JP2001176942A - Pattern defect inspector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device which cancels problems such as oscillation failure, etc., due to heat generation or oscillations of an ultraviolet laser light source and detects a fine circuit pattern with high resolution. SOLUTION: An ultraviolet laser device is installed separately from an optical system, and an observing means and a correcting means for visualizing variations of laser beams are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-resolution optical system used for inspection and observation of fine pattern defects and foreign matters typified by a semiconductor device manufacturing process and a flat panel display manufacturing process, and a defect inspection apparatus using the same. About.

[0002]

2. Description of the Related Art As semiconductors become more highly integrated, circuit patterns tend to be finer. Among them, masks and reticles used when manufacturing semiconductor elements in a photolithographic process,
Pattern defects on a wafer to which a circuit pattern formed on these is transferred by exposure are required to be detected with increasingly higher resolution. As a technique for increasing the resolution, there is a method of shortening the wavelength of the illumination light from visible light to ultraviolet light. Conventionally, a mercury lamp has been used as a light source, and only a required wavelength is optically selected from various emission lines of the mercury lamp.

[0003]

However, the emission line of the mercury lamp has a wide emission spectrum width and it is difficult to correct chromatic aberration of the optical system. In order to obtain sufficient illuminance, there is a problem that the light source becomes large and the efficiency is low.
In recent years, an exposure apparatus equipped with a KrF excimer laser apparatus having a wavelength of 248 nm has been developed as a light source for an exposure apparatus in semiconductor manufacturing. However, since the excimer laser light source is large and uses a fluorine gas, a predetermined light source is used. There are problems such as the need for safety measures. As an ultraviolet laser light source,
For example, there are a laser device in which a solid YAG laser beam is wavelength-converted by a nonlinear optical crystal, an Ar-Kr laser device, and the like, and a laser beam with a wavelength of 266 nm to 355 nm can be obtained. Conventionally, these laser devices have an advantage in that the output is large as compared with a lamp that has been used as a light source.
This is to increase the size of the device or to generate the third or fourth harmonic of the fundamental wave using a ring-shaped resonator, and the inside of the resonator has a fairly complicated structure. For this reason, mounting on a pattern inspection or measurement device cannot be installed on the same base as the optical system, unlike a conventional lamp, in consideration of the effects of heat generation and vibration of the mechanical part. Further, when used as an illumination light source, ultraviolet light is invisible, so there is a problem that it is difficult to handle such as optical axis adjustment.

It is an object of the present invention to provide a pattern defect inspection apparatus which solves the above problems, realizes stable and efficient illumination using ultraviolet laser light as a light source, and can detect a fine pattern of a semiconductor element or the like with high resolution. Is to do.

[0005]

