JP2002090313A - Pattern defect inspection equipment - Google Patents

Abstract

(57) [Summary] (with correction) [PROBLEMS] To provide an apparatus for detecting a fine circuit pattern with high resolution by solving the problem of coherence reduction in illumination using an ultraviolet laser light source. A laser light source (3) is installed separately from an optical system, an ultraviolet laser beam (L1) is multi-spotted by a coherence reduction device (6) provided in an optical path to oscillate and illuminate a semiconductor wafer (1), and a laser speckle is obtained. The occurrence of is suppressed. Invisible UV laser light on screen 2
5. Visualize at 5, monitor the illumination condition on the pupil of the objective lens 11, and correct the illumination system so that the optimal illumination condition is obtained.
At the same time, the illumination angle and the illumination area are made variable so that the semiconductor wafer 1
The above pattern was made efficient illumination and detectable.

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 is wavelength-converted by a nonlinear optical crystal, an Ar-Kr laser device, and the like, and a laser having a wavelength of 266 nm to 355 nm can be obtained. These laser devices are advantageous in that they have a higher output than lamps that have been used as light sources in the past. However, the size of the devices has been increased, or the third or fourth fundamental wave using a ring-shaped resonator has been used. It generates harmonics, and the inside of the resonator has a rather 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.

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

[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, 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 pupil of the objective lens. Means for measuring and averaging the reflected light intensity from the surface of the object to be inspected 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 is used as a light source to perform high-luminance illumination in a DUV region. 2 is X, Y, Z, θ
A stage having a degree of freedom in direction, on which a semiconductor wafer 1 as an example of a pattern to be inspected is mounted as a sample.
The laser L1 emitted from the laser light source 3 is incident on the objective lens 11 via the mirror 4, the beam expander 5, the coherence reducing optical system 6, the lens 7, the polarizing beam splitter 9, and the polarizing element group 10, and the laser L1 Irradiation is performed on the semiconductor wafer 1 as an example. The beam expander 5 expands the laser to a certain size. The expanded laser L1 is condensed by the lens 7 near the pupil 11a of the objective lens 11, and is then Koehler-illuminated onto the sample. The reflected light from the sample is applied to the objective lens 11, the polarizing element group 10, the polarizing beam splitter 9,
It is detected by the image sensor 13 via the imaging lens 12. The polarization beam splitter 9 has a function of reflecting when the polarization direction of the laser is parallel to the reflection surface, and transmitting when the polarization direction is perpendicular to the reflection surface.

The laser used as the light source is originally a polarized laser, and the polarized beam splitter 9 is installed so that the laser 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 polarizing element group 10 includes:
The polarization direction of the laser illumination light and the reflected light is controlled, and the reflected light is transmitted to the image sensor 13 by the pattern shape and the density difference.
It has a function of adjusting so as not to reach uneven brightness, and is constituted by, for example, 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 the reflected light from the semiconductor wafer 1, which is an example of the pattern to be inspected. 14
Denotes an A / D converter, which converts 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 an 8-bit gradation converter, for example, which performs gradation conversion on the digital image signal output from the A / D converter 14 as described in JP-A-8-320294. It is something to give. That is, the gradation converter 1
Numeral 5 is for performing logarithmic, exponential, polynomial conversion and the like to correct uneven brightness of an image generated by interference between a thin film formed on the semiconductor wafer 1 in the process and the laser. 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 mismatched 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 capable of easily securing a short wavelength is used as a light source.

Recently, a solid Y light source has been used as an ultraviolet laser light source.
The third harmonic (355 nm) of the fundamental wave or the fourth harmonic (266 n) of the fundamental wave is obtained by wavelength-converting the AG laser using a nonlinear optical crystal or the like.
There are devices that generate m), and it is conceivable to use these devices. 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, in the present invention, as shown in FIG. 10, the laser light source 3 is installed separately from the optical system 85, and the mechanical vibration generated by the stage and the like is transmitted to the laser device, and the laser light is transmitted from the laser device. The heat conduction to the 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 L1 emitted from the laser light source 3 is turned in the Z direction by the mirror 4, and reaches the optical system 85 via the mirror 90 and the beam expander 5.

