Description of Related Art A conventional photolithography projection system projects a UV light pattern on a semiconductor device to expose selected portions of a light sensitive coating (photoresist) on the device. The photoresist coating can then be developed to create a mask for a fabrication process such as etching or doping of the device. The photolithography systems commonly employ a photomask or reticle that controls which portions of the device are illuminated. For integrated circuit manufacturing, the photomask must have a precise pattern that the projection transfers, with or without demagnification, to the integrated circuit device.

Figs. 1 A to 1 C illustrate a method for making a hard-surface photomask such as commonly employed in integrated circuit fabrication. Photomask making begins with an optically transparent substrate 110 that is often made of quartz. One side of substrate 110 is coated with an opaque film 120 of a material such as chromium or another relatively heavy metal. A photoresist layer 130 is then applied to opaque film 120, and a pattern generation process exposes a pattern on photoresist layer 130 to light or electron bombardment. Various types of pattern generation equipment are known. For example, scanning systems can be programmed with a digitized image or pixel pattern that corresponds to the desired pattern to be exposed on photoresist layer 130. The scanning system typically directs light or electron beams at the areas to be exposed. The scanning system exposes only the photoresist areas that correspond to the pixels having pixel values indicating that the areas should be exposed.

As shown in Fig. 1B, developing of photoresist layer 130 creates a photoresist pattern 132 with openings 134 that expose the underlying opaque layer 120. Photoresist pattern 132 and openings 134 have a critical or minimum feature size that depends on the pattern generation equipment used to expose photoresist layer 130. An etching process

using photoresist pattern 132 as an etching mask removes portions of opaque layer 120 to create an opaque pattern 122 having openings 124. Following the etch, photoresist pattern 132 is stripped from substrate 110, leaving a hard photomask 100 consisting of opaque pattern 122 and substrate 110 as shown in Fig. 1D. The photomask is then measured, inspected and repaired if necessary.

A problem with the convention process for making a hard photomask is that the completed photomask has a critical feature size that is larger than the feature size that the pattern generation equipment can create. In particular, the etching process using photoresist pattern 132 as etch mask is typically a wet chemistry etch (or isotropic) process. An isotropic, wet chemistry etch process has historically been desirable because a wet etch process is inexpensive and relatively defect free. However, the isotropic etching undercuts photoresist pattern 132 by about the thickness of opaque layer 120 or more and makes openings 124 in opaque pattern 122 larger than the original openings 134 in photoresist pattern 132. For previous generations of semiconductor devices, the undercutting, while not desirable, was acceptable. The digitized pattern for photoresist pattern 132 can be tailored to accommodate an expected etch undercut, and the etching process forming opaque pattern 122 can be tightly controlled to achieve the expected etch undercut. However, as feature sizes become smaller, the size of the undercut becomes more difficult to accommodate, and future generations of integrated circuits having smaller feature sizes may find the undercutting unacceptable and unaccommodable.

To overcome this difficulty, the mask making community has, largely unsuccessfully, explored many options and approaches such as the use of anisotropic (dry) etch of the opaque material (chromium). Heavy metal compounds, that the dry etch processes inherently form, accrete and precipitate to create a high probability of defects.

Adding to the difficulty is a relatively poor etch selectivity (between photoresist pattern 132 and opaque layer 120) that results in an effective undercut because the dry etch widens openings 134. Further, variations in the side wall angle of openings 134 in photoresist pattern 132 perturb the final etch dimensions of openings 124. Many alternative opaque materials have also been explored with similar results.

As the feature size of integrated circuits decreases, the difficulty in creating precisely patterned photomasks increases. At the"180 nm"device generation, the typical size of a critical feature for a 4X photomask is around 700 nm. At this size, and with a typical thickness of 100 to 125 nm for opaque layer 120, the roughly 200 to 250 nm of etch

undercut is extremely difficult to accommodate. Accordingly, methods for creating an accurate photomask and eliminating the problems of etch undercut is sought.

SUMMARY In accordance with an aspect of the invention, a photomask fabrication method processes photoresist on the photomask into a conformal and hardened material that is not stripped off but rather becomes an optical and structural component of the photomask. As such, the photoresist optically defines the dimension of critical features. This eliminates the dimensional uncertainty that arises from etching of underlying opaque material, and the mask fabrication process no longer needs to account for the etch undercut. The process thus provides improved feature fidelity that is attendant with iso-focal pattern generation.

