Abstract:
A novel interlocked automatic chemical mixing system and method of use which is particularly well-suited to preparing a final diluted HF (hydrofluoric acid) mixture of desired concentration for the post-cleaning rinsing of semiconductor wafer substrates. The automatic chemical mixing system includes a mixing tank having a normal level sensor and a mixing level sensor above the normal level sensor. A mixing system is provided for thoroughly mixing the liquid precursor components in the mixing tank. In typical application, DI water is introduced into the mixing tank until the DI water reaches the level of the mixing sensor. The precursor aqueous HF is then introduced into the mixing tank until the level of the HF reaches the normal level sensor.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to apparatus for mixing precursor chemicals to obtain a final chemical mixture having selected ratios or percentages of the precursor chemicals. More particularly, the present invention relates to an automatic chemical mixing system for mixing liquid chemical precursors in selected ratios to obtain a final liquid mixture, particularly for use in semiconductor wafer processing.  
         BACKGROUND OF THE INVENTION  
         [0002]    Generally, the process for manufacturing integrated circuits on a silicon wafer substrate typically involves deposition of a thin dielectric or conductive film on the wafer using oxidation or any of a variety of chemical vapor deposition processes; formation of a circuit pattern on a layer of photoresist material by photolithography; placing a photoresist mask layer corresponding to the circuit pattern on the wafer; etching of the circuit pattern in the conductive layer on the wafer; and stripping of the photoresist mask layer from the wafer. Each of these steps, particularly the photoresist stripping step, provides abundant opportunity for organic, metal and other potential circuit-contaminating particles to accumulate on the wafer surface.  
           [0003]    In the semiconductor fabrication industry, minimization of particle contamination on semiconductor wafers increases in importance as the integrated circuit devices on the wafers decrease in size. With the reduced size of the devices, a contaminant having a particular size occupies a relatively larger percentage of the available space for circuit elements on the wafer as compared to wafers containing the larger devices of the past. Moreover, the presence of particles in the integrated circuits compromises the functional integrity of the devices in the finished electronic product. Currently, mini-environment based IC manufacturing facilities are equipped to control airborne particles much smaller than 1.0 μm, as surface contamination continues to be of high priority to semiconductor manufacturers. To achieve an ultra clean wafer surface, particles must be removed from the wafer, and particle-removing methods are therefore of utmost importance in the fabrication of semiconductors. Cleaning the surface of a wafer is particularly important prior to the growth of a thermal oxidation layer on a wafer, since ultra thin oxide layers must start with a completely clean wafer surface area.  
           [0004]    The most common system for cleaning semiconductor wafers during wafer processing includes a series of tanks which contain the necessary cleaning solutions and are positioned in a “wet bench” in a clean room. Batches of wafers are moved in sequence through the tanks, typically by operation of a computer-controlled automated apparatus. Currently, semiconductor manufacturers use wet cleaning processes which may use cleaning agents such as deionized water and/or surfactants. Other wafer-cleaning processes utilize solvents, dry cleaning using high-velocity gas jets, and a megasonic cleaning process, in which very high-frequency sound waves are used to dislodge particles from the wafer surface. Cleaning systems which use deionized (DI) water currently are widely used in the industry because the systems are effective in removing particles from the wafers and are relatively cost-efficient. Approximately 4.5 tons of water are used for the production of each 200-mm, 16-Mbit, DRAM wafer.  
           [0005]    The most widely used process for the wet-cleaning of wafers is the RCA clean process, which includes sequential immersion in two different chemical baths known as Standard Clean 1 (SC-1) and Standard Clean 2 (SC-2). SC-1 uses an alkaline solution including ammonium hydroxide, hydrogen peroxide and DI water and is capable of removing particles and organic materials from the surface of the wafer. SC-2 uses an acidic solution including hydrochloric acid, hydrogen peroxide and DI water and is capable of removing metals from the surface of the wafer. Over time, modifications to the RCA process have been made to save the high volumes of process chemicals and ultrapure water which are characteristic of the original process. For example, dilute cleaning chemistries that utilize the original chemical components in up to 100 times more dilute concentrations are currently in widespread use to both improve the safety and health of facility personnel as well as reduce chemical usage and disposal. The RCA wet-clean process continues to be widely used due to the availability of ultrapure water and chemicals.  
