BRIEF DESCRIPTION OF THE DRAWINGS
The disclosure will be better understood from the following detailed description with reference to the drawings, in which:
FIG. 1 is a schematic diagram illustrating a cross-sectional view of a semiconductor structure having potential contact profiles achievable using conventional contact lithography, in accordance with one embodiment of the present disclosure;
FIG. 2 is a schematic diagram similar to FIG. 1 illustrating a cross-sectional view of the embodiment of the semiconductor structure of FIG. 1;
FIG. 3 is a flow diagram illustrating an embodiment of a method of forming a semiconductor structure in accordance with the present disclosure;
FIGS. 4 and 5 are schematic diagrams illustrating a cross-sectional view of a partially completed semiconductor structures of the present disclosure;
FIG. 6 is a top view of the partially completed structure of FIGS. 4 and 5;
FIG. 7 is a schematic diagram illustrating a cross-sectional view taken along section line A of FIG. 6;
FIG. 8 is a schematic diagram illustrating a cross-sectional view taken along section line B of FIG. 6; and
FIGS. 9-12 are schematic diagrams illustrating a cross-sectional view of a partially completed structure of the present disclosure.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present disclosure and the various features and advantageous details thereof are explained more fully with reference to the nonlimiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the present disclosure. The examples used herein are intended merely to facilitate an understanding of ways in which the present disclosure may be practiced and to further enable those of skill in the art to practice the present disclosure. Accordingly, the examples should not be construed as limiting the scope of the present disclosure.
Referring to FIG. 1, a semiconductor structure of the present disclosure is illustrated and is designated generally as structure 100. As device densities within semiconductor structure 100 increase, overlay tolerances between contact (e.g., source/drain contact 180) and gate 111-112 lithography levels are difficult to achieve. In particular, when using conventional contact lithography there is a margin of error in the placement of the contact hole 180a directly above the silicide region 140. When the contact hole is etched (e.g., by using a reactive ion etching (RIE) process), lack of selectivity in the RIE process in combination with this margin of error, may cause the contact hole to punch through the dielectric spacers 121 or 122 on either side of the silicide region 140 (e.g., see potential contact profiles 180b and 180c). Consequently, when the contact is formed it may contact unsilicided active silicon in a substrate 105 and, thereby, degrade device performance. In order to compensate for this margin of error and lack of selectivity in the RIE process, source/drain contact sizes are often reduced (i.e., the contact diameter is scaled) so as to ensure that the contacts do not short the source/drain diffusion region to the gate electrode 111, 112. However, scaling the contact size substantially decreases the contact area between the contact and the silicide region and increases the resistance of the contact. Therefore, there is a need in the art for a semiconductor structure with a device contact that provides optimal resistance without impacting device yield.
In view of the foregoing, embodiments of a semiconductor structure are disclosed having a partially self-aligned device contact in which the bottom diameter of the contact is significantly enlarged to maximize the contact area between the contact and the silicide region and to minimize contact resistance. Since the contact is self-aligned to gate electrodes, the size expansion does not intrinsically impact device yield. Additionally, an embodiment of the present disclosure also allows for the integration of a thick middle-of-the-line (MOL) nitride stress film to enhance carrier mobility in the devices within the semiconductor structure. Embodiments of the method of forming the structure of the present disclosure comprise forming a sacrificial section (i.e., a disposable plug) in the intended location of the contact. This plug is patterned so that it is self-aligned to the gate electrodes and only occupies space that is intended for the future contact. Dielectric layer(s) (e.g., an optional nitride stress layer followed by an interlayer dielectric) may be deposited once the plug is in place. Conventional contact lithography is used to etch a contact hole through the dielectric layer(s) to the sacrificial section. The sacrificial section is, then, selectively removed and the contact is formed therein.
Referring to FIG. 2, embodiments of a semiconductor structure 200 of the present disclosure is illustrated and comprises at least two parallel gate electrodes 211, 212 (i.e., parallel gate electrode lines) on a semiconductor substrate 205 (e.g., a bulk wafer, an SOI wafer, etc.), sidewall spacers 221, 222 adjacent to each of the gate electrodes 211, 212, a silicide region 240 on the semiconductor substrate 205 between the gate electrodes 211, 212 and at least one dielectric layer 251, 252 over the gate electrodes 211, 212.
In one embodiment, the parallel gate electrodes 211, 212 and the semiconductor substrate 205 may be designed and configured to form multiple transistors for dense multi-transistor devices, such as, for example, static random access memory (SRAM) cells, complementary metal oxide semiconductor (CMOS) devices, or similar devices that have a minimum contacted pitch.
