a result of various processes such as film formation and etching. Accordingly, an internal structure or a temperature of a semiconductor wafer has been measured by using various conventional methods such as a focused ion beam-scanning electron microscope (FIB-SEM) and a fluorescent thermometer.
Recently, a measuring technology using a low-coherence interferometer which can directly measure an internal structure or a temperature of a semiconductor wafer which is difficult to do with the conventional temperature measuring methods has been developed. Also, as the measuring technology using the low-coherence interferometer, a technology has been suggested in which a light from a light source is divided into a measurement light for temperature measurement and a reference light by a first splitter, the measurement light is divided into n measurement lights by a second splitter, the n measurement lights are emitted to n measurement points, and interference between reflected lights of the n measurement lights and a reflected light of the reference light reflected by a reference light reflecting unit is measured to simultaneously measure temperatures of the plurality of measurement points (refer to, for example, Patent Document 1).
In a conventional technology which emits a light from a light source to an object to be measured and measures a physical state of the object to be measured by using a reflected light as described above, it is necessary to select a wavelength of the light emitted from the light source according to the object to be measured and the physical state to be measured. For example, in the conventional technology, in order to measure a temperature of a semiconductor wafer, it is necessary to use a light having a wavelength (for example, a wavelength equal to or greater than 1000 nm) which passes through silicon (Si) of which the semiconductor wafer is formed. Accordingly, it is necessary to use a light receiving element (for example, an InGaAs photodiode) having a sensitivity to a light having a wavelength equal to or greater than 1000 nm as a light receiving unit. However, since the InGaAs photodiode has a lower responsiveness than a Si photodiode, the physical state of the object to be measured cannot be measured at a high speed.
Considering the problems of the conventional technology, an objective of the present invention is to provide a physical state measuring apparatus and a physical state measuring method which can measure a physical state of an object to be measured at a speed higher than that of a conventional apparatus and method even when a light having a long wavelength equal to or greater than 1000 nm should be used.
According to an aspect of the present invention, there is provided a physical state measuring apparatus including: a light source; a transmitting unit which transmits a light from the light source to a measurement point of an object to be measured; a nonlinear optical device which changes a wavelength of the light reflected by the measurement point to a wavelength that is different from the wavelength of the light before the changing; a light receiving unit which receives the light whose wavelength has been changed; and a measuring unit which measures a physical state of the object to be measured at the measurement point based on a waveform of the light received by the light receiving unit.
According to another aspect of the present invention, there is provided a physical state measuring method including: transmitting a light from a light source to a measurement point of an object to be measured; changing a wavelength of the light reflected by the measurement point to a wavelength that is different from the wavelength of the light before the changing; receiving the light whose wavelength has been changed; and measuring a physical state of the object to be measured at the measurement point based on a waveform of the received light.
The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. Also, in the specification and drawings, components having substantially the same functions are denoted by the same reference numerals, and a repeated explanation thereof will not be given. Also, although a semiconductor wafer is exemplarily explained as an object to be measured and a temperature of the semiconductor wafer is exemplarily explained as a physical state, the object to be measured is not limited to the semiconductor wafer and various other objects may be measured. Also, the physical state is not limited to the temperature and various other physical states (for example, an internal structure) may be measured.
The CW light source 110 is a light source which generates a continuous light. Although the CW light source 110 can use an arbitrary light as long as interference between a measurement light and a reference light can be measured, since a temperature of a semiconductor wafer is measured as a temperature of the object to be measured W in the first embodiment, a light whose reflected light from a distance (it is generally in a range of 800 to 1500 μm) between a surface H and a rear surface R of the semiconductor wafer which is the object to be measured W does not cause interference may be used.
Specifically, a low-coherence light may be used. A low-coherence light refers to a light having a short coherence length. A center wavelength of a low-coherence light may be equal to or greater than 1000 nm so that a low-coherence light can pass through silicon (Si) which is a main component of the semiconductor wafer which is the object to be measured W. Also, a coherence length may be, for example, in a range of 0.1 to 100 μm, and a coherence length may be equal to or less than 3 μm. Since the CW light source 110 uses such a low-coherence light, obstruction due to unnecessary interference can be avoided, and interference with a reference light based on a reflected light from an inner layer or the surface of the wafer can be easily measured.
The splitter 120 is, for example, an optical fiber coupler. However, the present embodiment is not limited thereto, and for example, an optical waveguide type branching filter or a semi-transmissive mirror, may be used as the splitter 120 as long as it can split a light to a reference light and a measurement light.
Examples of the reference light reflecting unit 130 may include a corner cube prism and a plane mirror. From among the corner cube prism and the plane mirror, considering that a reflected light is parallel to an incident light, a corner cube prism may be used. However, the present embodiment is not limited thereto, and the reference light reflecting unit 130 may include, for example, a delay line as long as the delay line can reflect a reference light.
