Apparatus and method for reducing contamination in immersion lithography

An apparatus for reducing contamination in immersion lithography includes a wafer chuck assembly having a wafer chuck configured to hold a semiconductor wafer on a support surface thereof. The wafer chuck has a gap therein, the gap located adjacent an outer edge of the wafer, and the gap containing a volume of immersion lithography fluid therein. A fluid circulation path is configured within the wafer chuck so as to facilitate the radial outward movement of the immersion lithography fluid in the gap, thereby maintaining a meniscus of the immersion lithography fluid at a selected height with respect to a top surface of the semiconductor wafer.

BACKGROUND

The present invention relates generally to semiconductor device manufacturing, and, more particularly, to an apparatus and method for reducing contamination in immersion lithography.

Lithography is one of the most important techniques utilized in semiconductor manufacturing, and is particularly used to define patterns, such as those employed in a wiring layer patterning process or a doped-region defining process for example. A lithography process generally includes an exposure step and a development step, wherein the exposure step utilizes a light source to irradiate a photoresist layer directly or through a photomask to induce chemical reactions in exposed portions. The development step is conducted to remove the exposed portion in positive resist (or the unexposed portion in negative resist) and form photoresist patterns, thus completing the transfer of photomask patterns or virtual patterns to the resist material.

Immersion lithography (IL) is rapidly emerging as the technique of choice for printing sub-100 nm photoresist structures while still using 193 nm exposure sources. By increasing the index of refraction of the medium between the last lens element of the exposure tool and the resist-coated substrate, the numerical aperture of the lithography system is increased and thus the printable minimum feature size for a given exposure wavelength can be reduced in accordance with the well-known Rayleigh equation. Accordingly, existing immersion lithography processes are conducted in a liquid phase environment, and thus a higher resolution is achieved since the refractive index of the immersion liquid (e.g., ultra pure water) is higher than that of air (about 1.47 versus 1.0). Therefore, the dimensions of the formed IC devices can be further scaled using an immersion lithography technique.

However, one drawback associated with immersion lithography stems from the physical contact between the immersion fluid and the resist material, which can potentially lead to partial image integrity failure and contamination embedded in or below the resist. More specifically, evaporation of the immersion fluid off the resist surface on the trailing edge of the shower head during exposure can lead to the concentration of trace contaminants, which can be transferred during the subsequent processing steps and finally affect device yield and performance in a severe manner. For example, traces of colloidal silica present in the immersion fluid can be concentrated in areas where immersion fluid evaporation is verified.

In addition, the trailing edge of the water pool contained by the showerhead can easily leave behind a residual immersion fluid layer, or eventually break down into droplets of variable size, under specific scanning conditions. For example, with typical wafer stage speeds in the order of 500-1000 mm/s, any discontinuity present on the scanned surface will affect the mechanical stability of the fluid pool and lead to the formation of fluid droplets. Similarly, a low contact angle between the immersion fluid and the scanned surface will increase the shape and size of the trailing fluid edge, thus increasing the chances of forming a residual fluid layer. Either the presence of a residual fluid layer or droplets can easily lead to the formation of defects. Extractable components from the topcoat or resist layers (e.g., oligomeric material, photoacid generator, photogenerated acid, base quencher) can be extracted by the immersion fluid and result in micromasking or watermark-like defects upon fluid drying.

Accordingly, it would be desirable to be able to reduce or eliminate altogether the contamination left behind by immersion lithography.

SUMMARY

The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by an apparatus for reducing contamination in immersion lithography. In an exemplary embodiment, the apparatus includes a wafer chuck assembly including a wafer chuck configured to hold a semiconductor wafer on a support surface thereof. The wafer chuck has a gap therein, the gap located adjacent an outer edge of the wafer, and the gap containing a volume of immersion lithography fluid therein. A fluid circulation path is configured within the wafer chuck so as to facilitate the radial outward movement of the immersion lithography fluid in the gap, thereby maintaining a meniscus of the immersion lithography fluid at a selected height with respect to a top surface of the semiconductor wafer.

