Method for checking phase shift angle of phase shift mask, lithography process and phase shift mask

A method for checking a phase shift angle of a PSM is described. A calibration curve of a characteristic value of lithography performance with respect to the phase shift angle of a type of PSM is acquired. The patterns of a PSM of the type to be checked are transferred to a photoresist layer to form photoresist patterns, and the characteristic value is measured. The real phase shift angle of the PSM is derived based on the characteristic value according to the calibration curve.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the lithography technology. More particularly, the present invention relates to a method for checking a phase shift angle of a phase shift mask (PSM), to a lithography process utilizing the method for optimization, and to a phase shift mask of which the phase shift angle can be derived based on the method.

2. Description of the Related Art

In accompany with the requirement in increasing the integration degree of IC devices, the linewidths of advance semiconductor processes are mostly below the wavelength of the exposure light. When the linewidth is reduced to about a half of the wavelength or below, a phase shift mask is required to improve the resolution of the pattern transfer. Generally, a PSM utilizes a phase angle difference between adjacent light-transmittable regions to reduce the light amplitude at the portion of the photoresist layer requiring insufficient exposure to decrease the exposure dose thereat. Thus, the exposure contrast can be improved to enhance the resolution.

However, because the phase shift angle of a PSM is controlled usually by adjusting the substrate or film thickness in the transparent regions but the thickness is difficult to precisely control, the phase shift angle easily deviates from the required one. Consequently, the resolution is lowered and errors are caused in some characteristic values of the lithography performance including the pattern positions and/or the focus center as well as the depth of focus (DOF) and/or the critical dimension (CD) of the photoresist patterns. To solve the problem, a PSM has to be checked for its phase shift angle after being received from the vendor, wherein a proximity-type probed is usually used to measure the substrate or film thicknesses in the regions of different phase angles. If the phase shift angle is found to be erroneous, the exposure conditions can be adjusted accordingly to compensate the error in phase shift angle, or the PSM is returned to the manufacturer, who will calibrate the PSM and fabricate a new one accordingly.

Nevertheless, the above method of using a proximity-type probe to measure the thicknesses not only consumes much time, but also possibly damages the surface of the PSM to degrade the quality of pattern transfer.

SUMMARY OF THE INVENTION

In view of the foregoing, one object of this invention is to provide a method for checking a phase shift angle of a phase shift mask (PSM), which can save much time and doest not possibly damage the surface of the PSM.

Another object of this invention is to provide a lithography process that utilizes the above method of this invention for optimization.

Still another object of this invention is to provide a phase shift mask, of which the phase shift angle can be derived based on the above method of this invention.

The method for checking a phase shift angle of a PSM of this invention is described as follows. A calibration curve of a characteristic value of lithography performance with respect to the phase shift angle of a type of PSM is acquired. The patterns of a PSM of the type to be checked are transferred to a photoresist layer through exposure to form photoresist patterns, and the characteristic value is measured. The real phase shift angle of the PSM is derived based on the characteristic value according to the calibration curve.

In the above method, the characteristic value may be the depth of focus (DOF), the position of focus or the critical dimensions of the photoresist patterns, for example. The PSM of the type is possibly a half-tone (HT) PSM, an alternating PSM (Alt-PSM), a chromeless PSM or any other type of photomask with a phase-shift design. The photoresist patterns may include parallel line patterns or an array of opening patterns, while the exposure step for transferring the patterns of the PSM may be a dry exposure step or a wet exposure step.

In addition, when the photoresist patterns include parallel line patterns, the characteristic value used may be the position of focus or the DOF. When the PSM is a HT-PSM and the photoresist patterns include an array of opening patterns, the characteristic value used may be the position of focus or the DOF. When the PSM is a chromeless PSM and the photoresist patterns include an array of opening patterns, the characteristic value may also be the depth of focus (DOF) or the position of focus. Moreover, the calibration curve may be acquired with the following steps. A series of standard PSMs with known and different phase shift angles are used in lithography, and a series of the characteristic values corresponding to different standard PSMs are measured. Thereafter, the series of the characteristic values are plotted with respect to the phase shift angle.

The lithography process of this invention is described as follows. A calibration curve of a characteristic value of lithography performance with respect to the phase shift angle of a type of PSM is acquired. A PSM of the type is then used to conduct an exposure step, and the characteristic value is measured. The real phase shift angle of the PSM is derived based on the characteristic value according to the calibration curve. The exposure conditions are then adjusted according to the real phase shift angle to make the characteristic value achieve a predetermined value as required.

