US 20080179755 A1
A process and structure for enabling the creation of reliable electrical through-via connections in a semiconductor substrate and a process for filling vias. Problems associated with under etch, over etch and flaring of deep Si RIE etched through-vias are mitigated, thereby vastly improving the integrity of the insulation and metallization layers used to convert the through-vias into highly conductive pathways across the Si wafer thickness. By using an insulating collar structure in the substrate in one case and by filling the via in accordance with the invention in another case, whole wafer yield of electrically conductive through vias is greatly enhanced.
1. An insulated through via structure formed in a substrate, comprising:
an insulated pad region perforated with metal contacts formed in said substrate;
an annular structure formed in said substrate so as to surround the insulated pad region, said annular structure formed to a depth into the substrate which is greater than that of a plane defined by the insulated pad region;
a backside through-via centered on, and contained entirely within, lateral dimensions of the annular structure, and opened to expose a front side metal contact; and
an insulating layer completely covering side walls of the through-via, but not covering the metal contacts formed in said substrate.
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9. A method for forming an insulated through via structure in a substrate, comprising:
forming an insulated pad region perforated with at least one metal contact on said substrate;
forming an annular structure in said substrate so as to surround said insulated pad region, said annular structure formed to a depth into the substrate which is greater than that of a plane defined by the insulated pad region;
forming a backside through-via centered on, and contained entirely within, lateral dimensions of the annular structure, so as to expose said at least one metal contact; and
forming an insulating layer completely covering side walls of the through-via, but not covering the metal contacts formed in said substrate.
10. A method for coating deep vias in a substrate, comprising:
forming a mask on said substrate, said mask having opening therein aligned with respective ones of said vias, said openings having diameters smaller than that of said respective vias; and
depositing material to coat said vias through said openings in said mask.
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18. An insulating substrate for making electrical connections, comprising:
a plurality of blind vias extending from a first surface toward a second surface of said substrate, said vias having varying heights,
an electrically conductive coating lining said vias, respective coatings of said vias having different thicknesses at bottoms of respective vias;
an electrically conductive porous material inside a space defined by said coatings;
thicknesses of said coatings at said bottom of said vias being sufficient so that when material is removed from said second surface to expose all of said coatings, said electrically conductive porous material is not exposed.
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This invention was made with Government support under Contract No. NBCH30390004 and Contract No. MDA972-03-3-004 awarded by the Defense Advanced Research Projects Agency. The Government has certain rights in the invention.
This invention is directed to processes and structures for enabling the creation of reliable electrical through-via connections in a semiconductor substrate. More particularly, it is directed to processes and structures which mitigate problems associated with under etch, over etch and flaring of deep Si RIE etched through-vias, thereby vastly improving the integrity of the insulation and metallization layers used to convert the through-vias into highly conductive pathways across the Si wafer thickness. This invention also relates to techniques for filling vias with an electrical conductor. Thus this invention greatly enhances whole wafer yield of electrically conductive through vias.
There are many advantages to using silicon as a substrate for electronic packaging, rather than traditional ceramic and organic laminate packaging. Key features of the silicon carrier include: the ability to create high performance wiring and joining at much finer pitch than typical packaging, the ability to join heterogeneous technologies or different generation technologies for high speed applications, the ability to integrate passives, MEMs (micro electromechanical systems) or optical fibers, the ability to add silicon functionality to the carrier package in addition to wiring, the ability to dramatically increase the I/O density, and for many applications, the ability to reduce overall system cost when compared to other system on package (SOP) approaches which do not use Si as the carrier.
