METHOD OF ACHIEVING TOP ROUNDING AND UNIFORM ETCH DEPTHS WHILE ETCHING SHALLOW TRENCH ISOLATION FEATURES

Abstract: A method of etching a trench in a silicon layer is disclosed. The silicon layer is disposed below a hard mask layer having a plurality of patterned openings. The etching takes place in a plasma processing chamber. The method includes flowing a first etchant source gas into the plasma processing chamber, forming a first plasma from the first etchant source gas and etching through a first portion of the silicon layer with the first plasma at a first etch rate. The first etch rate being sufficiently slow to form an effective top-rounded attribute in a portion of the trench. The method further includes flowing a second etchant source gas into the plasma processing chamber, forming a second plasma from the second etchant source gas and etching through a second portion of the silicon layer with the second plasma, wherein the etching with the second plasma extends the trench into the silicon layer without unduly damaging the top rounded attribute.

Description: METHOD OF ACHIEVING TOP ROUNDING AND UNIFORM ETCH DEPTHS WHILE ETCHING SHALLOW TRENCH ISOLATION FEATURES Cross Reference to Related Application The present application is a Continuation - In - Part of U.S. Patent Application titled, "METHOD OF ACHIEVING TOP ROUNDING AND UNIFORM ETCH DEPTHS WHILE ETCHING SHALLOW TRENCH ISOLATION FEATURES", U.S. Patent Serial No. 09/346,563, Attorney Docket No. LAM1P109/P0517, filed June 30, 1999, which is incorporated herein by reference in its entirety for all purposes. Background of the Invention The present invention relates to the fabrication of semiconductor integrated circuits (IC's). More particularly, the present invention relates to methods and apparatuses for etching through an IC's layer stack to produce a top rounded isolation feature. During the manufacture of a semiconductor-based product, for example, a flat panel display or an integrated circuit, multiple deposition and/or etching steps may be employed. By way of example, one method of etching is plasma etching. In plasma etching, a plasma is formed from the ionization and dissociation of process gases. The positively charged ions are accelerated towards the substrate where they, in combination with neutral species, drive the etching reactions. In this manner, etched features such as vias, contacts, or trenches may be formed in the layers of the substrate. Recently, shallow trench isolation (STI) has grown in popularity as a preferred method for forming a trench that can, among other applications, electrically isolate individual transistors in an integrated circuit. Electrical isolation is needed to prevent current leakage between two adjacent devices (e.g., transistors). Broadly speaking, conventional methods of producing a shallow trench isolation feature include: forming a hard mask over the targeted trench layer, patterning a soft mask over the hard mask, etching the hard mask through the soft mask to form a patterned hard mask, and thereafter etching the targeted trench layer to form the shallow trench isolation feature. Subsequently, the soft mask is removed (e.g., stripped) and the shallow trench isolation feature is back-filled with a dielectric material. Figs. 1 A - 1C are cross section views of the conventional process steps that maybe used to form shallow trench isolation features. Referring initially to Fig. 1 A, there is shown a typical layer stack 10 that is part of a substrate or semiconductor wafer, (not drawn to scale for ease of illustration). A silicon layer 12 is located at the bottom of layer stack 10. A pad oxide layer 14 is formed above silicon layer 12 and a nitride layer 16 is formed above pad oxide layer 14. In most situations, the pad oxide layer is used as the interlayer that is disposed between the nitride layer and the silicon layer. Furthermore, in order to create a patterned hard mask with pad oxide layer 14 and nitride layer 16, a photoresist layer 18 is deposited and patterned using a conventional photolithography step over nitride layer 16. After patterning, soft mask openings 20 (narrow) and 22 (wide) are created in photoresist layer 18 to facilitate subsequent etching. The above-described layers and features, as well as the processes involved in their creation, are well known to those skilled in the art. Following the formation of layer stack 10, nitride layer 16 and pad oxide 14 are subsequently etched to create a hard mask, which includes a narrow hard mask opening 24 and a wide hard mask opening 26, as seen in Fig IB. The hard mask openings are used to pattern the trench during etching of the silicon layer. For the most part, etching stops after reaching silicon layer 12, however, a small portion 28 on the surface of silicon layer 12 is typically etched away during the etching of pad oxide layer 14. Moreover, a gas chemistry that includes CF4 is generally used to facilitate etching through the nitride and pad oxide layers. Typically, the CF4 chemistry etches the side walls of nitride layer 16, pad oxide layer 14 and small portion 28 of silicon layer 12 anisotropically (i.e., substantially straight down). Once hard mask openings are created through nitride layer 16 and pad oxide layer 14, silicon layer 12 is etched therethrough to form shallow trench isolation features, for example, a narrow feature 30 and a wide feature 32, as shown in Fig. lC. Typically, a gas chemistry that includes Cl 2 and/or HBr is used to facilitate etching through the silicon layer. Subsequent to the steps shown in Figs. 1B&1C, the mask layers are removed, leaving a trench disposed in the silicon layer. Typically, thereafter, the trenches are filled with a dielectric material such as an oxide (e.g., TEOS) to complete the formation of the shallow trench isolation. Typically, the shallow trench isolation features, etched in silicon, produce a sharp corner 34 in the vicinity of the interface of pad oxide layer 14 and silicon layer 12. If there is a sharp corner at the top of the feature when a dielectric material is deposited into the feature, the dielectric material may wrap around the corner creating a layer with varying thickness. Typically, the dielectric material (e.g., oxide) is the thinnest at the sharp corner. In some cases, the thin region at the sharp corner may cause a gate leakage problem. Adversely, gate leakage tends to cause malfunctions in the semiconductor device (e.g., threshold voltage problems). Additionally, in shallow trench isolation, the etch results tend to differ based on the geometry of the feature (e.g., narrow spaces and wide spaces). Typically, wide trenches have an increased etch rate over narrow trench etch rates. Correspondingly, differing etch rates tend to create a depth difference between the features. Referring to Fig. 1C, a depth difference 36 is shown between narrow feature 30 and wide feature 32. Trenches of varying depths may have adverse impacts on the functioning of the device. By way of example, if a depth non- uniformity exists between two features during subsequent deposition steps (e.g., filling the feature with dielectric), a non-level top surface may be encountered (i.e., more dielectric in one area and less dielectric in another area). Significant yield problems may be encountered when the top surface of the substrate is not flat or level. For this reason, it is desirable to create trenches that have substantially similar depths. Preferably, for a smooth functioning device, the depth difference is between 2 to 5 % or even lower. However, the semiconductor industry often sees more than a 12.5 % difference between the depths of the narrow features and the wide features. In view of the foregoing, there are desired improved techniques for etching features having rounded corners, substantially equal depths and similar profiles. Also, desired are etch techniques that may be implemented "in situ," in other words, in one plasma processing chamber. Historically, trench features have been etched with a sequential process flow, where the mask layers are etched in one chamber and the silicon trench is etched in another chamber. A significant improvement in productivity can be achieved if the mask and trench etch is performed in situ (in the same chamber). Summary of the Invention The invention relates, in one embodiment, to a method of etching a trench in a silicon layer. The silicon layer being disposed below a hard mask layer having a plurality of patterned openings. The etching taking place in a plasma processing chamber. The method includes flowing a first