Photomask

This application claims the benefit of Korean Patent Application No. 10-2009-0001244, filed on Jan. 7, 2009, in the Korean Intellectual Property Office.

The present inventive concept relates to photolithographic exposure equipment used in the manufacturing of semiconductor devices, LCDs and the like. More particularly, the present inventive concept relates to a photomask of photolithographic exposure equipment.

The manufacturing of electronic and optical devices, such as semiconductor devices and LCDs, includes a photolithography process in which the image of a very fine pattern such as a circuit pattern is transferred from a photomask to a substrate of the device under manufacture. An example of the substrate is a semiconductor wafer. To this end, exposure equipment is used. The exposure equipment is made up of several optical systems and thus, the equipment must possess high a degree of resolution to focus a precise image of the pattern of the photomask onto the wafer. Moreover, this process of transferring a pattern to a wafer is repeated many times on different areas (referred to as “shots”) of the wafer during a mass production process of, for example, manufacturing semiconductor chips. Therefore, various optical components, such as the photomask, a lens, and an aperture, etc., of the optical systems must offer a high degree of optical performance on a consistent and regular basis so that a critical dimension (CD) of a pattern in a particular shot is uniform and the CD is uniform from shot to shot.

The photomask is an optical component that bears the pattern whose image is to be transferred to the substrate of the electronic/optical device under manufacture. In addition, the photomask may have other patterns such as a blind pattern that enhances the precision of the optical transfer of the image to the substrate. A pellicle is attached to the photomask to protect the photomask. Furthermore, the maintaining of photolithographic exposure equipment entails cyclically removing the pellicle attached to the photomask, cleaning the photomask, and attaching a new pellicle to the photomask. Over time, however, such procedures reduce the thickness of the blind pattern of the photomask. In this case, the transmittance of the photomask increases, which causes a drop in shot uniformity on the wafer or other substrate.

According to an aspect of the inventive concept, there is provided a photomask comprising a photomask substrate having a front surface and a back surface and transparent to exposure light of a given wavelength, at least one mask pattern on the front surface of the photomask substrate, a blind pattern extending along the periphery of the photomask substrate on the front surface of the photomask substrate and opaque to the exposure light, and a light scattering element encapsulated in the medium of the photomask substrate. The mask pattern is the pattern whose image is to be transferred to a substrate of a device being manufactured by using the photomask in a photolithography process. The light scattering element is juxtaposed with the blind pattern in the direction of the thickness of the photomask substrate, and is configured to modify, e.g., reduce the intensity of, the exposure light that is incident on the back surface of the photomask as the incident exposure light propagates to the front surface of the photomask.

According to an aspect of the inventive concept, there is provided an optical component of photolithographic exposure equipment, comprising a substrate having a front surface and a back surface and transparent to exposure light of a given wavelength, at least one mask pattern adjacent the front surface of the transparent substrate, a blind pattern extending alongside an outer peripheral part of the optical component adjacent the front surface of the transparent substrate and opaque to the exposure light, and a light scattering element encapsulated in the medium of the transparent substrate. The mask pattern is the pattern whose image is to be transferred to a substrate of a device being manufactured by using the photolithographic exposure equipment. The light scattering element is juxtaposed with the blind pattern in the direction of the thickness of the transparent substrate, and is configured to modify, e.g., reduce the intensity of, the exposure light that is incident on the back surface of the transparent substrate as the incident exposure light propagates to the front surface of the transparent substrate.

According to another aspect of the inventive concept, the photomask or optical component is a product made by a process wherein a section of the photomask substrate is irradiated with energy which does not melt and/or vaporize the medium of the photomask substrate to form the scattering element. To this end, a femtosecond laser may be used.

Embodiments of the inventive concept will be more clearly understood from the following detailed description thereof, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a plan view of an embodiment of a photomask, according to the inventive concept;

FIG. 2 is a sectional view of the photomask;

FIGS. 3 and 4 are schematic diagrams used to explain, based on Fourier optics, changes in illumination produced by a scattering element of an embodiment of a photomask according to the inventive concept;

FIG. 5 is a sectional view of another embodiment of a photomask according to the inventive concept;

FIG. 6 is a sectional view of still of another embodiment of a photomask according to the inventive concept;

FIG. 7 is a plan view of one basic form of a photomask according to the inventive concept;

FIG. 8 is a plan view of another basic form of a photomask according to the inventive concept;

FIG. 9 is a plan view of first and second adjacent shots on a wafer; and

FIG. 10 shows a plan view of a photomask according to another exemplary embodiment.

