Abstract:
A wet etching system includes a tank for containing a chemical and having an open top portion, and a heater disposed in the tank for heating the chemical contained therein. A cover is arranged on the open top portion of the tank, and the cover includes a cooling apparatus formed therein. The wet etching method includes placing a semiconductor substrate, having a layer thereon to be etched, into the tank, and then driving the heater to maintain the chemical within a temperature range. Deionized water in the chemical evaporates when the temperature range is greater than a boiling point of the deionized water. The evaporated deionized water condenses on the cooler cover and then flows back into the tank to maintain a constant chemical concentration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of application Ser. No. 09/323,014, filed Jun. 1, 1999, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wet etching system for manufacturing semiconductor devices and a wet etching method using the same, and more particularly, to a wet etching system and method for etching a nitride layer, formed on a semiconductor substrate, for a short period of time while maintaining the concentration of phosphoric acid (H 3 PO 4 ) constant. 
     2. Description of the Related Art 
     Generally, semiconductor devices are manufactured by forming multiple insulating layers or conductive layers on a semiconductor substrate, and then forming a specific electrical pattern on the layers according to desired characteristics of the particular semiconductor device being manufactured. 
     The formation of the specific pattern is achieved by selectively removing designated portions of the insulating layers and/or the conductive layers by an etching process. There are two types of etching processes: dry etching using a plasma generated in a reaction chamber, and wet etching using a particular chemical that is capable of removing the layer to be etched. The wet etching process is carried out using a wet etching system employing a tank containing the chemical etchant. 
     FIG. 1 schematically illustrates a conventional etching system for manufacturing semiconductor devices, employing a chemical bath comprising a tank  10  for containing a chemical  16  for etching a certain layer on the semiconductor substrate. Normally, the chemical  16  is diluted with deionized water, with the degree of dilution being controlled depending on the etching process. 
     A heater  12  heats the chemical  16  inside the tank  10  to a certain temperature sufficient for the process conditions of the etching process. Typically, bar-shaped heaters  12  are disposed in the chemical  16  at opposing sides of the tank  10 . 
     A bubbler  14  is connected to the tank  10  and is used to generate bubbles in the chemical  16  inside the tank  10  in order to improve the reaction of the chemical  16  and a wafer placed in the chemical bath. 
     The temperature of the chemical  16  and the etching process time are important parameters in the etching process. However, heating the chemical  16  causes the deionized water, which was used to dilute the chemical, to evaporate. As a result, the concentration of the chemical  16  within the tank  10  is increased due to the evaporation of the deionized water, and thus, deionized water must be supplied in an amount equal to the evaporated amount in order to maintain the chemical  16  concentration constant. In some cases, additional amounts of chemical  16  are supplied to the tank  10  to maintain the chemical  16  concentration constant. 
     However, precisely controlling the concentration changes of the chemical due to the evaporation of the deionized water is not easy, despite the continuous supply of the deionized water, because the concentration of the chemical  16  inside a the tank  10  is not constant. 
     Although the chemical concentration changes can be detected by changes in the etch rate observed during the etching process, such observations occur after a particular wafer may have suffered a fatal defect caused by a failure of the pattern profile during the etching process. In other words, precise control of the etching process is difficult when using data generated after the etching process is performed. 
     Another drawback of the conventional wet etching system is that the temperature of the chemical  16  decreases as the deionized water is continuously supplied to the tank  10 , and thus the heater  12  must be continuously driven to heat the chemical  16  to the desired etching temperature, which shortens the life span of the etching system itself. 
     Referring to FIG. 1, the etching process for a nitride layer is described. First, a chemical  16  for etching the nitride layer, such as phosphoric acid (H 3 PO 4 ), is supplied in the tank  10 . The concentration of the phosphoric acid (H 3 PO 4 ) is 85%, with the remainder being deionized water. Although the phosphoric acid concentration is relatively high, the etch rate for the nitride layer is good at this concentration value. 
     The phosphoric acid (H 3 PO 4 ) is heated so as to maintain its temperature between about 170° C. to 174° C. according to the process conditions of the etching process, and then, the etching process is carried out. The etching process time depends on the thickness of the nitride layer. 
     However, as describe above, the required continuous supply of deionized water and the chemical to the tank  10  causes difficulties in trying to precisely control the chemical concentration inside the tank  10 ; which causes uneven etch rates. Further, because the heater  12  is bar-shaped, it takes a long time to heat the phosphoric acid (H 3 PO 4 ) and maintain its temperature, which may damage the heater and shorten the life time of the etching system. 