Elements and structures of semiconductor packages have been described in U.S. Pat. No. 5,998,292 to Black et al. and U.S. Pat. No. 6,593,644 to Chiu et al. The through-via structures described in these two cited works are constructed using a “vias first” process approach described below. In this method, deep blind vias (which may be up to several hundred micrometers in depth) are first etched into a silicon wafer, after which side wall insulation is deposited or thermally grown. The vias are then completely filled with a conductor, after which standard BEOL wiring levels are built on top of the silicon wafer. Up to this point in the process, the vias are still blind. They are converted to through-vias only when the whole wafer is thinned by backside grinding and/or etching to expose the via conductors on the backside. Once exposed, solder connections, such as C4 solder balls may then be built on the carrier back, and chips may be joined to the front, by any one of a number of conventional techniques, such as flip chip bonding.
The “vias first” process flow has the advantage that the side wall insulation process is fairly straightforward since a variety of techniques are available to ensure complete coverage of the side walls including, in many cases, thermal oxidation. Moreover, since the vias terminate in the silicon itself rather than on an etch stopping layer, there is little chance of side wall variation near the base of the via such as abrupt lateral etching or flaring. A smooth uniform side wall combined with thermal oxidation virtually assures high-yield via to via isolation.
On the other hand, in the “vias first” approach the filling of the through-vias must be absolutely complete, void-free and fully sealed at the top in order for wiring levels to be built on top and effectively connected down through the silicon wafer. Further, the conductive material used to fill the vias must be very well matched in coefficient of thermal expansion (CTE) to the silicon substrate otherwise mechanical deformation can take place during subsequent processing of the wiring levels leading to electrical and/or mechanical failure of the carrier by cracking of the silicon, the insulator or by upward pumping of material from within the via itself.
Another process approach is to build the devices and wiring according to the standard CMOS semiconductor process flow, adding the through-vias at the end of the process by driving them through from the back of the wafer. In this “vias last” approach the same techniques can be used to etch the vias e.g. deep anisotropic reactive ion etching (RIE) from an inductively coupled plasma source. However an important difference exists at the location of the via bottom. In the “vias first” process, the vias remains blind until the wafer is thinned to reveal the insulated conductors, at which time further processing must be done to add contact pads and insulate the back of the wafer. In the “vias last” approach the contact pads are built first on the front side of the wafer underneath the wiring levels, and are effectively uncovered at the back side by means of an etching process. This means that the contact pads, typically insulating layer of silicon oxide or silicon nitride perforated with conductive metal vias, act as etch stops.
This would be of little consequence in the case of perfect etch selectivity between the silicon and the etch stop, as well as perfect side wall passivation integrity at the contact pad to silicon interface. In practice, the etch chemistry available in deep Si RIE can be made very selective to common insulators and many metals used to create the contact pads. However, it is well known that side wall passivation in deep anisotropic etching has a tendency to become compromised at etch stop layers, which causes unintended lateral etching to begin at the foot of the via, resulting in reentrant features ranging from an outward flare to dramatic lateral undercutting. Since via insulation and metallization steps must take place after the via etching process, it is paramount that the via side wall be smooth from top to bottom and preferably vertical (slight positive slope is also acceptable) with no discontinuities or negatively flared regions which would make insulation and metal wall coating difficult or impossible.
Because all the wiring levels have already been built in the “vias last” process flow, the temperature cannot exceed a maximum of about 400 degrees C. which makes thermal oxidation impossible. Thus, low-temperature CVD or PECVD insulation with silicon oxide or nitride materials becomes a practical necessity in this case, and these do not coat reentrant via side walls (those having a negative slope or reverse taper) nearly as well as vertical walls, or walls having a slightly positive slope. Also, in the case where electroplating is to be used to coat the side walls with metallization, common liner and seed materials such as TaN, or metals such as Ta and Cu are typically sputtered into the vias, and once again smooth continuous surfaces without negative flare or undercut at the base are required to ensure complete coverage.
