Reflective mask, method of monitoring the same, and method of manufacturing semiconductor device

According to one embodiment, provided is a reflective mask having a substrate, a reflection layer that reflects EUV light formed above the substrate, and an absorption layer that absorbs the EUV light formed above the reflection layer. The reflective mask further includes a monitor pattern monitoring an attachment amount of contamination attached during exposure.

FIELD

Embodiments described herein relate generally to a reflective mask and a method for monitoring the same, and a method of manufacturing a semiconductor device.

BACKGROUND

In recent years, with intensive downsizing of semiconductor devices, a lithography using extreme ultraviolet (hereafter, referred to as “EUV”) light having a shorter wavelength has been considered in place of conventional ArF light. In the lithography using the EUV light, a mask is used under a certain degree of vacuum, and floating impurities are activated by the EUV light and may be attached onto the mask surface as contamination (dirt). When the contamination is attached onto the mask surface, the exposure amount is reduced and the dimension resulted after the exposure process may vary. Thus, a process is provided for cleaning up the mask that has been used for a certain time period to detach the contamination. Under the circumstances, however, the time until when the mask can be used is not known before the dimension actually changes, causing a problem.

DETAILED DESCRIPTION

In general, according to one embodiment, provided is a reflective mask having a substrate; a reflection layer, which reflects EUV light, formed above the substrate; and an absorption layer, which absorbs the EUV light, formed above the reflection layer. The reflection mask includes a monitor pattern for monitoring an attachment amount of a contamination attached during exposure.

Referring to the attached drawings, a reflective mask and a method for monitoring the same according to the embodiments, and a method of manufacturing a semiconductor device will be described below in detail. It is to be noted that the present invention is not limited by these embodiments.

First Embodiment

FIGS. 1A and 1Bare views schematically illustrating an example of a mask configuration applied to the first embodiment, in whichFIG. 1Ais a top view of the mask andFIG. 1Bis a cross-sectional view taken along A-A ofFIG. 1A. As illustrated inFIG. 1, a mask10to which the first embodiment is applied has a pattern forming area21in which a pattern to be transferred on the process object such as a wafer is formed, and a peripheral area22which serves as a dicing line on the process object and in which an alignment mark and the like for a positioning with respect to other layer is arranged.

A mask10has a reflection layer12for reflecting the EUV light that is provided above one of the primary surfaces of a substrate11and an absorption layer13absorbing the EUV light that is provided above the reflection layer12and patterned correspondingly to the pattern to be transferred on the process object.

It is desirable that the substrate11can be made of a material having a low thermal expansion coefficient in order to maintain pattern position accuracy in a high degree. For such substrate11, quartz glass, SiO2—TiO2based low thermal expansion glass, and the like may be used, for example.

The reflection layer12is configured with a material that reflects the EUV light used for the EUV exposure at a high reflection rate. Typically, used is a multi-layered reflection film in which a high refraction material layer and a low refraction material layer are stacked in an alternative manner for multiple times. For the reflection layer12, an Mo/Si multi-layered reflection film and the like may be used in which the Si layer whose refraction index is 0.999 at the wavelength of 13.5 nm is used as the high refraction material layer and the Mo layer whose refraction index is 0.924 at the wavelength of 13.5 nm is used as the low refraction layer, for example.

The absorption layer13is configured with the material that absorbs the EUV light. As the absorption layer13, a material having a primary component of Ta such as Ta, TaB, TaBN, and the like, Cr, a material having a primary component of Cr and containing at least one component selected from N, O, and C, and the like may be used.

In the first embodiment, a monitor pattern23monitoring the degree of attachment of the contamination is provided in at least one part of the peripheral area22of the mask10having the above-described configuration.FIG. 1Aillustrates an example in which the monitor patterns23are provided at four corners on the substrate11. The dimensions of the exposure pattern on which the monitor pattern23is transferred to the resist are measured by utilizing the fact that the reflection rate changes at a high sensitivity according to the film thickness of the contamination. Then, the timing of cleaning up the mask10is determined by using the attached amount of the contamination.

FIGS. 2A and 2Bare views schematically illustrating an example of the monitor pattern according to the first embodiment, in whichFIG. 2Ais a cross-sectional view of the monitor pattern andFIG. 2Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 15Afor the resist applied on the process object. It is to be noted that the monitor pattern23illustrated inFIG. 2Arepresents a state of immediately after the fabrication of the mask10or immediately after the cleaning of the mask10. Further, inFIG. 2B, the part with less reflection light from the mask10is provided with dark hatching, while the part with more reflection light is provided with light hatching. Such hatchings are common to the drawings of the exposure pattern presented in the first embodiment.

