Process for patterning local interconnects4657628Abstract A local interconnect system for VLSI integrated circuits. After titanium is deposited for self-aligned silicidation of exposed moat and gate regions in a nitrogen atmosphere, a hardmask is deposited and patterned over the titanium. When a conductive titanium nitride layer is formed overall, it will already be patterned according to this hardmask. Claims What is claimed is: Description BACKGROUND AND SUMMARY OF THE INVENTION
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Anodized Al
Si Electrode
Electrode
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TiN 2250 A/min 2600 A/min
TiSi.sub.2 850 A/min 1600 A/min
AZ-1400 (resist)
5500 A/min 8400 A/min
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The degradation in TiN:TiSi.sub.2 and TiN:resist etch rate ratios observed with the anodized Al electrode points toward the importance of a fluorine-deficient plasma in achieving good selectivities between these materials. Some etch rates for this chemistry with other materials include: thermal oxide, 4750 A/min; AZ 1400 5500 A/min; Kodak 820, 3300 A/min. The poor resist and oxide selectivities in these results prohibit doing dry etching only. However, increasing the slice substrate temperature from 50 to 70 degrees improves the resist selectivity by 25%. That is, a "fluorine-deficient" plasma is one in which the ratio of fluorine atoms to other atoms (or at least to atom species derived from the source gas flow) is less in the plasma discharge volume than it is in the source gas flow. For example, in the CF.sub.4 etch chemistry described, the ratio of fluorine atoms to carbon atoms in the source gas (where essentially all the carbon and all the fluorine is present as CF.sub.4) is 4:1, but in the plasma (which includes CF.sub.4 and also radical species such as free fluorine, CF.sub.2, CF.sub.3, etc.) the ratio is substantially smaller, e.g. 3.5 or less. (Note that the carbon atoms devolved from the substrate (e.g. volatiles outgassed from the photoresist) are not counted here.) Normally the etched surface itself will consume some fluorine to create a mild fluorine-deficiency; but one teaching of the present invention is that the fluorine-deficiency should be increased by increasing residence time and by using a fluorine sink in proximity to the glow discharge. The fluorine sink serves to preferentially remove fluorine from the plasma. For example, in the presently preferred embodiment, a silicon electrode serves as a fluorine sink, since it will be continually etched to form SiF.sub.4. (Other materials having volatile fluorides, such as graphite, could also be used.) Any fluorine discharge will etch TiN freely, but a simple fluorine discharge will also etch TiSi.sub.2 rapidly; the problem is to etch TiN with some selectivity to TiSi.sub.2. It should also be noted that the use of fluorocarbons (as opposed to inorganic fluorine sources such as SF.sub.6, NF.sub.3, etc.) is itself particularly advantageous. When CF.sub.x radicals impinge on the TiN the carbon can go off as cyanogen radicals CN, which rapidly recombine to form volatile species such as (CN).sub.2, HCN, or FCN, but when the CF.sub.x radicals impinge on the silicide there is no such easy way to get rid of the carbon: the surface carbon from adsorbed species will therefore compete with the titanium atoms for reaction with impinging fluorine atoms, thus retarding the rate of formation of TiF.sub.4 (and thus the rate of etching.) Thus, some fluorine centers (such as BF.sub.3 or SiF.sub.4) are unsuitable because they do not have a volatile nitride. Other fluorine sources (such as SF.sub.6) are unsuitable because they are very copious fluorine sources; for example, SF.sub.6 rapidly dissociates to form SF.sub.4, and may then further dissociate to SF.sub.3 radicals, etc. By contrast, CF.sub.4 is a much milder (less copious) source of fluorine. To achieve fluorine deficiency, it is preferable to use a mild (not copious) fluorine source. Moreover, the use of low gas flow rates increases the fluorine-deficiency of the plasma, but also increases the rate of polymer deposition. To keep polymer deposition from getting out of hand, the preferred embodiment uses elevated substrate temperatures (e.g. 70 C). At 70 C under the conditions given, an improved TiN:TiSi.sub.2 selectivity can be achieved. The flow rate is preferably such as to replace the equivalent volume of the glow discharge 2 times per second or less. Higher substrate temperatures are also advantageous because they increase the volatility of the TiF.sub.4 etch product, which tends to increase the etching throughput and also tends to give more selectivity to photoresist. Thus, some of the important features of the TiN etching method taught by the present application include: a mild fluorine source, such as CF.sub.4, is preferably used; a reducing electrode, such as silicon or graphite, is preferably used to scavenge fluorine; a relatively low flow rate is preferably used, to increase the fluorine deficiency of the plasma and thus increase the TiN:TiSi.sub.2 etching selectivity; and the substrate temperature is preferably allowed to rise into the range between 60 and 100 C during etching. It should also be noted that, if processing conditions are varied, the TiN layer may have a higher oxygen content and therefore may be difficult to remove with the wet etch given. In such embodiments it may be more preferable to use a dry etch chemistry of the type described as a blanket etch to strip the TiN layer, using higher pressure if needed to provide isotropy. A sample process flow for patterning the local interconnects according to one embodiment of the present invention is as follows: 1. Pattern TiN local interconnects: use 1400-27 or 1400-31 resist. 