Methods of defect inspection for photomasks

A method of defect inspection for a photomask is provided. According to the method, a light transmittance correction is performed to reduce a light transmittance of a calibration key pattern region of a photomask including a field region and the calibration key pattern region to the light transmittance of the field region. Light calibration is performed using the calibration key pattern region having corrected light transmittance. Defect inspection for the field region is performed by applying a result of the light calibration.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C 119(a) to Korean Application No. 10-2016-0020499, filed on Feb. 22, 2016, which is herein incorporated by reference in its entirety as set forth in full.

BACKGROUND

1. Technical Field

Various embodiments of the present disclosure relate to lithography technologies and, more particularly, to methods of defect inspection for a photomask.

2. Related Art

As semiconductor devices become more highly integrated, a wavelength of light generated from a light source used in photolithography processes has been continuously shorter and shorter to transfer finer patterns onto a wafer. In order to transfer finer patterns, mask patterns which are transferred onto the wafer need to have a small line width or a small line pitch. In case that pattern errors occur, such as overlay errors, after performing the photolithography processes using the photomask, a photomask correction process to correct or to improve such pattern errors may be performed. The photomask correction process may change light transmittance of a field region of the photomask, in which the mask patterns are located, thus inspection noise may be generated during the defect inspection process of the photomask.

SUMMARY

According to one embodiment, there is provided a method of defect inspection for a photomask. The method includes obtaining registration errors existing in a field region of the photomask, wherein the photomask includes the field region and a calibration key pattern region, obtaining a first light transmittance of the field region, performing registration correction by irradiating a first laser beam onto the field region, obtaining a first transmittance reduction in the field region by the registration correction, obtaining a second light transmittance in the field region using the first transmittance reduction, performing light transmittance correction to the calibration key pattern region so that the calibration key pattern region has light transmittance substantially equal to the second light transmittance, performing light calibration using the corrected calibration key pattern region to obtain a light calibration result, and performing defect inspection to the field region by using the light calibration result.

According to one embodiment, there is provided a method of defect inspection for a photomask. The method includes performing registration correction by irradiating a first laser beam onto a field region of the photomask to form a registration corrected field region, wherein the photomask includes the field region and a calibration key pattern region, performing light transmittance correction to reduce light transmittance of the calibration key pattern region, performing light calibration using the reduced light transmittance of the calibration key pattern region to obtain a result of the light calibration, and performing defect inspection to the registration corrected field region by applying the result of the light calibration.

According to one embodiment, there is provided a method of defect inspection for a photomask. The method includes performing light transmittance correction to reduce light transmittance of a calibration key pattern region of the photomask to a corrected light transmittance, wherein the corrected light transmittance is substantially equal to light transmittance of a field region, wherein the photomask includes the field region and the calibration key pattern region; performing light calibration using the calibration key pattern region having the corrected light transmittance to obtain a result of the light calibration, and performing defect inspection to the field region by applying the result of the light calibration.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The terms used herein may correspond to words selected in consideration of their functions in the embodiments, and the meanings of the terms may be construed to be different according to ordinary skill in the art to which the embodiments belong. If defined in detail, the terms may be construed according to the definitions. Unless otherwise defined, the terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the embodiments belong.

It will also be understood that when an element or layer is referred to as being “on,” “over,” “below,” “under,” or “outside” another element or layer, the element or layer may be in direct contact with the other element or layer, or Intervening elements or layers may be present. Other words used to describe the relationship between elements or layers should be interpreted in a like fashion for example, “between” versus “directly between” or “adjacent” versus “directly adjacent”.

The terminology “pattern” used herein may indicate a mask pattern such as a light blocking pattern or a phase shift pattern that is formed on a photomask to realize an element of an electronic circuit or an integrated circuit of a semiconductor device. The semiconductor device may correspond to a memory device or a logic device. The memory device may be a dynamic random access memory (DRAM) device, a static random access memory (SRAM) device, a flash memory device, a magnetic random access memory (MRAM) device, a resistive random access memory (ReRAM) device, a ferroelectric random access memory (FeRAM) device, or a phase change random access memory (PcRAM) device. The semiconductor device may be employed in communication systems such as mobile phones, electronic systems associated with biotechnology or health care, or wearable electronic systems.