Further,"additional chrome"defects, which are the major source of defects in known photomasks, are reduced or eliminated because the etch time and the magnitude of the etch undercut no longer need to be precariously controlled for minimization. As an additional benefit, the design-rule-checking of a digitized pattern prior to pattern generation, is no longer complicated by the accommodation of etch undercut because photomasks in accordance with embodiments of the invention have feature sizes that are substantially the same as the digitized feature sizes used in pattern generation. The photoresist's thickness and optical transmission can be selected to allow a phase shift of transmitted light at the boundary of all features to provide a rim shifter phasemask.

Further, the normal opaque material can be eliminated from the mask fabrication process altogether such that the above benefits accrue to a simplified mask making process that is also significantly less prone to defects. Here, proper selection of the photoresist's thickness and optical transmission allows a phase shift of transmitted light so that the resulting photomask is an attenuated phasemask.

In accordance with one embodiment of the invention, a photomask includes a transparent substrate and a hardened photoresist pattern overlying the transparent substrate.

An opaque pattern made of a material such as chromium can be between the hardened photoresist pattern and the transparent substrate or can be omitted. When an opaque pattern is used, the opaque pattern has openings that correspond to openings in the hardened photoresist pattern. Etch undercutting causes each of the openings in the opaque pattern to be larger than a corresponding opening in the hardened photoresist pattern.

However, the hardened photoresist pattern extends beyond edges of the openings in the

opaque pattern and conforms to the surface of the underlying structures to contact the transparent substrate through the openings in the opaque pattern. When the opaque pattern is omitted, the photoresist pattern can lie completely on the transparent substrate. In either case, dyes added to the photoresist pattern can make the hardened photoresist pattern opaque to a wavelength of light used in a photolithography system for which the photomask is designed.

In accordance with another aspect of the invention, the thickness and transmissivity of the photoresist pattern are such that interference of light transmitted through the openings and light transmitted through the hardened photoresist sharpens edges in a projection of the photomask, thereby making the photomask a phasemask.

In accordance with another embodiment of the invention, a method for manufacturing a photomask, includes: forming a photoresist layer overlying a transparent substrate; patterning the photoresist layer to create a photoresist pattern; and hardening the photoresist pattern for use as a permanent part of the photomask. The photoresist layer can be formed directly on the transparent substrate or one or more intervening structures can be between the photoresist layer and the transparent substrate. For example, the method may further include: forming an opaque layer on the transparent substrate, wherein the photoresist layer is on the opaque layer. The opaque layer can be etched using the photoresist pattern as an etch mask. When this etching undercuts the photoresist pattern, the method further includes heating or flowing the photoresist layer after patterning of the opaque layer. The flowing of the photoresist layer brings the photoresist layer into contact with the transparent layer where the etching undercut the photoresist layer.

When the photoresist layer is directly on the transparent substrate, an anti-reflective layer can be formed on the opposite side of the transparent substrate or on top of the photoresist to reduce stray reflections during optical patterning of the photoresist layer.

The anti-reflective layer is removed after the patterning. When electron beams pattern the photoresist, a transparent conductive layer next to the photoresist layer can prevent charging effects.

BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 A, 1 B, 1 C, and 1 D shows cross-sections of structures created during a process of fabricating a conventional hard photomask.

Figs. 2A, 2B, 2C, and 2D show cross-sections of structures formed in a process for making a photomask in accordance with an embodiment of the invention.

Figs. 3A and 3B show cross-sections of structures formed in a process for making a photomask in accordance with another embodiment of the invention.

Fig. 4 shows a cross-section of a photomask in accordance with the invention protected by a pellicle.

Use of the same reference symbols in different figures indicates similar or identical items.

DETAILED DESCRIPTION In accordance with an aspect of the invention, a process for fabricating a hard photomask hardens a photoresist pattern instead of removing the photoresist pattern as in prior photomask fabrication processes. Accordingly, the accuracy with which the photoresist pattern was formed controls the accuracy and minimum feature size of the photomask, and etch undercut does not increase the feature size of the photomask.

Further, a dry etch process, which is more costly and defect prone, can be avoided.

Fig. 2D shows a cross-section of a portion of a photomask 200 in accordance with an embodiment of the invention. Photomask 200 includes an optically transparent substrate 110, an opaque pattern 222, and a photoresist pattern 236. Opaque pattern 222 is on substrate 110. Photoresist pattern 236 is on opaque pattern 222 but is also in contact with substrate 110 in openings 224 through opaque pattern 222. Opening 238 in photoresist pattern 236 correspond to the openings 224 in opaque pattern 222 but are generally smaller than the corresponding openings 224.

Photomask 200 is generally less opaque in the areas where photoresist pattern 236 is in openings 224 and more opaque in areas where both photoresist pattern 236 and opaque pattern 222 overlie substrate 110. However, typical photosensitive coatings used in integrated circuit manufacturing have non-linear responses to light exposure.

