Abstract:
The present invention provides integrated circuit fabrication with a silicon oxynitride antireflective layer for gate location plus patterned photoresist linewidth reduction for gate length definition followed by interconnect definition without patterned photoresist linewidth reduction. This has the advantages of an antireflective layer compatible with linewidth reduction and polysilicon etching.

Description:
This application is a provisional of No. 60/049,006 filed Jun. 9, 1997. 
   RELATED APPLICATIONS 
   The following patent applications disclose related subject matter Serial No. 08/678,847, filed Jul. 12, 1996 (TI-20565). These applications have a common assignee with the present application. 

   BACKGROUND OF THE INVENTION 
   The invention relates to electronic semiconductor devices, and, more particularly, to antireflective structures and fabrication methods for such structures. 
   Semiconductor integrated circuits with high device density require minimal size structures such as short gates for field effect transistors, small area emitters for bipolar transistors, and narrow interconnects between devices. The formation of such polysilicon or metal (or a stack of metals or a metal silicide on polysilicon) structures typically involves definition of the locations of such structures in a layer of photoresist on a layer of polysilicon or metal by exposure of the photoresist with radiation passing through a reticle containing the desired structure pattern. During the exposure, radiation reflected from the underlying material (e.g., polysilicon, metal, . . . ) can degrade the pattern developed in the photoresist, so include an antireflective coating or layer on the polysilicon or metal. Commercial antireflective coatings include organic materials and TiN which strongly absorb at the radiation wavelength, such as polymers with dye-groups for absorption. 
   After exposure and development of the photoresist, the underlying layers of antireflective coating plus material (polysilicon, metal, . . . ) are anisotropically etched using the patterned photoresist as the etch mask (or the antireflective coating may first be wet developed and then the underlying material anisotropically etched). Thus the minimal structural linewidth equals the minimal linewidth that can be developed in the photoresist. 
   One approach to overcome the linewidth limitation patterns photoresist and then isotropically etches the patterned photoresist to shrink its size and thereby emulate a smaller original linewidth. However, this approach has a problem of the isotropic etch of the patterned photoresist also etches the antireflective coating. Ogawa et al, 2197 Proc. SPIE 722 (1994) describes the use of silicon oxynitride as an antireflective coating on a oxide coated tungsten silicide layer and on an aluminum layer for i-line and deep ultraviolet radiation by quarter-wavelength thickness for reflective interference. 
   SUMMARY OF THE INVENTION 
   The present invention provides integrated circuit fabrication with a silicon oxynitride antireflective layer for gate location plus patterned photoresist linewidth reduction for gate length definition followed by interconnect definition without patterned photoresist linewidth reduction. 
   This has the advantages of an antireflective layer compatible with linewidth reduction and polysilicon etching. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings are heuristic for clarity. 
       FIGS. 1   a-m  are cross sectional elevation views of a preferred embodiment integrated circuit fabrication method steps. 
       FIG. 2  illustrates the optical constant variation with composition. 
       FIG. 3  shows net reflection. 
       FIG. 4  illustrates linewidth reduction. 
       FIGS. 5   a-d  are cross sectional elevation views of another preferred embodiment. 
       FIG. 6  shows a damascene preferred embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Overview 
   The preferred embodiments use a silicon oxynitride (Si x O y N z ) as an antireflective layer for patterning of photoresist on polysilicon followed by an isotropic etching of the patterned photoresist to reduce linewidth and then an anisotropic etcing of the oxynitride plus polysilicon to form gates and gate level interconnects; and lastly, overlying metal interconnects are formed using patterned photoresist without linewidth reduction but using a differing composition silicon oxynitride as antireflective layer due to the differing underlying material reflectivity. 
   The index of refraction and the extinction coefficient (real and imaginary parts of the square root of the dielectric constant) of silicon oxynitride vary with the composition (see FIG.  2 ). Thus, essentially select the composition to have optical constants that adjust the reflected waves amplitudes according to the underlying material reflectivity and select the oxynitride layer thickness to provide quarter-wave interference so the reflected waves cancel. 
   The resultant integrated circuit may include residual silicon oxynitride on the gates or interconnects or both or neither. 
