JPS6014163Y2 - Automatic focus mechanism in optical length measuring device - Google Patents

Description

[Detailed description of the invention] The present invention relates to an optical length measuring device, and in particular to an optical length measuring device that has been improved so that the object to be measured in the optical length measuring device can be easily focused and the length measurement accuracy can be improved. Regarding automatic focus mechanism.

In recent years, optical length measuring devices have been widely used to measure patterns on photomasks in the manufacture of electronic devices such as LSIs and ICs.

In other words, these optical length measuring devices form a transmitted light image of a photomask pattern as an object to be measured on a slit surface having an optical slit, and the formed light is received by a part of the optical monitor.
The optical slit is moved in a fixed direction through the transmitted light image of the pattern while being monitored, and the length of the photomask pattern is measured from the amount of movement of the optical slit corresponding to a change in the light reception level of a part of the light monitor. There is.

However, according to conventional optical length measuring devices, the formation of the transmitted light image on the optical slit surface of the photomask pattern is performed by the measuring operator's visual focusing, and the focusing accuracy depends on the quality of this focusing accuracy. This method has the drawback of being unstable, resulting in fluctuations in length measurement accuracy.

Therefore, an object of the present invention is to provide an automatic focusing mechanism for an optical length measuring apparatus that can eliminate the drawbacks of the conventional optical length measuring apparatus and perform stable and highly accurate length measurement.

It goes without saying that the automatic focusing mechanism in the optical length measuring apparatus according to the present invention can be applied not only to the length measurement of a photomask pattern but also to the length measurement of any object to be measured for which a transmitted light image can be obtained.

According to the present invention, a portion of the monitor receives light obtained by forming a transmitted light image of the object to be measured on an optical slit surface, and changes in the light reception level of the portion of the monitor corresponding to the movement of the optical slit are detected as described above. In an optical length measuring device that measures the length of a measured object from the amount of movement of the optical slit while monitoring with a part of the monitor, the optical slit is spaced apart from each other in the direction of movement, and the transmitted light image formed by the optical slit is spaced apart from each other in the direction of movement. two photoelectric converters forming two slits arranged in the region and outside the region and converting the light level of each transmitted light of the measured object received through the two slits into electrical signals; an arithmetic unit that calculates a calculation result by dividing the difference value between both electric signals indicating the light level detected by the photoelectric converter by an electric signal indicating one of the light levels; and a calculation unit that calculates the optical level of the transmitted light from the calculation result of the arithmetic unit. and a peak level detection unit that detects the maximum value of the difference, and by adjusting the focus to the focused point of the transmitted light image where the detection result of the peak level detection unit becomes the maximum value, the optical slit surface is The present invention provides an automatic focusing mechanism for an optical length measuring device, characterized in that an automatic focusing image is formed on the optical length measuring device.

Hereinafter, the present invention will be explained in more detail with reference to the accompanying drawings.

FIG. 1 is a mechanical diagram showing an embodiment in which an automatic focusing mechanism according to the present invention is applied to an optical length measuring device whose object to be measured is a photomask having a photomask pattern.

In FIG. 1, a photomask pattern 12 made of chromium oxide or the like is formed on a photomask 10 usually made of glass, and its width dimension 1 (W', for example) is precisely measured.

That is, when measuring the length, light emitted from a light source 14 provided, for example, above the top surface of the photomask 10 is applied to the photomask 10.
After passing through the objective lens 16 and the intermediate lens 18, it is reflected by the mirror 20, and the first optical slit 24
a, slit surface 22 with second optical slit 24b
project upwards.

At this time, by adjusting the objective lens 16, a transmitted light image of the photomask pattern 12 is formed on the slit surface 22 as an enlarged image 12a at a constant magnification.

This enlarged image 12a can be visually viewed by the length measuring operator through the mirror 28, 30 and the eyepiece 32.

Now, the slit surface 22 is movable in the direction of arrow T by an appropriate moving mechanism (not shown) such as a pulse motor, and the amount of movement thereof is configured to be detected by a length detector 26 consisting of a linear encoder or the like. Usually, a linear encoder is configured so that the amount of movement can be detected in units of 1/100 (microns).

Now, the width dimension W of the photomask pattern 12 is determined as follows.

That is, the first optical slit formed on the slit surface 22)2
4a or the second optical slit 24b as the slit surface 22 moves in one direction, for example, when the first optical slit 24a passes the enlarged image 12a along the left direction of the arrow T, The light level of the light transmitted through the first optical slit 24a and received by the light receiving monitor 36a is photoelectrically converted by the photoelectric converter 38a and detected as an electrical output signal Va. Since the relationship between the amount of movement Wa and the amount Wa shown in FIG. 2 is shown, (wa2W
By detecting the value of a1) using the length detector 26 described above, the width dimension W of the photomask pattern 12 can be measured.

Of course, by using the second optical slit 24b, the width dimension W of the photomask pattern 12 can be similarly measured from the amount of movement of the slit surface 22 corresponding to changes in the light level via the light receiving monitor 36b and the photoelectric converter 38b. It can be long.

