Silicon nitride barrier for capacitance maximization of tantalum oxide capacitor

A method of making a capacitor on a conductive surface, preferably on a polysilicon surface includes contamination cleaning the surface with a high density plasma (HDP) of a first gaseous agent, such as hydrogen, then growing a silicon nitride barrier layer on the surface using a high density plasma (HDP) of nitrogen. A layer of tantalum oxide is then deposited on the silicon nitride layer to form a capacitor dielectric layer. A second silicon nitride layer is then grown on the capacitor dielectric layer, also using an HDP nitrogen plasma with the addition of a silicon containing gas, such as silane. Finally, a conductive layer is deposited on the second silicon nitride layer to form the capacitor. The HDP plasma is heated using an inductively coupled radio frequency generator. The invention also includes a capacitor constructed on a conductive surface by the method of the invention.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to capacitors formed on conductive surfaces,
 preferably silicon surfaces, during the construction of integrated
 circuits. More specifically, the present invention relates to a method of
 making a capacitor in which silicon nitride barriers are formed between
 conductive surfaces forming the plates of a capacitor and a capacitor
 dielectric layer of tantalum oxide located between the conductive
 surfaces.
 2. Description of Related Art
 As the memory size of dynamic random access memory (DRAM) devices increases
 into the gigabit range, the dimensions of all memory structures and the
 space allotted on the semiconductor substrate for those structures have
 decreased. To produce capacitors of the necessary value within the limited
 space available, conventional oxide/nitride/oxide stack dielectrics have
 had to be constructed ever thinner and thinner. Such stack dielectrics are
 beginning to reach the practical limit of how thin they may be deposited.
 One solution to this is to use tantalum oxide to replace
 oxide/nitride/oxide stacks as the capacitor dielectric. Tantalum oxide
 (Ta.sub.2 O.sub.5) has a higher dielectric constant and produces a
 correspondingly higher capacitance in the resulting capacitor.
 A problem with the use of tantalum oxide, however, is that the tantalum
 oxide deposition process results in the formation of a thin silicon oxide
 interface when the tantalum oxide is deposited directly on a polysilicon
 bottom electrode.
 This silicon oxide interface reduces the overall capacitance and degrades
 device functionality. To prevent the formation of the undesirable silicon
 oxide interface, it is known to deposit a thin nitride layer over the
 polysilicon before depositing the tantalum oxide. This nitride layer has a
 higher dielectric constant (K.sub.SiN =7) than the silicon oxide
 (K.sub.SiO2 =4).
 In the prior art implementation of this deposition process, the nitride
 layer has been deposited using a rapid thermal nitridation (RTN) process.
 The RTN process, however, is a relatively slow and costly process. This
 results from the multiple processing steps required. Further, the RTN
 process results in a nitride layer that is not uniformly thick. This
 adversely affects the performance of the capacitor being constructed.
 Bearing in mind the problems and deficiencies of the prior art, it is
 therefore an object of the present invention to provide a method of making
 a capacitor in which the capacitor dielectric layer is separated from the
 conductive surface by a silicon nitride layer that is uniformly thick.
 It is another object of the present invention to provide a method of making
 a capacitor in which the capacitor dielectric layer is separated from the
 conductive surface by a silicon nitride layer that can be constructed with
 fewer steps and less expensively than when constructed with the RTN
 process.
 Still other objects and advantages of the invention will in part be obvious
 and will in part be apparent from the specification.
 SUMMARY OF THE INVENTION
 The present invention uses a high density plasma (HDP) nitridation method
 to produce a high quality silicon nitride layer for separating a tantalum
 oxide layer in a capacitor from the conductive surfaces on either side of
 the tantalum oxide. A particular benefit of using the HDP process is the
 elimination of the more costly RTN process and the improved wafer
 throughput.
