JPS6285211A - Automatic focus device for photomask microscope - Google Patents

Abstract

PURPOSE:To evade variation in focus when semiconductor laser light is on the boundary line between a chromium surface and a glass surface by providing a holding circuit in an automatic focus circuit which controls an automatic focus device and inhibiting forcibly an automatic focusing motor from operating when amplifiers are switched frequently. CONSTITUTION:The semiconductor laser light 1 is passed through a collimator 2 and then made to illuminate a beam splitter 3 and its reflected light is caused to illuminate a photomask 5 to be brought under focus control through an object 4. The light obtained by returning and passing the reflected light through the splitter 3 is split by a splitting mirror 6 into light beams to divided detectors 7 and 8, so that divided outputs are sent to a differential amplifier 9. Then, chromium and glass amplifiers 12a and 12b are connected as a circuit which controls the differential amplifier 9 to the detectors 7 and 8 and their outputs are applied to the holding circuit 16 and a motor driving circuit 17 through an amplifier selecting circuit 17. Thus, the circuit 17 is inhibited from operating when the gain value of the boundary between chromium and glass becomes unstable.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an automatic focusing device, and more particularly to an automatic focusing device for a photomask microscope.

(Prior art) Conventionally, a photomask microscope automatic focusing device uses a split detection optical system that uses a semiconductor laser as a light source, and an electric signal obtained from a semiconductor laser beam reflected from a chrome surface or a glass surface on a photomask. An autofocus circuit was originally provided that generated sequential control signals.

(Problems to be Solved by the Invention) The conventional photomask microscope automatic focusing device described above uses a semiconductor laser beam as a light source, but the beam diameter of the semiconductor laser beam on the photomask surface is in focus. In the case of about 5 scales, it became more than 10μ as the focus became blurred, so if the beam of the semiconductor laser light is on the boundary line between the chromium surface and the glass surface on the photomask surface when the focus becomes blurred, it is necessary to use split detection. The disadvantage of this method is that the electrical signals obtained from the two detectors are not stable and the autofocus oscillates.

Means for Solving Problem C] The photomask microscope automatic focusing device of the present invention shines a semiconductor laser beam on the boundary line between the chromium surface and the glass surface of the photomask, and uses the signal obtained from the detector for automatic focusing. A hold circuit that forcibly stops the up/down signal of the autofocus motor when the gain switching of the amplifier that amplifies the gain to the gain necessary to operate the motor is performed a predetermined number of times within a minute time. It is characterized by having the following.

Therefore, when the semiconductor laser beam is on the boundary line between the chrome surface and the glass surface, the autofocus is prevented from oscillating.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings. 6 Figure 1 is a block diagram of an optical system of an embodiment of the photomask microscope automatic focusing device of the present invention. However, a differential amplifier is also shown. It's been done.

Semiconductor laser light 1, which is turned on with continuous + and lt pulses, passes through collimator 2 and is irradiated from I and rear beam sub-interface 3 to objective lens 4. The reflected semiconductor laser light from the photomask 5 is divided by the splitting mirror 6 in proportion to the distance C, and the amount of the reflected semiconductor laser light that hits the split detector 97 and the split detector 8 is divided into the 0-split detector and the reflective semiconductor that hits the split detector 8. The difference in light intensity of the L laser beams is expressed as an electrical signal by the differential amplifier 9, and the position of the microscope focal point is automatically corrected so that the electrical output of the second differential amplifier 9 becomes 0.

FIG. 2 is a clock diagram of the autofocus circuit.9 The reflected semiconductor laser light is divided into two parts by a splitting mirror 6 in accordance with the focal position.
A laser beam is irradiated onto the divided detector 7.8. When it is in focus, the amount of light received by the split detector 7.8 is equal (
] However, if the focus is not correct, split detector 7.8
The amount of light received by is different. The amount of light irradiated by the divided detector 7.8C is converted into an electrical signal and sent to the preamplifier 11a.

11b, respectively. Chrome/glass amplifier 1
2a and 12b have two types of amplifier gains in order to obtain appropriate outputs when the semiconductor laser beam is on the chrome surface and for special numbers on the glass surface.

The amplifier gain values of the chrome/glass amplifiers 12a and 12b are determined by the amplifier selection circuit 13. The outputs of the chrome/glass amplifiers 12a and 12b are the same as those of the amplifiers 14a and 1.
After each of the light beams 4b and 4b is amplified, the comparator 15 generates an electric signal corresponding to the difference in the amount of light received by the divided detector 7.8. Based on the positive and negative signals of this electric signal, sequentially controlled automatic focusing is performed by the motor drive circuit 17. When the semiconductor laser beam is on the boundary line between the chrome surface and the glass surface, the signal of the amplifier selection circuit 13 becomes unstable in the chrome/glass amplifier 12a.

The +2b amplifier gain value is not constant and the up/down signal of the motor drive circuit 17 of the autofocus motor oscillates. The hold circuit 16 is a chrome/glass amplifier 12a,
Only when +2b gain switching occurs frequently, the up/down signal of the motor drive circuit 17 is forcibly held to prevent the focal position from moving.

Here, the hold circuit 16 forcibly (prevents) the signal of the motor drive circuit 17 when switching between the chrome amplifier 12a and the glass amplifier +2b is switched at least three times.
The switch is configured to be canceled if switching between the chrome amplifier 12a and the glass amplifier +2b is not performed for a certain period of time.

[The young fruits of invention]

As explained above, the present invention provides a halt circuit in the autofocus circuit to prevent the autofocus from vibrating when the semiconductor laser beam is on the boundary line 1 between the chrome surface and the glass surface. There are young fruits that can be produced.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an optical system of an embodiment of the photomask microscope automatic focusing device of the present invention, and FIG. 2 is a block diagram of the automatic focusing circuit. 1-11 laser beam, 2----collimator, 3-
Beam substitute, 4-objective lens, 5-m photomask, 6----
Split mirror, 7.8-m-split detector, 9-11 differential amplifier, 11a, l1b---preamplifier, 12a, l2b---chrome/glass amplifier, 13---
- Amplifier selection circuit, 14a, l4b---Amplifier, 15-11 converter, 16---Hold circuit, 17---Motor drive circuit.

Claims (1)

[Claims] Sequential control is performed using a split detection optical system using a semiconductor laser as a light source and electrical signals obtained by the reflected semiconductor laser light from the chrome surface of the photomask and the reflected semiconductor laser light from the glass surface. In a photomask microscope automatic focusing device having an automatic focusing circuit, a semiconductor laser beam is applied to the boundary line between the chromium surface and the glass surface on the photomask, and the signal obtained from the detector is used to generate the signal necessary to operate the automatic focusing motor. The present invention is characterized by comprising a hold circuit that forcibly stops the up/down signal of the automatic focusing motor when the gain switching of the amplifier that amplifies the gain is performed more than a predetermined number of times within a minute time. Photomask microscope automatic focus device.