PROCESS FOR MANUFACTURING MOSFETS USING
AMORPHOUS CARBON REPLACEMENT GATE AND
STRUCTURES FORMED IN ACCORDANCE THEREWITH
BACKGROUND OF THE INVENTION
Field of the Invention
Embodiments of the invention pertain to metal oxide semiconductor field effect transistors (MOSFETs), and in particular to the manufacture of MOSFETs using a replacement gate process.
Background Technology
Integrated circuits typically include large numbers of transistors such as MOSFETs. Figure 1 shows an example of a typical MOSFET 10. The MOSFET 10 includes gate structure comprised of a gate line 12 (hereinafter "gate Line" or "gate"), a gate insulator 14 and spacers 16. The gate line 12 and gate insulator 14 are formed over a channel region 18 of a semiconductor substrate 20. The gate structure is surrounded by an interlevel dielectric layer (ILD) 22 such as silicon oxide. The MOSFET 10 further includes composite source and drain diffusions comprised of overlapping lightly doped source and drain regions 24 and heavily doped source and drain regions 26.
Conventional gate lines were originally formed of polysilicon. However, in the face of increasing performance demands, polysilicon has proven to be too resistive. This led to the development of the
"replacement gate" process, in which the initial polysilicon gate is used as a "dummy" gate that is replaced with metal after formation of other MOSFET features. This is typically done by forming a MOSFET gate structure including a polysilicon dummy gate, depositing an ILD over the substrate and the gate structure, polishing back the ILD to expose the polysilicon dummy gate, etching to remove some or all of the polysilicon dummy gate, and then inlaying metal in place of the removed polysilicon to form a metal gate.
While the replacement gate process improves over the conventional polysilicon gate MOSFET, the process has undesirable features. One undesirable feature is that it is relatively difficult to pattern polysilicon with good profiles, particularly at small dimensions, and so formation of the polysilicon dummy gate is relatively difficult. Another undesirable feature is the difficulty of preventing damage to the gate insulator. Because the gate insulator is so thin, any damage to the gate insulator during removal of the polysilicon gate requires complete removal and replacement of the gate insulator. Thus it would be desirable to have a replacement gate process that avoids the difficulties of polysilicon dummy gate formation and removal.
SUMMARY OF THE INVENTION In accordance with preferred embodiments of the invention, the replacement gate process is improved by the use of an amorphous carbon dummy gate. Amorphous carbon is easily and accurately patterned and
removed through the use of a dry anisotropic oxygen or hydrogen plasma etch, referred to herein as "ashing." This treatment provides rapid removal and high selectivity.
DESCRIPTION OF DRAWINGS Figure 1 shows a conventional MOSFET structure.
Figures 2a, 2b, 2c, 2d, 2e, 2f and 2g show structures formed during processing in accordance with a preferred embodiment.
Figure 3 shows a process flow encompassing the preferred embodiment and alternatives thereto.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Structures formed during processing in accordance with a preferred embodiment of the invention are illustrated in Figures 2a-2g. Figure 2a shows a semiconductor substrate 30 having formed thereon a gate insulating layer 32 such as Si02 or SiON. Overlying the gate insulating layer 32 are an amorphous carbon layer 34, a hardmask layer 36 such as SiN, and a photoresist pattern 38. The amorphous carbon layer 34 is formed by a PECVD process using carbon containing precursors.
As shown in Figure 2b, the hardmask layer 36, and the amorphous carbon layer 34 are sequentially anisotropically etched to form an amorphous carbon dummy gate 40 on the gate insulator 32. The hardmask layer is typically patterned using a combination of Ar and CF4. The amorphous carbon layer is typically patterned using a combination of HBr, oxygen, and argon. After etching to form the dummy gate 40, lightly doped source and drain regions 44 are formed by implantation using the amorphous carbon dummy gate 40 to mask a channel region 46 in the substrate. Other areas of the substrate are suitably masked during implantation to define the lightly doped source and drain regions 44.