In the pattern inspection of the surface of the semiconductor wafer 1, the stage 2 on which the wafer 1 is mounted is scanned in the X and Y directions to inspect the entire surface. During the inspection, the center of gravity of the stage changes as the stage moves. The board tilts. 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 L1 are temporarily out of the optical axis. Therefore, in the present invention,
Mirror 90, lens 91, position detector 92 on surface plate 80
Is installed, whereby the amount of movement of the laser L1 is detected, the mirror 4 is moved using an actuator such as a piezo, and the optical path of the off-axis laser L1 is corrected at high speed. Here, the mirror 90 is coated with a reflective film so as to reflect slight light of the laser L1,
The lens 91 enlarges and projects the reflected light onto the position detector 92.
The position detector 92 has, for example, a light receiving element divided in the XZ direction, and detects a moving amount of the laser by calculating a detection signal of the light receiving element by an electric circuit (not shown). This allows the laser to stably enter the optical system 80.

The laser 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, it is necessary to reduce coherence. In order to reduce the coherence, it is only necessary to reduce either the temporal or spatial coherence. In the present invention, the laser beam is reduced by two scanning mirrors 61 and 64 orthogonal to each other as shown in FIG.
Scanning is performed dimensionally to reduce spatial coherence.

FIG. 3 is a schematic diagram of an illumination system. The laser beam L1 emitted from the laser light source 3 and expanded to a certain size by the beam expander 5 becomes a parallel light beam, is reflected by the mirror 61, is condensed by the lens 62, and then becomes a parallel light beam again by the lens 63. 7, the pupil 1 of the objective lens
The light is focused on 1a. 41 and 43 are scanning mirrors 61 and 6
4 shows the reflection position of the laser beam, which has 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 an electric signal, whereby the laser 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, and the objective lens 11 is changed by changing the frequency and amplitude of the input electric signal.
Scanning of various shapes on the pupil plane 11a 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 an ultraviolet laser was installed. Since the ultraviolet laser is invisible light, fluorescent light is generated on a screen 25, which is magnified by a lens 26 and can be observed by a TV camera 27.

FIG. 4 is a schematic view showing an example of an illumination pattern on the light receiving surface when the illumination laser applied to the screen 25 is imaged by the TV camera 27.
According to this, although the laser illumination pattern is displayed in black, the laser illumination part is actually displayed bright. 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.

The scanning of the illumination light by the scanning mirror (1)
Needless to say, the operation is performed within the accumulation time of the image sensor.

In inspection, illumination for accurately detecting a pattern image is very important. For this reason, the inventor makes the laser illumination on the pupil plane of the objective lens 11 a multi-point (multi-spot), makes it possible to illuminate the sample at any angle on the sample, and captures an image from the pattern clearly. did. According to this, there is an advantage that the illumination σ can be increased and the illumination can be efficiently performed.
FIG. 5 is a three-dimensional view in which a multi-lens array is arranged in the coherence reduction optical system 6, and FIG. 6A is a diagram showing a schematic configuration of an illumination optical system using this. 2 and 3 is that a plurality of light sources of the laser L1 are created by the multi-lens array and the lens 66 newly added in the illumination optical path, and consequently, a plurality of light sources are formed on the pupil plane 11a of the objective lens 11. A plurality of laser focus points as shown in FIG. 6 (b) are formed.

As means for producing a plurality of light sources, for example, a cylindrical lens array 71 shown in FIG.
Are arranged perpendicularly to each other (FIG. 6 (b)), or a lens array 73 in which small convex lenses are two-dimensionally arranged is arranged in the optical path. FIG. 4B shows a scanning state on the pupil of the objective lens 11. Pitches 110a and 110 of laser converging points on pupil plane 11a of objective lens 11
b can be freely changed by selecting the focal length of the lens 66 and the other lenses.

