Abstract:
A low temperature coefficient resistor (TCRL) has some unrepaired ion implant damage. The damaged portion raises the resistance and renders the resistor less sensitive to operating temperature fluctuations A polysilicon thin film low temperature coefficient resistor and a method for the resistor&#39;s fabrication overcomes the coefficient of resistance problem of the prior art, while at the same time eliminating steps from the BiCMOS fabrication process, optimizing bipolar design tradeoffs, and improving passive device isolation. A low temperature coefficient of resistance resistor (TCRL) is formed on a layer of insulation, typically silicon dioxide or silicon nitride, the layer comprising polysilicon having a relatively high concentration of dopants of one or more species. An annealing process is used for the implanted resistor which is shorter than that for typical prior art implanted resistors, leaving some intentional unannealed damage in the resistor. The planned damage gives the TCRL a higher resistance without increasing its temperature coefficient. A process for fabrication of the resistor is used which combines separate spacer oxide depositions, provides buried layers having different diffusion coefficients, incorporates dual dielectric trench sidewalls that double as a polish stop, supplies a spacer structure that controls precisely the emitter-base dimension, and integrates bipolar and CMOS devices with negligible compromise to the features of either type.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/345,929, filed Jul. 1, 1999, in now U.S. Pat. No. 6,351,021, and a conversion of U.S. Ser. No. 60/155,027 filed Sep. 20, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     Advanced wireless communications products demand integrated circuit technologies with high performance, high levels of system integration, low power and low cost. For wireless applications up to several GHz, silicon BiCMOS technology is uniquely suited to meet these requirements. Of critical important to RF design is the availability of high quality passive components. In particular, it is desirable to have implanted thin film resistors that have a low temperature coefficient of resistance. Unfortunately, existing techniques for polysilicon thin film resistors generally result in thin film resistors with relatively large temperature coefficients of resistance. 
     SUMMARY 
     The invention comprises a polysilicon thin film low temperature coefficient resistor and a method for the resistor&#39;s fabrication that overcome the coefficient of resistance problem of the prior art, while at the same time eliminating steps from the BiCMOS fabrication process, optimizing bipolar design tradeoffs, and improving passive device isolation. The low temperature coefficient of resistance resistor (TCRL) is formed on a layer of insulation, typically silicon dioxide or silicon nitride. The layer comprises polysilicon that has a relatively high concentration of dopants of one or more species, and has a substantial amount of unannealed implant damage. Contrary to prior art methods, the implanted resistor is annealed less than typical prior art implanted resistors in order to leave some planned unannealed damage in the resistor. The planned damage gives the TCRL a higher resistance without increasing its temperature coefficient. Thus, even though the temperature may increase, the relative value of the resistance remains the same. As such, the resistor is more precise than others produced with current methods, and may be used where precision requirements for high quality RF devices apply. A process for fabrication of the resistor is used which combines separate spacer oxide depositions, provides buried layers having different diffusion coefficients, incorporates dual dielectric trench sidewalls that double as a polish stop, supplies a spacer structure that controls precisely the emitter-base dimension, and integrates bipolar and CMOS devices with negligible compromise to the features of either type. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In order to highlight features of the invention while showing them in their proper context, the proportions shown in the figures are not to scale. 
     FIGS. 1-19 show sequential process steps in the formation of a TCRL in a BiCMOS process 
     FIGS. 20-25 show experimental results for the TCRL. 
     FIG. 26 shows a more-detailed cross-section of the NPN bipolar device formed in the invention&#39;s BiCMOS process. 
     FIG. 27 shows one embodiment of the present invention that includes two bipolar devices. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     All figures show the lateral division of the regions of the substrate into CMOS regions  100 , bipolar NPN regions  200 , and transition regions  150  between the CMOS and bipolar regions. The regional divisions are shown by dotted lines. 
     Refer first to FIG. 1. A P-type substrate has its upper surface covered with a suitable ion implantation mask such as deposited oxide, thermally-grown oxide or photoresist. Openings are made in the resist mask to define the N+buried layer regions  12 . 1  and  12 . 2 . Those regions are implanted with a first N-type dopant such as Arsenic. The implantation mask is then stripped. 
     The substrate is then covered with a second suitable ion implantation mask such as deposited oxide, thermally-grown oxide or photoresist. Openings are made in the mask to define other buried layer regions  12 . 3 , into which are implanted a second N-type dopant with a significantly different diffusion coefficient than the first. An example of another buried layer region  12 . 3  is illustrated in FIG.  27 . The two different buried layer dopants enable the fabrication of transistors with varying collector profiles, which can be tailored to address speed versus breakdown voltage tradeoffs in the RF devices. Two different collector profiles, coupled with the use of the selectively implanted collector, provide for an integrated circuit with four NPN devices. 