2. Hardbake the resist to 120 C 3. Plasma etch the TiN: Gas: CF.sub.4 (200 sccm)+He(50 sccm) Pressure: 1 Torr Time: 25 sec (90% of 1000 A TiN layer etched; i.e. the TiN etch rate is about 36 A/sec) Power: 200 W Substrate temperature: 50 C 4. Ti strip in Megasonic (i.e. an ultrasonically-driven wet etching bath) Solution: NH.sub.4 OH(3000 cc)+H.sub.2 O.sub.2 (1750 cc)+H.sub.2 O (24000 cc) Time: 9 min Power: 250 W Duty cycle: 10% to 20% with 60 sec period 5. Ash the resist: 60 min 6. Solvent resist strip: R-10, 50 C, 10 min 7. Cleanup Solution: HCl(37-38% concentration): H.sub.2 O=1:1 Time: 20 min 8. TiSi.sub.2 /TiN anneal: 30 min, atmosphere including N.sub.2 (e.g. forming gas), at 800 C 9. deposit MLO, e.g. 5000 A of PSG 10. Pattern and etch contacts 11. Deglaze: Solution: 1% (volumetric) buffered HF; buffered HF solution is a mixture of HF:NH.sub.4 F:H.sub.2 O=49:11 by weight. Time: 60 sec. 12. Remaining conventional steps, e.g. deposit and pattern metal, vias and second metal (if used), protective overcoat, etc. After the TiN layer 22 has been patterned as desired, to produce the structure of FIG. 22, a second annealing step is preferably used to lower the sheet resistance of the silicide layers 20 to one ohm per square or less. This second heat treatment step is preferably performed at 800 degrees C for 30 minutes in an argon atmosphere at atmospheric pressure, but these parameters may be varied. For example, the annealing temperature could be in the range of 750 degrees to 875 degrees, or even wider, and the other processing parameters of this step can also be widely varied. As noted, the titanium nitride layer 22 is not necessarily fully stoichiometric after the initial silicidation step. This is, this layer may contain excess titanium (or nitrogen). The presence of excess titanium is advantageous, in some embodiments, since it facilitates the selective removal of the titanium nitride layer 22. If it is desired not to leave excess titanium in the interconnects in the finished integrated circuit (since titanium is highly reactive), a thinner layer of titanium can be deposited initially. Alternatively, by performing the subsequent annealing step in a nitrogen atmosphere, any excess of unreacted titanium may be removed. In fact, the silicidation may optionally be performed as a transient heating step, using a flash lamp or laser heating or other transient heating methods. This tends to leave a higher atomic fraction of unreacted titanium in the titanium nitride layers, which as noted may facilitate patterning, and may be disposed of in the later anneal. After the final anneal which lowers the sheet resistance of the silicide has been performed, processing then proceeds in conventional fashion. For example, an interlevel dielectric could now be deposited, such as 1000 A of low pressure CVD pad oxide followed by a micron of borophosphosilicate glass. Contact holes would then be cut in the interlevel dielectric and a patterned metal layer formed to achieve a desired pattern of electrical interconnections. The only modification to subsequent processing which is preferably used is that the contact etch which cuts through the interlevel dielectric is preferably an etch chemistry which will stop on titanium nitride. This means that the titanium nitride layer 22 can be used as a diffusion barrier at the bottom of contacts, and also that the contacts to moat can be allowed to overlap onto the field oxide regions 26, since the titanium nitride 22 will prevent undercutting of exposed portions of field oxide 26 during the contact etch. Normal fluorine-based oxide etch chemistries, such as CF.sub.4 plus 5% O.sub.2, are reasonably selective to TiN. Another alternative way to get a patterned local interconnect layer is to deposit and pattern a thin hardmask before the step of reacting the titanium in a nitrogen atmosphere. For example, a 1000 A layer of plasma oxide (i.e. silicon oxides deposited in a plasma reactor at low temperature, e.g. 300 C.--this produces a fairly low density oxide) can be deposited over the titanium metal, etched to mask local interconnects in the desired pattern (using an oxide etch such as CHF.sub.3 +C.sub.2 F.sub.6 +O.sub.2 +He, which is selective to titanium, so that the titanium layer under the plasma oxide provides an etch stop) and left in place during the reaction which produces TiN and TiSi.sub.2 compounds. There will be significant reaction between the titanium metal and the plasma oxide, so that (in one example where rapid thermal annealing was used) Rutherford Backscattering Analysis has revealed that the titanium metal under the plasma oxide layer has an oxygen titanium atomic ratio of 0.69:1 (as compared to an oxygen to titanium atomic ratio, for titanium which lies on field oxide and is exposed to the nitrogen atmosphere during the reaction step, of around 0.25:1). This high oxygen content is not enough to destroy the metallic conduction properties of the titanium, but does impart substantial resistance to etching (TiO.sub.2 is an extremely stable compound which is very difficult to etch). The metallic titanium remaining can be converted to titanium nitrides and/or silicides by a subsequent reaction step. It has been experimentally found that this process does provide a local interconnect with most or all of the valuable properties of the process described above, although the interconnect thus provided is not initially TiN, at least where rapid thermal annealing is used for the silicidation (its atomic fraction of nitrogen is much less than 1%, except possibly at the corners of the plasma oxide mask). Note that the plasma oxide mask remains in place through the TiN etching step. The material under