JPH02660B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a device for detecting foreign matter on the surface of photomask substrates and dry plates, which are materials for LSI photomasks. The recent progress and development of semiconductor manufacturing technology, with VLSI at its peak, has been remarkable. Among the various factors that brought about progress and development, the functions of semiconductors,
The quality of photomasks has become extremely important as it has a significant impact on manufacturing yield. Therefore, the requirements for photomasks are defined in terms of both high integration and precision through fine patterns and high quality with fewer defects, and it is not uncommon for photomasks to exceed the limits of manufacturing equipment. Under these circumstances, when looking at mask manufacturing as a whole, inspections that actually measure and guarantee quality and accuracy are becoming increasingly important. By the way, pinhole defects in a blank mask patterned with chromium etc. on a glass substrate are
This affects the yield of devices such as ICs and bubbles, and requires highly accurate non-destructive full-scale inspection. Therefore, when a laser beam spot with good coherence, directivity, and high brightness is irradiated onto the pinhole portion of a blank mask, the intensity and spatial distribution of the reflected light diffracted light change depending on the degree of defects. Taking advantage of this property, a method is used in which the entire surface of a mask is inspected by sequentially moving a laser beam spot on a blank mask as shown in FIG. In Figure 1, 1 is a helium-neon (He-Ne) laser with a wavelength of 6328 Å, 2 is a scanning mirror, 3 is a scanning lens, 4 is a substrate, 6 is a resist, 7 is a moving table, and 8 and 8' are defects. This is a photoelectric detector consisting of a PIN silicon photodiode that detects scattered reflected light from the By the way, a photomask is manufactured through the following process. Chromium (Cr) on a transparent substrate such as glass,
After depositing a thin film such as chromium oxide (Cr ₂ O ₃ ),
A photoresist is spin-coated onto the substrate and dried.
The substrate coated with the resist is called a dry plate. Next, a photomask is placed on the dry plate and exposed to ultraviolet light. After exposure, development is performed using a developer to perform patterning. Next, using the resist as a mask, the exposed portions of the thin film of Cr, Cr ₂ O ₃ , etc. are removed by etching, and then the remaining resist layer is removed to complete the photomask. By the way, photoresist is applied at a relative humidity of 20
% or less using a spinner in a nitrogen atmosphere in a dust-free box. The work room is class 1000
The inside of the clean bench is kept clean to below 100, and the outside environment is also kept clean at all times. Apply photoresist adjusted to a viscosity of 25 centipoise for approximately 30 seconds at 5000 revolutions per minute. Next, pre-baking is performed at 80℃ for 30 minutes while being irradiated with an infrared lamp or purged in a nitrogen atmosphere using an electric oven, and the organic solvent used for viscosity adjustment is evaporated, resulting in a film thickness of approximately 7000Å on the substrate. A photoresist layer is formed. The surface of the photoresist layer on the substrate formed in the above process has a structure in which macroscopic undulations C as shown in FIG. 2A are generated during spin coating or evaporation of a solvent or the like. Figure 2a
This is a dry plate in which a chromium layer 5 is deposited on a glass substrate 4, and a resist layer 6 is applied thereon. A is a large foreign object, and B is a small foreign object. Conventionally, when inspecting the surface of photomask substrates and dry plates, which are the materials of photomasks, a device as shown in Fig. 1 is used to direct the laser beam from a laser 1 through a scanning mirror 2 and a scanning lens 3.
The sample surface is scanned and the surface inspected by focusing on the spot. FIG. 2b shows an output signal of scattered reflected light from the dry plate generated on the photoelectric detector when the dry plate shown in FIG. 2a is scanned from the left end to the right end with a laser beam. Since the photoresist layer has large undulations as shown in C in FIG. 2A, the photodetectors 8 and 8' detect scattered reflected light. It was found that the noise due to the waviness of the resist had a signal amplitude of 7 to 8 mV and a signal width of about 20 .mu.s, as shown by C' in FIG. 2b. If there is a large foreign object such as A in FIG. 2a in the resist, the irradiated laser beam is strongly scattered by the large foreign object A, and the photoelectric detector detects strongly scattered light such as A'.
Conventionally, if the amplitude of the signal was 10 mV or more, it was detected as a foreign object. However, if there is a small foreign object on the resist like B in Figure 2a, the irradiated light is scattered by the small object, but the intensity of the scattered reflected light is weak, so the signal amplitude is less than 10 mV as shown in B'. The signal amplitude is 7 to 8 mV due to the large waviness on the surface of the photoresist layer.
The noise overlapped with the noise C', making it impossible to detect small foreign objects. An object of the present invention is to provide an amplitude discrimination circuit and a pulse width discrimination circuit when inspecting the surface of a dry plate by scanning the surface of the dry plate with a laser beam and detecting scattered reflected light from the dry plate with a photoelectric detector. It is characterized by distinguishing between signals from foreign objects and noise from resist by performing level determination on both amplitude and signal width. Examples of the present invention will be described in detail below. The smaller the foreign matter that is present due to dust adhering to the surface of the dry plate, the more the signal value will be the same as the surrounding noise area, but the size of the foreign matter and the characteristics of the surrounding noise determined through experiments are as shown in the table. It was hot.

