Field effect transistor formed with wide-submicron gate

Abstract: A mesa comprising successive layers of bottom SiO₂ (22A/32), polysilicon pad (22A), top silicon nitride (26A) is first formed on a semiconductor substrate (20). Next, a first layer (34) of N type polysilicon and a second layer (36) of silicon nitride are conformally deposited thereon. Next, the said second layer is anisotropically etched to leave sidewall spacers (36A). A portion of said first layer at the top of the mesa is now exposed. The said exposed portion is then removed leaving lateral sidewalls (34A), whose top part are oxidized (38). Next, all the silicon nitride material is removed exposing the polysilicon pad (22A) and leaving very narrow sidewall spacers (34A) of N type polysilicon that are used as in-situ masks to deliverate very sub-micronic portions (32A) of the bottom SiO₂ layer to be subsequently used as the gate dielectric for the FETs.

Description: The present invention relates generally to the formation of small-dimensioned features in semiconductor structures and more specifically to the formation of a field-effect transistor including a very narrow, vertical gate structure. In the fabrication and use of field-effect transistors (FETs), it is generally known that short channel lengths yield high performance devices. Conventional photolithography is used in fabrication processes to produce high-performance FETs with submicron, i.e. in the range of 0.8-1.0 µm, channel lengths. It is further known that FETs having channel lengths in the 0.1 µm range, when operated at very low temperatures (on the order of 77 degrees Kelvin, or lower), provide even better performance. However, with the resolution of conventional photolithography currently limited to about 0.5 µm, no process is known for the reliable, large-scale manufacture of devices incorporating the desirably shorter channel lengths. Direct write electron beam lithography is capable of providing features in the desired sub-micron range. However, this process is slow, and not adaptable to large-scale manufacturing. X-ray lithography is also capable of providing such high resolution, but comprises a complex, expensive process which is not currently practical for large-scale manufacturing. Another method of forming sub-micron features is the use of what is termed a "sidewall image transfer" processes. In such a process, a sub-micron sidewall element is used as a mask to transfer, by etching, the sub-micron dimension into underlying materials. See, for example, U.S.-A. 4.502.914 to Trumpp et al. (assigned to the assignee of the present invention), which shows many different sidewall image transfer processes utilizing polymer materials. See also Japanese Kokai 63-307739 (by Fujitsu Ltd.) showing a similar process. Such processes are capable of fabricating features even smaller than those available with conventional photolithography. However, as the feature dimensions approach the 0.1 micron range, the process variations, particularly those related to the etching, are too large to provide the uniformity and reproducibility required for manufacturing. Yet another method of forming FET devices includes the use of a conductive sidewall as the actual FET gate. See, for example, Japanese Kokai 57-42151 (by Fujitsu K.K) wherein a polycrystalline sidewall is utilized as an FET gate. U.S.-A. 4.419.809 to Riseman et al. (assigned to the assignee of the present invention) shows a similar process, and further includes the use of additional sidewalls, formed on the sidewall gate, to act as successive masks for dopant implants which control the effective channel length of the completed device. See also U.S.A. 4.312.680 to Hsu wherein the gate sidewall is formed by lateral diffusion with an etch limiting element from an mesa/source. Selective etching is then used to leave the sidewall. One problem encountered in the use of such "sidewall gates" as are shown in Fujitsu K.K., Hsu, and Riseman et al. is that of the gates themselves being asymmetrical. More specifically, Fujitsu K.K. and Riseman et al. teach the use of a sidewall gate formed by conformal deposition of a thin, conductive layer, and directional etching whereby to remove horizontal features and leave vertical sidewalls. Hsu teaches doping of the sidewall, followed by selective etching to leave the sidewall. The vertical sidewalls formed by these