METHOD OF SELECTIVELY EXPOSING THE SIDEWALLS OF A TRENCH AND ITS USE TO THE FORMING OF A METAL SILICIDE SUBSTRATE CONTACT FOR DIELECTRIC FILLED DEEP TRENCH ISOLATED DEVICES

Abstract: Deep trenches (14, 15) are formed according to the desired pattern through the N epitaxial layer (13) and N⁺ subcollector region (12) into the P⁻ substrate (11) of a silicon structure (10). Where a substrate contact is needed, the trenches delineate a central stud (16) or mesa of silicon material. Channel stop regions (18) are formed e.g. by ion implantation of boron atoms at the bottom of trenches. SiO₂ and Si₃N₄ layers (17, 19) are then deposited on the whole structure. A substrate contact mask is applied end patterned to selectively expose one side of the trench sidewalls, the bottom of the trenches adjacent thereto and others areas if desired such as the top surface of the stud. The composite SiO₂/Si₃N₄ layer is then etched to leave exposed only the sidewalls of the stud, et least partially the bottom of the trenches adjacent thereto and the top surface of the stud. Platinum is deposited preferably via sputter deposition, conformally coating all regions of the structure. After sintering, the unreacted platinum is removed using wet chemical etch (aqua regia). Platinum silicide is left in all opened contacts and on the stud sidewalls where its defines a metal silicide lining (25) or cap, covering the stud. This lining connects the top surface (25a) of the stud, with the channel stop implanted regions (18) and thence forms the desired substrate contact.

Description: The present invention relates to a method for producing semiconductor devices and more particularly to a method for selectively exposing the sidewalls of a trench and its use in the formation of a metal silicide substrate contact for high density dielectric filled, deep trench isolated semiconductor devices. A reverse bias is commonly applied to the substrate in a semiconductor integrated chip for isolation purposes. In this case, the potential of the substrate is maintained at an appropriate value, so that the P-N junction of the isolation region and the substrate becomes reverse biased. Conventional methods for forming a substrate contact for a semiconductor device are well known, such as a method for forming an electrode at the backside of the substrate to which is applied an external bias. With the development of large scale integration, thick and low doped substrates are widely used. Silicon wafers have lightly doped substrates in order to significantly reduce collector-substrate capacitances, which, in turn, increase the operating frequencies of the devices. Unfortunately, when the impurity concentration of the substrate is low, the electrical path between the electrode and the substrate becomes highly resistive, resulting in active devices, such as PNP lateral transistors having poor characteristics. This problem has been eliminated by contacting the substrate, through the N type epitaxial layer, with a highly doped P region. An example of this technique, in combination with standard recessed oxide isolation, may be found in European Patent application bearing the publication number 29552 assigned to VLSI Technology Research Association. More recent trends in micro-electronics capitalize on a particular type of dielectric isolation where patterns of dielectrically filled trenches, extending from the surface of the integrated circuit to the interior thereof are used to isolate the devices (e.g. one bipolar transistor from another bipolar transistor) or portions (e.g. the base region from the collector region in a bipolar transistor) of the devices. A method of forming a substrate contact for a trench structure is taught in an article of the IBM Technical Disclosure Bulletin, vol. 25 No. 12, May 1983, pages 6611 through 6614, authored by S.A. Abbas et al. This article describes the formation of a polysilicon buried conductor for reach through to the P substrate for ohmic electrical contact thereto. Once the trenches and channel stoppers have been formed, and trenches protected with an insulating layer, the bottom of the trench, where a contact is needed, is opened to expose the channel stopper. A thin polysilicon layer is CVD deposited, covering the sidewalls in totality and providing a contact with the substrate. It is obvious that with this technique, the P⁺ substrate contact reach through is highly resistive, and not suitable for advanced bipolar devices. Also, a parasitic polysilicon-oxide-silicon capacitor is created which will degrade the circuit performance. A variant is described in US Patent No. 4256514 assigned to the assignee of the present invention. According to this patent, a doped, polysilicon stud is used to provide the electrical contact with the substrate. However, although doped polysilicon still remains a relatively resistive material, and this solution is not completely satisfactory, because it implies the use of wide