Inspection device

Abstract

Provided is an inspection device including: an illumination optical system having a light source for emitting illumination light for illuminating a sample surface; and an interference light detection system for photoelectrically converting, by a detector, interference light containing reflected light generated on the sample surface. The interference light detection system includes: a relay lens optical system for forming a conjugate surface optically conjugate with the sample surface; and an imaging optical system for imaging the conjugate surface on the detector. The inspection device is configured such that rays of light for generating the interference light are coherent with each other, and reflected stray light, which is incident on the detector after being reflected by an optical element of the imaging optical system without passing through the relay lens optical system from the light source, and the reflected light generated on the sample surface are incoherent with each other.

Description

Inspection Equipment

[0001] The present invention relates to an inspection apparatus for inspecting a sample such as a wafer in a semiconductor device manufacturing process or the like.

[0002] An inspection device using interferometry is known, which separates illumination light emitted from a light source into a measurement beam and a reference beam using a beam splitter, and measures the surface shape of a sample based on interference light obtained by superimposing the measurement beam reflected from the sample and the reference beam reflected from a reference surface (see Patent Document 1, etc.).

[0003] Patent No. 5536983

[0004] In an inspection device that measures the surface shape of a sample based on interference light, such as that illustrated in Patent Document 1, noise caused by weak reflected stray light from optical elements interfering with reflected light from the sample, etc., can be a factor in reducing sensitivity. A commonly used method is to spatially separate the interference light and reflected stray light by shifting the optical axis of the illumination light from the optical axis of the optical element by adjusting the angle of the optical element, etc., thereby suppressing noise, but in this case there is a concern that resolution may be reduced due to aberrations.

[0005] To deal with reflected stray light without shifting the optical axis, it is possible to set the coherence length of the light source so that the main beams that generate the interference light to be measured interfere with each other, but these beams do not interfere with the reflected stray light from the optical elements.

[0006] However, when the difference in optical path length between the main beam and the reflected stray light is small, it is practically difficult to realize a light source that satisfies the above condition. For example, this occurs when reflected stray light occurs in the objective lens. For high-resolution measurements of sample surfaces such as wafers, objective lenses with high NA and short focal lengths are often used due to the constraints of optical system size. In this case, the distance between the objective lens and the sample surface is extremely short, so the difference in optical path length between the reflected light from the sample and the reflected stray light is very small. Therefore, the conditions required for the coherence length of the light source become strict and difficult to achieve.

[0007] The present invention provides an inspection apparatus that can suppress the influence of reflected stray light from optical elements and measure the surface shape of a sample with high sensitivity and high resolution.

[0008] In order to achieve the above object, the present invention provides an inspection apparatus comprising an illumination optical system having a light source that emits illumination light to illuminate a sample surface, and an interference light detection system that photoelectrically converts interference light including reflected light generated at the sample surface using a detector, wherein the interference light detection system comprises a relay lens optical system that forms a conjugate plane that is optically conjugate with the sample surface, and an imaging optical system that images the conjugate plane on the detector, wherein the lights that generate the interference light are coherent with each other, and reflected stray light that originates from the light source and is reflected by an optical element of the imaging optical system and enters the detector without passing through the relay lens optical system, and the reflected light generated at the sample surface are configured to be incoherent.

[0009] According to the present invention, the influence of reflected stray light from optical elements can be suppressed, and the surface shape of a sample can be measured with high sensitivity and high resolution.

[0010] FIG. 1 is a schematic diagram of an inspection device according to a first embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of the configuration of an interference light measurement device and a scattered light measurement device provided in the inspection device according to the first embodiment of the present invention. FIG. 3 is a diagram showing an example of the scanning trajectory of a sample in the inspection device according to the first embodiment of the present invention. FIG. 4 is a schematic diagram showing an example of the configuration of an interference light detector provided in the inspection device according to the first embodiment of the present invention. FIG. 5 is an explanatory diagram of coherence length. FIG. 6 is a schematic diagram showing an example of the configuration of an interference light measurement device provided in the inspection device according to the second embodiment of the present invention. FIG. 7 is an explanatory diagram of the principle of an inspection device according to a third embodiment of the present invention. FIG. 8 is an explanatory diagram of the principle of an inspection device according to a fourth embodiment of the present invention. FIG. 9 is an explanatory diagram of the principle of an inspection device according to the fourth embodiment of the present invention.

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0012] (First embodiment) - Inspection apparatus - Fig. 1 is a schematic diagram of an inspection apparatus according to a first embodiment of the present invention, Fig. 2 is a schematic diagram showing an example of the configuration of an interference light measurement apparatus and a scattered light measurement apparatus provided in the inspection apparatus according to the first embodiment of the present invention, Fig. 3 is a diagram showing an example of the scanning trajectory of a sample in the inspection apparatus according to the first embodiment of the present invention, and Fig. 4 is a schematic diagram showing an example of the configuration of an interference light detector provided in the inspection apparatus according to the first embodiment of the present invention.

[0013] The inspection apparatus of this embodiment is an apparatus that inspects and manages the microroughness (microscopic flatness of the sample surface) of the surface of a sample 1 (hereinafter referred to as the sample surface) in, for example, a semiconductor device manufacturing process. The inspection apparatus of this embodiment also has a defect inspection optical system that measures scattered light from the sample surface and inspects the sample surface for defects based on the measured scattered light, as will be described later. A typical example of the sample 1 is a disk-shaped semiconductor silicon wafer, and in addition to bare wafers with no patterns formed thereon, patterned wafers and the like can also be targeted.

