A METHOD FOR ETCHING MOLYBDENUM

Abstract

The disclosure relates to a method for etching a molybdenum feature, comprising the steps of: a) oxidizing a thickness portion of the molybdenum feature using a thermal oxidation process to form a thermal molybdenum oxide layer, and b) dissolving the thermal molybdenum oxide layer using a wet chemistry.

Claims

1. A method for etching a molybdenum feature, comprising the steps of: a) oxidizing a thickness portion of the molybdenum feature using a thermal oxidation process to form a thermal molybdenum oxide layer, and b) dissolving the thermal molybdenum oxide layer using a wet chemistry.

2. A method according to claim 1, wherein an oxidizing ambient of the thermal oxidation process comprises O.sub.3.

3. A method according to claim 2, wherein the thermal oxidation process comprises heating the molybdenum feature to a temperature of at least 150? C.

4. A method according to claim 2, wherein the thermal oxidation process comprises heating the molybdenum feature to a temperature in a range from 180 to 300? C.

5. A method according to claim 2, wherein an O.sub.3 concentration is at least 50 g/m.sup.3.

6. A method according to claim 2, wherein an O.sub.3 concentration is in a range from 100 to 200 g/m.sup.3.

7. A method according to claim 2, wherein an O.sub.3 flow rate is at least 5 SLM.

8. A method according to claim 2, wherein an O.sub.3 flow rate is in a range from 18 to 20 SLM.

9. A method according to any to claim 2, wherein the molybdenum feature is subjected to the oxidizing ambient for a duration of at least 30 seconds.

10. A method according to claim 1, wherein an oxidizing ambient of the thermal oxidation process comprises O.sub.2.

11. A method according to claim 10, wherein the thermal oxidation process comprises heating the molybdenum feature to a temperature of at least 200? C.

12. A method according to claim 1, wherein the wet chemistry is a water-comprising liquid removing the thermal molybdenum oxide selectively to molybdenum.

13. A method according to claim 12, wherein the wet chemistry is selected from DIW, an alkaline solution, an ammonia solution or an aqueous solution of CO.sub.2W, HF or HCl.

14. A method according to claim 1, further comprising repeating a sequence of steps a) and b) a number of times.

15. A method according to claim 1, further comprising a step of pre-cleaning to remove potential contamination and the native oxide from the molybdenum feature prior to performing steps a) and b).

16. A method according to claim 1, wherein step a) comprises oxidizing the thickness portion of the molybdenum feature using the thermal oxidation process until the thermal molybdenum oxide layer reaches a self-limited thickness.

17. A method according to claim 16, wherein the self-limited thickness is 6 nm or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The above, as well as additional objects, features and advantages, may be better understood through the following illustrative and non-limiting detailed description, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

[0031] FIG. 1 illustrates molybdenum etching using conventional continuous wet etching, as a comparative example.

[0032] FIG. 2 schematically illustrates a method of etching a molybdenum feature according to an embodiment.

[0033] FIG. 3 shows XTEM images of a molybdenum film as deposited (top left), after thermal oxidation (top right and bottom left), and after an oxide dissolution step (bottom right).

[0034] FIG. 4 shows an example of a repeated cycle of thermal oxidation and wet-chemistry oxide dissolution.

[0035] FIG. 5 is an example comparison of thickness of a PVD molybdenum film without (FIG. 5a) and with (FIG. 5b) performing a pre-cleaning step.

[0036] FIG. 6 compares roughness of a molybdenum surface after being subjected to different processes conditions.

[0037] FIG. 7 illustrates recess amount of ALD and PVD molybdenum films as a function of number of cycles of thermal oxidation and oxide dissolution.

DETAILED DESCRIPTION

[0038] The present invention relates in an aspect to a method of etching a molybdenum feature comprising a step of a) oxidizing a thickness portion of the molybdenum feature using a thermal oxidation process to form a thermal molybdenum oxide layer, and dissolving the thermal molybdenum oxide layer using a wet chemistry. This sequence of steps a) and b) can optionally be repeated (ALE style) to obtain a desired etch amount.

