Abstract:
A pattern of parallel lines defines first regions where no conductive material is to be located, a distance between adjacent lines in the first regions being smaller than a predetermined distance, and defines second regions where conductive material is to be located, a distance between adjacent lines in the second regions being larger than the predetermined distance. A subsequent layer caps air gaps between lines in the first regions. Conductive material is then deposited and planarized to form lines of conductive material in the second regions.

Description:
BACKGROUND 
       [0001]    This application relates generally to non-volatile semiconductor memories of the flash memory type, their formation, structure and use. 
         [0002]    There are many commercially successful non-volatile memory products being used today, particularly in the form of small form factor cards, USB drives, embedded memory, and Solid State Drives (SSDs) which use an array of flash EEPROM cells. An example of a flash memory system is shown in  FIG. 1 , in which a memory cell array  1  is formed on a memory chip  12 , along with various peripheral circuits such as column control circuits  2 , row control circuits  3 , data input/output circuits  6 , etc. 
         [0003]    One popular flash EEPROM architecture utilizes a NAND array, wherein a large number of strings of memory cells are connected through one or more select transistors between individual bit lines and a reference potential. A portion of such an array is shown in plan view in  FIG. 2A . Although four floating gate memory cells are shown in each string, the individual strings typically include 16, 32 or more memory cell charge storage elements, such as floating gates, in a column. Control gate (word) lines labeled WL 0 -WL 3  and string selection lines, Drain Select Line, “DSL” and Source Select Line “SSL” extend across multiple strings over rows of floating gates. An individual cell within a column is read and verified during programming by causing the remaining cells in the string to be turned on hard by placing a relatively high voltage on their respective word lines and by placing a relatively lower voltage on the one selected word line so that the current flowing through each string is primarily dependent only upon the level of charge stored in the addressed cell below the selected word line. That current typically is sensed for a large number of strings in parallel, thereby to read charge level states along a row of floating gates in parallel. 
         [0004]    The top and bottom of the string connect to the bit line and a common source line respectively through select transistors (source select transistor and drain select transistor). Select transistors do not contain floating gates and are used to connect NAND strings to control circuits when they are to be accessed, and to isolate them when they are not being accessed. 
         [0005]    NAND strings are generally connected by conductive lines in order to form arrays that may contain many NAND strings. At either end of a NAND string a contact area may be formed. This allows connection of the NAND string as part of the array. Metal contact plugs (or “vias”) may be formed over contact areas to connect the contact areas (and thereby connect NAND strings) to conductive metal lines that extend over the memory array (e.g. bit lines).  FIG. 2A  shows bit line contacts BL 0 -BL 4  and common source line contacts at either end of NAND strings. Contacts to contact areas may be formed by etching contact holes through a dielectric layer and then filling the holes with metal to form vias. Metal lines, such as bit lines, extend over the memory array and in peripheral areas in order to connect the memory array and various peripheral circuits. Electrical contact between metal lines and vias occurs where horizontal metal lines intersect vertical contact plugs. These metal lines may be close together (particularly in the memory array area where bit lines may be very close) which tends to make processing difficult and provides a risk of capacitive coupling. The characteristics of such lines (e.g. resistance and coupling) and the quality of connections with vias may be significant factors for good memory operation. 
         [0006]    Thus, there is a need for a memory chip manufacturing process that forms uniform low resistance conductive lines, such as bit lines, in close proximity in an efficient manner. 
       SUMMARY 
       [0007]    According to an example of formation of a memory integrated circuit, air gap structures for bit line isolation are formed prior to deposition of bit line metal. Instead of forming bit lines and then removing material between bit lines to form air gaps, air gap structures may already be in place before bit lines are formed. Air gaps may be formed by sidewall spacers that are formed along sides of sacrificial mandrels. After sacrificial mandrels are removed, the air gaps are capped by a capping layer that has a high deposition rate over sidewall spacers. Narrow air gap structures that include a single air gap may separate bit lines. Larger air gap structures that include two or more air gaps may also be formed using the same process steps. 
