Method of flip-chip assembly of two electronic components by UV annealing, and assembly obtained

Abstract

The invention concerns a method of flip-chip assembly between first (1) and second (2) components each comprising connection pads (11, 21) on one of the faces of same, referred to as assembly faces, which involves transferring the components onto each other via the assembly faces of same in such a way as to create electrical interconnections between the pads of the first and second components. The invention involves transforming the copper oxide into copper by UV annealing, very locally, in the gap between the components, at least around the areas adjacent to the connection pads. The method according to the invention can be used for any component that is transparent to UV rays, including for substrates made from a plastic material such as substrates made from PEN or PET. The invention also concerns the assembly of two components obtained by the method.

Claims

1. A process for flip-chip assembly of a first component and a second component, each component including connection pads on one of their faces, called their assembly faces, in which the components are added one to the other via their assembly faces so as to produce electrical interconnections between the pads of the first and those of the second component, the process including the following steps: a/ producing the first component with its connection pads, the first component being made of a material that is transparent to ultraviolet (UV) radiation in zones, called emission zones, each of said emission zones being defined around a pad, the pads of the first component being made of a material that absorbs or reflects UV radiation; b/ depositing copper oxide (CuO) on the first component at least on its connection pads and in the emission zones; c/ producing the second component with its connection pads, the connection pads of the second component being made of a material that reflects UV radiation, each of said pads being suitable for reflectively focusing ultraviolet radiation arriving from an emission zone onto one of the pads of the first component, and, depending on the circumstances, onto each metal protuberance formed on a pad of the second component; d/ forming interconnection metal protuberances on at least some of the pads of the second component; e/ aligning the first and second components and adding them one to the other via their assembly faces; and f/ applying UV radiation through the emission zones of the first component to the deposited copper oxide so as to carry out a UV anneal that converts the deposited copper oxide into copper and, depending on the circumstances, makes the interconnection protuberances melt.

2. The assembly process as claimed in claim 1, in which the first component includes a substrate made of a material that is transparent to UV.

3. The assembly process as claimed in claim 2, wherein the substrate made of transparent material is surmounted, on the same side as its assembly face, with an electrically insulating layer made of a material that absorbs or reflects UV radiation, said layer defining the emission zones.

4. The assembly process as claimed in claim 3, wherein the constituent material of the layer defining the emission zones is chosen from a titanium oxide, such as TiO.sub.2, zinc oxide (ZnO), zirconium oxide (ZrO.sub.2) and molecules of pyrene dissolved in acetone.

5. The assembly process as claimed in claim 2, wherein the substrate made of transparent material is surmounted, on the same side as its assembly face, with a layer made of a material that absorbs or reflects UV radiation and electrically insulated from the emission zones and connection pads.

6. The assembly process as claimed in claim 5, wherein the constituent material of the layer electrically insulated from the emission zones and connection pads is a metal such as gold (Au), titanium (Ti) or nickel (Ni).

7. The assembly process as claimed in claim 1, wherein the first component is a substrate made of transparent material and in which the emission zones are defined by drops of a solvent that absorbs UV radiation added to copper oxide (CuO).

8. The assembly process as claimed in claim 7, wherein the solvent is chosen from water, acetonitrile, pentane, n-hexane, cyclohexane and cyclopentane.

9. The assembly process as claimed in claim 1, wherein step b/ is carried out by depositing CuO in the form of an ink in order to produce a film covering the assembly face of the first component.

10. The process as claimed in claim 9, wherein, between steps e/ and f/, a step e1/ of thermally annealing the copper oxide (CuO) ink is carried out between 90 and 100 C. for a time of 10 min to 30 min so as to thermally activate the copper oxide.

11. The assembly process as claimed in claim 1, wherein the UV radiation applied in step f/ consists in a photonic pulse of a duration comprised between 0.5 and 2 milliseconds (ms) having an energy comprised between 10 and 20 joules/cm.sup.2.