[0039] The molybdenum feature may be any feature/structure of molybdenum typically used in integrated circuit fabrication, such as a (thin) film, a layer, a horizontal or vertical interconnect e.g. of a back-end-of-line structure such as a conductive line or via, a buried power rail, a contact e.g. of a semiconductor device such as source/drain contact or gate.

[0040] The molybdenum feature may e.g. be formed of molybdenum deposited using PVD, CVD or ALD, or using any other conventional deposition process allowing deposition of molybdenum of high material quality. The deposited molybdenum may be annealed (e.g. in an inert atmosphere such as N.sub.2). An anneal may improve properties of the deposited molybdenum such as the resistivity. The molybdenum deposition may be followed by process steps (e.g. conventional lithography and etching patterning techniques) for defining an initial shape of the molybdenum feature, which is to be etched.

[0041] The molybdenum feature may be arranged on a substrate. The substrate may be a semiconductor substrate of a conventional type, e.g. a Si-substrate, a Ge-substrate, a SiGe substrate, a silicon-on-insulator substrate etc.) or some other known type of substrate suitable for the type of integrated circuit device of which the molybdenum feature is to form part.

[0042] FIG. 2 schematically illustrates a method 100 of etching a molybdenum feature 10. Reference sign 10a indicates a metallic surface of the molybdenum feature 10 which is to be etched back or recessed. The direction R indicates the direction along with the surface 10a will be recessed, and may hence be referred to as recess direction R. The direction R is oriented perpendicular to the surface 10a and into the molybdenum feature 10. The dashed horizontal line indicates a geometrical plane defined by the surface 10a prior to the etching, thus representing a reference surface with respect to which the surface 10a will be recessed.

[0043] The first row of FIG. 2 shows a progression of step a) of forming of a thermal molybdenum oxide layer 14 by oxidizing a thickness portion of the molybdenum feature 10 using a thermal oxidation process. The thermal molybdenum oxide layer 14 may hence form a surface layer on the molybdenum feature 10. The arrow H represents supplying thermal energy to the molybdenum feature 10 by heating. The heat may be supplied via a substrate holder, and e.g. controlled via the set temperature of the substrate holder. However, thermal energy may also or alternatively be supplied via the ambient. As may be seen, a thickness portion of the molybdenum feature 10 is gradually oxidized and hence consumed such that the metallic surface 10a (i.e. defining the metal-oxide interface) is recessed in relation to the reference surface. During the oxidation one or more atomic layers of molybdenum may be oxidized.

[0044] As schematically shown, a thickness of the thermal molybdenum oxide layer 14 may typically exceed the thickness of molybdenum consumed during the oxidation. The thermal oxidation process may be performed until the thermal molybdenum oxide layer 14 reaches the self-limiting thickness (dependent on the process conditions of the thermal oxidation process). However, if a smaller amount of recess is desired the thermal oxidation process may be stopped prior to reaching the self-limiting thickness.

[0045] Prior to commencing the etching method, a native molybdenum oxide 12 may as shown be present on the surface 10a. Although depicted as a layer of uniform thickness, it is to be noted that the native oxide 22 also may be formed in a non-uniform manner, e.g. such that a thickness of the native oxide may vary along the surface 10a and/or portions of the surface 10a may be free from oxide. The native oxide 12 may comprise predominantly MoOx with x=2, but may additionally comprise sub-oxides with x=1 and x=3. The thermal oxidation process may however result in conversion of MoOx.sub.(x?2) of the native oxide 12 into MoO.sub.3, as indicated in row a), wherein native oxide layer 12 is replaced by the thermal molybdenum oxide layer 14.

[0046] The second row of FIG. 2 shows a progression of step b) of dissolving of the thermal molybdenum oxide layer 14 using a wet chemistry, performed subsequent to step a). As may be seen, the thermal molybdenum oxide layer 14, is gradually dissolved and thus removed from the molybdenum feature 10a. The arrow W represents contacting the thermal molybdenum oxide layer 14 with the wet chemistry. The thermal molybdenum oxide layer 14 may be completely removed such that the recessed (metallic) surface 10a of the molybdenum feature 10 is revealed. However, as may be appreciated by the skilled person, the wet chemistry (e.g. a solute or a solvent thereof) may even if being relatively benign with respect to the molybdenum still cause some oxidation of the surface 10a such that also after the dissolving step b) a (thin) wet native molybdenum oxide is formed on the molybdenum surface, in FIG. 2 indicated by layer 16.

