Patent Publication Number: US-2023154743-A1

Title: Wafer cleaning apparatus, method for cleaning wafer and method for fabricating semiconductor device

Description:
This application is a divisional application of U.S. application Ser. No. 17/478,619, filed on Sep. 17, 2021, which claims priority to Korean Patent Application No. 10-2021-0020478, filed on Feb. 16, 2021 and all the benefits accruing therefrom under 35 U.S.C. § 119, the disclosure of each of these applications being incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present inventive concept relates to a wafer cleaning apparatus. More specifically, the present inventive concept relates to a wafer cleaning apparatus using a laser, a method for cleaning the wafer, and a method for fabricating a semiconductor device. 
     2. Description of the Related Art 
     A wet cleaning process, which is indispensable in the semiconductor fabricating process, is a process of cleaning a hard mask or the like on a wafer, using a high-temperature chemical. Such a wet cleaning process may be performed by a batch device. The batch device refers to a cleaning device that wet-cleans a set of wafers by depositing a plurality of wafers as a set in a chemical at the same time. 
     However, such a batch device may cause problems such as flow defect, poor drying and poor dispersion uniformity on the wafer. Therefore, a conversion to a single-wafer device is required to solve such a problem. The single-wafer device refers to a device that applies each wafer to the wet-cleaning process one by one. 
     On the other hand, in order to heat the wafer disposed on the single-wafer device, a laser module that irradiates the wafer with a laser may be used. However, there is a problem of difficulty in achieving a uniform temperature on an entire surface of the wafer, due to the influence of the air flow, reflectivity of the wafer and the like. 
     SUMMARY 
     Aspects of the present inventive concept provide a wafer cleaning apparatus in which a temperature variation on the entire surface of the wafer is improved and the performance is improved. 
     Aspects of the present inventive concept also provide a method for cleaning a wafer using a wafer cleaning apparatus in which the temperature variation on the entire surface of the wafer is improved and the performance is improved. 
     Aspects of the present inventive concept also provide a method for fabricating a semiconductor device using the wafer cleaning apparatus in which the temperature variation on the entire surface of the wafer is improved and the performance is improved. 
     However, aspects of the present inventive concept are not restricted to the one set forth herein. The above and other aspects of the present inventive concept will become more apparent to one of ordinary skill in the art to which the present inventive concept pertains by referencing the detailed description of the present inventive concept given below. 
     According to an aspect of the present inventive concept, there is provided a method of fabricating a semiconductor device, the method comprising disposing a wafer on a rotatable chuck, irradiating a lower surface of the wafer with a laser to heat the wafer, and supplying a chemical to an upper surface of the wafer to clean the wafer, wherein the laser penetrates an optical system including an aspheric lens array, the laser penetrates a calibration window, which includes a first window structure including a first light projection window including first and second regions different from each other, a first coating layer covering the first region of the first light projection window, and a second coating layer covering the second region of the first light projection window, and the first coating layer and the second coating layer have different light transmissivities from each other. 
     According to an aspect of the present inventive concept, there is provided a wafer cleaning apparatus comprising a chuck configured to receive a wafer, a chemical supply unit configured to supply a chemical onto an upper surface of the wafer, a laser module configured to irradiate a lower surface of the wafer with a laser, and a calibration window configured that the laser is transmitted through the calibration window between the wafer and the laser module, wherein the calibration window includes a light projection window including a lower surface facing the laser module, a first coating layer having a first light transmissivity on the lower surface of the light projection window, and a second coating layer having a second light transmissivity greater than the first light transmissivity on the lower surface of the light projection window. 
     According to an aspect of the present inventive concept, there is provided a wafer cleaning apparatus comprising a rotatable chuck configured to receive a wafer, a chemical supply unit configured to supply a chemical onto an upper surface of the wafer, a laser module configured to irradiate a lower surface of the wafer with a laser, an optical system including an aspherical lens array configured that the laser is transmitted through the optical system between the laser module and the wafer, and a calibration window configured that the laser is transmitted through the calibration window between the optical system and the wafer, wherein the calibration window includes a light projection window including a first region and a second region different from each other, a first coating layer covering the first region of the light projection window, and a second coating layer covering the second region of the light projection window, and the first coating layer and the second coating layer include different light transmissivities from each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects and features of the present inventive concept will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, in which: 
         FIG.  1    is a schematic cross-sectional view for explaining a wafer cleaning apparatus according to some embodiments. 
         FIG.  2    is a plan view for explaining rotation of a wafer of  FIG.  1   . 
         FIG.  3    is a conceptual diagram for explaining the operation of a first rotor unit and a second rotor unit of  FIG.  1   . 
         FIG.  4    is a schematic conceptual diagram for explaining an optical system of  FIG.  1   . 
         FIG.  5    is a cross-sectional view for explaining a calibration window of  FIG.  1   . 
         FIG.  6    is a plan view for explaining the calibration window of  FIG.  1   . 
         FIG.  7    is an exemplary cross-sectional view for explaining a first coating layer of  FIG.  5   . 
         FIG.  8    is an exemplary cross-sectional view for explaining a second coating layer of  FIG.  5   . 
         FIG.  9    is an exemplary graph showing intensity and temperature of the laser depending on the position of the wafer. 
         FIGS.  10  to  12    are various plan views for explaining the calibration window of the wafer cleaning apparatus according to some embodiments. 
         FIG.  13    is a cross-sectional view for explaining a calibration window of the wafer cleaning apparatus according to some embodiments. 
         FIG.  14    is a plan view for explaining a calibration window of the wafer cleaning apparatus according to some embodiments. 
         FIG.  15    is a cross-sectional view for explaining the calibration window of the wafer cleaning apparatus according to some embodiments. 
         FIGS.  16  to  18    are various cross-sectional views for explaining a wafer cleaning apparatus according to some embodiments. 
         FIG.  19    is a flowchart for explaining the method for cleaning a wafer using the wafer cleaning apparatus according to some embodiments. 
         FIG.  20    is a flowchart for explaining a method for fabricating a semiconductor device using the wafer cleaning apparatus according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a wafer cleaning apparatus according to the exemplary embodiment will be described referring to  FIGS.  1  to  8   . 
