Patent Application
20050062161
The present invention is related to integrated circuits and their fabrication. More specifically, the invention relates to a structure and method of fabricating a conductor line (e.g. gate conductor, wordline, etc.) for improving process tolerances in providing borderless contacts.
As described below, the present invention is directed to forming an improved conductor line structure of an integrated circuit (IC) which provides increased process tolerance for forming a borderless contact to a semiconductor substrate. The present invention is especially well-suited for fabrication of wordlines and borderless bitline contacts of a dynamic random access memory (DRAM).
Dynamic random access memories (DRAMs) include very densely packed arrays of semiconductor devices which are accessed by wordlines running in a first horizontal direction over the major surface of a substrate, and a set of bitlines which run in a second horizontal direction over the major surface of the substrate. In a typical DRAM, a wordline is activated by raising its voltage to access a row of storage cells of a data storage array. When a particular row is accessed by an activated wordline, a bitline is used to read a data bit signal of a storage cell of that accessed row. This is done by transferring a charge stored in the storage cell to a sense amplifier. A bitline is used to write a data bit signal of a storage cell of the accessed row by transferring and storing a charge from the sense amplifier to the storage cell. Storage cells of a DRAM require periodic refreshing to avoid loss of the data stored therein. To refresh a storage cell, the bitline transfers the charge stored in the storage cell to the sense amplifier, amplifies it there to a desirable level, and then rewrites the storage cell with the same data by transferring the amplified charge back to the storage cell.
In many DRAMs, wordlines have an encapsulated structure including a linearly extending conductor and an insulative structure encapsulating the top and sidewalls of the conductor. The insulative structure typically includes an insulating cap and insulating spacers. Silicon nitride is a typical material for providing an insulating cap. Silicon nitride and/or silicon dioxide are typically used for providing insulating spacers. The linearly extending conductor generally includes heavily doped polysilicon and/or one or more metals and/or one or more compounds of metals, especially metal silicides. One or more barrier layers and/or adhesion layers may also be included within the conductor for enhancing performance.
DRAMs having high integration density typically require wordlines to be encapsulated to support tight wordline pitch. Tight wordline pitch is also needed for best signal transfer on bitlines, because bitlines are shorter when wordlines have tight pitch, and shorter bitlines have less parasitic capacitance to interfere with signal transfer. When wordlines are spaced very close together, i.e. at any spacing less than about 1½ times the minimum lithographic feature size or spacing (F) for the technology, the bitline contacts must be self-aligned to the insulative material which encapsulates the wordlines. The wordline to wordline spacing is typically set to the minimum spacing of 1 F, i.e. at the critical dimension. Because of difficulties in achieving perfect alignment between the critical dimensioned bitline contact mask and the underlying critical dimensioned pattern, the minimum sized holes (of 1 F size) that are etched to form the bitline contacts would be subject to landing on tops and/or sidewalls of the conductive wordlines. The bitline contacts formed by depositing a conductive material in the contact holes would then short circuit to the wordlines. Hence, wordlines must have conductors which are encapsulated both on their sides and top to prevent such shorting. Bitline contacts are then formed between such encapsulated wordlines which are self-aligned to the opening between the insulative material that encapsulates the wordlines.
Such self-aligned process for forming bitline contacts between encapsulated wordlines is known as a borderless bitline contact process. Some processes for forming borderless bitline contacts are described in U.S. Pat. Nos. 6,319,840 and 6,261,933. As described in U.S. Pat. No. 6,319,840, a bitline contact is formed in a self-aligned manner between two encapsulated wordlines. As used herein, the term “horizontal” means any direction which is parallel to the major surface of a semiconductor substrate, whereas “vertical” means a direction which is perpendicular to that major surface. As shown in that patent in
To form the borderless bitline contact, a contact hole is first etched in a dielectric region between two wordlines 14 to reach the single-crystal semiconductor region 10 below the wordlines. The etch process is generally but not completely selective to the material of the insulating sidewall spacers 16 and the insulating cap 18 that together encapsulate the wordline. Thereafter, heavily doped polysilicon is deposited in contact with the exposed single-crystal silicon to form a borderless bitline contact that is self-aligned to the encapsulated wordlines.
