Patent Description:
Contact sticking, or stiction, is one of the dominant failure mechanisms in MEMS devices. Stiction is one of the key challenges in fabricating viable MEMS devices. Ruthenium contacts provide low resistance, durable contacts, but ruthenium contacts are susceptible to potential stiction events over operating life. <CIT> discloses a method of manufacturing a MEMS device, comprising: forming one or more electrical contact stacks comprising a contact surface of ruthenium, at least one of the one or more electrical contact stacks formed over at least one contact electrode; forming a beam structure over the one or more electrical contact stacks; removing sacrificial material to free the beam structure to move within a cavity, wherein at least one contact portion of the beam structure is capable of contacting the contact surface of the at least one contact electrode; and sealing the cavity.

There remains a need to have low resistance, durable contacts that are less susceptible to stiction events.

The present invention provides a method of manufacturing a MEMS device according to claim <NUM> and a MEMS device according to claim <NUM>. The MEMS device has a cavity in which a beam will move to change the capacitance of the device. After most of the device build-up has occurred, sacrificial material is removed to free the beam within the MEMS device cavity. Thereafter, exposed ruthenium contacts are etched back with an etchant comprising chlorine to remove the top surface of both the top and bottom contacts. Due to this etch back process, low contact resistance can be achieved with less susceptibility to stiction events. Stiction performance can be further improved by conditioning the ruthenium contacts in a fluorine based plasma. The fluorine based plasma process, or fluorine treatment, can be performed prior to or after etch-back process of the ruthenium contacts.

The present disclosure generally relates to a method of manufacturing a MEMS device. The MEMS device has a cavity in which a beam will move to change the capacitance of the device. After most of the device build-up has occurred, sacrificial material is removed to free the beam within the MEMS device cavity. Thereafter, exposed ruthenium contacts are etched back with an etchant comprising chlorine to remove the top surface of both the top and bottom contacts. Due to this etch back process, low contact resistance can be achieved with less susceptibility to stiction events. Stiction performance can be further improved by conditioning the ruthenium contacts in a fluorine based plasma. The fluorine based plasma process, or fluorine treatment, can be performed prior to or after etch-back process of the ruthenium contacts.

<FIG> is a schematic illustration of a MEMS device <NUM> prior to removing sacrificial material and releasing the beam. The MEMS device <NUM> includes a substrate <NUM>, such as a CMOS substrate that includes numerous layers for a semiconductor device. It is also contemplated that the substrate <NUM> may simply be a semiconductor substrate containing silicon, germanium, or other suitable semiconductor material.

Within the substrate, one or more contact electrodes 104A, 104B are present. The contact electrodes 104A, 104B may be RF conductors or RF electrodes. It is to be understood that while two contact electrodes 104A, 104B are shown, a single contact electrode, or even more than two contact electrodes is contemplated. The contact electrodes 104A, 104B may be comprised of any conductive material suitable for use in a semiconductor device such as copper, aluminum, titanium nitride, tungsten, and combinations thereof.

Additional conductive material may be present on or above the substrate <NUM> as well as the contact electrodes 104A, 104B. For example, anchor electrodes 106A, 106B are shown in <FIG>, as are numerous additional electrical contacts <NUM>. The anchor electrodes 106A, 106B are the electrodes for the beam structure <NUM>, and the electrical contacts <NUM> may be used for pull-in electrodes. The anchor electrodes 106A, 106B and the electrical contacts <NUM> may be comprised of any conductive material suitable for use in a semiconductor device such as copper, aluminum, titanium nitride, tungsten, and combinations thereof.

A dielectric layer <NUM> is present over the substrate <NUM>, including over the electric contacts <NUM>. It is contemplated that the dielectric layer <NUM> encompasses an electrically insulating material such as silicon oxide, silicon dioxide, silicon nitride, silicon oxynitride, or combinations thereof.

A first sacrificial layer <NUM> is present over the dielectric layer <NUM>. The first sacrificial layer <NUM> will ultimately be removed to free the beam structure <NUM>. The first sacrificial layer <NUM> comprises a different material than the dielectric layer <NUM>. Suitable material for the first sacrificial layer <NUM> includes spin-on material such as a carbon based material. The first sacrificial layer <NUM> may comprise carbon, hydrogen, nitrogen, and oxygen.