the oxide mask after a furnace anneal is not exactly known, and may well be different; it may be a Ti/TiO.sub.2 or Ti/TiN/TiO.sub.2 or TiN/TiO.sub.2 or Ti/TiN/TiSi.sub.2 /TiO.sub.2 composite. At any rate, it has been experimentally found that this process does provide a local interconnect with most or all of the valuable properties of the process described above. Note that the plasma oxide mask remains in place through the TiN etching step. The additional fraction of oxygen introduced by this alternative embodiment can be highly advantageous in at least two ways. First, since TiO.sub.2 is so chemically inert, it adds to the durability of the local interconnect layer as an etch stop at contact hole locations. Second, the etching of the TiN layer becomes simpler: a high fraction of oxygen in the layer under the hardmask makes it possible to wet etch the TiN without undercutting the hardmask patterned interconnects too much. A sample wet etch for this purpose would be NH.sub.4 OH+H.sub.2 O.sub.2 in water at room temperature, but many other wet etch chemistries could be used. That is, titanium silicide typically gives a rough surface, so it is a pleasant surprise that the titanium under the oxide mask reacts with the oxide mask to form titanium oxides as a surface sealant. This surface TiO.sub.2 -rich layer avoids in-migration of etchants during a wet etch and thus permits the simple wet etch process described. In an alternative version of this embodiment, the plasma oxide hardmask is stripped after the silicidation heating step (the 675 C step in the example given) and before the annealing step (the 800 C step in the example given). By ensuring that a nitrogen atmosphere is present during the high-temperature step, any possibility of silicon and dopant outdiffusion is suppressed, and the remaining metallic titanium which the oxide mask protected from the first heating step will substantially all be converted to nitrides, providing additional diffusion barrier advantages as described above. Another (less preferred) alternative way (for NMOS processes or some CMOS processes with two types of polysilicon) to convert the patterned titanium into a stable conductor is to perform the high-temperature anneal in an atmosphere which does not include nitrogen, so that silicon outdiffusion can convert the titanium metal in the local interconnect straps to titanium silicide. Note that the oxide hardmask does not strictly have to be stripped at all--optionally it can be left in place through the whole remainder of the process. A very important advantage of this embodiment is that no TiN etch is required--instead the TiN is simply dipped out using a standard clean-up solution. Thus this embodiment may be the most suitable of all for transfer into a manufacturing environment. Another advantage of this embodiment is that it is very scalable, since the geometry-limiting step is simply an oxide etch. An example of the device applicability of the present invention is shown in FIG. 3. FIG. 3 shows a CMOS inverter, wherein an input signal is provided on a polysilicon line 106 which gates both a PMOS transistor 110 in a PMOS moat region 104 and an NMOS transistor 112 in an NMOS moat region 102. Metal contacts 120 are used to connect one source/drain of the PMOS transistor 110 to power supply, and to connect one source/drain of the NMOS transistor 112 to ground. The output of the inverter is provided on a polysilicon line 122 which (in the portion shown) runs over field oxide. Note that the titanium nitride local interconnect layer 22 is used to connect the polysilicon output line 122 to NMOS moat 102 and to PMOS moat 104, eliminating the need for any buried contact. Note also that other patterned portions of titanium nitride layer 22 are emplaced under the metal contact locations 120, so that the titanium nitride there in place permits the metal contact 120 to overlap onto field oxide, and also provides a diffusion barrier at the bottom of the metal contact. FIG. 4 shows a sectional view of a two-stage inverter, as a sample of the incorporation of the present invention in more complex CMOS logic structures. Patterned local interconnect layer 202 connects N+source/drain region 204 in P-tank 208 to P+source/drain region 206 in N-tank 210, and also connects both of them to polycide line 212 which provides the input to transistors P2 and N2 in the next stage of logic. That is, the output of one stage (transistors P1 and N1) has been connected to the input of the next stage (transistors P2 and N2) WITHOUT ANY CONTACTS. Thus, the present invention provides the advantage of local interconnect, which can connect moat to poly or moat to moat, including p+ moat to n+ moat, with a simplified processing sequence. Moreover, the present invention provides the further advantage that a diffusion barrier and etch stop can be located at the bottom of contact holes, so that metal contact processing is simplified, and dangers of overetch through field oxide 26 (which could create a short circuit to substrate 10) are reduced. The conventional 6T SRAM cell comprises two inverters coupled back to back (in FIG. 5a the first inverter includes transistors P1 and N1, and the second inverter includes transistors P2 and N2), together with two pass transistors (transistors N3 and N4 in the example of FIG. 5a) to selectably connect the inverter outputs to a pair of bit lines BL and BL(bar). Note that in this conventional cell two metal jumpers (labeled MJ1 and MJ2 in this example) are used to connect the pull-up and pull-down transistors of each inverter together. This use of metal jumpers consumes large amounts of space. By contrast, in the sample SRAM cell layout according to the present invention shown in FIG. 5b, wherein