Abstract:
In the formation of semiconductor devices, a processing method is provided, including steps for forming an oxide layer. The embodied methods involve a series of oxidation steps, with optional interposed cleanings, as well as an optional conditioning step after oxidation. In a preferred embodiment, these steps are clustered and transportation between the clustered process chambers takes place in a controlled environment such as nitrogen or a vacuum. In some embodiments, the method provides an oxide layer to be used as part of the device, such as a tunnel oxide for a flash-EEPROM, or as a general gate oxide. Alternatively, the steps can be used to sculpt through oxidation various levels of a substrate, thereby allowing for embedded memory architecture. Cleaning between oxidation steps offers the advantage of providing a more defect-free oxide layer or providing access to a more defect-free level of substrate.

Description:
RELATED APPLICATIONS 
   This application is a divisional of application Ser. No. 09/652,723, filed Aug. 31, 2000, now U.S. Pat. No. 6,589,877; which is a divisional of application Ser. No. 09/017,453, filed Feb. 2, 1998, now U.S. Pat. No. 6,475,927. 

   TECHNICAL FIELD 
   The present invention relates to the formation of semiconductor devices. More particularly, the present invention relates to the formation of an oxide layer as part of a device or as used in the fabrication of the device. 
   BACKGROUND OF THE INVENTION 
   In the semiconductor industry, oxide films are used in a variety of applications. Oftentimes they are used for scratch protection and passivation purposes. Oxide films are also used as a dielectric or insulative layer, electrically separating various regions or structures. For example, an oxide film can be used as a dielectric between different levels of metal in a semiconductor device. Such a film could also be used for field isolation. Moreover, an oxide film can serve as a gate oxide, wherein the film is provided above an area, such as a semiconductor substrate, having a source region, a drain region, and an interposing channel region. A gate, in turn, is formed on the oxide film. As a result, the voltage applied to the gate must reach a particular threshold before overcoming the insulative effects of the oxide and allowing current to flow through the channel. When used as field isolation, an oxide is formed in order to electrically insulate one device, such as a transistor, from another. 
   Whether for field isolation purposes or for application in the gate stack of a transistor, providing the oxide typically begins by exposing designated oxide regions of a substrate to an oxidizing ambient through a patterned mask. The mask may be made, for example, of silicon nitride. For purposes of explaining the current invention, it is assumed that the substrate represents the surface of a wafer and is comprised generally of silicon. Nevertheless, this invention is understood to cover devices having a substrate comprising any construction made of semiconductive material, including but not limited to bulk semiconductive materials such as a semiconductor wafer (either alone or in assemblies comprising other materials thereon) and semiconductive material layers (either alone or in assemblies comprising other materials). Upon exposure to the oxidizing ambient, the unprotected portions of the silicon substrate oxidize into silicon dioxide (SiO 2 ). The silicon at and below the surface of the substrate that oxidizes is often referred to as having been “consumed.” It follows that the amount of silicon consumed can indicate the depth of SiO 2  beneath the substrate&#39;s original surface. As a result, greater consumption allows for a greater depth of SiO 2  and, thus, greater electrical isolation between devices or between active areas within a device. 
   The consuming effect of oxide films on silicon serves other purposes as well. For example, greater consumption in a particular area of the wafer allows access to a lower level of silicon within the substrate. Accordingly, removing the oxide results in a wafer topography having different elevations of silicon, depending upon the amount of prior oxidation in each area. This is particularly helpful in embedded dynamic random access memory (DRAM) processing, wherein the memory cell array should be embedded deeper within the wafer than other memory elements. 
   Oxidizing the exposed substrate, as discussed above, is often referred to as “growing” the oxide. Oxides can be grown in a “dry” process using oxygen (O 2 ) or in a “wet” process using steam as the oxidizing agent. As an alternative to growing, oxides can be deposited on the substrate with techniques such as sputter deposition or chemical vapor deposition (CVD). 
   Oxide layers have a large impact on device performance due to their role in isolating active device regions and in establishing voltage thresholds for devices. Thus, there is always a need in the art for high quality oxide films. Further, as the dimensions of semiconductor devices are scaled down to enhance circuit density and speed, the oxide films must advance accordingly. Therefore, those skilled in the art are constantly striving to provide oxide films that are thinner and that have a high dielectric constant. 
