Process for manufacturing a strained semiconductor device and corresponding strained semiconductor device

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 process for manufacturing a strained semiconductor device, comprising: coupling a semiconductor die to a support at a coupling temperature, the semiconductor die including elementary components of the semiconductor device and having a coefficient of thermal expansion, the support being of a material different from the die and having a coefficient of thermal expansion that is different from the coefficient of thermal expansion of the die; cooling the die and support to an operating temperature of the semiconductor device, the cooling resulting in a coupling stress that is a function of the different values of the coefficients of thermal expansion of the die and of the support and of the temperature difference between the coupling temperature (T.sub.da) and the operating temperature; and causing an additional stress on the die designed to add to the coupling stress in order to generate a resulting stress on the die at the operating temperature, wherein the causing the additional stress comprises: positioning the die within a housing opening of a housing structure at a positioning temperature, a separation gap being present between the die and the housing structure, and the housing structure having a coefficient of thermal expansion different from that of the semiconductor die; reducing the temperature down to a contact temperature, lower than the positioning temperature, at which the housing structure comes into contact with the die, and the gap reduces to zero; and reducing further the temperature down to the operating temperature so as to force the housing structure onto the die and thus cause the additional stress.

2. The process according to claim 1, wherein the support is made of a metal material, chosen from among aluminum, brass, and copper.

3. The process according to claim 1, wherein the temperature difference is comprised between 150° C. and 400° C.

4. The process according to claim 1, wherein coupling comprises forming a coupling material layer between the semiconductor die and the support, having an elastic modulus comprised between 40 GPa and 200 GPa, and a Poisson's ratio comprised between 0.1 and 0.4.

5. The process according to claim 1, wherein coupling comprises forming a coupling material layer between the die and the support using a hard die-attach technique.

6. The process according to claim 1, wherein causing the additional stress comprises applying to the support an external mechanical load, which creates a stress on the support, at a temperature higher than the coupling temperature; and releasing the external mechanical load after the die has been coupled to the support so as to generate the additional stress on the die.

7. The process according to claim 1, wherein the additional stress is a compressive stress.

8. The process according to claim 1, wherein the positioning temperature is not lower than the coupling temperature, and reducing the temperature down to the contact temperature is carried out after coupling the die to the support.

9. The process according to claim 1, wherein the housing structure is made of a metal material chosen from among aluminum, copper, and brass.

10. The process according to claim 1, wherein coupling the die to the support comprises positioning the die and teh housing structure on a top surface of the support, which has a substantially planar extension.

11. The process 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 a cavity that communicates with the housing opening and extends through a bottom surface of the frame, the process including positioning a projection portion of the support in the cavity, wherein coupling the die to the support comprises attaching the die to the projecting portion of the support while the projecting portion is in the cavity.

12. The process according to claim 1, wherein causing the additional stress comprises: positioning the die within a housing opening in a top portion of the support at a positioning temperature, a separation gap being present between the die and a sidewall of the housing opening, and the housing opening having a bottom defined by a bottom portion of the support; wherein coupling the die to the support comprises attaching the die to the bottom portion of the support at the coupling temperature, once the die has been positioned inside the housing; reducing the temperature down to a contact temperature, lower than the coupling temperature, at which the sidewall of the housing opening comes into contact with the die, and the gap reduces to zero; and reducing further the temperature down to the operating temperature so as to force the support onto the die and thus cause the additional stress.

13. The process according to claim 1, wherein the support includes a first layer and a second layer coupled to, and vertically stacked on top of, one another, and made of different materials; and wherein coupling the die to the support comprises attaching the die to a top surface of the first layer of the support.

14. A method, comprising: coupling a die of semiconductor material with a support, the die including elementary components of a semiconductor device, the support having a material different from the die; positioning the die and the support in a housing opening of a housing structure at a positioning temperature, the housing structure having a material different from the die, a separation gap being present between the die and the housing structure, and the housing structure having a coefficient of thermal expansion different from a coefficient of thermal expansion of the die; reducing temperature down to a contact temperature, lower than the positioning temperature, at which the housing structure comes into contact with the die, and the separation gap reduces to zero; and reducing further the temperature down to an operating temperature of the semiconductor device so as to force the housing structure onto the die and thus cause additional stress on the die.

