Abstract:
A buried interconnect can be incorporated into the starting semiconductor on insulator wafer during the early stages of the circuit fabrication process flow for use with semiconductor devices. The buried interconnect provides an additional interconnect layer enabling an overall reduction in the silicon real estate occupied by interconnections. The buried interconnect has low resistance and can prevent the formation of unwanted PN junctions through the use of silicides. The buried interconnect and its fabrication method include an S0I wafer that has an oxidation layer formed on top of a semiconductor layer by oxidation, followed by an nitride layer formed on top of the oxide layer which then is selectively etched to form two trenches with regions of different depths. Some regions of the trenches are etched to remove all of the semiconductor layer in the trench to expose the buried oxide layer. In other regions, a thin layer of semiconductor is left at the bottom of the trenches. Next, all of the exposed surfaces of the trenches are oxidized and the oxide at the trench bottom is removed to expose the underlying semiconductor material. The underlying semiconductor material is then silicided to form a buried interconnect. The wafer, including the trenches, is subsequently covered with oxide and chemical-mechanical polishing is used to remove excess oxide outside the trenches.

Description:
This application is a division of application Ser. No. 09/711,629 filed Nov. 13, 2000, now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to semiconductor on insulator processes, and more particularly to a method of forming buried interconnects on wafers and devices including the same. 
     BACKGROUND OF THE INVENTION 
     Conventional or bulk semiconductor devices include transistors, and other semiconductor devices, formed in a semiconductor substrate by implanting a well of either P-type or N-type conductivity silicon in a silicon substrate wafer of the opposite conductivity, plus a field oxide to prevent surface inversion. Transistor gates and source/drain diffusions are then manufactured using commonly known processes. These form devices known as metal-oxide-semiconductor (MOS) field effect transistors (FETs). Each of these must be electrically isolated from the others to avoid shorting the circuits. 
     A relatively large amount of surface area of bulk semiconductor logic circuits is needed for the electrical isolation of the various FETs which is undesirable because it inhibits transistor packing density. Additionally, junction capacitance between the source/drain and the bulk substrate slows the speed at which a device using such transistors operates. 
     In order to deal with the junction capacitance problem and circuit density, SOI technology has been employed. For example, one method of forming an SOI wafer is using conventional oxygen implantation techniques to create an insulating buried oxide layer at a predetermined depth below the surface of a bulk semiconductor wafer. The implanted oxygen combines with the bulk silicon to form silicon oxide as is well known in the art. A second method of forming an SOI wafer includes depositing an insulating layer of silicon oxide on the surface of a first wafer and then bonding the first wafer to a second wafer using a heat fusion process. 
     Utilizing SOI technology, an SOI FET includes a source region and a drain region of one semiconductor type on opposite sides of a channel region of the opposite semiconductor type. The SOI FET is isolated by etching a trench around an area, filling the trench with oxide and planarizing this oxide. The area is an island of semiconductor material formed from a semiconductor layer above an insulating buried oxide layer. The appropriate portions of the island are doped to form the source, drain and channel regions. The SOI FET will occupy less surface area on the substrate and will have a lower junction capacitance than an equivalent bulk semiconductor FET because of the insulating trench and the insulated buried oxide layer. This structure improves clock speeds and increases the number of circuit elements that can be placed at a given area. 
     Unfortunately, die size reduction is also inhibited by the interconnection of the FETS or other devices with each other and other circuit elements. For example, typical interconnections of known complimentary MOS (CMOS) devices are structured to interconnect both P-channel and N-channel FETs and conventionally are metal layers above the bulk substrate. The presence of the metal layers above the bulk substrate inhibits size reduction. 
     Accordingly, there is a strong need in the art to have an alternative to metal layers above the bulk substrate to reduce the die area occupied by interconnects. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a semiconductor on insulator structure including a plurality of active regions formed on an insulator layer, a plurality of isolation trenches formed between the active regions for substantially isolating adjacent active regions, and at least one interconnection trench that includes an interconnect formed at the bottom of the at least one interconnection trench for electrically connecting a first active region of the active regions to a second active region of the active regions. 