In another embodiment, a wafer chuck assembly includes a first chuck section configured to hold a semiconductor wafer on a support surface thereof, and a second chuck section removably attached to the first chuck section. The first chuck section has a gap therein, the gap located adjacent an outer edge of the wafer, and the gap containing a volume of immersion lithography fluid therein. A fluid circulation path is configured within the first chuck section so as to facilitate the radial outward movement of the immersion lithography fluid in the gap, thereby maintaining a meniscus of said immersion lithography fluid at a selected height with respect to a top surface of the semiconductor wafer.

In still another embodiment, a method for reducing contamination in immersion lithography includes retaining a semiconductor wafer on a support surface of a wafer chuck, the wafer chuck having a gap therein, the gap located adjacent an outer edge of the wafer, and the gap containing a volume of immersion lithography fluid therein. A fluid circulation path is provided within the wafer chuck so as to facilitate the radial outward movement of the immersion lithography fluid in the gap, thereby maintaining a meniscus of the immersion lithography fluid at a selected height with respect to a top surface of the semiconductor wafer.

DETAILED DESCRIPTION

Disclosed herein is an apparatus and methodology for reducing contamination associated with immersion lithography. Generally speaking, wafer contamination is left behind near the wafer edge, and in a manner that such contamination is produced as a result of interaction of the immersion fluid with the topography between the wafer edge and the chuck. Recent simulations have shown that reducing topography on the surface covered by the tool showerhead helps to maintain the meniscus, and to avoid bubble formation. Thus, it is advantageous to artificially extend the wafer surface, so as to make the transition between the wafer and the chuck as flat as possible without sharp transitions.

As is outlined in greater detail hereinafter, the exemplary embodiments herein maintain fluid meniscus integrity in the topography gap of a wafer chuck by introducing an internal fluid circulation path within the chuck. The circulation path includes the gap itself, thus flowing immersion fluid through the gap (between the outer edge of the wafer and the chuck) radially outwards, and maintaining the water level at same height as the wafer surface. Moreover, the fluid level may be maintained at the same height as wafer surface with active or passive control of the fluid circulation path. It should be noted that any suitable fluid may be used for the fluid circulation path within the chuck, so long as it optically matches the immersion fluid of the lithography process and does not damage the surrounding surfaces of the wafer.

Referring initially toFIG. 1, there is shown a partial cross-sectional view of a portion of a chuck assembly100that may be used in conjunction with, for example, immersion lithography. As is shown, a wafer chuck102has a semiconductor wafer104held thereupon, with the wafer104depicted as having a thin layer of photoresist material106formed thereon. It will be noted that the relative dimensions of the chuck102, wafer104and resist layer106are not to scale and are depicted as such for illustrative purposes only. However, as is shown inFIG. 1, there exists a substantial topography at the outer edge of the wafer104as a result of the air gap108formed within the chuck102.

Accordingly,FIG. 2is a partial cross-sectional view of a chuck assembly200configured for immersion lithography, in accordance with an exemplary embodiment of the invention. As is shown, a wafer chuck202is modified to include an immersion fluid supply line210and an outer channel204that facilitates the outward flow of immersion fluid from the gap108, thus forming a liquid surface that extends from the edge of the wafer104top surface to the wafer chuck surface, thereby filling the air gap with the added fluid. While the outermost portion206of the chuck has a height that roughly correlates to the height of the wafer104, it will be noted that the intermediate portion (lip)208of the chuck between the gap108and the outer channel204has a height that is lower than the top of the outermost portion206and the wafer104. This allows fluid to travel over the top of the lip208, leading to a reduction in splashing and thus contamination.

In addition to the outer channel204, a fluid circulation path is also formed within the chuck202. Particularly, an inlet path210allows a pressurized source of fluid to flow into the bottom of the gap108, while a negative pressure return path212originates from a sidewall formed within the outer channel204. In an exemplary embodiment, the fluid used and circulated through the fluid circulation path has the same optical characteristics as that used for the immersion lithography, so as to avoid any changes in optical characteristics from any mixing therebetween. Furthermore, in order to prevent optical fluid from entering beneath the surface of the wafer104where it contacts the chuck202, a seal214(e.g., an O-ring) is positioned between the bottom of the wafer and the bottom of the gap108. As will be discussed in further detail hereinafter, a variety of seal shapes and materials may be implemented.