Same reference numerals refer to same elements throughout the specification. Thus, even though a reference numeral is not mentioned or described with reference to a drawing, the reference numeral may be mentioned or described with reference to another drawing. In addition, even though a reference numeral is not shown in a drawing, it may be mentioned or described with reference to another drawing.

FIG. 1schematically illustrates a cross-sectional feature of a transmissive photomask100.FIG. 2schematically illustrates a planar feature of the photomask100.FIG. 3schematically illustrates a planar feature of a calibration key pattern170.

Referring toFIGS. 1 and 2, a transmissive photomask100may be prepared as a photolithography mask used in a photolithography process. The photomask100includes a substantially transparent substrate110. The transparent substrate110includes a front side or a first surface120and a back side or a second surface130opposite to the first surface120. The substrate110may be formed of a transparent material, for example, quartz. The transparent material may be a material capable of transmitting a deep ultra violet (DUV) light, for example, light with approximately a 193 nm wave length band, which may be used as an exposure light200.

Mask patterns160providing features to be transferred onto a wafer may be disposed on the first surface120of the photomask100. The features provided by the mask patterns160may be transferred onto the wafer by an exposure process. A region where the mask patterns160providing the features to be transferred onto the wafer are located is an active region and may be set as a field region140located in the photomask100. An outer region of the field region140is an inactive region and may be set as a frame region150.

The mask pattern160may include a layer of a light blocking material such as chrome (Cr), or may include a phase shift layer such as molybdenum silicon (MoSi) layer. The frame region150may include a frame layer180substantially including a light blocking layer181, or a double-layer of the light blocking layer181and a phase shift layer186. The exposure light200may not pass through the frame region150, thus the features in the frame region150cannot be transferred onto the wafer.

A pellicle190to protect the mask patterns160may be attached over the field region140. A pellicle supporting portion195supporting the pellicle190may be attached on the frame layer180. The pellicle190can prevent the mask patterns160from being exposed to an external environment.

After manufacturing the photomask100, a process of inspecting the photomask100is performed to determine whether defects are generated on the photomask100. For example, during performing an exposure process using the photomask100, defects known as haze may be generated on the photomask100, and a process for inspecting the defects may be performed. To improve inspection accuracy of the defect inspection process, sensitivity to detect the defects may be set by performing light calibration, and then, a defect inspection for the field region140may be performed. Calibration key patterns170for the light calibration may be disposed in an outer region of the field region140or in the frame region150.

A plurality of the calibration key patterns170may be disposed in a plurality of locations in the frame region150outside the field region140, as illustrated inFIG. 2. The calibration key pattern170may be formed by patterning the frame layer180in a predetermined feature.

As illustrated inFIG. 3, each of the calibration key patterns170may include a dark pattern175and a clear pattern171. The clear pattern171may be defined by the dark pattern175and be formed in a cross shape. The dark pattern175may be set by a portion of the frame layer180. The clear pattern171may be a quartz portion of the substrate110, which is exposed by the frame layer180. The clear pattern171may be a substantially transparent feature region.

The clear pattern171may be used as a clear level that can be a reference to determine the upper limit of transmittance that can be detected in the defect inspection. The dark pattern175may be used as a dark level that can be a reference of light blocked by the light blocking layer181forming the frame layer180, for example, a reference used to determine the lower limit of the transmittance that can be detected in the defect inspection.

By performing light calibration using the calibration key patterns170, data of a detection light detected by irradiating an inspection light onto the field region140of the photomask100, for example, light transmittance may be calibrated between the clear level and the dark level. The light calibration may be performed to improve the reliability of defect inspection results by adjusting the inspection sensitivity of the photomask100.

Referring toFIG. 1, the photomask100may allow an exposure wavelength of an extreme ultra violet (EUV) wavelength range, for example, approximately 13.5 nm to pass through. The EUV mask structure may include a multi-layered mirror structure for reflection of the EUV.

