PROCESS FOR FABRICATING SEMICONDUCTOR STRUCTURES

Abstract: A process for fabricating silicon-on-insulator structures on semiconductor wafers and planarizing the topology of the patterns formed from the silicon. The process provides for the formation of monocrystalline silicon islands (19, 21) electrically isolated by dielectric in substantially coplanar arrangement with surrounding dielectric. As described, substrate silicon regions (13, 14) are initially formed and capped, and thereafter used as masks to etch the silicon substrate (1) between the regions. During the oxidation which follows, the capped and effectively elevated silicon regions (13, 14) are electrically isolated from the substrate by lateral oxidation through the silicon walls exposed during the preceding etch step, thereby providing silicon islands (19, 21). The capped regions, however, remain substantially unaffected during the oxidation. With the electrically isolated silicon islands (19, 21) in place, a silicon dioxide layer (24) and a planarizing polymer layer (28) are deposited over the wafer. Processing is concluded with a pair of etching operations, the first removing polymer (28) and silicon dioxide (24) at substantially identical rates, and the second removing silicon dioxide (24) and monocrystalline silicon at substantially identical rates.

Description: PROCESS FOR FABRICATING SEMICONDUCTOR STRUCTURES Technical Field This invention relates to a process for fabricating silicon-on-insulator structures on a semiconductor wafer, including the steps of: etching a silicon substrate to form regions of silicon; and capping the tops and sides of the silicon regions with silicon etchant masks. The invention also relates to a process for planarizing the surface of a semiconductor wafer having silicon islands. B c roun. Art Processes suitable to form silicon-on- insulator (SOI) structures have been the subject of extensive investigation in recent years. The same situation exists for planarization techniques suitable to eliminate or substantially reduce the step coverage problems attributable to recessed isolation or multi¬ level structures in semiconductor chips. The ap- proaches derived have, however, had limited general acceptance by virtue of their cost, the additional process complexity, or their degrading effect on active device performance. U.S. Patent No. 4,361,600 discloses a process for forming electrically isolated silicon islands on a semiconductor wafer. This known process includes forming a silicon mesa structure supported on silicon and oxidizing the mesa structure with the surfaces of the mesa completely covered with silicon nitride. Though the known process provides a method of forming electrically isolated islands of monocrys¬ talline silicon, the process is constrained by the lateral oxidation characteristics of silicon structure employed to applications involving relatively narrow island dimensions and relatively wide island-to-island OMPI separations. Thus, in the known process, when the silicon substrate is oxidized the silicon dioxide must first be grown downward before it can progress later¬ ally underneath the mesa. The specialized processing required to effectively diffuse oxygen this additional distance is an admitted deficiency of the known proc¬ ess. U.S. Patent No. 4,407,851 discloses a method of forming a flat field region in a semiconductor substrate, including the steps of forming a recess in the substrate, forming a CVD oxide layer over the entire surface, selectively removing the oxide layer to form a V-shaped groove between the side of the recess and the first insulating layer, and filling the V-shaped groove with a second CVD oxide layer, fol¬ lowed by the deposition of a resist film. The entire surface of the structure is then etched by reactive ion etching. The etching rates of the resist film and the second CVD oxide layer can be made to be substan- tially the same. Disclosure of the Invention According to a first aspect of the present invention, there is provided a process for fabricating silicon-on-insulator structures on a semiconductor wafer, including the steps of: etching a silicon substrate to form regions of silicon; and capping the tops and sides of the silicon regions with silicon etchant masks, characterized by the steps of: etching the silicon substrate between the silicon regions to increase the relative height of the regions; oxidizing exposed silicon until the capped silicon is electri¬ cally isolated from the silicon substrate to form silicon islands; depositing a dielectric layer to a thickness greater than the height of the islands; forming a planarized polymer