trenches, completely filled with polysilicon to reduce the resistance of the stud, which in turn results in lower integration density. In addition, some other problems may result from this solution, due to, firstly different thermal coefficients between polysilicon and silicon, which may lead to undesired cracks in the filled trenches and, secondly, the resulting structure has very poor planarity. Planarity is a must when multilevel metallurgy (e.g. 4) is needed to assure the fan-in, and the fan-out functions of highly integrated circuits. An other alternate solution is described in "Plasma oxide filled deep dielectric isolation", J.S. Basi et al, IBM Technical Disclosure Bulletin, vol. 25 No. 8, Jan. 1983 pages 4405 and 4406. This article describes a method for forming a buried conductor for reach-through to the P⁻ silicon substrate of a chip from the surface of an epitaxial layer. The substrate contact is made by shorting the subcollector to the P channel stopper, at the bottom of the trenches and then to the top metallurgy via the collector reach through diffusion. The role of the metal is therefore to short the N⁻ epitaxy - P substrate/P⁺ channel stopper junction. In this reference (and in the aforementioned U.S. Patent 4,256,514 as well) , a part of the electrical path, between the contact and the substrate is made of highly doped silicon material. Therefore, this solution cannot also be considered as fully satisfactory. It is very important to remark that according to the teachings of this article, an isotropic etch is performed to expose all the sidewalls of a trench. Therefore, all devices adjacent to that trench, will have their N⁻ epitaxy - P⁻ substrate junction shorted by the Pt-Si contact. In others words, all these devices will be only devoted to provide substrate contact and therefore will be lost for logical implementation. This results in a non negligible waste of the silicon surface. It is, therefore, a primary object of the invention to provide a method of selectively exposing the sidewalls of the trenches, to allow selective processing (etching, metal deposition...) of the sidewalls which in turn will result in a significant saving of silicon room and therefore will allow a denser design than those obtained in the prior art. It is another object of the invention to provide a method of selectively exposing the sidewalls of trenches and its use to the forming of a substrate contact for polyimide filled deep trench isolated devices compatible with very narrow trenches therefore considerably increasing integration density. It is another object of the invention to provide a method of forming a substrate contact for high density polyimide filled deep trench isolated devices, comprised of a metal lining having very low resistance characteristics. It is still another object of the invention to provide a method of forming a substrate contact for high density polyimide filled deep trench isolated devices which significantly improves the planarity of the structure. In accordance with the most general concept of the present invention as defined in the claims, it is proposed thereafter, a broad method for selectively exposing the sidewalls of a trench. In others words, as a result, on sidewall will be exposed for further processing while the other will remain passivated. In accordance with a more specific aspect of the present invention, said method will be described thereafter in connection with the forming of a platinum silicide lining by sputter deposition of platinum, on only a part of the sidewall and on the bottom of an isolation trench, for making contact from the surface of the epitaxial layer to the buried substrate. Deep trenches according to the desired pattern are opened through the N epitaxial layer and N⁺ subcollector region into the P⁻ substrate of a silicon structure. Where a substrate contact is to be formed, the trenches delineate a central stud or mesa of silicon material. It is generally highly desirable that channel stop regions be formed, e.g. by ion implantation of boron atoms, at the bottom of trenches. SiO₂ and Si₃N₄ layers are then deposited on the whole structure. A substrate contact mask is applied and patterned to selectively expose the sidewalls, the trenches which correspond to the stud, the bottom of the trenches adjacent thereto, and the top surface of the stud. The composite SiO₂/Si₃N₄ layer is etched to leave exposed said sidewalls of the stud, the bottom of the trenches adjacent thereto, and the top surface of the stud. Platinum is deposited preferably via sputter deposition, conformally coating all regions of the wafer. After sintering, the platinum in excess is removed using wet chemical etch (aqua regia). Platinum silicide is left in all opened contacts (bottom of the trenches and top surface of the stud) and on the trench sidewalls where it defines a metal silicide lining or cap, covering at least partly the stud. This lining connects the