As the chip area occupied by a DRAM is reduced from one technology generation to the next, the minimum lithographic feature size F (equivalent to minimum spacing) is also reduced such that there is less space available for accommodating both the bitline contact and the sidewall spacers disposed to the sides of the bitline contact.
It is apparent that there is a minimum thickness for the sidewall spacers formed on the conductor line, below which poor results are obtained. While the bitline contact holes are formed by etching selectively to the material of the spacers, the selectivity is not complete, such that the thickness of the spacer is reduced somewhat during the etch. If the resulting sidewall spacers become too thin, then capacitance between the wordline and bitline can increase up to several times the desired maximum capacitance, which degrades the quality of the bitline signal. If a spacer becomes thinned, i.e., etched through, to the point that the bitline comes in conductive contact with the wordline, the bitline will short circuit to the wordline.
On the other hand, it is apparent that there is little tolerance for increasing the thickness of the sidewall spacers. When the sidewall spacers are too thick, poor contact (highly resistive) or nonexistent contact will be made between the bitline contact and the drain of the transistor that is formed in the single-crystal semiconductor region 110 of the substrate. Such condition, known as “bitline contact open” must be avoided.
The thickness of the sidewall spacer is also limited for other reasons. In addition to the memory array, every DRAM, including standalone DRAMs and DRAMs which are embedded in chips having additional function, e.g. a processor, includes other “support” transistors which are optimized for certain performance such as switching, drive and/or gain. The sidewall spacers of transistors in the memory array are formed at the same time and by the same process as the sidewall spacers of such support transistors. The support transistors require the sidewall spacers to remain relatively thin for performance reasons. For one, the sidewall spacers need to relatively thin in order to allow implants and other processes to be performed in close proximity to the channel regions of the transistors.
The spacer thickness is a very important process parameter for planar device performance. The spacer thickness affects the threshold voltage, leakage current, and drive current of planar transistors, e.g. planar passgate transistors for the memory cell array and support circuit devices. The sidewall spacers cannot be thickened in the memory cell array without impacting the performance of support transistors. Accordingly, the sidewall spacer thickness must be maintained within the tolerances for fabricating the support transistors.
However, this approach leaves much to be desired. When the upper layer of the wordline line is made narrower, higher resistance results because the portion of the wordline containing the metal(s) and/or metal compound(s) has smaller cross-sectional area. The higher resistance causes higher propagation delay along the wordline, which degrades the speed of the circuit. A second problem is that the performance of memory array transistors coupled to the wordline is sensitive to the thickness of the spacers 220. As described above, the sidewall spacer cannot be thickened without limit, in order to avoid affecting the performance of the support transistors. Rather, the sidewall spacer thickness must be maintained within the tolerances provided therefor, and little margin is provided for variation.
In view of the foregoing, it would be desirable to provide a structure and method of forming a conductor line which improves process tolerance for forming borderless bitline contacts.
It would further be desirable to provide a structure and method of forming a conductor line having a layer including a metal and/or metal silicide which provides improved process tolerance while maintaining resistance tolerably low.
According to an aspect of the invention, a structure and method are provided for a conductor line stack of an integrated circuit. The conductor line stack includes a layer of a first material such as heavily doped polysilicon and/or a metal silicide. A layer of a second material such as a metal is formed over the layer of first material, the layer of second material having an upper portion and a lower portion. A pair of first spacers is disposed on sidewalls of the upper portion, wherein the lower portion has width defined by a combined width of the upper portion and the pair of first spacers.
According to a preferred aspect of the invention, a pair of second spacers is formed on sidewalls of the first spacers, the lower portion and the layer of first material.
According to another preferred aspect of the invention, the conductor line stack structure is suited to formation of a borderless bitline contact in contact therewith.
According to yet another preferred aspect of the invention, a conductor contact structure is provided. The conductor contact structure includes a pair of conductor line stacks oriented in parallel, wherein each conductor line stack includes a layer of a first material selected from heavily doped polysilicon and a metal silicide; and a layer of metal formed on the layer of first material, the layer of metal having an upper portion and a lower portion.
