JPH02307046A - Face plate defect detection system by heterodyne system and inspection device - Google Patents

Abstract

PURPOSE:To improve the detection performance by applying the heterodyne method for face plate defect detection. CONSTITUTION:A face plate 1 to be inspected is mounted on a turntable 1a and rotated by a spindle 1b. Laser light L emitted by a laser light source 2 is split into two by a beam splitter 6a and one beam is passed through a mirror 2a and converged by a projection lens 3 to form a spot on the face plate face. When there is a defect in the face plate 1, its scattered light S is converged by a light receiving lens 4 and inputted to a beam multiplexer 6b through a mirror 2c. Then, the other split laser beam is inputted to an acoustooptic modulator 10a and shifted in frequency with the shift frequency of an ultrasonic wave band supplied from a modulating wave generator 10b. This laser beam is inputted to the beam multiplexer 6b and multiplexed with the scattered light S, so that a light receiver 5 outputs a beam signal. Noises that the output signal of the light receiver 5 contains are removed by an electric filter 11 to select and extract only the beat signal, and a signal processing part 12 generates a defect signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a face plate defect detection method using a heterodyne method and an apparatus therefor.

[Prior Art] Semiconductor ICs are manufactured from wafer faceplates made of silicon or the like. If the surface of the faceplate has defects such as protrusions, scratches, or adhered dirt or dust, the performance of the IC will deteriorate, so surface inspection is performed at the substrate stage before wiring patterns are set on the faceplate.

FIG. 4 shows the basic configuration of the surface defect inspection device.

The face plate 1 to be inspected is placed on a turntable 1a and rotated by a spindle 1b. Laser L from laser light source 2
is projected by the projection lens 3 via the mirror 2a, and the surface is scanned by a spot formed on the surface of the face plate. In the case shown in the figure, it is a spiral scan in which the face plate or optical system moves in the radial direction indicated by arrow C as it rotates.
In addition to this, an XY scanning method is also used in which a laser is swept in the X direction and the face plate is moved in the Y direction for linear scanning.

When there is a defect on the surface of the face plate 1, the spot is scattered, and the scattered light S is focused by the light receiving lens 4 and input to the light receiver 5, and a defect signal is output. The above is the basic configuration,
In practical machines, the light receiving optical system has been improved to efficiently collect scattered light, remove stray light, and classify defects.

[Department Report to be Solved] In the above scattered light reception method, the detection performance is limited to the scattered light S
Therefore, in order to detect minute defects, in addition to improving the light-receiving optical system described above, it is effective to use a spot with high intensity. For this purpose, it is possible to use a laser light source with a large output, but
This naturally has its limits. In addition, since the intensity of the spot is inversely proportional to the cross-sectional area, it is effective to reduce the spot diameter, but on the other hand, this increases the scanning time, so there is still a limit, and the defect detection performance of current practical devices is limited to fine particles. It is about 0.1 μm or more when converted into . Recently, the line width of wiring patterns has become finer due to the higher density of ICs, and in response to this, there is a need to detect even finer defects, and the output of the laser light source mentioned above has been increased. There is a need for an inspection method and device that has better detection performance than the conventional one, without the need for a conventional spot diameter.

Now, a method and device for measuring fine particles such as dust in the air are described in ``Japanese Patent Application Laid-open No. 63-83944.

There is a method and device for measuring dust in the conductor manufacturing process. In a sense, fine particles in the air and surface defects on the face plate have something in common, and I will quote this article for reference and give an overview of it.

FIG. 5 shows a basic configuration diagram of one embodiment, in which a laser L (ν0) of frequency ν0 from a laser light source 2 is divided into two by a beam splitter 6a. One of them is the scattered light S of particles perpendicular to the air flow in the detection area of the detection cell 4.
(ν0) is scattered. The scattered light enters the beam combiner 6C through the slit of the condenser lens 8av slit plate 9 and the condenser lens 8b. The other laser beam divided into two is shifted by the shift frequency Δν of the oscillator 10b in the modulator JOa, and becomes a laser L' having a frequency of (ν0+Δν).
The light beam passes through the mirror 2b, enters the beam combiner 6c, and is combined with the scattered light S. By combining both, the shift frequency Δ
A heat signal of ν is generated, which is detected by the photoreceiver 5.

