Abstract:
In an apparatus and method for treating a wafer-shaped article, a spin chuck is provided for holding and rotating a wafer-shaped article. A first liquid dispenser communicates with a supply of an organic liquid and is positioned so as to dispense the organic liquid onto a surface of a wafer-shaped article. A degasifying unit is positioned upstream of the first liquid dispenser and downstream of the supply. The degasifying unit is configured to reduce a dissolved gas content of the organic liquid to less than 20% of a saturation concentration at a pressure of 1 bar.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and apparatus for liquid treatment of wafer-shaped articles. 
     2. Description of Related Art 
     Liquid treatment includes both wet etching and wet cleaning, wherein the surface area of a wafer to be treated is wetted with a treatment liquid and a layer of the wafer is thereby removed or impurities are thereby carried off. A device for liquid treatment is described in U.S. Pat. No. 4,903,717. In this device the distribution of the liquid may be assisted by the rotational motion imparted to the wafer. 
     Techniques for drying a surface of a disc-shaped article are typically used in the semiconductor industry after cleaning a silicon wafer during production processes (e.g. pre-photo clean, post CMP-cleaning, and post plasma cleaning). However, such drying methods may be applied for other plate-like articles such as compact discs, photo masks, reticles, magnetic discs or flat panel displays. When used in semiconductor industry it may also be applied for glass substrates (e.g. in silicon-on-insulator processes), III-V substrates (e.g. GaAs) or any other substrate or carrier used for producing integrated circuits. 
     Various drying methods are known in the semiconductor industry, some of which utilize isopropyl alcohol to reduce surface tension of rinse water on a semiconductor wafer surface. See, e.g., U.S. Pat. No. 5,882,433. Improvements upon such methods, involving the use of heated isopropyl alcohol, are described in commonly-owned patent applications WO 2011/007287 and U.S. Ser. No. 12/914,802 (filed Oct. 28, 2010). 
     There remains a need, however, to develop improved methods for preventing pattern collapse in the submicroscopic structures formed on such semiconductor wafers, not only during such drying process but also during other liquid treatments. Pattern collapse can occur when the surface tension of a liquid moving radially outwardly across the surface of a rotating wafer applies a damaging or destructive force to the submicroscopic structures formed on the wafer surface. 
     The problem of pattern collapse becomes more serious as the diameter of semiconductor wafers increases. For example, the current generation of single wafer wet processing technology is designed for 300 mm diameter wafers, but the previous generation technology was designed for 200 mm wafers and a next generation may be designed for wafers of 450 mm or larger diameter. 
     The problem of pattern collapse also becomes more serious as the aspect ratio of the submicroscopic structures continues to increase. This is also an ongoing trend in the manufacture of semiconductor devices, as the pressure to reduce device dimensions in general applies more to the horizontal layout and less to the thickness direction. 
     SUMMARY OF THE INVENTION 
     The present invention was based in part on the recognition by the present inventors that the problem of pattern collapse during hydrophobic drying processes with isopropyl alcohol is due partly to outgassing of air that is dissolved in the isopropyl alcohol, as well as the formation of silicates on the device structures owing to the oxygen concentration of the isopropyl alcohol. 
     Thus, in one aspect, the present invention relates to an apparatus for treating a wafer-shaped article, comprising a spin chuck for holding and rotating a wafer-shaped article. A first liquid dispenser communicates with a supply of an organic liquid and is positioned so as to dispense the organic liquid onto a surface of a wafer-shaped article when positioned on the spin chuck. A degasifying unit is positioned upstream of the first liquid dispenser and downstream of the supply, the degasifying unit being configured to reduce a dissolved gas content of the organic liquid to less than 20% of a saturation concentration at a pressure of 1 bar. 
     In preferred embodiments of the apparatus according to the present invention, the degasifying unit comprises a semipermeable membrane that is permeable to gas coming out of solution from the organic liquid, and impermeable to the organic liquid. 
     In preferred embodiments of the apparatus according to the present invention, the degasifying operates at a subatmospheric pressure less than 500 mbar (absolute pressure), preferably less than 200 mbar, and more preferably less than 100 mbar. 
     In preferred embodiments of the apparatus according to the present invention, the degasifying unit comprises one or a plurality of tubular semipermeable membranes, and wherein the degasifying unit is configured to generate a subatmospheric pressure of less than 500 mbar inside the one or plurality of semipermeable membranes. 
     In preferred embodiments of the apparatus according to the present invention, the organic liquid is 2-propanol. 
     In preferred embodiments of the apparatus according to the present invention, a second liquid dispenser communicates with a supply of a second liquid, wherein the second liquid contains more than 90 wt.-% water (i.e. a diluted aqueous solution or water, e.g. deionized water). 