layer over the dielectric layer; simultaneously etching the polymer and dielec- OMPI tric layers to remove polymer and dielectric material at substantially equal rates until the polymer layer is absent; and simultaneously etching the dielectric layer and the electrically isolated silicon to remove dielectric material and silicon at substantially equal rates. It will be appreciated that a process according to the first aspect of the invention allevi¬ ates the dimensional restriction problem of the proc- ess of U.S. Patent No. 4,361,600 while providing the coplanar wafer topology commonly sought. The benefits are attained by etching the monocrystalline silicon substrate at an appropriate stage in the process to effectively elevate the capped island region of sili- con before oxidizing the exposed silicon. To reduce the topological excursions of the mesa-shaped struc¬ ture formed in the manner of the present process, the wafer is also subjected to a novel sequence of planar- ization steps in the course of leveling the active surface for conventional fabrication of active devices and electrical interconnects. In this way, the uneven topology produced by the earlier steps of fabrication is fully planarized during the latter steps in the process. These process features produce structures having excellent line width control between the island perimeters as defined by the fabrication masks and the silicon island perimeters actually .fo med. This result is attributable to the absence of any oxidation at the upper surfaces of the islands. As a further consideration, it should be understood and appreciated that a coplanar relation¬ ship between the silicon islands and the surrounding dielectric is particularly important with the increas- ing utilization of anisotropic etching to form the gate electrodes of semiconductor devices, most common¬ ly field effect transistors. If the silicon islands OMPI are allowed to protrude above the surrounding dielec¬ tric, an anisotropic etch of the gate electrode mate¬ rial will leave narrow filaments of the gate electrode material at the perimeters of the islands. Any such electrode material residuals create electrical short¬ ing paths at the perimeters of the islands. According to a further aspect of the invention, there is provided a process for fabricating silicon-on-insulator structures on a semiconductor wafer, including the steps of: etching a silicon substrate to form regions of silicon and capping the tops and sides of the silicon regions with silicon etchant masks, characterized by the steps of etching the silicon substrate between the silicon regions to increase the relative height of the silicon regions; and oxidizing exposed silicon until the capped silicon is electrically isolated from the silicon substrate to form silicon islands. According to yet another aspect of the invention, there is provided a process for planarizing the surface of semiconductor wafer having silicon islands, characterized by the steps of: depositing a dielectric layer to a thickness greater than the height of the islands; forming a planarized polymer layer over the dielectric layer; simultaneously etch¬ ing the polymer and dielectric layers to remove poly¬ mer and dielectric material at substantially equal rates until the polymer is absent; and simultaneously etching the dielectric layer and the silicon islands to remove dielectric material and silicon at substan¬ tially equal rates. When used to fabricate integrated circuits from monocrystalline silicon substrates the embodied process provides the capability of forming silicon islands which have precisely defined perimeter lines and are situated in close proximity to each other, without degrading the original quality of the crystal¬ line silicon substrate material. OMPI In one form of its practice, the invention commences with an anisotropic etch of a photoresist masked wafer having a monocrystalline silicon sub¬ strate covered in ascending order with a PAD silicon dioxide (PAD oxide) layer, a silicon nitride (nitride) layer, and a thicker oxide layer. The etching is continued until island-like structures of substrate silicon are formed. Thereafter, a brief oxidation is followed with a nitride layer deposition and an aniso- tropic nitride etch operation. Upon the conclusion of the nitride etch, the silicon islands are capped using nitride on the walls and a nitride/oxide composite layer on the top plateau. During the next stage of fabrication, the capped silicon islands are subjected to another aniso¬ tropic silicon etch, suitable to remove further sub¬ strate and effectively elevate the relative height of the capped