JP2534699B2 - Optical axis direction position correction method in mask / reticle processing apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention is applied to a mask / reticle processing apparatus that uses light to measure the size of a mask or reticle and detects defects. Correcting the positional drift of the sensor, which serves as a reference for accurately controlling the positional relationship between the objective lens and the object to be processed, and changing the refractive index of air with changes in atmospheric pressure, temperature, humidity, etc. The present invention relates to an optical axis direction position correction method in a mask / reticle processing apparatus which is adapted to correct surface drift.

(Prior Art) Normally, in a mask / reticle defect inspection apparatus, as shown in FIG. 6, an object to be processed b such as a mask or a reticle mounted on an XY stage a is irradiated with light from a lamp c by an illumination lens d. The light is condensed and connected to a pattern portion (a pattern of an LSI to be inspected for defects) on the lower surface of the object to be processed b, enlarged by an objective lens e below the image part, and imaged on an image sensor f to obtain imaging data. . Then, the image pickup data and the design data are compared with each other to find a defect which is a difference between them. At this time, the distance between the object b to be processed and the objective lens e is a Z sensor, that is, a light emitting element (LD) fixed to the objective lens e.
g and the light receiving element (PSD) i to detect the piezo element j
The above distance is kept constant.

By the way, the positional accuracy required for the positional relationship of the focal planes of the object b and the objective lens e with respect to the detection accuracy of the conventional device is about ± 0.5 to ± 0.7 μm. The focal plane drift of the lens is sufficiently small, and no special correction is required for this accuracy,
For this reason, in the conventional apparatus, the method of performing the position drift correction and the focal plane drift correction is not taken at all.

(Problems to be Solved by the Invention) However, in recent years, when the pattern of the object to be processed b becomes fine and fine defects (about 0.3 to 0.5 μm) are detected, the magnification of the objective lens e is increased to 50 to 80 times. Then, it becomes necessary to form an image on the image sensor f.

The objective lens e having a large magnification has an extremely shallow depth of focus. For example, if the depth of focus is about ± 0.25 μm, the light emitting element (LD) fixed to the objective lens e as in the conventional case.
The light from g is applied to the pattern surface of the object b to be processed like the optical axis h of the Z sensor and is reflected, and the incident position on the light receiving element (PSD) i is always changed by the displacement in the Z direction when the XY stage a is moved. It is difficult to adjust within a focal depth of ± 0.25 μm in the long term by vertically adjusting the objective lens e by the piezo element j so as to enter the optimum value (this is called Z-axis correction).

This is because the objective lens e, the light emitting element g, the light receiving element i, and the members fixing them are thermally deformed due to the temperature change. For example, the room temperature where the apparatus is placed is ± 0.
Even if the temperature was controlled to 1 ° C, it was very difficult to keep the depth of focus within the above range.

Also, the focal plane drift due to the change in the refractive index of air is greatly affected by the atmospheric pressure fluctuation, and the atmospheric pressure fluctuation is usually about ± 25 mb, which is a serious problem.

Of course, if the depth of focus is deviated, it may cause a defect inspection error. That is, it is determined that there is a defect even if there is no defect (called a pseudo defect), and the reliability of the device is reduced.

The present invention has been made based on the above circumstances, and an object thereof is to keep the positional relationship between the focal plane position of the objective lens required by the apparatus and the pattern surface of the object to be processed within the depth of focus (± 0.25 μm). It is intended to provide a method of correcting the position in the optical axis direction in the mask / reticle processing apparatus, which can maintain the temperature of the mask / reticle processing device with higher accuracy.

[Structure of the Invention] (Means for Solving Problems) In order to solve the above problems, the present invention irradiates an object to be processed such as a mask or a reticle mounted on a stage with light and transmits the transmitted light. Is enlarged by an objective lens to form an image on an image sensor, and the dimension and defect of the pattern are detected from the imaged data, and the distance between the objective lens generated by the movement of the stage and the pattern surface of the object to be processed is measured in the optical axis direction. In a mask / reticle processing apparatus that measures changes with a Z sensor and corrects the relative positional relationship between the objective lens and the object to be processed, it is a part of the image drawn on the object to be measured or inspected and transmitted. A pattern image is obtained by observing the change in the electric signal obtained by the image sensor when light is projected on the boundary portion with a different rate and the object to be processed and the objective lens are relatively moved in the optical axis direction. Know that the focal plane position of the object lens are matched, in which to perform the positional relationship correction of the optical axis direction at this time resulting Z subsequent objective lens and the object to be treated based on the output of the sensor.