Embodiments of a photomask according to the inventive concept will now be described more fully with reference to the accompanying drawings. Like reference numbers are used to designate like elements throughout the drawings. Also, in the drawings, the thicknesses of layers and regions may be exaggerated for clarity. In particular, the cross-sectional views of the embodiments of the photomasks described herein (and of the intermediate structures leading up to the photomasks) are schematic in nature. Accordingly, the shape, for example, of regions of the illustrated photomasks may vary from those in practice due to manufacturing techniques and/or tolerances. Thus, the drawings should not be construed as limiting with respect to the inventive concept. Furthermore, relative spatial terms such as above, below, vertically, left and right are used throughout the specification for ease in describing the orientations of the elements/regions of photomasks. In this respect, the drawings provide the frame of reference for the relative spatial terms employed throughout the specification or at the very least allow the relative spatial terms to be put into their proper context. Thus, the use of relative spatial terms is not limiting insofar as the orientations that the various elements/regions may assume at the time of a manufacture or use, for example. Also, as is typical, the term “photomask” is used throughout the photomask to denote any of the optical components bearing a pattern whose image is to be transferred to a substrate using photolithography, i.e., the term “photomask” may also be used to denote what is considered by some in the art to be a reticle.

Referring to FIGS. 1 and 2 , a photomask 30 , according to the inventive concept, includes a photomask substrate 31 having a front surface 31 FS and a back surface 31 BS, mask patterns 37 on the front surface 31 FS of the photomask substrate 31 , and a scattering element 38 a disposed within the medium of the photomask substrate 31 itself.

The photomask substrate 31 is formed of a material (i.e., the aforementioned medium) that is transparent to the exposure light that is used in the process of exposing a substrate during the manufacturing of an electronic device or the like. For example, the photomask substrate 31 is a quartz or glass substrate. (Such a substrate may be referred to hereinafter as an “electronic device substrate” to distinguish it from the photomask substrate 31 ). Also, the photomask substrate 31 has a chip area 34 including a peripheral circuit region 32 and a plurality of cell array regions 33 .

The mask patterns 37 are confined to the chip area 34 . Basically, images of the mask patterns 37 are transferred to the electronic device substrate by directing the exposure light onto a resist on the electronic device substrate via the photomask 30 and an illumination system (described later on) of the exposure equipment. In this respect, the mask patterns 37 may comprise light shielding film patterns that prevent some of the exposure light from being transmitted by the photomask 30 . For example, the mask patterns 37 may be half-tone film patterns whose features block (absorb) only some of the exposure light incident thereon, or 100% shielding film patterns such as chromium patterns whose chromium features block (absorb) essentially all of the exposure light incident thereon. The mask patterns 37 may each be a combination of a light shielding film pattern, and phase shift transmittance regions which shift the phase of the exposure light transmitted therethrough. Therefore, the photomask 30 may be a phase shift mask (PSM). As is known per se in the art, such phase shift transmittance regions may be constituted by recesses in a surface of the photomask substrate 31 , in which case the phase shift transmittance regions are formed by etching away portions of the photomask substrate 31 .

As mentioned above, the scattering element 38 a is disposed in the medium of the photomask substrate 31 . That is, the scattering element 38 a can be considered to be encapsulated in the medium of the photomask substrate 31 so as to be situated entirely between the front surface 31 FS and a back surface 31 BS of the photomask substrate 31 . Exposure light that is incident on the back surface 31 BS of the photomask substrate 31 and is transmitted to the mask patterns 37 is scattered and/or diffracted by the scattering element 38 a , so that the intensity of the exposure light is reduced. Hence, the intensity of the illumination incident on the mask patterns 37 is less than that of the exposure light incident on the back surface 31 BS of the photomask substrate 31 .

The scattering element 38 a does not affect the intrinsic characteristics of the medium of the photomask substrate 31 and will not in and of itself form or cause cracks to form within the photomask substrate 31 . To this end, the scattering element 38 a may be an array of spots 39 that are produced by irradiating the medium of the substrate 31 with energy which changes the refractive index of the medium without melting and/or vaporizing the medium.

For example, the scattering element 38 a is formed using a femtosecond laser whose pulses have durations of 10 −15 ms and are characterized as having a high-energy density. In general, a laser beam having an energy level of 1 mJ and pulses whose durations are on the order of less than 100 fs has an energy density of approximately 10 6 to 10 7 W/cm 2 .