In order to achieve the above object, a laser light source is provided separately from an optical system, and the fluctuation of the laser beam due to the laser light source is constantly monitored.
Means for correcting the amount of fluctuation are provided, and the occurrence of laser speckle is suppressed by a coherence reducing means provided in the optical path. Further, the observation means installed by branching the optical path visualizes the invisible ultraviolet laser light, monitors the illumination state on the pupil of the objective lens, corrects the illumination system so that the optimal illumination condition is obtained, and further corrects the objective lens. Means for measuring and averaging the intensity of light reflected from the surface of the inspection object on the pupil are provided so that a fine pattern such as a semiconductor element can be detected with high resolution.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a diagram showing an example of an apparatus according to the present invention. In the present invention, a DUV laser beam is used as a light source to perform high-luminance illumination in a DUV region. 2 is X, Y, Z,
A stage having a degree of freedom in the θ direction, on which a semiconductor wafer 1 as an example of a pattern to be inspected is mounted as a sample. The laser light L1 emitted from the laser light source 3 is applied to a mirror 4, a beam expander 5, a coherence reduction optical system 6, a lens 7, a polarizing beam splitter 9, and a polarizing element group 1.
The laser beam is incident on the objective lens 11 via the reference numeral 0 and is irradiated on the semiconductor wafer 1 which is an example of the pattern to be inspected. The beam expander 5 expands the laser light to a certain size. After the expanded laser light L1 is condensed by the lens 7 near the pupil 11a of the objective lens 11, it is Koehler-illuminated onto the sample. . The reflected light from the sample passes through the objective lens 11, the polarizing element group 10, the polarizing beam splitter 9, and the image forming lens 12 from above the sample, and passes through the image sensor 1.
3 is detected. The polarization beam splitter 9 has a function of reflecting when the polarization direction of the laser beam is parallel to the reflection surface and transmitting the laser beam when the polarization direction is perpendicular. The laser light used as the light source is originally a polarized laser, and the polarized beam splitter 9 is installed so that the laser light is totally reflected. on the other hand,
The pattern to be inspected formed on the wafer 1 by the semiconductor process has various shapes. Therefore, light reflected from the pattern has various polarization components. The polarization element group 10 has a function of controlling the polarization directions of the laser illumination light and the reflected light so as to adjust the reflected light so as not to reach the image sensor 13 with uneven brightness due to the pattern shape and density difference. For example, it is constituted by a wave plate for changing the phase of the illumination wavelength by 45 degrees or 90 degrees. The image sensor 13 outputs a grayscale image signal corresponding to the brightness (shade) of light reflected from the semiconductor wafer 1, which is an example of the pattern to be inspected. 14 is A /
It is a D converter for converting a grayscale image signal 13a obtained from the image sensor 13 into a digital signal. That is, while scanning the stage 2 to move the semiconductor wafer 1 as an example of the pattern to be inspected at a constant speed, the focus detection system (not shown) always detects the position of the surface of the semiconductor wafer 1 to be inspected in the Z direction. The stage 2 is controlled in the Z direction so that the distance from the objective lens 11 is constant, and the brightness information (shade image signal) of the pattern to be inspected formed on the semiconductor wafer by the image sensor 13 is accurately detected. To detect. Reference numeral 15 denotes, for example, an 8-bit gradation converter.
The digital image signal output from the D converter 14 is subjected to gradation conversion as described in JP-A-8-320294. That is, the gradation converter 15 performs logarithmic, exponential, polynomial conversion, and the like, and corrects uneven brightness of an image generated by interference between the thin film formed on the semiconductor wafer 1 in the process and the laser beam. . Reference numeral 16 denotes a delay memory which stores and delays an output image signal from the gradation converter 15 for one cell or one chip or one shot constituting the semiconductor wafer 1 by using the scanning width of the image sensor 13. is there.

Reference numeral 17 denotes a comparator which compares the image signal output from the gradation converter 15 with the image signal obtained from the delay memory 16 and detects a mismatch portion as a defect.

The comparator 17 compares an image delayed by an amount corresponding to a cell pitch or the like output from the delay memory 16 with a detected image, and an arrangement on the semiconductor wafer 1 obtained based on design information. Input means 18 composed of a keyboard, a disk, etc.
The CPU 19 creates defect inspection data based on the coordinates of the input arrangement data and the like on the semiconductor wafer 1 based on the comparison result by the comparator 17 and stores the defect inspection data in the storage device 20. This defect inspection data is
The information can be displayed on the display means 21 such as a display as needed, and can be output to the output means 22 so that the defect portion can be observed by another reviewing device, for example.

The details of the comparator 17 are disclosed in
For example, an image alignment circuit, a difference image detection circuit for an aligned image, a mismatch detection circuit for binarizing a difference image, and the area or the area from the binarized output may be used. It is composed of a feature extraction circuit that extracts length, coordinates, and the like.