A second dielectric layer <NUM> is present over the first sacrificial layer <NUM>, and the bottom portion of the beam structure <NUM> is present over the second dielectric layer <NUM>. The second dielectric layer <NUM> may comprise the same material as the first dielectric layer <NUM>. The beam structure <NUM> may comprise any conductive material suitable for use in a semiconductor device such as copper, aluminum, titanium nitride (TiN), tungsten, titanium aluminum nitride (TiAlN), tantalum nitride (TaN), and combinations thereof. The beam structure <NUM> additionally includes a middle portion, a top portion, and post portions. Dielectric layers <NUM> are present on the top and bottom surfaces of the beam portions.

Additionally, in the areas where the beam structure <NUM> is not located, additional sacrificial material <NUM> is present. The sacrificial material <NUM> may comprise the same material in all locations within the MEMS device <NUM>. In fact, sacrificial material <NUM> is present over the top portion of the beam structure <NUM>. On and in contact with the topmost sacrificial material <NUM>, an additional dielectric layer <NUM> is present. A pull-up electrode <NUM> is present over and on the additional dielectric layer <NUM>. A dielectric roof <NUM> is also present over the pull-up electrode <NUM>. Release holes <NUM> are present through the dielectric roof <NUM> and the top-most dielectric layer <NUM>. The release holes <NUM> extend through the roof <NUM> to expose the sacrificial material <NUM>.

The bottom of the beam structure <NUM> comprises two beam contact portions <NUM> that comprise ruthenium. Two electrical contact stacks containing contact surface <NUM> of ruthenium are present over each contact electrode 104A, 104B and aligned with the two beam contact portions <NUM>. The contact surfaces <NUM> comprising ruthenium are the landing location for the beam structure <NUM>, as will be discussed later. The beam contact portions <NUM> contact the contact surfaces <NUM> when the beam structure <NUM> has been freed and is in the pulled-down state, which is the maximum capacitance state. One or more additional electrical contact stacks containing contact surface <NUM> of ruthenium are disposed over a portion of the electrical contacts <NUM>.

In order to free the beam structure <NUM> to move the device <NUM>, the sacrificial material needs to be removed. <FIG> is a schematic illustration of the MEMS device <NUM> of <FIG> after the sacrificial material <NUM> has been removed and the beam <NUM> has been released. The sacrificial material <NUM> is removed by an etching process where the etchant, which may be a wet etchant or a dry etchant, is introduced through the release holes <NUM>. Once the sacrificial material <NUM> is removed, the location where the sacrificial used to be is considered to be a cavity <NUM>.

The beam structure <NUM> is free to move within the cavity <NUM> once the sacrificial material <NUM> is removed. The beam structure <NUM> may move from a first position spaced from both the pull-up electrode <NUM> and the electrical contact stacks containing contact surface <NUM> of ruthenium (shown in <FIG>) back and forth as needed to a second position in contact with the contact surfaces <NUM> and a third position disposed adjacent to the pull-up electrode <NUM> and spaced a greater distance from the contact surfaces <NUM> than in the first position. The terms "first position", "second position", and "third position" are not intending to be limiting, and the beam structure <NUM> may move to and from any of the three positions in any order.

As noted above, the sacrificial material <NUM> is removed, but everything else in the device <NUM> remains. Thus, the contact surfaces <NUM>, <NUM> are now exposed as are the contact portions <NUM> that contain ruthenium. As additionally noted above, ruthenium has low resistance and is a durable contact, but ruthenium contacts are susceptible to potential stiction events over operating life. Hence, it has been surprisingly found that additional treatment of the ruthenium will lead to less stiction and further lower the contact resistance of the device.

The ruthenium comprising elements of the device <NUM> (i.e., the beam contact portions <NUM> and the contact surfaces <NUM>, <NUM>) may be treated with a fluorine-based treatment. For example, a plasma containing fluorine and oxygen may be optionally introduced to the cavity <NUM> through the release holes <NUM>. In one embodiment, the plasma is formed from O<NUM> and CF<NUM>. It is contemplated that other fluorine based gases may be used such as NF<NUM>, SF<NUM>, and CHF<NUM>. In one embodiment, a Fluorocarbon-based Self Assembled Monolayer (SAM) may be utilized. Thus, the disclosure is not to be limited to CF<NUM>. An excessive amount of CxFy polymer formation formed during the introduction of the fluorine-based treatment may increase contact resistance to an unacceptable level. Excessive fluorine doping of ruthenium may increase contact resistance to an unacceptable level. Thus, as little polymer formation as possible is desired to maintain low contact resistance.