[Table] In order to detect foreign objects based on the above characteristics, the flowchart shown in FIG. 3 is applied, level judgment is performed on both amplitude and signal width, and noise signals are removed. Discriminates the wave height of the input signal from the photoelectric detector.
That is, it compares whether the input signal is 10 mV or more, and if it is 10 mV or more, it is output as a large foreign object. If the input signal is less than 10 mV, the input signal is subjected to pulse width discrimination. That is, if the input signal is
If it is 3 μsec or less, it is output as noise. For this purpose, the circuit shown in FIG. 4 is used. Input signal 1 from photoelectric detector to comparator 2
It is divided into those with an amplitude of 10 mV or more and those with an amplitude of less than 10 mV. If the amplitude is 10 mV or more, the output circuit 3 outputs a signal as large as a foreign object. If the amplitude is less than 10 mV, the waveform is shaped to some extent by a shaper 4, and then input to an AND gate 6 having two inputs. A 1MHz reference signal is inputted to the AND gate 6 from the reference oscillator 5. The signal width from AND gate 6 is compared with the set value by digital comparator 7, and the set value is
If the time is 3 μs or less, the output circuit 3 is made to output a signal indicating a small foreign object, and if the set value is 15 μs or more, the output circuit 3 is made to output a noise signal. As is clear from the above explanation, in the signal processing system,
By using the circuit of the present invention, which includes a pulse height discrimination circuit and a pulse width discrimination circuit, it is possible to separate small foreign objects, which could not be discriminated in the past, from noise caused by resist waviness. It has become possible to detect even foreign objects.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of a surface foreign matter detection device using laser light, Figure 2 a is a cross-sectional view of a dry plate, Figure 2 b
3 shows a change in the output of scattered reflected light corresponding to FIG. 2a obtained by scanning a laser beam, FIG. 3 is a signal processing circuit of the present invention, and FIG. 4 is a flowchart explaining the circuit of FIG. 3. 1: He-Ne laser, 2: Scanning mirror, 3: Scanning lens, 4: Substrate, 6: Resist, A: Large foreign matter on resist, B: Small foreign matter on resist, C: Waviness on resist surface, 7: Moving table, 8: Photoelectric detector.

Claims (1)

[Scope of Claims] 1. A laser light source that scans the surface of a dry plate with a laser beam, a scanning optical system, a photoelectric detector that detects scattered reflected light from the dry plate, and a first input signal from the photoelectric detector. a first and second comparator that discriminates the wave height between a signal that is above a threshold value and a signal that is below the first threshold value and above a second threshold value; and a shaper that shapes the waveform of the amplitude-discriminated signal that is below the first threshold value. an AND gate into which the signal shaped by the shaper and a reference signal from a high-frequency reference oscillator are input; a digital comparator that compares the signal width from the AND gate with a set value to discriminate pulse width; A signal from the first comparator that is equal to or greater than the threshold value is outputted as a large foreign object, a signal from the digital comparator that is less than or equal to the first set signal width is outputted as a small foreign material, and a signal that is equal to or larger than the second set signal width is outputted as a resist waviness. What is claimed is: 1. A surface foreign matter detection device, comprising: an output circuit that outputs an output signal.