regions. The thermal oxidation step which follows is continued until the laterally exposed silicon regions beneath the now-elevated capped is¬ lands are completely oxidized, to form dielectrically isolated islands of monocrystalline silicon over dielectric. The nitride cap protects the silicon islands from the oxidation ambient. The nitride and oxide capping materials are then stripped to expose the monocrystalline silicon islands for further pro¬ cessing. The island structures formed during these initial steps of fabrication are then covered with a layer of chemical vapor deposited oxide to a thickness greater than the level of the island plateaus. The oxide deposition is followed with the deposition of one or more polymer layers to planarize the wafer topology. The preferred polymer is characterized by a low viscosity and a low surface tension when in the molten state. The polymer coated wafer is then subjected to a first etch sequence, during which the etchant composition is adjusted to maintain an equal etch rate for the polymer and immediately underlying oxide. Thereby, the undulations in the oxide are removed concurrently with the polymer layer. At the conclu¬ sion of the polymer/oxide etch sequence, the silicon islands remain masked with thin layers of oxide. To remove the oxide and expose the silicon island sur- face, another etch is undertaken, this time performed with etchants which remove oxide and silicon at sub¬ stantially the same rate. When all the oxide over the silicon islands has been removed, etching is terminat¬ ed. As presently embodied, the completed structure is comprised of high quality monocrystalline silicon islands isolated by, and lying coplanar with, surrounding regions of dielectric material. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic cross-section of the substrate and covering layers masked by patterned photoresist to define the locations of the islands. Fig. 2 is a schematic cross-section after the initial etching of the island structure and a nitride deposition. Fig. 3 illustrates the shape after an anisotropic etch of the nitride. Fig. 4 shows the region after anisotropic etching of the substrate silicon. Fig. 5 shows the effects of thermal oxidation. Fig. 6 illustrates the shape of the structure after the deposition of planarizing oxide and polymer. Fig. 7 shows the effects of a polymer/oxide etch. Fig. 8 depicts the final structure, at the conclusion of the oxide/silicon etch. Best Mode for Carrying Out the Invention A review of the drawings illustrating an embodiment of the present invention will disclose that the composite process consists of two generally dis¬ tinguishable sequences. The first sequence, encom- passing Figs. 1-5, forms the silicon islands, while the second sequence, encompassing Figs. 6-8, planar- izes the islands and surrounding dielectric. Though each segment incorporates novel features in its own right, the unique combination provides a composite process in which the latter steps complement the structural effects of the earlier steps. To more fully appreciate the various facets of the process, consider the preferred embodiment as described herein¬ after. The schematic cross-section depicted in Fig. 1 shows monocrystalline silicon (silicon) substrate 1 covered with a nominal 30 nanometers of thermally grown PAD oxide layer 2, and a nominal 150 nanometers nitride layer 3. Any commonly employed process may be utilized to form layers 2 and 3. Nitride layer 3 is further covered with a nominal 200 nanometer thick layer of plasma enhanced chemical vapor deposited.- (CVD) oxide 4. Oxide layer 4 will serve as a shield during the subsequent application of reactive ion etching (RIE) to remove nitride and silicon. A photoresist (PR) is then deposited and photolithographically processed to retain PR segments 6 and 7, as shown in Fig. 1. Each PR segment defines a location of the island to be formed beneath. The nominal width of each PR segment preferably lies between one and two micrometers, whereas, the separa- tion between PR segments 6 and 7 is preferably at least one micrometer. With the photoresist mask in place, the wafer is subjected to RIE 8, for removing oxide 4, nitride 3, oxide 2 and silicon 1, until silicon 1 has been etched a nominal one micrometer in depth. This etch can be performed in an Applied Materials AM-8110 RIE reactor. The nominal etching conditions are presented in Table A for the oxide and nitride layers, TABLE A Gas and Flow: CHF3 at 75 cm 3 /min + O2 at 5 cm^/min. Pressure: 65 mTorr. Power: 1,200 Watts. Standard SJLΩZ. --iaJs± Etch rate 45 nm/min 40 nm/min 7 nm/min The etching of silicon 1 with RIE 8 