[0014] The inspection apparatus of this embodiment is configured to include a stage device 100, a scattered light measurement device 200, an interference light measurement device 300, and a signal processing device (not shown).

[0015] -Stage Device 100- The stage device 100 is a device that holds the sample 1 relative to the beam spot and moves the sample 1 in the direction of the sample surface, and is equipped with a sample holding stage 101, a rotation stage 102, and a linear stage 103. The sample holding stage 101 is, for example, a chuck table that holds the sample 1 by suction or the like. The linear stage 103 causes the rotation stage 102 to move linearly in the radial direction of the sample 1, and the rotation stage 102 causes the sample holding stage 101 to rotate (spin on its axis).

[0016] As shown in FIG. 3 , light is irradiated onto the surface of the sample 1 from the scattered light measurement device 200 and the interference light measurement device 300, forming a thin line-shaped beam spot 2. The beam spot 2 shown in the figure is formed by both the scattered light measurement device 200 and the interference light measurement device 300 at substantially the same coordinates (or coordinates whose distance from each other is within a predetermined distance). During inspection, the sample 1 is rotated in the circumferential direction of the sample 1 by the rotary stage 102, and the beam spot 2 rotates in a rotation direction θ on the surface of the sample 1. Simultaneously, the sample 1 is driven linearly by the linear stage 103, and the beam spot 2 moves linearly on the surface of the sample 1 in the radial direction r. By combining this rotational movement and linear movement, the stage device 100 scans the illumination light along a spiral trajectory to inspect the entire sample surface. In this embodiment, in order to inspect the entire surface of the sample at high speed, the illumination optical systems of the scattered light measurement device 200 and the interference light measurement device 300 each shape the illumination light into a line to irradiate a thin line-shaped beam spot 2 onto the sample surface, and line sensors are used as detectors for the scattered light measurement device 200 and the interference light measurement device 300.

[0017] 1 and 3, the stage device 100 may also be equipped with a linear stage having a movement axis (here, Y-axis) that horizontally intersects with the movement axis (here, X-axis) of the linear stage 103. In this case, the sample 1 can also be scanned along an orbit in which the beam spot 2 repeatedly moves in the X direction and the −X direction while being sequentially shifted in the Y-axis direction.

[0018] - Scattered Light Measuring Apparatus 200 - The scattered light measuring apparatus 200 is an apparatus that illuminates a sample 1 and measures scattered light generated at a beam spot 2 on the sample surface, and is configured to include an illumination optical system 210 and an oblique detection optical system 220.

[0019] The illumination optical system 210 includes a light source 211 for scattered light and an optical system 212 that guides light emitted from the light source to the beam spot 2. The optical system 212 includes a plurality of optical elements such as lenses and mirrors. In this embodiment, the illumination optical system 210 can perform oblique illumination, in which light is irradiated obliquely onto the surface of the sample 1, as shown by the optical path indicated by the dashed line in FIG. 2 , and can also switch to epi-illumination, in which light is irradiated perpendicularly onto the surface of the sample 1. In this embodiment, the optical path of the epi-illumination partially overlaps with the illumination optical path of the interference light measurement device 300. However, since the illumination light of the scattered light measurement device 200 and the interference light measurement device 300 has different center wavelengths, wavelength separation is performed using a dichroic mirror 213 installed at a position where the optical path of the interference light measurement device 300 joins the optical path of the epi-illumination, allowing interference light inspection and scattered light inspection to be performed simultaneously.

[0020] The oblique detection optical system 220 is an optical system that spatially divides the scattered light generated by the beam spot 2 on the sample surface and detects it using a scattered light detector (not shown). Multiple oblique detection optical systems 220 are provided with different azimuth and elevation angles relative to the beam spot 2 (only two are shown in FIG. 2 ). The optical axis of each oblique detection optical system 220 is tilted with respect to the normal to the sample surface that passes through the beam spot 2. The scattered light detector is a photoelectric conversion element, and a high gain is desirable for measuring weak scattered light. A photomultiplier tube or an avalanche photodiode array can be used. Other possible scattered light detectors include a photon counting array with multiple photon counting elements arranged in an array. Sensor types that can be used include a photomultiplier tube, SiPM, CMOS sensor, and CCD.

[0021] The intensity of the scattered light measured by each oblique detection optical system 220 is AD converted and processed by, for example, a signal processing device (not shown), thereby making it possible to inspect defects on the sample surface. The signal processing device is one or more computers that process signals from the scattered light measurement device 200 and the interference light measurement device 300.

[0022] - Interference light measurement device 300 - The interference light measurement device 300 is a device that measures the surface shape of the sample 1 by measuring interference light including reflected light generated at a beam spot 2 on the sample 1. In this embodiment, an interference light measurement device 300 that detects differential interference contrast (DIC) and calculates the height of the sample surface will be described as an example. By measuring interference light, the entire surface of the sample 1 can be measured at high speed. Examples of interference light measurement include optical interferometry, phase-shift interferometry, and wavelength-shift interferometry, and any of these methods may be used.

