Abstract:
A method of forming logic cell contacts, forming CMOS integrated circuit (IC) chips including the FETs and the IC chips. After forming replacement metal gates (RMG) on fin field effect transistor (finFET) pairs, gates are cut on selected pairs, separating PFET gates from NFET gates. An insulating plug formed between the cut gates isolates the pairs of cut gates from each other. Etching offset gate contacts at the plugs partially exposes each plug and one end of a gate sidewall at each cut gate. A second etch partially exposes cut gates. Filling the open offset contacts with conductive material, e.g., metal forms sidewall cut gate contacts and stitches said cut gate pairs together.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present invention is a divisional application of U.S. patent application Ser. No. 14/749,614 (Attorney Docket No. FIS920150093US1), “INTEGRATED CIRCUIT (IC) WITH OFFSET GATE SIDEWALL CONTACTS AND METHOD OF MANUFACTURE” to Injo Ok et al., filed Jun. 24, 2015, assigned to the assignee of the present invention and incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The present invention is related to manufacturing Integrated Circuits (ICs) with logic cells and more particularly to forming contacts from logic chip wiring layers to fin field effect transistor (finFET). 
         [0004]    Background Description 
         [0005]    A typical integrated circuit (IC) chip includes several stacked or sequentially formed layers of shapes of various materials, e.g., semiconductors, insulators and metals. For example, the intersection of two rectangles, a gate layer rectangle on a rectangular channel layer forms a simple Field Effect Transistor (FET). In a typical state of the art complementary insulated gate FET process, such as what is normally referred to as CMOS, the layers are formed on a wafer to form pairs of opposite type devices, N-type FETs (NFETs) and P-type FETs (PFETs), on a surface of the wafer, e.g., a silicon surface layer of a Silicon On Insulator (SOI) wafer or a bulk semiconductor wafer. One or more FET pairs may be connected together into logic blocks, memory cells and more complex circuits. 
         [0006]    Each layer of shapes, also known as a mask level or mask layer, may be created or printed optically through well-known photolithographic exposure and development steps. Typical such patterning steps may include, for example, etching, dopant implanting and material deposition. A mask for each layer may be stacked or overlaid precisely on a prior layer and patterned from the mask to form the shapes that define devices (e.g., FETs)) and connect the devices into circuits. Usually, unless steps are taken to avoid it, each layer forms an irregular surface on another irregular surface, which complicates forming features that reflect the original layout or design. 
         [0007]    IC chip density and performance are primary semiconductor technology development goals. Increased chip density (the average density of circuit transistors on a chip) and chip die size has increased the number of transistors packed on a single chip. Typically, chip density has been achieved by shrinking features sizes to pack more transistors in the same area. Chip density also has been increased by forming FETs vertically on narrow semiconductor surface ridges or fins. Minimum feature sized lines of semiconductor material (fins) are formed on the surface of a bulk wafer or from the surface layer of an SOI wafer. Gates on the fins form tightly packed vertical FETs, known as finFETs. The surface above these ridge type features (narrow metal gates orthogonally formed on minimum sized fins) can be very irregular. 
         [0008]    A typical logic array technology, e.g., standard cell or brick wall, includes a library of logic blocks or circuits that may range from simple elemental logic (e.g., inverters and NAND/NOR gates) to more complex functions, such as n-bit registers, adders and multipliers. Wiring from essentially straight line shapes on a wiring layer (above the gate layer) connects array cell FETs (e.g., finFETs) into library logic blocks. Normally, a logic array chip includes additional wiring layers for global wiring that connects the logic blocks together into higher order, more complex circuits, and connects the higher order circuits together into chip functions, and off chip. 
