Abstract:
A method for producing at least one pad assembly ( 32, 50 ) on a support ( 19, 43 ) for use in a method for self-assembling at least one element ( 10 ) on the support ( 19, 43 ), comprises fanning, on the support ( 19, 43 ), a layer ( 28, 48 ) of at least one fluorinated material around the location ( 30, 44 ) of the pad assembly ( 32, 50 ), the layer ( 28, 48 ) having a thickness greater than 10 nm. The layer ( 28, 48 ) and the location ( 30, 44 ) are exposed to an ultraviolet treatment in the presence of ozone to form the pad assembly ( 32, 50 ) at said location ( 30, 44 ), wherein a drop of liquid ( 16 ) having a static contact angle on the pad assembly ( 32, 50 ) less than or equal to 15°, after the exposure to the ultraviolet treatment, has a static contact angle on the layer ( 28, 48 ) greater than or equal to 100°.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a method for manufacturing pads on a support for the implementation of a method of self-assembly of an electronic circuit on the support. 
       DISCUSSION OF THE ART 
       [0002]    For certain applications, one or several electronic circuits, for example, integrated circuits, are bonded to a support. The support for example corresponds to another integrated circuit. The bonding is for example performed by molecular bonding or by thermocompression. 
         [0003]    Before bonding the integrated circuit to the support, it is necessary to properly arrange the integrated circuit with respect to the support. This may be done by a method of self-assembly of the integrated circuit on the support. 
         [0004]      FIGS. 1A to 1C  are partial simplified cross-section views of the structures obtained at successive steps of an example of a method of self-assembly of an integrated circuit  10  on a support  11  and  FIGS. 2A to 2C  are respective cross-section views of the structures shown in  FIGS. 1A to 1C . 
         [0005]    Support  11  comprises a substrate  12  of a material of low wettability, for example, silicon, and comprises, at the surface of substrate  12 , a pad  14  of a material of high wettability. Support  11  may comprise several pads  14 . A drop  16  of a liquid, for example, demineralized water, is placed on pad  14  ( FIGS. 1A and 2A ). Pad  14  may be obtained by forming a silicon oxide layer on substrate  12  and by etching the obtained layer to delimit pad  14 . The silicon oxide layer may have a thickness on the order of 50 nm. 
         [0006]    Generally, the wettability of a material may be characterized by angle θ of contact of a liquid drop on the material. The smaller the contact angle, the higher the wettability of the material. In the cross-section plane of  FIG. 1A , liquid/air interface  18  of drop  16  is in contact with pad  14  at contact points P and P′. Call T the tangent to interface  18  at contact point P (or P′). Angle θ of contact of drop  16  on pad  14  is the angle between tangent T and the surface of pad  14 , the surface of pad  14  being horizontal. When drop  16  is at rest, the measured angle θ is the static contact angle. 
         [0007]    Electronic circuit  10  is then brought towards pad  14  until it comes in contact with drop  16 . During this approach phase, integrated circuit  10  may be shifted and inclined with respect to pad  14  ( FIGS. 1B and 2B ). 
         [0008]    The forces exerted by drop  16  on integrated circuit  10  then displace integrated circuit  10  until it is in the desired alignment with respect to pad  14  ( FIGS. 1C and 2C ), without for any external action to be necessary. 
         [0009]    The bonding of integrated circuit  10  to support  11 , for example, by molecular bonding or by thermocompression, can then be implemented. 
         [0010]    For the self-assembly method to take place properly, drop  16  must remain confined on pad  14  all along the alignment of integrated circuit  10  with respect to support  11 . In the previously-described self-assembly method, the confinement of drop  16  on pad  14  all along the self-assembly is obtained by the difference in wettability between pad  14  and substrate  12 . The greater the wettability difference, the more drop  16  tends to remain confined on pad  14 . 
         [0011]    However, with known methods, it may be difficult to obtain a significant wettability difference between pad  14  and substrate  12 . There may then be a misplacement of drop  16  on support  11  and drop  16  may leave pad  14  during the self-assembly. 