Next, the light source will be described. In order to obtain high resolution, it is necessary to shorten the wavelength, and to improve the inspection speed, high-luminance illumination is required. As a conventional illumination light source,
For example, by using a discharge lamp of mercury xenon or the like, the amount of illumination is secured by using a wide range of the visible region of the emission spectrum (bright line) of the lamp. However, the emission spectrum of the lamp in the ultraviolet and deep ultraviolet regions is about several percent of that of visible light, and a large light source is required to secure desired luminance. When the size of the light source is increased, the problem is the influence of heat generation on the optical system. However, since the illumination light is guided from the light source by the lens system, there is a limit in moving the light source away from the optical system. From such a viewpoint, in the present invention, an ultraviolet laser beam capable of easily securing a short wavelength is used as a light source. As an ultraviolet laser light source, there has recently been a device that generates a third harmonic (355 nm) or a fourth harmonic (266 nm) of a fundamental wave by converting the wavelength of a solid YAG laser using a nonlinear optical crystal or the like. It is also conceivable to use. In order to generate harmonics, a resonator is provided inside the laser device. That is, the input fundamental wave is resonated by a mirror resonator called a cavity, and only a specific wavelength is output. Some mirrors in the resonator vibrate so as to be in stable resonance, and are electrically fed back. As an issue for applying these laser devices to a pattern defect inspection as a light source, it is necessary to consider cooling of the laser device and the influence of vibration on the laser device.

Therefore, as shown in FIG. 10, the present inventor installs the laser light source 3 separately from the optical system 85, propagates mechanical vibration generated by a stage or the like to the laser device, and transmits the mechanical vibration from the laser device. The heat conduction to the optical system was cut off. In this embodiment, a case is shown in which the laser light source is installed below the anti-vibration table 80. In this case, although not shown, local exhaust is required so that heat generated by the laser light source is not transmitted to the upper platen. The laser light L1 emitted from the laser light source 3 is turned in the Z direction by the mirror 4 and passes through the mirror 90 and the beam expander 5 to the optical system 85.
To reach. The pattern inspection of the surface of the semiconductor wafer 1 is performed by scanning the stage 2 on which the wafer 1 is mounted in the XY directions and inspecting the entire surface. During the inspection, the position of the center of gravity changes with the movement of the stage. I do. In this case, the surface plate is returned to a horizontal state by air servo or the like, but the laser diameter of the laser L1 emitted from the laser light source 3 is 1 mm.
In the following, it is expected that the optical axes of the optical system 85 and the laser beam L1 are temporarily out of the optical axis. For this reason, in the present invention, a mirror 90, a lens 91, and a position detector 92 are installed on the surface plate 80, thereby detecting the amount of movement of the laser light L1, and moving the mirror 4 using an actuator such as a piezo. The optical path of the off-axis laser beam L1 is corrected at high speed. Here, the mirror 90 is coated with a reflective film so as to reflect a small amount of the laser light L1, and the lens 91 enlarges and projects the reflected light to the position detector 92. The position detector 92 has, for example, a light receiving element divided and arranged in the XZ direction, and detects a moving amount of the laser light by calculating a detection signal of the light receiving element by an electric circuit (not shown). Thus, the laser light can be stably incident on the optical system 80.

