Abstract:
An immersion lithography apparatus includes an optical system having a liquid delivery unit. The liquid delivery unit is arranged to deliver a layer of an immersion liquid onto a surface of a wafer as well as an annulus of a barrier liquid adjacent an exterior wall of the immersion liquid. The presence of the barrier liquid prevents ingress to the immersion liquid of a gas external to the immersion liquid.

Description:
FIELD OF THE INVENTION 
     This invention relates to an immersion lithography apparatus of the type, for example, that projects a pattern through an immersion liquid onto a wafer. This invention also relates to a method of performing immersion lithography of the type, for example, that projects a pattern through an immersion liquid onto a wafer. 
     BACKGROUND OF THE INVENTION 
     In the field of semiconductor processing, photolithography is a widely employed technique to “pattern”, i.e. define a profile in one or more layer of semiconductor material, a semiconductor wafer. Using this technique, hundreds of Integrated Circuits (ICs) formed from an even larger number of transistors can be formed on a wafer of silicon. In this respect, for each wafer, the ICs are formed one at a time and on a layer-by-layer basis. 
     For about the last four decades, a photolithography apparatus, sometimes known as a cluster or photolithography tool, has been employed to carry out a photolithographic process. The cluster comprises a track unit that prepares the wafer, including providing layers of photosensitive material on the surface of the wafer prior to exposure to a patterned light source. To expose the wafer to the patterned light source, the wafer is transferred to an optics unit that is also part of the cluster. The patterned light source is generated by passing a beam of light through a chrome-covered mask, the chrome having been patterned with an image of a given layer of an IC to be formed, for example, transistor contacts. Thereafter, the wafer is returned to the track unit for subsequent processing including development of the layers of photosensitive material mentioned above. 
     The wafer, carrying the layers of photosensitive material, is supported by a movable stage. A projection lens focuses the light passing through the mask to form an image on a first field over the layers of photosensitive material where an IC is to be formed, exposing the field of the layers of photosensitive material to the image and hence “recording” the pattern projected through the mask. The image is then projected on another field over the layers of photosensitive material where another IC is to be formed, this field over the layers of photosensitive material being exposed to the projected image, and hence pattern. 
     The above process is repeated for other fields where other ICs are to be formed. Thereafter, the wafer is, as mentioned above, returned to the track unit, and the exposed layers of photosensitive material, which become soluble or insoluble through exposure depending upon the photosensitive materials used, are developed to leave a “photoresist” pattern corresponding to a negative (or positive) of the image of a layer of one or more ICs to be created. After development, the wafer undergoes various other processes, for example ion implantation, etching or deposition. The remaining layers of photosensitive material are then removed and fresh layers of photosensitive material are subsequently provided on the surface of the wafer depending upon particular application requirements for patterning another layer of the one or more ICs to be formed. 
     In relation to the patterning process, the resolution of a photolithography apparatus, or scanner, impacts upon the width of wires and spaces therebetween that can be “printed”, the resolution being dependent upon the wavelength of the light used and inversely proportional to a so-called “numerical aperture” of the scanner. Consequently, to be able to define very high levels of detail a short wavelength of light is required and/or a large numerical aperture. 
     The numerical aperture of the scanner is dependent upon the product of two parameters. A first parameter is the widest angle through which light passing through the lens can be focused on the wafer, and a second parameter is the refractive index of the medium through which the light passes when exposing the layers of photosensitive material on the wafer. 
     Indeed, to provide the increased resolution demanded by the semiconductor industry, it is known to reduce wavelengths of light used whilst also making lenses bigger to increase the numerical aperture. However, practical limits to the usable wavelengths of light are currently being reached, for example due to cost of having to use lenses formed from different materials compatible with the lower wavelengths of light, and scarcity of suitable lens materials. 
     Additionally, the above-described scanner operates in air, air having a refractive index of 1, resulting in the scanner having a numerical aperture between 0 and 1. Since the numerical aperture needs to be as large as possible, and the amount the wavelength of light can be reduced is limited, an improvement to the resolution of the scanner has been proposed that, other than by increasing the size of the lens, uses the scanner in conjunction with a medium having a refractive index greater than that of air, i.e. greater than 1. In this respect, the more recent photolithographic technique proposed, employing water and known as immersion lithography, can achieve higher levels of device integration than can be achieved by air-based photolithography techniques. 