Accordingly, the opacity of photoresist layer 236 and the exposure characteristics of the photosensitive coating being patterned can be selected so that openings 238 (not openings 224) control which areas of the photosensitive coating receive adequate exposure to cause the required photo-chemical change in the photosensitive coating.

Further, in some embodiments of the invention, photoresist pattern 236 has a thickness selected so that light of the wavelength to be used in a photolithography process

employing mask 200 under goes a 180° phaseshift relative to light passing through openings 238. The distance that photoresist pattern 236 extends into openings 224 can also be selected so that photomask 200 is a rim shifter phasemask. In the phasemask embodiments, light passing through photoresist pattern 236 and light passing through openings 238 interfere and create sharper edges in the projected pattern.

Figs. 2A to 2C illustrate the initial steps of the fabrication process for photomask 200. The fabrication process begins with depositing an opaque layer 220 on a substrate 110. Substrate 110 can be a quartz substrate or a substrate of another material that is transparent to a selected wavelength of light for which photomask 200 is designed.

Opaque layer 220 is material such non-reflective chrome or another material that is generally opaque to the selected wavelength. For pattern generation using electron beams, opaque layer 220 is preferably a metal layer or otherwise a conductive layer. However, opaque layer 220 can be thinner in photomask 200 than similar layers used in conventional photomasks because photoresist pattern 236 helps block transmission of light. The thickness for an opaque layer 220 of chrome would typically be between about 70 nm and about 250 nm. However, as described below, opaque layer 220 can be made of a transparent material or omitted altogether in alternative embodiments of the invention.

A photoresist layer 230 formed on opaque layer 220 has a thickness selected so that photoresist pattern 236 will provided a desired opacity and/or a desired phaseshift after hardening. Additionally the opacity of photoresist layer 230 can be modified through the addition of dyes so that the combination of the thickness and optical transmission allows a light transmission of less than 0.5% so that the photoresist acts as an opaque material as in a binary mask. Dyes that control the opacity of photoresist have previously been used in integrated circuit manufacture to control spurious reflections when exposing a photoresist layer. Such dyes can be included in photoresist layer 230 to provide an opacity less than about 0.5%. Alternatively, the combination of the thickness and the optical transmission of photoresist layer is selected so that harden photoresist mask 236 transmits about 10 to 20 % of incident light with an appropriate phase shift (e. g., about 180 degrees). With a correctly chosen phase shift, the edge definition of the mask features is enhanced during use as in a rim shifter phasemask.

Conventional pattern generation equipment such as used in the conventional process for forming photomasks can pattern photoresist layer 230. However, for photomask 200, the original digitized pattern for the pattern generation equipment is not

sized to accommodate the subsequent etch undercutting of photoresist pattern 236. This allows automated design rule checking (DRC) to be performed on the actual data that is submitted for pattern generation. The pattern generation equipment can write the pattern onto photoresist layer 230 with iso-focal patterning such that the developed photoresist feature sizes are substantially equal to the intended final feature sizes and relatively immune to focusing errors. Opaque layer 220 acts as an anti-reflective layer that stops stray reflections during optical pattern generation and acts as a conductive layer to control charge effects when an electron-beam pattern generator patterns photoresist layer 230.

Opaque layer 220 can be made very thin, for example, about 25 nm, and still serve the purpose of acting as an anti-reflective layer or a conductive layer that improves the pattern generation process. Fig. 2B shows a photoresist pattern 232 having openings 234 that developing of photoresist layer 230 creates.

Wet etching of opaque layer 220 with photoresist pattern 232 as an etch mask forms opaque pattern 222 shown in Fig. 2C. The maskmaker selected over-etch need not be precariously minimized for formation of photomask 200. Specifically, over-etching of opaque pattern 222 is not a great concern because photoresist pattern 232 will define the edges of photomask 200. Accordingly, etching can be for a sufficient time to ensure proper formation of openings 224, and to reduce or eliminate"additional-chrome"type defects. Further, opaque layer 222 can be intentionally over-etched to undercut photoresist pattern 232 by more than the minimum distance. Extending the undercut by a controlled amount allows selection of the distance that hardened photoresist pattern 236 extends into opening 224 and helps eliminate additional chrome defects.