         [0012]    There thus is a need to form self-assembly pads enabling to improve the confinement of liquid drops on a support for the implementation of a self-assembly method. 
         [0013]    It is further desirable for the pad manufacturing method to be compatible with conventional electronic circuit manufacturing techniques. 
       BRIEF SUMMARY 
       [0014]    An object of the present invention is to overcome all or part of the disadvantages of known pad manufacturing methods. 
         [0015]    Another object of the present invention is to improve the confinement of liquid drops on pads of a support for the implementation of a self-assembly method. 
         [0016]    Another object of the present invention is to improve the wettability difference between the pad and the substrate surrounding the pad, for the implementation of a self-assembly method. 
         [0017]    Another object of the present invention is for the pad manufacturing method to be compatible with conventional integrated circuit manufacturing methods. 
         [0018]    An embodiment of the present invention provides a method for manufacturing at least one assembly pad on a support intended for the implementation of a method of self-assembly of at least one element on the support, the manufacturing method comprising the successive steps of:
       (a) forming, on the support, a layer of at least one fluorinated material around the location of the assembly pad; and   (b) submitting the layer and the location to an ultraviolet processing in the presence of ozone to form the assembly pad at said location.       
 
         [0021]    According to an embodiment, the fluorinated material is a fluorocarbon material. 
         [0022]    According to an embodiment, step (a) comprises exposing the support to a plasma based on octafluorobutene, hexafluoroethane, carbon tetrafluoride, and/or trifluoromethane. 
         [0023]    According to an embodiment, the method further comprises the steps of:
       (c) forming a resin portion on the support at said location before step (a); and   (d) removing the resin portion before step (b).       
 
         [0026]    According to an embodiment, the method further comprises, after step (c) and before step (a), a step of etching the support to form a protrusion under the resin portion. 
         [0027]    According to an embodiment, the support comprises a semiconductor material substrate having the fluorinated material layer formed thereon, the method further comprising a step of forming an oxide layer of the semiconductor material on the substrate before step (a). 
         [0028]    According to an embodiment, the method further comprises the steps of:
       (e) depositing a drop of a liquid on the assembly pad; and   (f) bringing the element in contact with the drop, whereby the element is self-assembled with respect to the support.       
 
         [0031]    According to an embodiment, the element is an integrated circuit chip. 
         [0032]    According to an embodiment, the method further comprises the steps of:
       (g) drying the drop; and   (h) bonding the element to the support.       
 
         [0035]    According to an embodiment, step (h) comprises a step of molecular bonding of the element to the assembly pad. 
         [0036]    An embodiment of the present invention also provides a support intended for the implementation of a method of self-assembly of at least one element on the support, comprising:
       at least one assembly pad, a liquid drop having a static contact angle on the assembly pad smaller than or equal to 15°; and   a drop of at least one fluorinated material around the assembly pad, the liquid drop having a static contact angle on the layer greater than or equal to 100°.       
 
         [0039]    According to an embodiment, the liquid drop has a static contact angle on the assembly pad smaller than or equal to 10° and has a static contact angle on the layer greater than or equal to 110°. 
         [0040]    According to an embodiment, the material is a fluorocarbon material. 
         [0041]    According to an embodiment, the support further comprises a protrusion comprising sides and a top, the assembly pad being on the top and the layer covering the sides. 
         [0042]    According to an embodiment, the sides are inclined by more than 45° with respect to the top. 