 A further benefit of using HDP is that cleaning of the conductive surface
 and silicon nitridation can be performed as a single integrated process in
 a single chamber. The above and other objects and advantages, which will
 be apparent to ne of skill in the art, are achieved in the present
 invention which is directed to, in first aspect, a method of making a
 capacitor including the steps of providing a conductive surface,
 contamination cleaning the conductive surface in a chamber sing a high
 density plasma of a first gaseous agent, growing a first silicon nitride
 layer on the silicon surface using a high density plasma of nitrogen,
 depositing tantalum oxide on the silicon nitride layer to form a capacitor
 dielectric layer, growing a second silicon nitride layer on the capacitor
 dielectric layer using a high density plasma of nitrogen and a silicon
 containing gaseous agent, and depositing a conductive layer over the
 second silicon nitride layer to form the capacitor.
 The conductive surface forming the lower plate of the capacitor is
 preferably silicon, and most preferably, polysilicon. The first gaseous
 agent in the step of contamination cleaning is preferably hydrogen. The
 high density plasma of nitrogen used in the step of growing a first
 silicon nitride layer on the conductive surface is formed by heating
 nitrogen gas with an inductively coupled radio frequency generator.
 In the most highly preferred implementation of the method, the steps of
 contamination cleaning and growing a first silicon nitride layer are both
 completed in the same chamber without removing the conductive surface from
 the chamber. This speeds processing as compared to RTN processing.
 The first silicon nitride layer is constructed with a thickness of less
 than 100 Angstroms, and it is preferred to be between about 10 and 40
 Angstroms thick. The high density plasma used during the step of growing a
 first silicon nitride layer on the conductive surface is formed by heating
 with an inductively coupled radio frequency generator operating at between
 about 2000 and 4800 watts and the conductive surface is exposed to the
 nitrogen plasma for a period of time ranging from about 3 to 60 seconds.
 Similar exposure times and heating temperatures are used during the
 contamination cleaning step with the high density hydrogen plasma.
 During the step of growing the first silicon nitride layer on the silicon
 surface, argon may be added to the high density plasma of nitrogen. During
 the step of growing a second silicon nitride layer on the capacitor
 dielectric layer, silane may be used as the silicon containing gaseous
 agent.
 The invention also includes a capacitor constructed on a conductive surface
 by a method comprising the steps described above. The resulting capacitor
 includes a uniform (&lt;2%, 1 sigma) silicon nitride layer on each side of
 the tantalum oxide layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
 In describing the preferred embodiment of the present invention, reference
 will be made herein to FIG. 1. Features of the invention are not
 necessarily shown to scale in this drawing.
 FIG. 1 shows a cross-section through a capacitor 10 constructed on a
 conductive surface 12 according to the method of this invention. The
 surface 12 acts as a lower capacitor plate. The capacitor 10 includes a
 first silicon nitride layer 14 separating the conductive surface 12 from
 the capacitor dielectric layer 16. The capacitor dielectric layer 16 is
 formed of tantalum oxide (Ta.sub.2 O.sub.5). A second silicon nitride
 layer 18 separates the capacitor dielectric layer 16 from an overlying
 conductive layer 20 which forms the upper capacitor plate.
 The method of this invention produces a capacitor having a very thin and
 uniform dielectric composed of the two silicon nitride layers 14, 16, and
 the tantalum oxide layer 16. The method begins by contamination cleaning
 the exposed conductive surface 12 in a high density plasma chamber using a
 high density plasma of a first gaseous agent, which is preferably
 hydrogen. In some applications, argon or oxygen may also be used in
 combination with or instead of hydrogen to obtain a clean surface.
 In the preferred embodiment, surface 12 is a silicon surface, most
 preferably, polysilicon. However, other conventional conductive materials
 may also be used. After the step of contamination cleaning, the first
 silicon nitride layer 14 is grown on the silicon surface 12 using a high
 density plasma of nitrogen. Argon may also be used with the nitrogen. The
 high density nitrogen plasma reacts with the exposed silicon surface to
 form the silicon nitride layer 14. Tantalum oxide is then deposited on the
 silicon nitride layer 14 in a conventional manner to form the capacitor
 dielectric layer 16.
