Abstract:
An etch apparatus. The etch apparatus includes a tank coupled to a recirculating path that includes a dissolver. The dissolver includes a porous carbon matrix filter coated with silicon nitride. An etchant from the tank circulates through the recirculating path and performs a selective etching of a structure in the tank in contact with the etchant. The structure includes silicon nitride on a pad layer that includes silicon dioxide. The selective etching is characterized by the silicon nitride on the pad layer being selectively etched by the etchant relative to an etching by the etchant of the silicon dioxide. The etch apparatus further includes: means for dissolving the silicon nitride coated on the filter into the etchant at a controlled dissolution rate sufficient to cause the selective etching; and means for coating the silicon nitride onto the filter to facilitate the selective etching.

Description:
[0001]     This application is a continuation application claiming priority to Ser. No. 10/760,896, filed Jan. 20, 2004; which is a divisional application of U.S. Pat. No. 6,699,400, issued Mar. 2, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates, in general, to semiconductor device fabrication and, more particularly, to etch processes used in the fabrication of semiconductor devices.  
       BACKGROUND OF THE INVENTION  
       [0003]     The fabrication of semiconductor devices and/or integrated circuits often requires removing certain materials from a semiconductor wafer while leaving other materials on the wafer. This can be accomplished in a selective etch process that uses an etchant having different etch rates with respect to different materials. To characterize the selective etch process, an etch selectivity is defined as the ratio of the etch rate of one material to the etch rate of another material. For example, an aqueous phosphoric acid solution having a concentration of approximately 85 percent heated to a temperature between 165 degrees Celsius (° C.) and 185° C. is routinely used for removing silicon nitride structures from a semiconductor wafer while leaving exposed silicon dioxide structures on the wafer. At the temperature of 165° C., the phosphoric acid solution etches silicon nitride at a rate of approximately 6 nanometers per minute and etches silicon dioxide at a rate of no more than 0.25 nanometers per minute. The resulting etch selectivity is at least 24:1.  
         [0004]     The etch selectivity of an etch process depends on the temperature, concentration, and composition of the etchant. Consequently, the etch selectivity usually changes as more wafers are processed in the etchant. For example, the nitride to oxide etch selectivity of the etch process using the phosphoric acid etchant is approximately 24:1 when the etchant is fresh. After processing approximately 1000 wafers having silicon nitride thereon, the etch selectivity increases dramatically to 50:1 or greater. This selectivity variation adversely affects the efficiency, reliability, and yield of the semiconductor device and/or integrated circuit fabrication processes.  
         [0005]     Accordingly, it would be advantageous to have an etch process that has a stable etch selectivity and an apparatus for performing the etch process. It is desirable for the etch process to have a high etch selectivity. It would be of further advantage if the etch apparatus can be adapted from existing etch apparatuses.  
       SUMMARY OF THE INVENTION  
       [0006]     A general object of the present invention is to provide an efficient and reliable etch process and an apparatus for performing the etch process. It is a further object of the present invention for the etch process to be capable of producing semiconductor devices and/or integrated circuits having high performance, high reliability, and high yield. Another object of the present invention is to implement the etch process with modifications to existing etch apparatuses.  
         [0007]     These and other objects of the present invention are achieved by adjusting and controlling the composition of the etchant during the etch process. For example, a selective etch modifier can be introduced into the etchant. The selective etch modifier alters the etch rates of certain materials but has no significant effect on the etch rates of other materials, thereby modifying the etch selectivity of the etch process. By monitoring and controlling the concentration of the etch rate modifier in the etchant, a stable etch selectivity is maintained during the etch process. The etch rate modifier can be either a selective etch intensifier or a selective etch rate suppressor. The selective etch intensifier selectively increases the etch rate of certain materials. On the other hand, the selective etch rate suppressor selective decreases the etch rate of certain materials.  