With continued reference to FIG. 2, a contact 280 connects to the silicide region 240 through the dielectric layer(s) 252, 251. This contact 280 comprises a lower portion 281 and an upper portion 282. The lower portion 281 is below the dielectric layer(s) 251, 252 and has a bottom surface 284 that is adjacent to the silicide region 240 and a top surface 283 that is adjacent to the upper portion 282. In addition, the lower portion 281 of the contact is self-aligned between the gate electrodes 211, 212. In particular, the lower portion 281 extends between and follows the contours 226, 227 of the sidewall spacers 221 and 222, respectively, that are adjacent to the gate electrodes 211, 212, respectively, so as to maximize the contact area between the contact 280 and the silicide region 240. In one embodiment, the lower portion 281 and particularly, the bottom surface 284 of the lower portion 281 may be formed such that it is wider than the gate electrodes 211, 212.
The upper portion 282 of the contact 280 extends through the dielectric layer(s) 251, 252 to the lower portion 281 and is narrower than the lower portion 281. In particular, the bottom surface of the upper portion 282 is narrower than the adjacent top surface 283 of the lower portion 281. In one embodiment, the upper portion 282 may be centered directly above the lower portion 281 and the silicide region 240, as illustrated in contact hole profile 280a. Alternatively, due to the separate processes used to form the lower and upper portions 281, 282 (described below) which result in the upper portion 282 being narrower than the lower portion 281, the upper portion 282 may be offset slightly to either side (e.g., see the contact hole profiles 280b and 280c) without significantly impacting device performance.
With continued reference to FIG. 2, the sidewall spacers 221, 222 of the structure 200 are positioned adjacent to each of the gate electrodes 211, 212, respectively, in order to isolate the contact 280 from the gate electrodes 211, 212. These sidewall spacers may be multi-layered and can comprise a variety of dielectric materials, such as, for example, oxides, nitrides, low-k dielectrics, etc. Additionally, the sidewall spacers 221, 222 may also be tapered such that they are wider towards the bottom of the gate electrodes 211, 212 and narrower towards the top. Such tapered sidewall spacers allow the top surface 283 of the self-aligned lower portion 281 of the contact 280 to be formed wider than the bottom surface 284.
As described hereinabove, in each embodiment of the present disclosure the upper portion 282 of the contact 280 extends through at least one dielectric layer to the lower portion 281. For example, in one embodiment of the present disclosure the structure 200 can comprise a single dielectric layer (e.g., 252) above the gate electrodes 211, 212 and the lower portion 281 of the contact 280. In another embodiment of the present disclosure the structure 200 can comprise a dielectric stress layer 251 above the gate electrodes 211, 212 and the lower portion 281 of the contact 280 as well as another dielectric layer 252 above the stress layer 251.
The stress layer 251 can comprise a compressive nitride layer, a tensile nitride layer, or a dual-strain nitride layer and is used to optimize carrier mobility within the semiconductor structure devices (e.g., within the transistors comprising the gate electrodes 211, 212). Particularly, mechanical stress control of the channel regions may be used to enhance hole mobility in p-type MOSTFETs (p-FETs) and electron mobility n-type MOSFETs (n-FETs). For example, a compressive film over a p-FET structure enhances hole mobility to optimize p-FET performance. Alternatively, a tensile film over an n-FET structure enhances electron mobility to optimize n-FET performance. A dual-strain nitride layer is a nitride layer that has both tensile strain regions and compressive strain regions in order to simultaneously enhance carrier mobility in the channel regions of both n-FETs and p-FETs, respectively, on the same wafer.
With reference to FIGS. 3-6, a method of forming a semiconductor structure in accordance with the present disclosure will now be discussed in detail. The embodiments of the method of forming the semiconductor structures, described above, comprise completing the front end of the line (FEOL) processing (302). Particularly, during the FEOL processing at least two parallel gate electrodes 211, 212 (i.e., gate electrode lines) are formed separated by a gap 213 on semiconductor substrate 205 (304), as illustrated in FIG. 4. In addition, sidewall spacers 221, 222 (e.g., oxide and/or nitride sidewall spacers) are formed adjacent to the gate electrodes 211, 212, respectively (306), as illustrated in FIG. 4. In particular, and as described hereinbelow, tapered sidewall spacers may be formed so that the lower portion 281 of the contact 280, which is subsequently formed between these sidewall spacers 221, 222 using a self-aligned process, will be formed with a wider top surface 283 than bottom surface 284 (308). See FIG. 2. Additional FEOL processing can include, but is not limited to, doping the source/drain extensions, doping the source/drain diffusion regions, forming halos, etc.