The monitor pattern23has a shape such that the absorption layer13is patterned in a rectangle in a plane view. Further, the thickness of the absorption layer13of the monitor pattern23is substantially even and is thicker than the thickness of other areas of the absorption layer13. After the monitor pattern23ofFIG. 2Ais exposed on the resist, the exposure pattern31corresponding to the monitor pattern23is formed as shown inFIG. 2B.

In the EUV exposure apparatus, the mask10having the monitor pattern23is placed in the vacuum for the use of exposure. In the exposure, floating impurities in the vacuum are activated by the EUV light and attached onto the surface of the mask10as the contamination.FIGS. 3A and 3Bare views schematically illustrating a state where the contamination is attached to the monitor pattern ofFIG. 2A, in whichFIG. 3Ais a cross-sectional view of the monitor pattern andFIG. 3Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 3Afor the resist applied on the process object.

The attachment of contamination14to each pattern within the mask10causes the exposure amount to be reduced and the dimensions of each pattern to vary. As illustrated inFIG. 3A, the monitor pattern23also has an isotropic attachment of the contamination14and has larger dimensions compared to the case ofFIG. 2A. The attachment of the contamination14causes the reduced reflection rate. As a result, the exposure of the monitor pattern23results in an exposure pattern32in which the area having a lower exposure amount than the peripheral area is more spread compared to the case ofFIG. 2B.

A width D2 of an exposure pattern32corresponding to the monitor pattern23of after an exposure process for a predetermined time period as illustrated inFIG. 3Bis larger than a width D1 of an exposure pattern31corresponding to the monitor pattern23of immediately after the fabrication or immediately after the cleaning as illustrated inFIG. 2B. Then, the width D1 of the exposure pattern31of immediately after the fabrication or immediately after the cleaning and the width D2 of the exposure pattern32of after the exposure process are monitored, and the attachment amount of the contamination14is calculated from the difference D2−D1 by using a CD-SEM (Critical Dimension Scanning Electron Microscope) or an alignment shifting apparatus. Then, based on the attachment amount of the contamination14, it can be determined whether or not to clean up the mask10. When the dimension measurement is made by using the alignment shifting apparatus, the dimension of the monitor pattern23should be a size that can be optically recognized or larger.

For example, if the attachment amount of the contamination14is the amount with which no adverse effect is caused by transferring the pattern of the pattern forming area21to the resist, the mask10is continued to be used. On the other hand, if the attachment amount of the contamination14is the amount with which an adverse effect may be caused by transferring the pattern of the pattern forming area21to the resist, the use of the mask10is stopped and the mask is cleaned up to detach the contamination14. The attachment amount of the contamination14as a reference for the determination is derived by experiments in advance.

It is to be noted that, althoughFIG. 2Aillustrates the case where the height of the monitor pattern23is even, the embodiment is not limited to it.FIGS. 4A and 4Bare views schematically illustrating another example of the monitor pattern according to the first embodiment, in whichFIG. 4Ais a cross-sectional view of the monitor pattern andFIG. 4Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 4Afor the resist applied on the process object.

In the example ofFIG. 4A, while the monitor pattern23has the rectangular shape in the plane view, the cross section in the longitudinal direction of the monitor pattern23is the highest at or around the center and has the height decreasing toward the ends in an inclined manner. The use of such monitor pattern23results in that the exposure amount becomes smaller as the thickness in the area of the absorption layer13increases, while the exposure amount becomes larger as the thickness in the area of the absorption layer13decreases. As a result, an exposure pattern31illustrated inFIG. 4Bis obtained.

FIGS. 5A and 5Bare views schematically illustrating a state where the contamination is attached to the monitor pattern ofFIG. 4A, in whichFIG. 5Ais a cross-sectional view of the monitor pattern andFIG. 5Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 5Afor the resist applied on the process object.

As illustrated inFIG. 5A, the contamination14is attached to each pattern within the mask10and the dimensions of the monitor pattern23have increased. Then, as illustrated inFIG. 5B, the attachment of the contamination14causes the dimensions of the exposure pattern32of the monitor pattern23to increase compared toFIG. 4B.

FIGS. 6A and 6Bare views schematically illustrating yet another example of the monitor pattern according to the first embodiment, in whichFIG. 6Ais a cross-sectional view of the monitor pattern andFIG. 6Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 6Afor the resist applied on the process object.