         [0043]    According to an embodiment, the support further comprises a substrate, the assembly pad and the layer being formed on the substrate, the substrate being made of a semiconductor material, the assembly pad comprising an oxide of the semiconductor material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
           [0045]      FIGS. 1A to 1C , previously described, are cross-section views of structures obtained at successive steps of an example of a self-assembly method; 
           [0046]      FIGS. 2A to 2C , previously described, are respective top views of  FIGS. 1A to 1C ; 
           [0047]      FIGS. 3A to 3E  are partial simplified cross-section views of the structures obtained at successive steps of a first embodiment of a method for manufacturing according to the present invention a pad intended for the implementation of a self-assembly method; 
           [0048]      FIG. 4  is a partial simplified cross-section view of a liquid drop resting on a protrusion illustrating the anchoring of the drop to the protrusion; and 
           [0049]      FIGS. 5A to 5G  are partial simplified cross-section views of the structures obtained at successive steps of a second embodiment of a method for manufacturing according to the present invention a pad intended for the implementation of a self-assembly method. 
       
    
    
       [0050]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
       DETAILED DESCRIPTION 
       [0051]    According to the present invention, the pads intended for the implementation of a self-assembly method are made of a material of high wettability, for which the static contact angle of a liquid drop on the material is smaller than or equal to 15°, preferably smaller than or equal to 10°, and preferably still smaller than or equal to 5°. Each pad is surrounded with a region of a material of low wettability, for which the static contact angle is greater than or equal to 100°, preferably greater than or equal to 110°, and preferably still greater than or equal to 115°. 
         [0052]    According to the present invention, the pads of high wettability are formed after the regions of low wettability surrounding the pads. The method for manufacturing regions of low wettability thus causes no decrease in the wettability of the pads of high wettability. The high wettability of the pads is obtained by a processing under an ultraviolet radiation in the presence of ozone. For the method for manufacturing pads of high wettability not to increase the wettability of the regions of low wettability which have already been formed, the regions of low wettability are made by the forming of a layer of a fluorinated material, preferably a fluorocarbon material. Indeed, the present inventors have shown that the layer of the fluorinated material, preferably a fluorocarbon material, keeps a low wettability even after having been applied the ultraviolet radiation processing in the presence of ozone while such a processing generally increases the wettability of the surfaces to which it is applied. 
         [0053]      FIGS. 3A to 3E  show the structures obtained at successive steps of a first embodiment of a method according to the present invention for manufacturing pads on a support intended for the implementation of a self-assembly method. A single pad is shown in  FIGS. 3A to 3E . It should be clear that the first embodiment of the manufacturing method according to the present invention can be implemented to simultaneously form several pads. 
         [0054]      FIG. 3A  shows a support  19  comprising a substrate  20  covered with an insulating layer  22 . Substrate  20  may be a substrate of a semiconductor material, for example, silicon. Insulating layer  22  may then be a silicon oxide layer. The thickness of silicon oxide layer  22  may range between 5 nm and 200 nm, for example being on the order of 50 nm. The free surface of layer  22  is designated with reference numeral  24 . 
         [0055]      FIG. 3B  shows the structure obtained after the deposition of a portion  26  of a resist on surface  24 . As an example, the thickness of resist portion  26  may range between 500 nm and 3 μm, for example being on the order of 2.5 μm. Silicon oxide layer  22  especially eases the bonding of the resin. The deposition of resin portion  26  may be performed by conventional photo-lithography steps comprising:
       depositing a resin layer all over surface  24 ;   exposing the resin layer to a radiation through a mask to reproduce in the resist layer patterns of the mask defining the contour of portion  26 ; and   dissolving a portion of the resin layer to delimit portion  26 .       
 
         [0059]    The resin may be a “positive” resist. The resist portion exposed to the radiation then becomes soluble in a specific aqueous or organic solution, called development solution, and the resist portion which has not been exposed remains non-soluble in the development solution. The resin may be a “negative” resist. The resist portion exposed to the radiation then becomes non-soluble in the development solution, and the resist portion which has not been exposed remains soluble in the development solution. 
         [0060]    Examples of resin comprise the following compounds:
       phenol formaldehyde, for example, a mixture of diazonaphtoquinone (or DNQ) and of a novolak resin (phenol formaldehyde resin);   polyhydroxystyrene;   poly(methyl methacrylate) or PMMA;   poly(methyl glutarimide) or PMGI; and   epoxy-based polymer (for example, resin sold under trade name SU-8 by Micochem company).       