     Another object of the present invention is to provide a method of forming a semiconductor on insulator structure including forming trenches in a semiconductor material, some of the trenches extending through the semiconductor material to an insulation layer and some of the trenches extending to an intermediate depth; and processing a bottom portion of the trenches extending to the intermediate depth to make the bottom portion electrically conductive. 
     Another object of the present invention is to provide a method of forming a semiconductor on insulator structure including providing a silicon layer including active regions on an insulation layer, forming a silicon oxide layer on the silicon layer by oxidation, forming a silicon nitride layer on the silicon layer by nitride deposition, forming a trench mask over the silicon nitride layer, etching through the silicon oxide and silicon nitride layers and into the silicon layer with a timed etch, providing an interconnect mask over a portion of the etched semi-conductor layer with the timed etch, etching the etched silicon layer not covered by the interconnect mask with a high silicon to nitride selectivity etch to isolate at least two active regions from each other, stripping the interconnect mask, oxidizing surface silicon exposed by the high silicon to nitride selectivity etch and the timed etch by oxidation, anisotropically etching a portion of the oxidized surface silicon with an anisotropic oxide etch, siliciding the area silicon to form an interconnect, depositing an oxide layer and polishing the oxide layer to the silicon nitride layer, and removing the silicon nitride layer. 
    
    
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a semiconductor on insulator device having a buried interconnect. 
     FIG. 2 is a cross-sectional view of FIG.  1 . 
     FIG. 3 is a cross-sectional view of an unprocessed SOI wafer. 
     FIG. 4 is a cross-sectional view of FIG. 3 after completing a first processing step of an exemplary embodiment of the present invention. 
     FIG. 5 is a cross-sectional view of FIG. 4 after completing a second processing step of an exemplary embodiment of the present invention. 
     FIG. 6 is a cross-sectional view of FIG. 5 after completing a third processing step of an exemplary embodiment of the present invention. 
     FIG. 7 is a cross-sectional view of FIG. 6 after completing a fourth processing step of an exemplary embodiment of the present invention. 
     FIG. 8 is a cross-sectional view of FIG. 7 after completing a fifth processing step of an exemplary embodiment of the present invention. 
     FIG. 9 is a cross-sectional view of FIG. 8 after completing a sixth processing step of an exemplary embodiment of the present invention. 
     FIG. 10 is a cross-sectional view of FIG. 9 after completing a seventh processing step of an exemplary embodiment of the present invention. 
     FIG. 11 is a cross-sectional view of FIG. 10 after completing an eighth processing step of an exemplary embodiment of the present invention. 
     FIG. 12 is a cross-sectional view of FIG. 11 after completing a ninth processing step of an exemplary embodiment of the present invention. 
     FIG. 13 is a cross-sectional view of FIG. 12 after completing a tenth processing step of an exemplary embodiment of the present invention. 
     FIG. 14 is a cross-sectional view of FIG. 13 after completing an eleventh processing step of an exemplary embodiment of the present invention. 
     FIG. 15 is a flow chart showing an exemplary process for fabricating an exemplary interconnect according to the present invention. 
     FIG. 16 is a top view of a buried interconnect of the present invention having an alternative geometry. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring in detail to FIGS. 1-16 of the drawings, wherein like reference numerals refer to like parts in the several figures, a buried interconnect and a method of forming a buried interconnect in accordance with the present invention are generally indicated. FIGS. 1-14 show a silicon-on-insulator (SOI) wafer that is processed by an exemplary method according to the present invention, such as shown in FIG. 15, to produce a SOI wafer having buried interconnects. FIG. 16 shows a buried interconnect having another possible buried interconnect geometry. 
     FIG. 1 a top view of an exemplary embodiment of device  100  and shows the interconnection of a first FET  102  with a second FET  104  via the buried interconnect  106 . The first FET  102  includes source and drain electrodes  108  and a gate electrode  110 . Similarly, the second FET  104  includes source and drain electrodes  112  and a gate electrode  110 . One of the source/drain electrodes of  108  of the first FET  102  is connected by the buried interconnect  106  to one of the source/drain electrodes  112  of the second FET  104 . The buried interconnect  106  is covered by an oxide material  114  (not shown) and is connected to a contact  116 . 