         [0008]    An example of a method of forming air gaps in an integrated circuit includes: forming a plurality of parallel lines of dielectric of equal height on a first layer of dielectric; the plurality of parallel lines defining first regions where no conductive material is to be located, a distance between adjacent lines in the first regions being smaller than a predetermined distance; the plurality of parallel lines defining second regions where conductive material is to be located, a distance between adjacent lines in the second regions being larger than the predetermined distance; subsequently, depositing a second layer that caps air gaps between lines in the first regions that are less than the predetermined distance apart and that deposits the second layer on the first layer of dielectric in the second regions where adjacent lines are more than the predetermined distance apart; subsequently removing the second layer in the second regions to expose the surface of the first layer; subsequently depositing barrier metal and conductive material across the first and second regions; and subsequently planarizing to expose the second layer in the first regions and to form individual lines of conductive material in the second regions. 
         [0009]    The method of may also include: forming a third layer on the first layer of dielectric, the third layer formed of a material with a lower etching rate than the first layer; patterning the third layer to form core portions; depositing a fourth layer on top surfaces of the core portions, side walls of the core portions, and on a surface of the first layer; etching back the fourth layer to remove the fourth layer from the top surfaces of the core portions to expose the top surfaces of the core portions and to remove the fourth layer from the surface of the first layer; and removing the core portions while maintaining the fourth layer that was deposited on the sidewalls of the core portions. The core portions may have a tapered shape with bottom surface in contact with the first layer of dielectric, the bottom surface being wider than the top surface. The forming of the plurality of parallel lines may include: forming a third layer on the first layer of dielectric; forming a mask pattern on the third layer using a lithography process; and forming the plurality of line patterns from the third layer by patterning the third layer using the mask pattern and an anisotropic etch process. The second layer may have a first thickness in the first regions and a second thickness in the second regions, the first thickness being greater than the second thickness. The barrier metal may be a metal that has a significant etch rate when etching dielectric material. The barrier metal may be titanium. The planarizing may expose the second layer in the first regions where the second layer overlies air gaps, and the planarizing may not expose the air gaps. 
         [0010]    An example of a method of forming metal lines in an integrated circuit includes: forming a plurality of air gap mandrels on a dielectric layer; forming sidewalls on the plurality of air gap mandrels; subsequently, removing the plurality of air gap mandrels; subsequently, depositing a capping layer to enclose a plurality of air gaps where the plurality of air gap mandrels were removed; and subsequently depositing bit line metal to form a plurality of bit lines at locations between the plurality of air gaps. 
         [0011]    Large areas without metal may be formed by forming a plurality of elongated closely-spaced air gap mandrels in the large areas so that the capping layer forms a continuous covering over tops of sidewalls formed on the plurality of elongated closely-spaced air gap mandrels. Additional air gaps in the large areas may be enclosed, the additional air gaps formed between sidewalls formed on neighboring mandrels of the plurality of elongated closely-spaced air gap mandrels. Prior to depositing the bit line metal, the capping layer may be etched back to remove the capping layer at locations where the capping layer is in contact with the dielectric layer without exposing the plurality of air gaps or the additional air gaps. The bit line metal may be planarized to separate bit lines at locations between the plurality of air gaps. The planarizing may stop at a level above the plurality of air gaps so that air gaps remain enclosed. The plurality of air gap mandrels may be formed of a sacrificial material and may be removed using an etch that is selective to the sacrificial material, the etch having a higher etch rate for the sacrificial material than for material of the sidewalls and material of the dielectric layer. 
         [0012]    An example of a structure includes: a NAND memory array; a dielectric layer overlying the NAND memory array; a plurality of vias extending through the NAND memory array; a plurality of bit lines extending on a surface of the dielectric layer, each of the plurality of bit lines formed of metal that electrically contacts a corresponding via of the plurality of vias; a plurality of enclosed air gaps extending on the surface of the dielectric layer, an individual enclosed air gap located between neighboring bit lines; and a large air gap structure comprising a plurality of isolated air gaps extending on the surface of the dielectric layer. 
         [0013]    The individual enclosed air gap may be defined by a first sidewall and a second sidewall located on the surface of the dielectric layer and a capping portion that extends from the first sidewall to the second sidewall and that overlies the air gap. The first sidewall may extend parallel to the second sidewall along the surface of the dielectric layer and may be less than a threshold distance from the second sidewall. The large air gap structure may include four or more parallel sidewall spacers that are spaced apart by less than the threshold distance. The large air gap structure may also include a large capping portion that extends over the four or more sidewall spacers and over the plurality of isolated air gaps that are formed between the four or more parallel sidewall spacers. 