   First Preferred Embodiment 
     FIGS. 1   a-m  illustrate in cross sectional elevation views the steps of a first preferred embodiment fabrication methods for integrated circuits including field effect transistors (e.g., CMOS or BiCMOS). The example is of lithography which has 0.25 μm minimal linewidth and a circuit with 0.25 μm minimal metal interconnect width together with 0.18 μm minimal (silicided) polysilicon linewidth (gate length). The preferred embodiment includes the following steps: 
   (1) Start with a silicon (or silicon on insulator) wafer  102  with LOCOS or shallow trench isolation and twin wells for CMOS devices (optionally, plus memory cell array wells and bipolar device buried layers). Perform threshold adjustment implants (which may differ for cell transistors and various peripheral transistors), and form gate dielectric. Deposit layer  104  of polysilicon gate material of thickness 300 nm; see  FIG. 1   a.    
   (2) Deposit a 29 nm thick conformal layer  110  of (hydrogenated) silicon oxynitride by PECVD from a flow of silane and nitrous oxide; see  FIG. 1   b . The composition of the oxynitride and thickness are selected according to the reflectivity of the underlying material; in particular, for polysilicon  104  take the composition to have a ratio of silicon to oxygen to nitrogen equal 53 to 38 to 9; the oxynitride also contains roughly 10% bound hydrogen, but the exact hydrogen fraction is difficult to measure. This composition gives measured optical constants of n=2.11 and k=0.55 (theoretical n=2.15 and k=0.60) and thus a layer thickness of 29 nm provides a quarter wavelength at 248 nm. See the following section on oxynitride composition. 
   (3) Spin on 900 nm average thickness layer  112  of photoresist which is sensitive to 248 nm radiation (deep ultraviolet); see  FIG. 1   c.    
   (4) Expose photoresist  112  with 248 nm wavelength radiation through a reticle for gates and gate level interconnects; the exposed minimal linewidth may be about 250 nm. Silicon oxynitride  110  acts as an interference antireflective layerduring exposure of photoresist  112 . Develop photoresist  112 ; see  FIG. 1   d  which shows the differing heights of the patterned photoresist possible with LOCOS isolation. 
   (5) Shrink the patterned photoresist (reduce linewidth) with an isotropic etch which may be an oxygen-nitrogen plasma in a high density plasma reactor with small bias to limit ion bombardment for isotropy. The final minimal linewidth is 180 nm (although reduction to 60 nm may be achieved with good linewidth control). Thus the minimal linewidth photoresist shrinks from a 900 nm by 250 nm to roughly 850 nm by 180 nm; see  FIGS. 1   d-e  which shows (exaggerated) original patterned photoresist line  114  reduced to photoresist line  116 . 
   (6) Remove the exposed portion of silicon oxynitride  110  with an anisotropic plasma etch using CF 4  or some other fluorine source; this etch also acts as the breakthrough etch (to eliminate surface oxides) for polysilicon  104  in the next step and may be performed immediately before and in the same chamber as the polysilicon etch. This etch also reduces the photoresist height to very roughly 700 nm; see  FIG. 1   f.    
   (7) Use the remaining silicon oxynitride  110  plus photoresist as an etch mask for the anisotropic plasma etch of polysilicon  104  to form gates  106  and gate level interconnects  108 . The etch may be a HBr plus oxygen plasma which is very selective to oxide and oxynitride. Thus even if the polysilicon etch strips the photoresist, the oxynitride antireflective layer remains and provides sufficient etch masking because of the etch selectivity.  FIG. 1   g  shows oxynitride-topped gates  106  plus gate level interconnects  108 . The gate material could also provide a polysilicon emitter for bipolar devices which would require a prior base implant. Gates  106  are 300 nm high and 180 nm long ( FIG. 1   g  is a cross section along the gate length, and gates typically have widths much greater than their lengths). 
   (8) Perform lightly doped drain implants with the gate, oxynitride, and any residual resist as the implant mask; this also requires non-critical photolithographic masking for CMOS or BiCMOS circuitry which will also remove any residual resist. See  FIG. 1   h.    
   (9) Form sidewall dielectric spacers  120  on the gates (and gate level interconnects) by conformal deposition of a dielectric layer followed by anisotropic etching. The sidewall dielectric may be silicon nitride and the anisotropic etch a plasma of fluorine plus an inert gas. This sidewall spacer etch will also remove some (or all) of the oxynitride on the tops of the gates and gate level interconnects. Introduce dopants to form sources and drains; see  FIG. 1   i . A variation would be a self-aligned silicidation to create a silicide on both the gate top and the source/drains. This silicidation may be by stripping any remaining oxynitride from the gate tops and oxide from the source/drain surfaces, blanket metal (Ti or Co or Ni) deposition followed by reaction with underlying silicon, and then removal of unreacted metal (or TiN for the case of Ti silicidation in an nitrogen atmosphere).  Figure 1   j  shows the resulting silicide. 
