Abstract:
A process for selectively depositing a silicon oxide layer onto silicon substrates of different conductivity types is disclosed. The silicon oxide layer is formed by the ozone decomposition of TEOS at relatively low temperatures and relatively high pressures. Use of the process to produce layers, spacers, memory units, and gates is also disclosed, as well as the structures so produced.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the fabrication of semiconductor devices. More particularly, this invention relates to selective deposition of silicon oxide onto silicon substrates.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optimization of semiconductor fabrication sometimes requires a thicker nonconducting film on some components than on other components. For example, a thick oxide layer or spacer on a P-type silicon wordline may he desired because the boron implants diffuse readily to an adjacent layer. In contrast, an N-type polysilicon component may optimally require a thinner oxide layer or spacer since N-type dopants do not diffuse as readily. A simple process that provides different thickness nonconducting films and spacers is desired in semiconductor fabrication.  
           [0003]    Forming oxide layers and spacers of different thicknesses over varying silicon substrates using current methods requires the application of a first mask over select parts of the semiconductor device and then depositing a layer of silicon oxide over the unmasked parts of the semiconductor device. The first mask is then removed and a second mask is applied over the parts that have been coated with the first silicon oxide layer leaving other parts unmasked. Subsequently, a second silicon oxide layer is deposited on the unmasked parts. Finally, an etch is used to remove silicon oxide from select surfaces, leaving behind an oxide layer or spacers where desired. This process adds a number of steps to the manufacturing procedures thereby increasing the complexity of the fabrication. As such, semiconductors are typically manufactured oxide with oxide layers or spacers of an intermediate thickness that will work acceptably, although not optimally, for either P-type or N-type polysilicons substrate.  
           [0004]    A hallmark of the current invention is the provision of a process that selectively deposits silicon oxide based on the conductivity type of the underlying silicon substrate.  
         SUMMARY OF THE INVENTION  
         [0005]    The current invention is a method for selectively depositing silicon oxide onto a silicon-comprising surface wherein the selectivity is based on the conductivity type of the silicon. In one embodiment, the invention is a semiconductor processing method for selectively depositing silicon oxide onto silicon, the method comprising the steps of: (i) providing a silicon-comprising substrate having exposed regions of different type conductivity; (ii) contacting the substrate with ozone and tetraethylorthosilicate (TEOS) gases; and, (iii) reacting the ozone and TEOS in contact with the substrate to selectively deposit silicon oxide onto the substrate, such that, compared to the deposition rate on exposed regions of non-doped silicon, the silicon oxide deposits at a faster rate on exposed regions of P-type silicon and at a slower rate on exposed regions of N-type silicon.  
           [0006]    Another embodiment of the invention is a method for forming an oxide layer of varying thickness on a silicon-comprising substrate, the method comprising the steps of: (i) providing the silicon-comprising substrate having a surface and comprising at least a first and second region of different type conductivity; and (ii) depositing silicon oxide onto the substrate in a single process step, to form an oxide layer over the first and second conductivity regions; whereby oxide layer overlying the first conductivity region has a first thickness and the oxide layer overlying the second conductivity region has a second thickness that is greater than the first thickness.  
           [0007]    Another embodiment of the invention is a semiconductor processing method of forming spacers of variable thickness, the method comprising providing a silicon-comprising substrate having a surface comprising at least one first P-type silicon structure or protrusion and at least one second structure or protrusion, provided that: (1) when the first protrusion comprises P-type or non-doped silicon, then the second structure or protrusion comprises either non-doped silicon or N-type silicon; and (2) when the first protrusion comprises non-doped silicon, then the second structure or protrusion comprises N-type silicon. Next, TEOS is decomposed with ozone to selectively deposit silicon oxide over the silicon surface and both the first protrusion and the second protrusion, such that a greater thickness of silicon oxide is deposited on the first protrusion than on the second protrusion. Finally, the deposited silicon oxide is etched to remove the oxide from select areas and leave silicon oxide as a layer or as formed spacers of variable thickness around the first protrusion and the second protrusion.  
           [0008]    Another embodiment of the invention is a semiconductor processing method of forming wordlines with an oxide layer or formed spacers of variable thickness. The method of this embodiment comprises providing a silicon-comprising substrate having a surface comprising at least one first wordline comprising P-type silicon and at least one second wordline comprising N-type silicon. Next, TEOS is decomposed with ozone to selectively deposit silicon oxide over the substrate surface and over both the first wordline and the second wordline, such that a greater thickness of silicon oxide is deposited on the first wordline than on the second wordline. Then, the silicon oxide deposited on the substrate during the reaction step is etched to provide a silicon oxide layer or formed spacers of variable thickness around the first wordline and the second wordline.  