[0023] The interference light measuring device 300 includes an illumination optical system 310 having a light source 311 that emits illumination light to illuminate the sample surface, and an interference light detection system 320 that photoelectrically converts interference light including reflected light generated on the sample surface using an interference light detector 327 to measure the surface shape of the sample 1.

[0024] Illumination optical system 310 The illumination optical system 310 includes a light source 311 and a differential interference illumination system 312 including a beam spot shaping unit and an illumination lens. In this embodiment, the light source 311 is a laser light source, but an SLD, an LED light source, or the like can also be used as the light source 311. The light emitted from the light source 311 is shaped into a linear light by the differential interference illumination system 312 and irradiated onto the sample surface. The light passing through the differential interference illumination system 312 is linearly polarized light.

[0025] Interference light detection system 320 splits the illumination light emitted from the illumination optical system 310 into two light beams l1 and l2 having different polarization directions, irradiates them onto two irradiation areas (beam spots 2a and 2b) spaced a predetermined distance apart on the sample surface, and collects the reflected light of the two light beams l1 and l2 from the two irradiation areas to generate an image of combined interference light. Specifically, the interference light detection system 320 includes a half mirror 321, a quarter-wave plate 322, a Nomarski prism 323, an objective lens 324, an imaging lens 325, a polarizing beam splitter 326, an interference light detector 327, and the like.

[0026] Light emitted from the interference light measurement device 300 is incident on a quarter-wave plate 322 via a half mirror 321. The quarter-wave plate 322 is installed so that its fast axis forms an angle of 45° with respect to the direction of incident polarization. Light passing through the quarter-wave plate 322 becomes circularly polarized light. The light that has passed through the quarter-wave plate 322 is incident on a Nomarski prism 323.

[0027] The Nomarski prism 323 is a spectroscopic element that separates illumination light into two beams. It is made of a birefringent optical material and separates circularly polarized incident light into two linearly polarized beams l1 and l2 with orthogonal vibration planes, for example, in the X direction. The beams l1 and l2 are, for example, S-polarized and P-polarized, respectively. The beams l1 and l2 separated by the Nomarski prism 323 are incident on a DIC objective lens 324. This objective lens 324 is mounted on a stage (not shown), and its pupil position coincides with the separation position of the Nomarski prism 323. The two beams l1 and l2 that pass through the objective lens 324 travel parallel to each other and are irradiated perpendicularly onto the sample surface, forming a beam spot 2 consisting of two beam spots 2a and 2b.

[0028] The Nomarski prism 323 can be moved in the direction of separation of the illumination light (the X direction in this example) by a drive mechanism (not shown), and the phase difference between the separated light beams l1 and l2 can be adjusted by adjusting its position in the X direction. The separation width of the light beams l1 and l2 is called the shear amount δ.

[0029] If there is a step between the beam spots 2a and 2b (if there is a difference in height on the sample surface in the direction of travel of the light beams l1 and l2), the phase difference between the light beams l1 and l2 will change. In DIC measurement, a larger shear amount δ increases the contrast, but reduces the measurable height difference per unit distance in the horizontal direction. When contrast is important, the shear amount δ can be set larger than the optical resolution or sampling interval of the interference light measurement device 300, for example.

[0030] In DIC measurement, the height difference (differential height Δh) between beam spots 2a and 2b is measured based on the phase difference between light beams l1 and l2 reflected from the surface of sample 1. Light beams l1 and l2 reflected from the surface of the sample are collimated by objective lens 324 and recombined along the same optical path by Nomarski prism 323 to become interference light, which enters interference light detector 327 via imaging lens 325. In this embodiment, a polarizing beam splitter 326 is disposed after imaging lens 325 to split the interference light into two orthogonal polarization directions, and the interference intensities of these two interference lights are measured by different interference light detectors 327.

[0031] The interference light detector 327 is a photoelectric conversion element, like the scattered light detector, but because it detects light directly reflected from the sample surface, its gain may be lower than that of the scattered light detector. In this embodiment, the illumination light is shaped into a line, so that the interference light detector 327 uses a line sensor in which photodiode (PD) sensors 328 are arranged in a one-dimensional manner, as shown in FIG. 4 , in order to efficiently measure the interference light image 329 from the thin line-shaped beam spot 2. However, the interference light detector 327 may also be a multi-line sensor consisting of multiple line sensors, a point sensor, or an area sensor, and the sensor type may be a photodiode sensor, an avalanche photodiode sensor, or the like. Because the interference intensity of the interference light varies depending on the differential height Δh, the interference intensity of the interference light measured by the interference light detector 327 is AD converted and processed, for example, by a signal processing device (not shown), to measure the differential height Δh.

[0032] DIC measurement by the interference light measurement device 300 can be performed simultaneously (during the same scan) with scattered light measurement by the scattered light measurement device 200 by wavelength separation or spatial separation. This enables faster measurement of the surface of the sample 1. This also applies when a surface shape measurement method other than DIC measurement is applied to the measurement method of the interference light measurement device 300.