15. The method according to claim 14, wherein the coupling the die with the support including positioning a coupling material layer 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.

16. The method according to claim 14, wherein the material of the housing structure is a metal, chosen from among aluminum, copper, or brass.

17. The method according to claim 14, wherein the positioning the die and the support in the housing opening of the housing structure includes positioning the die and the support into the housing opening of the housing structure that has a substantially square or rectangular shape in top plan view, and the 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.

18. The method according to claim 14, wherein the 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 a cavity that communicates with the housing opening and extends through a bottom surface of the frame; and wherein the die is coupled to a projecting portion of the support that is positioned in the cavity.

19. The method according to claim 14, wherein: the support includes a first layer and a second layer coupled to, and vertically stacked on top of, one another, and made of different materials; and the die is coupled to a top surface of the first layer of the support.

20. A method, comprising: coupling a die of a semiconductor material in contact to a support at a first temperature, the die including elementary components of a semiconductor device, the first temperature higher than a second temperature at which the semiconductor device is configured to operate; positioning the die within a housing opening of a housing structure at a third temperature that is higher than the second temperature, a separation gap being present between the die and the housing structure, and the housing structure having a coefficient of thermal expansion different from a coefficient of thermal expansion of the die; reducing temperature down to a fourth temperature, lower than the third temperature and higher than the second temperature, at which the housing structure comes into contact with the die, and the separation gap reduces to zero; and reducing further the temperature down to the second temperature so as to force the housing structure onto the die and thus cause additional stress on the die, wherein the support includes a housing opening and the die is coupled in the housing opening of the support.

21. The method according to claim 20, wherein the housing opening has a substantially square or rectangular shape in top plan view, and the support has, at corresponding corners, empty recesses of a substantially circular shape, which extend inside the support at a distance from the die.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) 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:

(2) FIGS. 1A-1C show consecutive steps of a process for manufacturing a strained semiconductor device, according to a first embodiment of the present solution;

(3) FIGS. 2A-2C show schematic cross-sectional views of a strained semiconductor device, during successive steps of a manufacturing process according to a further embodiment of the present solution;

(4) FIGS. 3A-3E show schematic cross-sectional views of a strained semiconductor device, during successive steps of a manufacturing process according to yet a further embodiment of the present solution;

(5) FIG. 4 is a schematic top plan view of the semiconductor device of FIG. 3C;

(6) FIGS. 5A-5C show schematic cross-sectional views of a strained semiconductor device, during successive steps of a manufacturing process according to yet a further embodiment of the present solution;

(7) FIGS. 6A-6C show schematic cross-sectional views of a strained semiconductor device, during successive steps of a manufacturing process according to yet a further embodiment of the present solution; and

(8) FIGS. 7 and 8 show schematic cross-sectional views of further variant embodiments of the semiconductor device, according to further aspects of the present solution.

DETAILED DESCRIPTION

(9) 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.

(10) 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).

(11) 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.

(12) As shown schematically in FIG. 1A, the die 1 of the semiconductor device, made of a crystalline semiconductor material, such as silicon or germanium, is coupled to a support 2 (which may, for example, be a leadframe of a corresponding package, or any support or frame) by means of a die-attach technique, at a coupling temperature T.sub.da; the support 2 may, for example, be made of metal material, such as aluminum, brass, or copper.

(13) The coupling step results in attaching of the die 1 to the support 2, as illustrated in FIG. 1B, by means of a coupling material layer 3, which is formed between a bottom surface 1a of the die and a facing top surface 2a of the support 2. At the coupling temperature T.sub.da, the semiconductor device may be considered substantially free from mechanical stresses.

(14) 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 FIG. 1C, the support 2 undergoes a reduction in volume with respect to the initial condition (represented by a dashed line).

(15) 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.

(16) 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.

(17) In order to increase the amount of coupling stress σ.sub.s, 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.

(18) 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.

(19) Consequently, the coupling material layer 3 transmits in a substantially complete manner the coupling stress σ.sub.s to the die 1.

(20) For example, the hard die attach coupling may be implemented via a technique of eutectic diffusion soldering between a gold-ti

(21) 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.