Now, the scattered light S (ν0) and the laser L' (ν0+Δν
), let Es and ER be the respective electric fields.

Es = ΔEexp[j(2πνt+φ1)]・”−
(1) ER=Eo exl) [j(2πν' t+φ2
)]'・−−−−(2) shi′=shi0+Δν
...... (3), the intensity I of the combined heat signal is I=lEs+ERl2=lE0 12+
lΔE12+2EQΔEcos(2ffΔνt+φ2−
φ1)...(4) It is expressed as follows. The electric field Δ of the scattered light S is determined by the third term of equation (4).
E is 2E at the heat signal intensity I.

However, since Eo is much larger than ΔE, the output voltage of the photoreceiver 5 has better sensitivity than directly detecting the scattered light. At the same time, by selectively extracting only the heat signal using an appropriate method, optical wavelength noise such as stray light is cut out and the S/N ratio is improved. These methods make it possible to detect minute particles on the order of 0.01 μm, which has been considered difficult in the past, and this is the heterodyne method.

An object of the present invention is to provide a face plate defect detection method and an inspection apparatus that improve detection performance by applying the above-mentioned heterodyne method to face plate defect detection.

[Means for solving the problem] This invention scans the surface of a surface plate to be inspected such as a silicon wafer with a spot formed by projecting a laser beam, and detects scattered light due to surface defects. This is a face plate defect detection method and inspection device using the heterodyne method in defect inspection.

The face plate defect detection method splits the laser from the laser light source into two, forms a spot with one laser to receive the defect scattered light, and shifts the other laser with an appropriate shift frequency to create an interference laser. and the received scattered light are combined to generate a heat signal with a shifted frequency. Defects are detected by selectively extracting this heat signal using an electric filter.

The face plate defect inspection device is equipped with a laser light source, a spot scanning mechanism for the face plate to be inspected, a beam splitter that splits the laser beam from the laser light source into two, and a light projector that focuses one laser beam to form a spot on the face plate surface. an optical system, a light receiving optical system for the scattered light of the spot, a frequency shift means for the other divided laser, and a heat signal is generated by combining the interference laser whose frequency has been shifted by the means and the received scattered light. It consists of a beam combiner that generates a heat signal, and an electric filter that selects the frequency of the heat signal.

A first embodiment of the frequency shifting means in the above inspection apparatus includes an acousto-optic modulator that shifts the frequency of the other divided laser, and an acousto-optic modulator that generates a shift frequency of ultrasonic waves and supplies it to the acousto-optic modulator. It consists of a modulated wave generator. In addition, the second embodiment uses a laser light source of a semiconductor laser element, and includes a frequency oscillator that periodically and linearly changes the oscillation frequency of the laser light source by changing the injection current to the laser light source, and a frequency oscillator that periodically changes the oscillation frequency of the laser light source in a linear manner. A frequency shifting means is constituted by an optical fiber which is split into two parts and the other laser is delayed for a certain period of time to create an interference laser.

[Function] In the face plate defect inspection method using the heterodyne method of the present invention described above, similarly to the dust measurement method according to the patent publication described above, the scattered light S due to the defect is combined with the frequency-shifted interference laser. A heat signal with a much higher intensity than the scattered light is generated. Applying this to equation (4) above, the electric field EO of the interference laser is one-half of the laser from the light source, and compared to this, the electric field ΔE of the scattered light S due to a minute defect is very small. , the heat signal is increased by 2E, improving the detection sensitivity.

Furthermore, noise is removed from the heat signal by an electric filter, resulting in a good S/N ratio, making it possible to detect smaller defects than before.