[0033] 1 , the interference light detection system 320 in this embodiment includes a relay lens optical system 350 that forms a conjugate plane 390 that is optically conjugate with the sample surface of the sample 1 set on the stage device 100, and an imaging optical system 370 that images the conjugate plane 390 on the interference light detector 327. The imaging optical system 370 is a part that includes the half mirror 321, quarter-wave plate 322, Nomarski prism 323, objective lens 324, imaging lens 325, polarizing beam splitter 326, etc., as described above, and enlarges the conjugate plane 390 to form an image on the interference light detector 327. That is, in this embodiment, the interference light detection system 320 includes the relay lens optical system 350 in addition to the imaging optical system 370 that includes the half mirror 321, etc.

[0034] The relay lens optical system 350 is disposed between the objective lens 324 of the imaging optical system 370 and the sample 1 set on the stage device 100, and relays the sample surface to the focal plane (a position within a narrow depth of field) of the imaging optical system 370. The image does not need to be enlarged or reduced by the relay lens optical system 350, but it is acceptable if the image is enlarged or reduced somewhat via the relay lens optical system 350.

[0035] The relay lens optical system 350 includes an objective lens 351 and an imaging lens 352. The objective lens 351 and the imaging lens 352 are preferably coated with an anti-reflection coating. The focal length (first focal length) of the objective lens 351 of the relay lens optical system 350 is longer than the focal length (second focal length) of the objective lens 324 of the imaging optical system 370. For example, the first focal length can be ten times the second focal length.

[0036] If a height difference (differential height Δh) occurs between the beam spots 2a and 2b due to unevenness or inclination of the sample surface, an optical path length difference OPL1 occurs between the two separated light beams l1 and l2. The optical path length difference OPL1 is the difference between the distance A that one of the light beams l1 and l2 travels between the objective lens 351 and the sample surface and the distance B that the other of the light beams l1 and l2 travels between the objective lens 351 and the sample surface, plus the optical path length difference C that occurs in optical elements such as the quarter-wave plate 322 and the Nomarski prism 323 (OPL1 = |A - B| + C). The optical path length difference OPL1 is, for example, a value on the order of several tens of micrometers.

[0037] Furthermore, in the interference light detector 327, weak stray light (reflected stray light) reflected by an optical element (e.g., the objective lens 324) of the imaging optical system 370 may interfere with the interference light (main beam) of the light beams l1 and l2, which are the measurement target, reflected from the sample surface. In this embodiment, the difference between the optical path length of the interference light from the light source 311 reflected from the sample surface to reach the interference light detector 327 and the optical path length of the reflected stray light from the light source 311 reflected by an optical element (e.g., the objective lens 324) to reach the interference light detector 327 is defined as an optical path length difference OPL2. The optical path length difference OPL2 corresponds to twice the distance between the optical element that reflects the reflected stray light and the sample surface. In the case of reflected stray light reflected by the objective lens 324, the optical path length difference OPL2 is twice the distance between the objective lens 324 and the sample surface. If the relay lens optical system 350 were not present and the sample surface were located at the position of the conjugate plane 390, the optical path length difference OPL2 would be on the order of several mm. However, in this embodiment, the relay lens optical system 350 is interposed between the imaging optical system 370 and the sample surface, so that the optical path length difference OPL2 can be increased to the order of several hundred mm.

[0038] In this embodiment, a relay lens optical system 350 is provided to ensure a sufficient optical path length difference OPL2, and a light source 311 is used that emits illumination light whose coherence length ΔL satisfies the condition OPL1<ΔL<OPL2.

[0039] The interference light detection system 320 is configured so that reflected stray light that is emitted from the light source 311 and reflected by an optical element (e.g., the objective lens 324) of the imaging optical system 370 without passing through the relay lens optical system 350 and then enters the interference light detector 327, and the reflected light generated on the sample surface are incoherent. Specifically, the interference light detection system 320 is configured so that, where the coherence length of the illumination light emitted by the light source 311 is ΔL, the condition OPL1<ΔL<OPL2 holds for the optical path length differences OPL1, OPL2, and the coherence length ΔL.

[0040] In this case, when the central wavelength λo of the illumination light is 532 [nm], the lower limit of the spectral width (wavelength width) of the illumination light is typically on the order of pm. In order to suppress the influence of interference noise due to reflected stray light while maintaining the accuracy of the interference light measurement, it is desirable to configure the light source 311 so that the central wavelength λo of the emitted illumination light satisfies OPL1<λo<OPL2.

[0041] - Coherence Length - FIG. 5 is an explanatory diagram of the coherence length.

[0042] In this embodiment, the coherence length is a measure of the difference in optical path length at which interference occurs between two light beams, and becomes narrower as the spectral width of the light source 311 becomes wider. The degree of interference between two light beams can be expressed by the visibility V. The visibility V is defined as follows, where Imax is the maximum intensity of the interference fringes and Imin is the minimum intensity: V = (Imax - Imin) / (Imax + Imin).

[0043] The spectrum of the light source 311 has a Gaussian shape with a spectral width Δλ, F(λ)=exp(−2((λ−λo) / Δλ) ２ ), where the optical path difference between the two light beams is L, the visibility V(L) with respect to the optical path difference L also has a Gaussian shape as shown in FIG. 5. The visibility V(L) shown in FIG. 5 is expressed as follows: V(L)=exp(-(1 / 2)(πLΔλ / λo ２ ) ２ The coherence length ΔL is expressed as follows: ２ It can be defined as the optical path difference such that

[0044] When the spectrum of the illumination light emitted by the light source 311 is Gaussian, the optical path length difference L is 2λo２ / (πΔλ), the visibility becomes 1 / e ２ Therefore, the interference light measurement device 300 determines the optical path length difference OPL1 between the two light beams l1 and l2, the optical path length difference OPL2 of the interference light reflected stray light, and the spectral width Δλ of the illumination light emitted by the light source 311 as follows: ２ The optical amplifier is configured so that the condition of / (πOPL2)<Δλ is satisfied.

