FIELD OF THE INVENTION
The present invention relates to the manufacture of semiconductor devices, and in particular, to the manufacture of conductor layers utilized in advanced semiconductor products, such as memories.
Semiconductor memory devices are comprised of an array of memory cells. Each memory cell is comprised of a capacitor, on which the charge stored represents the logical state of the memory cell. A charged capacitor corresponds to a logical state of “1” and an uncharged capacitor corresponds to a logical state of “0.” Word lines activate access transistors, so that the logical state of a memory cell can be read. Gates of multiple transistors are formed as one word line.
An example of a word line's application is in a dynamic random access memory (DRAM). In a DRAM, a common word line, used to access memory cells, is fabricated on a p-type silicon substrate coated with a thin film of silicon dioxide (SiO2), known as gate oxide. Then, a word line is formed on the gate oxide layer as a two-layer stack, comprising silicon (or polysilicon), coated with a conductor material. The most common two-layer stack used in the industry is a layer of polysilicon, coated with a tungsten silicide layer. Tungsten silicide is used because of its good integration properties, such as providing good thermal stability, stability during source/drain oxidation, and stability during dry etching, as well as having a low resistivity. Although titanium silicide is approximately 75% less resisitive than tungsten silicide, it has not been used extensively in two-layer stacks because it is not as thermally stable. Titanium silicide tends to agglomerate during subsequent high temperature processing steps. Alternatively, a metal is used instead of a silicide for the conductor layer.
Forming high conductivity films on word lines is one attempt to decrease the resistivity of a word line. Such films are commonly formed of a refractory metal silicide, such as titanium silicide (TiSi2). Titanium is preferably used as the refractory metal component because it has the ability to reduce oxygen, which remains on surfaces in the form of native oxides. Native oxides are reduced to titanium oxide by titanium. Native oxides degrade interface stability, and often cause device failure if not removed.
However, several problems occur with the use of TiSi2 in ULSI applications. At higher temperatures subsequent processing temperatures, TiSi2 has a tendency to agglomerate into two different phases, C54 and C49, which have different lattice structures. The C54 phase agglomerates at the interfaces between C49—TiSi2 and silicon (or polysilicon). While this is undesirable due to the increased resistance associated with agglomeration, the TiSi2 phase formed at higher temperatures, C54, is more stable and has a much lower resistivity than the C49 metastable phase formed at lower temperatures.
Another problem with using TiSi2 at higher temperatures is that the high-temperature phase, C54, has a grain size typically ranging from 0.3 to over 1.0 microns, which prohibits it from being used in sub-0.25 micron word line applications. However, it is always desirable to form a phase having the lowest free energy at a particular grain size, so that it is the most stable. FIG. 1 illustrates how free energy, ΔGf(γ), is a function of grain size, r. Free energy, ΔGf(γ), as a function of grain size, r, is divided into three regions: A, B, and C. The most stable phase at a given grain size is that which has the lowest free energy. Both regions A and B are in the sub-micron range. In the sub-micron range, the free energy of C54 is greater than that of C49, due to the larger surface energy of C54.
- SUMMARY OF THE INVENTION
In ultra large scale integrated (ULSI) circuits, a highly conductive word line is necessary to improve circuit density and performance. To date, word line resistance is one of the primary limitations of achieving faster ULSI circuits. A method for decreasing the resistivity of word lines is needed for use in ULSI applications. In order to use the C54—TiSi2 phase in ULSI circuits, particularly in 256 Megabit DRAMs and other devices requiring sub-0.25 micron line widths, it is necessary that the grain size be reduced, so that it will be more stable. Due to the increased sensitivity of ULSI circuits, it is important to maintain low resistivity in ULSI devices. There is a need for a stable, low resistivity TiSi2 phase which can be used in sub-0.25 micron word line applications.