The optical path length changing unit 140 includes a driving unit such as a motor for driving the reference light reflecting unit 130 which includes, for example, a reference mirror, in one direction parallel to an incident direction in which a reference light is incident. As such, an optical path length of a reference light reflected from the reference mirror can be changed by driving the reference mirror in one direction.
The wavelength changing unit 150 changes wavelengths of the measurement light reflected by the measurement point P of the object to be measured W and the reference light reflected by the reference light reflecting unit 130 to wavelengths (specifically, wavelengths less than 1000 nm) which can be received by the light receiving unit 161.
The wavelength changing unit 150 may be, for example, a nonlinear optical crystal which radiates a second-harmonic wave having a half (λ/2) of a wavelength λ of an input light. Since the nonlinear optical crystal is used, a wavelength of a light can be changed while a phase of the light is maintained. Examples of the nonlinear optical crystal may include a lithium niobate (LiNbO3) crystal, a potassium titanyl phosphate (KTP) crystal, a β-barium borate (BBO) crystal, a lithium triborate (LBO) crystal, a AgGaS2 crystal, a AgGaSe2 crystal, and a periodically poled lithium niobate (PPLN) crystal.
The light receiving unit 161 converts the reflected lights of the measurement light and the reference light whose wavelengths are changed by the wavelength changing unit 150 to electrical signals. In the first embodiment, the light receiving unit 161 includes a charged-coupled device (CCD) image sensor using a Si photodiode.
As described above, since an InGaAs photodiode having a sensitivity to a light having a wavelength equal to or greater than 1000 nm has a responsiveness lower than that of a Si photodiode, a temperature of the object to be measured W cannot be measured at a high speed. Accordingly, in the first embodiment, a temperature is measured at a high speed by changing a wavelength to a wavelength which can be received by the light receiving unit 161 including the CCD image sensor using the Si photodiode by using the wavelength changing unit 150. Also, since the CCD image sensor using the Si photodiode can form a photodiode at high density, a resolution, that is, a number of samples can be improved. Also, likewise, a resolution can be improved even by using a complementary metal-oxide-semiconductor (CMOS) image sensor instead of the CCD image sensor. Also, a sampling speed can be increased, a compact design can be achieved, and power consumption can be reduced.
The signal obtaining unit 101 obtains a waveform signal from the light receiving unit 161 and a driving distance signal of the reference light reflecting unit 130 from the optical path length changing unit 140.
The memory unit 102 is, for example, a nonvolatile memory such as a flash memory or a ferroelectric random-access memory (FeRAM). Properties and equations for calculating a temperature of the measurement point P are stored in the memory unit 102. In detail, a linear expansion coefficient α and a temperature coefficient β refractive index change according to a temperature of the object to be measured W, and equations that will be explained below are stored.
The temperature computing unit 103 calculates a temperature of the measurement point P of the object to be measured W based on the waveform signal from the light receiving unit 161 and the driving distance signal of the reference light reflecting unit 130 from the optical path length changing unit 140 by referring to the memory unit 102. A detailed calculating method will be explained in (Temperature Measuring Method Based on Interference Light) that will be explained below.
(Operation of Physical State Measuring Apparatus)
As shown in
The other light (reference light) obtained by the splitter 120 is emitted from the collimator fiber F2 and is reflected by the reference light reflecting unit 130. Then, a reflected light of the reference light is incident on the splitter 120, is combined with a reflected light of the measurement light, and is wavelength-changed by the wavelength changing unit 150. An interference waveform is detected by the signal processing apparatus 160, and a temperature of the measurement point P is calculated based on the interference waveform.
(Specific Example of Interference Waveform Between Measurement Light and Reference Light)
Here, a specific example of an interference waveform obtained by the physical state measuring apparatus 100 is shown in
Referring to
(Temperature Measuring Method Based on Interference Light)
Next, a method of measuring a temperature based on an interference wave between a measurement light and a reference light will be explained. A temperature measuring method based on an interference wave is, for example, a temperature converting method which uses an optical path length change based on a temperature change. Here, a temperature converting method using a misalignment of the interference waveform will be explained.
Since, when the object to be measured W is heated due to a heater or the like, the object to be measured W is expanded and a refractive index of the object to be measured W is changed, there is a misalignment of an interference waveform between before a temperature change and after the temperature change, and thus a width between peaks of the interference waveform is changed. The temperature change can be detected by measuring the width between the peaks of the interference waveform of the measurement point. For example, in the physical state measuring apparatus 100 shown in
If a thickness and a refractive index of an object whose temperature is to be measured are respectively d and n, a misalignment of an interference waveform is dependent on a unique linear expansion coefficient α of each layer for the thickness d, and is dependent mainly on a unique temperature coefficient β of refractive index change of each layer for the change of the refractive index n. It is known that the misalignment of the interference waveform is also dependent on a wavelength for the temperature coefficient β of refractive index change.