         [0014]    Various aspects, advantages, features and embodiments are included in the following description of examples, which description should be taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram of a prior art memory system. 
           [0016]      FIG. 2A  is a plan view of a prior art NAND array. 
           [0017]      FIG. 2B  shows a cross section of the NAND array of  FIG. 2A . 
           [0018]      FIG. 2C  shows another cross section of the NAND array of  FIG. 2A . 
           [0019]      FIG. 3  illustrates an example of formation of air gaps between bit lines. 
           [0020]      FIGS. 4  shows a cross section of a portion of a NAND memory die at an intermediate stage of fabrication including mandrels. 
           [0021]      FIG. 5  shows the structure of  FIG. 4  after formation of a sidewall layer on mandrels. 
           [0022]      FIG. 6  shows the structure of  FIG. 5  after etching back to form sidewalls along sides of mandrels. 
           [0023]      FIG. 7  shows the structure of  FIG. 6  after removal of mandrels. 
           [0024]      FIG. 8  shows the structure of  FIG. 7  after deposition of a capping layer. 
           [0025]      FIG. 9  shows the structure of  FIG. 8  after etching back to expose vias. 
           [0026]      FIG. 10  shows the structure of  FIG. 9  after deposition of bit line metal and barrier layer. 
           [0027]      FIG. 11  shows the structure of  FIG. 10  after planarization. 
           [0028]      FIG. 12  shows the structure of  FIG. 11  after deposition of an interlayer dielectric. 
           [0029]      FIG. 13  illustrates steps in the formation of bit lines separated by air gap structures. 
           [0030]      FIG. 14  shows sidewalls with different spacing. 
           [0031]      FIG. 15  shows the structure of  FIG. 14  with a capping layer. 
           [0032]      FIG. 16  shows the structure of  FIG. 15  after etching back. 
           [0033]      FIG. 17  shows the structure of  FIG. 16  after deposition of bit line metal and a barrier layer. 
           [0034]      FIG. 18  shows the structure of  FIG. 17  after planarization. 
           [0035]      FIG. 19  shows the structure of  FIG. 18  after removal of additional material. 
           [0036]      FIG. 20  shows the structure of  FIG. 19  after deposition of an interlayer dielectric. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Memory System 
       [0037]    Semiconductor memory devices include volatile memory devices, such as dynamic random access memory (“DRAM”) or static random access memory (“SRAM”) devices, non-volatile memory devices, such as resistive random access memory (“ReRAM”), electrically erasable programmable read only memory (“EEPROM”), flash memory (which can also be considered a subset of EEPROM), ferroelectric random access memory (“FRAM”), and magnetoresistive random access memory (“MRAM”), and other semiconductor elements capable of storing information. Each type of memory device may have different configurations. For example, flash memory devices may be configured in a NAND or a NOR configuration. 
         [0038]    The memory devices can be formed from passive and/or active elements, in any combinations. By way of non-limiting example, passive semiconductor memory elements include ReRAM device elements, which in some embodiments include a resistivity switching storage element, such as an anti-fuse, phase change material, etc., and optionally a steering element, such as a diode, etc. Further by way of non-limiting example, active semiconductor memory elements include EEPROM and flash memory device elements, which in some embodiments include elements containing a charge storage region, such as a floating gate, conductive nanoparticles, or a charge storage dielectric material. 
         [0039]    Multiple memory elements may be configured so that they are connected in series or so that each element is individually accessible. By way of non-limiting example, flash memory devices in a NAND configuration (NAND memory) typically contain memory elements connected in series. A NAND memory array may be configured so that the array is composed of multiple strings of memory in which a string is composed of multiple memory elements sharing a single bit line and accessed as a group. Alternatively, memory elements may be configured so that each element is individually accessible, e.g., a NOR memory array. NAND and NOR memory configurations are exemplary, and memory elements may be otherwise configured. 
         [0040]    The semiconductor memory elements located within and/or over a substrate may be arranged in two or three dimensions, such as a two dimensional memory structure or a three dimensional memory structure. 