12. An assembly of a first component and a second component, each component including connection pads on one of their faces, called their assembly faces, in which: the first component is made of a material that is transparent to ultraviolet radiation (UV) at least in zones, called emission zones, and includes connection pads made of a material that absorbs or reflects UV radiation, said pads each being bounded by emission zones; and the second component includes connection pads made of a material that reflects UV radiation, the assembly including: interconnection zones made of copper (Cu) each filling the space at least between the emission zones of the first component and a pad of the second component; and depending on the circumstances, metal protuberances each reflowed in an interconnection zone between a pad of the first component and a pad of the second component.

13. The assembly as claimed in claim 12, the first component including a substrate made of a material that is transparent to UV.

14. The assembly as claimed in claim 13, the substrate being made of polyethylene naphthalate (PEN), polyethylene terephthalate (PET) or of glass.

15. The assembly as claimed in claim 12, the connection pads of the second component being made of a material that reflects UV radiation and having a recessed, Isosceles-triangle shape relative to the assembly face.

16. The assembly as claimed in claim 15, the angle at the base, angle formed between the two sides of the Isosceles triangle and the assembly face of the second component, being smaller than 40.

17. The assembly as claimed in claim 12, the connection pads of the second component being made of a material that reflects UV radiation and having a concave shape relative to the assembly face, the concave shape consisting of a parabolic or cylinder segment.

18. The assembly as claimed in claim 12, the connection pads of the first component being made of a material that reflects UV radiation.

19. The assembly as claimed in claim 12, the connection pads of the second component being produced in the form of a metal layer the metal of which is chosen from aluminum (Al), gold (Au), titanium (Ti), nickel (Ni) and platinum (Pt), or of a metal bilayer chosen from nickel surmounted with gold (Ni/Au) and nickel surmounted with platinum (Ni/Pt).

20. The assembly as claimed in claim 12, the constituent material of the interconnection protuberances being chosen from indium (In), an aluminum-copper alloy (AlCu), tin (Sn) and a gold-tin alloy (AuSn).

21. The assembly as claimed in claim 12, the second component being a chip, in particular a silicon chip, and the first component being a substrate made of transparent material surmounted, on the same side as its assembly face, with a layer that absorbs UV radiation, defining the emission zones.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and features of the invention will become more clearly apparent on reading the detailed description of the invention, which is given by way of nonlimiting illustration, with reference to the following figures, in which:

(2) FIG. 1 is a schematic cross-sectional view illustrating an assembly of two components after they have been assembled according to the invention;

(3) FIG. 1A is a detail view of FIG. 1;

(4) FIGS. 2 and 3 are schematic cross-sectional views illustrating the optical paths of rays resulting from the reflection by reflection of UV radiation in a step of the process according to the invention, the step being implemented with connection pads made of reflective material in the shape of an Isosceles triangle and of a cylinder segment, respectively;

(5) FIGS. 4A to 4F are schematic cross-sectional views illustrating various foci for UV radiation reflected in a step of the process according to the invention, the step being implemented with connection pads made of reflective material in the shape of an Isosceles triangle having various angles at the base;

(6) FIGS. 5A and 5B are schematic cross-sectional views illustrating various foci for UV radiation reflected in a step of the process according to the invention, the step being implemented with connection pads made of reflective material in the shape of a cylindrical segment; and

(7) FIGS. 6A to 6G are schematic cross-sectional and detail views illustrating the various steps of a process according to the invention.

DETAILED DESCRIPTION

(8) FIGS. 1 and 1A show an assembly of two electronic components 1, 2, such as a component 1 with a flexible substrate 10 made of PEN or PET and an electronic chip 2 made of silicon, before it has been subjected to UV radiation according to the invention.

(9) The component 1, which is the flipped component, includes a substrate 10 that is transparent to UV radiation, on the assembly face 12 of which connection pads 11 of equal height have been produced. By way of example, the width W of a connection pad 11 is about 40 m.

(10) Between two connection pads 11, a layer 13 made of an electrically insulating material and that absorbs UV radiation is deposited. This layer 13 defines, around each connection pad 11, an emission zone 14 able to let UV radiation pass.