In such preferred aspect of the invention, each conductor line stack further includes an insulating cap formed over the layer of metal; and a pair of first spacers disposed on sidewalls of the upper portion and the insulating cap, the lower portion having width defined by a combined width of the upper portion and the pair of first spacers. A pair of second spacers is disposed on sidewalls of the first spacers, on sidewalls of the lower portion and on sidewalls of the layer of first material.
According to a preferred aspect of the invention, a borderless bitline contact to a single-crystal semiconductor region is disposed below the pair of conductor line stacks, the bitline contact contacting sidewalls of the second spacers of the conductor line stacks, such that a conductor line stack of the pair is separated from the single-crystal semiconductor region by an array top oxide layer and another conductor line stack of the pair is conductively coupled to a gate conductor of a vertical pass-gate transistor of a dynamic random access memory.
By the present invention, an improved conductor line structure is provided which decreases the proximity between the metal layer of the conductor line and a borderless contact that is formed adjacent to the conductor line. In such manner, the tendency of the conductor line to become short-circuited to the borderless contact as a result of the minimum feature size of the technology being reduced.
The conductor line is improved in this manner while maintaining resistance within a tolerable range. This is accomplished by processing the metal layer of the conductor line in two portions. An upper portion of the metal layer has reduced width such that proximity between the upper portion and the bitline contact is increased. Resistance is kept within tolerable limits by fabricating a lower portion of the metal layer having a larger width which is preferably equal to the width of a polysilicon layer of the conductor line that is formed below the lower layer.
Embodiments of the invention will now be described in relation to the figures which illustrate preferred structures and methods for implementing the invention.
Substrate 310 employed in the present invention can include any semiconducting material, including, but not limited to: Si, Ge, SiGe, GaP, InAs, InP and all other III/V compound semiconductors. The substrate may also be composed of a layered semiconductor such as Si/SiGe, Si/SiO2/Si or silicon-on-insulator (SOI) having a single-crystal semiconductor region as an uppermost layer. The substrate may be of n-type or p-type conductivity depending on the type of device or devices to be fabricated. Moreover, the substrate may contain various isolation regions such as shallow trench isolation (STI) regions, LOCOS (local oxidation of silicon) regions or other like isolation regions either formed in the substrate or on a surface thereof. Typically, a dielectric layer 312 separates the conductor lines 302 from the surface of the substrate 310. The substrate most preferably includes an upper region of single-crystal silicon in contact with the dielectric layer 312. In an embodiment in which the conductor lines 302 function as gate conductors of insulated gate field effect transistors (“IGFETs” or commonly “FETs”), the dielectric layer 312 is a thin layer which functions as a gate dielectric for planar transistors of a storage cell array. In another embodiment, the dielectric layer 312 can be a thicker array top oxide layer which functions as an isolation layer for drain regions of vertically oriented transistors of a storage cell array.
Each conductor line preferably includes a layered stack including a lowest layer of doped polysilicon 313 and one or more layers 314 of metals and/or one or more compounds of metals, especially metal silicides. For ease of reference, such layer or layers 314 of metal(s) and/or metal compound(s) is hereinafter referred to as a “metal layer.” An optional barrier layer comprising tungsten nitride (WN), titanium nitride (TiN) or tantalum nitride (TaN), for example, may be formed between the doped polysilicon layer 313 and the conductive metal and/or metal compound layer(s) 314.
As further shown in
As further shown in
Also, since the metal layer 314 has a lower portion 317 which spans the width of the polysilicon layer 313 below, the metal layer 314 can be formed to have lower resistance, in two ways, than the metal layer 214 of the conductor line structure described above relative to
In a particular embodiment, substrate 310 may include trench capacitors, such as deep trench capacitors. Trench capacitors can be used for a number of purposes. Notably, trench capacitors are used as storage capacitors in storage cells of a DRAM. Other purposes exist, as well, such as for providing decoupling capacitance.