     N+buried layers  12 . 1  and  12 . 2  are driven in with a suitable annealing operation and an N-type epitaxial layer  11  is grown on top of the substrate  10 . As a result, the substrate  10  is patterned into CMOS regions  100  that are separated from bipolar NPN regions  200  by transition regions  150 . The N-type buried layers  12 . 1  and  12 . 2  are formed beneath regions that will receive P-type wells. No buried layer is required for the N-type wells. 
     The initial trench formation step is shown in FIG.  2 . Isolation trenches are formed between transition region  150  and the NPN transistor region as well as in other locations as needed for improved lateral isolation. A trench photoresist mask  20  is uniformly deposited and patterned on the substrate  10 . The photoresist is developed to expose the trench regions  21 . A suitable wet or dry etch operation etches the trenches  21  down to a level below the N+buried layers  12 . 1  and  12 . 2 . The bottoms of the trenches are then implanted with a suitable P+channel stop  22 . 
     As shown in FIG. 3, the next step includes stripping the photoresist  20 , performing thermal oxidation on the trench sidewalls and depositing and patterning a sidewall dielectric layer  23  such as an nitride layer. Oxidation layer  23  is densified, providing a polish stop for planarization. Nitride in this layer has the feature of closely matching the thermal characteristics of silicon. The layer is formed at a thickness which is thin enough to prevent any overhang of the trench cavity, thereby allowing complete trench fill during subsequent deposition steps. Oxidation layer  23  also provides a pad oxide for LOCOS at a later stage. The combination of thermal oxidation, oxide deposition and oxide densification allows the trench sidewall to match the thermal expansion rate of the silicon substrate. 
     An alternate embodiment would be to deposit the sidewall dielectric layer in such away that would cause subsequent trench fill to form a void in the trench which is below the surface of the silicon substrate. This feature provides stress relief and eliminates silicon defect generation in the silicon adjacent the trench. 
     The substrate  10  is then subjected to a polysilicon deposition step that deposits a polysilicon layer  24  over the substrate  10  and epitaxial layer It and fills the trenches  21 . The undoped polysilicon fill is a semi-insulating material, which provides a favorable electrical characteristic for RF parasitic capacitances. 
     FIG. 4 shows completion of the trenches. The substrate  10  and epitaxial layer  11  are planarized to remove the layers of polysilicon  24  and the thermal oxide  23  from the surface of the substrate  10  and epitaxial layer  11  in all areas except above the trenches. Such planarization is accomplished with a conventional chemical mechanical polishing operation. The nitride underneath the polysilicon serves as a hard stop during the polish operation and protects the underlying oxide and silicon from damage. The thinness of the oxide nitride sandwich also assures the precise match of the polished trench polysilicon surface to the original silicon surface. 
     It is important both to protect the trenches  21  and to cover the NPN region  200  during formation of the CMOS devices. Likewise, it is a goal of this process to combine as many of the CMOS and bipolar processing steps as possible. Accordingly, turning to FIG. 5, the trenches are initially protected from the subsequent CMOS processing steps. This protection includes forming a pad oxide layer  51  over the trenches. Pad oxide layer  51  is followed by an N+sinker photoresist deposition, patterning, and implantation step to form the N+sinker  52  for the future collector of the NPN transistors  200 . Next, a layer of silicon nitride  54  is deposited over the pad oxide  51  on the surface of the substrate  10  and epitaxial layer  11 . The silicon nitride is initially patterned to expose local oxidation (LOCOS) regions  50 . Following LOCOS patterning, a conventional LOCOS operation fabricates LOCOS regions  50  that provide surface lateral isolation of the NMOS and PMOS devices  100  and separate the sinker diffusion  52  from the rest of the NPN transistor  200 . The silicon nitride is stripped from the rest of the surface of the substrate  10  and epitaxial layer  11  except for regions above the trenches  21 . 
     During the LOCOS operation, a ‘skin’ layer of silicon dioxide forms on the surface of the nitride oxidation mask. This skin layer is patterned using conventional photoresist and wet etch, leaving the skin layer over the trench regions. After photoresist removal, the nitride is removed in a suitable wet etch chemistry except for regions above the trenches  21  The use of this oxide layer allows simultaneous protection of the trench areas and removal of the nitride in a manner completely benign to the underlying pad ox and silicon substrate regions. Protection of these regions from further stress-generating thermal oxidation is important to the successful fabrication of shallow transistor structures, which follows as taught in U.S. Pat. No. 5,892,264. 
     The pad oxide is then removed from the surface of substrate  10  and epitaxial layer  11  to expose the surface for further fabrication. 