           [0006]    Another mixture commonly used to remove organic and metallic impurities from a wafer surface is the Pirhana mixture, which combines sulfuric acid and hydrogen peroxide. The Pirhana composition is used at different steps in the cleaning process, such as prior to the SC-1 and SC-2 cleaning steps of the RCA-process. Typically, the wafers are immersed in the solution at 125 degrees C for about 19 minutes, followed by a thorough DI-water rinse.  
           [0007]    Frequently, hydrofluoric acid (HF), diluted in deionized (DI) water, is applied to the wafer surface in a final step after the other cleaning steps are completed. Fluoride-containing chemistries are also used to clean prime semiconductor wafers, or wafers which have not yet undergone ion implantation or device fabrication. The HF removes native oxides from the wafer surface. As a result of the exposure to HF, the wafer surface is completely terminated with hydrogen atoms and is highly resistant to re-oxidation in air. HF-dispensing steps may be carried out in a SEZ spin wet cleaning tool available from Semiconductor Equipment Zubehor (SEZ) of Villach, Austria, for example.  
           [0008]    In the HF-rinsing step of wafer cleaning, the concentration of aqueous HF applied to the wafer is important for adequate removal of oxides from the wafer. The aqueous HF may be supplied to the SEZ tool from a Fab facility supply system at a predetermined and non-adjustable concentration, such as about 24.5%, which is less than optimal for the HF rinsing step. Currently, the HF rinsing step requires an aqueous HF solution which contains a concentration of typically about 49% HF in DI water. Accordingly, a chemical mixing system which is suitable for mixing the aqueous HF and DI water in proper ratios to obtain a diluted HF mixture for optimum HF rinsing, is needed.  
           [0009]    It is therefore an object of the present invention to provide a novel chemical mixing system which is suitable for mixing two or more components of a mixture in selected ratios.  
           [0010]    Another object of the present invention is to provide a novel interlocked chemical mixing system which may be adapted to automatically mix two or more liquid precursor components of a mixture in selected ratios to obtain a final liquid mixture.  
           [0011]    Still another object of the present invention is to provide a novel automatic chemical mixing system which is capable of mixing aqueous HF of selected concentration with DI water to prepare a diluted HF solution for the rinsing of semiconductor wafer substrates.  
           [0012]    Yet another object of the present invention is to provide a novel automatic chemical mixing system which is suitable for mixing a variety of liquids for various purposes.  
           [0013]    A still further object of the present invention is to provide a novel interlocked automatic chemical mixing system which utilizes a mixing tank for receiving the precursor liquid chemicals, multiple liquid level sensors for automatically stopping the introduction of the chemicals into the mixing tank, and a mixing system for mixing the chemicals and achieving a desired mixture of the chemicals in the mixing tank.  
           [0014]    Still another object of the present invention is to provide an interlocked automatic chemical mixing system for obtaining a final liquid mixture having components of selected percentages or ratios by providing precursor components of selected concentration or percentage in a mixing tank and mixing the precursor components to obtain the final mixture.  
           [0015]    Yet another object of the present invention is to provide an interlocked automatic chemical mixing system which utilizes multiple, vertically-spaced level sensors on a mixing tank to facilitate obtaining a final liquid mixture by filling the tank with a first liquid component to one of the sensors, filling the tank with a second liquid component to the next highest sensor, and then mixing the components to obtain a final mixture having a selected percentage of the first and second liquid components.  
           [0016]    A still further object of the present invention is to provide a method of obtaining a final liquid mixture which contains first and second precursor components each having a selected composition concentration or percentage, including the steps of providing a mixing tank having multiple, vertically-spaced liquid level sensors; filling the tank with the first liquid precursor component to one of the sensors; filling the tank with the second liquid precursor component to the next highest sensor; and then mixing the components to obtain a final mixture having a selected percentage or ratio of the first and second liquid precursor components.  