etchant source gas into the plasma processing chamber, forming a first plasma from the first etchant source gas and etching through a first portion of the silicon layer with the first plasma at a first etch rate. The first etch rate being sufficiently slow to form an effective top-rounded attribute in a portion of the trench. The method further includes flowing a second etchant source gas into the plasma processing chamber, forming a second plasma from the second etchant source gas and etching through a second portion of the silicon layer with the second plasma, wherein the etching with the second plasma extends the trench into the silicon layer without unduly damaging the top rounded attribute. The invention relates, in another embodiment, to a method of etching a trench in a layer stack. The layer stack including a silicon layer, a pad oxide layer and a nitride layer. The silicon layer being disposed underneath the pad oxide layer, and the pad oxide layer being disposed underneath the nitride layer. The layer stack including a plurality of patterned openings, which are opened to at least the pad oxide layer. The etching taking place in a plasma processing chamber. The method includes flowing a first etchant source gas into the plasma processing chamber. The etchant source gas including a first gas suitable for etching the pad oxide layer and a second gas suitable for silicon etching. The method further includes forming a first plasma from the etchant source gas, and etching through the pad oxide layer to about the silicon layer with the first plasma. The method also includes removing the first gas from the first etchant source gas to form a modified first plasma, and thereafter etching through a portion of the silicon layer with the modified first plasma. The etching being performed at a first etch rate that is sufficiently slow to form an effective top-rounded attribute on a portion of the trench. Brief Description of the Drawings The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: Fig. 1 A illustrates a cross-sectional view of a layer stack, representing some of the layers formed during the fabrication of a shallow trench isolation. Fig. IB shows the same layer stack of Fig. 1A after a hard mask has been etched through the nitride and pad oxide layers. Fig. 1C depicts the same layer stack of Fig. 1 A after a shallow trench isolation feature has been etched through the silicon layer. Fig. 2 is a simplified schematic diagram of a plasma reactor that may be suitably employed with the inventive etch process, according to one embodiment of the present invention. Fig. 3 A illustrates a cross-sectional view of a layer stack, prior to being etched using the inventive etch process, according to one embodiment of the present invention. Fig. 3B shows the same layer stack of Fig. 3A after a hard mask has been etched through the nitride layer, according to one embodiment of the present invention. Fig. 3C shows the same layer stack of Fig. 3 A after the pad oxide and a portion of the silicon layer have been etched using the inventive etch process that employs either the CH F or CHF 3 etchant source gas, according to one embodiment of the present invention. Fig. 3D shows the same layer stack of Fig. 3 A after a shallow trench isolation feature has been etched through the silicon layer using the inventive etch process, according to one embodiment of the present invention. Fig. 4A illustrates a broken away side view of a shallow trench isolation feature that has a small radius of curvature at the shoulder of the shallow trench isolation feature. Fig. 4B illustrates a broken away side view of a shallow trench isolation feature that has a moderate radius of curvature at the shoulder of the shallow trench isolation feature. Fig. 4C illustrates a broken away side view of a shallow trench isolation feature that has a large radius of curvature at the shoulder of the shallow trench isolation feature. Fig. 5 A illustrates a cross-sectional view of a typical layer stack with hard mask openings, prior to being etched using the inventive etch process, according to one embodiment of the present invention. Fig. 5B shows the same layer stack of Fig. 5 A after a top portion of the silicon layer has been etched using the inventive etch process that employs CH 2 F 2 / HBr etchant source gas, according to one embodiment of the present invention. Fig. 5C shows the same layer stack of Fig. 5A after a shallow trench isolation feature has been etched through the silicon layer using the inventive etch process, according to one embodiment of the present invention. Detailed Description of Preferred Embodiments The present invention will now be described in detail with reference to a few preferred embodiments thereof and as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention. There are certain key attributes of the shallow trench isolation etch that are required to make sure that the device will perform properly. By way of example, for shallow trench isolation features etched into silicon, it is necessary to provide a rounded comer at the top of the trench, to prevent current leakage in the semiconductor device. Typically, a rounded comer produces a smooth contour that enables subsequent layer deposition steps to have an even thickness around the comer. A comer with more radius is less likely to wrap around and therefore tends to reduce gate leakage problems. As the term is employed herein, an effective top rounded feature is a top rounded feature that is sufficiently contoured, i.e. lacking sharp comers, to substantially eliminate gate leakage problems. In accordance with one aspect of the present invention, top-rounded features in shallow trench isolation are achieved "in situ" by etching a nitride layer (e.g., Si x N y ) with a first etchant source gas chemistry and etching a pad oxide layer with a second etchant source chemistry that is substantially selective to silicon in a single plasma processing chamber. The etchings preferably take place while the substrate is disposed on a chuck within a single plasma processing chamber. Typically, the first etchant source gas is flowed into the chamber, and the pressure is stabilized. After the pressure has been stabilized the plasma is ignited, thereby generating ions from the etchant source gas. Next the plasma is stabilized and the ions are accelerated towards the substrate where they assist in etching the nitride layer, substantially stopping on the pad oxide layer or shortly after reaching the nitride/pad oxide interface. As mentioned, the etched nitride layer may be patterned with a photoresist mask or another suitable mask. The first etchant source gas is then evacuated from the chamber and the second etchant source gas is flowed into the chamber. Preferably, a chemistry having a relatively high selectivity to silicon is used. Preferably, the chemistry has a pad oxide:silicon selectivity at least about 1.2:1, more preferably at least 2:1, and preferably at least 3:1. After the pressure has been stabilized, the plasma is ignited, thereby generating ions from the second etchant source gas. The ions are accelerated towards the substrate where they assist in etching the pad oxide layer. Advantageously, as the pad oxide is etched through, the oxide etch process starts to round the comer of the silicon at the pad oxide/silicon interface. That is, the top rounding in silicon occurs as the oxide etch process reaches the pad oxide/silicon interface. While not wishing to be bound by theory, it is believed that the top rounding occurs because of the high silicon selectivity in the etchant source gas employed in the oxide etch step and due to the build up of polymer along the side walls of the hard mask and above the silicon. In other words, as the oxide etchants etch away the silicon slowly, a passivating film is built up along the vertical side walls. Because the slow downward etch (due to the high silicon selectivity of the oxide etchant) through silicon is accompanied by simultaneous lateral growth of the passivating film, the silicon etch profile near the pad oxide/silicon interface has both a vertical downward component and a lateral sideway component as the etch progresses. Accordingly, the shoulder that is formed thereby is a gradual slope instead of an abrupt drop off, i.e., a top rounded feature. Moreover, the passivating film that forms a protective mask along the side walls of the nitride and pad oxide layer sits above the top rounded feature in the silicon layer, so that during subsequent steps, the top rounding is preserved. If the passivating