     Refer now to FIG.  6 . In the next step, a sacrificial oxidation is performed on the surface of epitaxial layer  11 . The oxidation is atypical first step in the formation of N-wells and P-wells for the CMOS devices  100 . Suitable photoresist masks and implants  62  provide the N-wells and P-wells for the CMOS devices. A heavier P-type implant  64  provides junction isolation to separate PMOS and NMOS devices. Following removal of the sacrificial oxide, a gate oxide layer  65 , typically a thermal oxide, is grown on the surface of epitaxial layer  11 . That step is followed by uniform deposition of a layer of polysilicon which is subsequently patterned and doped to form polysilicon gates  66 . 
     The next stage in the fabrication of the CMOS transistors is shown in FIG.  7 . Next, the NMOS and PMOS drains receive a typical lightly-doped drain implant  72  (N) or (P) respectively (the P-type implant is not shown here) for forming the N-type lightly-doped drain regions and the P-type lightly-doped drain regions. An annealing step drives the lightly doped drains slightly under the sidewall of the gates. The lightly doped drain regions use the sidewalls of the gate as masks. These regions are self-aligned in a conventional manner using the gate as masks followed by suitable P-type and N-type implants. Following that step, in a region not shown in the figure, a typical P+resistor is formed in the N-type epitaxial region  11  using a suitable photoresist and implant. Next, an NPN protection spacer oxide layer  78  is uniformly deposited over epitaxial layer  11 . The spacer oxide  78  covers the transition region  150  and NPN region  200  of layer  11 . Without this spacer oxide coverage, subsequent CMOS processing steps would interfere with the formation of the NPN transistor. The spacer oxide layer over the gate  66  is patterned and removed to leave sidewall spacers  70 . 1 ,  70 . 2  at the edges of the gate  66 . 
     The spacer oxide layer  78  not only provides the sidewall spacers for the CMOS devices but also provides a hard mask and surface isolation for the active elements of the NPN transistor. Performing this deposition step early in the process saves one or more deposition and masking steps later in the process. As a result, the spacer oxide layer  78  forms the mask for the self aligned sources and drains of the CMOS devices and the mask for the collector and emitter openings  126 ,  127 , respectively. See FIG. 12 for these later process effects 
     The next CMOS processing step is shown in FIG. 8. A screen oxide layer  80  is deposited and patterned to cover the lightly doped source and drain regions of the CMOS device. Those regions are then suitably implanted with either P+or N+ions to form sources  81  and drains  82 . The respective P-type and N-type sources and drains are then subjected to an annealing operation where the diffusion time is set to adjust the depth of the sources and drains. While the figures show only one MOS device, those skilled in the art understand that the process disclosed herein can be used to form multiple transistors including pluralities of NMOS, PMOS and bipolar devices (see FIG.  27 ). 
     Having completed the formation of the CMOS transistors, the process protects the CMOS transistors while fabricating the NPN transistors. As a first step, a CMOS nitride etch stop protection layer  90 , as shown in FIG. 9, is uniformly deposited over epitaxial layer  11 . On top of the nitride protection layer, there is deposited a CMOS oxide protection layer  92 . Since the two protection layers can be selectively etched with respect to each other, the combination of deposited layers in two sequential steps saves a substantial number of future process steps by using the two layers as different etch stops. 
     A photoresist layer  94  is deposited and patterned to cover the CMOS devices and at least part of the LOCOS region that extends from the transition region  150  into the CMOS region  100 . The CMOS oxide protection layer  92  and nitride protection layer  90  are stripped from the exposed NPN region  200  using suitable wet etchings. As a result of sequential etching operations, the spacer oxide layer  78  is exposed as shown in FIG.  10 . 
     Turning to FIG. 11, a, photoresist layer  110  is uniformly deposited over spacer oxide layer  78  and patterned to have openings  112  and  114  in the NPN section  200 . With the photoresist  110  in place, the spacer oxide in exposed regions  112  and  114  is removed in order to expose the surface of the sinker diffusion  52  and the surface of the subsequent NPN transistor  200 . 
     In the formation of the NPN transistor, the process forms the extrinsic base first, then the intrinsic base, and finally the emitter. The extrinsic base comprises a stack of layers that are deposited on the epitaxial layer  11  Turning to FIG. 12, these layers include a doped polysilicon layer  120 , a tungsten suicide layer  121 , a polysilicon cap layer  122 , an inter-poly oxide layer  123  and a titanium nitride anti-reflective coating  124 . The polysilicon layer  120 , WSi layer  121  and polysilicon cap layer  122  are deposited followed by an implant of boron that will form the doping for the extrinsic base  222 . The polysilicon cap layer is included to prevent the boron doping from segregating heavily at the top of the poly/WSi layer and not adequately diffusing into the silicon to create the extrinsic base. It also prevents unwanted sputtering of the WSi layer during the boron implant, which could potentially contaminate the implant tool with heavy metallics. 