         [0008]     In a preferred embodiment of the present invention, a hot phosphoric acid solution is used as the etchant for etching the silicon nitride on a semiconductor wafer. A recirculating path is established for the hot phosphoric acid etchant. A high surface area structure such as, for example, a carbon matrix filter is coated with silicon nitride. The carbon matrix filter is installed in the recirculating path for the etchant. As the etchant in the recirculating path flows through the carbon matrix filter, it dissolves the silicon nitride coated on the carbon matrix filter. The dissolved silicon nitride significantly reduces the etch rate of silicon dioxide on the semiconductor wafer. The etch rate of the silicon nitride on the semiconductor wafer is substantially unaffected by the presence of the silicon nitride in the etchant. Therefore, the silicon nitride dissolved in the hot phosphoric acid etchant functions as an etch rate modifier that enhances the etch selectivity of the etch process. More particularly, the dissolved silicon nitride functions as a selective etch rate suppressor that substantially inhibits the etch of silicon dioxide on the semiconductor wafer. The concentration of silicon nitride in the etchant can be monitored and adjusted to maintain a stable etch selectivity of the etch process. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic diagram of an etch apparatus in accordance with the present invention;  
         [0010]      FIG. 2  is a flow chart schematically illustrating an etch process in accordance with the present invention;  
         [0011]      FIG. 3  is a schematic diagram of another etch apparatus in accordance with the present invention; and  
         [0012]      FIG. 4  is a schematic diagram of yet another etch apparatus in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     Preferred embodiments of the present invention are described herein with reference to the figures. It should be noted that the figures are not necessarily drawn to scale. It should also be noted that elements having similar functions are labeled using the same reference numerals in the figures.  
         [0014]      FIG. 1  is a schematic diagram of an apparatus  10  used in a wet etch process in accordance with the present invention. Apparatus  10  is also referred to as a wet etcher or simply an etcher. Etcher  10  includes a tank  11  filled with an etchant  12 . Tank  11  and etchant  12  form an etchant bath for etching an object, e.g., a semiconductor wafer  15 , submerged in the etchant bath. Tank  11  filled with etchant  12  is also referred to as a bath  11  of etchant  12 . A heating element  16  such as, for example, a filament is immersed in etchant  12  for adjusting and maintaining the temperature of etchant  12  during the etch process. Tank  11  has a drain  18 . In a semiconductor device fabrication process, old and contaminated etchant is periodically removed from tank  11  through drain  18  and tank  11  is then filled with new etchant. A spout  19  is connected to a source of deionized water (not shown) and provides tank  11  with deionized water, thereby adjusting the concentration of etchant  12 . Etcher  10  also includes a chamber  21  attached to a sidewall  14  of tank  11 . Chamber  21  has an outlet  22  at its bottom. A conduit  23  couples outlet  22  of chamber  21  to an inlet  24  of a filtering system  25 . Another conduit  27  has a first end connected to an outlet  26  of filtering system  25  and a second end mounted adjacent to tank  11 . During an etch process, etchant  12  in tank  11  overflows sidewall  14  into chamber  21 . Etchant  12  in chamber  21  is pumped back to tank  11  through conduit  23 , filtering system  25 , and conduit  27 . Therefore, chamber  21 , conduit  23 , filtering system  25 , and conduit  27  form a recirculating path for etchant  12  in tank  11 . The second end of conduit  27  serves as an outlet  29  of the recirculating path. Because etchant  12  in tank  11  reaches chamber  21  by overflowing sidewall  14 , sidewall  14  is also referred to as an overflow sidewall and chamber  21  is also referred to as an overflow chamber or an overflow compartment. Overflow chamber  21  communicates with tank  11  through overflow sidewall  14 .  