   However, during the deposition or growth of oxides, defects in the oxide can occur due to the presence of certain constituents within the layer, such as contaminants exposed to the oxide. For example, particulate matter in the process atmosphere is one source of contamination. Even when the oxide or other layers are developed in a “clean room” environment, wherein filters and other techniques attempt to remove particles from the environment, particles that are too small for these techniques to handle may nevertheless end up within the oxide layer. Further attempts at reducing defects have been made by clustering together the chambers for several wafer processes in an environment isolated from and even more controllable than the clean room atmosphere. Transferring the wafers between the clustered chambers can involve the use of a wafer carrier capable of maintaining a vacuum or a nitrogen atmosphere. See, for example, U.S. Pat. Nos. 5,613,821 and 5,344,365. Nonetheless, there is a constant need in the art for further lowering the number of defects in oxide films, including a need for methods of handling contaminants that find their way to the wafer despite the controlled environment. 
   SUMMARY OF THE INVENTION 
   Accordingly the current invention concerns methods for providing an oxide layer during the processing of a semiconductor device. One exemplary embodiment relates to a method wherein an oxide is provided on a substrate surface and is then subjected to a cleaning process, followed by a provision of still more oxide. The oxide in either step could be grown or deposited. Moreover, the cleaning step may be used to remove all or some of the first provision of oxide. This embodiment has the advantage of removing any oxide that may carry constituents such as contaminants that were part of the underlying substrate. Thus, this embodiment can be used to provide a more contaminant-free oxide for a semiconductor device. Alternatively, this embodiment can be used to selectively consume portions of a substrate, thereby allowing memory structures such as embedded memories to be formed within the lower elevations of the substrate. 
   Another exemplary embodiment allows for providing a gate dielectric having a high dielectric constant. Such dielectrics include oxides such as tantalum pentoxide (Ta 2 O 5 ), or layers produced through rapid thermal nitridation (RTN), such as oxynitrides. In this embodiment, a layer of oxide or oxynitride serves as an adhesion layer between the substrate and the subsequently deposited Ta 2 O 5 . A cleaning step between providing the adhesion layer and providing the Ta 2 O 5  layer is optional. One advantage of this embodiment is that leakage current can be improved. 
   Yet another exemplary embodiment covers a range of steps for processing the semiconductor device, including a vapor clean, an initial oxide growth or deposition, a subsequent oxide growth or deposition, an optional second vapor clean between the two oxide steps, an oxide hardening, and the formation of an electrode over the second oxide. In a more preferred version of this embodiment, these steps are clustered, wherein transportation between the various processes are performed in a common controlled environment, such as a nitrogen atmosphere or a vacuum. The cluster process environment lowers the amount of contaminants having access to the in-process semiconductor device, and the cleaning steps help to negate the effects of any contaminants that appear within the device despite the attempts to control the environment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a process flow diagram of one exemplary embodiment of the present invention. 
       FIGS. 2   a  through  2   e  illustrates a prior art process for forming an embedded DRAM. 
       FIGS. 3   a  through  3   c  depicts a second exemplary embodiment of the current invention. 
       FIGS. 4   a  through  4   e  demonstrates a third exemplary embodiment of the current invention. 
       FIG. 5  shows a portion of a flash-electrically erasable programmable read only memory. 
       FIG. 6  illustrates an in-process semiconductor device that is electrically isolated using a shallow trench isolation process. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As seen in  FIG. 1 , at least one embodiment of the current invention comprises a cleaning stage  20 , a stage involving forming an oxide (or an oxynitride)  22 , and a stage of forming a structure  24 . These basic stages,  20 ,  22 , and  24 , further comprise more detailed steps. For example, cleaning stage  20  includes a vapor clean  20   b . In many cases this vapor clean  20   b  is performed in at least five seconds and occurs at a temperature ranging between 50° and 75° C. This vapor clean  20   b  may take the form of an ultraviolet-chlorine clean, wherein ultraviolet light excites and dissociates a gas containing chlorine. As a result, chlorine radicals are generated. These chlorine radicals act as gettering agents, penetrating the oxide layer and bonding with constituents therein, such as contaminants. These radicals are particularly useful in neutralizing the harm caused by metallic constituents within an oxide layer. This ultraviolet-chlorine clean generally takes between ten and sixty seconds and may occur at pressures less than, greater than, or equal to 760 torr. The temperature for this process is generally between 50° and 150° C. but is preferably between 60° and 80° C. 
