Abstract:
A laser device for vaporizing a metal requires a source for generating a laser beam having a predetermined power density at a point on the laser beam. A solid metal target material is then moved along a path, and through the point, relative to the laser beam. This is done to sequentially transition the target material from a solid to a liquid, and from a liquid to a vapor. In this process there is minimal liquid ejection.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains generally to systems and methods for using a laser to vaporize materials. More particularly, the present invention pertains to systems and methods for using lasers to vaporize metals and metal compounds. The present invention is particularly, but not exclusively, useful for creating a metal vapor in a vapor jet production process, while minimizing the ejection of liquid material from the process before it can be vaporized.  
       BACKGROUND OF THE INVENTION  
       [0002]     Metal vapors can be useful for many industrial purposes, such as in vapor deposition procedures. For example, such procedures may be particularly applicable where it is desirable to achieve a substantially uniform metallic coating on a substrate. In any event, and for whatever reason, whenever a metal vapor is being generated, it is generally desirable that the vapor has certain determinable characteristics or attributes. For one, it may be desirable that there be very little, if any, ionization in the metal vapor. In particular, this attribute is desirable in applications where the resultant metal vapor is to be subjected to a magnetic field. For another, it is desirable that as much of the target material as possible be actually vaporized.  
         [0003]     An important consideration for the creation of a metal vapor involves the selection of a system that can be used to effectively vaporize the metal. For some applications, the use of an oven may be appropriate. Ovens, however, can be cumbersome and allow for uneven vapor jet production of the target material. This is particularly so if it is mixed or heterogeneous. For this reason, among others, various irradiation systems have been suggested as an alternative to ovens.  
         [0004]     Commercially available microwave radiation is known to be capable of generating the heat loads that are required to vaporize metals. The electric field that is associated with microwave radiation, however, induces an ionization of the metal vapor that in some applications may cause a reflection of the microwave radiation before it reaches the target. This, of course, will reduce the efficiency of the system. Laser radiation, however, is also known to be effective for the purpose of vaporizing metals. Importantly, laser irradiance and wavelength can be controlled to minimize ionization of the resultant vapor.  
         [0005]     In addition to the selection of a heating source, a critically important factor for consideration when using a metal to create a vapor is the target material itself. Specifically, for such a process, vaporization temperatures (T v ) above 3500° K are typically required. Of particular concern here is the fact that when a metal target is heated for vaporization, it will transition from a solid to a liquid, and then from liquid to a gas (vapor). Also, because metals normally have melting point temperatures (T m ) that are more than one thousand degrees Kelvin below their vaporization temperature (T v &gt;&gt;T m ), and because they have relatively high coefficients of thermal conductivity (κ) for both their solid and liquid phases, the liquid phase needs to be reckoned with. In particular, there will likely be a significant amount of target material in the liquid phase. The important consequence here is that, for an efficient metal vaporization process, the loss of liquid droplets needs to be minimized.  
         [0006]     With the above in mind, when considered in terms of throughput, (Γ), a metal vaporization process can be expressed as: 
 
Γ s =Γ l +Γ v   [eqn. 1]
 
 where Γ s  is the solid throughput, Γ l  is the liquid throughput, and Γ v  is the vapor throughput. In the optimal case for metal vaporization, all of the metal is vaporized and Γ l =0. Further, to keep the evaporating surface stationary, it is necessary that: 
 
 n   s   u=n   v   v   v   [eqn. 2]
 
 where “n s ” and “n v ” are the respective number densities of the solid metal and metal vapor, “u” is the feed velocity of the solid metal, and “V v ” is the vapor velocity. 
 
         [0007]     As a practical matter, during a metal vaporization process, Γ l  may not equal zero. An important reason for this is that the vapor pressure, p v , generates substantial forces on the liquid phase of the material as the metal is vaporized. These forces may then cause droplets to be ejected from the liquid before they can be vaporized. When this happens, the ejected droplets constitute the liquid throughput, Γ l . Consequently, as the liquid throughput (Γ l ) increases, the overall efficiency of the vapor jet production process is diminished.  
         [0008]     For an appreciation of several factors that are involved in the vaporization of a metal, consider the one-dimensional case wherein the metal target material is formed as a cylindrical rod having a radius “a”. Further, consider the target material is being axially advanced along a path at the feeding velocity “u”, and through a point on the path where a heating device (e.g. a laser beam) generates a determinable vaporization power density (Hn v v v ). In this expression for power density, H is the heat of vapor jet production per atom. Due to the power density of the heating source, the target material will sequentially transition from a solid to a liquid, and from the liquid to a gas (vapor). During these transitions, the melt zone where the target material is in its liquid phase will have a depth “d”. It happens that this depth “d” is related to characteristics of the vapor by the expression: 
 
 d =κ/(3 n   v   v   v   k )  [eqn. 3]
 
 wherein “κ” is thermal conductivity of the metal, and “k” is the Boltzmann constant. 
 
         [0009]     In the one-dimensional case, the vapor pressure (p v ) pushes against the liquid metal with an axially directed force that tends to eject liquid droplets from the melt zone. Specifically, this ejection of liquid droplets from the melt zone occurs before the droplets can be vaporized and will generally be in a radial direction. Droplet ejection, however, is resisted by forces that are generated in the liquid due to; 1) surface tension; 2) inertia; and 3) viscosity. Conditions for “d” (i.e. the depth of the melt zone), wherein these resistive forces minimize droplet ejection, can be respectively expressed as: 
 
 d&lt;χ/p   v   [eqn. 4]
 
 where “χ” is the surface tension of the liquid; 
 
 d&lt;Mn   s   u   2   a/p   v   [eqn. 5]
 
 for a condition where “M” is the mass of the atom, and wherein the radial velocity of the liquid due to inertia is less than the feed velocity “u” of the target material; and 
 
 d   2   &lt;ηau/p   v   [eqn. 6]
 
 where the viscosity of the liquid is influenced by the liquid/solid interface in the metal target material. 
 
         [0010]     It can be shown that if any one of the conditions set forth in eqns. 4, 5 or 6 above is satisfied, the loss of liquid droplets from the target feed material will be minimized. Accordingly, plots of the respective expressions (eqns. 4, 5 and 6) are set forth in  FIG. 1  as a function of the vapor throughput characteristics n v v v .  
         [0011]     A comparative evaluation of the plots for zirconium is presented in  FIG. 1  to indicate that inertial conditions in the liquid phase of the metal target material will allow for an increase in “d” with an increase in the product of vapor characteristics n v v v , under certain conditions. Thus, by combining eqn. 3 with eqn. 5, an expression for the number density (n v ) of a useable vapor can be obtained. The obtained value for n v  can then be used to determine an appropriate power density for the heating source. With the above in mind, the expression for the number density (n v ), derived by combining eqns. 3 and 5, is: 
 
 n   v   &gt;[κn   s /3 kv   v   a]   1/2   [eqn. 7]
 
         [0012]     And, the expression for the power density required for vaporization becomes 
 
 Hn   v   v   v   =H[κn   s   v   v/ 3 ka]   1/2   [eqn. 8]
 