[0045] The effect of interference noise due to reflected stray light on DIC measurement varies depending on the intensity ratio between the interference light (main beam) that is the measurement target and the reflected stray light, as well as the difference in relative polarization angle. For example, if the polarization angle difference between the interference light and the reflected stray light is 45 degrees, the interference noise affects only one of the interference light intensities detected by the two interference light detectors 327.

[0046] -Effects- (1) In this embodiment, when comparing the interference light measurement device 300 with a typical interference light measurement device (conventional device), the conjugate plane 390 corresponds to the position where the sample surface is located in the conventional device. In this case, the optical path length difference OPL2 is twice the gap between the objective lens 324 of the imaging optical system 370 and the conjugate plane 390, which is only about a few millimeters as described above. Therefore, in a configuration in which the sample surface is located at the conjugate plane 390, the difference between the optical path length differences OPL1 and OPL2 is small, and the range of spectral width allowable for the light source 311 is narrow and difficult to achieve, making it difficult to satisfy the condition OPL1 < ΔL < OPL2 and suppress interference noise caused by reflected stray light. Furthermore, the objective lens 324 often has a high NA and a short focal length. In such cases, the lens surface of the objective lens 324 has a large curvature, making it difficult to use an anti-reflection coating to sufficiently reduce the intensity of the reflected stray light to a level where interference noise does not affect the measurement.

[0047] In contrast, in this embodiment, a relay lens optical system 350 is interposed between the imaging optical system 370 and the sample surface, a conjugate plane 390 optically conjugate with the sample surface is formed by the relay lens optical system 350, and the conjugate plane 390 is imaged onto the interference light detector 327 by the imaging optical system 370. This ensures a sufficient distance between the imaging optical system 370 and the sample surface, and the optical path length difference OPL2 can be increased significantly (for example, on the order of several hundred mm). In this way, by ensuring a sufficient optical path length difference OPL2 between the reflected stray light and the main beam of the interference light to be measured by the relay lens optical system 350, the range of the spectral width allowable for the light source 311 can be expanded, and an interference light detection system 320 that satisfies the condition OPL1<ΔL<OPL2 can be realized.

[0048] According to this embodiment, by using a light source 311 that expands the optical path length difference OPL2 between the main beam of interference light and the reflected stray light so that the main beam and the reflected stray light are incoherent with each other, the two separated light beams l1 and l2 can interfere with each other, while the reflected stray light and the main beam can be prevented from interfering with each other. This makes it possible to suppress the influence of reflected stray light without taking measures such as shifting the optical axis of the illumination light and the optical axis of the optical elements. Therefore, the influence of reflected stray light from the optical elements of the imaging optical system 370 can be suppressed, and the surface shape of the sample can be measured with high sensitivity and high resolution.

[0049] Note that reflected stray light can also occur in the optical elements included in the relay lens optical system 350, but because the optical elements of the relay lens optical system 350 have a longer focal length than the optical elements of the imaging optical system 370, the difference in optical path length between the reflected stray light reflected by the optical elements of the relay lens optical system 350 and the interference light to be detected is also large. Therefore, the influence of reflected stray light from the optical elements included in the relay lens optical system 350 can be easily suppressed.

[0050] Furthermore, although there are multiple optical elements in the imaging optical system 370, the optical element closest to the sample surface is the objective lens 324. Therefore, among the reflected stray light reflected by each optical element in the imaging optical system 370, the optical path length difference OPL2 for the reflected stray light reflected by the objective lens 324 is the shortest. Therefore, if the condition OPL1<ΔL<OPL2 is satisfied for the reflected stray light from the objective lens 324, the condition is also satisfied for the reflected stray light from the other optical elements.

[0051] (2) The effect of interference noise on the differential height Δh (difference in height between the beam spots 2a and 2b) measured by the interference light measurement device 300 varies depending on the difference in polarization angle between the main beam and the reflected stray light. For example, when the difference in polarization angle is 45 degrees, the differential height Δh can be calculated using the following formula: Δh = (λ / 4π) sin －１ ((Ip-Is) / (Ip+Is)) Ip=(T ２ / 2) sin(1-φdef) Is=R ２ +RT・cos(φm)+RT・sin(φm+φdef)+(T ２ / 2) sin(1+φdef) where R and T are the amplitude reflectance and transmittance of the objective lens 324, φdef is the phase difference between the light rays l1 and l2, and φm is the phase difference between the main beam and the reflected stray light.

[0052] In this case, the term [RT·cos(φm)+RT·sin(φm+φdef)] included in the equation expressing Is is the part where the intensity fluctuation of the interference light occurs due to interference. This term changes due to minute up and down movements or vibrations of the sample surface, and becomes noise in the measurement of the interference light intensity and differential height Δh. Since the coherence length of a normal single-mode laser is long, this term becomes noise almost as it is.