         [0041]    In a two dimensional memory structure, the semiconductor memory elements are arranged in a single plane or a single memory device level. Typically, in a two dimensional memory structure, memory elements are arranged in a plane (e.g., in an x-z direction plane) which extends substantially parallel to a major surface of a substrate that supports the memory elements. The substrate may be a wafer over or in which the layer of the memory elements are formed or it may be a carrier substrate which is attached to the memory elements after they are formed. As a non-limiting example, the substrate may include a semiconductor such as silicon. 
         [0042]    The memory elements may be arranged in the single memory device level in an ordered array, such as in a plurality of rows and/or columns. However, the memory elements may be arrayed in non-regular or non-orthogonal configurations. The memory elements may each have two or more electrodes or contact lines, such as bit lines and word lines. 
         [0043]    A three dimensional memory array is arranged so that memory elements occupy multiple planes or multiple memory device levels, thereby forming a structure in three dimensions (i.e., in the x, y and z directions, where the y direction is substantially perpendicular and the x and z directions are substantially parallel to the major surface of the substrate). 
         [0044]    As a non-limiting example, a three dimensional memory structure may be vertically arranged as a stack of multiple two dimensional memory device levels. As another non-limiting example, a three dimensional memory array may be arranged as multiple vertical columns (e.g., columns extending substantially perpendicular to the major surface of the substrate, i.e., in the y direction) with each column having multiple memory elements in each column. The columns may be arranged in a two dimensional configuration, e.g., in an x-z plane, resulting in a three dimensional arrangement of memory elements with elements on multiple vertically stacked memory planes. Other configurations of memory elements in three dimensions can also constitute a three dimensional memory array. 
         [0045]    By way of non-limiting example, in a three dimensional NAND memory array, the memory elements may be coupled together to form a NAND string within a single horizontal (e.g., x-z) memory device levels. Alternatively, the memory elements may be coupled together to form a vertical NAND string that traverses across multiple horizontal memory device levels. Other three dimensional configurations can be envisioned wherein some NAND strings contain memory elements in a single memory level while other strings contain memory elements which span through multiple memory levels. Three dimensional memory arrays may also be designed in a NOR configuration and in a ReRAM configuration. 
         [0046]    Typically, in a monolithic three dimensional memory array, one or more memory device levels are formed above a single substrate. Optionally, the monolithic three dimensional memory array may also have one or more memory layers at least partially within the single substrate. As a non-limiting example, the substrate may include a semiconductor such as silicon. In a monolithic three dimensional array, the layers constituting each memory device level of the array are typically formed on the layers of the underlying memory device levels of the array. However, layers of adjacent memory device levels of a monolithic three dimensional memory array may be shared or have intervening layers between memory device levels. 
         [0047]    Then again, two dimensional arrays may be formed separately and then packaged together to form a non-monolithic memory device having multiple layers of memory. For example, non-monolithic stacked memories can be constructed by forming memory levels on separate substrates and then stacking the memory levels atop each other. The substrates may be thinned or removed from the memory device levels before stacking, but as the memory device levels are initially formed over separate substrates, the resulting memory arrays are not monolithic three dimensional memory arrays. Further, multiple two dimensional memory arrays or three dimensional memory arrays (monolithic or non-monolithic) may be formed on separate chips and then packaged together to form a stacked-chip memory device. 
         [0048]    Associated circuitry is typically required for operation of the memory elements and for communication with the memory elements. As non-limiting examples, memory devices may have circuitry used for controlling and driving memory elements to accomplish functions such as programming and reading. This associated circuitry may be on the same substrate as the memory elements and/or on a separate substrate. For example, a controller for memory read-write operations may be located on a separate controller chip and/or on the same substrate as the memory elements. 
         [0049]    In other embodiments, types of memory other than the two dimensional and three dimensional exemplary structures described here may be used. 
         [0050]    An example of a prior art memory system, which may be modified to include various structures described here, is illustrated by the block diagram of  FIG. 1 . A planar memory cell array  1  including a plurality of memory cells is controlled by a column control circuit  2 , a row control circuit  3 , a c-source control circuit  4  and a c-p-well control circuit  5 . The memory cell array  1  is, in this example, of the NAND type similar to that described above in the Background. A control circuit  2  is connected to bit lines (BL) of the memory cell array  1  for reading data stored in the memory cells, for determining a state of the memory cells during a program operation, and for controlling potential levels of the bit lines (BL) to promote the programming or to inhibit the programming. The row control circuit  3  is connected to word lines (WL) to select one of the word lines (WL), to apply read voltages, to apply program voltages combined with the bit line potential levels controlled by the column control circuit  2 , and to apply an erase voltage coupled with a voltage of a p-type region on which the memory cells are formed. The c-source control circuit  4  controls a common source line (labeled as “c-source” in  FIG. 1 ) connected to the memory cells (M). The c-p-well control circuit  5  controls the c-p-well voltage. 
