Abstract:
The present invention discloses an integrated circuit (IC) comprising a bond pad ( 160 ); a substrate stack carrying a first layer ( 130 ) comprising conductive regions ( 135 ); and an interconnect layer ( 140 ) over the first layer ( 130 ) comprising a dielectric material portion ( 400 ) between the bond pad ( 160 ) and the substrate stack, said portion comprising a plurality of air-filled trenches ( 345 ) defining at least one pillar ( 340 ) of the dielectric material ( 400 ), at least said air-filled trenches ( 345 ) being capped by a porous capping layer ( 440 ). The interconnect layer ( 140 ), which typically is one of the uppermost interconnect layers of the IC, has an improved resilience to pressure exerted on the bond pad ( 160 ). The present invention further teaches a method for manufacturing such an IC.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of manufacturing an integrated circuit (IC) comprising providing a substrate stack carrying a first layer comprising conductive regions and forming a interconnect layer over the first layer, said interconnect layer comprising a portion to be oriented between a bond pad and the substrate stack. 
         [0002]    The present invention further relates to an IC comprising a bond pad; a substrate stack carrying a first layer comprising conductive regions; and a layer of a dielectric material over the first layer, said dielectric material comprising a portion between the bond pad and the substrate stack. 
       BACKGROUND OF THE INVENTION 
       [0003]    In deep submicron IC technology, one of the main design challenges is the provision of metal interconnects that, although being located in close vicinity to neighboring metal interconnects, experience little detrimental influence, e.g. capacitive coupling, from their neighbors. Several solutions have been proposed. For instance, low-k materials have been introduced in the buried interconnect layers between the metal tracks of such layers to reduce the amount of capacitive coupling. Recently, proposals even include the replacement of the low-k material in such buried interconnect layers with air gaps to further reduce the capacitive coupling between neighboring interconnects. An example of the introduction of air gaps is for instance disclosed in “Advanced Cu interconnects using air gaps” by L. G. Gossett et al. in Microelectronic Engineering, 82 (2005), pages 321-332 and in “The evolution of multi-level air gap integration towards 32 nm node interconnects” by R. Daamen et al. in Microelectronic Engineering, 84 (2007), pages 2177-2183. 
         [0004]    However, such measures are not appropriate for the uppermost interconnect layer because this layer is typically exposed to significant mechanical forces during the back-end processing steps in the IC manufacturing. This is for instance demonstrated in  FIG. 1 , which shows a cross-section of an integrated circuit. The substrate  100 , which typically includes the semiconductor devices such as transistors, diodes and so on, is covered by a number of interconnect layers  120 , each comprising patterned metal portions  125  separated by a suitable dielectric material. The interconnect layers  120  are typically separated by via layers  130 , including vias  135  that interconnect metal portions  125  in different interconnect layers  120 . The uppermost interconnect layer  140  is the interconnect layer that is closest to the bond pads  160  of the IC, which are typically embedded in a passivation layer  150 . In  FIG. 1 , the uppermost interconnect layer  140  has a dielectric material portion  145  located underneath the bond pad  160 . In  FIG. 1 , the bond pad  160  is shown with a metal cap  170 . 
         [0005]    Upon wire-bonding the bond pad  160 , the mechanical forces on the bond pad  160  to ensure a good connection between the bond pad  160  and a wire (not shown) can cause damage to the dielectric material portion  145 , e.g. delamination, cracking or even collapse of the interconnect layer  140 . Similar mechanical forces may for instance be experienced during some packaging steps, e.g. molding. Such damage is likely to cause electrical failures within the IC at some point during its lifetime. For this reason, relatively brittle low-k materials are avoided in the uppermost interconnect layers of an IC. However, even more robust dielectric materials, e.g. SiO 2 , can still suffer damage when exposed to such mechanical forces. 
