Abstract:
A structure and a method for forming the same. The method includes (a) providing a structure which includes (i) a dielectric layer, (ii) an electrically conducting bond pad on and in direct physical contact with the dielectric layer top surface, (iii) a first passivation layer on the dielectric layer top surface and on the electrically conducting bond pad, wherein the first passivation layer comprises a first hole directly above the electrically conducting bond pad, and (iv) an electrically conducting solder bump filling the first hole and electrically coupled to the electrically conducting bond pad; and (b) forming a second passivation layer on the first passivation layer, wherein second passivation layer is in direct physical contact with the electrically conducting solder bump, and wherein the electrically conducting solder bump is exposed to a surrounding ambient immediately after said forming the second passivation layer is performed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to flip-chip technologies, and more specifically, to soft error issue in flip-chip technologies.  
         [0003]     2. Related Art  
         [0004]     In flip-chip technologies, solder bumps are typically formed on top of a semiconductor chip (i.e., integrated circuit IC). Each solder bump is formed directly on a bond pad of the chip. The chip is then flipped face down and then aligned to a package/substrate so that the solder bumps are bonded directly, simultaneously, and one-to-one to the pads of the package/substrate (called package/substrate pads). After that, an adhesive underfill material is used to fill the empty space between the chip and the package/substrate. Once in place, the adhesive underfill material is cured at a high temperature so as to form a solid underfill layer that tightly bonds the chip to the package/substrate. In some applications, multiple chips can be attached (i.e., bonded) to a single substrate using flip-chip technologies so as to form a multi-chip module (MCM). However, if one of the chips in the MCM is later found defective through testing, the entire MCM is wasted because with current materials it is not possible to replace the defective chip of the MCM (because all the chips of the MCM are tightly bonded to the substrate by the solid underfill layer). One way to solve this problem is to forgo the solid underfill layer by omitting the underfill process. However, for ceramic substrates, without the solid underfill layer separating the substrate from the chips, alpha particles (large subatomic fragments consisting of 2 protons and 2 neutrons) that continuously emit from the substrate would easily enter the chips of the MCM resulting in a substantially larger number of soft errors in the MCM during the normal operation of the MCM.  
         [0005]     Therefore, there is a need for a structure (and a method for forming the same) that allows omission of underfill for chip replacement without substantially increasing the soft error rate in the structure during the normal operation of the structure.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a structure formation method, comprising providing a structure which includes (a) a dielectric layer including a dielectric layer top surface that defines a reference direction essentially perpendicular to the dielectric layer top surface, (b) an electrically conducting bond pad on and in direct physical contact with the dielectric layer top surface, (c) a first passivation layer on the dielectric layer top surface and on the electrically conducting bond pad, wherein the first passivation layer comprises a first hole directly above the electrically conducting bond pad, and (d) an electrically conducting solder bump filling the first hole and electrically coupled to the electrically conducting bond pad; and forming a second passivation layer on the first passivation layer, wherein second passivation layer is in direct physical contact with the electrically conducting solder bump, and wherein the electrically conducting solder bump is exposed to a surrounding ambient immediately after said forming the second passivation layer is performed.  
         [0007]     The present invention also provides a structure formation method, comprising providing a structure which includes (a) a dielectric layer including a dielectric layer top surface that defines a reference direction essentially perpendicular to the dielectric layer top surface, (b) N electrically conducting bond pads on and in direct physical contact with the dielectric layer top surface, N being an integer greater than one, (c) a first passivation layer on the dielectric layer top surface and on the N electrically conducting bond pads, wherein the first passivation layer comprises N holes which are one-to-one directly above the N electrically conducting bond pads, (d) N electrically conducting solder bumps filling one-to-one the N holes and electrically coupled one-to-one to the N electrically conducting bond pads; and forming a second passivation layer on the first passivation layer, wherein second passivation layer is in direct physical contact with the electrically conducting solder bump, and wherein the electrically conducting solder bump is exposed to a surrounding ambient immediately after said forming the second passivation layer is performed.  