As a result of irradiating the photomask substrate 31 with the laser beam emitted by a femtosecond laser having the characteristics described above, avalanche photon absorption and ionization, i.e., a multi-photon phenomenon, occurs in the medium of the photomask substrate 31 . According to this phenomenon, a predetermined part of the medium of the photomask substrate 31 is changed into plasma without being melted and/or vaporized. The high temperature of the plasma produces a high level of pressure which expands ions at high speed. However, the duration of the pulses is less than the time needed for a photon to transfer heat to peripheral lattice structures. Therefore, the plasma cools rapidly and a shock wave is produced between the rapidly cooled plasma and the adjacent relatively cold media. As a result, regions of the medium, corresponding to the spots 39 , acquire a completely different morphology from the remainder of the medium of the photomask substrate 31 . In this example, this change in morphology results in the index of refraction of the spots 39 being greater than that of the remainder of the medium of the photomask substrate 31 . Thus, light passing through the spots 39 is diffracted.

In accordance with Fourier transforms for optics, if the shape of a mask pattern 37 as a function of location is f(x), a distribution of the energy field of the illumination that is transmitted by the photomask 30 and is projected onto a pupil of the illumination system is g(k)=F {f(x)}. That is, based on the shape of the mask pattern 37 the energy field distribution of the pupil can be predicted by a Fourier transform. That is, the change in the illumination may be designed for based on a Fourier transform.

The change in the illumination as provided for by the scattering element 38 a will now be described in more detail. The change in the illumination with respect to the scattering element 38 a may be expressed as a convolution of the illumination transmitted by the photomask if the scattering element 38 a were not provided and a Fourier spectrum (a power spectrum) with respect to the scattering element 38 a.

Referring to FIG. 3 , if the phase of light transmitted through the spots 39 of the scattering element 38 a is shifted by 180° relative to the phase of light transmitted through the remainder of the medium, the scattering element 38 a may be represented as a convolution of patterns I and II. A modification of the pattern I is shown in FIG. 4 . And if the spots 39 each have a diameter d and are arrayed at a pitch p, the Fourier spectrum (the power spectrum) of the illumination transmitted by the scattering element 38 a can be expressed by equation 1 below.

E (energy field)=1-2 d 2 /p 2   (1)

The intensity of the transmitted illumination which is equal to the square of E (energy field) is expressed by equation 2 below.

I (illumination intensity)=[1-2 d 2 /p 2 ] 2 ≈1-4 d 2 /p 2   (2)

If the intensity of the illumination has no high order diffraction term and the spots 39 have a low density (d 2 /p 2 ), equation 2 yields an approximation that the maximum drop in the illumination intensity is 4 times the density (d 2 /p 2 ) of the spots 39 . However, the high order diffraction term cannot be 0 during an actual exposure. Therefore, the drop in the illumination intensity with regard to the density (d 2 /p 2 ) of the spots 39 is smaller than 4.

It can thus be concluded that the density (d 2 /p 2 ) of the spots 39 needs to be increased to reduce the illumination density. The density (d 2 /p 2 ) of the spots 39 may be reduced by making the diameter d of the spots smaller relative to the pitch p of the spots 39 . However, the smaller the diameter d of each of the spots 39 , the greater the diffraction effect and the smaller the high order diffraction term become. Also, the smaller the diameter d of each of the spots 39 , the less the shape of the pupil of the illumination system changes.

Therefore, the scattering element 38 should be designed, in terms of producing a change in the illumination intensity, so that the spots 39 have the greatest pitch p and a minimum diameter d.

Referring back to FIGS. 1 and 2 , the cell array regions 33 each have a second width D 2 and are spaced apart from each other by a first distance D 1 in a first direction. The photomask 30 also has a blind pattern 45 extending along the periphery of the photomask substrate 31 . The blind pattern 45 has a width D 3 . More specifically, the lateral surface 31 LS or edge of the photomask substrate 31 and the chip region 34 are spaced from one another by a distance D 3 . The scattering element 38 is juxtaposed with the blind pattern in the direction of the thickness of the photomask substrate 31 . The width S 1 of the scattering element 38 a , namely, the dimension of the scattering element in the same direction as D 1 , D 2 and D 3 , may be the same as or smaller than the width D 3 of the blind pattern 45 . Moreover, a first thickness t 1 of the medium of the photomask substrate 31 exists between the scattering element 38 a and the front surface 31 FS of the photomask substrate 31 . The first thickness t 1 depends on the thickness t 2 of the blind pattern 45 and parameters of the optical process/equipment in which the photomask 30 is used.

