Patent Description:
Integrated circuits typically comprise power rails (e.g. for VSS and VDD supply voltage distribution). Conventionally, power rails are encapsulated within a back-end-of-line (BEOL) interconnect structure located above the level of the active physical devices (e.g. transistors). In contrast, a "buried" power rail (BPR) is at least partly lowered into the substrate, such that the power rail may be located at a level below the active physical devices. Burying power rails enables the cross-section of the power rails to be increased (e.g. for reduced line resistance) without occupying valuable space in the BEOL interconnect structure which can be used for other purposes (e.g. signal lines). As an example in the context of finFET technology, BPR formation may involve etching trenches in the substrate at positions between pairs of adjacent fins. The trenches may subsequently be filled with metal to form the BPRs.

<CIT> discloses a prior art method of manufacturing a semiconductor device includes forming a pillar-shaped active region by etching a portion of a semiconductor substrate, forming a blocking film selectively exposing a sidewall of a lower portion of the pillar-shaped active region, and forming a bit-line selectively on the exposed sidewall of the lower portion of the pillar-shaped active region.

It is envisaged that use of the BPR will be challenging in future smaller technology nodes, since it may be difficult to maintain a sufficiently low line resistance when the BPR line width (the critical dimension, CD, of the BPR) is reduced. The effect of reduced BPR CD on line resistance may be offset by using metals providing lower resistance and/or by increasing the depth of the BPR trenches (and correspondingly the height of the BPRs), however only to some extent: The available space for metal fill is already small when the BPR CD is scaled to <NUM> or less and increased trench depths is associated with a corresponding increase of trench aspect ratio, which eventually may make trench etching and also trench filling more challenging.

In view of this, it is an objective of the present inventive concept to provide an improved method for forming buried power rails, or more generally buried metal lines. More specifically, it is an objective to enable formation of buried metal lines with improved or at least maintained line resistance without requiring an increase of the substrate surface area occupied. Further and alternative objectives may be understood from the following.

According to an aspect of the present inventive concept there is provided a method for forming a buried metal line in a substrate, the method comprising:.

The inventive method allows an upper trench portion CD to be decoupled from a lower trench portion width, such that a lower trench portion may be widened below the level of the base of the semiconductor structures. In particular, the lower trench portion may be formed with a width exceeding a separation between the pair of semiconductor structures (e.g. a semiconductor fin or pillar spacing). A wider lower trench portion allows an increased width / cross-section of the metal line, which in turn allows a line resistance to be improved. A related advantage is that a wider metal line may provide a greater landing area for a back-side through-silicon via (TSV) contacting approach.

By the spacer masking the sidewall surfaces of the upper trench portion during the etching of the lower trench portion, the lower trench portion may be widened with a reduced risk of the etching extending into the base portions of the semiconductor structures from below. This contributes advantageously to the afore-mentioned decoupling of the upper trench portion CD and the width of the lower trench portion.

Furthermore, the relaxed CD implied by the wider lower trench portion may facilitate filling the trench with metal during the metal line formation.

The method is applicable to forming buried metal lines in any applications where line resistance and/or back-side TSV landing area is of importance. As may be appreciated, the method is particularly useful for forming a BPR, where line resistance typically is a major design consideration.

The method is particularly useful for forming a BPR, where line resistance typically is a major design consideration.

As used herein, the term "buried metal line" is used to refer to a metal line structure which is at least partially embedded in the substrate. As will be further set out herein, the metal line may be formed with a height less than a height (i.e. depth) of the metal line trench, wherein the metal line may be completely embedded / buried in the substrate. The metal line may also be formed with a height exceeding a height of the metal line trench, wherein the metal line may be partially embedded / buried in the substrate.

The pair of semiconductor structures may be formed by a pair of semiconductor bodies, such as a pair of semiconductor fins (e.g. for finFETs) or a pair of horizontal semiconductor nanowire or nanosheet stacks (e.g. for horizontal nanowire or nanosheet FETs).

As may be appreciated, the pair of semiconductor structures may comprise a pair of mutually facing sidewall surfaces (i.e. a pair of sidewalls surfaces in a mutually facing relationship), which in the following may be referred to as the pair of mutually facing sidewall surfaces of the pair of semiconductor structures. The pair of mutually facing sidewall surfaces of the pair of semiconductor structures may be formed on mutually opposite sides of the metal line trench to be formed.