film were not covering the top rounding during the silicon trench etch, the silicon trench etch would have etched straight through the top rounding, leaving a sharp comer at the pad oxide/silicon interface without top rounding. Furthermore, when the pad oxide etchant encounters the silicon interface, the etch rate slows down (e.g., due to the high silicon selectivity) such that the wide features do not etch deeper into the silicon than the narrow features. That is, the amount (e.g., depth) of silicon removed during the pad oxide etch is substantially the same between the narrow and wide features. In this manner, the depth difference is substantially reduced and depth uniformity between feature sizes is substantially maintained during the subsequent silicon etch. By the time the silicon etch is completed, the depth among the features are more closely balanced. Effectively balanced etch depths in narrow and wide regions in the silicon layer means that the etch depths are balanced enough to substantially eliminate silicon etch-depth related yield problems. In accordance with one important aspect of the present invention, the pad oxide etch is allowed to prolong for a sufficient amount of time to allow the top rounded feature to form and/or to substantially balance the etch depths between the narrow and wide areas after the pad oxide layer is etched through. Subsequent to the etching of the hard mask opening through the nitride and pad oxide layers, which forms the top rounding of the silicon features and the a protecting passivation layer, the second etchant source gas is evacuated from the chamber and a third etchant source gas, which is configured to etch the silicon layer is flowed into the chamber. After the pressure has been stabilized the plasma is ignited, thereby generating ions from the third etchant source gas. The ions are accelerated towards the substrate where they assist in etching the silicon layer thereby forming the shallow trench isolation feature. As mentioned, the passivation layer advantageously protects the top rounding at the silicon/pad oxide interface. In one embodiment, a first etchant source chemistry that includes a fluorocarbon (e.g., CF 4 , C 2 F 6 and/or CHF 3 ) is preferably employed to etch features through a nitride layer down to an underlying pad oxide layer. Furthermore, a second etchant source chemistry that includes a hydro-fluorocarbon (e.g., CHF 3 , CH 2 F , CH 3 F,CFl4, C 4 F 8 , C 2 F 6 , C 2 HF 5 ) is preferably employed to etch features through the pad oxide layer down to an underlying silicon layer. Preferably, the hydro-fluorocarbon etchant source chemistry has a high selectivity with respect to the underlying silicon layer. A third etchant source chemistry that includes a chlorine (e.g., Cl 2 ) and/or bromine (e.g., HBr) is employed to etch through the silicon layer to form a shallow trench isolation feature. In a preferred embodiment, the present invention is practiced in a high density plasma reactor, such as the inductively coupled TCP™ 9400 plasma reactor, which is available from Lam Research Corporation of Fremont, CA. Fig. 2 illustrates a simplified schematic of the TCP™ 9400 plasma reactor. Plasma reactor 100 includes a plasma processing chamber 102 with a chamber wall 103. Chamber wall 103 is preferably made from anodized aluminum. To provide an electrical path to ground, the chamber wall of chamber 102 is typically grounded. Above chamber 102, there is disposed an inductive electrode 104 (represented by a coil). Inductive electrode 104 is powered by a first RF power source 106 via a matching network (not shown in Fig. 2 to simplify the illustration). Preferably, a dielectric window 108 is disposed below inductive electrode 104. Typically, a gas port 110 is provided within chamber 102. Gas port 110 is preferably disposed around the inner periphery of chamber 102 and is arranged for releasing gaseous source materials, e.g., the etchant source gases, into the RF-induced plasma region between dielectric window 108 and a substrate 112. Alternatively, the gaseous source materials may also be released from ports built into the walls of the chamber itself or through a shower head arranged in the dielectric window. Substrate 112 is introduced into chamber 102 and disposed on a chuck 114, which acts as a bottom electrode and is preferably biased by a second RF power source 116 (also typically via a matching network). Chuck 114 may optionally include a focus ring (not shown to simplify illustration), preferably made from silicon oxide or aluminum oxide ceramic, positioned around the bottom electrode. Chuck 114 may represent, for example, an ESC (electrostatic) chuck, which secures substrate 112 to the chuck's surface by electrostatic force. Still referring to Fig. 2, an exhaust port 118 is typically disposed on one side of chamber 102. Exhaust port 118 is coupled to a turbomolecular pump (not shown), typically located outside of chamber 102. For the most part, the turbomolecular pump maintains the appropriate pressure inside chamber 102. Although, the high density, inductively coupled reactor is employed as the preferred apparatus for forming shallow trench isolation features and has shown particularly good results, any plasma reactor that is suitable for forming a plasma, such as a capacitively coupled reactor or wave excited reactor, e.g., ECR (microwave) or helicon resonator, may be used. In accordance with one embodiment of the present invention, an inventive etch process employing a first etch chemistry that includes CF 4 , a second etch chemistry that includes CHF 3 and/or CH 2 F 2 and a third etch chemistry that includes Cl 2 is employed to achieve top-rounded features with good depth uniformity. Both CHF 3 and CH 2 F 2 etch chemistries are fluorocarbon gases that provide a pad oxide to silicon selectivity of greater than about 2:1. Fig. 3A illustrates a typical layer stack 200 that may be etched with the inventive etch process to form a shallow trench isolation feature, in accordance with one embodiment of the present invention A photoresist mask 210 with a patterned narrow mask opening 212 and wide mask opening 213 is disposed above a nitride layer 214. Nitride layer 214 is disposed above a pad oxide layer 216. Typically, the nitride layer and the pad oxide layer form a hard mask for the silicon etch step. For the most part, the nitride and pad oxide layers are sacrificial layers that will be removed in subsequent processes and therefore the thickness of the nitride and pad oxide layers tend to vary. Furthermore, pad oxide layer 216 is disposed above a silicon layer 218 (monocrystalline or epitaxcal). Preferably, the silicon layer is used to form the shallow trench isolation features. Fig. 3B shows the layer stack 200 of Fig. 3 A after nitride layer 214 is etched using a CF 4 -based etchant source gas, in accordance with one embodiment of the present invention. Through the patterned narrow mask opening 212 and wide mask opening 213, etchants (formed by a plasma created with the CF 4 -based etchant source gas) react with the material of nitride layer 214 to form an etched feature in nitride layer 214. During this etching process, the plasma tends to anisotropically etch a narrow nitride opening 220 and a wide nitride opening 221. The plasma also tends to erode away some of photoresist layer 210 from an initial thickness. Etching with the first etchant source gas is substantially stopped on pad oxide layer 216 or shortly thereafter, as shown. Fig. 3C shows the layer stack 200 of Fig. 3 A after pad oxide layer 216 is etched using a CH 2 F 2 and/or CHF 3 -based etchant source gas, in accordance with one embodiment of the present invention. Through the patterned mask openings 212 and 213, and patterned nitride openings 220 and 221, etchants (formed by a plasma created with the CH 2 F 2 or CHF 3 -based etchant source gas) react with the material of the pad oxide layer 216 to form an etched feature in pad oxide layer 216. During this etching process, the plasma tends to anisotropically etch a narrow pad oxide opening 222 and a wide pad oxide opening 223. The plasma also tends to erode away some more of photoresist mask 210. Further, a narrow top portion 224 and a wide top portion 225 of silicon layer 218 is etched during the etching of pad oxide layer 216. The etching of the silicon layer forms a top rounded attribute 226 at the shoulders of top portions 224 and 225, which starts at about the silicon layer/pad oxide layer interface, as shown. More specifically, the process etches through the pad oxide and begins to etch the