     In preferred embodiments of the apparatus according to the present invention, the first and second liquid dispensers are each configured to dispense liquid from a central region of a wafer-shaped article toward a peripheral region of a wafer-shaped article, thereby to directly displace the second liquid with the organic liquid. 
     In preferred embodiments of the apparatus according to the present invention, the degasifying unit is configured to reduce a dissolved gas content of the organic liquid to less than 10% of a saturation concentration, and more preferably to less than 5% of a saturation concentration. 
     In another aspect, the present invention relates to a method for treating a wafer-shaped article, comprising positioning and rotating a wafer-shaped article on a spin chuck, degasifying an organic liquid to reduce a dissolved gas content of the organic liquid to less than 20% of a saturation concentration at a pressure of 1 bar, and to produce a degasified organic liquid, and dispensing the degasified organic liquid onto a surface of the wafer-shaped article rotating on the spin chuck. 
     In preferred embodiments of the method according to the present invention, the degasifying comprises contacting the organic liquid with a semipermeable membrane that is permeable to gas coming out of solution from the organic liquid, and impermeable to the organic liquid. 
     In preferred embodiments of the method according to the present invention, the degasifying comprises subjecting the organic liquid to a subatmospheric pressure less than 500 mbar, preferably less than 200 mbar, and more preferably less than 100 mbar. 
     In preferred embodiments of the method according to the present invention, the degasifying comprises contacting the organic liquid with one or a plurality of tubular semipermeable membranes, while maintaining a subatmospheric pressure of less than 500 mbar inside the one or plurality of semipermeable membranes. 
     In preferred embodiments of the method according to the present invention, the organic liquid is 2-propanol. 
     In preferred embodiments of the method according to the present invention, a second liquid is dispensed onto the surface of the wafer-shaped article adjacent to the organic liquid, wherein the second liquid contains more than 90 wt.-% water, thereby to directly displace the second liquid with the organic liquid. 
     In preferred embodiments of the method according to the present invention, 2-propanol and deionized water are dispensed from a central region of the wafer-shaped article toward a peripheral region of the wafer-shaped article. 
     In preferred embodiments of the method according to the present invention, the degasifying is performed so as to reduce a dissolved gas content of the organic liquid to less than 10% of a saturation concentration at a pressure of 1 bar, and more preferably to less than 5% of a saturation concentration at a pressure of 1 bar. 
     In preferred embodiments of the method according to the present invention, deionized water is dispensed simultaneously with the organic liquid onto a same surface of the wafer-shaped article, and the degasifying is performed so as to prevent formation of bubbles at an interface between the organic liquid and the water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the invention will become more apparent after reading the following detailed description of preferred embodiments of the invention, given with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of an apparatus according to a first embodiment of the invention; 
         FIG. 2  is schematic view of the degasifying unit used in the apparatus of  FIG. 1 ; 
         FIGS. 3 a , 3 b , 3 c  and 3 d    illustrate a sequence of silicate formation and pattern collapse that can occur with conventional drying techniques; 
         FIGS. 4 a , 4 b , 4 c  and 4 d    illustrate interface conditions between isopropyl alcohol and deionized water in conventional drying processes and according to embodiments of the present invention; 
         FIG. 5  presents data showing the influence of various drying techniques on pattern collapse; 
         FIG. 6  schematically illustrates a possible supply configuration for deionized water and isopropanol; and 
         FIG. 7  is a schematic view of an apparatus according to a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIG. 1  depicts a spin chuck  1  that holds a wafer W thereon in a predetermined orientation, which is preferably such that the major surfaces of disposed horizontally or within ±20° of horizontal. Spin chuck  1  may for example be a chuck that operates according to the Bernoulli principle, as described for example in U.S. Pat. No. 4,903,717. 
     Chuck  1  is typically present in a process module for single wafer wet processing of semiconductor wafers, and may or may not be positioned within a chamber C. Two liquid dispensers are positioned above the chuck  1 , namely, an IPA dispense nozzle  3  for dispensing isopropanol, and a DIW dispense nozzle  4  for dispensing deionized water onto the upper surface of wafer W. 
     IPA dispense nozzle  3  receives isopropanol from IPA reservoir  8  via IPA supply conduit  7 , whereas DIW dispense nozzle  4  receives deionized water from DIW reservoir  9  via DIW supply conduit  6 . 
     In this embodiment, the isopropanol from IPA reservoir  8  is passed through a degasifying unit  10  prior to being fed to the IPA dispense nozzle  3 . The degasifying unit  10  includes a semipermeable membrane  12 , and a vacuum pump  14  is connected to this degasifying unit at vacuum connections  16  (see  FIG. 2 ) to assist in bringing dissolved oxygen in the IPA out of solution and into gaseous form, to be separated from the IPA before the IPA is dispensed onto wafer W. 
