JP2000155409A - Method for correcting photomask - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct a photomask on a substrate with <=1 μm precision without damaging the substrate and to remove black defects. SOLUTION: When a photomask consisting of a thin chromium film 2 and a chromium oxide film 3 formed on a substrate 1 is corrected by laser beam machining, the photomask is irradiated with laser light having 3-16 ps pulse width. The wavelength of the laser light is preferably 600-1,100 nm. When the photomask is irradiated with laser light having 3-16 ps pulse width, the thin chromium film 2 and the chromium oxide film 3 are selectively machined without damaging the substrate 1 and a machined part having a sharp edge is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for correcting a black defect of a photomask used for patterning an LSI, a liquid crystal or the like.

[0002]

2. Description of the Related Art A circuit having a required structure is written in an ultrafine component such as an LSI or a liquid crystal by photoetching. In photoetching, the surface of a workpiece is covered with a photomask and exposed according to a predetermined pattern.
As a photomask, a metal thin film such as chromium and titanium formed on a quartz glass substrate, and a chromium oxide laminated on a chromium thin film for corrosion prevention are generally used. In the following description, a “metal thin film” is used in a meaning including an oxide film. Photomasks are generally manufactured by depositing a metal thin film on a quartz substrate, applying a resist, performing pattern exposure using an electron beam or the like, and chemically etching the metal thin film. Black defects are likely to occur due to the non-uniformity of If the photomask has a black defect, the portion to be photoetched is not irradiated with light, which may cause a circuit defect in an LSI, a liquid crystal or the like. Therefore, it is necessary to correct the black defect. In general, a laser is used to repair the photomask. When the laser is irradiated, the metal thin film in that portion is heated and evaporated to remove black defects. The normal correction accuracy is about 1 μm, but a higher correction accuracy is required in accordance with a tendency to require a more complicated and fine circuit configuration.

In order to improve the correction accuracy, a method of making the intensity distribution of the laser beam uniform and performing uniform correction by slit transfer processing (Japanese Patent Application Laid-Open No. 56-164345),
A method of precisely controlling the width of the slit to improve the correction accuracy (Japanese Patent Laid-Open No. 3-27042) is introduced. The correction includes Nd generating a wavelength of 1064 nm:
YAG, Nd: YVO and the like are used as typical lasers, and the second harmonic of 532 nm and the third harmonic of 355 nm are used.
In some cases, harmonics are used. Nd: YAG, Nd: Y
Laser light such as VO is pulse-driven by a Q switch and has a pulse width of several hundred ps to several tens ns and is processed by two or more pulses (NEC Technical Report 1977, Vol. 50, 3-1).
1 page).

For repairing a photomask, an apparatus (FI) for repairing a photomask using a focused ion beam is used.
B apparatus) has been developed. In the FIB apparatus, ions such as Ga are accelerated and irradiated on a photomask placed in a vacuum chamber to remove black defects. The FIB exhibits a high correction accuracy of 0.5 μm or less as compared with the conventional laser correction accuracy, and thus is suitable for correcting a photomask for LSI fine wiring. For example, in JP-A-7-28227, Cr on a substrate is selectively processed according to this method. In addition, a femtosecond pulse laser using Ti sapphire crystal or the like has been developed, and commercialized by Coherent Inc., Spectra Physics, Inc. in the United States [Spectra Physics Product Catalog, page 10 (1998)
Year)]. An ultrashort pulse laser can emit light with a pulse width on the order of femtoseconds when using a wavelength conversion system.
It is possible to oscillate in a wide wavelength range of nm to 3 μm.

Research on material processing using an ultrashort pulse laser has been conducted in LHZ and the like in Germany (Applied Physics A, 1996, 63: 106-110), and glass, silica, enamel, stainless steel, aluminum,
Examples of processing materials such as copper, steel, silicon, and aluminum nitride have been reported. When irradiating with an ultrashort pulse, if the pulse width is shorter than the heat conduction time of the material that absorbed the laser light, the material in the irradiated part evaporates instantaneously without the laser energy propagating around the irradiated part. . As a result, it is possible to perform processing while suppressing thermal deterioration.