 
         [0066]      FIG. 3C  shows the structure obtained after the deposition of a layer  28  of low wettability over the entire structure shown in  FIG. 3B , that is, over resin portion  26  and the portions of silicon oxide layer  22  which are not covered with resin portion  26 . Advantageously, the deposition is conformal. The thickness of layer  28  for example ranges between 10 nm and 300 nm. Layer  28  is based on fluorinated compounds. Preferably, layer  28  is based on fluorocarbon compounds. As an example, layer  28  is based on fluorocarbon compounds of C x F y  type, where x and y are real numbers, with x ranging from 1 to 5 and y ranging from 1 to 8. 
         [0067]    The deposition of layer  28  may be performed by placing the structure shown in  FIG. 3B  in a passivation plasma formed of octafluorobutene (C 4 F 8 ), hexafluoroethane (C 2 F 6 ), carbon tetrafluoride (CF 4 ), and/or trifluoromethane (CHF 3 ). The time of exposure of the structure shown in  FIG. 3B  to the plasma may range from 3 to 30 seconds. 
         [0068]    As an example, C 4 F 8 , C 2 F 6 , CF 4 , and/or CHF 3  gases may be introduced into a reactor chamber. A plasma is then formed at ambient temperature, for example, at approximately 20° C., by application of an adapted radio frequency current to the chamber. The pressure in the reactor chamber for example ranges from 10 to 100 Pa. The plasma based on C 4 F 8 , C 2 F 6 , CF 4 , and/or CHF 3  gas may be formed in an inductively-coupled plasma or ICP reactor with a radio frequency generator. 
         [0069]    The obtained layer has a low wettability. The static contact angle is greater than or equal to 110°, preferably greater than or equal to 115°. 
         [0070]    The wettability angle may be measured by means of the measurement device sold by GBX under trade name Digidrop—MCAT. The measurement comprises depositing a drop of demineralized water, from 2 to 10 μl, on a surface of the material to be examined, acquiring an image of the drop by means of an image acquisition device, and determining the contact angle by computer analysis of the acquired image. 
         [0071]      FIG. 3D  shows the structure obtained after removal of resin portion  26  to expose the portion  30  of silicon oxide layer  22  which was covered with resin portion  26 . The removal step may be carried out by placing the structure obtained at step  3 C in a bath of a solvent capable of dissolving the resin. The processing time in the bath for example ranges from 40 seconds to 1 minute. The solvent for example is acetone or N-methyl-2-pyrrolidone. Advantageously, ultrasounds are applied to support  19  while it is dipped in the bath. As an example, the wavelength of the ultrasounds ranges between 45 kHz and 47 kHz. 
         [0072]      FIG. 3E  shows the structure obtained after a processing causing an increase in the wettability of the exposed portion  30  to form a pad  32  of high wettability intended for the implementation of a self-assembly method. The processing comprises exposing the entire structure shown in  FIG. 3C  to an ultraviolet radiation in the presence of ozone. The ultraviolet radiation comprises a first ultraviolet radiation at a first wavelength ranging between 182 nm and 187 nm, for example, on the order of 184.9 nm, and a second ultraviolet radiation at a second wavelength ranging between 250 nm and 255 nm, for example, on the order of 253.7 nm. The first radiation especially enables to alter the hydrocarbon compounds present on portion  30 . The second radiation especially enables to form active radicals from ozone, such active radicals reacting with the hydrocarbon compounds to form volatile compounds which are evacuated. The processing may be implemented at a temperature on the order of 80° C. in an enclosure initially containing ozone. This UV processing may be performed at the atmospheric pressure. 
         [0073]    The contact angle of a liquid drop, for example, demineralized water, on pad  32  is smaller than or equal to 15°, preferably smaller than or equal to 10°, and preferably still smaller than or equal to 5°, while the contact angle of a liquid drop, for example, demineralized water, on portion  30  before the ultraviolet radiation processing in the presence of ozone ranges between 20° and 30°. The wettability difference between portion  32  and adjacent layer  28  is thus greater than or equal to 85°, preferably greater than or equal to 90°, and preferably still greater than or equal to 100°, which provides a proper confinement of a liquid drop, especially demineralized water, on pad  32 . 