         [0008]     The present invention also provides a structure, comprising (a) a dielectric layer including a dielectric layer top surface that defines a reference direction essentially perpendicular to the dielectric layer top surface; (b) N electrically conducting bond pads on and in direct physical contact with the dielectric layer top surface, N being a positive integer; (c) a first passivation layer on the dielectric layer top surface and on the N electrically conducting bond pads, wherein the first passivation layer comprises N holes which are one-to-one directly above the N electrically conducting bond pads, and wherein the first passivation layer comprises a first dielectric material; (d) N electrically conducting solder bumps filling one-to-one the N holes and electrically coupled one-to-one to the N electrically conducting bond pads, wherein the N electrically conducting solder bumps comprise a first electrically conducting material; (e) N electrically conducting ball limiting metallization (BLM) regions, wherein, for k=1, 2, . . . , N, the k th  electrically conducting BLM region of the N electrically conducting BLM regions (i) is sandwiched between and in direct physical contact with the k th  electrically conducting solder bump of the N electrically conducting solder bumps and the k th  electrically conducting bond pad of the N electrically conducting bond pads, and (ii) is sandwiched between and in direct physical contact with the k th  electrically conducting solder bump of the N electrically conducting solder bumps and the first passivation layer, and wherein the N electrically conducting BLM regions comprise a second electrically conducting material different from the first electrically conducting material; and (f) a second passivation layer on the first passivation layer; and wherein second passivation layer comprises a second dielectric material, and wherein second passivation layer is in direct physical contact with the N electrically conducting solder bumps.  
         [0009]     The present invention provides a structure (and a method for forming the same) that allows omission of underfill for chip replacement without substantially increasing the soft error rate in the structure during the normal operation of the structure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIGS. 1-8  illustrate the fabrication of a solder bump structure, in accordance with embodiments of the present invention.  
         [0011]      FIG. 9  illustrates one way to use the solder bump structure of  FIG. 8  to bond a semiconductor chip to a ceramic substrate so as to form a module, in accordance with embodiments of the present invention.  
         [0012]      FIG. 10  shows a plot illustrating the benefit of adding an additional passivation layer on top of the solder bump structure of  FIG. 8 , in accordance with embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIGS. 1-8  illustrate the fabrication of a solder bump structure  100 , in accordance with embodiments of the present invention. More specifically, with reference to  FIG. 1 , in one embodiment, the fabrication of the structure  100  starts with a dielectric layer  110  at top of a semiconductor chip (not shown for simplicity) with an electrically conducting line  120  (comprising copper (Cu) in one embodiment) embedded in the dielectric layer  110 . It should be noted that the Cu line  120  is a part of a top interconnect layer (not shown) of the semiconductor chip. There may be additional interconnect layers (not shown) beneath the top interconnect layer, but these additional interconnect layers are also not shown for simplicity.  
         [0014]     Next, with reference to  FIG. 2 , in one embodiment, a portion of the dielectric layer  110  is removed so as to create an opening  205  such that a top surface  122  of the Cu line  120  is exposed to the surrounding ambient via the opening  205 .  
         [0015]     Next, with reference to  FIG. 3 , in one embodiment, a bond pad  310  (comprising aluminum (Al) in one embodiment) is formed on top of the Cu line  120  and the dielectric layer  110  such that the Al bond pad  310  is electrically coupled to the Cu line  120 . Illustratively, the Al bond pad  310  can be formed by (a) forming an Al layer (not shown) on the entire structure  100  of  FIG. 2 , then (b) directionally and selectively etching back the Al layer stopping at the dielectric layer  110 . The directional and selective etching in step (b) may be performed using a traditional lithographic process such that what remains of the Al layer after the directional etching is the Al bond pad  310 .  
         [0016]     Next, in one embodiment, a patterned passivation layer  320  (comprising polyimide and having a thickness  321  of about 4 μm in one embodiment) is formed on top of the Al bond pad  310  and the dielectric layer  110  by, illustratively, a photo lithographic process. The patterned passivation layer  320  is formed with an opening  325  directly above the Al bond pad  310  such that a top surface  312  of the Al bond pad  310  is exposed to the surrounding ambient.  
         [0017]     Next, with reference to  FIG. 4 , in one embodiment, a ball limiting metallization (BLM) film  410  is formed on top of the entire structure  100  of  FIG. 3  by, illustratively, sputter deposition. Illustratively, the BLM film  410  comprises multiple layers of copper (Cu), chrome (Cr), and gold (Au).  
         [0018]     Next, with reference to  FIG. 5 , in one embodiment, a patterned photoresist layer  510  is formed on top of the structure  100  of  FIG. 4  with an opening  505  directly above the Al bond pad  310 . As a result, the opening  505  and the opening  325  are aligned and therefore can be collectively referred to as the opening  505 , 325 .  
         [0019]     Next, with reference to  FIG. 6 , in one embodiment, a solder bump  610  (comprising lead (Pb) and tin (Sn) in one embodiment) is formed in the opening  505 , 325  by, illustratively, electroplating. More specifically, in one embodiment, the structure  100  of  FIG. 5  is submerged in a solution (not shown) containing tin and lead ions. The BLM film  410  is electrically coupled to the cathode of an external dc (direct current) power supply (not shown), while the solution is electrically coupled to the anode of the dc supply. Under the electric field created in the solution by the dc power supply, tin and lead ions in the solution arrive at the exposed surface  412  ( FIG. 5 ) of the BLM film  410  and deposit there forming the solder bump  610 .  
