Electronic chip comprising strained transistors

Abstract

Electronic chip comprising strained transistors The present description relates to a method for manufacturing an electronic chip comprising the successive steps of:- providing a semiconductor layer (120) located on an insulator (130) covering a semiconductor substrate (110);- oxidizing first and second portions of the semiconductor layer down to the insulator, so as to form first oxidized portions (140) and second oxidized portions (150) on the insulator; - generating stresses (310) in a third portion (210) of the semiconductor layer not crossed by the first and second oxidized portions, the third portion extending continuously between the second oxidized portions (150);- forming cavities (410) extending at least to the semiconductor substrate through the second oxidized portions and the insulator; and- forming first field effect transistors in and on the third portion (210).Figure for abstract: Fig. 4A.

Description

Title of the invention: Electronic chip comprising strained transistors Technical field

[0001] The present description relates generally to the field of electronic chips and more particularly to the field of electronic chips comprising transistors with a constrained channel region, and their manufacturing methods.

[0002] The present description may, for example, relate to electronic chips further comprising a phase change memory circuit, and their manufacturing methods. Prior art

[0003] A strained channel region transistor, or strained transistor, is a field effect transistor in which a channel-forming semiconductor region has mechanical constraints. The presence of constraints in the channel-forming region (channel region) makes it possible to increase the speed of the transistor. This type of transistor is generally used in digital circuits.

[0004] A phase change memory is a type of memory in which a phase change material can assume two states, typically crystalline and amorphous states, each corresponding to a stored bit value. Typically, the memory consists of phase change memory points each storing the value of a bit.

[0005] There is a need for improvement of electronic chips comprising transistors with a strained channel region, and in some cases also a phase change memory circuit, as well as methods of manufacturing such electronic chips.

[0006] In particular, there is a need to improve, or not to relax, the constraints in the transistors with a constrained channel region, in particular during the manufacturing process of the electronic chips. There is a need to improve the performance of the transistors with a constrained channel region of the electronic chips. Summary of the invention

[0007] One embodiment overcomes all or part of the drawbacks of known electronic chips.

[0008] One embodiment overcomes all or part of the drawbacks of known methods for manufacturing electronic chips.

[0009] One embodiment provides a method of manufacturing an electronic chip, the method comprising the successive steps of: - providing a semiconductor layer located on an insulator covering a semiconductor substrate; - oxidizing first and second portions of the semiconductor layer to the insulator, so as to form first oxidized portions and second oxidized portions on the insulator; - generating stresses in a third portion of the semiconductor layer not crossed by the first and second oxidized portions, the third portion extending continuously between the second oxidized portions; - forming cavities extending at least to the semiconductor substrate through the second oxidized portions and the insulator; and - forming first field effect transistors in and on the third portion.

[0010] One embodiment provides an electronic chip comprising: - first, second and third portions of a semiconductor layer located on an insulator covering a semiconductor substrate, the first and second portions of said semiconductor layer being oxidized to the insulator, the third portion being constrained, not being crossed by the first and second oxidized portions, and extending continuously between first parts of the second oxidized portions; - cavities extending at least to the semiconductor substrate through the second oxidized portions and the insulator; and - first field effect transistors located in and on the third portion.

[0011] According to one embodiment, fourth portions of the semiconductor layer are formed next to the third portion, each fourth portion being between two of the first oxidized portions or between one of the first oxidized portions and one of the second oxidized portions; and - second field effect transistors are formed in and on the fourth portions.

[0012] According to one embodiment, first grids are formed above the third and fourth portions.

[0013] According to one embodiment, second grids are formed above the first oxidized portions and insulating grids are formed in the extension of the second grids above the third portion.

[0014] According to one embodiment, the fourth portions are positioned next to the third portion in the width direction of the field effect transistors.

[0015] According to one embodiment, the first transistors are constrained P-channel, and the second transistors are N-channel, for example unconstrained N-channel.

[0016] According to one embodiment, insulating trenches extend through the semiconductor layer and the insulator to a level located within the semiconductor substrate, the insulating trenches comprising a first insulating trench between the third portion and the fourth portions.

[0017] According to one embodiment, the stress of the third portion results from a modification of the composition of the semiconductor layer in said third portion, for example comprising the formation of a silicon-germanium layer on the third portion, followed by a heat treatment.

