Abstract:
In a photoresist dispensing apparatus for use in manufacturing a semiconductor device, to coercively emit photoresist from a bottle by using a dispensing pump and to pass it through a supply line and a filter to obtain a filtering operation, and to spray the filtered photoresist to a wafer through a spraying nozzle; a bubble removal unit is equipped with the supply line, before the dispensing pump. Large and micro bubbles generated in the midst of flow of photoresist, and foreign substances, are substantially filtered off so as to supply photoresist of a good quality. A floating load in a foreign substance removal filter is substantially removed, thus spraying photoresist under an always uniform and stabilized pressure by using a dispensing pump, to cover a wafer with photoresist in a uniform thickness and obtain a precise pattern formation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the manufacturing of semiconductor devices or the like. More particularly, the present invention relates to the dispensing of photoresist onto a wafer or other substrate.  
         [0003]     2. Description of the Related Art  
         [0004]     In general, the manufacturing of semiconductor devices involves subjecting a wafer to a series of processes such as diffusion, photolithography, etching, ion implantation and deposition processes. In particular, the photolithography and etching processes are carried out to form a pattern, e.g., a circuit pattern, on the wafer. The photolithography process entails forming a mask on a wafer to expose portions of a layer underlying the mask. The etching process entails removing the portions of the layer exposed by the mask.  
         [0005]     The photolithography process begins by dispensing a chemical, namely a photoresist, onto a surface on a wafer, and forming a uniform thin layer of the photoresist over the entire surface. The layer of photoresist is then exposed by light directed through a reticle. The reticle has a pattern corresponding to the pattern to be formed on the wafer. Accordingly, an image of the pattern of the reticle is transferred to the layer of photoresist. Then, the layer of photoresist is developed. The developing process removes either the exposed or non-exposed portions of the photoresist to thereby form a photoresist pattern, i.e., a mask, on the wafer.  
         [0006]      FIG. 1  illustrates a conventional photoresist dispensing apparatus of semiconductor device manufacturing equipment.  
         [0007]     As shown in  FIG. 1 , the dispensing apparatus includes bottles  10  containing photoresist, a supply line  20  connected to the bottles  10 , a pump  30  and a filter  40  disposed in the supply line  20 , and a spray nozzle  50  connected to an end of the supply line  20 . The photoresist is withdrawn from the bottles  10  and is forced through the supply line  20  by the pump  30 . Then, the photoresist is sprayed onto a wafer W by the spray nozzle  50 . The most important aspect of spraying the photoresist onto the wafer W is that the photoresist cover the wafer W uniformly.  
         [0008]     However, even if the layer of photoresist covering the wafer W has a uniform thickness, the photomask formed from the layer of photoresist will be defective if the photoresist itself contains foreign substances. A defective photomask will, in turn, cause defects in the pattern formed on the wafer by the subsequent etching process. In view of this, the filter  40  is disposed in the supply line  20  to remove foreign substances from the photoresist before the photoresist is dispensed onto the wafer W.  
         [0009]     Furthermore, bubbles generated in the photoresist flowing through the supply line  20  affect the quantity of photoresist sprayed by the nozzle  50 . Accordingly, bubbles in the photoresist can cause defects in the mask and hence, in the pattern formed by the etching process. In fact, even micro-bubbles entrained in the photoresist can severely influence the quality of a pattern formed according to the design rule of today&#39;s highly integrated semiconductor devices. Therefore, the filter  40  is also designed to remove bubbles from the photoresist in the supply line  20 .  
         [0010]     However, the bubbles removed form the photoresist by the filter  40  form a film that floats on the liquid in the filter  40 . The film acts as a load on the photoresist flowing through the filter  40 . Thus, the rate at which the photoresist passes through the filter  40  rapidly decreases as bubbles and foreign substances are simultaneously filtered. That is, the bubbles offer resistance to the photoresist passing through the filter  40 , thereby reducing the volume of photoresist sprayed through the nozzle  50 .  
