PROCESS FOR MANUFACTURING A STRAINED SEMICONDUCTOR DEVICE AND CORRESPONDING STRAINED SEMICONDUCTOR DEVICE
20210335730 · 2021-10-28
Assignee
Inventors
- Santo Alessandro Smerzi (Catania, IT)
- Michele CALABRETTA (Giarre, IT)
- Alessandro Sitta (Giardini Naxos, IT)
- Crocifisso Marco Antonio RENNA (Floridia, IT)
- Giuseppe D'Arrigo (San Giovanni la Punta, IT)
Cpc classification
H01L2924/00014
ELECTRICITY
H01L23/06
ELECTRICITY
H01L2224/83001
ELECTRICITY
H01L2924/00014
ELECTRICITY
H01L2224/83805
ELECTRICITY
H01L2924/177
ELECTRICITY
H01L2924/177
ELECTRICITY
H01L23/564
ELECTRICITY
H01L23/04
ELECTRICITY
H01L23/14
ELECTRICITY
H01L29/7842
ELECTRICITY
International classification
H01L23/06
ELECTRICITY
H01L23/14
ELECTRICITY
Abstract
A process for manufacturing a strained semiconductor device envisages: providing a die of semiconductor material, in which elementary components of the semiconductor device have been integrated by means of initial front-end steps; and coupling, using the die-attach technique, the die to a support, at a coupling temperature. The aforesaid coupling step envisages selecting the value of the coupling temperature at a value higher than an operating temperature of use of the semiconductor device, and moreover selecting the material of the support so that it is different from the material of the die in order to determine, at the operating temperature, a coupling stress that is a function of the different values of the coefficients of thermal expansion of the materials of the die and of the support and of the temperature difference between the coupling temperature and the operating temperature. Furthermore, additional stress can be enhanced by means of different embodiments involving the support, such as ring or multi-layer frame.
Claims
1. A strained semiconductor device, comprising: a die of semiconductor material including integrated circuit components of the semiconductor device; a support coupled to the die and of a material different from said die; a housing structure forced-coupled in contact around said die and made of a material different from said die, said die being arranged in a housing opening defined in said housing structure.
2. The device according to claim 1, further comprising a coupling material layer positioned between the die and support, the coupling material layer having an elastic modulus comprised between 40 GPa and 200 GPa, and a Poisson's ratio comprised between 0.1 and 0.4.
3. The device according to claim 1, wherein the material of said housing structure is a metal, chosen from among aluminum, copper, and brass.
4. The device according to claim 1, wherein said housing opening has a substantially square or rectangular shape in top plan view, and said housing structure has, at corresponding corners, empty recesses of a substantially circular shape, which extend inside the housing structure at a distance from the die.
5. The device according to claim 1, wherein said housing structure is a frame having a top portion and a bottom portion, the top portion including the housing opening in which the die is positioned and the bottom portion includes defines a cavity that communicates with the housing opening and extends through a bottom surface of the frame; wherein the die is coupled to a projecting portion of said support that is positioned in said cavity.
6. The device according to claim 1, wherein said support includes a first layer and a second layer coupled to, and vertically stacked on top of, one another, and made of different materials; wherein said die is coupled to a top surface of the first layer of said support.
7. A strained semiconductor device, comprising: a die of semiconductor material, in which integrated circuit components of the semiconductor device are integrated; and a support forced-coupled in contact around the die and of a material different from said die, the die being arranged in a housing opening defined in said support.
8. The device according to claim 7, wherein said housing opening has a substantially square or rectangular shape in top plan view, and said support has, at corresponding corners, empty recesses of a substantially circular shape, which extend inside the support at a distance from the die.
9. A strained semiconductor device, comprising: a support; a semiconductor die coupled to the support, the semiconductor die including integrated circuit components and having a first coefficient of thermal expansion, the support being of a material different from the die and having a second coefficient of thermal expansion that is different from the first coefficient of thermal expansion of the semiconductor die; and a stress-inducing housing structure on a surface of the support, the housing structure including a housing opening, the semiconductor die being positioned in the housing opening, a sidewall surface of the housing opening abutting the semiconductor die.