[0018] According to embodiments: - the first oxidized portions are substantially parallel to each other, for example parallel to the direction of the width of the field effect transistors; and / or - the second oxidized portions are substantially parallel to each other, for example parallel to the direction of the width of the field effect transistors; and / or - the first oxidized portions are positioned between at least two of the second oxidized portions.

[0019] According to one embodiment, the semiconductor layer is a silicon layer.

[0020] According to one embodiment, the first transistors are of the FDSOI type.

[0021] According to one embodiment, bipolar transistors are formed in at least a first part of the cavities, and phase change memory points connected to the bipolar transistors are formed.

[0022] According to one embodiment, doped emitter, base and collector semiconductor zones of the bipolar transistors are formed in first epitaxially grown semiconductor portions in the at least a first part of the cavities.

[0023] According to one embodiment, third gates are formed on second parts of the second oxidized portions between the bipolar transistors.

[0024] According to one embodiment, additional field effect transistors are formed in and on second epitaxially grown semiconductor portions in a second part of the cavities. Brief description of the drawings

[0025] These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

[0026] [Fig.1A] represents, by partial and schematic views from above and in section, a structure obtained at a stage of an embodiment of a method of manufacturing an electronic chip;

[0027] [Fig.lB] represents two partial and schematic sectional views of the structure of [Fig.lA];

[0028] [Fig.2A] represents, by partial and schematic views from above and in section, a structure obtained at another stage of the manufacturing process;

[0029] [Fig.2B] represents two partial and schematic sectional views of the structure of [Fig.2A];

[0030] [Fig.3A] represents, by partial and schematic views from above and in section, a structure obtained at another stage of the manufacturing process;

[0031] [Fig.3B] represents two partial and schematic sectional views of the structure of [Fig.3A];

[0032] [Fig.4A] represents, by partial and schematic views from above and in section, a structure obtained at another stage of the manufacturing process;

[0033] [Fig.4B] represents two partial and schematic sectional views of the structure of [Fig.4A];

[0034] [Fig.5A] represents, by partial and schematic views from above and in section, a structure obtained at another stage of the manufacturing process;

[0035] [Fig.5B] represents two partial and schematic sectional views of the structure of [Fig.5A];

[0036] [Fig.6A] represents, by partial and schematic views from above and in section, a structure obtained at another stage of the manufacturing process; and

[0037] [Fig.6B] represents two partial and schematic sectional views of the structure of [Fig.6A]. Description of the embodiments

[0038] The same elements have been designated by the same references in the different figures. In particular, the structural and / or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

[0039] For the sake of clarity, only the steps and elements useful for understanding the described embodiments have been shown and are detailed. In particular, steps for manufacturing phase-change memory points are not described in detail, the described embodiments being compatible with the usual steps for manufacturing phase-change memory points. Similarly, steps for manufacturing bipolar or field-effect transistors are not described in detail, the described embodiments being compatible with the usual steps for manufacturing bipolar or field-effect transistors.

[0040] Unless otherwise specified, when referring to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") between them, this means that these two elements can be connected or linked through one or more other elements.

[0041] In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made unless otherwise specified to the orientation of the sectional views.

[0042] Unless otherwise specified, the expressions "about", "approximately", "substantially", and "of the order of" mean to within 10% or 10°, preferably to within 5% or 5°.

[0043] Unless otherwise specified, ordinal numeral adjectives, such as "first", "second", etc., are used only to distinguish elements from one another. In particular, these adjectives do not limit the devices and methods described to a particular order of these elements.

[0044] Figures 1A, 2A, 3A, 4A, 5A and 6A each represent, by partial and schematic top views T and in section SA along a plane AA, a structure obtained at a step of an embodiment of a method for manufacturing an electronic chip. Views T and SA correspond.

[0045] Figures 1B, 2B, 3B, 4B, 5B and 6B each represent two partial and schematic sectional views of the respective structures of Figures 1A, 2A, 3A, 4A, 5A and 6A. Each of Figures 1B and 2B represents a sectional view SB along a plane BB parallel to the plane AA and a sectional view SC along a plane CC orthogonal to the planes AA and BB. The views SB and SC correspond. Each of Figures 3B, 4B, 5B and 6B represents a sectional view SB along the plane BB and a sectional view SD along a plane DD parallel to the plane CC. The views SB and SD correspond.