     FIG. 2 shows a cross-sectional view of a device  100  of FIG.  1 . The device  100  includes the silicon layer  120  which is doped to form P+ regions  122  and N+ regions  124  that are part of the CMOS FET. The first trench  126  is filled with the oxide material  114  and the second trench  130  is filled with a field oxide material  132 . The oxides  114 ,  132  in the trenches  126 ,  132  are formed simultaneously by the same oxide deposition step. Gate electrodes  110  and gate oxides  118  are placed on the surface of silicon layer  120  between adjacent P+ regions  122  and between adjacent N+ regions  124 . The P+ regions  122  and the gate  110  form the gate, source, and drain of a P type MOS FET  134  and the N+ regions  124  together with the adjacent gate  110  form the gate, source, and drain of an N type MOS FET  136 . The P type MOS FET  134  and the N type MOS FET  136  are connected together by the buried interconnect  106  and form the CMOS circuit. As shown in this embodiment, the adjacent drain/source electrodes  122 ,  124  of the two FETs  134 ,  136  are connected together by the buried interconnect  106 . 
     The device  100  of FIG. 2 is fabricated by processing an unprocessed semiconductor on insulator wafer  138  that includes an insulating layer  140  on a semiconductor substrate and a semiconductor layer  120  such as shown in FIG.  3 . Semiconductor on insulator wafers are well known within the art and silicon-on-insulator (SOI) wafers are the most commonly used type of semiconductor on insulator wafer. An SOI wafer has a buried oxide layer  140  as the insulating layer and a silicon layer  120  as the semiconductor layer. For simplicity, the following processing steps are described using a silicon semiconductor although the present invention may be applied to any kind of similar semiconductor on insulator wafer including those made from materials other than silicon. Additionally, the term layers is used to describe area of similar or common composition or properties and is not intended to be limited to discreet sheets of material. For example, a sheet of material which has another material implanted therein such that there are multiple compositions in the sheet may be described as having multiple layers. A layer may also have a varying composition and still be described as a layer. 
     Initially, the unprocessed wafer  138  of FIG. 3 is oxidized to form a wafer having a silicon oxide layer  142 . This results in the wafer of FIG.  4 . The silicon oxide layer  142  is preferably silicon dioxide but may also be non-stoichiometric silicon oxide. 
     Next, the wafer of FIG. 4 undergoes nitride deposition to form a silicon nitride layer  144  on the silicon oxide layer  142 . This results in the wafer of FIG.  5 . The silicon nitride layer  144  may be composed of any type of silicon nitride and may include other elements such as oxygen. The silicon nitride layer  144  alternatively can be formed by any process capable of forming the silicon nitride layer  144 . 
     Next, a source/drain mask  146  is applied to the wafer of FIG. 5 to selectively cover portions of the silicon nitride layer  144  thereby forming active regions and field regions. This results in the wafer of FIG.  6 . The source/drain mask  146  includes openings  148  that correspond to the field regions and masked portions the correspond to the active regions. Those regions which are covered by the mask  146  will become the active regions of the resultant device while those regions not covered by the mask  146  will become the field regions. The active regions define the silicon islands whereas the field regions define regions which either insulate the active regions or include the buried interconnect of the present invention to interconnect elements of the active regions as will be appreciated. The source/drain mask  146  may be formed by any known masking technique. For example, a layer of positive or negative photoresist is formed and patterned using conventional lithographic techniques. 
     Next, trench etching is performed on the wafer of FIG. 6 at the openings  148  of the source/drain mask  146  to etched through the silicon nitride layer  144 , the silicon oxide layer  142  and into the silicon layer  120 . The trench etching forms the first and second trenches  126 ,  130  and results in the wafer of FIG.  7 . The trench etch is controlled so as to remove all but a portion of the silicon underneath the mask openings  148  from the silicon layer  120 . The trench etch may be, for example, a timed etch. 