FIG. 9 shows adjacent regions of a wafer onto which the mask patterns 37 of a photomask embodied according to the inventive concept is transferred. In particular, FIG. 9 shows first and second shots A and B formed in sequence using a wafer scanner in which the photomask is loaded, and the wafer and the photomask are moved relative to each other over increments corresponding to the shots. Referring to FIG. 9 , the first shot A has a chip region 34 A and a blind pattern region 45 A corresponding to those regions of the photomask. The second shot B also has a chip region 34 B and a blind pattern region 45 B corresponding to those of the photomask. In general, regions of the wafer to which no useful pattern is transferred are minimized to increase the yield of the mass-production process.

Therefore, as can be seen in FIG. 9 , the shots are produced so that the chip regions 34 A and 34 B of the first shot A and the second shot B contact each other. Accordingly, the first and second shots A and B have overlapping regions X, Y, W, and Z. In particular, a part of the blind pattern region 45 B of the second shot B overlaps at region X a part of the chip region 34 A of the first shot A.

Meanwhile, a defect referred to as Haze occurs on a photomask in a wafer scanner after the photomask has transmitted a certain amount of the exposure light during the course of forming numerous shots. Accordingly, preventive maintenance (PM) is regularly performed to eliminate the Haze and thereby prevent the photolithographic process from being degraded. As was alluded to in the Background section, the PM includes cleaning the photomask after a pellicle has been removed from the photomask. However, the thickness of the blind pattern (e.g., a ring of chromium) formed on the photomask substrate is reduced due to a frequent cleaning of the photomask during the PM. In this case, the transmittance of the photomask is reduced due to the reduction of the thickness of the blind pattern.

More specifically, assuming again that the first shot A is followed by the second shot B, the chip region 34 A of the first shot A and the blind pattern region 45 B of the second shot B overlap each other in the region X. If the illumination has not been completely blocked in the blind pattern region 45 B of shot B due to a reduction in the thickness of the blind pattern 45 of the photomask 30 , the critical dimension (CD) of the patterns in the chip region 34 A of shot A may be affected by the exposure light transmitted through the blind pattern 45 of the photomask 30 when the second shot B is illuminated. Hence, there is non-uniformity in the critical dimension of the pattern formed in the chip region 45 A. According to the inventive concept, the scattering element 38 a is formed in the medium of the photomask substrate 31 below the blind pattern 45 , and causes loss in the exposure light propagating through the photomask 30 from the back surface 31 BS towards the blind pattern, thereby decreasing the transmittance of the exposure light by the blind pattern 45 in the case, for example, in which the blind pattern 45 becomes thin enough to transmit the exposure light. Accordingly, the uniformity of the shot is enhanced by a photomask embodied according to the inventive concept.

FIG. 5 shows of another embodiment a photomask according to the inventive concept.

Referring to FIG. 5 , a scattering element 38 b includes the spots 39 that are arrayed in each of three layers. However, the inventive concept is not so limited and encompasses embodiments in which the spots 39 are arrayed in each of various other numbers of layers. That is, according to the inventive concept, the scattering element 38 b can have a multi-layered array of spots 39 . Otherwise, the photomask 30 of the embodiment of FIG. 5 is substantially the same as that of the embodiment of FIGS. 1 and 2 .

FIG. 6 shows still another embodiment of a photomask according to the inventive concept.

Referring to FIG. 6 , a scattering element 38 c formed in the medium of the photomask substrate 31 is configured so as to allow light rays of the exposure light L to propagate obliquely at a predetermined angle θ toward the mask patterns 37 . More specifically, the length S 1 of the scattering element 38 c is smaller than the width D 3 of the blind pattern 45 , and the length S 1 of the scattering element 38 c and the thickness tl of the photomask substrate 31 between the front surface 31 FS of the photomask substrate 31 and the light scattering element 38 c are such that the rays of the exposure light L which are incident on the photomask substrate below the blind pattern 45 and which propagate obliquely toward the mask patterns 37 will pass around the light scattering element without having their intensity reduced. In some photolithography processes, it is preferable to illuminate the light rays of the exposure light L obliquely at a predetermined angle θ toward the mask patterns in order to improve the quality of image transferred to a substrate of a device being manufactured by using the photomask in the photolithography process.

Note, embodiments of the photomask have been described above with reference to FIGS. 2 , 5 , and 6 , as each having a particular scattering element 38 a , 38 b , and 38 c but the inventive concept is not limited thereto. Rather, a photomask 30 embodied according to the inventive concept may have a scattering element that combines the effects of the scattering elements 38 a , 38 b , and 38 c shown in FIGS. 2 , 5 , and 6 . To be more specific, in another example of a photomask according to the inventive concept, the scattering element is made up of spots arrayed in each of multiple layers (as in the embodiment of FIG. 5 ), and spots configured so oblique rays of the exposure light can be incident on the mask patterns 37 (as per the embodiment of FIG. 6 ).