Figure 2c shows the structure of Figure 2b after formation of spacers 48 surrounding the amorphous carbon dummy gate 40. The spacers 48 are typically made of silicon oxide or silicon nitride and are formed by depositing a conformal layer of silicon oxide over the substrate, followed by anisotropically etching the silicon oxide to leave spacers 48 as shown. After the spacers 48 are formed, heavily doped source and drain regions 50 are formed by an implantation process that uses the dummy gate 40 and spacers 48 to mask the channel region 46 and extension portions of the lightly doped source and drain regions 44. Other areas of the substrate are suitably masked during implantation to define the heavily doped source and drain regions 50. Figure 2d shows the structure of Figure 2c after formation of a conformal layer of an interlevel dielectric 52 such as silicon oxide over the substrate. As shown in Figure 2e, a portion of the interlevel dielectric layer 52 is removed by chemical mechanical polishing to expose the amorphous carbon dummy gate 40.
Figure 2f shows the structure of Figure 2e after removal of the amorphous carbon dummy gate 40 by an ashing process to leave a void 54. Ashing is preferably performed using a dry oxygen or hydrogen plasma. The ashing process consumes amorphous carbon by forming volatile products such as carbon dioxide, carbon monoxide, or methane that are evacuated from the chamber.
Figure 2g shows the structure of Figure 2f after inlaying of a metal gate 56 into the void 54 left by the removed amorphous carbon dummy gate. The metal gate 56 is preferably formed of a metal that exhibits relatively low resistivity and low diffusion, such as tungsten or aluminum. Other metals that may be used include, but are not limited to, TiN, WN, TaN, and Ru02. Inlaying is typically performed by depositing a conformal layer of metal by sputtering to fill the void 54, followed by polishing to remove an overburden portion of the sputtered metal to leave the inlaid metal gate 56 as shown in Figure 2g.
After the structure of Figure 2g is formed, additional conventional processing may be performed, such as formation of source and drain contacts by an inlay process and silicidation of the contacts, or formation of a protective layer over the ILD. While the process flow of Figures 2a-2g is presently considered to be the preferred embodiment of the invention, a wide variety of alternative embodiments in accordance with the invention may be formulated. For example, in one alternative embodiment, more than one set of spacers may be sequentially formed in conjunction with sequential implantations to form source and drain diffusions that are a composite of three or more implanted regions. In another alternative embodiment, source and drain contacts may be formed concurrently with inlaying of the metal gate.
In addition to the aforementioned alternatives, various additional combinations of the features of the preferred embodiment and its aforementioned alternatives may also be implemented. It will also be apparent to those having ordinary skill in the art that the tasks described in the preferred embodiment and the aforementioned alternatives are not necessarily exclusive of other tasks, but rather that further tasks may be incorporated in accordance with the particular structures to be formed. For example, processing tasks such as seed layer formation, seed layer enhancement, formation and removal of passivation layers or protective layers between processing tasks, formation and removal of photoresist masks and other masking layers such as antireflective layers, trimming of photoresist masks and other masking structures, formation of isolation structures, as well as other tasks, may be performed along with the tasks specifically described above. Further, the process need not be performed on an entire substrate such as an entire wafer, but rather may be performed selectively on sections of the substrate.
Figure 3 shows a process flow for producing a MOSFET that encompasses the preferred embodiment and its aforementioned alternatives, as well as other alternative embodiments that are not explicitly discussed here but will be apparent to those of ordinary skill in the art. Initially an amorphous carbon layer is formed over a substrate (60). The substrate is preferably a semiconductor layer that is covered by a gate insulating layer. A gate structure comprising an amorphous carbon dummy gate is then formed from the amorphous carbon layer (62). This is preferably accomplished by patterning the amorphous carbon using a photoresist mask and a hard mask, followed by formation of spacers. The dummy gate and spacers are preferably used as masks for one or more implantations that define source and drain diffusions. An interlevel dielectric layer is then deposited over the substrate and gate structure (64), and a portion of the interlevel dielectric layer is then removed to expose the amorphous carbon dummy gate (66). The amorphous carbon dummy gate is then removed by an ashing process (68). Ashing is preferably performed
using oxygen or hydrogen plasma. A metal gate is then inlaid in place of the amorphous carbon dummy gate (70). The metal gate may be aluminum or tungsten, or another metal as described above.
While the embodiment illustrated in the figures and described above is presently preferred, it should be understood that the presently preferred embodiment and its aforementioned alternatives are offered by way of example only. The invention is not limited to a particular embodiment, but extends to various modifications, combinations, and permutations that fall within the scope and spirit of the appended claims.