         [0051]    The data stored in the memory cells are read out by the column control circuit  2  and are output to external I/O lines via an I/O line and a data input/output buffer  6 . Program data to be stored in the memory cells are input to the data input/output buffer  6  via the external I/O lines, and transferred to the column control circuit  2 . The external I/O lines are connected to a controller  9 . The controller  9  includes various types of registers and other memory including a volatile random-access-memory (RAM)  10 . 
         [0052]    The memory system of  FIG. 1  may be embedded as part of the host system, or may be included in a memory card, USB drive, or similar unit that is removably insertible into a mating socket of a host system. Such a card may include the entire memory system, or the controller and memory array, with associated peripheral circuits, may be provided in separate cards. The memory system of  FIG. 1  may also be used in a Solid State Drive (SSD) or similar unit that provides mass data storage in a tablet, laptop computer, or similar device. Memory systems may be used with a variety of hosts in a variety of different environments. For example, a host may be a mobile device such as a cell phone, laptop, music player (e.g. MP3 player), Global Positioning System (GPS) device, tablet computer, or the like. Such memory systems may be inactive, without power, for long periods during which they may be subject to various conditions including high temperatures, vibration, electromagnetic fields, etc. Memory systems for such hosts, whether removable or embedded, may be selected for low power consumption, high data retention, and reliability in a wide range of environmental conditions (e.g. a wide temperature range). Other hosts may be stationary. For example, servers used for internet applications may use nonvolatile memory systems for storage of data that is sent and received over the internet. Such systems may remain powered up without interruption for extended periods (e.g. a year or more) and may be frequently accessed throughout such periods. Individual blocks may be frequently written and erased so that endurance may be a major concern. 
         [0053]      FIGS. 2A-2C  show different views of a prior art NAND flash memory. In particular,  FIG. 2A  shows a plan view of a portion of such a memory array including bit lines and word lines (this is a simplified structure with a small number of word lines and bit lines).  FIG. 2B  shows a cross section along A-A (along a NAND string) showing individual memory cells that are connected in series. Contact plugs, or vias, are formed at either end to connect the NAND strings in the memory array to conductive lines (e.g. connecting to bit lines at one end and to a common source line at the other end). Such a via may be formed of metal that is deposited into a contact hole that is formed in a dielectric layer.  FIG. 2C  shows a cross section along B-B of  FIG. 2A . This view shows metal contact plugs extending down through contact holes in a dielectric layer to make contact with active areas (“AA”) in the substrate (i.e. with N+ areas of  FIG. 2B ). STI regions are located between active areas of different strings to electrically isolate an individual NAND string from its neighbors. Bit lines extend over the memory array in a direction perpendicular to the plane of the cross section shown. Alternating bit lines are connected to vias in the cross section shown. (It will be understood that other vias, that are not visible in the cross section shown, connect the remaining bit lines to other active areas). In this arrangement, locations of vias alternate so that there is more space between vias and thus less risk of contact between vias. Other arrangements are also possible. 
         [0054]    As memories become smaller, the spacing between bit lines tends to diminish. Accordingly, capacitive coupling between bit lines tends to increase as technology progresses to ever-smaller dimensions.  FIG. 2C  shows an example of bit lines formed in a dielectric material. For example, copper bit lines may be formed by a damascene process in which elongated openings, or trenches, are formed in the dielectric layer and then copper is deposited to fill the trenches. When excess copper is removed (e.g. by Chemical Mechanical Polishing, CMP) copper lines remain. A suitable dielectric may be chosen to keep bit line-to-bit line capacitance low. 
         [0055]    One way to reduce bit line-to-bit line coupling is to provide an air gap between neighboring bit lines. Thus, rather than maintain dielectric portions between bit lines, the bit lines are formed in a sacrificial layer which is then removed to leave air gaps between bit lines. 