[0053] In contrast, in this embodiment, 2λo ２ Since the condition of / (πOPL2)<Δλ is satisfied, the influence of the above term can be reduced by, for example, 1 / e ２ It can be reduced to the following:

[0054] (3) The light source 311 is a laser light source. The illumination optical system 310 is configured to shape the illumination light into a linear beam and irradiate it onto the sample surface. By narrowing the illumination width in this way, the sample surface can be measured with high resolution and the entire sample surface can be inspected at high speed. To narrow the illumination width in an inspection device like this, it is desirable to use a stable and highly monochromatic laser light source. However, compared to the use of inspection light with a wide spectral width, such as LED light, laser light generally has a long coherence length, making it difficult to satisfy the above condition OPL1<ΔL<OPL2. Thus, providing a relay lens optical system 350 is highly significant in realizing an optical system that satisfies the above condition.

[0055] (4) Furthermore, when broadband light such as that from an LED is used, the accuracy of the measurement decreases in DIC, which uses wavelength to calculate the differential height Δh as described above. In contrast, by using a laser light source as the light source 311 and using narrow-band laser light as the illumination light, the accuracy of measurement of the sample surface by interference measurement can be improved.

[0056] Second Embodiment In the second embodiment, a case where a Michelson interferometer is used as the interference light measurement device 300 will be described as an example.

[0057] 6 is a schematic diagram showing an example of the configuration of an interference light measurement device provided in an inspection device according to a second embodiment of the present invention. In FIG. 6, elements that are the same as or correspond to those in the first embodiment are given the same reference numerals as those in the previously mentioned drawings, and descriptions thereof will be omitted as appropriate.

[0058] The interference light measurement device 300 of this embodiment is equipped with an objective lens optical system 331 including an objective lens 324. The objective lens optical system 331 includes a beam splitter 333 and a reference surface (reflection mirror) 334. The objective lens optical system 331 is driven by a drive device 332 and is displaced, for example, in a direction toward or away from the sample 1. The relay lens optical system 350 of this embodiment is interposed between the beam splitter 333 and the objective lens 324 in the objective lens optical system 331. It should be noted that the interference light measurement device 300 of this embodiment does not include a Nomarski prism 323 ( FIG. 2 ).

[0059] In this embodiment, the light source 311 is, for example, a white light source. Light emitted from the light source 311 passes through a half mirror 321 and enters an objective lens optical system 331, where it is split into two beams by a beam splitter 333. One of the split beams is directed to a predetermined irradiation area on the sample surface, and the other beam is directed to a reference surface 334. In this embodiment, the other beam is directed to the reference surface 334, but it may be directed to the sample surface separately from the first beam. The beams reflected by the sample surface and the reference surface 334 are combined by the beam splitter 333 to generate interference light. This interference image of the sample surface and the reference surface 334 is relayed to a conjugate plane 390 by a relay lens optical system 350, further focused by an objective lens 324 of the imaging optical system 370, and imaged on an interference light detector 327 by an imaging lens 325.

[0060] In this embodiment, the interference light detector 327 is a two-dimensional sensor. The interference light intensity of the reflected light from the sample 1 and the reflected light from the reference surface 334 is measured by the interference light detector 327 while the objective lens optical system 331 is displaced by the driving device 332. The surface shape of the sample 1 is calculated by the signal processing device based on the interference light intensity distribution measured by the interference light detector 327.

[0061] In this embodiment, two-dimensional height information of the surface of the sample 1 within the field of view can be obtained at once. As a method for scanning the entire surface of the sample, similar to the first embodiment, scanning combining rotation and linear movement (FIG. 3) or scanning combining linear movement in two directions can be applied.

[0062] Other configurations of the inspection device of this embodiment are the same as those of the inspection device of Embodiment 1. Although the scattered light measurement device 200 is not shown in Figure 6, the inspection device of this embodiment can also be equipped with the scattered light measurement device 200 in combination with the interference light measurement device 300.

[0063] As in the first embodiment, the inspection apparatus of this embodiment also includes a relay lens optical system 350 that includes a lens with a relatively long focal length and relays the sample surface to a conjugate plane 390, and an imaging optical system 370 that includes a lens with a relatively short focal length and magnifies and images the conjugate plane 390 onto the interference light detector 327. In this embodiment, the difference in optical path length between a light ray reflected by the reference surface 334 and a light ray reflected by the sample surface corresponds to the optical path length difference OPL1. Furthermore, the difference between the optical path length from the light source 311, for example, reflected by the objective lens 324 and reaching the interference light detector 327 without passing through the relay lens optical system 350, and the optical path length from the light source 311 and reflecting from the sample surface and reaching the interference light detector 327 corresponds to the optical path length difference PL2. As in the first embodiment, this embodiment is also configured so that the coherence length ΔL of the illumination light emitted by the light source 311 satisfies the condition OPL1<ΔL<OPL2. Furthermore, the spectral width Δλ of the illumination light emitted by the light source 311 is 2λo ２ The condition / (πOPL2)<Δλ is satisfied.

[0064] Therefore, in this embodiment as well, the use of a light source makes it possible to suppress stray light caused by interference noise.

