JPH10288734A - Automatic focusing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automatic focusing device capable of shortening a focusing time when an object is replaced by another. SOLUTION: The AF(automatic focusing) device is provided with plural sensor mechanisms (AF optical system AFS and slit 3, etc.) for detecting a focusing state and a focus actuator (Z table 8) for moving the wafer 7 of the object in the optical axis direction. In the case an absolute value of a difference between focusing signals sent from plural sensor mechanisms exceeds a prescribed value, the action of the focus actuator 8 is stopped by a focus signal deciding part 21, and in the case the absolute value of the difference between the focusing signals from plural sensor mechanisms is equal to or below the prescribed value, the action of the focus actuator 8 is restored.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus (AF) device for automatically focusing an optical system such as a microscope or a semiconductor circuit inspection device on an object. In particular, the present invention relates to an autofocus device that can shorten the focusing time when exchanging an object without taking a complicated interface with peripheral equipment.

[0002]

2. Description of the Related Art The prior art will be described by taking a semiconductor device inspection apparatus as an example. 2. Description of the Related Art After a process such as lithography in the manufacture of semiconductor devices, inspection of the shape accuracy of a photoresist layer or a process layer formed on a wafer is performed. The equipment that performs this test, in brief,
It is a device in which a CCD camera and an image processing device are attached to a microscope. The sample stage of this inspection device is a stage that is scanned in the optical axis direction (Z direction) and in the plane perpendicular to the optical axis (X, Y directions). A wafer is placed on this stage via a vacuum chuck or an electrostatic chuck, and inspection is performed while replacing the wafers one after another.

An inspection apparatus of this type is provided with an AF apparatus for automatically focusing an optical system of a microscope. Although there are various AF methods, many of them have a mechanism of irradiating an AF light to an object and detecting a reflected light thereof. In this case, a plurality of points around the observation field of view for inspection
F light is applied, reflected light from the plurality of points is detected to obtain a plurality of AF signals, the signals are processed to enhance the reliability of the signals, and the stage is controlled based on the processed AF signals. Move in the axial direction to focus.

[0004]

When the test object is a semiconductor wafer, the height of the peripheral portion (edge) of the upper surface is reduced by 10 μm or more due to so-called polishing sag. Here, when the wafer is moved out of the optical system while the AF is functioning at the time of wafer exchange or the like, the AF device follows the sagging of the edge and greatly changes the focal position. After that, when the next wafer is inspected, it takes a long time to focus on a legitimate part to be inspected (for example, 3
４4 seconds) Inspection efficiency decreases.

As a solution to this problem, it is conceivable to stop the AF operation before moving the test object when moving the test object outside the inspection stage. However, in this case, the control of the AF apparatus depends on the control of the entire inspection apparatus (system) in which the AF apparatus is incorporated. Once the operation of the AF apparatus is stopped, the focus instruction from the system is performed when the AF apparatus is operated again. , The operation is started, and it takes time until focusing. In addition, signals are exchanged with a system other than the AF system (an XY table or a control device for the entire inspection apparatus), and the processing becomes complicated. If possible, it is desirable to be able to deal with the situation in the AF device itself.

The present invention relates to an auto-focus (AF) device for automatically focusing an optical system such as a microscope or a semiconductor circuit inspection device on an object without using a complicated interface with peripheral equipment. It is another object of the present invention to provide an auto-focus device capable of shortening a focusing time when exchanging an object.

[0007]

In order to solve the above problems, an AF device according to the present invention is an autofocus device for focusing an optical system on an object; a plurality of sensor mechanisms for detecting a focus state. A focus actuator that adjusts a focus position by moving an object or an optical system in an optical axis direction; a calculation unit that calculates a focus position based on focus signals from the plurality of sensor mechanisms; and the plurality of sensors A control unit including a determination unit that determines whether a focus signal from the mechanism is in a predetermined specific state.

In the AF apparatus according to the present invention, when the absolute value of the difference between the focus signals from the plurality of sensor mechanisms exceeds a predetermined value, the determining unit determines that the state deviates from a specific state. In response, the control unit stops the operation of the focus actuator, and the determination unit returns to a specific state when the absolute value of the difference between the focus signals from the plurality of sensor mechanisms becomes equal to or less than a predetermined value. It is preferable that the control unit restores the operation of the focus actuator. When the test object is moved out of the automatic focus position detection range and returned to the detection range again, or when the test object is replaced, an efficient operation for position detection is performed when automatically detecting a new focus position. Will be

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram schematically showing a configuration of an optical system (knife edge method) and a control system of a semiconductor inspection apparatus having an autofocus device according to one embodiment of the present invention. This inspection apparatus forms an enlarged image of a semiconductor circuit layer and a photoresist layer formed on the upper surface of the wafer 7 as an object,
It is a device that inspects the shape and dimensions of those layers. Wafer 7
Is fixed on the wafer stage 8 via a holder such as a vacuum chuck or an electrostatic chuck. Wafer stage 8
Is scanned in the optical axis direction and in a vertical plane thereof (Z direction, XY plane).