A number of methods and structures have been described whereby through vias may be etched through silicon from the back in such a fashion as to land on a pad structure on the active side of the wafer. In U.S. Pat. No. 5,608,264 to S. J. Gaul a “vias first” process is described wherein vias begin as blind vias which are metal lined and completely filled with doped polysilicon, and are later exposed by etching the backside of the wafer. In one embodiment of this work, the polysilicon is later removed by wet KOH etching from the backside to create an open through via terminating on an oxide layer under a pad on the active front side. Such a structure avoids the problem of deep etch undercut at the pad interface since the via fill is entirely removed by highly selective wet etch. However, such a process is not a viable “vias last” process but instead is a time-consuming and potentially very expensive “vias first” process. Time and expense can be very high, especially when via diameter is large since the entire via must be first completely filled with CVD polysilicon in order to build the wiring levels, only to have all the polysilicon removed from the backside in a later etching step. As such, one of the most expensive and time consuming CVD steps turns out to be entirely sacrificial in this case.
U.S. Pat. No. 6,608,371 to Kuashima et al. discloses a true “vias last” process flow wherein the metal contact pads are first formed on the front side of a silicon wafer after which tapered through vias are formed using a laser. The vias are insulated, in the preferred embodiment, using a resin which could be made to fill the entire via, then opened again using a second laser etching step to produce a narrower diameter through hole coincident with the first. The via would then be metallized, where the metal could be applied in the form of a metal paste or solder. This method has a process flow which is more akin to board level packaging rather than silicon CMOS processing. In particular, the processing time and cost of using a laser system to create the through-vias grows in proportion to the number and density of vias required. For high-performance applications where several thousand through vias are required per die, and there are several hundred die per wafer, the raw processing time per wafer could be intolerably high. Further, the process so described specifies ranges for the through via diameter on a 50 μm thick wafer of 60 to 80 μm with the inner diameter of the insulated region ranging from about 30 μm to 40 μm. It is clear that vias of outer diameter of about 20 μm or less are difficult to produce or completely inaccessible to this method, whereas diameters down to 1 μm are feasible using deep anisotropic etching.
A CMOS-compatible “vias last” process having many steps generally comparable to the method described herein, but not having the novel steps and structure of the invention described below, can be found in U.S. Pat. No. 6,569,762 to Kong. In Kong the layer where the deep Si RIE etch lands is a blanket layer. For this reason, a second etch step with an oxide selective chemistry must be used to open this insulator and expose the contacts beneath. As has been well noted in the literature, it is extremely difficult in practice to maintain a vertical side wall at such an interface once all Si has been etched from the via. Recognizing the need for a smooth, continuous vertical side wall at the bottom of the via, U.S. Pat. No. 6,569,762 states explicitly that “maintaining the anisotropy of the etch and removal of any polymer material formed after the etch is important to the forming of holes from the back of the wafer to the front oxide. The etching of the holes is stopped at the front oxide by either a timed etch or by detecting a diminishing Si signal using an optical end point detector.”
Simple timing and/or optical methods, while useful, simply cannot provide for a robust wafer scale process because center to edge variation in RIE etch rate is typically seen in all etch systems, which means that vias at different locations will always see a varying degree of over etch. Moreover, wafer thinning techniques such as backside grinding and spin etching are not perfectly uniform processes, especially when the wafers to be thinned are bonded to handling substrates other than Si wafers (e.g. borofloat glass) since these may have a total thickness variation on the order of 10 μm. The compounding effect of center to edge RIE etch variation and non-uniform starting thickness makes it practically impossible to etch each via to the same degree, and thus some attack of the side wall near the via bottom must be anticipated, when the vias are targeted to be fully etched during the first RIE etch step.
The present inventors have recognized that there is a need for structures and methods which enable high-yield, electrically robust through via connections to be built using CMOS-compatible silicon wafer processing tools. In particular, there is a need for highly manufacturable deep via etching processes with wide process windows.
It is therefore an aspect of the present invention to provide structures and methods for reliably creating deep silicon through-vias with smooth side wall surfaces free of discontinuities which would otherwise prevent uniform, high-yield insulation and metallization of the through-vias.