In the example ofFIG. 6A, while the monitor pattern23has the rectangular shape in the plane view, the cross section in the longitudinal direction of the monitor pattern23is the highest at both ends and has the height decreasing toward the center in an inclined manner. As illustrated inFIG. 6B, the exposure pattern31of such monitor pattern23results in that the exposure amount becomes smaller as the thickness in the area of the absorption layer13increases, while the exposure amount becomes larger as the thickness in the area of the absorption layer13decreases. As a result, the exposure pattern31is divided into two parts as illustrated inFIG. 6B. In this case, the distance between the two divided exposure patterns31is measured.

FIGS. 7A and 7Bare views schematically illustrating a state where the contamination is attached to the monitor pattern ofFIG. 6A, in whichFIG. 7Ais a cross-sectional view of the monitor pattern andFIG. 7Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 7Afor the resist applied on the process object.

As illustrated inFIG. 7A, the attachment of the contamination14to each pattern within the mask10causes the dimensions of the monitor pattern23to increase. Then, as illustrated inFIG. 7B, the attachment of the contamination14has an effect of causing the dimensions of the exposure pattern32of the monitor pattern23to increase compared toFIG. 6B. As a result, the distance between the two divided exposure patterns32is shorter than the case ofFIG. 6B.

In the first embodiment, at least one monitor pattern23monitoring the attachment amount of the contamination14is provided in the peripheral area22of the mask10used in the EUV exposure apparatus. The size of the exposure pattern31obtained by exposing the monitor pattern23of immediately after the fabrication or immediately after the cleaning up of the mask10on the resist, and the size of the exposure pattern32obtained by exposing on the resist the monitor pattern23of after the use of mask10in the EUV exposure apparatus are measured and, based on the difference between them, the attachment amount of the contamination14is calculated. This allows for the advantage that the timing of cleaning up the mask10can be determined before the subsequent processing is affected.

Second Embodiment

In the first embodiment, the change in the dimensions of the exposure pattern is measured to determine the timing of cleaning up the mask. In the second embodiment, the case that the change in the alignment deviation between the two patterns obtained before and after the exposure process is measured to determine the timing of cleaning up the mask will be described.

FIGS. 8A to 8Care views illustrating an example of the monitor pattern according to the second embodiment, in whichFIG. 8Ais a cross-sectional view of the monitor pattern,FIG. 8Bis a top view of the monitor pattern, andFIG. 8Cis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIGS. 8A and 8Bfor the resist applied on the process object. InFIG. 8C, the part with more reflection light from a mask10is provided with dark hatching, while the part with less reflection light is provided with light hatching.

As illustrated inFIGS. 8A and 8B, a monitor pattern23according to the second embodiment has a pair of outer patterns23aand23b, and an inner pattern23cdisposed between the pair of outer patterns23aand23b. In the example ofFIGS. 8A and 8B, the outer patterns23aand23bhave a line shape and the inner pattern23chas a rectangular shape. While the cross section of the outer patterns23aand23bin the direction orthogonal to a substrate11has a rectangular shape and has an even height, the inner pattern23chas a configuration in which the height at the cross section in the direction orthogonal to the substrate11decreases in an inclined manner from the end of the outer pattern23atoward the end of the outer pattern23b. Further, the monitor pattern23here is configured by processing a reflection layer12, but no absorption layer13is provided thereon. The monitor pattern23illustrated inFIGS. 8A and 8Brepresents a state immediately after the fabrication or immediately after the cleaning of the mask10.

Such monitor pattern23can be formed by the processing using an FIB (Focused Ion Beam). For example, the typical lithography technique is used for the patterning so that the reflection layer12is patterned and thus the inner pattern23cis disposed between the pair of outer patterns23aand23b. At this time, the outer patterns23aand23band the inner pattern23care the same in height. Subsequently, the inner pattern23cis etched by the FIB so as to be inclined. It is to be noted that, in the case where an alignment shifting apparatus is used to measure the dimensions, the dimensions of the monitor pattern23should be a size that can be optically recognized or larger.

The monitoring method using such monitor pattern23will be described. First, the resist is applied onto the process object, the mask10that has not been used or just cleaned up is used for exposure on the resist by the EUV exposure apparatus, and development is made. Thereby, the exposure pattern33illustrated inFIG. 8Cis obtained.