         [0009]    Typically, these additional wiring layers alternate wiring direction, e.g., a horizontally (x) oriented wiring layer beneath a vertically oriented (y) wiring layer beneath another horizontally oriented wiring layer and so on. Thus, wires on each layer may be oriented in a single direction with direction changes made through interlevel vias connecting wires on one layer to wires above or below that layer. Wires deviating from the normal layer direction cross other wiring channels, blocking those channels and increasing congestion. The more logic a chip includes, the more likely severe routing congestion becomes. A typical logic chip, for example, may use all available space on one or more wiring layers. Thus essentially, chip wiring congestion limits logic chips circuit/function density. 
         [0010]    In what is known as physical design, various optical and resist patterning tools convert a layout into layout data, the layout data into masks used for printing chip layers. These patterning tools and materials have associated non-linearities that cause shape interactions, commonly known as optical effects. Thus, it is common to lose design shape fidelity from design to printing, both from the tools and from other adjacent shapes in the vicinity. For example, straight lines (e.g., wires) tend to print such that the ends pull back in what is known as “line-end shortening” effects. Rectangular corners, both inside and outside, print and etch rounded in what is known as “corner rounding” effects. 
         [0011]    FinFET logic cell libraries may use what are known as in-line gate pickups. In-line gate pickups are logic block connecting wires on the lowest wiring layer in channels that are superimposed on (in-line with) gates they connect. While in-line pickups connect to finFET gates, connecting externally requires bi-directional metal wiring (x and y) for interlevel vias, jogging from the level wiring direction to connect to subsequent wiring layers. Bi-directional metal wiring incurs tip-to-side (T2S) printability issues that reduce chip yield. T2S printability issues arise from the coincidence of line-end shortening (tip/end pull-back) and corner rounding at inside corners. T2S printability trades off shorts for opens and vice versa, e.g., from line-ends shorting to the sides of adjacent lines and, line opens from biasing lines to maintain separation with adjacent ends. 
         [0012]    When T2S issues are severe enough FinFET logic cell libraries use what are known as offset drops that allow stacking vias on gate contacts to maintain unidirectional wiring layers. Offset drops are, essentially, elongated rectangular contacts with one side over a finFET gate and the other connecting a wire and/or a via, more or less between gates. However, because offset drops effectively occupy double wide channels, they tend to push the limit on gate to contact spacing with shorts to adjacent gates occurring more frequently. Gate to contact shorts also reduce chip yield. Regardless of the cause, reduced chip yield increases production cost. 
         [0013]    Thus, there is a need for improved yield from dense logic chip block wiring, and more particularly, for logic chip block wiring drops that do not require intra-block bidirectional wiring within gate to contact spacing limits. 
       SUMMARY OF THE INVENTION 
       [0014]    It is a purpose of the invention to reduce wiring channel blockage in finFET logic circuits; 
         [0015]    It is another purpose of the invention to allow vias to land directly on finFET gate contacts in logic circuit; 
         [0016]    It is yet another purpose of the invention to form irregular-shaped offset contacts in finFET logic circuits that connect directly to finFET gate sidewalls without jogging outside of defined adjacent wiring channels; 
         [0017]    It is yet another purpose of the invention to form offset gate contacts in finFET logic circuits that connect directly to finFET gate sidewalls to one side without jogging outside of defined adjacent wiring channels and allow for vias landing directly on the offset gate contacts. 