         [0011]     Also, the pressure generated at the inlet of the dispensing pump  30 , which acts to withdraw photoresist from a bottle  10 , becomes weaker than the pressure generated by the dispensing pump  30  at its outlet to force photoresist through the nozzle  50 . The effect of this pressure difference manifests itself along the entire length of the supply line  20  from the bottles  10  to the dispensing pump  30 . Consequently, even more bubbles are created in the photoresist flowing through the supply line  20 . In fact, the problem can exacerbate until an entire portion of the supply line  20  is occupied by bubbles, i.e., a gap that is devoid of photoresist may be produced in the supply line  20 . Thus, the rate at which the photoresist sprayed through the spraying nozzle  50  fluctuates and, as a result, the thickness of the layer of photoresist covering the wafer is not uniform. As mentioned above, the end results are a defective mask, and defects in the pattern formed on the wafer by an etching process that uses the mask.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, a general object of the present invention is to provide a method of and apparatus for dispensing photoresist at a constant rate such that a substrate can be covered with a uniformly thick layer of the photoresist.  
         [0013]     Another object of the present invention is to provide a method of and apparatus for delivering photoresist to a spray nozzle in a manner that prevents or minimizes the production of bubbles in the photoresist.  
         [0014]     Yet another object of the present invention is to provide a method of and apparatus for dispensing photoresist using a pump wherein the volume of photoresist provided for the pump is maintained within a given range.  
         [0015]     The dispensing apparatus of the present invention includes at least one bottle containing photoresist, a spray nozzle, a supply line connecting the spray nozzle to the at least one bottle, a first (foreign substance removal) filter disposed in the supply line at an end of the supply line proximate the spray nozzle, and a dispensing pump for delivering photoresist through the first filter and spray nozzle. The dispensing pump is connected to the supply line proximate an end of the supply line to which the spray nozzle is connected.  
         [0016]     According to one aspect of the present invention, a bubble removal filter is disposed in the supply line downstream of the charge pump and upstream of the first filter to remove air bubbles from the photoresist. Preferably, the bubble removal filter has a housing, an inlet pipe and an outlet pipe that extend from the housing, and a membrane disposed in the housing. The inlet and outlet pipes may extend from opposite sides of the housing so as to be disposed in series. In this case, the housing has an inner diameter larger than each of the inner diameters of the inlet and outlet pipes. An exhaust pipe may be connected to the housing of the bubble removal filter so that air formed from an accumulation of the bubbles in the housing can be exhausted from the housing.  
         [0017]     According to another aspect of the present invention, a charge pump is connected to the supply line upstream of the dispensing pump to coerce photoresist into the supply line from the at least one bottle, and a buffer is tank disposed in the supply line between the charge pump and the dispensing pump so as to store a volume of the photoresist for delivery to the spray nozzle by the dispensing pump.  
         [0018]     In this case, and according to another aspect of the invention, the charge pump has a higher capacity, in terms of pressure generated, than the dispensing pump. Thus, the charge pump can deliver photoresist that is substantially free of bubbles to the buffer tank.  
         [0019]     Still further, and according to another aspect of the present invention, a bubble removal filter is disposed in the supply line downstream of the charge pump and upstream of the foreign substance removal filter, and the buffer tank is disposed between the bubble removal filter and the dispensing pump. The membrane of the bubble removal filter is stronger, under a given fluid pressure, than the membrane of the foreign substance removal filter.  
         [0020]     According to yet another aspect of the present invention, a method of dispensing photoresist onto a wafer comprises operating a dispensing pump to pump phototoresist at first rate through a spray nozzle, maintaining a volume of the photoresist in a buffer tank connected to an inlet of the dispensing pump so that a constant supply of the photoresist is provided for the dispensing pump as the dispensing pump is operated, and operating a charging pump to pump photoresist into the buffer tank at a second rate that is greater than the first rate.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     These and other objects, features and advantages of the present invention will become more fully understood from the detailed description thereof made below with reference to the accompanying drawings, of which:  
         [0022]      FIG. 1  is a schematic diagram of a prior art photoresist dispensing apparatus for use in manufacturing a semiconductor device;  
         [0023]      FIG. 2  is a schematic diagram of a first embodiment of a photoresist dispensing apparatus according to the present invention;  
         [0024]      FIG. 3  is a schematic diagram of a bubble removal filter of the photoresist dispensing apparatus according to the present invention; and  
         [0025]      FIG. 4  is a schematic diagram of a second embodiment of a photoresist dispensing apparatus according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to FIGS.  2  to  4 . Like elements are designated by like reference numerals throughout the drawings.  