FIG. 3illustrates an optional feature that may be introduced within the chuck assembly200ofFIG. 2, for the purpose of eliminating waves as the chuck moves about. More specifically, a plurality of side grooves216is formed (at periodic intervals) within the lip208of the chuck202. Thereby, an additional level of fluid level control is achieved since, in addition to flowing over the top of the lip, the optical fluid can also flow through each of the side grooves216.FIG. 4is a partial side cross-sectional view, taken along the lines4-4ofFIG. 3, illustrating in further detail one of the grooves216formed in the lip208.

Referring now toFIG. 5, there is shown a partial cross-sectional view of a chuck assembly500configured for immersion lithography, in accordance with an alternative embodiment of the invention. As is shown, a wafer chuck502is modified to include an interior fluid circulation path. However, in contrast to the embodiment ofFIGS. 2 through 5, the interior fluid circulation path of the chuck502inFIG. 5is configured directly beneath the existing gap108. As such, when immersion fluid within the gap108is accelerated, it is caused to flow from the direction of the pressurized inlet path510, over a lip508beneath the gap108, to the return path512. Moreover, the fluid level of the embodiment ofFIG. 5is maintained through an active control approach in that a pressure sensor504is configured within the chuck502in order to sense the fluid pressure within the path. In this manner, the positive pressure of the inlet path510and the negative pressure of the return path512may be independently controlled to adjust for changes in pressure in the immersion fluid in the gap, thereby maintaining water level110and meniscus integrity of the passing immersion fluid.

One particular advantage associated with the embodiment ofFIG. 5is a simpler flow of fluid within the chuck502. As is the case withFIG. 2, the chuck502also includes a sealing ring514to prevent immersion fluid from coming between the bottom surface of the wafer104and the chuck502.

In addition to active control, the fluid level within an immersion lithography chuck assembly can also be maintained through passive control means. For example,FIG. 6is partial cross-sectional view of a chuck assembly600configured for immersion lithography, in accordance with still another embodiment of the invention. As is shown, the passive control embodiment provides a modified wafer chuck602that incorporates a first water column (i.e., the existing gap108) and a second column604formed at an outer location with respect to the radius of the chuck602. Similar to the earlier embodiments, a fluid circulation path is once again provided within the chuck602for maintaining the integrity of the fluid meniscus110.

The fluid circulation path, including inlet path610and return path612, is directed through the second column604, which further includes an overflow lip608. Thus, fluid traveling in an outward direction will flow over the lip608and into the return path612of the second column604. The passive control of the fluid level in the gap108is achieved through the control of the second column604, since the gap108is fluidly connected to the second column604through passage606formed within the chuck602. In addition, a Venturi tube607is formed at the bottom of the gap108, connecting the gap108to the inlet path610and thus allowing for the circulation of fluid through the gap108as well. As is the case with the embodiments ofFIGS. 2 and 5, the chuck602also includes a sealing ring614to prevent immersion fluid from coming between the bottom surface of the wafer104and the chuck602.

Accordingly, in operation of the passively controlled chuck assembly600, fluid passes by the first column (i.e., gap108) and through the Venturi tube607, which sucks fluid from the gap108. Thereby, the excess fluid left over from the passing of the meniscus110over the gap108is removed from the gap108. Thereafter, the excess fluid joins the inlet path610where it then flows over the lip608and into a drain (i.e., return path612), thus maintaining the level of the fluid at the top surface of the chuck602. Because the two columns (gap108, second column604) are connected (e.g., through passage606) in zones of equal pressure at equal height, the fluid in the gap108will be maintained at the same level as that present in the second column604. It will be noted that the flow of fluid within the chuck602need not be continuous, and may instead be made to occur at selected locations along the circumference of the chuck602.