(22) In general, the characteristics of the resulting coupling material layer 3 are such as to render permanent the coupling stress σ.sub.s thus generated, throughout the range of operating temperatures T.sub.o at which the semiconductor device can be used.

(23) In detail, the coupling stress σ.sub.s 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.

(24) 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. In a possible embodiment, as illustrated schematically in FIG. 2A, an external mechanical load is applied to the support 2, causing a tensile stress σ.sub.t; in particular, the tensile stress σ.sub.t is in this case generated by clamping opposite longitudinal ends of the support 2 to clamps 5, arranged laterally with respect to the same support 2, and is represented by a uniaxial stress in a longitudinal direction (parallel to the top surface 2a of the support 2).

(25) 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.

(26) As illustrated in FIG. 2B, the die 1 is then coupled to the support 2, as described previously, by means of the die-attach technique at the coupling temperature T.sub.da.

(27) Next, as illustrated in FIG. 2C, the external mechanical load is removed (in the example, the support 2 is released from the clamps 5) so that a strain is generated and an additional compressive stress σ.sub.c acting on the die 1 (caused by the relief of tensile stress σ.sub.t at which the support 2 had been pre-tensioned).

(28) 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).

(29) 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.

(30) In one embodiment, the aforesaid additional stress σ.sub.c is exerted directly by the support 2 on the die 1.

(31) As shown in FIG. 3A, the support 2 defines in this case a housing 6, open towards the outside at its top surface 2a. In other words, the support 2 has a top portion 7, at the aforesaid top surface 2a, that is ring- or collar-shaped and defines inside it side walls of the aforesaid housing 6, and a compact bottom portion 7′, which defines a bottom wall 6a of the same housing 6 (the bottom wall 6a defining part of the top surface 2a of the support 2).

(32) As shown in FIG. 3B, the housing 6 is adapted to house the die 1, which in this case is arranged inside the housing 6 using the so-called pick-and-place technique, at a positioning temperature T.sub.p generally higher than the coupling temperature T.sub.da (and in any case not lower than the coupling temperature T.sub.da).

(33) As shown also in the top plan view of FIG. 4, the housing 6 is larger (in the horizontal plane) than the die 1, with a dimensional tolerance compatible with the aforesaid pick-and-place technique; in particular, a separation gap g, for example of 5-20 μm, is present between the side walls of the housing 6 and the die 1.

(34) As shown in FIG. 3C, the die 1 and the support 2 are then cooled and brought to the coupling temperature T.sub.da, at which the die 1 is coupled to the support 2 by means of the die-attach technique, and the aforesaid coupling material layer 3 is formed, in this case interposed between the bottom wall 6a of the housing 6 and the bottom surface 1a of the die 1.

(35) 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.

(36) 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 FIG. 3D.

(37) 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 σ.sub.t 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 FIG. 3E.

(38) 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.

(39) As shown in the aforesaid FIG. 4, the top portion 7 of the support 2 has, at the corners of the housing 6 (which has a substantially rectangular or square shape in the horizontal plane) empty recesses 6′, of a generically circular shape in the horizontal plane, which extend inside the top portion 7 of the support 2, at a distance from the die 1. The presence of said recesses 6′ allows to prevent generation, by the support 2, of excessive mechanical stresses at the corners of the die 1, during the aforesaid forced coupling, which might damage the corresponding semiconductor material.

(40) A different embodiment envisages that the aforesaid additional stress σ, 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.

(41) With reference first to FIG. 5A, a frame 8 is thus provided, made, for example, of metal material, such as aluminum, copper, or brass, having a ring or collar conformation and defining inside it a housing, once again designated by 6, inside which the die 1 of the semiconductor device is to be arranged, for example, using the pick-and-place technique (in a way substantially similar to what has been discussed previously; in particular, reference may again be made to the aforesaid FIG. 4 as regards the tolerances and dimensions).

(42) 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 FIG. 5B.