         SUMMARY OF THE INVENTION  
         [0017]    In accordance with these and other objects and advantages, the present invention is generally directed to a novel interlocked automatic chemical mixing system and method of use which is particularly well-suited to preparing a final diluted HF (hydrofluoric acid) mixture of desired concentration for the post-cleaning rinsing of semiconductor wafer substrates. The automatic chemical mixing system includes a mixing tank having a normal level sensor and a mixing level sensor above the normal level sensor. A mixing system is provided for thoroughly mixing the liquid precursor components in the mixing tank. In typical application, DI water is introduced into the mixing tank until the DI water reaches the level of the mixing sensor. The precursor aqueous HF is then introduced into the mixing tank until the level of the HF reaches the normal level sensor. Finally, the mixing system thoroughly mixes the DI water and the aqueous HF in the mixing tank to prepare a final diluted HF mixture in preparation for rinsing the substrates with the diluted HF, for example. Depending on the concentration of the precursor aqueous HF, various concentrations of the diluted HF may be obtained. Various liquid components other than DI water and aqueous HF may be mixed in the mixing tank to obtain a final liquid mixture having a selected percentage or ratio of the precursor liquid components.  
           [0018]    The mixing system may include a recirculation mixing loop which pumps the liquid components to be mixed from the mixing tank and back into the mixing tank. A filter may be provided in the recirculation mixing loop for filtering particles from the liquids. Alternative mechanisms known by those skilled in the art for mixing the liquid components in the tank may be used. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1 is a schematic view of an illustrative embodiment of the automatic chemical mixing system of the present invention;  
         [0021]    [0021]FIG. 2A is a schematic illustrating initial introduction of a first liquid into the mixing tank of the present invention as a first step in typical application of the invention;  
         [0022]    [0022]FIG. 2B is a schematic illustrating introduction of a second liquid into the mixing tank of the present invention as a second step in typical application of the invention; and  
         [0023]    [0023]FIG. 2C is a schematic illustrating mixing of the first and second liquids by operation of a re-circulation mixing loop as a third step in typical application of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    The present invention has particularly beneficial utility in the preparation of a diluted HF (hydrofluoric acid) mixture for the rinsing of semiconductor wafer substrates with the diluted HF, such as after an RCA wafer clean process is carried out on the substrates, for example. However, the invention is not so limited in application, and while references may be made to such diluted HF and semiconductor wafer substrates, it is understood that the present invention is more generally applicable to obtaining a final liquid mixture by mixing precursor liquid chemicals in a variety of industrial and mechanical applications.  
         [0025]    Referring initially to FIG. 1, an illustrative embodiment of the automatic chemical mixing system  10  of the present invention includes a mixing tank  12 , typically having a bottom panel  13 , end panels  14 , and side panels  15  spanning the end panels  14 , defining a tank interior  16 . A level sensor support  18  is connected to one of the end panels  14  typically through a connector  19 . A low liquid level sensor  20  and a normal liquid level sensor  24  are provided on the level sensor support  18 , with the low liquid level sensor  20  located just above the bottom panel  13  of the mixing tank  12 . A mixing liquid level sensor  22  is provided on the level sensor support  18 , typically substantially equidistant between the low liquid level sensor  20  and the normal liquid level sensor  24 . An alarm liquid level sensor  26  is typically further provided on the level sensor support  18 , above the normal liquid level sensor  24 . The low liquid level sensor  20 , the mixing liquid level sensor  22 , the normal liquid level sensor  24  and the alarm liquid level sensor  26  may be any type of liquid level sensor known by those skilled in the art and suitably adapted for sensing respective levels of a liquid or liquids in the mixing tank  12 , and may be conventional. The sensors  20 ,  22 ,  24  and  26  may alternatively be provided on the end panel  14  or on one of the side panels  15  of the mixing tank  12 , in which case the level sensor support  18  may be omitted, as desired, or in any other location which facilitates their sensing of the respective levels of the liquids in the mixing tank  12 .  
