Silicon etch process with tunable selectivity to SiO2 and other materials

A tunable plasma etch process includes generating a plasma in a controlled flow of a source gas including NH3 and NF3 to form a stream of plasma products, controlling a flow of un-activated NH3 that is added to the stream of plasma products to form an etch gas stream; and controlling pressure of the etch gas stream by adjusting at least one of the controlled flow of the source gas and the flow of un-activated NH3 until the pressure is within a tolerance of a desired pressure. An etch rate of at least one of polysilicon and silicon dioxide by the etch gas stream is adjustable by varying a ratio of the controlled flow of the source gas to the flow of un-activated NH3.

TECHNICAL FIELD

The present disclosure applies broadly to the field of plasma processing equipment. More specifically, systems and methods for managing selectivity of plasma etches are disclosed.

BACKGROUND

Semiconductor processing often utilizes plasma processing to etch, clean or deposit material on semiconductor wafers. Plasma etching often targets a particular material, but may also affect other materials that are present on the same wafer, or other workpiece. Selectivity of a plasma etch is often defined as a ratio of an etch rate of a target material to the etch rate of the same etch to another material. It is often advantageous for selectivity to be very high, such that the target material can be etched with little effect on other materials.

SUMMARY

In an embodiment, a tunable plasma etch process includes generating a plasma in a controlled flow of a source gas including NH3and NF3to form a stream of plasma products, controlling a flow of un-activated NH3that is added to the stream of plasma products to form an etch gas stream; and controlling pressure of the etch gas stream by adjusting at least one of the controlled flow of the source gas and the flow of un-activated NH3until the pressure is within a tolerance of a desired pressure. An etch rate of at least one of polysilicon and silicon dioxide by the etch gas stream is adjustable by varying a ratio of the controlled flow of the source gas to the flow of un-activated NH3.

In an embodiment, a plasma etch processing system that etches silicon and silicon dioxide with tunable selectivity includes a first NH3flow controller, an NF3flow controller, and a remote plasma source configured to apply RF energy to a plasma source gas stream controlled by the first NH3flow controller and the NF3flow controller, to generate a plasma product stream from the plasma source gas stream. The processing system further includes a second NH3flow controller configured to control an un-activated NH3gas stream, apparatus for adding the un-activated NH3gas stream to the plasma product stream to form an etch gas stream, and a process chamber configured to expose a workpiece to the etch gas stream. Etch rates of at least one of polysilicon and silicon dioxide by the etch gas stream on the workpiece are adjustable at least by varying a ratio of the plasma source gas stream to the un-activated NH3gas stream.

In an embodiment, a plasma etch processing system includes a process chamber configured to expose a workpiece to an etch gas stream, and a plasma source that generates a plasma product stream from a source gas stream that includes NH3and NF3. The system also includes means for controlling a first NH3flow and an NF3flow of the source gas stream, and means for controlling a second NH3flow of an un-activated NH3gas stream. The system also includes a diffuser plate disposed between the plasma source and the process chamber that allows the plasma product stream to flow through the diffuser plate toward the process chamber, and adds the un-activated NH3gas stream only on a process chamber side of the diffuser plate, to form an etch gas stream. The system also includes a controller for controlling the plasma source, the means for controlling the first NH3flow and the NF3flow of the source gas stream, and the means for controlling the second NH3flow of the un-activated NH3gas stream, such that the controller adjusts etch rates of at least one of polysilicon and silicon dioxide by the etch gas stream on the workpiece by varying a ratio of the un-activated NH3gas stream to the source gas stream.

DETAILED DESCRIPTION

FIG. 1schematically illustrates major elements of a plasma processing system100, according to an embodiment. System100is depicted as a single wafer, semiconductor wafer plasma processing system, but it will be apparent to one skilled in the art that the techniques and principles herein are applicable to plasma generation systems of any type (e.g., systems that do not necessarily process wafers or semiconductors). It should also be understood thatFIG. 1is a simplified diagram illustrating only selected, major elements of system100; an actual processing system will accordingly look different and likely contain additional elements as compared with system100.

