Patent Publication Number: US-6657213-B2

Title: High temperature EUV source nozzle

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a nozzle for an extreme ultraviolet (EUV) lithography source and, more particularly, to a nozzle for an EUV source that employs a target delivery tube within the nozzle to thermally isolate the target material from the heat generated by the plasma. 
     2. Discussion of the Related Art 
     Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process that is well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current state of the art for photolithography light sources generate light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13.4 nm). 
     Different devices are known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Krypton, and combinations of gases, are also known for the laser target material. The gas is forced through a nozzle, and as the gas expands, it condenses and converts to a liquid spray. The liquid spray is illuminated by a high-power laser beam, typically from an Nd:YAG laser, that heats the liquid droplets to produce a high temperature plasma which radiates the EUV radiation. U.S. Pat. No. 5,577,092 issued to Kubiak discloses an EUV radiation source of this type. 
     FIG. 1 is a plan view of a known EUV radiation source  10  including a nozzle  12  and a laser beam source  14 . A gas  16  flows through a neck portion  18  of the nozzle  12  from a gas source (not shown). The gas  16  is accelerated through a narrowed throat portion and is expelled through an exit collimator of the nozzle  12  as a jet spray  26  of liquid droplets. A laser beam  30  from the source  14  is focused by focusing optics  32  on the liquid droplets. The energy of the laser beam  30  generates a plasma  34  that radiates EUV radiation  36 . The nozzle  12  is designed so that it will stand up to the heat and rigors of the plasma generation process. The EUV radiation  36  is collected by collector optics  38  and is directed to the circuit (not shown) being patterned. The collector optics  38  can have any suitable shape for the purposes of collecting and directing the radiation  36 . In this design, the laser beam  30  propagates through an opening  40  in the collector optics  38 . 
     It has been shown to be difficult to produce a spray having large enough droplets of liquid to achieve the desired efficiency of conversion of the laser radiation to the EUV radiation. Because the liquid droplets have too small a diameter, and thus not enough mass, the laser beam  30  causes some of the droplets to break-up before they are heated to a sufficient temperature to generate the EUV radiation  36 . Maximum diameters of droplets generated by a gas condensation EUV source is on the order of 0.33 microns. However, droplet sizes of about 1 micron in diameter would be desirable for generating the EUV radiation. Additionally, the large degree of expansion required to maximize the condensation process produces a diffuse jet of liquid, and is inconsistent with the optical requirement of a small plasma size. 
     To overcome the problem of having sufficiently large enough liquid droplets as the plasma target, U.S. Pat. No. 6,324,256, issued Nov. 27, 2001, titled “Liquid Sprays as the Target for a Laser-Plasma Extreme Ultraviolet Light Source,” discloses a laser-plasma, extreme ultraviolet light source for a photolithography system that employs a liquid spray as a target material for generating the laser plasma. In this design, the EUV source forces a liquid, preferably Xenon, through the nozzle, instead of forcing a gas through the nozzle. The geometry of the nozzle and the pressure of the liquid propagating through the nozzle, atomize the liquid to form a dense spray of liquid droplets. Because the droplets are formed from a liquid, they are larger in size, and are more conducive to generating the EUV radiation. 
     Another problem exists in the known EUV sources that causes some of the liquid target material to vaporize prior to being energized by the laser. The plasma generation area is typically about 2 mm away from the nozzle exit, and is generating heat at about 200,000° K. Because the EUV radiation source nozzle is positioned so close to the plasma generation area, the heat from the plasma heats the nozzle and thus the target material therein. The nozzles are typically subjected to thermal inputs up to 10 kW/cm2. Warming the target material at the expansion aperture of the nozzle leads to reduced target production and to the formation of EUV absorbing vapors. Particularly, heating of the nozzle to such high temperatures causes some of the liquid target material to vaporize reducing the liquid density of the target. Further, particles from the plasma generation process cause a sputtering effect on the nozzle which adversely affects the EUV generation. It is known in the art to make the nozzle out of graphite to reduce the sputtering effects, although other materials may be used for better erosion resistance. However, graphite is a good thermal conductor which enhances heating of the cold target material within the nozzle. 