     The stack is suitably patterned to form the emitter opening  127 . As a result of thermal processing, dopants from layer  120  form the extrinsic base  222 . A further boron implant through the emitter opening forms the intrinsic base  220 . With the patterning mask for the stack still in place, a SIC (Selectively Implanted Collector) implant  224  is also made through the intrinsic base  220  and the emitter hole  127 . The stack pattern mask helps mask the high energy SIC implant and creates a perfect self-alignment of the SIC to the transistor. The SIC implant  224  contacts the N+buried layer  12 . 2 . The SIC implant  224  is annealed, the emitter surface is oxidized and a P-type implant completes the intrinsic base  220 . 
     Turning to FIG. 13, a layer of base spacer oxide  130  is deposited to mask the base region. A nitride spacer layer  131  is deposited and etched to open the emitter region. The base spacer oxide is etched with suitable hydrofluoric acid. The structure of the composite spacer allows the emitter-to-extrinsic-base spacing, and hence, speed-versus-breakdown device tradeoffs, to be varied easily by changing the nitride spacer deposition thickness, the base spacer oxide etch time, or both. Next, an emitter polysilicon layer  132  is deposited and patterned to form the emitter contact  134  and the collector contact  133 . In a subsequent annealing operation (see FIG.  17 ), the N-type dopants from the emitter poly layer  132  diffuse into the surface of the epitaxial layer  11  in order to form the collector surface contact and the emitter of the NPN transistors  200 . 
     FIGS. 14 and 15 show the formation of the polysilicon resistor with a relatively low temperature coefficient of resistance (TCRL) resistor  141 . As a first step, a protective oxide  140  is deposited over the emitter polysilicon layer  132 . This layer protects any exposed emitter polysilicon layer  132  from etching when the TCRL regions are defined. A polysilicon layer  142  is deposited in the opening. Next, the polysilicon layer is implanted with a BF 2  implant  143  Finally, the TCRL  141  is covered with a photoresist and etched to its suitable size As shown in FIG. 15, the TCRL layer  141  is then covered with a protective oxide  144  The oxide is suitably patterned and masked to protect the underlying portion of the TCRL  141 , while uncovering the contact regions of the resistor. It will be noted that the TCRL poly layer is deposited late in the process. As such, it is possible to deposit an amorphous silicon film and then adjust its resistivity by adding dopants. 
     This process of the invention forms a TCRL resistor  141  that has a resistance of 750 ohms per square and a temperature coefficient of resistance that is less than 100 parts per million (ppm). The resistor is formed using a non-selective BF 2  implant to dope the polysilicon layer. A 900° C. rapid thermal annealing (RTA) step activates the resistor implant and sets the final doping profiles for the bipolar and MOS devices  200 ,  100 . It will be noted that a TCRL poly layer is deposited late in the process. The invention&#39;s process deposits an amorphous silicon film and then adjusts its resistivity by adding dopants. A non-selective BF 2  implant is used to dope the film. A mask is used to clear oxide from all contact areas and a 900° C. RTA step activates resistor implants to set the final doping. Resistor contacts are consequently silicided before final back end processing. 
     The TCRL resistor  141  separates the resistance from temperature sensitivity. In the prior art, it was assumed that high resistivity resulted in a greater temperature sensitivity. Antecedents to the inventive process attempted to separate those two characteristics by providing a relatively thin film with dopings adjusted to set the resistivity to 750 ohms per square. As BF 2  implants approach a high level, an unanticipated and counter-intuitive increase in resistance was observed. This behavior was not observed when only boron was used to dope this film. Normal expectations were that higher implant levels would decrease resistance, not increase it It appears that the heavier ion (BF 2 ) in high doses creates a large amount of damage in the polysilicon film and that this damage cannot be annealed at a relatively low temperature (900° C.) with short thermal annealing (RTA) to activate the implants. The implant damage apparently creates additional trapping sites for carriers resulting in increased resistance at higher implant doses. It is believed that co-implantation of other ions could produce similar results making it possible to use the same high dose boron implant to produce even higher value resistors as well as emitters for PNP&#39;s or low resistivity extrinsic bases for NPN&#39;s or the sources and drains of MOS devices. In our preferred embodiment, the polysilicon layer  142  has a thickness of 70 nm and may be in a range of from 65 nm to 75 nm. The implant concentration of boron ions  142  is 1.3×10 16  and may be in a range from 9×10 15  to 1.5×10 16 . 
     Early in the invention&#39;s development, three film thicknesses with a medium boron dose were chosen for evaluation. As shown in table 1, the thinnest film came the closest to the objective of 750 ohms per square. However, the TCRs of all cells were above the goal of 100 ppm. A second set of tests left the film thickness at the thin setting and varied the implant dose over more than one decade with the expectation that the higher doses would result in lower sheet resistances and lower TCRs. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 TCR/RS vs. Poly Thickness 
               