When measuring the width W of the photomask pattern 12 as described above, the light reception level by the light reception monitor 36a or 36b is determined only when the enlarged image 12a is precisely focused on the slit surface 22. It shows a steep change as shown in Figure 2, and therefore the amount of movement of the slit surface 22 (
Wa2-Wa□) is detected as the only accurate value.

However, in the conventional optical length measuring device, only a single optical slit 24a or 24b is formed on the slit surface 22, and the enlarged image 12a that the length measuring operator visually observes through the eyepiece 32 is simply used as an object. Since a focusing mechanism was adopted by adjusting the lens 16, there was a natural difference in the focusing accuracy. Also, the change in light level obtained by the method cannot produce a sharp result as shown in FIG.

As a result, the amount of movement of the slit surface 22 is detected by the length detector 2.
Width dimension W of photomask pattern 12 obtained by detection in step 6
The accuracy of length measurement also had to be uneven.

Now, according to the present invention, by forming the first and second optical slits 24a and 24b on the slit surface 22, the enlarged image 12a of the photomask pattern 12 is precisely focused and focused on the slit surface 22. It became possible to visualize it.

That is, 1st. Since the second optical slits 24a and 24b are formed in advance to be spaced apart along the moving direction (arrow T) of the slit surface 22, for example, the second optical slits 24a and 24b are As shown in FIG. 1, one first optical slit 24a is placed on the enlarged image 12a, and the second
The slit surface 22 is moved so that the optical slit 24b is located at a position away from the area of the enlarged image 12a.
The light levels La, t, and b received at 36b are photoelectrically converted by photoelectric converters 38a and 38b, respectively, and taken out as electrical signals Va and Vb.

Next, these electric signals Va and Vb are inputted to the calculation section 40, and Va Vbx are calculated as Vb, Vb,
The calculation of Va is performed and the calculation result is input to the peak level detection section 42.

If the length measuring operator adjusts the objective lens 16 after constructing the above-described mechanism and detects when the output of the peak level detecting section 42 becomes maximum, the most precise magnification can be achieved on the slit surface 22. When the image 12a is formed, the difference in the level of light passing through the first and second optical slits 24at24b becomes maximum, so that precise focused imaging becomes possible automatically.

After completing the autofocus imaging of the photomask pattern 12 in this way, the width dimension W of the pattern 12 is detected by the length detector 26 using the first or second optical slits 24a and 24b as described above. This means that highly accurate length measurement results can always be obtained.

As is clear from the above explanation, according to the present invention, a precise and unique focused imaging state is automatically obtained, so there is no variation depending on the length measurement operator, and length measurement is always stable. It becomes possible to implement.

Therefore, by using the photomask 10 that has been judged pass/fail through such precision length measurement, it is possible to expect improvements in the quality of electronic devices such as LSIs and ICs.

Note that the first. Second optical slit 24a.

It goes without saying that the distance between the two optical slits 24b should be appropriately selected depending on the measured length of the object to be measured, and the two optical slits should be formed so that one optical slit can be located in each of the two regions having different amounts of light transmission.

Therefore, for example, in the case of a photomask pattern 12 that normally has a width dimension W of several microns, the first . The second optical slits 24a and 24b are slit surfaces 22
They may be formed so that they are spaced apart from each other by about 10 microns.

Furthermore, in the mechanism shown in FIG. If the peak level detecting section 42 uses a microcomputer device that has been widely used in recent years, it is possible to configure the automatic focusing mechanism according to the present invention without using a specific arithmetic circuit or a detecting section circuit.

[Brief explanation of the drawing]

FIG. 1 is a mechanical diagram showing one embodiment of an automatic focusing mechanism in an optical length measuring device according to the present invention, and FIG. 2 is a graph diagram illustrating changes in signal level obtained by a part of the same mechanism. In addition, in the figure, 10... photomask, 12...
...Photomask pattern, 14...Light source,
16...Objective lens, 22...Slit surface, 24at24b...1st. Second optical slit, 26...Length detection device, 32...
・Eyepiece lens, 36a, 36b・Song・Light reception monitor, 3
8a, 38b...Photoelectric converter, 40...
- Arithmetic unit, 42... Zeke level detection unit.

Claims (1)

[Claims for Utility Model Registration] A part of the monitor receives light obtained by forming a transmitted light image of the object to be measured on an optical slit surface, and a change in the light reception level of the part of the monitor corresponding to the movement of the optical slit. In an optical length measuring device that measures the length of an object to be measured based on the amount of movement of the optical slit while monitoring the length of the object using a part of the monitor, the optical slit is spaced apart from each other in the direction of movement of the optical slit, and the transmitted light image formed by the optical slit is two photoelectric converters having two slits arranged in a region and outside the region, and converting the light level of each transmitted light of the object to be measured received through the two slits into electrical signals; an arithmetic unit that calculates a calculation result by dividing the difference value between both electric signals indicating the light level detected by the two photoelectric converters by an electric signal indicating the light level of one; and a peak level detection section that detects the maximum value of the light level difference, and the optical slit is configured by adjusting the focus to the focal point of the transmitted light image where the detection result of the peak level detection section becomes the maximum value. An automatic focusing mechanism in an optical length measuring device, characterized in that an automatic focusing image is formed on a surface.