         [0015]     In addition, etcher  10  includes a substance dissolving system  32  in the recirculating path for etchant  12 . During an etch process, substance dissolving system  32  introduces a substance into etchant  12  to modify the characteristics of etchant  12 , thereby achieving a desired result such as, for example, a high etch selectivity, a stable etch selectivity, a contamination free etch process, etc. The type and quantity of the substance introduced into etchant  12  depend on the composition of etchant  12  and the desired result. Substance dissolving system  32  is installed between the two ends of conduit  27  and includes a split valve  34 , a dissolver  35 , a bypass conduit  37 , and a merge valve  36 . Split valve  34  has one inlet and two outlets. Merge valve  36  has two inlets and one outlet. The inlet of split valve  34  is coupled to the outlet  26  of filtering system  25  via a section of conduit  27 . Dissolver  35  has an inlet connected to the first outlet of split valve  34  and an outlet connected the first inlet of merge valve  36 . The second outlet of split valve  34  is coupled to the second inlet of merge valve  36  via bypass conduit  37 . The outlet of merge valve  36  is coupled to outlet  29  of the recirculating path via another section of conduit  27 . Split valve  34  and merge valve  36  control the partition of etchant  12  flowing through dissolver  35  and through bypass conduit  37 .  
         [0016]     Preferably, dissolver  35  has a high surface area coated with the substance to be introduced into etchant  12  during the etch process. In a preferred embodiment, dissolver  35  is formed by depositing the substance on a porous structure such as, for example, a carbon matrix filter. When etchant  12  in the recirculating path flows through dissolver  35 , the substance deposited on the porous filter is dissolved in etchant  12 .  
         [0017]     The dissolution rate at which etchant  12  dissolves the substance deposited on the porous filter can be controlled by adjusting the temperature of etchant  12  flowing through the porous filter. Preferably, substance dissolving system  32  includes a temperature controller (not shown), e.g., a heating coil and a cooling coil, for adjusting the temperature of etchant  12  flowing through the porous filter. An alternative method for controlling the dissolution rate is changing the surface area of the porous filter exposed to etchant  12 . This can be achieved by partially submerging the porous filter in etchant  12  flowing through substance dissolving system  32  and adjusting the extent to which the porous filter is submerged in etchant  12 . The dissolution rate can also be controlled by adjusting the rate at which etchant  12  flows through the porous filter. The flow rate of etchant  12  through the porous filter can be controlled by adjusting split valve  34 , merge valve  36 , and a pump (not shown) in the recirculating path. Further, etchant  12  adjacent dissolver  35  may be saturated with the substance dissolved from the surface of the porous filter. This may adversely affect the etch process. Therefore, substance dissolving system  32  preferably includes a flushing system (not shown) that can be periodically turned on to flush dissolver  35 .  
         [0018]      FIG. 2  is a flow chart schematically illustrating an etch process  50  in accordance with the present invention. By way of example, etch process  50  is a wet etch process using etcher  10  of  FIG. 1  for etching silicon nitride structures and/or polycrystalline silicon structures on semiconductor wafer  15 . In a semiconductor device fabrication process, silicon nitride structures are formed on semiconductor wafer  15 . Typically, a pad layer of silicon dioxide is formed between the silicon nitride structures and the surface of semiconductor wafer  15  to relieve the tension on the wafer surface. Other structures such as, for example, polycrystalline silicon structures can also be formed on semiconductor wafer  15 . Preferably, the silicon dioxide layer on semiconductor wafer  15  remains in place after etch process  50  so that it can protect the underlying films or retain a uniform thickness for consistent performance of the semiconductor devices fabricated on semiconductor wafer  15 . Therefore, etch process  50  preferably has a high and stable etch selectivity so that and the etch of the silicon dioxide pad layer on semiconductor wafer  15  is substantially inhibited.  