silicon with a tapered profile (e.g., less than 80 degrees) profile, which forms the top rounded attribute. By controlling the gas chemistry and reactor conditions, a passivating film 228 is deposited on the wall of the features (e.g., nitride , pad oxide and silicon), while the silicon etches. Typically, the passivation layer is formed from remnants of the pad oxide, silicon and photoresist layers. Preferably, the pad oxide etch is allowed to continue for an effective amount time after the pad oxide layer is etched through to allow the formation of an effective top rounded feature and/or to balance the etch depths in the silicon layer to a desired level. Advantageously, the passivating film is also deposited above the top rounded attribute. In this manner, passivating film 228 acts as a mask that protects top rounded attributes 226 during the subsequent silicon trench etch. The passivation layer may be subsequently stripped off (e.g., photoresist strip sequence). Fig. 3D shows the layer stack 200 of Fig. 3 A after silicon layer 218 is etched using a Cl -based etchant source gas, in accordance with one embodiment of the present invention. Through the patterned mask openings 212 and 213, the patterned nitride openings 220 and 221, and patterned pad oxide openings 222 and 223, etchants (formed by a plasma created with the Cl -based etchant source gas) react with the material of the silicon layer 218 to form a narrow shallow trench isolation feature 230 and a wide shallow trench isolation feature 231 in silicon layer 218. During this etching process, the plasma tends to anisotropically etch shallow trench isolation features 230 and 231. Typically, a profile angle between about 75 degrees and 89 degrees is formed. The etch profile is measured as an angle formed by the etch side wall with a plane parallel to the top surface of the silicon layer. The plasma also tends to erode away some of photoresist layer 210. As discussed, top rounded attributes 226 are kept intact by the passivating film 228 during the this silicon etching process. The resultant trench features, after removal of the passivating film, are silicon trench features with top rounding starting at about the pad oxide/silicon interface. Additionally, because the depth of the top portion of the wide and narrow features (Fig. 3C) before the silicon etch are substantially the same, depth uniformity is better achieved between the narrow features and wide features. As shown, the difference 232 between the narrow and wide features is relatively insignificant. In one exemplary application of this invention, a 200 mm substrate having thereon a silicon layer (e.g., monocrystalline or polycrystalline), a pad oxide layer (e.g., SiO 2 ), and a nitride layer (e.g., Si 3 N 4 ), is etched in the aforementioned TCP™ 9400 plasma processing system. The pad oxide has a thickness on the order of 80 to 150 angstroms. The nitride layer has a thickness on the order of 1000 to 1800 angstroms. The thickness of the photoresist layer is typically between about 6,000 to 8,000 angstroms. The smallest trench has a feature size of 0.18 μm and the largest trench has a feature size of greater than 2.0 μm. However, it should be noted that this invention is not limited to a particular sized substrate, a particular feature size or a particular plasma processing system. In this application, the nitride layer is etched using an etch process that employs the CFψΗBr-based chemistry. The pressure within the aforementioned plasma processing reactor system is preferably maintained from about 5 milliTorr (mT) to about 50 mT, and preferably at about 10 mT during nitride etching. The top RF power is preferably maintained from about 200 watts (W) to about 800 W, and preferably at about 600 W. The bottom electrode power is preferably maintained from about 40 watts (W) to about 300 W, and preferably at about 120 W. The flow rate of CF 4 is preferably from about 20 standard cubic centimeters per minute (seem) to about 200 seem, and preferably at about 100 seem during nitride etching. Preferably, the flow rate of CF 4 is between about 50% to 80% of the total flow. The flow rate of HBr is preferably from about 0 standard cubic centimeters per minute (seem) to about 100 seem, and preferably at about 50 seem during etching. The ratio of the HBr flow rate to the CF 4 flow rate is preferably from about 0% to 75% and preferably at about 50%. It should be noted that the above chemistry may include CHF 3 . The flow rate of CHF 3 may be as little as 0 standard cubic centimeters per minute (seem) to about 100 seem or about 0% to about 100% of the CF 4 flow. Furthermore, the etch source gas described above may also include a diluent gas, such as Argon. The pad oxide is etched using an etch process that employs the CHF 3 /CF 4 based chemistry. It should be noted that CH 2 F 2 may be used with a similar etch recipe as CHF 3 , although with differing shallow trench isolation feature results. Both chemistries provide good selectivity to silicon - CHF 3 is at about 2 to 1 and CH 2 F 2 is about 10 to 1. Generally, a higher silicon selectivity offers better depth uniformity between feature sizes. CH 2 F tends to form a more contoured top rounded feature, typically, because it is more selective to silicon and therefore the vertical component of the etch is less and the CH 2 F 2 also tends to deposit more polymer. The pressure within the aforementioned plasma processing reactor system is preferably maintained from about 5 milliTorr (mT) to about 70 mT, and preferably at about 30 mT during pad oxide etching. The top RF power is preferably maintained from about 200 watts (W) to about 800 W, and preferably at about 600 W. The bottom electrode power is preferably maintained from about 50 watts (W) to about 300 W, and preferably at about 200 W. The flow rate of CF 4 is preferably from about 10 standard cubic centimeters per minute (seem) to about 70 seem, and preferably at about 25 seem during pad oxide etching. Preferably, the flow rate of CF 4 is between about 15% to 50% of the total flow. The flow rate of CHF 3 is preferably from about 20 standard cubic centimeters per minute (seem) to about 120 seem, and preferably at about 100 seem during etching. The ratio of the CHF 3 flow rate to the CF 4 flow rate is preferably from about 100% to 500% and preferably at about 350%. It should be noted that the above chemistry may include O 2 . The flow rate of O may be as little as 0 standard cubic centimeters per minute (seem) to about 10 seem, or about 0% to about 50% of the CF 4 flow. Furthermore, the etch source gas described above may also include a diluent gas, such as Argon or Helium (e.g., 0 - 300 seem or 0% to about 1000% of the CF 4 flow). To facilitate the formation of an effective top-rounded feature and/or to sufficiently balance the etch depths in the silicon layer, the pad oxide etch is preferably continued for at least 5 seconds after the pad oxide layer is etched through and more preferably at least 15 seconds. The termination of the pad oxide etch may be performed using any endpoint or time etch technique. The silicon layer is etched, in this example, using an etch process that employs the Cl 2 /O 2 /HBr-based chemistry. The pressure within the aforementioned plasma processing reactor system is preferably maintained from about 5 milliTorr (mT) to about 30 mT, and preferably at about 10 mT during silicon etching. The top RF power is preferably maintained from about 250 watts (W) to about 800 W, and preferably at about 450 W during silicon etching. The bottom electrode power is preferably maintained from about 80 watts (W) to about 300 W, and preferably at about 120 W during silicon etching. The flow rate of Cl 2 is preferably from about 20 standard cubic centimeters per minute (seem) to about 70 seem, and preferably at about 35 seem during etching. Preferably, the flow rate of Cl 2 is between about 10% to 40% of the total flow. The flow rate of O 2 is preferably from about 2 standard cubic centimeters per minute (seem) to about 12 seem, and preferably at about 8 seem during etching. The ratio of the O flow rate to the Cl 2 flow rate is preferably from about 10% to 30% and preferably at about 20%. The flow rate of HBr is preferably from about 100 standard cubic centimeters per minute (seem) to about 200 seem, and preferably at about 150 seem during etching. The ratio of the HBr flow rate to the Cl flow rate is preferably from about 130% to 600% and preferably at about 400%. Furthermore, the etch source gas described above may also include a diluent gas, such as Argon. It should be