         [0074]    According to a second embodiment of the method for manufacturing the self-assembly pad, the pad is formed at the top of a protrusion having its sides forming a sharp edge with the pad. This enables to benefit, in addition to the wettability difference between the pad and the region surrounding the pad, from an effect of anchoring of the drop to the top of the protrusion to maintain the drop confined on the pad. 
         [0075]      FIG. 4  illustrates the anchoring effect and shows a protrusion  34  comprising a planar horizontal top  36  and an inclined edge  38 . In the cross-section plane of  FIG. 4 , top  36  and side  38  join at point Q. Side  38  is inclined by an angle α with respect to the horizontal direction. Angle α is equal to 0° in the absence of inclination.  FIG. 4  shows the liquid-air interface  40  of a drop  42  when angle α is equal to 0°. At rest, the angle between top  36  and the tangent to interface  40  at point Q is then equal to static contact angle θ. In the absence of inclined edge  38 , if liquid is added to drop  42 , interface  40  tends to move and to cross point Q, the contact angle remaining constant. In the presence of inclined edge  38 , angle δ between top  36  and the tangent to interface  43 , for which drop  42  crosses point Q, is equal to the sum of angles θ and α. All occurs as if drop  42  remained bonded to top  36  of protrusion  34 . 
         [0076]      FIGS. 5A to 5G  illustrate the structures obtained at successive steps of the second embodiment of a method for manufacturing according to the present invention pads of a support  43  intended for the implementation of a self-assembly method. A single pad is shown in  FIGS. 5A to 5G . It should be clear that the second embodiment of the manufacturing method according to the present invention may be implemented to simultaneously form several pads. 
         [0077]      FIGS. 5A and 5B  are identical to previously-described  FIGS. 3A and 3B . 
         [0078]      FIG. 5C  shows the structure obtained after a step of etching of silicon oxide layer  22  around resin portion  26 . A portion  44  of silicon oxide layer  22  then remains under resin portion  26 . The etching may be a reactive ion etching or RIE. The etch gas may be hydrofluoric acid (HF) or ammonium fluoride (NH 4 F). Silicon substrate  20  may behave as an etch stop layer. 
         [0079]      FIG. 5D  shows the structure obtained after a step of etching of substrate  20  to form a protrusion  46  which prolongs resin portion  26  and silicon oxide portion  44 . The height of protrusion  46  may range from 500 nm to 80 μm. Protrusion  46  comprises inclined sides  47  substantially perpendicular to the upper surface of portion  44 . As a variation, sides  47  may be inclined with respect to the support surface of portion  44  by an angle α (see  FIG. 4 ) ranging between 0° and 90°, preferably between 45° and 90°. 
         [0080]    The etching is an anisotropic etching, resin portion  26  behaving as an etch mask. It may be a deep reactive ion etching or DRIE. The etch gas may be sulfur hexafluoride (SF 6 ). In this case, the height of protrusion  46  is preferably smaller than a few micrometers. The etching may be performed according to the Bosch method. In this case, the height of protrusion  46  is preferably greater than a few micrometers. 
         [0081]    The Bosch method comprises exposing support  43  to an alternation of an etch plasma and of a passivation plasma at ambient temperature. The etch plasma is formed from sulfur hexafluoride (SF 6 ). Fluorinated radicals form and react with the silicon to create volatile species, especially tetrafluorosilane (SiF 4 ). The passivation plasma is formed from octafluorobutene (C 4 F 8 ), hexafluoroethane (C 2 F 6 ), carbon tetrafluoride (CF 4 ), and/or trifluoromethane (CHF 3 ). The etch plasma isotropically etches the silicon, with resin portion  26  behaving as an etch mask. After an exposure of support  43  to the etch plasma, support  43  is exposed to the passivation plasma, which causes the deposition of a protection layer on the sides of the etched area and on the bottom of the etched area by deposition of a layer of a fluorocarbon polymer. The thickness of the passivation deposition is conditioned by the parameters of the passivation plasma. The next etch step destroys the passivation layer at the bottom of the etched area with the ion bombarding. The silicon which is not protected by the passivation layer is then etched in this same step by the fluorinated radicals coming from the etch plasma. By increasing the number of alternations of the etch and passivation steps, a deep anisotropic etching can then be obtained. 