[0047] If a target recess amount/depth exceeds the amount of recessing obtainable by applying a single sequence of steps a) and b) to the molybdenum feature 10, the sequence may be repeated a number of times until reaching the target recess amount.

[0048] The thermal oxidation process may be conducted using conventional equipment as is known in the art, e.g. in a furnace for thermal oxidation.

[0049] The thermal oxidation process of step a) may be performed in an 03 (ozone gas) ambient/atmosphere. The thermal oxidation process may be performed at a temperature of at least 150? C. Although thermal oxide growth in O.sub.3 may be observed also at lower temperatures (e.g. an onset may be observed at about 60? C.) a temperature of at least 150? C. may increase the yield of MoO.sub.3 (which may be quickly dissolved in the wet chemistry) and allow forming of a thermal molybdenum oxide layer 14 of self-limiting thickness in a shorter time (e.g. in 30 to 300 seconds). A temperature in a range from 180 to 300? C. may further contribute to the growth rate and yield of MoO.sub.3. For example, the self-limiting thickness of the thermal molybdenum oxide layer 14 may be 1.8 nm at a temperature of 180? C., and 6 nm at a temperature of 290? C. A concentration of O.sub.3 may be at least 50 g/m.sup.3. An O.sub.3 flow rate may be at least 5 SLM. For example, an O.sub.3 concentration may be a range from 100 to 200 g/m.sup.3, and an O.sub.3 flow rate may be in a range from 18 to 20 SLM. Although a concentration and/or flow in these ranges may provide suitable process conditions for the thermal oxidation, the temperature has been observed to have a greater impact on the thermal oxidation process. It is hence contemplated that also lower concentration and/or flow of O.sub.3 may be used.

[0050] Alternatively, the thermal oxidation process may be performed in an O.sub.2 (gas-phase oxygen) ambient/atmosphere. The thermal oxidation process may be performed at a temperature of at least 200? C. Although thermal oxide growth in O.sub.2 may be observed also at lower temperatures (e.g. an onset may be observed at about 60? C.) a temperature of at least 200? C. or above may increase the yield of MoO.sub.3 and allow forming of a thermal molybdenum oxide layer 14 of self-limiting thickness in a shorter time (e.g. 20 minutes or less).

[0051] Oxidation may for example be observed in an 02 ambient at atmospheric pressure. An O.sub.2 flow rate may for example be 10 SLM or more. Although a concentration and/or flow in these ranges may provide suitable process conditions for the thermal oxidation, the temperature has been observed to have a greater impact on the thermal oxidation process.

[0052] After the oxidation step the substrate (with the molybdenum feature 100) may be submerged and/or rinsed with the wet chemistry, e.g. in a tank.

[0053] The wet chemistry of step b) may be a water-comprising liquid of a composition such that the thermal molybdenum oxide may be removed selectively to the molybdenum forming the metallic surface 10a. Examples of such liquids include an ammonia solution (dNH.sub.4OH) or other aqueous solution of CO.sub.2W, HF or HCl. A dilution ratio of the solutions may as a non-limiting example be 1:100, but smaller as well as higher dilution ratios are also possible as long it is ensured that the wet chemistry does not cause appreciable etching or oxidation of the molybdenum. However other aqueous solutions are also possible like (diluted) alkaline solutions. Other examples include DIW and UPW. It is contemplated that also non-aqueous solutions, such as an inorganic solvent, may be used.

[0054] Optionally, the sequence of step a) and b) may be preceded by a pre-cleaning step to remove a native oxide 12 from the molybdenum feature 10, as represented by arrow P in FIG. 2, first row. The native oxide 12 may if present inhibit the thermal oxidation process. The pre-cleaning may be performed by contacting the native oxide 12 with e.g. any of the above-mentioned (diluted) aqueous solutions.