Referring toFIG. 1again, wafer patterns such as resist patterns (not illustrated) may be formed by transferring images of the mask patterns160on the wafer (not illustrated) using the photomask100. The formed resist patterns may not exactly correspond to a predetermined structure as originally intended. This type of errors can be referred to as registration errors. Such registration errors may generally be exemplified in a two-dimensional (2D) map230. See arrows inFIG. 4.

The registration correction process may include a step of measuring the registration errors shown in the registration error map230ofFIG. 4, and performing a correction process for the photomask100using a laser to compensate for the registration errors. The registration correction process using a laser, as illustrated inFIG. 5, may be performed using a registration correction (RegC) system300.

Referring toFIG. 5, the registration correction system300includes a chuck350on which the photomask100is seated. In an embodiment, the chuck350may be movable in three dimensions. The registration correction system300includes a pulsed laser source310generating a pulsed beam, an optical pulsed beam or a laser beam320.

The laser source310may generate optical pulses. Although the registration correction system300is exemplified to include only one laser source310in this embodiment, the registration correction system300may include one or more laser sources generating a laser beam of different wavelength bands or the same wavelength band.

A steering mirror330and a focus objective lens340may be arranged between the laser source310and photomask100. The steering mirror330directs the pulse laser beam320emitted from the laser source310to the focus objective lens340. The pulse laser beam320is focused on the photomask100through the focus objective lens340. In an embodiment, the focus objective lens340may be arranged to be movable in three dimensions. The registration correction process may be performed by irradiating a laser beam onto the photomask100through the registration correction system300. As the laser beam is irradiated onto the photomask100, local errors of the photomask100can be corrected.

FIG. 6illustrates the registration correction process according to an embodiment. Referring toFIG. 6, a first laser beam301for registration correction is irradiated onto the substrate110of the photomask100using the registration correction system300ofFIG. 5. A beam spot of the first laser beam301may be irradiated to the field region140of the substrate110. The first laser beam301may be scanned to the substrate110to be irradiated to a plurality of local regions of the substrate110.

As illustrated in the registration error map230ofFIG. 4, the errors located in the field region140may have different sizes and different directions. To correct the errors illustrated in the registration error map230ofFIG. 4, the first laser beam301may be irradiated with different beam spot densities or different energies to the local regions corresponding to the field region140of the substrate110.

The first laser beam301irradiated onto the substrate110of the photomask100may be incident through the second surface130to the substrate110. The first laser beam301may cause first deformation elements360in the quartz material constituting the substrate110. The first deformation element360may have a three dimensional volume and may have a morphological organization of atoms. That is, the deformation element360is relatively less dense in packing structure or has a lower density than the quartz material therearound. The acting direction or size of the first deformation elements360may vary depending on pulse energy, pulse length, a repetition rate, or the number of repeated scans of the first laser beam301. The registration errors generated differently with respect to the local regions can be corrected by inducing the first deformation elements360.

The first deformation elements360applied to correct the registration errors may change a light transmittance of the field region140. The light transmittance of the local regions in the field region140may be dropped in different amounts depending on density of the first deformation elements360. This may cause an overall drop of the light transmittance in the field region140. The overall drop of the light transmittance of the field region140may be evaluated as an average value of drops in the light transmittance of the local areas.

The average drop of the light transmittance in the field region140of the photomask100caused by the registration correction process may cause unexpected difficulties to the defect correction for the photomask100. The average light transmittance of the field region140of the photomask100after the registration correction can be relatively decreased compared with the average light transmittance in the field region140of the photomask100before the registration correction.

Furthermore, the light transmittance of the calibration key pattern region170after the registration correction may not be varied or may be relatively slightly varied. Accordingly, the difference between the light transmittance of the calibration key pattern region170and the average light transmittance of the field region140of the photomask100after registration correction can be quite large.

When performing light calibration based on the calibration key pattern region170having relatively high light transmittance, defect inspection sensitivity for the field region140, which has relatively low light transmittance, may increase. Accordingly, an error may occur of misrecognizing a normal pattern as a defect. Erroneous correction to the normal pattern may be cause.

