SUBSTRATE AND METHOD FOR MANUFACTURING THE SAME

Abstract: The present invention is directed to a method for manufacturing a substrate especially for SOI technologies. Thereby, the new method comprises the steps of: a) providing a semiconductor wafer, having a frontside where active devices are to be located or are located and a backside opposite to said frontside; b) producing at least a trench in the backside of said wafer, said trench extending from the backside of the wafer to a level having a predetermined distance from the frontside of the wafer; and c) producing an insulation structure in said trench, so that a vertical insulation for active devices located on said frontside of the wafer is provided.

Description: Description Substrate and method for manufacturing the same The present invention is directed generally to the field of device insulation and more specifically to a substrate and a method for manufacturing a substrate especially for SOI technologies . Semiconductor devices, especially silicon devices, have problems with inherent parasitic circuit elements due to junction capacitances. One way to circumvent the problem is to fabricate semiconductor devices on an insulating substrate. One of the initial approaches was to grow silicon epitaxially on a substrate of sapphire (called silicon-on-sapphire, or SOS) . A more recent approach is the so called silicon-on-insulator (SOI) technology. The components or devices are thereby realized in a SOI substrate. A SOI substrate has normally an insulating layer of Si0 on a single-crystal silicon wafer and a single-crystal silicon layer on the insulating layer. This single-crystal silicon layer is the surface of the SOI substrate. The insulating layer of the SOI substrate assures the vertical insulation, whereas the lateral insulation of the components is realized for example by trenches filled with insulating material or local oxidation of silicon (LOCOS) . The advantages of the silicon-on-insulator technology are widely recognized: higher circuit speed, lower power consumption, greater immunity to radiation-induced errors, and com- patibility with existing IC fabrication processes. However, the consistency and cost of the SOI substrates and their availability remains a problem. In order to form SOI substrates, any of a variety of methods may be used, see for example „ hin-film SOI emerges" by M.L. Alles, IEEE Spectrum Magazine, June 97, pages 37 to 45. In the prior art, the main manufacturing options are the so called „separation by im- plantation of oxygen" (Simox) and the so called „wafer bonding", each with a number of process variants. The Simox process produces a SOI substrate by first implant- ing a silicon substrate with a relatively high dose of oxygen ions and then performing a high temperature anneal at about 1300°C for several hours, in order to form a buried layer of silicon oxide. The wafer bonding process is a method of producing SOI substrates in which two silicon wafers each with oxide on the surface, are bonded together and annealed at temperatures higher than 800°C to strengthen the bond. The bulk of one of the two wafers is then removed by any of several methods - grinding, etching, or polishing, for example. Unfortunately, the conventional SOI technologies are quite expensive to realize. Therefore, SOI substrate prices typically are several times those of bulk silicon substrates, preventing SOI substrates from being used in cost-sensitive applications as low-end consumer electronics and DRAMs . Accordingly, there is need for a new, simpler and more economical way of achieving circuit size reductions, speed improvements, and power reduction-operation at lower voltage. The object is inventively resolved by a method for manufacturing a substrate as presented in independent claim 1 and by the substrate as presented in independent claim 10. Thereby, the new method comprises the steps of: a) providing a semiconductor wafer, having a frontside where active devices are to be located or are located and a backside opposite to said frontside; b) producing at least a trench in the backside of said wafer, said trench extending from the backside of the wafer to a level having a predetermined distance from the frontside of the wafer; and c) producing an insulation structure in said trench, so that a vertical insulation for active devices located on said frontside of the wafer is provided. Preferably, the step c) of producing an insulation structure in said trench is further defined by filling the trench with insulating material, or by providing the surfaces inside said trench with a layer made of insulating material. Preferably, said insulating material is selected from the group consisting of Si0 and Si 3 N . In case the sidewalls of said trench are provided with a layer made of insulating material it is preferred that the remaining part of said trench is filled with a fill suitable of providing mechanical stability, preferably a silicon fill or a metal fill . In case the remaining part of said trench is to be filled with a silicon fill, it is preferred that the filling is done by the deposition and re-etching of a silicon layer. Preferably the trench is produced by etching the wafer using a trench mask, said trench mask preferably having a layer se- quence with a lower layer of thermal Si0 , a middle layer of CVD Si 3 N 4 and an upper layer of CVD Si0 2 . For reasons of mechanical stability it is also preferred that the step b) of producing at least one trench and the step c) of producing an insulating structure are repeated until a predetermined percentage of the backside of the wafer is made of said insulating structure. The invention shall be set forth in greater detail below with reference to an exemplary embodiment and to the figures. FIG. 1 illustrates a conventional silicon substrate. FIG. 2 illustrates the silicon substrate after deposition of a trench mask. FIG. 3 illustrates the silicon substrate after the trench etching. FIG. 4 illustrates the silicon substrate after deposition of an insulation layer and filling the remaining part of the trench with a silicon fill. A silicon wafer 1 has a monocrystalline bulk 3 and a monoc- rystalline epitaxial layer 2 (see FIG. 1) . The monocrystalline silicon bulk 1, for example is p+ -doped with a dopant concentration of, for example, 5.0 10 19 boron/cm 3 . The monocrystalline epitaxial layer 2, for example, is lightly n-doped with a dopant concentration of, for example, 4.0 10 14 phosphorous/cm 3 and has a thickness of, for example, 20 μm. The surface of the epitaxial layer 2 forms the frontside of the wafer where active devices are to be located, whereas the surface of the monocrystalline silicon bulk 3 forms the backside of the wafer. A trench mask 4 is formed on the surface of the monocyrstal- line silicon bulk 3. To that end, the surface of the monocrystalline silicon bulk 3 is first thermally oxidized. A lower layer 41 of thermal Si0 2 having a thickness of, for example 50 nm results. A middle layer 42 of a CVD Si 3 N 4 having a thickness of, for example, 150 nm is produced thereon. An upper layer 43 of a CVD Si0 is deposited on the middle layer 42 with a thickness of, for example, 1000-1600 nm. The layer sequence consisting of the lower layer 41, the middle layer 42 and the upper layer 43 is structured using a photoresist mask 5, for example in a CHF 3 /0 2 dry-etching process (see FIG. 2) . After the removal of the photoresist mask 5, a trench 6 is etched into the monocrystalline silicon bulk 3 (see FIG. 3) . For example, the etching of the trench 6 occurs in an ani- sotropic dry-etching process with a Cl /0 2 chemistry. The etching of the trench is done in such a manner that the trench extends from the backside of the wafer to a level having a predetermined distance from the frontside of the wafer. In the present example, the trench extends to monocrystalline epitaxial layer 2. After the removal of etching products in a HF dip, the trench 6 is present with clean sidewalls 61 on the silicon bulk 3. Furthermore, the trench mask 4 is removed . Thereafter, an insulation layer 7 of Si0 2 is applied onto the surfaces of the trench 6 in a CVD process, for example with a TEOS method. The remaining part of said trench is then filled by depositing and re-etching of a silicon layer 8 (see FIG. 4) . Thereby, an insulation structure 9 is produced in said trench 6, so that a vertical insulation for active devices is provided. The active devices are to be located in the monocrystalline epitaxial layer 2 on said frontside of the wafer 1. The resulting SOI substrate may now be further processed, in order to form integrated circuits thereon. Thereby, the SOI substrate shown in FIG. 4 is turned around, so that the monocrystalline epitaxial layer 2 is located on top of the remaining silicon bulk 3 and the insulation structure 9. Only conventional methods for trench etching and trench filling from CMOS fabrication are used in the method of the invention. Since the CMOS fabrication is already necessary for producing the logic components, a synergy effect is thereby exploited. It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is, therefore, intended that such changes and modifications be covered by the appended claims .