[0055] Although FIG. 2 shows the surface 10a in a horizontal orientation, this does not imply that the surface 10a during the etching method needs to be horizontally oriented in an absolute sense. Indeed, the etching method may be applied to a surface 10a oriented either parallel to or at an angle with respect to the substrate (e.g. a horizontal top surface or a vertical sidewall surface of a molybdenum feature 10 for instance forming a contact or line on the substrate). It is further to be noted that two or more differently oriented surfaces of the molybdenum feature 10 may be recessed simultaneously, e.g. by exposing each of these surfaces to the etching method. It is further to be noted that in a typical wafer-scale application a plurality of like molybdenum features may be recessed simultaneously by exposing them to the etching method.

[0056] FIG. 3 cross-sectional transmission electron microscopy (XTEM) image of a molybdenum film deposited on a wafer using PVD. The molybdenum film is subjected to a single cycle of thermal oxidation (bake in an 100 g/m.sup.3 O.sub.3 ambient with 18 SLM at 180? C. for 90 seconds) and wet-chemistry oxide dissolution (in dNH.sub.4OH in a 1:100 dilution for 30 seconds).

[0057] It is contemplated that the improved results in terms of surface roughness obtained in this example over the continuous wet etching results depicted in FIG. 1, in part may be attributed to an improved process uniformity since a thermal oxidation process in gas is less grain-boundary dependent and less diffusion-limited than a wet chemical oxidation process in a solution. Also, continuous wet etching of molybdenum in solution with a strong oxidizer, like hydrogen peroxide, has an electrochemical mechanism accompanied by undesirable formation of slightly soluble molybdenum hydrates that may passivate the metallic surface. A wet etch comprising conventional (strongly) acidic O3 solution may also result in MoOx(x>2) enrichment at the metal-oxide interface, at the expense of MoO3.

[0058] FIG. 4 is an example of a repeated cycle of thermal oxidation and wet-chemistry oxide dissolution, applied to a molybdenum feature (a PVD deposited film, twice annealed in N.sub.2) with an initial thickness of 50 nm. FIG. 4 illustrates a sequence of three repeated cycles of thermal oxidation (bake in a 100 g/m.sup.3 O.sub.3 ambient at 290? C. for 30 seconds) followed by oxide dissolution (in dNH.sub.4OH in a 1:100 dilution for 30 seconds). The recess amount per cycle was about 4.7 nm.

[0059] FIG. 5a shows the thickness of a PVD molybdenum film along a wafer radius before (dots with no fill) and (filled dots) after one cycle of thermal oxidation and wet-chemistry oxide dissolution, without performing a pre-clean step. FIG. 6b shows the thickness of a similar PVD molybdenum film before (dots with no fill) and (filled dots) after one cycle of thermal oxidation and wet-chemistry oxide dissolution, with a preceding pre-clean step. The greater molybdenum loss of the molybdenum film enabled by a pre-clean may be readily observed in FIG. 5b. In both examples, the thermal oxidation was performed at 180? C. for 30 seconds in 100 g/m.sup.3 O.sub.3 ambient with 18 SLM. The pre-cleaning as well as the dissolution step was performed in dNH4OH (dilution 1:100) for 30 seconds.

[0060] FIG. 6 compares roughness of a molybdenum surface with an initial roughness (REFERENCE) resulting after wet oxidation in acidified ozonated water, thermal oxidation in O.sub.3, and a full recess sequence of thermal oxidation followed by oxide dissolution, respectively. As may be observed, there is a decrease of roughness after thermal oxidation compared to the reference. At a temperature of 180? C. surface roughness is relatively insensitive to duration and O.sub.3 concentration and flow rate. There is a further additional decrease of roughness after the full recess sequence, compared to oxidation only. Increasing the number of cycles from 3 to 6 yields a further reduction of roughness. Wet oxidation in acidified ozonated water results in significant roughness increase.

[0061] FIG. 7 illustrates recess amount of ALD and PVD molybdenum films as a function of number of cycles of thermal oxidation (bake in a 100 g/m.sup.3 O.sub.3 ambient at 180? C. for 30 seconds) followed by oxide dissolution (in dNH.sub.4OH in a 1:100 dilution for 30 seconds). As may be seen there is an approximately linear relation between recess amount and number of cycles. There is no significant difference of recess amount for the different deposition methods.

[0062] In the above the inventive concept has mainly been described with reference to a limited number of examples. However, as is readily appreciated by a person skilled in the art, other examples than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims.