JP2530081B2 - Mask inspection equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mask inspection apparatus for inspecting defects in a mask for exposure, and more particularly to a mask inspection apparatus for inspecting defects by the intensity distribution of transmitted illumination light of the mask.

[0002]

2. Description of the Related Art FIG. 2 shows the structure of a general reduction projection type exposure apparatus. In this system, light R emitted from a light source is condensed by a condenser lens system and projected onto an exposure mask, and transmitted light from this mask is reduced and projected onto a wafer by an objective lens system. Here, the magnification of the reduced projection image is the distance z ₁ between the mask and the objective lens system and the distance z between the objective lens and the wafer.
_It is determined by the ratio with ₂ (z ₁ / z _2, for example 1/5).

From the prior art, the following methods are conceivable for inspecting the pattern defects of the mask used for this exposure. That is, (1) an image pickup means such as a CCD camera is arranged at the image forming position of the reduced projection image to directly observe the image of the mask.

(2) A wafer after exposure and development is directly observed with a microscope or the wafer is cut and its cross section is a scanning electron microscope (hereinafter referred to as SE).
Let M. ) And so on.

(3) The mask is irradiated with light and the mask image is magnified by a conventional microscope (FIG. 3 ). The intensity distribution pattern of the transmitted light is compared with the design pattern of the mask and both are compared. Defects are extracted according to the degree of coincidence.

[0006]

However, while the image of the mask projected by reduction is composed of a fine pattern of about 0.3 μm, a solid-state image sensor (Charge Coupled D) is used.
evice. Hereinafter referred to as CCD. The resolution of () is about 7 μm. That is, since there is no imaging device having a spatial resolution suitable for direct observation, the method (1) is impossible.

Further, when the transmitted light of the mask is reduced and projected on the wafer, the defect of the mask is imaged at a size larger than the reduction magnification due to the diffraction phenomenon. Therefore, when directly observing the mask itself, it is necessary to inspect with a detection sensitivity higher than an allowable value, and the inspection time becomes long.

The method (2) is generally used, but it is unavoidable due to these intermediate processing steps such as development. In addition, the work such as cutting the wafer is troublesome, and the wafer after the inspection cannot be used as a product.

[0009] In the method (3), by <br/> urchin conventional exposure mask, to form a light-dark pattern opaque film using the transmitted light, such as metal film and the design pattern Defects may be easily detected by comparing However, the phase Shifutomasu click that has been underway in recent years development,
The intensity distribution pattern of the transmitted interference light is formed by the arrangement of fine irregularities and the groove depth thereof, and the defect of the mask includes not only the defect of the pattern but also the cause of the phase shift formed by the irregularity. It is difficult to include such phase information in the design pattern, and it is extremely difficult to accurately perform the defect inspection of the phase shift mask.

In actual IC manufacturing, thin films are repeatedly stacked many times, and in the exposure process, it is often the case that the focus is deviated by the thickness of the resist and reduced projection is performed. Conventionally, the influence of such defocus is also caused by SEM.
Although it was carried out by observing the fractured surface, it was difficult to make an accurate evaluation due to the influence of the intermediate treatment step as in the above. Therefore, it is desirable to be able to evaluate the influence of defocus on the image formation in the mask defect inspection process. In particular, when using a phase shift mask that can take a deep depth of focus, the influence of defocus becomes important.

Therefore , an object of the present invention is to provide a mask inspection apparatus which can evaluate the influence of defocus on image formation. [Constitution of Invention]

[0012]

Means and operation for solving the problems] The present invention has been made in consideration of the above problems, an illumination means for emitting an inspection light having a wavelength of Jo Tokoro region, focusing lens for focusing said test beam A system, a stage section on which a mask is placed, an objective lens system that magnifies and projects the inspection light that has passed through the mask, and an aperture stop unit that adjusts the spread of the light flux of the inspection light that has passed through the objective lens system, An image pickup means for picking up an enlarged light image from the objective lens system and outputting an image signal, an image pickup position displacing means capable of displacing the image pickup means in the optical axis direction of the inspection light, and a position displacement amount of the image pickup means are shown. The image pickup position measuring means for measuring, the calculating means for calculating the image signal, and the operation of the image pickup position displacing means are controlled to focus the image pickup means.
A mask inspection apparatus, comprising: a control unit that shifts the position to capture the magnified optical image .