BRIEF DESCRIPTION OF THE DRAWINGS
A method for forming a word line, which is used in ultra-large scale integrated (ULSI) circuits, produces a lower resistivity word line than those formed using prior art techniques. Resistivity is lowered and grain size of the conductor layer is modified using barrier elements to form a low dose matrix in silicon, or polysilicon. Subsequently annealing the word line stack forms a preferential C54-titanium silicide (TiSi2). C54—TiSi2 is more stable at high temperatures than C49—TiSi2. Use of C54—TiSi2 minimizes problems with C49—TiSi2 agglomerating at higher temperatures. Furthermore, C54—TiSi2 has a much lower resistivity than C49—TiSi2. Sub-0.25 micron word lines are able to be silicided with C54—TiSi2 using the invention, due to modification of TiSi2 grain sizes using the implanted matrix. Previously, the relatively large size of C54—TiSi2 grains, greater than 0.3 microns, prohibited its use in sub-0.25 micron word line applications.
FIG. 1 is a graph illustrating free energy of C54—TiSi2 and C49—TiSi2 as a function of grain size.
FIGS. 2A to 2F are cross-sectional representations of a method for forming a word line stack in accordance with the invention, using a preformed matrix to modify grain size during silicidation.
- Modification of Conductor Layer Resistivity During Word Line Silicidation
The following description describes a method for forming a word line which is used in ultra-large scale integrated (ULSI) circuits. While the invention is described with reference to it being a word line, other conductors may also be formed by this method for various purposes, especially where reduced resistance is needed. For example, interconnects can be silicided according to the method of the invention in further embodiments of the invention.
Resistivity and grain size of the conductor layer, TiSi2, is modified using barrier elements to form a low dose matrix in silicon, or polysilicon, which modifies the C49—TiSi2 phase to C54—TiSi2, increasing chemical and thermal stability. When using the method to form a word line stack, a p-type silicon substrate is coated with a thin film of silicon oxide (SiO2), known as gate oxide. Then, a word line is formed on the gate oxide layer as a two-layer stack, comprising silicon (or polysilicon), coated with a conductor material. The most common two-layer stack used in the industry utilizes a bottom layer of polysilicon. When using the method of the invention to form a conductor layer in a word line stack, the method is performed subsequent to formation of the bottom silicon layer, and diffusion barrier layers, if any.
This invention is particularly advantageous in the formation of sub-0.25 micron word lines. The advantages to using this invention are that the conductor layer resistivity is lowered; C54 grain size is modified so that it is finer than the word line width, allowing it to be used in sub-0.25 micron applications; the agglomeration temperature of TiSi2 is increased; and, conventional two-step TiSi2 annealing is potentially condensed into a single step. This invention is crucial in the fabrication of 256 Megabit DRAMs and other devices that require line widths of less than 0.25 microns. As line widths decrease in the low sub-micron range, sheet resistance of titanium silicided polysilicon, or silicon, increases significantly. The agglomeration temperature also decreases with decreasing line width. Agglomeration is unwanted because it causes break-up spots, which lead to device failure.
As is well known in the art, TiSi2 forms various phases. One common phase is the base-centered orthorhombic structure, C49—TiSi2. C49 does not require a two-step annealing process for its formation. Deposited titanium is annealed once, between approximately 650 to 750 degrees Celsius, to form the C49 phase. However, use of the C49 phase has many drawbacks. One drawback of the C49 phase is that it has a much higher resistivity (a factor of approximately three) than the face-centered orthorhombic C54 phase. Furthermore, it is a metastable phase, compared to the thermally-stable C54 phase. C49 is also an undesirable TiSi2 phase because it has poor self-passivating properties against tetraethyloxysilicate (TEOS) spacer formation and borophosphosilicate glass (BPSG) reflow during subsequent process steps. Furthermore, agglomeration usually occurs at the C49—TiSi2/polysilicon (or Si) interface due to decomposition of C49 at higher temperatures. If the layer is comprised of the C54 phase, then agglomeration will not occur at higher temperatures.
In comparison, the C54—TiSi2 is a high-temperature bulk phase with low resistivity. It is widely used in metal oxide semiconductor (MOS) device applications, such as silicidation of gates and source/drain regions. The silicide, C54—TiSi2, is usually formed using a two-step annealing process after deposition of titanium on the region to be silicided. Such a region can comprise either polysilicon or silicon, doped or undoped. Traditionally, to form the C54 phase, a two-step annealing process is required. The first annealing step is at a low temperature (between approximately 650 to 750 degrees Celsius), in a nitrogen, or argon, ambient. This step forms the high resistivity, metastable phase C49—TiSi2. Next, unreacted titanium (Ti) and titanium nitride (TiN) are removed with a wet etch. The second annealing step comprises a high temperature (between approximately 750 to 850 degrees Celsius) anneal in a nitrogen (N2), or argon (Ar), ambient. This step transforms the C49—TiSi2 phase to C54—TiSi2. Due to the undesirable properties of C49—TiSi2, complete phase transformation is crucial. It is also crucial to maintain uniformity of phase, as small, nonstoichiometric areas form between boundaries of C49 and C54 phases. Such nonstoichiometric areas are susceptible to oxide growth during subsequent formation of TEOS spacers, resulting in undesirable high resistivity at such boundary areas.