Processing system100includes a housing110for at least a wafer interface115, a user interface120, a plasma processing unit130, a controller140, one or more flow controller(s)156and one or more power supplies150. Processing system100is supported by various utilities that may include gas(es)155, electrical power170, vacuum160and optionally others; within system100, controller140may control use of any or all of such utilities. Internal plumbing and electrical connections within processing system100are not shown, for clarity of illustration.

Processing system100is illustrated as a so-called indirect, or remote, plasma processing system that generates a plasma in a first location and directs the plasma and/or plasma products (e.g., ions, molecular fragments, free radicals, energized species and the like) to a second location where processing occurs. Thus, inFIG. 1, plasma processing unit130includes a remote plasma source132that supplies plasma and/or plasma products for a process chamber134. Process chamber134includes one or more wafer pedestals135, to which wafer interface115delivers and retrieves workpieces50(e.g., a semiconductor wafer, but could be a different type of workpiece) for processing. Wafer pedestal135is heated and/or cooled by a wafer heating/cooling apparatus117which could include, in embodiments, resistive heaters, other types of heaters, or cooling fluids. In operation, gas(es)155are introduced into plasma source132and a radio frequency generator (RF Gen)165supplies power to ignite a plasma within plasma source132. Plasma and/or plasma products pass from plasma source132through at least a diffuser plate137to process chamber134, where workpiece50is processed. An actual plasma system may provide many other optional features or subsystems through which plasma and/or plasma products flow and/or mix between plasma source132and process chamber134, and may include sensors118that measure parameters such as pressures, temperatures, optical emissions and the like at wafer pedestal135, within process chamber134as shown, and possibly at other locations within processing system100.

The elements illustrated as part of system100are listed by way of example and are not exhaustive. Many other possible elements, such as: pressure and/or flow controllers; gas or plasma manifolds or distribution apparatus; ion suppression plates; electrodes, magnetic cores and/or other electromagnetic apparatus; mechanical, pressure, temperature, chemical, optical and/or electronic sensors; wafer or other workpiece handling mechanisms; viewing and/or other access ports; and the like may also be included, but are not shown for clarity of illustration. In particular, flow controllers may be, or include, mass flow controllers, valves, needle valves, and pressure regulators. Various control schemes affecting conditions in process chamber105are possible. For example, a pressure may be maintained by monitoring the pressure in process chamber134and adjusting all gas flows upwards or downwards until the measured pressure is within some tolerance of a desired pressure. In embodiments herein, the pressure may be controllable within a range of about 0.5 Torr to 10 Torr; in certain embodiments, controlling pressure around 2 Torr or 6 Torr may be advantageous. Temperatures can be controlled by adding heaters and temperature sensors to process chamber134and/or wafer pedestal135. Optical sensors may detect emission peaks of plasmas as-generated and/or as they interact with workpieces.

Internal connections and cooperation of the elements illustrated within system100are also not shown for clarity of illustration. In addition to RF generator165and gases155, other representative utilities such as vacuum160and/or general purpose electrical power170may connect with system100. Like the elements illustrated in system100, the utilities illustrated as connected with system100are intended as illustrative rather than exhaustive; other types of utilities such as heating or cooling fluids, pressurized air, network capabilities, waste disposal systems and the like may also be connected with system100, but are not shown for clarity of illustration. Similarly, while the above description mentions that plasma is ignited within remote plasma source132, the principles discussed below are equally applicable to so-called “direct” plasma systems that create a plasma in a the actual location of workpiece processing.