     Therefore, scanners employing this improvement (immersion scanners) continue to use low wavelengths of light, but the water provides a refractive index of approximately 1.4 at a wavelength of 193 nm between the lens and the wafer, thereby achieving increased depth of focus and effectively increasing the numerical apertures of the immersion lithographic apparatus. 
     Further, the refractive index of the water is very close to that of quartz from which some lenses are formed, resulting in reduced refraction at the interface between the lens and the water. The reduced refraction allows the size of the lens to be increased, thereby allowing advantage to be taken of the higher available numerical aperture. 
     However, with the introduction of immersing lithography come technical challenges to be overcome if immersion lithography is to be a viable lithographic technique for defining sub-45 nm features. 
     One known immersion lithography apparatus comprises an illumination system to serve as a source of electromagnetic radiation. The illumination system is coupled to a support structure for holding a mask, the support structure being coupled to a first translation apparatus to position the illumination system accurately. A wafer table is disposed beneath the illumination system and is coupled to a second translation apparatus to position accurately the wafer table. A projection system is disposed adjacent the wafer table and projects light from the illumination system onto a wafer located on the wafer table. The projection system comprises an immersion head, which when in use, delivers and maintains an immersion liquid between the immersion head and the wafer. 
     In operation, the immersion lithography apparatus has a scan mode in which the wafer table is synchronously translated relative to the support structure, the immersion liquid having a leading edge corresponding to a direction of travel of the immersion liquid. 
     In order to increase wafer yields in connection with photolithographic processing of wafers, it is desirable to increase a velocity of translation of the wafer table relative to the support structure, i.e. to take less time to pattern the wafer. However, as scan rates increase to about 350 mms −1 , it has been found that the leading edge of the immersion liquid rolls under the immersion liquid causing bubbles to form in the immersion liquid, which are then printed. Indeed, bubbles are known to be a significant hindrance to successful implementation of the immersion lithography technique. One known solution is to limit the scan rate, but this, of course, impacts negatively upon the achievable wafer yields. 
     Additionally, it is becoming desirable to use so-called “high n”, or high refractive index liquids as the immersion liquid. However, high n liquids need to be used in an oxygen-free environment, otherwise the refractive index of the high n begins to change rapidly. Further, due to the relative expense of high n liquids, the high n liquid is likely to be recycled in a closed system. 
     STATEMENT OF INVENTION 
     According to the present invention, there is provided an immersion lithography apparatus and a method of performing immersion lithography as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       At least one embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a liquid delivery unit of an immersion lithography apparatus constituting an embodiment of the invention; 
         FIG. 2  is a schematic diagram of a part of the liquid delivery unit of  FIG. 1 , but in greater detail. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Throughout the following description identical reference numerals will be used to identify like parts. 
     Referring to  FIG. 1 , a semiconductor wafer  100  having layers of photosensitive material disposed thereon (not shown in  FIG. 1 ), the layers of photosensitive material having an upper surface  102 , is disposed upon a substrate stage (not shown) of an immersion lithography apparatus arranged to carry the wafer  100 . In this example, the immersion lithography apparatus is a modified TWINSCAN™ XT:1250i lithography scanner available from ASML. The lithography scanner is a complex apparatus having many parts, the structure and operation of which, are not directly relevant to the embodiments disclosed herein. Consequently, for the sake of clarity and conciseness of description, only the parts of the lithography scanner of particular relevance to the embodiment described herein will be described. 
     The immersion lithography apparatus comprises an optical exposure (projection or catadioptric) system  104  connected to a liquid delivery unit  106 , sometimes known as a “showerhead”. A so-called immersion liquid  107  is disposed between the bottom of the optical exposure system  104  and the surface  102  of the layers of photosensitive material. 
     The liquid delivery unit  106  comprises immersion liquid inlet/outlet ports  108  in fluid communication with a reservoir  109  defined by an inner peripheral surface  110  of the liquid delivery unit  106  and the upper surface  102 . A source of a barrier liquid  112  is coupled to barrier liquid inlet\outlet ports  114 . A vacuum pump (not shown) is coupled to vacuum ports  116 , the vacuum ports  116  being in fluid communication with a first channel loop  118 . A compressor (not shown) is coupled to air supply ports  120 , the air supply ports  120  being in fluid communication with a second channel loop  122 . 