Reference may herein be made to a "vertical" direction to denote a direction along a normal to the substrate (i.e. a normal to a main/upper surface of the substrate). Meanwhile, "vertical" qualifiers such as "below" and "above" may be used to refer to relative positions with respect to the vertical direction, and does hence not imply an absolute orientation of the substrate. Accordingly, the term "below" may be used to refer to a relative position closer to a main surface of the substrate. The term "above" may be used to refer to a position farther from a main surface of the substrate. For example, a first level or element located below a second level or element implies that the first level or element is closer to the main surface of the substrate than the second level or element is. Conversely, a first level or element located above a second level or element implies that the first level or element is farther from the main surface of the substrate than the second level or element is.

The term "horizontal" may meanwhile be used to denote a direction or orientation parallel to the substrate (i.e. to a main plane of extension or main surface thereof), or equivalently transverse to the vertical direction. Further, a lateral direction may be understood as a horizontal direction.

The etching of the lower trench portion may comprise a wet etch step. A wet etch step may provide a simultaneous deepening and widening of the trench. As may be understood from the above discussion, the presence of the spacer may counteract etching of the semiconductor structures from below. The spacers may further counteract a widening of the upper trench portion during the isotropic etch step.

The etching of the lower trench portion may further comprise a dry etch step prior to said wet etch step. A "dry" etch step may provide a vertical etching of the substrate, i.e. an etch having a major etch rate component oriented in a thickness direction of the substrate. In a dry etch step, the etching may thus proceed only or at least predominantly in a (downward) vertical direction (i.e. thickness direction) with respect to the substrate. Accordingly, the etching of the lower trench portion may initially proceed predominantly in a thickness direction of the substrate. Thereby, "deepening" the preliminary metal line trench defined by the upper trench portion prior to the widening (and further deepening) wet etching. This may increase an etch margin towards the respective base portions of the semiconductor structures for the subsequent wet etch step.

The etching of the upper trench portion may comprise a dry etch step. Hence, the upper trench portion may proceed predominantly in a thickness direction of the substrate. Advantageously, the upper trench portion may be formed with a width not exceeding a separation between the pair of semiconductor structures.

Forming the spacer may comprise conformally depositing a spacer layer, and vertically etching the spacer layer to expose the bottom surface of the upper trench portion. A thickness of the spacer may hence be precisely controlled via controlling a thickness of the deposited spacer layer. Owing to the conformal deposition, the spacer layer may be deposited on the sidewall surfaces and the bottom surface of the upper trench portion, and along sidewall surfaces of the pair of semiconductor structures (e.g. along the afore-mentioned pair of mutually facing sidewall surfaces of the pair of semiconductor structures). Portions of the spacer layer on horizontally oriented surfaces may subsequently be removed by the vertical etch to expose the bottom surface. Portions of the spacer layer on vertically oriented surfaces may remain to form the spacer. The spacer may act as an etch mask during the etching of the lower trench portion, both for the sidewalls of the upper trench portion and for the semiconductor structures.

The method may further comprise, prior to forming the metal line trench, forming an insulating liner on the pair of semiconductor structures, wherein the insulating liner acts as an etch mask for the semiconductor structures during the etching of the upper trench portion. The liner may thus mask the semiconductor structures during etching of the upper trench portion. The liner may be formed at least on the afore-mentioned pair of mutually facing sidewall surfaces of the pair of semiconductor structures. The subsequently deposited spacer (or spacer layer) may accordingly be formed on the liner.

The method may further comprise removing the spacer prior to forming the metal line. The full width of the upper trench portion may hence be made available for the subsequent metal line formation.

The method may further comprise, prior to forming the metal line, forming an insulating trench liner in the trench. The metal line may thus be electrically isolated from the semiconductor trench sidewall and bottom surfaces by the trench liner. The trench liner may further be formed along sidewall surfaces of the pair of semiconductor structures (e.g. along the afore-mentioned pair of mutually facing sidewall surfaces of the pair of semiconductor structures.

Hence, both the lower and upper trench portions may be used for the metal line. By stopping the etch back at a level above the semiconductor structure base a metal line of further improved line resistance may thus be obtained.