Although an indirect plasma processing system is illustrated inFIG. 1and elsewhere in this disclosure, it should be clear to one skilled in the art that the techniques, apparatus and methods disclosed herein may also be applicable to direct plasma processing systems—e.g., where a plasma is ignited at the location of the workpiece(s). Similarly, in embodiments, the components of processing system100may be reorganized, redistributed and/or duplicated, for example: (1) to provide a single processing system with multiple process chambers; (2) to provide multiple remote plasma sources for a single process chamber; (3) to provide multiple workpiece fixtures (e.g., wafer pedestals135) within a single process chamber; (4) to utilize a single remote plasma source to supply plasma products to multiple process chambers; and/or (5) to provide plasma and gas sources in serial/parallel combinations such that various source gases may be activated (e.g., exist at least temporarily as part of a plasma) zero, one, two or more times, and mixed with other source gases before or after they enter a process chamber, and the like. Gases that have not been part of a plasma are sometimes referred to as “un-activated” gases herein.

FIG. 2schematically illustrates major elements of a plasma processing system200, in a cross-sectional view, according to an embodiment. Plasma processing system200is an example of plasma processing unit130,FIG. 1. Plasma processing system200includes a process chamber205and a plasma source210. As illustrated inFIG. 2, plasma source210introduces gases155(1) directly, and/or gases155(2) that become plasma products in a further remote plasma source202, as plasma input gas212, through an RF electrode215. RF electrode215includes (e.g., is electrically tied to) a first gas diffuser220and a face plate225that serve to redirect flow of the source gases so that gas flow is uniform across plasma source210, as indicated by small arrows226. After flowing through face plate225, an insulator230electrically insulates RF electrode215from a diffuser235that is held at electrical ground (e.g., diffuser235serves as a second electrode counterfacing face plate225of RF electrode215). Surfaces of RF electrode215, diffuser235and insulator230define a plasma generation cavity where a capacitively coupled plasma245is created when the source gases are present and RF energy is provided through RF electrode215. RF electrode215and diffuser235may be formed of any conductor, and in embodiments are formed of aluminum (or an aluminum alloy, such as the known “6061” alloy type). Surfaces of face plate225and diffuser235that face the plasma cavity or are otherwise exposed to reactive gases may be coated with yttria (Y2O3) or alumina (Al2O3) for resistance to the reactive gases and plasma products generated in the plasma cavity. Insulator230may be any insulator, and in embodiments is formed of ceramic.

Plasma products generated in plasma245pass through diffuser235that again helps to promote the uniform distribution of plasma products, and may assist in electron temperature control. Upon passing through diffuser235, the plasma products pass through a further diffuser260that promotes uniformity as indicated by small arrows227, and enter process chamber205where they interact with workpiece50, such as a semiconductor wafer, atop wafer pedestal135. Diffuser260includes further gas channels250that may be used to add one or more additional, un-activated gases155(3) to the plasma products as they enter process chamber205, as indicated by very small arrows229. Diffuser260provides un-activated gases155(3) only on a downstream, or process chamber side, as shown.

Embodiments herein may be rearranged and may form a variety of shapes. For example, RF electrode215and diffuser235are substantially radially symmetric in the embodiment illustrated inFIG. 2, and insulator230is a ring disposed against peripheral areas of face plate225and diffuser235, for an application that processes a circular semiconductor wafer as workpiece50. However, such features may be of any shape that is consistent with use as a plasma source. Moreover, the exact number and placement of features for introducing and distributing gases and/or plasma products, such as diffusers, face plates and the like, may also vary. For example, it would be possible to substitute an arrangement for generating an inductively coupled plasma, for the capacitively coupled plasma arrangement illustrated. Also, in a similar manner to diffuser260including gas channels250to add un-activated gas155(3) to plasma products from plasma245as they enter process chamber205, other components of plasma processing system200may be configured to add or mix gases155with other gases and/or plasma products as they make their way through the system to process chamber205.