(11) The component 2, which is the flipped component, includes, for its part, a substrate 20 made of silicon, on the assembly face 12 of which connection pads 21 of equal height have been produced. The connection pads 21 are made of a material that reflects UV. The geometric shape of each connection pad is an Isosceles triangle with an angle at the base preferably smaller than 40, as will be explained below. By way of example, the width L of the base of the triangle of a pad 21 is about equal to 60 m and the height H of the triangle of a pad 21 is equal to 10 m.

(12) Each of the connection pads 21 is surmounted by a metal connection protuberance 4 in the form of a bump made of meltable material. Typically, it is a question of an indium bump 4. Between two connection pads 21 is deposited a layer 23 made of an electrically insulating material and that absorbs UV radiation.

(13) The interconnection pitch between two consecutive pads 21 is at least equal to 40 m.

(14) As may be best seen in FIG. 1A, before the components were added one to the other a continuous layer 3 of copper oxide CuO was deposited on the connection pads 11 and on the layer 13 made of an electrically insulating material and that absorbs UV.

(15) According to the invention, to finalize the assembly once the flipped component 2 has been added to the other component 1, UV radiation is applied through the emission zones 14 of the first component 1 to the deposited copper oxide 3 so as to implement a UV anneal converting the deposited copper oxide into copper, and to make the interconnection protuberances 4 melt. The UV radiation that passes through the emission zones 14 is reflected from each pad 21, the shape of which is adapted to focus as best as possible the UV radiation onto the protuberance 4 and onto the connection pad 11.

(16) Thus, in the interconnection zones 3, 4, localized conversion of the CuO into Cu is obtained while the protuberances 4 are simultaneously melted.

(17) Therefore, in the end, an assembly having a good electrical contact made of copper is obtained while at the same time a good mechanical and electrical contact is obtained via the molten material of the protuberances 4. A good dissipation of heat due to heating of the component 2 in operation is also obtained by way of the layer 3 of CuO.

(18) It is possible to vary the shape of the connection pads 21 in order to obtain a good focus on a connection protuberance 4, in order to make it melt, and on the subjacent connection pad 11. Thus, it may advantageously be a question of an Isosceles-triangle shape (FIG. 2) or a cylinder-segment shape (FIG. 3).

(19) The impact of the concave shape of the connection pads 21, of the angle at the base made between the two sides of the Isosceles triangle and the assembly face 22 of the second component 2 when the pads 21 are triangle-shaped, and lastly of the material that reflects and absorbs UV, on their focus of UV radiation onto the connection bumps 4, and therefore on whether or not it is possible to make the latter melt, has been studied.

(20) This study was carried out using the commercial ray-tracing software package Trace pro 6.0. This software package was developed by Lambda Research Corp.

(21) Firstly, simulations were carried out with pads 21 having an Isosceles-triangle shape, then their angle at the base , at the base formed between the two sides of the Isosceles triangle and the assembly face 22 of the second component 2, was varied.

(22) These simulations are shown in FIGS. 4A to 4F, respectively.

(23) Table 1 below indicates the conditions corresponding to each of these figures and results qualifying and quantifying the focus, if one were obtained, of the UV radiation on the interconnection zone 4 in question, i.e. the zones simulating the location of an indium bump 4.

(24) TABLE-US-00001 TABLE 1 Value of the angle FIG. Results 4A 45 No reflected UV rays reach the zone 4 in question 4B 40 No rays reflected onto the pad 11. In contrast, good focus on the zone 4 in question. However, many rays transmitted into the layer 3 of CuO after reflection from the reflecting pad 21 4C 30 48% of the UV flux reaches the zone 4 in question. A few rays are transmitted into the layer 3 of CuO but the effect is marginal (reflection from the corner of the structure) 4D 5 60% of the flux reaches the zone 4 in question provided that the pad 11 also reflects UV 4E 10 63% of the flux reaches the pad 11 and the zone 4 in question, with a pad 11 that reflects UV 4F 10 10% of the flux reaches the pad 11, with a pad 11 that absorbs UV. However, the rays do not make it to the center of the zone 4 in question

(25) Thus, it is clear from table 1 that the angle at the base of the Isosceles triangle formed by the pad 21 must be smaller than 40 and that the pad 11 must preferably be made of a material that reflects UV.

(26) The same type of simulations were carried out but this time with pads 21 having a cylinder-segment shape.

(27) These simulations are shown in FIGS. 5A and 5B, respectively.

(28) Table 2 below indicates the conditions corresponding to each of these figures and results qualifying and quantifying the focus, if one were obtained, of the UV radiation on the interconnection zone 4 in question, i.e. in the zones simulating the location of an indium bump 4.

(29) It will be noted that the dimensions of the cylinder-segment shape 21 are the same in FIGS. 5A and 5B, namely a radius of curvature R equal to 50 m and a height equal to 10 m.

(30) TABLE-US-00002 TABLE 2 Material of FIG. the pad 11 Results 5A reflects UV 62% of the flux reaches the pad 11 5B absorbs UV 22% of the flux reaches the pad 11 and furthermore only its center

(31) Thus, it is clear from table 2 that with a cylinder-segment-shaped pad 21 the pad 11 must preferably be made of a material that reflects UV radiation.

(32) The various assembly steps according to the first alternative of the invention are now described with reference to FIGS. 6A to 6G. It will be noted that in these figures only those portions of the components 1, 2 in which a mechanical and electrical interconnection is obtained in the assembly process have been shown.

(33) It goes without saying that it is possible to produce a large number, typically equal to about ten thousand, of interconnections between the two components, by implementing the assembly process according to the invention.

(34) Step a/: the first component 1 is produced with its connection pads 11 on its assembly face 12, from a flexible substrate 10 made of PEN or of PET.

(35) To do this, firstly, on the assembly face 12 of the substrate 10, a metal layer that reflects ultraviolet radiation and that is from 30 to 300 nm in thickness is deposited using a physical vapor deposition (PVD) technique.

(36) The constituent material of the layer may be aluminum (Al) aluminum (Al). It may also be a question of gold (Au), titanium (Ti), nickel (Ni) or platinum (Pt).

(37) Once the deposition has been carried out, the layer is etched so as to form the connection pads 11 (FIG. 6A).

(38) The etching may be wet etching or plasma etching or laser etching.

(39) Next, again on the assembly face 12, a layer 13 of an electrically insulating material that absorbs or reflects UV radiation is deposited, said layer defining, around the connection pads, emission zones 14 through which UV radiation will subsequently be emitted to finalize the assembly (FIG. 6B).

(40) The layer 13 may be the same thickness as the connection pads 11. The layer 13 may be made from a titanium oxide, such as TiO.sub.2. It may also be a question of a layer 13 made of zinc oxide (ZnO), zirconium oxide (ZrO.sub.2), or in the form of pyrene molecules dissolved in acetone.

(41) Step b/: next, copper oxide CuO in ink form is deposited in order to form a film 3 covering the assembly face 12 of the first component 1 (FIG. 6C).

(42) The CuO ink may be deposited by screen printing. By way of example, it may be a question of the ink sold under the denomination Metalon ICI-020 or even Metalon ICI-021 by Novacentrix.

(43) The thickness of the deposited CuO ink may advantageously be comprised between 2 m and 10 m.

(44) Step c/: the second component 2 is produced with its connection pads 21 on its assembly face 22, from a silicon chip 20.

(45) Firstly, Isosceles-triangle shapes are formed in the silicon chip 20. These shapes are produced either by isotropic etching, wet etching, i.e. wet etching in potassium hydroxide (KOH), for example, or laser etching, for example with a femtosecond laser.

(46) Next, a layer made of a material that reflects UV radiation is deposited by PVD and subsequently etched to form the connection pads 21, which closely follow the Isosceles-triangle shapes.

(47) The thickness of the deposited layer 21 may advantageously be comprised between 30 nm and 2 m.

(48) Next, once the connection pads 21 have been formed, a passivation layer 23 is deposited. It may be a question of a conventional passivation using a layer of silicon oxide (SiO.sub.2) obtained by thermal oxidation or deposited by PECVD or a layer of silicon nitride (Si.sub.3N.sub.4).