         [0018]    The present invention relates to a method of forming logic cell contacts, forming CMOS integrated circuit (IC) chips including the FETs and the IC chips. After forming replacement metal gates (RMG) on fin field effect transistor (finFET) pairs, gates are cut on selected pairs, separating PFET gates from NFET gates. An insulating plug formed between the cut gates isolates the pairs of cut gates from each other. Etching offset gate contacts at the plugs partially exposes each plug and one end of a gate sidewall at each cut gate. A second etch partially exposes cut gates. Filling the open offset contacts with conductive material, e.g., metal forms sidewall cut gate contacts and stitches said cut gate pairs together. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: 
           [0020]      FIG. 1  shows an example of a method for forming integrated circuit (IC) logic chips with offset contacts on active Field Effect Transistor (FET) gates according to a preferred embodiment of the present invention; 
           [0021]      FIGS. 2A-C  show a plan view, an exploded view and a cross sectional example of a logic cell after RMG; 
           [0022]      FIG. 3  shows example of the gates capped with insulator/dielectric caps; 
           [0023]      FIGS. 4A-C  show a plan view, an exploded view and a cross sectional example of cutting gates for selected finFET pairs and filling the cuts between the gates with dielectric; 
           [0024]      FIGS. 5A-D  show a plan view, an exploded view and a cross sectional example of opening offset contacts to the cuts gates; 
           [0025]      FIGS. 6A-C  show a plan view, an exploded view and a cross sectional example of the open contacts filled with metal contacting the gate pair sidewalls, stitching the pairs back together; 
           [0026]      FIG. 7  shows an example of a wafer with completed IC chips after normal BEOL. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0027]    Turning now to the drawings and, more particularly,  FIG. 1  shows an example of a method  100  for forming integrated circuit (IC) logic chips with offset contacts on active Field Effect Transistor (FET) gate sidewalls according to a preferred embodiment of the present invention. Essentially, insulator caps and selectively separate paired N-type finFET (NFET) and P-type finFET (PFET) gates that are rejoined by a conductive offset contacts. At the wiring surface the offset contacts accept direct interlevel via placement. Thus subsequent wiring on overlying layers connect to the offset contacts in the same line of sight without jogs blocking wiring channels. 
         [0028]    Fabrication begins in step  102  with providing a typical semiconductor wafer. In step  104  chip devices (FETs) are defined, e.g., by defining and forming fins in/on a surface layer, forming a gate dielectric layer on the surface, a sacrificial gate layer on the gate dielectric layer and defining gates on the fins. After forming gate sidewall spacers, source/drain regions may be formed, e.g., implanting suitable dopant. In step  106  the sacrificial gates are replaced with metal for replacement metal gate (RMG) devices. 
         [0029]    In Step  108 , the metal gates are recessed below the upper end of the gate sidewall spacers, and capped with insulator/dielectric. In step  110  gates for selected finFET pairs are separated (“cut”), and the cut is filled or plugged with insulator/dielectric. In step  112  a dielectric (contact) layer is formed on the wafer covering the capped gates and patterned for contacts, and opened into cut gate sidewalls and plugs defining an irregular shape for preferred offset contacts. Offset contacts are formed in step  114  by filling the open contact pattern with metal to contact the gate pair sidewalls and stitch the pairs back together. Thereafter, in-line vias land on, and connect directly to, preferred offset contacts. Fabrication continues normally in step  116  with standard middle of the line (MOL) and back end of the line (BEOL) steps, e.g., wiring devices together and wiring circuits together, to pads and off chip to complete Integrated circuit (IC) chip definition  118 . 
         [0030]      FIG. 2A  shows a plan view example of a logic cell  120  after RMG in step  106  of  FIG. 1 , with an exploded view of region B in  FIG. 2B  and a cross section through C-C in  FIG. 2C . The logic cell  120  includes multiple P-type fins  122 P and N-type fins  122 N formed on the surface of a wafer  124  provided in step  102 . The wafer  124  may be a silicon on insulator (SOI) wafer or a bulk wafer of silicon (Si) or a III-V semiconductor material. Metal gates  126 , preferably copper, on the fins  122 P,  122 N define cell finFET pairs. PFETs and NFETs. Sidewall spacers  128  remain on the metal gates  126  from FET source/drain definition in step  104 . The gates  126  are a suitable metal. Preferably, sidewall spacers  128  are of a low-k dielectric, e.g., SiBCN, SiOCN or SiOC. An insulating material  130 , e.g., oxide, covers the wafer  124  between the gates  126 . 