As further shown in
As in the embodiment shown and described above relative to
A method of forming conductor lines and a borderless contact to the substrate between the conductor lines will now be described, with reference to 3A through 10. As shown in
The layered stack 334 further includes a “metal layer” 314 including one or more metals and/or one or more compounds of metals, for example, a metal silicide. A preferred metal for use in the metal layer 314 is tungsten (W), because chemical vapor deposition (CVD) processes exist for depositing tungsten, while many metals cannot be deposited by such technique. CVD is advantageous over available alternative deposition techniques such as physical vapor deposition (PVD) techniques for reasons related to time, cost and/or quality of processing. When the metal layer 314 includes only a metal silicide, tungsten silicide is a preferred metal silicide for the same or similar reasons that tungsten is a preferred metal. As indicated above, the metal layer 314 may include one or more additional layers such as a barrier layer and/or adhesion layer (or dual function barrier/adhesion layer) between different materials of the metal layer 314 and/or between the metal layer 314 and the polysilicon layer 313 that underlies it. Such barrier/adhesion layer can be composed of titanium nitride (TiN) or tantalum nitride (TaN), for example. In addition thereto, a deposition of titanium (Ti) having very short duration, e.g. less than one minute in duration, may be performed to create a “flash” titanium layer between respective layers of the metal layer 314 and/or between the metal layer 314 and the underlying polysilicon layer 313.
A dielectric material layer 319 is formed over the metal layer 314 for the purpose of being patterned to provide an insulating cap over the conductor line and for serving as a hardmask when the layered stack 334 is etched as follows to form conductor lines. The dielectric material layer 319 is preferably composed of silicon nitride. In an alternative embodiment, the dielectric material layer 319 can be composed of silicon dioxide instead.
Next, as illustrated in
There are many ways to accomplish stopping the etch at a particular level 324. For one, a timed etch can be employed. The timed etch has the advantage that it does not require not require finely controlled depositions of certain material layers within the metal layer 314 for use in endpointing the etch. Alternatively, positive endpointing can be used which involves formation of a particular layer within metal layer 314 which can be distinguished during the etch process. For example, a distinguishable material can be provided at a certain point within metal layer 314 which produces a particular chemical signal when the etching process reaches it. For example, a nitride, e.g. TiN, or TaN, can be distinguished by a nitrogen-containing chemical signal (e.g. ammonia NH4) when the etchant is selected appropriately. An advantage of a timed etch is that it does not require any such distinguishable layer to be formed within the metal layer 314, making it less costly to implement in terms of processing complexity. A disadvantage of the timed etch is that it is less well-controlled than other processes that provide positive endpointing, such that the timed etch can result in a greater margin of error for controlling the thicknesses of the upper portion and lower portion of the metal layer 314.
A third possible way of etching metal layer 314 only down to the desired level 324 is to perform etching selective to a distinguishable layer within metal layer 314 such that the etch process effectively slows to a very slow rate and/or stops on the distinguishable layer. This process can also be used in conjunction with a timed etch such that the two methods complement each other, the timed etch shutting down etching if the endpoint is not reached there prior. Such distinguishable layer can again be composed of a conductive nitride such as of TiN and/or TaN. An advantage to this approach is that the level 324 can be more precisely reached than by a timed etch alone.
Next, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Thereafter, a further dielectric material layer 337 is deposited over the gap fill material 336. When the sidewall spacers 320, 321 are formed of silicon nitride, the dielectric material layer 337 preferably includes an oxide for distinguishing etch selectivity for the sidewall spacers 320, 321 which are to remain after the holes for the borderless bitline contacts are etched. Such dielectric material layer 337 is commonly referred to as an oxide cap 337.
As further shown in
Thereafter, with reference to
As a result of the etching, the spacers 321 and possibly spacers 320 that insulate the metal layer 314 and polysilicon layer 313 are subject to being eroded. However, since in the present embodiment, more space is available between the outer (second) spacers 321 of the conductor lines 302, there is less chance that the protective spacers covering the corners 346 will be eroded to the point that a borderless contact formed in such contact hole will short-circuit to one or more of the conductor lines 302.
A layer of one or more materials generally including polysilicon deposited to contact substrate 310, followed by one or more of metals and/or metal compounds such as metal suicides are then deposited into the contact hole to form a conductive contact to the substrate, as shown in
While the invention has been described in accordance with certain preferred embodiments thereof, those skilled in the art will understand the many modifications and enhancements which can be made thereto without departing from the true scope and spirit of the invention, which is limited only by the claims appended below.