 Next, a second silicon nitride layer 18 is formed on the capacitor
 dielectric layer 16. Because the tantalum oxide layer contains no silicon,
 the step of growing the second silicon nitride layer on the capacitor
 dielectric layer uses a high density plasma of nitrogen and a gaseous
 agent containing silicon, such as silane (SiH.sub.4).
 After the second silicon nitride layer is grown, the upper capacitor plate,
 comprising conductive layer 20, is deposited. The conductive layer may be
 any conventional conductive material suitable for capacitors, including
 metals and compounds containing metals such as platinum, tungsten and tin,
 oxides such as ruthenium oxide, and conductive materials such as silicon
 and polysilicon. These materials may also be used for the lower capacitor
 plate to form surface 12, in which case the step of forming the first
 silicon nitride layer on the surface 12 will use a silicon containing gas,
 such as silane, in combination with nitrogen in the high density plasma.
 An example of a suitable method for making a capacitor according to this
 method starts by contamination cleaning a silicon surface in an HDP
 chamber according to the following processing parameters:
 H.sub.2 Heating Step for Contamination Cleaning:

Low Frequency power: 2000-4800 Watt
 H.sub.2 gas: 200-500 sccm
 Stationary deposition time: 3-60 seconds
 Wafer temperature: 300-450.degree. C.
 In the contamination cleaning step described above, the hydrogen is used in
 a plasma reduction step which cleans the bare silicon wafer surface and
 removes contaminants. When the H.sub.2 heating/contamination cleaning step
 is completed the first nitridation step takes place, preferably in the
 same HDP chamber. Example nitridation process parameters are as follows:
 Nitridation Step:

Low Frequency power: 2000-4800 Watt
 N.sub.2 gas: 200-500 sccm
 Ar gas: 0-200 sccm
 Stationary deposition time: 3-300 seconds
 Deposition temperature: 300-500.degree. C.
 Where the underlying conductive surface contains no silicon, the
 nitridation step adds a gaseous agent containing silicon, such as silane
 (SiH.sub.4) at a sufficient rate for the supplied nitrogen gas which may
 be about 0-500 sccm (standard cubic centimeters per minute) for the
 process described above.
 The nitridation step produces a silicon nitride layer that is less than 100
 Angstroms thick. Preferably, the silicon nitride layer is between about 10
 and 40 Angstroms thick. In both the nitridation and the contamination
 cleaning steps, it is preferred for the HDP plasma to be formed by heating
 the gaseous agent with an inductively coupled radio frequency generator.
 When the nitridation step is completed, the HDP chamber is switched to an
 idle plasma state. The idle plasma state consists of the parameter set
 described in the contamination cleaning step. In this idle plasma state,
 the wafer is removed from the HDP deposition chamber. The wafer is then
 ready for the tantalum oxide (Ta.sub.2 O.sub.5) deposition in the next
 step of device fabrication.
 Tantalum oxide deposition occurs in the conventional manner to form a
 capacitor dielectric layer. Following tantalum oxide deposition, the
 tantalum oxide layer is cleaned. Cleaning may be accomplished using the
 same contamination cleaning step described above. The second silicon
 nitride layer is then grown on the clean tantalum oxide layer. The second
 silicon nitride layer is grown using the same parameter set described
 above, with the silane gas (or other silicon containing gaseous agent), to
 supply the necessary silicon.
 While the present invention has been particularly described, in conjunction
 with a specific preferred embodiment, it is evident that many
 alternatives, modifications and variations will be apparent to those
 skilled in the art in light of the foregoing description. It is therefore
 contemplated that the appended claims will embrace any such alternatives,
 modifications and variations as falling within the true scope and spirit
 of the present invention.