Patent Application
20090026249
The present invention relates to the field of microelectronics and, more especially, techniques for the hybridization of two different components using interconnection bumps which are also referred to as “solder bumps”. The invention therefore relates to the general field of microcomponents, more conventionally referred to as a chips, but it also has applications for microsensors, microactuators such as Micro-Electro-Mechanical Systems (MEMS) and optoelectronic components of the semiconductor Vertical Cavity Surface Emitting Laser (VCSEL) type, etc.
Within the meaning of the invention, the term “component” is understood to be an electronic sensor such as a chip, a mounting structure for an electric or electronic circuit or a passive electrical component of the integrated-circuit lid type or even a sensor for characterizing a physical quantity.
The microcomponents in question dealt with in the context of the present invention are conventionally deposited on a substrate of an appropriate nature, for example a semiconductor type substrate (monocrystalline silicon, sapphire, etc.) for electronic components.
These substrates have conductors, made of aluminum for example, that radiate from the microcomponent towards the periphery of the substrate in order to make it possible, above all, to process and analyze the signals that said component is expected to generate or even to control the functions that it incorporates, besides making it possible to provide the required electrical power supply for the component, if applicable.
One of the techniques which are widely used nowadays to assemble different components together is the hybridization technique using solder bumps which is referred to as “flip chip”. Briefly, this technology involves:
Thus, during the assembly process, the solder material that constitutes the bumps assumes a globular shape.
Soldering of indium, the material conventionally chosen to produce these bumps, on a wettable surface which itself is conventionally made of gold, requires the use of deoxidizing agents in liquid or gaseous form, the liquid flux also being referred to as “solder flux” in this case.
Generally speaking, these solder fluxes are inherently corrosive and after oxide reduction are also capable of attacking the actual solder metal.
Experience shows that this undesirable corrosion is made worse by two particular configurations:
The galvanic effect can be explained as follows in relation to
Component 1 comprises a host substrate 3, such as silicon, on which there are conductors 4, made of aluminum for example. Vertically above and below the hybridization areas there are, on substrate 3 and on the chip to be hybridized 2 respectively, wettable surfaces 7 and 8 which are also referred to as Under Bump Metallizations (UBM) and are typically made of gold.
At the same time, component 1 also comprises a plurality of external bonding pads 5. These pads actually consist of an electrically conductive surface which is capable of ensuring connections between the component to be hybridized and its external environment. Such a pad conventionally consists of a metallic surface onto which a wire, typically made of gold, may be thermocompressed.
During hybridization, after depositing the solder or braze material intended to constitute hybridization bump 6, deoxidizer 9 (
The gold layer of external bonding pad 5 is linked by an electrical connection consisting of aluminum conductor 4 to solder bump 6 so that, because of the presence of solder flux 9 which acts as an electrolyte, a closed electrical circuit is created, thereby causing an electrochemical phenomenon which results in reduction of the material which constitutes the hybridization bump in accordance with the well-known principle of a battery cell due to the different electrochemical potentials of the metals used. This being so, reduction of the material which constitutes the bump is observed and this results in it being dissolved, at least partially, in the solder flux and, consequently, impairment of the sought-after hybridization between chip 2 and component 1.
In order to overcome this drawback, suggestions were initially made to cover the external bonding pads with an insulating layer, for example a resin layer, in order to insulate the electrochemical circuit described above or to replace the gold of the external bonding pad by a metal having the same electrochemical potential as the material that constitutes the bump. However, both these solutions make the process of fabricating the host circuit, in this case the component, more complex, not only because of an additional stage which results in reduced fabrication yields, but also because of higher fabrication costs.
The use of cathodic protection has also been suggested:
In both cases, cathodic protection makes the fabrication process more complex and, in the latter case, makes the management of signal inputs and outputs more complicated.
It should, however, be emphasized that this technical problem which is inherent in the electrochemical consumption of the material that constitutes the hybridization bumps, is not actually critical if hybridization as such can be performed in a shorter time.
However, said hybridization operations tend to be systematically performed collectively in order to reduce fabrication costs. Thus, hybridization is performed using the collective reflow technique on hundreds or even thousands of components on a single host substrate. The corresponding hybridization sequence may then last several hours and, given this fact, the electrochemical corrosion or consumption phenomena described above can no longer be tolerated because they worsen over time.
The use of inert gas soldering has also been suggested. Using this method, inert gas can be used to ensure soldering. In this case, during the stage when the components are mounted on top of each other in order to ensure hybridization, the bumps and the metallic contacts are no longer electrically connected by an electrolyte. Known inert gases which are used include a N2+HCOOH or N2H2 mixture. However, these gases have somewhat high activation temperatures, in excess of 200° C. and 350° C. respectively. These temperature rises can be disadvantageous, the reader being reminded that, for indium, a thermal soldering cycle with a brief peak at 170-180° C. is sufficient.
The object sought after by the present invention is to overcome these various technical problems without thereby making the hybridization process more complex.
The invention relates to a method for soldering or hybridizing two components to each other by means of a solder material, at least one of said components, referred to hereinafter as the “first component”, comprising one or more metallic conductors linked to an equal number of external bonding pads.