In the inspection apparatus, a laser beam from a laser light source is split into two by a beam splitter, one of which is input to a projection optical system to form a spot, and the surface of the face plate is scanned by a spot scanning mechanism. The other laser becomes an interference laser whose frequency has been shifted by the frequency shifting means, and this is combined with the received scattered light of the defect in the beam combiner to generate a heat signal. The heat signal is output after noise is cut by an electric filter.

A first embodiment of the frequency shifting means shifts the frequency of a laser from a light source by supplying an ultrasonic shift frequency from a modulated wave generator to an acousto-optic modulator.

Next, the frequency shifting means in the second embodiment will be explained with reference to FIG. A t-conductor laser element is used as a laser light source, and by changing its injection current, the oscillation frequency νS is periodically and linearly changed with respect to time t, as shown in FIG. This light is divided by a beam splitter and one part is supplied to a projection optical system for detection of scattered light. The other beam is delayed by a certain time Δ using an optical fiber to form an interference beam with a frequency νd. Frequency difference Δ between the two at any time
A heat signal is generated with ν corresponding to the above shift frequency.

[Embodiment 2] FIG. 2 shows a schematic configuration diagram of a first embodiment of a face plate defect detection method and inspection apparatus using the heterodyne method of the present invention. The figure is for the rotation scanning method, and the face plate to be inspected 1
is placed on a turntable 1a and rotated by a spindle 1b. The laser light source 2 is a gas laser with a stable oscillation frequency, such as a helium-neon laser tube, and the laser L (ν0) from this is split into two by a beam splitter 6a, one of which is sent through a mirror 2a to a projection lens 3.
is focused to form a spot on the surface of the face plate. When there is a defect in the face plate 1, the scattered light S(ν0) is collected by the light receiving lens 4, passes through the mirror 2c, and is sent to the beam combiner 6.
Enter in C. Next, the other laser divided into two by the beam splitter 8a is input to the acousto-optic modulator 10a, and the frequency is shifted (ν0+Δν) by the shift frequency Δν of the ultrasonic band supplied from the modulated wave generator 10b. becomes. In this case, Δν is set to an appropriate frequency (for example, several MHz
), it is also possible to perform two-stage modulation using two sets of modulator 10a and generator 10b.

This laser is input to the beam combiner 6C via the mirror 2b, where it is combined with the above-mentioned scattered light S(v0), and a heat signal with a frequency Δv is output from the light receiver 5. The output signal of the photoreceiver 5 contains noise such as components of the original frequency ν0° (ν0+Δν) and stray light, but these are removed by the electric filter 11 and only the heat signal is selectively extracted. A defect signal is created in the processing section 12. The defect signal is processed by the data processing section 13, and the processed data is displayed or printed on the output section 14.

The processing methods of the signal processing section 12 and the data processing section 13 described above have been conventionally used, and the explanation thereof will be omitted here.

FIG. 3 is a schematic configuration diagram of a second embodiment of a face plate defect detection method and inspection apparatus using the heterodyne method of the present invention. In this case, a semiconductor laser element 2' is used as the laser light source. The semiconductor laser element 2' can change the oscillation frequency by injection current, and is connected to the power supply 1.
The current injected from 5c is controlled by the frequency fs of the sawtooth wave oscillator 15b, so that the frequency νS of the laser L is periodically and linearly varied as shown in FIG. laser beam νS)
is divided into two by a beam splitter 6a, one of which is used for the projection/reception optical system as in FIG. 2, and the scattered light S(νS) is guided to the beam combiner 6c. The other beam is delayed by a certain time Δt shown in FIG. 1 through the optical fiber 18a by the coupling lens teb, and further outputted from the coupling lens IGc and input to the beam combiner 6c. optical fiber l
Let ea be a length corresponding to the delay time of Δt. Due to this delay time, the frequency at any time t becomes νd, which is smaller than νS by Δν, as shown in FIG.
A heat signal with a frequency Δν is output from C. As described above, as in FIG. 2, noise is removed by the electric filter 11 and processed by the signal processing section 12 and data processing section 13, and the resultant signal is output to the output section 14.