     Moreover, after the etching process is completed, the disposal and the cleaning of the remaining waste solution, including the deionized water, the chemical, etc. due to the continuous supply thereof remains a difficult problem. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a wet etching system for manufacturing semiconductor devices and a wet etching method using the same, which provides efficient and precise execution of the etching process while maintaining the concentration of the chemical etchant constant. 
     Another object of the present invention is to provide a wet etching system for manufacturing semiconductor devices and a wet etching method using the same, which decreases the process temperature of the chemical to one that is lower than the conventional art, and increases the etch rate of the etching process, while maintaining the concentration of the chemical constant to thereby reduce the etching process time. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the wet etching system for manufacturing semiconductor devices comprises an open-topped tank containing a chemical diluted with deionized water to wet-etch a certain layer on a semiconductor substrate. A heater is disposed in the tank, and a cover is employed to cover the open top of the tank. The cover includes a cooling apparatus that condenses the deionized water evaporated by the heating of the heater. 
     The lower surface of the cover gradually slopes downwardly from opposing edges of the cover toward either a central lateral or longitudinal axis thereof, which facilitates collection of the condensed deionized water. A discharge opening is provided in the cover, in flow communication with the ambient outside of the tank, so that gas can be easily discharged from the tank. 
     The tank and the cover may be made of a chemically-resistant material, such as TEFLON™, and the surface of the heater may be coated with TEFLON™ as well. 
     In another aspect of the present invention, there is provided a wet etching method comprising supplying a chemical diluted with deionized water into a tank having a top portion with a cover arranged thereon, the cover having a cooling apparatus formed therein. A semiconductor substrate, having a layer thereon to be etched, is placed into the tank. A heater disposed within the tank maintains the chemical within a temperature range. The deionized water evaporates when the temperature range is greater than a boiling point of the deionized water. The deionized water then condenses on the cooler cover, before flowing back into the tank. The etching process is carried out while the evaporation and condensing steps are performed. 
     Where the layer to be etched is a nitride layer, the chemical supplied into the tank is a phosphoric acid (H 3 PO 4 ) diluted with deionized water, the concentration of the phosphoric acid (H 3 PO 4 ) being from 80% to 90%. The chemical is heated to maintain its temperature between about 153° C. to 157° C. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a schematic illustration of a conventional wet etching system for manufacturing semiconductor devices; 
     FIG. 2 is a perspective view of an embodiment of the wet etching system for manufacturing semiconductor devices according to the present invention; 
     FIG. 3 is a side view of the cover of FIG. 2; 
     FIG. 4 is a graphical representation comparing the heating temperature as a function of the concentration of the phosphoric acid (H 3 PO 4 ) as used in the present invention; and 
     FIG. 5 is a graphical representation comparing the etch rate and the etch thickness as a function of the etch time using the wet etching system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 2 illustrates a wet etching system according to one embodiment of the present invention. As used herein, the term chemical is used generically to refer to either a single chemical, a solution of two or more chemicals, or a solution of a chemical and a diluent, such as deionized water. 
     The wet etching system comprises a tank  20  having an open top. The tank  20  is preferably made of a chemical and acid-resistant material, such as TEFLON™ (Dupont Co.), which eliminates problems cause by chemical corrosion and the like. A chemical  46  for etching a semiconductor substrate is contained within the tank  20 , and the chemical is preferably a solution comprising a diluent of deionized water. 
     A heater  22  is provided in the lower portion of the tank  20  for heating the chemical  46  to a certain temperature. The heater  22  is ring-shaped or annular, which facilitates the uniform heating of the chemical  46  in the lower portion of the tank  20 . 
     In close proximity to the upper portion of the tank  20 , there is provided a temperature detecting device  30 , for example, a thermocouple or other suitable temperature detecting device. A controller  32  controls the operation of the heater  22  according to the temperature of the chemical  46  detected by the temperature detecting device  30 . 
     A chemical supply line  38  passes through an upper side wall of the tank  20  for supplying the chemical  46 , which comprises a diluent of deionized water, to the tank  20 . A filter  42  is provided along the chemical supply line  38  before it enters the tank  20  to prevent the intake of contaminants. A chemical discharge line  40  passes through a lower side wall of the tank  20  for draining out the chemical  46  after it has been used for a designated time period. A filter  42  is provided on the chemical discharge line  40  after it exits the tank  20 . 