         [0027]     Photoresist dispensing apparatus according to the present invention have a configuration similar to that of a conventional dispensing apparatus. More specifically, the dispensing apparatus include a bottle  10  for storing a predetermined volume of photoresist, a supply line  20  through which the photoresist flows from the bottle  10 , a dispensing pump  30  that pumps photoresist through the supply line  20 , a foreign-substance removal filter  40  for filtering foreign-substances from the photoresist, and a spray nozzle  50  for spraying the photoresist onto a wafer W. The foreign-substance removal filter  40  may comprise a porous membrane of polytetrafluoroethylene by which a sieving of the photoresist takes place.  
         [0028]     In the embodiment shown in  FIG. 2 , the photoresist dispensing apparatus also includes a bubble removal unit that is disposed in the supply line  20  upstream of the dispensing pump  30 . The bubble removal unit includes a charge pump  60 , a bubble removal filter  70  and a buffer tank  80 .  
         [0029]     The charge pump  60  coerces photoresist from the bottle  10 . Preferably, the charge pump  60  has a larger capacity than the dispensing pump  30 . In other words, the charge pump  60  delivers photoresist from the bottle  10  to the dispensing pump  30  at a relatively high rate, whereas the dispensing pump  30  pumps the photoresist at a relatively low rate sufficient to generate the pressure required to force the photoresist through the spray nozzle  50 . Accordingly, the pumping action of the dispensing pump  30  does not generate bubbles in the supply line  20 .  
         [0030]     The photoresist coerced from the bottle  10  by the high pressure generated by the charge pump  60  is passed through the bubble removal filter  70 . As shown in  FIG. 3 , the bubble removal filter  70  has a housing, an inlet pipe  72   a  and an outlet pipe  72   b  extending from the housing at opposite sides thereof and communicating with the interior of the housing, and a membrane  72   c  disposed in the housing as interposed between the inlet pipe  72   a  and the outlet pipe  72   b.  The interior of the housing occupied by the membrane  72   c  has a diameter larger than those of the inlet pipe  72   a  and the outlet pipe  72   b.  Also, the membrane  72   c  of the bubble removal filter  70  is preferably stiffer and stronger than the membrane of the foreign substance removal filter  40  seeing that the pressure generated by the charge pump  60  is greater than the pressure generated by the dispensing pump  30 .  
         [0031]     The membrane  72   c  serves to trap bubbles removed from photoresist passing through the housing from the inlet pipe  72   a  to the outlet pipe  72   b.  For example, the membrane  72   c  may be of UPE (ultra-high molecular weight polyethylene). The photoresist induced through the inlet pipe  72   a  is under a relatively high pressure because of the relatively small inner diameter of the inlet pipe  72   a.  The pressure of the photoresist then decreases rapidly as the photoresist passes through the membrane  72   c  because the housing in which the membrane  72   c  is disposed has an inner diameter larger than that of the inlet pipe  72   a.  Both large and micro-bubbles are released from the photoresist and cling to membrane  72   c  due to the decrease in pressure in the photoresist.  
         [0032]     Then, the photoresist from which the large and micro bubbles have been removed is discharged through the outlet pipe  72   b.  The bubble removal filter  70  may also have an exhaust pipe  72 d connected to the housing. Air from the bubbles removed from the photoresist is discharged from the housing through the exhaust pipe  72   d.  On the other hand, a drain pipe may also be connected to the housing of the bubble removal filter  70 . A valve  71  is disposed in the drain pipe. Accordingly, photoresist can be drained from the bubble removal filter  70  when the valve  71  is opened.  
         [0033]     Referring again to  FIG. 2 , the buffer tank  80  stores a given volume of the photoresist that has passed through the bubble removal filter  70 . Accordingly, the buffer tank  80  provides a stable supply of photoresist for the dispensing pump  30 . Also, a plurality of level sensors  82   a,    82   b  and  82   c  are installed in the buffer tank  80  to sense for the presence of photoresist at the top, bottom and center of the buffer tank  80 , respectively. Still further, a drain line is connected to the buffer tank  80 . A valve  81  is disposed in the drain line so that the buffer tank  80  can be drained when the valve  81  is opened.  