top surface of the stud with the channel stop implanted regions and thence forms the desired substrate contact. According to standard techniques, polyimide is then applied and appropriately etched to fill the trenches. It is an important point of the invention that the isolation trench provides two functions : first, the classic isolation of the device pocket containing the device (e.g. a NPN transistor) and second, through a continuous low resistive thin metal lining, an access to the buried substrate from the surface. In the accompanying drawings forming a material part of the disclosure : FIGS 1-9 illustrate cross sectional view of an integrated structure undergoing fabrication according to the method of the present invention. The present invention will be explained with reference to the attached figures. In the following, the present invention which basically consist in a method of selectively exposing the sidewalls of a trench will be described in detail with reference to a method of forming a metal silicide substrate contact for high density dielectric filled deep trench isolated bipolar semiconductor devices. However, it should be understood that various other embodiments or uses may be set forth. Lastly, it is to be understood that while various dopant, coating and/or masking layers, processing conditions and the like are disclosed herein, the present invention is not limited thereto. The usual bipolar semiconductor process is followed through the formation of trenches and passivation thereof with a composite Si₃N₄/SiO₂ layer. With reference to FIG.1, the initial structure 10 is comprised of a P⁻ silicon substrate 11, having a blanket N⁺ subcollector 12 and N⁻ epitaxial layer 13 thereon. Deep narrow trenches 14 and 15 have been formed by etching through the N⁻ epitaxial layer 13 and the N⁺ subcollector 12, into the P⁻ substrate 11. Many well known etching techniques may be employed for the trench formation, such as illustrated in US patent 4,381,953 to Ho et al. Trenches are about 6 micrometre deep and 2 micrometre wide. The lay out design of the trenches it such that it delineates a square shaped central stud 16 or mesa between trenches 14 and 15, (the other trenches in the orthogonal direction are not represented). Following trench formation, silicon dioxide is regrown by thermal oxidation, to provide a SiO₂ layer 17 of a thickness in the range of 50 to 300 nm. Then, a standard ion implantation is conducted and boron is introduced at the bottom of the trenches to create channel stoppers 18. An additional layer 19 of Si₃N₄ is CVD deposited of the order of 20 to 150 nm thick onto the whole structure. Of course, active devices such as bipolar transistors and/or passive devices have been previously formed in the different isolated epitaxial pockets such as 13a and 13b, but have not been represented for sake of clarity. However, no device have been formed in the epitaxial pocket 13c corresponding to the central stud (16). As will be further apparent to the man skilled in the art, many more deep and narrow trenches forming an isolation pattern have been formed simultaneously with trenches 14 and 15 at the surface of the structure for isolation purposes. It is clear that the structure as shown in FIG.1 results only from conventional manufacturing techniques. Now, with reference to FIG.2, an organic underlay material such as polyimide (2.0-4.0 µm thick) is first applied onto the Si₃N₄ layer 19 by spin coating to form layer 20. An inorganic barrier layer 21, for example SiO x , with a thickness in the 100 to 300 nm range is subsequently deposited in a CVD plasma. Polyimide is preferred over photoresist because it fills trenches without voids, is optically transparent, therefore allowing precise mask alignment, and lastly is thermally stable during deposition of the barrier layer. This step is followed by the application of a conventional photoresist such AZ 1350J (a trade name of Shipley Company), by spin coating. The photoresist layer 22, is exposed according to a desired pattern to ultraviolet light through a mask (not represented) and subsequently developed, in a conventional manner. The imaging layer 22 of photoresist is typically .7-1.5 µm thick to optimize exposed image quality. It is important in our particular embodiment that the opening 23 in the imaging photoresist layer 22 be wider than the square shaped stud as apparent from FIG.2. This will assure subsequently that the desired portions of the stud sidewalls, the bottom of the trenches adjacent thereto and the top surface of the stud be exposed. The steps described with reference to FIGS. 3 to 5 following, are typically performed in-situ in a parallel plate reactive ion etch tool. Etching of the exposed area of the inorganic barrier layer 21 is accomplished using CF₄. Over-etch of 50-100% of the barrier layer is used to assure complete removal in the exposed regions. Typical etch parameters are : Power density 0.24 watts/cm² Flow (CF₄) 30 Scm³/min. (S stands for Standard) Pressure 5.3 Pa (40 µm) Etch rate SiO x 45 nm/min. Etch rate Polymide 32.5 nm/min. The resulting structure is shown in FIG.3. The removed portion of the inorganic barrier layer correspond to opening 23 of the imaging photoresist layer 22. The reactive ion etch of the organic underlay 20 is accomplished in an O₂ ambient in preferably a two step process for reasons explained thereafter. First the under lay material is etched to the bottom of the trench under complete anisotropic conditions as follows : Power density 0.17 watts/cm² Pressure 0.4 Pa (3 µm) Flow O₂ 100 Scm³/min. Etch rate SiO x .7 nm/min. Etch rate Polymide 75 nm/min. Selectivity of N 100 : 1 is achieved which permits full removal of 8-10 µm of polyimide with minor erosion (N 100 nm) of the SiO x barrier layer 21. The second step of the process aims to perform a sidewall clean, to remove retained under lay material where trench sidewalls have negative slopes. Total layer removal is 100 - 300 nm thick. Etching conditions read as follows : Power density 0.23 watts/cm² Pressure 6.7 Pa (50 µm) Flow O₂ 30 Scm³/min. Etch rate Polymide 200 nm/min. The under-cut of the barrier layer 21 which defines now opening 23, is the result of the sidewall clean step. The remaining portions of the resist layer 22 are eliminated, when the exposed organic underlay layer is RIE etched in the O₂ plasma, leaving unprotected only the central stud 16 and a significant portion of the bottom of the trenches, which is contiguous to the sidewalls of said central stud. As previously mentioned, etching of the exposed portions of the Si₃N₄ layer 19 is performed in-situ following the barrier and underlay etch steps. The etch process is "semi isotropic" when etched in a parallel plate reactive ion etch tool as follows : Power density 0.23 watts/cm² Pressure 26.7 Pa (200 µm) Flow (CF₄+10% O₂) 150 Scm³/min. Etch rate Si₃N₄ 70 nm/min. Etch rate SiO₂ 40 nm/min. Etch rate Polymide 45 nm/min. This process step is particularly important. Fully isotropic Si₃N₄ etch processes are not workable in this structure because mechanical stresses in the organic underlay result in separation of the underlay from the Si₃N₄ trench liner. Isotropic processes result in etch extensions beyond the defined image in the underlay. The underlying thermal SiO₂ layer serves as an etch stop to prevent attack of the silicon. The resulting structure is shown in FIG.5. Now turning to FIG.6, the barrier layer 21 of SiO x is stripped in a solution of 10 : 1 H₂O : HF which removes SiO X N 5x faster than thermally grown SiO₂ constituting layer 17. The underlay 20 is then stripped using conventional O₂ ash processes in a barrel type plasma etcher. The exposed portions of the thermal SiO₂ layer 17 are finally etched using a buffer HF solution while the remaining portions of the Si₃N₄ over-coat layer 19 serve as a mask for this step. This completes the sidewall imaging process which selectively exposes the sidewalls and bottoms of trenches 14 and 15 encompassing the central stud. As a result, only the central stud sidewalls, the portions of the bottom of the trenches which are adjacent to it and the top surface of the stud, are now exposed and ready for further processing. A thin layer 24 of an appropriate contact metal is blanket deposited on the whole structure by sputter deposition. A silicide forming metal selected in the group comprised of platinum, palladium and the like, is selected. The preferred metal with a thickness in the range of 30-100 nm is platinum. Sputter deposition is the preferred method because unlike others techniques such vacuum evaporation, the metal is deposited on both horizontal and vertical surfaces of the structure. In others words, metal will coat conformally all the exposed regions of the structure. FIG.7 illustrates the structure at this stage. The structure is then heated at about 550°C to produce a platinum silicide layer 25 by sintering wherever the platinum layer 24 contacts silicon. Unreacted platinum is removed by wet etching (in aqua regia). FIG.8 shows the result of such a process wherein a platinum silicide contact is formed with the channel stoppers 18 at the bottom of the trenches, and with all the horizontal and vertical surfaces of the stud, forming a metallic lining or cap 25 which completely covers the silicon stud 16 and providing a buried ohmic contact with the P⁻ substrate 11 through channel stoppers 18. Polymide material 26 is spin applied to fill the trenches. It is baked out at 400°C and reactive ion etched in an O₂ plasma as seen in FIG.9. The horizontal portion of the Pt-Si layer which remains exposed (see numeral reference 25a) will be used as the substrate contact for the subsequent metallization steps. Electrical contacts are then made to the elements such as NPN transistors, diodes, resistors... in addition to that substrate contact by conventional deposition, lithography and etching techniques.