10. The device according to claim 9 wherein the support includes one or more of aluminum, brass, or copper.
11. The device according to claim 9 wherein the stress-inducing housing structure introduces a compressive stress to the semiconductor die.
12. The device according to claim 9 wherein the housing structure includes one or more of aluminum, copper, and brass.
13. The device according to claim 9 wherein the semiconductor die and the housing structure are positioned on a surface portion of the support, the surface portion of the supporting having a substantially planar extension.
14. The device according to claim 9 wherein the housing structure is a frame having a top portion and a bottom portion, the top portion including the housing opening, and the bottom portion including a cavity that communicates with the housing opening and extends through a bottom surface of the frame.
15. The device according to claim 14 wherein the support includes a projection portion, the projection portion extending into the cavity of the bottom portion of the housing structure.
16. The device according to claim 14, comprising a contact structure that extends into the cavity and is electrically coupled to the semiconductor die.
17. The device of claim 16 wherein the contact structure is coupled to the semiconductor die via an attachment layer.
18. The device according to claim 9 wherein the support includes a first layer and a second layer coupled to, and vertically stacked on top of, one another, the first layer and the second layer being made of different materials.
19. The device according to claim 9 wherein the housing structure and the semiconductor die are both coupled to the support via a coupling layer in a first direction.
20. The device according to claim 19 wherein the housing structure and the semiconductor die are adjacent to one another only in a second direction that is transverse to the first direction.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0016] For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example, with reference to the attached drawings, wherein:
[0017]
[0018]
[0019]
[0020]
[0021]
[0022]
[0023]
DETAILED DESCRIPTION
[0024] As will be clarified in detail hereinafter, the present solution envisages manufacturing a strained semiconductor device, using, for generation of the stresses, back-end process steps, which are altogether compatible with packaging of the semiconductor device.
[0025] In particular, one aspect of the present solution envisages generation of stresses in a die of the semiconductor device (in which the corresponding integrated elementary components, for example, transistors, have been previously provided by means of front-end steps of the manufacturing process, of a known type, not described in detail herein) by exploiting a step of coupling or attaching, the so-called die attach, to a support, or frame (this step may advantageously be part of the back-end process for manufacturing of the package of the semiconductor device).
[0026] This coupling step generates in the die, at an operating temperature of use of the semiconductor device (for example, comprised in a range of values between −40° C. and +150° C.), a substantially planar stress. This stress is due to a difference between the aforesaid operating temperature and the temperature at which the coupling step occurs (for example, of between 350° C. and 400° C.) and to the different values of the coefficient of thermal expansion (CTE) of the materials of the die and the support, which cause a different contraction/expansion of the same materials.
[0027] As shown schematically in
[0028] The coupling step results in attaching of the die 1 to the support 2, as illustrated in
[0029] Instead, at an operating temperature T.sub.o of use of the semiconductor device, generally much lower than the coupling or die-attach temperature T.sub.da, the die 1 and the corresponding semiconductor device are subjected to a stress, caused by the different coefficients of thermal expansion of the die 1 and of the support 2, i.e., by the different contraction/expansion of the corresponding materials as the temperature varies. In the example illustrated schematically in
[0030] In particular, the temperature difference ΔT between the coupling temperature T.sub.da and the operating temperature T.sub.o is preferably comprised between 150° C. and 400° C.
[0031] The resulting stress, denoted by σ.sub.s and referred to as “coupling stress”, is substantially planar and biaxial (acting in a horizontal plane parallel to the bottom surface 1a and to a top surface 1b of the die 1). As is known, a biaxial compressive stress is particularly advantageous for N-channel devices of a vertical type, such as power MOSFETs or IGBTs; for example, such stress, as a result of the piezoresistive effect, causes a decrease in the on-state resistance R.sub.DSon of the semiconductor device.