(Operation) That is, the present invention utilizes the fact that the output of the image sensor changes in accordance with the out-focus state of the boundary between glass and chrome, etc., of the pattern imaged on the image sensor, and thus the object to be processed and the objective lens are Positive relationship ± 0.2 ~ 0.4
Change by about μm, and store the point where the rise or fall of the image sensor output at the portion corresponding to the boundary between glass and chrome is the sharpest as the just focus point, and correct it to maintain the output of the Z sensor at this point. Therefore, as in the conventional case, the focal plane generated by incorporating the system in which the relationship between the positions of the light-emitting element, the light-receiving element and the mirrors on the optical axis fixed to the lens holder and the focal point of the objective lens is fixed is fixed. Positional deviation can be prevented, and higher precision processing can be performed.

(Embodiment) An embodiment of the present invention will be described below with reference to FIGS. 1 to 5.

FIG. 1 shows the entire structure, in which 1 is a surface plate, and the surface plate 1 is elastically supported on a base 2 via shock absorbers 3.

An XY stage 5 for holding an object to be processed 4 such as a mask or a reticle is mounted in the center of the upper surface of the surface plate 1, and an illumination system 8 having a lamp 6 and an illumination lens 7 is provided above the XY stage 5. Are attached via a support bracket 9.

Further, an objective body 12 incorporating an objective lens 10 and an image sensor 11 is provided in a state of penetrating the surface plate 1 and the upper base portion 2a of the base 2. The objective body 12 is configured to be vertically movable by a piezo element 13.

Further, the objective lens 10 is provided above the objective body 12.
In addition to the optical system 14 including the image sensor 11, the light emitting element (LD) 15, the mirror 16, the mirror 1 as shown in FIG.
An optical system (Z sensor) 21 including a mirror 18, a mirror 19, and a light receiving element (PSD) 20 is incorporated. The optical system 21 is in a state of being surrounded by a casing 22 having an opening 22a in a portion facing the objective lens 10.

Then, the light emitted from the light emitting element 15 is reflected by the mirrors 16, 1
The light is guided to the object to be processed 4 via 7 and is incident on the point a of the light receiving element 20 via the reflected light mirrors 18 and 19 from the object to be processed 4. This state corresponds to the focal plane position of the objective lens 10 and the pattern surface of the object to be processed 4.

Next, when the workpiece 4 fixed on the stage 5 moves, the positional relationship between the objective lens 10 and the workpiece 4 changes due to the vertical movement of the stage 5 or the flatness of the workpiece 4. It is detected by the position change of the incident light on the light receiving element 20.

Therefore, the feed voltage to the piezo element 13 is changed and adjusted so that the light always enters the same position, that is, the position a. However, in reality, even if the positional relationship between the object to be processed 4 and the objective lens 10 does not change, the mirror 19 is thermally deformed to the position 19 'shown by the chain double-dashed line as shown in FIG. When the position becomes a ', the objective lens 10 is driven by the piezo element 13 and adjusted so as to be incident on the point a, and then the focus position is deviated.

Further, even if the relative positional relationship of the constituents does not change, if the atmospheric pressure changes or the wavelength of the light source changes, the focal plane position of the objective lens shifts.

If this is within the depth of focus of the objective lens 10, there is no practical problem, but if the magnification of the objective lens 10 is high, the depth of focus becomes extremely shallow, which becomes a problem.

Therefore, in the present invention, this problem is solved by adding a method for automatically detecting a point where the pattern surface of the object to be processed 4 and the focal plane position of the objective lens 10 coincide with each other without significantly changing the existing system. I am trying to solve it.

This method will be described in detail. When the illumination light 23 is projected onto the boundary surface between the chrome surface 24 and the glass surface 25 from above the object 4 to be processed as shown in FIG.
Then, it is incident on the image sensor 11. It should be noted that FIG. 3 does not show the enlarged view but shows it as one-to-one.

The image sensor 11 includes a plurality of elements 11a, 11b,
11c ... 11d, each of which can individually obtain an electric signal corresponding to the amount of incident light.

In this embodiment, this electric signal is A / D converted to obtain a digital signal at a level of 0-64. The obtained data are plotted in FIGS. 4 and 5. That is, the level of light passing through the glass surface 25 is 64, and the level of light passing through the chrome surface 24 is 0. Then, the intermediate portion becomes data between 0 and 64, and when the pattern surface of the object to be processed 4 comes to the focus position, the rising or falling curve becomes steep as shown in FIG. 4, and when it deviates, as shown in FIG. It becomes a gentle curve.