utilizes the conditions prescribed in Table B. TABLE B Gas and Flow: NF3 at 20 cm^/min. Pressure: 40 mTorr. Power: 450 Watts. Si Standard PR Etch rate 40 nm/min 15 nm/min In progressing to the structure depicted in Fig. 2, the wafer is next thermally oxidized to grow a nominal 30 nanometers of new PAD oxide 9 on any ex- posed silicon 1. Thereafter, a nominal 150 nanometers of low pressure CVD nitride 11 is formed on the semi¬ conductor wafer. The nitride layer will protect the sidewalls of the silicon island from direct thermal oxidation. The concluding structure has the general appearance of the composite in Fig. 2. The succeeding step in the process is com¬ prised of RIE 12, to anisotropically remove at least 150 nanometers of nitride and 30 nanometers of PAD oxide without exposing the top or sidewalls of the silicon islands at locations 13 and 14. This etch can be performed in the same reactor under the conditions recited in Table C. TABLE C Gas and Flow: NF3 at 20 cm^/min. Pressure: 40 mTorr. Power: 900 Watts. Etch Rate: 70-80 nanometers per minute for Si3N4» The structure produced is depicted in Fig. 3. Note that nitride sidewalls 11 remain substan¬ tially intact because of the anisotropic etch charac¬ teristics. Notwithstanding the former practice in the art, the present process now prescribes the applica- tion of another RIE, 16, to the structure in Fig. 3. As presently embodied, anisotropic etching of sub¬ strate silicon 1 is continued until an additional layer of silicon, 1-2 micrometers thick, is removed. Note that the depth of the additional etch is propor- tional to the width of the islands established earlier by PR masks 6 and 7. RIE 16 is preferably performed with a Drytek DRIE-100 reactor according to the con¬ ditions prescribed in Table D. OMPI TABLE p Gas and Flow: CF3CI at 75 cm3/min. Pressure: 0.2 Torr. Power: 700 Watts. Etch Rate 50 nm/min 5 nm/min As shown in Fig. 4, the silicon islands at 13 and 14 are not only capped with oxide, nitride and PAD oxide layers 2, 3, 4, 9 and 11, but are elevated from silicon substrate surface 17 to expose lateral walls 18 of underlying silicon to subsequent process ambients. The structure depicted in Fig. 4 is there¬ after subjected to a thermal oxidation step. The oxidation replaces selected regions of substrate silicon with oxide in the course of forming elec¬ trically isolated islands of monocrystalline silicon. The outcome of the preferred process is illustrated in Fig. 5, where silicon islands 19 and 21 are formed and isolated from substrate silicon 1 by oxide 22. Though the oxidation process consumes some of the lower island silicon, and slightly lifts the lower edges of the nitride caps, the silicon at the top of each island remains intact. In this manner, the upper surface perimeter of each island remains in precise correspondence to the pattern established in masking nitride 2. Dashed lines 23 depict the general outline of the structure before oxidation. The voids in the silicon substrate are filled during the oxidation by the 2:1 volumetric increase in material when mono¬ crystalline silicon is converted to silicon dioxide. Note that the oxidation also penetrates downward into substrate 1. The magnitude of the oxidation beneath OMPI each island ensures not only consistent and reliable dielectric isolation of islands 19 and 21 but also provides adequate dimensional separation to completely eliminate electric field coupling between substrate 1 and islands 19 and 21. An ambient suitable to perform the thermal oxidation prescribed above involves an O2 plus H2O (gas) environment at 950-1100 degrees C for 10-20 hours. With the islands formed, and separating dielectric in place, the wafer is subjected to an oxide and nitride strip operation. Preferably, the strip operation is comprised of a deglaze with buf¬ fered HF to remove layer 4 and an etch with hot H3PO4 to remove residuals of nitride capping layers 3 and 11. Planarization of the mesa structure follows. Initially a plasma enhanced CVD oxide 24, as shown in Fig. 6, is applied to form a layer 120 nanometers thick over oxide 22 and monocrystalline silicon is¬ lands 19 and 21. The important process constraint at this stage of the fabrication is to ensure that the thickness of deposited oxide 24 reaches level 26, which is nominally at least 10 nanometers above pla- teau level 27 of silicon islands 19 and 21. It should be understood that the magnitude of this difference in levels is influenced by considerations such as the etching uniformity, the planarization efficacy, and the uniformity in the thickness of oxide 24. The impact of these factors will be more fully appreciated upon