         [0019]     Etchant  12  for etching silicon nitride and/or polycrystalline silicon on semiconductor wafer  15  is preferably an aqueous solution of phosphoric acid having a concentration of approximately 85 percent and a temperature between approximately 165 degrees Celsius (° C.) and approximately 185° C. Silicon nitride is deposited on a high surface area structure such as, for example, a carbon matrix filter, which serves as dissolver  35  in substance dissolving system  32 . Etchant  12  dissolves the silicon nitride deposited on the carbon matrix filter as it flows through dissolver  35 . In tank  11 , the silicon nitride dissolved in etchant  12  alters the composition and modifies the characteristics of etchant  12 . More particularly, the silicon nitride reacts with the phosphoric acid in etchant  12  in a chemical reaction: 
 
Si 3 N 4 +7H 3 PO 4 →2(NH 4 ) 2 HPO 4 +H2Si(PO 4 ) 2 +HSi 2 (PO 4 ) 3    (1) 
 
 The silicon phosphate acid compounds formed in the reaction are not volatile, so etchant  12  does not lose significant amounts of the silicon phosphate acid compounds through evaporation. However, the silicon phosphate acid compounds are unstable. They react with the water in etchant  12  as described in the following equations: 
 
H 2 Si(PO 4 ) 2 +2H2O→2H 3 PO 4 +SiO 2    (2) 
 
HSi 2 (PO 4 ) 3 +4H 2 O→3H 3 PO 4 +2SiO 2    (3) 
 
 Therefore, the series of chemical reactions described by equations (1), (2), and (3) can be described by the following equation: 
 
Si 3 N 4 +2H 3 PO 4 +6H 2 O→2(NH 4 ) 2 HPO 4 +3SiO 2    (4) 
 
         [0020]     The silicon dioxide formed in etchant  12  suppresses the etch of the silicon dioxide on semiconductor wafer  15  and has no significant effect on the etch rate of silicon nitride and polycrystalline silicon. The etch selectivity of etch process  50  is significantly increased. Therefore, the silicon nitride deposited on the carbon matrix filter in substance dissolving system  32  functions as a selective etch rate suppressor during etch process  50 .  
         [0021]     Etch process  50  starts with preparing an etchant bath (step  51 ) by filling tank  11  in etcher  10  with etchant  12  so that etchant  12  overflows sidewall  14  and spills into overflow chamber  21 . Heating element  16  in tank  11  maintains etchant  12  at a desired temperature, e.g., approximately 165° C., at which temperature the phosphoric acid solution loses its water component through evaporation. Spout  19  continuously adds deionized water into tank  11  to make up the water lost through evaporation, thereby maintaining a substantially constant concentration of etchant  12 .  
         [0022]     A pump (not shown) pumps etchant  12  in chamber  21  through filtering system  25  and substance dissolving system  32  to establish a recirculating path for etchant  12  (step  52 ). The pump also controls the recirculating rate of etchant  12 . Filtering system  25  reconditions etchant  12  throughout etch process  50  by filtering out contaminants that may be present in etchant  12 .  
         [0023]     When etchant  12  flows through dissolver  35  in substance dissolving system  32 , the silicon nitride deposited on the carbon matrix filter is gradually dissolved in etchant  12  and introduced into tank  11  through outlet  29  of the recirculating path (step  53 ). The silicon nitride dissolved in etchant  12  changes the characteristics of etchant  12 . More particularly, the silicon nitride functions as a selective etch rate suppressor to enhance the etch selectivity of etchant  12 .  
         [0024]     The introduction of the silicon nitride into etchant  12  continues while semiconductor wafer  15  is submerged in tank  11  of etchant  12 . The concentration of the silicon nitride selective etch rate suppressor in etchant  12  determines the etch selectivity of etch process  50 . Preferably, the concentration of the selective etch rate suppressor is sufficiently high to substantially quench or inhibit the etch of silicon dioxide on semiconductor wafer  15 . It should be noted that a very high silicon nitride concentration in etchant  12  may produce too much silicon dioxide in etchant  12 , thereby causing an undesirable effect of silicon dioxide precipitating on semiconductor wafer  15 . Preferably, an equilibrium between the consumption and production of silicon dioxide in etchant  12  is maintained at an appropriate level to achieve an etch selectivity approaching infinity while substantially inhibiting any silicon dioxide deposition on semiconductor wafer  15 . A desired equilibrium is achieved when the selective etch rate suppressor concentration in etchant  12  is, by way of example, approximately 0.5 milligram of silicon nitride per milliliter of the phosphoric acid solution. At this concentration, the etch selectivity of etch process  50  approaches infinity to one and there is no significant silicon dioxide precipitation on semiconductor wafer  15  during etch process  50 .  