Despite the fact that critical dimension uniformity is not substantially changed by the registration correction, inspection result data containing a large number of defects may be obtained. The detection sensitivity, which is overly high, erroneously determines a non-defect to be a defect. As a result, a noise due to such erroneous detection may increase significantly. For example, sub-resolution assist features (SRAFs) may be detected as a defect that is a noise in the result data. The increase of noise in the defect inspection result data may not only make defect inspection itself erroneous but also acts as a constraint when applying the registration correction process.

Scan of the first laser beam301to correct the registration errors may affect the light transmittance or luminance of the calibration key pattern region170. The first deformation elements360caused to the field region140by the first laser beam301may cause a local and irregular expansion of the photomask100. Accordingly, the light transmittance or luminance of the calibration key pattern region170may vary differently depending on the location of the calibration key patterns170.

When the calibration key patterns regions170have different light transmittances from each other, the accuracy and effectiveness of the light calibration may be degraded. The reliability of the light calibration is degraded and the defect inspection result may include a large number of noises, thus the reliability of the defect inspection may be degraded. The light transmittance difference between the field region140subjected to registration correction and calibration key pattern region170can be compensated for by reducing the light transmittance of the calibration key pattern region170.

FIG. 7illustrates a process of reducing the light transmittance in the calibration region including the calibration key patterns170of the photomask100. Referring toFIG. 7, a second laser beam307for reducing light transmittance is irradiated onto the calibration key pattern region170of the photomask100using the registration correction system300ofFIG. 5. A beam spot of the second laser beam307is irradiated onto the calibration key pattern region170and may cause second deformation elements370. The second deformation elements370may have a three dimensional volume and may have a morphological organization of atoms having a relatively less dense packing structure and a lower density than the quartz material therearound.

The light transmittance of the calibration key pattern region170may be reduced by the second deformation elements370. The second deformation elements370may reduce the light transmittance of the calibration key pattern region170so that the light transmittance of the calibration key pattern region170is similar to or substantially equal to the light transmittance of the registration corrected field region140.

Since a degree of reduction of light transmittance can be varied by the density or size of the second deformation elements370, desired light transmittance reduction can be obtained by adjusting the energy of the second laser beam307or the density of a beam spot of the second laser beam307causing the second deformation elements370.

For example, as illustrated inFIG. 6, the average initial value of a first light transmittance for the field region140can be obtained before irradiating the first laser beam301ofFIG. 6for registration correction. Then, an amount of the first light transmittance reduction caused by the process of irradiating the first laser beam301ofFIG. 6for registration correction to the field region140can be obtained from a correlation between the average initial value of the first light transmittance for the field region140and the light transmittance reduction after the irradiation of the first laser beam301.

The average second light transmittance for the field region140after the registration correction can be obtained from an amount of the detected first transmittance reduction. After obtaining the initial value of a third light transmittance for the calibration key pattern region170prior to the registration correction, a difference between the third light transmittance for the calibration key pattern region170before the registration correction and the average second light transmittance for the field region140after the registration correction can be obtained as the amount of a second transmittance reduction.

Parameters for the second laser beam307to obtain the amount of the second transmittance reduction may be set from the correlation between light transmittances according to irradiation of the second laser beam307. The second laser beam307may be irradiated onto the calibration key pattern region170by applying the obtained parameters for irradiation of the second laser beam307, for example, the type of the second laser beam307, the energy or density of the beam spot, and so on. Accordingly, the calibration key pattern region170may be induced to have a fourth light transmittance that is substantially equal to or similar to the average second light transmittance for the field region140after the registration correction.

Since the corrected fourth light transmittance of the calibration key pattern region170has a reduced value compared to the third light transmittance, the calibration key pattern region170whose light transmittance is corrected may provide a new reference of light calibration. By performing light calibration using the corrected calibration key pattern region170having reduced light transmittance, references such as the sensitivity of defect inspection may be reset or updated and defect inspection may be performed for the field region140on which the registration correction is performed.