The light source emits light including inspection light in a predetermined wavelength range. In addition, the size of the light source image on the rear focal plane of the objective lens system can be adjusted by adjusting the spread of the light beam by the condenser lens system. Further, the numerical aperture of the objective lens system can be adjusted by the aperture stop means. Since the imaging performance of the projected image is determined by these optical parameters, the present apparatus can make the imaging performance of the enlarged projected image of the mask on the imaging surface the same as that of the reduced projected image at the time of exposure. Therefore, the reduced projection image and the enlarged projection image similar in shape can be observed, so that it is not necessary to increase the detection sensitivity and the inspection speed can be improved . Also, since the imaging means is freely displaced in the optical axis direction, it is possible to assess the effect of the projected image of the defocus to easily.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. 1 is a configuration diagram of a mask inspection apparatus according to a first embodiment of the present invention. First, the configuration and operation of this device will be described.

This apparatus comprises an illumination unit (110) which emits an inspection light R ₁ having a wavelength λ in a predetermined region, and a condenser lens which condenses the inspection light R ₁ and irradiates it on a mask (100) to be inspected. system
(120) and stage part (130) for mounting the mask (100)
And the numerical aperture (hereinafter referred to as NA) of the objective lens system (140) for enlarging and projecting the inspection light R ₂ transmitted through the mask (100) and the inspection light R ₃ transmitted through the objective lens system (140). An aperture stop (150) to be adjusted, an image pickup section (160) for picking up an enlarged light image from the objective lens system (140) and outputting an image signal S, and an image pickup
Imaging position where the part (160) can be displaced in the optical axis direction of the inspection light R ₄
Measure the amount of positional displacement between the displacement means (180) and the image pickup unit (160).
Image pickup position measuring means (190) for calculating the image signal S and controlling the operation of the whole apparatus to focus the image pickup means.
Arithmetic control unit for capturing the magnified optical image by shifting from the point position
(170) and.

The illumination unit (110) is provided with a light source (111) such as a mercury lamp that can obtain high brightness and an optical filter (112) such as an interference filter that selectively transmits light in a specific wavelength range. Therefore, the inspection light R ₁ having the wavelength λ in the predetermined region can be extracted by combining the respective layers of the interference filter.

The condenser lens system (120) is provided with a condenser lens (121), an aperture stop (122) and a condenser lens (123) on the optical axis of the inspection light R ₁ , and a stage section ( The inspection light R ₁ is applied to the mask (100) placed on the (130). The condensing lens (121), the aperture stop (122), and the condenser lens (123) limit the spread of the ray bundle of the inspection light R ₁ , and the light source image for the pupil on the pupil plane of the objective lens system (140). It is possible to adjust the size ratio (this is called the coherence degree σ of the illuminating unit, which will be hereinafter referred to as σ value). In FIG. 1, when the aperture stop (122) is circular, σ is expressed by the following equation (1).

[0018] _{σ = {sin (θ 3/} 2)} / {sin (θ 4/2)} (1) stage portion (130) is detachably a mask (100) for IC exposure to be inspected It has a stage (131) to be placed and a stage drive section (132) capable of moving the stage (131) in the in-plane directions (XY directions), so that the inspection light R ₁ may pass through the mask (100). It is arranged on the optical axis.

The objective lens system (140) is arranged on the optical axis of the inspection light R ₂ transmitted through the mask (100), and the transmission intensity distribution of the inspection light R ₂ is displayed on the image pickup surface of the image pickup section (160). It is designed to be enlarged and projected. The magnification of this magnified optical image is
0) and objective lens system (140) distance z ₁ and objective lens system
Ratio of the principal point of (140) to the distance z ₂ between the imaging surface (z ₁ / z
₂ ) determined by.

The aperture stop (150) is usually composed of a light-shielding plate with a circular hole, and is located at the focal point or principal point of the objective lens system (140) so that the circular hole is substantially concentric with the optical axis. It is arranged. The relative position of the aperture stop (150) on the optical axis and the diameter of the circular hole limit the spread of the light beam bundle of the objective lens system (140). In FIG. 1, if the refractive index of air is n ₀ and the solid angle of the light beam bundle limited by the aperture stop (150) is θ ₄ , then n ₀ · sin (θ ₄ /
The value represented by 2) is the numerical aperture NA of the objective lens system (140).

The resolution of this mask inspection device depends on the image forming performance on the image pickup surface. To obtain this image forming performance, the diffraction by the condenser lens system (120) and the objective lens system (140) are performed.
Diffraction due to must be taken into account. According to HHHopkins' partial coherence theory,
The imaging performance Γ of the projected image is expressed by the following equation.