The laser light L 1 guided to the optical system 80 enters the coherence reduction optical system 6. In general, a laser has coherence (has coherence), and when the wafer 1 is illuminated with the laser, it causes speckle noise to be generated from a circuit pattern. Therefore, in laser illumination,
Coherence needs to be reduced. In order to reduce the coherence, it is only necessary to reduce either the temporal or the spatial coherence. In the present invention, the laser beam is two-dimensionally converted by two scanning mirrors 61 and 64 which are perpendicular to each other as shown in FIG. To reduce spatial coherence. FIG. 3 is a schematic diagram of an illumination system. The laser light L1 emitted from the laser light source 3 and expanded to a certain size by the beam expander 5 becomes a parallel light flux, is reflected by the mirror 61, is condensed by the lens 62, and is condensed by the lens 62.
The light again becomes a parallel light flux and is condensed by the lens 7 on the pupil 11a of the objective lens. 41 and 43 are scanning mirrors 6
The positions of reflection of the laser light at 1 and 64 are shown, and have a conjugate positional relationship with the surface of the wafer 1. Reference numeral 42 denotes a first pupil conjugate plane conjugate with the pupil plane 11a of the objective lens 11. The scanning mirrors 61 and 63 are oscillating mirrors that rotate or lift in response to electric signals, whereby the laser light L1 is two-dimensionally scanned on the pupil plane 11a of the objective lens 11. The electric signal input to the scanning mirrors 61 and 64 is, for example, a triangular wave or a sine wave. By changing the frequency and amplitude of the input electric signal, various shapes of scanning on the pupil plane 11a of the objective lens 11 can be performed. It is possible. For this reason, in the present invention, as a second embodiment, a mirror 24 is arranged in the illumination optical path to split the illumination light amount that does not hinder the illumination of the wafer 1 and to be conjugate with the pupil plane 11a of the objective lens 11. A screen that emits fluorescence when irradiated with ultraviolet laser light was installed. Since the ultraviolet laser light is invisible light, fluorescent light is generated on the screen 25, and the fluorescent light is magnified by the lens 26 and can be observed by the TV camera 27. FIG. 4 is a schematic diagram showing an example of an illumination pattern on the light receiving surface when the TV camera 27 captures an image of the illumination laser light applied to the screen 25. According to this, although the laser illumination pattern is displayed in black, the laser illumination portion is actually displayed brightly, so that the pixels of the TV camera are added in the Y direction as shown in FIG. , It is also possible to obtain the shift amount ΔX of the illumination light with respect to the center Xo of the pupil diameter 34 of the objective lens 11, the shift amount is calculated by the signal processing circuit 81, and the coherence is determined by a command from the control circuit 82. The correction can be made by driving the scanning mirrors 61 to 64 provided in the reduction optical system 6. Further, the area of the illumination can be calculated by binarizing the image received by the TV camera 27 and adding pixels having a certain brightness or higher, and the illumination condition (illumination σ) can be set to an optimal value. It is possible.

Needless to say, the scanning of the illumination light by the scanning mirror is performed within the accumulation time of the image sensor.

As another embodiment of the illumination conditions, the illumination on the pupil plane of the objective lens 11 may be multi-spot. According to this, there is an advantage that the scanning time of the scanning mirror can be reduced because the illumination σ can be increased. FIG. 5 is a three-dimensional view in which a multi-lens array is arranged in the coherence reduction optical system 6, and FIG. 6 is a schematic view of an illumination system using the same. 2 and 3 is that a plurality of light sources of the laser light L1 are created by the multi-lens array and the lens 66 newly added in the illumination light path, and consequently the objective lens 1
A plurality of laser focus points are formed on one pupil plane 11a. As a means to create multiple light sources,
For example, two cylindrical lens arrays 71 shown in FIG. 7A are arranged orthogonally (FIG. 7B), or a lens array 7 in which small convex lenses are two-dimensionally arranged.
3 in the optical path. Objective lens 11
FIG. 4C shows a scanning state on the pupil. Objective lens 11
The pitch 110 of the laser focus on the pupil plane 11a can be freely changed by selecting the focal length of the lens 66 and the other lenses.

Here, the laser light used as the light source has linearly polarized light. Since the resolution of the optical system changes depending on the polarization state of illumination or detection, in the present invention, the polarizing elements 10a (for example, a half-wave plate) and 10b (for example, 1
/ 4 wavelength plate) to be rotatable, and by controlling and detecting the polarization state of the reflected light emitted from the circuit pattern formed on the wafer 1 by the semiconductor process, the performance of the optical system is improved. I try to improve. That is, the mirror 28 and the lens 2 provided in the optical path from the polarizing beam splitter 9 to the image sensor 13
9. The detector 30 detects an aerial image of the pupil plane of the objective lens 11.