     What is needed is a nozzle for a laser-plasma EUV radiation source that provides thermal isolation between the nozzle body and the target material traveling therethrough to enhance the EUV radiation generation. It is therefore an object of the present invention to provide such an EUV radiation source nozzle. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a nozzle for a laser-plasma EUV radiation source is disclosed that provides thermal isolation between the nozzle body and the target material flowing therethrough. A separate target material delivery tube protrudes through the nozzle body with limited tube/nozzle surface contact such that proper tube/nozzle alignment is achieved while providing thermal isolation. In one embodiment, the delivery tube is made of a material having low thermal conductivity, such as stainless steel, so that heating of the nozzle body from the plasma does not heat the liquid target material being delivered through the delivery tube. The delivery tube has an expansion aperture positioned behind an exit collimator of the nozzle body. The expansion aperture has a smaller diameter than the known exit collimators to deliver less material to the plasma generation area. 
     Additional objects, advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a known laser-plasma, gas condensation, extreme ultraviolet light source; and 
     FIG. 2 is a cross-sectional view of a nozzle for a laser-plasma, extreme ultraviolet radiation source employing a target material delivery tube, according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a nozzle for an EUV radiation source is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     FIG. 2 is a cross-sectional view of a nozzle  46  for an EUV source, according to the invention, and is applicable to replace the known nozzle  12  discussed above. The nozzle  46  includes a graphite body portion  48  having a size and shape suitable for the purposes described herein. The nozzle  46  includes a cylindrical exit collimator  50  through which the liquid target material exits the nozzle  46  under suitable pressure. The collimator  50  collimates the liquid spray so that it is directed towards the plasma generation area. A heat exchanger  54  is threaded into a threaded opening  56  in the body portion  48 . The heat exchanger  54  includes a base portion  58  and stem portion  60  that is threaded within the threaded opening  56 . The heat exchanger  54  provides cooling for the body portion  48 , and further provides support for the nozzle  46 . A bore  62  extends through the heat exchanger  54 , and is in communication with a narrowed bore  64  in the body portion  48 . The bore  64  is in fluid communication with the exit collimator  50 , and forms a shoulder  70  therebetween. 
     In the known nozzles for EUV sources, the target material would flow through the bores  62  and  64  and exit the nozzle  46  through the collimator  50 . However, heating of the graphite body portion  48  from the plasma generation would affect the liquid target within the body portion  48 , causing some vaporization and target loss. According to the present invention, an elongated target material delivery tube  72  extends through the bores  62  and  64  and abuts against the shoulder  70 , as shown. The tube  72  includes a wide portion  74  and a narrow end portion  76 . The tube  72  is positioned to provide a gap between the delivery tube  72  and the heat exchanger  54 , and a gap between the delivery tube  72  and the internal walls of the body portion  48  within the bore  64 . The delivery tube  72  includes an expansion orifice  80 , or an array of orifices, at the end of the narrowed portion  76  so that the orifice  80  is positioned proximate to the shoulder  70 . 
     The liquid target material is delivered from a suitable target source (not shown) through the delivery tube  72  and enters the exit collimator  50  under pressure. The delivery tube  72  provides thermal isolation from the heated graphite body portion  48  during plasma generation. Additionally, the gap between the delivery tube  72  and the body portion  48  is at low pressure because the process occurs under vacuum pressure, and serves to further insulate the cold target material within the delivery tube  72  from the heated body portion  48 . The cold liquid target material is delivered at the desired operating pressure and temperature to the collimator  50  across which it undergoes supersonic expansion to yield particles of either solid or liquid target material. The diameter of the orifice  80  can be about 50 microns in one embodiment so that it provides the desirable size liquid droplets. Additionally, the delivery tube  72  provides structural integrity to the nozzle  46  so that the size of the body portion  48  can be minimized. 
     In one embodiment, the delivery tube  72  is made of a suitable stainless steel. However, this is the way of a non-limiting example in that other materials can be used, preferably thermally non-conductive materials, such as nickel and ceramic. Although it is desirable that the delivery tube  72  be made of a thermally non-conductive material, because of the gap, the contact area between the tubes  72  and the body portion  48  is minimal so that even thermally conductive delivery tubes will provide a reduced heating of the cold target material. 
     The foregoing discussion describes merely exemplary embodiments of the present invention. One skilled in the art would readily recognize that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.