     In operation, a quantity of an immersion liquid  107  is delivered to the reservoir  109  via the immersion liquid inlet/outlet ports  108 , a layer  111  of the immersion liquid  107  lying between the surface  102  and the liquid delivery unit  106 . The immersion liquid  107  is a high refractive index (high-n) liquid having a refractive index between about 1.5 and about 1.8, i.e. greater than the refractive index of water, for example between about 1.6 and 1.7. In this example, the immersion liquid  107  is Dupont® IF132 having a refractive index of close to 1.65. However, the skilled person will appreciate that liquids having a lower refractive index, for example water, can be used. 
     Turning to  FIG. 2 , a quantity of the barrier liquid  200  is deposited adjacent the layer  111  of the immersion liquid  107  as an annulus, in this example, to surround the layer  111  of the immersion liquid  107  and “cap” the layer  111  of the immersion liquid  107 . The barrier liquid  200  is, in this example, an aqueous-based liquid. However, the skilled person will appreciate that the barrier liquid  200  can be non-aqueous-based. The barrier liquid  200  has a density of a value so that the barrier liquid  200  does not mix with the immersion liquid  107  during translation of the wafer  100  relative to the liquid delivery unit  106 . The density of the barrier liquid  200  can be greater than that of the immersion liquid  107 . The density of the barrier liquid  200  can be between about 700 kgm −3  and about 1600 kgm −3 , for example oil or dodecane. The density of the barrier liquid  200  can be between about 800 kgm −3  and about 1500 kgm −3 . In this example, the barrier liquid  200  is dodecane and has a density of about 750 kgm −3  at 25° C. In this example, the barrier liquid  200  is also hydrophobic, though the skilled person will appreciate that hydrophilic liquids can be used. 
     The optical exposure system  104  coupled to the liquid delivery unit  106  scans the surface  102  of the layers of photosensitive material  202  by translation of the wafer beneath the optical exposure system  104  in order to project a pattern onto the layers of photosensitive material  202  in a manner known for the lithography scanner. A vacuum provided to the first channel loop  114  via the vacuum ports  112  and pressurised air expelled into the second channel loop  118  via the air supply ports  116  prevent egress of the immersion liquid  107  and the barrier liquid  200  through a clearance  126  between the liquid delivery unit  106  and the surface  102 , thereby serving as a seal and a means of preventing spurious deposition of the immersion liquid  107  and the barrier liquid  200  on the surface  102 . 
     Whilst the layer  111  of the immersion liquid  107  moves relative to the surface  102  of the wafer  100 , a first exterior edge or front  204  of the layer  111  of the immersion liquid  107  “rolls” in a direction of travel of the layer  111  of the immersion liquid  107  relative to the surface  102  of the wafer  100 . The annular quantity of the barrier liquid  200  also moves relative to the surface  102  of the wafer  100  with the layer  111  of the immersion liquid  107 . Independently of the first exterior surface  204 , a second exterior edge or front  206  of the barrier liquid  200  also rolls in the direction of travel of the layer  111  of the immersion liquid  107  relative to the surface  102  of the wafer  100 . 
     As a result of the presence of the barrier liquid  200  surrounding the layer  111  of the immersion liquid  107 , the immersion liquid  107  is isolated from environmental gases, for example oxygen, that are usually present in an atmosphere in which the scanner is disposed. Additionally, the provision of the barrier liquid  200  also prevents the layer  111  of the immersion liquid  107  from enveloping quantities of the environmental gases. 
     It is thus possible to provide an immersion lithography apparatus and a method of performing immersion lithography that prevents absorption of oxygen by the immersion liquid. Additionally, the formation of bubbles in the immersion liquid is mitigated at elevated scanning rates, for example above 500 mms −1 . Of course, the above advantages are exemplary, and these or other advantages may be achieved by the invention. Further, the skilled person will appreciate that not all advantages stated above are necessarily achieved by embodiments described herein. 
     It should be appreciated that quoted refractive indices herein are quoted with respect to a given wavelength of electromagnetic radiation, for example 193 nm.