Claims: Claims : 1. Method for manufacturing a substrate especially for SOI technologies, said method comprising the steps of: a) providing a semiconductor wafer, having a frontside where active devices are to be located or are located and a backside opposite to said frontside; b) producing at least a trench in the backside of said wafer, said trench extending from the backside of the wafer to a level having a predetermined distance from the frontside of the wafer; and c) producing an insulation structure in said trench, so that a vertical insulation for active devices located on said frontside of the wafer is provided. 2. Method according to claim 1, wherein said step c) of producing an insulation structure in said trench is further defined by filling the trench with insulating material. 3. Method according to claim 1, wherein said step c) of pro- ducing an insulation structure in said trench is further defined by providing the surfaces inside said trench with a layer made of insulating material . 4. Method according to one of claims 2 or 3 , wherein said in- sulating material being selected from the group consisting of Si0 2 and Si 3 N . 5. Method according to claim 3, further comprising the step of filling the remaining part of said trench with a fill suitable of providing mechanical stability. 6. Method according to claim 5, further comprising the step of filling the remaining part of said trench with a silicon fill or a metal fill. 7. Method according to claim 6, said step of filling the remaining part of said trench with a silicon fill is further defined by deposition and re-etching of a silicon layer. 8. Method according to one of the preceding claims, wherein said step b) of producing at least one trench is further defined by etching said trench using a trench mask, said trench mask preferably having a layer sequence with a lower layer of thermal Si0 2 , a middle layer of CVD Si 3 N 4 and an upper layer of CVD Si0 2 . 9. Method according to one of the preceding claims, wherein said step b) of producing at least one trench and said step c) of producing an insulating structure are repeated until a predetermined percentage of the backside of the wafer is made of said insulating structure. 10. A substrate especially for SOI technologies being characterized in that the substrate is obtainable by a method according to one of the preceding claims .

Inventors: SUDARSAN PARTHASARATHY S, SUDARSAN, PARTHASARATHY, S.
Applicants: SIEMENS AG, SUDARSAN PARTHASARATHY S, SIEMENS AKTIENGESELLSCHAFT, SUDARSAN, PARTHASARATHY, S.
Application year: 1999
Main classification: H01L21
IPC codes: H01L21