     A cover  24  is used to seal the open top edges of the tank  20 . The cover  24  prevents the deionized water used for diluting the chemical from being discharged to the ambient atmosphere, after the deionized water within the tank  20  evaporates when the temperature of the chemical  46  exceeds the boiling point of the deionized water. 
     As shown in FIG. 3, the lower surface  24   a  of the cover  24  gradually tapers downwardly, toward the bottom of the tank, from the opposing edges of the cover to a central region of the cover  24 . The lower surface  24   a  shown in FIG. 3 illustrates a downward taper from the opposing lateral edges  24   e  of the cover  24  toward a central region along the lateral axis of the cover  24 . Of course, the lower surface  24   a  may taper downwardly from the opposing longitudinal edges  24   d  (see FIG. 2) of the cover  24  toward a central region along the longitudinal axis of the cover  24 . In still another embodiment, the lower surface  24   a  may simultaneously taper downwardly from the opposing longitudinal and lateral edges  24   d ,  24   e  of the cover  24  toward a central region along the longitudinal and lateral axis of the cover  24 . 
     As shown in FIG.  2  and FIG. 3, a cooling water line  28  is formed within the cover  24  to cool the cover  24  by flowing cooling water through the cooling water line  28 . Therefore, for any of the above-described cover  24  embodiments, when the deionized water contained in the chemical  46  evaporates while the chemical  46  is heated to its proper processing temperature, the evaporated deionized water contacts the cooling water line  28  and the relatively cooler cover  24  and condenses, and the condensed deionized water flows down the tapered lower surface  24   a  and back into the tank  20 . 
     A discharge opening  26  extending through the cover  24 , preferably at a central longitudinal/lateral location, discharges any unwanted gases which accumulate inside the tank  20 . 
     The cover  24  can be hinge-coupled using hinges  27  on one side of the tank  20  to allow easy opening of the cover  24 . The cover  24  is also preferably made of a chemical and acid-resistant material such as TEFLON™. More preferably, the surface of the heater  22  is coated with a chemical and acid-resistant material such as TEFLON™. 
     A bubbler  34  is connected via a gas supply line to a lower portion of the tank  20  for bubbling the chemical  46  contained inside the tank  20  using a particular gas in order to improve the etching efficiency during the etching process. A gas supply nozzle  37  of the bubbler  34  is provided on the lower side of the heater  22 . The bubbling gas can be selected from the group consisting of chloride (Cl 2 ) gas, fluorine (F) gas, or ozone (O 3 ) gas, or a mixture of at least any two of these. The gas supplied from the bubbler  34  is discharged out of the tank  20  through the discharge opening  26  provided on the cover  24 . 
     In order to minimize decreases in the temperature of the chemical  46  due to the bubbling gas supplied as described above, a gas heating device  36  is provided on the bubbling gas supply line to preheat the bubbling gas to a predetermined temperature of about 70° C. to 80° C. before being supplied into the tank  20 . 
     A support member  48  is provided within the tank  20 , above the heater  22 , to support a wafer to be etched, or a cassette of wafers to be etched. If the wet etching system of the present invention is used to etch a nitride layer formed on a semiconductor substrate, the tank would contain a chemical  46  comprised of from 80% to 90% by weight phosphoric acid (H 3 PO 4 ), with the reminder being deionized water. Of course, the apparatus and method of the present invention can be used to etch any type of insulating, conductive, or semiconductive layer, using various types of conventional etchants. 
     Experiments were conducted to determine the optimum process conditions for etching the nitride layer. Eight different samples, having phosphoric acid (H 3 PO 4 ) concentrations of 40%, 50%, 60%, 65%, 70%, 75%, 80% and 85%, were prepared and heated from 100° C. to 180° C. FIG. 4 is a graphical representation comparing the upper heating temperature limit depending on the concentration of the phosphoric acid (H 3 PO 4 ) used. The upper heating temperature limit is the temperature of the phosphoric acid (H 3 PO 4 ) concentration sample which does not increase any further, even when heated for two or three hours. 
     As shown in FIG. 4, in case of the 40% phosphoric acid (H 3 PO 4 ) concentration sample, the temperature increase stops at 113° C., and in case of the 85% phosphoric acid (H 3 PO 4 ) concentration sample, the temperature increase stops at 157° C. The upper heating temperature limit exponentially increases with increasing phosphoric acid (H 3 PO 4 ) concentration. 