noted, however, that the flow rates of the component source gases may be scaled as needed when smaller or larger substrates are involved. Thus, while the relative flow rate among the various component gases (which may be expressed as a percentage of the CF 4 flow rate, for example) is important, the absolute flow rate for each component gas may be modified as needed to accommodate different substrate sizes. In another embodiment, an alternate process flow would be to etch the nitride layer in a separate chamber, stopping at about the pad oxide layer or shortly thereafter. The pad oxide could then be etched in situ with the trench etch in another chamber, using the above mentioned chemistries. Alternatively, the nitride layer and the pad oxide layer may be etched together in situ, with the bulk silicon etch being performed in a separate chamber. As can be seen from the foregoing, the present invention offers numerous advantages over the prior art. By way of example, the invention substantially eliminates sharp comers at the shoulder of a shallow trench isolation feature. The invention forms a top rounded feature that is located at about the interface between the pad oxide and silicon. Correspondingly, the top rounded feature reduces the possibilities of gate leakage at the shallow trench isolation features in the semiconductor device. Further, the inventive technique improves substrate throughput by not requiring any additional process steps to produce the trench feature . By way of example, it is now possible to form the top rounded attribute during the hard mask etch step. Furthermore, the invention eliminates the need for oxidation steps that are usually required to overcome the gate leakage problem, which is caused by sharp comers. Still further, the invention allows an in situ approach to achieving top rounding of the silicon trench. For example, the hard mask etch, the forming of the top rounded attribute and the silicon etch can be done in the same chamber. Correspondingly, the "in situ" approach reduces handling and additional processing steps and machines that may add to the time and cost of making a semiconductor device. Additionally, a protective passivating film is formed around the top rounded attribute. In this manner, the passivating film acts as a mask that protects the top rounded attribute during the subsequent silicon etching. Furthermore, the invention reduces the depth difference that is usually encountered between narrow and wide features during shallow trench isolation etching. By way of example, the invention is capable of providing a depth difference between about 2% to about 5% or even lower. Sometimes the "in situ" approach described above is not available to some manufacturers. That is, separate systems are used to create the hard mask opening and the shallow trench isolation. For example, one system may be implemented to open the nitride layer and/or the pad oxide layer, another system may be implemented to remove the photoresist, while even another system may be implemented to form the silicon shallow trench isolation. Therefore, an "ex situ" method for producing top rounded features in shallow trench isolation etch is provided. In accordance with a another aspect of the present invention, top-rounded features in shallow trench isolation are achieved by etching a first portion of a silicon layer with a first etchant source gas chemistry and etching a second portion of a silicon layer with a second etchant source gas chemistry. Preferably, the first etchant source gas has a first etch rate that is slower than a second etch rate of the second etchant source gas. Correspondingly, the first etchant source gas is used to form the top rounding and the second etchant source gas is used to form the shallow trench isolation. The etchings preferably take place while the substrate is disposed on a chuck within a single plasma processing chamber. Typically, the first etchant source gas is flowed into the chamber, and the pressure is stabilized. After the pressure has been stabilized the plasma is ignited, thereby generating ions from the etchant source gas. Next the plasma is stabilized and the ions are accelerated towards the substrate where they assist in etching the first portion of the silicon layer. As the first portion of the silicon layer is etched through, the first etch process starts to round the comer of the silicon. That is, the top rounding in silicon occurs about the time the first etch process starts to etch the silicon. Preferably, the etching starts at about the pad oxide/silicon interface or on an unetched surface of the silicon layer. However, because prior steps were used to open the hard mask (e.g., nitride layer and/or pad oxide layer), some of the silicon layer may have been removed. As is well known to those skilled in the art, some tools are better suited than others for stopping directly on the surfaces of the substrate. As long as the silicon layer removed is not excessive, top rounding may still be achieved. For the most part, the first etch should preferably take place as close as possible to the pad oxide/silicon interface. While not wishing to be bound by theory, it is believed that the top rounding occurs because of the slow etch rate in the first etchant source gas and due to the build up of polymer along the side walls of the hard mask and above the silicon. In other words, as the first etchants etch away the silicon slowly, a passivating film is built up along the vertical side walls. Because the slow downward etch through silicon is accompanied by simultaneous lateral growth of the passivating film, the silicon etch profile near the pad oxide/silicon interface has both a vertical downward component and a lateral side way component as the etch progresses. Accordingly, the shoulder that is formed thereby is a gradual slope instead of an abrupt drop off, i.e., a top rounded feature. Advantageously, the radius of curvature of the top rounding at the shoulder may be controlled by controlling the amount of polymer (passivating film) that is generated by the first etch. By controlling the radius of curvature, near equal behavior in narrow (dense regions) and wide (isolated regions) features may be achieved. That is, the invention may reduce the radius of curvature difference that is usually encountered between narrow and wide features during shallow trench isolation etching. Furthermore, with increased control, a wider range of curvature may be obtained for shallow trench isolation features. For example, the process engineer may be capable of producing a radius of curvature between about 100 and 600 angstroms. Alternatively or additionally, the radius of curvature of the top rounded feature may be controlled by adjusting the temperature of the bottom electrode on which the substrate is disposed. Alternatively or additionally, the radius of curvature of the top rounded feature may be controlled by adjusting the length of time of etching with the first etchant source gas. For the most part, the radius of curvature is a potential tradeoff between how close the features are packed together, the desired critical dimensions of the features, and the possibilities of gate leakage. If the features are closely packed together, a smaller curvature that cuts off sooner may be used. However, if the critical dimensions are more important, then a more moderate curvature should be used. Further, if gate leakage is a problem then a larger curvature may be needed. For exemplary purposes, Figs. 4A-4C illustrate a broken away side view of a typical layer stack 400 with three different radius of curvature that may be obtained using the present invention. In general, the radius of curvature is determined using an arc length model. For example, a template gauge is used to measure the arc length of the rounding and a conversion factor is used to determine the radius of curvature. Each of the figures includes a silicon layer 402, a pad oxide layer 404, a nitride layer 406 and a shallow trench isolation feature 408 having a shoulder 409. Fig 4A illustrates a small radius of curvature 410 (e.g., 100 angstroms) at shoulder 409 that has a deep angle 412 below the pad oxide/silicon interface. Fig 4B illustrates a moderate radius of curvature 414 (e.g., 400 angstroms) at shoulder 409 that has a moderate angle 416 below the pad oxide/silicon interface. Fig 4C illustrates a large radius of curvature 418 (e.g., 600 angstroms) at shoulder 409 that has a shallow angle 420 below the pad oxide/silicon interface. While not wishing to be bound theory, it is believed