Abstract:
A hybrid interconnect structure that possesses a higher interconnect capacitance in one set of regions than in other regions on the same microelectronic chip is described. Several methods to fabricate such a structure are provided. Circuit implementations of such hybrid interconnect structures are described that enable increased static noise margin and reduce the leakage in SRAM cells and common power supply voltages for SRAM and logic in such a chip. Methods that enable combining these circuit benefits with higher interconnect performance speed and superior mechanical robustness in such chips are also taught.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention pertains to the formation of back end of the line (BEOL) interconnect structures for advanced microelectronic chips of the type used in microprocessors, microcontrollers, communications, graphics and the like. In particular, methods pertinent to improving the performance of SRAM cells in such chips by enabling a higher capacitance interconnect environment selectively in the SRAM regions of such chips are described.  
       BACKGROUND  
       [0002]     Limited by array leakage and read stability, CMOS SRAM cell device threshold voltages (VT) have scaled less aggressively than logic requiring a separate, higher supply voltage (VDD-CELL) to enable higher static noise margin (SNM) and read current (IREAD), and lower read current variability, in the presence of increasingly severe, random VT fluctuations in small geometry SRAM cell transistors. However, a second array of power supply translates into more input output (I/O) requirements and fewer metal wiring tracks for other chip functions such as power, global signal and clock distribution—all of which directly impact cost. Secondly, higher/dual cell power supplies result in (i) more leakage due to all components from unaccessed Subarrays of large L 2  caches during active mode and (ii) more switching power, adversely impacting battery lifetime for portable applications as well as the cost of packaging for high performance desktop and server products.  
         [0003]     New circuit techniques were reported in a co-pending application YOR9200300292US1 by some of the present authors that enable a single VDD SRAM to operate at logic compatible voltages with a cell read current and cell static noise margin typically seen with higher/dual VDD SRAMs. The schematic circuit implementations are shown in  FIGS. 1   a ,  1   b  and  2 .  FIG. 3  shows the physical implementation of the SRAM cell interconnects schematically where the array of word lines (WL) and the associated power lines (PL) are shown. Implemented in a 65 nm CMOS SOI process with no alterations to the CMOS processes and materials or to a conventional, single VT SRAM cell, the voltage across power rails of the selected SRAM cells self-biases to permit a higher-than-VDD voltage during word line (WL) active periods and a lower than 2VT voltage at all other times. Based on circuit simulations data,  FIG. 4 , this “bootstrapping” has been shown to increase the supply voltage by about 17% under this circuit implementation. Bootstrapping the cell row power supply and regulating the cell Subarray virtual ground voltage enables the above ‘Transregional’ SRAM operation resulting in near-subthreshold data storage and superthreshold access, lowering total leakage by over 10× and improving IREAD and SNM by 7% and 18% respectively with a total area overhead of less than 13%.  
         [0004]     It is therefore clear that ‘Transregional’ SRAM operation enables logic VDD compatibility, lower leakage, and higher cell read stability without degradation of performance. The benefits of this bootstrapping effect can be significantly enhanced over those shown in prior art by enabling a stronger capacitive coupling between the WL that selects a row of SRAM cells and the power line(PL) pair that supply power to that row. Incorporation of a higher k dielectric material between the WL and PL lines in an SRAM subarray, to enable a stronger capacitive coupling between WL and PL is a way to achieve this end. However, using a higher k dielectric at other the regions of the chip (areas of logic and interconnects) is detrimental to chip performance as increased capacitance in these regions translates to increased RC delay and hence slower interconnect speeds. In this invention, we teach a structure that enables higher capacitive coupling in the SRAM cell areas and lower capacitance elsewhere so as to overcome this problem. Several methods to fabricate such a structure are also taught.  
         [0005]     It is therefore an object of this invention to describe a chip interconnect structure that will enable a hybrid system of inter-metal dielectrics (IMD): a high k or ultra-high k dielectric in the inter-metal gaps between the WL and PL features,  FIG. 5 , while providing low k or ultra low k dielectric separating the interconnect lines elsewhere. For the purpose of convenience in the descriptions below we arbitrarily define high k as 7&lt;k&lt;4 and ultra high k as k&gt;7 and low k as 4&lt;k&lt;2.0 and ultra low k as k&lt;2. It is further the object of this invention to achieve the optimum benefits in the SRAM performance metrics as outlined above while maintaining the mechanical robustness of the chip and the low effective interconnect capacitance to minimize interconnect delay elsewhere on the chip. It is further an object of this invention to describe methods to fabricate the above described hybrid interconnect structure.  