         [0006]    A known solution is disclosed in  FIG. 2 , which shows a cross-section of another IC design. Here, the area  245  of the interconnect layer  140  under the bond pad  160  is filled with metal interconnect portions to ensure that the metal, which typically is more rigid than the dielectric material in the interconnect layer  140  in between the metal portions, absorbs most of the pressure forces exerted on the IC during subsequent process steps. However, the introduction of such a design rule significantly reduces the flexibility of the interconnect design, which may prohibit the adequate routing of the required interconnections in the design stage of the IC. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention seeks to provide a method of manufacturing an IC having improved structural integrity. 
         [0008]    The present invention further seeks to provide an IC having improved structural integrity. 
         [0009]    According to an aspect of the present invention, there is provided a method of manufacturing an integrated circuit comprising providing a substrate stack carrying a first layer comprising conductive regions; forming an interconnect layer over the first layer, said interconnect layer comprising a dielectric portion to be oriented between a bond pad and the substrate stack; forming a plurality of trenches in said portion; filling said trenches with a thermally degradable polymer; forming a further layer over the interconnect layer; thermally decomposing said polymer; and forming the bond pad over said portion. 
         [0010]    In accordance with the method of the present invention, a portion of a dielectric material in an interconnect layer underneath a bond pad is patterned such that a plurality of air gaps are formed, thereby defining one or more pillars of the dielectric material underneath the bond pad area. The pillars provide structural support for preventing damage to the interconnect layer under pressure exerted on the bond pad. The first layer may be a metal layer, which may be patterned. 
         [0011]    The formation of pillars of the dielectric material under the bond pad improves the elastical flexibility of the portion of the interconnect layer under the bond pad compared to a continuous dielectric material portion as shown in  FIG. 1 . So even though less dielectric material may be present under the bond pad, the resilience of the interconnect layer is nevertheless improved. The dielectric material may be chosen to be any material that has a high enough density to withstand the mechanical forces exerted e.g. during the wire-bonding process, e.g. silicon oxide or a dense low-k material. 
         [0012]    The introduction of the air-filled trenches has the additional advantage that the dielectric constant of the dielectric material has become less critical because the air-filled trenches help to reduce the capacitive coupling between neighboring metal tracks. This enables the choice of dielectric materials for the uppermost interconnect layer that would not have been considered previously because of the performance issues associated with the capacitive coupling between neighboring metal portions in this interconnect layer. 
         [0013]    The dielectric material may be used in combination with a fluorosilicate glass for the via layers, which further improves the structural integrity of the IC. 
         [0014]    The trenches may be formed by forming a hard mask over the interconnect layer; depositing a resist over the hard mask; patterning the resist; and etching the plurality of trenches. The resist may be removed after said etching step. Any suitable hard mask and resist material may be used. 
         [0015]    The trenches may be filled by depositing the thermally degradable polymer over the interconnect layer, thereby filling said trenches; and subsequently removing excess thermally degradable polymer. Any suitable deposition technique such as spin-coating may be used. 
         [0016]    In an embodiment, the method further comprises forming, in said portion, at least one further trench over at least one of said regions following said filling step, and wherein forming the further layer comprising forming a further metal layer over the interconnect layer, said forming step including filling the at least one further trench with the further metal. In this case, the portion underneath the bond pad comprises both air-filled trenches and metal-filled, e.g. copper-filled trenches, with the air-filled trenches typically located between a pair of metal-filled trenches, with at least one of said metal-filled trenches being located in said portion. 
         [0017]    The at least one further trench may be formed by depositing a further hard mask over the interconnect layer; depositing a further resist over the further hard mask; patterning the further resist; and etching the at least one further trench. The further resist may be removed after forming the further trench in order to facilitate deposition of the filler material over the interconnect layer. 