titanium nitride is used to make the interconnection, these metal jumpers are not needed. This is, the local interconnects LI shown in FIG. 5b provide a capability for local interconnection of polysilicon to moat; from a circuit designer's point of view, they provide a capability very similar to that provided by first contacts (also known as buried contacts), and the use of titanium nitride interconnects has other advantages too. In the prior art, buried contacts have been used in such layouts to eliminate the need for metal jumpers. Buried contacts are formed by patterning the gate oxide, before the polysilicon gate level is deposited, to form windows where the polysilicon will contact the moat. However, due to the dopant outdiffusion from the polysilicon, the traditional buried contact process to increase packing density becomes less attractive as MOS technology is scaled to the submicron regime. Furthermore, buried contact can only be used for N-channel devices if phosphorus doped polysilicon is used. Moreover, the fact that the gate oxide itself is being directly patterned is an inherent source of processing difficulties. These limitations make the traditional buried contact process incompatible with advanced VLSI CMOS processing, and a replacement is required. FIG. 7 shows the equivalent circuit diagram of the configuration of FIGS. 5a, 5b, and 6. FIGS. 6a through 6e show a complete layout for an SRAM cell using titanium nitride local interconnect according to the present invention. Note that this layout is not quite the same as that shown in FIG. 5b. In FIG. 5b transistors N2 and N4 share a common moat, but, in the layout of FIG. 6, as may be seen in FIG. 6b and 6c, transistors N2 and N4 are in separate moats from each other. Thus, within the single cell layout of FIG. 6c, portions of six different moats are seen. Although a single moat region may contain active devices from more than one cell, the two active pull-down devices in each cell are separated by field oxide (field isolation). This provides important speed advantages, as will be discussed below. The layouts of FIGS. 5 and 6 include some features which are shared with the neighboring memory cells. That is, the cell of FIG. 6 would be bordered on its left and right sides by a left-right-reversed mirror image of itself, at its upper and lower edges by a top-bottom-reversed mirror image of itself, and diagonally by another image of itself which is both left-right and also top-bottom reversed. Thus, the actual scale of repeating geometries is in blocks of four SRAM cells. Moreover, the moat region including transistor P2, shown at the upper right of FIG. 6b and 6c, is only a portion of the actual patterned moat region. That is, the contact to the upper right of transistor P2 (which connects to the V.sub.DD Supply) is shared among four adjacent SRAM cells, and each of those cells has its own transistor P2 close to that contact. Thus, the actual shape of the patterned moat, of which only a quarter is shown, is roughly the shape of a capital H lying on its side. Similarly, the contact to V.sub.DD shown at the upper left corner is also connected through a moat to four closely adjacent transistors P1 and four adjacent SRAM cells. The contacts to V.sub.SS near transistors N1 and N2 are also shared, but these contacts are only shared among two adjacent cells. Similarly, the contacts shown at the bottom of the layout in the orientation shown in FIG. 6c are also each shared among two adjacent SRAM cells. Note that the FIG. 6c the local interconnect level LI is not only used to link the poly level to moat, but also is provided underneath the locations of each of the contacts K. The use of the patterned titanium nitride in these areas provides an etch stop which, as discussed above, helps prevent overetching and digging into the substrate when the contact holes are being etched. FIG. 6a shows the moat and tank levels only, for a single cell. (The tank regions are n-type regions where PMOS devices may be formed; the moat regions define regions of the substrate which are not covered with field isolation). FIG. 6b shows the moat and poly levels (the poly level shows the location of polysilicon lines, which will create active device channel regions, by screening the moat regions from subsequent source/drain implants, wherever they cross a moat). FIG. 6c shows the moat and poly levels of FIG. 6b, together with the local interconnect locations LI (where titanium nitride is left on the surface), and contact regions K (where a contact to substrate or to poly is opened by etching through a thick insulating layer (MLO) (which is typically a silicate glass which has been deposited overall and reflowed and possibly planarized, and which will insulate the first metal layer from the underlying polysilicon wiring and moat regions except where contact holes K have been opened)). Note that the conventional layout of FIG. 5a includes some contacts to poly as well as some contacts to substrate. However, the layout of FIG. 6 does not include any contacts to poly. This is advantageous, for the following reason: Because the thickness of the MLO (silicate glass) overlying the polysilicon wiring will typically be much less than that overlying the substrate regions, the contact etch (which cuts contact holes through the MLO to both poly and to substrate simultaneously) must normally be very highly selective to polysilicon, if it is not to erode the polysilicon (or polycide, or whatever is used for the poly level) during the time after the polysilicon level contacts are opened until the contact holes for the substrate have been cleared. Since the SRAM cell according to the present invention does not require contacts to poly in the array, this