(43) As illustrated in FIG. 5C, the die 1 is also in this case coupled to the support 2, at the coupling temperature T.sub.da, using the die-attach technique, by means of the coupling material layer 3 interposed between the bottom surface 1a of the die 1 and the top surface 2a of the support 2 (which, in this case, has a planar conformation). As discussed previously, cooling down to the operating temperature T.sub.o entails also in this case generation of the coupling stress σ.sub.s, which once again adds to the additional stress σ.sub.c, thus determining the resulting stress σ.sub.s′ on the die 1.

(44) In a variant embodiment, illustrated first with reference to FIG. 6A, the frame 8, which also in this case is made of a metal material, such as aluminum, copper or brass, is constituted by: a top portion 8a that has a ring or collar conformation and defines inside it the housing 6, inside which the die 1 of the semiconductor device is to be arranged using the pick-and-place technique (in a way substantially similar to what has been discussed previously; in particular, see again FIG. 4); and moreover a bottom portion 8b, which defines inside it a cavity 9, which traverses it throughout its thickness and communicates at the top with the aforesaid housing 6 and terminates at the bottom at a bottom surface of the frame 8.

(45) 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 FIG. 6A).

(46) 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 FIG. 6B. It should be noted, in particular, that a corner portion of the bottom surface 1a of the die 1 rests on the step defined between the housing 6 and the cavity 9, when the die 1 is forced into contact with the frame 8.

(47) 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.

(48) As illustrated in FIG. 6C, and as discussed previously, cooling down to the operating temperature T.sub.o entails also in this case generation of the coupling stress σ.sub.s, which once again adds to the additional compressive stress σ.sub.c, thus determining the resulting stress σ.sub.s′ on the die 1.

(49) 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.

(50) The present Applicant has performed several tests and simulations to verify proper operation of the solution described.

(51) For example, with reference to the embodiment shown in FIGS. 5A-5C, the behavior was simulated with different materials of the frame 8 (aluminum, brass, or copper) and different dimensions of the die 1, considered square in top plan view, with a side of length a.

(52) 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.

(53) The table below shows the results of the simulation.

(54) TABLE-US-00001 CTE ΔT Tc Material (ppm/K) (° C.) (° C.) Aluminum 23 165 215 Brass 19 210 170 Copper 17.6 225 155

(55) 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.

(56) 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.

(57) The table below, referring to Cu frame, shows the results of the simulation.

(58) TABLE-US-00002 a(mm) ΔT(° C.) Tc(° C.) 3 225 155 4 170 210 5 135 245 10 70 310

(59) 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.

(60) 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).

(61) The table below shows the results of the simulation.

(62) TABLE-US-00003 3 mm 5 mm Stress R.sub.DSon Stress R.sub.DSon Material (MPa) Variation (MPa) Variation Al −90 −4.7% −120 −6.3% Cu −70 −3.6% −115 −6.1% Brass −80 −4.2% −125 −6.5%

(63) 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.

(64) 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.

(65) The advantages of the present solution are clear from the foregoing description.

(66) 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.

(67) 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.

(68) The back-end process steps introduced are simple and do not require a substantial increase in the manufacturing costs.

(69) The mechanical stresses introduced in the die of the semiconductor device, encapsulated in the corresponding package, are moreover permanent.

(70) 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.

(71) 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.

(72) For example, a first variant embodiment may envisage, as illustrated in FIG. 7, that the support 2 to which the die 1 of the semiconductor device is coupled is of a multi-layer type, i.e., comprising at least a first layer 2′ and a second layer 2″ coupled and vertically stacked on top of one another, made of different materials, preferably both metal, and having appropriate thicknesses (the same as or different from one another). In this case, the die 1 is coupled via die attach to the top surface 2a of the first layer 2′, and the layer of coupling material 3 is interposed between the bottom surface 1a of the die and the aforesaid top surface 2a.

(73) 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.

(74) 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.

(75) Clearly, further layers for the multi-layer support 2 may possibly be provided.

(76) Yet a further variant embodiment, shown in FIG. 8, may envisage the presence of a rear-contact element 14, made of metal material, designed to be inserted in the cavity 9 defined inside the bottom portion 8b of the frame 8, and to be coupled, by means of die attach, to the bottom surface 1a of the die 1. This rear-contact element 14 may advantageously be connected, on the outside of the package of the semiconductor device, for example to a printed-circuit board (PCB).

(77) 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.).

(78) 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.