It is another aspect of the present invention to provide structures and methods for increasing the overall mechanical robustness and reliability of the electrical through via and contact pad assembly.
In accordance with one aspect, useful in a “vias last” process, the invention is directed to a method and structure for reliably landing deep RIE etched vias from one surface of a silicon wafer onto contact pad structures on the opposing surface comprises the steps of building a collar structure on the active side of a silicon wafer, the collar structure being filled with a material which acts as an etch stop to deep Si etch chemistry; building devices and wiring levels on the active side of the wafer; attachment of the wafer to a handler wafer to allow wafer thinning; patterning of the backside with the desired through via pattern taking care to align the via pattern such that it lands within the space defined by the collar on the opposite side; etching the via use deep silicon anisotropic etching, where vias may be slightly under etched or over etched; switching to a second etch chemistry, preferably a wet anisotropic (e.g. KOH, TMAH) or isotropic etch (e.g. HNO3+HF mixture) in order to complete the etch where needed, and to remove any sharp discontinuities from the base of the via by propagating the via side wall to or beyond the collar side wall.
It is another aspect of the invention, useful in a “vias first” process, to provide a method for filing the vias with an electrical conductor. In this respect, another aspect of the present invention provides a high aspect ratio, porous paste filled via, comprising: a semiconductor substrate having a high aspect ratio via defined therein, the via having a bottom and at least one side wall; a layer of an electrical insulator lining the via bottom and the at least one side wall, wherein the insulator electrically isolates the via from the substrate; a layer of an adhesion promoter lining the insulator; a layer of a seed material lining the adhesion promoter; a solid metal lining the via at few micrometer thickness; and porous paste filling the final space in the via.
An aspect of the present invention provides a high aspect ratio via having a depth of from about 100 to 300 micrometers and an aspect ratio of from about 3:1 to about 10:1.
An aspect of the present invention provides varied diameter vias properly tailored from edge to center so as to nullify the edge-to-center depth difference of the deep vias.
An aspect of the present invention provides a via within which the very bottom thickness of copper is increased to overcome the via depth difference across the wafer.
An aspect of the present invention provides a method to increase the bottom copper thickness in a preset manner thus overcoming the problem of via depth difference across the wafer.
An aspect of the present invention provides a method of fabricating a high aspect ratio deep via with controlled via bottom taper, which taper is controlled to give significantly thicker via bottom copper thickness.
An aspect of the present invention provides a method of fabricating a high aspect ratio deep via with flat bottom and tailored bottom copper thickness using contact or shadow mask to deposit the jet plasma copper into the deep via bottoms.
Thus, in general, these aspects of the invention may be directed to insulated, conductive through-vias formed in a carrier substrate that is thick enough to be self-supporting. Fabrication process practicalities place lower limits on via diameters. For realistic substrate thickness (e.g. 100-700 micrometers) and realistic via aspect ratios (e.g. 3:1 to 10:1), via diameters must range from about 10 micrometers to greater than 200 micrometers. Via fill methods common to integrated circuit interconnects become technically problematic or impractical when trying to fill such large blind holes (to be opened later). While there are techniques that overcome these practical issues by teaching a multitude of different methods to fill vias with metal-ceramic pastes, paste-filled vias are associated with an undesirably high degree of porosity. Extensive processing is required to seal any via surface containing exposed porosity as described. The present invention avoids these problems.
The advantageous aspects and features of the present invention will become apparent upon further consideration of the following detailed description of the invention when read in conjunction with the drawing figures, which unless otherwise indicated are cross sectional views, and in which:
Multiple conducting contacts 28 are used to increase reliability and to decrease total resistance, if a single connection is to be made. In other cases, the conducting contacts 28 may act as conductors to various parts of a circuit, such as for the distribution of a signal, or for the distribution of power to the wiring levels built on the front surface of the silicon substrate.