   (10) Form a planarized premetal dielectric layer  130  (such as reflowed BPSG or a stack of conformal and planarized layers, and the planarization may be by chemical mechanical polishing (CMP) or resist etch back). Then photolithographically define and etch vias through dielectric layer  120  for contact to the source/drains and the gate/interconnects; see  FIG. 1   j . The antireflective layer for this lithography may also be silicon oxynitride, but the transparency of underlying oxide makes the deeper reflections significant. 
   (11) For a structure with an embedded memory cell array using one-transistor one-capacitor memory cells, the bitlines and cell capacitors may be formed next. For clarity such steps are not illustrated and attendant additional dielectric layers deposited on dielectric  130  will just be considered part of dielectric  130  in the Figures. 
   (12). Blanket deposit (including filling vias) a metal stack such as 50 nm of Ti, 50 nm of TiN, 300 nm of W or Al (doped with Cu and Si), and 50 nm of TiN; the bottom Ti and TiN form a diffusion barrier and the top TiN forms an antireflective coating for lithography. Prior to the W or Al deposition the bottom Ti may be reacted with the source/drain to form a silicide to stabilize the metal-to-silicon contact. The Ti and TiN may be deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD) (e.g., TiCl4+NH 3 →&gt;TiN+HCl); the aluminum may be deposited by PCD and then forced into the vias under high pressure or by CVD; and W may be deposited by CVD. Alternatively the vias may be filled with W by a CVD blanket deposition followed by an etchback to leave W only in the vias, and then an Al blanket deposition. 
   (13) Apply silicon oxynitride antireflection layer  150  on blanket metal  140 ; see  FIG. 1   k . Again, select the oxynitride composition to give optical constants according to the reflectivity of the metal and take the oxynitride layer thickness to yield a quarter-wavelength. For example, with underlying aluminum and deep ultraviolet (248 nm) exposure, take n=2.16 and k=0.83; this implies a thickness of 248 nm/4*2.16=29 nm. Note that the extinction coefficient is much larger than that of the oxynitride for the polysilicon; this is due to the greater reflectivity of the aluminum. 
   (14) Spin on photoresist and pattern it to define the first level metal interconnects; see  FIG. 1   l  with patterned resist  152  illustrating minimal linewidth of 250 nm. Again, a brief fluorine based plasma etch strips the exposed oxynitride without removing much patterned resist. Because the interconnects are wider than the gates, no resist linewidth reduction is needed. 
   (15) Use the patterned resist and oxynitride as the etch mask to anisotropically plasma etch the metal and thereby form interconnects  142 ; chlorine-based plasmas with some fluorine for copper-doped aluminum and TiN, and fluorine-based plasmas for tungsten. 
   (16) Deposit a 50 nm thick conformal oxide liner  150  on interconnects  142  by plasma-enhanced decomposition of TEOS with oxygen or ozone. Liner  150  passivates the metal surfaces and prevents diffusion of metal atoms into the intermetal level dielectric, which may be a low dielectric constant material such as porous silica or fluorinated parylene. Then deposit the intermetal level dielectric over the liner-coated interconnects; the intermetal level dielectric may be a stack of two or more materials, such as porous silica between the first level interconnects and fluorinated silicon dioxide over the porous silica and interconnects. 
   (17) Form vias in the intermetal level dielectric analogous to the via formation of step (10). Then form second level interconnects on this intermetal level dielectric by a repetition of the foregoing steps (12)-(16). Similar repetitions may be used to form third, fourth, fifth, . . . level interconnects. 
   Silicon Oxynitride Composition 
   Silicon oxynitride has an index of refraction and extinction coefficient depending upon composition as illustrated in  FIG. 2  with increasing nitrogen content increasing n and k. Thus silicon oxynitride as an antireflective layer provides two parameters (layer thickness and composition) to adapt to the underlying material reflectivity to yield a net zero reflectivity from the antireflectivity layer. In particular, consider the simplified situation of single reflections: radiation of intensity 1 passing through the photoresist impinges on the antireflective layer and a fraction r (typically about 5-10%) reflects (with a phase shift of π) back into the photoresist and 1−r passes through the antireflective layer with attenuation by a factor e −kt  where k is the extinction coefficient and t is the thickness of the antireflective layer. At the interface with the underlying material, a fraction R reflects (plus phase shifts by π) and the remainder transmits through and/or absorbs in the underlying material. The reflected fraction R is also attenuated by e −kt  as it passes back through the antireflective layer and reenters the photoresist. Thus the portion reflected from the underlying material is R(1−r)e −2kt  and this is to cancel the initially reflected portion r. To cancel, the amplitudes must equal (r=R(1−r)e −2kt ) and the phases must differ by 7 which implies the thickness t must be a quarter wavelength. The wavelength in antireflective layer is the radiation&#39;s in vacuo wavelength λ divided by the index of refraction n of the antireflective layer. Thus take t=λ/4n, and select the composition of the oxynitride to have the ratio kin satisfying:
 
 r=R (1 −r ) e   −kλ/4n  
 
 k/n =4/λ[log( R )+log((1   31  r )/ r )]
 
Of course, the reflectivity r depends upon n and k and the optical constants of the photoresist, but is relatively constant over a range n and k. Similarly, once the underlying material is known, R is approximately known (it also depends upon n and k), and the composition to give the desired k/n can be selected.