[0032] In order to increase the amount of coupling stress as, in addition to acting on the value of the temperature difference ΔT and on the difference between the CTE coefficients of the materials of the die 1 (CTE.sub.1) and of the support 2 (CTE.sub.2), a particular aspect of the present solution envisages the use of so-called hard die-attach techniques, which lead to definition, for example by means of diffusion-soldering or sintering techniques, of a coupling material layer 3 having high hardness and stiffness characteristics.
[0033] In particular, this coupling material layer 3 has an elastic modulus E comprised between 40 GPa and 200 GPa, and a Poisson's ratio v comprised between 0.1 and 0.4.
[0034] Consequently, the coupling material layer 3 transmits in a substantially complete manner the coupling stress σ.sub.s to the die 1.
[0035] For example, the hard die attach coupling may be implemented via a technique of eutectic diffusion soldering between a gold-ti
[0036] n (Au—Sn) alloy and copper (Cu) at a coupling temperature T.sub.da that is very high (360° C.), so as to obtain a coupling material layer 3 that is thin (around a few microns) and has the aforesaid high stiffness characteristics.
[0037] In general, the characteristics of the resulting coupling material layer 3 are such as to render permanent the coupling stress as thus generated, throughout the range of operating temperatures T.sub.o at which the semiconductor device can be used.
[0038] In detail, the coupling stress as may be expressed by the following relation of proportionality:
σ.sub.sα(CTE.sub.1−CTE.sub.2).Math.(T.sub.da−T.sub.o)
and can thus be optimized by choosing a high die-attach temperature T.sub.da (given the same operating temperature T.sub.o) and materials with substantially different coefficients CTE.sub.1, CTE.sub.2, and moreover by using a coupling material layer 3 with the aforesaid high stiffness characteristics.
[0039] A further aspect of the present solution envisages increasing the mechanical stress acting on the die 1 by generating an additional stress that adds, at the operating temperature T.sub.o of use, to the coupling stress σ.sub.s deriving from the die attach coupling, thus determining a resulting stress σ.sub.s′, of a value higher than the coupling stress σ.sub.s.
[0040] In a possible embodiment, as illustrated schematically in
[0041] In particular, the aforesaid external mechanical load is applied at a temperature higher than the coupling temperature T.sub.da and determines a pre-tensioning of the die 1, prior to coupling thereof to the support 2 via die attach.
[0042] As illustrated in
[0043] Next, as illustrated in
[0044] The die 1 and the support 2 are then brought to the operating temperature T.sub.o, lower than the aforesaid coupling temperature T.sub.da, with the resulting generation, for the reasons previously discussed, of the coupling stress σ.sub.s; advantageously, the additional stress σ.sub.c due to the relief of the external mechanical load thus adds to the coupling stress σ.sub.s, thus determining the resulting stress σ.sub.s′ (with σ.sub.s′>σ.sub.s).
[0045] In a different embodiment, the additional stress on the die 1 is generated by a housing structure, configured to interact with the die 1 by means of a forced coupling at the operating temperature T.sub.o, the presence of an external mechanical load not being required.
[0046] In one embodiment, the aforesaid additional stress σ.sub.c is exerted directly by the support 2 on the die 1.
[0047] As shown in
[0048] As shown in
[0049] As shown also in the top plan view of
[0050] As shown in
[0051] The die 1 and the support 2 are then cooled down to the operating temperature T.sub.o, which, for example, coincides with room temperature.