The metal line may be formed with a height exceeding a depth of the metal line trench. This may further reduce a line resistance of the metal line.

The metal line formation may comprise depositing a metal line material in the metal line trench, filling at least the lower trench portion, and advantageously further filling the upper trench portion, at least partly.

According to some embodiments, forming the metal line may comprise:.

The lower trench portion may thus be reliably filled with metal. By depositing the metal selectively on the metal adhesion layer (which has been etched back) a risk of "clogging" in the narrower upper trench portion by deposited metal, before obtaining a fill factor of the wider lower trench portion may be reduced. Especially, a risk of obtaining a metal line with voids in the lower trench portion may be mitigated. The forming of the sacrificial layer allows the metal adhesion layer portions covered by the sacrificial line to be masked, thereby providing accurate control during the metal adhesion layer etch back.

The method may further comprise depositing a metal filling at least the upper trench portion. The full height/depth of the trench may hence be used for the metal line, for the benefit of an improved line resistance.

The metal may be deposited to at least partially fill a space between the pair of semiconductor structures, and subsequently be etched back to a level at or above the respective base of the semiconductor structures. By stopping the etch back at a level above the semiconductor structure base a metal line of further improved line resistance may thus be obtained.

The method may further comprise, prior to forming the metal line trench:.

The insulating layer may accordingly, after opening and during the metal line trench formation and metal line formation, cover the substrate in regions where no metal line trenches are to be formed.

The trench opening may extend through the insulating layer and between the pair of semiconductor structures, more specifically between the afore-mentioned pair of mutually facing sidewall surfaces of the pair of semiconductor structures.

In case an insulating liner is formed on the pair of semiconductor structures prior to forming the insulating layer, the trench opening may be formed by etching the insulating layer selectively with respect to the insulating liner.

An etch mask may be formed on the insulating layer, the etch mask defining an opening above said position between the pair of semiconductor structures. The trench opening may subsequently be formed by etching the insulating layer via the mask opening.

According to a further aspect, there is provided a method for forming a buried metal line in a substrate, the method comprising:.

This method of this further aspect brings about the same advantages as those discussed in connection with the above aspect. Reference is therefore made to the above.

The etching of the substrate via the lower trench portion to widen the lower trench portion may comprise a wet etch step. As may be understood from the above discussion, the presence of the spacer may counteract etching of the semiconductor structures from below. The spacers may further counteract a widening of the upper trench portion during the isotropic etch step.

The etching of the initial trench may comprise a dry etch step. Hence, the upper trench portion may proceed predominantly in a thickness direction of the substrate. Advantageously, the upper trench portion may be formed with a width not exceeding a separation between the pair of semiconductor structures.

Forming the spacer may comprise conformally depositing a spacer layer, and anisotropically etching the spacer layer to expose the upper surface of the sacrificial line.

Methods for forming a metal line trench and a metal line in the metal line trench will now be described with reference to the figures. The figures all schematically show, in cross-section, a (preliminary) semiconductor device <NUM> comprising a substrate <NUM> and a pair of semiconductor structures formed by a pair of semiconductor fins (hereinafter "fins" <NUM>) protruding from the substrate <NUM>. The following methods will be described in relation to a single pair of fins <NUM> and for forming a single buried metal line trench. However, as may be appreciated the method steps may be applied in parallel at a plurality of positions along the substrate, to form a buried metal lines between a plurality of pairs of fins. It may further be noted that the relative dimensions of the shown structures, for instance the relative thickness of layers, is merely schematic and may, for the purpose of illustrational clarity, differ from a physical device structure.

The substrate <NUM> may be a semiconductor substrate, i.e. a substrate comprising at least one semiconductor layer. The substrate <NUM> may be a single-layered semiconductor substrate, for instance formed by a bulk substrate. The substrate may however also be a multi-layered substrate, for instance formed by an epitaxially grown semiconductor layer on a bulk substrate, or a semiconductor-on-insulator (SOI) substrate. The substrate <NUM> may for instance comprise a layer of silicon (Si), germanium (Ge) or silicon germanium (SiGe), to name a few.