processes exhibit a characteristic "shoulder," or sloping upper corner, inherently formed by the etching step. This asymmetrical structure of the gate makes it impossible to form symmetrical transistors, resulting in reduced-performance devices. A further problem encountered in the formation of such sidewall gate structures is that of unequal etching of the surfaces adjoining the gates. More specifically, upon the completion of the etch step used to form the sidewall gates, it is necessary to remove the mesa or form on which the sidewall was formed. The etch used to remove the mesa also removes part of the exposed surface not covered by the mesa and adjoining the sidewall gate. This unevenness in the surfaces adjoining the gate results in an asymmetrical device structure providing less than optimum performance. No prior art process is known to the present inventors which is suitable in reliability, affordability, and reproducibility for the large-scale fabrication of sub-micron channel FETs. It is an object of the present invention to provide a high-performance FET having a deep sub-micron gate and a symmetrical structure. It is a further object of the present invention to provide a method of fabricating the above-described FET. It is another object of the present invention to provide a method for fabricating a sub-micron feature in a semiconductor structure. In accordance with the present invention, there is provided a method of forming a semiconductor structure according to claim 1. These and other objects, features, and advantages of the present invention will be apparent through a consideration of the following detailed description of the invention, when considered in conjunction with the drawing Figures, in which: FIGS. 1-9 are cross-sectional views showing consecutive steps in the fabrication of an FET using a method in accordance with the present invention; FIGS. 10-13 are cross-sectional views showing consecutive, alternate steps for FIGS. 4-7, respectively, in accordance with an alternate embodiment of the present invention; and FIG. 14 is a top view of the FET structure shown in FIG. 9 with lateral gate edges removed (as described below). Referring now to FIG. 1, a P conductivity type substrate 20 is provided having a generally planar upper surface, sheet resistivity in the range of 1-2 ohm-cm and a crystal orientation, for example, of <100>. Boron ions (not shown) are preferably implanted into the upper surface of substrate 20 at doses in the range of 2-5x10 12 atoms/cm 2 , to control threshold voltage and inhibit punch-through. When used, such boron ion implants are preferably performed at two implant energies: 17 and 35 KeV. Continuing to describe FIG. 1, a layer 22 of silicon dioxide is formed conformally over the upper surface of substrate 20 by a process of thermal oxidation to a thickness of in the range of 3-5nm. A layer 24 of polycrystalline silicon (polysilicon) is formed conformally over oxide layer 22 by a conventional process of chemical vapor deposition (CVD) to a thickness of about 150nm. Polysilicon layer 24 is formed in situ doped or subsequently doped by ion implantation to a concentration in the range of 1-5x10 20 atoms/cm 3 . A layer 26 of silicon nitride is formed conformally over polysilicon layer 24, by a conventional process of plasma-enhanced CVD (PECVD) to a thickness of about 300nm. Alternatively, layer 26 can comprise CVD oxide, for example tetraethylorthosilicate (TEOS), of the same thickness. Referring now to FIG. 2, a conventional photolithographic process is used to form a pattern mask 28 of multi-layer photoresist material. Mask 28 is then used to form a generally rectangular mesa 30 comprising consecutive layer portions of oxide 22A, polysilicon 24A, and nitride 26A. More specifically, mask 28 is used with consecutive, anisotropic, reactive ion etching (RIE) processes, using CF 4 plasma to remove the unmasked portions of nitride layer 26, SF 6 /Cl 2 plasma to remove the unmasked portions of polysilicon layer 24, and CF 4 /O 2 plasma to remove the unmasked portions of oxide layer 22. Mesa 30 is seen to have substantially vertical sidewalls. Continuing with reference to FIG. 2, mask 28 is removed in a conventional manner. The structure is subjected to a cleaning operation using BHF, and a thin layer of gate oxide 32 is formed on the exposed surface of substrate 20. Gate oxide 32 