[0046] By electronic chip, or chip, is meant a portion of semiconductor substrate 110 or semiconductor wafer, and electronic circuits located in and on the semiconductor substrate 110. The chip obtained at the end of the method below will comprise, in a DIG part (digital part) of the chip, one or more digital circuits comprising constrained transistors, and in a PCM part of the chip, a phase change memory (in English "Phase Change Memory"). Preferably, a PW part of the chip will comprise other transistors. These other transistors may be used in power supply circuits intended to provide a voltage to the digital circuits of the chip, from a supply voltage applied to the chip to make it operate.

[0047] Although the DIG, PCM and possible PW parts are juxtaposed in the example shown, these parts may be separated in other examples. In In yet other examples, the chip may include multiple DIG parts and / or multiple PCM parts and / or multiple PW parts.

[0048] In the step of figures 1A and 1B, a semiconductor layer 120 is provided located on an electrical insulator 130 (or dielectric) covering the substrate 110. The semiconductor layer 120, the insulator 130 and the semiconductor substrate 110 can thus constitute a structure called silicon on insulator, SOI (from the English "Silicon On Insulator").

[0049] Preferably, the semiconductor layer 120 is a silicon layer, and the insulator 130 is a silicon oxide layer. The substrate 110 may be a portion of a silicon wafer. For example, the semiconductor layer 120 has a thickness of between 3 nm and 10 nm.

[0050] First and second portions, respectively 140 and 150, of the semiconductor layer 120 have been oxidized, forming first oxidized portions 140 and second oxidized portions 150. More precisely, the first and second portions of the semiconductor layer 120 are oxidized over the entire thickness of the semiconductor layer 120. For this, any usual step of oxidation of a semiconductor layer can be used, typically thermal oxidation. This step, being usual, is not described here in detail. In particular, a mask protecting from oxidation the portions of the semiconductor layer 120 that it is not desired to oxidize in this step is not shown. Due to the oxidation, the oxidized portions have a thickness greater than the thickness of the semiconductor layer 120.

[0051] The first oxidized portions 140 may be located in the digital part DIG of the future chip. Preferably, the first oxidized portions 140 have a direction of elongation in the width, corresponding to the direction of the CC cut. The first oxidized portions 140 then form oxide bars parallel to each other, which may have an electrical insulation function. The first oxidized portions 140 do not extend across the entire width of the semiconductor layer 120. The first oxidized portions 140 do not extend into a future strained portion of the semiconductor layer 120. Thus, in the example shown, the first oxidized portions 140 are seen in view T of [Fig.lA] in the lower half, but not in the upper half, and the first oxidized portions 140 are seen in view SB of [Fig.lB], but not in view SA of [Fig.lA], the AA cut being made in a region where there are no first oxidized portions. Although two first oxidized portions 140 are formed in the example shown, preferably, more than two first portions 140 are oxidized.

[0052] The second oxidized portions 150 may be located partly in the PCM part intended to contain the phase change memory. In the example where a PW portion is provided, the second oxidized portions 150 may be located at least partly in the PCM portion and in the PW portion.

[0053] Preferably, in the PCM portion and, optionally, the PW portion, the entirety of the semiconductor layer 120 is oxidized. In other words, the second oxidized portions 150 occupy the entirety of the PCM and optionally PW portions.

[0054] Second oxidized portions 150 may also be provided in the DIG portion, for example on two opposite sides of the DIG portion. In the example shown, the second oxidized portions 150 cover the entirety of the PCM and PW portions and extend onto the DIG portion.

[0055] The first oxidized portions 140 may be positioned between the second oxidized portions 150.

[0056] The first and second oxidized portions can form very shallow isolation trenches (SSTI, from the English "Super Shallow Trench Isolation"), to isolate future transistors from each other.

[0057] In the step of Figures 2A and 2B, silicon-germanium 220 was formed by epitaxy on a third non-oxidized portion 210 of the semiconductor layer 120. The third portion 210 is included in, for example corresponds to, the region of the semiconductor layer which does not include first oxidized portions 140.

[0058] More precisely, the epitaxy is carried out on the side of the upper face of the structure (in the orientation of the sectional views), also called the front face, which corresponds to the upper face of the semiconductor layer 120. For example, the thickness of the epitaxially grown silicon-germanium 220 is between 5 and 20 nm.

[0059] The third portion 210 extends between the second oxidized portions 150, that is to say extends from one of the second oxidized portions 150 to another of the second oxidized portions 150. In other words, the third portion 210 is delimited, on each of two opposite sides by one of the second oxidized portions 150. Furthermore, the third portion 210 extends continuously between the second oxidized portions 150. By continuous, it is meant that the third portion is not crossed by first oxidized portions.