         [0034]     The respective level sensors  82   a,    82   b  and  82   c  are connected to an auxiliary controller  90 . The auxiliary controller  90  is connected to the charge pump  60  so as to control the operation of the pump  60  based on signals received from the level sensors  82   a,    82   b  and  82   c.  The auxiliary controller  90  is also connected to a main controller. The overall operation of the dispensing apparatus is controlled by the main controller. With this configuration, the portion of the dispensing apparatus from the charge pump  60  to the foreign substance removal filter  40  proximate the spray nozzle  50  can be provided as a discrete charge/dispensing unit (as illustrate by the dashed line in  FIG. 2 ).  
         [0035]     The operation of the photoresist dispensing apparatus will now be described in more detail.  
         [0036]     When the apparatus begins operating, the charge pump  60  draws photoresist from the bottle  10  into supply line  20 , and pumps the photoresist through the bubble removal filter  70  and into the buffer tank  80 . Accordingly, the photoresist filling the buffer tank  80  is free of large and micro bubbles. The level sensor  82   b  senses when the buffer tank  80  is filled with photoresist by the charge pump  60 , i.e., senses when the level of photoresist arrives at the top of the buffer tank  80 . At this time, the auxiliary controller  90  shuts down the operation of the charge pump  60 .  
         [0037]     The photoresist is then continuously discharged through the spray nozzle  50 . That is, the photoresist is withdrawn from the buffer tank  80  by the dispensing pump  30 , and then is passed through the foreign substance removal filter  40  so that foreign substances are removed from the photoresist. The dispensing pump  30  also forces the photoresist through the spray nozzle  50  and thus, onto the wafer W.  
         [0038]     Accordingly, the volume of photoresist in the buffer tank  80  is rapidly reduced until the level of photoresist arrives at a central portion of the tank  80 . The level sensor  82   c  senses when the level of photoresist falls below the central portion of the tank  80 , whereupon the auxiliary controller  90  re-starts the charge pump  60  to fill the buffer tank  80  again with photoresist. The operation of the charge pump  60  is thus controlled so that a certain volume of photoresist is always maintained in the buffer tank  80 . Therefore, the dispensing pump  30  always discharges the photoresist through the spraying nozzle  50  in a uniform and stable manner. That is, the rate at which the photoresist is dispensed onto the wafer always remains constant.  
         [0039]     If the level of photoresist in the buffer tank  80  ever reaches the bottom of the buffer tank  80 , the auxiliary controller  90  transfers a drive stop signal to the main controller. The main controller stops the operation of the dispensing apparatus, i.e, stops the operation of the dispensing pump  30 , upon receipt of the drive stop signal from the auxiliary controller  90 . At the same time, the main controller opens a drain valve  81  disposed in a drain line connected to the buffer tank  80  to empty the buffer tank  80  of the photoresist remaining in the bottom of the buffer tank  80 .  
         [0040]     Moreover, the entire operation of the dispensing apparatus can be stopped when a different type of photoresist is to be dispensed by the apparatus or when a bottle  10  of the photoresist is to be exchanged. In this state, the main controller opens drain valves  41 ,  71  and  81  and operates a dedicated high capacity suction pump to remove photoresist from within the supply line  20  and from within the components of the charge/dispensing unit of the apparatus to which the drain lines containing the valves  41 ,  71  and  81  are connected.  
         [0041]     Next, a second embodiment of a photoresist dispensing apparatus according to the present invention will be described with reference to  FIG. 4 .  
         [0042]     The second embodiment has a configuration similar to that of the first embodiment. Specifically, the second embodiment of the photoresist dispensing apparatus includes at least one bottle  10  for storing a given volume of photoresist, a supply line  20  connected to the at least one bottle  10 , a dispensing pump  30  for drawing photoresist into the supply line  20  from the bottle  10  a foreign substance removal filter  40  downstream of the dispensing pump  30 , a spray nozzle  50  by which the photoresist is sprayed uniformly onto a wafer W, and a bubble removal unit disposed within the supply line  20 .  
         [0043]     However, unlike the first embodiment, the bubble removal unit of the second embodiment includes a first charge pump  60 , a first buffer tank  100 , a bubble removal filter  70 , a second charge pump  110  and a second buffer tank  80 .  