             
          
           
               
                   
                 Poly Th. 
                 Rs 
                 TCR 
               
               
                   
                   
               
               
                   
                 Thin 
                 650 
                 228 
               
               
                   
                 Med. 
                 532 
                 238 
               
               
                   
                 Thick 
                 431 
                 292 
               
               
                   
                   
               
             
          
         
       
     
     At first, as indicated in FIG. 20, there was very little change in sheet resistance and TCR with increasing doses. However, as the implant levels started to approach the highest levels, an unanticipated increase in resistance was observed while the TCRs experienced a sharp decline until they became negative at the highest dose. 
     Yamaguchi, et al. [Yamaguchi, et al., “ Process and Device Characterization for a  30 -GHz ft Submicrometer Double Poly-Si Bipolar Technology Using BF 2 -Implanted Base with Rapid Thermal Process ”, IEEE TED, August 1993.] observed the same relationship between TCR and sheet resistance. In this study, TCRs of boron-doped P-type polysilicon resistors fabricated with a 150 nm amorphous layer approach zero at sheet resistances of 600-800 ohms per square. However, within the range of doses in the cited investigation, resistance declines with increasing boron doses. 
     In a parallel experiment aimed at lowering TCR, boron and boron plus another species (BF 2 ) were implanted into a medium thickness film. The implant energies were adjusted to compensate for the different ranges of the species The results, once again, were quite unexpected: the average resistance of the boron by itself was 200 ohms per square with a TCR of 445 ppm while the values for the BF 2  resistors were  525  and  221  respectively 
     Based on these results, it is believed that the heavier ion and the extremely high doses create a large amount of damage in the polysilicon film which cannot be annealed by the relatively short 900° C. RTA. This damage creates additional trapping sites for the carriers resulting in increased resistance at higher implant doses. Therefore, it is believed that co-implantation of other ions could produce similar results thus making it possible to use the same high dose boron implant to produce high value resistors as well as the emitters for PNPs or low resistivity extrinsic bases for NPNs or the sources and drains of MOS devices 
     Table 2 shows the effects of RTA temperature on sheet resistance and TCR as a function of implant dose. Once again, the higher sheet resistances obtained with the lower temperature yield reduced TCRs except at the lower dose where a resistance of  763  results in a TCR of  168 . 
     This lends support to the theory that damage is a major part of the previously observed TCR behavior. The lower RTA temperature leads to suppressed carrier activation and higher sheets. Concurrently, there is less annealing of the implant damage. However, at the low dose, there is insufficient implant damage to degrade carrier mobility to the point where it becomes less sensitive to the temperature variations. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 TCR/RS vs. RTA Temp 
               