           [0009]    Another embodiment of the invention is a semiconductor processing method of forming gates with spacers of variable thickness. The method of this embodiment comprises providing a silicon-comprising substrate having a surface comprising at least one first gate comprising P-type silicon-comprising material and at least one second gate comprising N-type silicon-comprising material. Next, TEOS is decomposed with ozone to selectively deposit silicon oxide over the substrate surface and over both the first gate and the second gate, such that a greater thickness of silicon oxide is deposited on the first gate than on the second gate. Then, the silicon oxide deposited on the substrate during the reaction step is etched to leave a silicon oxide layer or formed spacers of variable thickness around the first gate and the second gate.  
           [0010]    Another embodiment of the invention is a memory device comprising at least a first wordline comprising P-type silicon-comprising material and at least a second wordline comprising N-type silicon-comprising material, wherein both the first wordline and the second wordline have nonconductive spacers comprising silicon oxide wherein the nonconductive layer or formed spacer for the first wordline is thicker than the nonconductive layer or spacer for the second wordline.  
           [0011]    Another embodiment of the invention is a multi-gate semiconductor device comprising at least one gate comprising (i) P-type silicon-comprising material, (ii) at least one second gate comprising N-type silicon-comprising material and, (iii) layer or a nonconductive layer or formed spacers around each of the first and second gates, wherein the nonconductive layer or spacer is thicker for the first gate than for the second gate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Preferred embodiments of the invention are described below with reference to the following accompanying drawings, which are for illustrative purposes only. Throughout the following views, reference numerals will be used in the drawings, and the same reference numerals will be used throughout the several views and in the description to indicate same or like parts.  
         [0013]    [0013]FIG. 1 is a bar graph comparing deposition rates and layer thicknesses for the selective deposition of TEOS decomposed by ozone on silicon-comprising substrates that have different conductivities.  
         [0014]    [0014]FIG. 2 is a cross-sectional view of a silicon-comprising substrate having an N-type silicon-comprising protrusion and a P-type silicon-comprising protrusion.  
         [0015]    [0015]FIG. 3 shows the substrate of FIG. 2 following selective depositing of silicon oxide.  
         [0016]    [0016]FIG. 4 shows the substrate of FIG. 3 following an etch processing step.  
         [0017]    [0017]FIG. 5 shows a scanning electromicrograph (SEM) of a silicon substrate demonstrating the selective deposition of silicon oxide onto silicon substrates of different conductivity types. 
     
    
     DETAILED DESCRIPTION  
       [0018]    In the following detailed description, references are made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention.  
         [0019]    The terms “wafer” or “substrate” used in the following description include any semiconductor-based structure having an exposed polysilicon or other silicon-comprising surface in which to form the silicon oxide deposition layer of this invention. Wafer and substrate are to be understood as including silicon-on-insulator (SO 1 ) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when references made to a wafer or substrate in the following description, previous process steps may have been used to form regions or junctions in the base semiconductor structure or foundation.  
         [0020]    [0020]FIG. 1 is a bar graph showing selective deposition of silicon oxide using ozone/TEOS on silicon that has been doped with an N-type dopant (arsenic; center bar) or a P-type dopant (boron; right bar) or not doped (left bar). The substrate is composed of a single crystal silicon wafer, which has been implanted with the specified dopant. The surface was subjected to a hydrogen fluoride dip prior to the ozone/TEOS deposition processing. A blanket layer of silicon oxide was deposited on the wafer surface by ozone decomposition of TEOS at a temperature of about 400° C. and a pressure of about 300 torr. Under these reaction conditions, about five liters per minute of oxygen, containing about 10% by weight ozone, and about 350 milligrams per minute of TEOS were supplied to the deposition vessel.  
         [0021]    As shown in FIG. 1, a P-type implant, in this case boron difluoride, in a silicon-comprising substrate (polysilicon) obtains a higher deposition rate (approximately 22% faster) of oxide and reaches a greater deposition thickness for a given time than non-doped silicon. In contrast, an N-type implant, in this case arsenic, in a silicon-comprising substrate (polysilicon) retards the deposition rate (approximately 14% slower) of oxide as compared to non-doped silicon and results in a lower thickness. Similar results are obtained when the N-type implant is phosphorous. As such, the oxide deposits approximately 33% faster on P-type silicon than on N-type silicon. The selectivity effect is more pronounced at higher concentrations of dopant. Additionally, the selectivity increases as the reaction temperature decreases and/or the reaction pressure increases.  
         [0022]    FIGS.  2 - 4  shows a typical embodiment of the process of this invention, in which two non-abutting structures or protrusions  21 ,  22  are arrayed on a silicon-comprising substrate  20  such as single crystal silicon, epitaxial silicon or polysilicon. Protrusion  21  has a P-type doped silicon layer  23 . Protrusion  22  has an N-type doped silicon layer  24 . Protrusions  21  and  22  each have a metalized film  25 , such as tungsten silicide, arrayed atop the doped polysilicon layers  23  and  24 , respectively.  