FIG. 8 is a schematic diagram showing a state where the spatial images 42 to 44 of the pupil 81 of the objective lens 11 are projected as bright images on the light receiving surface 40 of the detector 30. Pixels 41 as light receiving elements are two-dimensionally arranged on the light receiving surface of the detector 30. 42 is a bright image of the 0th-order reflected light from the circuit pattern, and 43 and 44 are bright images of the primary reflected light, respectively. Among them, the largest reflected light amount is the 0th order, and is mainly reflected light from the wafer 1 surface. on the other hand,
The primary reflected light is light that is diffracted at the pattern edge and is often generated in a region where fine patterns are densely packed, such as a memory cell portion, but has a small intensity due to a small amount of regular reflection components. Accordingly, if the detection sensitivity of the image sensor 13 is adjusted to the 0-order reflected light, the primary reflected light is hardly detected. Therefore, a specific area (n × n pixels: n) on the light receiving surface of the detector 30
Focusing on P1 to P4, the average brightness of each area is calculated by the image processing apparatus 100, and the polarizing element 10 is set so that the 0th-order and 1st-order reflected lights fall within the dynamic range of the image sensor 13. The motor 5 is controlled by a signal from the control circuit.
3 and the transmission means 50 drive the holder 55 to
Is set to the rotation angle experimentally obtained. The motor 53 is, for example, a pulse motor, and the origin position of the polarizing element 10 can be detected by the sensor 102, and the concave portion provided on the end face of the holder 55 is used as the origin. In this operation, the reflected light from the pattern reaching the image sensor 13 is measured by measuring the reflected light from the circuit pattern formed in advance on the inspection target wafer 1 using design data or the like and controlling the polarization. The intensity can be averaged, and an effect of obtaining stable defect detection sensitivity can be obtained.

[0018]

As described above, according to the present invention, the laser light source is provided separately from the optical system, and feedback is performed so that the laser light path is always constant. And the effect of mechanical vibration on the laser device can be prevented. In addition, the coherence reduction means and multi-spot illumination can reduce the coherence characteristic of laser light, and realize stable pattern defect inspection by detecting reflected light from the pattern and controlling its polarization. It works.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a defect tube inspection apparatus for a pattern to be inspected according to the present invention.

FIG. 2 is a diagram illustrating an example of an optical system that reduces spatial coherence of laser illumination.

FIG. 3 is a schematic diagram of an optical system for reducing spatial coherence of laser illumination.

FIG. 4 is an explanatory diagram for detecting an illumination state on an objective lens pupil by laser illumination.

FIG. 5 is a diagram illustrating an example of an optical system that reduces spatial coherence of laser illumination using a multi-spot.

FIG. 6 is a schematic diagram of a laser illumination system using a multi-spot.

FIG. 7 is a diagram illustrating an example of an optical element that forms a multi-spot.

FIG. 8 is a diagram illustrating a state of reflected light from a pattern on an objective lens pupil.

FIG. 9 is a diagram showing an example of a means for controlling reflected light from a circuit pattern.

FIG. 10 is a side view of a defect inspection apparatus for a pattern to be inspected according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Wafer, 2 ... Stage, 3 ... Laser light source, 6 ... Coherence reduction optical system, 9 ... Polarization beam splitter, 10
... a polarizing element, 11 ... an objective lens, 13 ... an image sensor, 23 ... a signal processing circuit.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Toshihiko Nakata 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside the Hitachi, Ltd. Production Technology Research Institute (72) Inventor Shunji Maeda 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa F-term in Hitachi, Ltd. Production Technology Research Laboratory (Reference) CD03 DA08 EA11 EB03 4M106 AA01 AA09 AA20 BA05 BA07 CA39 DB01 DB04 DB08 DB14 DJ04 DJ05 DJ23

Claims (3)

[Claims] Illuminating a semiconductor circuit pattern with ultraviolet light;
In a pattern defect inspection apparatus for detecting a defect of the circuit pattern, a laser light source, a coherence reducing unit for reducing coherence of laser light emitted from the laser light source, and a laser beam passing through the coherence reduction unit Condensing means for converging light on the pupil position of the objective lens, and disposing the laser light condensed on the pupil of the objective lens by the condensing means in an optical path from the condensing means to the objective lens. A pattern defect inspection apparatus characterized in that laser light condensed on an objective lens pupil can be observed by said observation means. 2. The pattern defect inspection apparatus according to claim 1, wherein said observation means visualizes and observes the wavelength of invisible light with a visible light conversion means. 3. The pattern defect inspection apparatus according to claim 1, wherein said laser light source is provided separately from mounting means of said circuit pattern detection system.