(49) Step d/: an indium bump 4 is deposited on each connection pad 21 made of a material that reflects UV (FIG. 6D).

(50) The deposition of the indium bumps 4 may be carried out using one of two known methods, namely dispensing or liftoff followed by evaporation.

(51) The unitary volume of an indium bump 4 may vary between 520 m.sup.3 and 34000 m.sup.3, this corresponding to diameters varying between 10 m and 40 m.

(52) Step e/: the first component 1 is flipped, then the two components 1, 2 are brought close together and aligned with each other so that each connection pad 11, 21 faces the other with a connection bump 4 in between (FIG. 6E).

(53) Lastly, the first component 1 is added to the second component 2 so that each connection bump 4 makes contact with a connection pad of the first component 1 (FIG. 6F).

(54) Step e1/: the copper oxide (CuO) ink is thermally annealed at between 90 and 100 C. for between 10 min and 30 min so as to thermally activate the copper oxide.

(55) Step f: next, ultraviolet radiation is briefly applied, via a photonic pulse in the UV wavelength range, through the transparent substrate 10 and the emission zones 14 (FIG. 6G).

(56) The UV wavelength of the photonic pulse may advantageously be comprised between 200 and 400 nanometers.

(57) The UV photonic pulse has a duration comprised between 0.5 and 2 milliseconds (ms) and an energy comprised between 10 and 20 joules/cm.sup.2. Preferably, the duration is 1.5 ms for an energy of 14 joules/cm.sup.2. With such energy values, an angle of about 10 and pads 11 made of aluminum, not only is the CuO reliably converted into Cu from the back face 15 (face opposite the assembly face 12) of a flexible substrate made of PEN or of PET of thickness of about 125 m, but the indium bumps 4 are also reliably melted.

(58) In particular, the UV radiation acts as a reducing agent on the deposited CuO layer 3 having undergone a thermal anneal in step e1/, by generating hydrogen H.sub.2 that will deoxidize the copper oxide and convert it into copper with formation of a water molecule.

(59) By way of example, the UV photonic pulse may be produced by the xenon UV flash lamp sold under the denomination XENON PulseForge by Xenon Cooperation.

(60) The distance separating the UV flash lamp from the substrate 10 may be comprised between 2 and 7 cm. Advantageously, the distance between the lamp and the back face 15 of the substrate 10 is about 2.5 cm. Such a distance is advantageous because it corresponds to the length required to focus the photonic pulse by the optical system of the UV flash lamp and therefore to the length at which a maximum amount of energy is delivered.

(61) FIG. 6G shows an assembly of the two components 1, 2 obtained using the assembly process just described.

(62) The indium bumps 40, which however have a melting point above the glass transition temperature of the substrate 10, have been completely reflowed via a reflective focus on the reflective metal layer of the pads 21, without adversely affecting the substrate 10.

(63) Simultaneously, the CuO ink 3 has been converted into Cu in the interconnection zones 30 defined between the reflective metal layer 21 and the emission zones 14.

(64) Thus, the interconnections obtained in the zones 30 are obtained with a good electrical contact made of copper and at the same time a good electrical and mechanical contact via the bumps 40 reflowed under the energy delivered by the UV flash(es).

(65) Furthermore, the assembly obtained allows any heat due to the heating of the silicon chip 1 in operation to be effectively removed by way of the continuous CuO layer 3. In particular, CuO has a thermal conductivity of 33 W/m.Math.K, much better than air, which has a conductivity of 0.02 W/m.Math.K.

(66) Provision may be made for other variants and improvements, without however departing from the scope of the invention.

(67) Thus, it may be envisioned to assemble, according to the invention, two printed circuit boards on transparent flexible substrates made of PEN or PET, or one component by way of detector on a transparent substrate and another component by way of read circuit, or a sensor and another sensor with one thereof on a transparent substrate.

(68) The invention is not limited to the examples just described; it is especially possible to combine features of the illustrated examples together in variants that have not been illustrated.