       FIG.  1    is a schematic cross-sectional view for explaining a wafer cleaning apparatus according to some embodiments.  FIG.  2    is a plan view for explaining rotation of a wafer of  FIG.  1   .  FIG.  3    is a conceptual diagram for explaining the operation of a first rotor unit and a second rotor unit of  FIG.  1   .  FIG.  4    is a schematic conceptual diagram for explaining an optical system of  FIG.  1   .  FIG.  5    is a cross-sectional view for explaining a calibration window of  FIG.  1   .  FIG.  6    is a plan view for explaining the calibration window of  FIG.  1   .  FIG.  7    is an exemplary cross-sectional view for explaining a first coating layer of  FIG.  5   .  FIG.  8    is an exemplary cross-sectional view for explaining a second coating layer of  FIG.  5   . 
     Referring to  FIGS.  1  to  8   , the wafer cleaning apparatus according to some embodiments includes a housing  100 , a laser module  110 , an optical system  120 , a reflector  130 , a calibration window  140 , a transparent window  150 , a chuck  160 , a bowl  180 , a drain guide portion  190 , and a chemical supply unit  200 . 
     A wafer W may be disposed on the chuck  160 . The chuck  160  may fix the disposed wafer W. The chuck  160  may be rotatable. As the chuck  160  rotates, the wafer W fixed on/to the chuck  160  may also rotate. For example, as shown in  FIG.  2   , the wafer W may rotate in a first rotation direction a 1  or a second rotation direction a 2 . The wafer W may include a central region We and an edge region We. 
     In some embodiments, the wafer W may include an exposed portion Wb and a non-exposed portion Wa. The exposed portion Wb may be a region which is irradiated with light in an exposure process of the wafer W, and the non-exposed portion Wa may be a region which is not irradiated with light in the exposure process. For example, an exposure mask (not shown) may be disposed on the wafer W. When light irradiates from the upper part of the exposure mask, the light that has passed through a transmission portion of the exposure mask may irradiate a part of the wafer W to form the exposure portion Wb. The other part of the wafer W that is not irradiated with light by a shielding portion of the exposure mask may form the non-exposed portion Wa. The wafer W including the exposed portion Wb and the non-exposed portion Wa may include, but is not limited to, a photoresist film. 
     In some embodiments, the chuck  160  may include a grip portion  161  and a side wall portion  163 . The grip portion  161  may fix the wafer W by coming into contact with the side surface of the wafer W. Further, the grip portion  161  may include a heat insulating material. When the wafer W is heated by the laser module  110 , the grip portion  161  may block transfer of heat to prevent thermal damage of other components (e.g., the side wall portion  163  of the chuck  160 ). The side wall portion  163  may surround a side surface of a housing  100  to be described below 
     In some embodiments, the rotating chuck  160  may be supported by a fixing portion  170  and a bearing  172 . The fixing portion  170  may fix the chuck  160  on the ground on which the wafer cleaning apparatus according to some embodiments is disposed. The fixing portion  170  may not rotate. The bearing  172  may be interposed between the fixing portion  170  and the chuck  160  to allow the chuck  160  to rotate. Accordingly, the chuck  160  may be configured to be rotatable despite it is supported by the fixing portion  170  that does not rotate. Although the bearing  172  is shown as only being interposed between the fixing portion  170  and the side wall portion  163  in  FIG.  1   , this is only an example, and if rotation of the chuck  160  is allowed, the position of the bearing  172  may be freely changed. 
     In some embodiments, the chuck  160  may be rotatable by a first rotor unit  165  and a second rotor unit  175 . The first rotor unit  165  may be fixed on/to the chuck  160 . The second rotor unit  175  may rotate the first rotor unit  165  in a magnetic levitation method. For example, the first rotor unit  165  and the second rotor unit  175  may each include a magnetic material, and may generate a rotational force, using a magnetic force. As the first rotor unit  165  rotates, the chuck  160  and the wafer W may rotate together. Although the first rotor unit  165  is shown as only being fixed on the side wall portion  163  of the chuck  160  in  FIG.  1   , this is only an example. As long as the first rotor unit  165  is configured to rotate the chuck  160 , the position of the first rotor unit  165  may be freely changed. 
     As an example, as shown in  FIG.  3   , the first rotor unit  165  may include a first magnetic pole region  165   a  and a second magnetic pole region  165   b , and the second rotor unit  175  may include a third magnetic pole region  175   a  and a fourth magnetic pole region  175   b . The first magnetic pole region  165   a  and the second magnetic pole region  165   b  may be alternately disposed in the first rotor unit  165 , and the third magnetic pole region  175   a  and the fourth magnetic pole region  175   b  may be alternately disposed in the second rotor unit  175 . The first magnetic pole region  165   a  and the second magnetic pole region  165   b  may have magnetic poles different from each other, and the third magnetic pole region  175   a  and the fourth magnetic pole region  175   b  may have magnetic poles different from each other. Also, the first magnetic pole region  165   a  and the third magnetic pole region  175   a  may have the same magnetic pole, and the second magnetic pole region  165   b  and the fourth magnetic pole region  175   b  have the same magnetic pole. As an example, the first magnetic pole region  165   a  and the third magnetic pole region  175   a  may be an N-pole, and the second magnetic pole region  165   b  and the fourth magnetic pole region  175   b  may be an S-pole. Each of the first to fourth magnetic pole regions  165   a ,  165   b ,  175   a , and  175   b  may be implemented as, but is not limited to, an electromagnet. 
     As the first to fourth magnetic pole regions  165   a ,  165   b ,  175   a , and  175   b  are alternately disposed, the first rotor unit  165  may rotate in the first rotation direction a 1  or the second rotation direction a 2 . For example, as the first to fourth magnetic pole regions  165   a ,  165   b ,  175   a , and  175   b  are alternately disposed, there may be a repulsive force between the first magnetic pole region  165   a  and the third magnetic pole region  175   a , and there may be an attractive force between the first magnetic pole region  165   a  and the fourth magnetic pole region  175   b . Similarly, there may be an attractive force between the second magnetic pole region  165   b  and the third magnetic pole region  175   a , and there may be a repulsive force between the second magnetic pole region  165   b  and the fourth magnetic pole region  175   b . Accordingly, the first rotor unit  165  may rotate in a direction in which the first magnetic pole region  165   a  and the third magnetic pole region  175   a  face each other (or a direction in which the second magnetic pole region  165   b  and the fourth magnetic pole region  175   b  face each other). Subsequently, the magnetic pole of the first rotor unit  165  (or the magnetic pole of the second rotor unit  175 ) may be reversed. Accordingly, the rotation of the first rotor unit  165  may be accelerated and the first rotor unit  165  may continuously rotate. 
     The chuck  160  may rotate the wafer W at a predetermined rotation speed. As an example, the rotation speed of the chuck  160  may range from about 100 rpm to about 300 rpm. If the rotation speed of the chuck  160  is not enough high, a chemical  210  provided to the wafer W may not be applied evenly. If the rotation speed of the chuck  160  is excessively high, the edge region We of the wafer W may be relatively cooled, and the temperature control may not be easy. 
     The housing  100  may be disposed under the wafer W, e.g., while the wafer W is processed in the wafer cleaning apparatus. For example, the upper surface of the housing  100  may face the lower surface of the wafer W. The housing  100  may fix and support a laser module  110 , an optical system  120 , a reflector  130 , a calibration window  140 , and a transparent window  150 , which will be described below. For example, the laser module  110 , the optical system  120 , the reflector  130 , the calibration window  140 , and the transparent window  150  may be fixed on and supported by the housing  100 . In some embodiments, the housing  100  may be spaced apart from the chuck  160  and the wafer W. Accordingly, the housing  100  may not rotate, even while the chuck  160  and the wafer W rotate together. However, this is merely an example, and the housing  100  may rotate with or separately from the chuck  160 . 
     The laser module  110  may irradiate a lower surface of the wafer W with a laser L. For example, the laser module  110  may be disposed inside the housing  100 . The laser L irradiated/emitted from the laser module  110  may penetrate the calibration window  140  and the transparent window  150  to be described later, and may reach the wafer W. The laser L that reaches the wafer W may be used to heat the wafer W. 
     In some embodiments, the laser module  110  may be provided with the laser L from a laser supply unit  111 . The laser supply unit  111  may be connected to the outside of the wafer cleaning apparatus according to some embodiments to form a path through which the laser L is supplied. The laser supply unit  111  may include, but is not limited to, for example, optical fibers. 
     A wavelength of the laser L provided from the laser supply unit  111  may be, for example, from about 100 nm to about 2000 nm. Preferably, the wavelength of the laser L provided from the laser supply unit  111  may be from about 400 nm to about 1,600 nm. The laser L provided from the laser supply unit  111  may have a single wavelength or may have multiple wavelengths. 
     The laser L provided from the laser supply unit  111  may be a continuous wave (CW) type or a pulse type. A continuous wave type laser may be a laser that is continuously irradiated/emitted without being turned on/off. A pulse-type laser may be a laser that is periodically turned on/off and irradiated/emitted discontinuously. The frequency of the pulse-type laser may be, for example, about 10 MHz to about 1,000 MHz. 
     The laser L irradiated/emitted from the laser module  110  may penetrate the optical system  120 . The optical system  120  may process the profile of the laser supplied from the laser module  110  and transfer it to the wafer W. For example, the optical system  120  may modify wavelength configurations and/or distributional configurations of the laser supplied form the laser module  110  before transferring it to the wafer W. For example, the optical system  120  may determine intensities of respective directions/angles to which the laser transfers. The laser L processed by the optical system  120  may heat the wafer W, e.g., the whole area of the wafer W. 
     In some embodiments, the optical system  120  may include an aspheric lens array. As an example, as shown in  FIG.  4   , the optical system  120  may include first to third aspherical lenses  122 ,  124 , and  126 . Although the optical system  120  is only shown as including three aspheric lenses  122 ,  124 , and  126  in  FIG.  4   , this is merely an example, and the number of aspheric lenses may be variously changed. For example, the optical system  120  may include more than three aspheric lenses or less than three aspheric lenses. In certain embodiments, a spherical lens may be interposed between the aspherical lenses  122 ,  124 , and  126 . 
     The first to third aspherical lenses  122 ,  124 , and  126  may process the profile of the laser L through refraction of the laser L. For example, the first to third aspherical lenses  122 ,  124 , and  126  may provide a flat-top type laser L (e.g., a top-hot beam of laser) to the wafer W. 
     Gaps between the first and third aspheric lenses  122 ,  124 , and  126  may be adjusted to provide the required/proper profile of the laser L. For example, a first gap g 1  between the first aspherical lens  122  and the second aspherical lens  124 , a second gap g 2  between the second aspherical lens  124  and the third aspherical lens  126  or a third gap g 3  between the third aspheric lens  126  and the wafer W may be adjusted. The first to third gaps g 1 , g 2 , and g 3  may be determined and fixed at the time of fabricating/manufacturing the wafer cleaning apparatus according to some embodiments, and may also be adjustable depending on the size and type of the wafer W. 
     The reflector  130  may be disposed inside the housing  100 . For example, the reflector  130  may be disposed around the laser module  110  and/or the optical system  120 . The reflector  130  may re-reflect the laser L that is irradiated/emitted from the laser module  110  and is reflected from the lower surface of the wafer W. Further, the reflector  130  may also block the laser L from reaching other components (e.g., the side wall portion  163  of the chuck  160 ). As an example, the reflector  130  may have a hemispherical shape whose inner surface faces the lower surface of the wafer W. Therefore, the reflector  130  may improve the efficiency of treatment of the wafer W by the laser L. 
     The hemispherical reflector  130  may define a hollow region  100 H. The laser L irradiated/emitted from the laser module  110  may progress through the hollow region  100 H to reach the wafer W. 
     The transparent window  150  may be disposed on the lower surface of the wafer W, e.g., while the wafer W is processed in the wafer cleaning apparatus. For example, the transparent window  150  may be disposed on the top of the housing  100 . The laser L irradiated/emitted from the laser module  110  may penetrate the transparent window  150 . For example, the transparent window  150  may be formed of a transparent material through which the laser L may penetrate. For example, the transparent window  150  may include or be formed of, but is not limited to, quartz. 
     The transparent window  150  may be disposed adjacent to the wafer W, e.g., while the wafer W is processed in the wafer cleaning apparatus. As a result, the outflow of the laser L penetrating the transparent window  150  to a region other than the wafer W may be minimized. In some embodiments, the transparent window  150  may not be in contact with the wafer W. Accordingly, the transparent window  150  may not rotate, even while the chuck  160  and the wafer W rotate together. 
     The size of the transparent window  150  may correspond to the size of the wafer W to heat the whole area of the lower surface of the wafer W. For example, the width/diameter of the transparent window  150  may be the same as the width/diameter of the wafer W. For example, the transparent window  150  may be formed/configured to expose the edge region We, as well as the center region Wc, of the wafer W to the laser L. 
     The calibration window  140  may be disposed inside the housing  100 . The calibration window  140  may be interposed between the laser module  110  and the wafer W, e.g., while the wafer W is processed in the wafer cleaning apparatus. As an example, the calibration window  140  may be interposed between the reflector  130  and the transparent window  150 . The calibration window  140  may adjust the transmissivity of the laser L irradiated/emitted from the laser module  110  for each region. 
     For example, as shown in  FIGS.  5  and  6   , the calibration window  140  may include a first region I and a second region II different from each other. As an example, the first region I may surround the second region II, e.g., in a plan view. At this time, a first light transmissivity of the first region I may be different from a second light transmissivity of the second region II. As an example, the first light transmissivity of the first region I may be smaller/less than the second light transmissivity of the second region II. As another example, the first light transmissivity of the first region I may be greater than the second light transmissivity of the second region II. Here, the light transmissivity may be a ratio of the laser L passing through the calibration window  140  to the laser L incident on the calibration window  140 . For convenience of explanation, a case where the first light transmissivity is smaller than the second light transmissivity will be mainly described below. 
     Therefore, the calibration window  140  may provide the calibrated laser L to the wafer W. For example, the calibration window  140  may transmit the calibrated laser L toward the wafer W. As an example, the first light transmissivity of the first region I may be smaller than the second light transmissivity of the second region II. In such a case, an amount of light of the laser L that penetrates the first region I and reaches the edge region We of the wafer W may be smaller than an amount of light of the laser L that penetrates the second region II and reaches the central region We of the wafer W. 
     In some embodiments, the annular (or donut-like) first region I may have a shape that shares a center with the circular second region II. However, the shapes, sizes, numbers, and the like of the first region I and the second region II are merely examples, and are not limited thereto. 
     In some embodiments, an annular (or donut-like) first coating layer  144  may surround a circular second coating layer  146 . As an example, as shown in  FIG.  6   , a diameter DM 2  of the second coating layer  146  may be from about 200 mm to about 350 mm, a diameter DM 1  of the first coating layer  144  may be greater than the diameter DM 2  of the second coating layer  146 . Preferably, the diameter DM 2  of the second coating layer  146  may be from about 250 mm to about 330 mm, and the diameter DM 1  of the first coating layer  144  may be about 400 mm or less. 
     In some embodiments, the calibration window  140  may include a lens barrel  148  and a window structure  141 . The lens barrel  148  may fix and support the window structure  141 . For example, the window structure  141  may be fixed on and supported by the lens barrel  148 . For example, the lens barrel  148  may surround the side surfaces of the window structure  141 . The lens barrel  148  may have, but is not limited to, for example, a cylindrical shape. The lens barrel  148  may include or be formed of, but is not limited to, for example, at least one of aluminum (Al) and steel use stainless (SUS). The window structure  141  may be disposed inside the lens barrel  148 . As an example, the window structure  141  may be, but is not limited to, a disk type. 
     A thickness TH 11  of the window structure  141  may be appropriately selected as needed. For example, the thickness TH 11  of the window structure  141  may be, but is not limited to, from about 1 mm to about 100 mm. Preferably, the thickness TH 11  of the window structure  141  may be from about 5 mm to about 50 mm. 
     The size of the window structure  141  may correspond to the size of the wafer W to heat the whole area of the lower surface of the wafer W. For example, the window structure  141  may be formed to expose to the edge region We of the wafer W by the laser L. As an example, when a 300 mm wafer W is used, the diameter of the window structure  141  (e.g., DM 1  of  FIG.  5   ) may be from about 250 mm to about 400 mm. 
     In some embodiments, the window structure  141  may include a light projection window  142 , a first coating layer  144 , and a second coating layer  146 . 
     The light projection window  142  may include a lower surface  142 S 1  and an upper surface  142 S 2  that are opposite to each other. In some embodiments, the lower surface  142 S 1  of the light projection window  142  may be disposed to face the laser module  110 , and the upper surface  142 S 2  of the light projection window  142  may be disposed to face the wafer W, e.g., while the wafer W is processed in the wafer cleaning apparatus. The light projection window  142  may include or be formed of, but is not limited to, for example, at least one of a borosilicate glass (e.g., BK 7 ) and a fused silica glass. 
     The first coating layer  144  and the second coating layer  146  may cover the light projection window  142 . The first coating layer  144  may cover the light projection window  142  of the first region I, and the second coating layer  146  may cover the light projection window  142  of the second region II. In some embodiments, the first coating layer  144  and the second coating layer  146  may extend along the lower surface  142 S 1  of the light projection window  142 . As an example, the first coating layer  144  may extend along the lower surface  142 S 1  of the light projection window  142  of the first region I, and the second coating layer  146  may extend along the lower surface  142 S 1  of the light projection window  142  of the second region II. 
     The first coating layer  144  and the second coating layer  146  may have light transmissivities different from each other. As an example, the first light transmissivity of the first coating layer  144  may be smaller than the second light transmissivity of the second coating layer  146 . As an example, the first light transmissivity may be from about 50% to about 95%, and the second light transmissivity may be from about 95% to about 99.9%. Preferably, the first light transmissivity may be from about 80% to about 95%, and the second light transmissivity may be from about 95% to about 99.9%. 
     A thickness TH 21  of the first coating layer  144  and a thickness TH 22  of the second coating layer  146  may be appropriately selected to achieve the required light transmissivity. For example, each of the thickness TH 21  of the first coating layer  144  and the thickness TH 22  of the second coating layer  146  may be, but is not limited to, about 100 nm to about 10,000 nm. Preferably, each of the thickness TH 21  of the first coating layer  144  and the thickness TH 22  of the second coating layer  146  may be about 500 nm to about 5,000 nm. More preferably, each of the thickness TH 21  of the first coating layer  144  and the thickness TH 22  of the second coating layer  146  may be about 900 nm to about 1,000 nm. 
     Although only a case where the thickness TH 21  of the first coating layer  144  and the thickness TH 22  of the second coating layer  146  are the same is shown in  FIG.  5   , this is merely an example. Unlike the shown case in  FIG.  5   , the thickness TH 21  of the first coating layer  144  may be greater or smaller than the thickness TH 22  of the second coating layer  146 . For example, the thicknesses TH 21  and TH 22  may be different from each other. 
     The first coating layer  144  and the second coating layer  146  may each include or be formed of, for example, an oxide. As an example, the first coating layer  144  and the second coating layer  146  may include or be formed of, but are not limited to, at least one of a silicon oxide and a hafnium oxide. 
     In some embodiments, the first coating layer  144  may include or be formed of a high-reflection coating (HR coating) material. The high-reflection coating material may provide high reflectivity and low transmissivity, using a constructive interference of Fresnel reflection. 
     For example, as shown in  FIG.  7   , the first coating layer  144  may include first and second sub-coating layers  1441  and  1442  that are alternately stacked on the light projection window  142 . A refractive index n 1  of the first sub-coating layer  1441  may be lower than a refractive index n 2  of the second sub-coating layer  1442 . As an example, the first sub-coating layer  1441  may include or be formed of silicon oxide, and the second sub-coating layer  1442  may include or be formed of hafnium oxide. Therefore, an example, a first reflected light RW 11  generated on or reflected from the surface of the second sub-coating layer  1442  may achieve the constructive interference with a second reflected light RW 12  generated on or reflected from the surface of the first sub-coating layer  1441 . 
     The thickness of each of the first and second sub-coating layers  1441  and  1442  may be appropriately selected depending on the wavelength X of the laser L to induce the constructive interference. As an example, the thickness of the second sub-coating layer  1442  may be ¼X. 
     In some embodiments, the second coating layer  146  may include an anti-reflection coating (AR coating) material. The anti-reflection coating material may provide low reflectivity and high transmissivity, using the destructive interference of Fresnel reflection. 
     For example, as shown in  FIG.  8   , the second coating layer  146  may include third to sixth sub-coating layers  1461  to  1464  that are sequentially stacked on the light projection window  142 . The refractive indexes n 21  to n 24  of the third to sixth sub-coating layers  1461  to  1464  may decrease in a direction receding from the light projection window  142 . As an example, the fifth sub-coating layer  1463  may include or be formed of hafnium oxide, and the sixth sub-coating layer  1464  may include or be formed of silicon oxide. Therefore, as an example, a third reflected light RW 21  generated on or reflected from the surface of the sixth sub-coating layer  1464  may achieve the destructive interference with a fourth reflected light RW 22  generated on or reflected from the surface of the fifth sub-coating layer  1463 . 
     The thickness of each of the third to sixth sub-coating layers  1461  to  1464  may be appropriately selected depending on the wavelength X of the laser L to induce the destructive interference. As an example, the thickness of the sixth sub-coating layer  1464  may be ¼X. 
     In some embodiments, both the first coating layer  144  and the second coating layer  146  may include the anti-reflection coating material. As an example, the first coating layer  144  may include an anti-reflection coating material having a first light transmissivity, and the second coating layer  146  may include an anti-reflection coating material having a second light transmissivity greater than the first light transmissivity. 
     Although  FIGS.  5  to  8    show that the first coating layer  144  and the second coating layer  146  are formed on the lower surface  142 S 1  of the light projection window  142 , this is merely an example. Unlike the shown cases in the figures, the first coating layer  144  and the second coating layer  146  may be formed on the upper surface  142 S 2  of the light projection window  142 , and may be formed on both the lower surface  142 S 1  and the upper surface  142 S 2  of the light projection window  142 . 
     Although  FIG.  1    shows that the calibration window  140  is in contact with the reflector  130  and the transparent window  150 , this is merely an example. Unlike the shown case in  FIG.  1   , the calibration window  140  may be spaced apart from the reflector  130  or from the transparent window  150  in certain embodiments. 
     In some embodiments, the calibration window  140  may cover the reflector  130 . For example, the calibration window  140  may vertically overlap the whole area of the reflector  130 . In such cases, the hollow region  100 H may be isolated from the outside by the reflector  130  and the calibration window  140 . The reflector  130  and the calibration window  140  may prevent the hollow region  100 H where the laser L progresses from being contaminated by a fume generated from the chemical  210  to be described below. In some embodiments, the hollow region  100 H may be provided and/or maintained with vacuum state. The hollow region  100 H provided with vacuum state may be beneficial for and/or facilitate the progress of the laser L. 
     The chemical supply unit  200  may be disposed over the chuck  160 . The chemical supply unit  200  may supply the chemical  210  to the upper surface of the wafer W. The chemical  210  may include various substances for cleaning the wafer W. For example, the chemical  210  may include, but is not limited to, at least one of phosphoric acid, aqueous ammonia and TMAH (Tetramethylammonium hydroxide). 
     In some embodiments, the chemical  210  may perform a developing process on the wafer W. For example, the chemical  210  supplied to the wafer W may remove a portion of photoresist layer disposed on either the exposed portion Wb or the non-exposed portion Wa. 
     As an example, by the exposure process of the wafer W, the solubility of the exposed portion Wb in the chemical  210  may be increased compared to the solubility of the non-exposed portion Wa in the chemical  210 . In such a case, the chemical  210  supplied to the wafer W may remove the exposed portion Wb, and the non-exposed portion Wa may remain to form a photoresist pattern. For example, a positive tone development (PTD) process may be performed. 
     As another example, the exposed portion Wb may be cured by the exposure process of the wafer W. In such a case, the chemical  210  supplied to the wafer W may remove the unexposed portion Wa, and the cured exposed portion Wb may remain to form a photoresist pattern. For example, a negative tone development (NTD) process may be performed. 
     The chemical supply unit  200  may be, but is not limited to, for example, a nozzle. Although the chemical  210  is only shown as being supplied to the center of the wafer W in  FIG.  1   , this is merely an example, and the chemical  210  may be supplied from the edge of the wafer W in certain embodiments. 
     The chemical supply unit  200  may provide the chemical  210  at a predetermined flow rate. As an example, the chemical supply unit  200  may provide the chemical  210  at a flow rate of about 0.1 L/min to about 1 L/min. When the flow rate of the chemical  210  is not sufficiently large/great, the cleaning speed may be slow, and when the flow rate of the chemical  210  is excessively large/great, heating of the wafer W by the laser L may be slow or may be insufficient. 
     As the wafer W rotates along with the chuck  160 , the chemical  210  provided from the chemical supply unit  200  may spread along the upper surface of the wafer W. Accordingly, the chemical  210  may clean the entire upper surface of the wafer W. In some embodiments, a first flow F may be applied in the direction toward the upper surface of the wafer W for fixation of the wafer W and uniform spreading of the chemical  210 . For example, the first flow F may be a gas flow, and the gas flow may be controlled by a pressure difference (e.g., vacuum level difference) of the gas. The first flow F allows the chemical  210  to move from the central region We of the wafer W to the edge region We of the wafer W. The first flow F may include or be formed of, but is not limited to, an inert gas such as nitrogen (N 2 ) gas. 
     The bowl  180  may be disposed outside the chuck  160  to surround the chuck  160 , e.g., in a plan view. Also, the bowl  180  may be disposed to be higher than the chuck  160  and the wafer W. The bowl  180  may block the outflow of the chemical  210  and/or the vaporized fume of the chemical  210  to the outside. 
     The drain guide portion  190  may guide a drain path of the chemical  210  and/or the fume. As an example, the chemical  210  moved to the edge region We of the wafer W by the first flow F and/or centrifugal force generated by the rotation of the wafer W may reach the drain guide portion  190  via the side wall portion  163  of the chuck  160 . The chemical  210  that has reached the drain guide portion  190  may be discharged to the outside as a drain chemical  210   d . In some embodiments, the drain guide portion  190  may be disposed to be lower than the bowl  180 . Also, in some embodiments, the drain guide portion  190  may be spaced apart from the wafer W farther than the chuck  160  and the housing  100 , e.g., in a plan view. This configuration may be beneficial to prevent other components (e.g., the chuck  160 ) from being damaged by the drain chemical  210   d.    
     In some embodiments, a second flow C 1  and a third flow C 2  may be applied toward the drain guide portion  190 . The second flow C 1  and the third flow C 2  may be gas flows. As an example, as shown in  FIG.  1   , the second flow C 1  may be provided between the side wall portion  163  and the drain guide portion  190  of the chuck  160 , and the third flow C 2  may be provided between the bowl  180  and the drain guide portion  190 . The second flow C 1  and the third flow C 2  may prevent the chemical  210  and/or the fume from flowing backward (e.g., toward the wafer W), and may cool the wafer cleaning apparatus according to some embodiments. Each of the second flow C 1  and the third flow C 2  may include, but are not limited to, a refrigerant such as nitrogen (N 2 ) gas. 
     In some embodiments, the second flow C 1  may be directed toward the drain guide portion  190  via a space between the first rotor unit  165  and the second rotor unit  175 . Such a second flow C 1  may prevent the first rotor unit  165  and the second rotor unit  175  including a magnetic material from being excessively heated, by cooling the first rotor unit  165  and the second rotor unit  175 . 
     Hereinafter, the effects of the wafer cleaning apparatus according to the exemplary embodiment will be described referring to  FIGS.  1  to  9   . 
       FIG.  9    is an exemplary graph showing intensity and temperature of the laser depending on the position of the wafer. For reference, a horizontal axis (x-axis) of  FIG.  9    indicates a distance from the center of the wafer W, and a vertical axis (y-axis) of  FIG.  9    indicates intensity (or an amount of light) of the laser L reaching the wafer W and the temperature of the wafer W. 
     Referring to  FIG.  9   , the intensity (or an amount of light) of the laser L reaching the wafer W and the temperature of the wafer W may not be uniform on the entire surface of the wafer W. As an example, the intensity of the laser L and the temperature of the wafer W in the edge region We (e.g., 100 mm to 150 mm) of the wafer W may be higher than the intensity of the laser L and the temperature of the wafer W in the central region We (e.g., —100 mm to 100 mm) of the wafer W. This may be caused by various causes such as the characteristics of the laser L, a significant difference in reflectivity generated on the lower surface of the wafer W, an influence of airflow generated in the outside of the wafer W, and a significant difference in an oil film thickness formed on the upper surface of the wafer W. 
     However, the wafer cleaning apparatus according to some embodiments may minimize the intensity variation of the laser L and the temperature variation of the wafer W, by including the calibration window  140 . As an example, as described above, the first light transmissivity of the first region I of the calibration window  140  may be smaller/less than the second light transmissivity of the second region II of the calibration window  140 . For example, the amount of light of the laser L that penetrates the first region I and reaches the edge region We of the wafer W may be smaller/less than the amount of light of the laser L that penetrates the second region II and reaches the central region We of the wafer W. Accordingly, the temperature variation on the entire surface of the wafer can be improved to provide the wafer cleaning apparatus with improved performance. 
     Hereinafter, a wafer cleaning apparatus according to the exemplary embodiment will be described referring to  FIGS.  10  to  18   . 
       FIGS.  10  to  12    are various plan views for explaining the calibration window of the wafer cleaning apparatus according to some embodiments. For convenience of explanation, repeated parts of contents explained above using  FIGS.  1  to  9    will be briefly described or omitted. For example, above descriptions with respect to various components, elements, parts and features may also be applied to corresponding components, elements, parts and features of embodiments illustrated in  FIGS.  10  to  12   . 
     Referring to  FIGS.  10  to  12   , in the wafer cleaning apparatus according to some embodiments, the first region I and the second region II are variously formed as needed. 
     For example, as explained above using  FIGS.  1  to  8   , the first coating layer  144  that provides the first region I, and the second coating layer  146  that provides the second region II may be formed in various ways as needed. Accordingly, the wafer cleaning equipment according to some embodiments may provide various customized calibration windows  140 , depending on various intensity profiles of the laser L and various temperature profiles of the wafer W. 
     As an example, as shown in  FIG.  10   , the light transmissivity of the calibration window  140  may gradually change. For example, the light transmissivity of the first region I may gradually decrease in a direction receding from the second region II. Alternatively, for example, the light transmissivity of the first region I may gradually increase in a direction receding from the second region II. Although only a case where the light transmissivity of the first region I gradually changes is explained, this is merely an example. For example, the light transmissivity of the second region II may gradually change, and both the light transmissivity of the first region I and the light transmissivity of the second region II may gradually change. 
     As another example, as shown in  FIG.  11   , the first region I and/or the second region II of the calibration window  140  may be formed radially. For example, the first region I and the second region II are formed radially and may be disposed alternately. For example, the first region I and the second region II may have a fan shape, and a plurality of fanwise first regions I and a plurality of fanwise second regions II may be alternately arranged in an azimuthal direction as shown in  FIG.  11   . Although the calibration window  140  is only shown as including four first regions I and four second regions II in  FIG.  11   , this is merely an example, and the number of first regions I and the number of second regions II may be various. For example, the calibration window  140  may have more than four first regions I and more than four second regions II. In certain embodiments, the calibration window  140  may have less than four first regions I and less than four second regions II. Also, unlike the shown case in  FIG.  11   , at least a part of the plurality of first regions I and/or at least a part of the plurality of second regions II may be continuously disposed. 
     As still another example, as shown in  FIG.  12   , the calibration window  140  may include a plurality of second regions II that are spaced apart from each other. The shape and size of each second region II and the number of the second region II are merely examples, and are not limited thereto. For example, at least a part of the second region II may have a form other than a circular shape, e.g., an oval, an ellipse, a rectangle, a triangle, etc. 
     In some embodiments, the calibration window  140  may also be rotatively disposed on the reflector  130 . Therefore, the calibration window  140  may provide various forms for each region. 
       FIG.  13    is a cross-sectional view for explaining a calibration window of the wafer cleaning apparatus according to some embodiments.  FIG.  14    is a plan view for explaining a calibration window of the wafer cleaning apparatus according to some embodiments. For convenience of explanation, repeated parts of contents explained above using  FIGS.  1  to  12    will be briefly explained or omitted. 
     Referring to  FIGS.  13  and  14   , in the wafer cleaning apparatus according to some embodiments, the calibration window  140  may include a plurality of window structures  141   a ,  141   b , and  141   c.    
     As an example, the calibration window  140  may include first to third window structures  141   a ,  141   b , and  141   c  that are stacked sequentially. The laser irradiated/emitted from the laser module may sequentially penetrate the first to third window structures  141   a ,  141   b , and  141   c . Although the calibration window  140  is only shown as including the three window structures  141   a ,  141   b , and  141   c  in  FIG.  13   , this is merely an example, and the number of window structures may vary. In some embodiments, the number of stacked window structures  141   a ,  141   b , and  141   c  may be about 50 or less. Preferably, the number of stacked window structures  141   a ,  141   b , and  141   c  may be about 30 or less. 
     Although the thicknesses of the first to third window structures  141   a ,  141   b , and  141   c  are only shown as being the same as each other in  FIG.  13   , this is merely an example, and if necessary, the thicknesses of each of the first to third window structures  141   a ,  141   b , and  141   c  may differ from each other. 
     The first to third window structures  141   a ,  141   b , and  141   c  may respectively include light projection windows  142   a ,  142   b , and  142   c , first coating layers  144   a ,  144   b , and  144   c  and second coating layers  146   a ,  146   b , and  146   c.    
     In some embodiments, the first coating layers  144   a ,  144   b , and  144   c  of the respective first to third window structures  141   a ,  141   b , and  141   c  may have areas different from each other. For example, as shown in  FIG.  14   , the calibration window  140  may include first to fourth regions I to IV different from each other. As an example, a first region I may surround a second region II, the second region II may surround a third region III, and the third region III may surround a fourth region IV. At this time, for example, the first coating layer  144   a  of the first window structure  141   a  may cover (e.g., vertically overlap) a light projection window  142   a  of the first region I, the first coating layer  144   b  of the second window structure  141   b  may cover (e.g., vertically overlap) a light projection window  142   b  of the first and second regions I and II, and the first coating layer  144   c  of the third window structure  141   c  may cover (e.g., vertically overlap) a light projection window  142   c  of the first to third regions I to III. 
     In such a case, the first to fourth regions I to IV may have light transmissivities different from each other. As an example, the first light transmissivity of the first region I may be smaller/less than the second light transmissivity of the second region II, the second light transmissivity of the second region II may be smaller/less than the third light transmissivity of the third region III, and the third light transmissivity of the third region III may be smaller/less than the fourth light transmissivity of the fourth region IV. 
     In some embodiments, the annular (or donut-like) first to third regions I to III may have a shape that shares a center with the circular fourth region IV. However, the shapes, sizes, and the like of the first to fourth regions I to IV, and the number of regions are merely examples, and are not limited thereto. In some embodiments, as the number of regions of the calibration window  140  (e.g., first to fourth regions I to IV) increases, the temperature variation on the entire surface of the wafer may be improved more finely. 
       FIG.  15    is a cross-sectional view for explaining the calibration window of the wafer cleaning apparatus according to some embodiments. For convenience of explanation, repeated parts of contents explained above using  FIGS.  1  to  14    will be briefly explained or omitted. 
     Referring to  FIG.  15   , in the wafer cleaning apparatus according to some embodiments, at least some of the plurality of window structures  141   a ,  141   b , and  141   d  include a plurality of first coating layers  144   a ,  144   b  and  144   d.    
     As an example, the plurality of window structures  141   a ,  141   b , and  141   d  may include a fourth window structure  141   d . Each of the window structures  141   a ,  141   b , and  141   d  may include light projection windows  142   a ,  142   b  and  142   d , first coating layers  144   a ,  144   b  and  144   d , and second coating layers  146   a ,  146   b  and  146   d . At this time, the fourth window structure  141   d  may include a plurality of first coating layers  144   d.    
     Although the fourth window structure  141   d  is only shown as including two first coating layers  144   d  in  FIG.  15   , this is merely an example, and the number of aspheric lenses may vary. Further, if necessary, the first window structure  141   a  may include a plurality of first coating layers  144   a  or the second window structure  141   b  may include a plurality of first coating layers  144   b.    
       FIGS.  16  to  18    are various cross-sectional views for explaining a wafer cleaning apparatus according to some embodiments. For convenience of explanation, repeated parts of contents explained above using  FIGS.  1  to  15    will be briefly explained or omitted. 
     Referring to  FIG.  16   , the wafer cleaning apparatus according to some embodiments does not include a transparent window  150 . 