[0060] The third portion 210 is located, in top view, in a first strip 240. Another third portion, not shown, may be located in another strip parallel to the first strip 240. Other third portions may each be located in another strip parallel to the first strip 240.

[0061] During epitaxy, a mismatch between crystal lattices causes, in the epitaxially grown silicon-germanium 220, compressive stresses parallel to the front face (i.e. horizontal in the orientation of the sectional views).

[0062] Preferably, next to the third portion 210, for example next to it in width, the semiconductor layer 120 comprises fourth non-oxidized portions 230 and not covered with epitaxially grown silicon-germanium. The fourth portions 230 are included in, for example correspond to, a region of the semiconductor layer crossed by the first oxidized portions 140.

[0063] Each fourth portion 230 extends between some of the first and second oxidized portions, i.e. extends from one of the first and second oxidized portions 140 and 150 to another of the first and second oxidized portions 140 and 150. In other words, each of the fourth portions 230 is delimited, on two opposite sides, by two of the first and second oxidized portions 140 and 150. The fourth portions 230 are located, in top view, in a second strip 250 parallel to the first strip 240. The first and second strips 240 and 250 are located side by side, for example side by side in width. The second band 250 is substantially perpendicular to the first oxidized portions 140. Other fourth portions, not shown, may be located in another band parallel to the second band 250. Other fourth portions may be located in other bands parallel to the second band 250.

[0064] The fourth portions 230, not intended to be covered with epitaxial silicon-germanium, may be covered, during epitaxy, with any usual mask suitable for protecting at least these fourth portions, for example a mask suitable for protecting regions of the semiconductor layer 120 comprising the first and second oxidized portions 140, 150 and the fourth portions 230 while leaving the third portion 210 uncovered, so that the epitaxial silicon-germanium grows from the third portion but does not grow from the fourth portions.

[0065] In the step of Figures 3A and 3B, a heat treatment has been carried out, so as to oxidize the epitaxially grown silicon-germanium. Generally, the regions which are not to be treated are protected by a protective layer, for example a silicon nitride (SiN) layer. The silicon of the silicon-germanium 220 oxidizes preferentially, and the germanium of the silicon-germanium 220 migrates towards the third portion 210 of the semiconductor layer 120. This results in an intensification and a transfer into the third portion 210 of the stresses initially present in the silicon-germanium 220.

[0066] As a result, compressive stresses 310 have been generated in the third portion 210. The compressive stresses 310 are in both horizontal directions (in the orientation of the sectional views). To generate the stresses 310, the composition of the third portion 210 has been modified. In other words, the third portion 210 acquires a different composition from the fourth portions 230, namely, preferably, the third portion 210 is made of silicon-germanium, and the fourth portions 230 are made of silicon.

[0067] The fact that the third portion 210 is not crossed by the first oxidized portions 140 makes it possible to avoid a relaxation of the constraints 310, a relaxation which could occur if the first oxidized portions crossed the third portion. Furthermore, if the first oxidized portions were present within the third portion, all or part of the oxide of these first portions could be consumed during the epitaxy of silicon-germanium and during the removal of the silicon oxide layer obtained by the heat treatment, which would at least partially cancel the insulation function of these first portions.

[0068] Preferably, the front face of the structure is then cleaned, so as to remove what remains of the silicon oxide layer obtained during the heat treatment.

[0069] The particular example described above of generating stresses in the third portion 210 is not limiting. The embodiments described are compatible with the usual methods of generating stresses in one or more portions of a semiconductor layer.

[0070] In the step of FIGS. 4A and 4B, cavities 410 extending at least as far as the substrate 110 through the second oxidized portions 150 and the insulator 130 are etched from the front face of the semiconductor layer 120, in particular from the front faces of the second oxidized portions 150. The cavities 410 reach the substrate 110 and can penetrate into the substrate 110.

[0071] Preferably, in the PCM part intended to comprise the future phase change memory, stacks 430 of parts 130A of the insulator 130 and of parts 150A (second parts) of the second oxidized portions 150 are left in place. The cavities 410 delimit the stacks 430.

[0072] Although only two stacks 430 are provided in the example shown, the number of stacks 430 is preferably greater than two. Preferably, the stacks 430 have, in top view, strip shapes parallel to the first oxidized portions 140. The stacks 430 then form electrically insulating bars.