Here, the laser used as the light source has linearly polarized light. Since the resolution of the optical system changes according to 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), and make each rotatable configuration,
The performance of the optical system is improved 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. That is, from the polarization beam splitter 9 to the image sensor 1
The aerial image of the pupil plane of the objective lens 11 is detected by a mirror 28, a lens 29, and a detector 30 provided in the optical path to 3. FIG. 8 shows the light receiving surface 40 of the detector 30.
FIG. 3 is a schematic diagram showing a state where spatial images 42 to 44 of the pupil 81 of the objective lens 11 are projected as bright images. 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 0.
The following 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 minute patterns are densely arranged, such as a memory cell portion, but has a small intensity due to a small regular reflection component. 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 region (n × n) on the light receiving surface of the detector 30
Pixels: n is an integer) Paying attention to P1 to P4, the average brightness of each area is calculated by the image processing apparatus 100, and the polarizing element is set so that the 0th-order and 1st-order reflected lights fall within the dynamic range of the image sensor 13. 10 by the signal from the control circuit
The holder 55 is driven by the motor 53 and the transmission means 50, and the polarization element 10 held by the holder 55 is set to an experimentally obtained rotation angle.

The motor 53 is, for example, a pulse motor, and the origin of the polarizing element 10 can be detected by the sensor 102. The origin is a concave portion provided on the end face of the holder 55. This work, for example, using design data
By measuring the reflected light from the circuit pattern formed on the inspection target wafer 1 in advance and controlling the polarization, the intensity of the reflected light from the pattern reaching the image sensor 13 can be averaged, and the stable defect detection sensitivity can be improved. The effect of being able to obtain is obtained.

[0027]

As described above, according to the present invention, the laser light source is installed separately from the optical system, and feedback is performed so that the laser 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 the laser, and it can realize stable pattern defect inspection by detecting the reflected light from the pattern and controlling the polarization. The effect is obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a schematic configuration of a defect tube inspection apparatus for a pattern to be inspected according to the present invention.

FIG. 2 is a perspective view of an optical system for explaining an example of an optical system for reducing spatial coherence of laser illumination.

FIG. 3 is a front view showing a schematic configuration of an optical system for reducing spatial coherence of laser illumination.

FIG. 4 is a schematic diagram of an illumination pattern on a light receiving surface when a laser beam applied to a screen is imaged by a TV camera.

FIG. 5 is a perspective view of an optical system for explaining an example of an optical system for reducing spatial coherence of laser illumination using a multi-spot.

6A is a front view showing a schematic configuration of a laser illumination optical system using a multi-spot, and FIG. 6B is a pupil plane showing a state of a plurality of condensing points formed on a pupil plane of an objective lens. It is a schematic diagram.

FIG. 7 shows an optical element (a) for forming a multi-spot.
It is each a perspective view of (b) and (c).

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

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

FIG. 10 is a side view showing a schematic configuration of a pattern defect inspection apparatus 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 ... Polarizing beam splitter 10 ... Polarizing element 11 ... Objective lens 13 ... Image sensor 23 ... Signal processing circuit 11
0 ... Multi filter

 ──────────────────────────────────────────────────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 Laboratories F-term (reference) DB12 DB13 DB14 DB19 DJ04 DJ06 DJ11 DJ15 DJ18 DJ21

Claims (3)

[Claims] 1. A pattern defect inspection apparatus for illuminating a semiconductor circuit pattern formed on a substrate with ultraviolet light and detecting a defect in the circuit pattern, comprising: a laser light source; and a laser emitted from the laser light source. Means for forming multiple light emitting points; coherence reducing means for reducing the coherence of the laser; and condensing means for condensing the laser passing through the coherence reducing means at the pupil position of the objective lens. Scanning the multi-emission laser formed on the pupil of the objective lens by the light condensing means with the coherence reducing means to reduce the coherence of the laser, and A pattern defect inspection apparatus characterized in that illumination light is configured to be uniform. 2. The pattern according to claim 1, wherein said multiple light emitting point forming means is provided with a plurality of transmitting portions and light shielding portions to reduce illumination noise due to stray light on the sample. Defect inspection equipment. 3. The objective lens pupil, wherein a projection image of the multiple emission points of the laser by the multiple emission point forming means is provided to the objective lens pupil by a variable magnification optical means arranged in the optical path from the multiple emission point formation means to the objective lens. 2. The pattern defect inspection apparatus according to claim 1, wherein the magnification is variable at the position.