         [0056]    Removing sacrificial material between bit lines generally requires some form of etching which may expose bit lines to etch-related damage. While a suitable combination of sacrificial material and etch chemistry may be chosen so that sacrificial material is etched at a higher rate than bit line metal and/or barrier material, some etching or corrosion of bit line metal and/or barrier metal may occur and bit lines may be damaged accordingly. 
         [0057]      FIG. 3  shows formation of air gaps between bit lines  303   a - e  by etching away the sacrificial material between bit lines. Such etching may expose bit lines to etch damage (i.e. bit line materials such as copper, and barrier layer materials such as titanium may be corroded or removed by such etching). Damaged bit lines may provide higher resistance or may be shorted out if they are etched through. In addition, over-etching in such a step may undercut bit lines so that the bit lines are no longer attached and may lift off from the substrate resulting in failure. To avoid such over-etching, the etch depth may be reduced which may lead to a significant amount of dielectric remaining between bit lines with the result that coupling is not reduced as much as if the air gap extended further. 
         [0058]    Examples presented here form bit lines using a damascene process in which air gap structures are formed before bit line material is deposited. These air gap structures provide a pattern for damascene metal patterning. However, unlike a sacrificial layer, they remain in place so that no etching is required after bit lines are formed and thus the bit lines are not exposed to damaging etching. 
         [0059]      FIG. 4  shows an example of a portion of a memory die at an intermediate stage of fabrication with vias  407   a - c  extending through dielectric layer  419 . Underlying structures such as STI portions and active areas are omitted from this and subsequent figures for clarity. It will be understood that any suitable circuits, including planar NAND flash memory, may underlie the portions shown. A sacrificial layer  413  extends over dielectric layer  419  and is patterned into mandrels  413   a - d  that are used as sacrificial core portions to form air gap structures. Mandrels  413   a - e  are separated by trenches  415   a - e . Mandrels  413   a - e  extend in the direction perpendicular to the plane shown in cross section at locations where air gaps between bit lines are to be formed. Trenches  415   a - e  are located where bit lines are to be formed (e.g. overlying vias  407   a - c ). Trenches are shown having a tapered shape so that they are wider at the top than at the bottom. Accordingly, mandrels have an inverted tapered shape so that they are wider at the bottom than at the top. In general, the profile of etched trenches can be selected by tuning the etch process so that a desired profile (such as a tapered profile) may be obtained. Profiles of mandrels are determined by the trenches on either side so that these profiles are also selectable. Sacrificial layer  413  may be formed of a suitable material such as amorphous silicon or photoresist to allow for later removal without damaging adjacent structures. 
         [0060]      FIG. 5  shows the structure of  FIG. 4  after deposition of a sidewall layer  517  that extends along sides of mandrels  413   a - d  (and may also extend along tops of mandrels and bottoms of trenches). Sidewall layer may be formed of a suitable dielectric material such as silicon oxide (e.g. SiO2 or derivatives containing fluorine, carbon, hydrogen), silicon nitride (SiN), silicon oxynitride (SiON), or other suitable material 
         [0061]      FIG. 6  shows the structure of  FIG. 5  after etching back sidewall layer  517  to expose mandrels  413   a - d  and to expose vias  407   a - c . Anisotropic etching (e.g. Reactive Ion Etching “RIE”) may be used so that etching removes sidewall layer  517  where it lies substantially perpendicular to the etching direction (perpendicular to the vertical direction in this case, i.e. along a plane parallel to the plane of the substrate). Portions of sidewall layer  517  remain as sidewalls along sides of mandrels  413   a - d.    
         [0062]      FIG. 7  shows the structure of  FIG. 6  after removal of mandrels  413   a - d . For example, selective etching using a suitable wet or dry etch may remove amorphous silicon (e.g. wet etching using hot trimethyl-2 hydroxyethyl ammonium hydroxide “TMY”). Photoresist mandrels may be removed by ashing. The areas where mandrels are removed become air gaps. 
         [0063]    In some cases, sidewall spacers may tend to collapse because of forces during etching and deposition steps. A sacrificial layer may be formed over sidewall layer  517   FIG. 5  to provide extra physical support during removal of mandrels so that sidewalls  517   a  - j  may have a two layer structure. For example, a sacrificial layer of silicon nitride may be deposited to support silicon oxide sidewalls. The sacrificial layer may later be removed. 