         [0031]      FIG. 3  shows example of gates  126 ′ capped with insulator/dielectric caps  132  in Step  108  of  FIG. 1 , with reference to  FIGS. 2A-C  with like features labeled identically. After replacing dummy polysilicon gates (not shown) with metal the gates  126  in  FIG. 2C , are recessed 5-50 nanometers (nm) below the upper end of the gate sidewall spacers  128 , and preferably 25 nm below. Then, the recess is filled with dielectric, preferably with self-aligned contact SAC dielectric, which form dielectric caps  132  on the metal gates  126 ′. 
         [0032]      FIG. 4A  shows a plan view example of cutting gates in step  110  for selected finFET pairs and filling the cuts with dielectric in exploded view of  FIG. 2B , and cross sections through B-B and C-C in  FIGS. 4B and 4C , respectively. A cut pattern  134  formed on the wafer selects gate pairs  136  for cutting that separates PFET and NFET gates. Preferably, the cut pattern  134  is done photolithographically, using a typical photolithographic mask, followed by a suitable wet or dry etch to cut the metal gates  136 . Cutting opens gaps that are 20% of the gate width, or ˜15-20 nm, between the separated PFET and NFET gates. The gaps are filled with dielectric, preferably silicon nitride (SiN). 
         [0033]      FIG. 5A  shows a plan view example of forming a contact layer on the gates and patterned  138  for offset contacts in step  112  with reference to  FIGS. 4A-C  with like features labeled identically, and cross sections through B-B and C-C in  FIGS. 5B and 5C , respectively. Preferably, oxide  140  is grown on the wafer above the gate caps  132  to a level selected for the particular fabrication technology. Then, offset contacts pattern  138  is done again photolithographically, using a typical photolithographic mask. A suitable wet etch, preferably selective to oxide, opens the oxide, stopping on and exposing upper surface/ends of the cut dielectric  134  and sidewalls  128 . It should be noted that the exposed portions of the gate upper surface/ends need not be sufficient to satisfy normal gate contact requirements (e.g., ground rules) because the preferred offset contacts are formed to contact gate sidewalls. 
         [0034]    As shown in  FIG. 5D , a second, subsequent etch removes upper portions of the cut dielectric  134  and sidewalls  128 . Removing upper portions of the cut dielectric and sidewalls exposes upper portions of the separated PFET and NFET gate pairs through the offset contact pattern  138  and forms an irregular shaped contact orifice. 
         [0035]      FIG. 6A  shows a plan view example of irregular shaped offset contacts  140  formed in step  114  with reference to  FIG. 5D  with like features labeled identically, and cross sections through B-B and C-C in  FIGS. 6B and 6C , respectively. Offset contacts  140  may be formed, for example, by forming a metal layer, preferably tungsten (W), on the wafer to fill the open offset contact pattern  138 . Horizontal portions of the metal layer are removed, e.g., with a chemical mechanical (chem-mech) polish (CMP). After CMP metal irregular shaped offset contacts  142  remain between the cut gates, stitching the separated PFET and NFET gate pairs back together. 
         [0036]      FIG. 7  shows an example of a wafer  150  with completed IC chips  152  after normal BEOL and chip definition ( 116  and  118  in  FIG. 1 ). After contacting gates with preferred irregular shaped offset contacts  142 , in-line vias may be placed to land on, and connect directly to, preferred offset contacts  142 , enabling unidirectional metal wiring at book level and above. Subsequent wiring layers are formed using a suitable metal formation process, e.g., in a dual damascene metal line formation step, to connect finFETs  116  into circuits. The metal lines on upper layers (not shown) wire chip FETs into chip circuits  154  and chip circuits  154  together. 
         [0037]    Advantageously, preferred irregular shaped offset contacts maintain minimum required wiring level spacing without jogs blocking adjacent wires, and further, allowing in-line vias to land on, and connect directly to the contacts. The wiring layer contacts may be placed such that the gate upper surface/ends do not sufficiently satisfy normal gate contact requirements (e.g., ground rules) because the preferred offset contacts are formed to contact gate sidewalls. Thus, wiring density and congestion are reduced for improved chip function capacity and yield. 
         [0038]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.