         [0025]     The concentration of the selective etch rate suppressor in etchant  12  (step  54 ) is monitored. In one embodiment, the concentration of the selective etch rate suppressor is monitored by measuring the etch rates of the silicon nitride structures and the silicon dioxide structures on monitoring wafers (not shown) in etchant  12 . In another embodiment, the concentration of the selective etch rate suppressor is monitored by measuring the ammonium cation concentration in etchant  12 . As described in equations (1) and (4) above, the ammonium cation concentration in the phosphoric acid solution depends on the dissolved silicon nitride concentration in etchant  12 . Methods for measuring the ammonium cation concentration include cation ion chromatography and ammonia selective electrode measurement.  
         [0026]     Adjustments are made to etchant  12  if the monitoring scheme indicates that the concentration of the selective etch rate suppressor therein is not optimal. If the concentration of the selective etch rate suppressor in etchant  12  is too low, the dissolution rate of the silicon nitride deposited on the carbon matrix filter is increased. This can be accomplished by increasing the temperature of etchant  12  flowing through dissolver  35 , increasing the surface area of dissolver  35  in etchant  12 , and/or increasing the flow rate of etchant  12  through dissolve  35 . If the concentration of the selective etch rate suppressor in etchant  12  is too high, the temperature and/or the flow rate of etchant  12  through dissolver  35  are decreased to reduce the dissolution rate of the silicon nitride deposited on the carbon matrix filter in etchant  12 . The dissolution rate can also be reduced by decreasing the surface area of dissolver  35  exposed to etchant  12  flowing through substance dissolving system  32 . The temperature of etchant  12  flowing through dissolver  35  is adjusted using a temperature adjusting element or an etchant temperature controller (not shown), e.g., heating coil and a cooling coil, in substance dissolving system  32 . The flow rate of etchant  12  through dissolver  35  can be controlled by adjusting the recirculating rate of etchant  12 . The flow rate of etchant  12  through dissolver  35  can also be controlled by adjusting split valve  34  and merge valve  36  to alter the ratio of etchant  12  flowing through dissolver  35  with respect to that flowing through bypass conduit  37 . Split valve  34  and merge valve  36  are preferably capable of directing all etchant  12  in the recirculating path through dissolver  35 , thereby maximizing the dissolution rate of silicon nitride into etchant  12 . Likewise, split valve  34  and merge valve  36  are also preferably capable of directing all etchant  12  flowing in the recirculating path through bypass conduit  37 , thereby achieving a substantially zero dissolution rate of silicon nitride into etchant  12 . The dissolution rate of the silicon nitride deposited on dissolver  35  into etchant  12  can also be adjusted by periodically flushing dissolver  35  with deionized water.  
         [0027]     After an appropriate silicon nitride concentration in etchant  12  is achieved, semiconductor wafer  15  is submerged in etchant  12  in tank  11  (step  56 ). Usually, semiconductor wafer  15  is mounted on a cassette (not shown). The cassette includes a plurality of wafers mounted thereon. The wafers mounted on a cassette are referred to as a batch of wafers. By way of example, a batch typically includes between 15 and 20 wafers. Preferably, the wafers in a batch are substantially identical to each other. In tank  11 , the silicon nitride and/or polycrystalline silicon structures on semiconductor wafer  15  are etched by the hot phosphoric acid. The etch of silicon dioxide on semiconductor wafer  15  is greatly suppressed or substantially inhibited by the selective etch rate suppressor in etchant  12 .  
         [0028]     When a desired etch result is achieved, etch process  50  ends by removing semiconductor wafer  15  from tank  11  of etchant  12  (step  57 ). Preferably, steps  52 ,  53 , and  54  described herein above and shown in the flow chart of  FIG. 2  continue after semiconductor wafer  15  is removed from tank  11  to maintain etchant  12  in tank  11  in an optimal condition. Etcher  10  is ready for receiving the next batch of wafers. If etchant  12  is so contaminated that its continual use may adversely affect the performance, reliability, or yield of the semiconductor devices on semiconductor wafer  15 , it is discharged from etcher  10  through drain  18  at the bottom of tank  11 . Tank  11  is then filled with new and clean etchant  12 . Filtering system  25  and dissolver  35  may also need replacement from time to time. Further, the whole apparatus of etcher  10 , which includes tank  11 , chamber  21 , conduits  23  and  27 , filtering system  25 , and substance dissolving system  32 , may need to be cleansed after a prolonged use.  
         [0029]      FIG. 3  is a schematic diagram of another etch apparatus  60  in accordance with the present invention. Apparatus  60  is also referred to as a wet etcher or simply an etcher. Etcher  60  is structurally similar to etcher  10  shown in  FIG. 1  and includes a tank  11  filled with an etchant  12  and a deionized water supply spout  19 . Etcher  60  also includes a recirculating path comprised of a chamber  61 , a conduit  23 , a filtering system  25 , and a conduit  27 .  
         [0030]     A difference between etcher  10  of  FIG. 1  and etcher  60  is that substance dissolving system  32  installed between filtering system  25  and outlet  29  of the recirculating path of etcher  10  is absent in etcher  60 . Instead, etcher  60  includes a dissolver  65  in chamber  61  adjacent to outlet  22 . Like dissolver  35  in etcher  10 , dissolver  65  preferably includes a high surface area object coated with the substance to be introduced into etchant  12 . For example, when etcher  60  is used for etching silicon nitride on semiconductor wafer  15 , dissolver  65  can include a carbon matrix filter with silicon nitride deposited thereon. The silicon nitride dissolved in etchant  12  during an etch process functions as a selective etch rate suppressor to substantially inhibit the etch of silicon dioxide on semiconductor wafer  15 . In an etch process using etcher  60 , dissolution rate of the silicon nitride on dissolver  65  is controlled by adjusting the temperature and the recirculating rate of etchant  12 .  
         [0031]     Another difference between etcher  10  shown in  FIG. 1  and etcher  60  is that in etcher  60 , chamber  61  is attached to a permeable sidewall  64  of tank  11 . Etchant  12  in tank  11  flows into chamber  61  either through permeable sidewall  64  or by overflowing permeable sidewall  64 . In other words, chamber  61  communicates with tank  11  through permeable sidewall  64 . Additional differences include the locations of deionized water supply spout  19  and outlet  29  of the recirculating path. In etcher  60 , deionized water supply spout  19  and outlet  29  of the recirculating path are located in chamber  61 . Therefore in etcher  60 , recirculated etchant  12  and deionized water are supplied to tank  11  via chamber  61  and through permeable sidewall  64  between chamber  61  and tank  11 .  
         [0032]      FIG. 4  is a schematic diagram of yet another etch apparatus  70  in accordance with the present invention. Apparatus  70  is also referred to as a wet etcher or simply an etcher. Etcher  70  is structurally similar to etcher  10  shown in  FIG. 1  and includes a tank  11  filled with an etchant  12  and a deionized water supply spout  19 . Etcher  70  also includes a recirculating path comprised of a chamber  71 , a conduit  23 , a filtering system  25 , and a conduit  27 .  
         [0033]     A difference between etcher  10  of  FIG. 1  and etcher  70  is that substance dissolving system  32  installed between filtering system  25  and outlet  29  of the recirculating path of etcher  10  is absent in etcher  70 . Instead, etcher  70  includes a substance dissolving system  72  installed between outlet  22  of chamber  71  and inlet  24  of filtering system  25 . Substance dissolving system  72  is comprised of a split valve  74 , a dissolver  75 , and a bypass conduit  77 . Split valve  74  has one inlet and two outlets. The inlet of split valve  74  is coupled to the outlet  22  of chamber  71  via a section of conduit  23 . An inlet of dissolver  75  is connected to the first outlet of split valve  74 . Another section of conduit  23  couples an outlet of dissolver  35  to inlet  24  of filtering system  25 . Bypass conduit  77  is coupled between the second outlet of split valve  74  and inlet  24  of filtering system  25 . Split valve  74  controls the partition of etchant  12  in the recirculating path flowing through dissolver  75  and through bypass conduit  77 .  