         [0064]      FIG. 8  shows the structure of  FIG. 7  after formation of a capping layer  823  that encloses air gaps  825   a - d . A capping layer may be formed using a suitable process that provides a high deposition rate to pinch off openings such as at the tops of sidewalls. Deposition rates on sides of sidewalls and at bottoms of trenches are lower. This results in the thick caps over air gaps  825   a - d  as shown. 
         [0065]      FIG. 9  shows the structure of  FIG. 8  after etching back capping layer  823  to expose vias  407   a - c  to allow bit lines to contact vias. Capping layer  823  is reduced in thickness over air gaps  825   a - d  but because of the thickness of capping layer  823  at these locations, air gaps  825   a - d  remain unexposed and thus are protected from subsequent processing. If a sacrificial layer is used to physically strengthen sidewalls then it may be removed at this stage. 
         [0066]      FIG. 10  shows the structure of  FIG. 9  after deposition of bit line metal  131  and barrier layer (not separately shown). For example, a barrier layer may include one or more of the following materials: titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN). Bit line metal  131  may be copper (Cu), tungsten (W), aluminum (Al), or other metal. 
         [0067]      FIG. 11  shows the structure of  FIG. 10  after planarization to remove excess bit line metal  131 . For example, Chemical Mechanical Polishing (“CMP”) may be used to remove bit line metal  131  down to a uniform level. This separates bit line metal in different trenches into separate bit lines  131   a - e . Bit lines  131   a - e  have a tapered shape because of the tapered mandrels that established air gap structure shapes. Capping layer material is also removed in this step. However, enough capping layer material remains to maintain air gaps  825   a - d  intact (i.e. inner volume is not opened to ambient conditions). 
         [0068]      FIG. 12  shows the structure of  FIG. 11  after formation of a cover layer  121 . This may be a suitable dielectric layer to provide electrical insulation over the bit lines and to protect the bit lines and air gaps from further processing. For example silicon carbon nitride (SiCN) may be used to form such a cover layer. 
         [0069]      FIG. 13  illustrates process steps that may be used to form bit lines separated by air gaps. A sacrificial material such as amorphous silicon or photoresist is deposited and patterned to form core portions, or mandrels  125  that are used to form air gap structures. Mandrels are located where air gaps are to be formed and trenches between mandrels are located where bit lines are to be formed (trenches overlie vias). Sidewalls are formed  127  by depositing and etching back a suitable material such as silicon oxide. Mandrels are then removed  129 . A capping layer is then deposited  132 , which caps air gaps and protects them from subsequent processing. The capping layer is etched back  133  to expose vias. Then a barrier layer and bit line metal are deposited  135  to fill trenches between air gap structures. Excess material is then removed  137  without exposing air gaps. A cover layer is then deposited  139  for protection. 
       Different Sized Air Gap Structures 
       [0070]    While the above example shows air gap structures that contain a single air gap, larger air gap structures may be formed that contain two or more air gaps. In this way larger patterns may be formed when performing damascene patterning of a metal layer. Wide conductive lines may be formed by leaving wide spaces that are filled with metal. Wide areas without metal may be formed by large air gap structures that contain multiple air gaps close together. Process steps used to form air gap structures can form both smaller and larger structures so that a variety of patterns may be created using a common set of process steps. 
         [0071]      FIG. 14  shows an example of a substrate with parallel lines of dielectric (e.g. line  151  which is a sidewall in this example, but may also be formed by any other suitable patterning method) formed different distances apart to form small and large air gap structures. Sidewalls have a width=a, which may be in the range of three to fifty nanometers (3 nm-50 nm), e.g. 5 nm. Some sidewalls are spaced apart a distance=c which is close enough to ensure that a capping layer encloses an air gap. Mandrels may be formed with a width of about c and may be separated from neighboring mandrels by a distance that is about c to form a series of sidewalls spaced apart from their neighbors on either side a distance c. Generally the distance c is in the range of five to fifty nanometers (5 nm-50 nm), e.g. 10 nm. Other sidewall spacers are spaced apart a distance=b. The distance b is generally more than ten nanometers (&gt;10 nm), e.g. 20 nm. It will be understood that the dimensions are examples and that the sizes of openings that do or do not become capped depends on the process used. All sidewalls have a vertical dimension=d in this example. The dimension d may be thirty to two hundred nanometers (30 nm -200 nm) e.g. 40 nm. Sidewalls may be formed of silicon oxide, for example silicon dioxide (SiO2) formed by CVD using TEOS. Alternatively, sidewalls may be formed of silicon nitride (SiN), silicon oxynitride (SiON), or a modified form of SiO2, for example containing fluorine (F), carbon (C), hydrogen (H), or methyl (CH3). Dielectric layer  153  may be formed of any of these materials or may be formed of silicon carbon nitride (SiCN), porous SiO2, or a suitable low-k dielectric. Vias may be formed of a suitable metal such as tungsten (W), copper (Cu), or aluminum (Al). 