As indicated in <FIG>, the fins <NUM> protrude in a vertical direction Z, or equivalently in parallel to a normal direction with respect to the substrate <NUM>. <FIG> further indicates first and second mutually perpendicular horizontal directions X and Y, each extending along the substrate <NUM>. The pair of fins <NUM> are spaced apart along the first horizontal direction Z. A longitudinal direction of the fins <NUM> extends along the second horizontal direction Y. The fins <NUM> extend in parallel to each other. The pair of fins <NUM> is formed by a first and a second fin. The first and second fins may define a pair of mutually facing sidewall surfaces <NUM>. The pair of sidewalls surfaces <NUM> may as shown be located on opposite sides of the metal line trench which is to be formed. The fins <NUM> may for example comprise Si, Ge or SiGe. The semiconductor structures <NUM> may be homogenous, single-layered semiconductor bodies, e.g. patterned in a single semiconductor layer of the substrate <NUM>. The fins <NUM> may be formed on the substrate <NUM> in a fin patterning process in a conventional manner. As is known in the art, semiconductor fins may be used for forming horizontal channel devices, such as finFETs extending across the fins.

<FIG> illustrate forming a metal line trench <NUM> at a position between the pair of semiconductor structures, i.e. the fins <NUM>, at a level below a base thereof. In <FIG>, an upper trench portion <NUM> has been etched in the substrate <NUM> in a vertically oriented dry etch step. In <FIG> the substrate <NUM> has been further etched to form a lower trench portion <NUM> by a sequence of a further vertically oriented dry etch step followed by a wet etch step, thereby completing formation of the metal line trench <NUM>. The first dry etch step may as shown in <FIG> be stopped at a first depth D<NUM> in the substrate <NUM>. The further dry etch step may as shown in <FIG> be stopped at a second depth D<NUM> > D<NUM> in the substrate <NUM>. The wet etch step may as shown in <FIG> be stopped at a third depth D<NUM> > D<NUM>, which may define the final depth of the metal line trench <NUM>. Between the steps of etching the upper trench portion <NUM> and the lower trench portion <NUM>, a spacer <NUM> has been formed along sidewall surfaces <NUM> of the upper trench portion <NUM>, shown in <FIG>. The spacer <NUM> may accordingly mask the sidewall surfaces <NUM> of the upper trench portion <NUM> during the etching of the lower trench portion <NUM>, thereby counteracting etching the semiconductor structures <NUM> from below.

Examples of dry etching (e.g. for the predominantly "vertical" etching steps of the metal line trench formation) include reactive ion etching (RIE) and ion beam etching (IBE). Example chemistries include SF<NUM> or CF<NUM> comprising etchants, however other conventional dry etching chemistries suitable for etching Si and/or Ge-comprising semiconductors are also possible. By orienting the bias field to be directed towards and transverse to the substrate <NUM>, the major etch rate component may be aligned with the thickness direction of the substrate <NUM>, to achieve a vertical and anisotropic etching. It may be noted that a dry etch step may also provide a non-zero lateral (i.e. horizontally oriented) etch rate component. However, since a magnitude of the vertical etch rate component exceeds a magnitude of the lateral etch rate component (typically by one or more orders of magnitude), the etching may still proceed predominantly in the thickness direction of the substrate <NUM>. A dry etch step is preferable for the etching of the upper trench portion <NUM> as well as the etch step applied in <FIG>. However, it is envisaged that that the etch step for forming the upper trench portion <NUM> also may be a wet etch step, provided the upper trench portion <NUM> is formed with a shallow enough depth, i.e. using a short duration etch.

Examples of wet etching (e.g. for the widening etch step applied in <FIG>) include KOH, TMAH and NH<NUM>OH comprising etchants, however other conventional wet etching chemistries suitable for etching Si and/or Ge-comprising semiconductors are also possible. Wet etching may generally be isotropic, i.e. provide lateral and vertical etch rate components of matching or equal magnitudes. Wet etching may however also result in directional etching, i.e. with different etch rates along different crystal directions. However, also directional etching of the semiconductor substrate <NUM> may involve both lateral and a vertical etch rate components of corresponding magnitudes (typically differing by less than an order of magnitude).