     For example, the calibration window  140  may be disposed adjacent to the wafer W. Accordingly, it is possible to minimize the outflow of the laser L passing through the calibration window  140  to a region other than the wafer W. For example, the calibration window  140  of  FIG.  16    may replace a combined structure of the transparent window  150  and the calibration window  140  of  FIG.  1   . In some embodiments, the calibration window  140  may not be in contact with the wafer W. Accordingly, the calibration window  140  may not rotate, even while the chuck  160  and the wafer W rotate together. 
     Referring to  FIG.  17   , in the wafer cleaning apparatus according to some embodiments, the transparent window  150  is interposed between the reflector  130  and the calibration window  140 . 
     Although the calibration window  140  is only shown as being in contact with the transparent window  150  in  FIG.  17   , this is merely an example. Unlike the shown case in  FIG.  17   , the calibration window  140  may be spaced apart from the transparent window  150 . 
     In some embodiments, the transparent window  150  may cover the reflector  130 . For example, the transparent window  150  may vertically overlap the whole area of the reflector  130 . In such a case, the hollow region  100 H may be isolated from the outside by the reflector  130  and the transparent window  150 . The reflector  130  and the transparent window  150  may prevent the hollow region  100 H in which the laser L progresses from being contaminated by the fume generated from the chemical  210 . In some embodiments, the hollow region  100 H may be provided in vacuum. The hollow region  100 H provided in vacuum may facilitate the progress of the laser L. For example, the transparent window  150  and the reflector  130  may be airtightly attached. 
     Referring to  FIG.  18   , in the wafer cleaning apparatus according to some embodiments, at least a part of the calibration window  140  is disposed in the reflector  130 . 
     For example, the calibration window  140  may be disposed inside the hollow region  100 H. In some embodiments, the transparent window  150  may cover the reflector  130 . For example, the transparent window  150  may vertically overlap the whole area of the reflector  130 . In such a case, the hollow region  100 H in which the calibration window  140  is disposed may be isolated from the outside by the reflector  130  and the transparent window  150 . Although the calibration window  140  is only shown as being in contact with the transparent window  150  in  FIG.  18   , this is merely an example. Unlike the shown case in  FIG.  18   , the calibration window  140  may be spaced apart from the transparent window  150 . 
     Hereinafter, a method for cleaning a wafer according to exemplary embodiments will be described referring to  FIGS.  1  to  19   . 
       FIG.  19    is a flowchart for explaining the method for cleaning the wafer using the wafer cleaning apparatus according to some embodiments. For convenience of explanation, repeated parts of contents explained above using  FIGS.  1  to  18    will be briefly described or omitted. 
     Referring to  FIG.  19   , the method for cleaning the wafer using the wafer cleaning apparatus according to some embodiments may include measurement (S 10 ) of a temperature gradient of the wafer W, provision (S 20 ) of the calibration window  140  using the measured temperature gradient, and cleaning (S 30 ) of the wafer W using the calibration window  140 . 
     The measurement (S 10 ) of temperature gradient of the wafer W may be performed by measuring the temperature of the lower surface of the wafer W heated by the laser L in the wafer cleaning apparatus according to some embodiments. The measurement of the temperature of the lower surface of the wafer W may include, but is not limited to, for example, usage of a pyrometer and/or a charged-coupled device (CCD) camera. Accordingly, as an example, a graph as shown in  FIG.  9    may be provided. 
     The provision (S 20 ) of the calibration window  140  may be performed by adjusting the transmissivity of the calibration window  140  to the laser L for each region based on the measured temperature gradient. As an example, when a graph as shown in  FIG.  9    is provided, the calibration window  140  including the first region I having the first light transmissivity and the second region II having the second light transmissivity greater than the first light transmissivity may be provided. 
     The cleaning (S 30 ) of the wafer W may be performed by utilizing the wafer cleaning apparatus described above referring to  FIGS.  1  to  18   . This will be described more specifically below in the description of  FIG.  20   . Accordingly, a method for cleaning a wafer with improved performance may be provided. 
     Hereinafter, the method for fabricating the semiconductor device according to an exemplary embodiment will be described referring to  FIGS.  1  to  20   . 
       FIG.  20    is a flowchart for explaining a method for fabricating a semiconductor device using the wafer cleaning apparatus according to some embodiments. For convenience of explanation, repeated parts of contents explained above using  FIGS.  1  to  19    will be briefly explained or omitted. 
     Referring to  FIG.  20   , the method for fabricating the semiconductor device using the wafer cleaning apparatus according to some embodiments includes disposition (S 40 ) of the wafer W on the chuck  160 , heating (S 42 ) of the wafer W by irradiating/emitting the laser L, and supplying (S 44 ) of the chemical  210  to the wafer W. 
     The disposition (S 40 ) of the wafer W on the chuck  160  may be performed by fixing the wafer W to the grip portion  161  of the chuck  160 . As the chuck  160  rotates, the wafer W fixed on/to the chuck  160  may also rotate. In some embodiments, the wafer W may be a wafer subjected to the exposure process. For example, the wafer W may include an exposed portion Wb and a non-exposed portion Wa. The wafer W may include, but is not limited to, a photoresist film. 
     The heating (S 42 ) of the wafer W may be performed by irradiating the lower surface of the wafer W with the laser L. The irradiation/radiation of the laser L to the lower surface of the wafer W may be performed using the laser module  110 . As described above using  FIGS.  1  to  8   , the laser L irradiated/emitted from the laser module  110  may penetrate the optical system  120  and the calibration window  140 , and reach the lower surface of the wafer W. 
     The supplying (S 44 ) of the chemical  210  to the wafer W may be performed, using the chemical supply unit  200 . The chemical  210  supplied from the chemical supply unit  200  may be provided to the upper surface of the wafer W. As a result, the wafer W may be cleaned. In some embodiments, the cleaning of the wafer W may be performed by a puddle method that utilizes the surface tension of the chemical  210 . 
     In some embodiments, the chemical  210  supplied to the wafer W may remove either the exposed portion Wb or the unexposed portion Wa. As an example, the chemical  210  supplied to the wafer W may remove the exposed portion Wb, and the non-exposed portion Wa may remain to form a photoresist pattern. As another example, the chemical  210  supplied to the wafer W may remove the non-exposed portion Wa, and the exposed portion Wb may remain to form a photoresist pattern. Accordingly, a developing process of the wafer W may be performed to fabricate a semiconductor device including a predetermined pattern. 
     While the present inventive concept has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present inventive concept as defined by the following claims. It is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.