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Thus, it has been shown that the structure of the via itself and the processes used to create the via differ in substantive respects from the prior art, and that these differences are extremely significant in increasing production yield. A significant departure from the prior art is in the etch stop oxide layer. In the method described herein this step in unnecessary since the contacts are exposed at the same time, or even before the field oxide. However, the etch collar structure surrounding the via where it lands on the oxide of the active layer provides a major advantage over prior art structures.
One obvious option would be to deposit a thicker copper liner in the vias. However, the volume of copper needs to be controlled so as to avoid thermal expansion coefficient mismatch caused problems during high temperature processing, such as copper pumping. For electrical purposes, few micrometer thick copper is all that is needed, which thinner copper would keep the deposition time under control, reducing processing turn-around-time and thus cost.
To solve the problem discussed above, the novel filling technique described below may be utilized.
This invention is directed to insulated, conductive through-vias formed in a self-supporting carrier substrate. The requirement that the substrate be self-supporting implies a thickness of from about 100 to 700 micrometers. The vias are formed as blind vias. However, it is intended that they be opened to form through-vias. For purposes of the present invention, via cavities may be formed by any of a variety of methods known to the art.
Fabrication process practicalities place lower limits on via diameters. Given realistic substrate thicknesses of up to 700 micrometers and realistic via aspect ratios (e.g. 3:1 to 10:1), via diameters must range from about 10 micrometers to greater than 200 micrometers.
In view of the problems associated with the deep via depth uniformity across the wafer this aspect of the invention relates to increases in via diameter of about 1 micrometer for every 2 micrometer depth correction required across the entire wafer. Thus when via diameter goal is 70 micrometers, even with 20 micrometer via depth non-uniformity from center to edge of the wafer, the diameter variance center to edge would be about 10 micrometers or about 15%, using the this aspect of the invention. The values given here are intended only as examples. The correct tailoring of via mask is dependent on via etch method and tooling used.
Another aspect of the present invention provides a method of fabricating a high aspect ratio deep via with straight side walls and thick bottom copper film using laminated Riston (or another such organic) coating 300 on the surface of the wafer 200 with the metal lined 330 deep vias 320. The pattern is aligned with the deep vias and via diameter in the mask used may be slightly reduced as compared to the actual deep via dimension. Added copper is then deposited at via bottom using a line-of-sight jet plasma deposition technique with a preprogrammed pattern of deposition allowing tailoring of the required bottom copper thickness from sub-micrometer thickness to few micrometer thickness.
It is clear that the two options shown in
Yet another aspect of the present invention provides a method to tailor the taper of the deep via 260 as is known in the art in such a manner that when a certain thickness of copper is deposited onto the side walls of the via, the bottom thickness of the copper is adequate to compensate for the via depth difference across the wafer.
As an example, using the schematic shown in
h=solid copper height (μm)
r2=via side wall copper thickness (μm)
r=via bottom radius (μm)
A=taper angle, which can be calculated:
Results of this experiment are shown in Table 1.
Table 1 shows that if the taper is designed to be approximately 5.7 degrees the top via radius will be about 34 micrometer. If higher taper is preferred at the bottom of the via, then taper will be tailored to vary from top to bottom so as to result in required via diameter at the top and at the bottom of the via, which dimensions are guided by the required copper thickness at the via bottom to compensate any via depth differences experienced due to deep via RIE process.
Thus, the advantageous via filling method in accordance with the invention secures the integrity of the deep via without exposure of the paste porosity during the wafer thinning and polishing that exposes the conducting bottom of the deep via.
Variations described for the present invention may be realized in any combination desirable for each particular application. Thus particular limitations, and/or embodiment enhancements described herein, which may have particular advantages to the particular application need not be used for all applications. It will be clear to those skilled in the art that other modifications to the disclosed embodiments can be effected without departing from the spirit and scope of the invention. Thus, it should be understood that the embodiments have been provided as an example and not as a limitation. The scope of the invention is defined by the appended claims.