   Alternatively, the vapor clean  20   b  may comprise an HF vapor clean. As one example of the parameters needed for an HF vapor clean, the pressure should be 1 mtorr, the temperature should be less than 200° C., and the exposure to HF should last five to ten seconds. The pH can be established at any level between 1 and 10. Still other alternatives include using tetramethylammonium hydroxide (TMAH) in the vapor clean process to assist in cleaning. For purposes of explaining the current invention, the TMAH is assumed to have a pH of 3, although one of ordinary skill in the art can appreciate that the pH level can vary according to the particular passivation requirements of a given embodiment. Also for purposes of explanation, it is assumed that a wafer is undergoing the methods of the current invention, although this is not required to practice the invention, as individual devices could also be processed. In general, the cleaning stage helps to remove residue from etches executed earlier in the device-formation process. 
   As a part of the cleaning stage  20 , an optional wet clean  20   a  may be performed in addition to the vapor clean  20   b . Preferably, the wet clean  20   a  is performed before the vapor clean  20   b . This wet clean step  20   a  may be an RCA clean, which involves cleaning solutions developed by RCA. These solutions employ hydrogen peroxide chemistry, the two most common solutions being “standard clean 1” (SC-1) and “standard clean 2” (SC-2). The solution SC-1 typically comprises ammonium hydroxide, hydrogen peroxide, and deionized water in the following volume ratio: 1 NH 4 OH:1 H 2 O 2 :5 H 2 O. The solution SC-2 generally has the following composition: 6 H 2 O:1 H 2 O 2 :1 HCl . Typically wafers are immersed in the RCA cleaning solutions for 10 minutes at 20° to 80° C. for each solution, with deionized water rinses between immersions. There is also a final deionized water rinse after all immersions. The wet clean  20   a  serves to passivate the wafer by allowing for a termination of the silicon surface with hydrogen atoms. In this way, a hydrophilic or a hydrophobic surface can be prepared. 
   Once the wafer surface has been cleaned, it is ready for oxide or oxynitride formation, as indicated by stage  22 . In many embodiments, this stage  22  initially involves growing an oxide on the wafer surface  22   c , which consumes the silicon down to a particular level within the substrate. However, the current invention includes embodiments wherein oxide is deposited  22   b  at the initiation of this stage  22 . 
   In at least one embodiment, oxide growth can be accomplished through a rapid thermal oxidation (RTO) process  22   d  at a temperature generally ranging from 800° to 1100° C. This RTO process  22   d  is carried out until it has produced an oxide thickness of at least generally 10 angstroms. 
   In addition to this initial step of growing an oxide, there are additional steps which may take place to enhance the oxide formation stage  22 . For instance, after the vapor clean  20   b  has been performed, yet before the RTO process  22   d  begins, it may be beneficial to induce chemical oxide growth on the wafer through an ultraviolet-ozone treatment  22   a , wherein ultraviolet radiation is used to enhance the oxidation rate of the silicon substrate in an ozone environment. This treatment  22   a  is preferably carried out long enough to provide a high-quality oxide layer having a thickness generally ranging from 10 to 15 angstroms. Regardless of whether the oxide is provided in one step or a plurality of steps, it is preferable to have approximately 30 to 40 angstroms of oxide once step  22   d  has been completed. 
   Once the ultraviolet-ozone treatment  22   a  and the RTO process  22   d  have been performed, there is an option to provide additional amounts of oxide  22   g . Further growth  22   k  from the consumption of silicon may be used to provide oxide similar to that resulting from the initial growth. Alternatively, an oxide, such as Ta 2 O 5  ( 22   h ), could be deposited. Regardless of the precise methods of providing oxides in these steps  22   d ,  22   a , and  22   g , they are understood to cooperate in achieving a given thickness of oxide at the end of the oxide formation stage  22 . Thus, a greater oxide contribution in one step may relieve the need to produce more oxide in other steps. This “given thickness” of oxide varies depending on the structures formed, as one skilled in the art can appreciate. Exemplary thicknesses are discussed below. 
   Achieving this given thickness of oxide is further aided by an optional additional cleaning step  22   e , such as a second vapor clean  22   f  performed after an oxidation step and, preferably, interposed between oxidation steps such as  22   d  and  22   g . As a result of this vapor clean  22   f , some or all of the oxide present may be removed. If a gate dielectric such as Ta 2 O 5  is deposited, some of the oxide provided in a prior step should remain to act as an adhesion layer between the substrate and the gate dielectric. In addition to curing any problems that might result from an inadvertent overgrowth of oxide, this second vapor clean  22   f  also serves to remove defects that would otherwise exist in the finished device. Defective oxides can arise if there are certain constituents, such as contaminants, in the silicon from which the oxide grows. As the silicon is consumed in steps such as  22   d  and  22   a , the resulting oxide may retain those contaminants. By removing all or part of this grown oxide, the second vapor clean  22   f  also removes those contaminants associated with the removed oxide. 
