Abstract:
Isolation trenches and capacitor trenches containing vertical FETs (or any prior levels p-n junctions or dissimilar material interfaces) having an aspect ratio up to 60 are filled with a process comprising: applying a spin-on material based on silazane and having a low molecular weight; pre-baking the applied material in an oxygen ambient at a temperature below about 450 deg C.; converting the stress in the material by heating at an intermediate temperature between 450 deg C. and 800 deg C. in an H20 ambient; and heating again at an elevated temperature in an O2 ambient, resulting in a material that is stable up to 1000 deg C., has a compressive stress that may be tuned by variation of the process parameters, has an etch rate comparable to oxide dielectric formed by HDP techniques, and is durable enough to withstand CMP polishing.

Description:
BACKGROUND OF INVENTION  
       [0001]     The field of the invention is that of filling high aspect ratio trenches in integrated circuit processing.  
         [0002]     As ground rule dimensions shrink in integrated circuits, the problem of filling high aspect ratio trenches increases, in particular for isolation trenches used in the shallow trench isolation process, STI, that is commonly used in advanced processing.  
         [0003]     The industry-standard filling material and process has been silicon oxide, SiO2, deposited with the high density plasma, HDP, technique. This method has been widely adopted because it produces a high quality material that has good filling properties. Designers of integrated circuits have adapted their structural and material specifications to this process and material.  
         [0004]     Since silicon is piezo-electric, the properties of field effect transistors, FETs, are affected by the stress on the transistor converts the stress in the material from tensile to compressive.  
         [0005]     Another aspect of the invention is a stress conversion step followed by an anneal in a dry ambient.  
         [0006]     Another aspect of the invention is a first heating step in a steam ambient followed by the anneal in a dry ambient.  
         [0007]     Another aspect of the invention is the ability to relate total allowed device temperature budget and the annealing step to tune the stress and wet etch rate of the final material. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]      FIGS. 1A-1C  illustrate a portion of a substrate in the course of the invention.  
         [0009]      FIGS. 2A, 2B  illustrate an optional step of applying an HDP cap layer.  
         [0010]      FIG. 3  illustrates the filling of an aperture with a reentrant profile.  
         [0011]      FIG. 4  shows a graph of material stress resulting from different treatments.  
         [0012]      FIG. 5  shows a graph of relative etch rates resulting from different treatments.  
         [0013]      FIG. 6  shows a graph of relative etch rates resulting from different treatments.  
         [0014]      FIG. 7  shows a graph of material composition resulting from different treatments. 
     
    
     DETAILED DESCRIPTION  
       [0015]     The basic sequence in an SOD application is illustrated in  FIG. 1 , in which a silicon substrate  10  having a pad nitride/oxide  20  has a set of apertures that have been etched in it by any convenient process, e.g. a reactive ion etch. Two sizes of apertures are shown to illustrate that actual wafers will have a variety of values for the spacing between apertures and the width of the apertures, as well as different depths. The variation may result from fluctuations in a single etching process that may depend, for example, on the pattern density or it may result from the simultaneous filling of apertures formed in two different processes.  
         [0016]     On the right of  FIG. 1A , aperture  34  has a width  48  and a depth  46 , giving an aspect ratio of  46 / 48 . Apertures  32  have a width  44  and a depth  42 , giving an aspect ratio  42 / 44 . It is evident that the aspect ratio of the apertures  32  will be higher than that of aperture  34 , so that the process in question will need to accommodate a range of aspect ratios.  
         [0017]     In the course of the process, a trench dielectric material filling substance, referred to as a spin-on dielectric (SOD) or spin-on glass (SOG) will be applied to the wafer, which is rotated to spread the material quite uniformly over the surface. The material, which has a suitably low viscosity, will penetrate into the various apertures and overfill, with a top surface  52  above the top surface of pad  20 . The result is shown in  FIG. 1B .  
         [0018]     The wafer is then planarized by CMP or by an etchback step to remove the excess overfill material, as shown in  FIG. 1C .  
         [0019]      FIG. 1C  also shows an extra thickness of oxide  57  formed along the sides of the apertures by annealing the material in an oxidizing ambient. A conventional step of annealing in an ambient containing water vapor converts the Nitrogen and Hydrogen in the silazane to ammonia and molecular Hydrogen that escapes from the material, leaving a residue that is largely silicon oxide. The presence of oxygen in the ambient assists in converting Si—N bonds to Si—O bonds. The properties of the final material will depend on the degree to which this conversion has been accomplished.  
         [0020]      FIGS. 2A and 2B  illustrate an optional aspect of the invention, in which the silazane fill  54  is recessed by any conventional etch and a cap layer  62  of HDP is deposited by standard techniques and planarized with CMP.  
         [0021]     Those skilled in the art will appreciate that the process will be more consistent and the results better if the stress of the silazane layer  54  is close to that of HDP layer  62 .  
         [0022]     Spin-on materials have the well known property that they fill various aperture profiles that a process such as chemical vapor deposition cannot fill. Referring now to  FIG. 3 , there is illustrated an aperture  310  having a reentrant profile, meaning that there is at least one level where the transverse dimension B is less than the corresponding transverse dimension A at or near the top of the aperture. This occurs by design in the case of a trench capacitor or by accident when a divot has been formed on a wall that is intended to be smooth.  
         [0023]      FIG. 3C  illustrates the result of a CVD application, with a void  325  that has been formed in the body of material  320 ′. In contrast,  FIG. 3B  shows the result of a spin-on application that fills the aperture uniformly.  
         [0024]     It is well known in the art that the industry standard material for isolation trenches, referred to as shallow trench isolation, or STI, is High Density Plasma-assisted oxide (HDP oxide), which will not fill apertures uniformly when the aspect ratio is more than about 4 (assuming a vertical trench sidewall profile).  
         [0025]     In modern processing, there is very strong pressure to increase the density of features on a chip, so that the aspect ratio is constantly increasing. Process engineers are now working on filling trenches with aspect ratios in the range of about 10 and planning on filling trenches with much higher aspect ratios.  
         [0026]     Various complex and expensive schemes are in use to fill high aspect ratio trenches that involve depositing a portion of the total material, cleaning out the upper portion of the aperture so that new material is not blocked, then filling a second portion, etc.  
         [0027]     Process engineers go to this effort in spite of the much better filling properties of spin-on materials because no spin-on material is currently acceptable. In the case of isolation trenches, it is not enough that the filled trench insulate—i.e. not conduct current. Process integration requires that the steps in the process and the properties of the resulting material must be consistent with the rest of the process and structure.  
         [0028]     Since the processes in use at the present time have been developed to be consistent with the properties of HDP oxide, it would require a great deal of effort to alter those current processes. It would be highly advantageous if a spin-on process could be developed that would emulate the material properties of the HDP material.  
         [0029]     The inventors have realized that it is possible to tune the stress, composition and wet etch resistance of a spin-on material to be similar to those of HDP oxide.  
         [0030]     Poly-silazane processed according to the manufacturer&#39;s recommendations produces a final material that has properties very different from those of HDP, having poor etch resistance, tensile stress and low density.  
         [0031]     According to the invention, the sequence of processing steps is:  
         [0032]     Spin-on silazane having a molecular weight in the range of 2000-4000 dissolved in a solvent such as dibutyl ether;  
         [0033]     Pre-bake in O2 ambient (400-700 Torr) at a temperature in the range 100-450 deg C. for a period of 20-120 min;  
         [0034]     Perform a first anneal in water vapor at a temperature in the range 450-800 deg C. for a period of 20-120 min;  
         [0035]     In case temperature budget of prior levels allows: Perform a second anneal in an oxygen ambient without water vapor (400-700 Torr) at a temperature between 800-1200 deg C. for a period of 20 -120 min.  
         [0036]     Planarize the resulting film by CMP.  
         [0037]     For deep STI trenches (aspect ratio&gt;6), an additional step is an extra anneal in water vapor at a temperature in the range 450-800 deg C. for a period of 20-120 min after CMP to ensure that material at the bottom of the trench is oxidized and converted from Si—N bonding to Si—O bonding.  
         [0038]     It is an advantageous feature of the invention that the parameters of the final product can be varied by varying the processing parameters. Illustratively, the final material is high quality oxide with low impurity contamination; has a wet etch removal ratio (WERR) at 900 deg C. of less than 1.5; has compressive film stress in the range of 0.1 to 2 Gdyne/cm2; can be planarized by CMP with a standard slurry; is thermally stable at temperatures greater than 1000 deg C.; and has thickness uniformity (&lt;1% sigma) superior to HDP oxide.  
         [0039]     Referring now to  FIG. 4 , the graph shows the material stress in a silazane film annealed with steam and with oxygen ambients. Point  410  shows a typical value for HDP oxide. Curve  420  shows the result of a steam anneal between 700 and 900 deg C. As can be seen, the points at 800 deg C. and 900 deg C. are quite close to HDP, while the result of films annealed in an oxygen ambient (curve  430 ) have a tensile stress.  
         [0040]      FIG. 5  shows corresponding curves for the wet etch rate ratio i.e. the ratio of the etch rate of the film in question to the rate of thermal oxide. The etch material is buffered hydrofluoric acid (BHF). The results, on curve  520 , of the steam anneal at 800 and 900 deg C. are again very similar to the results of HDP oxide, point  510 , while the results of the oxygen anneal, curve  530 , are much higher (having less etch resistance).  
         [0041]      FIG. 6  shows corresponding results for the refractive index of the resulting oxide. The refractive index is a measure of material density as well as silicon to oxygen stoichometric ratio.  
         [0042]     Again, curve  620  (the steam anneal) is closer to point  610 , typical of HDP oxide, while the points on curve  630 , the oxygen anneal, are considerably different.  
         [0043]      FIG. 7  shows the composition of films after anneals at two different temperatures (in steam). The residual concentration of C (curve C), N (curve NSi) and hydrogen (Curve H) are much higher at 700 deg C. than at 1000 deg C.  
         [0044]     While very high temperature steam anneal (1000 C) produces the film with low impurity levels it is obviously can not be used if prior levels junctions have lower temperature budget restrictions. In the case of a vertical transistor, the STI is often done after the transistor has been formed and for example any prolonged (&gt;10 min) anneal above 900 C is prohibited assuming 100 nm vertical DRAM groundrule.  
         [0045]     Another well known issue with high temperature steam oxidation of silicon is the formation of defects which decrease activation energy for dopant diffusion (such as B, P, As, etc) and subsequent increase in their diffusion coefficients (so called oxygen enhanced diffusion (OED)). This effect in turn causes even bigger unwanted changes in prior p-n junction profiles.  
         [0046]     It has been also found, that steam oxidation above a temperature of 800 deg C. produces excessive oxidation of the silicon in the wafer (either bulk, epitaxial or SOI) that can degrade the parameters of the transistor that will be formed in the active area. Thick oxide film (&gt;300 A) grows on an STI sidewall (especially the top corner portion) and creates stress in adjacent active areas as well as increases the number of defects. Defects are easily formed in the top STI corner, after pad SiN strip and subsequent etch and anneal processing steps. The stress in the active areas may vary according to the circuit design. What is beneficially provided by the present invention is that the new trench material does not change the stress that the transistor was designed for.  
         [0047]     Once a silazane film is subjected to a first 800 C steam anneal it is beneficial to proceed with a second anneal in oxygen at temperatures&gt;800 C. Such an anneal further decreases the wet etch rate ratio (to about 1.1 of that of thermal oxide) and keeps film stress compressive within the range of 0.5-2 Gdyne/cm2.  
         [0048]     The properties of the final film can be adjusted or tuned by varying the time of the steam anneal and of the oxygen anneal.  
         [0049]     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.