In semiconductor processing, plasma etch processes for etching polycrystalline silicon (hereinafter sometimes referred to as “polysilicon” or “poly”) are often advantageously selective to silicon over silicon dioxide (hereinafter sometimes referred to as “oxide”). One process scenario in which a plasma etch is highly selective to poly over oxide is as follows. Polysilicon is often utilized as a “gate” electrode in so-called “MOS” transistors (MOS standing for Metal-Oxide-Semiconductor, although poly, instead of metal, is usually used for the gate electrode material). To form planar MOS transistors, a “source” and “drain” are formed in a silicon wafer, with the gate defining separation of, and influencing electrical conduction between, the source and drain. A poly layer may be deposited over a dielectric layer, typically SiO2. The SiO2layer is typically very thin, to maximize field strength in this layer in the final transistor when a voltage is applied. The poly layer is typically thicker than the SiO2layer, and poly depositions are often conformal such that minor crevices on the wafer surface fill with poly. When the poly layer is patterned, the poly in these crevices must be removed to prevent adjacent transistor gates from shorting to one another. The SiO2layer may need to protect other parts of the wafer during the fabrication process, especially as portions of the poly layer are etched away to form the transistor gates. Therefore, a poly etch with low selectivity to oxide (e.g., a poly etch that etches oxide, as well) might etch through the SiO2layer and damage the underlying silicon wafer areas that are intended to form the source and drain of the transistor.

There are, however, device geometries and processes that benefit from a less selective poly etch. For example, recent technologies have moved away from the original, planar MOS process in which gates always overlie thin dielectric layers on a generally flat silicon wafer surface, and/or form capacitors with conductors across a thin dielectric layer. Certain processes etch trenches into silicon surfaces, then grow or deposit oxides on the trench surfaces to form trench devices, including trench capacitors having much larger capacitance per unit area than would be possible on the flat wafer surface. These and/or other processes generate silicon fins by, for example, generating vertical steps on a wafer, depositing poly conformally, and anisotropically etching back the poly in a vertical direction, clearing the poly from horizontal surfaces but leaving the fins behind as residual features against the vertical steps. Many other arrangements are in use or under study to reduce wafer area needed for fabrication of complex devices, to improve device performance, density and/or yield of working circuits per wafer. In such cases, a plasma etch having a selectivity that could be adjustable, or “tunable” from highly selective to poly over oxide, to less selective, or even to being selective to oxide over poly, is advantageous.

Plasma etch embodiments herein have tunable selectivity, for example selectivity to Si over oxide. These embodiments typically utilize NF3and NH3gases as plasma source gases, and/or as gases that are mixed with plasma products before reaching the workpiece. Certain other gases such as He or Ar may additionally be used as carrier gases but generally do not take part in reactions; also, small percentages (less than about 10% of total gas flow) of oxygen may be added to help with plasma ignition and to boost reaction rates in the plasma. Generally speaking, in the plasma environment NF3and NH3dissociate, and/or some of the generated products thereof can combine, to form plasma products such as free H and F radicals, HF, N2, NH4F and/or NH4F.HF. Of these plasma products, plasma activation favors formation of free H and F radicals, HF and N2, while addition of un-activated NH3favors formation of NH4F and/or NH4F.HF. When Si (e.g., polysilicon) is present, F and Si can react to form SiF4, while H and Si can react to form SiH4. When SiO2is present, NH4F or NH4F.HF can react with SiO2to form (NH4)2SiF6and H2O; importantly, (NH4)2SiF6generally forms and remains as a solid, but heat can cause it to dissociate to form SiF4, NH3and HF. Also when SiO2is present, NH4F and NH4F.HF can react with SiO2to form SiH4F, H2O and NH3. Certain of these source gases may participate in reactions even when provided in un-activated form, for example, when provided as un-activated gas flow155(3), in addition to plasma products from plasma245,FIG. 2.

Etches described herein are also tunable with respect to selectivity of poly over silicon nitride. (The classic formulation of silicon nitride is Si3N4, but stoichiometry of silicon nitride can vary depending on the conditions in which it is generated. In this disclosure, Si3N4and other stoichiometric variations of silicon nitride are referred to herein simply as SiN.) When SiN is present, NF3and SiN can react to form NF and SiF4, while NF3, NH3and SiN can react to form SiH4.HF and NH4F.HF.