     As shown in  FIG. 2 , the semipermeable membrane  12  is preferably a gas-permeable membrane tube, which can degasify the IPA based on a pressure differential between the inside and outside of the tube. Typically, a vacuum less than 500 mbar (absolute) is needed to degasify the IPA. 
     In  FIG. 3 a   , non-degassed IPA  24  is shown between a pair of adjacent device features  22  formed on a semiconductor wafer that is undergoing treatment. As the features  22  begin to dry, the resultant forces displace the features toward one another, as shown in  FIG. 3 b   . As drying continues, silicates begin to precipitate on the device features  22 , the silicate precipitates being designated  26  in  FIG. 3 c   . Finally, as the drying becomes complete, the precipitated silicates may aggregate with one another as shown in  FIG. 3 d   , causing further distortion and collapse of device features  22 , and adversely affecting the performance of the device. 
     In  FIG. 4 a   , device features  22  are again shown in the context of a drying process, wherein deionized water  40  has first been used to rinse the wafer, and isopropanol is then dispensed in order to displace the remaining deionized water from the wafer surface. 
     In conventional processes of this type, the present inventors have discovered that the oxygen dissolved in the IPA tends to come out of solution as bubbles  38  (see  FIG. 4 b   ), in a region  36  proximate the water  40 , owing to the lower solubility of oxygen in water as compared to IPA. Indeed, a larger bubble  39  may form at the interface of the IPA and DIW, which blocks contact between the two liquids altogether. 
     Typical hydrophobic drying processes are based on hydrophobization of a substrate (wafer) by treatment with diluted HF followed by rinse with DI water and IPA. With the ever decreasing size of the device features on the wafer more and more pattern damage or leaning is observed. One mechanism for this effect is the formation of silicates and adhesion between device features caused by dissolved oxygen and/or remaining water, as discussed above in connection with  FIGS. 3 a -3 d   . Another mechanism would be the formation of gas bubbles caused by the different gas dissolving behavior of IPA and water, as discussed above in connection with  FIGS. 4 a -4 d   . IPA can dissolve more oxygen than water, and the disparity in oxygen solubility in these solvents increases with increasing temperature. Thus, when the dissolved oxygen in the IPA starts to outgas in the device features, this also causes the leaning. 
     However, when the IPA is first degassed according to the methods and apparatus of the present invention, the foregoing problems are averted. That is, bubble formation is suppressed, as is the precipitation of silicates, owing to the reduced concentration of oxygen. Thus, as shown in  FIG. 4 c   , the IPA  32  and DIW  40  are free to blend homogeneously in an intermediate region  34 , such that the water  40  can be readily displaced and replaced by IPA  32 , as shown in  FIG. 4   d.    
       FIG. 5  shows the effect of preferred embodiments of the present invention in relation to conventional techniques, by counting defects across the radius of 300 mm semiconductor wafers. The baseline data reflects non-degassed IPA and DIW. The hood data uses the same drying media in a nitrogen hood to provide a non-oxidizing ambient. The degassed IPA data reflects the techniques of the present invention, and the hood+low O2 media data used a nitrogen hood as well as degassed IPA. 
     The use of degassed IPA resulted in a very substantial decrease in defects relative to conventional techniques, which was most pronounced about halfway between the center and the outer periphery of the wafer. Degassed IPA in a nitrogen hood showed somewhat less improvement than degassed IPA without a nitrogen hood, which suggests that nitrogen bubbles might have interfered with drying. 
     In  FIG. 6 , a possible configuration of DIW and IPA supply is shown, wherein the DIW passes through venture  15 , and wherein the venture  15  is connected to the degasifying unit  10  by a line that include check valve  13  and vacuum sensor  11 . 
     In the alternative embodiment of  FIG. 7 , a single liquid dispense nozzle  4  is provided, and DIW from DIW reservoir  9  supplied through conduit  6  and IPA from IPA reservoir  8  supplied through conduit  7  are premixed at mixing junction  5 , whereafter the mixture of DIW and IPA is passed through the degasifying unit  10  as described in connection with the preceding embodiments. 
     Reference to DIW and IPA in the preceding embodiments in by way of example only, as the methods and apparatus of the invention may be employed with any suitable pair of liquids, whether dispensed individually or in admixture. Advantageously, one liquid is an organic liquid and the other is aqueous, the aqueous liquid preferably being at least 90% by weight water, such as pure deionized water or mixtures of water and other liquids miscible therewith. 
     While the present invention has been described in connection with various preferred embodiments thereof, it is to be understood that those embodiments are provided solely to illustrate the invention, and should not be used as a pretext to limit the scope of protection conferred by the true scope and spirit of the appended claims.