         [0026]    One end of a DI water conduit  29  is confluently connected to a DI water pump and supply mechanism  28  which contains a supply of DI water  30 . The opposite end of the DI water conduit  29  is positioned in the tank interior  16 , near the bottom panel  13  of the mixing tank  12 . In similar fashion, one end of an HF conduit  33  is confluently connected to an HF pump and supply mechanism  32  which contains a supply of aqueous HF  34  of selected concentration. The opposite end of the HF conduit  33  is positioned in the tank interior  16 , near the bottom panel  13  of the mixing tank  12 . Wiring  23  connects the mixing liquid level sensor  22  to the pumping component (not shown) of the DI water supply  28  and to the pumping component (not shown) of the HF pump and supply mechanism  32 . Similarly, wiring  25  connects the normal liquid level sensor  24  to the pumping component (not shown) of the HF supply  32 . The normal liquid level sensor  24  may be further operably connected to the pump  38  of a recirculation mixing loop  36 , which will be hereinafter described. Accordingly, the mixing liquid level sensor  22  is capable of terminating operation of the DI water pump and supply mechanism  28  and initiating operation of the HF pump and supply mechanism  32  when the level of the DI water  30  in the mixing tank  12  reaches the mixing liquid level sensor  22 . In similar fashion, the normal liquid level sensor  24  is capable of terminating operation of the HF pump and supply mechanism  32  and initiating operation of the pump  38  of the recirculation mixing loop  36  as the HF  34  is added to the mixing tank  12  and the level of the HF  34  and DI water  30  in the mixing tank  12  reaches the normal liquid level sensor  24 , as hereinafter described.  
         [0027]    The automatic chemical mixing system  10  typically further includes a recirculation mixing loop  36  for mixing the diluted HF  42  (FIG. 2B) after both the DI water  30  and the aqueous HF have been added to the mixing tank  12 , as hereinafter described. The recirculation mixing loop  36  includes an outlet conduit  37  which extends from the bottom panel  13  of the mixing tank  12 , in confluent communication with the tank interior  16 . A pump  38  is provided in the outlet conduit  37 , and a filter  39  may be connected to the outlet of the pump  38  through a connecting conduit  40 . A return conduit  41  extends from the outlet of the filter  39  and terminates in the bottom portion of the tank interior  16 , adjacent to the bottom panel  13 . Accordingly, by operation of the pump  38 , liquid is drawn from the tank interior  16  through the outlet conduit  37 , the pump  38 , the connecting conduit  40 , the filter  39 , and back into the tank interior  16  through the return conduit  41 . In this manner, the liquid components separately added to the tank interior  16  in preceding steps are thoroughly mixed to provide a substantially homogenous liquid mixture, as hereinafter described. It is understood that various other mechanisms known by those skilled in the art for mixing the liquid components in the tank interior  16  may be used as an alternative to the recirculation mixing loop  36 , as desired.  
         [0028]    Referring next to FIGS.  2 A- 2 C, in typical application the automatic chemical mixing system  10  is used to thoroughly mix DI water  30  with precursor aqueous HF  34  of selected concentration or percentage in order to obtain a final diluted HF  42  having a selected concentration or percentage. The diluted HF  42  is then distributed to a process chamber (not shown) such as a SEZ wet spin cleaner to rinse a substrate (not shown) in the cleaner, for example, after the substrate is subjected to an RCA rinsing process. Accordingly, DI water  30  is initially pumped from the DI water pump and supply mechanism  28  through the DI water conduit  29  and into the tank interior  16  until the level of the DI water  30  rises to the level of the mixing liquid level sensor  22 , as shown in FIG. 2A. At that point, the mixing liquid level sensor  22  terminates operation of the DI water pump and supply mechanism  28 , by sending a termination signal  44  through the wiring  23 , to prevent further flow of the DI water  30  into the tank interior  16 . Next, the HF pump and supply mechanism  32  pumps aqueous HF  34  into the tank interior  16  through the HF conduit  33 , until the diluted HF  42 , which includes both the DI water  30  and the aqueous HF  34 , reaches the level of the normal liquid level sensor  24 , as shown in FIG. 2B. At that point, the normal liquid level sensor  24  terminates operation of the HF pump and supply mechanism  32 , by sending a termination signal  45  through the wiring  25 , to prevent further flow of the aqueous HF  34  into the tank interior  16 . Accordingly, the DI water  30  and the aqueous HF  34  are present in substantially equal volumes in the tank interior  18  in the embodiment of the system  10  in which the mixing liquid level sensor  22  is located equidistant or midway between the low liquid level sensor  20  and the alarm liquid level sensor  26 . However, it is understood that the mixing liquid level sensor  22  may be positioned at any desired location between the low liquid level sensor  20  and the normal liquid level sensor  24  to facilitate the placement of corresponding relative volumes of the DI water  30  and the aqueous HF  34  in the tank interior  16 .  