[0065] 7 is a diagram illustrating the principle of an inspection apparatus according to a third embodiment of the present invention. In FIG. 7, the horizontal axis represents time, and the vertical axis represents the intensity of light detected by the interference light detector 327.

[0066] The inspection device of this embodiment differs from the inspection devices of the first and second embodiments in that a light source that emits a pulsed laser as illumination light is used as the light source 311. The inspection device of this embodiment is configured so that the condition OPL1<cΔt<OPL2 holds true, where OPL1 and OPL2 are optical path length differences, Δt is the pulse duration of the pulsed laser emitted by the light source 311, and c is the speed of light.

[0067] In other respects, this embodiment is similar to the first or second embodiment.

[0068] Most of the pulsed laser emitted from the light source 311 is reflected by the sample surface or the reference surface and reaches the interference light detector 327 via the relay lens optical system 350 (main beam L0 of interference light that is the object of measurement). However, some of the pulsed light of the pulsed laser emitted from the light source 311 is reflected by a predetermined optical element (e.g., the objective lens 324) and may reach the interference light detector 327 as reflected stray light Lx without passing through the relay lens optical system 350.

[0069] In this case, if the optical path length difference between the reflected stray light Lx and the main beam L0 is OPL2 and the speed of light is c, as shown in Figure 7, the main beam L0 arrives at the interference light detector 327 a time OPL2 / c later than the reflected stray light Lx. Therefore, if the pulse duration Δt of the pulsed laser serving as illumination light is shorter than OPL2 / c, the reflected stray light Lx and the main beam L0 will be incoherent. For example, if the relay lens optical system 350 ensures an optical path length difference OPL2 of about 30 cm, interference noise can be suppressed by using a pulsed laser with a pulse width of less than 1 ns.

[0070] From the above, the condition for making the reflected stray light Lx and the main beam L0 incoherent when a pulsed laser light source is used as the light source 311 can be defined as OPL1<cΔt<OPL2. As described above, the inspection device of this embodiment is configured to satisfy this condition, and therefore can obtain the same effects as those of the first and second embodiments.

[0071] 8 and 9 are diagrams illustrating the principle of an inspection device according to a fourth embodiment of the present invention. In Fig. 8, the horizontal axis represents wavelength, and the vertical axis represents light intensity. In Fig. 9, the horizontal axis represents optical path length difference, and the vertical axis represents visibility.

[0072] The inspection apparatus of this embodiment differs from the inspection apparatuses of the above-described embodiments in that a light source that emits a longitudinal multimode laser as illumination light is used as the light source 311. The inspection apparatus of this embodiment is configured so that the optical path length difference OPL2 between the main beam of interference light and the reflected stray light satisfies the condition described below for the visibility V determined from the spectrum of the longitudinal multimode laser.

[0073] In other respects, the inspection device of this embodiment is similar to the inspection devices of the above-described embodiments.

[0074] 8, the spectrum of a longitudinal multimode laser includes N narrow spectra of wavelengths λ1, ... λi, ... λN (i = 1, 2, ... N) spaced at regular intervals. Each narrow spectrum is called a longitudinal mode, and the longitudinal mode spacing δλ, which is the spacing (wavelength difference) between adjacent longitudinal modes, is determined by the center wavelength λo of the longitudinal multimode laser and the resonator length of the light source 311. When a longitudinal multimode laser is used as the light source 311, its visibility V is determined mainly by the envelope shape of the light intensity of the longitudinal modes and the longitudinal mode spacing δλ.

[0075] If the central wavelength of the longitudinal multimode laser is λo and the width (spectral width) of the envelope of the light intensity of each longitudinal mode is Δλ, the peak of the visibility V of the longitudinal multimode laser has a width dL according to the central wavelength λo and the spectral width Δλ, as shown in Fig. 9. Then, the peak having this wavelength width dL is repeated at a period P according to the central wavelength λo and the longitudinal mode spacing δλ.

[0076] When the spectral shape of each longitudinal mode and the envelope of the light intensity of each longitudinal mode are Gaussian, the spread width dL of the visibility V and the repetition period P can be calculated as follows using the center wavelength λo, the spectral width Δλ, and the longitudinal mode spacing δλ: dL=2λo ２ / (πΔλ) P=λo ２ / δλ The peaks of visibility V shown in FIG. 9 are distributed with a period P between the coherence lengths corresponding to the spectral widths of the respective longitudinal modes.

[0077] From the above, by designing the optical system of the interference light measurement device 300 so that the optical path length difference OPL2 between the main beam of the interference light and the reflected stray light does not fall within the range from an integer multiple of the period P to dL, it is possible to make the main beam and the reflected stray light incoherent. For example, if the resonator length Lr=10 [cm], the center wavelength λo=532 [nm], and the spectral width Δλ=0.03 [nm], then the longitudinal mode spacing δλ=(λo ２ / 2cL) ~ 1.4e －３In this case, the wavelength width dL is about 6 mm and the period P is about 20 cm.

[0078] By selecting the light source 311 according to the optical path length difference OPL2 ensured by the relay lens optical system 350, or by setting the focal length and the like of the relay lens optical system 350 according to the parameters of the light source 311 so that the inspection device satisfies the above conditions, it is possible to suppress interference noise in this embodiment as in the previously described embodiments.