         [0006]     These and other aspects of our invention are described below in detail along with the following set of illustrative figures.  
     
    
     DESCRIPTION  
       [0007]      FIG. 1 . Schematic circuit diagram of a Transregional SRAM implementation showing PL bootstrapped to be above VDD and with VGND regulated with a PFET diode stack. (Prior art patent application YOR9200300292US1 by some of the present inventors).  
         [0008]      FIG. 2 . Bootstrap approach shown schematically in circuit implementation form on PL. (Prior art patent application YOR9200300292US1 by some of the present inventors).  
         [0009]      FIG. 3 . Physical implementation of a bootstrap on to a pair of PL lines. (Prior art patent application YOR9200300292US1 by some of the present inventors).  
         [0010]      FIG. 4 . Circuit simulations demonstrating Transregional SRAM operation with the bootstrap using the same insulating material between WL &amp; PL as anywhere else. (Prior art patent application YOR9200300292US1 by some of the present inventors).  
         [0011]      FIG. 5 . Proposed enhanced physical implementation of the bootstrap approach according to the present invention using higher k dielectric between WL and PL tracks and low k dielectric elsewhere in the chip area (not shown).  
         [0012]      FIG. 6 . Schematic description of the etch back and gap fill integration scheme (U.S. patent application 200040087135A1 by some of the preset authors).  
         [0013]      FIG. 7 . Schematic description of the first inventive method to fabricate the hybrid SRAM interconnect structure using EBGF and a block out lithography step.  
         [0014]      FIG. 8 . Schematic description of the second inventive process described below.  
         [0015]      FIG. 9 . Schematic description of the third inventive process described below.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     We teach the fabrication of the hybrid structure described above using an etch back and gap fill (EBGF) integration scheme (U.S. patent application 200040087135A1 by some of the preset authors) or variants thereof.  
         [0017]     In the EBGF scheme, schematically shown in  FIG. 6 , the interconnect wires  500  are first fabricated in a dense dielectric medium  600  (typically a material of higher k such as oxide, SiCOH, dense spin on glasses and the like),  FIG. 6   a . The dense dielectric is then etched from between the inter-line gaps using the lines themselves as an etch mask to produce the structure shown in  FIG. 6   b . Then a lower k dielectric (typically a low k or very low k dielectric material  550  often porous so as to have k values as low as 1.6) is deposited so as to overfill the etched recesses as shown in  FIG. 6   c . The structure is then planarized by a chem-mech polish (CMP) process and capped with a passivation dielectric  650  resulting in an EBGF structure shown in  FIG. 6   d . The dense dielectric  600  is considered a support dielectric and is used to provide mechanical robustness as the low k IMD  550  is usually mechanically fragile.  
         [0018]     In the first inventive method, depicted schematically in  FIG. 7 , the EBGF method is used in conjunction with a simple block out lithography step that protects the SRAM cell regions where WL and PL lines are located with a photoresist mask so that the dense dielectric is left in tact between these lines.  FIG. 7 . 1  shows the top down view of the structure after the standard build in a dense and robust BEOL dielectric  600 . Interconnect lines are not shown for the sake of simplicity. Following this, as shown in  FIG. 7 . 2 , the SRAM cell regions are protected by photoresist  100  using an exposure with a blockout mask. Following this, as shown in  FIG. 7 . 3 , the robust BEOL dielectric is etched back in the regions not protected by the blockout mask and followed by a resist strip. Finally, the etched back regions are gap filled and planarized with a dielectric  550  with a lower k than the robust BEOL dielectric  600  as shown in  FIG. 7 . 4 . For example, the mechanically robust BEOL dielectric  600  can be selected from the group comprising silicon oxide, fluorinated silicon oxide, organosilicate dielectrics comprising silicon, carbon, oxygen and hydrogen. The gap fill dielectric  550  can be porous or dense versions of organosilicates and organic dielectrics such as polyimides and polyarylene ethers and porous silica with k&lt;2.5 and preferably even lower than 2. Additionally, regions such as the chip kerf sites, bond or probe pads and the dicing channels can also be protected by the block out lithography to preserve the support dielectric to prevent dicing and bonding induced cracks. The block out mask and the associated lithography can be of fairly relaxed in ground rules compared to the minimum ground rule of the technology used to fabricate the cells and thus will not be significant cost adder. In this manner, it is possible to make mechanically robust hybrid structures wherein the k of the IMD in the WL/PL gaps is as high as 4 and the k of the IMD in the remaining interconnect regions is as low as 1.6. This enables an increased bootstrapping voltage in the SRAM cells enabled by the higher Cc values from the high k material while maintaining the high speed (low capacitance) for the interconnect wiring elsewhere on the chip.  