Result data of defect inspection may be calibrated to values between the clear level and dark level which are reset or updated by the light calibration using the calibration key pattern region170having the reduced light transmittance after irradiation of the second laser beam307. Since the light transmittance of the corrected calibration key pattern region170is reduced, light calibration using the corrected calibration key pattern region170may function to reset the inspection sensitivity for the field region140to meet the reduced light transmittance of the field region140. Accordingly, a noise which is caused by erroneously correcting a non-defective pattern may be avoided.

The light calibration using the corrected calibration key pattern region170can eliminate noises and improve the reliability of the defect inspection result by re-adjusting the inspection sensitivity for the photomask100. By excluding noises, it is possible to overcome a burden for the defect inspection and to overcome the limitations of applying the registration correction process.

The light transmittance correction using the second laser beam307may be performed to the calibration key pattern regions170which are located in different locations from each other. As a result, the calibration key pattern regions170may have substantially the same fourth light transmittance. Accordingly, the phenomenon that the calibration key pattern regions170located in different locations from each other have different light transmittances can be compensated for. Since the calibration key pattern regions170located in different locations from each other can have substantially the same fourth light transmittance by the light transmittance correction using the second laser beam307, the reliability of the light calibration process based on the calibration key pattern regions170can be improved and the reliability of the defect inspection can be improved.

FIG. 8is a flow chart illustrating a method of defect inspection for a photomask according to an embodiment. Referring toFIG. 8, defect inspection for the photomask according to an embodiment may be performed by fabricating the photomask100as illustrated inFIG. 1, and measuring registration error data for the field region140and obtaining the registration error map230as illustrated inFIG. 4(S801). An initial value of the first light transmittance for the field region140may be measured (S802). The third light transmittance for the calibration key pattern region170may be measured (S803). The registration correction process may be performed by irradiating the first laser beam301onto the field region140of the photomask100ofFIG. 1, as illustrated inFIG. 6, using the registration correction system500ofFIG. 5illustrated inFIG. 5(S804).

The first transmittance reduction associated with the registration correction for the field region140ofFIG. 1may be calculated (S805). The second light transmittance for the field region140ofFIG. 1after the registration correction may be calculated from the first light transmittance and the first transmittance reduction (S806). As illustrated inFIG. 7, the second laser beam307may be irradiated onto the calibration key pattern region170to reduce a difference between the light transmittance of the calibration key pattern region170and the light transmittance of the field region using the second transmittance reduction (S808).

The light calibration may be performed using the corrected light transmittance of the calibration key pattern region170(S809). Then, defect inspection for the field region140may be performed by applying the light calibration result (S810). Defects detected in the defect inspection may be foreign substances such as haze or particles, which may be attached to or created by the photomask100by the exposure light source.

In the method of defect inspection for the photomask, as described with reference toFIG. 6, registration correction may be performed by irradiating the first laser beam301onto the field region140of the photomask100ofFIG. 1, and, as illustrated inFIG. 7, the light transmittance correction to reduce the light transmittance of the calibration key pattern region170of the photomask100may be performed.

In the method of defect inspection for the photomask, as illustrated inFIG. 1, the photomask100includes the field region140and the calibration key pattern region170. When a difference between the light transmittances of the field region140and the calibration key pattern region170is great, the light transmittance correction process may be performed to reduce the light transmittance of the calibration key pattern region170to be substantially equal to the light transmittance value of the field region140, as illustrated inFIG. 7.

The methods according to the aforementioned embodiments and structures formed thereby may be used in photolithography processes for fabricating integrated circuit (IC) chips. The IC chips may be supplied to users in a raw wafer form, in a bare die form or in a package form. The IC chips may also be supplied in a single package form or in a multi-chip package form. The IC chips may be integrated in intermediate products such as mother boards or end products to constitute signal processing devices. The end products may include toys, low end application products, or high end application products such as computers. For example, the end products may include display units, keyboards, or central processing units (CPUs).

The embodiments of the inventive concept have been disclosed above for illustrative purposes. Those of ordinary skill in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the inventive concept as disclosed in the accompanying claims.