Thus, this invention forms C54 at sub-micron grain sizes in order to decrease the resistivity of the word line stack due to C54's lower resistivity than C49. As illustrated by FIG. 1, in order to form C54 as a stable phase at sub-micron grain sizes, the total energy of the C54 phase needs to be reduced. One method for decreasing the total energy of the C54 phase is to create an implanted matrix in the polysilicon or silicon layer before deposition of titanium. Various barrier elements form a matrix, which provide nucleation sites and grain boundary stabilizers. The roles of different matrix elements are determined by implanting at low dose levels, in a systematic approach.
For each line width, a process window of implant doses is determined, according to different roles of matrix elements. Such a process window comprises a dose and depth for each line width. Group VIA metals, such as Mo and W, form titanium alloys (e.g., TiW and TiMo) with very low heat of formation. Such alloys are good transition materials between titanium and silicon due to their similar covalent radii. They are able to be formed epitaxially on titanium and silicon surfaces, resulting in a low interface energy due to less broken bonds and lower interfacial stress. Therefore, Group VIA elements are good candidates for their roles in creating nucleation sites and their ability to stabilize grain boundaries.
The depth of a matrix depends on the initial titanium layer thickness and the required TiSi2 thickness. Typically, the TiSi2 thickness is below 1,000 angstroms in sub-micron applications. Therefore, a low ion energy implant is utilized.
The dose of a matrix depends on the grain size desired. It is determined using a simple cubic estimation. For example, 0.05 micron grains need 4×1012 nucleation sites/cm2 for a 500 angstrom thick matrix. Ion implant elements are uniformly distributed in a substrate with a precise depth. The effectiveness of a given element, therefore, can be determined by implanting a known dose of ions and measuring the resulting C54 grain size. Thus, the invention is not limited to implantation of Group VIA elements, when forming the matrix. Any element can be tested according to the above method to determine whether it is suitable for use as a matrix implant element. A process window is determined by varying the ion dose. A wide range of ion doses should be examined in determining a process window, including low doses of 1011 to 1013 ions/cm2. The best candidate for a matrix implant element should have a coalescence threshold (or range) with the highest dose, resulting in the finest grain size.
Certain Group VB nonmetal elements, such as: N, P, As, and Sb, can react with titanium to form barrier materials (e.g., TiN, TiP, TiAs, and TiSb). While such elements increase the agglomeration temperature, too many Ti—VB ions at C49—TiSi2/Si (or polysilicon) interfaces hinders transformation of C49 to C54. Group VB elements also have large mismatch covalent radii compared to titanium, particularly As and Sb. This tends to drive the barrier material to the top surface of the TiSi2 film at high temperatures, resulting in agglomeration. High solubility of Group VB in TiSi2 may also affect Group VB element's effectiveness as nucleation sites or boundary stabilizers. However, tests for determining the best matrix implant element can be performed to determine if combining elements of both Group VIA and Group VB is a good choice for a matrix implant element.
As shown in FIG. 2A, a polysilicon or silicon film, doped or undoped, 214 is formed over a semiconductor substrate 210 and gate oxide 212. Next, one Group VIA element, such as W or Mo, is implanted at a low implant energy into the polysilicon or silicon layer 214 to a depth 226 according to the determined process window and to a low dose of approximately 5×1011 to 1×1014 ions/cm2, to form an implanted matrix 224.
Polycide lines are then formed by depositing titanium 228, as shown in FIG. 2B, on the implanted region, to a depth 230 determined by the desired thickness of TiSi2 and the depth 226 of the matrix 224. A subsequent anneal forms C54—TiSi2 232, as shown in FIG. 2C. C54—TiSi2 232 has a finer grain size than the word line width, on which it is formed. If the process is well-controlled, C49 completely transforms into C54, and there is no need for a second annealing step.