         [0019]     In the second inventive method, we propose the fabrication of the interconnect structures with a low k dielectric  700  (such as porous and dense versions of organosilicates and organic dielectrics such as polyimides and polyarylene ethers and porous silica), protecting all the regions EXCEPT the WL/PL gap areas with a block out resist mask  100 , etching the low k dielectric from between the WL/PL lines and gap filling and planarizing with a high k dielectric  750 . This process flow is shown schematically in  FIG. 8  (again omitting the interconnect lines for simplicity). In this case it is possible to gap fill with very high k materials such as titania, zirconia, hafnia and their silicates, barium strontium titanate, barium zirconium titanate and the like which can be deposited by sol gel processing using metal alkoxide solutions, for example. k values as high as 20-40 are possible in these films significantly increasing the capacitance attainable between WL and PL. The overall interconnect capacitance and mechanical robustness will be determined by the low k dielectric used to fabricate the original structure before EBGF.  
         [0020]     In the third inventive method, shown schematically in  FIG. 9 , the interconnects are fabricated using a robust support dielectric  600  with a moderate k, typically in the 2.5 to 4.0 range,  FIG. 9 . 1 (note: interconnect lines are omitted for simplicity of illustration). This robust support dielectric  600  can be selected from the group comprising silicon oxide, fluorinated silicon oxide, organosilicate dielectrics comprising silicon, carbon, oxygen and hydrogen. In the first block out lithography, only the WL and PL regions of the SRAM areas are exposed while the remaining area is blocked out with a photoresist pattern  100  as shown in  FIG. 9 . 2 . A first etch back of the robust support dielectric  600  ( FIG. 9 . 3 ) is performed followed by a gap fill with high k or ultra high k dielectrics  750  as in the first variant above followed by CMP planarization,  FIG. 9 . 4 . Dielectric  750  can thus be selected from the group comprising very high k materials such as titania, zirconia, hafnia and their silicates, barium strontium titanate, barium zirconium titanate and the like which can be deposited by sol gel processing using metal alkoxide solutions, for example. Next, a second block out photoresist pattern  110  is formed that protects all the SRAM WL/PL regions and the dicing channels and bond pads,  FIG. 9 . 5 . An etch back of the robust support dielectric  600  in the regions not protected by the block out pattern  110  ( FIG. 9 . 6 ) followed by gap fill with low k or ultra low k dielectric  550  and planarization leads to interconnect areas which are very low in capacitance and hence wiring delay,  FIG. 9 . 7 . Dielectric  550  can be selected from the group comprising porous or dense versions of organosilicates and organic dielectrics such as polyimides and polyarylene ethers and porous silica with k &lt; 2 . 5  and preferably even lower than 2. The net structure of  FIG. 9 . 7  combines the mechanical robustness afforded by the robust support dielectric  600 , significantly reduced interconnect delay enabled by the ultra low k dielectric gap fill  550 , and the very large capacitive coupling in the SRAM cell regions achieved through the high k or ultra high k gapfill dielectric  750  in that region. The cost of this variant is likely to be slightly more than the other two variants due to the additional steps required but a higher level of overall performance is achieved and will be justified where a cost premium for higher performance is acceptable.  
         [0021]     In the fourth inventive method the low k or ultra low k IMD regions in the SRAM cell area alone are modified using a suitable exposure method selected from ion implantation, photon irradiation, chemical infiltration from liquid, vapor or supercritical fluid based delivery media followed by an optional thermal annealing. The base interconnect structure itself can be fabricated by the standard dual damascene technique or the EBGF technique as described earlier. Following the build of the interconnect structure, a block out lithography is performed to protect all the areas other than the SRAM cell area with a photoresist. Then the modification process is carried out that enables the conversion of the IMD to a higher k material. The block out resist is stripped and the process of additional layer build is continued with the dielectric modification step for the SRAM area included in additional interconnect levels as needed.  
         [0022]     The structures resulting from the above described inventive methods have the higher interconnect capacitance in the cell areas of the SRAM desirable for the low voltage operation and the low to ultra low interconnect capacitance in the other areas desirable for high speed signal propagation and low interconnect power dissipation. Further, they incorporate a mechanically robust IMD in the dicing channels, bond pads and under all the interconnect lines thereby providing superior chip robustness.