This method involves:
According to the invention, this method involves defining wettability areas which are differentiated in terms of the liquid solder flux so as to insulate, at least electrically, the volumes of flux which are in contact with those areas of said first component which receive the solder material and those defined by the external bonding pad or pads.
In other words, the invention involves creating an electrical discontinuity at the level of the solder flux between a specific area where the solder material is positioned and the external bonding pads, in order to prevent the electrochemical reaction which is inherent in the different redox potentials of the metals used.
According to one particular embodiment of the invention, an area which is not wettable by the solder flux is defined on said first component, for example by depositing a thin anti-wetting coating. This coating consists of SiOC or C4F8 for example.
This area which is slightly wettable or not wettable by the flux can be located vertically above the external pads or between those areas which receive the solder material and said external pads.
The way in which the invention can be implemented and its resulting advantages will be made more readily understandable by the descriptions of the following embodiments, given merely by way of example, reference being made to the accompanying drawings.
As already stated,
The hybridization method according to the invention is described more especially in relation to
These show the hybridization of a chip 2 on an electronic component 1. As in the description of
Such external bonding pads require the use of a localized layer which encourages electrical conduction and, in this case, consists of a gold wire.
The hybridization bumps which ensure connection and interfacing and are also referred to as “useful bumps” are typically made by depositing a quantity of solder material 6, especially indium, on a wettability area 7, which is also made of gold and positioned so that it is in contact with conductors 4 at the chosen hybridization location. In a known manner, chip 2 also comprises such a wettability area which is not represented in relation to
The substrate complete with conductors can also be coated with a passivation layer made of SiO2 or Si3N4.
According to a first aspect of the invention, a coating 8 which is not wettable by the solder flux used during the hybridization step is deposited on external bonding pad 5. This coating is thin, typically 10 to 500 nm, and consists, for example, of SiOC or C4F8.
This being so, when solder flux 9 is deposited, it does not cover non-wettable area 8 and consequently does not come into contact with external bonding pad 5.
In order to make sure that said solder flux 9 does not come into contact with external bonding pad 5, the coating which is not wettable by solder flux 8 has a surface area which is greater than that of external bonding pad 5.
This being so, there can no longer be a closed electric circuit consisting of external bonding pads 5, metallic conductors 4, hybridization bumps 6 and solder flux 9. Consequently, there can no longer be any electrochemical reaction whatsoever between these various elements where solder flux 9 would be capable of acting as an electrolyte which is associated with corrosion phenomena, with respect to the indium which constitutes said bumps, 6 as demonstrated in the description of the prior art.
It is not imperative to protect external bonding pad 5 by this coating which is slightly wettable or not wettable 8. The feature which is absolutely imperative is the absence of any electrical contact between the external bonding pad and the solder flux which is in contact with the hybridization bumps.
Thus, in a version of the invention shown in relation to
The attraction of the method according to the invention is readily apparent.
Firstly, it eliminates any risk of corrosion of hybridization bumps 6 by eliminating the above-mentioned electrochemical reaction and does so regardless of the time that it takes to hybridize the components. Moreover, this method makes it possible to use a liquid solder or hybridization flux and, consequently, use a low hybridization temperature which is necessary if indium is used as a solder material.
What is more, it is also apparent that this hybridization method does not introduce any particular complications compared with methods according to the prior art and amounts to depositing a thin coating which is slightly wettable or not wettable by the solder flux.
This coating can be deposited, for example, by Plasma Enhanced Chemical Vapor Deposition (PECVD) by using a precursor, in this case Octamethylcyclotetrasiloxane (OMCTS) and a carrier gas such as argon or hydrogen.
Depending on the plasma power, pressure, initial precursor concentration and the plasma deposition time, one can control the thickness of the deposited layer and its SiOx1Cy1Hz1 stoichiometry. The formulation of material SiOx1Cy1Hz1 is such that 0.3≦x1≦0.4 and 0.15≦y1≦0.2 and 0.4≦z1≦0.55.
Obviously, this deposition method and the corresponding treatments can be extended to other types of siloxanes having a similar formulation such as, for example, tetraethylcyclotetrasiloxane (TNCTS).
This material can also be treated and surface converted into a material having the general formulation SiOx2Cy2Hz2 where x2>>x1, y2<<y1 and z2<<z1, or more simply SiOx, where x tends towards 2 in order, in particular, to increase its surface energy value, in particular its polar surface energy, and produce extremely good wettability by liquids (a contact angle of the order of 5° for water) and a very large increase in adhesion and cellular development (a generation time of 40 hours for unprocessed OMCTS compared with 20 hours for treated OMCTS).
The treatments used are either of the plasma type, namely a helium plasma or 02, SF6+O2, CHF3+O2 . . . ) oxygen plasmas or UV type plasmas in ambient air or in an oxygen enriched atmosphere.
This method therefore offers the possibility of easily localizing high surface-energy areas by treatment using either mechanical masks or optical masks. This being so, it is possible to localize droplets of various chemical substances, ranging from relatively non-viscous liquids (water, biological molecules or oil) to viscous liquids (adhesive, resin, paint) in a pattern determined by the mask and also to localize the adhesion of living cells (and the adsorption of biological molecules) in these areas.
The great attraction of the method according to the invention in simplifying the means used in order to ensure the hybridization of two components is readily apparent.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07 05329 | Jul 2007 | FR | national |