             
          
           
               
                   
                 Dose 
                 Rs 
                 TCR 
                 RTA 
               
               
                   
                   
               
             
          
           
               
                   
                 Low 
                 637 
                 293 
                 900 C. 
               
               
                   
                 Low 
                 763 
                 168 
                 800 C. 
               
               
                   
                 Med. 
                 628 
                 271 
                 900 C. 
               
               
                   
                 Med. 
                 849 
                 76 
                 800 C. 
               
               
                   
                 High 
                 726 
                 90 
                 900 C. 
               
               
                   
                 High 
                 832 
                 22 
                 800 C. 
               
               
                   
                   
               
             
          
         
       
     
     Characterization Results 
     FIG. 21 is a scatter plot of a 30×30 micron resistor showing the relationship of TCR to sheet resistance at 50° C. was chosen as the lowest measurement point The TCR is calculated by fitting a line to values measured from 50-125° C. at 25° intervals The dashed lines denote the objectives that were set for this development project. 
     Parts from two different runs were packaged and measured from −50 to 150° C. FIG. 22 shows average changes in sheet resistance for nine parts measured over this temperature range while FIG. 23 is a plot of the calculated TCRs for this set of measurements. The solid line represents a linear fit while the dashed line is a polynomial fit. The upward “hook” observed at lower temperature is typical to that of diffused resistors. 
     Since matching is of particular interest to analog and mixed signal designers, FIG. 24 shows the percent mismatch as a function of length for a fixed width resistor and FIG. 25 represents the same parameter as a function of width with a fixed length. The data, as expected, show improved matching with increasing dimensions. 
     The feasibility of fabricating a high value polysilicon resistor with low TCR has been demonstrated. The investigation has uncovered a relationship between ion species, sheet resistance and TCR which can result in reduced process complexity. Since 800° C. RTA is a benign temperature for present bipolar processes, it is possible if desired to de-couplet he resistor activation step from the RTA used to set the device electrical parameters. 
     With the Bipolar and TCRL components processed to this point, it is now appropriate to remove the protection layers from the CMOS portions of the wafer so that the remaining metalization operations can be performed on all devices. Turning next to FIG. 16, the TCRL resistor  141  and the NPN transistor regions  200  are protected with a layer of photoresist  160 . The photoresist is patterned to open a region above the CMOS devices  100 . Next, the protective oxide  92  (FIG. 15) is removed. 
     Now refer to FIG.  17 . The photoresist layer  160  is removed, followed by removal of the nitride protect layer  90 . At this time, the emitter  170  and the resistor  141  are subjected to an RTA step. The step is carried out at approximately 900° C. for 0.5 minutes, and completes the fabrication of the emitter first prepared in the steps shown previously in FIG. 13 
     The screen oxide layer  80  over the lightly doped source and drain regions of the CMOS device is then removed. As shown in FIG. 18, the exposed polysilicon regions of the resistor  141 , the gate  66 , the source and drain regions, and the collector and emitter contacts  133 ,  134  are silicided with platinum  180  to form a platinum silicide layer on the exposed polysilicon. As shown in FIG. 19, a sidewall spacer oxide  190  is applied to the sidewalls of the emitter contact  134  and the collector contact  133  The rest of the spacer oxide is etched and removed. Thereafter, the substrate is subjected to suitable metallization layers, including the formation of three metal layers separated from each other by suitable insulating layers and separate layers being selectively interconnected, one to the other, by the formation of vias that are filled with conductive material. After metallization the entire device is covered with a passivation layer, typically silicon nitride, and a substrate including the integrated circuits and devices made thereon are then further processed for testing and assembly. 
     Having thus disclosed preferred embodiments of the invention, those skilled in the art will appreciate that further modifications, changes, additions and deletions may be made to that embodiment without departing from the spirit and scope of the appended claims.