considering the concluding steps in the composite process. With the deposited oxide 24 in place, pla¬ narization continues with the application of a low molecular v/eight, low surface tension polymer solu¬ tion, such as poly ethylmethacrylate or polystyrene, to the wafer surface. This may be accomplished spin coating the wafer with a solution of the com¬ position. To ensure that the polymer reflows and planarizes during baking, the polymer composition and temperature should be selected so that the molten polymer has both a viscosity below 5 centistokes and a relatively low surface tension. Coating and baking steps may be repeated until the desired degree of surface planarization is achieved. In Fig. 6, the polymer composition is shown at 28. The polymer planarized wafer is then sub¬ jected to a pair of etching steps, steps suitable to expose silicon islands 19 and 21 while maintaining a uniform surface plane with the surrounding dielectric. The wafer is first subjected to RIE 29. The etch parameters are chosen to remove polymer 28 and oxide 24 at a substantially equal rate. Thereby, when the etch descends to a level between 26 and 27 the wafer surface is substantially planar and yet absent of all polymer. The etch process may be monitored for poly- mer residuals to establish an end point for the first segment of the etch sequence. A suitable set of conditions for performing this etch includes the use of an Applied Materials AM-8110 reactor with the parameters appearing in Table E. TABLE E Gas and Flow: CHF3 at 30-70 cm 3 /min + O2 at 30-70 cm^/min. Pressure: 60 mTorr. Power: 1200 Watts. It will no doubt be recognized that the exact parameters are intimately related to the chemi¬ cal composition of the selected polymer and, there¬ fore, can only be established by particularized tests. Nonetheless, the underlying objectives remain intact. OMPI Namely, to adjust the reaction rates of θ and CHF3 with polymer and Siθ2 to remove both materials from the wafer surface at substantially equal rates. At the conclusion of RIE 29 the surface appears as shown in Fig. 7. The purpose for the oxide between levels 26 and 27 in Fig. 6 is now apparent. Thin oxide layers 31 and 32, over respective silicon islands 19 and 21, allow sufficient overetching of oxide to ensure the complete removal of polymer 28 without subjecting the silicon itself to the etching of RIE 29. Furthermore, residuals of oxide layers 31 and 32 are excellent protective masks for any in-process handling and storage of the wafers. Fabrication of the islands is concluded with RIE 33. In this etch, the parameters are selected to ensure that oxide 24 and silicon 19 and 21 are etched at substantially identical rates. A suitable set of conditions for the above-noted AM-8110 reactor are presented in Table F. TABLE F Gas and Flowr CHF3 at 75 cm 3 /min + O2 at 40-50 cm 3 / in. Pressure: 65 mTorr. Power: 1200 Watts. Upon completing the etch of the wafer in Fig. 7 to remove oxides 31 and 32, the structure appears as depicted in Fig. 8. Note that surfaces 34 and 36 are coplanar with surrounding oxide regions 24, that monocrystalline silicon islands 19 and 21 are electrically isolated from the substrate silicon by oxide 22, and that islands 19 and 21 retain the orig¬ inal crystalline quality of the substrate silicon. It should also be appreciated that silicon islands 19 and 20 have precisely defined perimeter lines, are rela- OMPI tively wide in dimension, and lie in close proximity to each other. These latter features provide for efficient utilization of chip area. In considering the process as a whole, note that the structural features enumerated above are achieved with a single masking operation and a se¬ quence of steps which are relatively insensitive to process variables. In that regard, critical fabrica¬ tion steps are expressly avoided. It should be understood and appreciated that the operations performed in the sequence of steps depicted from Fig. 1 to Fig. 5 may conclude with the stripping of the silicon cap shown in Fig. 5. Though the wafer topology would not be planarized, active devices can be created in the silicon islands as formed. A similar independence can be ascribed to the sequence of operations depicted in Figs. 6-8. Therefore, the planarization steps are applicable to other topological structures. For instance, it is equally possible that the region designated 22 in Figs. 6-8 be monocrystalline silicon, resulting in a topology of silicon regions separated by dielectri- cally filled grooves or recesses. The presently disclosed planarization segment of the process is fully applicable to the fabrication of such struc¬ tures.