Subsequently, the alignment shifting apparatus is used to measure the alignment deviation of the inner pattern23cwith respect to the outer patterns23aand23b. Specifically, respective center positions P1 and P2 of exposure patterns33aand33bcorresponding to the outer patterns23aand23bare measured, and a position P3 that is the center between the exposure patterns33aand33bis measured from the positions P1 and P2. Further, a position P6 that is the center of the exposure pattern33ccorresponding to the inner pattern23cis measured. In the measurement of the inner pattern23c, while the reflection rate is higher at the area having a larger thickness of the reflection layer12, the reflection rate is lower at the area having a smaller thickness of the reflection layer12. Therefore, the threshold of the light amount is defined at the time of measuring the exposure pattern33c, and the exposure pattern33cis formed with the threshold as a border. Then, the width of the exposure pattern33ccorresponding to the inner pattern23cis measured. In the example ofFIG. 8C, the position of the end of the exposure pattern33cat the side of the exposure pattern33ais defined as P4, the position of the end at the side of the exposure pattern33bis defined as P5, and then the position P6 that is the center between the two positions P4 and P5 is measured. Then, a distance d1 between the position P3 and the position P6 is determined as the alignment deviation amount of the inner pattern23cwith respect to the outer patterns23aand23b.

Next, for the mask pattern23to which the contamination14is attached after the predetermined times of exposure process, the alignment deviation of the inner pattern23cwith respect to the outer patterns23aand23bis measured by using the alignment shifting apparatus.FIGS. 9A and 9Bare views schematically illustrating a state where the contamination is attached to the monitor pattern ofFIGS. 8A and 8B, in whichFIG. 9Ais a cross-sectional view of the monitor pattern andFIG. 9Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 9Afor the resist applied on the process object. InFIG. 9B, the part with more reflection light from the mask10is provided with the dark hatching, while the part with less reflection light is provided with the light hatching.

As illustrated inFIG. 9A, a repetition of the exposure processes causes the contamination14to attached to the mask10. The contamination14is attached to the outer patterns23aand23bin a conformal manner and thus the dimensions become larger, and the contamination14is attached onto the inclined surface in the inner pattern23c. In the outer patterns23aand23bto which the contamination14is attached in an isotropic manner, the center position of the outer patterns23aand23bdoes not change between before and after the contamination14is attached. On the other hand, in the inner pattern23c, the reflection amount is reduced in the entire area and the width of the exposure pattern34cdecreases.

Then, similarly to the case ofFIG. 8C, respective center positions P11 and P12 of exposure patterns34aand34bcorresponding to the outer patterns23aand23bare measured, and a position P13 that is the center between the exposure patterns34aand34bis measured from the positions P11 and P12. Positions P14 and P15 of the ends of the exposure pattern34ccorresponding to the inner pattern23care also measured, and a position P16 that is the center between the positions P14 and P15 is measured. Then, a distance d2 between the position P13 and the position P16 is determined as the alignment deviation amount of the inner pattern23cwith respect to the outer patterns23aand23b.

FIGS. 10A and 10Bare views illustrating a simulation of the relationship between the alignment deviation amount and the reflection rate according to the difference in the attachment amount of the contamination, in whichFIG. 10Bis an enlarged view of an area S ofFIG. 10A. As illustrated inFIG. 10B, when the film thicknesses of the contamination14at a certain reflection rate are 0 nm and 5 nm, the difference in the position of the inner pattern23cis 38 nm. That is, the measurement of the alignment deviation amount of the inner pattern23callows for the accurate detection of the attachment amount of the contamination14.

Subsequently, it is determined whether or not to clean up the mask10similarly to the first embodiment by using the difference d2−d1 that is the difference between the alignment deviation amount d1 of immediately after the fabrication or immediately after the cleaning of the mask10and the alignment deviation amount d2 resulted after predetermined times of exposure processes. For example, if the attachment amount of the contamination14that is obtained from the difference in the alignment deviation amount is the amount with which no adverse effect is caused by transferring the pattern of the pattern forming area21to the resist, the mask10is continued to be used. On the other hand, if the attachment amount of the contamination14that is obtained from the difference in the alignment deviation amount is the amount with which an adverse effect may be caused by transferring the pattern of the pattern forming area21to the resist, the use of the mask10is stopped and the mask is cleaned up to detach the contamination14.

FIG. 11is a view illustrating a simulation of the relationship between the film thickness of the contamination and the difference in the alignment deviation amount. For example, when the film thickness of the contamination14is 2 nm, the difference in the alignment deviation amount is 18 nm. This means that the change of 1 nm in the film thickness of the contamination14causes the change of 9 nm in the difference in the alignment deviation amount. That is, the measurement of the difference in the alignment deviation amount allows for detecting the change less than 1 nm in the film thickness of the contamination14.