For the above lithography process of this invention, other features about the type of PSM, the type of the characteristic value, the type of the photoresist patterns to be formed and the method for acquiring the calibration curve may be the same as mentioned above.

The phase shift mask of this invention includes a transparent substrate, an IC pattern area on the transparent substrate, and at least one phase-shift-angle test pattern on the transparent substrate at the periphery of the IC pattern area. Each of the IC pattern area and the phase-shift-angle test pattern contains multiple first regions and second regions with a phase difference from the first regions, wherein the arrangement of the first and second regions in the IC pattern area is the same as that in the phase-shift-angle test pattern.

The transparent substrate usually has a rectangular shape, and there may be four phase-shift-angle test patterns disposed respectively at the four corners of the substrate. The number of phase-shift-angle test patterns is not restricted to four, while the phase-shift-angle test patterns may alternatively be disposed at the four edge portions of the transparent substrate.

Moreover, the above PSM may be a HT-PSM, an Alt-PSM, a chromeless PSM or any other type of photomask with a phase-shift design, and each of the IC pattern area and the phase-shift-angle test pattern contains patterns for forming parallel lines or an array of openings. When each of the IC pattern area and the phase-shift-angle test pattern contains patterns for forming an opening array or parallel lines, the PSM can be a HT-PSM or a chromeless PSM. In addition, the transparent substrate in the second regions may be recessed to cause a phase shift relative to the first regions.

Since the method of this invention derives the real phase shift angle of a PSM based on the measured characteristic value of lithography performance according to a calibration curve acquired previously, much time can be saved and the surface of the PSM will not be damaged. Meanwhile, since the characteristic value like DOF or position of focus can be measured after the exposure step to derive the real phase shift angle, in the lithography process of this invention, the exposure conditions can be adjusted quickly to compensate the error of the phase shift angle and make the characteristic value achieve a predetermined value as required.

Moreover, since at least one phase-shift-angle test pattern is disposed at the periphery of the IC pattern area on the PSM of this invention, the phase shift angle of the PSM can be checked simply with the phase-shift-angle test pattern. Therefore, the phase shift angle can be checked easily, and the transfer of the IC patterns will not be affected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows a process flow of a method for checking the phase shift angle of a PSM according to this invention. Referring toFIG. 1, a calibration curve of a characteristic value of lithography performance with respect to the phase shift angle of a type of PSM is acquired in step100. The characteristic value is preferably the depth of focus (DOF), the position of focus or the critical dimension (CD) of the photoresist patterns, while the type of PSM is, for example, half-tone type, alternating type, chromeless type or any other type with a phase-shift design. In addition, the calibration curve may be acquired by the following steps. A series of standard PSMs with known and different phase shift angles are used in lithography, and a series of the characteristic values corresponding to different standard PSMs are measured. Then, the series of the characteristic values are plotted with respect to the phase shift angle. In this method, the phase shift angles of the standard PSMs can be precisely determined with the above-mentioned proximity-type probe.

In next step110, the patterns of a PSM of the type to be checked are transferred to a photoresist layer through exposure to form photoresist patterns, and the characteristic value is measured. When the characteristic value to be measured is the position or depth of focus, the measurement can be done after the exposure step for transferring the patterns. When the characteristic value to be measured is the CD of the photoresist patterns, the measurement must be done after the development step. The exposure step can be a dry exposure step or a wet exposure step, in which a liquid medium is filled between the object lens of the exposure optical system and the photoresist layer to reduce the refraction of the light incident into the photoresist layer and thereby improve the resolution. In addition, the patterns transferred onto the photoresist layer may be parallel line patterns or an array of opening patterns.

Thereafter, in step120, a real phase shift angle of the PSM is derived based on the characteristic value according to the calibration curve.

FIG. 2shows a flow chart of a lithography process of this invention. In step200, a calibration curve of a characteristic value of lithography performance with respect to the phase shift angle of a type of PSM is acquired, wherein the type of PSM, the type of the characteristic value, the type of the photoresist patterns to be formed and the method for acquiring the calibration curve may be the same as above. In next step210, a PSM of the type is used to conduct an exposure step, wherein the characteristic value is measured.

Thereafter, in step220, the real phase shift angle of the PSM is derived based on the characteristic value according to the calibration curve, and the exposure conditions are adjusted according to the real phase shift angle to make the characteristic value achieve a predetermined value as required in step230.