[0006]

The laser currently used for repairing a photomask is a laser of the order of ns pulse, and it is difficult to repair it with an accuracy of 1 μm or less. In other words, with conventional lasers, the absorbed energy propagates to the surroundings due to heat conduction, which inevitably causes thermal deterioration around the processed part, and melts the metal thin film that could not be ablated and melts inside and around the processed part. Will solidify. Therefore, the edge of the processed portion is not sharpened, and the correction accuracy is limited to about 1 μm. Further, a part of the molten metal solidifies and covers the processed bottom surface, thereby lowering the transmittance of the glass. In order to improve the accuracy of the correction processing to 1 μm or less and not reduce the transmittance, the influence of heat on the surroundings irradiated with the laser is suppressed as much as possible to prevent melting of the metal thin film and to sharpen the edge of the processed part. Is required.

[0007] Even in the case of the FIB apparatus having a high correction accuracy, the quartz substrate is easily damaged, and it is necessary to etch the glass after correcting the photomask. Moreover,
Since the sample needs to be set in the vacuum chamber, it takes time to perform the processing before and after the correction, the apparatus cost and the running cost are high, and the apparatus size has to be increased. In this regard, there is a demand for a method of repairing a photomask at normal pressure without damaging the substrate.

On the other hand, when a metal thin film is processed in an air atmosphere using an ultrashort pulse laser, a processing residue is deposited in a conical shape having a diameter of several tens to several hundreds nm and a height of about 100 nm on a laser irradiated portion. The deposition of processing residues is caused by the fact that the pulse width is shorter than the heat conduction transmission time, so that the intensity distribution of the laser beam generated by interference or the like is not uniformized due to heat conduction, and a non-ablated portion remains in a mountain shape. . That is, the mountain-shaped tapered portion is not processed because the angle with respect to the incident laser is increased and the laser light is reflected. Since the ablated metal is deposited on this, a conical residue is grown upward. In order to repair a photomask with an ultrashort pulse laser without using a vacuum chamber, it is necessary to remove conical residues. Furthermore, when processing a photomask in which a metal thin film of a different material is integrated on a glass substrate with an ultrashort pulse laser, both the glass substrate and the metal thin film are simultaneously processed because the laser power density is extremely high. It is very difficult to selectively ablate only a metal thin film. In this regard, it is desirable to increase the difference between the processing thresholds of the substrate and the metal thin film to selectively process the metal thin film.

[0009]

SUMMARY OF THE INVENTION The present invention has been devised to meet such a demand, and the use of an ultra-short laser with a regulated pulse width for repairing a photomask makes it possible to reduce the atmospheric pressure. The metal thin film constituting the photomask is selectively ablated without damaging the quartz glass, etc., which is the substrate of the photomask in the above, and is subjected to 1 μm.
An object is to correct a photomask with a correction accuracy of m or less. According to the present invention, in order to achieve the object, when a photomask made of a metal thin film provided on a substrate is modified with a laser beam, the metal thin film is selectively processed by irradiating a laser beam having a pulse width of 3 to 16 ps. Then, a black defect of the photomask is removed. The wavelength of the laser light is 600
Those in the range of 〜1100 nm are preferred. When a photomask composed of a chromium thin film and a chromium oxide film having a total thickness of 30 to 120 nm provided on a quartz glass substrate is modified by laser processing, an energy density is 4 to 5 J / cm ² , a pulse width is 3 to 16 ps, and a wavelength is 600. When one laser pulse of １1100 nm is irradiated, the photomask is corrected with an accuracy of 1 μm or less.