         [0029]    Finally, as shown in FIG. 2C, the DI water  30  and the aqueous HF  34  in the diluted HF  42  are thoroughly mixed to substantially homogenize the diluted HF  42 . This is accomplished by operation of the pump  38  of the recirculation mixing loop  36 , in which the diluted HF  42  is drawn from the tank interior  16 , through the outlet conduit  37 , the pump  38 , the connecting conduit  40 , the return conduit  41  and back into the tank interior  16 , respectively. As the diluted HF  42  is drawn through the filter  39 , particles (not shown) which may contaminate the diluted HF are removed therefrom. This recirculation step is carried out for a time period of typically about 5 minutes, after which the pump  38  is turned off and the diluted HF  42  in the tank interior  16  is thoroughly mixed and substantially homogenous. Finally, the diluted HF  42  is drawn from the tank interior  16  to the process tool, according to the knowledge of those skilled in the art. In the process tool, the diluted HF  42  may be used to rinse the substrate (not shown) typically having been previously subjected to a multi-step RCA rinsing process, for example.  
         [0030]    In the event that the normal liquid level sensor  24  malfunctions and the diluted HF  42  rises in the tank interior  16  to the level of the alarm liquid level sensor  26 , the alarm liquid level sensor  26  may activate an alarm (not shown) or may terminate operation of the DI water pump and supply mechanism  28 , the HF pump and supply mechanism  32 , or both, to prevent overflow of the diluted HF  42  from the mixing tank  12 . Conversely, in the event that the diluted HF  42  drops beneath the level of the low liquid level sensor  20 , the low liquid level sensor  20  may activate an alarm (not shown) or initiate operation of the DI water pump and supply mechanism  28  to begin preparation of another batch of the diluted HF  42 .  
         [0031]    For the embodiment of the system  10  described herein above, in which the mixing liquid level sensor  22  is located equidistant between the low liquid level sensor  20  and the normal liquid level sensor  26 , substantially equal volumes of the DI water  30  and the aqueous HF  34  are mixed together to define the diluted HF  42 . Depending upon the concentration of the precursor aqueous HF  34 , the final diluted HF  42  prepared according to the method described above may be any desired concentration between about 1% and about 50%, depending upon the intended application for the final diluted HF  42 . For example, to achieve a final diluted HF  42  of 25% concentration, the DI water  30  is mixed with a precursor aqueous HF  34  of equal volume and having a concentration of 50%. To achieve a final diluted HF  42  of 49% concentration, the DI water  30  is mixed with a precursor aqueous HF  34  of equal volume and having a concentration of 98%. Concentrations of the diluted HF greater than 50% are possible in embodiments in which the mixing liquid level sensor  22  is located closer to the low liquid level sensor  20  than to the normal liquid level sensor  24 , in which a greater volume of the aqueous HF  34  as compared to the volume of the DI water  30  is poured in the tank interior  16  by operation of the HF pump and supply mechanism  32  and the DI water pump and supply mechanism  28 , respectively, for mixing.  
         [0032]    While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.