         [0082]    As an example, gases SF 6 , C 4 F 8 , C 2 F 6 , CF 4 , and/or CHF 3  may be introduced into a reactor chamber. A plasma is then formed at ambient temperature, for example, at approximately 20° C., by application of an adapted radio frequency current to the chamber. The pressure in the reactor chamber for example ranges from 10 to 100 Pa. The partial pressure of SF 6  etch gas is, for example, on the order of 20 mTorr (2.67 Pa). The etch plasma based on SF 6  gas and the passivation plasma based on C 4 F 8 , C 2 F 6 , CF 4 , and/or CHF 3  gases may be formed in an inductive coupling plasma reactor. 
         [0083]      FIG. 5E  shows the structure obtained after the deposition of a layer  48  of low wettability over the entire structure shown in  FIG. 5D . In particular, layer  48  covers sides  47  of protrusion  46 . The deposition of layer  48  may be performed as described previously in relation with  FIG. 3C  for the deposition of layer  28 . The thickness of layer  48  for example ranges between 10 nm and 300 nm. Layer  48  is based on fluorinated compounds. Preferably, layer  48  is based on fluorocarbon compounds. As an example, layer  48  is based on fluorocarbon compounds of C x F y  type, where x and y are real numbers, with x ranging from 1 to 5 and y ranging from 1 to 8. 
         [0084]      FIG. 5F  shows the structure obtained after a step of removal of resin portion  26  to expose portion  44 . This may be done as previously described in relation with  FIG. 3D  by placing the structure shown at step  5 E in a bath of a solvent and by applying ultrasounds to support  43  during its dipping in the bath. 
         [0085]      FIG. 5G  shows the structure obtained after a processing causing an increase in the wettability of the exposed portion  44  to form pad  50  intended for the implementation of a self-assembly method. This may be done as previously described in relation with  FIG. 3E  by exposing the entire structure shown in  FIG. 5F  to an ultraviolet radiation in the presence of ozone. 
         [0086]    The angle of contact of a drop of a liquid, for example, demineralized water, on pad  50  is smaller than or equal to 15°, preferably smaller than or equal to 10°, and preferably still smaller than or equal to 5°. Further, the angle between sides  47  covered with layer  48  of low wettability and pad  50  enables to benefit from the previously-described anchoring effect. The confinement of a liquid drop on pad  50  is thus improved. 
         [0087]    The manufacturing method according to the present invention is particularly suitable in the case where the bonding of the integrated circuit to support  19 ,  43 , which is performed after the self-assembly, is performed by molecular bonding. Indeed, for this type of bonding, surfaces with an adapted roughness must be provided. The previously-described manufacturing method examples provide pads  32 ,  50  having an RMS roughness for example ranging between 0.2 nm and 0.5 nm (measured on a 1-μm 2  measurement surface area), that is, adapted to a molecular bonding method. Further, since portion  50  has a high wettability, it is adapted to the performing of a hydrophilic molecular bonding. 
         [0088]    Specific embodiments of the present invention have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, in the previously-described embodiments, substrate  20  is made of a semiconductor material, for example, silicon. As a variation, substrate  20  may be a glass substrate or a metal substrate. In this case, there may be no insulating layer  22 . Substrate  20  may be a composite substrate, comprising at its surface elements of a semiconductor material, elements of a metallic material, and/or elements of an insulating material. In this case, insulating layer  22  may only cover the semiconductor material elements.