[0052] During cooling, due to the higher coefficient of thermal expansion CTE of the support 2 (in particular, of the corresponding top portion 7) as compared to the die 1, the top portion 7 shrinks more than the die 1. Consequently, the gap g decreases progressively, until it becomes zero at a certain temperature T.sub.c, intermediate between the coupling temperature T.sub.da and the operating temperature T.sub.o, at which the top portion 7 of the support 2 comes into contact with the die 1, as shown in
[0053] As cooling continues, the top portion 7 is mechanically forced onto the die 1, and consequently a planar compressive stress is generated on the die, which in this case determines the additional compressive stress σ.sub.c (and a corresponding tensile stress at in the support 2); this additional stress σ.sub.c adds, at the operating temperature T.sub.o, to the coupling stress σ.sub.s, thus determining the resulting stress σ.sub.s′ as shown in
[0054] Also in this case, the resulting stress σ.sub.s′ is thus the combination of two different mechanical sources of stress, with a first contribution due to the die-attach operation and to the temperature difference between the coupling temperature T.sub.da and the operating temperature T.sub.o (in addition to the different coefficients of thermal expansion CTE.sub.1, CTE.sub.2 of the materials), and a second contribution, which adds in a synergistic manner to the first one, due to the compressive stress exerted by the portion of support 2 that is forced onto the die 1.
[0055] As shown in the aforesaid
[0056] A different embodiment envisages that the aforesaid additional stress σ.sub.c is exerted by a frame, distinct from the support 2 and in this case constituting the aforesaid housing structure adapted to exert a forced coupling onto the die 1.
[0057] With reference first to
[0058] The die 1 is arranged in the housing 6 defined by the frame 8 at the positioning temperature T.sub.p higher than (or equal to) the coupling temperature T.sub.da, and, then, the temperature is reduced, so as to cause compression of the frame 8 on the die 1 and generate, starting from the contact temperature T.sub.c, the additional compressive stress σ.sub.c, as shown in
[0059] As illustrated in
[0060] In a variant embodiment, illustrated first with reference to
[0061] In particular, the cavity 9 has an extension, in the horizontal plane, smaller than that of the housing 6 so that a step is formed between the inner walls of the cavity 9 and the inner walls of the housing 6 (visible in the aforesaid
[0062] In a manner similar to what has been discussed previously, also in this variant embodiment, the die 1 is arranged in the housing 6 defined by the top portion 8a of the frame 8 at the positioning temperature T.sub.p higher than the coupling temperature T.sub.da. The temperature is then reduced, so as to determine the forced coupling of the top portion 8a of the frame 8 onto the die 1 and generate the additional stress σ.sub.c, as shown in
[0063] The die 1 is in this case coupled to the support 2, at the coupling temperature T.sub.da, using the die-attach technique and via formation of the coupling material layer 3 interposed between the bottom surface 1a of the die 1, in this case at the aforesaid step, and the top surface 2a of the support 2.
[0064] As illustrated in
[0065] It should be noted that, in this embodiment, the support 2 has, at the top surface 2a, a projecting portion 12, adapted to be inserted in the cavity 9 defined inside the bottom portion 8b of the frame 8, for the purpose of attaching to the die 1.
[0066] The present Applicant has performed several tests and simulations to verify proper operation of the solution described.
[0067] For example, with reference to the embodiment shown in
[0068] In particular, the temperature difference ΔT for forcing the frame 8 onto the die 1 (i.e., for obtaining a zero gap g) was evaluated, starting from the positioning temperature T.sub.p, assumed as being 380° C., in this case considering a die 1 having a side of length a of 3 mm.
[0069] The table below shows the results of the simulation.
TABLE-US-00001 CTE ΔT Tc Material (ppm/K) (° C.) (° C.) Aluminum 23 165 215 Brass 19 210 170 Copper 17.6 225 155
[0070] Aluminum, as a result of having the highest coefficient CTE from among the materials considered, obtains contact between the frame 8 and the die 1 at a smaller temperature difference ΔT. In other words, the contact temperature T.sub.c is highest in the case of aluminum.
[0071] In addition, the effect of the size of the die 1 (considering different values of the side a) on the values of the aforesaid temperature difference ΔT was evaluated, in this case considering a copper frame and again a positioning temperature T.sub.p of 380° C.
[0072] The table below, referring to Cu frame, shows the results of the simulation.