[0073] Preferably, in the DIG part intended to comprise the future constrained transistors, parts 150B (first parts) of the second oxidized portions 150 are also left in place against the third portion 210 and the fourth portions 230. In other words, the third and fourth portions 210 and 230 are delimited in the length, corresponding to the direction of the sections AA and BB, by the parts 150B of the second oxidized portions 150.

[0074] A semiconductor was then formed by epitaxy in the cavities 410. The epitaxially grown semiconductor is typically the same as that of the substrate 110, namely, preferably, silicon. Each cavity 410 is filled with an epitaxially grown semiconductor portion 420. Preferably, the epitaxially grown semiconductor portions 420 reach a level located above (in the orientation of the sectional views) the upper level of the third portion 210 and the upper level of the fourth portions 230. In other words, the epitaxially grown semiconductor portions 420 exceed the levels of the front faces of the third and fourth portions 210 and 230. In the PCM part, the epitaxially grown semiconductor portions 420 are electrically isolated from each other by the stacks 430.

[0075] In the step of Figures 5A and 5B, preferably, all the elements of the structure of Figures 4A and 4B located above the level of the upper face of the fourth portions 230 have been removed. For this, a usual step of chemical-mechanical polishing can be implemented.

[0076] Preferably, insulating trenches 510 have been formed, i.e. trenches filled with an electrical insulator, preferably silicon oxide. In other words, a step of etching the trenches is successively implemented, followed by a step of filling with the electrical insulator. These steps are not described here in detail, the embodiments described being compatible with the usual etching and filling steps for forming insulating trenches. The insulating trenches may form shallow trench isolation (STI) trenches. Preferably, the insulating trenches 510 separate the DIG and PCM portions, and the DIG and PW portions, from each other.

[0077] The insulating trenches 510 extend from the front face of the semiconductor layer 120, pass through the level occupied by the first, second, third and fourth portions 140, 150, 210, 230 of the semiconductor layer 120, and through the insulator 130, to a level located inside the substrate 110.

[0078] The locations of the insulating trenches 510 are chosen such that the insulating trenches 510 surround (in top view) regions of the chip. The etching of the insulating trenches 510 leaves in place a central portion of each of the third and fourth portions 210 and 230, in other words, the insulating trenches 510 reduce the dimensions of the third and fourth portions. The etching leaves in place portions 140A of the first oxidized portions 140 and portions 430A of the stacks 430.

[0079] Among the regions of the chip surrounded by the insulating trenches 510, one or more regions 540 located in the PCM part comprise semiconductor portions 420A (first epitaxially grown semiconductor portions) constituted by a part of the epitaxially grown semiconductor portions 420. Doped semiconductor zones respectively of emitter, base and collector (not shown in detail) of a bipolar transistor 545 are formed in each of the semiconductor portions 420A. These zones can be formed by doping during epitaxy, or, preferably, by doping the semiconductor portions 420A. bipolar transistors 545 are for example of the NPN type. The doped zones of the bipolar transistors 545 are insulated from each other by the parts 430A of the stacks 430.

[0080] Among the regions surrounded by the insulating trenches 510, a region 520 located in the DIG part comprises the central part of the third portion 210. Preferably, the region 520 comprises one or more semiconductor portions 420C constituted by a part of the epitaxially grown semiconductor portions 420. The semiconductor portions 420C of the region 520 are electrically insulated from the third portion 210 by parts 150C of the second oxidized portions 150. The parts 150C correspond to the parts 150B of the second oxidized portions 150 which remained in place in the region 520 after formation of the insulating trenches 510.

[0081] Among the regions surrounded by the insulating trenches 510, a region 530 located in the DIG part comprises the parts 140A of the first oxidized portions 140 and the central part of the fourth portions 230. Preferably, the region 530 comprises one or more semiconductor portions 420D constituted by a part of the epitaxially grown semiconductor portions 420. The semiconductor portions 420D of the region 530 are isolated from the fourth portions 230 by parts 150D of the second oxidized portions 150. The parts 150D correspond to the parts 150B of the second oxidized portions 150 which remained in place in the region 530 after formation of the insulating trenches 510.

[0082] Preferably, among the insulating trenches 510, an insulating trench 510A extends, in top view, parallel to the first and second strips 240 and 250 and straddling the strips 240 and 250. The insulating trench 510A is thus located straddling the locations of the third portions 210 and the fourth portions 230, or between the central parts of the third and fourth portions.