[0010]

When the pulse width of the ultrashort pulse laser is gradually increased, the heat conduction distance per pulse time in the irradiated metal thin film gradually increases. When the heat conduction distance is about the period of the intensity unevenness of the laser power, the metal thin film can be ablated without processing residues. However, with an excessively long heat conduction distance,
The edge of the processed part melts due to the thermal effect. The present inventors have found that by selecting a pulse width under such a premise, processing residues and melting are suppressed, and processing with sharp edges and corners becomes possible. With a quartz glass substrate, the Cr thin film on the substrate can be ablated with a correction accuracy of 1 μm or less without processing residues by adjusting the pulse width of the ultrashort pulse laser between 3 and 16 ps.

In order to ablate a metal thin film with an ultrashort pulse laser without damaging the substrate,
It is necessary to increase the difference between the ablation thresholds of the substrate and the metal thin film. The ablation threshold is related to the absorption properties of the material, which is wavelength dependent. Specifically, the greater the difference in absorption coefficient for a certain wavelength, the greater the difference in ablation threshold. For example, in quartz glass used as a substrate, the absorption edge of the absorption characteristic is about 200 nm, and 200 nm to several μm.
There is almost no absorption in the range of m. On the other hand, chromium used as a metal thin film and chromium oxide laminated on the chromium thin film show a flat absorption characteristic of about 30% in the range of 200 to 1100 nm, and have a reflectance of nearly 100% at a wavelength exceeding 1100 nm. Therefore, the absorption approaches zero.

Considering only linear absorption, quartz glass and C
The difference between the absorption coefficients of the r thin films is almost the same in the range of 200 to 1100 nm, but irradiation with an ultrashort pulse laser having a very large peak power is affected by multiphoton absorption. For example, when quartz glass is irradiated with light of 400 nm, two photons are absorbed, and at 600 nm, three photons are absorbed, and the absorption coefficient increases. Therefore, if the wavelength used for laser processing is set to be between 600 nm or more where three-photon absorption does not occur in glass and 1100 nm which shows an absorption coefficient of 30% in a chromium thin film, the absorption coefficient including the multiphoton absorption of quartz glass and the chromium thin film is set. Can be increased, resulting in a larger ablation threshold difference. That is, the wavelength is set to 600 to 11
By using a laser adjusted to the range of 00 nm, a chromium thin film formed on a quartz glass substrate can be selectively processed and damage to the substrate can be suppressed.

When correcting the photomask, if the number of laser pulses is increased, there is a strong tendency that the processing accuracy is reduced even by a slight fluctuation of the beam. This type of reduction in processing accuracy can be prevented by adjusting the energy density at which a 80-120 nm-thick chromium thin film provided on a quartz glass substrate, which is a general photomask specification, can be ablated with one pulse. . Specifically, the pulse width is 3 to 1
The chromium thin film can be processed with one pulse by adjusting the energy density per pulse to 6 to 5 J / cm ² with 6 ps, wavelength from 600 to 1100 nm, and correction accuracy to 0.5 μm or less. It is possible to do.

[0014]

Embodiment 1 In this embodiment, as shown in FIG. 1, a 100-nm-thick chromium thin film 2 is provided on a quartz glass substrate 1 having a thickness of 2 mm, and a chromium oxide film 3 having a thickness of 20 μm is further provided.
Was used as a processed sample. The laser processing has an oscillator amplifier system and a harmonic generation system using Ti sapphire crystal as an oscillation material, and has a wavelength of 266 to 2000 nm and a pulse width of 120 fs.
100 ps light is repeated at 1 kHz and average output is 100 ~
An apparatus capable of outputting 800 mW was used. As shown in FIG. 2, a beam having a diameter of 8 mm emitted from the laser
Adjust to 0 nm, and adjust the beam diameter with lens pair 5 to 3 to 30 n.
m was adjusted to an appropriate size. After adjusting the laser power with the attenuator 6, only the central portion having good beam quality was transmitted through a square slit of 0.5 mm square, and
The light was introduced into a zero-magnification objective lens 8 and irradiated onto a photomask 9 arranged at an image forming position.