Referring again to <FIG>, prior to forming the metal line trench <NUM>, a liner <NUM> may as shown be formed on sidewall surfaces <NUM> of the fins <NUM>. The liner <NUM> may be a conformally deposited liner <NUM>, covering in addition to the fins <NUM> also an upper surface of the substrate <NUM> between the fins <NUM>. The liner <NUM> may be an insulating liner, for instance of SiN, SiO2, SiC, SiCO or SiOCN. An nitride or oxide liner <NUM> may for example be deposited by ALD, however a thermally deposited oxide is also possible for fins of Si.

As further shown, the fins <NUM> may be embedded in an insulating layer <NUM>. The insulating layer <NUM> may comprise or be formed of an insulating material, such as a silicon oxide or some other suitable dielectric. Forming the insulating layer <NUM> may comprise depositing an insulating layer covering the substrate <NUM> and the fins <NUM>, e.g. by chemical vapor deposition (CVD). The insulating layer <NUM> may be formed as a planarizing layer. The deposited insulating material may be planarized and reduced in thickness by polishing, e.g. chemical mechanical polishing (CMP), to form the final insulating layer <NUM>. Optionally, the thickness of the insulating layer <NUM> may be further reduced by an etch back of the (planarized) upper surface of the insulating layer <NUM>, to bring the upper surface of the insulating layer <NUM> to a desired level. The insulating layer <NUM> may be formed to be flush with an upper surface of the fins <NUM>, with the liner <NUM> covering an upper surface of the fins <NUM>, or as shown in <FIG> with caps <NUM> on the fins <NUM>. The caps <NUM> may be formed by respective mask portion (e.g. of a hard mask material) remaining from a fin patterning process.

A trench opening <NUM> has been formed in the insulating layer <NUM>. The trench opening <NUM> extends through the insulating layer <NUM> and exposes the substrate <NUM> (i.e. an upper surface portion thereof) at the position between the fins <NUM>. The trench opening <NUM> is formed by etching through an etch mask <NUM> formed above or on the insulating layer <NUM>. An opening defining the width and longitudinal dimension of the trench opening <NUM> is formed in the etch mask <NUM>. The etch mask <NUM> may be of a conventional type such as resist-based mask patterned using lithography. However, more complex lithographic layer stacks may also be used, such as spin-on-glass / spin-on-carbon stacks or SiOC / patterning film stacks, as per se is known in the art.

A width of the trench opening <NUM> (and accordingly a width of the opening in the etch mask <NUM>) may be such that the trench opening <NUM> extends along the sidewall surfaces <NUM> of the fins <NUM>. Provided the liner <NUM> is formed of a material different from the insulating layer <NUM>, the etching of the insulating layer <NUM> may advantageously be selective to the insulating layer <NUM> such that the liner <NUM> may act as an etch mask for the fins <NUM>. Thus, the trench opening <NUM> may expose the liner <NUM> formed on the sidewall surfaces <NUM> of the fins <NUM>. In case the liner <NUM> has been formed to cover also the substrate <NUM> between the fins <NUM>, the forming of the trench opening <NUM> may further comprise opening the liner <NUM> between the fins <NUM> to expose the upper surface portion of the substrate <NUM> between fins <NUM>. The liner <NUM> may be opened using a (vertical) dry etch step.

It is envisaged however that the trench opening <NUM> may be formed with an etch which is non-selective such that also the liner <NUM> is removed, or alternatively that no liner <NUM> has been formed prior to forming the insulating layer <NUM>. In such a case, the trench opening <NUM> may expose the sidewall surfaces <NUM> of the fins <NUM>. A liner <NUM> may subsequently be formed on sidewalls of the trench opening <NUM>, comprising the sidewall surfaces <NUM> of the fins <NUM>. The liner <NUM> may be formed by conformally depositing a liner layer and subsequently the liner layer may be opened between the fins <NUM> to expose the upper surface portion of the substrate <NUM> between fins <NUM>. The liner layer may be opened using a (vertical) dry etch step.

Subsequent to forming the trench opening <NUM>, the upper trench portion <NUM> may be etched in the manner set out above, by etching the substrate <NUM> via the trench opening <NUM>. During the etching of the substrate <NUM> the liner <NUM> may act as an etch mask, counteracting etching of the fins <NUM>. As shown in <FIG>, the upper trench portion <NUM> comprises a pair of mutually opposite sidewall surfaces <NUM> and a bottom surface <NUM>. The upper trench portion <NUM> of the metal line trench <NUM> extends between and along the pair of fins <NUM>.