[0083] The insulating trenches 510 are preferably formed after generation of the stresses 310. As a result, the third portion 210 can elongate in the insulating trench 510A before filling the trenches with the insulator, which releases the stresses 310 in the direction orthogonal to the first strip 240. However, due to the absence of the first oxidized portions 140 in the third portion 210, as explained further above, it is advantageously avoided that the stresses in the direction of the first strip 240 are released.

[0084] Thus, after formation of the insulating trenches 510, 510A, the third portion 210 has compressive stresses 310L in the longitudinal direction of the first strip 240, and are substantially not stressed in the transverse direction of the first strip 240.

[0085] Preferably, among the insulating trenches 510, an insulating trench 510B is located relative to the third portion 210 on the side opposite the insulating trench 510A, extends parallel to the first strip 240 and delimits the third portion 210. Thus, the region 520 is located in a central portion 240A of the first strip 240.

[0086] Preferably, among the insulating trenches 510, an insulating trench 510C is located relative to the fourth portions 230 on the side opposite the insulating trench 510A, extends parallel to the second strip 250 and delimits the fourth portions 230. Thus, the region 530 is located in a central part 250A of the second strip 250.

[0087] Preferably, among the regions of the chip surrounded by the insulating trenches 510, a region 550 located in the PW part comprises semiconductor portions 420B (second epitaxially grown semiconductor portions) constituted by a part of the epitaxially grown semiconductor portions 420.

[0088] In the example shown, the insulating trenches 510A, 510B and 510C, which extend parallel to the central portions 240A, 250A of the first and second strips 240, 250, extend into the PCM and PW portions. As a result, the PCM portion includes a region 540 in each of the central portions 240A, 250A, and the PW portion includes a region 550 in each of the central portions 240A, 250A.

[0089] In the step of Figures 6A and 6B, field effect transistors 610 are formed in and on the third portion 210. More specifically, gates 620 (first gates) are formed on the third portion 210, preferably elongated orthogonally to the direction of the first strip 240. Since there are no first oxidized parts 140 within the third portion 210, the third portion can form a continuous active zone ("continuons RX" in English). The transistors 610 can then be isolated from each other by isolation gates 650 ("gate-tie", or "tying gate" in English) positioned above the third portion and in the extension of the first oxidized parts 140 in the width direction. Between two isolation gates 650, the third portion 210 may comprise several transistors 610, for example transistors having a common drain-source terminal.Preferably, the grids 620, 650 are regularly spaced, that is to say that substantially the same space can be included between two grids 620 or between a grid 620 and an insulation grid 650.

[0090] The parts of the third portion 210 located under the gates 620 constitute channel-forming regions of the transistors 610. Due to the presence of the constraints 310L, the transistors 610 are strained transistors. Preferably, the parts of the third portion 210 located on either side of the channel-forming regions are P-type doped to form drain and source regions of the transistors 610. The transistors 610 are thus P-channel transistors (PMOS). In the channel-forming region, the presence of compressive constraints in the drain-source direction (length direction of the transistors) and, substantially, the absence of constraint in the width direction of the transistors makes it possible to obtain particularly fast 610 P-channel transistors.

[0091] Preferably, the grids 620 also extend above the fourth portions 230.

[0092] Preferably, field effect transistors 630 are formed in and on the fourth portions 230. More specifically, the portions of the fourth portions 230 located under the gates 620 constitute channel-forming regions of the transistors 630. The transistors 630 may be isolated from each other by the first oxidized portions 140 which may form very shallow isolation trenches (SSTI). In addition, gates 625 (second gates) are formed above the first oxidized portions 140 between the fourth portions 230. Thus, the isolation gates 650 are positioned in the extension of the gates 625 in the width direction, but are preferably separated from the gates 625 by a gap. Preferably, the parts of the fourth portions 230 located on either side of the channel forming regions are N-type doped to form drain and source regions of the transistors 630.The 630 transistors are thus N-channel transistors (NMOS), and allow, with the 610 P-channel transistors, to form digital circuits. The 630 transistors are not transistors with a constrained channel forming region.

[0093] Preferably, the transistors 610 and 630 are of the fully depleted SOI (FDSOI) type. By FDSOI transistor, it is meant here that the thickness of their channel forming region is less than 10 nm.