     As described earlier, using the conventional wet etching system shown in FIG.  1  and an 85% concentration of the phosphoric acid (H 3 PO 4 ) to etch a nitride layer, the etching process is carried out at a temperature between 170° C. to 174° C., while continuously supplying the deionized water and the phosphoric acid (H 3 PO 4 ) as an etchant. However, at a 170° C. processing temperature, the actual phosphoric acid (H 3 PO 4 ) concentration is greater than 85%, due to the evaporation of the deionized water through the open top of the tank, which is confirmed by the graph in FIG.  4 . In the conventional case, attempts are made to compensate for the evaporated deionized water by continuously supplying the deionized water and the phosphoric acid (H 3 PO 4 ) into the tank  10 . Such a procedure has been found to be inadequate to precisely control the phosphoric acid (H 3 PO 4 ) concentration, and accordingly, different etch rates are experienced by those practicing the conventional method. 
     The wet etching system shown in FIG. 2 solves the above problems, and maintains a constant phosphoric acid (H 3 PO 4 ) concentration inside the tank  20  by condensing the evaporated deionized water on the cover  24  and allowing it to flow back into the tank  20  along the tapered lower surface  24   a  of the cover  24 . 
     The wet etching method of the present invention is useful in optimizing the concentration of the phosphoric acid (H 3 PO 4 ) and the heating temperature, while maintaining the concentration of the phosphoric acid (H 3 PO 4 ) constant, thereby improving the etch rate in the etching process. Typically, the etch rate for the nitride layer by the phosphoric acid (H 3 PO 4 ) is inversely proportional to the concentration of the phosphoric acid (H 3 PO 4 ), and proportional to the heating temperature at the particular concentration value thereof. 
     In the embodiment of the present invention, phosphoric acid (H 3 PO 4 ) at from 80% to 90% concentration, which is commercially available, is supplied into the tank  20  and heated to about 153° C. to 157° C., and preferably 155° C. A semiconductor substrate, having a nitride layer formed thereon with a thickness of 1500 Å, is placed in the tank  20  while maintaining the above temperature. 
     FIG. 5 is a graphical representation comparing the etch rate and the etch thickness as a function of the etch time in the etching process of the present invention. 
     Referring to FIG. 5, the etch rate of the nitride layer is about 40 Å/min. to 45 Å/min. for the first 30 minutes. The reason that the etch rate decreases after 35 minutes is that only a small amount of the nitride layer is still remaining after about 35 minutes, so the etching process is nearly complete after 35 minutes. Compared with the conventional case, where it takes about 70 minutes to etch a nitride layer with a thickness of 1500 Å while maintaining the 85% concentration of the phosphoric acid (H 3 PO 4 ) at 170° C., the etch time in the present invention is reduced by almost half. 
     While the case involving a phosphoric acid (H 3 PO 4 ) concentration of from 80% to 90% was highlighted in the above example, depending on the operator&#39;s purpose and the process conditions, the phosphoric acid (H 3 PO 4 ) can be prepared with various concentrations, and the method of the present invention can be utilized for these various concentrations. Referring to FIG. 4, the optimized temperature conditions according to the concentration of phosphoric acid (H 3 PO 4 ) can be detected. 
     Moreover, although the etching of a nitride layer using phosphoric acid (H 3 PO 4 ) has been highlighted in the above example, it is understood that the present invention can be applied to etch various layer types, such as an oxide layer, and various etchants may be used, such as nitric acid, sulfuric acid and fluoric acid. 
     Therefore, according to the present invention, the changes in the chemical concentration within the tank, due to the evaporation of the deionized water, is minimized or prevented. As a result, changes in the etch rate are minimized so that the etching process can be more easily and precisely controlled and the profile of the pattern after the etching can be improved. 
     Further, according to the present invention, the desired etch rate can be maximized or optimized so that the etching process time is shortened, and the productivity for semiconductor devices is improved. 
     Further, using the wet etching system of the present invention, the etching process does not require continuous additional supplies of deionized water or chemical, such that the heating time and the process time can be reduced, thereby saving power. This results in less damage to the heater  22  and a greater life span. 
     While the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made thereto without departing from the spirit and scope of the invention as defined by the appended claims.