Claims: Process of selectively exposing the desired portions of a passivated trench (14,15) formed in a semiconductor structure (10) comprising the steps of : depositing an underlay material to form an underlay layer (20) to coat the structure and fill the trench; depositing a barrier layer (21) above said underlay layer; patterning said barrier layer (21) to form at least one opening which overlaps only partially the structure and the trench area, thereby delineating the desired portions of the sidewalls, of the bottom of the trench and of the top surface of the structure, adjacent thereto, to be exposed; anisotropically etching the underlay material which is exposed through said opening, thereby selectively exposing said desired portions for further processing. The process of claim 1 wherein said underlay material is an organic material. The process of claim 2 wherein said barrier layer (21) is formed by an inorganic material such as SiO x . The process of claim 3 wherein said step of patterning includes a conventional photolithographic step with an overetching of the barrier layer to ensure its complete removal in the exposed region. The process of claim 4 wherein said step of completely etching the underlay material is effected in a parallel plate reactive ion etch tool in an O₂ plasma. The process of claim 5 wherein said step is a two step process, comprised of a first anisotropic etch, to remove the underlay material to the bottom of the trench, then a cleaning step to remove retained underlay material where sidewalls have negative slopes, the latter step is accomplished at a higher pressure. The process of claim 6 wherein said two-step process operating condition read as follows : 1) Power density 0.17 watts/cm² Pressure 0.4 Pa (3 µm) Flow 100 Standard cm³/min. Etch rate SiO x .7 nm/min. Etch rate polyimide 75 nm/min. 2) Power density 0.23 watts/cm² Pressure 6.7 Pa (50 µm) Flow (O₂) 30 Standard cm³/min. Etch rate polyimide 200 nm/min. The process of claim 1 wherein said trench is passivated by a composite SiO₂Si₃N₄ layer (17, 19). The process of claim 8 wherein the exposed areas of the Si₃N₄ layer forming part of said composite masking layer are removed to expose the underlying SiO₂ layer. The process of claim 9 wherein said removal is accomplished in a CF4 + 10% O₂ plasma, in etching conditions reading as follows: Power density 0.23 watts/cm² Pressure 26.7 Pa (200 µm) Flow (CF4 +10% O₂) 150 Standard cc/min. Etch rate Si₃N₄ 70 nm/min. Etch rate SiO₂ 40 nm/min. Etch rate polyimide 45 nm/min. The process of claim 10 wherein the remaining portions of the barrier layer are removed in a 10 : 1 H₂O : HF solution. The process of claim 11 wherein the exposed portions of the SiO₂ layer are removed in a buffer HF solution. The process of claim 12 wherein said trench is part of a pattern of isolation trenches to isolate devices or part of devices. The process of claim 13 wherein said structure is comprised of a P⁻ silicon substrate (11) having a N⁺ subcollector (12) and N epitaxial layer (13) thereon, and said trenches are formed through said subcollector and epitaxial layer into said substrate. The process of claim 13 wherein the bottom of said trenches is provided with channel stopper regions (18). The process of claim 15 wherein said pattern of trenches defines a central stud (16) of silicon having all its sidewalls, the bottoms of trenches adjacent thereto and its top surface exposed. The process of claim 16 further comprising the steps of : blanket depositing a layer (25) of a silicide forming metal; sintering the structure to produce metal silicide wherever said metal contacts silicon; and, removing the unreacted metal, leaving a metal silicide cap covering the stud and channel stopper regions, therefore providing an electrical contact between the top side of the structure and the substrate. The process of claim 17 wherein said depositing step is accomplished by sputtering. The process of claim 18 wherein said metal is platinum or palladium. The process of claim 19 further comprising the step of filling the trenches with polyimide and reactively ion etching in an O₂ plasma the polyimide in excess to let exposed the top part of said metal silicide cap to be used as a substrate contact. The process of any of the preceding claims wherein said underlay material is an organic material selected from the polyimide group.

Inventors: GOTH, GEORGE RICHARD, HANSEN, THOMAS ADRIAN, MAKRIS, JAMES STEVE
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L21, H01L29