processing constraint is minimized. FIG. 6d shows the moat and contact patterns shown in FIG. 6c, together with the metal 1 pattern. The horizontal strip of metal 1 shown at the top of FIG. 6d is a V.sub.DD supply line, the middle horizontal strip is a V.sub.SS supply line, and the bottom two metal 1 portions will make contact (using the VIA pattern as shown in FIG. 6e) to the bitlines BL and BL(bar) in second metal, which are shown in FIG. 6e. A sample flow to produce the local interconnects using the embodiments of FIGS. 5b and 6 would be as follows. After source/drains have been formed, the titanium is deposited overall and is reacted in a nitrogen atmosphere, to form titanium silicides over moat regions and titanium nitride elsewhere. Photoresist is then patterned to protect the areas where it is desired to keep the local interconnects LI, as shown e.g. in FIG. 6c, and plasma etching is then performed. A sample set of etch conditions is 200 sccm of carbon tetrafluoride (CF.sub.4) plus 50 sccm of helium at a total pressure of 1 Torr for 25 seconds. An initial titanium nitride layer of 1000 A will be 90% etched away under these conditions, i.e. the resultant TiN etch rate is about 36 Angstroms per second. In this sample embodiment, this step is performed in a single plate plasma reactor with a power of 200 Watts at a substrate temperature of 50 degrees C. Next, a wet etch is used to strip the remainder of the undesired titanium nitride, to avoid shorts to the filaments. A solution of ammonium hydroxide (3000 cc) plus hydrogen peroxide (1750 cc) plus water (24 liters) is ultrasonically agitated for 9 minutes, using a duty cycle of 10 to 20 percent per 60 seconds, at an ultrasonic power of 250 Watts. Next, a conventional ashing step is used, e.g., 60 minutes in an asher, followed by a solvent resistant stripping operation, e.g. 10 minutes in R--10 (a commonly used organic resist stripping solution, consisting of about 70% diethyleneglycolmonobutyl ether in ethanol) at 50 degrees C., followed by a cleanup in dilute HCl (37% HCl solution diluted 1 to 1 with water) for 20 minutes. Next, an anneal to enhance the silicide quality is performed, e.g. 30 minutes at 800 degrees C. Next, the MLO layer is deposited conventionally, and the contacts are patterned and etched. However, after the contacts are etched, the deglazing step (which removes unwanted oxides from the bottom of the contact hole, which can degrade contact resistance or even destroy the contact) is performed using buffered HF. It has been found that buffered 1% HF, at least if buffered with ammonium fluoride, will not attack the titanium nitride local interconnect layers. A sample of the solutions with which this has been performed is 1% HF:NH.sub.4 F:H.sub.2 O in the ratio of 49 to 40 to 11 by weight. The processing then proceeds conventionally, with first metal deposition and patterning, interlevel oxide deposition, via patterning, second metal deposition and patterning, and protective overcoat patterning, in a normal sequence. It should be noted that, instead of performing the titanium nitride etch as described above, before the high temperature anneal which gives the clad moats good low sheet resistance, it is alternatively possible to ramp directly up the 800 degrees C. anneal immediately after the initial silicidation reaction. It has been found that, surprisingly, using this process sequence, the titanium nitride can still be stripped efficiently from areas where it is unwanted. It should also be noted that the cleanup conditions prior to the furnace reaction are quite important; the use of dilute HCl as noted above works well, but with some other plausible cleanups titanium nitride filaments may be found to run along the gate edges, and create shorts from moat to moat. Another advantage of using the local interconnect layer in the contact holes is that, as is well known, titanium nitride is quite a good diffusion barrier. Thus, using the titanium local interconnect layer at these locations provides better device yield and reliability, by reducing diffusion effects between aluminum and silicon. It should be noted that, not only does the area of the cell of the present invention provide an area advantage of about 20% over the prior art cell of identical geometries, but moreover the area of the sample cells in FIGS. 5b and 6 is limited by the pitch of the second layer metal. Thus, if the second metal pitch can be decreased, this cell could be further shrunk, which would have the potential to further increase the area advantage of this cell over the conventional cell. A major advantage of the present invention is reduced moat capacitance. Since the contact pattern is not required for strapping moat to poly, the moat area can be kept strictly to its minimum value, and therefore the moat junction parasitic capacitance is reduced. This reduced parasitic loading capacitance directly results in an improved switching speed for the cell. The present invention is believed to provide approximately a 35% reduction of moat junction capacitance over conventional SRAM cell layouts, with the corresponding improvement in switching speed. A further advantage of the titanium nitride in contact holes is that many a good contact etch, such as C.sub.2 F.sub.6 +CHF.sub.3 +O.sub.2 +He, is more selective to titanium nitride than it is to titanium silicide. Thus, degrading resistance of the contacts by increasing the sheet resistance of the clad moat underneath the contact is a reduced risk. Thus, it is believed that the present invention is the first to provide a 6 transistor SRAM cell having compact geometries in which no two devices within any one cell share a common continuous moat region. This is related to the advantages of