It has been discovered that in embodiments, certain ones of the input gases (e.g., NF3and NH3) have different effects on the etching, as does the proportion of input gases that are activated by forming plasma therefrom, to un-activated gases (e.g., the proportion of input gas212to gas155(3), seeFIG. 2), so as to provide etches with tunable selectivity. Temperature and pressure have also been discovered to have effects. These effects can be utilized to tune an etch that fundamentally uses the same types of input gases to have different selectivities, as discussed below. As noted above, it is possible to adjust ratios of plasma products to un-activated gas (e.g., by adjusting ratios of input gas212, which forms plasma245, to gas155(3), seeFIG. 2) to further vary these effects. Furthermore, in embodiments, the etches disclosed can be performed in such a way as to maintain very high selectivity to other materials that may be present on a wafer and should be left undisturbed, such as TiN or W. Still furthermore, the etches disclosed can, in embodiments, have differing etch rates such that an etch recipe can be tuned for high etch rate during one part of an etch sequence, and a different etch rate during another part of the etch sequence.

In an embodiment, a first variation of a plasma etch process (or “recipe”) has an initial ratio of NH3to NF3as a plasma source gas (e.g., input gas212) that forms plasma products in plasma245, and adds further NH3as an un-activated gas (e.g., gas155(3)) resulting in a ratio of input gas212to gas155(3) of about 10:1. The first recipe is performed at a pressure of 2 Torr at the wafer (e.g., workpiece50in process chamber205). The pressure of 2 Torr may be maintained, for example, by monitoring the pressure in process chamber205and adjusting all gas flows upwards or downwards until the measured pressure is within some tolerance of 2 Torr. The first recipe has a poly over oxide selectivity of about 20:1. In the first recipe, the dominant plasma reactions include NF3and NH3providing substantial amounts of free F and H radicals, which react aggressively with Si but less aggressively with SiO2.

A second variation of the etch recipe flows the same ratio of NH3to NF3as input gas212, but at a lower volume, and/or adds more NH3as un-activated gas155(3), as compared to the first recipe. The gas flows of the second etch recipe result in a ratio of input gas212to gas155(3) of about 4:1 or 5:1. The second etch recipe is also performed at a pressure of 2 Torr. The resulting poly over oxide etch rate selectivity is about 2:1. The reduced etch rate selectivity is due to the adjusted gas flows favoring production of NH4F and NH4F.HF, which substantially etch SiO2, over production of F and H radicals, which preferentially etch Si.

A third variation of the etch recipe flows the same ratio of NH3to NF3as input gas212, and NH3as un-activated gas155(3), as compared to the second recipe, but is performed at a pressure of 6 Torr. The increased pressure reduces the mean free path of available F and H radicals, which again favors the etching of SiO2over the etching of Si. The third etch recipe actually etches oxide faster than poly, with a resulting etch rate selectivity of oxide over poly of about 3:1.

The etch recipes noted above may proceed faster at relatively high wafer temperatures, such as greater than 100 C. The etch recipes noted above can also be highly selective to materials such as TiN and W that are commonly utilized in semiconductor processing at the so-called “frontend” of the process (e.g., layers where transistors and gates are formed and interconnected). That is, the etch rates of TiN and W can be negligibly low. Achieving high selectivity to TiN requires that temperature of a wafer being etched be maintained at less than about 125 C, at which temperature fluorine radicals begin to attack TiN.

Because the etches described above continuously vary the ratio of NH3and NF3plasma products (e.g., free F and H radicals) to un-activated NH3that is added, and the pressure at the workpiece (e.g., in process chamber205), it is believed that the selectivity of poly to oxide etch rates can be continuously adjusted at least between the extremes noted above; that is, at least from 20:1 poly over oxide to 3:1 oxide over poly. This generates new possibilities for etch recipes, for example, the ability to tailor a plasma etch to provide a selectivity that accommodates process requirements for certain device geometries, or to change the etch selectivity to accommodate changes in device geometry. It may also simplify the deployment of plasma equipment for multiple, similar etch steps in semiconductor process flows, since the input chemistry utilizes the same gases in different ratios to produce different results. Because the types of input gases do not change, process aspects such as conditioning of chamber surfaces is expected not to change significantly, such that a single piece of processing equipment could be shifted rapidly from one process to another with little or no re-conditioning of the surfaces.