   While the amount of oxide at the end of this stage  22  may have a generally constant thickness, such a limitation is not necessary for the present invention. In fact, providing for variations in oxide thickness is actually desired in certain circumstances, such as in providing the topography needed for embedded DRAM memory devices.  FIG. 2  illustrates how this topography is achieved in the prior art. As seen in  FIG. 2   a , a thin layer of SiO 2    30  is grown on the silicon substrate  32 .  FIG. 2   b  indicates that the SiO 2  layer  30  is then covered with a mask layer  34 , such as one made of silicon nitride. The mask layer  34  is patterned to expose an area  36  where further oxide growth is desired. Additional oxide growth is then carried out in  FIG. 2   c , resulting in more silicon being consumed in the exposed area  36 , while most of the SiO 2  layer  30  under the silicon nitride mask  34  retains its original thickness. In  FIG. 2   d , the mask  34  and SiO 2  layer  30  are removed, leaving behind a bi-level topography for the substrate  32 . It is preferred that the resulting levels be parallel, but differences in slopes are allowable to the extent that the devices would still be functional. Further oxidation and masking steps are carried out to form structures such as the field oxide isolation regions  38  and gate oxide regions  40  and  41  depicted in  FIG. 2   e . This prior art method, however, depends on the higher levels of the substrate  32  being relatively free of constituents such as contaminants. Should the substrate  32  be contaminated in an area under the mask layer  34 , then the gate oxide  40  formed by consuming silicon in that area will also be contaminated. 
   At least one embodiment of the current invention is free of such dependency.  FIG. 3   a  depicts the unoxidized substrate  32  having contaminants  42 . Some of these contaminants  42  become part of the grown SiO 2  layer  30  after oxidation, as depicted in  FIG. 3   b . The effect of the second vapor clean step  22   f , shown in  FIG. 1 , can be seen in  FIG. 3   c , wherein the SiO 2  layer  30 , along with its associated contaminants  42 , is removed. Subsequent oxidation, masking, and removal steps may then be taken to achieve the bi-level topography needed for embedded DRAM devices. Assuming some contaminants  42  remain in the substrate  32 , the subsequent oxidation and removal steps will preferably eliminate most if not all of these contaminants  42 . 
   Moreover, it is not necessary to completely remove the initial SiO 2  layer  30 , as shown in the embodiment depicted in  FIGS. 4   a  through  4   e .  FIG. 4   a  once again shows the substrate  32  with the SiO 2  layer  30  grown on top.  FIG. 4   b  depicts the in-process wafer after the second vapor clean  22   f , wherein only part of the SiO 2  layer  30  has been removed as a result of that cleaning. Preferably, this partial oxide removal or subsequent oxide removals will eliminate most if not all of any existing contaminants or other undesired constituents within the SiO 2  layer. Assuming an exemplary embedded memory topography is desired, it is preferred that approximately 40 angstroms of the SiO 2  layer  30  remain after the second vapor clean  22   f . The mask layer  34  is then provided and patterned to expose the area  36 . If desired, further cleaning steps could be taken to remove more of the exposed SiO 2  layer  30  before additional oxide is grown. Alternatively, one may retain the existing SiO 2  layer and continue to grow more oxide, as demonstrated in  FIG. 4   d . Regardless of whether this latest cleaning is performed, it is preferred that the final thickness of the SiO 2  layer  30  in the exposed area  36  be within a range of approximately 70 to 90 angstroms for this exemplary embedded memory structure. Removing the SiO2 layer  30  and the mask layer  34  reveals the bi-level topography of the wafer of  FIG. 4   e . Field oxide regions and gate oxide regions may then be formed by the oxidation/cleaning steps described in the above embodiments or by a method known in the art. 
   If an oxynitride layer is desired, then a nitridation step  22   j  (seen in  FIG. 1 ) may be performed on the oxide layer. One possible way to carry out this step  22   j  involves rapid thermal nitridation (RTN), wherein ammonia (NH 3 ) is introduced to the wafer environment and heated with a source, such as a halogen lamp, to a temperature generally ranging from 850° to 1050° C. As a result of this process, the oxide film is rapidly changed to an oxynitride film. 