<FIG> is a zoomed-in schematic illustration of a portion of the MEMS device <NUM> of <FIG> after the sacrificial material <NUM> has been removed and/or after the optional fluorine-based treatment. As shown in <FIG>, an interstitial layer <NUM> or layer of residue may be disposed on the top surface <NUM> of the contact surface <NUM>. The interstitial layer <NUM> or layer of residue may form on the contact surfaces <NUM>, <NUM> during the various formation operations of the MEMS device <NUM>, such as during fabrication or construction, during removal of the sacrificial material <NUM>, or during the optional fluorine-based treatment.

Following the optional fluorine-based treatment, a chlorine (Cl) etch process is performed. In one embodiment, the chlorine etch process is performed prior to the optional fluorine-based treatment. Thus, the optional fluorine-based treatment may occur before or after the chlorine etch process. The chlorine etch process may occur following the removal of the sacrificial material <NUM>. Etchant comprising chlorine, such as hydrochloric acid (HCl), boron trichloride (BCl<NUM>), or chlorine gas (Cl<NUM>), is introduced to cavity <NUM> through the release holes <NUM>. The chlorine etch-back process may be a "wet" or a "dry" process. In addition to chlorine, the dry etch-back process may include oxygen and/or a combination of oxygen and fluorine. The chlorine containing etchant has high etch selectivity such that the etchant etches only ruthenium comprising elements of the device <NUM> and residues such as the interstitial layer <NUM>. Thus, the chlorine containing etchant etches a portion of the contact electrode surfaces <NUM>, <NUM> and/or a bottom portion of the beam contact surfaces <NUM> without etching the beam structure <NUM> or the dielectric layers <NUM>. The chlorine containing etch-back process has a very high selectivity (<NUM>:<NUM>) towards dielectrics, titanium, titanium nitride, and titanium aluminum nitride, which enables the etch-back process to selectively etch-back the contact electrode surfaces <NUM>, <NUM> without negatively impacting other components inside the cavity.

<FIG> is a zoomed-in schematic illustration of the MEMS device <NUM> of <FIG> after the exposed ruthenium comprising elements of the device <NUM> (i.e., the beam contact portions <NUM> and the contact surfaces <NUM>, <NUM>) have been etched. In <FIG>, a top portion or surface <NUM> of the contact surfaces <NUM>, <NUM> and a bottom portion or surface <NUM> of the beam contact portions <NUM> have been partially removed through the chlorine etch process. While <FIG> illustrates the removal of ruthenium from both the contact surfaces <NUM>, <NUM> and the beam contact portions <NUM>, the removal may occur only to the contact surfaces <NUM>, <NUM>, only to the beam contact portions <NUM>, or to both the contact surfaces <NUM>, <NUM> and the beam contact portions <NUM>.

Etching a bottom portion or surface <NUM> of the beam contact portions <NUM> recesses the beam contact portions <NUM> into the beam structure <NUM>, and etching a top portion or surface <NUM> of the contact surfaces <NUM>, <NUM> reduces the overall height of the contact electrodes. As shown in <FIG>, the exposed ruthenium comprising elements etched during the chlorine etch process (i.e., the top surface <NUM> of the contact surfaces <NUM>, <NUM> and/or the bottom surface <NUM> of the beam contact portions <NUM>) may have a surface with elevated roughness. In other words, the top surface <NUM> of the contact surfaces <NUM>, <NUM> and/or the bottom surface <NUM> of the beam contact portions <NUM> have a surface roughness from being etched with the etchant containing chlorine. In one embodiment, the top surface <NUM> and the bottom surface <NUM> each have a roughness root mean square of about <NUM> to about <NUM>, such as about <NUM> to about <NUM>. The surface roughness may increase with extended exposure of the surfaces <NUM>, <NUM> to the etchant containing chlorine. Moreover, following the chlorine etch process, any residues or interstitial layers <NUM> that may have formed during formation of the device <NUM> are removed from the exposed ruthenium comprising elements by pumping an inert gas, such as argon, into the cavity.

The chlorine containing etchant has a ruthenium etch rate of about <NUM>/min to about <NUM>/min, such as about <NUM>/min, and a selectivity ratio for etching ruthenium to surrounding exposed materials, such as oxide, TiAlN, and TiN, of about <NUM>:<NUM> to about <NUM>:<NUM> (i.e., the chlorine containing etchant may etch ruthenium <NUM> to <NUM> times faster than the chlorine containing etchant etches oxide, TiAlN, and TiN). The etchant containing chlorine may be present in the cavity <NUM> for about <NUM> minute to <NUM> minutes, such as about <NUM> minutes to <NUM> minutes. The chlorine etch process may remove about <NUM> to <NUM> of ruthenium from each of the contact surfaces <NUM>, <NUM> and the beam contact portions <NUM>. By etching the ruthenium from the contact surfaces <NUM>, <NUM> and the beam contact portions <NUM>, the contact resistance of the MEMS device <NUM> is effectively lowered. In one embodiment, the chlorine etching process lowers the contact resistance of the device <NUM> by a factor of about <NUM> as compared to conventional MEMS devices.

Etching a portion of the contact surfaces <NUM>, <NUM> from each of the electrical contact stacks and/or a portion of each beam contact portion <NUM> cleans the ruthenium surfaces that were in contact with the sacrificial material <NUM>. The chlorine etch process further removes any residues, interstitial layers, or interstitial impurities that may have formed on the surfaces while forming the MEMS device <NUM>, such as during fabrication or construction, during removal of the sacrificial material <NUM>, or during the optional fluorine containing plasma treatment. For example, the chlorine etch process may remove any CxFy polymer formation that may have formed during the optional fluorine containing plasma treatment.

Once the sacrificial material <NUM> has been removed and the exposed ruthenium has been treated, the device <NUM> is ready to be sealed. <FIG> is a schematic illustration of the MEMS device <NUM> of <FIG> after the MEMS device has been sealed. As shown in <FIG>, a seal <NUM> is formed to seal the release holes <NUM>. The seal <NUM> extends down to contact the top-most dielectric layer <NUM> disposed on the beam structure <NUM>. The seal <NUM> may comprise a dielectric material such as silicon oxide, silicon dioxide, silicon nitride, silicon oxynitride, or combinations thereof.

By treating the exposed ruthenium surfaces after the sacrificial material has been removed, the resulting MEMS device will have contact surfaces that have low resistance, are durable, and are less susceptible to stiction events.

In one embodiment, a method of manufacturing a MEMS device comprises: forming one or more electrical contact stacks comprising a contact surface of ruthenium within a cavity; forming a beam structure over the one or more electrical contact stacks within the cavity, wherein the cavity contains sacrificial material; removing the sacrificial material from the cavity to free the beam to move within the cavity; etching a portion of the contact surface of ruthenium using an etchant comprising chlorine; and sealing the cavity.

The etchant may further comprise oxygen or fluorine based gasses. The etchant may be present in the cavity for a period of between about <NUM> minutes and <NUM> minutes. About <NUM> to about <NUM> of the contact surface of ruthenium may be removed through the etching. The beam structure may include at least one contact portion comprising ruthenium. The etching the portion of the contact surface of ruthenium using the etchant comprising chlorine may further comprise etching a bottom portion of the at least one contact portion comprising ruthenium. Etching the portion of the at least one contact portion comprising ruthenium may recess the at least one contact portion comprising ruthenium into the beam structure. The MEMS device may include at least one contact electrode. At least one of the one or more electrical contact stacks comprising the contact surface of ruthenium may be formed over the at least one contact electrode.

Claim 1:
A method of manufacturing a MEMS device (<NUM>), comprising:
forming one or more electrical contact stacks comprising a contact surface (<NUM>) of ruthenium, at least one of the one or more electrical contact stacks formed over at least one contact electrode (104A, 104B);
forming a beam structure (<NUM>) over the one or more electrical contact stacks;
removing sacrificial material (<NUM>) to free the beam structure to move within a cavity (<NUM>),
wherein at least one contact portion (<NUM>) of the beam structure is capable of contacting the contact surface of the at least one contact electrode;
etching a portion of the contact surface (<NUM>) of ruthenium within the cavity (<NUM>) using an etchant comprising chlorine; and
sealing the cavity.