Furthermore, in each of the embodiments of a photomask according to the inventive concept as described above with reference to FIGS. 2 , 5 , and 6 , the scattering element is uniform across the photomask substrate 31 , that is, up and down and to the left and right in the figures. However, the inventive concept is not so limited.

FIG. 10 shows a plan view of a photomask according to another exemplary embodiment.

Referring to FIG. 10 , the chip region 34 including a cell array region and a peripheral circuit region is located at the center of the photomask 30 . The blind pattern 45 has a predetermined width (D 3 ) and extends along the periphery of the photomask substrate ( 31 in FIG. 2 ). The region shaded by oblique lines is a region occupied by the spots 39 ′ a to 39 ′ c of the scattering element. In an example of an embodiment of a photomask according to the inventive concept, the spots 39 ′ a (the spots 39 of a scattering element 38 a of the type shown in and described with reference to FIG. 2 ) are disposed in the left hand portion of the photomask substrate 31 , the spots 39 ′ b (the spots 39 of a scattering element 38 b of the type shown in and described with reference to FIG. 5 ) are disposed in the right hand portion of the photomask substrate 31 , and the spots 39 ′ c (the spots 39 of a scattering element 38 c of the type shown in FIG. 6 ) are disposed in the upper and lower portion of the photomask substrate 31 .

FIG. 7 shows a basic form that a photomask 30 may take according to the inventive concept.

Referring to FIG. 7 , the chip area 34 including a cell array region and a peripheral circuit region is located at the center of the photomask 30 . The blind pattern 45 has a predetermined width (D 3 ) and extends along the periphery of the photomask substrate ( 31 in FIG.2 ). The region shaded by oblique lines is a region occupied by the scattering element 38 d . Thus, the scattering element 38 d is situated 38 e in the medium of the photomask substrate below the blind pattern 45 . The region where the scattering element 38 d is not present below the blind pattern 45 has a width S 2 . In some embodiments, the width S 1 of the scattering element 38 d is smaller than the width D 3 of the blind pattern 45 (D 3 =S 1 +S 2 ). However, the precise width S 1 of the scattering element 38 d depends on various conditions or parameters of the process in which the photomask is being used. Thus, in some embodiments, the width S 1 of the scattering element 38 d is the same as the width D 3 of the blind pattern 45 (S 2 is 0).

FIG. 8 is a plan view of another example of the basic form of a photomask embodied according to the inventive concept.

Referring to FIG. 8 , the blind pattern 45 has a predetermined width D 3 and extends along the periphery of the photomask substrate ( 31 in FIG. 2 ). The region shaded by oblique lines is a region occupied by the scattering element 38 e . Thus, the scattering element 38 e is situated in the medium of the photomask substrate below the blind pattern 45 . The width S 1 of the scattering element 38 e may be the same as or smaller than the width D 3 of the blind pattern 45 . However, the scattering element 38 e has a scattering section H that protrudes from the band of the scattering element having width S 1 , at a corner of the photomask substrate, toward the chip region 34 . For example, the scattering element H corresponds to the region W of the second shot B in FIG. 9 . Also, and as shown in FIG. 8 , the scattering element 38 e can include a respective scattering section H at each corner of the photomask substrate.

As described above, a photomask or equivalent optical component according to the inventive concept has a scattering element encapsulated in a substrate medium, and configured to adjust/filter the intensity, shape, and/or components of light that is incident onto the back surface of the substrate and propagates through the substrate. In particular, the scattering element may be configured to reduce the transmittance and intensity of exposure light that propagates towards the blind pattern. The intensity distribution of the light may be easily designed for by selecting a particular density of the spots making up the scattering element.

The photomask or equivalent optical component according to the inventive concept may thus be used to manufacture various electronic devices which require precise patterns, examples of which include a central processing unit (CPU), a digital signal processor (DSP), a processor combining a CPU and a DSP, an application specific integrated circuit (ASIC), a microelectromechanical (MEM) device, an optoelectronic device, and a display device such as an LCD.

Finally, embodiments of the inventive concept have been described herein in detail. The inventive concept may, however, be embodied in many different forms and should not be construed as being limited to the embodiments described above. Rather, these embodiments were described so that this disclosure is thorough and complete, and fully conveys the inventive concept to those skilled in the art. Thus, the true spirit and scope of the inventive concept is not limited by the embodiments described above but by the following claims.