that a shallow angle helps to reduce gate wrap around, while a deep angle tends to increase the chances of gate wrap around. Further, it is recognized that there can be a maximum radius of curvature in narrow features that can be achieved without compromising characteristics such as critical dimension (CD) control. Accordingly, it is generally preferable to use a moderate radius of curvature to balance the aforementioned effects. However, it should be understood that this is not a limitation and that the radius of curvature may vary according to the specific needs of each feature. Moreover, the passivating film that forms a protective mask along the side walls of the nitride and pad oxide layer sits above the top rounded feature in the silicon layer, so that during subsequent steps, the top rounding is preserved. If the passivating film were not covering the top rounding during the second silicon trench etch, the second silicon trench etch would have etched straight through the top rounding, leaving a sharp comer at the pad oxide/silicon interface without top rounding. Furthermore, the first etchant etch rate is typically time dependent. That is, the etch rate slows down as the first etchant etches deeper into the silicon. As a result, the wide features do not etch deeper into the silicon than the narrow features. That is, the amount (e.g., depth) of silicon removed during the first etch is substantially the same between the narrow and wide features. In this manner, the depth difference is substantially reduced and depth uniformity between feature sizes is substantially maintained during the subsequent second silicon etch. By the time the second silicon etch is completed, the depth among the features are more closely balanced. Effectively balanced etch depths in narrow and wide regions in the silicon layer means that the etch depths are balanced enough to substantially eliminate silicon etch-depth related yield problems. In accordance with one important aspect of the present invention, the first etch is allowed to prolong for a sufficient amount of time to allow the top rounded feature to form and/or to substantially balance the etch depths between the narrow and wide areas after the first portion of the silicon layer is etched through. Furthermore, subsequent to the etching of the first portion of the silicon layer through the hard mask layers for a certain amount of time, which forms the top rounding of the silicon features and the a protecting passivation layer, the first etchant source gas is evacuated from the chamber and a second etchant source gas, which is configured to etch the second portion of the silicon layer is flowed into the chamber. After the pressure has been stabilized the plasma is ignited, thereby generating ions from the second etchant source gas. The ions are accelerated towards the substrate where they assist in etching the second portion of the silicon layer thereby forming the shallow trench isolation feature. As mentioned, the passivation layer advantageously protects the top rounding at the silicon/pad oxide interface. In accordance with another aspect of the present invention, a first etchant source chemistry that includes a hydro-fluorocarbon and bromine is preferably employed to etch the first portion of the silicon layer to form top rounding, and a second etchant source chemistry that includes a chlorine (with or without an oxygen) is employed to etch through the second portion of the silicon layer to form a shallow trench isolation feature. The hydro-fluorocarbon etchant source chemistry preferably has a slow etch rate through silicon, and the chlorine etchant source chemistry preferably has a fast etch rate through silicon. Preferably, the etch rate ratio between the second etchant source chemistry and the first etchant source chemistry is between about 3 :2 to about 6:1. More preferably, the etch rate ratio between the second etchant source chemistry and the first etchant source chemistry is about 3:1. In a preferred embodiment, the first etch employs a first etch chemistry that includes CH 2 F and HBr, and the second etch employs a second etch chemistry that includes Cl 2 and O 2 to achieve top-rounded features in shallow trench isolation. The CH 2 F 2 and HBr chemistry provides an etch rate in silicon between about 1000 to about 2000 angstroms per minute. The Cl 2 and O 2 chemistry provides an etch rate in silicon between about 3000 to about 6000 angstroms per minute. In order to form the top rounded feature, the first etch (e.g., using CH 2 F 2 and HBr) is preferably allowed to prolong for a sufficient amount of time, for example, between about 10 to about 25 seconds. More preferably, the first etch (e.g., using CH 2 F 2 and HBr) is allowed to prolong for about 15 seconds. Furthermore, the gas flow ratio between CH 2 F 2 and HBr is preferably between about 10:1 to about 1 :1. More preferably, the gas flow ratio is between CH F 2 and HBr is about 3:1. By using the aforementioned chemistries, times and ratios, a top rounded shallow isolation trench may be formed that has a top rounded radius of curvature between about 100 angstroms and about 600 angstroms. Further, by using the aforementioned chemistries and times, the etch depths, radius of curvature and slope of the shallow trench isolation tend to be substantially balanced between the narrow (e.g., dense) and wide (isolated) features. The features and advantages of the present invention may be better understood with reference to the figures and discussions that follow. Fig. 5 A illustrates a typical layer stack 500 that may be etched with the inventive etch process to form a shallow trench isolation feature, in accordance with one embodiment of the present invention. A nitride layer 502 is disposed above a pad oxide layer 504. The nitride layer and the pad oxide layer preferably form a hard mask for the silicon etch steps. For the most part, the nitride and pad oxide layers are sacrificial layers that will be removed in subsequent processes and therefore the thickness of the nitride and pad oxide layers tend to vary. Furthermore, pad oxide layer 504 is disposed above a silicon layer 506, which may be, for example, a monocrystalline silicon layer or an epitaxcal silicon layer. Preferably, silicon layer 506 is used to form the shallow trench isolation features. Layer stack 500 also includes patterned narrow mask openings 508 and wide mask openings 510, which are disposed in nitride layer 502 and pad oxide layer 504. For the most part, mask openings 508, 510 were defined and formed in previous steps, which are well known to those skilled in the art. Preferably, mask openings 508, 510 are anisotropically formed and substantially stopped on silicon layer 506. Furthermore, the photoresist which was used to pattern the hard mask openings has been removed. Fig. 5B also shows layer stack 500 of Fig. 5A after a top portion 512 of silicon layer 506 is etched using a CH 2 F and HBr-based etchant source gas, in accordance with one embodiment of the present invention. Through the patterned narrow mask opening 508 and wide mask opening 510, etchants (formed by a plasma created with the CH 2 F 2 and HBr-based etchant source gas) react with the material of silicon layer 506 to form top rounding and to start a narrow feature 514 and a wide feature 516. Etching with the first etchant source gas is allowed to prolong for a certain amount of time to form a top rounded attribute 518 at the shoulders of narrow feature 514 and wide feature 516, which starts at about the silicon layer/pad oxide layer interface. More specifically, the first etch begins to etch the silicon with a tapered profile (e.g., less than 80 degrees) profile, which forms the top rounded attribute. By controlling the gas chemistry and reactor conditions, a passivating film 526 is deposited on the wall of the features (e.g., nitride , pad oxide and silicon), while the silicon etches. Typically, the gas chemistry and reactor conditions (e.g., process pressure, bias power, gas ratios, etc.) effect the amount of polymer being created (i.e., passivating film), which as a result effects the radius of curvature of the top rounding. Further, the passivating film is generally formed from remnants of the silicon layer, and gas chemistry polymers, and oxidation that forms at the exposed silicon layer. Preferably, the silicon etch is allowed to continue for an effective amount time to allow the formation of an effective top rounded feature and/or to balance the etch depths in the silicon layer to a desired level. Advantageously, the passivating film