FIG. 3is a flowchart of a tunable plasma etch process300. Plasma etch process300may be implemented, for example, by plasma processing system100,FIG. 1, and/or other plasma processing systems that utilize plasma processing system200,FIG. 2. In step302, a plasma is generated in a controlled flow of a source gas that includes NH3and NF3. An example of step302is flowing a mixture of NH3and NF3as input gas212,FIG. 2, and generating plasma245therefrom. Plasma245will include plasma products such as H and F free radicals, HF and N2, as shown in reaction (1) above. In step306, a controlled flow of un-activated NH3is added to the stream of plasma products, to form an etch gas stream. An example of step306is flowing NH3as un-activated gas155(3),FIG. 2. Step310controls pressure of the etch gas stream by adjusting the source gas and un-activated NH3flows until the etch gas pressure is within a tolerance of a desired pressure. An example of step310is controlling pressure of the etch gas stream in process chamber205,FIG. 2, and adjusting flows of input gas212and un-activated gas155(3) until the pressure in process chamber205is within a tolerance of the desired pressure. Step312adjusts a ratio of source gas to un-activated NH3to vary etch rates of polysilicon and/or silicon dioxide to achieve the desired selectivity. Step312is done in concert with step310; that is, an adjustment to one of the source gas and un-activated NH3will typically be performed with an adjustment to the other, so that the etch gas pressure and flow ratio are maintained simultaneously.

FIG. 4shows a graph330that illustrates test etch rate results of a Si etch having tunable selectivity to SiO2. In the etch illustrated, NF3and NH3may be supplied either as source gases155(2) in the apparatus illustrated inFIG. 2, and activated in remote plasma source202, or may be supplied as source gases155(1) and activated in plasma245. In the test data illustrated, TiN etch rate was measured but remained at zero across all tested process conditions. When the un-activated NH3flow rate was zero, Si had a low etch rate while SiO2(labeled as “Oxide” in graph330) had an etch rate of almost zero. Therefore with an un-activated NH3flow rate of zero, the etch conditions were extremely selective to Si over oxide. As the un-activated NH3flow rate increased, both Si and oxide etch rates increased, therefore the corresponding etch conditions were less selective to Si over oxide. At the highest un-activated NH3flow rate tested, the oxide etch rate increased enough that the Si over oxide selectivity was only about 2:1.

FIGS. 5, 6 and 7illustrate a process application that advantageously utilizes a Si etch with tunable selectivity to oxide, as described herein.

FIG. 5illustrates an SiO2region350on a wafer section340, forming trenches380,385that are lined with TiN360. Atop TiN layer360is a poly layer370. A region labeled as A inFIG. 5is illustrated in greater detail inFIG. 6.

FIG. 6illustrates that poly layer370sits atop a region of SiO2375that includes a significant fraction of Si. It is important in this process that SiO2375not be etched significantly, so when SiO2375is exposed, an etch that is highly selective to poly over oxide should be employed, to minimize attack on SiO2375. Thus, the following discussion begins with generating a plasma in a controlled flow of source gas that includes NH3and NF3, to form a stream of plasma products, and adding a controlled flow of un-activated NH3to form an etch gas stream that interacts with poly layer370and SiO2375. Referring toFIG. 4, this suggests processing with an un-activated NH3flow rate of zero, or nearly zero, to maximize selectivity of the etch to poly over oxide. However, low flows of NH3are also associated with somewhat lower Si etch rate, which will make the etch process take longer if the low or zero flow rate of un-activated NH3is used continually. A compromise that provides high etch throughput and high selectivity when SiO2375is exposed, is provided by utilizing the etch with tunable selectivity as described herein. Since the poly etch rate is at least approximately known from graph330, and the thickness of poly layer370is known, the etch can start out with a relatively high un-activated NH3flow rate. For example, a flow rate of 150 to 250, in the arbitrary units illustrated inFIG. 4, can be used. This provides a high poly etch rate, and this etch rate is maintained for a time calculated as corresponding to etching partially, but not completely, through the region of poly370labeled as390inFIG. 6. Then, either in a separate etch step or without interrupting the etch, the un-activated NH3flow rate is brought down to zero, or near zero, while etching the region of poly370labeled as395. This causes the poly etch rate to drop, but the corresponding oxide etch rate also drops to near zero, as illustrated inFIG. 4, so the etch can completely clear poly370without significant attack on Si rich SiO2375. All of the process conditions are also highly selective to TiN such that a reasonable overetch can be used to clear poly in topologically difficult locations such as sidewalls of trenches380and385while leaving TiN360intact.FIG. 7illustrates the result of etching wafer section340to form wafer section340′.