is formed using a conventional thermal oxidation process, and to a thickness in the range of 3-5nm. Referring now to FIG.3, a layer 34 N-type of polysilicon is deposited conformally over the structure, using a conventional process of CVD, and to a thickness of about 150nm. Polysilicon layer 34 can be formed in situ doped, or subsequently doped by ion implantation, to a dopant concentration in the range of 1-5x10 20 atoms/cm 3 . Referring now to FIG. 4, a layer 36 of silicon nitride is formed conformally over the structure, using a conventional PECVD process, to a thickness of about 300nm. As shown in FIG. 5, layer 36 is subjected to an anisotropic RIE process, using CF 4 /O 2 plasma, to remove horizontal portions of the layer, leaving sidewall spacer 36A adjoining the vertical walls of polysilicon layer 34. Referring now to FIG. 6, the exposed portions of layer 34, that is those portions not masked by spacer 36A, are subjected to an anisotropic RIE process, using SF 6 /Cl 2 , stopping on oxide layer 32, to selectively etch layer 34 and leave L-shaped wall region 34A. The structure is then subjected to a thermal oxidation environment so as to oxidize cap regions 38 and 40 on the upper and lower exposed surfaces of polysilicon wall 34A, respectively. These oxide caps are formed to a thickness of about 100nm. Layer 32 protects the surface of substrate 20 from further oxidation. In accordance with a key feature of the present invention, spacer 36A and nitride layer portion 26A are of generally equal vertical thicknesses. The bottom L-leg of polysilicon wall 34A is generally of equal thickness as polysilicon layer portion 24A. Similarly, gate oxide 32 is of generally equal thickness as oxide layer portion 22A. Thus, as these corresponding layers of like materials are simultaneously etched, the underlying surfaces of substrate 20 adjoining the subsequently formed FET gate (described below) are not over-etched so as to be asymmetrical. This symmetry of the surfaces adjoining the gate is an important advantage of the present invention. More specifically, the multilayer stack consisting of spacer 36A, the bottom L-leg of sidewall 34A, and the underlying portion of oxide layer 32, will etch (as described below) at substantially the identical rates as corresponding layers 26A, 24A, and 32 (below 24A). The bottom L-leg of sidewall 34A extends laterally about 300nm (the thickness of spacer 36A) from what will become the gate structure of the FET, ensuring that the topography of substrate 20 in the active region of the subsequently formed FET will be symmetrically flat. Referring now to FIG. 7, the device is subjected to a wet etch, using H 3 PO 4 acid at 170 degrees centigrade, to simultaneously remove nitride spacer 36A and nitride layer portion 26A from mesa 30. The structure is then subjected to an anisotropic RIE process, using SF 6 /Cl 2 , with oxide cap 38 functioning as a mask, to simultaneously remove the polysilicon bottom L-leg of wall 34A, and polysilicon layer portion 24A from mesa 30. An anisotropic RIE process, using a CF 4 plasma, is then used to simultaneously remove caps 38 and 40, the exposed portion of gate oxide layer 32 (i.e. that portion not under the remaining vertical portion of wall 34A), and oxide layer portion 22A. The result of these last-described etching processes, as shown in FIG. 8, is the stand-alone, vertical gate 42 spaced from the upper, major, planar surface of substrate 20 by remaining gate oxide 32A. It will be understood that, because mesa 30 (FIGS 2-6) is rectangular in nature, gate 42 circumscribes the outline of a rectangle. (See FIG. 14 below.) In accordance with another key feature of the present invention, it is seen that, when vertical gate structure 42 is formed in accordance with the present invention, it is squared-off at its upper surface. That is, it does not have the characteristic sidewall shoulder described with respect to the background art shown and discussed above. This feature contributes in providing the current invention with the significant advantage of an FET (described below) of highly symmetric structure. This symmetric structure yields a high-performance device. Continuing to describe FIG. 8, Sb ions are implanted (shown schematically by arrows 43) into the surface of substrate 1 at a power in the range of 10KeV and a dose in the range of 1-2x10 14 atoms/cm 3 . N+ source/drain regions 44 (inside sidewall 42), 46 (to the left of the sidewall), and 47 (to the right of the sidewall)are thus formed to a peak concentration in the range of 1-5x10 20 atoms/cm 3 . This first ion implantation is formed with relatively heavy ions to define the effective channel length of the to-be-completed FETs. Referring now to FIG. 9, insulating sidewalls 50 and 52 are formed on opposite sides of vertical gate 42 (and gate oxide 32A). Sidewalls 50, 52, comprising, for example, silicon dioxide, silicon nitride, or multiple layers thereof, are formed by the conventional method of conformal deposition followed by anisotropic etching to remove horizontal layer portions and leave the sidewall spacers. Continuing to describe FIG. 9, a second ion implantation is performed to form N+ regions 54 (inside sidewall 50), 56 (to the left of sidewall 52), and 58 (to the right of sidewall 52). This second ion implant comprises Arsenic ions 59 implanted at an energy of about 20 KeV and a dosage in the range of 2-5x10 15 atoms/cm 2 . Regions 54, 56 thus have a peak dopant concentration in the range of 1-5x10 21 atoms/cm 3 . The device is subjected to a conventional thermal anneal. The device is then subjected to a conventional silicide process so as to form silicide contact 60 on vertical gate 42, and silicide contacts 62, 64, and 66 over doped regions 54, 56, and 58, respectively. Referring now also to FIG. 14, the parallel, lateral edges of vertical gate 42 and sidewalls 50, 52 are removed by etching, these removed areas being shown in dotted line as 42A, 50A, 52A, respectively. With the lateral edges of gate 42 removed, there remain two gate structures, a left-most gate structure including a silicide cap 60 over a sidewall 42 (not visible in FIG. 14), and sidewalls 50B, 52B; and a right-most gate structure including a silicide cap 60 over a sidewall 42 (not visible in FIG. 14), and sidewalls 50C, 52C. There are thus formed two FET devices sharing a common source/drain region 54: a first FET having drain region 56, left-most gate 42, and source 54, and a second FET having source 54, right-most gate 42, and drain 58. Source contact 70, drain contact 72, and drain contact 74 are provided to regions 54, 58, and 56, respectively, by patterning and etching overlying oxide regions (not shown), and depositing these metal contacts in a conventional manner. Conductive landing pad 75 and metal contact 76 are provided in a conventional manner for making electrical contact to left-most gate 42/silicide 60. Similarly, conductive landing pad 77 and metal contact 78 are provided for making electrical contact to right-most gate 42/silicide 60. Referring now to FIGS. 10-13, alternate, low-temperature process steps are illustrated for the corresponding process steps shown in FIGS. 4-7. Like features are indicated by like reference numbers. Describing first FIG. 10, a layer 80 of polyimide is spin-coat deposited over layer 34 to a peak thickness of about 1.0 micrometer, and baked at 400 degrees centigrade. This structure is then subjected to an etch-back, using a RIE etch such as O 2 plasma, to expose the upper surface of polysilicon layer 34 (over mesa 30), yielding the structure shown in FIG. 11. Referring next to FIG. 12, the exposed surface of polysilicon layer 34 is removed using the same process described above with respect to FIG. 6. In contrast to the oxidation step described with respect to FIG. 6, the exposed, upper edges of wall 34A are coated with tungsten (W) 82, using a conventional, selective W deposition process. Such a process is typically performed at a temperature in the range of 300-400 degrees centigrade. With reference to FIG. 13, the remaining exposed portions of polyimide layer 80 are removed using an O 2 ash process. Using tungsten cap 82 as a mask, exposed nitride layer portion 26A is removed with a RIE process, using CF 4 plasma. To provide the structure shown in FIG. 8, tungsten cap 82 is used as a mask, while the exposed polysilicon in layers 24A and 34A (i.e. the unmasked, horizontal portions of these layers) is removed as described with respect to FIG. 8 above. Tungsten cap 82 is again used as a mask to remove the exposed, horizontal portions of oxide in layers 22A and 32, also as described with respect to FIG. 8, above. Finally, tungsten cap 82 is removed using a wet etch of NH 4 OH/H 2 O 2 . The resulting structure is substantially identical to that shown in FIG. 8, and processing is continued in a manner identical to the steps described with respect to FIGS. 8, 9 and 14, above. (Alternatively, tungsten cap 82 can be left in place to lower the resistance of the subsequently formed gate structure. Processing is otherwise continued in an identical manner to that described with respect to FIGS. 8, 9, and 14.) The process steps described with respect to FIGS. 4-7 require the formation of oxide caps 38, 40, typically at a temperature in the range of 750-900 degrees centigrade. In contrast, the process steps described in this alternate embodiment of the invention (i.e. steps 10-13) use instead tungsten cap 82, formed at a temperature of about about 400 degrees centigrade. These alternate steps thus yield the advantage of providing a relatively lower-temperature process. There is thus provided a method for forming a semiconductor structure which can be used to form a high-performance FET transistor having a deep sub-micron dimension gate, the gate being formed by a unique process forforming a deep sub-micron dimension semiconductor structure. This process for forming the sub-micron gate yields a highly symmetric, vertically oriented gate, having a square upper surface instead of the characteristic, asymmetrical sidewall shoulder of the background art. Further, this sub-micron vertical gate is formed in a manner so as to avoid damaging the adjoining substrate surfaces, providing planar surfaces for the active source and drain regions. The vertical gate structure is limited in thickness only by the conformal layer deposition process used to form layer 34 (see FIG. 3). The resulting FET structure is highly symmetrical, yielding high-performance operation. The present invention is useful in the formation of high-performance field-effect transistors, and particularly in the formation of very-large scale integration (VLSI) circuits incorporating such transistors.

Claims: A method of forming a semiconductor structure comprising the steps of: a) providing a substrate (20) including a planar surface; b) forming a mesa (30) consisting of a bottom layer (24A) of a polysilicon material and a top layer (26A) of an insulating material, said uses a having two vertical walls over said planar surface; said materials having different etch rates ; c) forming a first conformal layer (34) of material of a polysilicon material over said mesa and said planar surface so as to form a vertical sidewall on each of said vertical walls; d) forming a second conformal layer (36) of an insulating material onto the resulting structure; e) anisotropically etching said second conformal layer until the underlying first conformal layer is exposed while leaving spacers (36A) adjoining the vertical walls of the first conformal layer; f) removing the horizontal portions of said first conformal layer that are exposed; g) oxidizing or siliciding the exposed portions of said first conformal layer to form an in-situ hard, mask (38) selectively at least on the upper portion of said vertical sidewalls; and, h) using said hard mask to remove all the unmasked portions of the resulting structure in order to leave said two vertical sidewalls of said first conformal layer. A method in accordance with claim 1 further comprising the step of: i) removing said hard mask (38). A method in accordance with claim 1 or 2 further comprising the step of : j) implanting ions to form regions (44, 46...) of desired conductivity type in said substrate using said sidewalls as an in-situ mask. A method in accordance with claim 1, 2 or 3 wherein said mesa further comprises a thin silicon dioxide layer (22) underneath said bottom layer; A method in accordance with claim 1 wherein steps d) and e) are replaced by steps : k) depositing a layer (80) of a planarizing medium onto the resulting structure to cover said mesa; and l) etching back the structure to expose said first conformal layer on top of said mesa. A method in accordance with claim 5 wherein: said insulating material comprises silicon nitride; and said planarizing medium comprises polyimide.

Inventors: HSIEH CHANG-MING, HSU LOUIS LU-CHEN, KUMAR SHANTHA ARAKERE, TIEN ZU-JEAN, HSIEH, CHANG-MING, HSU, LOUIS LU-CHEN, KUMAR, SHANTHA ARAKERE
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1992
Main classification: H01L21
IPC codes: H01L29, H01L21