 
     FIG. 3  shows the computed net reflection back into resist as a function of the optical constants of the oxynitride with more precise modeling of reflections at the interfaces of resist-oxynitride and oxynitride-polysilicon for an oxynitride layer thickness of 29 nm. Inside the central contour shows the less than 1% net reflection into the resist. Also,  FIG. 3  shows the n-k relation curve of silicon oxynitride as in  FIG. 2  superimposed on the computed net reflections; the curve passes through the center of the region of less than 1% net reflection which indicates that targeting the composition at the center will give robust oxynitride parameters. 
   Linewidth Reduction 
     FIG. 4  shows the robustness of the patterned resist linewidth reduction using oxynitride antireflective layer with a fixed oxygen resist etch (which removes 80 nm of resist) for various initial resist linewidths.  FIG. 4  indicates the standard deviation observed in the reduced linewidth by the three sigma curves in FIG.  4 . Thus a reduction from 220 nm to 60 nm has a standard deviation of roughly 2-3 nm. This robust large linewidth reduction using the silicon oxynitride antireflective layer permits the original resist linewidth to be patterned in a range where the lithography is well controlled. 
   Disposable Gate Preferred Embodiment 
     FIGS. 5   a-d  illustrate a disposable gate method of integrated circuit fabrication which uses the silicon oxynitride antireflective layer. In particular, follow the foregoing steps (1)-(9) to have a polysilicon dummy gate with sidewall spacers and source/drains formed in the substrate. Use the patterned resist reduction to make the dummy gate length only 60 nm; this can be achieved by an original dummy gate length of 220 nm followed by 80 nm resist etch (see FIG.  4 ). The resulting structure is analogous to that of  FIG. 1   i.    
   Deposit 500 nm thick dielectric, such as TEOS oxide, and planarize, such as by CMP, to expose the top of the polysilicon dummy gate.  FIG. 5   a  shows dummy gate  505  and dielectric  530 ; dummy gate  505  may be about 200 nm high and 60 nm long. 
   Etch out dummy gate  505  with an HBr+O2 plasma. Optionally, strip the gate oxide and thermally grow a new gate oxide at the bottom of the groove left by the removal of dummy gate  505 . Next, blanket deposit gate material, such as TiN or a stack of different metals, to fill the groove plus cover dielectric  530 ; see  FIG. 5   b  showing 200 nm thick metal gate material  507 . 
   Deposited 29 nm thick silicon oxynitride with composition (optical constants) adapted to the gate material, followed by spin on of photoresist sensitive to 248 nm radiation. Pattern the photoresist to define a gate top of length 250 nm, and use the patterned photoresist (without linewidth reduction) to etch gate material  507  to form T-shaped gate  506 ; see  FIG. 5   c.    
   Continue as in foregoing step (10)-(17) by dielectric deposition, via formation, first level metal interconnect formation, and so forth. See  FIG. 5   d.    
   Damascene Preferred Embodiment 
   The metal interconnect structure of the foregoing preferred embodiments can be replaced with a damascene interconnect structure because the photoresist linewidth reduction is only used in the gate level. In particular, repeat the foregoing steps (1)-(12) with the variation in step (12) of first fill the vias, then deposit a second dielectric level, photolithographically define locations for grooves in the dielectric, next etch the dielectric to form the grooves, and then blanket metal with CMP or etchback to leave metal only in the grooves and thereby form the interconnects.  FIG. 6  illustrates the damascene structure for one metal level with gate  606 , premetal level dielectric  630 , filled via  644 , and first metal level dielectric  650 . 
   Modifications 
   The preferred embodiments can be modified in various ways while retaining the features of silicon oxynitride antireflective layer for resist linewidth reduction and linewidth reduction for gates together with no linewidth reduction for interconnects. 
   For example, the photoresist may be exposed at different radiation wavelengths, such as 193 nm, with corresponding adjustment in antireflective layer thickness.