         [0023]    The substrate  20  (single crystal) and protrusions  21  and  22  are contacted with gaseous ozone and gaseous TEOS under conditions where a silicon oxide layer  30  is deposited over the substrate and protrusions as shown in FIG. 3. At the proper reaction conditions, the silicon oxide will deposit selectively onto the substrate and protrusions in a single process step. The selectivity of this single process step avoids the necessity of masking and performing multiple photolithographic steps to form a suitably thick oxide layer or spacer  30  over the component layers of the protrusions  21 ,  22  and the substrate  20 . As shown a thicker layer  26  is formed over the P-type layer  23 . An intermediate thickness layer  27  is deposited over non-doped silicon substrate  20 . A thinner layer  24  is deposited over the N-type silicon layer  24 . An intermediate thickness layer  29  is deposited over metalized silicide film layer  25 .  
         [0024]    Appropriate reaction conditions for the selective deposition of silicon oxide over materials with different type doping is similar to the reaction conditions used in conventional methods to obtain selective deposition on silicon versus silicon nitride. Such reaction conditions are known in the art as shown in U.S. Pat. No. 5,665,644, incorporated herein by reference. Typically, the reaction temperature is greater than about 200° C. up to about 500° C., preferably up to about 400° C. Generally, the selectivity of the deposition is more pronounced at lower reaction temperatures. The reaction pressure is at least about 10 torr, preferably at least about 300 torr up to about atmospheric pressure, more preferably up to about 600 torr  
         [0025]    An exemplary reaction supplies about five liters per minute of oxygen containing about 10% by weight ozone and about 350 milligrams per minute TEOS. The oxygen: ozone ratio may typically vary from about 2 parts oxygen: 1 part ozone to about 20 parts oxygen: 1 part ozone. The ozone: TEOS ratio typically varies from about 0.5: 1 to about 200: 1. Reaction times will vary depending on the desired thickness of the deposited layer, generally about 2-3 minutes.  
         [0026]    Optionally, the surface to receive the oxide layer may be wet cleaned in a dip prior to depositing the oxide layer. A hydrofluoric acid (HF) wet-clean dip provides a marginal enhancement of the selectivity of the deposition. Other wet-clean dips, such as sulfuric acid or non-fluorine type etchants, have not been found to enhance the selectivity of the deposition and may negatively affect the subsequent deposition.  
         [0027]    Following the deposition of the oxide layer  30 , the portion of the oxide layer  27  overlying the substrate  20  is selectively etched to expose the substrate  20 , resulting in the structure of FIG. 4 having the oxide layers  26 ,  28  remaining over the protrusions  21 ,  22 , respectively. Any suitable oxide etching method may be used to remove the oxide layer  27  and expose the substrate  20 . Preferably, the method provides an anisotropic etch. Suitable etching methods include directional methods such as reactive ion etching (RIE). An exemplary etching process is by RIE using a mixture of carbon tetrafluoride (CF 4 ) at a flow of about 15 standard cubic centimeters per minute (sccm), and methylene trifluoride (CHF 3 ) at 25 sccm for thirty seconds at about 200 millitorr and a power of 100 watts.  
         [0028]    In one preferred embodiment, the protrusions  21 ,  22  of FIG. 2 represent wordlines of different conductivity. In this embodiment, layer  23  represents a wordline comprising P-doped silicon and layer  24  represents a wordline comprising N-doped silicon. These wordlines can be incorporated into a memory unit, such as a dynamic random access memory (DRAM), by any suitable means known in the art.  
         [0029]    In another preferred embodiment of the invention, the protrusions  21 ,  22  represent a dual gate structure. In this embodiment, layer  23  in FIG. 2 represents a gate comprising P-doped silicon and layer  24  represents a gate comprising N-doped polysilicon.  
         [0030]    In another embodiment of the invention, blanket layers of oxide using ozone/TEOS deposition processing are deposited over a silicon substrate having differentially doped areas. FIG. 5 is a SEM photomicrograph showing a cross-section of a silicon substrate  100  upon which this invention has been enacted. A transistor  114  is disposed on the surface of the substrate  100 . The portion  102  of substrate  100  has been doped with a P-type conductivity enhancing dopant such as boron, and portion  104  of the substrate  100  has been doped with an N-type dopant such as phosphorus. The intermediate (dark) layer  106  immediately above the substrate  100  and the transistor  114  is an oxide layer  106  formed from an ozone/TEOS deposition. The outermost (white) layer  112  above the oxide layer  106  is a deposited titanium nitride cap layer. As shown in FIG. 5, the silicon oxide layer  106  deposited as a significantly thicker layer  108  over the P-type doped portion  102  of the silicon substrate  100  compared to the thinner layer  110  deposited over the N-type doped portion  104  of the silicon substrate  100 .  
         [0031]    The methods and devices of the current invention are useful whenever semiconductors are fabricated with silicon-comprising regions or structures having different type conductivities. Examples of useful applications include memory arrays, such as DRAM and static random access memory (SRAM), logic circuitry, and combinations of memory and logic, such as a system-on-chip array.  
         [0032]    In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.