[0094] The other elements of the field effect transistors are not described, the embodiments described being compatible with the usual methods of forming field effect transistors on a portion of a semiconductor layer. In particular, it may be provided that conductive zones in electrical contact with the drain and source regions, that is to say contact zones or contact points, are formed, for example epitaxially, on the third portion 210 and on the fourth portions 230, at least on either side of the gates 620. Furthermore, preferably, the gates are insulated from the semiconductor layer 120 by a gate insulator.

[0095] Preferably, in the PCM part, phase-change memory points 640 have been formed. The memory points 640 are for example located in insulating layers, not shown, covering the structure. Each memory point is connected, preferably connected, to one of the bipolar transistors 545. More precisely, the memory point 640 and the bipolar transistor 545 are electrically in series. For each memory point 640, a via 645 can connect the memory point 640 and the associated bipolar transistor 545.

[0096] In the chip in operation, during a step of writing and / or reading the phase change memory, the memory point(s) 640 concerned by the writing or reading are selected by turning on the bipolar transistor(s) 545 in series with this or these memory points 640. The bipolar transistors 545 are thus called selection transistors.

[0097] In the chip 600 obtained by the above method, the electrical insulations between the NMOS transistors 630, constituted by the parts 140A of the first oxidized portions 140, and the electrical insulations between the bipolar transistors 545 for selecting the phase-change memory, constituted by the parts 150A of the second oxidized portions 150, result from a single step of oxidation of the semiconductor layer 120. Compared to a method in which the electrical insulators would have been obtained at different steps, this makes it possible to reduce the number of steps for forming these insulators.

[0098] Furthermore, the method allows, by not forming first oxidized portions in the third portion, as mentioned above, that the stresses 310L are not released in the length direction of the stressed PMOS transistors 610. This allows for higher mechanical stresses in the channel forming region of these PMOS transistors, and can thus allow for better performance of the PMOS transistors. Thus, the PMOS transistors 610 are not isolated from each other by SSTI type trenches, and the third portion 210 forms a continuous active area, the PMOS transistors 610 being able to be isolated from each other by the isolation gates 650.

[0099] Preferably, in the case where the semiconductor portions 420C and 420D have been provided in the respective regions 520 and 530, these portions are doped, at this step or at a previous step, with the same conductivity type (among the two types N and P) as the substrate 110 or as a doped well formed under the insulator 130 under the respective regions 520 and 530. In the case where wells (not shown) are formed under the respective regions 520 and 530, these wells can be electrically isolated from each other by means of the insulating trench 510A.

[0100] Each of the semiconductor portions 420C and 420D can then be surmounted by a contact point, forming polarization zones of the substrate ("bulk strap", or "bulk tap" in English). The application of a potential to the contact point makes it possible to polarize the substrate 110 or the well under the region 520, 530 concerned. This polarization has the role of electrostatically controlling the respective transistors 610, 630, through the insulator 130. Such control is desirable, for example, to modify the threshold voltage of the transistors 610, 630 according to a desired compromise between speed and consumption of the transistors.

[0101] As indicated above, the semiconductor portions 420C of the region 520 are isolated from the third portion 210 by the parts 150C of the second portions oxidized 150, and the semiconductor portions 420D of the region 530 are isolated from the fourth portions 230 by the parts 150D of the second oxidized portions 150. The second oxidized portions 150 form very shallow type isolations (SSTI), so that regions 111A and 111B, respectively forming the wells of the transistors 610 and 630, can be biased independently because they are isolated from each other by the insulating trenches 510A.

[0102] Preferably, at the same time as the gates 620, gates 620A (third gates) have been formed on the stacks 430. In the case where electrical contact zones with the emitter, base and / or collector regions of the bipolar transistors 545 are formed, for example epitaxially, on the semiconductor portions 420A, the gates 620A make it possible to form these zones without risking unwanted conductive bridges forming on the stacks 430 and causing short circuits between neighboring bipolar transistors 545.

[0103] Preferably, in the PW portion, a transistor 660, having a gate 620B, has been further formed in and on the semiconductor portion 420B. In other words, the transistor 660 has a channel-forming region located in a portion of the epitaxially grown portion 420 located under the gate 620B and separated from the gate 620B by a gate insulator (not shown). Preferably, the transistor 660 has a gate insulator thickness greater than that of the transistors 610 and 630. This allows the transistor 660 to have a maximum gate-source voltage (beyond which the transistor could be damaged) greater than that of the transistors 610 and 620. The gate 620B is preferably formed at the same time as the gates 620 and 620A.