         [0044]     The first charge pump  60  coerces photoresist from the bottle  10 . The dispensing pump  30  forces photoresist through the spray nozzle  50  and onto to a wafer W. Preferably, the first charge pump  60  has a larger capacity, i.e., generates a greater amount of pressure, than the dispensing pump  30 .  
         [0045]     The first buffer tank  100  is filled by the first charge pump  60 . Also, a volume of photoresist within a predetermined range is maintained within the first buffer tank  100 . To this end, a plurality of level sensors are provided to detect the amount of photoresist in the first buffer tank  100 . In particular, a bottom level sensor  102   a,  a top level sensor  102   b  and a center level sensor  102   c  are disposed on the buffer tank at positions corresponding to top, bottom and central portions of the tank  100 , respectively.  
         [0046]     The bubble removal filter  70  is of the type shown in  FIG. 3 . That is, the bubble removal filter  70  has a housing, an inlet pipe  72   a  and an outlet pipe  72   b  extending from the housing at opposite sides thereof and communicating with the interior of the housing, and a membrane  72   c  disposed in the housing as interposed between the inlet pipe  72   a  and the outlet pipe  72   b.  The interior of the housing occupied by the membrane  72   c  has a diameter larger than those of the inlet pipe  72   a  and the outlet pipe  72   b.  Also, the membrane  72   c  of the bubble removal filter  70  is preferably stiffer and stronger than the membrane of the foreign substance removal filter  40  to withstand the pressure generated by the charge pump  60  which pressure is greater than the pressure generated by the dispensing pump  30 . Accordingly, the bubble removal filter  70  removes any bubbles from the photoresist stored in the first buffer tank  100 .  
         [0047]     The second charge pump  110  is disposed downstream of the first buffer tank  100  to withdraw photoresist rapidly from the tank  100 . Thus, bubbles are not re-generated in the photoresist that has been passed through the bubble removal filter  70 .  
         [0048]     The second buffer tank  80  stores a volume of photoresist delivered thereto by the second charge pump  110 . The level of photoresist in the second buffer tank  80  is detected by a plurality of level sensors, namely a bottom sensor  82   a,  a top level sensor  82   b  and a center level sensor  82   c.  The respective level sensors  82   a,    82   b  and  82   c  are connected to the auxiliary controller  90  so as to issue signals, indicative of the amount of photoresist in the second buffer tank  80 , to the auxiliary controller  90 . The auxiliary controller  90  is also connected to a main controller. The operation of the first charge pump  60  and the second charge pump  110  are controlled by the auxiliary controller  90  based on the signals issued by the respective level sensors  82   a,    82   b  and  82   c,    102   a,    102   b  and  102   c.  The general operation of the dispensing apparatus is controlled by the main controller based, in part, on a signal issued to the main controller by the auxiliary controller  90 . With this configuration, the portion of the dispensing apparatus comprising the first charge pump  60 , the first buffer tank  100 , the bubble removal filter  70 , the second charge pump  110  and the filter  40  proximate the spray nozzle  50  can be provided as a discrete unit for assembly into the dispensing apparatus.  
         [0049]     Next, the operation of the second embodiment of the dispensing apparatus according to the present invention will be described in more detail.  
         [0050]     Photoresist is coerced from the bottle  10  by the first charge pump  60 , and a certain volume of the photoresist is stored in the first buffer tank  100 . From there, the photoresist is forced through the bubble removal filter  70  to remove bubbles from the photoresist.  
         [0051]     The photoresist is then pumped into the second buffer tank  80  by the second charge pump  110 . Next, the photoresist is withdrawn from the second buffer tank  80 , and is passed through the filter  40  by the dispensing pump  30  so that foreign substances are removed from the photoresist. Subsequently, the photoresist is sprayed onto a wafer W through the spray nozzle  50 .  
         [0052]     During this process, photoresist filling the first buffer tank  100  and the second buffer tank  80  is gradually consumed. Eventually, the level of the photoresist in the buffer tanks  100 ,  80  reaches central portions of the tanks  100 ,  80 . The center level sensors  101   c,    82   c  sense a drop in the level of the photoresist from the central portions of the tanks  100 ,  80 , and issue signals to the controller  90 . As a result, the controller  90  operates the first charge pump  60  and the second charge pump  110  to fill the first and second buffer tanks  100  and  80 .  