The inspection apparatus shown in FIG. 1 includes an observation optical system OBS disposed immediately above the wafer 7, an illumination optical system ILS and an autofocus optical system AFS disposed beside the optical system OBS. . In the illumination optical system ILS, a light source 1, a condenser lens 2, an AF slit plate 3, a projection lens 4, and a half mirror 5 are arranged in this order on the optical axis. The illumination light IL emitted from the light source 1 is condensed through the condenser lens 2 and then enters the AF slit plate 3. A slit for autofocus is formed in a peripheral portion of the AF slit plate 3. The central portion of the AF slit plate 3 is transparent. The illumination light emitted from the AF slit plate 3 is given an appropriate illumination magnification by the projection lens 4. Illumination optical system ILS and observation optical system OBS
A first half mirror 5 is placed at a point where the optical axes of the two cross. The illumination light IL exiting the projection lens 4 strikes the half mirror 5 and is reflected downward, and then illuminates the wafer 7 via the first objective lens 6.

In the observation optical system OBS, a first objective lens 6, a first half mirror 5, a second half mirror 9, a second objective lens 10, and an imaging lens 1
One and two-dimensional photoelectric conversion means (CCD camera) 12 are arranged. Among these, the first objective lens 6 is shared with the autofocus optical system AFS and the illumination optical system ILS.

The illumination light incident on the wafer 7 is reflected on the upper surface of the wafer 7. The reflected light RL is directed upward, passes through the first objective lens 6 and the first half mirror 5, and reaches the second half mirror 9. The second half mirror 9 receives the reflected light RL
Are passed upwards and partially reflected laterally. The transmitted light OL is used for observation or measurement, passes through the second objective lens 10 and the imaging lens 11, and forms an enlarged image on the photoelectric conversion surface of the CCD camera 12.

The light FL reflected by the second half mirror 9 is
Enter the autofocus optical system AFS. In the autofocus optical system AFS, a relay lens 13, a mirror 14, a knife edge 15, an imaging lens 16, a cylindrical lens 17, and an AF photoelectric conversion unit 18 are arranged in order from the half mirror 9. The knife edge 15 is arranged substantially at the pupil position of the autofocus optical system AFS, and its tip coincides with the optical axis. Photoelectric conversion means 18
Is an autofocus optical system A including the first objective lens 6
In the FS, it is at a position conjugate with the pupil where the wafer 7 and the knife edge 15 are arranged. Cylindrical lens 1
7 compresses the autofocus light FL in the width direction and forms an image on the photoelectric conversion means 18. The photoelectric conversion means 18 in this device is a linear CCD sensor having a width of about 10 μm and a length of 20 mm. Eventually, the portion of the AF slit image projected on the wafer 7 that passes through the tip of the knife edge 15 is formed on the AF photoelectric conversion unit 18.

A signal relating to the AF slit detected by the AF photoelectric conversion means 18 is sent to a focus position calculating section 19 in the control section of the inspection apparatus and processed (details will be described later), and a normal focus position is detected. . The signal relating to the focus position is sent to the focus actuator driving unit 20, which sends the signal to the Z stage of the wafer stage 8.
The actuator for driving the table is driven to move the wafer 7 up and down, and the upper surface of the wafer 7 is positioned at a regular focus position.

FIG. 2 is a plan view showing an example of the positional relationship between the observation field of view of the semiconductor inspection apparatus of FIG. 1 and the AF sensor mechanism (slit and AF optical system). Circular (diameter 200 μm)
2), two AF sensor mechanisms 33 are arranged at opposing positions on both sides of the observation field of view 31). When the observation field deviates from the wafer, one of the sensor mechanisms first falls on the wafer edge, and the balance of the position detection signal with the other sensor mechanism is lost. Therefore, by processing the signal as described later, it is determined that the wafer is being removed, and the driving of the focus actuator is stopped. Note that the number of sensor mechanisms may be three or more.

FIGS. 3 and 4 show AF in the apparatus shown in FIG.
3 shows an image profile of an AF slit on a photoelectric conversion unit for use. FIG. 3 shows a state where both of the two sensor mechanisms are substantially in focus. FIG. 4 shows a state in which the balance between the signals of the two sensors is significantly distorted. The horizontal axis represents the longitudinal position of the photoelectric conversion means, and the vertical axis represents the output of the photoelectric conversion means at the position. Each figure shows two peaks corresponding to the two slits. In the figure,
The two dashed lines CL in the vertical direction are lines that bisect the area of the profile at the time of focusing, and have already been given at the time of adjustment of the apparatus. Focus position calculator 19
Receives an input of a signal representing the profile of the photoelectric conversion unit 18 and receives the area a,
b is calculated. Then, the amount of defocus, that is, the dimension for moving the wafer up and down to the focus position is calculated from the difference between the two areas.