reduced junction parasitic capacitance discussed above. Of course, SRAM cells of various kinds can embody the novel concepts taught by the present invention: full-CMOS 6T cells (NMOS pull-down and pass (access) transistors, and PMOS pull-up transistors), "upside-down" full-CMOS 6T cells (i.e. with PMOS pass transistors), NMOS or pseudo-NMOS cells (using a linear or nonlinear resistance, or even an SOI transistor, as a load element), etc. Another very important and novel class of embodiments taught in the present application, which were not specifically disclosed in the parent application, provides small-geometry CMOS integrated circuits having two conductivity types of polysilicon (or analogous material) for the gate level, with the two conductivity types being electrically connected by a local interconnect, substantially comprising titanium nitride, as taught in the parent application. FIG. 8 shows an example of this embodiment. The SRAM cell shown as a sample layout is very similar to the SRAM cell of FIG. 5b, except that the polysilicon is not continuous from the p-channel transistors to the n-channel transistors: the gates of transistors P1 and P2 are p+ polysilicon, and the gates of transistors N1, N2, N3, and N4 are n+ polysilicon. The polysilicon over the PMOS areas is exposed to the p+ source/drain implants, and the polysilicon over the NMOS areas is exposed to the n+ source/drain implants. If the source/drains are counterdoped, the polysilicon will be also. The polysilicon may be originally deposited undoped, or it may be subjected to a slight blanket doping (either p- or n-type) to balance the conductivities of p+ and n+ poly lines as desired. It may be desirable (though not necessary) in this embodiment to use thinner polysilicon and/or higher dose source/drain implants than would otherwise be the case, since (for example) the dose which will dope a 1500 A deep source/drain to an average concentration of 1E18 cm.sup.-3 will only dope a 5000 A layer of polysilicon to about 3E17 cm.sup.-3 (or less, depending on how much of the dopant species is gettered by the grain boundaries and fails to ionize). Another very advantageous use of the local interconnect structure is, in a double-poly process, to provide a triple interconnect for first and second polysilicon layers. That is, a single direct-react silicidation process can be used to silicide moat, poly 1 (partially), and poly 2 at the same time, and the TiN local interconnect level formed during this silicidation step can be patterned to connect first poly, second poly, and moat, in any desired combination, all in one single interconnect level. This local interconnect improvement can readily be inserted into the double-poly processes commonly used in analog parts, where the poly 2 layer is used as the primary gate layer, and the poly 1 layer is used for poly-to-poly capacitors, and/or (sometimes) resistors. At present such processes normally do not permit direct connection of poly 1 to poly 2, but metal jumpers must be used instead. It would be highly advantageous to be able to do direct local interconnect between poly 1 and poly 2 and moat, and the present embodiment of the TiN process flow permits this. In analog processes the poly-to-poly dielectric is thin, and the coupling capacitance is very high, but this interconnect improvement can also be used with much thicker poly-to-poly dielectrics, so that crosstalk between poly 1 and poly 2 is not large. For example, a poly-to-poly dielectric of 1000 A is thick enough for many applications, and this thickness of oxide can be cleared during the sidewall oxide filament overetch. This not only facilitates layout in analog parts, and similar places where a double-poly process is already in use; it also provides designers with a full independent layer of interconnect, with minimal additional processing overhead and without intruding into the metal layer. This is highly advantageous for logic layout generally. It is particularly advantageous in custom and semi-custom layouts, where segmentation of the design process makes it desirable to leave the metal layers free as much as possible. It is also useful in memory array layout. This improvement is applicable to most processes where the poly 2 level is used as a silicided gate layer, regardless of whether the poly 1 layer is used for gates, or for capacitors to moat, or for poly-to-poly capacitors, or simply for a fully independent interconnect layer. It may also be possible to use this process with a refractory metal interconnect layer in place of the poly 1 layer (although encapsulation of this layer then becomes more difficult). For example, in advanced SRAM cell layouts the second metal pitch can become a constraint on compaction. Use of a poly 1 layer as a buried interconnect means that the bitlines can be shared, because doubled wordlines can be used so that each wordline accesses only alternate cells. The problem of routing the duplicate wordline through the cells where it is not needed is solved by using the additional routing capability of the poly 1 level to route this line through the wasted space over the tank boundary. This capability is also useful in scaled CMOS, where the poly 1 layer may be doped p+ and used for PMOS gates, and the poly 2 layer doped n+ and used for NMOS gates. The diffusion barrier characteristics of the TiN mean that short lengths of p+ and n+ poly can be pieced together using TiN straps to form electrically efficient routings, without worrying about counterdiffusion. The advantages of using TiN to link p+ and n+ polycide gates for submicron CMOS were pointed out above; the additional suggestion in this embodiment is merely that the p+ and n+ polycide gate portions can be provided by two