         [0053]     Thus, a steady volume of photoresist is always maintained in the first and second buffer tanks  100  and  80 . Therefore, photoresist is always discharged through the spray nozzle  50  at a constant rate by the dispensing pump  30 .  
         [0054]     If the level of photoresist in the respective first and second buffer tanks  100  and  80  ever reaches the bottom of the tanks, signals are issued by the bottom level sensors  101   a  and  82   a  to the auxiliary controller  90 . The controller  90  responds to these signals by issuing a drive stop signal to the main controller. As a result, the the main controller shuts down the operation of the dispensing apparatus to prevent defects that would otherwise result from a lack of photoresist in the system.  
         [0055]     Once the operation of the dispensing apparatus is stopped, respective drain valves  101  and  81  are opened and photoresist is drained from the first buffer tank  100  and the second buffer tank  80  through the drain lines connected to the tanks. The draining of the photoresist is facilitated by a dedicated pump connected to the drain lines. The valves  101  and  81  are closed once the buffer tanks  100  and  80  have been drained.  
         [0056]     Then the first charge pump  60  and the second charge pump  110  are operated to fill the first buffer tank  100  and the second buffer tank  80 , respectively. The top level sensors  101   b,    82   b  sense when the level of photoresist in the buffer tanks  100 ,  80  reaches the top of the tanks. At this time, the top level sensors  101   b,    82   b  issue signals indicative of the tanks  100 ,  80  being filled to the auxiliary controller  90 . In response, the controller  90  shuts down the first charge pump  60  and the second charge pump  100 .  
         [0057]     Meanwhile, the operation of the apparatus can be shut down as in the first embodiment when the apparatus is to dispense another type of photoresist and/or the bottle(s) is/are to be exchanged. In this case, photoresist in the supply line  20  and in the filters  70 ,  40  and the buffer tanks  100 ,  80  can be drained through respective drain lines by opening the valves  41 ,  71 ,  81 , and  101 . Again, a dedicated high capacity suction pump is used to withdraw the photoresist through the drain lines.  
         [0058]     In such a photoresist dispensing process according to the present invention, photoresist flows more rapidly through the first charge pump  60  and the second charge pump  110  than through the dispensing pump  30 , to prevent bubbles from being produced in the flow of photoresist. Regardless, any bubbles that are produced are filtered off by the bubble removal filter  70 . Therefore, the foreign substance removal filter  40  is very effective at removing foreign substances from the photoresist. Thus, only photoresist of a good quality is dispensed onto the wafer W.  
         [0059]     At the same time, the first and second buffer tanks  100  and  80  buffer the photoresist. The tanks  100 ,  80  thus prevent supply and process defects caused by an oversupply or lack of photoresist.  
         [0060]     As described above, according to the present invention, photoresist is withdrawn at a relatively high rate from a bottle  10 , thereby substantially preventing or minimizing the amount of bubbles (both large bubbles and micro bubbles) produced in the photoresist. Moreover, even if bubbles are generated, the bubbles are removed from the photoresist by a bubble removal filter  70  before the photoresist arrives at a foreign substance removal filter  40 . Thus, the following advantages are provided. First, the foreign substance removal filter  40  receives photoresist that is substantially devoid of bubbles. Accordingly, the foreign substance removal filter  40  is very efficient at removing foreign substances from the photoresist. Accordingly, photoresist of a high quality is dispensed onto a wafer W. In addition, the photoresist does not experience a significant load in the foreign substance removal filter  40 . Thus, the dispensing pump  30  delivers the photoresist under a uniform pressure. As a result, the wafer W is covered with a layer of photoresist having a uniform thickness. Therefore, a precise pattern can be formed on the wafer W when the layer of photoresist is patterned and used as a mask during an etching process.  
         [0061]     Finally, although the present invention has been described above in connection with the preferred embodiments thereof, the present invention is not so limited. Rather, the disclosed embodiments can be modified and varied as will be apparent to those of ordinary skill in the art. Accordingly, changes to and modifications of the disclosed embodiments are seen to be within the true spirit and scope of the invention as defined by the appended claims.