Γ = f (σ, λ, NA) where σ is the coherence degree of the illumination unit (110), λ is the wavelength of the inspection light, and NA is the numerical aperture of the objective lens system (140).
Therefore, according to the above equation, the imaging performance Γ of the projected image is σ,
Since it depends only on each parameter of λ and NA, if the values of σ, λ, and NA of the mask inspection apparatus are the same as those of the exposure apparatus, the same image forming performance as the reduced projection image at the time of exposure is obtained. It is possible to obtain an enlarged projection image having In consideration of this, in the present embodiment, the optical filter (112), the condenser lens system (120), and the aperture are set so that the mask (100) has the same σ, λ, and NA as those used during IC exposure. Aperture (150)
Has been set. (For example, in a commercially available g-line stepper, λ = 436 nm, NA = 0.108, σ = 0.5.) In the imaging unit (160), for example, a sensor element such as CCD is on a straight line (X direction). A plurality of line sensors (161) are arranged on the mask (100) and are arranged at the image forming position of the magnified light image of the mask (100). Each sensor element outputs an electric signal S ₀ according to the amount of received light.
61) can read the electric signal S ₀ from each sensor element in order according to the arrangement. Now, by moving the mask (100) in a direction (Y direction) orthogonal to the arrangement of the sensor elements by the stage driving unit (132) and reading the electric signal S ₀ in synchronization with this movement, the imaging unit
(160) can output an image signal S ₁ as two-dimensional image information corresponding to the enlarged projection image of the mask (100). The image pickup unit (160) may pick up a two-dimensional image by an area sensor in which the sensor elements are arranged in a matrix instead of the line sensor, and the electric signal S may be synchronized with the movement of the mask (100). Instead of outputting _{0s one} by one, the two-dimensional image data may be temporarily stored in the image memory and then processed. Furthermore, the mask (100) may be scanned with laser light, and the transmitted light or reflected light may be imaged and used as an image signal.

The image pickup position displacing means ( 180) has an axis extending in the optical axis direction (Z direction) of the inspection light and bearings at both ends (not shown).
The feed screw (182 ) pivotally supported by, the motor (1 83) connected to one end of the feed screw (1 82) and the feed screw (182 ) are screwed to support the imaging unit (1 60) on the side surface. It is composed of a table (181 ) to be fixed, and the image pickup section (1 60) is connected to the objective lens system (1 4).
(0) is installed in a position to receive the enlarged projection image from
The image pickup unit (1 60) can be moved in the Z direction by the rotational drive of (1 83).

The image pickup position measuring means ( 190) includes a table (18
1 ) Linear scale adjacent to the side and extending in the Z direction (1 9
It is composed of 1) and outputs an electric signal according to the detection position of the table (181 ).

The arithmetic control unit (1 70) includes an arithmetic processing circuit (1 71)
A table position detection circuit ( 172), a table drive control circuit ( 173), and a central processing unit ( 174). Processing unit (1 71) includes an amplifier circuit, A / D conversion circuit, comparator circuit, our Ri imaging section have a like delay circuit an image signal S ₁ inputted from the (1 60) and processing A defect detection signal is output.

The amplifier circuit amplifies the image signal S ₁ into an amplified image signal S ₂ , and the A / D conversion circuit A / D converts it into a digital image signal S ₃ . On the other hand, the arithmetic circuit, with reference to the design data of the mask (100) has prepared a standard digital video signal S _s, the standard image signal S _s
And the input digital image signal S ₃ are compared and calculated. If the difference between the two signals S _s and S ₃ is equal to or larger than the predetermined threshold value, it is determined that the mask (100) has a defect and the defect detection signal S _d is output. This inspection result is displayed on the display (1
It is output to the outside by 75).

As the standard image signal S _s , an image signal S _{3 ′} of a mask having no defect may be used. If two or more IC patterns are drawn on one mask, each image signal may be used as a standard image signal.

In equation (1), σ is calculated assuming that the aperture stop (122) is circular, but the shape of the aperture stop (122) is not necessarily limited to a circle. Also, in this case, σ is equal means that the shape of the aperture stop (122) is similar to that of the exposure machine, and the size ratio of the aperture stop (122) and the objective lens system (140) is the exposure. It means equal to that of the machine.

The table position detection circuit ( 172) receives the electric signal from the linear scale ( 191) and performs A / D conversion to calculate the current position of the table (181 ), and the table drive control circuit (1 72). 73).

The table drive control circuit ( 173) is a table drive control circuit.
A closed loop control system is formed between the position detection circuit (172 ) and the image pickup section ( 160 ) on the table (181 ) so that it can be aligned with a desired position. That is, the central processing unit
A signal indicating the displacement of the table is input from (1 74), and the table (1 8) is input from the table position detection circuit (1 72).
The signal indicating the current position in 4) is input and an electric signal that eliminates the difference between the two signal values is output to the motor (183).