FIG. 12is a view illustrating a simulation of the relationship between the difference in the mask width (ACD (Critical Dimension) and the difference in the alignment deviation amount. In this figure, for example, when the ΔCD is 10 nm, the difference in the alignment deviation amount is approximately 42 nm. Therefore, the sensitivity of the detection of the difference in the alignment deviation amount is fourfold compared to the detection of the change in the dimensions of the monitor pattern23due to the attachment of the contamination14. That is, the change in the film thickness of the contamination14can be detected at a fourfold sensitivity compared to the method of the first embodiment.

That is, while in the first embodiment the attachment amount of the contamination14can be detected only when it is equal to or greater than the size by which the change in the dimensions of the monitor pattern23can be sensed by the measuring equipment, in the second embodiment the attachment amount of the contamination14can be detected even when it is less than the size by which the change in the dimensions of the monitor pattern23can be sensed by the measuring equipment. This results in the advantage of allowing for the measurement with a high degree of accuracy.

In the above example, although it has been described that the inner pattern23cformed in the reflection layer12continuously decreases in its height, the embodiment is not limited to it.FIG. 13is a cross-sectional view schematically illustrating another example of the monitor pattern according to the second embodiment. In this example, the inner pattern23cis configured such that the height decreases stepwise from the end at the side of the outer pattern23atoward the end at the side of outer pattern23b.

Further, although the monitor pattern23is formed above the reflection layer12in the example described above, the monitor pattern23may be formed above the absorption layer13.FIGS. 14A and 14Bare views schematically illustrating yet another example of the monitor pattern according to the second embodiment, in whichFIG. 14Ais a cross-sectional view of the monitor pattern andFIG. 14Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 14Afor the resist applied on the process object.FIGS. 15A and 15Bare views schematically illustrating a state where the contamination is attached to the monitor pattern ofFIG. 14A, in whichFIG. 15Ais a cross-section view of the monitor pattern onto which the contamination is attached andFIG. 15Bis a plane view illustrating the exposure pattern obtained by exposing the monitor pattern ofFIG. 15Afor the resist applied on the process object. InFIGS. 14B and 15B, the part with less reflection light from the mask10is provided with the dark hatching, while the part with more reflection light is provided with the light hatching.

The processes for forming the monitor pattern23above the absorption layer13and measuring the monitor pattern23are the same as those in the case for forming the monitor pattern23above the reflection layer12as described above. However, since the absorption layer13absorbs the light from the light source in the lithography process, the light amount is larger at the area where no absorption layer13is present or the absorption layer13is thin, while the light amount is smaller at the area where the absorption layer13is present or the absorption layer13is thick. That is, as described in the first embodiment, the alignment deviation of the inner pattern23cwith respect to the outer patterns23aand23bis measured by using the exposure patterns33ato33cand34ato34cwith which the light amount is equal to or below a predetermined value, and the difference of the alignment deviation amounts between before and after the attachment of the contamination14is measured.

The semiconductor device can be manufactured by using the mask10having the monitor pattern23described in the second embodiment. Specifically, a resist is applied onto the process object such as a wafer, the exposure process is performed with the mask10that has not been used or has just been cleaned up, and the development is performed. Thereby, the resist pattern is formed. At this time, the exposure pattern corresponding to the monitor pattern23is used to measure a first alignment deviation amount. Subsequently, the etching process of the process object is performed with the resist pattern using as the mask. Next, the resist is applied to another process object, the exposure process is performed with the above mask10, and the resist pattern is formed after development. At this time, the exposure pattern corresponding to the monitor pattern23onto which the contamination14is attached is used to measure the second alignment deviation amount. It is then determined whether the difference between the second alignment deviation amount and the first alignment deviation amount may affect the operation of the device formed in the process object and, if not, the etching process of the process object is performed with the resist pattern using as the mask. Further, if it is determined that the difference of the alignment deviation amount may affect the operation of the device, the mask10is cleaned up. In this case, after the resist pattern on the process object is removed, the resist is applied again onto the process object, and the above-described process is made by using the cleaned-up mask10. It is to be noted that, although the description has been made here by referring to the example of the case of the second embodiment, the semiconductor device can be manufactured also by the similar method to the case of the first embodiment.

In the second embodiment, the alignment deviations of the inner pattern23cwith respect to the outer patterns23aand23bboth before and after the attachment of the contamination14are measured, and the timing of cleaning up the mask10is determined by using the difference of the alignment deviation amounts. This allows for detecting the dimensional changes which cannot be obtained by the measurement of the dimensions of the monitor pattern23, which results in the advantage that the determination of the timing of cleaning up the mask can be made with a high degree of accuracy.