         [0020]     In one embodiment, the solder bump  610  is formed such that its top surface  612  is at a lower level than a top surface  512  of the patterned photoresist layer  510 . Alternatively, the solder bump  610  is formed such that its top surface  612  is at a higher level than a top surface  512  of the patterned photoresist layer  510 .  
         [0021]     Next, in one embodiment, the patterned photoresist layer  510  is removed, and then the BLM film  410  is etched by, illustratively, a plasma etch process such that a BLM region  410 ′ ( FIG. 7 ) (i) sandwiched between the solder bump  610  and the patterned passivation layer  320  and (ii) sandwiched between the solder bump  610  and the Al bond pad  310  is carved out from the BLM film  410 .  
         [0022]     Next, with reference to  FIG. 7 , in one embodiment, the solder bump  610  is reflowed so as to have a spherical shape at its top portion. Illustratively, the solder bump  610  of  FIG. 6  is reflowed by subjecting it to a temperature lower than 400° C. In one embodiment, the resulting solder bump  610  has a height  614  in a range of 100-125 μm.  
         [0023]     Next, with reference to  FIG. 8 , in one embodiment, a passivation layer  810  (comprising polyimide and having a thickness  812  of about 20 μm or more in one embodiment) is formed on top of the entire structure  100  of  FIG. 7 . Illustratively, the polyimide passivation layer  810  is formed by (i) spin-applying liquid polyimide on top of the structure  100  of  FIG. 7  so as to form a liquid polyimide layer (not shown) on top of the structure  100  of  FIG. 7  and then (ii) curing the liquid polyimide layer at a temperature in a range of 350-370° C. for 1-2 hours so as to form the polyimide passivation layer  810 . Because the polyimide passivation layers  810  and  320  are in direct physical contact with each other, they can be collectively referred to as the polyimide passivation layer  810 , 320 .  
         [0024]     Next, in one embodiment, a thin polyimide film  810 ′ (a part of the polyimide passivation layer  810 ) that covers the solder bump  610  is removed so as to expose the solder bump  610  to the surrounding ambient by, illustratively, an ashing process (i.e., using a strongly oxidizing ambient such as oxygen plasma to bombard the thin polyimide film  810 ′. Alternatively, the thin polyimide film  810 ′ is removed by a mechanical process such as brushing (i.e., using rotating brushes).  
         [0025]     As can be seen in  FIG. 8 , the polyimide passivation layer  810 , 320  has a thickness  814  which is the sum of the thickness  812  of the polyimide passivation layer  810  and the thickness  321  of the polyimide passivation layer  320 . In one embodiment, the polyimide passivation layers  810  and  320  are formed such that the thickness  814  is at least 24 μm.  
         [0026]     In the embodiments described above, polyimide is used to form the passivation layer  810 . Alternatively, any other material can be used to form the passivation layer  810  provided that the other material has the characteristic of preventing some or all the alpha particles from passing through it. For example, the following materials can be used to form the passivation layer  810 : any spin-on dielectric materials, polymers including BCB (benzocyclobutene) and SiLK (an aromatic hydrocarbon polymer), and inorganics including spin-on glasses such as SiO 2  or C-doped SiO 2 .  
         [0027]      FIG. 9  illustrates one way to use the solder bump structure  100  of  FIG. 8  to bond a semiconductor chip  930  to a ceramic substrate  920  so as to form a module  900 , in accordance with embodiments of the present invention.  
         [0028]     More specifically, in one embodiment, while the chip  930  is still on a wafer with other chips (not shown), Al bond pads (not shown, but similar to the Al bond pad  310  of  FIG. 8 ) are simultaneously formed on all the chips (including the chip  930 ) on the wafer.  
         [0029]     Next, in one embodiment, the patterned passivation layer  320  is formed on the entire wafer. The patterned passivation layer  320  comprises openings (not shown, but similar to the opening  325  of  FIG. 3 ) directly above the Al bond pads.  
         [0030]     Next, in one embodiment, the BLM film  410  ( FIG. 4 ) is formed on the entire wafer. Next, in one embodiment, the patterned photoresist layer  510  ( FIG. 5 ) is formed on the entire wafer. The patterned photoresist layer  510  comprises openings (not shown, but similar to the opening  505  of  FIG. 5 ) directly above the Al bond pads. As a result, the openings in the patterned photoresist layer  510  and the openings in the patterned passivation layer  320  are aligned.  