         [0013]     In light of the above, it is an object of the present invention to provide a system and method for vaporizing a metal with a laser beam that minimizes liquid losses during the creation of the vapor. Another object of the present invention is to provide a system and method for vaporizing a metal with a laser beam that avoids ionization of the resultant vapor. Still another object of the present invention is to provide a system and method for vaporizing a metal that is simple to use, is relatively easy to manufacture, and is comparatively cost effective.  
       SUMMARY OF THE INVENTION  
       [0014]     In accordance with the present invention, a device for vaporizing a solid metal target material includes a source for generating a laser beam, and an optical apparatus for directing the beam along a beam path. Specifically, the laser beam is generated to establish a predetermined power density over an area at a predetermined point (i.e. focal point) on the beam path. Preferably, this predetermined power density will be in a range between approximately ten gigawatts per square meter and about one hundred gigawatts per square meter (10-100 GW/m 2 ). With this in mind, the area at the point on the laser beam where this power density is generated will be less than a square millimeter (area≦1 mm 2 ) and will, typically, be around one half square millimeter.  
         [0015]     Insofar as the solid metal target itself is concerned, it can either be a pure metal or a metal compound. Further, the target metal can be formed as a brick (i.e. block) with a substantially flat surface, or it can be formed as a cylindrical rod. In the latter case, the cross sectional area of the rod will be approximately the same as the point on the beam path where the laser power density is measured (e.g. cross sectional area&lt;1 mm 2 ).  
         [0016]     In the operation of the present invention, the target material is somehow moved relative to the laser beam, or vice versa with a velocity “u”. In each case the purpose is to sequentially transition the target material from a solid to a liquid, and from a liquid to a vapor. In this transition, the liquid portion (i.e. liquid phase) of the target material is maintained at a substantially constant depth “d”. Preferably, this depth is on the order of a few microns (d&lt;10 μm).  
         [0017]     In specific cases where the target material is formed as a cylindrical rod, the optical apparatus holds the laser beam stationary while directing the laser beam to the target material. The rod is then advanced along a laser path and through the point on the laser beam where the desired laser power density is being generated. There the target material is vaporized. In the case where the target material is formed as a block having a substantially flat surface, the point on the laser beam where the desired laser power density is being generated is maintained coincident with the surface of the target material. In this latter case, the optical means also moves the point on the laser beam over the surface of the target material. Preferably, this movement is made along a Lissajous&#39; curve.  
         [0018]     For the specific case wherein the target material is a cylindrical rod having a radius “a”, as the disclosure above in the BACKGROUND OF THE INVENTION indicates, eqn. 5 is controlling. With reference to eqn. 5,  FIG. 1  then shows an operable region between the melt thickness that is attainable for a given laser power, (i.e. “d”) and the inertial forces in the molten target material that resist a so-called “splatter” of the target material. Within this operable region, the practical limitation for a vapor jet production process is the feed velocity “u” that can be sustained.  
         [0019]     As intended for the present invention, vaporization of the target metal creates a vapor with a throughput in a range between approximately one one-tenth of a mole per second and one mole per second (0.1-1 mole/sec). Importantly, adjustments in the power density level of the laser beam is selected to minimize the creation of any liquid throughput (i.e. avoid liquid splatter), and to avoid creating a plasma from the vapor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0021]      FIG. 1  is a graph comparing exemplary data for melt layer thickness and vapor throughput for a vapor jet production process in accordance with the present invention;  
         [0022]      FIG. 2  is a perspective view of a device in accordance with the present invention, with portions broken away for clarity;  
         [0023]      FIG. 3  is a side, elevation view of a rod-like, cylindrical-shaped target metal material for use with the embodiment shown in  FIG. 2 ;  
         [0024]      FIG. 4  is a perspective view of an alternate embodiment of the present invention, shown with an optical steering mechanism, and with portions broken away for clarity; and  
         [0025]      FIG. 5  is a cross sectional view of the target metal material as seen along the line  5 - 5  in  FIG. 4 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     Referring initially to  FIG. 2 , a device for vaporizing a metal in accordance with the present invention is shown, and is generally designated  10 . As shown, the device  10  includes a laser source  12  which is coupled with appropriate optics  14 . Specifically, the laser source  12  can be of any type well known in the pertinent art that is capable of generating a continuous laser beam  16 . Further, the optics  14  can be of any type well known in the pertinent art that is capable of focusing the laser beam to a focal spot with a power density that is in a range of about ten to one hundred gigawatts per square meter (10-100 GW/m 2 ).  
         [0027]      FIG. 2  also shows that the device  10  includes a vessel  18  which receives a metal target material  20  for vaporization. For the embodiment of the present invention shown in  FIG. 2 , the metal target material  20  is substantially cylindrical shaped and has a radius “a” (see  FIG. 3 ). Further,  FIG. 2  shows that the metal target material  20  is supplied from a reel  22 , and is advanced into the vessel  18  by counter-rotating feed rollers  24   a  and  24   b . To do this, the feed rollers  24   a,b  are simultaneously counter-rotated by a drive unit  26 . For the present invention, the metal target material  20  can be any metal or metal compound.  