is also deposited above the top rounded attribute. In this manner, passivating film 526 acts as a mask that protects top rounded attributes 226 during the subsequent silicon trench etch. The passivating film may be subsequently stripped off (e.g., photoresist strip sequence). Fig. 5C shows layer stack 500 of Fig. 5A after a bottom portion 520 of silicon layer 506 is etched using a Cl and O 2 -based etchant source gas, in accordance with one embodiment of the present invention. Through patterned mask openings 508 and 510, etchants (formed by a plasma created with the Cl and O 2 -based etchant source gas) react with the material of silicon layer 506 to form a narrow shallow trench isolation feature 522 and a wide shallow trench isolation feature 524 in silicon layer 506. During this etching process, the plasma tends to anisotropically etch shallow trench isolation features 522 and 524. Typically, a profile angle between about 75 degrees and 89 degrees is formed. The etch profile is measured as an angle formed by the etch side wall with a plane parallel to the top surface of the silicon layer. As discussed, top rounded attributes 518 are kept intact by a passivating film 526 during the second silicon etch. The resultant trench features, after removal of the passivating film, are silicon trench features with top rounding starting at about the pad oxide/silicon interface. Additionally, because the depth of the top portion of the wide and narrow features (Fig. 5B) before the second silicon etch are substantially the same, depth uniformity is better achieved between the narrow features and wide features. For the most part, the depth difference between the narrow and wide features is relatively insignificant after using the inventive etch process. Additionally, a 3 to 5% difference in slope between the narrow and wide features is achieved when using the aforementioned inventive process. The slope or profile angle is generally measured as the angle formed by the etch side wall with a plane parallel to the top surface of the silicon layer. For most processes, the photoresist remains on the substrate through the trench etch. The presence of the photoresist during silicon etching can degrade some of the trench features, particularly for profile control between narrow and wide features. The typical result for profile slope variation between narrow (e.g., dense lines) and wide features (e.g., open lines) is 8 - 12 degrees. While not wishing to be bound by theory, it is believed that by removing the photoresist before the silicon etch, the trajectory angles of the polymer tend to be more closely matched between the wide and narrow features. That is, the deposition coming out of the plasma can see the side walls of the narrow feature more clearly. By removing the photoresist, the polymer tends to be deposited in a more even manner between the narrow and wide features. Accordingly, the absence of the photoresist results in better control of the profile angle. By way of example, the wide features (isolated regions) may have a slope of about 80 and the narrow features (dense regions) may have a slope between about 83 and 85 degrees. In an exemplary application of this invention, a 200 mm substrate having thereon a silicon layer (e.g., monocrystalline or polycrystalline), a pad oxide layer (e.g., SiO ), and a nitride layer (e.g., Si 3 N 4 ), is etched in the aforementioned TCP™ 9400 plasma processing system. However, it should be noted that this invention is not limited to a particular sized substrate, or a particular plasma processing system. Furthermore, the exemplary application achieves particularly good results. For example, the smallest trench may have a feature size of less than 0.18 μm and the largest trench may have a feature size of greater than 2.0 μm. Further, the depth of the feature may be between 1500 and 5000 angstroms, and the radius of curvature of the top rounding may be between 100 and 600 angstroms. Further still, substantially equal depths, substantially equal radius of curvatures and substantially equal slopes (e.g., between about 3% to 5%) may be obtained between smaller and larger trenches. In this application, the first portion of the silicon layer is etched, in this example, using an etch process that employs the CH 2 F /HBr-based chemistry. Preferably, the process pressure is selected to help produce a sufficient amount of polymer, which helps to generate the rounded radius at the top of the trench. Therefore, the pressure within the aforementioned plasma processing reactor system is preferably maintained from about 10 milliTorr (mT) to about 70 mT, and more preferably at about 50 mT during silicon etching. The top RF power is preferably maintained from about 400 watts (W) to about 800 W, and more preferably at about 700 W during silicon etching. The bottom electrode power is preferably maintained from about 20 watts (W) to about 80 W, and more preferably at about 40 W during silicon etching. For the most part, a low level of bias power is preferred to ensure some degree of silicon etch rate is maintained while allowing for a sufficient level of passivation to build up at the bottom comers of the side walls. Furthermore, the temperature of the bottom electrode is preferably maintained from about 15 to about 65 degrees Celsius, and more preferably at about 30 degrees Celsius during silicon etching. The flow rate of CH 2 F 2 is preferably from about 20 standard cubic centimeters per minute (seem) to about 100 seem, and more preferably at about 60 seem during etching. Preferably, the flow rate of CH F 2 is between about 50% to 100% of the total flow. The flow rate of HBr is preferably from about 0 standard cubic centimeters per minute (seem) to about 60 seem, and more preferably at about 20 seem during etching. Preferably, the flow rate of CH 2 F 2 is between about 0% to 50% of the total flow. Preferably, the ratio of CH 2 F 2 to HBr is selected to help produce a sufficient amount of polymer during etching, which helps to generate the rounded radius at the top of the trench. Therefore, the ratio of the CH F 2 flow rate to the HBr flow rate is preferably from about 10:1 to about 1 :1 and more preferably at about 3:1. Furthermore, the first etch is allowed to etch for a sufficient amount of time. Preferably, the first silicon etch is conducted for about 10 to about 25 seconds. More preferably, the first silicon etch is conducted for about 15 seconds. The second portion of the silicon layer is etched, in this example, using an etch process that employs the Cl 2 /O 2 -based chemistry. The pressure within the aforementioned plasma processing reactor system is preferably maintained from about 30 milliTorr (mT) to about 80 mT, and more preferably at about 50 mT during silicon etching. The top RF power is preferably maintained from about 200 watts (W) to about 800 W, and more preferably at about 500 W during silicon etching. The bottom electrode power is preferably maintained from about 100 watts (W) to about 300 W, and more preferably at about 250 W during silicon etching. Furthermore, the temperature of the bottom electrode is preferably maintained from about 0 to about 60 degrees Celsius, and more preferably at about 20 degrees Celsius during silicon etching. The flow rate of Cl 2 is preferably from about 50 standard cubic centimeters per minute (seem) to about 180 seem, and more preferably at about 120 seem during etching. Preferably, the flow rate of Cl 2 is between about 90% to 100% of the total flow. The flow rate of O 2 is preferably from about 0 standard cubic centimeters per minute (seem) to about 14 seem, and more preferably at about 7 seem during etching. Preferably, the flow rate of O 2 is between about 0% to 10% of the total flow. The ratio of the O 2 flow rate to the Cl 2 flow rate is preferably from about 5% to 24% and more preferably at about 15%. Furthermore, the etch source gas described above may also include HBr and/or a diluent gas, such as Argon. It should be noted, however, that the flow rates of the component source gases may be scaled as needed when smaller or larger substrates are involved. Thus, while the relative flow rates among the various component gases (which may be expressed as a ratio of the flow rates of the gases or percentage of the total flow rate, for example) are important, the absolute flow rate for each component gas may be modified as needed to accommodate different substrate sizes. In an alternative embodiment, the pad oxide is left on the layer stack prior to using the inventive etch process. That is, the nitride layer is etched