different levels poly 1 and poly 2, instead of by implanting a single poly layer. This capability may also be very useful in high-voltage CMOS circuits, where one level of poly is used for gates for high-voltage transistors and another level is used for gates of normal logic. For example, the triple interconnect process described here permits poly 1 gates of low-voltage transistors to be connected locally to moats of low-voltage or high-voltage transistors, and poly 2 gates of high-voltage transistors to be connected locally to moats of low-voltage or high-voltage transistors, and also permits interconnection of low-voltage transistors' source/drain regions to source/drain regions of high-voltage transistors, and also permits poly 1 gates to be connected directly to poly 2 gates. The high-voltage transistors will be formed to have higher punchthrough voltage from source to drain, and/or higher breakdown voltage across the source/drain junction, and/or higher gate-to-source breakdown voltage, than the low-voltage transistors. To achieve this, for example, poly 2 may be patterned so that the high-voltage transistors have longer effective channel length, and an additional V.sub.T or LDD implant may be performed after poly 1 has been patterned. In a further alternative embodiment, if sidewall oxides are formed both on poly 1 and poly 2 levels, the layout and oxide filament deposition may be adjusted so that the poly 1 gates have two sidewall oxide filaments, and therefore the high-voltage transistors (in poly 1, in this embodiment) have LDD regions which have a greater spacing between the gate edge and highly doped source/drain regions than the low-voltage transistors do. Moreover, the gate oxides can easily be made thicker for the high-voltage devices, since the gate oxides for the two levels are grown in separate steps. Alternatively, a masked tank implant may be used, so that the high-voltage transistors (at least the NMOS ones) will have a different substrate concentration in the channel than the low-voltage transistors will. For example, counterdoping may be used with tank implants or with source/drain implants, so that one type of NMOS transistors have counterdoped source/drains or tanks (i.e. are exposed to the corresponding implants for the PMOS devices as well), and the other type of NMOS transistors are masked from the PMOS (counterdoping) implants. Such circuits may include EPROMs, high-voltage drivers (to control analog, high-voltage, and/or power devices off-chip), and/or control logic. A sample process flow for this embodiment is as follows: 1. Form moats and field isolation regions. 2. Grow gate oxide, and deposit, dope and pattern poly 1 3. Strip moats and grow a new gate oxide, e.g. 200 A. This step will also grow a thicker oxide on the poly 1 layer, e.g. 750 A. 4. Deposit poly 2, e.g. 4000 A thick POCl.sub.3 -doped. 5. Pattern and etch poly 2. 6. Perform LDD implants if desired. 7. Sidewall oxides: Deposit conformal oxide and overetch to leave sidewall oxide filaments and also clear oxide from on top of exposed portions of poly 1. (Overetches to clear up to 1000 A of oxide during the filament overetch are well known.) 8. Perform source/drain implants (self-aligned to poly 1 and poly 2 gates), masked if for CMOS. 9. Deposit titanium overall, and heat it for 30 minutes in N.sub.2 at 675 C to form TiSi.sub.2 on exposed moats and polysilicon lines, and TiN elsewhere. Poly 2 will now be silicide clad everywhere; poly 1 will be silicided everywhere except where it is crossed by poly 2; and moat will be silicided everywhere except at poly 1 and poly 2 locations. 10. Pattern TiN to form local interconnects between moat, poly 1, and poly 2 in whatever pattern is desired. 11. Anneal at 800 C to activate S/D implants and lower silicide sheet resistance. 12. Processing continues conventionally with MLO, contact patterning, metal, etc. In an alternative embodiment, using a nitride dielectric for poly-to-poly capacitors, a sample process flow is as follows: 1. Form moats and field isolation regions. 2. Deposit and dope first poly. 3. Form a protective dielectric over first poly by, e.g., growing and stripping sacrificial oxide, growing 100 A oxide, and depositing 100 A of LPCVD nitride. The nitride will provide protection against uncontrolled oxide growth over poly 1 during the second gate oxide growth step. 4. Pattern and etch oxide/nitride/polysilicon stack. 5. Strip exposed moat portions (short oxide etch so as not to remove protective dielectric from first poly) 6. Grow a new gate oxide, e.g. 200 A. (This will also convert 50 A or so of nitride to oxide on the surface of the protective dielectric over poly 1, and will also help oxidize and passivate any pinholes in the poly 1 encapsulation.) 7. Deposit second polysilicon, e.g. 4000 A thick POCl.sub.3 -doped. Less preferably, this level may be made substantially thicker than normal, to provide more margin in the filament overetch. 8. Pattern and etch poly 2. 9. Perform LDD implants if desired. 10. Sidewall oxides: Deposit conformal oxide and overetch. The filament overetch will also clear the thin oxide/nitride/oxide stack from the first polysilicon layer. This step will provide sidewall oxides on both poly 2 and poly 1. 11. Perform source/drain implants, masked if for CMOS. 12. Deposit titanium overall, e.g. 1000 A sputtered at room temperature, and heat it 30 minutes in N.sub.2 at 675 C to form TiSi.sub.2 on exposed moats and polysilicon lines, and TiN elsewhere. 13. Pattern TiN to form local interconnects between moat, poly 1, and poly 2, in whatever pattern is desired. 14. Anneal at 800 C to activate S/D implants and lower silicide sheet resistance. 