In FIG. 3, the areas a and b of the profiles on the left and right of the line CL are equal, and both the sensor mechanisms are in focus. FIG. 4 shows a defocus state in which the areas a and a 'of the profiles on both outer sides of the line CL are larger than the areas b and b' of the center profile. Note that the profile has a curve in which the left and right sides where the knife edge exists are sharply dropped. This is because when defocusing occurs, light passing through the knife edge side is blocked by the knife edge, while light passing through the anti-knife edge side passes through to the photoelectric conversion means. is there.

Next, referring to FIG.
The restoration operation (the operation of the focus signal determination unit 21 in FIG. 1) will be described. FIG. 5 is a flowchart for explaining the focus stop / restore operation in the autofocus apparatus of the present embodiment. First, in step 101, the areas a, b, a ', b' on both sides of the center line of the focus signal profile shown in FIGS.

Next, in step 102, the following equation is calculated for each area, and is compared with a predetermined value k. | (Ab) − (a′−b ′) |> k As an example of the selection of the value of k, the difference between the heights of the target portions of the two sensor mechanisms is the depth of focus of the observation optical system. .8
μm, that is, 8 μm,
Give as.

If step 102 is YES, the process proceeds to step 103, in which it is assumed that the target portion of any of the sensor mechanisms has fallen on the edge of the wafer, and a signal for stopping the operation of the focus actuator is output to the focus actuator driving unit (FIG. 1). To stop the AF operation of the actuator (Z stage).

If step 102 is NO, step 1
Go to 04 and check if {(ab) + (a'-b ')} 2 = 0. If this is the case, the AF operation ends. In the case of NO, the process proceeds to 105, where the focus actuator is operated to adjust the position of the upper surface of the wafer to the focus position.

[0022]

As is clear from the above description, according to the present invention, when the test object is moved out of the automatic focus position detection range and returned to the detection range again, or when the test object is replaced, When a new focus position is automatically detected, an efficient operation for position detection is performed.

[Brief description of the drawings]

FIG. 1 is an optical system (knife edge method) of a semiconductor inspection device having an autofocus device according to one embodiment of the present invention.
FIG. 2 is a diagram schematically illustrating a configuration of a control system.

FIG. 2 is a plan view showing an example of a positional relationship between an observation field of view of the semiconductor inspection apparatus of FIG. 1 and an AF sensor mechanism (slit and AF optical system).

FIG. 3 is a view showing A on the photoelectric conversion means for AF in the apparatus of FIG. 1;
3 shows an image profile of an F slit.

FIG. 4 is a view showing A on a photoelectric conversion means for AF in the apparatus of FIG. 1;
3 shows an image profile of an F slit.

FIG. 5 is a flowchart for explaining a focus stop / restore operation in the autofocus apparatus of the present embodiment.

[Explanation of symbols]

Reference Signs List 1 light source 2 condenser lens 3 AF slit 4 projection lens 5 first half mirror 6 first objective lens 7 wafer 8 wafer stage 9 second half mirror 10 second objective lens 11 imaging lens 12 two-dimensional photoelectric conversion means 13 relay lens DESCRIPTION OF SYMBOLS 14 Mirror 15 Knife edge 16 Imaging lens 17 Cylindrical lens 18 Photoelectric conversion means for AF 19 Focus position calculation unit 20 Focus actuator drive unit 21 Focus signal determination unit 22 Control unit 31 Observation field of view 33 AF sensor mechanism

Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 21/30 526A

Claims (4)

[Claims] 1. An auto-focus device for focusing an optical system on an object; a plurality of sensor mechanisms for detecting a focus state; and a focus position by moving the object or the optical system in an optical axis direction. A focus actuator to be adjusted; a calculation unit for calculating a focus position based on focus signals from the plurality of sensor mechanisms; and whether or not the focus signals from the plurality of sensor mechanisms are in a predetermined specific state. An autofocus device comprising: a control unit including: a determination unit that determines: 2. The auto-focusing device according to claim 1, wherein the control unit stops the operation of the focus actuator according to the determination of the determination unit. 3. The autofocus device according to claim 2, wherein the control unit restores the operation of the focus actuator according to the determination of the determination unit. 4. The method according to claim 1, wherein the determining unit determines that the absolute value of the difference between the focus signals from the plurality of sensor mechanisms exceeds a predetermined value and deviates from a specific state.
The control unit stops the operation of the focus actuator, and the determination unit determines that the state has returned to a specific state when the absolute value of the difference between the focus signals from the plurality of sensor mechanisms is equal to or less than a predetermined value. 2. The auto-focusing device according to claim 1, wherein the control unit restores the operation of the focus actuator accordingly.