FIGS. 8A, 8B and 8Cschematically illustrate a process sequence in which trace amounts of SiO2are not successfully removed by prior art processing.FIG. 8Ashows a wafer section400that includes a portion of a silicon substrate410and overlying structures. A spacer structure450is part of a fin-FET process. Certain recesses within spacer structure450contain inter-layer dielectric oxides430that are generally to be retained in the etch sequence to be described. Other recesses contain polysilicon420that is to be removed; however the polysilicon is partially oxidized along grain boundaries thereof, shown as oxide traces425, near the surface. Depth of oxide traces425is known with reasonable accuracy. Below the poly, adjacent to silicon substrate410in the same recesses of spacer structure450are thin layers of high dielectric constant oxide (“high-K oxide” hereinafter)440that are not to be etched at all.

FIG. 8Bschematically illustrates partial etching of wafer section400with a highly selective poly etch to form a partially processed wafer section401. The poly etch used is highly selective so as not to etch high-K oxide440, but because of this selectivity, oxide traces425are also not etched. Wafer section401shows how wafer section401will look as poly420is about one third etched away.

FIG. 8Cschematically illustrates further etching of wafer section400with a highly selective poly etch to form a partially processed wafer section402. Some oxide traces425remain in their original locations, while others, having lost all mechanical connection with spacer structure450, migrate about the recesses to other locations, as shown. At least some of oxide traces425cause electrical defects in the finished semiconductor product, as they interfere with the structures that should be present at their locations, and/or cause photolithographic defects in further processing.

FIGS. 9A, 9B, 9C and 9Dschematically show how the structure shown as wafer section400can be more successfully processed with a Si etch with tunable selectivity to oxide, according to embodiments herein. Thus, the following discussion begins with generating a plasma in a controlled flow of source gas that includes NH3and NF3, to form a stream of plasma products, and adding a controlled flow of un-activated NH3to form an etch gas stream that interacts with the features of wafer section400.FIG. 9Ashows wafer section400(identical to that shown inFIG. 8A).FIG. 9Billustrates partial etching of wafer section400with the etch gas stream, to form a partially processed wafer section403. The etch gas stream used to form wafer section403includes enough NH3to provide a nonzero oxide etch rate, that is, to make the etch only partially selective to poly over oxide. As shown inFIG. 9B, oxide traces425are etched along with poly420in this part of the etch process. Once the etch penetrates past oxide traces425(for example, after a time at which a measured or calculated etch rate ensures etching to or beyond the known maximum depth of oxide traces425) NH3flow can be reduced to zero or near zero, to increase selectivity of poly over oxide.FIG. 9Cillustrates partial etching of wafer section400with the etch, to form a partially processed wafer section403. The etch proceeds in this manner until poly420is completely removed, as shown in wafer section405,FIG. 9D. The high selectivity of the Si etch having tunable selectivity to oxide ensures that high-K oxide440is not damaged as poly420is removed.

As used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a process” includes a plurality of such processes and reference to “the electrode” includes reference to one or more electrodes and equivalents thereof known to those skilled in the art, and so forth. Also, the words “comprise,” “comprising,” “include,” “including,” and “includes” when used in this specification and in the following claims are intended to specify the presence of stated features, integers, components, or steps, but they do not preclude the presence or addition of one or more other features, integers, components, steps, acts, or groups.