TABLE-US-00002 a(mm) ΔT(° C.) Tc(° C.) 3 225 155 4 170 210 5 135 245 10 70 310
[0073] It should be noted in particular that the temperature difference ΔT decreases (and the value of the contact temperature T.sub.c increases) as the size of the die 1 increases; this result is explained by the fact that the tolerance of the pick-and-place operation is assumed constant (in the example, 10 μm), whereas shrinking due to the temperature (given by the product CTE.Math.ΔT.Math.a) increases as the size of the die 1 increases.
[0074] Via simulation, the value of the resulting stress σ.sub.s′ acting on the die 1 and the resulting reduction of the value of the resistance R.sub.DSon were moreover calculated, considering three different materials for the frame 8 (once again aluminum, copper, and brass) and two possible lengths for the side of length a of the die 1 (3 mm and 5 mm).
[0075] The table below shows the results of the simulation.
TABLE-US-00003 3 mm 5 mm Stress R.sub.DSon Stress Material (MPa) Variation (MPa) R.sub.DSon Variation Al −90 −4.7% −120 −6.3% Cu −70 −3.6% −115 −6.1% Brass −80 −4.2% −125 −6.5%
[0076] Considering a die 1 with a side of length a of 3 mm, the aluminum frame 8 generates the highest stress on the die 1 and consequently the greatest variation of R.sub.Dson resistance; if the size of the die 1 is increased, instead, R.sub.DSon resistance reduction becomes similar for the three considered material.
[0077] This result can be explained by the fact that, as the size of the die 1 increases, mechanical forcing of the frame 8 on the die 1 occurs at a greater temperature difference ΔT, and this entails a greater role of the stiffness of the materials (in terms of Young's modulus) in determining the stress induced in the semiconductor material of the die 1.
[0078] The advantages of the present solution are clear from the foregoing description.
[0079] In any case, it is once again emphasized that the present solution for providing strained semiconductor devices can advantageously be applied at the end of the steps of manufacture of the corresponding integrated elementary components (for example, transistors), without additional steps or modifications in the front end of the manufacturing process being required.
[0080] In fact, the present solution envisages exclusively back-end process steps, being moreover altogether compatible with the techniques and steps of packaging of the semiconductor device, and thus suited to industrial manufacturing processes. In a manner that will be evident to a person skilled in the art, these packaging steps may, for example, envisage electrical connection of the die 1 to leads, coating of the die 1 with a packaging material, and so on.
[0081] The back-end process steps introduced are simple and do not require a substantial increase in the manufacturing costs.
[0082] The mechanical stresses introduced in the die of the semiconductor device, encapsulated in the corresponding package, are moreover permanent.
[0083] The above solution may be applied irrespective of the type or technique of manufacturing of the integrated elementary components of the semiconductor device, for example whether they are short-channel or long-channel transistor devices.
[0084] Finally, it is clear that modifications and variations may be made to what is described and illustrated herein, without thereby departing from the scope of the present disclosure.
[0085] For example, a first variant embodiment may envisage, as illustrated in
[0086] The advantage of this multi-layer embodiment is the availability of a further source of stresses, due to the different coefficient of thermal expansion CTE of the materials constituting the first and second layers 2′, 2″ of the support 2, which may represent an additional contribution that further increases the resulting stress σ.sub.s′ acting on the die 1 at the operating temperature T.sub.o.
[0087] Moreover, the use of a dedicated technique of brazing, at an appropriate brazing temperature, between the first and second layers 2′, 2″ of the support 2 may contribute to inducing a further stress, in particular a compressive stress, on the die 1.
[0088] Clearly, further layers for the multi-layer support 2 may possibly be provided.
[0089] Yet a further variant embodiment, shown in
[0090] The present solution in general finds advantageous application for any semiconductor device, whether a discrete device (for example, a power MOSFET, an IGBT, a VIPower, etc.) or an integrated circuit (for example, a microprocessor, a memory, a BCD device, an HCMOS device, etc.) and for any type of support 2 to which the die 1 of the semiconductor device is coupled (for example, a leadframe, a PCB, etc.).
[0091] The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.