     Next, an interconnect mask  150  is applied to the wafer of FIG. 7 to cover the first trench  126  to prevent removal of the silicon which will eventually be processed into the buried interconnect  106 . The source/drain mask  146  is stripped off of the silicon nitride layer  144  prior to formation of the interconnect mask  150 . This results in the wafer of FIG.  8 . The interconnect mask  150  protectively covers at least those exposed portions of silicon layer  120  in which etching is not desired and may also protectively cover any other layer except where etching is to be performed. However, it is unnecessary to place the interconnect mask  150  anywhere other than over the first trench  126  since the silicon nitride layer  144  will also protect against etching in the next step. 
     Next, high silicon-to-nitride selectivity etching of the wafer of FIG. 8 is performed to remove a remaining portion  152  of the silicon layer  120  in the second trench  130  to expose the buried oxide  140 . The interconnect mask  150  is removed after completion of the etch. This results in the wafer of FIG.  9 . Exposed silicon portions  154  of the silicon layer  120  are located along the sides and bottom of the first trench  126  and along the sides of the second trench  130 . There is no exposed silicon portion  154  along the bottom of the second trench  130 . Alternatively, any kind of etch which removes the remaining portion  152  from the bottom of the second trench  130  without etching other areas may be used. 
     Next, oxidation of the exposed silicon portions  154  of the silicon layer  120  of the wafer of FIG. 9 is performed to form silicon oxide sides  156  in the second trench  130 , and silicon oxide sides  158  and a silicon oxide bottom  160  in the first trench  126 . This results in the wafer of FIG.  10 . The oxidation may be a thermal oxidation or any other kind of oxidation which will oxidize the exposed silicon portions  154 . The silicon oxide sides  156 ,  158  are used to prevent silicidation later in the fabrication process. 
     Next, an anisotropic oxide etch is performed on the wafer of FIG. 10 to remove the silicon oxide bottom  160 . However, the silicon oxide  156 ,  158  along the walls of the trenches  126 ,  130  is not removed by the anisotropic oxide etch because of the directionality of the etch. This results in the wafer of FIG.  11 . The selective removal of the silicon oxide bottom  160  exposes the silicon underneath the silicon oxide bottom  160  and allows silicidation of the exposed silicon portions  154  by depositing a metal, e.g., Ti, Co or W followed by a rapid thermal anneal. The silicidation increases the conductivity of the exposed silicon thereby forming a buried interconnect  106  as shown in the cross-sectional view of FIG.  12 . The silicidation is performed until the exposed silicon becomes a good conductor. However, any method by which the conductivity of the buried interconnect  106  is greatly increased so as to provide a conductive path may be used instead of the silicidation. For example, the buried interconnects  106  could be made of three different elements instead of being a binary compound which normally results from silicidation. 
     Next, any unreacted material is removed. This results in the buried interconnect containing SOI wafer  138  of FIG. 12 which offers an additional method of electrically connecting circuit elements besides the traditional backend of line interconnects. As a result, the overall area occupied by interconnects may be reduced. 
     Next, oxide  143  is deposited over the wafer of FIG. 12 to fill the trenches and form the wafer of FIG.  13 . The wafer of FIG. 13 is then polished down to the silicon nitride layer  144  by chemical mechanical polishing or any other polishing method to remove excess oxide  143 . This results in all of the oxide outside the trenches  126 ,  130  being polished away leaving oxides  114 ,  132  and forms the wafer of FIG.  14 . The polishing does not remove any of the others layers because the silicon nitride layer  144  acts as a polishing stop layer. The silicon nitride layer  144  is then stripped from the wafer of FIG.  14 . The wafer of FIG. 14 then undergoes further fabrication, such as the formation of transistors, to complete the circuit. 
     FIG. 15 is a flow chart  200  showing an exemplary method of fabricating a buried interconnect  106  according to the present invention. The flow chart  200  begins with the step of providing a silicon-on-insulator wafer  202 . Alternatively, the wafer may be a semiconductor on insulator wafer composed of a material other than silicon (e.g., germanium). However, solely for brevity, a silicon-on-insulator wafer will be described with regard to the flow chart  200 . 