The results of this invention were confirmed using several standard test methods, well known to one skilled in the art. The results obtained from these tests are illustrative of the benefits of the invention, but exact results may vary upon duplication, depending on the equipment and parameters used by one performing the tests. Optical microscope measurements of a TiSi2 conductor layer 232 , as shown in FIGS. 2C to 2F, formed in accordance with the present invention illustrated that a preformed matrix 224 decreases the surface roughness of the resulting TiSi2 232, resulting from a finer grain size. Measurements were taken, using a 350 angstrom TiSi2 232 surface for titanium 228 deposited on a preformed polysilicon matrix 224, comprised of Mo at an approximate dose of 1×1014 ions/cm2 and for a 350 angstrom TiSi2 surface for titanium 228 deposited on non-implanted (zero matrix) polysilicon 214. Identical results were obtained using a 500 angstrom TiSi2 232 surface for titanium 228 deposited on a polysilicon matrix 224, comprised of Mo at an approximate dose of 1×1014 ions/cm2 and a 500 angstrom TiSi2 surface for titanium 228 deposited on non-implanted (zero matrix) polysilicon 214. Identical results were obtained for implanted tungsten (W) preformed matrices 224.
Results of thermal wave, reflectivity, and JEOL transmission electron microscopy (TEM) measurements are consistent with the data described above. UV 1050 reflectivity measurements show a gradual improvement in reflectivity with increasing dose concentration for preformed W and Mo matrix wafers. Decreasing reflectivity corresponds to decreasing grain sizes, due to a smoother surface. Reflectivity is improved even more dramatically between TiSi2 232, as shown in FIGS. 2C to 2F, thicknesses of 300 to 500 nanometers. Parallel results from thermal wave, JEOL, and optical microscope measurements confirm the reflectivity results.
Furthermore, atomic force microscopy (AFM) studies illustrate how TiSi2 232 grain size and surface morphology change with matrix 224 concentration changes. A 500 angstrom TiSi2 layer 232 was formed on both a zero matrix and a 1×1014 Mo ions/cm2 preformed matrix 224. High resolution AFM reveals that small TiSi2 grains tend to coalesce to form large grains in the case of normal silicidation with a zero matrix. This coalescence results in drastic variation of TiSi2 film thickness, as well as surface roughness. Grain sizes of 0.4 microns or larger, resulting from forming TiSi2 on a zero matrix, were measured using AFM. These large grain sizes are undesirable in sub-0.25 micron word line widths. The results of these measurements are very close to those reported by other non-AFM C54 grain size studies. However, a preformed matrix 224 tends to decrease surface roughness by uniformly nucleating and stabilizing individual TiSi2 232 grains. Coalescence is drastically reduced for a 1×1014 Mo ions/cm2 preformed matrix 224, resulting in finer grain sizes. This results in more uniform grain size distribution and a much smoother surface.
Finally, a cap 220 of one or more dielectric materials, such as silicon oxide or silicon nitride, is applied to the word line stack 236 layers, if needed, according to conventional techniques and shown in FIG. 2D, and the line width 234 is defined by a dry etch, which defines at least one word line stack 236. The width 234 of a word line stack 236 is equal to the line width 234. TiSi2 conductor layers 232 formed in accordance with this embodiment of the invention, are comprised of the preferred C54—TiSi2 phase, with a grain size finer than that of the line width 234. It is preferable to have C54—TiSi2 for the conductor layer 232 due to its lower resistivity than C49—TiSi2. After wafer cleaning, a spacer 222 is formed, as shown in FIG. 2E, and conventional source/drain (S/D) implantation forms doped S/D regions 260 aligned with the spacers 238, as shown in FIG. 2F.
While the method of the invention has been described with reference to forming a conductor layer in a word lines stack, this method is applicable to forming a titanium silicide layer on any semiconductor substrate or device. For example, silicided interconnects are formed using the method of the invention in a further embodiment of the invention. Furthermore, source/drain regions are silicided in even further embodiments of the invention. More variations will be apparent to one skilled in the art.