Therefore, the positional relationship between the object to be processed 4 and the objective lens 10 is positively moved up and down in a minute range by using the piezo element 13, and the data obtained as shown in FIG. 4 or FIG.
64 level) is loaded into the CPU (not shown) and the slope is calculated. Then, it is moved up and down in a minute range (for example, a range of ± 0.5 μm at intervals of 0.1 μm), and when it becomes the steepest, it is incident on the position 20 ′ of the light receiving element 20, and the output corresponding to the position is received. From the piezo element so that the reflected light is incident on the position a ′.
Control the power supply voltage to 13.

This adjustment is detected at regular intervals (for example, every 10 minutes, every 30 minutes, every time the object to be processed is exchanged, when there is a constant atmospheric pressure change or a wavelength change of the light source, etc.), and the light of the objective lens 10 is detected. If the axial position is corrected, the pattern surface of the object to be processed 4 can always be held within the depth of focus range of the objective lens 10.

Therefore, even if the Z sensor position for measuring the relative position variation between the object to be processed 4 and the objective lens 10 drifts or the focal plane position of the objective lens changes, the objective lens is always automatically corrected to the optimum value at regular intervals. The pattern surface can be held within the depth of focus of 10.

In terms of hardware, conventional ones may be used for both mechanical and electrical, and software for obtaining the inclination of the curve of the sensor signal and software for comparing the data up and down the minute range may be added.

As another method, a rising or falling curve of the image sensor 11 when the pattern surface obtained in advance and the focal plane position of the objective lens 10 match, and the objective lens 10 can be obtained by moving the objective lens 10 using the piezo element 13. There is a method in which the obtained curve is compared, and when the difference becomes equal to or less than a certain value, the reference (coincidence point) is used for subsequent correction. Further, the mask / reticle defect inspection apparatus has been described, but it goes without saying that it can be applied to a measurement apparatus.

In addition, it goes without saying that the present invention can be variously modified and implemented without departing from the spirit of the present invention.

As described above, according to the present invention, the positional relationship between the focal plane position of the objective lens required by the apparatus and the pattern surface of the object to be processed can be always maintained within the depth of focus. It is possible to provide an optical axis direction position correction method in the mask / reticle processing apparatus described above.

[Brief description of drawings]

1 to 5 show an embodiment of the present invention.
FIG. 1 is a schematic configuration diagram of a mask / reticle defect inspection apparatus, FIG. 2 is an enlarged view of a main part, and FIG. 3 illustrates a state in which light is irradiated onto an object to be processed and is incident on an image sensor through an objective lens. FIGS. 4 and 5 are diagrams in which the obtained electric signals are digitized to levels of 0 to 64 and plotted, and FIG. 6 is a schematic configuration diagram of a conventional device. 4 ... Object to be processed (mask or reticle), 5 ... Stage, 6 ... Lamp, 10 ... Objective lens, 11 ... Image sensor, 11a to 11d ... Light receiving element, 13 ... Piezo element,
15 …… Light emitting device (LD), 16 to 19 …… Mirror, 20 …… Light receiving device (PSD), 21 …… Optical system (Z sensor), 24 …… Chrome surface, 25 …… Glass surface.

Claims (4)

(57) [Claims] 1. An object to be processed such as a mask or a reticle placed on a stage is irradiated with light, the transmitted light is enlarged by an objective lens to form an image on an image sensor, and the pattern size and A mask / reticle processing apparatus that detects a defect and measures the distance variation in the optical axis direction between the objective lens and the pattern surface of the object to be processed with a Z sensor and corrects the relative positional relationship between the objective lens and the object to be processed. , The light is projected on the boundary part of the image drawn on the object to be measured or inspected, which has a difference in transmittance, and the object and the objective lens are relatively moved in a small range in the optical axis direction. At that time, by observing the change in the electric signal obtained by the image sensor, the point where the pattern image coincides with the focal plane position of the objective lens is known, and the output of the Z sensor obtained at this time is used as a reference for the subsequent objective laser. The optical axis direction position correcting method in the mask / reticle processing device and performing's and positional relationship correction of the optical axis of the object. 2. An optical axis in a mask / reticle processing apparatus according to claim 1, wherein the image sensor comprises a plurality of light receiving elements, and when receiving light, an electric signal according to the amount of received light is obtained. Directional position correction method. 3. As a method of detecting a point where the pattern surface and the focal plane position of the objective lens coincide with each other, a point at which the rise or fall of the electric signal of the image sensor due to the difference in the transmittance of the image on the pattern surface becomes the steepest or its point. The optical axis direction position correcting method in the mask / reticle processing apparatus according to claim 2, wherein the method is in the vicinity. 4. A method for detecting a point where the pattern surface and the focal plane position of the objective lens coincide with each other, which is obtained by comparing an electric signal obtained from the image sensor with a known electronic signal of the image sensor at just focus. An optical axis direction position correction method in a mask / reticle processing apparatus according to claim 2.