In the first and second embodiments described above, the scanning method of the face plate is a rotation method, and the laser projection/receiving optical system is modeled on a basic and simple configuration. It goes without saying that the present invention can be similarly applied to an improved fist receiving optical system as long as it uses a scattered light receiving method.

[Effects of the Invention] As is clear from the above explanation, the face plate defect detection method and inspection device using the heterodyne method of the present invention applies the heterodyne method in the above-mentioned patented method for measuring dust in the air to face plate defects. Almost the same improvement in detection performance for face plate defects is expected, and it can contribute to the detection and inspection of minute face plate defects of 011 μm or less, which has been difficult in the past.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of the principle of the frequency shifting means using optical fiber in the face plate defect detection method and inspection apparatus using the heterodyne method of the present invention, and FIG. 3 is a schematic basic configuration diagram of the second embodiment, FIG. 4 is a basic configuration diagram of a face plate defect detection method using scattered light reception, and FIG. 5 is a patent FIG. 1 is a schematic configuration diagram of a dust measuring method and device disclosed to the public. 1... Face plate to be inspected, Ia... Turntable, ■b... Spindle, 2... Laser light source, 2'
... Semiconductor laser element, 2a, 2b, 2c... Mirror 3... Light emitting lens, 4... Light receiving lens,
S... Light receiver, 6a... Beam splitter, 6b... Beam combiner, 7... Detection cell, g
a, gb...Condensing lens, 9...Slit plate, IO
a...Acousto-optic modulator, 10b...Modulated wave generator, ■...Electric filter,
I2...Signal processing unit, 13...Data processing unit, Lt...Output unit, 15a...Injection current control circuit, +
5b... Sawtooth wave oscillator, 15c... Power supply, lGa
...Optical fiber, lGb, IGc...coupling lens.

Claims (4)

[Claims] (1) In defect inspection, the surface of a face plate to be inspected such as a silicon wafer is scanned with a spot formed by projecting a laser beam onto the surface, and the scattered light due to defects existing on the surface is detected. The laser from the light source is divided into two, one of the two divided lasers forms a spot to receive the light scattered by the defect, and the other divided laser is shifted by an appropriate shift frequency. A face plate using a heterodyne method, characterized in that a laser and the received scattered light are combined to generate a heat signal of the shifted frequency, and the heat signal is selectively extracted by an electric filter to detect the defect. Defect detection method. (2) A beam splitter that is equipped with a laser light source and a spot scanning mechanism for the face plate to be inspected, and splits the laser beam from the laser light source into two, and focuses one of the two split lasers to create a spot on the surface of the face plate. a light-emitting optical system for forming a light emitting light, a light-receiving optical system for the scattered light of the spot, a frequency shifting means for the other of the two divided lasers, and combining the interference laser frequency-shifted by the means with the received scattered light. 1. A face plate defect inspection apparatus using a heterodyne method, comprising a beam combiner that generates the heat signal, and an electric filter that selects the frequency of the heat signal. (3) In the above, an acousto-optic modulator that shifts the frequency of the other of the two divided lasers, and a modulated wave generator that generates the shifted frequency of the ultrasonic wave and supplies it to the acousto-optic modulator, constituted the shifting means,
A face plate defect inspection device using a heterodyne method according to claim 2. (4) In the above, the laser light source includes a semiconductor laser element, a frequency oscillator that periodically changes the oscillation frequency linearly by changing the current injected into the laser light source, and the other laser obtained by dividing the laser from the laser light source into two. 3. A face plate defect inspection apparatus using a heterodyne method according to claim 2, wherein said frequency shifting means is constituted by an optical fiber which delays said interference beam by a certain period of time and forms said interference beam.