Accordingly, a thickness d′ and a refractive index n′ of a wafer after a temperature change at a certain measurement point P may be defined as shown in Equation 1. Also, in Equation 1, ΔT denotes an amount of temperature change of the measurement point, α denotes a linear expansion coefficient, and β denotes a temperature coefficient of refractive index change. Also, d and n respectively denote a thickness and a refractive index at the measurement point P before the temperature change.
[Equation 1]
d′=d·(1+αΔT), n′=n·(1+βΔT) (1)
As shown in Equation 1, an optical path length of a measurement light which passes through the measurement point P varies according to the temperature change. An optical path length is generally obtained by multiplying the thickness d by the refractive index n. Accordingly, if an optical path length of a measurement light which passes through the measurement point P before a temperature change is L and an optical path length after a temperature of the measurement point P is changed by ΔT is L′, the optical path lengths L and L′ are defined as shown in Equation 2.
[Equation 2]
L=d·n, L′=d′·n′ (2)
Accordingly, a difference (L′−L) between the optical path length L before the temperature change and the optical path length L′ after the temperature change at the measurement point is defined as shown in Equation 3 by referring to Equations 1 and 2. Also, in Equation 3, small terms are omitted in consideration of α·βα, α·β
β.
Here, an optical path length of a measurement light at a measurement point corresponds to a width between peaks of an interference waveform with a reference light. Accordingly, if a linear expansion coefficient α and a temperature coefficient β of refractive index change are obtained in advance, a width between peaks of an interference waveform with a reference light at a measurement point is measured to be converted to a temperature of the measurement point by using Equation 3.
As such, if an interference wave is converted to a temperature, since an optical path length between peaks of an interference waveform varies according to a linear expansion coefficient α and a temperature coefficient β of refractive index change as described above, the linear expansion coefficient α and a temperature coefficient β of refractive index change need to be obtained in advance. A linear expansion coefficient α and a temperature coefficient β of refractive index change of a material including a semiconductor wafer may be generally dependent on a temperature in a certain temperature range. For example, in general, since a linear expansion coefficient α is not much changed when a temperature ranges from 0 to 100° C., the linear expansion coefficient α may be regarded as constant. However, according to materials, since a linear expansion coefficient α increases as a temperature increases when a temperature is equal to or higher than 100° C., a temperature dependency of the linear expansion coefficient α cannot be disregarded in this case. Likewise, there are cases where a temperature dependency of a temperature coefficient β of refractive index change cannot be disregarded in a certain temperature range.
For example, it is known that a linear expansion coefficient α and a temperature coefficient β of refractive index change of Si constituting a semiconductor wafer approximate to, for example, a quadratic curve in a temperature range of 0 to 500° C. As such, since a linear expansion coefficient α and a temperature coefficient β of refractive index change are dependent on temperature, a temperature can be more accurately calculated by obtaining a linear expansion coefficient α and a temperature coefficient β of refractive index change according to temperature in advance and obtaining a temperature in consideration of the obtained linear expansion coefficient α and temperature coefficient β of refractive index change.
Also, a temperature measuring method based on an interference wave between a measurement light and a reference light is not limited to the above-described method, and for example, a method using an absorbance intensity change based on a temperature change may be used or a method which combines an optical path length change based on a temperature change and an absorbance intensity change based on a temperature change may be used.
As described above, since the physical state measuring apparatus 100 includes the wavelength changing unit 150 which changes wavelengths (equal to or greater than 1000 nm) of a measurement light reflected by the measurement point P of the object to be measured W and a reference light reflected by the reference light reflecting unit 130 to wavelengths which can be received by the light receiving unit 161 including the CCD image sensor using the Si photodiode, the physical state measuring apparatus 100 can measure a temperature at a higher speed. Also, since the CCD image sensor using the Si photodiode ensures high density, a resolution, that is, a number of samples can be improved. Also, the same effect can be achieved even when a CMOS image sensor instead of the CCD image sensor is used.
In the first embodiment, a temperature of a measurement point of the object to be measured W (for example, a semiconductor wafer) is measured by dividing a light generated by the CW light source 110 to a measurement light and a reference light, and causing the measurement light reflected by the measurement point P of the object to be measured W and the reference light reflected by the reference light reflecting unit 130 to interfere with each other. A second embodiment in which a reference light is not used will be explained.