         [0031]     Next, in one embodiment, multiple solder bumps  610  (similar to the solder bump  610  of  FIG. 8 ) are simultaneously formed on all the Al bond pads of all the chips of the wafer (including the chip  930 ).  
         [0032]     Next, in one embodiment, the polyimide passivation layer  810  is formed on top of the entire wafer (including the chip  930 ). The polyimide passivation layer  810  merges with the patterned passivation layer  320  to form the polyimide passivation layer  810 , 320 . The polyimide passivation layer  810  forms thin polyimide films (not shown, but similar to the thin polyimide film  810 ′ of  FIG. 8 ) on top of the multiple solder bumps  610 .  
         [0033]     Next, in one embodiment, an ashing process is performed to remove the thin polyimide films on top of the multiple solder bumps  610 . Next, in one embodiment, a dicing process is performed to separate the chips (including the chip  930 ) from the wafer.  
         [0034]     Next, in one embodiment, the chip  930  is flipped face down and then aligned to the ceramic substrate  920  as shown in  FIG. 9  so that the solder bumps  610  of the chip  930  are bonded directly, simultaneously, and one-to-one to the pads (not shown) of the ceramic substrate  920 .  
         [0035]     With the presence of the thick polyimide passivation layer  810  (with the thickness  812  being at least 20 μm in one embodiment), a larger number of alpha particles (large subatomic fragments consisting of 2 protons and 2 neutrons) that continuously emit from the ceramic substrate  920  are prevented from entering the chip  930  of the module  900  resulting in a lower number of soft errors in the module  900  during the normal operation of the module  900  (compared with the case where the additional polyimide passivation layer  810  is omitted).  
         [0036]     In one embodiment, other chips (not shown) beside the chip  930  are also formed on the same ceramic substrate  920 . As a result, the module  900  is called a multi-chip module (MCM). Because no underfill layer is formed between the chips and the ceramic substrate  920 , if one of the chips is later found defective during testing, the defective chip can be easily removed from the ceramic substrate  920  and replaced by another chip.  
         [0037]      FIG. 10  shows a plot  1000  illustrating a relationship between the additional passivation thickness  812  ( FIG. 9 ) and the normalized alpha particle passing rate, in accordance with embodiments of the present invention. The normalized alpha particle passing rate is defined as the ratio of the number of alpha particles passing through both the passivation layers  810  and  320  ( FIG. 9 ) to the number of alpha particles passing through only the passivation layers  320  ( FIG. 9 ).  
         [0038]     For example, as can be deducted from the plot  1000 , when the thickness  812  ( FIG. 9 ) is 10 μm, the normalized alpha particle passing rate is 0.6, meaning the number of alpha particles passing through both the passivation layers  810  and  320  is 60% of the number of alpha particles passing through only the passivation layers  320 . In one embodiment, the relationship between the additional passivation thickness  812  ( FIG. 9 ) and the normalized alpha particle passing rate is determined from empirical data obtained through testing.  
         [0039]     In one embodiment, with reference to  FIGS. 9 and 10 , the thickness  812  of the polyimide passivation layer  810  is at least a minimum thickness such that the normalized alpha particle passing rate is lower than a pre-specified normalized alpha particle passing rate. The minimum thickness can be determined based on the pre-specified normalized alpha particle passing rate and the plot  1000 .  
         [0040]     For example, assume that the normalized alpha particle passing rate is pre-specified at a target value of 40% (i.e., 0.4). Then, it can be determined from the plot  1000  that the minimum thickness is 20 μm. In other words, in order to have the number of alpha particles passing through both the passivation layers  810  and  320  (i.e., the passivation layer  810 , 320 ) not exceeding 40% of the number of alpha particles passing through only the passivation layers  320 , the passivation layer  810  must be formed with the thickness  812  being at least 20 μm.  
         [0041]     In one embodiment, the normalized alpha particle passing rate is pre-specified at any target value in the range of 0 to 1.0 (e.g., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc.).  
         [0042]     In summary, the polyimide passivation layer  810  is formed on top of the patterned passivation layer  320  resulting in the thicker polyimide passivation layer  810 , 320 . As a result, when the chip  930  is later attached to the ceramic substrate  920 , the number of alpha particles that enter the chip  930  from the ceramic substrate  920  is reduced compared with the case the polyimide passivation layer  810  is not formed. Therefore, soft error rate during the normal operation of the chip  930  is also reduced.  
         [0043]     While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.