   Regardless of the particular devices being developed on the wafer, once the correct amount of oxide or oxynitride is present, an optional conditioning step  22   i  may be performed. In at least one embodiment, this conditioning step  22   i  comprises hardening the oxide in a nitridizing ambient, such as NO or N 2 O. In other embodiments, particularly those involving high dielectric constant materials such as Ta 2 O 5 , the conditioning step can alternatively include exposure to an ambient comprised of NH 3 ; dichlorosilane (SiCl 2 H 2 ) and NH 3 ; an oxygen environment, such as ozone; N 2 ; Ar; or H 2 . This exposure would last for at least ten seconds at preferably five to fifteen atmospheres and at a temperature generally between 500° to 750° C. Another conditioning alternative involves steam oxidation, wherein the dielectric is exposed to a mixture of H 2  and O 2 . Those skilled in the art understand that such a mixture may be generated by using a plasma torch. The current invention, however, also includes a conditioning step wherein a plasma torch is not used; rather, H 2  and O 2  are brought together within the process chamber at a pressure of generally one atmosphere or lower and at a temperature generally ranging from 500° to 900° C. 
   If an embodiment of the current invention is used to provide a gate oxide, one advantage will be to reduce the current leakage of a device or, at least, counteract factors that could increase the leakage. Moreover, the cleaning, oxidizing, and conditioning steps described above have uses in addition to altering the topography of the silicon substrate and to providing some form of gate oxide. These steps, for example, can be used to form the tunnel oxide  44  of a flash memory cell  46  depicted in  FIG. 5 , as well as the entire oxide  48  for that cell  46 . The flash memory cell  46  pictured is more specifically a flash-electrically erasable programmable read only memory (flash-EEPROM) cell formed in a p-type substrate  50  having n-type source and drain regions  52 ,  54 . Over the substrate  50 , a floating gate  56  is provided within the oxide  48 . A control gate  58  is also within oxide  48  and is located over the floating gate  56 . The floating gate  56  is separated from the substrate by the tunnel oxide  44 . This tunnel oxide  44  is a relatively thin oxide layer; while it can be as thin as thirty to forty angstroms, it is preferably closer to seventy to one hundred angstroms thin. The tunnel oxide  44  is so named because memory programming and erasing is accomplished by way of Fowler-Nordheim tunneling through this tunnel oxide  44 . Thus, through an embodiment of the current invention, the tunnel oxide  44  could be provided through an initial oxidation step with a cleaning to reduce the oxide to a desired thickness. Alternatively, the oxide could be completely cleaned away, allowing a subsequent oxidation step to form the tunnel oxide  44  at a lower level of the substrate  50 . The rest of the oxide  48  could be formed by an additional oxidation step or by still more oxidation/cleaning cycles. 
   As another example, these cleaning, oxidizing, and conditioning steps can be used to provide a gate oxide for devices isolated using a shallow trench isolation (STI) process. Such a device in-progress appears in  FIG. 6 , wherein an etched substrate  60  defines trenches  62  that electrically isolate one cell site from another. After providing a trench fill  64 , the gate oxide  66  is grown in a high-pressure oxidation environment using the steps described above. For instance, the gate oxide  66  could be grown at a pressure generally ranging from five to fifteen atmospheres. 
   Given these varying environments for the oxide, the specific steps taken within the stage of forming a structure  24  will depend in part on the context in which the oxide is formed—such as whether it is formed for a gate in an embedded DRAM or to sculpt the substrate to accommodate such a gate, for a floating gate in a tunnel oxide, for a gate in an STI cell, or for other structures. In many instances, the stage of forming a structure  24  will involve a step of forming a gate  24   a  which, in turn, is often formed by depositing a polysilicon film. Moreover, a silicon-germanium film may be provided as an option by doping the polysilicon film with germanium, wherein the germanium concentration may generally range from 2% to 25%. Other alternatives to depositing polysilicon include depositing tantalum nitride, titanium nitride, and tungsten nitride. 
   Once the stage of forming a structure  24  has been completed, the wafer may undergo further processing, such as steps taken to form word lines. It should also be noted that, in a preferred embodiment, it is desirable to cluster the steps from the first vapor clean  20   b  to the stage involving forming a structure  24 . Of course, in an even more preferred embodiment, every step would be clustered. This would serve to further reduce the constituents such as contaminants that might appear in an oxide as well as other layers. 
   Finally, one skilled in the art can appreciate that, although specific embodiments of this invention have been described above for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. For example, providing oxide under the current invention is not limited to at most two oxidation steps; any number of oxidation steps and cleaning steps as needed are covered, as shown by the arrow in  FIG. 1  looping from step  22   g  back to  22   e . Further, any masking steps that might be needed to allow oxidation and cleaning in selected areas are also included. Accordingly, the invention is not limited except as stated in the claims.