The optical circulator 170 includes three ports A through C. A light input to the port A is output from the port B, a light input to the port B is output from the port C, and a light input to the port C is output from the port A. That is, a measurement light input from the CW light source 110 is emitted to the object to be measured W through the collimator fiber F1, and a reflected light from the object to be measured W is input to the light receiving unit 181 of the signal processing apparatus 180.
The signal receiving unit 201 obtains a discrete signal from the light receiving unit 181.
The DFT unit 202 performs DFT on the discrete signal obtained by the signal obtaining unit 201. Due to the DFT, the discrete signal from the light receiving unit 181 is converted to information regarding an amplitude and a distance.
The optical path length calculating unit 203 calculates an optical path length based on the information regarding the amplitude and the distance obtained by the DFT unit 202. In detail, a distance between a peak A and a peak B shown in
A relationship between an optical path length and a temperature shown in
Also, a relationship between an optical path length and a temperature shown in
The temperature computing unit 205 calculates a temperature of the object to be measured W from the optical path length calculated by the optical path length calculating unit 203 by referring to the memory unit 204.
As described above, since the physical state measuring apparatus 200 according to the second embodiment calculates an optical path length by converting a reflected light from the measurement point P to a discrete signal by using the light receiving unit 181 and performing DFT on the discrete signal, and since a reference mirror does not need to be mechanically operated unlike a case where an optical path length is calculated by using interference with a reflected light from a reference mirror, a temperature of the measurement point can be very rapidly measured and thus can be efficiently measured. Other effects are the same as those of the physical state measuring apparatus 100 according to the first embodiment.
Also, intervals between the wavelengths of the light generated by the multi-wavelength CW light source 110A, that is, the wavelengths λ1 through λm, may be different from one another. This is because when second-harmonic waves are generated by the wavelength changing unit 150, difference frequency waves are simultaneously generated to prevent a signal-noise ratio (SNR) from decreasing.
Also, although reflected lights having the wavelengths λ1 through λm from the measurement points P1 through Pm are combined and then input to the light receiving unit 181, only a reflective light having a wavelength desired to be processed by the temperature calculating unit 182 may be extracted from the CCD image sensor 181b.
Besides, a specific wavelength may be selected from a multi-wavelength light (reflected light) input from the wavelength changing unit 150 and may be received by the CCD image sensor 181b by rotating the diffraction grating 181a as shown in
Also, a PPLN (periodically poled lithium niobate) crystal which can select a wavelength of a second-harmonic wave may be used as the wavelength changing unit 150 as shown in
As described above, since the physical state measuring apparatus 300 according to the third embodiment uses the multi-wavelength CW light source 110A, a multi-wavelength light (measurement light) generated by the multi-wavelength CW light source 110A is wavelength-divided into a plurality of measurement lights respectively having wavelengths λ1 through λm by the branching filter 190, and the measurement lights obtained by the branching filter 190 are respectively emitted to the different measurement points P1 through Pm, temperatures of the plurality of measurement points can be simply measured.
Also, although physical states of the different measurement points P1 through Pm of a specific object to be measured W are measured in the third embodiment, physical states of different objects to be measured W may be measured by using measurement lights respectively having wavelengths λ1 through λm obtained by the branching filter 190.
Since the physical state measuring apparatus 400 according to the fourth embodiment uses the multi-wavelength CW light source 110A, a multi-wavelength light generated by the multi-wavelength CW light source 110A is wavelength-divided into plurality of measurement lights respectively having wavelengths λ1 through λm by the branching filter 190, and the measurement lights obtained by the branching filter 190 are respectively emitted to different measurement points P1 through Pm, temperatures of the plurality of measurement points can be simply measured. Other effects are the same as those of the physical state measuring apparatus 100 according to the first embodiment.
Also, in order to extract a specific wavelength from the plurality of wavelengths λ1 through λm, the light receiving unit 161 may be used or a diffraction grating may be provided in front of the light receiving unit 161, as described in the third embodiment. Also, a PPLN crystal which can select a wavelength of a second-harmonic wave may be used as the wavelength changing unit 150.
Also, the present invention is not limited to the embodiments, and various changes in form and details may be made therein without departing from the scope of the present invention. For example, a temperature of the object to be measured W is measured in the above embodiments. If layers or structures having different refractive indices exist in the object to be measured W, a measurement light causes interference by being reflected by the layers or the structures. Accordingly, it is possible to measure other physical states (for example, an internal structure) of the object to be measured W. Also, if a wavelength of a measurement light is changed, physical states of various other structures (for example, a human body) instead of a semiconductor wafer formed of Si may be measured.
According to the present invention, a physical state measuring apparatus and a physical state measuring method can measure a physical state of an object to be measured at a higher speed than that of a conventional apparatus and method.
While this invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-205402 | Sep 2010 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| 61386132 | Sep 2010 | US |