         [0018]    According to a further aspect of the present invention, there is provided an integrated circuit comprising a bond pad; a substrate stack carrying a first layer comprising conductive regions; and an interconnect layer over the first layer, said interconnect layer comprising a dielectric material portion between the bond pad and the substrate stack, said portion comprising a plurality of air-filled trenches defining at least one pillar of the dielectric material. Such an IC benefits from improved structural integrity under its bond pads, as previously explained. Such an IC may be integrated in an electronic device, which can benefit from improved reliability of its functionality controlled by the IC of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Embodiments of the invention are described in more detail and by way of non-limiting examples with reference to the accompanying drawings, wherein: 
           [0020]      FIG. 1  depicts a typical bond pad design on a substrate stack; 
           [0021]      FIG. 2  depicts another typical bond pad design on a substrate stack; 
           [0022]      FIG. 3  depicts a bond pad design in accordance with an embodiment of the present invention; and 
           [0023]      FIGS. 4   a - m  depict the various steps of a method in accordance with an embodiment of the present invention. 
       
    
    
       [0024]    It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts. 
       DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]      FIG. 3  shows a cross-section of an IC according to the present invention. Compared to  FIG. 1 , the uppermost interconnect layer  140  comprises a plurality of air-filled trenches  345  in the region under the bond pad  160 , such that a number of pillars  340  of the dielectric material are formed. The pillars  340  ensure that the interconnect layer  140  can absorb mechanical forces exerted on the bond pad  160 , e.g. during wire-bonding the bond pad  160  to a wire (not shown), because the pillars  340  have the ability to deform under the mechanical forces. 
         [0026]    It will be appreciated that the introduction of the pillars  340  is not limited to the uppermost interconnect layer  140  but may be extended to any interconnect layer that has to absorb such mechanical forces. For this reason, a further air-filled trench  325  is shown in the interconnect layer immediately below the uppermost interconnect layer  140  in  FIG. 3 . 
         [0027]    It will also be appreciated that the materials and process steps chosen for realizing the various components of the IC of the present invention other than the formation of the air-filled trenches are not critically important to the present invention. Any suitable material and/or process steps may be used. 
         [0028]    An embodiment of the method of the present invention facilitating the formation of air gaps in a layer such as the uppermost interconnect layer  140  is given in  FIGS. 4   a - m.    
         [0029]    As shown in  FIG. 4   a , a substrate  100  including a number of interconnect layers  120  and via layers  130  is provided. The substrate  100  typically comprises semiconductor devices such as diodes, transistors, ESD protection and so on. The interconnect layers typically comprise a number of metal portions  125  that are separated by air gaps and/or dielectric material, as is well-known in the art, and the via layers  130  typically comprise a number of vias  135  interconnecting metal portions  125  in different interconnect layers  120 . It is reiterated that such a stack may be formed in any suitable way. 
         [0030]    In accordance with the present invention, a dielectric layer  400  is deposited over the substrate stack, which will serve to form an interconnect layer  140  in accordance with the present invention. The dielectric material, e.g. a high density low-k material or silicon oxide (SiO 2 ) may be deposited in any suitable way, e.g. by means of a vapor deposition technique or spin-on coating. Alternative techniques will be apparent to the skilled person. It is pointed out that in  FIG. 4   a  the layer thickness of dielectric material  400  is exaggerated for the purpose of clearly demonstrating the method of the present invention. The thickness of the dielectric material layer may be any suitable thickness and may for instance be similar to the thickness of lower interconnect layers  120 . 
         [0031]    In a next step shown in  FIG. 4   b , a hard mask  410  is deposited over the dielectric layer  400 . It will be appreciated that this step is optional, because if a material such as SiO2 is used for the dielectric layer  400 , the material itself may act as a hard mask. In case a low-k material is used for the dielectric layer  400 , a hard mask  410  such as a SiO 2  mask may be formed over the dielectric layer  400 . Any suitable deposition method may be applied. 