     Next the wafer undergoes oxidation  204  to form the silicon oxide layer  142  which is then followed by a nitride deposition step  206  to form the silicon nitride layer  144 . Next, the step of forming a source/drain masking  208  is performed. Next, the trenches  126 ,  130  are formed by a trench etch  210 . The trenches  126 ,  130  will eventually be processed into two different types of structures. The first type of trench  126  will include the buried interconnects  106  while the second type of trench  130  will only contain the field oxide  132 . 
     After completion of the trench etch  210 , the source/drain mask  146  is stripped by a resist strip step  212  and the interconnect mask  150  is provided by an interconnect masking step  214 . The interconnect mask  150  covers those trenches which will eventually include the buried interconnects  106 . 
     A high silicon-to-nitride selectivity etch  216  results in differentiation of trenches into two kinds of trenches. The first trench  126  includes a silicon portion at the trench bottom while the second trench  130  is stripped clear of silicon to expose the buried oxide  140  at the trench bottom. 
     Next the interconnect mask  150  is removed in a resist strip step  218  to expose part of the silicon layer. The exposed parts  154  of the silicon layer  120  are oxidized in a liner oxidation step  220  so as to transformed the exposed parts  154  into silicon oxide. The buried interconnect  106  is formed at the bottom of the trench  126  by an anisotropic oxide etch  222  followed by silicidation performed by a metal deposition and rapid thermal anneal step  224 . The metal deposition and rapid thermal anneal step  224  deposits a thin metal film of Ti, Co, W or any other material useful for silicidation of the buried interconnect  106  and then performs a rapid thermal anneal to achieve silicidation of the buried interconnect  106 . The unreacted metal or other material is then removed in the removal of unreacted material step  226 . 
     Next, the trench oxides  114 ,  132  are formed in an oxide deposition and planarization step  228 . This step  228  coats the wafer of FIG. 12 with an oxide  143  that fills the trenches  126 ,  130 . Chemical mechanical polishing is then performed until the silicon nitride layer  144  is reached. The properties of the silicon nitride  144  enable it to act as a stop layer for the chemical mechanical polishing leaving the surface planarized with the silicon nitride layer  144 . The silicon nitride  144  is then removed in a nitride strip step  230 . At this point, a buried interconnect containing wafer  138  has been completely formed. Now a desire device incorporating a buried interconnect  106  can be fabricated by performing the remaining processing steps  232  necessary to complete the fabrication of the desired device. 
     Certain modifications to the steps of flow chart  200  shown in FIG. 15 can be made. For example, those of ordinary skill in the art will understand that other additional steps may be incorporated into the present invention. 
     FIG. 16 a top view of another exemplary embodiment of a device  100  having a buried interconnect  106  with another possible buried interconnect geometry. The buried interconnect  106  of FIG. 16 is substantially the same as that shown in FIG. 1 except that instead of the exterior and not the adjacent source/drain electrodes  110 ,  112  are connected by the buried interconnect  106 . The buried interconnects  106  also can be used to connect three or more circuit elements, if needed. The geometries of the buried interconnect  106  show in FIGS. 1 and 16 are exemplary geometries and numerous other geometries are possible. All such geometries are intended to be within the scope of the invention. 
     The foregoing device has certain advantages. For example, the additional interconnect layer may be incorporated into devices without greatly increasing their complexity or production cost and is compatible with conventional CMOS fabrication technologies that use shallow-trench isolation. This interconnect layer of the present invention has the further advantage of having high conductivity. Furthermore, the presence of the silicide prevents the formation of unintentional PN junctions. The additional interconnect layer provided by the buried interconnect also enables the reduction of the silicon real estate occupied by the interconnects, thus allowing the overall reduction in the die size, especially in a circuit where the die size is determined by the interconnects. The use of the high nitride-to-silicon selectivity etch and the use of the nitride layer  144  as a mask removes the need to have extremely good overlay during interconnect masking. 
     What has been described above are preferred embodiments of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit of the scope of the appended claims.