The irradiation conditions were as follows: the transfer pattern size was adjusted to ² μm square, the irradiation energy density was adjusted to 1.2 J / cm ² , and the pulse width was changed in the range of 120 fs to 100 ps, and four pulses were irradiated. . Pulse width 3-16ps
As shown in FIG. 3, there was no processing residue on the surface 10 of the quartz glass substrate 1 to which the laser was transferred, and a sharp 2 μm square pattern was formed on the processing edge. In the correction of the photomask, the sides and corners of the transfer pattern are irradiated on the black defect portion of the metal thin film and removed, so that a black defect having a size smaller than the pattern can be corrected. Incidentally, the pattern of FIG. 3 could be corrected with an accuracy of 1 μm or less.

Comparative Example 1: Pulse width of 120 fs to 3 ps
4A, laser processing was performed under the same conditions as in Example 1. As shown in FIG. 4A, conical processing residues 11 having a diameter of several μm and a height of about 10 to 100 nm were found in and around the transfer pattern. Was observed. Since the transmittance of the quartz glass substrate 1 was reduced by about 30% due to the processing residue 11, the pulse width 12
At 0 fs to 3 ps, the photomask could not be corrected. When laser processing was performed while changing the pulse width to 16 to 100 ps, as shown in FIG. 4B, a residue 12 which was once melted and then solidified occurred at the bottom of the processing hole. Since the transmittance of the quartz glass substrate 1 was reduced by about 20% due to the melting and solidification residue 12, the pulse width of 16 to 10 was required for correcting the photomask.
The 0 ps laser could not be used. In this case, sagging further occurred at the edge of the processed portion, and a correction accuracy of 1 μm or less could not be obtained.

Embodiment 2: Irradiation energy density is 1.2 J /
cm ² , the pulse width is fixed to 16 ps, the number of pulses is fixed to 4
The photomask was laser-processed in the same manner as in Example 1 except that the wavelength was changed in the range of 600 to 1100 nm. Under this condition, the chromium thin film 2 on the quartz glass substrate 1 is not damaged at all.
And the chromium oxide film 3 was selectively ablated, and the correction accuracy was 1 μm or less.

Comparative Example 2: When a photomask was processed under the same conditions as in Example 2 except that the wavelength was changed in the range of 400 to 600 nm, the quartz glass of the transfer pattern was damaged as shown in FIG. 13 occurred. Damage 13 is caused when a part of the quartz glass is ablated due to multiphoton absorption and becomes non-smooth.
Was reduced. Therefore, the energy density is set to 0.5
When processing was carried out at J / cm ² , similar damage occurred after 8 irradiations. The occurrence of damage can be prevented by further lowering the energy density. However, in order to process the chromium thin film 2 and the chromium oxide film 3 into the required patterns, eight or more pulses are required. Specifically, 1 laser pulse energy density 0.4 J / cm ²
When 6 shots were applied, the chromium thin film 2 and the chromium oxide film 3 were ablated, but the shift 14 of the transfer pattern was unavoidable, and the processed edge was blurred as shown in FIG. 5B.

From the above results, it was found that the photomask could not be corrected with an accuracy of 1 μm or less at a wavelength of 400 to 600 nm. Further, when processing was performed with the wavelength adjusted to 1100 to 1500 nm under the same conditions as in Example 2, the chromium thin film 2 and the chromium oxide film 3 were hardly processed by four irradiations with an energy density of 1.2 J / cm ² , The chromium thin film 2 and the chromium oxide film 3 could not be completely ablated by eight irradiations at an energy density of 1.5 J / cm ² . Although the ablation processing could be performed by irradiation of about 32 shots, the pattern deteriorated as in FIG.
At a wavelength of 100 to 1500 nm, a processing accuracy of 1 μm or less could not be obtained.