The mask is then baked in a convection or hot plate oven to flow the photoresist overhanging the edges of opaque mask 222 down onto substrate 110. Depending on the photoresist type and thickness, this will occur within several minutes of achieving an optimal temperature that would typically be in the range of 100 to 130 °C. The flowing photoresist 236 causes the size of openings 238 to differ slightly from the size of openings 234. The difference depends on the thickness of opaque pattern 222 and the undercutting of photoresist pattern 232. However, this change in size is much less than the change etch undercutting causes in prior photomasks. Additionally, making opaque pattern 222 thinner or eliminating opaque pattern 222 altogether can reduce or eliminate the change. After flowing, the photoresist is"hardened"with ultraviolet curing, additional high temperature baking, ion implantation or some optimized combination these processes. For example,

photoresist pattern 236 can be UV hardened to set the pattern, and then high temperature baked to remove solvents. The hardening increases the durability of photoresist pattern 236 and removes volatile compounds that might otherwise evaporate from photomask 200 during use in a photolithography system.

When hardened photoresist pattern 236 is sufficiently durable, photomask 200 is high pressure cleaned, inspected, and repaired, if necessary. Photomask 200 may not benefit from some of current photomask repair techniques since current techniques are optimized for chrome photomask, not masks including photoresist. However, techniques such as precision shaving of a chrome pattern to remove excess material are expected to be applicable with few changes to hardened photoresist patterns. Further, industry data shows that the majority of first-inspection defects of conventional photomasks are of the additional-chrome type and that most of these defects arise from attempts to minimize the etch undercut. Consequently, photomasks manufactured with processes in accordance with embodiments of this invention can be expected to have much better first-inspection results.

Figs. 3A and 3B illustrate a process of making a photomask 300 that lacks the opaque pattern 222 of photomask 200. As shown in Fig. 3A, a photoresist layer 330 is coated directly onto a transparent substrate 110 without an intervening opaque layer. In one embodiment, the thickness and optical transmission of photoresist layer 330 (as modified with the use of dyes) allows a transmission of less than about 0.5 % of the light used in photolithography, and photoresist layer 332 acts as an opaque material as in a binary mask. Alternatively, the combination of the thickness and the optical transmission of photoresist layer 330 is selected to transmit an appropriate fraction (e. g., about 10 to 20%) of the incident light, with an appropriate phase shift (e. g., about 180°) so that the edge definition of features projected using photomask 300 is enhanced as in an attenuated phasemask.

A pattern generator transfers a digitized pattern to photoresist layer 330. For optical or laser beam pattern generators, an anti-reflective layer above and/or below substrate 110 can inhibit spurious reflections during patterning. An anti-reflective layer on photoresist layer 330 may also improve patterning. Fig. 3A shows an optional anti- reflective layer 340, which is on a backside of substrate 110. In one embodiment, anti- reflective layer 340 is a non-reflective chrome coating on the backside of substrate 110, and substrate 110 and chrome 340 form a conventional photomask blank that is used

upside down. Anti-reflective layer 340 is completely removed after optical pattern generation but before use of photomask 300 for photolithography. For an electron beam pattern generator, anti-reflective layer 340 is not required. However, to better manage charging effects, photoresist layer 330 can include a conductive photoresist or have an underlying or overlying conducting layer (not shown). If a transparent conductive material such as indium-tin oxide is on photoresist layer 330, the conductive layer can be removed completely after pattern generation. If a conductive material such as chromium that is not transparent underlies photoresist layer 330, the underlying conductive layer is etched to increase light transmission through openings 334 in photoresist pattern 332.

After patterning, developing of photoresist layer 330 forms photoresist mask 332 with openings 334 that expose portions of substrate 110. Photoresist mask 332 is then hardened using ultraviolet curing, high temperature baking, ion implantation or a combination these processes. Such hardening processes are well known in integrated circuit manufacturing and used, for example, to improve the photoresist's adhesion or resistance to etching solutions. UV hardening is also used in integrated circuit manufacture to fix a photoresist pattern to prevent the photoresist from flowing during thermal processes.

The mask making process for photomask 300 is simpler than the process of Figs.

2A to 2D because photomask 300 does not require etching of an opaque layer or flowing of photoresist. Avoiding the flowing process leaves the photoresist pattern with substantially the same pattern that pattern generation formed. Photoresist pattern 332 after hardening is sufficiently durable for cleaning and testing, and a pellicle can protect photoresist pattern 332 during use in a photolithography system.

Fig. 4 shows a completed photomask 400 having a protective pellicle 410. Pellicle 410 includes a pellicle frame 412 and a transparent sheet 414 overlying mask material 420 on a transparent substrate 110. Mask material 420 includes harden photoresist in a structure such as photomask 200 of Fig. 2D or photomask 300 of Fig. 3B. Mask material 420 is absent from the boarder of substrate 110 to provide a durable edge for handling of photomask 400 and mounting of photomask 400 in a photolithography system. The original pattern generation and development process for mask material 420 can easily remove photoresist from the border area of substrate 110.

Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.