Claims: CLAIMS : 1. A process for fabricating silicon-on- insulator structures on a semiconductor wafer, includ¬ ing the steps of: etching a silicon substrate to form regions (13, 14) of silicon and capping the tops and sides of the silicon regions (13, 14) with silicon etchant masks, characterized by the steps of etching the silicon substrate between the silicon regions (13, 14) to increase the relative height of the regions (13, 14); oxidizing exposed silicon until the capped silicon is electrically isolated from the silicon substrate to form silicon islands (19, 21) ; depositing a dielectric layer (24) to a thickness greater than the height of the islands (19, 21) ; forming a planar¬ ized polymer layer (28) over the dielectric layer; simultaneously etching the polymer (28) and dielectric (24) layers to remove polymer and dielectric material at substantially equal rates until the polymer layer (28) is absent; and simultaneously etching the dielec¬ tric layer (24) and the electrically isolated silicon to remove dielectric material and silicon at substan¬ tially equal rates. 2. A process according to claim 1, characterized in that said steps of etching the sili¬ con substrate between the silicon regions (13, 14) to increase the relative height of the silicon regions (13, 14) includes an anisotropic etch to a depth proportional to the width of the silicon regions (13, 14). 3. A process according to claim 2, characterized in that said step of forming a planar¬ ized polymer layer (28) over the dielectric layer (24) includes depositing and reflowing polystyrene or polymethylmethacrylate. OMPI 4. A process according to claim 3, characterized in that said step of depositing a die¬ lectric layer (24) includes a plasma enhanced chemical vapor deposition of silicon dioxide. 5. A process according to claim 4, characterized in that said step of depositing a die¬ lectric layer (24) is preceded by the step of strip¬ ping the capping material from the silicon islands (19, 21). 6. A process according to claim 5, characterized in that said step of oxidizing exposed silicon until the capped silicon is electrically isolated from the silicon substrate includes thermal oxidation to grow silicon dioxide to the level of the capped silicon. 7. A process according to claim 6, characterized in that said step of capping the tops and sides of the silicon regions (13, 14) includes: forming a silicon nitride layer (11) over silicon regions (13, 14) already having PAD oxide (2), silicon nitride (3) and silicon dioxide (4) on the tops there¬ of; and anisotropically etching the silicon nitride layer (11) to expose the silicon dioxide (4) on the tops of the regions (13, 14) and the silicon substrate (1) between the silicon regions (13, 14). 8. A process for fabricating silicon-on- insulator structures on a semiconductor wafer, includ¬ ing the steps of: etching a silicon substrate to form regions (13, 14) of silicon; and capping the tops and sides of the silicon regions (13, 14) with silicon etchant masks, characterized by the steps of etching the silicon substrate between the silicon regions (13, 14) to increase the relative height of the silicon regions (13, 14); and oxidizing exposed silicon until the capped silicon is electrically isolated from the silicon substrate to form silicon islands (19, 21) . 9. A process for planarizing the surface of a semiconductor wafer having silicon islands (19, 21), characterized by the steps of: depositing a dielectric layer (24) to a thickness greater than the height of the islands (19, 21); forming a planarized polymer layer (28) over the dielectric layer (24) ; simultaneously etching the polymer (28) and dielectric (24) layers to remove polymer and dielectric material at substantially equal rates until the polymer layer is absent; and simultaneously etching the dielectric layer (24) and the silicon islands (19, 21) to remove dielectric material and silicon at substantially equal rates. OMPI

Inventors: SOBCZAK ZBIGNIEW PAUL, SOBCZAK, ZBIGNIEW, PAUL
Applicants: NCR CO, NCR CORPORATION
Application year: 1984
Main classification: H01L21
IPC codes: H01L21