Embodiment 3: A pulse width of 3 ps and a wavelength of 80
The photomask was processed under the same conditions as in Example 1 except that the energy density was changed by fixing the pulse number to 0 nm and the number of pulses to one, and without damaging the quartz glass substrate 1, the chromium thin film 2 and the chromium oxide film The conditions under which 3 were completely ablated were investigated. As a result, the energy density becomes 4 ~
When it was set to the range of 5 J / cm ² , processing could be performed with one pulse, and as shown in FIG. 6A, a pattern in which the edge 15 of the processed portion was sharply formed at almost 90 degrees was formed.
When we tried to repair the photomask by one irradiation,
The correction accuracy was 0.5 μm or less, which was almost the same value as the processing using the FIB apparatus. Comparative Example 3 A photomask was processed under the same conditions as in Example 3 except that the energy density was 4 J / cm ² and irradiation was performed twice. The edge 16 formed by the processing had a large taper as shown in FIG. 6B, and the correction accuracy was 0.7 μm.

[0021]

As described above, according to the present invention, when the photomask is modified by laser processing, the sharp edge is adjusted by adjusting the pulse width of the laser beam to be used in the range of 3 to 16 ps. The machined part is formed, and correction with an accuracy of 1 μm or less is enabled. Moreover, the photomask can be corrected with high precision without requiring a vacuum chamber and without damaging the substrate. Since the photomask modified in this way has an extremely high-precision pattern, it is suitable for the manufacture of LSIs, liquid crystals, and the like, for which demands for higher density and higher integration are increasing.

[Brief description of the drawings]

FIG. 1 Cross-sectional structure of chrome photomask

FIG. 2 Photomask repair device used for laser processing

FIG. 3 is a processing pattern of a chrome photomask when a laser beam having a pulse width of 3 to 16 ps is irradiated.

FIG. 4 shows a processing residue (a) and a melting and solidification residue (b) generated when processing is performed with a laser beam having a pulse width of 3 to 16 ps.

FIG. 5 is a diagram showing damage (a) and a transfer pattern with sagging (b) caused by processing with laser light having a wavelength of 400 to 600 nm.

FIG. 6 shows an edge (a) of a sharp machined portion formed by one laser pulse and a tapered edge (b) formed by two laser pulses.

[Explanation of symbols]

1: quartz glass substrate 2: chromium thin film 3: chromium oxide film 4: laser light source 5: lens system 6: attenuator 7: square slit 8: objective lens
9: Photomask 10: Quartz surface after processing 1
1: Processing residue 12: Melted and solidified residue 13: Damage 14: Transfer pattern 15: Sharp edge of processed portion 16: Edge of processed portion with dripping

Continued on the front page (72) Inventor Hiromi Kondo 2-28-303, Tsurumainishi-cho, Nara City, Nara Prefecture (72) Inventor Kazuyuki Hirao 3-58, Kizugawadai, Kizu-cho, Kizu-cho, Soraku-gun, Kyoto F-term (reference) 2H095 BD32 BD34 5F072 AB20 JJ20 QQ02 RR01 RR03 SS08 YY08

Claims (3)

[Claims] When a photomask made of a metal thin film provided on a substrate is modified with a laser beam, a pulse width of 3 to 16 is used.
The metal thin film is selectively processed by irradiating the laser light of ps,
A method of repairing a photomask, which comprises removing black defects of the photomask. 2. The method according to claim 1, wherein a laser beam having a wavelength of 600 to 1100 nm is used. 3. When a photomask made of a chromium thin film and a chromium oxide film having a total thickness of 30 to 120 nm provided on a quartz glass substrate is modified by laser processing, an energy density of 4 to 5 J / cm ² and a pulse width of 3 are used. ~ 16ps, wavelength 60
A method for repairing a photomask, which comprises irradiating one laser pulse of 0 to 1100 nm.