With continued reference to FIGS. 3 and 4, a silicide region 240 is then formed on the semiconductor substrate 205 in the gap 213 between the gate electrodes 211, 212 (310, see FIG. 4). In particular, the silicide region 240 is formed (e.g., using conventional silicide formation processes) on the exposed portion of the semiconductor substrate 205 that extends between the sidewall spacers 221 and 222 that are adjacent to each of the gate electrodes 211, 212.
With reference to FIGS. 3 and 5, once the silicide region 240 is formed (at process 310), a sacrificial section 290 (i.e., a disposable plug, self-aligned disposable mandrel, etc.) is formed adjacent to the sidewall spacers 221, 222 in the gap 213 between the gate electrodes 211, 212 (312). In particular, the sacrificial section 290 is formed at a pre-selected location on the silicide region 240 where the contact 280 is to be formed (i.e., at a desired contact location). This sacrificial section 290 may be formed by filling the gap 213 between the electrodes 211, 212 with a sacrificial material (314). The sacrificial material must comprise a material that has a very high etch selectivity to the materials used to form the spacers 221, 222 and the subsequently formed dielectric layer(s) 251, 252 (316). For example, the sacrificial material may be a low-k constant (i.e., k<3) dielectric material (e.g., SiLK™) that has a very high etch selectivity to nitrides and/or oxides (318). The sacrificial material may be deposited, spun-on, etc. The sacrificial material may be self-planarizing or may be planarized and/or recessed so that that the top surface 292 of the sacrificial material is level with or below the top 214 of the gate electrodes 211, 212.
With reference to FIGS. 3 and 6, once the gap 213 is filled (at process 316), the sacrificial material is patterned (e.g., lithographically patterned) (320) with a line/space feature. This line/space feature forms the sacrificial section 290 in the location of the semiconductor structure where the contact is to be positioned (322). This sacrificial material is particularly patterned so that the sacrificial section 290 extends within the gaps 213 between the sidewall spacers 221, 222, is perpendicular to the gate electrodes 211, 212 and is in a desired location for the contact 280. Additionally, the sacrificial material is patterned so that portions 291 of the semiconductor substrate 205 on either side of the sacrificial section 290 are exposed (324). Consequently, the sacrificial sections 290 and thus, the contact 280 locations are designed in a grid pattern.
After the sacrificial material is patterned to form the sacrificial section 290 (at process 312), at least one dielectric layer is formed over the gate electrodes, the sacrificial section and the exposed portions of the semiconductor substrate (326-334). For example, in one embodiment of the present disclosure a single dielectric layer 252 (e.g., interlayer dielectric) is formed over the gate electrodes 211, 212, the sacrificial section 290 and the exposed portions 291 of the semiconductor substrate 205 (334). In another embodiment of the present disclosure, an optional dielectric stress layer 251 (i.e., a middle of the line (MOL) stress liner) is first formed over the gate electrodes 211, 212, the sacrificial section 290 and the exposed portions 291 of the semiconductor substrate 205.
FIGS. 7 and 8 illustrate cross-sections A and B, respectively, of the structure of FIG. 6 following the deposition of an optional dielectric stress layer 251 (at process 324). Particularly, FIG. 7 illustrates the stress layer 251 over the electrodes 211, 212 and the sacrificial section 290. FIG. 8 illustrates the stress layer 251 on the electrodes 211, 212 and within the gap 213 on the exposed portions 291 of the substrate 205 on either side of the sacrificial section 290 (not shown). Thus, the sacrificial section 290 prevents deposition of the nitride layer 251 in the region of the semiconductor substrate 205 where the contact is to be formed.
The optional stress layer 251 may be used to apply mechanical stress to the channel regions of devices in the semiconductor structure (e.g., transistors that incorporate the gate electrodes 211, 212 and the semiconductor substrate 205) in order to enhance hole mobility in p-type MOSTFETs (p-FETs) or electron mobility n-type MOSFETs (n-FETs). In particular, known techniques may be used to form a compressive nitride layer (330), a tensile nitride layer (328) or a dual-strain nitride layer (332). Forming a compressive film over a p-FET structure enhances hole mobility to optimize p-FET performance. Forming a tensile film over an n-FET structure enhances electron mobility to optimize n-FET performance. Forming a dual-strain nitride layer that has both tensile strain regions and compressive strain regions can enhance mobility in both n-FETs and p-FETs that are formed on the same semiconductor substrate. With particular reference to FIG. 9 and with continued reference to FIG. 3, once the stress layer 251 is deposited, another dielectric layer 252 (e.g., an interlayer dielectric) may be formed (e.g., deposited and planarized) over the optional dielectric stress layer 251 (334).