         [0032]    A resist layer  420  is applied to the hard mask  410 , and subsequently patterned, as shown in  FIG. 4   c . Any suitable resist material may be used, such as a negative or positive photoresist, e.g. DNQ or Novolac. The photoresist may be applied in any suitable way, e.g. spin-coating. 
         [0033]    Next, trenches  425  are etched in the hard mask  410  (if present) and the dielectric layer  400 . This is shown in  FIG. 4   d . The trenches  425  are formed in a region of the dielectric material over which a bond pad will be formed in a subsequent processing step. Such an etch step is routine practice for the skilled practitioner and will not be explained in further detail for reasons of brevity only. The number of trenches  425  is not critical, as long as one or more pillars  340  of the dielectric material  400  are formed to ensure that the interconnect layer  140  will have sufficient flexibility to withstand mechanical forces associated with subsequent processing steps, as previously explained. The resist  420  may be removed after the trench formation. 
         [0034]    As shown in  FIG. 4   e , the trenches  425  are filled with a thermally decomposable polymer (TDP), which may be applied in any suitable way, e.g. spin-coating. A TDP is a polymer that decomposes in the gaseous compounds when exposed to a high enough temperature. The exact temperature depends on the type of TDP used. Any suitable TDP may be used. Non-limiting examples include polynorbornene-based polymers, polyacrylate-based polymers and XP0733 marketed by Rohm Haas™. As shown in  FIG. 4   f , excess TDP is removed, for instance by means of a chemical mechanical planarization (CMP) step, such that the hard mask  410  if present or the dielectric material  400  is exposed. 
         [0035]    Next, another hard mask  440  is formed over the exposed hard mask  410  or the dielectric material  400 . This is shown in  FIG. 4   g . This further hard mask  440  must be a porous material because the decomposition products of the TDP in the trenches  425  must be able to escape the trenches  425  through this hard mask  440 . Suitable materials for the hard mask  440  include SiOCH based materials, amongst others. The further hard mask  440  may be deposited using any suitable deposition technique, as already explained for the hard mask  410 . Preferably, when using SiOCH-based materials, the further hard mask  440  is deposited by means of plasma-enhanced chemical vapor deposition (PE-CVD). 
         [0036]    In the step shown in  FIG. 4   h , a further resist  450  is deposited over the further hard mask  440  and subsequently patterned. The further resist may be the same resist material as used for resist  420 , or may be a different resist material. In a subsequent step, as shown in  FIG. 4   i , further trenches  455  are etched through the hard masks  410  and  440  and the dielectric material  400  using any suitable etching technique. Following the etching step, the further resist  450  may be removed, thereby exposing the further hard mask  440 . 
         [0037]    The further trenches  455  are subsequently filled with a metal to form metal tracks through the dielectric material  400 . In a preferred embodiment, the metal is copper, which may be deposited in any suitable way, e.g. by means of a Damascene process, which is completed by the removal of the excess copper over the further trenches  455 , e.g. by means of CMP, such that the further hard mask  440  is exposed again. This is shown in  FIG. 4   k.    
         [0038]    The interconnect layer  140  of the present invention can now be formed by decomposing the TDP in the trenches  425 . To this end, the substrate stack is exposed to a thermal budget that is sufficient to decompose the TDP, causing the decomposition products to escape the trenches  425  through the porous further hard mask  440 . Upon completion of the TDP decomposition, an interconnect layer  140  comprising metal portions  125  and air-filled trenches  325  is formed, as shown in  FIG. 4   l.    
         [0039]    The manufacture of the IC of the present invention may now be completed, as shown in  FIG. 4   m . In accordance with the present invention, this includes the formation of a bond pad  160  over the portion of the interconnect layer  140  comprising the air-filled trenched  325 . Other further processing steps, which may or may not include the formation of a final via layer  130  over the interconnect layer  140 , the formation of the planarization layer  150  and the formation of a metal cap  170  on the bond pad  160  are not essential to the present invention and may be implemented in any suitable way. 
         [0040]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.