[0079] (Modifications) The present invention is not limited to the above-described embodiments and may include various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and are not necessarily limited to those including all of the described configurations. It is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is also possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

[0080] 1... sample, 2a... beam spot (irradiation area), 2b... beam spot (irradiation area), 310... illumination optical system, 311... light source, 320... interference light detection system, 323... Nomarski prism (spectroscope), 324... objective lens (optical element, lens having a first focal length), 327... interference light detector (detector), 333... beam splitter (spectroscope), 334... reference surface, 350... relay lens optical system, 351... objective lens (second a lens having a second focal length), 352...imaging lens (lens having a second focal length), 370...imaging optical system, 390...conjugate plane, c...speed of light, I1...light ray, I2...light ray, OPL1...optical path length difference, OPL2...optical path length difference, δ...shear amount (predetermined distance), δλ...longitudinal mode spacing of longitudinal multimode laser, ΔL...coherence length, Δt...pulse duration of pulse laser, Δλ...spectral width of illumination light, λo...center wavelength of longitudinal multimode laser

Claims

1. An inspection device comprising: an illumination optical system having a light source that emits illumination light to illuminate a sample surface; and an interference light detection system that photoelectrically converts interference light including reflected light generated at the sample surface using a detector, said interference light detection system comprising: a relay lens optical system that forms a conjugate plane that is optically conjugate with said sample surface; and an imaging optical system that images said conjugate plane on said detector, wherein the lights that generate the interference light are coherent with each other, and wherein reflected stray light that is reflected from the light source by an optical element of the imaging optical system without passing through the relay lens optical system and enters said detector, and reflected light generated at the sample surface are configured to be incoherent.

2. An inspection device according to claim 1, wherein the interference light detection system is provided with a spectroscopic element which separates the illumination light into two beams, and is configured such that the condition OPL1<ΔL<OPL2 is satisfied with respect to the optical path length difference OPL1 of the two beams, the optical path length difference OPL2 being the difference between the optical path length of the interference light from the light source until it is reflected by the sample surface and reaches the detector and the optical path length of the reflected stray light from the light source until it is reflected by the optical element and reaches the detector, and the coherence length ΔL of the illumination light emitted by the light source.

3. In the inspection device of claim 1, the interference light detection system is provided with a spectroscopic element that separates the illumination light into two beams, and an optical path length difference OPL1 of the two beams, an optical path length difference OPL2 being the difference between the optical path length of the interference light from the light source until it is reflected by the sample surface and reaches the detector, and the optical path length of the reflected stray light from the light source until it is reflected by the optical element and reaches the detector, and a spectral width Δλ of the illumination light emitted by the light source, ２ 2. An inspection apparatus characterized in that the inspection apparatus is configured so that the condition: / (πOPL2)<Δλ is satisfied.

4. An inspection apparatus according to claim 1, wherein said illumination optical system shapes said illumination light into a linear form and irradiates said sample surface.

5. The inspection device according to claim 1, wherein the detector is a line sensor.

6. The inspection apparatus according to claim 1, wherein the illumination light is scanned to inspect the entire surface of the sample.

7. An inspection device according to claim 1, characterized in that the relay lens optical system includes a lens having a first focal length, and the imaging optical system includes a lens having a second focal length shorter than the first focal length, and magnifies the conjugate plane to form an image on the detector.

8. The inspection apparatus according to claim 7, wherein the optical element is a lens having the second focal length.

9. An inspection device according to claim 1, characterized in that the interference light detection system separates the illumination light into two beams having different polarization directions, irradiates these onto two illumination areas on the sample surface that are a predetermined distance apart, and collects and combines the reflected light from the two beams to generate the interference light.

10. An inspection apparatus according to claim 1, characterized in that the interference light detection system separates the illumination light into two beams, irradiates one of the two beams onto a specified irradiation area on the sample surface while irradiating the other of the two beams onto the sample surface or a reference surface, and combines the reflected light of the two beams to generate the interference light.

11. The inspection device according to claim 1, wherein the light source is a laser light source.

12. The inspection device according to claim 11, wherein the laser light source emits a pulsed laser.

13. An inspection apparatus according to claim 12, wherein the interference light detection system is provided with a spectroscopic element which separates the illumination light into two beams, and is configured such that the following condition is satisfied with respect to the optical path length difference OPL1 of the two beams, the optical path length difference OPL2 being the difference between the optical path length of the interference light from the light source until it is reflected by the sample surface and reaches the detector and the optical path length of the reflected stray light from the light source until it is reflected by the optical element and reaches the detector, the pulse duration Δt of the pulsed laser, and the speed of light c: OPL1<cΔt<OPL2.

14. The inspection device according to claim 11, wherein the laser light source emits a longitudinal multimode laser.

15. In the inspection device according to claim 14, an optical path length difference between the optical path length of the interference light from the light source to be reflected by the sample surface and reach the detector and the optical path length of the reflected stray light from the light source to be reflected by the optical element and reach the detector is OPL2, the central wave of the longitudinal multimode laser is λo, the spectral width of the light intensity envelope of each longitudinal mode of the longitudinal multimode laser is Δλ, and the longitudinal mode interval of the longitudinal multimode laser is δλ, and OPL2 is (λo ２ / δλ) to 2λo ２ 2. An inspection apparatus according to claim 1, wherein the detection signal is outside the range of πΔλ.

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