In <FIG>, a spacer <NUM> has been formed along the sidewall surfaces of <NUM> of the upper trench portion <NUM>. The spacer <NUM> exposes the bottom surface 122B of the upper trench portion <NUM>. Forming the spacer <NUM> may comprise conformally depositing a spacer layer, and subsequently opening the spacer layer at the bottom surface 122B to expose the bottom surface 122B. The liner layer may be opened using a (vertical) dry etch step. The spacer <NUM> may be formed for instance by an oxide or nitride layer, for example deposited by atomic layer deposition (ALD). Example materials for the spacer <NUM> include those mentioned in connection with the liner <NUM>, preferably selected to be different from the liner <NUM> to allow later selective removal of the spacer <NUM>. By the dry etch step, the spacer layer may be removed from horizontally oriented surfaces, to remain as a (sidewall) spacer <NUM>, e.g. on vertically oriented surfaces. Due to the conformal deposition, the spacer <NUM> may form also on the sidewalls of the trench opening <NUM>, in particular on the liner <NUM> on the sidewall surfaces <NUM> of the fins <NUM>.

In <FIG>, the deepened preliminary trench <NUM>' has been formed by further etching back the bottom surface 122B of the upper trench portion <NUM>, e.g. by a dry etch step. During the further etching, the spacer <NUM> remains on the sidewall surfaces <NUM> of the upper trench portion as an etch mask. The sidewall surfaces <NUM> now form upper sidewall surface portions of the preliminary trench <NUM>', above lower sidewall surface portions of the preliminary trench <NUM>'. The spacer <NUM> covers the upper sidewall surface portions of the sidewall surfaces <NUM> and exposes the lower sidewall surface portions.

In <FIG>, the lower trench portion <NUM> of the final metal line trench <NUM> has been formed in a widening (and deepening) etching of the preliminary trench <NUM>', e.g. by a wet etch step. During the etching, the lower sidewall surface portions may be laterally etched back. As may be appreciated, the etching of the lower trench portion <NUM> may further deepen the trench <NUM> by an amount corresponding to a width difference between the lower trench portion <NUM> and upper trench portion <NUM>. The amount of deepening may however depend on the relative magnitudes of the lateral and vertical etch rates of the etching. Subsequent to forming the lower trench portion <NUM>, the spacer <NUM> may be removed e.g. using a nitride- or oxide selective wet etch.

As shown in <FIG>, the finished metal line trench <NUM> comprises an upper trench portion <NUM> with a width WU and a lower trench portion <NUM> with a width WL > WU. Purely by way of example, for a fin spacing x of about <NUM>, the upper trench portion <NUM> may be etched to a depth D<NUM> of about <NUM>. A width WU of the upper trench portion <NUM> may correspond to the fin spacing. The preliminary trench <NUM>' may be etched to a depth D<NUM> of about <NUM>. A width WL of the lower trench portion <NUM> may be about <NUM> or greater. A final depth of the trench <NUM> may be about <NUM> or greater.

Subsequent to forming the metal line trench <NUM>, the metal line <NUM> may be formed in the metal line trench <NUM>. One or more metals may be deposited to fill the metal line trench <NUM>, preferably the lower as well as the upper trench portion <NUM>, <NUM> thereof. Examples of metals for the metal line <NUM> include Cu, W, Ru, Ni and Al. The metal(s) may be deposited by conventional deposition techniques such as CVD or ALD. The metal(s) may be deposited with a thickness exceeding a depth of the metal line trench <NUM>, thereby filling, at least partially, also a space between the fin <NUM>. The deposited metal(s) may be recessed (e.g. by CMP and/or etch back) to form a metal line <NUM> of a desired height, such as meeting or exceeding the depth D<NUM> of the metal line trench <NUM>. The metal line <NUM> may thereafter be covered by one or more insulating layers, for instance comprising a line capping layer and an insulating layer (e.g. of a same material as the insulating layer <NUM>).

<FIG> illustrates in more detail one approach for forming the metal line <NUM>.

In <FIG>, an insulating trench liner <NUM> has been formed in the trench <NUM>. The trench liner <NUM> may for instance be an ALD oxide or nitride, such as SiO<NUM>, SiN, SiCO, SiOCN or SiCN. A metal adhesion layer <NUM> has been conformally deposited on the trench liner <NUM>, covering sidewall surfaces of the lower as well as upper trench portions <NUM>, <NUM> (i.e. the "trench walls"). The metal adhesion layer <NUM> may be a layer of TiN, deposited for instance by ALD. A layer of TiN may additionally serve as a metal barrier layer, counteracting metal diffusion into surrounding the semiconductor material. However the metal adhesion layer <NUM> may also be formed by any other metal with sufficient adhesion properties, and if barrier properties are absent be supplemented by a separate barrier layer underneath. Due to the conformal deposition, the trench walls of the metal line trench <NUM> may be reliably covered, also the widened lower trench portion <NUM> thereof.

In <FIG>, a sacrificial layer <NUM> has been formed in the lower trench portion <NUM>. The sacrificial layer <NUM> may be formed by filling the metal line trench <NUM> with a sacrificial material, such as spin-on-carbon (SOC) or some other spin-on material. The deposited sacrificial material may thereafter be recessed/etched back to form the sacrificial layer <NUM> of a desired height, such as filling the lower trench portion <NUM> but not the upper trench portion <NUM>. The sacrificial layer <NUM> may hence be formed as a sacrificial "line", filling the lower trench portion <NUM>. The sacrificial material may be etched back in an (vertical) dry etch step. After forming the sacrificial layer <NUM>, the metal adhesion layer <NUM> has been etched back while the sacrificial layer <NUM> masks the metal adhesion layer <NUM> in the lower trench portion <NUM>. The portions of the metal adhesion layer <NUM> outside of the lower trench portion <NUM> may thus be removed while the portion of the metal adhesion layer <NUM> in the lower trench portion <NUM> may be preserved. The metal adhesion layer etch back may be achieved by a wet etch step, for instance any suitable wet etch allowing metal (e.g. TiN) to be etched selectively to the sacrificial layer <NUM> (and the trench liner <NUM>).

In <FIG>, the sacrificial layer <NUM> has been removed to expose a remaining metal adhesion layer portion <NUM>' in the lower trench portion <NUM>. Suitably, an oxygen or reducing chemistry plasma may be employed. A (first) metal 150a has thereafter been selectively deposited on the remaining metal adhesion layer portion <NUM>' in the lower trench portion <NUM>. The metal 150a may for example be W or Ru, which may be selectively deposited by ALD or CVD on the metal adhesion layer <NUM> while not nucleating on the insulating material (e.g. the oxide or nitride trench liner <NUM>) covering the sidewalls <NUM> of the fins <NUM>. The metal 150a may be deposited until the lower trench portion <NUM> is filled, preferably without any voids, at least with a reduced occurrence of voids.

In <FIG>, a (second) metal 150b has been deposited over the (first) metal 150a. The metal 150b may as shown be deposited to fill at least the upper trench portion <NUM>, but may fill a space between the fins <NUM>. The metal 150b may for instance be W or Ru, and may be deposited by CVD or PVD. The deposition of the metal 150b may be preceded by deposition of a metal barrier layer <NUM>, for instance of a same material as the metal adhesion layer <NUM>. As discussed above, the deposited metal material may be recessed (e.g. by CMP and/or etch back) to a level at or above the respective base of the pair of semiconductor structures <NUM>, and subsequently be covered by one or more insulating layers.

After the metal line formation, the process may continue with conventional front-end-of-line processing to form FET devices followed by contact and back-end-of line processing. For example, further method steps include gate patterning, gate spacer formation and fin recessing, source/drain formation, and insulating layer deposition to cover the active source/drain regions. The buried metal line <NUM> may be contacted by etching a contact trench exposing the metal line <NUM> and filling the contact trench with metal. Additionally, the buried metal line <NUM> may be contacted by one or more backside TSVs, formed through the substrate <NUM>.