With reference to FIGS. 3, 10 and 11, after forming the dielectric layer(s) (at processes 326-334), a contact hole 285 is patterned (336-338) (e.g., using conventional contact lithography) and etched (e.g., by a conventional reactive ion etching (RIE) process) through the dielectric layer(s) 251, 252 to the top surface 292 of the sacrificial section 290 (340). The contact hole 285 may be particularly patterned so that it is narrower than the sacrificial section 290 and particularly, narrower than the top surface 292 of the sacrificial section 290. Thus, the sacrificial section 290 creates a landing pad for the contact hole 285 etch process that is wider than the contact hole and thereby provides a greater tolerance for over etching. If multiple dielectric layers (e.g., a dielectric stress layer 251 and another dielectric layer 252) were previously formed (at process 326-334), this etching process may require multiple stages designed to selectively etch through each of the dielectric layers 251, 252.
Once the top surface 292 of the sacrificial section 290 is exposed, the sacrificial section 290 is selectively removed forming a cavity 293 and exposing the silicide region 240 without removing or damaging the surrounding dielectrics (e.g., the dielectric materials used to form the sidewall spacers 221, 222 and the dielectric layer(s) 251, 252) (342). See FIG. 12. For example, if SiLK™ is used to form the sacrificial section 290, the sacrificial section 290 may be ashed out using a selective plasma etch process.
Once the silicide region 240 is exposed and the cavity 293 is formed, the contact 280 is formed (346-348) by forming an optional conductive contact liner (e.g., a titanium nitride liner) against the outer surfaces 294 of the cavity 293 (i.e., the surfaces that were exposed by removing the sacrificial section 290, such as the silicide region 240, the sidewall spacers 221, 222, etc.) and the sidewalls 286 of the contact hole 285. Then, a conductive fill material (e.g., a metal, such as tungsten or copper) is deposited (e.g., using a high-aspect-ratio fill or plating method) through the contact hole 285 until the cavity 293 and contact hole 285 are filled. Thus, as illustrated in FIG. 2, a lower self-aligned portion 281 of the contact 280 is formed between sidewall spacers 221, 222 that are adjacent to each of the gate electrodes 211, 212 and an upper portion 282 is formed within the contact hole 285 above the lower portion 281 using conventional RIE lithography. By self-aligning the lower portion 281 of the contact 280, this method both maximizes the contact area between the contact 280 and the silicide region 240 so as to minimize contact resistance and eliminates the junction leakage found in typical contacts resulting from contact etch through the sidewall spacers adjacent to the gate electrodes. The method also allows for line/space lithography of the lower portion 281 of the contact, thereby, increasing lithography fidelity. Additionally, as described hereinabove, if the sidewall spacers 221, 222 are tapered, this self-aligned process ensures that the top surface 283 of the lower portion 281 of the contact 280 is wider than the bottom surface 284. The wider top surface 283 not only decreases contact resistance but also improves the RIE process margin for the upper portion 282 of the contact since there is a larger landing pad for the etch process and thus, little, if any, over etch of the gate sidewall spacers. Following contact liner and fill deposition (at process 346) and contact planarization (at process 348), back end of the line (BEOL) processing continues to interconnect the active components (transistors, resistors, etc.) on the substrate with wiring, etc.
Therefore, disclosed above are embodiments of a semiconductor structure with a partially self-aligned contact integration scheme in which the diameter of the lower portion of the contact is significantly enlarged to reduce resistance. Since the contact is self-aligned to gate electrodes, the size expansion does not intrinsically impact device yield. Additionally, an embodiment of the structure allows for the integration of a thick middle-of-the-line (MOL) nitride stress film to enhance carrier mobility. Embodiments of the method of forming the structure comprise forming a sacrificial section (i.e., a disposable plug) in the intended location of the contact. This plug is patterned so that it is self-aligned to the gate electrodes and only occupies space that is intended for the future contact. Dielectric layer(s) (e.g., an optional stress layer followed by an interlayer dielectric) may be deposited once the sacrificial section is in place. Conventional contact lithography is used to etch a contact hole through the dielectric layer(s) to the sacrificial section. The sacrificial section is, then, selectively removed to form a cavity and the contact is formed in the cavity and contact hole.
While the present disclosure has been described in terms of preferred embodiments, those skilled in the art will recognize that the present disclosure may be practiced with modification within the spirit and scope of the appended claims.
Patent Application
20080026513