Thus configured, the passively controlled chuck assembly600allows for very fast control of fluid levels adjacent to the wafer104, by minimizing the topography that the meniscus110crosses in a stable manner. Since the Venturi tube607provides for circulation of immersion fluid through the chuck gap108, contamination of the fluid is less likely to accumulate, which in turn results in a smaller probability of contaminant particles being deposited on the surface of the wafer104. A second advantage of having a series of Venturi tubes607and connecting passages606at a specified intervals is that they provide for a method to prevent undue increase in fluid pressure in the gap108during chuck acceleration.

By way of further illustration,FIG. 7is a cross-sectional view of a chuck assembly700of the passive control type illustrated inFIG. 6, and depicts an exemplary two-piece construction embodiment of the same. As is shown, a first chuck section702aincludes the interior plumbing for the chuck, as well as the surface to which the wafer104is held. In particular, the first chuck section702aincludes the fluid inlet path610and return path612described above, as well as the Venturi tubes607at the bottom of the gap108. Plugs704may be inserted into the first chuck section702ato prevent the fluid from leaking outside the chuck.

Further, the first chuck section702ais removably attached (e.g., by means of bolts706) to a second chuck section702b. The second chuck section702b, once attached, also serves to define a barrier between the first column (gap108) and the second column604for passive fluid level control. As can been seen, the second chuck section702balso includes the interior passage606so as to bring the first and second columns in fluid communication with one another, and achieve the passive control of the gap fluid. O-rings708may also be used to seal the first and second chuck sections together, as also shown inFIG. 7. It will be noted that the detailed exemplary embodiment ofFIG. 7does not illustrate the sealing rings that prevent fluid from coming between the bottom of the wafer104and the first chuck section702a.

FIG. 8is a top view of the chuck assembly700ofFIG. 7. In addition to illustrating an exemplary fluid distribution path,FIG. 8also shows one possible example of the relative number and positioning of inlet and outlet ports in the first column604with respect to the inlet and return fluid paths610,612. An exemplary distribution of Venturi tubes607within the gap108is further illustrated, although it will be appreciated that a different number and location of tubes can also be implemented.

As stated earlier, and regardless of the particular chuck assembly embodiment utilized, it is desirable to prevent immersion fluid (e.g., water) from getting beneath the wafer, between the bottom of the wafer and the chuck surface. More specifically, since there is vacuum holding down the wafer, the immersion fluid will have a tendency to seep towards the lower pressure. As such, it is advantageous to block this path by (for example) placing a sealing ring at the outer edge of the wafer support. To this end, several types and shapes of such a sealing ring are available, and from various materials.

For example,FIGS. 9(a) through9(d) illustrate various possible cross-sectional shapes for the sealing rings discussed above. In particular, the sealing ring may be of a delta shape as shown inFIG. 9(a), an X-shape as shown inFIG. 9(b); an O-ring as shown inFIG. 9(c); and a square ring as shown inFIG. 9(d). Other cross-sectional shapes, however, are also contemplated. Furthermore, the sealing rings may be made from any suitable material including, but not limited to: nitrile (buna), silicone, fluorosilicone, hydrogenated nitrile, fluorocarbon (e.g., Viton® by DuPont), neoprene, ethylene propylene, butyl, polyurethane, ethylene acrylic (e.g., Vamac® by DuPont), polyacrylate, and tetrafluoroethylene-propylene (e.g., Aflas® by Asahi Glass).

Finally,FIG. 10depicts an alternate location of the sealing ring902with respect to the wafer edge support portion904of a chuck assembly. Whereas the previously described embodiments illustrate the sealing ring positioned outside of the wafer edge support904(with respect to the center of the wafer), the sealing ring902inFIG. 10is disposed on the inside of the wafer edge support904. If the sealing ring902is located outside the wafer edge support, there is a small force present on the wafer (due to the positive external seal pressure) that could possible bend the wafer upwards and cause defocus errors near the edge of the wafer. Although this condition is less likely with small overlap distances between the wafer edge and the wafer edge support, the inside placement of the sealing ring902with respect to the wafer edge support904would eliminate any such deflection.