15. Processing continues conventionally with interlevel dielectric, contact etch, metal, etc. One possible constraint on this process is topography: it may be appropriate to have a design rule requiring a minimum offset between poly 1 edges and poly 2 edges. Optionally, the second polysilicon layer may be deposited to be substantially thicker than normal, e.g. up to 10,000 A, to provide more margin in the filament overetch and avoid oxide filaments where poly 2 crosses poly 1--such filaments can cause a break in the silicide layer which overlies the poly 2 level, and thus increase the series resistance of poly 2. If the poly 2 layer is to be made extremely thick, the grain size can be kept less than the minimum linewidth (in one alternative embodiment) by depositing the poly 2 as a layered structure; for example, a very thin layer of a leaky dielectric (thin enough that conduction can easily occur by tunneling, e.g. 30 A of nitride oxide) may be included in the middle of the thick polysilicon layer by changing the gas flow briefly during deposition. Low temperature deposition (e.g. as low as 560 C) may also be useful to keep grain size small. In this embodiment, it may also be desirable to use a polysilicon etch which has an extremely high selectivity to oxide, so that the overetch portion of the polysilicon etch does not cut through the gate oxide where poly crosses moat; for example, etches using bromine-based chemistries (such as HCl plus HBr) readily provide selectivities of 30:1 or better in production environments. Another optional feature of this class of embodiments is to impose a design rule which forbids making contact from metal directly to poly 2. (In the present invention, such a design rule is much less onerous than it would otherwise be, since a short local interconnect run can be used to link poly 2 to a contact pad on field oxide or on a plug of poly 1. Usually it will be preferable to forbid contact from metal to poly 2 where it crosses over poly 1, even if poly 2 is not made unusually thick. In future alternative embodiments, the filament etch can be done using a chemistry which is slightly selective to oxide, e.g. NF.sub.3 (which in an 8110 gives about a 2:1 nitride/oxide ratio) or SiF.sub.4 (which will etch nitride about 4:1 over oxide). With a 2:1 nitride/oxide ratio, and a 1000 A overetch during the sidewall oxide filament etch, as much as 2000 A of nitride could be cleared from exposed flat surfaces of poly 1. It should also be noted that the present invention permits not only local contacts to moat, but also to buried diffusions, i.e. diffusions which are covered by thick enough oxide (e.g. 1000 A) that an active device does not necessarily occur at every point where polysilicon crosses it. For example, the filament overetch of the poly gate level can be operated with enough overetch to clear the buried diffusion in all exposed regions, so that silicidation will occur and TiN straps can link buried diffusions to poly 1, poly 2, etc. This embodiment of the invention may also be advantageous as a way to configure SRAM cells where a resistor is used to separate the PMOS latch from the NMOS latch, to minimize the chance of upset by transient events. For such structures, first poly can be used to configure compact resistors over the tank boundary, and the TiN local interconnect can be used to make local connection to these resistors without requiring contacts to metal. This embodiment of the invention also provides one way (albeit somewhat inelegant) to solve a problem which has been pervasive in double-poly processes: formation of the sidewall oxide filaments on the poly 2 level also tends to leave oxide filaments where poly 2 crosses over poly 1, and these unwanted filaments will cause breaks in the siliciding of poly 2. These breaks in a regular pattern can cause excessive series resistance. In many processes, these filaments can be avoided by putting sidewall oxides on poly 1, but, if need be, these breaks can be bridged using the present invention, by strapping them with TiN jumpers. Moreover, while the present application has referred primarily to titanium-based thin film compositions, other metals could be substituted in. For example, any metal which (1) reacts with silicon to form a conductive and stable silicide, and (2) whose nitride is (2a) conductive and (2b) stable and (2c) a reasonable diffusion barrier, can be used for direct-react silicidation and simultaneous formation of a conductive nitride which can be used to form local interconnects according to the present invention; candidates include molybdenum, tungsten, vanadium, cobalt, and others. Of course the poly gate layer (or poly 1 and poly 2 gate layers) so frequently referred to in the present application does not have to be strictly polysilicon, but can be other materials which are substantially polycrystalline or amorphous and contain a large fraction of silicon; in this usage silicides and polysilicon/silicide sandwich structures are certainly comprehended, and future sandwich structures which have analogous deposition and electrical properties in future processes to the place of polysilicon in present processes are intended to be comprehended also. It should also be noted that the present invention is applicable to a tremendous variety of processes which are quite different in many respects from that described; for example, the present invention is perfectly well applicable to SOI processes, or to processes using trench transistors, as well as to the more conventional bulk-CMOS processes primarily described. As will be appreciated by those skilled in the art, the present invention can be widely modified and varied, and its scope is not limited except as specified in the allowed claims.
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