On the other hand,FIGS. 3A and 3Billustrate two examples of the PSM according to this invention. The PSM300includes an integrated circuit (IC) pattern area302and at least one phase-shift-angle test pattern310at the periphery of the IC pattern area302, wherein the phase-shift-angle test pattern310contains the same phase shift patterns in the IC pattern area302and is formed simultaneously with the IC pattern area302to have the same phase shift angle. Since the phase-shift-angle test pattern310is disposed at the periphery of the IC pattern area302, the photoresist pattern corresponding to the phase-shift-angle test pattern310is located in the scribe line region of the wafer (not shown).

In the case ofFIG. 3A, four phase-shift-angle test patterns310are disposed respectively at the four corners of the rectangular PSM300. In the case ofFIG. 3B, multiple phase-shift-angle test patterns310are disposed at the four edge portions of the rectangular PSM300. The number of the phase-shift-angle test patterns310disposed is not restricted to 4, and the phase-shift-angle test patterns310can alternatively be disposed at any other positions at the periphery of the integrated circuit pattern area302if not conflicting with the alignment marks and other types of test patterns that are usually disposed in the prior art.

FIGS. 4A-4Ceach illustrates a top view and a cross-sectional view of a local structure of an exemplary phase-shift-angle test pattern on a different type of PSM for forming parallel line patterns.

FIG. 4Aillustrates a top view and a cross-sectional view of a local structure of an exemplary phase-shift-angle test pattern on a HT-PSM for forming parallel line patterns. The phase-shift-angle test pattern310includes linear transparent regions312and linear semi-transparent regions314with a phase shift of 180 degrees, wherein the transparent regions312and the semi-transparent regions314are arranged alternately and the transparency of the semi-transparent regions314is usually 6%. The low transparency of the semi-transparent regions314may be caused by a semi-transparent ultra-thin Cr or molybdenum silicide (MoSi) film314aformed on the transparent substrate30. In general, the semi-transparent regions314utilize the thickness of the metal film314ato produce a phase shift approaching 180 degrees, and the substrate30in the transparent regions312is shallowly etched to form trenches312ain the etching step for patterning the metal film314a. By controlling the depth of the trenches312ain combination with the thickness of the metal films314ato produce an optimal phase shift angle, the exposure contrast can be increased to improve the resolution.

FIG. 4Billustrates a top view and a cross-sectional view of a local structure of an exemplary phase-shift-angle test pattern on an Alt-PSM for forming parallel lines. The phase-shift-angle test pattern310may include linear transparent regions322, linear transparent regions324with a phase shift of 180 degrees and linear opaque regions326, wherein the transparent regions322and324are arranged alternately and any two adjacent transparent regions322and324are separated by an opaque region326. The opaque regions326may include a sufficiently thick Cr film326ato block the exposure light. The phase shift angle may be caused by etching the substrate30in the transparent regions322to form trenches322aand produce a substrate thickness difference between the transparent regions322and324.

FIG. 4Cillustrates a top view and a cross-sectional view of a local structure of an exemplary phase-shift-angle test pattern on an chromeless PSM for forming parallel line patterns. The phase-shift-angle test pattern310includes linear transparent regions332and narrower linear transparent regions334with a phase shift of 180 degrees, wherein the transparent regions332and334are arranged alternately. Since the transparent regions334are narrower and have a phase shift of 180 degrees from the transparent regions332, the portions of the photoresist layer corresponding to the transparent regions334and the two portions of each transparent regions332near the two adjacent transparent regions334are insufficiently exposed to form patterns (when positive photoresist is used) or to be removable (when negative photoresist is used). The phase shift angle may be caused by etching the substrate30in the transparent regions334to form trenches334a, or by etching the substrate30in the transparent regions332to form corresponding trenches (not shown), to produce a substrate thickness difference between the transparent regions332and334.

FIG. 5Aillustrates a top view and a cross-sectional view of a local structure of an exemplary phase-shift-angle test pattern on a half-tone PSM for forming an array of opening patterns. The phase-shift-angle test pattern310includes transparent regions342corresponding to the opening patterns and a grid-like semi-transparent region344with a phase shift of 180 degrees, wherein the transparency of the semi-transparent region344is usually 6%. The low transparency of the semi-transparent region344may be caused by a semi-transparent ultra-thin Cr or MoSi film344aformed on the substrate30. As in the case ofFIG. 4A, the semi-transparent region344utilize the thickness of the metal film344ato produce a phase shift approaching 180 degrees, and the substrate30in the transparent regions342is shallowly etched to form recesses342a. By controlling the depth of the recesses342ain combination with the thickness of the metal films344ato produce an optimal phase shift angle, the exposure contrast can be increased to improve the resolution.