         [0072]      FIG. 15  shows the structure of  FIG. 14  after deposition of a capping layer  157 , which may be formed of silicon oxide, e.g. silicon dioxide deposited by plasma CVD using silane (SiH4). Other capping layer materials may also be used including silicon nitride (SiN), silicon carbon nitride (SiCN), and alternative forms of silicon oxide such as porous SiO2, or SiO2 with fluorine (F), carbon (C), hydrogen (H), or methyl (CH3). High growth rate of provides capping layer thickness=e over air gap structures. Lower growth rate on dielectric layer produces a capping layer thickness=f, which is less than e. For example, thickness e may be in the range of ten to fifty nanometers (10 nm-50 nm), e.g. 30 nm. Dimension f may be in the range of two to twenty nanometers (2 nm-20 nm), e.g. 5 nm. 
         [0073]      FIG. 16  shows the structure of  FIG. 15  after etching back to remove some capping layer material so that the dielectric layer  153  and vias  155   a - c  are exposed. Etching back may be sufficient to remove a thickness greater than or equal to f so that exposure of vias is ensured (i.e. some over-etching may be used to ensure that no capping layer material remains over vias). Thus, remaining thickness of capping layer material over air gap structures, dimension=g, is generally somewhat less than e-f (g&lt;e−f). For example, dimension g may be ten to forty nanometers (10 nm-40 nm), e.g. 25 nm. Both small air gap structures  161   a - b  and larger air gap structure  163  are formed at this stage. 
         [0074]      FIG. 17  shows the structure of  FIG. 16  after deposition of a barrier layer  165  and bit line metal  167 . Barrier layer  165  may be formed of a suitable material including, but not limited to, one or more of: titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), ruthenium (Ru). Bit line metal may be copper (Cu), tungsten (W), aluminum (Al), or other suitable metal. A barrier layer may be deposited to a thickness of one to five nanometers (1-5 nm), e.g. 2 nm. Bit line metal may be deposited to a thickness of one hundred to one thousand nanometers (100 nm-1000 nm), e.g. 300 nm. 
         [0075]      FIG. 18  shows the structure of  FIG. 17  after CMP to remove excess bit line metal. CMP may stop at the barrier layer  165  over air gap structures (e.g. using end-point detection). Bit line metal is separated into separate bit lines in this step. Other conductors may also be formed where larger open spaces remain between air gaps. While small single air gap structures  161   a - b  separate bit lines, larger multi air gap structures (e.g. air gap structure  163 ) provide larger areas without metal. 
         [0076]      FIG. 19  shows the structure of  FIG. 18  after a subsequent CMP step to remove additional material including barrier layer material, capping layer material, and bit line metal. The remaining thickness of capping layer=j in this example, where j is sufficient to ensure that air gaps arc not exposed. For example, dimension j may be five to thirty nanometers (5 nm-30 nm), e.g. 10 nm. 
         [0077]      FIG. 20  shows the structure of  FIG. 19  after deposition of an interlayer dielectric layer  171  over the bit lines and air gap structures. Interlayer dielectric layer  171  may be formed of any suitable material, for example one or more of: silicon carbon nitride (SiCN); silicon carbide (SiC); silicon nitride (SiN); various forms of silicon oxide including porous SiO2 and SiO2 containing fluorine (F), carbon (C), hydrogen (H), methyl (CH3); or other low-k dielectric materials. 
       CONCLUSION 
       [0078]    Although the various aspects have been described with respect to examples, it will be understood that protection within the full scope of the appended claims is appropriate.