<FIG> show a variation of a method of forming the metal line trench <NUM>. The method proceeds in a similar manner as outlined in relation to <FIG> however differs in that a wet etch step for forming the lower trench portion <NUM> is applied directly after the dry etch step for forming the upper trench portion <NUM>. Hence, the widening etch step is not preceded by a further deepening of the preliminary trench. As shown in <FIG>, the finished metal line trench <NUM> comprises an upper trench portion <NUM> with a width WU and a lower trench portion <NUM> with a width WL > WU. Purely by way of example, for a fin spacing x of about <NUM>, the upper trench portion <NUM> may be etched to a depth D<NUM> of about <NUM>. A width WU of the upper trench portion <NUM> may correspond to the fin spacing. A width WL of the lower trench portion <NUM> may be about <NUM> or greater. A final depth of the trench <NUM> may be about <NUM> or greater. The method may thereafter proceed with formation of the metal line <NUM> as set out above.

<FIG> show a further variation of a method of forming a metal line trench <NUM>. The method generally proceeds in a similar manner as outlined in relation to the above. However the method differs in the sequence of steps applied immediately prior and subsequent to the forming of the spacer <NUM>. More specifically, the method comprises etching an initial trench <NUM>' in the substrate <NUM>, e.g. by a dry etch step. Compared to the method shown in <FIG>, the initial trench <NUM>' may be formed with a greater depth than the upper trench portion <NUM>, such as a depth D<NUM> of about <NUM>.

Subsequently, a sacrificial layer or "line" <NUM> has is formed in a lower trench portion <NUM>' of the initial trench <NUM>'. The sacrificial line <NUM> may be formed by filling the metal line trench <NUM> with a sacrificial material, such as spin-on-carbon (SOC) or some other spin-on material. The deposited sacrificial material may thereafter be recessed to form the sacrificial line <NUM> of a desired height, such as filling the lower trench portion <NUM>' but not an upper trench portion <NUM> of the initial trench <NUM>'. The sacrificial material may be etched back in an anisotropic etch step.

Subsequently a spacer <NUM> is formed on sidewall surfaces of <NUM> of the upper trench portion <NUM> of the initial trench <NUM>', above the sacrificial line <NUM>. The spacer <NUM> exposes an upper surface 231B of the sacrificial line <NUM>. The spacer <NUM> may be formed in a similar manner as the spacer <NUM> as discussed above.

In <FIG>, the sacrificial line <NUM> has been removed, thus exposing the sidewall surfaces <NUM> of the lower trench portion <NUM>'. While the spacer <NUM> masks the sidewall surfaces <NUM> of the upper trench portion <NUM>, a wet etch step may be applied via the lower trench portion <NUM>' to widen the lower trench portion <NUM>' to form a widened (and possibly deepened) lower trench portion <NUM> of a width WL exceeding a width WU of the upper trench portion <NUM>. The method may thereafter proceed with formation of the metal line <NUM> as set out above.

In the above, the methods have been disclosed with reference to semiconductor structures in the form of fins. However, the methods are applicable also for forming a buried metal line between a pair of semiconductor structures in the form of a pair of horizontal semiconductor nanowire or nanosheet stacks. A nanowire or nanosheet stack may for example comprise alternatingly arranged layers of Si and SiGe; or SiGe-layers of different compositions; or Si or SiGe layers separated by an insulating layer. As is known in the art, stacks of horizontal nanowires and nanosheets may be used for forming horizontal channel devices, such as horizontal gate-all-around FETs, i.e. with gates wrapping around the horizontally oriented channel structures.

Claim 1:
A method for forming a buried metal line (<NUM>) in a substrate (<NUM>), the method comprising:
at a position between a pair of semiconductor structures (<NUM>) protruding from the substrate (<NUM>), forming a metal line trench (<NUM>) in the substrate (<NUM>) at a level below a base of each semiconductor structure of the pair (<NUM>), wherein forming the metal line trench (<NUM>) comprises:
etching an upper trench portion (<NUM>) in the substrate (<NUM>),
forming a spacer (<NUM>) on sidewall surfaces of (<NUM>) of the upper trench portion (<NUM>), the spacer (<NUM>) exposing a bottom surface (122B) of the upper trench portion (<NUM>), and
while the spacer (<NUM>) masks the sidewall surfaces (<NUM>) of the upper trench portion (<NUM>), etching a lower trench portion (<NUM>), comprising etching the substrate (<NUM>) via the upper trench portion (<NUM>) such that a width of the lower trench portion (<NUM>) exceeds a width of the upper trench portion (<NUM>), and
forming the metal line (<NUM>) in the metal line trench (<NUM>), wherein the buried metal line (<NUM>) forms a buried power rail.