FIGS. 5B and 5Ceach illustrates a top view and a cross-sectional view of a local structure of an exemplary phase-shift-angle test pattern on a chromeless PSM for forming an array of opening patterns. The phase-shift-angle test patter310ofFIG. 5Bcontains a conventional pattern design on a chromeless PSM for forming an opening array, including transparent regions352corresponding to the opening and a grid-like transparent region354with a phase shift of 180 degrees. The phase shift angle may be caused by etching the substrate30in the grid-like transparent region354to form a grid-like trench354a, or by etching the substrate30in the transparent regions352to form corresponding recesses (not shown), to produce a substrate thickness difference between the transparent regions352and354.

However, when the conventional pattern design ofFIG. 5Bis used, the amplitude of the light through a portion of the transparent region354between four adjacent transparent regions352often cannot be negated effectively, so that a small hole is easily formed between four adjacent openings in the photoresist layer. Therefore, in the special pattern design ofFIG. 5C, the region between four adjacent transparent regions362corresponding to four openings is changed to a transparent region366having the same phase of the transparent regions362. The phase of the transparent region366is different, by 180 degrees, from that of the transparent regions364each between two adjacent transparent regions362, so that the amplitude of the light through the transparent region366can be effectively negated by the four transparent regions364around the transparent region366to prevent formation of extra small holes in the photoresist layer. Similarly, the phase shift angle may be caused by etching the substrate30in the transparent regions364to form short trenches364a, or by etching the substrate30in the transparent regions362and366to form corresponding large and small recesses that are connected at their corners, to produce a substrate thickness difference between the transparent regions362/366and364.

FIGS.6/7/8shows the computer simulation results of the variations of DOF/CD/“position of focus” with phase shift angle for three types of PSM for forming parallel line patterns as well as for two types (dry and wet) of exposure methods.

Referring toFIG. 6, in this case, no matter the PSM is a half-tone, alternating or chromeless PSM or the exposure method is of dry or wet type, the depth of focus is insensitive to the change of the phase shift angle. Therefore, in the cases wherein parallel line patterns are to be formed, the depth of focus is not a suitable parameter for deriving the phase shift angle.

Referring toFIG. 7, in this case, the critical dimension is slightly more sensitive to the change of phase shift angle than the depth of focus is, and can barely be used to derive the phase shift angle. However, as shown inFIG. 8, the position of focus is linearly dependent on the phase shift angle, and the sensitivity of the position of focus to the phase shift angle is mostly above 5 nm/degree. Therefore, in the cases wherein parallel line patterns are to be formed, the position of focus is a preferred parameter for deriving the phase shift angle.

FIGS.9/10shows the computer simulation result of the variation of DOF/“position of focus” with the phase shift angle for a half-tone PSM for forming an array of opening patterns.

Referring toFIG. 9, in this case, the DOF is insensitive to the change of phase shift angle, and is therefore not a suitable parameter for deriving the phase shift angle. On the contrary, as shown inFIG. 10, the position of focus is quite sensitive to the phase shift angle, and is therefore a preferred parameter for deriving the phase shift angle.

FIGS.11/12shows the computer simulation result of the variation of DOF/“position of focus” with phase shift angle for a chromeless PSM for forming an array of opening patterns, wherein a local structure of the PSM is shown inFIG. 5C.

Referring toFIGS. 11 and 12, in this case, the DOF and the position of focus both are quite sensitive to the phase shift angle. Therefore, both of them are preferred parameters for deriving the phase shift angle when a chromeless PSM for forming an array of opening patterns is used.

As mentioned above, the method of this invention derives the real phase shift angle of a PSM based on a measured characteristic value of lithography performance according to a calibration curve acquired previously, so that much time can be saved and the surface of the PSM will not be damaged.

Moreover, since the characteristic value like DOF or position of focus can be measured after the exposure step to derive the real phase shift angle, in the lithography process of this invention, the exposure conditions can be adjusted quickly to compensate the error of the phase shift angle and make the characteristic value achieve a predetermined value as required. In addition, since at least one phase-shift-angle test pattern is disposed at the periphery of the IC pattern area on the PSM of this invention, the phase shift angle of the PSM can be checked simply with the phase-shift-angle test pattern. Therefore, the phase shift angle can be checked easily, and the transfer of the IC patterns will not be affected.