The central processing unit ( 174) is usually composed of a microcomputer and various hardware circuits,
It is designed to control the whole operation. That is, the operation processing result from the operation processing circuit (1 71) is input and displayed externally, and the next table displacement command is driven by the table based on the operation processing result and the electric signal from the table position detection circuit (1 72). It is designed to output to the control circuit ( 173).

Next, a mask inspection method using the mask inspection apparatus according to the first embodiment will be described. The mask (100) to be inspected is placed on the stage ( 131) with the matrix directions of the IC patterns aligned with the XY directions.

The inspection light R ₁ emitted from the illuminating section (110) includes a condenser lens system (1 20), a mask (100) mounted on a stage section (1 30), an objective lens system (1 40), After passing through the aperture stop ( 150), a magnified projection image of the mask (100) is formed on the imaging surface. The enlarged projection image is the same imaging performance and reduced projection image during exposure light. In addition, enlarged projection image, La-sensor (1 61)
An image is picked up by and the arithmetic processing circuit ( 171) performs processing such as comparison calculation to inspect defects on the mask (100).

Here, a method of evaluating the influence of defocus of the reduced projection image at the time of exposing the mask (100) will be described. The central processing unit (1 74) is a table drive control circuit (1 73)
On the other hand, a displacement command for intentionally shifting the imaging unit ( 160) from the focus position is sent. The spherical aberration amount in the imaging optical system is, for example, in FIG. 1 , the movement amount Δz / sin ^{2 of the} imaging position.
Proportional to (θ _6/2). Therefore, if displacing the position of the imaging plane Delta] z in _{^{'= {sin 2 (θ 5}} /2) / sin 2 (θ 6/2)} · Δz (2) consisting Delta] z' only Z-direction in the mask inspection apparatus, {Σ, λ, N
In the exposure apparatus in which A} is equivalent, the wafer is
An enlarged projection image similar in shape to the reduced projection image obtained when displaced by z can be obtained. Therefore, in order to evaluate the defocus amount Δz at the time of exposure, the central processing unit ( 174) takes into consideration the optical characteristics of the present mask inspection apparatus and calculates Δz ′ from the above equation (2).
Table displacement control circuit
Send (1 73).

A table drive control circuit (1
73) positions the image pickup surface at a position deviated from the focus position by Δz 'by closed loop control with the table position detection circuit ( 172).

[0036] After completion of positioning, expanding from the lighting unit (1 10) such place inspection light R _1, is on the imaging surface of similar shape and reduced projection image defocus in the optical system of the characteristics equivalent exposure apparatus A projected image is formed.

Therefore, the image pickup section (100) picks up this enlarged projection image. The central processing unit ( 174) processes the image signal by the same operation as described above. That is, the influence of defocus of the mask projection image is evaluated by comparing the image signal created by the designed mask pattern and the image signal obtained by imaging before displacement. The configuration of the present invention is not limited to the above-mentioned embodiments, and can be modified without changing the gist of the invention.

[0038]

According to the mask inspection apparatus according to the present invention as Shoki according to the present invention above, the characteristics of optical science system examines the enlarged projection image of the blurred reduced projection image similar to the shape of the focus in equivalent exposure apparatus be able to.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a mask inspection apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of an exposure apparatus.

FIG. 3 is a diagram showing a configuration of a conventional mask inspection device.

[Explanation of symbols]

100 ... Mask, 110 ... Illumination section, 121 ... Condensing lens, 130 ... Stage section, 140 ... Objective lens system, 15
0 ... Aperture stop, 160 ... Imaging unit, 170 ... Arithmetic control unit.

Claims (1)

(57) [Claims] 1. An illuminating device that emits inspection light having a wavelength in a predetermined region, a condenser lens system that condenses the inspection light, a stage portion on which a mask is mounted, and the inspection light that has passed through the mask are enlarged and projected. An objective lens system, an aperture stop means for adjusting the spread of the light beam of the inspection light transmitted through the objective lens system, and an image pickup means for picking up an enlarged light image from the objective lens system and outputting an image signal. Imaging position displacement means capable of displacing the imaging means in the optical axis direction of the inspection light, imaging position measuring means for measuring the amount of positional displacement of the imaging means, arithmetic means for arithmetically processing the image signal, and the imaging position The operation of the displacement means is controlled to shift the image pickup means from the focus position before
A mask inspection apparatus comprising: a control unit that captures the magnified optical image .