down to the pad oxide layer and the resist is stripped from above the nitride layer before using the inventive etch process. In this embodiment, CF 4 is preferably added to the CH 2 F 2 - HBr chemistry before implementing the first silicon etch so that the pad oxide layer may be effectively etched. Once the pad oxide layer is significantly close to the silicon, the CF 4 is removed, and the etch continues with the inventive CH 2 F 2 - HBr chemistry In another alternative embodiment, the CH 2 F is replaced with CHF 3 , CH 3 F, or CH 4 . All three of these gases may produce an effective top rounded attribute. However, they tend to provide less control for the radius of curvature than the preferred embodiment of CH 2 F 2 . By way of example, CHF 3 typically provides a radius of curvature between about 100 angstroms and 400 angstroms. As can be seen from the foregoing, the advantages of the second aspect of the invention are numerous. Different embodiments or implementations may have one or more of the following advantages. One advantage of the invention is that it substantially eliminates sharp comers at the shoulder of a shallow trench isolation feature and forms a top rounded feature that is located at about the interface between the pad oxide and silicon. As a result of eliminating sharp comers and forming top rounded features, the possibilities of gate leakage at the shallow trench isolation features in the semiconductor device are reduced. Another advantage of the invention is that it provides a method for controlling the radius of curvature of the top rounded feature. As a result, process engineers tend to have more process control, and therefore the process engineer may reduce variations that tend to lead to device failure. By way of example, substantially similar radius of curvatures may be obtained between narrow and wide features. More specifically, the invention is capable of providing a radius of curvature difference between about 50 angstroms to about 100 angstroms or even lower. Moreover, for tightly packed features the radius of curvature can be decreased or for features susceptible to gate leakage the radius of curvature can be increased. Additionally, the invention provides a method for creating top rounded shallow trench isolation features with different tools and layers. For example, the inventive etch chemistry provides top rounding without having the presence of a photoresist layer. Importantly, the inventive etch chemistry does not appreciably etch the nitride layer, which is uncommon for most fluorocarbon etch chemistries. This is important because the nitride layer is needed for subsequent processing steps such as chemical-mechanical polishing (CMP). Moreover, a protective passivating film is formed around the top rounded attribute. In this manner, the passivating film acts as a mask that protects the top rounded attribute. Further, the invention reduces the depth difference that is usually encountered between narrow and wide features during shallow trench isolation etching. By way of example, the invention is capable of providing a depth difference between about 2% to about 5% or even lower. Further still, the invention reduces the profile angle difference between narrow and wide features. For example, the invention is capable of providing a profile angle difference between about 3 degrees to about 5 degrees or even lower. As a result of reducing both the depth and profile angle difference, process engineers tend to have more process control, and therefore the process engineer may reduce variations that tend to lead to device failure. While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. By way of example, an oxy-nitride or TEOS layer may be disposed on top of the nitride layer to help protect the nitride layer. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the tme spirit and scope of the present invention.

Claims: What is claimed is: 1. In a plasma processing chamber, a method of etching a trench in a target layer, comprising: flowing a first etchant source gas into the process chamber; forming a first plasma from at least the first etchant source gas; and etching a trench through a first portion of the target layer with the first plasma at a first etch rate and for a period of time, the first etch rate and the period of time being configured to form an effective top-rounded attribute at the shoulder of the trench. 2. The method of claim 1 wherein the target layer is a silicon layer. 3. The method of claim 2 wherein the plasma is substantially selective to silicon. 4. The method of claim 1 wherein the etchant source gas includes a fluorocarbon. 5. The method of claim 4 wherein the etchant source gas includes a hydro-fluorocarbon. 6. The method of claim 5 wherein the etchant source gas includes CHF 3 . 7. The method of claim 5 wherein the etchant source gas includes CH 2 F 2 . 8. The method of claim 1 wherein both narrow and wide trenches are formed in the target layer, and wherein the period of time is sufficiently long to effectively balance etch depths between the narrow and wide trenches. 9. The method of claim 8 wherein the period of time is between about 10 to about 25 seconds. 10. The method of claim 1 wherein a passivating film is formed adjacent to the top-rounded attribute during etching, the passivating film being arranged to substantially protect the top- rounded attribute from subsequent etching processes. 11. The method of claim 1 wherein etching includes simultaneously depositing a protective layer of etch product on the side walls of the trench, and wherein the deposited etch product is of sufficient resiliency to protect the top-rounded attribute during subsequent trench etching. 12. The method of claim 1 further comprising, flowing a second etchant source gas into the plasma processing chamber; forming a second plasma from at least the second etchant source gas; etching through a second portion of the target layer with the second plasma, wherein the etching with the second plasma extends the trench into the target layer without unduly damaging the top rounded attribute. 13. The method of claim 12 wherein the second etchant source gas includes chlorine. 14. The method of claim 12 wherein the second etchant source gas includes oxygen. 15. The method of claim 12 wherein the second etchant source gas includes bromine. 16. The method of claim 12 wherein etching through the second portion of the target layer with the second plasma is conducted at a second etch rate, and wherein an etch rate ratio between the second etch rate and the first etch rate is about 3 to 1. 17. The method of claim 16 wherein the first etch rate is between about 1000 to about 2000 angstroms per minute, and the second etch rate is between about 3000 to about 6000 angstroms per minute. 18. The method of claim 12 wherein the first plasma and the second plasma are formed in a single plasma processing chamber. 19. The method of claim 1 wherein the plasma processing chamber is an inductively- coupled plasma processing chamber, a capacitively coupled plasma processing chamber or a wave excited plasma processing chamber. 20. The method of claim 1 wherein the target layer is disposed below a hard mask layer having a plurality of patterned openings. 21. The method of claim 20 wherein the hard mask is formed from a plurality of layers including a pad oxide layer and a nitride layer, the pad oxide layer being disposed over the target layer, and the nitride layer being disposed over the pad oxide layer. 22. The method of claim 21 wherein the method further includes etching the patterned openings in the hard mask before etching the target layer, the method comprising, flowing a third etchant source gas into the plasma processing chamber; forming a third plasma from at least the third etchant source gas; etching substantially through the nitride layer with the third plasma; and etching through the pad oxide layer with the first plasma. 23. The method of claim 22 wherein the third etchant source gas includes a fluorocarbon. 24. The method of claim 22 wherein the third etchant source gas includes CF . 25. The method of claim 22 wherein the third etchant source gas includes HBr. 26. The method of claim 20 wherein the first plasma, the second plasma and the third plasma are formed in a single plasma processing chamber.

Inventors: MILLER ALAN J, VAHEDI VAHID, MILLER, ALAN, J., VAHEDI, VAHID
Applicants: LAM RES CORP, MILLER ALAN J, VAHEDI VAHID, LAM RESEARCH CORPORATION, MILLER, ALAN, J., VAHEDI, VAHID
Application year: 2000
Main classification: H01L21
IPC codes: H01L21