         [0028]     Still referring to  FIG. 2 , it is seen that the optics  14  of the device  10  direct the laser beam  16  from the laser source  12 , through a window  28  in the vessel  18 . Further, the laser beam  16  is focused by the optics  16  to a point  30  inside the vessel  18 . Importantly, the laser beam  16  is focused to a focal spot at the point  30  that has an area which is substantially the same as the area of the exposed end  32  (see  FIG. 3 ) of the cylindrical shaped metal target material  20  (i.e. area=πa 2 ). For most applications, the area of the focal spot (i.e. πa 2 ) will be less than one square millimeter and, typically, will be around one half square millimeter (πa 2 =0.5 mm 2 ). Recall, the power density over this area will be in an approximate range between ten and one hundred gigawatts per square meter (10-100 GW/m 2 ).  
         [0029]     For purposes of the present invention, it is to be appreciated that the metal target material  20  will inherently have a relatively high thermal conductivity. This characteristic of the metal target material  20  will cause it to successively progress through three noticeably different phases within the vessel  18 . As shown in  FIG. 3 , these are: a solid phase  34 , a liquid phase  36 , and a vapor (gas) phase  38 . As discussed above, however, it is desirable that little, if any, of the target material  20  be lost during the liquid phase (i.e. liquid throughput is preferably zero: Γ 1 =0). Stated differently, it is desirable that the vapor throughput, Γ v , be equal to the solid throughput, Γ s  (i.e. Γ v =Γ s ). To this end, the metal target material  20  is fed through the point  30  in vessel  18  along a path  40  in the direction of arrow  42  at a feed velocity “u”.  
         [0030]     As the metal target material  20  is being fed into the vessel  18  for vaporization, several aspects of the process are particularly important. For one, it is desirable that the metal target material  20  be advanced (fed) into the vessel  18  with a sustainable velocity. With this in mind, the feed velocity “u” and the power density (Hn v v v : see eqn. 8) of the laser beam  16  at the point  30  need to be reconciled in view of eqn. 5: namely, d&lt;Mn s u 2 a/p v  where p v =kT v n v . In this context, care should be taken to ensure that the vapor  38  will not be ionized by the laser beam  16 . Another important aspect of the vaporization process is that, if there is any liquid throughput (Γ l ), the particulates of this throughput should have diameters as small as possible and, preferably, less than about one micron. Regardless of the value of “d”, however, an optimal condition for vaporization is realized whenever the vapor throughput equals the solid throughput (Γ v =Γ s ).  
         [0031]      FIG. 4  shows an alternate embodiment for the device  10  of the present invention wherein the metal target material is formed as a brick (block)  44 . As shown, the brick  44  is formed with a substantially flat surface  46  and is positioned in a protective receptacle  48  for vaporization. Similar to the embodiment discussed above with reference to  FIGS. 2 and 3 , for the alternate embodiment, the laser beam  16  is also focused to a focal spot at the point  30 . Again, the power density over the area at point  30  will be in an approximate range between ten and one hundred gigawatts per square meter (10-100 GW/m 2 ). For the alternate embodiment, however, it is necessary that the point  30  of laser beam  16  be somehow moved over the surface  46  to vaporize the metal target material of brick  44 . Alternatively, the point  30  can be held stationary while the brick  44  is moved.  
         [0032]     As indicated in  FIG. 4 , a steering mechanism can be provided for movement of the point  30  of laser beam  16 . Specifically, this mechanism may include a mirror  50  that is positioned for rotation around an axis  52  through an angle “α”. The mechanism may also include a mirror  54  that is positioned for rotation around an axis  56  through an angle “φ”. Further, as shown, the mirror  54  is effectively positioned at a distance “L” above the surface  46  of the metal target material brick  44 . In this combination the axis  52  is oriented perpendicular to the axis  56 . Consequently, independent rotations of the mirrors  50  and  54  will respectively result in movements of the point  30  on surface  46  in “x” and “y” directions. For purposes of the present invention, the mirrors  50  and  54  can be of any type well known in the pertinent art, such as galvanometric or piezoelectric mirrors.  
         [0033]     For the vaporization of metal target material in brick  44 , the point  30  of laser beam  16  is moved over the surface  46  along a curve  58 . More specifically, the point  30  is moved along curve  58  with a linear velocity “w” and in a variable direction that, for purposes of disclosure, is indicated by the arrow  60 . Preferably, the curve  58  is a Lissajous&#39; curve. Further, it will be appreciated that the result of this movement is a vaporization of metal target material on the surface  46  that forms a trench having a depth “h” and a width “2a” (see  FIG. 5 ). With this in mind, and referring to  FIG. 5 , various geometrical relationships that are pertinent to the movement of the point  30  can be determined. In general, using approximations, the variables “w”, “L”, “h”, “a”, “θ”, “φ”, and “α” can be used to describe both dimensional and dynamic relationships for the device  10 . In this context, it can be dimensionally shown that: tan θ=u/w=h/a. Dynamically, it can be shown that: d(φ; α)/dt=W/L. Using these relationships, it is possible to manipulate the mirrors  50  and  54  to appropriately move the point  30  of laser beam  16  for the selected power density. Importantly, as with other embodiments of the present invention, it is desirable to minimize any liquid throughput (i.e. material loss due to particulate ejection) and to avoid ionizing the resultant vapor  38 . It will be appreciated that the optics  14  for this embodiment of the present invention can either be positioned as shown in  FIG. 4 , or the optics  14  can be appropriately positioned on the path of laser beam  16  between the mirrors  50 ,  54  and the brick  44 .  
         [0034]     While the particular System and Method for Vaporizing a Metal as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.