[0104] Various embodiments and variations have been described. Those skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to those skilled in the art. In particular, an electronic chip with a phase-change memory circuit comprising memory points connected to bipolar transistors has been described, but the embodiments may be applied to other electronic chips, which do not necessarily include points of non-volatile memory.

[0105] Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art from the functional indications given above.

Claims

Claims

1. A method of manufacturing an electronic chip (600), the method comprising the successive steps of: - providing a semiconductor layer (120) located on an insulator (130) covering a semiconductor substrate (110); - oxidizing first and second portions of the semiconductor layer down to the insulator, so as to form first oxidized portions (140) and second oxidized portions (150) on the insulator; - generating stresses (310) in a third portion (210) of the semiconductor layer not crossed by the first and second oxidized portions, the third portion extending continuously between the second oxidized portions (150); - forming cavities (410) extending at least to the semiconductor substrate through the second oxidized portions and the insulator; and - forming first field effect transistors (610) in and on the third portion (210).

2. An electronic chip (600) comprising: - first (140), second (150) and third (210) portions of a semiconductor layer (120) located on an insulator (130) covering a semiconductor substrate (110), the first and second portions of said semiconductor layer being oxidized to the insulator, the third portion (210) being constrained, not being crossed by the first and second oxidized portions, and extending continuously between first parts (150B) of the second oxidized portions; - cavities (410) extending at least to the semiconductor substrate through the second oxidized portions and the insulator; and - first field effect transistors (610) located in and on the third portion (210).

3. The method of claim 1, wherein: - fourth portions (230) of the semiconductor layer (120) are formed adjacent to the third portion (210), each fourth portion being between two of the first oxidized portions (140) or between one of the first oxidized portions and one of the second oxidized portions (150); and - second field effect transistors (630) are formed in and on the fourth portions.

4. The method of claim 3, wherein first grids (620) are formed above the third and fourth portions.

5. The method of claim 3 or 4, wherein second grids (625) are formed above the first oxidized portions (140) and insulating grids (650) are formed in the extension of the second grids above the third portion (210).

6. A method according to any one of claims 3 to 5, wherein the fourth portions (230) are positioned adjacent to the third portion (210) in the width direction of the field effect transistors.

7. A method according to any one of claims 3 to 6, wherein the first transistors (610) are constrained P-channel, and the second transistors (630) are N-channel, for example unconstrained N-channel.

8. The method of any one of claims 3 to 7, wherein insulating trenches (510) extend through the semiconductor layer (120) and the insulator (130) to a level within the semiconductor substrate (110), the insulating trenches comprising a first insulating trench (510A) between the third portion (210) and the fourth portions (230).

9. A method according to any one of claims 1, 3 to 8, wherein the stress (310) of the third portion (210) results from a modification of the composition of the semiconductor layer (120) in said third portion, for example comprising the formation of a silicon-germanium layer (230) on the third portion, followed by a heat treatment.

10. Method according to any one of claims 1 and 3 to 9, in which: - the first oxidized portions are substantially parallel to each other, for example parallel to the direction of the width of the field effect transistors; and / or - the second oxidized portions are substantially parallel to each other, for example parallel to the direction of the width of the field effect transistors; and / or - the first oxidized portions are positioned between at least two of the second oxidized portions.

11. A method according to any one of claims 1 and 3 to 10, wherein the semiconductor layer (120) is a silicon layer.

12. Method according to any one of claims 1 and 3 to 11, wherein the first transistors (610) are of the FDSOI type.

13. A method according to any one of claims 1 and 3 to 12, wherein bipolar transistors (545) are formed in at least a first portion of the cavities (410), and phase-change memory points (640) connected to the bipolar transistors are formed.

14. The method of claim 13, wherein doped emitter, base, and collector semiconductor regions of the bipolar transistors are formed in first epitaxially grown semiconductor portions (420A) in the at least a first portion of the cavities (410).

15. The method of claim 13 or 14, wherein third gates (620A) are formed on second portions (150A) of the second oxidized portions (150) between the bipolar transistors (545).

16. A method according to any one of claims 1 and 3 to 15, wherein additional field effect transistors (660) are formed in and on second epitaxially grown semiconductor portions (420B) in a second portion of the cavities (410).

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