         [0034]     Like dissolver  35  in etcher  10 , dissolver  75  preferably includes a high surface area structure coated with the substance to be introduced into etchant  12  during the etch process. In a preferred embodiment, dissolver  75  is formed by depositing the substance on a porous filter such as, for example, a carbon matrix filter. When etchant  12  in the recirculating path flows through dissolver  75 , the substance deposited on the porous filter is dissolved in etchant  12 .  
         [0035]     The dissolution rate can be controlled by adjusting the temperature of etchant  12  flowing through dissolver  75 . Like substance dissolving system  32  in etcher  10 , substance dissolving system  72  preferably includes a temperature controller (not shown), e.g., a cooling coil, for adjusting the temperature of etchant  12  flowing through dissolver  75 . The dissolution rate can also be controlled by adjusting the rate at which etchant  12  flows through dissolver  75 . The flow rate of etchant  12  through dissolver  75  can be controlled by adjusting split valve  74  and/or a pump (not shown) in the recirculating path. Further, substance dissolving system  72  preferably includes a flushing system (not shown) that can be periodically turned on to flush etchant  12  near dissolver  75 .  
         [0036]     Another difference between etcher  10  shown in  FIG. 1  and etcher  70  is that in etcher  70 , chamber  71  is attached to a permeable sidewall  64  of tank  11 . Etchant  12  in tank  11  flows into chamber  71  either through permeable sidewall  64  or by overflowing permeable sidewall  64 . In other words, chamber  71  communicates with tank  11  through permeable sidewall  64 . Additional differences include the locations of deionized water supply spout  19  and outlet  29  of the recirculating path. In etcher  70 , deionized water supply spout  19  and outlet  29  of the recirculating path are located in chamber  71 . Therefore in etcher  70 , recirculated etchant  12  and deionized water are supplied to tank  11  via chamber  71  and through permeable sidewall  64  between chamber  71  and tank  11 .  
         [0037]     By now it should be appreciated that an etch process and an apparatus for performing the etch process have been provided. In accordance with the present invention, a selective etch rate suppressor is introduced into the etchant bath during the etch process to increase the etch selectivity of the etch process. For example, in an etch process using hot phosphoric acid etchant to etch silicon nitride on a semiconductor wafer, silicon nitride is introduced into the etchant as the selective etch rate suppressor. The silicon nitride in the phosphoric acid etchant significantly decreases the etch rate of silicon dioxide on the semiconductor wafer. Preferably, the silicon nitride is introduced into the etchant using a filter coated with the silicon nitride and installed in the recirculating path for the etchant. The silicon nitride is dissolved in the etchant while the etchant in the recirculating path flows through the filter. When the etchant flows back to the etchant bath, the silicon nitride is substantially completely dissolved in the etchant, thereby substantially eliminating the particulate deposition of the silicon nitride on the semiconductor wafer. The silicon nitride concentration in the etchant is monitored and maintained at a desirable level by adjusting the temperature and flow rate of the etchant through the filter. The etch process of the present invention is efficient and reliable. The increased and stabilized etch selectivity improves the performance, reliability, and yields of semiconductor devices and/or integrated circuits fabricated using the etch process of the present invention.  
         [0038]     While specific embodiments of the present invention have been shown and described, further modifications and improvements will occur to those skilled in the art. For example, the selective etch rate suppressor is not limited to being coated on a porous filter in the recirculating path of the etchant and dissolved in the etchant as the etchant flows through the filter. The selective etch rate modifier can be introduced into the etchant in powder form. The powder can be either added directly into the etchant bath, or introduced into the etchant in the recirculating path. Further, the application of the present invention is not limited to enhancing the etch selectivity of an etch process. The principle of the present invention is applicable to other processes whose characteristics are improved by introducing a material not required for the process itself. This process improvement is not limited to etch selectivity enhancement. For example, in a hydrofluoric acid based etch process for etching silicon dioxide on a semiconductor wafer, silicon can be coated on a filter installed in the etchant recirculating path and introduced into the etchant as the etchant flows through the recirculating path. The silicon serves to getter copper contamination. More particularly, the silicon removes the copper from the etchant, thereby avoiding the copper being deposited on the exposed silicon on the semiconductor wafer and contaminating the wafer surface.